Gain and saturation intensity of the green Ho:ZBLAN upconversionfiber amplifier and laser

The gain and saturation intensity of the green Ho-doped fluorozirconate (ZBLAN) glass fiber amplifier and laser, pumped in the red (643 ⩽ λ_p ⩽ 649 nm; ⁵F₅ ← ⁵I₈), have been measured. For a 2.4-μm core diameter fiber 45 cm in length, the single-pass gain at 543.4 nm exceeds 12 dB for 90 mW of pump power at 643.5 nm. The saturation power for the ⁵F₄, ⁵S₂ → ⁵I₈ lasing transition was determined from gain measurements to be 970 ± 175 μW, which corresponds to a saturation intensity of 19.8 ± 3.5 kW · cm^-2 , and a stimulated emission cross section approximately one order of magnitude larger than theoretical estimates     

Lasing in Cs at 894.3 nm pumped by the dissociation of CsAr excimers

Lasing on the 6/sup>2P_1/2 /spl right arrow/ 6²S_1/2 (D₁) resonance transition of atomic Cs at 894.3 nm has been realised in mixtures of Ar, ethane, and Cs vapour by the photoexcitation of ground state Cs?Ar collision pairs and subsequent dissociation of diatomic, electronically-excited CsAr molecules (excimers). The blue satellites of the alkali D₂ lines provide a pathway for optically pumping atomic alkali lasers on the principal series (resonance) transitions with broad linewidth (/spl geq/ 2 nm) semiconductor diode lasers.     

The influence of velocity-changing collisions on resonantdegenerate four-wave mixing

The phase-conjugate signal observed in resonant degenerate four-wave mixing on the 6³P₂ to 7³ S₁ transition of atomic mercury in a Hg-Ar discharge is investigated. At a fixed argon pressure the variation of the signal with pump powers is explained by a model that includes the effects of velocity-changing collisions (VCCs). As the argon pressure was varied from 0 to 1 torr an increase in the phase conjugate signal was observed and is ascribed to a change in the discharge dynamics with argon pressure and to the influence of VCCs. To further clarify the role of collisions and optical pumping, degenerate four-wave mixing spectra are examined as a function of pump power. Line shapes are briefly discussed     

Phase conjugation of CO2 laser radiation in a mediumwith thermal nonlinearity

An investigation of reflectivity and quality of phase conjugation of CO₂ laser radiation by four-wave mixing in a medium with thermal nonlinearity is discussed. High reflectivity produced by four-wave mixing in CCl₄ was demonstrated both for pulsed radiation (180%) and for CW radiation (20%). A self-pumped phase-conjugate mirror based on four-wave mixing with feedback was realized. Energy, spatial, spectral, temporal, and polarization characteristics of radiation generated by four-wave mixing with feedback in CCl₄ were investigated. The minimum threshold energy of the input signal was ≃8 mJ; the maximum energy reflectivity was about 5. The spatial structure of radiation generated in the scheme of four-wave mixing with feedback was investigated theoretically, and the conditions for phase conjugation, both for regular and for speckled signal beams, were determined     

Ba原子6s2 1S0 - 6s6p3P1跃迁Hanle效应实验研究

Ba原子是光频标的候选者之一,对其进行有效的激光冷却与囚禁需要相关能级的寿命和跃迁几率的信息。Ba原子激发态6s6p 3P1能级在激光冷却实验中很重要,通过Hanle效应实验测量了这一能级的寿命和自发辐射率,从理论和实验上研究了探测激光有限线宽和光强对Ba原子基态6s2 1S0与激发态6s6p 3P1之间跃迁（波长791 nm）的Hanle效应的荧光信号的影响。在考虑了激光线宽和光强因素后所得到的激发态6s6p 3P1的能级寿命和自发辐射率与其他方法给出的结果很好符合。     

Frequency up-conversion to the VUV in Hg vapor

The efficient generation (up to 1 percent, 10 kW) of tunable, narrow-band radiation in the spectral ranges 1219-1221 Å, 1228- 1235 Å, and 1247-1255 Å, using two-photon resonant, four-wave sum mixing in Hg vapor, is reported, using both the planewave and the tight focusing situations. Oscillator strengths are derived from the planewave experiments for the6s^{1}S_{0}-9P, 10p, and11p^{1}P_{1}transitions with an accuracy of 3 percent.     

