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

Microresonator single- frequency pump laser

spectrum

ω

spectrum spectrum

Si3N4 nanowaveguide ω

spectrum

Modelocked laser χ(3) interaction

• Origin of combs can be traced to four-wave mixing (FWM)
• Requires small anomalous group-velocity dispersion

Chip-Based Comb Generation

Microresonator single- frequency pump laser

spectrum

ω

spectrum spectrum

Si3N4 nanowaveguide ω

spectrum

Modelocked laser χ(3) interaction

• Origin of combs can be traced to four-wave mixing (FWM)
• Requires small anomalous group-velocity dispersion

Microresonator-Based Parametric Combs

Si nitride

Levy et al., Nat. Photon. (2010). Ferdous et al., Nat Photon. (2012). Herr et al., Nat. Photon. (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)

silica µ-toroids

Del’Haye et al., Nature (2007). Del’Haye et al., PRL (2008).

silica µ-spheres

Agha et al., Opt. Express (2009).

CaF2, MgF2, & quartz

Savchenkov et al., PRL (2008). Liang et al., Opt. Lett. (2011). Papp & Diddams, PRA (2011). Herr et. al., Nat. Phot. (2012).

diamond

Hausmann et al., Nat. Photon. (2013).

Al nitride

Jung et al., Opt. Lett. (2013).

silicon

Griffith et al., (2014).

Microresonator-Based Parametric Combs

Si nitride

Levy et al., Nat. Photon. (2010). Ferdous et al., Nat Photon. (2012). Herr et al., Nat. Photon. (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)

silica µ-toroids

Del’Haye et al., Nature (2007). Del’Haye et al., PRL (2008).

silica µ-spheres

Agha et al., Opt. Express (2009).

CaF2, MgF2, & quartz

Savchenkov et al., PRL (2008). Liang et al., Opt. Lett. (2011). Papp & Diddams, PRA (2011). Herr et. al., Nat. Phot. (2012).

diamond

Hausmann et al., Nat. Photon. (2013).

Al nitride

Jung et al., Opt. Lett. (2013).

silicon

Griffith et al., (2014).

Microresonator Comb Spectral Coverage

[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).

λ 0.5 μm [1] [2] [3] [4] [5] [7] MgF2 Si Si3N4 4600 nm 4600 nm 3400 nm 2550 nm 2350 nm 2170 nm 1540 nm 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 [6] Si3N4 Si3N4 SiO2 MgF2 CaF2 [8] 4700 nm

Silicon-Based Microresonators for Parametric Comb Generation

• CMOS-compatible material
• Fully monolithic and sealed structures and couplers
• High-Q resonators à Si3N4 Q = 7×106 [Luke, et al., Opt. Express (2013).]

Si Q ~ 106 [Lee, et al., (2013).]

• High nonlinearity à n2 ~ 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).]]

cross-section

Si3N4 thermal SiO2 deposited SiO2

Si3N4 µ-resonator

Tailoring of GVD in Si-Based Waveguides

l

GVD can be tuned by varying waveguide shape and size.

l

Same chip can operate w/ different pump wavelengths.

SiO2

Foster, Turner, Sharping, Schmidt, Lipson, and Gaeta, Nature 441, 960 (2006). Turner, et al. Gaeta, and Lipson, Opt. Express 14, 4357 (2006).

anomalous normal

Si3N4 Si

Si/Si3N4

l

Oxide cladding limits generation < 5 µm (?)

n ~ 3.5 (SiN: n ~ 2.1)

Octave-Spanning Comb in Si3N4

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 Si3N4

• 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 Si3N4

• 950 x 2700 nm waveguide
• Fully filled in comb spanning 2.3 - 3.4um
• Pth ~ 80 mW, FSR = 99GHz

2250 2500 2750 3000 3250 3500

• 60
• 30

Power (dBm) Wavelength (nm)

Luke, et al., Gaeta & Lipson, Opt. Lett. (2015)

theory experiment

Silicon as a Mid-IR Material

Advantages:

• Large 3rd order

nonlinearity

• Transparent to ~ 8 um
• High refractive index

Problem:

• Need to pump > 2 µm
• Three-photon absorption
• Significant above 1 Watt

circulating power

ωo ωo

Three Photon Absorption

ωo

e

Free carrier

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 ~106
• 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

ωo ωo Three Photon Absorption ωo

e

Free carrier

• Pump wavelength 3095 nm
• Comb spans > octave
• Wavelength range: 2165 – 4617 nm
• Comb exhibits low RF noise

