2.2.1.

Butler’s interferometer

Butler^2,3 developed the first OFS for hydrogen detection in 1984. The device consists of a Mach–Zehnder fiber interferometer [Fig. 1(a)]. It combines two single-mode optical fibers (ITT-1601) with optical elements to achieve the interferometer. The light of a He-Ne laser, 0.5 mW, is split by a beam splitter and injected into both optical fibers by lenses. One is sensitive to hydrogen, whereas the second is insensitive. The jacket is mechanically removed for both fibers over a length of 3 cm. A Pd layer of $10\text{-}\mu \mathrm{m}$ thickness is sputtered in Ar with an initial 10-nm thick Ti (Titanium) layer (providing a good adhesion between the metal and the optical fiber) on the first unjacketed fiber, whereas a Pt (Platinum) layer is deposited on the second one in order to maintain, if possible, the same temperature dependence between both fibers. Both optical fibers are glued to a fused quartz plate to rigidify the structure. The expansion of the Pd upon hydrogenation results in stretching one fiber in both axial and radial directions and changing its effective path length. The change in optical path length results in a fringe pattern at the output of the interferometer, visually observable and recordable with a simple photodetector. The shift of the fringe is then related to the hydrogen concentration. The signal resolution is limited by the noise which comes from the sensitivity of the sensor to vibrations and sound.

The hydrogenation was carried out with different carrier gases (Ar, ${\mathrm{N}}_2$, and dry air) showing different response times. They observed that the response time, in the order of a few minutes, was longer than the diffusion constant for hydrogen in Pd. From these observations, they suggested that the kinetics is dominated by the surface reactions at room temperature.

Finally, they estimated a range from $10{}^{-4}\%$ (1 ppm) to 3% ${\mathrm{H}}_2$ for the sensitivity. The lower limit is determined by temperature fluctuation. The upper limit corresponds to the $\beta$-phase, because they considered only the regime of the $\alpha$-phase in order to relate the hydrogen partial pressure to the hydride composition (Sieverts equation). In 1988, they reported a response in a range from $2{\times 10}^{-6}\%$ (20 ppb) to 3% ${\mathrm{H}}_2$ for different film thicknesses deposited by sputter and electrodeposition³ [Fig. 1(a)].

For a concentration above 0.1% ${\mathrm{H}}_2$, the experimental result follows the Sieverts law, explained by the linear relationship between the lattice constant and hydrogen content.⁴ At lower concentrations, a large deviation is observed. The observed fringe shift, which is larger than expected from even the enhanced Sievert component, suggests an additional trappings site for hydrogen with a binding energy larger than the anomalous bulk behavior [Fig. 2]. The authors assumed that the fringe shift observed is likely due to the binding at a free metal surface.

Fig. 2

The fringe shift for a fiber sensor as a function of hydrogen concentration. The vertical dashed line indicates the upper edge of the $\alpha$-phase boundary. The straight solid line is the Sieverts law component of the response, and the curved line is the response from the deeply bound sites (these sites correspond likely to binding at a free-metal surface) described by a Langmuir isotherm. The dashed line is the sum of these two contributions.³

Theoretically, the optical path length in the fiber core is related to the strain in the fiber core by the elasto-optic equation:⁵

2.2.2.

Interferometer Development

Faralhi et al.⁶ demonstrated the feasibility of an all single-mode optical fiber Michelson interferometer for ${\mathrm{H}}_2$ sensing [Fig. 1(b)]. (A Michelson interferometer is a Mach–Zehnder interferometer that has been folded back upon itself.) A He-Ne laser, 0.2 mW, is split into two single-mode fibers (SMFs) and recombined by a fiber direction coupler (FC). The measurement is based on a pseudoheterodyne scheme⁷ to eliminate the differential drift in the arms of the SMF interferometer. A part of the reference fiber is wound onto a driven piezoelectric cylinder. The signal is, therefore, modulated at a frequency of the applied voltage. On the sensing fiber, a Pd wire, with a length and diameter, respectively, of 6 cm and $500\mu \mathrm{m}$, is attached on the optical fiber (without the jacket) using quick-setting epoxy resin.

