In the optical fiber communication system, some optical devices without power supply are required, which are collectively referred to as passive optical devices, such as optical fiber connectors , optical fiber couplers , optical attenuators, optical isolators, and optical switches. The feature that passive optical devices do not require power not only brings a lot of convenience to construction and maintenance, but also can greatly reduce the cost of network construction, so the passiveization of optical communication devices represents the development direction of optical communication devices. This article mainly introduces the working principle and technical indicators of various optical attenuators.
The optical attenuator is a device for attenuating optical signals. When the output optical signal power of the optical fiber is too strong and affects the test results, an optical attenuator should be added to the optical fiber test link to obtain accurate test results. Optical attenuators are mainly used for index measurement of optical fiber systems, signal attenuation of short-distance communication systems, and system experiments, such as adjusting the line loss of the relay section, evaluating the sensitivity of the optical system, and calibrating the optical power meter. Optical attenuators are required to be light in weight, small in size, high in precision, good in stability, and easy to use.
Optical attenuators can be divided into variable optical attenuators and fixed optical attenuators according to the change of their attenuation. The fixed optical attenuator is mainly used to attenuate the light energy in the optical path by a fixed amount, and its temperature characteristics are excellent, as shown in Figure 1 . In system debugging, it is often used for corresponding attenuation of optical signals after passing through a section of optical fiber or for reducing redundant optical power in relay stations to prevent saturation of optical receivers; it can also be used for calibration of optical test instruments. For different line interfaces, different fixed attenuators can be used; if the interface is a pigtail type, a pigtail type optical attenuator can be used to fuse between two sections of optical fibers in the optical path; if there is a connector during system debugging interface, it is more convenient to use a converter type or a converter type fixed attenuator.
Figure 1 Fixed optical attenuator
In practical applications, an optical attenuator whose attenuation can be changed according to user needs is often required, so the variable attenuator has a wider range of applications. The actual variable attenuator is shown in Figure 2 . For example, because the design margins of EDFA and CATV optical systems are not exactly the same as the optical power margins in the actual system, a variable optical attenuator must be inserted into the system when evaluating the BER of the system to prevent receiver saturation; In addition, variable attenuators will also be used in the measurement and calibration of optical power meters or OTDRs .
Figure 2 Variable optical attenuator
From the perspective of market demand, on the one hand, optical attenuators are developing toward miniaturization, serialization, and low price; Optical attenuator, high return loss optical attenuator, etc.
The methods of attenuating optical power include: reflecting a part of light, absorbing a part of light, blocking a part of light in space, or using a polarizer to select the polarization plane of light, etc. In terms of working principle, commonly used optical attenuators can be divided into four categories: displacement optical attenuators, direct coating attenuators, attenuating plate optical attenuators, and liquid crystal optical attenuators. The specific classification is shown in Figure 3 . .
Figure 3 Classification of optical attenuators
The displacement optical attenuator uses the principle that the attenuation of the optical fiber changes with its centering accuracy. It is intended to cause a certain displacement between the optical fibers when the optical fibers are connected, so as to attenuate a certain amount of light energy. When making a displacement optical attenuator Lateral displacement method and longitudinal displacement method can be used.
1 ) Lateral displacement optical attenuator
As we all know, when two sections of optical fiber are connected, a fairly high alignment accuracy must be achieved so that the optical signal can be transmitted with less loss. Conversely, if the alignment accuracy of the fiber is properly adjusted, its attenuation can be controlled. Based on this principle, the displacement optical attenuator intentionally displaces the optical fiber when it is docked, so that the light energy is lost, so as to achieve the purpose of controlling the attenuation. Its working principle is shown in Figure 4 .
Figure 4 Principle of lateral displacement optical attenuator
According to the above principles, the lateral displacement parameters corresponding to different losses can be designed and realized by a certain mechanical positioning method to obtain the required optical attenuator. Under normal circumstances, since the order of magnitude of the lateral displacement parameters is in the micron level, it is generally not used to make variable attenuators, and is only used to make fixed attenuators, and welding or bonding methods are used.
The lateral displacement method is a relatively traditional method, and it still has a large market. Its advantage is that the return loss is very high, usually greater than 60dB .
2 ) Axial displacement optical attenuator
The gap between the fiber end faces will also cause the loss of light energy. Even for a 3dB attenuator, the corresponding gap is more than 0.1mm . The process is simple and easy to implement. Its working principle is shown in Figure 5 .
