Upper limit of bandwidth improvement by optical equalization of graded-index fibres

Optical equalization was suggested as a means for increasing the bandwidth of optical fibre communication links when non-optimal graded-index fibre profiles are available. In this paper it is shown that by proper choice of the length as well as of the index of the compensating fibre one can significantly increase the bandwidth capability of the link. An upper limit of this improvement is derived, which is much larger than that obtained with alternating fibres of equal lengths.     

Enhancement of chaotic carrier bandwidth in laser diode transmitter utilizing external light injection

Semiconductor laser with optical feedback emitting chaotic optical signal can be treated as chaotic carrier transmitter. Based on laser rate equations, we numerically study the effect of external injection light on the bandwidth of chaotic carrier transmitter. Our numerical simulation shows that the bandwidth of the chaotic carrier transmitter can be enhanced significantly by external photons injection. Compared with the 2 GHz relaxation oscillation frequency of a solitary laser diode without optical injection, the bandwidth of a chaotic carrier transmitter is expanded to 14.5 GHz with injection parameter at k_inj = 0.39. Simulation results also demonstrate that the enhanced bandwidth depends obviously on the frequency detuning between the external injection laser diode and the chaotic carrier transmitter. The maximum bandwidth of the chaotic transmitter can be obtained when the frequency of the injected light is higher than the central frequency of the carrier transmitter between 2 GHz and 4 GHz.     

Design of Broadband Single Fundamental Mode Hollow Core Bragg Fibre

The condition of the single fundamental mode （HE11） transmission in hollow core Bragg fibres is investigated theoretically by the transfer matrix method. The influences of core size and cladding parameters on the single HE11 mode bandwidth are analysed, showing that the maximal bandwidth is more sensitive to the core size than the cladding. The numerical results show that sufficiently broad bandwidth of single HE11 mode transmission can be achieved by proper fibre design. A simple and fast method based on improved hollow metal waveguide model is proposed to optimize fibre structure parameters for the maximal single HE11 mode bandwidth.     

Design and analysis of a novel single-mode single-polarization photonic crystal fibre based on polarization-dependent coupling and absorption effect

A novel single-mode single-polarization (SMSP) photonic crystal fibre has been proposed and analysed based on the polarization-dependent coupling and absorption effect via a full-vector finite element method with perfectly matched layers. The numerical results predict that very efficient SMSP operation can be achieved with both high bandwidth and high extinction ratio at low loss penalty. Effects of the fibre structural parameters on the SMSP bandwidth and extinction ratio have been explored, which will provide useful guide for the design and fabrication of the fibre. The results obtained will be instructive for the realization of new SMSP fibres with high performance.     

Demonstration of the possibility of >100 Gbit/s transmission over 77 km of standard fibre using a super-step-chirped fibre grating dispersion compensator.

3.8 ps pulses at 1556 nm are reconstructed after transmission over 77 km of standard fibre using a 20 cm long super-step-chirped fibre Bragg grating. We show that good results can be obtained despite the fact that the grating was untrimmed and unapodised, even when the full 1.5 nm reflection bandwidth of the grating is used.     

Low-pass filter characteristics of multimode graded index fibres

On the basis of ray model solutions, the response of a lossless multimode graded-index fibre to a modulated source is calculated. The results demonstrate great sensitivity of the bandwidth of the fibre to small variations in index profile.     

Millimetre-wave fibre–wireless transmission systems with reduced effects of fibre chromatic dispersion

This paper describes two techniques for the implementation of millimetre-wave (mm-wave) wireless communication systems incorporating optical fibre distribution networks. Fibre chromatic dispersion can exhibit severe effects on the transportation of mm-wave frequencies over fibre, resulting in greatly reduced post-detection rf powers. Each mm-wave fibre–wireless system demonstrated here incorporates a method for the generation and modulation of an optical mm-wave carrier, which enables the effect of fibre dispersion to be significantly reduced. One set-up is a direct-detection scheme where optical single-sideband (SSB) with carrier modulation is used to overcome fibre dispersion effects. The other system employs a self-heterodyne arrangement in which a dual-frequency optical source generates a low phase-noise mm-wave beat signal. Data transmission is achieved by externally modulating the dual-mode signal and reduced effects of fibre dispersion are observed.     

