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Osram PL530 - Tiny 530nm OPSL

paul1598419

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Chris, by expander lens do you mean a collimator lens that is further away giving the divergence of a BE? I see you got more of these OPSL laser modules.
 



Alaskan

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Yep, those six have a tiny expander lens assembly glued to the output. Since the divergence of these modules without an expander lens is 4 to 8 mRad, the distance needed between a collimation lens to allow the beam to expand enough to lower the divergence to an acceptable or low enough amount is too far, I’d rather suffer the loss of adding one more lens than have an uber long pointer. Even if only 5 mm beam diameter, without a expander lens, its just too far way for my likes for the placement of the collimation lens.
 

zmaster

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Yes, the short focus concave lens (~-2mm) allready glued to the units from holokidd makes it very easy to achieve an excellent collimated beam, allmost diffraction limited! I added a +15mm lens for a ~3mm beam used in a single mode projector. I got 90mW - 110mW of power for different units at spec. current max (480mA, 23°C, Heater = current optimized at ~60mA). With the built in expander lens only it gives a very wide divergence that may be very usefull for holography: ~0,5m beam diameter at 2m distance.
 

diber

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Super nice build video and info in this thread ! I bought 2 of these from holokid some time ago to build into my raman spectrometer setup. They work super for that purpose ! The beams are fully polarized and can hence be combined with a PBS by mounting one laser horizontal and the other vertical in order to get double beam power. Both lasers had exactly the same wavelength, within the resolution of my spectrometer (top of peaks within 0.1 nm from eachother). Maybe interesting as well: no 808 nm IR beam coming out of the aperture either. No need for additional IR filter. The optimal current/voltage for the heater was quite different for both lasers. Individual tuning of the heater current was necessary to get highest output power.

Edit: if anyone has any idea how to tune the output WL by a few 1/10ths of a nm (heating does not seem to have any effect), I would be very interested (I would like to try some raman spectroscopy techniques that use slightly different excitation WL's to tease out better signal).
 
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paul1598419

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Thanks. I was dubious that the heater could be controlled by setting the current for a standard amount instead of measuring the temperature and setting it that way.
 

diber

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That is not exactly what I meant with the tuning of the heater. I did not actively control the heater voltage/current, and certainly not based on the temperature. I set it at the beginning (after a short powering up time) to achieve optimum beam intensity and then left it untouched. Under the environmental conditions on my desk in a heated room, with the laser mounted on a heat sink (thus relatively stable thermal environment), the laser output remained quite stable, even after power off/power on again (always same LD current and after a short warm-up time).

What I meant was that the optimum heater voltage was quite different from one laser to the other... hence, do not pre-assume that the heater setting is the same for each laser. The datasheet contains the following graph:

63506

When changing the voltage of the heater (or sometimes when changing the LD current, both changing the heater temperature and as a result its resistance), you clearly see the laser sweeping through the different peaks (more than 2 by the way). Once set and in a relatively stable environment, the laser remained operating within one peak. Whether it actually remained at the absolute top of the peak, I do not think so (If this would remain valid when taking the laser outside into the cold, I do not know either). I think controlling the heater based on measured output power (or maybe as you suggested based on a lookup table of measured optimum power as a function of environment temperature ?) could possibly be needed to squeeze out the maximum beam power.
 
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diber

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I just realised that I have probably come to a wrong conclusion in my post of 2 days ago about the need for an IR filter: I do not see any peak in the spectrum of the laser at 808 nm... however, higher up in the thread, there are 2 different WL mentioned for the pumping diode : 808 nm and 1060 nm (the latter being probably the right one, correct ?). My spectrometer only goes up to 900nm, hence there might still be a lot of 1060nm light leaking out of the laser aperture, I just can't see it.
 
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paul1598419

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These units use a thermistor as the heating element and as a temperature measuring device. That is what I meant by measuring the temperature.
 

Zenodilodon

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Heater voltage is 1.5V-2.0V. Try 3.0V at your own risk.

Not all of them have the output expander lens, mine didn't but another user on Discord did have one on his.
I did use the original expanding lens, it looks like I have a long focal range but collimating is actually about 8 mm from the expanding lens. The reason why it looks like I have a long range is because the pointer chassis is extended to make room for the door cover. Otherwise I could of cut another 30mm or so off the total build length.
 

Zenodilodon

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Another thing to note about heater questions. I believe the material is PTC so it should be best voltage regulated with feedback. PCT will increase resistance when heated, thus if you try to run them in current mode you can't really lock in a good temp. There are 2 ways to best do this. First method is to use PWM to run the heater and during off times get the resistance of the PTC ( Resistance = Thermal value ) like a thermsistor. There is the other option of running the heater in a linear regulated fashion either using a photosensor to monitor output or by carefully mounting a thermsistor on the crystal for feedback.

Other things to note. under the microscope is does not look like a VSECL was used, looks to be something right after I could not really identify along with what appears to be a photosensor between the pump laser and the twin aspheric lens assembly that is right on the silicon base. Looks also that the OPS chip might have a PTC heater as well, I am not quite able to see all the traces but it would make logical sense for the OPS to have a matched heater as well.

Enjoy these limbs of speculation.
 

diber

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Did some further testing with the 2 units I have.
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I got 90mW - 110mW of power for different units at spec. current max (480mA, 23°C, Heater = current optimized at ~60mA).
Same power for mine... LD running at 400 mW and heater current optimized, I got close to 100 mW (see picture... highest point I have seen was 108 mW). However, you really have to look for the best maximum since there are several other local maxima which are considerably lower.
63617
Concerning stability over time, I left the lasers running for a while, LD current controlled at 400 mW and heater controlled at a constant voltage which was giving highest output on one of the previous runs... results: see attached plots (x-axis range is 15 minutes, y-axis range 150 mW, first plot includes power up). I did not touch the setup during the measurements... just doing other stuff in the room (thus slight thermal variations possible). At one point, the output decreased considerably over a short amount of time (sampling every 6 seconds and decrease happened over max a few samples),... it partially regained output a bit later. I guess, but might be wrong, that some laser modes are kicking in/out? Not shown on the graph: re-tuning the heater voltage establishes again peak power.

6361863619
I was dubious that the heater could be controlled by setting the current for a standard amount instead of measuring the temperature and setting it that way.
Fully confirmed by the above measurements: active heater control seems necessary if you really want max (or lower but stable) output power. On the other hand, the modules stay "bright" for quite some time without active heater control, even with power downs/up and over multiple days.
 

lazeristasUVISIR

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Late to the party :)

If really a stable operation is needed, you shall have an active temperature controller for the whole unit and a stable voltage supply for the heater. The laser driver shall be stable too (I think that is obvious). Even that is not enough for a long term. An external photodiode feedback reduces a noise considerably.

If the laser power level is changed, the heater voltage should be adjusted too.

The laser has a great beam (think of M2 almost as 1). A proper expander or a collimating lens would make a nice collimating beam.

As I recall the laser leaks some infrared maybe 808 nm or 1064 nm. For some applications a filter would be needed.
 




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