Page 449 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 409

to result in a1 fairly short cavity. If a longer cavity is needed in order to obtain a nar-
rower linewidth, the requisite path length can be filled with prisms to expand the
filled depth in proportion to the total length. A 1300-nm external cavity containing
a GRIN rod collimator followed by two silicon prisms resulted in a 7-cm-long cav-
ity with about three modes in the grating passband [61]. Single-mode operation
was achieved over a 50-nm tuning range with a linewidth of 100 kHz or less.

10. PHASE-CONTINUOUS TUNING

Phase-continuous tuning [ 1061 means tuning without mode hopping or other-
wise intempting the phase of the oscillation. A prerequisite for phase-continuous
tuning is thlat the mode number-the number of half-wavelengths between the
laser mirrors-remains constant as the wavelength is varied. Phase-continuous
tuning is required for locking the oscillation to a wavelength reference. It is also
useful for interferometric measurements, optical frequency synthesis schemes,
and in general whenever fine control of the laser wavelength is needed.
Two regimes of phase-continuous tuning can be distinguished: short range
and long range. Short-range phase-continuous tuning over several euternal-cal,,ity
mode spacings (Le., up to a €ew tens of gigahertz) can be accomplished by chang-
ing the cavity length while keeping the position of the filter peak fixed [MI.
Eventually the lasing mode moves too far from the filter peak and a mode hop
occurs to a new mode with lower loss. Long-range phase-continuous tuning over
many nanometers requires precisely coordinated simultaneous shewing of the
axial-mode frequencies in synchronism with the filter peak. The requirements for
this type of tuning are considered next,

1 0.1 General Requirements for Long-Range PhaseContinuous Tuning
In phase-continuous tuning. the longitudinal modes must be shifted at
exactly the same rate as the filter peak so that the same mode maintains its status
as the lowest loss mode and thus continues to oscillate. Assume that the u’th lon-
gitudinal mode of the ECL starts out exactly at the peak of the filter passband.
The frequency of the q’th mode is given by

;70j

where q is an integer, c is the speed of light, and