Ultra-Short-Pulse Laser Technology and Applicationsinfo.tuwien.ac.at/tubiomed/de/aktuell/Wintner_Laser_Lecture_5p153.pdf · Ultra-Short-Pulse Laser Technology and Applications: Lecture

Ultra-Short-Pulse Laser Technology and Applications: Lecture 5

153

Lectures on

Ultra-Short-Pulse Laser Technology and Applications

Fifth Lecture

Applications: materials processing, dental lasers, optical coherence tomography,

nonlinear microscopy, rapid-prototyping

5.1 Three general aspects of applications 155 5.2 High temporal resolution by optical pump-probe

experiments 156 5.3 Material ablation by ultra-short pulses 159 5.4 Medical applications of ultra-short pulses 168

5.4.1 Applications to hard-tissue ablation in dentistry 168 5.4.2 Applications to soft-tissue ablation: examples 177 5.4.3 Optical coherence tomography using ultra-short pulses 178 5.4.4 Three-dimensional resolution by non-linear interactions

within the focus of ultra-short pulses 183 5.4.5 Advanced applications in optical communications 186

References (Lecture 5) 187

Ultra-Short-Pulse Laser Technology and Applications: Lecture 5 154


155

5.1 Three general aspects of applications

Modern applications of ultra-short (i.e. picosecond and femtosecond) laser pulses

represent a fascinating wealth of new options becoming more and more mature, also

for commercial use. One may divide them into three categories according to the

dominant feature for the specific type of application, some of them being already

discussed above:

• Ultra-short time duration

• Ultra-broad spectral bandwidth

• High-average power

Ultra-short laser pulses of even modest average power in the tens of mW regime

permit outstanding temporal resolution reaching down to the sub-fs time domain.

Amplified pulses of shortest duration (i.e. nowadays few-cycle duration) allow the

most efficient generation of ultra-short wavelengths in the XUV and X-ray re-gime, especially when carrier-envelope phase-stabilized, as was shown earlier in

chapter 4.3. Ultra-high intensities can be attained with setups of table size giving

the opportunity to discover a new regime of non-linear optics associated with much

higher photon energies than in the visible, also explained in detail in chapter 4.

Another aspect of application is quite new, whereby the ultra-short laser is only a tool

for the generation of extremely broad spectra organized in well-defined and equally spaced frequencies represented by the longitudinal modes. On this basis, optical frequency standards have been revolutionized, as was explained in chapter 4.1. In

case of solely taking advantage of the short coherence length associated with ultra-

short pulses, optical coherence tomography (OCT) allows unprecedented three-dimensional imaging. This aspect of application will be discussed below. Even

ultra-broadband communication takes advantage of the high number of well-defi-

ned longitudinal modes in ultra-short pulse lasers.

High-average power systems represent a new tool for efficient and precise manu-facturing of even hardest materials without the necessity of resonantly absorbed

wavelengths creating negligible damage to the surrounding and remaining material

which can be of anorganic or biological nature. Taking advantage of non-linear


Fig.5.1: Optical pump-probe experi-ment of the absorption dyna-mics as an example for the application of ultra-short pul-ses for time-resolved measu-rements in the 20 ps-regime. PR prism, RR retroreflector, PM photomultiplier, PO polarizer

processes confined to the focus of a mode-locked train of ultra-short pulses yields

the possibility of three-dimensional resolution, being well-known in non-linear microscopy since some time, but representing an innovative solution to three-di-mensional structuring within the context of rapid-prototyping.

5.2 High temporal resolution by optical pump-probe experiments

Fig.5.1 shows an example of a pump-probe experiment (compare the lecture notes

“ultra-short light pulses”, chapter 7 Applications) for the measurement of absorption

dynamics in a semiconductor performed by the author [1]. The experiment involved

an actively mode-locked Nd:YAG laser emitting ~80 ps pulses whose repetition rate

in the order of 80 MHz was reduced by a modulator. The pulse duration was control-

led simultaneously by a specially designed autocorrelator for long delays of more

than 1 ns [2]. In a beam splitter about 90% of the power was separated to form the

pump beam propagating along the path through a λ/2-plate and a chopper to become

recombined with the probe beam which travelled via a folded delay stage to become

collinear with the pump again in second beam splitter. Both beams, being orthogon-

ally polarized with respect to each other, were focused onto the sample. The pump

beam increased the carrier density by becoming absorbed, but very shortly after,

bleaching started to be effective by absorption saturation. The probe beam transmit-

ted through the sample (its contribution to absorption was considered to be negligib-

le) which could be delayed by up to 1 ns, allowed to measure the amount of bleach-

ing being effective at any time within the measurement time span. For this purpose, a


157

Fig.5.2: Bleaching of the absorption in GaAs dep-ending on carrier density. Full lines, measuement data, dotted lines, numerical simulation in order to yield parameters A (Auger), B (bimolecular) and C (normal recombination)

polarizer filtered out the transmitted pump light and, additionally for the increase of

measurement accuracy, a lock-in technique in association with the chopper was

employed. The time resolution of this experiment was ∼∼∼∼ 20 ps being not outstanding

but sufficient for the problem. Fig.5.2 exhibits the typical result of such a measure-

ment: the graphs collected at

different intensities represent

the evolution of bleaching in a sample of GaAs. From

this finding, via a rather com-

plicated numerical iterative

evaluation procedure, based

on the basic formula for the

lifetime of carriers decaying

by three different processes

given in the figure, the Auger recombination coefficient could be derived. This was a

quite important problem at

that time when attempts

were made to explain the

much stronger time-depend-

ence of the threshhold cur-rent in quaternary semi-conductor lasers (of the

type GaInAsP employed in

the communication business) with respect to GaAs diode lasers. The answer yielded

by such experiments to the question which became widely known as T0-problem

was the much higher Auger recombination efficiency represented by the coefficient A

being a material property [3]. It could only become reduced by lowering the carrier

density at threshold by chosing semiconductor media of higher gain like quantum

wells.

Fig.5.3 depicts another type of time-resolving measurement technique: the cross-correlation method. In this case, the effect of two ultra-short pulse laser beams on a

sample is studied under a varying temporal delay relative to each other. The signal


Fig.5.3: Time-resolving measurement of lumines-cence in GaInAs by cross-correlation of 100 fspulses of a CPM dye laser with λ ≈ 620 nm. The achievable time resolution is around 10 fs.

Fig.5.4: Pump-probe measurement of the „tunneling time“ of optical pulses through multilayer dielectric mirrors yielding an accuracy better than 0.3 fs

yielded by the consecutive or

cumulative effect of the be-

ams is taken and accordingly

evaluated. In the case of this

figure, the correlation signal

of luminescence in two GaInAs samples, one being

undamaged and the other af-

ter He+ bombardement, was

studied with respect to time

delay. The laser employed

was a CPM dye laser with

λ ≈ 620 nm and a pulse dur-

ation τ = 100 fs. The result-

ing time resolution in this ca-

se is in the order of ∼ 10 fs [4].

A record temporal resoluti-on at the time this work has

been carried out, is illustra-

ted in Fig.5.4. It reveals the

setup of a measurement of

the “tunnelling time” of op-

tical pulses through multilayer dielectric mirrors (1D photonic bandgaps) [5]. In this

case, being easily understandable with respect to the measurement procedure, the

accuracy was better than 0.3 fs de-

monstrating the capacity of sub-fem-tosecond resolution for ultrafast spectroscopy. The result demanding

the propagation of a pulse faster than

the speed of light has been a very hot


159

Fig.5.5: Price of diode laser optical po-wer versus year (extrapolation for 2005)

topic for a certain time. The explanation is centered on the fact that the pulse is atte-

nuated by about 100 times in an asymmetric way so that shifts of the pulse center

contribute to the speed of propagation.

