Generation of flat-top waveform in the time domain based on stimulated Brillouin scattering

A method of generating a flat-top waveform in the time domain based on stimulated Brillouin scattering (SBS) is proposed. The transmitted pulses are simulated with pump wavelength at 1064 nm and 532 nm, respectively, and validated in the experiment performed with the Nd:YAG seed-injected laser. The experimental results agree well with the theoretical simulation. With 532 nm pump wavelength, the top of the transmitted pulse is almost a platform, while there is a peak in the front and a platform in the back with 1064 nm pump wavelength. The mechanism behind the generation of flat-top waveform with 532 nm pump wavelength is analyzed in details. PACS 42.65-k; 42.65.Es; 42.65.Hw     

All-fibre wavelength conversion based on four-wave mixing in a ring erbium-doped fibre laser

A new scheme based on four-wave mixing is presented using a ring erbium-doped fibre laser and tuneable fibre Bragg gratings that can effectively perform wavelength conversion in a wide wavelength range. An application of this wavelength-conversion architecture to optical communications is also presented. PACS 42.65.-k; 42.65.Ky     

Ultrabroad gain in an optical parametric generator with periodically poled KTiOPO4

An optical parametric gain bandwidth of 115 THz at full-width half maximum is generated from a picosecond Ti:sapphire pumped degenerate optical parametric generator. This ultrabroad bandwidth could be obtained by first identifying the wavelength where the nonlinear optical material has zero group-velocity dispersion (GVD). By pumping at half this wavelength the degenerate signal–idler pairs can accommodate ultrabroad bandwidths. The explanation for this is that the group velocities of the signal and the idlers are approximately matched and the GVD is small. However, in order to thoroughly investigate the degeneracy region around 1700 nm we fabricated several periodically poled KTiOPO₄ (PPKTP) crystals with different periods, and also one periodically poled RbTiOPO₄ (PPRTP). Both collinear and noncollinear configurations were employed for broadband parametric generation in this region. It was found that the optimum pump wavelength is in the region between 800 nm to 850 nm for PPKTP, and we could also conclude that a similar performance was found for PPRTP. This work will allow the design of optical parametric devices for generating few-cycle pulses in the spectral region between 1.1 μm and 3.8 μm. PACS 42.65.Re; 42.65.Ky; 42.65.-k     

Broadband supercontinuum generation in a single potassium di-hydrogen phosphate (KDP) crystal achieved in tandem with sum frequency generation

We present here an enhanced broadband supercontinuum generation in a potassium di-hydrogen phosphate (KDP) crystal. The enhancement in the bandwidth of the white light is obtained towards the shorter wavelength regime (<400 nm) by employing supercontinuum generation and sum frequency generation in tandem. The tunability in the blue region of the spectrum with angle is demonstrated. The bandwidth of supercontinuum achieved spans from 350 nm to 1300 nm. Further, we show the excellent polarization maintenance of continuum generated in KDP in comparison to that generated in water and BK-7 glass. PACS 42.65.Ky; 42.65.Jx; 42.65.Re; 42.25.Ja     

Characterization of the nonlinear refractive index of the laser crystal Yb:KGd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We present results of the characterization of the nonlinear refractive index of the laser crystal Yb:KGd(WO₄)₂ using a z-scan technique over the 800–1600 nm wavelength range. Based on our experimental and theoretical results we conclude that Yb:KGW crystal is a good candidate for efficient Kerr-lens mode locking. PACS 42.65.An; 42.65.Hw; 42.55.Rz; 42.65.Re     

One-dimensional incoherently coupled grey solitons in two-photon photorefractive media

We show that grey solitons, grey–grey soliton pairs, and multi-component grey solitons can be realized in two-photon photorefractive media. The results for soliton pairs and multi-component solitons are derived under the assumption that the carrier beams share the same polarization, wavelength, and are mutually incoherent. PACS 42.65.Tg; 42.65.Hw; 42.70.Nq     

Accurate time delay determination for femtosecond X-ray diffraction experiments

Intensity correlations and noise reduction are observed and characterized in the broadband supercontinuum generated by spatio-temporal solitons propagating in air, i.e., in filamentation of ultrashort laser pulses. Large correlations and reduction of the laser noise are observed already at the first steps of the filamentation process, while further propagation results in cascaded χ⁽³⁾ broadening processes and yield complex correlation maps. The spectral range yielding an optimal laser noise reduction of 3.6 dB is found to cover 10 nm around the fundamental wavelength. PACS 42.65.Jx; 42.65.Tg; 42.65.Ky; 42.50.Lc     

Polarization Insensitive Wavelength Conversions by a LiNbO3 Waveguide Using a Multi-Ring Configuration

A novel configuration is proposed for polarization insensitive wavelength conversions using a LiNbO₃ waveguide with a periodically domain inverted structure. The proposed configuration consists of multiple rings, which are connected by a polarization sensitive wavelength converter such as the LiNbO₃ waveguide described. One ring containing a pump light source is used to pump the wavelength converter bidirectionally, while the other rings containing a polarization beam splitter and a 90° polarization rotator are used to couple in the input signal and extract the converted light. The proposed method is proven experimentally by the wavelength conversion based on sum frequency generation.     

