1ControlNumber p-n junctions: forward bias Effectively injecting electrons into n-type, holes into...

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p-n junctions: forward bias

Effectively injecting electrons into n-type, holes into p-type– Electrons repelled from contact with battery, move to

junction– Holes repelled from contact, move to junction

Recombination continuous at junction ~ conduction occurs– No depletion zone– Free electrons lose quantum of energy when

recombining with holes• “Injection luminescence”

Source: Dutton

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p-n junctions: reverse bias

Electrons sucked out of n-type region, holes out of p-type region– Depletion zone increases– No conduction; device insulates

Source: Dutton

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Laser technical parameters

Spectral width Linewidth Coherence length and coherence time Power Stability Switching time and modulation Tuning range (tunable lasers only)

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Spectral width

Semiconductor lasers do not produce light at a single wavelength– Cannot do so, since this would violate the Uncertainty

Principle of physics– Produce range of wavelengths, called “spectral width”

of laser– Usually produce around 8 frequencies or “modes”– Result from fact that resonating cavity is long enough

for several different multiples of wavelength– Width ~ 6 to 8 nm– Not produced simultaneously, but laser jumps randomly

among them• In each mode for a few nanoseconds

– Laser output power does not vary—just wavelength

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Spectral width (continued)

Importance– Spectral width determines chromatic dispersion

• Better for lasers than LEDs– In WDM systems, wavelengths must be packed closely

together, requiring narrow spectral width– Narrow spectral width signals can be subject to

nonlinear effects which are undesirable

Source: Dutton

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Linewidth

Width of individual frequencies discussed in connection with spectral width

Referred to as “lines” Affects modulation and detection techniques

– Frequency, phase modulation, coherent detection require linewidth to bandwidth ratio of 1:100

– Newer methods using optical amplifiers have mitigated this requirement somewhat

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Coherence time

Time that laser emits a given wavelength (line) Distance light travels in that time called “coherence length” Times

– LED: ~0.5 x 10-12 second– Simple laser: ~0.5 x 10-9 second– High quality laser: ~10-6 second

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Frequency or wavelength stability

Refers to changes in emitted wavelength of a laser with time, temperature changes, etc.

Not as important in single-channel systems with incoherent detection

Critical for WDM systems Fabry-Perot lasers can very 0.4 nm/degree Lasers modulated by on-off keying (OOK) produce “chirp”

at beginning of each pulse– Transient frequency shift up to several gigahertz

Operation of laser causes heating of cavity and changes in its parameters

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Switching time and modulation

Methods– On-off keying (OOK): switching laser on and off to

generate pulses• Up to thousands of teraHz ~ 0.5 fsec

– Some can be by frequency shift keying (FSK)• Changing frequency of laser by varying bias current• Requires coherent detection

– External modulation techniques which operate on the light beam after it is generated• Used in systems operating faster than 1 Gbps• Employ crystals which change optical properties in

response to electrical signals

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Tuning range

Some newer lasers can be tuned– Not fast– Tuning cannot be used as modulation technique– Not continuous: laser jumps between modes

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Laser operation—energy levels

Source: Dutton

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Laser operation—population inversion

Stimulated emission not enough to make a laser Problem is that electrons in ground state will absorb

photons at same wavelength at which those in higher state emit them– No net release of photons

But probabilities of the two are different Must fix this problem by having number of electrons in

higher energy x probability of emission > than number in lower energy state x probability of absorption

N(eh) x pe > N(el) x pa

– Called “population inversion”

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Laser operation—sequence of events

Energy applied—electrons in high state appear

Spontaneous emission begins, most is lost

Some hits mirrors at correct angle, is reflected back

Photons bouncing back and forth stimulate others

Number quickly builds up Some leaves through partially

reflecting mirror Power increases until amount

leaving = input power – losses Reflectivity ~6% in

semiconductor lasers

Fig 65

Source: Dutton

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Fabry-Perot lasers

Simplest kind of semiconductor laser

LED + pair of mirrors Distance between mirrors is integral

multiple of half wavelengths Wavelengths not resonant

encounter destructive interference Frequencies:

– 630-650 nm (pointers)

– 790 nm (CD players)

– 850, 1310, 1550 nm (fiber optics) Size: a few hundred microns

Source: Dutton

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Fabry-Perot lasers (continued)

Wavelength produced can be calculated as

= 2nCl / x

where x = 1, 2, 3…; Cl is cavity length, n is RI of active medium

Typical cavity length ~ 100-200 microns– Several hundred wavelengths

Source: Dutton

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Fabry-Perot lasers (continued)

Produces wide spectral width due to its construction– 5-8 nm– Not suitable for the most critical applications

• Extended distances• Coherent detection• WDM

– Emerging beam tends to diverge, requiring focusing

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Fabry-Perot lasers (continued)

Performance can be improved by modifying design to eliminate unwanted frequencies– Before reaching lasing threshold– Common way: put diffraction grating in cavity

• Effect is to deflect all but a narrow range of frequencies so that they do not hit mirrors at correct angle

• Linewidth of 0.2-0.3 nm possible– Can use external cavity with diffraction grating on one

mirror• Linewidth of 10 MHz

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Glitches in real lasers

Line broadening Turn-on delay Mode hopping Chirp Relaxation oscillations Relative Intensity Noise (RIN) Phase noise Intercavity noise Drift

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Line broadening

Can’t make a “perfect” laser– Single line– Infinitely narrow wavelength range

Line broadening occurs– Homogeneous

• Mainly quantum mechanical effects• t x E > h/2• Gives rise to range of energies and frequencies

