Chip-Based Optical Frequency Combs Alexander Gaeta Department of - PowerPoint PPT Presentation

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 spectrum single- frequency spectrum pump laser χ (3) interaction ω Microresonator spectrum Si 3 N 4 spectrum Modelocked nanowaveguide laser ω • Origin of combs can be traced to four-wave mixing (FWM) • Requires small anomalous group-velocity dispersion

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

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

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

Microresonator Comb Spectral Coverage CaF 2 [8] 4700 nm [7] 4600 nm MgF 2 [6] 4600 nm Si [5] 3400 nm Si 3 N 4 MgF 2 [4] 2550 nm Si 3 N 4 [3] 2350 nm SiO 2 [2] 2170 nm [1] 1540 nm Si 3 N 4 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 deposited SiO 2 Si 3 N 4 µ -resonator Si 3 N 4 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).] Q ~ 10 6 [Lee, et al., (2013).] Si • 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 GVD can be tuned by varying l waveguide shape and size. SiO 2 Same chip can operate w/ different l pump wavelengths. n ~ 3.5 Si/Si 3 N 4 (SiN: n ~ 2.1) Si 3 N 4 Si anomalous normal Oxide cladding limits generation < 5 µm (?) l 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- µ m Pump in Si 3 N 4 • 690 x 1400 nm cross section, 46- µ m 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 0 Power (dBm) experiment -30 -60 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: Problem: Large 3 rd order • Need to pump > 2 µm • nonlinearity • Three-photon absorption • Transparent to ~ 8 um Significant above 1 Watt • circulating power High refractive index • Three Photon Absorption Free carrier ω o ω o ω o e

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 è Bandwidth 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 ω o Three Photon Absorption ω o ω o e

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

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

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 400 Cross section 690 x 900 nm ß 2 [ps 2 /km] 200 0 600 800 1000 1200 1400 1600 Wavlength [nm]

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 1-GHz cavity Yb:CALGO laser [ Klenner et al. , Opt. Express Yb:CALGO (2014)] multimode SESAM • 92-fs input pulses, 1055 nm center output pump diode coupler wavelength Power [dB] -30 -40 37 pJ coupled pulse energy -50 (37 mW average power) -60 600 800 1000 1200 1400 1600 Wavelength [nm]

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

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

Carrier Envelope Offset Frequency Detection Using Silicon Nitride Waveguide • Carrier envelop offset frequency (f ceo ) beatnote from f -to-2 f interferometry • Spectrum at 1360 nm is frequency doubled and overlapped with spectrum 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 spectrum Pump Properties spectrum • single-frequency • P > 200 mW • CEO control Comb Properties ω • • Tuning for modelocking (?) 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?) Pump Properties spectrum spectrum • Modelocked • < 200 fs for coherent comb ω Comb Properties • CEO & comb Nanowaveguide Properties • Spacing > 20 GHz spacing control • Passive • > 100 nW/line • P ~ 40 mW • Waveguide dispersion • Stabilized > Octave tailored longitudinally • Visible – mid-IR

Compact Solid-State 5-GHz Modelocked Laser