Broadband Light Source Research Articles

Improvement of angle random walk of fiber-optic gyroscope using polarization-maintaining fiber ring resonator.

For an interferometric fiber-optic gyroscope (IFOG), the angle random walk, which represents the sensitivity of rotation detection, is mainly limited by the relative intensity noise (RIN) of a broadband source. Using a single-mode fiber ring resonator (SM-FRR) to filter the spectrum of a broadband light source is a common strategy for reducing the RIN at the proper IFOG frequency. However, this method depends on the polarization cross-coupling within the SM-FRR. We model the effect of polarization cross-coupling on the SM-FRR. Then, to further reduce the RIN, we introduce a polarization-maintaining fiber ring resonator (PM-FRR), which mitigates the effect of polarization cross-coupling on the SM-FRR. Using the PM-FRR as a spectrum filter, the RIN is reduced to -143 dB/Hz, with a reduction ratio of 25 dB, and the angle random walk in the IFOG is improved by over five times from 1.17 to 0.223 mdeg/h1/2 using a 2.1 km sensing coil.

Multicore Fiber Bending Sensors with High Sensitivity Based on Asymmetric Excitation Scheme.

Bending sensing was realized by constructing a tapered four-core optical fiber (TFCF) sensor. The four-core fiber (FCF) between the fan-in and fan-out couplers was tapered and the diameter became smaller, so that the distance between the four cores arranged in a square became gradually smaller to produce supermodes. The two ends of the TFCF were respectively connected to the fan-in and fan-out couplers so that the individual cores in the FCF could link to the separate single-mode fibers. A broadband light source (superluminescent diodes (SLD)) spanning 1250–1650 nm was injected into any one of the four cores, and the orientation was thus determined. In the tapering process, the remaining three cores gradually approached the excitation core in space to excite several supermodes based on the tri-core structure first, and then transited to the quadruple-core structure. The field distributions of the excited supermodes were asymmetric due to the corner-core excitation scheme, and the interference thus resulted in a higher measurement sensitivity. When the diameter of the TFCF was 7.5 μm and the tapered length was 2.21 mm, the sensitivity of the bending sensor could reach 16.12 nm/m−1.

Ultraviolet supercontinuum generation driven by ionic coherence in a strong laser field

Supercontinuum (SC) light sources hold versatile applications in many fields ranging from imaging microscopic structural dynamics to achieving frequency comb metrology. Although such broadband light sources are readily accessible in the visible and near infrared regime, the ultraviolet (UV) extension of SC spectrum is still challenging. Here, we demonstrate that the joint contribution of strong field ionization and quantum resonance leads to the unexpected UV continuum radiation spanning the 100 nm bandwidth in molecular nitrogen ions. Quantum coherences in a bunch of ionic levels are found to be created by dynamic Stark-assisted multiphoton resonances following tunneling ionization. We show that the dynamical evolution of the coherence-enhanced polarization wave gives rise to laser-assisted continuum emission inside the laser field and free-induction decay after the laser field, which jointly contribute to the SC generation together with fifth harmonics. As proof of principle, we also show the application of the SC radiation in the absorption spectroscopy. This work offers an alternative scheme for constructing exotic SC sources, and opens up the territory of ionic quantum optics in the strong-field regime.

A Panda fiber temperature sensor up to 900°C

The use of Panda-type polarization-maintaining (PM) fiber for the localized sensing of high temperatures was analyzed with simulations and experiments up to 900°C. Accuracy and repeatability of the results started to decline above 800°C. Fused silica optical fiber melts at 1700°C, which sets an ultimate limit for measurable temperatures. In practice, optical fiber birefringence restricts the maximum temperature to 1060°C where PM fiber loses its ability to maintain polarization. Three sensor fibers (4, 5 and 10 cm long) were spliced at 45° angles to input/output fibers and calibrated in an oven from room temperature to 850–900°C temperature range. Two superluminescent light-emitting diodes (SLEDs) were coupled together as a broadband light source. Birefringence-induced change of the polarization in the sensor fiber was measured with a polarization splitter and an optical spectrum analyzer (OSA) as a function of the wavelength. Temperature-dependent birefringence generates a sinusoidal reflection spectrum from the input polarization mode to the orthogonal output polarization mode. Temperature changes could be concluded from variations in these spectra. Finally, a small fusion device, NORTH, at DTU, Denmark was successfully used as a testbed to make sure that the sensors can handle transportation and the instrumentation required for vacuum operation and still produce sensible data from a harsh environment.

