1

Lecture 12: Lasers

• Intro to Lasers

• Laser is an oscillator

• Conditions for laser oscillation

• Laser threshold

• Optical resonators

• Laser modes

• Gain saturation

• Steady-state laser internal photon flux density

• Brief discussion on laser systems

Reading: Senior 6.2.4-6.2.5

2

Before the laser

• Before the invention of the laser the available light sources were essentially either thermal (e.g. tungsten filament lamp) or spontaneous emission from atoms and molecules (e.g. gas discharge)

• In either case their brightness was limited by the temperature of the emitter

E.g. the broadband white light of solar radiation is limited in the brightness of the spectral lines by the temperature of the gases (recall black-body radiation)

• Light from a thermal or spontaneous emission source is incoherent – individual atoms radiating at random without any relation to one another

3

Coherent radiation

• In a laser, however, the emission from individual atoms is synchronized, giving coherent radiation.

• The process of synchronization is stimulated emission – a concept introduced by Einstein in 1916.

• The essential effect of stimulated emission is the coherent emission of radiation from excited atoms – adding precisely in phase and with the same direction and polarization

f

k

E

4

Stimulated emission

• One photon arrives at the excited atom, and two photons leave, with the same energy, traveling together and in phase

• The stimulated photon has the same momentum as the incident photon, and hence travels in the same direction

• Both photons can then repeat the stimulated emission process at other excited atoms

=> the resulting chain reaction causes the light wave to grow exponentially!

• To make such an amplifier into a self-excited oscillator – the light must be fed back into the laser material. This is attained by enclosing the lasing material between mirrors, forming a resonant cavity.

5

From coherent microwave to coherent lightwave

• The first use of stimulated emission to achieve coherent radiation was in the microwave spectrum – known as “maser” (microwave amplification by stimulated emission radiation)

• In 1953, James Gordon and Charles Townes demonstrated stimulated emission between the two lowest levels of ammonia molecules, giving a narrow emission line at a wavelength of 12.6 mm.

• In 1960, the first laser was demonstrated by Ted Maiman.

• LASER – light amplification by stimulated emission radiation

• Maiman generated red laser light at a wavelength of 694.3 nm from the chromium ions in a ruby crystal.

Laser turns 51 years old in 2011!

6

1958 Arthur L Schawlow and Charles H Townes (USA). Published a paper titled “Infrared and Optical Masers” in which it was proposed that the maser principle could be extended to the visible region of the spectrum to give rise to what later became known as a “laser” [Physical Review. 112(6), p1940, 1958]

1960 Theodore H Maiman (USA). Demonstrated the first laser. The laser was built at the Hughes Research Laboratories and used a rod of synthetic ruby as the lasing medium [Nature. 187, p493, 1960]

• 1964, Nobel prize for the development of lasers: C. H. Townes, A. M. Prokhorov, N. G. Basov

• 1964 -, nonlinear optics, fiber optics, light emission from semiconductor, etc… (the beginning of the photonics age!)

The Beginning of the Laser

7

Charles Townes, James Gordon and the

MASER

How to obtain gain?

Ref. Reflections on the first maser, James Gordon,

OPN Optics & Photonics News, pp. 34-41, May 2010

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The first laser

Ref. Lasers and the glory days of industrial research, Jeff Hecht, OPN Optics & Photonics News, pp. 20-

27, May 2010

9

Laser light is

• Coherent

• Quasi-monochromatic (nearly single frequency), from soft x-ray (few nm) to mid-IR (e.g. 10.2 mm)

• Directional

• Polarized

• Can be high-power (e.g. kilo Watts)

• Can be continuously operated (continuous wave) or pulsed with narrow pulse widths (picosecond, femtosecond, attosecond)

• Can be generated from gas, liquid, solid medium (almost anything can become a laser if you pump it hard enough!)

10

More on stimulated emission

• Recall that the stimulated emission photon is an exact copy of the seed photon (identical frequency, phase, polarization and direction).

