High-speed optical coherence tomography using ... - OSA Publishing

High-speed optical coherence tomography using fiberoptic acousto-optic phase modulation Tuqiang Xie, Zhenguo Wang, and Yingtian Pan Department of Biomedicine Engineering, State University of New York at Stony Brook HSC T18, Room 030, Stony Brook, NY 11794-8181 [email protected], [email protected], [email protected]

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 highspeed Doppler OCT techniques. 2003 Optical Society of America OCIS codes: (170.4500) Optical coherence tomography; (170.4090) Modulation technique, (170.3880) Medical and biological imaging; (170.7230) Urology; (170.3890) Medical optics instrumentation.

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Received September 16, 2003; Revised November 14, 2003

1 December 2003 / Vol. 11, No. 24 / OPTICS EXPRESS 3210

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1. Introduction Optical coherence tomography (OCT) is a recently developed optical technique that enables non-invasive, high resolution in vivo imaging in turbid biological tissue. Since its first introduction to intraocular imaging in early 1990s [1], OCT has found widespread applications in diagnosing diseases in various biological tissues. In recent years, significant technological advances have been made including polarization OCT [2, 3], Doppler OCT (DOCT) [4], spectral OCT [5] and ultra-high-resolution OCT [6]. With the commercialization of ultra-broadband laser technology, OCT will have the potential to become a subcellularresolution diagnostic imaging tool for ‘optical biopsy’. In addition to ultra-broadband lasers for subcellular OCT imaging, rapid and precise pathlength scanning or optical delay still remains a technical challenge for high-performance real-time OCT. Early attempts for optical delay included a DC motor or voice coil driven translating mirror or retroreflector [7]; however, the maximum scan velocity for these OCT systems was less than 40 mm/s at a repetition rate of 30Hz, too slow for many applications. Piezoelectric fiber stretchers achieved a repetition rate of 1.2 kHz; but the scanning range was limited and adverse dynamic birefringence effect was induced [7]. Grating-lens-based rapid-scanning optical delay line (RSOD) has been widely used by OCT researchers to achieve high-speed linear axial scan and independent control of the phase- and group-delay rates [8, 9]. Although it has been reported to perform high-duty-cycle axial linear scan over 3.0mm at a repetition rate of 2 kHz, the low phase delay rate and unstable Doppler shift and thus the resultant incomplete demodulation of interferograms may comprise the OCT image rate and fidelity. In addition, delay scanners with a rotating tilted mirror array and with a multi-pass cavity and electromagnetic actuation have demonstrated high duty-cycle scan at rates of 2.4 kHz and 2 kHz, respectively [10, 11]. However, unlike RSOD, these two scanners are unable to independently control the phaseand group-delay scan rates. As a consequence, dispersion compensation critical to OCT imaging may not be easily managed, and the resultant Doppler frequency is too high (e.g., >10MHz) to achieve high-performance heterodyne detection, which is important for OCT and particularly DOCT of turbid biological tissues. It has been reported that velocity sensitivity and spatial resolution are coupled in DOCT. Increasing velocity sensitivity may result in a decrease in spatial resolution; at the same time, increasing image frame rate also decreases velocity sensitivity [12]. Recently, incorporation of electro-optic (EO) phase modulation into grating-lens-based optical delay has been reported by our group and others to improve the performance of high-speed reference scan [12, 13]. Although our initial test shows promising #3032 - $15.00 US

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results, the use of a resonant EO phase modulator exhibits residual harmonic frequency components, and therefore compromises the envelop demodulation of the OCT detection [13]. Recent studies show that the use of a fiberoptic broadband EO phase modulator can effectively enhance the performances of the RSOD for high-speed OCT [13, 15]. As an alternative, we present in this paper a new rapid-scanning optical delay by using fiberoptic acousto-optic modulation (AOM) to further improve the performances of high-speed OCT and OCDT. Compared to EO phase modulation requiring a high-frequency and high-voltage broadband driving source, a direct frequency modulation at 2MHz (tunable) is implemented in the reference beam of the fiberoptic interferometer driven by two simple low-voltage resonant signals [19]. Because of direct frequency modulation to the OCT signal by using a pair of differential AO modulators [14, 19], this technique allows for generation of an ultra-stable interference fringe carrier frequency in the proper frequency range (e.g., 1-2MHz) compared to the group delay (e.g., bandwidth) so that the optically heterodyned OCT signal can be completely demodulated at an enhanced signal-to-noise ratio. As no pivot offset is needed to induce phase shift, the servo mirror size can be reduced to decrease mechanic inertia and thus to increase OCT frame rate. Also because of direct frequency modulation, a resonant servo mirror can be used for group delay without changing the carrier frequency. This offers a great potential to drastically improve OCT and OCDT frame rate without compromising the signalto-noise performance. 2.

