Transcript ULTRALOW THRESHOLD VCSELS FOR EFFICIENT OPTICAL …

Dense Integration of Novel Optical
Functionalities Using Photonic
Crystals
B. Momeni, A. Jafarpour, C. Reinke, J. Hunag, M. Askari, M. Soltani,
S. Mohammadi, and A. Adibi
Center for Advanced Processing-tools for Electromagnetic/acoustic
Xtals (APEX)
School of Electrical and Computer Engineering
Georgia Institute of Technology
Outline
• Introduction: Applications of Photonic Crystals
• Photonic Crystal Structures using Dispersion Engineering
– Optimal Waveguides
– Optimal Wavelength Demultiplexers
–On-Chip Integrable PC Spectrometers
• Conclusions
Introduction to Photonic Crystals
• Photonic Crystals:
Periodic dielectric structures
• Photonic Bandgap (PBG):
Frequency range with no
electromagnetic mode allowed
Photonic Crystal Devices Using
Photonic Bandgap
Cavity
Waveguide
Bends
Discrete Functionalities:
Filters, lasers, guides, delay lines, couplers, combiners
One key aspect of this research is the integration of these
functionalities into one single substrate.
Photonic Crystal Devices Using
Anomalous Dispersion
Band Structure
Demultiplexers
1
6
0.22
2
0.2
4
0.18
kya
w a/2pc
0.16
2
3
n
4
0.14
0
-2
0.12
1
0.1

5
6
0.08
-4
0.06
0.04
-6
Diffraction Control
-6
-4
-2
0
2
kxa
4
6
7
8
Functionalities:
Wavelength MUX/DEMUX, Pulse Shaping, Frequency to Space
mapping, Time to Space (Spectroscopy), Time to frequency
mapping (Chirping, Coding)
More Complex Functionalities
Waveguide Couplers
2r’
Delay Lines
Control Waveguide
Bend
Signal Waveguide
Coupling Waveguide
Nonlinear PC Structures
• Selective infiltration of PC holes with nonlinear polymers
Distance
• Functionalities:
Tunable structures (lasers, cavities, filters, delay lines),
switching, modulation
Advantages of Photonic Crystals
•
•
•
•
•
•
Photonic bandgap
Dispersion control through geometry
Nonlinearity independent of dispersion
Anomalous dispersion (superprism effect)
Devices can be made by adding defects
Compatibility with electronics substrates
Integrated Photonic Crystal
Structure
• Other Possible Applications:
Ultra-compact optical packet switching, Compact transmitters and
receivers for secure communications, Adaptive filters, Optical
sensing, Lab-on-a-chip, ...
Dispersion Engineering in Photonic
Crystal Waveguides (PCWs)
• Conventional PCWs: One row of air holes is removed.
• The waveguide has two guided modes in the bandgap.
• Single-mode PCWs are essential for practical applications.
Frequency, ωa/(2πc)
Mode Dispersion (TM)
Wavevector, ka/π
Design of Single-Mode PCWs
• By increasing the size of the air holes next to the guiding
region, the odd mode can be pushed out of the PBG.
• The guiding bandwidth is limited due to mode flattening.
a : Lattice Constant
r : Radius
r’ : Modified Radius
Odd Mode
0.4
0.4
Normalized Frequency, wa/(2pc)
Normalized Frequency, wa/(2pc)
Frequency, ωa/(2πc)
Even Mode
0.38
0.36
0.34
0.32
0.3
0.28
PBG
0.26
0.24
0
0.5
1
1.5
2
2.5
Normalized Phase, K a
x
3
0.38
0.36
0.34
0.32
0.3
0.28
PBG
0.26
0.24
0
0.5
1
1.5
2
2.5
Normalized Phase, Kxa
Normalized Phase Shift, ka
3
Design of Biperiodic PCWs
• Mode flattening is cause by distributed Bragg reflection
(DBR) due to the periodicity in the guiding direction.
