Fiber Optic Tutorials

This is a continuation from the previous tutorial - junction photodiodes.   The avalanche photodiode (APD) is the solid-state counterpart of the PMT (photomultiplier tube) [refer to the photoemissive detectors tutorial]. An APD versus an ordinary junction photodiode [refer to the junction photodiodes tutorial] is similar to a PMT versus a vacuum photodiode [refer to the photoemissive detectors tutorial]. However, the high-gain and low-noise characteristics of PMTs are difficult for conventional APDs to match. An internal gain is built into an APD to multiply the photogenerated electrons and holes. The physical process responsible for the internal gain in an APD is avalanche multiplication...

This is a continuation from the previous tutorial - photoconductive detectors.   Every junction diode has a photoresponse that can be utilized for optical detection. Junction photodiodes are the most commonly used photodetectors in the photonics industry. They can take many forms, including semiconductor homojunctions, semiconductor heterojunctions, and metal-semiconductor junctions. Similarly to that of a photoconductor, the photoresponse of a photodiode results from the photo generation of electron-hole pairs. In contrast to photoconductors, which can be of either intrinsic or extrinsic type, a photodiode is normally of intrinsic type, in which electron-hole pairs are generated through band-to-band optical absorption. Therefore,...

This is a continuation from the previous tutorial - photoemissive detectors.   Photoconductive detectors are based on the phenomenon of photoconductivity. The conductivity of a photoconductor, which can be an insulator but is usually a semiconductor, increases with optical illumination due to photogeneration of free carriers. The conductivity of a semiconductor that has electron and hole concentrations of \(n\) and \(p\), respectively, is \[\tag{14-63}\sigma=e(\mu_\text{e}n+\mu_\text{h}p)\] where \(e\) is the electronic charge and \(\mu_\text{e}\) and \(\mu_\text{h}\) are the electron and hole mobilities, respectively. In the absence of optical illumination, the conductivity, known as the dark conductivity, \(\sigma_0=e(\mu_\text{e}n_0+\mu_\text{h}p_0)\) because the electron and hole...

This is a continuation from the previous tutorial - photodetector performance parameters.   Photoemissive detectors are based on the external photoelectric effect. Photoelectrons are emitted when the surface of a metal or a semiconductor, known as a photocathode in this situation, is illuminated with light of a sufficient photon energy. The lowest vacuum energy level, \(E_\text{vac}\), for an electron freed from the confinement of a material is higher than the Fermi level in the material. For either a metal or a semiconductor, the energy barrier between the lowest vacuum level and the Fermi level is defined as the work function,...

This is a continuation from the previous tutorial - photodetector noise.   Several parameters are commonly used to define the performance characteristics of photodetectors. These parameters can be considered as the figures of merit of a photodetector. They are used for comparing one photodetector with another and for determining the suitability of a photodetector for a particular application.     Spectral Response Because the response of a photon detector is wavelength dependent, a given photodetector is responsive only within a finite, specific range of the optical spectrum. The spectral range of response for a photodetector is determined by the material,...