Polarisation characteristics of a 12 fibre coupler under femtosecond pulsed and continuous wave illumination

We report on the experimental investigation carried out in the polarisation characteristics of femtosecond pulsed and continuous wave beams propagating through a 1×2 fibre coupler. It is demonstrated that the polarisation states of pulsed and continuous wave illumination over the broad wavelength range can be preserved in a conventional fused fibre coupler. Furthermore, the fibre coupler acts as a low-pass spectral filter in the visible wavelength range with a splitting ratio between 99.6/0.4 and 99.7/0.3. The result is of importance for fibre-optic second harmonic generation microscopy which requires the polarisation preservation to extract molecular organization of a sample.     

The spectral sensitivity of long period gratings fabricated in elliptical core D-shaped optical fibre

Long period gratings (LPGs) were written into a D-shaped optical fibre that has an elliptical core with a W-shaped refractive index profile and the first detailed investigation of such LPGs is presented. The LPGs’ attenuation bands were found to be sensitive to the polarisation of the interrogating light with a spectral separation of about 15 nm between the two orthogonal polarisation states. A finite element method was successfully used to model many of the behavioural features of the LPGs. In addition, two spectrally overlapping attenuation bands corresponding to orthogonal polarisation states were observed; modelling successfully reproduced this spectral feature. The spectral sensitivity of both orthogonal states was experimentally measured with respect to temperature and bending. These LPG devices produced blue and red wavelength shifts depending upon the orientation of the bend with measured maximum sensitivities of −3.56 and +6.51 nm m, suggesting that this type of fibre LPG may be useful as a shape/bend orientation sensor with reduced errors associated with polarisation dependence. The use of neighbouring bands to discriminate between temperature and bending was also demonstrated, leading to an overall curvature error of ±0.14 m⁻¹ and an overall temperature error of ±0.3 °C with a maximum polarisation dependence error of ±8 × 10⁻² m⁻¹ for curvature and ±5 × 10⁻² °C for temperature.     

全光纤激光多普勒测速技术的研究

介绍一种全部采用普通单模光导纤维作为光路的激光多普勒测速系统.在该系统中采用普通单模光纤声光频率偏移装置实现被测流场速度方向的判别;以单模光纤偏振控制器控制光纤中激光偏振态,与常规激光多普勒测速系统相比,该系统结构简单,体积小、调整方便,而且测速探头具有极好的移动灵活性.     

Compensation for depolarisation by a fibre coil in the presence of self-phase modulation

We report on the dependence of the degree of polarisation of an 80 fs pulsed laser beam propagating through a length of a single-mode birefringent fibre on the illumination power. Due to the birefringence and the nonlinear effect of self-phase modulation, the measured depolarisation dependence is oscillatory. It is, however, demonstrated that the oscillatory depolarisation can be compensated for by introducing a loop into the fibre geometry, which maintains the degree of polarisation of the pulsed beam as high as 90%.     

基于双折射效应的普通单模光纤频率偏移器和偏振控制器

本文介绍了利用普通单模光导纤维弯曲产生的双折射效应而研制成的频率偏移装置和偏振控制装置.频率偏移装置的单边带抑制达30dB以上,它与光纤偏振控制装置一起是构成全光纤功能型传感器比较理想的光纤元件.     

A new and original practical method for detecting the spectral properties of monolayers of molecular material on homogeneous inhomogeneous substrates

A theoretical method is described of using power reflection spectroscopy to detect the spectral properties of layers of molecular material on homogeneous or inhomogeneous solid substrates. Results are presented for non-reflective, partially reflecting and highly reflecting substrates at various angles of incidence in σ and π polarisation. At angles near the Brewster angle of the substrate the nature of the spectrum in pi polarisation is such that a spectral feature due to a surface layer only 1.0 Å thick is visible in the form of a small “blip” on a constant background. For non-metallic and metallic substrates the spectrum is extremely sensitive to the thickness of the liquid layer, polarisation, and angle of incidence, so much so that this method provides a large amount of new information per experiment, much more so than conventional absorption spectroscopy.     

