Integrated Linewidth Reduction of Rapidly Tunable Semiconductor Lasers

Widely tunable lasers with fast tuning speeds have applications in dense wavelength division multiplexing (DWDM), optical sensing and optical packet switching. In DWDM, tunable lasers can greatly reduce inventory costs, increase manufacturing efficiency, and increase flexibility. For this application, tunable lasers must meet stringent requirements in terms of linewidth, SMSR, RIN, etc. As coherent detection moves to higher modulation formats to increase spectral efficiency, linewidths on the order of 100 kHz will be required. In FMCW LIDAR, the sensing range is directly coupled to the coherence length, i.e. linewidth, of the laser, and the resolution is determined by the tuning range of the laser. A laser with a 40 nm tuning range and 100 kHz linewidth can lead to a LIDAR system with 30 microm of resolution at a 1.5 km range. The above motivations demonstrate the need for a laser that is widely tunable, with tuning speeds in the nanosecond regime, a 100 kHz linewidth and small form factor. Many different approaches have been taken to achieve a low linewidth laser, generally with the trade-off of slower tuning speeds or larger size. Typically, the widely tunable mirrors used to create a highly agile laser are noisy. In our approach we use negative feedback along with an InGaAsP/InP photonic integrated circuit (PIC) to reduce the linewidth of a widely tunable SG-DBR laser. The SG-DBR laser has a 40 nm tuning range, ns tuning speeds and is 1.5 mm long. Typically the linewidth is in the MHz range due to carrier induced frequency fluctuations. We use an asymmetric Mach Zehnder integrated on the same PIC to monitor and convert the laser frequency fluctuations to amplitude fluctuations. This error signal is fed back through a stabilizing loop filter to the phase tuning section of the SG-DBR laser to reduce the laser linewidth. Through integration of all the optical components, the loop delay is minimized and loop bandwidths upwards of 600 MHz have been achieved. Using this technique, we demonstrate an SG-DBR laser with the linewidth suppressed from 19 MHz to 150 kHz, which is the lowest linewidth yet for an SG-DBR laser.