As for Butler’s sensor, the Pd expansion upon hydrogenation causes a change of the fiber sensing length, which results in a phase retardance. The thermally induced effect of the hydrogenation (exothermic reaction) is small compared with the axial strain when the reaction takes place at room temperature and at a low concentration ($\alpha$-phase). The sensor responses to a range of 0.0054% (54 ppm) to 1.5% ${\mathrm{H}}_2$ in ${\mathrm{N}}_2$ at 1 atm, with a response time of a few minutes. The resolution is 2 Pa limited by the differential thermal fluctuations.

In order to avoid polarization problems and to detect a single axial strain, in 1994, Zeakes et al.⁸ developed an extrinsic Fabry–Perot (FP) interferometric sensor [Fig. 1(c)]. The device consists of putting a multimode fiber (MMF) at a distance $S$ from an SMF in a glass alignment tube to achieve a FP cavity. A $2\text{-}\mu \mathrm{m}$ layer of Pd is sputtered on this tube.

Upon hydrogenation, the Pd expansion changes the length of the cavity $S$. The output intensity is described by

The hydrogen interferometer sensors based on the Pd expansion presented here use thick films. Pd thick films show a poor reproducibility over ab-desorption cycles. After a few cycles, irreversible microcracks appear. Moreover, the observed response time is too slow due to the diffusion limitation. Therefore, this technology has been put aside during a decade. Recently, interferometers have received renewed attention due to their high sensitivity potential. New designs based on thin films have been studied.

Innnuzzi et al.⁹ proposed a hydrogen fiber top cantilever sensor. The sensor consisted of achieving an FP cavity between the fiber tip and the cantilever [Fig. 1(e)]. The light scattered back into the fiber encounters an optical FC, which couples 50% of the reflected beam into another fiber that is aligned with an infrared photodiode. A 150-nm Pd film with a 10-nm chromium substrate layer is deposited on the cantilever. The output of the photodiode $W$, as for Zeakes’s sensor, is given by

Maciak et al. developed a simple optical FP fiber interferometer. It consisted of a deposited layered sensing structure made of 145-nm ${\mathrm{TiO}}_2$ and 10-nm Pd at the tip of fiber, as depicted in Fig. 1(e). The first mirror is achieved on the ${\text{fiber}/\mathrm{TiO}}_2$ boundary, whereas the Pd layer plays the role of the second mirror and promotes the dissociation of hydrogen. The ${\mathrm{TiO}}_2$ layer is the resonance cavity of the interferometer and the sensitive layer. According to the authors, the contribution of the Pd layer to the optical signal is minimum.

The sensor responds from 1% to 3.5% ${\mathrm{H}}_2$ in synthetic air with a response time that is faster than 1 min. The response is reproducible. A large change is obtained at 2% after which the signal response is saturated.

We conclude from the hydrogen response that the sensor response is likely due to the hydrogenation of the Pd layer.

Lately, Kim et al.¹⁰ proposed a Mach–Zehnder fiber optic interferometer achieved by two identical long-period gratings (LPGs); each had a $500\text{-}\mu \mathrm{m}$ period and was characterized by a length of 20 mm and a center-to-center distance of 50 mm. A 50-nm thick Pd film was uniformly deposited over the cladding of an SMF. This sensor is sensitive to the change of the refractive index of the Pd. During Pd hydrogenation, the effective index of the cavity changes, which result in the changing of the effective optical path length. The response sensor was only measured at a concentration of 4% ${\mathrm{H}}_2$ in ${\mathrm{N}}_2$. The response time is approximately 8 min.

2.4.1.

Micromirror sensors

Micromirror sensors^12–15 probably constitute the most mature and simple hydrogen sensing devices. A Pd thin film is deposited on one cleaved end of a multimode optical fiber. The other end of the optical fiber is connected to a Y coupler, which allows the illumination of the sensing film and the collection of the reflected light from the latter [Fig. 3(a)].

Fig. 3

(a) Schematic of optical fiber hydrogen sensor based on a micromirror; (b) response of the Butler’s micromirror sensor to different [${\mathrm{H}}_2$] from Ref. 12; (c) and (d) hydrogen concentration versus reflectivity change for 10-nm Pd film at (c) two different temperatures and (d) for clamped and unclamped films. The dashed line represents the theoretical stress in the clamped film (from Ref. 13).

