Thermal Power Sensors (C-Series)

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Internal SM05 (0.535"-40) threading on the S405C's aperture (left) accepts the included external SM05 to SM1 (1.035"-40) thread adapter (right). Most sensors include an SM1 externally threaded adapter that allows accessories like fiber adapters to be mounted.

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S401C Connected to the PM100D Console

Features

• Broad Spectral Ranges with Relatively Flat Spectral Responses (See Plots Below)
• Individually Calibrated with NIST- and PTB-Traceable Certificate of Calibration 
• Free-Space and Fiber-Based Applications Supported
  • All Sensors Accept Free Space Input
  • Majority Accept Fiber Adapter (Available Below) for Fiber-Based Input
• C-Series Connector
  • Enables Quick Sensor Connection to Our Power Meter Consoles
  • Embedded EEPROM Contains Sensor and Calibration Data
• Ten Models Feature Over-Temperature-Alert Sensor (See Specs Tab for Details)

Thorlabs' C-Series Thermal Power Sensors are collectively able to detect power ranges from 10 µW to 200 W and wavelength ranges from 190 nm to 20 µm. These thermopile-based sensors are ideal choices for measuring broadband spectra from amplified spontaneous emission (ASE) sources, light emitting diodes (LEDs), filament lamps, swept-wavelength lasers, and other sources. In addition, thermal power sensors do not saturate, which makes them well suited to measuring pulsed sources with high pulse peak powers or long-duration pulses. These thermal power sensors also exhibit low dependency on the angle and position of the incident light beam. They are preferred for applications that cannot tolerate the strong wavelength dependencies and/or saturation thresholds of photodiode sensors. However, thermal power sensors generally have lower power resolutions and longer response times. We offer a wide range of thermal power sensors, with each including different design features.

Mounting
The sensors sold here (except the S175C) can be mounted on our Ø1/2" Posts using an 8-32, M4, or M6 tap. A 60 mm or 75 mm tall post is included with select sensors. Additionally, many of our thermal sensors are compatible with 30 mm cage systems, Ø1" lens tube systems, and our fiber adapters (sold below). Please refer to the Specs tab for more information.

Calibration
Each sensor head is individually calibrated and is shipped with a NIST- and PTB-Traceable Calibration Certificate. The calibration and identification data is stored in the electrically erasable programmable read-only memory (EEPROM) built into the connector of the sensor and is downloaded automatically to the connected power meter console. The EEPROM also contains sensor model and serial numbers, wavelength range, information on the built-in thermistor (when present), and the calibration date. For more information on sensor calibration, please see the Calibration tab on our Power Meter and Sensor Tutorial.

Power Meter Compatibility
All sensors are connected to the power meter console via the C-Series connector, which offers quick sensor exchange. These sensors are compatible with our current power meter console offering but cannot be used with our previous generation of consoles. For our sensors with natural response times greater than 1 second, these power meters can use the data stored in the connector to predict the incident power after a single time constant of the sensor. Please see the Operation tab for more information.

Recalibration Services
Thorlabs offers recalibration services for our thermal power sensors. To ensure accurate measurements, we recommend recalibrating the sensors annually. This service can be ordered below (see Item # CAL-S200).

Sensor Upgrade and Service
All C-Series Sensors are incompatible with former generation power meter consoles with non-C-Series connectors. We offer a sensor upgrade service if you want to use your existing sensors with a new power meter console with a C-Series connector. Note: upgraded sensors will be incompatible with old power meter consoles with non-C-Series connectors. Please contact our Tech Support team for details.

• Combined Range for All Sensors in the Respective Category

Thorlabs' Thermal Power Sensors (C-Series)

Click on the links in the following list or Selection Guide to move to the indicated specification tables.

• High Resolution
• Max Power: 10 W
• Max Power: 40 W to 200 W 
• High Max Power Density for Pulsed Lasers
• Microscope Slide Thermal Sensor

High Resolution

• Twenty-minute maximum exposure time for the S401C. The S405C saturates for optical input powers >5 W.
• Measurement taken with the legacy PM200 console for the S401C and the PM400 console for the S405C. In both cases, the acceleration circuit was switched off. Resolution performance will be similar with our other power meter consoles.
• Measurement uncertainty during calibration at the specified wavelengths for a beam diameter > 1 mm. The ±3% specification was determined by laser calibration, and the ±5% specification was determined through spectral calibration, in which values were interpolated using the laser calibration data and the absorption curve for the absorber. Calibration can be performed at 10.6 µm upon request.
• Typical natural response time (0 - 95%). Our power consoles can provide estimated measurements of optical power on an accelerated time scale (typically <1 s). See the Operation tab for additional information. 
• Distance is measured from the detector to the front face of the housing.

