K6 Optical Sources and Amplifierspeople.ee.ethz.ch/~fyuriy/oe/oe_optcom_chapters/...Basic concepts...

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Electronics Laboratory: Optoelectronics and Optical Communication 18.02.2009

6-1

diode laser- crystal

Heatsink (Diamant)

SOA

I

InP-LED, Light Emitting Diode with integrated lens

6 Optical Sources and Amplifiers 12/02/2009 ← Fiber Semiconductor Optical Amplifier with fiber coupling (SOA, courtesy OptoSpeed SA)

Diode Laser with salt crystal as a comparison

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6-2

Goals of the chapter: • Functional concepts, design and analysis of semiconductor based components for light

generation and amplification

• Light Emitting Diodes (LED) as incoherent sources based on spontaneous light emission

• Light Amplification by Stimulated Emission of Radiation (LASER) as a light oscillator and major current-modulated source of coherent mono-chromatic light from semiconductor pn-heterojunction diodes

• Semiconductor and Solid State Optical Amplifier (SOA) as a light amplifier with THz-bandwidth Methods for the solution:

• LED, Diode-Lasers and SOAs can be treated in the frame-work of the rate equation formalism • Calculation of static emission vers. current P-I of LED and LASERS and amplification characteristics gain

vers. current of SOAs • Small and large signal modulation and noise analysis models are developed

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6-3

6 Optical Sources and Amplifiers:

Schematic functional blocks and devices of a modern 40 Gb/s (4x10Gb/s) fiberoptic TDM Link:

data input

data output

80Gb/s MUX with InP/InGaAs-DHBT, I. Schnyder, IfE, ETHZ

waveguideUTC

SOAwaveguideUTC

SOA

2ps-pulse generating InGaAsP-diode laser, H-J., Lohe, R. Scoillo, H.J. Lohe IfE, ETHZ

PIN-Photo-diode

TransimpedanceAmp

9 μm

PIN-Photo-diode

TransimpedanceAmp

9 μm

• 40 Gb/s fiber-optics is commercial

and being deployed • 80-160 Gb/s TDM-systems are de-

monstrated for feasibility but the electronics is not yet commercial.

No fundamental bottle-neck. DRLM DRLM

Frequency-Phase Locked Loop

VCO

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6-4

lense

free-space wave

active media

Introduction:

Fiberoptic application depend on compact, fast and easy to modulate (10-40 Gb/s) nearly monochromatic light sources adapted to the optimal fiber transmission wavelength bands of 850nm , 1200 – 1700nm (IR=infra red).

with the realization of fast (GHz – several 10 GHz) optical sources, such as LEDs and in particular of diode lasers, operating at low current densities, the optoelectronic communication technology achieved a breakthrough in the 70-ties.

The basic achievement was the invention of the double heterojunction (DH) diode structure, providing the solution for a 1000-fold reduction of the operation and threshold currents of diode LASERs in the order of a few 10uA.

The introduction of the optical amplifier in the 90-ties, eliminating the electronic „bottle-neck“ for amplification and opened the path to extremely broadband, optically transparent networks.

Basic concepts of optical / electronic interaction in active optoelectronic devices:

a) Active planar (wafer plane) waveguides b) free-space and guided beam vertically emitting devices - Long interaction length, high intensity, small dimensions - short interaction length, weak intensity, larger dimensions - Current flow vertical to the light propagation - current flow co-linear to the light propagation

Pump current

Pump current

Light emission

t

Light emission

t

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6-5

Confinement of Electrons and Photons: Device Concepts Maximizing the interaction of electrons and photons in a device requires confining them into the same space (eg. active core of WG) at high density

- guiding photons by dielectric waveguides (WG, resonators) or free space optics requires optical confinement by dielectric contrasts

- localized injection or extraction of electrons into or from potential boxes in SC requires electrical carrier confinement by potential barriers

Active waveguides: the dielectric SC-WG contains the “active” (interacting) SC-material (often in the WG core)

charge carriers confined for high density by - potential barriers at heterojunction interfaces and - local electrical contacts

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6-6

Carrier Injection / Extraction Processes in pn-junctions

In optoelectronic devices the resonant interaction with photons generates e-h-pairs (absorption, PD) or destroys e-h-pairs by stimulated (gain, SOA, LASERs) or spontaneous emission (LED).

Generated or required e-h-pairs are extracted or supplied, resp. injected into the active WG material

Carrier Injection or Extraction is realized by forward- or backward biased pn-junctions:

Carrier extraction: reverse biased pn-junction (PD) Carrier injection: forward biased pn-junction (SOA, LED, LASER)

Strong electric field E in the depletion layer w transports by drift the photogenerated electron to the n-contact and holes to the p-contact

photogenerated “reverse” photocurrent Iph carrier density n,p → 0 in the depletion layer

Electrons (holes) diffuse from the n- and p-contact through the depletion layer w to the p(n)-contact where they recombine radiatively as minority carriers

injection current generates photon flow minority carrier density are high population inversion and optical gain

hv

e

h

hID

e

ehID

hv

e

h

p n p n

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6-7

Optoelectronic Device Requirements: LED, LASER and SOA

a) Fundamental requirements for Light Sources in fiberoptic applications:

1) Emission-wavelength at dispersion or attenuation minimum of optical glass fibers: λ=1.3 and 1.5μm (Eg~ 0.80, 0.93 eV) 2) Minimal spectral width (Fourier-limit) Δλ, resp. Δω~1/B (B=data rate) of the optical emission spectrum for minimal

pulse broadening and high data rates 3) Simple and efficient current-modulation of the light intensity or light frequency (phase) at high modulation-bandwidth,

high linearity and with low optical noise 4) Compact, efficient monolithic device realization and low operation power 5) Small emission area, resp. high spatial and temporal coherence of the light emission for diffraction limited focusing

down into the μm-sized core of optical fibers and adapted to the fiber NA 6) Small dynamic emission wavelength λ shift (frequency-chirp) during modulation for minimal dispersion in the fiber

• Diode laser fulfils all requirements almost ideally with the exception of 6). • LEDs fail in 2), partially 3), 5) and 6), LED are therefore only suited for short distances and MM-fibers

b) Requirements for Optical Amplifiers:

1) High gain per length at: λ=1.3 and 1.5μm 2) Low insertion losses 3) Efficient pumping and simple, compact pumping mechanism 4) High optical bandwidth and low optical noise 5) Low intermodulation between different wavelength channels

• EDFA (Erbium doped fiber amplifiers) fail 3)

• SOAs (semiconductor optical amplifiers) fail on 2) and 5)