Numerical analysis of the broadband interferometric sensor in the planar gradient-step index configuration

In the paper the model of an optical sensor based on a two-mode interference in a planar gradient-step index configuration is presented. The analyzed structure consists of a single-mode gradient index waveguide, made by Ag+-Na+ ion exchange, which is partially covered by a step-index SU-8 polymer layer. This step and gradient index structure forms a two-mode waveguide which is excited by a broadband light source from a selected spectral range. The refractive index variation of measured external surrounding affects the modal properties of multimode waveguide and the spectral field distribution at the output of the structure. Full Text: PDF ReferencesM. Błahut, "Numerical analysis of planar multimode optical sensor with active nanolayer", Opt. Appl. 50, 377 (2020). CrossRef M. Nordstrom et al J. Light. Technol. 25, 1284 (2007). CrossRef K. Gut, "Study of a Broadband Difference Interferometer Based on Low-Cost Polymer Slab Waveguides", Nanomaterials, 9, 729 (2019). CrossRef M. Błahut, "Numerical characteristics of the polarimetric interferometer made by K+-Na+ ion exchange", Opt. Appl. 27, 3 (1998). CrossRef https://www.filmetrics.com/refractive-index-database/BK7/Float-Glass DirectLink M. Błahut, "Model of the broadband interferometric optical biosensor in a planar configuration", Phot. Lett. Pol. 12, 28 (2020). CrossRef K. Gut, "Broad-band difference interferometer as a refractive index sensor", Opt. Express, 25, 31111 (2017). CrossRef

Ultra-broadband long-wave-infrared pulse production using a chirped-pulse difference-frequency generation.

We present a broadband light source based on near-infrared chirped-pulse difference-frequency mixing that is suitable for seeding long-wave-infrared (LWIR) optical parametric chirped-pulse amplification (OPCPA). A nitrocellulose pellicle is used in a Ti:sapphire regenerative amplifier to generate dual-frequency output pulses, which are subsequently mixed in a 0.4-mm thick AgGaS2 crystal. LWIR pulses with ∼1 µm full width at half maximum (FWHM) bandwidth centered at 10.5 µm are generated by mixing transform-limited pulses. Assisted by genetic algorithm optimization, the bandwidth is broadened to ∼3 µm FWHM within the 8-12 µm atmospheric transmission window. The seed source paves the path towards tabletop ultrafast terawatt-class passively carrier-envelope-phase stabilized OPCPA in the LWIR region.

Real-Time Temperature Monitoring under Thermal Cycling Loading with Optical Fiber Sensor.

A fiber optic sensing system consisting of a fiber Bragg grating (FBG) sensor, optical circulator, optical band pass filter and photodetector is developed to monitor the real-time temperature response of a structure under a dynamic thermal loading. The FBG sensor is surface-bonded on a test specimen and integrated with an optical band pass filter. As a broadband light source transmits into a FBG sensor, a specific wavelength is reflected and transmitted into an optical band pass filter. The reflected wavelength is significantly affected by the temperature, while the output light power from the optical band pass filter is dependent on the wavelength. By measuring the light power with a photodetector, the wavelength can be demodulated, resulting in the determination of the temperature. In this work, the proposed optical sensing system was utilized to monitor the dynamic temperature change of a steel beam under a thermal cycling loading. To verify the accuracy of the fiber optic sensor, a thermocouple was adopted as the reference. The experimental results illustrate a good agreement between the fiber optic sensor and thermocouple. Electronic packages composed of various components such as a solder joint, silicon die, mold compound, and solder mask are frequently subjected to a thermal cycling loading in real-life applications. Temperature variations’ incorporation with mismatches of coefficients of thermal expansion among the assembly components leads to crack growth, damage accumulation and final failure. It is important to monitor the temperature to prevent a thermal fatigue failure. A fast response and easy implementation of the fiber optic sensing system was proposed for the real-time temperature measurement under thermal cycling loading.

Research of continuous fiber laser sustained Xe plasma

High-speed optical inspection of wafers requires a high-brightness and broadband light source. The brightness of traditional arc discharge lamps is difficult to increase, which is limited by the cathode current density. Laser sustained plasma (LSP) is a promising method to overcome the limitations of existing light source technologies. Then the characteristics of fiber laser sustained plasma in a xenon bulb were investigated, the absorption coefficient was measured, and the spectrum was analyzed. The results show that with the increase of laser power, the volume of plasma gradually increases and moves towards the direction of laser incident. With the increase of Gaussian beam focusing number F, the threshold laser power required to maintain the stable existence of plasma decreases gradually. There are upper and lower limits for laser power at which the LSP could be sustained. When the laser incident at the optimal incident angle, the laser propagation in the plasma is the optimal path for laser energy deposition, and the plasma has higher absorption coefficient, temperature and brightness. When F ≥ 10, the Xe plasma will show spatial instability due to refraction effect. Compared with the traditional short-arc xenon lamp, the LSP spectrum has a higher proportion in short wavelength.