• Each stimulated emission photon could stimulate more photon emissions, leading to the build-up of a coherent wave of very large intensity.

• This requires the number of atoms in the higher energy level N2 to exceed the number in the lower level N1, a condition known as population inversion,

=> the rate of stimulated emission exceeds the rate of absorption.

11

Three-level lasers

• In three-level lasers, the atoms in the active medium have three energy levels involved in the laser action

• Absorption raises the energy from level 1 to level 3 (this process is called pumping)

• Spontaneous decay (or non-radiative transition) reduces the energy to level 2, which is a longer lived or metastable state

• Stimulated emission occurs between levels 2 and 1

• The accumulation of excited atoms in the metastable state results in population inversion (compared to the ground state level 1).

e.g. The ruby laser is an example of a three-level laser in which the active species is the chromium Cr3+ ion

12

Population inversion in a three-level laser

• The energy supply used to create the population inversion is often referred to as a pump, which can be light absorbed between a ground level E1 and an excited level E3.

• If the excitation of this level is short-lived, and it decays to a lower but longer-lived level E2, the process leads to an accumulation and overpopulation of atoms in level E2 compared with E1. Stimulated emission, fed by energy from a pump, is the essential process in a laser.

E1 (ground)

E2 (long-lived)

E3 (short-lived)

pump

Spontaneous decay

Laser transition

13

Oscillators

• An oscillator is an amplifier with positive feedback at a particular

frequency.

e.g. For radio-frequency (RF) oscillators, an electronic amplifier

provides the signal gain, a filter determines the frequency, and feedback

results from connecting the amplifier output back to its input.

frequency-selective feedback

amplifier

Power supply

output

RF oscillator

Vo Vi

b

14

Overall gain of the electronic oscillator

• The input and output voltages of the amplifier are Vi and Vo. The overall gain of the system is A, where A = Vo/Vi.

Vo = Ao(Vi + bVo)

=> Vo = AoVi / (1 – bAo)

And A = Ao / (1 – bAo)

• If bAo = +1 then the gain of the circuit would apparently become infinite, and the circuit would generate a finite output without any input.

• In practice electrical “noise,” which is a random oscillatory voltage generated to a greater or lesser extent in all electrical components in any amplifier system, provides a finite input. (Oscillators are “noise-start.”)

15

Oscillates by amplifying noise at specific frequencies

• Because bAo is generally a function of frequency the condition bAo = +1 is generally satisfied only at one frequency.

• The circuit oscillates at this frequency by amplifying noise at this same frequency that appears at its input.

• However, the output does not grow infinitely large, because as the signal grows, Ao falls --- this process is called saturation.

• This phenomenon is fundamental to all oscillator systems.

16

Laser is an optical-frequency oscillator

• A laser is an optical-frequency oscillator constructed from an optical-frequency amplifier with positive feedback.

• Light waves which become amplified on traversing the amplifier are returned through the amplifier by the reflectors and grow in intensity, but this intensity growth does not continue indefinitely because the amplifier saturates.

• Spontaneous emission photons serve as “noise” to start the optical oscillator.

mirror

Active medium

Partially transmitting mirror

Laser output

Optical-frequency oscillator

17

• In the case of the laser, the “gain” medium provides the light

amplification by stimulated emission.

• The gain medium also determines the frequency. It does so through its

characteristic energy levels and transitions between levels.

• Mirrors provide the feedback. Photons bounce off the mirrors and

return through the gain medium for further amplification. The resulting

standing waves favor the growth, and therefore oscillation, of cavity

resonance frequencies.

• One of the mirrors is partially transmitting to allow a fraction of the

generated light to output-couple. This results in coherent lasing emission.

Amplification, feedback and oscillation

18

To make a laser

1. Population inversion – a criterion to provide gain

2. Stimulated emission

3. Optical feedback

mirror partially transmitting

mirror

L

gain medium Optical

output

19

Population inversion: a criterion to provide gain

• Population inversion is the basic condition for the presence of an optical gain.

