BLM-50 DPSS Blue Laser

JohnFP · Post by **JohnFP** » Wed Sep 05, 2007 2:27 pm

I have had the great fortune of coming across a DPSS laser to play with. It has made holograms before but it seems it does mode hope. I set up a rail interferometer and separated the two legs by 5” (10” coherence length). I get really nice contrasty fringes but I do see a blur of the fringes followed by a definite fringe jump sometimes. I assume this is the laser getting two lasing lines to compete and if the one not previously dominate wins the fringes jump. Sometimes I get the blurriness without the actual hop. Bad still.

So I need to query the talent on the forum for some information.

First, the laser is 473 rated at 50mW but I measure it putting out 74mW. It is a nice Tem00 mode beam profile. It has nice coherence as I mentioned above. The specs on the module claim it is TEC cooled (specs at bottom). Now what does that mean?

Does it have the thermoelectric module placed at the Diode?

Does it have a non sophisticated TEC controller in the power supply that just doesn’t have a fine enough resolution?

I read that the heat has to be taken away from the hot end of the thermoelectric module. So, if I simply place a nice heat sink on the hot side of the Thermo module, will that help in stabilizing the laser? Of course I have to find out how the where that hot side is in relation to the head. The 315 has it on the bottom. So that is probably where it is.

Do, I need to get a better TEC controller for the unit?

Is it simply that I have to dial down the power somehow to get it more stable and if so, is there a pot in the power supply that can do it? It is rated at 50mW and I am getting ~75mW out.

Sorry, lots of question and I am not in a big hurry as it seems after about 40 minutes, the laser stabilizes enough that I can time my exposures in between hops. It seems there is a pretty nice block of time in between the hops that the laser acts very stable. I will simply have to steal a little light with a piece of glass after the laser, and send it down the interferometer.

Then to get fancy, I could set up the fringe locker such that the shutter only opens when the fringes are in the exact location. That is, if they move or fuzz out the shutter closes then reopens when the fringes return to their exact location. I would have to separate the two diode detectors quite a bit in the modified locker to accommodate a hop to another near by set of fringes. But I believe as a last resort it may work

So, does anyone have any advice for this laser, The BLM-50?

Thanks in advance,

John

Specifications: BLM-10 BLM-30 BLM-50 BLM-75 BLM-100

Output Power 10mW 30mW 50mW 75mW 100mW

Wavelength: 473nm

Beam Diameter (1/e2): ~2mm @ output mirror

Beam Divergence: <1.5mrad

Beam Quality (M2): <1.2

Cooling Means: Thermo Electric Cooling (TEC)

Dimensions (Head): 116 x 73 x 46mm

Dimensions (PSU): 130 x 120 x 65mm

Expected Lifetime: >10,000hrs

Frequency: 1-10kHz (Optional up to 50kHz)

Linewidth: <0.1nm

Operating Current: <1A

Operating Temperature: 10-35oC

Operating Voltage: 90-240VAC, 50~60Hz

Output Mode: CW, Analog or Modulation (TTL)

Pointing Drift: <0.05mrad per hour

Stability: +/-2% @ 4hrs

Transverse mode: TEMoo

Warm-up time: <10mins

Weight (Laser Head): 0.645Kg

Weight (Power Supply): 1Kg

Warranty: 1 Year

JohnFP · Post by **JohnFP** » Wed Sep 05, 2007 2:29 pm

Whoops, here is the laser.

http://www.extremelasers.com/BLM.htm

Joe Farina · Post by **Joe Farina** » Wed Sep 05, 2007 3:32 pm

John, even though interferometers are useful, I am always skeptical until actual holograms are made.

The intermittent exposure control idea is interesting, though. I think I mentioned before that this idea has been published, and I finally found this among my papers a while back. I'm a bit pressed for time at the moment, but as soon as I get the chance, I will email it to you.

JohnFP · Post by **JohnFP** » Wed Sep 05, 2007 4:55 pm

Wow Joe. Do you know how amazing it is to have a thought and then see it completely done on paper...lol. That's exactly what I have in mind. What gave me the idea was the years of working with a fringe locker to lock the expanded fringes in place.

I will implement this idea immediately manually. With that I mean, I will be using a SBR with the plate lying right on the object. Then steal some light and set up the interferometer. Open the shutter only at the most stable of times and make sure no hop happens in between shots.

Again, even with a nice electronic apparatus, an actual hop would not be compensatable unless the # of fringes was somehow counted between a certain distance and if that number of fringes changed (the laser actually hopped) the shutter would not be opened back up until the original line was relasing and the number of fringes went back to the original number (sheesh, hope that made sense).

I will keep all apprised of my findings and hope other will comment if they have any experience with these lasers also. What really excites me is the mention that these lasers are TEC controlled.

Thank you for your time,

John

Colin Kaminski · Post by **Colin Kaminski** » Wed Sep 05, 2007 5:46 pm

A hop can not be compensated for because the distance between the fringes changes not the placement. The only reasonable fix for mode hopping is temperature control.

Joe Farina · Post by **Joe Farina** » Wed Sep 05, 2007 6:28 pm

I don't see any reason why an intermittent exposure control (either manual or automatic) can't be tried for a laser that is mode-hopping. In other words, blocking the beam when the fringes start wavering or washing out. Hasn't this already been done successfully by a forum member? I seem to recall that it has, but I can't remember if it was described in a personal message, or posted on the forum.

Colin Kaminski · Post by **Colin Kaminski** » Wed Sep 05, 2007 7:01 pm

The big problem would be when the laser settles down to a new frequency after the mode hops. I have made exposures that use a microscope slide as an etelon in an unused leg to detect mode hops and just placed the card in the beam when the laser gets noisy. This only works on really long exposures and it is not 100%. This idea was given to me be an old forum member, Jonathan Head.

Joe Farina · Post by **Joe Farina** » Wed Sep 05, 2007 7:26 pm

Colin Kaminski wrote:The big problem would be when the laser settles down to a new frequency after the mode hops.

I agree that would be a problem, but there's a chance (?) a blue DPSS laser (with a different cavity structure, of course, in comparison to a diode laser) might be different in this respect.

BobH · Post by **BobH** » Wed Sep 05, 2007 8:23 pm

I set up a rail interferometer and separated the two legs by 5” (10” coherence length). I get really nice contrasty fringes

This doesn't necessarily mean you have long coherence length. You might have very short coherence length repeating so often that you have a hard time getting right in a minimum. Joe's right, make a hologram with it to test coherence length!

Colin Kaminski · Post by **Colin Kaminski** » Thu Sep 06, 2007 2:52 am

BobH wrote:
I set up a rail interferometer and separated the two legs by 5” (10” coherence length). I get really nice contrasty fringes
This doesn't necessarily mean you have long coherence length. You might have very short coherence length repeating so often that you have a hard time getting right in a minimum. Joe's right, make a hologram with it to test coherence length!

This is what I found with a greeen laser pointer I played with.