Analysing the impact of non-parallelism in Fabry-Perot etalons through optical modelling

Fabry-Perot (FP) etalons, composed of two parallel mirrors, are used widely as optical filters and sensors. In certain applications, however, such as when FP etalons with polymer cavities are used to detect ultrasound, the mirrors may not be perfectly parallel due to manufacturing limitations. As little is known about how the mirrors being non-parallel impacts upon FP etalon performance, it is challenging to optimize the design of such devices. To address this challenge, we developed a model of light propagation in non-parallel FP etalons. The model is valid for arbitrary monochromatic beams and calculates both the reflected and transmitted beams, assuming full-wave description of light. Wavelength resolved transmissivity simulations were computed to predict the effect that non-parallel mirrors have on the sensitivity, spectral bandwidth and peak transmissivity of FP etalons. Theoretical predictions show that the impact of the non-parallel mirrors increases with both mirror reflectivity and incident Gaussian beam waist. Guidelines regarding the maximum angle allowed between FP mirrors whilst maintaining the sensitivity and peak transmissivity of a parallel mirror FP etalon are provided as a function of mirror reflectivity, cavity thickness and Gaussian beam waist. This information, and the model, could be useful for guiding the design of FP etalons suffering a known degree of non-parallelism, for example, to optimize the sensitivity of polymer based FP ultrasound sensors

Simulating optical memory effects and the scanning of foci using wavefront shaping in tissue-like scattering media

Wavefront shaping could enable focussing light deep inside scattering media, increasing the depth and resolution of imaging techniques like optical microscopy and optical coherence tomography. However, factors like rapid decorrelation times due to microscale motion and thermal variation make focusing in living tissue difficult. A way to ease the requirements could be exploiting prior information provided by memory effects. For example, this might allow partially or wholly scanning a focus. To study this and related ideas, a computational model was developed to simulate the generation and correlations of foci formed by WFS in scattering media. Predictions of the angular memory range were consistent with experimental observations. Furthermore, correlations observed between optical phase maps required to focus at different positions suggested correlation-based priors might enable accelerated focussing. This work could pave the way to faster optical focussing and thus deeper imaging in living tissue

A computational framework for investigating the feasibility of focusing light in biological tissue via photoacoustic wavefront shaping

Photoacoustic (PA) wavefront shaping (WFS; PAWS) could allow focusing light deep in living tissue, increasing the penetration depth of biomedical optics techniques. PAWS experiments have demonstrated focusing light through rigid scattering media. However, focusing deep in tissue is significantly more challenging. To examine the scale of this challenge, a computational model of the propagation of coherent light in tissue was developed to simulate the focusing of light via PAWS. To demonstrate the model, it was used to simulate focusing in an 800 µm thick tissue-like medium. To show the utility of the model, the focusing was repeated in different conditions illustrative of simplified PAWS experiments involving different spatial resolutions. As expected, a finer spatial resolution led to a brighter focus. By providing a simulation platform for studying PAWS, this work could pave the way to developing systems that can focus light in tissue

Rapid Spatial Mapping of Focused Ultrasound Fields Using a Planar Fabry-Pérot Sensor

Measurement of high acoustic pressures is necessary in order to fully characterise clinical high-intensity focused ultrasound (HIFU) fields, and for accurate validation of computational models of ultrasound propagation. However, many existing measurement devices are unable to withstand the extreme pressures generated in these fields, and those that can often exhibit low sensitivity. Here, a planar Fabry-Pérot interferometer with hard dielectric mirrors and spacer was designed, fabricated, and characterised and its suitability for measurement of nonlinear focused ultrasound fields was investigated. The noise equivalent pressure of the scanning system scaled with the adjustable pressure detection range between 49 kPa for pressures up to 8 MPa and 152 kPa for measurements up to 25 MPa, over a 125 MHz measurement bandwidth. Measurements of the frequency response of the sensor showed that it varied by less than 3 dB in the range 1 - 62 MHz. The effective element size of the sensor was 65 μm and waveforms were acquired at a rate of 200 Hz. The device was used to measure the acoustic pressure in the field of a 1.1 MHz single element spherically focused bowl transducer. Measurements of the acoustic field at low pressures compared well with measurements made using a PVDF needle hydrophone. At high pressures, the measured peak focal pressures agreed well with the focal pressure modelled using the Khokhlov-Zabolotskaya-Kuznetsov equation. Maximum peak positive pressures of 25 MPa, and peak negative pressures of 12 MPa were measured, and planar field scans were acquired in scan times on the order of 1 minute. The properties of the sensor and scanning system are well suited to measurement of nonlinear focused ultrasound fields, in both the focal region and the low pressure peripheral regions. The fast acquisition speed of the system and its low noise equivalent pressure are advantageous, and with further development of the sensor, it has potential in application to HIFU metrology

The visible and near-infrared optical absorption coefficient spectrum of Parylene C measured by transmitting light through thin films in liquid filled cuvettes

