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Title: Semiconductor Waveguides for Nonlinear Optical Signal Processing


1
Semiconductor Waveguides for Nonlinear Optical
Signal Processing
  • Paveen Apiratikul
  • Department of Electrical and Computer
    Engineering
  • University of Maryland, College Park
  • PhD Dissertation defense
  • November 13rd, 2009

Committee members Prof. Thomas E. Murphy (
Chair, Academic advisor) Prof. Julius
Goldhar Prof. Mario Dagenais Prof. Gary M.
Carter Prof. John T. Fourkas (Deans
representative)
2
Acknowledgements
  • Collaborators
  • Gary Carter (Professor of CSEE, UMBC)
  • William Astar (Postdoctoral researcher, UMBC)
  • Andrea Rossi (Istituto Nazionale di Ricerca
    Metrologica, Italy)
  • Others
  • Shu Zee Alencious Lo, Reza Salem, Zhifu Liu,
    Christopher Richardson, Dyan Ali, Warren Berk,
    Dan Hinkel, Scott Horst

3
Achievements
  • Goal Study and characterize nonlinearities in
    semiconductor waveguides for optical signal
    processing
  • Porous silicon waveguides
  • First measurement of nonlinearities including
    instantaneous and free-carrier effects in porous
    silicon waveguide at 1550 nm
  • GaAs waveguides
  • First demonstration of 10-Gb/s wavelength
    conversion using nonlinear effects in a GaAs
    waveguide.
  • Optical sampling
  • - First demonstration of an ultrafast optical
    sampling using nonlinear absorption in a GaAs
    photodetector.

4
Publications
  • Journal papers
  • 1. P. Apiratikul, W. Astar, G. M. Carter and T.
    E. Murphy, Demonstration of 10-Gb/s wavelength
    conversion using four-wave mixing in GaAs
    waveguide, submitted to IEEE Photon. Technol.
    Lett. .
  • 2. W. Astar, P. Apiratikul, T. E. Murphy, and G.
    M. Carter, Wavelength conversion by XPM of 10
    Gb/s RZ-OOK utilizing a passive GaAs/AlGaAs
    bulk-waveguide and a detuned filter, submitted
    to IEEE Photon. Technol. Lett. .
  • 3. P. Apiratikul, and T. E. Murphy,
    Background-suppressed ultrafast optical sampling
    using nondegenerate two-photon absorption in a
    GaAs photodiode, submitted to IEEE Photon.
    Technol. Lett.
  • 4. P. Apiratikul, A. M. Rossi, and T. E. Murphy,
    Nonlinearities in porous silicon optical
    waveguides at 1550 nm, Opt. Express 17,
    3396-3406 (2009).
  • Conference papers
  • 1. P. Apiratikul, and T. E. Murphy, Ultrafast
    optical sampling using nondegenerate two-photon
    absorption in a GaAs photodiode, Frontiers in
    Optics, FTuI2, San Jose, Oct. 2009.
  • 2. P. Apiratikul, A. M. Rossi, and T. E. Murphy,
    Characterization of free-carrier nonlinearities
    in porous silicon waveguides, CLEO, CThU7,
    Baltimore, Jun. 2009.
  • 3. P. Apiratikul, A. M. Rossi, and T. E. Murphy,
    Measurement of two-photon absorption in porous
    silicon waveguides at 1550 nm, Frontiers in
    Optics, FThF5, Rochester, Oct. 2008.

5
Outline
  • Motivation
  • Characterization of nonlinearities in porous
    silicon waveguides
  • Wavelength conversion using nonlinear effects in
    a GaAs waveguide
  • Ultrafast optical sampling using two-photon
    absorption in a GaAs photodiode

6
Optical Signal Processing
  • Control light with light using ultrafast
    nonlinear effects

7
Why Optical Signal Processing ?
  • Speed
  • Optical nonlinear processes can respond on a
    timescale faster than the fastest electronics
  • Short optical pulses are easier to create than
    short electrical pulses
  • Cost / Simplicity
  • Avoid costly O-E-O conversion(Optical
    Electrical Optical)

8
Why Waveguide ?
  • Diffraction limits the nonlinear interaction
    length in bulk devices.
  • Light is confined in optical waveguides with a
    small cross-sectional area over long interaction
    length.

