Frequency-resolved Optical Gating Technique Research Articles

Background-free coherent Raman spectroscopy by detecting the spectral phase of molecular vibrations

We propose and demonstrate a new approach to subtracting high nonresonant background in coherent anti-Stokes Raman scattering spectroscopy. The method is based on the retrieval of the spectral phase of molecular vibrations using the technique of frequency-resolved optical gating of Raman scattering. In the presence of high nonresonant background the retrieved phase corresponds directly to the background-free spectrum of the coherent Raman response.

Ultralow-power second-harmonic generation frequency-resolved optical gating using aperiodically poled lithium niobate waveguides [Invited

We discuss ultralow-power second-harmonic generation (SHG) frequency-resolved optical gating (FROG) in the telecommunication C-band using aperiodically poled lithium niobate (A-PPLN) waveguides as the nonlinear medium. A key theme of this work is that the phase-matching curve of the nonlinear medium is engineered to obtain an optical bandwidth adequate for measurement of subpicosecond pulses while retaining the optimum nonlinear efficiency consistent with this constraint. Our experiments demonstrate measurement sensitivity (defined as the minimum product of the peak and average pulse powers at which a reliable nonlinear signal can be detected) of 2.0×10−6 mW2 in a collinear SHG FROG geometry, approximately 5 orders of magnitude better than previously reported for any FROG measurement modality. We also discuss asymmetric Y-junction A-PPLN waveguides that permit background-free SHG FROG and a polarization-insensitive SHG FROG technique that eliminates the impairment that frequency-independent random polarization fluctuations induce in the FROG measurement. Finally, we applied these SHG FROG techniques in chromatic dispersion and polarization mode dispersion compensation experiments. In these experiments the FROG data enabled complete correction of distortions incurred by subpicosecond pulses passing through optical fibers; these results also demonstrate the ability to retrieve extremely complex pulses with high accuracy.

Supercontinuum generation and soliton timing jitter in SF6 soft glass photonic crystal fibers

We use broadband frequency-resolved optical gating (FROG) and cross-correlation FROG (XFROG) techniques to study the details of the supercontinuum generated in extruded soft glass SF6 photonic crystal fibers pumped with envelope-modulated 100 fs pulses at telecom wavelengths. Strong temporal jitter of solitons is observed with highly non-Gaussian statistics, which is related to the statistics of the pump pulse envelope shape fluctuations. The ripples present on the input pulse seed the modulation instability at high pump powers, affecting soliton fission. Numerical modeling confirms strong sensitivity of the soliton fission process to the presence of ripples on the pump pulse envelope.

Full Characterization of Low-Power Picosecond Pulses From a Gain-Switched Diode Laser Using Electrooptic Modulation-Based Linear FROG

We use a linear frequency-resolved optical gating technique based on electrooptic modulation to fully characterize for the first time the highly chirped pulses from a 1.06-m Fabry-Perot laser diode and design a chirped fiber Bragg grating to provide high-quality pulse compression. With this grating, we achieved 18-ps pulses at a repetition rate of 1.35 GHz with a time-bandwidth product of 0.7.

Experimental analysis of phase conjugation properties of four-wave mixing in an SOA after probe broadening due to fibre dispersion

Experimental results are presented which analyze the phase conjugation properties of four-wave mixing signals generated due to the beating between probe pulses broadened by a length of fibre and narrow pump pulses. This results in four-wave mixing pulses significantly narrower than the injected probe pulses albeit with reduced phase conjugation properties, which are examined. The pulses are completely characterized using the second-harmonic generation frequency resolved optical gating technique. The probe pulse is initially broadened due to propagation through 40m of dispersion compensating fibre. This causes the probe pulse to be much wider than the injected pump pulse, in contrast to previously reported results. The four-wave mixing signal is therefore both wavelength converted and compressed, due to the gating properties of four-wave mixing, with respect to the initial probe signal. The phase conjugation properties of the converted signal are discussed and this signal is then passed through a second length of dispersion compensating fibre in order to further compress the pulse and to examine in more detail the phase conjugation properties.