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6-8

6.1 SC pn-Junction devices for light generation

Important material requirements for SC-materials for light generation / amplification are:

• Bandgap Eg sets emission at telecom wavelength of (0.8), 1.3, 1.55μm, resp. Eg~ (1.45), 0.93 - 0.80 eV, using (GaAs/AlGaAs) and InP/InGaAsP

• Direct optical transitions for gain (not necessary for absorption in photodetectors) • Optical Confinement in waveguides requires a strong dependence of refractive index n on the composition of the ternary /

quaternary compound SC crystal (high dielectric contrast Δn)

• Carrier Confinement in light emitters requires strong dependence of bandgap Eg on crystal composition and suitable band alignment and off-sets ΔEc, ΔEv for efficient Double Heterojunction (DH) diodes

• Long carrier lifetime for low threshold currents (low crystal defect density, long spont. lifetime) Generic InP-InGaAsP-InP-pin-diode DH-structure with a planar ridge-waveguide:

I

Popt front mirror

ridge waveguide optical farfield

Barrier ΔEc for electron confinement

Barrier ΔEv for hole confinement

Lateral Rib- Stripe- Waveguide Contact w ~3-5μm

Lateral optical and Current Confinement

Light field (mode)

Active layer

Bottom ohmic contact

Planar InP-InGaAsP-InP 3-layer waveguide

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6-9

Most optoelectronic devices must provide by geometrical and material design

1) 2 dimensional waveguide structure vertical and lateral optical confinement (Δnlateral, Δnvertical)

2) DH- and stripe-contact structure for vertical and lateral carrier confinement (ΔΕC, ΔΕV, lateral contact width w)

6.1.1 Material requirements for SC for optoelectonic devices

Direct optical Band-to-Band-transition

For a non-zero matrix element vc uru the wavevector k of the conduction band minima and the valence band maxima must be identical k-conservation rule .

Indirect SC (Si, Ge) Direct SC (III-V): GaAlAs, GaInAsP, GaN… II-VI: ZnSe)

Bild

k

For details on materials / fabrication technology of optoelectronic devices see literature [11,7].

electrons , e direct transition at k=0 (energy minimum/maximum for e and h) holes, h

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6-10

Material systems for specific mission-wavelength and bandgap Eg

Because the gain-maximum gmax(Emax) occurs slightly above bandgap Eg, the Emission Wavelength λ of a direct SC (for Laser, LED) is determined mainly by the Bandgap Eg. Eg determines also the absorption edge of the SC (for Photodiodes, Modulators)

The bandgap Eg is a function of crystal composition: GaAs : λ=0.85μm → Eg=1.45eV

InxGa1-xAs : λ=0.98μm → Eg=1.26eV GaxIn1-xAsyP1-y : λ=1.35μm → Eg=0.92eV : λ=1.55μm → Eg=0.80eV (AlxGa1-x)yIn1-yP : λ=0.65μm → Eg=1.90eV

GaN : λ=0.35μm → Eg=3.53eV

AlN : λ=0.35μm → Eg=3.53eV UV Visible IR Communication- FIR wavelengths

[ ]g 0 gE c / eVλ≅

The Bandgap Eg and electron affinity χ determine the also:

potential-barriers in conduction- and valence band ΔEC, ΔEV at the heterojunctions.

Direct Compound-SC-crystals for different wavelengths regions:

Emission wavelength of direct compound semiconductor material systems:

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6-11

Control of the refractive index n by the crystal composition

For the realization of 2D-dielectric waveguides in optoelectronic components use is made of the property of SCs, that the refractive index n together with the bandgap Eg can be changed spatially by modification of the material composition x,y by the material growth (epitaxy), etching processes and regrowth of material

Bandgap Eg and refractive index n of Ga1-xAlxAs and GaxIn1-xAsyP1-y versus composition x,y:

x, y = crystal compositions

x 1 xgroup Vgroup III

x 1 x y 1 y

group III group V

Al Ga As

Ga In As P

−

− −

Trend: n ~1/Eg

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6-12

Lattice matching (Gitteranpassung) for different crystal compositions:

To grow epitaxially layers of semiconductors with different n and Eg onto a monocrystalline substrate (eg. GaAs, InP, GaP) without crystal defects, the lattice constants a of the different layers and the substrate have to be matched closely (Δa/a< 10-4).

Bandgap versus lattice constant diagram for various ternary material systems:

GaAs-Substrate InP-Substrate (automatic matching) (concentration (x,y) dependent matching)

AlGaAs, Al-Variation

InGaAsP, P-Variation

InP (binary)

GaAs (binary)

InAs (binary) Ga0.47In0.53As (ternary)

In1-xGaxAsyP1-y

(quaternary)

InGaP (ternary)

Increasing Ga Increasing P

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6-13

Suitable Band-Offsets ΔEC, ΔEV for Carrier-Confinement in Hetero-Junction pn-Diodes

For the realization of an effective vertical Carrier-Confinement in the active layer (Eg2) of optoelectronic heterojunction-devices the following conditions for n and Eg must be fulfilled simultaneously:

g1 g3 g 2

1 3 2

1 ) E , E E and

2 ) n , n n

>

< 2: active layer, 1,3: cladding layer

But also the resulting band-barriers ΔEC (C-band) and ΔEV (V-band) must properly aligned and have large enough values ΔE >>6-7kT

Anderson-Model for single Heterojunction interface: (unbiased pn Heterojunction, equilibrium)

n1 , Eg1

n3 , Eg3

n2 , Eg2

ΔE12 ΔE23

InP GaAlAs InGaAs GaAs InP GaAlAs

P depletion layer w N

Band-offset ΔE depends on (no proof)

- bandgaps Eg and

- electron affinities χ:

( )

( ) 21 2

21

10

0

6V g

CE

see append x

E

i

;

E χ

χΔ = −Δ

Δ = −Δ + Δ

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6-14

active layer d d electron-barrier hole-barrier

Forward Bias: FQ,n FQ,p DE E V−

Degenerate n and p doping for reaching FQ,n C V FQ,pE E 0 und E E 0− > − > (eg. cladding layers for laser diodes)

The doping in the layers (in particular for SC-laser diodes) must be high enough to reach the

inversion requirement of Barnard-Durafough:

FQ,n FQ,p gE E E− >

Bandalignment in forward biased (VD>0) P-AlGaAs – i-GaAs – N-AlGaAs Double Heterojunction (DH)-Diode:

ID

d

Quasi-neutrality n=p (assumption)

eVD

ID

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6-15

6.2 Light Emitting Diodes, LEDs Forward biased Light Emitting Diodes (LED) are the simplest optoelectronic light source generating incoherent radiation from electron-hole recombination by spontaneous light emission in current pumped direct SC pn-junctions. As each injected e-h-pair generates ideally one photon the optical power Pout is proportional to the diode current ID.

