TÉCNICAS DE PROCESAMIENTO DE RECOLECCIÓN DE

The research work presented in this thesis focused on the investigation and characteriza- tion of different novel concepts of fast semiconductor-based wavelength-swept light sources, which are characterized by a very high wavelength sweep speed, a large spec- tral sweep range and a narrowband instantaneous spectrum. The most prominent and successful application is swept source based optical coherence tomography (SS-OCT), where the demand for high imaging speed drove the research in the field of wavelength- swept light sources in the last years. Conventional semiconductor-based wavelength- swept lasers suffer from a fundamental sweep speed limit [5] which is determined by the laser cavity length and caused by the finite build-up time of lasing from amplified spontaneous emission (ASE). The most obvious solution to this problem is a minimiza- tion of the cavity length. In this way, ultra-high OCT imaging speeds have been achieved with optically pumped MEMS-based tunable vertical-cavity surface-emitting lasers (VCSEL) [183, 184], providing single line operation at sweep rates approaching the MHz range. However, the fastest high-quality OCT imaging up to now was realized with a completely different approach, overcoming the fundamental sweep speed limita- tion. In Fourier domain mode locked (FDML) lasers [6], the laser cavity length is ex- tended up to a few kilometers by inserting optical fiber enabling a repetitive wavelength tuning synchronous to the round-trip time of light in the laser cavity. In this way, SS-OCT imaging at sweep rates exceeding several MHz [7, 8] has been demonstrated. One main objective of this thesis was the research on novel concepts of wavelength- swept light sources improving performance and applicability for SS-OCT. Regarding this, an important issue was the investigation of two novel modes of operation in FDML lasers.

Almost all wavelength-swept light sources utilized for SS-OCT exhibit non-linear time-frequency sweep characteristics, which has several disadvantages in the applica- tion for OCT. Since equidistant frequency sampling is required prior to Fourier trans- formation, additional numerical resampling is usually applied complicating data pro- cessing. Apart from that, a non-linear sweep characteristic can result in a slightly re- duced sensitivity or dynamic range. Within the framework of this thesis, the first k-space linear FDML laser has been demonstrated at 1300 nm with a sweep rate of 57 kHz, overcoming these drawbacks. As shown, a very high linearity, i.e. an integrat- ed relative optical frequency error on the order of 10-5 or smaller, is required to enable high-quality OCT imaging without numerical resampling. The crucial point to achieve k-space linear operation is the correct drive of the piezo-actuated tunable Fabry-Pérot filter. Based on numerical simulations, different strategies were introduced to determine

the suitable control waveform comprising the fundamental FDML frequency and sever- al higher harmonics. Here, the consideration of the frequency-dependent elec- tro-mechanical amplitude and phase response of the Fabry-Pérot filter played an im- portant role. Adapting the filter drive waveform, the k-space linear FDML laser has also been used to compensate for unbalanced chromatic dispersion in the OCT setup, which worked very well within at least a small imaging range. Moreover, k-space linear opera- tion was the prerequisite for a new approach of en-face OCT , which was introduced by our group [231] and enabled real-time visualization of en-face images without the need for Fourier transformation. In the future, k-space linear FDML operation at considerably higher imaging speeds could be achieved using the technique of optical buffering. The gained knowledge about k-space linear FDML operation could also be transferred to other non-FDML swept sources facilitating the linearization of the wavelength sweeps. The second introduced novel mode of operation is subharmonic FDML (shFDML), which was demonstrated at 1300 nm. A reflective tunable Fabry-Pérot filter is placed at the beginning of the linear delay line of a sigma-ring cavity and thus can play the role of an optical switch. In this way, light is recirculated in the delay line a preselected number of times yielding multiple passes through the same delay fiber during each round-trip in the cavity. The main advantage is that shFDML lasers provide the inherent possibility to multiply the sweep rate and increase OCT imaging speed with a minimum of optical fiber required and without the need for an additional optical buffer stage. This can be accomplished by extracting a part of the optical power within each pass of the fiber delay in combination with appropriate gain switching. In this way, a multiplication by a factor of 10 has been achieved reaching a sweep rate of 570 kHz. However, there are currently two main drawbacks which limited the achievable spectral sweep band- width. On the one hand, there is parasitic lasing, which was suppressed best possibly using a polarization maintaining setup. On the other hand, the double pass of light through the Fabry-Pérot filter during each cavity round-trip increasingly complicates shFDML operation when raising the sweep speed due to the finite length of the short ring. Future improvements, like the application of an one-sided reflective filter in order to avoid parasitic lasing and a further reduction of the short ring length, will be neces- sary. Alternatively, an extension of this length, making the short ring resonant, in com- bination with a suitable gain modulation could have the potential to overcome both drawbacks. In the future, shFDML lasers could become an attractive alternative for ul- tra-high speed OCT, in particular if high filter drive frequencies are not feasible requir- ing a high sweep rate multiplication factor.

