Bragg grating technology augments dense WDM communications
The ability of Bragg gratings to preselect and fix wavelengths over fiber offers a stable, reliable and cost-effective improvement in dense wavelength-division multiplexing
Paul Sanders, Gary Ball and Laura A. Weller-brophy
Bragg grating technology offers increased performance and capacity of fiber-optics-based telecommunications and sensor systems through the incorporation of fiber gratings in transmission lines and device pigtailing, and in combination with fiber-related components.
Advances in fiber development and manufacturing processes are moving Bragg grating technology from the laboratory into product development. In addition, commercial Bragg grating products are being developed in accordance with customer demands for higher-capacity installed fiber-optic telecommunications networks.
Projections made by BNR Europe Ltd. indicate that bandwidth requirements for major networks are expected to exceed 100 gigabits per second by the year 2000. To handle those high speeds, dense wavelength-division multiplexing, or DWDM, is emerging as the viable transmission technology for increasing the available bandwidth of the installed fiber base.
Fiber Bragg gratings are anticipated to facilitate the implementation of DWDM networks by providing a calibrated means to preselect and fix wavelengths for transmitters, receivers, routers and channel filters. They are also projected for use in optimizing the performance of erbium-doped fiber amplifiers, or EDFAs. Moreover, fiber Bragg gratings are figured to provide the dispersion compensation required in the high-bit-rate operation of the installed base of non-dispersion-shifted fiber over the 1550-nanometer EDFA wavelength band.
The implementation of fiber Bragg gratings into a DWDM network involves the use of frequency-stabilized laser diodes that operate at approximately 1550 nm and are externally modulated onto a single fiber with wavelength-multiplexed signals.
Amplification of all wavelengths is performed using EDFAs with gain flattening. At the receiver end of the network, the signals are demultiplexed, dispersion compensation is done using fiber grating compensators, and the signals are coupled to receivers. Laser diodes, wavelength multiplexers, fiber amplifiers and dispersion compensators constitute--in part--fiber Bragg gratings.
Fiber gratings are made by exposing a singlemode fiber to a periodic pattern of intense ultraviolet light. The exposure produces a permanent increase in the refractive index of the fiber core, thereby creating a fixed index modulation according to the exposure pattern.
A small amount of light is reflected at each periodic modulation of refractive index. The contributions of light reflected by each period of the grating coherently add in phase when the wavelength of light in the fiber is equal to twice the grating period. This wavelength is referred to as the Bragg wavelength according to Bragg`s law (after 19th century scientist, Sir William L. Bragg).
Only the wavelengths that satisfy the Bragg condition are efficiently reflected by the periodic grating. The other wavelengths propagate through the grating with minimal attenuation. Grating reflectivity and bandwidth are adjusted through control of the grating length, grating period and amplitude of index modulation.
When properly fabricated, the grating spectrum remains fixed to a tight tolerance and does not drift significantly over the lifetime of the grating. Accelerated aging experiments suggest that fiber gratings suffer negligible change in peak reflectivity and spectral bandwidth after the gratings are properly annealed. Because the grating fabrication and recoating processes retain the high strength of the optical fiber, mechanical wavelength tuning over several nanometers has been successfully demonstrated through mechanical strain (stretching of the fiber) and by heating (thermal expansion).
Fiber Bragg gratings find widespread applications in fiber-optic networks and provide the frequency selection and stabilization needed for WDM usage. These applications include the frequency stabilization of external cavity semiconductor lasers, spectral filtering, fiber-amplifier optimization, dispersion compensation, fiber lasers and fiber sensors.
Gratings are being used to achieve wavelength and mode control of semiconductor diode lasers by serving as an external cavity reflector in the device`s fiber pigtail. This wavelength-selective grating establishes the lowest loss lasing mode, suppresses undesired resonances and narrows the spectrum of the lasing mode.
Frequency control of a low-cost Fabry-Perot laser diode has been demonstrated using this approach. Also, the demonstration of a grating-controlled 1.2-Gbit/sec directly modulated laser-diode experiment showed a 50-kilohertz source linewidth and a laser chirp of less than 500 kHz. Although the round-trip travel time of light in the external fiber cavity limits the direct modulation transmission rate of this device to several gigabits per second, high-speed transmission can be achieved by operating the diode in continuous-wave mode and using external modulation.