Basic Properties:

- LEDs generate light by spontaneous emission of radiation in current pumped pn- or pin-junction diodes.

- the emission wavelength is determined by the bandgap Eg of the active layer

- the spontaneous radiation is temporal and spatial incoherent, unpolarized and broadband and can not be focused to the diffraction limit (only for MM-fibers)

- IR-LED are important low-cost, low power (~100μW) light sources for low bandwidth (~100MHz 1 GHz) applications with multimode-fibers over short distances

- Visible LEDs are important for highly efficient, long-life display and illumination applications

6.2.1 Spontaneous Light Emitter with current-pumped pn-Diodes

Concept of carrier injection in forward biased pn-junctions • Electrical carrier injection in pn-junction is technological simple, efficient and very fast.

• Spontaneous emission from injected e-h-pair recombination is the simplest form of light generation in SC.

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6-16

Electrical Pumping: Carrier Injection in pn-Homojunctions junctions

Drawback of the simple pn-Homojunction diode:

- the thickness d of the active injection volume is large, comparable to the minority carrier diffusion lengths d~(Lp+Ln)~2-4μm and the carrier densities remain low. - the bandgap Eg is constant for the whole pn-structure the light can be reabsorbed in cladding layers and substrate !

Injection volume V=dA where non-equilibrium minority carrier, resp. e-h-pair density is high

The e-h-pair injection rate or pump rate Rpump caused by the diode current ID is: Assuming all injected e-h-pairs recombine by photon emission the emitted isotropic total photon flux φ from the volume V is:

( )0 0

pump D

equilibrium e and h densities

Ad R I / e

n p n , p − −

=

= >> =

( ) ( )spont pump Dopt,internal D

Ad R Ad R I / e and

P hv hv / eI

φ

φ

= = =

= =

ID

A

d

hv

ep

eVD

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6-17

Carrier Confinement in Heterojunctions Diodes

• Potential barriers ΔEV and ΔEC confine the electron to a thickness d

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6-18

6.2.2 Light Emitting Diodes (LED):

Control of broadband emission wavelength by:

The bandgap of the active layer Eg, resp. material composition of the SC determine the emission wavelength

6.2.2.1 Planar structures of LEDs

Example: p-AlGaAs/i-GaAs/n-AlGaAs-DH-LED:

2-

80μm

electron-barrier ΔEc

Hole barrierΔEv

EFQ,C

EF,V

A

Current I

Light Popt

Band diagram for forward bias:

spontaneous e-h-pair-recombination

VD ~ Eg/e ≅(EFQ,n – EFQ,p)/e

Pout

Planar, lensed InP-LED:

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6-19

Advantages of LEDs:

1) relative small active (typ. 50x50x0.1μm) volume (GaAs) and transparent, nonabsorbing (AlGaAs-) cladding layers resulting in relative high quantum efficiency

2) cheap and simple planar device structure Drawbacks of LEDs:

1) broad emission spectrum, several 10nm high signal dispersion 2) modest modulation bandwidth < 1GHz (carrier lifetime limited) 3) low to moderate light emission efficiency, large emission area, not suited for SM-fibers large (50-100mA) operation currents

6.2.2.2 Rate-equation description and PI-Characteristics • Spontaneous e-h-pair recombination (Rspont) converts the electrical excitation energy by current pumping ID into light Pmax. • Pmax=(ηihv/e) ID • Stimulated emission (Rcv,net) and amplification is small because the photon density sph and gain-length ~d is small. • spontaneous emission occurs non-directional into all spatial modes (Lambertian radiation pattern).

The conduction- and valence-bands of the active volume Ad are considered as pumped “reservoirs” for electrons and holes with densities n and p.

The active region is assumed to be undoped such that n=p (quasi-neutrality)

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6-20

( )

pump 21,net spont

21,net spont spont

pump spont

D Di D i

spontspont

with the assumption of a constant spontaneous lifetime

V active volume and nV N number of electrons in V

with

0 R R R

R R n /

R R 0 dA V

I N I0 N I nVe e

τ

η η ττ

= = =

= + − −

= =

spontR

int

Ad)

int ernal quantum efficiency ~ 80 90%η = −

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6-21

φkrit

n2 ~1 (air)

n1~3.5 (SC)

total reflection angle:n2

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6-22

VD-ID-Characteristic of LEDs (voltage-current characteristic) Knowing n(ID)=p(ID) in the active volume, we can in principle determine the Voltage-Current-characteristic VD(ID) from the dependence of quasi-Fermi-levels EFQ,n(n) , EFQ,p(p) on the carrier density n,p: Concept: in non-equilibrium the external diode voltage VD must be equal to the the separation of the quasi-Fermi-levels. Procedure: n, p(ID) → EFQ,n(n) ; EFQ,p(p) → VD(ID)=EFQ,n(n)/e + EFQ,p(p)/e

( ) ( ) ( ) ( )

Dspont

D FQ,n FQ,p eff ,n 1/ 2 FQ,n eff ,n 1/ 2 FQ,p

1 1FQ,n C 1/ 2 V QF ,p 1/ 2

eff ,n eff ,p

In p quasi-neutralityV e

V E n E p with n N F E and p N F E

n nE E kT F , and analog E E kT FN N

τ

− −

= =

= − = =

⎛ ⎞ ⎛ ⎞− = − =⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟

⎝ ⎠ ⎝ ⎠

( ) ( ) ( )1 1D DD D 1 / 2 1 / 2 geff ,n eff ,p

n I n IV I kT F kT F E

N N− −⎛ ⎞ ⎛ ⎞= + +⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟

⎝ ⎠ ⎝ ⎠

V-I-characteristic (intern) ( + voltage drop ΔV across access resistance RS ΔV= ID Rs )

As the equation can not be solved analytically one often approximates the pn-heterojunction characteristic by an exponential:

D DD S D

S

V e kT II I exp 1 ideality factor V ln 1kT e I

ηηη

⎛ ⎞ ⎛ ⎞⎛ ⎞= − = = +⎜ ⎟ ⎜ ⎟⎜ ⎟

⎝ ⎠ ⎝ ⎠⎝ ⎠

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6-23

Spectral Properties:

• spontaneous emission represents “optical noise” (caused by random spontaneous emission processes) which is temporal and spatial incoherent (random)

• the lack of spatial coherence prevents focussing the radiation to the diffraction limit ( very small coupling efficiency to SM-fibers)

• the emission spectrum is relatively broad (typ. Δλ~100nm which is ~ 7% of the bandgap Eg) The large spectral width is determined by the current dependent band-filling of the C- and V-band by the injected electrons (n) and holes (p) (energy spread of the carriers)

The broadband spectrum is not ideal for low dispersion applications. Spectral broadening by band-filling and thermal spreading in LEDs:

EFQ,n EFQ,p

λmin λmaxλmin λmax

( )30 20E ~ kT ~ meV ~ nmΔ ≡

( )30 20E ~ kT ~ meV ~ nmΔ ≡

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6-24

Popt,out-ID-Characteristic of different long-wavelength LED with Fiber coupling:

LEDs produce spatially and temporally incoherent radiation from an area A>>λopt2, which can not be focused to the diffraction limit ~λopt.