The reflectance $R$ (and transmittance $T$) of the Pd film changes as a function of ${\mathrm{H}}_2$ uptake due to a decrease of the Pd hydride refractive index. By measuring the reflected intensity, the optical change is related to hydrogen concentration.

In 1991, Butler et al. described the response of a 10-nm Pd film deposited on an MMF end with a $125\text{-}\mu \mathrm{m}$ diameter. Figure 3(b) shows the reflected light for different hydrogen concentrations. The reflectance changes continuously depending on the hydrogen concentration.^13,16 In particular, they have observed that the reflectivity change is a measure of the composition of the Pd hydride. The plot of the relative change in reflectivity as a semilog function of ${\mathrm{H}}_2$ concentration shows the pressure composition isotherms (pcts) of Pd for different temperatures [Fig. 3(c)]. The slope of the plateau region observed is characteristic of thin films.

The sensor response is reproducible over hydrogen cycles, but different responses are obtained for identical sensors. The authors suggested that this difference comes from an uncontrollable parameter during the deposition process. They observed, in fact, two trends for the change in the relative reflectivity independent of the thickness of the film (10 to 70 nm). While some thin films exhibit reversible microblistering and microcracking, some films, such as thicker films (100-nm PD¹⁷) show permanent mechanical failure of film. The microblistering and microcracking are eliminated by depositing a 1- to 2-nm thick Ni (Nickel) underlayer as an adhesive agent, improving the sensor reproducibility. In addition, the change in reflectivity for the Ni/Pd film upon hydrogenation is reduced compared with the Pd film. The level and the width of the plateau region are different as depicted in Fig. 3(d) (circle symbol and line for the Pd/Ni and Pd film, respectively). The authors suggested that this difference is due to the clamping effect of the Pd on the substrate (quartz or Ni). Reversible microblistering, so-called buckles, and the clamping effect have now been well investigated and proven to be related to the stress relaxation.¹⁸

2.4.2.

Micromirror development

In continuation of Butler’s work, in 1998, Bevenot¹⁶ described the response of a 13-nm Pd deposited on an MMF, $400\text{-}\mu \mathrm{m}$-core diameter, in a wide range of temperatures: between $-196°\mathrm{C}$ and 23°C. As previously noted, the response change depends on the temperature and is explicitly related to the isotherm pressure equilibrium. A larger signal change is obtained at lower temperatures since the level of the pressure plateau decreases and its width increases. On the other hand, the response time is slower at low temperatures. A response time of 14 s and 100 min, respectively, is obtained at 23°C and $-45°\mathrm{C}$ for 4% ${\mathrm{H}}_2$ in ${\mathrm{N}}_2$. To improve the temperature range of the sensor, they proposed to heat the sample with a power laser diode in order to enhance the response time.

Kazemi et al. developed a micromirror based on colorimetric material.¹⁹ The colorimetric material consisted of inserted Pd into a porous substrate. This micromirror sensor showed a very good reproducibility and reliability. They proved the reliability of the sensor by performing the test of an Evolved Expendable Launch Vehicle (EELV)/Delta IV engine stand at NASA’s Stennis Space Center.

2.4.3.

Characterization of Pd thin film

In addition to the micromirror results, several studies have been reported to characterize the hydrogenation of Pd thin film deposited on a flat substrate. These studies emphasized the relation between the optical change and the pct. As these studies are based on flat substrate, they provide a simple tool to investigate the properties of Pd thin film and consequently optimize the performances of hydrogen sensors based on Pd.

In 1996, Garcia and Mandelis studied the transmittance of Pd thin film. The film deposited on a glass substrate is illuminated by an optical fiber, and a second fiber collects the transmitted light through the sample. As for the reflectance measurement, the graph of the optical transmittance as a function of hydrogen concentration on a logarithm axis [Fig. 4(a)] shows the equilibrium isotherm pressure curves,²⁰ in agreement with the literature.

Fig. 4

(a) Measured isotherm by transmittance for Pd at 25 deg, from Ref. 20, reported by J.A. Garcia et al.; (b) reflectivity change as a function of [${\mathrm{H}}_2$] for 1- and 3-nm Pd films; (c-d) relative change of the reflectivity versus [${\mathrm{H}}_2$] for different substrates.

