paper of Ashish Vengsarkar et al.  that introduced the long-period
fiber grating (LPG) technology has won the honor of being the most cited
paper ever published in the Journal of Lightwave Technology. The reason
for the frequent citations is simple. This paper introduced a very important
new optical device platform that represented a key functionality in
optical communication systems at an important time in the evolution
of wavelength division multiplexed systems. The paper also established
new applications for optical fiber devices in the growing field of optical
sensing. The paper is well written and covers both the formal description
of LPGs (based on coupled mode theory), details on their fabrication
(based on photo-induced refractive index change), detailed discussion
of their optical properties (such as sensitivity to environmental effects),
and their potential applications. Of particular importance, the paper
discusses the important application of LPGs in optical communication
systems for gain equalization, associated with the introduction of erbium
doped optical amplifiers, which exhibit a complicated gain profile.
The LPG spectral shaping approach proposed by Vengsarkar et al. represented
an important solution to this problem and was the first viable solution
for gain equalization in long-haul optical communication systems.
Long-period gratings are periodic structures that couple co-propagating
modes in optical fibers. They are related to the well-known grating
assisted couplers that had been studied for many years in the context
of integrated waveguide structures and are discussed in Amnon Yariv’s
famous textbook. In the context of optical fibers, LPGs provide phase
matching between the core guided mode (i.e. the fundamental LP01) and
higher-order modes that propagate in the cladding region. The higher-order
modes are confined by the outer air boundary of the fiber, although,
higher-order modes can also be confined within the cladding, e.g. within
photonic crystal fibers . Because the modes are co-propagating, they
have similar propagation constants, and thus the associated period of
the grating is typically hundreds of microns and sometimes even millimeters.
The devices are known as long period gratings to distinguish them from
typical optical fiber Bragg gratings, which typically have a period
smaller than 500 nanometers. Since the coupling changes with wavelength
,the LPG acts as a wavelength-dependent loss element. Phase matching
occurs at discrete wavelengths determined by the phase matching condition
so the wavelength-dependent loss occurs at a range of different wavelengths,
associated with the excitation of specific higher-order cladding modes.
Optical fiber Bragg gratings had been discovered in the late 1970s by
Hill and co-workers  and Meltz et al. , and were already under
development by the time Vengsarkar discovered the optical fiber LPGs.
A similar type of long-period grating structure had already been demonstrated
by other groups several years before the Vengsarkar paper. For example,
the Hill group  reported on mode-converting LPG that exploited the
same principle of grating-induced phase matching to convert light from
the fundamental (LP01) mode to a higher order “core guided”
mode, either the LP11 or LP02 mode. The motivation of this earlier demonstration
was to convert the fundamental mode to a higher-order mode that exhibits
desirable properties, e.g. strong dispersion. More than a decade later,
these applications are now being vigorously pursued by numerous research
groups, as discussed further below.
The Vengsarkar paper also discusses the sensitivity of LPGs to both
strain and temperature, opening up important new potential sensing applications,
which have been pursued by many groups around the world. Because the
phase matching condition for LPGs depends on the difference between
the effective indices of the different modes, they are inherently sensitive
to variations in these parameters. For example, LPGs are typically an
order of magnitude more sensitive to temperature than fiber Bragg gratings,
which makes them very useful but can also make them difficult to package
and stabilize. This sensitivity can be further enhanced by tailoring
the cladding properties. For instance, the cladding mode can be made
very sensitive to external refractive index if the surrounding region
has an index slightly below the index of the glass cladding ; this
scheme can be implemented elegantly in a microstructured optical fiber
LPGs have now been demonstrated in a range of different optical fibers,
including photonic crystal fibers, polymer optical fibers, and even
chalcogenide optical fibers. They have been implemented using a range
of different approaches, including micro-bend gratings, splice-induced
gratings, and acoustic gratings that are completely reconfigurable.
The acoustic gratings  are a very elegant approach that has seen
commercial success and predates the photo-induced LPGs introduced by
Vengsarkar et al. The most significant current application of LPGs is
probably the application to mode-conversion for dispersion compensation,
as initially considered by Hill et al.  and Poole . It has been
known for many years that higher-order modes can exhibit very high dispersion
that can be useful for dispersion compensation in long-haul optical
communication systems. The earlier papers by Ken Hill  and Craig
Poole  show that these researchers realized this fact but did not
have the grating strength and quality that can now be achieved. Poole
also only considered coupling to the asymmetric LP11 mode, which is
highly polarization dependent. The recent results of Ramachandran and
co-workers at OFS Laboratories highlights one of the most compelling
current applications of LPGs to dispersion compensation and more recently
to high-power fiber lasers . Other applications of mode-converting
LPGs are being envisaged by Ramachandran and other research groups.
The Vengsarkar paper continues to be frequently cited as researchers
explore new geometries and new applications for LPGs. We must thank
Ashish and his co-workers for this significant contribution to the field.
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