Simple Optical Alignment Research Articles

Sensitive carbon monoxide detection based on laser absorption spectroscopy with hollow‐core antiresonant fiber

AbstractA carbon monoxide (CO) sensing system based on hollow‐core antiresonant fiber (HC‐ARF) is developed using tunable diode laser absorption spectroscopy (TDLAS) technology. A HC‐ARF with length of 75 cm is applied as light transmission passage and gas chamber. Compared with conventional multipass cell, HC‐ARF has the advantages of compact volume, low cost, steady transmission, and simple optical alignment. Two techniques of direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS) were used respectively. In DAS, the minimum detection limit (MDL) was 12251.04 ppm. In WMS, the modulation depth of this HC‐ARF based CO‐TDLAS sensor was optimized to be 0.13 cm−1 and the R‐squared numeric of the linear relationship between the peak of the 2 f signal and the gas concentration was 0.99, indicating that the system has a nice linear responsiveness to the CO gas concentration. Finally, the MDL of the sensor was calculated to be 196.33 ppm.

Terabit Optical Wireless-Fiber Communication With Kramer-Kronig Receiver—Part I

An optical communication link is proposed for high speed point-to-point data transmission. The link incorporates both fiber and optical wireless technologies. Kramer-Kronig coherent detection is used due to its component and implementation simplicity. Spatial multiplexing, via a multi-core fiber followed by a wireless link, and wavelength division multiplexing allow the efficient exploitation of all available degrees of freedom. The link signal-to-noise ratio is estimated when all possible noise sources are taken into account. The impact of link loss and optical amplifier placement on the achievable SNR are studied theoretically. Finally, it is shown that over 1 Tb/s is achievable per wavelength with a 4-core fiber and a simple optical wireless alignment system. An important application of this design would be backhaul and intra-data center links.

Fiber Lithography: A Facile Lithography Platform Based on Electromagnetic Phase Modulation Using a Highly Birefringent Electrospun Fiber.

Lithography plays a key role in advancing manufacturing as well as the semiconductor industry. However, the currently available state-of-the-art lithography methods still require access to expensive tools and facilities. Herein, we suggest a novel lithography method based on electromagnetic phase modulation of ultraviolet using a highly birefringent electrospun fiber to overcome such limitations. The optical birefringent effect, by which the phase of incident ultraviolet electromagnetic fields is retarded when passing through optically anisotropic media, is combined with semicrystalline poly(ethylene oxide) (PEO)-poly(ethylene glycol) (PEG) polymeric microfibers patterned in a programmable form using near-field electrospinning. By positioning the mask between two linear polarizers that are perpendicular to each other, only the UV waves that are passing through the fibers can be selectively utilized to exhibit lithographic property. Therefore, the UV intensity on the photoresist (PR) surface follows the shape of the fiber pattern, enabling precisely controlled patterning of the photoresist. Zero- to two-dimensional key features of lithography are achieved, including straight, curved, array, and isolated patterns. Facile optical alignments without using dedicated alignment marks are successfully demonstrated, as well as various applications including micro- to macroscale serpentine, tree, and antenna circuit patterns on a flexible substrate. The presented approach, packed in a table-top scale, is expected to provide a practical and affordable lithography solution by leveraging the direct-writing capability and tunable optical functionality of polymers, scalability, and the simple optical alignment method.

ICL-based mid-infrared carbon dioxide sensor system for deep-sea natural gas hydrate exploration.

