摘要:

A theoretical model is presented, describing the relationship between the quality of a laser beam at the input and output of a multimode graded-index optical-fibre beam delivery system, as a function of the beam launching conditions. The launching conditions are described in terms of the diameter and numerical aperture of the laser beam at the input to the fibre, relative to the core diameter and acceptance angle of the fibre. The beam quality transfer function is shown to depend on the position-dependent divergence of the input beam, which is described in terms of its phase space. The theoretical model has been developed to accommodate phase-space distributions observed experimentally for a practical laser source and launch optics, and for the specific case of fibres having a quadratic index distribution. The model shows that an optimum launch condition exists. For the ideal case of an input beam with elliptical phase space used at the optimum launch condition, the ratio of output/input beam quality is unity. The model has been tested experimentally, using a high average power Nd: YAG laser of a type commonly used for material processing operations and a graded-index fibre of core diameter 600 μm. The output-beam quality was measured as a function of launch conditions, including a measurement of the input-beam phase space. The results were in accordance with the theoretical prediction, the optimum launching condition was identified, and an optimum ratio of output/input beam quality of 1.2 was demonstrated.

DOI:10.1049/ip-j.1993.0035

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The temperature dependences of the basic parameters of single-mode InGaAsP semiconductor lasers have been analysed with due regard for the Auger effect. Formulas have been derived for computing the temperature drifts of amplitude-frequency and phase-frequency characteristics (AFC, PFC) under direct high-frequency modulation to construct three-dimensional amplitude and phase surfaces. On the basis of the formulas derived, an analytical method has been designed for calculating the temperature drift of the AFC and PFC of semiconductors lasers under high-frequency modulation by small signals without any optical feedback. The proposed method yields results in good agreement with the experimental data over wide frequency (tens of GHz) and temperature (δT≃ 80 K) intervals. The PFC drifts have been studied as a function of different parameters and initial temperatures (thermostabilisation temperature) of lasers. The temperature drifts of lasers have been numerically estimated and criteria have been formulated for choosing the laser operation modes in phase stabilisation fibre-optic laser transmitter modules. The error of the AFC and PFC determination method is of the order of 0.2%/K.

DOI:10.1049/ip-j.1993.0036

出版商:IEE    

年代:1993

数据来源: IET

摘要:

Conditional artificial dielectrics are considered; these are composites of small semiconductor crystallites embedded in a polymeric host material. The dielectric properties of these materials were measured at microwave frequencies and under light illumination. It was found that the light affect mostly the phase velocity of a propagating microwave beam and with almost no loss.

DOI:10.1049/ip-j.1993.0037

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The transient statistics of an almost single-mode semiconductor laser biased below threshold is studied. We characterise the statistics of rare turn-on events in which the depressed mode carries significant output power in terms of output power at turn-on and mean power during duration of the optical pulse for the main and side modes. The turn-on time is defined as the time at which the total output power reaches a prescribed value. The statistics of power partition at turn-on time is obtained analytically and numerically for different excitation currents and gain differences. Numerical results for the statistics of mean power partition during pulse emission are presented. A simple relation between both characterisations is found.

DOI:10.1049/ip-j.1993.0038

出版商:IEE    

年代:1993

数据来源: IET

摘要:

Comparison between the F–P mode spectrum from a subthreshold-biased semiconductor diode and its measured counterpart with a grating monochromator has been made. It is shown that the modulation index measured from the recorded spectrum takes its minimum periodically when the slit width is set to allow the light from an integer number of modes to pass through, and the measured wavelengths at the valleys (or peaks) on the recorded mode spectrum are different from their true values by an amount equal to a half of the diode mode spacing if the passband width of the monochromator is larger than the mode spacing.

DOI:10.1049/ip-j.1993.0039

出版商:IEE    

年代:1993

数据来源: IET

摘要:

It is known that a dielectric waveguide with semiconductor cladding exhibits a maximum attenuation of a given polarisation of light when the cladding thickness corresponds to the cut-off of any guided mode of the same polarisation that the semiconductor layer supports itself. In this paper mode cut-off conditions in the semiconductor layer are analysed and analytic expressions for the layer cut-off thicknesses are developed. The results show good accuracy when compared with more sophisticated numerical analysis techniques consuming a large amount of computer time. Design curves for silicon-clad polarisers on Ti: LiNbO3and glass waveguides are developed from the presented theory.

DOI:10.1049/ip-j.1993.0040

出版商:IEE    

年代:1993

数据来源: IET

摘要:

A computer program is demonstrated for calculating the two-dimensional transversal vectorial field in longitudinally invariant isotropic optical waveguides by applying a simple finite difference approach using successive overrelaxation on a Gauss—Seidel algorithm and estimation of the propagation constant using the Rayleigh quotient. The method is formulated in terms of the transverse electric field that implicitly satisfies the boundary conditions to the order of the discretisation step. The implementation is such that it allows the computations, even of structures with complex refractive index, to be performed on a personal computer. Using this implementation technique (based on successively more accurate approximations of the field), the first few higher order modes can be obtained without difficulty, which is usually one of the problems with this method. The comparison of results is given for standard passive structures examined and for waveguides in the index gain guiding regime.

DOI:10.1049/ip-j.1993.0041

出版商:IEE    

年代:1993

数据来源: IET

摘要:

A theory of THz optical frequency conversion using highly nondegenerate four-wave mixing (FWM) in a semiconductor laser is presented by small signal analysis. The analytical expressions for the conversion efficiency and the bandwidth of the converted signal are given. The optical frequency conversion is attributed to the mechanism of cavity enhanced highly nondegenerate FWM in a semiconductor laser in a range of 1 THz detuning frequency between the pump and the probe waves when the probe frequency is tuned closed to one of the resonance modes. The frequency conversion is mainly introduced by the nonlinear gain effect when the detuning frequency is very high. The frequency conversion efficiency is asymmetrical wrt the zero detuning frequency, which is caused by the interaction between the nonlinear gain effect and the carrier beating, especially when the detuning frequency is not very high. The bandwidth of the converted signal can be increased by increasing the bias current and the nonlinear gain coefficient of the laser.

DOI:10.1049/ip-j.1993.0042

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The current work proposes the use of optical fibres as sensing elements for dose rate measurement of ionising radiation based on controlled light induced annealing or photobleaching. It shows that, under steady-state irradiation, the propagation characteristics of the fibre can be modulated by variations in the intensity of the photobleaching signal. Since the effected modulation was found to depend both on the intensity of the incident radiation and on the optical power density variation of the annealing signal, a suitable scheme can be devised to measure the dose rate of the incident radiation in an all-fibre ionising radiation detector.

DOI:10.1049/ip-j.1993.0043

出版商:IEE    

年代:1993

数据来源: IET