Polarized differential-phase laser scanning microscope

A polarized differential-phase laser scanning microscope, which combines a polarized optical heterodyne Mach-Zehnder interferometer and a differential amplifier to scan the topographic image of a surface, is proposed. In the experiment the differential amplifier, which acts as a PM-AM converter in the experiment, converting phase modulation (PM) into amplitude modulation (AM). Then a novel, to our knowledge, phase demodulator was proposed and implemented for the differential-phase laser scanning microscope. An optical grating (1800 lp/mm) was imaged. The lateral and the depth resolutions of the imaging system were 0.5 mum and 1 nm, respectively. The detection accuracy, which was limited by the reflectivity variation of the test surface, is discussed.     

Laser Interferometer for Space Gravitational Waves Detection and Earth Gravity Mapping

The idea of using space laser interferometer to measure the relative displacement change between two satellites has been considered for space gravitational waves detection and Earth gravity filed mapping in recent years. Some investigations on the key issues of laser interferometer in our working team have been presented in this paper. An on-ground laser interferometer prototype used for the demonstration of satellite-to-satellite ranging has been constructed, which is equipped with phasemeter, laser pointing modulation and laser phase-locking control. The experimental results show that path-length measurement sensitivity of the laser interferometer reaches 200 pm/√ Hz, and phase measurement precision achieves 2π × 10^??5 rad/√ Hz, and laser pointing modulation precision is better than 80 nrad/√ Hz, and laser phase-locking control precision attains 2π × 10^??4 rad/√ Hz within the frequency regime of 1 mHz–1 Hz. All of these demonstrate that the proposed laser interferometer has very promising feasibility to meet the requirement of the Taiji, TianQin and Space Advanced Gravity Measurement (SAGM) missions which are put forward by Chinese scientists.     

Quantum-limited optical phase detection at the 10(-10)-rad level.

Interferometric detection of gravitational waves at a level of astrophysical interest is expected to require measurement of optical phase differences of < or = 10(-10) rad. A fundamental limit to the phase sensing is the statistics of photon detection--Poisson statistics for light in a coherent state. We have built a laboratory-scale interferometer to achieve and investigate the phase detection sensitivity required for the Laser Interferometer Gravitational-Wave Observatory. With 70 W of circulating power, we have obtained a phase sensitivity of 1.28 x 10(-10) rad/square root(Hz) at frequencies above 600 Hz, limited by quantum noise. Below 600 Hz, excess noise above the quantum limit is seen, and we present our investigations into the sources of this excess. Compared with the results of previous such experiments, the phase sensitivity over the full 100-Hz-10-kHz band of interest has been improved by factors of up to 100, with a factor-of-2.5 improvement in the quantum-limited level.     

A Stabilized Fiber Laser for High-Resolution Low-Frequency Strain Sensing

We frequency stabilize a fiber laser for use in low-frequency sensing applications. Using a radio frequency locking technique, an Erbium-doped single longitudinal mode fiber laser is stabilized to a Mach–Zehnder interferometer. The low-frequency fiber laser noise was suppressed by more than 1.5 orders of magnitude at frequencies below 300 Hz reaching a minimum of 2 ${rm Hz}/sqrt {rm Hz}$ between 60 and 250 Hz. The corresponding strain sensitivities are 2 ${rm p}epsilon /sqrt {rm Hz}$ at 1 Hz and 15 ${rm f}epsilon /sqrt {rm Hz}$ from 60 to 250 Hz.      

Intersatellite range monitoring using optical interferometry

We report on an interferometer designed to provide 1-10 nm/square root(Hz) displacement measurement resolution, in the range 0.01 Hz to 1 Hz, while in low Earth orbit. The interferometer comprises two units, each with its own laser and in separate satellites, which would be in the same orbit separated by approximately 50 km. We discuss the requirements on the interferometer subsystem and describe the optical transponder distance measurement, including a phase locking method to generate a heterodyne beat signal between the two lasers. Design, fabrication, and testing of a "flightlike" engineering model interferometer is outlined, and results from environmental and performance tests are reported.     

