Output radiation focusing in curved-grating distributed Bragg reflector laser

Output radiation focusing in AlGaAs/GaAs quantum-well curved-grating distributed Bragg reflector (c-DBR) lasers has been theoretically and experimentally studied. It is established that the focal spot size for c-DBR lasers is determined to a considerable extent by the spectral perfection of laser radiation.     

Effect of argon buffer and the UV preionization regime on the electric-discharge XeCl laser parameters

The gas discharge and output radiation parameters were studied in XeCl lasers with various buffer gas compositions (argon vs. neon) and different types of excitation regimes (fast vs. quasi-stationary) in the preionization source. The intervals of specific energy for the neon-and argon-based buffer gas mixtures are 10–125 and 45–360 J/(l atm), respectively. The specific parameters of radiation in the neon-and argon-based buffer gas mixtures depend on the energy required for the UV preionization. For high-power UV preionization sources, the output parameters of lasers using argon-based gas mixtures may exceed those obtained with neonbased mixtures.     

Output-coupling optimization of Nd-doped fiber lasers

A simple theoretical modeling of the static properties of a fiber laser that includes distributed losses and inhomogeneous pumping is presented. Closed-form expressions for both the output and the backward (at the input mirror) intensities are obtained. The model is based on an extended formulation of the Rigrod's theory. It is shown that the laser responds differently depending on the length of the fiber. In particular, we show that for long (short) lasers optimal output power is achieved with low (high) output-coupler reflectivities. Experimental evidence of these results is obtained with Nd-doped fiber lasers with various lengths.     

Phase effects in a semiconductor laser with diffraction extraction of radiation

Phase effects arising in a semiconductor laser with diffraction extraction of radiation and a distributed Bragg reflector on the substrate side are taken into account exactly quantitatively and the possibilities of using these effects in lasers is analyzed. It is shown that the phase effects studied can be used to increase the laser efficiency substantially. Pis’ma Zh. Tekh. Fiz. 23, 7–11 (September 26, 1997)     

Phase Locking of CO(2) Lasers

A method of phase locking two CO(2) lasers by radiation exchange is presented. This phase-locking was achieved by use of a copper prism as a beam folding device in the resonators and extraction of the output radiation by a common output coupler. Energy exchange led to a phase-locked state if several locking conditions were fulfilled. The amount of radiation injected from one resonator to the second cavity could be adjusted by movement of the prism. The influence of the strength of coupling on the locking range was studied. The beat signal between the two unlocked lasers could be measured, whereas in the case of phase-locked operation twice the intensity was detected. Despite the inclusion of several assumptions, a simplified mathematical model delivered good agreement between calculated and experimental results.     

Long-term frequency stabilization of a continuous-wave tunable laser with the help of a precision wavelengthmeter

For the first time to our knowledge we experimentally demonstrate an efficient method for the reduction of long-term radiation line drift in single-frequency cw Ti:sapphire and dye lasers that relies on a fast and precise wavelengthmeter together with a digital-analog feedback system. Generation line drift of lasers is reduced approximately by an order of magnitude down to 40 MHz/h, which corresponds to the residual drift in readings of the wavelengthmeter itself. The implemented automatic frequency control system allows us to lock the laser generation frequency to a specified absolute value. This approach may be used in single-frequency lasers of different types (solid-state, fiber, diode, dye lasers, etc.) and allows reduction by an order of magnitude or more of the long-term generation line drift in lasers that are not equipped with other systems for long-term stabilization of output radiation frequency.     

Superfocusing of mutimode semiconductor lasers and light-emitting diodes

The problem of focusing multimode radiation of high-power semiconductor lasers and light-emitting diodes (LEDs) has been studied. In these sources, low spatial quality of the output beam determines theoretical limit of the focal spot size (one to two orders of magnitude exceeding the diffraction limit), thus restricting the possibility of increasing power density and creating optical field gradients that are necessary in many practical applications. In order to overcome this limitation, we have developed a method of superfocusing of multimode radiation with the aid of interference. It is shown that, using this method, the focal spot size of high-power semiconductor lasers and LEDs can be reduced to a level unachievable by means of traditional focusing. An approach to exceed the theoretical limit of power density for focusing of radiation with high propagation parameter M ₂ is proposed.     

Near-infrared distributed feedback solgel lasers by intensity modulation and polarization modulation

Near-infrared distributed feedback (DFB) laser actions of Oxazine 725 dye in zirconia thin films and in silica bulks were investigated. Intensity modulation and polarization modulation were used to generate the DFB lasing. Wideband tuning of the output wavelength was achieved by varying the period of the modulation generated by a nanosecond Nd:YAG laser at 532 nm. Tuning ranges were 716-778 nm and 724-813 nm for the thin film lasers and the bulk lasers, respectively. The laser output showed different polarization characteristics and threshold energy variation when the feedback mechanism was changed from intensity modulation to polarization modulation.     

