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Position Tuning Mode in the OPM Software GUI

Features

• Segmented Quadrant Position Sensors
• Broad Wavelength Range with Relatively Flat Spectral Response (See Plot on Specs Tab)
• Wide Optical Power Ranges with High Precision
• C-Series Connector
  • Enables Quick Sensor Connection to PM400 Console or PM102 Series Interfaces
  • Embedded EEPROM Contains Sensor and Calibration Data
• Post Mountable via 8-32 / M4 Universal Taps

These position sensors use thermopile sensor elements to obtain high-resolution measurements of a beam's position and power. They incorporate our OEM thermal position detectors, which consist of four sensor quadrants that calculate the beam's position by comparing the intensity measured in each quadrant. See the Operation tab for details.

The S440C detector uses the TD4HR18XP PCB-mounted thermopile sensor and is optimized for high sensitivity. The housing features four Ø6 mm through holes for compatibility with 30 mm cage systems, as well as an 8-32 / M4 universal tap for post mounting. The S442C detector incorporates the TD4HP18XA aluminum-plate-mounted thermopile sensor and can measure power levels up to 50 W. The housing includes a heat sink for superior heat dissipation, as well as two 8-32 / M4 universal taps for post mounting.

We also offer thermal sensor heads for a wide range of applications, as well as photodiode-based position sensing detectors, which offer faster response times that make them suitable for automated alignment.

Compatible Power Meter Console and Power Meter Interfaces
Both detectors feature C-Series connectors and can be controlled using either the PM400 power meter console or the PM102 series power meter interfaces (all control units sold separately). The PM400 console features a touchscreen display for the power and position readout. The PM102 series requires a PC with the OPM software in order to operate the sensor in a power meter system; depending on the PM102 model used, operation can be through USB 2.0, RS-232, or UART. 

There are a number of different display options that can be viewed either on the PM400 console display screen or in the OPM software GUI:

• Position Tuning: a visual representation of the beam's location on the sensor. The visible area can be adjusted between a centered Ø1 mm circle and Ø6 mm circle. The left side shows the power measured by the entire sensor surface and the X and Y coordinates of the beam. An optional setting can show a visual trace of the position over time (Position Trace Display).
• Graph: a graph of the X and Y positions over time, or a graph of total power over time. The user can touch a single button to toggle between the position and power graphs.
• Statistics: a table of measurement statistics. The unit takes a number of sequential samples and then displays the mean, median, standard deviation, and more.
• Numerical: a digital readout of the overall power measurement for the entire sensor, displayed in W, V, or dB. The beam position coordinates are also shown in the upper right of the screen.

See the PM400 full web presentation and manual for more information on the touchscreen power meter console. See the PM102 series full web presentation and manual for details on the power meter interfaces.

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Numerical Display on the PM400 Power Meter Console

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Graph Display on the PM400 Power Meter Console

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Statistics Display on the PM400 Power Meter Console

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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.

Operating Principle

These thermal power detectors are based on thermopile sensors. 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 such as the thermal quadrant detectors on this page can achieve high resolutions in the microwatt range while providing relatively fast response times. These sensors detect optical powers up to 50 Watts, which is limited mostly by the thickness of the absorbing material.

Figure 2: The sensor area consists of four thermopile sensors arranged as quadrants of a square.

Quadrant Detector Beam Position Calculation
Each thermal position detector consists of four individual thermopile sensors arranged in a 2 x 2 matrix, as shown in Figure 2. The quadrants are mechanically coupled by one aluminum plate but electrically isolated; thus, 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. The beam's coordinates X and Y are calculated using the voltage outputs of all four detectors:

where V1, V2, V3, and V4 are the output voltage signals from Q1, Q2, Q3, and Q4, respectively, in Figure 2. Since the sensor response is nonlinear, the detector uses a calibration factor for each axis, Cal_x and Cal_y, to calculate the absolute position.

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Figure 3: Natural Response of the S440C and S442C Detectors. The dotted line is at 0.99 and the red square indicates 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 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 3 shows the natural response measured for the thermal position sensing detectors. 

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 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. 

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.