I am successfully running the head with my home-built switchmode power supply without problems.
It runs pretty stable with less than 1-2% noise in the light output and has
been so far tested up to 30A incl 3A magnet current. The magnet is simply
put parallel to the tube at 160V (ie, between the center-tap of the filament
transformer and the anode). For this it seems important that the idle voltage
(before laser ignition) is set not too high in order not to stress the magnet
coil (I use ca 220VDC - just high enough that the laser starts reliably).
8V needed to trigger the starter relay.
It is very important to make sure that the lexel head never runs with
less than 3-4l/min cooling water. Therefore I have built a simple water-flow
controlled interlock that switches the SMPS off immediately when the cooling
water flow drops below an adjustable value. The circuit diagram is here. The input comes from
a water flow sensor, and the output connects to the shutdown switch in the
SMPS circuit via an extra little
relay (it shorts the capacitor when the relay is without power). It is mounted
in a box right next to the starter unit on the lexel head, making use of the
nearby two holes in the steel base plate. It also supplies 8V for the lexel
starter relay.
As for water: it is important to run the head with a pressure reducer
close to it, in order to prevent a pressure wave (eg, from a pumped-up garden
hose) entering it. An ordinary household reducer (preferably equipped with
valve and a manometer) from the local home improvement store will do the
job. Mine keeps the pressure below a fixed value of 3 bar, but since the
head is leaking above 1 bar, I have to carefully adjust the water flow with
the valve.
For holography one needs monochromatic light, so the first thing one needs
in addition is either single line mirrors, or a wavelength selecting prism;
I was lucky in that an original lexel prism incl mount came with the laser,
but in general it is quite hard to find them on the surplus market or via
ebay. It may be possible to use a prism from a different manufacturer (typically
Coherent), but that requires setting up a suitable mount that fits in the
HR mirror holder.
Here you see the laser running with open resonator, the prism is on an
extra adjustable mount to the right - by slightly tilting the mount, one
can easily tune to the individual argon lines, from violet to green.
The original OEM Lexel88 optics are not strictly for TEM00 mode,
which is however needed for holography (the different multimode beam components
are not coherent with respect to each other). By careful adjustment one may
achieve TEM00 operation at lower power levels, but when tuning up the power
(ie., the tube gain), the beam profile can jump suddenly, indicating that
new transverse modes have appeared. Because the resonator is not very stable,
it is advisable to check the mode structure from time to time, to be sure
that it didn't go into multimode .... I lost an entire session of holographic
work because of this !
I found some spare OC and HR mirrors (from a Coherent CR-2) with a larger
focusing radius, and with them achieving TEM00 mode is no problem even at
>500mW single line output. Due to the larger radius the adjustment is however
more critical as compared to the original mirrors, and often I even don't
have a beam after a few days of rest. In general replacing the mirrors by
ones made for a different laser gives sub-optimal results unless one is
lucky, so getting original Lexel 88 TEM00 optics is highly preferable, though
finding them cheaply is difficult - as said, the OEM units do not come with
them.
The laser, like most external mirror argon lasers, is vertically polarized.
To minimize/avoid disturbing interference patterns in holographic images,
one needs to illuminate the film/plate under the Brewster angle (ca 56 degrees
with respect to the normal to the film); for this the light must be horizontally
polarized. This can be achieved by inserting a half-wavelength waveplate
into the beam. This allows to rotate continuously the polarization, and get
rid off the unwanted reflections. Note that the waveplate must be appropriate
to the wavelength one uses. I got mine from Sterling Resale Optics. One can also build
waveplates by oneself, eg from mica chips.
The coherence length (=max depth of hologram) I got for my setup
is more than10cm which is not bad for an argon laser and sufficient for a
start. In order to improve this, I have fitted the resonator with an etalon
(a good explanation how this works is here
and here).
It gives single longitudinal mode operation, which increases the coherence
length from a few cm to dozens of meters. This allows to make holograms with
great depth, and to avoid painstakingly matching the lengths of object and
reference beams.
