LPFG Inscription Monitoring using SLED Broadband Light Source

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LPFG Inscription Monitoring using SLED Broadband Light Source

SLED Broadband light sources can take advantage from broadband emission and high power as it is described in our previous post about SLEDs. These characteristics are very interesting in many different applications and particularly for Long Period Fiber Grating (LPFG) interrogation and monitoring as it is mentioned here.

LPFGs periodic structures were developed in parallel to Fiber Bragg Gratings (FBGs), with the first LPFG-based refractometric application published in 1996 [1]. The ability of LPFGs to couple light from guided modes propagated through the optical fiber core to cladding modes opened the door to numerous applications, such as notch and equalization filters and sensors that exploit spectrally selective losses as a function of the outer medium as it is depicted in Figure 1.

In this sense, LPFG sensing applications comprise the three physical parameters LPFGs are sensitive to, such as strain, temperature and refractive index as well as biochemical sensing applications, which require additional sensitization processes in order to improve the selectivity of the device.

Schematic representation of the sensing mechanisms of LPFGs.
Figure 1: Schematic representation of the sensing mechanisms of LPFGs.

The profound study in the last decades of the principles and parameters that govern LPFGs (the dispersion turning point, the mode transition, and cladding diameter reduction) as well as the utilization of non-standard fibers and the combination with other structures has enabled to increase the sensitivity and application of these devices as it is described in [2].

In contrast to FBGs, the period of LPFG is of the order of a fraction of millimeter, which simplifies enormously the manufacturing process. In particular, LPFGs can be obtained using an ample range of techniques that can modify periodically the fiber structure. These techniques include irradiation using CO2 laser, electric arc discharges, femtosecond lasers, UV lasers as well as mechanical modifications of the fiber structure [3].

Concerning the manufacturing techniques, CO2 laser irradiation method presents a low cost and versatile technique compared to other options. Figure 2 represents a typical LPFG fabrication scheme using a CO2 laser with a cylindrical plano-convex lens that permit to focus the beam onto the fiber.

Here, the selection of the adequate optical fiber characteristics (photosensitive, tapered, photonic crystal fibers, polymeric optical fibers, etc.) permits to adjust the LPFG performance. An electronic shutter is used here to easily implement the adjustment of the duty cycle (optical power) while the translation stage shifts the fiber during the LPFG inscription process. LPFG interrogation is performed using a SLED based broadband excitation light source at the input of the fiber and collecting the optical power at the output using an optical spectrum analyzer that permits to measure the resonant wavelength location and the band attenuation [4].

LPFG fabrication and interrogation system.
Figure 2: LPFG fabrication and interrogation system.

[1]. V. Bhatia and a M. Vengsarkar, “Optical fiber long-period grating sensors.,” Opt. Lett. 21, 692–694 (1996).

[2] S.W. James, R.P. Tatam, “Optical fibre long-period grating sensors: characteristics and  application,” Meas. Sci. Technol. 14, R49–R61 (2003).

[3] Min, Rui; Marques, Carlos; Bang, Ole; Ortega, Beatriz, “LPG inscription in mPOF for optical sensing,” Proc. SPIE 10681, Micro-Structured and Specialty Optical Fibres V, 1068108 (9 May 2018); doi: 10.1117/12.2306451.

[4] Renan Sebem, Andr´e Ricardo Herbst and Aleksander S. Paterno, “Customizing the CO2 laser inscription of LPG sensors to enhance the sensitivity to refractive index,” International Conference on Optical Fibre Sensors (OFS24), (September 2015); doi:10.1117/12.2195294.

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