Optical power detector with broad spectral coverage, high detectivity, and large dynamic range.

Optical power measurements are needed in practically all technologies based on light. Here, we report a general-purpose optical power detector based on the photoacoustic effect. Optical power incident on the detector's black absorber produces an acoustic signal, which is further converted into an electrical signal using a silicon-cantilever pressure transducer. We demonstrate an exceptionally large spectral coverage from ultraviolet to far infrared, with the possibility for further extension to the terahertz region. The linear dynamic range of the detector reaches 80 dB, ranging from a noise-equivalent power of 6 n W/H z to 600 mW (independent of signal averaging time).

Incoherent broadband cavity enhanced absorption spectroscopy using supercontinuum and superluminescent diode sources.

We investigate incoherent broadband cavity enhanced absorption spectroscopy using a tailored supercontinuum source. By tailoring the supercontinuum spectrum to match the high reflectivity bandwidth of the mirrors, we achieve an unprecedented spectral brightness of more than 7 dBm/nm at wavelengths where the effective absorption path length in the cavity exceeds 40 km. We demonstrate the potential of the source in spectrally broadband measurement of weak overtone transitions of carbon dioxide and methane in the near-infrared 1590 nm - 1700 nm range and evaluate its performance against that of a typical superluminescent diode source. Minimum detectable absorption coefficients (3σ) of 2.2 × 10(-9) cm(-1) and 6.2 × 10(-9) cm(-1) are obtained with the supercontinuum and the superluminescent diode sources, respectively. We further develop a spectral fitting method based on differential optical absorption spectroscopy to fully and properly account for the combined effect of absorption line saturation and limited spectral resolution of the detection. The method allows to cope with high dynamic range of absorption features typical of real-world multi-component measurements.

Long distance active hyperspectral sensing using high-power near-infrared supercontinuum light source.

A hyperspectral remote sensing instrument employing a novel near-infrared supercontinuum light source has been developed for active illumination and identification of targets. The supercontinuum is generated in a standard normal dispersion multi-mode fiber and has 16 W total optical output power covering 1000 nm to 2300 nm spectral range. A commercial 256-channel infrared spectrometer was used for broadband infrared detection. The feasibility of the presented hyperspectral measurement approach was investigated both indoors and in the field. Reflection spectra from several diffusive targets were successfully measured and a measurement range of 1.5 km was demonstrated.

Periodic interactions between solitons and dispersive waves during the generation of non-coherent supercontinuum radiation.

We present a numerical study of interactions between dispersive waves (DWs) and solitons during supercontinuum generation in photonic crystal fibers pumped with picosecond laser pulses. We show how the soliton-induced trapping potential evolves along the fiber and affects the dynamics of a DW-soliton pair. Individual frequency components of the DW periodically interact with the soliton resulting in stepwise frequency blue shifts. In contrast, the ensemble blue shifts of all frequency components in the DW appear to be quasi-continuous. The step size of frequency up-conversion and the temporal separation between subsequent soliton-DW interactions are governed by the potential well which confines the soliton-DW pair and which changes in time.

Optical detection of the anesthetic agent propofol in the gas phase.

The anesthetic agent propofol (2,6-diisopropylphenol) is the most widely used intravenously administered drug in general anesthesia. However, a viable online capability to monitor metabolized levels of propofol in patients does not currently exist. Here we show for the first time that optical spectroscopy has good potential to detect metabolized propofol from patients' exhaled breath. We present quantitative absorption measurements of gas phase propofol both in the ultraviolet and middle-infrared spectral regions. We demonstrate that a detection limit in the subparts-per-billion concentration range can be reached with photoacoustic spectroscopy in the UV spectral region, paving the way for the development of future optical monitors.

Phase-sensitive method for background-compensated photoacoustic detection of NO2 using high-power LEDs.

