Topi Uusitalo Single- and Dual-Frequency Laser Diodes based on Surface Gratings Julkaisu 1556• Publication 1556 Tampere 2018 Tampereen teknillinen yliopisto. Julkaisu 1556 Tampere University of Technology. Publication 1556 Topi Uusitalo Single- and Dual-Frequency Laser Diodes based on Surface Gratings Thesis for the degree of Doctor of Science in Technology to be presented with due permission for public examination and criticism in Sähkötalo Building, Auditorium SA203, at Tampere University of Technology, on the 3rd of August 2018, at 12 noon. Tampereen teknillinen yliopisto - Tampere University of Technology Tampere 2018 Doctoral candidate: Topi Uusitalo, M.Sc. Laboratory of Photonics / ORC Faculty of Natural Sciences Tampere University of Technology Finland Supervisor: Mihail Dumitrescu, Doc. Laboratory of Photonics / ORC Faculty of Natural Sciences Tampere University of Technology Finland Instructors: Mircea Guina, Prof. Laboratory of Photonics / ORC Faculty of Natural Sciences Tampere University of Technology Finland Jukka Viheriälä, Doc. Laboratory of Photonics / ORC Faculty of Natural Sciences Tampere University of Technology Finland Pre-examiners: Trevor Hall, Prof. School of Electrical Engineering & Computer Science Faculty of Engineering University of Ottawa Canada Jari Turunen, Prof. Institute of Photonics Department of Physics and Mathematics University of Eastern Finland Finland Opponent: Martijn Heck, Prof. Department of Engineering Aarhus University Denmark ISBN 978-952-15-4165-0 (printed) ISBN 978-952-15-4173-5 (PDF) ISSN 1459-2045 i ABSTRACT The thesis covers the development of single and dual longitudinal mode dis- tributed feedback (DFB) lasers with surface gratings. These gratings were studied since they enable regrowth-free fabrication and easy implementa- tion of arbitrary longitudinal effective refractive index variations. Advanced transverse and longitudinal surface grating structures were modeled, simu- lated, and experimentally tested. Procedures for optimizing the laser struc- tures across a wide range of parameter variations were developed. The main targets were narrow linewidth emission for optical pumping of Rubidium atomic clocks, increased direct amplitude modulation for optical communi- cations, and photonic generation of widely tunable electrical signals in the millimeter wave band for the next generation of wireless communications. Laterally-coupled ridge-waveguide (LC-RWG) surface gratings with alter- nating lateral protrusions were developed to circumvent the etching aspect ratio limitation that prevents the fabrication of low-order short-wavelength surface gratings. The single longitudinal mode DFB lasers with alternating LC-RWG gratings emitting around 780 nm demonstrated improved perfor- mances, particularly important for the space borne target applications. The dual longitudinal mode emission was achieved by periodically placing phase shifts along the grating. The experiments showed that the direct amplitude modulation bandwidth is extended when the photon-photon res- onance associated with dual-mode emission is properly placed with respect to the carrier-photon resonance and when both resonances are adequately damped. Difference frequency range controlled by structural variations and 40 GHz difference frequency tuning by bias were demonstrated, as well as high speed difference frequency modulation. Linearly apodized LC-RWG surface gratings enabled a lower threshold current, a higher output power, and a broader range of difference frequency tunability by bias. Also the apodization and the complex-coupling of the surface gratings enabled the use of higher facet reflectivities, leading to narrower intrinsic short time- scale linewidths. The achieved performance improvements indicate that the studied laser types are promising solutions for their target applications. ii iii PREFACE This study was carried out at Tampere University of Technology (TUT) Op- toelectronics Research Centre (ORC), Laboratory of Photonics. The work is the result of a combined effort contributed by many researchers and support staff. I wish to acknowledge the financial support from Business Finland in project Excelsior (230336), and from the European Space Agency in project EARLY (4000110645/13/NL/HB). I would like to express my sincere gratitude to my supervisor Doc. Mi- hail Dumitrescu, whose support and guidance have been invaluable for this thesis’ work and for many other projects. I would also like to thank my colleague Dr. Heikki Virtanen for all the support, especially with device processing and with other tasks we have carrier out together: without his work ethic and attention to detail many of the results in this thesis would not have been published. Prof. Mircea Guina has been the driving force behind the whole