Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Dynamics & Modulation Properties...

Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center

Dynamics & Modulation Properties of Multi-Transverse-Modes

Semiconductor Vertical-Cavity Surface-Emitting

Lasers

Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Yinon Satuby - M.Sc. Thesis

2

Outline

VCSEL - an introduction Single-mode VCSEL dynamics Multi-transverse-modes VCSEL dynamics Dynamic response to an optical,

parasitic-free excitation Characterization and dynamics of VCSEL

grown on a patterned wafer Summary


3

VCSEL Vs. Edge Emitting Laser

No need for cleavage: 2-D arrays Cheaper device On chip testing

Length cavity single longitudinal mode. Epitaxial mirrors R=0.999 high photon density. Symmetric “wavequide” with broad lateral area:

High order transverse modes. Easy coupling to a multi-mode fiber.

N

PI

Current

Light

(b)

N

P

I

TopMirror

Bottom Mirror

Current

Light

(a)VCSEL

Edge Emitting


4

(b)

Bottom emitting mesa

(d)

Intra-cavity mesa

(f)

Buried ion layer

(h)

Grown on a patterned wafer

(g)

Oxide confined

(c)

Top emitting mesa

(e)

Ion implanted device

VCSEL Device GeometriesP

NI

Oxide IsolatorDielectric

Mirror

(a)

Etched well


5

VCSEL Main Characteristics Thermal Red Shift. Substrate feedback induced ripples on L-I curve Multi-Transverse modes appearance

949950951952953954955956Wavelength [nm]

A6

B

C

D

E

F

A5

A4

A3

A2

A1

0

0.5

1

1.5

2

0 4 8 12 16 20 24Current [mA]

Po

we

r [

mW

]

10um*10um square VCSEL - buried layer

A1

B

C D

E

F

A2

A3

A5

A4

A6

Spectrally Resolved Near Field


6

Ion-Implantation-Based VCSEL Advantages

Easier and cheaper to manufacture. Large area contact pads. Planar surface.

Surrounding material: Better heat dissipation Less recombination centers at the periphery

Higher efficiency Gain guided mechanism - fewer transverse modes

Advantage ?

Fabrication:


7

VCSEL Main Application - Optical Interconnections Systems

The problem: Multi-mode fibers tend to generate modal noise

The solution: Usage of a less coherent light source:

i.e. multi-mode VCSEL

? What are the modulation characteristics of a

multi-mode VCSEL ?

Optical interconnection systems are based on: Array of independent VCSEL Multi-mode fiber ribbons


8

The Experimental Set-up

Variable Attenuator

CCDCCD

VCSEL

Fast GaInAs Detector

Bias - T

DC Current Source

Temp.Controller

Two Options

Two Options

RFGenerator

RF spectrum Analyzer

NetworkAnalyzer

RemovableSilicon PIN

Detector

X-Y RecorderL-I curve

microscope

ImagingSpectrometer

BS

RF probe

Near Field Image

Spectrally Resolved

Near Field

Spectrally Resolved

Near Field

Removable Mirror

Near Field Image


9

I - Modulation of a Single Mode VCSEL

Direct modulation of semiconductor laser. Modulation of a 10m diameter VCSEL defined by

buried proton layer - experimental: MCEF - modulation coefficient efficiency factor Max -3db B.W. & Intrinsic max B.W. Novel study of the transport time across the device


10

Laser Dynamics - Basic Model

2 conjugate poles response - resonance & damping factor.

I

S

P NInjection

-10

-5

0

5

10

15

1 10 100w [Grad/sec]

Vo

ut

/ Vin

[d

B]

- 40 dB/dec

R L

C VoutVinEquivalent

Circuit

2R

2

Rp f2

fj

ff

1

1

)f(I

)f(S)f(H

SS)(G

dt

dS

S)(GN

Vq

I

dt

dN

S,N

S,N

Assumptions: Neglecting transport effects Lumped QWs - uniform carrier density Single lasing mode


11

Laser Dynamics - Including Transport effects

3 poles response - roll-off pole in addition the to resonance & damping factor .

I

S

P NN

Equivalent Circuit

Vout

R L

CVin

-10

-5

0

5

10

15

1 10 100w [Grad/sec]

Vo

ut

/ Vin

[d

B]

- 60 dB/dec

Roll-off pole

- 3 dB B.W.

