Synthesizing multi-frame high-resolution fluorescein angiography images from retinal fundus images using generative adversarial networks.

BACKGROUND: Fundus fluorescein angiography (FA) can be used to diagnose fundus diseases by observing dynamic fluorescein changes that reflect vascular circulation in the fundus. As FA may pose a risk to patients, generative adversarial networks have been used to convert retinal fundus images into fluorescein angiography images. However, the available methods focus on generating FA images of a single phase, and the resolution of the generated FA images is low, being unsuitable for accurately diagnosing fundus diseases. METHODS: We propose a network that generates multi-frame high-resolution FA images. This network consists of a low-resolution GAN (LrGAN) and a high-resolution GAN (HrGAN), where LrGAN generates low-resolution and full-size FA images with global intensity information, HrGAN takes the FA images generated by LrGAN as input to generate multi-frame high-resolution FA patches. Finally, the FA patches are merged into full-size FA images. RESULTS: Our approach combines supervised and unsupervised learning methods and achieves better quantitative and qualitative results than using either method alone. Structural similarity (SSIM), normalized cross-correlation (NCC) and peak signal-to-noise ratio (PSNR) were used as quantitative metrics to evaluate the performance of the proposed method. The experimental results show that our method achieves better quantitative results with structural similarity of 0.7126, normalized cross-correlation of 0.6799, and peak signal-to-noise ratio of 15.77. In addition, ablation experiments also demonstrate that using a shared encoder and residual channel attention module in HrGAN is helpful for the generation of high-resolution images. CONCLUSIONS: Overall, our method has higher performance for generating retinal vessel details and leaky structures in multiple critical phases, showing a promising clinical diagnostic value.

Enhance the delivery of light energy ultra-deep into turbid medium by controlling multiple scattering photons to travel in open channels.

Multiple light scattering is considered as the major limitation for deep imaging and focusing in turbid media. In this paper, we present an innovative method to overcome this limitation and enhance the delivery of light energy ultra-deep into turbid media with significant improvement in focusing. Our method is based on a wide-field reflection matrix optical coherence tomography (RM-OCT). The time-reversal decomposition of the RM is calibrated with the Tikhonov regularization parameter in order to get more accurate reversal results deep inside the scattering sample. We propose a concept named model energy matrix, which provides a direct mapping of light energy distribution inside the scattering sample. To the best of our knowledge, it is the first time that a method to measure and quantify the distribution of beam intensity inside a scattering sample is demonstrated. By employing the inversion of RM to find the matched wavefront and shaping with a phase-only spatial light modulator, we succeeded in both focusing a beam deep (~9.6 times of scattering mean free path, SMFP) inside the sample and increasing the delivery of light energy by an order of magnitude at an ultra-deep (~14.4 SMFP) position. This technique provides a powerful tool to understand the propagation of photon in a scattering medium and opens a new way to focus light inside biological tissues.

Microscope integrated optical coherence tomography system combined with augmented reality.

One of the disadvantages in microscope-integrated optical coherence tomography (MI-OCT) systems is that medical images acquired via different modalities are usually displayed independently. Hence, surgeons have to match two-dimensional and three-dimensional images of the same operative region subjectively. In this paper, we propose a simple registration method to overcome this problem by using guided laser points. This method combines augmented reality with an existing MI-OCT system. The basis of our idea is to introduce a guiding laser into the system, which allows us to identify fiducials in microscopic images. At first, the applied voltages of the scanning galvanometer mirror are used to calculate the fiducials' coordinates in an OCT model. After gathering data at the corresponding points' coordinates, the homography matrix and camera parameters are used to superimpose a reconstructed model on microscopic images. After performing experiments with artificial and animal eyes, we successfully obtain two-dimensional microscopic images of scanning regions with depth information. Moreover, the registration error is 0.04 mm, which is within the limits of medical and surgical errors. Our proposed method could have many potential applications in ophthalmic procedures.

Interplane bulk motion analysis and removal based on normalized cross-correlation in optical coherence tomography angiography.

Bulk motion seriously degrades the image quality of optical coherence tomography angiography (OCTA). Conventional correction methods focus on in-plane displacement, while the bulk motion component perpendicular to B-scans also introduces noise. This work first presents an evaluation of this component using a specific scan protocol and an approximate expression derived from peak-normalized cross-correlation values, and then quantitatively assesses how interplane bulk motion noise reduce the sensitivity of cross-sectional angiograms. Finally, we developed a repetitive bulk motion correction method based on the estimated displacements and redundant volume scans. The correction does not require registration and angiogram reconstruction of low flow sensitivity frames, and the results of in vivo mice skin OCTA imaging experiments show that the proposed method can effectively reduce bulk motion noise caused by cardiac and respiratory motion and occasional shaking, and improve OCTA image quality, which has practical significance for clinical OCTA diagnosis and analysis.

Polarization dynamics of dissipative-soliton-resonance pulses in passively mode-locked fiber lasers.

