Cavity-ring-down principle for fiber-optic resonators: experimental realization of bending loss and evanescent-field sensing

422 Million intrinsic quality factor planar integrated all-waveguide resonator with sub-MHz linewidth

High quality-factor (Q) optical resonators are a key component for ultra-narrow linewidth lasers, frequency stabilization, precision spectroscopy and quantum applications. Integration in a photonic waveguide platform is key to reducing cost, size, power and sensitivity to environmental disturbances. However, to date, the Q of all-waveguide resonators has been relegated to below 260 Million. Here, we report a Si3N4 resonator with 422 Million intrinsic and 3.4 Billion absorption-limited Qs. The resonator has 453 kHz intrinsic, 906 kHz loaded, and 57 kHz absorption-limited linewidths and the corresponding 0.060 dB m-1 loss is the lowest reported to date for waveguides with deposited oxide upper cladding. These results are achieved through a careful reduction of scattering and absorption losses that we simulate, quantify and correlate to measurements. This advancement in waveguide resonator technology paves the way to all-waveguide Billion Q cavities for applications including nonlinear optics, atomic clocks, quantum photonics and high-capacity fiber communications.

Spatially resolved fibre cavity ring down spectroscopy

This paper presents a fibre cavity ring down spectroscopy probed by Rayleigh scattering optical frequency domain reflectometry (OFDR), which provides spatial location of stimuli and improved signal to noise ratio for distributed sensing measurements. A section of optical fibre was integrated into an active fibre ring cavity with optical gain and interrogated by the OFDR system for 11 cycles with a single laser scan. Through the cavity ring down configuration, root-mean-squared (RMS) noise of distributed temperature and strain measurements was reduced to 6.9 mK and less than 0.1 με, respectively for 1-cm spatially resolved measurements. Our work shows that the active fibre cavity configuration can be combined with distributed fibre sensing schemes to achieve both high spatial resolution and high sensitivity measurements.

Absorption path extension tunable diode laser absorption spectroscopy system with a dual fiber loop configuration

Increasing absorbance by lengthening the absorption path is a direct and effective approach to improve the signal-to-noise ratio of infrared gas absorption spectroscopy. However, once the absorption path is extended by designing and optimizing the gas cell structure to a certain extent, a bottleneck will appear due to the difficulties in the optical alignment and the interference effect. A modified tunable diode laser absorption spectroscopy system with a dual fiber loop configuration is proposed that can extend the effective absorption path length of the original multipass cell several times. The relevant theoretical model has been established and its effectiveness has been verified through experiments.

Trapping light into high orbital momentum modes of fiber tapers

A tapered cylindrical dielectric optical waveguide acts as a high quality factor white-light cavity providing high field concentration as well as long optical group delay. It is possible to optimize shape of a lossless taper to suppress reflection of the input light and to achieve infinitely high field concentration. These tapers can be used in sensing and optoelectronics applications instead of conventional microcavities.

High-sensitivity ring-down evanescent-wave sensing in fiber resonators

We report on optical-fiber cavity ring-down spectroscopy (CRDS) in the liquid phase using a laser emitting at telecommunication wavelengths. A fiber-ring cavity, comprising a short evanescent-wave coupler for radiation-matter interaction, is used as a sensor while its resonance modes are frequency locked to the laser. Exploiting the intrinsic sensitivity and noise immunity of the CRDS technique, we show that liquid absorption can be detected down to a level that is nearly a factor of 20 above the shot noise limit. We provide a thorough comparison between the experimental results and various noise contributions and address different expressions that can be used to calculate the shot noise equivalent absorbance. As a proof of principle, polyamine detection in aqueous solutions is carried out demonstrating a minimum detectable absorbance of 1.8×10(-7)  Hz(-1/2), which, to our knowledge, is the best sensitivity limit reported to date for evanescent-wave sensors.

