F-12
Measurement Considerations
Model 2520 User’s Manual
The propagation speed of an electrical current pulse through a conductive material is a
function of the material’s impedance. Any change in propagation speed as a signal passes
between different materials or impedances will result in coupling loss and reflections. As
with the optical realm, reflections can result in constructive or destructive interference.
The resulting signal can exceed the desired level or require considerable settling time and
impact system integrity. Electrical design engineers have developed techniques to identify
and manage impedance mismatches to optimize signal coupling and minimize unwanted
reflections. This is fundamental in the application of high-speed pulses to laser diodes dur-
ing testing. Impedance matching and transmission line effects must be understood and
managed to prevent damage or optimize test results.
Laser diode impedance matching
The typical laser diode has a characteristic impedance of 2
Ω
to 6
Ω
. This impedance is
considerably lower than that of common coaxial cables, which typically have characteris-
tic impedances in the range of 50
Ω
to 75
Ω
. Matching a laser diode to the impedance of the
coaxial cable would require adding 48
Ω
to 44
Ω
of series resistance. However, sourcing a
5A pulse across 50
Ω
load would generate a 250V potential. Such potentials are not only a
safety hazard, but will also damage the laser diode under test.
To enhance impedance matching, low-impedance cables, such as the 15
Ω
coaxial cables
supplied with the Model 2520 should be used. This cable was designed for fast transient
high-current response and reduces the mismatch between the laser diode and transmission
line.
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