ultrafast coherent optical signal processing … · · 2016-04-04ultrafast coherent optical...

Ultrafast Coherent Optical Signal

Processing using Stabilized Optical

Frequency Combs from Mode-

locked Diode Lasers Peter J. Delfyett

CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816-2700

[email protected]

University of California

Santa Barbara, CA

December 5, 2012

2

Outline

• Motivation – Background

• Key Technologies

– Stabilized Optical Frequency Combs

– Arcsine Phase & Linear Intensity Modulators w/ Comb Filter

– Direct Phase Detection (w/o external local oscillator) w/ Comb Filter

• Applications

– Arbitrary Waveform Measurements

– Arbitrary Waveform Generation

– Pattern Recognition using Matched Filtering Techniques

• Summary and Conclusions

3

Motivation

Why Diode Based Fiber Lasers? • Diode lasers are small (100’s microns), electrically efficient

(>70%), wavelength agile (300 nm to >10 microns via

bandgap engineering).

• Robust, no moving / mechanical parts

• Broad bandwidth potential for large tuning bandwidth.

• Operates over very broad temperature ranges.

• Cost effective, direct electrically (battery) pumped.

• Can engineer the cavity Q to be >> than conventional cavities

• Potential for photonic integrated circuits, e.g., electronics,

lasers, modulators & detectors – full functioning

optoelectronic systems on a chip

computing & signal processing at the speed of light!

4

Ultrawideband Communications

Synthetic Aperture Imaging Sensing, Detecting and Response

Applications Enabled By Optical Frequency Combs

Advanced Waveform Generation/Measurement

5

Time Interleaved Pulse Trains Time Overlaid Pulse Trains

Interleaved Supermode Spectra Overlaid Supermode Spectra

P

ow

er

Time

Po

wer

Optical Frequency

Am

pli

tud

e

Time

Po

wer

Po

wer

Time

Optical Frequency

Am

pli

tud

e

Po

wer Time

ei

ei2

E(-)

E(-2)

E()

2

Po

we

r

eit

eit2t

fML

c/L

TC=L/c

c/L

fML

T= 1/fML

A1=1

A2=1

A3=0.5

Harmonic Modelocked Lasers Schematic Representations

6

0 200 400 600 800 1000 12000

50

100

150

200

250Intensity of Optical Pulse Train

Time

Inte

nsity

0 100 200 300 400 500 600 700 800 900 1000

195

200

205

210

215

220

225

230

235

Intensity of Optical Pulse Train

Time

Inte

nsity

210 220 230 240 250 260 270 280 290 300 3100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Optical Spectrum of Pulse Train

Frequency

Watt

s/H

z

20 40 60 80 100 120 140

-80

-60

-40

-20

0

20

RF Power Spectrum of Pulse Train

Frequency

dB

/Hz

Supermode Noise Spurs

(a)

(c)

(b)

(d)

Optical Pulse Train Intensity Optical Pulse Train Intensity

Optical Spectrum of Pulse Train RF Power Spectrum of Pulse Train

7

Low Noise Modelocked Diode Lasers

Via

Stabilization of the Frequency Comb

8

Fundamentally Modelocked Lasers

Time

Optical Frequency

fmod=c/L

=10 GHz

L

c/L

T=100 ps

~

Po

wer

Po

wer

Log Frequency

RF Power Spectrum

Corner frequency moves to

large offset frequencies w/ short cavities

1 pulse in the cavity

Corner

Frequency

SOA

System Noise Floor

RF Power Spectrum

Frequency

9

Harmonically Modelocked Lasers

Time

Optical Frequency

fmod=Nc/L

=10 GHz

L

c/L

T=100 ps

~

SOA

Po

wer

Po

wer

Log Frequency

RF Power Spectrum

Supermodes

System Noise Floor

Example: Ring Laser

Mode Spacing=10 MHz

fmod= 10 GHz

N=1000

N pulses in the cavity

N Independent longitudinal

mode groups

Coupled Modes

Corner

Frequency

10GHz RF Power Spectrum

10

Harmonic Modelocking & Supermode

Suppression

Fmod=nc/L

= 10GHz

L

T=100 psec

~

Time

Optical Frequency

10GHz

T=100 psec

P

ow

er

Po

wer

Time

Optical Frequency

10GHz

T=100 psec

Po

wer

Po

wer

SOA

=10GHz

Fmod=nc/L

=10GHz

L

~

Supermode

Suppression Filter

SOA

11

Ii

T

1 R exp i d i

64

0.0 I2i

T2

1 R2 exp i di

8

0.0

0

0.2

0.4

0.6

0.8

1

1.2

0

0.2

0.4

0.6

0.8

1

1.2

Frequency

Tran

smis

sio

n

Frequency

Tran

smis

sio

n

(a)

(b)

Nested Optical Cavities

R1=R2=90%; T1=T2 =10%; FSR2 / FSR1 =8

Cavity Product Identical to R=99%; T=1%

12

Harmonically Mode-locked Lasers &

Supermode Suppression

ν

Modulation rate

The etalon free spectral range must match the mode-locking rate.

Laser cavity modes must coincide with etalon transmission peaks.

