Fourier Domain Mode-locked Laser Research Articles

4D optical coherence tomography-based micro-angiography achieved by 16-MHz FDML swept source

We demonstrate the use of an ultra-high-speed swept-source optical coherence tomography (OCT) to achieve optical micro-angiography (OMAG) of microcirculatory tissue beds in vivo. The system is based on a 1310-nm Fourier domain mode-locking (FDML) laser with 1.6-MHz A-line rate, providing a frame rate of 3.415KHz, an axial resolution of ∼10 μm and signal to noise ratio of 102dB. Motion from blood flow causes change in OCT signals between consecutive B-frames acquired at the same location. Intensity-based inter-frame subtraction algorithm is applied to extract blood flow from tissue background without any motion correction. We demonstrate the capability of this 1.6-MHz OCT system for 4D OMAG of in vivo tissue at a volume rate of 4.7 volumes/s (volume size: 512×200×720 voxels).

Ultra-widefield retinal MHz-OCT imaging with up to 100 degrees viewing angle.

We evaluate strategies to maximize the field of view (FOV) of in vivo retinal OCT imaging of human eyes. Three imaging modes are tested: Single volume imaging with 85° FOV as well as with 100° and stitching of five 60° images to a 100° mosaic (measured from the nodal point). We employ a MHz-OCT system based on a 1060nm Fourier domain mode locked (FDML) laser with a depth scan rate of 1.68MHz. The high speed is essential for dense isotropic sampling of the large areas. Challenges caused by the wide FOV are discussed and solutions to most issues are presented. Detailed information on the design and characterization of our sample arm optics is given. We investigate the origin of an angle dependent signal fall-off which we observe towards larger imaging angles. It is present in our 85° and 100° single volume images, but not in the mosaic. Our results suggest that 100° FOV OCT is possible with current swept source OCT technology.

Wide-Field Megahertz OCT Imaging of Patients with Diabetic Retinopathy.

Purpose. To evaluate the feasibility of wide-field Megahertz (MHz) OCT imaging in patients with diabetic retinopathy. Methods. A consecutive series of 15 eyes of 15 patients with diagnosed diabetic retinopathy were included. All patients underwent Megahertz OCT imaging, a close clinical examination, slit lamp biomicroscopy, and funduscopic evaluation. To acquire densely sampled, wide-field volumetric datasets, an ophthalmic 1050 nm OCT prototype system based on a Fourier-domain mode-locked (FDML) laser source with 1.68 MHz A-scan rate was employed. Results. We were able to obtain OCT volume scans from all included 15 patients. Acquisition time was 1.8 seconds. Obtained volume datasets consisted of 2088 × 1044 A-scans of 60° of view. Thus, reconstructed en face images had a resolution of 34.8 pixels per degree in x-axis and 17.4 pixels per degree. Due to the densely sampled OCT volume dataset, postprocessed customized cross-sectional B-frames through pathologic changes such as an individual microaneurysm or a retinal neovascularization could be imaged. Conclusions. Wide-field Megahertz OCT is feasible to successfully image patients with diabetic retinopathy at high scanning rates and a wide angle of view, providing information in all three axes. The Megahertz OCT is a useful tool to screen diabetic patients for diabetic retinopathy.

Simultaneous dual-band optical coherence tomography for endoscopic applications.

Dual-band optical coherence tomography (OCT) can greatly enhance the imaging contrast with potential applications in functional (spectroscopic) analysis. A new simultaneous dual-band Fourier domain mode-locked swept laser configuration for dual-band OCT is reported. It was based on a custom-designed dual-channel driver to synchronize two different wavelength bands at 1310 and 1550 nm, respectively. Two lasing wavelengths were swept simultaneously from 1260 to 1364.8 nm for the 1310-nm band and from 1500 to 1604 nm for the 1550-nm band at an A-scan rate of 45 kHz. Broadband wavelength-division multiplexing was utilized to couple two wavelength bands into a common catheter for circumferential scanning to form dual-band OCT. The proposed dual-band OCT scheme was applied to endoscopic OCT imaging of mouse esophageal wall ex vivo and human fingertip in vivo to justify the feasibility of the proposed imaging technique. The proposed dual-band OCT system is fast and easy to be implemented, which allows for in vivo high-speed biomedical imaging with potential applications in spectroscopic investigations for endoscopic imaging.

Fiber-optic catheter-based polarization-sensitive OCT for radio-frequency ablation monitoring.

An all-fiber optic catheter-based polarization-sensitive optical coherence tomography system is demonstrated. A novel multiplexing method was used to illuminate the sample, splitting the light from a 58.5 kHz Fourier-domain mode-locked laser such that two different polarization states, alternated in time, are generated by two semiconductor optical amplifiers. A 2.3 mm forward-view cone-scanning catheter probe was designed, fabricated, and used to acquire sample scattering intensity and phase retardation images. The system was first verified with a quarter-wave plate and then by obtaining intensity and phase retardation images of high-birefringence plastic, human skin in vivo, and untreated and thermally ablated porcine myocardium ex vivo. The system can potentially in vivo image of the cardiac wall to aid radio-frequency ablation therapy for cardiac arrhythmias.

