Nondestructive volumetric optical analysis of corroded copper oxidation using 1700nm swept-source optical coherence microscopy.

This study presents a novel nondestructive analysis method for precise characterization of corroded copper oxidation using optical coherence microscopy (OCM). By exploiting the partial light transmission through metallic oxide layers, we employed a specialized OCM system with a wavelength of 1700nm and enhanced the analysis accuracy compared to conventional optical coherence tomography (OCT). The developed OCM system featured a numerical aperture (NA) of 0.15, providing improved surface profiling and higher lateral resolution than OCT. we developed a peak-finding algorithm to accurately determine the thickness of the copper oxide layer from the acquired interference data with zero padding. Our method was validated by comparing the measured thickness profiles with those obtained from scanning electron microscope (SEM) images of corroded metals. The copper oxidation specimens were prepared after heat treatment for 1, 2, 4, and 8 h in an alumina tube furnace at a temperature of 900 °C to find the correlation between the OCM thickness measurement. Additionally, the acquired enface 3D images enabled the identification of local corrosion distribution within a 4 mm × 4 mm area. The en-face mapping images are utilized to analyze the uniformity of the metal oxidation process across the imaging area of the copper oxidation specimens. With an increase in heat treatment time, the median value of the thickness histogram for the copper oxide within the area consistently remained around 10 µm. However, the thickness variation ranged from -2 µm to 5 µm. This indicates that as the heat treatment time progresses, the thickness of the copper oxide becomes more non-uniform. Our technique holds great potential for nondestructive and noncontact detection of metal corrosion and assessment of corrosion rates in various industrial applications. Future research efforts could focus on expanding the application of OCM to different metals and exploring its commercialization prospects for practical implementation in diverse industries.

Optically tunable fiber Bragg grating based on a photo-mechanical tuning mechanism.

We present an optically tunable fiber Bragg grating (FBG) based on a photo-mechanical tuning mechanism. Azobenzene containing polymer was utilized as a photo-mechanical tuning agent coated over a bare section of FBG. Controlled by ultraviolet (UV) irradiation, the polymer coating acts as a micro-actuator that deforms the fiber structure. Reversible tunability was obtained by the coating locally stretching the embedded FBG fiber by the intensity and the spatial distribution of the UV light. We found that our tunable FBG could linearly tune its center wavelength as large as 2.2 nm in a symmetric UV irradiation. We also found that a chirp of grating periods could be produced by an asymmetric irradiation, where the center of the UV irradiation profile is offset to that of the FBG device. The spectral width measured at 10 dB could be broadened from 0.4 to 1.2 nm by simply adjusting the relative position of the UV light source. The direction of the chirp, in either normal or anomalous dispersion, could be easily switched as well. Our observations demonstrated that our device provides a versatile means of tunable filters or dispersion generators, which is useful in various applications.

Analysis of phototoxin taste closely correlates nucleophilicity to type 1 phototoxicity.

Pigments often inflict tissue-damaging and proaging toxicity on light illumination by generating free radicals and reactive oxygen species (ROS). However, the molecular mechanism by which organisms sense phototoxic pigments is unknown. Here, we discover that Transient Receptor Potential Ankyrin 1-A isoform [TRPA1(A)], previously shown to serve as a receptor for free radicals and ROS induced by photochemical reactions, enables Drosophila melanogaster to aphotically sense phototoxic pigments for feeding deterrence. Thus, TRPA1(A) detects both cause (phototoxins) and effect (free radicals and ROS) of photochemical reactions. A group of pigment molecules not only activates TRPA1(A) in darkness but also generates free radicals on light illumination. Such aphotic detection of phototoxins harboring the type 1 (radical-generating) photochemical potential requires the nucleophile-sensing ability of TRPA1. In addition, agTRPA1(A) from malaria-transmitting mosquitoes Anopheles gambiae heterologously produces larger current responses to phototoxins than Drosophila TRPA1(A), similar to their disparate nucleophile responsiveness. Along with TRPA1(A)-stimulating capabilities, type 1 phototoxins exhibit relatively strong photo-absorbance and low energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. However, TRPA1(A) activation is more highly concordant to type 1 phototoxicity than are those photochemical parameters. Collectively, nucleophile sensitivity of TRPA1(A) allows flies to taste potential phototoxins for feeding deterrence, preventing postingestive photo-injury. Conversely, pigments need to bear high nucleophilicity (electron-donating propensity) to act as type 1 phototoxins, which is consistent with the fact that transferring photoexcited electrons from phototoxins to other molecules causes free radicals. Thus, identification of a sensory mechanism in Drosophila reveals a property fundamental to type 1 phototoxins.

