2:Oopm - 2:15pm WDM3.2 A Six Wavelength Laser Array with Integrated Amplifier and Modulator M.G. Young, U. Koren, B.I. Miller, M. Chien, T.L. Koch, D.M. Tennant, K. Feder, K. Dreyer, and G. Raybon AT&T Bell Laboratories, Holmdel, NJ 07733 (908) 949-5974 To take advantage of the large bandwidth of optical fiber, many researchers are considering semiconductor laser arrays as a compact and reliable choice for wavelength-division- multiplexed (WDM) transmitters[ 1-41. Previous results address the need for a device that will simultaneously transmit a number of channels through an optical fiber. Future WDM architectures may also require transmitters that allow the arbitrary selection of a particular WDM channel. We report here a laser array designed for transmitting a single, selectable wavelength. A schematic of the device is shown in Figure 1. A passive combiner guides the signal from each of six W6shifted distributed feedback (DFB) lasers into a single output waveguide where it passes through an optical amplifier to compensate for the inherent losses of the combiner. The signal is then encoded using an electroabsorption modulator which is also integrated into the output waveguide. Both anti-reflection coatings and a window section prior to the output facet are employed to reduce the reflections as required for low-chirp operation. The use of back-facet detectors allows the monitoring of the operation of the lasers, and also eliminates the need for back facet coating. Since the device is meant to only have one laser operating at a given time, there is no concern of electrical crosstalk among adjacent lasers. This allows the spacing between lasers to be relatively small (80 pm), which has a direct affect on the length of the combiner. The total width of the device is only 500 pm, and the total length is 3.5 mm. We believe that substantial reductions in the length will be possible with further reductions in laser spacing and improved combiner design. Previously, gratings for laser arrays have been done by either repeated holographic exposures, or direct e-beam writing. For this device, we have used a recently developed lithographic technique to print the U6shifted gratings. The e-beam generated phase mask simultaneously prints all the required pitches in a single photolithographic step[5]. The lasers were designed to have a 200 GHz channel spacing. Figure 2 shows the output spectrum of a device with 50 mA bias on the lasers, and zero bias on the amplifier and modulator. The inset shows a typical spectrum from a laser processed using these techniques just below threshold, showing the stop-band characteristics inherent to a U4-shifted grating. Figure 3 shows the L-I curves for all six lasers with an amplifier bias current of 75 mA, displaying thresholds typically in the 20 mA range. As the amplifier current is increased further up to 175 mA, 4 dB additional gain is available beyond the values shown in Fig. 3. However, larger amplifier gain beyond that required for combiner compensation places increasingly stringent output facet anti-reflection requirements. For high amplifier gain in the present device, this manifests itself in observable 1A-level spectral shifts and associated kinks in L-I characteristics. Figure 4 shows the response of the device with one laser actuated (laser 4) when reverse bias is applied to the modulator, and the amplifier is held at high gain with a bias of 150 mA. As the voltage is varied from 0 to -3 volts, the detected optical power, measured here with a broad- area detector, is reduced by a factor of three. When coupled to a single mode fiber, the extinction ratio of this modulator will be greatly improved. The high amplifier gain induced kinks mentioned above disappear as facet reflection is attenuated, and also there is a reduction in the ASE as it becomes absorbed by the modulator. In conclusion, we have demonstrated a six-wavelength laser array with an integrated amplifier and modulator designed for transmission of a single selectable wavelength. The single- 24 1