Faraday rotation in highly birefringent optical fibers

All-optical polarimeter for laser Stokes vector measurement using self-induced nonlinear phase modulation

This paper utilizes an analytical model of polarization dependent frequency sideband generation via the Kerr effect in a highly nonlinear fiber to determine the state of polarization (SOP) of a laser by all-optical means. Theoretical analysis shows that the power of the n^th order sideband generated by the propagation of two lasers with distinct frequencies in the nonlinear medium is proportional to cos ²ⁿ(α/2), where α is the angle between the normalized Stokes vectors representing the SOPs of the lasers on the Poincaré sphere. By tailoring the SOP of one laser acting as a reference and experimentally measuring the power of the first order sideband, the SOP of the laser under test is recovered with an error smaller than 10.22° on the Poincaré sphere corresponding to 0.8% the sphere's total area. Comparing the SOPs of two lasers without referencing them to fixed polarizers enables potential applications in remote environmental sensing, novel polarization division multiplexing schemes for enhanced telecommunication data rates, and scientific instrumentation.

Sensitivity enhancement of fiber optical polarimetric sensors using self-induced nonlinear phase modulation via the Kerr effect

This work presents an analytical model accounting for the impact of optical polarization on the generation of frequency sidebands by the Kerr effect in a highly nonlinear fiber. Theoretical analysis shows that for a relative polarization angle, α, between two input lasers expressed on the Poincaré sphere, the optical power of the n^th order sideband is proportional to cos ²ⁿ(α/2). This theoretical result enables a novel all-optical technique for interrogating changes in polarization with higher sensitivity than conventional measurement schemes using linear polarizers. The predicted theoretical relationship between the sideband power and the relative polarization angle is verified experimentally and sensitivity enhancement by a factor of 1.45 compared to a conventional polarimetric sensor is demonstrated for the 3rd order sideband. This novel nonlinear approach, which allows dynamic range to be traded for an enhanced ability to detect small polarization variations, has potential applications in fusion reactor monitoring, instrumentation and material characterization.

Wavelength-dependent orientation of the principal axes of photonic crystal fibers measured by windowed Fourier-transform spectral interferometry

We present a novel polarization alignment technique based on windowed Fourier-transform (WFT) spectral interferometry to determine the wavelength-dependent orientation of the principal polarization axes of photonic crystal fibers (PCFs). To test the technique, a commercially available, 82.5-cm-long HC-800-02 type hollow-core PCF was measured. The angles belonging to the fast and the slow principal axes of the fiber were determined from the peak intensity values of the ridges in the WFT map at different wavelengths. We demonstrate that the orientation of the principal polarization axes of the tested PCF is wavelength-dependent. The precision of the angle measurement was better than 0.3°.

Novel configuration for an enhanced and compact all-fiber Faraday rotator with matched birefringence

We propose a novel configuration for an improved and compact all fiber Faraday rotator based on phase matching between the Faraday rotation and bend-induced birefringence. The device utilizes a coiled fiber within two electro-magnetic toroids, such that the fiber length required for getting the beat length is quite long and several rounds of fiber are needed. Analysis of the capabilities of the proposed device and its sensitivity to different parameters is presented. Faraday rotation of 13° was experimentally measured in six meters of single mode silica fiber, with a magnetic field of about 0.06T at a wavelength of 1064nm. We show that phase matching between the two phenomena significantly improves the polarization rotation by a factor of 4-10. In addition, we demonstrate the ability to achieve higher rotation by using Fabry Perot resonator in low terbium doped glass.

Analysis of Faraday effect in multimode tellurite glass optical fiber for magneto-optical sensing and monitoring applications

The design and fabrication of a tellurite glass multimode optical fiber for magneto-optical applications are presented and discussed. The analysis of the polarization shows that an optical beam, linearly polarized at the fiber input, changes to elliptically polarized with an ellipticity of 1∶4.5 after propagating down the fiber. However, the elliptical distribution remains unchanged with or without an applied magnetic field, demonstrating that no circular dichroism occurs within the fiber. The Verdet constant of the tellurite glass in the fiber is measured to be 28±0.5  rad·(T·m)-1, diverging by less than 3% from the Verdet constant found on the same glass composition in bulk form. These results demonstrate the feasibility to develop reliable tellurite glass fibers by the preform drawing method for magneto-optical applications.

Fiber-optic Sagnac interferometer for noncontact structural monitoring in power plant applications

The design of a noncontact fiber-optic sensor is described for the detection of acoustic emission for structural integrity monitoring in high-temperature power plant applications. The sensor is based on a Sagnac interferometer and produces an output proportional to target velocity, without the need for active phase stabilization. It is inherently insensitive to low-frequency perturbations of the instrument or the target and incorporates an environmentally insensitive downlead, which may be of arbitrary length. It is shown that the sensor is capable of meeting the specifications for structural integrity monitoring of high-temperature power plant components based on acoustic emission detection and has a velocity resolution of 50 nm s(-1) Hz(-1/2).