A commercial application of fiber gratings deals with the wavelength stabilization of 980-nm EDFA pump lasers. The output gain spectrum and the subsequent performance of this amplifier depends heavily on the wavelength of the pump source. Fiber-grating wavelength stabilization in this application offers a solution in setting and stabilizing the lasing wavelength and the added benefit of allowing relaxed device manufacturing tolerances for high-production yield. Another potential application focuses on the wavelength stabilization of uncooled semiconductor lasers.
Fiber Bragg gratings provide efficient bandpass, bandstop and channel add/drop filtering functions in other applications. A single grating acts as a wavelength-selective filter by reflecting wavelengths around the Bragg resonance. Several network components can be constructed in combination with other gratings and fiber-based components. For example, gratings written in the output segments of a fused biconic taper 1300/1550-nm wavelength-division multiplexer can improve channel isolation to expand the device`s role into more demanding applications.
In another application, the placement of identical gratings in the output segments of a 3-decibel coupler helps to configure a special bandpass filter. This filter passes only the band around the Bragg resonance through the adjacent input line and passes all other nonreflected bands through the output lines of the coupler. Adding a second coupler to these segments provides a Mach-Zehnder arrangement for a four-port channel add/drop filter. Channel add/drop filters can also be made by writing the gratings in the output lines of a circulator to redirect or drop reflected WDM channels.
In another fiber Bragg gratings application, light can be coupled out of the core of a fiber by tilting or blazing a grating at angles to the fiber axis. In this manner, light at the Bragg resonance is tapped out of the guided mode into a continuum of fiber-radiation modes. Besides coupling to radiation modes, long-period gratings that couple light into fiber cladding modes have been demonstrated. This approach provides bandstop filtering with ultralow backreflections.
In an approach analogous to the blazed grating-mode converter, output side-tap fiber gratings can be used to flatten the gain spectrum of fiber amplifiers. Fiber gratings used in all-optical feedback techniques have demonstrated better than a 0.1-dB gain flatness over a wavelength range of 1532 to 1560 nm.
The combination of passive filtering with gratings and optical gain control allows the development of high-linear amplifiers. Intracavity gratings can also be used to backreflect pump energy (pump folding) to improve gain efficiency.
Chirped or aperiodic gratings that reflect different wavelengths at different points along the grating have been proposed to compensate for dispersion penalties in operating the 1550-nm band on the installed base of nondispersion-shifted, 1300-nm singlemode fiber. To this end, the 30-picosecond compensation of 400-femtosecond pulses operating at 100 Gbits/sec have been demonstrated.
Fiber laser sources
Densely packed WDM, fiber-optic communications and sensor systems are expected to require arrays of low-noise lasers operating at a prespecified wavelength or in a specified comb of wavelengths. In addition to wavelength selection, convenient wavelength tunability is desired.
Although it is difficult to achieve the required wavelength specific to distributed feedback semiconductor diode lasers because of substrate inconsistency and manufacturing tolerances, fiber Bragg gratings can be used to conveniently define wavelength and limit lasing to a single frequency in both rare-earth-doped fiber lasers and external-cavity semiconductor lasers. Single-frequency linear fiber lasers that use intracore Bragg reflectors for cavity feedback and mode selectivity are important for their wavelength selectivity during manufacturing and for their engineering simplicity. To date, an externally modulated erbium-doped fiber grating laser has transmitted 2.5 Gbits/sec over 654 kilometers with a 10-9 bit-error-rate.
Bragg gratings serve as excellent fiber-optic sensing transducer elements. To accomplish this function, they are integrated into the light-guiding core of the fiber and are then arc-wavelength-encoded. This scheme eliminates the problems of amplitude and intensity variations that plague other types of fiber sensors.
These transducer elements are also conveniently multiplexed in a fiber-optic network because of their narrowband wavelength reflection. Fiber gratings have been embedded in composite-material smart structures and then monitored and tested with civil structures to monitor load levels. They have also been tested successfully as acoustic sensing arrays.
Applications for fiber grating sensors are estimated to emerge soon for the utility, process-control and aerospace industries. u
Paul Sanders is product manager of advanced products at 3M Specialty Optical Fibers in West Haven, CT. Gary Ball is general manager of Bragg grating technologies at 3M Specialty Optical Fibers in Bloomfield, CT. Laura Weller-Brophy is a research specialist in the 3M Telecom Systems Division`s Fiber Optic Laboratory in Austin, TX.