Dual modal compressive photocurrent and optical imaging through a multimode fibre

Wavefront shaping (WFS) techniques have been used as a powerful tool to control light propagation in complex media, including multimode fibers. In this paper, we propose a new application of WFS for multimode fiber-based sensors. The use of a single multimode fiber alone, without any special fabrication, as a sensor based on the light intensity variations is not an easy task. The twist effect on multimode fiber is used as an example herein. Experimental results show that light intensity through the multimode fiber shows no direct relationship with the twist angle, but the correlation coefficient (CC) of speckle patterns does. Moreover, if WFS is applied to transform the spatially seemingly random light pattern at the exit of the multimode fiber into an optical focus. The focal pattern correlation and intensity both can serve to gauge the twist angle, with doubled measurement range and allowance of using a fast point detector to provide the feedback. With further development, WFS may find potentials to facilitate the development of multimode fiber-based sensors in a variety of scenarios.

When light propagates through complex medium, such as biological tissue and multimode fiber, refractive index inhomogeneity causes multiple scattering and distortion. This phenomenon is usually seen as obstacles for biomedical imaging, telecommunications, photodynamic therapy and so on. Thus, manipulation of the incident wavefront to compensate for the wavefront distortion due to multiple scattering has been an interdisciplinary subject of interest. Fortunately, wavefront shaping technologies have emerged to provide versatile solutions to minimize the influence of light scattering. By modulating the incident light into a special wavefront with a spatial light modulator, focusing through scattering medium is obtained. To date, several wavefront shaping techniques have been proposed, mainly including transmission matrix inversion, feedback based iterative optimization, and digital optical phase conjugation. Unlike a planar wavefront, the modulated light with special wavefront is transformed into a bright optical focus spot or a desired focus pattern after the scattering medium. Among the proposed approaches, the transmission matrix is considered as a significant tool to characterize a multiple scattering medium with the purpose of manipulating light propagation through it, which contains all the information related to the input field and the scattered output field. In this work, we experimentally measure the transmission matrix of scattering media based on self-reference interference method with a digital micromirror device. Unlike the conventional setup, which divides the incident wavefront into a signal part and reference part, in the self-reference interference method, the reference light is superimposed directly on the signal light to form a new set of input light fields. This self-reference interference method effectively improves the degree of freedom of optical field modulation. Moreover, the intensity ratio between the signal light and the reference light can be adjusted conveniently. In our experiment, this superimposed field is generated by a digital micromirror device with superpixel method. We measure the Hadamard basis and the OAM-basis transmission matrices of scattering medium, respectively. With the measured transmission matrices, single-spot, multi-spot and vortex focusing are achieved after scattering medium, verifying the accuracy of the measured transmission matrices. The strong diagonal presented in the norm of focusing operator also proves the accuracy of the measured transmission matrices. The proposed method may have potential applications in optical imaging and optical communication under scattering environment.

Wavefront shaping techniques are being actively developed to achieve optical focusing through and inside opaque scattering media. These techniques promise to revolutionize biophotonics by enabling deep-tissue non-invasive optical imaging, optogenetics, optical tweezing, and light-based therapy. Among the existing wavefront shaping techniques, optical time-reversal-based techniques determine the optimum wavefront globally based on the principle of time reversal, without the need to perform time-consuming iterations to optimize each mode in sequence. In all previous optical time-reversal-based wavefront shaping experiments, Nyquist sampling criterion was followed so that the scattered light field was well-sampled during wavefront measurement and wavefront reconstruction. In this work, we overturn this conventional practice by demonstrating that a high-quality optical focus can still be achieved even when the scattered light field is under-sampled. Even more strikingly, we show both theoretically and experimentally that the focus achieved by the under-sampling scheme can be one order of magnitude brighter than that achieved by the well-sampling schemes used in previous works, where 3×3 to 5×5 pixels sampled one speckle grain on average. Moreover, since neighboring pixels were uncorrelated in feedback-based wavefront shaping, introducing the concept of sub-Nyquist sampling in time-reversal-based wavefront shaping makes the optimal phase maps obtained using these two different methods consistent. We anticipate that this newly explored under-sampling scheme will transform the understanding of optical time reversal and boost the performance of optical imaging, manipulation, and communication through opaque scattering media.