Mode spacing

Etalon transmission

Laser cavity

10.24 GHz

SOA

IM

PC I

PC

DCF

DC

etalon

PC

PC

DCF

FL

SOA: semiconductor optical amplifier

PC: polarization controller

IM: intensity modulator

I: isolator

DCF: dispersion compensating fiber

FL: fiber launcher

FL

13

Setup

SOA

VOD OPS

IM

I I

PC PC

PC

Output

DC

PC

Free Space

Optics FPE

PM

Cir

PBS

PID

O PS

PC

PC

PD

640 MHz

Laser Cavity

PDH Loop

I: isolator

SOA: semiconductor optical

amplifier

OPS: Optical phase shifter

PD: photodetector



PBS: polarization beam splitter

FPE: Fabry-Perot etalon

PID: PID controller

PM: phase modulator

Cir : optical circulator

OPS: Optical Phase Shifter

VOD: Variable Optical Delay

DCF: Dispersion Comp. Fiber

PDH: Pound Drever Hall

Ultra-low noise osc.

at 10.287GHz

14

Laser is constructed on a optical breadboard and thermally and

acoustically isolated with foam insulation.

Actively MLL with intracavity 1000 Finesse

etalon

15

The pulses are compressed to 1.1 ps autocorrelation FWHM by using a

dual grating compressor.

Sampling scope and autocorrelation traces


etalon

16

•The 10 dB spectral width of the optical spectrum is ~8.3nm.

•The comb line has a ~50dB signal-to-noise ratio

Optical spectrum


etalon

High Resolution – Comb Line

17

Timing jitter and amplitude noise:


etalon

• Integrated timing jitter (1 Hz – 100 MHz) is ~3fs

and up to Nyquist it is 14fs.

• Integrated amplitude noise (1 Hz – 100

MHz) is 230ppm.

Note the overall dynamic range of the measurement 1016 )

18

•The linewidth of the laser with the 1000 Finesse etalon was measured as ~ 500 Hz

(Note the relative ratio of the carrier frequency to the linewidth ~ 1012)

•Stability of 150 kHz over 30 sec

(NB: Measurements are limited by the CW laser linewidth & stability)

MLL

CW laser

PC RFSA

OSA

-20 -10 0 10 20-70

-60

-50

-40

-30

-20

-10

0

Am

plit

ude

(d

Bm

)

Frequency (GHz)

High Resolution Spectrum Analyzer

CW laser

Stabilized Frequency Comb lines

Optical linewidth/stability measurement.


etalon

Stability

19

Low Noise Modelocked Diode Lasers

The Effect of Intracavity Power

20

SCOW Amplifier SCOWA – Slab-Coupled Optical Waveguide Amplifier

J. J. Plant, et. al. IEEE Phot. Tech. Lett., v. 17, p.735

(2005)

W. Loh, et. al. IEEE J. Quant. Electron., v. 47, p. 66

(2011)

0 5 10 15 20 25 300

3

6

9

12

15

Pout

(dBm)

Ga

in (

dB

)

1 A

2 A

3 A

4 A

21

Etalon stabilized HMLL Experimental setup

CIR: Circulator DBM: Double Balanced Mixer FPE: Fabry-Perot Etalon ISO: Isolator LPF: Low-Pass Filter OC: Output Coupler (Variable) PC: Polarization controller PD: Photodetector PID: Proportional-Integral-Differential Controller PM: Phase Modulator PS: Phase Shifter PZT: Piezoelectric Transducer (Fiber Stretcher) SOA: Semiconductor Optical Amplifier (SCOWA) VOD: Variable Optical Delay

Pound-Drever-Hall Loop

Optical Path

Electrical Path

SCOWA

IM

PC

PC

ISO ISO

FPE (FSR = 10.287 GHz)

OC

PS

PID

DBM

PD

CIR

LPF

PM

PC

PC PC

10.287 GHz

500 MHz

PC

Laser Output

Ultra-low

noise oscillator

Long fiber cavity provides narrow resonances

Fabry-Pérot Etalon provides wide mode spacing

Pound-Drever-Hall loop locks both cavities

An ultra-low noise oscillator is used to drive the laser

VOD PZT

I. Ozdur, et. al., PTL, v. 22, pp. 431-433 (2010)

F. Quinlan, et. al., Opt. Express 14, 5346-5355 (2006)

PBS

22

-80

-70

-60

-50

-40

-30

-20

Pow

er

(dB

m)

Frequency (100 MHz/div)

Span: 1 GHz

Res. BW: 1 MHz

~60 dB

High-Resolution Optical Spectrum Optical Spectrum

1544 1546 1548 1550

-70

-60

-50

-40

-30

-20

-10

Pow

er

(dB

m)

Wavelength (nm)

~60 dB

10.24 10.26 10.28 10.30 10.32-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Rela

tive P

ow

er

(dB

)

Frequency (GHz)

Span: 100 MHz

Res. BW: 3 kHz

Radio-Frequency Spectrum

1 10 100 1k 10k 100k 1M 10M 100M

-170

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70 Residual Phase Noise

Noise Floor

Poseidon Oscillator Absolute Noise

L(f

) (d

Bc/H

z)

Frequency Offset (Hz)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Inte

gra

ted T

imin

g J

itte

r (f

s)

Single sideband phase noise spectrum

Etalon-stabilized

laser (10.287 GHz)

Etalon-stabilized

laser (10.285 GHz)

Real-time Spectrum Analyzer

Real-time spectrogram

Tim

e (3

5 s

)

4 2 0 -2 -4

Frequency Offset (MHz)

Optical Frequency Stability Measurement

Etalon-based Ultralow-noise Frequency

Comb Source

23

Oscillator characterization

-40 -30 -20 -10 0 10 20 30 40

0.0

0.5

1.0

Compressed AC

Transform Limited AC

AC

Tra

ce (

a.u

.)