Assumptions: Single lasing mode Lumped QWs - uniform carrier density - N

T

C

Lumped barrier - uniform carrier density - NB

Adding time constant, s, which consists of: t ; c ; parasitic

Rp

Cp

x1


12

Laser Dynamics - Small Signal Analysis

P

thg

thg

eS

BSCH

eSCHS

B

SCH

iB

SS

S1

NNav

dt

dS

SS1

NNavNNN

V

V

dt

dN

N

V

VN

Vq

I

dt

dN

Rate equations:

Small Signal Analysis

S

2R

2

R

1

1

1

1

f2j1

1

f2

fj

ff

1

1

)0(S

)0(I

)f(I

)f(SfH

fK

1S av1

2

1f

02

R

e

s

P

g2

2R

Higher photon density in VCSEL larger B.W.

At higher injection levels, limits max. B.W.


13

Modulation of a 10m Diameter VCSEL (Single Mode Operation Regime)

Max B.W. - 14.5 GHz ; limited by the emerging of multi-mode lasing regime.

All curves were fitted to the a 3 pole transfer function, extracting:B.W. ; Fr ; ; s

0

0.5

1

1.5

0 5 10 15 20 25

Current [mA]

Po

we

r [

mW

]

10um diametr VCSEL

-12

-6

0

6

12

0 4 8 12 16Frequency [GHz]

20*l

og

10(|

H(w

)|)

[d

B]

4.8mA

5.4mA

5.65mA

6.3mA

7.15mA

7.85mA

8.2mA


14

Extracting Modulation Coefficient Efficiency Factor

As long as: << R

The roll-off pole influence can be neglected

Since

RRdb3 f55.1f21f

thR IIf

th

R

th

db3

II

f55.1

II

fMCEF

Vq

av1

2

55.1

II

fMCEF ig

th

db3

0

4

8

12

16

0 0.4 0.8 1.2 1.6 2

sqrt (I-Ith) [mA^0.5]

f_3d

b

[GH

z]

MCEF = 7.3832 GHz/mA^0.5

MCEF = 7.38 GHz / mA The best reported for ion implanted VCSEL

? What are the limiting factors ? (beside multi-mode lasing)


15

Maximum Intrinsic Modulation B.W.

When: The roll-off pole influence

can be neglected However, ~ R

Assuming Maximum B.W. Is achieved at: = 2*R

K

22f

MAXdb3

K = 0.11 nSec Maximum Intrinsic f-3dB= 80 GHzThe best reported for VCSEL

0

5

10

15

0 25 50 75 100 125 150

Fr^2 [GHz^2]

Dam

pin

g F

acto

r [

Gra

d/s

ec]

0

0.4

0.8

1.2

1.6

2

Po

we

r [

mW

]

Gamma=0.11*fr^2+0.6875 P = 0.0063 * fr^2

2R0

2R fKfK

? Yet, What is the influence of the transport effects …


16

-12

-6

0

6

12


Res

po

nse

[d

b]

FITTING RESULTS:fr = 11.15 GHz = 16.14 Grad/secf_transport = 9.37 GHz

10m dia. VCSELI = 8.2 mAPout = 0.75 mW

f_3dB = 14.5 GHz

Transport Effect on the Modulation Response

-12

-6

0

6

12


Res

po

nse

[d

b]

FITTING RESULTS:fr = 11.15 GHz = 16.14 Grad/secf_transport = 9.37 GHz

10m dia. VCSELI = 8.2 mAPout = 0.75 mW

f_3dB = 14.5 GHz

Theoretical -without transport


17

Extracting the Transport Time: The roll-off pole time

constant is composed of: The intrinsic transport &

capture time. The diode & Bragg

Mirrors, current depended, RC time constant

Phenomenological approximation:

0

10

20

30

40

50

0.1 11/I [1/mA]

s [

pS

ec]

I

1RCtranss

Carrier’s Transport & Capture time constant trans = 15pSec Extracted for VCSEL for the first time !

I

I

q

TKR

V)(V

1

1CC B

d

0

Diode

0dep


18

I - Modulation of a Single Mode VCSELConclusions

Medium area, ion implanted VCSEL exhibit high modulation B.W. , As long as single mode operation is maintained.

The MCEF & the max. B.W. , are the highest measured for ion implanted device.

An intrinsic max B.W. Of 80GHz was demonstrated.

The carrier transport time was extracted:

trans = 15psec , and its limitations on modulation B.W. were as illustrated.


19

II - Modulation of a Multi-mode VCSEL

The Theoretical Model. The model Small signal modulation frequency response for different mode

combinations Experimental Results

Modulation of a 20m VCSEL defined by buried proton layer : Frequency response of a multi-mode VCSEL modulation 2nd harmonic distortion

Modulation of a VCSEL array

Y. Satuby and M. Orenstein,“Modulation Characteristics and Harmonic Distortion of VCSEL Arrays and Multi Transverse Mode VCSELs” , LEOS Annu. Meeting, Nov. 1997, ThA2


20

The Model

)y,x(S)y,x(g)y,x(NB)y,x(N

dq

)y,x(J)y,x(ND

dt

)y,x(dN

PPG

dt

dP

2

nri

2T

p

iii

i

Intensity distribution of the modes is assumed to be known.

