![]() ![]() Often, it is possible to exchange the chopper disc in order to adapt the device to different operation needs. Figure 2: Transmittance of the chopper wheel of Figure 1 for operation with 30 rotations per second and use of the outer row with a beam radius of 500 μm. 10 kHz) without requiring excessive rotation frequencies. On the other hand, chopper discs with hundreds of slits are used to obtain very high chopping frequencies (e.g. In such cases, one may prefer to use a wheel with a lower number of slits, operated with accordingly higher rotation frequency and a larger beam diameter. This can be problematic, however, for beams with high optical power due to the concentrated deposition of heat to the wheel surface. For maximum switching speed, as required for some applications, one can use a tightly focused beam. The switching speed depends on the beam radius and the rotation frequency. The used beam radius influences the switching speed, but also the power handling capability. ![]() In other cases, it is important that the wheel can withstand substantial optical intensities and powers. Problems with scattered light may also be greatly reduced with an appropriate enclosure. Some applications involve the simultaneous use of both rows with dual beams, typically with a frequency ratio of 2.įor some applications, it is important that the coating of a chopper disc is made such that it causes only a minimum amount of light scattering. Normally, the outer row has the highest number of slits. Due to the integer frequency ratios, the term harmonic blades is sometimes used, or otherwise dual frequency blades. The wheel is driven by a small electric motor.Īs shown in Figure 1, there can be two (and rarely more) rows of slits, often with different numbers of holes, so that different chopping frequencies can be achieved with a given rotation frequency. Its central part is a chopper disc (also called chopper blade) which is typically made of a black coated metal, having the shape of a circle with some pattern of holes or slits (slots). The most common form of optical chopper is that of a rotating disc chopper. Rotating Disc Choppers Figure 1: A chopper wheel with two rows of holes. Besides, they may produce disturbing mechanical vibrations of an optical setup. On the other hand, mechanical choppers are limited in frequency and switching speed, and they output partially truncated beams during the switching. Also, simple mechanical choppers can be far cheaper than electronically controlled modulators. This is in contrast to the use of various kinds of intensity modulators, which may exhibit incomplete transmission, beam distortions, effects of nonlinearities etc. The simple mechanical operation principle – letting a beam pass or block it with some blade – makes such devices work for basically any optical wavelength, and beam distortions in the open state are avoided. See below for some examples of applications. ![]() Usually, they are used for periodically modulating the optical power of a beam. There are different kinds of optical chopper devices which can be used for periodically transmitting and interrupting a light beam – for example, a laser beam – with mechanical means. ![]()
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