National Institute of Standards and Technology Towards Chip-Integrated Low-Noise Lasers William Loh The Laser is Everywhere National Institute of Standards and Technology Laser Invention ‒ 1958 Wavelength: 150 nm >250 μm Excimer Quantum Cascade Power: 1 mW > 1000 W Laser Pointer Industrial Cutting Surgery Spectroscopy Imaging Laser Tag Welding Communications Countermeasures Holography CDs Ranging Guidance Lasers Cooling Sensing Photolithography Printers 2 The Lasers of Tomorrow National Institute of Standards and Technology Greater Higher Tuning Power Laser Smaller Lower Size Noise 3 Laser Configuration National Institute of Standards and Technology Two Conditions T for Oscillation 1) Gain > Loss 2) Tm 2 Modes (a) Phase Noise (b) Linewidth Power Spectral Density Spectral Power Full-Width Half-Max Frequency 4 Applications of Low-Noise Lasers National Institute of Standards and Technology Coherent Fiber-Optic Sensing Spectroscopy Communications Q I Optical Development of LIDAR Frequency advanced optical- Metrology atomic clocks Phase Detection Sharp Resonance Need low Need narrow phase noise linewidth 5 Applications of Low-Noise Lasers National Institute of Standards and Technology Optical Frequency Division T. Fortier et al. Ultrastable cavity (Q ~ 1011) Nature Photon. 2011 CW laser n ~ 50,000 1 Hz linewidth νn = n fr+fo νn - fo fr f = r n δνn - δfo δf = r n 1/fr Optical phase noise Noise sidebands reduction ~ 90 dB Spectrum analysis f fr 6 Low-Noise Lasers by Feedback National Institute of Standards and Technology Stabilize Erbium-doped fiber laser Sharp Cavity Resonance Q ~1011 7 Current Technology for Low-Noise Lasers National Institute of Standards and Technology External- Cavity- Diode DFB Fiber Cavity Stabilized Laser Laser Laser Laser 6 4 5 3 4 Linewidth 10 Hz 10 ‒10 Hz 10 ‒10 Hz 1 Hz Narrower Linewidth Size 0.5 mm 10 mm 100 mm 1000 mm Larger Size Less Tunability Tunable Want: Narrow Linewidth Compact Size Wavelength Tunable 8 The SBS Microresonator Laser National Institute of Standards and Technology Stimulated Brillouin Scattering (SBS) Pump Phonon R. W. Boyd, Nonlinear Optics SBS SBS Nonlinearity SBS Lasing D = 6 mm Output Grudinin et al., PRL, 2009 PumpLee et al., Nature Photon., 2012 9 SBS Versus Conventional Lasers National Institute of Standards and Technology Spontaneous Stimulated Emission Emission Conventional Laser Pump Output Thermal Stimulated Excitation Brillouin Scattering SBS Laser Output Pump 10 The SBS Process ‒ Gain National Institute of Standards and Technology Pump Phonon SBS 1. Pump scatters off of acoustic phonon to create SBS 2. Pump interferes with SBS to reinforce phonon E~ eiPPSS t k z e i t k z 11 The SBS Process ‒ Noise National Institute of Standards and Technology Pump Phonon SBS h Average : Thermal Energy h kT energy e 1 per mode h 1 kT Phonon kT … … SBS process limited by thermal noise EE 11 GHz fcutoff ~ 6 THz reflected pump phonon 12 Microcavity SBS Laser National Institute of Standards and Technology 1. Pump Length ~ 10 m Pump ~ 100 mW Output Pump Loss High-Q Microresonator 2. 6 mm Easy to achieve Output Gain > Loss Pump Fiber Taper Pump ~ 1-10 mW 13 The Importance of SNR National Institute of Standards and Technology SBS lasers achieve large SNR Low Noise! Im{E} To achieve low phase noise, Noise want large signal to noise: P Signal SBS PNoise Re{E} Schawlow-Townes linewidth ~ 1/P 14 Low Noise SBS Lasing National Institute of Standards and Technology PSBS Diode Lasers SBS Lasers PNoise -1 Want large signal-to-noise αm = 45.6 cm -1 αi = 5 cm Low Loss Nonlinear Process Q ~ 5×103 Q > 108 Large SNR P = 1 mW P = 1 mW Δν = 1 MHz Δν < 100 Hz Low Noise Moderate Power (Noise Clamps (mW range) at Threshold) Hertz-class intrinsic linewidth 15 SBS Laser Configuration National Institute of Standards and Technology • Separate SBS and Pump • Lock Pump to Cavity Pound-Drever-Hall High-Q Microresonator Pump Fiber Taper Phase Photodiode Modulator 1 mm Output H. Lee et al., Nature Photon., 2012 Fiber Taper Fabricated by Vahala group at Caltech 16 SBS