Because the emission area A (φ ~10-100 μm) of LEDs is considerably larger than the core of Single-Mode (SM)-Fibers (φ~8μm), coupling also by optical means (eg. lenses) produces large coupling losses.

In additions LEDs behave almost as Lambertian-radiation with large beam angles (typ.~900), which is much larger than the

acceptance angle of fibers (typ. NA~0.1 –0.2)

Thermal heating

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6-25

6.2.3 LED-Device Design and Properties

6.2.2.1 Surface and Edge-emitting LEDs

Surface Emitting LEDs: Edge-Emitter LED:

(with planar waveguide and stripe contact)

• High optical power/area ratio • Small emission area, (~stripe contact width) • Waveguide with mirrors (avoiding lasing !) • Relative high fiber coupling efficiency • High modulation bandwidth (small

capacitance) • Light emission vertical to current flow

Direct MM-fiber butt-coupling

• Poor optical power/area ratio • Relative large emission area • Simple planar construction, no mirrors • Slow response and large junction

capacitance (external RC time constant) • Light-emission through the surface

and parallel to the current flow

I

Lens-MM-fiber coupling

LED with ball-lens for better fiber coupling by beam collimation (focusing)

NA

50-60μm

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6-26

6.2.2.2 Small Signal Equations and Modulation-Transfer function M(ω)

Concept: The modulated light flux is proportional to the modulated carrier density. Due to the finite carrier lifetime the carrier density can not follow the injection rate immediately.

The lightpower Popt of a LED is modulated by a current ID(t), in particular by an additional small harmonic current modulation Δ

j tDI e

ωΔ ( ) ( ) ( )j t j t j tD D D opt optI t I I e , P t P Pe , n t n neω ω ω= + Δ = + Δ = + Δ small signal assumption

Small signal Light-Current Modulation-Transfer function ( ) ( )opt ,outP

MI

ωω

Δ=

Δ:

From the rate equation we calculate only the time-dependent parts Δn(t) and the modulated optical power ΔPopt(t)=ηemV Δn(t)/τspont ħω

( )

pump 21,net spont

stim spont

Di opt em

spont spont

n R R R dA V V active volumet

R R and N nV total carrier number in V

I t N NN with P and jt e t

η η ω ωτ τ

∂= + − − ∗ = =

∂

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6-27

( )

pump spont

Di D

spont

Static part : 0 R n /

I Nj N N Ie

τ

ω ητ

= −

Δ ΔΔ = − → Δ Δ

sponti

D spont

opt ,outem i

D spon

3dB sp

t

ont1/N 1 LP withI e 1 j

P 1I e 1 j

ωτ

ηωτ

ωη ηωτ

τ−Δ

=Δ +

Δ=

Δ +

=

Lowpass Modulation-Transfercharacteristic LEDs show in general:

• modest bandwidth B (100- max. 1000 MHz), data rates ~100Mb/s • small optical power, a few 10-100μW in MM-fibers, short distances 1- ~100m only efficient coupling into MM-fibers with large diameters • large operation currents (50-150mA) and heating effects • large optical bandwidth (Δλ~50 - 100 nm) leading to dispersion • low cost in comparison to lasers

LEDs are interesting for low-cost, short-distance (m – 100m) links with MM-Fibers with modest bit-rates

The intrinsic (without the external RC-time constant due to the series resistance RS and the diffusion-capacitance CD) Bandwidth B=1/τspont of LEDs is carrier-lifetime limited and reaches values of a few 100 MHz.

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LED Applications

The emission wavelength of LEDs is determined by the bandgap Eg depending on the material composition. • Current LEDs cover the total spectral range LED-Illumination: from blue-to-red:

“White” LED = Blue LED + phosphorus converter

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LEDs have become very efficient emitters by eliminating reabsorption in the crystal (transparent substrates, back-side reflectors) and total reflection (structured surfaces).

Typical LED Applications:

LEDs are successful for display application and illumination at visible wavelengths

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6.3 Optical Amplifiers (semiconductor optical amplifier, SOA and Erbium doped fiber amplifier, EDFAs) During propagation in fibers lightwave signals suffer from reversible and irreversible distortions:

a) Reversible distortions: attenuation and dispersion signal can be restored by amplification or dispersion compensation

b) Accumulative, irreversible distortions: noise and random fluctuations, which can not be eliminated

Optoelectronic repeaters are used to restore (amplitude, pulse width and postion) the digital signal after a max. propagation distance (typ. 10-100km) L, before the signal is no longer recoverable with out substantial information loss (bit errors).

3R-Repeater Functions: Reamplification, Regenerate, Retime (today mostly electronically)

optical electronic optical

O/E-Converter (PD)/amplifier Electronic (Regeneration) E/O-Converter (LED/LASER/moduator)

In communication systems of 1. - 3. generation optical signals had to be converted into electrical signals and processed by an electronic 3-R-repeater before being retransmitted in the optical domain again.

For data rates >40 Gb/s the electronic circuits became a severe bottle-neck of bandwidth.

A breakthrough was the transition to:

amplification in the optical domain by optical amplifiers (OA) optical amplification with THz-bandwidth (parallel or serial format) is possible

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6.3.1 Concepts of SC and Fiber Optical Amplifiers (SOAs and EDFAs) Generic requirements for OAs:

• High optical gain (20-50bB) • Low insertion losses • High optical bandwidth (10 – 50nm, ~2-10 THz) • High output power for power OA • Polarization independent gain • Lowest noise figure F (4-5dB) for preamp-OA

1) Semiconductor Optical Amplifiers

SOAs are devices performing a coherent amplification with THz bandwidth of an optical wave at frequency ω.

