Upload
others
View
3
Download
0
Embed Size (px)
Citation preview
The CECOM Center for Night Vision and Electro-Optics
OPTOELECTRONIC WORKSHOPS
00
OPTO-ELECTRONICS IN IIl-V SEMICONDUCTORS:
MATERIALS AND DEVICES
May 3, 1988
sponsored jointly by
ARO-URI Center for Opto-Electronic Systems ResearchThe Institute of Optics, University of Rochester
*UNCLASSIFIEDSECURITY CLASSIFICATIOD OF THIS PAGE
REPORT DOCUMENTATION PAGEIs. REPORT SECURITY CLASSIFICATION 1b. RESTRICTIVE MARKINdS
2&. SECURITY CLASSIFICATION AUTHIORITY 3. DISTR11UTIO)iIAVAjLABIuTy OF REPORT
2b. DECLASE FCATION/OOWWJ6RADING SCHEDULE Approved for public release;distribution unlimited.
4. PERFORMING ORGANIZATION REPORT NUMBER(S) 5. MONITORING ORGANIZATION REPORT KUMER(S)
6a. NAME OF PERFORMING OPCGANIZATION 6b. OFFICE SYMBOL 7a. NAME OF MONITORING ORGANIZATION(If applicable)
University of Rochester U.S ryRserhOfc6c. ADDRESS (Oty. State, a&d ZIP Code) 7b. ADDRESS (Cty, State, and ZIP Code)The Insitute of Optics P. 0. Box 12211Rochester, NY 14627 Research Triangle Park, NC 27709-2211
Be. NAME OF FUNDISPONSORING 6 b. OFFICE SYMBOL 9. PROCUREMENT INSTRUMENT IDENTIFICATION NUMBERORGANIZATION O (f ipplcbe)
U. S. Army Research Office 19403-? k67Sc. ADDRESS (City, State, and ZIP Code) 10. SOURCE OF FUNDING NUMBERS
P. 0. Box 12211 PROGRAM PROJECT NO.K ION UNO.
Research Triangle Park, NC 27709-2211 ELEMENT NO. INO.NOr SNN.
11. TITLE (Includ SeCurity C~alafako)Optoelectronic Workshop III: Opto-Electronics in IIl-V Semiconductors: Materials
and Devices12. PERSONAL. AUTHOR(S) Gr ik
13a. TYPE OF REPORT 13ab. TIME COVERED 14. DATE OF REPORT (YearMontADay S1. PAGE COUNTTechnical I FROM TO ____I May 3, 1988 1'
16. SUPPLEMENTARY NOTATION The view, opinions and/or findings contained in this report are those
of h auh r) an shuld nbef"A cos a n fficial Deartuent of the Army position,17. COSATI CODES 1S. SUBJECT TERMS (Condinu on rewi Nf neceaavy end ilentify by block number)
FIELD GROUP SUB-GROUWorkshop: III-V Semiconductors: Materials and Devices
'9. ABSTRACT (Contu an reve'se If necenaiy and Ident by block number)
'T his workshop on pto-Electronics in Ill-V Semiconductors: Materials and Deviceskrepresents the third of a series of intensive academic/ government interactions inthe field of advanced electro-optics, as part of the Army sponsored UniversityResearch initiative., By documenting the associated technology status and dialogueit is hoped that thi\,b aseline will serve all interested parties towards providing asolution to high priority Army requirements. Responsible for program andprogram execution areDr. Nicholas George, University of Rochester (ARO-URI) andDr. Rudy Buser, NVEOC. * A--'
I f\Lo t
20. DISTRIBUTION/ AVAELAIUY OF ABSTRACT 121. ABSTRACT SECURITY CLASSIFICATIONO3UNCLASSIFIEDJNLIMTED - 0 SAME AS RPT. C) OTIC USERS Unclassified
22a. NAME OF RESPONSIBLE INDIVIDUAL I22b. TELEPH4ONE (include Arts Code) 1 22c. OFFICE SYMBOLNicholas George I716-275-2417
DD FORM 1473, B4 MAR 83 APR edition may be used until eidtauted. --- SECURITY gLSSFICATION OF TIS PAGE'AX other editions are obsoWee UNCLASSIFIED
OPTOELECTRONIC WORKSHOP
ON
