Dichroic Beam Splitter Research Articles

One Sensor for Multiple Colors: Fluorescence Analysis of Microdroplets in Microbiological Screenings by Frequency-Division Multiplexing.

High-speed multiwavelength fluorescence measurements are of paramount importance in microfluidic analytics. However, multicolor detection requires an intricate arrangement of multiple detectors and meticulously aligned filters and dichroic beamsplitters that counteract the simplicity, versatility, and low cost of microfluidic approaches. To break free from the restrictions of optical setup complexity, we introduce a simpler single-sensor setup based on laser-frequency modulation and frequency-division multiplexing (FDM). We modulate lasers to excite the sample with four non-overlapping frequency signals. A single photomultiplier tube detects all the modulated emitted light collected by an optical fiber in the microfluidic chip. Signal demodulation is performed with a lock-in amplifier separating the emitted light into four color channels in real time. This approach not only reduces complexity and provides setup flexibility but also results in improved signal quality and, thus, higher signal-to-noise ratios that translate into increased sensitivity. To validate the setup for high-throughput biological applications, we measured multiple signals from different microorganisms and fluorescently encoded droplet populations for exploring beneficial or antagonistic roles in microbial cocultivation systems, as is the case for antibiotic screening assays.

Smart laser osteotomy: integrating a pulsed 1064nm fiber laser into the sample arm of a fiber optic 1310nm OCT system for ablation monitoring.

Real-time depth metrology during material removal via laser ablation is useful in many forms of laser machining. Until now, coaxial optical coherence tomography (OCT) metrology was achieved by the coupling of an OCT imaging beam and ablating beams using a dichroic filter. We present an alternative design with all fiber delivery that is more suitable for surgical laser ablation applications. The novel system design integrates a high peak-power pulsed Yb-doped fiber laser (1064nm) coupled directly into the sample arm of a swept-source OCT system (λc = 1310nm). We measured the OCT signal degradation due to dispersion and attenuation through the ablation fiber laser cavity. Ablation progression is measured in real-time using M-mode OCT. The mean depth targeting error was found to range from 10µm to 80µm in phantom ablation experiments and 21µm to 60µm in bone ablation. A number of issues have been solved, including point-spread function (PSF) peak broadening due to signal delay and dispersion, high bending loss due to dissimilar fiber used throughout the design, and problems due to the extremely high ablation power to swept-source power ratio (> 2×104 peak to average power). To our knowledge, this is the first demonstration of thermal-mediated laser ablation drilling integrated with coaxial OCT imaging through a single-mode, single-cladded output fiber, without using dichroic beam splitters or free-space optic filters anywhere in the optical path and with this high ablation laser power to OCT source power ratio. The removal of bulk optics compared to existing designs opens a new path for compact integration of the entire system. Also, since the ablation laser and OCT feedback system exist along the same fiber path, the need for maintenance and repair are greatly reduced since spatial beam alignment and the potential open-air contamination of optical surfaces are virtually eliminated. We believe that this integrated system is a great candidate for adoption in depth-controlled surgical ablation applications.

Instrumentation for the upgrade to the JET core charge-exchange spectrometers.

Charge-exchange spectroscopy on JET has become particularly challenging with the introduction of the ITER-like wall. The line intensities are weaker and contaminated by many nuisance lines. We have therefore upgraded the instrumentation to improve throughput and allow the simultaneous measurement of impurity and fuel-ion charge exchange by splitting the light between two pairs of imaging spectrometers using dichroic beam splitters. Imaging instruments allow us to stack 11 × 1 mm diameter fibres on the entrance slits without cross talk. CCD cameras were chosen to have 512 × 512 pixels to allow frame transfer times <0.2 ms which with minimum exposure times of 5 ms give tolerable smearing even without a chopper. The image plane is optically demagnified 2:1 to match the sensor size of these cameras. Because the image plane of the spectrometer is tilted, the CCD must also be tilted to maintain focus over the spectrum (Scheimpflug condition). To avoid transverse keystoning (causing the vertical height of the spectra to change across the sensor), the configuration is furthermore designed to be telecentric by a suitable choice of the lens separation. The lens configuration is built almost entirely from commercial off-the-shelf components, which allowed it to be assembled and aligned relatively rapidly to meet the deadline for in-vessel calibration in the JET shutdown.

