Phase retrieval for the generation of arbitrary intensity distributions using an optofluidic phase shifter.

An optofluidic phase shifter can be used to generate virtually arbitrary intensity patterns, but only if the phase shift generated by the controllably deformed fluidic surface can be appropriately defined. To enable this functionality, we present two phase retrieval algorithms based on neural networks and least-squares optimization which are used to determine the necessary phase profile to generate a desired target intensity pattern with high accuracy. We demonstrate the utility of the algorithms by showing experimentally the ability of an optofluidic phase shifter to generate arbitrary complex intensity distributions.

Simulation of light interaction with seedless grapes.

BACKGROUND: Spectroscopic techniques are widely used for the non-destructive maturation and quality monitoring of different fruits. To develop new sensor devices for this purpose, knowing the optical properties of the agricultural sample is crucial for enabling the prediction of the interaction of the incident light with the fruit. RESULTS: In the present study, the optical properties of three different seedless grape varieties (ARRA15, Tawny and Melody/Blagratwo) were determined from 400 to 1000 nm using a UV-visible/near-infrared spectrometer with an integrating sphere and subsequent calculation of the absorption and scattering coefficients and the anisotropy factor using the inverse adding doubling method. The results indicate that the optical properties of different grape varieties have significant differences, especially in the visible wavelength region, whereas these are less distinct in the near-infrared range. Independent of grape variety, the grape berry skin has a higher scattering coefficient and scattering occurs predominantly in the forward direction. Based on the optical properties of the grape berries, a three-dimensional grape berry model is generated within OpticStudio (Zemax, LLC) for the different varieties that can be used in optical illumination simulations. The bulk scattering inside the fruit is modeled by the Henyey-Greenstein distribution. A comparison of the simulated values for the total transmission and the specular reflection determined experimentally shows that realistic optical grape models can be created within OpticStudio. CONCLUSION: Overall, the procedure for creating optical grape models presented here will be helpful for the development of optical applications used in pre- and post-harvest food quality monitoring. © 2022 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Analysis of assembly tolerance compensation in microscope objectives with a free-form element at the aperture stop.

We analyze the feasibility of using refractive free-form phase plates at the aperture stop of microscope objectives as an alternative to active alignment to compensate for assembly tolerances. The method involves the determination of misalignment-induced aberrations at the exit pupil, and transferring them to the aperture stop while taking pupil aberrations into consideration. We demonstrate that despite being able to correct only for field-independent aberrations, this method can restore near-diffraction-limited imaging performance of passively aligned systems with practical tolerances, given that the as-designed system is highly corrected. We confirm the results via numerical simulations for two different commercial objective designs.

Electro-optically tunable single-frequency lasing from neodymium-doped lithium niobate microresonators.

Tunable light sources are a key enabling technology for many applications such as ranging, spectroscopy, optical coherence tomography, digital imaging and interferometry. For miniaturized laser devices, whispering gallery resonator lasers are a well-suited platform, offering low thresholds and small linewidths, however, many realizations suffer from the lack of reliable tuning. Rare-earth ion-doped lithium niobate offers a way to solve this issue. Here we present a single-frequency laser based on a neodymium-doped lithium niobate whispering gallery mode resonator that is tuned via the linear electro-optic effect. Using a special geometry, we suppress higher-order transverse modes and hence ensure single-mode operation. With an applied voltage of just 68 V, we achieve a tuning range of 3.5 GHz. The lasing frequency can also be modulated with a triangular control signal. The freely running system provides a frequency and power stability of better than Δ ν=20MHz and 6 %, respectively, for a 30-minute period. This concept is suitable for full integration with existing photonic platforms based on lithium niobate.

Accelerated electrowetting-based tunable fluidic lenses.

One of the limitations in the application of electrowetting-based tunable fluidic lenses is their slow response time. We consider here two approaches for enhancing the response speed of tunable fluidic lenses: optimization of the properties of the fluids employed and modification of the time-dependent actuation voltages. Using a tubular optofluidic configuration, it is shown through simulations how one may take advantage of the interplay between liquid viscosities and surface tension to reduce the actuation time. In addition, by careful designing the actuation pulses, the response speed of both overdamped and underdamped systems may be increased by over an order of magnitude, leading to response times of several ten milliseconds. These performance improvements may significantly enhance the applicability of tunable optofluidic-based components and systems.

