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We have developed a real-time ultrasound needle tracking system based on a fibre-optic hydrophone integrated into an intraoperative needle. The system is intended to track the location of a needle tip during minimally invasive surgical procedures that rely on handheld ultrasound guidance. During these procedures, clear visibility of the needle tip is essential to reach the procedure target and avoid adverse events due to erroneous needle placement. The ultrasound emitted by the handheld probe is detected by the hydrophone and used to draw a cursor marking the position of the needle tip within the B-mode ultrasound image. The tracking accuracy was assessed by comparison to manual labelling of the location of the needle tip within the ultrasound images and found to be ±2.2 mm.

Dual‐modality imaging employing complementary modalities, such as all‐optical ultrasound and photoacoustic imaging, is emerging as a well‐suited technique for guiding minimally invasive surgical procedures. Quantum dots are a promising material for use in these dual‐modality imaging devices as they can provide wavelength‐selective optical absorption. The first quantum dot nanocomposite engineered for co‐registered laser‐generated ultrasound and photoacoustic imaging is presented. The nanocomposites developed, comprising CuInS2 quantum dots and medical‐grade polydimethylsiloxane (CIS‐PDMS), are applied onto the distal ends of miniature optical fibers. The films exhibit wavelength‐selective optical properties, with high optical absorption (> 90%) at 532 nm for ultrasound generation, and low optical absorption (< 5%) at near‐infrared wavelengths greater than 700 nm. Under pulsed laser irradiation, the CIS‐PDMS films generate ultrasound with pressures exceeding 3.5 MPa, with a corresponding bandwidth of 18 MHz. An ultrasound transducer is fabricated by pairing the coated optical fiber with a Fabry–Pérot (FP) fiber optic sensor. The wavelength‐selective nature of the film is exploited to enable co‐registered all‐optical ultrasound and photoacoustic imaging of an ink‐filled tube phantom. This work demonstrates the potential for quantum dots as wavelength‐selective absorbers for all‐optical ultrasound generation., Bilayer nanocomposites comprising CuInS2 quantum dots within micron‐scale polydimethylsiloxane host layers are microfabricated with dip‐coating on the distal ends of optical fibers. The wavelength‐selective optical absorption (high from 400 to 600 nm; low from 700 to 1200 nm) is well‐suited to multimodality optical ultrasound and photoacoustic imaging of biological tissue structures such as atherosclerotic plaque.

Many percutaneous needle-based procedures such as foetal interventions, tumor biopsies, nerve blocks, and central venous catheterizations are guided by ultrasound (US) imaging to identify the procedural target and to visualize the needle. A key challenge associated with ultrasound-guided needle insertions is accurate and efficient identification of the needle tip, as thin needles can readily stray from the imaging plane and can have poor visibility at large insertion angles. Ultrasonic tracking is a method for localising the needle tip relative to the imaging plane in real-time, using an ultrasonic transmitter or receiver integrated into the needle that is in communication with an external ultrasound imaging probe. This study had two foci. The first was to increase the sensitivity with which ultrasound reception was performed, using a custom fiber optic hydrophone with a high-finesse Fabry-Perot cavity based ultrasound sensor. This sensor, which comprised of a polymer layer sandwiched between dielectric mirrors, was interrogated continuously during insertions into tissue. The second focus of the study was to develop a custom needle stylet into which the fiber optic hydrophone was integrated, which was fully compatible with clinical practice and which could be adapted to different needles. We tested the sensitivity of the sensorized stylet across a wide range of needle angulations, depths and insertion angles in different biological tissues. We demonstrated, for the first time, needle tip localization in ex-vivo tissues at depths beyond 6 cm and insertion angles steeper than 80°. We conclude that ultrasonic tracking with high-finesse Fabry-Perot fiber optic hydrophone is very promising for use in clinical practice.

