Fabrication of long period fiber gratings of subnanometric bandwidth

Significance Statement

Gratings in optical fibers have been identified as periodic modulations of the fiber structure that couple energy between different modes. They have been classified in two categories, which are Bragg gratings with periodicity matching half a guided wavelength, and long period gratings with a period longer than the wavelength. Bragg gratings couple energy at selected wavelengths between counter propagating modes and to radiation modes. Long period gratings couple energy between co-propagating modes.

The bandwidth of the Bragg gratings is in the range of a few pm to hundreds of pm. On the other hand, long period gratings have broader spectra due to the fact that the number of periods of the gratings is smaller. Their bandwidth ranges between a few nanometers to hundreds of nanometers. To reduce the rejection bandwidth below 1nm, association of gratings in coaxial interferometric configuration is needed. However, individual gratings possess a larger bandwidth while the spectral sub-bands cannot be designed independently.

The need for designing long period fiber gratings with narrow rejection bands for core—cladding conversion is still outstanding. These grating can be implemented in signal processing, rejection filters for telecommunications, and DWDM components. Therefore, researchers led by Dr. José Luis Cruz at the University of Valencia in Spain demonstrated a long period grating written in an optical fiber with 3dB bandwidth below the frontier of 1nm at 1527 nm and more than 100 nm of free spectral range. In their work, they discussed the fabrication procedure, grating attributes and sensitivity to mechanical and thermal perturbations. Their work is published in Optics Letters.

In their experiments the authors used a fiber with NA of 0.29. The selected fiber had a modal field diameter of 4.2µm and a cutoff wavelength of 1387nm. They wrote a Bragg grating in order to get the information about the indices of the cladding modes. They then inscribed the Bragg grating over the fiber through a phase mask. A laser beam at 244nm was focused with lens, and the flux of energy in the fiber approximated as 160J/mm2. A number of couplings to cladding modes appeared at short wavelengths. The main series of resonances corresponded to HE1m and EH1m cladding modes. A secondary series of notches appeared between notches of the main series. This series corresponded to modes TE0m, TM0m and HE2m. These resonances have been observed to appear in strong gratings owing to fiber imperfections as well as fabrication insufficiencies.

The research team demonstrated the narrowband long period gratings fabrication implementing a high NA fiber with core and cladding modes having differences between indices larger than those of low NA fibers. The period of the gratings was short in this fiber, therefore, the number of periods for a selected length was higher, and as a result, the bandwidth was reduced.

Bandwidths of 0.83nm were realized in 15cm long gratings and were observed to have excellent spectral quality. The bandwidth was reduced to 0.68nm in 20cm long gratings, but the spectral quality was a little bit poor. The spectrum had a 125nm free spectral range. The resonance that corresponded to the HE1,18 mode exhibited superior temperature and strain sensitivities. The stability of the UV beam and room temperature in the course of fabrication forms the basis for further improvement of the main limitations of the study.

Fabrication of long period fiber gratings of subnanometric bandwidth

About The Author

Antonio Díez-Cremades is Associate Professor in the Department of Applied Physics, University of Valencia, Spain. He is a member of the Laboratory of Fiber Optics. He received his PhD degree in physics in 1998 from the University of Valencia, Spain, where he was involved in the design and fabrication of optical fiber sensors. While doing his PhD, he did several stays at the Optoelectronic Research Centre, Southampton, U.K. In 1999 he joined the Optoelectronics Research Group at the University of Bath, U.K, for two years, where he was engaged in research on acousto-optic effects in standard fibers and in photonic crystal fibers.

His current research interests include photonic crystal fibers, fiber lasers, optical fiber devices for signal processing, and optical sensors. He is the author or coauthor of over 80 papers on waveguide propagation and optical fiber technology and holds 6 patents.

About The Author

Professor Jose Luis Cruz was born in Cuenca (Spain) in 1964. He received the Ph. Degree in Physics from the University of Valencia (Spain) in 1992. Initially his career focused on microwave devices for radar applications, afterwards, he joined the Optoelectronics Research Center of the University of Southampton (UK) where he was working in optical fiber fabrication and he is currently Professor of the Applied Physics Department at the University of Valencia where he is conducting research on fiber lasers and amplifiers fiber gratings,  microwave photonics and sensors.

He is co-inventor of six patents related with the above mentioned topics, has published over 140  papers in international journals and more than 200 in conferences.  He has also been Vice-Dean of the Physics Faculty and was Director of the Applied Physics Department for thirteen years.

About The Author

Luis Poveda-Wong was born in San José, Costa Rica, in 1987. He received the Electrical Engineering degree from the Universidad de Costa Rica in 2011 and Ph.D. degree in Physics from the Universidad de Valencia (Spain) in 2017. During his stay with the Universidad de Valencia’s fiber optics group from 2013 to 2017, his work involved fiber bragg grating design and fabrication for all-optical fractional calculus operators. His interests include fractional calculus applications, fiber bragg grating sensors and ultrafast signal processing.

About The Author

Miguel V. Andrés was born in Valencia (Spain) in 1957 and, at present, he is Professor at the Department of Applied Physics of the University of Valencia, Spain. He received the B.Sc. and Ph.D. degrees in physics from the University of Valencia, Spain, in 1979 and 1985, respectively. Since 1983, he has successively served as Assistant Professor, Lecturer, and Professor in the Department of Applied Physics, University of Valencia, Valencia, Spain. After a postdoctoral stay (1984-1987) at the Department of Physics, University of Surrey, U.K., he founded the Laboratory of Fiber Optics at the University of Valencia (www.uv.es/lfo).

His current research interests include photonic crystal fibers, in-fiber acousto-optics, fiber lasers and new fiber-based light sources, fiber sensors, optical microcavities, microwave photonics, and waveguide theory. His research activity includes an increasing number of collaborations with Latin America universities and research institutes of Mexico, Argentina and Brazil among others. From 2006 to 2016, he was member of the External Evaluation Committee of the Centro de Investigaciones en Óptica, México. In 1999, he was awarded the Premio Cooperación Universidad-Sociedad 1999 of the Universidad de Valencia. Since 2009, he is member (Académico Correspondiente) of the Real Academia de Ciencias de Zaragoza.

About The Author

Martina Delgado-Pinar received the Licenciada en Física and the Ph.D. degrees from the Universitat de València (Spain) in 2002 and 2008, respectively. In 2008-2012 she worked as a postdoctoral research assistant at the Centre for Photonics and Photonic Materials of the University of Bath (UK). In 2012 she joined back to the Optical Fiber Laboratory of the Universitat de València as a Juan de la Cierva researcher.

Currently, she is Assistant Teacher of the Departamento de Física Aplicada at this university. Her research topics include whispering gallery modes sensors, biosensors, tapered fibers, acousto-optic interaction, all-fiber lasers, nonlinear optics and fabrication of photonic crystal fibers.

About The Author

Xavier Roselló-Mechó received the Physics degree and the MSc in Advanced Physics from the University of Valencia (Spain) in 2013 and 2014, respectively. In 2014 he joined the Fiber Optics Group of the University of Valencia as a PhD Student. His research topics include whispering gallery modes sensors, tapered fibers, fiber gratings and nonlinear optics.

 Reference

L. Poveda-Wong, J. L. Cruz, M. Delgado-Pinar, X. Roselló-Mechó, A. Díez, And M. V. Andrés. Fabrication of long period fiber gratings of subnanometric bandwidth. Vol. 42, No. 7 / April 1 2017 / Optics Letters.

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