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

A Novel Mach-Zehnder Interferometer Using Eccentric-Core Fiber Design for Optical Coherence Tomography

A novel Mach-Zehnder interferometer using eccentric-core fiber (ECF) design for optical coherence tomography (OCT) is proposed and demonstrated. Instead of the commercial single-mode fiber (SMF), the ECF is used as one interference arm of the implementation. Because of the offset location of the eccentric core, it is sensitive to directional bending and the optical path difference (OPD) of two interference arms can be adjusted with high precision. The birefringence of ECF is calculated and experimentally measured, which demonstrates the polarization sensitivity of the ECF proposed in the paper is similar to that of SMF. Such a structure can replace the reference optical delay line to form an all-fiber passive device. A mirror is used as a sample for analyzing the ECF bending responses of the system. Besides, four pieces of overlapping glass slides as sample are experimentally measured as well.

Single-camera full-range high-resolution spectral domain optical coherence tomography

We developed spectral domain optical coherence tomography using a dual-channel spectrometer for complex conjugate artifacts (CCA) suppression. We used a three-line charge coupled device to simultaneously detect two interferometric spectra with 2π/3 phase difference. The complex interferometric signal was reconstructed by trigonometric manipulation of two real interferometric spectra, and then full-range images were obtained by use of inverse Fourier transform. Artifacts at direct current (DC) and the ghost remnant of the CCA are common issues with the previously reported two-spectrometer method because the slight mismatching between two spectral detection channels had strong negative effects on CCA suppression and appeared to be the limiting factor on system performance. This novel dual-channel spectrometer uses the same spectrometer optics for the two spectral detection channels and, therefore, achieves better matching between two spectral detection channels and consequently better performance in CCA suppression as compared with the dual spectrometer solution. Full-range imaging with CCA suppression up to ∼25  dB was demonstrated when imaging an attenuated reflector. The efficacy of both CCA and DC suppressions also was validated by imaging the anterior segment of a rat eye ex vivo. The quality of CCA-suppressed images was significantly improved with regard to those obtained with the dual-spectrometer design.

Quantitative phase microscopy with off-axis optical coherence tomography

We have developed a modality for quantitative phase imaging within spectral domain optical coherence tomography based on using an off-axis reference beam. By tilting the propagation of the reference beam relative to that of the sample beam, a spatially varying fringe is generated. Upon detection of this fringe using a parallel spectral domain scheme, the fringe can be used to separate the interference component of the signal and obtain the complex sample field. In addition to providing quantitative phase measurements within a depth resolved measurement, this approach also allows elimination of the complex conjugate artifact, a known limitation of spectral interferometry. The principle of the approach is described here along with demonstration of its capabilities using technical samples.

Quantitative upper airway endoscopy with swept-source anatomical optical coherence tomography

Minimally invasive imaging of upper airway obstructions in children and adults is needed to improve clinical decision-making. Toward this goal, we demonstrate an anatomical optical coherence tomography (aOCT) system delivered via a small-bore, flexible endoscope to quantify the upper airway lumen geometry. Helical scans were obtained from a proximally-scanned fiber-optic catheter of 820 μm outer diameter and >2 mm focal length. Coupled with a long coherence length wavelength-swept light source, the system exhibited an SNR roll-off of < 10 dB over a 10 mm range. Operating at 10 rotations/s, the average accuracy of segmented cross-sectional areas was found to be -1.4 ± 1.0%. To demonstrate the capability of this system, aOCT was performed on a pediatric airway phantom and on ex vivo swine trachea. The ability for quantitative endoscopy afforded by this system can aid in diagnosis, medical and surgical decision making, and predictive modeling of upper airway obstructive disorders.