5.3 Material ablation by ultra-short pulses

Ablation, i.e. vaporization of material without substantial heating of the bulk, was

successfully tested with all kinds of matter so far. Hence this method allows to shape

metals, semiconductors and insulators as well as biological substance leading

to interesting aspects for commercial applicability. This aspect puts several limiting

requirements onto the source of the ultra-short pulses, i.e. the mode-locked laser.

1. Compactness: e.g. 40 × 40 × 30 cm3

2. Moderate cost: e.g. ≤ 50 000 €

3. Design: simple, robust, stable

4. Relevant specifications:

• pulse length

• wavelength

• average power

• repetition rate

• pulse quality and stability

Besides important aspects of laser oscillator and amplifier design, as they have been

discussed in detail in the previous chapters, the price of diode laser optical pump power is a major limitation. This is illustrated in Fig.5.5 following data and estimati-

ons by D. Scifres of Spectra Diode

Labs (SDL) [6]: the price of diode pump

power is plotted versus the year indi-

cating a decrease in $/Watt of 60% per

year. This allows a very optimistic ex-trapolation of 1 $/W to be reached

around 2005. It might be a question of

precision for such a forecast, but there

is no question that such values are

going to be reached. Unfortunately,


Fig.5.6: Femtosecond ablation of metals by (a, top) femtosecond and (b, bottom) nanosecond pulses

according to Momma et al. [7].

prices for diode-pumped solid-state laser like Nd:YAG of today are still higher than for

flashlamp-pumped ones.

For many industrial fabrication processes the shaping of metals is of great import-

ance. A very impressive comparison between the effect of femtosecond pulses drilling holes in steel and nanosecond pulses of even higher energy is depicted in

Fig.5.6a und b [7]. The main advantage of the ultra-short pulse application can be re-

cognized very easily: it is the

regularly shaped crater rims with no trace of dam-age in the surrounding ma-

terial. In more scientific

terms, there is no evidence

of thermal or shock-wave

collateral damage. The low-

er figure illustrating the im-

pact of the ns pulses dem-

onstrates just the contrary.

Nevertheless, there is still

discussion on this issue fac-

ing the experimental result

that under specific circum-

stances even ns pulses may

yield smooth craters and

surrounded by absolutely

unmolten material, as shown

in Fig.5.7 [8].In general, sub-picosecond pulses are the

ones of choice for achieving

the best results in case that

the fluence onto the focal

area is selected carefully not

overheating the material

leading to heat diffusion by hot carriers into the surroundings. Fig.5.8 representing a

result of Laser Zentrum Hannover (by S. Nolte and B.N. Chichkov) demonstrates the


161

Fig.5.7: Laser ablated nozzle plate com-prising an array of 500 nm diameter slant holes at 20° to surface. τp = 20 - 40 ns, kHz repetition rate (Knowles et al. [8])

Fig.5.8: Microstructuring of tungsten by laser ablation with pulses of τp = 100 fs, Ep ≈1mJ, kHz rep. rate, λ=780 nm (Courtesy S. Nolte, B.N. Chichkov, Laser-Zentrum Hannover, Germany)

Fig.5.9: Multishot damage in fused silica by N=80 ultra-short pulses of different duration and flu-ence at λ=780nm yielding various depths as indica-ted in the figure. Fotos courtesy M. Lenzner, pre-sently at BAM, Berlin, Germany

capability of 100 fs pulses with Ep ≈ 1mJ for microstructuring of even hardest me-

tals like tungsten in this case.

The following set of SEM pictures in Fig.5.9 illustrates the effect of laser pulses of

different duration from 3 ps down to 5 fs on a high-bandgap dielectric like silica cre-


ating multi-shot damage [9]. The different shapes of the ablation craters depicted

in the four micrographs can be clearly seen. Picosecond ablation yields a very irregu-

lar hole, while shorter and shorter pulses give rise to eventually completely smooth

holes as the last photograph shows being created by the shortest 5 fs pulses.

Laser-induced breakdown resulting in damage to dielectrics has been subject of

extensive experimental and theoretical investigations since powerful lasers became

available [10,11,12]. It has been described in terms of three major processes: (i) the

excitation of electrons in the conduction band by impact and multiphoton ioniza-tion (MPI), (ii) heating of the conduction-band (hencefourth free) electrons by the

radiation, and (iii) transfer of the plasma energy to the lattice. For pulses of a few

picoseconds or shorter, heat diffusion is “frozen” during the interaction [13] and the

shock-like energy deposition leads to ablation.

Experiments being the background for Fig.5.9 by Lenzner et al. were carried out at

Photonics Institute a few years ago [9] as a comparison of two isotropic dielectrics,

fused silica (FS, Corning 7940), and barium aluminium borosilicate (BBS, Cor-

ning 7059). The surfaces of the samples were formed by a direct drawing process

from the melt; resulting in a residual surface roughness of < 13 nm. The bandgap

energies of the two materials are Eg ≈ 9 eV and Eg ≈ 4 eV, respectively. For a quan-

titative evaluation of ablation, the samples were investigated by light and scanning

electron microscopy. In order to make the ablated volume per pulse Va and the ab-

lation depth da accurately measurable, each site was exposed to 50 pulses at a given

fluence. As an example, Fig.5.10 depicts Va versus on-axis fluence F for two different

pulse durations in FS. The linearity of Va(F) is striking and found to be a general fea-

ture for the entire pulse width regime studied. This characteristic can be utilized for

determining the damage threshold fluence Fth by extrapolating the regression line

of Va on F to Va = 0. Fig.5.11 shows Fth determined in such a way for pulse durations

between 5 ps and 5 fs in FS and BBS. The error bars depict relative (random) errors,

the absolute (systematic) error of the measurements was less than ±15%. Figs.5.12

and 5.13 show the accumulated ablation depth as a function of the number of laser

shots for FS and BBS, respectively, revealing important differences (A) and similari-

ties (B) in the behavior of the two materials. (A) The comparable slopes of the regres-

sion lines in Fig.5.12 yield ablation depths da that exhibit hardly any dependence on


163

Fig.5.10: Volume Va of fused silica ablat-ed by one laser pulse versus energy flu-ence F for two different pulse durations

Fig.5.11: Threshold fluence in FS and BBS versus τ at λ = 780 nm. Each site was irradiated by 50 laser pulses [9]

the pulse duration in FS. In strong contrast, such a τ invariance is limited to the sub-

picosecond regime in BBS (Fig.5.13), whereas da rapidly decreases for decreasing

pulse durations as τ approaches the 10 fs regime. The reproducibility of ablation is,

in both materials, substantially higher in the 10 fs regime than in the subpicosecond

regime.

A few years ago, Stuart et al. [14] derived a simple rate equation for the evolution of

a free electron density n(t) in a dielectric medium exposed to intense laser radiation,

dn/dt = αI(t)n(t) + σkIk, (1)

where I(t) is the intensity of the laser pulse, α is the avalanche coefficient, and σk is

the k-photon absorption cross-section with the smallest k satisfying kћω ≥ Eg, where

ω is the laser angular frequency. The energy of the free electrons heated by the laser

is subsequently transferred to the lattice. This energy transfer leads to the ablation of

the heated zone, which is the major manifestation of femtosecond optical breakdown.

The experimental observations could be consistently interpreted in terms of Eq. (1).

