Optically-injection locked tunable multimode VCSEL for WDM passive optical networks (2008)

We review the progress of optical injection locking (OIL) of multimode (MM) VCSELs and discuss the potential of using tunable OIL-MM-VCSELs as low-cost high-performance transmitters for wavelength division multiplexed passive optical networks. The wavelength division multiplexed passive optical network (WDM-PON) is actively being developed to deliver bandwidth intensive applications to end users in a cost-effective and future-proof manner [1]. To meet the demands of low-cost wavelength-specific upstream transmitters, we have recently proposed a novel scheme that utilizes optical injection-locked (OIL) 1.55 mum vertical-cavity surface-emitting lasers (VCSELs) as directly modulated upstream transmitters, whereby the injection locking light is furnished by modulated downstream signals [2]. In that configuration, the distributed feeback (DFB) lasers that carry downstream signals from the central office (CO) also serve a second function as master lasers to injection-lock upstream slave VCSELs in the optical network units (ONUs) onto the WDM grid. Our scheme thereby removes the need for external injection locking optical sources, external modulators, and potentially wavelength stabilization circuitry. We showed that the OIL-VCSEL responds mainly to the carrier wavelength. Hence, it does not "see" to the modulated downstream signal. Hence, downstream signal does not adversely affect the upstream data flow yet can provide wavelength locking to the standard WDM grid. The VCSELs used in [2] and all of our past experiments [3], however, are single mode VCSELs. While very effective for OIL experiments, single mode VCSELs are intrinsically more expensive. Multimode (MM) VCSELs are attractive low-cost transmitters for high-speed short-reach fiber-optic networks [4,5]. The multimode nature introduces mode competition noise and modal dispersion, preventing these devices from very high speed modulation speed or long transmission distance. Recently, greatly enhanced bandwidth and transmission distance were reported using optical injection locking (OIL) as a technique to improve the direct-modulation response of MM VCSEL [6]. We showed that 54 GHz resonance frequency and 38 GHz 3-dB bandwidth can be obtained with a MM-VCSEL with a free-running 3dB-bandwidth of 3 GHz. These improvements are made possible by leveraging the unique properties of MM-VCSEL, having spatially and spectrally well-separated modes which facilitate efficient injection to preferentially enhance the fundamental transverse mode. We believe this result will be very important for low-cost upgrades of existing embedded local area networks (LANs) and indeed for WDM-PON. The final piece of puzzle to make OIL-MM-VCSEL for WDM-PON is wavelength tunability. As the injected power will be low, coming from CO through 10-25 km of standard fiber, the locking wavelength range is small. A tunable VCSEL will provide the flexibility to enable better locking. Furthermore, a tunable VCSEL can be a universal transmitter for the ONUs despite each ONU could be designated with a different wavelength. Finally, a tunable VCSEL will enable a wider temperature range for high speed operation, enabling not only colorless but coolerless WDM-PON. In this talk, I will review our recent work on tunable VCSEL using a high contrast grating with a very large fabrication tolerance.