波长间隔可调的可开关双波长掺铒光纤激光器

对两种波长间隔可调的可开关双波长掺铒光纤激光器结构进行了实验研究。一种结构应用保偏光纤中布拉格光栅作为波长选择器件，另一种结构则利用侧向压力下的常规单模光纤光栅作为波长选择器件。两种结构均利用了光纤光栅的双折射特性。在室温下，通过调整偏振控制器的状态可使两激光器工作在稳定的双波长状态或在两单波长之间转换。通过改变加在光纤光栅上侧向应力的大小和方向，可有效控制双波长激射的波长间隔。     

基于高双折射光子晶体光纤环镜的全光纤平顶梳状滤波器(英文)

A novel all-fibre flat-top comb filter based on a high birefringence photonic crystal fibre loop mirror is proposed and demonstrated. We simulate theoretically its output spectra and experimentally realize a flat-top output with a high extinction ratio. Compared to filters consisting of the conventional Panda polarization maintaining fibre, filters based on a high birefringence photonic crystal fibre loop mirror have better temperature stability. This kind of filter can be expected to be used widely in Wave...     

基于高双折射光子晶体光纤环镜的全光纤平顶梳状滤波器

A novel all-fibre flat-top comb filter based on a high birefringence photonic crystal fibre loop mirror is proposed and demonstrated. We simulate theoretically its output spectra and experimentally realize a flat-top output with a high extinction ratio. Compared to filters consisting of the conventional Panda polarization main- taining fibre, filters based on a high birefringence photonic crystal fibre loop mirror have better temperature stability. This kind of filter can be expected to be used widely in Wavelength-division-multiplexing（WDM） systems in the future.     

Erbium-doped twin-core fibre narrow-band filter for fibre lasers

Theoretical analysis of a recently experimentally demonstrated erbium-doped twin-core (ErTCF) fibre saturable-absorber-based narrow-band tracking filter is presented. Basic qualitative characteristics of the filter are deduced from an analogy with an ideally inhomogeneously broadened saturable absorber while the effect of wide band saturation of absorption due to spatial overlap of individual spectral components of the radiation is studied in detail using a numerical model based on coupled mode theory, resolved in space and frequency. The effect of the input power, twin-core fibre length and laser operating wavelength on the spectral characteristics of the filter is investigated.     

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.     

Tuneable Sagnac comb filter including two wave retarders

In this paper we study theoretically and experimentally a wavelength-tuneable Sagnac birefringence filter. The device is a Sagnac interferometer including a symmetric fibre coupler and a length of high-birefringence fibre in the loop. A wave retarder is inserted at each end of the birefringent fibre for absolute wavelength tuning. We show theoretically that wavelength tuning through wave plate orientation ensuring constant amplitude of the filtering function is possible only if a minimum of two wave retarders are included in the setup. The position of the transmission peaks then varies linearly with the angle of one of the retarders and can be adjusted over one entire channel spacing. This happens only when a quarter-wave retarder and a half-wave retarder are used, if the former is oriented at 45° with respect to the fibre birefringence axes, while the orientation of the latter serves as the adjustment parameter. The theoretical predictions are confirmed by the experimental results.     

基于受激布里渊散射的波长间隔可变多波长光纤激光器

提出了一个基于自激发受激布里渊散射的波长间隔可变多波长光纤激光器.利用单模光纤中自激发产生的非线性布里渊增益和掺铒光纤的线性增益组成混合增益光纤激光器,从而使光纤激光器在室温下产生稳定的多波长输出.改变双折射光纤环镜滤波器中保偏光纤的长度,可以实现波长间隔可变多波长激光产生,提高了多波长光纤激光器操作的灵活性和实用性.实验实现了波长间隔从0.8 nm至0.076 nm可变的多波长激光产生,波长数随波长间隔减小而增加,间隔为0.08 nm的激光波长数达86.     