High definition live 3D-OCT in vivo: design and evaluation of a 4D OCT engine with 1 GVoxel/s.

We present a 1300 nm OCT system for volumetric real-time live OCT acquisition and visualization at 1 billion volume elements per second. All technological challenges and problems associated with such high scanning speed are discussed in detail as well as the solutions. In one configuration, the system acquires, processes and visualizes 26 volumes per second where each volume consists of 320 x 320 depth scans and each depth scan has 400 usable pixels. This is the fastest real-time OCT to date in terms of voxel rate. A 51 Hz volume rate is realized with half the frame number. In both configurations the speed can be sustained indefinitely. The OCT system uses a 1310 nm Fourier domain mode locked (FDML) laser operated at 3.2 MHz sweep rate. Data acquisition is performed with two dedicated digitizer cards, each running at 2.5 GS/s, hosted in a single desktop computer. Live real-time data processing and visualization are realized with custom developed software on an NVidia GTX 690 dual graphics processing unit (GPU) card. To evaluate potential future applications of such a system, we present volumetric videos captured at 26 and 51 Hz of planktonic crustaceans and skin.

Compact piezoelectric transducer fiber scanning probe for optical coherence tomography

We developed a compact, optical fiber scanning piezoelectric transducer (PZT) probe for endoscopic and minimally invasive optical coherence tomography (OCT). Compared with previous forward-mount fiber designs, we present a reverse-mount design that achieves a shorter rigid length. The fiber was mounted at the proximal end of a quadruple PZT tube and scanned inside the hollow PZT tube to reduce the probe length. The fiber resonant frequency was 338 Hz using a 17-mm-long fiber. A 0.9 mm fiber deflection was achieved with a driving amplitude of 35 V. Using a GRIN lens-based optical design with a 1.3× magnification, a ∼6 μm spot was scanned over a 1.2 mm diameter field. The probe was encased in a metal hypodermic tube with a ∼25 mm rigid length and covered with a 3.2 mm outer diameter (OD) plastic sheath. Imaging was performed with a swept source OCT system based on a Fourier domain modelocked laser (FDML) light source at a 240 kHz axial scan rate and 8 μm axial resolution (in air). En face OCT imaging of skin in vivo and human colon ex vivo was demonstrated.

Frequency synchronization of Fourier domain harmonically mode locked fiber laser by monitoring the supermode noise peaks

In a harmonically mode locked laser, the supermode noise peaks in the RF spectrum can be observed directly because they are separated from the driving frequency and its harmonics of the active mode locker. Using a simple theoretical model, we showed that the intensities of the supermode noise peaks will decrease if the coherence of the laser output decreases. We harmonically mode locked a Fourier domain mode locked (FDML) fiber laser to the third order. We observed that the supermode noise peak intensities decrease significantly when the detune between the sweeping frequency of the tunable filter and the cavity resonant frequency increases. It is therefore possible to use the supermode noise peaks to monitor the frequency detune of the tunable filter for auto-calibration of FDML fiber lasers.

In Vivo OCT Imaging Based on La-Codoped Bismuth-Based Erbium-Doped Fiber

We demonstrate a Fourier domain mode-locked laser based on lanthanum-codoped bismuth-based erbium-doped fiber (Bi-EDF) for swept source optical coherence tomography (SS-OCT) imaging. Raman amplification is incorporated to suppress the gain competition and homogenous linewidth broadening effects of Bi-EDF. A wavelength sweeping bandwidth of 81 nm is generated under stable operation. Therefore, in vivo OCT imaging of human finger print and orange slices is enabled and the results are also presented. This scheme paves the way for doped fiber amplifiers to be employed to generate ultra-wideband SSs for OCT applications.

Multi-MHz retinal OCT

We analyze the benefits and problems of in vivo optical coherence tomography (OCT) imaging of the human retina at A-scan rates in excess of 1 MHz, using a 1050 nm Fourier-domain mode-locked (FDML) laser. Different scanning strategies enabled by MHz OCT line rates are investigated, and a simple multi-volume data processing approach is presented. In-vivo OCT of the human ocular fundus is performed at different axial scan rates of up to 6.7 MHz. High quality non-mydriatic retinal imaging over an ultra-wide field is achieved by a combination of several key improvements compared to previous setups. For the FDML laser, long coherence lengths and 72 nm wavelength tuning range are achieved using a chirped fiber Bragg grating in a laser cavity at 419.1 kHz fundamental tuning rate. Very large data sets can be acquired with sustained data transfer from the data acquisition card to host computer memory, enabling high-quality averaging of many frames and of multiple aligned data sets. Three imaging modes are investigated: Alignment and averaging of 24 data sets at 1.68 MHz axial line rate, ultra-dense transverse sampling at 3.35 MHz line rate, and dual-beam imaging with two laser spots on the retina at an effective line rate of 6.7 MHz.

