First, the principle classification of variable optical attenuator
Variable optical attenuators include mechanical technology, adjustable diffraction grating technology, MEMS technology, liquid crystal technology, magneto-optical technology, planar optical waveguide technology, etc. Mechanical VOA
This type of VOA also has many specific implementation methods. Figure 1 is a schematic diagram of a light-blocking optical attenuator, which drives a light-blocking element between two collimators to attenuate optical power. The light blocking element can be sheet-shaped or cone-shaped, the latter can be pushed by rotation, while the former needs to be pushed horizontally or through a certain mechanical structure to realize the conversion from rotation to horizontal push. The light-blocking optical attenuator can be made into a fiber optic adapter structure, or it can be made into an in-line structure as shown in the figure.
Similar to the light-blocking VOA mentioned above, there is also a mechanical-potentiometer EVOA solution. The principle is to use a stepping motor to drive the neutral gradient filter. When the light beam passes through different positions of the filter, the output optical power will change according to the predetermined attenuation law, so as to achieve the purpose of adjusting the attenuation. There is also a mechanically polarized light attenuator. The basic principle is that the light beam emitted from the entrance port is reflected by the reflector to the exit port, and the reflection coupling efficiency between the two ports is controlled by the inclination angle of the reflector, so as to realize the adjustment of light attenuation. The tilt of the reflector is controlled by a variety of different mechanisms.
The mechanical optical attenuator is a relatively traditional solution. So far, most of the VOAs that have been applied in the system use mechanical methods to achieve attenuation. This type of optical attenuator has the advantages of mature technology, good optical characteristics, low insertion loss, small polarization-related loss, and no need for temperature control; but its disadvantages are large volume, complex structure of components, low response speed, and difficulty in automatic production , Not conducive to integration, etc.
Polymer adjustable diffraction grating VOA
The fabrication of polymer tunable diffraction gratings is based on a thin film surface modulation technology. Originally, this technology was developed as a replacement for liquid crystal displays (LCDs) and digital light processors (DLPs) in projectors and projectors. The top layer of this tunable diffraction grating (Figure 1) is glass, the bottom layer is indium tin oxide (ITO), the middle is an array of air, polymer, and ITO, and the bottom layer is a glass substrate. When no electrical signal is applied, the interface between the air and the polymer layer is a plane parallel to the surface of the structure. When incident light enters this plane, no diffraction occurs. After applying an electrical signal, the interface of air and polymer changes periodically with the distribution of the electrode array, forming a sinusoidal grating. When incident light hits the surface, it is diffracted. Applying different electrical signals can form sinusoidal gratings with different degrees of phase modulation.
Polymer tunable diffraction grating.
The working mechanism of the VOA using polymer tunable diffraction grating is: by modulating a thin layer of polymer on the surface, the surface is approximately sinusoidal, forming a sinusoidal grating. Using this technique, it is possible to fabricate a sinusoidal grating with a period of 10 micrometers and a surface height h that varies with an applied electrical signal up to 300 nanometers. Diffraction occurs when light is incident on the surface being modulated. Different electrical signals are applied to change the amplitude of the sinusoidal grating, that is, when h is changed, different phase modulation degrees can be obtained, and the distribution of diffracted light intensity under different phase modulation degrees is different. When the phase modulation degree gradually increases from zero, the intensity of diffracted light shifts from zero order to light of higher diffraction order. This modulation can continuously change the light intensity of the zero-order light from 100% to 0%, thereby realizing the control of the attenuation. And the response time of this modulation is very fast, on the order of microseconds.
Magneto-optical VOA uses the change of optical properties of some substances under the action of a magnetic field, such as the magneto-optical rotation effect (Faraday effect) to achieve the attenuation of light energy, so as to achieve the purpose of adjusting the optical signal. A typical polarization-independent magneto-optical VOA structure is shown in the left figure of Fig. 2.
Polarization-independent magneto-optical VOA structure and optical path.
In the right figure of Figure 2, the mirrored light path in the left figure is drawn on the right side to facilitate the analysis and explanation of the principle. When the light is incident from one end of the dual-core fiber, after being collimated by the lens (the thickness of the beam is omitted), it enters the birefringent crystal (its optical axis is perpendicular to the paper surface), and is divided into two beams of O light and E light, and then Entering the Faraday rotator, the light is reflected by the total reflection mirror after exiting the Faraday rotator, then passes through the Faraday rotator, birefringent crystal and lens in turn, and finally outputs from the other end of the dual-core fiber. Therefore, by controlling the magnetic field by modulating the voltage, 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 of light are not parallel and cannot be combined, as shown by the dotted line in the figure. At this time, the degree of attenuation is the largest; in Faraday When the rotation angle is 45 degrees, the total Faraday rotation angle is 90 degrees, the O light becomes E light, and the E light becomes O light. The two beams of light are parallel and converged after being focused by the lens. At this time, the degree of attenuation is the smallest. .
The liquid crystal VOA utilizes the birefringence effect exhibited 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 changes in its light transmission characteristics (Figure 3).
The change of light transmittance of liquid crystal before and after electrification.
