Exploring The Two Main Wave Lengths Multiplexing WDM Technology: TFF (Thin-Film Filter) vs. AWG (Arrayed Waveguide Grating)

Wavelength Division Multiplexing (WDM) is a sophisticated method for bolstering the bandwidth and enhancing the transmission capability of optical fibers. It achieves this by simultaneously transmitting numerous optical signals, each operating at distinct wavelengths. Among the prevalent WDM technologies are Thin-Film Filters (TFF) and Arrayed Waveguide Gratings (AWG), both widely adopted for their efficacy in managing optical signals across different wavelengths.

Thin Film Filter-TFF Technology

Thin-film filter (TFF) technology stands as a prevalent choice in the realm of WDM device technology. TFF effectively segregates or combines optical signals of varying wavelengths by leveraging the optical characteristics of specialized thin-film materials. Typically crafted from multiple layers of films with diverse thicknesses, these filters boast a structured arrangement and precise reflectivity. This configuration enables specific wavelengths to reflect within the thin film while permitting others to traverse through, thereby achieving signal separation and multiplexing. Noteworthy advantages of TFF technology include its uncomplicated construction, compact form factor, cost-effectiveness, and robust reliability.

TFF Technology

A multi-layer dielectric film filter represents a class of high-reflection film structured with numerous layers. These layers can span from several to potentially hundreds, alternating between two distinct dielectric materials possessing varying refractive indices. Adjacent to the air, layers on the filter substrate exhibit a higher index of refraction. An interference filter with precise wavelength selection attributes can be synthesized by amalgamating multiple layers of diverse dielectric films. This facilitates the capability to segregate or amalgamate different wavelengths effectively.

The Thin-Film Filter (TFF) is the pivotal and relatively pricey component within the entire Wavelength Division Multiplexing (WDM) apparatus. A three-port WDM device comprises a dual-fiber collimator, a single-fiber collimator, and the TFF filter. Positioned at the end face of the collimating lens of the dual-fiber collimator, the TFF filter serves the primary role of signal transmission and reflection. Within WDM signals encompassing wavelengths λ1, λ2,...λn, originating from a shared source, the TFF filter permits the transmission of one wavelength, λn, while the rest undergo reflection. Consequently, λn emerges from the transmission section, while the remaining wavelengths exit from the reflection end. This division transforms one input optical signal into two distinct optical signal outputs, constituting demultiplexing. Conversely, two input optical signals converge into a single composite optical signal output, constituting multiplexing.

Multiple three-port devices must be linked sequentially to construct a Wavelength Division Multiplexing (WDM) module to demultiplex all wavelengths, as depicted in the illustration. Within this setup, each three-port device features TFF filters with distinct transmission wavelengths. The resulting WDM module serves dual roles: it can function as either a demultiplexer or a multiplexer, contingent upon the direction of signal transmission.

The Wavelength Division Multiplexing (WDM) module, constructed from cascaded three-port WDM devices, often exhibits a relatively substantial footprint, with dimensions typically around 130×90×13mm³ for an 8-channel WDM module, which may not be suitable for specific specialized applications. A compact WDM module has been developed to address this limitation, as illustrated in the figure. The TFF filters are affixed onto a glass bench in this setup, while the input/output fiber collimators are meticulously aligned in sequence. The compact module boasts significantly reduced dimensions, typically measuring 50×30×6mm³, rendering it much smaller. These compact Dense Wavelength Division Multiplexing (DWDM) and Coarse Wavelength Division Multiplexing (CWDM) modules are commonly called CDWDM and CCWDM, respectively.

The Compact Coarse Wavelength Division Multiplexing (CCWDM) offers a more condensed solution. Here, adjacent channels are cascaded, not relying on bulky fiber cascade configurations but operating in a free-space setup under collimated beam conditions. The underlying principle involves using an input lens to concentrate the optical signals of wavelengths λ1, λ2...λn onto the initial filter. Subsequently, the optical signal-bearing wavelength λ1 traverses through the initial filter and is directed to the first output lens. Within the first output fiber, the λ1 optical signal is isolated, while the remaining signals are reflected by the initial slide onto the subsequent one for further signal separation. This process iterates until all signals are successfully separated. The interconnection between the wavelength channels occurs through zigzag patterns of light.

