PhD Defense: "Integrated Linewidth Reduction of Rapidly Tunable Semiconductor Lasers"

Abirami Sivananthan

June 21st (Friday), 2:00pm
Engineering Science Building (ESB), Room 2001

Highly agile widely tunable lasers have applications in dense wavelength division multiplexing (DWDM), optical sensing and optical packet switching. In DWDM, tunable lasers can greatly reduce inventory costs and increase flexibility. For this application, the tunable lasers must meet the same requirements as currently used fixed frequency lasers, (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 μm of resolution at a 1.5 km range.

The above applications demand a laser with wide tunability, nanosecond tuning speeds, a 100 kHz linewidth and small form factor. Multiple techniques can achieve a low linewidth laser but with the trade-off of slower tuning speeds or larger size. In our approach we use negative feedback in an InGaAsP/InP photonic integrated circuit (PIC) to reduce the linewidth of a widely tunable SG-DBR laser. Integration of the optical components has allowed us to keep the advantages of size, weight and power inherent in semiconductor lasers and simultaneously attain low linewidth.

The SG-DBR laser has a 40 nm tuning range, ns tuning speeds and is 1.5 mm long but the linewidth is in the MHz range due to carrier induced frequency fluctuations. We use an asymmetric Mach Zehnder integrated on the PIC to monitor and convert the laser frequency fluctuations to amplitude fluctuations. This error signal is detected on-PIC and 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 reported for an SG-DBR laser.

Hosted by: Professor Larry Coldren