PHOTONICS RESEARCH GROUP
NONLINEAR OPTICS IN SILICON WIRE WAVEGUIDES: TOWARDS INTEGRATED LONG WAVELENGTH LIGHT SOURCES
B. Kuyken
1Photonics Research Group, Ghent University, Ghent, Belgium 2Center for Nano- and Biophotonics, Ghent, Belgium
PHOTONICS RESEARCH GROUP 1 Photonics Group – Ghent University - IMEC
Ghent Leuven
250 people (70 in photonics) Nanotechnology Research Center 2 identities: University and IMEC 1700 people Located at the university Independent Research Center III-V Processing Main Activity: R&D for CMOS scaling Photonic Characterisation Clean-room facilities for 200mm and Simulation and Design 300mm Silicon processing
PHOTONICS RESEARCH GROUP 2 Photonics Research Group
Facilities 600m2 Clean rooms (200m2 for photonics) III-V processing technology (GaAs, InP) Nanopatterning (Focused ion beam) Optical Characterisation Labs Design and Simulation Tools
PHOTONICS RESEARCH GROUP 3 IMEC
300mm clean room
IMEC 4
NanoTechnology Research Centre 200mm clean room Location: Leuven, Belgium Population: 1700 > 8000m2 Clean room space 200 and 300mm CMOS fabrication lines
IMEC 2 IMEC 1 IMEC 3
PHOTONICS RESEARCH GROUP 4 “Traditional” telecom silicon photonics in our group
PHOTONICS RESEARCH GROUP 5 Waveguides Etched wire in silicon 460 x 220 nm2 straight loss: 1.36dB/cm
460nm
220nm
1.36dB/cm Si
oxide
PHOTONICS RESEARCH GROUP 6 High-speed operation
Travelling-wave electrodes operation up to 40Gbps
28Gbit/s
PHOTONICS RESEARCH GROUP 7 Germanium Photodiodes Ge-on-Si PD: Vertical p-i-n
n+ Ge i-Ge p-type Ge xx x xx x p+ Si xx x xxxxxxxxx xx x p+ Si p-type Si
Ge epi on silicon BOX
Si substrate Integrated in waveguides
Selective Ge growth on Si Optical mode
PHOTONICS RESEARCH GROUP 8 Spectroscopy at longer wavelengths: Strong absorption features
x100
Absorption CO2 1E-17 x100 1E-18 x100
1E-19
.) 1E-20 a.u 1E-21 1E-22 1E-23
1E-24 Absorption ( Absorption
Absorption 1E-25 1E-26 1500 2000 2500 3000 3500 4000 4500 5000 5500 Wavelength (nm)
PHOTONICS RESEARCH GROUP 9 Mid-infrared is the fingerprint region
Widely tunable source in the mid-infared?
PHOTONICS RESEARCH GROUP 10 The Silicon-on-insulator platform for long wavelengths
SiO2
Silicon
0 1 2 3 4 5 Wavlength (um) “Standard” SOI waveguides “Almost standard” waveguides 900 nm 1400 nm
220 nm 400 nm
Loss: 1-2 dB/cm from 3.5-3.9 um Loss: 0.6 dB/cm from 2.0-2.5 um Waveguides are transparent up to 4.2 um
PHOTONICS RESEARCH GROUP 11 Outline
Very basic introduction to nonlinear optics
Phasematching in Silicon waveguides
Parametric amplification in Silicon waveguides
Supercontinuum generation in silicon waveguides
A silicon based optical parametric oscillator
What is next?
