At the present time ultrashort laser pulses are the basis of micromachining, medical diagnostic and therapy, and high precision time-resolved spectroscopy. Therefore, a reliable mode-locking mechanism is crucial. Additionally, with increasing laser power and increasing importance in industrial usage of pulsed lasers the requirements con-cerning damage threshold and robustness of mode-locking mechanisms increase, too.
For pulsed high-power lasers mode-locking with a semiconductor saturable absorber mirror (SESAM) is very common. Thus, a deeper understanding of damage mecha-nisms in SESAMs as well as new SESAM designs and even new mode-locking concepts to overcome these problems is essential to push the limits in mode-locked high-power lasers. These objectives were explored within this thesis based on the realization of a high-power mode-locked Yb:YAG thin-disk laser.
For the investigation of optical and structural properties of SESAMs and Bragg mirrors a variety of experimental setups was built. Static reflectivity spectra were measured with a FTIR-spectrometer and for the investigation of nonlinear reflectivity spectra even two setups were realized: Degenerate pump-probe spectroscopy allowing for time-resolved reflectivity transients as well as single pulse spectroscopy. At the latter case the comparison of the SESAM’s reflectivity with the reflectivity of a highly reflective mirror allows for absolute reflectivity spectra of the SESAM. Based on an appropriate data analysis of the time-resolved pump-probe transients both methods lead to the same result for the nonlinear reflectivity curve of a saturable absorber. This consis-tency is very important, since based on the nonlinear reflectivity curve key parameters of a SESAM such as the modulation depth, the saturation fluence, and the roll-over parameter can be extracted.
In future, the experimental methods for SESAM characterization could even be im-proved. Since experiments have shown that the nonlinear reflectivity curve depends on the SESAM temperature an in-situ time dependent characterization of a mode-locking SESAM would be very interesting. The intracavity mode-locked laser pulse could be used as pump pulse, whereas an external probe pulse must be coupled into the cavity and synchronized with the pump pulse. For different values of intracavity power tran-sients of different fluence values could be taken giving insight in the SESAM dynamics during mode-locking performance. Long-term investigations would even allow for a real-time observation of aging and damage effects.
During this PhD study, damage effects of SESAMs were analyzed based on the ob-servation of the consequences of laser induced damage of a SESAM on a mode-locked Yb:YAG thin-disk laser. Additionally, the damaged SESAMs were characterized in
extracavity setups. These observations showed that the mode-locking mechanism of a SESAM mode-locked laser is destabilized when the pulse intensities on the SESAM were too high, respectively when the SESAM was slightly damaged. In worst case the mode-locking regime even broke down and burn marks were visible to the naked eye.
Characterizing experiments of intentionally damaged SESAMs indicated that at the very beginning of the damage process the top layers of the SESAM were destroyed.
For example, if a coated SESAM is slightly damaged the coating layers are defective or even completely lost leading to a modified nonlinear reflectivity curve. This in turn results in a modified mode-locking mechanism explaining the destabilization of the pulse generation. If the damage was too strong the nonlinear reflectivity curve could not be measured at all. This indicated that in worst case the quantum well absorbers were completely destroyed. It is obvious that SESAMs exhibiting such strong damage effects can no longer be used for mode-locking lasers.
Since any changes in the mode-locking behavior or even mode-locking failure should be avoided, damage prevention is a main issue in the development of SESAM mode-locked high-power lasers. Hence, in this thesis two new design concepts for SESAMs are pre-sented that reduce the susceptibility to damage of the device. One idea utilizes the following two effects: Firstly, the temperature of the SESAM rises during the power-up phase of the laser due to increasing intracavity power and secondly, the band gap energy of semiconductor materials decreases with increasing temperature. To avoid the damage of high pulse intensities as they occur in the Q-switching regime during the power-up phase this design idea suppresses Q-switch pulses at all. An appropriate choice of the quantum well material allows for a band gap energy that is larger than the laser photon energy when the SESAM is still cool, as it is at the beginning of the power-up phase. Therefore, the quantum wells are transparent for the laser photon and no Q-switch pulses can be built. Only when the temperature of the device increases due to power increase during the power-up phase of the laser, the band gap energy reduces, the laser photon can be absorbed, and the SESAM initializes mode-locking.
The second design idea is based on sophisticated quantum engineering. This technique is already common for quantum cascade lasers and other complex multilayer quantum structures, but is not yet adopted for SESAMs. However, this way allows for a tailored spacing between the quantum states and paves the way for self-cooled SESAMs.
So far the state of the art SESAM design is optimized for non-resonant interband tran-sitions. In contrast, resonant absorption with respect to the laser wavelength combined with a tailored energy spacing in the conduction band leads to an effective cooling of the SESAM: When the initial and final states in the relaxation process within the con-duction band are separated by the energy of one longitudinal optical (LO) phonon in each relaxation process a phonon is absorbed resulting in a self-cooled device.
