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This tiny inexpensive device is quite stunning, and with reliable 100mW single longitudinal mode operation at 530nm it beats out, at least for the amateur holographer, much more expensive green lasers such as argon ion lasers and Coherent Compass lasers. So naturally it is interesting to investigate these little beasties in some more detail. Thanks go to George S. and Dave B. for providing me a few samples of these!
Also, thanks go to members of the holography forum for useful informations and inspiration. Most relevant info can be obtained there, and I will add here just a few extra notes and observations from my perspective.
First off, the data sheet of the PL530 can be found here, and a good background read about how it works is this.
What is important for the holographer is to identify and maintain an operation spot where single mode emission is stably maintained, without mode jumps over time. The relevant parameters are the diode current, I_LD, and the PPLN heater resistance, R_H. The latter is most crucial in that it needs to be fine tuned and kept very stable.
The case temperature is not very critical but it appears it should be kept low and relatively constant, say at 18C. So three drivers are necessary, one for the laser diode at about 500mA max, one for the PPLN heater at about 80mA, and a TEC driver for the peltier element against which the PL530 case is press-mointed (or glued with thermal adhesive). Running I_LD > ~480mA or I_H>~80mA can instantly destroy the device, so precautions need to be taken.
The data sheet mentions that the limiting factor for the PPLN heater is the resistance R_H, and the allowed maximum is given by R_H=1.27*R_25, where R_25 is the resistance at 25C. So the first thing to do to with a new device is to measure R_25, as it can widely vary between 27Ω and 37Ω. The 8 samples I got so far lie all around 28Ω, so one should keep R_H<36Ω for these.
From this it is clear that what needs to be kept constant during operation is the PPLN heater resistance, with acts like a PTC thermistor. Feeding it eg. with constant current can lead to a destructive runaway, in that if the resistance grows due to heatup, then due to P=I^2R the dissipated power grows too, and makes it heat up even faster. So the right thing to do is to use a bridge circuit that stabilizes the resistance and not the current, as mentioned in the data sheet.
On the other hand, changing the resistance setpoint also changes the current, so monitoring the current is still a good way to label the operating point. For the first device I tested, I found the following relationship:
This is just for orientation, it will change a bit in actual operating conditions when there is thermal input from the laser diode. The take-home message is that the current should be kept below of about 80mA, at least for the samples I got.
Here is a plot of the output power as a function of the heater current, where discrete jumps reflect transitions between different mode structures:
As for a concrete setup, I used for testing one of my existing LD driver/TEC controller boards, which can deliver up to I_LD=500mA and so didn't need any modification. The only new ingredient was a driver for the PPLN heater, which is based on a ratiometric resistor bridge that stabilizes R_H. For this I took more or less the same as the nice circuit proposed here. At first I had trouble making it work, since the current always shot up to the max. It turned out that due to mysterious circumstances the opamp AD822 I used got into an illdefined, latched-up state upon start, and all was working fine by exchanging it by an LT1013. So here is the circuit:
This is set up to allow easy digital control, and the response of the current sis:
Of course, the current T_H is easily monitored via the voltage drop over the resistors R1,2,3.
To be continued
(click for movie).
Precise measurements of spectral width, noise in laser output and power, as depending on diode and heater currents:
Above to the left: relatice line with, dark blue corresponds to single mode. In the middle: Noise which usually indicates instabilities. Dark blue denotes minimal noise. To the right: Output power -- fortunately maximal power coincides more or less with a clean spectrum!
Now spectral width and power, as depending on case temperature and heater current, for fixed diode current IL=460mA:
Thus lower case temperature seems moderately better, but this isn't critical. Fixing it for good around 17C seems fine.
Automated LabVIEW interface:
Clean single mode:
Not quite DPSS:
Current: Vers. 0.2 02-2020
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