PHOTONICS RESEARCH GROUP 12 Nonlinear vs linear optics: light matter interaction electron
Energy electron photon
Nothing really happens, the light is a bit slowed down
PHOTONICS RESEARCH GROUP 13 Nonlinear optics: Four-wave mixing
Energy electron
PHOTONICS RESEARCH GROUP 14 Interaction with charges: Polarization
Energy electron
(1) Induced Polarization has same PElin frequency as electric field
Energy electron
(3) Pnonlin EEE
PHOTONICS RESEARCH GROUP 15 Silicon as a nonlinear material
High confinement in Si nanophotonic waveguides enhances effective nonlinearity by : ~ 105 compared to single-mode fiber (SMF) ~ 104 compared to highly nonlinear fiber (HNLF)
~10 m ~1 mm HNLF fiber silicon waveguide
pump Chip-scale nonlinear optical applications using four-wave-mixing (FWM): Ultra-fast digital systems: Tbps all-optical signal processing Wavelength-provisioned networks: Wavelength conversion On-chip light sources: OPOs, supercontinuum generation signal idler Quantum computation: Entangled photon-pair generation
PHOTONICS RESEARCH GROUP 16 Outline
Very basic introduction to nonlinear optics
Phasematching in Silicon waveguides
Parametric amplification in Silicon waveguides
Supercontinuum generation in silicon waveguides
A silicon based optical parametric oscillator
What is next?
PHOTONICS RESEARCH GROUP 17 Nonlinear optics: Four-wave mixing
Energy electron
PHOTONICS RESEARCH GROUP 18 Constructive interference of generated photons?
PHOTONICS RESEARCH GROUP 19 Generated photons need to be in phase!
Photons are created along the waveguide
Phasematching ensures constructive interference
PHOTONICS RESEARCH GROUP 20 Another way of looking at it: conservation of momentum
Impulse is conserved p
Impulse needs to be conserved as well
Impulse:
Input: pinp= ħkpump+ ħkpump
Output: poutp= ħksignal+ ħkidler
PHOTONICS RESEARCH GROUP 21 Another way of looking at it: conservation of momentum
Input: pinp= ħkpump+ ħkpump
Output: poutp= ħksignal+ ħkidler
ħkpump+ ħkpump = ħksignal+ ħkidler+ (small) correction
Correction results from the fact that the refractive index is 2n k dependent on the power/intensity: n = n0 +n2I
PHOTONICS RESEARCH GROUP 22 Another way of looking at it: conservation of momentum
Input: pinp= ħkpump+ ħkpump
Output: poutp= ħksignal+ ħkidler
ħkpump+ ħkpump = ħksignal+ ħkidler+ (small) correction
2kpump = ksignal+ kidler+ (small) correction
This is very good because we can engineer the k vectors of the waves involved in a silicon photonic waveguide. This is called dispersion engineering
PHOTONICS RESEARCH GROUP 23 Phasematching in high index contrast waveguides
2k pump kidler ksignal (2P) k k k idler signal (P) pump 2
Small correction for SPM/XPM
High confinement in silicon allows for dispersion engineering n 2 Ac
Figure: MA Foster et al., “Broad-band optical parametric gain on a silicon photonic chip”, Nature, 2006
PHOTONICS RESEARCH GROUP 24 Dispersion engineering in a silicon waveguide
k k k k idler signal (P) pump 2
P
ωs ωp ωi ω
Δω1 Δω1
PHOTONICS RESEARCH GROUP 25 Dispersion engineering in a silicon waveguide
k k k k idler signal (P) pump 2
P
ωs ωp ωi ω
Δω1 Δω1
2k 0 2
PHOTONICS RESEARCH GROUP 26 Outline
Very basic introduction to nonlinear optics
Phasematching in Silicon waveguides
Parametric amplification in Silicon waveguides
Supercontinuum generation in silicon waveguides
A silicon based optical parametric oscillator
What is next?
