Polarization Beat Research Articles

Polarization Beat Length Estimation Based on the Statistical Properties of Brillouin Gain in SMF

A simple method is proposed to estimate the average polarization beat length of single mode fiber (SMF) through the analysis of statistical characteristics of stimulated Brillouin amplified signals. Through launching a variety of pump wave polarizations, the standard deviation of the Brillouin gain is found linearly related with the fiber beat length, within the normal range of beat length values for SMFs. Both the numerical simulations and the experimental results are presented to test the validity of the method. The technique reported here offers a fast, convenient, and low-cost solution to estimate fiber average beat length.

Polarization beating of random electromagnetic beams

We consider temporal interference of two stationary, quasi-monochromatic, partially polarized optical beams with different mean frequencies. We show that both the intensity and the polarization state, represented by the Stokes parameters, exhibit beating, i.e., periodic temporal variation with frequency specified by the difference of the mean frequencies. The contrasts, or visibilities, of the Stokes-parameter changes are characterized by the equal-time electromagnetic degree of coherence between the beams. If the beams are otherwise identical random processes, but with spectra centered at different frequencies, then the polarization modulation is characterized by the degree of polarization, consistently with a recent interferometric interpretation of this quantity. Our results provide insight into the role of polarization in beating of electromagnetic waves.

Characterization of Nonlinear Temperature Dependence of Brillouin Dynamic Grating Spectra in Polarization-Maintaining Fibers

Temperature dependencies of the Brillouin dynamic grating (BDG) spectra in polarization-maintaining fibers (PMF's) are rigorously investigated. PANDA fiber, Bow-tie fiber, and PM photonic crystal fiber (PM-PCF) with different parameters are tested in the temperature range of −30 to +150 °C, where the linear and nonlinear temperature dependencies are quantified for each fiber. It is observed that bare PANDA and Bow-tie fibers show similar shape and magnitude of the nonlinear temperature dependence, while the contribution of the acrylate coating appears differently for each fiber. Suppression of the temperature dependence is observed in the PM-PCF, where notable effects of the acrylate coating on the birefringence are observed only below the room temperature. We also present the result of comparative measurement of the polarization beat length using the BDG analysis and the periodic lateral force method, which confirms the validity of the BDG-based birefringence measurement.

Raman Effect on the Polarization State in Low-Birefringence Fiber

In low birefringence fiber, by solving the coupled nonlinear Schrodinger equations contained the Raman effect for satisfy right- and left-handed circularly po- larized light in the quasi-CW case, the relationship between the effective polarization beat length and changes of input power have been studied emphatically in low- and high input power situation considering the Raman effect or not. The results showed that the polarization state evolution cycles of the incident light can be changed and meanwhile the transmission distance of the incident light also can be changed due to Raman effect, regardless of the polarization of the incident light was along the slow axis or along the fast axis.

Polarimetric heterodyning fiber laser sensor for directional acoustic signal measurement

A DBR fiber grating laser acoustic sensor based on polarization beat signal modulation analysis has been demonstrated for directional acoustic signal measurement. The acoustic sensor was fabricated in birefringent erbium-doped fiber, and the influences of external-acoustic pressure on fiber grating laser sensor were analyzed, considering the effect of relative orientation of the acoustic wave on the degrees of birefringence modulation. In experiment, the birefringence in sensing fiber was modulated by ultrasonic pressure. Agreement between theoretical and experimental results was obtained for ultrasound wave propagating from different directions (0-360 degrees in 15 degrees intervals) corresponding to a nonlinearly change in beat frequency modulation rates. The results demonstrate that the DBR fiber grating laser acoustic sensor has an orientation recognizable ability, offering a potential for acoustic vector signal detection.

Erratum: Quasi-phase-matching of high-order-harmonic generation using polarization beating in optical waveguides [Phys. Rev. A85, 053823 (2012)

Erratum: Quasi-phase-matching of high-order-harmonic generation using polarization beating in optical waveguides [Phys. Rev. A<b>85</b>, 053823 (2012)

Quasi-phase-matching of high-order-harmonic generation using multimode polarization beating

The generalization of quasi-phase-matching using polarization beating and of multimode quasi-phase-matching (MMQPM) for the generation of high-order harmonics is explored, and a method for achieving polarization beating is proposed. If two (and in principle more) modes of a waveguide are excited, modulation of the intensity, phase, and/or polarization of the guided radiation will be achieved. By appropriately matching the period of this modulation to the coherence length, quasi-phase-matching of high-order-harmonic radiation generated by the guided wave can occur. We show that it is possible to achieve efficiencies with multimode quasi-phase-matching greater than the ideal square wave modulation. We present a Fourier treatment of QPM and use this to show that phase modulation, rather than amplitude modulation, plays the dominant role in the case of MMQPM. The experimental parameters and optimal conditions for this scheme are explored.

