As voice, video, and data traffic continue to increase, improving the flexibility and capacity of optical networks is a pressing challenge. Multiband optical filters have been used in applications as varied as astronomy, fluorescence spectroscopy, and 3D cinema.

The ability to combine several spectral bands into a single optical filter enables flexibility of network design and minimizes the number of components installed. In telecommunications, multiband optical filters provide the wavelength selectivity required for optical add-drop module (OADM) and wavelength division multiplexing (WDM) functionality.

Advancements in optical filters have supported the development of dual and multi band-pass filters in telecommunications. This filter design approach has also spawned hybrid gain-flattening filters that couple gain-flattening functionality with laser pump blocking. These components implement multiband optical filters to improve performance and save module build costs in telecom networks.

Use of Filters in Wavelength Division Multiplexing

Wavelength Division Multiplexing (WDM) technology has been in use since the late 1990s. WDM supports the transmission of multiple data protocols – using different wavelengths of light – over a single fiber pair. It eliminates the need for separate fiber pairs for voice, video and data.

WDM supported the enormous growth in bandwidth that was required for the World Wide Web. Designers used thin-film filters to add or drop channels in optical fibers. These thin-film filters transmit or reflect a single channel (or wavelength), or sometimes a group of channels using skip filters.

Inserting individual band-pass filters sequentially achieves the desired wavelength selectivity required in WDM. However, one filter component per band is inserted. This increases a module’s footprint, insertion losses and costs. Multiband optical filters are used to transmit two or more WDM channels, replacing two or more single band-pass filters with a single component.

Multiband optical filters are supporting the demands of 5G wireless transmission, and increased connectivity requirements amongst consumers, businesses, and other organizations. Multiband optical filters continue to play a major role in extending the lifetime of existing optical fiber infrastructures.

Multiband Optical Filters

An optical filter is a device that transmits light in a particular range of wavelengths, while reflecting or blocking the rest. An optical filter can pass long wavelengths, short wavelengths, or a band of wavelengths. Their optical properties are described by their frequency response, which is a measure of how the filter modifies the magnitude and phase of an optical signal.

Single band-pass filters have been traditionally used in telecommunications. They transmit one wavelength range while reflecting or blocking the rest. The core telecommunication range starts around 1260 nm, rising up to between 1650 nm and 1675 nm. Across this range, several single band-pass filters may be applied. For example, ITU stipulates a channel spacing of 20 nm for Coarse Wavelength Division Multiplexing (CWDM). So, starting with a central wavelength of 1270 nm, a network architect can insert 1290 nm, 1310 nm, 1330 nm, etc., filters up to 1670 nm.

It is worth noting that band-pass filters must be packaged into a component containing the filter, two collimating lenses, and a fiber lead out. If using two or more single band-pass filters within a module, a network designer must connect each component to the next. If, however, they use a dual band-pass filter, they would save at least one fiber connection. If they use a triple band-pass filter, they would save at least two connections. It is clear to see how this reduces the overall footprint for the equipment “closets” at wireless interconnect stations. This becomes critical as the number of stations increases in 5G networks and the cost of urban real estate rises.

Multiband optical filters simplify network design and reduce insertion losses, thus improving network performance. Further, the costs of using multiband optical filters do not rise linearly with the number of bands. In fact, the cost is proportional to the steepness of the dead band, which determines design thickness and how difficult it is to construct. Multiband optical filters are priced less than 20% more than single band-pass filters, despite doubling component capability and improving performance.

Multiband optical filters are used in wireless station interconnects for 4G, LTE and 5G systems. They are also applied in WDM networks, where minimizing component footprint is required. Dual or triple band-pass filters are the most typical configurations installed in the telecommunications wavelength range.

Multiband optical filters enable network designers to optimize the price-performance balance of their designs in WDM, PON and EDFA modules. They simplify design and offer comparable or better performance than single band-pass filters at little or no additional cost.

Iridian Multiband Optical Filters

Iridian broke ground in the use of multiband optical filters in telecom applications. Channel spacing is a key multi band-pass filter design difficulty. Iridian achieves consistent quality and performance, ensuring the stable high-volume supply of these multi band-pass filters.