A Mach–Zehnder interferometric humidity sensor based on waist-enlarged tapers

https://doi.org/10.1016/j.optlaseng.2013.07.023Get rights and content

Highlights

  • A novel humidity sensor based on sandwiching a multimode fiber in a single-mode fiber with two waist-enlarged fiber tapers is proposed and demonstrated.

  • The sensor has a linear response to humidity with enhanced sensitivity of −0.119 dB/%RH in the range of 35–90%RH.

  • The temperature experimental results show that the sensor is immune to temperature.

  • The sensor has advantages such as easy fabrication, cost-effective and temperature-immune, which could be used for high sensitivity humidity detection.

Abstract

A novel humidity sensor based on an in-fiber Mach–Zehnder interferometer (MZI) is proposed and demonstrated. The sensor head is formed by a single-mode–multimode–single-mode (SM–MM–SM) fiber structure through arc fusion splicing. The intermodal interference is achieved by two waist-enlarged fiber tapers at the coupling points of the multimode fiber and single-mode fibers. The sensor has a linear response to humidity with enhanced sensitivity of −0.119 dB/%RH in the range of 35–90%RH. Additionally, the sensor exhibited excellent temperature immunity in the temperature test. Such easily fabricated, cost-effective and temperature-immune fiber interferometer could be used for high sensitivity humidity sensing applications.

Introduction

Relative humidity (RH) is an important parameter in chemical engineering, health, medical, industrial and agricultural fields. In a number of these areas, the detection devices are required to be small-size, immune to electromagnetic interference and work in severely poisonous environments. Optical fiber sensors could satisfy these requirements better than traditional electrical humidity sensors, which makes the fibers good candidates for humidity sensing applications. Many types of optical fiber humidity sensors have been reported, such as fibre gratings [1], [2], [3], U-bend fibre structures [4], [5], photonic crystal fibre (PCF) structures [6], fiber taper structures [7], [8] and interference structures [9], [10], [11]. Generally, these sensors are coated with thin poly-film to improve humidity sensitivity. The sensitivity of the sensors is thus determined and limited by film quality and coating technology.

As one of the excellent fiber-based optical interferometers, in-fiber Mach–Zehnder interferometers (MZI) are widely used to measure temperature, strain, and refractive index [12], [13], [14], [15]. The key principle for this device is the interference between the core mode and cladding modes of a fiber. To excite the cladding modes from the fundamental core mode, the coupling technology is focused on three methods. One method is making fiber tapers through arc fusion splicing or flame brushing techniques, the second is making a small lateral offset in two pieces of fibers by arc fusion splicing, and the third is fusion two pieces of fibers with different core diameters. In MZI structures with mismatched core diameters, multimode fiber (MMF) has attracted researchers' interest because of its large core diameter. Wang et al., [16] proposed a fiber refractometer based on an MMF taper sandwiched between two single-mode fibers (SMF) that achieved high sensitivity by combining the second and third methods listed above. The MMF based MZI has also been exploited for measuring humidity. Shohei et al., [17] constructed an MZI humidity sensor by sandwiching a hetero-core fiber into an MMF. However, the sensitivity of that sensor remained low despite being coated with multiple hygroscopic polymer layers. Li et al., [18] demonstrated a hybrid structure of an MMF taper and fiber Bragg Grating with a polyvinyl alcohol film coating on the taper. However, the cross-sensitivity to temperature and humidity was not mentioned in their paper.

In this paper a novel humidity sensor based on an SM–MM–SM in-fiber structure is proposed and experimentally demonstrated. The sensor consists of two waist-enlarged tapers created through cleaving and arc fusion splicing. The advantages of this sensor are simple fabrication, compact structure and insensitivity to temperature. More importantly, the humidity sensitivity is greatly improved without requiring polymer coating.

Section snippets

Sensor principle and fabrication

The schematic diagram of the proposed MZI is shown in Fig. 1. A section of MMF is sandwiched in an SMF, while the two coupling sections are formed as waist-enlarged fiber tapers by arc fusion splicing. The cladding modes are excited from the fundamental mode at the first waist-enlarged taper and then propagate in the MMF. The cladding modes could recouple with each other or with the fundamental mode at the second coupling point. The distance between the two coupling points is taken as the

Measurement and discussion

The MZI with 53 mm interferometer length is chosen for the humidity measurement due to its clearer interference fringes and large dynamic range. Fig. 5 shows the schematic diagram of the experiment. The humidity environment is provided by a saturated salt solution in a beaker, which is placed in a closed vessel. The performance of this type of MZI is affected by bending loss, so the sensor is straightened and fixed on plexiglass that has a distribution of holes for moisture to pass through.

Conclusions

In this paper, a novel type of MZI based on a piece of MMF sandwiched in an SMF for humidity sensing was demonstrated. The coupling points are fabricated as waist-enlarged tapers, which excite multiple modes and cause intermodal interference in the core of the MMF. The theoretical analysis indicates that interference primarily occurs between the LP01 and LP1n modes in such an MZI. The linear humidity response with an enhanced sensitivity of −0.119 dBm/%RH is achieved over the humidity range of

Acknowledgments

This work is supported by the National Science Foundation of China under Grant F050304 and the National “863” Project of China under Grant 2009AA06Z203. We Thank Jing Zhang for useful discussions.

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