• Population inversion in a semiconductor can only be accomplished through pumping – populating the normally empty conduction band with electrons and the normally empty valence band with holes.

• Population inversion is a non-equilibrium state that cannot be sustained without active pumping.

• To maintain a constant optical gain we need continuous pumping (e.g. continuous injection) to keep the population inversion at a constant level.

20

Conditions for laser oscillation

• Two conditions must be satisfied for the laser to oscillate

(lase):

– The gain condition determines the minimum

population difference, and thus the pumping threshold

required for lasing

– The phase condition determines the frequency (or

frequencies) at which oscillation takes place

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To achieve “lasing,” we need: 1. optical gain, and 2. optical feedback

• Optical gain makes an optical amplifier (usually broadband).

• Optical feedback (frequency selective) converts an amplifier into

an oscillator.

Amplifier vs. Oscillator

wavelength

Semiconductor

optical amplifier

(broadband,

~30 – 50 nm)

Oscillator (narrow band, ~ nm –

sub-nm)

*Linewidth narrowing is one key

signature of oscillation

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Laser threshold: a key signature of oscillation

Lig

ht

outp

ut

(pow

er)

Current

Incoherent

emission (LED)

Coherent

emission

(Lasing)

Threshold current ith

much steeper than LED

(typically few 10 mA’s

using double heterostructures)

23

Laser threshold

• We can understand the concept of laser threshold by noting that a

certain fraction of photons generated by stimulated emission is lost

because of the resonator loss (absorption, scattering, extraction of laser

light).

The unsaturated optical gain needs to exceed the resonator loss

such that the photon population can build up. The resonator loss thus

sets the threshold gain.

The laser oscillation condition:

g0(u) > ar

where ar is the resonator loss coefficient (cm-1). The threshold gain

coefficient (cm-1) is therefore ar. For laser diodes, the injection current

that is needed to reach the threshold is called the threshold current.

24

The unsaturated gain must exceed the resonator loss

• sub-threshold

(incoherent emission)

• Threshold

(oscillation begins,

start to emit coherent

light)

• above-threshold

(increase in

coherent

output

power)

Resonator

loss (assume

constant) gain

x(mm) x(mm) x(mm)

x: dimension along an active layer

gain < loss gain = loss

loss loss

gain > loss

gain

gain

25

Optical resonators

• In practical laser devices, it is generally necessary to have certain positive optical feedback in addition to optical amplification provided by a gain medium.

• This requirement can be met by placing the gain medium in an optical resonator. The optical resonator provides wavelength selective feedback to the amplified optical field.

• In many lasers the optical feedback is provided by placing the gain medium inside a “Fabry-Perot” cavity, formed by using two mirrors or highly reflecting surfaces

reflectivity (R1 ~ 100 %) R2 < 100 %

Gain medium Light output (laser)

Fabry-Perot Lasing Cavity

A Fabry-Perot cavity consists

of two flat, partially reflecting

mirrors that establish a strong

longitudinal optical oscillator

feedback mechanism, thereby

creating a light-emitting

function.

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The distance between the adjacent peaks of the resonant wavelengths in a Fabry-Perot cavity is the free spectral range (FSR). If D is the distance between the reflecting mirrors in a device of refractive index n, then at a peak wavelength λ the FSR is given by

27

R1 R2

Pout

R1

R2

Pout

Pout

Pout Pout Pout

Pout Pout

Common optical resonator

configurations

Bragg grating

Fiber/waveguide

ring resonator

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A linear cavity with two end mirrors is known as a Fabry-Perot cavity because it takes the form of a Fabry-Perot interferometer. In the case of semiconductor diodes, the diode end facets form the two end mirrors with Fresnel reflection.

A folded cavity can simply be a folded Fabry-Perot cavity with a standing oscillating field.

A folded cavity can also be a non-Fabry-Perot ring cavity that supports two independent oscillating fields traveling in opposite directions (clockwise, counterclockwise). Ring cavity can be made of multiple mirrors in free space, or in the form of fiber/waveguide-based devices.

