Chemical sensing using fiber cavity ring-down spectroscopy

Broadband-Tunable Vanadium Dioxide (VO2)-Based Linear Optical Cavity Sensor

Sensors fabricated by using a silicon-on-insulator (SOI) platform provide promising solutions to issues such as size, power consumption, wavelength-specific nature of end reflectors and difficulty to detect ternary mixture. To address these limitations, we proposed and investigated a broadband-thermally tunable vanadium dioxide (VO2)-based linear optical cavity sensor model using a finite element method. The proposed structure consists of a silicon wire waveguide on a silicon-on-insulator (SOI) platform terminated with phase-change vanadium oxide (VO2) on each side to provide light confinement. A smooth transmission modulation range of 0.8 (VO2 in the insulator state) and 0.03 (VO2 in the conductive phase state) in the 125 to 230 THz spectral region was obtained due to the of Fabry-Pérot (FP) effect. For the 3.84 μm cavity length, the presented sensor resulted in a sensitivity of 20.2 THz/RIU or 179.56 nm/RIU, which is approximately two orders of magnitude higher than its counterparts in the literature. The sensitivity of the 2D model showed direct relation with the length of the optical cavity. Moreover, the change in the resonating mode line width Δν of approximately 6.94 THz/RIU or 59.96 nm/RIU was also observed when the sensor was subjected to the change of the imaginary part k of complex refractive index (RI). This property of the sensor equips it for the sensing of aternary mixture without using any chemical surface modification. The proposed sensor haspotential applications in the areas of chemical industries, environmental monitoring and biomedical sensing.

Broadband cavity enhanced UV-VIS absorption spectroscopy for picolitre liquid samples

Absorption spectroscopy is a widely used analytical technique due to its label-free nature, however its application to small liquid samples is hampered by the associated short absorption pathlengths, which limit sensitivity. A novel concept for the development of an ultrasensitive broadband absorption spectrometer optimised for thin liquid films is presented here. To enhance sensitivity of the absorbance measurements an optical cavity is implemented on a fibre-based absorption spectrometer (CEASpec). Light is circulated multiple times through the sample of interest to increase sensitivity. The bandwidth of the instrument is chosen by the choice of the dielectric mirrors forming the optical cavity spectra and, in this implementation, has been set to be 200 nm wide (250-450 nm). The sensing volume of the spectroscope is prescribed by the choice of optical fibres employed to deliver light to the sample, and in this implementation fibres of 400 μm in diameter were employed, giving a sensing volume of 630 picolitres for a thin film of 5 μm in thickness. Amphotericin B, a broad light absorber in the 280-450 nm region of the spectrum, was used here to prove the capabilities of the proposed cavity enhanced absorption spectroscope. Cavity enhancement factors, the equivalent pathlength increase over classical absorption spectroscopy, in the range of 200× have been achieved across a broad wavelength range.

Optical sensor using space-domain active fiber cavity ringdown technique

A novel active fiber cavity ringdown (FCRD) technique using frequency-shifted interferometry (FSI) is proposed for the first time. Using this scheme, external parameters can be monitored in the space domain by measuring the ringdown distance instead of ringdown time. A bidirectional erbium-doped fiber amplifier (Bi-EDFA) is employed to compensate the inherent cavity loss for achieving higher sensitivity. And two band-pass filters are used to reduce the amplified spontaneous emission (ASE) noise of the Bi-EDFA. Compared with the well-known time-domain active FCRD scheme, our proposed method enables us to avoid using pulsed laser needed in time-domain active FCRD, it uses continuous-wave laser to inject into the fiber cavity and stabilize the optical power in the fiber cavity, which can suppress the baseline drift of ringdown signal caused by the gain fluctuations of the EDFA and thus improve the detecting precision. Moreover, this novel method enables us to use differential detection method for further reducing the ASE noise, and thus eliminating the baseline drift of ringdown signal. A magnetic field sensor was developed as a proof-of-concept demonstration. The experimental results demonstrate that the proposed sensor with a sensitivity of 0.01537 (1/km·Gs) was achieved. This is the highest magnetic field sensitivity compared to the time-domain active FLRD method. Due to the reduced ASE noise, the stability of the proposed sensing system was also greatly improved.

