Ring-down Time Research Articles

Fiber-loop ring-down interrogated refractive index sensor based on an SNS fiber structure

In this paper, a fiber-loop ring-down interrogated refractive index sensor based on a single-mode-no-core-single-mode (SNS) fiber structure is reported. The influence of the wavelength and pulsewidth on the performance of the sensor are systematically investigated. Results show that the performance of the sensing system is strongly dependent on the interrogation wavelength and a narrower pulsewidth is helpful to avoid the disturbance of the relaxation oscillation. A method to determine the optimal interrogation wavelength was proposed. After determining the interrogation wavelength and pulsewidth, refractive index (RI) measurement was experimentally performed in the RI range of 1.3330–1.3539. And a cubic polynomial fitting was established between RI and ring-down time. In the linear region, a sensitivity of −3271μs/RIU was achieved.

Q-Band Loop-Gap Resonator for EPR Applications with Broadband-Shaped Pulses

Recently, Q-band-pulsed electron paramagnetic resonance spectroscopy has strongly advanced its performance by the introduction of high-power microwave amplifiers and the use of shaped pulses. For such applications, the resonator Q value has to be low enough to achieve sufficient bandwidth for short microwave pulses and to reduce the ring-down time after the pulses. However, a low Q value reduces the detection sensitivity as well as the conversion efficiency of the microwave input power to the magnetic field strength at the sample position. Therefore, the resonator Q value has to be optimized for a given microwave input power and specific application. We designed a three-loop/two-gap resonator using CST Microwave Studio for such applications, and tested its performance in comparison with a standard Bruker D2 Q-band microwave resonator by accomplishing broadband SIFTER experiments on a nitroxide model compound.

Fiber-loop ring-down magnetic field and temperature sensing system based on the principle of time-division multiplexing

A combined magnetic field and temperature sensing system based on the principle of time-division multiplexing was developed. A fiber-loop ring-down technique was introduced to enhance the stability and sensitivity of the system. Magnetic sol-gel was coated onto a single-mode fiber with its cladding partly etched to form the magnetic field sensor head. A temperature-deformation converter was employed in the temperature sensing unit. A delay fiber was used to adjust the decaying pulses of the two sensing units. The influence of temperature on the magnetic field sensor head was experimentally investigated. The magnetic field strength exhibited a well-defined linear relationship with respect to the inverse of the ring-down time over a range of 25–70mT with a sensitivity of 12.7ns/mT, and the temperature showed a well-defined linear relationship between 20.3°C and 79.7°C with a sensitivity of 3.53ns/°C when the magnetic field intensity was 25mT.

Vibration damping materials with enhanced loss due to microstructural nonlinearity

One conventional approach to attenuating structure-borne waves or reduce the ringdown time of modes in structural elements is to attach damping layers or patches to vibrating components. Common patch and layer materials are specially formulated polymers or engineered polymeric composites that demonstrate elevated viscoelastic loss factors in the frequency range of interest. Recent research has shown that small volume fractions of negative stiffness inclusions embedded in a lossy material generate effective loss factors that exceed that of the host material. The ability to generate negative stiffness behavior, however, is often the result of nonlinear inclusion material response. Presented here is a multiscale model of a particulate composite material consisting of a nearly incompressible host material containing small-scale heterogeneities with a nonlinear elastic stress-strain response. We investigate the nonlinear dynamic behavior of the heterogeneous medium for small harmonic perturbations about several pre-strain states to demonstrate the influence of the microscale dynamic response on the macroscopically observable mechanical loss. Of primary interest is the energy dissipation capabilities of the composite which can be tuned using inclusion pre-strain. Loss for composites with nonlinear inclusions is compared to conventional composites containing air voids or steel inclusions. [Work supported by ONR.]

Cl2 Elimination in 248 nm Photolysis of (COCl)2 Probed with Cavity Ring-Down Absorption Spectroscopy.

Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl)2 at 248 nm obtaining a primary Cl2 elimination channel. A ratio of vibrational population is estimated to be 1:(0.12 ± 0.03):(0.011 ± 0.003) for the v = 0, 1, and 2 levels. The quantum yield of Cl2 molecular channel is obtained to be 0.8 ± 0.4 initiated from the X̃1Ag ground state surface (COCl)2 via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl)2 + hν → 2CO + Cl2) and secondary reactions (dominated by Cl + COCl → Cl2 + CO). The former is estimated to share a yield of >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl2 is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ring-down time to 70 μs, thus allowing for the contribution from 17% up to 66% of the total Cl2 yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, the primary Cl2 dissociation channel eliminated from (COCl)2 is observed for the first time.

Microwave-assisted frequency domain measurement of fiber-loop ring-down system.

A new frequency domain measurement method for a fiber-loop ring-down system is proposed in this Letter. Compared to traditional time domain measurement, this method uses a microwave modulated continuous wave (CW) laser as a light source, making full use of the duty cycle to achieve enhanced measurement efficiency. By measuring the amplitude modulation over a frequency span, this technique can be used to determine the ring-down time in the frequency domain, which will then be used to calculate the loss in the ring.

Simultaneous mapping of reflectance, transmittance and optical loss of highly reflective and anti-reflective coatings with two-channel cavity ring-down technique.

We demonstrate the use of a two-channel cavity ring-down (CRD) technique for simultaneously measuring/mapping the reflectance R, transmittance T and optical loss L (absorption plus scattering losses) of highly reflective (HR) and anti-reflective (AR) laser components. High reflectance/transmittance of HR/AR components is measured with the ring-down time of CRD signals, while the low residual transmittance/ reflectance of HR/AR components is determined by the amplitude ratio of two CRD signals, and the optical loss is then determined via L = 1-R-T. Experiments are performed to measure and map R, T, and L of HR mirrors with different transmittance levels from below 1ppm to about 70 ppm (part-per-million) and of one AR window at 635nm. For a 4 ppm-transmittance HR mirror, the measured R, T, and L at one position are 99.99821 ± 0.00004%, 4.042 ± 0.008 ppm and 13.9 ± 0.4 ppm, respectively. For the AR sample, the measured T, R, and L at one position are 99.99279 ± 0.00004%, 50.0 ± 0.7 ppm and 22.0 ± 0.4 ppm, respectively. The sub-ppm standard deviations for R, T, and L indicate the high accuracy of the two-channel CRD technique for the simultaneous measurements of reflectivity, transmittance and optical loss of HR and AR components. High-resolution mappings of R, T, and L of both HR and AR samples are demonstrated. The simultaneous measurements/mappings of reflectance, transmittance, and optical loss with sub-ppm accuracy are of great importance to the preparation of high-performance laser optics for applications such as gravitational-wave detection and laser gyroscopes.

Fiber loop ring-down cavity integrated U-bent single-mode-fiber for magnetic field sensing

A novel magnetic field sensing system based on the fiber loop ring-down technique is proposed in this paper. In the fiber loop, a U-bent single-mode-fiber structure coated with magnetic fluid (MF) serves as the sensing head, and an erbium-doped fiber amplifier (EDFA) is introduced to compensate for the intrinsic loss of the cavity. The ring-down time of the system varies with the change of applied magnetic field due to the tunable absorption coefficient and refractive index of the MF. Therefore, measurement of the magnetic field can be realized by monitoring the ring-down time. The experimental results show that the performance of the system is extremely dependent on the interrogation wavelength, because both the gain of the EDFA and the loss of the sensing head are wavelength dependent. We found that at the optimal wavelength, the ratio of the gain to loss attained its maximum. The sensing system was experimentally demonstrated and a sensitivity of −0.5951 μs/Oe was achieved.

Fiber loop ringdown humidity sensor.