Determination of the optimum electrical bandwidth for spectral slicing

The implementation of wavelength division multiplexing (WDM) in communication access networks is dependent upon the availability of low cost optical sources. One possible alternative to lasers is to employ slices of a broadband optical source obtained via optical filters, which may be tuneable. This paper considers the behaviour of such a spectrally sliced optical transmission system as a function of the bandwidth of a realistic receiver electrical filter for the first time. The treatment includes distortion from optical fibre dispersion over several kilometres of fibre. The optimum ratio between the electrical bandwidth and the optical bit rate is found to be ~0.6 for a fifth-order Bessel filter     

Spectral-domain intermodal interference under general measurement conditions

Spectral-domain intermodal interference is analysed theoretically at the output of a few-mode optical fibre alone and at the output of the optical fibre in a tandem configuration with a Michelson interferometer. The theoretical analysis is performed under general measurement conditions when a light source of a given spectral bandwidth and a spectrometer of a given spectral bandpass are considered and when first- and second-order intermodal dispersion effects in the optical fibre are taken into account. Thus, the effect of a variable spectral bandpass of a low-resolution spectrometer on the spectral fringe visibility functions for a two-mode optical fibre of known intermodal dispersion is specified.     

Transfer function of a multimode step-index fibre: a geometrical optical investigation

The transfer function of a multimode step-index fibre has been calculated by means of geometrical optics taking into account mode coupling and leaky rays. The light source may have axial symmetry (light emitting diode) or be line shaped (semiconductor laser). Radial displacement of the line source relative to the fibre axis is allowed. Calculations performed for a large numerical aperture fibre (N.A.=0.475) with a coupling length of 2 km show that leaky rays and radial displacement of the line source are significant for fibre lengths shorter than 200m. The influence of the width of the entrance beam is significant for lengths up to 5 km.Varying coupling length and steady state angle for a fibre length of 150 m calculations show that the bandwidth varies between 15 and 95 MHz for penalty losses between 8 and 18 dB.Calculations have been compared with direct baseband frequency measurements for a high-loss fibre with a large numerical aperture and excellent agreement has been found.     

Enhanced Raman amplifier flatness with nonlinear broadening over non-standard transmission fibre

Nonlinear CW pump broadening over non-standard transmission fibre is used for the first time to achieve improved gain flatness in a single-pump broadband Raman amplifier. As an illustration of the benefits that can be obtained from this approach, a threefold increase in the bandwidth for 0.1 dB gain variation is reported when the broadened pump is used to produce 9.2 dB on-off gain over 25 km LEAF fibre.     

High efficiency, dual-wavelength fibre Raman pump laser for U-band fibre Raman amplifiers

A dual-wavelength, continuous wave, high efficiency fibre Raman ring laser for pumping U-band (1625–1675 nm) fibre Raman amplifiers is demonstrated. Two fibre Bragg gratings were used as wavelength-selective elements resulting in lasing lines at 1529.7 and 1558.7 nm with respective maximum powers of 209.0 and 390.9 mW. A simple U-band Raman amplifier was constructed to test the dual-wavelength pump laser and on:off gain with a peak of 7.3 dB and a 3-dB bandwidth of 73 nm was obtained.     

Transmission properties of cabled multimode optical fibres

The study of real multimode fibres suggests a division into two main classes: very-low-dispersion fibres, employed in high-capacity transmission systems, for which a bandwidth limitation occurs in the repeater spacing; and other fibres. Surprisingly, for the fibres in the first class, the cabling process need not be too sophisticated and expensive, provided that no bandwidth limitations arise from material dispersion. In fact, in this case, the bandwidth limitation can be partly relaxed owing to the fibre perturbation.     