SOA are based on the process of current or optical pumped stimulated emission of radiation R21,net(ω,n).

The optical device gain G(ω) is a complex value an d the amplification is not noise-less (amplified spontaneous emission).

SOAs are key for:

- low-noise preamplifiers (F~6dB) with THz-bandwidth for optical signal amplification (no electronic bottle-neck !) - power amplifiers in fiberoptic signal distribution (Pout ~ 15dBm) - LASERs (Light Amplification by Stimulated Emission of Radiation) for optical wave generation

However signal regeneration and retiming is still done electronically doday.

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Active current-pumped WGs

For strong interaction between photons and e/h pairs SOAs are often realized as current pumped active wave guides.

- active layer provides optical gain by stimulated emission

- active layer forms part of a current pumped pn-junction - active layer provides part of the high refractive index core of a planar waveguide structure

Concept and Realization of SOAs: Input Pin (r

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Advantages SOAs: small (mm) size, efficient current pumping, high gain, low cost

Drawbacks SOAs: high fiber-SOA coupling loss (3-5dB), “high” noise figure F~6dB, polarization sensitivity of G

Waveguides and optical confinement (opt. confinement factor Γ) Active WG consist of a high index (ncore=n1) active core with the lowest bandgap defining the wavelength of the gain G(λ) and low-index (nclad=n2,n3), transparent high bandgap top, bottom and lateral cladding layers.

Only a fraction Γ is confined to the active layer and amplified as G=eΓL.

confinement factor Γ

22 2

2

1d /

active

mod ed /

AE dx / E dxA

+ +∞

− −∞

Γ = ≤∫ ∫

Aactive, n1

n2

dn3 n3

wn2 Amode

Aactive, n1

n2

dn3 n3

wn2 Amode

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2) Fiber Optical Amplifiers (eg. Er-doped Fiber Amplifier, EDFA at 1.55μm)

FOA consist of 10 – 100m of optical glass fiber, containing active Erbium-ions in its core. Er+-ions form a 3-level system which is optically pumped by a diode pump-lasers producing with ~100-200 mW pump power at 980 or 1480 nm. Advantage of EDFA: 1) high gain G~40dB, 2) good fiber-coupling 3) low cross-talk between WDM-channels, 4) low noise.

Drawbacks of EDFA: 1) expensive and large size, 2) low electrical efficiency.

Schematic EDFA: Gain ≈ 40dB at 1.55 μm, bandwidth ≈ 5nm Erbium-doped Fiber:

InGaAs/AlGaAs Pump-Diode Lasers: λ= 980 or 1480nm Popt ~100-200mW

coupler: λ= 980 /1550nm

Energy-levels of Er-ions in Glass (3-level system):

980nm 1550nm Pump gain transition

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6.3.2 Optical material gain g in SCs vs. carrier density n and pump-current ID

The small-signal material gain g(ω,n) in SC is often approximated to first order as a linear gain function with a “parabolic” frequency dependence.

Practically the confinement factor Γ reduces the effective optical gain further

geff=gΓ. Linear small-signal gain g

( )( )

( )

L

0

g n z dzg n L

homogeneouspumping

G e e∫

= =

Non-linear, saturated gain g

In SC the optical gain g(n,ω,P) at high power levels depends for large optical P also on the power itself mainly due to the carrier depletion processes by R21,net > Rspont, resp. P >Psat (saturation power as parameter) Gain Satturation

( ) ( )( )

( ) ( ) ( )

( )

sat

s

tr 2 20

max 0 trsat

assumption

tr

tr

at s

1 1 Pg ,n a n nP z1 / P /

1g n,P g ,n a n n1 P / P

optical ban

P

P P saturation power ;dwidth , a n n unsaturated gainconfinement factor , n transparency carrier den

ωω ω ω

ω ω

ω

∂Γ ≅ Γ − =

∂+ − Δ +

Γ = Γ = ≅ Γ −+

Δ = − =

Γ =

= =

= sity

g(ω,n)gmax(n)

2Δω

n

ωo ω

gmax(n,P)P/Psat=0)

P/Psat

ntr n

g(ω,n)gmax(n)

2Δω

n

ωo ω

g(ω,n)gmax(n)

2Δω

n

ωo ω

gmax(n,P)P/Psat=0)

P/Psat

ntr n

gmax(n,P)P/Psat=0)

P/Psat

ntr n

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6-36

( ) ( )

( ) ( )

( ) ( ) ( ) ( )

D spont0 0 0 tr tr

in out

D spont0 tr

g z optical power P z

I 1g ,n ,z a n n a n PeLwd P z

boundary conditions : P 0 P , P L P

IP ,z P 0 e P 0 exp a n z ;

eLwd

with z L

τω

τω Γ

⎛ ⎞ ∂Γ ≅ Γ − = Γ − =⎜ ⎟ ∂⎝ ⎠

= = →

⎛ ⎞⎛ ⎞= = Γ −⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠=

( ) D spontout0 0 trin

IPG ,I exp a n LP eLwd

τω

⎛ ⎞⎛ ⎞= = Γ −⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠

6.3.2.1 Rate-equation description of SOAs

Goal: determine the small signal (P

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6.3.2.2 Gain Saturation in SOAs (large signal gain)

unsaturated device gain G0

Including the local carrier depletion n(z)PSat as function of the SOA length L If g is constant we obtain the result of chap.4.3.4

If P(z) > Psat then n(z)

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Observe: 1) n0 is the carrier density without saturation or carrier depletion (small signal) 2) in the above expression for G(ω0, n0), the local power P(z), the boundary values at z=0 and L Pin and Pout are contained implicitly in the integral and can not be expressed analytically.