Later, Kalli et al.²¹ studied the reflectivity of a Pd film varying from a thickness of 1 to 30 nm and characterized the $\alpha$- and $\beta$-phase transitions after both single and multiple gas cycles from the reflectivity measurement. In particular, they demonstrated that the change of reflectivity depends on the Pd thickness and on the substrate.

Figures 4(b) and 4(c) show the reflectivity for 1- and 3-nm thick Pd film, respectively, upon hydrogen exposure. Remarkably, the reflectivity increases upon hydrogenation for a thickness inferior to 3 nm, whereas the reflectivity decreases upon hydrogenation for a thicker film as previously described. Figure 4(d) shows the influence of the used substrate on the hydrogen response. A higher change is obtained for ${\mathrm{SiO}}_2$ as a substrate. Moreover, they observed that the structural changes of the Pd film on the silicon nitride, induced via exposure to hydrogen due to surface energy effects, resulted in the creation of nanostructures.

Lately, Matelon et al.²² showed that the response time is also strongly dependent on the different substrates used. Maximum values of 700 s for Pd/Si and 3700 s for ${\mathrm{Pd}/\mathrm{Al}}_2{\mathrm{O}}_3$ were, for instance, recorded at pressures corresponding to Pd hydride $\alpha$- to $\beta$-phase transition. They attributed this difference to the substrate roughness which modifies the Pd subsurface microstructure.

Recently, the optical transmittance study of metal/metal hydride thin film allows investigating and identifying new hydrogen storage materials.^23,24

2.4.4.

Evanescent wave hydrogen sensor responses

The principle is presented in Fig. 5.

Fig. 5

Schematic of the optical fiber hydrogen sensor based on evanescent field. Pd coated on the (a) cladding, (b) core, and (d) tapered core of MMF (multimode fiber) or SMF (single-mode fiber). (c) Pd coated on heterostructure.

In 1999, Tabib-Azar et al.²⁵ described an optical fiber hydrogen sensor which takes advantage of the evanescent fields associated with the guided light. The device consisted of removing the optical cladding of an MMF ($50\mu \mathrm{m}$) and replacing it by a 10-nm Pd film, with an interaction length of 1.5 cm. The optical fiber is illuminated with a light source with a wavelength of 650 nm. In the presence of hydrogen (between 0.2% and 0.6% ${\mathrm{H}}_2$ in ${\mathrm{N}}_2$), the transmitted intensity increases with a response time of about 30 s [Fig. 6(a)]. The intensity change is a function of hydrogen concentration [Fig. 6(b)]. According to the authors, the increase in intensity is explained by the decrease of the imaginary part of the Pd refractive index upon hydrogenation. In fact, the transmitted light is less attenuated through the Pd hydride section, since the Pd hydride is less absorbing than Pd. In addition, the authors claimed that the change in the real part of the Pd refractive index results in an effective phase change in the guided light.

Fig. 6

(a) and (b) Unclad MMF’s response;²⁵ (c) and (d) multimode tapered fiber’s (MMTF’s) response:²⁶ (c) time response for a taper diameter of 30 (solid line) and $60\mu \mathrm{m}$ (dashed line), and (d) $I/I_{{\mathrm{H}}_2}$ versus ${\mathrm{H}}_2$ for samples of different taper diameters: 30 (squares), 40 (rhombs), 50 (triangles), and $60\mu \mathrm{m}$ (dots);²⁶ (e) and (f) single-mode tapered fiber’s (SMTF’s) response; (g) and (h) nanotapered fiber’s hydrogen response: time and dynamical ranges.²⁷

In this case, the situation is, in fact, slightly different. Since the optical cladding is removed and replaced by a Pd film, the Pd layer plays now the role of the optical cladding. Therefore, a new numerical aperture (NA) is defined for the sensing section. Upon hydrogenation, the change of the real part of the refractive index also results in a change of the transmitted intensity since the NA of the sensing section will be modified.