ORC for a long time and I wish to extend my gratitude for giving me the chance to carry out the research presented in this work. Doc. Jukka Viheriälä has been invaluable in coordinating the laser diode processing workflow, M.Sc. Antti Aho in drawing the nanoimprint masters, and M.Sc. Maija Karjalainen in processing the devices: thank you all. I also wish to thank Dr. Sanna Ranta for growing the epitaxial structures used in this thesis. My thanks also go to Prof. Paolo Bardella for assistance in simulations of laser dynamics. Additionally I wish to thank Dr. Antti Laakso for his assistance in the be- ginning of my career, and later, especially with various modeling problems. I would also like to thank all the colleagues I have had the pleasure of knowing and working together along the years, having shared ideas and coffee, and sometimes perhaps coming up with bright ideas. Without the extensive and professional support staff of ORC this thesis would not be complete: thank you all. With the bureaucratic and other matters Mrs. Anne Viherkoski has been invaluable to myself and most likely almost everybody having worked at ORC: thank you for your patience, understanding and never-ending pos- itivity during the time I have had the honor of working at ORC. Special iv thanks go to my student comrades who have made studying at Tampere University of Technology more memorable and enjoyable. Last I want to thank my family for all the support and understanding along the rocky road of higher education over the years. And Essi, whose love, support and voice of reason remind me that there indeed is life even after laser diodes inevitably die. Tampere 8.5.2018 Topi Uusitalo v CONTENTS Abstract . .i Preface . iii Glossary . vii Acronyms . xi List of Publications . xv Author’s Contribution . xvii 1. Introduction . .1 1.1 Approach and motivation . .5 1.2 Theoretical background . .6 1.2.1 Single frequency laser diodes . .9 1.2.2 Dual frequency laser diodes . 10 2. Design and simulation . 13 2.1 Epilayer structure . 13 2.2 Transverse and grating structure . 17 2.2.1 Transverse mode discrimination . 20 2.2.2 Coupling coefficient . 23 2.2.3 Wider LC-RWG grating trenches by alternating place- ment of the lateral grating protrusions . 29 2.3 Optimization of single frequency gratings . 31 2.3.1 Linewidth . 32 vi 2.3.2 Modulation frequency . 34 2.3.3 Single longitudinal mode and yield . 35 2.4 Dual frequency gratings . 35 2.4.1 Difference frequency generation . 37 3. Experimental results and discussion . 41 3.1 Single frequency lasers . 41 3.2 Dual frequency lasers . 43 3.2.1 Modulation of dual frequency lasers . 46 3.2.2 Difference frequency linewidth . 51 4. Conclusions . 55 References . 58 Original papers . 81 Distributed Feedback Lasers with Alternating Laterally-Coupled Ridge-Waveguide Surface Gratings . 83 Transverse Structure Optimization of Distributed Feedback and Distributed Bragg Reflector Lasers with Surface Gratings . 89 Analysis of the Photon-Photon Resonance Influence on the Direct Modulation Bandwidth of Dual-Longitudinal-Mode Distrib- uted Feedback Lasers . 101 Dual-Mode DFB Laser Diodes with Apodized Surface Gratings . 117 vii GLOSSARY th Am Fourier coefficient for m order grating G modal gain Ith threshold current I bias current K Petermann K factor M number of grating periods between phase shifts Np photon density P0 power contained in the fundamental mode P number of phase shifts in the grating S stopband width fitting parameter V volume of gain material ∆νBragg frequency separation between the Bragg reso- nances of two stopbands ∆νFWHM spectral linewidth full width at half maximum ∆νmodes frequency separation between two grating modes ∆νsb node-to-node width of a stop band ∆ν difference frequency ∆n refractive index contrast Γ + under-the-ridge confinement factor Γ − not-under-the-ridge confinement factor Γg optical confinement factor in the grating area Γ optical confinement factor in the gain material Λ1 grating protrusion width Λ grating period Ω+ region of gain material under the ridge Ω− region of gain material not under the ridge Ω integration domain αeff effective linewidth enhancement factor αint optical internal losses αm mirror losses viii β0 propagation constant of the fundamental quasi- TE mode ηi internal quantum efficiency γ grating fill factor κL coupling strength of a DFB or DBR grating κr real coupling coefficient between forward and backward traveling waves κ coupling coefficient between forward and back- ward traveling waves λ wavelength νBragg Bragg frequency ν laser frequency ωr relaxation resonance frequency ψ quasi-TE mode amplitude ρ modal loss τp photon lifetime inside laser cavity θ angle of lateral current spreading c speed of light in vacuum dg/dN differential gain dn/dN change in refractive index with carrier