One parameter rate equation for the photon number of each mode. Rate + Continuity equation for a two dimensional distribution of the carrier density -

N(x,y)

(x,y) dxdyFg(x,y)G ii

Modal gain is attributed to the overlap between the gain distribution and the mode profile

i

2

i F1(x,y) dxdyF

)y,x(S1

1]N)y,x(N[av)y,x(g th0g

Device geometry is defined through J(x,y)

i

ii

d

)y,x(FP)y,x(S Photon density is the incoherent sum

for all modes


21

Example - Two Non-Overlapping Transverse Modes 20um diameter device LPmn modes are assumed, (according to experimental results):

LP21 LP01 - smaller in diameter (compare to device diameter) due to:

Spatial hole burning (self focusing) Thermal lensing

I=14mA

-15

0

14-15

0

14

0

1E+14

2E+14

3E+14

4E+14

5E+14

6E+14

Photon Density

microns microns-1

5

0

14-15

0

14

0

1E+14

2E+14

3E+14

4E+14

5E+14

6E+14

Photon Density

microns microns-1

5

0

14-15

0

14

0

2E+18

4E+18

6E+18

8E+18

1E+19

Carrier Density

microns microns

0.7 mW 0.43 mW

? How does the Dynamic response look like …


22

Dynamics of Two Non-Overlapping Transverse Modes

-20

-10

0

10

0 1 2 3 4 5 6 7 8Frequency [GHz]

Total Response

-0.05

0

0.05

0 1 2 3 4 5Time [nSec]

Total Response

Impulse Response Frequency Response

The modes behave as two independent lasers.

? How do current level & diffusion coefficient modify the dynamic response ?


23

As current increases the power of each mode increases linearly fr of each mode changes according to the power of the mode

0

1E+14

2E+14

3E+14

4E+14

5E+14

10 15 20 25Current [mA]

Me

an

Ph

oto

n D

en

sity

[c

m^

-3]

0

6

12

18

24

30

36

fr^

2

[GH

z^2

]

Ph. Dens. LP01

Ph. Dens. LP21

Fr^2 LP01Fr^2 LP21

"Kink"


24

Diffusion coefficient is not well known. Thus, calculation are made for a wide range of it

As diffusion coefficient increases, (at constant current of 14mA), the basic mode becomes dominant

fr of each mode changes according to the power of the mode

0.0E+00

5.0E+13

1.0E+14

1.5E+14

2.0E+14

0 5 10 15 20 25 30 35 40

Diffusion Coff. [cm^2/sec]

Me

an

Ph

oto

n D

en

sity

[c

m^

-3]

0

4

8

12

16

fr^

2

[GH

z^2

]

Ph. Dens. LP01Ph. Dens. LP21Fr^2 LP01Fr^2 LP21


25

Dynamics of Two Overlapping Transverse Modes

The modes behave as “coupled” oscillators.

Impulse Response Frequency Response

-20

-10

0

10

0 1 2 3 4 5 6 7 8Frequency [GHz]

Total Response

-0.1

0

0.1

0 1 2 3 4 5Time [nSec]

Total Response

? How do current level & diffusion coefficient modify the dynamic response ?

According to experimental results, the modes of a non-linear laser cavity are taken as: LP01

A combination of LP02+LP21

I=15mA , D=30


26

When the higher mode emerges, the power of the basic mode is almost clamped.

The resonance frequencies can not be related to a specific mode

0

1E+14

2E+14

3E+14

4E+14

5E+14

6E+14

10 15 20 25Current [mA]

Me

an

Ph

oto

n D

en

sity

[c

m^

-3

]

0

6

12

18

24

30

36

fr^

2

[GH

z^2

]

Ph. Dens. LP01

Ph. Dens. LP21+LP02

fr^2 Higher Resonanse

fr^2 Lower Resonance

The resonance frequencies do not follow the power of the modes - an “Avoided Crossing” phenomena is observed:Despite of crossing of the photon density of the two modes, the resonance frequencies do not cross


27

As Diffusion Coefficient increases, (at constant current of 15mA), the basic mode becomes dominant

0.0E+00

5.0E+13

1.0E+14

1.5E+14

2.0E+14

0 10 20 30 40 50

Diffusion Coff. [cm^2/sec]

Mea

n P

ho

ton

Den

sity

[cm

^-3

]

