Publikationsserver der Universitätsbibliothek Marburg

Titel:Investigating self-mode-locking in VECSELs: Nonlinear lensing and frequency-modulated combs
Autor:Kriso, Christian David
Weitere Beteiligte: Koch, Martin (Prof. Dr.)
URN: urn:nbn:de:hebis:04-z2021-04956
DDC:530 Physik
Titel (trans.):Untersuchung der Selbstmodenkopplung in VECSELn: Nichtlinearer Linseneffekt und frequenzmodulierte Kämme


Modenkopplung, Semiconductor Laser, Mode-locking, Nichtlineare Optik, Halbleiterlaser, Nonlinear Optics

Optically pumped semiconductor disk lasers, also known as vertical-external-cavity surface-emitting lasers (VECSELs), stand out by their high output power, excellent beam quality and a multitude of available emission wavelengths that can be obtained by advanced semiconductor epitaxy. Apart from running in continuous-wave operation, these lasers can also generate pulses with durations of only a few hundreds of femtoseconds. This is called mode-locking because the laser modes are coupled to each other by a nonlinear optical element and therefore become phase-stable. Usually, this intracavity element is a semiconductor saturable absorber mirror (SESAM), which, similar to a VECSEL, consists of one or more quantum wells, or quantum dot layers, and a distributed Bragg reflector (DBR), but is not optically pumped. Therefore, due to the added saturable absorption in the cavity, short pulses with high peak power experience less loss than continuous-wave operation which favors pulsed operation. Such kind of pulsed lasers are useful for many applications such as two-photon microscopy or frequency-comb spectroscopy. Besides ultrashort-pulse generation with SESAMs, also spontaneous pulse emission from VECSELs was observed by several groups in the last years. This phenomenon is called self-mode-locking. Up to now, it is controversial, what the origin of this phenomenon is and whether it really leads to a stably mode-locked state. One possible cause of self-mode-locking might be that the intrinsic nonlinear refractive index of the VECSEL chip leads to self(de-)focusing of the intracavity beam. By inserting a slit into the cavity or reducing the pump spot size on the chip, the losses in pulsed operation might be reduced as is the case for Kerr-lens mode-locking of solidstate lasers. However, in contrast to solid-state lasers, the nonlinear lens in a VECSEL chip generally depends on the optical excitation as well as the pulse length. Therefore, one part of this work deals with the experimental investigation of the nonlinear refractive index of VECSEL chips as a function of the wavelength, the excitation fluence and the pulse length. For this purpose, the measurement methods Z-scan and ultrafast beam deflection are used. A nonlinear refractive index in the order of magnitude of −10e−16 m^2/W is obtained for laser pulses shorter than one picosecond. For long pulse lengths of a few picoseconds and large excitation fluences, the nonlinear refractive index becomes positive and increases strongly in magnitude (∼ +10e−15 m^2/W). These measurements confirm some reports of self-mode-locking where the sign and the magnitude of the nonlinear refractive index can be estimated from the cavity geometry. There, it is assumed that for efficient Kerr-lens mode-locking an intracavity beam width modulation of a few percents is required. Furthermore, this thesis investigates whether so-called frequency-modulated combs can be generated by VECSELs. Thereby, the laser modes are locked. However, the phase relations between adjacent modes are not approximately zero but vary between 0 and 2π. This leads to a situation where the emitted laser power is temporally nearly constant but the frequency varies periodically. Such kind of mode-locking is generated by resonant four-wave mixing processes in the semiconductor gain medium. Particularly for quantum cascade and diode lasers, it is a fundamental operation regime that can be used for applications such as dual comb spectroscopy. With a novel measurement technique called shifted-wave-interference Fourier-transform spectroscopy (SWIFTS), the intermode phases can directly be measured and the phase stability can be assessed . In this thesis, it is demonstrated by using SWIFTS that also VECSELs can run in a frequency-modulated comb state. This is quite surprising as elements such as spatial hole burning and large outcoupling losses, that are characteristic for edge-emitting diode and quantum cascade lasers, and have been considered to be important for frequency-modulated comb generation, are lacking in VECSELs. However, it shows how universal the phenomenon of frequencymodulated combs is and it can serve as motivation to understand it also better from the theoretical side.

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