Delay (ps)

p = 930 fs

10.24 10.26 10.28 10.30 10.32

-100

-80

-60

-40

-20

0

Re

lative

Po

we

r (d

B)

Frequency (GHz)

Span: 100 MHz

Res. BW: 3 kHz

Pulses are compressible to close to the transform limit

Photodetected RF tone has >90 dB dynamic range

Intensity Autocorrelation RF Power Spectrum

24

Amplification Output power and spectral characteristics

-60

-40

-20

-60

-40

-20

1552 1554 1556 1558 1560 1562 1564

-60

-40

-20 I=4A, P

out=320 mW

I=4A, Pout

=214 mW

Directly from MLL

Op

tica

l P

ow

er

(dB

m)

Wavelength (nm)

25

1 10 100 1k 10k 100k 1M 10M 100M

-170

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70(iv)

(iii)

(ii) (i) All-anomalous Cav.

(ii) Disp. Comp. Cav.

(iii) All-anomalous and Covega

(iv) Poseidon Oscillator

Noise Floor

L(f

) (d

Bc/H

z)


(i)

0

2

4

6

8

10

In

teg

rate

d J

itte

r (f

s)

and SCOWA

Timing Jitter SSB Phase Noise Comparison

26

Outline






• Applications




– High Precision Laser Radar w/ Unambiguous Ranging &

Velocimetry


27

General Ideas for OFC Modulation

Desirable Modulator Qualities for real time OFC applications:

Current methods of modulating light intensity:

– Direct modulation of diode driving current → Frequency chirp

– External modulation:

• Electro-optic modulators (EOM) → Nonlinear modulation transfer function

and Relatively high Vπ

• Electro-absorption modulators (EAM) → Poor optical power handling,

High insertion loss and Sensitive to temperature and wavelength

Proposed concept for OFC modulation:

Injection locking a resonant cavity w/ gain (VCSEL) arcsine phase modulation

NB: Linear intensity modulator in an interferometric configuration

- Linear modulation transfer function

- Large modulation bandwidth

- Low Insertion Loss (negative..?)

- Low Vπ

- Good power handling capability

- Comb filtering, tunable, arrays

28

Injection-Locked Resonant Cavity as an “Arcsine Phase

Modulator”

ω1

ω0

Master laser

ω1

Slave laser ω0

Adler’s equation*:

φ ∆ω = 𝑠𝑖𝑛−1∆𝜔

𝜔𝑚

∆ω = 𝝎𝟏 −𝝎𝟎

2𝜔𝑚: locking range

*A. E. Siegman, Lasers, 1986

φ ∆ω

∆ω = 𝝎𝟏 −𝝎𝟎

𝛑

𝟐

−𝛑

𝟐

Locking range

ω0

ω1

29

V

f(t) ~

π/2

Iin

V

T(V)

))((sin 1 tf

I’0 ,ω1 )2

)(1(

tfII inout

Resonant cavity linear modulator • Phase response

• Stable locking range

• Calculate SFDR

f(t) ~

Iin

T(V)

)2

)cos(1(

inout II

Electro-optic Mach-Zehnder modulator

VtV /)(0

Resonant Cavity Interferometric Modulator Comparison to a Conventional MZ Modulator

outI

outI

30

Filtering &

Modulation

Optical Spectrum RF Spectrum

f1

VCSEL

Bias T

AC Modulation; f1,

DC current= I1

Phase Modulation & Filtering -Channel selection concept

I(ω)

ω f

P(f)

Ch. 1

DC=I1

Ch. 2

DC=I2

…

Ch. 1

Ch. N

Ch. 2

…

Comb Modulated Output

0 ω= ω + f1

31

Filtering &

Modulation

VCSEL

Bias T

AC Modulation; f2

DC current= I2


…

Ch. 1

Ch. N

Ch. 2

…


RF spectrum

f2 f

P(f) ω= ω + f2 I(ω)

ω Ch. 1

DC=I1

Ch. 2

DC=I2

0

Optical Spectrum 8.6 8.7 8.8 8.9 9.0

193.405

193.410

193.415

193.420

193.425

193.430 Measurement

Linear fit

Fre

quency (

TH

z)

DC Driving Current (mA)

Slope ~ 50 GHz/mA

Frequency vs. Current

32

Linear Modulator Experimental Results

0 1 2 3 4 5 6

-80

-75

-70

-65

-60

-55

-50

Po

we

r (d

Bm

)

Frequency (GHz)