Delay-constrained anycast routing problem in WDM networks with wavelength conversions

Anycast routing and wavelength assignment problem has been intensively studied in wavelength-division multiplexing (WDM) networks, however, real-time application where packets have explicit expiration deadlines is not taken into consideration. In a WDM network, each link is associated with limited wavelengths available for channel connections. Data transmission over one wavelength to another requires wavelength conversion, which causes a long delay of message transmission. When the network and the set of anycast routing requests are given, the delay-constrained wavelength conversions anycast routing (DWCAR) problem is to find a set of light-paths, one for each source, for anycasting message to one member of the anycast destination group. The objective is to find a minimum number of wavelength conversions with the constraint that the sum of delays along every path must not exceed a given delay-bound. In this paper, we present a Tabu-search based delay-constrained anycast routing algorithm (TSDA) to solve this NP-complete problem, and compare it to a simple K shortest path based anycast algorithm (KSPA). Simulation results show TSDA achieves an average 40–50% improvement in required wavelength conversions.     

Conical four-wave mixing in sodium vapour excited by femtosecond laser pulses

Broadband (sometimes exceeding 1500 cm^-1) red-shifted (with respect to the sodium 3S–3P transition frequency) conical emission has been observed with the pump wavelength tuned in the range between 540 and 589 nm. Such broadband emission was attributed to the generation and amplification of light at the Rabi sideband frequencies in the field of intense femtosecond laser pulses. It has been shown that the cone angle of the emitted radiation is determined by the process of four-wave mixing under the conditions of longitudinal (Cherenkov-type) phase matching. PACS 42.65.Ky; 42.65.Re; 42.50.Hz     

Optimization of a femtosecond pulse self-compression region along a filament in air

We have identified the pulse self-compression region in a filament produced by 55 fs, 4 mJ, 805 nm radiation propagating in air without geometrical focusing. In our experiment the pulse self-compression region is attained by the propagation distance, where the shortest wavelength in the supercontinuum blue wing reaches a minimum, and the growing conversion efficiency to white light has a large gradient. Numerical tracking of the pulse along the filament shows a single-peak 9 fs pulse with a flat spectral phase at the optimum compression distance. PACS 42.65.Jx; 42.65.Re; 42.65.Ky     

Parametric amplification in a microstructure fiber

We report an observation of parametric amplification with a gain greater than 30 dB in a 1.7-m microstructure fiber. It is found that there exists a peak pump power P_s. For peak pump powers less than P_s, the amplified twin (signal: longer wavelength, and idler: shorter wavelength) pulses are highly correlated, and the bandwidth of the amplified signal pulse broadens monotonically with pump power. For peak pump powers exceeding P_s, the correlation between the amplified twin pulses drops when the pump power increases, the bandwidth of the amplified signal pulse becomes narrower when the pump power increases, and the central wavelength of the amplified signal (idler) pulse shifts towards longer (shorter) wavelength, respectively, due to phase matching. PACS 42.65.Yj     

Quasi-phase matching in LiNbO3 using nonlinear coefficients in the XY plane

We report quasi-phase-matched second-harmonic generation in periodically poled lithium niobate (PPLN), where both fundamental and second-harmonic waves are ordinary waves. It provides a lower-limit value for d₂₂ of 1.1 pm/V. The measured temperature and wavelength bandwidth of the second-harmonic signal are in good agreement with the theoretical predictions. Since the d₂₂=d_YYY nonlinear coefficient of LiNbO₃ changes its sign as a result of electric field periodic poling along the Z direction, we deduce that all tensor components of the second-order susceptibility χ⁽²⁾ of trigonal 3m crystals are reversed, thereby expanding the quasi-phase-matching possibilities in these crystals. Furthermore, it enables the realization of all-optical processes based on the nonlinear coefficients in the XY plane, such as all-optical polarization rotation in PPLN, as well as multipartite entanglement experiments based on simultaneous phase matching using different elements of χ⁽²⁾ in a single LiNbO₃ crystal. PACS 42.65.-k; 42.65.Ky; 42.65.Lm     

Yellow nanosecond sum-frequency generating optical parametric oscillator using periodically poled LiNbO<Subscript>3</Subscript>

