Study of second harmonic generation by non-linear crystals with phase conjugation

We present the correction of the phase distortion which occurs during the second harmonic generation by non-linear crystals, such as KDP and KTP at high average power laser. This is due to the optical quality and thicknesses of the crystals which in turn influence the quality of the incident laser beam. This phase distortion is corrected by reflecting back the laser beam into the crystal using a phase conjugate mirror. It is found that the conversion efficiency of second harmonic generation without phase conjugation is more than that with phase conjugation. Far field pattern shows that the distortion of the laser beam can be corrected by using the phase conjugate mirror. Fidelity of the beam profile increases significantly with phase conjugation in the case of KDP crystal.     

Heat dissipative solitons in optical fibers

We propose a one-dimensional model governing the propagation of heat waves in an optical fiber (the “fiber fuse”). The model has solutions in the form of high temperature localized waves moving towards the input end of the fiber, fueled by the laser power. These waves can be ignited by local heating at any point along the fiber. The effect of such a wave is irreversible damage to the fiber core. The phenomenon was observed earlier by Hand and Russell, when locally heating a fiber through which CW light of modest intensity was propagating. This induced self-destruction of the optical fiber core.     

Generation of subpicosecond infrared pulses tunable between 5.2 μm and 18 μm at a repetition rate of 76 MHz

2 (2 mm thick) and GaSe (1 mm thick) crystals are applied as nonlinear media. GaSe has a larger tuning range and is more efficient in the whole spectral range than AgGaS₂. The average IR power reaches up to 2.0 mW at 8.5 μm when the GaSe crystal is used, and up to 1.3 mW at the same wavelength when AgGaS₂ is applied for the frequency conversion. Received: 17 February 1997/Revised version: 14 July 1997     

Directivity influence of signals propagation through EDFA gain medium of Brillouin-erbium fiber laser

We experimentally demonstrate a multiwavelength Brillouin-erbium fiber laser in two configurations; uni-directional and bi-directional propagation of Brillouin pump and Brillouin Stokes signals through an Erbium-doped fiber gain. The influence of these configurations on the performance of the output parameters in terms of lasing threshold, output channel generation and tuning range of the generated output channels are investigated. We discovered that there is a trade-off between these two fiber laser configurations. The uni-directional amplifier configuration provides greater tuning range of 46.8 nm against 23 nm at maximum Brillouin pump power of 2 mW and 1480-nm pump power of 130 mW. On the other hand, the bi-directional amplifier configuration provides 13 output channels against 6 output channels obtained from the uni-directional amplifier configuration at the same pumping powers. Nevertheless, the bi-directional amplifier configuration requires much lower pump power to initiate lasing.     

Experimental investigation and numerical simulation of the influence of resonator-length detuning on the output power,pulse duration and spectral width of a cw mode-locked picosecond optical parametric oscillator

We report on a cw mode-locked non-critically phase matched KTP optical parametric oscillator synchronously pumped by a picosecond Ti:Sapphire laser. High average signal output power of up to 950 mW over a large tuning range has been achieved. For this OPO the influence of resonator-length detuning on the output power, pulse duration and spectral bandwidth has been investigated. The measured data are in good agreement with the results of a numerical simulation using a split-step Fourier method which considers the group-velocity mismatch, the group-velocity dispersion and the self-phase modulation. The numerical simulation also describes the measured strong pump depletion and its influence on the OPO output and efficiency.     

Pulsed solid-state laser system with fiber phase conjugation and 315 W average output power

Solid-state laser sources with high average output power and high beam quality are required for numerous applications in industry and science. To improve the beam quality, stimulated Brillouin scattering (SBS) mirrors based on optical phase conjugation are used to compensate for phase distortions in master oscillator power amplifier (MOPA) systems. SBS phase conjugation in commercially available silica optical fibers facilitates the construction of high-power solid-state laser systems with high beam quality. Here we present a flash lamp-pumped, passively Q-switched Nd:YAP system which delivers an average output power of 315 W with M²=2.6 at a wavelength of 1.08 μm. Received: 11 July 2001 / Revised version: 28 September 2001 / Published online: 23 November 2001     

Threshold reduction of stimulated Brillouin scattering (SBS) using fiber loop schemes

SBS mirrors as self-pumped and easy to handle non-linear optical devices are frequently used in high-power laser systems for improving the beam quality based on optical phase conjugation. Because of the non-linear behaviour, a certain pulse energy or power of incident light is needed to generate enough reflectivity for practical purposes. Therefore, reducing this “threshold” is still a main topic in the development of new schemes for optical phase conjugation. In addition to the taper concept reported earlier, this paper deals with loop schemes for reducing the power requirements. A reduction of the so-called “threshold” by a factor of between two and four was obtained with the schemes investigated using liquids and fibers. Received: 4 September 2001 / Revised version: 22 October 2001 / Published online: 23 November 2001     