PHOTONICS RESEARCH GROUP 27 Parametric Amplification in a 2 cm silicon waveguide
Silicon waveguide chip
2 cm waveguide with a loss of 2.5 dB/cm
300 um 300
700 um
PHOTONICS RESEARCH GROUP 28 Parametric Amplification in a silicon waveguide
Silicon waveguide chip
Pump: ~2 ps, 13.5W peak power at 2170 nm Gain: > 40 dB FWM gain ~ 580 nm gain bandwidth
B. Kuyken, et al., Mid-infrared broadband modulation instability and 50dB Raman assisted parametric gain in silicon photonic wires. Opt. Letters (2011)
PHOTONICS RESEARCH GROUP 29 Extensive dispersion engineering
k 2 4 k k 2 0 4 0 k idler signal (P) pump 2
P
ω ω ω ωs s p i ωi ω
Δω1 Δω1 Δω2 Δω2
PHOTONICS RESEARCH GROUP 30 Up-Conversion of Mid-IR Signals to Telecom Band
Pump: ~2 ps, 37W peak power at 1950 nm Gain: > 19.5 dB FWM gain
X. Liu, B. Kuyken, et al,” Bridging the mid-infrared to telecom gap with A silicon wavelength translator”, Nat. Photonics (2012)
31 PHOTONICS RESEARCH GROUP 31 Generation of long wavelength light
Pump: ~2 ps, 37W peak power at 1950 nm Gain: > 8.5 dB FWM gain
PHOTONICS RESEARCH GROUP 32 More Extensive dispersion engineering
2 4 k 0 0 2 4
P
ω ωs p ωi ω Δω2 Δω2
There is no phase matching anymore close to the pump
PHOTONICS RESEARCH GROUP 33 Phasematching as a function of pump wavelength 1650 nm
400 nm
PHOTONICS RESEARCH GROUP 34 Conversion over more than a octave in silicon waveguide
Pump: 20W @2190 nm To FTIR
Signal: CW laser @1565 nm
Idler generated @ 3635nm
B. Kuyken, et al, “Mid-Infrared Generation by Frequency Down-Conversion Across 1.2 Octaves in a Normally-Dispersive Silicon Wire“. to CLEO 2013
PHOTONICS RESEARCH GROUP 35 Amplification of telecom signals
Pump: 20W @2190 nm To fast photo detector Signal: CW laser @1565 nm
1.4 13 dB amplification
1.2 C.W. light 1.0
0.8
0.6
power (a.u.) power 0.4
0.2 0 100 200 300 400 500 Time (ps)
PHOTONICS RESEARCH GROUP 36 Outline
Very basic introduction to nonlinear optics
Phasematching in Silicon waveguides
Parametric amplification in Silicon waveguides
Supercontinuum generation in silicon waveguides
A silicon based optical parametric oscillator
What is next?
PHOTONICS RESEARCH GROUP 37 Supercontinuum generation in silicon waveguides
PHOTONICS RESEARCH GROUP 38 Supercontinuum generation in silicon waveguides
900 nm
220 nm
-20 Output -30 Pump: 12.7 W @2120nm Input -40 -50 -60 -70
-80 OutputSpectrum(dB) 1600 1800 2000 2200 2400 Wavelength (nm) B. Kuyken, et al., Mid-infrared to telecom-band supercontinuum generation in highly nonlinear silicon-on insulator wire waveguides,Optics Express, (2011)
PHOTONICS RESEARCH GROUP 39 Supercontinuum generation in silicon waveguides
Very interesting as a broadband light source, but very noisy.
Every pulse amplifies background noise, which is very different from pulse to pulse.
PHOTONICS RESEARCH GROUP 40 Coherent supercontinuum generation
Pumping a waveguide with short ~50 fs pulses.
Supercontinuum generated through the process of soliton fission and dispersive wave generation. Is deterministic, not noise driven
All pulses generate the same spectrum
PHOTONICS RESEARCH GROUP 41 Coherent supercontinuum generation
1600 nm Pump: at 2290 nm, 60 fs, 300W
400 nm
0.01 Output Spectrum Input Spectrum
1E-3
1E-4
Intensity 1E-5 More than an octave
1E-6
1E-7 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 Wavelength (um)
PHOTONICS RESEARCH GROUP 42 How is a broadband source coherent?
PHOTONICS RESEARCH GROUP 43 Is the supercontinuum a frequency comb?
PHOTONICS RESEARCH GROUP 44 Is the supercontinuum a frequency comb?
Sweep
-50 73 kHz -30 -60 Continuous laser at 1586 nm -70 -80 -40 Intensity(dBm) 7.0 7.5 8.0 8.5 9.0 RF Frequnecy (MHz) =28.4MHz beat =100MHz =71.4MHz beat beat
-50 Intensity (dBm)
-60
0 20 40 60 80 100 RF Frequency (MHz)
PHOTONICS RESEARCH GROUP 45 Is the supercontinuum a frequency comb?
