Micro-Cantilever Based Fiber Optic Hydrophone

Endoscopic photoacoustic imaging probe is becoming increasingly important for many clinical photoacoustic imaging applications in which the target tissue can only be accessed by introducing an endoscopic probe percutaneously or through a natural orifice. Miniature fiber optic hydrophone (FOH) has become an attractive choice for endoscopic photoacoustic imaging application. Fiber optic hydrophone has many proven advantages, including small size, light weight, immunity to electromagnetic interference, low cost for single-use application and capability of integration of excitation light source and acoustic wave receiver. This dissertation demonstrates an open cavity, micro-cantilever based fiber optic Fabry-Perot interferometer (FPI) hydrophone. A fused silica micro-cantilever beam as the sensing element is directly fabricated by femtosecond (fs) laser micromachining system. The theoretical analyses and experimental verifications were all applied for evaluation of the proposed cantilever based FOH. A rectangular micro-cantilever based FOH is presented, which has a narrow bandwidth but high response and high sensitivity around its resonant frequency, and has many advantages as a good potential candidate for endoscopic photoacoustic imaging application. As a key parameter of the hydrophone, the resonant frequency can be adjusted by changing the dimensions and shapes of the micro-cantilever. In order to increase the resonant frequency of the rectangular micro-cantilever based FOH, and without loss in sensitivity, V-shaped and triangular cantilever based FOHs are investigated and compared with the rectangular cantilever based FOH theoretically and experimentally. The resonant frequency of the triangular cantilever based FOH has been doubled without loss in sensitivity compared with the rectangular cantilever based FOH. Cantilever based 45° angled FOH was proposed for a new choice for sideway looking detection except forward looking detection for endoscopic imaging in vessels. It consists of a fiber with a 45° angled endface and an fs laser fabricated micro-cantilever. The 45° angled endface would steer the optical axis by 90° via total internal reflection, and send the input light to the sensing part. This configuration could be applied for cross-axial sensing application. The proposed FOHs were all theoretically analyzed and experimental tested. Experimental results agree well with the simulated frequency responses of the proposed FOHs

An All-Fiber-Optic Combined System of Noncontact Photoacoustic Tomography and Optical Coherence Tomography

We propose an all-fiber-based dual-modal imaging system that combines noncontact photoacoustic tomography (PAT) and optical coherence tomography (OCT). The PAT remotely measures photoacoustic (PA) signals with a 1550-nm laser on the surface of a sample by utilizing a fiber interferometer as an ultrasound detector. The fiber-based OCT, employing a swept-source laser centered at 1310 nm, shares the sample arm of the PAT system. The fiber-optic probe for the combined system was homemade with a lensed single-mode fiber (SMF) and a large-core multimode fiber (MMF). The compact and robust common probe is capable of obtaining both the PA and the OCT signals at the same position without any physical contact. Additionally, the MMF of the probe delivers the short pulses of a Nd:YAG laser to efficiently excite the PA signals. We experimentally demonstrate the feasibility of the proposed dual-modal system with a phantom made of a fishing line and a black polyethylene terephthalate fiber in a tissue mimicking solution. The all-fiber-optic system, capable of providing complementary information about absorption and scattering, has a promising potential in minimally invasive and endoscopic imaging

Multimodal photoacoustic remote sensing (PARS) microscopy combined with swept-source optical coherence tomography (SS-OCT) for in-vivo, non-contact, functional and structural ophthalmic imaging applications

