Raman-shifted wavelength-selectable pulsed fiber laser with high repetition rate and high pulse energy in the visible

Efficient tunable cascaded Raman source with all-silica fibers based on 2-µm DSR pulse pumping

We present an efficient tunable all-silica-fiber 2nd-order cascaded Raman pulse laser utilizing 2-µm dissipative-soliton-resonance (DSR) rectangular pulses for pumping and highly GeO2-doped silica fiber as Raman gain medium. When pumped at 1966.5 nm, the maximum 1st-order Raman optical conversion efficiency is up to 64.4% at 2153 nm, with 92.4% spectral purity and 0.39-W average power. The maximum 2nd-order Raman optical conversion efficiency is 19.3% at 2370 nm, with 39.2% spectral purity and 0.25-W average power. To our knowledge, these conversion efficiencies and spectral purities represent the highest levels achieved in a mid-infrared all-silica-fiber cascaded pulsed Raman laser. Additionally, by adjusting the central wavelength of the DSR seed pulse, the 2nd-order Raman light can be tuned within a range of 41 nm (2354∼2395 nm). Our system provides a simple and easy-to-implement solution for realizing efficient tunable cascaded pulsed Raman lasers in the 2.4-µm band.

High-speed photoacoustic microscopy: A review dedicated on light sources

In recent years, many methods have been investigated to improve imaging speed in photoacoustic microscopy (PAM). These methods mainly focused upon three critical factors contributing to fast PAM: laser pulse repetition rate, scanning speed, and computing power of the microprocessors. A high laser repetition rate is fundamentally the most crucial factor to increase the PAM speed. In this paper, we review methods adopted for fast PAM systems in detail, specifically with respect to light sources. To the best of our knowledge, ours is the first review article analyzing the fundamental requirements for developing high-speed PAM and their limitations from the perspective of light sources.

Recent Technical Progression in Photoacoustic Imaging—Towards Using Contrast Agents and Multimodal Techniques

For combining optical and ultrasonic imaging methodologies, photoacoustic imaging (PAI) is the most important and successful hybrid technique, which has greatly contributed to biomedical research and applications. Its theoretical background is based on the photoacoustic effect, whereby a modulated or pulsed light is emitted into tissue, which selectively absorbs the optical energy of the light at optical wavelengths. This energy produces a fast thermal expansion in the illuminated tissue, generating pressure waves (or photoacoustic waves) that can be detected by ultrasonic transducers. Research has shown that optical absorption spectroscopy offers high optical sensitivity and contrast for ingredient determination, for example, while ultrasound has demonstrated good spatial resolution in biomedical imaging. Photoacoustic imaging combines these advantages, i.e., high contrast through optical absorption and high spatial resolution due to the low scattering of ultrasound in tissue. In this review, we focus on advances made in PAI in the last five years and present categories and key devices used in PAI techniques. In particular, we highlight the continuously increasing imaging depth achieved by PAI, particularly when using exogenous reagents. Finally, we discuss the potential of combining PAI with other imaging techniques.

Wide-field polygon-scanning photoacoustic microscopy of oxygen saturation at 1-MHz A-line rate

We report wide-field polygon-scanning functional OR-PAM that for the first time achieves 1-MHz A-line rate of oxygen saturation in vivo. We address two technical challenges. The first is a 1-MHz dual-wavelength pulsed laser that has sufficient pulse energy and ultrafast wavelength switching. The second is a polygon-scanning imaging probe that has a fast scanning speed, a large field of view, and great sensitivity. The OR-PAM system offers a B-scan rate of 477.5 Hz in a 12-mm range and a volumetric imaging rate of ∼1 Hz over a 12 × 5 mm2 scanning area. We image microvasculature and blood oxygen saturation in a 12 × 12 mm2 scanning area in 5 s. Dynamic imaging of oxygen saturation in the mouse ear is demonstrated to monitor fast response to epinephrine injection. The new wide-field fast functional imaging ability broadens the biomedical application of OR-PAM.

Wave of single-impulse-stimulated fast initial dip in single vessels of mouse brains imaged by high-speed functional photoacoustic microscopy

SIGNIFICANCE

The initial dip in hemoglobin-oxygenation response to stimulations is a spatially confined endogenous indicator that is faster than the blood flow response, making it a desired label-free contrast to map the neural activity. A fundamental question is whether a single-impulse stimulus, much shorter than the response delay, could produce an observable initial dip without repeated stimulation.

AIM

To answer this question, we report high-speed functional photoacoustic (PA) microscopy to investigate the initial dip in mouse brains.

APPROACH

We developed a Raman-laser-based dual-wavelength functional PA microscope that can image capillary-level blood oxygenation at a 1-MHz one-dimensional imaging rate. This technology was applied to monitor the hemodynamics of mouse cerebral vasculature after applying an impulse stimulus to the forepaw.

RESULTS

We observed a transient initial dip in cerebral microvessels starting as early as 0.13 s after the onset of the stimulus. The initial dip and the subsequent overshoot manifested a wave pattern propagating across different microvascular compartments.

CONCLUSIONS

We quantified both spatially and temporally the single-impulse-stimulated microvascular hemodynamics in mouse brains at single-vessel resolution. Fast label-free imaging of single-impulse response holds promise for real-time brain-computer interfaces.

