Wavefront Division Interferometer Research Articles

Compact, low cost, large field-of-view self-referencing digital holographic interference microscope

Digital holographic microscopes provide quantitative phase images of transparent objects like biological cells. These phase images can in turn provide a host of parameters based on the object morphology, which could be used for object identification. But one of the biggest challenges in digital holographic microscopy is the design of compact setups with minimal optical elements providing high quality images and nanometer level temporal stability. Self-referencing interferometers in which the reference beam is obtained from the object beam is ideal for implementing compact devices with only few optical components. But they suffer from reduced field of view and has restriction on the sparsity of object distribution. Here we describe the development of a low-cost large field of view self-referencing digital holographic microscope based on wavefront division interferometer utilizing Lloyd’s mirror geometry. The device takes away the restriction on the sparsity of the sample also. The developed device does not have any translation stage and the focusing is achieved post-recording of the holograms. Off the shelf low-cost optics, a laser pointer and a webcam were used in development of the device. The developed device was tested on glass microspheres as well as was used to study human erythrocytes. To the best of our knowledge this is the first time such low-cost configuration using webcam is demonstrated for quantitative phase imaging of cells.

Super-resolution x-ray imaging using interaction between periodic structure of object and standing wave generated with total-reflection-mirror interferometer

A super-resolution method in projection-type x-ray imaging is proposed. In this method, interference fringes generated with a two-beam interferometer are used for detecting the fine periodic structure of the object. When the sample has a fine periodic structure, the structure can be detected as interaction between the periodic structure of object and the standing wave formed by the two-beam interferometer. Feasibility studies have been carried out using wavefront-division interferometer with total-reflection-mirror optics and a resolution test chart as a model sample. The fine structures with a period up to 100 nm were detected as modulation of transmitting x-ray intensity at 11.5 keV.

Interaction between periodic structures of object and X-ray standing wave generated by wavefront-division interferometer.

An x-ray imaging method that utilizes interaction between standing wave and periodic structure of object is proposed. When a periodic object is placed in a standing wave field, the interaction is modulated by relation between the periodic structure of object and the standing wave period. Then, the structure of object can be observed as a modulation of transmission image, and the existence of fine periodic structures can be detected with an imaging detector of low spatial resolution. Feasibility studies have been performed using a standing wave generated by a wavefront-division x-ray interferometer with refractive prism optics.

Wave front division interferometer based optical coherence tomography for sensitivity optimization

Optical coherence tomography (OCT) based on a wave front division (WD) interferometer is demonstrated for the first time. Unlike a conventional amplitude division interferometer based OCT, WD OCT utilizes different wave front portions of the light source for sample and reference arm. Due to easy manipulation of the wavefront portions for sample and reference arm, it is feasible for WD OCT, provided that the source power of light is fixed, to optimize the ratio of light power for sample and reference arm to achieve both high power efficiency and high system sensitivity. The system sensitivity as a function of the ratio of light power directed to the two arms is experimentally studied and the result found to be well consistent with the theoretical analysis. In our current time-domain WD OCT, we have achieved the system sensitivity as high as 115 dB at a total source power of only 7.5 mW with 8% to the reference arm. Real-time WD OCT imaging of bio-tissues in vivo is also demonstrated.

Temporal coherence and spectral linewidth of an injection-seeded transient collisional soft x-ray laser

The temporal coherence of an injection-seeded transient 18.9 nm molybdenum soft x-ray laser was measured using a wavefront division interferometer and compared to model simulations. The seeded laser is found to have a coherence time similar to that of the unseeded amplifier, ~1 ps, but a significantly larger degree of temporal coherence. The measured coherence time for the unseeded amplifier is only a small fraction of the pulsewidth, while in the case of the seeded laser it approaches full temporal coherence. The measurements confirm that the bandwidth of the solid target amplifiers is significantly wider than that of soft x-ray lasers that use gaseous targets, an advantage for the development of sub-picosecond soft x-ray lasers.

Longitudinal coherence and spectral profile of a nickel-like silver transient soft X-ray laser

The spectral width of a Ni-like silver transient soft X-ray laser (4d-4p λ= 13.9 nm) was experimentally inferred from the measurement of its temporal coherence, performed with a novel wavefront division interferometer. The measured coherence time of 3 ps corresponds to a spectral linewidth of 0.7 mA which is narrow. We compare this experimental result to numerical simulations of the amplified line profile, performed along two different approaches. Both calculations predict a spectral linewidth that is about 3 times larger than the experimental value. We discuss several effects that might explain this discrepancy. We show that, due to the short duration of the X-ray laser pulse, the assumption of a stationary electromagnetic field used in both the experimental analysis and in the calculations need to be reconsidered.

