Broadband Optical Pulses Research Articles

Dynamics of triple quantum beats retrieved from coupled four-wave mixing signals

A theoretical model of coupled four-wave mixing (FWM) processes comprising five atomic energy levels was proposed to investigate the coherent characteristics of atomic wave packets using perturbation theory. As several states in atoms are coherently excited by a two-photon transition, triple coupled FWM can be produced, and triple quantum beats associated with the atomic energy levels are embedded in the FWM signals. Triple coupled FWM processes were achieved experimentally in Rb vapor by two-photon excitation using broadband optical pulses. A pump–probe scheme was utilized to record the time-varying FWM signals, and Fourier transform was applied to retrieve the quantum beats. Furthermore, short-time Fourier transform was implemented to investigate the dynamics of quantum beats. The results show that the coherent characteristics of atomic wave packets, which are intimately related to the dynamics of quantum beats, can be probed by coupled nonlinear optical wave mixing.

Compressive sensing based high-speed time-stretch optical microscopy for two-dimensional image acquisition.

In this paper, compressive sensing based high-speed time-stretch optical microscopy for two-dimensional (2D) image acquisition is proposed and experimentally demonstrated for the first time. A section of dispersion compensating fiber (DCF) is used to perform wavelength-to-time conversion and then ultrafast spectral shaping of broadband optical pulses can be achieved via high-speed intensity modulation. A 2D spatial disperser comprising a pair of orthogonally oriented dispersers is employed to produce spatially structured illumination for 2D image acquisition and a section of single mode fiber (SMF) is utilized for pulse compression in the optical domain. In our scheme, a 1.2-GHz photodetector and a 50-MHz analog-to-digital converter (ADC) are used to acquire the energy of the compressed pulses. Image reconstructions are demonstrated at a frame rate of 500 kHz and a sixteen-fold image compression is achieved in our proof-of-concept demonstration.

Enhancement cavities for zero-offset-frequency pulse trains.

The optimal enhancement of broadband optical pulses in a passive resonator requires a seeding pulse train with a specific carrier-envelope-offset frequency. Here, we control the phase of the cavity mirrors to tune the offset frequency for which a given comb is optimally enhanced. This enables the enhancement of a zero-offset-frequency train of sub-30-fs pulses to multi-kW average powers. The combination of pulse duration, power, and zero phase slip constitutes a crucial step toward the generation of attosecond pulses at multi-10-MHz repetition rates. In addition, this control affords the enhancement of pulses generated by difference-frequency mixing, e.g., for mid-infrared spectroscopy.

Ultrahigh-Resolution Photonic-Assisted Microwave Frequency Identification Based on Temporal Channelization

Real-time ultrahigh-resolution microwave frequency identification is paramount for widespread applications, such as communications, radar, and electronic warfare. Photonics-assisted microwave frequency identification can be achieved using an optical channelizer. While this technique enables simultaneous measurement of multiple frequencies, it has poor measurement resolution due to the large channel spacing, which is usually greater than 1 GHz. Here, we introduce a new channelizer-based microwave frequency measurement technique that offers nearly 500 times higher spectral resolution. This method employs largely dispersed broadband optical pulses to encode the time-domain characteristics of the modulating signal to the optical spectral domain. An optical channelizer is employed to slice the spectrum, which is equivalent to performing temporal sampling of the time-domain waveform. The unknown microwave signal is then reconstructed and its spectral distribution is analyzed by a digital processor. To evaluate the proposed technique, frequency measurements of a single-tone, a multiple-tone, and a frequency-hopping microwave signal are demonstrated. A measurement resolution as high as 55 MHz is achieved using an optical channelizer with a channel spacing of 25 GHz.

Coherent-control of linear signals: Frequency-domain analysis

The dependence of various types of linear signals on the phase profile of broadband optical pulses is examined using fundamental time translation invariance symmetry of multipoint correlation functions. The frequency-domain wave-mixing analysis presented here unifies several arguments made earlier with respect to the conditions whereby coherent control schemes may be used.

Broadband phase coherence between an ultrafast laser and an OPO using lock-to-zero CEO stabilization

The carrier-envelope-offset frequencies of the pump, signal, idler and related second-harmonic and sum-frequency mixing pulses have been locked to 0 Hz in a 20-fs-Ti:sapphire-pumped optical parametric oscillator, satisfying a critical prerequisite for broadband optical pulse synthesis. With outputs spanning 400 - 3200 nm, this result represents the broadest zero-offset comb demonstrated to date.

