Modeling and simulation of polarization errors in Sagnac fiber optic current sensor

Sagnac fiber optic current sensor (S-FOCS) is a kind of optical interferometer based on Sagnac structure, optical polarization states of sensing light wave in Sagnac fiber optic current sensor are limited. However, several factors induce optical polarization error, and non-ideal polarized light waves cause the interference signal crosstalk in sensor, including polarizer, quarter-wave retarder, splice angular, birefringence and so on. With these errors, linearly polarized light wave in PM fiber and circularly polarized light wave in sensing fiber become elliptically polarized light wave, then, nonreciprocal phase shift induced by magnetic field of the current is interrupted by wrong polarization state. To clarify characteristics of optical polarization error in fiber optic current sensor, we analyze the evolution process of random optical polarization state, linear optical polarization state and circular optical polarization state in Sagnac fiber optic current sensor by using Poincare sphere, then, build optical polarization error models by using Jones matrix. Based on models of polarization state in Sagnac fiber optic current sensor, we investigate the influence of several main error factors on optical polarization error characteristics theoretically, including extinction ratio in polarizer, phase delay in quarter-wave retarder, splice angular between quarter-wave retarder and polarization maintaining fiber. Finally, we simulate and quantify nonreciprocal phase shift to be detected in fiber optic current sensor related with optical polarization errors. In the end, we demonstrate S-FOCS in test. The results show that transfer matrix errors are induced by inaccurate polarization properties during polarization state conversion, then, the stability and accuracy of the S-FOCS are affected, and it is important to control the polarization properties at each step of the polarization state conversion precisely.     

Switchable and tuneable multi-wavelength Er-doped fibre ring laser using Sagnac filters

We demonstrate experimentally a simple configuration to perform wavelength-tuning and multi-wavelength operation in a fibre ring laser that is composed of an Erbium-doped fibre and includes a fibre-optic Sagnac interferometer as a spectral filter. We consider three different Sagnac filters including a 3 dB coupler, two wave retarders (WRs) and respectively 7 cm, 1 and 7 m of high birefringence fibre. The transmission spectrum of each filter is sinusoidal with a 70, 6, and 0.85 nm period, and its maxima can be shifted over one period by adjusting the angle of the retarders in the loop. By adjusting the WRs included in the filter and in the laser ring, tuneable single wavelength and several two- or three-wavelength lasing regimes were observed. In particular, fine adjustments of the WRs allow observing two- and three- wavelength operation with wavelength separations well below the homogeneous bandwidth of the gain medium. The stability of these modes of operation however strongly depends on environmental conditions at room temperature.     

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.     

Tunable fiber-optic parametric oscillator

We report operation of a tunable optical parametric oscillator that employs a nonlinear-fiber Sagnac interferometer as a parametric amplifier. The amplifier, which consists primarily of dispersion-shifted fiber that has zero dispersion at 1538 nm, is synchronously pumped with 7.7-ps pulses at 1539 nm. The wide bandwidth of the parametric gain permits tuning of the output signal pulses over a 40-nm range centered on the pump wavelength. The Sagnac interferometer decouples the pump wave from the oscillator cavity while a bandpass filter in the cavity transmits only the signal wave, thereby creating a singly resonant parametric oscillator that is phase insensitive. Whereas we demonstrate tuning over almost the entire bandwidth of Er-doped-fiber amplifiers, one could construct a similar device that operates near the 1310-nm zero-dispersion wavelength of standard telecommunication fiber.     

Equal channel spacing Sagnac filter using high-birefringent photonic crystal fibers

An equal channel spacing Sagnac filter is proposed using a high-birefringent photonic crystal fiber （PCF） with birefringence square to the operation wavelength. A PCF with four smaller central air holes sur-rounded by larger air holes is applied in a Sagnac filter with optimized parameters to achieve equal channel spacing of 0.8 nm. Given that the birefringence of this PCF is square to the operation wavelength, the channel spacing of the proposed filter changes by only 0.03 nm within a wavelength range from 1400 to 1650 nm; this is about one order of magnitude less than that constructed with conventional high-birefringent fibers.     

