Periodic Reporting for period 1 - ICOFAS (Improved Coherence Fast Swept Source Lasers for Optical Imaging Applications)
Berichtszeitraum: 2019-02-01 bis 2021-01-31
ICOFAS targeted developing theoretical and experimental methods to improve coherence of swept source lasers (SSL) and design and fabricate new devices at shorter wavelengths (850nm) using multi-section architecture. The major application of such lasers is Optical Coherence Tomography (OCT) imaging and the coherence of the laser is crucial for the image quality. The project proposed a new approach to apply the optical feedback to swept source lasers in order to improve their coherent properties. Realisation of 850nm SSL makes OCT technology comparable with silicon technologies to allows faster imaging at higher resolution. Implementation of the objectives of the project requires multi-disciplinary approach that brings together laser science, theory of dynamics of nonlinear systems, fabrication of semiconductor devices, OCT technology. The European community benefits from the ICOFAS as it addressed innovative technology of SSL lasers for prioritised area of medical and industrial applications.
The main exploitable results comprise:
1) Experimental and theoretical study of coherence limitations in semiconductor lasers based on fibre cavity. The results confirm the impact of the intra-cavity filter on the laser dynamics. In particular, the transmission bandwidth should be carefully considered when designing the laser for applications such as imaging, where any deviation from stable operation deteriorates image quality.
2) Developing of the theoretical model and experimental testing to demonstrate the influence of optical feedback in swept source lasers. It was shown that the optical feedback can be used as a tool to lock the phase of the successive modes. Also, the feedback can lead to the sliding frequency mode-locking regime demonstrating sub-nanosecond pulses, interesting for nonlinear optics applications. The power of the proposed technique has been demonstrated on the model of the multi-section semiconductor lasers. It can also be adapted to other swept source configurations exhibiting mode hops, or serve as a tool to couple several swept sources to increase the sweeping range with maintained coherence.
The achieved results were disseminated via publications and presentations at conferences and obtained visibility, which lead to new collaboration between the Host institute and companies developing new optical devices.
The details of the work performed are as follows:
First, we investigated the mechanisms of coherence limitations of ring cavity SSL with semiconductor gain medium and a tunable optical filter in normal and anomalous dispersion regimes. Such lasers can operate in a regime of synchronisation, when the filter tuning speed is synchronous to the cavity roundtrip time (FDML regime). FDML lasers currently demonstrate the fastest tuning rate. It was previously shown that the dynamics of the lasers operating in the FDML regime is equivalent to the laser dynamics operating at a static filter transmission for each wavelength, and FDML lasers can exhibit similar coherence limitations. Experimentally, we studied the dynamics of the laser operating in the static regime using the electric field reconstruction technique. The technique is based on simultaneous recording of the laser and three outputs of the 3x3 fiber interferometer using a fast detector and a fast real time oscilloscope. Temporal evolution of the phase of the electric field was obtained using the output signals of this interferometer. This measurement revealed that at the bias current close to threshold the laser emitted stable intensity drops whose frequency was out of the filter transmission resonance. Therefore these drops experienced additional losses and could not be compensated by the amplifier and remained stable for many roundtrips. The presence of these drops limits the coherence of such lasers. Theoretically, a delay differential equation model was used and numerical simulation revealed that these structures followed heteroclinic connections between two stable modes. These structures are similar to those predicted by Ginsburg-Landau equation (Nozaki-Bekki holes). The dynamics of these structures depended on the bias current and the filter transmission bandwidth. In addition, it was shown that unstable Nozaki-Bekki holes could generate turbulent puffs which deteriorated the coherence in an FDML laser when synchronisation was disturbed. These results lead to two publications. In addition, the effect of the fiber dispersion on laser dynamics was analysed experimentally and theoretically. One manuscript is under review, one in preparation.
Second, we performed experimental study of formation of such coherent structures during the laser turn on. The bias current of the semiconductor gain was quickly turned on with an electrical signal, whose rise time was much shorter than the cavity roundtrip time, and the laser intensity time trace was recorded using fast electronics. The turn on transient of the laser was studied at various laser parameters, including the bias current, filter width and the dispersion regime. The phase measurement technique, based on a 3x3 fiber coupler, allowed for reconstruction of evolution of the optical spectrum during the laser turn on. The results obtained were confirmed using the same delay differential equations model. The manuscript is now under preparation.
The ring cavity including a VCSEL, used for wavelength tuning, and a semiconductor gain was tested.
The theoretical model was developed to study the effect of optical feedback in swept source lasers. The model have shown that it is possible to lock the phase of successive modes of a multi-section swept source semiconductor laser using optical feedback. As a result of locking, the coherence of the laser does not deteriorate after the mode hops. The model predicted four dynamical regimes of the laser depending on the optical feedback strength and the sweeping rate of the filter: beating, chaotic, periodic pulsations and periodic pulsations with doubled frequencies. Experimental study of the feedback on SSL was conducted using tunable VCSELs. The advantage of such devices is the absence of mode hops during the sweep over several tens of nm. Optical feedback was implemented using an external mirror and using the feedback strength and the sweep rate parameters predicted by the model.
The prototypes of 850nm lasers were fabricated.
1) Experimental and theoretical study of coherence limitations in semiconductor lasers based on fibre cavity. The results confirm the impact of the intra-cavity filter on the laser dynamics. In particular, the transmission bandwidth should be carefully considered when designing the laser for applications such as imaging, where any deviation from stable operation deteriorates image quality.
