Tunable Lasers For Photonic Integrated Circuits

Prior to 1991, the most advanced tunable semiconductor laser was the “Distributed Bragg Reflector” or DBR laser. DBR lasers employ a cavity design which constrains the fractional wavelength tuning Ahlh to be no more than the achievable fractional index change Aplp in a semiconductor waveguide. The maximum fractional index shift is just under 1 %, resulting in maximum tuning ranges around 10 nm at 1.5 microns (pm). Since 1991, however, three different cavity geometries have been demonstrated, with much wider tuning ranges. Figure 1 shows the Y-cavity geometry [ 11, versions of which have demonstrated on the order of 50 nm tuning [ 1,2]. Figure 2 shows the grating-assisted co-directional coupler laser (GACC) [3], which has also demonstrated more than 50 nm tuning. Lastly, Fig. 3 shows the sampled grating DBR laser, which we describe in more detail below. The sampled grating laser was first proposed by us in 1990 [4], and demonstrated in 1991 [5]. As shown in Fig. 3, the device relies on two DBR gratings modulated by an on-off sampling function, resulting in periodic reflection spectra. The periods of the two mirrors are slightly mismatched, and lasing occurs where two mirror maxima are aligned. Tuning one mirror relative to the other causes the alignment position to shift to adjacent maxima, resulting in wide-range “vernier effect” tuning. Inducing identical index changes in both mirrors allows coverage between mirror maxima. Figures 4,5, and 6 show our most recent sampled grating DBR results [6]. Figure 4 shows 73 nm tuning with 62 nm continuous wave range. Figure 5 shows light-current properties. Figure 6 shows some continuous-wave spectra, indicating very large suppression of spurious mirror resonances. The sampled grating DBR laser has also been implemented using periodically chirped gratings. This approach has also resulted in very impressive results, with tuning ranges of up to 100 nm demonstrated [7].