OPTO-ELECTRONICS IN IIl-V SEMICONDUCTORSMATERIALS AND DEVICES
Organizer: ARO-URI-University of Rochesterand CECOM Center for Night Vision and Electro-Optics
1. INTRODUCTION
2. SUMMARY -- INCLUDING FOLLOW-UP
3. VIEWGRAPH PRESENTATIONS
A. Center for Opto-Electronic Svstems ResearchOrganizer -- Gary Wicks
Optical Properties and Technologies of III-V MaterialsGary Wicks
Superlattice DisorderingSusan Houde-Walter
Optical Interactions in Indirect Bandgap:Il-V Semiconductors and Silicon
Dennis Hall
III-V Optoelectronics for Optical Communication.Thomas Brown
B. CECOM Center for Night Vision and Electro-OpticsOrganizer-- L. N. Durvasula
4. LIST OF ATTENDEES Accesslon 'c r
NTIS GRA&IDTIC TAB
Justifioatlon
Di stribut on/
Availabllty Codes
Dist Speo lal
1. INTRODUCTION
This workshop on "Opto-Electronics in Ill-V Semiconductors: Materials andDevices" represents the third of a series of intensive academic/ governmentinteractions in the field of advanced electro-optics, as part of the Armysponsored University Research Initiative. By documenting the associatedtechnology status and dialogue it is hoped that this baseline will serve allinterested parties towards providing a solution to high priority Armyrequirements. Responsible for program and program execution areDr. Nicholas George, University of Rochester (ARO-URI) and Dr. Rudy Buser,CCNVEO.
2. SUMMARY AND FOLLOW-UP ACTION
Rudy Buser started the workshop with a short talk. He mentioned two areas ofIll-V s which are of interest at NVEOC: 'ocal plane arrays of 10 pm detectors; andmonolithic integration--sources; detectors, etc.
Ward Trussel spoke about NVEOC's interest in high power laser diode arrays forYAG pumping
Several NVEOC personnel expressed interest in high power/high efficiency visible(green) lasers with good beam quality. One way of doing this is by doubling thediode pumped YAG lasers. Dennis Hall received some intercst when hementioned the possibility of green GaP lasers,
George Simonis spoke of interest in optoelectronics at Harry Diamond Labs.Their activities include: doping superlattices for optical modulation; resonanttunneling (mainly theory); piezoreflectance studies of Ill-V heterostructures, andlightwave research for microwave radar applications.
The original plan was for L. N. Durvasula and I to get together for a short whileat the end of the workshop to decide how future interactions might proceed.Time did not allow this, however, and we agreed to discuss these items on thephone. I tried several times without success during the week following theworkshop to contact Dr. Durvasula. Finally I sent him a letter stating a fewspecific areas where there appeared to be good overlap between research areasat UR and interests at NVEOC. Also stated in the letter was my feeling that theformal presentation structure was appropriate for the first workshop, butsubsequent interactions should consist of smaller, less formal groups and moredialogue.
I would estimate that 30 NVEOC personnel attended the workshop.
L1
2o
J
w
0
Z-J
.-
0
Outfin
*Crystal growth technologies
*Bulk materials properties
*Properties of quantum wells and superlattices
*Device fabrication technologies
