Narrow linewidth millimetre-wave generation by optical heterodyning of two vertical-cavity surface-emitting lasers

This paper demonstrates a dual VCSEL system with optical feedback, capable of generating a narrow linewidth RF beat signal over a broad frequency range by optical heterodyning. The cavity diameter of the devices used in this work is 10μm and their threshold currents are approx.10mA. Each device emits light in a single stable polarisation at a wavelength of approx.853nm. Single mode powers of 1mW are achievable in each device. The two collimated beams are spatially combined using a 50/50 beam-splitter. The light from one of the beam-splitter outputs passes through an optical isolator and is coupled to a high speed detector and RF spectrum analyser. To provide optical feedback, an external cavity is formed using the second output beam. Beat notes are generated up to 25GHz, limited by the detector speed, by varying the bias current of one of the lasers and a tuning rate of the beat note frequency of approx.5GHz/0.1mA is obtained. Since the lasers are independent and the noise of each is uncorrelated, broad 3dB linewidths of typically 200-300MHz are obtained. The beat note conversion efficiency is estimated to be as high as 70%. However, when optical feedback is applied to the lasers, the linewidths narrow considerably to ultra-low values of typically 200kHz, limited by the resolution of the measurement. This also results in higher peak beat note peak powers. Conversion efficiencies of typically 60-70% are calculated. External optical feedback has previously been observed to narrow the VCSEL linewidth. It is also found to be essential for the generation of the narrowest linewidth highest peak power electrical signals in this work. Further results and analysis, with and without feedback, are presented.