Injection-locking for Coherent Detection of Multi-wavelength Systems

Multi-wavelength transmission has become the accepted solution to enhancing the capacity of optical communi-cations systems. The use of coherent detection in multi-wavelength systems offers improved receiver sensitivity and wavelength selectivity. In addition to these fundamental advantages, coherent detection also allows the direct appli-cation of sophisticated multiple access protocols such as CDMA (code division multiple access) which are common in radio frequency communications. For instance, the capacity of a high-bandwidth local area network can be en-hanced by employing a hybrid CDMA/WDMA protocol [1]. Description of the multi-wavelength coherent system Techniques developed for radio frequency multiple access often use phase modulation with coherent detection. In optical systems, the laser phase noise can be considerable when using inexpensive distributed feedback (DFB) lasers. We propose the use of a set of centralized narrow linewidth, highly stable reference sources which are circu-lated through the network, thus amortizing their cost among the base of network subscribers. Individual users would employ these reference signals to injection-lock their DFB lasers for either transmission or reception. Although the reference signals are attenuated by distance, the output of the injection-locked laser will be on the order of a few milliwatts. Wavelength selection is provided by activating one of the user’s bank of DFB lasers with nominal center frequency on the desired wavelength. Injection-locking leads to the DFB laser adopting the absolute frequency and frequency purity of the reference source. The use of injection-locking therefore provides many benefits such as power amplification, wavelength selection and transfer of spectral purity and stability