Silicon photodiode

Silicon photodiodes exhibit maximum sensitivity at about 800 nm and they can be used in the whole visible range however their sensitivity drops by several times at the blue region. Special structures can be made with enhanced blue sensitivity (so-called blue or UV diodes). Germanium photodiodes are capable to detect radiation from 600 nm up to 1700 nm. In telecommunication applications InGaAs elements are widely used. 

The construction of the optoelectronic interface can be based on a silicon photodiode since analytical and reference wavelengths are from the visible and the IR regions, respectively. The signals can be filtered out by optical filters (then two photodiodes are required) or one photodiode can be synchronised with modulation waves of the LEDs used. Finally, silica optical fibres can be used as light waveguides. The choice between single fibre or bundle is determined by the application of the sensor. 

This interferometric dilatometer consists of a rather simple and small Michelson interferometer, in which the two arms are parallel, and of a 4He cryostat, in which the sample to be measured is hold. The sample is cooled to 4 K, and data are taken during the warm up of the cryostat. The optical path difference between the two arms depends on the sample length hence a variation of the sample length determines an interference signal. The Michelson interferometer consists of a He-Ne stabilized laser (A = 0.6328 xm), two cube corner prisms, a beam splitter, three mirrors and a silicon photodiode detector placed in the focal plane of a 25 mm focal length biconvex lens (see Fig. 13.1). 

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A p-type electrode is in depletion if a cathodic bias is applied. Illumination generates one electron per absorbed photon, which is collected by the SCR and transferred to the electrolyte. It requires two electrons to form one hydrogen molecule. If the photocurrent at this electrode is compared to that obtained by a silicon photodiode of the same size the quantum efficiencies are observed to be the same for the solid-state contact and the electrolyte contact, as shown in Fig. 4.13. If losses by reflection or recombination in the bulk are neglected the quantum efficiency of the electrode is 1. 

Figure 7.6. Detector efficiency for photodiodes. Spectra for (A) a silicon photodiode and (B) a PIN silicon photodiode are shown. The peak wavelength response of both detectors is at 960 nm,...

The first measurement we make when starting a fluorescence study is not usually a fluorescence measurement at all but the determination of the sample s absorption spectrum. Dual-beam differential spectrophotometers which can record up to 3 absorbance units with a spectral range of 200-1100 nm are now readily available at low cost in comparison to fluorimeters. The wide spectral response of silicon photodiode detectors has made them preeminent over photomultipliers in this area with scan speeds of a few tens of seconds over the whole spectral range being achieved, even without the use of diode array detection. 

The use of a linear detector array in the image plane of a polychromator in place of the fluorescence monochromator in Figure 12.1 enables the parallel data accumulation of complete fluorescence spectra. Silicon photodiode arrays, operated in a CCD mode(34) are the most widely used detector elements. The spectral response of the diodes enables fluorescence to be detected from the near-UV up to ca. 1100 nm with a peak response in the near-IR. Up to 8192 elements are now available commercially in a single linear array at low cost. However, the small length of each element (ca. 10 [im) presently limits sensitivity and hence cylindrical lens demagnification is often necessary. 

Alternatively, a physical detector, such as a silicon photodiode, a vacuum photodiode, or a thermopile can be chosen for the measurement of light intensity. Such instruments may either monitor the incident light... 

The classical silicon photodiode linear array manufactured by Reticon was the first detector marketed successfully. Similar solid state linear array detectors based on charge coupled devices, or charge.injection devices may also be of interest. Typical of the families of detectors, the Reticon detectors are built in a number of elements/array sizes. Commercially available units have anywhere from 128 to 1024 elements/array. Each individual element in the array is 1 x 1 mil to 1 x 100 mil in area, and are spaced on approximately 1 mil centers. The spectrometer system discussed in this article was built using Reticon-type devices. 

We have used a Reticon photodiode array and an Optical Multi-channel Analyzer to measure optical density changes Induced in photovisual materials using this technique. The Reticon is a device which incorporates a linear high density monolithic array of silicon photodiodes with integrated scanning circuits for serial readout. The array consists of 128... 

Detection of the particles is accomplished by means of their attenuation of a light beam provided by a halogen lamp and collimated by two slit assemblies. A silicon photodiode and an operational amplifier circuit are used to monitor the light intensity. Adjustable gain and offset potentiometers are located on the control module. The signal can be displayed on a stripchart recorder and is monitored by means of a digital panel meter. 

As an example, linear arrays of silicon photodiodes (photodiode arrays) are available as a package complete with the necessary circuitry to read out the array following exposure. Arrays are available with up to 2048 elements, and they are read out on an element by element basis. The scanning circuitry can access an element in 10 - 25 /rs, which suggests that the entire array can be read out in approximately 20 - 50 ms. Thus modulation frequencies can not exceed 50 Hz. However, as was discussed earlier, PEMs are driven at a resonant frequency which usually varies between 50 and 100 kHz, and this exceeds the readout rate of the photodiode array by three orders-of-magnitude. 

A 100-W immersion heater (such as VWR Scientific Inc. 33897-140 a 200-W to 300-W tea/coffee immersion heater can also be used), controlled by a Variac magnetic stirrer and stir bar 1000-mL beaker thermometer with a resolution of 0.02 K (platinum resistance thermometer or calibrated thermistor such as Vernier model TMP-BTA) 1- to 5-mW He-Ne laser (633 nm) or a red (545 mn) or green (532 nm) laser — a battery-operated laser pointer is suitable, but a module with a separate 3-volt power supply, available from Z-Bolt and other sources, will give better power stability for long rans two Polaroid sheets in rotation holders photodetector such as photomultiplier tube or Thorlabs model 201/579-7227 silicon photodiode analog-to-digital data collection system such as Vernier LabPro. 

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