ENERGY GAP (in eV and /im) VERSUS LATTICECO.NSTANT AT 300K FOR COMPOUND SEMICONDUCTORS
.0-
ZnSe
AlP 0.5
.0-
:Si 1n
0
0eG~
-2
-H 105.4 5.6 5.8 6.0 6.2 6.4 6.6
a0(A)
Heterojunction Energy Bands
*~jjjujd~ConductionAEC Ooeeo Ban d
EA~~ Gaas as adsa
Quantum Well
T~na a Iz
~n=Z E
AMGaAs GaAs ANGaAs
Superlattice (type 1)
Important Epitaxiol Structures
LosersDouble Heterostructure (DH) Al concentration
p GaAs
p AlGoAs 121,m
GaAs 0.2,&Lmmbn AIGoAs 121mn GaAs ___
Graded refractive index- Al concentrationseparate confinementheterostructure (GRINSCH)
p GaAsp AIGaAs
groded AIGoAs
n AIGoAsn GaAs
Other Versionsmulti-quantum wellconstant composition, seporai confinement heterostructure
Ill-V Materials for Lasers
x = 2.2 A~m - GaInAsSb/AIGaAsSb DH
x' *=1.7 -1.2 Am GaInAsPAnP DH
x = 1.2 -0.9 A~m -pseudomorphic
GaInAs/AIGaAs OW's
* X = 0.9 0.68 Am - AIxGaj-xAs/AIyGai 1yAs OW's
e x = 0.65 pm - Gajswtn.48P/AIGaInP
IIl-V's for Long WavelengthPhotodetectors
@Lowest bandgap bulk Ill-V is InAs Sb6 l
eStrained InAsSb Quantum Wells
x,< 12 pm
*Intraband transitions in the conduction band ofsuperlattices
2 A/Watx = 10 pm
note: only radiation polarized along growth axis isdetected
Crystal G rowh Tech noI
*Bulk Growth Techniques
-Bridgeman
-Uquid Encapsulated Czochralski (LEC)
9Epitaxial Growth Techniques
-Uquid Phase Epitaxy (LPE)
-Vapor Phase Epitaxy (VPE)
-Molecular Beam Epitaxy (MBE)
-Metal Organic Chemical Vapor Deposition (MOCVD)
VAPOR PHASE EPITAXY
Goci
AsC 3 As2 A5. - Chloilide VPEHtECI -
2 Go H2 GoAs
T, > T
GHCIHyrdVEH-2, H C Hydride VPE
Go Ha - GAs
T constont
hot wll
_ __H3 MON MOCVD
GoAs
cold wall
0.00
Ad b
"4 P
'I 4W40
060
0o 0
P-4 C OC b0 0 U 0( 0 ae m)
Ad c to0 9-
.C 40 w Wbi 64A00 0 Ik.k 004
Wo 4' c c
0~A 0 be 0 0)0 0
01 CO 4) 9-4 u44'l 0)
00 C
. ).Cbe um
MOLECULAR SEAM EPITAXY
CRVOPANELLINO
SUBSTRATENEATING fLOCX
GGAS WAFER
EFFUUSNICEL
CELLS MUTTER
IIIcc T ePOte[je 0 0s 6 0 S 0 0
thea ~~ccoupSW'oX4 AS5P;NFL.i~tic'. mteia__________________VACUUM cucbl
Mo~ciA RBam Eotax (MBE)
MAIN ORO~T CHAMBERCHME
FUSIO CELS ILED. MASS SPEC
SMAM SOUTRCS ALMANIPLATO (LQaD NITON E
II SENSTRAjMOEFFUSION~.I GCELL
6as Source MBE SystemMOMBE
Standard MBEgrowth chamber
DiffusionPump
*Pressure control of
qroup V gases
* Crack AsHf and PH
in same cell
Computer Controlled K- Cells L PUP I I /:3 j
CIHAMBER I (a ,Al, As,
SOLID SOURCES Be, Si)
Optical PortsandRED
Residual GasAnalyzer
High Temperature
3 Waer TackCleaning Stage3" Wafer Track --- '
Syatem
Expansion o o
0 Dual e-Beame-gun Metallization
Optical Ports Residual Gas
and Analyzer
RED N
CHAMBER # 2 (s3P3GAS SURCESTEGa, TEIn,
Al, Be, Si)
Computer Controlled Gas Sources
Densityof
States 2
n-1
Energy
Electrooptic Effects in Ill-V's
Bulk materials - Franz-Keldysh Effect
EC
AEGv
A & lOKV/cm
Shift Absorption Edge * Change refractiv index:
An- 5 x1O1
AE lOKV/cm
Electrooptic Effects an HIl 'sQuantum Wells - Excitonic Effects
Case 1. Electric field // Growth axis
Ec
Ev
*Change shape of potential well 4* shift exciton energy levels 4 shiftabsorption edge =# change refracive index.
Case 2. Electric field I Growth axis
*Field ionize exciton * remove excitonic features from absorptionspectrum 4 change refracive Index.