An optical fiber relative humidity (RH) sensor based on the evanescent field-fiber loop ringdown (EF-FLRD) technique is demonstrated. The sensor was placed inside a chamber that provides a humidity reference and is monitored by a humidity meter. The presence of moisture in the chamber changes the refractive index of the medium; thus the ringdown time changes due to a change in the EF scattering loss induced in the sensor head. The sensor demonstrated a fast response (∼1 s), high sensitivity, and excellent reproducibility and reversibly. The EF-FLRD sensor can measure RH in a wide dynamic range of 4% to 100% at a constant temperature of 20±1°C.

Fiber cavity ring down and gain amplification effect

The effect of an erbium-doped fiber amplifier (EDFA) placed inside the fiber ring of a cavity ring down (CRD) configuration is studied. The limitations and advantages of this configuration are discussed, and the study of the ring-down time as a function of the current applied and gain to the EDFA is also presented. In this case, the power fluctuations in the output signal are strongly dependent on the cavity ring-down time with the EDFA gain.

Pressure sensor based on the Sagnac effect and fiber loop ringdown spectroscopy

A fiber-optic sensing scheme of measuring pressure is described here. The high reflective mirror in a laser cavity is replaced by a Sagnac loop in this scheme. The method combining the Sagnac effect with fiber loop ringdown technology fully embodies the advantages of both. The working principle is discussed in detail, and the whole sensing performance is demonstrated by applying sensing force to the sensor area. The sensing force can be obtained by measuring the ringdown time. The pressure measurement range of this device is 40 to 350 N. The detection sensitivity 0.4 ns/N can be realized in this system. The experimental curve reveals a close relationship between the sensing strain and the ringdown time.

Electromechanical transition in quantum dots

The strong coupling between electronic transport in a single-level quantum dot and a capacitively coupled nano-mechanical oscillator may lead to a transition towards a mechanically-bistable and blocked-current state. Its observation is at reach in carbon-nanotube state-of-art experiments. In a recent publication [Phys. Rev. Lett. 115, 206802 (2015)] we have shown that this transition is characterized by pronounced signatures on the oscillator mechanical properties: the susceptibility, the displacement fluctuation spectrum and the ring-down time. These properties are extracted from transport measurements, however the relation between the mechanical quantities and the electronic signal is not always straightforward. Moreover the dependence of the same quantities on temperature, bias or gate voltage, and external dissipation has not been studied. The purpose of this paper is to fill this gap and provide a detailed description of the transition. Specifically we find: (i) The relation between the current-noise and the displacement spectrum. (ii) The peculiar behavior of the gate-voltage dependence of these spectra at the transition. (iii) The robustness of the transition towards the effect of external fluctuations and dissipation.

Monolithic Cylindrical Fused Silica Resonators with High Q Factors.

The cylindrical resonator gyroscope (CRG) is a typical Coriolis vibratory gyroscope whose performance is determined by the Q factor and frequency mismatch of the cylindrical resonator. Enhancing the Q factor is crucial for improving the rate sensitivity and noise performance of the CRG. In this paper, for the first time, a monolithic cylindrical fused silica resonator with a Q factor approaching 8 × 105 (ring-down time over 1 min) is reported. The resonator is made of fused silica with low internal friction and high isotropy, with a diameter of 25 mm and a center frequency of 3974.35 Hz. The structure of the resonator is first briefly introduced, and then the experimental non-contact characterization method is presented. In addition, the post-fabrication experimental procedure of Q factor improvement, including chemical and thermal treatment, is demonstrated. The Q factor improvement by both treatments is compared and the primary loss mechanism is analyzed. To the best of our knowledge, the work presented in this paper represents the highest reported Q factor for a cylindrical resonator. The proposed monolithic cylindrical fused silica resonator may enable high performance inertial sensing with standard manufacturing process and simple post-fabrication treatment.

Extinction measurement with open-path cavity ring-down technique of variable cavity length.

