Laser saturation grating phenomena

Spatial hole burning of population differences by standing waves is considered in terms of Bragg-like gratings. This philosophy is used to explain increased saturation in two-mirror, single-mode laser operation with stationary active systems, modified mode coupling in the corresponding two-mode operation, an intensity dip that might occur in flowed standing wave lasers, bistable unidirectional ring laser operation, and increased saturation in distributed feedback lasers. The analysis is developed for both stationary media and for those moving with respect to the standing wave. The latter treatment is used to interpret washout of grating contributions in Doppler broadened media, and to determine level decay constants in stationary media. This last application constitutes a stationary system analog to the Doppler medium's Lamb dip spectroscopy and can be called saturation grating spectroscopy. Knowledge of the decay constants is particularly important in laser studies involving coherent mode couplings such as due to saturation grating scattering and population pulsations. It is further shown that the same equations result for probe and saturating waves propagating in the same direction. One then obtains a signal absorption with a heterodyne advantage. Inasmuch as diffusion affects the two methods quite differently, this phenomenon should be easily examined. Work supported in part by a U.S. Senior Scientist Award (administered by the Alexander von Humboldt Stiftung) and in part by the Space and Missile Systems Organization, Los Angeles, California.     

Self-pulsing phenomena in (GaAl) as double-heterostructure injection lasers

Repetitive self-pulsations under CW operation in the frequency range of 200 MHz–2 GHz are observed in many (GaAl)As double-heterostructure injection lasers. About 100 lasers of both low-mesa stripe geometry and with proton-implanted structures have been studied and about half of them exhibited more or less pronounced self-induced pulsations. The experimental results on three typical lasers are discussed more in detail on the basis of existing theories. Self-induced pulsations have often been observed in the non-linear region of the laser light output characteristic. The most probable reason for the instabilities appears to be a repetitive Q-switching process.     

Polarization instability in lasers with weakly anisotropic cavities

A theoretical study is made of the influence of the active medium and the cavity parameters, as well as that of an external magnetic field, on the properties of spontaneous pulsation regimes of lasers with weakly anisotropic cavities. Our results provide a physical interpretation of recently reported experimental data for spontaneous intensity pulsations of orthogonal components of the electromagnetic field generated by a single-mode, Fabry-Perot, HeNe gas laser at λ = 3.3922 μm and by a HeXe laser at λ = 3.51 μm.     

Laser intracavity absorption spectroscopy

Emission spectra of multimode lasers are very sensitive to spectrally selective extinction in their cavity. This phenomenon allows the quantitative measurement of absorption. The sensitivity of measurements of intracavity absorption grows with the laser pulse duration. The ultimate sensitivity obtained with a cw laser is set by various perturbations of the light coherence, such as quantum noise, Rayleigh scattering, four-wave mixing by population pulsations, and stimulated Brillouin scattering. It depends on the particular laser type used, and on its operative parameters, for example pump power, cavity loss, cavity length, and length of the gain medium. Nonlinear mode-coupling dominates the dynamics of lasers that feature a thin gain medium, such as dye lasers, whereas Rayleigh scattering is more important in lasers with a long gain medium, such as doped fibre lasers, or the Ti:sapphire laser. The highest sensitivity so far has been obtained with a cw dye laser. It corresponds to 70000 km effective length of the absorption path. The ultimate spectral resolution is determined by the spectral width of mode emission, which is 0.7 Hz in this dye laser. High sensitivity and high temporal and spectral resolution allow various practical applications of laser intracavity spectroscopy, such as measurements and simulations of atmospheric absorption, molecular and atomic spectroscopy, process control, isotope separation, study of free radicals and chemical reactions, combustion diagnostics, spectroscopy of excited states and nonlinear processes, measurements of gain and of spectrally narrow light emission. Intracavity absorption in single-mode lasers shows enhanced sensitivity as well, although not as high as in multimode lasers. Received: 10 May 1999 / Published online: 29 July 1999     

Very large-core, single-mode, gain-guided, index-antiguided fiber lasers

Lowest-order, single-mode laser oscillation is reported in gain-guided index-antiguided fiber lasers having core diameters from 100 to 400 microm. A model is presented explaining how to select resonator mirrors to assure single-mode operation.     

