Coupling Pulse Research Articles

Numerical investigation and experimental development on VM-PT cryocooler operating below 4 K

Vuilleumier coupling pulse tube (VM-PT) cryocooler is a novel kind of cryocooler capable of attaining liquid helium temperature which had been experimentally verified. Depending on different coupling modes and phase shifters, VM-PT cryocooler can be designed in several configurations. This paper presents a numerical investigation on three typical types of VM-PT cryocoolers, which are gas-coupling mode with room temperature phase shifter (GCRP), gas-coupling mode with cold phase shifter (GCCP) and thermal-coupling mode with cold phase shifter (TCCP). Firstly, three configurations are optimized on operating parameters to attain lower no-load temperature. Then, based on the simulation results, distributions of acoustic power, enthalpy flow, pressure wave, and volume flow rate are presented and discussed to better understand the energy flow characteristics and coupling mechanism. Meanwhile, analyses of phase relationship and exergy loss are also performed. Furthermore, a GCCP experimental system with optimal comprehensive performance among three configurations was built and tested. Experimental results showed good consistency with the simulations. Finally, a no-load temperature of 3.39K and cooling power of 9.75mW at 4.2K were obtained with a pressure ratio of 1.7, operating frequency of 1.22Hz and mean pressure of 1.5MPa.

Coherently controlled emissions |4P3/2,1/2〉 ↔ |4S1/2〉 from a femtosecond Λ-type excitation scheme in potassium atom

The combined excitation of high density potassium (K) vapour by 100 fs pump-coupling pulses is experimentally studied. The intense pump pulse excites the two-photon transition and internally generated emissions are initiated along the atomic paths: (path-1) and, (path-2). The temporally delayed coupling pulse coherently drives the transitions, in a Λ-type excitation scheme. The competing axial and conical emission components of the well-resolved transitions (D2 and D1 lines of K) are substantially enhanced and controlled, for appropriate detunings and pump-coupling temporal delays. The coherence relaxation time (CRT) of the two-photon excited state is determined by exploiting the temporal delay in the pulse sequence. The effect of the pulse delay and the fs pulse bandwidth on the system dynamics is discussed as well as the role of dephasing collisions between K and buffer gas atoms. The proposed scheme can be employed in radiative multi-level systems, for the direct estimation of coherence relaxation rates of various states.

Modulation instability dynamics of coupling pulses with different powers in nonlinear fibers

In the paper, the modulation instability (MI) of the pulses with different powers is studied based on the modified coupled nonlinear Schrödinger equations in nonlinear fiber. By analyzing MI gain spectrum with different power ratios, it shows that the increase of power ratio leads to the diminution in the range of strongly unstable frequency and the decrease in the gain of MI spectra. With a fixed power ratio, the increased relaxation time is equally suppressing overall MI.

Nearly transform-limited sub-20-fs pulses at 1065 nm and >10 nJ enabled by a flat field ultrafast pulse shaper.

Using a low-nonlinearity fiber and pulses from a nonlinear fiber amplifier seeded by a modelocked Yb-doped fiber oscillator, we generate 19-fs pulses centered at 1065nm with 11.5nJ of pulse energy (700mW average power). The short (<15 cm) 10-μm core, polarization maintaining fiber minimizes deleterious nonlinear effects and eliminates fiber damage, while still producing pulse bandwidths well beyond the Yb gain bandwidth limit. A flat-field pulse shaper, utilizing a Plössl lens, compresses the pulse to within 92% of the transform-limited peak power. The total power transmission efficiency is as high as 67%, including fiber coupling losses and pulse shaper transmission, due to the novel pulse shaper design allowing the incorporation of a high-efficiency transmission grating.

