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The 1996 paper of Ashish Vengsarkar et al. [1] 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 [2]. 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 [3] and Meltz et al. [4], 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 [5] 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 [6]; this scheme can be implemented elegantly in a microstructured optical fiber geometry [2].
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 [7] 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. [5] and Poole [8]. 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 [5] and Craig Poole [8] 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 [9]. 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.
[1] A.M. Vengsarkar, P.J. Lemaire PJ, J. B. Judkins, V. Bhatia, T. Erdogan, J. E. Sipe, “Long-period fiber gratings as band-rejection filters,” Journal of Lightwave Technology, vol. 14, 58-65 (1996).
[2] B. J. Eggleton, C. Kerbage, P. S. Westbrook, R. S. Windeler, A. Hale, “Microstructured optical fiber devices, “ Optics Express vol. 9, 698-713 (2001).
[3] K. O. Hill, Y. Fuji, D.C. Johnson and B.S. Kawasaki, “Photosensitivity in optical fiber waveguides - application to reflection filter fabrication,” Applied Physics Letters vol.32, 647-649 (1978).
[4] G. Meltz, W.W. Morey and W. H. Glenn, “Formation of Bragg gratings in optical fibers by transverse holographic method,” Opt. Lett. vol. 14, 823-825 (1989).
[5] F. Bilodeau, K. O. Hill, B. Malo, D.C. Johnson and I. Skinner, “Efficient narrowband LP01 to LP02 mode converters fabricated in photosensitive fiber: Spectral response,” Electron. Lett. vol. 27, 682-684 (1991).
[6] H. J. Patrick, A. D. Kersey, F. Bucholtz, “Analysis of the response of long period fiber gratings to external index of refraction,” Journal of Lightwave Technology, vol.16, 1606–1612 (1998).
[7] H. E. Engan, B. Y. Kim, J. N. Blake, H. J. Shaw, “Propagation and optical interaction of guided acoustic waves in two-mode optical fibers,” Journal of Lightwave Technology, vol. 6, 428-436 (1988).
[8] C. D. Poole, “Optical fiber-based dispersion compensation using higher-order modes near cutoff, Journal of Lightwave Technology,” vol. 12, 1746 (1994).
[9] S. Ramachandran, “Dispersion-tailored few-mode fibers: A versatile platform for in-fiber photonic devices,” Journal of Lightwave Technology, vol. 23, 3426-3443 (2005).

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