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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 3 — Feb. 10, 2014
  • pp: 2632–2655
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Akinetic all-semiconductor programmable swept-source at 1550 nm and 1310 nm with centimeters coherence length

M. Bonesi, M. P. Minneman, J. Ensher, B. Zabihian, H. Sattmann, P. Boschert, E. Hoover, R. A. Leitgeb, M. Crawford, and W. Drexler  »View Author Affiliations


Optics Express, Vol. 22, Issue 3, pp. 2632-2655 (2014)
http://dx.doi.org/10.1364/OE.22.002632


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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 <2 mrad (<160 pm) were observed without external phase referencing, indicating that the laser operated close to the shot noise limit (~2 × factor); constant percentile wavelengths variations of sliding RIN and ortho RIN <0.2% could be demonstrated, ~5 times better as compared to other swept laser technologies.

© 2014 Optical Society of America

1. Introduction

Optical coherence tomography (OCT) is a well-established non-invasive, biomedical optical diagnostic imaging modality that enables in vivo cross-sectional tomographic and 3D visualization of internal microstructure and functional information in biological systems [1

1. Optical coherence tomography: technology and applications – Vol. 1, W. Drexler and J. G. Fujimoto eds. (Springer 2008).

]. This technology is nowadays used for biomedical imaging and has found successful applications in ophthalmology, cardiology and dermatology, including numerous technology advances. When considering the key aspect for a successful transfer of a new biomedical imaging technology into clinical practice, the technology must provide not only the required biomedical information the investigator is looking for in a clear and easily understandable manner, but it must also include some practical and functional characteristics that justify the effort spent in learning, training and/or implementing the new technology. Typical requirements in biomedical imaging include real-time, non-invasive, in-vivo imaging with micron-scale isotropic resolution and sufficient penetration, short measuring time (especially when fast in-vivo functional biological processes are under investigation), simple and convenient interfacing of the device with the sample under test, integration of dedicated delivery probes and achievement of meaningful imaging results. Recent OCT technology is capable of fulfilling most of the requirements needed by lab development and/or clinical practice [1

1. Optical coherence tomography: technology and applications – Vol. 1, W. Drexler and J. G. Fujimoto eds. (Springer 2008).

]. Following these guidelines for new technology adoption, from its beginnings [2

2. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science 254(5035), 1178–1181 (1991). [CrossRef] [PubMed]

] to date, OCT technology has rapidly improved and evolved from time-domain OCT (TD-OCT), to spectral/Fourier-domain OCT (SD/FD-OCT) and to swept-source OCT (SS-OCT). Advances in OCT technology went hand-in-hand with broadband light sources development. The light source represents a key technological element for OCT systems and the specific laser features directly define OCT system design and performance, specifically in terms of imaging speed and resolution. Various tunable source technologies are currently used in SS-OCT. Swept source OCT takes advantage of simultaneous high scanning speed and high axial resolution, together with the advantage of simple, cost-effective system and detector architecture [1

1. Optical coherence tomography: technology and applications – Vol. 1, W. Drexler and J. G. Fujimoto eds. (Springer 2008).

], when compared with other OCT technologies, such as SD/FD-OCT. State-of-the-art swept sources are based on mechanical movements, e.g. Fabry-Perot tunable filter [3

3. R. Huber, M. Wojtkowski, and J. G. Fujimoto, “Fourier domain mode locking (FDML): a new laser operating regime and applications for optical coherence tomography,” Opt. Express 14(8), 3225–3237 (2006). [CrossRef] [PubMed]

], MEMS tunable filters [4

4. High speed 1310nm swept source for OCT,” datasheet #2010–0230, Axsun Technologies Inc. (2009); http://www.axsun.com/PDF/OCT-SS1310-datasheet-update-7-12-13.pdf.

] or spinning mirrors [5

5. High speed scanning lasers,” datasheet, Santec Corp. (2013); http://www.santec.com/en/products/oct/lightsource-for-octsystem?gclid=CMCZkrLwlboCFQZZ3godk3UAUA.

] to achieve the desired sweep range and speed. New optically-pumped, externally-amplified MEMS tunable Vertical Cavity Surface Emitting Laser technology enables 100 nm tuning range at 1065 nm and 1310 nm central wavelength with single mode operation, resulting in several centimeters coherence length [6

6. V. Jayaraman, J. Jiang, H. Li, P. J. S. Heim, G. D. Cole, B. Potsaid, J. G. Fujimoto and A. Cable, “OCT imaging up to 760 kHz axial scan rate using single-mode 1310nm MEMS-tunable VCSELs with >100nm tuning range,” CLEO 1–2 (2011).

,7

7. B. Potsaid, V. Jayaraman, J. G. Fujimoto, J. Jiang, P. J. S. Heim, and A. E. Cable, “MEMS tunable VCSEL light source for ultrahigh speed 60kHz - 1MHz axial scan rate and long range centimeter class OCT imaging,” Proc. SPIE 8213, 82130M (2012). [CrossRef]

]. The mechanical movement, which represents the key aspect around which the above mentioned laser technology designs are based on, could also represent the limiting factor of the technology itself, limiting, by consequence, laser performance and imaging quality. Swept-wavelength laser technologies rely on highly repeatable, periodical and very stable sweep actuation; in simple terms, the more these criteria are met, the better the laser performs. All mechanical laser tuning mechanisms (such as tunable filters or MEMS mirrors) require accumulation and depletion of momentum for its completion, which can be subject to hysteresis (e.g. due to the mechanical movement) or experience undesired drifts of operational regime, e.g. due to external sources, such as thermal drift due to friction. Unstable or drifting mechanical movements can result in overall degradation of source performance. Other aspects that can influence swept laser performance are cavity length and laser optics design. Relatively long cavity length and complex laser source optics might result in cavity and/or mechanical instability (e.g. unwanted vibrations of the system), introducing another degradation mechanism in laser performance. In this paper we present – to the best of our knowledge for the first time – results from a novel akinetic, all-semiconductor swept laser source from Insight Photonic Solutions, Inc. (Lafayette, Colorado, USA). The innovative technology implemented in the laser involves no mechanically-moving parts (akinetic) to generate the sweep; the laser is based on integrated semiconductor opto-electronic design without the need of external cavity coupling. The all-semiconductor laser cavity is ~2 mm in length and is monolithically-constructed within the semiconductor. The all-semiconductor design enables a full electronic control of laser operation. The akinetic, all-semiconductor technological approach and design allows the akinetic laser to overcome most of the limitations encountered with mechanical sweep-based design implementations.