0

4

8

12

16

fr^

2 [

GH

z^2]

Ph. Dens. LP01Ph. Dens. LP21+LP02fr^2 Higher Resonancefr^2 Lower Resonance

The “Avoided Crossing” is illustrated again


28

20m Diameter VCSEL Defined by Buried Proton Layer (Higher Dose) - Experimental

0

0.5

1

1.5

2

0 10 20 30 40

Current [mA]

Po

we

r [

mW

]

Device I

A

B

C

D

E F

G

H

955956957958959960961

Wavelength [nm]

B

C

D

A

E

F

G

H

Frequency ResponseSpectrally Resolved Near FieldL - I Curve

-15

-9

-3

3

9

0 1 2 3 4 5 6 7 8 9Frequency [GHz]

FGH


29

20m Diameter VCSEL Defined by Buried Proton Layer (Lower Dose) - Experimental

Frequency ResponseSpectrally Resolved Near FieldL - I Curve

954955956957958959960

Wavelength [nm]

A

B

C

D

E

F

G

H

m20

-15

-9

-3

3

9

0 1 2 3 4 5 6 7 8 9Frequency [GHz]

G

H

0

0.5

1

1.5

2

0 10 20 30 40Current [mA]

Po

we

r [

mW

]

Device II

A

B

C

D

E

F

G

H

Lower dose A wider active area

B , D , F are local minima on the L-I curve


30

-36

-24

-12

0

0 1 2 3 4 5 6 7 8 9Frequency [GHz]

Resp

onse

[db]

Point E - 2nd. Har. generation

Point E - 1st. Har.

20m Diameter VCSEL Defined by Buried Proton Layer (Lower Dose) - 2nd Harmonic Distortion - Experimental

Single mode operation, 2nd harmonic level is-24dbc

Two transverse mode regime - 2nd harmonic peaks at: Excitation at the two

resonance frequencies Excitation at half the

resonance frequencies Excitation at half the

notch frequency


31

Modulation of a VCSEL Array - Experimental

Multi-mode operation is maintained throughout the whole L-I curve

L - I Curve

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40

Current [mA]

Po

wer

[m

W]

VCSEL array of three 6 micron dia. elements

A

B

C

DE

Array is defined using mirror patterning

Triangular array - producing modes similar to the large area VCSEL


32

Frequency ResponseSpectrally Resolved Near Field

Modulation response with two resonance was measured - regardless of local minima or maxima on the L-I curve

Modulation response with three resonance was obtained for three mode operation

2nd Harmoic Distortion peaks: At the resonances & their half frequencies At half the notch frequency (stronger response than

excitation at the notch itself)

946947948949950951952Wavelength [nm]

A

C

D

E

B

m20

-15

-9

-3

3

9

Respon

se [db

]

E

Array Modulation - Continue


33

II - Modulation of a Multi-mode VCSELConclusions

A theoretical model for the dynamics of multi-transverse-mode VCSEL was presented: A multi-mode laser is characterized by a multi-resonance frequency

response to a small signal current modulation For two modes - one contained in the other, the resonance frequencies

exhibited an “avoided crossing” like phoneme as modal power changed

Experimental results demonstrated: The multi-resonance behavior for multi-mode VCSEL A “flattened” frequency response for multi-higher-transverse-mode

operation regime

Modulation of a VCSEL array further confirmed the results

A strong second harmonic distortion was measured, when frequency response was not spectrally uniform


34

III - Parasitic-Free Response to a Pulsed Optical Excitation of a Large Area VCSEL

BS

CCD

x50

BS

ElectricalPulser

Fast GaInAs Detector

Variable Attenuator

CCD

VCSEL

FastSampling

Oscilloscope

microscope

OpticalSpectrumAnalyzer

Pulsed Ti-SaLaser


35

Parasitic-Free Response Along the Current Pulse

Optical excitations along pulse

0

5

10

15

20

0 50 100 150 200 250Time [nSec]

Powe

r [a

.u.]

12

3 4 56 7 8 9 10

11

150nSec 80mA current pulse

Excitation by 1pSec 810nm pulses

Two time constants: Relaxation-oscillation

of 8GHz Second pulse

generation after 0.35nSec (3GHz)

Second pulse generation is time depended


36

IV - Characterization and Dynamics of VCSEL Grown on a Patterned Wafer

A novel method of “ready to use” VCSEL fabrication

Unique modal behavior Dynamic properties:

Theoretical analysis Experimental results

M. Orenstein, Y. Satuby, U. Ben-Ami, J. P. Harbison, “Transverse modes and lasing characteristics of selectively grown vertical cavity semiconductor lasers” . Appl. Phys. Lett. 69(1996), pp. 1840-1842.