Max Power: 10 W

• Two Minute Maximum Exposure Time
• Measurement taken with the PM400 with the acceleration circuit switched off. Resolution performance will be similar with our other power meter consoles.
• Measurement uncertainty during calibration at the specified wavelengths for a beam diameter > 1 mm. The ±3% specification was determined by laser calibration, and the ±5% specification was determined through spectral calibration, in which values were interpolated using the laser calibration data and the absorption curve for the absorber. Calibration can be performed at 10.6 µm upon request.
• Typical natural response time (0 - 95%). Our power consoles can provide estimated measurements of optical power on an accelerated time scale (typically <1 s). As the natural response times of the S415C and S425C are fast, these do not benefit from accelerated measurements and this function cannot be enabled. See the Operation tab for additional information.
• Distance is measured from the detector to the front face of the housing.

Max Power: 40 W to 200 W

• Two Minute Maximum Exposure Time
• Measurement taken with the PM100D console, except for the S425C-L in which the PM400 was used. In all cases, the acceleration circuit was switched off. Resolution performance will be similar with our other power meter consoles.
• Measurement uncertainty during calibration at the specified wavelengths for a beam diameter > 1 mm. The ±3% specification was determined by laser calibration, and the ±5% specification was determined through spectral calibration, in which values were interpolated using the laser calibration data and the absorption curve for the absorber. Calibration can be performed at 10.6 µm upon request.
• Typical natural response time (0 - 95%). Our power consoles can provide estimated measurements of optical power on an accelerated time scale (typically <1 s) for the S350C and S322C. As the natural response time of the S425C-L is fast, the S425C-L does not benefit from acceleration and this function cannot be enabled. See the Operation tab for additional information.
• 12 VDC power supply included.
• Distance is measured from the detector to the front face of the housing.

Operational Principle

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Figure 1: A thermal sensor with radially configured thermocouples, which is depicted as seen from the top. Light is incident on the absorbing layer at the center, and heat flows through the thermocouples to the heat sink.

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Figure 2: 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.

Thorlabs' Thermal Power Sensors are based on thermopiles. The top layer of the sensor consists of a light-absorbing material. A region filled with multiple thermocouples, which are connected in series, is immediately adjacent to the absorber. Thermocouples are made by bringing two dissimilar metals into contact, and their point of contact is called a junction. On the other side of the thermocouples is 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.

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 those the adjacent junctions placed close to the heat sink. This arrangement takes advantage of the thermoelectric (Seebeck) effect, in which a temperature difference between the adjacent junctions generates a proportional voltage difference. By connecting multiple thermocouples in series, the magnitude of the generated voltage is increased. The thermocouples are often arranged in a radial (also called a disk) configuration, as illustrated in Figure 1, or in an axial (also called a matrix) configuration, which is diagrammed in Figure 2. Thermal power sensors with both types of configurations are available from Thorlabs.

Radial Configuration of Thermocouples
A diagram of a thermal power sensor with a radial thermopile is shown in Figure 1, viewed from the top. This construction places the light absorber at the center. It is surrounded by a concentric ring of thermocouples connected in series, which are surrounded by a concentric heat sink. Light incident on the absorber generates heat that flows in a radial direction through the thermocouples and towards the heat sink. The heat sink must be specially designed so that it is in good mechanical contact with the outer ring of thermocouple junctions, without being in thermal contact with the light absorber or the inner ring of thermocouple junctions. The area behind the absorber cannot be in thermal contact with anything that will divert the heat flow from its intended radial path of flow. 

A benefit of the radial construction is that sensors can be designed to measure power levels as high as kilowatts. This high upper limit is made possible both by the thickness of the sensor disk and by displacing the thermocouples from the absorber, which protects them from the conditions in the laser impact area. Disadvantages of the radial thermopiles include the use of a heat sink with a special design, which adds complexity when customizing the sensor head, and a sensor head which is generally a least twice the diameter of the active detector area. Resolution for radial thermal power sensors is typically limited to around 10 mW.