9
Outline
  • Motivation
  • Characterization of nonlinearities in porous
    silicon waveguides
  • Wavelength conversion using nonlinear effects in
    a GaAs waveguide
  • Ultrafast optical sampling using two-photon
    absorption in a GaAs photodiode

10
Porous Silicon
  • Nanoscale pore size ltlt wavelength of light
  • Wide range of refractive index (nair lt nPS lt
    nSi)
  • Simple fabrication electrochemical etching
  • Large internal surface area reduce
    free-carrier lifetime
  • Several groups have measured the third-order
    nonlinearities on thin film porous silicon below
    1064 nm 1-3.
  • We report the first measurement of instantaneous
    and carrier-based nonlinear effects in porous
    silicon waveguides at 1550 nm.

1 F. Z. Henari et al, Appl. Phys. Lett. 67 (3),
323-325, (1995) 2 S. Lettieri et al, Optics
Communications,168, 383-391, (1999) 3 F. Z.
Henari, Laser Physics, 15, 1634-1636 (2005)
11
Fabrication of Porous Silicon Waveguide
Laser
  • P silicon substrate (1020 cm-3)
  • Electrochemical etching in HF solution

Porous silicon Cladding 80 porosity
PS Cladding
  • Multi-layer structure is formed by changing
    etching current

Porous silicon Core 70 porosity
PS Core
  • Porous silicon is locally oxidized by focused
    Argon Krypton laser 1

Porous silicon Cladding 80porosity
PS Cladding
  • Remove oxidized porous silicon in HF solution

P silicon
1 A. M. Rossi et al, Appl. Phys. Lett., 78
(20), 3003-3005, (2001)
12
Linear Properties of Porous Silicon Waveguide
TE
TM
  • Mode profile inferred from far-field diffraction
  • Effective area Aeff 20 µm2
  • Waveguide is single mode
  • TE mode a 8.6 dB/cm
  • TM mode strongly absorbed

13
Nonlinear Pulse Propagation
Transmitted power
Transmitted spectrum
Free-carrier dispersion (FCD)
Optical Kerr effect
Free-carrier absorption (FCA)
Linear loss
Two-photon absorption (2PA)
?nFCD and ? aFCA depend on carrier density (?N),
generated through 2PA.
Equations are solved numerically.
tc is measured by pump-probe experiment.
1 L. Yin et al, Opt. Lett., 32 (14), 2031-2033,
(2007)

14
Measurement of Free-carrier Lifetime
Non-degenerate 2PA (fast)
(
)
Free-carrier absorption (slow tc )
tc can be estimated from slow recovery time of
the transmitted probe.
15
Free-carrier Lifetime in Porous Silicon Waveguide
  • Initial dip 2PA
  • The slower recovery free-carrier absorption
  • tc 0.2 ns

16
Experimental Setup for Measuring Nonlinearities
  • Transmitted power and spectrum are measured for
    both SOI waveguide and PS waveguide.
  • Nonlinear parameters n2, ß2PA,sFCA and
    kFCDare determined by matching simulation to
    measurements.