The optical cavity can also comprise a distributed Bragg grating with distributed feedback. Distributed Feedback (DFB) diode lasers are the most common single-mode laser diodes for optical communications.

Common optical resonator

configurations

29

Fabry-Perot cavity resonances

• Only standing waves at discrete wavelengths exist in the cavity.

=> the laser wavelengths must match the cavity resonance wavelengths.

The resonance condition: 2nd = q l

where q is an integer, known as the longitudinal mode order,

k = 2pn/l

d

refractive index n

or 2kd = 2pq

30

• The modes along the cavity axis is referred to as longitudinal modes.

• Many l’s may satisfy the resonance condition => multimode cavity

The longitudinal mode spacing (free-spectral range):

Dl = l2 / 2nd

l

intensity

q q-2 q+2

Dl

q+1 q-1

….. …..

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e.g. A semiconductor laser diode has a cavity length 400 mm

with a refractive index of 3.5. The peak emission wavelength from

the device is 0.8 mm. Determine the longitudinal mode order

and the frequency spacing of the neighboring modes.

• The longitudinal mode order q = 2nd/l ~ 3500

• The frequency spacing Du = c/2nd ~ 100 GHz

• The longitudinal mode frequencies:

u = uq = qc/2nd

• The mode spacing (free-spectral range) in frequency unit:

Du = c/2nd

32

Resonator loss • The resonator contributes to losses. Absorption and

scattering of light in the gain medium introduces a power loss per unit length (attenuation coefficient as in cm-1)

• In traveling a round trip through a resonator of length L, the photon-flux density f is reduced by the factor

R1R2 exp(-2asL)

where R1 and R2 are the reflectances of the two mirrors

• The overall power loss in one round trip can be described by a total effective resonator loss coefficient ar in cm-1

exp(-2arL) = R1R2exp(-2asL)

33

Loss coefficients

ar = as + am1 + am2

am1 = (1/2L) ln(1/R1)

am2 = (1/2L) ln(1/R2)

where am1 and am2 represent the contributions of mirrors 1 and 2. (i.e. assuming the lumped mirror loss is distributed over a cavity round-trip length of 2L)

• The contribution from both mirrors

am = am1 + am2 = (1/2L) ln(1/(R1R2))

We can consider

34

Photon lifetime and resonator linewidth

• Define photon lifetime (cavity lifetime) tc as the 1/e lifetime for photons inside the cavity of index n:

exp(-ar tcc/n) = exp(-1)

tc = n/arc

• The resonator linewidth (FWHM) du is inversely proportional to the cavity lifetime (think Fourier transform)

du = 1/2ptc

• The cavity quality factor Q at resonance frequency um is

Q ≈ um/du

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• The finesse of the resonator

F ≈ Du/du

• When the resonator losses are small and the finesse is large

F ≈ p/(arL)

du

um-1 um

um+1 u

Du = c/2nL

Finesse of the resonator

36

• An active medium provides optical gain (stimulated emission) only

within the gain bandwidth. In the case of semiconductors, the gain

bandwidth is about 10 - 20 THz.

Only cavity resonant wavelengths that lie within the gain curve may

oscillate.

l

intensity gain

d ~ 100 mm

multimode

cavity

~nm

~10’s nm

37

Loss ar

u Allowed

modes

u Resonator

modes

B u

Du

g0(u)

• Laser oscillation can occur only at frequencies for which the

unsaturated gain coefficient exceeds the resonator loss

coefficient.

Only a finite number of oscillation frequencies (u1, u2,…, um)

are possible.

u1 u3

u2

38

Laser modes

• The number of possible laser modes

M ≈ B/Du

• However, of these M possible modes, the number of modes that actually carry optical power depends on the nature of the spectral lineshape broadening mechanism.

• For a homogeneously broadened medium (e.g. semiconductor) these modes compete, rendering fewer modes (ideally single mode) to oscillate.

• For an inhomogeneously broadened medium (e.g. HeNe gas, Er3+-doped glass) all M modes may oscillate (albeit at different powers).

