Thermopile laser sensor

Thermopile laser sensors (Fig 1) are used for measuring laser power from a few μW to several W (see section 2.4). The incoming radiation of the laser is converted into heat energy at the surface. This heat input produces a temperature gradient across the sensor. Making use of the thermoelectric effect a voltage is generated by this temperature gradient. Since the voltage is directly proportional to the incoming radiation, it can be directly related to the irradiation power (see section 2.1). Unlike photodiodes, thermopile sensors can be used for a broad spectrum of wavelengths ranging from UV to MIR (depending on the characteristics of the absorption coating at different wavelengths). Further, photodiodes are reverse biased and saturate for optical powers above a certain value (typically in mW), making thermopile sensors suitable for high power measurements. Pyroelectric sensor and calorimeter are commonly used for measuring the energy of laser pulses. Pyroelectric sensor can measure low to medium energies (mJ to J) and are prone to microphonic effects. Calorimeters are capable of measuring high energies (mJ to kJ) but have large response times.

Working principle and structure

As shown in Fig 2, a thermopile laser sensor consists of several thermocouples connected in series with one junction type (hot junction at temperature T1) being exposed to an absorption area and the other junction type (cold junction at temperature T2) being exposed to a heat sink. When a laser beam hits the surface of a thermopile sensor, the incident radiation is absorbed within the coating layer and transformed into heat. This heat then induces a temperature gradient across the sensor given as

d T d x = T 2 − T 1 t {\displaystyle {\frac {dT}{dx}}={\frac {T_{2}-T_{1}}{t}}} [K/m], where t is the thickness of the sensor. Due to the thermoelectric effect, the temperature difference causes an electrical voltage to build up within each thermocouple. 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.

Surface absorber For surface absorbers the thickness of the absorption layer is very thin (0.1 – 100 μm) and so is the total absorption length. It is used for power measurements of lasers with long pulse length (generally for CW laser). If a laser with pulse length in the range of 10−7 – 10−4 sec is used the sensor can be damaged by either dielectric break-down or thermal effects. In case of thermal damage, heat is deposited in a short time and cannot be dissipated until the next pulse arrives. This leads to an accumulation of energy in a thin layer leading to partial vaporization. For dielectric breakdown, the peak energy density during a pulse is high enough to locally ionize the sensor surface.

Volume absorber To protect the sensor from damages by short optical pulses, volume absorbers are used with absorption lengths in the order of millimetres. This enables volume absorbers to withstand higher pulse energy densities, since the optical power is absorbed over a considerable depth of material.

Sensor geometry There 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). All thermocouples are electrically connected in series with one junction at the circumference of the inner area which is illuminated and the other junction at the outer circumference. The absorption coating in the illuminated area converts radiation into heat which flows radially outwards generating a temperature gradient between inner and outer ring and thus a thermoelectric voltage.

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. This arrangement permits a reduction of the total sensor thickness to 0.5 mm (Fig 4).

Cooling/Heat management It is crucial to dissipate the incoming heat in order to establish a stable temperature gradient across the sensor. 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. The incident power can be calculated by measuring the sensor voltage and using the formula:

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