Function Of Optical Path Difference Research Articles

Unified description on principles of fourier transform infrared spectroscopy and terahertz time-domain spectroscopy

Fourier transform and terahertz time-domain (TD) spectroscopic techniques are widely used in infrared and terahertz frequency bands, and they are usually considered as two different techniques. Here, however, we show that the two techniques are based on the same basic principle. In a Fourier transform infrared spectroscopy (FTIR), the infrared radiation from a continuous incoherent light source passes through a Michelson interferometer, and the interferogram as a function of optical path difference is recorded. The spectral information is decoded from the interferogram by using the Fourier transform process. In a terahertz TD spectrometer, the terahertz transient electrical pulse train from a femtosecond-laser-pumped photoconductive antenna (PCA) is sampled by another PCA (or an electro-optical crystal) gated by a probe femtosecond laser beam from the same femtosecond laser. In this paper, the principle of terahertz TD spectroscopy is described in Fourier domain. We show that the recorded TD terahertz “electrical waveform” can be regarded as an interferogram also. The FTIR and the terahertz TD spectroscopic techniques can be described within the same framework. The asynchronous optical sampling (dual comb) terahertz time-domain technique, a variant form of the traditional terahertz TD spectroscopic technique, can also be described by the same principle. Such a unified description on principles of FTIR and terahertz TD spectroscopies is useful for improving the performance of terahertz emitters and detectors, and developing new terahertz spectroscopic techniques based on femtosecond lasers.

Effect of input spectrum on the spectral switch characteristics in a white-light Michelson interferometer

We report here a detailed experimental study to demonstrate the effect of source spectral characteristics such as spectral bandwidth (Deltalambda), peak wavelength (lambda(0)), and shape of the spectrum on the spectral shifts and spectral switches measured due to temporal correlation in a white-light Michelson interferometer operated in the spectral domain. Behavior of the spectral switch characteristics such as the switch position, switch amplitude, and switch symmetry are discussed in detail as a function of optical path difference between the interfering beams. The experimental results are compared with numerical calculations carried out using interference law in the spectral domain with modified source spectral characteristics. On the basis of our results we feel that our study is of critical importance in the selection of source spectral characteristics to further improve the longitudinal resolution or the measurement sensitivity in spectral-domain optical coherence tomography and microscopy.

Ground-Based Solar Absorption FTIR Spectroscopy: Characterization of Retrievals and First Results from a Novel Optical Design Instrument at a New NDACC Complementary Station

Abstract The authors describe the optical design of a high-resolution Fourier Transform Spectrometer (FTS), which serves as the primary instrument at the University of Toronto Atmospheric Observatory (TAO). The FTS is dedicated to ground-based infrared solar absorption atmospheric measurements from Toronto, Ontario, Canada. Instrument performance is discussed in terms of instrumental line shape (ILS) and phase error and modulation efficiency as a function of optical path difference. Typical measurement parameters are presented together with retrieval parameters used to derive total and partial column concentrations of ozone. Retrievals at TAO employ the optimal estimation method (OEM), and some impacts of the necessary a priori constraints are examined. In March 2004, after participating in a retrieval algorithm user intercomparison exercise, the TAO FTS was granted the status of a Complementary Observation Station within the international community of high-resolution FTS users in the Network for the Detection of Atmospheric Composition and Change (NDACC). During this exercise, average differences between total columns retrieved from the same spectra by different users were below 2.1% for O3, HCl, and N2O in the blind phase, and below 1% in the open phase, when all retrieval constraints were identical. Finally, a 2.5-yr time series of monthly mean stratospheric ozone columns agrees within 3% with those retrieved from Optical Spectrograph and Infrared Imager System (OSIRIS) measurements on board the Odin satellite, which is within the errors of both measurement platforms.

