Light Sources – IIThe Laser and External Modulation

EE 8114-Xavier Fernando

A better light source

LED has:– Large line width (large material dispersion)– Large beam width (low coupling to the fiber)– Low output power – Spontaneous emission (random polarization,

phase, direction etc.)A better light source addressing all these issues

were needed.– The Laser is designed to address all these issues

The LASERLight Amplification by ‘Stimulated Emission’ and Radiation

• Laser is an optical oscillator. It comprises a resonant optical amplifier whose output is fed back into its input with matching phase. Any oscillator contains:

1- An amplifier (with gain-saturation mechanism) 2- A positive feedback system 3- A frequency selection mechanism 4- An output coupling scheme

Fundamental Lasing Operation

• Absorption: An atom in the ground state might absorb a photon emitted by another atom, thus making a transition to an excited state.

• Spontaneous Emission: random emission of a photon, which enables the atom to relax to the ground state.

• Stimulated Emission: An atom in an excited state might be stimulated to emit a photon by another incident photon.

Spontaneous & Stimulated Emissions

LASER

• In laser, the light amplifier is the pumped active medium (biased semiconductor region) where emitted photons stimulate more photon emission.

• Feedback is obtained by placing some kind of reflector (mirror/filter) in the optical resonator.

• Frequency selection is achieved by the resonators, which admits only certain modes.

• Output coupling is accomplished by making one of the resonator mirrors partially transmitting.

Lasing in a pumped active medium• In thermal equilibrium the stimulated emission is

essentially negligible, since the density of electrons in the excited state is very small. This is LED like operation with mostly spontaneous emission.

• Stimulated emission will exceed absorption only if the population of the excited states is greater than that of the ground state. This condition is known as Population Inversion. Population inversion is achieved by various pumping techniques.

• In a semiconductor laser, population inversion is accomplished by injecting electrons into the material to fill the lower energy states of the conduction band.

How a Laser Works

In Stimulated Emission incident and stimulated photons will have

Attribute Result

Identical Energy Narrow line width

Identical Direction Narrow beam width

Identical Phase Temporal Coherence

Identical Polarization Coherently polarized light

Fabry-Perot Laser (resonator) cavity

Fabry-Perot Resonator

[4-18]

R: reflectance of the optical intensity, k: optical wavenumber

1,2,3,.. :modesResonant mmkL

A

BL

M1 M2 m = 1

m = 2

m = 8

Relative intensity

m

m m + 1m - 1

(a) (b) (c)

R ~ 0.4R ~ 0.81 f

Schematic illustration of the Fabry-Perot optical cavity and its properties. (a) Reflectedwaves interfere. (b) Only standing EM waves, modes, of certain wavelengths are allowedin the cavity. (c) Intensity vs. frequency for various modes. R is mirror reflectance andlower R means higher loss from the cavity.

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Fabry-Perot Lasing CavityA Fabry-Perot cavity consists of two flat, partially reflecting mirrors that establish a strong longitudinal optical oscillator feedback mechanism, thereby creating a light-emitting function.

The distance between the adjacent peaks of the resonant wavelengths in a Fabry-Perot cavity is the modal separation. If L is the distance between the reflecting mirrors & the refractive index is n, then at a peak wavelength λ the MS is given by

nL2 Separation Modal

2

Laser Diode Characteristics• Nanosecond & even picosecond response time (GHz BW)• Spectral width of the order of nm or less• High output power (tens of mW)• Narrow beam (good coupling to single mode fibers)

• Laser diodes have three distinct radiation modes namely, longitudinal, lateral and transverse modes.

• In laser diodes, end mirrors provide strong optical feedback in longitudinal direction, so by roughening the edges and cleaving the facets, the radiation can be achieved in longitudinal direction rather than lateral direction.

Laser Operation & Lasing Condition• To determine the lasing condition and resonant frequencies, we

should focus on the optical wave propagation along the longitudinal direction, z-axis. The optical field intensity, I, can be written as:

• Lasing is the condition at which light amplification becomes possible by virtue of population inversion. Then, stimulated emission rate into a given EM mode is proportional to the intensity of the optical radiation in that mode. In this case, the loss and gain of the optical field in the optical path determine the lasing condition.