Open-path cavity ring down (OPCRD) technique with variable cavity length was developed to measure optical extinction including scattering and absorption of air in laboratory environment at 635 nm wavelength. By moving the rear cavity mirror of the ring-down cavity to change cavity length, ring-down time with different cavity lengths was experimentally obtained and the dependence of total cavity loss on cavity length was determined. The extinction coefficient of air was determined by the slope of linear dependence of total cavity loss on cavity length. The extinction coefficients of air with different particle concentrations at 635 nm wavelength were measured to be from 10.46 to 84.19 Mm-1 (ppm/m) in a normal laboratory environment. This variable-cavity-length OPCRD technique can be used for absolute extinction measurement and real-time environmental monitoring without closed-path sample cells and background measurements.

Cantilever Ringdown Dissipation Imaging for the Study of Loss Processes in Polymer/Fullerene Solar Cells

We use dissipation imaging to probe local changes in electronic properties of nanostructured semiconductor films due to local photochemistry. We make quantitative maps of electrostatic dissipation due to photogenerated carriers by measuring the ringdown time of an oscillating atomic force microscope cantilever. Using organic photovoltaic materials as a testbed, we study macroscopic device degradation as a function of photooxidation for three different film morphologies comprising the conjugated polymer poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and the fullerene derivative [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM). We find that, judged by device performance, the stability of the macroscopic devices is sensitive to processing conditions, with films processed with the solvent additive 1,8-diiodooctane being the most stable. At the microscopic level, we compare the evolution of cantilever power dissipation as a function of photochemical degradation for three different polymer/fullerene blend morphologies and show that the changes in local power dissipation correlate with device stability. Using ringdown imaging to look at local dissipation in a highly phase-separated PTB7:PC71BM film morphology, we show that cantilever power dissipation increases more rapidly over large fullerene aggregates than in well-mixed polymer/fullerene regions, suggesting that local photochemistry on the fullerene contributes strongly to the dissipation signal.

Continuous wave external-cavity quantum cascade laser-based high-resolution cavity ring-down spectrometer for ultrasensitive trace gas detection.

A high-resolution cavity ring-down spectroscopic (CRDS) system based on a continuous wave (cw) mode-hop-free (MHF) external-cavity quantum cascade laser (EC-QCL) operating at λ∼5.2 μm has been developed for ultrasensitive detection of nitric oxide (NO). We report the performance of the high-resolution EC-QCL based cw-CRDS instrument by measuring the rotationally resolved Λ-doublet e and f components of the P(7.5) line in the fundamental band of NO at 1850.169 cm-1 and 1850.179 cm-1. A noise-equivalent absorption coefficient of 1.01×10-9 cm-1 Hz-1/2 was achieved based on an empty cavity ring-down time of τ0=5.6 μs and standard deviation of 0.11% with averaging of six ring-down time determinations. The CRDS sensor demonstrates the advantages of measuring parts per billion NO concentrations in N2, as well as in human breath samples with ultrahigh sensitivity and specificity. The CRDS system could also be generalized to measure simultaneously many other trace molecular species within the broad tuning range of cw EC-QCL, as well as for studying the rotationally resolved hyperfine structures.

Ultralow-Noise SiN Trampoline Resonators for Sensing and Optomechanics

In force sensing, optomechanics, and quantum motion experiments, it is typically advantageous to create lightweight, compliant mechanical elements with the lowest possible force noise. Here we report wafer-scale batch fabrication and characterization of high-aspect-ratio, nanogram-scale Si$_3$N$_4$ "trampolines" having quality factors above $4 \times 10^7$ and ringdown times exceeding five minutes (1 mHz linewidth). We measure a thermally limited force noise sensitivity of 16.2$\pm$0.8 aN/Hz$^{1/2}$ at room temperature, with a spring constant ($\sim$1 N/m) 2-5 orders of magnitude larger than those of competing technologies. We also characterize the suitability of these devices for high-finesse cavity readout and optomechanics applications, finding no evidence of surface or bulk optical losses from the processed nitride in a cavity achieving finesse 40,000. These parameters provide access to a single-photon cooperativity $C_0 \sim 8$ in the resolved-sideband limit, wherein a variety of outstanding optomechanics goals become feasible.

