1.7-μm spectroscopic spectral-domain optical coherence tomography for imaging lipid distribution within blood vessel

Broadband wavelength-swept Cr4+:YAG crystal fiber laser

We present a broadband wavelength-swept laser using a 16-µm-core-diameter Cr4+:YAG crystal fiber as the gain medium. The laser-diode-pumped crystal fiber laser has a threshold of only 102 mW due to the low propagation loss and high heat dissipation efficiency. The laser achieves a sweeping wavelength range of 134 nm, centered around 1425 nm, with a scanning speed of 163 k nm/s. Notably, the cross-polarization-coupled excited state absorption of the signal wavelength constrained the long-wavelength lasing limit. This laser has the potential for swept source optical coherence tomography applications, providing an axial resolution of 11.4 µm.

Review on Laser Technology in Intravascular Imaging and Treatment

Blood vessels are one of the most essential organs, which nourish all tissues in our body. Once there are intravascular plaques or vascular occlusion, other organs and circulatory systems will not work properly. Therefore, it is necessary to detect abnormal blood vessels by intravascular imaging technologies for subsequent vascular treatment. The emergence of lasers and fiber optics promotes the development of intravascular imaging and treatment. Laser imaging techniques can obtain deep vascular images owing to light scattering and absorption properties. Moreover, photothermal and photomechanical effects of laser make it possible to treat vascular diseases accurately. In this review, we present the research progress and applications of laser techniques in intravascular imaging and treatment. Firstly, we introduce intravascular optical coherent tomography and intravascular photoacoustic imaging, which can obtain various information of plaques. Multimodal intravascular imaging techniques provide more information about intravascular plaques, which have an essential influence on intravascular imaging. Secondly, two laser techniques including laser angioplasty and endovenous laser ablation are discussed for the treatment of arterial and venous diseases, respectively. Finally, the outlook of laser techniques in blood vessels, as well as the integration of laser imaging and treatment are prospected in the section of discussions.

Optimization of light scattering enhancement by gold nanoparticles in fused silica optical fiber

A conventional distributed fiber optic sensing system offers close to linear sensitivity along the fiber length. However gold nanoparticles (NP) have been shown to be able to enhance the contrast ratio to improve the quality of signal detection. The challenge in improving the contrast of reflected signals is to optimise the nanoparticle doping concentration over the densed sensing length to make best use of the distributed fiber sensing hardware. In this paper, light enhancement by spherical gold NPs in the optical fibers was analyzed by considering the size-induced NP refractive index changes. This was achieved by building a new model to relate backscattered light from a gold NP suspension between the optical fiber end tips and backscattered light from gold NPs in the core of the optical fiber. The paper provides a model to determine the optimized sizes and concentrations of NPs for sensing at different desired penetration depths in the optical fiber.

Intravascular Polarimetry: Clinical Translation and Future Applications of Catheter-Based Polarization Sensitive Optical Frequency Domain Imaging

Optical coherence tomography (OCT) and optical frequency domain imaging (OFDI) visualize the coronary artery wall and plaque morphology in great detail. The advent of these high-resolution intracoronary imaging modalities has propelled our understanding of coronary atherosclerosis and provided enhanced guidance for percutaneous coronary intervention. Yet, the lack of contrast between distinct tissue types and plaque compositions impedes further elucidation of the complex mechanisms that contribute to acute coronary syndrome (ACS) and hinders the prospective identification of plaques susceptible to rupture. Intravascular polarimetry with polarization-sensitive OFDI measures polarization properties of the coronary arterial wall using conventional intravascular imaging catheters. The quantitative polarization metrics display notable image contrast between several relevant coronary plaque microstructures that are difficult to identify with conventional OCT and OFDI. Tissues rich in collagen and smooth muscle cells exhibit birefringence, while lipid and macrophages cause depolarization. In this review, we describe the basic principles of intravascular polarimetry, discuss the interpretation of the polarization signatures, and outline promising avenues for future research and clinical implications.

Cascaded telecom fiber enabled high-order random fiber laser beyond zero-dispersion wavelength

Four-wave mixing induced spectral broadening near the zero-dispersion wavelength (ZDW) of the fiber is a bottleneck factor that limits the further wavelength extending in cascaded random fiber lasers (RFLs). In this Letter, we successfully suppress the spectral broadening near the ZDW of the fiber in the cascaded RFL by simply combining two kinds of commercial telecom fibers with different ZDWs, G655C fiber with ZDW around 1.52 µm and G652D fiber with ZDW around 1.31 µm. As a result, an 8th order Stokes light component at 1721 nm with a maximum output power of 2.1 W and a spectral purity of 96.94% is realized in this telecom-fiber-based cascaded RFL. This work provides a reference of nonlinear effect management in fiber lasers as well as affords a cost-effective way with great potential of realizing high-power widely tunable fiber lasers.