Materials and methods

The schematic of our fiber-optic AOM-based OCT system is shown in Fig. 1, in which a broadband light source was used to illuminate the fiberoptic Michelson interferometer. The pigtailed output power is 13mW, and its central wavelength λ0 and spectral bandwidth ∆λ are 1310nm and 78nm, respectively, thus the corresponding coherence length LC is 9.8µm. The light from the broadband source was split equally into the reference (R) and the sample (S) arms of the fiberoptic Michelson interferometer and recombined in the detection fiber (B). In the reference arm, a grating-lens-based rapid-scanning optical delay line (RSOD) is used to vary the optical path length or to provide axial scan. As has been reported previously [9, 15], this technique allows separation of the phase and the group delays, permitting an independent control over Doppler frequency shift fD and bandwidth ∆f of the OCT signal. For a RSOD without additional EO or AO modulation, the detected OCT signal can be given, based on our previous modeling, as [16],

[

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I( ∆L r / g , ∆L r / p ) ≈ 2 I s I r ⋅ η R ( ∆L r / g ) ⊗ C A ( ∆L r / g ) ⋅ cos k∆L r / p

(1)

where R(Ls) is the pathlength-resolved backscattered reflectance, CA(Ls) is the modular part exp − 4( Ls Lc ) 2 of the low-coherence function of exp −4 ( L s L c ) 2 cos kL s assuming that

[

]

the light source has a Gaussian spectral profile. Here η is the coefficient related to the speckle effect as a result of summation of local light fields with randomized phase distribution, i.e., η = R ( LS ) ⊗ C A ( LS ) R ( L S ) ⊗ C( ∆L S ) . The phase and group delays can be derived as

[

][

]

∆Lr/p=4ωxt and ∆Lr/g=4ωxt-4ωfλt/p, and the resultant Doppler frequency shift fD and the bandwidth ∆f of the OCT signal can be derived as [15]:

f D = 4 xω λ 0

(2)

∆f = 4 xω∆λ λ20 − 4ωf∆λ pλ 0 ≈ − 4ωf∆λ λ 0 p

(3)

where ω is the angular speed of the tilting mirror, x is the mirror pivot offset from the optical axis, f is the focal length, and p is the pitch of the diffraction grating. It can be seen that fD and

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∆f can be constant only when the servo mirror moves at a uniform speed ω, and more importantly, fD can be parametrically regulated by the pivot offset x. Phase Control Delay Line Galvanometer Mirror CM Acousto-optic modulator

BBS 1320 nm LD

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Scan Head X z Analog Signal Processing

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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.

Although up to 2kHz scan rate has been reported by utilizing a fast galvanometric mirror driven with a triangular waveform, problems encountered in RSOD include: 1) for component parameters of p-1=560/mm, f=60mm, ∆λ=78nm, even at a large pivot offset with x=2mm, the quality factor allowed for bandpass filtering, fD/∆f≈xp/f∆λ1kHz), because of mechanical inertial and bandwidth limitation of the servo scanner, frequency instability δf may vary substantially even with a reduced duty cycle, resulting in

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further increased bandwidth B for bandpass filtering and thus drastically reduced signal-tonoise ratio. On the other hand, high-speed resonant scanner has been successfully used for real-time OCT [17]. Our test result in Fig. 3(a) shows that if driven with a 500Hz sinusoid waveform, δf changes from 236kHz to 379kHz (56%) so that a broader bandwidth B>600kHz has to be adopted to avoid cutting off the signal band. These results suggest that a new technique is desirable to provide a more stable and appropriately elevated Doppler frequency shifted signal to implement a high-performance and complete envelop demodulation for highspeed OCT without compromise between frame rate and image fidelity.

∆T=5.35µs

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. ∆T relates to the measured coherence length Lc. Artifacts such as serve ripples resulted from incomplete demodulation is obvious.

To provide a stable and appropriately enhanced Doppler frequency shift fD for highperformance OCT imaging, a fiberoptic AOM is inserted into the reference arm prior to RSOD as shown in Fig. 1. Unlike EO phase modulation, the optical frequency of light passing through the AOM will be modulated from ν0 to ν0 ± fAOM according to Bragg diffraction theory, where fAOM is the frequency of the modulating signal applied to the AOM crystal and frequency upshift or downshift depends on the propagation angel between the light wave and the acoustic wave. Here, different from commonly used AOM which operates in the RF frequency range of fAOM=40-200MHz, this custom made AOM consists of two AO modulators in series, one of which is upshifted to ν1=ν0 + fAOM1 whereas the other is downshifted to ν2=ν0 − fAOM2, yielding an adjustable roundtrip beat frequency fAOM=2(fAOM1-fAOM2). This pigtailed polarization insensitive fiberoptic AOM uses 2 0.3m long Corning SMF-28 fibers terminated

