1. Introduction

Tapered fibers with micro-scale diameters are a commonplace in telecommunications and sensing. Recently, however, sub-wavelength diameter tapered fibers (nano tapers) have received much attention by many researchers around the world. In a nano taper a considerable amount of the energy of the guided light is in form of evanescent waves [1–4]. These waves are very sensitive to changes occurring in the external environment which makes nano tapers ideal for the development of highly-sensitive sensors [5,6]. Moreover, owing to the small dimensions of a taper very small samples are required to carry out the sensing task. In spite of the potential applications of nano tapers for sensing, so far, no sensor has been demonstrated yet with nano tapers. The increasing demands for miniaturization and the rapid development of nanotechnology in various fields makes sensors based on nano tapers or other sub-wavelength diameter waveguides attractive. Nano optical sensors are an alternative to other nano electrical sensors based on nano wires and nano tubes [7–9].

 

Fig. 1. (a) Illustration of a tapered optical fiber. ρ0 is the initial diameter, typically 125 µm. (b) Schematic cross section of the device. ρ is the waist diameter, L₀ is the length of the waist, and t is the maximum thickness of the palladium film (shadowed area). λ refers to wavelength.

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Here, we report a miniature hydrogen sensor which consists of a nano taper coated with an ultra thin palladium film. The evanescent waves that are present in a nano taper interact with the palladium film. The exposure of the device to hydrogen converts the palladium layer into a palladium hydride film. As a consequence, the evanescent waves suffer attenuation changes which can be monitored with a simple detection scheme. The dimensions of our sensor are very small. The taper had a diameter of 1300 nm and the Pd-coated zone, i.e., the interaction length, was only 2 mm. The wavelength employed to test the sensor was 1550 nm. Our device responds in approximately 10 seconds and is suitable for sensing low concentrations of hydrogen at normal conditions. The miniature optical hydrogen sensor proposed here is an alternative to other nano electrical hydrogen sensors reported so far [10–12].

2. Sensor fabrication and operation principle

In Fig. 1 we show an schematic representation of a tapered fiber which can be fabricated by stretching the fiber while it is heated with an oscillating flame torch [13]. By an accurate control of the elongation distance and the length of oscillation of the flame torch, tapered fibers with any value of L₀ and ρ can be fabricated. Moreover, standard telecommunications optical fiber (Corning SMF-28) is needed to fabricate such tapers. It is important to point out that from uniform-waist tapered fibers, like that shown in Fig. 1, one can obtain high-quality unclad sub-wavelength diameter fibers (nano fibers), also called nano silica wires [5,6], of length L₀.

In Fig. 2 we show the experimental results of the tapering of a Corning SMF-28 fiber. The tapering conditions have been reported elsewhere [14]. The plot shows the transmission of the fiber as a function of time. The tapering was stopped when the fiber had a diameter of 650 nm which was calculated according to [15]. The length of oscillation of the flame torch, i.e., L₀, was 4 mm and the wavelength injected into the fiber was 1550 nm from a laser diode. The output light was monitored with a photodetector which was controlled by a personal computer through an RS-232 interface. As we can see from the figure the losses introduced by the tapering were 0.05 dB which are negligible for practical purposes. About 50 nano tapers were fabricated to get familiar with their handling and to study their losses. In the majority of the samples fabricated, with ρ=650 nm, the losses were less than 0.1 dB. Tapers with diameters inferior to 650 nm were also fabricated but with higher losses owing to some technical limitations. Note that the diameter of our nano tapers is ~2.3 smaller than the wavelength of the guided light.

 

Fig. 2. Experimental results on the fabrication of a nano taper. The transmission was measured during the whole fabrication process. The monitored wavelength was 1550 nm.

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Once we ensured the fabrication of high-quality nano tapers were proceeded to coat them with ultra-thin palladium layers to obtain the final structure, see Fig. 1. Special mechanical mounts were designed to secure the nano tapers and to transport them from one side to another. The palladium layer was deposited only on one side of the nano taper, see Fig. 1(b). The palladium layer was deposited in a vacuum chamber with an electron beam source. Although the tapers had a length of 4 mm only a half of it or less was coated. It is important to point out that the palladium layer introduces high attenuation losses therefore a long interaction length is impractical. The maximum thickness of the palladium layer, indicated as t in Fig. 1(b), was 4 nm. It should be mentioned that the evaporation of the palladium layer on the nano taper was carried out immediately after the fabrication of the taper. With this procedure we avoided any contamination of the tapers.

A nano taper coated with an ultra thin palladium film can be used for the detection of hydrogen. It is well known that a thin palladium film has the ability to selectively absorb hydrogen [16–18]. Basically, when a thin palladium film is exposed to hydrogen it is converted to a palladium hydride film whose optical properties are different from those of a hydrogen-free Pd film. The hydration of palladium causes that both the real and imaginary parts of the palladium refractive index to change. This in turn causes attenuation changes of the evanescent waves. To a good approximation the transmission power P_t (mW) of a Pd-coated nano taper when it is exposed to hydrogen can be expressed as

P₀ is the transmitted optical power (mW) when no hydrogen is present. r (arbitrary units) can be taken as the ratio of the power of the evanescent waves to the total power of the light guided by a nano taper. Δα=α’-α, where α’ is the absorption coefficient (mm^-1) of the Pd hydride film and α is the absorption coefficient of the hydrogen-free Pd film. L is the interaction length (mm), i.e., the section of the nano taper that is coated with palladium. When the device is not exposed to hydrogen Δα=0, and P_t=P₀.