Characterization of a Turbo-Switch SOA Wavelength Converter Using Spectrographic Pulse Measurement

We use the frequency-resolved optical gating (FROG) technique to determine the pulsewidth and chirp characteristics of wavelength-converted pulses from a turbo-switch and asymmetrical Mach-Zehnder filter combination under a variety of operating conditions. The output pulses were found to be near transform limited and had a high tolerance to line rate and input power variations.

Pump–Probe Detuning Dependence of Four-Wave Mixing Pulse in an SOA

Four-wave mixing (FWM) between 2-ps pulses in a multiquantum-well semiconductor optical amplifier (SOA) is presented. The conjugate pulses are fully characterized using the frequency-resolved optical gating technique. The detuning between the pump and probe is varied, leading to a compression of the FWM signal from 3.71 to 2.77 ps as the detuning is increased from 5 to 25 nm. The output conjugate pulse is always broader than the injected probe signal due to gain saturation effects. A reshaping of the conjugate pulse is also measured. However, large nonlinearities are introduced to the frequency chirp across the pulse for large detunings which may degrade the performance of four-wave-mixing-based all-optical processing applications in SOAs.

The study of numerical character for femtosecond pulse shaping

This paper describes the generation of shaped femtosecond multiple pulses by using the phase-only Dammann filters in 4f femtosecond shaper and gives the experimental result of femtosecond pulse characterization by the frequency-resolved optical gating (FROG) technique. With the theoretical simulation, it concludes that the quality of the generated output array is relevant to the number of pixels and the spacing between the components.

Investigation of noise suppression, pulse intensity and chirp of an actively mode-locked semiconductor fiber ring laser

An actively mode-locked fiber ring laser that utilizes an EDFA and semiconductor optical amplifier (SOA) is presented. The SOA enhances noise suppression. Active mode-locking was demonstrated at repetition rates up to 20 GHz. The effects of intra-cavity power and SOA current on noise suppression are studied for two types of commercially available SOA. The frequency resolved optical gating technique is used to characterize the output pulse stream intensity and frequency chirp.

Generation of shaped ultraviolet pulses at the third harmonic of titanium-sapphire femtosecond laser radiation

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located.

Phase structure of soliton molecules

Temporal optical soliton molecules were recently demonstrated; they potentially allow further increase of data rates in optical telecommunication. Their binding mechanism relies on the internal phases, but these have not been experimentally accessible so far. Conventional frequency-resolved optical gating techniques are not suited for measurement of their phase profile: Their algorithms fail to converge due to zeros both in their temporal and their spectral profile. We show that the VAMPIRE (very advanced method of phase and intensity retrieval of $E$-fields) method performs reliably. With VAMPIRE the phase profile of soliton molecules has been measured, and further insight into the mechanism is obtained.

Investigation of polarization dependent gain dynamics in a bulk SOA

The polarization dependence of the gain dynamics in a bulk semiconductor optical amplifier is examined using two experimental setups. The first experimental investigation is based on four-wave mixing. Using a low-pass filter model it is possible to retrieve information on the polarization dependence of the linewidth enhancement factors and recovery timeframes associated with carrier density pulsation and carrier heating, respectively. These experimental results have been confirmed using a second setup. Linearly polarized pulses (2ps) are launched into the same SOA as a function of the angle of injection. The propagated pulse is analyzed in the frequency and time domains using the frequency resolved optical gating technique. It is found that the SOA presents a polarization dependence when the device is in the non-linear regime.

Selective excitation of CARS by adaptive pulse shaping based on genetic algorithm

Selective excitation of coherent anti-Stokes Raman scattering (CARS) from the benzene solution is experimentally demonstrated by shaping femtosecond laser pulses based on genetic algorithm. Second harmonic generation frequency-resolved optical gating (SHG-FROG) technique is adopted to characterize the original and optimal laser pulses, and the mechanisms for the coherently selective excitation of coherent anti-Stokes Raman scattering (CARS) is discussed. The results show that two-pulse coherent anti-Stokes Raman scattering (CARS) has good signal-to-background ratio and high sensitivity, and it has attractive potential applications in the complicated molecular system, such as biomolecules and materials.

