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Airborne Distributed Coherent Aperture Radar (ADCAR) is one of the most promising next-generation radars to significantly improve target detection and discrimination abilities. However, time and phase synchronization among unit radars should be done before an ADCAR is intended to cohere on a potential target. To address this problem, a time and phase synchronization technique using clutter observations is proposed in this paper. Clutter returns from different azimuths and elevations on the surface of the earth are employed to calibrate system uncertainties. Two stages are mainly considered: a scene registration among range-Doppler units from different transmit/receive pairs is performed to enhance the clutter coherence in the first stage, followed by a joint estimation of those synchronization errors in the second stage. To relieve the computational burden, a novel Separable and Sequential Estimation (SSE) method is provided to separate the unknowns at the sacrifice of a range-Doppler unit. Moreover, performance analyses including the clutter coherence ability, estimation lower bound, and signal coherence loss are also performed. Finally, simulation results indicate that ADCAR time and phase synchronization is realized by using our methods.

Quantum coherence in quantum optics is an essential part of optical information processing and light manipulation. Alkali metal vapors, despite the numerous shortcomings, are traditionally used in quantum optics as a working medium due to convenient near-infrared excitation, strong dipole transitions and long-lived coherence. Here, we proposed and experimentally demonstrated photon retention and subsequent re-emittance with the quantum coherence in a system of coherently excited molecular nitrogen ions (N2+) which are produced using a strong 800 nm femtosecond laser pulse. Such photon retention, facilitated by quantum coherence, keeps releasing directly-unmeasurable coherent photons for tens of picoseconds, but is able to be read-out by a time-delayed femtosecond pulse centered at 1580 nm via two-photon resonant absorption, resulting in a strong radiation at 329.3 nm. We reveal a pivotal role of the excited-state population to transmit such extremely weak re-emitted photons in this system. This new finding unveils the nature of the coherent quantum control in N2+ for the potential platform for optical information storage in the remote atmosphere, and facilitates further exploration of fundamental interactions in the quantum optical platform with strong-field ionized molecules.

It has been observed that in a pre-excited medium produced by femtosecond laser pulses, nonlinear interaction can be dramatically enhanced. The underlying mechanism is still under debate because of the lack of detailed information on the species, energy states, and temporal dynamics of the excited medium. Here, we report a dramatic enhancement of seventh-harmonic generation in pre-excited argon atoms. By measuring the evolution of the enhanced signal with the propagation distance, we are able to separate the contributions from the improved phase matching and the enhanced nonlinear susceptibility. The time- and spectrum-resolved measurements clearly show that generation of excited states is responsible for the enhanced nonlinear susceptibility. The maximum population of excited states on the picosecond and nanosecond timescales can be attributed to two possible pathways.

We report a combined theoretical and experimental investigation on the unambiguous role of electronic coherence underlying the generation of laserlike coherent emission from ${{\mathrm{N}}_{2}}^{+}$ ions launched in a strong laser field. The coherence manifests itself by giving rise to a dramatic enhancement of ${{\mathrm{N}}_{2}}^{+}$ laserlike radiation with two temporally separated pump laser pulses centered at 800- and 1580-nm wavelengths, respectively. We further show that coherent control of ${{\mathrm{N}}_{2}}^{+}$ lasing in a subfemtosecond timescale can be achieved by replacing the single 800-nm laser pulse with a pair of 800-nm laser pulses of adjustable time delay. The experimental results are well reproduced in theory using a three-level quantum model of ${{\mathrm{N}}_{2}}^{+}$ ions under the density matrix representation.

We experimentally investigate air lasing emission from singly ionized nitrogen molecules exposed to bicircular two-color (BTC) femtosecond laser pulses. The light driven electron recollision is suppressed in the corotating BTC field as compared to the counter-rotating one. It allows us to straightforwardly examine the role of electron recollision on the air lasing generation by simply changing the rotating sense of the BTC fields. Lasing emission is observed for both corotating and counter-rotating BTC fields without noticeable difference when the rotating sense is switched from one to the other case. It indicates a minor contribution of electron recollision on the air lasing emission. In addition to the well-observed linear polarization, we demonstrate that the polarization of the newborn lasing emission can be remotely manipulated from linear to circular by controlling the polarization of the driving second-harmonic field which at the same time acts as the seed.

We developed an 808 nm diode laser pumped composite Holmium laser via the integration of Tm-doped and Ho-doped YAG crystals into a single bulk structure. A maximum output power of 1.8 W at 2122 nm with near diffraction-limited beam quality was achieved for an absorbed pump power of approximately 7 W. Given that the pump wavelength is approximately 20 nm away from the absorption peak of the Tm-doped region of the crystal, pump wavelength adaptability of the Tm/Ho composite laser was effectively demonstrated. In this case, a pump range width in excess of 40 nm at approximately 800 nm was predicted by defining the pump spectrum of the gain medium. The temperature adaptability of the Ho laser was demonstrated by varying the cooling temperature from 9 °C to 27 °C. This resulted in Ho laser operation above 1.5 W with a power deviation below 10%. This work presents the experimental results for a compact assessable 2.1 μm laser without pump wavelength and operation temperature restrictions.