Laser diode-pumped holmium-doped fluorozirconate glass fiber laserin the green (λ~544-549 nm): power conversion efficiency, pumpacceptance bandwidth, and excited-state kinetics

The green (544-549 nm) Ho-doped fluorozirconate (ZBLAN) glass fiber laser, pumped in the red (λ~6;15 nm) by a high-power (~30 mW) InGaAlP laser diode or a ring dye-laser, has been characterized with regard to power conversion efficiency, fiber core-diameter and length, cavity output coupling, and pump acceptance bandwidth. Fibers doped with ~1200 ppm (by weight) of Ho and having core diameters of 1.7, 3, and 11 μm, and lengths ranging from 12.5 to 86 cm, have been studied in Fabry-Perot resonators having output couplings ranging from 1.545 to 96%. For a 1.7-μm core-diameter fiber, 21 cm in length, the threshold-launched pump power for the diode-pumped fiber laser is 1.9 and 3.5 mW for cavity output couplings of 1.5% and 24%, respectively. These values are the lowest for any upconversion-pumped fiber laser reported to date. Also, the noise and threshold-pumping power properties of the diode-pumped fiber laser are superior to those for its dye-laser-pumped counterpart. The highest laser slope efficiency (>22% with respect to launched pump power) was measured for a 3-μm core-diameter fiber and a cavity output coupling of 24%. The spectral interval over which the launched threshold pump power for this laser is <10 mW is almost 20 nm (637-656 nm). Studies of the fiber laser waveform as a function of pump power reveal competition for population between the ⁵S² and ⁵F₄ states and among the Stark sublevels of the ⁵F₄ manifold. Also, measurements of the output power on individual laser lines of the ⁵F₄, ⁵S₂→ ⁵I₈ (ground) transitions of Ho³⁺:ZBLAN as a function of pump power demonstrate the existence of a loss mechanism at the fiber laser wavelength, presumably due to absorption from ground or the ⁵I_y, ⁶S₂ or ⁵F₄ excited states of the ion     

The sublinear relationship between index change and carrier densityin 1.5 and 1.3 μm semiconductor lasers

The carrier-induced index change was measured using a novel injection-reflection technique in combination with differential carrier lifetime data. The observed relation between index change and injected carrier density at bandgap wavelength is nonlinear and is approximately given by δn_act=-6.1×10^-14 ( N)^0.66 for a 1.5-μm laser and δn _act=-1.3×10^-14 (N)^0.68 for a 1.3-μm laser. The carrier-induced index change for a 1.3-μm laser at 1.53-μm wavelength is smaller and is given by δn _act=-9.2×10^-16 (N)^0.72      

Stimulated Raman effect and four-wave mixing in a hollow waveguide

A theoretical and experimental investigation of stimulated Raman scattering and four-wave mixing in a gas-filled hollow waveguide was performed. The four-wave mixing processes, specifically the generation of the third Stokes component at ω₃ when ω₃ =2ω₂-ω₁, show a strong resonance-like behavior as a function of wavelength. These resonances are caused by the phase matching required for efficient four-wave mixing, because the hollow waveguide supports only certain propagation directions leading to the development of waveguide modes. The theoretical results are compared with experimental data for TEM₀₀ and TEM₁₀ fundamental dye-laser beams generating third Stokes wavelengths between 2.6 and 7.5 μm     

A new extrapolation algorithm for band-limited signals using theregularization method