Optimisation of superimposed chirped fibre Bragg gratings for the generation of ultra-high speed optical pulse bursts

Compact superstructure fibre Bragg gratings for the generation of ultra-high speed optical pulse bursts are reported. The gratings are equivalent in response to multi-pass superimposed chirped fibre Bragg gratings (SI-CFBGs), but an arbitrary sequence of consecutive gratings can be replicated in a single fabrication step using a simple trigonometric amplitude-phase function. SI-CFBGs are fabricated in photosensitive optical fibre and the time-resolved reflection of a 10GHz picosecond pulse train is measured via frequency-resolved optical gating and optical sub-harmonic sampling detection techniques. The experimental results are compared to the theoretically simulated grating response and good agreement is found.

Coherent pulse propagation in GaSe

Coherent optical pulse transmitted through GaSe is investigated in both energy and time domains by using the frequency-resolved optical gating technique. The free induction decay signal due to the coherent emission from excitons is observed. The temporal evolution of the intensity of optical pulse passing through the sample exhibits a coherent dip as a result of a destructive interference between the coherent emission and the input optical field. Furthermore, the dispersion of the refractive index at the excitonic resonance is obtained.

Reconstruction of the phase of matter-wave fields using a momentum-resolved cross-correlation technique

We investigate the potential of the so-called cross-correlation frequency-resolved optical gating (XFROG) technique originally developed for ultrashort laser pulses for the recovery of the amplitude and phase of the condensate wave function of a Bose-Einstein condensate. Key features of the XFROG method are its high resolution, versatility, and stability against noise and some sources of systematic errors. After showing how an analog of XFROG can be realized for Bose-Einstein condensates, we illustrate its effectiveness in determining the amplitude and phase of the wave function of a vortex state. The impact of a reduction of the number of measurements and of typical sources of noise on the field reconstruction are also analyzed.

Measurement of electric field by interferometric spectral trace observation

We present a new methodology that obtains, in an analytical way, the complex electric field of ultrashort pulses. This methodology is based only on Fourier analysis of the frequency components of spectrally resolved interferometric collinear autocorrelations. We present an experimental demonstration of this technique and the results are compared with the conventional second-harmonic generation frequency-resolved optical gating technique.

Characterization of a supercontinuum generated from a photonic crystal fiber and its application to coherent Raman spectroscopy.

The temporal and spectral profiles of supercontinuum radiation generated from a photonic crystal fiber are evaluated with a polarization-gate frequency-resolved optical gating technique. The supercontinuum is then applied to coherent inverse Raman spectroscopy. A stimulated Raman signal of cyclohexane is observed as an induced absorption signal with an instantaneous response. The Raman signal has a peak at a slight negative delay time, which is explained by perturbed Raman-induced coherence.

On the designing of densely dispersion-managed optical fiber systems for ultrafast optical communication

We present some theoretical and experimental results which suggest the possibility of constructing a non-empirical methodology of designing optical transmission systems with ultra high bit-rate per channel. Theoretically, we present an average dispersion decreasing densely dispersion-managed (A4dm) fiber system, which exhibits many advantages over the densely dispersion-managed fiber system, such as the possibility of transmitting chirp-free Gaussian pulses at 160 Gbit/s per channel over transoceanic distances, with a reduced energy and minimal intra-channel interaction. Experimentally we present generation of a 160-GHz picosecond pulse train at 1550 nm using multiple four-wave mixing temporal compression of an initial dual frequency beat signal in the anomalous-dispersion regime of a non-zero dispersion shifted fiber. A complete intensity and phase characterization of the pulse train by means of a frequency-resolved optical gating technique is achieved, showing generation of transform-limited pedestal-free Gaussian pulses.

Self-induced transparency in InGaAs quantum-dot waveguides

We report the experimental observation and the theoretical modeling of self-induced-transparency signatures such as nonlinear transmission, pulse retardation and reshaping, for subpicosecond pulse propagation in a 2-mm-long InGaAs quantum-dot ridge waveguide in resonance with the excitonic ground-state transition at 10 K. The measurements were obtained by using a cross-correlation frequency-resolved optical gating technique which allows us to retrieve the field amplitude of the propagating pulses.