Effective Finesse Research Articles

Multi-beam interferometric Rayleigh scattering technique for simultaneous 2-D quantitative measurement of multi-parameters in high speed flow field

In order to diagnose the high-speed compressible turbulent flow field, a two-dimensional (2-D) multi-parameters measurement method based on multi-beam interferometric Rayleigh scattering (IRS) technique was proposed. Under the beam splitting of etalon, the Rayleigh scattering signals generated by the interaction between multi-beam laser probes and the flow field gas molecules formed 2-D interference measurement lattice. Each measurement point contained the thermodynamic and velocity information of the flow field corresponding to a specific spatial location. By calibrating the effective finesse and transmittance of the etalon at different incident angles, the measurement accuracy of temperature and density of high-order interferometric points was improved. A 2-D IRS measurement system was established by combining multi-beam laser probes, amplification imaging system and solid quartz etalon. A high speed jet was used to verify the technology. The distributions of temperature, density and velocity at different positions along the radial direction and axial direction of the nozzle were obtained. The results shown that the technique could be used to simultaneously measure the 2-D multi-parameter information of the flow field.

Quantum Noise in a Fabry-Perot Interferometer Including the Influence of Diffraction Loss of Light

The DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is designed to detect gravitational waves at frequencies between 0.1 and 10 Hz. In this frequency band, one of the most important science targets is the detection of primordial gravitational waves. DECIGO plans to use a space interferometer with optical cavities to increase its sensitivity. For evaluating its sensitivity, diffraction of the laser light has to be adequately considered. There are two kinds of diffraction loss: leakage loss outside the mirror and higher-order mode loss. These effects are treated differently inside and outside of the Fabry-Perot (FP) cavity. We estimated them under the conditions that the FP cavity has a relatively high finesse and the higher-order modes do not resonate. As a result, we found that the effects can be represented as a reduction of the effective finesse of the cavity with regard to quantum noise. This result is useful for optimization of the design of DECIGO. This method is also applicable to any FP cavities with a relatively small beam cut and the finesse sufficiently higher than 1.

MEMS-based filter integrating tunable Fabry–Perot cavity and grating

We present a novel Micro-Electro-Mechanical-Systems (MEMS) filter integrating a tunable Fabry–Perot cavity and a grating (FP-G). It fuses the functions of the Fabry–Perot (FP) cavity and grating into a single device so as to realize both narrow spectral linewidth given by FP cavity and wide spectral range given by grating. A MEMS FP-G filter with 200μm cavity length was designed and fabricated. It is based on a W-shaped-section micro-bridge structure by which the electrostatic actuator is independent on the FP cavity and a long cavity length can be configured. A grating etched in the back of a cavity mirror serves to spatially separate the transmitted spectral lines corresponding to different interference orders of FP cavity. Experimental results indicated that the MEMS FP-G filter obtained a broader operation spectral range compared with the traditional MEMS FP filter, and achieved 1.2 nm spectral linewidth, which could be narrower by improving the effective finesse of the FP cavity in the fabrication process. Compared with the simulation results, there was considerable room for improvement in device performance by the optimal technique.

Feedback-free optical cavity with self-resonating mechanism

We demonstrated the operation of a high finesse optical cavity without utilizing an active feedback system to stabilize the resonance. The effective finesse, which is a finesse including the overall system performance, of the cavity was measured to be 394 000 ± 10 000, and the laser power stored in the cavity was 2.52 ± 0.13 kW, which is approximately 187 000 times greater than the incident power to the cavity. The stored power was stabilized with a fluctuation of 1.7%, and we confirmed continuous cavity operation for more than two hours. This result has the potential to trigger an innovative evolution for applications that use optical resonant cavities such as compact photon sources with laser-Compton scattering or cavity enhanced absorption spectroscopy.

Slow-light interferometry: practical limitations to spectroscopic performance

We investigate how the use of slow-light methods can enhance the performance of various types of spectroscopic interferometers under practical conditions. We show that, while in ideal cases the enhancement of the spectral resolution is equal to the magnitude of the group index of the slow-light medium, the ratio between the associated gain or loss and the group index of the slow-light medium actually determines the spectral resolution under more-general conditions. Moreover, the dispersion of this ratio leads to frequency-dependent spectral resolution, which limits the useful working bandwidth of the interferometer. We also evaluate the performance of interferometers using three specific slow-light processes in terms of the achievable spectral resolution and the effective working finesse. We show that the spectral resolution is typically limited by the characteristic linewidth of each slow-light process and that there is no fundamental upper limit for the effective working finesse.