A simplified approach to digital optical receiver design

A simplified theory for the performance of a digital optical receiver is developed. The receiver sensitivity is calculated in terms of circuit parameters, received and equalized pulse shapes, photodiode parameters and bit-rate. An excellent agreement between this theory and a more complicated analysis by Personick [4] is demonstrated. It is shown that the receiver sensitivity may be improved by launching reduced-width pulses into the fibre, particularly if fibre bandwidth is a significant limitation. Reduced-width pulses bring benefits in source power consumption and lifetime, and in timing recovery.     

Long-range optical fibre backscatter loss signatures using t wo-point processing

The ability of the two-point technique to obtain accurate backscatter loss signatures for fibre systems with long repeater spacing is investigated. A systematic analysis of the attenuation range capability of two-point processing is presented and a predicted measurement range of 40 dB one-way fibre attenuation verified by measurement. The analysis takes into account the effect of receiver bandwidth, quantization noise and A–D converter nonlinearity.     

Polarimetric fibre laser sensors using Er-doped fibre

A polarimetric fibre laser sensor with narrow polarization mode beat frequency bandwidth is demonstrated using a polarization-maintaining Er-doped fibre. The slope coefficients of 124kHz°C^-1 cm^-1 and 137kHz m^-1 are obtained for temperature change and longitudinal strain measurements, respectively. A new configuration of polarimetric fibre laser sensor is also demonstrated using a conventional Er-doped fibre as a gain medium and a short length of polarization-maintaining fibre as a sensing part.     

Fibre delay equalization by carrier drift in the detector

The transmission of optical multimode carriers via clad glass fibres has many advantages, but in general introduces severe signal distortion because of mode delay differences. We describe a simple semiconductor receiver capable of equalizing these delay differences at the fibre end. A tenfold increase in bandwidth (data rate) should be possible by this means if mode coupling in the fibre is not excessive.     

Carrier transport effects in quantum well lasers: an overview

Early theoretical predictions and later experimental work have shown that lasers with quantum well active areas have enhanced differential gain over bulk lasers. The resonance frequency in a semiconductor laser is proportional to the square root of the differential gain. The resonance frequency is directly related to the modulation bandwidth, and the enhancement in the intrinsic differential gain led to theoretical predictions of increased modulation bandwidth in quantum well lasers. This enhancement in the modulation bandwidth proved to be elusive initially, and later it was realized that other factors, namely carrier transport effects, played a more dominant role in the high-speed properties of quantum well lasers. Carrier transport effects, in addition to bandfilling, affect a wide range of static and dynamic properties of the quantum well lasers. This paper will present an overview of our present understanding of the carrier transport processes and their effects in quantum well lasers.     

Theoretical Design of Single-Polarization Single-Mode Microstructured Polymer Optical Fibres

Using the full-vector plane-wave expansion method, a kind of PMMA-based polarization-maintaining microstructured optical fibre （PM-mPOF） is theoretically studied. Dependence of the cutoff wavelengths of the two orthogonal polarization states （polarized along the two principal axes of PM-mPOF） on the structure parameters of the fibre is investigated in detail A single-polarization single-mode （SPSM） PM-mPOF working in the visible region is designed and optimized with the result of the maximum SPSM bandwidth of 140nm.     

Pulse Evolution Characteristics in Self-Similar Mode-locked Fibre Laser

A self-similar mode locked fibre laser is studied based on a numerical model. By introducing a dimensionless factor k to characterize the pulse shape, the self-similar pulse evolution, formation and the temporal and spectral shape changes due to the elements in the cavity are investigated throughout the laser cavity. The results show that in the self-similar mode locked fibre laser, self-similar pulse is first formed in the single-mode fibre, which is then amplified in the gain fibre. Gain bandwidth has a small influence on pulse shape, so high energy self-similar pulse can be obtained after amplification. Because net cavity dispersion directly influences the pulse width as well as peak power after compression by a pair of gratings, which can determine the pulse self-similar evolution, it is very important to control the net cavity dispersion to a certain range to obtain self-similar pulses.