Frequency-shifted interferometry--a versatile fiber-optic sensing technique

Fiber-optic sensing is a field that is developing at a fast pace. Novel fiber-optic sensor designs and sensing principles constantly open doors for new opportunities. In this paper, we review a fiber-optic sensing technique developed in our research group called frequency-shifted interferometry (FSI). This technique uses a continuous-wave light source, an optical frequency shifter, and a slow detector. We discuss the operation principles of several FSI implementations and show their applications in fiber length and dispersion measurement, locating weak reflections along a fiber link, fiber-optic sensor multiplexing, and high-sensitivity cavity ring-down measurement. Detailed analysis of FSI system parameters is also presented.

Novel gas sensor combined active fiber loop ring-down and dual wavelengths differential absorption method

A novel active fiber loop ring-down gas sensor combined with dual wavelengths differential absorption method is proposed. Two Distributed Feedback Laser Diodes (DFB LDs) with different wavelengths are employed. One LD whose wavelength covered with the absorption line of target gas is used for sensing. Another LD whose wavelength is centered outside the absorption line is used for reference. The gas absorption loss can be obtained by differencing the reference signal and sensing signal. Compared with traditional method of one wavelength employed, it can eliminate the influence of the cavity loss variety and photoelectric device drift in the system efficiently. An Erbium Doped Fiber Amplifier (EDFA) with Automatic Gain Control (AGC) is used to compensate the loss of the light in the ring-down cavity, which will increase the cavity round trips and improve the precision of gas detection. And two fiber Bragg gratings (FBGs) are employed to get rid of amplified spontaneous emission (ASE) spectrum noise as filters. The calibrating ethyne samples of different concentrations are measured with a 65 mm long gas cell in order to evaluate the effect of reference. The results show the relative deviation is found to be less than ± 0.4% of 0.1% ethyne when a certain additional loss from 0 to 1.2dB is introduced to the cavity and the relative deviation of measured concentration is less than ± 0.5% over 24 hours.

Integrated fiber optic incoherent broadband cavity enhanced absorption spectroscopy detector for near-IR absorption measurements of nanoliter samples

An integrated fiber-optic sensor is described that uses incoherent broadband cavity enhanced absorption spectroscopy for sensitive detection of aqueous samples in nanoliter volumes. Absorption was measured in a 100 µm gap between the ends of two short segments of multimode graded-index fiber that were integrated into a capillary using a precision machined V-grooved fixture that allowed for passive fiber alignment. The other ends of the fibers were coated with dielectric mirrors to form a 9.5 cm optical resonator. Light from a fiber-coupled superluminescent diode was directly coupled into one end of the cavity, and transmission was measured using a fiber-coupled silicon photodiode. Dilute aqueous solutions of near infrared dye were used to determine the minimum detectable absorption change of 2.4×10(-4) under experimental conditions in which pressure fluctuations limited performance. We also determined that the absolute minimum detectable absorption change would be 1.6×10(-5) for conditions of constant pressure in which absorption measurement is limited by electronic and optical noise. Tolerance requirements for alignment are also presented.

Evanescent field-fiber loop ringdown glucose sensor

Evanescent field-fiber loop ringdown (EF-FLRD) is a relatively new hybrid sensing technique which combines a versatile sensing mechanism with a sensitivity-enhanced ringdown detection scheme. An array of low cost, fast response, and high sensitivity biosensors based on the EF-FLRD technique can be developed. In this work, new fiber loop ringdown glucose sensors using refractive index-difference evanescent field attenuation effect as a sensing mechanism are described. The sensor head consists of either a section of partially-etched bare single mode fiber or a section of the etched fiber with glucose oxidase (GOD) immobilized on the etched fiber surface. Effects of the sensor head, with and without the immobilized GOD, on the sensor's performance are comparatively examined. The sensors' responses to standard glucose solutions and synthetic urines in different glucose concentrations ranging from 50 mg/dl to 10 g/dl are studied. The sensors, with or without the immobilized GOD, showed a linear response to glucose concentrations in the range of 100 mg/dl to 1 g/dl, but a nonlinear response in the higher glucose concentration ranging from 1 to 10 g/dl. The detection sensitivities of the sensors for the glucose solutions and artificial urine samples are 75 and 50 mg/dl respectively, and the sampling rate of the sensors is 10 to 100 Hz. Estimated theoretical detection sensitivity of the EF-FLRD glucose sensors is 10 mg/dl, which is approximately 17 times lower than the glucose renal threshold concentration.

