High-quality Pulses Research Articles

Generation of high-quality soliton pulses using all-fiber optical compressor

In this paper, optical pulse compression using a dispersion shift fiber and a standard singlemode fiber is demonstrated both analytically and experime nta lly. The effects of fiber and input pulse parameters on the compressing quality are discussed through numerical simulation. Experimental results of compressing puls es generated from a harmonically modelocked fiber laser verify that highqual ity pulses can be obtained using this compression method.

2 ~ 5 times tunable repetition-rate multiplication of a 10 GHz pulse source using a linearly tunable, chirped fiber Bragg grating

We experimentally demonstrate a simple scheme for the tunable pulse repetition-rate multiplication based on the fractional Talbot effect in a linearly tunable, chirped fiber Bragg grating (FBG). The key component in this scheme is our linearly tunable, chirped FBG with no center wavelength shift, which was fabricated with the S-bending method using a uniform FBG. By simply tuning the group velocity dispersion of the chirped FBG, we readily multiply an original 8.5 ps, 10 GHz soliton pulse train by a factor of 2 ~ 5 to obtain high quality pulses at repetition-rates of 20 ~ 50 GHz without significantly changing the system configuration.

Generation of pedestal-free ultrashort soliton pulses and optimum dispersion profile in real dispersion-decreasing fibre

In this paper, we study the generation of high-quality ultrashort soliton pulses in dispersion-decreasing fibres with the inclusion of fibre loss in the integrable case. Five closely shaped profiles, namely linear, hyperbolic, exponential, logarithmic, and Gaussian, have been considered. The effects of these dispersion profiles and fibre loss on the performance of pulse compression in dispersion-decreasing fibres have been analysed. The pulse is compressed adiabatically by keeping the soliton property of conservation of pulse area provided the fibre loss is less than the critical value. Both analytical and graphical results show that, out of the five different profiles considered, the Gaussian profile is the optimum profile for achieving maximum pulse compression.

Soliton-like pulse train generation using a nonlinear optical loop mirror constructed from dispersion decreasing fiber

A nonlinear optical loop mirror constructed from dispersion decreasing fiber is used to generate a high-quality pulse train from a dual frequency beat signal. Numerical simulations show that, unlike previous techniques based on adiabatic compression in dispersion decreasing fibers which limited the repetition rates to >50 GHz, the proposed method does not use adiabatic compression and can be extended to repetition rates <20 GHz using reasonable fiber lengths.

Adiabatic Enhanced Compression of Ultrashort Pulses in Fibreswith Slowly Decreasing Dispersion

We investigate the adiabatic enhanced compression of ultrashort pulsesfrom a distributed feedback laser operated at 1550 nm in fibres with slowlydecreasing dispersion (FSDD). High-quality pulse compressions from4.4 ps down to 1.1 ps in a 100 m FSDD and from 4.4 ps down to830 fs in a 500 m FSDD are obtained. The Raman self-frequency shift offemtosecond pulses in the compression is observed. Theoretical analysesare in good agreement with experimental results.

Multiwavelength pulse source for OTDM/WDM applications based on arrayed waveguide grating

A 13-wavelength channel 10-GHz pulse source is demonstrated using a novel scheme incorporating a dispersion-imbalanced fiber loop mirror and an arrayed waveguide grating. The output high-quality pulses are 5 ps wide with the pedestal suppression in excess of 25 dB.

Pedestal suppression from compressed femtosecond pulses using a nonlinear fiber loop mirror

The compression of higher order soliton pulses in dispersion decreasing fibers (DDF's) generates ultrashort fundamental soliton-like pulses accompanied by a broad low-intensity pedestal component. In this paper, we investigate the subsequent removal of the pedestal using a nonlinear optical fiber loop mirror (NOLM) to produce high-quality pulses. The influence of higher order effects such as stimulated Raman scattering and third-order fiber dispersion on the pedestal removal from ultrashort pulses are considered, and the effectiveness of using dispersion-flattened fiber within the loop is shown. The technique is experimentally demonstrated for 540-fs pulses generated by compression in DDF. In this case, high-quality pulses are produced by pedestal removal using a NOLM constructed from dispersion-shifted fiber.

