1DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

Optoelectronic Oscillators for

Communication Systems

Bruno Romeira* and José M. L. Figueiredo

Centro de Electrónica, Optoelectrónica e Telecomunicações,

Departamento de Física, Faculdade de Ciências e Tecnologia,

Universidade do Algarve, 8005-139 Faro, Portugal*E-mail: bmromeira@ualg.pt

Collaborations: � University of Glasgow, UK (Prof. Charles Ironside)� University of Seville, Spain (Prof. José Quintana)

Support:

DoCEIS’10 Caparica – Lisboa 23/02/2010 2DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� Contribution to technology innovation: Photonic RF Systems.

� Resonant Tunneling Diode Optoelectronic Oscillator (RTD-OEO).

� RTD-OEO optical-electro conversion.

� RTD-OEO electro-optical conversion.

� RF Photonics Interfaces for Telecommunications.

� RTD-OEO Self-Injection Locking.

� Conclusion and Future Work.

Outline

3DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� Optoelectronic oscillators for microwave photonics systems: novel

technologies for pico/femtocellular transmission systems.

� Key elements of microwave photonics systems: optical sources

and optical detectors or optically controlled microwave devices.

Photodiode

Laser

Uplink

RF signal

Downlink

RF signal

Uplink

optical

signal

Passive pico/femtocell

Downlink

optical

signal

Communication link using radio-over-fiber transmission

Contribution to Technology Innovation

4DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

Schematic of a

Photonic System

Oscillator

� Photonic RF systems involve RF signals in both optical and

electrical domains and control of oscillator by both electrical

and optical signals.

X. Yao and L. Maleki,

IEEE JQE 32 (7), 1141.

OEO Functional Diagram

Optical out

POUT

Optical fiber

OpticalCoupler

BIAS

PIN

Photodetector

RF amplifier

Electrical in

Electrical out

Fiberstretcher

RF driving

port

RF coupler

Filter

Optical in

“Classical” Optoelectronic Oscillators

5DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� The optoelectronic oscillator (OEO) consists of a resonant tunneling

diode (RTD) embedded in a integrated-optical waveguide (OW) and a

laser diode (LD) and has both optical and electrical input and output

ports. The RTD can operate as a voltage controlled oscillator (VCO)

Novel Optoelectronic Oscillators (OEO)

Schematic of RTD-OEO configuration

6DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� Resonant Tunneling Diodes (RTDs) are nonlinear devices that use

quantum effects to produce negative differential resistance (NDR).

n InGaAlAs

n InGaAlAs

Emittern+ InGaAs

Substrate InP

AlAs

AlAsInGaAsDouble-Barrier{n+ InP

Collector

DBQW-RTD Structure

~10

nm

Conduction

band

profile

2 nm

2 nm

6 nm

Collector

0.5 mm

0.4 mm

RTD-OW die top view

Resonant Tunneling Diode

Electron transmission probabilitysilica

Ridge Waveguide

gold

Slope<0 (NDR)

V

I

NDR ���� Pdiss<0

(GAIN)

RTD I-V characteristic

7DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� The effect of bias on the conduction band profile.

NDRR=1/G<0

Typical I-V characteristic

Zero Bias (i) Off Resonance (iii)Resonance (ii)

E0 V=VvV=VpEF EF

EF

EC EC

EC

EF

EC

EF

EC

EC

EF

I

V

Conduction band

profile under

applied voltage

Conduction band profile

Ip

Iv

Vp Vv

How does an RTD works?

8DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

Operation as photo-detector (RTD-OWPD)

Current voltage (I-V)

characteristic (valley point)

Energy-band diagram

Light outLight in RF out

RTD – Optical Waveguide (RTD-OW)

� Embedding an RTD within an optical waveguide core we obtain a

monolithic integrated RTD photodetector.

9DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

R=1/G <0

slope<0

V

I

Nonlinear

VZ+

- I=GV

I

G: Conductance

Transistors

linear

Pdiss<0 = GAIN

Amp-ops

831 GHz

Conventional electronic oscillators

S. Susuki et al.,

Appl. Phys. Express 2 (2009)

Novel RTD based solid-state electronic oscillators

Voltage Controlled Oscillators

10DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� Combination of a RTD-OW and a laser diode (LD) in a optoelectronic

circuit providing electrical-to-optical (E/O), optical-to-electrical (O/E),

and optical-to-optical (O/O) conversion in the same circuit layout.

LD

Light out

RF in

Auwire

RF out

(to the PCB

strip line)

0.5 mm

DC current-voltage

(I-V) characteristics

Light inRTD-OW-LD

top view

RTD-OW

Hybrid RTD-OEO Prototype

11DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� The RTD relaxation oscillator undergoes repetitive switching between

the lower and upper positive-differential-resistance (PDR) regions.

Self-sustained relaxation oscillations

1/ 2f LCπ=

Characteristic frequency:

~ 1.2 GHz

RTD Optoelectronic VCO (1)

L~5 nH

C~3 pF

RTD oscillator

12DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� The optoelectronic voltage controlled oscillator (VCO) shows self-

sustained GHz oscillations in the optical and electrical domains

controlled by DC bias voltage.

� Electrical RF output power up to 1 mW at GHz frequencies.

VCOElectrical and optical Spectra

RTD Optoelectronic VCO (2)

13DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

The optical control signal (λλλλ=1530 nm -

1570 nm) was intensity modulated up to

12 dB extinction ratio using a 10 Gb/s

external modulator.