A PON Monitoring Scheme for Online Fault Detection and Localization

We propose and experimentally demonstrate an online PON monitoring scheme that can simultaneously achieve the objectives of fault detection and localization. Fault detection and localization share the same transmitter module and perform in sequence. Based on a simple algorithm, one or several fiber Bragg gratings with different central reflection wavelengths are allocated to each end-user for fault detection. A large number of wavelength combinations are generated by several fiber Bragg gratings with different center wavelengths, and are finally identified in the time domain, which is very suitable for fault detection of large-capacity and high-density PON. An additional module is added before the power splitter/combiner to ensure the online troubleshooting. The model equation is established to locate the fault using two unmodulated broadband light sources. The calculated results of the fault localization method show a good agreement with the OTDR measurement results. The proposed scheme provides an integrated online troubleshooting solution for the detection and localization of PON link faults.

Highly Sensitive Integrated Photonic Sensor and Interrogator Using Cascaded Silicon Microring Resonators

Silicon photonic sensors have formed a favorable platform for various sensing applications due to the great advantages of high-performance, compactness, and large-scale integration capability. However, interrogation systems for reading out the encoded sensing information from an integrated sensor are still bulky and inflexible for various portable applications. In this paper, we propose a monolithic silicon refractive index sensor and interrogator by utilizing the cascaded microring resonators. A novel dithering and bias scan interrogation technique is designed for highly sensitive refractive index sensing with a broadband light source. The reflective index for aqueous solutions of ethanol with different concentrations are measured in the experiment, verifying that the sensitivity of the sensor is 125.1 nm/RIU and the detection accuracy of the whole system is 7.76<inline-formula><tex-math notation="LaTeX">$\boldsymbol{\times 10^{-6}}$</tex-math></inline-formula> RIU. This integrated photonic sensor and interrogator will be a promising solution for large-scale applications in biological and chemical sensing scenarios.

Optical Fiber Thermal Anemometer With Light Source-Heated Fabry–Perot Interferometer

An optical fiber thermal anemometer with a light source-heated Fabry–Perot interferometer (FPI) is proposed. The FPI is fabricated on the tip of a single-mode fiber using an ultraviolet-cured adhesive. A broadband light source acts as a heating light source as well, eliminating the need for a heating resistor or a pump laser which are usually required in optical fiber thermal anemometers. The interference fringe of the FPI shifts with airflow velocity because airflow not only reduces temperature of the FPI but also introduces strain due to wind force. Airflow velocity is therefore measured by detecting wavelength shift of the interference fringe. In the experiment, a high sensitivity up to −3.13 nm/(m·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ) was achieved at the low velocity region, reducing to ∼−0.2 nm/(m·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ) at the high velocity region within the measurement range of 0–7 m/s. The response and recovery time is 250 and 580 ms, respectively. It is worth noting that the anemometer maintains a relatively high sensitivity at the high velocity region due to the contribution of wind force effect that makes it outperform most of the fiber thermal anemometers.

Broadband Quantum Dot Superluminescent Diode with Simultaneous Three-State Emission.

Semiconductor superluminescent light-emitting diodes (SLEDs) have emerged as ideal and vital broadband light sources with extensive applications, such as optical fiber-based sensors, biomedical sensing/imaging, wavelength-division multiplexing system testing and optoelectronic systems, etc. Self-assembled quantum dots (SAQDs) are very promising candidates for the realization of broadband SLED due to their intrinsic large inhomogeneous spectral broadening. Introducing excited states (ESs) emission could further increase the spectral bandwidth. However, almost all QD-based SLEDs are limited to the ground state (GS) or GS and first excited state (ES1) emission. In this work, multiple five-QD-layer structures with large dot size inhomogeneous distribution were grown by optimizing the molecular beam epitaxy (MBE) growth conditions. Based on that, with the assistance of a carefully designed mirror-coating process to accurately control the cavity mirror loss of GS and ESs, respectively, a broadband QD-SLED with three simultaneous states of GS, ES1 and second excited-state (ES2) emission has been realized, exhibiting a large spectral width of 91 nm with a small spectral dip of 1.3 dB and a high continuous wave (CW) output power of 40 mW. These results pave the way for a new fabrication technique for high-performance QD-based low-coherent light sources.

