F_pirzio Tesi Dott

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UNIVERSITA DEGLI STUDI DI PAVIAFACOLTA DI INGEGNERIA

Dottorato Di Ricerca in Ingegneria Elettronica,

Elettrica ed Informatica - XIX CICLO

Picosecond mode-locked laser sources

for fundamental physics investigations

Supervisor:

Prof. A. Agnesi

Ph. D. Thesis

of Federico Pirzio

Anno Accademico 2006


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Contents

Introduction 1

1 Motion Induced Radiation (MIR) experiment 5

1.1 Theoretical situation . . . . . . . . . . . . . . . . . . . . . . . 6

1.2 Experimental approaches . . . . . . . . . . . . . . . . . . . . 7

1.2.1 The mechanical motion approach . . . . . . . . . . . . 7

1.2.2 A novel experimental approach . . . . . . . . . . . . . 8

1.2.3 Experiment feasibility discussion . . . . . . . . . . . . 9

1.3 Layout for the detection of Casimir radiation . . . . . . . . . 11

1.3.1 The laser system - conceptual scheme . . . . . . . . . 12

1.3.2 Preliminary calculations . . . . . . . . . . . . . . . . . 14

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2 High rep-rate passively mode-locked solid state lasers 21

2.1 Review of the theory of picosecond lasers . . . . . . . . . . . 22

2.1.1 SEmiconductor Saturable Absorber Mirrors . . . . . . 25

2.1.2 Critical energy criterion . . . . . . . . . . . . . . . . . 27

2.2 Critical energy reduction by Inverse Saturable Absorption . . 30

2.3 Cavity design of a multi-GHz laser resonator . . . . . . . . . 31

2.3.1 325MHz Nd:YVO4 laser resonator . . . . . . . . . . . 31

2.3.2 720MHz Nd:YVO4 laser resonator . . . . . . . . . . . 32

2.3.3 1.4GHz Nd:GdVO4 laser resonator . . . . . . . . . . . 33

2.3.4 2.6GHz rep-rate Nd:GdVO4 widely tunable laser res-

onator . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

i


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ii Contents

2.3.5 Tunability of the 2.6GHz Nd:GdVO4 with SHG-ISA . 41

2.3.6 Nd:YVO4 based 4.8GHz rep-rate resonator . . . . . . 42

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3 IRENE - A laser source for photoconductivity measure-

ments 53

3.1 State of the art ofJ level picosecond sources . . . . . . . . . 54

3.1.1 Multi-pass amplifiers . . . . . . . . . . . . . . . . . . . 54

3.1.2 Regenerative amplifiers . . . . . . . . . . . . . . . . . 55

3.1.3 Cavity dumping . . . . . . . . . . . . . . . . . . . . . 57

3.1.4 Grazing incidence Nd:YVO4 slab amplifiers . . . . . . 583.2 Laser system setup . . . . . . . . . . . . . . . . . . . . . . . . 59

3.2.1 Overwiev of system functioning . . . . . . . . . . . . . 60

3.2.2 The 1064nm Master Oscillator . . . . . . . . . . . . . 61

3.2.3 The pulse picking stage . . . . . . . . . . . . . . . . . 66

3.2.4 Amplification stage . . . . . . . . . . . . . . . . . . . . 68

3.2.5 The Second Harmonic Generation stage . . . . . . . . 72

3.2.6 The Optical Parametric Generation (OPG) stage . . . 75

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

A Guidelines for a model of a grazing incidence single-pass

QCW amplifier 87

A.1 Grazing incidence single pass amplifier . . . . . . . . . . . . . 87

A.2 1st order consideration while designing a single pass amplifier 90

B Critical parameters for efficient harmonic and parametric

generation 93

B.1 Second harmonic efficient generation in LBO . . . . . . . . . 93

B.2 Optical parametric generation in KTP . . . . . . . . . . . . . 97

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Issues and workshops 103


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List of Figures

1.1 (a) Mirror effective motion: a composite mirror changes its reflec-

tion properties (under intermittent laser light irradiation), and the

microwave reflecting surface switches its position between P1 and

P2 accordingly. (b) Arrangement of the composite mirror in a mi-

crowave resonant cavity. The semiconductor is irradiated by an

optical fiber piercing the cavity . . . . . . . . . . . . . . . . . . . 9

1.2 Detailed experimental setup. There are three main parts: the elec-

tromagnetic cavity already shown in Figure 1.1, the electronic chain

and the laser system. This block diagram displays the interrelations

between laser and radio frequency generator for the control of para-

metric resonance . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.3 Laser system blocks scheme . . . . . . . . . . . . . . . . . . . . 13

2.1 Instantaneous and average laser power versus time for (a) a stable

cw mode-locked laser and for (b) a mode-locked laser exhibiting

large Q-switching instabilities. The average laser power (thick line)

is the same for both lasers . . . . . . . . . . . . . . . . . . . . . 23

2.2 Measured data (filled points) and fitted (solid) curve according to

Eq. (2.8) for the nonlinear reflectivity R(FP,A) of a SESAM as a

function of the pulse energy fluence FP,A = EP/Aeff,A. . . . . . . 26

2.3 Setup of the cavity operating @375MHz . . . . . . . . . . . . . . 32

2.4 Setup of the cavity operating @720MHz . . . . . . . . . . . . . . 32

2.5 Autocorrelation trace of the 720MHz frep laser cavity . . . . . . . 33

2.6 RF spectrum analyzer trace for cw-ML (a) and QML (b) . . 34

iii


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iv List of Figures

2.7 720MHz cavity setup, a picture . . . . . . . . . . . . . . . . . . 34

2.8 Diode pump output power characteristic . . . . . . . . . . . . . . 35

2.9 Layout of the diode-pumped 1.4 and 2.6 GHz Nd:GdVO4 laser . . 36

2.10 Two mirror plano-concave output power versus incident pump power

characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.11 Fresnel loss from the quasi-Brewster interface as a function of the

angle offset. The inclination of the uncoated face yielding exact

Brewster incidence is = 24.52 . . . . . . . . . . . . . . . . . . 38

2.12 Critical output power calculated for cw mode-locking . . . . . . . 39

2.13 Critical Pout calculated for cw mode-locking (a) and waist radii (b)as a function of frep. The waist radii on the SAM (wa) as well as

on the gain medium (wg) are calculated (both for the tangential (t)

and the sagittal (s) planes), near the 2.6GHz edge of the stability

region. Actually, the tangential waist radius within the laser crystal

has to be multiplied by the refractive index n=2.192 . . . . . . . . 40

2.14 Non-collinear background-free second-harmonic autocorrelation and

spectrum of the passively mode-locked laser (inset) . . . . . . . . 41

2.15 Experimental setup for the oscillator operating around 2.5GHz . . 42

2.16 Cavity setup with LBO placed near SAM . . . . . . . . . . . . . 432.17 Autocorrelation trace of the 2.41GHz cw mode-locking pulses emerg-

ing from the cavity of Figure 2.16 . . . . . . . . . . . . . . . . . 43

2.18 Output power versus input current characteristic of the new pump

diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

2.19 Plano-concave Nd:YVO4 output power performances . . . . . . . 46

2.20 Picture of the 4.8GHz oscillator, white continuous line shows the

cavity path, dash line is the output . . . . . . . . . . . . . . . . 46

2.21 cw Mode-Locking oscilloscope traces with long (a) and short

(b) time span . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

2.22 sech2 shaped autocorrelation trace for the 4.74GHz cw mode-locking

laser pulses. The conversion coefficient for the FWHM pulse dura-

tion is 29.4ps/ms which gives a duration of about 9.7ps . . . . . . 47

2.23 RF spectrum of the cw mode-locking . . . . . . . . . . . . . . . . 48


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List of Figures v

3.1 In a multipass amplifier, the beam passes through the gain medium

several times, at a slightly different angle each time . . . . . . . . 54

3.2 A typical regenerative amplifier setup . . . . . . . . . . . . . . . 56

3.3 Nd:YVO4 passively mode-locked cavity dumped oscillator: pulse

extraction is realized through an Electro-Optical Modulator (EOM)

in combination with a Thin Film Polarizer (TFP) . . . . . . . . . 58

3.4 Amplifier module in side-pumped grazing incidence configuration . 59

3.5 Layout of the diode-pumped oscillator-amplifier system. L1, L2,

L3: lenses; PD: photodiode generating the 56-MHz reference clock;

AOPP: acousto-optic pulse-picker; BD: beam dump; LD: quasi-cw

laser diode arrays; HWP: half-wave plate; slabs: Nd:YVO4 graz-

ing incidence high-gain modules; LBO: SHG crystal; HS: harmonic

separator; KTP: OPG crystal . . . . . . . . . . . . . . . . . . . 60

3.6 Temporal operations sequence of the laser system: the timing is

set by the high frequency ( 56 MHz) clock signal provided by themaster oscillator; the low frequency repetition rate is set reducing

or increasing the idle time . . . . . . . . . . . . . . . . . . . . . 61

3.7 Output power characteristic of the diode pump . . . . . . . . . . 62

3.8 Master Oscillator cavity setup: R1=R2=250mm, OC=98%, M1and M2 High Reflectivity plane mirrors . . . . . . . . . . . . . . 62

