Isolator-Free 840-nm Broadband SLEDs for High-Resolution OCT

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1 Isolator-Free 840-nm Broadband SLEDs for High-Resolution OCT M. Duelk *, V. Laino, P. Navaretti, R. Rezzonico, C. Armistead, C. Vélez EXALOS AG, Wagistrasse 21, CH-8952 Schlieren, Switzerland ABSTRACT We have developed ultra-broadband Super-Luminescent Emitting Diodes (SLEDs) at 840 nm with a 3-dB bandwidth of nm. The SLEDs show high robustness against back-reflections of up to 50% with little change in coherence length, sidelobe suppression ratio and secondary peak suppression over a wide range of back-reflections. First long-term measurements do not show any signs of device degradation. Hence, these SLEDs can be employed in OCT systems without costly broadband optical isolators. Keywords: Super-Luminescent Emitting Diodes, SLD, SLED, back-reflections, return loss, OCT 1. INTRODUCTION The vast majority of commercial OCT systems today are used in the field of ophthalmology. These systems operate in the wavelength range of 750 to 900 nm and use SLEDs as broadband light sources [1]. A common center wavelength for retinal diagnostics is 840 nm, for example. State-of-the-art SLEDs achieve a 3-dB bandwidth of ~50 nm with a flat-top spectral shape, which results in a coherence length of 7-8 µm in air or ~5 µm in tissue. Next-generation 840-nm SLEDs will offer up to 75 nm of bandwidth, resulting in a resolution improvement of more than 30%. It is expected that the race for more bandwidth and higher resolution [2] will continue over the next years, envisioning SLED devices achieving up to 150 nm in bandwidth, corresponding to a coherence length of µm in air. The architecture of current OCT systems is based on a Michelson interferometer, featuring a reference arm and a sample arm. Light from both arms is reflected and is interfering in the detector arm that comprises an optical receiver or a spectrometer. However, a certain fraction of the optical power is also always reflected back to the source, i.e. to the broadband SLED. SLEDs are semiconductor devices that generate a large amount of amplified spontaneous emission (ASE). In order to do that, they incorporate high-power gain sections in which seeding spontaneous emission is amplified with high gain factors of 30 db or more. It is therefore natural that even small amounts of back-reflections are amplified inside the SLED chip in a similar manner, producing optical power levels of several tens of milliwatts at the back facet, which may destroy the SLED device. For this reason it is common practice to use SLEDs in OCT systems in combination with broadband optical isolators. These isolators require multiple optical stages in order to achieve sufficient optical isolation over a wide wavelength range, which results in larger footprint, higher insertion losses and increased system costs. We have fabricated ultra-broadband SLEDs at 840 nm with a 3-dB bandwidth of nm (more details on the device fabrication and performance can be found at [3]) that show an inherent robustness against optical back-reflections (BRs). This might allow for using them in ophthalmic OCT systems without optical isolators. We measured and characterized the optical output of these SLEDs under varying return loss (RL) from -40 db (0.01% BR) to -4 db (40% BR) and demonstrate that key optical parameters like coherence length, sidelobe suppression ratio and secondary peak suppression are not changing significantly over this wide range of back-reflections. Long-term measurements with a RL of -14 db (4% BR) do not reveal any sign of degradation. 2. RETURN LOSS MEASUREMENTS For our experiments we choose a high-power (9-10 mw output power in fiber) broadband (45-nm bandwidth) 840-nm SLED that we test under the influence of back-reflections. The experimental setup is shown in Fig. 1. A 10:90 power splitter is used to tap off light in the forward and return path to monitor the SLED output with an optical spectrum analyzer (OSA) and the return loss with an optical powermeter (OPM). A variable optical attenuator (Attn) adjusts the * phone ; fax ;

