Intravascular photoacoustic-ultrasound (IVPA-US) imaging is an emerging hybrid modality for the detection of lipidladen plaques by providing simultaneous morphological and lipid-specific chemical information of an artery wall. The clinical utility of IVPA-US technology requires real-time imaging and display at speed of video-rate level. Here, we demonstrate a compact and portable IVPA-US system capable of imaging at up to 25 frames per second in real-time display mode. This unprecedented imaging speed was achieved by concurrent innovations in excitation laser source, rotary joint assembly, 1 mm IVPA-US catheter, differentiated A-line strategy, and real-time image processing and display algorithms. By imaging pulsatile motion at different imaging speeds, 16 frames per second was deemed to be adequate to suppress motion artifacts from cardiac pulsation for <i>in vivo</i> applications. Our lateral resolution results further verified the number of A-lines used for a cross-sectional IVPA image reconstruction. The translational capability of this system for the detection of lipid-laden plaques was validated by <i>ex vivo</i> imaging of an atherosclerotic human coronary artery at 16 frames per second, which showed strong correlation to gold-standard histopathology.
Lipid deposition inside the arterial wall is a hallmark of plaque vulnerability. Overtone absorption-based intravascular photoacoustic (IVPA) catheter is a promising technology for quantifying the amount of lipid and its spatial distribution inside the arterial wall. Thus far, the clinical translation of IVPA technology is limited by its slow imaging speed due to lack of a high-power and high-repetition-rate laser source for lipid-specific excitation at 1.7 μm. Here, we demonstrate a potassium titanyl phosphate-based optical parametric oscillator (OPO) with output pulse energy up to 2 mJ at a wavelength of 1724 nm and with a repetition rate of 500 Hz. This OPO enabled IVPA imaging at 1 frame per sec, which is about 50-fold faster than previously reported IVPA systems. The IVPA imaging system was characterized by a pencil lead and a lipid-mimicking phantom for its imaging resolution, sensitivity, and specificity, respectively. Its performance was further validated by ex vivo study of an atherosclerotic human femoral artery and comparison to gold standard histology.
We exploit photothermal effect in gas-filled hollow-core photonic bandgap fibers, and demonstrate remarkably sensitive all-fiber (acetylene) gas sensors with noise equivalent concentration of 1-3 parts-per-billion and an unprecedented dynamic range of nearly six orders of magnitude. These results are two to three orders of magnitude better than previous direct absorption-based optical fiber gas sensors. The realization of photothermal spectroscopy in fiber-optic format will allow a new class of sensors with ultra-sensitivity and selectivity, compact size, remote and multiplexed multi-point detection capability.
Sensitive detection of nitric oxide (NO) at ppbv concentration levels has an important impact in diverse fields of
applications including environmental monitoring, industrial process control and medical diagnostics. For example, NO
can be used as a biomarker of asthma and inflammatory lung diseases such as chronic obstructive pulmonary disease.
Trace gas sensor systems capable of high sensitivity require the targeting of strong rotational-vibrational bands in the
mid-IR spectral range. These bands are accessible using state-of-the-art high heat load (HHL) packaged, continuous
wave (CW), distributed feedback (DFB) quantum cascade lasers (QCLs). Quartz-enhanced photoacoustic spectroscopy
(QEPAS) permits the design of fast, sensitive, selective, and compact sensor systems. A QEPAS sensor was developed
employing a room-temperature CW DFB-QCL emitting at 5.26 μm with an optical excitation power of 60 mW. High
sensitivity is achieved by targeting a NO absorption line at 1900.08 cm<sup>-1</sup> free of interference by H<sub>2</sub>O and CO<sub>2</sub>. The
minimum detection limit of the sensor is 7.5 and 1 ppbv of NO with 1and 100 second averaging time respectively . The
sensitivity of the sensor system is sufficient for detecting NO in exhaled human breath, with typical concentration levels
ranging from 24.0 ppbv to 54.0 ppbv.
Hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) is a relevant molecular trace gas species, that is related to the oxidative capacity of the
atmosphere, the production of radical species such as OH, the generation of sulfate aerosol via oxidation of S(IV) to
S(VI), and the formation of acid rain. The detection of atmospheric H<sub>2</sub>O<sub>2</sub> involves specific challenges due to its high
reactivity and low concentration (ppbv to sub-ppbv level). Traditional methods for measuring atmospheric H<sub>2</sub>O<sub>2</sub>
concentration are often based on wet-chemistry methods that require a transfer from the gas- to liquid-phase for a
subsequent determination by techniques such as fluorescence spectroscopy, which can lead to problems such as sampling
artifacts and interference by other atmospheric constituents. A quartz-enhanced photoacoustic spectroscopy-based
system for the measurement of atmospheric H<sub>2</sub>O<sub>2</sub> with a detection limit of 75 ppb for 1-s integration time was previously
reported. In this paper, an updated H<sub>2</sub>O<sub>2</sub> detection system based on long-optical-path-length absorption spectroscopy by
using a distributed feedback quantum cascade laser (DFB-QCL) will be described. A 7.73-μm CW-DFB-QCL and a
thermoelectrically cooled infrared detector, optimized for a wavelength of 8 μm, are employed for theH<sub>2</sub>O<sub>2</sub> sensor
system. A commercial astigmatic Herriott multi-pass cell with an effective optical path-length of 76 m is utilized for the
reported QCL multipass absorption system. Wavelength modulation spectroscopy (WMS) with second harmonic
detection is used for enhancing the signal-to-noise-ratio. A minimum detection limit of 13.4 ppb is achieved with a 2 s
sampling time. Based on an Allan-Werle deviation analysis the minimum detection limit can be improved to 1.5 ppb
when using an averaging time of 300 s.
