Page 442 - Tunable Lasers Handbook

402 Paul Zorabedian grating
,&

optical
isolator
Ele + \

plate
\
nonpolarizing
output beam splitter

beam polarizing splitter w optical
mirror
amplifier
FIGURE 32 Grating-tuned ring ECL. (Reproduced with permission from Peng and Su [SSJ.)

to -30 dB and reduced the homodyne linewidth to -50 kHz. The wavelength
range over which these improved results could be obtained was not mentioned.

8.2 Interference-Filter Tuning

An interference filter can be used as the sole tuning element in an ECL
because the blocking layers can be designed to allow only one transmission
order within the gain bandwidth of the semiconductor. The advantage of an
interference filter is that it is compatible with the degenerate-resonator extended
cavity configuration in which the feedback strength is very insensitive to tilt of
the external mirror and lateral drift of the gain diode. Interference filter tuning of
a 1300-nm extended-cavity laser in a quasi-degenerate-resonator configuration
with a high degree of angular misalignment tolerance has been demonstrated
[141 (Fig. 33).

8.3 Etalon Tuning
By reducing the mirror spacing. the need for blocking layers is eliminated
and thin etalons can be used to tune ECLs. Some examples follow.
Kahn and coworkers [89] constructed a pair of high-stability etalon-
controlled ECLs. For this design, the gain element was a 400-pm-long dual-
electrode buried-heterostructure InGaAsP laser diode with one HR and one AR
facet. The extended cavity comprised a 100-pm air gap etalon and an output