Quasi-self-imaging planar waveguide lasers with high-power single-mode output

We describe high-power planar waveguide laser which can achieve single-mode output from a multi-mode structure. The planar waveguide is constructed with incomplete self-imaging properties, by which the coupling loss of each guided mode can be discriminated. Thermal lens effects are evaluated for single-mode operation of such high-power diode-pumped solid-state lasers.     

Raman fiber laser pumped by a semiconductor disk laser and mode locked by a semiconductor saturable absorber mirror

A 1.6μm mode-locked Raman fiber laser pumped by a 1480nm semiconductor disk laser is demonstrated. Watt-level core pumping of the single-mode fiber Raman lasers with low-noise disk lasers together with semiconductor saturable absorber mirror mode locking represents a highly practical solution for short-pulse operation.     

Optical modeling of laterally-corrugated ridge-waveguide gratings

The paper presents some model improvements for the optical simulation of laterally-corrugated ridge-waveguide distributed feedback lasers. Simulation results are discussed and design principles for achieving single-longitudinal-mode operation are outlined. The effects of the laterally-corrugated ridge geometry both on the coupling coefficient and on the Bragg wavelength of different transverse modes are presented. The improved modeling has been used to design 980 nm distributed feedback lasers with laterally-corrugated ridge-waveguide third-order gratings. The lasers fabricated using nanoimprint lithography exhibited single-mode operation with 50 dB side-mode suppression ratio.     

均匀加宽注入锁定环形激光器的半经典理论—小信号分析

本文用拉姆(Lamb)的半经典理论分析了在小信号情况下均匀加宽注入锁定环形激光器的单模运转问题.     

Intensity and polarization self-pulsations in vertical-cavity surface-emitting lasers

We implement a dynamic model that describes the polarization behavior in vertical-cavity surface-emitting lasers that contain an absorbing region surrounding the active zone. We find four regions of qualitatively different behavior: stable linearly polarized operation, intensity pulsations of a linearly polarized component, pulsations of both total-intensity and polarization components, and polarization self-pulsation with constant total intensity. We characterize the four regions by computing the polarization-resolved optical and power spectra. The predicted behavior agrees with recent experimental results.     

Single mode operation of cw dye lasers

This paper describes an investigation of the special problems of single mode operation in cw dye lasers. Single mode operation in homogeneously broadened lasers can be obtained, if the gain due to spatial hole burning is compensated by dispersive elements, as prisms, Fabry-Pérot etalons and single dielectric interfaces within the resonator. The gain and the influence of the dispersive elements were calculated for two commonly used types of resonators. The calculations make it possible to design simple continuously tunable single-mode systems.     

Low-frequency pulsations in class-B solid-state lasers with delayed feedback

We report on the numerical observation of short-duration low-frequency pulsations in class-B solid-state lasers subject to delayed feedback. The period of these pulsations is much larger than either the delay induced by the feedback or the relaxation oscillation period of the laser without feedback. A link is established between the low-frequency pulsations and the low-frequency fluctuations observed in semiconductor lasers subject to coherent optical feedback.     

Near-infrared sources in the 1–1.3 μm region by efficient stimulated Raman emission in glass fibers

Low-loss glass fiber waveguides are found to be excellent media for Raman lasers and amplifiers in the near-infrared region of the spectrum. Multiwavelength emission in the 1–1.3 μm range is readily obtained by efficient stimulated Raman scattering in single-mode silica fibers. With a 1.064 μm pulsed pump of 250 W in a 175-m, 6-μm diameter single-mode silica fiber we observed four orders of Stokes radiation at 1.12 μm, 1.18 μm, 1.23 μm and 1.3 μm, respectively. Our results imply that pulsed tunable stimulated Raman emission in this wavelength region is possible using kW tunable infrared dye lasers near 1 μm as pumps. These sources are useful for studying the dispersion of glass fibers as well as for other spectroscopic applications.     

