If you want a stable laser system—whether you’re using it for scientific or engineering purposes or anything in between—you need certain factors to stay consistent. To achieve this, you must consider controlling the system’s temperature.
While written with laser systems in mind, many of the considerations of this article apply to any precision thermal system.
Continue reading to learn about the important temperature control fundamentals: stability, accuracy, and noise.
What Goes Into a Temperature Control System?
The first step in understanding the importance and application of a temperature control system in a larger laser system is to recognize the different parts of a temperature controller. Without a temperature controller, your laser will not stay at a consistent temperature. Rather, it will experience all kinds of variability. After understanding each element, it will be much easier to pinpoint variables such as stability, accuracy, and noise, and determine ways to improve each one.
The Actuators
Within a temperature control system, there are two different types of actuators commonly used to control the temperature of your system. The first kind of actuator is a thermoelectric cooler, sometimes called a Peltier cooler or TEC, while the other is a resistive heater. When the temperature controller is in use, it sends power through these types of actuators based on feedback it receives from the temperature sensor.
Temperature Sensors
To ensure the signal being sent through the actuators is correct, you need to have a quality temperature sensor that can accurately measure even the most minute differences in temperature. Depending on the application of your system, you may select from a few different types of temperature sensors. Although, in many cases, your temperature sensor will be dictated by the sensor integrated into the laser. The most common ones are thermistors, but for wide temperature range applications, you might use an RTD (see our article Thermistors vs. RTDs – 4 Key Differences To Consider for the difference between thermistors and RTDs). Some specialty applications might use IC sensors like the LM335 or AD590, but these are less common. These temperature sensors are precise, and when you use them, you can often achieve stabilities between 0.01°C and 0.001°C.
Electronics
Modern temperature controllers utilize precise power supplies to change temperature and measurement circuits for the temperature sensor—these are just as critical for keeping your system stable, accurate, and noise-free. Temperature control systems typically use proportional-integral-derivative (PID) control loops to govern how the temperature controller reacts to changes in temperature.
Other Considerations
Beyond the obvious are other considerations—cables for carrying the signals from the controller to the mount, the quality of the thermal interface between the laser and the mount, how well-tuned the PID parameters are for the control system, and location and ambient sensitivity of the temperature sensor are just a few of the many other elements that can impact performance.
What Happens Without Temperature Control?
After learning about some of the most important components of the temperature controller, you can begin to understand why the system is so crucial to your laser setup. Without a controller to regulate the temperature, you will experience many variabilities that affect the accuracy, noise, and stability of your laser system. Any of these variables are disastrous for your system and could result in the following:
- Inaccurate results
- Damaged equipment or components
- Degraded performance
- Unreliability
Because of these potential issues, you should do everything you can to prioritize stability, accuracy, and the elimination of noise, which temperature controllers can help you achieve.
What Is Noise?
To understand how the accuracy and stability of a thermal system are affected by a temperature controller, you must first understand the various sources of noise. It can be user settings on the controller, ambient impacts, fixture design, mounting of the laser, or electrical noise, to name a few. Short-term instability can also be considered a source of noise. When working with laser systems, this noise translates into variations in the optical output of the laser, but regardless of the type of device being tested, noise is something you want to minimize.
While noise cannot be completely eliminated, a properly designed temperature control system will reduce this variability as much as possible. Without doing this, noise will create inaccurate, non-repeatable results in your application. Consider the following sources of noise:
PID Settings
The proportional-integral-derivative (PID) settings of the temperature controller can have a big impact on temperature stability. Some controllers feature an “auto-tune” process where PID parameters are automatically determined, but adjustments of these PID settings can often further improve loop stability. This does take time—adjusting PID settings is a notoriously slow and time-consuming process but can often lead to better performance.
Laser Mounting
The ability to efficiently move heat away from your laser can directly impact noise. A poor thermal interface between the laser and the mount effectively increases the time constant of the system—this is the time between making a change in the output power of the temperature controller and when that change is measured at the laser. Longer time constants make it harder for the temperature controller to react to a change; the longer the time constant, the longer in the past that event actually happened and the harder it is to adjust for it.
Sensor Placement
Similarly, the further the temperature sensor is from the laser, the longer the time constant. Placing the temperature sensor as close to the laser as possible lowers the time constant and improves the ability of the temperature controller to sense, and therefore react, to a change.
Electrical Noise
Lasers are sensitive devices, and electrical noise from the temperature controller can be coupled into the signal from the laser driver. Because the laser is a high bandwidth device, the electrical noise is converted into optical noise. Using a temperature controller with a low-noise output will minimize this impact (see the article Direct Current vs. Pulse Width Modulation for more details on noise considerations with temperature controllers).
What About Stability?
Noise is the short-term variations, measured in seconds, while stability quantifies the long-term performance of the system, measured in minutes, hours, and days. Some of the elements that impact noise can also impact stability, but in different ways. The following are some elements that will influence the operation of your system over longer periods of time:
Ambient Temperature
Perhaps one of the largest impacts on long-term performance is changes in ambient temperature. The air around the laser will act as a heat sink, injecting heat into or drawing heat away from the laser. Air-conditioning systems can move ambient temperatures by 1 to 2 degrees within a few minutes, which will influence the laser and temperature sensor and force the temperature controller to respond, perhaps incorrectly. Where possible, shielding these changes—placing inside an enclosure, a cover over the laser, preventing air from directly blowing onto the mount, etc.—will help.
Sensor Placement
Making sure the temperature sensor is well protected from ambient is also a crucial step. Embedding the sensor at least 1 inch (2.5cm) into the cold plate will greatly reduce the impact of ambient temperature changes on the sensor’s measurement. Placing the sensor as close to the laser as possible will also improve stability.
Temperature Controller
All electronics shift with temperature, so selecting a temperature controller with a low temperature coefficient means a change in ambient temperature will have minimal change in the actual measured temperature. Ambient temperature changes may still affect your laser, but those effects will not be compounded by errors from the temperature controller itself.
And Accuracy?
Accuracy in a thermal system is far more challenging than most people give it credit for. There are many elements that influence total accuracy, including:
- Accuracy of the temperature controller
- Accuracy of the temperature sensor
- Errors caused by resistance in cables and connectors
- Error between the point of measurement and laser under test, which can be through multiple thermal interfaces
To arrive at total system accuracy, all of these individual contributors must be added together, and some, such as the point of measurement error, can be very difficult to quantify. Further, changing the operating conditions, such as temperature set point, operating power, and ambient conditions, will also influence the accuracy of the system.
As previously mentioned, noise and instability are to be expected when working with a thermal system, but you want to minimize the impact. This can be achieved in multiple ways because there are multiple sources of noise and instability. Remember, the more unstable your thermal control system is, the more inaccurate your results will be.
Devices like temperature controllers vary from use to use. There is a lot you can do just within the realm of lasers, so you must ensure you have the right setup. If you want your results to be accurate, you need to eliminate as much noise as possible—a good, precise temperature controller is one way to do just that.
Understanding the temperature control fundamentals of stability, accuracy, and noise, and how to better account for each of these variables, is critical for ensuring your laser system works as intended. If you’re not focused on maintaining the temperature of your system, your laser system is liable to produce incorrect results or even break down. At Arroyo Instruments, we have the equipment you need, like temperature controllers, to keep the temperature of your system in check.