*The excltonic absorption In quantum wells leads to very largeelectroopticeffects:
A n - 5 x 10-2A-F lOKVlcm
ha c 100 cm- ,
lOKVlcm
Edge Emitting Laser
Epiv= .Layers
Substrate 1
Dry Etching Technius optoelectronic Devi
*Chemically Assisted Ion Beam Etching
•Reactive Ion Beam Etching
eReactive Ion Etching
Applications
*Integrated lasers no cleaved mirrors
*Other integrated devices- lenses- mirrors- modulators
L97W
IAJQ
0
Iui
LU
0U.
zwj
Experimental:ref: Y. Suzuki and H. Okamoto, J. Elec. Mats. 12,397 (1983)
Compare refractive index (dispersion) of bulkAlo.3Gao.oAs to GaAs/AlAs SL's with various barrierthicknesses:38
3.7 -j LE
w 3.6, .0 - Et(e-hh) -
,,,3.5 "
3.4 -162,27)
w3.3,
1.3 1.4 1.5 1.6 1.7 IB
PHOTON ENERGY (eV)
Note: for a fixed average alloy composition, one canmake large changes in refractive index by tailoringheterostructure dimensions.
>>useful degrees of freedom in device designlaserswaveguides, lenses, modulatorsdetectors
Heterostructures are grown epitaxiallymolecular beam epitaxymetallorganic chemical vapor deposition
-,ilayers(2-6000 A)
GaAs substrate
INTEGRATION: grow one structure, use same basematerial for all components
define components(e.g. ridge waveguides,heterostructure lasers) by removing surroundingmaterial (ion beam etching, cleaving, etc.)
Disadvantage:scattering, band bending, not planar, etc.
Objective: change bandgap and refractive index locallyto define components
New Method: smooth abruptness of heterostructureslocally
GALLIUM COC.T
C,, 0
Ch
fCD.1%
A9)
co 0
00
.4DI
Mixing is enhanced by impurity transport
ref: W.D. Laildig, et al, Appl. Phys. Lett. 38,776 (1981)
MASKED Zn DIFFUSION INTOSL
Zn2As3Si3N4
AlAs/GaAs SL
small bandgap large bandgaphigh index low index
Lasers:
Use layer mixing to:1) tune emission wavelength
ref.: M. Camras, et a!, App!. Phys. Lett, 54y,5637(1983).
fgl-3s EC
*,-12
2.0l
2h
W'' 0 PWl
IS
CW CURRET (A)
THIS WORK:
lID involves electrically active species>>use applied electric field
>>more accurate diffusion coefficient measurements>>improved lateral resolution for component definition
Zn diffusion
1gin
Si diffusion - ---'---_
1gm
mask edgediffusion dominated
drive in mixing profile;little time for sidewaysdiffusion.
field-dominated
Use evaporated source or gettering layer:
Zn2As3Si3N4
V
AlAs/GaAs SL
Use to study lID mechanism, fabricate components.
z0
0z
=0
CAZ
0 Lp
Liu<
0M
LU
=z0-
Z0
OPTICAL INTERACTIONS IN INDIRECT BANDGAP
Ill-V SEMICONDUCTORS AND SILICON
" Gallium Phosphide, GaPOptical Emission
* SiliconOptical EmissionWaveguides
ENERGY GAP (in eV and /m) VERSUS LATTICEONSTANT AT 300K FOR COMPOUND SEMICONDUCTORS
ZnSe
AlP CdS 0.5.0
All~b
-AISb CdTe - )
GaAs_"Si "I
' Ge GaSb" ---2--0
S_. 11I I .L~ -110
.4 5.6 5.8 6.0 6.2 6.4 6.6a,(A)
GALLIUM PHOSPHIDE, GaP
INDIRECT GAP: - 2.27 eV AT T =300K.
IMPURITY - RELATED LUMINESCENCE
B C N, X
Al Si P S
NZn Ga Ge As S e
Cd in S n Sb To
Hg TI Pb Bi, Po
ISOVALENTWITH
PHOSPHORUS
ISOVALENT ( ISOELECTRONIC) IMPURITY
GaP: N or GaP :Bi
ISOVALENT ( ISOELECTRONIC ) MOLECULE
GaPZn-O or GaP:Cd-O0
(Nearest-Neighbor Donor-Acceptor Pair)
ENERGY BAND STRUCTURE
GaAs - DIRECT ENERGY GAP
E Conduction
Band
SphotonE=,r - 1.4ev 1
ValenceBand
k 0 Crystal Momentum, k
Silicon - INDIRECT ENERGY GAP
't'I CB
EIS -1.1 OVj
k=kk -0
I I1k- CONSERVATION SELECTION RULE
*UNASSISTED , BAND - TO - BAND, RADIATIVE RECOMBINATION OF ELECTRONS
AND HOLES IS FORBIDDEN.
