Swept Laser Source Research Articles

Optical-domain subsampling for data efficient depth ranging in Fourier-domain optical coherence tomography

Recent advances in optical coherence tomography (OCT) have led to higher-speed sources that support imaging over longer depth ranges. Limitations in the bandwidth of state-of-the-art acquisition electronics, however, prevent adoption of these advances into the clinical applications. Here, we introduce optical-domain subsampling as a method for imaging at high-speeds and over extended depth ranges but with a lower acquisition bandwidth than that required using conventional approaches. Optically subsampled laser sources utilize a discrete set of wavelengths to alias fringe signals along an extended depth range into a bandwidth limited frequency window. By detecting the complex fringe signals and under the assumption of a depth-constrained signal, optical-domain subsampling enables recovery of the depth-resolved scattering signal without overlapping artifacts from this bandwidth-limited window. We highlight key principles behind optical-domain subsampled imaging, and demonstrate this principle experimentally using a polygon-filter based swept-source laser that includes an intra-cavity Fabry-Perot (FP) etalon.

Dual-Band Time-Multiplexing Swept-Source Optical Coherence Tomography Based on Optical Parametric Amplification

We report a high-speed time-multiplexing dual wavelength band swept laser source based on an optical parametric amplifier. A dual-band swept-source optical coherence tomography (OCT) system is implemented to demonstrate the advantage of a second wavelength band for fast spectroscopic OCT (SOCT). The innovative time-multiplexing architecture greatly reduces the complexity of the coupling and detecting configuration in comparison with the previous dual-band swept-source setup. We demonstrate the optical parametric amplification’s characteristics as a dual-band generator and applied the source to firstly achieve the SOCT around 1550 nm.

Real-time speckle variance swept-source optical coherence tomography using a graphics processing unit

: Advances in swept source laser technology continues to increase the imaging speed of swept-source optical coherence tomography (SS-OCT) systems. These fast imaging speeds are ideal for microvascular detection schemes, such as speckle variance (SV), where interframe motion can cause severe imaging artifacts and loss of vascular contrast. However, full utilization of the laser scan speed has been hindered by the computationally intensive signal processing required by SS-OCT and SV calculations. Using a commercial graphics processing unit that has been optimized for parallel data processing, we report a complete high-speed SS-OCT platform capable of real-time data acquisition, processing, display, and saving at 108,000 lines per second. Subpixel image registration of structural images was performed in real-time prior to SV calculations in order to reduce decorrelation from stationary structures induced by the bulk tissue motion. The viability of the system was successfully demonstrated in a high bulk tissue motion scenario of human fingernail root imaging where SV images (512 × 512 pixels, n = 4) were displayed at 54 frames per second.

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.

Differential phase-contrast, swept-source optical coherence tomography at 1060 nm for in vivo human retinal and choroidal vasculature visualization

Human retinal and choroidal vasculature was visualized by a differential phase-contrast (DPC) method using high-speed, swept-source optical coherence tomography (SS-OCT) at 1060 nm. The vasculature was recognized as regions of motion by creating differential phase-variance (DPV) tomograms: multiple B-scans of individual slices through the retina were collected and the variance of the phase differences was calculated. DPV captured the small vessels and the meshwork of capillaries associated with the inner retina in en-face images over 4 mm(2). The swept-source laser at 1060 nm offered the needed phase sensitivity to perform DPV and generated en-face images that capture motion in the inner choroidal layer exceeding the capabilities of previous spectrometer-based instruments. In comparison with the power Doppler phase-shift method, DPV provided better visualization of the foveal avascular zone in en-face images.

High Speed MEMS Scanner Based Swept Source Laser for SS-OCT

High Speed MEMS Scanner Based Swept Source Laser for SS-OCT

Single mode fiber based polarization sensitive optical coherence tomography using a sweptlaser source

We report the development of a near real time single mode fiber-optic based polarizationsensitive optical coherence tomography (PSOCT) setup utilizing a swept source. The setuphas a polarization modulator (PM) and polarizer in a sample arm to serve as a polarizationstate generator and analyzer. Since only intensity information is used to retrieve thepolarization parameters of the sample, the problem of phase jitters present in the sweptsource is avoided. Further, the use of polarization elements after the sample armfiber makes the approach insensitive to the random birefringence of fibers. Theswept source based PSOCT (SS-PSOCT) setup was calibrated by carrying outmeasurements on a quarter-wave plate whose fast axis orientation was rotated from0° to360° in stepsof 10°. The setup has been used for imaging birefringence of mice tail tendon, thigh muscle exvivo and human nail fold in vivo.