Solar CIV vacuum-ultraviolet Fabry-Perot interferometers

Aims: A tunable, high spectral resolution, high effective finesse, vacuum ultraviolet (VUV) Fabry-Perot interferometer (PPI) is designed for obtaining narrow-passband images, magnetograms, and Dopplergrams of the transition region emission line of CIV (155 nm). Methods: The integral part of the CIV narrow passband filter package (with a 2-10 pm FWHM) consists of a multiple etalon system composed of a tunable interferometer that provides high-spectral resolution and a static low-spectral resolution interferometer that allows a large effective free spectral range. The prefilter for the interferometers is provided by a set of four mirrors with dielectric high-reflective coatings. A tunable interferometer, a VUV piezoelectric-control etalon, has undergone testing using the surrogate F2 eximer laser line at 157 nm for the CIV line. We present the results of the tests with a description of the overall concept for a complete narrow-band CIV spectral filter. The static interferometer of the filter is envisioned as being hudt using a set of fixed MgF2 plates. The four-mirror prefilter is designed to have dielectric multilayer n-stacks employing the design concept used in the Ultraviolet Imager of NASA's Polar Spacecraft. A dual etalon system allows the effective free spectral range to be commensurate with the prefilter profile. With an additional etalon, a triple etalon system would allow a spectrographic resolution of 2 pm. The basic strategy has been to combine the expertise of spaceflight etalon manufacturing with VUV coating technology to build a VUV FPI which combines the best attributes of imagers and spectrographs into a single compact instrument. Results. Spectro-polarimetry observations of the transition region CIV emission can be performed to increase the understanding of the magnetic forces, mass motion, evolution, and energy release within the solar atmosphere at the base of the corona where most of the magnetic field is approximately force-free. The 2D imaging of the full vector magnetic field at the height of maximum magnetic influence (minimum plasma beta) can be accomplished, albeit difficult, by measuring the Zeeman splitting of the CIV resonance pair. Designs of multiple VUV FPIs can be developed for integration into future orbiting solar observatories to obtain rapid cadence, spectral imaging of the transition region.

Fabrication of vertical optical plane using DRIE and KOH crystalline etching of (1 1 0) silicon wafer

The present study aims at obtaining a novel fabrication method to realize smooth vertical sidewalls for optical micro/nanoelectromechanical system applications. To reduce the surface defects of the vertically etched sidewall, KOH crystalline etching is employed after silicon deep reactive ion etching process of a (1 1 0) silicon-on-insulator wafer. The effects of additional KOH etching on the sidewall morphology are investigated with respect to the etching temperature and concentration of KOH. The optimized process conditions of KOH etching are found to be 45 wt.% KOH concentration and 70 °C temperature. To evaluate the morphological characteristics of the vertical sidewall, optical resonators consisting of a single silicon plate are fabricated using an optimized process with additional KOH crystalline etching. The experimental effective finesse of the fabricated optical resonator is compared with theoretically simulated finesse and is found to be improved by 21.8% compared with a reference sample fabricated only by the deep reactive ion etching process.

Fabrication and analysis of a Fabry-Perot cavity with a micromechanical wet-etching process.

We present a novel low-cost and batch fabrication method to fabricate a Fabry-Perot (FP) cavity with a micromechanical wet-etching process, through which FP cavities can be achieved with a cavity length of from several micrometers to tens of micrometers. The parallelism of mirror elements can be well achieved without electrostatic control. The quality of an etched surface can be greatly improved by the oxidation polish process. FP cavities with a finesse of approximately 50 are achieved. Analysis shows that the effective finesse is dominated mainly by the quality of the etched surface.

Spatiospectral transmission of a plane-mirror Fabry-Perot interferometer with nonuniform finite-size diffraction beam illuminations.

The transmission of a plane-mirror Fabry-Perot (PFP) interferometer is theoretically modeled and investigated by treating the spatial and spectral features in a unified manner. A spatiospectral transfer function is formulated and utilized to describe the beam propagation and the multiple-beam interference occurring in an ideal one-dimensional strip PFP interferometer with no diffraction loss. The spatial-frequency filtration of a finite-size beam input not only determines the transmitted spatial beam profile but also plays a crucial role in affecting the overall spectral transmittance. The inherent deviations of the spectral transmittance from what we know as the standard Airy's formula are revealed in diverse aspects, including the less-than-unity peak transmittance, the displacement of a resonance peak frequency, and the asymmetric detuning profile. Our theoretical analysis extends to the misaligned PFP interferometers, such as the cases in which non-normal-incidence beams or wedge-aligned mirrors are used that could severely degrade the effective interferometer finesse.

