高功率光纤激光器中ASE噪声的测量方法（英）

高峰值功率脉冲光纤激光器在激光雷达系统中有着广泛的应用。然而,光纤激光器中放大自发辐射噪声（ASE）严重影响了系统的探测性能。提出一种测量高峰值功率脉冲光纤激光器中ASE噪声的方法。在该方法中,首先对高峰值功率的激光脉冲衰减,然后在时域分别测量和计算ASE噪声和激光脉冲的相对能量。给出了光纤激光器在驱动电流分别为6 A,7 A和8 A时衰减后的ASE噪声廓线以及ASE噪声占激光脉冲能量的比例。     

高功率光纤激光器中ASE噪声的测量方法（英）

高峰值功率脉冲光纤激光器在激光雷达系统中有着广泛的应用。然而,光纤激光器中放大自发辐射噪声（ASE）严重影响了系统的探测性能。提出一种测量高峰值功率脉冲光纤激光器中ASE噪声的方法。在该方法中,首先对高峰值功率的激光脉冲衰减,然后在时域分别测量和计算ASE噪声和激光脉冲的相对能量。给出了光纤激光器在驱动电流分别为6 A,7 A和8 A时衰减后的ASE噪声廓线以及ASE噪声占激光脉冲能量的比例。     

Dynamic mode interaction in multi-mode laser diodes

Dynamic characteristics of laser diodes are numerically calculated on multi-mode rate equations, including spatial relaxing interaction causing mode competitions or jumps. It is revealed that the interaction effect is classified into three cases [“independent (or inhomogeneous)”, “competition (or intermediate)”, and “monopoly (or ideally homogeneous)”], that an optimum relaxing coefficient value exists to minimize relaxation oscillations in rise and decay transients, and that the optimum value is related to the relaxation period. Calculated results are discussed in comparison with the experimental results.     

Influence of optical field emission on the nonlinear photoelectric effect induced by ultrashort laser pulses

A decrease in the order of nonlinearity of photoelectron emission from a metal cathode was found experimentally with mode-locked laser pulses at sufficiently high intensities. This phenomenon is interpreted as due to the theoretically predicted appearance of optical field emission.     

Theory of noise in semiconductor laser emission

For the complex light field amplitude of a semiconductor laser an equation of a generalized van der Pol oscillator with a fluctuating driving term is derived from first principles. This equation is shown to be valid for optical band-to-band transitions with and withoutk-selection rule. Neglecting the nonlinearity in the saturation higher than of second order and also the intensity dependence of the dispersion this equation reduces to the standard van der Pol equation with a noisy driving term. From the general equation the linewidth and the noise of the intensity of the laser emission is calculated above and below threshold. The results are in agreement with the experimental data ofArmstrong andSmith.     

Broadband enhancement of spontaneous emission in a photonic-plasmonic structure

We demonstrate that a broadband enhancement of spontaneous emission can be achieved within a photonic-plasmonic structure. The structure can strongly modify the spontaneous emission by exciting plasmonic modes. Because of the excited plasmonic modes, an enhancement up to 30 times is observed, leading to a 4 times broader emission spectrum. The reflectance measurement and the finite-difference time-domain simulation are carried out to support these results.     

Bosonic enhancement of spontaneous emission near an interface

We show how the spontaneous emission rate of an excited two-level atom placed in a trapped Bose-Einstein condensate of ground-state atoms is enhanced by bosonic stimulation. This stimulation depends on the overlap of the excited matter-wave packet with the macroscopically occupied condensate wave function, and provides a probe of the spatial coherence of the Bose gas. The effect can be used to amplify the distance-dependent decay rate of an excited atom near an interface.     

Amplitude and phase control of spontaneous emission in a double-driven four-level atom

The spontaneous emission property of a coherent microwacve-driven four-level atomic system is investigated. It is shown that a few interesting phenomena due to microwave generated coherence (MGC) such as spectral narrowing, partial cancellation, and spectral-line enhancement can be achieved by varying the Rabi frequency and the phase of microwave field coupling the two upper levels in the atomic system.     

Theory of intensity noise in semiconductor laser emission

The intensity fluctuations expected in the output of cw semiconductor lasers are studied analytically using linearized multimode rate equations. Spontaneous emission causes intrinsic fluctuations of the laser wave. A power-independent contribution to the intensity fluctuations may occur due to inversion-induced modulation noise. The correlation of the low-frequency noise of different laser modes is investigated. The results are in agreement with experimental data reported in the literature.     

