Gain flattening

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FBG-based gain flattening filters aid optical amplifiers
by Arkell W. Farr, Product Manager, Amplification Division
20-360 Franquet, Sainte-Foy (Qc) Canada G1P 4N3 Toll free: 1 877 TERA sales@ teraxion.com
Erbium-doped fibre amplifiers (EDFA) enabled the broad deployment of DWDM networks. Limitations of first-
generation EDFAs, however, necessitated frequent signal regeneration in the network. Advancements in EDFA
performance have allowed for longer fibre links between regenerators. To reduce the cost of regeneration efforts
are ongoing to improve amplifier performance.
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 optic 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.
TERA03

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ERROR FUNCTION SPECIFICATION
GFFs have a significant impact on the achievable levels of gain-uniformity in an EDFA. Recent advancements 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 Fig.1).
Amplifier components need 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 of a GFF.
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 the 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 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.

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-8
-7
-6
-5
-4
-3
-2
-1
0
1530 1535 1540 1545 1550 1555 1560 1565
Wavelength (nm)
Transmission(dB)
Target
Dielectric filter
-0.4
-0.2
0
0.2
0.4
1530 1535 1540 1545 1550 1555 1560 1565
Wavelength (nm)
ErrorFunction(dB)
Figure 1: Error function of a GFF. The top graph compares the target profile to a
typical filter response, as obtained using thin-film dielectrics; the bottom graph shows
the resulting error function when taking the difference between the two.

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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),
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 shifting 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 of the
FBG ensures that performance is maintained through the end-of-life of the amplifier.
Gain flattening filters based on advanced fibre Bragg gratings allow amplifier manufacturers to improve gain
flatness. Advanced FBGs can be used to replace other GFF technologies in current- generation amplifier designs
as a simple means 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.
Comparison of commercial GFF technologies
Typical
chirped
Bragg
grating
Advanced
chirped
Bragg
grating
Slanted
Bragg
grating
Long-period
grating
Typical
thin-film
filter
Advanced
thin-film
filter
Error ±0.20 dB < ±0.10 dB ±0.25 dB ±0.20 dB ±0.30 dB ±0.25 dB
PDL 0.1 dB <0.03 dB 0.2 dB 0.1 dB 0.1 dB 0.1 dB
Wavelength
shift over 75°C <75 pm <30 pm <150 pm <250 pm <150 pm <100 pm

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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 (see Fig.2).
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 (see Fig.3).
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 chirped fibre Bragg gratings, 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 advancements 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 end-of-life.

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Figure 2: Effect of cascading dielectric-based GFFs. The graph compares the error of
a single dielectric GFF to the accumulated error of five similar dielectrics.
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
1530 1535 1540 1545 1550 1555 1560 1565
Wavelength (nm)
ErrorFunction(dB)
Total error of 5 dielectric filters
Error of 1 dielectric filter
-0.40
-0.30
-0.20
-0.10
0.00
0.10
0.20
0.30
0.40
1530 1535 1540 1545 1550 1555 1560 1565
Wavelength (nm)
ErrorFunction(dB)
Total error of 5 FBG filters
Error of 1 FBG filter
Figure 3: Effect of cascading FBG-based GFFs. The graph compares the
error of a single FBG GFF to the accumulated error of five similar FBGs.
Published in Lightwave Europe June 2002

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