Investigation of a heterodyne receiver with open structure mixer at 324 GHz and 693 GHz

The performance of a submillimeter heterodyne receiver using an HCOOH laser local oscillator and an open structure mixer with a Schottky barrier diode has been optimized for 693 GHz. Working at room temperature a single sideband (SSB) system noise temperature of 7,300 K, a mixer noise temperature of 6,100 K and a conversion loss of 12 dB has been achieved. The same receiver system has been investigated at 324 GHz using an HCOOD laser local oscillator yielding a noise temperature of 3,100 K (SSB), a mixer noise temperature of 2,400 K (SSB) and a conversion loss of 10 dB (SSB). An acousto-optical spectrometer has also been constructed, with 1024 channels and a channel-bandwidth of 250 kHz. The system NEP per channel was 2.5×10^–17 W/Hz^1/2 at 324 GHz and 5.0×10^–17 W/Hz^1/2 at 693 GHz.     

An optical system for monitoring torsion in a power transmission shaft in realtime

An optical system has been proposed and verified experimentally for monitoring the torsion of a power transmission shaft in realtime. The system consists of a pair of lasers, mirrors and light receivers as a sensor head, and logic circuit, high-frequency oscillator, and computer as a data processing system. The smallest measurable angle of torsion can be expressed by ω/f, where ω is the rotational frequency of the power transmission shaft and f is the frequency of the oscillator. The experimental error was found to be a few percentage points.     

Automatic thermal frequency control of microwave oscillator with dielectric disk resonator

Microwave oscillators that are stabilized by sapphire disk resonators have good short-term relative frequency stability (()=10^–10 over averaging time 1 sec) but inadequate long-term stability. This is mainly due to the high temperature coefficient of frequency of the disk resonator. An automatic frequency-control system based on thermal control of the frequency of the disk resonator was developed to improve the long-term frequency stability of the oscillators. The parameters of the automatic frequency-control system were optimized by computer, which involved solution of the nonstationary problem of the temperature distribution in the disk resonator and examination of the stability of the automatic control system for diffuse propagation of the control signal. Recommendations are given for selection of the heater geometry and frequency-control method. An oscillator with the thermal frequencecy-control system had a long-term relative frequency instability of (0.3–1)·10^–8 per day, which was determined by the stability of the crystal-controlled reference oscillator. The automatic frequency-control system did not appreciably change the noise characteristics of the oscillator.Institute of Problems of Safe Development of Atomic Energy, Russian Academy of Sciences. Translated from Izvestiva Vysshikh Uchenbykh Zavedenii, Radiofizika, Vol. 36, No. 11, pp. 1043–1052, November, 1993.     

Double XeCl laser with lateral UV preionization

We present a very compact and very reliable laser system formed of two discharge sections both with lateral UV preionization. The discharge electrodes are contained in a single vessel and form an oscillator-amplifier system. A single spark gap switches on both circuits. By applying a generalized self-filtering unstable resonator to the oscillator and injecting the laser beam into the single pass amplifier, an output beam of 105 mJ, with a brightness of 5.5×10¹³ W cm^–2Sr^–1 has been obtained. Moreover, by applying a pulse-forming network in one discharging circuit, the duration of the discharge breakdown has been increased and output laser pulses of 40 ns FWHM, and 80 ns base width duration (at 10% points) and of energy 100 mJ are obtained.     

Optical diagnostics of dispersing He plasma

An experimental study has been made of the characteristics of radiation from a dispersing helium plasma. From measurements of the space-time dependence of the line intensities of the helium 2³P — n³D series we obtained the distribution of the populations of the n³D levels for n=3–10. The study has shown that over a wide range of conditions the high-lying levels (n=5–10) are in equilibrium with electrons while a substantial deviation from equilibrium is observed for levels n=3, 4. From the energy distribution of the populations of the upper levels we determined the space-time dependence of the electron temperature, which reflects the process of effective electron cooling during dispersal of the plasma. The gas temperature was estimated from the populations of helium singlet and triplet levels with n=5. The time distribution of the electron density in the plasma was found from the Stark broadening of the spectral lines.Deceased.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 15–19, November, 1992.     