The ill-posedness of the extrapolation problem in the presence of noise is considered. A stable algorithm is constructed by solving a Fredholm equation based on a regularization method. The algorithm appears relatively robust, since the noise η_δ(t ) is taken as a function in L²[-T,T](T>0) such that the error energy ∫|η_δ(t)|² dt⩽δ², where integration is from - T to T, and the constructed extrapolation uniformly converges to the desired signal over (-∞, +∞) as δ→0. An estimate for the error energy of the constructed extrapolation over (-∞, +∞) and for the absolute error between the constructed extrapolation and the desired signal over (-∞, +∞) are presented     

Q-switch regime of 3-μm Er:YAG lasers

A mathematical model, based exclusively on spectroscopic data concerning radiative, nonradiative, and energy transfer processes, is proposed and used to simulate the Q-switch regime of a 3-μm Er:YAG laser. The connection between the main energy transfer mechanisms that make possible generation on the self-saturated transition ⁴I _11/2→⁴I_13/2 (upconversion from ⁴I_13/2 and ⁴I_11/2 and cross relaxation from ⁴S_3/2) and the giant pulse characteristics are discussed. The radiative as well as nonradiative losses during optical pumping and giant pulse generation are defined and evaluated. A particular attention is given to the frustrated total internal reflection (FTIR) Q-switch which demonstrated real qualities for 3-μm erbium lasers. The reasons responsible for experimental performances of Q-switched Er:YAG lasers inferior to those predicted by the mathematical modeling are analyzed     

FDTD simulation of radiation characteristics of half-volume HEM and TE-mode dielectric resonator antennas

Cylindrical and rectangular dielectric resonator antennas (DRAs) using HEM_11δ, TE_11δ, and TE_01δ mode were examined to see the radiation pattern, impedance, field distribution and resonant frequency that were achieved when the DRAs were bisected through an image plane by a conducting sheet. The resultant half DRAs are smaller in volume and have a more directional radiation pattern. The elevation angle of maximum radiation was lowered in some cases. Finite-difference time-domain simulation techniques were used     

New CW CO2 laser lines: the 9-μm hot band

We have made what we think is the first observation of the oscillation of the 9-μm hot-band lines 01¹1→[11¹ 0,03¹0]_II of CO₂. We have observed 40 lines with a maximum power of 8 W. They will provide a new source of laser radiation for spectroscopy. The set of lines has been positively identified by directly measuring the frequencies of two of the lines with a heterodyne technique using a CO₂ laser standard     

Energy transfer up-conversion and excited state absorption of laserradiation in Nd:YLF laser crystals

Experiments at room temperature were carried out to study the higher order processes that produced the yellow and blue fluorescence observed in Nd:YLF laser crystals, which were pumped by lasers with wavelengths between 785 nm and 811 nm, and with the pump power density ranging from 10 to 30 kW/cm², or the stored energy density, 15-45 J/cm³. The analysis of the experimental results provided an explanation for the performance degradation of a Q-switched laser with increasing pump intensity. These higher order processes were energy transfer up-conversion and excited state absorption of both pump and laser radiation. A model was also developed based upon rate equations to describe these higher order processes. This model allowed us to discriminate between these processes and to determine which one played the most important role. The experimental results agreed well with the predictions of the model. It is concluded that the energy transfer up-conversion process is the dominant mechanism for the population of the ⁴G_7/2 multiplet. It is also concluded that excited state absorption of the 1.05 μm or 1.3 μm radiation from the ⁴G_7/2 multiplet play a significant role in populating the ²P_3/2, ² D(1)_5/2, and ²P_½ manifolds, and in the performance of Nd:YLF lasers operating in the pulsed mode. The effect of excited state absorption of the pump radiation from the ⁴F_½ multiplet is much less important     

Emission Regimes of a Green Er:YLiF4 Laser

We present an analysis of the emission regimes of an Er:YLiF₄ laser on the transition ⁴S_3/2 rarr ⁴I_15/2, under direct pumping and various upconversion pumping mechanisms. The analysis is based on the study of the steady-state solutions of a rate equation system. The thresholds of the various emission regimes are calculated. The influence of the pump mechanism and the excited-state absorption losses on the emission regime is discussed.     