3.9 Oscilloscope trace of the cw Mode-Locking pulse train . . . . . . . 63

3.10 Output power versus pump current characteristic of the Master

Oscillator; each output beam carries out 50% of the total output

power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.11 Optical spectrum of the cw mode-locking . . . . . . . . . . . . . 64

3.12 SHG non collinear sech2 shaped autocorrelation trace . . . . . . . 65

3.13 Frequency down-scaling stage . . . . . . . . . . . . . . . . . . . 65

3.14 Single pulse selection . . . . . . . . . . . . . . . . . . . . . . . . 67

3.15 Amplification stage setup: a couple of Quasi Continuous-Wave

(QCW) 150W peak power laser diodes pump two slabs of Nd:YVO4;

a collimation lens and Half Wave Plate (HWP) provides the right

polarization and pump spot dimension on the amplifier crystal face 68


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vi List of Figures

3.16 Output energy versus input current caracteristic for the QCW 150W

peak power diodes . . . . . . . . . . . . . . . . . . . . . . . . . 69

3.17 Typical 120s-long pump pulse, a conversion factor of 20A/V gives

a pump current amplitude of 135A . . . . . . . . . . . . . . . 703.18 Background free, non collinear SHG autocorrelation trace of the

amplified pulses . . . . . . . . . . . . . . . . . . . . . . . . . . 71

3.19 Comparison between the seed and an amplified pulse train optical

spectrum; slight narrowing and central wavelength shift for the

amplified pulses can be appreciated . . . . . . . . . . . . . . . . 71

3.20 Critical phase matching of SHG in LBO. The polarization directions

of fundamental () and second-harmonic generated wave (2) are

perpendicular to the beam direction, and to each other, the crystal

is cut with the angle for phase-matching at 1064nm . . . . . . . 72

3.21 Phase-matching angle for critical phase matching of frequency dou-

bling in LBO at room temperature, configuration Ordinary - Ordi-

nary - Extraordinary in the XY plane . . . . . . . . . . . . . . . 74

3.22 SHG stage setup . . . . . . . . . . . . . . . . . . . . . . . . . . 75

3.23 A picture of the second harmonic beam . . . . . . . . . . . . . . 75

3.24 Oscilloscope traces of the undepleted fundamental and second har-

monic pulses, normalized so that the ratio of the peaks corresponds

to the observed conversion efficiency. In case of 2 pulse, also the

adjacent small pulses are strongly depressed by the non-linear process 76

3.25 Type-II phase matching in the XZ plane for KTP crystal. The

polarization directions of pump (p), signal (s) and idler (i) are also

reported . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

3.26 Signal (blue) and idler (green) wavelength as a function of crystal

tilting angle in the vertical plane . . . . . . . . . . . . . . . . . . 77

3.27 Side view of the Optical Paramatric Generation setup . . . . . . . 78

3.28 Spectra of the OPG pulses, obtained at several tuning angles . . . 79

3.29 System setup, see Table 3.2 for legend . . . . . . . . . . . . . . . 81

A.1 Geometry definitions for the side-pumped grazing incidence slab

amplifier medium . . . . . . . . . . . . . . . . . . . . . . . . . . 89


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List of Figures vii

A.2 Fluence of ASE F in normalized with respect to the saturation flu-

ence Fsat as a function of the single pass small signal gain g0 for

an emission solid angle = 4 104sterad. Dashed curve is ob-tained with the approximation F in Fsat, dotted line refers tothe approximation of F in Fsat . . . . . . . . . . . . . . . . . 91

B.1 Walk-off angle as a function of wavelength in LBO . . . . . . . . . 94

B.2 Group Velocity Dispersion in LBO as a function of pump wavelength 95

B.3 Angular acceptance in LBO around 1.064m . . . . . . . . . . . . 96

B.4 Temperature acceptance in LBO around 1.064m . . . . . . . . . 96

B.5 Walk-off angle for signal in KTP as a function of wavelength . . . 98B.6 GVD in KTP for signal with respect to pump (blue curve) and to

idler (green curve) for a signal wavelength ranging from 0.6 to 1 m 99

B.7 Angular acceptance in KTP with respect to signal wavelength in

the range 0.6-1m . . . . . . . . . . . . . . . . . . . . . . . . . 99

B.8 Signal (blue curve) and idler (green curve) spectral bandwidth ver-

sus wavelength in KTP. The signal bandwidth in the range 0.6-

1m is lower than the 1nm maximum resolution of the Ocean Op-

tics USB2000 spectrometer employed the OPG characterization re-

ported in Figure 3.28, as expected by the measurements results . . 100


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viii List of Figures


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List of Tables

2.1 Tunability range of 1.4GHz V-folded cavity . . . . . . . . . . 38

2.2 Results obtained employing LBO . . . . . . . . . . . . . . . . 44

3.1 Constructor specification for the Acousto-Optical Modulator 66

3.2 Legend of Figure 3.29 . . . . . . . . . . . . . . . . . . . . . . 80

A.1 Physical parameters in our working condition (active medium

Nd:YVO4) for a 1st order estimation of g0 . . . . . . . . . . . 92

B.1 Working Conditions: Crystal length l = 1.5cm, = 1.064m 97

ix


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x List of Tables


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Introduction

Since the discovery of the first laser in 1960, this device knew an enor-

mous growth, becoming very important in many industrial applications,

like telecommunication systems, material processing or remote sensing, as

well as in scientific fields like medicine, chemistry and physics.

Laser that employees a crystal, a ceramic or a doped glass as active

medium are named solid state lasers. Thanks to the relative simplicity

of their complete structure and to their limited cost, these lasers cover a

relevant part of the laser source market and play an active role in the devel-

opment of many fields where they found application.

During the 1990s, the impressive increase of telecommunications market

gave incentive to the spread of optical devices and to the improvements

in semiconductor growth technology, allowing a decisive step forward to

semiconductor diode laser performances.

The availability of new compact, reliable and efficient diode pump mod-

ules made easier the rapid growth of DPSSL (Diode Pumped Solid State

Laser) systems, which became more and more competitive with respect to

flash lamp pumped system not only in laboratory research field, but also in

commercial applications.

In parallel with the development in the field of pump sources, also new

semiconductor based devices such as SEmiconductor Saturable Absorber

Mirrors (SESAMs) became available on the market. Since their introduc-

tion, the pulse durations, average powers, pulse energies and pulse repetition

rates of compact ultrafast solid-state lasers have improved by several orders

of magnitude.


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Introduction 3

ability of intense ultrashort-pulse lasers to fabricate microstructures in solid

targets is very promising, and the quality of ablated holes and patterns is

much better using femtosecond or picosecond pulses instead of nanosecond

pulses.

The Laser Source Laboratory of the University of Pavia is a well ex-

perienced group in the development of innovative solutions in the field of

DPSSL. In the last decade our group has accumulated experience in side-

pumping as well as in end-pumping with diode lasers; various aspects of

pumping schemes, resonator modeling, thermal problems, active and pas-

sive Q-switching and mode-locking techniques have been intensively inves-

tigated.

During these three years of my Ph.D. fellowship (2003-2006) I had the

opportunity to be involved in an exciting project in collaboration with the

INFN, MIR (Motion Induced Radiation) experimental team. The object

of the collaboration, that will be explained better in the first Chapter of

this thesis, is the realization of innovative and highly customized picosecond

mode-locked laser sources for both the MIR experiment itself and spectro-

scopic characterization of semiconductor materials.

Along the way to the goal I had the opportunity to explore the limits of

the state of the art regarding this kind of laser systems and to manage witha lot of different topics such as passively mode-locked resonators modeling

and design, picosecond pulse continuous and quasi-continuous-wave amplifi-

cation and non-linear conversion processes, just to mention the mains. There

are many other fundamental aspects of this job, I learnt in these years; they

can not be easily enumerated, and are related to being daily a part of a

research group. As well as the matter itself, surely I found that an exciting

experience.

The content of this thesis work is organized as follows:

in Chapter 1 the Motion Induced Radiation (MIR) experiment is de-

scribed. It concerns the detection of the dynamical Casimir effect, a


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4 Introduction

fundamental physics phenomenon related to point zero energy fluctu-

ations. The experimental setup is currently under development at the

INFN (Istituto Nazionale di Fisica Nucleare) Labs located in Legnaro

(PD). A particular attention is dedicated to the role and the specifica-

tion of the laser system that will be inserted in the experimental setup

and is currently under construction at the Laser Source Laboratory of

the Electronics Department of the University of Pavia. The study and

realization of such a device occupied a relevant part of my research

activity and it is currently not yet concluded;

in Chapter2, after an introduction to the critical design parameters forpassively cw mode-locked high repetition frequency laser sources, the

experimental work and the final cavity design of the master oscillator

for the Casimir experiment laser system is shown. All the experimental

work we carried out in order to achieve the required performances and

the scientific relevant results we obtained are here reported;

in Chapter 3 is described realization, functioning and performances of

IRENE (InfraRed ENergy Emitter), the laser source we realized for the

INFN group of Legnaro in order to investigate the photoconductivity

properties of the semiconductor materials candidate to be inserted in

the Casimir experiment cavity. Since in literature are not present

data about semiconductor mobility and recombination time under

vacuum at cryogenic temperature, the selection of the proper material

to employ in the experiment undergoes to a direct measurements of

such intrinsic properties actually carried out at the INFN national

Labs in Legnaro (PD);

in Appendix A and B an analytical model for the grazing incidence

Quasi-cw single pass amplifier employed in the IRENEs setup and a

brief description of the critical parameters for an efficient non-linear

conversion stage design, are respectively shown.


