Burleigh WAVEMETER JR WA-2500, Operating Manual, Page: 24 / 47

Wavemeter
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Operating Manual 3. Operating Instructions
Burleigh Instruments, Inc. (04659-M-00 Rev E) page 3-15
Burleigh Park, Fishers, NY 14453
3.14 OPERATION WITH MODE-LOCKED AND OTHER LOW COHERENCE LASERS
The Wavemeter
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can measure the wavelength of most mode-locked CW lasers when
the following considerations are met.
NOTE: Mode-locked lasers produce peak powers that are typically 1000-10,000 times
greater than their average output power. These lasers can permanently damage
the fiber optic cable when such high power is focused inside the fiber by the beam
receiver. To avoid such damage the average power incident on the beam receiver
head should be kept under 10 milliwatts. Reflecting the beam off one or more
uncoated glass beamsplitters provides one simple means of reducing the beam
power to an acceptable level.
The short pulse output of a mode-locked laser results in a broad spectral linewidth
as depicted in the middle pattern of Figure 3.3. The reduced coherence associated
with this broad linewidth produces a fringe envelope observed on the MONITOR
signal that is short in duration. The maximum of the fringe envelope occurs at the
point in the scan where both paths through the interferometer are equal in length.
For proper operation of the Wavemeter
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, the TRIGGER pulse should start near the
maximum of this fringe envelope. This condition is factory set for a laser source
operating at 800 nm, but may require slight readjustment for other wavelengths if
the MONITOR signal envelope is very short.
First display the MONITOR signal for the broad linewidth laser and the TRIGGER
signal simultaneously on an oscilloscope. Determine if the TRIGGER pulse begins
before or after the peak of the MONITOR fringe envelope and by how much in
milliseconds.
Refer to Figure 3.6 and use the following procedure to set the correct optical path
length:
1. Turn off the Wavemeter
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and disconnect the AC line cord.
2. Remove the top cover of the chassis secured by two black screws on each
side. Carefully detach the ribbon cable connector between the main circuit
and the display panel.
3. Remove the silver dust cover from the interferometer that is secured by one
top screw.
4. Locate the knurled mirror holder in the adjustable mirror mount next to the
rear panel. Mark the existing 12 o’clock position on the face of the knurled
mirror holder with a felt tipped marker pen.
5. On the side of the mirror mount facing the transformer, use a 1/16” hex
wrench to loosen the set screw that secures the threaded section of the
knurled mirror holder.
6. If the TRIGGER pulse began before the peak of the MONITOR envelope,
rotate the mirror holder ½ turn counterclockwise for each millisecond
difference. If the TRIGGER pulse began after the peak of the MONITOR
envelope, rotate the mirror holder ½ turn clockwise for each millisecond
difference.
7. Secure the mirror holder by tightening the set screw on the side of the
mount.
8. Introduce a HeNe laser input to the beam receiver and realign the
interferometer as described in Section 3.11.
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