Thermopile laser sensor

. This output voltage is directly proportional to the power of the incoming radiation. Since a large number of thermopiles are typically connected in series, voltages of several μV to V are reached. In general, a thermopile sensor consists of three elements: an absorber, the sensor element and a cooling body to dissipate the incoming heat. Absorber Depending on the thickness of the absorption layer, the thermopile sensor can be classified into two categories. For dielectric breakdown, the peak energy density during a pulse is high enough to locally ionize the sensor surface. Axial sensor with 0.5 mm thicknessThere are two main types of thermopile laser sensors which can be classified according to the geometric arrangement of the thermocouples inside the sensor element. Radial thermopile sensor/Thermopile discs Thermopile discs have thermocouples deposited onto an aluminium plate in a radial arrangement as shown in Fig 3(a). Axial thermopile sensor Fig 3(b) shows the cross sectional view of the axial sensor where the temperature difference is established between the top and bottom surfaces. Thermocouples are embedded into a matrix and aligned parallel with respect to the heat flow, forming junctions at top and bottom. Therefore, the cold side of the sensor needs to be thermally coupled to a heat sink. Passive cooling In this method of cooling the cold side of the sensor is mounted onto a heat conductor (usually an aluminium heat sink), and heat is dissipated to the surrounding by conduction (through heat conductor) and convection (air flow). Active cooling In this method of cooling the heat is actively transferred to the environment. This is usually done by mounting a fan on the heat sink of a passively cooled detector or by pumping water through a channel system to cool the sensor. The preferred choice depends on the amount of heat to be dissipated and thus on the detector power. == Characteristics ==

Sensitivity The sensitivity S [V/W] is the ratio of voltage U [V] generated due to the incident laser power P [W] on the sensor. The voltage generated depends on the Seebeck coefficient of the thermoelectric material; hence it is a material specific constant. leading to increase in output voltage. Spectral range The spectral range depends on the absorption characteristics of the coating material. Typically, a flat absorption spectrum across a broad wavelength range is desired. It can also be tailored to a wavelength range or to a particular wavelength. File:Risetime both2-01.jpg|thumb|326x326px|Figure 5: == Sources of measurement errors ==

Temperature error The sensitivity of the sensor varies with the mean sensor temperature. This is due to the temperature dependence of the Seebeck coefficient (see section 2.1). Since the dependence is quasi linear, the temperature error can be corrected by multiplying the measured value by a temperature dependent correction factor Background error If the sensor temperature is different from the ambient temperature heat flows directly to the surrounding without contributing to the detected temperature gradient therefore effectively reducing the sensor output. This type of error is on the order of few mW and is thus significant only at low incident powers The background error can be minimized by keeping the sensor at ambient temperature and avoiding convective air flows. It can also be corrected by subtracting the signal of a non-illuminated sensor (dark measurement). == Applications ==

Thermopile laser sensors find their use mainly where sensitivity to a wide spectral range is needed or where high laser powers need to be measured. Thermopile sensors are integrated into laser systems and laser sources and are used for sporadic as well as continuous monitoring of laser power, e.g. in feedback control loops. Some of the applications are Medical systems According to EU standard (EN6001-1-22), every medical laser system needs to be equipped with a redundant power measurement unit. For procedures such as precise tissue cutting and ablation the laser power can be measured before operation or even continuously throughout the process. One possible means of integrating a thermopile sensor in a medical system is by using a shutter or beam reflector (Fig 6) which can be flipped into and out of the beam path for short measurement periods of the full laser power. Power meters For sporadic measurements outside the laser system (e.g. during maintenance) a separate measuring unit is beneficial. For such a power meter, the sensor element is usually integrated into a metal housing for mechanical and thermal stability. The signal is recorded and processed in a read-out unit which displays the measured laser power (Fig 8). Position sensor, with different quadrant as shown in the image An arrangement of several thermally coupled thermopile sensors similar to a quadrant photodiode design (Fig 9) can be used to detect beam position as well as beam power. This is useful for beam alignment purposes or for processes where a correct beam position is crucial for high production yield. == Comparison between different types of detectors. ==