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Chapter 1

Motion Induced Radiation

(MIR) experiment

For any quantum field, the vacuum is defined as its ground state. Differently

than in the classic case, this ground state, due to the uncertainty principle, is

not empty, but filled with field fluctuations around a zero mean value. More-

over this vacuum state depends on the field boundary conditions: if they

change, there will be a correspondingly different vacuum (whose fluctua-

tions will have a different wavelength spectrum). Thus a quantum vacuum

state may be equivalent to real particles of a new vacuum after a change in

boundary conditions. If we consider the electromagnetic field, the peculiar

nature of the quantum vacuum has experimentally observable consequences

in the realm of microscopic physics, such as natural widths of spectral lines,

Lamb shift, anomalous magnetic moment of the electron and many more.

It is perhaps even more striking that there exist also observable effects at

a macroscopic level. The Casimir force (static Casimir effect[1][2]) is one

of these macroscopic effects which has been observed experimentally. A

dynamic Casimir effect is also predicted to occur when one boundary is

accelerated in a nonuniform way, as, for instance, when a metal surface un-

dergoes harmonic oscillations. In this case a number of virtual photons from

the vacuum are converted into real photons (Casimir radiation), while the

moving metal surface loses energy[3][4].


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6 Motion Induced Radiation (MIR) experiment

It is worth notice that, whereas the static Casimir effect has been ob-

served by several experiments[5][6], the Casimir radiation is to date unob-

served, in spite of the abundant theoretical work done in this field[7][8][9]

(see [8] for a historical review and a bibliography of the relevant studies).

1.1 Theoretical situation

The simplest system that can produce Casimir radiation is a single mirror,

harmonically oscillating in a direction perpendicular to its surface. In this

case the number N of created photons should be[10]:

N =t

2

vc

2(1.1)

where:

is the angular frequency of the mirror motion;

t is the duration of the motion;

v is the maximum speed reached in the oscillation;

c is the speed of light.

Even if we stretch all parameters to their utmost values ( 1010rad/s,t 1s and v/c 108), the number of produced photons is not detectable.

A great theoretical progress was to realize that when the oscillating mir-

ror is a wall of an electromagnetic resonant cavity, the cavity itself behaves

as a multiplier for the produced radiation if the frequency of the moving

wall is twice one of the proper electromagnetic cavity frequencies (paramet-

ric resonance). It is however disappointing that the formulae developed so

far using different approaches (in the case of parametric resonance) are not

the same and even irreconcilable. Apart from minor differences, the for-

mulae for the produced photons found in literature[8][9][11] can be brought

back to either of two forms:

N =t

2

vc

2Q (1.2)


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1.2 Experimental approaches 7

sinh2 t vc (1.3)where Q is the quality factor of the cavity.

1.2 Experimental approaches

One possible experimental solution for detection of the Dynamical Casimir

Effect is based on the mechanical motion of a resonant cavity wall. We will

now show that this approach is nowadays impracticable.

1.2.1 The mechanical motion approach

The highest frequency attainable for mechanical motion is in the gigahertz

range[12] and following the parametric amplification request this implies

microwave cavities with dimensions ranging from 1cm to 1m. The motion

of a single wall of such a cavity requires a huge amount of power. In fact a

wall of volume V, made of a material with mass density , vibrating at an

angular frequency 0, with an amplitude x, has a maximum kinetic energy

E =1

2V 20x

2 which vanishes in a time of order2

0. If we estimate the

required power for = 3

103kg/m3, V = 3cm

3cm

0.1mm = 9

108m3,02

= 2GHz, x = 1nm, we obtain about 3 108W.At present there are two known ways to make a body oscillate at gi-

gahertz frequencies and both of them have some disadvantages precluding

their use in a dynamic Casimir experiment.

The first way would exploit acoustic waves in solids. Waves at gigahertz

frequencies were produced in the 60s by Bommel and Dransfeld in a quartz

rod placed inside a microwave resonant cavity[13]. What makes this tech-

nique ineffective for our purpose is that a large microwave power is needed

and that the rod motion has a maximum displacement x much less than

1nm. A small amplitude x implies a small maximum oscillation speed v (for

a harmonic motion v = 0x where 0 is the oscillation angular frequency).

Hence the number of photons produced by a mechanical oscillation with

such a speed would be undetectable, as is readily seen from eq.(1.2) and

(1.3).


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The second technique is the one applied in acoustic microscopes[12]. A

resonant vibrating mode in a sapphire block is excited at a typical frequency

around 3GHz. The use of a mechanical system with high quality factor Q

reduces power requests in this case. For sapphire at a temperature of 4.3K

the product of the cavity quality factor Q by the oscillation frequency f is

about Qf 1014Hz[14]. Therefore if f 109Hz, Q can be as high as 105.The same oscillation amplitude as in a nonresonant system can be reached

with a power 105 times smaller. But again the oscillation amplitude x is

about 1010m and the moved area is quite small (about 100m2).

1.2.2 A novel experimental approach

Another possible experimental approach is to realize an oscillating mirror

without mechanical methods. The notion of using laser pulses to quickly

change the dielectric properties of a semiconductor can be found in litera-

ture. In 1989 Yablonovitch[15] proposed the use of laser pulses to change the

refraction index of a semiconductor very rapidly. Another work by Lozovik,

Tsvetus and Vinograd[16] studied the parametric excitation of electromag-

netic waves using a dense plasma layer in a cavity; the layer was created by

irradiating a semiconductor film with femtosecond laser pulses.

In the MIR experimental scheme mirror motion is simulated by changing

the actively reflecting surface of a composite mirror. The mirror consists of

a metal plate with a semiconductor wafer fixed on one side (see Figure

1.1 (a)). The semiconductor reflectivity is driven by irradiation from laser

light, with photon energy corresponding to the semiconductor energy gap,

so that it can switch from completely transparent to completely reflective for

microwaves. By sending a train of laser pulses at a given frequency we get

a mirror oscillating from position P1 to position P2. An advantage of this

method is that the distance between P1 and P2 can be made of the order of

a millimeter, compared to about 1nm obtainable by mechanical oscillations.

This leads to a layout as represented in Figure 1.1(b). The composite

mirror becomes a wall of a superconducting cavity. The laser pulses are

guided into the cavity via an optical fiber. A small pickup antenna is also


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1.2 Experimental approaches 9

Figure 1.1: (a) Mirror effective motion: a composite mirror changes its reflec-

tion properties (under intermittent laser light irradiation), and the

microwave reflecting surface switches its position between P1 and P2

accordingly. (b) Arrangement of the composite mirror in a microwave

resonant cavity. The semiconductor is irradiated by an optical fiber

piercing the cavity

introduced in the cavity and the signal fed to high sensitivity electronics.

1.2.3 Experiment feasibility discussion

A number of points need to be checked in order to state that the method

shown could be effective:

1. Is the mirror created in P2 as good as the one in P1?

2. Is the Q of the cavity influenced by the presence of the semiconductor?

3. Is the sensitivity of the pickup electronics good enough to detect the

predicted number of created photons?

4. Is it actually feasible to make the mirror appear and disappear in P2

at gigahertz frequencies?


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Experiments carried out at the INFN Legnaro (PD) National Labs gave

an answer to the first three questions.

1. Inserting a semiconductor layer in a waveguide and measuring the

reflected and the transmitted power under laser irradiation, it was

shown that the semiconductor can reflect microwaves as effectively as

copper. This test yields also another important parameter, that is

the laser power needed to make a good mirror. This question arises

from the fact that one needs to build a plasma of thickness equal to

at least three skin depths (for the given microwave frequency) in order

that it may be fully reflective. The energy needed was estimated tobe approximatively 1J/cm2 per pulse in the microwave range[17].

2. Measurements of the Q value of a niobium cavity brought to 4.6K,

were performed. Determining the decay time of the loaded cavity, a

value of Q 2 106 was obtained. Once the semiconductor wafer wasinserted in the cavity no difference in the decay time (hence in Q) was

detected.

3. In order to answer question (3) a complete electronic chain was con-

nected to the pickup antenna inserted in the cryogenic cavity. Thefirst amplification stage was placed near the cavity at liquid helium

temperature[19]. The cavity was then loaded with microwave pulses

of decreasing power in order that the minimum detectable signal could

be reached. The minimum signal detected had an energy of 0.1eV, cor-

responding to about 104 microwave (2.5GHz) photons. By taking 100

measurements one arrives at 103 photons. Further improvements in

the electronic chain should allow to detect even feebler signals.