Propagation of short soliton pulses through a parabolic index fiber with dispersion decreasing along length

A parabolic index dispersion decreasing fiber (DDF) has been designed and optimized to produce high capacity soliton communication system. Variation of different fiber parameters such as core radius, effective core area and GVD factor along the 25 km of DDF length has been carried out to optimize a best possible DDF which can sustain the propagation of fundamental soliton. The variation of non-linearity with length along with the conventional power and GVD factor variation has been included in the generalized non-linear Schrodinger equation (NLSE). This NLSE has been solved numerically by split step Fourier method for shorter pulse propagation, incorporating the term for third order dispersion and intrapulse Raman scattering. Stable soliton pulses in transmission system have been achieved by our simulation, when a correction factor due to Raman induced soliton mean frequency shift is incorporated to the GVD profile predicted by the fundamental soliton condition. The interaction of neighboring soliton pulse pair through the proposed fiber has also been studied.     

SPM-induced spectral compression of picosecond pulses in a single-mode Yb-doped fiber amplifier

In a fiber amplifier, spectral compression due to self-phase modulation is demonstrated for ultrashort pulses. We report the generation of near-transform-limited picosecond pulses with peak powers of several kW at a repetition rate of 74 MHz and diffraction-limited beam quality in a Yb-doped fiber amplifier when seeding with a negative chirped pulse. Received: 17 September 2001 / Revised version: 22 November 2001 / Published online: 17 January 2002     

Ultra-fast spectroscopy and extreme nonlinear optics by few-optical-cycle laser pulses

Many fundamental processes of radiation–matter interaction, which take place on the ultra-short time scale, can now be directly investigated by using few-optical-cycles light pulses with duration <10 fs. We discuss two techniques for generating such pulses: broad-band parametric amplification, which allows the generation of pulses in the visible range suitable for spectroscopy, and compression of high-energy light pulses in a hollow fiber. As an example of application in time-resolved spectroscopy we report results of pump–probe experiments in a prototypical conjugated molecule, namely sexithiophene. The new laser sources, due to their characteristics of peak power and coherence, also allow the exploration of new fields of experimental physics, such as extreme nonlinear optics. We focus on high-order harmonics, showing that a high-energy bunch of photons, up to the X-ray-energy region, with coherence typical of laser radiation and time duration comparable to or shorter than the exciting pulses, can be generated. Received: 31 July 2000 / Revised version: 19 September 2000 / Published online: 8 November 2000     

Nonlinear optical correction of the pulse shape from a directly modulated DFB laser

In this work, the effect of modulation instability (MI) in optical fiber is used to reshape nanosecond pulses form a directly modulated diode laser. Our configuration includes a fiber where MI causes the side lobes in the signal spectrum and a filter at the fiber output rejecting the side lobes. Simulations show abrupt drop of the transmission of the setup if pulse power is above some critical value. We investigated the transmission for fibers with lengths in the range between 62-m and 4.5-km. The critical power was found to be inversely proportional to the fiber length. An average scaled critical power is 2.16 W km. We demonstrated the application of the method for rejection of the transient peak in a directly modulated diode laser.     

Generation of a wide discrete visible spectrum in a frequency-doubling optical fiber

We describe some special features of frequency doubling in a Ge-doped SiO₂ optical fiber. The generation of a multi-frequency visible spectrum in a single short fiber pumped with 1.06 µm radiation is demonstrated. This effect is the result of an interplay between the processes of frequency doubling and third-order nonlinear frequency mixing. Most of the new components coincide with the characteristic stimulated four-wave-mixing spectrum of the fiber, although the power of the internally generated 0.532 µm pump was more than an order of magnitude below the respective thresholds.     

Decryption characteristics in message modulation type chaos secure communication system using optical fiber ring resonators

Message modulation type chaos secure communication system using optical fiber ring resonators is analyzed numerically. It is found that the decryption characteristics degrades monotonically as the differences in parameters which are not included in the linear phase term becomes large, while they degrade periodically with a period of 2π as the differences in parameters included in the linear phase term increase. When a noise is added in the input light, the error dispersion in the message demodulation is determined by the noise level in the small parameter difference region, while the parameter differences it dominates in the large parameter difference region. Best decryption characteristics are always obtained at the minimum input power for the chaos generation, i.e., at the lowest Lyapnov exponent in chaos state in the message modulation type chaos secure communication system using optical fiber ring resonators.     

