Laser Operation: Temperature Control and Mount SelectionThe best choices will help you manage the heat generated by laser operation and ensure a robust laser system.
Paul Corr and Patrick Klima, Arroyo Instruments LLCAs you operate your laser, only a portion of the electrical power sourced from your laser driver converts to light energy. The remainder turns to heat, and the buildup of that thermal energy presents a number of issues for the laser system as a whole, and for your laser in particular.
Within your laser setup, the temperature control system is responsible for managing the heat generated by the operation of the laser. In addition to your temperature controller, careful selection of the application-appropriate laser mount will be critical to a robust laser system.
Ultimately, the most pressing issue regarding temperature control is that a change in temperature affects laser light quality, particularly wavelength. If left unchecked, excessive heat may also damage the laser’s light-emitting facets, degrading the quality and quantity of the light produced.
The two basic strategies for dissipating the heat produced by a laser are known as passive cooling and active cooling. In addition, this article will cover unconventional thermal management approaches for high-power applications and for applications that require a heated laser mount.
Figure 1. Anatomy and heat flow of a passively cooled laser mount.
A passive heat sink conducts heat away from the laser and dissipates it into the ambient air (Figure 1). Because this type of laser mount is simply a large thermal heat sink, it is unavoidable that the mount temperature, and that of the laser, will increase. A passively cooled laser mount is designed so that this temperature increase occurs in an incremental and predictable manner.
The thermal performance of such a mount is rated as thermal resistance, in units of °C/W. This rating represents the amount of temperature increase, in degrees Celsius, the laser mount will exhibit at thermal equilibrium for every watt of waste heat generated by the laser.
A fan will improve the thermal performance of a passively cooled laser mount. Typically, the manufacturer will provide ratings for the laser mount with and without an auxiliary fan. Even with a fan, the performance and power range of a passive heat sink are limited to low- and medium-power applications, or applications where higher operating temperatures are acceptable.
Active cooling is a more comprehensive, and also a more complex, approach to thermal management. A device called a Peltier cooler, built into the laser mount or directly into the laser package, operates to cool the laser (Figure 2).
Figure 2. Temperature control loop response in an actively cooled laser mount.
A Peltier device, also known as a thermoelectric cooler (TEC), is a small, flat, thermally conductive ceramic that uses the electrical power supplied by the temperature controller to cool one of its surfaces while heating the opposing surface. The laser mount is responsible for acting as a heat sink to one side of the Peltier device. The other side of the Peltier device is applied to an aluminum or copper cold plate, which makes contact with the laser package case.
To complete the control loop, a temperature sensor provides a feedback signal to the temperature controller, which regulates the power provided to the Peltier device. In many cases, the laser mount will also be equipped with a fan to maximize thermal performance.
The thermal performance of an actively cooled laser mount is referred to as thermal capacity and is rated in watts. This rating represents the amount of thermal power the laser mount can absorb while maintaining a stable temperature. This rating typically applies when the cold plate temperature of the mount matches the ambient temperature. For mount temperatures away from ambient, the manufacturer can typically provide a thermal performance curve as a function of plate temperature.
Notably, a laser mount equipped with a Peltier device will be capable of heating as well as cooling. This allows faster stabilization and response times. In addition, if you are characterizing the performance of an LED or laser device, this capability also allows you to stabilize the system above and below ambient temperature. Since output wavelength is correlated to laser temperature, this also provides a convenient way to precisely control the optical performance of the laser.
Functional considerations for mount selection
Aside from the basic question of sufficient thermal capacity, there are three areas of function that affect the utility of a laser mount. These are the thermal conductivity, the flexibility of the wiring harness, and the mechanical mounting of the laser.
The thermal conductivity of the laser mount, and of the cold plate in particular, is an important design aspect. While aluminum is sufficient for some applications, the preferable material for a cold plate is copper. The thermal properties of copper are superior to those of other materials and will provide a more uniform temperature across the cold plate.
For optimal versatility, consider the flexibility of the wiring harness built into the mount and, by extension, into the laser driver and the temperature controller. Ideally, the manufacturer should provide standard prefabricated cables from the instrumentation to the laser mount. From the laser mount to the laser, connections should be easy to establish and easy to change, using a screw terminal or some other simple means. Soldered connections, or connectors that require significant setup time, are less preferable.
The same principle applies to the mechanical connection between the laser and the mount. That this connection should provide a good thermal interface goes without saying. In addition, it should provide easy disconnections and a measure of versatility for a variety of laser packages. Some manufacturers will offer customizable cold plates, allowing you to specify the mounting hole pattern you require.
Beyond laser mounts with an integrated fan and a Peltier cooler, managing higher thermal output levels becomes more challenging. If an air-cooled mount proves insufficient, then the next option is a water-cooled mount (Figure 3). Water offers a substantial increase in thermal capacity at the cost of complexity and responsiveness.
Figure 3. Layout for a water-cooled, high-power laser mount.
While a water-cooled plate is effective at transferring large amounts of heat, several drawbacks exist. First, your temperature set point must fall between the boiling and freezing points of water. Second, a water system requires a chiller, a pump, a customized laser mount and plumbing, which will add setup time and cost. Third, some water systems may have an error range of several tenths of a degree and do not react quickly to changes in temperature. This may be unsuitable for high-precision applications.
To improve the precision of a water system, a hybrid system combining a TEC with a water-cooled laser mount works well. This system relies on the TEC for fine temperature control, and uses the water cooling system to move heat away quickly. This approach is common in high-power laser applications that require good temperature stability.
As mentioned earlier in the article, if you are characterizing the performance of a device across a temperature range, or working with applications that require higher temperatures, such as LEDs, the heating capability of a Peltier device may prove useful. Higher mount temperatures require a different type of temperature sensor and a TEC suitable for high-temperature operation, so discuss your application with the laser mount manufacturer. Some laser mounts also incorporate resistive heaters, although this arrangement is obviously only suitable for heat-only applications. In this circumstance, the rest of the thermal management system can remain the same, provided that the temperature controller has the capability to power resistive heaters.
Selecting the right mount for your laser system will save time and effort while improving overall performance. Beyond the decision to use passive or active cooling, pay special attention to the additional features of the laser mount. Easy mounting, flexible electrical connections and good materials selection are important factors to consider. In the end, the best course may be to call the manufacturer directly and ask questions regarding its mount performance for your particular application.