37

Selective Growth Over Openings in a Si3N4 Mask

A novel “ready to use” VCSEL structure grown by MBE over GaAs patterned wafer Over the Si3N4 layer an insulating

polycrystalline material was grown. Through the 20m20m

openings growth of a monocrystalline VCSEL structure was achieved.

Unisotropic growth process, material is less packed along (011) direction

The only required process, is the formation of contact layers

SEM pictures of cleaved device’s facets


38

Top View of the Selective Grown VCSEL

Top view: (a) Optical photo (b) AFM scan of a single VCSEL (c) Corresponding height profile along

the [011] axis The final device area is 15m15m

due to 2m migration of the interfaces


39

0

1

2

3

4

0 10 20 30

Current [mA]P

ow

er [

mW

]

Pulsed L-I

Pulsed Operation Characteristics

The dominant mode was always a one dimensional transverse mode aligned along 011 axis. with 3-5 lobes

(2)

TEM30 lasing Mode

(3)

TEM31 lasing Mode

Pulsed L-I Curve,Ith=7mA , =14%

? What will SRNF image revile at higher current levels ?

(1)

Spontaneous emission

Near field patterns:


40

Transverse Modes During Pulsed Operation

964965966

Wavelength [nm]

( I )

( II )

( III )

m

A 10nSec current Pulse to avoid thermal wavelength sweeping

( I ) 23 mA

( II ) 40mA

( III ) 58mA

The TEM30 & TEM00

modes, polarized

perpendicularly to each

other, are the dominant

modes Non-typical, the lower

modes emerge at higher current levels

SRNF Images

Remark: At CW operation, the lower-order modes are the dominant !


41

CW Operation of the Selective Grown VCSEL

0

0.1

0.2

0.3

0.4

0 10 20 30

Current [mA]

Po

wer

[m

W]

0

2

4

6

8

Vo

ltag

e [v

]

Power

Voltage

CW L-I & V-I

968969970971972973

Wavelength [nm]

A typical CW L-I curve is achieved

V-I curve demonstrates a typical 50 resistance

The fundamental modes become the dominant ones

? How would the dynamics & modulation response look like ?


42

Theoretical Response

The model described earlier was used.

A 15m15m

square current

injection profile

Modes TEM00 &

TEM10 were

assumed. (highly

overlapping

modes)

-15

-5

5

15

0 1 2 3 4 5 6 7 8Frequency [GHz]

Total Response

-0.1

-0.05

0

0.05

0.1

0 1 2 3 4 5Time [nSec]

Total Response

P = 1.18 mW

Single Resonance Response


43

Experimental Response

-12

0

12

0 1 2 3 4 5 6 7 8 9Frequency [GHz]

Res

po

nse

[d

b] 18mA

20mA

963964965966967968

Wavelength [nm]

20mA

18mA

A single resonance response in accordance to theory Multi-transverse TEM m0 modes operation


44

Carrier Life Time Measurement? Does the polycrystalline material induce shorter life time,

due to traps at the periphery ?

0

0.3

0.6

0.9

1.2

0 0.2 0.4 0.6 0.8 1

Ln( I / (I-Ith) )

t_ri

se [

nSec

]

1.82 nSec

Carrier life time nr=1.8 nsec , as for proton implanted VCSEL

thon

offonnrise II

IIlnt

Using the large signal response relation:


45

IV - VCSEL Grown on a Patterned Wafer Conclusions

A simple selective growth method for VCSEL fabrication was demonstrated.

The lasers exhibited similar characteristics to VCSEL fabricated using conventional methods

A unique transverse mode behavior, attributed to strain induced by the growth boundaries was observed .

The traps induced by the growth process at the boundaries, did not modify carrier life time

The modulation scheme for such a modal behavior was calculated & measured to yield a single resonance frequency response


46

Summary The dynamics of a single mode operated VCSEL was

analyzed, and transport time across the device was measured

The dynamics of a multi-transverse-mode VCSEL was studied: A theoretical model has been presented, and a number of cases were

examined : Two non-overlapping modes respond as two independent lasers Two modes, one contained in the other acts as two “coupled oscillators”

having two resonance response In case of two highly overlapping modes, single resonance modulation

response is expected Experimental results confirmed the results

The use of optical excitation to achieve a dynamic parasitic-free VCSEL’s response was illustrated

A VCSEL fabricated by novel method of using selective growth was introduced and characterized

Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Dynamics & Modulation Properties...

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Transcript of Technion-I.I.T., EE Dep., Advanced Optoelectronics Research Center Dynamics & Modulation Properties...