Lidar detection using a dual-frequency source

A new technique of dual-frequency Doppler-lidar measurement is investigated. This technique is based on the use of a coherently locked, tunable, dual-frequency laser source and is shown to accurately measure velocities as small as 26 mum/s. It is generated by exploiting the nonlinear dynamics of a semiconductor laser through a proper combination of optical injection and operating conditions.     

Improved spatial phase detection for profilometry using a TDI imager

Performance of a time delay and integration (TDI) imager is analogous to a drum/periphery camera. Coupled to a modulated line generating laser diode, it is possible to use a TDI imager to encode the shape profile of a rotating cylindrical object as deformed gratings in a digital image. Among the various methods for extracting the phase from a deformed grating, the spatial phase detection (SPD) method is fast and easy to use. Errors are introduced in this method if the phase gradient is high over a grating interval. Here we propose a method to reduce the phase gradient by using a TDI feature which makes it possible to record images at variable lateral magnification.     

Underwater tunable organ-pipe sound source

A highly efficient frequency-controlled sound source based on a tunable high-Q underwater acoustic resonator is described. The required spectrum width was achieved by transmitting a linear frequency-modulated signal and simultaneously tuning the resonance frequency, keeping the sound source in resonance at the instantaneous frequency of the signal transmitted. Such sound sources have applications in ocean-acoustic tomography and deep-penetration seismic tomography. Mathematical analysis and numerical simulation show the Helmholtz resonator's ability for instant resonant frequency switching and quick adjustment of its resonant frequency to the instantaneous frequency signal. The concept of a quick frequency adjustment filter is considered. The discussion includes the simplest lumped resonant source as well as the complicated distributed system of a tunable organ pipe. A numerical model of the tunable organ pipe is shown to have a form similar to a transmission line segment. This provides a general form for the principal results, which can be applied to tunable resonators of a different physical nature. The numerical simulation shows that the "state-switched" concept also works in the high-Q tunable organ pipe, and the speed of frequency sweeping in a high-Q tunable organ pipe is analyzed. The simulation results were applied to a projector design for ocean-acoustic tomography.     

Proposal for a loophole-free Bell test using homodyne detection

We propose a feasible optical setup allowing for a loophole-free Bell test with efficient homodyne detection. A non-Gaussian entangled state is generated from a two-mode squeezed vacuum by subtracting a single photon from each mode, using beam splitters and standard low-efficiency single-photon detectors. A Bell violation exceeding 1% is achievable with 6 dB squeezed light and a homodyne efficiency around 95%. A detailed feasibility analysis, based upon the recent experimental generation of single-mode non-Gaussian states, suggests that this method opens a promising avenue towards a complete experimental Bell test.     

A white-light interferometer using a lamp source and heterodyne detection with acousto-optic modulators

A heterodyne technique for white-light interferometer with a lamp source, which uses two acousto-optic modulators, is developed for high-sensitivity detection of weak light. By using converging input beams input into a Michelson interferometer with spherical mirrors, the spectral dependence of the modulators is canceled, and the white-light heterodyne interference fringes are generated at 200 kHz. Using a tandem interferometer, the object surface which has a low surface reflectivity of less than 10⁻⁴ was detected with a good signal-to-noise ratio.     

Cantilever enhanced photoacoustic detection of carbon dioxide using a tunable diode laser source

Recently introduced cantilever enhanced photoacoustic sensing has been applied to the tunable diode laser-based trace gas detection. The pressure variations due to the photoacoustic signal are detected with a miniature silicon cantilever, whose displacement is measured with a compact Michelson type laser interferometer. The system has been used to detect carbon dioxide (CO₂) at 1572 nm with a distributed feedback diode laser. With a new photoacoustic cell, that was optimized for the laser sources, a normalized noise equivalent sensitivity of 1.7×10^-10 cm^-1W/ at atmospheric pressure was realized. The result obtained in the non-resonant operation mode is at least 10 times better than in previous reports. The future improvements of the technique are also discussed. PACS 07.07.Df; 42.55.Px; 82.80.Kq     

