Polarization-independent Techniques For Optical Signal Processing

Polarization-Independent Techniques for Optical Signal Processing Reza Salem Electrical and Computer Engineering Dept. University of Maryland, College Park PhD Dissertation Defense August 29th , 2006 Committee Members: Dr. Thomas Murphy (chair, academic advisor) Dr. Christopher Davis Dr. Julius Goldhar Dr. Ping-Tong Ho Dr. Wendell Hill (dean’s representative) 1

Acknolwedgements Collaborators • Gary Carter (Professor of CSEE, UMBC) • Gaston Tudury (Research Associate, UMBC) • Anthony Lenihan (Research Assistant Professor, UMBC) • Timothy Horton (Laboratory for Physical Sciences) • Amir Amadi (Undergraduate Student, ECE, UMD)

Others Paveen Apiratikul, Arash Komaee, Kuldeep Amarnath William Astar, Naoki Sugimoto (Asahi Glass Co.) 2

Outline • Introduction – Optical vs. Electrical Signal Processing – Polarization Dependence

• Two-Photon Absorption in Silicon Photodiodes – Polarization Independent Clock Recovery

• Cross-phase modulation in Fiber – Polarization Independent Demultiplexing 3

Simplified Diagram of OTDM Network

high-speed data

Green blocks deal with high-speed data 4

Electrical vs. Optical Signal Processing  Electrical Signal Processing (conventional method) Based on O/E conversion + Polarization- and wavelength-independent + Easy to Integrate – Limited speed (usually < 40 Gb/s)

Optical Data Signal

O/E converter

Electrical Signal Processing

5

Electrical vs. Optical Signal Processing  Optical Signal Processing (controlling light with light) Usually Based on Nonlinear Processes + Higher speed (up to 640 Gb/s demonstrated) – Can be polarization- and wavelength-sensitive – Integration is often challenging Optical Data Signal

Optical Signal Processing

Optical Control Signal 6

The Problem of Polarization Dependence • Optical fiber is (nominally) symmetric – No preferred polarization axis • Even when input polarization is known – The output polarization is unpredictable – and it can vary on a µs timescale – Polarization evolution depends on bending, temperature, manufacturing imperfections, etc. • Photodetectors are polarization insensitive

PROBLEM: Nonlinear Processes are Polarization Dependent 7

Signal Processing Tasks • Clock Recovery • Demultiplexing • Regeneration • Performance Monitoring (Sampling, PMD Measurement, OSNR Measurement, etc.) • Logic Gate • Wavelength Conversion 8

OTDM Clock Recovery and Demultiplexing

9

Polarization-Independent Clock Recovery Using Two-photon Absorption in a Silicon Photodiode

10

Linear vs. Nonlinear Absorption Linear Absorption

Two-Photon Absorption

Quadratic nonlinearity: Similar to second-harmonic generation 11

Two-Photon Absorption in Silicon Photodiode

TPA is observed when: 1100 nm < λ < 2200 nm 12

Applications of Two-Photon Absorption • TPA autocorrelation (TPAA) and cross-correlation Two-photon absorption has been used to characterize pulses as short as 20 fs (Ripin et. al., Optics Letters, 27(1), p. 61, 2002).

• Ultrafast optical thresholding and pulsewidth measurement Two-photon absorption can distinguish between optical pulses with the same average power and different peak powers (Inue et. al., Electron. Lett., 38(23), p. 1459, 2002).

Very few cases of using TPA in telecommunication applications have been reported in the literature 13

Optical Cross-Correlation using TPA

Background Level

Cross-Correlation

14

Clock Recovery using Two Photon Absorption

Salem et al., IEEE Photon.Technol.Lett. 17(9), 1968-1970 (2005) S. Takasaka et al, ECOC, Th 1.3.6 (2005) 15

Details of the Clock Recovery System

Clock Avg. Power = 6mW, Data Avg. Power = 3mW Clock and data pulse widths are both about 4ps PLL filter designed to get 6kHz closed loop bandwidth

Loop step resp.

Light is focused to 3 μm spot size on detector surface

16

80 Gb/s Transmission Experiment To Receiver

80 Gb/s data after transmission

17

Bit Error Rate Measurements

• Measured with recovered clock, after 110 km transmission 18

Effect of OSNR on Clock Recovery

• Q is measured as a function of OSNR • OSNR is controlled by injecting noise 19

Effect of OSNR on Clock Recovery

back-to-back (with original clock) back-to-back (with recovered clock) after 110 km (with recovered clock)

20

Longer Transmission Distances • Longer distances can be achieved by recirculating fiber loops • Up to 840 km error free (Q>6) transmission is achieved at 80 Gb/s • No polarization control is needed

21

Polarization Dependence of TPA (One input) Optical Input

TPA

TPA photocurrent

In silicon photodiode, the TPA photocurrent varies with the degree of ellipticity of the light polarization.