4. The answer to question (4) can b e found in literature[18]. The mir-

ror appearance at P1 is fast enough for gigahertz frequencies, since the

transition time of the electrons is some femtoseconds, so that the dom-

inant factor is the rise time of the laser pulse, which is in the hands of

the experimenter. However the disappearance of the mirror depends

on the recombination time of the electrons, which is a property of the


22/116

1.3 Layout for the detection of Casimir radiation 11

semiconductor only. If one uses semi-intrinsic semiconductors one can

obtain recombination times as low as 510ps[18].

One important expect is the determination of the semiconductor to em-

ploy as vibrating mirror. Since its recombination time is an important

issue for the experiment feasibility, this property has to be measure in order

to determine the best material. These measurements are carried out at the

Legnaro INFN National Labs and in Chapter 3 the laser system we realized

for the experiments is described.

1.3 Layout for the detection of Casimir radiation

On the basis of these results a general layout for the detection of Casimir

radiation is shown in Figure 1.2. A niobium cavity at cryogenic tempera-

ture is placed in a vacuum vessel. A cryogenic amplifier is connected by a

transmission line to an inductive pickup loop coupled with the cavity in crit-

ical matching. A directional coupler is inserted between the cavity and the

cryogenic amplifier to enable measurements of the resonance cavity reflec-

tion coefficient and calibration of the electronic chain. The signal output by

the cryogenic amplifier is further amplified at room temperature, then pro-

cessed by a superheterodyne receiver and eventually integrated over time.

The laser light carried by the optical fiber is tuned in the near infrared and

modulated in amplitude at a frequency exactly double the cavity resonance

frequency. The generator drives a frequency doubler whose output turns to

a low power laser master oscillator. The master oscillator yields a continu-

ous signal from which a pulse picker selects the number of pulses required

in each excitation stage. The total energy stored in the laser is limited, so

must be the number of available pulses. The present estimate is between

103 and 104 pulses for each run.

This experimental setup leaves open the possibility of changing many

configuration parameters to help distinguishing real from spurious signals.

The master laser frequency can be changed and thus the oscillation mirror

frequency to slightly detune the parametric resonance condition. Also the

cavity temperature can be varied in order to study possible contributions


23/116


Figure 1.2: Detailed experimental setup. There are three main parts: the electro-

magnetic cavity already shown in Figure 1.1, the electronic chain and

the laser system. This block diagram displays the interrelations be-

tween laser and radio frequency generator for the control of parametricresonance

from thermal radiation. Mirrors made with different semiconductor samples

and with different thickness can be tried.

1.3.1 The laser system - conceptual scheme

In Figure 1.3 is represented the conceptual scheme of the laser system in-

serted in the experimental setup described in Figure 1.2.

Master Oscillator: it should provide a low energy pulsed laser

beam with a repetition frequency slightly tunable around the para-

metric resonance of the microwave cavity ( 5GHz with the actualcavity geometry) and a pulse duration less than 20ps. The laser source

consists in a cw-ModeLocked oscillator operating at 1064nm.


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AMPLIFICATION STAGES OPTICAL FREQUENCY

DOUBLING STAGE

OUTPUT

10100mJ PULSE BURST @780820nm

MASTER OSCILLATOR

5 GHz

HIHG REPRATE LOW POWER PULSE TRAIN @1064nm

NON LINEAR WAVELENGTH

CONVERSION STAGE

SELECTOR

PULSE BURST

100010000PULSEsBURSTLOWENERGY

Figure 1.3: Laser system blocks scheme

Pulse Burst Selector: from the continuous pulse train, burst

made by 103-104 single pulses should be picked up. Within the single

burst the pulses maintains the same time spacing given by the mas-ter oscillator. The selector relies on an Acousto-Optical Modulator

(AOM).

Amplification Stages: the low energy single burst is amplified in a

double stages amplifier. The first is a diode pumped pre-amplification

stage, the second a flash lamp pumped power amplifier. An energy

level of about 50-500J per single pulse (hence a total energy of 50-

500mJ for a burst made by 103 single pulses) is expected.

Optical Frequency Doubling Stage: the amplified laser beamat a wavelength of 1064nm is then frequency doubled in order to pump

an optical parametric generation stage.

Nonlinear Wavelength Conversion Stage: an optical paramet-

ric generator provides the output at the desired wavelength of 780-


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820nm, selected in function of the semiconductor material deposed on

the vibrating microwave cavity wall. A total energy ranging from 10

to 100mJ per burst is expected.

1.3.2 Preliminary calculations

In order to understand if this scheme leads to observable results it is nec-

essary to insert real numbers in the theoretical formulae and compare the

predicted number of photons with the apparatus sensitivity. Several phys-

ical parameters are essentially already chosen, since a niobium cavity and

an electronic chain have been used satisfactorily in the tests carried on to

answer questions (3) and (4). The niobium cavity has transverse dimensions

of 71mm and 22mm, and length x = 110mm. The cavity mode chosen was

TE101 with eigenfrequency around 2.5GHz. The semiconductor was GaAs

with thickness 2x = 0.6mm. The excitation time duration for a single run, at

5GHz, according to the number of pulses, can be between 0.2-2s. Typically

a run can be repeated after a few seconds.

The following data can be used to estimate the number of photons pro-

duced by dynamic Casimir effect:

t = 106s;

02

= 2.5 109s1;

v

c=

x

x=

0.3mm

110mm= 3 103;

Q = 2 106

.

With formula (1.2), which is the more pessimistic, a number of 4 104microwave photons, well above apparatus sensitivity, turns out.

A good knowledge of quantum vacuum is of great importance in cosmol-

ogy, both to the recurrent question of Einsteins cosmological constant[20],


26/116


with its significance to the dark matter problem; and to the critical question

of the birth of density inhomogeneities, ancestors of galaxies, from inflated

quantum vacuum fluctuations[21]. Moreover a sound grasp of quantum vac-

uum dynamics is crucial in understanding some issues on the nature of quan-

tum particles and on the relationships among vacuum noise, the concepts of

information and entropy, and gravitation[21].


27/116



28/116

Bibliography

[1] H. B. G. Casimir, D. Polder, The Influence of Retardation on the

London-van der Waals Forces, Phys. Rev. 73, 360 (1948)

[2] M. Bordag, U. Mohideen, V. M. Mostepanenko, New develop-

ments in the Casimir effect, Phys. Rep. 353, 1 (2001)

[3] G. T. Moore, Quantum theory of the electromagnetic field in a

variable-length one-dimensional cavity, J. Math. Phys. 9, 2679 (1970)

[4] S. A. Fulling, P. C. W. Davies, Radiation from a moving mirror

in two dimensional space-time - Conformal anomaly, Proc. R. Soc.

London A 348, 393 (1976)

[5] S. K. Lamoreaux, Demonstration of the Casimir Force in the 0.6 to

6 m Range, Phys. Rev. Lett. 78, 5 (1997)

[6] G. Bressi, G. Carugno, R. Onofrio, G. Ruoso, Measurement

of the Casimir Force between Parallel Metallic Surfaces, Phys. Rev.

Lett. 88, 041804 (2002)

[7] A. Lambrecht, M. T. Jaekel, S. Reynaud, Motion Induced Radi-

ation from a Vibrating Cavity, Phys. Rev. Lett. 77, 615 (1996)

[8] V. V. Dodonov, Modern Nonlinear Optics, edited by M.W. Evans,Adv. Chem. Phys. Ser. Vol. 119, p. 309 (Wiley, New York, 2001)

[9] M. Crocce, D. A. R. Dalvit, F.D. Mazzitelli, Resonant photon

creation in a three-dimensional oscillating cavity, Phys. Rev. A 64,

013808 (2001)


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[10] M. T. Jaekel, A. Lambrecht, S. Reynaud, Relativity of Motion

in Quantum Vacuum, Proceedings of the Ninth Marcel Gross-

mann Meeting, edited by V.G. Gurzadyan, R.T. Jantzen and

R. Ruffini, p. 1447 (World Scientific, 2002)

[11] G. Schaller, R. Schutzhold, G. Plunien, G. Soff, Dynamical

Casimir effect in a leaky cavity at finite temperature, Phys. Rev. A

66, 023812 (2002)

[12] Z. Yu, S. Boseck, Scanning acoustic microscopy and its applications

to material characterization, Rev. Mod. Phys. 67, 863 (1995)

[13] H. E. Bommel, K. Dransfeld, Excitation and Attenuation of Hy-

personic Waves in Quartz, Phys. Rev. 117, 1245 (1960)

[14] V. B. Braginsky, C. M. Caves, K. S. Thorne, Laboratory experi-

ments to test relativistic gravity, Phys. Rev. D 15, 2047 (1977)

[15] E. Yablonovitch, Accelerating reference frame for electromagnetic

waves in a rapidly growing plasma: Unruh-Davies-Fulling-DeWitt radi-

ation and the nonadiabatic Casimir effect, Phys. Rev. Lett. 62, pp.

1742-1745 (1989).

[16] Yu. E. Lozovik, V. G. Tsvetus, E. A. Vinograd, Parametric

excitation of vacuum by use of femtosecond laser pulses, JETP Lett.