Laser Performance National Institute of Standards and Technology 2 • > 102 SBS noise reduction at high frequencies • SBS noise comparable or better than commercial solid-state and fiber lasers Gorodetsky et al., JOSA B, 2004 Thermorefactive Noise: Matsko et al., JOSA B, 2007 17 Dual-Microcavity SBS Laser National Institute of Standards and Technology Pound-Drever-Hall Microdisk SOA Pump Phase Photodiode Modulator Heat Output SBS Pump 18 SBS Tuning National Institute of Standards and Technology Tuning by microdisk Tuning by other temperature resonant modes Laser tuning up to Terahertz 19 Dual-Microcavity SBS Laser National Institute of Standards and Technology Pound-Drever-Hall Microdisk SOA Pump Microrod Phase Photodiode Modulator Photodiode Pound-Drever-Hall Papp, PRX 2013 Del’Haye, APL, 2013 20 Microrod Resonators National Institute of Standards and Technology Microrod Fab: Fused Silica Papp, PRX 2013 Del’Haye, APL, 2013 Q = 0.9 Billion 21 Microrod Resonator Fabrication National Institute of Standards and Technology Video Courtesy of Dr. Pascal Del’Haye 22 Dual-Microcavity SBS Laser Noise National Institute of Standards and Technology SBS noise improved through locking to a second microresonator 23 Dual-Microcavity SBS Laser Spectrum National Institute of Standards and Technology Pump Laser: 10 kHz linewidth SBS Microdisk: 4 kHz linewidth Dual-microcavity SBS Laser: 90 Hz linewidth 24 SBS Laser Spectroscopy National Institute of Standards and Technology 4-kHz cavity resonance only resolved by the locked SBS laser W. Loh et al., Noise and dynamics of stimulated Brillouin scattering microresonator lasers, PRA 2015 W. Loh et al., Dual-microcavity narrow-linewidth Brillouin laser, Optica 2015 25 Current Technology for Low-Noise Lasers National Institute of Standards and Technology External- Cavity- Dual- Diode DFB Cavity Fiber Laser Stabilized Microcavity Laser Laser Laser SBS Laser Linewidth 106 Hz 104‒105 Hz 103‒104 Hz 1 Hz 100 Hz Size 0.5 mm 10 mm 100 mm 1000 mm 10 mm • Size comparable to an external cavity laser • Linewidth many orders of magnitude better 26 Takeaways National Institute of Standards and Technology 1. There is a real need for low noise lasers that are both compact and tunable 2. SBS lasing in a microresonator allows for large ratios of signal to noise at low pump powers Low noise lasers 3. Obtain four orders of magnitude noise reduction by locking to a microrod 27 Combining Two Resonators into One National Institute of Standards and Technology Pound-Drever-Hall MicrorodMicrodisk SOA Pump Microrod Phase Photodiode Modulator Photodiode ChallengesPound-Drever -Hall 1. Dimensions 2. Power/Area 28 SBS Microrod Laser Noise National Institute of Standards and Technology SBS microrod laser is 102 better than microdisk laser at low freq. 29 SBS Microrod Laser Spectrum National Institute of Standards and Technology Pump Laser: 10 kHz linewidth SBS Microdisk: 4 kHz linewidth Dual-microcavity SBS Laser: 90 Hz linewidth SBS Microrod: 400 Hz linewidth 30 Future Directions ‒ SBS Laser National Institute of Standards and Technology Stabilize 6 mm Δf3dB = 90 Hz 1 Hz Cesium Clock: Δf/f ~ 2×10-15 Optical-Atomic Clock: Δf/f ~ 2×10-18 N. Hinkley et al. Science 2013 10 Hz Stabilize 1 m 31 Future Directions ‒ SBS Laser National Institute of Standards and Technology Optical Frequency Division T. Fortier et al. Ultrastable cavity (Q ~ 1011) Nature Photon. 2011 CW laser 6 mm n ~ 50,000 1 Hz linewidth νn = n fr+fo νn - fo fr f = r n δνn - δfo T. Herr et al. Nature Photon. 2014 δf = r n 1/fr Optical phase noise Noise sidebands reduction ~ 90 dB Spectrum analysis f fr 32 Acknowledgments National Institute of Standards and Technology • Postdoctoral Advisors – Dr. Scott Diddams and Dr. Scott Papp • NRC Postdoctoral Fellowship, DARPA Pulse NIST OFM Lab Caltech Vahala Group Adam Green Hansuek Lee Joseph Becker Kerry Vahala Fred Baynes Daniel Cole Frank Quinlan Pascal Del’Haye Aurélien Coillet Katja Beha Scott Papp Scott Diddams 33 Questions? National Institute of Standards and Technology 34 .