Active mode locking with hybrid lasers

We present the results of a numerical study of active mode locking with hybrid lasers which contain an inhomogeneously broadened laser medium and an homogeneously broadened laser medium. The spectral, pulse, and gain characteristics of actively mode-locked hybrid lasers and the influence of the unsaturated gain, the saturation power, and the homogeneous linewidth on the pulse coherence and bandwidth are studied. The simulations show that coherent and shorter pulses are generated as compared to that by either an inhomogeneously broadened medium or a homogeneously broadened medium alone. Varying the unsaturated gain or the saturation power of the gain medium are two equivalent ways to obtain the same maximal coherent pulse bandwidth for given gain media. When different gain media can be selected, a larger pulse bandwidth can be obtained with the use of a broadband homogeneously broadened medium     

Application of spectral-hole burning in the inhomogeneously broadened gain of self-assembled quantum dots to a multiwavelength-channel nonlinear optical device

A nonlinear device capable of handling multiple wavelength channels simultaneously by using spectral-hole burning in the inhomogeneous broadening of the gain in self-assembled quantum dots is proposed. The multichannel property of this device stems from the localized absorption or gain change at the pump wavelength within the inhomogeneously broadened gain spectrum. The spectral width of the modulation and its time evolution was also evaluated. The result indicates the applicability of this device to multichannel operation.     

Dispersion effects in an actively mode-locked inhomogeheously broadened laser

We theoretically and experimentally studied the dispersion effects in an actively mode-locked inhomogeneously broadened laser. In the positive group velocity dispersion (GVD) region, the laser generates incoherent pulses. Self-phase modulation and resonant dispersion impede reduction of the pulse duration when the GVD is small or near zero. A single, stable, soliton-like pulse can be generated only when the GVD is within a certain range of negative values. When the negative GVD is too small, the laser generates only multiple soliton-like pulses because of excess gain filtering. When the negative GVD is too large, the soliton pulse-shaping process fails, and the laser generates only incoherent pulses due to insufficient gain filtering. In the experiment, we characterized an actively mode-locked inhomogeneously broadened Nd:silicate glass laser and confirmed this GVD dependence. The laser generated self-sustaining, soliton-like pulses as short as 77 fs.     

Continuous-wave lasing of hybrid lasers

The continuous-wave (CW) lasing of hybrid lasers, which contain a broad-band inhomogeneously broadened laser medium and a narrow-band homogeneously broadened laser medium in a single cavity, is analyzed theoretically. The interactions of the laser modes and two gain media are solved self-consistently using the coupled rate equations. The spectral, gain, and power characteristics under different gain conditions are simulated. It is shown that a small gain from a second narrowband laser medium can effectively shape the lasing spectrum and improve the spectral concentration of the hybrid laser. The total saturated gain profile of the hybrid laser is relatively smooth, due to saturation of the gain of the narrow-band medium to a smaller and below-threshold level     

高增益激光的饱和效应和最佳输出耦合

本文在均匀和非均匀谱线加宽情况下,计算了高增益介质在不同小信号增益系数和吸收系数下的激光输出特性,并讨论了饱和效应和最佳输出耦合。     

Description of laser pulses using Fourier analysis

The output of lasers in various modes of operation is described in a unified way, using Fourier analysis. A novel model for treating an active Fabry-Perot cavity, including the case of high gain, is developed resulting in a more exact expression for the spectral width of the cavity modes. Simple mathematical tools are given to relate the observed pulse length with the average gain and with the initial population inversion in the laser medium. It is shown that lasers with homogeneously broadened spectral lines have cavity modes of equal width and that if the modes are locked together the resulting ultrashort pulses are of equal duration. For lasers with inhomogeneously broadened spectral lines the cavity modes vary in width along the gain curve and, when mode-locked, the ultrashort pulses increase monotonically in duration along the emitted pulse train.     

连续波DF激光器光腔温度和相对粒子数范围估算

通过讨论连续波DF激光器谱线小信号增益随光腔温度和相对粒子数变化，指出随光腔温度和相对粒子数增加，谱线小信号增益最大值转动量子数和短波方向零小信号增益转动量子数向高转动态移动，且与一定范围的光腔温度和相对粒子数对应。整数零小信号增益转动量子数较难取得，结合实测光谱给出了替代条件:谱线短波方向相对强度最小截止转动量子数JL小信号增益大于零和假定转动量子数JL-1谱线小信号增益小于零。根据DF化学激光器实测光谱三个谱带所表征光腔温度范围相等，估算出某连续波DF激光器光腔温度范围为305～368 K，1P、2P和3P谱带相对粒子数范围分别为:1.77～2.09、1.65～2.05和1.19～1.72。     