10 dB

00 00 00 000 00 00 00 000

0

0

0

0

0

0

0

0

0

0

0 Measurement

Fit

Sta

tic p

hase (

rad

ian

)

DC Current Deviation (mA)

1 GHz

1.0001 GHz

CW

laser

PID

RFSA

VCSEL

High-res

OSA

+

Bias

Tee

EDFA

IDC RF

VOA PC

50/50 Iso

PD

PD

90/10

PZT

VCSEL: vertical cavity surface emitting laser

Iso: isolator

VOA: variable optical attenuator


PZT: piezoelectric transducer

PD: photo detector

PID: proportional-integrated-differential controller

CIR: circulator

OSA: optical spectrum analyzer

RFSA: RF spectrum analyzer

CIR

Spur free dynamic range of ~130 dB.Hz2/3

Very low Vπ of ~ 2.6 mV

Multi-gigahertz bandwidth (~ 5 GHz)

Possible gain

PC

-80 -70 -60 -50 -40 -30 -20

-160

-140

-120

-100

-80

-60

-40

-20

0

Fundamental

IM3

Fu

nd

am

enta

l &

in

term

odu

latio

n

pow

er

(dB

m)

RF Input (dBm)

Noise floor

SFDR = 130

dB.Hz2/3

33

Outline






• Applications




– High Precision Laser Radar w/ Unambiguous Ranging &

Velocimetry


34

Direct demodulation of phase

modulated signals

• Operating principle: Detecting light-induced changes in the

forward voltage of an optically injection locked VCSEL operating

above threshold.

• Physical origin: Voltage change is due to the change in the

carrier density in the active region of the VCSEL when driven by

an external phase modulated light.

V(ω)

Δω ωh

ωl ωo

I (ω) &

ψ(ω)

ω

Locking

range N. Hoghooghi, et. al, IEEE Photonics Technology

Letters, 22(20), pp. 1509-1511, 2010.

35

Phase

detector

0 π

0 π

Optical spectrum RF spectrum

0 π

0 π

f1 f2

VCSEL

Bias T

AC voltage

DC voltage

Channel filtering concept

I(ω)

ω f

P(f)

Ch. 1

fmod=f1

Ch. 2

fmod=f2

…

Ch. 1

Ch. N

Ch. 2

…

36

Demodulation & channel filtering with

an injection-locked VCSEL


PM: phase modulator


CIR: circulator



CW

laser IM

PM

PM

PM

VCSEL

12.5 GHz

Ch.3

(1 GHz)

Ch.2

(0.9 GHz) Ch.1

(0.8 GHz)

RFSA

OSA

DC

RF

CIR Bias T

WD

M

filter N

x1

co

mb

iner

PC

PC

PC

PC

Electrical path

Optical path

1538.2 1538.4 1538.6-60

-50

-40

-30

-20

-10

0

Po

wer

(dB

)

Wavelength (nm)

Ch.1 Ch.2 Ch.3

VCSEL

37

Experimental results of three channel

system

1538.1 1538.2 1538.3 1538.4 1538.5-60

-50

-40

-30

-20

-10

0

Wavelength (nm)

Po

we

r (d

B)

1538.1 1538.2 1538.3 1538.4 1538.5-60

-50

-40

-30

-20

-10

0

Po

we

r (d

B)

Wavelength (nm)

1538.1 1538.2 1538.3 1538.4 1538.5-60

-50

-40

-30

-20

-10

0

Po

we

r (d

B)

Wavelength (nm)

Ch.1 Ch.2 Ch.3

750 800 850 900 950 1000-95

-90

-85

-80

-75

-70

-65

RBW 30 kHz

Span 270 MHz

Po

we

r (d

Bm

)

Frequency (MHz)

SNR ~ 60

dBc/Hz

750 800 850 900 950 1000-95

-90

-85

-80

-75

-70

-65

Po

wer

(dB

m)

Frequency (MHz)

RBW 30 kHz

Span 270 MHz

SNR ~ 60

dBc/Hz

750 800 850 900 950 1000-95

-90

-85

-80

-75

-70

-65RBW 30 kHz

Span 270 MHz

Po

we

r (d

Bm

)

Frequency (MHz)

SNR ~ 62

dBc/Hz

Optical

spectra

Corresponding

detected RF

spectra

First demonstration of direct demodulation and channel filtering of

phase modulated signals with SNR of 60 dBc/Hz.

38

Linear Modulator Concept for “Pulsed” Light

Received RF signal

- A resonant cavity (Fabry-Perot) with multiple resonances, injection locked by a mode-

locked laser as the frequency comb.

- By simultaneous modulation of the period combs, one imparts arcsine phase modulation

on each injected comb.

