Pulse Energy Fluctuations Research Articles

Resonance Frequency Shift of a Two-Level Atom in a Pulsed Optical Trap

We consider the effect of a periodic pulsed perturbation on the shift of resonance frequency for a Rabi oscillation. Unlike a continuous perturbation, where the frequency shift is proportional to the perturbation strength, the shift under a pulsed perturbation shows periodic behavior. In fact, when the pulse height and the pulse duration are set such that a fictitious spin completes a 2π precession around the principal quantization axis, there is no frequency shift. This phenomenon can be used for optical frequency metrology by trapping reference atoms in a properly controlled pulsed optical trap. We estimate the residual systematic shift and line broadening due to practical limits from pulse energy fluctuation, atomic motion, and finite pulse duration and find that it is on the order of 100 Hz.

High-Power fiber amplifier with widely tunable repetition rate, fixed pulse duration, and multiple output wavelengths

We report a pulsed, fiber-amplified microchip laser providing widely tunable repetition rate (7.1 - 27 kHz) with constant pulse duration (1.0 ns), pulse energy up to 0.41 mJ, linear output polarization, diffraction-limited beam quality (M(2) < 1.2), and < 1% pulse-energy fluctuations. The pulse duration was shown to minimize nonlinear effects that cause temporal and spectral distortion of the amplified pulses. This source employs passive Q-switching, single-stage single-pass amplification, and cw pumping, thus offering high efficiency, simplicity, and compact, rugged packaging for use in practical applications. The high peak power and high beam quality make this system an ideal pump source for nonlinear frequency conversion, and we demonstrated efficient harmonic generation and optical parametric generation of wavelengths from 213 nm to 4.4 mum with Watt-level output powers.

Increased Stokes pulse energy variation from amplified classical noise in a fiber Raman generator

We present an experimental and theoretical study of the transition from linear to nonlinear amplification of classical pump noise in a fiber Raman generator. In particular, we focus on the conversion of fluctuations in the fine temporal structure of Q-switched pump pulses into Stokes pulse energy fluctuations. We show that there is a distinct pump power domain where large scale fluctuations in the Stokes pulse energy result from the amplification of fluctuations in the temporal structure of pump pulses with stable energies. Dramatic changes in the shape of the Stokes pulse energy probability distribution also occur as the pump power is swept through the domain of large scale energy fluctuations.

Effect of beam diameter on the propagation of intense femtosecond laser pulses

This paper describes the effects observed during the propagation of intense femtosecond laser pulses in air following the modification of the laser beam diameter with a pair of convex–concave lenses placed/mounted in a telescopic configuration. We observed that by reducing the diameter of the beam the detected back-scattered nitrogen fluorescence from the filaments becomes more stable on a shot-to-shot basis while, for a larger beam size, greater fluctuations are observed that are not correlated to shot-to-shot fluctuations in the laser pulse energy. This result leads to a new method to control the fluorescence signal which can be very important in remote-sensing applications.

Ultraviolet high-gain harmonic-generation free-electron laser at BNL

We report the first experimental results on a high-gain harmonic-generation (HGHG) free-electron laser (FEL) operating in the ultraviolet. An 800 nm seed from a Ti-Sapphire laser has been used to produce saturated amplified output at the 266 nm third-harmonic. The results confirm the advantages of the HGHG FEL: stable central wavelength, narrow bandwidth and small pulse energy fluctuation. The harmonic output at 88 nm, which accompanies the 266 nm radiation, has been used in an ion pair imaging experiment in chemistry.

Quasi-continuous-wave diode array side-pumped Q-switched Yb:YAG slab laser

A quasi-continuous-wave diode array side-pumped Q-switched Yb:YAG slab laser has been demonstrated. A KD*P crystal is used as an electro-optic Q-switch and 3.07-mJ output energy with a pulse width as short as 56.27 ns (FWHM) is obtained. The Yb:YAG crys- tal doped density is 10 at.% with the dimension 83331 mm. The pulse width and energy fluctuations are 3.44% and 3.36% respectively. The relative Q-switched theory is analyzed and rate equations are calculated. The influence of Q-switch delay time on pulse performance is also stud- ied. The final experimental results coincide well with the theoretical results. © 2004 Society of Photo-Optical Instrumentation Engineers.

