The diffraction tomography theorem is adapted to one-dimensional length measurement. The resulting spectral interferometry technique is described and the first length measurements using this technique on a model eye and on a human eye in vivo are presented.

Measurement of intraocular distances by backscattering spectral interferometry

We demonstrate a new technique for frequency-swept laser operation--Fourier domain mode locking (FDML)--and its application for swept-source optical coherence tomography (OCT) imaging. FDML is analogous to active laser mode locking for short pulse generation, except that the spectrum rather than the amplitude of the light field is modulated. High-speed, narrowband optical frequency sweeps are generated with a repetition period equal to the fundamental or a harmonic of cavity round-trip time. An FDML laser is constructed using a long fiber ring cavity, a semiconductor optical amplifier, and a tunable fiber Fabry-Perot filter. Effective sweep rates of up to 290 kHz are demonstrated with a 105 nm tuning range at 1300 nm center wavelength. The average output power is 3mW directly from the laser and 20 mW after post-amplification. Using the FDML laser for swept-source OCT, sensitivities of 108 dB are achieved and dynamic linewidths are narrow enough to enable imaging over a 7 mm depth with only a 7.5 dB decrease in sensitivity. We demonstrate swept-source OCT imaging with acquisition rates of up to 232,000 axial scans per second. This corresponds to 906 frames/second with 256 transverse pixel images, and 3.5 volumes/second with a 256x128x256 voxel element 3-DOCT data set. The FDML laser is ideal for swept-source OCT imaging, thus enabling high imaging speeds and large imaging depths.

Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography.

State-of-the-art retinal optical coherence tomography

Since its introduction, optical coherence tomography OCT technology has advanced from the laboratory bench to the clinic and back again. Arising from the fields of low coherence inter- ferometry and optical time- and frequency-domain reflectometry, OCT was initially demonstrated for retinal imaging and followed a unique path to commercialization for clinical use. Concurrently, sig- nificant technological advances were brought about from within the research community, including improved laser sources, beam delivery instruments, and detection schemes. While many of these technolo- gies improved retinal imaging, they also allowed for the application of OCT to many new clinical areas. As a result, OCT has been clinically demonstrated in a diverse set of medical and surgical specialties, in- cluding gastroenterology, dermatology, cardiology, and oncology, among others. The lessons learned in the clinic are currently spurring a new set of advances in the laboratory that will again expand the clinical use of OCT by adding molecular sensitivity, improving image quality, and increasing acquisition speeds. This continuous cycle of laboratory development and clinical application has allowed the OCT technology to grow at a rapid rate and represents a unique model for the translation of biomedical optics to the patient bedside. This work presents a brief history of OCT development, reviews current clinical applications, discusses some clinical translation challenges, and re- views laboratory developments poised for future clinical application.

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Optical coherence tomography: a review of clinical development from bench to bedside

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.

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

We demonstrate a high-speed, frequency swept, 1300 nm laser source for frequency domain reflectometry and OCT with Fourier domain/swept-source detection. The laser uses a fiber coupled, semiconductor amplifier and a tunable fiber Fabry-Perot filter. We present scaling principles which predict the maximum frequency sweep speed and trade offs in output power, noise and instantaneous linewidth performance. The use of an amplification stage for increasing output power and for spectral shaping is discussed in detail. The laser generates ~45 mW instantaneous peak power at 20 kHz sweep rates with a tuning range of ~120 nm full width. In frequency domain reflectometry and OCT applications the frequency swept laser achieves 108 dB sensitivity and ~10 mum axial resolution in tissue. We also present a fast algorithm for real time calibration of the fringe signal to equally spaced sampling in frequency for high speed OCT image preview.

Amplified, Frequency Swept Lasers for Frequency Domain Reflectometry and OCT Imaging : Design and Scaling Principles

In one aspect the invention relates to a frequency varying wave generator. The generator includes a gain element adapted to amplify a wave having a wavelength; a time varying tunable wavelength selective filter element in communication with the gain element, the tunable Filter element adapted to selectively filter waves during a period T; and a feedback element in communication with the tunable filter element and the gain element, wherein the tunable wavelength selective filter element, the gain element and the feedback element define a circuit such that the roundtrip time for the wave to propagate through the circuit is substantially equal to a non-zero integer multiple of the period T.

Mode locking methods and apparatus

A high speed, tunable laser using Fourier Domain Mode Locking is demonstrated for OCT imaging. Record sweep speeds up to 290 kHz, 3 cm coherence length and 145 nm range at 1300 nm are achieved.

Fourier Domain Mode Locked Lasers for OCT Imaging at up to 290 kHz Sweep Rates

We demonstrate a high-speed tunable, continuous wave laser source for Fourier domain OCT. The laser source is based on a fiber coupled, semiconductor optical amplifier and a tunable ultrahigh finesse, fiber Fabry Perot filter for frequency tuning. The light source provides frequency scan rates of up to 20,000 sweeps per second over a wavelength range of >70 nm FWHM at 1330 nm, yielding an axial resolution of ~14 μm in air. The linewidth is narrow and corresponds to a coherence length of several mm, enabling OCT imaging over a large axial range.

/pdf/high-speed-frequency-swept-light-source-for-fourier-domain-4z0by5h1yi.pdf

High-speed frequency swept light source for Fourier domain OCT at 20 kHz A-scan rate

A new type of laser operation, Fourier Domain Mode Locking (FDML), is demonstrated for high performance, frequency swept light sources. FDML achieves superior sweep speeds, coherence lengths and bandwidths compared to standard bulk or fiber lasers. At 1300 nm a sweep range up to 145 nm, up to 4 cm delay length, and sweep rates up to 290 kHz were achieved. This light source is demonstrated for swept source OCT imaging.

Kenji Taira

Papers

Amplified, Frequency Swept Lasers for Frequency Domain Reflectometry and OCT Imaging : Design and Scaling Principles

Mode locking methods and apparatus

Fourier Domain Mode Locked Lasers for OCT Imaging at up to 290 kHz Sweep Rates

High-speed frequency swept light source for Fourier domain OCT at 20 kHz A-scan rate

Fourier domain mode-locked lasers for swept source OCT imaging at up to 290 kHz scan rates