Easily tuneable nonlinear optical loop mirror including low-birefringence,highly twisted fibre with invariant output polarisation

We investigate theoretically the operation of a versatile nonlinear optical loop mirror (NOLM) structure to be used in optical communication systems. The proposed device is a fibre Sagnac interferometer that includes a low-birefringence, highly twisted fibre and a quarter-wave plate retarder in the loop. We study, both analytically and numerically, the evolution of the intensity-dependent NOLM transmission for both output polarisation components, using different models for the NOLM. From this analysis, we propose an easy way to adjust the position of the NOLM maximum transmission, simply by tuning the angle of the retarder. This procedure is particularly useful for amplitude regularisation of an optical signal. We also demonstrate that, if a tuneable optical attenuator is inserted in the loop, the positions of both maximum and minimum transmission can be tuned separately, using a perfectly reproducible procedure. It is therefore possible to optimise the NOLM transmission for both pedestal and amplitude fluctuations removal in an optical pulse train. For a circular input polarisation, this procedure ensures the highest possible contrast between minimum and maximum transmission, and an output polarisation state that is linear and independent of the input power. Finally, we demonstrate that the transmission characteristic of this NOLM is robust to environmentally induced changes in the fibre birefringence. Thanks to its versatility, robustness and polarisation invariance, this device is thought to be of primary interest for applications such as passive mode locking, pulse compression and pedestal suppression, amplitude regularisation in harmonically mode-locked, rational-harmonically mode-locked or subharmonic synchronous mode-locked lasers, as well as damping of relaxation oscillations.     

Optical pulse shaping at moderate power using a twisted-fibre NOLM with single output polarisation selection

We demonstrate theoretically and experimentally that efficient signal shaping operation can be obtained at moderate power by using the transmission characteristic of a power-symmetric nonlinear optical loop mirror (NOLM) including highly twisted fibre and operating through nonlinear polarisation rotation, when the circular polarisation state orthogonal to the input polarisation is selected at the NOLM output. By adjusting the angle of the quarter-wave retarder inserted in the loop, the phase bias of the transfer characteristic can be adjusted precisely to enable proper signal shaping for moderate values of input power, remaining well below switching power. The tolerance of the procedure to deviations of the input polarisation from the ideal circular case is investigated numerically. We demonstrate experimentally the capabilities of this setup for both power equalisation and extinction ratio enhancement. Finally, we show that this setup is also useful to shape ultrashort optical pulses from the relaxation oscillations of a DFB semiconductor laser. In comparison with other NOLM-based techniques, the proposed approach allows to reduce by a factor of 8-10 the peak power required for pulse shaping, for the same fibre length and Kerr coefficient.     

Strain and vibration analysis by fibre based speckle shearing interferometry

An optical fibre based, speckle shearing interferometer is described. The instrument uses a highly birefringent optical fibre to illuminate a test object with equal intensities of light guided by the orthogonal polarisation eigenstates of the fibre. A Wollaston prism is used to obtain two sheared images with adjustable shear. Optical phase changes between the sheared images are readily achieved, without mechanical movement of components, by straining the optical fibre. Object strain determination, by fringe analysis with phase stepping techniques, is readily achieved. Vibration analysis by heterodyning is also reported.     

Origin of the variability of the mechanical properties of silk fibres: 3. Order and macromolecule orientation in Bombyx mori bave,hand‐stretched strings and Nephila madagascarensis spider fibres

The comparison of the low wavenumber of polarized Raman spectra (50–300 cm^–1) from Bombyx mori (fresh cocoons fibres, hand‐stretched ‘Crins de Florence’ strings from the gland content, dried gland, regenerated silk films) and Nephila madagascarensis silks reveals the high polarisation of fibre modes and the absence of polarisation for dried gland and regenerated silk films. This is consistent with X‐ray diffraction measurements. The orientation of the fibroin/spidroin chains is due to the stretching during production, as for advanced synthetic fibres. The bandwidth of the ‘ordered chains’ signature is almost the same for the different fibres. However, the degree of polarisation seems to be higher in the case of spider fibre. The huge bandwidth of low wavenumber components of regenerated films indicates high disorder. Measurements along the fibre point out conformation changes with a periodicity (~20 mm) related to the silkworm head motion during the fabrication of the cocoon. Copyright © 2012 John Wiley & Sons, Ltd.     

86-km optical link with a resolution of 2 × 10<Superscript>-18</Superscript> for RF frequency transfer

RF frequency transfer over an urban 86 km fibre has been demonstrated with a resolution of 2×10^-18 at one day measuring time using an optical compensator. This result is obtained with a reference carrier frequency of 1 GHz, and a rapid scrambling of the polarisation state of the input light in order to reduce the sensitivity to the polarisation mode dispersion in the fibre. The limitation due to the fibre chromatic dispersion associated with the laser frequency fluctuations is highlighted and analyzed. A preliminary test of an extended compensated link over 186 km using optical amplifiers gives a resolution below 10^-17 at 1 day.     