Beating with a naarow line width source at 2420 nm and 2580 nm
beatnote at 2580 nm -40 -30 -45 =4.6MHz -60 beat 51 kHz 53 kHz -60 =100MHz beat -50 -40 -75
-75 Intensity(dBm) =33.7MHz Intensity(dBm) 30.0 30.5 31.0 31.5 32.0 10.0 10.5 11.0 11.5 12.0 beat RF Frequnecy (MHz) =100.2MHz RF Frequnecy (MHz) -60 beat -50
=66.6MHz beat =94.5MHz beat
-70 -60
Intensity (dBm) Intensity (dBm)
-80 -70
0 20 40 60 80 100 0 20 40 60 80 100 RF Frequency (MHz) RF Frequency (MHz)
PHOTONICS RESEARCH GROUP 46 The supercontinuum is a frequency comb
0.01 Output Spectrum Input Spectrum
1E-3
1E-4
Intensity 1E-5
1E-6
1E-7 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 Wavelength (um)
Mid infrared frequency combs Extremely interesting for spectroscopy Extremely interesting for metrology Nobel prize in 2005 for T. Haens and J. Hall
PHOTONICS RESEARCH GROUP 47 Outline
Very basic introduction to nonlinear optics
Phasematching in Silicon waveguides
Parametric amplification in Silicon waveguides
Supercontinuum generation in silicon waveguides
A silicon based optical parametric oscillator
What is next?
PHOTONICS RESEARCH GROUP 48 Building an Silicon based tunable light source: Silicon based optical parametric oscillator
Optical Parametric Oscillator
Image: RP photonics
PHOTONICS RESEARCH GROUP 49 Building an Silicon based OPO
Optical Parametric Oscillator
Use robust fiber
Use silicon chip
PHOTONICS RESEARCH GROUP 50 Design of the silicon based OPO
Fiber length ~5m
Due to lack of WDM (DE)MUX beyond 2000 nm
PHOTONICS RESEARCH GROUP 51 Single Pass gain in the silicon chip
. Single Pass gain of almost 60 dB can compensate high losses
70 Gain Conversion Losses at 2100 nm 60 Round trip loss Coupling 2x 7dB 50 Fiber 1 dB Delay line 6 dB 40
splitter 10 dB Gain (dB)Gain 30 Pol Rot 1 dB Waveguide 2 dB 20
10 2000 2050 2100 2150 2200 2250 2300 2350 Wavelength (nm)
Pump: 2ps, 78 MHz rep rate, 24W peak at 2170 nm
PHOTONICS RESEARCH GROUP 52 Output of the OPO with correct delay
. When the delay in the fiber is tuned such that the round trip time in the cavity becomes equal to the repetition rate of the pump pulses, the OPO starts oscillating
. Output of the OPO at 2075 nm when pumped at 2175 nm
Output spectrum at output chip Output Energy (pJ) at output chip
PHOTONICS RESEARCH GROUP 53 Using dispersion in fiber to tune the OPO
12.8 ns
PHOTONICS RESEARCH GROUP 54 The OPO is a widely tunable source
Output spectra for different delays Pulse Energy (pJ) at output of the chip 0 2 4 6 8 10
1
0,1
Powerlinear scale (a.u) >70 nm
0,01 2040 2060 2080 2100 2020 2040 2060 2080 2100 2120 2140 Wavelength (nm) Wavelength (nm)
B. Kuyken, et al., “Widely Tunable Silicon Mid-Infrared Optical Parametric Oscillator”, accepted for publication in optics express
PHOTONICS RESEARCH GROUP 55 Outline
Very basic introduction to nonlinear optics
Phasematching in Silicon waveguides
Parametric amplification in Silicon waveguides
Supercontinuum generation in silicon waveguides
A silicon based optical parametric oscillator
What is next?
PHOTONICS RESEARCH GROUP 56 What is next?