Pico-strain resolution fiber-optic sensor with white-light interferometry.

The fiber Bragg grating (FBG) sensor is well known for simple fabrication, absolute measurement, inherent multiplexing capability, etc. To date, most FBG sensors that use a broadband light source for demodulation can only achieve resolution at the µɛ level. In this Letter, we propose a white-light-driven self-reference sensing system with FBGs, using a Mach-Zehnder interferometer (MZI) and a white light source, to realize multiplexed high-resolution vibration sensing with a very simple system configuration. A strain resolution of 35p ε/Hz at 1 kHz is demonstrated, which is several orders of magnitude better than the current FBG sensor systems with white light sources. The performance of the sensing system is analyzed, and multiplexing capability is also experimentally evaluated; there is no observable cross talk.

Simple approach to broadband mid-infrared pulse generation with a mode-locked Yb-doped fiber laser.

Broadband mid-infrared (MIR) molecular spectroscopy demands a bright and broadband light source in the molecular fingerprint region. To this end, intra-pulse difference frequency generation (IDFG) has shown excellent properties among various techniques. Although IDFG systems pumped with 1.5- or 2-µm ultrashort pulsed lasers have been extensively developed, few systems have been demonstrated with 1-µm lasers, which use bulky 100-W-class high-power Yb thin-disk lasers. In this work, we demonstrate a simple and robust approach of 1-µm-pumped broadband IDFG with a conventional mode-locked Yb-doped fiber laser. We first generate 3.3-W, 12.1-fs ultrashort pulses at 50 MHz by a simple combination of spectral broadening with a short single-mode fiber and pulse compression with chirped mirrors. Then, we use them for pumping a thin orientation-patterned gallium phosphide crystal, generating 1.2-mW broadband MIR pulses with the -20-dB bandwidth of 480 cm-1 in the fingerprint region (760-1240 cm-1, 8.1-13.1 µm). The 1-µm-based IDFG system allows for additional generations of ultrashort pulses in the ultraviolet and visible regions, enabling, for example, 50-MHz-level high-repetition-rate vibrational sum-frequency generation spectroscopy or pump-probe spectroscopy.

Profile Morphology of Fiber Bragg Gratings During Initial Cracking: A Case Study on Proof Load Testing of an Arch Spandrel Segment

The occurrence of “initial cracking” in bridges during proof load testing (PLT) is a relatively brief procedure, taking only a few hours. Its long-term effects may play a role in future understanding of the future behavior of the bridge. In this paper, we document a brief period of PLT via interrogation an FBG sensor (central wavelength of 1554.2nm) in an arch spandrel bridge which was constructed before opening for public use. We logged data before, during and after the load testing session using a broadband light source and a tunable laser. Based on the data, we discuss the wavelength shift profiles, and its morphology during the period. The main findings of this study reveals that the initial cracking procedure exhibits unique behavior indicated by the sudden changes of the FBG spectra, though exhibiting some latency. Prior to the PLT session (acclimatizing window), there were already perturbations to structure causing up to 0.2nm shift in the FBG central wavelength, while during the PLT session, the central wavelength snapped between ~1554.1nm to ~1554.3nm. The fluctuations stopped after the PLT, confirmed with tunable lasers’ profile as well. These findings demonstrates that not only initial crackings most likely can be detected via PLT, but it may also occur before the PLT exercise itself due to its own intrinsic load.

Detection of ionized air using a photonic-crystal nanocavity excited by broadband light from a superluminescent diode.

It has been shown that silicon photonic crystal nanocavities excited by spectrally narrow light can be used to detect ionized air. Here, to increase the range of possible applications of nanocavity-based sensing, the use of broadband light is considered. We find that the use of a superluminescent diode (SLD) as an excitation source enables a more reproducible detection of ionized air. When our photonic-crystal nanocavity is exposed to ionized air, carriers are transferred to the cavity and the light emission from the cavity decreases due to free carrier absorption. Owing to the broadband light source, the resonance wavelength shifts caused by the carriers in this system (for example, due to temperature fluctuations) do not influence the emission intensity. SLD-excited cavities could be useful to determine the density of ions in air quantitatively.

High Sensitivity Strain Sensors Using Four-Core Fibers through a Corner-Core Excitation.