Dispersion control over 150 THz with chirped dielectric mirrors

Ultrabroad-band chirped multilayer dielectric mirrors providing nearly constant negative group delay dispersion over the wavelength range of 640-950 nm and high reflectance between 590 and 970 nm are demonstrated. A key to this performance has been an improved design method, which also substantially reduces the computing time needed for ultimate optimization. The presented devices constitute an enabling technology for producing high-quality terawatt pulses in the sub-10-fs regime. The generation of 5-fs 0.1-TW pulses by using exclusively these mirrors as negative delay line demonstrates this potential.

Optimization of power extraction in a high-power soliton fiber ring laser containing a chirped fiber grating

Power extraction from a high-power soliton fiber ring laser containing a chirped fiber grating is investigated with a variable output coupler. Comparisons are made between the intracavity pulse and the rejection port output pulse. It is shown that a good design for extracting high quality pulses at high power is to insert an intracavity filter to suppress spectral sideband generation and to take the output from the rejection port. The results should apply in general to soliton fiber lasers. Gain saturation is shown to be the main mechanism in the laser that compensates for changes in cavity loss.

Ultra high-bit-rate optical sources and applications

Various techniques are described for the generation of high repetition rate high-quality soliton pulse trains in the range 10 GHz-1 THz. Emphasis is placed on the technique of nonlinear conversion of an optical beat signal in dispersion-modified fibre. This can be achieved by a number of methods, including tapered or dispersion-decreasing fibre, step-like dispersion-profiled fibre and comb-like dispersion-profiled fibre. In addition, self-Raman scattering has been demonstrated as a powerful technique for ultrashort pulse generation and excellent mark space ratio operation. Techniques based on phase-modulated sideband extraction and direct modulation using electroabsorption modulators operating in the 10-60 GHz regime are also described.

Dual-wavelength source of high-repetition-rate, transform-limited optical pulses for soliton transmission

I show how removal of the central optical frequencies from the output of an externally phase-modulated, single-frequency laser results in a dual-wavelength source of high-quality pulse trains for wavelength-division-multiplexed soliton transmission and describe successful experimental demonstration of such sources in the 2.5-12.6-Gbit/s range.

Generation of nearly transform limited fs-pulses by pulse compression of an APM YAG laser

High-quality pulse compression is reported of a continuous wave Nd:YAG laser with additive pulse modelocking. Pulses with a duration of 310 fs and a peak power of 3.7 kW were obtained. A time-bandwidth product of 0.36 was achieved by spectral windowing techniques.

Clinical value of antenatal fetal pulse oximetry

The aim of this prospective observational study was to see if high quality fetal pulse oximetry signals can be obtained from the fetus before labour. It was carried out in Labour Ward, Clarendon Wing, Leeds General Infirmary and St. James Hospital, Leeds. We studied Caucasian term fetuses antenatally and 20 fetuses during labour. Antenatal fetal oxygen saturation was measured using a sensor passed through the cervix onto the membranes covering the fetus before the onset of labour. The quality of these signals was compared to data obtained from 20 fetuses monitored during active labour after amniotomy. High quality pulse oximetry signals were obtained for less than 1% of the monitoring time before the onset of labour (95% CI 0-2.5%). High quality data was seen 38.6% of the time once the membranes were ruptured and the head descendend into the pelvis. With present technology fetal pulse oximetry will not contribute to antenatal fetal assessment.

Femtosecond soliton propagation in fibers with slowly decreasing dispersion

We obtain the adiabatic compression of femtosecond solitons in fibers with slowly decreasing dispersion (FSDD’s) and discover the effect of soliton pulse-width stabilization in FSDD’s. A theoretical model for the description of femtosecond-pulse propagation in FSDD’s is presented. The model takes into account higher-order nonlinear and dispersive effects in fibers. We present numerical simulations of a generalized nonlinear Schrodinger equation and the adiabatic soliton approximation theory as well. Good quantitative agreement between the theory and the experiments is seen. The effect of soliton pulse-width stabilization in FSDD’s is described as a result of the combined action of the third-order dispersion and Raman self-scattering effects. We also show the possibility of obtaining high-quality pulses with less than a 15-fs pulse width in the 1.6-μm spectral region by soliton compression in FSDD’s.

