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
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 , 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) , spectral OCT  and ultra-high-resolution OCT . With the commercialization of ultra-broadband laser technology, OCT will have the potential to become a subcellular-resolution 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 ; 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 . 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 phase-and 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 . 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 results, the use of a resonant EO phase modulator exhibits residual harmonic frequency components, and therefore compromises the envelop demodulation of the OCT detection . 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 . 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 signal-to-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 Δλ 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 Δf 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 ,
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 [-4Ls/Lc)2] cosk̄Ls 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., . The phase and group delays can be derived as ΔLr/p=4ωxt and ΔLr/g=4ωxt-4ωfλt/p, and the resultant Doppler frequency shift fD and the bandwidth Δf of the OCT signal can be derived as :
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 Δf 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.
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, Δλ=78nm, even at a large pivot offset with x=2mm, the quality factor allowed for bandpass filtering, fD/Δf≈xp/fΔλ<2, is too low to implement a complete envelop demodulation for optical heterodyne detection in our current OCT systems (especially when employing linear demodulation), as indicated in Fig. 2. To increase fD to permit a complete demodulation (e.g., fD/Δf≥3–5), the pivot offset x has to be increased and a larger mirror has to be used, which will in turn compromise the scanning rate of the servo mirror because of increased mechanical inertia for real-time OCT. 2) The stability of fD depends on the linearity of ω of the servo mirror. For instance, if a modified triangle waveform at 500 Hz was used to drive the servo mirror, the Doppler frequency shift δf, i.e., the instability of the interference fringe carrier frequency for optical heterodyne detection, varied slightly from 210 kHz to 270 kHz (25%) with the scanning depth within the duty cycle of 80%, as shown in Fig. 3(a). This relatively stable Doppler signal allows the use of a narrow bandpass filtering (ultimately limited by Δf) to reject off-band noise and thus ensure a high signal to noise detection of OCT signal (e.g., 95-100dB). However, the frequency instability range increased drastically 140 kHz to 270 kHz (63%) at 90% duty cycle due to acceleration and deceleration at the edges of the modified triangular waveform. This results in a broader bandwidth (e.g., B≥δf+Δf) for OCT signal processing and thus a reduced signal to noise ratio or dynamic range [7, 9, 15]. To make things worse, for high-speed OCT (e.g., with A-scan>1kHz), 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 further increased bandwidth B for bandpass filtering and thus drastically reduced signal-to-noise ratio. On the other hand, high-speed resonant scanner has been successfully used for real-time OCT . 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 high-speed OCT without compromise between frame rate and image fidelity.
To provide a stable and appropriately enhanced Doppler frequency shift fD for high-performance 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 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
which is independent of servo mirror speed ω and can be adjusted by the AOM. Thus, the detected OCT signal changes to
From Eq. (5), the advantages of using an AOM for frequency modulation are obvious. Although the group delay ΔLr/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 (ΔfD/AOM/fD/AOM) is eliminated to permit an effective signal band filtering and thus noise reduction. fD/AOM/Δf 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, Δf=180kHz, fD/AOM/Δf≈10; for 16 frames/s OCT, fD/AOM/Δf≈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.
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 Δf≈180kHz according to Eq. (3), it can be seen in Fig. 2 that because the carrier frequency fD≈270kHz is too low compared with Δf, 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<Δf, it cuts off signal band and results in broadening of the demodulated envelop or the detected OCT signal, i.e., the measured coherence length Lc of the OCT system. In other words, compared with Fig. 4(b), about 25% of reduced axial resolution is observed in Fig. 2 in order to reduce the artifacts such as ripples caused by incomplete demodulation. The frame rate of current high-speed OCT is determined by scanning speed ω of the servo mirror. This result suggests that an appropriate increase of the Doppler frequency shift will result in complete linear demodulation and thus enhanced signal to noise performance of optical heterodyne detection. Also, because the carrier frequency is ultrastable within 95% of the duty cycle, this increase in carrier frequency will not further broaden the signal bandwidth which is determined primarily by ω according to Eq. (3), there will be no reduction in signal to noise ratio.
