Incoherent Light Source Research Articles

Study on information content of light field on the object surface under various coherent states illumination

A novel approach for the measurement of information content of light field on the object surface illuminated by various coherent states source based on the statistical optics theory and Shannon’s information theory has been proposed. Firstly, we propose that the information content of light field on the object surface can be regarded as the product of the spacial freedom degrees of the object surface itself and that of the illuminating light field arriving at the object surface. Then, calculate the distribution of complex degree of coherence on the object surface under various coherent states illumination and then obtain the number of uncorrelated areas of light field on the object surface which can be used to determine the number of freedom degree of the light field. At the same time, taking into account the structure of the object itself, the freedom degree of the light field on the object surface in completely coherent, partially coherent and incoherent illumination are derived respectively.The specific algorithms for the information content of the light field illuminated by a 1D linear incoherent light source and uniform quasi monochromatic incoherent surface light source with arbitrary shape are also presented.

LED-Based Photoacoustic NO2 Sensor with a Sub-ppb Detection Limit.

A high-sensitivity light-emitting diode (LED)-based photoacoustic NO2 sensor is demonstrated. Sensitive photoacoustic gas sensors based on incoherent light sources are typically limited by background noise and drifts due to a strong signal generated by light absorbed at the photoacoustic cell walls. Here, we reach a sub-ppb detection limit and excellent stability using cantilever-enhanced photoacoustic detection and perform a two-channel relative measurement. A white-light LED is used as a light source, and the spectrum is divided into two wavelength channels with a dichroic filter. The photoacoustic signals generated by the two wavelength channels are measured simultaneously and used to solve the NO2 concentration. The background signal is highly correlated between the two channels, and its variations are suppressed in the relative measurement. A noise level below 1 ppb is reached with an averaging time of 70 s. This is, to the best of our knowledge, the first time a sub-ppb detection limit is demonstrated with an LED-based photoacoustic NO2 sensor. As LEDs are available at a wide selection of emission wavelengths, the results show great potential for development of cost-effective and sensitive detectors for a variety of other trace gasses as well.

Attaining quantum limited precision of localizing an object in passive imaging

We investigate our ability to determine the mean position, or centroid, of a linear array of equally-bright incoherent point sources of light, whose continuum limit is the problem of estimating the center of a uniformly-radiating object. We consider two receivers: an image-plane ideal direct-detection imager and a receiver that employs Hermite-Gaussian (HG) Spatial-mode Demultiplexing (SPADE) in the image plane, prior to shot-noise-limited photon detection. We compare the Fisher Information (FI) for estimating the centroid achieved by these two receivers, which quantifies the information-accrual rate per photon, and compare those with the Quantum Fisher Information (QFI): the maximum attainable FI by any choice of measurement on the collected light allowed by physics. We find that focal-plane direct imaging is strictly sub-optimal, although not by a large margin. We also find that the HG mode sorter, which is the optimal measurement for estimating the separation between point sources (or the length of a line object) is not only suboptimal, but it performs worse than direct imaging. We study the scaling behavior of the QFI and direct imaging's FI for a continuous, uniformly-bright object in terms of its length, and find that both are inversely proportional to the object's length when it is sufficiently larger than the Rayleigh length. Finally, we propose a two-stage adaptive modal receiver design that attains the QFI for centroid estimation.

Polypyrrole-Gold nanocomposites as a promising photothermal agent: Preparation, characterization and cytotoxicity study

Photothermal nanomaterials with near-infrared absorption and high energy conversion efficiency have recently attracted significant interest. Polypyrrole-gold nanocomposites (PPy-Au NCs) as photothermal nanoagents are synthesized using ex-situ polymerization method of the modified pyrrole monomers. Microscopic and spectroscopic characterization techniques are used to reveal the surface structure, composition variation and photoelectric properties of PPy-Au NCs, gold nanorods (Au NRs) and polypyyrole nanoparticles (PPy NPs). Their cytotoxic effects on the viability of Ehrlich Ascites Carcinoma cells in the dark are demonstrated. The surface coating of Au NRs with PPy NPs shows an enhancement in the photothermal efficiency of the proposed photothermal nanoagent. The photothermal conversion of nanomaterials are examined using polarized polychromatic incoherent low-energy light source (the energy density of the light is 2.4 J/cm2 per minute and the specific power density is 40 mW/cm2).

