Position Sensing Devices (PSDs)

  1. What are position sensors?
  2. What are the different types of available PSDs?
  3. What are Segmented PSDs?
  4. What are Lateral-Effect PSDs?

  5. Lateral-Effect PSDs:
  6. When does one choose segmented PSDs vs. lateral effect PSDs and vice versa?
  7. What are the different types of Lateral Effect PSDs?
  8. What are the primary differences between the tetra-lateral and the duo-lateral structure PSDs?
  9. What is the Position Resolution?
  10. What is the Position Detection Error (PDE)?
  11. What is position linearity?
  12. What does the position resolution depend on?
  13. Does accuracy (PDE) depend on the spot profile, shape or intensity?
  14. How about spot size?
  15. I notice a drift in the position detection over a period time. What is going on?
  16. Does resolution and/or position detection error change with the reverse bias?
  17. Is the position detection error for position sensitive detectors repeatable?
  18. Why do I need to bias the PSD?

  19. Segmented PSDs:
  20. How uniform is the response between the segments?
  21. How does the gap size affect the measurements?
  22. What is the resolution for a typical segmented PSD?
  23. How does accuracy and resolution vary with shot noise?


1. What are position sensors?
Position Sensing Devices, better known as PSDs, are single substrate photodiodes capable of finding or locating a beam within a defined sensing area. The are used in a variety of applications from human eye movement monitoring, mirrors or machine tool alignment to vibration analysis, beam position control and more.

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2. What are the different types of available PSDs?
Position Sensing Devices are divided into two families: the segmented PSDs and the lateral-effect PSD's.

3. What are Segmented PSDs?
Segmented PSDs are common substrate photodiodes divided into either two or four segments, separated by a gap or dead region. A symmetrical optical beam generates equal photocurrents in all segments, if positioned in the center.

4. What are Lateral-Effect PSDs?
Lateral PSDs are continuous single element planar diffused photodiodes with no gaps or dead areas. Theses types of PSDs provide direct readout of a light spot displacement across the entire sensing area.

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Lateral-Effect PSD:

5. When does one choose segmented PSDs vs. lateral effect PSDs and vice versa?
Segmented PSDs offer position resolutions better than 0.1mm and accuracy higher than lateral effect PSDs due to superior responsivity match between elements. With S.N.R. (Signal to Noise Ratio) not effecting the resolution, very low light level detection is possible. They exhibit excellent stability over time and temperature and fast response times necessary for pulsed applications. They are excellent for nulling or beam centering applications.

Lateral effect PSDs have a wide dynamic range, they can measure the light position all the way to the edge of the sensor. They are independent of the light spot profile and can resolve positions better than 0.5 mm. Their resolution is detector / circuit S.N.R. dependent.

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6. What are the different types of Lateral Effect PSDs?
OSI Optoelectronics manufactures two types of lateral effect PSDs. Duo-Lateral and Tetra-Lateral structures. Both structures are available in one and two-dimensional configurations.

7. What are the primary differences between the tetra-lateral and the duo-lateral structure PSDs ?
In Duo-Lateral PSDs, there are two resistive layers, one at the top and the other at the bottom of the photodiode for two directional movements of X and Y. This type of structure can resolve light spot movements of less that 0.5 mm. They exhibit excellent position linearity over the entire active area.

The Tetra-Lateral PSDs, however, consist of a single resistive layer, in which the photocurrent is divided in two or four parts for one or two-dimensional sensing respectively. They show smaller dark currents and faster response times compared to duo-lateral PSDs.

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8. What is the Position Resolution?
Position resolution is defined as the minimum detectable displacement of a spot of light on the detector active area.

9. What is the Position Detection Error (PDE)?
Position Detection Error (PDE) or Accuracy is defined as the geometric variation between the actual position and the measured position of an incident spot of light. For all calculations, the zero is defined as the electrical center. This is the point at which I1=I2. The PDE is calculated as follows:

where I1 and I2 are the photocurrents at the ends of the PSD, L is the sensing area half-length in mm, and X (mm) is the actual displacement of the light spot from the electrical center. PDE is smaller for a small active area device.

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10. What is position linearity?
Position linearity refers to the accuracy of the measured position compared to the actual position of an incident light spot. Good position linearity means low Position Detection Error.

11. What does the position resolution depend on ?
The position resolution depends on the light intensity, the detector/electronics noise and bandwidth. In lateral-effect PSDs, the position resolution is limited by the signal to noise ratio of the system.

It varies from device to device; however, position resolutions in excess of one part in ten million have been achieved with OSI Optoelectronics lateral effect PSDs.

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12. Does accuracy (PDE) depend on the spot profile, shape or intensity?
In the lateral-effect PSDs, the input light spot profile and intensity do not affect the accuracy of the devices. The incoming beam may be any size and shape, since the position of the centroid of the light spot is measured which provides output signals proportional to the displacement from the center.

13. How about spot size?
In lateral effect PSDs, neither position resolution nor accuracy is dependent on the spot size. However, in segmented PSDs, the light spot has to overlap all segments at all times and it can not be smaller than the gap between the segments. Furthermore, in segmented PSDs, the position linearity is dependent on beam profile uniformity.

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14. I notice a drift in the position detection over a period time. What is going on?
It could be related to fluctuations of the ambient temperature or instability of the power supply source.

15. Does resolution and/or position detection error change with the reverse bias ?
Position Detection Error (PDE) does not change whether the device is operated in PV mode or PC mode. However, since Position resolution is SNR dependent, the additional dark current (shot noise) due to biasing does in fact reduce the resolution.

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16. Is the position detection error for position sensitive detectors repeatable ?
It is repeatable, but only within the maximum percentage specified for the device.

17. Why do I need to bias the PSD ?
Similar to a regular photodiode applying a reverse bias reduces the rise time. Furthermore, it increases the high end of dynamic range.

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Segmented PSD:

18. How uniform is the response between the segments?
The response uniformity between the segments is almost perfect with XX% maximum non-uniformity between the elements.

19. How does the size of the gap affect the measurement?
Crosstalk between the elements is affected by the gap size between the elements and the wavelength that's being measured. A larger gapsize will result in less cross talk and shorter wavelengths produce less crosstalk. Quantifiable results depend on the specific silicon device structure.

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20. How does accuracy and resolution vary with shot noise?
Since in segmented photodiodes the relative position of the spot is determined by the ration between the photocurrents, the accuracy and resolution does not depend on the photodiode dark current, as long as the signal to noise ratio is above one.

21. How does accuracy and resolution vary with shot noise?
Since in segmented photodiodes the relative position of the spot is determined by the ration between the photocurrents, the accuracy and resolution does not depend on the photodiode dark current, as long as the signal to noise ratio is above one.

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