Download Frequency Conversion

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)
Fiber Optic Link
(b)
Frequency
Conversion
KTP
CW Laser
(1.5 µm)
EO Modulator
ΔV = 1GHz
• Frequency conversion step required
− Best components built for 1.5 μm
− SSH requires visible light
(c)
S2 Crystal
Frequency Conversion
Frequency, Energy
Second
Harmon
Generation
Sum
Frequency
Generation
X2 =
+
χ2 Processes
=
• Conversion efficiency:
50%+
• AdvR developed highpower 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
Frequency-Converted Output
Waveguides
2 µm x 2 µm
2 mm
Focused Input Beam
Not to scale!
Wavelength Spectra
Optical Power (mW)
Optical Signal Analyzer displays:
1
1
0.1
0.01
0.1
1E-3
0.01
1E-4
1E-5
1E-3
791.4
791.6
791.8
792.0
792.2
1580
1582
1584
1586
Wavelength (nm)
• Scale and signal spacing exaggerated for clarity
• Experimental data taken with lasers spaced much
closer together
Optical Power (mW)
Wavelength Spectra
1
1
2 Tone Signal
Pump
0.1
0.01
0.1
1E-3
0.01
1E-4
1E-5
1E-3
791.4
791.6
791.8
792.0
792.2
1580
1582
1584
1586
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
Optical Power (mW)
Wavelength Spectra
1
Pump + Signal 1
1
Pump + Signal 2
2 Tone Signal
Pump
0.1
0.01
0.1
1E-3
0.01
1E-4
1E-5
1E-3
791.4
791.6
791.8
792.0
792.2
Wavelength (nm)
1580
1582
1584
1586
Optical Power (mW)
Wavelength Spectra
1
Pump + Signal 1
1
Pump + Signal 2
2 Tone Signal
Pump
0.1
0.01
0.1
2nd Order Spurs
1E-3
0.01
1E-4
1E-5
1E-3
791.4
791.6
791.8
792.0
792.2
Wavelength (nm)
1580
1582
1584
1586
Optical Power (mW)
Wavelength Spectra
1
Pump + Signal 1
1
Pump + Signal 2
2 Tone Signal
Pump
0.1
0.01
0.1
0.01
3rd Order Spur
2nd Order Spurs
1E-3
3rd Order Spur
1E-4
1E-5
1E-3
791.4
791.6
791.8
792.0
792.2
Wavelength (nm)
• Intentionally Large Spurs
1580
1582
1584
1586
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 Detection
ΔV = 1GHz
Relative
Power
Local
Oscillator
Relative
Amplitude
793
Wavelength (nm)
5
10
Frequency (GHz)
15
Heterodyne Detection
ΔV = 1GHz
Relative
Power
Local
Oscillator
Relative
Amplitude
793
Wavelength (nm)
5
10
Frequency (GHz)
15
Heterodyne Detection
ΔV = 1GHz
Relative
Power
Local
Oscillator
Relative
Amplitude
793
Wavelength (nm)
ΔV = 1GHz
5
10
Frequency (GHz)
15
Heterodyne Detection
ΔV = 1GHz
Relative
Power
Local
Oscillator
Relative
Amplitude
793
Wavelength (nm)
ΔV = 1GHz
5
10
Frequency (GHz)
15
Heterodyne Detection
ΔV = 1GHz
Relative
Power
Local
Oscillator
Relative
Amplitude
793
Wavelength (nm)
ΔV = 1GHz
5
10
Frequency (GHz)
15
Spur Free Dynamic Range
Spur Free Dynamic Range (dB)
70
• Pump power fixed at 22 dBm in
waveguide
• Varied the signal power
• Slope = -2 dB/dBm
65
60
55
50
45
40
35
0
2
4
6
8
10
12
Sum Frequency output power (dBm)
14
Three-wave Mixing Model
• Three-wave mixing ( 𝜒 (2) 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
TwoToneSFG (1586 nm Frequencies)
TwoToneSFG (793 nm Frequencies)
20
Power Out [dBm]
Power Out [dBm]
20
0
-20
-40
-60
-10
0
-20
-40
-60
0
10
Signal Power In [dBm]
20
-10
0
10
Signal Power In [dBm]
Black lines are various spurs
20
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
Questions
Acknowledgements:
• MBRCT #15-14
• AdvR Inc.
• S2 Corp.