A dual-loop Sagnac interferometer

A new type of all-fibre Sagnac interferometer composed of two loops is proposed and analysed in detail. It can be used with a very long transmission line while maintaining excellent performance characteristics due to the automatic compensation of any birefringence effects in the trunk fibre. Preliminary experiments at 1310~nm wavelength with a 70~km long trunk fibre demonstrated an interference visibility as high as 98%, indicating that this scheme has promising potential applications.     

基于低双折射光子晶体光纤Sagnac干涉仪的超低温度系数扭曲传感器

研究了低双折射光子晶体光纤中由光纤扭曲造成的圆双折射效应,并应用Sagnac干涉仪结构设计了扭曲传感器.在Sagnac环中的光子晶体光纤上施加机械压力引入初始线双折射并产生正弦干涉光谱,再扭曲光纤产生圆双折射使干涉光谱随扭曲角度移动.光谱峰值波长随扭曲角度变化符合Sinc函数关系,理论分析与实验相符.传感器灵敏度为1.00 nm/°,分比率为0.01°,并具有超低的温度系数-0.5 pm/℃.     

高双折射Sagnac环透射特性的理论与实验研究

 采用传输矩阵分析法、数值模拟和实验研究相结合的方法研究了由光耦合器（OC）、偏振控制器（PC）和保偏光纤（PMF）组成的高双折射Sagnac干涉环的透射特性。理论分析了PMF特性和PC状态对输出特性的影响,并通过数值模拟得到了PMF长度、双折射率（Δn）和PC状态不同时所对应的透射光谱。最后通过实验验证得出了波长间隔只由PMF的长度和双折射率Δn决定,透射率大小只由PC状态决定,而透射谱的峰值位置由PMF和PC状态共同决定的规律。实验结果与理论分析结果非常一致,对Sagnac环透射规律的研究对这种可调谐滤波器在多波长光纤激光器中的应用具有一定的参考价值。     

Tunable optical filter based on Sagnac phase-shift using single optical ring resonator

In this paper, a single optical ring resonator connected to a Sagnac loop is used to demonstrate theoretically a novel narrow band optical filter response that is based on Sagnac phase-shift Δφ. The given filter structure permits the Sagnac rotation to control the filter response. It is shown that by changing the Sagnac rotation rate, we can tune the filter response for desired bandwidths. To increase the wavelength selectivity of the filter, the Sagnac phase-shift should be as small as possible that is limited by the loop length. For Δφ=0.1 rad, the obtained FWHM is 2.63 MHz for tuning loop length of 2 m. The simulation response agrees fairly with the recently reported experimental result.     

Polarization properties of elliptical core non-hexagonal symmetry polymer photonic crystal fibre

In this paper, polarization properties and propagation characteristics of polymer photonic crystal fibres with elliptical core and non-hexagonal symmetry structure are investigated by using the full vectorial plane wave method. The results show that the birefringence of the fibre is induced by asymmetries of both the cladding and the core. Moreover, by adjusting the non-symmetrical ratio factor of cladding $\eta$ from 0.4 to 1 in step 0.1, we find the optimized design parameters of the fibre with high birefringence and limited polarization mode dispersion, operating in a single mode regime at an appropriate wavelength range. The range of wavelength approaches the visible and near-infrared which is consistent with the communication windows of polymer optical fibres.     

