4112 Paul Zorabedian
ARBITRARY PIVOT y
~,f ~~~, . _~ 9 ot PIVOT ARM, LENGTH,~R ] ;'x
+
/ (91/
OPTIC AXIS .-'"""" /
-'" GRATING L.
.,'~ tanO 1
9 sssodSSo 9149149
,-'"'.. OPTIMUM PIVOT
.;, L 3 ~-
FIGURE 37
Location of optimum pivot point in phase-continuous tuned grating extended-
cavity laser. (Reproduced with permission from Trutna and Stokes [ 112]. 9 1993 IEEE.)
11. CHARACTERIZATION METHODS FOR EXTERNAbCAVITY LASERS
To design and construct external lasers with well-characterized and reliable
behavior, routine measurement of the laser's properties is essential. Table 7 indi-
cates several commonly measured quantities and the measurement techniques
for each.
12. MEASUREMENT OF FACET AND EXTERNAL-CAVITY REFLECTANCES
12.1 Determination of Gain and Loss Parameters for Solitary
Diode Lasers
To determine the internal gain and loss parameters of the gain medium, the
minimum information required is the threshold current / and the original lasing
wavelength ~'o of the solitary diode laser prior to antireflection coating. The soli-
tary threshold current is given by
8 Tunable External-Cavity Semiconductor Lasers 4 | 3
TABLE 7
Parameter
Quantities and Measurement Techniques for ECLs
Instrument or method Resolution/sensitivity
Wavelength
Power
Threshold current
Sidemode ratio
Linewidth
Optical spectrum analyzer
Michelson interferometer wavelength meter
Optical power meter
Measurement of L-I curve under computer
control [ 115]
Scanning Fabry-Perot interferometer
Lightwave signal analyzer [24]
Delayed-self-heterodyne method with
Mach-Zender interferometer [ 116] (Fig. 38)
Resolution --43.1 nm
resolution--5 ppm
resolution --0.01 dBm
0.05-mA resolution
requires 2 to 3 sec per
wavelength
Sensitivity ~- 15 to -20 dB
Sensitivity ~-60 dB
minimum resolvable
linewidth--V2
x a,
where
x a
is
the differential delay of the
interferometer
+l 1 'n/r /l +'tr
Io =
a int "1- ~int
(77)
One way to determine the internal parameters of the gain media is to sacrifice a
sampling of them by coating them with a partially reflective coating r 1 on one
facet. The reflectance of the test film is measured from a witness sample. The
threshold current becomes
+-I 1,nIro+,)l+,tr.
11 = o~
int "1-
(78)
Assuming the laser diode still lases at ;L o, the following relations for the internal
gain and loss parameters are obtained
1 ln(ro/q)
'Y=tin t (11__I0) "
(79)
and
[~Itr (~ o )+ 0~] Lint =
ln(ro/r,)/o
/,-/o
+ ln(r 2) . (80)
414 Paul Zorabedian
EXT I
CAV i
LASER
FIGURE 38
POLARIZATION ~ SPECTANAL
CONTROLLER
Delayed self-heterodyne interferometer for linewidth measurement.
Typical values for a GalnAsP buried crescent device with
Lin t =
300 lam at 1300
nm
are
[71tr(~o)
+ ~]Lin t =
0.7 and
~/-'int =
0.12 mA-1.
12.2 External-Cavity Reflectance
The following calculations pertain to the basic extended-cavity configura-
tion. They can readily be modified for the double-ended ECL or ring ECL con-
figurations. It is relatively straightforward to determine the transmission losses
of the various intracavity elements separately. However, the mode-conversion
loss due to the coupling lens is not easy to determine directly. Therefore, the
external-cavity reflectance usually must be deduced from changes in the thresh-
old current caused by modifications to the feedback. To determine the external
feedback strength, place the laser diode in the external cavity, tune to the vicinity
of the solitary lasing wavelength Xo, and measure the external-cavity threshold
current versus wavelength over one period of its ripple pattern. Compute the
average external-cavity threshold current
_89
,man/
lext
ImaX+-extJext
,
(81)
which is related to the wavelength-averaged external-cavity reflectance
rex t
by
- +1 ',n
/ext'- (X int + Z--~n t
9
(82)
At this point, assuming
~tin t
is independently known from test devices, the external-
cavity reflectance can now be determined from
Io l"ext 1 In
rext
- = -- . (83)
]tLint
r o

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