High-finesse mid infrared microcavities based on lead salts

Microcavities for the mid-infrared range with very high quality factors were fabricated by molecular beam epitaxial growth of IV–VI semiconductor layers. The samples consist of PbTe/EuTe and Pb0.95Eu0.05Te/EuTe Bragg mirrors with three to five layer pairs rendering reflectivities well above 99%. The transmission spectrum of a Pb0.95Eu0.05Te2λ cavity shows three Lorentzian shaped Fabry–Perot resonances in the mirror stop band, whereas a PbTe λ/2 microcavity exhibits one narrow resonance maximum at 1336cm−1 (λ=7.48μm) with a full-width at half-maximum of 0.7cm−1. This corresponds to a very high effective cavity finesse of 1100, which is to the best of our knowledge the highest value for the mid infrared spectral region.

Ultra-high-finesse IV–VI microcavities for the midinfrared

IV–VI semiconductor-based midinfrared microcavities with very high-quality factors were grown by molecular-beam epitaxy. The structures consist of PbTe/EuTe Bragg mirrors with three to five layer pairs rendering reflectivities in excess of 99.7%. The PbTe resonator layer between the reflectors has an optical length of λ/2, which yields a first-order cavity. The transmission spectra of the microcavities show a very narrow Lorentzian-shaped Fabry–Perot resonance at 1877 cm−1 (λ=5.32 μm) with a full width at half maximum of 0.63 cm−1 78 μeV. This corresponds to an ultrahigh effective cavity finesse of 1700.

Backconversion in a pulsed optical parametric oscillator: evidence from injection-seeded sidebands

A pulsed optical parametric oscillator (OPO) is pumped at 355 nm and injection seeded at two distinct idler wavelengths at ∼855 nm; these are separated by a frequency interval Δ falling within the 350-GHz free-running OPO bandwidth. Above-threshold operation of this dual-frequency seeded OPO causes sidebands to be observed at multiples of Δ in the spectrum of the 607-nm signal radiation. The sideband spacing varies smoothly as Δ is tuned (with the injection-seeded OPO cavity misaligned to reduce its effective finesse), and corresponding sidebands are observed on the transmitted 355-nm pump radiation. This is interpreted as direct evidence of backconversion of signal and idler waves in a pulsed OPO, consistent with previous temporal observations and other aspects of OPO performance. Signal-wave sidebands are observed well beyond the regular free-running OPO gain profile, which is understood in terms of the different phase-matching conditions for the various OPO pump and idler (seed) frequencies that are involved.

Fast charge exchange spectroscopy using a Fabry-Perot spectrometer in the JIPP TII-U tokamak

A new charge exchange spectroscopic technique using a Fabry-Port spectrometer has been developed to increase the photon flux at the detector, and to improve the time resolution of ion temperature and plasma rotation velocity measurements. The spectral resolution is obtained by arranging two-dimensional fiber-optics and a two-dimensional detector at the focal plane of a coupled lens located on both sides of a Fabry-Perot spectrometer. The effective finesse (the ratio of the spectral resolution to the free spectral range) of the Fabry-Perot interferometer in this system is 14. The time evolution of the ion temperature is obtained with a time resolution of 125 gs and with a spatial resolution of 3 cm (eight channels).

High-speed continuously tunable liquid crystal filter for WDM networks

We describe a high-speed wavelength tunable liquid crystal filter which can be utilized as the tuning element at the receiving end of wavelength division multiaccess (WDMA) optical networks. The filter uses chiral smectic A electroclinic liquid crystals as the active cavity material in a Fabry-Perot etalon in order to obtain a microsecond switching speed. Using the commercially available BDH764E liquid crystal material, we demonstrate a tunable optical filter both in a bulk Fabry-Perot and a fiber Fabry-Perot (FFP) configuration. A bandwidth of about 0.7 mm and effective finesse of 70 were obtained in the FFP configuration. A FFP tuning range of 13 nm with a switching time of less than 10 /spl mu/s were measured at the operating wavelength of 1.55 /spl mu/m. A theoretical analysis of the expected filter performance in the FFP configuration is given. Diffraction in the Fabry-Perot cavity is identified as the dominant loss factor, resulting in reduced throughput and finesse broadening. It is calculated theoretically that an effective finesse of 130 and a throughput loss of 2.2 dB are achievable for a mirror finesse of 200 and a liquid crystal cavity thickness of 5 /spl mu/m. A short waveguide piece is assumed to be included in the cavity. Other expected loss sources for the filter in the FFP configuration have been calculated, showing negligible effect on the filter performance.