– Inhomogeneous• Thermal vibration of atoms• Impurities

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Turn on delay

Delay from application of power to production of coherent light

Spectrum sharpens as full power reached

Source: Dutton

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Mode hopping Cause by “hole burning”

– After short time in operation, laser depletes excited atoms in center of cavity (dominant path)

– Not possible to get power to all of active regions at an even rate

– “Hole” is burned in path of dominant mode• Reduces its power• Other modes gain power• Occurs in 10s of psec

Source: Dutton

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Chirp

Most serious of transient effects RI of cavity changes after turn on

– Density of charge carriers drops– Temperature in cavity abruptly rises

Results in rapid change in center wavelength produced Downward “chirp” produced

– Wavelength shifts to longer wavelength than at start of pulse

Requires use of external modulators for extremely high speed transmission rates (> 1 Gbps)

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Relaxation oscillations

Short term fluctuations in intensity of light produced Result from depletion of high energy electrons

– Lasing action reduced or disappears High energy electrons have to build up again Usually damps out, but if laser not properly designed, will

continue indefinitely

Source: Dutton

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Relative intensity noise (RIN)

Random intensity fluctuations in output of laser– Due to random nature of spontaneous emissions– Some spontaneous emissions can resonate and are

amplified

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Phase noise

Related to RIN New spontaneous emissions different in phase from

previous emissions– Leads to random changes in phase of emitted light– Cannot be suppressed as it is a consequence of way

lasers operate Not important in amplitude modulated systems

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Intercavity noise

Caused by reflections from components other than mirrors at ends of cavity– Because reflections are of the correct wavelength, they

are amplified– Leads to undesired fluctuations in light production

Sources– Nearby: laser-to-fiber coupling– More distant: Optical components down the fiber

• Can be suppressed with optical isolator which prevents such reflections from passing through

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Drift

After a period of operation, laser operation will change because critical parameters change– Known as “drift”

Temperature rises, changing cavity length and therefore resonant wavelength

Age of device

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Construction of real lasers: simple laser

Made (grown) from a single crystal– Planes of crystal exactly parallel– Cleaved instead of cut along planes of crystal

• Gives exactly parallel mirrors at ends• No silvering required: interface of semiconductor

medium (RI ~ 3.5) and air effectively forms mirror No lasing in vertical or lateral modes Lasing across width of active region

– Difficult to get light into fiber

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Simple laser (continued)

Source: Dutton

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Construction of real lasers: gain guided

Gain guided operation Basic idea: control lasing region by controlling entry of

power into active region– Limit area of electrical contact by inserting (growing)

insulator Improved performance

– Narrow beam– Spectral width 5-8 nm– 8-20 lines– Linewidth .005 nm

Source: Dutton

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Construction of real lasers: index guided

In addition to insulator, reduce width of active region– Put strips of semiconductor material with high bandgap

energy on either side of active region– Active region bounded on all sides by material of lower

RI– Call “index guided”

Improved performance– Spectral width 1-3 nm– 1-5 lines– Linewidth 0.001 nm

Source: Dutton

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Operational characteristics

Minimum, maximum levels (0, 1 states)– At 0, laser set just above lasing threshold– At 1, laser set just below maximum threshold– Extinction ratio: light at full power / light at 0 level

• Quoted in db Temperature control

– Needed for long-term stability– High performance communications lasers incorporate

thermoelectric coolers and associated control circuitry Power control

– Monitor light level, adjust output with feedback circuit– Monitor diode at back of laser

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Distributed feedback lasers (DFB)

Designed to solve problems of standard FP lasers– Spectral width too wide– Too much mode hopping

Put diffraction grating (Bragg grating) into laser cavity– Effectively selects certain wavelengths through

constructive interference• Period of corrugations is multiple of desired

– Grating actually put just below cavity• Too much attenuation if put in cavity• Still works because of E field penetration into

adjacent layers

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DFB laser (continued)

Chirp problem still exists, but much smaller than FB lasers because grating determines wavelength, not energy gap

Advantages– Narrow linewidths ~50 kHz– Low chirp– Low RIN

Source: Dutton

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Problems with DFB lasers

Extremely sensitive to reflections– Cause widening of wavelength– Requires integrated isolator

Sensitive to temperature variations– Average temperature– Rapid changes produced by certain bit patterns

Significant fluctuations in output– Stabilized by feedback circuit with PIN diode

Relatively high cost

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Improving switching speed

Inherent device physics limits switching speed Extremely high speed devices use external modulator Modulator can be integrated with laser

– Common type referred to as “Integrated absorption modulators” or “Electro-absorption modulators” (EML)

Source: Dutton

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Ericsson PGT 204 01

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Q-switching

Similar to previous case, except mirror moved to right hand end

When laser in OFF state, active medium pumped– Can turn on very quickly– Generates high power pulse– Can be used to generate solitons

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Tunable lasers

Under development One method: adjust Bragg grating in DBR

– Current can change parameters of Bragg grating– Results in selection of different wavelength

Source: Alcatel

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Tunable lasers (continued)

Source: Alcatel

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Vertical Cavity Surface Emitting Laser (VCSEL)

Emit from surface instead of edge– Better light pattern for coupling into fiber

Low power (~1 mw) but much higher than most LEDs Size 12-20 microns 850 nm or 950 nm Low threshold currents Low modulation currents High stability—no special circuitry required Very high modulating bandwidth—up to 2.4 GHz

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VCSEL (continued)

Source: Dutton

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