From Eq. (1) we can note that the performance of our sensor depends critically on r. In a core-exposed or tapered optical fiber with diameters larger than the wavelength of the guided light r is typically below 20%. This low value of r is compensated with large values of L, i.e., with long interaction lengths, to improve the sensitivity of the evanescent waves sensors. However, in a nano fiber r may reach a value close to 100% [1–6]. The term Δα depends on several parameters, such as, the thickness and the refractive index of the Pd film, the index of refraction of the external medium, and the wavelength of the guided light. Δα also depends on r and L. We next show some experimental results.

 

Fig. 3. Diagram of the experimental set-up used to test the sensors. MFC stands for mass flow controllers, LD for laser diode, and SMF for singlemode optical fiber.

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3. Experimental results and discussion

In Fig. 3 we show the experimental set-up that was used to test the sensors. The light source employed was a power-stabilized laser diode emitting light of wavelength of 1550 nm and 1 mW of maximum optical power. The gas chamber was made of a section of aluminum tube with an inlet and an outlet to allow the gases flow in and out. As a carrier gas we used nitrogen. The flow rates of hydrogen and nitrogen gases were individually controlled with mass flow controllers. The flow rate of nitrogen was 750 sccm and was kept fixed in all the experiments carried out. The flow rate of hydrogen was adjusted according to the required concentration. All the experiments reported here were carried out at normal conditions. It is important to point out that a flow of nitrogen of 750 sccm did not cause any change in the sensor transmission. The detection system consisted of a single photodetector attached to a personal computer. This allowed us to collect data as a function of time.

 

Fig. 4. (a) Time response of a sensor when it was exposed to different hydrogen concentrations. (b) Transmission versus hydrogen concentration. Sensor parameters: ρ=1300 nm, L=2 mm, and t=4 nm.

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In Fig. 4(a) we show the transmission of a sensor as a function of time for different concentrations of hydrogen which are indicated in the figure. The data were obtained with a sensor that had diameter ρ of 1300 nm and an interaction length L of 2 mm. The thickness t of the palladium layer was 4 nm. The sensor was tested immediately after the evaporation of the palladium layer. Although we were able to fabricate thinner tapers we have preferred to work with thicker tapers because their handling is easier. From successively experiments we obtained a calibration curve which is shown in Fig. 4(b). The non linear behavior is characteristic of evanescent wave sensors [19,20]. Figures 4(a) and 4(b) show us that the sensor transmission changes remarkably for hydrogen concentrations below 2% and that the sensor saturates at a concentration of about 5%. It is worth noting that the maximum transmission change is about 35%. With thinner tapers such a change can be higher which means that the interaction length can be even shorter, probably of a few hundred microns. The fluctuations of our laser diode were below 0.5% which means that the minimum concentration of hydrogen that can be detected with our sensor is about 0.05%.

Other test that was carried out to the sensor was its response to consecutive exposures to hydrogen that allowed us to determine the response time (the time required for the sensor to reach 90% of transmission change). Such an experiment has also allowed us to study the repeatibility and aging of our devices. In Fig. 5 we show the transmission of our sensor as a function of time when it was exposed successively to a 3.9% concentration of hydrogen. From the plot we calculated a response time of ~10 s. The response time of our nano taper-based hydrogen sensor is between 3 to 5 times faster than that of other optical hydrogen sensors reported so far [17,19–23], and about 15 times faster than that of some electrical nano hydrogen sensors [11,12]. However, our hydrogen sensor is much slower than the sensors based on the hydrogen-induced lattice expansion in palladium reported in [10]. We believe that the fast response of our sensor is owing to the ultra low volume of palladium that is rapidly filled with hydrogen. It is important to mention that our sensor has preserved its characteristics, sensitivity and fast response time, after some months of its fabrication.

 

Fig. 5. Time response of the sensor described in Fig. 4 upon consecutive cycles from a pure nitrogen atmosphere to a mixture of 3.9% hydrogen in nitrogen.

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To end this section we would like to point out that hydrogen gas has a number of important applications. For example, it is utilized in soldering, hydrogenation processes, petroleum transformation, etc. As an energy carrier hydrogen is considered ideal since it is clean, sustainable, and abundant. Hydrogen can be converted to electricity which can be stored in fuel cells which may soon be widely employed in the automotive industry and even at our homes. Like any other gas, fuel, or energy carrier, the handling, storing or transportation of hydrogen poses some risks since it is extremely flammable and very volatile. Moreover, hydrogen is colorless and odorless and has a large flame propagation velocity. The range of combustion of hydrogen is from 4%, called the lower explosive limit, to 75%, called the upper explosive limit. A number of hydrogen sensors are needed for the applications mentioned above. This imposes some requirements on the devices, such as, low cost, small size, durability, reliability, etc. Therefore, hydrogen sensors based on nano fibers or nano tapers seem to be attractive.

4. Conclusions

In this paper we have demonstrated for the first time a sensor based on nano fiber tapers. The sensed parameter was hydrogen. A nano taper coated with an ultra thin palladium film allowed us to detect low concentrations of hydrogen with fast response time. The dimensions of our sensor were very small. The sensor diameter was 1300 nm and the interaction length was only 2 mm. Owing to the ultra thin palladium film the sensor’s response time was approximately 10 s. Such a response time is several times faster than that of several optical and electrical hydrogen sensors reported so far. The sensor was tested in a simple transmission measurement setup that consisted of a laser diode emitting at 1550 nm and simple photodetector. Our experimental results showed that a Pd-coated nano taper is suitable for the detection of hydrogen around the lower explosive limit. Moreover, the sensor is reversible. The experiments shown here may be useful to design other sensors for gases or chemical parameters based on nano tapers coated with suitable ultra thin metallic films or films made of other materials.