For deep-sea natural gas hydrate exploration, highly sensitive detection of the dissolved gas in seawater near the seabed is significant because it requires the sensor system to be small in size, low in power consumption, and high in sensitivity. A mid-infrared sensor system was developed to detect dissolved carbon dioxide (CO2) in sea-water, while employing a 4319 nm continuous-wave interband cascade laser (ICL) and a multi-pass gas cell (MPGC) with a 29.8 m optical path length. A compact rectilinear optical structure was proposed by using the free-space-emitting ICL and tunable laser absorption spectroscopy (TLAS). This leads to a minimized sensor size and a simple optical alignment for deep-sea operation. A strong CO2 absorption line, located at 2315.19 cm-1 and a weak 2315.28 cm-1 line and at a low pressure of 40 Torr, was targeted for low- and high-concentration CO2 detection within a concentration range of 0-1000 parts per billion by volume (ppbv) and 0-40 parts per million by volume (ppmv), respectively. The limit of detection (LoD) was assessed to be 0.72 ppbv at an averaging time of 2 s, and the response time was measured to be ~30 s at a flow rate of ~180 standard cubic centimeters per minute (sccm). Deployment of the CO2 sensor combined with a gas-liquid separator was carried out for the CO2 detection in the gas extracted from water, which validated the reported sensor system's potential application for deep-sea natural gas hydrate exploration.

Non-invasive low charge electron beam time-of-arrival diagnostic using a plasmonics-enhanced photoconductive antenna

The use of a plasmonics-enhanced photoconductive antenna (PCA) optically gated by a near infrared (NIR) pulse enables non-invasive time-of-arrival measurements of a low charge electron beam with respect to the NIR reference, achieving picosecond resolution. The measured signal values show the expected scaling with the beam charge and distance from PCA to the beam axis, as the PCA samples the electric field of the passing electron beam. We operate the device with an NIR spot size much larger than the PCA active-area, resulting in a very simple optical setup and alignment procedure, making the plasmonics-enhanced PCA a preferred alternative to more complex timing diagnostics for applications requiring non-invasive picosecond or sub-picosecond timestamping.

Optical alignment device for two-photon microscopy.

Two-photon excitation fluorescence microscopy has revolutionized our understanding of brain structure and function through the high resolution and large penetration depth it offers. Investigating neural structures in vivo requires gaining optical access to the brain, which is typically achieved by replacing a part of the skull with one or several layers of cover glass windows. To compensate for the spherical aberrations caused by the presence of these layers of glass, collar-correction objectives are typically used. However, the efficiency of this correction has been shown to depend significantly on the tilt angle between the glass window surface and the optical axis of the imaging system. Here, we first expand these observations and characterize the effect of the tilt angle on the collected fluorescence signal with thicker windows (double cover slide) and compare these results with an objective devoid of collar-correction. Second, we present a simple optical alignment device designed to rapidly minimize the tilt angle in vivo and align the optical axis of the microscope perpendicularly to the glass window to an angle below 0.25°, thereby significantly improving the imaging quality. Finally, we describe a tilt-correction procedure for users in an in vivo setting, enabling the accurate alignment with a resolution of <0.2° in only few iterations.

Two-photon imaging of multiple fluorescent proteins by phase-shaping and linear unmixing with a single broadband laser

Imaging multiple fluorescent proteins (FPs) by two-photon microscopy has numerous applications for studying biological processes in thick and live samples. Here we demonstrate a setup utilizing a single broadband laser and a phase-only pulse-shaper to achieve imaging of three FPs (mAmetrine, TagRFPt, and mKate2) in live mammalian cells. Phase-shaping to achieve selective excitation of the FPs in combination with post-imaging linear unmixing enables clean separation of the fluorescence signal of each FP. This setup also benefits from low overall cost and simple optical alignment, enabling easy adaptation in a regular biomedical research laboratory.

Plasmonic interferometers for label-free multiplexed sensing

We report a plasmonic interferometric biosensor based on a simple slit-groove metallic nanostructure that monitors the phase changes of surface plasmon polaritons resulting from biomolecular adsorptions. The proposed sensing scheme integrates the strengths of miniaturized plasmonic architectures with sensitive optical interferometry techniques. Sensing peak linewidths as narrow as 7 nm and refractive index resolutions of 1 × 10(-5) RIU were experimentally measured from a miniaturized sensing area of 10 × 30 µm(2) using a collinear transmission setup and a low-cost compact spectrometer. A high-density array of such interferometric sensors was also fabricated to demonstrate its potential for real-time multiplexed sensing using a CCD camera for intensity interrogation. A self-referencing method was introduced to increase the sensitivity and reduce sensor noise for multiplexing measurements. The enhanced sensing performance, small sensor footprint, and simple instrumentation and optical alignment suggest promise to integrate this platform into low-cost label-free biosensing devices with high multiplexing capabilities.