Signal-to-background ratio and lateral resolution in deep tissue imaging by optical coherence microscopy in the 1700 nm spectral band

We quantitatively investigated the image quality in deep tissue imaging with optical coherence microscopy (OCM) in the 1700 nm spectral band, in terms of the signal-to-background ratio (SBR) and lateral resolution. In this work, to demonstrate the benefits of using the 1700 nm spectral band for OCM imaging of brain samples, we compared the imaging quality of OCM en-face images obtained at the same position by using a hybrid 1300 nm/1700 nm spectral domain (SD) OCM system with shared sample and reference arms. By observing a reflective resolution test target through a 1.5 mm-thick tissue phantom, which had a similar scattering coefficient to brain cortex tissue, we confirmed that 1700 nm OCM achieved an SBR about 6-times higher than 1300 nm OCM, although the lateral resolution of the both OCMs was similarly degraded with the increase of the imaging depth. Finally, we also demonstrated high-contrast deep tissue imaging of a mouse brain at a depth up to 1.8 mm by using high-resolution 1700 nm SD-OCM.

Tunable random Raman fiber laser at 1.7 µm region with high spectral purity

We demonstrate a tunable, high order cascaded random Raman fiber laser (RRFL) with high purity at 1.7 µm band by using a high power amplified spontaneous emission source (ASE) with both wavelength and linewidth tunability as pump source. The influence of the spectral bandwidth of the ASE source on the spectral purity of the output at 1.7 µm band is investigated. By adjusting the spectral bandwidth of the ASE source to the optimized 20 nm, output power >14 W with spectral purity up to 98.29% at 1715 nm is achieved. As far as we know, this is the highest spectral purity ever reported for a RRFL at 1.7 µm region. Furthermore, by adjusting the central wavelength of ASE source, the output of the RRFL can be tuned from 1695 to 1725 nm with >10 W output power. What's more, the spectral purity is above 92% over a tuning range from 1705 to 1725 nm.

A high-peak-power orthogonally-polarized multi-wavelength laser at 1.6-1.7 µm based on the cascaded nonlinear optical frequency conversion

We demonstrated a high-peak-power orthogonally polarized multi-wavelength laser at 1.6-1.7 µm based on the intracavity cascaded nonlinear optical frequency conversion (CNOFC). This CNOFC can be characterized by a configuration of "series-parallel optical circuit", where the OPO pumped Raman conversion operates in the "series-mode" and orthogonally-polarized waves are simultaneously converted in the "parallel-mode". The fundamental wave at 1064 nm was first simultaneously converted to orthogonally-polarized 1534 and 1572 nm by the x-cut KTA and KTP optical parametric oscillation (OPO), respectively. Then the x-cut KTA and KTP acted as the Raman crystal to each other with the X(ZZ)X Raman configuration, converting the OPO signals to multi-order Raman emissions with the wavelengths at 1601, 1632, 1673, 1697, 1752 and 1767nm. A maximum total average output power of 1.2 W and a minimum pulse width of 10.5 ns were achieved from this CNOFC-based laser, corresponding to a pulse peak power of 28.5 kW and a pulse energy of 0.3 mJ.

High-spatial-resolution deep tissue imaging with spectral-domain optical coherence microscopy in the 1700-nm spectral band

We present three-dimensional (3-D) high-resolution spectral-domain optical coherence microscopy (SD-OCM) by using a supercontinuum (SC) fiber laser source with 300-nm spectral bandwidth (full-width at half-maximum) in the 1700-nm spectral band. By using low-coherence interferometry with SC light and a confocal detection scheme, we realized lateral and axial resolutions of 3.4 and 3.8  μm in tissue (n  =  1.38), respectively. This is, to the best of our knowledge, the highest 3-D spatial resolution reported among those of Fourier-domain optical coherence imaging techniques in the 1700-nm spectral band. In our SD-OCM, to enhance the imaging depth, a full-range method was implemented, which suppressed the formation of a coherent ghost image and allowed us to set the zero-delay position inside the samples. We demonstrated the 3-D high-resolution imaging capability of 1700-nm SD-OCM through the measurement of an interference signal from a mirror surface and imaging of a single 200-nm polystyrene bead and a pig thyroid gland. Deep tissue imaging at a depth of up to 1.8 mm was also demonstrated. This is the first demonstration of 3-D high-resolution SD-OCM in the 1700-nm spectral band.

Efficient 1.7 μm light source based on KTA-OPO derived by Nd:YVO4 self-Raman laser

An intra-cavity optical parametric oscillator (OPO) emitting at 1.7 μm derived by Nd:YVO₄ self-Raman laser is demonstrated in this Letter, with a KTiOAsO₄ (KTA) crystal used as nonlinear optical crystal. A laser diode end-pumped acousto-optic Q-switched Nd:YVO₄ self-Raman laser at 1176 nm was employed as the pump source. At an incident pump power of 12.1 W and a pulse repetition frequency of 60 kHz, average output power up to 1.2 W signal light at 1742 nm was obtained, with diode-to-signal conversion efficiency of 10%. The pulse width was about 11 ns and spectral line width was less than 0.5 nm for the signal light. The results show that compact intra-cavity KTA-OPO derived by Nd:YVO₄ self-Raman laser is an efficient method for 1.7 μm waveband laser generation, with potential applications in biological imaging, laser therapy, special materials processing, etc.

