Miniaturized Thermopile-Based Laser Power Sensors - Introducing the gRAY Sensor Range

Interview conducted by Jake WilkinsonFeb 23 2016

In this interview, AZoSensors spoke to Philipp about greenTEG's new product range - the gRAY sensors.

Please could you give our readers a brief overview of the gRAY sensors?

In general, when you regard any laser mediated process the power of a laser is usually the most important parameter to be considered. This makes being able to measure laser power critically important.

Our gRAY product line provides a laser power sensing solution for a wide range of applications. The sensors are optimized for original equipment manufacturers (OEMs) because they are robust, easy to handle and integrate into other technologies. This is very important for OEMs as they don't want complicated systems that are difficult to integrate into their original designs

Our whole product line can be essentially divided into 3 groups on the basis of the powers they are capable of sensing.

Firstly, we offer lower power sensors, which can detect up to 5 W. These are very sensitive sensors that can detect power down to 10 μW. At the same time they are the most compact thermal sensors available with sizes down to 2x2 mm².

Secondly, we have a higher power group where the power goes up to 100 W. These sensors are very fast. They have a signal rise time of 300 milliseconds which is around 5 times faster than conventional detectors.

Finally, there are position sensors which can detect the position of the laser beam on the sensor whilst also providing information on its power.

The B05-MC PCB Sensor from greenTEG

What wavelengths and powers are gRAYs capable of detecting?

In principle the wavelength range is determined by the coating on the sensor. We use specifically broadly absorbing coatings. The coating is characterized by a very flat absorption spectrum over an extremely wide wavelength range. This allows our sensors to be sensitive from 190 nm to 15  μm which covers every wavelength from UV to the mid infrared.

The measurable power ranges from few uW to 100 W depending on the detector type. In general, the detectors cover up to 5 orders of magnitude in power meaning that the dynamic range of the detectors is very large.

What are the typical applications of your sensors?

Laser power measurements are needed in research environments where lasers are used, but these tend to be our customers’ customers. Our customers are mostly OEM, i.e. either laser system manufacturers or laser source manufacturers. They integrate the detector at specific locations in the beam path to control the laser power and ensure stable operation.

The laser system manufacturers are again divided; mainly into the medical industry and the materials processing industry where they use lasers for cutting, welding and marking. In these applications, the laser processes need to be highly reproducible to prevent harm of patients or waste production.

Laser source manufacturers are usually working with diode lasers, fiber lasers or CO2 lasers. Since a stable output power is one of the quality criteria for a laser source, power detectors are integrated to provide the feedback signal for a closed control loop.

Why is the regular monitoring of a laser’s power important?

There are three important reasons why laser power must be measured and monitored.   

The first reason is for laser diagnostics; which involves taking a reference measurement at different points in the laser’s lifetime. The aim of this is to monitor how the laser is aging, a process which usually involves a decrease in power. This decrease in power is common and is due to several factors ranging from contamination of the optics to the laser itself losing efficiency.

Differences in the power output of a laser over the course of a year can be used to identify if the laser is in need of repairs.  

This brings us to second reason; which is laser maintenance. When people are, for example, readjusting the beam parts or anything, then you need a sensing device to quickly check the power of the beam just to ensure it's not too high for safety reasons.

The last reason is to ensure the power output is stable. Using a gRAY sensor you can read out the laser output in real time and then adjust the power of the laser accordingly. This means you can create a feedback loop with the aim of increasing the stability of the laser.

Industries involved in laser marking or welding benefit from this because the quality of the mark or weld decreases when the laser beam power is unstable. An unstable laser ruins the uniformity of the marking and welding. By stabilizing the power there is less waste production and a higher quality end product; which, in the end, translates into saved money for the customer.

What features make gRAY the best choice for monitoring laser power?

Firstly, our sensors are extremely compact. The sensor modules themselves have a thickness of only 0.5 millimetres. We also fabricate sensors down to areas of 2 x 2 mm if space is limited but still an accurate power measurement at affordable costs is required.

There is also the speed. Some of our sensors have signal rise-times of 300 milliseconds, which is around a factor of 5 faster than conventional thermal power sensors.

Finally, there is  the high resolution that our low power detectors have. Measuring powers as low as 10 uW is lower than most other thermopile detectors.

The B0.5-SC sensor is smaller than the size of a fingerprint, allowing OEM's to easily integrate it into their new or existing systems.

You mentioned medical applications earlier. What role do lasers play in surgery? What wavelengths and power ranges are commonly used for surgical lasers?

Surgery using a laser is very attractive because it solves a lot of the problems associated with conventional surgery. If tissue is cut with a laser it is instantly sterilized because the germs are evaporated from the high laser power. At the same time the laser can be used to either cut the tissue or, when at a lower focus, it can be used to cauterize blood vessels which leads to a reduced postoperative recovery time.