Propagation of two short laser pulse trains in a Λ-type three-level medium under conditions of electromagnetically induced transparency

We investigate the dynamics of a pair of short laser pulse trains propagating in a medium consisting of three-level Λ-type atoms by numerically solving the Maxwell–Schrödinger equations for atoms and fields. By performing propagation calculations with different parameters, under conditions of electromagnetically induced transparency, we compare the propagation dynamics by a single pair of probe and coupling laser pulses and by probe and coupling laser pulse trains. We discuss the influence of the coupling pulse area, number of pulses, and detunings on the probe laser propagation and realization of electromagnetically induced transparency conditions, as well on the formation of a dark state.

(swap)α gate in the presence of spin–orbit coupling in coupled quantum dots

In this paper we study the influence of the Dzyaloshinskii–Moriya (DM) interaction on the (swap)α operation of spin qubits in two coupled quantum dots. It is shown that a sequence of single-qubit rotations as well as properly chosen width and strength of the exchange coupling pulse are effective to correct the error caused by the DM term. Depending on the amplitude of the DM interaction, the operation error is either fully or partly correctable.

Emissions enhancement in a pump–coupling V-type coherently controlled four-level atomic system

The nonlinear interaction of a four-level potassium system with a strong pump and a weak coupling laser is investigated. A strong pump pulse excites the two-photon transition , causing prompt parametric emissions along the path-1 and delayed emissions in the second available de-excitation path-2 . A weak coupling pulse is introduced to coherently excite either the transition (path-1) or the one (path-2) in a V-type scheme. Results are presented for the emissions generated under the combined action of the pump–coupling pulses as a function of their delay for certain peak pulse intensities and coherence relaxation times (CRTs) related to dephasing collisions. We find that emissions in path-1 are considerably enhanced for negative delays (counterintuitive pulse sequence) of the order of the CRT. An approach is suggested for the estimation of CRTs. Emissions in path-2 are enhanced and temporally shifted, suggesting an approach to control this path by varying the pulse delay.

Hidden identity in a generic Λ system: applications to coherent population trapping

A generic three-level Λ system is the source of a group of interesting quantum optical phenomena from coherent population trapping (CPT) to electromagnetic induced transparency. However, in contrast to its cousin, the two-level system, analytic solutions are difficult to acquire, except certain special cases. Here, we show that there exists a hidden identity in a generic Λ system as far as the rotating wave approximation is valid. This identity links the squares of the probability amplitudes in three states through the laser detuning phase (product of the detuning and time) and the Rabi phase. It is independent of the laser pulse wave forms and time delay. An application to CPT not only recovers all the previous asymptotic results, but also explains analytically why in the stimulated Raman adiabatic passage, the Stokes pulse must precede the pump pulse. More importantly, our results show that while a longer probe pulse is always favoured, a not too short probe pulse is still able to induce CPT because the time delay between the probe and coupling pulses can make up the difference in the formation of dark states. These can be directly tested experimentally.

Multimodal Nonlinear Microscopy by Shaping a Fiber Supercontinuum From 900 to 1160 nm

Nonlinear microscopy has become widely used in biophotonic imaging. Pulse shaping provides control over nonlinear optical processes of ultrafast pulses for selective imaging and contrast enhancement. In this study, nonlinear microscopy, including two-photon fluorescence, second harmonic generation, and third harmonic generation, was performed using pulses shaped from a fiber supercontinuum (SC) spanning from 900 to 1160 nm. The SC generated by coupling pulses from a Yb:KYW pulsed laser into a photonic crystal fiber was spectrally filtered and compressed using a spatial light modulator. The shaped pulses were used for nonlinear optical imaging of cellular and tissue samples. Amplitude and phase shaping the fiber SC offers selective and efficient nonlinear optical imaging over a broad bandwidth with a single-beam and an easily tunable setup.

Dependence of the molecular iodine B-state predissociation induced by a femtosecond laser pulse on pulse phase modulation

The processes of pumping and laser-induced predissociation of B-states of the I2 molecule under the action of femtosecond laser pulses are considered theoretically. An analytical formula is derived, which describes the dependence of the predissociation on such parameters of femtosecond pulses as spectral chirp, spectral width and delay time between pulses. The formula is used to calculate numerically the dependence of the predissociation yield on the parameters of the phase modulation of the pump pulse and coupling pulse.

