连续太赫兹波数字全息相衬成像

太赫兹波具有独特的低能性、高穿透性、惧水性等成像特性,将其应用于相衬成像能够反映物体的内部结构和更加丰富全面的生物信息,在生物医学检测等领域具有重要的应用。其中,太赫兹波数字全息成像是一种可以给出定量的振幅和相位信息的非接触、全场相衬成像方法,是太赫兹成像技术领域的重要研究方向之一。本文基于连续太赫兹源,从离轴式和同轴式数字全息成像的相衬成像原理、光路系统和再现算法多个方面,介绍了相关技术的研究现状,分析了太赫兹源、再现算法等因素对成像分辨率的影响,并对太赫兹数字全息的发展趋势进行了展望。     

Optical frequency-modulated continuous-wave interferometers

I discuss optical frequency-modulated continuous-wave (FMCW) interferometers, including their principles, characteristics, specific requirements, procedure for their construction, optical configurations, primary applications, optical sources, resolution, measurement range, and stability.     

Potential for detection of explosive and biological hazards with electronic terahertz systems

The terahertz (THz) regime (0.1-10 THz) is rich with emerging possibilities in sensing, imaging and communications, with unique applications to screening for weapons, explosives and biohazards, imaging of concealed objects, water content and skin. Here we present initial surveys to evaluate the possibility of sensing plastic explosives and bacterial spores using field-deployable electronic THz techniques based on short-pulse generation and coherent detection using nonlinear transmission lines and diode sampling bridges. We also review the barriers and approaches to achieving greater sensing-at-a-distance (stand-off) capabilities for THz sensing systems. We have made several reflection measurements of metallic and non-metallic targets in our laboratory, and have observed high contrast relative to reflection from skin. In particular, we have taken small quantities of energetic materials such as plastic explosives and a variety of Bacillus spores, and measured them in transmission and in reflection using a broadband pulsed electronic THz reflectometer. The pattern of reflection versus frequency gives rise to signatures that are remarkably specific to the composition of the target, even though the target's morphology and position is varied. Although more work needs to be done to reduce the effects of standing waves through time-gating or attenuators, the possibility of mapping out this contrast for imaging and detection is very attractive.     

Influence of terahertz pulse width on two-dimensional terahertz spectroscopy

The influence of the width of terahertz (THz) pulses on two-dimensional THz spectroscopy (2DTS) has been studied theoretically via a classical method in which the expressions for the second-order nonlinear polarizations with different nonlinear sources are derived by using a perturbation approach. Compared to the common disposal method in which the THz pulse is treated as a delta function, some terms that were of unknown physical meaning or vanished will come into force when the width is considered. Three types of nonlinear sources, i.e. anharmonicity, nonlinear damping and nonlinear coupling, are considered for a single mode system. The simulation results demonstrate that the width of the incident THz pulse can markedly affect the properties of 2DTS and that different sources have different influences. This study reveals a more practical insight for 2DTS and could provide much information, such as the optimal width and interval of THz pulses, to guide possible future experiments.     

High-bandwidth laser-pulse generator using continuous-wave lasers

A laser-pulse generator for the transfer of continous-wave (cw) absolute radiometric calibrations to high-speed pulse photodetectors is described. The cw input lasers are modulated by a spinning polygon mirror in a ring cavity to produce high-speed, constant-amplitude, constant-shape laser pulses. Constantamplitude outputs range temporally from cw to 15-ns full-width at half-maximum (FWHM) Gaussian pulses. An all-reflective optical design permits operation of the device from the visible to the far-infrared spectral regions. Design and performance estimates for the pulse generator as well as experimental verification are presented.     

Heterodyne wavelength meter for continuous-wave lasers

A wavelength meter based on a heterodyne interferometer is presented. A single-wavelength test laser beam is modulated to two orthogonal linearly polarized components with different frequencies by a pair of acousto-optic modulators. Then the modulated laser beam and a two-wavelength laser beam are sent to a heterodyne interferometer in a common path. The ratio of two laser interference phase shifts in the heterodyne interferometer is equal to the ratio of their wavelengths. The heterodyne technique measures the heterodyne interference phase but not the interference intensity, which means that it could measure a light source whose intensity is not stable. The heterodyne interference signal is an alternating signal that can easily magnify and process the circuit that makes up the heterodyne wavelength meter and could be used to measure the low-intensity light source even when there are environmental disturbances. A tunable diode laser wavelength range of 630-637 nm has been measured to an accuracy of 5 parts in 10(7).     