2. Methods

2.1 Akinetic laser technology

The Insight laser technology uses semiconductor structures integrated within a single laser chip for wavelength tuning, therefore eliminating the need for mechanical tuning elements, as traditionally used in state-of-the-art OCT swept sources. The parameters that define laser performance are software-controlled and the most relevant of them, e.g. sweep rate, sweep tuning range, output power and spectral output profile, are accessible to and modifiable by the user. This gives the laser a unique flexibility to adapt to specific user requirements and imaging performance necessities. The sweep tuning mechanisms of the all-semiconductor laser are based on controlled changes of the refractive index of the semiconductor materials composing the laser cavity. The semiconductors’ refractive index value is an expression of the electronic current flowing through it [10

10. B. R. Bennett, R. A. Soref, and J. A. Del Alamo, “Carrier-induced change in refractive index of InP, GaAs and InGaAsP,” IEEE J. Quantum Electron. 26(1), 113–122 (1990). [CrossRef]

]; a precise control and synchronization of the currents flowing through the cavity semiconductor elements yields precise control of laser tuning.

2.2 Duty cycle vs. sweep efficiency

2.3 Experimental setup

Figure 3
Fig. 3 Schematic representation of the imaging setup as used in the experiments. laser – Insight laser source; cr1, cr2 – circulator; 50/50 – fiber-based single mode optical coupler, 50/50 splitting ratio; p – polarization controller; c – collimator; R – retroreflector; M – mirror; xy scan – 2-axis galvo scan unit; L – imaging lens; PD – dual-balanced photodetector; DAQ – digitizer; PC – personal computer. See text for details.
shows a schematic representation of the imaging engine as used in the experiments. The system implemented a standard Michelson interferometer scheme with dual-balanced photodetection. The laser output was coupled into port 1 of a circulator (cr1) and delivered into the 50/50 single mode optical coupler (50/50) via port 2 of cr1. There the probing radiation was split according to the coupler’s splitting ratio – i.e. equally split, in our system – and diverted towards the reference and sample arm respectively via collimating optics (C).

The back-reflected (reference) and backscattered (sample) portion of the light coupled back into the optic fiber, crossed back through the 50/50 coupler, where it split and interfered. The split outputs from the coupler travelled through ports 3 of the respective circulator (cr1 and cr2) and were directed towards the dual-balanced photodetector (PD, Exalos AG, Schlieren, Switzerland; mod. EBR Balanced Receiver). The converted optic-to-electric interferometric signal was first low-pass filtered with a 190 MHz LP filter (Minicircuits, mod. SLP-200 + , not shown in Fig. 3) and then acquired by the 12-bit A/D digitizer (DAQ, Alazartech Technologies, Inc., Canada, mod. ATS9350) at 400 MS/s and sent to the personal computer (PC) to be converted into OCT image. The DAQ sample clock signal was derived from the Insight laser Sample Clock output, to synchronize laser sweep (generation of new cavity optical state or wavelength) with A-scans sample acquisition. The second circulator (cr2) was inserted in the system to create (as much as possible) symmetrical optical paths to be crossed by the interfered probing radiation while back-propagating from the 50/50 coupler towards the photodetectors.

2.4 Sources characterization and performance

Three different prototypes of the Insight akinetic all-semiconductor swept laser source were evaluated and used for high-speed SS-OCT imaging. Results from two swept laser sources at 1550 nm central wavelength and different sweep range (Δλ), and from one at 1310 nm central wavelength were obtained. Table 1 summarizes the main specifications of these sources as used in the experiments.

Based on the imaging engine schematic as illustrated in Fig. 3, two similar imaging systems were built for the three sources; one optimized for the 1550 nm central wavelength light sources and the other optimized for the 1310 nm central wavelength light source.

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 cross- correlation 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
Fig. 6 Schematic representation of the algorithms applied to evaluate the laser (auto-correlation) and system (cross-correlation) phase stability. Si represents the i-th spectrum of the ensemble (raw data); ℱ{} the FFT operator; |•| the module operator; ∠ the phase operator; ℱ−1{} the inverse FFT; F^ 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.
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. Cross-correlation 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.

These calculations gave an indication of laser (auto) and system (cross) phase jitter (φ) and displacement sensitivity (Δφ). In particular, σΔφ represents σD,i) shown in the cross-correlation 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 auto-correlation 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 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).

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 φiφ¯fit, i = 1, … N, was applied to quantify the sweep linearity. Figure 7
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 φ¯fit = φfit; c) evaluation of sweep linearity: standard deviation of the differences between single sweep phases, φi, andφ¯fit; and d) evaluation of sweep repeatability: standard deviation of the differences between single sweep phases φi, and φ¯.
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 (<0.06%) along almost the whole sweep range. Since the phase of a single sweep differs from its linear approximation by less than 0.06%, linearity and repeatability estimations offered very similar results (Figs. 7(c) and 7(d)). What can be observed from Figs. 7(c) and 7(d) is that an increase of the laser sweep rate yields an increase in phase variations (larger standard deviation) during the sweeps, degrading or altering linearity and repeatability. Our calculations revealed a standard deviation of the order of milliradians over a span of ~335 rad per sweep. We observed larger overall variations <2 mrad for the worst case at higher sweep rate, with the only exception of the observed peak of ~8 mrad at ~1558 nm wavelength for the 200 kHz sweep rate case. The presence of the ~8 mrad standard deviation peak in Fig. 7(c) is a consequence of the laser calibration process, whose performance is governed by the laser internal algorithms. It is expected these anomalies will be reduced or eliminated with improved versions of laser firmware. These plots are compared with the minimum detectable phase (difference) model valid under shot noise limit conditions [14

14. B. Park, M. C. Pierce, B. Cense, S. H. Yun, M. Mujat, G. J. Tearney, B. E. Bouma, and J. de Boer, “Real-time fiber-based multi-functional spectral-domain optical coherence tomography at 1.3 microm,” Opt. Express 13(11), 3931–3944 (2005). [CrossRef] [PubMed]

]:
σsnσΔφ,shotnoise=1SNR,
where the SNR is calculated, for each selected sweep rate, as the ratio between the auto-correlation PSF peak value and the standard deviation of the background noise measured in the PSF neighbor area. Computed values of σsn in the range from 0.32 to 1.15 mrad for sweep rates from 20 kHz to 200 kHz respectively (see Table 2

Table 2. Estimation of phase (σφ), phase differences of consecutive spectrums (σΔφ) and phase under shot noise conditions (σsn, model) of power spectrum FFTs of auto-correlation (left side) and cross-correlation (right side) data sets using the Insight 1550 nm, 40 nm sweep range laser source. FFT phases are evaluated at cover glass front surface (PSF front peak position). The cover glass is 1 mm thick.