Waveguideloss (dB/cm) as a function of time @ 2.2 um
2.5
2.0
1.5
1.0
0.5
2010 2011 2012
Q factors > 100000
PHOTONICS RESEARCH GROUP 57 Still room for improvement:
S. Selvaraja, et al., ”Advanced 300-mm Waferscale Patterning for Silicon Photonics Devices with Record Low Loss and Phase Errors”,17th OptoElectronics and Communications Conference (OECC 2012), South Korea, p.PDP2-2 p15 (2012)
PHOTONICS RESEARCH GROUP 58 What is next? CW gain in waveguides Amplification as a function of pump power
Air
Si 220 nm
SiO2 900 nm
CW gain for pump powers as low as 40 mW
PHOTONICS RESEARCH GROUP 59 Second order nonlinearities?
Energy electron
(1) PElin
Energy electron
(3) Pnonlin EEE
PHOTONICS RESEARCH GROUP 60 Second order nonlinearities? Three wave mixing
Energy electron (2) Pnonlin EE
Is this possible in silicon?
PHOTONICS RESEARCH GROUP 61 Converting χ(3) to χ(2)
Using a strained silicon nitride layer a DC electric field is induced in the waveguide
(3) (2) Pnonlin () E dc EE Pnonlin () EE
N Hon, “Periodically-Poled Silicon”, CLEO 2010
PHOTONICS RESEARCH GROUP 62 Three wave mixing
Can potentially very efficient. For now experimental results showing the potential of strained silicon
M. Cazzanelli, “Second-harmonic generation in silicon waveguides strained by silicon nitride”, Nat Materials (2012)
PHOTONICS RESEARCH GROUP 63 Also more advanced schematics have been proposed
Efficient conversion to very long wavelengths
N. Hon, “Periodically poled silicon”, Apl Phys Lett (2009)
PHOTONICS RESEARCH GROUP 64 Alternative platforms?
Materials with a built in χ(2) nonlinearity
SiC has recently been proposed. First integrated circuits demonstrated
AlN circuits have been shown and second harmonic generation is also demonstrated
X. Lu, “Silicon carbide ring resonator” Opt. Expr. (2013) M. Radulaski, “Photonic crystal cavities in cubic (3C) polytype silicon carbide films” Opt. Expr. (2013) H. Pernice, “Second harmonic generation in phase matched aluminum nitride waveguides” Archive (2012)
PHOTONICS RESEARCH GROUP 65 Conclusion
The silicon-on-insulator platform is an excellent platform for doing nonlinear optics
High nonlinearity Dispersion engineering
Enables complex sources such as frequency combs
Way is open for CW experiments
χ(2) nonlinear integrate optics in strained silicon, AlN or SiC
PHOTONICS RESEARCH GROUP 66 CMOS compatible amorphous-Si:H platform
Device layer deposited by PECVD
n~3.6 220 nm a-Si:H
500 nm Q>10000 Loss: 3.5-4.5dB/cm
A-Si:H has a higher bandgap than c-Si
Hence, lower TPA at 1550 nm
S. Selvaraja et al., Optics Communications (2009)
PHOTONICS RESEARCH GROUP 67 Nonlinear properties of a-Si:H waveguides
Measurement of real and imaginary part of gamma through self-phase modulation experiment
PHOTONICS RESEARCH GROUP 68 Results for a 1.1 cm waveguide
1 exp(L)Leff 2IP exp(L) T
I =-28/Wm R =770/Wm FOM>2 (c-Si<~0.5) B. Kuyken et al, Photonics Society Annual Meeting (2010)
PHOTONICS RESEARCH GROUP 69 Parametric amplification in 1.1 cm a-Si:H waveguide Gain/conversion as a function of wavelength at peak power of 5.2W 30 Conversion Gain Max On/off gain 26.5 dB 25 Max On chip gain 23.0 dB
20 c-Si: on/off gain 4.2 dB on chip gain 1.8 dB 15
10
5
0 On/off Gain and conversion (dB) conversion and Gain On/off 1460 1480 1500 1520 1540 1560 1580 1600 Wavelength (nm)
B. Kuyken, et al., Optics Letters (2011)
PHOTONICS RESEARCH GROUP 70 320 Gbit/s sampling of data in 4 mm a-Si:H waveguide
Succesful sampling with conversion efficiency of +12 dB! (c-Si -7.5dB)
H. Ji et al., ECOC (2011)
PHOTONICS RESEARCH GROUP 71 Supercontinuum generation in 900 nm wide waveguide
Waveguide: 1cm Source: Thulium doped mode locked fiber laser at 1950 nm Power: 6, 7,9,15 W
PHOTONICS RESEARCH GROUP 72 extra
PHOTONICS RESEARCH GROUP 73 What is next? Amplification as a function of pump power
Air
Si 220 nm
SiO2 900 nm
CW gain for pump powers as low as 40 mW
PHOTONICS RESEARCH GROUP 74 What is next?