Emulator with inverse trend in first-order and second-order polarisation mode dispersion

We design a polarisation mode dispersion (PMD) emulator through subdividing a polarisation maintaining fibre (PMF) of 22 m in length. The aim of this emulator design is to show that first-order and second-order PMD can be inversely proportional to each other. Furthermore, the emulator is also used to show that the magnitude of PMD is independent to whether its statistics approach theoretical distributions or not, of most importance is the degree of mode coupling. The same (as former) applies to its autocorrelation function (ACF). The PMD control mechanism for the emulator is not in real time.     

Propagation of a polarised polychromatic beam through a birefringent plate

The effect of propagation of a polarised polychromatic beam having a truncated gaussian spectral profile has been studied. The degree of polarisation of the output beam is seen to be less than of the of the standard deviation of the profile on the degree of polarisation of the output has been considered. Finally, the special case of a flat-top spectral profile has been discussed.     

Polarisation-sensitive four-wave mixing and soliton self-frequency shift effect in the highly birefringent photonic crystal fibre

By adjusting the polarisation state of the pump at 805 nm parallel to slow (x) and fast (y) axes of the highly birefringent photonic crystal fibre with zero dispersion wavelengths 790 nm and 750 nm, this paper demonstrates the efficient polarisation-sensitive four wave mixing involved in pump, anti-Stokes and Stokes signals and soliton self-frequency shift effects induced by the phase-matching between red-shifted solitons and blue-shifted dispersive waves. If the reduction of coupling efficiency to the circular pump laser mode or other circular fibres due to asymmetry of the core is neglected, more than 98% of the total input power is kept in a single linear polarisation. Controlled dispersion characteristic of the doublet of fundamental guided-modes results in achieving light field strongly confined in principal axes of photonic crystal fibre, and enhancing the corresponding nonlinear-optical process through the remarkable nonlinear birefringence.     

光纤激光器的偏振开关操作

本文阐述了采用光子注入法实现光纤激光器的偏振开关操作.这种新颖的全光开关器件具有极低的阈值(0.1μW).对操作机理、实验实现和器件特性.     

Development of a new generation of high permeability non-oriented silicon steels

The paper describes a new generation of the high permeability fully processed silicon steel grades developed by Acesita, with core loss (W_1.5/60) in range of 3.10–4.20 W/kg and polarisation (J₅₀) from 1.71 to 1.75 T, respectively. The new grades have lower (Si+Al) content and a better crystallographic texture, with lower fractions of [1 1 1]||ND fibre and higher fractions of [0 0 1]||RD fibre. The new grades have better mechanical and magnetic properties than conventional grades.     

An integrated optical coupler used in a fibre interferometry system for on-line surface measurements

In this paper, an integrated optical (IO) coupler produced using the technique of silica-on-silicon was described. A fibre interferometry system using the IO coupler was then developed and its performance was tested. The environmental noise and disturbances in this system were reduced significantly after combining a reference interferometer and an all fibre polarisation scrambler so as to produce good stability and repeatability. Furthermore, the measurement mirror was mounted on a piezoelectric transducer (PZT) and disturbances were introduced in the system in order to simulate on-line surface measurements. Both a commercial fibre and the IO couplers were compared using this method. Better experimental results were achieved when the system using the IO coupler than using the fibre one.     

单模光纤相移DSPI测量二维面内位移的方法

本文提出将相移技术应用于2-D面内位移场的分离,用单模光纤相移数字散斑干涉计量技术快速、定量地同时测量2-D面内位移场。     

Broadening free spectral range of an evanescent-wave pumped Whispering-Gallery-Mode fibre laser by Vernier effect

Evanescent-wave pumping scheme has been successfully used to excite Whispering-Gallery-Mode fibre laser. The free spectral range of the lasing spectrum from a single optical fibre is found to be very narrow, to increase the free spectral range, a coupled cylinder-cavity structure has been fabricated by binding two bare optical fibres together, based on Vernier effect, the free spectral range of the coupled cylinder-cavity structure has been effectively broadened over 15 times compared with that of a single optical fibre. The measured free spectral ranges for both coupled cylinder-cavity structure and the single optical fibre are consistent with theoretical prediction.     

Optical fibre amplifiers: physical model and design issues

The development of optical fibre amplifiers has caused an impressive evolution in optical telecommunications systems since the end of the 1980s. The widespread application of active fibre amplifiers requires accurate tools to design and simulate these devices in very different operating conditions. The present work describes some physical aspects of active fibre amplifiers and a numerical model for the analysis of active optical fibres that can be applied to investigate spatial and spectral properties of amplifiers for the transmission windows of optical fibre telecommunications. Finally, various design issues relevant to the investigation of optimal fibre design of silica, fluoride erbium-doped and fluoride praseodymium-doped fibres are discussed and illustrated by means of examples.