Thermopile Sensors for Laser Power Measurement

Interview conducted by Will SoutterMar 19 2015

 

In this interview with AZoSensors, Susanne Dröscher from greenTEG talks about their new gRAY power detectors, and the important applications of power measurement in industry and research.

Why is laser power measurement important?

The importance of power control can be easily illustrated with the everyday example of cooking pasta. You want to increase the heat enough to keep the water boiling. At the same time you want to avoid too much heat, which would let it overboil and cause a mess in the kitchen. For a satisfying result, you therefore need to adjust the power carefully.

Laser systems are used in industrial processing for e.g. cutting, welding or marking. Just like for pasta cooking, the careful adjustment of the power of a laser system needs to be ensured in order to achieve high-quality results and reproducibility. A part might not be cut properly if the power is too low, whereas the raw edge might be rough at overly high powers. Monitoring the laser power is therefore crucial for an effective process control.

What are the typical applications of your new gRAY sensors?

The main field of application for the gRAY power detectors is in laser systems to provide a key feedback parameter for stable processes. Typically, the detectors are integrated into the systems (e.g. inside the processing head) and monitor the power continuously. Both the user as well as the manufacturer benefit from this. The operation scheme can be logged and malfunction is easier tracked down.

Further, laser systems used in medical applications have a need for power monitoring to ensure the safety of the patient and staff. In many countries, the integration of laser power detectors is obligatory.

The heart of every laser system is the laser source itself. Power instabilities or drift caused by the laser source influence the whole system operation. A power detector inside the laser source is therefore important.

Are there possibilities for more research-focused applications as well?

Yes, our gRAY detectors are used in research applications as well. Since they are all thermal detectors, the wavelength range in which they are sensitive stretches all the way from UV to MIR. Even THz radiation can be detected, which is currently a big research topic.

Further, one class of our detectors (type B) detects powers as low as 10 µW. When looking at the performance of conventional power detectors, no solution exists for IR-radiation at such low powers. Researchers either integrate our sensor components into their setups directly or use the Thorlabs S401C detector, which contains our detector elements.

The second detector class (type C) is designed for larger powers of up to 50 W. Besides the accurate measurement results, research users mainly appreciate the fast rise time. With only 200 ms signal rise time from 0 to 95%, the detector is the fastest of its kind on the market. Our housed detectors have a normalized output signal, which allows researchers simple read-out.

How do greenTEG's laser power detectors work?

For our detectors we make use of the thermoelectric effect, more specific called the Seebeck effect. In thermoelectric materials, an electric voltage is generated as soon as a temperature gradient is present across the material. Materials like copper, constantan and platinum are well known for their thermoelectric properties, and are commonly used in thermocouple thermometers.

For a temperature measurement, two wires of different materials are connected at one end. A voltage builds up, if this junction is at a different temperature to the open end. The magnitude of the electrical signal can be related to the temperature.

The gRAY detectors contain a large number of such thermocouples, all connected in series and embedded in a matrix. The junctions are formed on the bottom and the top side of the detector alternately.

On the top surface of the detector, an inorganic absorber coating is deposited. If the detector is illuminated, the light gets absorbed in the coating layer and is converted into heat. As a consequence, the upper side of the detector gets warm and a temperature difference across the detector is established. This, in turn, induces a voltage at each of the thermocouples. The total voltage at the detector output is the sum of all individual thermocouple signals.

The measurement signal hence is an analog voltage, which is directly proportional to the power of the incoming light. The corresponding power value is calculated using a determined calibration constant.

In summary, the detector first converts light into heat and then heat into electrical voltage.

How does the greenTEG thermopile technology compare to other solutions for laser power measurement?

The two key specifications for a power meter are its power range and its wavelength range. Within this parameter space, we have identified a region where no convenient detectors exist currently. This gap is located at large wavelengths (beyond the sensitivity spectrum of photodiodes) and low powers (below the resolution limit of conventional thermopile discs).