级联光栅结合Sagnac环的可调谐光纤激光器

提出并验证了一种单-双波长可调谐掺铒光纤激光器。利用级联光纤布拉格光栅(Cascaded Fiber Bragg gratings,Cascaded FBGs)结合Sagnac环结构所产生的复合滤波效应,实现较高精细度滤波,并通过调节环内偏振控制器(Polarization Controller,PC),引入双折射效应,得到波长可调谐的光纤激光器。基于耦合模理论并使用传输矩阵法对该结构的传输特性进行了分析,在此基础上搭建实验系统,验证了理论分析的正确性。实验结果表明:通过调节PC,激光器输出激光的波长范围约为1 555.644～1 556.112 nm,双波长间隔的可调范围约为0.108～0.452 nm,单-双波长的边模抑制比(SMSR)均高于40 dB;在稳定性测试中,输出单-双波长激光的波长最大漂移量小于0.008 nm。该方法具有结构简单、调谐方便、易于实现且精细度较高的优点,可应用于密集波分复用及全光通信系统等领域。     

基于低双折射光子晶体光纤Sagnac干涉仪的超低温度系数扭曲传感器

研究了低双折射光子晶体光纤中由光纤扭曲造成的圆双折射效应,并应用Sagnac干涉仪结构设计了扭曲传感器.在Sagnac环中的光子晶体光纤上施加机械压力引入初始线双折射并产生正弦干涉光谱,再扭曲光纤产生圆双折射使干涉光谱随扭曲角度移动.光谱峰值波长随扭曲角度变化符合Sinc函数关系,理论分析与实验相符.传感器灵敏度为1....     

Tunable single-longitudinal-mode Ytterbium all fiber laser with saturable-absorber-based auto-tracking filter

A broadband tunable, single-longitudinal-mode (SLM) Ytterbium fiber laser with unpumped Ytterbium-doped Sagnac loop is proposed and demonstrated experimentally. The unpumped Ytterbium-doped Sagnac loop is employed as a saturable absorber based auto-tracking filter to ensure single-longitudinal-mode oscillation. And a tunable band pass optical filter with large tuning range is applied to achieve broadband tuning ability. With 1-m Ytterbium-doped fiber as the gain medium, the SLM operation is achieved with over 60-nm wavelength tuning range at 160-mW pump power. The laser is very stable with output power of about 3 dBm and optical signal to noise ratio of higher than 50 dB in all the 60-nm tuning range.     

Analysis of a polarisation-maintaining NOLM switch for OTDM applications

We propose a polarisation-maintaining NOLM switch design to be used as optical regenerator or wavelength converter in dense optical time-division multiplexed (OTDM) systems. The Sagnac loop is made of a piece of high birefringence fibre which is cut and cross-spliced in the middle. If pump and probe polarisations are linear and aligned in the co-propagating direction, the cross-splice ensures that the counter-propagating probe beam will be orthogonal to the pump, so that the parasitic cross-phase modulation between counter-propagating beams is minimised. This architecture also allows easy control of the optical phase bias, through squeezing a short section of the fibre, without any other modification of the setup. The performances of the proposed architecture are studied analytically and numerically, and compared with those of conventional schemes. It appears that, although the proposed setup reduces the interaction between counter-propagating beams only by a factor 3, it yields an extinction ratio improvement of a factor 10 or higher in comparison with conventional schemes. If there is substantial walkoff between pump and probe, a 10-fold reduction of the relative intensity noise of the emerging signal is also obtained when the mark ratio of the incoming data is variable.     

一种新型侧漏型光子晶体光纤的研制及其传输特性研究

通过引入椭圆掺锗芯和侧向泄露通道, 提出并研制出一种侧漏型光子晶体光纤(photonic crystal fiber, PCF). 应用结构重构全矢量有限元数值分析法分析了设计结构和实际研制的侧漏型PCF的传输特性. 研制的侧漏型PCF, 在波长1550 nm处基模的平均模场直径为9.275 μm, 与G652标准单模光纤具有很好的适配性, 模式双折射为0.837× 10^-4, 群双折射约为1.508× 10^-4. 基于研制的侧漏型PCF光纤构建了Sagnac干涉仪, 对其群双折射进行了测量. 测量结果表明:当侧漏型PCF光纤达到一定长度时, 在1450–1750 nm波长范围内, 二阶模在光纤中不能成为有效传输模式, 光纤可以实现单模传输; 另外, 研制的侧漏型PCF群双折射实验的测量平均值, 与数值分析结果相符合. 侧向泄露通道的引入, 增强了侧漏型PCF光纤对外界参量变化的敏感性, 提高了其在扭转、弯曲、压力等参量的光纤传感和高性能光纤激光器构建等方面的应用潜能. 关键词： 光子晶体光纤 侧向泄露通道 双折射 Sagnac干涉仪     