Temperature dependence of electrical and optical modulationresponses of quantum-well lasers

We present theory and experiment for high-speed optical injection in the absorption region of a quantum-well laser and compare the results with those of electrical injection including the carrier transport effect. We show that the main difference between the two responses is the low-frequency roll-off. By using both injection methods, we obtain more accurate and consistent measurements of many important dynamic laser parameters, including the differential gain, carrier lifetime, K factor, and gain compression factor. Temperature-dependent data of the test laser are presented which show that the most dominant effect is the linear degradation of differential gain and injection efficiency with increasing temperature. While the K-factor is insensitive to temperature variation for multiple-quantum-well lasers, we find that the carrier capture time and nonlinear gain suppression coefficient decreases as temperature increases     

Phase variations due to dispersion in inhomogeneously broadened lasers

One-dimensional analytical solutions for irradiance and phase are obtained for a two-level ring resonator model of a long inhomogeneously broadened chemical laser with strong velocity changing collisions (momentum cross relaxation). The solutions give the longitudinal mode spectrum of the laser line in terms of the ratio of elastic to inelastic collision rate, the Doppler linewidth, the product of small-signal gain times the length of the gain medium, and the resonator loss. The intensity is given in terms of the width of the mode spectrum, the saturation intensity, and the gain-length product. The phase in a single pass is given in terms of the mode spectrum, the loss, and the ratio of homogeneous to inhomogeneous width. For typical conditions the phase difference relative to line center for a detuning frequency near the edge of a laser line is less than about one twentieth of a wavelength for a single pass through the resonator. Application to unstable resonator geometries suggests that the total phase difference at the edge of such a laser may reach a few tenths of a wave, although most of the power in the line has a much smaller phase variation.     

The impact of energy band diagram and inhomogeneous broadening on the optical differential gain in nanostructure lasers

We present a general theoretical model for the optical differential gain in semiconductor lasers. The model describes self assembly quantum dots (QDs), self assembly quantum wires (QWRs) and single quantum-well lasers. We have introduced the inhomogeneous broadening due to size fluctuations in the assembly cases. At each dimensionality, we have considered the carrier populations in the excited states and in the reservoirs, where conduction and valence bands are treated separately. We show that for room temperature operation the differential gain reduction due to increased size inhomogeneity is more pronounced in QDs than in QWRs. We show this reduction to be smaller than the one-order reduction attributed to state filling in conventional dot and wire assemblies operating at room temperature. The integration prefactor coefficient of the differential gain in zero-dimensional cases exceed one- and two-dimensional coefficients only for low temperatures where the homogenous broadening is considerably smaller than the thermal energy. The differential gain of QDs, QWRs, and compressively strained single quantum-well lasers operating at room temperature and close to equilibrium is nearly the same.     

Saturation effects in semiconductor lasers

This paper describes a theory for a semiconductor active medium interacting with a laser field. In a semiconductor laser, the charge carrier transitions are inhomogenously broadened, and electron-electron and electron-phonon collisions tend to dephase the laser transitions and maintain thermal equilibrium among the carriers. These properties cause semiconductor lasers to frequency tune as though they are inhomogeneously broadened and to saturate as though they are homogeneously broadened. A theory that contains these two aspects of semiconductor laser behavior is presented. From it, we are able to calculate the loaded gain, efficiency, intensity, and carrier-induced refractive index of a semiconductor active medium.     

Theory of line narrowing and frequency selection in an injection locked laser

We apply the multimode semiclassical laser theory to the problem of line narrowing and frequency selection in an injection locked laser. Our goal is to predict from the spectroscopic properties of a given active medium, the behavior of a broad-band laser that is under the influence of a narrow-band injected signal. Analytic expressions for the injected signal strength necessary for single-mode operation are obtained for both homogeneously and inhomogeneously broadened lasers, and for small-signal and strong-signal regimes. We found that under certain experimental conditions, an injection locked inhomogeneously broadened laser behaves significantly different from a homogeneously broadened lasel.     