Theory

22

Polarization Dependence of TPA (Two Inputs) Pulse 1 S = (S1,S2,S3) Pulse 2 S’ = (S’1,S’2,S’3)

Question:

TPA

Cross-Correlation Signal

How do the background level and cross-correlation component vary with the polarization state of the two pulses? 23

Polarization Sensitivity of Cross-Correlation

If pulse 1 is circularly polarized (S1=S2=0), the cross-correlation becomes independent of S’ vector

R. Salem et al., Opt. Lett. 29(13) 1524–1526, 2004. 24

Polarization Sensitivity

• If one polarization state is fixed CIRCULAR, the cross-correlation is independent of the other state 25

Effect of Polarization Fluctuations

τmin < τ0 < τmax Zero-crossing time

• Statistical distribution of τ 0 (zero-crossing time) is calculated • Standard deviation of the distribution gives an estimate for timing jitter:

σrms ≈ 208 fs 26

Timing Jitter of Recovered Clock

• Enables measurement of low-frequency jitter (drift) below 100 Hz • Limited by electronic jitter of instrument 27

Jitter Measurement

28

Effect of Polarization Fluctuation Received Data

Pol. Scrambler

Receiver

Scrambling Rate: DC – 5 kHz (within loop bandwidth) 10 GHz Electrical Clock

OFF

50 ps

ON

σPOL ≈ 190 fs In agreement with theory

29

System Improvement • An offset-free scheme for the clock recovery can make the system even less sensitive to polarization • Such a scheme also increases the power dynamic range Our approach: Optical time dithering of the clock signal (Ahmadi et. al., OFC 2006, OWW4) 30

Polarization-Independent Demultiplexing Using Cross-Phase Modulation in Nonlinear Fibers

31

Cross-Phase Modulation in Optical Fiber • Cross-Phase Modulation (XPM) in optical fibers is an ultra-fast process • Spectral filtering of the XPM spectrum can be used for wavelength conversion and demultiplexing at speeds beyond 40 Gb/s

Olsson et. al., IEEE Photonics Technology Letters, 13(8), p. 875, 2001

32

Polarization Independent XPM • Polarization-independent XPM can be achieved if pump is circularly polarized – Requires twisted or spun fibers to maintain circular polarization

Lou et. al., IEEE Photonics Technology Letters, 12(12), p. 1701, 2001

• Circular polarization is difficult to maintain in highlynonlinear fibers (non-zero birefringence) Is polarization-independent XPM possible without using circular polarization?

33

Bismuth-Oxide-Based Highly Nonlinear Fiber • Fiber made by Asahi Glass Co., Japan • Fiber length = 2 m • Spliced to standard SMF pigtails Fiber Parameter

Bi2O3 Fiber

Std. SMF

Unit

Dispersion

−260

17

ps/nm.km

Loss

3

5 x 10-5

dB/m

Effective Area (Aeff )

3

80

μm2

Kerr Nonlin. Coefficient (n2)

8.2 x 10-15

2.2 x 10-16

cm2/W

Nonlinear Coefficient (γ)

1100

1.1

W-1 .km-1

DGD for 2 m

0.15

0

ps

Lee et. al., Optics Letters, 30(11), p. 1267, 2005 34

Demultiplexing Using XPM • Blue- or Red- Shifting of the data pulses (probe) is used to distinguish between adjacent channels • Two 10 GHz pulse sources are used as pump and probe signals to investigate polarization dependence

35

XPM-Induced Wavelength Shift INPUT:

OUTPUT: HNL Fiber

36

Blue Shift In 10 GHz Probe Spectrum Experiment

Simulation

2.5nm Polarization Insensitive

Key parameters to choose are: • Pump power (Move crossing point to the desired wavelength) • Band-pass filter location (tune to the crossing point) 37

Polarization-Insensitive Demultiplexing A

B

C

Point A data

clock

Point B (data and clock temporally aligned) Point B (data and clock temporally misaligned) Point C (demux data)

38

160 Gb/s to 10 Gb/s Demultiplexed Eye Diagrams

Scrambling OFF

Scrambling ON (Low Pump Power) (Optimum Pump Power)

39

Sensitivity Measurements at 160 Gb/s

40

16 Channels in the Presence of Polarization Fluctuations

100 ps

41

Photonic Crystal Fiber (Crystal-Fiber Co.) K. P. Hansen, Opt. Express 11,1503-1509 (2003)