Multiline absorption spectroscopy for methane gas detection

A multiline absorption spectroscopy technique was investigated based on the single-line absorption spectroscopy technique. An open-path methane-detecting system was designed. An LED was used as a broadband source, and a Fabry-Perot interferometer whose transmission peaks matched the methane R-branch absorption lines was used to enhance the detectable sensitivity. We demonstrate a minimum-detectable concentration of 7600 +/- 10% ppm (parts per million) with a multiline differential absorption spectroscopy technique and a concentration of 1000 +/- 10% ppm with a multiline wavelength modulation spectroscopy technique.     

Alignment of an interferometric gravitational wave detector

Interferometric gravitational wave detectors are designed to detect small perturbations in the relative lengths of their kilometer-scale arms that are induced by passing gravitational radiation. An analysis of the effects of imperfect optical alignment on the strain sensitivity of such an interferometer shows that to achieve maximum strain sensitivity at the Laser Interferometer Gravitational Wave Observatory requires that the angular orientations of the optics be within 10(-8) rad rms of the optical axis, and the beam must be kept centered on the mirrors within 1 mm. In addition, fluctuations in the input laser beam direction must be less than 1.5 x 10(-14) rad/ radicalHz in angle and less than 2.8 x 10(-10) m/ radicalHz in transverse displacement for frequencies f > 150 Hz in order that they not produce spurious noise in the gravitational wave readout channel. We show that seismic disturbances limit the use of local reference frames for angular alignment at a level approximately an order of magnitude worse than required. A wave-front sensing scheme that uses the input laser beam as the reference axis is presented that successfully discriminates among all angular degrees of freedom and permits the implementation of a closed-loop servo control to suppress the environmentally driven angular fluctuations sufficiently.     

Two-longitudinal-mode He-Ne laser for heterodyne interferometers to measure displacement

We propose a new configuration for a high-resolution heterodyne interferometer that employs a two-longitudinal-mode He-Ne laser with an intermode beat frequency of 600-1000 MHz. The high beat frequency is downconverted to 5 MHz such that the phase change of the interferometer output is precisely measured with a displacement resolution of 0.1 nm. A thermal control scheme is adopted to stabilize the cavity length of the He-Ne plasma tube such that a frequency stability of 2 parts in 10(9) is obtained by suppression of frequency drifts caused by t he phenomena of frequency pulling and polarization anisotropy. This two-longitudinal-mode He-Ne laser yields a high output power of 2.0 mW, which permits multiple measurements of as many as 10 machine axes simultaneously.     

基于干涉解调技术的光纤激光器水声传感系统

分析了基于Michelson干涉解调技术的光纤激光器水声传感的原理,在一段掺铒光纤中写入具有π相移的光纤光栅构成光纤激光器,水声压力作用在激光器上引起激光工作波长的变化;采用基于3×3耦合器的偏振无关的非平衡光纤Michelson干涉仪将激光波长变化转化为干涉仪的相位变化;干涉仪的输出由光电探测器转换后使用DSP进行信号解调.针对3×3耦合器分光比不对称的问题,本文提出利用实时调整幅度的2路干涉信号进行解调的方案,该方案不需要3×3耦合器有严格的分光比,消除了外界环境对解调输出的非线性影响.水声探测实验表明,光纤激光器水声传感系统的声压灵敏度为-166.5 dB(参考值1 rad/μPa),解调结果与水声信号具有良好的线性关系.     

角位移Fabry-Perot干涉测量

提出了一种基于Fabry-Perot板干涉的角位移测量新方法。此方法采用函数近似,只需将初始入射角确定在40°到50°之间,即可由角位移与干涉信号条纹数变化间的函数关系,高精度测量角位移。解决了采用F-P板干涉法测量角位移需精确确定入射光初始角的问题。使用计算机处理采集的干涉信号,对干涉条纹进行细分,实现干涉信号相位测量的高分辨力。理论模拟和实验结果得出本方法可以实现精度为10-5rad数量级的角位移测量。该方法的测量装置采用带尾纤的半导体激光作为光源,由自聚焦透镜准直,出射光束直径为0.5mm,使探测头为小光斑。该装置结构简单,能实现小型化。     