Temperature effects on supercontinuum generation using a continuous-wave Raman fiber laser

We describe the effect of temperature variations on supercontinuum (SC) generation in optical fibers using a continuous-wave (CW) Raman fiber laser as a pump. We achieve supercontinuum generation by pumping only ∼2 W of power into a 7 km-long nonzero dispersion-shifted fiber (NZDSF) in the region of small anomalous dispersion. In these conditions, the supercontinuum builds up basically on modulational instability and Raman. At room temperature, the supercontinuum covers effectively the S, C and L transmission bands defined by the International Telecommunication Union (ITU). Temperature tuning of the fiber environment provides a means of tuning the fiber dispersion, and thus a means of changing the width and shape of the supercontinuum spectrum. We demonstrate a 27% increase in the 10-dB SC width. We believe that the application of this new tuning mechanism to other experimental configurations using pulsed sources might be used to produce extremely broad supercontinuums.     

Third-order dispersion for generating optical rogue solitons

We theoretically and numerically evidence that optical rare and strong temporal events generated in fiber supercontinua originate from convective modulational instabilities. This convective nature is induced by higher-order terms (odd-order dispersion and stimulated Raman scattering) that break the time reversal symmetry of the nonlinear Schrödinger equation. We demonstrate (i) analytically that the third-order dispersion term alone turns the system to be convectively unstable and (ii) numerically that the sign of the curvature of the tail of the probability density function changes (in logarithmic scale) when the third-order dispersion term is added. This latter feature results in more powerful rare events. If, in addition, stimulated Raman scattering is taken into account, both the convective instabilities and the power of extreme events are further enhanced giving rise to a probability density function with a more pronounced curvature.     

The ultrahigh-peak-power laser: present and future

19 to 10²⁰-W/cm² range. With some refinements and with superior energy storage materials, even higher peak power in the petawatt range should be possible from tabletop systems. In this paper we show the ultimate achievable power and intensity, as well as their applications in science and technology. Their applications cover a wide variety of fields, such as precision surgery, micromachining, coherent and incoherent X-ray generation, thermonuclear ignition, particle acceleration, and nonlinear quantum electrodynamics. Received: 5 May 1997/Revised version: 2 June 1997     

An all-solid-state tunable 214.5-nm continuous-wave light source by using two-stage frequency doubling of a diode laser

3 and β-BaB₂O₄ crystals. The power of the generated 214.5-nm light amounts to more than 100 μW. This light source will be used for laser cooling of Cd⁺ ions. Received: 4 August 1997/Revised version: 28 October 1997     

Diode-pumped intra-cavity doubled Nd:LuVO4 laser at 458 nm

We have demonstrated a diode-pumped intra-cavity frequency doubling Nd:LuVO₄ laser operating at 916 nm with a Z-folded cavity. A 10-mm long LBO crystal, cut for critical type I phase matching at 912 nm, is used for the experiment. A maximum output power of 330 mW at 458 nm has been achieved at pump power of 22 W. The optical-to-optical conversion efficiency and slope efficiency is 1.5% and 2.3%, respectively. The power instability at the maximum output power in 30 min is better than 3%.     

Four-wave mixing of picosecond pulses in hollow fibers: expanding the possibilities of gas-phase analysis

Third-harmonic, difference-frequency, and sum-frequency generation processes in hollow fibers are experimentally studied with 30-ps pulses having an energy of several millijoules. The experimental dependence of the difference-frequency signal on the pressure of the gas filling the fiber agrees well with the results of calculations when the contribution of higher order waveguide modes is taken into consideration, thus indicating the importance of nonlinear-optical processes involving higher order waveguide modes of a hollow fiber. Hollow fibers are also shown to expand the possibilities of nonlinear-optical analysis of gases by allowing the generation of third-harmonic and sum-frequency signals, which vanish in the regime of tight focusing in a medium with normal dispersion. Received: 26 September 2000 / Revised version: 15 January 2001 / Published online: 30 March 2001     

Cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber

Copropagating fundamental-wavelength and second-harmonic femtosecond pulses of Cr: forsterite laser radiation are used to study cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber. Parametric instability of the second-harmonic probe pulse induced through cross-phase modulation by the fundamental-wavelength pump pulse gives rise to distinct sidebands in the spectrum of the probe field transmitted through the fiber. The wavelength of these sidebands was tuned in our experiments within approximately 100 nm by varying the peak power and the delay time of the pump pulse, suggesting a convenient way of controlled parametric spectral transformation of ultrashort laser pulses.This revised version was published online in March 2005. In the previous version, the published online date was missing