An optical attenuator is one of very important elements of an optical circuit for an optical communication to control an optical signal transmission. The optical attenuator performs an add/drop function to equalize an optical power of channels of a wavelength division multiplexing (WDM) system. Optical attenuators provide a variety of useful functions. One function of attenuators is to reduce the intensity of optical signals entering a photosensitive device to preclude device damage and/or overloading. Photosensitive components are affected by variations in light intensity. Therefore, an attenuator causes the light intensity to be within the dynamic range of the photosensitive components. By using an attenuator, damage to the component is precluded. Optical attenuators are used to reduce the power of optical signals from an input fiber to an output fiber, and especially to balance optical power between several lines of an optical system. As the optical communication technology advances, the optical communication is conducted through a longer optical path and a more complex line. An optical line taking various paths is required to be connected to an optical communication apparatus and an optical transmission apparatus to process the optical signal. The intensity of light varies according to the path taken by the line. If the lines which differ from each other in the amount of light are connected together, the deterioration of the S/N ratio may occur to reduce the optical transmission quality. Many optical attenuators are also capable of actively attenuating an optical signal. In other applications, attenuators may serve as noise discriminators by reducing the intensity of spurious signals received by an optical device to a level below the device response threshold. Attenuators are also used to simulate the operation of optical systems without the need for long lengths of optical fibers. Optical signal attenuation can be accomplished in a variety of ways through diverting all or a portion of an optical signal from an original pathway. The diverted optical signal can be discarded for performing such operations as in filtering or can be transferred to one or more additional pathways for performing such operations as switching, splitting, and coupling. Optical attenuators that are currently used in operating optical systems either change the gap between two fiber optic cable ends or change the alignment between two fiber optic cable ends. The amount of attenuation obtained by these procedures depends on a number of different factors. These factors include fiber optic cable end shape and surface finish. The optical attenuator includes a fixed optical attenuator which attenuates a fixed amount of the light, and a variable optical attenuator which attenuates a variable amount of the light. Optical attenuators are classified into two types, a fixed optical attenuator for reducing an optical power by a fixed amount of attenuation and a variable optical attenuator (VOA) capable of attenuating an optical power by a varied amount of attenuation based on user's requirements. Fixed attenuators, however, are oftentimes undesirable for attenuator applications as the commercially available attenuation values only approximate the precise attenuation required and/or many different valued fixed attenuators are required.

A variable optical attenuator (VOA) is a device designed to attenuate an intensity or power level of an input optical beam in a controlled manner to produce an output optical beam with different attenuated intensities. Variable optical attenuators play an important role in the implementation of modern information networks having optical interconnects. In fiber optic communication systems, variable optical attenuators are broadly employed to regulate the optical power levels to prevent damages to the optical receivers caused by irregular optical power variations. As the optical power fluctuates, a variable optical attenuator is employed, in combination with an output power detector and a feedback control loop, to adjust the attenuation and to maintain the optical power inputted to a photo-receiver at a relatively constant level. A variable optical attenuator (VOA) is typically used to control the intensity of selected light signals and thereby to maintain each light signal at substantially the same light intensity. Variable optical attenuators are used to attenuate light beams in optical systems such as fiber optic communication systems. Controllably attenuating a light beam can be achieved by various methods, such as by passing the beam through a variable-attenuation filter, radially bending a fiber loop to vary the optical loss within the loop, thermally changing the refractive index of cladding material, and partially blocking the beam by inserting a beam block into the light beam path. VOAs are used for gain control of optical amplifiers in wavelength division multiplexed (WDM) networks, for dynamic channel power regulation and equalization in cross-connected nodes, channel blanking for network monitoring, and signal attenuation to prevent detector saturation. A variable optical attenuator may be placed to regulate the optical output of a laser diode transmitter without changing its driving current so that the current-dependent laser wavelength and line width are not changed by such power regulation. A VOA may be placed between two erbium doped fiber amplifiers (EDFA) to compensate for the gain tilt caused by the variation in the pumping power. VOAs may be used with a demultiplexer to individually adjust the power levels of different optical channels to form a dynamic gain equalizer for applications such as maintaining signal quality at all optical channels in entire optical networks, especially in long haul networks where multiple stages of optical amplifiers are installed. Furthermore, VOAs may be used in connection with optical add/drop multiplexer (OADM) and optical cross-connect (OXC) switch sites to compensate for the power variation caused by a change in the optical channel number. A variable optical attenuator is generally designed to adjust the power ratio between a light beam exiting the device and a light beam entering the device over a variable range. With a VOA, the amount of attenuation may be controllably varied. Variable optical attenuators are commonly formed of a blocking structure disposed in a free space region between an input waveguide and an output waveguide. The position of the blocking structure within the free space region determines the amount of attenuation. A variable optical attenuator can be either an electrical or a manual attenuator. An electrical variable optical attenuator comprises an electrical controlling cell. The electrical variable optical attenuator is more widely used in optical transmission systems and optical networks, because it can control reduction of intensity of optical signals more precisely than a manual variable attenuator. The electrical variable optical attenuator typically comprises an electrical controlling unit.

Variable optical attenuators have been developed with a variety of technologies. Currently, there are several types of commercially available variable optical attenuators in the market, they are opto-mechanical VOA devices using stepping motor or magneto-optical crystal, and variable optical attenuators based on waveguide technology, using liquid crystal (LC) technology, and using micro-electro-mechanical systems (MEMS) technology. An MEMS optical attenuator is realized by forming a microstructure acting as an actuator on a substrate such as silicon by using a thin film processing technology. Mechanical variable optical attenuators adjust optical attenuations through controlling optical coupling efficiency by moving fibers, mirrors, polarizers, and so forth. A MEMS variable optical attenuator generally includes one or more mirrors manufactured on a chip. A mirror on the MEMS device is generally in a tilting angle that can be controlled by a control variable, such as a voltage variable or a current variable. Variable optical attenuators controlled by micro electromechanical system (MEMS) provide downsized arrays of variable optical attenuators, which can be applied to a miniaturized module. The opto-mechanical optical attenuator utilizes a technique which spaces two optical fibers at a small interval from one another and mechanically moves a variable absorption filter or a shielding film over the interval, to thereby absorb or shut off a portion of incident light, or a technique which collocates two side-polished optical fibers with each other and controls an interval therebetween, to thereby adjust an intensity of light to be transmitted through. The opto-mechanical variable optical attenuators are capable of providing consistent and stable attenuation by using stepping motor or magneto-optic crystal to drive a shutter or light blocker into a light beam to obstruct part or all of the light power. However, they can not be minimized to meet the needs of high channel-count integration due to the bulky size of the stepping motor or the electromagnetic coil. Electro-optic attenuators or thermo-optic attenuators modify optical attenuations according to refractive indices of specific materials therein altered by imposing different electric fields or different temperatures, respectively. Hence, one of the utmost issues for manufacturing electro-optic or thermo-optic attenuators is to supply adequate optical applied materials. The thermo-optic optical waveguide type of optical attenuator is implemented through the use of operation characteristics of a thermo-optic modulator or thermo-optic switch, which may be used in an integrated optical circuit. The thermo-optic tunable attenuator using silica or polymer has been published, which can precisely control an optical attenuation degree in an operation speed of milliseconds, a polarization independence and powers less than several hundreds mW. The waveguide and liquid crystal variable optical attenuators, while being suitable for high channel-count integration, are lack of consistent and stable attenuation expressed in the form of high insertion loss (IL), high polarization dependent loss (PDL), high polarization mode dispersion (PMD) and sensitivity to ambient temperature.