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In this study, we demonstrated a wide tuning range wavelength-swept laser with a single semiconductor optical amplifier (SOA) at 1020 nm for ultrahigh resolution, Fourier-domain optical coherence tomography (UHR, FD-OCT). The wavelength-swept laser was constructed with an external line-cavity based on a Littman configuration. An optical wavelength selection filter consisted of a grating, a telescope, and a polygon scanner. Before constructing the optical wavelength selection filter, we observed that the optical power, the spectrum bandwidth, and the center wavelength of the SOA were affected by the temperature of the thermoelectric (TE) cooler in the SOA mount as well as the applied current. Therefore, to obtain a wide wavelength tuning range, we adjusted the temperature of the TE cooler in the SOA mount. When the temperature in the TE cooler was 9 °C, our swept source had a tuning range of 142 nm and a full-width at half-maximum (FWHM) of 121.5 nm at 18 kHz. The measured instantaneous spectral bandwidth (δλ) is 0.085 nm, which was measured by an optical spectrum analyzer with a resolution bandwidth of 0.06 nm. This value corresponds to an imaging depth of 3.1 mm in air. Additionally, the averaged optical power of our swept source was 8.2 mW. In UHR, FD/SS-OCT using our swept laser, the measured axial resolution was 4.0 μm in air corresponding to 2.9 μm in tissue (n = 1.35). The sensitivity was measured to be 93.1 dB at a depth of 100 μm. Finally, we obtained retinal images (macular and optic disk) and a corneal image.
©2011 Optical Society of America
1. Introduction
Optical coherence tomography (OCT) has provided in vivo cross-sectional images and three-dimensional volumetric images of biological tissues with high axial resolution in real time since it was introduced by Huang et al. [1]. In ophthalmology, OCT has become a powerful imaging tool for diagnosing age-related macular degeneration, glaucoma, and diabetic macular edema because it is not possible to take a biopsy of the retina [2–6]. Recently, Fourier-domain OCT based on either a wavelength-swept laser (FD/SS-OCT) or a spectrometer (FD/SD-OCT) has been actively used because FD/SS-OCT and FD/SD-OCT can achieve a higher acquisition speed and sensitivity than time-domain OCT [7–11]. FD/SD-OCT has limited imaging depth range and fast sensitivity roll-off due to the limited number of camera pixels [12,13]. In contrast, FD/SS-OCT can achieve a slow sensitivity roll-off as it realizes a long imaging depth and reduced fringe washout [14,15]. In addition, FD/SS-OCT reaches a faster acquisition speed since the rise time of the photodiode is faster than that of InGaAs line-scan cameras at 1.0 μm or 1.3 μm [16].
Using ultrahigh resolution OCT (UHR-OCT) with enhanced axial resolution below 3 μm, it is possible to obtain in vivo cross-sectional images close to the level of histology. The axial resolution of an OCT system is determined by the center wavelength and the full-width at half-maximum (FWHM) of the light source. Specifically, a light source of a broad FWHM is needed to get an ultrahigh axial resolution because the axial resolution is inversely proportional to the FWHM of the light source. Drexler et al. first introduced UHR-OCT using a Ti:sapphire laser with a FHWM of 260 nm at 800 nm and obtained OCT images with an axial resolution of 1 μm of an African frog tadpole [17]. They also applied UHR-OCT with an axial resolution of 2–3 μm to obtain retinal images [18,19]. Leitgeb et al. [20] and Cense et al. [21] demonstrated UHR, FD/SD-OCT systems with resolutions of 2.5 μm and 3.5 μm in retinas, using a Ti:sapphire laser at 800 nm and a commercialized ultra-broadband superluminescent diode (SLD) at 890 nm, respectively.
Recently, a light source at 1060 nm has been actively used for ophthalmic applications because it has a local absorption minimum and zero dispersion near 1060 nm. In addition, the visualization of morphological features in superficial layers of the choroid should improve due to decreasing light scattering [22–25]. Additionally, according to the ANSI standard or ICNIRP guidelines, the maximum insertion power in the eye increases from 1.7 mW at 800 nm to 5 mW at 1060 nm for 10-s continuous wave exposure [23]. However, in UHR-OCT for ophthalmology, a spectral-domain method with an axial resolution of 3 μm has only been demonstrated at 1060 nm [22,25,26]. An InGaAs line-scan camera, which is used in a spectrometer at 1060 nm, is limited by the acquisition speed (92 klines/s) and number of pixels (1024 pixels). The acquisition speeds of FD/SS-OCT have increased greatly since a wavelength-swept laser based on the Fourier-domain mode-locking (FDML) method was introduced by Huber et al. [27,28]. Although the acquisition speed of FD/SS-OCT using a FDML swept laser is very fast, the wavelength tuning range and the FWHM are limited by the semiconductor optical amplifier (SOA) bandwidth and tuning range of the fiber Fabry–Perot tunable filter (FFP-TF). Because the FFP-TF is driven by an electronic piezoelectric actuator, the tuning range decreases as the tuning speed increases [29]. FD/SS-OCT using an FDML laser at 1060 nm with a scan rate of 249 kHz and an 8-μm axial resolution in tissue has been performed [30]. Kuznetsov et al. demonstrated a swept laser with a tuning range of 100 nm and a tuning speed of 100 kHz near 1060 nm, which was constructed with a silicon microelectromechanical system (MEMS) reflective tunable Fabry–Perot filter [31]. FD/SS-OCT using a MEMS reflective Fabry-Perot tunable laser at 1050 nm with a scan rate of 200 kHz and a 5.3-μm axial resolution in tissue has been used to obtain ophthalmic images [32].
In this paper, we demonstrate a wide tuning range wavelength-swept laser at 1020 nm for UHR-OCT. Wide tuning range wavelength-swept lasers with two SOAs at 1310 nm have been demonstrated [33,34]. However, we only used a single SOA with a wide bandwidth (>140 nm). In addition, we adjusted the temperature of the thermoelectric (TE) cooler in the SOA mount to maximize the tuning range. The optical wavelength selection filter consisted of a grating and a polygon scanner [35]. When the temperature in the SOA mount was 9 °C, our swept laser had a tuning range of 142 nm, a FWHM of 121.5 nm, and a scan speed of 18 kHz. The measured axial resolution of the UHR, FD/SS-OCT system was 4 μm, corresponding to 2.9 μm in tissue. Finally, we obtained retinal images (macular and optic disk) and a corneal image.
2. Experimental setup
2.1. Wavelength-swept laser
Figure 1 shows a schematic of the wavelength-swept laser, which was constructed on the external line-cavity [35]. The optical wavelength selection filter was based on the Littman configuration [35–38]. The SOA is a commercial broadband gain module based on a double-pass superluminescent diode gain chip similar to the AR/HR-coated laser diode gain chips (SOA-521, Superlum Ltd., Co. Cork, Ireland). The optical wavelength selection filter consisted of a volume phase holographic transmission grating (1007-1, Wasatch Photonics Inc., UT, USA), two achromatic doublet lenses (Thorlabs Inc., NJ, USA), and a polygon scanner (SA34, Lincoln Laser, AZ, USA) with a 30-facets mirror as shown in Fig. 1. The optical fiber of the SOA was spliced onto a 60:40 optical fiber coupler. The 60% beam of the total power from the optical fiber coupler was collimated by an aspheric collimation lens (NA = 0.50) and incident onto the optical wavelength selection filter. The back-reflected beam from the optical wavelength selection filter was amplified by the SOA. A polarization controller was inserted into the laser cavity because the SOA is polarization dependent. The 40% port of the optical fiber coupler was used to the output of the line-cavity of the swept laser. The free spectral range (FSR, λFSR) and FWHM (δλ) of an instantaneous wavelength in the filter are respectively determined by the following equations [36–38]:
where p, α0, and θ are the grating pitch, incident angle at the center wavelength (λ0), and the facet-to-facet polar angle of the polygon scanner (θ = 2π/N, N: number of facets). Additionally, m is the diffraction order, and W is the 1/e2 width of the Gaussian beam at the collimation lens. In this experiment, we selected optical components with the following parameters: p = 1/600 mm, α0 = 0.311 rad, f1 = f2 = 30 mm, θ = 0.209 rad (N = 30), W = 3.1 mm. The theoretical FSRs of the filter with these parameters were calculated to be λFSR = 332 nm and δλ = 0.27 nm. The rotational rate of the polygon scanner was set to 3600 rpm, corresponding to a filter repetition rate of 18 kHz.
Fig. 1 Schematic of the wavelength-swept laser based on the Littman configuration. SOA: semiconductor optical amplifier, PC: polarization controller, Col. Lens: collimation lens.
Table 1 shows a look-up table of temperature versus thermistor resistance in the TE cooler of the SOA mount, which was provided by the manufacturing company. Other temperatures versus resistances were estimated with a second-order polynomial curve fitting. We adjusted the temperature of the TE cooler in the SOA mount to maximize the tuning range of the wavelength-swept laser.
Table 1. Resistance vs. Temperature (provided by Superlum Ltd.)
2.2. Fourier-domain OCT
We constructed a UHR, FD/SS-OCT system using our wide tuning range swept laser, as shown in Fig. 2 . Thirty percent of the light of the total optical power from the swept laser was incident onto a Mach–Zehnder interferometer, which consisted of two 50:50 optical couplers for making the sampling clock. Seventy percent of the light from the swept laser was used for the Michelson interferometer of the FD/SS-OCT system. Light from the optical circulator was incident on a 2 × 2 optical coupler and was split into the sample and reference arms with a 60:40 ratio. To obtain retinal images, the sample arm consisted of 2-D galvanometers with silver-coated mirrors (TS8203, Beijing Century Sunny Technology Co., Ltd, China), an achromatic doublet lens (f = 50 mm, Thorlabs Inc., NJ, USA), and a double aspheric ocular lens (40 D, Volk Optical Inc., OH, USA), as shown in Fig. 2. The double aspheric ocular lens of 40 D can provide a wide field of angle, i.e., 90°. When a corneal image was obtained, only the achromatic doublet lens without the ocular lens was used. We used a neutral density (ND) filter in the reference arm to reduce the source intensity noise [39]. Recombined light from the sample and reference arms was incident onto the InGaAs-balanced photodiode (PDB130C, Thorlabs Inc., NJ, USA). The interference signals from both the Mach–Zehnder and Michelson interferometers were converted by a high-speed digitizer with 12-bit and 100 MS/s
Fig. 2 Schematic of the FD-OCT system using a wide tuning range wavelength-swept laser at 1020 nm. Col. Lens: collimation lens, PC: polarization controller, ND: neutral density filter, DP: dispersion compensation prism pair, BD: balanced detector.
(PCI-5124, National Instruments Corp., TX, USA), which has a maximum sampling rate of 200 MS/s. A total of 5555 samples were acquired per A-scan and channel of the digitizer at 18.0 kHz.
Since the swept laser was driven as a function of the wavelength, we performed numerical resampling as a function of the wavenumber (k). We used the zero-crossing method to achieve equal sampling space in the k-domain [40]. The zero-crossing points (or time) of the intensity from the interference signal of the Mach–Zehnder interferometer were found by the linear interpolation method. After finding the zero-crossing points, the intensity of the Michelson interference signal was selected at each one of the zero-crossing points. The intensity values at the resampled positions were also calculated by the linear interpolation method. After resampling into the k-domain, the total number of resampling points per A-scan was reduced to 1777 points. The data were zero-filled so that the total number of data points per A-scan was 2048 points.
3. Results and discussion
The optical power and spectrum shape of the SOA depend on the applied current and temperature of the TE cooler. Figure 3 shows the SOA spectra according to changes of the applied current and temperature of the TE cooler before the construction of the laser cavity. The SOA used in our swept source has two gain peaks near 960 nm and 1040 nm. The threshold current for lasing is typically 40 mA. When a current of 100 mA was applied, there was a gain peak near 1040 nm, as shown in Fig. 3(a). Following the current increase, the spectra near 960 nm were enhanced as shown in Figs. 3(b) and 3(c). Additionally, the temperature of the TE cooler of the SOA mount affects the spectrum and the optical power of the SOA. In Figs. 3(a), 3(b), and 3(c), we could observe that the optical power and the spectrum bandwidth of the SOA increased and the spectrum shifted to a shorter wavelength. Figure 3(d) shows the spectra of SOA at 9 °C when the applied currents were 100 mA, 200 mA, and 255 mA.
Fig. 3 Spectra of the SOA according to changes in the applied current or temperature of the TE cooler before constructing the laser cavity. (a) Apply current of 100 mA, (b) apply current of 200 mA, (c) apply current of 255 mA, and (d) at a temperature of 9 °C in the TE cooler.
Figure 4 shows the performance of the swept laser. Figure 4(a) shows the peak-held spectra of the swept laser measured by an optical spectrum analyzer according to temperature changes of the TE cooler. When a current of 255 mA was applied to the SOA and the temperature was decreased from 30 °C to 9 °C, the gain near 970 nm was more enhanced. Consequently, the center wavelength was linearly shifted to a shorter wavelength and the tuning range was also linearly broadened, as shown in Fig. 4(b). Although the tuning ranges were almost 140 nm at 15 °C and 10 °C, the FHWM values were measured to be 98.1 nm and 102 nm, respectively, due to the valley of the spectrum (red arrow), as shown in Fig. 4(a). When the valley of the spectrum increased a little at 9 °C, the FHWM rapidly broadened to 121.5 nm with a tuning range of 142 nm. When the polygon scanner was stopped, the measured instantaneous spectral bandwidth (δλ) was 0.085 nm, which was measured by an optical spectrum analyzer with a resolution bandwidth of 0.06 nm. This value corresponds to an imaging depth of 3.1 mm in air. Figure 4(c) shows the time-domain output trace of the swept laser at 18 kHz. We calculated a theoretical duty cycle of 42.7% from the measured tuning range and the theoretical FSR in section 2.1. The measured duty cycle was approximately 44.6% from Fig. 4(c). In addition, the averaged optical power of our swept source was 8.2 mW. In this study, we used an old type of polygon scanner with a low rotational rate (3600 rpm). In addition, the light was reflected once from the polygon scanning mirror. When the light is reflected four times from the polygon scanning mirror, which is similar to the configuration used by Oh et al. [38], the FSR theoretically increases to four times, corresponding to a duty cycle of 11.5%. When seven wavelength scans are sequentially delayed, and then interleaved with the fiber delay line with a two-stage cascaded Mach–Zehnder interferometer, eight wavelength sweeps can be achieved during a single facet rotation of the polygon scanner. In addition, if we use a new version of the polygon scanner with a high rotational rate of 5460 rpm, the wavelength-sweeping rate can be increased to 218.4 kHz.
Fig. 4 Performance of the wavelength-swept laser. (a) Spectra of the swept laser according to temperature changes of the TE cooler, (b) changes of the center wavelength and FHWM with respect to temperature changes of the TE cooler, and (c) time-domain output trace of the swept laser: the center wavelength and the FWHM are 1020 nm and 121.5 nm at 18 kHz, respectively, when the temperature of the TE cooler was set to be 9 °C.
To satisfy the safe ocular exposure limits set by the American National Standards Institute (ANSI), an averaged optical power of 1.6 mW was incident onto the sample arm. Figure 5 shows the performances of a UHR, FD/SS-OCT system using our swept laser. The sensitivity was measured to be 93.1 dB at a depth of 100 μm. The R-number of the sensitivity roll-off was around 11.18 dB/mm, as shown in Fig. 5(a). Although the sensitivity decayed fast due to the short imaging depth (or short coherence length of the swept-laser), the R-number value of our system is almost the same as or a little bit larger than that of the previous FD/SD-OCT system [25,41]. The theoretical resolution at the conditions of Gaussian shape and FHWM of 121.5 nm is 3.7 μm. When a mirror was used in place of the sample, the measured axial resolution was 4.0 μm in air, corresponding to 2.9 μm in tissue (n = 1.35). Figure 5(b) shows axial resolution variations in air as a function of the depth. We observed that the axial resolution was maintained below a depth of 2.0 mm. The graph in Fig. 5(c) is a point-spread function of the axial resolution near a depth of 0.7 mm.
Fig. 5 Performance of FD/SS-OCT. (a) Sensitivity as a function of depth: R-number values of sensitivity roll-off were found to be 11.1 dB/mm; (b) axial resolution as a function of the depth; and (c) point-spread function of the axial resolution near 0.7-mm depth.
Figure 6 shows UHR, FD/SS-OCT images of the retina. Figure 6(a) was selected with a region of interest (ROI) of 1000 (lateral) × 550 (axial) pixels. We applied a Wiener filter and a median filter with 3 × 3 matrices to reduce any white noise and speckle noise. We believe that the axial resolution in the retina increased up to 4.0 μm due to water absorption in the human eye [25,42]. Figures 6(b) and 6(c) are magnified images of optic disk (red) and macular (blue) areas, respectively, in Fig. 6(a). In Fig. 6(a), the external limiting membrane (ELM), which is the gray layer below the outer nuclear layer (ONL), can be seen. This layer is not visible with an OCT system using an ordinary broadband light source [21]. In addition, we distinguished three layers below the ELM: photoreceptor inner segment/outer segment junction (IS/OS), photoreceptor outer segments (PR OS), and retinal pigment epithelium (RPE). Below the RPE, the choroid-capillaries and the choroid can be seen. In Fig. 6(d), small highly reflecting white dots (red circles) can be seen in the plexiform layer. We believe that these white dots are capillaries because they consistently appear at the same location [25,42]. As shown in Figs. 6(a) and 6(d), a significantly deeper penetration below the RPE and into the choroid could be achieved and visualized.
Fig. 6 UHR, FD/SS-OCT images of a retina with 18 000 A-scans/s at 1020 nm. (a) A wide field of view image with 1000 (lateral) × 550 (axial) pixels from optic disk to macular area; (b) magnification of optic disk area (red box) in the retina; (c) magnification of macular area (blue box) in the retina; and (d) large blood vessel (blue circle) and capillaries (red circles) in the retina. NFL: nerve fiber layer, GCL: ganglion cell layer, IPL: inner plexiform layer, INL: inner nuclear layer, OPL: outer plexiform layer, ONL: outer nuclear layer, ELM: external limiting membrane, IS/OS: photoreceptor inner segment/outer segment junction, PR OS: photoreceptor outer segments, RPE: retinal pigment epithelium, and CH: choroid.
The accurate measurement of corneal thickness is an important diagnostic tool in monitoring corneal changes after laser in situ keratomileusis (LASIK) or extended contact lens wear [43–45]. Figure 7 shows UHR, FD/SS-OCT images of the cornea. We selected a ROI with an area of 500 (lateral) × 550 (axial) pixels. When the apex region (red box) in Fig. 7(a) was magnified, structural layers such as the epithelium, Bowman’s membrane, stroma, and endothelium could be observed. In particular, as shown in Fig. 7(b), Bowman’s membrane and the interface (red arrow) between Bowman’s membrane and stroma are very clear. Figure 7(c) shows intensity profiles as a function of the depth at the center of the cornea. When the epithelium and Bowman’s membrane areas were magnified, the coherence lengths (axial resolutions) were measured to be 4.1 μm and 4.2 μm in air (corresponding to 3.0 μm and 3.1 μm in tissue), respectively, as shown in Figs. 7(d) and 7(e). The axial resolution in the retina was larger than the calculated value (2.9 μm) but the axial resolution in the cornea was close to the calculated value.
Fig. 7 UHR, FD/SS-OCT images with 18 000 A-scans/s at 1020 nm. (a) The cross-sectional image of the cornea with 550 (depth) × 1000 (lateral) pixels; (b) magnification of the apex region (red box) of the cornea; (c) intensity profile as a function of the depth at the center of the cornea; (d) magnification of epithelium area (blue box in Fig. 7(c)); and (e) magnification of Bowman’s membrane area (red box in Fig. 7(c)). EP: epithelium, BM: Bowman’s membrane, S: stroma, and EN: endothelium.
4. Conclusions
In this study, we demonstrated a wide tuning range wavelength-swept laser at a center wavelength of 1020 nm with a single SOA for UHR, FD/SS-OCT. The wavelength-swept laser was constructed based on an external line-cavity. The optical wavelength selection filter was based on the Littman configuration with a grating, a telescope, and a polygon scanner. To obtain a wide wavelength tuning range, we adjusted the temperature of the TE cooler in the SOA mount. The SOA used in our swept source has two gain peaks near 960 nm and 1040 nm. When the temperature of the TE cooler decreased, the gain near 960 nm increased. Consequently, the center wavelength of the swept laser shifted to shorter wavelength and the tuning range broadened. At 9 °C, our swept laser had a tuning range of 142 nm and a FWHM of 121.5 nm at a scan speed of 18 kHz. In addition, the averaged optical power was 8.2 mW. When our swept laser was used in UHR, FD/SS-OCT, we measured a sensitivity of 93.1 dB and an axial resolution of 4.0 μm in air, corresponding to 2.9 μm in tissue. Finally, we obtained in vivo cross-sectional images of the retina and cornea of a healthy volunteer. We observed the structural layers of macular and optic disks in the retina. In the corneal image, Bowman’s membrane and the interface (red arrow) between Bowman’s membrane and stroma could be clearly seen.
Acknowledgments
This work was supported by the ETRI Support Program of the Ministry of Knowledge Economy (10035602).
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