SignificanceExtending the photoacoustic microscopy (PAM) into the mid-infrared (MIR) molecular fingerprint region constitutes a promising route toward label-free imaging of biological molecular structures. Realizing this objective requires a high-energy nanosecond MIR laser source. However, existing MIR laser technologies are limited to either low pulse energy or free-space structure that is sensitive to environmental conditions. Fiber lasers are promising technologies for PAM for their potential to offer both high pulse energy and robust performance, which however have not yet been used for PAM because it is still at the infant research stage.AimWe aim to employ the emerging gas-filled anti-resonant hollow-core fiber (ARHCF) laser technology for MIR-PAM for the purpose of imaging myelin-rich regions in a mouse brain.ApproachThis laser source is developed with a high-pulse-energy nanosecond laser at 3.4μm, targeting the main absorption band of myelin sheaths, the primary chemical component of axons in the central nervous system. The laser mechanism relies on two-order gas-induced vibrational stimulated Raman scattering for non-linear wavelength conversion, starting from a 1060-nm pump laser to 3.4 μm through the two-stage gas-filled ARHCFs.ResultsThe developed fiber Raman laser was used for the first time for MIR-PAM of mouse brain regions containing structures rich in myelin. The high peak power of ∼1.38 kW and robust performance of the generated MIR Raman pulse addressed the challenge faced by the commonly used MIR lasers.ConclusionsWe pioneered the potential use of high-energy and nanosecond gas-filled ARHCF laser source to MIR-PAM, with a first attempt to report this kind of fiber laser source for PAM of lipid-rich myelin regions in a mouse brain. We also open up possibilities for expanding into a versatile multiwavelength laser source covering multiple biomarkers and being employed to image other materials such as plastics.
The emerging mid-infrared photoacoustic microscopy (MIR-PAM) is a potential imaging modality in revealing special biomolecules compositions in thick samples by utilizing the light-excited ultrasound signals. The development of a nanosecond and high-energy MIR fiber laser source is still at an early age, facing the challenges of either low peak power or large footprint. This work aims to develop a new Raman laser source for MIR-PAM based on the gas-filled anti-resonant hollow core fiber (ARHCF) technology. As a proof of concept, a MIR laser source at 3.4 μm is developed and combined with PAM for the first time targeting at the lipid-rich mouse brain sample due to main absorption band of myelin sheaths. This laser source is based on the cascading of two ARHCFs, where a high-energy (~26.5 μJ) Raman Stokes line at 1409 nm is generated through the 1st-stage nitrogen-filled ARHCF with a pump fiber laser at 1060 nm. The output Raman laser from the 1st stage ARHCF is used as a pump for the 2nd-stage hydrogen-filled ARHCF, to generate the Raman laser at 3.4 μm with ~2.7 μJ pulse energy. Our label-free ex-vivo imaging depicted the lipid-rich myelin region in the mouse brain, showing the feasibility of extending the novel gas-filled laser platform into PAM imaging modalities.
Unraveling the scientific and technological importance of the mid-infrared (mid-IR) region remains yet a long-standing challenge. Despite the significant efforts on mid-IR light sources, development of high-energy, narrow-linewidth and compact lasers still constitutes the main obstacle towards novel spectroscopic, imaging and sensing devices. Photoacoustic modality is known as one of the most powerful tools enabling high signal-to-noise ratio gas detection and albeit its wide use in the mature near-infrared (near-IR) region, further research has to be carried out in the mid-IR in order to “unlock” its full potential. In this work, we aim on tracing CO2 based on the innovative combination of the emerging gas-filled mid-IR silica anti-resonant hollow-core fiber (ARHCF) Raman laser technology with the powerful photoacoustic modality. The laser source adopts the stimulated Raman scattering effect of H2 filled in a piece of ARHCF, to enable the generation of first-order vibrational Raman Stokes from a 1533 nm Er-doped fiber laser pump. With this configuration, a nanosecond laser pulses with micro-joule level pulse energy is achieved at ~ 4.25 μm wavelength, which is located within the strongest absorption band of CO2. The laser’s linewidth is estimated to be tens GHz level. This laser source is used to drive an in-house developed photoacoustic sensor, revealing a 1.78 ppm level CO2 detection limit in laboratory condition. This work provides a valuable reference for the development of high-sensitivity gas detectors.