Besides advances in FDML laser technology, the research work presented in this thesis also included the introduction and characterization of a novel concept of ultrafast wave- length-swept light sources, referred to as wavelength-swept ASE sources. This approach can overcome the fundamental sweep speed limit and, unlike other swept sources, is based on a setup with no optical feedback. In order to guarantee a sufficiently high out- put power, ASE light alternately passes a cascade of different gain elements and differ- ent optical filters, which are required to prevent the amplification of unfiltered ASE

background. As verified experimentally, optimum operation can only be achieved if the different spectral filters are driven with a precisely adjusted phase delay in order to compensate for the transit time of light between the filters. It was demonstrated that the required phase accuracy and the sensitivity roll-off are mainly determined by the spec- tral transmission functions of the filters. Within the framework of this thesis, different implementations of wavelength-swept ASE sources have been investigated enabling in-vivo OCT imaging of the human finger at 1300 nm and, additionally, of the human retina at 1060nm, where very high average output powers exceeding 40 mW were achieved using Yb-doped fiber post-amplification. The effective sweep rate of 340 kHz, which has been realized with this novel approach, was the highest sweep rate demon- strated for retinal OCT imaging at this time. Wavelength-swept ASE sources provide several advantages compared to other swept light sources, like highly repeatable tem- poral tuning characteristics or equal performance of forward and backward sweep. Since no long optical fiber delay is required, this technique could be an attractive alter- native for future high speed SS-OCT systems in wavelength ranges where high disper- sion, high loss or large polarization effects in the fiber complicate FDML operation. Each setup was based on a cascade of two filtering and three amplification steps, how- ever, for future applications the number of elements could be extended, if necessary. An automated regulation of the filter drive parameters will be required. Furthermore, an optimization of the spectral bandwidths of the different filters, using non-equal values, could improve the roll-off performance and relax the required filter drive accuracy. In summary, the introduced novel approaches, which were investigated within the framework of this thesis, have the potential to facilitate and improve SS-OCT in the future. From today’s perspective, it is highly probable that swept source based technol- ogy will become the technique of choice for all future OCT systems, however, it re- mains exciting to see which swept source technique will prevail.

The second main objective of this thesis was the demonstration of a completely novel approach of short pulse generation based on subsequent temporal compression of the sweeps from an FDML laser. This has been accomplished using a 15 km long dispersion compensation fiber (DCF) placed after a dispersion compensated FDML laser, which was operated at 1560 nm and 390 kHz. Moreover, this approach offered the possibility to gain an insight into the coherence properties of the FDML laser. This new concept is of high interest due to different reasons. On the one hand, this technique has the poten- tial to enable considerably higher pulse energies compared to conventionally mode locked semiconductor lasers, since energy is stored optically in the laser cavity and not as population inversion in the gain medium. On the other hand, the generation of almost time-bandwidth limited pulses might become possible in the future, since ideal FDML operation can be seen as a new mode of stationary laser operation, where sequential sweeps are mutually coherent and the laser modes are phase locked [6].

In the research work presented in this thesis, temporal pulse widths of 60-70 ps at a repetition rate of 390 kHz have been demonstrated. Due to uncompensated higher order chirp, the bandwidth was limited to 6 nm. Therefore and due to loss in the DCF, the

pulse energies were restricted to 140 pJ. Erbium-doped fiber amplification (EDFA) prior to temporal compression enabled pulse energies of 5.6 nJ at a peak power of 96 W. In the future, the additional application of chirped fiber Bragg gratings [185] could enable temporal compression of several tens of nm, which would raise the achievable pulse energy to several nJ without using EDFA amplification.

The minimum achievable temporal pulse width of 60 -70 ps, as realized under current- ly given experimental conditions, is limited by the internal coherence properties of the laser and not due to imperfect temporal compression. The crucial role of coherence in the laser has been impressively demonstrated by the strong dependence of the pulse width on the drive frequency of the tunable filter, requiring a relative accuracy of _{. Moreover, equivalent pulse compression experiments have been performed} with a per se incoherent wavelength-swept ASE source, showing considerably longer pulse widths. The experimental results were in good agreement with a theoretical mod- el, which was introduced describing pulse generation with fully incoherent swept light sources. On the basis of this model, it has been shown that there occurs at least partially coherent superposition of the different spectral components of the FDML sweeps within the temporal compression process.

Analyzing the interference signal, obtained by superposing one sweep with a delayed copy, high intra-sweep coherence of the FDML laser has been observed. Moreover, these measurements indicate an ultra-stable operation similar as presented in [239] and suggest that the FDML sweeps show sections with a very high coherence which are interrupted by wavelength/phase discontinuities. Further investigations using a consid- erably higher detection bandwidth will be required. Moreover, different studies which were performed in our research group and were based on superposing different succes- sive wavelength sweeps showed mutual inter-sweep coherence [222].

In collaboration with the research group of Dr. Jirauschek, pulse generation with FDML lasers has been described using numerical simulations based on a theoretical model of the FDML laser [15]. The simulations have largely reflected the dependencies of the different parameters that were observed in the experiment and predicted significantly narrower pulses in case of considerably reduced spectral filter widths, which are cur- rently not feasible in the experiment due to restrictions of the filter performance. More- over, using zero residual cavity dispersion and very small filter widths, the simulations yielded almost time-bandwidth limited pulses. In the future, an extension of the under- lying theory, considering different polarization states, can become reasonable since the polarization in the fiber seems to play a non-negligible role for FDML dynamics.

All these findings give justified hope that the generation of considerably shorter, or even almost bandwidth-limited pulses might become possible in the future by improv- ing the coherence properties of the FDML laser. Active frequency feed-back stabiliza- tion, improved tunable filter performance or reduced residual dispersion in the cavity could be important steps on this way. A deeper understanding of FDML laser physics will play a crucial role. Therefore, this field provides many exciting challenges remain- ing for the future.

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