Low-threshold lasing in stimulated Raman lasers with nanosecond pumping

We have studied the conditions resulting in maximum lowering of the excitation threshold for pulsed stimulated Raman (SRS) lasers. It has been shown theoretically that in order to achieve the lowest possible values of laser radiation pulse energy needed to excite lasing in SRS lasers, we need high reflection of the cavity mirrors and low losses at the wavelength of the 1st Stokes component, high reflection of the output mirror at the wavelength of the pump radiation, and also matching of the confocal parameters for the exciting laser radiation and the cavity with each other and with the length of the Raman-active medium. The experimentally achieved excitation threshold for an SRS laser based on a barium nitrate crystal was 6 μJ, which quantitatively corresponds well to the calculation results. Lasing of up to five Stokes components simultaneously occurred. The efficiency for conversion of the laser radiation to one component was as high as 39%. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 284–290, March–April, 2008.     

Design and performance of two-channel EUV multilayer mirrors with enhanced spectral selectivity

In this paper, we present a study on two-channel multilayer mirrors which can operate at two wavelengths in Extreme Ultraviolet (EUV) spectral range. We propose a new method to design two-channel EUV multilayer mirrors with enhanced spectral selectivity. The mirror structure is a stack of two periodic multilayers separated by a buffer layer. We have defined the main parameters which allow adjustment of the distance between different order Bragg’s peak and of wavelength positions of reflectivity minima. Two mirrors have been designed and deposited for solar EUV telescope applications by using this method. The first mirror reflects Fe IX–X line (17.1 nm) and Fe XVI (33.5 nm) lines with attenuation of the He II line (30.4 nm). The second mirror reflects Fe IX–X and He II lines with attenuation of Fe XV (28.4 nm) and Fe XVI lines. Measurements with synchrotron radiation source confirm that, in both cases, for these mirrors, we are able to adjust reflectivity maxima (Bragg peak position) and minima. Such multilayers offer new possibilities for compact design of multi-wavelength EUV telescopes and/or for high spectral selectivity.     

Nonlinear dynamics of free electron terahertz lasers with bragg mirrors based on coupling of traveling and quasi-critical waves

Combination of various modifications of Bragg structures makes it possible to control electromagnetic fluxes in the interaction space, thus ensuring spatial coherence of radiation for high superdimensionality in two transverse coordinates required, which is for the operation of free electron lasers with distributed feedback in the submillimeter range. We propose that coupling between traveling and quasi-critical waves in the input Bragg mirror be used for mode selection in the “narrow” transverse coordinate directed along the normal to the conductors forming a planar waveguide. A traditional Bragg structure coupling copropagating and counterpropagating wave flows can be used as the output mirror.     

Competitive relaxation of the lumped-distributed feedback intensity in dye lasers

Relaxation oscillations of intensity in lumped-distributed feedback dye lasers are observed. These are caused by a competition between distributed feedback (DFB) and a feedback caused by an external mirror cavity. The effect is interpreted in terms of rate equations which include the time delay of radiation reflected from external mirrors relative to radiation due to periodic structure.     

Phase Correlations and Polarization Characteristics of Dye-Solution Laser Radiation

The influence of phase correlations of components on the energy and polarization parameters of output radiation have been calculated within the framework of the formalism of polarization components. It is shown that these correlations have a marked effect only on the energy characteristics. At the same time, an analysis of the polarization characteristics of a probe wave provides rich information for the analysis of the phase-polarization structure of radiation. An analysis of experimental data allows the inference that the correlation of components is relatively weak even for lasers with a high-Q resonator and at a high excess of threshold values. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 5, pp. 591–597, September–October, 2005.     

Experimental Investigation of High Power Erbium-Ytterbium Co-doped Double Clad Fibre Laser

All-fibre high power erbium-ytterbium co-doped double clad fibre lasers are proposed and demonstrated. By using different back-cavity mirrors, the different double clad fibre lasers are constructed. It is experimentally found that the output behaviour of laser can be controlled by a back-cavity mirror. The lower the reflectivity of the back-cavity mirror, the higher the output power and the high the slop efficiency. The maximum output power is about 1.6 W and the slop efficiency is 27.6%.     

Multilayer X-ray mirrors based on La/B<Subscript>4</Subscript>C and La/B<Subscript>9</Subscript>C

Multilayer X-ray mirrors that are based on La/B₄C and La/B₉C and intended for the reflection of X-ray radiation in the spectral region near λ = 6.7 nm are prepared and studied. Reflection coefficients at a level of 40–60% are achieved for mirrors with various periods. The difference in the interlayer roughnesses reconstructed from the data measured in the hard and soft X-ray spectral regions is explained using a structural model with an asymmetric permittivity profile in a mirror period. A proximate technique is developed to estimate the permittivity profile in a multilayer-structure period using reflectometry data. The effect of antidiffusion Sn, Cr, and Mo barriers on the reflection coefficient of multilayer La/B₄C structures is studied experimentally     

Fabrication and characterization of 1.55 μm single transverse mode large diameter electrically pumped VECSEL

