Why is AWG demanded?
As we know, DWDM technology enables transmission of dozens of wavelengths in a single fiber, which expands the capacity of optical fiber communication enormously. The first mux/demux modules for DWDM system are based on thin-file filters (TFFs), as shown in Fig.1 and Fig.2. Both are designed in serial structure. Different wavelengths travel different number of devices in the module and result in different power loss. The loss uniformity degrades with increment of port number. Meanwhile, the maximum loss at the last port is another limitation on the port number. Thus the TFF-based WDM modules are usually limited to be ≤16 channels.
However, a typical DWDM system needs to transmit 40 or 48 wavelengths in a single fiber. Multiplexer/demultiplexer with high port number is required. A serial structure will accumulate too much loss at the last ports. Thus a parallel structure is demanded, which can multiplex/demultiplex dozens of wavelengths at the same time. Arrayed waveguide grating (AWG) is such a device.
Structure of AWG
Structure of a typical AWG is shown in Fig.3. It consists of five parts: a transmitter waveguide, an input star coupler (FPR (free propagation region) in Fig.3), arrayed waveguides, an output star coupler and dozens of receiver waveguides. The lengths of the arrayed waveguides are in arithmetic progression. Given L0 as the length of the first waveguide, the length Li of the i-th waveguide is as follow.
The DWDM signals emit from the transmitter waveguide and are separated into the arrayed waveguides after free propagation in the input star coupler. The separation is colorless, which means that all the wavelengths are separated into the arrayed waveguides identically. The arrayed waveguides generate phase difference between the multiple optical beams. The phases of the multiple beams are in arithmetic progression, which is just like the traditional gratings. Thus the different wavelengths are dispersed and then focused at different positions in the output star coupler. The receiver waveguides are set at the focusing positions. Different wavelengths are received by different waveguides and thus parallel demultiplexing of DWDM signals are realized.
Principle of AWG
We begin with the principle of a concave grating to better understandard that of the AWG. The structure of a concave grating is shown in Fig.4. The gratings are aligned on a large circle with radius R=2r. There exists a small circle with radius r, which is half of the large circle. The small circle is called Rowland circle. The concave grating provides both functions of a traditional grating and a lens. Light emitting from any point P1 on the Rowland circle will be focused on another point P2 on the circle after diffraction by the concave grating. The diffraction angle θ and incident angle α are related by Eq. (2).
According to Eq. (2), the diffraction angle θ is wavelength dependent. When colorful light emitts from P1, different wavelengths will be focused on different points (near P2) on the Rowland circle.
Now we come to discussion on the AWG. The input/output star couplers have the similar structure as the concave grating. Fig.5 shows the structure of the output star coupler. The endface of the arrayed waveguides are aligned on a large circle with radius R=2r, while the endface of the receiver waveguides are aligned on a small circle (Rowland circle) with radius r. The structure of the input star coupler is similar with the receiver waveguides substituted by one transmitter waveguide at the central position C.
Let’s see the analogy between the concave grating and the star coupler, as shown in Fig.6. In the concave grating, colorful light emits from one point on the Rowland circle. Different wavelengths are focused on different positions on the Rowland circle. In the star coupler, DWDM signals emit from the central point C of the receiver waveguides, which is on the Rowland circle. If reflective diffraction happens in the arrayed gratings as in the concave grating, different wavelengths will be focused at different positions on the Rowland circle. Then the dispersed wavelengths are received by the receiver waveguides aligned on the Rowland circle. The key point is how can reflective diffraction happen at the arrayed grating.
As the input/output star couplers are the similar, we can fold the AWG as Fig.7. A mirror is placed to symmetrically divide the arrayed waveguides. The left half arrayed waveguides are mirror to the right half. The input star coupler is mirrored to the output star coupler. The transmitter waveguide is mirror to the central position of the receiver waveguides. Thus the operation of the AWG can be regarded as follow. DWDM signals emit from the central position C of the receiver waveguides and are separated into the arrayed waveguides after free propagation in the output star coupler. The multiple beams propagate in the right half of the arrayed waveguides and reflected by the mirror. The reflected multiple beams emit to the output star coupler. After free propagation in the star coupler, different wavelengths are focused on different positions and are received by different receiver waveguides. Thus demultiplexing of DWDM signals is realized.
For more details about WDM devices, you can refer to our previous post: WDM Devices — TFF-based WDM Devices.
Written by Zhujun Wan, HYC Co., Ltd
 Meint K. Smit, Cor Van Dam, PHASAR-based WDM-devices: principles, design and applications, IEEE Journal of Selected Topics in Quantum Electronics, 2(2):236-250, 1996