Long-Period Fiber Gratings as Band-Rejection Filters

Properties
In Figure 6, we show the transmission spectrum of a strong
grating of length 2.54 cm written with a grating period
= 402 μm. The maximum loss is 32 dB at a peak wavelength
λ p = 1517 nm, the 20 dB isolation point is
4 nm wide, the 3 dB point (λ) is 22 nm, and the
insertion loss is < 0.20 dB for wavelengths < 1480
nm and > 1545 nm. Viewed over a broader wavelength
range, a typical spectrum is shown in Figure
7 (for a FLEXCOR fiber with = 198 μm) which
clearly indicates the different cladding modes.
Several different gratings with periodicities in the
150–600 μm range were fabricated and the different
peaks in the spectrum were categorized according to
the different cladding mode orders. The experimental
results of the complete DSF characterization are
shown in Figure 8. One of the key features that distinguishes
the long-period gratings from their shortperiod
blazed counterparts is the ultra-low backreflection
properties. An optical coherence domain
reflectometer trace for a long-period grating is
shown in Figure 9. The backreflection numbers
are consistent with our estimate (based on detailed
growth and annealing experiments) of maximum
UV-induced n of 5 × 10 −4 . A measurement of
polarization properties of these devices shows the
polarization mode dispersion (PMD) < 0.01 ps and
polarization dependent loss (PDL) < 0.02 dB.
Applications
The most common use of the long-period gratings is
as a band-rejection filter and several applications
have been explored. The grating of Figure 6 can be
used as an ASE suppressor, presenting a near-transparent
path to the pump (1480 nm) and signal
wavelengths (> 1545 nm) in erbium-doped amplifiers.
A band-rejection filter is also used to remove
unnecessary Stokes’ orders in a cascaded Raman
amplifier/laser [18]. For example, the grating of
Figure 10 is used to reject extraneous 1310 nm light
while simultaneously allowing the 1240 nm pump
wavelength to be transmitted with minimum loss.
The band-gap filling effect [19] seen in Bragg-grating
stabilized 980 nm pump-diodes can also be
countered using a small band-rejection filter at
∼ 1 μm. This effect is shown in
Figure 11, where the tendency of the laser to split its
spectrum and lase at λ >980 nm (thus rendering it
ineffective for erbium pumping applications) is
curbed by the addition of the filter whose transmission
spectrum is shown in the inset [20]. In coarse
WDM applications, it is often critical to insert a
cleanup filter after the demultiplexer (at the receiver
end) to remove vestiges of the channel that is not
being monitored. For a 1310/1550 nm system, we
can fabricate this filter by splicing two gratings separated
by a few nanometers in peak wavelength.
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With concatenation, we have fabricated devices with a 15 dB
bandwidth of 20 nm, a 20 dB bandwidth of 10 nm
(centered around 1555 nm), with an insertion loss < 0.20 dB
at 1310 nm.
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Figure 11: Effect of band-rejection filters on grating-stabilized
980 nm pump diodes. (a) Bragg-grating stabilized semiconductor pump diode
shows a split in the spectrum. (b) The addition of a long-period grating moves
the peak lasing wavelength toward 980 nm. Inset shows transmission spectrum
of long-period grating [20].
June 2007 IEEE LEOS NEWSLETTER 17
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