102-nm, 44.5-MHz inertial-free swept source by mode-locked fiber laser and time stretch technique for optical coherence tomography

Normal central foveal thickness in a thousand eyes of healthy patients in sub Saharan Africa using fourier domain optical coherence tomography

Background

Optical coherence tomography provides high resolution in vivo images of the retina which are essential for diagnosis and follow up of patients with retina disorders like macula edema and exudative age-related macular degeneration. Establishing the normal range of central fovea values in our population provides vital baseline data for comparison.

Aim

To report the range of normal central fovea thickness measurements in eyes of healthy hospital patients in sub-Saharan Africa using a commercially available Fourier domain optical coherence tomography (OCT) scan.

Patients and Methods

A retrospective non-comparative review of case files of a thousand consecutive healthy patients who had retina OCT scans between January 2015 and December 2019 was done.

Results

Data from 1000 consecutive eyes of 500 healthy patients were used for the study. There were 181 females and 319 males. The mean central foveal thickness was 239.48 microns (μm), with a minimum thickness of 200.0 μm and maximum thickness of 297.0 μm. Males had significantly (P < 0.001) thicker mean CFT (mean CFT = 241.77 μm) compared with females (mean CFT = 235.43 μm). The mean CFT increased with age of participants by 0.139 μm (P < 0.001) for every year of life below 70.

Conclusion

The mean central foveal thickness (CFT) in eyes of healthy patients in our study was 239.48 μm with a range from 200 μm to 297.0 μm. Males had thicker mean CFT compared with females and there was a significant increase in mean CFT by 0.139 μm (P < 0.001) for every year of life below 70.

FBG array based wavelength calibration scheme for Fourier domain mode-locked laser with pm resolution and hourly stability

We demonstrate a fiber Bragg grating (FBG) array based wavelength calibration scheme for Fourier domain mode-locked (FDML) laser. The wavelength interval and the temperature feedback module of the FBG array are designed to ensure the reference stability of the wavelength calibration scheme. Combined with the calibration scheme, the FDML laser with a tunable wavelength range of ∼60 nm, a center wavelength of 1300 nm and a sweep frequency of 39.63 kHz is built up to demonstrate its feasibility. The FBG wavelength demodulation based on the calibrated FDML laser system shows a wavelength resolution of 2.76 pm and hourly stability of 10.22 pm.

Dual-comb based time-stretch optical coherence tomography for large and segmental imaging depth

Optical coherence tomography based on time-stretch enables high frame rate and high-resolution imaging for the inertia-free wavelength-swept mechanism. The fundamental obstacle is still the acquisition bandwidth's restriction on imaging depth. By introducing dual-comb with slightly different repetition rates, the induced Vernier effect is found to be capable of relieving the problem. In our work, a dual-comb based time-stretch optical coherence tomography is proposed and experimentally demonstrated, achieving a 1.5-m imaging depth and 200-kHz A-scan rate. Moreover, about a 33.4-µm resolution and 25-µm accuracy are achieved. In addition, by adjusting the frequency detuning of the dual-comb, the A-scan rate can be further boosted to video-rate imaging. With enlarged imaging depth, this scheme is promising for a wide range of applications, including light detection and ranging.

Highly coherent, flat, and broadband time-stretched swept source based on extra-cavity spectral shaping assisted by a booster semiconductor optical amplifier

We demonstrate a flat broadband time-stretched swept source based on extra-cavity spectral shaping. By adjusting the polarization-dependent gain profile and driving current of the booster optical amplifier (BOA), extra-cavity spectral shaping is optimized to generate output with a 1-dB bandwidth of ∼100 nm, 3-dB bandwidth of ∼140 nm and output power of ∼21.4 mW. The short-term and long-term stabilities are characterized. The average cross correlation of 183,485 round trips is 0.9997 with a standard deviation of 2×10^-5, indicating high single-shot spectral similarity and high coherence. The noise floor of relative spectral energy jitter is -141.7 dB/Hz, indicating a high short-term spectral energy stability. The proposed highly stable flat broadband time-stretched swept source is applied to an optical coherence tomography (OCT) system. The axial resolution is 10.8 µm. The proposed swept source can serve as excellent light sources in ultra-fast coherent detection systems for high precision sensing and imaging.

