Characterization of the wind imaging interferometer

The Wind Imaging Interferometer is a field-widened Michelson interferometer onboard the Upper Atmosphere Research Satellite. The characterization of the instrument required a pixel-by-pixel evaluation of the instrument performance. Some of the configurations, techniques, and results of the characterization are summarized. Throughput was excellent and equivalent to a total system quantum efficiency of ~10%. Localized spatial noise in response has been attributed to scattering from residual surface effects on the CCD. Instrument visibility factors greater than 90% were measured with distinct distribution patterns over the field of view that were different for the night and day apertures. The instrument phase for zero wind was determined with laboratory airglow sources.     

Characterization and use of an optical fiber interferometer for measurement of the electric wind

An optical fiber interferometer of the Mach-Zehnder type has proved to be a convenient and accurate method for measuring the electric wind in the active region of a corona discharge. The technique relies on the cooling effect of the wind on a small heated region of one arm of the interferometer, which has been remotely heated with an infrared CO(2) laser beam. Wind speeds of up to 5.5 m s(-1) have been measured near the generation region, and by the use of a mesh electrode, the wind has been detected on the axis up to 0.5 m away from the gap. A number of characterization experiments that show the interferometer to be a useful diagnostic tool in the quantitative analysis of the CO(2) laser beam have also been carried out, and good agreement between experimental results and theoretical calculations based on a simple heat-power balance equation for the fiber exists.     

Polar cap mesosphere wind observations: comparisons of simultaneous measurements with a Fabry-Perot interferometer and a field-widened Michelson interferometer

Polar cap mesospheric winds observed with a Fabry-Perot interferometer with a circle-to-line interferometer optical (FPI/CLIO) system have been compared with measurements from a field-widened Michelson interferometer optimized for E-region winds (ERWIN). Both instruments observed the Meinel OH emission emanating from the mesopause region (~86 km) at Resolute Bay, Canada (74.9 degrees N, 94.9 degrees W). This is the first time, to our knowledge, that winds measured simultaneously from a ground-based Fabry-Perot interferometer and a ground-based Michelson interferometer have been compared at the same location. The FPI/CLIO and ERWIN instruments both have a capability for high temporal resolution (less than 10 min for a full scan in the four cardinal directions and the zenith). Statistical comparisons of hourly mean winds for both instruments by scatterplots show excellent agreement, indicating that the two optical techniques provide equivalent observations of mesopause winds. Small deviations in the measured wind can be ascribed to the different zenith angles used by the two instruments. The combined measurements illustrate the dominance of the 12-h wave in the mesopause winds at Resolute Bay, with additional evidence for strong gravity wave activity with much shorter periods (tens of minutes). Future operations of the two instruments will focus on observation of complementary emissions, providing a unique passive optical capability for the determination of neutral winds in the geomagnetic polar cap at various altitudes near the mesopause.     

Nonlinear common-path interferometer: an image processor

A single-lens optical setup with a nonlinear medium placed in its geometrical focal plane is used to contrast a phase disturbance. This setup blends the robustness of phase-contrast methods with an optical nonlinear intensity-dependent medium and the usefulness of traditional interferometric techniques. We show that the ratio of the total illumination area to the phase-object area determines an adequate phase-disturbance contrast.     

Assessment of a multibeam Fizeau wedge interferometer for Doppler wind lidar

The Fabry-Perot interferometer is the standard instrument for the direct detection Doppler lidar measurement of atmospheric wind speeds. The multibeam Fizeau wedge has some practical advantages over the Fabry-Perot, such as the linear fringe pattern, and is evaluated for this application. The optimal Fizeau must have a resolving power of 10(6) or more. As the multibeam Fizeau wedge is pushed to such high resolving power, the interference fringes of the device become complicated by asymmetry and secondary maxima. A simple condition for the interferometer plate reflectance, optical gap, and wedge angle reveals whether a set of parameters will yield simple, Airy-like fringes or complex Fizeau fringes. Tilting of the Fizeau wedge improves the fringe shape and permits an extension of the regime of Airy-like fringes to higher resolving power. Sufficient resolving power for the wind lidar application is shown to be possible with a large-gap, low-finesse multibeam Fizeau wedge. Liabilities of the multibeam Fizeau wedge in the wind lidar application include a smaller acceptance solid angle and calibration sensitivity to localized deviations of the plates from the ideal.     

Mach-zehnder interferometer as a spectral analyzer for molecular Doppler wind lidar

The theoretical performance of a Mach-Zehnder interferometer used as a spectral analyzer for wind-speed measurement by direct-detection Doppler lidar is presented. The interferometer is optimized for measurement of wind velocity from the signal backscattered by the molecules. Two arrangements are proposed, involving two detection channels (DMZ) or four detection channels (QMZ). Using the assumption of a pure molecular signal with a Gaussian spectral profile, we derive an analytic expression for the standard deviation of the measurement error for each arrangement. They are then compared with the ideal spectral analyzer (ISA) and with the double-edge Fabry-Perot (DFP) in the case of a shot-noise-limited signal. The DMZ measurement error is shown to be only 1.65 times that of the ISA and is 1.4 times lower than that given by the DFP. The QMZ arrangement provides a measurement that is insensitive to the aerosol scattering contribution but gives a measurement error that is 1.4 times higher than that of the DMZ.     