Efficient operation of diode-pumped single-frequency thulium-doped fiber lasers near 2 micro m

Efficient operation of diode-pumped single-frequency fiber lasers at wavelengths from 1740 to 2017 nm has been demonstrated by using a very short piece of newly developed single-mode active fiber, i.e., heavily thulium-doped germanate glass fiber. At 1893 nm, the single-frequency fiber laser has a pump threshold of 30 mW, a slope efficiency of 35%, and maximum output power of 50 mW with respect to the launched power of single-mode pump diodes at 805 nm. To the best of our knowledge, this is the highest lasing efficiency achieved in single-frequency fiber lasers operating near 2 micro m. Frequency noise of the single-frequency fiber laser at 1893 nm has been characterized and compared with that of single-frequency fiber lasers at 1 and 1.55 micro m.     

A high-power single-mode cw dye ring laser

Due to spatial hole burning, standing-wave dye lasers require a large amount of selectivity inside the cavity for single-mode operation. The output power of these lasers is limited by losses caused by the frequency selecting elements. In a travelling-wave laser, on the other hand, spatial hole burning does not exist, thereby eliminating the need for high selectivity. A travelling-wave cw dye laser was realized by unidirectional operation of a ring laser, yielding single mode output powers of 1.2 W at 595 nm and of 55 mW in the UV-region with intracavity frequency doubling.     

单模光纤激光极限功率的数值研究

对光纤激光极限功率的探索和其受限因素的分析, 有利于为大功率光纤激光器的发展提供理论依据和实验指导. 本文考虑热效应、光效应、非线性效应和抽运亮度等因素对光纤激光极限功率的影响, 分析了掺镱和掺铥光纤的极限功率和受限因素. 在此基础上, 结合激光在光纤中单模传输的条件, 计算了单模掺镱和掺铥光纤激光的极限功率. 计算结果表明, 在现有技术条件下, 使用常规的976 nm和793 nm激光二极管抽运, 单模掺镱和掺铥光纤激光的极限功率分别为4.2 kW和7.8 kW, 其中单模掺铥光纤激光的功率水平还远低于它的极限功率的原因是受抽运亮度的限制. 最后分析指出减小纤芯的数值孔径和改进少模光束的光束质量是提升单模光纤激光极限功率的重要途径.     

Experimental study of phase-locking of two photonic crystal fiber lasers

Two photonic crystal fiber (PCF) lasers are phase-locked by using self-imaging resonator techniques. Obvious and steady interference strips are observed even without using spatial filter in high power operation, which demonstrates the advantages of pure single-mode PCF lasers in coherent beam combining. The maximum coherent output power of 95.8 W is obtained with coupling efficiency of 90.2%.     

Proposed enhancement of single mode operation in VCSELs using diffused quantum well structure

The use of interdiffused quantum wells to enhance single transverse mode operation in vertical cavity surface emitting semiconductor lasers is proposed and analysed theoretically. It is found that by introducing a step diffused quantum well structure inside the core region of a quantum-well active layer, the influence of self-focusing (due to carrier spatial hole burning and thermal lensing) on the profile of transverse modes can be minimized. Therefore, stable single-mode operation in vertical cavity surface emitting lasers can be maintained.     

Laterally-corrugated ridge-waveguide distributed feedback lasers at 980 nm

The paper presents a simulation-based analysis of laterally-corrugated ridge-waveguide distributed feedback semiconductor lasers emitting at 980 nm. The simulations were performed using software developed in-house and the PICS3D software package from Crosslight Software Inc. The effects of the corrugation geometry, phase-shift section, and mirror reflectivities on single longitudinal mode operation are discussed. The lasers, designed along the guidelines derived from the simulation results, were fabricated by using molecular beam epitaxy for wafer growth and low-cost nano-imprint lithography. They exhibited stable single-mode operation with up to 50 dB side-mode suppression ratio.     

Ultrashort-pulse sources based on single-mode rare-earth-doped fibers

An overview of ultrashort-pulse sources based on single-mode rare-earth-doped fibers is given. A wide range of pulse-generation schemes comprising mode-locked fiber lasers, parametric pulse sources and hybrid diode-fiber amplifier sources are discussed. Both actively and passively mode-locked fiber lasers are described and their specific merits and operation regimes are elucidated. Techniques for improving the spectral quality and the output powers of diode-based systems based on amplification in rare-earth-doped fibers are also reviewed. Finally, applications are discussed and directions for future research are indicated.