Velocity measurement by the laser speckle method using optical fibres

An optical device for in-plane velocity measurement is described. Laser light is focused by the use of an optical fibre onto the object, and the device utilizes the dynamic sparkle effect for velocity determination. The influence of different optical fibres on the speckle formation is investigated with special respect to fibre movement, and as the diameter of the aperture used for speckle detection also affects the measuring signal, an optimum value is found. A simple electronic circuit processes the measuring value proportional to the object velocity.     

Remote actuation by optical power transmission in fibres

Some preliminary experiments on the remote actuation of a servo-valve are described in which the power transmission medium is an optical fibre. Using a semiconductor laser or a Nd:YAG laser as the power source and silicon cells as the opto-electrical transducer, switching of the servo-valve has been observed at power levels of a few milliwatts over 800 m of fibre.     

高斯光束通过失调透镜系统的聚焦特性

为分析透镜系统中圆孔光栏和透镜失调对高斯光束聚焦特性的影响,利用椭圆光栏近似展开式和失调光学系统的广义衍射公式,推导高斯光束经含圆孔光栏失调透镜系统传输的近似解析式,得到输出光束光强极大值场分布与光束参量、孔径尺寸、光栏和透镜失调量之间的关系。针对特定透镜系统,定量分析失调量对输出光束聚焦特性的影响。结果表明:各元件的失调对输出光束聚焦特性均产生影响,在失调量较小时透镜横位移对输出光束聚焦特性的影响比透镜角位移对输出光束聚焦特性的影响更明显。     

冷原子穿越激光束的量子隧穿时间

原子通过激光冷却技术能够被制备在低温状态，这时冷原子云会展现出量子力学的波动性.研究了一束冷原子入射到一个蓝失谐的激光束上所表现出的量子力学隧穿效应.蓝失谐的激光束相对于冷原子而言等效于一个量子力学势垒.根据二能级模型，在理论上分析了具有内部结构的原子矢量物质波穿过激光束的量子力学反射与透射，特别是对原子穿越激光束所需的时间——量子隧穿时间进行了详细的研究.量子力学波动性使得冷原子穿越一个激光束时明显地展现出与经典粒子(热原子)不同的结果. 关键词： 冷原子 原子光学 量子隧穿     

Semiconductor optical fibres: progress and opportunities

This paper reviews recent progress in the nascent field of semiconductor optical fibres, from the fundamentals through to device demonstration. The incorporation of semiconductor materials into both the step‐index and microstructured fibre geometries provides a route to introducing new optoelectronic functionality into existing glass fibre technologies. Herein, the various fabrication methods that have been developed as of to date are described, and their compatibility with the different semiconductor materials and fibre designs discussed. Results will be presented on the optical transmission properties of several fibre types, with particular attention being paid to the observation of nonlinear propagation in silicon core fibres. Finally, some speculation regarding the future prospects and applications of this new class of fibre will be provided.     

Zero dispersion wavelength and dispersion slope control of hollow-core photonic bandgap fibres

This paper investigates the zero dispersion wavelength and dispersion slope control of hollow-core photonic bandgap fibres (PBGFs) by using a full-vector finite element method. By simulation we found that theoretically the zero dispersion wavelength can be tailored by respectively changing the rounded diameter of air holes, pitch, refractive index, normalized thickness of core rings, and hole diameter to pitch ratio. At the same time the tailoring of dispersion slope can also be realized by changing the rounded diameter of air holes or pitch or normalized thickness of core rings. To illustrate the reasonability of fibre designs, this paper also gives the variance of normalized interface field intensity which measures the scattering loss relatively versus wavelength for different designs. From the viewpoint of loss, varying the rounded diameter and the thickness of core ring could shift zero wavelength but it is difficult to get the required parameters within so tiny range in practical drawing of PBGFs, on the other hand, it is possible in practice to respectively alter the pitch and refractive index to shift zero wavelength. But varying hole diameter to pitch ratio is not worthwhile because they each induce large increase of loss and narrowness of transmission bandwidth. The zero dispersion wavelength can be engineered by respectively varying the rounded diameter of air holes, pitch, refractive index, and normalized thickness of core rings without incurring large loss penalties.     

