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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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