Alexander Gaeta Department of Applied Physics and Applied Mathematics Michal Lipson Department of Electrical Engineering

Chip-Based Optical Frequency Combs Alexander Gaeta Department of Applied Physics and Applied Mathematics Michal Lipson Department of Electrical Engineering KISS Frequency Comb Workshop Cal Tech, Nov. 2-5, 2015

Chip-Based Comb Generation singlefrequency pump laser ω χ (3) interaction Microresonator Modelocked laser ω Si 3 N 4 nanowaveguide Origin of combs can be traced to four-wave mixing (FWM) Requires small anomalous group-velocity dispersion

Microresonator-Based Parametric Combs silica µ-toroids Del Haye et al., Nature (2007). Del Haye et al., PRL (2008). silica µ-spheres Agha et al., Opt. Express (2009). CaF 2, MgF 2, & quartz Savchenkov et al., PRL (2008). Liang et al., Opt. Lett. (2011). Papp & Diddams, PRA (2011). Herr et. al., Nat. Phot. (2012). high-index glass µrings Razzari et al., Nature Photon. (2010). Pasquazi et al., Opt. Express (2013). silica disks & rods Li et al., PRL (2012) Papp, et al., PRX (2013) diamond Hausmann et al., Nat. Photon. (2013). silicon Griffith et al., (2014). Si nitride Levy et al., Nat. Photon. (2010). Ferdous et al., Nat Photon. (2012). Herr et al., Nat. Photon. (2012). Al nitride Jung et al., Opt. Lett. (2013).

Microresonator Comb Spectral Coverage CaF 2 MgF 2 [8] 4700 nm [7] 4600 nm Si [6] 4600 nm Si 3 N 4 [5] 3400 nm MgF 2 [4] 2550 nm Si 3 N 4 [3] 2350 nm SiO 2 [2] 2170 nm Si 3 N 4 [1] 1540 nm λ 0.5 μm 1 μm 1.5 μm 2 μm 2.5 μm 3 μm 3.5 μm 4 μm 4.5 μm 5 μm 5.5 μm 6 μm [1] Saha, et al., Lipson & Gaeta (2013); Luke, et al., Gaeta & Lipson, in preparation (2015). [2] Del Haye, et al., and Kippenberg, Phys. Rev. Lett. (2011). [3] Okawachi, et al., Lipson & Gaeta, Opt. Lett. (2011); Okawachi, et al., Lipson & Gaeta, Opt. Lett. (2013). [4] Wang, et al., and Kippenberg, Nature Comm (2012). [5] Griffiths, et al., Gaeta & Lipson, Nat. Comm. (2015). [6] Luke, et al., Gaeta and Lipson, in preparation (2015). [7] Lecaplain, et al., Kippenberg, arxiv (2015). [8] Savchenko, et al., Maleki, arxiv (2015).

Silicon-Based Microresonators for Parametric Comb Generation cross-section Si 3 N 4 µ-resonator Si 3 N 4 deposited SiO 2 thermal SiO 2 CMOS-compatible material Fully monolithic and sealed structures and couplers High-Q resonators à Si 3 N 4 Q = 7 10 6 [Luke, et al., Opt. Express (2013).] Si Q ~ 10 6 [Lee, et al., (2013).] High nonlinearity à n 2 ~ 10-100 silica Waveguide dispersion can be engineered [Foster, et al., Lipson, Gaeta, Nature 441, 960 (2006). Turner-Foster, et al., Gaeta, Lipson, Opt. Express 18, 1904 (2010).]]

Tailoring of GVD in Si-Based Waveguides l GVD can be tuned by varying waveguide shape and size. SiO 2 l Same chip can operate w/ different pump wavelengths. Si/Si 3 N 4 n ~ 3.5 (SiN: n ~ 2.1) Si 3 N 4 anomalous Si normal l Oxide cladding limits generation < 5 µm (?) Foster, Turner, Sharping, Schmidt, Lipson, and Gaeta, Nature 441, 960 (2006). Turner, et al. Gaeta, and Lipson, Opt. Express 14, 4357 (2006).

Octave-Spanning Comb in Si 3 N 4 l > 150 THz bandwidth l Stable, robust, highly compact comb source for clock applications l Modest power requirements (100 s of mw) Okawachi, et al., Lipson, and Gaeta, Opt. Lett. (2011).

Dispersion Engineering: Broadband Combs with 1-µm Pump in Si 3 N 4 690 x 1400 nm cross section, 46-µm resonator radius (500 GHz FSR) >2/3 octave of continuous comb bandwidth Saha, et al., Lipson, and Gaeta, Opt. Express (2012) Luke et al. Lipson, Gaeta, to be published (2014).