Ophthalmic imaging has long played an important role in the understanding, diagnosis, and treatment of a wide variety of ocular disorders. Currently, available clinical ophthalmic imaging instruments are primarily optical-based, including slit-lamp microscopy, fundus photography, confocal microscopy, scanning laser ophthalmoscopy, and optical coherence tomography (OCT). The development of these imaging instruments has greatly extended our ability to evaluate the ocular environment. Studies have shown that at least 40% of blinding disorders in the United States are either preventable or treatable with timely diagnosis and intervention. OCT is a state-of-the-art imaging technique extensively used in preclinical and clinical applications for imaging both anterior and posterior parts of the eye. OCT has become a standard of care for the assessment and treatment of most ocular conditions. The technology enables non-contact, high-speed, cross-sectional imaging over a large field of view with submicron resolutions. In eye imaging applications, functional extensions of OCT such as spectroscopic OCT and Doppler OCT have been applied to provide a better understanding of tissue activity. Spectroscopic OCT is usually achieved through OCT systems in the visible spectral range, and it enables the amount of light absorption inside the ocular environment to be measured. This indirect optical absorption measurement is used to estimate the amount of ocular oxygen saturation (SO2) which is a well-known biomarker in prevalent eye diseases including diabetic retinopathy, glaucoma, and retinal vein occlusions. Despite all the advancements in functional spectroscopic OCT methods, they still rely primarily on measuring the backscattered photons to quantify the absorption of chromophores inside the tissue. Therefore, they are sensitive to local geometrical parameters, such as retinal thickness, vessel diameters, and retinal pigmentation, and may result in biased estimations. Of the various optical imaging modalities, photoacoustic imaging (PAI) offers unique imaging contrast of optical absorption because PAI can image any target that absorbs light energy. This unique imaging ability makes PAI a favorable candidate for various functional and molecular imaging applications as well as for measuring chromophore concentration. Over the past decade, photoacoustic ophthalmoscopy has been applied for visualizing hemoglobin and melanin content in ocular tissue, quantifying ocular SO2, and measuring the metabolic rate of oxygen consumption (MRO2). Despite all these advantages offered by PAI devices, a major limitation arises from their need to be in contact with the ocular tissues. This physical contact may increase the risk of infection and cause patient discomfort. Furthermore, this contact-based imaging approach applies pressure to the eye and introduces barriers to oxygen diffusion. Thus, it has a crucial influence on the physiological and pathophysiological balance of ocular vasculature function, and it is not capable of studying dynamic processes under normal conditions. To overcome these limitations and to benefit from the numerous advantages offered by photoacoustic ophthalmoscopy, non-contact detection of photoacoustic signals has been a long-lasting goal in the field of ocular imaging. In 2017 Haji Reza et al. developed photoacoustic remote sensing (PARS) for non-contact, non-interferometric detection of photoacoustic signals. PARS is the non-contact, all-optical version of optical-resolution photoacoustic microscopy (OR-PAM), where the acoustically coupled ultrasound transducer is replaced with a co-focused probe beam. This all-optical detection scheme allows the system to measure the photoacoustic pressure waves at the subsurface origin where the pressure is at a maximum. In a very short time, PARS technology has proven its potential for various biomedical applications, including label-free histological imaging, SO2 mapping, and angiogenesis imaging. PARS is an ideal companion for OCT in ophthalmic applications, where the depth-resolved, detailed scattering information of OCT is well complemented by rich absorption information of PARS. This combined multimodal imaging technology has the potential to provide chromophore selective absorption contrast in concert with depth-resolved scattering contrast in the ocular environment. The main goals of this PhD project are to: • Develop a photoacoustic remote sensing microscopy system for in-vivo, non-contact ophthalmic imaging. This is the first time a non-contact photoacoustic imaging has been used for in-vivo imaging of the eye. • Develop a robust and temporally stable multiwavelength light source for functional photoacoustic imaging applications. • Develop a multimodal PARS-OCT imaging system that can image in-vivo and record, simultaneously, functional, and structural information in the anterior segment of a rodent eye. This is the first time a multiwavelength non-contact photoacoustic system is used for in-vivo measurement of oxygen saturation in the ocular environment. • Develop and modify the multimodal PARS-OCT imaging system for non-contact, in-vivo, functional, and structural imaging of the posterior part of the rodent eye