We cover this gap with our sensor components and PCB mounted detector, which have the additional advantage of being compact. Hence they can be mounted into any system where space is limited. Compared to photodiodes, no angular dependence of the sensitivity is observed, and due to the flat absorption spectrum of the coating, the spectral dependence of the sensitivity is very small.

With the development of our larger detectors, we have focused on the needs of OEMs. The robust package is designed for straightforward system integration. and the detector signal is amplified and normalized to facilitate read-out. The major differentiation to other thermopile detectors is, however, the signal rise time, which is a factor of 10 faster. This makes significant contribution to both faster control and faster processing.

How do your laser power detectors complement the other detectors in your product range?

With our other product lines, we also focus on improving measurement and control systems, by providing an important system parameter. Our gSKIN^® sensors measure heat flux, which is present in all thermal systems. Since heat flux describes the dynamics of a system, it is different from a simple temperature measurement.

In buildings, the quality of insulation (U-value) can be determined by measuring the heat flow through the walls; the body core temperature of humans can be monitored by recording heat flux through the skin; thermal influences onto high precision machines can be compensated when knowing the amount of heat going into the system; and the overcharging of batteries can be avoided by detecting the amount of heat flowing to the environment.

Can you give us a bit of background about greenTEG? How was the company started?

The first proof of principle for our current sensor technology was demonstrated in 2008 when our CEO Wulf Glatz defended his PhD thesis at ETH Zurich. Since then, a motivated team of researchers worked on steadily improving the processing methods until the company greenTEG was founded in 2009 as an ETH Zurich spin-off.

The main objective of the company is to develop, manufacture and market thermoelectric modules for sensing applications. The market for sensors is steadily growing as measurement and control gets ever more important.

With our sensors we provide a new parameter to measurement and control systems already today and have several partners in diverse fields of applications. Our main focus currently is on photonics applications though.

What are your plans for promoting this new product line this year?

We have started the promotion of the new gRAY line already. I've just returned from attending the Photonics West exhibition in San Francisco, where we presented the new products. We got great response to all our laser power detectors from both potential customers and distributors.

We will launch the gRAY line officially during the Laser World of Photonics in Shanghai at our booth N3.3660/9. Since this is our first exhibition in China, we are very excited about getting to know the Chinese photonics community and learning about the requirements of OEMs regarding power monitoring.

gRAY detectors are currently integrated by leading OEMs in DACH and the USA. To strengthen and expand our distribution network worldwide, we are looking for distributors and representatives in Asia (mainly Taiwan and China).  

How does the product development process happen at greenTEG? Are there any other new product announcements coming up that you can tell us about?

We regularly develop customized solutions for our OEM customers to meet their requirements since we always strive to find the most suitable solution for our customers. That’s why we are in constant exchange with them about their experience with our products. We provide suggestions for new detector units and plan developments in close collaboration with them. As a small company with an excellent R&D and production team, we can react to customization demands of customers immediately.

However, we are also working on new products that allow for new measurement parameters. The latest prototype is a position sensitive device which allows to measure both laser power and beam position simultaneously.

To provide an even more cost effective laser power detector for our large volume OEM customers, we will launch a detector that is just 2x2 mm² in summer. In IR laser systems, this will be an ideal complement for photodiodes, which can only be used at lower wavelengths.

Where can we find more information on the gRAY products?

On our product website (gRAY.greenteg.com), information on all laser power detectors is available.

For more specific questions, our team can be contacted directly as well.

About Susanne Dröscher

Susanne has a background in materials science and physics. She received her PhD from ETH Zurich in 2012 and afterwards joined the greenTEG AG as a Sales Engineer. As Product Manager she is currently responsible for all photonics products.

At greenTEG she coordinates the development and customization of laser power detectors, which are adapted to the user’s requirements. These projects are carried out with close contact to the customers involving technical discussions and consulting.