Detector Response Measurements Presented by Dr. Richard Young VP of Marketing & Science Optronic Laboratories, Inc.

Outline of Presentation  Detector Types  Response Uniformity  Measurements  Irradiance Response By substitution  Power Response By substitution By beam switching

Detector Types Detectors generally fall into two main types PhotodiodesPhotoconductors

Detector Types  Photodiodes  Can operate unbiased or biased  Are generally suited to DC amplification  Have low noise  Generally operate in the UV to near IR  Photoconductors  Must be biased to operate  Are usually operated in AC mode  Have low sensitivity and high noise  Generally operate in the visible to far IR

Response Uniformity Photodiodes often have uniform responsivity Note: the z-axis scale is in 0.1% increments

Response Uniformity Photoconductors often have non-uniform responsivity Note: the y-axis scale is in 10% increments

Response Uniformity Signal For the silicon photodiode, the signal is virtually the same regardless where on the chip the light spot falls.

Response Uniformity Signal For the PbS photoconductor, the signal is very different, depending where on the chip the light spot falls.

Power Response  Power is measured correctly at all locations on the chip  Power is measured correctly at all spot sizes  Provided the spot lies within the detector area  The spot location on the chip and spot size that was used in the detector calibration must be reproduced exactly for correct measurements Si Detector PbS Detector If the detectors were calibrated, we could measure the power [W] in the spot, but…

Irradiance Response If the spot is larger than the detector area, the detector measures PowerArea = Irradiance (watt/cm 2)

Irradiance Response Provided the intensity within the spot is uniform, the PbS detector also gives correct irradiance results and is not sensitive to alignment or spot size.

Detector calibration is usually done by direct substitution. For irradiance response calibration, the detector is over-filled with a uniform beam Calibrated Detector First, the irradiance [W/cm 2 ] is measured with the calibrated detector Test Detector Then the signal on the test detector gives its irradiance response [(A or V)/(W/cm 2 )] Irradiance Response

Power reponse calibration is also usually done by direct substitution. For power response calibration, the detector is under-filled with a small spot Calibrated Detector First, the power [W] is measured with the calibrated detector Test Detector Then the signal on the test detector gives its power response [(A or V)/W] Power Response

 However, if the detector is non-uniform, it needs to be positioned correctly each time.  Often the test detector and the calibrated detector require different holding fixtures. And changing these, with alignment adjustments, every time you need to calibrate can be time consuming.  An accessory with a beam switch can speed up the calibration/measurement cycle considerably.

Detector measurements with a beam switch requires one extra step. First, the calibration as normal… Calibrated Detector Power Response …then the beam is switched and the calibrated (or other) detector is placed at the switched position…

Calibrated Detector Power Response Measurement of the detector response in THIS position is called a transfer calibration.

Once this is done, the detector remains in this position. Calibration is achieved by applying the transferred values. Calibrated Detector Power Response

Measurement is the same as before with the mirror switched back. Calibrated Detector Power Response Test Detector