Tunable fluidic lens with a dynamic high-order aberration control.

Fluidic lenses based on electrowetting actuation are attractive for their wide focal tuning range, yet are limited by optical aberrations, either intrinsic to the lenses themselves or due to the optical imaging systems in which they are employed. However, the ability to control the meniscus shape that forms the lens refractive surface with a high degree of spatial accuracy will allow correction of and compensation for a wide range of these aberrations. We demonstrate here for what we believe, to the best of our knowledge, is the first time a tunable optofluidic lens controlled by 32 azimuthally placed electrodes for which most aberrations up to the fourth radial Zernike order may be corrected. Using both wavefront sensing and sensorless wavefront estimation techniques, it is shown that focal length tunability with a significant reduction in imaging aberrations and the ability to compensate for externally induced aberrations may be achieved using a single component.

Fully refractive adaptive optics fluorescence microscope using an optofluidic wavefront modulator.

Adaptive optics (AO) represents a powerful range of image correction technologies with proven benefits for many life-science microscopy methods. However, the complexity of adding a reflective wavefront modulator and in some cases a wavefront sensor into an already complicated microscope has made AO prohibitive for its widespread adaptation in microscopy systems. We present here the design and performance of a compact fluorescence microscope using a fully refractive optofluidic wavefront modulator, yielding imaging performance on par with that of conventional deformable mirrors, both in correction fidelity and articulation. We combine this device with a modal sensorless wavefront estimation algorithm that uses spatial frequency content of acquired images as a quality metric and thereby demonstrate a completely in-line adaptive optics microscope that can perform aberration correction up to 4th radial order of Zernike modes. This entirely new concept for adaptive optics microscopy may prove to extend the performance limits and widespread applicability of AO in life-science imaging.

Pockels-effect-based adiabatic frequency conversion in ultrahigh-Q microresonators.

Adiabatic frequency conversion has some key advantages over nonlinear frequency conversion. No threshold and no phase-matching conditions need to be fulfilled. Moreover, it exhibits a conversion efficiency of 100 % down to the single-photon level. Adiabatic frequency conversion schemes in microresonators demonstrated so far suffer either from low quality factors of the employed resonators resulting in short photon lifetimes or small frequency shifts. Here, we present an adiabatic frequency conversion (AFC) scheme by employing the Pockels effect. We use a non-centrosymmetric ultrahigh-Q microresonator made out of lithium niobate. Frequency shifts of more than 5 GHz are achieved by applying just 20 V to a 70-µm-thick resonator. Furthermore, we demonstrate that with the same setup positive and negative frequency chirps can be generated. With this method, by controlling the voltage applied to the crystal, almost arbitrary frequency shifts can be realized. The general advances in on-chip fabrication of lithium-niobate-based devices make it feasible to transfer the current apparatus onto a chip suitable for mass production.

Endoscopic pyrometric temperature sensor.

We demonstrate a pyrometric contact-less temperature sensor using a flexible fused silica fiber of 360 µm diameter able to measure down to 30°C with a precision better than 1°C at 10 Hz. Silica fibers, as opposed to dedicated mid-IR fibers, are non-degrading, low-cost, and bio-compatible. The large bandwidth (up to several kilohertz) and the broad temperature range (up to 235°C) of the sensor can be instrumental for time-resolved analysis and control of laser ablation and electrothermal surgery procedures.

Morpho-molecular ex vivo detection and grading of non-muscle-invasive bladder cancer using forward imaging probe based multimodal optical coherence tomography and Raman spectroscopy.

