Frequency-domain Interferometer Research Articles

All-Fiber Pulse-Train Optical Frequency-Domain Interferometer for Dynamic Absolute Distance Measurements of Vibration

In this paper, we propose an all-fiber co-axial optical frequency-domain interferometer (OFDI) in a pulse-train mode with a sample rate of 9 kHz for measuring the vibrations in an internal structure without any contact. It was subjected to a range of 4.555 mm and had an accuracy level of 0.006 mm, as confirmed by a linear motion experiment. Due to the precise time synchronization for reducing the background light leakage and suppressing the dynamic fuzziness, the proposed OFDI could realize the dynamic absolute distance measurements of the vibration process under harmonic excitation with frequencies ranging from 200 Hz to 1800 Hz. The characteristic parameters of vibration could be analyzed using the acquired distance results. Furthermore, the OFDI system obtained the frequency conversion as the time under anharmonic periodic excitation with a sweeping rate of 3600 Hz/s. The measurement performance for the vibration velocity compared with the displacement interferometer system for any reflector (DISAR) was demonstrated in a harmonic excitation experiment. The proposed method expands the application of all-fiber OFDI technology from static to dynamic scenes.

Effect of differential tip clearance on the performance and noise of an axial compressor

Rotor tip clearance (TC) shows significant effects on the stability and performance of aero-engine compressors. TC is also related to the producing of noise and tip leakage flow which is an indicator of critical operating conditions for rotating stall or surge. In this paper, the effect of TC on stall evolution process under different TC values is studied on a single-stage compressor with low speed and high load, and a frequency-domain interferometer diastimeter for high-resolution measurements is applied to accurately measure the TC value. In the present study, a soft-wall casing made of grindable material is applied to achieve the “zero clearance” condition, and casings with different diameters are adopted to reach different clearance values. Experimental results show that pressure rise and efficiency are improved with a small TC. With the TC varying from zero to a certain large scale, experimental results have proved that there is an optimal TC with the presence of maximum flow margin. In addition, eighty measurement points are evenly distributed around the casing circumferentially at the leading edge of the blade to analyze the circumferential flow characteristics of the tip flow field. And twenty wall-flush mounted microphones are evenly distributed circumferentially to monitor the induct sound field of the compressor, and the experimental results show that the sound pressure level (SPL) of the compressor is lowest at a relatively small TC rather than zero. The influence of different TC values on compressor is discussed based on the analysis of the pressure fields, stall process, and the acoustic signal. Furthermore, a detailed discussion of rotating instability (RI) brought about by large clearances is provided.

Refractive index measurement of compressed nitrogen using an infrared frequency-domain interferometer

Frequency-domain interferometry is used to obtain the refractive index of compressed nitrogen in the vicinity of 1550 nm as the pressure of nitrogen increases from 0 to 40 MPa at room temperature. By referring to the equation of state of nitrogen, the relation between the refractive index and the nitrogen density is obtained and the electric polarizability of a nitrogen molecule is thereby determined to be (1.82 ± 0.15) × 10−30 m−3 according to the Lorentz-Lorentz relation.

Frequency-domain Hong–Ou–Mandel interference

Hong-Ou-Mandel (HOM) interference unveils a distinct behavior of identical particles which cannot be distinguished from each other. Especially for bosons, two separated identical particles passing through a beamsplitter always go together into one of the output ports, but that is not the case with other particles including fermions or classical ones. So far many elemental properties of quantum physics and information have been discovered through the concatenated HOM effects, which has been demonstrated in photons and recently in plasmons, atoms and phonons. However, all demonstrations in optical region employed two particles in different spatial modes. Here we first report the HOM interference between two photons in a single spatial mode with different frequencies (energies) by using a partial frequency conversion. The demonstrated frequency-domain interferometer allows us to replace spatial optical paths by optical frequency multiplexing, which opens up a distinct architecture of the quantum interferometry.

Dynamic frequency-domain interferometer for absolute distance measurements with high resolution.

A unique dynamic frequency-domain interferometer for absolute distance measurement has been developed recently. This paper presents the working principle of the new interferometric system, which uses a photonic crystal fiber to transmit the wide-spectrum light beams and a high-speed streak camera or frame camera to record the interference stripes. Preliminary measurements of harmonic vibrations of a speaker, driven by a radio, and the changes in the tip clearance of a rotating gear wheel show that this new type of interferometer has the ability to perform absolute distance measurements both with high time- and distance-resolution.

