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1. High-speed wide-field multi-parametric photoacoustic microscopy

   https://doi.org/10.1364/OL.391824  

   Zhong, Fenghe ; Bao, Youwei ; Chen, Ruimin ; Zhou, Qifa ; Hu, Song ( May 2020 , Optics Letters)

   Capable of imaging blood perfusion, oxygenation, and flow simultaneously at the microscopic level, multi-parametric photoacoustic microscopy (PAM) has quickly emerged as a powerful tool for studying hemodynamic and metabolic changes due to physiological stimulations or pathological processes. However, the low scanning speed poised by the correlation-based blood flow measurement impedes its application in studying rapid microvascular responses. To address this challenge, we have developed a new, to the best of our knowledge, multi-parametric PAM system. By extending the optical scanning range with a cylindrically focused ultrasonic transducer (focal zone,$76\text{µ<\#comment/>}\mathrm{m}\times <\#comment/>4.5\mathrm{m}\mathrm{m}$) for simultaneous acquisition of 500 B-scans, the new system is 112 times faster than our previous multi-parametric system that uses a spherically focused transducer (focal diameter, 40 µm) and enables high-resolution imaging of blood perfusion, oxygenation, and flow over an area of$4.5\times <\#comment/>1{\mathrm{m}\mathrm{m}}^2$at a frame rate of 1 Hz. We have demonstrated the feasibility of this system in the living mouse ear. Further development of this system into reflection mode will enable real-time cortex-wide imaging of hemodynamics and metabolism in the mouse brain.
2. Picostrain-resolution fiber-optic sensing down to sub-10  mHz infrasonic frequencies

   https://doi.org/10.1364/JOSAB.395397  

   Hoque, Nabil Md Rakinul ; Duan, Lingze ( August 2020 , Journal of the Optical Society of America B)

   High-resolution strain sensing based on long, high-finesse fiber Fabry–Perot interferometers (FFPIs) has been demonstrated with a special focus on the infrasonic frequency range. A novel dual-FFPI scheme allows the large environment-induced background at low frequencies to be suppressed, permitting high strain resolution limited only by excess electronic noise. Noise-equivalent strain resolution of$257\mathrm{p}\text{ε<\#comment/>}/\surd <\#comment/>\mathrm{H}\mathrm{z}$has been achieved at 6 mHz, and the resolution improves to$\sim <\#comment/>200\mathrm{f}\text{ε<\#comment/>}/\surd <\#comment/>\mathrm{H}\mathrm{z}$between 4–20 Hz. Without the use of any additional optical frequency references and with only off-the shelf commercial components, these resolutions are much better than most in the prior reports. Especially, an improvement of a factor of 1.8 is achieved in comparison with the highest resolution reported so far near 5 Hz. The limiting factors of the current scheme have been analyzed in detail, and the application prospects have been demonstrated using an acoustic transducer. The work lays out the potential of using long FFPIs with high finesse for high-resolution fiber-optic sensing in the infrasonic frequency range.
3. Super-resolution imaging with an achromatic multi-level diffractive microlens array

   https://doi.org/10.1364/OL.410013  

   Banerji, Sourangsu ; Meem, Monjurul ; Majumder, Apratim ; Sensale-Rodriguez, Berardi ; Menon, Rajesh ( November 2020 , Optics Letters)

   Compound eyes found in insects provide intriguing sources of biological inspiration for miniaturized imaging systems. Inspired by such insect eye structures, we demonstrate an ultrathin arrayed camera enabled by a flat multi-level diffractive microlens array for super-resolution visible imaging. We experimentally demonstrate that the microlens array can achieve a large fill factor (hexagonal close packing with$\mathrm{p}\mathrm{i}\mathrm{t}\mathrm{c}\mathrm{h}=120\text{µ<\#comment/>}\mathrm{m}$), thickness of 2.6 µm, and diffraction-limited ($\mathrm{S}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{h}\mathrm{l}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}=0.88$) achromatic performance in the visible band (450 to 650 nm). We also demonstrate super-resolution imaging with resolution improvement of$\sim <\#comment/>1.4$times by computationally merging 1600 images in the array.
4. Needle-based deep-neural-network camera

   https://doi.org/10.1364/AO.415059  

   Guo, Ruipeng ; Nelson, Soren ; Menon, Rajesh ( March 2021 , Applied Optics)

   We experimentally demonstrate a camera whose primary optic is a cannula/needle ($\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{r}=0.22\mathrm{m}\mathrm{m}$and$\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}=12.5\mathrm{m}\mathrm{m}$) that acts as a light pipe transporting light intensity from an object plane (35 cm away) to its opposite end. Deep neural networks (DNNs) are used to reconstruct color and grayscale images with a field of view of 18° and angular resolution of$\sim <\#comment/>{0.4}^{\circ <\#comment/>}$. We showed a large effective demagnification of$127\times <\#comment/>$. Most interestingly, we showed that such a camera could achieve close to diffraction-limited performance with an effective numerical aperture of 0.045, depth of focus$\sim <\#comment/>16\text{µ<\#comment/>}\mathrm{m}$, and resolution close to the sensor pixel size (3.2 µm). When trained on images with depth information, the DNN can create depth maps. Finally, we show DNN-based classification of the EMNIST dataset before and after image reconstructions. The former could be useful for imaging with enhanced privacy.
5. Twist-free ultralight two-photon fiberscope enabling neuroimaging on freely rotating/walking mice

   https://doi.org/10.1364/OPTICA.422657  

   Li, Ang ; Guan, Honghua ; Park, Hyeon-Cheol ; Yue, Yuanlei ; Chen, Defu ; Liang, Wenxuan ; Li, Ming-Jun ; Lu, Hui ; Li, Xingde ( June 2021 , Optica)

   Lightweight and head-mountable scanning nonlinear fiberscope technologies offer an exciting opportunity for enabling mechanistic exploration of ensemble neural activities with subcellular resolution on freely behaving rodents. The tether of the fiberscope, consisting of an optical fiber and scanner drive wires, however, restricts the mouse’s movement and consequently precludes free rotation and limits the freedom of walking. Here we present the first twist-free two-photon fiberscope technology for enabling neuroimaging on freely rotating/walking mice. The technology equips a scanning fiberscope with active rotational tracking and compensation capabilities through an optoelectrical commutator (OEC) to allow the animal to rotate and walk in arbitrary patterns during two-photon fluorescence (TPF) imaging of neural activities. The OEC provides excellent optical coupling stability ($<<\#comment/>\pm <\#comment/>1\mathrm{\%<\#comment/>}$fluctuation during rotation) and an extremely high torque sensitivity ($<<\#comment/>8\mathrm{m}\mathrm{N}\cdot <\#comment/>\mathrm{m}$). In addition, the new technology is equipped with a custom grating and prism to effectively manage the temporal properties of the femtosecond excitation pulses through the fiber-optic system, which improved neuroimaging signal by more than$2\mathrm{X}$. This TPF fiberscope imaging platform has been tested forin vivoimaging, and the results demonstrate that it enablesmore »reliable recording of calcium dynamics of more than 50 neurons simultaneously in the motor cortices of freely behaving mice in a twist-free fashion. With active tracking function of the OEC enabled, we observed considerable increase in both behavior and neural activities in the motor cortices of the mice during freely behaving neuroimaging experiments.

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