High-speed phase- and group-delay scanning with a grating-based phase control delay line

Long-range frequency-domain optical delay line based on a spinning tilted mirror for low-cost ocular biometry

Optical biometers are routinely used to measure intraocular distances in ophthalmic applications such as cataract surgery planning or myopia monitoring. However, due to their high cost and reduced transportability, access to them for screening and surgical planning is still limited in low-resource and remote settings. To increase patients' access to optical biometry we propose a novel low-cost frequency-domain optical delay line (FD-ODL) based on an inexpensive stepper motor spinning a tilted mirror, for integration into a time-domain (TD)-biometer, amenable to a compact footprint. In the proposed FD-ODL, the axial scan range and the A-scan rate are decoupled from one another, as the former only depends on the spinning mirror tilt angle, while the A-scan rate only depends on the motor shaft rotational speed. We characterized the scanning performance and specifications for two spinning mirror tilt angles, and compared them to those of the standard, more expensive FD-ODL implementation, employing a galvanometric scanner for group delay generation. A prototype of the low-cost FD-ODL with a 1.5 deg tilt angle, resulting in an axial scan range of 6.61 mm and an A-scan rate of 10 Hz was experimentally implemented and integrated in a dual sample beam optical low-coherence reflectometry (OLCR) setup with a detour unit to replicate the measurement window around the anterior segment and the retina. The intraocular distances of a model eye were measured with the proposed low-cost biometer and found to be in good agreement with those acquired by a custom swept-source optical coherence tomography (SS-OCT) system and two commercial biometers, validating our novel design.

ADC-Net: An Open-Source Deep Learning Network for Automated Dispersion Compensation in Optical Coherence Tomography

Chromatic dispersion is a common problem to degrade the system resolution in optical coherence tomography (OCT). This study is to develop a deep learning network for automated dispersion compensation (ADC-Net) in OCT. The ADC-Net is based on a modified UNet architecture which employs an encoder-decoder pipeline. The input section encompasses partially compensated OCT B-scans with individual retinal layers optimized. Corresponding output is a fully compensated OCT B-scan with all retinal layers optimized. Two numeric parameters, i.e., peak signal to noise ratio (PSNR) and structural similarity index metric computed at multiple scales (MS-SSIM), were used for objective assessment of the ADC-Net performance and optimal values of 29.95 ± 2.52 dB and 0.97 ± 0.014 were obtained respectively. Comparative analysis of training models, including single, three, five, seven and nine input channels were implemented. The mode with five-input channels was observed to be optimal for ADC-Net training to achieve robust dispersion compensation in OCT.

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.

Sensitive detection of radio-frequency field phase with interacting dark states in Rydberg atoms

An efficient scheme of phase measurement of a radio-frequency (RF) field is proposed by interacting dark states. Under the condition of electromagnetically induced transparency (EIT), the four-level Rydberg atom exhibits two windows. Compared with the transmission spectrum on resonance, the linewidths of absorption peaks off resonance are very narrow due to the interaction of double dark states. It is interesting to find that the distance of absorption peaks shifts approximately linearly with the phase of an RF field, which can be used to measure the RF field phase. Simulation results show that the linewidth of an absorption peak can be narrowed by more than one order of magnitude, and a narrow linewidth improves the detectable minimum phase difference by more than six times. It helps to reduce analyzation complexity and increase sensing resilience. The dependence of phase measurement on the control field and RF field is also investigated.

Optical coherence tomography

Optical coherence tomography (OCT) is a non-contact method for imaging the topological and internal microstructure of samples in three dimensions. OCT can be configured as a conventional microscope, as an ophthalmic scanner, or using endoscopes and small diameter catheters for accessing internal biological organs. In this Primer, we describe the principles underpinning the different instrument configurations that are tailored to distinct imaging applications and explain the origin of signal, based on light scattering and propagation. Although OCT has been used for imaging inanimate objects, we focus our discussion on biological and medical imaging. We examine the signal processing methods and algorithms that make OCT exquisitely sensitive to reflections as weak as just a few photons and that reveal functional information in addition to structure. Image processing, display and interpretation, which are all critical for effective biomedical imaging, are discussed in the context of specific applications. Finally, we consider image artifacts and limitations that commonly arise and reflect on future advances and opportunities.

Optical Coherence Tomography for Ophthalmology Imaging

Optical coherence tomography (OCT) is a depth-resolved imaging modality, which is able to achieve micrometer-scale resolution within biological tissue noninvasively. In the past 30 years, researchers all around the world had made several essential efforts on techniques relevant to OCT. OCT has become a routine process for eye diseases with different types. In this chapter, the three important stages in the development of OCT are briefly illustrated, including the time domain OCT (TD-OCT), the frequency domain OCT (FD-OCT) and the optical coherence tomography angiography (OCTA). Each of the technique has made great progress for use on living human eye imaging in clinical applications. TD-OCT was first proposed and commercialized, which is able to achieve acceptable 2D depth-resolved cross-sectional images of human retina in vivo. FD-OCT was the upgraded OCT technique compared with TD-OCT. By capturing the coherent signal within the Fourier space, the FD-OCT could improve the image sensitivity compared with TD-OCT, and achieve dozens of kilo hertz imaging speed. OCTA is the newest developments of OCT technique, which is able to visualize the micro vasculature networks of human retina in vivo. With OCTA technique, the newest ophthalmologic OCT system is able to achieve detailed diagnosis for both micro-structure and vasculature abnormalities for clinical applications. The further development of OCT technique on imaging speed, contrast, resolution, field of view, and so on will make OCT to be a more powerful tool for clinical usages.

