Bandwidth Of Light Source Research Articles

(Invited) Graded Index Silicon Germanium Photonics Circuits in the Mid-Infrared

Mid-infrared (mid-IR) spectroscopy can be used to identify chemical and biological substances. Indeed, most molecules absorb light at specific wavelengths in the mid-infrared (mid-IR). Mid-IR photonic circuits on silicon chips have recently gained a lot of attention. Indeed, they can provide high-performance with a low power consumption, while being cheap, compact, and light. Germanium (Ge) and silicon-germanium (SiGe) alloys with a high Ge concentration are particularly interesting because of the wide transparency window of Ge extending up to 15 µm. It has been demonstrated for a few years now that a Ge-rich graded SiGe platform relying on a graded SiGe layer epitaxially grown on a Si substrate can be used as a photonics platform for mid-IR operation in a wide spectral range.In this invited talk, recent developments concerning graded SiGe photonic integrated circuits will be presented. First, passive devices will be reviewed. It will be shown that graded-SiGe waveguides can be used in an unprecedented spectral range, e.g. up to 11 µm. Mach Zehnder interferometers, resonators and integrated Fourier transform spectrometers will be discussed. Then, a large bandwidth light source based on non-linear optical effects in SiGe waveguides [1] will be presented, together with results on integrated mid-IR optoelectronic modulators [2, 3]. Tunable electro-optical frequency-comb generation around 8 µm wavelength based on such integrated modulators will also be discussed [4]. Finally, the recent scale-up of this platform will be shown, with the first demonstration of low-loss mid-infrared waveguides fabricated on a Ge-rich SiGe strain-relaxed buffer grown on an industrial-scale 200 mm wafer [5].Acknowledgment:This work was co-funded by ANR Light-up Project (ANR-19-CE24-0002-01) and by European Union (ERC, Electrophot,101097569). This work was partly supported by the French RENATECH network.

Dual-convolutional neural network-enhanced strain estimation method for optical coherence elastography.

Strain estimation is vital in phase-sensitive optical coherence elastography (PhS-OCE). In this Letter, we introduce a novel, to the best of our knowledge, method to improve strain estimation by using a dual-convolutional neural network (Dual-CNN). This approach requires two sets of PhS-OCE systems: a high-resolution system for high-quality training data and a cost-effective standard-resolution system for practical measurements. During training, high-resolution strain results acquired from the former system and the pre-existing strain estimation CNN serve as label data, while the narrowed light source-acquired standard-resolution phase results act as input data. By training a new network with this data, high-quality strain results can be estimated from standard-resolution PhS-OCE phase results. Comparison experiments show that the proposed Dual-CNN can preserve the strain quality even when the light source bandwidth is reduced by over 80%.

Reliability of Retinal Layer Annotation with a Novel, High-Resolution Optical Coherence Tomography Device: A Comparative Study.

Optical coherence tomography (OCT) enables in vivo diagnostics of individual retinal layers in the living human eye. However, improved imaging resolution could aid diagnosis and monitoring of retinal diseases and identify potential new imaging biomarkers. The investigational high-resolution OCT platform (High-Res OCT; 853 nm central wavelength, 3 µm axial-resolution) has an improved axial resolution by shifting the central wavelength and increasing the light source bandwidth compared to a conventional OCT device (880 nm central wavelength, 7 µm axial-resolution). To assess the possible benefit of a higher resolution, we compared the retest reliability of retinal layer annotation from conventional and High-Res OCT, evaluated the use of High-Res OCT in patients with age-related macular degeneration (AMD), and assessed differences of both devices on subjective image quality. Thirty eyes of 30 patients with early/intermediate AMD (iAMD; mean age 75 ± 8 years) and 30 eyes of 30 age-similar subjects without macular changes (62 ± 17 years) underwent identical OCT imaging on both devices. Inter- and intra-reader reliability were analyzed for manual retinal layer annotation using EyeLab. Central OCT B-scans were graded for image quality by two graders and a mean-opinion-score (MOS) was formed and evaluated. Inter- and intra-reader reliability were higher for High-Res OCT (greatest benefit for inter-reader reliability: ganglion cell layer; for intra-reader reliability: retinal nerve fiber layer). High-Res OCT was significantly associated with an improved MOS (MOS 9/8, Z-value = 5.4, p < 0.01) mainly due to improved subjective resolution (9/7, Z-Value 6.2, p < 0.01). The retinal pigment epithelium drusen complex showed a trend towards improved retest reliability in High-Res OCT in iAMD eyes but without statistical significance. Improved axial resolution of the High-Res OCT benefits retest reliability of retinal layer annotation and improves perceived image quality and resolution. Automated image analysis algorithms could also benefit from the increased image resolution.