Analysis of optical frequency-modulated continuous-wave interference

I systematically analyze the theory of optical frequency-modulated continuous-wave (FMCW) interference. There are three different versions of optical FMCW interference, discussed in detail: sawtooth-wave optical FMCW interference, triangular-wave optical FMCW interference, and sinusoidal-wave optical FMCW interference. The essential concepts and technical terms are clearly defined, the necessary simplifications are introduced according to the characteristics of optical waves, and the formulas used to calculate the signal intensities under two different situations (static and dynamic) are properly derived. Advantages and limitations of each version of optical FMCW interference are also discussed.     

Periodically locked continuous-wave cavity ringdown spectroscopy

We demonstrate a simple periodically locked cw cavity ringdown spectroscopy technique that enables a very large number of ringdown events to be rapidly acquired. An external cavity diode laser is locked to a high-finesse cavity, and as many as 16,000 ringdown events per second are obtained by periodically switching off the light entering the high-finesse cavity. Following each ringdown event, the light to the cavity is switched back on and cavity lock is rapidly reacquired. Limited only by our relatively modest digitization rate, we obtained a minimum detectable absorption loss of 4.7 x 10(-9) cm(-1), but we show that faster digitization could provide a sensitivity of 5.9 x 10(-10) cm(-1) Hz(-1/2).     

LD抽运355nm连续紫外激光器

报道了一台LD端面抽运Nd:YVO4晶体的内腔三次谐波转换的全固态连续紫外激光器.在谐振腔内,1064 nm的基频波经Ⅱ类相位匹配KTP晶体进行二倍频产生532 nm波长激光,二者再经Ⅰ类相位匹配LBO晶体进行和频获得355 nm紫外激光输出.在Nd:YVO4晶体的外端镀1064 nm/532 nm双波长高反膜作为输入镜,其与输出镜构成平-凹腔结构,并考虑到Nd:YVO4晶体所产生的热透镜效应,对腔长进行了详细的分析计算.当LD抽运功率为3W时,获得4.2 mW连续运转的355nm紫外激光.     

The thermal regime of solid continuous-wave lasers

A method is presented for the calculation of the temperature field of continuous-wave lasers, given constant thermophysical characteristics for the active medium and that these are dependent on temperature.     

Continued analysis of optical frequency-modulated continuous-wave interference

I continue to analyze systematically the theory of optical frequency-modulated continuous-wave (FMCW) interference. Two special cases, multiple-beam optical FMCW interference and multiple-wavelength optical FMCW interference, are discussed in detail. Multiple-beam optical FMCW interference generates a signal with multiple frequencies because of mutual interference among the waves. Multiple-wavelength optical FMCW interference produces a signal whose amplitude is modulated by a synthetic wave. The applications of both types of optical FMCW interference are also discussed.     

Low-loss terahertz ribbon waveguides

The submillimeter wave or terahertz (THz) band (1 mm-100 microm) is one of the last unexplored frontiers in the electromagnetic spectrum. A major stumbling block hampering instrument deployment in this frequency regime is the lack of a low-loss guiding structure equivalent to the optical fiber that is so prevalent at the visible wavelengths. The presence of strong inherent vibrational absorption bands in solids and the high skin-depth losses of conductors make the traditional microstripline circuits, conventional dielectric lines, or metallic waveguides, which are common at microwave frequencies, much too lossy to be used in the THz bands. Even the modern surface plasmon polariton waveguides are much too lossy for long-distance transmission in the THz bands. We describe a concept for overcoming this drawback and describe a new family of ultra-low-loss ribbon-based guide structures and matching components for propagating single-mode THz signals. For straight runs this ribbon-based waveguide can provide an attenuation constant that is more than 100 times less than that of a conventional dielectric or metallic waveguide. Problems dealing with efficient coupling of power into and out of the ribbon guide, achieving low-loss bends and branches, and forming THz circuit elements are discussed in detail. One notes that active circuit elements can be integrated directly onto the ribbon structure (when it is made with semiconductor material) and that the absence of metallic structures in the ribbon guide provides the possibility of high-power carrying capability. It thus appears that this ribbon-based dielectric waveguide and associated components can be used as fundamental building blocks for a new generation of ultra-high-speed electronic integrated circuits or THz interconnects.     

Coherence analysis of optical frequency-modulated continuous-wave interference

I analyze the coherence of optical frequency-modulated continuous-wave (FMCW) interference. With a simple model modified from the classical coherence theory, I successfully derive the relationships among the frequency bandwidth, coherence length, and coherence time of the practical optical source, and the contrast of the beat signal in optical FMCW interference.     