17
Nonlinear Absorption in Porous Silicon Waveguide
  • ß2PA 0.80.1 cm/GW
  • sFCA 10020x10-17 cm2

18
Nonlinear Spectral Shift in PS Waveguide
Measurements
Simulations
  • Spectrum is blue-shifted as the intensity
    increases due to free-carrier dispersion effect.
  • n2 2.30.7x10-14 cm2/W
  • kFCD 9015x10-21 cm3

19
Silicon and Porous Silicon parameters

SOI waveguide PS
waveguide Free carrier lifetime (tc)
1.1 ns
0.2 ns 2PA coefficient (ß2PA) 10.25 cm/GW
0.80.1 cm/GW Kerr coefficient (n2)
4.20.8x10-14 cm2/W 2.30.7x10-14
cm2/W FCA cross-section (sFCA)
1.45x10-17 cm2 1 10020x10-17 cm2 FCD
coefficient (kFCD) 5.52x10-21
cm3 2 9015x10-21 cm3
Literature
Measurement
1 A. Cutolo et al, J. Lightwave Technol., 15
(3), 505-518, (1997) 2 S.R.Giguere et al, J.
Appl. Phys., 68 (10), 4964-4970, (1990)
20
Summary
  • First measurement of nonlinearities in porous
    silicon waveguide at 1550 nm
  • Free-carrier effects including FCA and FCD are
    faster and stronger in porous silicon compared to
    silicon.
  • 2PA coefficient (ß2PA) and Kerr coefficient (n2)
    in PS waveguide are comparable to those in
    silicon waveguide.

21
Outline
  • Motivation
  • Characterization of nonlinearities in porous
    silicon waveguides
  • Wavelength conversion using nonlinear effects in
    a GaAs waveguide
  • Ultrafast optical sampling using two-photon
    absorption in a GaAs photodiode

22
Wavelength Conversion
  • Wavelength conversion is a process to convert
    encoded signal from one wavelength to another.
  • - Provide flexibility for optical
    interconnection and increase capacity in all
    optical dense wavelength division multiplexing
    networks (DWDM)

Conventional optical-electrical wavelength
conversion slow speed
All-optical wavelength conversion using
nonlinear effects
23
GaAs/AlGaAs Waveguides for Optical Signal
Processing
GaAs waveguide 1
AlGaAs micro-ring 2
GaAs nanowire 3
  • Nonlinear effects that have been observed
  • Two-photon absorption (2PA)
  • Self-phase modulation (SPM)
  • Cross-phase modulation (XPM)
  • Four-wave mixing (FWM)

AlGaAs nanowire 4
GaAs photonic crystal 5
  • No reports - Eye
    diagram - Bit-error rate (BER)

1 J. Lu et al, J. Vac. Sci. Technol. A, 22(3),
10581061, (2004). 2 J. E. Heebner et al,
Opt. Lett., 24(7), 769771, (2004). 3 D.
Lauvernier et al, Electron. Lett., 42 (4),
217219, (2006). 4 G. A. Siviloglou et
al, Opt. Express, 14 (20), 93779384, (2006). 5
S. Combrié et al, Opt. Lett., 33 (16),
1908-1910, (2008).
24
Why GaAs/AlGaAs ?
  • Large optical Kerr coefficient (n2)

GaAs
Standard fiber Silicon n2
(cm2/W ) 2.9 X 10-13 0.002 X
10-13 0.45 X 10-13
  • Flexibility to engineer the bandgap and
    incorporate heterostructure multilayers
  • Suppress 2PA
  • Eg gt 2h?
  • potential for integration with optically active
    devices such as lasers, modulators and detectors

25
GaAs Waveguide
  • Photolithography
  • ICP etching using a BCl3/Cl2 gas mixture
  • Properties
  • Linear Loss 4-6 dB/cm
  • 2PA coefficient 15 cm/GW
  • n2 2.9 10-13 cm2/W
  • Aeff 1.8 µm2
  • Leff 3.3 mm
  • tc 250 ps

26
Cross-Phase Modulation (XPM)
  • Strong pump pulses induce the refractive index
    change in the waveguide

- Phase-shift of the pump ( SPM)
spectral broadening
- Phase-shift of the probe ( XPM)
spectral pedestal
27
Experimental Setup for XPM-Based Wavelength
Conversion
  • Eye diagram
  • BER measurement
  • Input power 14 dBm for pump pulses
  • 12 dBm for CW probe.