Experimental and kinetic model evaluation of HONO production from surface nitrate photolysis

Solar irradiation plays a key role in promoting nitrous acid (HONO) production from surface enhanced photochemical reactions such as photolysis of nitrate. Nitrate anions are ubiquitous in ambient particles and on various outdoor and indoor surfaces, with reported surface density from 10−7 to 10−3 mol m−2. In this work, we measured HONO and NO2 production from surface nitrate photolysis under eight narrow bandwidth light sources with peak emission wavelengths ranging from 300 to 605 nm. We defined the probability of HONO and NO2 release per surface nitrate absorbing unit photon density of particular wavelength as “reactive cross section” of surface nitrate. Relative to the absorption cross section of gaseous HNO3, the reactive cross section of surface nitrate was found enhanced by up to 970-fold. The reactive cross section determined in our laboratory decreased from (6.3 ± 0.5) × 10−20 cm2 molecule−1 at 300 nm to (1.0 ± 0.3) × 10−24 cm2 molecule−1 at 605 nm, and showed a nonlinear relationship with the surface density of nitrate. Outdoor HONO production was mostly attributed to solar UV irradiation as expected, while underappreciated indoor HONO accumulation was also feasible considering HONO production induced by visible light and large specific surface area indoors. We then constructed a dynamic model to further evaluate the indoor HONO source in typical indoor environments. Photochemical properties of this HONO source lead to diurnal profiles in indoor HONO accumulation, with daily average HONO accumulation ranging from 1.1 to 17.8 ppb under various conditions. The model calculation confirmed that visible light substantially promotes surface nitrate photolysis and indoor HONO accumulation, with an average daily strength that is 0.28–0.34 mg h−1 at surface nitrate of 10−4 mol m−2 under simulated sunlight illumination. Our results call for direct measurements of indoor HONO in a variety of real indoor environments.

1030 nm All-Fiber Closed-Loop Fiber Optic Gyroscope with High Sensitivity

Angle random walk (ARW) is a critical noise component of a typical gyroscope alongside with bias instability noise. ARW has dominant effects especially in short-term accuracy. The measurement uncertainty degrades with the deterioration in ARW resulting in the lowered overall gyroscope accuracy. Many inertial navigation applications such as satellite control and gyro-compassing require low ARW. For an interferometric fiber optic gyroscope, lowest detectable rotation is proportional to scale factor of sensor and inversely proportional to optical bandwidth of light source used in the gyroscope. In this study, using a novel pump laser control closed-loop method, gyroscope performance is able to significantly enhance including signal-to-noise ratio (SNR). Fiber optic gyroscope includes an Yb-doped amplified spontaneous emission (ASE) source with broad emission spectra of 15 nm bandwidth used as light source in order to improve gyroscope sensitivity. The final ARW performance is about 0.008°/√hr for a fiber coil of 150 m length.

16.4: Bandwidth effect correction used for wide color gamut display measurement

With the development of display technology, several new displays have attracted much attention such as OLED display, QLED display, Laser display and Mini/MicroLED display. Displays with high brightness, wide color gamut, high efficiency and long lifetime are our ultimate goal. Defect inspections and color measurements are important for us to evaluate display quality. To obtain wide color gamut, we usually use narrow spectral bandwidth light source. However, it's not easy for traditional color camera or filter camera to measure narrow bandwidth light source because the accuracy is limited. In this paper, we simulate color measurement accuracy by color camera and filter camera. We analyze the influence of bandwidth effect on the color measurement and carry out bandwidth effect correction method. With the correction, the measurement accuracy improves obviously. In addition, different displays with different spectral bandwidth have been measured to validate the corrections. The results show significant improvement in color measurement accuracy. Our bandwidth effect correction can be used for wide color gamut display measurement.

Ghost imaging based on the control of light source bandwidth

A scheme to improve the quality in ghost imaging (GI) by controlling the bandwidth of light source (BCGI) is proposed. The theoretical and numerical results show that the reconstruction result with high quality can be obtained by adjusting the bandwidth range of the light source appropriately, and the selection criterion of the bandwidth is analyzed by the power distribution of the imaging target. A proof-of-principle experiment is implemented to verify the theoretical and numerical results. In addition, the BCGI also presents better anti-noise performance when compared with some popular GI methods.

