Cooling kit keeps lasers on the right wavelength

20 Mar 2003

Too much heat can cripple or kill a laser system. Oliver Graydon profiles the cooling technologies that can help give optical sources a long life and a stable performance.

From Opto & Laser Europe April 2003

Regardless of whether you're operating a tiny laser-diode or a giant argon-ion laser, picking an appropriate temperature-control system is vital for optimizing the performance of your optical source.

Controlling the temperature of a low-power semiconductor laser can help to extend its lifetime, reduce fluctuations in the power of its output beam and improve its emission wavelength. And for high-power scientific and industrial lasers, an efficient cooling system is essential for removing the large amount of heat that the laser generates, and preventing it from burning up.

So what kind of cooling system does your laser need? The answer is nearly always dictated by the heat-load that the laser generates. This figure, usually quoted in watts, represents the amount of heat a cooling system must remove to keep the laser at ambient temperature. A system with a larger cooling capacity will be needed if you wish to cool the laser below the temperature of its surroundings. The maker of your laser system should be able to supply you with values for the heat-load of your laser.

Two main cooling technologies offer accurate temperature control. A thermoelectric cooler, based on semiconductor technology, is usually ideal for integrating with semiconductor diode lasers. For larger and more powerful lasers such as argon-ion or CO₂ lasers, a recirculating water chiller, which relies on an evaporator, compressor and pump is often more appropriate. Both methods can usually stabilize the temperature of a laser to within 0.1 °C.

The two technologies are very different. Opto & Laser Europe gives a rundown of each below.

1. Thermoelectric coolers (TECs) Also known as Peltier coolers, these semiconductor-based devices act as miniature heat-pumps and rely on a phenomenon called the Peltier effect. When an electrical current flows through a semiconductor p-n junction it induces a temperature difference between the two sides of the junction. As the current flows, heat is pumped through the TEC, with the result that one side warms up while the other cools down. The greater the current (drive currents are typically a few amps at the most), the more heat is pumped.

By placing the cool side of the TEC in contact with the housing of a semiconductor laser diode, the diode's temperature can be stabilized. It is important that the warm side of the TEC is connected to a metal heat-sink so that the pumped heat is dissipated efficiently. In more demanding applications the heat sink is used in conjunction with a fan or water-cooling system to stop it from becoming too hot.

Today, TECs are invariably made from bismuth telluride surrounded by a ceramic, and are available in many sizes and specifications. Devices are typically square and while the smallest are just a few millimetres in length with a cooling rate of around 1 W, the largest can be up to 5 cm long and have a cooling rate of more than 130 W.

This range of performance is more than sufficient to cool most laser diodes, with the exception of very high-power diode arrays, by up to 15 °C below room temperature. If a more powerful cooling solution is required, several TECs can be used, or a multi-stage TEC (a TEC made from a stack of several Peltier devices) can be considered.

TECs are often used in conjunction with electrical thermometers known as thermistors that have a resistance that relates to the temperature of their surroundings. This enables the temperature of the laser to be accurately monitored.

Dedicated power supplies/controllers that interface directly with both the TEC and the thermistor are readily available. These allow on-screen read-out of the temperature and TEC current and often come with a range of safety features, including over-temperature alarms and maximum current settings that can be used to protect your laser from accidental damage.

If you are selecting and using TECs for the first time, the following tips may help:

• It's all too easy to underestimate the cooling capacity you will need. Make sure you check the performance you require with a sales engineer and build in a safety margin.

• Make sure that sufficient physical space is available for attaching not only the Peltier element, but also its associated heat-sink.

• Be careful not to wire up the TEC in reverse (i.e. reverse the direction of current flow) - this will heat the laser diode instead of cooling it, which could destroy it.

An interesting recent development has been the use of TEC technology to create compact stand-alone bench-top water-cooling systems with a cooling capacity of up to 400 W. Here TECs are used instead of conventional evaporator/compressor technology (see below) to cool a recirculating liquid.

2. Recirculating liquid chillers If you need a cooling rate of more than 300-400 W, which is typically the case with large scientific lasers, it's time to invest in a refrigerated liquid cooling system (chiller). Such systems pump coolant (cooled water) through a closed circuit of pipes via the laser head. At the heart of the system is a refrigeration unit that uses a built-in evaporator and compressor in much the same way as a domestic refrigerator to control the temperature of the circulating coolant.

Chillers can have cooling capacities of up to several kilowatts and often a wide variety of pumps (centrifugal, turbine, positive-displacement) that support different pressures and flow rates are available. You will need to match the flow rate, pressure and temperature required by your laser to the performance of the chiller and pump.

It's important to note that the performance of a chiller is influenced by both ambient and set-point temperatures. A warmer room or a lower set-point (output temperature of the coolant) will require the chiller to work harder and reduce its cooling capacity. Make sure you know the operating conditions for the performance that you're quoted by the vendor. Most chillers are designed to work with distilled or tap water and it is worth considering the use of additives to inhibit algae growth, lower the freezing point or prevent corrosion.

Once you've made a shortlist of products that meet your raw performance needs, it's time to look for extra features that may be useful. Visible/audible alarms and safety shut-down circuits safeguard the laser and chiller against problems caused by low flow-rate, leaks or abnormally low or high coolant temperatures. Another sensible precaution is the use of filters to protect the unit from damage from contaminant particles.

Other handy features include computer-control via an RS232 or similar interface, and castors for easy movement. Finally, find out how noisy the unit is. If you're going to work alongside it for years, you'll want a chiller that is as quiet as possible.