Laser speckle contrast imaging system using nanosecond pulse laser source

SIGNIFICANCE

Nanosecond-pulsed laser has proven to be used to obtain the velocity of blood using the speckle contrast method. Without the scanning time, it has potential for achieving fast two-dimensional blood flow images in a photoacoustic imaging system with the same pulsed laser.

AIM

Our study aimed to evaluate the qualities of regional cerebral blood flow (rCBF) obtained in a laser speckle contrast imaging (LSCI) system using continuous wave (cw) and nanosecond pulse laser sources.

APPROACH

First, a LSCI system consisting of a cw laser with a wavelength of 632.8 nm and a cw laser/nanosecond pulse laser with a wavelength of 532 nm was developed. This system was used to obtain rCBF images of mouse in vivo with two different laser sources.

RESULTS

Continuous wave lasers (532 and 632.8 nm) show different imaging characteristics for rCBF imaging. The rCBF images obtained using 532-nm nanosecond pulse laser showed higher resolution than those using 532-nm cw laser. There was no significant difference in the results using nanosecond pulse laser among various pulse widths or repetition rates.

CONCLUSIONS

It is proved that a nanosecond pulse laser could be used for LSCI.

Single-shot photoacoustic microscopy of hemoglobin concentration, oxygen saturation, and blood flow in sub-microseconds

We present fast functional optical-resolution photoacoustic microscopy (OR-PAM) that can simultaneously image hemoglobin concentration, blood flow speed, and oxygen saturation with three-pulse excitation. To instantaneously determine the blood flow speed, dual-pulse photoacoustic flowmetry is developed to determine the blood flow speed from photoacoustic signal decay in sub-microseconds. Grueneisen relaxation effect is compensated for in the oxygen saturation calculation. The blood flow imaging is validated in phantom and in vivo experiments. The results show that the flow speed can be measured accurately in sub-microseconds by comparing the dual-pulse flowmetric method with photoacoustic Doppler flowmetry. Wide-field OR-PAM of hemoglobin concentration, blood flow speed, and oxygen saturation are demonstrated in the mouse ear. This technical advance enables more biomedical applications for fast functional OR-PAM.

Optical-resolution photoacoustic microscopy of oxygen saturation with nonlinear compensation

Optical-resolution photoacoustic microscopy (OR-PAM) of oxygen saturation (sO2) offers high-resolution functional information on living tissue. Limited by the availability of high-speed multi-wavelength lasers, most OR-PAM systems use wavelengths around 532nm. Blood has high absorption coefficients in this spectrum, which may cause absorption saturation and induce systematic errors in sO2 imaging. Here, we present nonlinear OR-PAM that compensates for the absorption saturation in sO2 imaging. We model the absorption saturation at different absorption coefficients and ultrasonic bandwidths. To compensate for the absorption saturation, we develop an OR-PAM system with three optical wavelengths and implement a nonlinear algorithm to compute sO2. Phantom experiments on bovine blood validate that the nonlinear OR-PAM can improve the sO2 accuracy by up to 0.13 for fully oxygenated blood. In vivo sO2 imaging has been conducted in the mouse ear. The nonlinear sO2 results agree with the normal physiological values. These results show that the absorption saturation effect can be compensated for in nonlinear OR-PAM, which improves the accuracy of functional photoacoustic imaging.

Highly efficient nanosecond 560 nm source by SHG of a combined Yb-Raman fiber amplifier

We demonstrate a nanosecond 560 nm pulse source based on frequency-doubling the output of a combined Yb-Raman fiber amplifier, achieving a pulse energy of 2.0 µJ with a conversion efficiency of 32% from the 976 nm pump light. By introducing a continuous-wave 1120 nm signal before the cladding pumped amplifier of a pulsed Yb:fiber master oscillator power amplifier system operating at 1064 nm, efficient conversion to 1120 nm occurs within the fiber amplifier due to stimulated Raman scattering. The output of the combined Yb-Raman amplifier is frequency-doubled to 560 nm using a periodically poled lithium tantalate crystal with a conversion efficiency of 47%, resulting in an average power of 3.0 W at a repetition rate of 1.5 MHz. The 560 nm pulse duration of 1.7 ns and the near diffraction-limited beam quality (M²≤1.18) make this source ideally suited to biomedical imaging applications such as optical-resolution photoacoustic microscopy and stimulated emission depletion microscopy.

2 MHz multi-wavelength pulsed laser for functional photoacoustic microscopy

Fast functional photoacoustic microscopy requires multi-wavelength pulsed laser sources with high pulse repetition rates, short wavelength switching time, and sufficient pulse energies. Here, we report the development of a stimulated-Raman-scattering-based multi-wavelength pulsed laser source for fast functional photoacoustic imaging. The new laser source is pumped with a 532 nm 1 MHz pulsed laser. The 532 nm laser beam is split into two: one pumps a 5 m optical fiber to excite a 558 nm wavelength via stimulated Raman scattering; the other goes through a 50 m optical fiber to delay the 532 nm pulse by 220 ns. The two beams are combined and coupled into an optical fiber for photoacoustic excitation. As a result, the new laser source can generate 2 million pulses per second, switch wavelengths in 220 ns, and provide hundreds of nanojoules pulse energy for each wavelength. Using this laser source, we demonstrate optical-resolution photoacoustic imaging of microvascular structures and oxygen saturation in the mouse ear. The ultrashort wavelength switching time enables oxygen saturation imaging of flowing red blood cells, which is valuable for high-resolution functional imaging.