Phase-stabilized optical frequency domain imaging at 1-µm for the measurement of blood flow in the human choroid

In optical frequency domain imaging (OFDI) the measurement of interference fringes is not exactly reproducible due to small instabilities in the swept-source laser, the interferometer and the data-acquisition hardware. The resulting variation in wavenumber sampling makes phase-resolved detection and the removal of fixed-pattern noise challenging in OFDI. In this paper this problem is solved by a new post-processing method in which interference fringes are resampled to the exact same wavenumber space using a simultaneously recorded calibration signal. This method is implemented in a high-speed (100 kHz) high-resolution (6.5 µm) OFDI system at 1-µm and is used for the removal of fixed-pattern noise artifacts and for phase-resolved blood flow measurements in the human choroid. The system performed close to the shot-noise limit (<1dB) with a sensitivity of 99.1 dB for a 1.7 mW sample arm power. Suppression of fixed-pattern noise artifacts is shown up to 39.0 dB which effectively removes all artifacts from the OFDI-images. The clinical potential of the system is shown by the detection of choroidal blood flow in a healthy volunteer and the detection of tissue reperfusion in a patient after a retinal pigment epithelium and choroid transplantation.

Dual-core ytterbium fiber amplifier for high-power 1060 nm swept source multichannel optical coherence tomography imaging

A novel (to our knowledge) dual-core ytterbium (Yb(3+)) doped fiber, as an optically pumped amplifier, boosts the output power from a 1060 nm swept source laser beyond 250 mW, while providing a wavelength tuning range of 93 nm, for optical coherence tomography (OCT) imaging. The design of the dual-core Yb-doped fiber amplifier and its multiple wavelength optical pumping scheme to optimize output bandwidth are discussed. Use of the dual-core fiber amplifier showed no appreciable degradation to the coherence length of the seed laser. The signal intensity improvement of this amplifier is demonstrated on a multichannel in vivo OCT imaging system at 1060 nm.

Swept-Source Laser Based on Quantum-Dot Semiconductor Optical Amplifier—Applications in Optical Coherence Tomography

We present a swept-source laser based on a quantum-dot semiconductor optical amplifier (QD SOA) (designed and manufactured by Innolume Gmbh, Germany) as the gain medium. The properties of the laser are as follows: center wavelength is 1220 nm, sweep range is 94 nm (up to 120 nm in manual mode), the peak laser power is 16 mW, and the sweep speed is 100 Hz. We apply the laser in an optical coherence tomography (OCT) configuration and present images of in vivo skin. QD SOA are well suited to OCT and we show the setup can be improved for power, sweep range, and sweep speed.

Birefringence measurement of the retinal nerve fiber layer by swept source polarization sensitive optical coherence tomography

A Swept Source Polarization-Sensitive Optical Coherence Tomography (SS-PS-OCT) instrument has been designed, constructed, and verified to provide high sensitivity depth-resolved birefringence and phase retardation measurements of the retinal nerve fiber layer. The swept-source laser had a center wavelength of 1059 nm, a full-width-half-max spectral bandwidth of 58 nm and an A-line scan rate of 34 KHz. Power incident on the cornea was 440 µW and measured axial resolution was 17 µm in air. A multiple polarization state nonlinear fitting algorithm was used to measure retinal birefringence with low uncertainty. Maps of RNFL phase retardation in a subject measured with SS-PS-OCT compare well with those generated using a commercial scanning laser polarimetry instrument. Peak-to-valley variation of RNFL birefringence given here is less than values previously reported at 840nm.

FDML 방식 스위핑 광원을 사용한 SS-OCT 구현

In this paper, the swept source-optical coherence tomography system using frequency domain mode locked(FDML) laser has realized. The FDML swept source laser showed 55.03 kHz sweeping speed, 125 nm sweeping range, and 9 mW output optical power, which are the superiority of FDML laser compared to previous swept source lasers. Also, through the cross-sectional image captured at 5 frames per second for a mirror, a 1 mm-thickness glass plate, and a thumb bottom, the performance of the system has demonstrated.