Fth can be predicted by postulating a threshold electron density nth associated with

the onset of permanent damage and solving the rate equation. In strong contrast with

long-pulse damage, the density of seed electrons for the avalanche does not have to

be postulated because it no longer relies on thermal excitation of impurity states, but


Fig.5.12: Ablation depth in fused silica FS for increasing number of pulses and different pulse durations, measured at fluence levels of 5±J/cm2 [9].

Fig.5.13: Ablation depth in barium aluminum borosilicate BBS for increa-sing number of pulses and different pulse durations at fluence levels of 6.2±0.7 J/cm2 [9].

can be produced by MPI with rapidly increasing efficiency for decreasing pulse dura-

tion. The dramatically increased reproducibility of ablation in the 10 fs regime report-

ted above is a direct consequence of the strongly increased deterministic seed elec-

tron production for the avalanche (Fig.5.9 basically illustrates this finding in a two-di-

mensional way). Another interesting finding derived from theory applies just for 5 fs

pulses, where photoionization is dominated by tunnelling as the breakdown threshold

is approached at this pulse duration.

Quantitative prediction of the penetration depth of the incident radiation, and hen-

ce that of da, would call for solving the coupled rate and wave equations for n(t,z)

and E(z,t). Nevertheless, the influence of the pulse duration on da, which is expected

to scale inversely proportional to the free electron density, can be qualitatively asses-

sed by inspecting rate Eq. (1). For a regime in which carrier generation is dominated

by impact ionization, Eq. (1) predicts an electron density, and hence da, that is in-

dependent of τ at a fixed fluence. By contrast, in an MPI-dominated regime, da is

expected to rapidly decrease for decreasing pulse durations (at F is constant). The

theoretically accessible values of the avalanche and MPI coefficients allow predicting

the qualitative behavior of da (τ) and its comparison with the data in Figs.5.12 and

5.13. The fraction of the critical density produced by photoionization at F = Fth is

calculated as np(500 fs)/nth ≈ 4×10-8 and np(50 fs)/nth ≈ 1.5×10-4 for FS, and


165

Fig.5.14: Scheme of interaction of laser pulses with a metal exciting first the elec-tron gas which is coupled to the lattice in a delayed fashion

np(500 fs)/nth ≈ 0.09 and np(50 fs)/nth ≈ 0.35 for BBS. These data suggest that opti-

cal breakdown in fused silica is dominated by the avalanche process down to the

10 fs regime, whereas in BBS having much smaller bandgap, multiphoton ioniza-tion takes over for pulse durations below 100 fs. This finding is conclusively confir-

med by the data in Figs.5.12 and 5.13. As a matter of fact, da is virtually independent

of pulse duration for FS in the entire femtosecond regime. In BBS, this applies only to

the subpicosecond regime, with da becoming subject to a rapid decrease with de-

creasing pulse duration for τ approaching the 10 fs regime. These results show that,

even for a bandgap as large as ∼ 9 eV, MPI produces some 10 orders of magnitude

higher seed electron density in the 10 fs regime than available in thermal equilibrium.

As a result, sub-10 fs laser ablation can be accomplished with a precision correspon-

ding to a few tens of atomic layers. Maybe this will allow precise microstructuring

in the future also for dielectrics by applying 10 fs pulses in the mJ regime.

The comparison of ablation results for

metals and dielectrics given above

clearly reveals the fact that the former

are much easier to shape by ablation

as sufficient densities of free elec-

trons to be heated are around. For a

quantitative treatment, there exist a

number of theories of different com-

plexity to describe the relevant para-

meters. In the following, a simple two-temperature model [7] is pre-

sented by its basic aspects. Fig.5.14

schematically depicts the situation where laser radiation penetrates a solid thereby

first heating the electrons. The hot electron gas is coupled (via a coupling coefficient

γ) to the lattice transferring thermal energy there. Of course, the electron gas may

also loose heat via diffusion, especially via hot electrons. Eventually, the lattice un-

dergoes ablation as soon as the temperature exceeds the boiling point, and hence

atoms being expelled off the surface. The table in Fig.5.15 contains the coupled rate

equations for electron temperature Te and lattice temperature Ti based on the

heat balance in the electron gas (1) and the lattice (2). All the symbols used are ex


Fig.5.16: Ablation depth per pul-se versus fluence for Ti:sapphire laser pulses with λ = 780 nm and τp = 150 fs. The lines repre-sent the best fits to the corres-ponding equations in Fig.5.15 indicating two ablation thres-holds and efficiencies.

Fig.5.15: Metal ablation:Theoreti-cal des-cription by a simple two-tem-perature model [7]

plained in the figure. There exist two special solutions of these coupled differential

equations: in the first case, the characteristic diffusion length of electrons l is much

smaller than the optical penetration depth δ. In the second case l is much larger than

δ. This fact yields different ablation efficiencies as depicted in Fig.5.16. Calculations


167

based on the formulae derived above show the following corresponding results for

two cases of different laser specifications:

Ablation Depth in Metals

Pulse energy 10 µµµµJ, repetition rate 1 MHz, average power 10 W

Focal diameter d ∼ 10 µm d ∼ 50 µm per pulse per ms per pulse per ms

Al 320 nm 0.32 nm 43 nm 43 µm Au 310 nm 0.31 nm 36 nm 36 µm Cu 270 nm 0.27 nm 13 nm 13 µm

Pulse energy 1 mJ, repetition rate 10 kHz, average power 10 W

Focal diameter d ∼ 10 µm d ∼ 50 µm per pulse per ms per pulse per ms

Al 710 nm 7.1 µm 430 nm 4.3 µm Au 700 nm 7.0 µm 430 nm 4.3 µm Cu 640 nm 6.4 µm 380 nm 3.8 µm

In case the average power is kept constant, the results suggest to employ rather

higher repetition rates and smaller pulse energies to achieve the best ablation

efficiency. Of course, the repetition rate is limited by the time, the ablated plasma

plume needs to leave the area of laser irradiation yielding an upper limit for repetition

in the regime between 100 kHz and 1 MHz.

Fig.5.17a and b shows two other examples of high-precision ultra-short pulse ma-chining of metals, in this case of copper: (a) represents a scanning electron micro-

graph of a micro-gear wheel out of Cu. (b) demonstrates the reproducibility of abla-

tion by femtosecond pulses.

Fig.5.17: Femtosecond ablation of copper: (a, left) scanning electron micrograph of a micro-gear wheel. (b, right). Array of holes in order to demonstrate the repro-ducibility. λ = 390 nm, τp = 120 fs, F = 0.8 J/cm2, number of pulses N = 5000 [7]


5.4 Medical applications of ultra-short pulses

5.4.1 Applications to hard-tissue ablation in dentistry

Already shortly after the first demonstration of the laser in the year 1960, first at-

tempts to ablate dental hard-tissue were undertaken. As soon as in 1967, however,

it became evident that ruby, Nd:YAG and CO2 lasers are not suitable for such appli-

cations because by their interaction a damaging temperature rise within the pulp oc-

curred. Hence, only dental technologic applications (prosthesis etc.) of these lasers

could be successfully carried out. The critical limit for a reversible temperature rise

within the pulp lies around 5°C above body temperature. If this limit is exceeded irre-

versible damage is caused [15].

The Ho:YAG and the Er:YAG became the next candidates for dental lasers. Becau-

se of the danger of overheating the pulp they are only used in a pulsed mode mostly

under application of a cooling water spray onto the location of treatment on the tooth.