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) indicate that the system is, with good approximation, close to shot noise limit or, in other words, that the laser does not generate significant phase noise. Table 2 summarizes the computed results of power spectrum (raw data) FFT phase stability for both the auto- and cross- correlation acquired data sets, using the 1550 nm, 40 nm sweep range laser source at 20 kHz, 100 kHz and 200 kHz sweep rates respectively.

Auto-correlation shot-noise limited phase sensitivity measurements were obtained without the need for any external wavelength references, such as a temperature-stabilized Bragg grating or a gas cell [15

15. R. V. Kuranov, A. B. McElroy, N. Kemp, S. Baranov, J. Taber, M. D. Feldman, and T. E. Milner, “Gas-cell referenced swept source phase sensitive optical coherence tomography,” IEEE Photon. Technol. Lett. 22(20), 1524–1526 (2010). [CrossRef]

]. Estimations of standard deviation of phase (σφ, σφD), phase differences (σΔφ, σΔφD) between consecutive spectrums and phase under shot noise conditions (σsn) of the computed power spectrum FFT were referred to the cover glass front surface (PSF front peak position) for the cross- correlation data sets and to the cover glass front surface referred to cover glass back surface for the auto-correlation data sets. Direct application of the FFT algorithm on the recorded interference pattern (raw data) yields Fourier-transform limited peak widths. The slight deviations observable at the boundaries of the sweep (e.g. ref. Figure 7(b)) do not cause any observable broadening of the computed PSF, ascribing instead their presence mainly to the applied digital processing. Figure 8
Fig. 8 Direct FFT computation of acquired spectra (raw data) for (a) auto-correlation and (b) cross-correlation data sets. The peaks in (b) relate, from left to right, to the front and back surface of the cover glass respectively. Data acquired with Insight 1550 nm, 40 nm sweep range at 200 kHz sweep rate.
shows the computed FFT for the auto-correlation (Fig. 8(a)) and cross-correlation (Fig. 8(b)) data set, acquired at 200 kHz.

Similar quality figures were obtained from the 20 kHz and 100 kHz data sets. The auto-correlation PSF peak shown in Fig. 8(a) is broadened due to dispersion experienced by the probing beam while crossing the 1 mm cover glass. Figure 8(b) shows two cross-correlation interferometric peaks, associated (from left to right) to the front and the back surface of the cover glass. The front surface peak (left peak in Fig. 8(b)) is composed by one single sample, indicating that the PSF is Fourier-transform limited. This is an expression of simultaneous good laser phase linearity and that the system is, with good approximation, dispersion-free. The cover glass back surface peak (right peak in Fig. 8(b)) is broadened due to dispersion introduced by the sample. A direct comparison of the shape (envelope) of this peak with the auto-correlation peak shape (envelope) suggests that the main contribution to peak broadening is originated by dispersion introduced by the sample and not by the system or by non-linear phase changes introduced by the laser.

2.5 Relative intensity noise

Relative intensity noise (RIN) is the measure of undesired fluctuations in the laser output power. The laser output power can be expressed as P = P0 + ΔP(t), where P0 represents the average optical output power, and ΔP(t) represents the time-dependent fluctuation with zero mean, i.e. the time-averaging value of the fluctuation is zero. The power of the fluctuation is characterized via the mean of its squared deviation, which can be expressed in the power spectral density function SΔP(f) [16

16. A. Yariv, “Optical electronics in modern communication,” (Oxford University, 1997).

]. We measured the RIN of the 1550 nm, 40 nm sweep range, 5.5 mW output power akinetic laser at different laser sweep rates. In our analysis, the power spectral density (PSD) function was calculated by computing the Fourier transformation of normalized recorded time series of power fluctuations. The measurements were performed for the 20 kHz, 100 kHz and 200 kHz laser sweep rates and with no active sweep. Power fluctuations traces of ~1.5 million points and ~3.5 million points for the 200 kHz and 100 kHz cases respectively, and ~19 million points for the 20 kHz and no-sweep cases respectively were Fourier transformed, and the square of the amplitudes displayed as power spectrum. The analysis also included the laser non-sweeping regime to estimate the amplified spontaneous emission (ASE), regarded as reference background noise, generated by the system and captured by the digitizer.

The spectrums were normalized to units of dBc/Hz by adding a constant on the logarithmic scale such that the DC component assumed the value of 10·log(1Hz/RBW), with RBW being the equivalent RF-resolution bandwidth defined as digitizer sampling frequency/trace length. RBW values of ~250 Hz, ~110 Hz and ~20 Hz were obtained for the 20 kHz, 100 kHz and 200 kHz sweep rates respectively. During the measurements, the laser output power was set to 5.5 mW, and a fraction of it, ~110 µW, was sent to the photodetector. Figure 9
Fig. 9 Normalized RF PSD of Insight 1550 nm, 40 nm sweep range. (a-c) ASE with no active sweep; (d-f) PSD at 20 kHz sweep rate; (g) PSD at 100 kHz sweep rate; (h) PSD at 200 kHz sweep rate. Red traces in d-f indicate previously published data for a FDML swept laser source (ref [17].). The computed detector’s shot noise limit (~110 µW incident power) was −145.8 dBc/Hz.
shows the normalized power spectra of the laser during ASE (9(a)) and sweeping regimes (9(d), 9(g), 9(h)) at the different sweep rates. The ASE shows an approximately linear increase of the noise floor over the detection bandwidth; similar result was also obtained when no laser output was connected to the photodetector, therefore ascribing the cause of the rising of the noise floor due mainly to the photodetector. Figure 9(b) and Fig. 9(c) show a magnification of the PSD in ASE regime. With the laser connected to the PD and in sweep regime, the computed PSD show an increase in the noise floor level while approximately preserving a linear variation along the detection bandwidth, with the exception of the very low frequency region where a slight non-linear increase is observed. Figure 9(d) shows the computed PSD for the 20 kHz sweep rate; Fig. 9(e) and Fig. 9(f) show a magnified portion of the PSD at 20 kHz. The overlapped red traces in Fig. 9(d)-9(f) represent previously published data obtained with another OCT swept laser (ref [17

