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Generation of Spurious Signals in Nonlinear Frequency Conversion
Tyler Brewer, Russell Barbour, Zeb Barber
Introduction
•S2 Corp. investigating spatial-spectral holography
•Ultra-high bandwidth signal detection•Their research requires:–Electro-Optic Modulators–Nonlinear frequency conversion
Spatial-Spectral Signal Analysis
(a)
S2 Crystal(c)
EO ModulatorCW Laser
(1.5 µm)
Fiber Optic Link
KTP
(b)Frequency Conversion
ΔV = 1GHz
• Frequency conversion step required− Best components built for 1.5 μm− SSH requires visible light
Frequency Conversion
X 2 =+ =
SecondHarmon
Generation
Freq
uenc
y, E
nerg
y
χ2 Processes
SumFrequencyGeneration • Conversion efficiency:
50%+
• AdvR developed high-power capable waveguides to produce 400 mW of Second Harmonic Generation
Questions
• Undesired signals (“spurs”) produced when undergoing nonlinear frequency conversion
• Where do they come from: the EOM, amplifier, or the waveguides?
• How do we reduce or eliminate them?
Approach
• Combine and focus 3 lasers into the KTP– Pump– Two-tone signal (Δv = 1GHz)
These tones interfere to create a 1 GHz RF “beat”
• Combine output with 4th laser (the “local oscillator”)• Measure signal interference with a precision RF detector
Frequency Conversion
• Nonlinear Optical χ(2) Process
• Potassium Titanyl Phosphate (KTP)
• 2 μm x 2 μm x 2 cm waveguides contain light to maximize χ2 process
• Periodic poling for quasi-phase matching
• Waveguide technology developed by AdvR Inc.
• Applicable to many interest groups: Quantum Networks, Cold Atom Sensors, Ladar/Lidar applications, etc.
Frequency Conversion
Focused Input Beam
Frequency-Converted OutputWaveguides2 µm x 2 µm
2 cm
Not to scale!
2 m
m
2 mm
Wavelength Spectra
1580 1582 1584 1586
1E-5
1E-4
1E-3
0.01
0.1
1
791.4 791.6 791.8 792.0 792.2
1E-3
0.01
0.1
1
Opt
ical
Pow
er (
mW
)
Wavelength (nm)
Optical Signal Analyzer displays:
• Scale and signal spacing exaggerated for clarity• Experimental data taken with lasers spaced much
closer together
1580 1582 1584 1586
1E-5
1E-4
1E-3
0.01
0.1
1
Wavelength Spectra
791.4 791.6 791.8 792.0 792.2
1E-3
0.01
0.1
1
Opt
ical
Pow
er (
mW
)
Pump 2 Tone Signal
Wavelength (nm)ΔV = 1GHz
• Interference of tones produces 1 GHz RF signal• This RF signal emulates the behavior of a 1 GHz
frequency modulation from an EOM
Wavelength Spectra
1580 1582 1584 1586
1E-5
1E-4
1E-3
0.01
0.1
1
791.4 791.6 791.8 792.0 792.2
1E-3
0.01
0.1
1
Opt
ical
Pow
er (
mW
)
Pump
Wavelength (nm)
Pump + Signal 1 Pump + Signal 2 2 Tone Signal
Wavelength Spectra
1580 1582 1584 1586
1E-5
1E-4
1E-3
0.01
0.1
1
791.4 791.6 791.8 792.0 792.2
1E-3
0.01
0.1
1
Opt
ical
Pow
er (
mW
)
Pump
Wavelength (nm)
Pump + Signal 1 Pump + Signal 2
2nd Order Spurs
2 Tone Signal
Wavelength Spectra
1580 1582 1584 1586
1E-5
1E-4
1E-3
0.01
0.1
1
791.4 791.6 791.8 792.0 792.2
1E-3
0.01
0.1
1
Opt
ical
Pow
er (
mW
)
Pump
Wavelength (nm)
Pump + Signal 1 Pump + Signal 2
3rd Order Spur3rd Order Spur
2nd Order Spurs
• Intentionally Large Spurs
2 Tone Signal
Heterodyne Detection
• Interference between two optical waves produces a detectable RF frequency
• Closely-tuned lasers produce RF frequency equal to difference between laser frequencies
• RF detection has high dynamic range, allowing reduced noise and detection of weak spurs
Heterodyne DetectionR
ela
tiv
eA
mp
litu
de
Local Oscillator
Frequency (GHz)
105 15
Wavelength (nm)793
ΔV = 1GHz
Re
lati
ve
Po
we
r
Heterodyne DetectionR
ela
tiv
eA
mp
litu
de
Local Oscillator
Frequency (GHz)
105 15
Wavelength (nm)793
ΔV = 1GHz
Re
lati
ve
Po
we
r
Heterodyne DetectionR
ela
tiv
eA
mp
litu
de
Local Oscillator
Frequency (GHz)
105 15
Wavelength (nm)793
ΔV = 1GHz
Re
lati
ve
Po
we
r
ΔV = 1GHz
Heterodyne DetectionR
ela
tiv
eA
mp
litu
de
Local Oscillator
Frequency (GHz)
105 15
Wavelength (nm)793
ΔV = 1GHz
Re
lati
ve
Po
we
r
ΔV = 1GHz
Heterodyne DetectionR
ela
tiv
eA
mp
litu
de
Local Oscillator
Frequency (GHz)
105 15
Wavelength (nm)793
ΔV = 1GHz
Re
lati
ve
Po
we
r
ΔV = 1GHz
Spur Free Dynamic Range
0 2 4 6 8 10 12 1435
40
45
50
55
60
65
70
Spu
r F
ree
Dyn
amic
Ran
ge (
dB)
Sum Frequency output power (dBm)
• Pump power fixed at 22 dBm in waveguide
• Varied the signal power• Slope = -2 dB/dBm
• Three-wave mixing ( nonlinearity)• 1D numerical propagation of nonlinear ODE system along waveguide• Pump, two-tone signal, and generated outputs treated as different modes• 20 modes required to track all signals observed at output of waveguide• Coupling terms automatically calculated based on momentum and energy conservation
• All pump-depletion and back conversion terms included• Phase-matching handled parametrically based on differential linear dispersion
Three-wave Mixing Model
-10 0 10 20
-60
-40
-20
0
20
Signal Power In [dBm]
Pow
er O
ut [
dBm
]
TwoToneSFG (793 nm Frequencies)
-10 0 10 20
-60
-40
-20
0
20
Signal Power In [dBm]
Pow
er O
ut [
dBm
]
TwoToneSFG (1586 nm Frequencies)
Black lines are various spurs
Why do these matter?
• Unfilterable
• Close proximity to main signal
• Still above noise floor (RF detectors have large dynamic range, >60dB)
Conclusions
• Nonlinear frequency conversion responsible for undesired spurs
• Spur Free Dynamic Range depends on pump power vs. signal power (More pump power allows better range)
• S2 uses nonlinear conversion in SSH systems
QuestionsAcknowledgements:•MBRCT #15-14•AdvR Inc.•S2 Corp.
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