Avalanche Photodiode

Definition: Avalanche photodiodes utilize the avalanche multiplication process to achieve higher electric output. The avalanche photodiode resembles a PIN photodiode except for the insertion of an additional p-type multiplying region. Most of the incident photons are absorbed in the intrinsic (‘I’) region and electron-hole pairs are first generated in this region as in the PIN photodiode. The holes drift toward the p-type region (‘P’) and the electrons toward the n-type (‘N’). The electrons, before reaching the n-type region, pass through a p-type multiplying region. If the electrons have undergone sufficient acceleration on reaching the multiplying region, new electron-hole pairs are generated by the collision- ionization process. The newly generated electron-hole pairs themselves generate more electron-hole pairs. The drawbacks to avalanche photodiodes are the high bias voltage needed, and the temperature compensation necessary for stable operation. Definition: Avalanche photodiodes utilize the avalanche multiplication process to achieve higher electric output. The avalanche photodiode resembles a PIN photodiode except for the insertion of an additional p-type multiplying region. Most of the incident photons are absorbed in the intrinsic (‘I’) region and electron-hole pairs are first generated in this region as in the PIN photodiode. The holes drift toward the p-type region (‘P’) and the electrons toward the n-type (‘N’). The electrons, before reaching the n-type region, pass through a p-type multiplying region. If the electrons have undergone sufficient acceleration on reaching the multiplying region, new electron-hole pairs are generated by the collision- ionization process. The newly generated electron-hole pairs themselves generate more electron-hole pairs. The drawbacks to avalanche photodiodes are the high bias voltage needed, and the temperature compensation necessary for stable operation.

Alternative definition: An avalanche photodiode (APD) is a type of semiconductor photodetector used to detect low levels of light in applications such as fNIRS. It is based on the principle of avalanche multiplication, which occurs when an electron in the photodiode collides with an atom and generates a cascade of secondary electrons, resulting in a much larger current than would be caused by a single photon.

Synonym:

References: E. Hui Pan. Fiber Optics Weekly Update. Information Gatekeepers Inc.

Related terms: Photomultiplier tube, Detector, Sensor, Silicon Photomultiplier  

Optode

Definition: An optode is the distal end of an fNIRS system which interfaces with the surface of the tissue being measured. Optodes can contain a single source (source optode), a single detector (detector optode), or a source accompanied by a short-separation detector.Definition: An optode is the distal end of an fNIRS system which interfaces with the surface of the tissue being measured. Optodes can contain a single source (source optode), a single detector (detector optode), or a source accompanied by a short-separation detector.

Alternative definition:

Synonym:

References: https://doi.org/10.1016/j.neuroimage.2005.08.008

Related terms: source; detector; emitter; emission, receiver, transmitter  

Optode array

Definition: Relative arrangement of linked sources and detectors on a region of interest. Definition: Relative arrangement of linked sources and detectors on a region of interest.

Alternative definition: A schematic arrangement of sources and detectors to create a pattern of measurement channels.

Synonym: Probe array; fNIRS array; Channel configuration; fNIRS-sensor configuration; Source–detector array, fNIRS Montage

References: https://doi.org/10.1117/1.NPh.5.3.035010

Related terms: Probe design; Optode placement, topographic layout, Montage  

Power supply

Definition: Power supply is an electronic device that provides a stable voltage, current, and frequency required for other devices to function correctly. This device maintains a stable rated wattage output consistently.Definition: Power supply is an electronic device that provides a stable voltage, current, and frequency required for other devices to function correctly. This device maintains a stable rated wattage output consistently.

Alternative definition:

Synonym:

References: https://doi.org/10.1109/TPEL.2019.2941528

Related terms:  

Probe

Definition: A probe or probeset is the arrangement of optodes on the head. Probes can be placed according to the international 10-20 system and/or other anatomical landmarks. The optodes (emitters and detectors) are typically located in a grid with fixed inter-optode-distances for long-distance and short-distance channels.Definition: A probe or probeset is the arrangement of optodes on the head. Probes can be placed according to the international 10-20 system and/or other anatomical landmarks. The optodes (emitters and detectors) are typically located in a grid with fixed inter-optode-distances for long-distance and short-distance channels.

Alternative definition:

Synonym: probeset

References:

Related terms: topographic layout, optode placement, fNIRS caps, inter-optode-distance  

Sampling frequency

Definition: The number of samples per second collected from a continuous signal through analog-to-digital conversion.Definition: The number of samples per second collected from a continuous signal through analog-to-digital conversion.

Alternative definition: Number of data samples collected per unit of time (in Hz) from each channel.

Synonym: Sampling Rate

References: https://doi.org/10.3389/fpsyg.2017.00093

Related terms:  

Saturation

Definition: In terms of a photodiode, if the incident optical power continues to increase but the photocurrent output stops growing, it indicates that the photodiode has reached saturation. The saturation limit can be adjusted by reducing the resistance seen by the load or changing the reverse bias voltage applied. The saturation point represents the upper boundary of the photodiode’s linear range, while the lower boundary of that linear range is determined by the noise. Beyond this range, the photodiode operates in a nonlinear manner. Definition: In terms of a photodiode, if the incident optical power continues to increase but the photocurrent output stops growing, it indicates that the photodiode has reached saturation. The saturation limit can be adjusted by reducing the resistance seen by the load or changing the reverse bias voltage applied. The saturation point represents the upper boundary of the photodiode’s linear range, while the lower boundary of that linear range is determined by the noise. Beyond this range, the photodiode operates in a nonlinear manner.

Alternative definition:

Synonym: photocurrent output limit

References: https://doi.org/10.1364/OE.422478

Related terms: Photodiode  

Source-detector separation

Definition: Source-detector separation (SDS) refers to the physical distance between each fNIRS source-detector pair of optodes. It determines the signal-to-noise ratio (SNR), the number of measurable photons, depth sensitivity to the change in hemoglobin, the allowed amount of sources/detectors/channels, and the size of covered regions. Depending on the population being studied (e.g., adults vs. infants) and the purpose of the channels (e.g., measuring oxygenated hemoglobin in the brain vs. removing the confounding effects of superficial tissue physiology), the optimal SDS will vary. In a reflection geometry, larger SDS increases depth sensitivity (i.e., to deep regions further from the surface), at the cost of reduced total detected intensity. Definition: Source-detector separation (SDS) refers to the physical distance between each fNIRS source-detector pair of optodes. It determines the signal-to-noise ratio (SNR), the number of measurable photons, depth sensitivity to the change in hemoglobin, the allowed amount of sources/detectors/channels, and the size of covered regions. Depending on the population being studied (e.g., adults vs. infants) and the purpose of the channels (e.g., measuring oxygenated hemoglobin in the brain vs. removing the confounding effects of superficial tissue physiology), the optimal SDS will vary. In a reflection geometry, larger SDS increases depth sensitivity (i.e., to deep regions further from the surface), at the cost of reduced total detected intensity.

Alternative definition: The inter-optode distance is the spatial distance between a source and a detector. In circular geometries it is defined as the length of the chord between the optodes.

Synonym: Source-detector separation range, Source-detector separation, inter-optode distance

References: https://doi.org/10.1117/1.NPh.2.2.025005

Related terms: short-channel separation, source-detector pair  
en_USEnglish