J
[email protected]
- Jan 1, 1970
- 0
Since the other thread ("replacement for 741') is dragging on, I wanted
to post a description of a fringe-locker circuit. But first, let me
say that this forum is incredible. I'm not used to such an active
forum with so many knowledgeable participants. Thank you!
Years ago, I wanted to try fringe-locking. Commercial fringe-lockers
are in the 2k range, so I wanted to build my own. Even though I did a
lot of reading, there wasn't a single schematic (or even a patent) for
a fringe-locker. With the help of a friend and some guesswork, I
managed to build a working circuit. I was really surprised and happy,
the cost in parts was probably under $100. This circuit is shown on
the holography forum, and as far as I know, it's the only circuit
diagram available for a fringe-locker, on-line or off.
http://www.holographyforum.org/HoloWiki/index.php/Equipment#Fringe_Lockers
When a certain kind of hologram is made (called a split-beam hologram)
laser light in split into two beams. These beams are eventually
re-combined at the hologram to record an interference pattern in the
holographic emulsion. The path lengths of each beam need to remain
exactly the same for the length of the exposure, even a variation of
one micron will degrade the hologram. (Holographers do everything they
can to use rigid mounts, vibration isolation tables, draft exclusion,
temperature control, etc.) to minimize this, and these are known as
"passive" methods to keep the path lengths exactly the same.
But sometimes for long exposures, "active" means are needed, and this
is where a fringe-locker comes in. Exactly how the fringe-locker is
used is difficult to explain, but a "secondary" interferometer (with
fringes) is included in the setup which "mimics" the actual fringes
which form in the holographic emulsion. If this secondary
interferometer is locked in place, then the hologram fringes will also
be locked, to a certain extent at least.
The photodiodes shown above are placed in the fringes of the secondary
interferometer. These fringes are large, alternating bright and dark,
with a width of maybe 1/4 inch for each fringe. The photodiodes
straddle a bright fringe, or a number of fringes. The differential
amplifier circuit drives a transducer (small speaker with a mirror on
it) to move back and forth over small distances. This mirror is placed
in one the beam-paths. After turning the locker on, the potentiometers
are "tuned" and the system will lock.
The circuit sends a singal to the transducer which increases (or
decreases) the path of one beam, so that it matches the other exactly.
It actually works. But sometimes my locker will "lose" its lock, and
this may be because the speaker cannot move far enough to make the
necessary corrections. I think commercial fringe-lockers use a piezo
stack for the transducer, but they typically require 100 to 150 volts,
and I don't know how to modify the above circuit for this. I have only
had a brief opportunity to test my homemade locker, but want to build a
couple more. I thought the 741 might be too ancient, that's why I
asked about it.
to post a description of a fringe-locker circuit. But first, let me
say that this forum is incredible. I'm not used to such an active
forum with so many knowledgeable participants. Thank you!
Years ago, I wanted to try fringe-locking. Commercial fringe-lockers
are in the 2k range, so I wanted to build my own. Even though I did a
lot of reading, there wasn't a single schematic (or even a patent) for
a fringe-locker. With the help of a friend and some guesswork, I
managed to build a working circuit. I was really surprised and happy,
the cost in parts was probably under $100. This circuit is shown on
the holography forum, and as far as I know, it's the only circuit
diagram available for a fringe-locker, on-line or off.
http://www.holographyforum.org/HoloWiki/index.php/Equipment#Fringe_Lockers
When a certain kind of hologram is made (called a split-beam hologram)
laser light in split into two beams. These beams are eventually
re-combined at the hologram to record an interference pattern in the
holographic emulsion. The path lengths of each beam need to remain
exactly the same for the length of the exposure, even a variation of
one micron will degrade the hologram. (Holographers do everything they
can to use rigid mounts, vibration isolation tables, draft exclusion,
temperature control, etc.) to minimize this, and these are known as
"passive" methods to keep the path lengths exactly the same.
But sometimes for long exposures, "active" means are needed, and this
is where a fringe-locker comes in. Exactly how the fringe-locker is
used is difficult to explain, but a "secondary" interferometer (with
fringes) is included in the setup which "mimics" the actual fringes
which form in the holographic emulsion. If this secondary
interferometer is locked in place, then the hologram fringes will also
be locked, to a certain extent at least.
The photodiodes shown above are placed in the fringes of the secondary
interferometer. These fringes are large, alternating bright and dark,
with a width of maybe 1/4 inch for each fringe. The photodiodes
straddle a bright fringe, or a number of fringes. The differential
amplifier circuit drives a transducer (small speaker with a mirror on
it) to move back and forth over small distances. This mirror is placed
in one the beam-paths. After turning the locker on, the potentiometers
are "tuned" and the system will lock.
The circuit sends a singal to the transducer which increases (or
decreases) the path of one beam, so that it matches the other exactly.
It actually works. But sometimes my locker will "lose" its lock, and
this may be because the speaker cannot move far enough to make the
necessary corrections. I think commercial fringe-lockers use a piezo
stack for the transducer, but they typically require 100 to 150 volts,
and I don't know how to modify the above circuit for this. I have only
had a brief opportunity to test my homemade locker, but want to build a
couple more. I thought the 741 might be too ancient, that's why I
asked about it.