All Single-Mode-Fiber Supercontinuum Source Setup for Monitoring of Multiple Gases Applications

In this paper, a gas sensing system based on a conventional absorption technique using a single-mode-fiber supercontinuum source (SMF-SC) is presented. The SC source was implemented by channeling pulses from a microchip laser into a one kilometer long single-mode fiber (SMF), obtaining a flat high-spectrum with a bandwidth of up to 350 nm in the region from 1350 to 1700 nm, and high stability in power and wavelength. The supercontinuum radiation was used for simultaneously sensing water vapor and acetylene gas in the regions from 1350 to 1420 nm and 1510 to 1540 nm, respectively. The experimental results show that the absorption peaks of acetylene have a maximum depth of approximately 30 dB and contain about 60 strong lines in the R and P branches, demonstrating a high sensitivity of the sensing setup to acetylene. Finally, to verify the experimental results, the experimental spectra are compared to simulations obtained from the Hitran database. This shows that the implemented system can be used to develop sensors for applications in broadband absorption spectroscopy and as a low-cost absorption spectrophotometer of multiple gases.

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.

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 30 mT with a sensitivity of 95.5 ns/mT.

Double-looped Mach-Zehnder interferometer for achieving multiple ring-down interferograms

We put forward a double-looped Mach-Zehnder interferometer for acquiring continuous ring-down interferograms with two fiber-loop cavities with slightly different optical path lengths. Each group of pulses through the sample and reference loops creates a ring-down pulse train with equal time intervals in Vernier fashion, and interferes with each other to produce multiple ring-down interferograms successively by scanning of a delay line. The system requires a scanning range of only a few millimeters to obtain multiple ring-down interferograms. In a proof-of-concept demonstration, the intrinsic losses of two loops are estimated. The measured combined-loss of both loops is compared to the sum of the loop losses measured separately with a conventional fiber-loop ring-down system. The result obtained using the proposed system exhibits a difference of only 0.06 dB with that of the reference system.

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.

Monitoring adsorption and sedimentation using evanescent-wave cavity ringdown ellipsometry

We monitor the adsorption of Rhodamine 800, and the sedimentation of a polytetrafluoroethylene (PTFE) suspension at the surface of a fused-silica prism, by measuring both the absorption and s-p phase shift Δ of a 740 nm probe laser beam, using evanescent-wave cavity ringdown ellipsometry (EW-CRDE). The two systems demonstrate the complementary strengths of EW-CRDE, as the progress of adsorption of the Rhodamine 800 dye can only be observed sensitively via the measurement of absorption, whereas the progress of sedimentation of PTFE can only be observed sensitively via the measurement of Δ. We show that EW-CRDE provides a sensitive method for the measurement of Δ and demonstrates precision in Δ of about 10(-4) deg.

Fiber loop ringdown sensor for potential real-time monitoring of cracks in concrete structures: an exploratory study

A fiber loop ringdown (FLRD) concrete crack sensor is described for the first time. A bare single mode fiber (SMF), without using other optical components or chemical coatings, etc., was utilized to construct the sensor head, which was driven by a FLRD sensor system. The performance of the sensor was evaluated on concrete bars with dimensions 20 cm × 5 cm × 5 cm, made in our laboratory. Cracks were produced manually and the responses of the sensor were recorded in terms of ringdown times. The sensor demonstrated detection of the surface crack width (SCW) of 0.5 mm, which leads to a theoretical SCW detection limit of 31 μm. The sensor's response to a cracking event is near real-time (1.5 s). A large dynamic range of crack detection ranging from a few microns (μm) to a few millimeters is expected from this sensor. With the distinct features, such as simplicity, temperature independence, near real-time response, high SCW detection sensitivity, and a large dynamic range, this FLRD crack sensor appears promising for detections of cracks when embedded in concrete.

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.

Evanescent-Wave Cavity Ring-Down Ellipsometry

We introduce the new technique of evanescent-wave cavity ring-down ellipsometry (EW-CRDE), used for the measurement of ellipsometric angles of samples at a solid-gas or solid-liquid interface, and achieve phase-shift measurements with precision of ∼0.01°. We demonstrate the technique by measuring the time-dependent refractive index of methanol-water mixtures and thin films at the liquid/fused-silica interface, showing that the monitoring of monolayers on microsecond time scales using EW-CRDE should be achievable.

Fiber loop ringdown DNA and bacteria sensors

We report a new type of refractive index-based biosensor using a fiber loop ringdown evanescent field (FLRD-EF) sensing scheme, in which the sensing signal is a time constant and detection sensitivity is enhanced by the multipass nature of the ringdown technique. Bulk index-based detections of three different single strand DNAs and one type of bacteria are demonstrated for the FLRD-EF sensors that utilize a partially-etched single mode fiber as the sensor head. Stepwise coating of the sensor head with poly-L-lysine and a probe DNA has enabled surface index-based label-free target DNA sensing. We expect an array of FLRD-EF biosensors to be created, which are superior to counterparts in terms of simplicity, low cost, and high sensitivity.