Characterization of the dynamics of an FDML laser during closed-loop cavity length control

In Fourier domain mode locked (FDML) lasers, extremely precise and stable matching of the filter tuning period and light circulation time in the cavity is essential for ultra-low noise operation. During the operation of FDML lasers, the ultra-low noise mode can be lost due to temperature drifts of the already temperature stabilized cavity resulting in increased intensity noise. Until now, the filter frequency is continuously regulated to match the changing light circulation time. However, this causes the filter frequency to constantly change by a few mHz and leads to synchronization issues in cases where a fixed filter frequency is desired. We present an actively cavity length controlled FDML laser and a robust and high precision feedback loop algorithm for maintaining ultra-low noise operation. Instead of adapting the filter frequency, the cavity length is adjusted by a motorized free space beam path to match the fixed filter frequency. The closed-loop system achieves a stability of ~0.18 mHz at a sweep repetition rate of ~418 kHz which corresponds to a ratio of 4×10-10. We investigate the coherence properties during the active cavity length adjustments and observe no noise increase compared to fixed cavity length. The cavity length control is fully functional and for the first time, offers the possibility to operate an FDML laser in sweet spot mode at a fixed frequency or phase locked to an external clock. This opens new possibilities for system integration of FDML lasers.

[1]  J. Fujimoto,et al.  Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles. , 2005, Optics express.

[2]  Wolfgang Draxinger,et al.  Ultra low noise Fourier domain mode locked laser for high quality megahertz optical coherence tomography , 2018, Biomedical optics express.

[3]  R. Huber,et al.  Dispersion, coherence and noise of Fourier domain mode locked lasers. , 2009, Optics express.

[4]  Armin Wolf,et al.  Combined in-depth, 3D, en face imaging of the optic disc, optic disc pits and optic disc pit maculopathy using swept-source megahertz OCT at 1050 nm , 2017, Graefe's Archive for Clinical and Experimental Ophthalmology.

[5]  D. Adler,et al.  Extended coherence length Fourier domain mode locked lasers at 1310 nm. , 2011, Optics express.

[6]  Wolfgang Draxinger,et al.  Beating of two FDML lasers in real time , 2020, LASE.

[7]  Armin Wolf,et al.  Megahertz ultra-wide-field swept-source retina optical coherence tomography compared to current existing imaging devices , 2014, Graefe's Archive for Clinical and Experimental Ophthalmology.

[8]  J. Fujimoto,et al.  Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography. , 2006, Optics express.

[9]  Muhammad Kamran,et al.  Implementation of improved Perturb & Observe MPPT technique with confined search space for standalone photovoltaic system , 2020 .

[10]  Wolfgang Draxinger,et al.  MHz-OCT for low latency virtual reality guided surgery: first wet lab experiments on ex-vivo porcine eye , 2019, European Conference on Biomedical Optics.

[11]  A. Kampik,et al.  Multi-MHz retinal OCT. , 2013, Biomedical optics express.

[12]  Wolfgang Wieser,et al.  Wide-Field Megahertz OCT Imaging of Patients with Diabetic Retinopathy , 2015, Journal of diabetes research.

[13]  R. Huber,et al.  K-space linear Fourier domain mode locked laser and applications for optical coherence tomography. , 2008, Optics express.

[14]  R. Huber,et al.  Recent developments in Fourier Domain Mode Locked lasers for optical coherence tomography: Imaging at 1310 nm vs. 1550 nm wavelength , 2009, Journal of biophotonics.

[15]  S. Sanders,et al.  Ultrastable Fourier domain mode locking observed in a laser sweeping 1363.8 – 1367.3 nm , 2009, 2009 Conference on Lasers and Electro-Optics and 2009 Conference on Quantum electronics and Laser Science Conference.

[16]  Wolfgang Wieser,et al.  Multi-megahertz OCT: High quality 3D imaging at 20 million A-scans and 4.5 GVoxels per second. , 2010, Optics express.

[17]  Robert Huber,et al.  High-resolution retinal swept source optical coherence tomography with an ultra-wideband Fourier-domain mode-locked laser at MHz A-scan rates. , 2018, Biomedical optics express.

[18]  Wolfgang Wieser,et al.  Real time en face Fourier-domain optical coherence tomography with direct hardware frequency demodulation. , 2008, Optics letters.

[19]  Wolfgang Wieser,et al.  Analysis of FDML lasers with meter range coherence , 2017, BiOS.

[20]  Wolfgang Wieser,et al.  Live video rate volumetric OCT imaging of the retina with multi-MHz A-scan rates , 2019, PloS one.

[21]  Deepkiran Tirkey,et al.  Maximum Power Point Tracking Using Perturb & Observe Algorithm and Compare With another Algorithm , 2013 .