Thorlabs' thermal power sensors featuring a radial configuration include the S350C and S322C, which are all designed for mid-power range applications.

Axial Configuration of Thermocouples
A diagram of a thermal power sensor with an axial configuration of thermocouples is shown in Figure 2. In this design, the thermocouples are arranged between two flat layers. One layer is the light absorber, and the other is the heat sink. As the heat flows directly from the front surface to the back side, the dimensions of these sensor packages can be made compact. The sensor housing can be approximately the same size as the active detector area.

The new generation of axially-designed sensors achieves high resolutions in the microwatt range while providing relatively fast response times. These sensors detect optical powers up to several Watts, which limited mostly by the thickness of the absorbing material. The performance of the newly designed sensors, which includes the S401C and S405C, contrasts with sensors of previous generations, which have slower response times.

Heat sink shapes and dimensions are much less constrained for axially, as compared with radially, configured thermopiles. Heat sinks for axial designs can be as simple as a block of aluminum attached with thermal glue, or as sophisticated as a metal-core PCB that is soldered to the sensor. Our S415C, S425C, and S425C-L thermal power sensors have heat sinks can be easily removed and replaced. This enables the user to upgrade the heat sink, potentially to one with fans or water cooling, or to integrate the sensor into a custom setup. Please note that the heat sink must provide heat dissipation adequate for the application.

Volume Absorbers for Pulsed Lasers
Volume absorbers are alternatives to surface absorbers, which sustain damage when subjected to highly energetic and short pulses of nanosecond duration. Unlike surface absorbers, which suffer damage as a consequence of absorbing the pulse energy within a localized region, volume absorbers collect the heat from the light pulse and distribute it throughout a volume. Heat generated throughout the volume flows across the thermocouples and dissipates in the heat sink. Thorlabs offers two thermal power sensors with volume absorbers, S370C and S470C, which are both designed for the detection of Nd:YAG laser pulses, among other applications. In these axially-constructed sensors, the Schott glass volume absorber replaces the surface absorber of the other axial sensors. The response times of sensors with volume absorbers are slower than those with surface absorbers, as the thermal mass of the volume absorber is larger. The S470C is faster than the S370C, as its glass volume is smaller and other design changes have resulted in a faster response of the axial thermopile.

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Figure 3: 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.

Natural Responses, the Sensor Time Constant, and Power Measurement Predictions

The typical natural response of the S415C thermal sensor to an instantaneous transition from darkness to being steadily illuminated is shown in Figure 3. This step function illumination stimulus produces a response that can be modeled using an exponential function and is similar to the function describing the rate at which a capacitor charges.

The sensor time constant is defined in terms of how long it takes for the sensor response to reach 99% of its maximum response. When the sensor has reached the 99% level, a time period equal to five sensor time constants has elapsed. In Figure 3, 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. Thorlabs' power meter consoles calculate and display predictions of the stabilized power reading when Thorlabs sensors with sensor time constants ≥0.5 s (natural response times >1 s) are connected. Prediction is implemented using the sensor information stored in the EEPROM built into the C-Series connectors. The S415C, S425C, and S425C-L are fast enough, with sensor time constants <0.2 s (natural response times <0.6 s), that prediction is not necessary and is not enabled. Prediction is enabled for the other sensors.

When prediction is active, the first prediction is made after a time duration equal to a single sensor time constant, and this prediction is updated at time intervals of one sensor time constant until a total time duration of seven sensor time constants has elapsed. Prediction is then turned off; the power reading after seven time constants is 99.9% of the final reading. As there is uncertainty associated with the predicted measurements, they can exhibit some ripple. The faster the sensor, the less the uncertainty. After prediction is turned off, the gradient of the power reading is monitored, and prediction is re-enabled if an increase is detected which exceeds a defined threshold. 

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.

Thermal power sensors operate by measuring a temperature differential, which is converted to a voltage signal. The sensor design assumes that heat generated in the absorber flows towards the heat sink. If the operator is in contact with the sensor housing during operation, body heat may transfer to the sensor and make spurious contributions to the power measurement. For example, if the sensor is held by the heat sink, heat transferred from the hand to the heat sink will flow towards the absorber. If no light is incident on the absorber, this will result in a negative power reading. If there is light incident on the absorber, it will result in an inaccurate power reading.