|Wavelength division multiplexing (WDM)1 was suggested2 quite early on as a way to overcome the bandwidth limitation in fiber optical system. However, its applications were not widely accepted until significant progress had been made for optical amplifiers3 in the early 1990s. Erbium-doped fiber amplifiers (EDFAs) allow direct amplification of the optical signal of all avelengths. This makes WDM a cost effective way to increase the capacity of fiber optical communication systems by adding wavelengths (channels) to the existing system. The number of channels one can add to the system in large part depends on the filter performance. Earlier WDM systems typically used 2, 4, or 8 channels4. The rapid progress in the filtration technologies in the late 1990s made dense wavelength division multiplexing (DWDM) systems possible. DWDM systems with more than 160 channels in the C- and L-bands have been demonstrated and made commercially available.
Fig. 1 is a schematic of long haul, point-to-point DWDM systems. The system basically consists of ransmitters, modulators, multiplexer modules, optical amplifiers (EDFA), demultiplexer modules, and receivers. As indicated in the schematic optical filters are used in the Mux/Demux modules and optical amplifiers, both of which are the heart of a DWDM system.The following types of filters are used in DWDM systems:
• DWDM filters – this type of filters include 50 GHz, 100 GHz and 200 GHz filters. They are used to build multiplexer (Mux) and demultiplexer (Demux) modules on the ITU channel spacing of 50, 100, and 200 GHz. DWDM Muxes and Demuxes are the building blocks of DWDM systems, which are deployed in the long-haul and metro networks.
• Gain flattening filters or GFF – One of the most important components used in an erbium doped fiber amplifier (EDFA). GFFs flatten the gain of EDFAs, allowing multiple amplifications of all channels in a DWDM system over a long distance.
• High performance band splitters – They are key to the architect for high channel count DWDM systems and provide solutions for managing the bandwidth with great efficiency and flexibility. They are also extensively used in EDFAs.
• Wide band WDM filters – They are used in EDFAs as pump couplers and supervisory channel monitors. This family of filters covers a wide variety of other filters. Their applications range from CWDM (Coarse WDM), to bi-directional transceivers, to 1310/1490/1550 nm tri-band filters for fiber to the home (FTTH).
There are three competing filtration technologies: Thin Film Filters (TFF), Array Waveguides (AWG), and Fiber Bragg Gratings (FBG). Thin film filters were adopted very early on and have been widely deployed since because they have the unique attributes that meet the stringent requirements of optical communication systems.
These attributes include:
• Environmentally stable
• Thermally stable – its completely passive nature offers simple system integration
• Superior optical properties including
o Low insertion loss
o Wide and flat passband
o Excellent isolation, and
o Small polarization dependent loss
• Modularity and scalability
• Batch manufacturing processes
• Flexibility – sometimes TFF is the only viable solution
(To be continued)