For the solution we eliminate g to obtain the saturated Gain G=Pout/Pin at ω=ω0:

( ) ( )( ) ( ) ( )( )

( )

( )out

in

00

sat sat

P out in out out in0 0 P

sat sat in sat

PL outdz dP

0 Pin

we take the exponential func

P zP 1 gfrom the definition of g P = P g P z P P z P g zz P z 1 P z / P 1 P z / P

1 1 P P P P Pg z P g L ln P lnP z P P P P

=∫ ∫=

∂ ∂→ = = → ∂ = ∂ →

∂ ∂ + +

⎡ ⎤ − −∂ = + ∂ ⎯⎯⎯⎯⎯⎯→ = + = +⎢ ⎥

⎣ ⎦

in

outout in out outsat

0 sat out sat sat

out in

P1P 1

P P P P1 1 G 1P 1g L P P G P G Pout out out out

0 0using the definitionin in in inG=P / P

tion of both sides of the above equation:

P P P PG e e e e e ; G unsaturatedP P P P

−

− −⎛ ⎞ ⎛ ⎞−⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠= = ⋅ = ⋅ = ⋅ = ⋅ = gain

( ) ( )

( )

out outout out in 0 0

sat sat

0 0 satwe get an implicite equation for G, with

G 1 P G 1 Pwith G L,P P / P exp g L / exp G / exp ; G saturated gainG P G P

G exp g L = small signal gain (P

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6-39

( )0 in insat

G Pln G implicit equation for saturated gain G PG P

⎛ ⎞ ≅ ⇒⎜ ⎟⎝ ⎠

( )

( ) ( ) ( )

out,sat

0 out ,sat 0

out ,sat 0 0 0

We are looking for the saturated output-power P where the amplification G of the whole amplifiers is reduced

to half of the small-signal-gain G : G P G / 2 :

1G P G / 2 exp g L exp g L2

=

= = = ( )

( )

out ,sat out ,sat00

sat 0 sat

ln out ,sat0

0 sat

0out ,sat sat

0

out ,sat satG 1o

P PG 1 G / 2 1/ exp exp g L / expG P G / 2 P

PG 2ln2G P

G ln2P P Output Saturation Power of the amplifiersG 2

P ln2 P high gain OA>>

⎛ ⎞ ⎛ ⎞− −=⎜ ⎟ ⎜ ⎟

⎝ ⎠ ⎝ ⎠−

⎯⎯→ =

= −−

⎯⎯⎯⎯→ =

Saturation of the output power as a function of unsaturated Gain Go and Psat As derived before the implicit eq. for G(Pin) is:

( )0 in insat

G Pln G G PG P

⎛ ⎞ ≅ →⎜ ⎟⎝ ⎠

Approximated saturated Gain G as a function of the input power (implicit equation)

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6-40

From numerical simulation of the implicit equation for G:

G as a function of Pout: Gain-Reduction ΔG=G0-G as a function of Pin: To obtain a high saturated output power Pout,sat of an amplifier, we need a high local saturation power Psat. Pout,sat is almost independent of the small signal gain G0. Remark: We will see that gain saturation is important also in Lasers diodes at the lasing threshold, for amplitude stabilization (nonlinearity) and for the spectral characteristics (see chap.4.4)

Pout ≈ PS G0/2

Pout/PSATPin/PSAT

~PSAT

Pout ≈ PS G0/2

Pout/PSATPin/PSAT

~PSAT

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6.3.2.3 Gain Saturation by Carrier depletion (self-study)

Homogenous broadening is assumed, so total (distributed in energy) carrier density n is depleted by a single discrete transition.

In the following we determine the saturation power Psat caused by the reduction of the carrier density n(z)=n(P(z)) < n0 originating from the non-negligible local stimulated emission rate R21,stim(z). We also prove that gain

saturation can be represented as assumed as ( ) sat1

1 P z / P+

(We consider only the static case and not dynamic gain saturation for short optical pulses).

( )

( )

21,net ph gr 21,net opt 21,net mod e opt

opt mode

21,net opt opt mode

/ t 0 stationary and using the relation g R / s v R / I R A / P

I intensity P / A

R g I / g P / A

ω ω

ω ω

∂ ∂ = = = =

= =

→ = =

( ) optDpump stim spont tr 0mod e spont

opt

Rate eq. including stimulated emissionPI nn R R R a n n 0 with n n

t eLwd A

we assume that L is so small that P does not change in the active volume:

multiplication wit

ω τ∂

= + − − = + − − − = <∂

→ ( )

( )D tr 0mode spont

D spontDtr 0

mode mode spont

h the active volume .... V .... LwdI P Na N N 0 N saturated carrier number ; N unsaturated carrier numbere A

II P P 1N aN / a Ne A A e

ω τ

τω ω τ

=

− − − = →

⎛ ⎞⎛ ⎞= + + < = →⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

i i

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For P=0 we obtain and P=∞ N=Ntr. Insertion of N(P,I) in g(P) leads to:

0 spontN N I / eτ= =

( ) ( ) Dtr tr tr trmode mode spont

D trtr tr

mode mode spont mode spont

D tr

spont mode

1 1 I P P 1g a n n a N N a aN / a NV V e A A

1 I P P N P 1a aN aN / aV e A A A

1 I N Pa / aV e A

ω ω τ

ω ω τ ω τ

τ

⎛ ⎞⎛ ⎞⎛ ⎞= − = − = + + − =⎜ ⎟⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟⎝ ⎠ ⎝ ⎠⎝ ⎠

⎛ ⎞⎛ ⎞ ⎛ ⎞= + − − +⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟⎝ ⎠ ⎝ ⎠⎝ ⎠

⎛ ⎞= −⎜ ⎟⎜ ⎟

⎝ ⎠

( ) ( )

0

D spont sponttr

spont mode

spont0 0 o tr

mode sat sat

N

I P1 1 a N / a 1V e A

a P Pg P g / 1 P g / 1 a n n / 1 qed.A P P

Reduction of g by reduction of the

τ τω τ ω

τω

⎛ ⎞⎛ ⎞⎛ ⎞ ⎜ ⎟⎛ ⎞ ⎜ ⎟ ⎛ ⎞

+ = − + =⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟ ⎝ ⎠⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠ ⎜ ⎟⎝ ⎠⎝ ⎠⎛ ⎞ ⎛ ⎞ ⎛ ⎞

= + = + = − +⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠

carrier density n

modesat

spont

AP Saturation Energya

ωτ

→ = Saturation Energy is a material parameter

Amplifiers with high saturation power require:• large mode crossection • short carrier lifetime • small differential gain

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6.3.4.2 Bandwidth reduction as a function of gain

Similar to electronic amplifiers, SOAs also decrease their device gain bandwidth ΔΩ with increasing total linear device gain G in comparison to the material gain g bandwidth Δω. ( ) ( )

( )

( )( )

( )

( ) ( )

spont0 0 tr o2 2

0

0

D spontout0 D tr 2 2

in 0

spont0

I1 1g ,n a n n P with nP z eLwd1 /

spectral total gain G :

IP 1G ,I exp a n LP eLwd 1 /

3dB Bandwidth 2 L at amplifier exit:

IG , I 1G , I exp a2 2 eL

τω

ω ω ω

τω

ω ω ω

τω

∂≅ − = =

∂+ − Δ

→

⎛ ⎞⎛ ⎞= = −⎜ ⎟⎜ ⎟⎜ ⎟+ − Δ⎝ ⎠⎝ ⎠

− ΔΩ

±ΔΩ = =( )

sponttr tr 2 2

I 1n L exp a n L solve for wd eLwd 1 /

τ

ω

⎛ ⎞⎛ ⎞⎛ ⎞ ⎛ ⎞− = − → ΔΩ⎜ ⎟⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟+ ΔΩ Δ⎝ ⎠ ⎝ ⎠⎝ ⎠ ⎝ ⎠

( )( )

( ) ( )

( )( )

( ) ( )

( )

0 0 2 2

0 0 02 22 2

2 22 22 2

0 02 2

0 0

1 1exp g L exp g L2 1 /

1 1 1 1ln g L g L ln ln2 g L 12 21 / 1 /

/ln2 g L ln2 ln2 / g L /

1 /

L ln2 ln2ln2 g L G ln2

ω

ω ω

ωω ω

ω

ω

⎛ ⎞= ⎜ ⎟⎜ ⎟+ ΔΩ Δ⎝ ⎠

⎛ ⎞+ = → = − = − =⎜ ⎟⎜ ⎟+ ΔΩ Δ + ΔΩ Δ⎝ ⎠

⎛ ⎞ΔΩ Δ− = − → − − ΔΩ Δ = − ΔΩ Δ⎜ ⎟⎜ ⎟+ ΔΩ Δ⎝ ⎠

ΔΩ=

Δ − + −

G(ω) g(ω)

SOA-Gain G(ω)

Gmax(ω0)

2ΔΩ

2Δωmaterial-Gain

g(ω)

G(ω) g(ω)

SOA-Gain G(ω)

Gmax(ω0)

2ΔΩ

2Δωmaterial-Gain

g(ω)

SOA-Gain G(ω)

Gmax(ω0)

2ΔΩ

2Δωmaterial-Gain

g(ω)

Bandwidth reduction vs. gain G

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6.3.2.5 Nonidealities of SOAs (self-study)

Polarization dependence of gain: Because the WG are 1) not symmetric and 2) because the optical matrix elements in Quantum Wells is anisotropic the gain depends on polarisation (gTM, gTE). Undesired because standard optical fibers Do not maintain polarization → gain fluctuations in the SOA.

Noise Properties of SOAs:

Noise in SOAs is caused by spontaneous emission PASE, resp. amplified spontaneous emission (ASE) which is superposed to the coherently amplified signal beam Pout=GPin, resp. optical signal mode. Spontaneous emission is optical noise and not directional and can be partially filtered out, except for the part emitted in the spatial angle of the amplified beam. Optical SOAs have much lower noise Figures F~6dB than most electronic high frequency amplifiers. G

Amplified spontaneous emission

PASE(z)PASE(L)

Pout(L) Pin(0)

PASE(L)

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SOA with Noise and Signal detection by a PD (optical square law detector)

Photodiode PD: i~E2 ~Popt

The optical ASE random field in a bandwidth Δω is mixed in the “square law” detection process of the PD with the signal field and with itself to produce the photocurrent containing “down-mixed” random noise (beat noise) + the intrinsic detection shot-noise:

As a measure of signal quality we introduce the Signal/Noise Power Ratio S/N, (SNR) after detection in the current i(t) of the PD: Time domain i(t):

iS(t) =signal-photocurrent, harmonic intensity modulation at ωs

in(t) = noise-current resulting from mixing in PD:

1.) Shot-noise of the photodetection , 2.) Interference- or „Beat“-noise of ASE and ASE 3.) interference of signal and ASE

Frequency domain for i(t) PSD Si(f):

( ) ( ) ( ) ( ) ( ) ( ) ( )i s n s shot ASE / Signal ASE / ASESIGNAL NOISE

S f S f S f S f S f S f S f= + = + + +

Pin

Amode

dVol

Pout

i(t)

in,PD(t) Rauschen

iph(t)

APD

θ Oeffnungswinkel, Mode

Pspont, mode

S/N ?

verstärkteSignalwelle

verstärkteSpontanemissions-Welle

i(t)=is(t) + in(t) signal noise

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6-46

Noise Figure F is then defined as usual for the conversion: optical input current output (SOA+PD)

Definition of SNR of the source: (input-noise) SNR at the SOA output:

( ) ( )in S ,in n,inB B

Def : SNR S f df / S f df= ∫ ∫ ( ) ( )out S ,out n,outB B

Def : SNR S f df / S f df= ∫ ∫

The Noise Figure F of the SOA is defined as the ration of the output SNR to the input SNR (after detection by a square-law PD):

1SNRSNRF

out

in ≥=

Ps,out = G Ps,in Pn,out > Pn,in G + PASE

( ) ( )( ) ( )sps,in s,in ASE / Sin outout out,quelle s,in

2 G 1 n2eRGP B 4GRP RS B 2 G 1SNR N 1 1F .SNR N G G G G2eRP B

−+ −= = = ≅ <

It is obvious that the noise figure F is always ≥2 (3dB), because nsp>1. For low F operate the SOA at high G !

Summary: Optical Amplifiers

• OA allow a direct optical signal amplification with THz-band width Bopt • OA combined with an optical feedback mechanism are the basic building blocks for LASER-oscillators • Increasing pump current increases the gain G and the noise figure F but decreases the optical bandwidth ΔΩ • SOAs and EDFAs become nonlinear at high output powers resulting in a gain-saturation • The spontaneous emission in OA (and LASERs) is the fundamental source of noise in optical amplification

Ps,in Ps,out=G Ps,in Signal SOA, G Pn,in Pn,out=G Pn,in + Pn,SOA Noise

Internal noise

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6.4 Optical LASER Oscillators and Laser diodes

LASER (Light Amplification by Stimulated Emission of Radiation)

Light waves at ~200 THz can not be amplified by transistor electronics based on free carrier transport because transit frequencies fT are

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6.4.1 Analogy: electronic and optical oscillators (coherent light wave generator)

A laser is a generator of almost coherent light waves by stimulated emission.

It converts for electrically pumped diode laser DC-electrical power from the pump-mechanism into optical AC-power with a frequency of several 100 THz.

