Intensity Modulation of a HeNe laser in combination with Interferometry

[olliH]

Hi,

i would like to know if it is possible to measure the length
(interferometric) with HeNe laser whoese intensity is modulated and
how it is possible to modulate a HeNe laser.

The modulation is nececary to compensate ambient light. A filter is
not sufficient enough.
The modulation frequency should lie in the range of approx. 200 Hz to
max. 10 kHz. The intensity of the HeNe laser should change from
approx. 40% to 100%.

Who has an idea?

Regards

Olli

[Phil Hobbs]

The usual approach would be an acousto-optic modulator.

Why do you need to compensate ambient light with laser power? Are you
trying to surf a fringe in bright field? If so, it's probably going to
be easier to use a polarizing beam splitter oriented at 45 degrees to
the laser polarization, and two photodiodes to detect the two resulting
beams. Subtract the photocurrents and servo around zero, and you'll
have at least as good ambient rejection plus near-total immunity to
laser noise. You can do this by wiring the PDs in inverse parallel, or,
better, wire them in series between + and - bias supplies, and connect
the midpoint to a transimpedance amp.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

[Salmon Egg]

In article
<f4d82500-ed14-4f25...@f11g2000yql.googlegroups.com>,

Surveyors have used such equipment for decades. HeNe lasers have been
largely replaced because of scintillation arising out of their
coherence. They go under the name of distance measuring equipment or
DME. The same was done earlier using microwaves under the tradename
Tellurometer.

The first I know about was by Spectra Physics as an altitude profiler. I
had some experience with HeNe DME made by AGA.



t

--

Sam

Conservatives are against Darwinism but for natural selection.
Liberals are for Darwinism but totally against any selection.

[olliH]

Thank you for your answers.

The interferometer isn't the problem. We split the laser into two
beams and use the one beam for measurements with a PSD the other beam
is for the interferometer. We have band-pass filter on top of the PSD
but it would be much better to modulate the laser too.

The acousto-optic modulator can't reach my frequency range. The cutoff
frequency is ~30 kHz.

But I found a way to modulate the laser in my frequency range. A
Pockels cell should do the job.

Now I just need to find the right one... :)

Regards

Olli


On 16 Jan., 17:14, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net>
wrote:

[Ron Capik]

On 1/17/2012 4:12 AM, olliH wrote:
> Thank you for your answers.
>
> The interferometer isn't the problem. We split the laser into two
> beams and use the one beam for measurements with a PSD the other beam
> is for the interferometer. We have band-pass filter on top of the PSD
> but it would be much better to modulate the laser too.
>
> The acousto-optic modulator can't reach my frequency range. The cutoff
> frequency is ~30 kHz.
>
> But I found a way to modulate the laser in my frequency range. A
> Pockels cell should do the job.
>
> Now I just need to find the right one... :)
>
> Regards
>
> Olli
>

Have you thought about mechanical choppers?
Maybe something like:
< http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=287 >
or
<
http://www.boselec.com/products/documents/OpticalChoppersLock-InsWWW1-10-11.pdf
>


Later...
Ron Capik
--

[Samuel M. Goldwasser]

I've seen several replies to this but could you describe the problem
you are trying to solve? HeNes have been used for several decades in
interferometric displacement measuring systems without intensity
modulation.

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[Phil Hobbs]

olliH wrote:
>
> Thank you for your answers.
>
> The interferometer isn't the problem. We split the laser into two
> beams and use the one beam for measurements with a PSD the other beam
> is for the interferometer. We have band-pass filter on top of the PSD
> but it would be much better to modulate the laser too.
>
> The acousto-optic modulator can't reach my frequency range. The cutoff
> frequency is ~30 kHz.

AOMs can modulate at megahertz rates. Maybe you're looking at a
photoelastic modulator?

>
> But I found a way to modulate the laser in my frequency range. A
> Pockels cell should do the job.

Pockels cells aren't terribly cheap, however, and they generally have a
lot of birefringence drift.

[olliH]

@Ron: With a mechanical chopper i would loose my laser complelty. I
would like to modulate the laser's intensity from 40% to 100%.
@Phil Hobbs: AOMs are to fast for me. Birefringence drift of a pocels
cell doesn't sound good. How fast can a photoelastic modulator
modulate the laser?

Is it possible to modulate the laser and measure the distance with
interferometry without loosing contrast and any other bad influenc to
the distance measurements?




On 18 Jan., 05:06, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net>
wrote:

[Phil Hobbs]

Photoelastics are slow and narrowband, because they rely on an acoustic
resonance in the cell.

The Pockels effect (i.e. the linear electrooptic effect) only occurs in
birefringent materials, because you have to start with a broken symmetry
to get a linear effect. Linear effects change sign when you invert the
drive voltage, and if the material were symmetric, it wouldn't know
which way to go. That's why isotropic materials can exhibit a quadratic
electro-optic effect, called the Kerr effect, but not the Pockels
effect.

Thus any simple linear electrooptic device will have a lot of
birefringence, except perhaps in one special direction. A large static
birefringence almost always implies a large polarization drift with
temperature, so Pockels cells are basically laboratory devices.
(Lateral Pockels cells, as used in telecom modulators, are very much
better in this regard, but you aren't working at 1.55 um.)