Dynamic characteristics of double-clad fiber amplifiers for high-power pulse amplification

Based on numerical modeling, the dynamic characteristics of high-power ytterbium-doped double-clad fiber amplifiers during the amplification of nanosecond pulses with kilohertz repetition rates are comprehensively analyzed. The temporal pulse energy, power, upper-level population distribution, amplified spontaneous emission, stored energy, pulse waveform evolution, etc., are emphasized in this paper. Some practical issues in amplification processes, such as the impacts of reflected pulses from the external surfaces and the fluctuation of input pulse energy on the amplifier performance, are also discussed. The models and results are important for the design and development of high-power double-clad fiber amplifiers.

First ultraviolet high-gain harmonic-generation free-electron laser.

We report the first experimental results on a high-gain harmonic-generation (HGHG) free-electron laser (FEL) operating in the ultraviolet. An 800 nm seed from a Ti:sapphire laser has been used to produce saturated amplified radiation at the 266 nm third harmonic. The results confirm the predictions for HGHG FEL operation: stable central wavelength, narrow bandwidth, and small pulse-energy fluctuation.

An accumulative x-ray streak camera with sub-600-fs temporal resolution and 50-fs timing jitter

We demonstrated that the shot-to-shot timing jitter of an x-ray streak camera was reduced to close to 50 fs when it was triggered by a standard kilohertz laser with 1.2% rms pulse energy fluctuation. This was achieved by improving the response time of deflection plates and the rise time of the ramp pulse generated by a photoconductive switch, and by operating the photoconductive switch at optimum conditions. Furthermore, after reducing the angular distribution of electron bunch, the temporal resolution of the x-ray streak camera operating in accumulation mode was measured to be better than 600 fs.

Reduction of optical intensity noise by means of two-photon absorption

We demonstrate an all-optical intensity noise reduction technique by using two-photon absorption of ZnSe in a Z-scan configuration. We measure the nonlinear transmission and the shot-to-shot energy fluctuations of 860-nm intense femtosecond laser pulses. At a 40% nonlinear transmission level the noise in the pulse train is reduced to 24%. This yields an additional 40% reduction in the relative pulse energy fluctuation. A realistic theoretical treatment is presented and is found to be in good agreement with experimental results.

Multiframe observation of an intense femtosecond optical pulse propagating in air

We have demonstrated femtosecond time-resolved and picosecond time-interval successive observations of a single femtosecond optical pulse propagating in air with ultrafast self-modulation such as filamentation. A quadruple femtosecond probe pulse crossing an intense propagating pulse at picosecond intervals was able to capture directly four successive images of the propagating pulse as in a movie but with femtosecond time resolution. From this observation, we can directly analyze and recognize the propagation process, which is significantly affected by pulse-energy fluctuation, and (or) atmospheric turbulence from shot to shot.

Status of the Beijing IR Free Electron Laser facilities

Beijing free electron laser (BFEL) is a Compton type mid-infrared FEL facility. By using a thermionic cathode RF gun as an injector, 30 MeV s-band RF linac and two wiggler-optical cavity beam lines generate mid- and far-infrared radiation in the wavelength range of 5-50 µm. BFEL facility possesses the conditions for a user's facility. The running is stable, intensity fluctuation of laser is small, and wavelength adjustment is flexible. The stability test showed that the pulse energy fluctuation is about 1% within 30 minutes, and wavelength fluctuation is less than 0.1%. In this year, the lasing time used for users has reached 1000 hours. More than 250 samples have been studied and many application items have been concerned. Five user's stations will be established. Among them, two key experimental stations; FEL spectrum analysis station, and FEL steady state analysis station, have been constructed. For the purpose of user's applications, special equipments have been provided. Some experimental results of the application items are also presented.

Stability of multiple pulses in optical fibers with phase-sensitive amplification and noise

We demonstrate via numerical simulations the long-term stability of bound 2- and 3-pulses in a fiber-optic communication or storage system where loss is compensated by periodic, lumped, phase-sensitive amplification. The effects of attenuator quantum noise induced by linear loss in the fiber and of guided acoustic-wave Brillouin scattering caused by coupling to acoustic modes of the fiber are included in the computations. The first noise source is shown primarily to produce a small amount of intra-pulse timing jitter, while the second is shown mainly to cause pulse energy fluctuations. For amplifier spacings that are not too large, however, both effects are smaller than in systems using phase-insensitive amplification. It is suggested that these 2- and 3-pulses could be used in a novel encoding scheme to improve transmission system bit rates.

