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.