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Thermal Optical Power Detectors, Unmounted and Mounted
Unmounted SMD Package
Mounted on Aluminum Plate
PCB-Mounted Position Detector
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These detectors provide uniform absorbance from the UV into the MIR.
These Thermal Optical Power Detectors include unmounted and mounted single-thermopile sensors, as well as mounted position detectors comprising four individual thermopile sensors that are mechanically integrated into a quadrant configuration. The advantage of these thermal detectors over photodiodes is that the thermal detectors have a spectrally flat response over a broad wavelength range that extends from the UV through the MIR. Additionally, the thermal detectors have negligible dependency on the angle of incidence.
The three unmounted, single-thermopile thermal detectors have sensors optimized for high sensitivity and offer considerable flexibility in both mounting and making electrical connections. All have backing layers of solderable copper that facilitates mounting them on a PCB. Each unmounted sensor is also available mounted on a PCB, which provides mechanical stabilization and convenient copper solder pads for making electrical connections. The TD2XP's PCB has through-contacts and ground layers, while the TD4XP and TD10XP are mounted on metal-core PCBs and include an integrated thermistor.
The TD15A has a sensor that has been designed to accept higher input power levels, which enables its maximum optical power working range to extend up to 50 W. Its Ø15.0 mm sensor is mounted on an aluminum plate to assist with heat dissipation and also includes copper solder pads.
The four thermopile sensors composing the active area of the position detectors are mechanically integrated so that heat from the incident beam flows across the entire active area. As the heat intensity in each quadrant depends on the position of the incident beam, the position of the beam can be determined by comparing the magnitudes of the voltage signals from each quadrant. Position detectors are available mounted on a PCB or an aluminum plate. The sensor of the TD4HR18XP is optimized for high sensitivity and is mounted on a metal-core PCB, while the sensor of the TD4HP18XA is compatible with higher power levels and is mounted on an aluminum plate for enhanced heat dissipation.
Mounting and Handling
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Figure 1: A thermal sensor with axially configured thermocouples, which is depicted as seen from the side. Light is incident on the top, and heat flows down through the thermocouple layer and dissipates in the heat sink below.
These unmounted and mounted thermal power detectors are based on thermopiles. The top layer of the sensor, which appears gray in color, consists of a light-absorbing material. Located immediately behind the sensor, and in thermal contact with it, is a layer containing multiple thermocouples. Thermocouples are made by bringing two dissimilar metals into contact, and their point of contact is called a junction. The other side of the thermocouple layer must be thermally coupled to a heat sink. The thermocouples are connected in series, and the placement of the junctions alternates from being in close proximity to the absorber to being in close proximity to the heat sink. This axial (or matrix) configuration of thermocouples is diagrammed in Figure 1.
The absorber converts incident light energy into heat. The heat flows from the absorber, across the thermocouples, and to the heat sink, where it dissipates. The temperatures of the thermocouple junctions placed close to the absorber are higher than the adjacent junctions placed close to the heat sink. This arrangement takes advantage of the thermoelectric (Seebeck) effect, in which a temperature difference between adjacent junctions generates a proportional voltage difference. By connecting multiple thermocouples in series, the magnitude of the generated voltage is increased.
Axially-configured sensors, including those used in these unmounted and mounted thermal power detectors, can achieve high resolutions in the microwatt range while providing relatively fast response times. These sensors detect optical powers up to several Watts, which is limited mostly by the thickness of the absorbing material.
Mounting the Detector to a Heat sink
Thermal detectors must be both mechanically stabilized and mounted on an appropriate heat sink, which dissipates heat from the absorbed incident laser light. To ensure sufficient cooling of the detector, choose a heat sink with high thermal conductivity and follow the thermal integration instructions in Chapter 2 of the Handling Instructions manual. Mounting methods include thermally conductive tape, thermally conductive glue, and soldering. After the thermal detector is mechanically stabilized and mounted to its heat sink, electrical connections can be made to the detector.
Unmounted Thermal Detectors
If the chosen heat sink can provide adequate mechanical stabilization, an unmounted thermal detector can be mounted directly onto the heat sink.
Mounted Thermal Detectors
Natural Responses, the Sensor Time Constant, and Power Measurement Predictions
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Figure 2: Natural response of the S415C with the dotted line at 99% and the red square indicating the point on the curve corresponding to a single sensor time constant.
The typical natural response of a thermal sensor is its measured response to being instantaneously and steadily illuminated after being held in total darkness. This step function illumination stimulus produces a measured response that can be modeled using an exponential function and is similar to the function describing the rate at which a capacitor charges. Figure 2 shows the natural response measured for our S415C thermal power detector. (The S415C is mounted to a heat sink, calibrated, and includes a C-Series Connector that allows it to be used with our power meter consoles.)
The sensor time constant is defined in terms of how long it takes for the sensor response to reach 99% of its maximum response. The definition used by Thorlabs' power meter consoles is that when the sensor has reached the 99% level, a time period equal to five sensor time constants has elapsed. In Figure 2, the dotted line corresponds to the 99% level and the red square to the response after a single sensor time constant has elapsed. When the sensor's natural response characteristic function is known, it is possible to use it to model and predict the final power reading well before the sensor reading has stabilized.
Protect Thermal Power Sensors from Thermal Disturbances
For the most accurate results, thermal power sensors should be protected from air flow and other thermal disturbances during operation. Otherwise, measurements will drift. This is of particular importance for low power sensors with high resolution. Handheld use is not recommended for any of the thermal power sensors, as body heat transferred to the sensor or heat sink can negatively impact the accuracy of the measurements.
The compact sizes of these unmounted thermal detectors makes them particularly suitable for integration into setups where space is constrained. Mechanical and electrical integration methods can be chosen based on the needs of the application, provided that adequate cooling is provided. The bottom layer of solderable copper is convenient for attaching these detectors to a heat sink, and the copper solder pads on the TD4X and TD10X facilitate making electrical connections.
These mounted thermal detectors include a thermopile-based sensor mounted on either a PCB or aluminum plate. Each includes solder terminals for electrical connections and mechanical features for mounting the devices to a heat sink. Their compact size is a benefit for applications where space is limited. The TD2XP features a high-sensitivity sensor, and it is mounted on a PCB with through-contacts and ground layers. The TD4XP and TD10XP also feature high-sensitivity sensors, and they are mounted on metal-core PCBs that include thermistors. The high-power sensor used in the TD15A is compatible with incident optical powers up to 50 W, assuming an appropriate heat sink is used, and the aluminum mounting plate assists with thermal management.
The mechanical integration and electrical separation of the four constituent thermopile-based sensors forms a position sensitive device (PSD); heat is free to flow across the entire active area, but the signal from each quadrant measures the response in only that quadrant's thermopile. When the beam spot of the optical input is contained in one quadrant, the output signal from this quadrant will be higher than the output signals of the other three quadrants. If the beam spot moves across the active area towards the center of the PSD, the signals from the four quadrants will become more similar. The X and Y position of the beam is determined by comparing the signal intensities of all four quadrants.
The TD4HR18P features a high-resolution sensor mounted on a metal-core PCB, which has four Ø3.4 mm mounting holes. The TD4HP18XA's high-power sensor is mounted on an aluminum plate, which assists with thermal management and has four 1.6 mm radius mounting points.