The influence of dichroic beam splitter on the airborne multiband co-aperture optical system

The multiband co-aperture optical system with compact structure can achieve full and effective integration of multi- source intelligence information, which is one of the development direction of the optical system. Dichroic beam splitter is a vital optical component to make several systems with different bands share one aperture. The effect of the dichroic beam splitter on the multiband co-aperture optical system is analyzed by matrix optics method and primary aberration theory. The results indicate that the reflection angle of the dichroic beam splitter as a reflector changes imaging direction, and the wedge angle of the dichroic beam splitter as a transmission component increases some aberrations.

Development and characterization of a highly sensitive fluorometric transducer for ultra low aqueous ammonia nitrogen measurements in aquaculture

The accurate measurement of ammonia nitrogen concentration is of great significance for the study of aquatic ecological processes. Unfortunately, the commonly used colorimetric and ammonia sensitive electrode methods suffer from poor consistency, especially when the concentration of ammonia nitrogen is low. This paper presents the development and characterization of a highly sensitive fluorometric transducer, intended for measuring ultra-low ammonia nitrogen concentration in natural waters. The design of the transducer is based on molecular fluorescence analysis and focused on the ammonia-orthophthaldialdehyde-sulfite complexation reaction in alkaline aqueous solutions. LED excitation and photodiode detection are integrated in a compact pressure hull, with a small dichroic beam-splitter installed on the optical path. The beam-splitter reflects a small percentage of ultraviolet light into the reference photodiode to evaluate variations of the excitation energy and simultaneously guide fluorescence light into the sample photodiode by total internal refection. As a major innovation, pseudo-random sequence modulation and self-calibration are introduced into the amplitude measurement of the fluorescence for the first time. Specifications include a limit of detection (LOD) of 0.16 μg·L−1, a measurement range of 0–419.31 μg·L−1, and the R-square of 0.99. Two sets of algorithms with inputs of pH, temperature, salinity, and atmospheric pressure are embedded into the device for the high-precision measurement.

Polarization modeling and predictions for DKIST part 3: focal ratio and thermal dependencies of spectral polarization fringes and optic retardance

Data products from high spectral resolution astronomical polarimeters are often limited by fringes. Fringes can skew derived magnetic field properties from spectropolarimetric data. Fringe removal algorithms can also corrupt the data if the fringes and object signals are too similar. For some narrow-band imaging polarimeters, fringes change the calibration retarder properties and dominate the calibration errors. Systems-level engineering tools for polarimetric instrumentation require accurate predictions of fringe amplitudes, periods for transmission, diattenuation, and retardance. The relevant instabilities caused by environmental, thermal, and optical properties can be modeled and mitigation tools developed. We create spectral polarization fringe amplitude and temporal instability predictions by applying the Berreman calculus and simple interferometric calculations to optics in beams of varying F/ number. We then apply the formalism to superachromatic six-crystal retarders in converging beams under beam thermal loading in outdoor environmental conditions for two of the world’s largest observatories: the 10-m Keck telescope and the Daniel K. Inouye Solar Telescope (DKIST). DKIST will produce a 300-W optical beam, which has imposed stringent requirements on the large diameter six-crystal retarders, dichroic beamsplitters, and internal optics. DKIST retarders are used in a converging beam with F/ ratios between 8 and 62. The fringe spectral periods, amplitudes, and thermal models of retarder behavior assisted DKIST optical designs and calibration plans with future application to many astronomical spectropolarimeters. The Low Resolution Imaging Spectrograph with polarimetry instrument at Keck also uses six-crystal retarders in a converging F / 13 beam in a Cassegrain focus exposed to summit environmental conditions providing observational verification of our predictions.

Volumetric imaging of rod and cone photoreceptor structure with a combined adaptive optics-optical coherence tomography-scanning laser ophthalmoscope.