Doppler-free three-photon resonant six-wave mixing

We study the three-photon resonant six-wave mixing (SWM) with phase-conjugation geometry in Doppler system. The theoretical calculation indicates that the signal spectrum is Doppler-free when the laser line width is narrow. This technique provides a new spectroscopic tool for studying the highly excited atomic or molecular states with high resolution. It has advantages of uncritical phase matching condition, excellent spatial signal resolution and simple optical alignment.

Lens-free heterodyne detection for transient grating experiments

An enhanced technique of heterodyne detection for transient grating experiments has been developed. This technique features a very simple optical alignment because no lenses are used to focus beams on the samples. It was applied to a vapor-deposited gold thin film (thickness: 30 nm), and the detected signal was confirmed to consist of a heterodyne component based on the dependence of the signal intensity on the pump intensity. Furthermore, this technique was shown to provide easy control of the phase difference between the probe and reference beams.

Precise and simple optical alignment method for double-sided lithography

A method for aligning a photolithographic mask at the top of a transparent wafer that has a pattern on its bottom side is presented. The method is based on optical self-imaging of special alignment marks and provides submicrometer accuracy. The method is simple and robust and can conveniently be implemented on laboratory mask aligners for contact or proximity printing.

A New Ellipsoidal Mirror-Type Reflectometer for Measuring Normal-Incident Hemispherical Reflectance Spectrum.

A new reflectometer system is developed for measuring the radiative characteristics of solid materials from a thermal engineering point of view. This system employs an ellipsoidal mirror in a simple optical alignment, and measures the spectrum of the normal-incident hemispherical reflectance in a wavelength region from 0.35 to 1.10μm. A single spectral scanning offers the hemispherical reflectance spectrum which completely covers both of the specular and diffuse components of reflection. The reflectance is directly interpreted into the absorptance for the normal incidence. The system also measures the angular dependence of reflection readily. The system is applied to a systematic study on the reflection characteristics of rough metallic surfaces. The predominant contribution of the specular component in the angular characteristics is demonstrated.

A New Ellipsoidal Mirror-type Reflectometer for Measuring Normal-Incident Hemispherical Reflectance Spectrum.

A new reflectometer system is developed for measuring the radiative characteristics of solid materials from a thermal engineering point of view. This system employs an ellipsoidal mirror in a simple optical alignment, and measures the spectrum of the normal-incident hemispherical reflectance in a wavelength region from 0.35 to 1.10μm. A single spectral scanning offers the hemispherical reflectance spectrum which completely covers both of the specular and diffuse components of reflection. The reflectance is directly interpreted into the absorptance for the normal incidence. The system also measures the angular dependence of reflection readily. The system is applied to a systematic study on the reflection characteristics of rough metallic surfaces. The predominant contribution of the specular component in the angular characteristics is demonstrated.

Ellipsometric monitor for process control

A process monitor that uses ellipsometry is described. It is provided with simple automatic azimuth calibration and optical alignment mechanisms. Calibration way determines azimuth angle errors and corrects errors of psi and Delta derived from those errors after crude manual adjustment or a change in plane of incidence. The alignment method is also composed of manual adjustment and automatic correction. Performance tests were carried out and they indicated that the two correction methods have practically no problems although they use approximations.

