Page 459 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Lasers 41 9
~~~~i~~ Laser Diode Directional Coupler

PZT
Fiber
Delay
Line
Laser
Output

FlGu RE 4 1 Interferometric apparatus for mode-hopping suppression in an extended-cavity
laser. (Reproduced with permission from Ohtsu et a!. [120]. 0 1989 IEEE.)

Open-loop control is an alternative to a closed-loop servo system. Synchro-
nism of the peak transmission wavelengths of an intracavity etalon and a grating
can be maintained in open-loop fashion by means of careful calibration. This is
the basis for maintaining single-mode oscillation in the Hewlett-Packard
8 B67.4/8 16SA tunable laser source products [93 j.

1 3.2 Electrical Multimode Suppression Methods
As mentioned previously, multimoding is correlated with the v;avelengrhs of
the residual diode laser internal Fabrj-Perot modes. Therefore, it should be pos-
sible to suppress multimoding bj controlling the positions of the residual solitary
cavity longitudinal modes. In a conventional diode laser, this is possible by chang-
ing the injection current or the temperature. Using these controls is undesirable
because changing the injection current changes the output power, and changing the
temperature is slow. Use of a two-section diode laser with separate gain and phase-
control terminals solves this problem by providing a way to shift the internal
modes with, only minimal influence on the gain. Elimination of multimodin, 0 in . a
grating-tuned ECL with a two-section diode laser by varying the phase-control
current has been demonstrated [121] (Fig. 42). Without using the phase-control
current, the laser showed periodic regions of multimoding separated by the inter-
nal-mode spacing. By varying the phase-control current from 0 to 1 mA. single-
mode operation could be obtained in any of the previously multimode regions. A