Polarization Mode Beat Frequency Research Articles

High-sensitivity and directional-identification fiber magnetic field sensor via polarimetric fiber ring laser with a fiber Faraday rotator

A highly sensitive fiber magnetic field sensor with directional identification utilizing multi-longitudinal-mode fiber ring laser (FRL) based on polarization-mode beat frequency (PMBF) is experimentally demonstrated. A bismuth-doped iron garnet (BIG), as a kind of fiber Faraday rotator, is embedded in the FRL and serves as a fiber magnetic field probe, whose birefringence shows strong dependence on the external magnetic field due to magneto-optical Faraday effect, leading to the frequency drift of PMBF. Thus, the external magnetic field can be characterized by monitoring the change in a certain PMBF. Additionally, the drift direction of the tracked PMBF is closely related to the applied magnetic field direction, which is attributed to the non-reciprocity of Faraday magneto-optical rotation. Therefore, it shows the good ability to distinguish the applied forward (positive) and reverse (negative) magnetic field. The experimental results reveal that the tracked PMBF signal near 100 MHz has opposite frequency drift signs in the forward (positive) range of 0–20 mT and reverse (negative) range of 0 - −20 mT, which can be used to identify the axial orientation of the applied magnetic field. Additionally, it shows the maximum magnetic field sensitivities of −100.76 and −105.73 kHz/mT in the positive and negative magnetic field ranges, respectively. The designed device makes full use of the advantages of the fiber laser that rapidly demodulates the magnetic field with the help of high-speed digital processing technology, as well as fast response, high integration and low cost of fiber Faraday rotator, offering potentials in high-sensitivity magnetic field measurement in a confined space.

Polarimetric fiber laser for relative humidity sensing based on graphene oxide-coated D-shaped fiber and beat frequency demodulation.

We first propose and demonstrate a polarimetric fiber laser system for relative humidity (RH) sensing based on the beat frequency demodulation. A graphene oxide-coated D-shaped fiber (GDF) with a low insertion loss of 0.8 dB was embedded into a laser cavity to form an RH sensing probe. The output of the fiber laser could generate mode splitting between two orthogonal polarization modes due to birefringence of the GDF device. Hence, two types of beat signals, i.e., longitudinal mode beat frequency (LMBF) and polarization mode beat frequency (PMBF) could be generated synchronously. The experimental results indicated that the LMBFs of the fiber laser had almost no response to the ambient humidity, and the PMBFs of the fiber laser were very sensitive to the various RH levels. There was a good linear relationship between the PMBF and RH changes in the range of 30% to 98%. This fiber-optic RH sensor exhibited a sensitivity of 34.7 kHz/RH% with a high quality of fit (R2>0.997) during the ambient RH increase and decrease. Moreover, the average response and recovery times of the fiber-optic RH sensor were measured to be about 64.2 ms and 97.8 ms, respectively. Due to its long stability, reversibility, quick response time and low temperature cross-sensitivity (i.e., 0.12 RH%/°C), the proposed fiber-optic RH sensor could offer attractive applications in many fields, such as biology, chemical processing and food processing, etc.

Fiber Birefringence Measurement of Single-Mode Fiber by a Polarimetric Fiber Ring Laser

A novel and low-cost fiber birefringence measurement scheme is proposed and studied by a polarimetric fiber ring laser interrogated by the beat frequency technique. For the fiber ring laser, modes splitting occur between two orthogonal polarization modes because of intrinsic fiber birefringence. As a result, two types of beat frequency signals (i.e., longitudinal mode beat frequency and polarization mode beat frequency signals) are generated simultaneously. The principle of measurement of fiber birefringence by the fiber ring laser has been analyzed theoretically in detail. Two alternative expressions for fiber birefringence change are obtained, which is very convenient for users to choose different order longitudinal and polarization mode beat frequency signals. The variation of fiber birefringence caused by applied strain has been calculated in terms of beat frequency shift. To validate the proposed sensing system, the strain-induced fiber birefringence change in commercial Erbium-doped fiber is investigated and obtained by measuring frequency shift of two types of beat frequency signals. The experimental results show that the fiber birefringence experiences a linear decrease when the applied strain is increased. A strain response coefficient of –1.756 × 10–11/μe is obtained. The proposed sensing scheme has obvious advantages of simple structure, low cost, and high sensitivity and it will provide great convenience for measuring fiber birefringence for short-length single mode fiber.