Highly Birefringent Elliptical-Hole Microstructure Fibers With Low Confinement Loss

A novel kind of elliptical-hole highly birefringent microstructure optical fibers (MOFs) with two defects and six rings is analyzed and proposed. Simulation results show that geometrical birefringence of this elliptical-hole MOF is two times higher than that of the circular-hole ones, which found to be about 3.5×10-3. The polarization beat length of such fiber is calculated to be 0.44 mm. Besides, this kind of MOF with two defects can couple with single mode fibers efficiently and the confinement loss is almost near zero when the cladding ring number reaches 6. Such proposed fiber with high birefringence and low confinement loss can find really potential applications in the fields of light transmission and optical sensing.

Quasi-phase-matching of high-order-harmonic generation using polarization beating in optical waveguides

A scheme for quasi-phase-matching high-harmonic generation is proposed in which polarization beating within a hollow core birefringent waveguide modulates the generation of harmonics. The evolution of the polarization of a laser pulse propagating in a birefringent waveguide is calculated and is shown to periodically modulate the harmonic generation process. The optimum conditions for achieving quasi-phase-matching using this scheme are explored and the growth of the harmonic intensity as a function of experimental parameters is investigated.

Imaging of Surface Plasmon Polariton Fields by Femtosecond Laser Excited Photoemission Electron Microscopy

Femtosecond laser excited two-photon photoemission electron microscopy facilitates the studies on optical properties and dynamics of surface plasmon polariton (SPP) wave packets beyond the diffraction limit. In this article we review our recent work on imaging of SPP wave packets excited by an obliquely incident 10 fs laser pulse at a single slit fabricated in a thin silver film. We image the forward propagating polarization beat that is formed by the interference of the excitation laser light and the SPP wave packet fields. By systematically varying the slit width from sub- to multiple-wavelength scale, we observe modulated increase of the beat intensity, which is phenomenologically accounted for by the enhanced light-SPP coupling efficiency. By using a time-resolved pump-probe technique, we also show the temporal evolution of SPP wave packet that is imaged as the delay-time dependent displacements in space of the beat pattern.

Differential Rayleigh scattering method for measurement of polarization and intermodal beat length in optical waveguides and fibers

We propose a modification of the Rayleigh scattering method, which allows for measurement of polarization and intermodal beat length in single-mode and few-mode channel waveguides and optical fibers. A significant sensitivity increase is achieved by taking two high-resolution photographs in oblique scattered light of π-shifted intensity distributions produced by interference of polarization or spatial modes and applying Fourier analysis to the differential image. In the case of polarization beat length measurements, the π-phase shift is obtained by switching the polarization state at the fiber input, while in intermodal measurements, the π-phase shifting is realized by changing the excitation conditions. The usefulness of the method for characterization of channel waveguides and optical fibers is demonstrated in several examples. Moreover, we show that the combination of the spectral interferometry method with the proposed method allows for broadband measurements of differential phase and group effective indices.

Extremely short distributed Bragg reflector fibre lasers with sub-kilohertz linewidth and ultra-low polarization beat frequency for sensing applications

We report the fabrication, characterization and application of extremely short linear-cavity distributed Bragg reflector fibre lasers (DBR-FLs) written in commercial erbium-doped fibres. The DBR-FL has a total length of 7 mm, of which the nominal cavity is 0.4 mm in length, making it the shortest DBR-FL being reported. Two distinct spectral properties are observed: ultra-narrow linewidth and ultra-low polarization beat frequency, which are measured to be 0.22 kHz and 18.9 MHz, respectively. For the former, the FLs find applications in high sensitivity interferometric sensing, optical frequency metrology and coherent optical communications, whereas for the latter, such a low beat frequency is beneficial to the construction of low-cost and simple demodulator electronics for sensor signal demodulation. The relative intensity noise measurement showed that the relaxation oscillation frequency and relative peak are 120 kHz and −75 dB Hz−1, respectively. The FL exhibited low-noise characteristics, with an intensity noise of −107 dB Hz−1 at 1 MHz. We demonstrate the use of these extremely short FLs in the construction of an efficiently pumped and serially multiplexed FL sensor array suitable for practical sensing applications. Two array configurations were tested, namely with individual FLs fabricated in short sections of EDF and connected via passive fibres and with all FLs fabricated along a single EDF.

矩形晶格微结构光纤偏振拍长稳定性的优化

矩形晶格微结构光纤偏振拍长稳定性的优化

Ultra-short distributed feedback fiber laser with sub-kilohertz linewidth for sensing applications

A low-noise, ultra-short linear-cavity distributed feedback fiber laser (DFB-FL) with extremely narrow linewidth is presented. The FL has a total length of 17 mm, which is, to our knowledge, the shortest DFB-FL being reported. It has a measured linewidth of merely 250 Hz without active stabilization. It has a polarization beat frequency of 101 MHz, which is several times lower than that of most FLs. The relaxation oscillation frequency and relative peak are 110 kHz and −76.5 dB/Hz, respectively. The FL exhibited low-noise characteristics, with an intensity noise of −107 dB/Hz at 1 MHz. Due to low dopant concentration of the EDF and low splice loss with ordinary single-mode fibers, the net insertion loss amounts to only 0.45 dB. Such low pump power loss greatly enhances the capability of multiplexing a large number of FL sensors. Thus, these ultra-short DFB-FLs open up new opportunities for the development of compact-sized point sensor array systems for large-scale high sensitivity sensing applications.