• Idea: Change the period of the air holes next to the guiding
region to modify the DBR frequency.
Optimization of Guiding Bandwidth
in Biperiodic PCWs
• Pushing the DBR peak frequency upward [1]
• Guiding over the full PBG for a’ < 0.7a
• Similar results by increasing a’, guiding over PBG for a’>1.25 a
0.32
0.8
0.6
a’/a=0.7
2a’
a’/a=0.93
0.4
0.2
a’/a=1.0
0.26
0.28
0.30
Frequency, a/λ
Frequency, a/λ
Transmission
1.0
a
0.32
[1] A. Jafarpour et al., Physical Review B, vol. 68, p. 233102 (2003)
0.31
0.30
0.29
0.28
Increased
Group Velocity
0.27
0.26
1.2
No Modegap
1.4
1.6
1.8
Phase Constant, ka/π
2.0
Transmission Properties of the
Bi-periodic PCW
• Loss for a’/a=0.7, r/a=0.3, r’/a=0.25 is as low as 3 dB/mm over a
bandwidth of 60 nm.
• Loss of a conventional PCW on the same substrate is 66 dB/mm.
L
a
2r
a’
2r’
[1] A. Jafarpour et al., Applied Physics B, 79, 409, 2004.
Superprism-Based Photonic Crystal
Demultiplexers
• Anomalous dispersion effects of PCs outside the bandgap
y
4
x
kya
0.24
Superprism
effect
Negative
refraction
0.22
0.20
3
0.18
0.16
Self-guiding
0.14
2
0.12
0.10
Negative
effective index
1
0.08
0.06
0.04
0
-4
-3
-2
-1
0
1
2
3
4
kxa
• Goal: Engineering PC dispersion for optimum demultiplexing
performance
Conventional Superprism-Based PC
Demultiplexers
• Collimated input beam at optimal incidence angle
1
Dqg
2
Cross-talk (dB)
PC
0
Cross-talk between adjacent channels
0.9
1.0
1.1
-20
-40
1.5
-60
d
Dqg/d = 2.0
-80
0
0.5
1
1.5
2
2.5
Propagation length (normalized to z0)
3
• Due to beam diffraction in side PC, device size is large and
varies as N4 with N being the number of channels.
Preconditioned PC Demultiplexers
• Diffraction compensation and superprism effect inside PC
Negative
effective index
Low 3rd-order
diffraction
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
1
kya
4
3
Strong superprism
effect
2
2
1
0
-4
-3
-2
-1
0
1
2
3
4
kxa
• Using the model for higher-order effective indices, device size
varies as N2.5 with N being the number of channels.
Working in Negative Refraction Regime
• To eliminate unwanted contributions from stray signals
(unwanted polarization or wavelengths not in the operation range)
1
2
2
1
Fabrication of the PC Demultiplexer
on SOI
• 70nm SiO2 hard-mask; 220nm Si; 3μm SiO2; on Si substrate
• 45°-rotated square lattice PC (length: only 100 μm)
• A series of output waveguides for high resolution detection
Fabricated Structure
Preconditioning region
Input
waveguides
100mm
Beam blocks
Three unique effects combined
– Negative diffraction
(focusing)
– Superprism effect
– Negative refraction
Output
waveguides
mm
Input beam
WG#
WG#
2r=180nm
PC
a=367nm
Beam block
WG#
50mm
Measurement Setup
IR-Camera
• Free space end-coupling
• Lock-in measurement
Tunable laser
SM fiber
Polarizer
Microscope
DUT
40x
GRIN
lens
Iris
Beam
splitter
CCD
Camera
20x
5-D Stage
Detector
Lock-in amplifier
Measurement Results
• Imaging the output waveguides on the camera
Ch#1 Ch#2 Ch#3 Ch#4 Ch#5
1591 nm
1580 nm
1568 nm
1557 nm
1545 nm
• 5-channel demultiplexer with >6.5dB isolation and 10 nm
spacing.