Parametric spectro-temporal analyzer (PASTA) for real-time optical spectrum observation

Real-time optical spectrum analysis is an essential tool in observing ultrafast phenomena, such as the dynamic monitoring of spectrum evolution. However, conventional method such as optical spectrum analyzers disperse the spectrum in space and allocate it in time sequence by mechanical rotation of a grating, so are incapable of operating at high speed. A more recent method all-optically stretches the spectrum in time domain, but is limited by the allowable input condition. In view of these constraints, here we present a real-time spectrum analyzer called parametric spectro-temporal analyzer (PASTA), which is based on the time-lens focusing mechanism. It achieves a frame rate as high as 100 MHz and accommodates various input conditions. As a proof of concept and also for the first time, we verify its applications in observing the dynamic spectrum of a Fourier domain mode-locked laser, and the spectrum evolution of a laser cavity during its stabilizing process.

A Fourier domain mode-locked fiber laser based on dual-pump fiber optical parametric amplification and its application for a sensing system

A Fourier domain mode-locked (FDML) fiber laser based on dual-pump fiber optical parametric amplification (FOPA) is proposed and demonstrated. The output spectrum of the proposed FDML fiber laser covers a sweeping wavelength range from 1540.8 to 1559.8 nm with a sweeping frequency of 31.688 kHz. A comparison of two FDML fiber lasers which are based on dual-pump FOPA and one-pump FOPA is also presented. A novel sensing system based on the FDML laser and a fiber Bragg grating, by which the sensing signal can be measured in the time domain instead of the frequency domain, is also demonstrated.

Swept-Source OCT System Based on a Polarization-Dependent Semiconductor Optical Amplifier-Enabled Fourier Domain Mode Locked Laser

We demonstrate a swept-source OCT system based on a polorization-dependent semiconductor optical amplifier (SOA)-enabled Fourier Domain Mode Locked laser (FDML). The broad bandwidth and high saturation power of the semiconductor optical amplifier enables single semiconductor optical amplifier operation with enough bandwidth and output power. The FDML generates ~32mW output power at 45kHz sweep rates, with a tuning range of 115 nm centered at 1326 nm. Using the SOA-enabled FDML laser, ~12.9μm axial resolution,9μm transverse spot size and 105dB sensitivity are achieved. OCT imaging of biological sample is also demonstrated.

Long distance fiber Bragg grating strain sensor interrogation using a high speed Raman-based Fourier domain mode-locked fiber laser with recycled residual Raman pump

We propose a novel fiber Bragg grating (FBG) sensor interrogation using a Raman-based Fourier-domain mode locking (FDML) fiber laser for a high speed and long distance measurement. A residual Raman pump after the generation of the Raman-based FDML fiber laser is recycled for secondary signal amplification in a 2-m erbium-doped fiber (EDF) to further enhance the output power. The chromatic dispersion is precisely controlled to suppress the phase noise in the FDML laser cavity, resulting in the improvement of an R-number of 1.43 mm/dB. After recycling residual pump, we achieve the 40-km round trip transmission of the sensing probe signal with a high scan rate of 30.8 kHz. With 205-mW residual pump power, the bandwidth and the maximum gain are measured to be more than 50 nm, 10.3 dB at 1550 nm, respectively. The sensitivity of the proposed Raman-based FDML fiber laser to strain is also measured, which are 0.81 pm/μstrain in the spectral domain and 0.19 ns/μstrain in the time domain, respectively.

Picosecond pulses from wavelength-swept continuous-wave Fourier domain mode-locked lasers

Ultrafast lasers have a crucial function in many fields of science; however, up to now, high-energy pulses directly from compact, efficient and low-power semiconductor lasers are not available. Therefore, we introduce a new approach based on temporal compression of the continuous-wave, wavelength-swept output of Fourier domain mode-locked lasers, where a narrowband optical filter is tuned synchronously to the round-trip time of light in a kilometre-long laser cavity. So far, these rapidly swept lasers enabled orders-of-magnitude speed increase in optical coherence tomography. Here we report on the generation of ~60-70 ps pulses at 390 kHz repetition rate. As energy is stored optically in the long-fibre delay line and not as population inversion in the laser-gain medium, high-energy pulses can now be generated directly from a low-power, compact semiconductor-based oscillator. Our theory predicts subpicosecond pulses with this new technique in the future.