28
Spectra XPM-Based Wavelength Conversion
  • Spectral pedestal of the probe is due to XPM
    induced by the pump pulses.

29
Output Spectrum Before and After Filter
Filter is detuned to 0.5 nm relative to the
carrier wavelength of CW probe to select the
red-shifted sideband of the probe.
30
Eye-Diagram of XPM-Based Wavelength Conversion
Converted RZ-OOK _at_?probe
Baseline RZ-OOK _at_ ?pump
  • Baseline eye diagram bypass the waveguide and
    filters
  • Converted RZ-OOK eye diagram measured at a
    receiver power of -35 dBm.
  • Data is efficiently transferred from pump to
    probe wavelength.

31
BER Measurement of XPM-Based Wavelength Conversion
  • Receiver power penalty of lt 1 dB at 10-9-BER
    relative to the baseline

32
Four-Wave Mixing (FWM)
Third-Order Nonlinearity
  • Refractive index of material is periodically
    modulated by optical Kerr effect at the beat
    frequency ?? ?p ?s .
  • For the partially-degenerate FWM, pump is phase
    modulated at ??.
  • Two sidebands at ?p?? are created. ( signal and
    Idler)

33
Experimental Setup for FWM-Based Wavelength and
NRZ-RZ Format Conversion
  • Eye diagram
  • BER measurement

Input 10 Gb/s NRZ-OOK _at_ 1545 nm 23 dBm
10 GHz Clock _at_ 1553.5 nm 13.6 dBm
Output 10 Gb/s RZ-OOK _at_ 1536.5 nm
34
Output Spectrum of FWM-Based Wavelength Converter
35
Eye-Diagram of FWM-Based Wavelength Conversion
Baseline RZ-OOK _at_ ?s
Converted RZ-OOK _at_ ?i
Baseline NRZ-OOK _at_ ?p
  • Baseline NRZ-OOK eye diagram bypass the
    waveguide and filters
  • Baseline RZ-OOK eye diagram modulate clock and
    bypass the waveguide and filters
  • Data is efficiently transferred from pump to
    idler wavelength.

36
BER Measurement of FWM-Based Wavelength Conversion
  • The pre-amplified receiver was pre-optimized for
    RZ-OOK reception.
  • Penalty relative to baseline RK-OOK 1 dB

37
Summary
  • First demonstration of 10-Gb/s wavelength
    conversion using nonlinear effects in a GaAs-bulk
    waveguide.
  • XPM and FWM based wavelength conversions have a
    BER penalty less than 1 dB.
  • Future
  • Use an AlGaAs waveguide suppress 2PA
  • Develop low loss and compact waveguides.

38
Outline
  • Motivation
  • Characterization of nonlinearities in porous
    silicon waveguides
  • Wavelength conversion using nonlinear effects in
    a GaAs waveguide
  • Ultrafast optical sampling using two-photon
    absorption in a GaAs photodiode

39
Review of Optical Sampling Techniques
Optical sampling - to directly monitor
high-speed optical waveforms with a fast temporal
resolution.
  • Four-wave mixing (FWM) 3,4
  • Cross-phase modulation (XPM) 5
  • Coherent mixing 1
  • Sum frequency generation (SFG) 2

1 C. Dorrer et al, IEEE Photon. Technol. Lett.,
15 (12), 1746, (2003) 2 N. Yamada et al, IEEE
Photon. Technol. Lett., 16 (4), , (2001) 3 J.
Li et al, IEEE Photon. Technol. Lett., 13 (9),
987, (2001) 4 C.-S. Brès et al, IEEE Photon.
Technol. Lett., 20 (14), 1222, (2008) 5 C.
Schmidt et al, in Eur. Conf. Optical
Communications (ECOC) 2002, paper 2.1.3
40
Two-Photon Absorption (2PA)
  • Two photon absorption (2PA) is a nonlinear
    process in which two photons are simultaneously
    absorbed and generate a single electron-hole pair.

