MONOLITHIC INTEGRATION OF AN ARRAYED WAVEGUIDE GRATING AND OPTICAL AMPLIFIERS ON SEMICONDUCTOR

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MONOLITHIC INTEGRATION OF AN ARRAYED WAVEGUIDE
GRATING AND OPTICAL AMPLIFIERS ON SEMICONDUCTOR

• AUTHORS
• Kameron Rausch, Nasuhi Yurt,
• Xiaolan Sun, Chia-Hung Chen,
• David Geraghty, Seppo Honkanen,
• Nayer Eradat, Alan Kost
• Optical Science Center

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INTRODUCTION

• Project
• Design and realize an AWG monolithically
  integrated with multiple SOAs on semiconductor
  for use in Course Wavelength Division
  Multiplexing (CWDM).
• CWDM is a cheap alternative to DWDM for short to
  medium haul networks.

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WDM (WAVELENTGH-DIVISION-MULTIPLEXING)A
(NON-INTEGRATED) REVOLUTION
SOA
OPTICAL MODULATOR
LASER
PHOTODIODE
l1
l1
li
l1, l2 ln
ADD
l2
l2
FIBER
IN-LINE AMP
PRE- AMP
POWER AMP
DROP
lj
lm
ln
Goal of our project
MULTIPLEXER
DE-MULTIPLEXER
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ARRAYED WAVEGUIDE GRATING WITH SEMICONDUCTOR
OPTICAL AMPLIFIERS
Response of AWG

• 8 channels
• 20nm Channel Spacing centered on 1.54 mm
• 150 mm input/ouput waveguide spacing
• 21 array waveguides
• 5.25 mm array waveguide spacing (C2C)
•  
• Minimum bend radius 960 mm
• 7849 mm lt Path Length lt 9456 mm

De-multiplexing, amplification, and equalization
on one chip
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OPTIMIZATION OF WAVEGUIDE FOR AWG(BEND RADIUS
AND ETCH DEPTH)

• t 1.4 mm
• h 0.1 mm
• w 2.5 mm

Our Lowest Bend Radius

• Etch to within 0.1 mm of the guiding
• layer required for acceptable bend loss
• Simple processing
• Reduced sidewall scattering

• Becomes multimode
• when w 2.7 mm

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SEMICONDUCTOR OPTICAL AMPLIFIER
HIGH INDEX REGION (OPTICAL CONFINEMENT)
Electrons
P-type
N-type
Barriers
cladding
cladding
Quantum wells can lower current requirement for
SOAs but here we use them to facilitate band
gap modification
Quantum Wells For amplification near 1.55 ?m
Holes
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OUR APPROACH FOR BANDGAP MODIFICATIONION
IMPLANTATION
IMPLANT IONS
Rapid Thermal Anneal
DEFECT
SOA
AWG
Requirement for AWG material is different than
for SOAs. AWG needs to be transparent and the
SOAs need to be absorbing at the operating
wavelengths.
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USING MONTE CARLO SIMULATIONS WITH SRIM
2000(Stopping Range of Ions in Matter)
We calculate ion distribution, lateral range of
Ions, ionization events, number of damage events,
sputtered ions, backscattered ions, transmitted
ions, and more. Studying these data allow us to
choose the optimum ion energy, ion type,
impinging angle and ion dose for a safe level of
defect introduction without causing essential
damages to the structure or doping it.
An example calculation Ion tye B Ion Energy
100 KeV Impinging Angle 7o Target GaAs
Thickness 450 nm
Software By J. F. Ziegler and J. P. Biersack
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VARYING THE AMOUNT OF BANDGAP MODIFICATION
DEFECT
Ions, which create defects, penetrate deeper into
the semiconductor wafer with a thinner mask.
Substrate
By varying the depth of formation of defects, the
amount of bandgap shift can also be varied.
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LASER MATERIAL SYSTEMS REQUIREMENTS FOR CWDM
APPLICATIONS

• large blue shift in the bandgap for monolithic
  integration purposes
• Lasing over the desired bands 1.3-1.6 mm
• Making cheap VCSELs based on developed
  technologies GaAs.
• High temperature stability for making un-cooled
  lasers.
• Low threshold current density