Faraday effect in standard optical fibers: dispersion of the effective Verdet constant

We have measured the Faraday effect in silica standard optical fibers in the wavelength range 458-1523 nm. An effective Verdet constant Vef that exhibits a linear dependence on the square of the optical frequency ν is defined: V(ef) = (0.142 ± 0.004) × 10(-28) ν(2) rad T(-1) m(-1). We demonstrate that the negative effects of a small linear birefringence can be minimized by adjustment of the input polarization to an optimum state.

Characteristics and applications of birefringent-fiber Kerr-couplers

We consider the effects of the pump power and the interaction length on the efficiency of polarization coupling induced by the optical Kerr effect in high-birefringence fibers. An efficiency of up to 100% is predicted for suitable combinations of the length and the strength of the birefringence grating in the fiber. The device is rapidly tunable, highly selective in wavelength, and independent of any thermal drift in the fiber. Potential applications include real-time tuning in wavelength-division demultiplexers, wavelength selection in optical spectrum analyzers, and band rejection in tunable notch filters.

Phase-matched polarization coupling in high-birefringence fibers through the optical Kerr effect

We describe the use of the optical Kerr effect in a simple and novel way to write a transient polarization coupler in high-birefringence fiber. A conversion efficiency of as much as 0.4% is demonstrated, with a coupling bandwidth of approximately 0.4 nm. The method is fast and independent of any thermal drift. Potential applications include real-time tuning in wavelength-division demultiplexers and wavelength selection in optical spectrum analyzers.

All-optical switching and intensity discrimination by polarization instability in periodically twisted fiber filters

Self-induced birefringence effects in a periodically modulated nonlinear medium are investigated. An input beam linearly polarized along the axis of a fiber polarization-rocking filter undergoes a spatial instability as the input power crosses a threshold value that scales linearly with the coupling strength. We describe the potential use of this effect for relatively low-power intensity discrimination, all-optical polarization switching, and small-signal amplification using linearly polarized beams. The wavelength for maximum polarization conversion between the axes can be tuned by varying the input power, and the filter bandwidth narrows with increasing power.

Internal rotation of the birefringence axes in polarization-holding fibers

An internal rotation of the birefringence axes has been measured in a variety of polarization-holding fibers. The rotation of the axes causes coupling of the major-field components of the fundamental modes, which limits the polarization-extinction ratio in short lengths of birefringent fibers to -45 dB in some cases. A practical consequence of the rotation of the axes is a reduction of the polarization-holding ability of devices such as fiber couplers that are made with these fibers.

In-line fiber-polarization-rocking rotator and filter

An in-line polarization rotator has been built into a single-mode birefringent fiber. The rotator utilizes periodic twists of the fiber's principal axes, which were formed by rocking the preform as the fiber was drawn. The polarization conversion between the principal axes is wavelength dependent, with a bandwidth inversely proportional to the number of twist periods. The bandwidth of the present rotator was 4.8 nm for 100% conversion in a fiber length of 170 cm.

Polarization holding in birefringent single-mode fibers

Perturbations in highly birefringent single-mode fibers couple the two polarization modes and degrade the polarization-holding ability. With a broadband source we demonstrate wavelength averaging of the power in either mode, permitting a simple measurement of the power transfer to the cross-polarized mode as a function of fiber length. Using this technique, we confirm experimentally the theory of random mode coupling between the polarization modes.

Birefringence and polarization mode-dispersion in spun single-mode fibers

A theoretical and experimental analysis of the polarization properties of twisted single-mode fibers is presented. It is shown that whereas a conventionally twisted fiber possesses considerable optical rotation, a fiber which has a permanent twist imparted by spinning the preform during fiber drawing exhibits almost no polarization anisotropy. It is thus possible to virtually eliminate the commonly observed fiber linear birefringence. As a consequence, fibers made in this way are ideally suited for use in the Faraday-effect current transducer. It is further shown that a permanent twist of a few turns/meter effectively eliminates polarization mode-dispersion. The technique therefore appears attractive for enhancing the bandwidth of very long unrepeatered telecommunication links.

Fiber Faraday circulator or isolator

We demonstrate a Faraday circulator or isolator that uses a silica-core, single-mode, birefringent fiber as the active medium and small permanent magnets for the magnetic field. Circulators were constructed for wavelengths of 632.8 and 830 nm using about 2 m of fiber. This is the first description to our knowledge of such potentially useful working devices made in birefringent fiber. Bandwidth and temperature dependence were also investigated.