Multimode optical fibers support low-loss transmission of multiple spatial modes, allowing for the transport of high-dimensional, spatially encoded information. In particular, encoding quantum information in the transverse shape of photons may boost the capacity of quantum channels while using existing infrastructure. However, when photons propagate through a multimode fiber, their transverse shape gets scrambled because of mode mixing and modal interference. This is usually corrected using free-space spatial light modulators, inhibiting a robust all-fiber operation. In this work, we demonstrate an all-fiber approach for controlling the shape of single photons and the spatial correlations between entangled photon pairs, using carefully controlled mechanical perturbations of the fiber. We optimize these perturbations to localize the spatial distribution of a single photon or the spatial correlations of photon pairs in a single spot, enhancing the signal in the optimized spot by over an order of magnitude. Using the same approach, we show a similar enhancement for coupling light from a multimode fiber into a single-mode fiber.

Raman fiber lasers (RFLs) have broadband tunability due to cascaded stimulated Raman scattering, providing extensive degrees of freedom for spectral manipulation. However, the spectral diversity of RFLs depends mainly on the wavelength flexibility of the pump, which limits the application of RFLs. Here, a spectrally programmable RFL is developed based on two-dimensional spatial-to-spectral mapping of light in multimode fibers (MMFs). Using an intracavity wavefront shaping method combined with genetic algorithm optimization, we launch light with a selected wavelength(s) at MMF output into the active part of the laser for amplification. In contrast, the light of undesired wavelengths is blocked. We demonstrate spectral shaping of the high-order RFL, including a continuously tunable single wavelength and multiple wavelengths with a designed spectral shape. Due to the simultaneous control of different wavelength regions, each order of Raman Stokes light allows flexible and independent spectral manipulation. Our research exploits light manipulation in a fiber platform with multi-eigenmodes and nonlinear gain, mapping spatial control to the spectral domain and extending linear light control in MMFs to active light emission, which is of great significance for applications of RFLs in optical imaging, sensing, and spectroscopy.

We present a new approach for shaping light at the output of a multimode fiber by modulating the transmission matrix of the system rather than the incident light. We apply computer-controlled mechanical perturbations to the fiber and obtain a desired intensity pattern at its output resulting from the changes to its transmission matrix. Using an all-fiber apparatus, we demonstrate focusing light at the distal end of the fiber and dynamic conversion between fiber modes in the few-mode regime. Since in this approach the number of available degrees of control scales with the number of spectral channels and can thus be larger than the number of fiber modes, it potentially opens the door to simultaneous control over multiple inputs and at multiple wavelengths.

Modal interference in a multimode fiber (MMF) has been utilized to develop simple sensors for various physical parameters. Herein, first, we investigate the dependences of the core diameter and the numerical aperture (NA) of the MMF on the performance of multimode-interference-based strain and temperature sensing. We find that a larger core diameter leads to higher temperature sensitivity but lower strain sensitivity (absolute value) and that higher NA does not influence the temperature sensitivity but results in higher strain sensitivity (absolute value). Subsequently, using the obtained low strain sensitivity, we demonstrate strain-insensitive temperature sensing with a sensitivity of approximately 6.1 pm °C−1.

Strain sensing based on modal interference in multimode fibers (MMFs) has been extensively studied, but no experimental or theoretical reports have been given as to how the system works when strain is applied not to the whole MMF but only to part of the MMF. Here, using a perfluorinated graded-index polymer optical fiber as the MMF, we investigate the strain sensing characteristics of this type of sensor when strain is partially applied to fiber sections with different lengths. The strain sensitivity dependence on the length of the strained section reveals that this strain sensor actually behaves as a displacement sensor.