39

Growth of oscillation in an ideal homogeneously

broadened medium

ar

go(u)

g(u)

g(u)

uo uo uo

• Immediately following laser turn-on, all modal frequencies for which

the gain coefficient exceeds the loss coefficient begin to grow, with the

central modes growing at the highest rate. After a transient the gain

saturates so that the central modes continue to grow while the peripheral

modes, for which the loss has become greater than the gain, are

attenuated and eventually vanish. Only a single mode survives.

u

u

40

Gain and loss profiles in semiconductor lasers

The saturated gain of the longitudinal mode near to the gain peak

equals the loss.

frequency

saturated gain g (cm-1)

Loss ar (cm-1)

Longitudinal

modes uq

Lasing

mode

41

Laser linewidth and the coherence length

• Singlemode or multimode lasing determines the laser

linewidth du.

• Singlemode lasing gives relatively narrow linewidth which

is only limited by the resonator loss.

• Multimode lasing gives relatively broad linewidth which is

given by the number of lasing modes and their mode

spacing.

• The concept of coherence length:

Lc = ctc ≈ c/du

e.g. A singlemode laser with a relatively narrow linewidth of

300 kHz gives a Lc ~1 km, while a multimode laser with a

relatively wide linewidth of 0.3 THz gives a Lc ~1 mm.

(where tc is coherence time)

42

Gain saturation

• In the case of injection diode lasers, when the laser current density is

increased above its threshold value (i.e. J > Jth), the peak gain coefficient

gp exceeds the loss coefficient ar. (more discussion in Lect. 13)

=> Stimulated emission then outweighs absorption and other resonator

losses so that oscillation begins and the photon flux f in the resonator

increases.

• However, saturation sets in as the photon flux becomes larger.

the population difference (initial injected carrier density) becomes

depleted.

=> The gain coefficient then decreases until it becomes equal to the loss

coefficient, whereupon steady state is reached.

43

• The saturated gain coefficient (for homogeneously broadened media)

g(u) = g0(u)/(1 + f/fs(u))

The gain coefficient is a decreasing function of the photon-flux density f.

When f equals its saturation value fs(u), the gain coefficient is reduced to half its unsaturated value.

0.01 0.1 1 10

0.5

1

0 f/fs(u)

g/g0(u)

44

• At the moment the laser lases, f = 0 so that g(u) = g0(u).

• As the oscillation builds up in time, the increase in f causes g(u) to drop through gain saturation.

• When g(u) reaches ar, the photon-flux density ceases its growth and steady-state conditions are attained.

• The smaller the ar (or larger the g0(u)), the greater the values of steady-state photon flux density f.

0.01 0.1 1 10 0

f/fs(u)

g0(u) g(u)

Laser

turn-on

ar loss coefficient

f

steady-state

Steady-state oscillation condition

Gain clamping

45

Steady-state laser internal photon flux density

• Gain clamping at the value of the loss.

• The steady-state laser internal photon flux density f is therefore determined by equating the saturated gain coefficient to the loss coefficient

g(u) = g0(u) / (1 + f/fs(u)) = ar

f = fs(u) (g0(u)/ar – 1), g0(u) > ar

= 0, g0(u) ≤ ar

• This is the mean number of laser photons per second crossing a unit area in both directions – laser photons traveling in both directions contribute to the saturation process. The photon-flux density for laser photons traveling in a single direction is thus f/2.

46

Round-trip gain and threshold gain

coefficient • Consider a cavity made up of mirrors M1 and M2 with

reflectivities R1 and R2 and spaced by a distance L

• A beam of irradiance I0 starting at M1 on reaching M2 has become

I1 = I0 exp {(g - as) L},

where g and as are the gain and loss coefficients (cm-1) within the active medium

• On reflection from M2 and traveling in return through the medium and undergoing reflection at M1, the irradiance becomes

I2 = I0R1R2 exp{2(g – as) L}

M1 M2

L I0

I2

47

Round-trip gain and threshold gain

coefficient

• The round-trip gain G is defined as

G = I2/I0 = R1R2 exp{2(g – as) L}

• The threshold condition for steady-state laser oscillation is G = 1

R1R2 exp{2(gth – as) L} = 1

where gth is the threshold gain coefficient, at which the laser begins to oscillate

gth = as + (1/2L) ln (1/R1R2) = ar

• The first term is the loss within the cavity. The second term is the loss due to the mirror transmission including the useful laser output.

