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Register a new userSimultaneous measurement of quality factor and wavelength shift by phase shift microcavity ring down spectroscopy
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Optical resonant microcavities with ultra high quality factors are widely used for biosensing. Until now, the primary method of detection has been based upon tracking the resonant wavelength shift as a function of biodetection events. One of the sources of noise in all resonant-wavelength shift measurements is the noise due to intensity fluctuations of the laser source. An alternative approach is to track the change in the quality factor of the optical cavity by using phase shift cavity ring down spectroscopy, a technique which is insensitive to the intensity fluctuations of the laser source. Here, using biotinylated microtoroid resonant cavities, we show simultaneous measurement of the quality factor and the wavelength shift by using phase shift cavity ring down spectroscopy. These measurements were performed for disassociation phase of biotin-streptavidin reaction. We found that the disassociation curves are in good agreement with the previously published results. Hence, we demonstrate not only the application of phase shift cavity ring down spectroscopy to microcavities in the liquid phase but also simultaneous measurement of the quality factor and the wavelength shift for the microcavity biosensors in the application of kinetics measurements.
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Liquid phase sensing applications at 1550~nm are highly desirable due to widely available off-the-shelf components. Generally, liquids at 1550~nm induce a high absorption loss that limits the overall sensors sensitivity and detection limit. One solution is to use an active fiber loop in conjunction with cavity ring down spectroscopy to overcome these absorption losses. However, the amplifier inside the fiber loop suffers from inherent gain fluctuations that limit the sensing systems overall performance. Here, we provide a novel sensor using the wavelength-scanned phase shift-cavity ring down spectroscopy (PS-CRDS) in conjunction with a linear active fiber cavity that potentially offers a more sensitive solution than traditional fiber loop sensors. We use a tapered fiber as a sensing head inside the active cavity built from fiber Bragg gratings. We derive a theoretical phase shift expression for our system and simulate it using the finite element method to determine optimum tapered fiber diameter for glucose sensing in DI water. Compared to a non-amplified system, we find that our amplified system can increase the sensitivity by fourteen times via the amplifier gain tuning. We also conduct experimental measurements using 0-15.5~mM glucose solutions and find them in excellent agreement with our theoretical predictions. Experimentally we obtain the sensors sensitivity of 0.768~$^o$/mM (1164~$^o$/RIU) and detection limit of 0.75~mM ( 4.5~$times$~10$^{-4}$~RIU) without any temperature stabilization in the system. We anticipate that the present work will find a wide range of sensing applications in fiber cavities, ring resonators, and other microcavity structures.
Cavity ring-down spectroscopy is a ubiquitous optical method used to study light-matter interactions with high resolution, sensitivity and accuracy. However, it has never been performed with the multiplexing advantages of direct frequency comb spectroscopy without sacrificing orders of magnitude of resolution. We present dual-comb cavity ring-down spectroscopy (DC-CRDS) based on the parallel heterodyne detection of ring-down signals with a local oscillator comb to yield absorption and dispersion spectra. These spectra are obtained from widths and positions of cavity modes. We present two approaches which leverage the dynamic cavity response to coherently or randomly driven changes in the amplitude or frequency of the probe field. Both techniques yield accurate spectra of methane - an important greenhouse gas and breath biomarker. The high sensitivity and accuracy of broadband DC-CRDS, shows promise for applications like studies of the structure and dynamics of large molecules, multispecies trace gas detection and isotopic composition.
Aflatoxin M1 (AFM1) is a carcinogenic compound commonly found in milk in excess of the WHO permissible limit, especially in developing countries. Currently, state-of-the-art tests for detecting AFM1 in milk include chromatographic systems and enzyme-linked-immunosorbent assays. Although these tests provide fair accuracy and sensitivity however, they require trained laboratory personnel, expensive infrastructure, and many hours for producing final results. Optical sensors leveraging spectroscopy have a tremendous potential of providing an accurate, real time, and specialists-free AFM1 detector. Despite this, AFM1 sensing demonstrations using optical spectroscopy are still immature. Here, we demonstrate an optical sensor that employs the principle of cavity attenuated phase shift spectroscopy in optical fiber cavities for rapid AFM1 detection in aqueous solutions at 1550 nm. The sensor constitutes a cavity built by two fiber Bragg gratings. We splice a tapered fiber of $<$ 10 $mu$m waist inside the cavity as a sensing head. For ensuring specific binding of AFM1 in a solution, the tapered fiber is functionalized with DNA aptamers followed by validation of the conjugation via FTIR, TGA, and EDX analyses. We then detect AFM1 in a solution by measuring the phase shift between a sinusoidally modulated laser input and the sensor output at resonant frequencies of the cavity. Our results show that the sensor has the detection limit of 20 ng/L (20 ppt) which is well below both the US and the European safety regulations. We anticipate that the present work will lead towards a rapid and accurate AFM1 sensor, especially for low-resource settings.
Optical imaging modalities are non-ionizing methods with significant potential for non-invasive, portable, and cost-effective medical diagnostics and treatments. The design of critical parameters of an optical imaging system depends on a thorough understanding of optical properties of the biological tissue within the purposed application. Integrating sphere technique combined with inverse adding doubling algorithm has been widely used for determination of biological tissue ex vivo. It has been studied for tissues typically with a large sample size and over a spectral range of 400 nm to 1100 nm. The aim of this study is to develop a methodology for calculating optical absorption and reduced scattering of small size biological tissues from reflectance and transmittance measurements at a wide spectral range of 400 to 1800 nm. We developed a small sample adaptor kit to allow integrating sphere measurements of samples with small sizes using a commercial device. We proposed a two-tier IAD algorithm to mitigate the profound cross-talk effect in reduced scattering using IAD. We evaluated the two-tier IAD with both simulated data by Monte Carlo Simulation and data obtained from phantom experiments. We also investigated the accuracy the proposed work flow of using small sample kit and condensed incident light beam. We found that the small sample measurements despite with condense beam size led to overestimated absorption coefficient across the whole wavelength range while the spectrum shape well preserved. Our proposed method of a two-tier IAD and small sample kit could be a useful and reliable tool to characterise optical properties of biological tissue ex vivo particularly when only small size samples are available.
Nanophotonic technologies offer great promise for ultra-low power optical signal processing, but relatively few nonlinear-optical phenomena have yet been explored as bases for robust digital modulation/switching~cite{Yang07,Fara08,Liu10,Noza10}. Here we show that a single two-level system (TLS) coupled strongly to an optical resonator can impart binary phase modulation on a saturating probe beam. Our experiment relies on spontaneous emission to induce occasional transitions between positive and negative phase shifts---with each such edge corresponding to a dissipated energy of just one photon ($approx 0.23$ aJ)---but an optical control beam could be used to trigger additional phase switching at signalling rates above this background. Although our ability to demonstrate controlled switching in our atom-based experiment is limited, we discuss prospects for exploiting analogous physics in a nanophotonic device incorporating a quantum dot as the TLS to realize deterministic binary phase modulation with control power in the aJ/edge regime.
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