We have designed and implemented a dual-mode adaptive optics (AO) imaging system that combines spectral domain optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO) for in vivo imaging of the human retina. The system simultaneously acquires SLO frames and OCT B-scans at 60Hz with an OCT volume acquisition time of 4.2s. Transverse eye motion measured from the SLO is used to register the OCT B-scans to generate three-dimensional (3-D) volumes. Key optical design considerations include: minimizing system aberrations through the use of off-axis relay telescopes, conjugate pupil plane requirements, and the use of dichroic beam splitters to separate and recombine the OCT and SLO beams around the nonshared horizontal scanning mirrors. To demonstrate system performance, AO-OCT-SLO images and measurements are taken from three normal human subjects ranging in retinal eccentricity from the fovea out to 15-deg temporal and 20-deg superior. Also presented are en face OCT projections generated from the registered 3-D volumes. The ability to acquire high-resolution 3-D images of the human retina in the midperiphery and beyond has clinical importance in diseases, such as retinitis pigmentosa and cone-rod dystrophy.

Design of a multi-channel polarization-preserving optical system for the KSTAR motional Stark effect diagnostic

An optical system capable of preserving, or minimizing the change of, the polarization properties of incident light has been designed and fabricated for the motional Stark effect diagnostic system which measures internal magnetic field structures inside the tokamak for the Korea Superconducting Tokamak Advanced Research. A dual photoelastic modulator (PEM) with a linear polarizer is included in the optical train with four lenses, a mirror and a dichroic beam splitter. Particular cares have been taken for the polarization properties of the delivered light to be perturbed as little as possible under a strong magnetic field and high vacuum. The lenses are made of STIH-6, a material with low Verdet constant, which minimizes the Faraday rotation. The residual Faraday rotation that takes place in the non-STIH-6 optical elements such as the vacuum window and the optical apertures of the PEM is calibrated out from the in-situ measurements using an in-vessel reference polarizer while energizing the magnetic field coils. In the lens design, the object plane is rotated by 44.8° from the optical axis because of the tilted setup-port. The image surface has a finite curvature to reduce the aberration from the four lenses. The fiber dissector is designed based on the focal plane that aligns the focal points from 25 lines-of-sight, each of which constitutes a bundle of 19 fibers. The fibers run about 35m from the front optics in the tokamak vacuum vessel to the detector in the diagnostic area remote from the tokamak hall. The footprint images at the intersections of the lines-of-sight on the neutral beam trajectory confirms the imaging quality is sufficient to the diagnostic requirement with the designed magnifications.

A catadioptric infrared dual band optically passive athermal optical system

The paper proposes a catadioptric infrared dual band optical system based on catadioptric configuration and optically passive athermal design method. The system shares primary mirror, secondary mirror and collimating lens group, and achieves abruption in longwave infrared and midwave infrared using a dichroic beam splitter. The influence of temperature on optical parameters, the design theory and design method are analyzed. A design example is presented. The detailed optimization design method and some considering factors in engineering design moment are discussed. The design results illustrate that the system can obtain good image quality in environment temperature of 10–40 °C, which could satisfy the design requirement. The actual test result not only satisfies the design requirement, but also validates the availability of the method in the paper.

Design of combined system of helium charge exchange spectroscopy and hydrogen beam emission spectroscopy in VEST

A 468.6-nm HeII charge exchange spectroscopy (CES) system combined with 656.3-nm H beam emission spectroscopy (BES) system is designed to measure helium impurity density as well as toroidal rotation and ion temperature in Versatile Experiment Spherical Torus (VEST). A dichroic beam splitter is used to measure CES and BES spectra simultaneously with sharing the same observation and transmission components which include the line of sight, lenses, and fiber bundles. In the combined system, we employ the transmission gratings with high diffraction efficiency for CES and BES in order to obtain high throughput and spectral resolution. Spectra simulation results show that the signal to noise ratio (SNR) of this diagnostic system is no less than 10 with the injection of a diagnostic neutral hydrogen (beam energy and current are 40keV and 0.4A, respectively).