Ultrasound is frequently used for guiding minimally invasive procedures, but visualizing medical devices is often challenging with this imaging modality. When visualization is lost, the medical device can cause trauma to critical tissue structures. Here, a method to track the needle tip during ultrasound image-guided procedures is presented. This method involves the use of a fiber-optic ultrasound receiver that is affixed within the cannula of a medical needle to communicate ultrasonically with the external ultrasound probe. This custom probe comprises a central transducer element array and side element arrays. In addition to conventional two-dimensional (2D) B-mode ultrasound imaging provided by the central array, three-dimensional (3D) needle tip tracking is provided by the side arrays. For B-mode ultrasound imaging, a standard transmit-receive sequence with electronic beamforming is performed. For ultrasonic tracking, Golay-coded ultrasound transmissions from the 4 side arrays are received by the hydrophone sensor, and subsequently the received signals are decoded to identify the needle tip's spatial location with respect to the ultrasound imaging probe. As a preliminary validation of this method, insertions of the needle/hydrophone pair were performed in clinically realistic contexts. This novel ultrasound imaging/tracking method is compatible with current clinical workflow, and it provides reliable device tracking during in-plane and out-of-plane needle insertions.

Ultrasound is frequently used for guiding minimally invasive procedures, but visualizing medical devices is often challenging with this imaging modality. When visualization is lost, the medical device can cause trauma to critical tissue structures. Here, a method to track the needle tip during ultrasound image-guided procedures is presented. This method involves the use of a fiber-optic ultrasound receiver that is affixed within the cannula of a medical needle to communicate ultrasonically with the external ultrasound probe. This custom probe comprises a central transducer element array and side element arrays. In addition to conventional two-dimensional (2D) B-mode ultrasound imaging provided by the central array, three-dimensional (3D) needle tip tracking is provided by the side arrays. For B-mode ultrasound imaging, a standard transmit-receive sequence with electronic beamforming is performed. For ultrasonic tracking, Golay-coded ultrasound transmissions from the 4 side arrays are received by the hydrophone sensor, and subsequently the received signals are decoded to identify the needle tip's spatial location with respect to the ultrasound imaging probe. As a preliminary validation of this method, insertions of the needle/hydrophone pair were performed in clinically realistic contexts. This novel ultrasound imaging/tracking method is compatible with current clinical workflow, and it provides reliable device tracking during in-plane and out-of-plane needle insertions. ispartof: JOVE-JOURNAL OF VISUALIZED EXPERIMENTS issue:138 ispartof: location:United States status: published

Intravascular (IV) imaging in percutaneous coronary interventions can be invaluable to treat coronary artery disease, to facilitate decision making and to guide stent placement. Intravascular ultrasound (IVUS) and optical coherence tomography (OCT) are both established IV imaging modalities. However, achieving contrast for specific structures such as lipid plaques can be challenging; with OCT, visualisation is typically limited to tissue depths less than 2 mm. Photoacoustic (PA) imaging provides contrast that is complementary to those of IVUS and OCT, and with previous demonstrations, visualisation of lipid plaques at depths greater than 4 mm has been achieved. In this study, we developed an intravascular PA probe that comprises a commercial OCT catheter and a high sensitivity miniature fibre optic ultrasound sensor with a Fabry-Perot cavity. This probe, which can provide both PA imaging and OCT, had a maximum width of 1.2 mm. The PA excitation sources included both pulsed and modulated lasers at different wavelengths. The omni-directionality of the US sensor allowed for three-dimensional PA images. The PA-OCT probe was characterised using a series of resolution phantoms, including fine carbon fibres. It was found that with PA imaging, the probe can provide a lateral resolution better than 25 µm and an axial resolution better than 100 µm at the optical focus. Co-registered PA and OCT images of blood vessels ex-vivo with stents and lipid injections were acquired. We conclude that PA imaging with OCT catheters is viable and that it has strong potential to guide clinical interventions.