Design of a Linear Wavenumber Spectrometer for Line Scanning Optical Coherence Tomography with 50 mm Focal Length Cylindrical Optics.

Optical coherence tomography (OCT) has a wide range of uses in bioimaging and nondestructive testing. Larger bandwidth light sources have recently been implemented to enhance measurement resolution. Increased bandwidth has a negative impact on spectral nonlinearity in k space, notably in the case of spectral domain OCT (SD-OCT). This nonlinearity reduces the depth-dependent signal sensitivity of the spectrometers. A grating and prism combination is extensively used for linearizing. In an earlier study, we used a combination of the reflective grating and prism, as well as a cylindrical mirror with a radius of 180 mm, to achieve a high SR ratio with low nonlinearity. A creative design for a spectrometer with a cylindrical mirror of radius 50 mm, a light source with a center wavelength of 830 ± 100 nm (μm−1 − 6.756 μm−1 in k-space), and a grating of 1600 lines/mm is presented in this work. The design optimization is performed using MATLAB and ZEMAX. In the proposed design, the nonlinearity error reduced from 157 μm to 10.75 μm within the wavenumber range considered. The sensitivity research revealed that, with the new design, the SR ratio is extremely sensitive to the imaging optics’ angles. To resolve this, a spectrometer based on Grism is introduced. We present a Grism-based spectrometer with an optimized SR ratio of 0.97 and nonlinearity of 0.792 μm (). According to the sensitivity study, the Grism-based spectrometer is more robust.

High-precision co-phase method for segments based on a convolutional neural network

In order to achieve the resolution comparable to the resolution of a monolithic primary mirror telescope and make the imaging quality of the imaging system reach or approach to the diffraction limit, the submirrors of the segments telescope should ensure co-phase splicing. To solve the problem of phase error detection, a high-precision piston error detection method is proposed based on convolutional neural network (CNN). By setting a mask with a sparse multi-subpupil configuration on the exit pupil of the imaging system, a point spread function (PSF) image dataset that is extremely sensitive to the piston error is constructed. According to the characteristics of this dataset, a high-performance CNN model is built. And the best detection range of CNN is tested. The simulation results show that a single network can accurately output the piston error of one or more submirrors in the capture range slightly less than one wavelength. When the single network is applied to the six-submirror imaging system, the detection precision of the piston error reaches an RMS value of 0.0013&lt;i&gt;λ&lt;/i&gt; (here, RMS stands for root mean square). And the method has good robustness to residual tip-tilt error, wavefront aberration, and CCD noise, light source bandwidth. The method is simple and fast, and can be widely used to detect the piston error of the segments.

Implementation of Underground Water for Wireless Optical Communication System using CDMA

Optical wireless communications is a powerful and cost-effective approach for high-speed wireless links that have been tightly guarded For underwater optical wireless communication, the following three optical code division multiple access (CDMA) techniques have been used. systems are associated, investigated, and presented in this paper, such as AC-biased optical CDMA (ACO-CDMA), symmetrically-SCO-OFDM (clipped optical OFDM), and unipolar CDMA (U-CDMA). Peak power constraints, light source bandwidth tag, there are so many factors to recognize, such turbulence, fading underwater signals, and channel estimation error. Advocate for a bit loading algorithm and a simplified modulation index that determines signal magnitude is being used to minimize the achievable data propagation distance. In this optimization procedure, the signal-to-noise ratio and the clipping distortion triggered by the peak power limitation are balanced (SNR). The SNR and clipping effects of the three compared CDMA techniques are represented in this paper. When the transmitted bit index is greater than the channel bandwidth, ACO-OFDM outperforms SCO- and UCDMA, according to the determined model. U-CDMA, on the other end, has a longer propagation distance but needs less transmitted power.

Measuring color capability of wide color gamut near‐eye displays

AbstractNear‐eye displays often use narrow spectral bandwidth light sources to create virtual images. These sources provide wider color gamuts. However, the measurement instrument's spectral bandwidth and spectral stray light need to be considered. We evaluate these aspects in characterizing the display's color capability.

51‐2: Distinguished Paper: Measuring Color Capability of Wide Color Gamut Near‐Eye Displays

Near‐eye displays often use narrow spectral bandwidth light sources to create virtual images. These sources provide wider color gamuts. However, the measurement instrument's spectral bandwidth and spectral stray light need to be considered. We evaluate these aspects in characterizing the display's color capability.