We report on the generation of electronic quantum coherence in the ionized nitrogen molecules. The coherence is revealed by observing N2+ air lasing with a near-infrared laser and a delayed mid-infrared laser.

We report generation of cascaded rotational Raman scattering up to 58th orders in coherently excited CO_2 molecules. The high-order Raman scattering, which produces a quasiperiodic frequency comb with more than 600 sidebands, is obtained using an intense femtosecond laser to impulsively excite rotational coherence and the femtosecond-laser-induced N_2^+ lasing to generate cascaded Raman signals. The novel configuration allows this experiment to be performed with a single femtosecond laser beam at free-space standoff locations. It is revealed that the efficient spectral extension of Raman signals is attributed to the specific spectra-temporal structures of N_2^+ lasing, the ideal spatial overlap of femtosecond laser and N2+ lasing, and the guiding effect of molecular alignment. The Raman spectrum extending above 2000 cm^-1 naturally corresponds to a femtosecond pulse train due to the periodic revivals of molecular rotational wavepackets., Comment: 17 pages, 4 figures

We investigate nonlinear interaction of nitrogen molecules with a two-color laser field composed by an intense 800 nm laser pulse and a weak 400 nm laser pulse. It is demonstrated that the spectrum of 400 nm pulses is dramatically broadened when the two beams temporally overlap. In comparison, the spectral broadening in argon is less pronounced, although argon atoms and nitrogen molecules have comparable ionization potentials. We reveal that the dramatic spectral broadening originates from the greatly enhanced nonlinear optical effects in the near-resonant condition of interaction between the 400 nm pulses and the nitrogen molecular ions.

We investigate free-space lasing actions from molecular nitrogen ions (N2+) at the wavelengths of ~391 nm and ~428 nm. Our results show that pronounced gain can be measured at either 391 nm or 428 nm laser wavelength with a pump laser centered at 800 nm wavelength, whereas the gain at 391 nm laser wavelength completely disappears when the wavelength of the pump laser is tuned to 1500 nm. Our theoretical analysis reveals that the different gain behaviors can be attributed to the vibrational distribution of populations in X2Σg+(v=0) and X2Σg+(v=1) states as the N2+ ions are generated by photoionization in the laser fields, giving rise to more robust (i.e., less sensitive to the pump laser wavelength) population inversion for generating the 428 nm laser.

It is known that in an intense laser field of a sufficiently high strength combined with a sufficiently long wavelength, i.e., in the regime of the Keldysh parameter γ

We observe a dramatic enhancement in third harmonic generation (THG) when intense femtosecond laser pulses travel through CO2-laser-induced spark in air. By further investigating the dependence of the enhancement factor on the delay between the femtosecond laser pulse and CO2 laser pulse in various gases at different pressures, we reveal that the enhancement in THG results from the suppression of Gouy-phase-induced destructive interference, which can be attributed to a fast and a slow process. Our analyses show that the plasma generated in CO2 laser field plays the dominant role in the fast process whereas hydrodynamics of the gas heated by the CO2 laser contributes to the slow process.

Atmospheric lasing has aroused much interest in the past few years. The 'air–laser' opens promising potential for remote chemical sensing of trace gases with high sensitivity and specificity. At present, several approaches have been successfully implemented for generating highly coherent laser beams in atmospheric condition, including both amplified-spontaneous emission, and narrow-bandwidth stimulated emission in the forward direction in the presence of self-generated or externally injected seed pulses. Here, we report on generation of multiple-wavelength Raman lasers from nitrogen molecular ions (), driven by intense mid-infrared laser fields. Intuitively, the approach appears problematic for the small nonlinear susceptibility of ions, whereas the efficiency of Raman laser can be significantly promoted in near-resonant condition. More surprisingly, a Raman laser consisting of a supercontinuum spanning from ~310 to ~392 nm has been observed resulting from a series near-resonant nonlinear processes including four-wave mixing, stimulated Raman scattering and cross phase modulation. To date, extreme nonlinear optics in molecular ions remains largely unexplored, which provides an alternative means for air–laser-based remote sensing applications.

Circuit breaker is a very important control and protective equipment in the field of grid system. When the electric equipment or circuit are in accident, the relay protection and circuit breaker will cut the fault section from grid rapidly and guarantee the grid running safely. However, there are a mount of high voltage circuit breakers. According to the traditional planned overhaul model to inspect circuit breaker, there will be some phenomenons such as large workload, high expense and so on. At the same time, different circuit breakers need different signal sources. During maintenance, this character will lead to the waste of human resource and cause artificial errors. Therefore, the paper presents a new standard signal source of circuit breaker. It realized the integrated, informational and intelligent reform for the various kinds of signal sources. Utilize standard signal source to realize the calibration for common equipment can reduce the maintenance workload and improve the pertinence of maintenance. It is very helpful for improving the running reliability and economy of electric equipment.