Fiber Bragg gratings stretch metro applications

by Changzun Zhou, Peter Chan, Jian Yang, and Peter Kung

Effective deployment of tunable filters will help reduce the cost of metro networks. Fiber Bragg grating filters based on thermomechanical, piezoelectrical, and actuator/stepper motor stretching techniques are promising means of achieving tunability.

The conventional WDM architecture for metro network systems, in which each node is equipped with scores of lasers, is too costly to be realistic. One potentially successful architecture is a structure of multiple logical rings based on simple wavelength tuning.¹ The logical ring network can be easily reconfigured without physically altering existing network components, and has practical potential to reduce overall cost of operations while increasing performance. Such a network would be enabled by several critical elements: switches, tunable lasers, low-cost broadband optical amplifiers, and tunable filters—among which tunable fiber Bragg grating (FBG) filters are an integral part of the solution.

Fiber Bragg gratings are wavelength-selective, reflective filters with steep spectral profiles, created by exposing a core of germanium-doped silica fiber to ultraviolet light through a phase mask (see Fig. 1). The resulting interference pattern is recorded as a periodic index change along the fiber (see Fig. 2). The two inventing organizations of FBG technology, Communications Research Centre and United Technologies, license the technology to about 30 companies, supporting a large variety of Telcordia-qualified products such as pump lockers, WDM filters, gain-flattening filters, and dispersion-compensation filters. Fiber Bragg gratings are considered a mature technology and have been deployed in high-reliability submarine networks and long-haul applications.

The tunable grating filter is an extension of this mature technology. The optical fiber is a stretchable medium. As the fiber with the grating inside is stretched, the index perturbation period is lengthened linearly, changing the center wavelength of the filter, λ_c (see Fig. 3). According to the Bragg grating equation, before stretching,

λ_c = 2n_eff Λ_c

where n_eff is the effective index of the fiber containing the gratings and Λ_c is the period size of the index perturbation. By stretching the fiber grating through a distance ΔΛ, we get a corresponding change in the center wavelength (without considering stress effect on the effective index):

Δλ_c = 2n_eff ΔΛ

If the stretching is far below the elastic limit, this process is perfectly linear and recoverable. In fact, many grating manufacturers have used this effect to form the temperature-compensation packaging for multiplexing/demultiplexing, gain-flattening, and dispersion-compensation devices.

This stretching concept can be extended to tunable filter operation. The manufacturing of fiber gratings is well-established, user-friendly, and flexible. In a very short time, a custom grating can be developed as a special filter. Filters can be made for the C-, L-, and S-bands, and for operation at 1310 nm. The key to success is to develop a high-pull-strength stretching process for fiber—the higher the pull-strength, the higher the elastic limit.

STRETCHING METHODS
There are several ways to stretch a fiber grating: thermomechanically, piezoelectrically, and using an actuator/stepper motor (see Fig. 4). For example, thermomechanical stretching can use a bimetal differential expansion element that is susceptible to temperature change. A mechanical amplifier in the device is designed to magnify the expansion. A Pelletier heating/cooling element attached to the bimetal structure controls the amount of stretching, and therefore the resolution, of the filter. This method is low-cost, but has limitations: the tuning is slow, the temperature of the bimetal structure takes time to stabilize, and environmental temperature can be a factor.

FIGURE 4. Fiber Bragg gratings can be tuned by thermomechanical stretching in a bimetal structure, in which the high expansion bar stretches more than the low expansion frame with increasing temperature (and contracts with decreasing temperature; top). The piezoelectric method uses a piezo stack that expands or contracts the grating with increasing or decreasing current (center). A stepper-motor can push or pull one end of the structure, which tunes the grating (bottom).

The thermomechanical method also offers limited spectral range, up to 10 nm. This mechanism has been tested for 300,000 cycles with a center wavelength shift of less than 10 pm—within the measurement limit of the optical spectrum analyzer used in the reliability testing. This method, once refined, should be suitable for low-cost tunable transponders for metro-area networks. In particular, for customer premises equipment, it will be useful for applications such as the front end of an optical PBX where speed is not paramount.

Instead of using the bimetal structure and the Pelletier cooler, one can substitute a piezoelectric stack in conjunction with the mechanical amplifier structure. The controller for the piezo stack is more complex, requiring high voltages. The piezoelectric effect provides fine resolution but limited range. A larger range can be obtained by adding several piezo stacks on top of one another; however, this adds to the size and cost. The piezoelectric effect has inherent hysteresis, necessitating the deployment of an external reference, which also adds to the cost and complexity. The only other attribute of the piezoelectric method is a speed enhancement. The influence of environmental conditions remains. Like an arrayed waveguide, which is very sensitive to temperature, the stretched grating must be maintained at an elevated temperature.