Femtosecond laser induced luminescence in hierarchically structured NdIII, YbIII, ErIII co-doped upconversion nanoparticles: Light-matter interaction mechanisms from experiments and simulations

Fundamental studies of light-matter interactions are important for basic knowledge and in applications. Thanks to advances in experimental and theoretical methods, nowadays it is possible to perform such studies in a broad dynamic range, covering timescales from that of elementary interactions to real time. In the present work, we perform an experimental-theoretical study of light intensity-dependent femtosecond and CW-laser induced frequency upconversion in hierarchically structured core-multishell nanoparticles co-doped with NdIII, YbIII, and ErIII. Upconversion spectra recorded with CW and femtosecond excitation are qualitatively similar whereas the intensity dependence of upconversion depends on excitation mode (CW or femtosecond). To further assess the observed intensity dependence, we perform light-matter interaction simulations in the dynamic range from 100 fs to 3 ms for 18-level system describing the UCNPs, including nine levels of the NdIII, two of the YbIII, and seven of the ErIII ions and a classical model for the excitation source. The calculated time- and intensity-dependent energy level population are compared with measured spectra to understand CW vs femtosecond laser-induced upconversion. To further discuss the differences between CW and femtosecond laser-induced light-matter interactions for the systems studied here, we perform semi-classical pulse propagation simulations and ultrafast pump-probe measurements to study how the light source bandwidth, relative to the absorption linewidth, influence light absorption and transmission and further connect these results with the intensity dependence. Overall, we report our progress toward mechanistic studies of light-matter interaction and photophysical pathways following femtosecond excitation and UCNPs.

Resolving nonuniform temperature distributions with single-beam absorption spectroscopy. Part I: Theoretical capabilities and limitations

Absorption spectroscopy is traditionally used to determine the average gas temperature and species concentration along the laser line-of-sight by measuring the magnitude of two or more absorption transitions with different temperature dependence. Previous work has shown that the nonlinear temperature dependence of the absorption strength of each transition, set by the lower-state energy, E″, can be used to infer temperature variations along the laser line-of-sight. In principle, measuring more absorption transitions with broader bandwidth light sources improves the ability to resolve temperature variations. Here, we introduce a singular value decomposition framework in order to explore the theoretical limits to resolving temperature distributions with single-beam line-of-sight absorption measurements. We show that in the absence of measurement noise or error, only the first ∼14 well-selected absorption features improve the temperature resolution, and a Tikhonov regularization method improves the accuracy of the temperature inversion, particularly for recovery of the maximum gas temperature along the laser beam. We use inversion simulations to demonstrate that one can resolve a selection of temperature distributions along a laser beam line-of-sight to within 3% for the sample cases analyzed. In part II of this work, we explore the influence of measurement noise and error, and experimentally demonstrate the technique to show that there is benefit to measuring additional absorption transitions under real conditions.

Chromatic response of a four-telescope integrated-optics discrete beam combiner at the astronomical L band.

We show the results of simulation and experimental study of a 4-telescope zig-zag discrete beam combiner (DBC) for long-baseline stellar interferometry working at the astronomical L band (3 - 4 µm) under the influence of a narrow bandwidth light source. Following Saviauk et al. (2013), we used a quasi-monochromatic visibility-to-pixel matrix (V2PM) for retrieving the complex coherence functions from simulated and experimentally measured power at the output of the device. Simulation and coefficient of determination (R2) measurements show that we are able to retrieve the visibility amplitudes with >95 % accuracy of our chromatic model source up to a bandwidth of 100 nm centred at 3.5 µm. We characterized a DBC manufactured by 3D ultra-fast laser inscription (ULI) written on gallium lanthanum sulphate (GLS). Experimental results showed retrieval of visibility amplitude with an accuracy of 80-90 % at 69 nm bandwidth, validating our simulation. The standard deviation of experimental phase residuals are between 0.1-0.4 rad, which shows that the retrieval procedure is sufficient to get good quality images, where phase perturbations of less than 1 rad are expected under good seeing conditions for astronomical applications.