Continuous-wave fiber cavity ring-down measurements using frequency-shifted interferometry

We present a spatial-domain fiber cavity ring-down (CRD) technique that does not require optical pulses for time-resolved detection. Instead of measuring the intensity decay rate of an optical pulse in the time domain as in conventional CRD experiments, we measure the decay rate of a CW signal in a ring-down cavity (RDC) using frequency-shifted interferometry in the spatial domain, as a function of distance traveled by the light. As a proof-of-concept demonstration, we measured fiber bend loss in a loop RDC using this technique, and a cavity loss change as low as 0.0135 dB induced by every fiber turn on a mandrel was unambiguously discerned.

Absorption detection using optical waveguide cavities

Cavity ring-down spectroscopy is a spectroscopic method that uses a high quality optical cavity to amplify the optical loss due to the light absorption by a sample. In this presentation we highlight two applications of phase-shift cavity ring-down spectroscopy that are suited for absorption measurements in the condensed phase and make use of waveguide cavities. In the first application, a fiber loop is used as an optical cavity and the sample is introduced in a gap in the loop to allow absorption measurements of nanoliters of solution at the micromolar level. A second application involves silica microspheres as high finesse cavities. Information on the refractive index and absorption of a thin film of ethylene diamine on the surface of the microresonator is obtained simultaneously by the measurements of the wavelength shift of the cavity mode spectrum and the change in optical decay time, respectively.

Optical fiber sensing based on reflection laser spectroscopy

An overview on high-resolution and fast interrogation of optical-fiber sensors relying on laser reflection spectroscopy is given. Fiber Bragg-gratings (FBGs) and FBG resonators built in fibers of different types are used for strain, temperature and acceleration measurements using heterodyne-detection and optical frequency-locking techniques. Silica fiber-ring cavities are used for chemical sensing based on evanescent-wave spectroscopy. Various arrangements for signal recovery and noise reduction, as an extension of most typical spectroscopic techniques, are illustrated and results on detection performances are presented.

Chemical sensing using fiber cavity ring-down spectroscopy

Waveguide-based cavity ring-down spectroscopy (CRD) can be used for quantitative measurements of chemical concentrations in small amounts of liquid, in gases or in films. The change in ring-down time can be correlated to analyte concentration when using fiber optic sensing elements that change their attenuation in dependence of either sample absorption or refractive index. Two types of fiber cavities, i.e., fiber loops and fiber strands containing reflective elements, are distinguished. Both types of cavities were coupled to a variety of chemical sensor elements, which are discussed and compared.

Fabry-Perot resonances in birefringent YAlO3 analyzed at terahertz frequencies

We present transmission and phase measurements on a birefringent YAlO(3) crystal that is illuminated with linearly polarized terahertz radiation in a broad frequency range (10 cm(-1) to 16 cm(-1)). The spectra exhibit pronounced and complex Fabry-Perot resonances that depend on the polarization orientation of the incoming light. We present a theoretical description to analyze these data and to obtain the optical constants of the crystal in the terahertz range.

Sensitivity enhancement of fiber loop cavity ring-down pressure sensor

We present a theoretical and experimental study on sensitivity enhancement of a fiber-loop cavity ring-down pressure sensor. The cladding of the sensing fiber is etched in hydrofluoric acid solution to enhance its sensitivity. The experimental results demonstrate that the pressure applied on the sensing fiber is linearly proportional to the difference between the reciprocals of the ring-down time with and without pressure, and the relative sensitivity exponentially increases with decreasing the cladding diameter. When the sensing fiber is etched to 41.15 microm, its sensitivity is about 36 times that of nonetched fiber in the range of 0 to 32.5 MPa. The measured relative standard deviation of the ring-down time is about 0.15% and, correspondingly, the least detectable loss is about 0.00069 dB.