Photonic crystal fibers (PCF) have been demonstrated for various novel sensing applications for which conventional optical fiber sensors either fail to provide a solution or have low sensitivity issues. A combination of liquid crystal (LC) materials with their tunable optical properties and PCFs with their air-hole microstructure provide the unique advantage to fabricate all-fiber tunable photonic devices. Liquid crystal infiltrated photonic crystal fibers (LCPCF) have been demonstrated for various tunable photonic devices such as filters, switches, polarizers, in-line fiber attenuators, polarization controllers etc. Fiber optic based electric field (e-field) and voltage sensors have been demonstrated using conventional fibers and fiber sensors such as fiber bragg gratings for applications in electric power industry. Due to the natural immunity of the fibers to electromagnetic fields, external transducers or actuators have to be used to induce e-field sensitivity resulting in sensors with low sensitivity. On LC infiltration PCF propagation properties become sensitive to external parameters such as electric fields. The inherent sensitivity of the LCPCF structure and the large e-field induced birefringence of the LC allows to fabricate all-fiber e-field sensor probes with high sensitivity and large measurable e-field range. The book chapter on "Liquid crystal photonic crystal fiber for sensing applications" focuses on the use of LCPCFs for electric field sensing. The book chapter will discuss miniature e-field probes with capability to detect and measure all the parameters of an unknown e-field such as amplitude, frequency and direction. The work presented in this book chapter includes authors' original contribution in this area. With a brief mention of the tunable properties of liquid crystals and the electric field sensing mechanism of LCPCFs, the book chapter will present details on fabrication and characterization of various LCPCF e-field probes. The capability of the e-field probe to simultaneously measure e-field intensity and frequency will be presented. The chapter will discuss a polarimetric e-field sensing scheme combined with LC infiltration length control to fabricate sensor probes customized for measurement in a particular e-field range. A true all-fiber directional e-electric field sensor using LCPCF will be discussed. The chapter will provide an insight into the future research and application areas of LCPCF such as an all-fiber sensor for both electric and magnetic fields. Fiber optic sensors with capability to simultaneously measure both electric current and voltage for practical applications in electric power industry and smart grids will also be discussed briefly.

The frequency dependence of a nematic liquid crystal infiltrated photonic crystal fiber (LCPCF) subjected to a time varying electric field is studied experimentally in the frequency range from 50 Hz-1 kHz. Due to the millisecond order response times of nematic liquid crystals the transmittance response of the infiltrated fiber is found to be highly dependent on the frequency of the input electric field. The time varying transmission response of the infiltrated fiber is found to vary sinusoidally with a frequency that is twice the frequency of the input electric field. It is demonstrated that the time varying transmission response of the LCPCF can be approximated using an appropriate sinusoidal function. It is shown that a fitted function can be used to extract the parameters of the electric field such as frequency and amplitude. The potential application of these structures as all-fiber sensors serving as the core of an electrical signal frequency and amplitude monitoring device is demonstrated and discussed.

The operation of an all-fiber polarimetric electric field sensor based on nematic liquid crystal infiltrated photonic crystal fiber is demonstrated. A section less than 1 mm long of a selectively infiltrated polarization maintaining photonic crystal fiber is used as the probe. The transmission properties are studied for different lengths of the infiltrated section of the fiber subjected to an electric field. It is shown that the phase birefringence of the fiber can be controlled by adjusting the length of the infiltrated section subjected to an electric field, which allows for the sensor to have a linear transmittance response with the applied electric field intensity. The sensor is operated in the telecommunication window at 1550 nm and can be used for the fabrication of all-fiber electric field sensors for high and low electric field environments with an appropriate choice of liquid crystal material for infiltration and with optimization of the infiltration length.