Amplified spontaneous emission contrast of CPA laser

The influence on the level of amplified spontaneous emission （ASE） contrast resulting from the different seed pulse energies has been experimentally studied in a 10-TW-level femtosecond Ti：sapphire chirped pulse amplification laser. It is found that, with the seed pulse energy increasing, the ASE pedestal is suppressed more efficiently, and the ASE contrast is improved with a saturable tendency. The measurement of -2×10^-8 ASE contrast in the range of more than 50 ps before the main pulse is achieved in the laser.     

Nonlinear photoelectron emission from metal surfaces induced by short laser pulses: the effects of field enhancement by surface plasmons

Nonlinear electron emission processes induced by surface plasmon oscillations have been studied both experimentally and theoretically. The measured above-threshold electron spectra extend up to high energies whose appearance cannot be explained solely by standard non-perturbative methods, which predict photon energy separated discrete energy line spectra with the known fast fall–plateau–cutoff envelope shape, even when taking the large field enhancement into account. The theoretical analysis of our data, based on the concept of plasmon-induced surface near-field effects, gives a reasonably good explanation and qualitative agreement in the whole intensity range.     

Spatially inhomogeneous saturation of gain in organic solutions caused by amplified spontaneous emission

A theoretically predicted phenomenon, the spatially inhomogeneous saturation of the gain caused by amplified spontaneous emission, was investigated. The direct measurements of the population of excited states are in good qualitative agreement with the theoretical calculations.     

Experimental characterization of nonlinear harmonic radiation from a visible self-amplified spontaneous emission free-electron laser at saturation

Nonlinear harmonic radiation was observed using the VISA self-amplified, spontaneous emission (SASE) free-electron laser (FEL) at saturation. The gain lengths, spectra, and energies of the three lowest SASE FEL modes were experimentally characterized. The measured nonlinear harmonic gain lengths and center spectral wavelengths decrease with harmonic number, n, which is consistent with nonlinear harmonic theory. Both the second and third nonlinear harmonics energies are about 1% of the fundamental energy. These experimental results demonstrate for the first time the feasibility of using nonlinear harmonic SASE FEL radiation to produce coherent, femtosecond x rays.     

Abnormal properties of spontaneous emission in a Kerr nonlinear blackbody

We reexamine the atomic spontaneous decay in a Kerr nonlinear blackbody by contrast with our previous paper [M. Yin, Z. Cheng, Phys. Rev. A 78 (2008) 063829]. In the process of deriving the atomic decay rate, we use the temperature-dependent velocity of photons to take the full nonlinearity of a KNB into account. It is found that below a transition temperature T_c, the atomic spontaneous emission in a KNB might be enhanced or inhibited compared with that in a normal blackbody whose interior is filled with a nonabsorbing linear medium. The physical origin of the enhancement and inhibition of spontaneous emission is also discussed.     

Subnanosecond amplified spontaneous emission pulses by a nitrogen pumped dye laser

Pulses of 100 ps duration and peak power up to 100 kW are obtained with a dye laser pumped by an atmospheric pressure nitrogen laser of 0.5 ns duration. The shortening of the dye laser pulses is attributed to amplified spontaneous emission.     

Giant enhancement of photoluminescence emission in monolayer WS2 by femtosecond laser irradiation

Monolayer transition metal dichalcogenides have emerged as promising mat erials for opt oelectTonic and nanophotonic devices.However,the low photoluminescence(PL)quantum yield(QY)hinders their various potential applications.Here we engineer and enhance the PL intensity of monolayer WS₂by femtosecond laser irradiation.More than two orders of magnitude enhancement of PL intensity as compared to the as-prepared sample is determined.Furthermore,the engineering time is shortened by three orders of magnitude as compared to the improvement of PL intensity by continuous-wave laser irradiation.Based on the evolution of PL spectra,we attribute the giant PL enhancement to the conversion from trion emission to exciton,as well as the improvement of the QY when exciton and trion are localized to the new-formed defects.We have created microstructures on the monolayer WS₂based on the enhancement of PL intensity,where the engineered structures can be stably stored for more than three years.This flexible approach with the feature of excellent long-term storage stability is promising for applications in information storage,display technology,and opto electronic devices.