Low-aberration high-speed-compatible optical delay line

We describe a simple approach to dispersion-free optical delay line design that provides very low aberration over an extended delay range. In this approach, we minimize aberrations by directing non-axial beam displacements along a line of symmetry built into the apparatus. We show improved performance and significant reduction of wavefront aberrations by comparing simulation and experimental results with a similar delay line that lacks this line of symmetry. The new design facilitates transform-limited recovery of spectral resolution in Fourier transform coherent anti-Stokes Raman scattering, and accordingly, we demonstrate 3.5cm^-1 spectral resolution with a 10 ps delay scan range.

Detection and compensation of dispersion mismatch for frequency-domain optical coherence tomography based on A-scan's spectrogram

Balanced dispersion between reference and sample arms is critical in frequency-domain optical coherence tomography (FD-OCT) to perform imaging with the optimal axial resolution, and the spectroscopic analysis of each voxel in FD-OCT can provide the metric of the spectrogram. Here we revisited dispersion mismatch in the spectrogram view using the spectroscopic analysis of voxels in FD-OCT and uncovered that the dispersion mismatch disturbs the A-scan's spectrogram and reshapes the depth-resolved spectra in the spectrogram. Based on this spectroscopic effect of dispersion mismatch on A-scan's spectrogram, we proposed a numerical method to detect dispersion mismatch and perform dispersion compensation for FD-OCT. The proposed method can visually and quantitatively detect and compensate for dispersion mismatch in FD-OCT, with visualization, high sensitivity, and independence from sample structures. Experimental results of tape and mouse eye suggest that this technique can be an effective method for the detection and compensation of dispersion mismatch in FD-OCT.

Dispersion correction for optical coherence tomography by the stepped detection algorithm in the fractional Fourier domain

Dispersion in optical coherence tomography (OCT) leads to a series of wavelength-dependent phase distortions, which cause degradation of axial resolution. Due to the lack of prior information or the complexity of an exhaustive search calculation, all-depth dispersion suppression can hardly be realized in practical cases, especially for high-speed processing and irregular-structure samples. This paper explores the understanding of the depth-dependent dispersion in the fractional Fourier domain (FRFD) and addresses a new method for dispersion correction based on the FRFD stepped detection algorithm that is able to adaptively compensate the dispersion at all depths of the sample. For the detection of each dispersion component, a coarse search followed by a localized fine search is presented in our algorithm to reduce the calculation complexity with high accuracy guaranteed. A signal separation method utilizing FRFD filtering is also designed to avoid the interference between the dispersion from different depths of the sample, which allows all-depth dispersion correction. The proposed algorithm is verified to be effective through the stratified media of ZnSe. The application of the proposed algorithm in OCT imaging of onion and human coronary artery also demonstrates the feasibility of our algorithm for dispersion correction in bio-tissues.

Gabor domain optical coherence microscopy combined with laser scanning confocal fluorescence microscopy

We report on the development of fluorescence Gabor domain optical coherence microscopy (Fluo GD-OCM), a combination of GD-OCM with laser scanning confocal fluorescence microscopy (LSCFM) for synchronous micro-structural and fluorescence imaging. The dynamic focusing capability of GD-OCM provided the adaptive illumination environment for both modalities without any mechanical movement. Using Fluo GD-OCM, we imaged ex vivo DsRed-expressing cells in the brain of a transgenic mouse, as well as Cy3-labeled ganglion cells and Cy3-labeled astrocytes from a mouse retina. The self-registration of images taken by the two different imaging modalities showed the potential for a correlative study of subjects and double identification of the target.

Simple and robust calibration procedure for k-linearization and dispersion compensation in optical coherence tomography

In Fourier-domain optical coherence tomography (FD-OCT), proper signal sampling and dispersion compensation are essential steps to achieve optimal axial resolution. These calibration steps can be performed through numerical signal processing, but require calibration information about the system that may require lengthy and complex measurement protocols. We report a highly robust calibration procedure that can simultaneously determine correction vectors for nonlinear wavenumber sampling and dispersion compensation. The proposed method requires only two simple mirror measurements and no prior knowledge about the system's illumination source or detection scheme. This method applies to both spectral domain and swept-source OCT systems. Furthermore, it may be implemented as a low-cost fail-safe to validate the proper function of calibration hardware such as k-clocks. We demonstrate the method's simple implementation, effectiveness, and robustness on both types of OCT systems.

