Variable optical attenuator working principle

Variable optical attenuator (VOA) has a wide range of applications in optical communications. In recent years, a variety of technologies for manufacturing variable optical attenuator have emerged, including mechanical VOA, magneto-optical VOA, liquid crystal VOA, MEMS VOA, thermal Optical VOA and acousto-optic VOA, etc.

Variable optical attenuator (VOA) has a wide range of applications in optical communications, and its main function is to reduce or control optical signals. The most basic characteristic of the optical network should be adjustable, especially with the application of DWDM transmission system and EDFA in optical communication, gain flattening or channel power equalization must be carried out on multiple optical signal transmission channels. The end must carry on the dynamic saturation control, the optical network also needs to control other signals, these all make VOA an indispensable key component among them. In addition, VOA products also have the characteristics of combining with other optical communication components and pushing them to higher-end modules.

Mechanical VOA

There are also many specific implementations for this type of VOA. Figure 1 is a schematic diagram of a light-blocking fiber attenuator. The light-blocking element is driven to be blocked between two collimators to achieve optical power attenuation. The light blocking element can be sheet-shaped or cone-shaped, the latter can be propelled by rotation, and the former needs to be pushed horizontally or through a certain mechanical structure to realize the conversion from rotation to horizontal pushing action. The light-blocking fiber optic attenuator can be made into an optical fiber adapter structure, or it can be made into an in-line structure as shown in FIG. 1.

Similar to the light-blocking VOA mentioned above, there is also an EVOA solution in the form of a mechanical potentiometer. The principle is to use a stepper motor to drag a neutral gradient filter, and when the light beam passes through different positions of the filter, its output light power will change according to a predetermined attenuation law, so as to achieve the purpose of adjusting the attenuation. There is also a mechanically polarized fiber attenuator. The basic principle is that the light beam emitted from the inlet port is reflected by the reflector to the outlet port, and the reflection coupling efficiency between the two ports is controlled by the tilt angle of the reflector, thereby realizing the adjustment of the light attenuation. The tilt of the reflector is controlled by a variety of different mechanisms.

Mechanical fiber attenuators are a more traditional solution. So far, most of the VOAs that have been used in systems use mechanical methods to achieve attenuation. This type of fiber optic attenuator has the advantages of mature technology, good optical characteristics, low insertion loss, low polarization-related loss, and no need for temperature control; but its disadvantages are large size, complex structure of components, low response speed, and difficulty in automated production , Is not conducive to integration and so on.

Magneto-optical VOA

Magneto-optical VOA utilizes the changes in optical properties exhibited by some substances under the action of a magnetic field. For example, the magneto-optical rotation effect (Faraday effect) can also achieve the attenuation of light energy, thereby achieving the purpose of adjusting the optical signal. A typical polarization-independent magneto-optical VOA structure is shown in Figure 2.

In Figure 2, (a) is the actual optical path. In order to better explain its principle, we use the mirrored optical path in (b). When the light enters from one end of the dual-core fiber, it is collimated by the lens (omitting the thickness of the beam), enters the birefringent crystal (its optical axis is perpendicular to the paper), and is divided into two beams of O light and E light, and then Entering the Faraday rotator, the light exits the Faraday rotator and is reflected by the total reflection mirror, then passes through the Faraday rotator, birefringent crystal and lens in turn, and finally output from the other end of the dual-core fiber. Therefore, by modulating the voltage to control the magnetic field, the polarization state of the polarized light entering the Faraday rotator can be rotated. When the Faraday rotation angle is 0 degrees, the O light is still O light, and the E light is still E light. The two beams are not parallel and cannot be combined. As shown by the dotted line, the attenuation degree is the largest at this time; in Faraday rotation When the angle is 45 degrees, the total Faraday rotation angle is 90 degrees, the O light becomes E light, and E light becomes O light. The two beams are parallel and combined after being focused by the lens. At this time, the degree of attenuation is minimal. When the Faraday rotation angle is controlled to continuously change between 0 degrees and 45 degrees, continuous adjustment of the attenuation can be achieved.