Wavelength-dependent characteristics of modulo Nλ0 optical wavefront control

A Fourier analysis treatment of the wavelength dependences associated with modulo lambdar optical path control, treating the general case of a modulo lambdar optical path difference function OPD (x, y) mod lambdar in which the reset wavelength lambdar, is allowed to be an integer multiple of a nominal wavelength lambda0, lambdar=Nlambda0, is presented. Equations are derived describing the wavelength-dependent characteristics of the diffraction efficiency and the wavefront errors associated with all diffracted orders. The results are applied to the cases of diffraction of a planar wavefront and compensation of large aberrations and show that over an extended spectral bandwidth the output field consists of multiple diffracted orders with a range of fractional wavefront errors that diminishes with increasing reset multiple N, but does not reach the diffraction limit associated with conventional optics.

レーザ干渉を利用した射出成形品表面のレコード溝状フローマークの発現プロセスの解明

The generation of wave-like flow marks on an injection molded polymer strip was experimentally studied using laser interferometry with a He-Ne laser during the filling process. A microscope camera system was used to observe the interference fringe patterns through a calcium fluoride observation window of an experimental mold. A geometric analysis of the flow marks was calculated with a simple optical theory that gave the surface shape change as a function of optical path difference. The variation of local flow mark depths was obtained as a function of the distance from the melt flow front, and the time after the passage of the flow front. It was found that the polymer started to shrink when it contacted the mold wall surface, and this deformation caused a warp on the molded surface that was the origin of flow marks. It was also shown that the hill and valley shape of the flow marks grew with time. In a previous paper, one of the authors reported a model that described the wave-like flow mark generation from a thermal engineering point of view. The results obtained in this study correspond to the surface deformation that was discussed in the previous paper. Therefore the present authors believe that the flow mark generation model proposed in the previous paper is proper.

Spectral Based Estimation of Optical Path Difference of Interference Colors

A spectral intensity distribution of interference colors which is a function of optical path difference (OPD) is represented as a pattern vector on a subspace. A new algorithm to design a color scale on the subspace to estimate OPD values is proposed. In experiments, interference colors generated by a differential interference microscope (DIM) were examined. A subspace spanned by four orthogonal bases was constructed to make a color scale to determine OPD values in the range between 0 nm and 1000 nm. Estimated OPD values almost coincided with these expected. In an experiment on two-dimensional interference colors obtained by a spherical surface of a ballbearing observed by DIM, the resultant OPD distribution coincided well with that expected within a standard deviation of 12.8 nm.

Synthetic wavelength stabilization for two-color laser-diode interferometry

The phase ambiguity in conventional interferometers can be removed by using two laser diodes of different optical frequencies to generate a synthetic wavelength. However, the stability requirements for a two-color interferometric laser gauge that must provide unambiguous determination of the optical fringe order over a large distance can be severe. We derive upper limits on the optical wavelength uncertainty and express them as a function of optical path difference between the object and reference beams, phase measurement errors, and the synthetic wavelength. A simple stabilization arrangement is proposed, involving simultaneous servo control of both lasers with a single Fabry-Perot étalon. The experimental implementation of the proposed system demonstrates its effectiveness for long-term (16-h) stabilized two-color interferometry over a distance of 250 mm, with a 15-mm synthetic wavelength and a repeatability of 40 nm. For periods of < 1000 s, the repeatability was 8 nm.

Phase noise of single-mode diode lasers in interferometer systems

Measurements have been made of the phase noise of six different types of single-mode diode laser in an unbalanced Michelson interferometer, as a function of optical path difference. The frequency dependence of the phase noise has also been determined. Possible origins of the frequency fluctuations which result in the phase noise are discussed. The detrimental effect of the phase noise on the sensitivity and dynamic range of optical fiber interferometer sensors is briefly discussed.

Single-mode diode laser phase noise

Measurements have been made of the phase noise of a single-mode diode laser in an unbalanced Michelson interferometer, as a function of optical path difference. The noise increased linearly with increasing optical path difference. The origin of the phase noise is discussed.