PHONON - ASSISTED RADIATIVE TRANSITIONS CAN AND DO OCCUR.
I ..o
EXCITONS BOUND TOISOELECTRONIC IMPURITIES
CB
electron -
UCOMPLEX gap photon
hole
PROBABILITY OF NONRADIATIVE AUGER RECOMBINATION IS LOW.
PROBABILITY OF RADIATIVE RECOMBINATION IS HIGH.
GaP:NGaP 4
- ~~NNW ~IT N- -E T ON, BOUND O
2 A
,,, -- A --o SINGLE.N"IMP' URITY
NN ' PAIRS OF/q, jb P.s
2.30 2.305 a.t 2.315 2.32PHOTON ENERGY (ev)
ATOMS/CC or -;?GEd
d * d0 @ 4 3 '
40 1.6"K F 7 o{
20 NN/ ,. Rev. p__, (toa (I a ID).
I to-1 uo
4 4- SLOPE 2
2 z
S. a
4
06-
06 6 .0L4 -
2 4 6 a 10 20 00 60 t00
ABSORPTION AT 2.32eeV ON PHONON WING OF A (cu.)
G p~ N2R
24 Lc~oUlil~LL
G'ree~ ~ 20
LED16 Absorption
02.0 2.1 2.2 2.3 2.4
Photon energy', eV
LASER /AC7)ON
V.S. Pateyit 3,'71)3'7 (1973)
Le~em , Loan NoAIor'8) and Shaklee
"Losers io. Xvidmipet0- Samdpap Sernicondvdht.
crstgas Voec WA Xsoe)ectronh-. T. r 1 ,:.
G; in GAP i= ~sre
RoM T7EMEATOT
Gap: zn Cd-O Zn-O UI
z0-.4TK
24
3 a-
0 . 18 92
StiuIiteJ ETmiSSjon and4 LASerOrverati..st (CtW, '77K) Associite4 ~.
WAt pe ISOc)eci'.n~c Tra~s.li
N. He-fornoi' +7*vies
TL~optIV Ail *MSe 1fVSedletebO fo1 ORIO1 W/cnil'l
IN.arr~vd\ at -fie FeSS~nIM_______cov"C~s;ons 5ha 16f We arcceAI(W 7.2 7.0 6.8 6.6 6.4 62
ik ' +he Nfrmiur'e r.VnctrmIA Woveength (103A)
in~rc sewikesduct-ars.controve'sia L
WHAT ABOUT SILICON ?
ISOELECTRONIC FSOEECTONCPAIRS SERIES
II IV V VI
B C N
191986
Cu Zn
1979.Ag Cd In Sn Sb Te
Au Hg ~ Pb Bi Po
W. P. DUMKEIBM, YORKTOWN HEIGHTS
"INTERBAND TRANSITIONS AND MASER ACTION"PHYS. REV. 127, 1559 (1962).
FIRST LINE OF THE PAPER
Since the initial operation of the ruby maser, there has been
considerable speculation concerning the possibility of observing maser
action in semiconductors such as Ge and Si.
ABSTRACT OF THE PAPER
The possibility of using interband transitions to achieve maser action is
considered. The criterion for maser action Is presented in a way which allows the most
direct use of optical absorption data. The absorption constant for interband transitions,
which is negative corresponding to induced emission when a population inversion
exists, is related to the normal absorption constant for direct, indirect, and indirect
exciton transitions. Using available absorption data, it is shown that in Ge (S )maser action, using either the Indirect or indirect exciton transitions, would
be prevented by absomtion due to free carriers. In GaAs, or other
materials with a direct gap, however, it is entirely possible that maser
action could be achievbd.