Mid-IR Comb in Si 3 N 4 Power (dbm) 0-30 -60 experiment 2250 2500 2750 3000 3250 3500 Wavelength (nm) theory 950 x 2700 nm waveguide Fully filled in comb spanning 2.3-3.4um P th ~ 80 mw, FSR = 99GHz Luke, et al., Gaeta & Lipson, Opt. Lett. (2015)

Silicon as a Mid-IR Material Advantages: Large 3 rd order nonlinearity Transparent to ~ 8 um High refractive index Problem: Need to pump > 2 µm Three-photon absorption Significant above 1 Watt circulating power Three Photon Absorption Free carrier ω o ω o e ω o

Fabricated Silicon Device 510,000 intrinsic quality factor at 2.6 um 0.8 db/cm loss Wavelength (nm)

Mid-IR Parametric Frequency Comb 500 1400 nm etchless silicon microresonator with p-i-n structure Q-factor ~10 6 Measurement with FTIR OSA èbandwidth limited by dynamic range of OSA 2608-nm pump 750-nm bandwidth 125-GHz FSR (100 μm radius) Griffith, et al., Gaeta and Lipson, Nat. Comm. (2015)

Comb Generation without Carrier Extraction Free carrier Three Photon Absorption ω o ω o e ω o

Near Octave-Spanning Mid-IR Comb Generation in Si Microresonator Pump wavelength 3095 nm Comb spans > octave RF signal RF analyzer noise floor Wavelength range: 2165 4617 nm Comb exhibits low RF noise

Waveguide Design for Octave-Spanning Coherent SCG at 1 μm Engineer dispersion by tailoring waveguide cross section Design broad region of anomalous group velocity dispersion (β 2 ) around 1-μm pump Coherent SCG with 100-fs pump through self-phase modulation and dispersive wave emission ß 2 [ps 2 /km] 400 200 0 Cross section 690 x 900 nm 600 800 1000 1200 Wavlength [nm] 1400 1600

Supercontinuum Generation with Diode-Pumped Solid-State Laser Collaboration w/ Ursula Keller s group (ETH-Zurich) Pump with 1-GHz repetition rate SESAM-modelocked diode-pumped Yb:CALGO laser [ Klenner et al., Opt. Express (2014)] 1-GHz cavity Yb:CALGO 92-fs input pulses, 1055 nm center wavelength SESAM output coupler multimode pump diode Power [db] -30-40 -50-60 600 800 37 pj coupled pulse energy (37 mw average power) 1000 1200 Wavelength [nm] 1400 1600

Supercontinuum Coherence Measurement Power [db] 0-50 -100 600 800 OSA sweep records ensemble average (1) Coherence related to visibility g 12 [Nicholson and Yan, Opt. Express (2004); Gu et al., Opt. Express (2011)] V (λ) = I max(λ) I min (λ) I max (λ) + I min (λ) 1000 1200 Wavelength [nm] V (λ) 1400 V (λ) = 2 g (1) 12 [ I 1 (λ)i 2 (λ)] 1/ 2 [ I 1 (λ)+i 2 (λ)] Perform coherence measurement in 100-nm increments 1600 1 0.8 0.6 0.4 0.2 0 Visibility

Coherent Supercontinuum for f-to-2f Interferometry (a) 0 1 Power [db] -50 0.8 0.6 0.4 0.2 Visibility -100 600 800 1000 1200 Wavelength [nm] 1400 1600 0 (b) Power [db] -60-80 680 700 720 Wavelength [nm] 1 0.5 0 Visibility (c) Power [db] -20-40 1360 1400 1440 Wavelength [nm] 1 0.5 0 Visibility

Carrier Envelope Offset Frequency Detection Using Silicon Nitride Waveguide Carrier envelop offset frequency (f ceo ) beatnote from f-to-2f interferometry Spectrum at 1360 nm is frequency doubled and overlapped with at 680 nm f ceo signal-to-noise ratio > 30 db Much lower noise level (10 db) than w/ PCF f CEO SNR > 30 db [ Mayer et al., Opt. Express (2015)]

Comparison of Comb Generation Schemes Pump Properties single-frequency P > 200 mw CEO control Tuning for modelocking (?) ω Spacing > 20 GHz Microresonator Properties > 200 µw/line Thermal issues important Stabilized ~ 2/3 Octave Comb spacing control (thermal) Near-IR mid-ir Modelocking (Thermal?) Comb Properties Pump Properties Modelocked < 200 fs for coherent comb CEO & comb spacing control P ~ 40 mw ω Nanowaveguide Properties Passive Waveguide dispersion tailored longitudinally Comb Properties Spacing > 20 GHz > 100 nw/line Stabilized > Octave Visible mid-ir

Compact Solid-State 5-GHz Modelocked Laser

Comparison of Comb Generation Schemes Pump Properties single-frequency P > 200 mw CEO control Tuning for modelocking (?) ω Spacing > 20 GHz Microresonator Properties 1-200 µw line Thermal issues important Stabilized ~ 2/3 Octave Comb spacing control (thermal) Near-IR mid-ir Modelocking (Thermal?) Comb Properties Pump Properties Modelocked < 200 fs for coherent comb CEO & comb spacing control P ~ 40 mw ω Nanowaveguide Properties Passive Waveguide dispersion tailored longitudinally Comb Properties Spacing > 20 GHz 1-200 µw line Stabilized > Octave Visible mid-ir

Spectrally Efficient Octave-Spanning Spectrum For applications (e.g., frequency synthesizer) that are particularly power sensitive. Simulations [ Okawachi et al. Lipson & Gaeta (2015)]