61, 723 (1995)

[17] C. Braggio, G. Bressi, G. Carugno, A. Lombardi, A. Palmieri,

G. Ruoso, D. Zanello, Semiconductor microwave mirror for a mea-

surement of the dynamical Casimir effect, Rev. of Sci. Instr. 75,

4967 (2004)

[18] J. Mangeney, N. Stelmakh, F. Aniel, P. Boucaud, J.-M. Lour-

tioz, Temperature dependence of the absorption saturation relaxation

time in light- and heavy-ion-irradiated bulk GaAs, Appl. Phys. Lett.

80, 4711 (2002)


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Bibliography 19

[19] R. F. Bradley, Cryogenic, low-noise, balanced amplifiers for the

300-1200MHz band using hetreostructure field-effect transistors, Nucl.

Phys. B (Proc. Suppl.), 72 137 (1999)

[20] M. Fukugita, The Dark Side, Nature 422, 489 (2003)

[21] P. C. W. Davies, Quantum vacuum noise in physics and cosmology,

Chaos 11, 539 (2001)


31/116

20 Bibliography


32/116

Chapter 2

High rep-rate passively

mode-locked solid state lasers

Diodepumped, iondoped solidstate lasers are well known for their poten-

tial to deliver highpower modelocked pulse trains in diffractionlimited

beams[1][2]. They feature efficient, robust, compact and reliable opera-

tion. In highspeed electrooptic sampling[3][4], photonic switching or op-

tical clocking [5][6], highcapacity telecommunication systems or free space

data links[7] and timeresolved ultrafast spectroscopy[8], highrepetition

rate pulse generating lasers are a desirable tool. Even in electron accel-

erators, high repetition rate lasers are used to generate polarized electron

beams[9]. Although the field of current and potential applications is rather

diversified, laser sources for these applications have to meet common require-

ments: compact, reliable and efficient lasing operation is a key goal. Wave-

length tunability and/or phase locking to an external microwave reference

source is often desired. Low phase and amplitude noise are therefore another

musthave. Depending on the specific type of application, multigigahertz

pulse trains with average output powers between tens of milliwatts to sev-

eral watts, delivered in a diffractionlimited beam, are required.

In high repetition rate applications, passive mode-locking is preferred

against active mode-locking, because potentially shorter pulses and thus a

higher extinction ratio between the pulses can be achieved, besides the fact


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22 High rep-rate passively mode-locked solid state lasers

that the modulators add costs and complexity to the setup and limit the

maximum achievable cavity compactness.

Short cavities need to be built in order to generate high repetition rate

(frep) pulse trains, since the round-trip time TR =1

frepfixes the time spacing

between two consecutive pulses. Besides mechanical and geometrical prob-

lems which can arise from building very small laser cavities, Qswitched

mode locking (QML) becomes the main problem. QML means that the out-

put pulse train consists of pulses of different energies instead of pulses that

all have the same energy. An unwanted regime of operation for most applica-

tions of course. Because of their typically low emission crosssections (com-

pared to semiconductor gain media, for example), passively modelocked

iondoped solidstate lasers show an increased tendency for Qswitching

instabilities when the repetition rate is increased.

We will now briefly review the theory describing the transition from

Qswitched mode-locking to continuous wave mode-locking, in order to

give some practical design parameters for passively mode-locked picosecond

lasers.

2.1 Review of the theory of picosecond lasers

Figure 2.1 illustrates qualitatively the two laser-operation regimes of inter-

est. The instantaneous laser power is shown versus time. In the cw mode-

locking regime (Figure 2.1(a)) the laser generates a train of mode-locked

pulses with high amplitude stability, while Q-Switching-Mode-Locking (Fig-

ure 2.1(b)) means that the pulse energy is modulated with a strongly peaked

Q-switching envelope. To derive a stability criterion against QML we start

from the rate equations for the intracavity power, gain, and saturable ab-

sorption. We call stable the operating conditions in which the relaxationoscillations are damped.

The rate equations for the mode-locked laser can be written as[10]:

dP

dt=

g l qP(EP)TR

P (2.1)


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2.1 Review of the theory of picosecond lasers 23

Figure 2.1: Instantaneous and average laser power versus time for (a) a stable

cw mode-locked laser and for (b) a mode-locked laser exhibiting large

Q-switching instabilities. The average laser power (thick line) is the

same for both lasers

dg

dt= g g0

L P

Esat,Lg (2.2)

dq

dt= q q0

A P

Esat,Aq (2.3)

where:

P is the average intracavity laser power;

TR = 1/frep is the cavity round-trip time;

EP = P TR is the mode-locked intracavity pulse energy;

g is the time-dependent round-trip power gain and g0 the correspon-

dent value for P = 0;

qis the time-dependent round-trip saturable absorption coefficient and

q0 the correspondent value for P = 0;

l is the linear loss per round-trip;


35/116


L,A are the upper-state lifetime of laser medium and absorber recovery

time respectively;

Esat,L = Fsat,L Aeff,L is the saturation energy of the gain, whichis defined as the product of saturation fluence Fsat,L =

h

mand the

effective laser mode area inside the active medium Aeff,L = w2L, with

m the number of passes through the gain element per cavity round trip;

Esat,A = Fsat,A Aeff,A is the absorber saturation energy and is de-fined by the product of absorber saturation fluence Fsat,A and effective

laser mode area on the saturable absorber Aeff,A

= w2A

. Fsat,A

cor-

responds to the pulse fluence that is necessary to bleach the saturable

absorption to 1/e of its maximum amount q0.

It is worth notice that while eq. (2.1) and eq. (2.2) describes long time

scale phenomena (many round-trip), eq. (2.3) has to be solved in the laser

pulse time scale.

qP(EP) in eq. (2.1) represents the round-trip loss in average laser power

(or pulse energy) introduced by the saturable absorber for a given intra-

cavity pulse energy. In our conditions we can make two assumptions to to

determine qP. First, we have a slow absorber, i.e., the duration P of themode-locked pulses is shorter than the absorber recovery time A, although

the results remain valid even for A P[10]. Second, A is much shorterthan the cavity round-trip time TR. With these assumptions we can neglect

the relaxation term in Eq. (2.3) during the time necessary for a mode-locked

pulse to pass the saturable absorber and we can assume that the absorber

is always fully recovered before it is hit by the next pulse. Then, for qP(EP)

we obtain:

qP

(EP

) = q0

Fsat,AAeff,A

EP 1 expEP

Fsat,AAeff,A (2.4)Therefore we can now describe the mode-locked laser by the following two

coupled rate equations:

TRdEP

dt= [g l qP(EP)]EP (2.5)


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dg

dt=

g g0L

EP

Esat,LTRg (2.6)

By linearizing these equations for small deviations EP and g from the

steady-state values EP and g we obtain the criterion for stability against

QML:

EP

dqPdEPEP

Fsat,LFsat,AR Aeff,LAeff,A (2.14)P > frep

Fsat,LAeff,LFsat,AAeff,AR (2.15)

With respect to the experimental verification of the theory, it is helpful

to introduce the QML parameter relating Esat,L, Esat,A, R, because it

contains all the parameters that determine the laser dynamics. We then

define the critical intracavity pulse energy EP,c as the square root of the

QML parameter:

EP,c = (Esat,LEsat,AR)1/2

This is the minimum intracavity pulse energy which is required for ob-

taining stable cw mode-locking; i.e., for EP > EP,c we obtain stable cw

mode-locking, and for EP < EP,c we have to expect QML instability. Note

that, if we neglect the lifetime dependent term in relation (2.11) and set

the bracketed term in eq. (2.9) as 1, both approximations lead to a slightly

stricter stability criterion: a laser fulfilling the stability condition with these

approximations will always fulfill the exact condition.

As can be seen from eq. (2.15), the intracavity power required for sta-

ble cw mode-locking regime increases linearly with the laser repetition fre-

quency. At higher pulse repetition rates the tendency for QML will increase.

In addition, for very short laser cavities we also have to take into account

the tendency for pure Q-switching[11], which is negligible in 100-MHz-type

laser oscillators with a saturable absorber recovery time A TR. Sincethe pulse energy scales inversely with the pulse repetition rate (for funda-

mental mode locking and a given intracavity power), a pulse repetition rate

that is ten times higher requires an intracavity laser power that is ten times

higher, if we leave the QML parameter fixed. At the same pump level, the

intracavity power can be increased with reduced output coupling, but only

at the expense of efficiency and output power.

We can reduce the absorber modulation depth R by using a thinner

absorber layer. However, this leads to longer pulses and to a weaker self-

starting tendency of the mode-locking process, neglecting the fact that by

now SESAM with R < 0.7% are not commercially available. Tighter


40/116


focusing onto the SESAM reduces the absorber saturation energy since,

Esat,A = Aeff,AFsat,A. The tradeoffs are that operation of the laser at pulse

energies far above the absorber saturation energy can lead to pulse breakup

or can damage the device.

Another variable that affects the QML parameter is the gain saturation

energy. Esat,L is determined by the gain cross section of the laser material,

the laser mode area inside the gain medium, and the type of laser resonator.

To minimize the gain saturation energy it is desirable to use a laser ma-

terial with a large gain cross section, e.g., Nd:YVO4, or Nd:GdVO4 rather

than Nd:YAG or Nd:YLF. Broadband gain media, suitable for subpicosec-

ond pulse generation, usually have low emission cross sections (with the

exception of Ti:sapphire) and thus have a stronger tendency for QML.