RF analyzer noise floor RF signal

Near Octave-Spanning Mid-IR Comb Generation in Si Microresonator

Chip-Based Comb Generation

Microresonator single- frequency pump laser

spectrum

ω

spectrum spectrum

Si3N4 nanowaveguide ω

spectrum

Modelocked laser χ(3) interaction

• 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 (β2)

around 1-μm pump

• Coherent SCG with 100-fs pump through self-phase modulation and

dispersive wave emission

ß2 [ps2/km] 1600 1400 1200 1000 800 600 Wavlength [nm]

690 x 900 nm

400 200

Cross section

1600 1400 1200 1000 800 600 Wavelength [nm] Power [dB]

• 60
• 50
• 40
• 30

Supercontinuum Generation with Diode-Pumped Solid-State Laser

• Pump with 1-GHz repetition rate

SESAM-modelocked diode-pumped Yb:CALGO laser [ Klenner et al., Opt. Express

(2014)]

• 92-fs input pulses, 1055 nm center

wavelength

37 pJ coupled pulse energy (37 mW average power) Collaboration w/ Ursula Keller’s group (ETH-Zurich)

multimode pump diode Yb:CALGO SESAM

• utput

coupler

1-GHz cavity

Supercontinuum Coherence Measurement

• OSA sweep records ensemble average
• Coherence related to visibility

[Nicholson and Yan, Opt. Express (2004); Gu et al., Opt. Express (2011)]

• Perform coherence measurement in 100-nm increments

V(λ) = Imax(λ) − Imin(λ) Imax(λ) + Imin(λ) g12

(1)

V(λ)

V(λ) = 2 g12

(1) I1(λ)I2(λ)

[ ]

1/ 2

I1(λ)+I2(λ)

[ ]

1 0.8 0.6 0.4 0.2 Power [dB]

• 100
• 50

1600 1400 1200 1000 800 600 Wavelength [nm] Visibility (a)

Coherent Supercontinuum for f-to-2f Interferometry

1 0.8 0.6 0.4 0.2 Power [dB]

• 100
• 50

1600 1400 1200 1000 800 600 Wavelength [nm] Visibility 720 700 680 Power [dB] Wavelength [nm] Visibility 1 0.5

• 80
• 60

1440 1400 1360 Power [dB] Visibility 1 0.5

• 40
• 20

Wavelength [nm] (a) (b) (c)

Carrier Envelope Offset Frequency Detection Using Silicon Nitride Waveguide

fCEO SNR > 30 dB

[ Mayer et al., Opt. Express (2015)]

• Carrier envelop offset frequency (fceo) beatnote from f-to-2f interferometry
• Spectrum at 1360 nm is frequency doubled and overlapped with spectrum

at 680 nm

• fceo signal-to-noise ratio > 30 dB
• Much lower noise level (10 dB) than w/ PCF

Comparison of Comb Generation Schemes

Microresonator Properties

• Thermal issues important
• Comb spacing control (thermal)
• Modelocking (Thermal?)

Pump Properties

• single-frequency
• P > 200 mW
• CEO control
• Tuning for modelocking (?)

spectrum

Comb Properties

• Spacing > 20 GHz
• > 200 µW/line
• Stabilized ~ 2/3 Octave
• Near-IR – mid-IR

ω

spectrum spectrum

ω

spectrum

Pump Properties

• Modelocked
• < 200 fs for

coherent comb

• CEO & comb

spacing control

• P ~ 40 mW

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

Microresonator Properties

• Thermal issues important
• Comb spacing control (thermal)
• Modelocking (Thermal?)

Pump Properties

• single-frequency
• P > 200 mW
• CEO control
• Tuning for modelocking (?)

spectrum

Comb Properties

• Spacing > 20 GHz
• 1 - 200 µW line
• Stabilized ~ 2/3 Octave
• Near-IR – mid-IR

ω

spectrum spectrum

ω

spectrum

Pump Properties

• Modelocked
• < 200 fs for

coherent comb

• CEO & comb

spacing control

• P ~ 40 mW

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

[ Okawachi et al. Lipson & Gaeta (2015)]

• For applications (e.g., frequency synthesizer) that are particularly power

sensitive. Simulations

Comparison of Comb Generation Schemes

Microresonator Properties

• Thermal issues important
• Comb spacing control (thermal)
• Modelocking (Thermal?)

Pump Properties

• single-frequency
• P > 200 mW
• CEO control
• Tuning for modelocking (?)

spectrum

Comb Properties

• Spacing > 20 GHz
• 1 - 200 µW line
• Stabilized ~ 2/3 Octave
• Near-IR – mid-IR

ω

spectrum spectrum

ω

spectrum

Pump Properties

• Modelocked
• < 200 fs for

coherent comb

• CEO & comb

spacing control

• P ~ 40 mW

Nanowaveguide Properties

• Passive
• Waveguide dispersion

tailored longitudinally Comb Properties

• Spacing > 20 GHz
• 1 - 200 µW line
• Stabilized > Octave
• Visible – mid-IR