Both design concepts will revolutionize the SESAM development once they are im-plemented. During the time of this PhD study the realization of the design ideas was not possible. Thus, in future steps the realization of the presented new design concepts could result in huge progresses in SESAM development. As a basis, the tem-perature dependence of conventional SESAMs was observed by heating them up to a temperature of 100◦C. To cover the range of different SESAMs a SESAM with a small
modulation depth (3 %) and one with a high modulation depth (20 %) were charac-terized. Both SESAMs showed similar effects when heated, even the strength of the effects was different. The static reflectivity spectra showed a redshift due to the tem-perature dependence of the refractive index of GaAs and AlAs leading to a reduction of the linear reflectivity at the laser wavelength. This decrease of the linear reflectiv-ity was also visible in the nonlinear reflectivreflectiv-ity spectra of the SESAMs. Additionally, the nonsaturable reflectivity showed nearly no temperature depending changes. Thus, these two effects lead to an increase of the effective modulation depth with increasing SESAM temperature. In contrast, the time constant of the recombination process of the SESAM was not affected by the heating. These experimental results permit two main conclusions: Firstly, adjusting the temperature of the SESAM allows for the adjustment of the modulation depth and even an in-situ control of the mode-locking mechanism based on the modulation depth. Secondly, the investigated temperature range was too low to affect the recombination processes leading to a change of the time constants of the SESAMs. These findings show that indeed the device temperature is a parameter to control the behavior of the SESAM and optimize its mode-locking performance within a laser resonator.
If the temperature dependence of the modulation depth is combined with the suppres-sion of Q-switch pulses by temperature management this allows for the development of a robust SESAM with reduced vulnerability and capability for in-situ optimization of the mode-locking performance. In the future such new SESAM ideas are necessary for the advancement of high-power mode-locked lasers.
An important part of SESAMs and semiconductor lasers are Bragg mirrors. Fur-thermore, rapid thermal annealing is a very common post-growth process to tailor the amount of defect states of the device, which in turn affects its time constants. Thus, the question of interest was, whether and to what extend heating processes change the properties of semiconductor Bragg mirrors. Therefore, the optical and structural parameters of GaAs/AlAs Bragg mirrors that were post-growth annealed at different temperatures between 450◦C and 750◦C were investigated. The optical properties were measured with a FTIR-spectrometer and simulated with the transfer-matrix method.
Even the static spectra changed due to the annealing process no uniform trend could be extracted. However, the stopband region was nearly unaffected by the annealing process suggesting that post-growth annealing does not change the performance of the Bragg mirror at the laser wavelength.
For the investigation of the layer structure a new metrological method based on co-herent acoustic phonon spectroscopy was used. This technique is already common for structural analysis of short-periodic heterostructures but not yet for long-periodic structures. By comparing the experimentally obtained phonon modes with theoret-ically calculated phonon modes the superlattice constant of the heterostructure can be extracted. Coherent phonons were excited and detected with a pump-probe setup with asynchronous optical sampling technique, the calculation of phonons in hetero-structures was based on the Rytov model [Jus89]. Indeed, it was possible to analyze the Bragg mirror by means of coherent phonon spectroscopy resulting in a superlattice
constant of 156.3 nm with an uncertainty of 1 nm. The reliability of this technique was proofed with X-ray spectroscopy. These measurements got to the result of a su-perlattice constant of 155.9 nm. This remarkable accordance allows for the conclusion that structural analysis of multilayer systems can be performed by coherent acoustic phonon spectroscopy, an all-optical technique with precision in the nanometer range.
After this proof of principle different annealed Bragg mirrors were analyzed, too, but no changes in the detected phonon spectra could be found. This indicated that the superlattice constant of the Bragg mirrors was not affected by the annealing process.
This assumption was confirmed by further X-ray measurements.
In further experiments coherent acoustic phonon spectroscopy could be used for struc-tural analysis of SESAMs. In this case the calculation of the detected phonons is more complex, since the effects of quantum wells, spacing layers, and coatings must be taken into account. However, it is a promising method for structural investigations and even an in-situ analysis of SESAMs in a laser cavity can be considered. In combination with an in-situ pump-probe experiment for nonlinear reflectivity curves an all-inclusive insight to the structural changes as well as to carrier dynamics and important reflec-tivity parameters of a mode-locking SESAM could be given. By means of long-term investigations a real-time observation of aging and damage effects could be achieved.