A weakly-coupled multicore fiber can generate supermodes when the multi-cores are closer to enter the evanescent power coupling region. The high sensitivity strain sensors using tapered four-core fibers (FCFs) were demonstrated. The fan-in and fan-out couplers were used to carry out light coupling between singlemode fibers and the individual core of the FCFs. A broadband lightsource from superlumminescent diodes (SLDs) was launched into one of the four cores arranged in a rectangular configuration. When the FCF was substantially tapered, the asymmetric supermodes were produced to generate interferences through this corner-core excitation scheme. During tapering, the supermodes were excited based on a tri-core structure initially and then transited to a rectangular quadruple-core structure gradually to reach the sensitivity of 185.18 pm/μԑ under a tapered diameter of 3 μm. The asymmetric evanescent wave distribution due to the corner-core excitation scheme is helpful to increase the optical path difference (OPD) between supermodes for improving the strain sensitivity.

Evaluating interproximal and occlusal lesion severity with a dual SWIR transillumination/reflectance probe.

We have developed a clinical probe capable of acquiring simultaneous, multispectral short wavelength infrared (SWIR) reflectance and occlusal transillumination images of lesions on tooth proximal and occlusal surfaces to reduce the potential of false positives and enhance diagnosis. The dual probe was 3D printed and the imaging system uses an InGaAs camera and broadband light sources at 1310 nm for occlusal transillumination and 1600 nm for cross-polarization reflectance measurements. In this study a mathematical model to estimate the penetration depth of "hidden" occlusal lesions from the SWIR images was developed. We compared the model's estimated lesion depth on 18 extracted teeth with lesions against microCT measurements. Although the model estimated depth deviates from that measured in microCT at higher depths, there is a good linear correlation (R2 = 0.93) between the estimated depth from SWIR images and the measured depth using microCT. SWIR occlusal transillumination images at 1300 nm also provide information about interproximal lesion penetration depth which can be directly viewed from the occlusal surface. SWIR occlusal transillumination and reflectance depth measurements on 49 natural interproximal lesions were compared with microCT measurements. There was significant correlation between the depths measured with SWIR occlusal transillumination (R2 = 0.81) and reflectance (R2 = 0.19) compared with the depths measured with microCT.

Controllable Dispersive Wave Radiation from Pearcey Gaussian Pulses

AbstractThe process of dispersive waves (DWs) emitted from unique Pearcey Gaussian (PG) pulses with both temporal and spectral asymmetric shapes in the anomalous dispersion region of optical fiber is investigated. In contrast to symmetric pulses such as fundamental soliton (Sech) or Gaussian, the radiation amplitude and energy conversion efficiency of DWs are significantly improved by adjusting the asymmetry of the spectral distribution and the temporal oscillation structure of PG pulses. It is shown that, for the case of PG pulses, the minimum value of third‐order dispersion parameter required for the emergence of DWs emission is nearly reduced 50% compared with the case of symmetric pulses. It is also shown that the radiation frequency of DWs emitted from PG pulses can be predicted by a modified phase matching condition. The theoretical predictions are in good agreement with the numerical simulations. The impact of Raman scattering effects on the DWs emission is also disclosed. The results of this study clearly reveal the impact of the inherent behaviors of PG pulses on the DWs emission, which should be relevant for applications requiring broadband light sources based on the mechanism of DWs emission.

Optical absorption spectroscopy of reactive oxygen and nitrogen species in a surface dielectric barrier discharge

A twin surface dielectric barrier discharge (SDBD) ignited in a dry synthetic air gas stream is studied regarding the formation of reactive oxygen and nitrogen species (RONS) and their impact on the conversion of admixed n-butane. The discharge is driven by a damped sinusoidal voltage waveform at peak-to-peak amplitudes of 8 kVpp–13 kVpp and pulse repetition frequencies of 250 Hz–4000 Hz. Absolute densities of O3, NO2, NO3, as well as estimates of the sum of the densities of N2O4 and N2O5 are determined temporally resolved by means of optical absorption spectroscopy using a laser driven broadband light source, suitable interference filters, and a photodiode detector. The measured densities are acquired across the center of the reactor chamber as well as at the outlet of the chamber. The temporal and spatial evolution of the species’ densities is correlated to the conversion of n-butane at concentrations of 50 ppm and 400 ppm, measured by means of flame ionization detectors. The n-butane is admixed either before or after the reactor chamber, in order to separate the impact of short- and long-lived reactive species on the conversion process. It is found that, despite the stationary conversion at the selected operating points, at higher voltages and repetition frequencies the densities of the measured species are not in steady state. Based on the produced results it is presumed that the presence of n-butane modifies the formation and consumption pathways of O3. At the same time, there is no significant impact on the formation of dinitrogen oxides (N2O4 and N2O5). Furthermore, a comparatively high conversion of n-butane, when admixed at the outlet of the reactor chamber is observed. These findings are discussed together with known rate coefficients for the reactions of n-butane with selected RONS.