Development of a tunable IR lidar system

A differential absorption lidar system (DIAL) based on a continuously tunable optical parametric amplifier (OPA) pumped by a Nd : YAG laser (200 mJ at λ=355 nm) operating at a maximum pulse repetition rate of 100 Hz has been developed. The system provides continuously tunable coherent radiation in the Visible–near IR range (0.4–2.5 μm), allowing to perform DIAL measurements in a spectral region where most of atmospheric constituents and pollutants display absorption lines. The spectral width of the OPA system is line-narrowed by using a master oscillator dye laser as seeder, achieving a linewidth of 0.04 cm⁻¹ (FWHM), a spectral purity larger than 99% and a frequency stability better than 1 pm h⁻¹, with an output energy in the IR of 1–10 mJ. The OPA system was used to perform DIAL measurements in the lower troposphere. Preliminary results in terms of water vapor content and aerosol backscattering profiles are presented and discussed.     

Linear Theory of Free Electron Laser with a Plasma-Loaded Cylindrical Waveguide

The dispersion characteristics of plasma–loaded free-electron laser has been analyzed using linear fluid model. The device under consideration consists of the cylindrical metallic waveguide, completely filled with background plasma and a relativistic electron beam which passes through a helical wiggler magnetic field. The result predicts that reasonable plasma density tends to improve the growth rate of the low-frequency optical wave of FEL and causes an shiftup in the operating frequency, However it has little effect on the growth rate of the high-frequency wave. In the plasma–loaded FEL, for the FEL oscillator, it may be tuned by varying the plasma density; and for the FEL amplifier, the wider frequency bandwidth is gained. A critical density n ^c _p for the background plasma density is found.     

Development of injection-seeded, pulsed optical parametric generator/oscillator systems for high-resolution spectroscopy

We report the development and application of pulsed optical parametric generator (OPG) and optical parametric oscillator (OPO) systems that are injection seeded with near-infrared distributed feedback diode lasers. The OPG is injection seeded at the idler wavelength without the use of a resonant cavity. Two counter-rotating, beta-barium-borate (β-BBO) crystals are used in the OPG. These crystals are pumped by the third harmonic, 355-nm output of an injection-seeded Nd:YAG laser. An OPO version of the system has also been developed by placing two flat mirrors around the two β-BBO crystals to form a feedback cavity at the signal wavelength. The OPO cavity length is not actively controlled. The output signal beam from the OPG or OPO is amplified using an optical parametric amplifier (OPA) stage with four β-BBO crystals. The frequency bandwidths of the signal and idler laser radiation from OPG/OPA and OPO/OPA systems have been determined to be slightly greater than 200 MHz. The temporal pulses from each system are smooth and near-Gaussian. High-resolution optical absorption measurements of acetylene (C₂H₂) were performed as another check of the frequency spectrum of the idler beam. The frequency-doubled signal output of the OPO/OPA system was used to perform high-resolution, single-photon, laser-induced fluorescence (LIF) spectroscopic studies of the (0,0) vibrational band of the A ²Σ⁺−X ²Π electronic transition of nitric oxide (NO) at low pressure. Excellent agreement was obtained between the theory and the experiment. The signal output of the OPG/OPA system was also used for sub-Doppler, two-photon LIF spectroscopic studies of the same vibration–rotation manifold of NO.This revised version was published online in August 2005 with a corrected cover date.     

Investigation of the redistribution of the current in a nanosecond volume discharge after the appearance of a cathode spot

An investigation was made of the process of contraction of a nanosecond volume discharge in air with a homogeneous field distribution and a current density 500 A/cm² when the gap was 0.7 cm and the pressure 76 torr. It was found that the appearance of a cathode spot in this volume discharge resulted in an effective contraction of the current into a spot and formation of a diffuse channel. A high-conductivity channel emerging from the cathode spot appeared when the current density in the diffusion channel above this spot reached (1–3)·10⁵ A/cm².Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 16–18, January, 1981.     

