Optimization of a Broadband Gain Element for a Widely Tunable High-Power Semiconductor Disk Laser

Abstract

The layer structure of the gain element in an optically pumped semiconductor disk laser was parametrically optimized with respect to a target function specifying a desired unsaturated reflectance over a desired wavelength range at a constant pump intensity. Spectral threshold pump intensity measurements confirmed the efficacy of the design, showing a much wider low-threshold regime than a conventional nonbroadband gain element, in good agreement with simulations. This evaluation avoids the possible influence of additional factors under high-power operation. Nonetheless, having a high and nearly constant broadband unsaturated reflectance at low pump intensity is a key to obtain good high-power performance, as evidenced by the obtained continuous tuning from 967 to 1010 nm with a maximum output power of 2.6 W.Index Terms-Birefringent filter (BRF), continuous tuning, high-power laser, InGaAs, optically pumped semiconductor disk laser (OP-SDL), vertical-external-cavity surface-emitting laser (VECSEL).

Optically pumped VECSELs: review of technology and progress

“…It should also be noted that the micro-cavity resonance narrows the gain bandwidth and introduces significant group delay dispersion. This can be detrimental for mode locking [26,27], wavelength tunability [28] and single-frequency operation [29]. Consequently, the VECSEL gain structures are sometimes designed to be anti-resonant at the emission wavelength; such an approach inevitably reduces the gain and makes the laser operation less tolerant to cavity losses [26,27,30].…”

Semiconductor Disk Lasers: Recent Advances in Generation of Yellow-Orange and Mid-IR Radiation

“…Along the way, many demonstrations concerned widely tunable [35][36][37][38] and narrow band lasers [39][40][41]. In terms of available output power, the 10 W level has been reached and exceeded using both single and multiple gain elements [29,30,42,43].…”

Direct measurement of the spectral reflectance of OP-SDL gain elements under optical pumping

“…The design of the broadband GE used in the measurements is described in [4]. In this section we briefly review the design, to illustrate significant differences and additional features compared to a standard GE for a fixed-wavelength continuous wave OP-SDL.…”

“…This is a straight-forward measurement method but reveals little information on the actual performance of the GE under pump excitation. Other methods are based on measurements on the entire OP-SDL, thus including also the external cavity, such as the pump threshold intensity measurements of a tunable OP-SDL in [4], and thus only provide indirect information on the spectral properties of the GE since cavity and saturation effects influence the measurements.…”

“…This method avoids adverse influences from external cavity effects and enables a direct comparison with design simulations. This comparison is very valuable for assessing different GE designs, particularly those for more advanced functions, such as designs for wide tuning [4,5] or short-pulse generation [6]. Under optical pumping, the AMR is typically just above 100%, with small variations from this value having a large impact on the performance of the OP-SDL.…”