强度调制532 nm激光水下测距

激光水下探测在水下目标搜寻、资源勘探等领域具有重要的应用,而散射是激光水下探测面临的主要挑战.载波调制激光雷达具有抗散射、抗干扰的优点,本文利用自行研制的532 nm强度调制激光源,在3 m长的水箱中搭建激光水下探测系统. 532 nm激光源最大输出功率为2.56 W,强度调制范围为10.0 MHz—2.1 GHz,光束发散角约0.5 mrad.通过在水箱中添加氢氧化镁(Mg(OH)_2)粉末,测量了不同浑浊度下水的衰减系数.采用相位测距的方法,目标反射光的调制信号为探测信号,对激光源进行调制的电信号作为参考信号,利用相关运算获得激光的延时时间,进而可以获得水下目标的距离.最大调制频率为500 MHz时,实现了距离为4.3个衰减长度目标的探测,测距误差约12 cm.探测距离越远,测距误差越大,调制频率越高,测距精度越高.     

Microwave generation using a superluminal source

The power produced by existing sources of microwave radiation falls off with decreasing wavelength. To solve this problem a new concept is proposed for generating microwave radiation, based on the use of a superluminal source formed when electrons are emitted into vacuum from a medium and the emission front propagates along the surface with a speed greater than that of light. Such generators are shown to have a number of completely unique properties: they radiate extremely short pulses (as short as picoseconds); their power exceeds that of existing sources by orders of magnitude; and unlike existing sources, it increases as the wavelength is reduced. Zh. éksp. Teor. Fiz. 115, 1689–1707 (May 1999)     

XBIC using a laboratory X-ray source

Results from using the current method, in which a current is induced by a focused X-ray beam (XBIC = X-ray beam induced current) on a laboratory source are presented. It is shown that by reducing the X-ray probe sizes and improving the resolution, there is a gain in contrast increase for obtained images of grain boundaries in a crystal.     

Ultrabright femtosecond source of biphotons based on a spatial mode inverter

A method of enhancing the efficiency of entangled biphoton sources based on a type II femtosecond spontaneous parametric downconversion (SPDC) process is proposed and implemented experimentally. Enhancement is obtained by mode inversion of one of the SPDC output beams, which allows the beams to overlap completely, thus maximizing the number of SPDC photon pairs with optimum spatiotemporal overlap. By use of this method, biphoton count rates as high as 16 kHz from a single 0.5-mm-long beta-barium borate crystal pumped by second-harmonic radiation from a Ti:sapphire laser were obtained.     

Multipass interferometer using a filamentary spatial filter

A multiple pass interferometer using a filamentary spatial filter is developed. We can change the sensitivity of the interference fringe pattern by selecting the order of multiply reflected image without reduction of the fringe visibility.     

Speckle shearography using a multiband light source

Applications for optical metrology usually use lasers as light sources, because of the excellent spatial and temporal coherence of the emitted light. By comparison, the demands of speckle shearography concerning the coherence of the light source are low. This enables certain white-light sources to be an option.

In this paper, the feasibility of low coherence shearography is shown. For this purpose an experimental setup is designed, composed of a mercury arc lamp, a spatial filter and a Michelson interferometer. With respect to speckle shearography, important characteristics of the light source are described and the mercury arc lamp is shown to be suitable. Finally, some experimental investigations of an object under load are presented.     

Nuclear physics merely using a light source

The interaction of ultra-intense focused laser beams with solid targets is a new field of research resulting in the production of exotic plasma conditions similar to the conditions which exist in the interior of some stellar objects. The lasers generate very high energy electrons and ions which can subsequently produce gamma-rays, positrons, neutrons and pions. The results obtained from these studies have major implications to fundamental plasma physics and high energy accelerator physics as well as important technological potential for the production of compact sources of neutrons, positrons and isotopes.     