Figure 5 Principle of axial displacement optical attenuator
When using the principle of axial displacement to make an optical attenuator, in terms of process design, as long as the two optical fibers are separated by a certain distance in a mechanical way for alignment, the purpose of attenuation can be achieved. This principle is mainly used in the manufacture of fixed optical attenuators and some small variable optical attenuators.
The direct-coating attenuator is an attenuator that directly coats a metal absorbing film or a reflective film on the end face of an optical fiber or a glass substrate to attenuate light energy. Commonly used evaporated metal films include: Al film, Ti film, Cr film, W film, etc. If the A1 film is used, a thin film of SiO 2 or MgF 2 is often plated on it as a protective film. FIG. 6 is a schematic structural diagram of a direct coating type optical attenuator.
Figure 6 Schematic diagram of the direct coating optical attenuator
The attenuating sheet type optical attenuator directly fixes the attenuating sheet with absorption characteristics on the end face of the optical fiber or in the optical path to attenuate the optical signal. This method can be used not only to make fixed optical attenuators, but also to make variable optical attenuators .
The production method is to directly fix the attenuation sheet in the collimation optical path through a mechanical device. When the optical signal passes through the attenuation sheet after being collimated by a quarter-pitch self-focusing lens, the light energy is attenuated, and then by the second A self-focusing lens focuses the coupling into the fiber.
By using attenuation sheets with different attenuation values, optical attenuators with corresponding attenuation values can be obtained. Materials commonly used for attenuation sheets include infrared colored optical glass, crystals, and optical films. Two optical attenuators based on this principle are further introduced below.
1 ) Stepping two- wheel variable optical attenuator
The Dual Wheel Variable Optical Attenuator utilizes a pair of single-mode fiber collimators consisting of quarter-pitch self-focusing ( GRIN ) lenses and single-mode fiber. When it collimates the beam transmitted in the optical fiber, a certain distance is allowed in the middle of its coupling structure. The optical attenuator just takes advantage of its characteristics and inserts an attenuation unit in this optical path distance to attenuate the optical power.
The optical path of the stepping double-wheel variable optical attenuator adopts the parallel optical path emitted by the collimator, and two attenuating disks with fixed attenuation are inserted in the optical path, and each attenuating disk is equipped with 0 , 5 , 10 , 15 , 20 , 25dB six attenuators, by rotating the two disks, different attenuators on the two disks can be combined with each other to obtain ten levels of attenuation such as 5 , 10 … 50dB . Of course, if you want to obtain a step attenuator with other attenuation ranges, you only need to make corresponding changes to the filter and position on the attenuation disk, and you can easily achieve the desired purpose. Its structure is shown in Figure 7.
Figure 7 Structural diagram of the step-by-step double-wheel variable optical attenuator
2 ) Continuous two-wheel variable optical attenuator
The overall structure and working principle of the continuous double-wheel variable optical attenuator are similar to the synchronous double-wheel variable optical attenuator, but it is composed of a step attenuator and a continuous attenuator. The attenuation of the step attenuator There are six steps of 0 , 10 , 20 , 30 , 40 , and 50dB , and the attenuation of the continuous attenuation sheet is 0-15dB . Therefore, the total attenuation adjustment range is 0-65dB . In this way, the purpose of continuously attenuating light energy can be achieved through the joint action of the coarse gear of the step attenuator and the fine gear of the continuous attenuator, and its structure is shown in Figure 8.
Figure 8 Structural diagram of continuous double-wheel variable optical attenuator
The continuous attenuation sheet is made by coating a metal absorption film on a circular optical glass sheet by vacuum coating method. During evaporation, a special fan-shaped device is used to cover the glass substrate. Since this special covering device can continuously and uniformly change its opening angle, the thickness of the evaporated film layer can be gradually and uniformly changed, so that it can be achieved. The purpose of continuously changing the attenuation. Except for the non-coated area, the remaining fan-shaped areas are evenly distributed by coating layers of different thicknesses, and the perimeter of each 1dB coating area is equal, so the uniform change of attenuation can be guaranteed.
Obviously, this kind of attenuator has high requirements on the uniformity of the coating layer of the attenuator. Two points should be considered when designing:
( 1 ) The larger the radius of the attenuation sheet, the higher the resolution of the attenuation.
( 2 ) Try to avoid too large attenuation range. Because the attenuation range is too large, that is, the thickness of the film layer in the fan-shaped area per unit length varies too much, which will affect the improvement of resolution and increase the difficulty of the coating process.