17. B. R. Biedermann, W. Wieser, C. M. Eigenwillig, T. Klein, and R. Huber, “Dispersion, coherence and noise of Fourier domain mode locked lasers,” Opt. Express 17(12), 9947–9961 (2009). [CrossRef] [PubMed]

].), regarded as the best term of comparison. It is interesting to notice that fluctuations associated with sweep repetition rate of the akinetic laser do not appear in the PSD data, unlike what can be observed in mechanically tuned swept sources. The three peaks observed in the PSD curves for the three sweep rates are originated by the calibration process (particularly the peak observed at ~80 MHz) and other laser internal processes. Similarly as commented for the phase stability analysis, these processes are software controlled and the reduction or elimination of these peaks might be expected with improved releases of laser internal firmware. The RIN analysis was based on the definition of sliding RIN and ortho RIN as from [17

17. B. R. Biedermann, W. Wieser, C. M. Eigenwillig, T. Klein, and R. Huber, “Dispersion, coherence and noise of Fourier domain mode locked lasers,” Opt. Express 17(12), 9947–9961 (2009). [CrossRef] [PubMed]

], which can explicitly express the relative intensity noise of the swept laser as a function of the wavelength. A detailed description of the definitions and methods that defines the sliding and ortho RIN is reported in [17

17. B. R. Biedermann, W. Wieser, C. M. Eigenwillig, T. Klein, and R. Huber, “Dispersion, coherence and noise of Fourier domain mode locked lasers,” Opt. Express 17(12), 9947–9961 (2009). [CrossRef] [PubMed]

]. In the case of the Insight swept laser, the wavelength-sampled point sweep relation is obviously straightforward (see section 2.1 and section 2.6), making the definition of Biedermann et al. an ideal evaluation method for the akinetic all-semiconductor laser types.

2.6 Considerations on integrating the Insight laser source into OCT imaging engines

The all-semiconductor akinetic laser technology impacts most aspects of the performance and design of a SS-OCT system, including the way the optical signal generated by the interferometer must be conveniently acquired, and how the subsequent digitized data is treated to obtain meaningful OCT information. In this section we describe the aspects of data acquisition and conditioning as imposed by the Insight laser source. The optical frequency versus time relation in mechanically tuned swept lasers often lack sufficient linearity to enable a direct conversion of the acquired interferometric signal (raw data) into an OCT image. To compensate this disadvantage, most SS-OCT systems require an external optical k-clock for variable-rate sampling [18

18. M. A. Choma, K. Hsu, and J. A. Izatt, “Swept source optical coherence tomography using an all-fiber 1300-nm ring laser source,” J. Biomed. Opt. 10(4), 044009 (2005). [CrossRef] [PubMed]

,19

19. J. Xi, L. Huo, J. Li, and X. Li, “Generic real-time uniform K-space sampling method for high-speed swept-source optical coherence tomography,” Opt. Express 18, 9511 (2010).

] or the acquisition of a similar k-clock signal that can be analyzed to properly resample the even-in-time acquired raw data [20

20. Y. Yasuno, V. D. Madjarova, S. Makita, M. Akiba, A. Morosawa, C. Chong, T. Sakai, K. P. Chan, M. Itoh, and T. Yatagai, “Three-dimensional and high-speed swept-source optical coherence tomography for in vivo investigation of human anterior eye segments,” Opt. Express 13(26), 10652 (2005).

,21

21. R. Huber, M. Wojtkowski, K. Taira, J. G. Fujimoto, and K. Hsu, “Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles,” Opt. Express 13(9), 3513–3528 (2005). [CrossRef] [PubMed]

]. The akinetic all-semiconductor laser is built to inherently generate linear sweeps, removing the requirement for an external optical k-clock. It enables data that is linearly spaced in optical frequency (k-space) to be sampled at a constant sampling rate by the digitizer. This means that the acquisition of a reference rescaling signal to compensate for laser sweep non-linearity is no longer necessary, as it is no longer necessary to resample the acquired interferometric signal for k-space linearization, significantly reducing raw data-to-image computing time. No external k-clock was needed in our SS-OCT system, since this was provided from a digital sample clock output signal from the laser, with a sweep-to-sweep linearity deviation <0.002%, resulting in sampling triggers accurate within 300 fs – 10 × better than any optical k-clock. The digital sample clock provided by the laser source ensured optimal synchronization between the laser’s optical state changes, occurring each 2.5 ns (valid and invalid sweep points, see section 2.1), and data acquisition (sampling rate) during one sweep. At each new laser optical state, the clock must correspond to the acquired sample from the digitizer. During the full completion of one sweep/data acquisition, this synchronized one-to-one relation between optical states and acquired samples must be maintained, yielding acquisition of samples at constant rate of both valid and invalid optical laser states (see section 2.1). The effective sampling rate at which the digitizer must be set is imposed by the laser; it must match the laser master clock frequency, which was, in our experiments, 400 MS/s (or 2.5 ns sampling interval). This constraint requires that the digitizer must be driven by an external sampling clock signal – the Sample Clock signal supplied by the laser: Once acquired, and before proceeding with conventional data processing (e.g. fixed pattern noise removal, windowing, FFT, etc.), the sampled data from each sweep, i.e. each A-scan, must be decimated at specific samples’ index positions in order to remove those samples associated with laser transitions (invalid points) occurring during the sweep. To illustrate this concept, Fig. 11
Fig. 11 Effect of the decimation process on acquired (digitized) raw data to remove the invalid points. The inset in (a) represents the scales of the horizontal and vertical axes for each plot in the figure. Plots (a-f) show recorded power vs. sample traces before (left column) and after (right column) the removal of the invalid points. The insets in (c) show magnified portions of the trace. The red arrows in the 40 points (pts.) inset (leftmost) indicatively show the beginning (left arrow) and the end (right arrow) of the transition interval between two valid points subsets. The blue arrow in the 5000 points inset (rightmost) in (c) and its analogous in the 5000 points inset in (d) for the equivalent magnified portion of the traces, illustrate the effect of an incorrect reconstruction of the data set with invalid points removed. The 200 points insets in (g) and (h) illustrate the effect of the removal of invalid points on interferometric fringes signal.
reports recordings of power spectra output traces with no active sweep (Figs. 11(a)-11(b)) and with the laser sweeping at very low sweep rate (~8 kHz) at 1 mW (Figs. 11(c)-11(d) and 2 mW (Figs. 11(e)-11(f)) output power; and recordings of interferometric signal (Figs. 11(g)-11(h)) using a mirror as a sample in the sample arm (2 mW laser output power, interferometric signal power attenuated before the photodetector to avoid saturation).