Laser Linewidth • Noise arising from spontaneous emission effects

results in a finite spectral width or linewidth Δν for

the lasing output.

• In terms of the optical output Pout, the group velocity

Vg, the photon energy hν, the threshold gain gth, the

cavity loss αt, the linewidth enhancement factor α,

and the spontaneous emission factor nsp, the

linewidth is

48

For DFB lasers the linewidth ranges from 5 to 10 MHz (or, equivalently, around 10–4 nm).

49

Summary: Conditions for laser oscillation

Two conditions must be satisfied for the laser to oscillate:

• The amplifier unsaturated gain must exceed the loss in the feedback

system so that net gain is incurred in a round trip through the feedback.

• The total phase shift in a single round trip must be a multiple of 2p so that the feedback input phase matches the phase of the original

input.

*As the power in the oscillator grows, the amplifier gain saturates.

A stable condition is reached when the reduced gain is equal to the

resonator loss. Steady-state oscillation then prevails.

50

Brief discussion on laser

systems

51

Characteristics of some laser gain media

Gain medium Classification Wavelength

(mm)

Gain bandwidth

Du

HeNe Gas 0.6328 1.5 GHz

Ruby (Cr3+:Al2O3) Solid-state 0.6943 330 GHz

Nd:YAG Solid-state 1.064 150 GHz

Nd:glass Solid-state 1.054 6 THz

Er:fiber Solid-state

(fiber)

1.53 5 THz

Ti:sapphire

(Ti:Al2O3)

Solid-state 0.66-1.1 100 THz

Diode Semiconductor 0.37-1.65 10-20 THz

52

Gas lasers

Gas lasers may be divided into atomic, ionic and

molecular, depending on the active amplifying

species in the gas

Generally gas lasers are excited by an electrical

discharge in which excitation of the gas atoms or

molecules is by collision with energetic electrons

Optical excitation of a gas is usually

inappropriate as the absorption lines of gases

are very narrow (in contrast to solids)

53

Example gas lasers: helium-neon

lasers

The helium-neon (HeNe) laser was the

first gas laser to be operated, and was the

first continuously operating laser

It is still one of the most common lasers,

operating on the 632.8 nm wavelength

Used in many applications requiring a

relatively low power, visible, continuous

and stable beam

54

Gas lasers

DIAMOND series CO2 laser

(Coherent Inc.)

Power 20W ~1000W

Applications:

Metal cutting

Material processing (plastic,

glass, paper, cloth)

Precision manufacturing

55

Solid-state lasers

A solid-state laser, such as the ruby laser, may be in the simple form of a transparent rod with mirrors formed directly on the ends

The gain medium contains active ions in a host crystalline solid or glass.

The active ions may be substituted into the crystal lattice or may be doped as an impurity into the glass host

There are many combinations of dopant ion and host materials which provide a wide range of laser wavelengths

The doped solids exhibit broad absorption bands which makes them amenable to optical excitation from continuous or pulsed lamps or from semiconductor diode lasers (known as diode-pumped solid state (DPSS) lasers, e.g. green laser pointers)

56

Doped medium

• The dopant ion should fit readily into the crystal host by matching the size and valency of the element for which it is substituting

• The optical quality of the doped medium should be high s.t. there is low loss for the amplifying beam

• Refractive index variations, scattering centers and absorption can contribute to loss processes

• Suitable host media are garnets (complex oxides), sapphire (Al2O3), aluminates and fluorides (e.g. Ti:Al2O3 lasers emitting in 800 nm range)

57

Dopant ions

The dopant ions are usually from the transition metals and lanthanide rare earths

The Nd:YAG laser operating at 1064 nm is one of the most used solid-state lasers

Neodymium ions Nd3+ provide the laser action, and yttrium aluminum garnet (YAG) is the usual crystal host

The crystal has a relatively high thermal conductivity which enables it to distribute heat efficiently following optical pumping.