Comparison of tissue dispersion measurement techniques based on optical coherence tomography

The effects of dispersion on optical coherence tomography (OCT) images have long been documented. The imbalance of spectral broadening, caused by dispersion mismatches in the two arms of the OCT interferometer, can result in significant resolution degradation. Efforts to correct this phenomenon have resulted in improved image quality using various techniques. However, dispersion is also present and varies in tissues. As a result, group velocity dispersion (GVD) can be used to detect changes in tissues and provide useful information for diagnosis. Several methods can be utilized to measure the GVD from OCT images: (i) the degradation of the point spread function (PSF), (ii) the shift (walk-off) between images taken at different wavelengths, (iii) the changes in the second derivative of the spectral phase, as well as two new methods, which do not require a reflector and are applicable in intact tissues, i.e., using (iv) the speckle degradation, and (v) the speckle cross correlation. A systematic, experimental, evaluation of these methods is presented to elucidate the capabilities, the limitations, and the accuracy of each technique when attempting to estimate the GVD in scattering samples. The most precise values were obtained from the estimation of the PSF degradation, whereas using the phase derivative method was only applicable to minimally scattering samples. Speckle broadening appears to be the most robust method for tissue GVD measurements.

Phase-controlled Fourier-transform spectroscopy

Fourier-transform spectroscopy (FTS) has been widely used as a standard analytical technique over the past half-century. FTS is an autocorrelation-based technique that is compatible with both temporally coherent and incoherent light sources, and functions as an active or passive spectrometer. However, it has been mostly used for static measurements due to the low scan rate imposed by technological restrictions. This has impeded its application to continuous rapid measurements, which would be of significant interest for a variety of fields, especially when monitoring of non-repeating or transient complex dynamics is desirable. Here, we demonstrate highly efficient FTS operating at a high spectral acquisition rate with a simple delay line based on a dynamic phase-control technique. The independent adjustability of phase and group delays allows us to achieve the Nyquist-limited spectral acquisition rate over 10,000 spectra per second, while maintaining a large spectral bandwidth and high resolution. We also demonstrate passive spectroscopy with an incoherent light source.

Optical coherence tomography in Optics Express [Invited]

Optical coherence tomography (OCT) is one of the most successful technologies in the history of biomedical optics. Optics Express played an important role in communicating groundbreaking technological achievements in the field of OCT, and, conversely, OCT papers are among the most frequently cited papers published in Optics Express. On the occasion of the 20th anniversary of the journal, this review analyzes the reasons for the success of OCT papers in Optics Express and discusses possible motivations for researchers to submit some of their best OCT papers to the journal.

Low-coherence interferometry for phase-sensitive measurement of optical rotation

We present phase-sensitive measurement of optical rotation using spectral-domain and time-domain low-coherence interferometry. The method utilizes two decorrelated polarization states and simultaneous dual-channel detection provided by polarization-maintaining fiber-based implementation. The sample is placed between polarization optics to control and switch left- and right-handed circular states that experience the sample in forward and backward directions. Phase difference between two interferometric signals yields the optical rotation. Results from glucose and fructose samples are presented for validation.

Rapid scanning optical delay line based on a diffraction grating pair for a low-coherence reflectometer

We present a simple low-coherence time-domain interferometric reflectometer with a rapidly scanning optical delay line (RSODL) based on a non-parallel diffraction grating (DG) pair. The novelty of the solution is that the lightwave in the reference channel is focused on a galvo-mirror in a sub-mm static spot, which allows implementation of fast microelectromechanical systems scan optics. It is shown that the DG pair can be operated as a non-dispersive element that provides dynamic group delay of a reference lightwave. The DG pair system is also capable of tuning the RSODL dispersion from negative to positive values. The experimental depth range in air was obtained as large as 2.5 mm for axial resolution of 20 μm.

Determination on the Coefficient of Thermal Expansion in High-Power InGaN-based Light-emitting Diodes by Optical Coherence Tomography

The coefficient of thermal expansion (CTE) is a physical quantity that indicates the thermal expansion value of a material upon heating. For advanced thermal management, the accurate and immediate determination of the CTE of packaging materials is gaining importance because the demand for high-power lighting-emitting diodes (LEDs) is currently increasing. In this study, we used optical coherence tomography (OCT) to measure the CTE of an InGaN-based (λ = 450 nm) high-power LED encapsulated in polystyrene resin. The distances between individual interfaces of the OCT images were observed and recorded to derive the instantaneous CTE of the packaged LED under different injected currents. The LED junction temperature at different injected currents was established with the forward voltage method. Accordingly, the measured instantaneous CTE of polystyrene resin varied from 5.86 × 10⁻⁵ °C⁻¹ to 14.10 × 10⁻⁵ °C⁻¹ in the junction temperature range 25–225 °C and exhibited a uniform distribution in an OCT scanning area of 200 × 200 μm. Most importantly, this work validates the hypothesis that OCT can provide an alternative way to directly and nondestructively determine the spatially resolved CTE of the packaged LED device, which offers significant advantages over traditional CTE measurement techniques.

Compact flexible multi-pass rotary delay line using spinning micro-machined mirrors

We propose a new method to extend the path length tunability of rotary delay-lines. This method was shown to achieve a duty cycle of >80% and repetition rates of over 40 kHz. The new method relies on a new multi-segmented micro-machined mirror and serial injection of a single reflection onto separate segments of this mirror. The tunability is provided by the relative positioning of each reflective point on the mirror segments. There are two distinct modes of operation: synchronous and asynchronous. By simply manipulating the spatial position of the returning paths over the respective mirror segments, we can switch between increasing the repetition rate (asynchronous mode) or the total delay path (synchronous mode). We experimentally demonstrated up to 8 m/s scans with repetition rates of up to 42.7 kHz. Furthermore, we present numerical simulations of 18 reflection points to illustrate possibility of achieving a scan speed of up to 80 m/s. Through intermediate combinations of synchronous and asynchronous operation modes with 4 or more passes, we also show that the system can simultaneously increase both repetition rate and scan depth.