1/frep

MLL

Fabry-Perot Laser FSR=frep

FP Optical

Frequency

FP resonances

Corresponding phase responses

Injected comb lines from the MLL

Imparted phase on each injected combs

39

Outline






• Applications





40

Multi-heterodyne detection of

frequency combs Motivation

• Extremely complex arbitrary waveforms can be generated with

frequency combs

• Instantaneous bandwidth in the order of several THz

W

D

M

W

D

M

PM AM

PM AM

… … Mode-locked

laser

dt ~ 1/BW

f

A

f f

A

f

BW

WDM

41

ff(1)rep

2f(1)rep

f(2)rep

δ

δ

Δ

RF

Po

we

r Sp

ect

ral D

en

sity

½f(1)rep

Photo-detection

ν

Δ Δ+δ Δ+2δ

f(1)rep

ν

f(2)rep

Op

tica

l P

ow

er

Spe

ctra

l De

nsi

ty

Comb Source

Comb Source

D

Oscilloscope

RFSA

Diagnostics

frep

PLLLPF

(a)

(b)

Multi-heterodyne Detection of

Frequency Combs (Optical Sampling)

•Each pair of comb-lines generates a unique RF beat-note

•The RF beat-note retains the relative phase between the comb-lines

42


frequency combs Experimental results – Mode-locked laser combs

Effective repetition rate detuning ~600 kHz

Total Optical BW ~ 17nm → ~2.12THz

Compression factor ~ 17,000x

10 GHz spacing optical comb is mapped into a 600 kHz spacing RF comb

Optical spectra

1500 1600 1475 1575 1550 1525 Wavelength (nm)

Po

wer

(5 d

B/d

iv.)

First two sets of RF beat notes

50 250 0 200 150 100 Frequency (MHz)

Pow

er (

dB

m) -50

-60

-70

Frequency (MHz) 140 160 180

-50

-60

-70

-80 Pow

er (

dB

m)

43

Pulse Combs – Time Domain Experimental Results (10 GHz & 250 MHz)

As the optical pulse is stretched and compressed, the RF

waveform does the same → Optical waveform is mapped to RF

waveform

Normal

Anomalous

Dispersion

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-50

-40

-30

-20

-10

0

10

20

30

40

time [s]

Vo

lta

ge

[m

V]

1 2 3 4 0 5 Time (μs)

Am

plit

ude (

a.u

.)

Time domain waveform

Stretched

Direct output

Compressed

44

Phase Modulated CW Combs Experimental Results

Real time

oscilloscope

RFSA CW Laser Phase

Modulator

~10 GHz

Erbium Fiber

Mode-locked

Laser

45


frequency combs Experimental results – Phase modulation combs

1 2 3 4 Time (s)

0

0

-20

-40

20

40

Am

pli

tude

(mV

)

Time domain waveform

1 2 3 4 Time (s)

0

70

60

80

90

Inst

anta

neo

us

Fre

quen

cy (

MH

z) Instantaneous frequency

65 70 75 80 Frequency (MHz)

60

Am

pli

tude

(a.u

.)

Fourier transform

Phas

e (

)

-1

0

1

The optical waveform chirp is mapped to the

RF waveform

Spectral phase information can be retrieved

46

Outline






• Applications





47

Optical DACs using Frequency

Comb Filtering – Static Approach

T

T

1/T

1/T

Time

Time

Frequency

Frequency

Inte

nsi

ty

Inte

nsi

ty

Inte

nsi

ty

48

Modelocked

Comb Generator

N Combs

WDM DeMux Modulator Array

Maximum Modulation Rate F~

WDM Mux

Arbitrary Waveform

Instantaneous Bandwidth Nx

Ultra-Pure CW channels Modulated CW Channels

Comb Spacing

Temporal

Gate

Pulse Shaping at the Highest Possible Spectral Resolution

Challenges: Some waveforms require phase modulation well beyond 2

Optical DACs using Frequency

Comb Filtering – Dynamic Approach

49 49

A Novel Concept for Ultra-High-Speed Optical Pulse Shaping

• Our novel idea is different than the conventional approaches in 4 ways:

Instead of manipulating the existing optical combs, we regenerate the optical

combs with the desired amplitudes and phases

The refresh rate is limited by the modulation speed of the VCSELs (10s of GHz)

The channel count can easily be scaled by going from 1-D array into 2-D array

geometry

Simultaneous modulation and amplification

Phase / Amplitude

> 106 increase in

the refresh rate !!!

50

High-speed Reconfigurable Optical

Arbitrary Waveform Generation

• Four optical comblines, independently modulated and

coherently combined

• Wavelength demux and mux pair

– 6.25 GHz channel spacing

• Each modulator – Injection-locked VCSEL with

current modulation

51

1538

.80

1538

.85

1538

.90

1538

.95

1539

.00

1539

.05

1539

.10

1539

.15

1539

.20

1539

.25

1539

.30

1539

.35

1539

.40

1539

.45

1539

.50

1539

.55

1539

.60

-70

-60

-50

-40

-30

-20

-10

0

Pow

er

(dB

m)

Wavelength (nm)

0.0 0.1 0.2 0.3 0.4 0.5-5.0

0.0

5.0

10.0

15.0

20.0

25.0

30.0160 ps

Vo

lta

ge

(m

V)

Time (ns)