On the Determination of Spectral Properties of Non-stationary Radiation

The measurement of the time-dependent spectrum of non-stationary radiation is of interest, for instance, in the field of ultrashort-time spectroscopy using picosecond lasers. The incident non-stationary radiation is characterized by the time-dependent input spectrum. The electric field strength of the radiation in the exit plane of a grating spectrometer and of a prism spectrometer is analysed in terms of its dependence on time and on the spectral coordinate. The action of the grating spectrometer is regarded as a special case of diffraction at an arbitrary diaphragm. The intensity registered by a square-law detector as a function of time and the spectral coordinate (output spectrum) is calculated as a function of the input spectrum. The input spectrum can be determined from the output spectrum by mathematical operations for cases important in practise (very short light-pulses, intrinsically stationary radiation). Moreover, the conditions are investigated under which the output spectrum resembles the input spectrum, taking into account the influence of noise. Two examples of intrinsically stationary radiation (bandwidth-limited pulse, fluorescence of a two-level system) are studied in more detail. For experimental verification the ouput spectrum of a single ultrashort light pulse has been measured under the condition of different spectrometer time constants.     

Wavelength tuning and spectral properties of distributed feedback diode lasers with a short external optical cavity

We report on the wavelength tuning and spectral properties of distributed feedback (DFB) diode lasers operated with a plane external cavity (XC) mirror positioned as close as possible to the diode-laser front facet. These lasers generate single-frequency near IR radiation at wavelengths of 1392, 1580, 1602, and 1653 nm. A piezoelectric variation of the XC length provided continuous single-frequency tuning to as high as 19 GHz. A further benefit of XC DFB lasers is a residual amplitude modulation per gigahertz tuning of less than 10(-3). The XC feedback also suppresses residual side-mode oscillations to less than 60 dB. The laser's total intensity noise is close to the shot noise limit. The laser linewidth (measured in a beat note experiment) is less than 90 kHz within an acquisition time of 40 ms. The advantageous properties of XC DFB lasers for molecular spectroscopy are demonstrated by recording R(3) 2nu(3) overtone spectra of methane by single-scan single-pass absorption or frequency-modulation spectroscopy.     

Multipass pumped Nd-based thin-disk lasers: continuous-wave laser operation at 1.06 and 0.9 microm with intracavity frequency doubling

The laser performances of the 1.06 microm (4)F(3/2) --> (4)I(11/2) four-level transition and of the 0.9 microm (4)F(3/2) --> I(9/2)4 quasi-three-level transition were investigated using multipass pumped Nd-based media in thin-disk geometry. When pumping at 0.81 microm into the (4)F(5/2) level, continuous-wave laser operation was obtained with powers in excess of 10 W at 1.06 microm, in the multiwatt region at 0.91 microm in Nd:YVO(4) and Nd:GdVO(4), and at 0.95 microm in Nd:YAG. Intracavity frequency-doubled Nd:YVO(4) thin-disk lasers with output powers of 6.4 W at 532 nm and of 1.6 W at 457 nm were realized at this pumping wavelength. The pumping at 0.88 microm, which is directed into the (4)F(3/2) emitting level, was also employed, and Nd:YVO(4) and Nd:GdVO(4) thin-disk lasers with ~9 W output power at 1.06 microm and visible laser radiation at 0.53 microm with output power in excess of 4 W were realized. Frequency-doubled Nd:vanadate thin-disk lasers with deep blue emission at 0.46 microm were obtained under pumping directly into the (4)F(3/2) emitting level.     

Anisotropic polarization of radiation in quantum-confinement whispering-gallery-mode lasers

The polarization of lasing and spontaneous emission from half-disk- and disk-shaped quantum-confinement lasers operating on whispering-gallery modes with wavelengths within 2.18–2.28 μm has been studied. It is established that electric vector E of output radiation is oriented predominantly in the p-n junction plane, which is explained by the fact that radiation is generated due to electron-heavy hole transitions. Modulation of the polarized spontaneous radiation spectra has been observed.     

腔面镀膜分布反馈量子级联激光器

报道了后腔面蒸镀高反射率镀膜工艺及其对分布反馈量子级联激光器性能影响的研究结果.与没有腔面镀膜的器件相比,采用了腔面镀膜工艺的器件,室温下的阈值电流密度降低了20%,前腔面出光峰值功率提高了50%,斜率效率提高了44%.通过对比镀膜和未镀膜器件的闻值电流密度,估算出器件的波导损耗约为7.25cm-1.     

室温准连续多量子阱二极管激光器调谐特性

为了将应变补偿多量子阱（SC-MQW）激光器用于调谐激光吸收光谱（TDLAS）技术进行气体分子检测,研究了SC-MQW激光器在室温、准连续工作模式下的调谐特性.从半导体激光器的速率方程出发,分析了SC-MQW激光器输出波长与温度和注入电流的关系;使用傅里叶变换红外光谱仪（FT-IR）测量了激光器分别在不同电流、温度条件下的调谐特性,得到SC-MQW激光器在允许工作电流范围内的调谐范围接近20nm,电流调谐率约为0.03nm/mA;温度调谐率约为0.25nm/℃;电流和温度联合调谐的波长覆盖范围达50nm.结果表明,该激光器在带状吸收谱的挥发性有机物气体检测方面有很好的应用前景.     