GaAsN-on-GaAs MBE using a DC plasma source

A new dc plasma source for MBE growth of GaAsN layers is suggested. The efficiency of nitrogen incorporation, crystal perfection, surface morphology, and luminescent properties of the epilayers vs. operation conditions of the source are studied.     

Underwater moving noise source identification based on hybrid near-field acoustical holography

Near-field acoustical holography (NAH) is a powerful tool for identifying noise sources and visualizing acoustic field. By recording the acoustic pressures in the near-field, the acoustic quantities in the whole 3-D field can be reconstructed and predicted. However, the current theory of NAH is not applicable to tracking large scale moving noise sources. Therefore, the hybrid near-field acoustical holography is developed for reconstructing acoustic radiation, which is derived from statistically optimized ne...     

The experimental study of fatigue crack detection using scanning laser point source technique

For the purpose of better understanding the interaction of Rayleigh wave and the fatigue crack in a metallic sample, a set of experimental setups is built, based on the scanning laser source (SLS) technique, utilizing a point source to take place of the line source to generate surface acoustic waves (SAWs), and an interferometer is to detect the SAWs signal. The information of the crack (such as position and length) can be obtained by utilizing a two-dimensional scan of the material surface. This paper focuses on the detection of visible and invisible fatigue crack by using this point-source-based scanning laser source technique, and comparing the results with those of conventional pitch-catch technique. The result shows that with two-dimensional scanning, and analyzing the amplitude of the generated SAWs, not only the visible fatigue can be identified, but also the invisible fatigue crack can be discriminated. As a result, the sensitivity of the scanning point laser source technique is higher than the conventional pitch-catch technique.     

Hybrid multinary modulation using a phase modulating spatial light modulator and a low-pass spatial filter

We propose a method for performing binary intensity and continuous phase modulation of beams with a spatial light modulator (SLM) and a low-pass spatial filtering 4-f system. With our method it is possible to avoid the use of phase masks in holographic data storage systems or to enhance the phase encoding of the SLM by making it capable of binary amplitude modulation. The data storage capabilities and the limitations of the method are studied.     

Underwater three-dimensional imaging using narrowband MIMO array

To obtain high cross-range resolution, the underwater 3-D acoustic imaging system usually requires a rectangular array with a great number of sensors and a large physical size. To reduce the sensor number and the array physical size simultaneously, this paper proposes a new underwater 3-D acoustic imaging approach based on a novel multiple-input multiple-output (MIMO) array. Specifically, the MIMO array is composed of four uniform linear arrays (ULAs) located on four sides of a rectangle. The transmitting array composed of two ULAs is located on a pair of opposite sides, and the receiving array composed of another two ULAs is located on the other two sides. Furthermore, narrowband waveforms coded with orthogonal polyphase sequences are employed as transmitting waveforms. When the subcode numbers in the polyphase coded sequences are sufficient, the MIMO array has the same 3-D imaging ability as a rectangular array, which has a two-time bigger size than that of the former. Consequently, the MIMO array can not only save a great number of sensors, but halve the array size, when compared to a rectangular array with the same cross-range resolution. Computer simulations are provided to demonstrate the effectiveness of the proposed imaging approach.     

Probabilistic characteristics of signal detection by a spatial correlation filter

Results of theoretical and model studies of the probabilistic characteristics of signal detection by a spatial correlation filter are described. A mathematical model of signal processing in the spatial correlation filter is presented. Expressions for estimating the detection probability, missed detection probability, false alarm probability, and threshold level, as well as expressions for choosing the optimal filter parameters to obtain preset probability characteristics, are derived on the basis of this model. Results of computer simulation of the probabilistic characteristics of signal detection by spatial correlation filters with random arrays are shown to corroborate the theoretical findings obtained within the model.