High-Performance Semi-Polar InGaN/GaN Green Micro Light-Emitting Diodes

Semi-polar micro-LEDs have gain increasing interests due to the advantages of polarization control and quantum efficiency improvement. In this work, a novel semi-polar (20–21)-plane micro-LEDs array has been designed and manufactured. In comparison with c-plane micro-LEDs, semi-polar micro-LEDs indicate better electrical and optical performance. The relative EQE of semi-polar micro-LEDs remains at 62% under the injected current density of 775.6 A/cm2, which indicates a reduced efficiency droop due to less polarization in MQWs. It has been further proved by a significant reduction of 55% in emission peak blue-shift under the injected current density from 11.1 A/cm2 to 775.6 A/cm2. In addition, the carrier recombination dynamics and spatial light distribution of semi-polar micro-LEDs with different pixel sizes have been studied. Fast recombination lifetime in smaller size semi-polar micro-LEDs indicates a promising way to be used as a high modulation bandwidth light source. Stable and uniform light distribution in a wider range of spatial azimuths further supports for the semi-polar micro-LEDs as a strong candidate for the applications of high-resolution display and high-speed visible light communication.

Wavelength diversity detection for phase-modulation holographic data storage system

The wavelength diversity detection (WDD) method for a holographic data storage system (HDSS) actively utilizes scattering from the media to suppress speckle noise with a finite bandwidth light source. In this study, we analytically and numerically revealed signal degradation in the case of a phase-modulation HDSS under WDD in practical conditions, e.g. recording medium position shift; and we proposed an oscillator page recording method to suppress such degradation. Using phase modulation, an oscillator beam must retrieve the recorded phase from interference between the oscillator beam and the reproduced signal beam. The proposed method also records the oscillator beam as a hologram, and simultaneously reproduces the two beams. Simulation results indicate that the proposed method can compensate the degradation and drastically improve the signal-to-noise ratio from 6.5 to 39.1 dB at the largest possible media shift. Therefore, we confirmed feasibility to utilize WDD in PSK under the practical condition.

High-Speed Interrogation Approach for FBG Sensors Using a VCSEL Array Swept Source

This paper presents a fiber Bragg grating (FBG) interrogator based on vertical surface cavity emitting lasers (VCSEL). A Fabry–Perot filter technique is developed to directly calibrate the dynamic wavelength behavior of VCSELs at both low and high sweep rates. A broad bandwidth light source is constructed by multiplexing five VCSELs together to increase the number of FBGs that can be tracked. Scanning of the VCSEL-array is accomplished by a high-speed electronic switch circuit. The developed interrogator achieves a sweep bandwidth of 10 nm at a scanning rate of 4 kHz. Low-velocity impact testing of a composite plate with a surface mounted FBG sensor shows that the wavelength detection error was 2.7% when compared with the FBG strain obtained by a high-speed swept laser.

Modeling and Analysis for Modulation of Light-Conversion Materials in Visible Light Communication

Visible light communication (VLC) is a kind of high-efficiency wireless communication technology. As one of the key issues in VLC, the modulation bandwidth of light-conversion materials based light-emitting diodes (LEDs) has gained considerable attention in recent years. However, few people focus on theoretical analyses of modulation on light-conversion materials, which show significant influence on the modulation bandwidth of white LED. In this work, we proposed low-pass filter based models of light-conversion materials, including YAG:Ce phosphor and CdSe/ZnS quantum dots, to build the relationship between fluorescence lifetime of these light-conversion materials and their modulation bandwidth. Furthermore, the modulation bandwidth of light source with polychromatic light was obtained and analyzed based on the proposed model. The experimental results of the modulation bandwidth showed a highly consistency with the results from simulation models. These analyses were believed to help realizing white-light illumination with high bandwidth. To the best of our knowledge, this is the first time the modulation mechanisms of light-conversion materials in VLC have been comprehensively discussed in literature.

Mid-Infrared Spectral Compression of Soliton Pulse in an Adiabatically Suspended Silicon Waveguide Taper

Spectral compression (SPC) can be used for generating narrow bandwidth and wavelength-tunable light sources, which have important applications in optical communication system, spectroscopy, and nonlinear microscopy. In this paper, we numerically demonstrate the high-degree SPC of the chirp-free femtosecond pulse at wavelength 2.4 μm in a 6-cm long adiabatically suspended silicon waveguide taper. The silicon waveguide taper is designed with a dispersion-increasing profile along the propagation distance z . Simulation results show that the SPC factor can be up to 10.9, along with the brightness-enhanced factor of 8.0 and negligible sidelobe. The impacts of the higher order dispersion, higher order nonlinearity, losses (including linear and nonlinear loss), and variation of Kerr nonlinear coefficient along z on the SPC are also investigated. It is found that variation of Kerr nonlinear coefficient γ ( z ) and linear loss are the dominant perturbation to the degradation of the SPC performance.