Theory of spin-flip laser modes--I: Threshold modulation and tuning behavior

Theoretical calculations of threshold intensity and output frequency, as functions of magnetic field, are presented for the InSb spin-flip Raman laser (SFRL). These are based on a numerical solution of Maxwell's equations for the case of a pump beam of Gaussian profile, and various analytical results for more approximate models. The results are used to discuss nonlinear tuning characteristics, mode jumping, and the dependence of threshold power upon beam size. These are the first calculations to include the effects of finite beam size, which are found to be quite significant and consist chiefly of a marked asymmetry of the variation of threshold power and tuning rate across each mode. This asymmetry is in qualitative agreement with recent experimental observations. The model may be readily generalized to include higher modes and extended to the case of constant input power.     

Spin saturation and pump depletion in continuous spin-flip Raman oscillation

A saturation model of stimulated Raman scattering, of general applicability, is put forward. The model is developed with reference to spin-flip electronic Raman scattering in InSb, where the number of excitations available for Raman scattering may be small by comparison with the incident photon numbers. A rate equation technique is used to evaluate the steady-state Stokes intensity and output powers for a Gaussian pump beam. The model is extended to take account of pump depletion and the relative importance of depletion, and saturation in limiting the conversion efficiency under different conditions is brought out. By comparison with continuous wave spin-flip measurements, values are obtained for the spin-relaxation time (τs) associated with spin reversal in InSb. For 10¹⁶free carriers per cubic centimeter at 36 kG,tau_{s} sim 12ns and for 10¹⁵cm^-3at 16.9 kG,tau_{s} sim 1.3ns.     

Higher order transverse modes of an unstable-cavity laser

We present measurements of the higher order transverse modes of an unstable-cavity HeXe gas laser. We have studied the one-dimensional strip-resonator modes and the two-dimensional square- and circular-resonator modes. The experimental intensity profiles showed generally good agreement with theoretical profiles based upon virtual-source diffraction theory. We also measured the quantum-limited linewidths and excess-noise factors of several of these modes     

On the saturation effects and start jump of Gaussian modes in oscillators

The present paper deals with the interaction of a Gaussian mode with a homogeneous and an inhomogeneous laser transition. The interaction of the beam with the medium in a laser is treated by taking into account the spatial distribution of both radiation and gain. The intensity characteristics are very different from those obtained for a one-dimensional interaction of a plane wave with a saturating medium. In the presence of a small-signal gain profile the threshold condition requires much higher inversion densities along the optic axis than by ignoring this profile. For gas lasers, for instance, having a small-signal gain profile that is approximately described by a zero-order Bessel function, the threshold inversion density can be about 50 percent higher. For high-power systems the saturation of the medium by the Gaussian intensity distribution results in a considerable amount of radial radiation transport. For homogeneous transitions this amount is about equal to the stimulated emission and for inhomogeneous transitions it is about half of it, independent of the beamwidth. Further, it is found that if one slowly passes the threshold condition for laser action, the intensity jumps from zero to a certain value and vice versa. This effect has also been verified experimentally.     

Enhanced polarisation self-switching in a vertical-cavitysurface-emitting laser by gain saturation of transverse modes

Enhanced polarisation self-switching is observed in a vertical-cavity surface-emitting laser by involving gain saturation due to transverse mode competition. Switching between orthogonal polarisations is triggered by a quarter-wave plate external cavity and does not change the total output power     

A theoretical study of saturation effects in three-level laser systems

Exact numerical solution of a time-space (one dimensional) three-level lasing system (homogeneously broadened) is used to study the relevance of steady-state models to such a system. This study revealed: 1) the existence of two disconnected (quasi) steady-state regions-the small-signal gain region (slow pumping rate) and the high saturation region (large pumping rate), and 2) the spatially averaged equations for the high saturation region have a form similar to those usually employed in the small-signal gain region, except that a constant which in the latter case is unity becomes smaller than 1 in the high saturation region.     

Gain saturation effects in supermodes of phase-locked semiconductor laser arrays

A basic modal analysis that includes gain saturation effects in phase-locked semiconductor laser arrays is presented. For a particular supermode operation, different lasers in the array emit different amounts of light, and hence their (waveguide) propagation constants are modified differently. Solving the lasers' rate equations self-consistently with the coupled-mode wave equations seems to provide an answer that is in much better agreement with experimental results than the result using only the coupled-mode analysis.     

Gain dispersion and saturation effects in four-wave mixing insemiconductor laser amplifiers

This paper addresses four-wave mixing (FWM) in semiconductor laser amplifiers from the point of view of a propagation problem. The gain dispersion effect, i.e., the difference of the gain factors for the pump, probe and signal waves is shown to be significant in the case of large detunings >1 THz. It is given an analytical solution of the FWM problem including gain dispersion and saturation effects. Considering the saturation behaviour, it is shown that the linear gain factors for the different waves and the nonlinear susceptibilities associated with the different nonlinear effects must be characterized by different carrier densities at transparency. A comparison of our theory with a numerical model, with previous approaches and with experimental data is given     

Gain saturation in Nd:doped laser materials

Saturation of gain in Nd:doped silicate, phosphate, and fluorophosphate glasses, and in doped crystals of YAG and YLF was observed during amplification of 1064 and 1053 nm pulses with durations of 1.0, 9.0, and 50 ns. Saturation fluences were computed from measured external parameters by means of the Frantz-Nodvik model for a homogeneous amplifier. The resulting values of saturation fluence increase with increasing output fluence and were only slightly greater at 50 ns than at 1 ns, suggesting that the lower level lifetime is less than 1 ns.     

