Polarization effect: Squeeze out wider communication bandwidth in the optical fiber

With the rapid development of communication technology, telecom operators are constantly improving the transmission rate of single channels in WDM systems to meet people's needs for communication bandwidth. At present, a single wavelength WDM System with a transmission rate of 10 Gb/s is under construction, and a WDM System with a transmission rate of 40 Gb/s has entered our field of view.

At the same time as the transmission rate is improved, the communication system performs polarization mode dispersion (PMD) on the optical fiber, polarization-related modulation (PDM) in the electro-optic modulation, and polarization-related gain (PDG) in the optical amplifier) and a series of damage caused by polarization is becoming more and more sensitive. These damages are mainly caused by defects of the optical fiber. In the idealized optical fiber, the polarization SOP of the transmitted light will not change, these damages caused by polarization effects can also be easily eliminated. In the standard communication optical fiber used in practice, the polarization state of the transmitted light is constantly changing along the optical fiber. Generally, the output light of the common optical fiber is Elliptical Polarized light, and the degree of elliptic is constantly changing, the main axis is at any angle relative to the reference direction). The reason for this change is that the fiber is caused by various factors such as thermal stress, mechanical stress, and the irregularity of the core. What's worse, the refraction effect in the optical fiber varies with temperature, pressure, stress, and other environmental factors, which greatly increases the unpredictability of polarization-related damages. Since polarization-related damages change over time, the method for eliminating them must be dynamic and adaptive to random changes.

Dynamic Polarization Control

The dynamic polarization Controller Used for PMD compensation is the most important device to overcome these damages. It can convert any given polarization state to any desired polarization state. In addition to the advantages of low insertion loss and high return loss, the ideal dynamic polarization controller should also have the following important performance parameters:

1. High response speed is an essential element for tracking rapidly changing polarization states. The external environment will have different influences on the laid optical fiber cables. For example, the vibration when the train passes through will have a great impact on the optical fiber cables laid along the railway and the sea waves, the polarization of Optical Fiber Transmission changes rapidly. Currently, on-site measurement using the PMD recorder has observed rapid ups and downs in several milliseconds. Therefore, the response time of the dynamic polarization Controller Used for PMD compensation must be less than 1 ms. In practical application, the response time of the dynamic polarization controller must be less than 100 μs.

2. startup loss: it measures the insertion loss introduced when the polarization controller is started. It is defined as the difference between the maximum insertion loss and the minimum insertion loss under all possible startup conditions. All Polarization-related damages are compensated by the feedback signal to activate the polarization controller for dynamic Polarization Control. Therefore, the loss and Fluctuation Caused by Controller startup may lead to feedback signal errors, which directly leads to performance reduction of the instrument. In addition, the startup loss also limits the resolution and accuracy of the measurement by using polarization controllers for PDL measurement. Similarly, the PDL of the polarization controller may make the feedback signal wrong, making the software and hardware design of compensation very complicated.

3. Wide Bandwidth is very important for Dense Wavelength Division Multiplexing (DWDM) systems. A sufficient bandwidth can make the polarization controller have the same performance in different channels. This not only simplifies the system design, reduces system costs, but also makes system bandwidth expansion possible.

4. Non-disruptive adjustment of the polarization controller is also an important feature. In optical networks, any reset of polarization may cause unexpected signal interruptions.

Currently, commercial polarization controllers can be divided into three types based on their technical principles: one is composed of multiple fixed latencies and variable azimuth wave slices; the other is composed of a single wave with adjustable latency and variable azimuth. The other is composed of multiple fixed azimuth and adjustable latencies.

Among them, the polarization controller based on fixed delay wave is wavelength sensitive, and the adjustment speed of the polarization Controller Based on mechanical rotation is very slow. In addition to these inherent limitations, the above three methods are feasible in principle, but the specific implementation means will directly determine the performance, cost and reliability of the product.

 

Figure 1 is a typical structure of the polarization controller. It consists of three rotatable wave slices, and a λ/2 HWP) is in the middle of two λ/4 QWP) wave slices, each wave can rotate freely along the optical axis relative to other wave slices. The first λ/4 wave is used to convert arbitrary input polarized light into linear polarized light, and then the λ/2 wave is used to rotate the linear polarized light to any desired polarization direction, so the second λ/4-wave film can convert the polarized light into any desired output polarization state. In this implementation method, the bandwidth delay is fixed, but the relative angle of the bandwidth is variable.