Recent advances in wavefront shaping techniques have enabled so-called lensless endoscopes using fiber probes. Unlocking the full potential of such endoscopes call for the capability of optically sectioned and/or label free imaging. Or in other words, imaging through fibers must retain the functionality of a nonlinear microscope. This is a key challenge due to the temporal broadening of ultrashort pulses in fibers owing to modal dispersion. Here, we detail the first ever demonstration of two photon fluorescence imaging at the distal tip of a conventional graded index (GRIN) multimode fiber. GRIN fibers possess a high mode density, excellent throughput and limited temporal broadening. These features, in addition to its ready availability, make them attractive candidates for ultrathin endoscopes. In our approach, we apply the transmission matrix formalism and treat these fibers akin to highly scattering media. This lets us retrieve combinations of input modes that would generate intense focal spots throughout the field of view. Furthermore, we identify a regime where the modal dispersion in the fiber is minimal and two-photon excitation with femtosecond light pulses is possible. This allows us to perform two-photon imaging with ultrashort pulses in an epi-detection configuration analogous to conventional nonlinear microscopes. Finally, these concepts are validated by acquiring optically sectioned two photon fluorescence images of 3D samples with cellular resolution. We believe this first report of an ultrathin rigid endoscope of only 125 µm thickness would further accelerate the development of novel tools for demanding applications in biological imaging and opto-genetics.

Optical scattering is a fundamental problem in biomedical optics and limits most optical techniques to shallow operating depths less than 1 millimeter. However, although the scattering behavior of tissue scrambles the information it contains, it does not destroy it. Therefore, if you can unscramble the scattered light, it increases the accessible imaging depths up the absorption limit of light (several centimeters deep). One such way to beat optical scattering is using wavefront shaping. Borrowing ideas from adaptive optics in astronomy and phased arrays in radar and ultrasonic imaging, the basic concept of wavefront shaping is to control the phase and amplitude of the light field in order to harness scattered light. Using wavefront shaping techniques, scattered light can be used to form focal spots or transmit information through or inside optically scattering media. Furthermore, even without correcting for scattering directly by shaping the input light field, the properties of the scattered light can be analyzed to recover information about the structure and dynamic properties of a sample using methods from diffuse optics. The main contributions of this thesis are along these two lines of research: moving wavefront shaping toward more practical applications and developing new techniques to recover useful physiological information from scattered light. This is developed through three main projects: (1) an investigation of how dynamic samples impact the scattering process and the practical implications of these dynamics on wavefront shaping systems, (2) the development of a wavefront shaping system combining light and ultrasound to focus light inside acute brain slices to improve light delivery for optogenetics, (3) a novel method to sensitively detect the dynamics of scattered light and use it to tease out information about the flow of blood within the tissue sample of interest.

Quantitative phase microscopy (QPM) is a label-free imaging technique often employed for long-term, high-contrast imaging of live bio-samples. Yet, QPM is not specific to a certain subcellular organelle. As a remedy, fluorescence microscopy can visualize specific subcellular organelles once being labeled with fluorescent markers. In this paper, a high-resolution phase/fluorescence dual-modality microscopic imaging method based on structured light illumination is proposed. In a dual-modality microscopic system, periodic stripes are generated by a digital micromirror array (DMD), and are used as the common illumination for both modalities. For QPM imaging, the holograms of the sample under structured light illuminations from different directions and phase shifts are recorded, from which a quantitative phase image with resolution enhancement can be reconstructed via a synthetic aperture procedure. Furthermore, a numerical approach is proposed to compensate for the environmental disturbances that often challenge aperture synthesis of phase imaging. This method determines each time the phase distortions caused by environmental disturbances through using the spectrum of the 0th order of the structured light illumination, and the phase distortions are removed from the phase distributions of the waves along the 0th and the ±1st diffraction orders. Resolution enhancement of QPM imaging is realized by synthesizing the spectra of all the waves along different diffraction orders of the structured light illuminations of different orientations. With phase images, three-dimensional shapes, inner structures, or refractive index distributions of transparent and translucent samples can be obtained. For fluorescence imaging, intensity images (morie patterns) of the sample under different structured light illuminations are recorded. The spectra along different diffraction orders are separated by using a phase shifting reconstruction algorithm, and are shifted to their original positions, forming a synthesized spectrum that is much broader than the spectra of raw intensity images (NA-limited spectra). An inverse Fourier transform on the synthesized spectrum yields a super-resolution fluorescence image of the sample. With the reconstructed fluorescence images, specific subcellular organelles labeled with fluorescent markers can be visualized. The combination of quantitative phase microscopy and fluorescence microscopy can obtain multidimensional information about the sample. In this dual-mode imaging system, the spatial resolution of quantitative phase imaging and fluorescence imaging are 840 nm and 440 nm, respectively. The proposed dual-mode microscopy imaging technique has been demonstrated for imaging fluorescent beads, fly wings, spring/rice leaves, mouse tail transection, and fluorescence-stained SiHa cells. We envisage that this method can be further applied to many fields, such as biomedicine, industry, and chemistry.