The pulse durations thereby are in a range between 4 µs and several 100 µs and the

repetition rates typically are around 1-20 Hz. While the laser energy in case of the Er-

laser predominantly is coupled in via absorption of water within the dental hard tis-

sue, in case of the Ho-laser due to the weaker coupling of radiation to water absorp-

tion mostly thermal ablation takes place. Therefore, already in 1993 this type of

laser was considered to be not suited for the treatment of dental hard tissue.

The wavelength of the Er-laser is located close to an absorption maximum of water

thus allowing to couple energy into the dental tissue very effectively. Thereby, via im-

mediate heating and vaporization of the water content within the focus on a tooth,

micro-explosions take place blowing out pieces of tissue [16]. By this approach, ab-

lation rates become feasible which allow practical application of this technology. As-

sociated with this mechanism of ablation, however, micro-cracks within the dental

hard-tissue occur penetrating down to 0.3 mm depth within the tooth. Conventional

turbine based cavity preparation, for comparison, only causes cracks up to 0.02 mm

depth.

Already at the beginning of the 1990s, investigations on ablating dental hard tissue

(enamel and dentine) were carried out with ultra-short pulses [17,18,19] involving

pulse durations around 30 ps. The results achieved with respect to shape of faces,


169

quality of the crater rims and absence of damage within the remaining material (key

issues: free of cracks as well as of molten, re-solidified zones along rims and faces of

cavities) were clearly superior to conventional IR laser systems. In case of suitably

selected parameters even complete absence of damage could be achieved. The rea-

son for this improvement can be seen in plasma-induced ablation, whereby dental

tissue is not removed by sudden vaporization of water as described for the case of

conventional IR laser systems, but via direct plasma formation and the consecutive

expulsion of material. The associated shock waves and the thermal load are signi-

ficantly smaller than in conventional laser cavity preparation. The mechanisms of plasma-induced ablation are as follows: at power densities >1011 W/cm² obtainable

routinely with ultra-short pulses of sufficient pulse energy at suitable focusing condi-

tions, an electric field of >107 V/cm is generated at the focus spot. A micro-plasma is

induced with an absorption coefficient much higher than that of enamel. Conse-

quently, the laser beam is absorbed totally by the plasma. The ablation itself is caus-

ed by the ionization of the enamel and the shock wave generated by this. The validity

of this model was proven by numerous publications and consequently elaborated for

the regimes of femto-, sub-pico- and picosecond pulse durations. In comprehensive

studies the interaction of ultra-short pulses with dental tissue and the ideal parame-ters for ablation were investigated. Thereby, out of reasons of system availability,

predominantly results covering the time interval between 120 fs and a few ps as well

as above 20-25 ps were reported. Spatial scanning over the area to be treated, hav-

ing been employed by some research groups right from the beginning on in order to

make handling for the dentist more efficient as opposed to small focal areas, was

soon also requested out of more scientific reasons, because laser shots impacting

repetitively onto the same spots yield unfavorable heat distribution and cavity shapes

[20] being undesirable in practical applications.

The details of ultra-short pulse tissue interaction are as follows:

i) Plasma formation by the impact of ultra-short pulses

The incident electromagnetic field of the laser radiation causes an avalanche-like generation of free electrons. The laser energy is absorbed that rapidly within the

material so that at pulse durations of ∼ 10 ps and less hardly any thermal or hydro-dynamic response can take place. The value of the fluence leading to the formation

of a plasma depends on several factors, like the wavelength, pulse duration, physical


Fig.5.18: Observed values of damage threshold for fused silica at λ = 1053 nm (full circles) and 825 nm (full triangles), and CaF2 at λ = 1053 nm (full squares). Solid lines are τ1/2 fits to long pulse results. Estimated absolute error in data is ± 15% [22].

tissue parameters and defect density [21]. Models to calculate the relevant quantities

in case of dental hard tissue follow quite exact calculations performed on silica as-

suming a situation matching to an insulator in many respects. Fig.5.18 gives relevant

data on damage threshold for fused silica at λ = 1053 nm and 825 nm as well as for

calcium fluoride CaF2 at λ = 1053 nm [22]. The trigger for the plasma formation is the

high intensity I of the incident radiation (starting at ∼ 0.1-10 TW/cm²). At such values

multi-photon absorption (MPA) is very likely to occur. A valence electron can gain

enough energy by the absorption of several photons (e.g. 8 for fused silica, 6 for

water @ λ = 1 µm) resulting in an effectively free electron. The probability for the

simultaneous absorption of m photons scales as Im also making the process quite

confined. The free electron faces additional acceleration in the electromagnetic field

of the laser pulse creating a hot electron. Collisions with the surrounding atoms lead

to impact ionization causing an avalanche process to start. Even transparent media

become highly absorbing in this way. In general, an initial electron concentration n0 is

present in all materials, especially in dental hard tissue. It can be easily shown (simi-

larly to description above)[23] that n0 is unimportant if it is small compared to the ratio

of the MPA source term to the avalanche rate. This is always the case due to the

high nonlinearity if I is sufficiently high. The electric conductivity can be derived in a

simplified model by assumptions on the electron-phonon coupling and hence on the

mobility of electrons being comparable in silica and dental enamel. According to the

Wiedemann-Franz law the thermal and the electric conductivity are mutually propor-

tional thus allowing to approximate the thermal diffusivity. Quantitative results of the

model thus are: the thermal diffusion length for heat induced by a 1 ps pulse is on

the order ~0.01 µµµµm in biological tissue. Hence no heat conduction for pulses with


171

Fig.5.19: Distribution of the absorbed laser energy density in di-rection of the beam for 4 different pulse durations in dental hard-tissue. The flu-ences were assumed to be 10% above ablation thres-hold. Nearly all the energy is deposited within a layer of 1 µm (λ = 1053 nm, µa li-near absorption coefficient)

Fig.5.20: Generation of free elec-trons (a) solely via MPA (dotted line) and (b) via MPA and impact ionization together (full line). The incident laser pulse is shown by the broken line for reference [23].

durations < 1 ps may be assumed (for comparison the mean-free path of an electron

in Si is ~1000 times larger, hence the corresponding diffusion length ~1 µm).

ii) Ultra-short pulse propagation in dental material

The propagation of an ultra-short laser pulse within a material in this context is gover-

ned by absorption and reflection at the layer of plasma generated by the pulse it-

self. Absorption is described in this case (when neglecting reflection as a first step)

by the dissipated energy per time employed for the generation of free electrons. Mea-

surements like the ones depicted in Fig.5.19 for fluences 10% above the ablation

threshold show that the values of absorption throughout the whole range of depths

within tissue do not differ significantly for pulse durations between 0.5 ps and 10 ps.

For all pulse durations within this regime nearly the whole energy is deposited within

a layer of ∼ 1 µm thickness [24].


Fig.5.21: The generation of high electron densities yields reflection of the laser pulse by the plasma. The full line depicts the intensity trans-mitted into the tissue un-derneath the plasma layer. The dotted line represents the shape of the incident pulse for reference [21].

Fig.5.20 describes the evolution of electron density at the material surface for a

pulse close to ablation threshold. From the quasi-parallel onsets of curve (a) and (b)

within the rising part of the pulse it becomes evident that MPA within the ultra-short

pulse temporal regime is the most important initiation effect for the increase of

carrier density. Because of its strong intensity dependence, in fact, it ends very soon

after the peak of the pulse indicated by horizontal part of curve (a) [23].