Broadband sum frequency generation spectroscopy of dark exciton states in hBN-encapsulated monolayer WSe2

We demonstrate that broadband sum frequency generation (SFG) spectroscopy based on a partially incoherent supercontinuum light source can elucidate dark p-series excitons in monolayer WSe2 encapsulated between hexagonal boron nitride (hBN) slabs. The observed 2p exciton peak energy is a few meV higher than that predicted by the Rytova-Keldysh potential model, which is originated from the Berry phase effect. Interestingly, although the radiative relaxation of the 2p exciton is weaker, the 2p exciton peak is broader than the 1s and 2s peaks, which indicates its faster dephasing than the 1s and 2s excitons. Measuring the excitation intensity and temperature dependence, we clarified that this broader linewidth is not caused by excitation- or phonon-induced dephasing, but rather by exciton-electron scattering.

Broadband Optical Activity Spectroscopy with Interferometric Fourier-Transform Balanced Detection.

Spectrally resolved measurements of optical activity, such as circular dichroism (CD) and optical rotatory dispersion (ORD), are powerful tools to study chiroptical properties of (bio)molecular and nanoplasmonic systems. The wider utilization of these techniques, however, has been impeded by the bulky and slow design of conventional spectropolarimeters, which have been limited to a narrowband scanning approach for more than 50 years. In this work, we demonstrate broadband measurements of optical activity by combining a balanced detection scheme with interferometric Fourier-transform spectroscopy. The setup utilizes a linearly polarized light field that creates an orthogonally polarized weak chiral free-induction-decay field, along with a phase-locked achiral transmitted signal, which serves as the local oscillator for heterodyne amplification. By scanning the delay between the two fields with a birefringent common-path interferometer and recording their interferogram with a balanced detector that measures polarization rotation, broadband CD and ORD spectra are retrieved simultaneously with a Fourier transform. Using an incoherent thermal light source, we achieve state-of-the-art sensitivity for CD and ORD across a broad wavelength range in a remarkably simple setup. We further demonstrate the potential of our technique for highly sensitive measurements of glucose concentration and the real-time monitoring of ground-state chemical reactions. The setup also accepts broadband pulses and will be suitable for broadband transient optical activity spectroscopy and broadband optical activity imaging.

Fresnel incoherent correlation hologram recording in real-time

Fresnel incoherent correlation holography (FINCH) showed its potential for three-dimensional images with super-resolution by using incoherent light sources. However, regular FINCH requires at least two times of holographic recording, and is not suitable for the instantaneous measurement of dynamic objects. Here, we propose the single-shot FINCH technology to realize real-time imaging. With three dual-lens patterns mixed regularly into a phase mask, the new system can capture a compound hologram, which contains three phase-shifted sub-holograms on different areas of image plane. We draw a comparison of both methods, and find it suitable for single-shot FINCH to realize holographic recording in real-time. A dice is used as a test object to record its dynamic rotational movement and holograms from different angles. We also clearly record the whole process of a drop of glue falling vertically under the force of gravity. Experimental results show the capability of single-shot FINCH system to record moving objects dynamically. This system provides a simple and flexible way to achieve real-time recording based on FINCH.

Performance of Vehicular Visible Light Communications under the Effects of Atmospheric Turbulence with Aperture Averaging.

In this paper, we investigate the performance of a vehicular visible light communications (VVLC) link with a non-collimated and incoherent light source (a light-emitting diode) as the transmitter (Tx), and two different optical receiver (Rx) types (a camera and photodiode (PD)) under atmospheric turbulence (AT) conditions with aperture averaging (AA). First, we present simulation results indicating performance improvements in the signal-to-noise ratio (SNR) under AT with AA with increasing size of the optical concentrator. Experimental investigations demonstrate the potency of AA in mitigating the induced signal fading due to the weak to moderate AT regimes in a VVLC system. The experimental results obtained with AA show that the link’s performance was stable in terms of the average SNR and the peak SNR for the PD and camera-based Rx links, respectively with <1 dB SNR penalty for both Rxs, as the strength of AT increases compared with the link with no AT.