Tunable dichroic polarization beam splitter created by one-step holographic photoalignment using four-beam polarization interferometry

A tunable dichroic polarization beam splitter (tunable DPBS) simultaneously performs the follow functions: 1. Separation of a polarized incident beam into multiple pairs of orthogonally polarized beams; 2. Separation of the propagation direction of two wavelength incident beams after passing through the tunable DPBS; and 3. Control of both advanced polarization and wavelength separation capabilities by varying the temperature of the tunable DPBS. This novel complex optical property is realized by diffraction phenomena using a designed three-dimensional periodic structure of aligned liquid crystals in the tunable DPBS, which was fabricated quickly with precision in a one-step photoalignment using four-beam polarization interferometry. In experiments, we demonstrated that these diffraction properties are obtained by entering polarized beams of wavelengths 532 nm and 633 nm onto the tunable DPBS. These diffraction properties are described using the Jones calculus in a polarization propagation analysis. Of significance is that the aligned liquid crystal structure needed to obtain these diffraction properties was proposed based on a theoretical analysis, and these properties were then demonstrated experimentally. The tunable DPBS can perform several functions of a number of optical elements such as wave plates, polarization beam splitter, dichroic beam splitter, and tunable wavelength filter. Therefore, the tunable DPBS can contribute to greater miniaturization, sophistication, and cost reduction of optical systems used widely in applications, such as optical measurements, communications, and information processing.

Optimization of a T-shaped optical grating for specific applications

A detailed analysis of the optical reflectivity of a monolithic, T-shaped surface relief grating structure is carried out. It is shown that by changing the groove depths and widths, the frequency-dependent reflectivity of the diffraction grating can be greatly modified to obtain various specific optical elements. The basic T-shaped grating structure is optimized for three specific applications: a perfect mirror with a wide maximal reflection plateau, a bandpass filter, and a dichroic beam splitter. These specific mirrors could be used to steer the propagation of bichromatic laser fields, in situations where multilayer dielectric mirrors cannot be applied due to their worse thermomechanical properties. Colored maps are presented to show the reflection dependency on the variation of several critical structure parameters. To check the accuracy of the numerical results, four independent methods are used: finite-difference time-domain, finite-difference frequency-domain, method of lines, and rigorous coupled-wave analysis. The results of the independent numerical methods agree very well with each other indicating their correctness.

Development of multi-spectral three-dimensional micro particle tracking velocimetry11Originally presented in 11th International Symposium on Particle Image Velocimetry, Santa Barbara, California, September 14–16, 2015.

The color-coded 3D micro particle tracking velocimetry system (CC3DμPTV) is a volumetric velocimetry technique that uses the defocusing digital particle image velocimetry (DDPIV) approach to reconstruct the 3D location of the particle. It is suited for microscopic flow visualization because of the single camera configuration. However, several factors limit the performance of the system. In this study, the limitation of the CC3DμPTV is discussed in detail and a new concept of a multi-camera 3D μ-PTV system is proposed to improve the performance of the original CC3DμPTV system. The system utilizes two dichroic beam splitters to separate the incoming light into 3 spectral ranges, and image with three monochrome image sensors. The use of a color-matched light source, off-center individual pinhole and monochrome image sensors allow the system to achieve better sensitivity and optical resolution. The use of coherent lasers light sources with high-speed cameras improves the velocity measurement dynamic range. The performance of the proposed multi-spectral system is first evaluated with a simulation model based on the finite element method (FEM). The performance is also compared numerically with the CC3DμPTV system. The test results show significant improvements on the signal to noise ratio and optical resolution.

Focus on lasers and imaging

Focus on lasers and imaging

CHIMERA: a wide-field, multi-colour, high-speed photometer at the prime focus of the Hale telescope

The Caltech HIgh-speed Multi-color camERA (CHIMERA) is a new instrument that has been developed for use at the prime focus of the Hale 200-inch telescope. Simultaneous optical imaging in two bands is enabled by a dichroic beam splitter centered at 567 nm, with Sloan u' and g' bands available on the blue arm and Sloan r', i' and z_s' bands available on the red arm. Additional narrow-band filters will also become available as required. An Electron Multiplying CCD (EMCCD) detector is employed for both optical channels, each capable of simultaneously delivering sub-electron effective read noise under multiplication gain and frame rates of up to 26 fps full frame (several 1000 fps windowed), over a fully corrected 5 x 5 arcmin field of view. CHIMERA was primarily developed to enable the characterization of the size distribution of sub-km Kuiper Belt Objects via stellar occultation, a science case that motivates the frame-rate, the simultaneous multi-color imaging and the wide field of view of the instrument. In addition, it also has unique capability in the detection of faint near-Earth asteroids and will be used for the monitoring of short duration transient and periodic sources, particularly those discovered by the intermediate Palomar Transient Factory (iPTF), and the upcoming Zwicky Transient Facility (ZTF).

