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High-speed optical coherence tomography using fiberoptic acousto-optic phase modulation

Tuqiang Xie, Zhenguo Wang, and Yingtian Pan

Open Access Open Access

Abstract

We report a new rapid-scanning optical delay device suitable for high-speed optical coherence tomography (OCT) in which an acousto-optic modulator (AOM) is used to independently modulate the Doppler frequency shift of the reference light beam for optical heterodyne detection. Experimental results show that the fluctuation of the measured Doppler frequency shift is less than ±0.2% over 95% duty cycle of OCT imaging, thus allowing for enhanced signal-to-noise ratio of optical heterodyne detection. The increased Doppler frequency shift by AOM also permits complete envelop demodulation without the compromise of reducing axial resolution; if used with a resonant rapid-scanning optical delay, it will permit high-performance real-time OCT imaging. Potentially, this new rapid-scanning optical delay device will improve the performance of high-speed Doppler OCT techniques.

©2003 Optical Society of America

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Figures (6)
Fig. 1.
Fig. 1. A sketch of an optical coherence tomographic imaging system using acousto-optic modulation. A fiberoptic acousto-optic modulator (AOM) is inserted in the reference arm to provide a stable 2MHz (tunable) frequency modulation. In combination with a RSOD, this allows for high-performance reference scanning for high-speed OCT imaging, BBS: broadband source: BBS; LD: aiming laser diode; PD: photo diode; CM: fiber-optic collimator.

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Fig. 2.
Fig. 2. Recorded modulated and linearly demodulated interferometric signals without using acousto-optic modulation. The servo mirror was driven with a 500 Hz triangular waveform, and the pivot offset x=2mm. ΔT relates to the measured coherence length Lc. Artifacts such as serve ripples resulted from incomplete demodulation is obvious.

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Fig. 3.
Fig. 3. Measured Doppler frequency changes with depth in OCT scanning system without using AOM. (a) Servo mirror was driven by a 500 Hz triangular waveform. Frequency variation δf is less than 25% for 80% duty cycle and increases 63% for 90% duty cycle. (b) Servo mirror was driven by a 500 Hz sinusoidal waveform. δf is 56% for 90% duty cycle.

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Fig. 4.
Fig. 4. Recorded modulated OCT transient signal (a) and demodulated interferometric signal (b) using AOM-mediated RSOD. The pivot offset x=0mm, and the AOM was modulated at fD/AOM=2MHz. The measured carrier frequency 1/ΔT=2MHz. Linear amplitude demodulation is clean and complete. ΔT related to the measured coherence length Lc.

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Fig. 5.
Fig. 5. Measured Doppler frequency changes with depth in scanning system using AOM mediated RSOD. (a) Servo mirror was driven by a 500 Hz triangular waveform. (b) Servo mirror was driven by sinusoidal waveform. The measured frequency instability is less than 0.39% over 95% duty cycle. The large error bar was primarily caused by measurement errors.

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Fig. 6.
Fig. 6. Porcine bladder imaged by OCT with AOM-mediated RSOD. U: normal urothelium, SM: submucosa, M: muscular layer. The 2D-OCT image size is 2×5 mm2 displayed in grayscale (linear demodulation). Signal level ranged from -40dB (bright) to -100dB (dark).

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Equations (5)

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

I ( Δ L r g , Δ L r p ) ≈ 2 I s I r . ⌊ η R ( Δ L r g ) ⊗ C A ( Δ L r g ) ⌋ . cos k ¯ Δ L r p
f D = 4 x ω λ 0
Δ f = 4 x ω Δ λ λ 0 2 − 4 ω f Δ λ p λ 0 ≈ − 4 ω f Δ λ λ 0 p
f D AOM = 4 x ω λ 0 + f AOM = f AOM
I ( Δ L r g , t ) ≈ 2 I s I r · ⌊ η R ( Δ L r g ) ⊗ C A ( Δ L r g ) ⌋ · cos ( f AOM t )
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