Mitigation of Scintillation in FSOC Using RSOA-Based Spectrum-Sliced Incoherent Light

We propose and experimentally demonstrate the mitigation of adverse effects of scintillation using a spectrum-sliced incoherent light source based on gain-saturated reflective semiconductor optical amplifiers (RSOAs). This incoherent light source, originally developed for the cost-effective realization of wavelength-division-multiplexed passive optical networks, employs RSOAs for both data modulation and the suppression of large excess intensity noise inherent in incoherent light sources. We show through experiment that this incoherent light source is effective in suppressing the power fluctuations induced by scintillation when compared to a coherent light source, and thus improves the receiver sensitivity of free-space optical communication systems under weak atmospheric turbulence.

Wavelength-dependent speckle multiplexing for imaging through opacity

Memory-effect-based speckle-correlated imaging through opacity has been exploited widely in recent years. Generally, spatially incoherent light sources are required to provide effective illumination that can eliminate the interference noise. Here, the wavelength-dependent speckle multiplexing (WDSM) technique is first proposed and demonstrated as an effective method to suppress the interference noise, thereby facilitating the use of partially spatially coherent light as an alternative promising illuminating source. In contrast to the conventional decoherent approach which often employs a rapidly rotating diffuser to reduce the spatial coherence of a spatially coherent light, the WDSM shows greatly enhanced structural robustness by combining a tunable laser and the speckle illumination using a multimode fibre. The proposed WDSM not only expands the scope of candidate light sources for speckle-correlated imaging based on the classical phase-retrieval algorithm, but also enables the multi-channel detection determined by the speckle spectral decorrelation, which would further facilitate the waveband selective light-matter detection through opacity. This work also combines the advantages of fibre optics and laser technology and enriches the potential applications of speckle-correlated imaging.

Simultaneous 4-phase-shifted full-field optical coherence microscopy.

A new method is presented for full-field optical coherence tomography imaging, which permits capturing single shot phase sensitive imaging through simultaneous acquisition of four phase-shifted images with a single camera using unpolarized light for object illumination. Our method retains the full dynamic range of the camera by using different areas of a single camera sensor to capture each image. We demonstrate the performance of our method by imaging phantoms and live cultures of fibroblast, cancer, and macrophage cells to achieve 59 dB sensitivity with isotropic resolution down to 1 μm, and displacement sensitivity down to 0.1 nm. Our method can serve as a platform for developing high resolution imaging systems because when used in conjunction with broadband spatially incoherent light sources, the resolution is not affected by optical aberrations or speckle noise.

Design and experiment of light field shaping system for three-dimensional extended light source used in photoacoustic spectrometer

Compared with infrared laser sources, the three-dimensional incoherent extended light source has the advantages of high power, wide spectral range, and low cost. It has extremely wide applications in high-precision and multi-component photoacoustic spectrometers. However, it encounters some problems about poor directivity, low energy density, irregular shape, light field shaping needed in the design of optical system. The photoacoustic spectrometer is required to collect and optimize the radiation of the centimeter-level three-dimensional extended light source to the whole space in a small volume. Through using a series of wavelength and frequency modulation elements, the final cylindrical light field distribution with millimeter-level radius and centimeter-level length is realized. According to the concept of optical expansion and the principle of edge light, this paper breaks through the traditional design mode based on point light source in the process of optical system design and optimization. The concept of extended light source is used throughout the design process. The luminous characteristics of the three-dimensional extended light source are directly acquired by the self-designed measurement method and device which is accurately reflected in the three-dimensional extended light source model in the form of micro-element. The design of the light field shaping system of the three-dimensional extended light source for the photoacoustic spectrometer is realized by the aspheric surface, and the relevant experimental verification is carried out. Taking the Hawkeye IR-Si272 light source for example, the experimental value of the light power at the entrance of the photoacoustic cell and the sidewall noise rate of the photoacoustic spectrometer have a small deviation from their corresponding simulation values. Compared with the original condenser system, the self-designed photoacoustic spectrometer light source system increases the value of the light power at the entrance of the photoacoustic cell from 0.86W to 1.32W, and reduces the value of the sidewall noise rate from 50.3% to 19.7%. The lower limit of detection of the concentration of trace gas in the order of ppm (parts per million) is also achieved.