A photoacoustic (PA) sensor has been developed for the detection of nitrogen dioxide (NO(2)). Ten amplitude-modulated high-power light emitting diodes (LEDs), emitting a total optical power of 9 W at 453 nm, are used to excite the photoacoustic signal in NO(2). The LEDs are attached to the circumference of a cylindrical PA cell. The induced longitudinal acoustics waves are detected using two electromechanical film stacks, located at the ends of the cell. Background signal cancelation is achieved by using phase-sensitive detection of the difference signal of the two pressure transducers. The phase-sensitive approach allows for improved dynamic range and sensitivity. A detection limit of 10 parts per billion by volume was achieved for flowing NO(2) gas sample in an acquisition time of 2.1 s, corresponding to a minimum detectable absorption coefficient of 1.6 × 10(-7) cm(-1) Hz(-1/2). The developed sensor has potential for compact, light-weight, and low-cost measurement of NO(2).

Photoacoustic characteristics of carbon-based infrared absorbers.

We present an experimental comparison of photoacoustic responsivities of common highly absorbing carbon-based materials. The comparison was carried out with parameters relevant for photoacoustic power detectors and Fourier-transform infrared (FTIR) spectroscopy: we covered a broad wavelength range from the visible red to far infrared (633 nm to 25 µm) and the regime of low acoustic frequencies (< 1 kHz). The investigated materials include a candle soot-based coating, a black paint coating and two different carbon nanotube coatings. Of these, the low-cost soot absorber produced clearly the highest photoacoustic response over the entire measurement range.

Sensitive method for the kinetic measurement of trace species in liquids using cavity enhanced absorption spectroscopy with broad bandwidth supercontinuum radiation.

A novel spectrometer for the rapid and sensitive detection of liquid phase analytes at trace concentrations is presented. Broad bandwidth supercontinuum radiation was coupled into a linear optical cavity incorporating an intracavity liquid-sample cuvette. Cavity enhanced absorption spectra of trace species covering more than 300 nm were acquired on time scales of milliseconds. Single shot acquisition times of 10-50 ms are demonstrated here. The effective absorption path length exceeds 2 m in sample volumes measuring 2.7 mL. A key feature of the instrument is that it can be calibrated using cavity ring-down spectroscopy without the requirement of changing the optical alignment. The sensitivity of the instrument is exemplified by measurements of trace concentrations of dye molecules and nickel sulfate dissolved in water. A minimum detectable absorption coefficient of 9.1 x 10(-7) cm(-1) Hz(-1/2) at 550 nm was obtained. The capability to capture broad bandwidth absorption spectra on short time scales permits kinetic studies of liquid phase reactions. We demonstrate this by recording the oscillatory behavior of a Belousov-Zhabotinsky reaction.

An adaptive filter for studying the life cycle of optical rogue waves.

We present an adaptive numerical filter for analyzing fiber-length dependent properties of optical rogue waves, which are highly intense and extremely red-shifted solitons that arise during supercontinuum generation in photonic crystal fiber. We use this filter to study a data set of 1000 simulated supercontinuum pulses, produced from 5 ps pump pulses containing random noise. Optical rogue waves arise in different supercontinuum pulses at various positions along the fiber, and exhibit a lifecycle: their intensity peaks over a finite range of fiber length before declining slowly.

Predicting supercontinuum pulse collisions with simulations exhibiting temporal aliasing.

Interactions between supercontinuum (SC) light pulses, produced by the propagation of rapidly sequenced picosecond pump laser pulses along a photonic crystal fiber, result in spectral broadening, which we attribute to interpulse soliton collisions. This phenomenon was measured experimentally, following our observation of spectral broadening in numerical simulations that exhibit so-called "pulse wraparound" or "temporal aliasing." This occurs in simulations with narrow time grids: as early parts of the SC pulse leave the computational time domain, they "reenter" at the beginning and so interact with later parts of the evolving SC pulse. We show that this provides an effective model to predict the experimentally observed spectral changes.

Following interfacial kinetics in real time using broadband evanescent wave cavity-enhanced absorption spectroscopy: a comparison of light-emitting diodes and supercontinuum sources.