In our AOM based OCT system, because x=0, a small 4mm servo mirror is used to facilitate fast axial scanning. Test results show that with the servo mirror scanning angle of 5° at a repetition rate of 1 kHz (driven by a linear triangular waveform), the axial scanning range of 2.8 mm can be implemented, thus allowing 2D OCT imaging at almost 4–8 frames/s depending on the lateral pixel resolution. The axial and transverse resolutions are roughly 10 µm, respectively . It was noticed in the experiment that, because AOM introduced additional dispersion, dispersion compensation was necessary to achieve the highest axial resolution and was implemented by moving the diffraction grating from the front focal plane of the Fourier lens to achieve close to transform-limited interferogram profiles. However, a complete compensation of the dispersion induced by AOM in reference arm requires insertion of like AOM crystals into the sample arm. To demonstrate the capability of our new system, a fresh porcine bladder was imaged as presented in Fig. 6. The image size is 1000×1000 pixels covering an area of a cross section of 6mm×2.8mm displayed in pseudocolor. Unlike other published high-speed OCT techniques using logarithmic demodulation, linear envelop demodulation has been implemented in our bladder cancer OCT imaging studies to enhance the imaged morphological details of the bladder tissue, especially that of the epithelium. The micro morphology of the porcine bladder such as the urothelium (U), the submucosa (SM) and the muscularis (M) can be delineated owing to their different backscattering signatures; therefore, early cancerous changes can be detected based on their morphological alternations, e.g., urothelial thickening and backscattering increase .
In this paper, we report the development of a high-speed OCT technique using acousto-optic modulation and present experimental results to compare the system performances with and without using acousto-optic modulation. Previously reported OCT techniques used pivot offset of the servo mirror x in the RSOD to adjust the Doppler frequency fD. It has been found that the carrier frequency fD is unstable and too low to implement complete amplitude demodulation, thus comprising the performances of high-speed OCT imaging. In contrast, the reported OCT setup employing an AOM-mediated RSOD uses a pair of AOMs to provide an ultra-stable and adjustable fD/AOM signal for high-performance optical heterodyne detection. Because the carrier frequency is appropriately lifted compared to the bandwidth Δf, a clean and complete linear demodulation can be easily implemented and has been proved by the presented measurement results. As indicated in Figs. 2–5, the AOM is successfully applied to yield a stable and high-frequency phase delay instead of increasing the offset x of the servo mirror in the RSOD, which will eventually slow down the mirror movement because of increased servo mirror size. It offers enormous advantages for high-speed OCT and DOCT. 1) the scanning mirror size can be minimized from currently 6–8mm to 3mm (limited by the linewidth D of projected spectral components. D=fΔλ/pcosθ, where θ is the diffraction angle), which will largely reduce the mechanical inertia of the servo mirror and thus increase the scanning rate and linearity. 2) because group and phase delays are decoupled, a resonant instead of a linear servo mirror can be used to facilitate 4-8kHz axial scanning, i.e., real-time OCT  without the compromise of reducing signal-to-noise ratio (no need to track fD or increase B to accommodate the variations of fD). The nonlinear group delay can easily be calibrated by virtue of simple digital image processing. 3) fD/AOM/Δf can be easily lifted appropriately to allow for complete and more accurate envelop demodulation, which will restore the signal amplitude and reduce the artifacts induced by incomplete demodulation. 4) For DOCT, independent phase modulation decouples spatial resolution and velocity sensitivity and largely increases imaging speed without compromising either spatial resolution or velocity sensitivity. In summary, we demonstrate a new rapid-scanning optical delay for high-speed OCT in which an AOM is used to provide a stable and higher-frequency Doppler signal at 2MHz for high-performance optical heterodyne detection. The measured Doppler frequency fD is highly stable with less than 0.39% fluctuation over 95% of the duty cycle. This technique has the potential to substantially improve the performance of a real-time OCT system by employing a resonant servo mirror to drive RSOD as well as the imaging speed and velocity sensitivity of a high-speed Doppler OCT system.
This research is supported in part by the Whitaker Foundation 00149 (Y.P.) and the National Institutes of Health RO1-1DK059265 (Y.P.). Address correspondence to Yingtian Pan (Yingtian.Pan@SUNYSB.edu)
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