WDM (Wavelength Division Multiplexing) is a technology that expands the optical fiber transmission bandwidth and improves network transmission capacity by transmitting multiple optical signals of different wavelengths in the optical fiber. TFF (thin film filter) and AWG (arrayed waveguide grating) are two commonly used WDM technologies. Here we will take an overview and comparison of these two WDM technologies.
- TFF technology
TFF (Thin-film filter) technology is a commonly used WDM device technology. It uses the optical properties of special thin-film materials to separate or multiplex optical signals of different wavelengths. Thin-film filters are usually composed of multiple film layers of different thicknesses, with a certain regularity and specific reflectivity, allowing specific wavelengths to reflect in the thin film, while other wavelengths pass through these layers, achieving the separation and multiplexing of signals. The advantages of TFF technology are its simple structure, small size, low cost, and high reliability.
Multi-layer dielectric film filter is a type of multi-layer high-reflection film. The number of film layers can reach dozens or even hundreds, alternately composed of two dielectric materials with higher and lower refractive indices. The filter substrate Layers adjacent to air have a higher index of refraction. Combining dozens of layers of different dielectric films to form an interference filter with specific wavelength selection characteristics can achieve the effect of separating or combining different wavelengths.
The TFF is the most essential and expensive component in the entire WDM device. The structure of a three-port WDM device includes a dual-fiber collimator, a single-fiber collimator, and a TFF filter. The TFF filter is attached to the end face of the collimating lens of the dual-fiber collimator, and its main function is to transmit and reflect signals. WDM signals include wavelengths λ1, λ2,...λn, which are input from the common end. The TFF filter allows one wavelength λn to be transmitted, while other wavelengths are reflected, so the wavelength λn is output from the transmission section, while other wavelengths are output from the reflection end. Wherein, one input optical signal is divided into two different optical signal outputs, which is demultiplex; two input optical signals are synthesized into one mixed optical signal output, which is multiplex.
In order to demultiplex all wavelengths, n three-port devices need to be connected in series to form a WDM module, as shown in the figure, where the TFF filters in each three-port device have different transmission wavelengths. The WDM module can be used as a demultiplexer or a multiplexer, depending on the direction of signal transmission.
The WDM module based on cascaded three-port WDM devices has a relatively large size (typical size for an 8-channel WDM module is 130×90×13mm3), which does not meet the requirement by some special applications. Compact WDM module is developed for such applications, as shown in Fig. The TFF filters are fixed on a glass bench and the input/output fiber collimators are aligned one by one. The size of the compact module is typically 50×30×6mm3, which is much smaller. The compact DWDM and CWDM modules are usually called CDWDM and CCWDM, respectively.
CCWDM is more compact. Adjacent channels are cascaded, not through a bulky fiber cascade solution but rather in free-space, under collimated beam conditions. The principle is to use an input lens to focus the optical signals of the wavelengths λ1, λ2...λn on the first filter; the optical signal with the wavelength λ1 passes through the first filter and is coupled to the first output lens. In the first output fiber, the optical signal of wavelength λ1 is separated; the remaining optical signals are reflected by the first slide to the next slide for optical signal separation; and so on, until all signals are separated. The coupling between the wavelength channels is achieved in the form of zigzag lines of light.
- AWG technology
In TFF WDM module, 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.
AWG (Arrayed Waveguide Grating) technology is also a commonly used WDM device technology. It is an arrayed waveguide grating fabricated on a chip substrate using PLC technology through a planar wavefront beam splitter on an optical fiber, to multiplex and separate optical signals of different wavelengths. AWG usually consists of a row of parallel waveguides with a specific regularity and lattice distribution on the optical waveguide. Each wavelengths will be guided out by a specific waveguide, so that multiplexing and separation of signals can be realized. Compared with TFF technology, AWG technology has higher wavelength isolation, channel count, and bandwidth, and can be used in higher-speed optical communication systems.
Structure of a typical AWG is shown in Fig. It consists of five parts: a transmitter waveguide, an input star coupler (FPR (free propagation region)), arrayed waveguides, an output star coupler and dozens of receiver waveguides.
The 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.
Both of these WDM technologies are widely used in optical communication systems. It is generally believed that AWG has a higher cost-effectiveness in long-distance, high-channel-capacity DWDM applications, while TFF is more ideal for low-channel-capacity CWDM metropolitan applications. TFF usually consists of multiple layers of films of different thicknesses, and the thin film filter is the key and most expensive component is the WDM devices. If a device with a large channel is required, the number of thin films needs to be increased, and therefore the price of TFF increases. By using AWG, 40 channels can be obtained simultaneously, but there is a disadvantage that you can't just choose one or two of them, which means that the cost of adding and dropping with 10 channels is the same as that with 40 channels. Therefore, AWG is more economical than TFF in the case of a large number of channels. Many sources regard 16 channels as the conversion point of the two technologies. Applications lower than 16 channels are suitable for TFF technology, while applications higher than 16 channels are suitable for AWG technology.
ABOUT HYC
With more than 20 years’ experience of WDM manufacturing, HYC can provide full series of WDM products, including CWDM, DWDM, FWDM, CCWDM, etc.