The application of a nematic liquid crystal infiltrated solid core photonic crystal fiber as a novel all-fiber in-line electronically controlled broadband fiber optic attenuator in the wavelength range from 1500 to 1600 nm is demonstrated.The device based on photonic bandgap transmission is electronically controlled and, due to its compact configuration, can be easily incorporated into an optical network. A low-insertion loss at zero attenuation state and a high-extinction ratio of ∼40 dB are obtained between the high and low transmission states of the attenuator. The device works on the application of a varying voltage applied perpendicular to the fiber axis. The attenuation varies linearly with an increase in voltage, with a ∼3.5 V increment required for 1 dB attenuation. The device is shown to have a flat spectral response for all attenuation states in the wavelength range. The device can be used as the core of a highly efficient in-line electronically controlled variable fiber optic attenuator. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:539–543, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25789

The third-order nonlinear optical and optical limiting properties of 2,3,9,10,16,17,23,24-octakis-(heptyloxy)-phthalocyanine, 2,3,9,10,16,17,23,24-octakis-(heptyloxy)-phthalocyanine zinc(II) (symmetric) and 2(3)-(butane-1,4-dioic acid)-9(10),16(17),23(24)-tri tert-butyl phthalocyanine zinc(II) (unsymmetrical) have been investigated using a continuous wave laser at 633 nm. We have employed the Z-scan technique to evaluate the sign and magnitude of nonlinear refractive index and the nonlinear absorption coefficient. Optical limiting based on nonlinear refraction was performed and limiting thresholds were estimated for all the three samples. The magnitude of the third-order nonlinearities measured were one of the highest reported in the cw regime till date. Unsymmetrical phthalocyanine had better limiting characteristics and its nonlinear coefficients were comparatively high. Results of the nonlinear optical studies performed with these organic dyes doped in Poly(methylmethacrylate) are also presented. Our studies indicate that these phthalocyanines are potential candidates for optical limiting applications in low power cw regime.

We present our results on nonlinear optical (NLO) and optical limiting properties of Tetra tert-butyl phthalocyanine and Zinc tetra tert-butyl phthalocyanine studied at 633 nm using a continuous wave laser. We have evaluated the sign and magnitude of the third-order nonlinearity from the closed aperture Z-scan data while the nonlinear absorption properties were assessed using the open aperture data. We have observed low power optical limiting, with low limiting thresholds, based on nonlinear refraction in both the samples. We also present results on the NLO properties of the same dyes doped in Polymethylmethacrylate (PMMA). These studies indicate that both the phthalocyanines are potential candidates for low power optical limiting applications.

There is considerable interest in the development of photoacoustic endoscopy probes (PAE) for applications in foetal medicine, interventional surgery and gastroenterology. However, most previous PAE probes employ a combination of mechanical scanning and piezoelectric transducers at the distal end which can be technically complex and pose challenges in achieving the required level of miniaturisation and acoustic performance. To overcome these limitations, we present two novel all-optical forward-viewing endoscopic probes that use coherent fibre bundles to address a Fabry-Perot polymer film ultrasound sensor.

A feasibility study on using polymer fiber Bragg sensors (PFBG) for providing force feedback to minimally invasive surgical devices is carried out. For this purpose a 3 mm long PFBG is fabricated and characterized for strain and temperature sensitivities. The PFBG sensor is then integrated onto a commercial laparoscopic clip applicator which is used as a proof of concept device. The force characterization of the clip applicator is carried out, with a replica setup which simulates the clip forming process of the device. An original clip is then formed without and with synthetic tissue samples of different hardness. The replica device force profile and original clip forming force profile follows the same pattern and thus the calibration data can be used to calculate the original force exerting on the tissues which can help in optimizing the clip formation process or can be used for providing force feedback capability to the device.

The directional sensitivity of nematic liquid crystal infiltrated polarization-maintaining photonic crystal fiber to an externally applied electric field is evaluated for two-dimensional electric field sensing. Selectively infiltrated and infiltration length optimized polarization-maintaining photonic crystal fiber is used as a probe. Using a polarimetric scheme, the polarized transmission properties of the infiltrated fiber are evaluated in terms of the orientation of the fiber polarization axis in an electric field. It is shown that the sensor has varying sensitivity to electric fields oriented at different angles with respect to the fiber polarization axis. The proposed low-cost all-fiber sensor can be used for two-dimensional directional electric field sensing.

