Rocking Filters Research Articles

Double pass technique for fabricatingcompact optical fibre rocking filters

By using the photosensitivity of Ge-doped birefringent fibres, efficient narrowband rocking filters have been fabricated that, to the authors' knowledge, are the shortest in length per unit beat length to date. This has been achieved by using a point-by-point external writing process in which a UV beam periodically exposes sections of an elliptically-cored fibre. Alternate sections are exposed from opposite sides of the fibre, effectively doubling the coupling strength per beat length. Alignment with the fibre axes is assisted by use of a keyed optical fibre.

Pressure sensitivity of rocking filters fabricated in an elliptical-core optical fiber

The pressure sensitivity of a UV-written rocking filter in an elliptical-core optical fiber was measured up to 50 MPa. Comparison with the response of a Bragg grating showed that the rocking filter was 100 times more sensitive to pressure.

Soliton propagation in twin-core fiber rocking filters

We investigate nonlinear pulse propagation in a twin-core fiber whose birefhingent axes have been periodically rocked at the beat length of the cores. We find that the four coupled-mode equations that describe this system can be reduced to a pair of coupled nonlinear Schrödinger equations under suitable conditions. Consequently we find two new types of compound soliton: one that propagates down both cores of the fiber simultaneously and another that couples completely between the cores of this structure without degradation. These solitons typically have a peak power of ~1 W and a length of ~10 ps.

Demonstration of demultiplexing with a rocking filter fiber

We show that the presence of a strong pump beam detuned from the center wavelength of a rocking filter fiber causes a signal beam at the filter resonance frequency to switch between polarization states. This process is used to switch out a pulse from a pulse train.

Photolytic Index Changes in Optical Fibers

The ability to change the core refractive index of fibers by optical pro­ cessing allows the fabrication of phase structures in the fibers-the simplest of which is a phase grating obtained by a periodic modulation of index along the core. Such in-line fiber phase gratings fall into two classes: short-pitch Bragg gratings, with periods of order 100 nm with applications including reflectors (1, 2), transmission filters (3), and strain sensors (4, 5), and long-pitch gratings, with periods of order 1 mm with applications including mode converters (6, 7) and polarization rocking filters (8, 9). These devices and applications are reviewed by K. Hill et al in this volume. This review is concerned with understanding the mechanism by which photolysis changes the index, with attention to the role of defects and thermoelastic stress. A photolytic refractive index change is the permanent change in the index induced by irradiation with visible/UV light. This terminology is used to distinguish the effect from photorefractivity, in which the index response to the optical fields is dynamic and reversible, rather than cumulative and irreversible, on the time scale of interest. The first observations of photolytic index changes in germanosilica fibers were reported by Hill and co-workers (I, 1 1 ) in 1978; they described permanent holographic gratings written into the cores of fibers by intense 488 nm laser light launched into the fiber. These gratings had a weak index modulation bn of order 105-106, which gave narrow-band reflection filters at the writing wavelength. Interest in the process was revived by Morey and co-workers (12) in 1 989 through their demonstration of side-

Compact Mach–Zehnder fiber interferometer incorporating photoinduced gratings in elliptical-core fibers

A new type of Mach-Zehnder interferometer has been fabricated in an elliptical-core high-birefringence fiber. The directional couplers of a conventional fiber interferometer are replaced by two photoinduced polarization rocking filters written on the same fiber. The resonant wavelength of the interferometer was 787 nm, with an operational bandwidth of 18 nm. The device can be used in two different modes for temperature sensing.

Long-length, long-period rocking filters fabricated from conventional monomode telecommunications optical fiber

Using fiber photosensitivity, we have made efficient, application-specific rocking filters from conventional monomode telecommunications optical fiber. Filters with a length of 10.89 m, a bandwidth of 58 nm, and a transmission of 99% at the resonant wavelength of switching 200-fs solitons.

Birefringent photosensitivity in monomode optical fibre: application to external writing of rocking filters

Efficient, narrowband rocking filters have been made in polarisation-maintaining monomode optical fibre by photoinducing periodic birefringent gratings along the length of the fibre. These gratings, fabricated with a period chosen to provide synchronous coupling between the two principal states of polarisation of the LP01 mode at a preselected wavelength, gently rock to and fro along the principal polarisation axes of the fibre. To photoinduce the grating, the fibre is exposed periodically along its length to a UV light beam which is incident at right angles to the geometrical axis of the fibre and is directed π/A rad with respect to the principal polarisation axes. Efficient polarisation mode conversion at 1.3 μm with the birefringent fibre grating is demonstrated.

Rocking filter formation in photosensitive high birefringence optical fibres

Parent et al. observed that the refractive index changes induced in optical fibres by Hill grating formation (at 488 and 514.5 nm) were birefringent. A wavelength filter, formed in a simple and novel way by exposing high birefringence fibre to linearly polarised 488 nm light, is reported. Narrow bandwidth operation and high conversion efficiencies are achieved experimentally between the two orthogonal polarisation states of the fibre.

Observation of ultrafast nonlinear polarization switching induced by polarization instability in a birefringent fiber rocking filter

Ultrafast all-optical switching between two orthogonal linear polarizations in a periodically twisted fiber filter is discussed. The dependence of the switching characteristics on the input power and operating wavelength is considered. >