Improved retrieval of gas abundances from near-infrared solar FTIR spectra measured at the Karlsruhe TCCON station

Abstract. We present a modified retrieval strategy for solar absorption spectra recorded by the Karlsruhe Fourier Transform Infrared (FTIR) spectrometer, which is operational within the Total Carbon Column Observing Network (TCCON). In typical TCCON stations, the 3800–11 000 cm−1 spectral region is measured on a single extended Indium Gallium Arsenide (InGaAs) detector. The Karlsruhe setup instead splits the spectrum across an Indium Antimonide (InSb) and InGaAs detector through the use of a dichroic beam splitter. This permits measurements further into the mid-infrared (MIR) that are of scientific interest, but are not considered TCCON measurements. This optical setup induces, however, larger variations in the continuum level of the solar spectra than the typical TCCON setup. Here we investigate the appropriate treatment of continuum-level variations in the retrieval strategy using the spectra recorded in Karlsruhe. The broad spectral windows used by TCCON require special attention with respect to residual curvature in the spectral fits. To accommodate the unique setup of Karlsruhe, higher-order discrete Legendre polynomial basis functions have been enabled in the TCCON retrieval code to fit the continuum. This improves spectral fits and air-mass dependencies for affected spectral windows. After fitting the continuum curvature, the Karlsruhe greenhouse gas records are in good agreement with other European TCCON data sets.

PlanetCam UPV/EHU: A Two-channel Lucky Imaging Camera for Solar System Studies in the Spectral Range 0.38–1.7 μm

We present PlanetCam UPV/EHU, an astronomical camera designed fundamentally for high-resolution imaging of Solar System planets using the “lucky imaging” technique. The camera observes in a wavelength range from 380 nm to 1.7 μm and the driving science themes are atmosphere dynamics and vertical cloud structure of Solar System planets. The design comprises two configurations that include one channel (visible wavelengths) or two combined channels (visible and short wave infrared) working simultaneously at selected wavelengths by means of a dichroic beam splitter. In this paper the camera components for the two configurations are described, as well as camera performance and the different tests done for the precise characterization of its radiometric and astrometric capabilities at high spatial resolution. Finally, some images of solar system objects are presented as well as photometric results and different scientific cases on astronomical targets.

Highly efficient oscillator for an optically pumped 192-μm far-infrared laser

We demonstrate an efficient CH3F oscillator based on an anti-reflection coated Ge dichroic beam splitter. When pumped by the 10R32 line of a CO2 laser (10.17 μm), 0.81-mJ far-infrared laser is obtained with the wavelength of 192 μm. The energy conversion efficiency of 0.16 % is the highest for an optically pumped 192-μm laser system to our knowledge. The beam quality factor of \(M_{x}^{2}\) and \(M_{y}^{2}\) is 1.53 and 1.57, respectively. Further, this oscillator can be extended to optically pumped far-infrared lasers with various wavelengths.

工作波段覆盖近紫外到近红外波的消偏振分色片的设计与研制

工作波段覆盖近紫外到近红外波的消偏振分色片的设计与研制

Concept Design of a Multiwavelength Aerosol Lidar System With Mitigated Diattenuation Effects and Depolarization-Measurement Capability

It is known that the retrieval of aerosol extinction and backscatter coefficients from lidar data acquired through so-called total-power channels – intended to measure the backscattered power irrespective of the polarization – can be adversely affected by varying depolarization effects produced by the aerosol under measurement. This effect can be particularly noticeable in advanced multiwavelength systems, where different wavelengths are separated using a system of dichroic beam splitters, because in general the reflection and transmission coefficients of the beam splitters will be different for fields with polarization parallel or perpendicular to the incidence plane. Here we propose a setup for multiwavelength aerosol lidars alleviating diattenuation effects due to changing depolarization conditions while allowing measure linear depolarization.