Phase and group modal birefringence of an index-guiding photonic crystal fibre with helical air holes

In this paper, we propose a novel photonic crystal fibre (PCF) with high phase birefringence and very low group birefringence. It is composed of a solid silica core and a cladding with helix-pattern air holes. Using a full-vector finite-element method, we study the phase and group modal birefringence of such PCF at various air-hole sizes, pitches and wavelengths. Owing to this innovative structure of air holes, a high phase to group modal birefringence rate is obtained. Its phase modal birefringence is as large as 10⁻⁴ magnitude; however, the group modal birefringence of this PCF is at 10⁻⁷-10⁻⁶. The phase birefringence is 2 orders of magnitude larger than group birefringence over a broad wavelength span, which means that the light with different polarization and effective index has almost a same group velocity. As a result, the group modal birefringence that closely relates to the polarization modal dispersion is negligible.     

通过外加应变于高双折射环镜实现掺铒光纤激光器的输出功率控制

本文提出一种基于外加应力于高双折射环镜的输出功率可调控的掺铒光纤激光器新颖结构。通过将高双折射光纤环镜中的部分双折射光纤粘贴于一个等强度梁的中性面上，在梁尾端作用不同的应力，由于双折射光纤的长度和双折射的改变可使环镜的反射率从0.8%变化到100%。以该环镜作为线形腔光纤激光器的输出腔镜，实现了动态范围达33dB的输出功率可调控的掺铒光纤激光器。该激光器不仅具有结构简单、成本低廉的优点，而且利用该机理可以很容易的找出最优腔镜反射率，从而使线形腔光纤激光器的输出性能实现最优化。     

Design and fabrication of Panda-type erbium-doped polarization-maintaining fibres

To obtain the stable operation of erbium-doped fibre laser, the simple and ideal technology is adopted by use of the erbium doped polarization maintaining fibre (EDPMF). The design criteria of the Panda-type EDPMF are presented, which take into account the cutoff wavelength, mode field diameter, modal birefringence and background loss. Four groups of optimum structural parameter combinations are determined in terms of the design criteria. Two kinds of the Panda-type EDPMFs are selected to be fabricated. The fabrication process and the parameter control of the Panda-type EDPMFs are presented in detail. Their refractive index profiles, birefringence and absorption spectra are experimentally investigated. The absorption coefficient of the EDPMF, whose core is co-doped with Bi, Ga, Al and Ge, is about 57.9dB/m at 1.53\mum. Co-doping Bi, Ga and Al can greatly increase the erbium concentration in the silica-based fibre. The high birefringence is obtained for the Panda-type EDPMF. The group birefringence of the EDPMF, whose outer cladding diameter is 125\mum, is about 4.8\ti10^-4.     

Equilateral pentagon polarization maintaining photonic crystal fibre with low nonlinearity

A new pentagon polarization maintaining photonic crystal fibre with low nonlinearity is introduced. The full vector finite element method was used to investigate the distribution and the effective area of modal field, the nonlinear properties, the effective indices of two orthogonal polarization modes and the birefringence of the new PM-PCF effectively. It is found that the birefringence of the new polarization maintaining photonic crystal fibre can easily achieve the order of 10^-4, and it can obtain higher birefringence, larger effectively mode-field area and lower nonlinearity than traditional hexagonal polarization maintaining photonic crystal fibre with the same hole pitch, same hole diameter, and same ring number. It is important for sensing and communication applications, especially has potential application for fibre optical gyroscope.     

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.     

Polarization-maintaining photonic crystal fibre

Photonic crystal fibres (PCFs) offer new possibilities of realizing highly birefringent fibres due to a higher intrinsic index contrast compared to conventional fibres. In this paper, we analyse theoretically the modal properties of a kind of polarization-maintaining PCF, namely the elliptical hole photonic crystal fibre (EHPCF) with a central small elliptical hole introduced in the core region. We demonstrate the possibility of achieving large birefringence with zero walk-off in the single-mode regime. This PCF also exhibits unusual dispersion properties. Several hundred nanometres of ultra broadband flattened dispersion near the wavelength 1.55μm is achieved in this EHPCF with a central small hole of d_c=0.4Λ.     

