Electro-Optical Sensors

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Electro-Optical Sensors
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Some sensors still need explaining. And there are new sensors coming out all the time that will need to be added.

1 Introduction to Electro-Optical Sensors

See Also

Electro-Optical sensors are a category of sensors that turn light energy into electrical energy. The sensors can be common such as the light/dark sensors on kids' night lights - I suspect these are just simple solar cells that tell the light to run on when the voltage is appproximately 0. Electro-Optical sensors can also be much more complicated. A common but complex electro-optical sensor is the CCD at the heart of digital cameras. CCDs are also at the heart of many scientific imaging satellites.

In my laser communications work I come across a variety of electro-optical sensors. The most common are LECs, Quad Cells, FPAs and CCDs.

2 A Quick Overview of Electro-Optical Sensors

2.1 LECs

This section is adapted from UDT's Position Sensing Photodiode Application Note.

Lateral effect cells (LECs) are Position Sensor Devices (PSDs) which are continuous single element planar diffused photodiodes with no gaps or dead areas.

LECs provide direct readout of a light spot displacement across the entire active area. The analog output is directly proportional to both the position and intensity of a light spot present on the detector active area. A light spot present on the active area will generate a photocurrent, which flows from the point of incidence through the resistive layer to the contacts. This photocurrent is inversely proportional to the resistance between the incident light spot and the contact. When the input light spot is exactly at the device center, equal current signals are generated. By moving the light spot over the active area, the amount of current generated at the contacts will determine the exact light spot position at each instant of time. These electrical signals are proportionately related to the light spot position from the center.

The main advantage of LECs is their wide dynamic range. They can measure the light spot position all the way to the edge of the sensor. They are also independent of the light spot profile and intensity distribution that effects the position reading in the segmented diodes. The input light beam may be any size and shape, since the position of the centroid of the light spot is indicated and provides electrical output signals proportional to the displacement from the center. The devices can resolve positions better than 0.5 μm. The resolution is detector / circuit signal to noise ratio dependent.

2.2 Quad Cells

This section is adapted from UDT's Position Sensing Photodiode Application Note.

Segmented PSD’s, like Quad Cells, are common substrate photodiodes divided into either two or four segments (for one or two-dimensional measurements, respectively), separated by a gap or dead region. A symmetrical optical beam generates equal photocurrents in all segments, if positioned at the center. The relative position is obtained by simply measuring the output current of each segment. They offer position resolution better than 0.1 μm and accuracy higher than LECs due to superior responsivity match between the elements. Since the position resolution is not dependent on the SNR of the system, as it is in LECs, 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 however, confined to certain limitations, such as the light spot has to overlap all segments at all times and it can not be smaller than the gap between the segments. It is important to have a uniform intensity distribution of the light spot for correct measurements. They are excellent devices for applications like nulling and beam centering.

2.3 CCDs

A specially developed CCD used for ultraviolet imaging in a wire bonded package.

Charge Coupled Devices (CCDs) are an array of photodiodes which can be thought of as a bucket for photons. The fuller the bucket gets the more current that bucket outputs when the readout circuitry empties the bucket. Often the array in a CCD is very large meaning larges amounts of parallel circuitry for emptying those buckets. It also leads to a readout rate of the full array in the 10s of Hz.

Additionally, CCD readout circuits typically readout a column of array pixels in sequencial order. Figure 3 presents a simple graphic of how the CCD reads out the data. To be specfic, the CCD reads the pixel at the bottom of the column first then moves the current from the buckets above down 1 pixel. Then the circuit empties the next pixel and moves the rest down 1. This process is repeated until all the pixel buckets are emptied.

Figure 3: Simplified CCD Readout Diagram

The way a CCD is readout creates a spatial noise across the imager. There is electrical noise in each pixel which is accumulated as the current from 1 pixel bucket down to the next. Also, light continues to shine on the CCD as the pixels are read out by the circuitry. This means that the pixel data from the pixel at the bottom of the column is read first and thus has less noise than the pixel at the top. This leads to even more spatial noise across the CCD.

CCD Applications

Digital Cameras

Space Imaging

Star Trackers

2.4 CMOS

Complementary Metal Oxide Semiconductor (CMOS) is an imager similar to CCDs. In CCDs each pixel gathers light and electrical potential is transferred to a number of pixels before becoming an analog voltage. This analog voltage then needs to converted to a digital number. In CMOS detectors each pixel has its own photon to voltage conversion. Many CMOS detectors have amplifiers, noise corrections and an analog to digital converter so that each pixel is output as a digital number.

In CCDs all of the pixel area can be devoted to gathering light making them more sensitive than a CMOS. Also, since all of the pixels are read out through a small number of charge to voltage and voltage to digital number converters the uniformity across a CCD is better than a CMOS detector.

2.5 FPAs

Focal Plane Arrays (FPAs) are similar to CMOS except that CMOS and CCDs are for visible light while FPAs are used for infrared. (Infrared light is invisible to CMOS and CCDs because they are made of silicon.) Like a CMOS, an FPA can have circuitry for each pixel. And like a CMOS uniformity across the array can be a problem.

To correct the non-uniformity of the FPA the FPA can be tested with a uniform light source. Each pixel can be read and compared to an ideal value. The non-uniformities come from a difference in electrical circuitry and responsivity to light from pixel to pixel. This is observed as a change in photon to voltage gain. Each pixel gain difference can be accumulated into a table. This table can then be applied as a correction factor.

3 A Quick Overview of Electro-Optical Sensor Noise

Figure 1: Generic Random Noise Model

All sensors have noise. Electro-optical sensor noise is generally an accumulation of Shot Noise, Johnson (thermal) Noise, Dark Current Noise, and others specific to the sensor such as Beat Noise. All noises are random but Johnson and Dark Current noise have a "fixed" RMS magnitude. Shot noise is signal strength dependent.

See Also

In most of my modeling the sim is too complex to let it run for more than a few seconds of sim time. These few seconds take 10s of minutes, sometimes hours, of real time to complete. In reality the RMS magnitude of the Dark Current and Johnson noise will be approximately constant. However, Shot noise is signal strength dependent and signal strength can change very rapidly. Firgure 1 show a generic random noise model with a fixed RMS magnitude.

For Dark Current and Johnson noise the model in Figure 1 is adequate when you set the Band Limited White Noise block to have an RMS of 1 and the constant block to the RMS value you desire. For Shot noise the constant needs to be replaced with a connection to a time varying source like a gain connected to a signal power measure.