Evanescent wave (EW) sensors are based on the alteration of the evanescent field of the propagating mode. In the general case of optical fiber (multimode, single-mode, tapered fiber, and so on), the transmission intensity $I_t$ of a fiber, when exposed to hydrogen, can be expressed by²⁶

$I_0$ is the transmitted optical power without hydrogen, and $r$ is the ratio of the power of the EWs on the total power of the guided light. $r$ depends on the configuration: $r$ is inferior to 1%, about 1%, inferior to 20%, superior to 50%, and can reach 100% for, respectively, a multimode, a heterostructure, a tapered MMF, an SMF, and a nanofiber. $L$ is the interaction length and $∆\alpha$ is the variation of absorption between the absorption coefficient of the Pd hydride and the Pd film. The sensitivity of the sensor is tuned by both parameters $r$ and $L$. The sensitivity of the sensor is enhanced when their values increase.

EW development: Villatoro et al. reported the response of a Pd-coated mulitmode tapered fiber (MMTF²⁶), a nanotapered fiber²⁷ and a single-mode tapered fiber (SMTF^28,29).

For SMTF,²⁸ a 12-nm Pd layer with a length of 1.5 cm was deposited on the tapered fiber. The transmitted intensity increased upon Pd hydrogenation since $\mathrm{\Delta }\alpha <0$. The intensity change was hydrogen dependent [Fig. 6(d)] in a range from 1.8% to 10% ${\mathrm{H}}_2$ in Ar. In addition, the sensor was polarization independent: the same response for TE (transverse electric) and TM (transverse magnetic) polarizations was obtained. The sensor was also wavelength dependent: a large intensity change is observed for the longer wavelengths in the spectral range of 900 to 1600 nm [Fig. 6(a)].

Although MMTF showed a lower sensitivity than SMTF, it was proposed by Villatoro et al. as a more robust and convenient sensor. The sensor consisted of depositing a 15-nm Pd layer on the taper waist over a length of 1 cm.²⁶ The waist was in the range of 30 to $70\mu \mathrm{m}$. Two Pd layers were deposited over the taper on the uniform section, with one layer every 180 deg. The measurements were carried out with an LED at a wavelength of 850 nm with an optical power of $20\mu \mathrm{m}$.

The transmitted intensity increased upon Pd hydrogenation ($\mathrm{\Delta }\alpha <0$) [Fig. 6(e)]. They detected hydrogen concentrations ranging from 0.3% to 3.5% ${\mathrm{H}}_2$. The minimal value was limited by the intensity noise present in the signal. Above 3.5% ${\mathrm{H}}_2$, the signal was saturated for all the taper waist sensors considered. Nevertheless, the sensor performance can be adjusted with the taper diameter [Fig. 6(f)] in the detected range. The maximal change was found for a thinner waist diameter because of the maximum evanescent field coupling in the Pd layer. A response time of 30 s was found at 2% with a recovery time of 90 s in Ar. The authors suggested that this response time was mainly driven by the diffusion of the hydrogen atoms into the Pd film. After cycling, the loading time and unloading time became 40 and 100 s. According to the authors, this increase was due to the degradation of the Pd over cycles. In spite of the possible degradation, the response is reproducible.

For nanotapered fibers,²⁷ a 4-nm Pd film was deposited on the nanotapered waist of an SMF. They used a standard telecommunication optical fiber (Corning SMF-28). The waist diameter was limited to 1300 nm for handling reasons. The length interaction was 2 mm. The source was a laser diode emitting at a wavelength of 1550 nm. The intensity decreased with the increase of the hydrogen concentration in a range from 0.8% to 5.2% ${\mathrm{H}}_2$ in Ar [Fig. 6(c)]. This change in intensity is opposite to the response reported for the EW sensor (where the decrease in the absorption coefficient of the Pd hydride causes an increase in intensity). The authors did not discuss the increase of the intensity observed.

The design of tapered fiber sensors allows for the decrease of the interaction length. This length is, for instance, only 2 mm for the nanotapered sensor. The authors estimate that this length can be decreased to a few hundreds of micrometers. Nevertheless, tapered optical fibers such as SMTF showed a poor reproducibility due to inaccuracies during the fabrication process.³⁰ The fabrication of the tapered fiber also made the sensor very fragile.

An optical heterostructure fiber was recently proposed by Luna-Moreno and Monzon-Hernandez³¹ to achieve a very robust sensor with a high sensitivity. The heterostructure was made of a 3 to 8-mm length SMF, coated with a 10-nm thick Pd and Pd alloy film, sandwiched between two MMFs. Due to the diameter mismatch of the optical fiber core, the light was guided by the cladding of the SMF. The evanescent field was hence strongly coupled in the sensitive layer. The intensity increased upon Pd hydrogenation from 0.8% to 4.6% ${\mathrm{H}}_2$ in Ar. The relative change in intensity increased with longer interaction length.