Fiber loop ringdown - a time-domain sensing technique for multi-function fiber optic sensor platforms: current status and design perspectives

Fiber loop ringdown (FLRD) utilizes an inexpensive telecommunications light source, a photodiode, and a section of single-mode fiber to form a uniform fiber optic sensor platform for sensing various quantities, such as pressure, temperature, strain, refractive index, chemical species, biological cells, and small volume of fluids. In FLRD, optical losses of a light pulse in a fiber loop induced by changes in a quantity are measured by the light decay time constants. FLRD measures time to detect a quantity; thus, FLRD is referred to as a time-domain sensing technique. FLRD sensors have near real-time response, multi-pass enhanced high-sensitivity, and relatively low cost (i.e., without using an optical spectral analyzer). During the last eight years since the introduction of the original form of fiber ringdown spectroscopy, there has been increasing interest in the FLRD technique in fiber optic sensor developments, and new application potential is being explored. This paper first discusses the challenging issues in development of multi-function, fiber optic sensors or sensor networks using current fiber optic sensor sensing schemes, and then gives a review on current fiber optic sensor development using FLRD technique. Finally, design perspectives on new generation, multi-function, fiber optic sensor platforms using FLRD technique are particularly presented.

Liquid-phase and evanescent-wave cavity ring-down spectroscopy in analytical chemistry

Due to its simplicity, versatility, and straightforward interpretation into absolute concentrations, molecular absorbance detection is widely used in liquid-phase analytical chemistry. Because this method is inherently less sensitive than zero-background techniques such as fluorescence detection, alternative, more sensitive measurement principles are being explored. This review discusses one of these: cavity ring-down spectroscopy (CRDS). Advantages of this technique include its long measurement pathlength and its insensitivity to light-source-intensity fluctuations. CRDS is already a well-established technique in the gas phase, so we focus on two new modes: liquid-phase CRDS and evanescent-wave (EW)-CRDS. Applications of liquid-phase CRDS in analytical chemistry focus on improving the sensitivity of absorbance detection in liquid chromatography. Currently, EW-CRDS is still in early stages: It is used to study basic interactions between molecules and silica surfaces. However, in the future this method may be used to develop, for instance, biosensors with high specificity.

Multiple-cladding fibers with reduced bend loss

We demonstrate that a highly bend-resistant fiber can be realized. It is shown theoretically that, by introducing both depressed and elevated rings into the cladding, bending loss can be reduced significantly. A fiber based on this design has been fabricated and characterized as a first step toward achieving this goal. The results show that a multiple-cladding fiber is highly bend resistant when compared with the standard telecom single-mode fiber.

Hemoglobin Adsorption to Silica Monitored with Polarization-Dependent Evanescent-Wave Cavity Ring-Down Spectroscopy

Evanescent-wave cavity ring-down spectroscopy was used to monitor the adsorption of human hemoglobin to a fused-silica surface from aqueous solution. An uncoated dove prism was situated in a ring-down cavity such that the beam entered and exited with a normal-incidence geometry. This afforded ring-down times as high as 5 mus and values of sigma(tau)/tau as low as 0.3%. Normal-incidence geometry permits the detection of both S- and P-polarized light, yielding some information of the orientation of adsorbates. The orientation of the adsorbed hemoglobin molecules is found to change as adsorption progresses, but with a different time profile than adsorption itself. The adsorption kinetics from a quiescent solution is consistent with a reaction-diffusion model that includes both reversible and irreversible adsorption operating in parallel. Systems behaving according to this model also seem to display adsorption isotherms, although the increased adsorption from more concentrated solutions is only a consequence of the system being under kinetic control. In some cases, this may be sufficient to explain the paradox of protein adsorption systems which seem to be both irreversible and consistent with isotherm models as well.

Evanescent-Wave Cavity Ring-Down Investigation of Polymer/Solvent Interactions

Evanescent-wave cavity ring-down spectroscopy (EW-CRDS) is used to measure interfacial phenomena when methanol or water is placed in contact with a film of poly(dimethylsiloxane) (PDMS), which is attached to the face of a fused-silica prism that constitutes part of a ring cavity. In the first few minutes after contact, the uptake of methanol is slower than that of water, but after this initial period the methanol diffuses more rapidly in the film than water does. Bulk weight-gain measurements confirm this result and yield diffusion coefficients of (25.1 +/- 0.7) x 10(-7) cm(2)/s for methanol in PDMS and (7 +/- 2) x 10(-7) cm(2)/s for water in PDMS. The interfacial optical losses found in the EW-CRDS measurements result primarily from scatter. In particular, we find that delamination of the film from the fused-silica substrate dominates the optical losses in the case of methanol. This conclusion is confirmed by separate surface plasmon resonance experiments.

