Cryogenic Cooling of Laser Diodes

All semiconductor lasers become super efficient at low temperatures. The first diode lasers could only be operated at the sub-freezing temperature of liquid nitrogen (minus 197 degrees Celsius). Only by the mid 1970's had they perfected the room temperature laser diode. By dipping a standard low power (1 to 5 mw) double heterostructure CW laser in a glass filled with liquid nitrogen, you can increase its operating efficiency by several hundred percent!

Try this experiment. Connect a laser diode as shown in the figure below, and dip it into a glass Pyrex measuring cup. Connect a meter to the monitor photodiode to register light output. Note the reading from the photodiode on the meter. Now slowly fill the cup with liquid nitrogen. The liquid will bubble violently as it boils. Watch the reading on the meter jump as the diode is cooled. In my experiments, the reading on the meter was some 850 times higher at cryogenic temperatures than it was at regular room temperature.

Although some of this increase is due to the higher efficiency of the photodiode, most of the increase is due to the higher output of the laser diode itself. This extra light can be readily seen not only because of the higher output of the laser, but because the laser operates at a lower wavelength (higher frequency / more toward visible red) when cooled. Instead of operating at the threshold of visible light (about 780 nm), the liquid nitrogen brought the operating wavelength to approximately 700 nm, in the far red region of visible light. Use a lab grade spectroscope to measure the exact wavelength of the emitted light.

Tips for obtaining and handling liquid nitrogen:

You can buy liquid nitrogen at welding, hospital, and medical supply outlets. The price is typically between $2 and $3 per liter.
Use a stainless steel or glass Thermos bottle to hold the liquid nitrogen (some outlets won't fill these canisters due to breakage and waste; check first). Drill a hole in the top of the Thermos to allow the nitrogen vapor to escape. Without the hole, the Thermos will explode.
The liquid nitrogen will last about a day in a Thermos. If you want to keep it longer, use a Dewar's flask or other approved container designed for handling refrigerated liquid gas.
Wear safety goggles and waterproof welder's gloves when handling liquid nitrogen. Never allow the liquid to touch your skin or you could get serious frostbite burns.
If the liquid touches clothing, immediately grasp the material at a dry spot and pull it away from your body.
While experimenting, fill liquid nitrogen only in Pyrex glass or metal containers. Avoid plastic and regular glass containers as they can shatter on contact with the extremely cold temperatures.

From: The Laser Cookbook, 88 Practical projects, Gordon McComb, Tab Books Inc., Blue Ridge Summit, Pa. 1988.

NOTE: Although liquid nitrogen is the ultimate laser cooling system, similar (very worthwhile but less spectacular) results can also be obtained by cooling the typical laser diode with a Peltier Junction. Unfortunately you may destroy a diode or two determining what the maximum power that is safely obtainable with this method is (unless the manufacturer will provide you with this data! :-^) so only try the above if you can afford to destroy the diode! Once the destruction point is noted you should be fairly safe operating another similar diode BELOW this power level.

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