Microcrystalline diamond cylindrical resonators with quality-factor up to 0.5 million

We demonstrate high quality-factor 1.5 mm diameter batch-fabricated microcrystalline diamond cylindrical resonators (CR) with quality-factors limited by thermoelastic damping (TED) and surface loss. Resonators were fabricated 2.6 and 5.3 μm thick in-situ boron-doped microcrystalline diamond films deposited using hot filament chemical vapor deposition. The quality-factor (Q) of as-fabricated CR's was found to increase with the resonator diameter and diamond thickness. Annealing the CRs at 700 °C in a nitrogen atmosphere led to a three-fold increase in Q, a result we attribute to thinning of the diamond layer via reaction with residual O2 in the annealing furnace. Post-anneal Q exceeding 0.5 million (528 000) was measured at the 19 kHz elliptical wineglass modes, producing a ring-down time of 8.9 s. A model for Q versus diamond thickness and resonance frequency is developed including the effects of TED and surface loss. Measured quality factors are shown to agree with the predictions of this model.

Magnetic field sensor using the fiber loop ring-down technique and an etched fiber coated with magnetic fluid

The fiber loop ring-down spectroscopy technique is introduced into the evanescent-field-based sensing scheme in order to create a new type of fiber-based magnetic field sensor. As a consequence, the sensitivity and stability of the magnetic field sensing system are significantly enhanced. The sensor head is constructed using a section of a single-mode fiber with its cladding partially etched. The process of fiber etching is described in detail, and the relationship between the diameter of the etched fiber and the etching time is experimentally investigated. After adopting the appropriate size of the etched fiber, the final experimental results show that the magnetic field strength has a well-defined linear relationship with the inverse of the ring-down time τ over a range of 30mT with a sensitivity of 95.5ns/mT.

Research and data processing of double locked cavity ringdown absorption spectroscopy

A continuous wave cavity ringdown spectroscopy based on a double-locking loop is proposed to improve the shortcoming of low acquisition rate of concentration in traditional scheme. A small portion of laser is separated to pass through a C2H2 reference cell, used to lock the laser frequency to the 1+3 band P(9)e absorption line of C2H2 at 6534.3634 cm-1 by the 1st harmonic demodulation of the frequency modulation spectroscopy. The remaining portion is incident on a high finesses cavity to observe the ringdown events. Meanwhile, the reflected light of cavity is used to extract the error signal to lock the laser based on the PDH frequency locking technique. As a consequence, the frequency drift of the laser and the jitter of the cavity length are improved, therefore a more relatively accuracy result is expected. The laser light is dual frequency modulated by a fiber coupled electro optic modulator (FEOM)in the above system. In order to optimize, to some extent, the asymmetry of the error signal caused by the residual amplitude modulation due to the inconsistency of the laser polarization direction with the extraordinary axis of the FEOM, the demodulation phase is adjusted carefully until the error signal is smoothed up and close to symmetry. Then, the effect of locking loop is examined. The frequency of laser, based on the measurement by a wavelength meter, is more stable and the relative frequency discrimination between the laser and the longitudinal mode of cavity is about 9.8 kHz. In addition, the PDH locking, ensuring the efficient coupling of the laser with the cavity, can gain a high acquisition rate of the concentration information. In order to obtain a complete ringdown event, the frequency of square wave to the fiber coupled acoustic optical modulator (FAOM) is limited to 30 kHz with the duty cycle of 85%, which is determined by the ringdown time and re-lock time. However, there exists a relatively large random noise in a series of ringdown time measurements of empty cavity, which is mainly caused by the errors of fitting and measurement. For the further improvement of the accuracy of experiment, an efficient digital filter, Kalman filter which can suppress the noise considerably at no expense of real-time capability, is used. The standard deviation of the ringdown time is reduced from 0.00333 to 0.00153. According to Allan variance analysis, the detection limit can reach 410-9 cm-1 for a 2 s integration time. Finally, the C2H2 gases with different concentrations from 100 ppb to 5 ppm are measured to demonstrate the linear response of this system.