fU HC4-
14
0Lr
co
(sLNnl aEIV) AJISNHJLNI HONDSNIFII
~~ k
I~~ ~~~ -- _New v v
wovOw
%#Mo
IMO
'I'3~woo
a a 2 a a m I ML
(Snmn BHY) AATISNW IDNzoszNmflfI
z=0
LOZ
u0
L
Oo
W.Jzo
0 LI
Ill-V OPTOELECTRONICS FOR OPTICAL
COMMUNICATIONS
1. Overview: Communications Systems
2. Overview: Modulation/DemodulationSchemes
3. Source Stability Requirements forAdvanced Optical Communications
4. Methods of Source Stabilization
Historical Perspective:
Free Space Optical Communications AncientGreeks
Morse Telegraph 1840's
Bell Telephone 1870's
Marconi's Wireless Telegraph 1900. a. Radio
Coherent Carrier 1940's
Modulation/Demodulation
Phased Locked Receivers 1950's
First Optical Mixing Observed 1955
Semiconductor Laser Invented 1963
Low Loss Optical Fiber 1970
Free-Space Optical Heterodyne 1970'sSystems
High" speed, low loss single-mode 1980'sfiber links
Coherent Optical Fiber Communications 1980'sproposed
Communications Systems:
Free Space Optical(Fiber
Telecommunications Data Transfer
Trunk Local Area Network Subscriber
There is current interest in:
Long repeaterless links.Extremely high capacity links.
Wide area distribution.
Direct Detection Systems
Traditional:
_n-F-u1+ Dispersi~Channel
IntensityModulated PIN-FET
Diode Laseror
Avalanche PhotodiodeReceiver
Wavelength-Division Multiplexed (WDM) Systems:
X1+ rnrLDispersive X2
X3 el-anne3
X4
X4t
Pulse-Position Modulation (PPM) Systems:
Low Dispersion na Detector
(lowest fundamental limit)
* Direct Detection Systems
Average Power Ps ideal detector rl =1
Average # of photons per time slot:
Ns- PsTB
?ico
Photo-electron statistics:
p(n)= e poisson processn!
Ideal photon counting: (no dark count)
prob. of error - PE = prob(n = 0)
= E-Ns
I Ns-20.7 for PE-- 99
The Ideal System: Receiver Sensitivity
* Heterodyne/Homodyne Systems- No phase noise, perfect phase
matching
- Coherent(Phase-estimation) D{ IncoerentDemod.Incoherent (Envelope)
Photodetector RF
Es(r, t) + Eo(r, t)
i 0~t)t
i (t) = r Es(r, t) + ELo(r, t) 2
i (t) = (rI e/hw) {Ps + PLO + 2 VPTPL Cos (&t + 4(t))}
ASK
representation: a(t) = 1 "mark"
(Binary) a(t) = 0 "space"
representation: a(t) = 1
(Quaternary) a(t) = .67a(t) = .33
a(t)= 0
Analog limit (minimum bandwidth)
* We are not interested in bandwidth
compression, so use binary.
I TB
t=O
ASK IMPLEMENTATION
• Same modulator technology as direct
detection systems.
* External modulators only, since direct
modulation of semiconductor lasers
results in frequency modulation.
(GHz/mA)
* Easy detection (in analogy to RF
systems)
MACH-ZEHNDER INTERFEROMETRIC MODULATOR
4- Phase Modulator 01
0 1 = Io/2 Cos 2 (AO/2)
- _Phase Modulator 02, AO=O6 _-e
A =1 62
* Identical waveguides, equal path
lengths yield constructive interference.
* Relative phase shift AO causes
destructive interference for AO = n
• Practical design considerations require
tuning for "on" and "off' states.
• Top speed > 17 GHz
[Ref. Gee and Thurmond OFC'84
R. C. Alferness, IEEE JQE 17,946 (1981)]
Receiver Sensitivity PenaltyASK Envelope Detection with
Optimized Post-detection Filtering
3.o
im.0
Analytical (single-pole IF filter)C
1.0 .O
-. Numerical (integrating IF filter)
7
/
0.0
0.0 .2 .4 . .9 1.0
Llnewidth/Data Rate
PSK
representation: a(t) = 1 mark"
(Binary) a(t) = -1 "space"
representation: a(t) = 1
(Quaternary) a(t) = 1a(t) -. 4=Levels
a(t)
Analog limit (minimum bandwidth)* Once again, use binary.
IIIII
I/
* PSK is a supp ressed carrier modulation
technique
SSE, E
, \ /, \,I/I
* No energy at ct = Co
$ Requires nonlinear phase recovery
scheme.
