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We present the design of a Modulated Grating Y-branch laser containing a splitter and two multi-peak reflectors. High selectivity is achieved using the additive Vernier effect.Widely tunable lasers find an increasing number of applications since they can contribute to flexibility in WDM systems. 1 The GCSR (Grating-assisted codirectional Coupler with Sampled Reflector) 2 and the SSG-DBR (Super Structure GratingDistributed Bragg Reflector) 3 lasers are two types of monolithic edge-emitting lasers based on multi-peak modulated reflector gratings such as the SSG or the sampled grating. Also the Modulated Grating Y-branch laser 4 (MGY) presented here ( Figure 1) is based on multi-peak reflectors and can be designed to cover the entire C-band (192)(193)(194)(195)(196). In our design the light is split by the use of a 60-µm long, 7-µm wide Multi-Mode Interferometer (MMI). Then 46-µm long S-bends with a 130-µm radius of curvature are used to increase the separation between the waveguides. Each arm ends with a 500-µm long multi-peak reflector with an effective coupling coefficient of 30/cm. In one arm there is an 80-µm long phase section that can be used to adjust the phase difference between the reflections. A common phase section of the same length is used to align the cavity mode with the reflector peaks. The design is to be manufactured as an InP/InGaAsP buried hetero structure, butting a 400 µm multi-quantum-well gain section to the passive sections made in a 0.35-µm thick InGaAsP layer with a photo-luminescence wavelength of 1.39 µm.The MGY is similar to the SSG-DBR in that it is based on the Vernier effect using two multi-peak reflection gratings. The SSG-DBR utilizes two such gratings, one at each end of the cavity. The reflection peak separations of the two gratings are slightly different and the Vernier effect is used to select one reflection peak for lasing. By tuning the two reflection sections and a phase section, any frequency within a wide range can be reached. One disadvantage with the SSG-DBR laser is that the output light has to pass through the front reflector, which inevitably suffers from free carrier absorption when tuned. This gives a higher power variation. The MGY is also comparable to the GCSR laser in the sense that it has all tuning sections on the same side of the gain section, so that the power can exit the cavity without absorption. Instead of using the Vernier effect, the GCSR laser uses a widely tunable coupler transmission peak to efficiently select one peak for lasing. One disadvantage for the GCSR is that it is typically a rather long chip.

Device and tuning characteristics of superstructure grating tunable twin-guide (SSG-TTG) laser diodes are presented. The devices are based on the distributed feedback (DFB) TTG laser, but comprise sampled or superstructure gratings in order to utilise Vernier effect tuning to extend the tuning range to potentially several tens of nanometres. In contrast to most other existing monolithic widely tunable semiconductor lasers, this device requires only two tuning currents. The first tuning characteristics show distinct regions of high SMSR where continuous wavelength tuning can be carried out. These regions of high SMSR are spaced in agreement with theoretical predictions that are based on the superstructure grating design. Hence, our observations clearly demonstrate Vernier effect tuning for the first time in a TTG-type laser diode.

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The widely tunable twin-guide laser diode with sampled gratings is a monolithically integrated device that utilizes a vertical integration scheme to realize an electronically and widely tunable distributed feedback (DFB)-type laser. Owing to its DFBtype structure, it possesses many advantages, such as large continuous tuning ranges, simple wavelength tuning scheme, and promising dynamic characteristics. In this paper, we detail on the design of the sampled grating tunable twin-guide laser and discuss its performance. Recent devices achieve tuning ranges over 40 nm along with high side-mode suppression (≥35 dB) as well as large output power (≥10 mW).