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Dynamics & Modulation Properties of Multi-Transverse-Modes Semiconductor Vertical-Cavity Surface-Emitting Lasers

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### Dynamics & Modulation Properties of Multi-Transverse-ModesSemiconductor Vertical-Cavity Surface-Emitting Lasers

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

Current

(a)

(b)

Top

Mirror

P

I

Bottom Mirror

P

N

I

N

Light

Light

VCSEL Vs. Edge Emitting LaserEdge Emitting

VCSEL

- 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.

P

(b)

(a)

I

N

Oxide Isolator

Dielectric Mirror

Etched well

Top emitting mesa

Bottom emitting mesa

(e)

(f)

(d)

Intra-cavity mesa

Ion implanted device

Buried ion layer

(g)

(h)

Grown on a patterned wafer

Oxide confined

VCSEL Device GeometriesA2

A3

A4

A5

A6

B

C

D

E

F

VCSEL Main CharacteristicsSpectrally Resolved Near Field

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

Ion-Implantation-Based VCSEL Advantages

- Fabrication:

- 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 ?

VCSEL Main Application - Optical Interconnections Systems

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

- 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 ?

Near Field Image

Spectrally Resolved Near Field

Spectrally Resolved Near Field

Two Options

RF

Generator

Network

Analyzer

Removable Mirror

RF spectrum Analyzer

Removable

Silicon PIN Detector

Two Options

X-Y Recorder

L-I curve

Fast GaInAs Detector

The Experimental Set-upmicroscope

DC Current Source

Temp.

Controller

RF probe

Bias - T

VCSEL

CCD

CCD

Imaging

Spectrometer

BS

Variable Attenuator

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

L

Vout

C

Vin

P

I

N

Injection

Equivalent Circuit

S

Laser Dynamics - Basic Model- Assumptions:
- Neglecting transport effects
- Lumped QWs - uniform carrier density
- Single lasing mode

- 2 conjugate poles response - resonance & damping factor.

x1

P

I

tT

N

N

tC

Cp

S

Laser Dynamics - Including Transport effects- Assumptions:
- Single lasing mode
- Lumped QWs - uniform carrier density - N

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

- Lumped barrier - uniform carrier density - NB
- Adding time constant, ts, which consists of: tt ; tc ; tparasitic

R

L

Vout

Equivalent Circuit

C

Vin

Laser Dynamics - Small Signal Analysis

- Rate equations:

Small Signal Analysis

- Higher photon density in VCSEL larger B.W.
- At higher injection levels, limits max. B.W.

Modulation of a 10mm 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

Extracting Modulation Coefficient Efficiency Factor

- As long as:
- << R
- The roll-off pole influence can be neglected

- Since

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

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

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

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

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:

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

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

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

Photon density is the incoherent sum for all modes

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

- 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)

- Device geometry is defined through J(x,y)

0.7 mW

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

- How does the Dynamic response look like …

Dynamics of Two Non-Overlapping Transverse Modes

Impulse Response

Frequency Response

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

- The modes behave as two independent lasers.

As current increases the power of each mode increases linearly

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

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

Frequency Response

Dynamics of Two Overlapping Transverse Modes

- 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

- The modes behave as “coupled” oscillators.

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

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

- 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

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

- The “Avoided Crossing” is illustrated again

B

C

D

E

F

G

H

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

Spectrally Resolved

Near Field

L - I Curve

Frequency Response

B

C

D

E

F

G

H

mm

20

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

Spectrally Resolved Near Field

Frequency Response

L - I Curve

- Lower dose A wider active area
- B , D , F are local minima on the L-I curve

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

- Array is defined using mirror patterning
- Triangular array - producing modes similar to the large area VCSEL

Modulation of a VCSEL Array - Experimental

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

B

C

D

E

mm

20

Array Modulation - Continue

Spectrally Resolved Near Field

Frequency Response

- 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)

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

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

CCD

microscope

Electrical

Pulser

x50

BS

CCD

VCSEL

Variable Attenuator

Fast

Sampling

Oscilloscope

Optical

Spectrum

Analyzer

BS

Pulsed Ti-Sa

Laser

Fast GaInAs Detector

Parasitic-Free Response Along the Current Pulse

- 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

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.

Selective Growth Over Openings in a Si3N4 Mask

SEM pictures of cleaved device’s facets

- 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

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

- Near field patterns:

- Spontaneous emission

- TEM30 lasing Mode

(2)

- TEM31 lasing Mode

(1)

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

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

- What will SRNF image revile at higher current levels ?

Transverse Modes During Pulsed Operation

SRNF Images

- 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

15mm

( I )

( II )

( III )

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

CW Operation of the Selective Grown VCSEL

- 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 ?

Theoretical Response

- The model described earlier was used.
- A 15m15m square current injection profile
- Modes TEM00 & TEM10 were assumed. (highly overlapping modes)

- Single Resonance Response

Experimental Response

18mA

20mA

- A single resonance response in accordance to theory
- Multi-transverse TEMm0 modes operation

Using the large signal response relation:

Carrier Life Time Measurement- Does the polycrystalline material induce shorter life time, due to traps at the periphery ?

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

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

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

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

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