PSK Implementation:
" Simplest possible modulator
configuration.- Electro-optic (Pockels) effect- Free carrier effect (semiconductors)
* External modulators only
" Detection is difficult
as(t) Cos (cost + 4(t))
weak signal/XI
Strong Local OscillatorCOS(COLOt - ( )
for error-free detection,4(t) = )(t)
PSK M -ER-oP'(I0E to51I-c-TIc
4.0
3.0-
A. 2.0-
1.0-
O0.0 , . , , , ,I , , ,, g , , , , t,*
0.0 0.05 0.1 0.15 0.2 0.25
NORMALIZED SPECTRAL WIDTH (/)
FSK
Binary Representation: a(t) = e i "mark"
(Ac channel spacing a(t) e cspace"
M-ary a(t) = ei(m+I)()ct
representation a(t) e iIct
#Channels ]M=2m .#e ( a(t) = e - i(m+ )Oct
Bandwidth.Expansion
technique
I Power Spectrum SE E ()
I( C
CIL) = o
Receiver Sensitivity PenaltyFSK Envelope Detection with
Optimized Post-detection Filtering3.0
Analytical (single-pole IF filter)
4D - Numerical (integrating IF filter)
1.0 .4
Linewidth/Data Rate
Ill-V Optoelectronic Devices: Limitations to SystemPerformance
Semiconductor Lasers:
Mode Partitioning(Direct Detection)
Phase Fluctuations/Line Broadening
(Coherent Detection)
Noise Mechanisms
Thermal Instabilities (Phase ShiftsIn
Quantum Fluctuations Optical Signal
How does this happen?
AX. Fabry-Perot Modes
n It
, I- -- I
- ,, e -Gain ProfileIIj
I I !I I
I I I IS
Wavelength
Large changes ..... Mode "partitioning"Small changes ..... Line Broadening
Mode Partitioning
1) Time Averaged Laser Spectruma0
a a+1
a 2 a+2
2) "Instantaneous" Measurementa-1
a0 a
+1a 2 a +2
0
Assuming the population adiabatically follows thequantum fluctuations:
+00XIa. = Constant
Constant flux Is dynamically "partitioned" among themodes.
What happens In a dispersive channel?
---- a° Dispersive W -W.. ao
a Channel .
Nearly Single Mode][SourceSolutions:
- Use the minimum dispersion point In fiber.- Stabilize the diode laser:
"Single-mode" lasers require a side modesuppression ratio of about 100:1.
Distributed Feedback (DFB) Lasers provide modestabilization.
Active Region
DFB Laser Structure
10-6 - --
k=0.18- - -(MODE RATIO= -31)
10-8 -------
o k m0. 15410 (MODE RATIO= -42)
0-1 V:ODE RATIO-Si) __
10-16L- - - -
0.6 .0.9 1.2 1.6 2.2INTERMODAL DELAY/BIT PERIOD
LINE BROADENING
Take a closer look at what Is driving the phasefluctuations:
IM[C] Stimulated SpontaneousEmission o°°° Emission
Re[s]
AO= A~irst + A0 relax
Al -- Im[An] -> Re[Anj -*A4
Figure of (de)merit for Gain medium:
cRe[An] - 5 for GaInAsIm[An]
$ 4) exhibits a "random walk"
o,2=Dt
t=0 p(;t)= 2/22
//- ",, t=tl
/ \ 4a/ t=t2>tl
R-Sp. emission rateD (1 +a 2) R./21
a increases o2 by -25 to 50.
Av = D/2n
- 0 SolutionsDFB Lasers (MHz)External Cavities (10-100 KHz)
Hew do we narrow the laser Iinewidth?
In an Ideal system, we can:
-Increase the pump power (% above threshold).
-Decrease the spontaneous emission rate.(External/Coupled Cavities)
-Introduce active stabilization.
-Decrease a
How can (x be reduced?
Bulk III-V Gain Medium
G Gai 71q" -- refractiveELos Index
Discrete Transitions (Multiple Quantum Well)
RefractiveIndex GaIn
Energy ..-
S a eS p
Reducing the Strong amplitude-phase coupling can
improve linewidths by 25 - 50.
Options for discrete transitions:
- Multiple Quantum Well (MOW) diode lasers.
- Rare-eerth resonant diode lasers.
Modulation Sensitivity Linewidth LaserTechnique (Photons/bit) Requirements of choice
Direct 400-4000 100:1 Good FPDetection side mode or
DFB
ASK Het. 80 0.1 x Data Rate DFB
FSK Het. 40 0.1 x Data Rate DFB
PSK Het. 18 10-3 x Data Rate ?
PSK Horn. 9 10-4 x Data Rate ?