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by FC/APC connectors. Thus, with the insertion of AOM and placing the pivot offset to x=0, the resultant Doppler frequency fD of the detected interferometric signal changes to 400

Doppler frequency (KHz)

Doppler frequency (KHz)

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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.

f D / AOM = 4xω λ 0 + f AOM = f AOM

(4)

which is independent of servo mirror speed ω and can be adjusted by the AOM. Thus, the detected OCT signal changes to

[

]

I( ∆L r / g , t ) ≈ 2 I s I r ⋅ η R ( ∆L r / g ) ⊗ CA ( ∆L r / g ) ⋅ cos(f AOM t )

(5)

From Eq. (5), the advantages of using an AOM for frequency modulation are obvious. Although the group delay ∆Lr/g=-4ωfλt/p still depends on the scanning speed ω of the servo mirror in the RSOD, the interference phase term cos(fD/AOMt) is completely decoupled from ω; therefore, frequency instability (∆fD/AOM/fD/AOM) is eliminated to permit an effective signal band filtering and thus noise reduction. fD/AOM/∆f can be easily increased to a proper range (e.g., >3-5) to permit a complete envelop demodulation without increasing the servo mirror size (x=0). Also, the maximum RF power used to drive the AOM is less than 1W with input signal level lower than 24VDC. In the experiment, the AOM parameters were set to fAOM1=55MHz and fAOM2=54MHz. Here, fD/AOM=2MHz was carefully chosen to ensure a complete demodulation (e.g., for 5 frames/s OCT, fD/AOM=2MHz, ∆f=180kHz, fD/AOM/∆f ≈10; for 16 frames/s OCT, fD/AOM/∆f≈3.5) and a high detection signal-to-noise ratio (the sensitivity of a low-noise photocurrent preamplifier module decreases with increasing bandwidth). Additional advantages of employing a fiberoptic AOM for high-speed OCT include low insertion loss. Our test results showed that the total double-pass insertion loss of the AOM is only about 4.5dB (including polarization loss) so that an additional 0.7ND filter has to be added in the reference arm for optimal signal to noise yield. Also unlike EO modulator which requires a high-frequency broadband ramp or triangular waveform for linear phase modulation, this AOM is driven by 2 phase-referenced resonant signals at fAOM1=55MHz and fAOM2=54MHz, which is narrow band and is therefore technically simpler for the driver circuit design. Also, for the design scheme, it is polarization insensitive. The diffractive angel is designed to 29mrad to allow for spectral separation over 200nm at 1.3µm wavelength range.

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3.

Results

Experiments have been performed to examine and compare the performance and advantages between the OCT systems using regular RSOD and using AOM-mediated RSOD for reference scanning. Figure 4(a) is a recorded transient signal measured from the AOM-based OCT system, which was modulated at fD/AOM≈2MHz. Because the interference fringes within the low-coherence envelop have increased to 18, complete linear demodulation can be easily implemented. The excessive dispersion induced by the AO crystals in the AOM is well compensated (although not completely eliminated) by RSOD and the measured coherence length Lc≈10.6µm is close to the theoretical value of 9.8µm. According to Eq. (5), one of the important advantages of an AOM-based OCT system is that the carrier frequency f of the OCT signal is independent of ω, i.e., the scanning instability of the mechanical servo scanner in the RSOD. To examine this, a 500Hz modified triangle waveform (broadband linear scan) and a 500Hz sinusoid waveform (resonant scan) were used to drive the servo mirror in the RSOD, and the measured results are shown in Figs. 5(a) and (b), respectively. It can be seen that the measured Doppler frequency shift remains highly stable and is independent of the servo mirror scanning. The Doppler frequency variation is less than 0.39% over 95% of the duty cycle. The variation could be further reduced if the servo mirror is placed more precisely to zero pivot offset. It must be noted that the relatively high error bars in Fig. 5 were caused primarily by the measurement errors due to the limited time base resolution of the digital oscilloscope used to measure the Doppler frequencies of the OCT signal. To further demonstrate the performance enhancement by using AOM, Fig. 2 and Fig. 4(b) compare the results of the demodulated OCT signals under a comparable system condition except that Fig. 2 was measured without AOM (x=2.2mm, B=100kHz) whereas Fig. 4(b) was measured with AOM (x=0, B=300kHz). Although the OCT signal bandwidth is ∆f≈180kHz according to Eq. (3), it can be seen in Fig. 2 that because the carrier frequency fD≈270kHz is too low compared with ∆f, artifacts such as ripples and unstable signal amplitude due to incomplete linear demodulation are observed. Figure 4(b) shows that the linear demodulation with AOM is clean, complete and with enhanced signal-to noise ratio (Incomplete demodulation reduces signal level 10-25% according to our simulation and measurement). It should be noted that because the detection bandwidth is lowered to B=100kHz to reduce incomplete demodulation, B