Nucleophile sensitivity of Drosophila TRPA1 underlies light-induced feeding deterrence.

Solar irradiation including ultraviolet (UV) light causes tissue damage by generating reactive free radicals that can be electrophilic or nucleophilic due to unpaired electrons. Little is known about how free radicals induced by natural sunlight are rapidly detected and avoided by animals. We discover that Drosophila Transient Receptor Potential Ankyrin 1 (TRPA1), previously known only as an electrophile receptor, sensitively detects photochemically active sunlight through nucleophile sensitivity. Rapid light-dependent feeding deterrence in Drosophila was mediated only by the TRPA1(A) isoform, despite the TRPA1(A) and TRPA1(B) isoforms having similar electrophile sensitivities. Such isoform dependence re-emerges in the detection of structurally varied nucleophilic compounds and nucleophilicity-accompanying hydrogen peroxide (H2O2). Furthermore, these isoform-dependent mechanisms require a common set of TRPA1(A)-specific residues dispensable for electrophile detection. Collectively, TRPA1(A) rapidly responds to natural sunlight intensities through its nucleophile sensitivity as a receptor of photochemically generated radicals, leading to an acute light-induced behavioral shift in Drosophila.

Femtosecond pulse shaping using wavelength-selective directional couplers: proposal and simulation.

The femtosecond pulse-shaping capabilities of wavelength-selective directional couplers are investigated. Numerical results show that, depending on the coupling length and coupling coefficient, one can achieve very different temporal shapes at the output of the directional couplers. For instance, temporal re-shaping of Gaussian-like pulses into Hermite-Gaussian pulses, parabolic pulses, square temporal waveforms and sequences of equalized multiple pulses with time widths down to the femtosecond range can be achieved using readily feasible fiber/waveguide designs. The detrimental influence of the second-order variation of the detuning factor in these pulse shapers is also numerically investigated.

TrpA1 Regulates Defecation of Food-Borne Pathogens under the Control of the Duox Pathway.

Pathogen expulsion from the gut is an important defense strategy against infection, but little is known about how interaction between the intestinal microbiome and host immunity modulates defecation. In Drosophila melanogaster, dual oxidase (Duox) kills pathogenic microbes by generating the microbicidal reactive oxygen species (ROS), hypochlorous acid (HOCl) in response to bacterially excreted uracil. The physiological function of enzymatically generated HOCl in the gut is, however, unknown aside from its anti-microbial activity. Drosophila TRPA1 is an evolutionarily conserved receptor for reactive chemicals like HOCl, but a role for this molecule in mediating responses to gut microbial content has not been described. Here we identify a molecular mechanism through which bacteria-produced uracil facilitates pathogen-clearing defecation. Ingestion of uracil increases defecation frequency, requiring the Duox pathway and TrpA1. The TrpA1(A) transcript spliced with exon10b (TrpA1(A)10b) that is present in a subset of midgut enteroendocrine cells (EECs) is critical for uracil-dependent defecation. TRPA1(A)10b heterologously expressed in Xenopus oocytes is an excellent HOCl receptor characterized with elevated sensitivity and fast activation kinetics of macroscopic HOCl-evoked currents compared to those of the alternative TRPA1(A)10a isoform. Consistent with TrpA1's role in defecation, uracil-excreting Erwinia carotovora showed higher persistence in TrpA1-deficient guts. Taken together, our results propose that the uracil/Duox pathway promotes bacteria expulsion from the gut through the HOCl-sensitive receptor, TRPA1(A)10b, thereby minimizing the chances that bacteria adapt to survive host defense systems.

A subset of enteroendocrine cells is activated by amino acids in the Drosophila midgut.

The intestine is involved in digestion and absorption, as well as the regulation of metabolism upon sensation of the internal intestinal environment. Enteroendocrine cells are thought to mediate these internal intestinal chemosensory functions. Using the CaLexA (calcium-dependent nuclear import of LexA) method, we examined the enteroendocrine cell populations that are activated when flies are subjected to various dietary conditions such as starvation, sugar, high fat, protein, or pathogen exposure. We find that a specific subpopulation of enteroendocrine cells in the posterior midgut which express Dh31 and tachykinin are activated by the presence of proteins and amino acids.

The Mosquito Repellent Citronellal Directly Potentiates Drosophila TRPA1, Facilitating Feeding Suppression.

