WDM Wave Lengths Multiplexing Technology: TFF (Thin-Film Filter) & AWG (Arrayed Waveguide Grating)

WDM (Wavelength Division Multiplexing) technology is a technique used to increase the bandwidth and improve the transmission capacity of optical fibers by transmitting multiple optical signals of different wavelengths. TFF (Thin-Film Filter) and AWG (Arrayed Waveguide Grating) are two commonly used WDM technologies.

TFF - Thin Film Filter (FWDM) Technology

Thin-Film Filter (TFF) technology, also known as thin-film filtering, is pivotal in WDM devices like CWDM mux demux. It exploits unique optical properties of thin-film materials to separate or combine optical signals of differing wavelengths. Typically comprising multiple layers of films with varied thicknesses, these filters feature specific reflectivity patterns that reflect certain wavelengths while allowing others to pass through, facilitating signal segregation and multiplexing. Compared to alternatives, TFF offers advantages such as simpler structure, compact size, lower cost, and heightened reliability.

Figure 1 : TFF Technology

Multilayer dielectric film filters are a type of high-reflection film with multiple layers, ranging from dozens to hundreds of layers. They are composed of two types of dielectric materials with different refractive indices, alternating between layers. The layers adjacent to the filter substrate and air have higher refractive indices. By combining several layers of different dielectric films, an interference filter with specific wavelength selectivity can be created, allowing for the separation or merging of different wavelengths.

Figure 2 : Multilayer Dielectric Film Filters

The core of TFF technology lies in its application within a three-port WDM device. This setup typically includes a dual-fiber coupler, a single-fiber coupler, and the TFF filter positioned at the end face of the dual-fiber coupler's collimating lens. The TFF filter's function is pivotal: it allows the transmission of a specific wavelength λn while reflecting all others. This capability enables the demultiplexing of WDM signals, separating one input signal into two distinct outputs based on wavelength, or conversely, combining two input signals into a single composite output. This process underpins the device's role in optical signal routing and wavelength management in telecommunications and optical networks.

Figure 3 : Three-Port WDM Device with TFF Technology Setup

In order to multiplex all wavelengths, multiple three-port devices need to be connected in series to form a WDM module, as shown in the figure below. Each TFF filter in each three-port device has a different transmission wavelength.

Figure 4 : Cascade Connection of Three-Port Devices for WDM Module

Traditional three-port WDM modules are large (about 130x90x13mm³). To meet space-constrained needs, compact versions like CDWDM and CCWDM have been developed. To optimize WDM module size without compromising functionality, compact solutions like CDWDM and CCWDM utilize TFF filters on a glass substrate. These filters are individually aligned and fixed with input/output collimators. Unlike traditional modules, compact designs employ a free space cascading method. Incoming signals are focused by an input lens onto the first filter, separating wavelengths (in1, in2, ..., inM). Each wavelength exits through an output lens into the first output fiber. Remaining signals are sequentially reflected by prisms for further separation, following a "Z" path configuration. This innovative approach significantly reduces module dimensions to approximately 50x30x6mm³, meeting stringent space requirements in specialized optical communication applications.

AWG - Arrayed Waveguide Grating Technology

With the increase of ports, the uniformity of the TFF-type DWDM mux and demux deteriorates. At the same time, the maximum loss generated at the last port is another factor restricting the number of ports. Therefore, DWDM mux demux based on TFF technology usually have no more than 16 channels. However, a typical DWDM system typically transmits 40 or 48 wavelengths in a single optical fiber, hence requiring DWDM mux demux with larger port counts. WDM mux demux with a serial structure accumulate too much power loss at the rear ports, so a parallel structure is needed to simultaneously multiplex/demultiplex dozens of wavelengths. Thus, an Arrayed Waveguide Grating (AWG) technology has been introduced.

AWG technology is also a commonly used WDM device technology. It is based on the optical waveguide and utilizes a planar wavefront beam splitter on the optical fiber. It is a technology that uses PLCQ (Planar Lightwave Circuit on Quartz) technology to fabricate an array waveguide grating on a chip substrate to multiplex and separate light signals of different wavelengths. This means that it efficiently combines multiple wavelengths (Mux Wave) for transmission and subsequently separates them (Demux Wave) at the receiving end. AWG technology typically consists of a row of parallel waveguides with a specific distribution pattern and lattice on the optical waveguide, incorporating AWG filters to precisely manage wavelengths. Each channel's wavelength is guided out by a specific waveguide, allowing for signal multiplexing and separation. Compared to TFF technology, AWG technology offers higher wavelength isolation, channel count, and bandwidth, making it suitable for higher-speed optical communication systems.

As shown in the diagram, AWG structure includes an input waveguide, an input star coupler (represented by the free propagation region FPR in the diagram), an array waveguide, an output star coupler, and multiple output waveguides.

Figure 5 : Parallel Structure of AWG

The signal enters the input star coupler from the input waveguide and is then distributed to the array waveguides after free transmission. This distribution process is wavelength-independent, and all wavelengths are evenly distributed to the array waveguides. The array waveguides introduce phase differences to the multiple beams, with the phase of each beam forming an arithmetic progression, similar to traditional gratings. Different wavelengths are dispersed and focused at different positions in the output star coupler. Different wavelengths are received by different waveguides, enabling parallel demultiplexing of DWDM signals efficiently.

Conclusion

In WDM, Thin-Film Filter (TFF) and Arrayed Waveguide Grating (AWG) technologies offer distinct advantages tailored to different needs in optical communication. TFF, known for simplicity and cost-effectiveness, is ideal for applications with fewer channels (up to 16 wavelengths), including compact solutions like CDWDM and CCWDM for space-limited environments. In contrast, AWG excels in handling higher channel counts with superior wavelength isolation and bandwidth efficiency, making it cost-effective for complex WDM deployments requiring more than 16 channels. Additionally, products like FS CWDM and DWDM mux demux provide robust options for various network demands, ensuring scalability and performance optimization across different optical network architectures through advanced WDM multiplexing solutions.