Two-reference swept-source optical coherence tomography of high operation flexibility

The significantly less stringent operation of a two-reference swept-source optical coherence tomography (OCT) system for suppressing the mirror image is demonstrated based on the spatially localized image processing method. With this method, the phase difference between the two reference signals is not limited to 90 degrees. Based on the current experimental operation, the mirror image can be effectively suppressed as long as the phase difference is larger than 20 degrees. In other words, the adjustment of the beam splitter orientation for controlling the phase difference becomes much more flexible. Also, based on a phantom experiment, the combination the spatially localized mirror image suppression method with the two-reference OCT operation leads to the implementation of full-range optical Doppler tomography.

Computation time-saving mirror image suppression method in Fourier-domain optical coherence tomography

The theory and experimental results of a computation time-saving mirror image suppression method in Fourier-domain optical coherence tomography, which utilizes the property of reversed system phase shift between the real and mirror images, for differentiating one from the other are demonstrated. By solving a set of two equations based on a reasonable approximation, the real image signal can be obtained. The theoretical backgrounds and the improved real image quality of the average and iteration procedures in this method are particularly illustrated. Also, the mirror image suppression ratios under various process conditions, including different process iteration numbers and different system phase shifts between two neighboring A-mode scans, are evaluated. Meanwhile, the mirror image suppression results based on our method are compared with those obtained from the widely used BM-scan technique. It is found that when a process procedure of two iterations is used, the mirror image suppression quality based on our method can be higher than that obtained from the BM-scan technique. The computation time of our method is significantly shorter than that of the BM-scan technique.

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

We correct an error in our previous paper [Biomed. Opt. Express 2, 1218 (2011)] which led to an erroneous conclusion that a dispersive optical delay line (DODL) used in a swept source optical coherence tomography (SSOCT) system generated a pure phase modulation allowing for complex conjugate artifact removal in Fourier domain OCT via optical heterodyning. We now understand that an alternate phenomenon known as coherence revival was responsible for the observed phase modulation, while the DODL provided a compact means of generating a large group delay with readily adjustable group velocity dispersion compensation.

Complex conjugate resolved heterodyne swept source optical coherence tomography using coherence revival

We describe a simple and low-cost technique for resolving the complex conjugate ambiguity in Fourier domain optical coherence tomography (OCT) that is applicable to many swept source OCT (SSOCT) systems. First, we review the principles of coherence revival, wherein an interferometer illuminated by an external cavity tunable laser (ECTL) exhibits interference fringes when the two arms of the interferometer are mismatched by an integer multiple of the laser cavity length. Second, we report observations that the spectral interferogram obtained from SSOCT systems employing certain ECTLs are automatically phase modulated when the arm lengths are mismatched this way. This phase modulation results in a frequency-shifted interferogram, effectively creating an extended-depth heterodyne SSOCT system without the use of acousto-optic or electro-optic modulators. We suggest that this phase modulation may be caused by the ECTL cavity optical pathlength varying slightly over the laser sweep, and support this hypothesis with numerical simulations. We also report on the successful implementation of this technique with two commercial swept source lasers operating at 840nm and 1040nm, with sweep rates of 8kHz and 100kHz respectively. The extended imaging depth afforded by this technique was demonstrated by measuring the sensitivity fall-off profiles of each laser with matched and mismatched interferometer arms. The feasibility of this technique for clinical systems is demonstrated by imaging the ocular anterior segments of healthy human volunteers.

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.

Method for suppressing the mirror image in Fourier-domain optical coherence tomography

A method, novel to our knowledge, for effective mirror image suppression in Fourier-domain optical coherence tomography based on a phase shift between neighboring A-mode scans is demonstrated. By realizing that the phase shifts of the real and mirror images are mutually reversed and assuming that the real image intensities of the two successive A-mode scans are the same, we can solve a set of two coupled equations to obtain the real image signals. The images based on the scanning of a high-resolution spectral-domain optical coherence tomography system are processed to show effective mirror image suppression results. Compared with a similar method of broad application, our approach has the advantages of shorter process time and higher flexibility in selecting the concerned image portions for processing.