~30 ps

Experimental Setup Optical Frequency Comb Source

• Generated by modulation

of CW laser

• Adjust DC bias voltages,

RF phases and amplitudes

to achieve five combs of

equal power

52

Experimental Setup Demux, Mux Specifications

• Essex Hyperfine WDM filters

– Fiber-pigtailed input and outputs

– Channel spacing of 6.25 GHz

– Adjacent channel isolation ~ 22

dB

– Gaussian shaped passband

– 3 dB channel bandwidth ~ 3.5

GHz

• Mux, Demux are a matched pair

53

Intensity Profile of Rapidly

Updated Optical Waveforms VCSEL 1 2 3 4

RF frequency (MHz) 4 5 6 7

VCSEL 1 2 3 4

RF frequency (MHz) 1562.5 3125 781.25 2343.75

5.120n 6.400n0

100m

200m

Voltage (

V)

Time (s)

0.00

1.28

n

2.56

n

3.84

n

5.12

n

6.40

n

7.68

n

8.96

n0

100m

200m

Voltage (

V)

Time (s)

Photodetected RF spectrum

0.00 6.25G 12.50G 18.75G 25.00G

-60

-40

-20P

ow

er

(dB

m)

Frequency (Hz)

54

VCSEL 1 2 3 4

RF frequency (MHz) 1562.5 3125 781.25 2343.75

194.

747

194.

754

194.

760

194.

766

194.

772

194.

778

194.

785

-60

-50

-40

-30

-20

-10

VCSEL 1 2 3 4

Freq.(MHz) 1562.5 3125 781.25 2343.75

RF (dBm) -13 +2 -19 -14

VCSEL 3 IL 2010-10-13-1.trc

Po

we

r (d

Bm

)

Frequency (THz)

Optical Spectrum

Intensity Profile of Rapidly

Updated Optical Waveforms

55

Reconfigurable Cross Connect Switch /

Pulse Shaping Code Reconfiguration

Information from any wavelength can be arbitrarily switched

between channelsat rates approaching channel spacing.

100 times faster that the existing MEMS technology.

DC3

DC4

DC1

DC2

DC1

DC2

DC3

DC4

56

Outline






• Applications





57

Matched Filtering using OFC’s

58

Comparison to OCDMA • Optical Code Division Multiple

Access (OCDMA)

– Spectral modulation, temporal

spread

• Decoding:

– Needs non-linear optical

thresholding because of slow

response time of photodetectors

• Coherent detection technique

is linear

– Requires less optical power

Heritage and Weiner, IEEE JSTQE, 2007

Jiang et al., IEEE PTL, 2004

59

Complete Experimental Setup

60

Interference using Orthogonal

Codes • Using orthogonal codes gives best contrast between

different binary sequences

- PD

PD

Differential

signal

λ

0,π,0,π

1 2 3 4

λ

0,0,0,0

1 2 3 4

λ 1 2 3 4

λ 1 2 3 4

0

- PD

PD

Differential

signal

λ

0,0,0,0

1 2 3 4

λ

0,0,0,0

1 2 3 4

1

λ 1 2 3 4

λ 1 2 3 4

1111

1111

1111

1111

DCode

C Code

BCode

ACode

61

5.11

mismatchmatch

mismatchmatchQ

301022

1

QerfcBER

, Summary of Results of Matched Filtering

62

Outline






• Applications





63

Summary • Demonstrated key technologies and applications using OFC’s


– Stabilized optical frequency combs (1.5 fsec jitter; <1kHz, 10Hz)

• Lowest noise mode-locked comb source at 10 GHz & 1550nm

– Linear intererometric intensity modulators & channel filtering

• First linear interferometric modulator (130 dB/Hz2/3 SFDR, Vπ =2.6mV)

– Direct phase detection and channel filtering (>60 dBc/Hz)

• Applications

– Arbitrary waveform measurements (A to D Converter )

• Reconstruction of Incoherent (Independent) Sources

– Arbitrary waveform generation (D to A Converter)

• Fastest true real-time waveform generation (Mod Rates: >3GHz; IB: >22 GHz)

– Matched filtering w/ differential photodetection (BER=10-30)

65

• Optical intensity and phase response vs. Δω

– Δω controlled via current modulation of VCSEL

• Intensity change is small

• Phase difference between master and slave light, φ0 :

Fundamentals of Injection Locking Using VCSELs as Modulators

Locking range = ωL

ωfr ω ω1

A.E. Siegman, Lasers, Chap. 29, University Science Books, 1986

F. Mogensen, et al., IEEE J. Quantum Electronics., vol. 21, 1985

Phase response

Output intensity

tansin 11

0

f

L

α – Linewidth enhancement factor

Ph

ase

Δω =ω1 – ωfr

f 1

0 cot2

Slave Laser

(VCSEL)

Master

Laser

66

Resonant Cavity Interferometric Modulator - Theory

)(sin)( 101

1

m

ω0: slave frequency

ω1: master frequency

)2

)cos(1(

inI

)2

)(1()

2

)2/))((cos(sin1(

1 tfI

tfII ininout

Linear Modulator

))((sin 1 tf

inI

inI outI

Mach-Zehnder Interferometer

Injection-locked laser phase response

ω1

π/2

-π/2

ωo

φ-φ1 Locking

Range

Put them together

67

V

f(t) ~

π/2

Iin

V

T(V)