Axial resolution and signal-to-noise ratio in deep-tissue imaging with 1.7-μm high-resolution optical coherence tomography with an ultrabroadband laser source

We investigated the axial resolution and signal-to-noise ratio (SNR) characteristics in deep-tissue imaging by 1.7-μm optical coherence tomography (OCT) with the axial resolution of 4.3  μm in tissue. Because 1.7-μm OCT requires a light source with a spectral width of more than 300 nm full-width at half maximum to achieve such high resolution, the axial resolution in the tissue might be degraded by spectral distortion and chromatic dispersion mismatching between the sample and reference arms. In addition, degradation of the axial resolution would also lead to reduced SNR. Here, we quantitatively evaluated the degradation of the axial resolution and the resulting decrease in SNR by measuring interference signals through a lipid mixture serving as a turbid tissue phantom with large scattering and absorption coefficients. Although the axial resolution was reduced by a factor of ∼6 after passing through a 2-mm-thick tissue phantom, our result clearly showed that compensation of the dispersion mismatching allowed us to achieve an axial resolution of 4.3  μm in tissue and improve the SNR by ∼5  dB compared with the case where dispersion mismatching was not compensated. This improvement was also confirmed in the observation of a hamster’s cheek pouch in a buffer solution.

Noise-bias and polarization-artifact corrected optical coherence tomography by maximum a-posteriori intensity estimation

We propose using maximum a-posteriori (MAP) estimation to improve the image signal-to-noise ratio (SNR) in polarization diversity (PD) optical coherence tomography. PD-detection removes polarization artifacts, which are common when imaging highly birefringent tissue or when using a flexible fiber catheter. However, dividing the probe power to two polarization detection channels inevitably reduces the SNR. Applying MAP estimation to PD-OCT allows for the removal of polarization artifacts while maintaining and improving image SNR. The effectiveness of the MAP-PD method is evaluated by comparing it with MAP-non-PD, intensity averaged PD, and intensity averaged non-PD methods. Evaluation was conducted in vivo with human eyes. The MAP-PD method is found to be optimal, demonstrating high SNR and artifact suppression, especially for highly birefringent tissue, such as the peripapillary sclera. The MAP-PD based attenuation coefficient image also shows better differentiation of attenuation levels than non-MAP attenuation images.

Optical coherence microscopy in 1700 nm spectral band for high-resolution label-free deep-tissue imaging

Optical coherence microscopy (OCM) is a label-free, high-resolution, three-dimensional (3D) imaging technique based on optical coherence tomography (OCT) and confocal microscopy. Here, we report that the 1700-nm spectral band has the great potential to improve the imaging depth in high-resolution OCM imaging of animal tissues. Recent studies to improve the imaging depth in OCT revealed that the 1700-nm spectral band is a promising choice for imaging turbid scattering tissues due to the low attenuation of light in the wavelength region. In this study, we developed high-resolution OCM by using a high-power supercontinuum source in the 1700-nm spectral band, and compared the attenuation of signal-to-noise ratio between the 1700-nm and 1300-nm OCM imaging of a mouse brain under the condition of the same sensitivity. The comparison clearly showed that the 1700-nm OCM provides larger imaging depth than the 1300-nm OCM. In this 1700-nm OCM, the lateral resolution of 1.3 μm and the axial resolution of 2.8 μm, when a refractive index was assumed to be 1.38, was achieved.

Preliminary study of optical coherence tomography imaging to identify microscopic extrathyroidal extension in patients with papillary thyroid carcinoma

BACKGROUND AND OBJECTIVES

We evaluated the feasibility of using optical coherence tomography (OCT), to identify microscopic extrathyroidal extension (mETE) in ex vivo thyroidectomy specimens of patients who underwent thyroidectomy for the treatment of papillary thyroid carcinoma (PTC).

METHODS

A total of 170 ex vivo OCT images of the tumor, were acquired just after completion of thyroidectomy in 17 patients. The OCT images of each patient were separately evaluated by two blinded investigators, and the outcomes were compared with the histopathology reports.

RESULTS

The sensitivity and specificity of mETE identification from the OCT images were 81.4% and 86.0%, respectively, for the first investigator, and 82.9% and 87.0%, respectively, for the second investigator. Substantial agreement between the investigators was verified by Cohen's κ (Cohen's κ = 0.772).

CONCLUSION

In this preliminary study of a limited series of ex vivo thyroidectomy specimens, we verified the feasibility of OCT as a method of identifying mETE in patients with PTC.

Tri-band spectroscopic optical coherence tomography based on optical parametric amplification for lipid and vessel visualization

A tri-band spectroscopic optical coherence tomography (SOCT) system has been implemented for visualization of lipid and blood vessel distribution. The tri-band swept source, which covers output spectrum in 1.3, 1.5, and 1.6  μm wavelength windows, is based on a dual-band Fourier domain mode-locked laser and a fiber optical parametric amplifier. This tri-band SOCT can further differentiate materials, e.g., lipid and artery, qualitatively by contrasting attenuation coefficients difference within any two of these bands. Furthermore, ex vivo imaging of both porcine artery with artificial lipid plaque phantom and mice with coronary artery disease were demonstrated to showcase the capability of our SOCT.