Proposed spin-qubit controlled-not gate robust against noisy coupling

We propose an implementation of the two-qubit gate in a quantum dot spin qubit system which is immune to charge noise problems. Our proposed implementation, if it could be realized in a physical system, would have the advantage of being robust against uncertainties and fluctuations in the tunnel coupling and barrier gate voltage pulse area. The key idea is to introduce an auxiliary dot and use an analog to the stimulated Raman adiabatic passage pulse sequence in three-level atomic systems, often referred to in the context of electron transport in quantum dot systems as Coherent Tunneling by Adiabatic Passage. Spin-dependent tunneling opens the possibility of performing two-qubit gate operations by this method.

Controlling the spectrum of light pulses by dynamical electromagnetically induced transparency

We present a theoretical and experimental study on the possibility of spectral manipulation of weak probe-laser pulses in the presence of dynamical electromagnetically induced transparency. We predict a spectral enlargement or narrowing process depending on whether the probe-laser pulse is overlapped by the rising or the falling edge of the coupling pulse, respectively. The results of an experiment in sodium atomic vapors confirm the theoretical predictions.

Phase distortion ratio: alternative to group delay dispersion for analysis and optimization of dispersion compensating optics

We present a new approach for designing dispersion-engineered optics based on a simple unitless spectral quantity we call the phase distortion ratio (PDR). In contrast to minimizing the group delay dispersion (GDD) deviation from the ideal, minimizing the PDR is optimal in the sense that it minimizes the fraction of pulse energy lost to phase distortions. As an example, a mirror system optimized via PDR is empirically found to result in significantly better compression of single-cycle pulses than a system designed in terms of GDD. In the context of coupling pulse trains to cavities, minimizing the PDR of the cavity is shown to maximize throughput.

All-optical modulation based on electromagnetically induced transparency

We numerically investigate the implementation of all-optical absorption modulation of electromagnetic pulses by a medium that exhibits electromagnetically induced transparency. The quantum system is modelled as a three-level Λ-type system that interacts with two electromagnetic pulses, a probe pulse and a coupling pulse. The dynamics of the system is described by the coupled Maxwell-density matrix equations, and we explore the dependence of the optical modulation efficiency on the parameters of the system.

Light storage in a doped solid enhanced by feedback-controlled pulse shaping

We report on experiments dealing with feedback-controlled pulse shaping to optimize the efficiency of light storage by electromagnetically induced transparency (EIT) in a Pr${}^{3+}$:Y${}_{2}$SiO${}_{5}$ crystal. A learning loop in combination with an evolutionary algorithm permits the automatic determination of optimal temporal profiles of intensities and frequencies in the driving laser pulses (i.e., the probe and coupling pulses). As a main advantage, the technique finds optimal solutions even in the complicated multilevel excitation scheme of a doped solid, involving large inhomogeneous broadening. The learning loop experimentally determines optimal temporal intensity profiles of the coupling pulses for a given probe pulse. The optimized intensity pulse shapes enhance the light-storage efficiency in the doped solid by a factor of $2$. The learning loop also determines a fast and efficient preparation pulse sequence, which serves to optically prepare the crystal prior to light-storage experiments. The optimized preparation sequence is 5 times faster than standard preparation sequences. Moreover, the optimized preparation sequence enhances the optical depth in the medium by a factor of $5$. As a consequence, the efficiency of light storage also increases by another factor of $3$. Our experimental data clearly demonstrate the considerable potential of feedback-controlled pulse shaping, applied to EIT-driven light storage in solid media.

