Abstract

Optically pre-amplified detection employing semiconductor laser amplifier at the input stage has attracted considerable interests for high speed, high sensitivity lightwave receiver systems.¹ However, the current hybrid method using discrete laser amplifier followed by a narrow band filter and detector incurs a high coupling loss (varies from 3.5 to 9 dB) and lacks integration capability. In the monolithic integration scheme,² on the other hand, suppression of residual optical reflection is difficult to achieve and the incorporation of an optical filter is difficult to implement. We have demonstrated in this work, a vertically coupled InGaAsP/InP laser amplifier at 1.5 μm which can circumvent these problems. The device schematic is shown in Fig. 1. A 4 μm wide ridge laser with a tapered rear end which extends to 10 μm is first fabricated by conventional method.³ A 45° mirror is then formed by focused ion beam etching on the tapered end of the device which deflects the amplified signal (incident from the front facet) onto the backside of the substrate for detection. The substrate is thinned down to 120-150 μm, polished on the backside and contacted electrically. The cavity of the gain medium, formed by cleaving the front facet of the laser amplifier, has a typical length (between the front facet and the 45° mirror) of 400 μm. Figure 1 also shows the beam size on the 45° mirror and on the backside of the substrate respectively. For a near field pattern of 8.1 μm by 3.4 μm, on the 45° mirror, a nearly diffraction-limited far field pattern of 15 μm by 32 μm is obtained on the substrate side. Figures 2 and 3 show the light versus current and the emission spectra of the device before and after anti-reflection (AR) coating at the front facet. The device functions as a multimode laser prior to AR coating and displays periodic intensity modulation in its longitudinal modes due to residual reflection between the 45° mirror and the polished backside of the substrate. These gain ripples are suppressed after AR coating at the front facet and an external gain (fiber to detector) of 15.4 dB at 1.490 μm (from a DFB laser) is obtained at an injection level of 170 mA with a gain ripple < 2 dB. Owing to the vertical propagation of the output beam and enlarged output spot size (15 μm by 30μm), the device has advantages for low insertion loss and compact integration with narrow band dielectric DBR filters⁴ (externally coated or epitaxially grown) and photodetectors (via hybrid flip-chip bonding⁵ or monolithic integration on the backside of the substrate) in high sensitivity, high bit-rate lightwave communication systems.

© 1991 Optical Society of America