Multifrequency laser probing

A method is developed for processing the results of measuring gain or absorption by multiple sounding of gaseous media. The accuracy of the determined parameters of gaseous media is analyzed as a function of the accuracy with which the gain or absorption is measured. The method was developed by solving model problems and used to reduce the measured values of the gain in media containing CO and CO₂ molecules. Recommendations are made for diagnostics of equilibrium and nonequilibrium gaseous media containing CO and CO₂ and conditions are stipulated with respect to the probing system and the measurement accuracy needed in measurements of the gain and absorption indices from which the parameters of the gaseous media are determined.Translated from Preprint No. 109 of the Lebedev Physics Institute, Academy of Sciences of the SSSR, 1989.     

A numerical analysis of gain characteristics of Er-doped Al2O3 waveguide amplifiers

The gain characteristics of erbium-doped Al₂O₃ waveguide amplifiers are investigated by solving numerically rate equations with upconversion effects and propagation equations. We obtained the dependence of gain of erbium-doped Al₂O₃ waveguide amplifiers on the waveguide length, erbium concentration and pump power at different pumping wavelengths (980 and 1480 nm). The performance of amplifiers pumping at 1480 and 980 nm are compared. It is shown that 980 nm pumping has higher gain and higher pumping efficiency. The parameters of waveguide amplifiers have been optimized. A optical gain of 43 dB can be achieved for a optimum waveguide length of 8.25 cm and 5.8 × 10²⁰ cm^–3 Er concentration pumped with 100 mW at 980 nm, that is a gain of 5.2 dB/cm.     

Optical Q-branch pumping of the P- and R-branch transitions in the ν3 mode of SF6

One consequence of pumping the Q-branch of the ν₃ mode of SF₆ heretofore not examined is the production of substantial gain on certain low J R-branch transitions. In the analysis, the small-signal gain equation is derived for an individual symmetry species within a rotational manifold, since the nuclear spin statistical weigfhts play a significant role in the gain. The large value for the gain is a consequence of the selection rules governing the symmetry species and the way the spin statistical weights vary with the rotational quantum number J. It is shown that the transitions with gain, although only a fraction of a wave number removed from the pumped Q-branch, should be observable under appropriate conditions. For example, at a temperature of 135 K and a pump intensity of 10 kW/cm², the transitions with gain are not obscured by either hot-band absorption or by power broadening of the Q-branch. This induced gain may be of importance for the multiple photon dissociation of SF₆.     

Saturation of the net electron excitation of the CO2(00°1) laser level as a function of discharge input energy

Measurements are made of the small signal gain in He:CO₂ and He:N₂:CO₂ gas mixtures as a function of pulsed discharge energy. In all case it is found that the gain saturates as the discharge energy is increased. Thermal effects cannot account for this gain saturation which appears to be a fundamental property of high-pressure CO₂ discharges.     

开放的V型三能级系统中原子注入和退出速率对位相相关的无反转激光增益和强度空间演化的调制作用

利用数值计算结果, 研究了具有自发辐射诱导相干的开放V 型三能级原子系统中的传播效应. 研究表明: 改变探测场和驱动场之间的相对位相对无反转激光(LWI)增益和 强度随传播距离的变化即空间演化具有重要的影响; 而原子的注入速率比(S)和退出速率(r₀)的改变将对位相相关的LWI增益和强度的空间演化产生明显的调制作用. 在S(r₀)的一定取值范围内, S(r₀)的值越大, LWI增益和强度越大且强度达到极大值需要的传播距离越长; 相应封闭系统中LWI增益和强度的最大值及能产生增益的传播距离都小于开放系统. Doppler效应对LWI增益和强度的空间演化也具有明显的影响, 存在Doppler效应时得到的LWI增益和强度明显小于无Doppler效应时的值.     

Phonon-assisted stimulated emission in 2H type PbI2

The stimulated emission in 2H type PbI₂ crystals has been observed under N₂ laser beam excitation at 4.2 K. The analysis of its gain spectrum shows that the emission is induced by the recombination of free exciton assisted by the emission of one longitudinal optical phonon. At a higher level of excitation, the gain spectrum peak shifts to lower energy side. Possible mechanisms of this spectral change are discussed.     