All these experimental observations were based on the realization of a high-power molocked Yb:YAG thin-disk laser optimized for long-term use. The resonator de-sign was based on the multipass geometry concept that was established from Neuhaus et al. [Neu08b] and further developed from Bauer et al. [Bau12a]. Whereas Bauer et al. pushed the limits in terms of pulse energy [Bau12b] the aim of the laser pre-sented in this thesis was long-term stability, stable output characteristics, and a high reliability in pulse generation. These objectives could be met with a diode-pumped SESAM mode-locked Yb:YAG thin-disk laser with 22 passes over the disk per cavity round trip. A SESAM with a modulation depth of 2.0 % and a saturation fluence of 45µJ/cm2 was used. Soliton mode-locking without any cw background could be achieved. The multipass concept allowed for output coupling (OC) rates in the range of 40-50 %, which could be continuously varied by an OC unit consisting of a thin film polarizer and an intracavity quarter wave plate. The maximum average output power was 44 W corresponding to an optical to optical efficiency of 31 %. The shortest pulses exhibit a pulse length of 0.87 ps and a spectral width of 1.3 nm resulting in a time-bandwidth product of 0.32 that is close to transform limited pulses. The central laser wavelength was 1030.0 nm and the repetition rate 3.47 MHz.
These output characteristics were constant over a time period of several weeks and months without any realignment of the laser. Nevertheless, due to a narrow stability window the alignment of such a laser is complicated and sophisticated and especially during the Q-switching regime damage on the SESAM was observed. Therefore, despite the long-term stability and reliability of the SESAM mode-locked laser, an alternative mode-locking mechanism should be found. Thus, first experiments were done with a nonlinear mirror (NLM), as it was introduced by Stankov et al. [Sta88b]. A nonlin-ear mirror consists of a nonlinnonlin-ear crystal and a dichroic OC mirror. During the first
pass through the crystal second harmonic is generated, whereas in the second pass second harmonic light is reconverted into fundamental again. Since second harmonic generation is a nonlinear, intensity dependent process and since the dichroic OC mir-ror exhibits a reflectivity of the second harmonic of 100 % and a reduced reflectivity
<100 % for the fundamental, the nonlinear mirror is a device with increasing reflectiv-ity with increasing incoming light. Therefore, it can be used for mode-locking.
After very successful extracavity wave mixing experiments with a LBO crystal the non-linear mirror was incorporated into the Yb:YAG thin-disk laser cavity. The resonator was slightly modified for output coupling by means of the dichroic output coupling mir-ror. Unfortunately it was not possible to start mode-locking only with the NLM, the assistance of the SESAM was necessary. One reason for this might be the conversion efficiency of second harmonic in cw mode that is too low to convert the LBO crystal in a nonlinear mirror. Thus, the SESAM was needed to start mode-locking. Therefore, a laser resonator was built with a nonlinear mirror and a SESAM. This mode-locking mechanism was called ’NLM assisted SESAM mode-locking’. With this combination perfect mode-locking could be realized.
Different numbers of passes through the multipass cell, different nonlinear crystals, and different output coupling rates were tested. The best results in terms of stability and pulse length could be achieved with 14 passes over the disk per cavity round trip by use of a 6 mm long LBO as a nonlinear crystal. The reflectivity of the dichroic mirror at the second harmonic was 70 %, resulting in an output coupling rate for the fundamental of 20 %. With this configuration pulses as short as 0.7 ps at a repetition rate of 5.22 MHz could be achieved. These were the shortest pulses emitted from the Yb:YAG thin-disk laser presented in this PhD thesis.
Extracting the LBO crystal destabilized the mode-locking mechanism, Q-switching in-stabilities occurred, and sometimes the mode-locking regime even broke up. Thus, for a qualitative analysis of the effects of the NLM on the mode-locking behavior the laser was also characterized without the LBO crystal. Different amounts of passes over the disk and variable output coupling (OC) rates were tested. Measurements for different OC rates were an important issue, since the incorporation of the NLM reduces the OC rate due to the fact that the fundamental light is partially converted into second harmonic. In fact these experiments showed that the Q-switch tendency depends on the appropriate output coupling rate rather than on the incorporation of the non-linear mirror. Nevertheless, the incorporation of the nonnon-linear mirror stabilized the mode-locking performance and made the resonator very insensitive to misalignment.
Therefore, NLM assisted SESAM mode-locking can be recommended if a robust, easy-to-use, and easy-to-adjust laser with sub-ps-pulses is needed.
However, stable mode-locking is a highly sensitive interplay of intracavity power, output coupling rate, self phase modulation, and negative dispersion. In future experiments toward a NLM mode-locked thin-disk laser without the assistance of a SESAM all these parameters have to be considered. Since best laser performance is an interplay of so many different parameters, computer simulations on the lasing and mode-locking behavior of a resonator with nonlinear mirror could be used to find the optimum con-figuration.