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Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles

R. Huber, M. Wojtkowski, K. Taira, J. G. Fujimoto, and K. Hsu

Open Access Open Access

Abstract

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 µm 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.

©2005 Optical Society of America

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Figures (8)
Fig. 1.
Fig. 1. Top: Schematic of the amplified frequency swept laser source. The laser uses a fiber coupled SOA and a FFP-TF. A SOA boosts the laser output. Bottom: Schematic of the OCT system using Fourier domain/swept source detection.

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Fig. 2.
Fig. 2. Left: Signal generation for recalibration. Right: Principle of the “nearest neighbor check” algorithm. The schematic shows the two synchronously acquired signal traces, the calibration trace (right top) and the sample trace (actual OCT signal) from the dual balanced detector (right bottom).

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Fig. 3.
Fig. 3. Concept of cavity tuning: build up of laser activity from ASE background.

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Fig. 4.
Fig. 4. Maximum output power of the forward- (from shorter to longer wavelengths) and backward- (from longer to shorter wavelengths) frequency scan versus the sweep frequency normalized to the power at low frequencies. The boxes mark the regions between the “saturation limit” and the “single roundtrip limit.” Top left: Energy of forward (black) and backward (red) scan without an external booster. Top right: Comparison of the relative drop in the maximum power output of the laser without (black) and with (blue) booster for the forward scan. Bottom left: Comparison of the relative drop in the maximum power output of the laser without (red) and with (green) booster for the backward scan. Bottom right: Comparison of the relative drop in the maximum power output of the laser with booster for the forward (blue) and the backward scan (scan).

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Fig. 5.
Fig. 5. Temporal intensity profile for different frequency sweep rates. Top left: Saturation regime: for sweep speeds significantly slower than the saturation limit (2 kHz drive waveform). Top right: Multiple-roundtrip regime with frequency sweep speed between the saturation limit and single roundtrip limit (20 kHz drive waveform). Bottom left: Frequency sweep speed at single roundtrip limit with (left) and without (right) booster (30 kHz drive waveform).

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Fig. 6.
Fig. 6. Top: Output spectra of the laser (red), the laser with an external booster amplifier identical to the laser gain (green), and the laser with an external booster amplifier with a gain spectrum, which is blue shifted compared to the laser gain (black). Bottom: Calculated pointspread functions for the different spectra on a linear scale (left) and a logarithmic scale (right).

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Fig. 7.
Fig. 7. Left: (colored lines) Pointspread functions for different delays, measured with -61 dB sample arm attenuation including losses. The scale is calibrated such that the peak values of the point spread functions match the sensitivity for the different depths. Sensitivity versus delay for the amplified and unamplified laser measured at 20 kHz frequency scan rates calculated by the drop in fringe contrast and calibrated to the sensitivity at 500 µm delay (diamonds and circles, respectively). Sensitivity of unamplified laser measured using Fourier transform of fringe signal (black line). Right: Axial resolution plotted as FWHM of the amplitude signal of the Fourier transformed fringe signal in air versus delay.

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Fig. 8.
Fig. 8. OCT images taken with swept source at different sweep rates. Top left: Nailfold imaged at 20 kHz line rate. Top right: Skin in the area of the palm imaged at 20 kHz line rate. Middle left: Human skin in the area of the finger tip imaged at 20 kHz line rate. Middle right: Human skin in the area of the finger tip imaged at 27 kHz line rate. Bottom: Hamster cheek pouch ex vivo images at 7 kHz line rate.

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Equations (7)

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n = log ( P sat P ASE ) log ( β ) ,
β = G · ρ ,
P ASE ≈ Δ λ Δ λ tuningrange · P ASEtotal ,
τ roundtrip = L · n ref c ,
v tuning ≈ Δ λ n · τ roundtrip ≈ log ( G · ρ ) · c · Δ λ log ( P sat · Δ λ tuningrange Δ λ · P ASEtotal ) · L · n ref .
f sweep ≈ v tuning · η Δ λ tuningrange ≈ log ( G · ρ ) · Δ λ · η · c log ( P sat · Δ λ tuningrange Δ λ · P ASEtotal ) · L · n ref · Δ λ tuningrange .
f sin gle = Δ λ · c · η Δ λ tuningrange · L · n ref .
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