• • • • •

Mode Diameter ~ 2.1 μm Nonlinear coefficient γ ~ 11 (W·km)-1 Spliced to standard SMF pigtails Loss ~ 8 dB/km DGD ~ 1.25 ps (30 m length)

42

XPM Polarization Dependence in Birefringent Fiber Signal

Slowly Varying Envelope Vector

Frequency

pump

A1(z,t)

ω1

probe

A2(z,t)

ω2

LB : beat length L : fiber length

Evolution of probe signal by NL Schrodinger Equations

equations are identical for A2x and A2y Δω = ω1 ─ ω2

if |A1x | = if|A1y | Negligible ifif and only Negligible LB/L < 1 LB/L < Δω/ω 43

Polarization-Independent XPM in Birefringent Fiber • Because of birefringence: clock and data polarization states evolve at different rates • System becomes independent of probe polarization if:

Δω.Δτ > 2π DGD Large Enough DGD

&

|A1x | = |A1y | Pump should equally excite fiber principle axes (circle on Poincaré sphere)

• For the photonic crystal fiber, ∆ ω ∆ τ ~ 10 • For the bismuth-oxide fiber, ∆ ω ∆ τ ~ 1 44

Pol. Dependence of XPM, Measurement and Simulation PUMP SOP 1

y x

|A1y | = 0 PUMP SOP 2

y x

|A1x | = |A1y | 45

80 Gb/s Optical Demultiplexing A

A

B

B

C

C

46

Polarization Independent Demultiplexing EYE DIAGRAMS AFTER DEMULTIPLEXING: CLOCK SOP 1 (worst)

y x

BER > 10–2

CLOCK SOP 2 (best)

y x

BER < 10–9

• To achieve polarization-independent performance: Must use the right CLOCK polarization • (Data polarization scrambled in both cases) 47

Receiver Performance

• Power penalty is about 2.5 dB for all channels (compared to back to back) • Polarization scrambled in all cases 48

What is the best CLOCK Polarization?

• Programmable polarization controller & analyzer on CLOCK • Measure BER as a function of CLOCK polarization • (Data polarization is still scrambled) 49

Optimal Polarization States (Measured) CLOCK POLARIZATION:

• Plotted points correspond to BER < 10–9

S3

• (Data polarization is still scrambled)

S1

S2

• Agrees with the theoretical analysis (circle of states)

50

Summary • GOAL: Replace high-speed electronics with nonlinear optical signal processing • PROBLEM: Most nonlinear optical processes are polarization dependent • Optical Clock Recovery based on Two-Photon Absorption in Silicon Detector – Inexpensive, ultrafast, wavelength- and polarization independent

• Optical Demultiplexing in Highly Nonlinear Fibers – Lengths as short as 2 meters – Up to 160 Gb/s speeds – Two new ways to overcome polarization dependence of XPM 51

Awards and Publications Awards • First Prize Recipient in Poster Competition, IEEE LEOS, Balt./Wash. Chapters, Apr 2006 • First Prize Recipient in Poster Competition, IEEE LEOS, Balt./Wash. Chapters, Mar 2006 • Invention of the Year Finalist, University of Maryland, College Park, MD, Apr 2004

Journal Papers • Salem et. al., “160 Gb/s Polarization-Independent Optical Demultiplexing in 2-m Bismuth-Oxide Fiber”, to appear in IEEE Photon. Technol. Lett. • Salem at. al., "Two-Photon Absorption for Optical Clock Recovery in OTDM Networks", to appear in J. Lightwave Technol. • Lenihan et. al., "All-Optical 80 Gb/s Time-Division Demultiplexing Using Polarization-Insensitive Cross-Phase Modulation in Photonic Crystal Fiber", IEEE Photon. Technol. Lett., 18(12), p. 1329, (2006). • Tudury et. al., "Transmission of 80 Gbit/s over 840 km in standard fibre without polarisation control", Electron. Lett. 41(25), 1394-1395, (2006). • Salem et. al., "Polarization-Insensitive Optical Clock Recovery at 80 Gb/s using a Silicon Photodiode", IEEE Photon. Technol. Lett., 17(9), 1968-1970, (2005). • Salem et. al., “Broadband Optical Clock Recovery System Using Two-Photon Absorption”, IEEE Photon. Technol. Lett., 16(9), 2141-2143, (2004). • Salem at. al.,“Polarization-Insensitive Cross-Correlation Using Two-Photon Absorption in a Silicon Photodiode”, Opt. Lett., 29(13), 1524-1526, (2004).

Conference Presentations TPA Cross-Correlation and Clock Recovery: CLEO 2004, Nonlin. Optics Meeting 2004, CLEO 2005, LEOS Summer Topical Meeting 2005, OFC 2006 Demultiplexing: OSA Annual Meeting 2006, CLEO 2006

52