Watt-level emission at 589 nm for biomedical applications
Fig. 1: P-U-I characteristics of a DBR-taper laser. Upper inset: scheme of the DBR-taper laser, lower inset: spectrum at 5 A.
Fig. 2: Power characteristics of a ridge waveguide crystal. Upper inset: beam quality of the frequency doubled at 0.86 W, lower inset: spectrum at 0.86 W.
Fig. 3: Micro-module with an emission at 589 nm
Micro-integrated modules with a small footprint of a couple of square centimeters basing on semiconductor laser-chips enable a flexible, energy-saving as well as low-maintenance operation. Hence, they apply cost-efficient to industry and research.
Direct emitting laser diodes featuring a great reliability are not available in the yellow spectral range. Therefore, laser light of the near infrared wavelength region is frequency doubled by means of a nonlinear crystal. However, efficient laser sources with an emission wavelength of more than 1100 nm, which are feasible for micro-integration, had to be developed. Recently, novel laser diodes basing on a quantum well with an emission at 1178 nm were introduced by the FBH. These lasers sources show a nearly diffraction limited narrow-band single-mode emission of 1 W (Fig. 1) which is indispensable for efficient frequency doubling in a nonlinear crystal.
For a rugged construction of micro-modules, we strive for single-pass operation of a nonlinear crystal. Periodically poled lithium niobate was utilized for frequency conversion. Thus, in preliminary investigations of the interplay between DBR-taper laser and nonlinear crystal the highest presented optical output power for a ridge waveguide crystal of 0.86 W at a pump power of 2.0 W could be demonstrated (Fig. 2). This process corresponds to an optical-to-optical conversion efficiency of more than 40 %. Contemporaneously, the frequency doubled laser light features a narrow linewidth (< 230 MHz) and is nearly diffraction limited (M² ≤ 1.1)(insets in Fig. 2). Since the DBR-taper laser possesses of a resistive heater at the DBR section and the quasi-phase matching condition of the crystal can be adapted with a thermo-electrical heater, both elements are tunable over more than 1 nm. Hence, both sodium D-lines at 589.0 and 589.6 nm can be addressed.
First steps towards micro-integration of laser diode and ridge waveguide crystal in a module have been undertaken and optical output powers of more than 100 mW at an emission of 589 nm have been demonstrated (Fig. 3). Therewith a laser source for biomedical applications is provided now, which can be used for medical treatment and diagnosis as well as for biomedical analysis.