3. Results

To validate laser performance, measurement at several sweep rates and at different coherence gate displacements were performed using three different Insight swept laser sources whose main specifications are described in Table 1. We applied the Insight 1550 nm, 40 nm sweep range laser source, first available in our labs, to investigate laser performances and imaging limits at different coherence gate displacements. For each selected sweep rate, the sample in the sample arm (SA) was fixed and positioned in the SA focal plane while varying the distance in the reference arm (RA). Data sets for 2D and 3D sample reconstructions were acquired at each coherence gate displacement, starting close to the zero delay up to the largest displacement allowed by the selected laser sweep rate and digitizer sampling rate (this latter was always set to 400 MS/s, as required by the laser source). Selecting a specific laser sweep rate means selecting a corresponding specific number of (valid) points per sweep (A-scan); combined with a fixed sampling rate, this also defines the upper limit of the coherence gate displacement for OCT imaging. The lower the sweep rate, the higher the number of points per sweep, which produces a higher limit of the coherence gate displacement from the zero point of the delay line. The sample used in the experiments was an ex vivo tooth; sequences of 2D tomograms acquired always from the same location on the sample for all the different configurations were processed and the results are presented in Fig. 12
Fig. 12 OCT imaging using the Insight 1550 nm, 40 nm sweeping range laser source, at different sweep rates and different displacements of the coherence gate. The tomograms show cross-sectional reconstruction of an ex vivo tooth. Data were acquired always on the same location on the sample for all the shown data sets. Each figure was averaged from 32 consecutives B-scans. Image size is 6 × 2.9 mm2 (width × depth, in tissue), corresponding to 1024 × 100 pixels (hor. × vert.) for the 20 kHz images (topmost row) and 2048 × 100 pixels for the remaining images. Per each selected laser sweep rate, imaging range spans from zero delay (z.d.) up to the largest coherence gate displacement allowed by the system. Incident power on the sample ~2 mW. Refractive index n = 1.44.
. Three different sweep rates were chosen for the measurements: 20 kHz, 100 kHz and 200 kHz, requiring 19000, 3500 and 1550 (valid) points per sweep respectively. The higher the number of points per sweep, the larger coherence gate displacement in RA it was possible to achieve before encountering the limits imposed by the system: first, the maximum mechanical displacement in the RA of the interferometer; second, the Nyquist limit (200 MHz) to the maximum analog signal that may be measured by our data acquisition system, clocked by the laser’s 400 MHz user sample clock. For a 20 kHz sweep, the RA displacement limited the maximum coherence gate. By contrast, at 200 kHz sweep rates the Nyquist limit of 200MHz signal frequencies is reached at coherence gate displacements well below the constraints of our particular reference arm mechanism. Fringe degradation could be observed from non-optimal laser calibration, i.e. non-optimal stitching of consecutive valid points in sweep sub-intervals, which might introduce unwanted frequency components in the interferometric signal falling within the selected imaging range.

Jitter noise also induced a rise in the noise floor of the OCT signal, producing a loss in image sensitivity at larger RA optical path distances. Occasionally, fringe corruption caused the appearance of PSF side lobes, which translated into ghosted replicas of the sample along the imaging range as selected by the coherence gate position in the final OCT tomograms.

Keeping the coherence gate displacement within the useful limits, the obtained images were clear and undistorted. Undesired image disturbances introduced by the calibration process were removed by repeating the calibration. At each selected sweep rate–more evident in the 20 kHz case of Fig. 12 – it can be noticed a drop in intensity due to a natural decay of the probing radiation intensity and also due to the rise in the noise floor that limited image dynamic range (see the background outside the tooth boundary). From Fig. 12 it is also possible to appreciate how an increase of the laser sweep rate does not significantly degrade OCT imaging performance for a fixed coherence gate displacement position. Main factors that limited imaging at longer coherence gate displacements were the available number of samples for FFT computation and digitizer sampling rate (sample clock of 400 MHz, fixed), both imposed by the laser source. Figure 13
Fig. 13 Single vs. averaged in vivo skin imaging at 1550 nm, 40 nm sweep range. a-c) single frame; d-f) averaged frame from 32 consecutive B-scans (M-series). Image size is 6 × 2.9 mm2 (width × depth, in tissue, n = 1.44), corresponding to 1024 × 100 pixels (hor. × vert.) for a and d (i.e. 20 kHz sweep) and 2048 × 100 pixels for the others. Coherence gate location close to zero delay. Scale bar = 1 mm. Incident power on sample ~2 mW.
shows a comparison of single vs. averaged tomograms of in vivo skin acquired with 1550 nm central wavelength, 40 nm sweep range laser at 20 kHz, 100 kHz and 200 kHz sweep rates. The sample was positioned in the sample arm focal plane and the coherence gate positioned close to the zero delay. M-series of 32 B-scans were acquired and processed. Each frame was acquired every ~50 µs (19424 × 1024 raw data pixels, vert. × hor., including invalid points), ~10 µs (3924 × 1024 raw data pixels, vert. × hor., including invalid points) and ~5 µs (1974 × 1024 raw data pixels, vert. × hor., including invalid points) for the 20 kHz, 100 kHz and 200 kHz sweep rates respectively. No motion correction was applied to the averaged tomograms. The left column (Figs. 13(a), 13(b) and 13(c)) shows single frame results obtained at different sweep rates; the right column (Figs. 13(d), 13(e) and 13(f)) shows the respective sweep rate data set averaged 32 times. Incident power on the sample was ~2 mW.