The laser can operate either pulsed or continuously

58

Neodymium-doped glass

Nd3+:glass is a four-level system.

0

3

2

1

pump

t32

t1

1.053 mm

59

Solid-state lasers

Ti: Sapphire (Ti:Al2O3) Laser

(Newport Inc.)

Ultra-short pulse width (< 40 fs)

High pulse energy (7 mJ)

Broad tuning range (from UV to

mid IR)

High beam quality

Pulse sharp, width, repetition

rate fully control

60

CPA: Chirped Pulse Amplification

Ref: Strickland and Mourou, Opt.Comm. 56,219(1985)

• With the same energy, the short pulse

has a high peak power, which induces

undesirable nonlinear effects

• Laser material damage

CPA technique:

Step 1. Pulse expansion

Step 2. Gain

Step 3. Pulse compression

Peak laser power and focused intensity can be increased by several orders of magnitude.

(pulse width: 20~1000fs, single pulse energy: 1J)

Why do we need CPA to amplify short pulses?

61

Fiber lasers

Usually pumped by a laser diode

Simple structures, low cost,

portable

Gain medium: rare-earth doped

fiber Erbium (Er), Ytterbium (Yb)

and Holmium (Ho) and so on.

Photonic crystal fiber laser

(“Supercontinuum” emitted from a fiber laser)

62

Fiber laser

• Fiber laser is one example of solid-state lasers

• Active ion is distributed throughout a long silica fiber

• The core of the silica fiber, which is several meters long and has resonator mirrors at each end, is doped with rare-earth ions (e.g. erbium Er3+ for 1.55 mm, ytterbium Yb3+ or neodymium Nd3+ for 1.06 mm, thulium for 2 mm)

• The pump is a semiconductor diode laser array (at a shorter wavelength than the fiber laser wavelength) focused on one fiber end

• Fiber laser can output few – kilo Watts

63

Erbium-doped silica fibers

Er3+:silica fiber is a three-level system.

3

2

1

pump

t32

1.55 mm

• Pumping at 980 nm using semiconductor InGaAs laser diodes

• The laser transition can also be directly pumped at 1.48 mm by

light from InGaAsP laser diodes – a quasi-two-level scheme

64

Semiconductor lasers

Semiconductor lasers (Sony

blu-ray laser)

65

Semiconductor lasers Semiconductor lasers are distinct from the solid-state

lasers in their pumping and photon generation processes.

They derive their energy from the electrical excitation of electrons within the semiconductor.

The lasers are essentially diodes so they are electrically pumped and compact.

Different laser lines can be generated by using different semiconductor gain media (e.g. GaN-based for blue light, GaAs-based for near-IR, InP-based for 1.55 mm telecom. wavelengths, etc.)

Some semiconductor lasers can generate high power (e.g. Spectra Physics ~kW continuous-wave per chip) for pumping other lasers such as solid-state lasers / fiber lasers.

We will see in Lecture 13 that semiconductor diode lasers have a lot in common with light-emitting diodes (LEDs).

66

NIF (National Ignition Facility)

The world´s largest and highest–energy laser, which has the goal of achieving

nuclear fusion and energy gain in the laboratory for the first time – in essence,

creating a miniature star on Earth. Lawrence Livermore National Laboratory

https://lasers.llnl.gov/

A total of 192 laser

beams focused onto

the target

67

NIF

Frequency Single beam

performance

Equivalent 192-beam

performance

1ω (infared) 21 kJ 4.0 MJ

2ω (green) 11.4 kJ 2.2 MJ

3ω (ultraviolet) 10.4 kJ 2.0 MJ*

https://lasers.llnl.gov/

1053nm