• The radiation intensity of a photon at energy varies exponentially with a distance z amplified by factor g, and attenuated by factor according to the following relationship:

)()(),( ztjezItzI

h

zhhgIzI )()(exp)0()( [4-20]

1R 2R

Z=0 Z=L

)2()()(exp)0()2( 21 LhhgRRILI [4-21]

2

21

21 t,coefficien absorption effective :

tcoefficiengain :g factor,t confinemen Optical :

nnnnRα

1n

2n

Lasing Conditions:

1)2exp()0()2(

Lj

ILI

[4-22]

Threshold gain & current density

21

1ln21

RRLgth

thgg :iff lase"" tostartsLaser

For laser structure with strong carrier confinement, the threshold current Density for stimulated emission can be well approximated by:

thth Jg

onconstructi device specificon dependsconstant :

Laser Resonant Frequencies• Lasing condition, namely eq. [4-22]:

• Assuming the resonant frequency of the mth mode is:

,...3,2,1 ,2L2 1)2exp( mmLj

n2

1,2,3,... 2

mLn

mcm

LnLnc

mm 22

2

1

Spectrum from a Laser Diode

widthspectral: 2

)(exp)0()( 20

gg

Semiconductor laser rate equations• Rate equations relate the optical output power, or # of photons per unit

volume, , to the diode drive current or # of injected electrons per unit volume, n. For active (carrier confinement) region of depth d, the rate equations are:

emission stimulatedionrecombinat sspontaneouinjectionrateelectron

lossphoton emission sspontaneouemission stimulatedratePhoton

CnnqdJ

dtdn

RCndtd

sp

phsp

densitycurrent Injection

timelifephoton

mode lasing theintoemission sspontaneou of rate

process absorption &emission optical theofintensity theexpressingt Coefficien

:

:

: :

J

RC

ph

sp

Threshold current Density & excess electron density

• At the threshold of lasing:

• The threshold current needed to maintain a steady state threshold concentration of the excess electron, is found from electron rate equation under steady state condition dn/dt=0 when the laser is just about to lase:

0 ,0/ ,0 spRdtd

thph

ph nC

nCn

10/ 25]-[4 eq. from

sp

thth

sp

thth nqdJnqdJ

0

Laser operation beyond the threshold

• The solution of the rate equations [4-25] gives the steady state photon density, resulting from stimulated emission and spontaneous emission as follows:

thJJ

spphthph

s RJJqd

)(

External quantum efficiency

• Number of photons emitted per radiative electron-hole pair recombination above threshold, gives us the external quantum efficiency.

• Note that:

)mA()mW(]m[8065.0

)(

dIdP

dIdP

Eq

gg

g

th

thiext

%40%15 %;70%60 exti

Laser P-I Characteristics (Static)

Threshold Current

External Efficiency Depends on the slope

25

Laser Optical Output vs. Drive CurrentSlope efficiency = dP/dIThe laser efficiency changes with temperature:

20° C 30° C

40° C

50° COpt

ical

out

put

Efficiencydecreases

Relationship between optical output and laser diode drive current. Below the lasing threshold the optical output is a spontaneous LED-type emission.

Modulation of Optical Sources

• Optical sources can be modulated either directly or externally.

• Direct modulation is done by modulating the driving current according to the message signal (digital or analog)

• In external modulation, the laser is emits continuous wave (CW) light and the modulation is done in the fiber

Why Modulation • A communication link is established by transmission

of information reliably• Optical modulation is embedding the information on

the optical carrier for this purpose• The information can be digital (1,0) or analog (a

continuous waveform)• The bit error rate (BER) is the performance measure

in digital systems• The signal to noise ratio (SNR) is the performance

measure in analog systems

Direct Modulation

• The message signal (ac) is superimposed on the bias current (dc) which modulates the laser

• Robust and simple, hence widely used• Issues: laser resonance frequency, chirp, turn on

delay, clipping and laser nonlinearity

29

Light Source LinearityIn an analog system, a time-varying electric analog signal modulates an optical source directly about a bias current IB. •With no signal input, the optical power output is Pt. When an analog signal s(t) is applied, the time-varying (analog) optical output is: P(t) = Pt[1 + m s(t)], where m = modulation index

For LEDs IB’ = IB

For laser diodes IB’ = IB – Ith

LED Laserdiode

Modulation of Laser Diodes

• Internal Modulation: Simple but suffers from non-linear effects.• Most fundamental limit for the modulation rate is set by the photon

life time in the laser cavity:

• Another fundamental limit on modulation frequency is the relaxation oscillation frequency given by:

thph

gnc

RRLnc

21

1ln211

2/1

1121

thphsp IIf

Laser Digital Modulation

Current (I)I(t)

IthI1

t

P(t)

tI2

th

spd IIIIt

2

12ln

Optical Power

(P)

• Input current– Assume step input

• Electron density– steadily increases

until threshold value is reached

• Output optical power – Starts to increase

only after the electrons reach the threshold

Turn on Delay(td)

Resonance Freq.(fr)

I1

I2

Turn on Delay (lasers)• When the driving current suddenly jumps from

low (I1 < Ith) to high (I2 > Ith) , (step input), there is a finite time before the laser will turn on

• This delay limits bit rate in digital systems• Can you think of any solution?

th

spd IIIIt

2

12ln

34

Relaxation Oscillation• For data rates of less than approximately 10 Gb/s (typically 2.5

Gb/s), the process of imposing information on a laser-emitted light stream can be realized by direct modulation.

• The modulation frequency can be no larger than the frequency of the relaxation oscillations of the laser field

• The relaxation oscillation occurs at approximately

2/1

1121

thphsp IIf

The Modulated Spectrum

Two sidebands each separated by modulating frequency

Twice the RF frequency

Limitations of Direct Modulation

• Turn on delay and resonance frequency are the two major factors that limit the speed of digital laser modulation

• Saturation and clipping introduces nonlinear distortion with analog modulation (especially in multi carrier systems)

• Nonlinear distortions introduce higher order inter modulation distortions (IMD3, IMD5…)

• Chirp: Unwanted laser output wavelength drift with respect to modulating current that result on widening of the laser output spectrum.

Laser Noise• Modal (speckle) Noise: Fluctuations in the

distribution of energy among various modes.• Mode partition Noise: Intensity fluctuations in

the longitudinal modes of a laser diode, main source of noise in single mode fiber systems.

• Reflection Noise: Light output gets reflected back from the fiber joints into the laser, couples with lasing modes, changing their phase, and generate noise peaks. Isolators & index matching fluids can eliminate these reflections.

External Modulation

The electro-optical (EO) phase modulator (also called a Mach-Zhender Modulator or MZM) typically is made of LiNbO3.

The optical source injects a constant-amplitude light signal into an external modulator. The electrical driving signal changes the optical power that exits the external modulator. This produces a time-varying optical signal.

Mach-Zhender Principle

mmm

LmL

of esother valu allfor result willmodulationintensity Light phase) (opposite ceinterferenen destructiv -- odd is If

(inphase) ceinterferen veconstructi --even is If

isoutput armlower in theshift Phase isoutput armupper in theshift Phase

:signals ginterferin twoebetween th difference phase relative Total•

Traveling Wave Phase Modulator

• Much wideband operation is possible due to the traveling wave tube arrangement (better impedance matching)

Electro Absorption Modulator• An EAM is a semiconductor external modulator based on the Franz–Keldysh

effect, i.e., a change in the absorption spectrum caused by an applied electric field, which changes the bandgap energy.

• Most EAM are made in the form of a waveguide with electrodes for applying an electric field in a direction perpendicular to the modulated light beam.

• EAM can operate with much lower voltages and at very high speed (tens of GHz)

• EAM can be integrated with a DFB laser diode on a single chip to form a data transmitter in the form of a photonic integrated circuit.

• EAM can also be used as Photo Detectors in the reverse mode

Distributed Feedback Laser (Single Mode Laser)

The optical feedback is provided by fiber Bragg Gratings Only one wavelength get positive feedback

Fiber Bragg GratingThis an optical notch band reject filter

DFB Output Spectrum

Laser Nonlinearity

...2coscos)(cos)(

210

tAtAAtytAtx

x(t) Nonlinear function y=f(x) y(t)

Nth order harmonic distortion:

1

log20AAn

Intermodulation Distortion

nmmn m,ntnmBty

tAtAtx

,21

2211

2,...1,0, )cos()(

coscos)(

Harmonics:21 , mn

Inter-modulated Terms:

,...2,2, 212121

47

Transmitter Packages• There are a variety of transmitter packages for different applications. • One popular transmitter configuration is the butterfly package.• This device has an attached fiber fly lead and components such as the

diode laser, a monitoring photodiode, and a thermoelectric cooler.

48

Transmitter PackagesThree standard fiber optic transceiver packages