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

R. Huber, M. Wojtkowski, and J. G. Fujimoto

Open Access Open Access

Abstract

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 3 mW 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 256×128×256 voxel element 3-D OCT data set. The FDML laser is ideal for swept-source OCT imaging, thus enabling high imaging speeds and large imaging depths.

©2006 Optical Society of America

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Supplementary Material (2)

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Figures (10)
Fig. 1.
Fig. 1. Left: Standard tunable laser. The light from a broadband gain medium is spectrally filtered by a tunable narrowband optical bandpass filter and coupled back to the gain medium. Only the longitudinal modes with frequencies within the spectral filter window of the optical bandpass are transmitted and have energy. Right: In Fourier domain mode locking (FDML), an entire frequency sweep is optically stored within the laser cavity. The tunable optical bandpass filter is periodically driven with a period matched to the round-trip time of the cavity or a harmonic. The synchronous drive of the optical tunable filter ensures that the light transmitted through the filter will return to the filter at a time when the filter is tuned to the same spectral position again. Therefore, lasing does not have to build up repeatedly from spontaneous emission.

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Fig. 2.
Fig. 2. Schematic diagram of the Fourier domain mode-locked (FDML), high-speed, frequency-swept laser.

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Fig. 3.
Fig. 3. Schematic of the swept-source OCT system (optics: blue; electronics: green).

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Fig. 4.
Fig. 4. Transient intensity profiles of the FDML laser for different effective sweep rates. The traces always show the transient intensity for one forward and one backward frequency sweep.

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Fig. 5.
Fig. 5. Integrated spectra of the FDML source for different effective sweep rates.

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Fig. 6.
Fig. 6. Axial OCT point spread function (PSF)-resolution in air 12.7 μm (FWHM of amplitude) corresponds to ~9 μm in tissue.

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Fig. 7.
Fig. 7. Measured OCT PSFs on a logarithmic scale for different delays, which is relative to the interferometer reference arm length of the Michelson interferometer. The depth scale of 0 mm to 7 mm in the graph reflects actual imaging depth and corresponds to a optical roundtrip delay of 0 mm to 14 mm. The scale is adjusted by a constant, such that the peak values reflect the sensitivity values at the different depth positions.

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Fig. 8.
Fig. 8. Image of human finger in vivo. The image size is 4096×1024 pixels and is acquired in 0.097 s, which corresponds to 42,000 axial scans/s and 10 frames/s.

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Fig. 9.
Fig. 9. Formalin-fixed hamster cheek pouch specimen in vitro. A 3-D OCT data set acquired in 0.8 s. The 512×512×200 pixel volume was recorded at 124,000 axial scans/s, 242 frames/s and 1.2 volumes/s. The animation shows a fly-through visualization of the 3-D OCT data set (2.0MB).

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Fig. 10.
Fig. 10. Human finger in vivo. 3-D data set acquired in 0.28 s. The 256×128×256 pixel volume was recorded at 232,000 axial scans/s, 906 frames/s, and 3.5 volumes/s. The animation shows a volume-rendered representation of the 3-D OCT data set (1.4MB).

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Equations (2)

Equations on this page are rendered with MathJax. Learn more.

τ gate = Δ λ η ∙ f drive ∙ Δ λ tuningrange
Δ τ disp = ( λ − 1313 nm ) 2 ∙ 0.086 ps km ∙ nm 2 ∙ L
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