摘要:

Twenty five years ago, modern optical communications was launched in the now famous paper outlining the theory of single mode fibres by Charles Kao and George Hockham. Almost exactly half that time ago, the first fibre systems carried telephone traffic (BTRL and STL) whereas today, fibre is the universal wideband transmission medium. However, having made almost unlimited bandwidth available at low cost, the problems have now moved firmly to the network nodes, the switches, and much interest centres on whether optics can contribute here. The paper will examine some of the many ways in which this might come about, ranging from ‘all optical networks’ through to ‘synchronous digital-optic freespace’ switches.

DOI:10.1049/ip-j.1992.0001

出版商:IEE    

年代:1992

数据来源: IET

摘要:

All-monolithic phase-locked arrays of antiguided diode lasers have recently demonstrated exceptionally high diffraction-limited powers: 0.5 W continuous wave (CW) and 2.1 W peak pulsed. An overview of theoretical and experimental work to date is presented. The performance of antiguided arrays is compared to the best results from conventional array types (evanescent-wave, Y-junction, diffraction). The two basic types of array modes: evanescent-wave and leaky-wave are discussed. Resonant leaky-wave coupling in antiguided arrays is explained and interpreted. One key new insight is revealed: when gain is placed in the low-index regions of large arrays (≥10 elements) the array mode favoured to lase is the leaky mode (in-phase or out-of-phase) closest to its respective resonance. Thus the ‘classic’ prediction of coupled-mode theory that high gain in low-index regions automatically favours in-phase mode operation is incorrect. The intrinsic array modal discrimination mechanisms (mode overlap with the gain regions [i.e. the Γ effect]; edge radiation loss, and interelement loss) as well as Talbot-type spatial filtering are briefly explained, and their respective effects on resonant and nonresonant devices are discussed. Results of rigorous modelling of antiguided arrays using two-dimensional analysis are discussed and compared, for the first time, to results obtained via the effective-index method. Finally, key electro-optical characteristics of 20-and 40-element arrays are described.

DOI:10.1049/ip-j.1992.0003

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The wavelength tuning and intensity modulation characteristics are presented for three types of twin-guide (TG) lasers entirely fabricated by metalorganic vapour phase epitaxy (MOVPE). A wide continuous tuning range of 5.3 nm is obtained for the tunable twin-guide (TTG) laser. The TTG lasers, however, have the drawbacks of output power reduction and spectral linewidth broadening during tuning. A new type of tunable laser, the complementary twin-active-guide (CTAG) laser, is proposed, and its tuning characteristics are presented for the first time. A wide continuous tuning range over 4 nm is obtained, although the thermal effect is more dominant than carrier effects. The CTAG structure enables suppression of the spectral linewidth broadening induced by the injected carrier recombination shot noise (IRSN) during tuning. The characteristics of the TG lasers modulated by reverse bias are presented, and the possibility of using these TG lasers for wavelength tuning is studied.

DOI:10.1049/ip-j.1992.0004

出版商:IEE    

年代:1992

数据来源: IET

摘要:

Investigations on InGaAs/InGaAsP multiquantum well separate-confinement lasers emitting in the 1.3 μm wavelength range requiring extremely thin wells of the order of 2 nm are presented. Multiquantum well laser structures with varying numbers of wells and different thicknesses of the barrier layers were grown using low-pressure MOVPE, and were intensively characterised in terms of the threshold current performance and temperature behaviour. Structures comprising 10–15 nm thick barriers and up to 12 wells gave threshold current densities for infinite cavity lengths in the range of 780–1030 A/cm2, being somewhat smaller than those values measured on comparable 1.3 μm bulk lasers. On the other hand, lasers with both smaller and larger barrier thicknesses as well as higher numbers of wells showed greatly increased threshold current densities which may be attributed to higher internal losses and inhomogeneous carrier injection, respectively. Independent of the number of wells and of the thickness of the barriers, all samples under investigation exhibited a characteristic temperatureT0, of about 60 K in the temperature range 5–50°C.

DOI:10.1049/ip-j.1992.0005

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The paper describes gain measurements in InGaAs/InGaAsP multiquantum-well lasers at 1.55 μm derived from the spontaneous emission under current injection. The gain spectrum in a broad-area laser device is determined from the spontaneous emission spectrum on using the measured differential quantum efficiency and the loss conditions at the lasing wavelength at threshold. Spectral measurements from 1.0 to 1.7 μm with intensity resolution over four orders of magnitude clearly exhibit contributions from the barriers, and indicate that a nonthermal equilibrium may exist between the well and barrier materials. Band-filling effects in the well can be seen in the absorption spectra for low injection currents. Gain spectra are obtained for carrier injection levels from 1% of threshold to 1.5 times threshold.

DOI:10.1049/ip-j.1992.0006

出版商:IEE    

年代:1992

数据来源: IET

摘要:

A simple formula is derived which is applied as a quasi-TE- or quasi-TM-mode corrector to the propagation constant values obtained from a scalar treatment of semiconductor rib waveguides. The resulting polarised propagation constants agree favourably with well established benchmark values for both types of mode. The present approach offers advantages in the speed and simplicity of solution.

DOI:10.1049/ip-j.1992.0007

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The paper outlines a model for an all-optical gate and presents results concerning its static and dynamic characteristics. The gate is an inhomogeneously pumped travelling wave amplifier operating with an optical information signal and an optical control signal. Optical gates with two and three homogeneously pumped regions are analysed. The model takes into account the longitudinal variation of photon flux and carrier density, and adopts a multiwavelength approach to consideration of the optical pumping due to spontaneous emission. It is extended to consider also the switching characteristics with applied electrical pulses. Criteria are introduced for the evaluation of the device performance, and, through numerical simulations, the trade-off between the desired static and dynamic characteristics is shown. It is found that, although the switch-on time can be strongly enhanced, the switch-off time is limited by the dynamics of the free carriers for the all-optical operation. Finally, it is verified that electrical switching can substantially improve the performance for the switch-off transient.

DOI:10.1049/ip-j.1992.0008

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The spontaneous emission noise of an axially inhomogeneous laser structure is evaluated by Green functions. Explicit analytical expressions are given for the field rate equation and linewidth in the case of soft variations of the complex effective index of refraction and are applied to a gain-lever laser.

DOI:10.1049/ip-j.1992.0009

出版商:IEE    

年代:1992

数据来源: IET

摘要:

A theoretical analysis of the properties of DBR filters and DFB amplifiers with gain gratings is presented. The results show that, contrary to the case of devices with only an index grating, no compromise is needed between selectivity and peak reflectivity. The properties of active DBR filters improve considerably if gain coupling is introduced.

DOI:10.1049/ip-j.1992.0010

出版商:IEE    

年代:1992

数据来源: IET

摘要:

A wavelength filter consisting of two dissimilar rib waveguides is analysed with respect to the waveguide response and tuning. An optimised structure with a 0.41 nm bandwidth and a 9 mm coupling length is designed. Two techniques are developed to model tapered filters, where the guide separation is varied to improve the wave-length response. A good agreement is found between both methods.

DOI:10.1049/ip-j.1992.0011

出版商:IEE    

年代:1992

数据来源: IET