Noncollinear optical parametric amplifiers with output parameters improved by the application of a white light continuum generated in CaF 2

We report the drastic improvement of output parameters of femtosecond noncollinear optical parametric amplifiers by applying supercontinuum seed pulses generated in CaF 2 instead of sapphire and imaging via all reflective optics. Features of the setup are broad amplification bandwidths up to 310 THz (470–910 nm), an almost Gaussian beam profile, a divergence with M 2 factor of less than 1.4, pulse to pulse energy fluctuations of less than 2.5% and compressibility of the pulses down to 16 fs. The high intensity of the supercontinuum seed pulse (80 nJ) yields a quantum efficiency for the conversion of pump to signal of more than 30%. Even for short wavelengths e.g. at 470 nm pulse energies above 30 μJ were obtained for 110 μJ of 400 nm pump light.

Stability of synchronous intensity modulation control of 40-Gb/s dispersion-managed soliton transmissions

The efficiency of transmission control of dispersion-managed solitons by means of in-line synchronous intensity modulation is numerically analyzed. We demonstrate that intensity modulation may lead to dramatic improvements in the transmission performance of 40-Gb/s dispersion-managed solitons. In particular, we identify the locations within the dispersion map where synchronous intensity modulation provides an efficient control of the pulse propagation. Namely, a stabilization of pulse energy fluctuations is achieved, with no need for additional in-line control by guiding filters. On the other hand, incorrectly placed intensity modulation may cause significant pulse instabilities.

Creating process margin in laser thermal processing: Application to formation of titanium silicide

Recently, we have demonstrated that, due to differential thermal budget, laser silicidation is an attractive alternative for deep submicron metal-oxide field effect transistors. In laser thermal processing, any spatial beam nonuniformities or pulse to pulse energy fluctuations lead to varying Si melt depth and hence variations in the silicide depth. In this letter, we report that amorphization is a possible solution for this problem. We demonstrate that stochiometric titanium disilicide can be fabricated using laser thermal processing. We also show that the depth of the silicide can be defined by amorphization and that process margin can be created in laser thermal processing.

Origin of Inhomogeneity in Pulsed Excimer-Laser Crystallized Silicon Films Studied by Computer Simulation

AbstractIn order to understand the occurrence and development of inhomogeneous poly-Si texture in the polycrystallization process of poly-SiTFT fabrication, we have simulated 1) the influence of pulse energy (PE) and optical axis (OA) fluctuations on the texture of poly-Si films and 2) the interference between both fluctuations, under the single-pulsed XeCl laser irradiation at the overlapping ratio 98%. The frequency of fatal irregular pulses in the PE fluctuation is much more than that in the OA fluctuation (pointing stability). Once the poly-Si texture suffers a fatal irregular pulse over the complete melting (CM) point, the inhomogeneous texture caused is never recovered by the following overlapping irradiation and no columnar structure is formed. However, when no irregular pulses exceed the CM point, the partial inhomogeneous texture caused in the poly-Si films is recovered by the thinning out of small Si grains in the texture and a columnar structure is formed. The key to the issue of fatal irregular pulses should be not only the reduction of PE and OA fluctuations but also the optimum selection of pulse energy not over the CM point.

Noise suppression in femtosecond mid-infrared light sources

An experimental and theoretical study of intrinsic correlations and noise-suppression mechanisms in two-stage femtosecond mid-IR light sources is presented. The setup, based on parametric amplification in BBO and subsequent difference-frequency mixing in AgGaS(2), delivers approximately 100-fs mid-IR pulses with 1-2-muJ energy. Exceptionally low pulse-energy fluctuations of only 0.2% in the mid-IR (lambda approximately 3-6 mum) are found, which are much smaller than the Ti:sapphire amplifer noise. The noise suppression is analyzed and found to stem from the interplay between dispersion and pump depletion.

Tuning of in-line filter position for dispersion-managed soliton transmission

We optimise the positioning of guiding filters in a fiber-optic transmission system with periodic dispersion compensation. It is shown that stabilization of pulse energy fluctuations and interaction reduction for adjacent pulses that propagate in the same channel may be achieved whenever the filters are placed at optimal points within the map.

Noise behavior of pulsed vertical-cavity surface-emitting lasers

We determine the pulse energy fluctuations of vertical-cavity surface-emitting lasers by measuring the number of photoelectrons produced when a laser pulse is incident upon a photodetector. We obtain probability distributions for the number of photoelectrons produced by the total pulse energy, as well as distributions produced by the energy in each of the two orthogonal laser polarizations. We find that the noise of the laser increases when each new laser mode comes above threshold and that the individual polarization outputs are noisier than the total output, which indicates negative correlations between the energies in the two polarizations. We also find that, while the statistics of the total output are Gaussian, this is not always true of the individual polarizations.