Using speckle to measure tissue dispersion in optical coherence tomography

Tissue dispersion could be used as a marker of early disease changes to further improve the diagnostic potential of optical coherence tomography (OCT). However, most methods to measure dispersion, described in the literature, rely on the presence of distinct and strong reflectors and are, therefore, rarely applicable in vivo. A novel technique has been developed which estimates the dispersion-induced resolution degradation from the image speckle and, as such, is applicable in situ. This method was verified experimentally ex vivo and was applied to the classification of a set of normal and cancerous colon OCT images resulting in 96% correct classification.

Intravascular optical coherence tomography [Invited]

Shortly after the first demonstration of optical coherence tomography for imaging the microstructure of the human eye, work began on developing systems and catheters suitable for intravascular imaging in order to diagnose and investigate atherosclerosis and potentially to monitor therapy. This review covers the driving considerations of the clinical application and its constraints, the major engineering milestones that enabled the current, high-performance commercial imaging systems, the key studies that laid the groundwork for image interpretation, and the clinical research that traces intravascular optical coherence tomography (OCT) from early human pilot studies to current clinical trials.

Stimulated Raman scattering microscopy and spectroscopy with a rapid scanning optical delay line

Stimulated Raman scattering (SRS) microscopy that is capable of both high-speed imaging and rapid spectroscopy will be advantageous for detailed chemical analysis of heterogeneous biological specimens. We have developed a system based on spectral focusing SRS technology with the integration of a rapid scanning optical delay line, which allows continuous tuning of SRS spectra by scanning a galvo mirror. We demonstrate SRS spectral measurements of dimethyl sulfoxide solution at low concentrations and multi-color imaging of rice pollens and HeLa cells with line-by-line delay tuning to reduce motion artifacts, as well as fast acquisition of SRS spectra at specific regions of interest.

High-speed OCT light sources and systems [Invited]

Imaging speed is one of the most important parameters that define the performance of optical coherence tomography (OCT) systems. During the last two decades, OCT speed has increased by over three orders of magnitude. New developments in wavelength-swept lasers have repeatedly been crucial for this development. In this review, we discuss the historical evolution and current state of the art of high-speed OCT systems, with focus on wavelength swept light sources and swept source OCT systems.

Two-dimensional phase unwrapping in Doppler Fourier domain optical coherence tomography

For phase-related imaging modalities using interferometric techniques, it is important to develop effective method to recover phase information that is mathematically wrapped. In this paper, we propose and demonstrate a two-dimensional (2D) method to achieve effective phase unwrapping in Doppler Fourier-domain (FD) optical coherence tomography (OCT), and recover the discontinuous phase distribution in retinal blood flow successfully for the first time in Doppler OCT studies. The proposed method is based on phase gradient approach in the axial dimension, with phase denoising performed through 2D window moving average in the sampled phase image using complex Doppler OCT data. The 2D unwrapping is carried out to correct phase discontinuities in the wrapped Doppler phase map, and the abrupt phase changes can be identified and corrected accurately. The proposed algorithm is computationally efficient and easy to be implemented.

Acousto-optic tunable filter for dispersion characterization of time-domain optical coherence tomography systems

A broadband supercontinuum light source with an acousto-optic tunable filter (AOTF) are used to characterize dispersion in two time-domain OCT systems, at 850 and 1300 nm. The filter is designed to sweep across two spectral ranges, which are restricted here from 800 to 900 nm and from 1200 to 1500 nm, respectively. Dispersion compensation for 850 nm was achieved with a spectral delay line. Dispersion compensation for 1300 nm was achieved using BK 7 rod glasses in the reference arm. The AOTF allows evaluation of dispersion in under as well as overcompensated systems. The AOTF method is based on wavelength dependence of the optical path difference corresponding to the maximum strength of the interference signal recorded using a mirror as object. Comparison is made between the AOTF method and the more usual method based on measurement of the full width at half-maximum of the autocorrelation peak. This comparison shows that the AOTF method is more accurate in terms of evaluation of the dispersion left uncompensated after each adjustment. The AOTF method additionally provides information on the direction of dispersion compensation.

Master/slave interferometry - ideal tool for coherence revival swept source optical coherence tomography

In this paper, we demonstrate that the master slave (MS) interferometry method can significantly simplify the practice of coherence revival swept source optical coherence tomography (OCT) technique. Previous implementations of the coherence revival technique required considerable resources on dispersion compensation and data resampling. The total tolerance of the MS method to nonlinear tuning, to dispersion in the interferometer and to dispersion due to the laser cavity, makes the MS ideally suited to the practice of coherence revival. In addition, enhanced versatility is allowed by the MS method in displaying shorter axial range images than that determined by the digital sampling of the data. This brings an immediate improvement in the speed of displaying cross-sectional images at high rates without the need of extra hardware such as graphics processing units or field programmable gate arrays. The long axial range of the coherence revival regime is proven with images of the anterior segment of healthy human volunteers.

