Lockerless tuneable DBR laser

A distributed Bragg reflector (DBR) laser has a phase section 502 in a cavity, a DBR section 501 comprising a frequency tuning system having a resistance heater, and a photodetector 509. A controller 510 is configured to cause the phase section 502 to apply a dither to the optical path length of the cavity, or cause the frequency tuning system to apply a dither to a Bragg frequency of the DBR section; to use the detector 509 to monitor light intensity transmitted from the laser cavity via the DBR section 501 during the dither; determine deviation from longitudinal mode centre operation based on monitored intensity; reduce the deviation by causing phase section 502 to adjust the optical path length of the cavity; and control laser output frequency by adjusting the power of the resistance heater. The laser is tuned without a frequency locker. Laser drifts are prevented, reducing mode hops.