The paper presents the results of experimental studies of the temperature dependence of a microfibre coupler (MFC) with a waist diameter of ~4 μm covered with a layer of liquid crystal (LC) material. The microfiber coupler is fabricated by fusing together and tapering of two standard telecom fibers using a microheater brushing technique, followed by partially embedding the structure in a low-refractive index UV curable polymer (Efiron PC-363) for stability and later by placing a thin heated LC layer over the polymer-free uniform taper waist region. The temperature dependence of the embedded in polymer MFC sensor before the application of the LC layer demonstrates a redshift of the coupler’s spectrum with an average sensitivity of ~0.5 nm/°C in the temperature range of 14-70 °C. The application of the LC overlay increases the average temperature sensitivity to ~0.7 nm/°C. The demonstrated device offers several advantages such as ease of fabrication and light coupling, the potential for better stability and the possibility of electric field tuning for realizing temperature, electric field, bio-, chemical sensors and tunable add-drop filters for fiber communication systems. Further work is ongoing to explore various tuning mechanisms of the MFC spectrum.

Miniature photonic crystal fiber strain sensors with a typical size ∼ 200 µm are fabricated and employed as force measurement sensors for a laparoscopic surgical device. The strain transfer characteristics and strain sensitivity of photonic crystal fiber interferometric sensors fixed onto the clipping arm of a commercial endoscopic clip applicator are studied and presented.

This paper demonstrates temperature-induced tuning of the optical spectrum of a microfiber coupler covered with a low-refractive index liquid crystal (LC) layer. The microfiber coupler with a minimum waist diameter of ~6 μm is fabricated by fusing together and tapering of two standard telecom fibers using a microheater brushing technique and by placing a thin heated LC layer over the uniform taper waist region. Repeatable and reversible tuning of the optical spectrum (~4 nm) of the microfiber coupler is demonstrated for the temperature range of 50-78 °C.

This paper reviews and discusses our latest developments in the area of fiber optic devices integrated with liquid crystals, presents new experimental results for a microfiber based liquid crystal device and discusses possibilities and directions for future developments.

The usage of Nematic Liquid Crystal (NLC) infiltrated Photonic Crystal Fibers (PCFs) for a low cost, all-fiber, high voltage sensing device is experimentally demonstrated. The infiltrated PCF which transmit light based on the photonic bandgap mechanism is employed in an intensity based all-fiber voltage measurement scheme, by applying the voltage perpendicular to the fiber axis. The output transmittance of the PCF varies linearly with increasing voltage. The PCF voltage sensor is capable of measuring high voltages in the range from 100 V to 850 V. The voltage sensing device is compact and exhibits accurate voltage measurement owing to its intrinsic nature with the NLC material infiltrated within the air-holes of the PCF microstructure.

The preliminary results of the characterization of nematic liquid crystal coated photonic crystal fiber (PCF) interferometers are presented in this paper. Two types of interferometer were fabricated; one by sandwiching a small section of PCF between standard single mode fibers and the second one is formed by tapering a small section of a PCF by collapsing the air holes, and thinning down the air hole collapsed region to a micron size. The interferometers are fabricated from different types of the photonic crystal fiber and temperature studies are carried out to select an interferometer to be coated with the liquid crystal material. The requirement for a suitable liquid crystal materials to be used for PCF interferometers are also discussed. The behaviour of liquid crystal (LC) coated interferometers with temperature and applied electric field is studied to determine the feasibility of using such interferometers in sensing applications.