Nanosecond yellow light has been generated through simultaneously phase matched sum-frequency generation and optical parametric oscillation in a periodically poled LiNbO₃ crystal. 300 mW of yellow light at a wavelength of 586 nm has been generated from 1.3 W of laser power from a Q-switched Yb:YAG laser operating at 1031 nm. The conversion efficiency of the device is 23%. PACS 42.65.Ky; 42.65.Yj     

Two-photon fluorescence properties of a styrylpyridinium derivative organic chromophore in different solvents

The fluorescence properties of a new two-photon absorption chromophore in dimethyl formamide, methanol, acetone, benzyl alcohol, methylene chloride and chloroform are reported. The lifetime, intensity and central wavelength of the fluorescence signal vary significantly in different solvents. The fluorescence properties are explained by using the twisted intra-molecular charge-transfer model, the viscosity of the solvents and the formation of the hydrogen bond. For the dye in all solvents, the longer the fluorescence lifetime, the higher the fluorescence intensity. Generally, the higher the dipole moments, the longer the central wavelengths of the dye. PACS 42.65.-k; 42.65.Ky; 87.64.Ni     

Detection of frequency red shifts and blue shifts for single-mode IR laser radiation in Rydberg matter

Stimulated emission and laser gain were recently reported for the excited condensed matter Rydberg matter (RM). This shows that RM is in an excited and inverted state. A low-intensity single-mode IR laser beam is here red shifted in reflection from a layer of RM, and blue shifted when it passes through a cloud of RM. The shifts of approximately 0.02 cm^-1 are observed with a temperature-stable Fabry–Pérot interferometer. The process giving the wavelength shifts is proposed to be stimulated Raman scattering. RM is here formed from Rydberg states of both K atoms and nitrogen molecules inside a vacuum chamber, and it may be in liquid or solid form. PACS 78.30.-j; 42.65.Dr; 42.55.Ye     

All-optical 40 Gb/s wavelength conversion using hybrid SOI-based MZI and SOAs configuration

In this paper, we present a compact combination of silicon-based Mach–Zehnder interferometer (MZI) and semiconductor optical amplifiers (SOAs) to perform all-optical return to zero (RZ)-to-RZ and non-return to zero (NRZ) wavelength conversions at a bit rate of 40 Gb/s. Using the proposed theoretical project, the attractive issue is that converted signals with inverted and noninverted formats can be simultaneously achieved at the same target wavelength. Moreover, both extinction ratio and eye-opening ratio of the converted RZ and NRZ signals can be significantly optimized by judiciously adjusting the system parameters and initial conditions.     

Carrier concentration dependence of optical Kerr nonlinearity in indium tin oxide films

Optical Kerr nonlinearity (n₂) in n-type indium tin oxide (ITO) films coated on glass substrates has been measured using Z-scans with 200-fs laser pulses at wavelengths ranging from 720 to 780 nm. The magnitudes of the measured nonlinearity in the ITO films were found to be dependent on the carrier concentration with a maximum n₂-value of 4.1×10^-5 cm²/GW at 720-nm wavelength and an electron density of N_d=5.8×10²⁰ cm^-3. The Kerr nonlinearity was also observed to be varied with the laser wavelength. By employing a femtosecond time-resolved optical Kerr effect (OKE) technique, the relaxation time of OKE in the ITO films is determined to be ∼1 ps. These findings suggest that the Kerr nonlinearity in ITO can be tailored by controlling the carrier concentration, which should be highly desirable in optoelectronic devices for ultrafast all-optical switching. PACS 42.65.An; 42.65.Hw; 78.40.Fy     

Study of the broad-band saturable absorption of indocyanine green J-aggregates in polymeric films using 10-fs laser pulses

Thin films of indocyanine green J-aggregates in polymers have been prepared and their saturable absorption properties studied in a high intensity regime using a unique ultrashort pulse Ti:sapphire laser (10 fs) with a very broad bandwidth (over 200 nm at -20 dB). The samples showed a surprisingly high stability to irradiation, strong absorption bleaching, and wavelength dependent differential absorption spectra. PACS 42.70 Jk; 42.65.-k; 78.66.Qu     

Ultrabroadband continuum generated in air (down to 230 nm) using ultrashort and intense laser pulses

We present results on supercontinuum generation extended up to 230 nm in air during the propagation of a powerful femtosecond laser pulse. The broad supercontinuum generated in air is contributed by self-phase modulation and self-steepening of the fundamental laser pulse, the third-harmonic pulse and their interaction. In particular, the strong interaction between the fundamental and the third-harmonic pulses leads to broad and efficient continuum generation of the third-harmonic pulse itself. The spectrum of the third-harmonic generated in air extends over several tens of nm and overlaps with the shorter wavelength extent of the fundamental continuum. PACS 42.65.Ky; 42.65.Jx; 52.35.Mw