Wang Joo Lee

A polarimetric current sensor using an orthogonally polarized dual-frequency fibre laser

We demonstrate a novel -doped fibre laser operating in orthogonally polarized dual-frequency modes and its application to electric current sensing with frequency read-out. A Faraday rotating mirror and spatial hole burning effects in a gain medium and in a saturable absorber are utilized to control the lasing mode and polarization. The polarization mode beat frequency changes linearly in response to the non-reciprocal circular birefringence induced by an external magnetic field. When the laser was applied to sensing an alternating electric current with a simple phase-locked loop signal processing scheme, the slope coefficient of 8.0 kHz per turn and the noise equivalent current of 460 per turn were obtained. The new current sensor is immune to perturbations in the lead fibres.

Fiber-optic sensor array based on Sagnac interferometer with stable phase bias

We propose and experimentally demonstrate a novel fiber sensor array based on a Sagnac interferometer with very simple electronic signal processing. A stable quadrature phase bias was obtained using a phase modulator, and the polarization-induced signal fading was suppressed by using a depolarizer and a broad-band source. A phase sensitivity of about 4.0 /spl mu/rad/sub rms///spl radic/Hz at 5 kHz was obtained using a two-sensor array.

Dual heterodyne polarization diversity demodulation for fiber-optic interferometers

A novel method is described for the simultaneous suppression of polarization and phase bias induced signal fading in fiber-optic interferometers based on differentially frequency shifted orthogonal polarization components. An experimental demonstration proved the operating principle with a high sensitivity of 28 /spl mu/rad//spl radic/(Hz).

Optical signal-to-noise ratio monitoring based on tracking of principal states ofpolarization

Feature Issue on Optical Performance Monitoring (OPM). We propose and demonstrate a simple method for monitoring optical signal-to-noise ratio (OSNR) of each channel in wavelength-division multiplexing networks based on tracking of principal states of polarization (PSP) and scrambling at the transmitter part. This technique reduces measurement error caused by the effect of PMD, which limits the performance of a previous polarization extinction method. PSP tracking uses bandpass-filtered power at clock frequency for a non-return-to-zero signal as a feedback signal to separate the two PSPs by aligning them to the axes of a polarization beam splitter (PBS). The scrambling makes it possible to extinguish the polarization of an optical signal from one output of the PBS and to provide maximum and minimum optical powers, thus enabling us to obtain OSNR regardless of the effect of PMD. In a transmission experiment over a 400-km single-mode fiber-dispersion-compensating fiber (SMF+DCF) link, the results showed that the difference between the OSNRs measured by use of the proposed method and by an optical spectrum analyzer (OSA) was less than 1.0 dB.

Novel type of PMD compensator based on separation of PSP and DGD controls

We propose and experimentally demonstrate a novel type of PMD (polarization mode dispersion) compensator (PMDC) that separates PSP (principal states of polarization) control from DGD (differential group delay) control with an automatically adaptive 40 Gb/s PMDC module manufactured on PCBs. The results showed that this method provided a very fast response time of /spl sim/2 /spl mu/s to PSP change and allowed a reduction in compensation time compared with the conventional method of alternately controlling PSP and DGD.

Dual-polarization single-mode fiber laser

radiation, 3) effective transfer of the pump radiation to excitation of the active material, and 4) short length. To meet these partly conflicting requirements we have introduced a new fiber design, cf. Fig. 1 and Ref. 1. In contrast to the wellknown double clad fiberszs3 this design has the advantage that the pump light is almost completely absorbed over a length of about 1 m. This compares favourably with a 10-50 m absorption lengths found for the double clad design (DCD). The specific output power and the slope efficiency are also higher in comparison to a laser with a DCD fiber. For optimizing the fiber design in order to increase the output power and the efficiency a theoretical analysis of the pump light absorption is required just because this point marks the important difference between the double clad and the M-profile fiber. The analysis ofthe laser field in such a structure gives valuable information concerning the mode selection in azimuthal and radial directions. Moreover, the discrimination of modes against each other can be treated, too, in the case of applying external mirrors. Hence, a numerical program was developed to evaluate the mode fields of the pump and laser radiation. This procedure of treating the waveguide holding a large number of modes (>1000) by a wave-optical method contrast markedly with simple ray-optical models (ray tracing approach3). By applying the program optimum design parameters for the M-profile fiber can be deduced concerning the matching of the numerical apertures of the pump radiation onto that of the fiber, the optimum fiber length, the ring diameter and its thickness. These findings may then be easily proofed by experiments. On the basis of the theoretical analysis we have optimized the geometry of our low loss neodymium-doped M-profile fiber. The fiber consists of a undoped core (+ 350 pm) a doped high refractive index ring (thickness 8 pm) an undoped cladding (thickness 15 pm) and a silicon rubber protecting (coating). The doping concentration is about 1300 ppm Nd,O,. The pump waveguide is made up of the core, ring, and cladding. The laser radiation is generated and guided in the ring. The results of our simulation are compiled in Fig. 2 where the absorbed pump power being normalized to the incident power is shown as a function of the coupling angle and the normalized radius of the ring p = Rring/Rfiber. The shortest fiber length for, say CTuG4 Fig. 2 Three-dimensional plot of the absorbed pump power (normalized to the incident power) as a function of the coupling angle and position of the ring within the fiber.