A discretely tunable Surface-Stabilized Ferroelectric Liquid Crystal based Lyot Filter, with tuning speeds in the order of microseconds, is demonstrated experimentally as a channel dropper for the demodulation of multiple Fibre Bragg Grating sensors. The 3-stage Lyot Filter designed and experimentally verified can be used together with the high-speed ratiometric wavelength measurement system employing a fibre bend loss edge filter. Such systems can be used for the demodulation of distributed Fibre Bragg Grating sensors employed in applications such as structural monitoring, industrial sensing and haptic telerobotic surgical systems.

The electronic tunability of ferroelectric liquid crystal filled photonic crystal fibers is experimentally demonstrated in the wavelength range of 1500 nm - 1600 nm. The tunability is achieved by applying electric field onto the ferroelectric liquid crystal infiltrated photonic crystal fiber. Tuning of the fiber propagation properties is achieved due to re-orientation of ferroelectric liquid crystal molecules on the application of the applied electric field. Such fibers could find applications in the fabrication of fast, low loss, cost effective and highly efficient in-fiber tunable devices to be used in the telecom wavelength range.

A ratiometric wavelength measurement system based on a fiber bend loss edge filter has been proposed and demonstrated previously. Such a system offers the advantage of a high resolution, simplicity and a high measurement speed. The applications of such a system for the demodulation of the outputs of multiple Fiber Bragg grating (FBG) sensors requires the use of a high speed tunable filter to separate responses from multiple sensors. Here we present the results of modelling and analysis for a discretely tunable ferroelectric liquid crystal filter, which could be used as a channel dropper in a WDM-based demodulation system containing multiple FBG sensors.

The application of nematic liquid crystal infiltrated photonic crystal fiber as a sensor for electric field intensity measurement is demonstrated. The device is based on an intrinsic sensing mechanism for electric fields. The sensor probe, which consists of a 1 cm infiltrated section of photonic crystal fiber with a lateral size of ∼125 μm, is very compact with small size and weight. A simple all-fiber design for the sensor is employed in an intensity based measurement scheme. The transmitted and reflected power of the infiltrated photonic crystal fiber is shown to have a linear response with the applied electric field. The sensor is operated in the telecommunication window at 1550 nm. The temperature dependence of the device at this operating wavelength is also experimentally studied and discussed. These structures can be used to accurately measure electric field intensity and can be used for the fabrication of all-fiber sensors for high electric field environments as both an in-line and reflective type point sensor.

An experimental investigation on liquid crystal coated tapered photonic crystal fiber (PCF) interferometer is presented in this paper. The interferometer is fabricated by tapering a small section of a PCF by collapsing the air holes and thinning down the air hole collapsed region to a micron size. Several interferometers are fabricated from different types of photonic crystal fibers and the temperature dependence studies are carried out to select an intrinsically temperature insensitive interferometer that can be used with liquid crystal (LC) materials. The properties of the LC materials to be used with the tapered PCF interferometers are also discussed. Temperature tuning of the spectral response of the tapered interferometers with different waist diameters coated with high index and low index LC materials are carried out and the characteristics are presented.

The electronic tunability of ferroelectric liquid crystal filled photonic crystal fibres is demonstrated for the first time. The tunability is obtained in the wavelength range of 1500 -1600 nm, with a free spectral range of ~ 20 nm. The electronic tunability is achieved by applying an electric field onto the infiltrated photonic crystal fibre which facilitates the photonic bandgap tuning owing to the microsecond order switching speeds of Ferroelectric Liquid crystals. The polarized transmission spectrum shows the behavior of a tunable notch filter with a change in applied voltage. The results obtained could pave the way for the fabrication of fast tunable all-in-fibre devices in the telecom wavelength range.

In this paper, we report the fabrication of Bragg gratings in singlemode polymer optical fibers using 248 nm irradiation through a careful tailoring of the ultraviolet pulsed laser parameters to achieve a grating in the bulk region without significant surface ablation.

Ambient temperature and refractive index insensitive miniature PCFI strain sensors are fabricated using precision cleaving and fusion splicing technique. These sensors can be used to provide force feedback for surgical staplers employed in laparoscopic surgeries.