Electronic polarization in SbSI near electrodes

An experimental study has been made of the residual-polarization charge [q] at various temperatures in darkness and with illumination. Illumination of the anode has the greatest effect on the q = f(E) dependence. The potential distribution in the sample and the q = f(T) dependences reveal a high electron density near the anode, giving rise to polarization charge of up to 10^-5 C/cm², in SbSI with In-Ga electrodes. The time dependence of the discharge current reveals three slow-relaxation components, with relaxation times of 1.4 · 10², 1.1·10³, and 2.1·10³ sec.Translated from Izvestiya VUZ. Fizika, No. 12, pp. 45–49, December, 1969.The authors thank M. S. Kosman and V. A. Izvozchikov for discussion of these results.     

一氧化碳分立跃迁的光学振子强度和微分散射截面研究

在电子入射能量1500eV、平均散射角为0°和能量分辨为60meV条件下，得到了一氧化碳在7—21eV能量区间的绝对光学振子强度密度谱，获得了电子态A¹Π，B¹Σ⁺，C¹Σ⁺及E¹Π的各振动能级的绝对光学振子强度，通过与已发表的各实验和理论数据作比较，分析了差异的原因，并讨论了偶极(e,e)方法确定光学振子强度的局限性.同时还给出在电子入射能量1500eV时上述四个电子态的各 关键词：     

35 GHz dielectric resonator stabilised gunn oscillator

A 35 GHz dielectric resonator oscillator(DRO) using GaAs Gunn diode in microstrip configuration has been designed and developed. The oscillator, with an integral waveguide-to-microstrip transition, delivered an output greater than 18 dBm. Phase noise of the oscillator is found to be better than –80 dBc/Hz at 100 KHz away from the carrier. A frequency drift of about ±25 MHz has been observed over the temperature range from –10 °C to 50 °C.     

A superheated Na cell for X-ray photoionization experiments

The performance of a superheated alkali vapor cell, with a geometry suitable for x-ray photoionization experiments, is described. An internal heater was used to superheat sodium vapor in a split-wick heat pipe. At a Na pressure of 20 Torr, the transmission through the cell in the region of theC–X molecular band of Na₂ increased to 50% from the 5% observed when the cell was operated in a conventional heat-pipe mode. In the presence of a Na atom density of 2×10¹⁷ cm^–3, the average molecule density over the length of the cell was measured to be 2.5×10¹⁵ cm^–3. In the hot central region of the superheated cell, the molecule density was predicted to have been reduced by a factor of 50 to 3×10¹⁴ cm^–3.     

Controlled fabrication of Si nanostructures by high vacuum electron beam annealing

Silicon nanostructures, called Si nanowhiskers, have been successfully synthesized on Si(1 0 0) substrate by high vacuum electron beam annealing (EBA). Detailed analysis of the Si nanowhisker morphology depending on annealing temperature, duration and the temperature gradients applied in the annealing cycle is presented. A correlation was found between the variation in annealing temperature and the nanowhisker height and density. Annealing at 935 °C for 0 s, the density of nanowhiskers is about 0.2 μm⁻² with average height of 2.4 nm grow on a surface area of 5×5 μm, whereas more than 500 nanowhiskers (density up to 28 μm⁻²) with an important average height of 4.6 nm for field emission applications grow on the same surface area for a sample annealed at 970 °C for 0 s. At a cooling rate of −50 °C s⁻¹ during the annealing cycle, 10–12 nanowhiskers grew on a surface area of 5×5 μm, whereas close to 500 nanowhiskers grew on the same surface area for samples annealed at the cooling rate of −5 °C s⁻¹. An exponential dependence between the density of Si nanowhiskers and the cooling rate has been found. At 950 °C, the average height of Si nanowhiskers increased from 4.0 to 6.3 nm with an increase of annealing duration from 10 to 180 s. A linear dependence exists between the average height of Si nanowhiskers and annealing duration. Selected results are presented showing the possibility of controlling the density and the height of Si nanowhiskers for improved field emission properties by applying different annealing temperatures, durations and cooling rates.     

The distribution of trapping states at the Al/InP-oxide interface

The volume density of trapping states is derived throughout the metal-dielectric interface. This has been facilitated by equating the dielectric loss component to the tunneling conductance using a new relaxation time formulation. Subsequently, the trap distribution at the Al/InPO₄ interface has featured a peak of 1.15×10¹⁹ cm^–3 at about 15 Å from the Al contacting electrode. The new approach could be extended to deal with semiconductor-dielectric interfaces.     