2) Developing of the theoretical model and experimental testing to demonstrate the influence of optical feedback in swept source lasers. It was shown that the optical feedback can be used as a tool to lock the phase of the successive modes. Also, the feedback can lead to the sliding frequency mode-locking regime demonstrating sub-nanosecond pulses, interesting for nonlinear optics applications. The power of the proposed technique has been demonstrated on the model of the multi-section semiconductor lasers. It can also be adapted to other swept source configurations exhibiting mode hops, or serve as a tool to couple several swept sources to increase the sweeping range with maintained coherence.
The achieved results were disseminated via publications and presentations at conferences and obtained visibility, which lead to new collaboration between the Host institute and companies developing new optical devices.
The details of the work performed are as follows:
First, we investigated the mechanisms of coherence limitations of ring cavity SSL with semiconductor gain medium and a tunable optical filter in normal and anomalous dispersion regimes. Such lasers can operate in a regime of synchronisation, when the filter tuning speed is synchronous to the cavity roundtrip time (FDML regime). FDML lasers currently demonstrate the fastest tuning rate. It was previously shown that the dynamics of the lasers operating in the FDML regime is equivalent to the laser dynamics operating at a static filter transmission for each wavelength, and FDML lasers can exhibit similar coherence limitations. Experimentally, we studied the dynamics of the laser operating in the static regime using the electric field reconstruction technique. The technique is based on simultaneous recording of the laser and three outputs of the 3x3 fiber interferometer using a fast detector and a fast real time oscilloscope. Temporal evolution of the phase of the electric field was obtained using the output signals of this interferometer. This measurement revealed that at the bias current close to threshold the laser emitted stable intensity drops whose frequency was out of the filter transmission resonance. Therefore these drops experienced additional losses and could not be compensated by the amplifier and remained stable for many roundtrips. The presence of these drops limits the coherence of such lasers. Theoretically, a delay differential equation model was used and numerical simulation revealed that these structures followed heteroclinic connections between two stable modes. These structures are similar to those predicted by Ginsburg-Landau equation (Nozaki-Bekki holes). The dynamics of these structures depended on the bias current and the filter transmission bandwidth. In addition, it was shown that unstable Nozaki-Bekki holes could generate turbulent puffs which deteriorated the coherence in an FDML laser when synchronisation was disturbed. These results lead to two publications. In addition, the effect of the fiber dispersion on laser dynamics was analysed experimentally and theoretically. One manuscript is under review, one in preparation.
Second, we performed experimental study of formation of such coherent structures during the laser turn on. The bias current of the semiconductor gain was quickly turned on with an electrical signal, whose rise time was much shorter than the cavity roundtrip time, and the laser intensity time trace was recorded using fast electronics. The turn on transient of the laser was studied at various laser parameters, including the bias current, filter width and the dispersion regime. The phase measurement technique, based on a 3x3 fiber coupler, allowed for reconstruction of evolution of the optical spectrum during the laser turn on. The results obtained were confirmed using the same delay differential equations model. The manuscript is now under preparation.
The ring cavity including a VCSEL, used for wavelength tuning, and a semiconductor gain was tested.
The theoretical model was developed to study the effect of optical feedback in swept source lasers. The model have shown that it is possible to lock the phase of successive modes of a multi-section swept source semiconductor laser using optical feedback. As a result of locking, the coherence of the laser does not deteriorate after the mode hops. The model predicted four dynamical regimes of the laser depending on the optical feedback strength and the sweeping rate of the filter: beating, chaotic, periodic pulsations and periodic pulsations with doubled frequencies. Experimental study of the feedback on SSL was conducted using tunable VCSELs. The advantage of such devices is the absence of mode hops during the sweep over several tens of nm. Optical feedback was implemented using an external mirror and using the feedback strength and the sweep rate parameters predicted by the model.
The prototypes of 850nm lasers were fabricated.
The project advanced the technology of swept source lasers via development of new techniques for coherence control of such lasers for high quality optical imaging systems for medical and industrial applications. The progress include
• Developing new techniques of coherence control in SSLs via optical feedback.
• Theoretical study of a new concept of coherence transfer between successively tuning modes in one SSL, or between two SSLs.
• Theoretical study of influence of optical feedback on SSL dynamics.
• Understanding of coherence limitations in long cavity SSL with single mode fiber cavity.
• Experimental study of turn on processes in lasers.
• Study of optical turbulence in lasers.
• Fabrication of prototype SSL at 850nm,
The impact of the project concerns the development of technologies in key application areas of lasers for imaging and sensing, medical devices, semiconductor lasers, as well as scientific research in the field of nonlinear dynamics, turbulence and complex systems.
• Developing new techniques of coherence control in SSLs via optical feedback.
• Theoretical study of a new concept of coherence transfer between successively tuning modes in one SSL, or between two SSLs.
• Theoretical study of influence of optical feedback on SSL dynamics.
• Understanding of coherence limitations in long cavity SSL with single mode fiber cavity.
• Experimental study of turn on processes in lasers.
• Study of optical turbulence in lasers.
• Fabrication of prototype SSL at 850nm,
The impact of the project concerns the development of technologies in key application areas of lasers for imaging and sensing, medical devices, semiconductor lasers, as well as scientific research in the field of nonlinear dynamics, turbulence and complex systems.