Broadband similariton generation in a mode-locked Yb-doped fiber laser

We experimentally demonstrated a broadband similariton generated from a mode-locked Yb-doped fiber laser with dispersion compensation. The broadest spectrum was obtained by pushing the net dispersion to its limitation and fully exploiting the gain bandwidth. The spectrum was 115 nm broad in 10 dB bandwidth and 36 nm broad in 3 dB bandwidth. The output was 105 mW optical power at 545 mW pump power. Simulation combined with experiment was performed to investigate and confirm the mode-locking regime of the laser. Experimental observations agreed well with the numerical simulation. We believe our study provides a practical route for designing broadband mode-locked fiber lasers.

Fourier Domain Mode Locked Laser and Its Applications

The sweep rate of conventional short-cavity lasers with an intracavity-swept filter is limited by the buildup time of laser signals from spontaneous emissions. The Fourier domain mode-locked (FDML) laser was proposed to overcome the limitations of buildup time by inserting a long fiber delay in the cavity to store the whole swept signal and has attracted much interest in both theoretical and experimental studies. In this review, the theoretical models to understand the dynamics of the FDML laser and the experimental techniques to realize high speed, wide sweep range, long coherence length, high output power and highly stable swept signals in FDML lasers will be discussed. We will then discuss the applications of FDML lasers in optical coherence tomography (OCT), fiber sensing, precision measurement, microwave generation and nonlinear microscopy.

High-speed wavelength-swept femtosecond source from 1055 to 1300 nm using a GHz femtosecond fiber laser

In this Letter, we demonstrate a high-speed broadband wavelength-swept femtosecond source (WFS) that leverages the soliton self-frequency shift (SSFS) and intensity-wavelength encoding technologies. The optical wavelength of the high-speed WFS can be continuously swept from 1055 nm to nearly 1300 nm at a sweeping rate of 100 kHz. This WFS is especially seeded by a femtosecond mode-locked all-fiber laser at 1055 nm that has a fundamental repetition rate of ∼1.0 GHz, a maximum output power of 7 W, and a compressed pulse width of 220 fs. It is anticipated that this high-speed broadband WFS can be a promising source for applications that require fast wavelength scanning and high-speed data processing.

114 nm broadband all-fiber nonlinear polarization rotation mode locked-laser and time-stretch optical coherence tomography

We propose and demonstrate an all-fiber Er-doped mode-locked laser with a 3-dB spectrum of 114 nm by using nonlinear polarization rotation (NPR), which to the best of our knowledge is the first realization to date of such a broad spectrum without any spatial optical devices. The repetition rate and pulse width of the laser are 183.6 MHz and 3.7 ps, respectively. Such an all-fiber NPR mode-locked laser is then applied in time-stretch optical coherence tomography. The axial resolution is 12.1 µm. The all-fiber high speed broadband swept laser based on the time stretching technique has compact structure and high stability, which is a promising source for frequency metrology and high resolution optical coherence tomography.

Enhanced medical diagnosis for dOCTors: a perspective of optical coherence tomography

SIGNIFICANCE

After three decades, more than 75,000 publications, tens of companies being involved in its commercialization, and a global market perspective of about USD 1.5 billion in 2023, optical coherence tomography (OCT) has become one of the fastest successfully translated imaging techniques with substantial clinical and economic impacts and acceptance.

AIM

Our perspective focuses on disruptive forward-looking innovations and key technologies to further boost OCT performance and therefore enable significantly enhanced medical diagnosis.

APPROACH

A comprehensive review of state-of-the-art accomplishments in OCT has been performed.