The laser mode area Ap inside the gain medium should be chosen relying

in the following considerations. In order to assure an efficient pump absorp-

tion, crystal length lc should be longer than the pump absorption depth; we

can assume lc 2p

, where p is the active medium absorption coefficient at

the pump wavelength p. Efficient pump absorption requires a pump waist

rayleigh range zRp W2p np

pM2pcomparable with the crystal length. As can be

seen zRp depends on the pump focusing (Wp), on the refractive index of theactive medium at the pump wavelength (np) and on the beam quality fac-

tor of the pump beam M2p which roughly increases proportionally to pump

power. Hence we can finally find that:

Ap W2p 2pM

2p

pnp

with the constrain that in order to suppress higher-order transverse modes

oscillation and to achieve maximum efficiency, wL Wp.

Therefore QML can be more difficult to suppress in lasers pumped by

high-power laser diodes (with their poor beam quality), while the use of

highly doped gain media can be advantageous because the reduced absorp-

tion length allows for tighter focusing.

When the stability condition given by eq. (2.15) is not achievable in

practice, it is necessary to investigate some method to obtain a reduction


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in critical energy. One possible strategy is to introduce inside laser cavity

some element capable to produce Inverse Saturable Absorption (ISA).

2.2 Critical energy reduction by Inverse Saturable

Absorption

As shown in [14], the critical energy required for starting cw mode-locking

can be reduced significantly in the presence of inverse saturable absorption

such as two-photon absorption, free-carrier absorption, and Second Har-

monic Generation (SHG). Dependending on the working conditions (solitonor non-soliton mode-locking regime, slow or fast saturable absorber) the

amount of reduction in critical energy is different. In the case of our in-

terest (slow absorber in non-soliton regime), eq. (2.13) in presence of ISA

becomes:

Ec =

Esat,AR2+

1

Esat,L

(2.16)

where, in case of Second Harmonic Generation induced ISA[15]:

=42Z0(deffL)2

3n3ASHG(2.17)

with:

Z0 = 377 is the vacuum impedance;

deff is the non-linear effective coefficient of the SHG crystal;

L is the minimum between the length of the crystal and the Rayleigh

range of the focused laser beam;

ASHG is the beam area in the SHG crystal;

is the pulse duration;

n and have the usual meaning.


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2.3 Cavity design of a multi-GHz laser resonator 31

If >1

EsatLsuppression of QML b ecomes relatively independent of the

properties of the gain medium. Independence of the gain medium will allow

suppression of QML in lasers with very large Esat,L, i.e. lasers with large

mode volumes such as diode-pumped high-powered lasers, lasers with a low

gain cross-section that is related to a long upper-state lifetime, as shown in

[15][16].

Both the direct nonlinear loss owing to SHG, as well as the gain reduction

due to self-phase modulation SPM contribute to the effective stabilization

of the cw mode locking while providing the minimum pulse duration allowed

by the passive mode locking alone[15][19][20].

Also, lasers with a limited intracavity pulse energy, which hardly reaches

the critical value Ec, as is the case for high-repetition-rate lasers, may greatly

benefit from ISA induced critical energy limiting.

2.3 Cavity design of a multi-GHz laser resonator

Since our workgroup had no experience before in design GHz repetition rate

passively mode-locked resonators, our strategy was to procede step by step

from hundred MHz regime, up to thousand MHz laser sources. In this pathwe tested many different laser cavity before determining our way to high

rep-rates. Ill discuss now of the principals steps we made.

2.3.1 325MHz Nd:YVO4 laser resonator

At first we set up a laser resonator employing an 1% doped, 3mm long a-

cut Nd:YVO4 crystal, pumped by a 750mW maximum output power laser

diode emitting around 807nm. The cavity scheme is shown in Figure 2.3.

The length L1 + L2

280mm while L3

100, hence the cw mode-locking

repetition frequency was 325MHz. The ABCD simulation of the TEM00intracavity resonating mode gives the following dimensions for the funda-

mental mode inside the active medium and over the saturable absorber mir-

ror: wL 180m, wA 60m. Employing the R = 1% saturable lossesmirror, with the saturation fluence given by the constructor, eq. (2.13) gives


43/116


OC98,4%

R1=150mm

SAM 1% /4%LBO

Nd:YVO 4PUMP L1

L2

L3

Figure 2.3: Setup of the cavity operating @375MHz

a critical energy Ec 30nJ per pulse.Without employing SHG-ISA method, we were not able to obtain stable

cw mode-locking. Hence we introduced a 15mm long LBO crystal near the

SAM, as shown in Figure 2.3. Opportunely tilting the LBO crystal out

of phase matching we obtain stable and self-starting cw mode-locking with

an intracavity pulse energy Eic 10nJ (significantly lower than the limitpredicted without SHG ISA) and average output power of 120mW (60mWfor each of the two laser cavity outputs).

We also tested the laser resonator with a R = 4% SAM, which further

increases of a factor 2 the critical intracavity energy. In these conditions we

experimented cw mode-locking with Eic 5nJ (more than a factor 10 belowthe critical energy) and an average output power of about 72mW (36mWfor each harm).

2.3.2 720MHz Nd:YVO4 laser resonator

OC98,4%

R1=80mm

SAM 1%LBO

Nd:YVO 4PUMP L1

L2

L3

Figure 2.4: Setup of the cavity operating @720MHz


44/116


Subsequently we scaled up the resonator repetition frequency oppor-

tunely reducing the optical cavity length and consequently reducing the

folding mirrors radius of curvature. With the Z-folded cavity shown in Fig-

ure 2.4 we obtained stable cw mode-locking at 720MHz.

The new simulated TEM00 mode dimensions inside active medium an

SAM were respectively wL 160m and wA 50m, also in these condi-tions we had to employ LBO SHG-ISA in order to avoid QML instabilities.

The average output power was 160mW, 80mW for each output.

Figure 2.5: Autocorrelation trace of the 720MHz frep laser cavity

In Figure 2.5 is shown the autocorrelation trace of the output pulses.

We measured a FWHM duration p 6.6ps with a time-bandwidth product 0.43, tipical of this kind of resonators in which the active mediumis placed on one end of the cavity.

We report in Figure 2.6(a) and Figure 2.6(b) respectively the radiofre-

quency spectrum analyzer trace in case of cw mode-locking and QML regime.

A picture of the cavity setup is shown in Figure 2.7.

2.3.3 1.4GHz Nd:GdVO4 laser resonator

The Z-folded cavity scheme employed for our resonators operating at 325 and

720MHz pulse repetition rates was not further scalable for shorter cavities,


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(a) Cw-Ml (b) Qs-Ml

Figure 2.6: RF spectrum analyzer trace for cw-ML (a) and QML (b)

Figure 2.7: 720MHz cavity setup, a picture

due to mechanical limitations. Therefore we decided for a V-folded cavity

employing a 1% doped, plane-brewster vanadate laser crystal, coated on one

side AR at 808nm and HR at 1063nm, whereas the second face was cut with

a slight offset from the Brewster angle. The available a-cut Nd:GdVO4

laser crystal was already investigated as a promising candidate for high

repetition-rate sources[17], owing to its superior thermal conductivity that

allows for power up-scaling, while the absorption peak is broader than that

of Nd:YVO4 and is especially attractive for diode-pumping.

The pump was a readily available 1W, 100m single emitter diode laser


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tuned at 808nm, beam-shaped for end-pumping with aspheric lenses and an

anamorphic prism pair. The pump spot radius was measured to be 36m.

The output power versus current characteristic is shown in Figure 2.8.

400 600 800 1000 12000

100

200

300

400

500

600

700

Ial (mA)

Pout(mW)

Figure 2.8: Diode pump output power characteristic

Laser resonator design criteria

Eq. (2.13) suggests straightforward criteria for achieving cw mode-locking[21]:

1. tightly focus the resonant mode both in the laser crystal and on the

SAM device;

2. use small R;

3. use a lowloss oscillator with high intracavity power (and pulse energy).

The smallest modulation depth R offered with commercial devices is

generally around 1% and cannot be reasonably reduced to a small frac-

tion of such a value without accepting large variations in production runs

and significantly increased costs. The ultimate low-loss oscillator for such

an application should employ only high reflectivity (HR) mirrors and no

anti-reflection (AR) optics, since these kind of coatings always brings in a

non-vanishing Fresnel loss of 0.1% - 0.2% per pass, of the order of the out-

put coupling that is generally tolerated, which is also comparable to the


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non-saturable losses of the SAM. These considerations led us to propose

the design for the high-frequency passively mode-locked oscillator shown in

Figure 2.9. L1 and L2 are aspheric lenses for collimation and focussing,

respectively, and APP is the prism pair for slow-axis expansion.

Figure 2.9: Layout of the diode-pumped 1.4 and 2.6 GHz Nd:GdVO4 laser

An HR concave mirror, with R = 50mm radius of curvature, folded the

nearly-symmetric resonator. The folding angle was kept as small as possible,

6, in order to allow maximum overlap of the tangential and sagittalstability regions. The SAM (supplied by BATOP Gmbh, Weimar, Germany)

was the second end-mirror: the saturable modulation depth was specified

to be R = 0.7% (nonsaturable loss 0.3%), with saturation fluence of

30J/cm2 and recovery time 10ps.