Note: A novel method to measure the deformation of diamond anvils under high pressure

A novel and simple method based on optical-fiber frequency domain interferometer to measure the deformation of diamond anvils under high pressure is presented. The working principle and application examples are given in this paper. The deformation of diamond anvils is obtained up to 37.7 GPa, our results verify that the deformation has an obvious difference between uploading and downloading at a given pressure, the maximum difference is up to 4.5 μm at 18.8 GPa, and the cupping effect is observed directly.

Optical-fiber frequency domain interferometer with nanometer resolution and centimeter measuring range

A new optical-fiber frequency domain interferometer (OFDI) device for accurate measurement of the absolute distance between two stationary objects, with centimeter measuring range and nanometer resolution, has been developed. Its working principle and on-line data processing method were elaborated. The new OFDI instrument was constructed all with currently available commercial communication products. It adopted the wide-spectrum amplified spontaneous emission light as the light source and optical-fiber tip as the test probe. Since this device consists of only fibers or fiber coupled components, it is very compact, convenient to operate, and easy to carry. By measuring the single-step length of a translation stage and the thickness of standard gauge blocks, its ability in implementing nanometer resolution and centimeter measuring range on-line measurements was validated.

Vibration Sensor Based on Frequency-domain Interferometer by Using Doppler Shift Effect

Vibration Sensor Based on Frequency-domain Interferometer by Using Doppler Shift Effect

Frequency-domain interferometer simulation with higher-order spatial modes

FINESSE is a software simulation allowing one to compute the optical properties of laser interferometers used by interferometric gravitational-wave detectors today. This fast and versatile tool has already proven to be useful in the design and commissioning of gravitational-wave detectors. The basic algorithm of FINESSE numerically computes the light amplitudes inside an interferometer using Hermite–Gauss modes in the frequency domain. In addition, FINESSE provides a number of commands for easily generating and plotting the most common signals including power enhancement, error and control signals, transfer functions and shot-noise-limited sensitivities. Among the various simulation tools available to the gravitational wave community today, FINESSE provides an advanced and versatile optical simulation based on a general analysis of user-defined optical setups and is quick to install and easy to use.

Direct measurement of coherent ultrahigh wakefields excited by intense ultrashort laser pulses in a gas-jet plasma

The coherent wakefield excited by 2 TW, 50 fs laser pulses in a gas-jet plasma around 1018 cm−3 is measured with a time-resolved frequency domain interferometer. The density distribution of the helium gas is measured with a time-resolved Mach–Zehnder interferometer to search for the optimum laser focus position and timing in the gas jet. The results show an accelerating wakefield excitation of 20 GeV/m with good coherency, which is useful for ultrahigh gradient particle acceleration in a compact system. This is the first time-resolved measurement of laser wakefield excitation in a gas-jet plasma.

White-Light Frequency-Domain Interferometry Using a Köosters Prism

An optical frequency-domain interferometer incorporating a Kosters prism is constructed to offer an excellent instrument for making a stable precision-measurement of material absorption and dispersion. The working stability is well retained over a long-term operation of 15 h, but the spectral resolution is limited to the resolving power of the spectrum analyzer used. The absorption and dispersion measurements are demonstrated by use of two samples of a BK-7 glass plate and rhodamine in ethanol.

Femtosecond phase spectroscopy of multi-level systems: Phthalocyanines

A femtosecond frequency-domain interferometer is applied to metal-free and vanadyl phthalocyanine (H2Pc and VOPc) thin films to measure time-resolved difference phase and transmission spectra simultaneously. For both samples, the phase-change dynamics is different from the transmission-change dynamics at 620 nm, reflecting that the phthalocyanines (Pc) cannot be modeled with a two-level system but by a multi-level or inhomogeneously broadened system, in which each level pair exhibits different relaxation dynamics. Because of this dynamical difference, a phase-change measurement is required to correct distortion of the transient spectra due to induced phase modulation of probe pulses. Near zero time delay, the phase and transmission changes show different growth behavior. This behavior is explained by antisymmetric amplitude and phase gratings which are produced by coherent coupling between frequency-chirped pump and probe pulses.

