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Semiconductor disc laser

Source: Anne Schroll
Figure 1: Laser setup for the characterization of high-performance VECSEL systems

Development and optimization of VECSELs and the expansion of their capabilities.

General facts:

  • In recent years, vertical-external-cavity surface-emitting lasers (VECSELs) have attracted a great deal of attention in the area of laser development. They may be seen as a hybrid system between semiconductor lasers and solid-state lasers, due to the combination of a semiconductor laser chip with an external cavity. 
    While conventional semiconductor lasers can, thanks to the current abilities in epitaxy, already cover a wide range of the electromagnetic spectrum with high efficiency, the beam quality and achievable power output is insufficient for many applications. This contrasts with both solid-state and gas lasers. Here the flexibility of the laser cavities as well as the excellent beam quality combined with high output powers of these laser systems allow their use in many areas, from basic research to material processing. However, this type of laser is strongly limited in the choice of wavelength, and therefore the optimum wavelength is not always available for application purposes. VECSELs allow combining the respective advantages of both laser families: high power operation with a good beam quality and wavelength versatility for various applications. For instance, VECSELs can be employed for spectroscopy in pump-probe experiments, for the generation of frequency combs, or microwave- and THz-radiation. Thereby, one can not only achieve high powers up to 106 W in continuous-wave (cw) operation but also remarkably high intracavity powers which can be used for efficient nonlinear frequency mixing. Particularly, the external cavity allows for the insertion of optical elements directly into the laser resonator, such as birefringent filters for a single-frequency operation or nonlinear crystals for difference frequency generation. Moreover, passively self-mode-locked VECSELs have been demonstrated, providing sources of ultrashort-pulsed radiation.
Source: Dalia Al Nakdali

Figure 2: Schematic diagram of a VECSEL with a V-shaped cavity and side pump.

  • The construction of a VECSEL requires both a gain medium based on quantum well or quantum dot structures as well as a semiconductor integrated, highly reflective Bragg mirror. This, combined with the gain medium, creates an active mirror. The total laser cavity is finally completed by employing one or more external mirrors. With this arrangement, it is possible to simultaneously achieve a high-power output in addition to a high degree of flexibility in the resonator configuration and excellent beam quality. The wavelength can be set relatively freely with the choice of the underlying semiconductor system.
Source: SFB1083/Tim van der Bovenkamp

Figure 3: The Team in 2014.

...and what we do:

Following topics are handled in the workgroup experimental semiconductor physics:

  • Intra-cavity frequency conversion (terahertz generation
  • (Self-)Mode-Locking of VECSELs

At the moment our workgroup has the world record for the highest power output from a VECSEL. 

see: B. Heinen, et al., 106 W continuous-wave output power from a vertical-external-cavity surface-emitting laser (VECSEL), IET Electronics Letters 48, 516, (2012) [DOI]

Source: AG Koch/AR-I

We demonstrated a saturable-absorber-free vertical-external-cavity surface-emitting laser which is harmonically mode-locked with repetition rates in the GHz range.

The picture to the left shows us doing alignment work for a pulsed Vecsel.

In our device, we observed fundamental mode-locking along with the second and third harmonic operations for increased optical pump powers, with pulse durations penetrating the femtosecond regime at a wavelength of 1014 nm. Such self-starting mode-locked VECSELs can be very useful for certain applications following further optimization steps and they can provide a compact and reasonably priced alternative to Titan-Sapphire oscillators.

More information: (DOI:10.1049/el.2014.0895)
M. Gaafar et al., Harmonic self-mode-locking of an optically pumped semiconductor disc laser, Electronics Letters 7/2014

Contact: Dr. Arash Rahimi-Iman, Prof. Dr. Martin Koch