To determine the amount of the loss from the Nd:GdVO4 quasi-Brewster

face, a simpler two-mirror plano-concave cavity was separately set up with

a 25mm radius of curvature, 2% transmissivity output coupler. The laser

emitted up to 250mW in TEM00 mode with 700mW of incident pump power,

whereas the external reflection from the Brewster face was measured to be 0.2% of the intracavity power. In Figure 2.10 the output power characteristic

versus pump power for the plano-concave cavity is reported.

Since the working tolerance for crystal cutting is often within 0.5 and

the reflectivity dependence from the offset angle is parabolic near the

Brewster condition, it is easy to specify some offset angle to introduce an

acceptable amount of output coupling (see Figure 2.11; see the inset for angle


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definitions). The shortcoming of this approach is the loss for the internal

reflection, and correspondingly reduced laser efficiency, but this is of little

concern as long as few tens of milliwatts can be considered a sufficient output

power as in our case.

Numerical computation results summarized in Figure 2.12 show the vari-

ation of the resonant mode size and of the critical output power (critical

energy multiplied by the repetition rate frequency), considering the 0.2%

effective coupling through the quasi-Brewster face, as a function of the rep-

etition rate. In agreement with the numerical modeling, cw mode-locking

could be readily achieved only near the edge of the stability region.

Experimental results

In Table 2.1, the results obtained for different pulse frep are reported. A

little tunability of the repetition frequency was experimented (frep/frep 3MHz 2 ). The output beam was, as expected, linearly polarized.

0 100 200 300 400 500 600 7000

50

100

150

200

250

300

Ppump

[mW]

Pout

[mW]

Measured dataLinear fitting

Slope 38%

Figure 2.10: Two mirror plano-concave output power versus incident pump power

characteristic


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1.39 1.4 1.41 1.42 1.43 1.44 1.45 1.46 1.4720

40

60

80

100

120

140

160

180

f GHz

Criticaloutputpower[mW]

Figure 2.12: Critical output power calculated for cw mode-locking

mirror are shown in Figure 2.13.

The output power from the quasi-Brewster face was up to 60mW, therepetition rate was centered at 2.6GHz with pulse duration of 4.4ps (Figure

2.14). The time-bandwidth product was 0.47, slightly above the sech2 limit

as often occurs in lasers with the gain element at the end [18]. The output

beam was TEM00 with horizontal polarization. The repetition rate could

be varied only slightly (few MHz) by translating the SAM longitudinally

without compromising the stability of cw mode-locking. Once the cw mode-

locking was started, the laser could be used several days without any damage

of the SAM.

We used a radio-frequency spectrum analyzer (HP 8562A) to monitor the

quality of mode-locking and to measure carefully the repetition frequency.

Owing to the limited photodetector sensitivity we were limited to a S/N

ratio of 30dB, with no trace of relaxation oscillations in the background

noise (the signal on a 500-MHz oscilloscope did not show any significant

train modulation at low frequency).

The flexibility of this cavity design can be appreciated when a broader

repetition rate tuning range is required. In fact, a significantly broader

tuning range, of 200 MHz, has been achieved by unbalancing the resonatorarms, varying the length of the SAM arm within 30% - 50% of the total


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intracavity path in air, which physically corresponds to right-left shifting

the graph in Figure 2.13(a). Such a tuning range is important, for example,

for matching the pulse frequency to the resonance of a given microwave test

device, and cannot be done with linear plano-concave resonators without

compromising the cw mode-locking stability.

Reducing the pump power we found that the critical output power for

cw mode-locking was 30mW, corresponding to 470mW of pump power(for comparison, the pump threshold for laser emission with the SAM was

Figure 2.13: Critical Pout calculated for cw mode-locking (a) and waist radii (b)as a function of frep. The waist radii on the SAM (wa) as well as on

the gain medium (wg) are calculated (both for the tangential (t) and

the sagittal (s) planes), near the 2.6GHz edge of the stability region.

Actually, the tangential waist radius within the laser crystal has to

be multiplied by the refractive index n=2.192


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Figure 2.15: Experimental setup for the oscillator operating around 2.5GHz

2.3.6 Nd:YVO4 based 4.8GHz rep-rate resonator

The results obtained with the Nd:GdVO4 based oscillators were the pre-

liminary condition for further steps. Since the 1063nm centered Nd:GdVO4

emission wavelength does not match the gain bandwidth of the amplification

stages of the laser system, based on Nd:YVO4 slabs, further cavity setups

rely on Nd:YVO4 active medium. A 1% doped, plane-brewster a-cut, 3mm

long crystal was chosen. Due to its higher emission cross section and con-

sequently lower saturation fluence, in according with eq. (2.14) Nd:YVO4

reduces the intracavity critical pulse energy. Also the pump diode was sub-

stituted with the aim to achieve a better absorption peak wavelength match-

ing and higher pump power. The diode output power versus input current

characteristic is reported in Figure 2.18, showing a 20% available pumppower in excess if compared with the characteristic shown in Figure 2.8.

The diode emission is centered at 808nm with a FWHM of about 2nm.

Once again, in order to determine the amount of the loss from the

Nd:YVO4 quasi-Brewster face, a two-mirror plano-concave cavity was set


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L1

Lg

HR,R=25mm

SAM 1%

L2

2a

a

LBO

4mm

Figure 2.16: Cavity setup with LBO placed near SAM

Figure 2.17: Autocorrelation trace of the 2.41GHz cw mode-locking pulses emerg-

ing from the cavity of Figure 2.16

up with a 25mm radius of curvature, 2% transmissivity output coupler. The

output power performances as a function of the incident pump power are

reported in Figure 2.19 and show a significant improvement with respect to

the previously tested Nd:GdVO4 setup (see Figure 2.10 for a comparison).

A slope efficiency of about 50% was achieved, while quasi-brewster face loss

was 1 .The conceptual scheme of the laser cavity operating at 4.8GHz repeti-

tion frequency is the same we employed for the Nd:GdVO4 based oscillator

descripted before (see Figure 2.9). The main difference is obviously relying


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frip[GHz] Ppump [mW] Pout Brewster[mW]

2.415 610 (cw ML thres.) 53

690 65

2.43 530 (cw ML thres.) 34

690 45

2.45 580 (cw ML thres.) 42

690 52

2.47 560 (cw ML thres.) 48

690 63

2.49 605 (cw ML thres.) 42690 47

2.51 630 (cw ML thres.) 40

690 43

2.53 690 (cw ML thres.) 53

2.545 690 (cw ML thres.) 50

2.57 690 (cw ML thres.) 45

2.60 690 (cw ML thres.) 48

Table 2.2: Results obtained employing LBO

on the cavity length and hence the curvature radius of the folding mirror. A

mirror coated HR at 1064nm with R=12mm was employed. The cavity opti-

cal length of 31mm ( 24mm in air) related to this frep needs an accuratemechanical design of the oscillator in order to minimize the folding angle

(and consequently HR mirror losses and induced astigmatism in the cavity

mode) and carefully manage the available space to put every component in

place. It is worth notice that we used only commercially available compo-

nents, without any expensive customization. In Figure 2.20 is reported a

picture of the oscillator.

Obtaining a stable cw mode-locking regime in these conditions was not

so simple. Only with tricky and accurate alignment of the mirrors very close

to the cavity stability edges we reached our goal. Once the laser operates

in cw mode-locking it shows good stability. It works for hours without any


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Figure 2.19: Plano-concave Nd:YVO4 output power performances

In Figure 2.23 is reported a typical Radio Frequency (RF) spectrum of

the cw mode-locking. A S/N ratio of 40dB, with no trace of relaxation

oscillations in the background noise was obtained.

Figure 2.20: Picture of the 4.8GHz oscillator, white continuous line shows the

cavity path, dash line is the output


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(a) 10s/div (b) 500ps/div

Figure 2.21: cw Mode-Locking oscilloscope traces with long (a) and short

(b) time span

Figure 2.22: sech2

shaped autocorrelation trace for the 4.74GHz cw mode-lockinglaser pulses. The conversion coefficient for the FWHM pulse duration

is 29.4ps/ms which gives a duration of about 9.7ps


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Figure 2.23: RF spectrum of the cw mode-locking


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[1] G. J. Spuhler, R. Paschotta, U. Keller, M. Moser, M. J. P.

Dymott, D. Kopf, J. Meier, K. J. Weingarten, J. D. Kmetec, J.Alexander, G. Truong: Diode-pumped passively modelocked Nd:YAG

laser with 10-W average power in diffraction-limited beam, Opt. Lett.

24, pp. 528-530 (1999)

[2] J. Aus der Au, S. F. Schaer, R. Paschotta, C. Honninger,

U. Keller, M. Moser: Highpower diode-pumped passively modelocked

Yb:YAG lasers, Opt. Lett. 24, pp. 1281- 1283 (1999)

[3] K. J. Weingarten, M. J. W. Rodwell, H. K. Heinrich, B. H.