In all three shown cases it was possible to obtain images with very similar quality and no noticeable intensity drop with increased sweep rate. Figure 14
Fig. 14 In vivo skin imaging at 1550 nm, 40 nm SR with ~20 µm isotropic resolution. a) 3D sample reconstruction; b) internal view of the structure; c) cross-sectional view; overlapped rectangle correspond to overlapped rectangle (vertical plane, blue) in b; d) en-face view; overlapped rectangle corresponds to overlapped rectangle (horizontal plane, green) in b. Volume size is 5 × 5 × 2 mm3 (width × height × depth, in tissue, n = 1.44), corresponding to 1024 × 256 × 180 pixels. Scale bars correspond to 0.50 mm. Incident power on sample ~2 mW.
demonstrates the capability of the system to perform very fast in vivo acquisitions of large 3D data sets and to generate 3D reconstruction of the sample. In the reported example, results from in vivo skin measurements are shown, acquired with the 1550 nm central wavelength, 40 nm sweep range swept laser source, sweeping at 109 kHz. The 3D volume data set of 1.5 GB was acquired in ~2.4 s with a 12-bit digitizer (Alazartech, mod. ATS9350). Figures 14(a) and 14(b) show the 3D reconstruction of the sample in its entirety and with a portion of the data removed to reveal the internal structure, respectively. Figure 14(c) shows a single tomogram extracted from the 3D volume corresponding to the vertical frontal tomogram as shown in Fig. 14(b) and delimited by the blue rectangle. Figure 14(d) illustrates an en face view of the sample, extracted from the 3D data set corresponding to the horizontal plane as shown in Fig. 14(b) and delimited by the green rectangle. The larger speckle patterns which can be noticed from the presented results are a consequence of the applied 3D rendering to the data. The latest generation of the Insight laser source at 1550 nm offers extended spectral bandwidth, up to 80 nm (sweep range @ 0 dB drop) in our tests, higher number of sampling points per unit depth and higher output power, up to 13.5 mW.

Figure 15
Fig. 15 Extended sweep range of the 1550 nm laser improves OCT axial resolution (Δz, in tissue) imaging. a), b) cross-sectional tomographies (average from 32 B-scans) of ex vivo tooth; images size is 7 × 2.9 mm2 (width × depth, in tissue, n = 1.44), corresponding to 2048 × 100 pixels and 2048 × 300 pixels (hor. × vert.) respectively. c), d) 1.5 × magnification of the rectangle areas in 12a and 12b respectively. Coherence gate close to zero delay. Scale bar = 1 mm. Incident power on sample ~2 mW.
demonstrates enhanced performance of the broad bandwidth light source, by comparing results obtained with the two 1550 nm laser sources (40 nm vs. 80 nm sweep ranges), both sweeping at 40 kHz, the highest achievable sweep rate with the broader sweep range Insight laser source at the time of the experiments. Figure 15(b) reveals sharper and more detailed morphological structure, not always evident in the correspondent image (Fig. 15(a)) acquired with the smaller bandwidth laser source. Figs. 16
Fig. 16 OCT imaging using the Insight 1310 nm, 30 nm sweep range laser source. a), b) single frame and 32 frames average of ex vivo tooth; image size is 6 × 2.9 mm2 (width × depth, in tissue). c) en face projection of ex vivo tooth 3D data set; image size is 6 × 8 mm2 (width × height). d), e) single frame and 32 frames average of in vivo skin; image size is 6 × 2.9 mm2 (width × depth, in tissue). f) en face projection of in vivo skin 3D data set; image size is 6 × 6 mm2 (width × height). Incident power on sample was 0.5 mW. Refractive index n = 1.44.
and 17
Fig. 17 OCT imaging using the Insight 1310 nm, 30 nm sweep range source. a) 3D reconstruction of ex vivo tooth; vol. size is 6 × 8 × 3 mm3 (width × height × depth, in tissue); b) internal view of the structure; c) cross-sectional tomography extracted from 3D data set (blue rectangle in b); d) en-face view extracted from 3D data set (green rectangle in b). Incident power on sample was 0.5 mW. Coherence gate close to zero delay. Refractive index n = 1.44. Scale bar = 1 mm.
introduce results obtained with the Insight 1310 nm laser source from ex vivo tooth and in vivo skin recorded at 100 kHz sweep rate. In all recordings, the samples were positioned close to the zero delay line. Figures 16(a) and 16(b) illustrate single frame vs. averaged frame 2D tomography of the tooth; Fig. 16(c) illustrates the en face projection of a 3D data set of the same sample. Figures 16(d) and 16(e) illustrate single frame vs. averaged frame 2D tomography of the skin. Figure 16(f) illustrates the en face projection of a 3D data set of the same sample.

Figure 17(a) shows a 3D reconstruction the ex vivo tooth; Fig. 17(b) shows the same data set of Fig. 17(a) with a portion of the data removed to reveal the internal structure of the sample. Figure 17(c) shows a 2D tomography extracted from the 3D volume corresponding to the blue vertical rectangle in Fig. 17(b). Figure 17(d) shows an en face view of the 3D data set corresponding to the green horizontal rectangle in Fig. 17(b).

Despite the lower power incident on the sample (~0.5 mW), image quality was comparable to images obtained with the 1550 nm sources (e.g. Fig. 16(b) vs. Fig. 12, 2nd row or Fig. 16(e) vs. Fig. 13(e)).

4. Conclusions

We demonstrated, for the first time to the best of our knowledge, the performances of new all-semiconductor, akinetic swept laser sources for SS-OCT at 1550 nm and 1310 nm with no need of k-clock implementations. Results from ex-vivo and in vivo samples were presented. The employed systems performed imaging at zero delay as well as at very large (>170 mm) coherence gate displacement while maintaining sufficient image dynamic range. Auto- (laser) and cross- (system) correlation phase linearity and repeatability analysis per sweep was performed at different sweep rates. Obtained results show good laser phase linearity and repeatability (<2 mrad or <160 pm displacement sensitivity in both cases) with the laser operating close to the shot noise limit for phase-sensitive detection. Relative intensity noise was also investigated, showing a RIN percentile variation <0.2% per sweep, about 5 times smaller when compared to other [17

17. B. R. Biedermann, W. Wieser, C. M. Eigenwillig, T. Klein, and R. Huber, “Dispersion, coherence and noise of Fourier domain mode locked lasers,” Opt. Express 17(12), 9947–9961 (2009). [CrossRef] [PubMed]