Emission cross sections and spectroscopy of Ho3+ laserchannels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho³⁺ laser channels in KGd(WO₄)₂ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho³⁺ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω₂=15.35×10^-20 cm², Ω ₄=3.79×10^-20 cm², Ω₆ =1.69×10^-20 cm². The 7-300-K lifetimes obtained in diluted (8·10¹⁸ cm^-3) KGW:0.1% Ho samples are: τ(⁵F₃)≈0.9 μs, τ( ⁵S₂)=19-3.6 μs, and τ(⁵F₅ )≈1.1 μs. For Ho concentrations below 1.5×10²⁰ cm^-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τ_ph ^-1(0)=βexp(-αΔE), where β=1.4×10^-7 s^-1, and α=1.4×10³ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho³⁺ emission cross section σ_EM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σ_EM values achieved for transitions to the ⁵I₈ level are ≈2×10^-20 cm² in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the ⁵I₇→⁵I₈ channel     

Measurement of the characteristics of the optical-microwavedouble-resonance effect of the 87Rb D1line for application as an atomic frequency standard

The OMDR (optical-microwave double resonance) spectrum of ⁸⁷ Rb with the aim of using a frequency-stabilized GaAs semiconductor laser instead of an Rb lamp as a pumping source in a gas-cell-type Rb frequency standard. Natural isotope ⁸⁷Rb was sealed in a glass cell with buffer gases (Ar/N₂=1.2, total pressure=39 torr). The double resonance signal in the 5P_1/2(F=2)←5S_1/2( F=1) transition appearing at the resonance to the F=2←1 hyperfine transition of the 5S_1/2 state was detected. The optimum operational cell temperature was 56°C. The peak-to-peak frequency width of the atomic hyperfine resonance discriminator used to stabilize the microwave frequency shifts induced by detuning of the laser frequency, changes in the laser and microwave powers, and temperature drift of the cell were investigated     

Nonlinear gain dynamics in quantum-dot optical amplifiers and itsapplication to optical communication devices

Ultrafast gain dynamics in quantum-dot (QD) optical amplifiers has been studied. It was found that there are at least three nonlinear processes, which are attributed to carrier relaxation to the ground states, phonon scattering, and carrier capture from the wetting layers into the QDs. The relevant time constants were evaluated to be ~90 fs, ~260 fs, and ~3 ps, respectively, under a 50-mA bias condition. The dephasing time was evaluated to be ~85 fs. The third-order optical susceptibility (χ⁽³⁾) has been evaluated by means of both nonlinear transmission and four-wave mixing experiments. The results show that the nonlinearity expressed by χ⁽³⁾/g₀ is quite similar to that of bulk and quantum wells, which can be explained by similar relaxation times. Applications to optical communication devices are also discussed     

Photon avalanche in Er:ZBLAN fibre pumped at 690 nm

A new photon avalanche mechanism has been observed in Er:ZBLAN fibre when pumped at 700 nm; this pump scheme corresponds to the excited state absorptions from ⁴I_13/2 and ⁴I _11/2. Experimental results of pump power dependence and build-up process are reported     

Photon avalanche upconversion in Er3+:YAlO3

The results of an investigation of photon avalanche upconversion pumping in Er³⁺:TiAlO₃ are reported. Five pump wavelengths corresponding to transitions from the metastable ⁴I_13/2 state to the ²H_11/2 state generated upconversion laser emission at 549.8 nm. The dependence of the laser output power on pump power near laser threshold is discussed in terms of a four-level kinetics model and is shown to reflect the threshold power requirement for photon avalanche. The maximum output power at 7 K was 33 mW, giving an optical conversion efficiency of 3.5% and a conversion efficiency of 28% based on absorbed power. Pumping Er:YALO by cross relaxation energy transfer produced 166 mW of laser output with an optical conversion efficiency of 17%