Although this method has been widely applied to commercial products, this technology has many shortcomings. First of all, it takes a lot of time and labor to align, align, and focus the light. Second, the wave, Micro Lens and other components are expensive, and the need to add a film, polishing angle to reduce back reflection. Again, because it is inevitable to couple the light from one fiber, and then focus it on another fiber, the insertion loss is high. Moreover, the wave itself is sensitive to the wavelength. Any fractional wave is determined for a fixed wavelength), so that this polarization controller is also sensitive to the wavelength. Finally, the use of motor or other mechanical devices to rotate the wave, will limit the control speed of the polarization controller.

Other options

Based on the same principle, all-optical fiber polarization controller 1b can reduce insertion loss and costs. In this device, the three optical fiber coils replace free-space delayed wave slices. The stress produced by the coil bending can produce a bilinear effect inversely proportional to the diameter of the coil. Adjust the diameter and the number of circles of the optical fiber coil to obtain any desired full-fiber wave.

Although insertion loss and production costs are both reduced, this polarization controller still fails to eliminate the disadvantages of wavelength sensitivity and slow control speed. In addition, in order to reduce the insertion loss introduced by the optical fiber bending, the optical fiber coil must have a large diameter, so that the size of this polarization controller is usually very large. Therefore, the polarization controller in the shape of "Mickey Mouse ears" is mainly used in the laboratory.

Speed is a key factor in the development of network technology. It is difficult for mechanical rotary wave sheets to meet the requirements of speed adjustment. As a result, people began to develop fast polarization controller 1c Based on LiNbO 3 material ). The polarization controller consists of three waveguide structures, two of which are used to act as λ/4-wave slices, and the other as λ/2 wave slices. No longer need to rotate the wave. Two Control voltages and photoelectric effects can determine the direction of the relative orientation of each wave ). Select the appropriate voltage to achieve unlimited rotation of each wave orientation. The following is an example of the control voltage of the three wave slices:

Among them, α, β and γ are three constantly adjusted parameters to determine the corresponding wave orientation, and V a to V I are nine preset voltage parameters.

 

However, unfortunately, the cost of using this method to increase the speed is unacceptable to network applications. Its main disadvantage is: High insertion loss ~ 3 dB), high polarization correlation loss ~ 0.2dB), high startup loss ~ 0.15dB) and expensive. In addition, this device requires Optimization of at least nine parameters, which is complex and cost-effective.

One alternative method is Babinet-Soleil compensator, which can convert any input polarization state to any desired output polarization state. The core device of this device is a composite wave chip 2a consisting of two wedge-shaped double-state crystals ). The variation of wave thickness corresponds to the total delay) can be achieved through the relative slide of the two crystals. At the same time, the orientation of the composite wave can be rotated around the optical axis.

Compared with the installation map 1a mentioned above, this device has the advantage of no sensitivity to the wavelength because it can achieve precise latency of any wavelength. However, this device has the disadvantages of high cost, high insertion loss, and slow adjustment speed.

To reduce costs and losses, a fully optical fiber polarization controller named PolaRITE 2b was developed in 1996. This polarization controller, based on the same principle as the Babinet-Soleil compensator, is composed of an optical fiber extruder that can rotate around the optical fiber. Apply pressure to the optical fiber to produce a linear bilinear, which is equivalent to a full Optical Fiber wave with the delay changing with the pressure. In this way, any desired output polarization state can be generated by any input polarization state through simple extrusion and rotation operations.

This device not only has low insertion loss and low cost, but also has the advantages of small size and no sensitivity to wavelength compared with the polarization controller in the shape of "Mickey Mouse ears. These advantages make this polarization controller useful for integration into WDM modules. However, similar to the polarization controller that relies on mechanical rotation, the device's adjustment speed is so slow that it cannot be applied to the optical fiber network's PMD compensation.

The polarization controller can also use several free-space wave slices with a 45 o angle to implement 3a ). The delay of each wave varies with the loading voltage, and the orientation of the wave is fixed. This variable delay wave can be made of liquid crystal, electro-optic crystal, electro-optic ceramic and other materials. The disadvantage of using liquid crystal materials is that the adjustment speed is slow, while electro-optic crystals generally require an extremely high operating voltage. This type of polarization controller generally has many disadvantages, such as high insertion loss, high cost, and the wide and narrow work caused by the film and microlens.

All-optical fiber Solution

As shown in Figure 3a, an all-optical fiber polarization controller with the same working principle can solve the problems of high insertion loss and high cost. The delay of the wave varies with the pressure applied by the optical fiber extruder. The key to this kind of device is how to improve the reliability, closeness and cost effectiveness of the device.