Optical coherence tomography for embryonic imaging: a review

Embryogenesis is a highly complex and dynamic process, and its visualization is crucial for understanding basic physiological processes during development and for identifying and assessing possible defects, malformations, and diseases. While traditional imaging modalities, such as ultrasound biomicroscopy, micro-magnetic resonance imaging, and micro-computed tomography, have long been adapted for embryonic imaging, these techniques generally have limitations in their speed, spatial resolution, and contrast to capture processes such as cardiodynamics during embryogenesis. Optical coherence tomography (OCT) is a noninvasive imaging modality with micrometer-scale spatial resolution and imaging depth up to a few millimeters in tissue. OCT has bridged the gap between ultrahigh resolution imaging techniques with limited imaging depth like confocal microscopy and modalities, such as ultrasound sonography, which have deeper penetration but poorer spatial resolution. Moreover, the noninvasive nature of OCT has enabled live imaging of embryos without any external contrast agents. We review how OCT has been utilized to study developing embryos and also discuss advances in techniques used in conjunction with OCT to understand embryonic development.

Demonstration of tolerance to dispersion of master/slave interferometry

A theoretical model is developed for the Master/Slave interferometry (MSI) that is used to demonstrate its tolerance to dispersion left uncompensated in the interferometer when evaluating distances and thicknesses. In order to prove experimentally its tolerance to dispersion, different lengths of optical fiber are inserted into the interferometer to introduce dispersion. It is demonstrated that the sensitivity profile versus optical path difference is not affected by the length of fiber left uncompensated. It is also demonstrated that the axial resolution is constant within the axial range, close to the expected theoretical resolution determined by the optical source bandwidth. Then the thickness of a glass plate is measured several times in the presence of dispersion and errors in measurements are evaluated using the MSI method and the conventional Fourier transformation (FT) based method using linearized/calibrated data. The standard deviation for thickness results obtained with the MSI is more than 5 times smaller than the standard deviation for results delivered by the conventional, FT based method.

Optical coherence tomography system requirements for clinical diagnostic middle ear imaging

Noninvasive middle ear imaging using optical coherence tomography (OCT) presents some unique challenges for real-time, clinical use in humans. We present results from a two-dimensional/three-dimensional OCT system built to assess the imaging requirements of clinical middle ear imaging, and the technical challenges associated with them. These include the need to work at a low numerical aperture, the deleterious effects of transtympanic imaging on image quality at the ossicles, sensitivity requirements for clinical fidelity of images at real-time rates, and the high dynamic-range requirements of the ear. We validated the system by imaging cadaveric specimens with simulated disorders to show the clinical applicability of the images. We also provide additional insight into the likely role of OCT in clinical otology.

Adaptive optics SLO/OCT for 3D imaging of human photoreceptors in vivo

We present a new instrument that is capable of imaging human photoreceptors in three dimensions. To achieve high lateral resolution, the system incorporates an adaptive optics system. The high axial resolution is achieved through the implementation of optical coherence tomography (OCT). The instrument records simultaneously both, scanning laser ophthalmoscope (SLO) and OCT en-face images, with a pixel to pixel correspondence. The information provided by the SLO is used to correct for transverse eye motion in post-processing. In order to correct for axial eye motion, the instrument is equipped with a high speed axial eye tracker. In vivo images of foveal cones as well as images recorded at an eccentricity from the fovea showing cones and rods are presented.

Dispersion compensation in Fourier domain optical coherence tomography using the fractional Fourier transform

We address numerical dispersion compensation based on the use of the fractional Fourier transform (FrFT). The FrFT provides a new fundamental perspective on the nature and role of group-velocity dispersion in Fourier domain OCT. The dispersion induced by a 26 mm long water cell was compensated for a spectral bandwidth of 110 nm, allowing the theoretical axial resolution in air of 3.6 μm to be recovered from the dispersion degraded point spread function. Additionally, we present a new approach for depth dependent dispersion compensation based on numerical simulations. Finally, we show how the optimized fractional Fourier transform order parameter can be used to extract the group velocity dispersion coefficient of a material.

Full-range swept source optical coherence tomography based on carrier frequency by transmissive dispersive optical delay line

A high speed swept source optical coherence tomography (SS-OCT) system capable of full-range imaging is presented. Wave-number carrier frequency is introduced into the spectral interference signal by a transmissive dispersive optical delay line (TDODL). High carrier frequency in the spectral interference signal corresponding to an equivalent distance-shift is exploited to obtain full-range OCT imaging. Theoretical development is conducted with the instantaneous coherence function introduced for a complete description of a spectral interference signal. Performance advantage of the TDODL-based method over the conventional approach where only one side (positive or negative path length difference) is used for imaging to avoid overlaying mirror artifacts is confirmed by the measured envelopes of spectral interference signal. Feasibility of the proposed method for full-range imaging is validated in a custom-built SS-OCT system by in vivo imaging of a biological sample.

Large tunable fractional delay of slow light pulse and its application to fast optical correlator

The slow light device based on photonic crystal coupled waveguide was fabricated, and a tunable delay of 72 ps was obtained for 2-ps-wide slow light pulses by local heating, which corresponds to a tunable fractional delay of 36. This value was further enhanced to 110 by compressing the output pulses through self-phase modulation and dispersion compensation in external fibers. We applied this device to optical correlator as a delay scanner, where the fractional delay determines the resolution of the delay scanning. Using this correlator, we successfully observed sub-picosecond pulses at a scan frequency up to 2 kHz, which is 100 times faster than that of mechanical scanners in conventional correlators.

