# M

### MBE (molecular beam epitaxy)

An epitaxial growth technology for forming a thin-film crystal on a heated substrate crystal by supplying molecular beams of crystal-constituent elements, which are created by evaporating each element from separate cells in an ultra-high vacuum.

### MCA (multichannel analyzer)

An instrument that displays the heights of multiple input pulses as a histogram.

### MEMS (micro-electro-mechanical systems)

MEMS is a system for integrating electronic circuits with microactuators, micromechanisms, and microsensors, etc. fabricated using fine processing technology.

### Microfocus X-ray source

This is an X-ray source with a very small focal spot on the order of micrometers. Microfocus X-ray sources allow capturing clear, sharp magnified X-ray images since edge blur is suppressed even when magnified.

### Minimum sensitivity

The minimum light input power required to acquire bit error rates within a certain level. The minimum sensitivity differs depending on the transmission bit rate, the required bit error rate, the type of pseudo-random codes used for measurement, and the extinction ratio, etc.

### MOCVD (metal organic chemical vapor deposition)

An epitaxial growth technology for forming a thin-film crystal on a heated substrate crystal by thermally decomposing and chemically reacting an organic metal supplied in the form of vapor, which is solid or liquid at normal temperatures and pressures.

### Modulation transfer function (MTF) - CCD

The modulation transfer function (MTF) is commonly used to quantify the resolution of an image sensor that reproduces the contrast at a certain spatial frequency of the scene. Since the photosensitive area of a CCD consists of discrete pixels, it exhibits a limiting resolution determined by the Nyquist limit based on the discrete sampling theorem. For example, when a black-and-white pattern is viewed with a CCD, the difference between the black and white signal levels decreases as the pattern becomes finer, finally reaching the point at which the pattern cannot be resolved. The ideal MTF is expressed as follows: $$MTF = sinc \biggl( \frac{\pi \cdot f}{2 \cdot fn} \biggr)$$ where f and fn are the spatial frequency and spatial Nyquist frequency of the scene, respectively.

However, because of the difficulty of creating an optical sine wave, a test pattern that provides a square wave is generally used. In this case, the spatial frequency response is called the contrast transfer function (CTF) to distinguish it from the MTF. (Note that the CTF can be converted into the MTF by means of a Fourier transform.)

Actual CCD resolution is determined by the extent of diffusion occurring before the signal charge collects inside the silicon. When the incident photons are absorbed within the depletion layer, the generated charge does not diffuse and is collected by the corresponding pixels. Consequently, the resolution does not deteriorate. In other words, the resolution depends on the depth in the silicon where the incident photons are absorbed. The longer the incident photon wavelength, the more the resolution deteriorates.

### MOEMS (micro-opto-electro-mechanical systems)

Optics systems using MEMS technology. Also called optical MEMS.

### MOST (Media Oriented Systems Transport)

An in-vehicle network standard mainly used in European vehicles. This is a ring type network using POF (plastic optical fibers), and its internode communication speeds are 25 Mbps and 150 Mbps. Besides in-vehicle applications, MOST is used for interphone networks in airports, etc.

### MPP (multi-pinned phase) operation

MPP is an operation mode in which all CCD channels under the MOS structure gates constituting the CCD electrodes are reversed. This mode is also called reverse operation and reduces the dark current because it drastically suppresses the generation of thermally excited electrons at the silicon-silicon dioxide interface.

### Multimode fiber

An optical fiber for transmitting light in multiple transverse mode (electromagnetic field distribution). Multimode fibers are not suitable for long-distance transmissions because the transmitted waveform is distorted due to differences in the signal light arrival time depending on the mode (modal dispersion). Compared to single-mode fibers, the core diameter is large so that connecting to a light emitter is easy. However, condensing a light beam onto a light receiver element for high-speed communication is difficult because the element must have a small light receiving area.

### Mu metal

A shield made up of high permittivity metal. It is used to shield a photomultiplier from magnetic fields.