RESULTS

The most disruptive future OCT innovations include imaging resolution and speed (single-beam raster scanning versus parallelization) improvement, new implementations for dual modality or even multimodality systems, and using endogenous or exogenous contrast in these hybrid OCT systems targeting molecular and metabolic imaging. Aside from OCT angiography, no other functional or contrast enhancing OCT extension has accomplished comparable clinical and commercial impacts. Some more recently developed extensions, e.g., optical coherence elastography, dynamic contrast OCT, optoretinography, and artificial intelligence enhanced OCT are also considered with high potential for the future. In addition, OCT miniaturization for portable, compact, handheld, and/or cost-effective capsule-based OCT applications, home-OCT, and self-OCT systems based on micro-optic assemblies or photonic integrated circuits will revolutionize new applications and availability in the near future. Finally, clinical translation of OCT including medical device regulatory challenges will continue to be absolutely essential.

CONCLUSIONS

With its exquisite non-invasive, micrometer resolution depth sectioning capability, OCT has especially revolutionized ophthalmic diagnosis and hence is the fastest adopted imaging technology in the history of ophthalmology. Nonetheless, OCT has not been completely exploited and has substantial growth potential-in academics as well as in industry. This applies not only to the ophthalmic application field, but also especially to the original motivation of OCT to enable optical biopsy, i.e., the in situ imaging of tissue microstructure with a resolution approaching that of histology but without the need for tissue excision.

In-process measurement of a keyhole using a low-coherence interferometer with a high repetition rate

The shape of an instance hole (keyhole) created via a high-power laser was measured using a low-coherence interferometer with the following parameters: repetition rate, 10 MHz; center wavelength, 1550 nm; absolute spatial resolution, 10 µm; and measurement range, 5 mm. The keyhole was created on a 3-mm-thick stainless-steel plate using a high-power laser with 8-kW peak power and 1070-nm center wavelength. The cross-sectional area of the keyhole was measured to be 0.42 mm × 0.78 mm (width × depth) using the interferometer, and its side dimension was 0.46 mm × 0.78 mm (width × depth).

1.1-µm Band Extended Wide-Bandwidth Wavelength-Swept Laser Based on Polygonal Scanning Wavelength Filter

We demonstrated a 1.1-µm band extended wideband wavelength-swept laser (WSL) that combined two semiconductor optical amplifiers (SOAs) based on a polygonal scanning wavelength filter. The center wavelengths of the two SOAs were 1020 nm and 1140 nm, respectively. Two SOAs were connected in parallel in the form of a Mach-Zehnder interferometer. At a scanning speed of 1.8 kHz, the 10-dB bandwidth of the spectral output and the average power were approximately 228 nm and 16.88 mW, respectively. Owing to the nonlinear effect of the SOA, a decrease was observed in the bandwidth according to the scanning speed. Moreover, the intensity of the WSL decreased because the oscillation time was smaller than the buildup time. In addition, a cholesteric liquid crystal (CLC) cell was fabricated as an application of WSL, and the dynamic change of the first-order reflection of the CLC cell in the 1-µm band was observed using the WSL. The pitch jumps of the reflection band occurred according to the electric field applied to the CLC cell, and instantaneous changes were observed.

400 MHz ultrafast optical coherence tomography

An ultrafast time-stretched swept source with a sweep rate of 400 MHz is demonstrated based on the buffering of a 100 MHz femtosecond laser pulse train. To the best of our knowledge, this is the highest sweep rate of swept sources for optical coherence tomography (OCT) that has been reported. With a 10 dB sweep range of ∼100nm, an axial resolution of 19 µm is obtained in the OCT. OCT imaging of high-speed rotating disks is demonstrated. A composite complex apodization method is proposed and demonstrated to enhance the signal to noise ratio in the OCT imaging.

