Coherent Injection Locking

Coherent pulse injection into a monolithic passive mode-locked laser

Arun S. Mampazhy, Arthur Liu, Shuo-Yen Tseng, Christopher J.K. Richardson and Julius Goldhar
(presented at CLEO '08)

We investigate the conditions required to optically coherent injection lock a monolithic mode-locked semiconductor laser. Proper operation requires repetition rate matching and mode alignment between the master and slave lasers as illustrated in Fig. 1.

While it is easy to measure the free running repetition rate of the slave laser, and to adjust the master repetition rate to be close to it (within ~10MHz), the mode alignment is more difficult to monitor. For that reason we developed an interferometric technique illustrated in Fig.2 . When modal detuning is small we observe destructive interference between the master laser pulse reflected from the slave laser facet and the pulses from the injection locked slave laser.

Current tuning through longitudinal modes is observed in CW injection locking experiment as shown below.

Output of the "master" laser is injected into the “slave”

Both spectrum and temporal pulse shape vary significantly as we tune through the dip. Very similar results were observed at the second dip.

Effects of modal and repetition rate detuning as well as dispersion and spectral filtering due to wavelength dependent gain is calculated in the frequency domain.                               


  1. Measurements and modeling indicate that the pulses emitted from the locked laser can be significantly different from the master laser
  2. The results indicate precision is required for coherently injection mode locking.  Small changes in repletion rate or mode alignment between the master and slave lasers can lead to the pulse shapes from the slave laser that have dramatically different characteristics.
  3. Large tuning range of repetition rate is explained by large group velocity variation and nonlinear pulse propagation [4]
  4. A simple numerical model predicts qualitative features of experimentally observed interferometric dips and shows appropriate  temporal and spectral shape changes

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