After MPA impact ionization dominates based on the acceleration of electrons in the

electric field of the laser leading to stochastic collisions of those electrons with atoms

giving rise to avalanche multiplication effects. The incident radiation is most effective-

ly absorbed in the vicinity of the critical electron density nc (see below). Radiation be-

ing not absorbed on its way towards this layer having nc is reflected there [21]. As

soon as the material within the surface layer is completely ionized, the additional inci-

dent radiation acts predominantly towards the increase of electron energy. The re-

flection over the temporal span of the incident pulse, however, is rather incomplete as

it is graphically demonstrated in Fig.5.21.

Generally, the reflection increases proportional to the plasma density, i.e. the

penetration depth decreases with the generation of the plasma. If the electron density

in the material reaches the critical value nc being defined as

2

20

4 emn c π

ω=


173

the plasma becomes highly reflective for the incident radiation (ω0 being the angular

frequency of the laser light). Thus, a dependence of the critical electron density

nc ∝ 1/λ² on the wavelength λλλλ follows representing the well-known shielding effect.

The higher the intensity within the pulse, the earlier reflection occurs, i.e. strong ab-

sorption of radiation happens at such electron densities n within a very thin layer

whose thickness is proportional to √n. After nc is reached, the penetration depth is

not reduced any further. As soon as total ionization has taken place, additional input

energy is used for heating the electrons causing a rather fast drop of absorption and

an increase of reflection. This leads to the conclusion that intensities being higher

than necessary for plasma formation are not useful. Hence, the existence of an optimum fluence for ultra-short pulse ablation can be derived.

Reflection plays only a minor role for fs pulses as due to their short duration only a

fraction of the trailing pulse edge can be reflected. For ps pulses stronger interaction

takes place because a substantial part of the pulse propagates into the plasma form-

ed by the leading section of the pulse, i.e. a larger fraction of the energy is employed

for the heating of the plasma or reflected.

Process of ablation

The ablation process proper happens after the end of the impacting laser pulse. As

all the absorbed energy is concentrated in a very thin layer, an energy density occurs

far above the binding energy per atom. Consecutively, the energy of the electrons is

transferred to the ions, a process lasting some tens of ps. The largest fraction of the

energy within the plasma is not used to overcome the binding energy but for acceler-ation of the material to be expelled [20]. The plasmafied material is thrown out ac-

cording to hydrodynamic laws whereby the layer thickness and the velocity of expul-

sion can be calculated according to the deposited energy. Ablation depths are

around ∼ 1 µm. An increase of pulse energy does not lead linearly to an increase of

penetration depth, due to the rising reflectivity of the plasma. The typical duration of

the expulsion process lies within a few ns [20]. The processes of material expulsion

and crater formation are to a large amount independent of the structure and molecu-

lar composition of the tissue. Behind the layer of thickness δ a shock wave propa-

gates into the material, heating up and removing also deeper lying layers. The trans-

fer of input energy into heat, however, is limited to ∼ 15 %. The expanding expelled


plasma cools down and recombines within a time duration of ~100 ps. Thereby the

exiting plasma wave undergoes a transition into a wave of evaporation. This evapo-

ration process of the irradiated tissue goes on until the temperature falls below the

evaporation point.

Effects on the surrounding tissue Only a small amount of energy stays in the surrounding material while most of it is

removed together with the ablated matter. Via the dissipation of the shock wave ener-

gy a rise of pressure and temperature is caused in deeper lying layers. Thus 3 zones

may be distinguished within the material:

1. Ablated zone (< 0.32 µm): the material is completely removed (1 - 30 ps

after the end of the pulse). In this case, the penetration of MPA amounts

to ∼ 0.1 µm, up to 0.18 µm depth plasma is generated by the electron

avalanche. Between 0.18 µm and ∼ 0.32 µm, i.e. within a layer where no

plasma is built up, ablation is caused by impact heating. The shock

wave penetrating into the tissue still deposits sufficient energy to enable

ablation there (i.e. evaporation).

2. Zone of irreversible physical modifications: shock heating, however with

no ablation (> 0.32 µm).

3. Zone influenced by mechanical and thermal interaction where reversible

effects take place with no indication for physical modifications.

Zones 1 and 2 together may have a thickness of < 5 µm in reality in case of certain

fluences [25]. Caused by high pressures on the surface of the material during the ab-

lation process (plasma expansion) a pressure wave penetrates into the material pro-

pagating at a velocity of ∼ 5 µm/ns (slightly faster than the speed of sound at room

temperature). According to simulations, the pressure peaks reach up to ∼ 10 kbar and

decay exponentially towards larger material depths. The simulations also report on

tensile stresses allowing to assume that ablation also may happen via splintering off

processes. The shock wave of the entering pressure wave has extremely high fre-

quency (1-10 GHz). Due to this, however, it cannot penetrate deeply into the tissue

and is absorbed in a very shallow layer under the surface (< 1 µm). The amplitude of

recoil pressure caused by the evaporation process is defined by the product of ex-

pelled mass times the expulsion velocity. The duration of this recoil pulse depends on


175

the temperature distribution within the irradiated medium. As the fraction of energy

contained within the high-frequency components of the recoil wave is much higher in

case of ultra-short pulses than in case of longer laser pulses (e.g. sub-ns pulses) the

shock wave of the former is damped significantly more.

A shock wave in the kbar regime can damage the tissue persistently. The steeper

the rising slope of the wave amplitude the higher the damage. Via nonlinear propa-

gation of acoustic waves of high amplitude their velocity can exceed the speed of

sound thereby generating very steep rising slopes. When comparing shock waves

caused by 350 ps pulses with corresponding ones originating from 350 fs pulses, the

former cause more substantial damage as the major part of their frequency spectrum

contains lower frequencies intruding into tissue with higher pressures. In comparison,

350 fs shock waves have only ¼ of the recoil amplitude and half of the gradient depth of the 350 ps waves.

Heat conduction is another very critical issue on the surrounding tissue and has to

be discussed also. The thermal relaxation time τrel is the characteristic time heat

needs to penetrate deeper into the material in order to reduce the local temperature

by a factor of 2. If the pulse duration is substantially shorter than τrel the heat input

depends only on the incident laser radiation and not on the heat conduction proper-

ties of the tissue. In order to avoid linear absorption and the associated strong heat-

ing, the laser pulses should have a risetime as short as possible to generate plas-

ma as early as possible. The trailing edge is much less critical as its energy is mostly

reflected by the plasma.

Out of all these findings one can derive the following conclusion: sub-picosecond pulses cause a lower heat input into the surrounding tissue, although in no case

picosecond pulses were declared to be incapable for the ablation of dental hard

tissue.

As a conclusion of this subsection, 3 electron micrographs (Fig.5.22a,b,c) are pre-

sented showing examples of successful dental enamel processing by femtose-cond pulses (A. Kasenbacher in cooperation with Laser-Zentrum Hannover). These

results have been acquired with a commercial laser system of rather great complexity

comparable to the setup depicted in Fig.5.23 not allowing commercial application of


Fig.5.22: Examples of successful femtose-cond ablation of dental enamel. The micrograph on the left bottom shows a detail of the larger section on the left top. The parameters are: depth d=321 µm, λ=780 nm, energy per pulse Ep=50µJ, τp=130 fs, fluence F= 4.2 J/cm2, Ipeak=3.23×1013 W/cm2, frep= 1kHz Courtesy A.Kasenbacher, Germany

this technique without the availability of a totally different new ultra-short pulse laser

design, maybe following one of the concepts presented in Lecture 2.

Fig.5.23: Typical setup and parameters of a standard femtosecond Ti:sapphire laser system with amplifier: λ=750-850 nm, τp=100 fs – 5 ns, Ep=1 mJ (350 µJ),

fr=1kHz (5kHz), beam quality 6 times diffraction limited.