Spatial Isolation of Unwanted
Polarization
WG# 15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
• Output power distribution:
TE-like
1540.5nm
1548.0nm
1557.6nm
1565.5nm
WG# 24
TM-like
• Focusing of desired channels is
visible.
1540.5nm
1548.0nm
1557.6nm
1565.5nm
12
4
1
Measurement Results
• Transmitted power measured at output waveguides
0
0.9
Normalized Transmission (dB)
Normalized Channel Response
1
0.8
0.7
0.6
0.5
0.4
0.3
0.2
-4
-8
-12
-16
0.1
0
-20
1470 1480 1490 1500 1510 1520 1530 1540 1550 1560 1570 1580
Wavelength (nm)
1520
1540
1560
Wavelength (nm)
4-channel demultiplexer, 6.5dB isolation, 8nm resolution
1580
Multiband Operation:
Incidence at Slightly Different Angles
13°
15°
17°
Normalized channel response
=13°
Input
waveguides
Output
waveguides
100mm
=15°
=17°
1500
1520
1540
1560
1580
Wavelength (nm)
1600
1620
1640
Operation band changes by the choice of incident angle
Comparison With Other Reported
Implementations
Method
PI’s
Cross-talk
Wavelength
Size (mm2)
(projected for 8nm
Spacing
spacing)
Conv. Superprism
in GaAs
Wu et al
JLT 2003
Conv. Superprism
in SOI
Lupu et al
Opt. Exp. <2dB
2004
Preconditioned
Current
Superprism in SOI work
~0dB (4 Ch.)
~10dB (2 Ch.)
~6.5dB (4 Ch.)
~10dB (2 Ch.)
~50nm
~1250
(~1.9×106)
~20nm
~6300
(~2.5×105)
~8nm
~4500
Demultiplexing performance at two-orders of magnitude smaller size
Controlling the Dispersion of
Optical Materials
• New design possibilities with controlled dispersion properties
B. Momeni and A. Adibi, “Demultiplexers harness photonic-crystal dispersion
properties,” Laser Focus World, vol. 42, no. 6, pp. 125-128, June 2006
Integrated Spectrometers
• Basic configuration
– Mapping from spectrum to space
– Post-processing to extract the spectrum
Input light
Optical device
N channels
• Requirements for an efficient implementation
– Strong dispersive properties
– Isolation of stray light
M detectors
Locating Spectral Features
• Along with frequency selective optical components
– Spectral-domain sensing
Superprism
spectrometer
Environmental
changes
Input
light
Correlator
• Correlation of the output spatial distribution for spectral pattern
recognition
Locating Spectral Features
• Correlation of detected power levels at the output by calibration
data is used to find the location of the peak
• Estimation error occurs in presence of detection noise 30 nm
operation bandwidth
1560
Estimated wavelength
Input peak wavelength
Number of events
Wavelength (nm)
1555
1550
1545
1540
1535
1530
1530
45
40
35
30
25
20
15
10
5
0
-1.5
1535
1540
1545
1550
Wavelength (nm)
1555
1560
Std dev. = 0.3 nm
-1
-0.5
0
0.5
1
Estimation error (nm)
1.5
Applications of Ultra-compact
Wavelength Demultiplexers
• Chip-scale WDM
• Spectroscopy (spatial-spectral mapping)
• Sensing: Wavelength separation properties are highly
affected by the material inside the air holes
• Lab-on-a-chip and integrated photonics circuits
Conclusions
• Photonic Crystals are excellent candidates for photonics
integrated circuits (for communications, information
processing, spectroscopy, sensing, …) due to the possibility
of dispersion engineering using geometry.
• Ultra-low loss wideband guiding and compact demultiplexing
with focusing are possible by combining some of the unique
dispersion properties of the photonic crystals.
• The possibility of designing electromagnetic modes
(dispersion, field profile, density of states,…) is a powerful
advantage of PCs, yet not highly utilized.