Using the magneto-optical effect of the material and combining with other technologies, an optical attenuator with high performance, small size, high response and relatively simple structure can be produced. This is an area to be further developed in the use of discrete micro-optic device technology to make optical attenuators.

 LCD VOA

Liquid crystal VOA utilizes the birefringence effect exhibited by the anisotropy of liquid crystal refractive index. When an external electric field is applied, the orientation of the liquid crystal molecules will be rearranged, which will cause its light transmission characteristics to change. Its working principle is shown in Figure 3.

The specific implementation of the liquid crystal VOA is shown in Figure 4. The light incident from the incident fiber is collimated by the collimator, enters the birefringent crystal, and is divided into O light and E light whose polarization states are perpendicular to each other. After passing through the liquid crystal, the O light becomes E light, and E light becomes O light. The beam is combined by another birefringent crystal, and finally output from the collimator. When a voltage V is applied to the transparent electrodes at both ends of the liquid crystal material, both the O light and the E light change a certain angle after passing through the liquid crystal. After the second birefringent crystal, each light is divided into O light and E light, forming 4 Two beams of light, the middle two beams are finally combined into one beam to exit from the second birefringent crystal and received by the collimator, and the other two beams are not received by the collimator after exiting from the second birefringent crystal, thereby achieving attenuation. Therefore, by applying different voltages on the two electrodes of the liquid crystal to control the change of light intensity, different attenuations can be achieved.

The liquid crystal VOA can realize the miniaturization and high response of the optical attenuator. But at the same time, the insertion loss of the liquid crystal material is relatively large, and the manufacturing process is relatively complicated, especially due to the greater influence of environmental factors. Its advantage is low cost, and it has been commercialized in batches. There are other functional materials that also change their optical properties under the action of a strong electric field, such as the electro-optical effect of lithium niobate (LiNbO3) crystals, so this is also a possible way to use. However, since electro-optical effects like this usually require a strong electric field of several thousand volts or even tens of thousands of volts, their application in the field of passive devices for optical communications has certain limitations, and there is little relevant information so far.

MEMS VOA

MEMS is a relatively new application technology in this field. After several years of development, the production process of MEMS Chip has become mature, which has strongly promoted the application of MEMS VOA. In the optical network application, products based on MEMS technology also have obvious price and performance advantages. MEMS VOA has reflective VOA and diffractive VOA, as shown in Figure 5.

The working principle of reflective VOA is shown in Figure 5(a), which is to fabricate a micro-mirror on a silicon substrate. Take the unblocking VOA as an example. The light enters through one end of the dual-fiber collimator and is incident on the micro-mirror at a certain angle. When a voltage is applied, the micro-mirror is twisted under the action of static electricity, the inclination angle changes, and the incident angle of the incident light changes. After the light is reflected Energy cannot be completely coupled into the other end of the dual-core collimator to achieve the purpose of adjusting the light intensity; when no voltage is applied, the micro-mirror is in a horizontal state, and the energy is completely coupled into the other end of the dual-core collimator after light reflection.

Diffractive VOA is based on dynamic diffraction grating technology, as shown in Figure 5(b). This kind of dynamic diffraction grating is composed of parallel micro-grid bar array. The upper surface of the micro-grid bar is coated with 200~300 nm thick aluminum film, which plays the dual role of electrode and reflected light. The lower surface is specially designed and formed by Si3N4 and SiO2 film. The double spring structure provides elastic force, and the thickness of the air gap etched below is related to the desired spectral band. When a voltage signal is applied, under the action of electrostatic force, the positions of the spaced moving gratings move downward to produce a diffraction grating effect. The working state is shown in Figure 5(b). The first-order diffracted light is controlled by adjusting the voltage to achieve the purpose of adjusting the attenuation of the optical signal. This kind of dynamic diffraction grating was first applied in imaging and display technology. It has the characteristics of fast response speed, high attenuation control accuracy, large extinction coefficient, fatigue resistance, etc., and can be used to make many other optical communication devices. Core components, such as optical switch arrays, etc.