] mechanically-tuned swept laser sources. During our experiments, robustness and wide customizability was experienced, allowing the user to select and/or adjust all the most important parameters, including laser output power, spectral bandwidth and sweep rate. The laser directly supplies a user sample clock (or electronic k-clock) signal and a start sweep (A-line trigger) signal for convenient integration of the laser source into the SS-OCT system and optimal synchronization with data acquisition hardware. One of the most attractive characteristics of the akinetic all-semiconductor laser technology is that laser performances and functionalities are defined and controlled by the laser source firmware, which can be updated as new releases became available. For example, at the time of the writing of this manuscript, a new release of the firmware was available and installed into the 1550 nm, 40 nm sweep range Insight laser source. This recent firmware release included several improvements and added functionalities, including e.g. the ability for users to specify sweeps not only by the number of (valid) points, but also by sweep rate or by optical frequency step size, offering to the user more significant or familiar set of parameters for defining the sweep characteristics. This release also included improvements in the calibration algorithms, which e.g. produced a significant reduction of power fluctuations during sweeps, with corresponding reduction of RIN.

Acknowledgments

This work is supported by the Medical University of Vienna, the European projects FAMOS (FP7 ICT 317744) and FUN OCT (FP7 HEALTH 201880), Macular Vision Research Foundation (MVRF, USA), Austrian Science Fund (FWF) project number S10510-N20 and the Christian Doppler Society (Christian Doppler Laboratory “Laser development and their application in medicine”).

References and links

1.

Optical coherence tomography: technology and applications – Vol. 1, W. Drexler and J. G. Fujimoto eds. (Springer 2008).

2.

D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science 254(5035), 1178–1181 (1991). [CrossRef] [PubMed]

3.

R. Huber, M. Wojtkowski, and J. G. Fujimoto, “Fourier domain mode locking (FDML): a new laser operating regime and applications for optical coherence tomography,” Opt. Express 14(8), 3225–3237 (2006). [CrossRef] [PubMed]

4.

High speed 1310nm swept source for OCT,” datasheet #2010–0230, Axsun Technologies Inc. (2009); http://www.axsun.com/PDF/OCT-SS1310-datasheet-update-7-12-13.pdf.

5.

High speed scanning lasers,” datasheet, Santec Corp. (2013); http://www.santec.com/en/products/oct/lightsource-for-octsystem?gclid=CMCZkrLwlboCFQZZ3godk3UAUA.

6.

V. Jayaraman, J. Jiang, H. Li, P. J. S. Heim, G. D. Cole, B. Potsaid, J. G. Fujimoto and A. Cable, “OCT imaging up to 760 kHz axial scan rate using single-mode 1310nm MEMS-tunable VCSELs with >100nm tuning range,” CLEO 1–2 (2011).

7.

B. Potsaid, V. Jayaraman, J. G. Fujimoto, J. Jiang, P. J. S. Heim, and A. E. Cable, “MEMS tunable VCSEL light source for ultrahigh speed 60kHz - 1MHz axial scan rate and long range centimeter class OCT imaging,” Proc. SPIE 8213, 82130M (2012). [CrossRef]

8.

M. P. Minneman, J. Ensher, M. Crawford, and D. Derickson, “All-semiconductor high-speed akinetic swept-source for OCT,” Proc. SPIE 8311, 831116 (2011). [CrossRef]

9.

J. Ensher, P. Boschert, K. Featherston, J. Huber, M. Crawford, M. P. Minneman, C. Chiccone, and D. Derickson, “Long coherence length and linear sweep without an external optical k-clock in a monolithic semiconductor laser for inexpensive optical coherence tomography,” Proc. SPIE 8213, 82130T (2012). [CrossRef]

10.

B. R. Bennett, R. A. Soref, and J. A. Del Alamo, “Carrier-induced change in refractive index of InP, GaAs and InGaAsP,” IEEE J. Quantum Electron. 26(1), 113–122 (1990). [CrossRef]

11.

V. Jayaraman, J. Jiang, H. Li, P. J. S. Heim, G. D. Cole, B. Potsaid, J. G. Fujimoto, and A. Cable, “OCT imaging up to 760 kHz axial scan rate using single-mode 1310nm MEMS-tunable VCSELs with >100nm tuning range,” PDPB2, CLEO 2011.

12.

“Inner vision: optical coherence tomography,” 2010 Vol. 1.1, p.8, Santec Corp.

13.

“Wide bandwidth 100kHz 1310nm swept source OCT,” datasheet #2013–0103, Axsun Technologies Inc. (2013).

14.

B. Park, M. C. Pierce, B. Cense, S. H. Yun, M. Mujat, G. J. Tearney, B. E. Bouma, and J. de Boer, “Real-time fiber-based multi-functional spectral-domain optical coherence tomography at 1.3 microm,” Opt. Express 13(11), 3931–3944 (2005). [CrossRef] [PubMed]

15.

R. V. Kuranov, A. B. McElroy, N. Kemp, S. Baranov, J. Taber, M. D. Feldman, and T. E. Milner, “Gas-cell referenced swept source phase sensitive optical coherence tomography,” IEEE Photon. Technol. Lett. 22(20), 1524–1526 (2010). [CrossRef]

16.

A. Yariv, “Optical electronics in modern communication,” (Oxford University, 1997).

17.

B. R. Biedermann, W. Wieser, C. M. Eigenwillig, T. Klein, and R. Huber, “Dispersion, coherence and noise of Fourier domain mode locked lasers,” Opt. Express 17(12), 9947–9961 (2009). [CrossRef] [PubMed]

18.

M. A. Choma, K. Hsu, and J. A. Izatt, “Swept source optical coherence tomography using an all-fiber 1300-nm ring laser source,” J. Biomed. Opt. 10(4), 044009 (2005). [CrossRef] [PubMed]

19.

J. Xi, L. Huo, J. Li, and X. Li, “Generic real-time uniform K-space sampling method for high-speed swept-source optical coherence tomography,” Opt. Express 18, 9511 (2010).

20.

Y. Yasuno, V. D. Madjarova, S. Makita, M. Akiba, A. Morosawa, C. Chong, T. Sakai, K. P. Chan, M. Itoh, and T. Yatagai, “Three-dimensional and high-speed swept-source optical coherence tomography for in vivo investigation of human anterior eye segments,” Opt. Express 13(26), 10652 (2005).

21.