2 amount of back-reflections coming from a broadband fiber reflector (Reflc) at 840 nm. A polarization controller (PolCtrl) is used to adjust the polarization of the back-reflected light. PolCtrl Reflc SLED 90% Attn OPM 10% OSA Fig. 1. Experimental setup to characterize SLEDs under varying optical return loss. We observe that the SLED output characteristics change under the influence of back-reflections. With increasing return loss the optical power density on the short-wavelength side of the ASE spectrum is decreasing, as shown on the left-hand side of Fig. 2. This results in a change of the spectral shape from flat-top (without back-reflections) towards a first-order Gaussian function. It also causes the optical bandwidth to decrease considerably from 44 nm FWHM without BR to ~31 nm with 40% BR, which is shown on the right-hand side of Fig. 2. This represents a reduction in optical bandwidth of 30% over this wide range of BR values ASE Power [rel.db/0.01nm] Polarization RL db RL db RL db RL db RL db RL db RL db 3-dB Bandwidth [nm] Wavelength [nm] 30 Fig. 2. Optical output spectrum of tested SLED under varying optical return loss (RL) from -40 db to -4 db (left side). As the back-reflections or the return loss is increased the power density on the short-wavelength side is decreased, resulting in a reduction of the optical power and of the optical bandwidth (right side). Typically, back-reflections in polarization (same as SLED) result in stronger changes as compared to back-reflections being polarized. We observe that the impact of BR on the optical performance of the SLED is stronger when the return signal has the same polarization as the SLED, which is in this case. The optical spectra under BR shown in Fig. 2 are only for backreflection in polarization. However, they look similar to those spectra measured for back-reflections in polarization, except that the same change in spectral shape is measured at a higher level of BR. This is also indicated by the right-hand side of Fig. 2 that shows similar curves for back-reflections in and polarization but being shifted by 1-2 db in return loss. Varying the polarization but not the level of the back-reflected light (fixed RL value), the corresponding change in optical bandwidth is less than 2.5%. Accompanied by the change in spectral shape, and in particular by the drop in optical power density on the shortwavelength side, is an increase of mean wavelength (up to ~6 nm or 7000 ppm for a RL change from -40 db to -4 db), as shown on the right-hand side of Fig. 3. We observe again that the change is lower for back-reflections in polarization than for polarization. The curves for the two polarizations look similar but are shifted by 3-4 db in return

3 loss. Varying the polarization but not the level of the back-reflected light (fixed RL value), the corresponding maximum change in mean bandwidth is below 1500 ppm. The drop in optical power density of the ASE light output of the SLED is causing a decrease in total optical output power. As shown on the left-hand side of Fig. 3, the output power of the SLED is reduced by up to 7 db (80 %) for a return loss of -4 db (40% BR) in polarization. As mentioned above, back-reflections in have lower impact on the device performance, though the general behavior is the same. The curves for the two polarizations look similar but are shifted by ~7 db in return loss. Varying the polarization but not the level of the back-reflected light (fixed RL value), the corresponding maximum change in optical power is about 1 db (~25%) for a RL of -20 db (1% BR) and about 2 db (~60%) for a RL of -10 db (10% BR). The higher dependence on the state of polarization of the back-reflected light is indicative for the fact that the SLED output power is, among all optical parameters, affected most by back-reflections, which is in agreement with the findings mentioned above Output Power [dbm] Mean Wavelength [nm] Fig. 3. Optical output power (left side) and mean wavelength (right side) of tested SLED under varying optical return loss (RL) from -40 db to -4 db. As the back-reflections or the return loss is increased the power density on the shortwavelength side is decreased, resulting in a reduction of the optical power and a red-shift of the mean wavelength. Typically, back-reflections in polarization (same as SLED) result in stronger changes as compared to back-reflections being polarized. Another conclusion from the results shown in Fig. 2 and Fig. 3 is that a return loss of -35 db (0.03% BR) or less does not cause any measurable change in optical performance of the SLED. For a common RL value of -14 db (4% BR) we observe a mean wavelength change of 3500 ppm, a 12% reduction in optical bandwidth and a power drop of 3 db. The latter is comparable to the insertion loss of a broadband optical isolator, which means that these isolator-free SLEDs will deliver the same optical power to the interferometer and to the sample. As the change in the wavelength or frequency domain may seem to be dramatic, we will show in the next section that the change in the spatial domain is rather minor and a loss in optical bandwidth is not necessarily translated into a loss of resolution in the OCT image. Lastly, when assessing the performance of SLEDs under back-reflections it is important to further distinguish between short-term exposure to high levels of BR and long-term exposure to medium levels of BR. For example, it is quite common that during system integration and manufacturing, or during calibration of the OCT engine, high levels of backreflections into the SLED are occurring. Here, it is crucial that the SLED is having a short-term tolerance towards RL values in the range of -5 db to -10 db. Then, during OCT measurements a constant lower level of back-reflections is present and it is hence equally important that the SLED is having a long-term tolerance towards RL values in the range of -10 db to -20 db.