MATERIAL SYSTEMS UNDER STUDY FOR BANDGAP
MODIFICATION

• GaInNAS/GaAs
• GaAsSb/GaAs
• GaSb/AlSb
• GaAlSb/AlSb

We will compare the amount of shift in theses
material systems to that of the well studied
InGaAsP/InGaAs systems.
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GaInNAs/GaAsADDITION OF N REDUCES THE BANDGAP IN
GaAs ALLOYS
Ordinary Semiconductor Alloys
GaAs
AlGaAs
ECB
ECB
Eg
Eg0
EVB
EVB
Addition of N Repulsive Interaction of the EN
and ECB States Conduction Band Forced to Lower
Energies E- A Second Conduction Band Formed at
E Decreasing the Bandgap
Addition of Small Constituents Reduction of the
Lattice Constant Increasing the Bandgap
Nitrogen Alloys Are Suitable Candidates for CWDM
Laser Sources Laser Wavelength Can be Tuned Over
a Wide Range By Varying the N Concentration
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GaInNAs/GaAs MULTIPLE QUANTUM WELLS PEAK
LUMINESCENCE AT 1.3-1.5 mm

• Growth Method MetalOrganic Vapor Phase
  Epitaxy (MOVPE)
• GrowerCollaborators at Helsinki University of
  Technology (HUT)
• Annealing Condition 650-7000C in As
  Atmosphere
• Expected Room Temperature PL Peak at 1.3 to
  1.5 mm After Annealing
• Composition x0.77, y0.025 (nominally)

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EXPERIMENTS
Photoluminescence (PL) Measurement As Grown
Ion Implantation 350KeV Boron ions Impinging
Angle 7 Ion Dose 10¹³(at/cm²) Ion Ranges Last
Quantum Well (Calculated with TRIM software)
Transmission Reflectance Measurements
Rapid Thermal Annealing (RTA) with GaAs
cap Temperature 650C Time 1, 5, 10 min
RTA Temperature 650C Time Different time
durations
PL Measurement
Transmission Reflectance Measurements
Compare Effect of the Ion implantation on the
speed of Interdiffusion and Amount of the blue
shift
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BANDGAP SHIFT DETERMINED BY PL MEASUREMENTS

• Excitation 488 nm CW Argon Laser, Max Power
  5w.
• Detection Range 500 nm to 2 mm.
• Gratings Blaze Wavelength 700 nm and 1.5 mm
• Data Acquisition Software LabView 5.1.
• Computer/Lock-in Amplifier Interface
  GPIB-USB port.
• Computer/Spectrometer Interface TTL card.

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PROCESS FLOW FOR AWG AND SOA
Reactive Ion Etching
100 µm
AWG Region
MQW









SOA Region
Process Flow

• SiO2 Patterning for Ion Implantation Masking
  (MASK I)
• Ion Implantation (Band-gap Shifting)
• Annealing
• Removal of SiO2
• Photolithography for SOA and AWG mesa formation
  (MASK II)
• Waveguide mesa Etch (RIE)
• Photolithography for Metallization (MASK III)
• Metallization
• Lift off
• Cleaving and measurements

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PROCESSING TECHNIQUES FOR RIDGE WAVEGUIDE
FORMATION
We explored three different processing techniques
. . .

• Wet Etching
• Argon Ion Beam Etching
• Reactive Ion Etching

Plasma Chamber
RIE Machine
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PROCESSING RESULTS USING RIE
Etch Depth 3.2 mm Etch Time 30 mins Etch Rate
1.8 nm/min
0.75 mm wide
2.5 mm wide

• RIE is the most promising technique in achieving
  
• vertical and smooth sidewalls
• good aspect ratio

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SETUP FOR CHARACTERIZING LOSS, GAIN, CROSS-TALK,
AND LASER SPECTRUM
Required for AWG and AWG/SOA devices
To check the mode profile of the amplified
signal, the right half of the setup will be
replaced with a CCD camera.
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MOTIVATION FOR CWDM

• The difference between CWDM and DWDM is channel
  spacing. In CWDM, the channel spacing is
  generally 10-25 nm whereas in DWDM, the channel
  spacing is less than a few nanometers.
• CWDM is a low-cost alternative to DWDM for use in
  short to medium haul networks.
• CWDM systems can be as much as one-third the cost
  of DWDM systems.
• Two reasons why CWDM systems are relatively
  cheaper are
• The use of lasers that dont have to be cooled
  and stabilized at the central wavelength for each
  channel.
• The use of passive devices, such as mux/demux and
  add\drop filters, that dont have such narrow
  filtering characteristics.

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ABSORPTION EDGE DIFFERENCEFOR AWG AND SOAs
Since the AWG needs to be transparent and the
SOAs need to be absorbing, the requirement for
AWG material is different than for SOAs.
Absorption edge in each SOA needs to be shifted
by different amounts so that its sensitive to
the wavelength in that channel
AWG
SOAs