Kim et al. presented in 2007 a side-polished SMF as a hydrogen sensor.³² A jacket-removed SMF was embedded in a quartz fiber holding groove and side-polished. The radius of curvature of the circular groove was 50 cm. After polishing, the cladding thickness left was $2\mu \mathrm{m}$. The interaction length was 2.46 mm. 20-, 100-, and 40-nm Pd films were, respectively, deposited on the side-polished surface by a thermal evaporator. The length of the fiber between the output polarizer and the transducer was only 2 cm in order to minimize the possible polarization entanglement. The hydrogen measurements were performed at 1% and 2% ${\mathrm{H}}_2$ in Ar at a wavelength of 1550 nm.

The sensor principle is based on the propagation losses of the guided mode. The intensity increased upon Pd hydrogenation for the TM polarization, but the TE response of the sensor was negligible. According to the authors, the TE polarization field cannot penetrate into the metal deeply enough to be affected by the metal film. Consequently, the TE polarization can serve as a reference. Finally, the response and recovery times were 100 and 150 s, respectively.

We conclude the review of EW hydrogen sensors by mentioning the work done by Barmenkov³³ to introduce the measurement of hydrogen concentration into the time domain. The author placed the Pd-tapered fiber as a hydrogen sensing element in a fiber laser cavity based on an Er-doped fiber laser. The attenuation of the sensing element upon hydrogenation decreased the cavity losses, which resulted in decreasing the buildup time. By measuring the changes in the laser buildup time, the author had access to the hydrogen concentrations.

2.4.5.

Surface plasmon hydrogen sensor response

Surface plasmon resonance (SPR) sensors can be considered as a variety of EW sensors. Because surface plasmon (SP) may propagate at the interface between a Pd thin film and a dielectric medium, the EWs presented above may sometimes be considered as SPR sensors. Strictly speaking, EW is considered as an SPR sensor when the guided light matches with the wavevector of the SP.

In 1998, Bevenot demonstrated an optical fiber hydrogen sensor based on SPR.¹⁶ The sensor consisted of a Pd layer deposited on the MMF core (diameter of $400\mu \mathrm{m}$, $\mathrm{NA}=0.48$), after removing the optical cladding over a section (2 cm). Note that the design is similar to Tabib-Azar’s sensor. The light was injected into the fiber with an appropriate angle of incidence and at a given wavelength (670 nm). The hydrogen concentration was determined by measuring the change in the transmitted intensity of the excited mode groups. The hydrogen response of the sensor depends strongly on the incident angle. From theoretical simulation, an optimal angle of 12 deg was found for 4% ${\mathrm{H}}_2$.

The measurements were carried out in a hydrogen range of 0.8% to 100%. The intensity increased and changed as a function of ${\mathrm{H}}_2$ in the same way as the pct of Pd. The response can be related to the two phases $\alpha$ and $\beta$, for the concentration ranges of 0.8% to 1% ${\mathrm{H}}_2$ and 3% to 100% ${\mathrm{H}}_2$, respectively. Between 1% and 3% ${\mathrm{H}}_2$, the response strongly varies representing the phase transition. The response time varies between 3 and 300 s for 100% ${\mathrm{H}}_2$. Finally, the authors achieved a two-point detection by inserting two sensing zones along an optical fiber.

One argument is to describe that the change is caused by the modification of the SPR due to the variation in the Pd complex permittivity. Although the SPR resonant angle occurs at an angle beyond the fiber NA, the modification is observable due to the presence of a wide SPR peak. As a result, the intensity changes only for the TM polarized light since the SP is only excited for TM polarization.

2.5.1.

Fiber Bragg grating sensor

Sutapun et al.³⁴ studied the design of a Fiber Bragg grating (FBG) sensor for hydrogen detection in 1999. The sensor was achieved by depositing a 560-nm Pd layer onto the optical cladding of an SMF (5 to $10\mu \mathrm{m}/125\mu \mathrm{m}$), where an FBG (with Bragg wavelength of 829.73 nm) was previously inscribed. A part of the cladding was etched, and the final cladding had a value of $35\mu \mathrm{m}$.