4 3. COHERENCE FUNCTION From the measured ASE spectra (in the wavelength domain) the autocorrelation or coherence functions are calculated using the Fast Fourier Transform (FFT) in combination with resampling, inter- and extrapolation of the ASE spectrum. The results for back-reflections in polarization are shown on the left-hand side of Fig. 4. We observe that, despite the large changes in the wavelength domain, the changes in the spatial domain are minor. As mentioned earlier, the optical spectrum changes towards a first-order Gaussian shape with increasing return loss. This compensates for the loss in optical bandwidth since, for example, a second-order flat-top Gaussian spectrum requires a ~27% larger optical 3-dB bandwidth compared to a first-order Gaussian spectrum in order to have the same coherence length. As shown on the right-hand side of Fig. 4, the coherence length is changing by only ± µm over the full RL range and by only ±0.1 µm for a common RL value of -14 db (4% BR). In other words, the resolution of the OCT system is maintained over a wide range of back-reflection values, even though the optical spectrum and its bandwidth are changing measurably. To be more precise, we measure a small reduction in coherence length (up to 2.5% or 0.2 µm for 40% BR) with increasing levels of back-reflections in polarization while observing a slight increase in coherence length (up to 3.8% or 0.3 µm for 40% BR) with increasing levels of back-reflections in polarization. The opposite behavior is originating from the slightly different spectral shapes under or back-reflections, respectively. Coherence Function [lin. a.u.] Polarization RL db RL db RL db RL db RL db RL db RL db Coherence Length in Air [µm] Relative Distance [µm] 7.6 Fig. 4. Calculated coherence function in air under varying optical return loss (RL) from -40 db to -4 db (left side). The 3-dB width is hardly affected while the sidelobes are actually disappearing with increasing return loss because the shape of the optical spectrum is approaching a first-order Gaussian function. Extracted coherence length (half-width at half maximum of coherence function) shows sub-micron change versus wide-range change in return loss (right side). The left-hand side of Fig. 4 demonstrates that with increasing return loss the auto-correlation or coherence function becomes smoother and sidelobes are changing. Sidelobe suppression and position have been analyzed and the results are plotted in Fig. 5. As shown, for RL values above -15 db (3.2% BR) under -polarized back-reflections or for RL values above -12 db (6.3% BR) under -polarized back-reflections, sidelobes have completely disappeared. In many SD-OCT systems, sidelobes of the coherence function are not an issue as their imaging artifacts are eliminated through signal processing at the receive side. What might be more important is the change of coherence function and related change of sidelobes due to variations of back-reflections, for example change of BR level or change in the BR polarization. Both parameters, i.e. attenuation and polarization in the reference arm of the interferometer, can typically be controlled in an OCT system such that changes in the coherence function can be minimized. Independent of that, we believe that for a typical system environment with reasonably fixed operating conditions the minor changes in the coherence function will be acceptable.

5 Sidelobe Suppression [db] Sidelobe Position [µm] Fig. 5. Extracted change in sidelobe suppression (left side) and sidelobe position (right side) as a function of the return loss for - and -polarized back-reflections. For RL values above -15 db (for ) or above -12 db (for ) the sidelobes are disappearing and the detection algorithm does not deliver any results. Another aspect in the OCT imaging quality is sufficient suppression of secondary peaks. Secondary peaks in the coherence function arise from any kind of modulation in the spectral shape of the ASE light output of the SLED. This kind of spectral ripple may come, for example, from reflections of the SLED chip or from reflections of optical components inside the SLED package such as micro-lenses or the fiber tip. The offset position of the secondary peaks from the peak of the coherence function correlates to the optical round-trip path of the cavity that is created due to these reflections. The amplitude of these secondary peaks, or the secondary peak suppression relative to the peak of the coherence function, depends on various aspects. For an SLED with a center wavelength of 840 nm, it can be calculated that spectral ripple of 0.1 db (2.3%) correspond to a suppression of secondary peaks by more than -30 db while spectral ripple of 0.01 db (0.23%) correspond to a suppression ratio of more than -40 db. Fig. 6 shows the secondary peak suppression as a function of the return loss for - and -polarized back-reflections. Typical values of these SLEDs are in the range of -35 db to -38 db. For back-reflections in, the secondary peak suppression is hardly changing and only an insignificant increase of secondary peaks may be seen for RL values above -10 db. For back-reflections in, a small increase of secondary peaks can be measured for RL values above -20 db. For a common RL value of -14 db an increase of secondary peaks by 1-2 db to -34 db can be measured. Over the whole RL range from -40 db to -4 db, the secondary peak suppression under -polarized back-reflections remains well below -30 db, as can be seen in Fig. 6. These results indicate that even under high back-reflections the ASE spectrum remains smooth and does not exhibit significant spectral ripple, lasing modes or other irregularities.