Therefore, in the design, the combination of adjustable continuous attenuation film and stepped attenuation film is often adopted, so that the optical attenuator has a large adjustable attenuation range and high resolution. In addition, the accuracy of the attenuation is also related to the processing accuracy and assembly of device parts. It is required that important parts must be precisely processed, and need to be wear-resistant, rust-proof, moisture-proof, and dust-proof. The assembly accuracy should be high and the sealing performance should be good.
As a condensed matter, liquid crystals are ordered fluids whose characteristics and structure are between solid crystals and isotropic liquids. From a macroscopic point of view, it not only has the fluidity and viscosity of liquid, but also has the anisotropy of crystal. It can produce birefringence, Bragg reflection, diffraction and optical rotation effect like crystal, and can also generate heat and light under the action of external electric field. , electro-optic or magneto-optical effects. When an electric field is applied to the liquid crystal, the alignment direction of the liquid crystal molecules will be changed, and the incident light with a certain polarization direction will undergo birefringence in the crystal, that is, the electrically controlled birefringence effect. The liquid crystal light attenuator uses the birefringence effect caused by the anisotropy of the refractive index of the liquid crystal. When an external electric field is applied, the orientation of the liquid crystal molecules is rearranged, which will lead to a change in its light transmittance. The change of light transmittance before and after the liquid crystal is powered is shown in FIG. 9 .
Figure 9 Comparison of light transmittance before and after the liquid crystal is powered on
The picture above is the working principle diagram of the traditional liquid crystal light attenuator, which uses the P -type liquid crystal whose molecular axis is twisted. The specific implementation of the liquid crystal optical attenuator is shown in Figure 10. After the incident light is collimated by the collimator, it 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, E light becomes O light , then combined by another birefringent crystal, and finally output from the collimator. When the voltage V is applied to the transparent electrodes at both ends of the liquid crystal material , the O light and the E light will change a certain angle after passing through the liquid crystal, and each beam of light will be divided into O light and E light through the second birefringent crystal, forming 4 The two beams in the middle are finally synthesized and one beam is emitted from the second birefringent crystal and received by the collimator, and the other two beams are not received by the collimator after emitting from the second birefringent crystal, so as to achieve attenuation. Therefore, different attenuation can be achieved by applying different voltages on the two electrodes of the liquid crystal to control the change of light intensity.
Figure 10 Working principle diagram of liquid crystal optical attenuator
If the light leakage of the liquid crystal is not considered, and I 0 is used to represent the total power of polarized light when no voltage is applied, then when the liquid crystal crystal is inclined at an angle θ, the polarized light power of the part where the deflection plane rotates is: I= I 0 cos θ . It can be seen that the larger the angle θ is, the smaller I is . Therefore, as the external electric field is continuously strengthened, the optical power of the part of the polarization plane rotated by 90 º is gradually reduced, that is, the optical signal coupled into the optical fiber by the self-focusing lens is also smaller and smaller, thereby realizing the attenuation of the optical signal .
In recent years, a variety of technologies for manufacturing variable optical attenuators have emerged, including MEMS VOA , thermal optical VOA , and high photoelectric coefficient material VOA .
1 ) MEMS VOA
At present, based on microelectronics technology and microfabrication technology, a new revolution has begun in the field of micro-machine manufacturing, thus giving birth to micro-electro-mechanical systems ( Micro-Electro-Mechanical Systems , MEMS ). MEMS technology has also played an important role in optical switching and optical connection technology, providing technical support for the miniaturization, large array and low cost of passive optical devices such as optical switches, optical attenuators and filters. possibility.
MEMS VOA mainly includes reflective optical attenuator and diffractive optical attenuator, as shown in Figure 11 , where ( a ) is a reflective VOA , and ( b ) is a diffractive VOA . The reflective VOA is to make a micro-mirror on a silicon base. The light enters through one end of the dual-core collimator and is incident on the micro-mirror at a certain angle. When a voltage is applied, the micro-mirror is twisted under electrostatic action. When the inclination angle changes, the incident angle of the incident light changes, and the energy after the light reflection cannot be fully 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 light reflection The final energy is fully coupled into the other end of the dual-core collimator.
Diffractive VOA is based on dynamic diffraction grating technology. When a voltage is applied, the position of the moving grating bars at the same interval moves downward under the electrostatic action to produce a diffraction grating effect, and the first-order diffracted light is controlled by voltage adjustment to achieve the purpose of adjusting the attenuation of the optical signal.
