Mode-Locking of High-Power Resonant-Optical-Waveguide Diode Laser Arriays

We report the first mode-locked operation of a resonant optical waveguide (ROW semiconductor laser array. Pulsewidths as short as 23 ps and peak powers of ower 1W are generated in a single-lobed beam. Mode-locked semiconductor lasers are attractive as compact sources of short optical pulses for use in physics measurements, for instrumentation systems, and for telecommunications applications. Such lasers operate with average output powers of typically a few milliwatts, which limits their use in applications where higher power is required. The use of laser arrays results in higher output powers, but such arrays tend to emit in multi-lobed far-field patterns. This makes their incorporation into external cavities with high coupling efficiencies difficult. Resonant-optical-waveguide (ROW) diode array lasers have recently demonstrated in-phase (single main-lobed) operation at cw output powers of up to 500 mW1. Such arrays are therefore promising candidat,es for high power semiconductor laser mode-locking. In this paper, we report the first mode-locked operation of such devices, with external cavity ccsup1i:ng efficiencies comparable to that typically obtained using single-element lasers. In this experiment, a 20-element array was used with a cleaved cavity length of 500 pm. The lasing wavelength was 850 nm. Half-wave Al,O, coatings were applied to both diode facets. The facet used t o couple into the external cavity was then additionally coated with an anti-reflective quarter-wave SiN,O, lalrer (index-1.83). Before AR coating, the device threshold current was 320 mA. The application of the AR coating resulted in an increase of threshold to 570 mA. The array was coupled to an external air cavity of approximately 15 cm in length, corresponding to a repetition rate of 1 GHz. As shown in Figure 1, three intra-cavity lenses were used to couple the beam into the external cavity. An ARcoated GRINROD lens is used at the laser because of its high collection efficiency and numerical aperture. The cylindrical lens is used t o compensate for astigmatism in the laser emission. The beam is focused onto the external cavity mirror using an achromatic doublet. This doublet is used because the noncircular beam profile fills most of the lens and would be subject t o the off-axis aberrations characteristic of a singlet lens, which are compensated in ;In achromat. As shown in Figure 2, the external cavity feedback reduces the