Three-dimensional terahertz wave imaging

Pulsed terahertz (THz) wave sensing and imaging is a coherent measurement technology. Like radar, based on the phase and amplitude of the THz pulse at each frequency, THz waves provide temporal and spectroscopic information that allows us to develop various three-dimensional (3D) terahertz tomographic imaging modalities. The 3D THz tomographic imaging methods we investigated include THz time-of-flight tomography, THz computed tomography (CT) and THz binary lens tomography. THz time-of-flight uses the THz pulses as a probe beam to temporally mark the target, and then constructs a 3D image of the target using the THz waves scattered by the target. THz CT is based on geometrical optics and inspired from X-ray CT. THz binary lens tomography uses the frequency-dependent focal-length property of binary lenses to obtain tomographic images of an object. Three-dimensional THz imaging has potential in such applications as non-destructive inspection. The interaction between a coherent THz pulse and an object provides rich information about the object under study; therefore, 3D THz imaging can be used to inspect or characterize dielectric and semiconductor objects. For example, 3D THz imaging has been used to detect and identify the defects inside a Space Shuttle insulation tile.     

High-power terahertz synchrotron sources

Electromagnetic waves, or light, are produced by accelerating electrons according to the formula assigned the name of Sir Joseph Larmor, who was secretary of The Royal Society from 1901 to 1912. For relativistic electrons the emission is enhanced by the fourth power of the increase in mass. Thus for 10 MeV electrons, for which the mass increases by a factor of 21, the enhancement is a factor of 200000. We have generated high-power broadband THz light using relativistic electrons and demonstrated that ca. 100 W can readily be produced in a band from 0.1 to 3 THz with possible extensions to 6 THz. The experiments use a new generation of light source in which bunches of electrons circulate once, but in which their energy is recovered. In such a machine the electron bunches can be very much shorter than those, say, in storage rings or synchrotrons.     

Towards terahertz near-field microscopy

We have detected sub-wavelength spot sizes in the near-field of a metal tip, which is illuminated with terahertz (THz) pulses. The THz near-field is detected by using electro-optic detection in a (100) oriented GaP crystal. Contrary to conventional electro-optic detection, which uses (110) oriented detection crystals, (100) crystals only allow the detection of THz light, polarized perpendicular to our crystal surface. This component is strongly localized near the apex of the tip, which has sub-wavelength dimensions. The detection process is blind to the incident THz radiation, which is polarized parallel to the crystal surface. As a result, a sub-wavelength THz spot size with an intensity full-width half maximum (FWHM) diameter of lambda/200 is observed.     

Mid-infrared w quantum-well lasers for noncryogenic continuous-wave operation

We review the recent progress of electrically injected and optically pumped mid-IR lasers based on antimonide quantum wells with the type II W configuration. W quantum-well diodes have achieved cw operation up to 195 K at lambda = 3.25 mum. Optically pumped devices that employ the diamond pressure bond heat sink have reached 290 K at 3 mum and 210 K at 6 mum. Pulsed power conversion efficiencies of up to 7% at 220 K have been attained by use of an optical pumping injection cavity approach, in which an etalon cavity for the pump beam significantly enhances its absorptance. The angled-grating distributed-feedback configuration has been used to obtain near-diffraction-limited output for an optical pumping stripe width of 50 mum.     

A full-wave Helmholtz model for continuous-wave ultrasound transmission

A full-wave Helmholtz model of continuous-wave (CW) ultrasound fields may offer several attractive features over widely used partial-wave approximations. For example, many full-wave techniques can be easily adjusted for complex geometries, and multiple reflections of sound are automatically taken into account in the model. To date, however, the full-wave modeling of CW fields in general 3D geometries has been avoided due to the large computational cost associated with the numerical approximation of the Helmholtz equation. Recent developments in computing capacity together with improvements in finite element type modeling techniques are making possible wave simulations in 3D geometries which reach over tens of wavelengths. The aim of this study is to investigate the feasibility of a full-wave solution of the 3D Helmholtz equation for modeling of continuous-wave ultrasound fields in an inhomogeneous medium. The numerical approximation of the Helmholtz equation is computed using the ultraweak variational formulation (UWVF) method. In addition, an inverse problem technique is utilized to reconstruct the velocity distribution on the transducer which is used to model the sound source in the UWVF scheme. The modeling method is verified by comparing simulated and measured fields in the case of transmission of 531 kHz CW fields through layered plastic plates. The comparison shows a reasonable agreement between simulations and measurements at low angles of incidence but, due to mode conversion, the Helmholtz model becomes insufficient for simulating ultrasound fields in plates at large angles of incidence.     

Sum-frequency generation of continuous-wave light at 194 nm

Over 2 mW of continuous-wave tunable 194-nm light is produced by sum-frequency mixing approximately 500 mW of 792-nm and 500 mW of 257-nm radiation in beta-barium borate (BBO). The powers in both fundamental beams are enhanced in separate ring cavities whose optical paths overlap in the Brewster-cut BBO crystal. Due to the higher circulating fundamental powers, the sum-frequency-generated power is nearly 2 orders of magnitude greater than previously reported values.