When you operate a laser, only a portion of the electrical power provided by the laser driver is converted to light energy. The rest is converted to heat energy, and the buildup of heat energy presents a number of problems for the laser system in general and the laser in particular.
In your laser setup, the temperature control system is responsible for managing the heat generated by the operation of the laser. In addition to the temperature controller, careful selection of a laser mount that is appropriate for the application is critical to a robust laser system.
Ultimately, the most pressing concern regarding temperature control is that temperature variations can affect laser quality, particularly wavelength. If not controlled, overheating can also damage the emitting face of the laser, reducing the quality and quantity of light produced.
The two basic strategies for dissipating heat generated by lasers are called passive cooling and active cooling. In addition, this article will describe non-traditional thermal management methods for high-power applications and applications that require heated laser mounts.
Passive Cooling
Passive heat sinks conduct heat away from the laser and dissipate it into the ambient air (Figure 1). Because this type of laser mount is simply a large heat sink, the mount temperature and the temperature of the laser will inevitably increase. Passively cooled laser mounts are designed so that the temperature increase occurs in a gradual and predictable manner.
The thermal performance of such a mount is rated as thermal resistance, in "C.N. This rating indicates the amount of temperature rise in the laser mount for each watt of waste heat generated by the laser, in degrees Celsius.
A fan will improve the thermal performance of a passively cooled laser mount. Typically, manufacturers provide ratings for laser mounts with and without auxiliary fans. Even with fans, the performance and power range of passive heat sinks is limited to low- to medium-power applications, or applications where higher operating temperatures are acceptable.
Active Cooling
Active cooling is a more comprehensive and complex approach to thermal management. A device called a Peltier cooler is built into the laser mount or directly into the laser package,
A Peltier device, also known as a thermoelectric cooler (TEC), is a small, flat, thermally conductive Ceramic, which uses power supplied by a temperature controller to cool one of its surfaces while heating the opposite surface. The laser mount is responsible for acting as a heat sink on one side of the Peltier device. The other side of the Peltier device is applied to an aluminum or copper cold plate that contacts the laser package housing.
To complete the control loop, a temperature sensor provides a feedback signal to the temperature controller, which regulates the power supplied 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 called the thermal capacity and is rated in watts. This rating indicates the amount of thermal power the laser mount can absorb while maintaining a stable temperature. This rating generally applies when the mount's cold plate temperature matches the ambient temperature. For remote Manufacturers can often provide thermal performance curves as a function of plate temperature.
It is worth noting that laser mounts equipped with Peltier devices will be able to be heated and cooled. This allows for faster stabilization and response times. In addition, if you are characterizing the performance of an LED or laser device, this feature can also allow the system to be stable both above and below ambient temperature. Since the output wavelength is related to the laser temperature, this also provides a convenient way to precisely control the optical performance of the laser.
Functional Considerations for Mount Selection
Beyond the basic issue of adequate thermal capacity, there are three functional areas that affect the usefulness of a laser mount. These are thermal conductivity, harness flexibility, and mechanical mounting of the laser.
Thermal conductivity of the laser mount, especially the cold plate is an important design aspect. While aluminum is adequate for some applications, the preferred material for the cold plate is copper. Copper has better thermal properties than 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 holder and, by extension, into the laser driver and temperature controller. Ideally, the manufacturer should provide standard pre-made cables from the instrument to the laser holder. When interfacing laser to laser, the connection should be easy to make and change, using wire terminals or some other simple method. Soldered connections or connectors that require extensive setup time are less desirable.
The same principle applies to the mechanical connection between the laser and the holder. It should go without saying that this connection should provide a good thermal interface. In addition, it should provide an easily disconnectable connection and a degree of versatility for a variety of laser packages. Some manufacturers will offer customizable cold plates that allow you to specify the desired mounting hole pattern.
High-Power Systems
Beyond laser holders with integrated fans and Peltier coolers, managing higher levels of thermal output becomes more challenging. If an air-cooled mount proves insufficient, the next option is a water-cooled mount (Figure 3). Water greatly increases heat capacity at the expense of complexity and responsiveness.
While water-cooled plates are effective at transferring large amounts of heat, there are several drawbacks. First, your temperature setpoint must be between the boiling and freezing points of water. Second, water systems require chillers, pumps, custom laser mounts, and plumbing, which will increase setup time and cost. Third, some water systems can have an error margin of a few tenths of a degree and do not react quickly to temperature changes. This may not be suitable for high-precision applications.
To improve the accuracy of water systems, hybrid systems that combine TECs with water-cooled laser mounts work well. This system relies on the TEC for fine temperature control and uses a water-cooling system to quickly dissipate heat. This approach is common in high-power laser applications that require good temperature stability.
High-Temperature Systems
As mentioned earlier in this article, the heating capabilities of a Peltier device may be useful if you are characterizing the performance of a device over a range of temperatures, or working with applications that require higher temperatures, such as LEDs. Higher mount temperatures require different types of temperature sensors and TECs suitable for high-temperature operation, so discuss your application with the laser mount manufacturer. Some laser mounts also contain resistive heaters, although this arrangement is obviously only suitable for heating-only applications. In this case, as long as the temperature controller can power the resistive heater, the rest of the thermal management system can remain unchanged.
Conclusion
Choosing the right bracket for your laser system will save time and effort while improving overall performance. In addition to deciding to use passive or active cooling, pay special attention to other features of the laser bracket. Ease of installation, flexible electrical connections, and good material selection are important factors to consider. In the end, the best course of action may be to call the manufacturer directly and ask questions about the performance of the mount for their specific application.
Reprinted from: Photon Bit
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