Femtosecond phase spectroscopy by use of frequency-domain interference

We present the principles, experimental procedures, applications, and theoretical analyses of femtosecond phase spectroscopy, which is complementary to femtosecond absorption spectroscopy. In femtosecond phase spectroscopy difference spectra of both phase and transmission are simultaneously measured with a frequency-domain interferometer, which is only slightly modified from the conventional pump–probe method. Femtosecond time-resolved dispersion relations for CdSxSe1−x-doped glass and CS2 are obtained with transform-limited pulses of 60-fs duration and 620-nm center wavelength. The results are theoretically analyzed and are well reproduced by numerical simulations. Although time-resolved data are not expected to satisfy the Kramers–Kronig (K–K) relations, the degree of discrepancy from the K–K relations is more substantial for CS2 than for CdSxSe1−x-doped glass. These results arise from the difference in the linear susceptibility and in the excited-population dynamics. The conditions for which the K–K relations are applicable to time-resolved spectra are obtained theoretically and verified experimentally. It is shown that induced amplitude and phase modulations of the probe pulses cause a deviation from the K–K relations.

Frequency-domain interferometer for measuring the phase and amplitude of a femtosecond pulse probing a laser-produced plasma

A frequency-domain interferometer for probing the variations of the dielectric constant of a plasma with sub-100- fs temporal resolution and lambda/2000 phase resolution is described. Imaging the plasma on the entrance slit of a spectrograph provides spatial resolution along a diameter of the focal spot. The technique is used to map out the expansion of the critical density surface of a femtosecond laser-produced plasma with subnanometer spatial resolution along the laser axis.

Frequency-domain interferometer for measurement of the polarization mode dispersion in single-mode optical fibers

A new technique based on frequency-domain interferometry has been used to measure the polarization mode dispersion of a birefringent optical fiber. This technique is based on the spectral interference of two, temporally delayed, broadband ultrashort pulses in the frequency domain. Two different configurations are discussed.

New observational techniques for studying the dynamics of the middle atmosphere using the Chung Li VHF radar

Taking advantage of the newly developed volume scattering model for MST radar and the unique features of the Chung Li VHF radar, several novel observational techniques have been developed and implemented. Techniques such as oblique spaced antenna (OSA), the frequency domain analysis of spaced antenna data, the full spectrum analysis (FSA) and the multifrequency frequency domain interferometer (FDI) will be discussed and experimental results will be presented. Potential applications of the new techniques to study the dynamics of the middle atmosphere will be discussed.

Femtosecond time-resolved dispersion relations studied with a frequency-domain interferometer.

Difference phase and transmission spectra (DPS and DTS) due to induced phase modulation of probe pulses in ${\mathrm{CS}}_{2}$ were measured with a femtosecond frequency-domain interferometer. Signals that satisfy the Kramers-Kronig (KK) relations with opposite signs were observed. At zero time delay, DTS and DPS were both even functions with respect to the probe center frequency due to the fifth- and third-order nonlinearities, respectively. The conditions for applying the KK relations to time-resolved data were obtained theoretically and verified experimentally.

Induced phase modulation of chirped continuum pulses studied with a femtosecond frequency-domain interferometer

The effect of rapid phase change on chirped continuum pulses is studied with a frequency-domain interferometer. Because of the chirp, temporal evolution of the optical Kerr response in CS(2) is projected into difference phase spectra. The chirped continuum shows spectral shifts that are due to induced phase modulation even when the continuum has a flat spectrum.

Frequency-domain interferometer for femtosecond time-resolved phase spectroscopy

A new femtosecond time-resolved interferometer was developed that utilizes interference fringes in the frequency domain to obtain simultaneously difference phase spectra (DPS) and difference transmission spectra with a multichannel spectrometer. For the first time to our knowledge, transient oscillations were observed in DPS and the spectral shift of a probe pulse was time resolved together with the rise in DPS, which is clear evidence for induced phase modulation in absorptive materials.

The uses of swept frequency microwaves

The microwave frequency domain interferometer can be used for measuring refractive index and thickness and for detecting flaws in dielectric materials. This pulse-echo technique has been used on sample thicknesses from less than 10mm to over 1m; it can resolve defects as small as 2mm in diameter or 0.025mm in thickness. In ndt applications the technique is characterized by low attenuation in most plastics, lack of coupling problems, and penetration of multiple laminar defects. Thickness measurements are more accurate than with ultrasonics for most plastics. Refractive index measurements are accurate and do not require cutting the samples for insertion into a waveguide.