Waveguideloss (dB/cm) as a function of time @ 2.2 um
2.5
2.0
1.5
1.0
0.5
2010 2011 2012
Q factors > 100000
PHOTONICS RESEARCH GROUP 75 Still room for improvement:
S. Selvaraja, et al., ”Advanced 300-mm Waferscale Patterning for Silicon Photonics Devices with Record Low Loss and Phase Errors”,17th OptoElectronics and Communications Conference (OECC 2012), South Korea, p.PDP2-2 p15 (2012)
PHOTONICS RESEARCH GROUP 76 Links with CUDOS
• Generating a long wavelength supercontinuum in silicon waveguides
Measurements planned in week of 11/02
• Design of waveguides for down conversion to longer wavelengths and spectrometers on SOS platform
PHOTONICS RESEARCH GROUP 77 Conclusion
The silicon-on-insulator platform is an excellent platform for doing nonlinear optics
Way is open for CW experiments
PHOTONICS RESEARCH GROUP 78 Reduction of carrier lifetime
Reducing the effective lifetime by sweeping the carriers away from the optical mode
SiO2
70 nm Si n+ p+ 220 nm 500 nm 500 nm 500 nm
200
150
100
50 EffectiveLifetime (ps) 0 10 ps 0 1 2 3 4 5 Voltage (V)
PHOTONICS RESEARCH GROUP 79 Effect ofReduction of carrier lifetime to 10ps
1,0
0,8 < 1dB/cm excess Loss
0,6
0,4
0,2 Nonlinear Loss (dB/cm) Loss Nonlinear
0,0 0,00 0,05 0,10 0,15 0,20 0,25 Power (W)
PHOTONICS RESEARCH GROUP 80 CW gain at 1550 nm at pump power of 250 mW
Negligible overlap with doped region, low linear Loss (0.3 dB/cm)*
-28 signal idler Net gain after -30 ~3 cm
-32
Power (dBm) Power -34
-36 0,00 0,01 0,02 0,03 Pump: 250 mW Propagation length (z) Phasemismatch of 0.5/m
*W. Bogaerts, S. Selvaraja, Compact Single-mode Silicon Hybrid Rib/Strip Waveguide with Adiabatic Bends,IEEE Phot. Journal p.422-232 (2011)
PHOTONICS RESEARCH GROUP 81 Low loss strip waveguides at ~2200nm
Strip waveguides with 0.6 dB/cm loss at 2200 nm Air
Si 220 nm
SiO2 900 nm
Microring resonators with intrinsic Q>150000 and modal volumes <70um3
F. Leo, B. Kuyken, N. Hattasan, R. Baets, G. Roelkens, Passive SOI devices for the short-wave infrared, accepeted for publication at ECIO (2012)
PHOTONICS RESEARCH GROUP 82 CW gain at 2200 nm at pump power of 300 mW
Perfect phasematching in these waveguides, no TPA
But lower confinement, 4X lower nonlinear parameter ~100 /Wm
-24 Idler -26 Signal
-28 Conversion Gain -30 after 4 cm Air
-32 Power (dBm) Power Si 220 nm -34
SiO2 900 nm -36 0,00 0,02 0,04 0,06 0,08 Propagation length (m)
PHOTONICS RESEARCH GROUP 83 CW conversion in dispersion engineered waveguide bij 300 mW pump
SiO2 Si 300 nm
900 nm
-24 Signal -26 Idler
-28
-30
-32 Power (dBm) Power
-34
-36 Conversion from 3.5 um to 1.6 um 0,00 0,02 0,04 0,06 0,08 Propagation length (m)
PHOTONICS RESEARCH GROUP 84