Characteristics of amine-based all-gas-phase iodine laser

The characteristics of an amine-based all-gas-phase iodine laser (AGIL) are studied. At constant flow rates of the chemical species, the small-signal gain and laser output power are measured at three different positions in the flow reactor. It is clarified that the positive gain exceeding the threshold (2.3×10^?3%/cm) is maintained over a region more than 80 mm long along the flow. The highest small-signal gain of 7.8×10^?3%/cm and laser output power of 50 mW are observed at 170 mm downstream from the mixing point of HI and H. Numerical simulations suggest that the long positive-gain region is attributable to the following characteristics of the present apparatus: inefficient mixing and inefficient H₂ dissociation. It is confirmed experimentally and numerically that these limitations are beneficial for extending the positive-gain region to the downstream; however, the optimum HI flow rate is limited a small value and results in a much lower gain than the theoretical limit. In order to achieve a higher gain, fast mixing and efficient H₂ dissociation must be simultaneously achieved so as to increase the optimum HI flow rate. Numerical simulation results suggest that a positive gain of 5.4×10^?2%/cm, a 7-times higher gain than that indicated by the present experimental results, should be obtained with the same NCl₃ flow rate.     

Stimulated emission effects in excitonic molecule luminescence of CuCl under resonant two-photon excitation

Luminescence spectra of excitonic molecules in CuCl resonantly created by nano-second dye laser excitation are measured at 4.2 K changing the lenghts of the excited region. The spectra are dominated by stimulated emission. The gain coefficient for the M_T line is saturated more easily than that for M_L, though the former is larger under unsaturated conditions. These gain characteristics explain well various stimulated emission effects observed.     

Effect of bias condition on heavy ion radiation in bipolar junction transistor

The characteristic degradations in silicon NPN bipolar junction transistor (BJT) of 3DG142 type are examined under the irradiation with 40-MeV chlorine (Cl) ions under forward, grounded, and reverse bias conditions, respectively. Different electrical parameters are in-situ measured during the exposure under each bias condition. From the experimental data, larger variation of base current (I_B) is observed after irradiation at a given value of base-emitter voltage (V_BE), while the collector current is slightly affected by irradiation at a given V_BE. The gain degradation is affected mostly by the behaviour of the base current. From the experimental data, the variation of current gain in the case of forward bias is much smaller than that in the other conditions. Moreover, for 3DG142 BJT, the current gain degradation in the case of reverse bias is more severe than that in the grounded case at low fluence, while at high fluence, the gain degradation in the reverse bias case becomes smaller than that in the grounded case.     

Low gain threshold density of a single InGaP quantum well sandwiched by digital alloy

A single In_0.49Ga_0.51P quantum well sandwiched by In_0.49Ga_0.51P/In_0.49(Ga_0.6Al_0.4)_0.51P digital alloy structures was investigated in terms of optical modal gain, where gain saturation effects were also considered for both changes in wavelength and stripe length by using a modal gain contour map analysis. We found the gain threshold density is considerably lower by an order of magnitude when compared to the Mott density (∼2 × 10¹² cm⁻²), which can be attributed to a carrier-harvesting effect through the mini-band of the digital alloy.     

Simulations of a photopumped X-ray laser using the H-like Cl−Li-like Se scheme

Calculations of the modal photon densities and gain in a photopumped Cl XVII-Se XXXII X-ray laser are presented. In this paper we undertake a realistic simulation of the generation of both Cl and Se plasmas, using a high-power optical laser, which includes radiation from both Ly-α fine-structure components of H-like Cl pumping the 2p_3/2-5d_5/2 transition in Li-like Se. The calculations are performed in two dimensions in a realistic geometry taking into account plasma gradients. This gives information about the spatial extent and time evolution of X-ray lasing gain on the 5-4 transitions in Li-like Se. We find that gain (about ) is expected only when the optical laser includes a pre-pulse. Calculations show that the absorption of pumping radiation in the pumped plasma can reduce the gain by 20%. Time-dependent calculations have shown that the gain is reduced by 30% in comparison to the steady-state calculations. The effect of the spectral profile and self-radiation of 5d_5/2-2p_3/2 transition in Li-like Se reduces the gain by about 2%.     

Albedo and gain threshold of a diffusive Raman random laser

The diffuse reflectance (albedo) and transmittance of a Raman random gain medium are derived from a diffusion equation with power dependent gain. The results show good agreement with the experimental data for barium nitrate powder. Both the Raman albedo A_R and Raman transmittance T_R diverge at a critical gain γ_c, interpreted as the threshold for diffusive Raman laser generation. The parametric dependence of the albedo and threshold gain on the scattering characteristics of the random medium is analysed and the feedback effect of Fresnel reflection at the gain boundaries evaluated. The addition of external mirrors, particularly at the pumped surface, significantly reduces the generation threshold.     