Hybrid Dispersion Laser Scanner

Laser scanning technology is one of the most integral parts of today's scientific research, manufacturing, defense, and biomedicine. In many applications, high-speed scanning capability is essential for scanning a large area in a short time and multi-dimensional sensing of moving objects and dynamical processes with fine temporal resolution. Unfortunately, conventional laser scanners are often too slow, resulting in limited precision and utility. Here we present a new type of laser scanner that offers ∼1,000 times higher scan rates than conventional state-of-the-art scanners. This method employs spatial dispersion of temporally stretched broadband optical pulses onto the target, enabling inertia-free laser scans at unprecedented scan rates of nearly 100 MHz at 800 nm. To show our scanner's broad utility, we use it to demonstrate unique and previously difficult-to-achieve capabilities in imaging, surface vibrometry, and flow cytometry at a record 2D raster scan rate of more than 100 kHz with 27,000 resolvable points.

Modeling of ultrabroadband and single-cycle phenomena in anisotropic quadratic crystals

We present a comprehensive model to describe the propagation of single-cycle and broadband optical pulses in anisotropic, dispersive, and nonlinear materials. Two nonlinear coupled wave equations describe the dynamics and interactions of optical pulses in uniaxial second-order nonlinear materials. The equations are first order in the propagation coordinate and are valid for arbitrarily wide pulse bandwidth, providing an accurate modeling of the evolution of ultrabroadband pulses also when the separation into different coupled frequency components is not possible or not profitable. We exploit this model to simulate recently observed femtosecond single-cycle multiterahertz transients in gallium selenide and to predict harmonic generation and spectral broadening in the visible and mid-infrared in lithium niobate.

Nonlinear envelope equation for broadband optical pulses in quadratic media

We derive a nonlinear envelope equation to describe the propagation of broadband optical pulses in second order nonlinear materials. The equation is first order in the propagation coordinate and is valid for arbitrarily wide pulse bandwidth. Our approach goes beyond the usual coupled wave description of $\chi^{(2)}$ phenomena and provides an accurate modelling of the evolution of ultra-broadband pulses also when the separation into different coupled frequency components is not possible or not profitable.

Uniform-field transverse electroreflectance using a mode-locked laser and a radio-frequency bias

We describe a technique for transverse electroreflectance that uses broadband optical pulses from a mode-locked laser and a rapidly oscillating bias with a frequency near half the laser repetition rate. Since the electrodes are electrically isolated from the sample, highly nonuniform trap-enhanced fields do not occur. Because the time-averaged electric field is zero, space charges do not build up. Experimental results are presented for semi-insulating GaAs. The technique is also useful in ultrafast experiments that require a uniform transverse electric field.

Broadband optical pulse scattering from a glass fiber

We report results of scattering measurements using femtosecond pulses to collect a wealth of information in a single experiment. Potential issues with particle scattering, such as variation in particle size, were avoided by using 9 and 50 microm diameter glass fibers. We first establish an angular scattering intensity baseline, and we show that the spectral width of very short pulses leads to smoothing of the angular scattering pattern, consistent with continuous broadband illumination. We then measure the angular scattering pattern from short pulses with a spectrometer and reveal an underlying spectral periodicity of broadband scattered light that is consistent with narrowband cw scattering. Our experimental results compare well with existing theory. We show that such two-dimensional experimental data and derived analytic solution can provide robust characterization of scattering objects even in the presence of noise.

Half-Brewster-prism pair for broadband mode-locked optical pulse compression

A new design, based on a prism pair, is proposed for broadband optical pulse compression. Our results show that it is possible to achieve shorter pulse duration, to have better stabilities of both the CEP and the pulse duration suffering from the fluctuation of the laser beam size and less variation of the CEP due to the fluctuation of the incident angles of the light pulses to the prism pair by using the novel prism pair instead of the conventional Brewster-prism pair for intracavity dispersion compensation. This design is quite suited for intracavity compression of broadband and low repetition rate optical pulses.