High-resolution wide-field OCT angiography with a self-navigation method to correct microsaccades and blinks

In this study, we demonstrate a novel self-navigated motion correction method that suppresses eye motion and blinking artifacts on wide-field optical coherence tomographic angiography (OCTA) without requiring any hardware modification. Highly efficient GPU-based, real-time OCTA image acquisition and processing software was developed to detect eye motion artifacts. The algorithm includes an instantaneous motion index that evaluates the strength of motion artifact on en face OCTA images. Areas with suprathreshold motion and eye blinking artifacts are automatically rescanned in real-time. Both healthy eyes and eyes with diabetic retinopathy were imaged, and the self-navigated motion correction performance was demonstrated.

Calibration-free time-stretch optical coherence tomography with large imaging depth

We demonstrate a calibration-free time-stretch optical coherence tomography (TS-OCT), based on an optical higher order dispersion compensation scheme, which substitutes the digital calibration with optical dispersion compensation. As a result, the acquired raw data can directly perform the Fourier transform, and data processing time is greatly reduced by 82%, compared with the digital calibration. Moreover, because of the high-sensitivity and calibration-free characteristics, the high-order dispersion compensation-based TS-OCT can increase sensitivity roll-off by 2.6 times to 6.91 mm/dB and effective imaging depth by 14.2% to 16 mm. The in vivo biological tissue imaging has been demonstrated, with the single-shot A-scan rate approaching 19 MHz. This higher order dispersion compensation scheme could provide a promising solution for the TS-OCT system to realize 3D imaging in real time and enhanced imaging quality.

Ultrafast discrete swept source based on dual chirped combs for microscopic imaging

An inertial-free, ultrafast frequency comb source based on two chirped optical frequency combs (OFCs) is proposed and experimentally demonstrated. The high linearity frequency sweeping is realized by the Vernier effect between the two OFCs rather than any mechanical motion component, so that good stability and reliability are ensured and no recalibration or resampling process is required. Swept rate up to 1 MHz is realized while keeping a narrow instantaneous linewidth of 0.03 nm, thanks to the extra-cavity frequency sweeping method. The wavelength step is proportional to the swept rate (3.8 pm at 10 kHz), and can be tuned by changing the repetition rate difference between the two OFCs. This swept source is applied for high-speed wavelength encoded imaging and achieves 4.4-μm spatial resolution at a 329-kHz frame rate. Compared with the traditional time-stretch microscopy, the signal acquisition bandwidth decreased from 3.8 GHz to below 90 MHz to achieve the same spatial resolution. Furthermore, the exposure time for a specific wavelength is much longer due to the discrete sweeping feature, which is a benefit for higher sensitivity. This discrete swept source provided a promising low-cost option for high-speed biomedical imaging systems and high-accuracy spectroscopy.

Down-conversion en-face optical coherence tomography

We present an optical coherence tomography (OCT) method that can deliver an en-face OCT image from a sample in real-time, irrespective of the tuning speed of the swept source. The method, based on the master slave interferometry technique, implements a coherence gate principle by requiring that the optical path difference (OPD) between the arms of an imaging interferometer is the same with the OPD in an interrogating interferometer. In this way, a real-time en-face OCT image can originate from a depth in the sample placed in the imaging interferometer, selected by actuating on the OPD in the interrogating interferometer, while laterally scanning the incident beam over the sample. The generation of the en-face image resembles time domain OCT, with the difference that here the signal is processed based on spectral domain OCT. The optoelectronic processor operates down-conversion of the chirped radio frequency signal delivered by the photo-detector. The down-conversion factor is equal to the ratio of the maximum frequency of the photo-detected signal due to an OPD value matching the coherence length of the swept source, to the sweeping rate. This factor can exceed 10⁶ for long coherence swept sources.