Quantum-cascade laser measurements of stratospheric methane and nitrous oxide

A tunable quantum-cascade (QC) laser has been flown on NASA's ER-2 high-altitude aircraft to produce the first atmospheric gas measurements with this newly invented device, an important milestone in the QC laser's future planetary, industrial, and commercial applications. Using a cryogenically cooled QC laser during a series of 20 aircraft flights beginning in September 1999 and extending through March 2000, we took measurements of methane (CH(4)) and nitrous oxide (N(2)O) gas up to ~20 km in the stratosphere over North America, Scandinavia, and Russia. The QC laser operating near an 8-mum wavelength was produced by the groups of Capasso and Cho of Bell Laboratories, Lucent Technologies, where QC lasers were invented in 1994. Compared with its companion lead salt diode lasers that were also flown on these flights, the single-mode QC laser cooled to 82 K and produced higher output power (10 mW), narrower laser linewidth (17 MHz), increased measurement precision (a factor of 3), and better spectral stability (~0.1 cm(-1) K). The sensitivity of the QC laser channel was estimated to correspond to a minimum-detectable mixing ratio for methane of approximately 2 parts per billion by volume.     

Holographic common-path interferometer for angular displacement measurements with spatial phase stepping and extended measurement range

A novel technique for extending the unambiguous measurement range for differential measurements of angular deflections is presented. The technique utilizes a common-path interferometer that simultaneously probes the out-of-plane displacement of three points on the object surface. The system is based on a single laser diode, and all the optical functions of the system are implemented in a dedicated holographic optical element (HOE). The HOE automatically provides spatially phase-stepped interference signals for real-time phase measurement. It is therefore not necessary to employ any polarizing optics or active elements to introduce the phase stepping. The common-path scheme combined with the HOE provides a system that is inherently stable, since the HOE operates as both transmitter and receiver in the system. The system is compact, is robust, and has the potential for being mass-produced at a low cost and is thus well suited for industrial use, such as in commercial vibrometers. The technique is demonstrated in a system for measuring angular deflections of a plane mirror. The technique, however, is not restricted to this use alone and can easily be configured to probe other types of surface displacements, e.g., the deflection of a diaphragm. In the present configuration, the system can measure angular deflections with a sensitivity of 2.5 x 10(-7) rad over a measurement range that is approximately 3.5 x 10(-3) rad, i.e., a dynamic range of approximately 1:14,000. Furthermore, the system can easily be reconfigured for a desired angular sensitivity and measurement range.     

The Development of Phasemeter for Taiji Space Gravitational Wave Detection

Taiji space gravitational wave detection utilizes the laser interferometer to convert the tiny distance change into the phase fluctuation of the beat note. As to realize the sensitivity of 1 pm/√ Hz, the phasemeter needs to calculate the phase with the precision of 2πμ rad/√ Hz in the frequency range of 0.1 mHz and 1 Hz. In this paper, we report recent progress of the phasemeter for Taiji. Noises which possibly affect the measurement sensitivity are tested and discussed, especially the sampling noise and the frequency jitter. Finally, the accuracy of the phasemeter is calibrated. The result shows that the sensitivity has reached the requirement of Taiji in the frequencies between 0.01 Hz and 1 Hz, 0.1 mHz–1 mHz. Noises in the range of 1 mHz and 0.01 Hz, which have not yet depressed well, are dominated by the clocking jitter and the thermal fluctuation.     

HBAR-Based 3.6 GHz oscillator with low power consumption and low phase noise

We have designed and built 2 oscillators at 1.2 and 3.6 GHz based on high-overtone bulk acoustic resonators (HBARs) for application in chip-scale atomic clocks (CSACs). The measured phase noise of the 3.6 GHz oscillator is -67 dBc/Hz at 300 Hz offset and -100 dBc/Hz at 10 kHz offset. The Allan deviation of the free-running oscillator is 1.5 × 10^-9 at one second integration time and the power consumption is 3.2 mW. The low phase noise allows the oscillator to be locked to a CSAC physics package without significantly degrading the clock performance.     