A white light-emitting diode (LED) with emission between 420 and 700 nm and a supercontinuum (SC) source with emission between 450 and 2500 nm have been compared for use in evanescent wave broadband cavity-enhanced absorption spectroscopy (EW-BB-CEAS). The method is calibrated using a dye with known absorbance. While the LED is more economic as an excitation source, the SC source is superior both in terms of baseline noise (noise equivalent absorbances lower than 10(-5) compared to 10(-4) absorbance units (a.u.)) and accuracy of the measurement; these baseline noise levels are comparable to evanescent wave cavity ringdown spectroscopy (EW-CRDS) studies while the accessible spectral region of EW-BB-CEAS is much larger (420-750 nm in this study, compared to several tens of nanometres for EW-CRDS). The improvements afforded by the use of an SC source in combination with a high sensitivity detector are demonstrated in the broadband detection of electrogenerated Ir(IV) complexes in a thin-layer electrochemical cell arrangement. Excellent signal to noise is achieved with 10 micros signal accumulation times at a repetition rate of 600 Hz, easily fast enough to follow, in real time, solution kinetics and interfacial processes.

Optical characterization of bisphenol-A-glycidyldimethacrylate-triethyleneglycoldimethacrylate (BisGMA/TEGDMA) monomers and copolymer.

OBJECTIVE: The fundamental optical characteristics of dental polymers as a function of curing time are exiguously reported in the literature. In this work the refractive index [1] and the extinction coefficient [2] of bisphenol-A-glycidyldimethacrylate-triethyleneglycoldimethacrylate (BisGMA/TEGDMA)-based polymers of different volume fractions have been studied as a function of the curing time. A comparison between the measured index of refraction and theoretical calculation based on molar refraction is made. Also the temporal behavior of both the refractive index and extinction coefficient is compared. METHODS: Index of refraction: BisGMA/TEGDMA monomer samples were cured with a commercial light-curing unit by using camphorquinone and DMAEMA as the photoinitiator system. Curing times of 0, 40, 60, 300, 900 and 1800 s were used. The refractive indices were determined with a refractometer at 470 nm wavelength, where the peak absorption of the camphorquinone photoinitiator is located. Extinction coefficient: BisGMA/TEGDMA samples having different lengths were cured and the relative attenuation of light at 470 nm was measured as a function of the curing time (0, 40, 60, 300, 900, 1800 s). RESULTS: The results show that in general the index of refraction increases during the photopolymerization process in a time frame from 0 to 40s and then approaches a steady-state value, which depends on the original composition of the admixture. The detailed refractive index change depends on the volume fractions of the constituent substances. The extinction coefficient decreases in a time frame from 60 to 900 s and reaches a steady-state value thereafter. SIGNIFICANCE: Knowledge of the fundamental optical characteristics of commonly used dental polymers is essential in the development of advanced light-initiated curing methods.

Evaluation of polymerization shrinkage and hydroscopic expansion of fiber-reinforced biocomposites using optical fiber Bragg grating sensors.

OBJECTIVES: Polymerization shrinkage of dental composite materials is recognized as one of the main reasons for the development of marginal leakage between a tooth and filling material. In addition, hydroscopic expansion due to water sorption is known to cause instability in dental materials. Several methods have been proposed to quantify the polymerization shrinkage and hydroscopic expansion. However, in the case of anisotropic materials, such as unidirectional fiber-reinforced composites (FRCs), the measurement method must allow for the discrimination of the shrinkage or expansion in the two orthogonal directions. In this work, optical fiber sensors were employed to study strains in dental materials induced by polymerization shrinkage and hydroscopic expansion. METHODS: Four dental materials were evaluated in this study: unfilled BisGMA/TEGDMA-based resin, Z250 dental restorative composite, unidirectional and bidirectional fiber reinforced composites. The changes in the linear strain due to the polymerization shrinkage and hydroscopic expansion were monitored in real-time using embedded optical fiber Bragg grating (FBG) sensors. The polymerization shrinkage was monitored during the light curing process. FBG sensors were also used to record the hydroscopic expansion of the samples which were immersed in water up to 132 days. RESULTS: Unfilled polymer resin had the highest polymerization shrinkage of 0.84%. Unidirectional FRC had a relatively high shrinkage in the transverse direction with respect to the reinforcing fibers (0.41%) whereas the shrinkage along the reinforcing fibers was small (0.02%). Bidirectional FRC showed a low shrinkage value (0.03%). For most tested materials the hydroscopic expansion seemed to compensate for the polymerization shrinkage. SIGNIFICANCE: Fiber Bragg grating sensors are suitable for accurate real-time monitoring of small internal strains of biomaterials, e.g., due to polymerization shrinkage and hydroscopic expansion. Detailed data on polymerization shrinkage and water sorption behavior of different dental materials can be used to optimize the mechanical properties of dental composite materials and to improve the longevity of a dental restoration.