))((sin 1 tf

I’0 ,ω1 )2

)(1(

tfII inout

Resonant cavity linear modulator • Phase response

• Stable locking range

• Calculate SFDR

f(t) ~

Iin

T(V)

)2

)cos(1(

inout II

Electro-optic Mach-Zehnder modulator

VtV /)(0

Resonant Cavity Interferometric Modulator - Comparison to a conventional MZ modulator

outI

outI

68

Filtering &

Modulation

Optical Spectrum RF Spectrum

f1

VCSEL

Bias T

AC Modulation; f1,

DC current= I1


I(ω)

ω f

P(f)

Ch. 1

DC=I1

Ch. 2

DC=I2

…

Ch. 1

Ch. N

Ch. 2

…


0 ω= ω + f1

69

Filtering &

Modulation

VCSEL

Bias T

AC Modulation; f2

DC current= I2


…

Ch. 1

Ch. N

Ch. 2

…


RF spectrum

f2 f

P(f) ω= ω + f2 I(ω)

ω Ch. 1

DC=I1

Ch. 2

DC=I2

0

Optical Spectrum 8.6 8.7 8.8 8.9 9.0

193.405

193.410

193.415

193.420

193.425

193.430 Measurement

Linear fit

Fre

quency (

TH

z)

DC Driving Current (mA)

Slope ~ 50 GHz/mA

Frequency vs. Current

70

Linear interferometric modulator setup

CW

laser

Piezo

driver

RFSA

VCSEL

High-res

OSA

Bias

Tee IDC

RF

VOA PC

50/50 Iso

PD PZT

CIR

PC

VCSEL: vertical cavity surface emitting

laser

Iso: isolator




PD: photo detector

CIR: circulator

High-res OSA: High resolution optical

spectrum analyzer


Electrical path

Optical path

0 1 2 3 4 5 6

-80

-75

-70

-65

-60

-55

-50

Po

wer

(dB

m)

Frequency (GHz)

10 dB

00 00 00 000 00 00 00 000

0

0

0

0

0

0

0

0

0

0

0 Measurement

Fit

Sta

tic p

hase (

rad

ian

)

DC Current Deviation (mA)

0.00 0.05 0.10 0.15 0.20 0.250.15

0.20

0.25

0.30

0.35

0.40

Vo

lta

ge

(V)

Time(sec)

Vπ ~ 2.6 mV

-10 dB bandwidth

~5 GHz

71

V(t)

F

ω

ω1 ω2 2ω2 2ω1 3ω1 3ω2

2ω2 –ω1 2ω1 –ω2

ω2

ω1

2ω1-ω2

2ω2-ω1

3ω2

3ω1

2ω2

2ω1

=

+

+

+

Noise floor

Spur-free

dynamic range

(SFDR)

ω

How to measure linearity of a modulator? -Two-tone experiment

Iin

Iout

Modulator

N. Hoghooghi and P. J. Delfyett, IEEE Journal of Lightwave

Technology, 29(22), pp.3421-342, 2011.

72

Analog link employing linear modulator

1 GHz

1.0001 GHz

CW

laser

PID

RFSA

VCSEL High-res

OSA

+

Bias

Tee

EDFA

IDC

RF

VOA PC

50/50 Iso

PD

PD 90/10

PZT

CIR

PC

VCSEL: vertical cavity surface emitting laser

Iso: isolator




PD: photo detector

PID: proportional-integrated-differential controller

CIR: circulator



1 km of

fiber

Electrical path

Optical path

73

Spur-free dynamic range measurement

of the link

-80 -70 -60 -50 -40 -30 -20

-160

-140

-120

-100

-80

-60

-40

-20

0

Fundamental

IM3

Fundam

enta

l &

inte

rmodula

tion

pow

er

(dB

m)

RF Input (dBm)

Noise floor

Power of the fundamental is a factor of >3,000,000^2

higher than third-order intermodulation power.

Order of the magnitude better than DARPA project

goal.

0.999 1.000 1.001 1.002

-60

-50

-40

-30

-20

-10

0

10

Po

we

r (d

Bm

)

Frequency (GHz)

Sample RF spectrum

10 20 30 40 50 60 70 80 90 100

-160

-150

-140

-130

-120

-110

-100

-90

-80

RIN

[d

Bc

/Hz]

10 20 30 40 50 60 70 80 90 1000

0.05

0.1

0.15

0.2

0.25

Frequency Offset [MHz]

Inte

gra

ted

RM

S R

IN (

%)

RIN

SFDR = 130 dB.Hz2/3

74

L

c/2L

Modelocking Basics A Review

Optical Cavity Allowed Modes

Laser Medium

Laser Cavity

Spontaneous Emission Spectrum

Laser Spectrum

75

c=

o=2

m o

o

o+ m o- m

T=2L/c

P=2/

E-Field

E-Field Spectrum

Modulated

E-Field

E-Field Spectrum

Modelocked Spectrum

Modulator

Modelocking Basics A Review

76

Coherent Optical Signal Processing &

Communications using Optical Frequency Combs

•What are optical frequency combs?

Coherent, stabilized cw optical frequencies generated on a periodic frequency grid, (e.g.,

a set of longitudinal modes from a modelocked laser).

Why re-visit coherent communications/signal processing?

