Microfiber in-line Mach–Zehnder interferometer for strain sensing

Abstract

An elegant way of achieving an ultracompact optical fiber in-line Mach-Zehnder interferometer is to create an inner air cavity in a section of microfiber. The sandwich structure splits the light propagating in the fiber into two beams: one passes through the inner air cavity and the other travels along the silica wall of the cavity before recombining at the cavity end, resulting in an interference fringe pattern. Such a device is applied for strain measurement with a high sensitivity of 6.8 pm∕με. In this Letter, an ultracompact microfiber in-line MZI based on an inner air cavity is fabricated by combining fs laser micromachining and fusion splicing techniques. Such a device can be operated as a high-sensitivity strain-sensor with advantages of ultracompact size, low cost, and easy fabrication.During the device fabrication process, fs laser pulses (120 fs∕1 kHz∕800 nm) were focused onto the fiber by an objective lens with an NA value of 0.5. A standard single-mode fiber-28 (SMF-28) (from Corning) was mounted on a computer-controlled X-Y -Z translation stage with a 40 nm resolution. The following steps were taken in the fabrication process:(a) A microhole of a few micrometers in diameter at the center of the cleaved fiber end facet was ablated by an fs laser with on-target power of ∼5 mW, as shown in Fig. 1(a). The size of the microhole determined the size and shape of the hollow sphere to be formed.(b) The fiber tip with the microhole structure was spliced together with another cleaved SMF tip by use of a fusion splicer with a fusing current of ∼16.3 mA and a fusing duration of ∼2.0 s. The two splicing parameters also play an important role in determining the size and the shape of the hollow sphere and hence the air cavity in the microfiber to be formed subsequently.(c) Since the air in the hole was suddenly heated, the hole rapidly expanded to an elliptical hollow sphere with a rather smooth surface. Its microscope side view and scanning-electron-microscope (SEM) image of a cross-section view are displayed in Figs. 1(b) and 1(c), respectively.(d) The SMF with a hollow sphere was mounted between two translation stages and drawn into microfiber by use of a flame-brushing technique. By appropriate control of the moving speed of the flame and the holders, microfiber of different diameters could be produced, with an inner air cavity along the fiber length. The transmission spectrum was monitored during the tapering process by use of a broadband light source and an optical spectrum analyzer.Figure 1(d) displays the microscope image of the microfiber with an inner air cavity that essentially forms a microfiber in-line MZI. In the tapered section, the input light beam is split into two portions denoted by I 1 and I 2 , respectively. While I 1 travels along the silica cavity wall, I 2 propagates via the inner air cavity, and the

Study of Interference Wavelength Characteristics of Fiber Bragg Grating Modulated All-Fiber Inline Mach–Zehnder Interferometer

“…The enveloping MZI transmission arrives from interference between core modes and several cladding modes, and is modulated by fluctuations in the core modes or cladding modes. (1)(2)(3) In our experiment, the lower order mode interference dip wavelengths were almost unchanged as the wavelength of the FBG shifted, and the higher order interference dip wavelength shifted with a FBG with a high modulation ratio (3.78). This kind of device could effectively improve the sensitivity of slight mode changes, which makes it attractive for many communication and sensing applications.…”

“…(1)(2)(3) Recently, an optical fiber Mach-Zehnder interferometer (MZI) cascaded or inscribed on an optical fiber Bragg grating (FBG) has attracted significant interest because of its special spectrum characteristics and refractive index sensing. New kinds of refractometers have been proposed based on a single modemultimode-single mode (SMS) fiber structure followed by a FBG or chirp FBG.…”

Tunable single wavelength erbium-doped fiber ring laser based on in-line Mach-Zehnder strain

“…Many kinds of optical fiber techniques have been used as strain sensors like Fiber Bragg Grating [7], Fabry-Perot interferometer [8], displacement sensor using multimode interference technique by splicing a section of multimode fiber to single mode fiber [9] and IMZI with an inner air cavity along the fiber length or photonic crystal fiber [10,11]. The principle of Mach-Zehnder interferometer depends on the interference induced by the phase deference between the core and cladding modes.…”

Tunable phase-shifted FBG based on an in-grating bubble

“…Thirdly, a pair of micro-channels was successively fabricated by fs laser drilling from both upper and lower fiber surface vertically to the bubble center to allow external liquid to flow in or out. Details of this fabrication process have been introduced in our previous paper [13,14]. …”