The stepper-motor method provides middle-of-the-road performance and cost. Actuators such as those used in stepper motors have been deployed for many years with large-capacity hard disks to support hundreds of millions of movements. As a result, they are reliable and inexpensive. Using stepper motors, access speed can also be optimized for the application. Finally, this method is capable of extended range—engineers can custom-design an actuator based on the required stretching force and size. For example, a load cell can provide accurate feedback on the amount of tension and can be correlated to physical displacement. It is capable of providing the smallest-size solution, a reasonable tuning speed, and optimized cost of ownership.

Since all the tuning mechanisms involve the stretching of fiber, tuning speed is always an issue. The current techniques are not likely to ever achieve subnanosecond tuning speed, which would be needed for reading optical headers. As for rapid provisioning, use of these methods for tunable laser and tunable filter applications is quite sufficient. One technology in development uses the electro-optic effect on polled sol gel material to achieve high-speed index modulation. In the future, such integrated photonics technologies will yield low-cost, multifunctional, fully integrated, arrayed tunable devices.

RELIABILITY
There are three emerging applications for tunable filters: network monitoring, bandwidth on demand, and applications complementary to tunable lasers. Each application will require a different mode of operation that determines the performance. Tunable grating filters offer full custom solutions but must meet many important challenges (see "Potential tunable filter applications and requirements," p. 36).

Mechanical failure can lead to complete loss of the optical performance of tunable fiber gratings. To improve endurance and minimize the chance of performance loss, it is important to start with strong gratings. Endurance is also affected by every aspect of production: fiber handling, stripping, grating printing, dust control in the production environment, and grating mounting. Precautions at each step of the way must be taken to prevent seed defects from forming, which can give rise to cracks that lead to failure.

When applied stress is not sufficiently high, mechanical failure occurs over time rather than instantaneously. This phenomenon is referred to as fatigue, characterized into two categories: static fatigue caused by constant applied stress, and cyclic fatigue caused by variable applied stress. For tunable filters, static fatigue is related to long-term activity on a set channel, and cyclic fatigue corresponds to tuning among different channels frequently. In some applications, tuning may be both static and cyclic, in which case residual grating life can be calculated by integrating the consumed portion of fatigue life under different tuning conditions.

In addition to the static or cyclic tuning mode, life endurance of a tunable filter can be affected by many other factors, the most important of which are tuning range, holding time at maximum, tuning speed, relaxation time, and environmental conditions such as temperature and humidity. In many different materials exhibiting cyclic fatigue, including metals and ceramics, the life endurance will increase exponentially with decreasing load. For tunable filters, this means the higher the tuning range, the shorter the lifetime. Tests have achieved 1.2 million cycles at a range of 30 nm, and 1200 hours have been reached at 37 nm for static tuning (stretch-hold).

Tunable filters maximize return on investment for network providers via rapid provisioning and scalability. Eventually, lower-cost tunable devices will be affordable for fiber-to-the-business applications. Tunable FBG filters are a disruptive technology capable of changing the photonics landscape, reducing the cost of systems by a factor of ten, and allowing carriers, as well as equipment builders, to increase revenue.

REFERENCE
1. R. Dhar and M. Lowry, WDM Solutions 3 (9), 83 (September 2001)

Changzun Zhou is reliability engineering manager, Peter Chan is product manager, Jian Yang is R&D engineer, and Peter Kung is CEO of Bragg Photonics, 880 Selkirk, Pointe Claire, Quebec, Canada H9R 3S3. Changzun Zhou can be reached at c.zhou@braggphotonics.com.

Potential tunable-filter applications and requirements

There are three emerging applications for tunable filters: network monitoring, bandwidth on demand, and complementary solutions to applications requiring tunable lasers. Each application will require a different mode of operation that determines the reliability performance of the module. Tunable grating filters offer full custom solutions but must meet the following challenges.

Reliability. The filter must meet the Telcordia guidance and support standards of at least 25 years of life.

Resolution. Depending on the application, one may want the tunable grating to jump from grid to grid (50 or 100 GHz), or to scan the whole spectrum with sufficient resolution to observe in-channel power, channel activities, stability of the transmitting wavelengths, and out-of-channel noise.

Repeatability. A tunable filter must be able to tune consistently to the specific wavelength and hold on it accurately.

Stability. A filter requires proper engineering so that it is immune to environmental changes such as temperature and humidity.

User-friendliness. A tunable filter must incorporate intelligence that allows carriers to monitor or provision the network from a central location.

Tuning range. For tunable gratings, there is a tradeoff between tuning range and reliability. A scanning mode, as used in network monitoring, involves stretching and relaxing the grating.

Footprint. Small-size and lightweight filters are important for portability and system design.

Power consumption. Low power consumption uses less power and requires less air ventilation in the cabinet.

High-power handling. Metro networks are tributaries of the long-haul network, receiving data in the form of DWDM. In the future, it's conceivable to have more than 100 channels in fiber. Thus, a tunable filter must be capable of handling up to 300 mW of total optical power.

Availability. The technology behind the tunable filter must be manufacturable. High volume will drive down the price of the tunable filter.

Flexibility. The technology must support various filter characteristics—in other words, each filter can be customized to suit a particular application.