Hydrostatic pressure sensors based on ultrashort distributed-Bragg-reflector fiber lasers

We propose and demonstrate, for the first time to our knowledge, simple hydrostatic pressure sensors based on ultrashort dual-polarization distributed-Bragg-reflector fiber lasers. A bare polarimetric laser is used directly to measure the high fluid pressure up to 20 MPa. By detecting the polarization beat signal change generated from the two orthogonal polarization modes in the laser, a 0.42-MHz/Mpa pressure sensitivity and ~5-kPa resolution are achieved. For effectively enhancing the pressure sensitivity, we also exhibit a simple and accurate low-pressure sensor by designing a structure to convert the external pressure to the lateral force on the laser. This sensor shows significant and considerably high sensitivity to hydrostatic pressure of 69.2 MHz/kPa, which is almost 5 orders of magnitude higher than that of a sensor via bare lasers.

Effects of O2–CO2 polarization beating on femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy of O2

Femtosecond coherent anti-Stokes Raman scattering (fs-CARS) spectroscopy has recently emerged as a promising laser-based temperature-measurement technique in flames. In fs-CARS, the broad spectral bandwidths of the pump and Stokes lasers permit the coupling of each ro-vibrational Raman transition via a large number of pump-Stokes photon pairs, creating a strong Raman coherence. However, the broad-bandwidth fs pulses also excite other molecular transitions that are in resonance. The polarization beating between these closely spaced Raman transitions can affect the coherence dephasing rate of the target molecule, making it difficult to extract accurate medium temperature. In a previous study our group investigated N2/CO polarization beating in N2 fs-CARS; in the present work we study O2/CO2 polarization beating in O2 fs-CARS. O2 fs-CARS can be particularly important for thermometry in non-air-breathing combustion in the absence of N2. The effects of O2/CO2 polarization beating are investigated in the temperature range 300–900 K at atmospheric pressure and also at 300 K for pressures up to 10 bar. Unlike in the N2/CO system, it was observed in the O2/CO2 system that the presence of CO2 can significantly alter the time evolution of the Raman coherence and, hence, affect the measured temperature.

Highly sensitive bending sensor based on Er^3+-doped DBR fiber laser

A short cavity Er(3+)-doped distributed-Bragg-reflector (DBR) fiber laser with a low polarization beat frequency has been demonstrated for bending measurement. The polarization beat frequency of the DBR laser is extremely sensitive to bending and can measure curvature changes as small as 1.8 x 10(-2) m(-1). Excellent agreement between experimental and theoretical results was obtained for bending curvatures from 0 m(-1) to 58.8 m(-1) with corresponding changes in beat frequency from 18.6 MHz to 253 MHz. The sensor is insensitive to temperature fluctuations and has a temperature coefficient of the beat frequency of -25.4 kHz/degrees C, making the temperature compensation unnecessary in most practical applications. The very low beat frequency of the DBR fiber laser makes frequency down-conversion unnecessary. This can greatly simplify the demodulation scheme and thus, allow the realization of low-cost but highly sensitive optical bending sensor systems.

Coexisting Brillouin, Rayleigh, and Raman-enhanced four-wave mixings

The coexisting Rayleigh and Brillouin attosecond sum-frequency polarization beats (ASPBs) and the coexisting Raman, Rayleigh, and Brillouin ASPBs have also been investigated via homodyne and heterodyne detections, respectively. The Raman, Rayleigh, and Brillouin-enhanced four-wave mixing processes strongly compete with each other in ASPBs. The heterodyne-detected signal of ASPBs offers the phase dispersion of the third-order nonlinear susceptibility in the homogeneous broadening material.

Transversal Loading Sensor Based on Tunable Beat Frequency of a Dual-Wavelength Fiber Laser

Microwave signal generation by using the photonic beating from a phase-shift fiber Bragg grating (PS-FBG)-based dual-wavelength laser is proposed and experimentally demonstrated. The dual-wavelength laser is formed by a linear cavity, in which a PS-FBG is used as a dual-wavelength selective component. Transversal loading on the PS-FBG enhances the birefringence of the optical fiber and consequently makes the transmission peak of the PS-FBG splitting into two sharp transmission peaks of orthogonal polarizations. The wavelength spacing between the two transmission peaks increases with the transversal loading on the PS-FBG, thus making the polarization beating frequency increase. This property is exploited in a transversal loading sensor.

Coexisting Raman- and Rayleigh-Enhanced Four-Wave Mixing in Femtosecond Polarization Beats

Based on the polarization interference of Raman- and Rayleigh-enhanced four-wave mixing processes, heterodyne detection of the Raman, Rayleigh and coexisting Raman and Rayleigh femtosecond difference-frequency polarization beats is investigated in the cw and the three Markovian stochastic models, respectively. These two processes exhibit asymmetric and symmetric spectra, respectively, and the thermal effect in them can be suppressed by a field-correlation method. Such studies of coexisting Raman- and Rayleigh-enhanced four-wave mixing processes can have important applications in coherence quantum control, and quantum information processing.