High speed Thermopiles

High speed Thermopiles for Blink sensor series

High speed thermal sensors based on this very new, patented technologies developed by LaserPoint are capable of measuring the energy of single laser pulses with repetition frequencies up to the MHz range, adding the possibility to also measure the output power of cw-lasers.

Moreover, the thermal nature of the sensor enables the capability to work in a broadband spectrum, from UV to THz as well as the possibility of operating in a broad-range (10^-3-10² W/cm²) of incident average optical power densities of the laser radiation, without the need of adopting optical filters nor other precautions.

Thermoelectric devices are subdivided into two different groups: the devices using the standard longitudinal thermoelectric effect induced at the junction of different types of materials and the devices based on the Laser Induced Transverse Voltage (LITV) effect.

Sensors based on the standard longitudinal thermoelectric effect are commonly designed adopting multiple electrically interconnected thermocouples, which can measure a heat flux axially across a suitable substrate. This type of sensors using the standard thermoelectric effect are an evolution of the common radial thermopile design. Working on a thermal principle, the spectral acceptance region of this kind of sensors is still broadband. However, the thermal design of this kind of sensors only allows for relatively slow response times (currently greater than 100 ms). Moreover, the design of multiple axial thermocouples often implies a scarce coverage of the active area of the sensor.

Sensors using the Laser Induced Transverse Voltage (LITV) effect also transduce a thermal gradient into an electric signal. Thin films of deposited suitable materials can show a transverse thermoelectric response to laser-irradiation. That is, if a thermal gradient is present along the normal direction to the film surface, a thermoelectric response is generated, longitudinally to the plane of the film surface. The adoption of the LITV effect has the intrinsic advantage of showing a good conversion efficiency of a thermal signal into an electric voltage, while showing response times in the nanosecond timescale. Another advantage of the LITV based devices over standard thermoelectric devices is the uniform coverage of the active area, with respect to a design based on axially disposed thermocouples.

The advantage of sensors using the LITV effect over pyroelectric sensors and photodiodes for laser radiation measurement is the combination of an overall fast response time, broadband spectral acceptance, high saturation threshold to direct laser irradiation and the possibility to measure pulsed as well as cw-laser sources.

Linearity of the response vs power

The response of the Blink sensor under cw-laser radiation, up to about 50 W, is shown in the picture below. Data show the output signal of the Blink detector without any post-linearization. The plotted line indicates the linear fit, which quantifies the linearity of the detector within the measured range with an R coefficient of 0.9998. Having a good linearity is crucial for the calibration of the detector once manufactured.

Linearity of the response vs pulse energy

The response of the detector to pulses of different energy and pulse duration is shown in the picture below. In the same figure, it is also shown the independency of the energy measurements from the pulse duration τP, which in turn indicates the independency of the measurements from power density.

The measurements of the energy of the incident pulses EP is independent from the spot size as well and the energy range of the detected pulses ranges from 10 uJ up to tens of mJ.

Response time

Different definitions can be used for the response time. We use here the 0-100% (τ_0-100) response time definition, which is the time needed by the detector to restore the signal output to the level of the baseline, after the incident laser pulse is terminated. Such definition was chosen because f_max = 1/τ_0-100 defines the maximum measurable frequency of a train of laser pulses by the detector.

In the below picture, it is shown the fast response of the detector to a train of laser pulses with pulse duration of 4ns and with a repetition rate of 1 MHz.

Optical response and damage threshold

The thermal nature of the LITV effect allows the detector to work in a broadband wavelength range of incident radiation, as long as the electromagnetic radiation is absorbed and transformed into heat by the absorption coating.

An important characteristic of the coating is its spectral flatness, which is of great importance for a simple calibration of the detector. The optical reflectance of the detector is shown in picture. Within the whole measurement range, i.e. from 250 nm to 1100 nm, the reflectance curve shows a smooth and overall relatively flat behavior.

It was found that the damage threshold of the detector in its fastest configuration, i.e. with 1 MHz bandwidth, was greater than 30 mJ/cm2/pulse for pulses of 200 ns.