Highly-sensitive magnetic field sensor based on fiber ring laser.

A highly sensitive magnetic field sensor based on a fiber ring laser has been proposed and experimentally demonstrated. The magnetic field sensor was fabricated by introducing a rotary apparatus modulated by an external magnetic field into the fiber cavity to twist one section of the fiber. Due to the remarkable birefringence change induced into the laser cavity, the beat frequency generated between two polarizations of the laser is sensitive to the variation of applied magnetic field intensity. Experimental results show that the polarization mode beat frequency linearly shifts with the increment of the magnetic field intensity and the sensitivity reaches up to 7.09 KHz/Oe in the range of 0 - 437 Oe. Therefore, it will be a promising candidate for the weak magnetic field applications including military, hazard forecast and biomedical fields.

Multimode fiber laser for simultaneous measurement of strain and temperature based on beat frequency demodulation

A multimode fiber laser sensor system for simultaneous measurement of strain and temperature is proposed and demonstrated. Because of the long cavity and birefringence, longitudinal mode beat frequency and polarization mode beat frequency are achieved in the beat frequency signals of the multimode fiber laser. The strain and temperature can be obtained by monitoring both of them for their different strain and temperature responses. The experimental measurement errors are within ± 16.2 με and ± 1.9 °C. The usage of only one fiber laser and one demodulation system makes the system simple, cost-effective and portable.

Simultaneous Measurement of Strain and Load Using a Fiber Laser Sensor

There are longitudinal-mode beat frequency and polarization-mode beat frequency in the beat frequency spectrum of a fiber laser. Thus, a fiber laser sensor for simultaneous measurement of strain and load by monitoring both the longitudinal-mode beat frequency and polarization-mode beat frequency is proposed and demonstrated in this paper. The maximum experimental errors of strain and load obtained are within and g over ranges of 0-960 and 0-600 g. The presented method makes full use of the sensing information of the fiber laser sensor and improves its sensing ability significantly.

Frequency-division-multiplexed polarimetric fiber laser current-sensor array

We demonstrate a novel frequency-domain-multiplexing technique for implementing polarimetric fiber laser current sensors. Each sensor operates at a different polarization-mode beat frequency that is modulated in response to applied electric current. A bank of bandpass filters can be used to separate signals from different sensors. A simple frequency-demodulation technique based on a phase-locked loop is used for signal processing.

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.

Response of fiber lasers to an axial magnetic field

The effects of an axial magnetic field applied to a fiber laser cavity on the polarization characteristics of the fiber laser output are investigated. The experimental and theoretical results show sensitive changes in the polarization mode beat frequency as a function of the applied magnetic field.

Polarimetric fibre laser sensors using Er-doped fibre

A polarimetric fibre laser sensor with narrow polarization mode beat frequency bandwidth is demonstrated using a polarization-maintaining Er-doped fibre. The slope coefficients of 124kHz°C-1 cm-1 and 137kHz μm-1 are obtained for temperature change and longitudinal strain measurements, respectively. A new configuration of polarimetric fibre laser sensor is also demonstrated using a conventional Er-doped fibre as a gain medium and a short length of polarization-maintaining fibre as a sensing part.

Polarization properties of a twisted fiber laser

The characteristics of the output polarization angle and the polarization-mode beat frequency of a twisted fiber laser are investigated. A theoretical model is used to describe successfully the polarization properties.