Generation of nanosecond terahertz pulses by the optical rectification method

The possibility of the generation of quasi-cw terahertz radiation by the optical rectification method for broad-band Fourier unlimited nanosecond laser pulses has been experimentally demonstrated. The broadband radiation of a LiF dye-center laser is used as a pump source of a nonlinear optical oscillator. The energy efficiency of terahertz optical frequency conversion in a periodically polarized lithium niobate crystal is 4 × 10⁻⁹ at a pump power density of 7 MW/cm².     

Spectral purity and phase coherence of narrow-band noise in NbSe3

An unusually simple narrow band noise spectrum with a single fundamental and seven harmonics due to depinned charge density wave motion in NbSe₃ is reported. The simplicity of the spectrum is attributed to a very uniform current distribution associated with a short, thin sample of high perfection. The distribution of harmonics is near to that of the sawtooth waveform generated by a relaxation oscillator. As indicated by the amplitude ratio of the fundamental to the first harmonic, this distribution has only a moderate variation over three orders of magnitude in frequency. An electrical relaxation oscillator analog which reproduces most of the properties of CDW motion in NbSe₃ is discussed. A search for phase memory of the narrow band noise in NbSe₃ showed it to be less than one period.     

Harmonic and sum-frequency generation of pulsed laser radiation in BBO,LBO, and KD*P

The optical properties of the nonlinear crystals lithium borate (LBO), barium borate (BBO) and deuterated potassium phosphate (KD*P) are compared for second and third harmonic generation of Nd:YAG laser radiation. In an experimental investigation the conversion efficiency has been measured as a function of the energy density of 8 ns long laser pulses, generated by a commercial Nd:YAG oscillator-amplifier system. In LBO and BBO the second harmonic generation saturates at an energy density of about 1.5 J cm^–2 at efficiencies of 55–60%. In KD*P comparable efficiencies (40–55%) require energy densities of 2–2.6 J cm^–2. Similar results are obtained for frequency tripling. In LBO and BBO saturated efficiencies of 20–25% are measured at an energy density of about 1.5 J cm^–2. In KD*P efficiencies of 20% are obtained at energy densities exceeding 2 J cm^–2. Besides for doubling and tripling of Nd:YAG laser radiation the phase-matching is calculated for frequency conversion of tunable laser light. The results demonstrate that in LBO and BBO phase-matched sum-frequency mixing of UV and infrared laser light generates tunable radiation at wavelengths as short as the transmission cut-off at 160 nm and 190 nm, respectively.     

Novel mid-IR laser based on hybrid AlGaAsSb/InAs/CdMgSe heterostructure

We report on the development of a novel design of a mid-IR laser combining III–V and II–VI compounds in a “hybrid” double heterostructure. It possesses large (1.5 eV) potential barriers both for injected electrons and holes, suppressing their leakage from the active region, and provides strong optical confinement. An AlGaAsSb/InAs/CdMgSe laser diode with a III–V/II–VI heterovalent interface at the 0.6 μm-InAs active region has been grown by molecular beam epitaxy on an InAs substrate. Despite a far from optimal defect density at the CdMgSe/InAs interface and high losses inherent for bulk active region of the laser, the structure demonstrates lasing at 2.8 μm (up to 100 K) in the pulsed regime with a threshold current density of 3–4 kA/cm². Type II InSb monolayer insertions into an InAs layer show bright photoluminescence at 3.8 μm (77 K), confirming the great potential of the InAs-based nanostructure active region for longer wavelength applications.     

Frequency-stabilized seed laser system for dial applications around 0.94 μm

Absolute frequency stabilization of an extended-cavity diode laser at 0.94 μm is reported. The diode laser was frequency locked against rovibrational absorption lines of water vapour by using the frequency modulation spectroscopy technique. The stabilized oscillator shows a short-term frequency stability level of 40 kHz for integration times of 1 s and a long-term frequency drift lower than 10 MHz for observation times longer than 10³ s. The frequency-stabilized oscillator system is mounted on a compact breadboard (75 cm×50 cm) and constitutes the seed laser system for the injection of a high-energy DIAL laser transmitter operating in the 0.94-μm spectral region.