Evaluating changes of blood flow in retina, choroid, and outer choroid in rats in response to elevated intraocular pressure by 1300 nm swept-source OCT

We report the development of a 1300 nm swept-source optical coherence tomography (SS-OCT) system specifically designed to perform OCT imaging and optical microangiography (OMAG) in rat eyes in vivo and its use in evaluating the effects of intraocular pressure (IOP) elevation on ocular circulation. The swept laser is operated in single longitude mode with a 90 nm bandwidth centered at 1300 nm and 200 kHz A-line rate, providing remarkable sensitivity fall-off performance along the imaging depth, a larger field of view of 2.5 × 2.5 mm2 (approximately 35°), and more time-efficient imaging acquisition. The advantage of the SS-OCT/OMAG is highlighted by an increased imaging depth of the entire posterior thickness of optic nerve head (ONH) and its surrounding vascular anatomy, to include, for the first time in vivo, the vasculature at the scleral opening, allowing visualization of the circle of Zinn-Haller and posterior ciliary arteries (PCAs). Furthermore, the capillary-level resolution angiograms achieved at the retinal and choroidal layers over a larger field of view enable a significantly improved quantification of the response of vascular area density (VAD) to elevated IOP. The results indicate that reduction in perfusion of the choroid in response to elevated IOP is delayed compared to that seen in the retina; while choroidal VAD doesn't reach 50% of baseline until ~70 mmHg, the same effect is seen for the retinal VAD at ~60 mmHg. The superior image quality offered by SS-OCT may allow more comprehensive investigation of IOP-related ocular perfusion changes and their pathological roles in glaucomatous optic nerve damage.

Sensitivity-enhanced ultrafast optical tomography by parametric- and Raman-amplified temporal imaging

To overcome the speed limitation of conventional optical tomography, a temporal imaging technique has been integrated with optical time-domain reflectometry to realize ultrafast temporally magnified (TM) tomography. In this Letter, the sensitivity of TM tomography has been further enhanced using optical parametric amplification and distributed Raman amplification, and this technique is named temporally encoded amplified and magnified (TEAM) tomography. As a result, a 78-dB sensitivity has been realized, comparable to ultrafast optical coherence tomography systems. In addition, an 86.7-μm axial resolution can be realized across a 67.5-mm imaging range. To demonstrate the significance of sensitivity improvement, tomographic imaging of a centimeter-thick phantom is provided at an A-scan rate of 44 MHz.

Video-rate centimeter-range optical coherence tomography based on dual optical frequency combs by electro-optic modulators

Imaging speed and range are two important parameters for optical coherence tomography (OCT). A conventional video-rate centimeter-range OCT requires an optical source with hundreds of kHz repetition rate and needs the support of broadband detectors and electronics (>1 GHz). In this paper, a type of video-rate centimeter-range OCT system is proposed and demonstrated based on dual optical frequency combs by leveraging electro-optic modulators. The repetition rate difference between dual combs, i.e. the A-scan rate of dual-comb OCT, can be adjusted within 0~6 MHz. By down-converting the interference signal from optical domain to radio-frequency domain through dual comb beating, the down-converted bandwidth of the interference signal is less than 22.5 MHz which is at least two orders of magnitude lower than that in conventional OCT systems. A LabVIEW program is developed for video-rate operation, and the centimeter imaging depth is proved by using 10 pieces of 1-mm thick glass stacked as the sample. The effective beating bandwidth between two optical comb sources is 7 nm corresponding to ~108 comb lines, and the axial resolution of the dual-comb OCT is 158 µm. Dual optical frequency combs provide a promising solution to relax the detection bandwidth requirement in fast long-range OCT systems.

Advances in Retinal Optical Imaging

Retinal imaging has undergone a revolution in the past 50 years to allow for better understanding of the eye in health and disease. Significant improvements have occurred both in hardware such as lasers and optics in addition to software image analysis. Optical imaging modalities include optical coherence tomography (OCT), OCT angiography (OCTA), photoacoustic microscopy (PAM), scanning laser ophthalmoscopy (SLO), adaptive optics (AO), fundus autofluorescence (FAF), and molecular imaging (MI). These imaging modalities have enabled improved visualization of retinal pathophysiology and have had a substantial impact on basic and translational medical research. These improvements in technology have translated into early disease detection, more accurate diagnosis, and improved management of numerous chorioretinal diseases. This article summarizes recent advances and applications of retinal optical imaging techniques, discusses current clinical challenges, and predicts future directions in retinal optical imaging.