The expansion of the Pd hydride upon hydrogenation stretched the fiber. The periodicity and the effective index were altered which resulted in shifting the Bragg wavelength. This FBG hydrogen sensor acts as a pure strain sensor. The determination of the Bragg wavelength makes it possible to quantify the ${\mathrm{H}}_2$ concentration. In this case, the resolution of the spectrometer of 0.14 nm was not adequate to measure the Bragg wavelength shift (less than 0.1 nm). They determined the shift by applying a spline-fitting routine to the data measured (Fig. 7).

Fig. 7

(a) Schematic of a typical hydrogen sensor based on Pd-coated fiber Bragg grating (FBG); (b) and (c) the response of (b) Wei et al.’s LPG (long-period grating) sensor³⁵: (b1) gives the resonance wavelength shift as a function of H2 concentration at 30°C, and (b2) at 100°C, 150°C and 200°C and (c) Buric et al.’s³⁶ FBG sensor. The sensor responses are described by the sensitivity considered as a function of hydrogen concentration.

The Bragg wavelength increased linearly as a function of ${\mathrm{H}}_2$ concentration in the range of 0.3% to 1.8% ${\mathrm{H}}_2$. After exposure to 1.8% ${\mathrm{H}}_2$, the Pd coating peeled off, which destroyed the sensor. The authors proposed to add a thin adhesion layer to overcome the Pd degradation.

2.5.2.

Grating development

FBG is insensitive to the surrounding medium refractive index since the resonance results from the core modes coupling. Meltz et al.³⁷ in 1996 proposed to use FBG written in D-fibers to make the FBG sensitive to the surrounding medium refractive index via EW interactions. In this direction, in 2008, Tien et al.³⁸ deposited a Pd film on a side-polished fiber over a length of 12 nm (SMF-28, $10\mu \mathrm{m}/125\mu \mathrm{m}$). A FBG was previously inscribed (grating period of $0.534\mu \mathrm{m}$, length of 20 mm: resonance 1545 nm) in the optical fiber. A 20-nm thick Pd film can be used thanks to the evanescent field interactions. The wavelength resonance shifts to a higher wavelength in the presence of ${\mathrm{H}}_2$. The shift increased with the increase of ${\mathrm{H}}_2$ for a range of 20% to 70%. In continuation of this work, Buric et al.³⁶ studied the response of such a sensor in a range of 1% to 5% ${\mathrm{H}}_2$ (Fig. 7). They pointed out that the evanescent coupling came from the excitation of the SP at the Pd air interface. Consequently, the sensor was TM polarization dependent. The response time is in the order of a few minutes.

Maier et al.³⁹ developed, in 2007, a new sensor package to amplify the strain caused by the Pd expansion. The sensor consisted of a semicircular cross-section Pd tube in which a fiber containing an FBG was attached coaxially under tension to a mounting element. Upon hydrogenation, the Pd sensor expanded a half tube. The axial component of this expansion was mechanically collected and redirected into a short section of fiber containing an FBG. A minimal thickness of $300\mu \mathrm{m}$ has to be chosen to ensure the stability.

In 2006, Trouillet et al.⁴⁰ experimentally replaced the short-period grating (FBG) by an LPG. The device consisted of depositing a 50-nm Pd layer on an SMF (Corning SMF 28) by thermal evaporation after removing the jacket. In contrast to an FBG which uses the fundamental mode propagating through the core, LPG was mainly based on the coupling between the cladding modes and the evanescent or SP at the interface between the Pd coating and the outer medium. Therefore, the sensor was sensitive to the change of the Pd refractive index. The determination of the shift of the loss peaks gave access to the hydrogen concentration [Fig. 7(d)]. At 4% ${\mathrm{H}}_2$, a shift in the order of $-5$ to $-7\mathrm{nm}$ toward the lower wavelengths was observed in comparison with a shift of 14 pm that was obtained for an FBG (50-nm Pd deposited on fiber with FBG).

In conclusion, depending on the selected OFS, the sensor measures the hydrogen concentration by measuring either the Pd hydride expansion, the Pd refractive index, or the heat release, as described in Table 1.

Table 1

Operating mechanism of the optical fiber hydrogen sensors.