Combinational reduction of graphene oxide via coherent and incoherent light irradiation for flexible supercapacitors

The effect of combinational reduction by coherent and incoherent light sources of graphene oxide (GO) in fabricated thin flexible film supercapacitors was studied. Coherent light irradiation produces equally divided cracks on a reduced GO (rGO) surface, followed sequentially by incoherent light irradiation from a xenon camera flashtube source that not only significantly increases the overall degree of cracking on the surface substantially but also swells the rGO sheet vertically. Combinational light irradiation composed of sequential coherent and incoherent light sources efficiently reduces GO and increases the surface area of rGO, which was investigated through Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The enlarged surface area of the cracked rGO flakes can improve the specific energy and power of the supercapacitor, which were evaluated using cyclic voltammetry and galvanostatic charging/discharging measurements. The specific energy and power of the combinational light irradiation rGO supercapacitor were approximately 1000% and 100% higher than those of solitary coherent and incoherent light irradiation rGO supercapacitors, respectively.

Implementation of an incoherent broadband cavity-enhanced absorption spectroscopy technique in an atmospheric simulation chamber for in situ NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; monitoring: characterization and validation for kinetic studies

Abstract. An incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) technique has been developed for the in situ monitoring of NO3 radicals at the parts per trillion level in the CSA simulation chamber (at LISA). The technique couples an incoherent broadband light source centered at 662 nm with a high-finesse optical cavity made of two highly reflecting mirrors. The optical cavity which has an effective length of 82 cm allows for up to 3 km of effective absorption and a high sensitivity for NO3 detection (up to 6 ppt for an integration time of 10 s). This technique also allows for NO2 monitoring (up to 9 ppb for an integration time of 10 s). Here, we present the experimental setup as well as tests for its characterization and validation. The validation tests include an intercomparison with another independent technique (Fourier-transform infrared, FTIR) and the absolute rate determination for the reaction trans-2-butene + NO3, which is already well documented in the literature. The value of (4.13 ± 0.45) × 10−13 cm3 molecule−1 s−1 has been found, which is in good agreement with previous determinations. From these experiments, optimal operation conditions are proposed. The technique is now fully operational and can be used to determine rate constants for fast reactions involving complex volatile organic compounds (VOCs; with rate constants up to 10−10 cm3 molecule−1 s−1).

Single-shot, shadowless total internal reflection fluorescence microscopy via annular fiber bundle.

We demonstrate a method of generating instantaneous and uniform total internal reflection fluorescence (TIRF) excitation by using an annular fiber bundle and spatially incoherent light sources. We show the flexibility of our method in that it can generate TIRF excitation with either a laser light source or an LED of different wavelengths, and facilitate switching between TIRF and epi illumination. In this report we detail the design of the fiber bundle, then demonstrate the performance via single-molecule imaging in the presence of high background and high throughput, and uniform TIRF imaging of cells over a large field of view. Our versatile method will enable quantitative shadowless TIRF imaging.

Effect of curing beam intensity and photoinitiator concentration on the geometrical features and optical transmission of light induced polymer waveguides

Self-written polymer wave guide was constructed between two single mode fibers by photopolymerization using monochromatic laser beam and incoherent light source. Geometrical and optical properties of polymer waveguides have been investigated by changing curing beam power and photoinitiator concentration. For longer bridges beyond 500 µm some surface degradation appeared which caused extra optical loss. We also compared characteristics of two bridges when curing beam power for the second one was reduced from 12.8 µW to 6.4 µW, maintaining all the other fabrication parameters similar. Illumination with high power and longer exposure time leads to a thick waveguide which is wider than the core of the fibers, whereas illumination with reduced beam power and shorter exposure time results in thin bridge with considerable mode mismatching with fibers. Thicker bridge has insertion loss of about 0.4 dB but it is about 2.7 dB for the thinner one. Regarding the effect of photoinitiator density, three different monomer systems were studied. Polymer bridges associated with Eosin concentration 0.5 wt% present optimum surface morphology and optical transmission.

Measuring source width and transverse coherence length using Fresnel diffraction from a phase step.

