Researchers at MIT’s Lincoln Laboratory in Lexington, Mass., have passively mode-locked an InGaAsP slab-coupled optical waveguide laser and generated 250 mW of average power at 1.5 μm in 10-ps pulses at a repetition rate of 4.29 GHz. The laser’s unusually high power and pulse energy make it promising for applications from telecommunications and laser ranging to nonlinear optics and research.

Unlike other edge-emitting semiconductor lasers, slab-coupled optical waveguide lasers, which were pioneered at the lab, feature a mode that is nearly circular in cross section, thanks to the unique ridge-waveguide geometry of their resonators (Figure 1).

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Figure 1. The resonator mode of a slab-coupled optical waveguide laser is nearly circular in cross section, resulting in a nearly symmetric, diffraction-limited output beam. The dimensions of the experimental laser are t = 4.9 µm, w = 5.8 µm and h = 4.5 µm. Images reprinted with permission of MIT Lincoln Laboratory.

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As a result, the output beam is nearly diffraction-limited in both transverse directions, and the lab researchers previously showed that the devices are capable of high average powers. They believe that the current laser is the first of its kind to be passively mode-locked.

Fabricated by metallorganic chemical vapor deposition on an InP substrate, the laser comprises separate gain and absorption sections. The geometry of the five 8-nm-thick quantum wells and the dimensions of the slab and the ridge waveguide ensure that higher-order modes are coupled into the slab, where they are below threshold so that only the fundamental mode lases.

The scientists first operated the laser in a free-running mode, applying identical biases to the gain and absorption sections. With dielectric coatings on both the laser’s facets (95 percent reflecting on the back facet, 5 percent on the output facet), they obtained 580 mW with a 3-A drive current.

In the near-field region, the output beam diameter was 5.3 and 7.0 μm at the 1/e² points in the two transverse directions, and the beam quality, M², was better than 1.2 in both directions.

By reverse-biasing the absorption section while maintaining a forward bias on the gain section, the researchers passively mode-locked the laser. With the same 3-A current through the gain section and a 1.8-V reverse bias on the absorption section, they observed 250 mW of average output power. They believe that this result is an order of magnitude greater than has been observed previously from 1.5-μm, electrically pumped, monolithic mode-locked semiconductor lasers. The beam quality of the mode-locked laser was essentially the same as that of the free-running laser.

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Figure 2. A sampling-scope trace of the mode-locked output pulses showed a pulse repetition frequency of 4.29 GHz. Autocorrelation measurements indicated the duration of the pulses to be 10 ps, and they could be compressed to 4 ps by passing them through 200 m of SMF-28 single-mode fiber.

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The 4.29-GHz pulse repetition frequency of the output (Figure 2) corresponded to the optical length of the 1-cm-long InGaAsP resonator. A second-harmonic autocorrelation measurement indicated that the duration of the individual, presumably Gaussian, pulses was 10 ps. Each pulse contained 58 pJ, corresponding to a peak power of 5.8 W.

The scientists believe that this pulse energy is the greatest achieved from an electrically pumped, mode-locked semiconductor laser and is two orders of magnitude greater than achieved from such a laser operating at 1.5.