00de8c7f
From: sam@repairfaq.org
olliH writes:
> 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
> wrote:
> > olliH wrote:
> >
> > > @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
> > > wrote:
> > > > 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.
> >
> > 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?
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.)
--
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