6 Secondary Peak Suppression [db] Fig. 6. Secondary peak suppression of the coherence function as a function of the level of back-reflections being either -polarized or -polarized. The suppression ratio remains high (better than -30 db) over the full RL range. 4. LONG-RM MEASUREMENTS Long-term measurements of up to 600 hours (25 days) with medium levels of back-reflections show stable SLED operation in terms of output power as well as spectral and spatial properties. For example, Fig. 7 shows the relative power change of the SLED during a long-term measurement with a BR level of 4% (RL -14 db). The polarization of the back-reflection was set to in order to capture the worst case in terms of device degradation. The SLED was operated with a driver running in automatic current control (ACC) mode, i.e. the drive current was stabilized and not the optical output power of the SLED. For up to ~550 hours we observe the power fluctuations to be within a ±0.5% range. These fluctuations are due to changes in the ambient temperature or due to residual fiber movements and vibrations, changing the polarization or the level of back-reflection by ±0.025 db (±0.6%). Even though the fibers were taped down on an optical bench a drift in polarization occurred in the last ~50 hours where the polarization was slowly moving away from. This resulted in a slow increase in optical power, as shown in Fig. 7. The measurement was hence terminated after 600 hours. Fig. 7. Long-term power measurement of up to 600 hours under a constant level of 4% back-reflections. Power fluctuations are due to small changes in the level or the polarization of the back-reflected light, or due to changes in the ambient temperature during the measurement.

7 Comparing the optical characteristics before and after the experiment does not reveal any signs of degradation. Similar results are found during short-term (up to 30 minutes) exposures of very high RL values up to 0 db (100% BR). These results indicate again high robustness of our SLEDs against long-term medium-level back-reflections (RL -14 db), for example in an OCT system, as well as short-term high-level back-reflections (RL -6 db), for example during system manufacturing and device handling. Further, though less scientific, evidence of our SLEDs featuring high robustness against back-reflections is coming from the fact that various customers have deployed EXALOS SLEDs in OCT and other optical systems over the past years where these SLEDs are exposed to medium-levels of back-reflections over several thousand hours of operation. Up to this day, no device failure could be attributed to the exposure to back-reflections. This is refuting the commonly made statements (e.g., [4]) that SLEDs require optical isolators in order to prevent device degradation, damage or shorter lifetime. 5. CONCLUSIONS We have developed and characterized ultra-broadband 840-nm SLEDs featuring high robustness against optical backreflections. Short-term measurements with back-reflections of up to 100% as well as long-term measurements with backreflections of 4% have been carried out. No evidence and no sign of device degradation due to back-reflections were found. This work indicates that these devices can be employed in (ophthalmic) OCT systems without the commonly used optical isolators, hence reducing system cost and complexity. REFERENCES [1] [2] [3] [4] PennWell Corporation & LaserFocusWorld, Optical Coherence Tomography Technology, Markets, and Applications , Market Report BioOptics World, (2008) Drexler, W., and Fujimoto, J.G., State-of-the-art retinal optical coherence tomography, Progress in Retinal and Eye Research, Vol. 27, pp (2008) Laino, V., Occhi, L., Navaretti, P. and Velez, C., Broadband superluminescent light-emitting device at 840nm with high performance stability, Proc. of SPIE, Vol. 6847, paper 2N (2008) Drexler, W., and Fujimoto, Optical Coherence Tomography Technology and Applications, Springer, chapter 9 by Shidlovski, V.R., Superluminescent Diode Light Sources for OCT, pp (2008)

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