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Optical sensing with photonic crystal fibers

Profile image of Francisco AraujoFrancisco Araujo

2008, Laser & Photonics Reviews

Abstract

A review of optical fiber sensing demonstrations based on photonic crystal fibers is presented. The text is organized in five main sections: the first three deal with sensing approaches relying on fiber Bragg gratings, long-period gratings and interferometric structures; the fourth one reports applications of these fibers for gas and liquid sensing; finally, the last section focuses on the exploitation of nonlinear effects in photonic crystal fibers for sensing.

Figures (6)
Two main classes of photonic crystal fibres (PCF): index-guiding PCF (a) and photonic bandgap PCF (b).
Two main classes of photonic crystal fibres (PCF): index-guiding PCF (a) and photonic bandgap PCF (b).
those physical parameters. The temperature sensitivity in an FBG written in PCF depends on the core size and presents a lower value when compared with the one obtained with a standard FBG. For simultaneous measurement of strain and temperature, the obtained resolutions were +1.5 °C and +10 ue over a measurement range of 100 °C and 2000 ie, respectively. Recently, Bragg gratings written in PCFs with different air-hole sizes were also studied (Fig. 2c); Han et al., [25]). The work reports a dependence of the strain sensitivity on the air-hole size — the values for the strain sensitivity are enhanced (~ 28.4%) when the air-hole size is increased because the area with silica in the fiber cross section decreases. However, due to the same material composition in PCFs, similar temperature sensitivities were obtained. The measurement of other parameters with Bragg gratings written in PCF, such as refractive index, has also been studied. Phan et al. [26] used two fiber structures to measure the refractive index of liquids in the range 1.29- 1.45; one fiber has a cross section with six holes, while the other has a two-ring triangular structure. The liquids were inserted in the holes, originating a shift in the FBG resonant wavelengths. For refractive index near that of water, a mea- surement resolution of 7x 10~* was obtained for the PCF with two-ring triangular geometry; a similar value was ob- tained for the six-hole fiber (4x 10~*). However, when the refractive index approached the effective refractive index
those physical parameters. The temperature sensitivity in an FBG written in PCF depends on the core size and presents a lower value when compared with the one obtained with a standard FBG. For simultaneous measurement of strain and temperature, the obtained resolutions were +1.5 °C and +10 ue over a measurement range of 100 °C and 2000 ie, respectively. Recently, Bragg gratings written in PCFs with different air-hole sizes were also studied (Fig. 2c); Han et al., [25]). The work reports a dependence of the strain sensitivity on the air-hole size — the values for the strain sensitivity are enhanced (~ 28.4%) when the air-hole size is increased because the area with silica in the fiber cross section decreases. However, due to the same material composition in PCFs, similar temperature sensitivities were obtained. The measurement of other parameters with Bragg gratings written in PCF, such as refractive index, has also been studied. Phan et al. [26] used two fiber structures to measure the refractive index of liquids in the range 1.29- 1.45; one fiber has a cross section with six holes, while the other has a two-ring triangular structure. The liquids were inserted in the holes, originating a shift in the FBG resonant wavelengths. For refractive index near that of water, a mea- surement resolution of 7x 10~* was obtained for the PCF with two-ring triangular geometry; a similar value was ob- tained for the six-hole fiber (4x 10~*). However, when the refractive index approached the effective refractive index
Figure 3 (online color at: www.lpr-journal.org) (a) Cross section of a PCF with inner and outer cladding where an LPG has been impressed in the core [31]; (b) PCF with total or partial hole filling with polymer [31].
Figure 3 (online color at: www.lpr-journal.org) (a) Cross section of a PCF with inner and outer cladding where an LPG has been impressed in the core [31]; (b) PCF with total or partial hole filling with polymer [31].
3.42 nm/MPa using a fiber length of 58 cm. Recently, with the Hi-Bi PCF structure shown in Fig. 4b), it was possible to demonstrate an FLM-based curvature sensor with resid- ual temperature and strain sensitivity (Frazao et al. [49]). Using a standard differential interferometric configuration, Ju et al. [50] also researched the temperature sensitivity of a two-mode PCF sensing head.
3.42 nm/MPa using a fiber length of 58 cm. Recently, with the Hi-Bi PCF structure shown in Fig. 4b), it was possible to demonstrate an FLM-based curvature sensor with resid- ual temperature and strain sensitivity (Frazao et al. [49]). Using a standard differential interferometric configuration, Ju et al. [50] also researched the temperature sensitivity of a two-mode PCF sensing head.
(MacPherson et al. [60]). At another level, the intrinsic sen- sitivity of standard PCF fiber to hydrostatic pressure was studied using interferometric techniques by Bock et al. [61], and PCF-based polarimetric sensing structures were studied for pressure and temperature measurements (Statkiewicz et al. [62] and Nasilowski et al. [63]).
(MacPherson et al. [60]). At another level, the intrinsic sen- sitivity of standard PCF fiber to hydrostatic pressure was studied using interferometric techniques by Bock et al. [61], and PCF-based polarimetric sensing structures were studied for pressure and temperature measurements (Statkiewicz et al. [62] and Nasilowski et al. [63]).

Key takeaways

  • Since the first publication by Knight et al. in 1996 on photonic crystal fibers (PCF) [1], the optical fiber community has been continuously engaged on R&D activity around these new fibers.
  • In the proposed structure, an LPG induces strong mode coupling from the fundamental core mode to the cladding modes of the PCF, while recoupling to the core mode is performed by collapsing the air holes in a particular point of the fiber.
  • By collapsing the air holes, a PCF zone is transformed into a multimode fiber.
  • In the optical fiber sensing domain, as is already clear from what was presented in previous sections, PCF essentially enables a substantial increase in design flexibility, allowing the possibility for new or improved sensing solutions relative to the situation where the choice of components and devices was limited to the standard optical fiber technology.
  • General issues have first been addressed and then PCF sensing approaches relying on specific fiber structures, such as Bragg www.lpr-journal.org and long-period fiber gratings have been presented.

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