Ultrafast control of donor-bound electron spins with single detuned optical pulses

A technique that controls electron spins using single optical pulses far detuned from the optical transition has been demonstrated. This approach may enable fast spin manipulation in a variety of solid-state systems. The ability to control spins in semiconductors is important in a variety of fields, including spintronics and quantum information processing. Due to the potentially fast dephasing times of spins in the solid state1,2,3, spin control operating on the picosecond timescale, or faster, may be necessary. Such speeds—which are not possible to reach with standard electron spin resonance techniques based on microwave sources—can be attained with broadband optical pulses. One promising ultrafast technique uses single broadband pulses detuned from resonance in a three-level Λ system4. This technique is robust against optical-pulse imperfections and does not require a fixed optical reference phase. Here we demonstrate, theoretically and experimentally, the principle of coherent manipulation of spins using this approach. Spin rotations with areas exceeding π/4 for a single pulse and π/2 for two pulses are achieved for donor-bound electrons. This technique might find applications from basic solid-state electron spin resonance spectroscopy to arbitrary single-qubit rotations4,5 and bang–bang control6 for quantum computation.

用光门产生非相干宽带光脉冲

In two-dimensional (2D) time-spreading/wavelength-hopping optical code division multiple access (OCDMA) systems, employing less coherent broadband optical pulse sources allows lower electrical operating rate and better system performance. An optical gate based scheme for generating weakly coherent (approximately incoherent) broadband optical pulses was proposed and experimentally demonstrated. In this scheme, the terahertz optical asymmetric demultiplexer, together with a coherent narrowband control pulse source, turns an incoherent broadband continuous-wave (CW) light source into the required pulse source.

Quantum Computers Based on Electron Spins Controlled by Ultrafast Off-Resonant Single Optical Pulses

We describe a fast quantum computer based on optically controlled electron spins in charged quantum dots that are coupled to microcavities. This scheme uses broadband optical pulses to rotate electron spins and provide the clock signal to the system. Nonlocal two-qubit gates are performed by phase shifts induced by electron spins on laser pulses propagating along a shared waveguide. Numerical simulations of this scheme demonstrate high-fidelity single-qubit and two-qubit gates with operation times comparable to the inverse Zeeman frequency.

Modulational instability of spatially broadband nonlinear optical pulses in four-state atomic systems

The modulational instability of broadband optical pulses in a four-state atomic system is investigated. In particular, starting from a recently derived generalized nonlinear Schrödinger equation, a wave-kinetic equation is derived. A comparison between coherent and random-phase wave states is made. It is found that the spatial spectral broadening can contribute to the nonlinear stability of ultrashort optical pulses. In practical terms, this could be achieved by using random-phase plate techniques.

Filamentational instability of partially coherent femtosecond optical pulses in air

The filamentational instability of spatially broadband femtosecond optical pulses in air is investigated by means of a kinetic wave equation for spatially incoherent photons. An explicit expression for the spatial amplification rate is derived and analyzed. It is found that the spatial spectral broadening of the pulse can lead to stabilization of the filamentation instability. Thus optical smoothing techniques could optimize current applications of ultrashort laser pulses, such as atmospheric remote sensing.

Optically controlled delays for broadband pulses

We propose a scheme that provides large controllable delays for broadband optical pulses. The system is based on the steep dispersion of a coherently driven medium, in which the narrow electromagnetically induced transparency (EIT) band is overcome by using spatial and temporal processing.

Modeling of femtosecond pulse interaction with inhomogeneously broadened media using an iterative predictor corrector FDTD method

We present a one-dimensional iterative predictor-corrector finite-difference time-domain method for modeling of broadband optical pulse propagation and interaction with inhomogeneously broadened materials. The simulator is used to demonstrate two- and three-pulse photon echoes resulting from bandwidth limited pulse and matched chirp interactions with a material modeled with hundreds of equally spaced, discrete spectral lines of detuning. The results are illustrated as Bloch-sphere evolution movies.

The dynamics of a THz Rydberg wavepacket

An optically excited Rydberg wavepacket can be generated by exciting the electron from a low-lying state to a coherent superposition of high-lying states with a short broadband optical pulse. A special kind of Rydberg wavepacket is generated in the case of a interaction of a weak THz half cycle pulse with a stationary Rydberg state, called the THz wavepacket. This THz wavepacket is a coherent superposition of the initial Rydberg state and its neighbouring states. We have investigated the time evolution of THz wavepackets by measuring the impact of two in time delayed half cycle pulses ( ≈ 200 V cm-1) on the population of a stationary (n = 40) Rydberg state in rubidium. The first half cycle pulse creates the THz wavepacket and the second half cycle pulse probes the dynamics of the THz wavepacket. We support our experimental data by numerically solving the Schrodinger equation and with a semi-classical picture. Whereas an optically excited wavepacket is initially localized, a THz wavepacket is initially delocalized and becomes localized after half a revival time.