Gas-filled anti-resonant hollow-core fiber (ARHCF) constitutes an efficient route towards development of high-energy fiber lasers in the near-infrared (NIR) and mid-infrared (MIR) spectral region. We will present our recent work on developing both vibrational and rotational Raman-active gas-filled fiber lasers spanning from the NIR up to around 4.3 μm wavelength. We will also show how such fiber lasers can be used for high-resolution photoacoustic gas sensing and imaging.
Here, we present a high pulse energy Raman laser at 1946 nm wavelength pumped with a 1533 nm linearly polarized fiber laser, with ∼92 μJ pulse energy, ∼60 pm linewidth, 8 kHz repetition rate, and 7 ns pulse duration. The Raman laser is based on the stimulated Raman scattering (SRS) effect in an 8-meter carbon dioxide (CO2) filled nested anti-resonant hollow-core fiber (ARHCF). The nested structure contributes to the significant reduction of the fiber loss caused by light leakage, surface scattering and bend, therefore allowing coiling the gas-filled ARHCF with a relatively small bend radius of just ~5 cm. When the pressure in the CO2-filled ARHCF increases from 1 to 17 bar, the pulse energy first reaches the maximum pulse energy level of 16.3 μJ (corresponding to 28 % quantum efficiency) at only 1.2 bar, and then rapidly decreases due to the pressure-dependent overlap of the Raman laser line with the absorption band of CO2 at 2 μm spectral range. The relative intensity noise (RIN) of the Raman laser reaches a minimum level (4%) when the pulse energy exceeds ∼8 µJ. Due to the low amount of heat release during the SRS process, the laser has a good long-term stability without significant drift. Our results constitute a novel and promising technology towards high-energy 2 μm lasers.
We will present our recent work using noble and Raman-active gas-filled anti-resonant hollow-core fiber (ARHCF) technology. First, we will present the generation of supercontinuum spanning from 200 nm to 4000 nm based on nonlinear effects of soliton self-compression and phase-matched deep‑ultraviolet (DUV) dispersive wave (DW) emission in Argon (Ar)-filled ARHCFs pumped at 2.46 μm wavelength with 100 fs pulses and ~8μJ pulse energy. Then we will discuss our recent work on stimulated Raman scattering (SRS) effect in a hydrogen (H2)-filled ARHCF, to achieve near- and MIR Raman lasers. By employing the single-stage vibrational SRS effect, a 4.22 μm Raman laser line is directly converted from a linearly polarized 1.53 μm pump laser. A quantum efficiency as high as 74% was achieved, to yield 17.6 µJ pulse energy. The designed 4.22 μm wavelength is well overlapped with the strongest CO2 absorption, therefore constituting a promising way for CO2 detection. In addition, we report a multi-wavelength Raman laser based on the cascaded rotational SRS effect. Four Raman lines at 1683 nm, 1868 nm, 2100 nm, and 2400 nm are generated, with pulse energies as high as 18.25 µJ, 14.4 µJ, 14.1 µJ, and 8.2 µJ, respectively. The energy of these Raman lines can be controlled by tuning the H2 pressure from 1 bar to 20 bar.
Silica anti-resonant hollow-core fiber (ARHCF) is a promising platform for optofluidic applications because it can act as fluid-cell, permitting intense fluid-light interaction over extended length with low optical loss from ultra-violet to midinfrared region. For this kind of applications, an all-fiberized and compact structure is necessary. However, a prerequisite for this purpose is that micro-channels must be created on the side of the fiber, to provide access for the diffusion of fluids (i.e. liquid or gas) into the core. Several attempts based on femtosecond laser micro-machining technology have been made to create micro-channels through the silica cladding, but significant loss could be induced due to the damage of the cladding capillaries of ARHCF. Here, we report a high-precision and repeatable micro-machining technique using focused ion beam (FIB) milling on a nodeless ARHCF. Ga+ ion beam is employed to bombard a 43 μm thick outer cladding of ARHCF for 30 minutes, to create a 50 μm deep fluidic channel. The micro-channel in the silica cladding is precisely drilled at the middle position of two adjacent capillaries with a 2.8 μm gap, providing direct access for liquid/gas to diffuse into the hollow-core region, while avoiding the damage of the capillaries. Corroborating results from simulation of such a structure are presented to demonstrate that no additional loss is induced by the milled structure.
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