177

Fig.5.24: Optical photomicrograph showinga 550 µm deep excision in bovine brain tis-sue. τp=140 fs, Ep=11 µJ, λ=800 nm, spot size dspot=20 µm, excision volume by abla-tion Va=0.37 mm3, number of applied pul-ses N=36000. Va/pulse=10.3×103 µm3. Thesample was stained with HE.

Fig.5.25: Corneal ultra-ac-curate surgery by femtosecond laser pulses: (top row) SEMs of in-vitro intrastromal cutt-ing in primate eye, (a) successful by femtosecond and (b) unsatisfactory by 60ps pulses. (bottom row) Intraoperative and 1-week post-oper-ative view of rab-bit eye treated with femtosecond laser pulses in a spiral pattern.

5.4.2 Applications to soft-tissue ablation: examples

Soft-tissue can be ablated by ultra-

short pulses very effectively, as the

example on bovine brain tissue given

in Fig.5.24 [26] shows. The precision of the cut and the complete absence of thermal collateral damage are in-

triguing. The laser operation data are

mentioned in the figure caption.

Also applications in ophthalmology

have been successfully tested. Corne-

al shaping usually is well-known in the

context of application of excimer laser

radiation. Fig.5.25 depicts a series of 4

scanning electron micrographs of in-vitro intrastromal cutting in a primate eye

(i.e. a cut achieved in the interior of the cornea by a two-dimensional series of optical

breakdowns) [27]. The first row allows a comparison between (a, left) a successfully


Fig.5.26: (a,left) Histological section of pig myocardium drilled by excimer (top) and USPL (bottom), illustrating extensive thermal damage or smooth-side holes, respecti-vely. (b,right) Luminescence signal for bone and spinal cord. The prominent Ca peaks

allow the distinction between bone and spinal cord or other soft tissue. (bottom,left) 1 ns ablation of enamel showing extensive thermal damage and cracking. (bottom,

right) Smooth hole in enamel with no thermal damage after drilling with a USPL.

lifted corneal flap and extracted lenticule, generated by a femtosecond laser, and (b,

right) unsatisfactory dissection when using longer 60 ps pulses. The background of

this procedure is the idea to keep an epithel layer of the cornea intact and remove

only internal layers. The second row in Fig.5.25 shows the interoperative (a, left) and

1-week post-operative (b, right) view of rabbit eye treated with femtosecond laser

pulses in a spiral pattern. Strong scattering at the cavitation bubbles is observed in

(a). Transparency returns to normal at approximately 3 days (b).

Ultra-short pulses have been applied for myocardium drilling as shown in Fig.5.26a

depicting a comparison between the results achieved by excimer laser (top) and

ultra-short pulse laser (bottom). The former approach is characterized by extensive

thermal damage surrounding the hole while the latter shows a smooth-sided hole free

of thermal damage to surrounding tissue (source U.S. Department of Energy, Medical

Technology Program Livermore National Laboratory). In Fig.5.26b the luminescence


179

Fig.5.27: Optical coherence tomographyfilling headlines of journals: tomogram ofXenopus laevis (African frog) with sub-cellular resolution.

signals of different tissue sections are identified by their spectroscopic lines. This

idea also has a great potential for identification feedback when ablating dental tissue.

5.4.3 Optical coherence tomography using ultra-short pulses

A very challenging rather new field of application of shortest femtosecond pulses is

optical coherence tomography [28] which was advertised in recent years in many pub-

lications, e.g. in the trade journal Biophotonics (October 1999) on the front page

(Fig.5.27). This method originally involved superluminescent diodes having a rather

short coherence length defining the resolution of measurement [29], originally of dist-

ances between optically different lay-

ers in the eye, like the thickness of

the cornea or the depth of the eye

down to the retina [30]. Soon, two-di-

mensional scanning of the back-

ground of the eye was achieved and,

furthermore, this method was applied

to a variety of scattering biological

media like skin tissue or dental tissue

[31]. A major step forward was made

by taking advantage of the short coherence length of femtosecond

pulses which is in the µm regime

and hence about one order of magni-

tude shorter than the one of superlu-

minescent diodes but associated with

excellent brightness of laser sources.

As a definition, optical coherence to-

mography allows non-invasive images of cross-sections (tomograms) in scattering

media, especially in biological tissue. Imaging through scattering media was an issue

since long time, i.e. since the early das of laser development. Fig.5.28 schematically

shows the propagation of a laser pulse through scattering media. The unscattered

photons are faster, but fewer and under normal conditions completely overwhelmed

by the scattered photons arriving later. If it is possible to establish a temporal gate


Fig.5.28: Scheme for imaging through scattering media (a) by employing a fast optical gate switch allowing just few unscattered but faster photons to reach the detection system generat-ing the image while the larger numbers of scattered photons are shut off (b).

Fig.5.29: (upper half) Scheme of a fiber-optic Michelson interferometer for optical coherence tomography consisting of measurement (S) and reference (R) arms. (lower half) Tomographic cross sections through an onion, taken (a) by high-resolution Ti:S OCT or (b) superluminescence diode OCTsystems [32].

switch to select only the unscattered photons, imaging becomes possible. This was

achieved by non-linear fast optical switches which, however, are very inefficient and

require large laser power. In the concept of OCT, the gate is realized as a coher-ence gate.

Fig.5.29 exhibits a very simplified scheme of a modern fiber-based optical coher-ence tomograph [32]. It

seems to be worth mentio-

ning that it is no problem or

even contradiction when

considering ∼ 10 fs pulses

with a bandwidth of about

120 nm propagating through

non-negligible lengths of

optical fiber associated with

a lot of dispersion. It is the


181

Fig.5.30: Comparison of bandwidth (top) and coherence length (bottom) for super-

luminescent diodes and femtosecond laser pulses

coherence length and bandwidth remaining unchanged even if the originally band-

width limited pulses out of the laser are torn apart by dispersion. A compensator

within the reference arm should provide approximately equal dispersion conditions in

both, the measurement and the reference arm. Fig.5.30 allows a comparison of

bandwidth and coherence length for superluminescent diodes and femtosecond laser

pulses being more or less self-explaining. Thus, superluminescence diodes yield

∼ 20 µm longitudinal resolution while femtosecond lasers achieve one order of

magnitude smaller values, i.e. ≥2 µm.

In this way, a resolution of ∼ 2 µm in the

longitudinal direction is achieved as it

can be seen in Fig.5.31 showing sub-cellular details of African frog tissue

(xenopus laevis [33,34]) with a lateral re-

solution of 3 µm. This most modern

version of OCT, although so far only

performed by Ti:sapphire pulses

around 800 nm penetrating about 1

mm into normal tissue, seems to be

ideally suited for the application of

diode-pumped femtosecond lasers like

Cr3+:LiSAF or LiSGaF as they can be

directly diode-pumped and hence are

substantially simpler and potentially

cheaper. Their penetration depth is

also somewhat deeper due to the

center wavelength being about 100 nm

longer. The average power needed for OCT lies in the sub-10mW regime, usually it

is 3-5 mW, representing a rather modest requirement in general. For ophtalmological

applications only 300 µW are needed and only ∼ 1mW would be permissible for safety

reasons [35]. The implementation of OCT together with compact and moderately priced femtosecond laser sources in several wavelength regimes would be highly

desirable: there are absorption windows around 950 nm, 1300 nm (yielding approxi-

mately 4 mm penetration depth) and 2200 nm. If taking also advantage of secondary


Fig.5.31: Optical coherence tomograms of African frog tadpole (Xenopus laevis) yielded by a Ti:sapphire femtosecond laser having τp=5.4 fs and 250 nm band-width revealing sub-cellular resolution. The image is 0.83 × 1 mm large, 1800 ×

1000 pixels. On the left picture, the olefactory tube (OT) and mitosis of 2 cell pairs(arrows) are shown. Courtesy of W. Drexler.

information like spatially resolved dispersion and absorption (compare Fig.5.32

[36]) this will revolutionize in-vivo non-destructive medical diagnosis being applicable

to many areas of interest via endoscopic methods.

