Acousto-optical tunable filter enables dynamic add/drop multiplexing

Sept. 1, 1998

Acousto-optical tunable filter enables dynamic add/drop multiplexing

Hector E. Escobar

Fujitsu Network Communications

The commercial availability of high-speed, high-capacity ultra dense wavelength-division multiplexing (dwdm) transmission systems requires service providers to manipulate and control their networks more efficiently at the optical level. This has created a need for optical equipment with greater functionality. Key to this functionality are the enabling technologies for optical add/drop multiplexers (oadms), which provide the ability to drop and add a certain number of wavelengths at an intermediate location where access to all the propagating optical channels is not required between dwdm transmission terminals. Advances in acousto-optic tunable filter (aotf) technology that enable the development of dynamically reconfigurable oadms represent one of the most important of these breakthroughs.

Current technologies

Currently, the two technologies being used for optical add/drop applications are optical thin-film interference filters and fiber Bragg gratings. Both fiber Bragg gratings and thin-film interference filters are based on passive devices that only allow oadm designs of fixed wavelengths. The wavelengths that can be dropped and added are hardware-dependent and do not provide flexibility for reconfiguring the network.

Thin-film interference filters work on the principle of transmission and reflection of energy, which is dependent in large part on the material used. The optical characteristics of the filter are determined by the layer thickness of the material and the number of cavities. The filter is designed to drop a specific wavelength or wavelength range. Thin-film interference filter technology is maturing, yet there are still technical challenges involved when fabricating very narrow and sharp band-pass filters for dwdm applications.

The developing technology of fiber Bragg gratings operates on the Bragg-effect principle. Here, periodic variations of refractive index are created to form mirrors longitudinally aligned inside the fiber core. The inscribed periodic mirrors reflect light at wavelengths that satisfy the Bragg condition, which corresponds to a wavelength twice the grating period times the effective index of refraction. Fiber Bragg gratings are used in conjunction with optical isolators to extract the reflected wavelength from the rest of the wavelengths transmitted by the grating.

Advanced dynamic technology

Research advancements have made aotf the only current technology that enables dynamically reconfigurable oadms. New aotf devices have been used successfully in a dynamic oadm design that provides a high level of functionality and flexibility to network operators and service providers. An aotf device is constructed by engraving titanium (Ti) waveguides in the lithium niobate (LiNbO3) semiconductor material (see Fig. 1). This device works on the acousto-optic-effect principle, in which a change in the refractive index of the material is caused by a strain generated by an acoustic wave. An electrical radio-frequency (RF) signal is applied to the LiNbO3 material, generating a surface acoustic wave that alters the refractive index of the semiconductor material and thus changes the optical propagation characteristics of the wdm signals.

Light at different wavelengths, containing the aggregate wdm channels, comes into the input port of the aotf. The polarization beam splitter (pbs) then separates the light into two polarization components--the transverse electric (TE) and transverse magnetic (TM) modes. These polarization modes branch out, propagating along two different paths.

The controlling RF signal`s frequency corresponds to a specific wavelength. When this signal is applied, the surface acoustic wave rotates the state of polarization of that wavelength by 90, changing the TE polarization to TM polarization (or vice versa). This is known as polarization-mode conversion. The output pbs then filters out the wavelength of light that has experienced a change in the polarization state.

Granular wavelength selectivity is accomplished by applying RF signals at different frequencies. These frequencies correspond to the different wavelengths (or wdm channels) that are propagating through the aotf and need to be dropped. The flexibility to drop a single wavelength channel or multiple channels in any combination, without restrictions, is provided by a single aotf device (see Fig. 2). This device allows complex configurations such as comb filtering, where every other wavelength of the aggregate wdm (wavelength) channels is dropped, and also has the capability of dropping all the wdm channels, if necessary. These capabilities provide two key features:

wavelength selectivity, which allows a simple software command to be sent to the aotf to select the wavelength to be dropped

remote reconfiguration, which allows the network to be reconfigured remotely, based on customer demand.

These features are advantageous, since oadms typically will be located at remote or intermediate sites within dwdm networks. Consequently, they reduce network operations and maintenance costs and accelerate service response time.

Note that adding or inserting channels into the transmission fiber is relatively easy compared to dropping optical channels at an oadm. When multiple channels are dropped, additional aotf devices are used in a cascade configuration to further perform the demultiplexing of wavelength channels into individual wavelengths. This demultiplexing produces accumulated internal equipment loss, which can be compensated for by using optical amplifiers.

The characteristics of the aotf are low insertion loss, bit-rate independence, and low power consumption. Recent technological advancements have made possible the development of temperature-stabilized aotf devices with high optical filtering performance. These devices provide narrow passband wavelengths with high channel isolation (which virtually eliminates crosstalk) and independent channel selectivity with an ample tuning range of about 80 nm within the 1550-nm wavelength region. The wavelength-selective switching speed is in the microsecond range.

aotf in action

An elegantly designed dynamic optical add/drop multiplexer has been developed using an aotf device. The capabilities of the dynamic oadm have been successfully demonstrated in an integrated optical network configuration, which included a 4-fiber bidirectional line-switched ring (blsr) and optical crossconnect systems. The demonstration network (see Fig. 3) was built around a 32-channel dwdm system, which operates at transmission speeds of up to OC-192 (10 Gbits/sec) per channel for a total capacity of 320 Gbits/sec. itu-compliant wavelength spacing of 100 GHz (0.8 nm) was used.

In this configuration, the dynamic oadm provides efficient wavelength routing and wavelength translation. Thus, optical input signals at any wavelength are converted to the appropriate 1550-nm wavelength region with the proper spectral width, laser-wavelength stability, and dispersion tolerance. Wavelength translation is an added feature that allows the oadm to interface with legacy and wavelength-incompatible systems for proper signal transmission over the wdm system, making the dynamic oadm wavelength- and bit-rate-transparent.

The aotf-based dynamic oadm permits a most efficient and elegant architecture (see Fig. 4) compared with other proposed dynamic oadm architectures, in which back-to-back wavelength- division multiplexers and demultiplexers are used in conjunction with a cross-bar optical switch on each wavelength. This latter configuration may dynamically drop the appropriate channels, but it requires a lot of equipment, is difficult to manage, and is very costly. The dynamic oadm based on aotf technology is suitable for use in long-haul wdm transmission as well as in short-haul metropolitan applications where network reconfiguration is most often performed.

The aotf technology promises an even more advanced dynamic oadm product in the near future, which includes sophisticated functionality that will be the basis for the survivable all-optical network. In addition to the fundamental application of dynamically dropping and adding wavelengths at an intermediate location between dwdm terminals, the dynamic oadm can be configured in an optical blsr architecture to provide a robust network protection scheme, all in the optical domain (see Fig. 5).

In this type of configuration, the dynamic oadm will, among other features and capabilities, perform the fundamental protection techniques of span switching in case of a transmission fiber break and ring switching in the event of an optical equipment failure of a certain node in a ring network architecture. As a result, the dynamic oadm will provide network protection functionality similar to that of the Synchronous Optical Network standard, which currently can be provided only in the electrical domain. This functionality will enable the network element intelligence to migrate from the electrical layer to the optical layer, giving way to low-cost, less-complex time-division-multiplexed electronic transmission equipment.

Advancements in aotf technology are permitting the design of highly functional oadm products for use in dwdm applications. These advancements also will enable new self-healing network architectures in the optical domain and make the long-envisioned all-optical network a reality. u

Hector E. Escobar is senior product manager of the optical networking group at Fujitsu Network Communications Inc. (Richardson, TX). He can be reached at Hector.gifscobar

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