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Why do we need AWG? We know that DWDM technology can transmit dozens of wavelengths in a single fiber, which greatly expands the transmission capacity of optical fiber communication systems. The earliest WDM/demultiplexing module used in DWDM system is based on dielectric membrane filter TFF, as shown in FIG. 1 and FIG. 2. Both of these are series structures, and different wavelengths experience different numbers of devices in the module, producing different power losses. With the increase of the number of ports, the loss uniformity of DWDM module deteriorates. At the same time, the maximum loss generated at the final port is another factor restricting the number of ports. Therefore, the DWDM module based on TFF technology usually has no more than 16 channels.
Figure 1. WDM module based on three-port WDM device
Figure 2. Mini WDM module structure
However, a typical DWDM system typically transmits 40 or 48 wavelengths in a single fiber, so a multiplexer/demultiplexer with a larger number of ports is required. WDM modules with a series structure accumulate too much power loss in the rear ports, so a parallel structure is needed to multiplexed/demultiplexed tens of wavelengths at once. Array waveguide grating AWG is one such optical device.
Array waveguide gratings are usually used in Optical (De)Multiplexers in WDM systems. These devices can combine many wavelengths of light into a single fiber to improve the propagation efficiency of fiber networks.
Typical structure of AWG
A typical AWG structure is shown in Figure 3 and consists of an input waveguide, an input star coupler (free transmission area FPR in the figure), an array of waveguides, an output star coupler, and dozens of output waveguides. The length of the array waveguide is an arithmetic series, the length of the first waveguide is L0, and the length of Li is
Figure 3: Typical AWG structure
The DWDM signal enters the input star coupler from the input waveguide and is distributed to the array waveguide after free transmission. This allocation process is wavelength independent, and all wavelengths are allocated to the array waveguide indiscriminately. The array waveguide produces phase difference for multiple beams, and the phase of each beam is in arithmetic order, which is similar to the situation in traditional grating. Different wavelengths are dispersed and focused at different locations in the output star coupler. Different wavelengths are received by different waveguides to achieve parallel demultiplexing of DWDM signals.
How does AWG works
In order to better understand the working principle of AWG, the concave grating is analyzed here. The concave grating structure is shown in Figure 4. The groove surface of the grating is distributed on a large circle with a radius of R=2r, and there is a small circle in front of the grating, whose radius r is half of the large circle. This small circle is called Roland circle. The concave grating has the functions of both traditional grating and lens. The beam emitted from any point P1 on the Rowland circle must be focused on another point P2 on the Rowland circle after diffraction by the concave grating. The relation (2) between the diffraction Angle θ and the incidence Angle α is satisfied.
According to equation (2), the diffraction Angle θ is wavelength dependent. When a polychromatic light wave is diffracted from point P1 by a concave grating, different wavelengths will be focused at different locations on the Rowland circle (near point P2).
Figure 4. Diffraction of concave grating
Now let's go back to the AWG discussion. The input/output star coupler has a structure similar to a concave grating. Figure 5 shows the structure of the output star coupler. The ports of the array waveguide are distributed on a large circle with a diameter of R=2r, while the ports of the output waveguide are distributed on a small circle with a radius of r (Roland circle). The structure of the input star coupler is similar to that of the star coupler, except that dozens of output waveguides are replaced by an input waveguide, and the port of the input waveguide is located in the center of the output waveguide.
Figure 5. Structure of the output star coupler
The analogy between a concave grating and a star coupler is shown in Figure 6. In a concave grating, a polychromatic beam is emitted from a point on the Rowland circle, and different wavelengths of light are focused at different points on the Rowland circle. In a star coupler, the DWDM signal is emitted from the central store C of the output waveguide (i.e. the mirror point of the input waveguide), which is on the Rowland circle. If reflective diffraction can occur in the array waveguide, as with a concave grating, then the different wavelengths will be focused at different points on the Rowland circle. The different wavelengths of dispersion expansion are then received by different output waveguides on the Rowland circle. The key point now is how to generate reflective diffraction in the array waveguide.
Figure 6. Analogy between concave grating and star coupler
Because the structure of the I/O star coupler is similar, we can fold the AWG, as shown in Figure 7. A mirror is arranged in the middle of the array waveguide to symmetrically separate the array waveguide. The left half of the array waveguide is mirrored to the right half, the input star coupler is mirrored to the output star coupler, and the input waveguide is mirrored to the center position C of the output waveguide. Therefore, the working process of AWG can be regarded as: DWDM signal input from the central position of the output waveguide C, through the output star coupler free transmission, distributed to the array waveguide; Multiple beams are transmitted to the mirror in the right half of the array waveguide, and the reflected beams enter the output star coupler. After the free transmission in the star coupler, the beam of different wavelengths is focused at different positions and received by the output waveguide, thus realizing the demultiplexing of the DWDM signal.
Figure 7. Folding operation of AWG
Array waveguide grating (AWG) is a key component of DWDM networks (dense wavelength division multiplexing systems) which are developing rapidly. AWG can obtain a large number of wavelengths and channels, achieve tens to hundreds of wavelengths of multiplexing and demultiplexing, and can flexibly with other optical devices to form multi-function devices and modules. The high stability and excellent cost performance are also one of the reasons why AWG is the preferred technology for DWDM.