摘要:

The method of path integration is used to study tapered, graded-index waveguides in the context of paraxial, scalar wave optics. Closed form analytic results are obtained for the propagator, or Green's function, and coupling efficiencies of such structures. The results of this general theory are applied to tapers of parabolic and inverse-square-law shapes to derive closed form expressions for the lowest-order mode-coupling efficiency of these tapers. The results are compared with those obtained for the linear taper (discussed in a previous paper) in order to establish the suitability of each taper geometry for use in practical optical components. The inverse-square-law taper is found to be the least suitable one for single-mode devices.

DOI:10.1049/ip-j.1992.0063

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The derivation of a detailed expression for the frequency spectrum of pulse interval and width modulation (PIWM) is presented. Close agreement has been obtained between spectral predictions from the formula and experimental results. Design details and performance figures are also included for a practical PIWM optical fibre transmission link intended for use with TV, video and instrumentation signals.

DOI:10.1049/ip-j.1992.0064

出版商:IEE    

年代:1992

数据来源: IET

摘要:

Semiconductor ring lasers with a threshold current as low as 12.5 mA have been fabricated in GaAs/AlGaAs. The low threshold current is due to a passivating layer of silicon nitride and an optimised fabrication process that results in very smooth sidewalls. The threshold current was analysed and it was found that a considerable part of the current is lost by spreading towards the inside of the pillbox structure. The remaining loss mechanisms were identified as internal loss (1 dB), coupling loss (7–8 dB) and scattering loss (2–4 dB). The scattering loss was found to be independent of the radius for radii between 30 μm and 145 μm. The L–I curve displays several kinks, which are explained by shifts of the mode position and thus the coupling ratio at the Y-junction due to temperature changes with pumping.

DOI:10.1049/ip-j.1992.0065

出版商:IEE    

年代:1992

数据来源: IET

摘要:

A theoretical model has been formulated using rate equations to analyse the response of a semiconductor optical amplifier to a single analogue amplitude-modulated input signal. The results are found to agree very closely with experimental results obtained from a bulk buried-heterostructure device, and show that signal distortion is significant even for input powers substantially below saturation. The level of distortion is strongly affected by both the saturation power of the device, and the Fabry-Perot resonance arising from residual facet reflectivity. On the basis of good agreement with experimental data, the theoretical model is used to assess a novel method of bias current feedback that reduces the effect of carrier density modulation by the modu lation of the input signal, and greatly improves the harmonic performance of the device. Using this feedback method, the calculated value of signal phase distortion has been reduced by a factor of up to 40, and the harmonic distortion by up to 11 dB.

DOI:10.1049/ip-j.1992.0066

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The effect of surface recombination on DBR sensitivity and the microwave reflection spectrum of laser-induced photoconductivity gratings in silicon coplanar waveguides is analysed quantitatively. Numerical results are supported by experimental results.

DOI:10.1049/ip-j.1992.0067

出版商:IEE    

年代:1992

数据来源: IET

摘要:

Two large-signal dynamic DFB laser models have been developed recently: the transmission-line laser model and the power matrix method. The paper presents quantitative comparisons between these two models and demonstrates good agreement between results for L—I characteristics, transient response, time-resolved spectra and spatial hole-burning of the longitudinal carrier density. The models are also compared in terms of applicability.

DOI:10.1049/ip-j.1992.0068

出版商:IEE    

年代:1992

数据来源: IET

摘要:

A semiconductor Raman laser with a waveguide in which a pump beam transmits with a fundamental mode, which can be used as an optical heterodyne demodulator, is discussed. As a result of the improved growth process and resonator construction, a threshold pump power as low as 300 mW has been achieved. The internal loss is discussed, and a tapered-waveguide semiconductor Raman laser is fabricated as a method to realise further lower pump power operation, and its lasing operation is demonstrated.

DOI:10.1049/ip-j.1992.0069

出版商:IEE    

年代:1992

数据来源: IET

摘要:

A novel approach for designing optimal multilayer filters based on a real-coded genetic algorithm is presented. Given the total number of layers in the filter, as well as the electrical properties of the materials constituting each layer, the algorithm iteratively constructs multilayers whose frequency response closely matches a desired frequency response. In contrast to existing iterative techniques, this method does not require a preliminary design using classical techniques. Also, the design procedure is independent of the nature of the multilayer as well as the characteristics of the incident and substrate media. The algorithm is applied to the design of various lowpass and high-pass optical filters, operating between practical terminal conditions. The performance of the resulting designs matches or improves on that for filters that were synthesised using semiclassical techniques.

DOI:10.1049/ip-j.1992.0070

出版商:IEE    

年代:1992

数据来源: IET