Parylene C (PPXC) is a polymer deposited from the gas phase to form optically clear thin films used in devices including waveguides and sensors. The performance of these devices depends on the visible and near infrared absorption coefficient of PPXC. However, the absorption coefficient is difficult to measure. This is because PPXC films are typically too thin to exhibit detectable absorption in conventional transmittance measurements. To address this challenge, a method involving measuring the transmittance of multiple films immersed together in a liquid filled cuvette was devised. This increased the sensitivity to absorption by increasing the path length in PPXC, while also minimizing reflections and surface losses. Using 200-500 µm thick films, this method was applied to measure the absorption coefficient of PPXC at wavelengths in the range 330-3300 nm. The coefficient was found to vary spectrally by more than two orders of magnitude from 0.025 mm-1 at 1562 nm to 7.7 mm-1 at 3262 nm. These absorption measurements could aid the design of PPXC based sensors and waveguides. The method could be useful for measuring the absorption coefficient of other thin, low-loss materials, particularly those for which it is challenging to obtain thick samples such as other polymers deposited from the gas phase in a similar manner to PPXC

Photoacoustic wavefront shaping with a long coherence length laser

Photoacoustic (PA) wavefront shaping (WS; PAWS) could allow focusing light deep in biological tissue. This could enable increasing the penetration depth of biomedical optical techniques including PA imaging. However, focussing at depth requires a light source of long coherence length (CL), presenting a challenge because the CLs of typical PA excitation lasers are short. To address this challenge, we developed a PAWS system based on an externally modulated external cavity laser with a long CL. The system was demonstrated by focussing light through rigid scattering media using both PAWS and optical WS. PAWS enabled focussing through diffusers with 8 × enhancements, while all-optical WS enabled focussing through various scattering media including a 5.8 mm thick tissue phantom. By enabling PAWS with increased coherence, the system could facilitate exploring the practical depth limits of PAWS, paving the way to focussing light deep in tissue

ABCD transfer matrix model of Gaussian beam propagation in plano-concave optical microresonators

Plano-concave optical microresonators (PCMRs) are optical microcavities formed of one planar and one concave mirror separated by a spacer. PCMRs illuminated by Gaussian laser beams are used as sensors and filters in fields including quantum electrodynamics, temperature sensing, and photoacoustic imaging. To predict characteristics such as the sensitivity of PCMRs, a model of Gaussian beam propagation through PCMRs based on the ABCD matrix method was developed. To validate the model, interferometer transfer functions (ITFs) calculated for a range of PCMRs and beams were compared to experimental measurements. A good agreement was observed, suggesting the model is valid. It could therefore constitute a useful tool for designing and evaluating PCMR systems in various fields. The computer code implementing the model has been made available online

Interrogating Fabry-Perot ultrasound sensors with Bessel beams for photoacoustic imaging

Photoacoustic Tomography (PAT) systems based on Fabry-Perot (FP) sensors provide high-resolution images limited by the system’s sensitivity. The sensitivity is limited by the optical Q-factor of the FP cavity (i.e., the optical confinement of the interrogation laser beam in the FP cavity). In existing systems, a focussed Gaussian beam is used to interrogate the sensor. While providing a small acoustic element required for high-resolution imaging, this interrogation beam naturally diverges inside the FP cavity, leading to the current sensitivity limit. To break this limit, a new approach of interrogating the FP sensor using a Bessel beam is investigated. The Noise Equivalent Pressure (NEP) and both axial and lateral PAT resolutions using Bessel beam interrogation were quantified. Bessel beam interrogation provided lower NEP, similar axial resolution, but lower lateral resolution. Thus, Bessel beam might be an alternative interrogation scheme for deep PAT imaging as high sensitivity is needed and the lateral resolution is limited by the aperture of the PAT system

Probing the optical readout characteristics of Fabry-Perot ultrasound sensors through realistic modelling

The Fabry-Perot interferometer (FPI) is widely used in photoacoustic imaging (PAI) as an ultrasound (US) sensor due to its high sensitivity to weak US waves. Such high sensitivity is important as it allows for increasing the depth in tissue at which PAI can access, thus strongly influencing its clinical applicability. FPI sensitivity is impacted by many factors including the FPI mirror reflectivity, focussed beam spot size, FPI cavity thickness and aberrations introduced by the optical readout system. Improving FPI sensitivity requires a mathematical model of its optical response which takes all of these factors into account. Previous attempts to construct such a model have been critically limited by unrealistic assumptions. In this work we have developed a general model of FPI optical readout which based upon electromagnetic theory. By making very few assumptions, the model is able to replicate experimental results and allows insight to be gained into the operating principles of the sensor

A method for measuring the directional response of ultrasound receivers in the range 0.3 MHz to 80 MHz using a laser generated ultrasound source

A simple method for measuring the directivity of an ultrasound receiver is described. The method makes use of a custom-designed laser ultrasound source which generates a large diameter (>1 cm) broadband monopolar plane wave with a continuous frequency content extending from ≤330 kHz to ≈80 MHz. The plane wave is highly uniform in amplitude (±5% over >8 mm) and phase (equivalent to <;λ/7 at 80 MHz over ≥11 mm). To measure directivity, the source is rotated around the receiver under test in a compact centimeter-scale setup. To demonstrate the method, it was used to measure the directivity of two broadband small aperture Fabry-Perot ultrasound sensors over an angular range of ±50° at frequencies up to 80 MHz. Measurements were found to be highly repeatable with an estimated typical repeatability <;4% in the range of 0.5-25 MHz. Due to the broad bandwidth, large size, and adjustable nature of the source, the method is widely applicable and could aid the characterization of receivers used in medical ultrasound, ultrasonic nondestructive testing. and ultrasound metrology