True logarithmic amplification of frequency clock in SS-OCT for calibration

With swept source optical coherence tomography (SS-OCT), imprecise signal calibration prevents optimal imaging of biological tissues such as coronary artery. This work demonstrates an approach using a true logarithmic amplifier to precondition the clock signal, with the effort to minimize the noises and phase errors for optimal calibration. This method was validated and tested with a high-speed SS-OCT. The experimental results manifest its superior ability on optimization of the calibration and improvement of the imaging performance. Particularly, this hardware-based approach is suitable for real-time calibration in a high-speed system where computation time is constrained.

Complete complex conjugate resolved heterodyne swept-source optical coherence tomography using a dispersive optical delay line

Swept-source optical coherence tomography (SSOCT) provides a substantial sensitivity advantage over its time-domain counterpart, but suffers from a reduced imaging depth range due to sensitivity falloff and complex conjugate ambiguity. Heterodyne complex conjugate-resolved SSOCT (HCCR-SSOCT) has been previously demonstrated as a technique to completely resolve the complex conjugate ambiguity, effectively doubling the falloff limited imaging depth, without the reduction in imaging speed associated with other CCR techniques. However, previous implementations of this technique have employed expensive and lossy optical modulators to provide the required differential phase modulation. In this paper, we demonstrate the use of a dispersive optical delay line (D-ODL) as the reference arm of an OCT system to realize HCCR-SSOCT. This technique maintains the existing advantages of HCCR-SSOCT in that it completely resolves the complex conjugate artifact and does not reduce imaging speed, while conferring the additional advantages of being low cost, maintaining system sensitivity and resolution, not requiring any additional signal processing, and working at all wavelengths and imaging speeds. The D-ODL also allows for hardware correction of unbalanced dispersion in the reference and sample arm, adding further flexibility to system design. We demonstrate the technique using an SSOCT system operating at 100kHz with a central wavelength of 1040nm. Falloff measurements performed using a standard OCT configuration and the proposed D-ODL demonstrate a doubling of the effective imaging range with no sensitivity or resolution penalty. Feasibility of the technique for in vivo imaging was demonstrated by imaging the ocular anterior segments of healthy human volunteers.

Dispersion management for controlling image plane in Fourier-domain spectrally encoded endoscopy

Spectrally encoded endoscopy (SEE) uses single optical fiber and miniature diffractive optics to allow imaging through a miniature probe. Utilizing Fourier-domain interferometry, SEE was shown capable of video-rate three-dimensional imaging, albeit at limited depth of field due to the limited spectral resolution of the detection spectrometer. We show that by using dispersion management at the reference arm of the interferometer, the tilt and curvature of the field of view could be adjusted without modifying the endoscopic probe itself. By controlling the group velocity dispersion, this technique is demonstrated useful for imaging specimen regions which reside outside the system's depth of field. This approach could be used to improve usability, functionality and image quality of SEE without affecting probe size and flexibility.

Electronically controlled agile lens-based broadband variable photonic delay line for photonic and radio frequency signal processing

To the best of our knowledge, proposed for the first time is the design of an optically broadband variable photonic delay line (VPDL) using an electronically controlled variable focus lens (ECVFL), mirror motion, and beam-conditioned free-space laser beam propagation. This loss-minimized fiber-coupled VPDL design using micro-optic components has the ability to simultaneously provide optical attenuation controls and analog-mode high-resolution (subpicoseconds) continuous delays over a moderate (e.g., <5 ns) range of time delays. An example VPDL design using a liquid-based ECVFL demonstrates up to a 1 ns time-delay range with >10 dB optical attenuation controls. The proposed VPDL is deployed to demonstrate a two-tap RF notch filter with tuned notches at 854.04 and 855.19 MHz with 22.6 dB notch depth control via VPDL attenuation control operations. The proposed VPDL is useful in signal conditioning applications requiring fiber-coupled broadband light time delay and attenuation controls.

Miniature 10 kHz thermo-optic delay line in silicon

The scanning delay line is a key component of time-domain optical coherence tomography systems. It has evolved since its inception toward higher scan rates and simpler implementation. However, existing approaches still suffer from drawbacks in terms of size, cost, and complexity, and they are not suitable for implementation using integrated optics. In this Letter, we report a rapid scanning delay line based on the thermo-optic effect of silicon at λ = 1.3 μm manufactured around a generic planar lightwave circuit technology. The reported device attained line scan rates of 10 kHz and demonstrated a scan range of 0.95 mm without suffering any observable loss of resolution (15 µm FWHM) owing to depth-dependent chromatic dispersion.

Time-domain Optical Coherence Tomography system with integrated delay line for surgical guidance applications

Optical Coherence Tomography is a high resolution imaging technique able to provide in-depth information about living tissue. Among all its applications, it can be argued that surgical guidance is one of the most demanding in terms of system reliability, footprint and cost. In order to enable faster adoption of this technology in that field, we had already developed and demonstrated a new type of scanning delay line based on the thermo-optic effect of silicon. By changing the temperature of an integrated waveguide, we are able to produce a variation in optical delay. In this paper, we demonstrate the inclusion of such a device in a complete system and the performance levels that can be achieved with this technique. In particular, we show a line scan rate of 2kHz with good linearity and a scan range of 0.95mm in air. These values meet the needs of most surgical guidance applications.