Arrayed Waveguide Grating-AWG Technology

In a Thin-Film Filter (TFF) Wavelength Division Multiplexing (WDM) module, varying wavelengths traverse various devices, resulting in distinct power losses. As the port count increases, the uniformity of loss deteriorates. Additionally, the maximum loss at the final port constrains the port count. Consequently, TFF-based WDM modules are typically restricted to ≤16 channels. However, a conventional Dense Wavelength Division Multiplexing (DWDM) system often necessitates transmitting 40 or 48 wavelengths through a single fiber. This demands multiplexer/demultiplexer units with high port counts. A serial structure would accumulate excessive loss at the terminal ports, prompting the need for a parallel structure capable of simultaneously multiplexing/demultiplexing dozens of wavelengths. The Arrayed Waveguide Grating (AWG) is a solution fulfilling this requirement.

Arrayed Waveguide Grating (AWG) technology is another prevalent Wavelength Division Multiplexing (WDM) device technology. It involves the fabrication of an arrayed waveguide grating on a chip substrate utilizing PLC (Planar Lightwave Circuit) technology. AWG serves to multiplex and separate optical signals of various wavelengths through a planar wavefront beam splitter on an optical fiber. Typically, AWG comprises a series of parallel waveguides arranged with specific regularity and lattice distribution on the optical waveguide. Each wavelength is directed through a designated waveguide, enabling the realization of signal multiplexing and separation. Compared with TFF technology, AWG technology offers superior wavelength isolation, channel count, and bandwidth. Consequently, it finds application in higher-speed optical communication systems.

The following image illustrates the typical structure of an AWG. It comprises five main components:

1. A transmitter waveguide

2. An input star coupler (FPR - free propagation region)

3. An array of waveguides

4. An output star coupler

5. Numerous receiver waveguides

The signals emanate from the transmitter waveguide and undergo separation as they propagate freely within the input star coupler, ultimately entering the arrayed waveguides. Notably, this separation process is colorless, meaning all wavelengths are uniformly distributed into the arrayed waveguides. A phase difference is induced among multiple optical beams within the arrayed waveguides. These beams' phases follow an arithmetic progression akin to traditional gratings. Consequently, the distinct wavelengths are dispersed and subsequently focused onto various positions within the output star coupler. Positioned at these focal points are the receiver waveguides. Each receiver waveguide corresponds to a specific wavelength, enabling the parallel demultiplexing of Dense Wavelength Division Multiplexing (DWDM) signals.

Both Wavelength Division Multiplexing (WDM) technologies have extensive applications in optical communication systems. Conventionally, Arrayed Waveguide Grating (AWG) technology is deemed more cost-effective for long-distance, high-channel-capacity Dense Wavelength Division Multiplexing (DWDM) scenarios, whereas Thin-Film Filter (TFF) technology is preferable for low-channel-capacity Coarse Wavelength Division Multiplexing (CWDM) metropolitan setups. 

Conclusion

TFF typically comprises multiple layers of films of varying thicknesses, with the thin film filter being the pivotal and priciest component of WDM devices. If a device requires more channels, the quantity of thin films must be augmented, thus elevating the TFF's price. Conversely, AWG allows for the simultaneous acquisition of 40 channels but lacks the flexibility to utilize individual channels selectively. Consequently, the cost of adding and dropping signals with ten channels remains equivalent to managing 40 channels. Hence, AWG proves to be more economical than TFF for scenarios involving many channels. Many sources cite 16 channels as the demarcation point between the two technologies. Applications requiring fewer than 16 channels are deemed suitable for TFF technology, while those necessitating more than 16 channels are better suited to AWG technology.