Citronellal, a well-known plant-derived mosquito repellent, was previously reported to repel Drosophila melanogaster via olfactory pathways involving but not directly activating Transient Receptor Potential Ankyrin 1 (TRPA1). Here, we show that citronellal is a direct agonist for Drosophila and human TRPA1s (dTRPA1 and hTRPA1) as well as Anopheles gambiae TRPA1 (agTRPA1). Citronellal-induced activity is isoform-dependent for Drosophila and Anopheles gambiae TRPA1s. The recently identified dTRPA1(A) and ag-TRPA1(A) isoforms showed citronellal-provoked currents with EC50s of 1.0 B1 0.2 and 0.1 B1 0.03 mM, respectively, in Xenopus oocytes, while the sensitivities of TRPA1(B)s were much inferior to those of TRPA1(A)s. Citronellal dramatically enhanced the feeding-inhibitory effect of the TRPA1 agonist N-methylmaleimide (NMM) in Drosophila at an NMM concentration that barely repels flies. Thus, citronellal can promote feeding deterrence of fruit flies through direct action on gustatory dTRPA1, revealing the first isoform-specific function for TRPA1(A).

Optical properties of the fiber-optic temperature sensor based on the side-hole fiber filled with indium.

A highly sensitive temperature sensor was made by use of a side-hole glass fiber filled with indium metal, and its optical properties were investigated. The temperature sensitivity of the fiber-optic temperature sensor was dλ/dT=-7.38 nm/K. The temperature sensitivity was also examined in sensors made by different lengths of the side-hole fiber and the indium-filled fiber region. The temperature sensitivity could be varied in the range of -1.83 to -7.38 nm/K by changing the relative length between the side-hole fiber and the indium-filled fiber region.

25-terahertz-bandwidth all-optical temporal differentiator.

An all-optical temporal differentiator with a record operation bandwidth of ~25 THz (~200 nm, at least one order of magnitude larger than any previously reported temporal differentiation technology) is experimentally demonstrated based on a simple and compact all-fiber wavelength-selective directional coupler. The fabricated directional coupler can be used to process optical signals with time features as short as a few tens of femtosecond. A Gaussian-like optical pulse with a time-width of 250-fs is experimentally differentiated with a processing error of 2.1%. As an application example, a chirp-free flat-top pulse with a time-width of 540-fs is also successfully generated.

Wavelength-selective directional couplers as ultrafast optical differentiators.

Wavelength-selective directional couplers with dissimilar waveguides are designed for ultrafast optical differentiation within the femtosecond regime (corresponding to processing bandwidths > 10 THz). The theoretically proposed coupler-based differentiators can be produced by wavelength matching of the propagation constants of two different waveguides in the coupler at the center wavelength. A single directional coupler can be designed to achieve either a 2nd-order differentiator or a 1st-order differentiator by properly fixing the product of coupling coefficient and coupling length of the coupler. We evaluated the differentiation errors (~2%) and energetic efficiency (~11% for 1st order differentiation) of the designed optical differentiators through numerical simulations. The proposed design has a strong potential to provide a feasible solution as an integrated differentiation unit device for ultrafast optical signal processing circuits.

Real-time complex temporal response measurements of ultrahigh-speed optical modulators.

We demonstrate a technique for direct, real-time characterization of the complex (amplitude and phase) temporal response of ultrahigh-speed (GHz-bandwidth) optical modulators. The demonstrated technique is based on pulse interferometry combined with time-frequency mapping processes using fiber linear dispersion. A new mechanism is incorporated to overcome the temporal resolution (bandwidth) limitation of the detectable modulation response in our previously reported setup. This mechanism, referred to as 'common-path temporal image magnification', lowers the required detection bandwidth by a factor of more than 10, enabling real-time single-shot waveform acquisition without loss of information using a conventional temporal digitizer. The design specifications of the proposed measurement setup are derived and discussed in detail. As a proof-of-concept experiment, real-time characterization of a complex electro-op c modulation temporal response with time features as fast as approximately 35 ps (modulation bandwidth > 40-GHz) was obtained and displayed at a video rate of 30 frames/sec.

All-optical temporal integration of ultrafast pulse waveforms.

An ultrafast all-optical temporal integrator is experimentally demonstrated. The demonstrated integrator is based on a very simple and practical solution only requiring the use of a widely available all-fiber passive component, namely a reflection uniform fiber Bragg grating (FBG). This design allows overcoming the severe speed (bandwidth) limitations of the previously demonstrated photonic integrator designs. We demonstrate temporal integration of a variety of ultrafast optical waveforms, including Gaussian, odd-symmetry Hermite Gaussian, and (odd-)symmetry double pulses, with temporal features as fast as ~6-ps, which is about one order of magnitude faster than in previous photonic integration demonstrations. The developed device is potentially interesting for a multitude of applications in all-optical computing and information processing, ultrahigh-speed optical communications, ultrafast pulse (de-)coding, shaping and metrology.

Photonic temporal integrator for all-optical computing.

