From: Reece, Lisa (lreece@utmb.edu)
Date: Mon Nov 16 1998 - 14:38:40 EST
Derek Schulze wrote:
Hello everyone!
We are having a problem with a our Spectra Physics laser (6W argon
Stabilite 2016 running at 100mW)). Essentially the current creeps up on it
over time when it is in power mode. This is easily corrected by adjusting
the mirrors until the current level drops sufficiently. The trouble is
that my alignment seems to change or laser output is not the same after I
adjust the mirrors ?? My light scatter changes and my FL means shift??
This shouldn't happen if the laser is in power mode! The laser is securely
bolted down and alignment does not shift when other parts of the laser are
gently pushed.
If I ignore the current drift my light scatter and FL means stay consistent.
In my ignorance I assume that a laser will continue to lase in the same
path that the tube is aiming regardless of how the mirrors are adjusted.
The mirrors should just affect the efficiency of light production. (??) Is
tube age an issue here? My fear is that the power mode circuit is not
operating properly (it indicates 100mW consistently).
Where have I gone wrong? Any thoughts? Any guru ideas?
I have also contacted Spectra Physics but they haven't called back in quite
some time :-(
Any thoughts would be much appreciated.
Derek Schulze
Queens University, Cancer Research Laboratories
Flow Cytometry and Confocal Microscopy Services
Kingston, Ontario, Canada
flow@post.queensu.ca
============================================================================
================
Hello Derek:
I, too run a Spectra Physics laser (Series 2000 Ion Laser System). Since I
don't know your background and experience, allow me to give some background
information. If you are already familiar with this, please don't take
offense. Most of this you can find in your Spectra Physics Instruction
Manual.
First let's remember what the term "laser" means. Laser is an acronym for
"light amplification by stimulated emission of radiation". A laser is
basically a resonant optical cavity that is defined by two mirrors at the
ends of the cavity (one mirror is the output coupler and the other is the
flat high reflector). As for the other basic parts of the laser, there is
typically a prism just prior to the rear mirror (i.e. the flat high
reflector) and you adjust the angle of the prism so that the path for one
wavelength between the mirrors will be selected and supported. All the
other wavelengths being made by the laser will be suppressed. The tilt of
the prism determines which line will amplify (or be selected) and the
dispersion of the prism allows for only one line to be perfectly aligned
with the high reflector. This is why you get a single line out of a laser
when there are multiple lines in it. When you turn the laser on, a voltage
pulse is sent down through the plasma to begin the lasing. A large
electromagnet is used and should be adjusted so to force free electrons of
the plasma away from the walls of the plasma tube and towards its center.
This results in laser light produced in the plasma tube and bouncing between
the two mirrors. To be clearer, the mirrors are used to amplify the wave
pattern of the resonant frequency of the emitted radiation. Both mirrors
are coated to reflect wavelengths of interest while transmitting all others.
The output coupler transmits a fraction of the energy that is stored within
the optical cavity while the escaping radiation becomes the output laser
beam. Photons of light that are emitted parallel to the cavity axis are
reflected by the flat high reflector mirror only to return to interact with
other excited ions. This "stimulated emission" increases geometrically
until an equilibrium is acheived between excitation and emission.
My boss gives a good analogy: As kids we probably played with soda bottles
that had different volumes of liquid in them. When you blew across the top
of these bottles, different notes would be played. Making music like this
is making a "standing wave pattern" going from the top of the bottle down to
the fluid level. You do the same thing optically with a laser. You adjust
the mirrors so that they reflect an integral multiple of the wavelength
range you want. As long as the cavity length between the two mirrors is an
integral number of the wavelength range of a given laser line, it will be
reinforcing itself (i.e. the ions will be geometrically increased) and a
standing wave pattern will be acheived.
TEMs (transverse electromagnetic modes) are variations of the
electromagnetic field perpendicular to the direction of travel of the wave.
These variations help to determine the power distribution across the laser
beam. Another important component of the laser is the aperture. This is
used to "squeeze" the laser down so that you eliminate the sideways bouncing
of the output beam (that emits in the directions perpendicular to the
direction of the wave). By adjusting the aperture, we can get what is called
the TEM00 mode, so that the light is distributed in a Gaussian beam profile
pattern equally in all of your cells. The aperture actually is controlling
the current path. By using special lenses, you can determine if you have the
TEM00 mode which is a round spot of light that is brighter in the center
than at its edges.
You must also keep in mind that you have a gaseous plasma that is heated.
The mirrors are mounted onto supports. After the lasing action has begun,
both the supports (graphite rods)and the coatings on the surfaces of the
mirrors will tend to heat up and be unstable for a few minutes. The heat in
the plasma tube can cause a change in the length of the optical cavity by
these thermal differences associated with the heating up of the rods and
mirrors, as well as mirror alignment. Any change in the length of the cavity
will result in changes of the resonant frequency. This is one of the reasons
why I always wait at least 15 minutes before I do an alignment so that I can
be sure that the mirrors' supports and their coatings are thermally stable.
So to answer your question about mirrors and light efficiency and to include
the question about the path and the plasma tube, just remember that the
mirrors must be in proper position to obtain equilibrium of the stimulated
emission and your electomagnet and aperture must be optimized so that your
laser is properly condensed toward the center and is in the TEM00 mode.
Yes, it does matter that your mirrors have to be adjusted properly because
due to the lowering of the power you are losing the standing wave pattern by
decreasing the stimulated emission of radiation. In addition, if mirrors are
moved slightly, this will cause a misalignment of the beam with the rest of
the optics. You can prove this by simply placing a piece of cardboard in
front of the laser beam and while holding it in place, move the mirrors.
When I begin an alignment, I use the course adjustments (vertical and
horizontal) to maximize the ouput which I read on the laser power meter
while in the current mode (all changes should be done in the current mode
because thermal equilibrium is stable). Dropping the current level means
that you are losing power to the laser beam and the mirrors will not be in
such a position that a good standing wave pattern can be acheived. I use
the fine adjustments in both directions to assure that I have maximal
output. I then adjust the power so that the current and power modes are
both at 500 mW (which is what I typically run at), so that the laser is not
fighting itself through the feedback circuits.
You must also be aware that if you make any changes to the mirrors, you are
affecting the emission equilibrium which in turn affects the resonant
frequency which in turn affects your laser power output. This is not
surprising, then, that your FL and scatter will be out of whack when
compared to earlier runs. Our laser is a rather old one and we have also
seen this current drift. I simply readjust the mirrors and perform another
quick alignment to ensure good results. I would believe that this would be
a function of the feedback electronics or power supply and not just the
optics.
Hope my 2 cents worth helps,
Lisa Reece
Research Associate
University of Texas Medical Branch
Molecular Cytometry Unit
Div. of Infectious Diseases, Dept. of Internal Medicine
301 University Blvd., Rte. 0835
Galveston, TX 77550
(409) 747-1932
FAX: (409) 772-6527
E-mail: lreece@utmb.edu
-----Original Message-----
This archive was generated by hypermail 2.1.6 : Thu Jan 01 2004 - 17:35:26 EST