Measurements of multiple gas parameters in a pulsed-detonation combustor using time-division-multiplexed Fourier-domain mode-locked lasers

Hyperspectral absorption spectroscopy is being used to monitor gas temperature, velocity, pressure, and H(2)O mole fraction in a research-grade pulsed-detonation combustor (PDC) at the Air Force Research Laboratory. The hyperspectral source employed is termed the TDM 3-FDML because it consists of three time-division-multiplexed (TDM) Fourier-domain mode-locked (FDML) lasers. This optical-fiber-based source monitors sufficient spectral information in the H(2)O absorption spectrum near 1350 nm to permit measurements over the wide range of conditions encountered throughout the PDC cycle. Doppler velocimetry based on absorption features is accomplished using a counterpropagating beam approach that is designed to minimize common-mode flow noise. The PDC in this study is operated in two configurations: one in which the combustion tube exhausts directly to the ambient environment and another in which it feeds an automotive-style turbocharger to assess the performance of a detonation-driven turbine. Because the enthalpy flow [kilojoule/second] is important in assessing the performance of the PDC in various configurations, it is calculated from the measured gas properties.

Fiber Fabry-Perot tunable filter based Fourier domain mode locking swept laser source

An all-fiber Fourier domain mode locking (FDML) swept laser source at 1300 nm for swept source optical coherence tomography is reported. The swept laser source is realized with power amplification and laser resonator which includes gain medium, tunable filter and dispersion managed delay line. FDML swept laser can realize high-speed tuning, and phase is stable since its highly stable mode locking operation. The turning range of fiber Fabry-Perot tunable filter (FFP-TF) based FDML swept laser is 130 nm, and the 3 dB bandwidth is 70 nm with an average output power of 11 mW. The tunable speed of FDML laser is 48.12 kHz compared with 8 kHz of short-cavity FFP-TF based swept laser. The axial resolution in OCT imaging of FDML swept laser is 7.8 μm (in tissue), which is improved by 1.9 μm compared with that of short-cavity swept laser.

Noninvasive imaging of heart chamber in Drosophila with dual‐beam optical coherence tomography

The heart chamber of an adult Drosophila is approximately 2 mm long and 0.5 mm wide, and the interwall separation of different heart portions during systole and diastole range from tens of micrometers to hundreds of micrometers. Furthermore, the heart chamber has a curved structure, which results in the larger differences in depth between the different heart portions. However, applying the wavelength calibration process before Fourier transform in an optical coherence tomography (OCT) system may cause degradation in system sensitivity and longitudinal resolution when the optical path difference between the reference and sample arms increases, which makes imaging the entire heart chamber difficult with OCT system. Additionally, since the heartbeat rate of Drosophila is approximately 6 beats/s, a high-speed OCT system is necessary to record the dynamics of the heat beats. In this study, we propose a new approach to visualize the entire heart chamber including the conical chamber and four ostia portions, and to observe the retrograde and anterograde beats. A buffered Fourier-domain mode-locked (FDML) laser is implemented to provide a high imaging speed. Two output ports of the buffered FDML laser are used simultaneously to scan the different heart portions of Drosophila, and the effective A-scan rate of the OCT system can be doubled. Then, the two scanned images are merged into a single B-mode scan. Furthermore, with dual-beam OCT system, the beating behaviors of the different heart portions from 7-day-old and 21-day-old flies are compared.

분광 영역 모드록킹 레이저를 이용한 공진형 광섬유 격자 센서

We report a resonance fiber Bragg sensor interrogation based on a Fourier domain mode-locking (FDML) laser. The FDML laser is constructed based on a conventional ring laser cavity configuration with fiber Fabry-Perot tunable filter (FFP-TF). There are two sensor parts which are composed with two FBGs inside the laser cavity. Each sensor part provides a separate laser cavity for the FDML laser. The resonance frequencies of the laser cavities are 46.687 kHz and 44.340 kHz, respectively. We applied a static and a dynamic strain on the FBG sensor system. The slope coefficients of the measured relative wavelength shift and relative time interval from the static strain are found to be and , respectively.

Extended coherence length megahertz FDML and its application for anterior segment imaging

We present a 1300 nm Fourier domain mode locked (FDML) laser for optical coherence tomography (OCT) that combines both, a high 1.6 MHz wavelength sweep rate and an ultra-long instantaneous coherence length for rapid volumetric deep field imaging. By reducing the dispersion in the fiber delay line of the FDML laser, the instantaneous coherence length and hence the available imaging range is approximately quadrupled compared to previously published MHz-FDML setups, the imaging speed is increased by a factor of 16 compared to previous extended coherence length results. We present a detailed characterization of the FDML laser performance. We demonstrate for the first time MHz-OCT imaging of the anterior segment of the human eye. The OCT system provides enough imaging depth to cover the whole range from the top surface of the cornea down to the crystalline lens.