Capturing reflected cladding modes from a fiber Bragg grating with a double-clad fiber coupler

Abstract

We present a novel measurement scheme using a double-clad fiber coupler (DCFC) and a fiber Bragg grating (FBG) to resolve cladding modes. Direct measurement of the optical spectra and power in the cladding modes is obtained through the use of a specially designed DCFC spliced to a highly reflective FBG written into slightly etched standard photosensitive single mode fiber to match the inner cladding diameter of the DCFC. The DCFC is made by tapering and fusing two double-clad fibers (DCF) together. The device is capable of capturing backward propagating low and high order cladding modes simply and efficiently. Also, we demonstrate the capability of such a device to measure the surrounding refractive index (SRI) with an extremely high sensitivity of 69.769 ± 0.035 μW/RIU and a resolution of 1.433 × 10(-5) ± 8 × 10(-9) RIU between 1.37 and 1.45 RIU. The device provides a large SRI operating range from 1.30 to 1.45 RIU with sufficient discrimination for all individual captured cladding modes. The proposed scheme can be adapted to many different types of bend, temperature, refractive index and other evanescent wave based sensors.

Surface plasmon resonance sensor interrogation with a double-clad fiber coupler and cladding modes excited by a tilted fiber Bragg grating

“…Figure 1 shows a schematic of our device. It consists of a DCFC fabricated by fusing and tapering two doubleclad fibers (DCFs) [5][6][7]. The DCF's single-mode core is excited by a SMF spliced to a branch (1) and connected to a broadband source in a wavelength range between 1525 and 1590 nm.…”

Evanescently coupled optical fiber refractometer based a tilted fiber Bragg grating and a D-shaped fiber

“…2 indicates the inscription direction of the TFBG and hence the orientation of the tilt plane, so in principle there is no need to search for the best orientation when packaging the devices as long as we keep track of the direction from which the laser beam was incident on the fiber when the TFBG was fabricated. The red line shows the re-coupled cladding spectrum has a broad wavelength range over 30 nm and the maximum spectral SNR is about 30 dB, which is much higher than early reports of in-fiber cladding-to-core re-coupling schemes [2][3][4][5][6][7]. The integrated SNR, of the total recoupled power (by PD2) is at least as high as well.…”

“…Various kinds of "in-fiber" configurations have been reported to re-couple the backward transmitted cladding modes into fiber core so as to allow this. These methods include a large lateral offset splicing [2], a core-diameter-mismatched fiber section [3], hybrid with a long period grating (LPG) [4][5][6] and a double-clad fiber coupler [7]. However, all above "in-fiber" TFBG refractometers can only re-couple a relatively narrow band cladding modes (the low order cladding modes [2,3] or those with resonances within the LPG spectral window), which limits the ranges for which SRI measurements can be made, especially for the low SRI (close to 1.33) encountered in biochemical solution measurements.…”

Distributed Tilted Fiber Bragg Grating and Surface Plasmon Polariton Sensors

“…In our experiments up to three concatenated TFBGs were measured simultaneously and independently. These sensors may be oriented such that their polarization response is orthogonal [7], so that by selecting a particular polarisation, only one sensor is interrogated, although all three may be measured simultaneously.…”