The simple way to amplitude modulate a laser is to make it a diode
laser, and change the bias current.

If you tell us more about your actual measurement, we can probably make
intelligent suggestions about it. Why do you care about ambient light?

[olliH]

Thanks a lot for your explanation.

@ Phil Hobbs
my actual measurement
I have a two frequency HeNe circular polarized 1 mw HeNe laser. The
laser points to a retroreflector and comes back to my receiver for the
length measurement.
On top of the retro i have a beam splitter which spilts the beam. The
splitted beam hits a PSD to detect the spot position.
The detected spot position of the psd is sensitive to ambient light.
Now I want to measure the spot position with the PSD as good as
possible. I use a band pass filter with 25 nm band width but I have
still to much ambient light on my psd. The next thing I want to
investigate is to use a poliraziation filter too. But even when I have
no abient light on my PSD modulation of my HeNe laser without losing
the interferometer would be nice.
The next thing is that my retroreflector with the beam splitter and
the PSD tilts in the range of +/- 15°. That means that the angle of
incidence on my PSD with the filter changes in the same range. My PSD
is big enough to measure in this range the spot position but the
environmental influences are a problem. To use a CCD or CMOS sensor
could help me but the signal processing is more complex and I can't
reach the same measuring rate.

The polarization filter on my psd is the next thing I want to
investigate but I have to use filter components that allow me to
measure in the range of +/- 10°.





On 19 Jan., 21:56, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net>

[George Herold]

> > hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -
>
> - Show quoted text -

If you are stuck with the HeNe laser. (I like the diode laser and
modulated current idea.) Then how about a chopper wheel made from
glass, you could have some partially reflective 'spokes' on it, or ND
absorptive 'spokes'.

As a 'crazy' idea maybe polarization modulation via a coil and Faraday
rotation.

George H.

[Ron Capik]

Ages ago Bulova made tuning fork choppers
that could fully or partially block the beam depending
on drive voltage. However, they're out of business
and I don't know if anyone makes such devices now.

Later...
Ron Capik
--

[Helpful person]

> > > hobbs at electrooptical dot nethttp://electrooptical.net-Hide quoted text -

>
> > - Show quoted text -
>
> If you are stuck with the HeNe laser.  (I like the diode laser and
> modulated current idea.)  Then how about a chopper wheel made from
> glass, you could have some partially reflective 'spokes' on it, or ND
> absorptive 'spokes'.
>
> As a 'crazy' idea maybe polarization modulation via a coil and Faraday
> rotation.
>
> George H.

Would not a rotating linear polarizer do a better job? Doesn't this
result in a single frequency for detection with better signal/noise
ration?

[Samuel M. Goldwasser]

I agree with this. One of the benefits of using a two-frequency laser
is that it is mostly insensitive to ambient light.

So, I'm not sure what going to all this trouble is going to do for you.

Are you using a commercial metrology laser (e.g., HP/Agilent) or something
you put together?

Having said that, the simplest way to electrically modulate the output
from a HeNe laser is to modulate the laser current. But this will only
get you between 80 or 90 percennt and 100 percent output. There may also
be some shift in optical frequency, which would be bad for position
measurements.

A mechanical chopper using a film which allows partial
transmission could get you anything from 0 to 100 percent modulation depth.
You can also use an AOM or AOD (acousto-optic deflector) using the zero-order
beam. (However, they will probably want a linearly polarized beam.)

[Bret Cannon]

"olliH" wrote in message
news:ad964cdf-1e6f-4848...@j14g2000vba.googlegroups.com...

I would expect that your two frequency circularly polarized He-Ne would have
one mode polarized with right-hand polarization and the other with left-hand
polarization, which would result in this pair of modes having orthogonal
linear polarizations after going through a 1/4 waveplate. Is this what you
want? My expectation is based on the least expensive single frequency He-Ne
lasers use a linear polarizer and the fact that adjacent linearly polarized
modes have orthogonal polarization, which minimizes the gain competition for
the two modes.

You say above that you have too much ambient light on your detector and are
using a 25 nm width filter. Why not use a narrower bandpass filter? For
example, http://www.thorlabs.us/thorProduct.cfm?partNumber=FL632.8-1, which
has a 1 nm full width at half maximum. For about $210 for the 1" diameter
version and $89 for the 1/2" diameter version, this seems much cheaper than
anything else you've discussed.

Bret Cannon

[George Herold]

> > > > hobbs at electrooptical dot nethttp://electrooptical.net-Hidequoted text -

>
> > > - Show quoted text -
>
> > If you are stuck with the HeNe laser.  (I like the diode laser and
> > modulated current idea.)  Then how about a chopper wheel made from
> > glass, you could have some partially reflective 'spokes' on it, or ND
> > absorptive 'spokes'.
>
> > As a 'crazy' idea maybe polarization modulation via a coil and Faraday
> > rotation.
>
> > George H.
>
> Would not a rotating linear polarizer do a better job?  Doesn't this
> result in a single frequency for detection with better signal/noise

> ration?- Hide quoted text -

>
> - Show quoted text -

Yes that might be better... did he say the beam was circularly
polarized?
The OP didn't want full modulation. Which is harder with a rotating
linear polarizer.

I haven't done any rotating chopper stuff in a long time.

George H.