A trace gas absorption sensor for formaldehyde (H2CO) detection was developed using a continuous wave, room
temperature, low-power consumption interband cascade laser (ICL) at 3.6 μm. The recent availability of ICLs with
wavelength ranged between 3−4 μm enables the sensitive detection of trace gases such as formaldehyde that possesses a
strong absorption band in this particular wavelength region. This absorption sensor detected a strong formaldehyde line at
2778.5 cm<sup>-1</sup> in its v1 fundamental band. Wavelength modulation spectroscopy with second harmonic detection (WMS-2f)
combined with a compact and novel multipass gas cell (7.6 cm physical length, 32 ml sampling volume, and 3.7 m optical
path length) was utilized to achieve a sensitivity of ~6 ppbv for H2CO measurements at 1 Hz sampling rate. The Allan-
Werle deviation plot reveals that a minimum detection limit of ~1.5 ppbv can be achieved for an averaging time of 140
Phase sensitivity of the fundamental mode of hollow-core photonic bandgap fiber to gas pressure applied internally to its core is investigated. The measured phase sensitivity for a 95-cm-long fiber is 9.92 rad/kPa, over two orders of magnitude higher than that to external pressure. The large phase sensitivity is attributed mainly to the pressure-induced refractive index change of air inside the fiber core. Such an effect may be exploited for high sensitivity pressure sensing and biochemical and environmental process analysis involving pressure variations.
The effects of modal interference (MI) on the performance of hollow-core photonic bandgap fiber (HC-PBF) gas sensors are investigated. By optimizing mode launch, applying wavelength modulation with proper modulation parameters as well as appropriate digital signal processing, an estimated lower detection limit of <1 ppmv acetylene is achieved with 13-m long HC-PBF. The impacts of drilling side-hole on the MI and response time are also studied. With a 62-cm long sensing HC-PBF drilled with multiple side-holes, an acetylene sensor with a lower detection limit of 11 ppmv and a recovery time of 2 minute is demonstrated.
The spectrophone performance for QEPAS is numerically investigated by using a finite element method. The effect of
varying system parameters such as the excitation frequency, relative position between the acoustic resonant tubes and the
quartz tuning fork, and the dimensions of resonant tubes are examined A pair of rigid tubes, each with a length of 5.1
mm and an inner diameter of 0.2 mm, positioned 0.6 μm down from the opening and 20 μm away from the edge of
tuning fork is suggested for optimal spectrophone performance.
Quartz-enhanced photoacoustic spectroscopy with a near infrared distributed feedback diode laser at 1.53 μm is
demonstrated for acetylene detection at atmospheric pressure and room temperature. The P(9) absorption line in the
ν<sub>1</sub>+ν<sub>3</sub> band of C<sub>2</sub>H<sub>2</sub> is selected for light absorption and photoacoustic pressure wave excitation. A pair of resonant tubes
with optimal dimensions is used in combination with a quartz tuning fork for photoacoustic signal enhancement. The
wavelength of diode laser is modulated at half of the resonant frequency of tuning fork for second harmonic signal
detection. The effect of residual amplitude modulation is theoretically analyzed and compared with the experimental
results. A noise-limited minimum detectable concentration (1σ) of 2 part-per-million (ppm) is achieved with a 7-mW
laser power and a 1-s lock-in time constant, corresponding to a normalized noise equivalent absorption coefficient of
5.4×10<sup>-8</sup> cm<sup>-1</sup> W/√Hz.
Proc. SPIE. 8351, Third Asia Pacific Optical Sensors Conference
KEYWORDS: Signal to noise ratio, Optical amplifiers, Single mode fibers, Fiber lasers, Scanning electron microscopy, Gas lasers, Tapered optical fibers, Photoacoustic spectroscopy, Acoustics, Absorption
Evanescent-wave gas sensing with tapered optical fibers (TOFs) and quartz-enhanced photoacoustic spectroscopy
(QEPAS) is reported. The evanescent field of TOFs with diameter down to sub-wavelength is utilized for photoacoustic
excitation in photoacoustic spectroscopy. A quartz tuning fork (QTF) with resonant frequency about ~32.75 kHz is used
to detect the generated pressure wave. A normalized noise equivalent absorption coefficient of 1.5×10<sup>-6</sup> cm<sup>-1</sup> W/√Hz is
achieved for acetylene detection with a fiber taper with a waist diameter of 1.1 μm. It is found that QEPAS with TOFs of
sub-wavelength diameters exhibit comparable sensitivities with open path QEPAS but with additional advantages of
lower insertion loss, easier alignment, and multiplexing capability.