Spatial and spectral response of a Fabry–Perot interferometer illuminated by a Gaussian beam

A generalized study has been done of the transmission characteristics of a Fabry-Perot interferometer (FPI) illuminated by a Gaussian light beam impinging on it at normal and non-normal incidence. The theoretical approach is based on a plane-wave, angular-spectrum representation of both the incident Gaussian beam and the transmitted beam. Expressions are obtained for the FPI instrumental function and for the spatial distribution of the transmitted beam. Numerical results are presented for the FPI maximum transmission, effective finesse, and spectral displacement of the interference maximum.

Molecular beam epitaxy growth of an ultrahigh finesse microcavity

A very high finesse Fabry-Pérot resonator consisting of a one wavelength long GaAs cavity and two AlAs/Al 0.1Ga 0.9As distributed Bragg reflectors is grown using molecular beam epitaxy, MBE. The measured Fabry-Pérot linewidth is 0.84 Å at 930 nm. This implies a record finesse in excess of 5500 and an effective finesse greater than 1450. Besides permitting the mirror reflectivity to be determined accurately, more detailed analysis of the reflectivity spectrum show that (i) the growth rates are stable to 0.25% over 10 h and (ii) the internal losses are less than 1 cm −1.

Wide-stopband chirped fibre moiré grating transmissionfilters

Efficient, wide-stopband fibre transmission filters with single narrow transmission bands produced using multiply exposed chirped fibre grating resonators are reported. Tailored combinations of these structures with additional chirped gratings have achieved single passband transmission with an effective finesse of 48. This demonstrates the feasibility of fabricating all-fibre transmission filters with arbitrary passband/stopband combinations.

A Fabry-Perot imager for near-infrared astrophysics at the Communications Research Laboratory 1.5 meter telescope, and imaging of hydrogen recombination lines in the Orion Nebula

We have developed a Fabry-Perot imager at the Nasmyth focus of the Communications Research Laboratory (CRL) 1.5 m telescope. The field of view of the camera is 4.0 min x 4.0 min, which is the largest among near-infrared Fabry-Perot imagers. In order to make the velocity shift in the field of view small, we chose a long focal length for the collimator (approximately 470 mm). The accurate parallelism of reflecting surfaces, 5 nm, is achieved by systems including a He-Ne laser and Si photodiodes. This gives an effective finesse of approximately 50. Wavelength calibration is carried out by using a Kr lamp. This calibration includes a correction for wavelength-dependent phase changes in the reflective coatings of the etalon. Using this Fabry-Perot imager, we obtained Br gamma and Pa beta images of the Orion Nebula, including the Bright Bar and a part of the Trapezium. From these images we obtained the distribution of extinction in this area. We found that there are regions with a tendency for the Br gamma/Pa beta intensity ratio to be larger at a place with a larger Br gamma intensity. We conclude that in such regions dust grains exist within the ionized region, causing the extinction.

Cascaded acoustooptic/fiber Fabry-Perot filter with finesse over 2000

A compound optical filter consisting of an integrated acoustooptic filter in series with a fiber Fabry-Perot filter possesses a very high effective finesse of over 2000 as well as the unique multichannel passband capability of the acoustooptic filter. The fine-grain element in this pair is a narrow (0.5-AA) passband Fabry-Perot filter, and the coarse-grain element is a wide tuning range (130-nm) polarization-independent integrated acoustooptic filter. The composite device has the narrow passband of the Fabry-Perot, the wide tuning range of the acoustooptic filter and the tuning speed of the faster acoustooptic filter (5 mu s).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Techniques of genital examination and vesical catheterization in female children

The physical examination of the female child should not be considered complete until there has been an examination of the external genitalia, but without an effective technique and finesse, this examination can become an ordeal for everyone involved. The examination usually can be conducted with the child in a frog-leg position on the mother's lap. Catheterization can be performed with the child similarly positioned without using gloves, an examination light, or preparatory washing of the genitalia.