Andersen, Peter E.

Biedermann, Benjamin

Huber, Robert

Klein, Thomas

Marschall, Sebastian

Pedersen, Christian

Wieser, Wolfgang

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 19, 2017Broadband Fourier domain mode-locked laser for optical coherence tomography at1060 nmMarschall, Sebastian; Klein, Thomas; Wieser, Wolfgang; Biedermann, Benjamin; Pedersen, Christian;Huber, Robert; Andersen, Peter E.Published in:Proceedings of Optical Sensors and Biophotonics IIIPublication date:2011Link back to DTU OrbitCitation (APA):Marschall, S., Klein, T., Wieser, W., Biedermann, B., Pedersen, C., Huber, R., & Andersen, P. E. (2011).Broadband Fourier domain mode-locked laser for optical coherence tomography at 1060 nm. In Proceedings ofOptical Sensors and Biophotonics IIIBroadband Fourier domain mode-locked laserfor optical coherence tomography at 1060 nmSebastian Marschalla, Thomas Kleinb, Wolfgang Wieserb, Benjamin Biedermannb,Christian Pedersena, Robert Huberb, Peter E. AndersenaaDTU Fotonik, Department of Photonics Engineering, Technical University of Denmark,P.O. Box 49, 4000 Roskilde, Denmark;bLehrstuhl fur BioMolekulare Optik, Ludwig-Maximilians-Universitat Munchen,Oettingenstr. 67, 80538 Munchen, Germany;1. INTRODUCTIONOptical coherence tomography (OCT) in the 1060 nm range is very interesting for imaging the human retina andthe underlying layers (choroid, sclera)1,2 due to low absorption in water and good penetration through the retinalpigment epithelium. Especially, swept source OCT (SS-OCT) enables ultra-high imaging speed if an appropriaterapidly tunable light source is used. For instance, with a state-of-the-art Fourier domain mode-locked (FDML)laser,3 densely sampled 3D dataset covering a wide eld of view can be acquired within a few seconds.4 Thetypical gain media for fast swept sources are semiconductor optical ampliers (SOAs) which feature fast gaindynamics enabling rapid wavelength tuning and a broad tuning bandwidth. These factors are important forOCT in order to achieve high imaging speeds and good depth resolution.Whereas SOAs for the telecom bands (1310 nm, 1550 nm) are well developed, SOAs for the 1060 nm rangehave been lacking output power and bandwidth. So far commercially available devices enable tuning ranges upto 80 nm with around 15mW output power. Although, one uses normally less than 2mW on the eye for retinalimaging, certain system implementations can require higher power levels. E.g. buering the light source outputfor multiplying the depth scan rate4,5 brings a power penalty of at least 6 dB, and some OCT interferometerssacrice a part of the input light in order to optimize the detection eciency.We have implemented an FDML laser using a new SOA featuring a broad gain bandwidth and high outputpower, even with a relatively low drive current. The gain spectrum is centered around 1060 { 1070 nm andcoincides thus perfectly with the wavelength range of minimal water absorption, making it very interestingfor retinal imaging. By modulating the SOA current during a wavelength sweep, we can optimize the outputspectrum in order to improve the depth resolution. With a total sweep range of 100 nm, we achieved a depthresolution of 8 µm in air. Output power levels well above 20mW are possible, and the performance is very similarfor both sweep directions.Figure 1. Schematic setup of the FDML laser. SOA: semiconductor optical amplier, FP-TF: ber-coupled Fabry-Perottunable lter, SMF: single-mode ber, ISO: optical isolator, PC: polarization controller. 0 10 20 30 40 50 0  1  2  3  4  5  6  7  8inst. power [mW]time [µs]wavelength increasing wavelength decreasing 0 10 20 30 40 50 0  1  2  3  4  5  6  7  8 0 100 200 300 400 500inst. power [mW] SOA current [mA]time [µs]wavelength increasing wavelength decreasing-0.6-0.4-0.2 0 0.2 0.4 0.6 1  2  3  4  5  6  7detector signal [V]time [µs]wavelength increasing wavelength decreasingwavelength increasing wavelength decreasing-0.6-0.4-0.2 0 0.2 0.4 0.6 1  2  3  4  5  6  7detector signal [V]time [µs]wavelength increasing wavelength decreasingwavelength increasing wavelength decreasingFigure 2. Top: instantaneous output power of the swept source during one sweep cycle (ca. 1{7 µs) for 250mA constantSOA current (left) and modulated SOA current (right). Bottom: corresponding exemplary interferograms from a Mach-Zehnder interferometer.2. SETUPWe tested the performance of the new SOA in an FDML laser running at 175 kHz repetition rate (Fig. 1). 