Figure 11 Schematic diagram of MEMS VOA structure
2 ) High photoelectric coefficient material VOA
High photoelectric coefficient material VOA uses a special ceramic photoelectric material, similar to lithium niobate ( LiNbO 3 ), but has a larger photoelectric coefficient than lithium niobate. Using this material with a sufficiently large photoelectric coefficient to make an optical attenuator does not need to be made into a waveguide, but can be made into a free-space structure, just like an isolator. As shown in Figure 12, the light is introduced through the input collimator, passes through an element made of special optoelectronic material, and then outputs from the output collimator. Adjust the voltage applied to the photoelectric material element to change its refractive index, thereby achieving attenuation.
Figure 12 High photoelectric coefficient material VOA
3 ) Thermal light VOA
Thermo-optic VOA mainly utilizes the optical property change characteristics of some materials in the temperature field, such as changes in the refractive index of thermo-optic materials caused by temperature changes. According to the different structures, it can be mainly divided into two types: leakage type and opening type.
The principle of leaky thermo-optic VOA is shown in Figure 13 ( a ). The principle is to first strip off the original cladding of some optical fibers and replace them with thermo-optic materials. When a temperature change is applied to the outer skin of the thermo-optic material, due to the change of its refractive index, the original light transmission characteristic, that is, the mode field diameter ( MFD ) changes, and part of the optical signal energy will escape from this place ( Radiation light), so as to achieve the purpose of adjusting the amount of light attenuation by controlling the temperature.
( a ) Leakage type ( b ) open light type
Figure 13 Working principle diagram of thermo-optic VOA
most typical one for open-type thermo-optic VOA is a principle based on Mach-Zehnder interferometer ( MZI ), and its specific structure is shown in Figure 13 ( b ). The main way of working is to add a thermo-optic material on one of the interference arms of the MZI , and place the thermo-optic material on a thin-film heater. The thermo-optic effect is used to change the refractive index of the material, thereby changing the length of the interference arm of the MZI , causing the two arms to produce different optical path differences, further changing the interference light intensity of the double beams, and realizing the control of the light attenuation . The MZI planar optical waveguide VOA is small in size and is conducive to high integration, but its technology is still in development and perfection. This method must split and couple the light beam, which will inevitably introduce a large insertion loss. At present, the performance of this VOA is still poor, and the packaging is difficult.
Due to the relatively complex heating and cooling devices of thermo-optic VOA , the mathematical function relationship between the temperature field and the refractive index of the light-guiding medium is complex and difficult to accurately quantify and control, especially the long response time hinders the application of thermo-optic VOA in modern optical communications. application.
Attenuation and insertion loss are important indicators of an optical attenuator. The attenuation indicator of a fixed optical attenuator is actually its insertion loss. In addition to the attenuation, the variable attenuator also has a separate insertion loss indicator. The insertion loss of the variable attenuator is below 1.0dB . Generally, the index of the ordinary variable attenuator can be used if it is less than 3dB .
The insertion loss of the optical attenuator mainly comes from the insertion loss of the fiber collimator, the transmittance accuracy of the attenuation unit and the coupling process, and the process focuses on the production of the fiber collimator. If the coupling between the optical fiber and the self-focusing lens and the two light collimators is good, the insertion loss of the entire optical attenuator can be greatly reduced. When actually selecting an adjustable attenuator, the insertion loss should be as small as possible.
Attenuation accuracy is an important indicator of optical attenuators. Usually, the attenuation accuracy of the mechanical adjustable optical attenuator is ± 0.1 times of its attenuation, and its size depends on the degree of precision processing of the mechanical components. The attenuation accuracy of the fixed optical attenuator is very high. Usually the higher the attenuation accuracy, the higher the price.
An important indicator affecting system performance among optical device parameters is return loss. The impact of returning light on optical network systems is well known, it will cause laser relative intensity noise, nonlinear chirp and lasing drift, which will deteriorate the communication system.
The return loss of an optical attenuator refers to the ratio of the light energy incident into the optical attenuator to the light energy reflected along the incident light path in the attenuator. The return loss of the high-performance optical attenuator is above 40dB . The return loss is caused by the reflection caused by the mismatch between the refractive index of each component and the air. Usually, the return loss caused by the planar component is about 14dB . Through sufficient anti-reflection coating and proper bevel polishing and assembly process, the echo of the entire device The loss can reach more than 50dB . In fact, due to technical reasons, the actual return loss of the attenuator is still far from the theoretical value. In order not to reduce the return loss of the entire line, a high return loss attenuator must be used in the corresponding line, and the optical attenuator is also required It has a wider temperature range and spectrum range.