•Allows the use of E(t) as compared to I(t) high spectral efficiency.

(80x -200xincrease)

•Coherent combs of stabilized optical frequencies are easily obtainable from mode-

locked lasers.

•Channel conditioning can be done simply ((frequency stabilization of the entire comb as

compared to individual lasers).

•Sets of combs at separate locations can be made coherent (frequency and phase)

Modelocked Spectrum

T=2L/c

P=2/ Modulator

Optical Frequency Combs

77

Ultrafast Photonics Group

Fundamental Physics Quantum Dot

Ultrafast Light- Matter

Dynamics

New Device Development Q-Dot Optical Amplifiers

Modulators & Photodetectors

Active Optical Filters

Systems Applications Optical Networks for Signal Processing &

Communications

Optical Sampling for A-to-D Converters

Arbitrary Waveform Generation

Precision Laser Radar

[email protected]

http://creol.ucf.edu

http://up.creol.ucf.edu

78

Stabilized Comb Source Specs

• 1.1ps pulse width with and 50 dB suppresion

to the next observable optical mode.

• 500 Hz optical linewidth and sub 150 kHz

maximum frequency deviation in 30 seconds.

• 3 fs integrated timing jitter from (1 Hz–100

MHz) and 14 fs timing jitter extrapolated to

Nyquist (1 Hz – 5.14 GHz).

Simultaneous optical frequency stabilization and supermode suppression

of a 10.287 GHz harmonically mode-locked laser with:

Ozdur I., et al,” A semiconductor based 10-GHz optical comb source with 3 fs integrated timing jitter

(1Hz-100MHz) and ~500 Hz comb linewidth” Photonic Technology Letters Vol. 22, No. 6, March 15, 2010.

79

Oscillator characterization Optical Spectra

1550 1555 1560

-80

-70

-60

-50

-40

-30

Pow

er

(dB

m)

Wavelength (nm)

Optical Spectrum

0 10 20 30 40 50 60

-0.2

-0.1

0

0.1

0.2

time (s)

Fre

quency O

ffset

(MH

z)

Spectrogram

-1.0 -0.5 0.0 0.5 1.0

-80

-70

-60

-50

-40

-30

-20

-10

0

Rel. P

ow

er

(dB

)

Frequency Offset (MHz)

Span: 2 MHz

Res. BW. 100 Hz

Single comb-line

beat-note

2 kHz FWHM Lorentzian

1 kHz FWHM Lorentzian

80

Phase and amplitude noise

1 10 100 1k 10k 100k 1M 10M 100M

-170

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

L(f

) (d

Bc/H

z)


0

2

4

6

8 Directly from MLL

Amplified (Pout

~ 200 mW)

In

teg

rate

d T

imin

g J

itte

r (f

s)

Single Sideband Phase Noise

1 10 100 1k 10k 100k 1M 10M 100M

-170

-160

-150

-140

-130

-120

-110

-100

-90

-80

-70

-60

M(f

) (d

Bc/H

z)


Directly from MLL

Amplified

0.00

0.02

0.04

0.06

0.08

0.10

In

teg

rate

d A

M N

ois

e (

%)

Pulse-to-pulse energy fluctuations

81

Oscillator characterization Pulses

-40 -30 -20 -10 0 10 20 30 40

0.0

0.5

1.0

Compressed AC

Transform Limited AC

AC

Tra

ce (

a.u

.)

Delay (ps)

p = 930 fs

10.24 10.26 10.28 10.30 10.32

-100

-80

-60

-40

-20

0

Re

lative

Po

we

r (d

B)

Frequency (GHz)

Span: 100 MHz

Res. BW: 3 kHz

Pulses are compressible to close to the transform limit

Photodetected RF tone has >90 dB dynamic range

82

Conclusions

• An optical comb source has been built with:

– Stable (instability < 300 kHz @ 194 THz over 60 s), low line-width (< 1

kHz) optical comb

– High repetition rate (10 GHz) optical pulse-train

– Short pulses generated from a dispersion compensated cavity (τp<1 ps)

• Power Amplification with a Slab-Coupled Optical Waveguide

Amplifier yields

– High optical power (up to 390 mW, > 5 mW per comb-line)

– No evident degradation in Phase (14 fs jitter integrated to Nyquist) and

Amplitude Noise (< 0.03%, 1 Hz to 100 MHz)

83

Linear Intensity Modulator

System Configuration

-Iso: Isolator

-PC: Polarization Controller

-PS: Optical Phase Shifter

-VOA: Variable Optical Attenuator

-TEC: Temperature Controller

-Cir : Circulator

-VCSEL: Vertical Cavity Surface Emitting Laser

-RFSA: Radio Frequency Spectrum Analyzer

-OSA: Optical Spectrum Analyzer

84

Concept of Photonic Arbitrary Waveform Generation

Static Fourier Analysis

K

k

kk tkAA

tf1

00 )cos(

2)(

k : periodic frequency components

Ak: amplitude of the kth frequency component

αk phase of the kth frequency component

Performance Characteristics

Limited to periodic signals

Minimum periodicity ~ Mode spacing - filter spacing

Accuracy determined by number of combs

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