MEMS VOA is very mature, and has been mass-produced and applied on a large scale. At the same time, because of the yield problem, the price is also facing challenges. In addition, because of the micro-electromechanical components, the reliability is sometimes not ideal. Early MEMS VOA used laser welding, which required a large investment in equipment, low production efficiency and high assembly costs. At present, the market has also introduced the all-glue process MEMS VOA, which has solved this problem well.

At present, foreign manufacturers that can mass produce MEMS VOA mainly include: Lightconnect (acquired by Neophotonics), JDSU, Oplink, Avanex, Santec, Lightwave2020, AFOP, etc. In China, Gaoyi Communication Co., Ltd. has the ability to mass produce MEMS VOA, and has a technology platform for laser welding and all-glue. The main products include single VOA devices, 4-channel and 8-channel VOA modules, as shown in Figure 6.

Thermo-optical VOA

Thermo-optic VOA mainly utilizes the optical properties of some materials in the temperature field, such as the change in refractive index of thermo-optic materials caused by temperature changes. According to the different structure, it can be divided into two major categories, leakage type and switch type VOA.

The principle of leaky thermo-optical VOA is shown in Figure 7(a). The principle is to strip off part of the original outer sheath of the optical fiber and replace it with thermo-optical materials to form the outer sheath. When a temperature change is applied to the outer skin layer of the thermo-optical material, the original optical transmission characteristic, namely the change of the mode field diameter (MFD) due to the change of its refractive index, will cause a part of the optical signal energy to escape from there ( Radiation light), so as to achieve the purpose of adjusting the amount of light attenuation by controlling the temperature.

The most typical of the open-light type thermo-optic VOA is based on the principle of the Mach-Zehnder interferometer (MZI), and its specific structure is shown in Figure 7(b). The main working method is to add thermo-optical material on one of the interference arms of the Mach-Zehnder interferometer, and place the thermo-optical material on the thin film heater. The thermo-optical effect is used to change the refractive index of the material, thereby changing the length of the interference arm of the MZI, so that the two arms produce different optical path differences, which further changes the interference light intensity of the double beam, and realizes the control of the light attenuation. . The MZI planar optical waveguide VOA is small in size, which is good for high integration, but its technology is still under development and improvement. This method must split and couple the beam, which will introduce a larger insertion loss, so the performance of this kind of VOA is still poor, and the packaging is difficult.

The thermo-optical VOA has relatively complicated heating and cooling devices, and the mathematical function relationship between the temperature field and the refractive index of the optical guide medium is complicated and difficult to accurately quantify and control. In particular, its long response time hinders its application in modern optical communications.

Sound and light VOA

The basic principle of this type of attenuator is to use the periodic strain produced by the acousto-optic crystal under the action of ultrasonic waves to cause the periodic change of the refractive index, which is equivalent to the establishment of a phase grating. modulation.

Some companies have already claimed to have developed an variable optical attenuator (called AVOA) using acousto-optic crystals. It is understood that there is no problem in obtaining acousto-optic crystal materials, but at this stage, the overall cost is relatively high, accounting for about 4-5% of it.

Concluding remarks

Variable optical attenuator (VOA) is one of the important optical devices in optical communication systems. For a long time, it has stayed at the mechanical level, because its large size is not conducive to integration, and it is generally only suitable for single-channel attenuation. With the development of DWDM systems and the potentially huge market demand for flexible and upgradeable reconfigurable optical add/drop multiplexers (ROADM), there is an increasing need for variable optical attenuator arrays with a large number of channels and a small volume. Especially some integrated VOA products. Traditional mechanical methods can no longer solve these problems. With the development of optical fiber networks, the development trend of VOA is: low cost, high integration, fast response time, and hybrid integration with other optical communication devices.