Effect of interstitial air holes on Bragg gratings in photonic crystal fibre with a Ge-doped core

The effect of interstitial air holes on Bragg gratings in photonic crystal fibre (PCF) with a Ge-doped core is numerically investigated by using the beam propagation method (BPM). It is shown that the interstitial air holes (IAHs) can make Bragg resonance wavelength λ_B shift a little towards short wavelengths and increase λ_B-λ₁ (the wavelength spacing between the main peak with Bragg resonance wavelength λ_B and the first side peak with wavelength λ₁ and the coupling coefficient к of Bragg resonance. Moreover, when the ratio of air hole diameter (d) to pitch (Λ), d/Λ, is small, IAHs can suppress the cladding mode resonance. When d/Λ is large, IAHs increase the number of mode that could strongly interact with the fundamental mode. By comparing the transmission spectral characteristics of PCF-based fibre Bragg grating (FBG) with IAHs with those without IAHs at the same air-filling fraction, it is clarified that the change of transmission spectral characteristics of PCF-based FBG with IAHs is not due to a simple change in air-filling fraction. It is also closely related to the distribution of interstitial air holes.     

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%.     

Er-doped twin-core fibre coupler as a saturable-absorber-based narrow-band filter for fibre lasers

We have proposed and demonstrated experimentally that an Er-doped twin-core fibre (ErTCF) can be employed as a tracking narrow-band filter in fibre ring lasers for line narrowing and enhancement of the spectral stability. The device mimics a saturable absorber with inhomogeneous broadening. The slow erbium ion saturation dynamics in the ErTCF prevents wavelength hopping whilst it enables the filter to track the operation wavelength on its tuning. We have improved the device operation by utilizing the inherent filtering effect of a twin-core fibre coupler. Presented at the 1st Czech-Chinesse Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998 This research has been in part supported by the Commission of the European Union in the frame of Copernicus programme, Project # 7882 and by the Ministry of Education of the Czech Republic under Contract OC 241.10/1998.     

Chirped fibre Bragg grating based multiplexer and demultiplexer for DWDM applications

A multiplexer/demultiplexer for 100 GHz channel spacing based on chirped fibre Bragg gratings with different bandwidths and optical circulators is presented. The spectral characteristics, specifications and operation of these passive devices are described, showing its potential use in dense wavelength division multiplexing (DWDM) applications.     

偏斜光线抽运下的回音壁模式光纤激光辐射特性

研究了偏斜光线抽运下的回音壁模式光纤激光辐射特性.实验发现,在抽运光以偏斜光线方式沿光纤的近轴向抽运时,回音壁模式的光纤激光辐射中,既存在光电矢量和光纤径向垂直的横电波(TE),也存在光电矢量和光纤径向平行的横磁波(TM);随增益包层染料溶液折射率的增加,径向模式数和角模式数相同的TE波和TM波之间的波长差单调减小,回音壁模式激光辐射的中心波长向短波方向移动,辐射的波长范围变窄.用回音壁模式激光的辐射理论结合激光染料的四能级模型,满意地解释了实验结果.     

Characterization of a dual wavelength fibre laser based-on dumbbell configuration

This paper reports the design of a dual wavelength fibre laser system using a dumbbell shaped configuration. The oscillation of the lasing wavelength is completed via fibre loop mirrors with a tuneable bandpass filter. The output power, side mode suppression ratio and wavelength spacing of the lasers were investigated against the pump power for forward and backward pumping schemes. The output powers of 4.99 and 3.69 dBm at 1545.08 and 1547.28 nm, respectively, for forward pumping scheme. On contrary, the output powers of 1.93 and 2.14 dBm at 1546.12 and 1551.48 nm, respectively, for backward pumping scheme. The peak output powers of less than 1 dB difference are obtained without any adjustment of the intra-cavity loss over a wider spectral range.     

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.