Miniaturization of free space optical systems

Coherent illumination enables not only integrated optics, but also miniaturized free-space optics that takes advantage of the amplitude and phase control afforded by optical microelectromechanical systems (MEMS) and photonic crystals. These technologies also provide a practical and cost-effective means for integration and packaging of optical systems. The properties of miniaturized optical systems based on optical MEMS and photonic crystals are described, and efficient analysis and design approaches to miniaturized optical scanners and tunable diffractive optical elements are demonstrated. The impact of photonic crystals on free-space micro-optics is discussed.

Quantitative FRET measurement by high-speed fluorescence excitation and emission spectrometer

Förster resonance energy transfer (FRET) is an important method in studying biochemistry reactions. But quantifying FRET rapidly is difficult to do because of crosstalk between free donor, free acceptor and FRET fluorescent signals when only excitation or emission property of a FRET sample is measured. If FRET is studied with excitation-emission matrix (EEM) measurements, because the fluorescence intensity maxima of donor, acceptor, and FRET emissions occupy different regions within the EEM, FRET fluorescence can be easily separated out by linear unmixing. In this paper, we report a novel high-speed Fourier Fluorescence Excitation Emission spectrometer, which simultaneously measures three projections of EEM from a FRET sample, which are excitation, emission and excitation-emission cross-correlation spectra. We demonstrate that these three EEM projections can be measured and unmixed in approximately 1 ms to provide rapid quantitative FRET in the presence of free donors and acceptors. The system can be utilized to enable real-time biochemistry reaction studies.

Robust High-Resolution Fine OCT Needle for Side-Viewing Interstitial Tissue Imaging

Fine optical coherence tomography (OCT) imaging needles that can be integrated with a standard biopsy needle have been developed with a new optics design to improve the optical quality and mechanical robustness, where a fiber-optic lens (that is spliced to a single-mode fiber) and a microreflector are encased within a microglass tube. The design also minimizes the cylindrical lens effect induced by the glass tube and eases the needle assembly process. Real-time cross-sectional OCT imaging of various tissue samples were performed ex vivo using the miniature-imaging needle along with a 1300-nm swept-source OCT system. The preliminary results demonstrate the improved mechanical and optical performance and suggest the potential of the fine OCT needle for minimally invasive interstitial imaging and image-guided biopsy.

In vivo investigation of human cone photoreceptors with SLO/OCT in combination with 3D motion correction on a cellular level

We present a further improvement of our SLO/OCT imaging system which enables to practically eliminate all eye motion artifacts with a correction accuracy approaching sub-cellular dimensions. Axial eye tracking is achieved by using a hardware based, high speed tracking system that consists of a rapid scanning optical delay line in the reference arm of the interferometer. A software based algorithm is employed to correct for transverse eye motion in a post-processing step. The instrument operates at a frame rate of 40 en-face fps with a field of view of approximately 1 degrees x 1 degrees. Dynamic focusing enables the recording of 3D volumes of the human retina with cellular resolution throughout the entire imaging depth. Several volumes are stitched together to increase the total field of view. Different features of the three dimensional structure of cone photoreceptors are investigated in detail and at different eccentricities from the fovea.

High-speed optical coherence tomography: basics and applications

In the past decade we have observed a rapid development of ultrahigh-speed optical coherence tomography (OCT) instruments, which currently enable performing cross-sectional in vivo imaging of biological samples with speeds of more than 100,000 A-scans/s. This progress in OCT technology has been achieved by the development of Fourier-domain detection techniques. Introduction of high-speed imaging capabilities lifts the primary limitation of early OCT technology by giving access to in vivo three-dimensional volumetric reconstructions on large scales within reasonable time constraints. As result, novel tools can be created that add new perspective for existing OCT applications and open new fields of research in biomedical imaging. Especially promising is the capability of performing functional imaging, which shows a potential to enable the differentiation of tissue pathologies via metabolic properties or functional responses. In this contribution the fundamental limitations and advantages of time-domain and Fourier-domain interferometric detection methods are discussed. Additionally the progress of high-speed OCT instruments and their impact on imaging applications is reviewed. Finally new perspectives on functional imaging with the use of state-of-the-art high-speed OCT technology are demonstrated.

High-velocity-flow imaging with real-time Doppler optical coherence tomography

We present a real-time time-domain Doppler optical coherence tomography (OCT) system based on the zero-crossing method for velocity measurements of fluid flows with attainable velocities up to 10 m/s. In the current implementation, one-dimensional and two-dimensional velocity profiles of fluid flows ranging from 1 cm/s to more than 3 m/s were obtained for both laminar and turbulent flows. The line rate was approximately 500 Hz, and the images were treated in real time. This approach has the advantage of providing reliable velocity maps free from phase aliasing or other artifacts common to several OCT systems. The system is particularly well suited for investigating complex velocity profiles, especially in the presence of steep velocity gradients.