Moving beyond Traditional UV−Visible Absorption Detection: Cavity Ring-Down Spectroscopy for HPLC

We describe the use of liquid-phase continuous-wave cavity ring-down spectroscopy for the detection of an HPLC separation. This technique builds on earlier work by Snyder and Zare using pulsed laser sources and improves upon commercially available UV-visible detectors by a factor of up to 50. The system employs a compact doubled-diode single-mode continuous-wave laser operating at 488 nm and a previously described Brewster's-angle flow cell. Ring-down time constants as long as 5.8 micros were observed with liquid samples in a 0.3-mm path length cell. The baseline noise during an HPLC separation was only 2 x 10(-7) absorbance units (AU) peak to peak, as compared to 1 x 10(-5) AU for a state-of-the-art commercial UV-visible detector.

Fiber loop ringdown for physical sensor development: pressure sensor

A new method of developing optical fiber pressure sensors by use of a fiber loop ringdown scheme is described. The fiber loop ringdown system is characterized in terms of the ringdown baseline stability, fiber transmission loss, and fiber refractive index. The overall sensor performance is demonstrated by use of sensing forces applied to the sensor head. The current device can sense pressures in the range of 0 to 9.8 x 10(6) Pa, converted approximately from the applied forces. The sensor's linear response, repeatability, detection sensitivity, measuring dynamic range, and temperature tolerance are explored.

Cavity ringdown strain gauge

Biconical tapered single-mode fiber, which is common in many telecommunications components, offers an alternative sensor to typical optical fiber strain gauges that are susceptible to temperature and pressure effects and require expensive and sophisticated signal acquisition systems. Cavity ringdown spectroscopy, a technique commonly applied to high-sensitivity chemical analysis, offers detection sensitivity advantages that can be used to improve strain measurement with biconical tapers. Combining these two technologies in a spatially extended resonator, we demonstrate a minimum detectable change in ringdown time of 0.08%, corresponding to a minimum detectable displacement of 4.8 nm, and a sensitivity to strain as small as 79 n epsilon/square root(Hz) over a 5-mm taper length.

Configuration of ring-down spectrometers for maximum sensitivity

Cavity ring-down (CRD) spectrometers used for analytical applications frequently have design requirements different from spectrometers used for gas-phase spectroscopic applications. A formalism that allows for maximization of the relative sensitivity by adapting the cavity length and absorption path through the sample is presented. These experimental configurations may not reduce the detection limit but do allow for a high sensitivity of the ring-down time measurement in the concentration range of interest. The formalism is applied to two common CRDS experimental configurations and to a fiber-loop ring-down experiment.Key words: cavity ring-down (CRD), absorption, detector, fiber-loop, sensitivity, detection limit.

Cavity-enhanced spectroscopy in fiber cavities

We discuss the relative merits of passive and active fiber cavities for ring-down. Ring-down times of approximately 2 micros were recently demonstrated in passive cavities, but operation is restricted to weak evanescent wave interaction. We report on active cavities with amplifiers used for loss compensation, permitting the use of open-path micro-optic cells. Ring-down times of tens of microseconds can readily be achieved and extended to several hundred microseconds in gain-clamped cavities, but relaxation oscillations and system drift impose limits on accuracy and repeatability. A cavity enhancement of 2 orders of magnitude is realistically possible, and sensitivity may be further enhanced if ring-down is combined with established spectroscopic methods.

Fiber ringdown pressure sensors

An exploratory study on a novel fiber ringdown pressure sensor is presented. With this technique, pressure measurements are achieved in a time domain by measurement of ringdown times. The proof-of-concept device consists of a diode laser light source, two 2 x 1 fiber couplers, a section of fused-silica single-mode fiber, a photodetector, and an electronic control. The sensor's performance in the areas of stability, repeatability, and dynamic range is explored. The results demonstrate the new concept of fiber pressure sensors and the technical feasibility of developing a new generation of fiber sensors for pressure measurements.