Fabry-Perot interferometer based Mie Doppler lidar for low tropospheric wind observation

Similar in principle to recent implementations of a lidar system at 355 nm [Opt. Lett. 25, 1231 (2000), Appl. Opt. 44, 6023 (2005)], an incoherent-detection Mie Doppler wind lidar at 1064 nm was developed and deployed in 2005 [Opt. Rev. 12, 409 (2005)] for wind measurements in the low troposphere, taking advantage of aerosol scattering for signal enhancement. We present a number of improvements made to the original 1064 nm system to increase its robustness for long-period operation. These include a multimode fiber for receiving the reference signal, a mode scrambler to allow uniform illumination over the Fabry-Perot interferometer, and a fast scannable Fabry-Perot interferometer for calibration and for the determination of outgoing laser frequency during the wind observation. With these improvements in stability, the standard deviation of peak transmission and FWHM of the Fabry-Perot interferometer was determined to be 0.49% and 0.36%, respectively. The lidar wind measurements were validated within a dynamic range of +/-40 m/s. Comparison experiments with both wind profiler radar and Vaisala wiresonde show good agreement with expected observation error. An example of 24 h continuous observations of wind field and aerosol backscatter coefficients in the boundary layer with 1 min and 30 m temporal and spatial resolution and 3 m/s tolerated wind velocity error is presented and fully demonstrates the stability and robustness of this lidar.     

Fringe-imaging Mach-Zehnder interferometer as a spectral analyzer for molecular Doppler wind lidar

The theoretical performance of a Mach-Zehnder interferometer used as a spectral analyzer for wind-speed measurement by direct-detection Doppler lidar is presented. The interferometer is optimized for the measurement of wind velocity from the signal that is backscattered by the molecules. In the proposed fringe-imaging Mach-Zehnder (FIMZ) interferometer, a pattern of equally spaced linear fringes is formed and detected by two conventional detector linear arrays. Assuming a pure molecular signal with Gaussian spectral profile, an analytic expression for the standard deviation of the measurement error is obtained and compared with the Cramer-Rao lower bound given by an ideal spectral analyzer (ISA) in the case of shot-noise-limited signal. The FIMZ measurement error is shown to be 2.3 times that of the ISA and is comparable with the error given by previously developed multichannel spectral analyzers that are based on Fabry-Perot interferometers that, in contrast, have the disadvantages of producing unequally spaced circular fringes and requiring dedicated detectors. The optimal path difference for a FIMZ operating at 355 nm is approximately 3 cm. The interferometer is shown to match important lidar beam étendues without significant performance reduction.     

Reflective grating interferometer: a folded reversal wave-front interferometer

We present a new, folded reversal interferometer. It is based on the reflective grating interferometer and can be applied for optical isolation and testing of coma aberration. The interferometer has several advantages in respect to other existing optical reversal configurations. A carrier can be easily added for phase retrieval in interferometric fringe patterns for mapping coma aberration. Furthermore, in an asymmetric optical configuration a lateral shear can also be added, transforming it in a reversal shearing interferometer. The principle of operation of the interferometer is described, and the application for measuring the coma aberration of a parabolic mirror used off axis is demonstrated.     

Influences of pyramid prism deflection on inversion of wind velocity and temperature in a novel static polarization wind imaging interferometer

The principle of the novel static polarization wind imaging interferometer (NSPWII) [Acta Opt. Sin. 28, 700 (2008)] based on a pyramid prism is described. Since the measured wind velocity and temperature depend on the transmittance of the pyramid prism, the deflection of the prism introduced by vibration would produce some measuring errors. In this paper, with an assumed deflection case, we analyze its influence on the derived wind velocity and temperature theoretically. The relative error of the inversion temperature and variety of the inversion velocity as they changed with the deflection angle are discussed.     

Sensitivity of an imaging space infrared interferometer

We study the sensitivities of space infrared interferometers. We formulate the signal-to-noise ratios of infrared images obtained by aperture synthesis in the presence of source shot noise, background shot noise, and detector read noise. We consider the case in which n beams are combined pairwise at n(n - 1)/2 detectors and the case in which all the n beams are combined at a single detector. We apply the results to future missions, Terrestrial Planet Finder and Darwin. We also discuss the potential of a far-infrared interferometer for a deep galaxy survey.     

Two-wavelength heterodyne interferometer with an expanded measurement range

Translated from Izmeritel'naya Tekhnika, No. 8, pp. 39–40, August, 1992.     

Absolute distance measurement with an optical feedback interferometer

An important use of the self-mixing effect inside a frequency-modulated single-mode laser diode is in laser velocimetry and range-finding applications. The optical beam reflected by a target and injected into the laser diode cavity modulated by a reshaped current is mixed with the light inside the cavity, causing variations of the optical output power. A theoretical analysis of this effect is proposed, based on the determination of the beat frequencies of the optical power variations, to improve the accuracy of laser distance measurement. A resolution of ?1.5 mm from 50 cm to 2 m is obtained when thermal effects are taken into account.     

A contact interferometer with an extended measuring range

Summary The experimental work carried out makes it possible to conclude that the extended range interferometer can be widely used for accurate measurements of small lengths. The interferometer can be easily adapted for checking microcators, opticators, ultraoptimeters, strain-gauge calibrators, optical micrometer screws, angles of small wedges, for measuring magnetostriction, determining the linear temperature expansion coefficients, and other purposes.     

Reflective grating interferometer: a folded reversal and shearing wave-front interferometer

The reflecting grating interferometer (RGI) is a folded and reversal wave-front interferometer sensitive only to asymmetrical aberrations such as third-order coma. The RGI can isolate and evaluate coma both in nearly collimated and in noncollimated beams. We propose a RGI with a different optical configuration that includes a lateral shearing in addition to folding and reversal operations. With lateral shear, the RGI also becomes sensitive to other terms of third-order aberrations such as defocusing, astigmatism, and spherical aberration. Optical path difference equations for interpreting interferograms and numerical simulations are presented to show how the interferometer works in the shearing configuration. Its potential applications are described and discussed.