Highly stable Mach-Zehnder interferometer using a single standard telecommunication optical fiber

Summary form only given. Sensors, filters and intermodal couplers have been demonstrated using the interference between modes in special optical fibers. Such systems are very useful, and could be employed more widely if the requirements on the multimode fiber could be relaxed. We describe a technique that makes possible constructing a monolithic fiber interferometer from most existing silica-based fibers. The technique is based on the possibility of increasing the refractive index of fibers by using heat and hydrogen treatment. In the present work, a section of the available fiber is H/sub 2/-loaded and heated to 1000/spl deg/C for -1 second, leading to massive OH formation (referred to as OH-flooding). This leads to a large refractive index increase, sufficient to render a single-mode fiber bimodal in the treated region. We illustrate the potential of the technique by building an in-fiber interferometer from a single piece of standard telecommunications fiber (Corning SMF28), and achieving contrast ratios exceeding 20 dB.