Effect of spin-orbit split-off bands on GaInP/AlGaInP strainedquantum well lasers

We theoretically analyzed the effect of the spin-orbit split-off band on GaInP/AlGaInP strained quantum well lasers using 6×6 Luttinger-Kohn Hamiltonian. Because of the small spin splitting energy of GaInP, we show that the spin-orbit split-off subbands strongly couple with heavy and light hole subbands, and linear gain properties are greatly different from those without the effect of spin-orbit split-off bands. The unstrained quantum well structure is most influenced by the spin-orbit split-off bands, and laser characteristics such as the differential gain and threshold current are degraded. The compressive-strained quantum well has the lowest threshold current, and the tensile-strained quantum well has largest differential gain, which is improved by the effect of the spin-orbit split-off bands     

Excitation spectra of the green Ho:fluorozirconate glass fiberlaser

Laser and fluorescence excitation spectra of the green Ho:ZBLAN glass fiber laser have been measured at room temperature by pumping 20-90-cm-long fibers in the 640-653-nm spectral region with a CW dye laser. Pronounced structure in the laser excitation spectrum appears to arise from Stark sublevels of the Ho³⁺⁵F₅ and ⁵I₈ states rather than excited state absorption. Because the overall gain profile is inhomogeneously broadened, the laser excitation spectrum width increases with pump power and is ~5.7-nm FWHM when the pump power is a factor of 2.3 above threshold. For a 22-cm fiber, the threshold pump power is 128±5 mW and, with 5% output coupling, more than 12 mW has been extracted in the green for 318 mW of 646.7 nm input power. The slope efficiency for this oscillator (accounting for the pump launch efficiency) exceeds 14%     

Automatic gain flattening in optical fiber amplifiers via clampingof inhomogeneous gain

An Er-doped fiber amplifier with a flattened gain spectrum is obtained by incorporating the amplifier into a ring laser and making use of inhomogeneous broadening at 77 K. The results indicate several potential benefits for doped-fiber material systems that are inhomogeneously broadened at room temperature     

Multiwavelength generation in an erbium-doped fiber laser using in-fiber comb filters

Simultaneous continuous wave multiwavelength operation of an erbium-doped fiber laser has been demonstrated using two types of recently developed in-fiber grating comb filters. The lasing wavelengths and linewidths were determined by the comb filter, which was inserted in a ring cavity with a single inhomogeneously broadened gain medium.     

Theoretical studies of GaInP-AlGaInP strained quantum-well lasersincluding spin-orbit split-off band effect

We studied the GaInP-AlGaInP strained quantum-well laser characteristics, taking into account the spin-orbit split-off bands. When the well width is kept constant at 85 Å, the threshold current of the unstrained quantum-well structure is most degraded by the effect of the spin orbit split-off subbands, due to the larger hole density of states near the band-edge. While the linear gain of the compressive-strained quantum well is slightly changed by the effect, it has the lowest threshold current with the lower threshold gain. In the tensile-strained quantum-well structure, the spin-orbit split-off bands improve the differential gain because they increase the density of states at the valence band-edge. When the lasing wavelength is fixed at 630 nm, the threshold current of the compressive-strained quantum well is the lowest as well. The tensile-strained quantum well has lower threshold current than the unstrained quantum well, and this phenomena is not observed in the analysis without the spin-orbit split-off bands. However, the reduction of threshold current of the tensile-strained quantum well is smaller than that of compressive-strained quantum well, The tensile strain is more preferable for high speed modulation because of its large differential gain, due to the mixing between the light hole and the spin-orbit split-off subbands     

A sampling series representation of the gain and refractive-index formulas for a combined homogeneously and inhomogeneously broadened laser line

The well-known sampling theorem of communication theory and the associated sampling-series representation of a function are used to derive series representations for the gain and refractive index associated with a laser line that may, in general, be both homogeneously and inhomogeneously broadened. The representations obtained provide the basis for an efficient and highly accurate algorithm for computing numerical approximations to the gain and refractive-index integrals.     

Optimization of long wavelength InGaAsP strained quantum-welllasers

A theoretical study of InGaAsP-InGaAsP multiple quantum-well lasers emitting at 1.55 μm has been carried out to investigate the variation of threshold current density and differential gain with strain, well width and well number. We show that the greatest scope for exploiting this quaternary alloy in laser structures is through the use of compressive wells with unstrained or tensile barriers. We consider structures with a fixed compressive strain of 1% but variable well width, and also with fixed well width but variable strain from 0% to 1.75%. For structures with 1% compressive wells and unstrained barriers we find that the optimum structure for lowest threshold current density with sizable differential gain consists of six 35-Å quantum wells. We find also that there is little benefit to having compressive strains greater than 1.2%. In addition we examine zero-net-strain (ZNS) structures with compressive wells and tensile barriers. We show how the conduction band offset can be significantly increased and valence band offset reduced in such structures. Our gain calculations suggest that the large modification in band offset can decrease the threshold current density compared to similar devices with unstrained barriers     

Theory of a Zeeman ring laser--Part II: Special cases

We apply the Zeeman ring laser theory to the case of four-mode operation in an inhomogeneously broadened medium in the Doppler limit. The coupling parameters between the four laser modes are studied quantitatively. Reductions to the standing wave laser and the scalar ring laser cases are made. The case of operation in a homogeneously broadened medium is also treated.