Absolute surface coverage measurement using a vibrational overtone

Determination of absolute surface coverage with sub-monolayer sensitivity is demonstrated using evanescent-wave cavity ring-down spectroscopy (EW-CRDS) and conventional CRDS by employing conservation of the absolute integrated absorption intensity between gas and adsorbed phases. The first C-H stretching overtones of trichloroethylene (TCE), cis-dichloroethylene, and trans-dichloroethylene are probed using the idler of a seeded optical parametric amplifier having a 0.075 cm(-1) line width. Polarized absolute adsorbate spectra are obtained by EW-CRDS using a fused-silica monolithic folded resonator having a finesse of 28 500 at 6050 cm(-1), while absolute absorption cross sections for the gas-phase species are determined by conventional CRDS. A measure of the average transition moment orientation on the surface, which is utilized for the coverage determination, is derived from the polarization anisotropy of the surface spectra. Coverage measurement by EW-CRDS is compared to a mass-spectrometer-based surface-uptake technique, which we also employ for coverage measurements of TCE on thermally grown SiO(2) surfaces. To assess the potential for environmental sensing, we also compare EW-CRDS to optical waveguide techniques developed previously for TCE detection.

Surface-plasmon-resonance-enhanced cavity ring-down detection

The cavity ring-down technique is used to probe the absolute optical response of the localized surface plasmon resonance (SPR) of a gold nanoparticle distribution to adsorption of trichloroethylene (TCE) and perchloroethylene (PCE) from the gas phase. Extended Mie theory for a coated sphere with a particle-size-dependent dielectric function is used to elucidate size-dispersion effects, the size-dependence of the SPR sensitivity to adsorption, and the kinetics of adsorption. An approximate Gaussian distribution of nanospheres with a mean diameter of 4.5 nm and a standard deviation of 1.1 nm, as determined by atomic force microscopy, is provided by the intrinsic granularity of an ultrathin, gold film, having a nominal thickness of approximately 0.18 nm. The cavity ring-down measurements employ a linear resonator with an intracavity flow cell, which is formed by a pair of ultrasmooth, fused-silica optical flats at Brewster's angle, where the Au film is present on a single flat. The total system intrinsic loss is dominated by the film extinction, while the angled flats alone contribute only approximately 5 x 10(-5)/flat to the total loss. Based on a relative ring-down time precision of 0.1% for ensembles averages of 25 laser shots from a pulsed optical parametric oscillator, the minimum detectable concentrations of PCE and TCE obtained by probing the SPR response are found to be 2 and 7 x 10(-8) mol/L, respectively, based on a 30 s integration time.

Fiber-optic cavity sensing of hydrogen diffusion

A novel type of fiber-optic cavity sensor for hydrogen diffusion into and out of fibers is presented. The sensor is an implementation of a cavity ringdown scheme in a silica-based single-mode fiber that has been exposed to gaseous hydrogen at normal pressure. The measured ringdown times during the H2 diffusion show good agreement with a theoretical diffusion model. This model allows the determination of the diffusion coefficient of hydrogen in silica, resulting in D = (3.02 +/- 0.07) x 10(-15) m2/s at 30 degrees C.

Cavity-enhanced spectroscopy in optical fibers

Cavity-enhanced methods have been extended to fiber optics by use of fiber Bragg gratings (FBGs) as reflectors. High-finesse fiber cavities were fabricated from FBGs made in both germanium/boron-co-doped photosensitive fiber and hydrogen-loaded Corning SMF-28 fiber. Optical losses in these cavities were determined from the measured Fabry-Perot transmission spectra and cavity ring-down spectroscopy. For a 10-m-long single-mode fiber cavity, ring-down times in excess of 2 ms were observed at 1563.6 nm, and individual laser pulses were resolved. An evanescent-wave access block was produced within a fiber cavity, and an enhanced sensitivity to optical loss was observed as the external medium's refractive index was altered.