We report the first experimental realization of an all-optical temporal integrator. The integrator is implemented using an all-fiber active (gain-assisted) filter based on superimposed fiber Bragg gratings made in an Er-Yb co-doped optical fiber that behaves like an 'optical capacitor'. Functionality of this device was tested by integrating different optical pulses, with time duration down to 60 ps, and by integration of two consecutive pulses that had different relative phases, separated by up to 1 ns. The potential of the developed device for implementing all-optical computing systems for solving ordinary differential equations was also experimentally tested.

Simple and highly sensitive optical pulse-characterization method based on electro-optic spectral signal differentiation.

A very simple self-referenced, linear pulse-characterization technique based on spectral phase reconstruction by frequency-domain signal differentiation is introduced. This technique can be implemented using electro-optic intensity modulation of the pulse under test with a synchronized RF sinusoid. The pulse spectral phase profile can be accurately and unambiguously reconstructed from only two measured energy spectra, i.e., at the input and at the output of the modulator, using a direct analytic equation. The method is experimentally demonstrated by precisely characterizing microwatt-power picosecond pulses after linear dispersion through short sections (50-700 m) of conventional single-mode fiber.

Stabilization of a fiber-optic two-arm interferometer for ultra-short pulse signal processing applications.

We experimentally demonstrate a stable ultrafast first-order temporal differentiator using a fiber-optic Michelson interferometer incorporating a simple feedback stabilization control, which is based on dithering a single wavelength cw reference. Feedback control signals are acquired by a phase-lock-loop and used for automatically adjusting and maintaining the resonance wavelength of the differentiator at the pulse center wavelength without dithering or disturbing the interferometer arms. Picosecond odd-symmetry Hermite-Gaussian waveforms using the implemented first-order differentiator have been stably generated. The demonstrated stabilization system should prove useful for a wide range of ultrafast pulse processing and analysis applications based on the use of two-arm interferometers.

Frequency-domain modal delay measurement for higher-order mode fiber based on stretched pulse interference.

We propose and demonstrate a novel modal delay measurement technique for a higher-order mode fiber (HOF) based on optical frequency-domain reflectometry (OFDR) using an extremely simple, entirely passive, and ultrafast wavelength sweeping mechanism, namely, dispersion-induced optical pulse stretching. We obtained a high temporal resolution of approximately 1.12 ps, which was sufficient for discerning the four excited modes in an HOF with a length of only approximately 5 m. The results from our measurements were very consistent with those obtained by using a traditional time-domain measurement method and a conventional OFDR measurement based on a tunable CW laser. Our proposed technique can be also easily adapted to perform conventional time-domain modal delay measurements for very long HOFs.

Direct time-response measurement of high-speed optical modulators based on stretched-pulse interferometry.

A simple technique for the direct measurement of the complex temporal response of a high-speed electro-optic (EO) modulator is proposed. This technique recovers the amplitude and phase temporal profiles of an instantaneous modulation over the duration of a chirped pulse (obtained by linear dispersion) using Fourier-transform interferometry, and it exploits the time-to-frequency mapping induced by the pulse dispersion process. The method can be implemented by using either time- or frequency-domain interferometric detection, allowing the characterization of modulation bandwidths in the tens-of-gigahertz range. The concept is demonstrated by accurately measuring the complex temporal response of a 2.5 Gbps intensity EO modulator.

Fast and accurate group delay ripple measurement technique for ultralong chirped fiber Bragg gratings.

A simple and very precise group delay ripple (GDR) measurement technique for linearly chirped fiber Bragg gratings (CFBGs) is proposed. It is based on real-time optical Fourier transformation of an ultrashort pulse directly induced by the CFBG dispersion. We have experimentally demonstrated highly accurate characterization of the GDR profile of a commercial 10-m-long CFBG with a dispersion of +2000 ps/nm, having achieved a remarkably small standard deviation in our measurements of about 4 ps over a bandwidth of 28 nm. The proposed method has the unique potential to provide real-time GDR monitoring (in the MHz range) by use of commercially available high-speed sampling electronics.

Analysis of nonlinear frequency sweep in high-speed tunable laser sources using a self-homodyne measurement and Hilbert transformation.

We have proposed and demonstrated a novel measurement technique for characterizing nonlinear frequency sweep in high-speed tunable laser sources by using a simple self-homodyne setup and Hilbert transformation. Measurement results, such as the variation in frequency scanning rate during a frequency sweeping process, are presented for a temperature-tuned distributed feedback laser diode and external cavity tunable laser. The time-varying optical phase of the incident light of a laser is calculated from the integration of the instantaneous optical frequency, and the tuning rate is obtained from its derivative.