In the commercialized PolaRITE II dynamic polarization controller, the piezoelectric actuator drives the extruder to change rapidly. Due to the full-fiber structure, this device not only has no back-to-back reflection, but also has extremely low insertion loss and polarization related loss. Its response speed is 30 μs, which is enough to track the fastest polarization state fluctuation in the fiber links laid in the wild. Use appropriate control procedures to achieve unrestricted Polarization Control without resetting.

The startup loss of this polarization controller is less than DB, so it is also applicable in the high-precision PDL Testing Instrument and the feedback loop of polarization-related damage compensation. Moreover, it is not sensitive to wavelengths and has a wavelength range of 1280nm ~ The signal within 1650nm has consistent good working performance.

System Application

The dynamic polarization controller (DPC) based on the optical fiber extruder has the advantages of low insertion loss, low polarization-related loss, low startup loss, low back-to-reflection, high speed and low cost. As shown in 5a-e, in the application of optical communication systems, it is an ideal choice to overcome polarization-related damages. DPC plays an important role in the following applications:

◆ PMD compensation: As shown in 5a, a typical first-order PMD compensator consists of a dynamic polarization controller and a fixed or variable differential group delay line DGD. The DOP parameter is measured using an online polarization tester to monitor the dependent information on the dependent information. Then the DOP signal was fed back to control DPC and DGD. The response time of a typical PMD test and DPC is 100 μs.

◆ Polarization optimization: many devices or modules in the transmission link are sensitive to polarization, such as photoelectric (E-O) and electrical absorption (EA) modulated devices, optical interference meters, and external optical receivers. Using a DPC (5b) in such a link can minimize the polarization sensitivity by optimizing the output power of the device or module. This scheme can also be used to reduce the PDL effect of many passive devices.

◆ Reduce polarization-related crosstalk: To improve the spectral efficiency of the DWDM system, two polarization-related transmission technologies are used: polarization multiplexing PDM, that is, the multiplexing of Two Orthogonal Polarization States with the same wavelength; polarization crossover technology, that is, the multiplexing of two adjacent WDM channels with the same polarization state orthogonal. As shown in figure 5c, the polarization staggered technique is followed by a initiator) used to reduce polarization-related crosstalk between two adjacent channels.

◆ Polarization disturbance: the dynamic polarization controller based on the optical fiber extruder can also be used as a disturbance splitter to obtain a high random polarization state. The Vibrator has a built-in resonant enhancement circuit. The half-wave voltage at the disturbance frequency is only a few volts. We have successfully obtained the polarization sensitivity of less than 0.05dB and the degree of polarization smaller than 1% by selecting the appropriate driving parameters. The main applications of the streamer include:

◆ Reduction of polarization-related gain: In the transmission system, 5d), the performance of the optical amplifier may be reduced due to polarization-related gain PDG. The polarization correlation gain is directly proportional to the degree of polarization. The low degree of polarization can reduce the polarization burning hole effect PHB) and reduce the polarization correlation gain 5. When the SOP disturbance frequency is greater than the MS level of the light amplifier response time), the degree of polarization can be minimized.

◆ Eliminate polarization sensitivity: the vibrator can be used to eliminate the polarization sensitivity of the instrument. Some optical instruments, such as spectrometer based on the diffraction grating principle, are sensitive to the polarization state of the input light. The polarization state of the disturbance input light can eliminate the measurement uncertainty caused by polarization sensitivity.

◆ Simplified PMD compensation: The splitters can be used to simplify the PMD compensation 6 in the communication system. A low residual phase adjustment system is critical to simplifying the compensation for PMD. The fiber-optic voltage divider is suitable for this application with its extremely low residual phase adjustment system.

◆ PDL monitoring and compensation: it is very important to monitor PDL quickly and accurately during the manufacturing process of optical devices. The dynamic polarization controller based on the optical fiber extruder is attractive to these applications due to its low PDL, low startup loss, and greatly improved PDL test accuracy. In system applications, in order to monitor and compensate the PDL along the link, you need to place the fast disturbing device after the optical emission module, by monitoring the devices or optical modules such as EDFA) to monitor the PDL of the system. Using feedback signals to control dynamic polarization controllers and devices that generate PDL can minimize power fluctuations as shown in 5e ).

All in all, the dynamic polarization controller based on the optical fiber extruder is a key element to overcome polarization-related damage in the optical transmission system and to monitor the polarization characteristics of the instrument.

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