We report on the design, fabrication, and characterization of InP-based 1.55 μm wavelength large diameter (50 μm) electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs). The hybrid device consists of a half vertical cavity surface emitting laser (1/2-VCSEL) structure assembled with a concave dielectric external mirror. The 1/2-VCSEL is monolithically grown on InP substrate and includes a semiconductor Bragg mirror and a tunnel junction for electrical injection. Buried (BTJ) and ion implanted (ITJ) tunnel junction electrical confinement schemes are compared in terms of their thermal and electrical characteristics. Lower thermal resistance values are measured for BJT, but reduced current crowding effects and uniform current injection are evidenced for ITJ. Using the ITJ technique, we demonstrate Room-Temperature (RT) continuous-wave (CW) single transverse mode laser operation from 50-μm diameter EP-VECSEL devices. We show that the experimental laser optical output versus injected current (L–I) curves are well-reproduced by a simple analytical thermal model, consistent with the thermal resistance measurements performed on the 1/2-VCSEL structure. Our results indicate that thermal heating is the main mechanism limiting the maximum CW output power of 50-μm diameter VECSELs, rather than current injection inhomogeneity.     

Double random mirror Hi–Bi photonic crystal fiber Sagnac based multiwavelength fiber laser

In the present work, a multiwavelength Raman fiber laser based in a Sagnac configuration is presented. The Sagnac configuration is composed of a Hi–Bi photonic crystal fiber in the middle of two dispersion compensation fibers. The pump power induces two random mirrors to work in opposite directions in the Sagnac, leading to the amplified signal and the multiple scattering signals from the random mirror traveling through the Hi–Bi photonic crystal fiber in a counter-propagating way interfering in the output port.     

Modulation Characteristics of Tunable Quantum-Well Heterolasers at the Radiation Frequency Tuning

The influence of the tuning of the radiation frequency of quantum-well heterolasers within the limits of their amplification band on the output laser radiation parameters at different frequencies of pump-current modulation has been numerically simulated using two optical models of an active medium under the assumption that the excess of the invariable component of the pump current over its threshold value at any point of the amplification band is constant. It has been established that the amplitude-detuning characteristics of the lasers studied are linear at high current-modulation frequencies and nonlinear at medium ones. The behavior of the nonlinear spectral response of a laser diode is explained by its multiresonance structure and is mainly determined by the ratio between the pump-current modulation frequency and the maximum resonance frequency of the laser. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 473–478, July–August, 2005.     

Art output mirror for fir laser

ATR prism was used firstly as the FIR laser output coupler. Such output mirror possesses good separation of the pump (λ≈10μm) and FIR radiation without losses. Transmittance of this mirror can be simply controlled by adjustment of gap between two output prisms. The dependence of mirror output characteristics on pump radiation polarization, prism refractive index, gap between prisms are under consideration.     

An all-diode-laser-based, dual-cavity AgGaS2 cw difference-frequency source for the 9–11 μm range

2 ) nonlinear crystal is reported. The device uses a standing-wave dual-resonator scheme to enhance both near-infrared lasers. For 7 mW of 778.2-nm and 275 mW of 842.5-nm input powers, 40 nW of 10.2-μm radiation is generated by a preliminary setup. A μW-level range output power is expected after optimization of the device optical components. This compact tunable source will allow Fabry–Pérot high-resolution Doppler-free spectroscopy of spherical molecules, such as OsO₄ or SF₆, in the 9–11 μm range as an alternative source to frequency-stabilized CO₂ lasers. Received: 17 March 1998     

Grazing incidence systems with multiple reflections

The theory of turning soft x-ray SXR radiation beams by means of concave surfaces through multiple reflections (the “whispering-gallery” effect) is developed. It is demonstrated that concave mirrors are capable of turning an SXR beam by an arbitrary angle determined by their aperture angle. The reflectivities are of the same order of magnitude as those of multilayer mirrors (20–80% at a turn by 90°). The turning mirrors feature a wide spectral transmission band (Δλ ∼ λ). The potentialities of concave mirrors and hollow cylindrical waveguides with respect to concentration and collimation of SXR radiation are considered. The requirements imposed on the smoothness of the reflecting surfaces are obtained and the optimal (as to their optical properties) materials are chosen for their fabrication. The theoretical results are compared to experimental data. Translated from Trudy Fizicheskogo Instituta im. P. N. Lebedeva (Proceedings of the Lebedev Physical Institute, Russian Academy of Sciences, Moscow), Vol. 196, pp. 143–167, 1989.     

Neodymium lasers with fiber-optic reflectors

The lasingin an Nd:YAG laser with a fiber-optic reflector in the form of a loop was experimentally investigated. Two designs of the laser with a fiber-optic loop reflector were considered. In the laser of the first type the reflector was formed by a fragment of a fiber guide the ends of which were placed in the focal plane of a microscope objective. In the laser of the second type the role of the output mirror was played by a loop-shaped fiber-optic reflector made on the basis of a V-shaped fused three-fiber splitter. It is shown that when such lasers operate in the regime of free-running lasing, in them there takes place the effect of nonlinear conversion of the generated radiation frequency in the Stokes and anti-Stokes regions of the spectrum. Institute of Applied Optics, National Academy of Sciences of Belarus, 11, Belynitskii-Birulya Str., Mogilev, 212793, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 259–262, March–April, 1999.     