Birefringence measurement in polarization-sensitive optical coherence tomography using differential-envelope detection method

In this research, we integrated two demodulating logarithmic amplifiers with one differential amplifier for use in a Mach-Zehnder interferometer so as to obtain a two-channel polarization-sensitive optical coherence tomography system. Birefringence signals can be acquired using this system along with a differential-envelope detection method. Because the two orthogonal polarizations are common-path propagation, common noise originating from background fluctuations or multiple scattering in turbid media can be reduced to improve the detection sensitivity and accuracy of birefringence measurement. Besides, this simple and effective technique is an analog detection method and is capable of providing high temporal response; it can also help obtain a high time-bandwidth product as compared to the conventional method of using a numerical method with a limited sampling rate. The feasibility of the proposed system is supported by theory and is also shown by performing experiments involving a human vessel, which is a highly scattering medium with weak birefringence.

Imaging port wine stains by fiber optical coherence tomography

We develop a fiber optical coherence tomography (OCT) system in the clinical utility of imaging port wine stains (PWS). We use our OCT system on 41 patients with PWS to document the difference between PWS skin and contralateral normal skin. The system, which operates at 4 frames with axial and transverse resolutions of 10 and 9 mum, respectively, in the skin tissue, can clearly distinguish the dilated dermal blood vessels from normal tissue. We present OCT images of patients with PWS and normal human skin. We obtain the structural parameters, including epidermal thickness and diameter and depth of dilated blood vessels. We demonstrate that OCT may be a useful tool for the noninvasive imaging of PWS. It may help determine the photosensitizer dose and laser parameters in photodynamic therapy for treating port wine stains.

High speed optical coherence microscopy with autofocus adjustment and a miniaturized endoscopic imaging probe

Optical coherence microscopy (OCM) is a promising technique for high resolution cellular imaging in human tissues. An OCM system for high-speed en face cellular resolution imaging was developed at 1060 nm wavelength at frame rates up to 5 Hz with resolutions of < 4 microm axial and < 2 microm transverse. The system utilized a novel polarization compensation method to combat wavelength dependent source polarization and achieve broadband electro-optic phase modulation compatible with ultrahigh axial resolution. In addition, the system incorporated an auto-focusing feature that enables precise, near real-time alignment of the confocal and coherence gates in tissue, allowing user-friendly optimization of image quality during the imaging procedure. Ex vivo cellular images of human esophagus, colon, and cervix as well as in vivo results from human skin are presented. Finally, the system design is demonstrated with a miniaturized piezoelectric fiber-scanning probe which can be adapted for laparoscopic and endoscopic imaging applications.

Assessment of artifacts and reproducibility across spectral- and time-domain optical coherence tomography devices

PURPOSE

To report the frequency of optical coherence tomography (OCT) scan artifacts and to compare macular thickness measurements, interscan reproducibility, and interdevice agreeability across 3 spectral-domain (SD) OCT (also known as Fourier domain; Cirrus HD-OCT, RTVue-100, and Topcon 3D-OCT 1000) devices and 1 time-domain (TD) OCT (Stratus OCT) device.

DESIGN

Prospective, noncomparative, noninterventional case series.

PARTICIPANTS

Fifty-two patients seen at the New England Eye Center, Tufts Medical Center Retina Service, between February and August 2008.

METHODS

Two scans were performed for each of the SD OCT protocols: Cirrus macular cube 512 x 128 (software version 3.0; Carl Zeiss Meditec, Inc., Dublin, CA), RTVue (E)MM5 and MM6 (software version 3.5; Optovue, Inc., Fremont, CA), Topcon 3D Macular and Radial (software version 2.12; Topcon, Inc., Paramus, NJ), in addition to 1 TD OCT scan via Stratus macular thickness protocol (software version 4.0; Carl Zeiss Meditec, Inc.). Scans were inspected for 6 types of OCT scan artifacts and were analyzed. Interscan reproducibility and interdevice agreeability were assessed by intraclass correlation coefficients (ICCs) and Bland-Altman plots, respectively.

MAIN OUTCOME MEASURES

Optical coherence tomography image artifacts, macular thickness, reproducibility, and agreeability.

RESULTS

Time-domain OCT scans contained a significantly higher percentage of clinically significant improper central foveal thickness (IFT) after manual correction (11-mum change or more) compared with SD OCT scans. Cirrus HD-OCT had a significantly lower percentage of clinically significant IFT (11.1%) compared with the other SD OCT devices (Topcon 3D, 20.4%; Topcon Radial, 29.6%; RTVue (E)MM5, 42.6%; RTVue MM6, 24.1%; P = 0.001). All 3 SD OCT devices had central foveal subfield thicknesses that were significantly more than that of TD OCT after manual correction (P<0.0001). All 3 SD OCT devices demonstrated a high degree of reproducibility in the central foveal region (ICCs, 0.92-0.97). Bland-Altman plots showed low agreeability between TD and SD OCT scans.

CONCLUSIONS

Out of all OCT devices analyzed, cirrus HD-OCT scans exhibited the lowest occurrence of any artifacts (68.5%), IFT (40.7%), and clinically significant IFT (11.1%), whereas Stratus OCT scans exhibited the highest occurrence of clinically significant IFT. Further work on improving segmentation algorithm to decrease artifacts is warranted.