Non-muscle-invasive bladder cancer affects millions of people worldwide, resulting in significant discomfort to the patient and potential death. Today, cystoscopy is the gold standard for bladder cancer assessment, using white light endoscopy to detect tumor suspected lesion areas, followed by resection of these areas and subsequent histopathological evaluation. Not only does the pathological examination take days, but due to the invasive nature, the performed biopsy can result in significant harm to the patient. Nowadays, optical modalities, such as optical coherence tomography (OCT) and Raman spectroscopy (RS), have proven to detect cancer in real time and can provide more detailed clinical information of a lesion, e.g. its penetration depth (stage) and the differentiation of the cells (grade). In this paper, we present an ex vivo study performed with a combined piezoelectric tube-based OCT-probe and fiber optic RS-probe imaging system that allows large field-of-view imaging of bladder biopsies, using both modalities and co-registered visualization, detection and grading of cancerous bladder lesions. In the present study, 119 examined biopsies were characterized, showing that fiber-optic based OCT provides a sensitivity of 78% and a specificity of 69% for the detection of non-muscle-invasive bladder cancer, while RS, on the other hand, provides a sensitivity of 81% and a specificity of 61% for the grading of low- and high-grade tissues. Moreover, the study shows that a piezoelectric tube-based OCT probe can have significant endurance, suitable for future long-lasting in vivo applications. These results also indicate that combined OCT and RS fiber probe-based characterization offers an exciting possibility for label-free and morpho-chemical optical biopsies for bladder cancer diagnostics.

Quantifying spatial alignment and retardation of nematic liquid crystal films by Stokes polarimetry.

Recently developed alignment techniques for liquid crystals enable the definition of arbitrary alignment patterns. We present a method to determine these two-dimensional spatial alignment distributions as well as the retardation of thin nematic liquid crystal films. The method is based on quantifying the influence of the birefringence of such a film on light with known input polarization by measuring the Stokes parameters of light. We show that we are able to distinguish arbitrary alignment patterns unambiguously. In addition, we demonstrate the ability to evaluate the homogeneity of the alignment as well as the thickness or birefringence of the film.

Extended field-of-view adaptive optics in microscopy via numerical field segmentation.

Sample-induced optical aberrations in microscopy are, in general, field dependent, limiting their correction via pupil adaptive optics (AO) to the center of the available field-of-view (FoV). This is a major hindrance, particularly for deep tissue imaging, where AO has a significant impact. We present a new wide-field AO microscopy scheme, in which the deformable element is located at the pupil plane of the objective. To maintain high-quality correction across its entirety, the FoV is partitioned into small segments, and a separate aberration estimation is performed for each via a modal-decomposition-based indirect wavefront sensing algorithm. A final full-field image is synthesized by stitching of the partitions corrected consecutively and independently via their respective measured aberrations. The performance and limitations of the method are experimentally explored on synthetic samples imaged via a custom-developed AO fluorescence microscope featuring an optofluidic refractive wavefront modulator.

Optimization-based real-time open-loop control of an optofluidic refractive phase modulator.

We present a novel open-loop control method for an electrostatically actuated optofluidic refractive phase modulator, and demonstrate its performance for high-order aberration correction. Contrary to conventional electrostatic deformable mirrors, an optofluidic modulator is capable of bidirectional (push-pull) actuation through hydro-mechanical coupling. Control methods based on matrix pseudo-inversion, the common approach used for deformable mirrors, thus perform sub-optimally for such a device. Instead, we formulate the task of finding driving voltages for a given desired wavefront shape as an optimization problem with inequality constraints that can be solved using an interior-point method in real time. We show that this optimization problem is a convex one and that its solution represents a global minimum in residual wavefront error. We use the new method to control both the refractive phase modulator and a conventional electrostatic deformable mirror, and experimentally demonstrate improved correction fidelity for both.

Scattering-loss reduction of ridge waveguides by sidewall polishing.

Ridge waveguides provide a large refractive index contrast and thus strong mode confinement, making them highly attractive for building compact photonic integrated circuits. However, ridge waveguides suffer from scattering losses. We demonstrate scattering-loss reduction of ridge waveguides made of lithium-niobate-on-insulator (LNOI) substrates by more than one order of magnitude. This is achieved by gently polishing of the ridge's sidewalls and simultaneous protection of the top surfaces by a metal layer. Whispering-gallery-resonator loss measurements reveal ultra-low losses down to 0.04 dB/cm of the processed waveguides. Our approach pushes ridge waveguides further towards their fundamental absorption-loss limit, enabling highly efficient integrated devices.

Optofluidic adaptive optics.

