[Laser] Modulation by quartz xtal

Mon, 15 Apr 2002 20:46:35 -0400

James,

Your best bet if you want FM is to go with a diode laser and modulate the
injection current (like with a microwave Gunn). You'll get wideband FM
(typical diode FM deviation would be 30 GHz/mA unstabilized and much less if
stabilized via an external feedback cavity (self-lock). You will of course
need to use a heterodyne detection scheme to receive.
I'm currently working on a setup like this - an FM laser transceiver using
FM of an external-cavity self-feedback stabilized diode (simple homemade
cavity), and heterodyne receiver, where the LO laser is both transmitter and
RX local oscillator - like the familiar microwave Gunnplexer transceivers.
In your method, the vibrating xtal may affect the laser beam, but it will
produce AM (or subcarrier "FM") rather than true FM. The vibration may not
be of sufficient magnitude to produce much detectable modulation unless
using interferometric techniques to detect.

73, Jon W2MXW

-----Original Message-----
From: James D. Goodwin <[email protected]>
To: [email protected] <[email protected]>
Date: Monday, April 15, 2002 12:20 PM
Subject: [Laser] Modulation by quartz xtal

I was recently given several xtals in glass holders.  They are in the 6 and
11 MHz range. A laser beam can be reflected from the quartz element or
passed through the element. It also can be reflected off the metal strips
that hold the xtal wafer.

I have attempted to detect rf modulation on a laser beam passed through an
oscillating xtal.  So far - no success. I have swept a wide span of
frequencies in this test.

>From research on the 'net I know that quartz is used to modify laser beams
in several ways.  Will the motion of the thin wafer in these oscillating
xtals affect the beam? If so, will the beam be modulated with the exact
frequency of the xtal or some other frequency? I am setting up to test a
xtal that is frequency modulated like those xtals in the early 2-meter rigs.
If a modulated laser beam can be detected, it could be amplified and
multiplied so there would be adequate f.m. swing to produce n.b.f.m. or even
wideband f.m.

Ideas anyone? Detection techniques?  It may be that I am trying to invent
the wheel only to find that it is square!

KF4BL

Any help with these questions will be appreciated:
1. Does the vibrating xtal affect the laser beam?
2. If so, what type of modu

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