Optical coherence tomography is al-

so, as mentioned above, used for

dental applications: Fig.5.33 illu-

strates this approach and its results

in an impressive way allowing to di-

stinguish the various features of a

tooth, aiding diagnosis (source U.S

Department of Energy, UCRL-MI-

129402).

Fig.5.32: Comparison of high-resolution (top) and spectroscopic (bottom) femtose-cond OCT. By exploiting linear optical properties like dispersion and absorption, ad-ditional contrast can be yielded which usually is depicted in false colors as the bar

below the pictures indicates [36].


183

Fig.5.33: Optical coherence tomography for dental applications. (left) Cross secti-onal images taken with OCT can be combined to give a 3-dimensional represen-tation. (right) In the OCT image (right part) the various features of a tooth can be

easily distinguished, aiding diagnosis.

Fig.5.34: Example of multi-photon non-linear microscopy depicting two-photon fluorescence of leukaemia cells of rats [37]

5.4.4 Three-dimensional resolution by non-linear interactions within the focus of ultra-short pulses

Ultra-short pulses having sufficient sin-

gle pulse energy and repetition rates in

the MHz regime allow the application of

high peak powers to the focal spots

reaching intensities as required for var-ious non-linear processes without

destroying the sample necessarily, to-

gether with sufficient speed for e.g.

sampling or scanning purposes. Multi-photon non-linear microscopy (e.g.

[37]) takes advantage of this aspect al-

lowing three-dimensional resolution

via two- and three-photon fluorescence

excitation or second and third harmonic

generation. In this case, the require-

ments with respect to short pulse dur-

ation or high-average power are not

very stringent. Therefore, this techni-

que has been started to be employed


Fig.5.35: Classical steps of rapid-prototyping employing monomers to become UV-hardened: first a cal-culated CAD-model is computer-decomposed into layers. In each of the layers laser-assisted gen-eration of cured rasin structures is carried out, which consecutively form the real 3D model. The fabrication of the single solidified layers involves a number of mechanical steps of motion like dip-ping in and wiping off, so that the overall process is rather time consuming.

already a number of years ago. Fig.5.34 represents a very successful example de-

picting two-photon fluorescence of leukaemia cells of rats [37]. The development of

compact diode-pumped sources might contribute to the wide spread of this technique

in an innovative way as well as the availability of a wide spectral range of ultra-short

pulses.

Related to this concept, with respect to localized non-linear intra-focal interaction,

but demanding for much more average power is a rather novel approach to rapid-prototyping with polymers. Traditionally, two-dimensional computer controlled expo-

sition of monomers to UV cw laser radiation allows to solidify them to polymeric struc-

tures. They have to be created layer by layer eventually yielding a sizable three-di-mensional body. Typical commercial exposition times for 30 cm sized figures are of

the order of 50 hours limited by UV laser power on the one hand, but also by many

mechanical steps like immersion into the liquid monomer, wiping off the excess liquid,

linear sequential exposition by a scanned focus and so forth. Fig.5.35 illustrates how

this technique works. 3D-prototyping, however, makes use of three-dimensional

confinement of non-linear interactions within the focus of an ultra-short pulse train.

Two- or three-photon absorption can expose a monomer [38,39] and create a hard

structure within the volume of the liquid [40] starting from a substrate. There are no

mechanical motions required any more, and therefore the process is potentially much


185

Fig.5.36: Example for stereo litho-graphy for medical appli-cations generated by poly-mer rapid-prototyping. The commercial fabrication process takes around 50 hours representing a po-tential for shortening by more advanced methods like 3D ultra-short pulse rapid-prototyping.

Writing progression

Top surface

60µµµµm 10µµµµm

Writing beam

Guided, coupled beams

faster than the traditional one depending basically on the average power of the ultra-

short pulse source. Laser-Zentrum Hannover so far is one of the first institutions pub-

lically demonstrating the method for rather small structures (1 cm dimension) [41].

In the field of integrated optics, higher and higher densities of package are neces-

sary and realized involving waveguide writing in three dimensions. Recently,

some work on photonic device fabrication in glass using non-linear materials pro-

cesssing with a femtosecond laser oscillator was completed successfully at M.I.T.

having the potential for revolutionizing the integrated device fabrication [42]. A cross-

section of the three-dimensi-

onally written waveguides by

ultra-short pulses is depicted

in Fig.5.37.

Fig.5.37: Scheme of 3D-writing of wave-guides by ultra-short pulses in-cident from the top on the 60 µm thick wafer. The ver- tical separation of the guides is 60 µm.


time

27.2 ns 24 ns

time

Power

271-bitword

time

λλλλ 27.2 ns

fs FiberLaser

36.7 MHz

4 km DCF (-340ps/nm)

9.94 Gb/s Data

Pulse PatternGenerator

InGaAsPModulator

Ext.Clock

271RF

harm.

st

9.94 GHz

TDM

λλλλ

<100 fs

101 10 ... 11" " " "11 0271-bitword

21 ns27.2 ns

Tx Output

(*) Dynamic biasadjustment

Fig.5.38: Schematic diagram of 206-channel chirped-pulse WDM transmitter by Boivin et al. [43]. The channels are spaced by ∼ 37 GHz, the channel bit rate is

36.7 Mbit/s and the aggregate bit rate is 9.942 Gbit/s. The device is operated by a single mode-locked femtosecond fiber laser and a single TDM modulator

5.4.5 Advanced applications in optical communications

Also in the field of optical communication femtosecond pulses become more and

more important and get used for wavelength division multiplexing (WDM) and

demultiplexing. Fig.5.38 illustrates a 206-channel chirped-pulse WDM transmitter as

published by Nuss, Knox and collaborators at Bell Laboratories about 4 years ago

[43]. This represented a record for the number of channels, using a single femtose-

cond laser (τp < 100 fs, fr =36.7 MHz @1.55 µm) and a single time-division multiplex-

ed electro-absorption modulator. The channel spacing is ∼ 37 GHz (0.3 nm), and the

bit rate in each channel is 36.7 Mbit/s. In the meantime, this approach has been dra-

matically improved to a 1021 channel WDM system [44]. Very recently, 80 Gbit/s de-

multiplexing via a semiconductor optical amplifier in an ultra-fast nonlinear interfero-

meter using two mode-locked fiber lasers (@ 1550 nm and 1545 nm) yielding a 5 ps

switching window has been achieved at M.I.T. Lincoln Laboratory [45].


