Total Solution for On-line

Measurement and Control System

Palma TechnologyResearch CenterTechnical Info
APD
¢ÙAPDs(Avalanche Photodiodes)

Avalanche Photodiode is a high velocity and sensitivity photo detector through internal gain mechanism using a reverse voltage. It is widely being used for measurement of a small quantity of light, such as remote distance measurement and high sensitivity communication using optic fiber, especially when light has single photons. If the photo energy is higher than band gap energy of an element when light reaches a detector, electron hole is formed. At this point, the photo energy,E (eV) is inversely proportional to wavelength.

   

When electron-hole in depletion layer is formed by reverse voltage applied on PN junction, light moves through electric field and PN junction between N+ region and P+ region. Its moving velocity depends on the movement of electron-hole based on electric field strength. With the electric field appropriately increasing, charge carriers approach crystal lattice moving at more than average movement velocity. This phenomenon occurs at about electric field of 104V/cm, and movement velocity of charge carriers get saturated at 107V/cm.

¡ªFig. 1. Schematic diagram of avalanche process

Dark current generates when surface leakage current (Ids) flows into PN junction and current inside of an element (Idg) emits into avalanche regions. Surface leakage current is not multiplied by not flowing into avalanche region, while current produced inside of element is multiplied by flowing into it. A total dark current of APD is related to the parameter, multiplication (M) as follows.

   

Figure 2 shows the dark current varies with reverse voltage. The reason why Ids does not increase along with increasing reverse voltage is because of its breaking-down.

¡ªFig. 2. Dark current vs. reverse voltage
¡ªFig. 3. Terminal capacitance vs. reverse voltage
¡ªFig. 4. Typical peripheral circuit
PMT
¢ÙPMT(Photomultiplier)

A PMT is called a photomultiplier tube, generally used for detection of visible lights. When electrons fall upon a solid surface, they reflect on the surface, which cause other electrons within the solid to eject from the surface, emitting energy. This phenomenon is referred to as secondary electron emission. The tube is an electron tube amplifying infinitesimal photo current through using this principle, called secondary electron multiplier tube. As shown in the figure, a PMT consists of a photocathode that emits electrons when exposed to light and a series of secondary electron emissions, (dynodes), such as CS-Sb, Cs-CsO-Ag, and MgO in an evacuated glass enclosure. Photoelectrons are sequentially directed toward secondary electron emissions from 1 to 9 as indicated by the arrows, multiplied up to 106 times and collected by the anode as an output signal. This method provides low noise output, compared to multiplication that can be done by using transistor or vacuum multiplier, and is fit for detecting infinitesimal light. It is used in wide variety of applications such as flying spot camera for transmission of films or slides in TV, spectrophotometer and other industrial measuring system.

¡ªSectional structure
| CPM
¢ÙCPM(Channel Photomultiplier)

There are a wide variety of methods of measuring light and types of sensor, which are selectively used depending on the purpose of use. Because wrong selection results in much trouble, it is very important to select the most optimal sensor. Among several detectors, CPM (Channel Photomultiplier) is a high sensitivity photo detector that can be replaced by PMT (Photomultiplier) and APD (Avalanche Photodiodes).

This product of a small size provides extremely high gain value and dynamic range, ultra low noise and fast response by using unique detector principle, and is characteristically used for analytical purpose in radiation spectrum, fluoroscopy, atomic absorption spectrometer and biochemical luminescence. Also this high performance product is available in wide applications: bioscience products, industrial and medical apparatus and high-energy physics. Compared to PMT, CPM enhances anode sensitivity while lowering noise from single photons in two orders. Noise level shows extreme stability to time without crack. Ultra low noise results in higher dynamic range than PMT, extending sensible limit to variety of applications.

CPM can be used in analog-DC mode, single photon counting mode, and nuclear spectroscopy. (When connected to luminous materials such as BGO, LSO, and Nal) It provides photocat hodes for selection of window materials and spectrum ranging from 115nm (UV range) to 850nm (NIR), and is of a capsule type of 10.mm in diameter, the same size as that of a chalk.

CPM, as do all the photomultiplier tubes, detects ultra low-level light at semi-transparent photocathode, which is connected to the inner side of window, causing it to emit electrons. The photoelectrons pass through narrow semi-conductor channel on their way from cathode to anode. Each time the electrons strike the sidewall of circular channel, many secondary electrons emit from the wall. This phenomenon repeatedly occurs along the passage, inducing multiplication effect of 108A/W above.

| CCD
¢ÙCCD(Charge Coupled Device)

A CCD consists of an array of infinitesimal pixels, which enable to convert the light that passes through lens into charges. Pixels function as condenser. The position of a pixel and the size of a charge, hence, enable it to obtain light intensity data. Attention should be paid to the fact that only light¡¯s intensity, not color information, can be inferred from CCD itself.

When CCD outputs data, analog data are only produced. So they require to be converted by A/D converter into digital data. This requires a certain time to convert it from imaging element depending on the scanning time in scanning mode. CCD accumulates charges and releases them once if directed to do so. Until that time, it keeps storing light. Using this is an electronic shutter. It accumulates charges for a desired time and then releases them instantly. This concerns the speed of shutter. However, because it is difficult to intercept light with CCD only, so a mechanical shutter can be used according to CCD scanning method.

CCD distinguishes the images by digitizing the difference in their charges. The resolution can vary with how much the difference is digitized. Negative charge is generated, even when light does not fall upon CCD. The higher CCD temperature, the more the negative charge generated. Even though negative charges are added, the methods of increasing absolute quantity of charges from light through lens can be considered so that the error may be enough disregarded. To reduce the error, a large quantity of light, anyhow, should be collected.

But too much quantity of light also brings about a trouble. Each pixel in CCD is allowed to have quantity of charges that can be accumulated at maximum. Too much light causes charges to get saturated, thus disabling to differentiate the difference between charges. Therefore, it is required CCD should have an optimal quantity of light, which determines sensitivity, necessitating the conditions of shutter speed, including diaphragm, in order to control the quantity.