Resolving quadrature fringes in real time

In many interferometers, two fringe signals can be generated in quadrature. The relative phase of the two fringe signals depends on whether the optical path length is increasing or decreasing. A system is developed in which two quadrature fringe signals are digitized and analyzed in real time with a digital signal processor to yield a linear, high-resolution, wide-dynamic-range displacement transducer. The resolution in a simple Michelson interferometer with inexpensive components is 5 x 10(-13) m Hz(-1/2) at 2 Hz.     

Frequency- and intensity-noise measurements of a widely tunable 2-/spl mu/m Tm-Ho:KYF laser

The measurement of the frequency and intensity noise in a novel single-mode 2-/spl mu/m Tm-Ho:KYF laser is presented. The laser frequency noise is measured by exploiting the fringe side of the transmission of a Fabry-Pe/spl acute/rot interferometer. The measured power spectral density of the frequency noise is principally characterized by a random-walk noise contribution, which sets an emission linewidth of /spl sim/ 600 kHz for the 2-/spl mu/m radiation. The relative intensity noise (RIN) reaches the quantum limit of -155 dB/Hz for Fourier frequencies above 1 MHz and shows a maximum level of -90 dB/Hz at the relaxation-oscillation frequency of 20 kHz.     

Noise and Sensitivity of Aluminum Kinetic Inductance Detectors for Sub-mm Astronomy

Kinetic inductance detectors are based upon high Q superconducting resonators. We have measured the electrical Noise Equivalent Power (NEP) of 100 nm thick 1/4λ coplanar waveguide Aluminum resonators at 100 mK using phase readout and radius readout. We find that the phase NEP is independent of the Q factor of the resonator, limited by excess noise in the KID and given by NEP at 100 Hz. It increases with roughly f ^−0.5 at lower frequencies. The amplitude NEP is strongly Q factor dependent, limited by the setup noise, nearly frequency independent and as low as NEP for a high Q resonator (Q=454.000). For lower Q resonators the amplitude NEP increases to values equal to or even larger than the phase readout.      

Differential wavelength-scanning heterodyne interferometer for measuring large step height

An interferometer based on the differential heterodyne configuration and wavelength-scanning interferometry for measuring large step heights is presented. The proposed interferometer is less sensitive to environmental disturbances than other interferometers and can accurately measure interference phases. A tunable diode laser is utilized to illuminate the interferometer and thus solve the phase ambiguity problem. Counting the interference fringes as the wavelength is scanned through a known change in wavelength directly determines the step height. Three gauge blocks of different lengths, 5, 10, and 50 mm, are individually wrung on a steel plate to simulate large step heights. Comparing the results measured by the proposed interferometer with those by the gauge block interferometer reveals that the accuracy is approximately 100 nm.     

Measurement of changes in optical path length and reflectivity with phase-shifting laser feedback interferometry

The operating characteristics of a novel phase-shifting interferometer are presented. Interference arises by reflecting the light from a sample back into the cavity of a cw He-Ne laser. Changes in phase and fringe visibility are calculated from an overdetermined set of phase-shifted intensity measurements with the phase shifts being introduced with an electro-optic modulator. The interferometer is sensitive enough to measure displacements below 1 Hz with a rms error of approximately 1 nm from a sample that reflects only 3% of the 28 muW that is incident on its surface. The interferometer is applied to the determination of cantilever bending of a piezoelectric bimorph.     

Micromachined low-finesse Fabry-Perot interferometer for the measurement of DC and AC electrical currents

A micromachined low finesse Fabry-Perot interferometer for measuring DC and AC electrical current is presented. Interrogation of the microcavity is achieved by a dual-wavelength fiber Bragg grating technique working in quadrature. A linear relation between the DC electrical current and the optical phase defined by the microcavity was detected. Large enhancement of the sensitivity of the microcavities is presented with the use of a planar coil instead of a power line. The sensitivity of the sensor with the planar coil configuration is 7.9 rad/A and resolution of /spl sim/0.18 mA//spl radic/Hz is achieved when the distance between the planar coil and the transducer head is 2 mm. The response of the sensor for AC measurements is 0.14 V/A with a resolution of 6 mA//spl radic/Hz when the distance between the power line and the transducer head is 5.5 cm.