Linewidth-determining Processes in Distributed Feedback Dye Lasers

Abstract

Processes determining the linewidths of distributed feedback dye lasers (DFDL) have been investigated. Time resolution of the frequency of the output pulse shows that the linewidth, averaged over a pulse, arises predominantly from a dynamic sweeping of the laser frequency during the course of the pulse. This sweeping results from refractive-index changes in the dye over the duration of the pumping pulse; either through thermal effects or dispersion associated with the saturated gain. Thermal effects may be minimized by suitable choice of solvent but the dispersive sweep is inherent in this type of laser. The magnitude of the dispersive sweep changes across the tuning range of the laser. By judicious choice of dye solvent and dye parameters we have developed a narrow linewidth DFDL of 140 MHz for τ = 3·2 ns pulses, which is close to the transform limit.     

Fabrication, characterization, and theoretical analysis of controlled disorder in the core of optical fibers

We present results of experimental and theoretical studies of polarization-resolved light transmission through optical fiber with disorder generated in its germanium-doped core via UV radiation transmitted through a diffuser. In samples longer than a certain characteristic length, the power transmitted with preserved polarization is observed to be distributed over all forward-propagating modes, as evidenced by the Rayleigh negative exponential distribution of the near-field intensity at the output surface of the fiber. Furthermore, the transmitted power becomes also equally distributed over both polarizations. To describe the optical properties of the fibers with the experimentally induced disorder, a theoretical model based on coupled-mode theory is developed. The obtained analytical expression for the correlation function describing spatial properties of the disorder shows that it is highly anisotropic. Our calculations demonstrate that this experimentally controllable anisotropy can lead to suppression of the radiative leakage of the propagating modes, so that intermode coupling becomes the dominant scattering process. The obtained theoretical expressions for the polarization-resolved transmission fit very well with the experimental data, and the information extracted from the fit shows that radiative leakage is indeed small. The reported technique provides an easy way to fabricate different configurations of controlled disorder in optical fibers suitable for such applications as random fiber lasers.     

Determining conditions for 13.5-nm radiation generation by a supersonic-xenon-jet laser plasma source

We propose and justify a simple calculation method for (i) evaluating the optimum conditions for the generation of extreme ultraviolet (EUV) emission by a laser plasma source employing a supersonic xenon jet and (ii) finding ways to increase in the efficiency of such sources. The main processes involved in the interaction of laser radiation with a xenon jet target, which account for the EUV emission with a wavelength of 13.5 nm, are taken into account. It is shown that one of the main factors that decreases the output efficiency of such EUV sources is the absorption of generated radiation by the gas target. Qualitative calculations of the optimum conditions for the generation at 13.5 nm have been performed for three lasers with various wavelengths. The most promising results can be expected for a CO₂ laser, for which the radiation conversion efficiency on the order of 3% at a xenon pressure of 10–15 Torr can be achieved.     

A Design and New Functionality of Antiwaveguiding Vertical-Cavity Surface-Emitting Lasers for a Wavelength of 850 nm

Vertical-cavity surface-emitting lasers (VCSELs) with an aperture limited by an oxide and a resonance cavity based on GaAlAs with high Al content provide a maximum γ factor (λ/2 design) and suppression of optical power beyond the aperture. A VCSEL with two coupled cavities provides additional sharp growth of the loss of high-order lateral modes by leakage to the oxidized region and provides single-mode laser generation for an aperture diameter of up to 5 μm. Single-mode antiwaveguiding VCSELs provide ultrafast data transmission with a rate of up to 160 Gbit/s. The structure in which the active medium is placed in the lower distributed Bragg reflector and the cavity and the upper distributed Bragg reflector are dielectric, reducing the temperature shift of the radiation wavelength by a factor of 2 (to ～0.03 nm/K).     

半导体激光器输出功率影响因素的研究进展

大功率半导体激光器是现代激光加工设备、激光再制造设备、激光医疗、激光显示以及国防设备中重要的关键基础元器件和核心组件,在工业和国防等领域有着广泛的应用.提高半导体激光器的输出功率首先需要确定影响功率输出的因素,然后通过优化外延材料、芯片结构和制备工艺来解决这些问题.因此,对大功率半导体激光器输出功率影响因素的研究具有重...     

Evaluation of quantum-cascade lasers as local oscillators for infrared heterodyne spectroscopy

We report experiments evaluating the feasibility of quantum-cascade lasers (QCLs) at mid-infrared wavelengths for use as local oscillators (LOs) in a heterodyne receiver. Performance tests with continuous-wave (cw) lasers around 9.6 and 9.2 microm were carried out investigating optical output power, laser linewidth, and tunability. A direct comparison with a CO2 gas laser LO is presented as well. The achieved system sensitivity in a heterodyne spectrometer of only a factor of 2 above the quantum limit together with the measured linewidth of less than 1.5 MHz shows that QCLs are suitable laser sources for heterodyne spectroscopy with sufficient output power to replace gas lasers as LOs even in high-sensitivity astronomical heterodyne receivers. In addition, our experiments show that the tunability of the lasers can be greatly enhanced by use of an external cavity.