Quantification of airway thickness changes in smoke-inhalation injury using in-vivo 3-D endoscopic frequency-domain optical coherence tomography

Smoke inhalation injury is frequently accompanied by cyanide poisoning that may result in substantial morbidity and mortality, and methods are needed to quantitatively determine extent of airway injury. We utilized a 3-D endoscopic frequency-domain optical coherence tomography (FD-OCT) constructed with a swept-source laser to investigate morphological airway changes following smoke and cyanide exposure in rabbits. The thickness of the mucosal area between the epithelium and cartilage in the airway was measured and quantified. 3-D endoscopic FD-OCT was able to detect significant increases in the thickness of the tracheal walls of the rabbit beginning almost immediately after smoke inhalation injuries which were similar to those with combined smoke and cyanide poisoning.

A NOVEL SWEPT LASER SOURCE BASED ON COMBINED TUNABLE FILTERS FOR OCT

A novel broad tunable bandwidth and narrow instantaneous line-width linear swept laser source using combined tunable filters working at 1,300 nm center wavelength is proposed. The combined filters consist of a fiber Fabry–Perot tunable filter and a tunable filter based on diffractive grating with scanning polygon mirror. In contrast to traditional method using single tunable filter, the trade-off between bandwidth and instantaneous line-width is alleviated. Parallel implementation of two semiconductor optical amplifiers with different wavelength range is adopted in the laser resonator for broadband light amplification. The Fourier domain mode locking swept laser source with combined tunable filters offers broadband tunable range with narrow instantaneous line-width, which is especially benefiting for high-quality optical frequency domain imaging. The proposed Fourier domain mode locking swept laser source provides a tuning range of 160 nm with instantaneous line-width of about 0.01 nm at sweeping rate of 15 kHz, a finesse of 16,000 is thus achieved.

宽带快速线性扫频激光光源的研制

宽带快速线性扫频激光光源的研制

宽带快速线性扫频激光光源的研制

宽带快速线性扫频激光光源的研制

Frequency comb swept lasers

We demonstrate a frequency comb (FC) swept laser and a frequency comb Fourier domain mode locked (FC-FDML) laser for applications in optical coherence tomography (OCT). The fiber-based FC swept lasers operate at a sweep rate of 1kHz and 120kHz, respectively over a 135nm tuning range centered at 1310nm with average output powers of 50mW. A 25GHz free spectral range frequency comb filter in the swept lasers causes the lasers to generate a series of well defined frequency steps. The narrow bandwidth (0.015nm) of the frequency comb filter enables a approximately -1.2dB sensitivity roll off over approximately 3mm range, compared to conventional swept source and FDML lasers which have -10dB and -5dB roll offs, respectively. Measurements at very long ranges are possible with minimal sensitivity loss, however reflections from outside the principal measurement range of 0-3mm appear aliased back into the principal range. In addition, the frequency comb output from the lasers are equally spaced in frequency (linear in k-space). The filtered laser output can be used to self-clock the OCT interference signal sampling, enabling direct fast Fourier transformation of the fringe signals, without the need for fringe recalibration procedures. The design and operation principles of FC swept lasers are discussed and designs for short cavity lasers for OCT and interferometric measurement applications are proposed.

Depth resolved holographic imaging with variable depth resolution using spectrally tunable diode laser

The application of a tunable diode laser for depth resolved photorefractive holographic imaging is demonstrated. Using a swept-source laser with a sweep time that is much shorter than the write time of the hologram allows the tunable laser to act like a broadband source with a bandwidth variable with the tuning range. This opens up the possibility of zooming into a three-dimensional object with variable depth resolution.

Swept laser source at 1μm for Fourier domain optical coherence tomography

A frequency swept source at 1μm wavelength for Fourier domain optical coherence tomography (FDOCT) was demonstrated. The source incorporates a fiber-coupled semiconductor optical amplifier gain module, a fiber Fabry-Pérot tunable filter, and fiber isolators to form a ring laser. The laser is capable of a scanning range of 64nm, 3dB sweep range of 31nm, and coherence length of 9.8mm at 2kHz sweep rate. Based on the swept source, a FDOCT system was developed which can achieve 12μm axial resolution in tissue. Imaging of pig retina was demonstrated with the FDOCT system.

Imaging through scattering medium by recording 3D “spatial-frequential” interferograms

Speed acquisition for image formation process through scattering medium is a challenge in optical coherence tomography (OCT) approach. Besides time domain (TD), spectral Fourier domain (FD) is now widely studied. By using a swept laser source, we demonstrate that a particular time domain OCT method (optical SISAM correlator) can be simultaneously implemented in a single set-up with the corresponding Fourier domain OCT approach (spectral interferometry). Then, FD-OCT and TD-OCT signals are obtained by processing a 3D “spatial-frequential” interferences pattern. We show that these two numerical approaches can be complementary when imaging in scattering medium is achieved.