Ultrashort-pulse propagation in optical fibers

A more exact model is suggested for the description of nonlinear light propagation in fibers. In addition to the previously discussed self-phase modulation, parametric, dispersion, self-steepening, and Raman self-scattering effects, this model also takes into account the Stokes losses associated with the material excitation, the dependence of nonlinear effects on the light frequency, and the frequency dependence of the fiber mode area. The self-steepening effect is taken into account more correctly in comparison with previous models. The effects influence considerably the femtosecond soliton propagation. The model is generalized for the case of various fiber dispersion properties along the fiber length. The possibility of obtaining high-quality pulses of less than 15-fsec duration by compression of fundamental solitons with approximately 100-fsec duration in fibers with slowly decreasing dispersion is shown.

Application of a phase-conjugate Brillouin mirror to generation of high-quality variable-duration KrF pulses

Phase conjugation by Brillouin scattering in SF/sub 6/ gas has been implemented on a two-pass KrF amplifier to provide near-diffraction-limited output pulses having a very low background level of amplified spontaneous emission. The pulse duration is variable between 1 and 10 ns by simply attenuating the input pulse. These pulses have also been used for the generation of much shorter duration pulses by truncated Brillouin scattering at the surface of cyclohexane. >

Accurate Determination of Vertical Air Velocities in Rain by Doppler-Radar

Vertical air velocity in rain is calculated from the minima in the reflectivity profile of the Doppler velocity spectra between succeeding range cells. The reflectivity minimum is converted to air velocity, assuming a negative exponential dropsize distribution. The scatter in the observed reflectivity minima can be lowered considerably by representing the reflectivity profile around the minima with a best-fit second order polynomial. A statistical method is developed to express the observed scatter in true fluctuations of the air velocity and a measurement error. In this way the accuracy of the method could be calculated and compared to other methods. Experimental verification has been made during a steady rain with a vertically directed FM-CW Doppler radar. This type of radar is very suitable for this kind of measurements because of its high sensitivity and range resolution. High quality pulse Doppler radars might also be used. Accuracies in the mean vertical air velocity of a few cm s−1 have been obtained. However, just below the bright band larger errors were found, due to deviations from the assumed dropsize distribution by some large, partly melted snowflakes. Turbulent fluctuations of the vertical air velocity in a space volume of 60 × 20 × 20 m and a sampling time of 3 s were determined with a rms accuracy of 0.11 m s−1. Even in the stratiform rain this measurement error appeared much smaller than the matter in the true air velocity and the errors that should be made by estimating the air velocity from the mean fall velocity. It is expected that these differences will be even more pronounced in turbulent precipitation.

PAM pulse generation using binary transversal filters

Modern narrow-band pulse-amplitude modulation (PAM) and pulse-code modulation (PCM) systems require the generation of precisely shaped pulses in order to avoid intersymbol interference. It is not a simple matter to design analog fiters having pulse responses with the requisite carefully positioned zeros, and this has led researchers to study other approaches. It is shown that the use of a binary transversal filter (BTF) coupled with a simple low-pass filter can generate extremely good pulses with small bandwidths. A BTF consists of a digital shift register and weighting resistor network to construct a staircase approximation to the desired pulse. The attendant circuitry is very simple, such that a highquality pulse generator can be built and adjusted with remarkable ease. The problem of pulse design using BTF's is considered in some generality, and a computational procedure is developed for adjusting the weighting network in the BTF to generate optimum pulse shapes. This method is then applied to several practical cases, and the circuitry required to generate excellent pulses is discussed. Test results on an implementation of such a generator using only five taps are presented, revealing the inherent simplicity and quality of the approach.

PAM Pulse Generation Using Binary Transversal Filters

Modern narrow-band pulse-amplitude modulation (PAM) and pulse-code modulation (PCM) systems require the generation of precisely shaped pulses in order to avoid intersymbol interference. It is not a simple matter to design analog filters having pulse responses with the requisite carefully positioned zeros, and this has led researchers to study other approaches. It is shown that the use of a binary transversal filter (BTF) coupled with a simple low-pass filter can generate extremely good pulses with small bandwidths. A BTF consists of a digital shift register and weighting resistor network to construct a staircase approximation to the desired pulse. The attendant circuitry is very simple, such that a highquality pulse generator can be built and adjusted with remarkable ease. The problem of pulse design using BTF's is considered in some generality, and a computational procedure is developed for adjusting the weighting network in the BTF to generate optimum pulse shapes. This method is then applied to several practical cases, and the circuitry required to generate excellent pulses is discussed. Test results on an implementation of such a generator using only five taps are presented, revealing the inherent simplicity and quality of the approach.