Resonant Modulation of Single-Contact Lasers

Chapters 9 and 10 above describe in detail the concept and performances of narrow-band subcarrier modulation of a laser diode at mm-wave frequencies based on the concept of mode-locking, alternatively known as “resonantly-enhanced modulation” (or simply “resonant modulation” for short). This technique is useful in applications where optical fiber is used for the remote transport of millimeter wave (>30 GHz) signals for phased-array antenna systems [107] and wireless personal communication networks [101]. Due to the fact that efficient longitudinal mode-coupling requires inhomogeneous modulation of the laser cavity in the longitudinal spatial dimension, previous demonstrations of monolithic resonant modulation such as those described in Chap. 10 were performed using split(multi)-contact laser diodes. Although the fabrication of a multi-contact structure itself is neither difficult nor intricate, the increased complexity is undesirable — considering the fact that it does require a non-standard fabrication process; rendering them unsuitable for seamless integration into a standard telecom laser fabrication line. Recent measurements of millimeter-wave propagation along the contact stripe of a semiconductor laser showed a high attenuation (~60 dB mm⁻¹ at 40 GHz) of the signal along the laser stripe [108]. In this chapter, it is demonstrated that the confinement of high frequency modulation current resulting from the high signal attenuation along the length of the laser stripe can be utilized to achieve resonant modulation at 40 GHz of a standard single contact monolithic semiconductor laser. This concept is illustrated schematically in Fig. 11.1 for a ridge waveguide structure. The injected modulation current is confined to a local region near the feed point, resulting in a (longitudinally spatial) partial modulation of the laser cavity. The experimentally measured modulated light output and the small signal response of the laser device at two different feed points along the stripe of a semiconductor laser are shown in Fig. 11.2. In Fig. 11.3, a maximum modulation efficiency (on-resonance at ~40 GHz) of −20 dB (relative to that at baseband) can be observed. Also, using a simple distributed circuit model of the laser in conjunction with conventional mode-locking theory, the characteristics and limitations of this technique are investigated.