70%of the light from the SOA output are coupled out of the resonator. The remaining fraction is guided througha delay line (1155m single-mode ber) to a ber-coupled Fabry-Perot tunable lter (FP-TF) which selects thewavelength that is fed back to the amplier. Optical isolators behind and in front of the tunable lter ensureuni-directional lasing in the ring-resonator. The length of the delay line is chosen in order to match the resonatorroundtrip frequency with a mechanical resonance frequency of the lter. The lter transmission linewidth isapproximately 170 pm.Using a fast SOA driver in conjunction with an arbitrary function generator, we modulated the SOA currentsynchronously with the wavelength sweep. By doing so, we could shape the output spectrum in order to exploitthe tuning bandwidth optimally. For evaluation of the light source performance, we generated interferogramsusing a Mach-Zehnder interferometer with dual balanced detection and calculated the corresponding point spreadfunctions for varying path length dierence.3. RESULTSWe compare the performance of the FDML laser with constant SOA current and modulated SOA current. Evenwith a relatively low constant current of 250mA, the swept source emits laser light without interruption over asweep range of 100 nm. The average output power is approximately 23mW. The instantaneous output power(Fig. 2 top left) has a high peak on the red side (longer wavelengths) and a at tail on the blue side. The samefeatures can be seen in the corresponding interferogram (Fig. 2 bottom left). Such a spectral shape is typicalfor these light sources, but not desirable, because the at tail does not signicantly improve the OCT depthresolution.In order to optimize the depth resolution, we modulated the SOA current, and thus the optical gain duringthe sweep cycle, in order to achieve and output power spectrum featuring a higher baseline and a at peakin the center (Fig. 2 top right) with the same average power (23mW). The current modulation waveform wascalculated by an iterative algorithm. The corresponding interferogram (Fig. 2 bottom right) shows a considerablymore uniform amplitude over the entire sweep range. Accordingly, the depth resolution (dened as FWHM ofthe point spread functions) is 8{9µm (in air) with modulated current, and 12{13µm with constant current,although the total sweep range is 100 nm in both cases. The sweep spectrum extends from 1015 to 1115 nm andis hence well centered in the range of minimal water absorption.-60-50-40-30-20-10 0 0  0.5  1  1.5  2  2.5  3signal [dB*]probing depth (air) [mm]forward sweep, constant current-60-50-40-30-20-10 0 0  0.5  1  1.5  2  2.5  3signal [dB*]probing depth (air) [mm]forward sweep, modulated current-60-50-40-30-20-10 0 0  0.5  1  1.5  2  2.5  3signal [dB*]probing depth (air) [mm]backward sweep, constant current-60-50-40-30-20-10 0 0  0.5  1  1.5  2  2.5  3signal [dB*]probing depth (air) [mm]backward sweep, modulated currentFigure 3. Point spread functions for dierent OCT probing depths.Remarkably, even with constant current the interferogram amplitudes of the forward sweep (increasing wave-length) and the backward sweep (decreasing wavelength) are very similar, while they are almost symmetric withcurrent modulation. This indicates that both sweep directions could be used for OCT imaging. The pointspread functions for dierent interferometer delays (Fig. 3) exhibit a relatively fast signal roll-o. For constantcurrent, the roll-o is 8.4 and 9.2 dB/mm (forw./backw. sweep); for modulated current, it is 9 or 10 dB/mm,respectively.4. DISCUSSION AND OUTLOOKUsing a new SOA, we implemented an FDML swept source for the 1060 nm wavelength band featuring 100 nmsweep range and more than 20mW average output power. By modulating the drive current, we optimized thespectral envelope and improved the axial resolution by 30% to 8µm in air. Both sweep directions exhibit similarperformance, and might thus both be suitable for OCT imaging. In this case a depth scan rate of 350 kHz wouldbe possible without buering the output. More detailed experiments are necessary to verify this hypothesis.Furthermore we will investigate in how far water absorption in the sample path aects the depth resolution ofthe system.REFERENCES1. B. Povazay, K. Bizheva, B. Hermann, A. Unterhuber, H. Sattmann, A. F. Fercher, W. Drexler, C. Schubert,P. K. Ahnelt, M. Mei, R. Holzwarth, W. J. Wadsworth, J. C. Knight, and P. S. J. Russel, \Enhanced visu-alization of choroidal vessels using ultrahigh resolution ophthalmic OCT at 1050 nm," Opt. Express 11(17),pp. 1980{1986, 2003.2. Y. Yasuno, Y. Hong, S. Makita, M. Yamanari, M. Akiba, M. Miura, and T. Yatagai, \In vivo high-contrastimaging of deep posterior eye by 1-m swept source optical coherence tomography and scattering opticalcoherence angiography," Opt. Express 15, pp. 6121{6139, May 2007.3. R. Huber, M. Wojtkowski, and J. G. Fujimoto, \Fourier domain mode locking (FDML): a new laser operatingregime and applications for optical coherence tomography," Opt. Express 14(8), pp. 3225{3237, 2006.4. T. Klein, W. Wieser, C. M. Eigenwillig, B. R. Biedermann, and R. Huber, \Megahertz oct for ultrawide-eldretinal imaging with a 1050 nm fourier domain mode-locked laser," Opt. Express 19, pp. 3044{3062, Feb 2011.5. R. Huber, D. C. Adler, and J. G. Fujimoto, \Buered fourier domain mode locking: unidirectional sweptlaser sources for optical coherence tomography imaging at 370,000 lines/s," Opt. Lett. 31(20), pp. 2975{2977,2006.dB is here dened as 20 log of the PSF amplitude.

Broadband Fourier domain mode-locked laser for optical coherence tomography at 1060 nm

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Broadband Fourier domain mode-locked laser for optical coherence tomography at 1060 nm

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