IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 23, DECEMBER 1, 2008 1899 Compact In-Fiber Interferometer Formed by Long-Period Gratings in Photonic Crystal Fiber Jian Ju, Wei Jin, Senior Member, IEEE, and Hoi Lut Ho Abstract—We reported the design and implementation of an in-fiber Mach–Zehnder interferometer (MZI) based on a pair of long-period gratings (LPGs) written on a photonic crystal fiber (PCF). The LPG was fabricated by using a pulsed CO laser to carve grooves periodically along the PCF. The MZI relies on the interference between the fundamental core mode and a cladding mode of the PCF. The MZI was further demonstrated as a tem- perature sensor and a strain sensor. The temperature and strain sensitivities were measured to be 42.4 pm/ C m and 2.6 pm/ , respectively. We also fabricated an MZI on a single-mode fiber, which has a temperature sensitivity of 1215.56 pm/( C m) and a strain sensitivity of 0.445 pm/ . Index Terms—Long-period gratings (LPGs), optical fiber sen- sors, photonic crystal fibers (PCFs). I. INTRODUCTION P HOTONIC crystal fiber (PCF) refers to an optical fiber that has a periodic array of air holes running along its length [1], [2]. PCFs have attracted great interest during the past decade because of their unique properties such as endless single-mode, large-mode area, and high nonlinearity. In-fiber Mach–Zehnder interferometers (MZIs) have been made on conventional optical fibers for wavelength filtering and sensing applications [3]. In the MZI configuration, two cascaded long-period gratings (LPGs) are commonly used with the first LPG couples part of the core mode power into a forward-propagating cladding mode and the second LPG com- bines the two modes, resulting in sharp interference fringes. The two LPGs function as beam-splitter/combiner and the core and the cladding modes travel through two independent paths along the same fiber. Lim et al. reported the first PCF-based MZI where periodic mechanical stress was applied to the PCF to form two nearly identical LPGs [4]. However, mechanically induced LPGs are not permanent and disappear when the stress is removed. Alternatively, Choi et al. demonstrated all-PCF MZIs by offset-splicing combined with partial collapsing of air holes [5], or by using a single LPG in combination with a short section of PCF where air holes are fully collapsed [6]. Villatoro et al. collapsed the air holes of a two-mode PCF to induce coupling between the two core modes and demonstrated an MZI based on the interference between the two core modes Manuscript received April 17, 2008; revised July 31, 2008. Current version published November 12, 2008. This work was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region of China under project PolyU 5176/05E. The authors are with the Department of Electrical Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong (e-mail: eejju@polyu.edu.hk; eewjin@polyu.edu.hk; eehlho@polyu.edu.hk). Digital Object Identifier 10.1109/LPT.2008.2005207 Fig. 1. Experimental setup for fabricating MZI using a high-frequency pulsed CO laser. [7], [8]. However, both air holes collapsing and offset-splicing introduce extra losses at the spot, which cause deterioration in the interference fringe contrast. In this letter, we report an in-fiber MZI formed by a pair of LPGs written directly on PCF by use of a pulsed CO laser. Compared with the MZI formed by mechanical stress and fusion splicing techniques, the present MZI is a compact, in-fiber device with very low insertion loss. We further measured the temperature and strain sensitivity of the MZI and compared the results with an MZI formed on a conventional single-mode fiber (SMF) by a pair of CO laser written LPGs on it. We identified that the MZI on PCF can be used for temperature-insensitive strain sensing [9] and as stable comb filters for enhancing sensitivity in gas concentration measurement with a broadband source [10]. II. PCF MZI FABRICATION The setup (Fig. 1) used for fabricating the in-fiber MZI is sim- ilar to that in [11]. Light from a high-frequency pulsed CO laser is focused to a spot of 35 m and the PCF with coating re- moved is placed on the focal plane of the focusing lens. The laser beam can be controlled by a computer to scan transversely across the fiber and longitudinally along the fiber. The computer also controls the CO laser to generate laser pulses with ap- propriate pulse energy and repetition rate, and to select scan- ning pattern, speed, and the number of scanning cycles. The PCF used in our experiment has four rings of air holes (see the inset in Fig. 1) and the air-hole diameter and pitch are 3.38 and 7.33 m, respectively. The PCF was spliced to conventional SMF at both ends, which are respectively connected to a broad- band source and an optical spectral analyzer (OSA), to monitor the transmission spectrum of the device. The LPG near the light source (input LPG) was written first with the output spectrum monitored continuously by the OSA. The written process stops when coupling efficiency reaches 3 dB. Fig. 2(a) shows the transmission spectrum of an input LPG with a pitch of 410 m and a length of 8.2 mm. Two resonant peaks (at 1505 and 1041-1135/$25.00 © 2008 IEEE