187

References (Lecture 5): [1] E. Wintner, J. Appl.Phys. 57, 1533 (1985) [2] E. Wintner, Optics and Laser Technology 17, 159 (1985) [3] E. Wintner and E.P. Ippen, Appl. Phys. Lett. 44, 999 (1984) [4] R.A. Höpfel, N. Sawaki, E. Wintner, Appl. Phys. Lett. 55, 460 (1989) [5] Ch. Spielmann, R. Szipöcs, A. Stingl, F. Krausz, Phys. Rev. Lett. 73, 2308 (1994) [6] D. Scifres, Conf. on Lasers and Electro-Optics, CLEO (1999) [7] C. Momma et al., Laser und Optoelektronik 29, 82 (1997) [8] M.R.H. Knowles, G. Rutterford, A.I. Bell, A.J. Andrews, G. Foster-Turner, A.J. Kearsley,

D.W. Coutts, D. Kapitan, C.E. Webb, CLEO 2000 San Francisco, paperCFD5, p 581 [9] M. Lenzner, J. Krüger, S. Sartania, Z.Cheng, Ch. Spielmann, G. Mourou, W. Kautek, F.Krausz,

Phys. Rev. Lett. 80, 4076 (1998) [10] N. Bloembergen, IEEE J. Quantum Electron. QE-10, 375 (1974) [11] R.M. Wood, Laser Damage in Optical Materials (Hilger, Boston, 1986) [12] S.C. Jones et al., Opt. Eng. 28, 1039 (1989) [13] M.H. Niemz, Appl. Phys. Lett. 66, 1181 (1995) [14] B.C. Stuart et al., Phys. Rev. Lett. 74, 2248 (1995) ; J. Opt. Soc. Am. B 13, 459 (1996) [15] L. Zack, G. Cohen, Pulp response on external applied heat, OS,OM & OP V 19, 515-430 (1965) [16] A. Mehl, L. Kremers, K. Salzmann, R. Hickel, 3D-volume-ablation rate and thermal side effects with

the Er:YAG and Nd:YAG laser, Dent. Mater. 13, 246-251 (1997) [17] G.B. Altshuler, N.R. Belashenkov, V.B. Karasev, A.V. Skripnik, A.A. Solunin, Application of ultra-

short laser pulses in dentistry, SPIE 2080, 77-81 (1993) [18] A. Mindermann, M.H. Niemz, L. Eisenmann, F.H. Loesel, J.F. Bille, Comparison of three different

laser systems for application in dentistry, SPIE 2080 Dental Application of Lasers, 68-76 (1993) [19] M.H. Niemz, Schmerzfreie Zahnpräparation mit dem Nd:YLF-Pikosekundenlase, Laser und

Optoelektronik 26, 68-73 (1994) [20] A.M. Rubenchik, L.B. Da Silva, M.D. Feit, S. Lane, R. London, M.D. Perry, B.C. Stuart, J. Neev,

Dental Tissue Processing with Ultra-Short Pulse Laser, SPIE 2672, 222-230 (1996) [21] W. Seka, J.D.B. Featherstone, D. Fried, S.R. Visuri, J.T. Wals, Laser ablation of dental hard tissue:

from explosive ablation to plasma-mediated ablation, SPIE 2672, 144-158 (1996) [22] M.D. Perry et al., Ultra-short pulse laser machining of dielectric materials, J. Appl. Phys. 65,

(1999) [23] M.D. Feit, A.M. Rubenchik, B.W. Shore, Unique Aspects of Laser Energy Deposition in the fs Pulse

Regime, SPIE 2672, 243-249 (1996) [24] A.A. Oraevsky, L.B. da Silva, A.M. Rubenchik, M.D. Feit, M.E. Glinsky, M.D. Perry, B.M. Mammini,

W. Small, B.C. Stuart, Plasma Mediated Ablation of Biological Tissues with Nanosecond- to Femtosecond Laser Pulses: Relative Role of Linear and Nonlinear Absorption, IEEE J. Sel. Top. Quantum Electron. 2, 801-809 (1996)

[25] J. Neev, D.S. Huynh, C.C. Dan, J.M. White, L.B. da Silva, M.D. Feit, D.L. Matthews, M.D. Perry, A.M. Rubenchik, B.C. Stuart, Scanning Electron Microscopy and Ablation Rates of Hard Dental Tissue Using 350 fs and 1 ns Laser Pulses, SPIE 2672, 250-261 (1996)

[26] F.H. Loesel et al., Non-thermal ablation of neural tissue with femtosecond laser pulses, Appl. Phys. B 66, 121 (1998)

[27] T. Juhasz, F.H. Loesel, C. Horvath, R.M. Kurtz, G. Mourou, invited talk, Digest Ultrafast Phenomena, Garmisch Partenkirchen (1998)

[28] Ultrafast Phenomena XII-OSA Technical Digest (Optical Society of America, Washington DC (2000); Ultrafast Phenomena X-Proceedings of the 10th International Conference, Del Coronado, CA (1996) (Springer Series in Chemical Physics 62); Ultrafast Phenomena XI-Proceedings of the 11th International Conference, Garmisch-Partenkirchen (1998) (Springer Series in Chemical Physics 63).

[29] For a comprehensive review: A.F. Fercher, J. Biomed. Opt. 1, 157 (1996) [30] W. Drexler, In vivo optical coherence tomography and topography of the fundus of the human eye,

PhD thesis, Univ. Technology Vienna (1995) [31] A. Baumgartner, Ch.K. Hitzenberger, S. Dichtl, H. Sattmann, A. Moritz, W. Sperr, A.F. Fercher,

Proc. SPIE 3248, 130 (1998) [32] W. Drexler, U. Morgner, F.X. Kärntner, C. Pitris, S.A. Boppart, X.D. Li, E.P. Ippen, J.G. Fujimoto,

Opt. Lett. 24, 1221 (1999) [33] W. Drexler, private communication (2000)


[34] U. Morgner, F.X. Kärntner, S.H. Cho, Y. Chen, H.A. Haus, J.G. Fujimoto, E.P. Ippen, V. Scheuer,

G. Angelow, T. Tschudi, Opt. Lett. 24, 411 (1999) [35] comp. European Standard EN 60825-1 about safety of laser products (1996); based on IEC 825-

1:1993+A11:1996 [36] U. Morgner, W. Drexler, F.X. Kärtner, X.D. Li, C. Pitris, E.P. Ippen, J.G. Fujimoto, Opt. Lett. 25,

111(2000) [37]. Ch. Xu, W. Zipfel, J. Shear, R. Williams, W. Webb, Proc. Natl. Acad. Sci. USA 93, 10763 (1996) [38]. Ch. Ondrusek, diploma thesis, Univ. Technology Vienna (2001) [39]. B.H. Cumpston, S.P. Ananthavel, S. Barlow, D.L. Dyer, J.E. Ehrlich, L.L. Erskine, A.A. Heikal,

S.M. Kuebler, I.-Y. Sandy Lee, D. McCord-Maughon, J Qin, H. Röckel, M. Rumi, Xiang-Li Wu, S.R. Marder, J.W.Perry, Nature 398, 51 (1999)

[40]. S. Kawata, Hong-Bo Sun, T. Tanaka, K. Takada, Nature 412, 697 (2001) [41]. B.N. Chichkov, private communication; announcement at LASER 2001 World of Photonics,

Munich 2001 [42]. K. Minoshima, A.M. Kovalevicz, I. Hartl, E.P. Ippen and J.G. Fujimoto, Opt. Lett. (in press, 2001) [43]. L. Boivin, M.C. Nuss, W.H. Knox, J.B. Stark, Electron. Lett. 33, 827 (1997) [44]. B.C. Collings, M.L. Mitchell, L. Boivin, W.H. Knox, IEEE Photon. Tech. Lett. 12, 906 (2000) [45]. E.P. Ippen, private communication 2001

Documents

Ultra-Short-Pulse Laser Technology and Applicationsinfo.tuwien.ac.at/tubiomed/de/aktuell/Wintner_Laser_Lecture_5p153.pdf · Ultra-Short-Pulse Laser Technology and Applications: Lecture