Analogous to electronic oscillator LASERs comprise an optical amplifier (SOA) and a feedback structure (DFB, FP). In contrast to electronic oscillators the LASER contains: - a SOA is a bidirectional amplifier based on quantum-mechanical transitions between excited electron states - a feedback structure providing attenuation and phase shift by distributed optical resonators L>>λ

Electronic Transmission-line Oscillator: • Non-inverting amplifier, matched at its input to the TL Rin=Z0 • the feed back element is a transmission line resonator:

L= resonator length γ(ω) =α + jβ = propagation constant of transmission line Γ = γL G(ω)=IGI=Vo/Vin = amplifier gain

For a self-consistent input – output relation (self-reproducing)

( ) ( ) ( ) ( ) ( )0 0 1iRoundtrip Gain

V G V G V Gω ω β ω ω β ω−

= = → =

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Multi-Frequency Traveling Wave Oscillators (Ring-Laser): feedback by transmission line

( ) ( )

( )( ) ( ) ( )

( )

0

0 2

2

1

2 1 2

2 1 2

ph

L jph

L

j L / vj nph

phres ,n

with j / j / v ; e and G G e

Amplitude condition : G e and

Phase condition : G e e e L / v n ; n , ...

vOscillation frequencies : n ; n , ... multiple resonances

L

γ

α

ω π

γ α π λ α ω β ω ω

ω

ω β ω ω π

πω

− −

−

−− −

= + = + = =

=

= = → = =

= =

G(ω) SOA

Mirror Mirror

Mirror

Mirrorsemi-transparent

opt. Isolator

L, ΓL, Γ

Z0

Vi V0

Rin=Z0 Rout=0

ΔΩ

Iμβ(ω)I

arg(μβ(ω)) φ

ω

ω

amplifier

transmission line

opt. feedback

μ:

β:

−π

−2π

ωres,1 ωres,2

Δωres=2πvph/L

1

Multi-Frequency Oscillation are possible !

(without frequency selective element if the bandwidth ΔΩ of G>>(Δωr))

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Amplitude stabilization by non-linear gain, gain-saturation of the amplifier:

• Because the amplifiers are assumed to be linear, the amplitude V0 drops from the oscillation condition and is undefined. • To define the amplitude V0 the oscillator needs a kind of nonlinearity (very often the nonlinear amplifier gain G). • Gain saturation in the SOA of the LASER leads to an amplitude dependent G(V0,ω) resulting in an amplitude condition for V0:

( ) ( )L L0 0 0G V e 1 G V e amplitude condition equation for Vα α− = → ≥ → nonlinear Standing Wave Distributed Oscillator: Leads to a very similar oscillation condition, but the field is not a rotating filed but a standing wave as a sum of 2 counter-propagating waves. The configuration represents the standard Fabry-Perot Resonator LASER (exception VCSEL), Reflector Bidirectional Gain Resonator-Transmissionline Reflector R1 Element G R2 Z0, L, Γ

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Laser Oscillator: (standing wave Fabry-Perot Resonator)

The Laser consists of: An optical amplifier SOA (eg. a ridge WG DH-Junction) inside a Fabry-Perot (FP) feedback Resonator (with cleaved or etched facets):

Operation Principle: A stationary optical standing wave between the mirror is possible if the the gain and mirror losses just cancel, resp. the round-trip gain is 1 and the field reproduce itself in amplitude and phase.

The FP-resonator consists of 2 semi-transparent, parallel mirrors with power reflections R1(2)=r1(2) r1(2)* and transmission T1(2)=1- R1(2) separated by a distance L . We assume propagation losses αWG and bidirectional gain g in the resonator between the mirrors

E+

E-

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For self-consistent field relations at the mirrors z=0 and L using k=2π/λ: (rotating field must reproduce itself after one roundtrip)

Round-trip gain condition Transmitted left Transmitted right Propagating wave E - Propagating wave E +

2-2

0 L zr1 g, αWG r2

E+(0)

E-(0)

r1 r2

E-(L)

E+(L)

0 L zr1 g, αWG r2

E+(0)

E-(0)

r1 r2

E-(L)

E+(L)

( ) ( ) ( ) ( ) ( )( )( ) ( ) ( )

( ) ( ) ( ) ( ) ( )( )

( ) ( ) ( )

21 1

21 2

2 21 2

21 2

0 0

0

1 1

WG

WG

WG WG

WG

g L/ jk L

g / jk L

g / g /jk L jk L

g j k L

E r E r E L e e

re e r E L

r r e e E e e

r r e e round trip gain

α ω

α ω

α αω ω

α ω

− −+ − −

− − +

− −− −+

− −

= = =

= =

=

= − =

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6.4.2 Generic Structures of Diode Lasers

6.4.2.1 Requirements and basic building blocks of diode lasers:

The LASER with a Fabry-Perot Resonator (2 parallel semitransparent mirrors (r) separated by the distance (L) represents a standing wave oscillator in the optical domain: • SOA, G(I) • Wave guide FP-resonator with mirror-separation L and opt. propagation constant Γ • Reflection coefficient of the mirrors r1, r2 (field) resp. R1, R2 (power)

Generic functions:

PIN-Double-Hetero-diode-structure for efficient Current-Injection of electrons and holes into the thin active layer d

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x

y

L

w

d

current z

Maximum spatial overlap between waveguide mode and current pumped active layer (large confinement factor Γ∼1, geff=Γg)

Small contact resistance Rs and low ohmic losses

Minimal thermal resistance Rth for efficient cooling of the active layer Laser Structures (epitaxial and by etching) realize

lateral carrier- and optical confinement in many ways:

Vertical Carrier (optical) Confinement is mainly realized because the bandgap Eg,clad (nclad) of the cladding layers (eg. AlxGa1-xAs) is larger (smaller) than Eg,active (nactive) of the active layer (eg. GaAs) → crystal composition (eg. Al-content x) Buried Heterostructure Laser Diode as a generic realization of the ideal SC-Diode Laser:

Vertical Carrier-Confinement

AlGaAs-GaAs-AlGaAs-HD-structure

Lateral Carrier-Confinement GaAs-AlGaAs-Diode-Isolation

Vertical and lateral optical Confinement AlGaAs-embedding of the active GaAs-wave guide The high index GaAs active layer and waveguide core is completely surrounded by low-index AlGaAS Front mirror (cleaved, etched)

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Examples of different lateral optical and carrier confinement:

During the past 3 decades many laser diode structures emerged for different material systems and growth techniques:

ridge waveguide

rib waveguide

etched mesa buried DH Double-channel planar buried DH

Planar buried DH strip buried DH

K6 Optical Sources and Amplifierspeople.ee.ethz.ch/~fyuriy/oe/oe_optcom_chapters/...Basic concepts...

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Transcript of K6 Optical Sources and Amplifierspeople.ee.ethz.ch/~fyuriy/oe/oe_optcom_chapters/...Basic concepts...