R. Huber, M. Wojtkowski, K. Taira, J. G. Fujimoto, and K. Hsu, “Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles,” Opt. Express 13(9), 3513–3528 (2005). [CrossRef] [PubMed]

OCIS Codes
(140.3600) Lasers and laser optics : Lasers, tunable
(170.0110) Medical optics and biotechnology : Imaging systems
(170.3880) Medical optics and biotechnology : Medical and biological imaging
(170.4500) Medical optics and biotechnology : Optical coherence tomography
(170.4580) Medical optics and biotechnology : Optical diagnostics for medicine
(230.1480) Optical devices : Bragg reflectors

ToC Category:
Imaging Systems

History
Original Manuscript: October 24, 2013
Revised Manuscript: December 19, 2013
Manuscript Accepted: December 27, 2013
Published: January 30, 2014

Virtual Issues
Vol. 9, Iss. 4 Virtual Journal for Biomedical Optics

Citation
M. Bonesi, M. P. Minneman, J. Ensher, B. Zabihian, H. Sattmann, P. Boschert, E. Hoover, R. A. Leitgeb, M. Crawford, and W. Drexler, "Akinetic all-semiconductor programmable swept-source at 1550 nm and 1310 nm with centimeters coherence length," Opt. Express 22, 2632-2655 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-3-2632


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References

  1. Optical coherence tomography: technology and applications – Vol. 1, W. Drexler and J. G. Fujimoto eds. (Springer 2008).
  2. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, J. G. Fujimoto, “Optical coherence tomography,” Science 254(5035), 1178–1181 (1991). [CrossRef] [PubMed]
  3. R. Huber, M. Wojtkowski, J. G. Fujimoto, “Fourier domain mode locking (FDML): a new laser operating regime and applications for optical coherence tomography,” Opt. Express 14(8), 3225–3237 (2006). [CrossRef] [PubMed]
  4. High speed 1310nm swept source for OCT,” datasheet #2010–0230, Axsun Technologies Inc. (2009); http://www.axsun.com/PDF/OCT-SS1310-datasheet-update-7-12-13.pdf .
  5. High speed scanning lasers,” datasheet, Santec Corp. (2013); http://www.santec.com/en/products/oct/lightsource-for-octsystem?gclid=CMCZkrLwlboCFQZZ3godk3UAUA .
  6. V. Jayaraman, J. Jiang, H. Li, P. J. S. Heim, G. D. Cole, B. Potsaid, J. G. Fujimoto and A. Cable, “OCT imaging up to 760 kHz axial scan rate using single-mode 1310nm MEMS-tunable VCSELs with >100nm tuning range,” CLEO 1–2 (2011).
  7. B. Potsaid, V. Jayaraman, J. G. Fujimoto, J. Jiang, P. J. S. Heim, A. E. Cable, “MEMS tunable VCSEL light source for ultrahigh speed 60kHz - 1MHz axial scan rate and long range centimeter class OCT imaging,” Proc. SPIE 8213, 82130M (2012). [CrossRef]
  8. M. P. Minneman, J. Ensher, M. Crawford, D. Derickson, “All-semiconductor high-speed akinetic swept-source for OCT,” Proc. SPIE 8311, 831116 (2011). [CrossRef]
  9. J. Ensher, P. Boschert, K. Featherston, J. Huber, M. Crawford, M. P. Minneman, C. Chiccone, D. Derickson, “Long coherence length and linear sweep without an external optical k-clock in a monolithic semiconductor laser for inexpensive optical coherence tomography,” Proc. SPIE 8213, 82130T (2012). [CrossRef]
  10. B. R. Bennett, R. A. Soref, J. A. Del Alamo, “Carrier-induced change in refractive index of InP, GaAs and InGaAsP,” IEEE J. Quantum Electron. 26(1), 113–122 (1990). [CrossRef]
  11. V. Jayaraman, J. Jiang, H. Li, P. J. S. Heim, G. D. Cole, B. Potsaid, J. G. Fujimoto, and A. Cable, “OCT imaging up to 760 kHz axial scan rate using single-mode 1310nm MEMS-tunable VCSELs with >100nm tuning range,” PDPB2, CLEO 2011.
  12. “Inner vision: optical coherence tomography,” 2010 Vol. 1.1, p.8, Santec Corp.
  13. “Wide bandwidth 100kHz 1310nm swept source OCT,” datasheet #2013–0103, Axsun Technologies Inc. (2013).
  14. B. Park, M. C. Pierce, B. Cense, S. H. Yun, M. Mujat, G. J. Tearney, B. E. Bouma, J. de Boer, “Real-time fiber-based multi-functional spectral-domain optical coherence tomography at 1.3 microm,” Opt. Express 13(11), 3931–3944 (2005). [CrossRef] [PubMed]
  15. R. V. Kuranov, A. B. McElroy, N. Kemp, S. Baranov, J. Taber, M. D. Feldman, T. E. Milner, “Gas-cell referenced swept source phase sensitive optical coherence tomography,” IEEE Photon. Technol. Lett. 22(20), 1524–1526 (2010). [CrossRef]
  16. A. Yariv, “Optical electronics in modern communication,” (Oxford University, 1997).
  17. B. R. Biedermann, W. Wieser, C. M. Eigenwillig, T. Klein, R. Huber, “Dispersion, coherence and noise of Fourier domain mode locked lasers,” Opt. Express 17(12), 9947–9961 (2009). [CrossRef] [PubMed]
  18. M. A. Choma, K. Hsu, J. A. Izatt, “Swept source optical coherence tomography using an all-fiber 1300-nm ring laser source,” J. Biomed. Opt. 10(4), 044009 (2005). [CrossRef] [PubMed]
  19. J. Xi, L. Huo, J. Li, X. Li, “Generic real-time uniform K-space sampling method for high-speed swept-source optical coherence tomography,” Opt. Express 18, 9511 (2010).
  20. Y. Yasuno, V. D. Madjarova, S. Makita, M. Akiba, A. Morosawa, C. Chong, T. Sakai, K. P. Chan, M. Itoh, T. Yatagai, “Three-dimensional and high-speed swept-source optical coherence tomography for in vivo investigation of human anterior eye segments,” Opt. Express 13(26), 10652 (2005).
  21. R. Huber, M. Wojtkowski, K. Taira, J. G. Fujimoto, K. Hsu, “Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles,” Opt. Express 13(9), 3513–3528 (2005). [CrossRef] [PubMed]

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