We present a comprehensive numerical investigation of the polarization dynamics of dissipative-soliton-resonance (DSR) pulses in a passively mode-locked Yb-doped fiber laser with a nonlinear optical loop mirror (NOLM). In our simulations, the NOLM's saturable absorption effect is modeled by its transmission function. Depending on the strength of cavity birefringence, various types of vector DSR pulses, including the polarization-locked DSR (PL-DSR), the polarization rotation-locked DSR (PRL-DSR), and the group velocity-locked DSR (GVL-DSR) states are obtained in the laser cavity. The characteristics of these vector DSR pulses are shown. We also analyze the polarization evolution and internal polarization dynamics of PRL-DSR pulses within the cavity. Our simulation results offer insight into the vector nature of DSR pulses in polarization-insensitive mode-locked fiber lasers.

Mechanism of dissipative-soliton-resonance generation in fiber laser mode-locked by real saturable absorber.

Generation of dissipative soliton resonance (DSR) is numerically investigated in an all-normal-dispersion Yb-doped fiber laser mode-locked by a real saturable absorber (SA). In the simulation model, the SA includes both the saturable absorption and reverse saturable absorption (RSA) effects. It is found that the RSA effect induced by the SA material itself plays a dominant role in generating the DSR pulses. We also systematically analyze the influence of key SA parameters on the evolution of DSR pulses in the cavity. Our simulation results not only offer insight into the underlying mechanism of DSR generation in mode-locked fiber lasers by means of real SAs, but also provide a guideline for engineering SA parameters to generate optical pulses with the highest possible energy.

High-repetition-rate all-fiber femtosecond laser with an optical integrated component.

We demonstrate a high-repetition-rate all-fiber soliton pulse laser mode-locked by the nonlinear polarization rotation technique. The laser cavity is effectively shortened by incorporating an optical integrated component possessing the hybrid functions of a polarization-dependent isolator, a wavelength-division multiplexer, and an output coupler. Resultant output soliton pulses have a fundamental repetition rate of 384 MHz, a 3-dB spectral bandwidth of 25.2 nm, and a dechirped pulse duration of 115 fs. By using an external power amplification and pulse recompression system, the average output power of the laser is boosted to 207 mW. The amplified pulses have a 2.33-ps duration, which is recompressed to 340 fs. Numerical simulations reproduce the generation of high-repetition-rate soliton pulses in the fiber laser. Such a simple and low-cost high-repetition-rate fiber laser is a potential laser source for various practical applications.

Stable multi-wavelength fiber laser with single-mode fiber in a Sagnac loop.

In this paper, we propose and experimentally demonstrate a stable multi-wavelength fiber laser at 1.5 µm with single-mode fiber (SMF). The Sagnac loop structure with a 48.6:51.4 coupler and 2 km SMF has an intensity-dependent loss, which contributes to suppress the mode competition in the cavity and leads to a steady multi-wavelength output. In the experiment, five stable lasing wavelengths are obtained with a pump power of 300 mW at 980 nm. The demonstrated multi-wavelength fiber laser has great potential for applications in optical communications and optical sensing systems.

Multi-wavelength Erbium-doped fiber laser based on four-wave-mixing effect in single mode fiber and high nonlinear fiber.

A multi-wavelength Erbium-doped fiber (EDF) laser based on four-wave-mixing is proposed and experimentally demonstrated. The 5 km single mode fiber in the cavity enhances the four-wave-mixing to suppress the homogenous broadening of the erbium-doped fiber and get the stable multi-wavelength comb. The lasing stability is investigated. When the pump power is 300 mW, the fiber laser has 5-lasing lines and the maximum fluctuation of the output power is about 3.18 dB. At the same time, a laser with 110 m high nonlinear fiber (HNFL) is demonstrated. When the pump power is 300 mW, it has 7-lasing lines (above -30 dBm) and the maximum fluctuation is 0.18dB.

Polarization-maintaining buffered Fourier domain mode-locked swept source for optical coherence tomography.

A polarization-maintaining buffered Fourier domain mode-locked (FDML) swept source with a center wavelength of 1300 nm is demonstrated. The scanning rate of the buffered FDML swept source is doubled without sacrificing the output power of the swept source by combining two orthogonally polarized outputs with a polarization beam combiner. The stability of the swept source is improved because the polarization state of the laser beam inside the laser cavity is maintained without the use of any polarization controllers. The swept source is capable of an edge-to-edge tuning range of more than 150 nm and a FWHM range of 95 nm at a 102 kHz sweeping rate and with an average power of 12 mW. A swept source optical coherence tomography (SSOCT) system is developed utilizing this buffered FDML swept source. The axial resolution of the SSOCT system is measured to be 9.4 µm in air. The sensitivity of the SSOCT system is 107.5 dB at a depth of 0.25 mm with a 6 dB roll-off at a depth of 2.25 mm.