RTD-OEO Optical-Electro Conversion

14DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� The I-V characteristic, RF injection locking capture and

responsivity of an optical signal modulated at 1.2 GHz (~9.5 dB

extinction ratio).

RTD-OEO Response to Optical Signals

Optical power level ~1 mW Optical power level ~1 mW and λλλλ=1550 nm

14 dB

15DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� The optical power was fixed at 2 mW (λλλλ=1550 nm) and modulated at

1.2 GHz with an extinction ratio of 0.5 dB and 10.5 dB.

Single Side Band (SSB) phase noise(SSB phase noise of RF reference source

was -118 dBc/Hz)

Spectra (RF electrical output)

Vb=2.537 V

Vb=2.530 V

RTD-OEO Optical Injection LockingLocking @ 1.2 GHz - O/E Conversion

16DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� The optical injection locking range was investigated for an optical

signal λλλλ=1550 nm varying the incident optical (in fiber) between 1 mW

and 10 mW and fixing the extinction ratio at 5 dB and 10 dB.

16

0

0

2=∆

P

P

Q

ff

inj

Adler’s equation

∆f – locking range

f0 – oscillator frequency

Pinj – power of injected signal

P0 – output power of oscillator

Q – oscillator quality factor

RTD-OEO Optical Injection LockingLocking range as a function of optical power level

cold cavity

bandwidth

17DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� In the electro-optical conversion the RTD oscillator locks to low power

microwave signals. The laser diode delivers the microwave locked

signals as an optical sub-carrier.

RTD-OEO Electro-Optical Conversion

18DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� RTD-OEO oscillations lock to very low power injected microwave

signals. The laser diode output shows the same modulation features

of the injected signal (phase, frequency, noise).

� The spectrum analyser resolution and video bandwidths were 1 kHz.

Laser output RF power spectra

E-O Injection Locking and Phase Noise

20 kHz

Single Side Band (SSB) phase noise(SSB phase noise of reference

source was -100 dBc/Hz)

19DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� RTD-OEO frequency locking structure: fundamental and

harmonic injection locking.

Arnold’s Tongues Map

(p=fin/f0 in the very weak injection condition)

Frequency Locking Range

0

0

2=∆

P

P

Q

ff

inj

Adler’s equation

∆f – locking range

f0 – oscillator frequency

Pinj – power of injected signal

P0 – output power of oscillator

Q – oscillator quality factor

cold cavity

bandwidth

20DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� The optoelectronic oscillator (OEO) consists of a resonant tunneling

diode (RTD) embedded in a integrated-optical waveguide (OW) and a

laser diode (LD) and has both optical and electrical input and output

ports. The RTD can operate as a voltage controlled oscillator (VCO)

Novel Optoelectronic Oscillators (OEO)

Schematic of RTD-OEO configuration

21DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

RF out

RF in Light out

Light in

Picocells for Radio-over-Fiber Systems� Next generation of wireless access networks will have short

range cells – each office in a building with its own cell and base

station.

� Lots of cells: RTD-OEO offers single chip solutions.

22DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� The self-phase locking in (a) optical-to-electrical (OE) and (b)

electrical-to-optical (E-O) schemes.

� In both cases the optical delay also controls the RTD-OEO, providing

low-noise E/O or O/E conversions.

(a) (b)

Self-Injection Locking

23DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� Demonstration of a novel microwave photonic interface that operates

as an optoelectronic voltage controlled oscillator.

� Optical-to-RF and RF-to-optical conversion using synchronization of an

RTD-OW oscillator.

� The RTD-OEO applications include: single chip platform with reduced

size for low-cost microwave photonic devices in telecommunications

(Radio-over-Fiber networks, clock recovery, etc).

� Further work includes the study of self-injection locking capabilities of

the RTD-OW-LD circuit design by introducing a long delay in the

oscillator loop.

Conclusion and Future Work

24DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� Nonlinear dynamical system based on the Liénard’s drivenoscillator and laser diode single mode rate equations.

( )τ

β

τε

NS

S

SNNgS

p

+−+

−=1

00&

( )S

SNNg

N

q

IN

ετϑ +−−−=

100

&

( )[ ]tfπVVVVRIL

I inAC+THDC 2sin-+--1

=&

[ ])(1

VFIC

V −=&

( ) ( ) ( ) ( ) sin(2 ),AC inV t H V V t G V V f tπ+ + =&& &

Electrical model:

Optical model:

Rate equations

Liénard’s driven oscillator

N – electron density

S – photon density

I – current through the laser

given by Liénard’s model

Damping

factor

Driving

signal

Optoelectronic Model

25DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� We used the Lyapunov Characteristic Exponents (LCEs) to

analyze the stability of the RTD-LD dynamical system.

�Chaotic regions (exponent > 0).

�RTD-LD frequency locking structure

showing the Arnold Tongues map.

Experiment and numerical model

26DoCEIS’10 Caparica – Lisboa 23/02/2010 Bruno Romeira

� When the RTD-LD emitter is not synchronized with the external force,either quasi-periodic or chaotic signals are observed.

Quasi-periodic

B. Romeira et al., IEEE Photonics Technology Letters, vol. 21, no. 24, pp. 1819-1821 (2009)

Chaos

Route

To chaos

Optical Chaos in the RTD-OEO