Wavefront shaping holds great potential for high-resolution imaging or light delivery either through or deep inside living tissue. However, one of the biggest barriers that must be overcome to unleash the full potential of wavefront shaping for practical biomedical applications is the fact that wavefront shaping, especially based on iterative feedback, requires lengthy measurements to obtain useful correction of the output wavefront. As biological tissues are inherently dynamic, the short decorrelation time sets a limit on the achievable wavefront shaping enhancement. Here we show that for wavefront shaping in thin anisotropic scattering media such as biological tissues, we can optimize the wavefront shaping quality by simply limiting the numerical aperture (NA) of the incident wavefront. Using the same number of controlled modes, and therefore the same wavefront measurement time, we demonstrate that the wavefront shaped focus peak to background ratio can be increased by a factor of 2.1 while the energy delivery throughput can be increased by a factor of 8.9 through 710 µm thick brain tissue by just limiting the incident NA.

Traditionally, large core (greater than 100 micron) step index multimode optical fiber has occupied a reactively small niche of applications in data communications. While the large diameter of this type of fiber makes it easy to align to optoelectronic devices, its bandwidth*distance (BW*D) product is low due to modal dispersion between the large number of modes supported by a fully filled fiber. Recently, interest has been renewed in using an underfilled launch to excite 62.5 micron core graded index multimode fiber as a way to improve its bandwidth performance. With the proper launch conditions, this same effect has been measured in large core fiber. A vertical cavity surface emitting laser (VCSEL) is used to provide a low numerical aperture launch into a large core fiber, which has a relatively large numerical aperture. The laser thus underfills the modes of the fiber, and a bandwidth enhancement for the fiber is obtained. Results of experiments performed on step and graded index large core multimode fibers using a direct VCSEL launch are presented. In addition, these relaxed alignment tolerance fibers allow the utilization of very low cost cabling and connectorization procedures for parallel optical fiber cables. Data, including skew, bandwidth, and insertion loss, are presented on these cables.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this letter, we proposed a compact multimode fiber (MMF)-based laser scanning endoscope (LSE) using wavefront shaping with a liquid crystal spatial light modulator. Experimentally, we demonstrated the lensless imaging capability of the LSE using a negative USAF-1951 test target placed near the distal MMF facet, obtaining ∼7000-pixel images with micro-level spatial resolution and high contrast across a 105 μm field of view. This study proves the feasibility of a single MMF imaging without mechanical scanning, and also paves the way for ultra-thin micro-endoscopic imaging.

The use of wavefront shaping has found extensive application to develop ultra-thin endoscopic techniques based on multimode optical fibers (MMF), leveraging on the ability to control modal interference at the fiber’s distal end. Although several techniques have been developed to achieve MMF-based laser-scanning imaging, the use of short laser pulses is still a challenging application. This is due to the intrinsic delay and temporal broadening introduced by the fiber itself, which requires additional compensation optics on the reference beam during the calibration procedure. Here we combine the use of a supercontinuum laser and an internal reference-based wavefront shaping system to produce focused spot scanning in multiple planes at the output of a step-index multimode fiber, without the requirement of a delay line or pulse pre-compensation. We benchmarked the performances of internal vs external reference during calibration, finding that the use of an internal reference grants better focusing efficiency. The system was characterized at different wavelengths, showcasing the wavelength resiliency of the different parameters. Lastly, the scanning of focal planes beyond the fiber facet was achieved by exploiting the chromato-axial memory effect.