Cross-phase modulation instability in optical fibres with exponential saturable nonlinearity and high-order dispersion

Utilizing the linear-stability analysis, this paper analytically investigates and calculates the condition and gain spectra of cross-phase modulation instability in optical fibres in the case of exponential saturable nonlinearity and high-order dispersion. The results show that, the modulation instability characteristics here are similar to those of conventional saturable nonlinearity and Kerr nonlinearity. That is to say, when the fourth-order dispersion has the same sign as that of the second-order one, a new gain spectral region called the second one which is far away from the zero point may appear. The existence of the exponential saturable nonlinearity will make the spectral width as well as the peak gain of every spectral region increase with the input powers before decrease. Namely, for every spectral regime, this may lead to a unique value of peak gain and spectral width for two different input powers. In comparison with the case of conventional saturable nonlinearity, however, when the other parameters are the same, the variations of the spectral width and the peak gain with the input powers will be faster in case of exponential saturable nonlinearity.     

Joint nonlinearity and chromatic dispersion pre-compensation for coherent optical orthogonal frequency-division multiplexing systems

This paper introduces a joint nonlinearity and chromatic dispersion pre-compensation method for coherent optical orthogonal frequency-division multiplexing systems. The research results show that this method can reduce the walk-off effect and can therefore equalize the nonlinear impairments effectively. Compared with the only other existing nonlinearity pre-compensation method, the joint nonlinearity and chromatic dispersion pre-compensation method is not only suitable for low-dispersion optical orthogonal frequency-division multiplexing system, but also effective for high-dispersion optical orthogonal frequency-division multiplexing transmission system with higher input power but without optical dispersion compensation. The suggested solution does not increase computation complexity compared with only nonlinearity pre-compensation method. For 40 Gbit/s coherent optical orthogonal frequency-division multiplexing 20×80 km standard single-mode fibre system, the suggested method can improve the nonlinear threshold (for Q > 10 dB) about 2.7, 1.2 and 1.0 dB, and the maximum Q factor about 1.2, 0.4 and 0.3 dB, for 2, 8 and 16 ps/(nm·km) dispersion coefficients.     

Investigation of mechanism of free electron laser through spontaneous radiation spectrum

In this paper, the mechanism of free electron laser with a guide magnetic field is investigated by analyzing spontaneous radiation spectrum of electrons. The physical relation between spontaneous radiation and stimulated radiation in FEL with a guide magnetic field is studied. It is found that this relation in FEL is similar to that in quantum lasers. The spontaneous radiation spectrum is discrete. The frequency of stimulated radiation in FEL is oneselected and amplified in the radiation spectrum.     

Raman oscillation with intracavity pumping of a fibre laser

We introduce the concept of cascaded resonant Raman pumping of fibre lasers. The pump scheme utilises the relatively large intracavity Stokes field that is generated within a Raman fibre laser to excite a lanthanide ion that is doped within the core of the fibre providing the Raman gain. In order to illustrate the general characteristics of the pump method and, to establish the design parameters necessary for the realisation of the pump scheme, calculations from a theoretical model that is used to simulate the generation of laser output from a fibre laser that is resonantly pumped with first Stokes radiation is presented. Specifically, the 2.1 μm output from a Ho³⁺-doped silica fibre laser that is pumped with 1.15 μm first Stokes radiation is calculated with the use of a relatively simple numerical model. For a launched pump power of 20 W at 1.07 μm, a fibre laser output of 3 W is predicted for a nominal intrinsic loss of 1.5 dB/km at 2.1 μm, however, this low value of the intrinsic loss at 2.1 μm can be significantly relaxed when the length of the Ho³⁺-doped silica fibre laser resonator is made considerably shorter than the Stokes resonator.     