Tuning proceeds by first removing the etalon and adjusting the laser to
TEM00 mode, and then reinserting it. Lasing is strongest when the etalon
is adjusted parallel to the laser mirrors, however then several modes may
be present. By detuning it by a very slight angle, single longitudinal mode
operation obtains, as can be checked by an interferometer with large path
difference.
First thing I tried was to convert a Coherent etalon for using in the Lexel
88 frame. For this I exchanged the HR and OC mirrors and extended the resonator
by an additional mirror mount (from a broken Lexel88), making good use of
the over-long invar rods. On the new mount I fixed the HR (inside the prism
assembly, that is), and in the original mirror holder next to the tube I
placed the etalon. The whole setup looked as follows:
However, I found this setup pretty unstable, it required constant readjustment,
and the power dropped to ca 20% - which means ca 100mW at full throttle, and
this was not satisfying. In fact the transmission of the etalon (as well as
the gap) needs to be optimized for the laser in question, and thus one cannot
expect good results with a non-genuine part.
Fortunately there is ebay and finally I could acquire an original Lexel model
503 etalon; its data are described here
and its workings here.
A minor problem was that the laser head needs to be equipped with he right
holes and mirror mounts in order to be able to put the etalon in - and the
OEM version is not of that kind. Fortunately I could modify the setup described
above, ie by using extra mounts, and it now looks like this:
To the left there is the original mirror mount which is left untouched. In
the middle there is the etalon casing attached to its own adjustable mount.
To the right there is then the prism assembly with the HR. Note that I fitted
the prism adjustments with micrometers, which makes them much easier to operate.
The whole things runs pretty stable despite the increased resonator length,
probably because the coherent mirrors I use have a relatively large curvature
radius.
With the etalon in place, there is some longitudinal mode hopping which
means fringes are a bit jumpy. As long as the object and reference beam lengths
are not too much different, a few fringe jumps do not matter (because the
spacing between two adjacent longitudinal modes is given by the resonator
length, so one mode jump will reduce the effective coherence length from ca
100m to the resonator length, which is well tolerable - this is in contrast
to diode lasers, for which one mode jump effectively reduces the coherence
length to a fraction of a mm, and thus cannot be tolerated).
At any rate, for holograms with very large depth, one needs single mode operation
over the whole exposure time (or at most very few mode hops), and that requires
a temperature stabilization of the etalon. For this reason, etalons
usually come in a heatable enclosure with temperature sensor, which allows
to stabilize the temperature up to ca 1/100 degrees.
For the Coherent etalon I had designed a simple circuit that keeps the temperature
so stable (at ca 40 degrees C) such that there is no mode jump within more
than 1 min (after more than 30min warm-up time). It makes use of built-in
resistors for heating and a thermistor for feedback; the circuit diagram is
here.
The Lexel etalon came fortunately with its own controller card, and it was
a simple matter to hook it up thanks to friendly people on alt.lasers who
sent me the schematics. This circuit, while still simple, is more advanced
since it uses pulse width modulation and not linear regulation, and works
very well - after considerable warmup time, there are very few mode jumps
to observe. See here for some
measurements with my USB data acquisition machinery.
Shown are fringes of an interferometer where the path difference is almost
4 meters (folded back and forward several times) - this means the coherence
length is at least of this order of magnitude.
Note added
I now have checked the longitudinal mode structure with my scanning Fabry-Perot
interferometer, see the results here. In brief,
there is clean single frequency operation at 514nm and 496nm, however things don't
work so well at 488nm.
Finally here some power measurements with an Ophir thermistor head:
Pink means TEM00 all line, and the other colors refer to 514, 488, 476nm,
resp, using the prism.) The plot on the right shows the single mode power when
using the Lexel etalon, the power loss is not at all bad !
Tube voltage versus current:
I had ruined the magnet by inadvertedly powering it up without
cooling - I had to get a surplus one, here are pics from the delicate
operation, where the tube had to be removed and stripped down to its
beryllium oxide core:
