Akinetic all-semiconductor programmable swept ... - OSA Publishing

Akinetic all-semiconductor programmable swept-source at 1550 nm and 1310 nm with centimeters coherence length M. Bonesi,1 M. P. Minneman,2,4 J. Ensher,2 B. Zabihian,1 H. Sattmann,1 P. Boschert,2 E. Hoover,2 R. A. Leitgeb,1 M. Crawford,2 and W. Drexler1,3 1

Center for Medical Physics and Biomedical Engineering, Medical University Vienna, Währinger Gürtel 18-20, 1090 Vienna, Austria 2 Insight Photonic Solutions, Inc., 300 S. Public Rd, Lafayette CO, 80026 USA 3 [email protected] 4 [email protected]

Abstract: We demonstrate, for the first time, OCT imaging capabilities of a novel, akinetic (without any form of movement in the tuning mechanism), all-semiconductor, all-electronic tunable, compact and flexible swept source laser technology at 1550 nm and 1310 nm. To investigate its OCT performance, 2D and 3D ex vivo and in vivo OCT imaging was performed at different sweep rates, from 20 kHz up to 200 kHz, with different axial resolutions, about 10 µm to 20 µm, and at different coherence gate displacements, from zero delay to >17 cm. Laser source phase linearity and phase repeatability standard deviation of 75% duty cycle, since this all-electronic tunable swept sources moved from any optical state to any other optical state in 2.5 ns. The OCT system’s probing beam spot size (diameter) was ~20 µm, measured in the sample arm’s focal plane. OCT system sensitivity >95 dB was accomplished with the 1550 nm sources and >90 dB for the 1310 nm source. Sensitivity values were acquired at 100 kHz sweep rate, with ~2 mW and ~0.5 mW incident power on the sample for the 1550 nm and 1310 nm laser source respectively. Further improvement is expected with improved detection and power levels from the source approaching 20 mW. The side mode suppression of the akinetic swept source was at least an order of magnitude better than that of other commercial lasers [11]. Most of the commercially available swept lasers offer (double pass) coherence lengths (@ −6 dB signal drop) between 3 and 15 mm, with one expensive polygon mirror unit that provides coherence lengths >50 mm, and an optically-pumped, externally amplified MEMS tunable VCSEL laser with coherence length of several centimeters [4,12,13]. The cavity of the akinetic swept source is ~2 mm long with a single internal longitudinal mode, therefore inherently offering long coherence length. In our SS-OCT setup employing akinetic swept sources, OCT imaging at 1550 nm was achieved with more than 170 mm double pass coherence gate displacement, with −1 dB signal drop within the first 40 mm and no significant changes between 20 and 200 kHz sweep speeds (see Fig. 5 and Fig. 12). To estimate laser performance, point spread function (PSF) roll-off curves, laser source phase stability and relative intensity noise (RIN) were evaluated. PSF peak value remained approximately constant for most of the displacement range, observing instead a rise in the noise floor at higher distances (>20 mm) from the zero delay.

#200144 - $15.00 USD Received 24 Oct 2013; revised 19 Dec 2013; accepted 27 Dec 2013; published 30 Jan 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.002632 | OPTICS EXPRESS 2640

Fig. 5. PSF roll-off curves of Insight 1550 nm 40 nm sweep range laser source recorded at (a) 20 kHz, (b) 100 kHz and (c) 200 kHz sweep rate. Plots (a), (b) and (c) show the PSF decay in the 0-180 mm, 0-40 mm and 0-20 mm depth range respectively, which represent the whole depth range allowed by the mechanical limitations of the reference arm for (a) and the associated selected sweep rates for all. The PSF peak values drop-off and the appearance of associated side lobes (some pointed by the black arrows as indications) were likely related to Nyquist limitations.

Figure 5 shows the roll-off plots for the 1550 nm 40 nm sweep range Insight laser source recorded at 20 kHz, 100 kHz and 200 kHz sweep rate respectively. To evaluate the system’s and laser’s sweep phase stability at different sweep rates, auto- and crosscorrelation measurements were performed. For both measurement types and for each selected sweep rate (20 kHz, 100 kHz and 200 kHz) a set of ~32000 consecutive spectra (sweeps) were collected from the same point on the sample (no xy scan active) using a 1 mm thick cover glass as sample in the sample arm, with the sample arm focal plane positioned inside the cover glass just beyond the front surface. Figure 6 reports a schematic representation of the algorithms applied to the acquired spectrums (raw data) for both cross- and auto-correlation phase


stability analysis. Auto-correlation analysis intended to estimate the laser’s sweep phase repeatability; in this case, data were obtained with the reference arm signal blocked while recording the self-interfering signal (front surface of the cover glass referred to its back surface) generated from the sample arm. Assuming that the self-interfered signal did not experience significant disturbances in the phase when travelling outside the laser source, the computed phase variations with the auto-correlation analysis (auto-correlation branch in Fig. 6) are attributed mainly to overall phase variations arising inside the laser source. Crosscorrelation analysis intended to estimate the laser phase linearity and phase stability of the system (laser + interferometer), which represents useful information in those cases where OCT imaging is obtained from the phase of the interferometric signal, such as Doppler OCT imaging. In this case measurements were conducted with the reference arm unblocked (conventional OCT imaging) while recording the interferometric signal. Standard deviation of phase differences of the recorded data evaluated at the PSF peak position coincident with the front surface of the cover glass were computed at the three sweep rates following the procedure illustrated in Fig. 6, “cross-correlation” branch.

Fig. 6. Schematic representation of the algorithms applied to evaluate the laser (autocorrelation) and system (cross-correlation) phase stability. Si represents the i-th spectrum of the ensemble (raw data); {} the FFT operator; |•| the module operator; ∠ the phase operator; −1{} the inverse FFT; Fˆ indicates the filtered PSF; φ the unwrapped phase; φD the phase difference of consecutive spectrums;

φ

the mean phase of the ensemble;

φ fit the linear fit of

the mean phase; σ(•) the standard deviation computation. See text for details.

These calculations gave an indication of laser (auto) and system (cross) phase jitter (φ) and displacement sensitivity (Δφ). In particular, σΔφ represents σ(φD,i) shown in the crosscorrelation branch of the algorithm schematic in Fig. 6, evaluated at the PSF peak position relative to the cover glass front surface. Following computational steps along the autocorrelation arm of the algorithm of Fig. 6, phases of each recorded power spectrum evaluated at the cover glass front surface, φi, were extracted from the raw data. Each φi was computed by unwrapping the phase of the inverse FFT signal, being previously filtered (peak extraction) to isolate the PSF values at the sample’s front surface. The digital filtering operation was #200144 - $15.00 USD Received 24 Oct 2013; revised 19 Dec 2013; accepted 27 Dec 2013; published 30 Jan 2014 (C) 2014 OSA 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.002632 | OPTICS EXPRESS 2642

obtained by multiplying the computed FFT of the recorded power spectrums (FSi) with a rectangular digital filter centered on the PSF peak and applying an additional Hanning window to the extracted portion of the FFT in order to reduce computational artifacts, e.g. induced oscillations due to Gibbs phenomena introduced by the application of the digital rectangular filter. The rectangular filter was defined as an array of zeros, except at the index position coinciding with the detected sample’s front surface PSF peak index position (derived from |FSi|) and few neighbor samples, where it assumed the value of 1. In our computation, the selected digital rectangular filter length was 20 samples, symmetrically centered on the PSF peak index position (i.e. 10 points before and 10 points after the PSF peak index).

Fig. 7. Estimation of phase stability for the Insight 1550 nm, 40 nm sweep range laser source at 20 kHz (blue curves), 100 kHz (red curves) and 200 kHz (black curves) sweep rates. a) φ = φmean mean phase, unwrapped; b) difference between mean unwrapped phase and associated linear fit curve

φfit

= φfit; c) evaluation of sweep linearity: standard deviation of the

differences between single sweep phases, φi, and φfit ; and d) evaluation of sweep repeatability: standard deviation of the differences between single sweep phases φi, and

φ.

From the collection of unwrapped phases, the mean phase, ϕ , was extracted and used as a reference curve to evaluate the laser’s sweep phase repeatability, quantified by computing the standard deviation of φi,− ϕ , i = 1, … N, where N indicates the total number of occurrences

(i.e. recorded spectrums) used in the calculations. A linear fit of ϕ , named ϕ fit, was also computed and used as a reference curve for the laser’s sweep phase linearity evaluation; also in this case, standard deviation computation of φi− ϕ fit , i = 1, … N, was applied to quantify the sweep linearity. Figure 7 reports the measured average phases per sweep (Fig. 7(a)) and quality figures of phase linearity (Fig. 7(c)) and repeatability (Fig. 7(d)) of the Insight 1550 nm, 40 nm sweep range laser source, evaluated at the three sweep rates of 20 kHz, 100 kHz and 200 kHz. In all three cases, the computed laser’s mean sweep phases exhibited, with a very good approximation, linear behavior. Sweep linearity is illustrated in Fig. 7(a), where the


three plotted curves of ϕ for the three selected sweep rates are quasi overlapping to each other. Figure 7(b) shows plots of the difference between the mean phase and its associated linear fit curve for the three sweep rates, where it is possible to appreciate a contained variation of the phase of ± 0.2 rad over a span of ~335 rad per sweep (