FBG-based gain flattening filters aid optical amplifiers


Optical amplification

Erbium-doped fibre amplifiers enabled broad deployment of DWDM networks but first-generation EDFAs need frequent signal regeneration. Advances have enabled longer links between regenerators and improved performance.

By Arkell W Farr, Product Manager Amplifier components, Teraxion

For optical amplifiers, gain flatness is necessary to mitigate optical signal-to-noise (OSNR) and non-linear effects in DWDM networks. To equalise gain, a suitable gain flattening filter with a spectral response matching the inverse gain profile is incorporated within the amplifier. Fixed gain flattening filters (GFF) are therefore widely employed in EDFAs to correct for the non-uniform gain spectrum.

Among the technologies available for fixed gain flattening, the most widely employed are based on thin-film dielectrics and fibre gratings. GFFs based on fibre gratings include chirped Bragg gratings, slanted Bragg gratings, and long-period gratings.

GFFs have a significant impact on the level of gain ripple amplifier manufacturers can specify for their devices. The accumulation of gain ripple in a fibre link spanning many amplifiers will require regeneration of the optical signal periodically across the network. Signal regeneration imposes significant cost and complexities on the network. Proper selection of fixed GFFs can allow amplifier manufacturers to reduce gain ripple, thus offering significant economic benefits.

Amplifier gain ripple
Optical networks are currently being designed to incorporate components that allow for bandwidth management and pulse re-shaping in the optical domain. Additional amplification will be required to compensate for the cumulative losses caused by the inclusion of devices such as optical add/drop and dispersion compensating modules. Increased need for amplification will necessitate more frequent signal regeneration.

New EDFA designs are therefore addressing the need to reduce OSNR degradation attributed to amplification. To meet this challenge amplifier manufacturers are seeking ways to achieve greater gain uniformity. This will allow for more amplifiers to be cascaded along a fibre link, reducing the need for electronic repeaters.

Amplifier parameters which contribute to gain non-uniformity in an EDFA include, but are not limited to, the error function of GFFs, temperature dependence of the erbium gain profile, inhomogeneity of erbium ions, and amplifier manufacturing tolerances.

Error function specification
GFFs have a significant impact on the achievable levels of gain-uniformity in an EDFA. Recent advances in FBG and thin-film dielectric GFF manufacturing have allowed a number of GFF suppliers to offer improved error functions, defined as the difference between the customer-defined target and the actual transmission function of a manufactured filter (see Figure 2).

Amplifier components need to operate reliably over a large temperature range - often as much as 75°C. It is common for amplifier manufacturers to design in function of the worst-case operating conditions. So, the ability of a GFF to maintain its attenuation profile across the entire operating temperature range is of great importance.

All GFF technologies exhibit wavelength shifting with changing temperature. This inherent temperature sensitivity must be accounted for when determining the effective error function.

To simplify this analysis some GFF manufacturers include the effect of temperature shifting in the error functions that they guarantee to the end-user. Similarly, polarisation-dependent loss (PDL) is another source of flattening error that should be considered when determining the effective error function of a gain flattening solution.

Since the GFF is a critical component in an amplifier's design it should ideally remain within its error function tolerance over the lifetime of an amplifier. Any long-term wavelength drift and environmental degradation will have a direct impact on the EDFA's gain stability over time.

However, Telcordia qualification alone does not guarantee that a GFF will remain within the tight boundaries of its error function for the lifetime of the device. Careful attention must therefore be paid to long-term environmental testing results. Severe wavelength shifting during damp heat and high-temperature testing could indicate that the error function will deteriorate during the life of the device.

GFFs for high-performance EDFAs
High-performance EDFAs require as exact gain flatness as possible without incurring prohibitive costs. Appropriate GFFs need to offer a small effective error function - low error, low PDL, minimal wavelength shifting, and long-term reliability. Based on the relative performance of available GFF technologies (see Table. right), advanced chirped fibre Bragg gratings (FBG) are the optimal choice for gain equalisation.

Long-period gratings (LPG) may have an error close to FBGs, but severe thermal wavelength shift renders them unsuitable for high-performance amplifiers. Similarly, slanted Bragg gratings (SBG) are inappropriate for amplifiers that require low gain ripple, exhibiting excessive thermal shift and high PDL.

Thin-film dielectric filters are the most ubiquitous gain flattening technology, but performance has typically been inferior to competing solutions. Recognising the need for improved error functions, some dielectric manufacturers have improved the temperature sensitivity and error in their devices. The effective error functions of newly developed dielectric filters are now superior to slanted and long-period grating technologies.

The error functions of GFFs are somewhat dependent on the desired attenuation profile. Thin-film, slanted, and long-period technologies will show larger error functions as the GFF attenuation profile becomes more complex. Fibre Bragg gratings do not show this limitation and are the ideal choice when the required attenuation profile is irregular.

To enable lower gain ripple amplifiers, advanced fibre Bragg gratings are now available, offering nearly exact flatness regardless of attenuation profile depth and complexity. The effective error function of these filters is <±0.15 dB - this includes the error attributable to PDL and thermal wavelength shifting. Proper packaging guarantees this specification at the end-of-life of the amplifier.

Gain flattening filters based on advanced fibre Bragg gratings allow amplifier manufactures to improve gain flatness. Advanced FBGs can be used to replace other GFF technologies in current-generation amplifier designs as a simple way to improve gain ripple. Similarly, new amplifier designs can take advantage of this technology to help push the performance of next-generation amplifiers to new heights.

Amplifier cascading
The accumulation of gain ripple across a network link will be more severe if the ripple in each amplifier is similar. Gain flattening technologies which exhibit systematic errors will hasten the accumulation of signal power imbalance in the network.

GFFs based on dielectrics have error functions that are mostly systematic, i.e. the filter error as a function of wavelength will be nearly identical from one component to the next.

This is a consequence of the batch manufacturing process used in dielectric production. A stack of thin films is deposited on a large wafer with uniform properties across its surface. The wafer is then diced into very small pieces and each piece is packaged along with some collimating optics to produce a single device. Cascading GFFs with systematic errors will cause a linear accumulation of error in the network link.

However, fibre Bragg grating GFFs exhibit a high degree of randomness in their error functions. The fact that each GFF is manufactured individually and not in a batch process ensures that slight random variations occur between each filter. This randomness is manifested primarily within the high-frequency ripple inherent to the FBG. Cascading GFFs with random errors will cause a statistical accumulation of error, reducing the power discrepancy among the strongest and weakest channels in a long cascade of amplifiers.

GFFs figure prominently in the magnitude and composition of EDFA gain ripple. Careful selection of a GFF allows manufacturers to improve the ripple of their amplifiers. The optimal choice of filter to minimise EDFA gain ripple is the chirped fibre Bragg grating, which are widely found in field-deployed optical amplifiers because of their lower error functions and proven reliability. In addition, the random nature of FBG error provides for additional performance benefits in long cascades of amplifiers.

Historically, fibre Bragg gratings have been regarded as the higher-cost/higher-performance technology, but improvements in FBG manufacturability now make them very cost-effective. Recent advances in FBG manufacturing provide improved flattening capability. Advanced gain flattening filters based on FBG technology are available which offer errors as low as ±0.10dB across all operating conditions and states of polarisation while maintaining their performance until their end-of-life.

Arkell W Farr
Product Manager
Amplifier Components
Arkell Farr is Product Manager for TeraXion's amplifier components division. He is based in Quebec City, Canada.

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