Gain saturation in semiconductor lasers: Theory and experiment

The semiconductor stimulated gain saturation model of Zee has been extended using reasonable approximations to obtain an analytical solution for the gain saturation process in PbSnTe and to determine the limit to single mode power directly from the gain expression, the intraband relaxation time, and device and material parameters. The theoretical results are compared with experimental observations for single transverse mode cavity narrow stripe buried heterostructure PbSnTe lasers. Those results are interpreted in terms of an intraband relaxation time on the order of2 times 10^{-12}s in the temperature range 20-80 K.     

Gain saturation of CW laser pumped FIR laser gases

The saturation behavior of the 70 μm and the 119 μm transitions in CH3OH has been investigated both experimentally and theoretically. This knowledge is essential for constructing a complete model of the CW laser pumped FIR laser. Saturated gain measurements made in a single pass amplifier cell were found to be in good agreement with the theory of a quantum-mechanical three-level system in resonant interaction with two coherent fields. The calculation of the gain using the density matrix formalism yields a pump intensity dependent saturation, a result which is not predicted by the rate equation models.     

激光模传输的方向性

用测不准关系⊿S⊿Ω≥λ~2可以对TEM_(oo)模激光方向性进行讨论,但却不适用于高阶模激光传输方向性的讨论。若把激光振荡看作光子在谐振腔中振荡的线性谐振子,高阶模的测不准关系便可求得,并且与已有实验结果一致。     

钛宝石激光器的多模式运转

在一台双光束泵浦的钛宝石激光器中实现了可调谐连续输出、独立自锁模、交叉锁模和多脉冲模运转式。分析了可调谐连续输出模式下两个激光腔的增益竞争,研究了飞秒和皮秒激光腔内的群速弥散、自相位调制、交叉相位调制以及自振幅调制。结果表明,可调谐连续输出模式下两个激光腔的输出主要取决于泵浦激光的功率,独立自锁模工作模式下,群速弥散和自相位调制决定了自锁模的稳定性;交叉锁模工作模式下,交叉相位调制是影响飞秒和皮秒脉冲之间同步性的主要因素。多脉冲工作模式下,脉冲自相关曲线的测量结果表明,飞秒脉冲仍然是单脉冲,而皮秒脉冲分裂为3个次脉冲。     

Gain saturation of the carbon dioxide laser

The saturated gain constant of the carbon dioxide laser is calculated using a thermodynamic approach. At low electron densities the gain constant is found to saturate according to a well-known and simple function. At higher electron densities, however, a more complicated form is assumed. The calculations are found to agree well with measured saturated gain.     

Gain and saturation power of resonant AlGaAs laser amplifier

Results of gain measurements on an AlGaAs laser amplifier with an optical isolator are reported. The maximum net gain was 25 dB when input power was less than ?40 dBm. By measuring internal loss, a saturation parameter was estimated to be 6 mW.     

Higher order modes in coupled striplines: prediction andmeasurement

Finite element and resonant cavity methods are used to predict and confirm the cut-off frequencies of higher order modes in stripline. Infinite elements are used to treat open-wall boundaries. Single lines, symmetrically coupled and asymmetrically coupled lines are treated in turn. In the tightly coupled situation the cut-off frequency of the lowest TE mode degenerates to that of an abutted combined linewidth. Established approximation methods for single lines are used where possible. Some practical observations are made and selected finite element field plots are included for completeness     

Competition and stimulated switching of transverse laser modes

A 6328-Å He-Ne laser operating in the fundamental transverse mode may be switched to second order mode operation by injection of a small second order mode signal. Experiments are reported and results compared with theory.     

Effect of gain saturation on the oscillating modes of optical masers

Because an active maser medium exhibits nonlinear gain saturation, the oscillating modes of an optical maser are expected to be somewhat different from those of the passive resonator. Statz and Tang [5] have obtained some numerical results for an active resonator with a pair of parallel-plane, infinite-strip mirrors. We have reformulated the problem for active resonators with circular mirrors of both parallel-plane and confocal geometries and have obtained numerical results using an iterative method of solution. We find the cardinal features of the active modes, such as mode patterns, diffraction losses, and resonant frequencies, to be essentially the same as those of the passive modes, even for unsaturated gains as high as three and a half dB per pass. The mode that predominates in an active Fabry-Perot resonator is found to be the lowest-order (TEM00) mode. However, the predominating modes in an active confocal resonator are found to depend on the Fresnel number; the larger the Fresnel number, the higher is the mode order. The study includes computations of field distributions, diffraction losses, and phase shifts of the steady-state predominating modes and of their output intensities as functions of unsaturated gain, saturation parameter, mirror transmissivity, scattering loss, and resonator geometry.     

钠激光导星的饱和效应分析

针对自适应光学系统波面探测的应用需求,从平衡态和非平衡态的角度讨论了钠激光导星的回光强度。利用热平衡态下的状态方程,分析了平衡态时因入射信标激光增强所导致的钠激光导星饱和效应问题。结果表明,原子分数随辐射功率谱密度的增强而增大,其变化速率的下降是钠激光导星回光效率降低的根本原因。分析了信标激光器线宽、探测装置子孔径大小、积分时间对共振荧光回光探测的影响,模拟计算并得到了特定条件下能够满足自适应光学系统波面探测的信标激光器功率指标要求。