Measurement of the source size and specifying its effect on the spatial coherence of propagating light are important for characterizing distant sources such as stars, and imaging with partially coherent light. The common method for measuring spatial coherence is Young's two-pinhole experiment. For characterizing spatial coherence along a line, one needs to change the location of the pinholes over a large number of pairs of points. But it requires many measurements, which takes significant time. In this paper, we use Fresnel diffraction from a step in reflection to measure the source width and transverse coherence length. It is shown theoretically and experimentally that these quantities are determined by specifying the location of minimum visibility on the diffraction pattern. We utilize a sodium vapor lamp with a variable slit in front of it as an extended one-dimensional incoherent light source. The measurements are made through recording only one diffraction pattern formed by the step. The study is applicable in 2D, and one can characterize weak starlight using highly sensitive equipment.

Band-edge-enhanced tunable random laser using a polymer-stabilised cholesteric liquid crystal

ABSTRACT Electrically tunable random laser action has been demonstrated in a cavity-free structure. The scattering structure was shaped by the treatment of AC, and a polymer network structure is introduced to fix it. The fluorescence of the laser dye is scattered internally and selectively amplified at the band-edge of the photonic crystal, due to the concentrate of the photonic density of state. As the electric field increased, the deformation of the polymer network causes the cholesteric liquid crystal pitch to form a gradient distribution, resulting in the widening of the bandgap, and the random laser wavelength changes accordingly. This random laser with a broad tuning range (∼100 nm) opens new possibilities for the design of highly efficient incoherent light source in a large area with a thickness of micron level.

Luminescent ion-doped transparent glass ceramics for mid-infrared light sources [invited

Glass ceramics (GCs), which consist essentially of a homogeneous solid state dispersion of nanocrystals (NCs) embedded in a chemically inert and mechanically robust glass matrix, appear to be an extremely promising class of solid state materials that can be easily tailored into arbitrary shapes, including a new generation of optical fibers, for efficient incoherent and coherent sources of mid-infrared (MIR) light emission. This unique capability not only stems from the fact that one can tailor the underlying glass matrix for optimal macroscopic physical properties and ultrahigh transparency at the wavelengths of interest (resulting in appropriate "transparent glass ceramics" or TGCs), but also stems from the fact that one can embed these matrices with size and structure-tailored NCs, which in turn can be doped with relatively high concentrations of MIR emitting rare-earth or transition metal ions. This potential is tantamount to the localization of these highly efficient MIR ionic emitters into carefully selected and highly favorable "process-engineered" custom crystalline host "nanocages," while insulating the ionic emitters from the emission-quenching glass host matrix, the latter being chosen largely because of its highly favorable macroscopic bulk properties, including its ductility and formability into near-arbitrary shapes (at appropriate temperatures). Such MIR TGCs appear to be very promising for numerous photonics applications, including compact and relatively efficient waveguide sensors, broadband incoherent MIR light sources, superluminescent light sources, advanced fiber-optic devices, and broadly wavelength-tunable and ultrashort pulse mode-locked fiber and bulk solid-state lasers. In this paper, we review past achievements in this field, starting with an overview of TGCs, followed by discussions of currently preferred methods of fabrication, characterization, and optimization of suitably doped oxyfluoride, tellurite, and chalcogenide TGCs and of our projections of anticipated future developments in this field at both the materials and device levels.

Structured illumination microscopy using digital micro-mirror device and coherent light source

Structured illumination microscopy (SIM) achieves doubled spatial resolution through exciting the specimen with high-contrast, high-frequency sinusoidal patterns. Such an illumination pattern can be generated by laser interference or incoherent structured patterns. Opto-electronic devices, such as a Spatial Light Modulator (SLM) or a Digital Micro-mirror Device (DMD), can provide rapid switch of illumination patterns for SIM. Although the DMD is much more cost-effective than the SLM, it was previously restricted in association with incoherent light sources, as its diffractive orders are related to the incident angle and the wavelength of coherent incidence. To extend its application with coherent illumination, here, we model the DMD as a blazed grating and simulate the effect with DMD pattern changes in the SIM. With careful analysis of the illumination contrast along different angles and phases, we report a fast, high-resolution, and cost-efficient SIM with DMD modulation. Our home-built laser interference-based DMD-SIM (LiDMD-SIM) reveals the nuclear pore complex and microtubule in mammalian cells with doubled spatial resolution. We further proposed the multi-color LiDMD-SIM concept by jointly employing the DMD ON/OFF states with different incident angles for different wavelengths, with high contrast and maximum resolution enhancement.