Microscopie interférentielle X-UV de plasmas créés par laser

The X-UV interferometry is a promising tools for plasma electron density measurement. We report on the results of such a plasma interferometry experiment using a soft x-ray laser as a probing beam (λ = 13.9 nm) and a new X-UV interference microscope. This device is a wavefront division interferometer coupled to a high resolution aspherical imaging mirror. Laser produced plasma electron density profiles were investigated at density as high as 10 20 cm -3 . The expanding plasma has been probed at different times of its evolution. Several nanosecond after its production, the plasma exhibits an index of refraction that is no more below one as expected for a pure plasma. The contribution of the bound electrons of plasma ions to the refractive index is discussed for that situation.

A FOURIER TRANSFORM SPECTROMETER WITHOUT BEAM SPLITTER FOR THE VUV–EUV RANGE

We describe a Fourier transform spectrometer designed to operate down to 60 nm on a synchrotron beam line. As far as we know, there is no such instrument available in the EUV (λ < 140 nm) partly because manufacturing accurate beam splitters remains the major difficulty at these wavelengths. We use a wave front division interferometer instead of an amplitude division one to overcome this difficulty. The interferometer is based on a modified Fresnel bimirror configuration, which is controlled by an original optical system. This system keeps the mirror tilt error to a negligible value during mirror translation, and provides a sensitive interferometric measurement of the mirror translation. The sampling interval is 29 nm (path difference), allowing one to record large band spectra down to λ = 58 nm with spectral resolution δσ = 0.33 cm -1 for 512 K samples (one-sided interferograms). We measured the apparatus function by recording an interferogram from a He–Ne stabilized laser. By studying the white noise in the corresponding spectrum, we found that the sampling error in the interferogram was about 0.4 nm rms, which produces a near-perfect apparatus function. Finally, we recorded the visible/near UV spectrum of an arc mercury lamp for illustration.

Fringe localization in wavefront division interferometers

Abstract When an amplitude division interferometer is illuminated by an extended incoherent source the fringes are localized. If the incoherent source is a periodic array of sources or if it continuous, there is a fundamental difference at the observation space and this is the appearance of several (more than one) localization surfaces. In the present paper it is stated that localized fringes can also be obtained in wavefront division interferometers provided the source is incoherent and periodic. As an example, the multiple localization planes in Fresnel's biprism are shown and experimental results are compared to the theoretical ones.

Soft-x-ray laser interferometry of a pinch discharge using a tabletop laser.

We have used a tabletop soft-x-ray laser and a wave-front division interferometer to probe the plasma of a pinch discharge. A very compact capillary discharge-pumped Ne-like Ar laser emitting at 46.9 nm was combined with a wave division interferometer based on Lloyd's mirror and Sc-Si multilayer-coated optics to map the electron density in the cathode region of the discharge. This demonstration of the use of tabletop soft-x-ray laser in plasma interferometry could lead to the widespread use of these lasers in the diagnostics of dense plasmas.

Interferograms obtained with a X-ray laser by means of a wavefront division interferometer

A wavefront division interferometer has been used for the first time with a soft X-ray laser (λ = 21.2 nm). The experiment aims to demonstrate X-ray laser interferometry in this configuration and to investigate the phase shifting measurement accuracy as well. The X-ray laser is generated in a neon-like zinc plasma in which it makes two passes thanks to a multilayer mirror half-cavity. The X-ray pulse duration is ≈50 ps. The beam has a very high brightness (≈4 × 10 15 W cm −2 sr −1 in 0.01% bandwidth) which allows us to place the interferometer far from the source (2.8 m) and thus to benefit by a large transverse coherence width. The interferometer consists of a Fresnel bi-mirror which adds coherently one part of the X-ray laser beam section to the other one. Single laser-shot interferograms of a reflecting sample provided with a λ 2 dephasing step (51.7 nm height) have then been successfully recorded. The phase shifting accuracy resulting from the smallest observable fringe change is about λ 20 .

Applications of wavefront division interferometers in soft x rays

The development of soft x-ray interferometry demands high quality optical elements and high brightness sources. However, the balance between these two requirements depends on the exact nature of the interferometer setups. Generally speaking, amplitude division interferometers require beam splitters, the optical quality of which remain a big technological issue. On the contrary, the high brightness sources needed by wavefront division interferometers are readily available, and will progress in the near future. Examples of such interferometers are given for various applications in development or in project.

Mesure de l'indice de réfraction par interférométrie en X mous

We propose a method for measuring directly the refractive index in the soft X-ray region using a wavefront division interferometer, the Fresnel twin mirrors. We discuss the design problems and requirements which are imposed by the application of this method to soft X-rays. This method of measurement has the advantage of being insensitive to absorption and is not based on any theory of condensed state. A simulation of the whole experiment is done

Fourier transform spectrometry in relation to other passive spectrometers

The relative merits of Fourier transform spectrometers and alternative passive spectrometers are explored qualitatively with emphasis on the quantity, quality, and cost of information flow. This includes a brief look at throughput, detector considerations and spectral coverage; resolution and accuracy in intensity and wavelength; and cost of construction and data reduction. Finally, the possibilities of multiple detectors and extensions of the technique to the vacuum ultraviolet by means of all-reflecting wavefront division interferometers are mentioned.