We introduce a transmissive refractive adaptive optics system featuring a deformable transparent optofluidic wavefront modulator and a sensorless wavefront error estimation algorithm. The wavefront modulator consists of a cavity filled with an optical liquid which is sealed by a deformable elastic polymer membrane. Deformation of the membrane is achieved through electrostatic actuation using 25 transparent indium tin oxide electrodes buried in the cavity. Modulation of the two-dimensional phase distribution generated by the system is performed using open loop control both with and without active wavefront sensing. For the latter, a progressive modal decomposition algorithm is used to estimate and correct distortion in the point-spread function (PSF) of the wavefront arising due to the optical system and other sources of wavefront distortion. Using this control method, we experimentally demonstrate high-fidelity recreation of Zernike modes up to the fourth order, and blind (sensorless) PSF correction in a wide-field microscope.

Reusable thin-film all-polymer couplers for vertical light coupling to single-mode waveguides.

A reusable all-polymer coupler using gratings and tapers for vertical light coupling to single-mode planar waveguides is demonstrated. Numerical simulations are performed to optimize the tapers and gratings to increase the coupling efficiency. A hot-embossing replication technique is used for the fabrication of waveguide sensors and gratings, which is adaptable to mass production, such as roll-to-roll processes. The external grating is reversibly bonded to the waveguide via van der Waals forces, and the thus realized re-usability reduces the fabrication costs. The utility of the structure is shown by using these grating couplers for testing of asymmetric Mach-Zehnder interferometers.

Cascaded second-order optical nonlinearities in on-chip micro rings.

We demonstrate cascaded Stimulated Raman Scattering (SRS), Second-Harmonic Generation (SHG), and Sum-Frequency Generation (SFG) in integrated on-chip whispering-gallery resonators (WGRs). These lithium niobate-based WGRs are fabricated using highly-parallel semiconductor manufacturing techniques coupled with specialized polishing as a post-processing step and thus represent a novel means for batch fabrication of this family of non-linear devices. We achieved record high Q-factors for on-chip lithium niobate WGRs reaching up to 3 × 106. Furthermore, we present a flexible but stable coupling scheme, which gives us the opportunity to optimize the coupling regarding the non-linear optical processes we observe.

All-liquid dual-lens optofluidic zoom system.

An all-liquid optical zoom system based on two independently controllable liquid lenses positioned inside a single sealed cylindrical housing is demonstrated. This system yielded a zoom ratio of 1.5 for an object distance of 200 mm, an image distance 37 mm, and a corresponding resolution of better than 5 line pairs/mm. With a diameter of 5 mm, a packaged system length of 9.88 mm, and a power consumption of 3.5 mW, the system represents a new generation of ultra-miniaturized optofluidic systems with high functionality, excellent imaging properties, and highly flexible tunability, all with no mechanically moving parts.

Spectral response of Bragg gratings in multimode polymer waveguides.

A means to calculate the multimodal spectral response of Bragg gratings in general non-circular multimode waveguides is proposed. To illustrate the power of the technique, the spectra of two Bragg temperature sensors are numerically calculated in which coupling between 100 modes is considered. It is shown how the Bragg wavelength in multimode Bragg grating waveguides is affected by the number of modes and energy distribution among them. Good matching of the simulated spectrum of a multimode Bragg grating on a planar inverted rib waveguide to the measured spectrum is seen.

Tubular astigmatism-tunable fluidic lens.

We demonstrate a new means to fabricate three-dimensional liquid lenses which may be tuned in focal length and astigmatism. Using actuation by electrowetting-on-dielectrics, astigmatism in arbitrary directions may be tuned independently, with almost no cross talk between orthogonal orientations. The lens is based on electrodes structured on planar polyimide foils and subsequently rolled, enabling high-resolution patterning of complex electrodes along the azimuthal and radial directions of the lens. Based on a design established through fluidic and optical simulations, the astigmatism tuning is experimentally verified by a change of the corresponding Zernike coefficients measured using a Shack-Hartmann wavefront sensor. It was seen that the back focal length can be tuned by 5 mm and 0° and 45° astigmatism by 3 µm through application of voltages in the range of 50 Vrms. It was observed that the cross talk with other aberrations is very low, suggesting a novel means for astigmatism control in imaging systems.