Eg/2 lt h?1 lt Eg
h?1 h?2 gt Eg
Advantages
  • Ultrafast response
  • Wide optical bandwidth (Eg/2 lt h? lt Eg)
  • No phase matching
  • No need for external optical filter or detector

Applications
  • Autocorrelation measurement of ultrashort pulses
    1
  • Optical clock recovery system 2
  • Waveform monitoring 3,4

1 J. K. Ranka et al, Opt. Lett., 22 (17),1344,
(1997) 2 R. Salem et al, J. Lightwave Technol.,
24 (9), 3353, (2006) 3 K. Kikuchi, Electron.
Lett., 34(13), 1354, (1988) 4 P. J. Maguire et
al, Electron. Lett., 41(8), 489, (2005)
41
Nonlinear Processes That Contribute to
Photocurrent
Signal pulse Eg/2 lt h?1 lt Eg
Sampling pulse Eg/2 lt h?2 lt Eg
Photocurrent
Background (constant)
Cross-correlation signal (function
of t)
42
Large Background Photocurrent
Signal pulse Eg/2 lt h?1 lt Eg
Sampling pulse Eg/2 lt h?2 lt Eg
Photocurrent
4 1 1
When
40 1 100
  • Large background current !!
  • Shot noise
  • Detector saturation

Background (constant)
Cross-correlation signal (function
of t)
2 1
0.4 1
43
Background Suppression Technique
Signal pulse Eg/2 lt h?1 lt Eg
Sampling pulse h?2 lt Eg/2 h?1 h?2
gt Eg
Background suppression using long-wavelength
sampling pulse below the half-bandgap
Photocurrent
Background (constant)
Cross-correlation signal (function
of t)
Recently used for infrared photon counting 1.
1 F. Boitier et al, Appl. Phys. Lett., 94 (8),
081112 (2009)
44
Wavelength-Dependent Background Photocurrent
GaAs photodiode (Hamamatsu G8522-01) -
bandgap _at_ 870 nm - 3 GHz bandwidth
Hamamatsu G8522-01
3PA photocurrent is much smaller than 2PA
photocurrent generated from the same input power
used in this experiment.
45
Photocurrent With Fixed Sampling Power

µ


i
ph
  • Fixed idler power at
  • 100µw (1 pA)
  • Maximum signal to background ratio at average
    signal power of 3 µw (1pA)
  • Maximum signal to background ratio of 301

46
Experimental Setup for Optical Sampling
Avg. signal power 2 dBm Avg. sampling
power 12 dBm
47
Measured Eye-Diagrams
  • Measured eye diagrams of an on-off keyed signal
    with relative delay t of
  • - 250 fs (quasi-4 Tb/s)
  • - 500 fs (quasi-2 Tb/s)
  • - 750 fs (quasi-1.3 Tb/s)
  • Small background current

48
Summary
  • Ultrafast optical sampling in a GaAs photodiode
    using non-degenerate 2PA
  • Use long-wavelength sampling pulses to suppress
    background photocurrent
  • Ultrafast response time (4 Tb/s)
  • Wide optical bandwidth
  • Compact and inexpensive
  • FUTURE
  • Develop waveguide-based detector for improved
    sensitivity
  • Study polarization dependence

49
Conclusions
  • Porous silicon waveguides
  • n2 and ß2PA comparable to Si
  • Faster and stronger free-carrier effects than Si
  • GaAs waveguides
  • 10-Gb/s wavelength conversion using nonlinear
    effects in a GaAs waveguide
  • BER penalty less than 1 dB for XPM and FWM based
    wavelength conversion
  • Optical Sampling using 2PA in a GaAs photodiode
  • - Background suppression
  • 4Tb/s response time limited by optical pulse
    width
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