Noise dark current

Readout resolution thermal detector noise dark current and amplifier noise... 

QE at lOfim with Vbias Photoconductive gain Gain dispersion Multiplexer read noise Dark current at 9 K Response variation... 

Consider a BIB detector illuminated with a quasi-monochxomatic background. The noise, JV, after collecting photons of energy hv for a time t is a combination of photon noise, dark current noise, and read noise. Taking photon noise to be shot noise (no boson statistics) then ... 

Ideal Performance and Cooling Requirements. Eree carriers can be excited by the thermal motion of the crystal lattice (phonons) as well as by photon absorption. These thermally excited carriers determine the magnitude of the dark current,/ and constitute a source of noise that defines the limit of the minimum radiation flux that can be detected. The dark carrier concentration is temperature dependent and decreases exponentially with reciprocal temperature at a rate that is determined by the magnitude of or E for intrinsic or extrinsic material, respectively. Therefore, usually it is necessary to operate infrared photon detectors at reduced temperatures to achieve high sensitivity. The smaller the value of E or E, the lower the temperature must be. 

Under Httle or no illumination,/ must be minimized for optimum performance. The factor B is 1.0 for pure diffusion current and approaches 2.0 as depletion and surface-mode currents become important. Generally, high crystal quality for long minority carrier lifetime and low surface-state density reduce the dark current density which is the sum of the diffusion, depletion, tunneling, and surface currents. The ZM product is typically measured at zero bias and is expressed as RM. The ideal photodiode noise current can be expressed as follows ... 

Thermoelectrical cooling of the photomultiplier tube at about — 30°C reduces the dark noise current to a very low level. However, as the quantum efficiency of the S-20 type decreases as rapidly as the dark current in the red region, cooling brings only modest increases in the signal-to-noise ratio 23). 

Detect 100% of photons Photon detected as a delta function Large number of pixels Time tag for each photon Measure photon wavelength Measure photon polarization No detector noise fr Up to 99% detected fr One electron for each photon fr Over 377 million pixels 0 No - framing detectors 0 No - provided by optics 0 No - provided by optics 0 Readout noise and dark current... 

Detector noise - The two most signihcant noise sources of a detector are readout noise and dark current. 

The electrons associated with the dark current are also released in a statistical manner and are associated with shot noise. The latter is very sensitive to temperature and also depends on the exposure time. Its contribution to the total noise is significant only at low signal levels. 

The main limitation of photoelectric detectors is the noise cansed by thermal excitation of the carriers from the valence band or from the impnrity levels. If there is a large dark current (a cnrrent generated by the detector in the absence of incident hght), the sensitivity of the photoelectric detector becomes poor (only very intense beams will indnce an appreciable change in the detector condnctivity). In order to rednce the dark cnrrent, photoelectric detectors are nsnally cooled dnring operation. 

As was mentioned before, noise is a term used to describe any random output signal that has no relationship with the incoming signal (the incoming light). In photomultipliers, noise can be classified, depending on its origin, into three types dark current, shot noise, and Johnson noise. The differences between these three classes are explained next ... 

Even in the absence of illumination (darkness) some electrons, excited by thermal energy, are emitted from the photocathode. Since photocathodes are materials with low working functions, the thermal energy can be high enough to induce the emission of electrons. These emitted electrons give rise to what is known as the dark current or, sometimes, the thermo-ionic current. The dark current varies randomly with time, so that it is considered as noise. It has been experimentally determined that the thermo-ionic current, U, due to photoelectrons emitted by a photocathode in the absence of illumination is given by... 

In the particular case of a photocathode, this fluctuation affects both the dark current it) as well as the illumination induced current (/lum)- In the absence of illumination, the only current generated in the photocathode is the dark current, and so the shot noise associated with it is Aif If the light-induced current, /lum. is smaller than the shot noise associated with the dark signal Ai,), then it will be not possible to distinguish any light-induced current. In these conditions, the incident light cannot be detected by the photomultiplier, as it is not possible to separate the noise and the signal. As a consequence, the shot noise associated with the dark current determines the minimum intensity that can be detected by a particular photomultiplier (or by a particular photocathode). This is clearly shown in the next example. 

As we found in Example 3.1, the dark current of this photocathode when operating at room temperature is I, T = 300 K) 2 x 10 a. The minimum current that can be measured is equal to the current dispersion caused by shot noise over the dark current, so that... 

It is important that a photomultiplier gives low noise and a low dark current, i.e. low background signal in the absence of photons, usually caused by thermionic emission of photons from the cathode material. 

Figure 18-6 Errors in spectrophotomefric measurements due to dark current noise and cell positioning imprecision in a research-quality instrument. [Data from L D. Rothman. S. R. Crouch, and J. D. Ingle. Jr.."theoretical and Experimental Investigation of Factors Affecting Precision in Molecular Absorption Spectrophotometry." Anal. Chem. 1975,47, 1226.]...

The slow-scan CCD, also called the scientific CCD, or in the spectroscopy literature simply CCD, is the detector of choice for most applications of Raman spectroscopy. A well-designed CCD has essentially zero dark current, very low readout noise, and high quantum efficiency (peak 45—70% near 700 nm) in the visible region of the spectrum. However, the response drops quickly above 800 nm and there is no photon response above 1.05 J m. For routine spectroscopy or process control, thermoelectrically cooled (to about —40° C) CCDs are adequate. Although these detectors are somewhat noisier than detectors operated at —100° C or lower, the former do not require liquid nitrogen cooling. The general properties and spectroscopic applications of the CCD have been reviewed (22). 

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