Mode-locked semiconductor disk laser for ultra-short pulses in the red and ultraviolet spectral range
Final Report Abstract
The main goal of this project was to realize a stable pulsed laser source in the visible red spectral range with the expansion towards shorter wavelength and the UV. The material system we chose is AlGaInP, which enables us to cover the wavelength range from approx. 600 nm to 700 nm, depending on the composition and the used low-dimensional structures (QW or QD) for the active region. The aimed short pulses were created first by mode locking with semiconductor saturable absorber mirrors (SESAM). These structures as well as the respective gain chips were fabricated by metal-organic vaporphase epitaxy. For the characterization of the semiconductor chips, photoluminescence spectroscopy and reflection measurements as well as structural method as scanning electron microscopy and X-ray diffraction was used. The fabricated samples were then combines into laser cavities, mainly in a V-shaped design. Thus the gain chip and formed one end mirror and the other the absorber. The folding mirror act as well as out coupler, so that two beams could be used for the characterization of the laser dynamics. Here we have chosen a fast photodiode in combination with a fast oscilloscope to record the temporal behavior of the pulse trains inside the cavity. Additionally if possible autocorrelation measurements for the determination of the pulse width were performed. In the spectral range around 660 nm we could create a stable pulsed laser with a SEASM with two QWs with a pulse width of only 223 fs. The oscilloscope traces and the derived frequency spectrum confirmed a harmonic mode locking behavior of the pulse. By changing the design of the absorber chip, the pulse duration could be changed as well from the before mentioned 223 fs to around 1 ps. This allows for a nice flexibility depending on the targeted application. The repetition rate was in most cases around 1 GHz, which depends on the cavity length and can as well be adjusted. We have implemented furthermore in the cavity a nonlinear crystal to frequency double the cycling pulses inside the cavity. With this extension, pulses with a width of 1.2 ps at 325 nm were achieved. The output power was not yet optimized but the potential was shown for a pulsed laser in the UV spectral range. These devices could be very useful for fast curing or as marker lasers even for opaque objects. Additionally to these progresses, we have invented the new concept of a semiconductor saturable absorber membrane (SEASME). This device is just a very thin (~ 100 nm) semiconductor foil that can be bonded to any kind of mirror, dielectric or metallic. With this concept, we can overcome the restriction of lattice-matched growth of distributed Bragg mirrors and improve the heat removal and thus the lifetime of the absorber structure. We have demonstrated the successful fabrication of the SEASAME and integrated it into a V-shaped cavity for mode locking. The first pulse train with such a cavity could be observed even though the output power was a bit too low for a determination of the pulse width. These promising first results motivate us to continue this work. Finally, we have applied the absorber free or the so-called self-mode-locking to the red spectral range. Here the theoretical description of the pulse formation is not completely understood, but most probably, the pulses are formed due to a nonlinear Kerr effect inside the gain structure. We have examined also here a V-shaped cavity but this time the VECSEL chip was the folding mirror. We found several operation regimes of the cavity. Mostly the pulses were noise like but also single pulses could be observed. Here the proper alignment of the cavity plays a very important role as well the control of all external noise sources. Overall, with this project a major step in the understanding of the SESAM mode locking as well as the self-mode-locking was achieved which opens the door for new future applications of pulsed semiconductor lasers.
Publications
- “Intracavity frequency-doubled mode-locked semiconductor disk laser at 325 nm”, Optics Express 23, 19947 (2015)
R. Bek, S. Baumgärtner, F. Sauter, H. Kahle, T. Schwarzbäck, M. Jetter, and P. Michler
(See online at https://doi.org/10.1364/OE.23.019947) - “Quantum dot based mode-locked AlGaInP-VECSEL”, Proc. SPIE 9349, Vertical External Cavity Surface Emitting Lasers (VECSELs) V, 93490G (2015)
R. Bek, G. Kersteen, H. Kahle, T. Schwarzbäck, M. Jetter and P. Michler
(See online at https://doi.org/10.1117/12.2077164) - “VECSEL and MECSEL: high power, wavelength versatility and ultra-short pulses”, Proc. SPIE 10515, Vertical External Cavity Surface Emitting Lasers (VECSELs) VIII, 105150B (2018)
R. Bek, H. Kahle, M. Jetter and P. Michler
(See online at https://doi.org/10.1117/12.2291744) - “Self-modelocked AlGaInP-VECSEL”, Applied Physics Letters 111, 182105 (2017)
R. Bek, M. Großmann, H. Kahle, M. Koch, A. Rahimi-Iman, M.Jetter, and P. Michler
(See online at https://doi.org/10.1063/1.5010689) - ”Towards self-mode locking of AlGaInP-VECSELs”, Proc. SPIE 10087, Vertical External Cavity Surface Emitting Lasers (VECSELs) VII, 100870N (2017)
R. Bek, Q. Duong-Ederer, M. Vaupel, H. Kahle, T. Schwarzbäck, A. Rahimi-Iman, M. Jetter, M. Koch and P. Michler
(See online at https://doi.org/10.1117/12.2252473) - “Microcavity-enhanced Kerr nonlinearity in a vertical-external-cavity surfaceemitting laser”, Optics Express 27, 11915 (2019)
C. Kriso, S. Kress, T. Munshi, M. Großmann, R. Bek, M. Jetter, P. Michler, W. Stolz, M. Koch, and A. Rahimi-Iman
(See online at https://doi.org/10.1364/OE.27.011914)