Ultrafast atomic phase modulation

We demonstrate a phenomenon very similar to electromagnetically induced transparency using a shaped ultrafast laser pulse. Pulse shaping allows us to produce probe and coupling pulses with arbitrary intensities and time delays. The coupling Rabi frequency and probe bandwidth are both on the order of 1 THz, which is much larger than the natural or Doppler broadened linewidth of the probe transition. Important dynamics associated with a time-dependent coupling field are discussed.

Demonstration of carrier envelope offset locking with low pulse energy

We demonstrate a carrier-envelope-offset (CEO)- locked frequency comb with 230-pJ fiber coupling pulse energy by using a passively mode-locked Er-fiber amplifier laser. For the generation of an octave-bandwidth spectrum in a highly nonlinear fiber and the second harmonic in a self-referenced interferometer with the lower pulse energy, we use a tellurite photonic crystal fiber and a direct-bonded quasi-phasematched LiNbO3 ridge waveguide, respectively. Our method is feasible for locking the CEO with a lower pulse energy to obtain a low-noise and highaccuracy optical frequency comb at telecommunications wavelengths.

Tellurite-based fibers and their applications to optical communication networks

This paper reviews tellurite-based fibers and their applications to optical communication networks. First, an investigation of tellurite-based glass and the fabrication of conventional step-index fibers and photonic crystal fibers (PCF) are described. By purifying the raw materials and employing a novel PCF fabrication process, low background losses were achieved for an Er3+-doped, an undoped tellurite-based fiber and a tellurite-based PCF. Second, the optical properties of Er3+-doped tellurite-based glass and fiber, and the gain characteristics of erbium doped tellurite fiber amplifiers (EDTFAs) were studied. A seamless, low noise and gain flattened C + L band EDTFA was realized, and an S + C + L band amplifier was constructed by combining an EDTFA and a thulium-doped fluoride fiber amplifier (TDFFA) in parallel. Third, it is confirmed that the Raman scattering characteristic of tellurite-based fiber has such a large gain coefficient and Stokes shift that it is possible achieve a wideband tellurite-based fiber Raman amplifier. To overcome several problems, a distributed/discrete hybrid tellurite- and silica-based fiber Raman amplifier (FRA) was constructed as an S + C + L band WDM repeater with a gain bandwidth of 127-nm. Fourth, Brillouin amplification and the simulated performance of slow light generation in a tellurite-based fiber were investigated. The fiber exhibits the largest time delay per unit power of 19.9 ns/mW. Finally, a carrier-envelope offset (CEO)-locked frequency comb with low fiber coupling pulse energy (230 pJ) was demonstrated by using a tellurite-based PCF. This method has the potential to lock the CEO with a lower pulse energy and thus provide a low-noise and high-accuracy optical frequency comb at telecommunication wavelengths. These tellurite-based fibers with low background loss thus offer attractive functions for applications in the optical communication field.

Suppression and coherent control of free-induction-decay emission in multilevel systems

In this paper, I study the coherent control and suppression of the free-induction-decay emission associated with the decay from an excited multilevel atom to its ground state. It is shown here that a strong and ultrashort coupling pulse, resonant with the excited states and a lower state other than the ground state, can induce destructive quantum interferences in the decay process. The suppression is temporary and the free-induction-decay signal reappears one quantum-beat period (of the excited states) later. A sequence of equally spaced, ultrashort coupling pulses can control the moment the free-induction-decay emission occurs.

Propagation of ultrashort pulses in multilevel systems under electromagnetically induced transparency

In this paper, I numerically investigate the propagation of ultrashort pulses through an extended collection of multilevel systems under the condition of electromagnetically induced transparency. The transparency of a weak probe pulse is induced by a much stronger coupling pulse. In the limit of a weak probe excitation and a large number of excited states, I show that the free-induction decay signal that would naturally follow the excitation pulse is strongly suppressed, and the envelope of the probe pulse suffers a distortion in its temporal profile as the pulse propagates into the medium. This distortion is characterized by a stronger absorption of the leading edge of the pulse than that of its trailing edge. The temporal phase of the pulse, however, remains constant throughout propagation.