From: SFPhysics@aol.com
Date: Thu Apr 19 2001 - 01:34:48 PDT
From: SFPhysics@aol.com Message-ID: <6f.1434a1df.280ffd28@aol.com> Date: Thu, 19 Apr 2001 04:34:48 EDT Subject: Laser Diode Amplitude Modulation
> Subject: Laser Diodes
> From: "MC elover" <mcelover@yahoo.com>
> Date: Tue, 17 Apr 2001 16:30:10 -0700 (PDT)
> Hello
> I had an interesting afterthought to the LED
> discussion: It came up during the discussion that
> diodes are slightly ohmic, meaning that more voltage
> gives you more current. Since laser pointers use
> diodes to create light, I was wondering weather it is
> possible to modulate the output intensity of a laser
> pointer by modulating the input voltage.
> Does anyone have any
> sugestions for a photoelctric demo?
--Eric
>>
Hi Eric:
Laser diodes operate on a very closely regulated voltage but may be AM
modulated by either of two simple methods. There is a narrow saturation
range because the laser diode junction is forward biased and the biasing
determines the depth of the photon emission layer thus the lasing action, so
the applied DC should be left alone. At this point in time laser diodes have
voltage control circuits attached and sometimes built into the diode itself.
Applying modulation to just the battery terminals may be ineffective.
For the older laser diode units you can just connect one leg of the diode
through the 8 Ohm winding of a center-tapped audio transformer then apply an
audio signal to the second section of the secondary winding or the primary.
This way the DC current to the diode is uneffected and the audio AC may
modulate the DC. I have used this simple autotransformer setup and it works
well.
The second method is to modulate the diode with a capacitor as a DC block
between the applied AC signal and the diodes DC operating voltage. The audio
is applied along with the DC but the blocking capacitor only lets through the
AC to modulate the diode and keeps the DC out of the audio circuitry. This
works also but the coupling capacitor must be of the electrolytic type and
polarities should be observed.
The laser diode voltage is critical because what makes for a monochromatic
light source in the diode is to have most of the electrons jumping across the
forward biased junction at the same potential through an extremely shallow
junction potential well. The biasing voltage directly controls the voltage
well so a very slight AC modulation will produce a moderate change in optical
output. At least that was my experience.
One note on regular LEDs, they should have a current limiter resistor in
series with them or they will self-destruct. As forward biased junctions
they can pass huge amounts of current until the Galium Arsenide junction pops
or melts.
Photovoltaic action can be observed where the hf is converted to displacement
of an electron in solar cells that can cheaply be purchased at Radio Shack.
Most of the action here is in the near infra-red region. I have Selenium
Cells and Cesium Cells that require higher photon energies. By using a small
arc lamp source of white light I have been able to separate the spectrum with
a prism and move the different cells across the spectrum to show how
different materials require different photo energies to liberate electrons.
Is this what you were looking for or you wanted to show how increasing energy
liberated higher frequency photons. For the latter I used a simple light
bulb on a variable power supply. You can go from no visible light seen by
the naked eye but detectable with a Silicon photocell to very bright light
registering on a Selenium Cell with a UV filter in front of it. There are
all kinds of things that can be done to show the relationship of wavelength
and energy.
Regards to all,
Al Sefl
Happy retired after 33 years teaching with a lousy little STRS pension.......
Hey Kids, don't forget to put money in that 403b tax sheltered annuity......
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