Measurement of birefringence in integrated optical waveguides by use of a microwave-modulated optical wave

We present a technique for measuring birefringence (B) in integrated optical waveguides by use of a microwave-modulated optical wave. It is shown that the technique is able to yield an accurate measurement of birefringence. In addition, an approximate estimate of birefringence dispersion (dB/dlambda) can be achieved by means of tuning the wavelength of the light source and measuring the dependence of the birefringence on the wavelength. As an example of the use of the technique, we show how to evaluate an innovative Ti:LiNbO (3) waveguide. The results show that such a Ti:LiNbO (3) waveguide has a birefringence of 0.0783+/-0.0001 and a birefringence dispersion of 0.05+/-0.01mum (-1) when the optical wavelength is approximately 1.560mum .     

Stable multiwavelength fiber ring laser with equalized power spectrum based on a semiconductor optical amplifier

We experimentally demonstrated a new structure of a multiwavelength semiconductor optical amplifier (SOA) ring laser based on a fiber Sagnac loop filter that can generate up to 25 stable output lasing wavelengths at room temperature. By varying the length of a polarization-maintaining (PM) fiber within the Sagnac loop filter, the wavelength spacing between the output lasing wavelengths can be changed to a desired value. By tuning a polarization controller (PC) within the Sagnac loop filter, stable multiwavelength 1550-nm operation with up to 17 lasing lines within 3 dB power level variation and with a wavelength spacing of ∼0.8 nm was achieved. The optical signal-to-noise ratios (OSNRs) of all the lasing wavelengths are greater than 40 dB.     

Demonstration of sharp switching from a power-symmetric NOLM and a polarizer

We demonstrate theoretically, experimentally and numerically that a steep all-optical transfer characteristic can be obtained from a power-symmetric NOLM, including a quarter wave retarder and highly twisted fibre, followed by a polarizer. We first develop a theoretical analysis to show that, if the input polarisation is chosen linear, then for a correct choice of the wave plate and input polarisation angles the transmission grows from zero with a 4 dB/dB slope as power is increased, two times faster than can be obtained with a NOLM alone. An experimental study is then realised, which supports the theoretical results. Steep switching is demonstrated for reasonably low peak power levels that do not exceed 8 W, less than one third of the NOLM critical power. In addition, using the same setup we demonstrate the possibility to obtain a sinusoidal transmission characteristic that cancels out for a particular nonzero value of input power, which can be easily adjusted simply through rotation of the wave plate and output polarizer. This setup would be very useful in the frame of optical pulse shaping and signal processing applications, among others.     

Electrically tunable Sagnac filter based on a photonic bandgap fiber with liquid crystal infused

We demonstrate an electrically tunable Sagnac filter based on a photonic bandgap fiber that is realized by infusing liquid crystal into a solid-core air-hole photonic crystal fiber. The filter enables an electrical tuning with a range of about 26 nm for one single interference minimum in the transmission bandgap from 1330 to 1650 nm wavelength. The tuning efficiency is averaged to 0.53 nm/V.     

Tunable chirped fiber Bragg grating based on the D-shaped fiber

A novel tunable chirped fiber Bragg grating is realized by bending a piece of D-shaped fiber Bragg grating into a specific shape like the character ‘Ω’. The principle of tuning is demonstrated by numerical simulation. The dispersion of the grating can be tuned while the center wavelength is kept nearly no shift. The experimental results match well with the numerical analysis. The influence from D-shaped fiber’s birefringence and polarization dependent loss is discussed theoretically and experimentally as well. This proposed tuning technique has potential applications in the optical communications and other fiber optics systems.