Exciting the optical response of nanowire metamaterial films on the tip of optical fibres

This Letter reports on the assembly on the tip of an optical fibre of a metamaterial film fabricated by a self‐assembly bottom‐up method, composed of silver nanowires embedded in an alumina matrix. By illuminating the film through the fibre in a reflection configuration, we observe experimentally the optical response of the metamaterial in agreement with theoretical predictions and interpreted as the excitation of surface plasmon‐polaritons in the cylindrical surface of the nanowires. These results pave the way for low‐cost optical fibre devices that incorporate metamaterial films.

     

Cs Faraday anomalous dispersion optical filter for blue pulsed laser

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Tm:Ho co-doped single mode optical fibre laser pumped by a 1600 nm Er fibre laser

An all-fibre Tm:Ho laser system is reported, using a 1600 nm Er fibre pump laser and 0.3 m length of the fibre through a two-stage optimization of both the pump source and laser configuration to achieve a low threshold operation. As a result a low threshold power of 33 mW and a slope efficiency of 0.6% have been achieved, in laser operation at a wavelength of 1870 nm and a cross-comparison with 785 nm laser diode pumping has been made.     

Time domain measurement of dispersion in optical fibres

A time domain dispersion measurement apparatus was successfully built and tested at the Indian Institute of Technology, Madras. This instrument can measure directly the pulse broadening in a multimode optical fibre. The basic dispersion mechanisms in optical fibres and the design details of the system are discussed.     

Comprehensive research on self phase modulation based optical delay systems

This paper comprehensively investigates the properties of self phase modulation based optical delay systems consisting of dispersion compensation fibre and highly nonlinear fibres.It researches into the impacts of power level launched into highly nonlinear fibres,conversion wavelength,dispersion slope,modulation format and optical filter bandwidth on the overall performance of optical delay systems.The results reveal that,if the power launched into highly nonlinear fibres is fixed,the time delay generally varies linearly with the conversion wavelength,but jumps intermittently at some conversion wavelengths.However,the time delay varies semi-periodically with the power launched into highly nonlinear fibres.The dispersion slope of highly nonlinear fibres has significant influence on the time delay,especially for the negative dispersion slope.The time delay differs with modulation formats due to the different combined interaction of nonlinearity and dispersion in fibres.The bandwidth of the optical filters also greatly affects the time delay because it determines the bandwidth of the passed signal in the self phase modulation based time delay systems.The output signal quality of the overall time delay systems depends on the conversion wavelength and input power level.The optimisation of the power level and conversion wavelength to provide the best output signal quality is made at the end of this paper.     

Self-mode-locking in a Q-switched Nd: doped silica fibre laser

Operating characteristics of a Q-switched Nd³⁺: doped single-mode silica fibre with a mechanical chopper as a Q-switching device are presented. Peak powers as high as 16 W with pulse duration of 146 ns are demonstrated. Self-mode-locking in Nd³⁺:doped fibre is also reported. An output power of 312 W with a centre pulse duration of 4 ns is observed.     

Reduction of beam divergence of a thermal lensing insensitive Nd:YAG laser

In a previous paper we proposed an active resonator designed especially for Nd:YAG lasers. The beam generated by this resonator had a flat-top beam profile and a divergence that was insensitive to the thermal lensing effect. The beam generated by this resonator was, however, too divergent to be focused into a fibre. With the help of numerical simulations, we have found a way to reduce this divergence, and to render the far-field of the laser beam fine enough to be coupled into a fibre with 600 μm core and NA = 0.2.     

Proposals for a standard for the safe use of light-emitting diodes, laser diodes, and optical fibre transmission systems

A new safety standard for light emitting diodes, laser diodes and fibre optical transmission systems is needed because of the difficulties in applying existing standards to these products. New proposals, generated by a BSI working group, include a new classification system which takes into account the inherent beam divergence of most of these products, and the need to classify at different locations within an extended optical system such as a fibre-optic link.