Raman-pumped Fourier-domain mode-locked laser: analysis of operation and application for optical coherence tomography

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.

Dental optical coherence tomography

This review paper describes the applications of dental optical coherence tomography (OCT) in oral tissue images, caries, periodontal disease and oral cancer. The background of OCT, including basic theory, system setup, light sources, spatial resolution and system limitations, is provided. The comparisons between OCT and other clinical oral diagnostic methods are also discussed.

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.

Chromatic polarization effects of swept waveforms in FDML lasers and fiber spools

We present detailed investigations of chromatic polarization effects, caused by fiber spools used in FDML lasers and buffering spools for rapidly wavelength swept lasers. We introduce a novel wavelength swept FDML laser source, specially tailored for polarization sensitive optical coherence tomography (OCT) which switches between two different linear polarization states separated by 45°, i.e. 90° on the Poincaré sphere. The polarization maintaining laser cavity itself generates a stable linear polarization state and uses an external buffering technique in order to provide alternating polarization states for successive wavelength sweeps. The design of the setup is based on a comprehensive analysis of the polarization output from FDML lasers, using a novel 150 MHz polarization analyzer. We investigate the fiber polarization properties related to swept source OCT for different fiber delay topologies and analyze the polarization state of different FDML laser sources.

Instantaneous lineshape analysis of Fourier domain mode-locked lasers

We present a theoretical and experimental analysis of the instantaneous lineshape of Fourier domain mode-locked (FDML) lasers, yielding good agreement. The simulations are performed employing a recently introduced model for FDML operation. Linewidths around 10 GHz are found, which is significantly below the sweep filter bandwidth. The effect of detuning between the sweep filter drive frequency and cavity roundtrip time is studied revealing features that cannot be resolved in the experiment, and shifting of the instantaneous power spectrum against the sweep filter center frequency is analyzed. We show that, in contrast to most other semiconductor based lasers, the instantaneous linewidth is governed neither by external noise sources nor by amplified spontaneous emission, but it is directly determined by the complex FDML dynamics.

Multi-megahertz OCT: High quality 3D imaging at 20 million A-scans and 4.5 GVoxels per second

We present ultra high speed optical coherence tomography (OCT) with multi-megahertz line rates and investigate the achievable image quality. The presented system is a swept source OCT setup using a Fourier domain mode locked (FDML) laser. Three different FDML-based swept laser sources with sweep rates of 1, 2.6 and 5.2MHz are compared. Imaging with 4 spots in parallel quadruples the effective speed, enabling depth scan rates as high as 20.8 million lines per second. Each setup provides at least 98dB sensitivity and approximately 10microm resolution in tissue. High quality 2D and 3D imaging of biological samples is demonstrated at full scan speed. A discussion about how to best specify OCT imaging speed is included. The connection between voxel rate, line rate, frame rate and hardware performance of the OCT setup such as sample rate, analog bandwidth, coherence length, acquisition dead-time and scanner duty cycle is provided. Finally, suitable averaging protocols to further increase image quality are discussed.

Wavelength-swept spectral and pulse shaping utilizing hybrid Fourier domain modelocking by fiber optical parametric and erbium-doped fiber amplifiers

We report the first Fourier domain modelocked (FDML) laser constructed using optical parametric amplifier (OPA) in conjunction with an erbium-doped fiber amplifier (EDFA), centered at approximately 1555 nm, to the best of our knowledge. We utilize a one-pump OPA and a C-band EDFA in serial configuration with a tunable Fabry-Perot interferometer to generate a hybrid FDML spectrum. Results demonstrate a substantially better spectral shape, output power and stability than individual configurations, with decreased sensitivity to polarization changes. We believe this technique has the potential to enable several amplifiers to complement individual deficiencies resulting in improved spectral shapes and power generation for imaging applications such as optical coherence tomography (OCT).

Wavelength swept amplified spontaneous emission source

We present a new, alternative approach to realize a wavelength swept light source with no fundamental limit to sweep speed. Amplified spontaneous emission (ASE) light alternately passes a cascade of optical gain elements and tunable optical bandpass filters. We show that for high sweep speeds, the control signal for the different filters has to be applied with a defined, precise phase delay on the order of nanoseconds, to compensate for the light propagation time between the filters and ensure optimum operation. At a center wavelength of 1300 nm sweep rates of 10 kHz, 100 kHz and 340 kHz over a sweep range of 100 nm full width and an average power of 50 mW are demonstrated. For application in optical coherence tomography (OCT), an axial resolution of 12 microm (air), a sensitivity of 120 dB (50 mW) and a dynamic range of 50 dB are achieved and OCT imaging is demonstrated. Performance parameters like coherence properties and relative intensity noise (RIN) are quantified, discussed and compared to the performance of Fourier Domain Mode Locked (FDML) lasers. Physical models for the observed difference in performance are provided.

Nonlinear optical frequency conversion of an amplified Fourier Domain Mode Locked (FDML) laser

We report on the highly efficient non-linear optical frequency conversion of the wavelength swept output from a Fourier Domain Mode Locked (FDML) laser. Different concepts for power scaling of FDML lasers by post-amplification with active fibers are presented. A two-stage post-amplification of an FDML laser with an amplification factor of 300 up to a peak power of 1.5 W is used to supply sufficient power levels for non-linear conversion. Using a single-mode dispersion shifted fiber (DSF), we convert this amplified output that covers the region between 1541 nm and 1545 nm to a wavelength range from 1572 nm to 1663 nm via modulation instability (MI). For this four wave mixing process we observe an efficiency of approximately 40%. The anti-Stokes signal between 1435 nm and 1516 nm was observed with lower conversion efficiency. In addition to shifting the wavelength, the effect of MI also enables a substantial increase in the wavelength sweep rate of the FDML laser by a factor of approximately 50 to 0.55 nm/ns.

Recent developments in Fourier domain mode locked lasers for optical coherence tomography: imaging at 1310 nm vs. 1550 nm wavelength

We report on recent progress in Fourier domain mode-locking (FDML) technology. The paper focuses on developments beyond pushing the speed of these laser sources. After an overview of improvements to FDML over the last three years, a brief analysis of OCT imaging using FDML lasers with different wavelengths is presented. For the first time, high speed, high quality FDML imaging at 1550 nm is presented and compared to a system at 1310 nm. The imaging results of human skin for both wavelengths are compared and analyzed. Sample arm optics, power on the sample, heterodyne gain, detection bandwidth, colour cut levels and sample location have been identical to identify the influence of difference in scattering and water absorption. The imaging performance at 1310 nm in human skin is only slightly better and the results suggest that water absorption only marginally affects the penetration depth in human skin at 1550 nm. For several applications this wavelength may be preferred.

Dispersion, coherence and noise of Fourier domain mode locked lasers

We report on the effect of chromatic dispersion on coherence length and noise of Fourier Domain Mode Locked (FDML) lasers. An FDML laser with a sweep range of 100 nm around 1550 nm has been investigated. Cavity configurations with and without dispersion compensation have been analyzed using different widths of the intra-cavity optical band-pass filter. The measurements are compared to non-FDML wavelength swept laser sources. Based on these observations, a simple model is developed providing a connection between timing, photon cavity lifetime and characteristic time constant of the filter. In an optimized configuration, an instantaneous laser linewidth of 20 pm is observed, corresponding to a 10x narrowing compared to the intra-cavity optical bandpass filter. A relative intensity noise of -133 dBc/Hz or 0.2% at 100 MHz detection bandwidth during sweep operation is observed. For optimum operation, the filter drive frequency has to be set within 2 ppm or 120 mHz at 51 kHz.