Polarizing and non-polarizing mirrors for the hydrogen Lyman-<Emphasis Type="Italic">α</Emphasis> radiation at 121.6 nm

Efficient polarizing as well as non-polarizing mirrors for a wavelength of λ=121.6 nm (hydrogen Lyman-α radiation) are necessary to achieve an experimental determination of the magnetic field in the solar corona through the Hanle effect. We have designed, realized and characterized such mirrors. These consist of glass coated with a thin-film stack. The coatings use the most reflective (Al) and most transparent (fluorides) materials at this wavelength. Different coatings were explored which involve an increasing number of films in the coating stack. At the incident angle of maximum polarization where the p-polarized reflectivity R _p is minimized, an s-polarized reflectivity R _s as high as 69% is experimentally obtained with a coating made of a Fabry–Pérot resonator. To our knowledge, this value is the highest ever reported for a polarizing mirror at this wavelength. Additionally, efficient non-polarizing mirrors have been designed and realized by using a two-layer coating (MgF₂/Al/glass). By optimizing the fluoride layer thickness, a mirror with non-polarizing properties in the whole range of incident angles was realized.     

Energetic Sn+ irradiation effects on ruthenium mirror specular reflectivity at 13.5-nm

Sn⁺ irradiations of Ru single-layer mirrors (SLM) simulate conditions of fast-Sn ion exposure in high-intensity 13.5 nm lithography lamps. Ultra-shallow implantation of Sn is measured down to 1–1.5 nm depth for energies between 1–1.3 keV at near-normal incident angles on Ru mirror surfaces. The Sn surface concentration reaches an equilibrium of 55–58% Sn/Ru for near-normal incidence and 36–38% for grazing incidence at approximately 63 degrees with respect to the mirror surface normal. The relative reflectivity at 13.5 nm at 15-degree incidence was measured in-situ during Sn⁺ irradiation. For near-normal Sn⁺ exposures the reflectivity is measured to decrease between 4–7% for a total Sn fluence of 10¹⁶ cm⁻². Theoretical Fresnel reflectivity modeling shows for the same fluence assuming all Sn atoms form a layer on the Ru mirror surface, that the reflectivity loss should be between 15–18% for this dose. Ex-situ absolute 13.5 nm reflectivity data corroborate these results, indicating that implanted energetic Sn atoms mixed with Ru reflect 13.5-nm light differently than theoretically predicted by Fresnel reflectivity models.     

An integrated optical system for the vacuum ultraviolet region

We use a 0.5 m Seya-Namioka monochromator and two gold plated cylindrical mirrors to set up an optical system which produces a collimated beam of plane polarized, monochromatic radiation in the vacuum ultraviolet region. The two mirrors compensate for the astigmatism of the Seya-Namioka monochromator, thereby increasing the spectral resolution drastically. The degree of linear polarization is about 0.97–1.00 in the spectral region of 6–11.5 eV.     

Tuning characteristics of asymmetric inhomogeneously pumped quantum-well lasers

An analysis of the power-current and tuning characteristics of asymmetric multiple-layer inhomogeneously pumped quantum-well lasers with mode selection in the external resonator was performed. It is shown that the choice of the parameters of the barrier and active layers of the laser heterostructure and optimization of the dispersion characteristics of the selective element of the external resonator make it possible to realize continuous tuning of the lasing wavelength in a wide spectral range at a practically constant output radiation power. Reported at the Second International Scientific and Technical Conference on Quantum Electronics, Minsk, November 23–25, 1998. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 687–692, September–October, 1999.     

腔镜失调对激光陀螺信号输出的影响

研究了不同腔镜结构不同失调情况对空间谐振式激光陀螺中光束空间位置的影响.利用Collins积分与Collins程函理论以及MATLAB软件工具,对腔镜失调前后激光陀螺中光束传输进行了仿真计算.结果表明,相同失调量的不同反射镜所产生的拍频频差是不同的,且分别从M1和M4镜探测到的拍信号频不相同|对于小增益激光器,使用球面镜的腔镜结构较平面镜的腔镜结构更易形成稳定光束空间位置分布|由所计算的空间坐标xn可以看出,双向行波光束的光斑中心发生了偏移,在一定的失调量下将会影响到激光陀螺的拍频输出.研究结果对激光陀螺的工程检测和腔镜调整有指导作用,有助于激光陀螺测量准确度的提高.     

Background noise suppression in wide-band lasers with a nonlinear mirror in the cavity

An effective method for suppressing free background generation is proposed. It is based on polarization separation of the narrow-band output radiation of a controlled laser with a nonlinear mirror in the cavity. An increase in the integral ratio between the energy of the narrow generation line and the integral value of the energy of the background over the entire gain band of a wide-band laser from 0.17 to 55.00 has been attained. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 169–171, March–April, 2000.