As 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, E light The light becomes O light, then combined by another birefringent crystal, and finally output from the collimator. When the liquid crystal material is loaded with a voltage V, both the O light and the E light change a certain angle after passing through the liquid crystal, and each beam of light is divided into O light and E light after passing through the second birefringent crystal, forming 4 beams of light, the two in the middle The beams 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 realize attenuation. Therefore, by applying different voltages on the two electrodes of the liquid crystal to control the change of light intensity, different attenuation can be achieved.
The principle of liquid crystal VOA.
MEMSVOA has reflective VOA and diffractive VOA (Figure 5).
The structure of MEMSVOA.
The reflective VOA is to make a micro-mirror on the 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, the inclination angle changes, and the incident angle of the incident light changes. After light reflection The energy 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 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. 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.
Planar optical waveguide VOA
There are also two types of planar optical waveguide VOA.
One is based on the principle of Mach-Zehnder interferometer (MZI), and uses the thermo-optic effect 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, and achieve optical alignment. Attenuation control (Figure 6). This method must split and couple the beam, which introduces a large insertion loss.
Planar optical waveguide VOA based on MZI principle
The other is directly based on electroabsorption (EA) modulation, which uses carrier injection to change the absorption coefficient to achieve optical power attenuation. As shown in Figure 7, a layer of single-mode optical waveguide layer is added between the PN junctions. When no power is applied, the single-mode light emitted from the fiber enters the single-mode optical waveguide layer and is still in the conduction mode, which is limited to This layer continues to propagate and output from another optical fiber; when the voltage is applied, due to the injection of carriers, the absorption coefficient of the single-mode optical waveguide increases, so that part of the light is absorbed. And as the voltage increases, the current flowing through the PN junction also increases, so that more photons are absorbed and the attenuation increases.
Plane Light Wave VOA Using Electroabsorption Modulation
High photoelectric coefficient material VOA
This VOA uses a special ceramic photoelectric material, similar to lithium niobate (LiNbO3), but has a larger photoelectric coefficient than lithium niobate. Using this material with a sufficiently large photoelectric coefficient to make VOA 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 8, light is introduced through the input collimator, passes through an element made of a special optoelectronic material, and then exits the output collimator. Adjust the voltage applied to the photoelectric material element to change its refractive index, thereby achieving attenuation.
Use high photoelectric coefficient materials to make VOA
Second, the comparison of various technologies
With the increasing application of VOA in optical communication, the requirements for its functions are also getting higher and higher. VOA should be able to precisely control the power of optical signals and provide stable attenuation for all communication wavelengths; in ultra-long-distance DWDM systems, VOA must also respond to signals that gradually change with environmental influences; on dynamic network nodes, The response time of VOA should be in ms level. The technical indicators of VOA mainly include: working wavelength range, dynamic range, insertion loss, polarization-dependent loss, response time, temperature characteristics, working temperature, etc. The following is a simple comparison of various technologies, see Table 1.
The polymer tunable diffraction grating VOA array has simple manufacturing process, good performance, dynamic range up to 20dB, small insertion loss, fast response time, little influence by ambient temperature, no temperature compensation, and optical power monitoring, which has high cost-effective.
The magneto-optical VOA is affected by the ambient temperature due to the change of the polarization state of the beam by the magneto-optic crystal, and the temperature characteristic is poor, so temperature compensation is required. In addition, when the magnetization of the magneto-optic crystal does not reach saturation, many magnetic domains will be generated in the magneto-optic crystal. The existence of magnetic domains makes the repeatability of the attenuation effect of the variable optical attenuator worse. difficult to control. At present, there are few companies that can provide this type of product in the market. Its advantage is that the response time is very fast, and it has been commercialized in small batches.
Because the liquid crystal VOA is easily affected by the ambient temperature, its temperature characteristics are very poor, and it needs to be supplemented by temperature calibration when using it. Another disadvantage is that its response time is very slow at low temperatures. It has the advantage of low cost and has been commercially available in batches.
MEMS VOA has been very mature, and has been mass-produced and applied on a large scale. The product is also greatly affected by the ambient temperature and requires temperature compensation. At the same time, due to the problem of yield rate, it faces challenges in terms of price. In addition, because it is a micro-electromechanical component, the reliability is sometimes not ideal.
The MZI-type planar optical waveguide VOA is small in size and is conducive to high integration. However, its technology is still in development and improvement, its performance is still poor, and its packaging is difficult. The EA-type planar optical waveguide VOA requires a large change in the carrier concentration, and the modulation area is very long, so it will increase the volume and power consumption of the device, and this VOA is also temperature-dependent, but it has the advantage of a very fast response time , and can even be used as a low-speed modulator. And because of the huge advantages of integration, with the development and maturity of technology, it is believed that planar optical waveguide VOA will be more and more widely used.
The free-space optoelectronic material VOA has a fast response time and can withstand high power, and has been used in some applications. Because it can be made into a free-space structure, it can make good use of the relatively mature micro-optical device platform at present. But because of the special materials it uses, the current price is relatively high.