From: Dr. T. D. Prospero (terryprospero@hotmail.com)
Date: Sat Nov 14 1998 - 01:35:15 EST
Dr. Terry Prospero terryprospero@hotmail.com ---------- > > 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 > > Dear Stephen I am not familiar with the specific model laser you have. However, I have used several other types of water-cooled (and air-cooled) lasers from Spectra and other suppliers. You are correct in asserting that in power mode (I assume you mean light power output rather than electrical power consumption ) the output (in mW of light) should be constant. The current, however, may drift for a number of reasons. This is more common in air-cooled lasers than in water cooled lasers. As the heat affects, e.g., the invar rods used to hold the mirrors in alignment, very small angular changes may occur in the mirror alignment. This will affect the light power output. In light power mode, the current will adjust to keep the light output constant. If you correctly aligned the mirrors to start with, this current drift must always be upwards. You should always run the laser at the output power you wish to use - especially after changing wavelength - for some time, say 20-30 minutes for a water cooled device - before finally "tweaking" the mirrors, and then aligning the cytometer's optics. If you tweak the mirrors during the run, the signals will most certainly change - the threads on the mirror alignment rods, etc., are too coarse to allow for the degree of precision you need at the stream or flowcell. If you try this while your laser is stable, you will quite literally see what I mean. You will, almost invariably, get a poorer signal until you tweak the nozzle or flowcell to re-centre the cell stream in the laser spot and adjust the detection optics to match. Lasers have extremely high gain factors. A minute change in mirror alignment may have a greater effect on light power output than on beam directionality or stability. As I understand your e-mail, although the current creeps up, your fluorecence and light scatter signals are unaffected. In that case, don't worry unless the current creeps up by more than say 10%. If the current continually creeps up there may be a laser problem. A laser will NOT point in the same direction if you shift the alignment of the mirrors. The mirrors are not plane. One or both is very slightly concave on the tube side. Although the tube bore is narrow, there is a kind of "double cone" of lightpaths that allow lasing without beam clipping, with the' pointy' ends of the cones meeting in the middle of the tube and thecone bases on the mirror surfaces. A small angular shift in alignment may not affect power output, but over several tens of centimetres of lightpath may shift the beam laterally and in focal point by the size of the stream! - check using trigonometry after measuring the total light path between the output mirror and your stream or flowcell. Since the laser mirrors may be relatively thick, there will also be issues of lateral beam displacement due to refraction in the output mirror. The exact effect will depend on your focussing optics, i.e. whether you have a simple cylindrical lens, or a crossed cylindrical pair to produce an elliptical, semi-slit scan laser spot. An truly excellent (and free!) source of information, advice, and laser/laser optics properties and optical equations is the Melles Griot catalogue. Every flow lab should have one. (And no, I don't have shares in the company!). Your laser manual should specify the beam-pointing stability in terms of angular drift per degree centigrade. When I set up a Facstar Plus in Germany 10 years ago, I had constant problems for a while with noisy signals - cell clusters wandering on the cytograms (dotplots). My 6W Coherent and 5W Lexel lasers both acted the same. The closed-circuit water chiller/recirculator had too coarse an on/off cooling cycle (hysteresis). The problem was worst at low power consumption - e.g.100mW light at 488nm - than at high power consumption (=heat output) when, e.g., using 100 mW of UV. I solved the problem by putting in a 50 gallon "buffer" tank in the chiller outflow before the lasers to smooth the temperature jumps as the chiller cut in and out. This was about DM 150, 000 cheaper than changing the cooler; or worse, installing heavy duty DC motor controllers with feedback loops. Beam pointing stability is very temperature dependent because of expansion effects on the mirror alignment rods, etc. Significant temperature drift of the coolant over time may thus also produce a rise in current for a given light output. A digital thermometer with a probe in the cooling water inlet will enable you to monitor if your coolant temperature risesor wobbles with time. Again, equlibrate your system for 20-30 minutes before making the final fine adjustments first to the laser(s, ) then the cytometer optics Plotting the the mean signal from beads, or a sample with a tight DNA cv, against time can give you graphic illustration of such problems.
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