Semiconductor photonic integrated circuit for high-density WDM light source

Recent progress in semiconductor optical devices and their fabricatin technique has brought about a possibility to develope various kinds of semiconductor photonic integrated circuits (SPICs) [1,23. We report, here, a 1.5um range four-channel light source SPIC, which consists of four wavelength-tunable LDs and four light intensity modulators and two waveguide couplers, aming at high-density WDM optical system application. The SPIC! is shown schematically in Fig.1. Wavelength-tunable three section DBR LD [3], Franz-Keldysh electro-absorption intensity modulator 141 and Y-branch waveguide were employed as each device structure. The SPIC wafer was grown with four MOVPE steps and one LPE step. Wavelength-tunable DBR LDs consisted of a 600pm long active region, a phase control region (lOOpm), and a DBR region (150pm). The active region included a four-well InGaAs/InGaAsP MQW active layer [5]. Modulators were 300pm long. Coupler waveguides were 300pm long and bent in a 1 mm radius. A 0.3pm thick non-doped optical confinement layer (1.35pm wavelength composition) was grown selectively by MOVPE, except for the active region. This layer acts as an absorber in the modulator region. Semi-insulating buried-hetero structure [6] was formed for transverse optical and current confinement, as well as for sufficient electrical isolation between these optical elements. The front end (coupler side) was antireflection coated with SiN film. DBR LDs operated with 37-55 mA threshold currents and maximum output power values were 2-3 mW from the front end under CW conditions. Figure 2 shows extinction ratio versus applied voltage characteristics for the modulators. Over 1 OdB extinction ratios were obtained at -5V applied voltage in all the modulators. Calculated modulator capacitance was 2 pF. 3 GHz modulation bandwidth was obtained experimentally. Wavelength tuning characteristics are shown in Fig.3. Lasing wavelengths were tuned over 80%l semi-continuously, by adjusting both DBR region current (IDBR) and phase con~trol current (I PC). Simultaneous four-channel wavelength control to govem 5A, 1OA and 2 0 a channel spacing was demonstrated successfully, as shown in Fig.4. 2.5Gb/s modulation was carried out with 5 V D P NRZ signal. A clear modulated light output waveform was obtained as shown in Fig.S(a). Figure 5(b) shows a time-averaged spectrum for modulated light. A CW spectrum was shown simultaneously , where lasing wavelength was detuned by 3 8, for easy comparisov. The modulated light spectrum at -3OdB level to the peak was found to be only 0.12A wider than the CW spectrum. This result implies the applicability of the present SPIC for high-density WDM systems with narrow channel spacing, which is hard to be achieved by LD direct modulation scheme. The c.hanne1 spacing will be reduced ideally to six times the bit-rate (for example, 1.25A in the case of 2.5-Gb/s modulation) without substantial spectral interference between adjacent channels. In conclusion, an SPIC for high density WDM systems was developed. Successful channel spacing control in wide wavelength range, and very low chirping 2.5Gb/s intensity modulation, were demonstrated.