Diode laser-based photoacoustic spectroscopy with interferometrically-enhanced cantilever detection: erratum.

An erratum is presented to correct the calculated figures of merit.

Diode laser-based photoacoustic spectroscopy with interferometrically-enhanced cantilever detection.

A novel sensitive approach to detect weak pressure variations has been applied to tunable diode laser-based photoacoustic spectroscopy. The sensing device consists of a miniature silicon cantilever, the deflection of which is detected with a compact Michelson-type interferometer. The photoacoustic system has been applied to the detection of carbon dioxide (CO2) at 1572 nm with a distributed feedback diode laser. A noise equivalent sensitivity of 2.8 x 10-10 cm-1WHz-1/2 was demonstrated. Potential improvements of the technique are discussed.

VCSEL based detection of water vapor near 940nm.

A vertical-cavity surface-emitting laser (VCSEL) was used to study the absorption spectrum of water vapor in the 940nm region. Measurements were performed in ambient air at room temperature and in a hydrogen-oxygen flame over the temperature range of 1500-1800K. Several rotational absorption lines within the 2v1 + v3 vibrational band were measured. The absorption spectra were well resolved, which demonstrates the feasibility of VCSEL-based spectroscopic measurements of water vapor at room and high-temperature in this spectral region. The results were in good agreement with the values obtained from the HITRAN-96 database.

Cavity enhanced spectroscopy of high-temperature H(2)o in the near-infrared using a supercontinuum light source.

In this paper we demonstrate how broadband cavity enhanced absorption spectroscopy (CEAS) with supercontinuum (SC) radiation in the near-infrared spectral range can be used as a sensitive, multiplexed, and simple tool to probe gas-phase species in high-temperature environments. Near-infrared SC radiation is generated by pumping a standard single-mode fiber with a picosecond fiber laser. Standard low reflectivity mirrors are used for the cavity and an optical spectrum analyzer is used for the detection of gas-phase species in combustion. The method is demonstrated by measuring flame generated H(2)O in the 1500 to 1550 nm region and room-temperature CO(2) between 1520 nm and 1660 nm. The broadband nature of the technique permits hundreds of rotational features to be recorded, giving good potential to unravel complex, convoluted spectra. We discuss practical issues concerning the implementation of the technique and present a straightforward method for calibration of the CEAS system via a cavity ringdown measurement. Despite the large spectral variation of SC radiation from pulse to pulse, it is shown that SC sources can offer good stability for CEAS where a large number of SC pulses are typically averaged.

Tunable external-cavity diode laser at 650 nm based on a transmission diffraction grating.

A tunable external-cavity diode laser (ECDL) based on a transmission diffraction grating in a Littrow mount has been developed and characterized. A single-transverse-mode diode laser at 650 nm is used in an external-cavity configuration in which the transmission grating is used as a dispersive element to select the single longitudinal mode. The transmission diffraction grating is made with electron-beam lithography. A tunable true single-mode cw output power of >20 mW is obtained from the ECDL. The total wavelength tuning range is 12 nm, and the mode-hop-free continuous tunability is >20 GHz.