Finally, when you do cut with a laser, it directly seals the nerve ends which means you have less postoperative pain.

Surgical lasers are designed to target a high absorption peak in water which is at approximately 2.9 μm. Since biological tissue is composed mostly of water, Erbium YAG lasers can be used which create laser light that has a wavelength of 2.94 μm.

Sometimes, especially when cutting hard tissue, a high power laser is required, these are often CO2 lasers, which emit 10.6 μm laser light.  

Conventional neodymium YAG lasers, which emit at 1064 nm, can also be used. The power output of these lasers is very diverse; they range from a couple of watts to 100 W.

Taking everything into account these lasers are all in the infrared range and with moderate power ranges. These properties mean that medical lasers are within the ideal range to be measured using a gRAY sensor.

The use of lasers in surgery is becoming increasingly common meaning the demand for suitable laser measurement systems is high. Dmitry Kalinovsky | Shutterstock

Can the gRAY also be used in industrial settings?

Yes. At first sight, our sensors only measure up to 100 watts and the lasers used to cut metals tend to be much more powerful. However, that does not rule out using gRAY detectors. The sensors can still be used behind a beam splitter where only a small part of the beam is measured

Inside a CO2 laser, for example, the back mirror of the laser cavity can be tuned to transmit around 1% of the laser power. This highly attenuated beam can then safely be detected using a gRAY sensor. Using this method allows continuous monitoring of the laser’s power as the sensor is constantly in the beam path.

If the beam is attenuated gRAY sensors are capable of accurately measuring the power of industrial lasers, such as those used to cut metal. Dmitry Kalinovsky | Shutterstock

In what instances is the use of thermal power based laser power sensors such as the gRAY preferred over photodiode based sensors?

Photodiode based sensors are perfect for the rapid measurement of low power laser beams which have a well-defined wavelength.

However, they have some big disadvantages. Photodiodes have an extremely limited spectral range; for example, you can only measure between 400 and 1100 nm using a Silicon photodiode and the response is depending on the specific wavelength meaning they require wavelength correction.

In contrast to this thermal sensors show homogenous absorption over a huge spectral range; from mid IR to UV. This makes thermal sensors ideal if you want to measure different lasers using the same sensor.

Photodiodes are also easily saturated meaning they are limited to measuring only low powers. If the power exceeds several milliwatts photodiode based sensors start to saturate.

Thermal sensors also have a geometric advantage; their response is independent of the incident angle of the laser beam and the location at which the beam hits the sensor. This is not the case for photodiode sensors which are highly sensitive to beam misalignment.

Until now, thermal sensors have always been bulky, which held back their use in lots of applications; however, the gRAY is extremely compact meaning it can substitute a photodiode sensor with no compromise on space. This small size means they can be produced in large volumes at an attractive price and they can easily be integrated into existing technologies, such as circuit boards and fibre lasers.

What can we expect from greenTEG in the coming year?

Our most recent development has been a sensor which can simultaneously measure both the position and the power of lasers with powers up to 50 W.

We have also improved our housed sensors to have digital output signals. This makes it even easier for customers to integrate our sensors into their products. At the same time, we eliminate one of the issues that all thermopile detectors exhibit: the temperature dependence of the output signal.

Furthermore, we developed a sensor suitable for measuring ultrafast lasers with pulse widths on the nanosecond scale and shorter.

The details of all of these sensors are still kept secret. The official launch will be at Laser World of Photonics in Shanghai. Anybody who is interested should pass by our booth - which is number W3.3122 in Shanghai.

Please could you tell our writers about greenTEG as a company.

greenTEG was founded in 2009 following the discovery of a new technique of sensor fabrication by our CEO, Wulf Glatz . All of our fabrication facilities are located in Zurich, Switzerland. Our basic products are thermoelectric modules for sensing applications. This not only comprises the photonics sector, we also provide researchers in corporate and academic labs as well as building energy consultants with sensor solutions customized for specific tasks. We have a highly skilled and interdisciplinary team which is always ready to take complex problems and customer requests.

Where can our readers find out more about your company and the gRAY sensor range?

A lot of information on our products and their application can be found on our website. Additionally, we offer a newsletter and are posting any news on LinkedIn.

We are happy to talk about your specific application if you contact us directly, all of the details are on our homepage. If you are in Zurich just pass by; at greenTEG we are always happy to have visitors.

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About Dr Philipp Eib

Dr. Philipp Eib studied physics at the ETH Zurich where he completed his masters.

This was followed by a PhD at IBM Research - Zurich, where he used laser light to study the magnetism of structures intended for future memory applications.

Following graduation Philipp joined greenTEG in 2015 as an application engineer.

As part of this position he finds solutions to customer’s problems and helps to support the development department of OEMs .