A Photodiode Array Fourier Transform Spectrometer Based on a Birefringent Interferometer

A new version of the photodiode array Fourier transform spectrometer for visible spectroscopy is described. In this version, a birefringent interferometer with a Wollaston prism produces a spatial interferogram of the radiation, which is detected by a linear photodiode array. In addition to the merit of having no mechanical moving parts, this spectrometer has the beneficial advantage of the simple and rugged optical setup and alignment due to its common-path and in-line interferometer formation. These advantages lend this spectrometer to field-use applications. Emission spectra of a low-pressure mercury lamp and a light-emitting diode and an absorption spectrum of a didymium glass filter measured by the developed spectrometer are presented.

Optical method for resolution enhancement in photodiode array Fourier transform spectroscopy

In this paper, we propose a method for optically improving the resolving power of the photodiode array Fourier transform spectrometer [ T. Okamoto S. Kawata S. Minami , Appl. Opt.23, 269 ( 1984)] rather than using mathematical superresolution techniques. The spatial frequency of the interferogram is modulated by the dispersing element added to the triangular common path interferometer so that the spectral resolution is enhanced in the effective spectral range. Since the modification of the method is only the addition of the dispersing optical element to the original optical setup, the advantages of the simple, stable, and easy optical alignment and the large optical throughput with the photodiode array Fourier transform spectroscopy are maintained. Experimental results demonstrate the resolution enhancement by the proposed method with the emission spectra of a low pressure mercury lamp and a cadmium lamp.

Optical alignment in magic-angle NMR

A simple optical alignment system is described for adjusting the angle of the axis of rotation of an Andrews-Beams rotor in magic-angle NMR expts. A low-power ruby laser mounted between the horns of the magnet cryostat is used a light source. The beam from the laser travels vertically down the superconducting magnet bore, is deflected first by the rotor cap, then by 2 mirrors located 1 cm from the rotor and on the laser source, resp., and is projected onto a screen 5 m distant. A deviation of +-0.1 Deg from the magic angle causes the image to move by +-1 cm. Slight instabilities, such as low-frequency nutation effects, become immediately apparent upon inspection of the laser image. 13C spectra of carbonyl in glycine powder of polycryst. p-dimethoxybenzene, detd. by using the app. are shown. [on SciFinder (R)]

Optical alignment of an x-ray collimator

The hard x-ray imaging spectrometer (HXIS) is one of the instruments aboard the NASA Solar Maximum Mission satellite. For the imaging of solar flares in hard x rays HXIS employs a grid collimator with an 8-sec of arc FWHM angular resolution. The collimator consists of ten grid plates in a sequence. The position accuracy of the grids in their planes must be better than a few microns. This paper describes a simple optical alignment method developed to perform measurements of the grid positions with a precision of 1 microm.

A NEW METHOD FOR MEASURING Sn←S1 ABSORPTION SPECTRA

Abstract A new method for measuring absorption spectra of transient species in the nanosecond time scale was developed. It is applicable to highly fluorescent systems and gives accurate results in spite of its rather simple optical alignment. The method was applied to the 1,2,4,5-tetracyanobenzene-methyl-substituted benzene and 1,4-dicyanobenzenepyrene systems.

Sensitive Acoustic Vibration Measurements by Optical Heterodyne Detection of Scattered Laser Light

A sensitive method for measurement of small vibrations at acoustical frequencies has been devised using optical heterodyne spectroscopy to detect the Doppler shifts of laser light diffusely scattered from the vibration object. Beginning with the geometry of a Michelson interferometer, we replace one mirror by the scatterer and mix the reference and scattered beam on a small-area photomultiplier cathode. With only simple optical alignment, acoustic vibration spectra can be measured without perturbations even on very small-size objects, since no attachment such as an interferometer mirror is required. Vibration amplitudes as low as 0.01 nm and up to ∼100 nm can be detected with a precision of a few percent in a few seconds averaging time by phase sensitive detection of the optical beat note at the excitation frequency. Application to mechanically unstable subjects such as the auditory organs of living biological specimens is facilitated by incorporation of low-frequency feedback control of the optical path length and provision for continuous amplitude calibration. The complete system has been used in measurements of tympanal membrane movements of acoustic receptor organs. [Supported by NSF and NIH].