Nonlinear analysis of a multi-cavity gyro-twystron

To preserve high gain, high efficiency and high power merits of gyroklystron, a gyro-twystron is designed using an electron beam with α(v_⊥/v_z) greater than unity. With a multi-cavity section of high gain, the length of the waveguide output section can be made shorter than the threshold length of the absolute instability without losing total system gain. Numerical simulations are carried out to analyze a ka-band gyro- twystron consisting of three TE₁₁₁ mode cavities and an output section of a TE₁₁ mode waveguide. Stability study is performed to ensure the tube without self-excited oscillations. With α=1.5, the 3-dB linear and saturated gain bandwidth in excess of 2 % can be obtained by stagger tuning for an 80 kV, 3 A electron beam with 5 % axial velocity spread. The maximum saturated gain is more than 55 dB at 33 % efficiency. By tapering the magnetic field of the last 2 cm of the interaction region, the efficiency can be increased to 43 % without degrading the bandwidth, which corresponds to an output power of 103 kW.     

Crystal orientation dependence of the SNR for signal beam amplification in photorefractive BaTiO3

In signal beam amplification by two-beam coupling in BaTiO₃ photorefractive crystals, beam fanning in the direction of the amplified signal reduces the signal-to-noise ratio (SNR). The dependence of the SNR and the signal beam gain on the crystal orientation are analysed using a HeNe laser. It is found that orientating the crystal for maximum gain gives poor signal-to-noise ratio. A compromise has to be made between the SNR and high gain for optimum signal amplification.     

Angular spectrum characters of high gain non-critical phase match optical parametric oscillators

The angular spectrum gain characters and the power magnification characters of high gain non-walk-off colinear optical parametric oscillators have been studied using the non-colinear phase match method for the first time.The experimental results of the KTiOAsO 4 and the KTiOPO 4 crystals are discussed in detail.At the high energy single resonant condition,low reflective ratio of the output mirror for the signal and long non-linear crystal are beneficial for small divergence angles.This method can also be used for other high gain non-walk-off phase match optical parametric processes.     

Dynamics and gain in highly-excited InGaN MQWs

The Kerr gate technique is used to time-resolve the gain in an In_0.02Ga_0.98N/In_0.16Ga_0.84N multiple quantum well sample. A new way of analyzing the data in such a variable stripe length method gain experiment is used to analyze both the time-integrated and time-resolved spectra. We confirm that the stripe length dependence of the gain in the multiple quantum wells under nanosecond excitation is caused by the change of the chemical potential along the excited stripe due to the interaction of the carrier and photon densities, and the gain threshold density is estimated. A trial function assuming a Lorentzian line shape for the stripe length dependence of the gain is compared with the edge emission intensity. This is found to fit very well with our data, even beyond the saturation region. Furthermore, we have extended the investigation to examine the dynamics of the emission and gain. These measurements suggest that the photoexcited carriers must localize (possibly at indium-rich sites) before strong stimulated emission is seen.     

Effect of laser dye deterioration on performance

A previous analysis that described loss of gain in a dye solution as a result of photochemical decomposition by pump photons is refined and is extended to the situation where impurities absorb at the pump wavelength. A limitation in the first paper is that it described only a single absorption depth in the medium at a constant pump fluxI _p. The present examination includes the fall-off ofI _p and integrates the total signal output with depth for a single-pass amplifier operated in the fully saturated gain condition. The equations are simpler, but as before loss of output, expressed in terms of a quantum yield of laser decayQ _L, is minimized for a photochemically bleachable dye, and for one with high gain. Pumping in the vicinity of the absorption maximum is also advantageous.     

Improved gain on the Cl2(D′3 П g −A′3 П u ) transition at 258 nm by halogen donor mixing

The temporal development of the small-signal gain on the Cl₂(D–A) transition at 258 nm has been investigated by means of an amplified spontaneous emission (ASE) technique. For electron beam pumped He/Cl₂ mixtures, the gain appears only at the end of the pumping pulse, whereas for He/Cl₂/CCl₄ mixtures the temporal gain profile coincides with the fluorescence pulse, and the maximum gain coefficient increases by about a factor of two. The observed effects are due to the mixing of both halogen donors and can be explained by considering the quenching of theD-state by electrons.     

Study of the gain saturation effect on the propagation of dark soliton in Er-doped, Ga5Ge20Sb10S65 chalcogenide fiber amplifier

We study the effect of gain saturation on the propagation of fundamental dark soliton in a nonlinear, dispersive and amplifying medium. The Er^+ 3-doped, Ga₅Ge₂₀Sb₁₀S₆₅ chalcogenide glass is used for dark and erbium doped silicon glass for bright solitons. The numerical simulations show that dark soliton doesn't split to subpulses unlike bright soliton and also the dark soliton is more stable in the presence of gain saturation and gain dispersion effects. So the chalcogenide glasses are suitable for designing all optical devices.