Kolner, B. M. Bloom: Direct electrooptic sampling of GaAs inte-grated circuits, Electron. Lett. 21, pp. 765-766 (1985)

[4] K. J. Weingarten, M. J. W. Rodwell, D. M. Bloom: Picosec-

ond optical sampling of GaAs integrated circuits, IEEE J. Quantum

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[5] D. A. B. Miller: Optics for low-energy communication inside digi-

tal processors: quantum detectors, sources, and modulators as efficient

impedance converters, Optics Letters 14, pp. 146-148 (1989)

[6] M. N. Islam, C. E. Soccolich, D. A. B. Miller: Lowenergy ul-

trafast fiber soliton logic gates, Opt. Lett. 15, pp. 909 (1990)

[7] R. Ramaswami, K. Sivarajan: Optical Networks: A Practical Per-

spective, Morgan Kaufmann Publishers (1998)


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[8] A. Bartels, T. Dekorsky, H. Kurz: Femtosecond Ti:sapphire ring

laser with 2-GHz repetition rate and its application in time-resolved

spectroscopy, Opt. Lett. 24, pp. 996- 998 (1999)

[9] A. Hatziefremidis, D. N. Papadopoulos, D. Fraser, H.

Avramopoulos: Laser sources for polarized electron beams in cw and

pulsed accelerators, Nucl. Instrum. Meth. A 431, pp. 46-52 (1999)

[10] C. Honninger, R. Paschotta, F. Morier-Genoud, M. Moser, U.

Keller: Q-switching stability limits of continuous-wave passive mode

locking, J. Opt. Soc. Am. B 16, pp. 46- 56 (1999)

[11] F. X. Kartner, L. R. Brovelli, D. Kopf, M. Kamp, I. Calasso,

U. Keller: Control of solidstate laser dynamics by semiconductor de-

vices, Opt. Eng. 34, pp. 2024-2036 (1995)

[12] U. Keller, K. J. Weingarten, F. X. Kartner, D. Kopf, B.

Braun, I. D. Jung, R. Fluck, C. Honninger, N. Matuschek,

J. Aus der Au, Semiconductor saturable absorber mirrors (SESAMs)

for femtosecond to nanosecond pulse generation in solid-state lasers,

IEEE J. Sel. Topics Quantum Electron. 2, 435453 (1996)

[13] L. R. Brovelli, U. Keller, and T. H. Chiu, Design and operation of

antiresonant FabryPerot saturable semiconductor absorbers for mode-

locked solid-state lasers, J. Opt. Soc. Am. B 12, 311322 (1995)

[14] T. R. Schibli, E. R. Thoen, F. X. K artner, E. P. Ippen, Sup-

pression of Q-switched mode locking and breakup into multiple pulses

by inverse saturable absorption, Appl. Phys. B 70, pp. 41-49 (2000)

[15] A. Agnesi, A. Guandalini, A. Tomaselli, E. Sani, A. Toncelli,

M. Tonelli, Diode-pumped passively mode-locked and passively stabi-lized Nd3+:BaY2F8 laser, Opt. Lett. 29, pp.1638-1640 (2004)

[16] A. Agnesi, A. Guandalini, A. Lucca, F. Pirzio, A. Tomaselli,

G. Reali, E. Sani, A. Toncelli, M. Tonelli, Passive stabilization

technique applied to continuous-wave picosecond mode-locked Yb:YAG


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and Nd:BaY2F8 lasers, Conference on Lasers and Electro-

Optics (CLEO) Baltimore, 2, pp. 784-785 (2005)

[17] L. Krainer, D. Nodop, G.J. Spuhler, S. Lecomte, M. Golling,

R. Paschotta, D. Ebling, T. Ohgoh, T. Hayakawa, K. Wein-

garten, U. Keller, Compact 10-GHz Nd:GdVO4 laser with 0.5-W av-

erage output power and low timing jitter, Opt. Lett. 29, 2629-2631

(2004)

[18] B. Braun, K. Weingarten, F.X. Kartner, U. Keller, Continuous-

wave mode-locked solid-state lasers with enhanced spatial hole-burning.

Part I. Experiments, Appl. Phys. B 61, 429-437 (1995)

[19] A. Agnesi, A. Guandalini, G. Reali, Self-stabilized and dispersion-

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86, 171105 (2005)

[20] S.L. Schieffer, D. Brajkovic, A.I. Cornea, W. Andreas

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[21] A. Agnesi, F. Pirzio, A. Tomaselli, G. Reali, C. Braggio, Multi-GHz tunable-repetition-rate mode-locked Nd:GdVO4 laser, Opt. Exp.

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Chapter 3

IRENE - A laser source for

photoconductivity

measurements

A crucial point regarding the feasibility of the Motion Induced Radiation

(MIR) experiment, as described in Chapter 1, is the possibility to make

appear and disappear at high frequency the end mirror of the Casimir ex-

periment microwave cavity. This p ossibility relies on physical properties of

the semiconductor layer that, optically switching its conductivity, acts as a

vibrating wall for the electromagnetic field. The experimental tests of the

photoconductive behavior of different semiconductors materials at cryogenic

temperatures are performed at the INFN National Labs of Legnaro (PD),

by using a laser source realized at the Laser Source Lab of the Electronics

Department of the University of Pavia. These measurements require a low

repetition rate source, able to provide short pulses (p < 10ps) with en-

ergy of the order of few microjoule at 532nm and tens of nanojoule around

790nm. In this Chapter, I will describe the laser system (IRENE - InfraRed

ENergy Emitter), its realization and its performances.


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54 IRENE - A laser source for photoconductivity measurements

3.1 State of the art ofJ level picosecond sources

Many laser systems consist of an oscillator followed by one or more ampli-

fier stages. These optical amplifiers can take several very different forms,

including that of a single-pass amplifier, a multipass amplifier or a regen-

erative amplifier. A number of interdependent factors determine which of

these configurations is the best suited for a particular application. These

factors include the amplification required, the gain and saturation properties

of the active medium, the input power from the oscillator, the desired beam

quality, system cost, complexity and reliability. Microjoule-level picosecond

pulses are interesting for a variety of applications, including micromachin-ing and nonlinear optics. Laser systems delivering microjoule pulse energy

have been reported relying either on multi-pass gain elements[1] or, more

often, on regenerative amplification[2][3]. Also less complex soultions that

in principle do not need amplification stages like cavity dumping have been

employed[4]. We will now briefly go into each of these techniques in order

to point out their capabilities and drawbacks.

3.1.1 Multi-pass amplifiers

A possible scheme for multipass amplification systems is represented in Fig-

ure 3.1.

Figure 3.1: In a multipass amplifier, the beam passes through the gain medium

several times, at a slightly different angle each time


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3.1 State of the art of J level picosecond sources 55

In this setup optics (usually mirrors) are arranged so that input beam

makes several passes through the amplifier gain medium before exiting. In

practice, each pass through the gain medium may travel through the same

optically pumped spot in the center of the amplifier material, but with

different paths. A folded path allows the beam to enter and exit the amplifier

after a finite number of passes. The optimum number of passes and hence

the number of folds in the amplifier path, depends on the gain per pass,

the amount of overall gain required, the saturated gain coefficient for the

material and the amount of optical complexity tolerable. The greater the

number of passes needed the more complex the design must be. The design is

limited also in the difficulty of focusing each pass through the gain medium.

Typically four to eight passes are made, with cascading multipass amplifiers

used for a greater number of passes. The technique is desirable as it is

relatively inexpensive, but it needs time-consuming adjustments. The gain

medium must also be used close to the damage threshold to have a high gain

per pass ratio.

3.1.2 Regenerative amplifiers

Practical issues of optical complexity limit the number of passes feasiblefor a multipass amplifier, so the net gain of such an amplifier cannot be

increased beyond a certain level. For some applications, this gain level is

not sufficient. This happens when the input signal from the laser oscillator is

very weak, or when several passes are not enough to reach saturation. Both

instances are often the result of a low cross-section for stimulated emission.

One possible alternative solution is represented by regenerative ampli-

fiers. The operation principle can be understood as follows:

first, the gain medium is pumped for some time, so that it accumulates

energy;

then, the initial pulse is injected into the cavity through a port which

is opened for a short time (shorter than the round-trip time) with an

electro-optic (or sometimes acousto-optic) switch;


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56 IRENE - A laser source for photoconductivity measurements

after that, the pulse can undergo many (possibly hundreds) of cavity

round trips, being amplified to a high energy level;

finally, the pulse is again released from the cavity. This can either hap-

pen with a second electro-optic switch, or with the same one previously

used for coupling in.

This principle allows to achieve very high gain and thus pulse energies in

the millijoule range with amplifiers of moderate size, or even higher ener-

gies with larger devices. Typical pulse repetition rates are of the order of

1kHz (although repetition rates of 100kHz are sometimes possible), while

the highest pulse energies are achieved at lower repetition rates.

The pulse is usually trapped using a Pockels cell and a broadband po-

lariser. Figure 3.2 represent a typical set-up of a regenerative amplifier. In

PUMP

ACTIVE MEDIUM

OUT

IN

POLARIZER

POCKELLS CELL

Figure 3.2: A typical regenerative amplifier setup

terms of output characteristics, one of the m

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