1ControlNumber 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

2ControlNumber 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

3ControlNumber Laser technical parameters Spectral width Linewidth Coherence length and coherence time Power Stability Switching time and modulation Tuning range (tunable lasers only)

4ControlNumber 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

5ControlNumber 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

6ControlNumber 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

7ControlNumber Coherence time Time that laser emits a given wavelength (line) Distance light travels in that time called “coherence length” Times –LED: ~0.5 x second –Simple laser: ~0.5 x second –High quality laser: ~10 -6 second

8ControlNumber 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

9ControlNumber 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

10ControlNumber Tuning range Some newer lasers can be tuned –Not fast –Tuning cannot be used as modulation technique –Not continuous: laser jumps between modes

11ControlNumber Laser operation—energy levels Source: Dutton

12ControlNumber 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(e h ) x p e > N(e l ) x p a –Called “population inversion”

13ControlNumber 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

14ControlNumber 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: – nm (pointers) –790 nm (CD players) –850, 1310, 1550 nm (fiber optics) Size: a few hundred microns Source: Dutton

15ControlNumber Fabry-Perot lasers (continued) Wavelength produced can be calculated as = 2nC l / x where x = 1, 2, 3…; C l is cavity length, n is RI of active medium Typical cavity length ~ microns –Several hundred wavelengths Source: Dutton

16ControlNumber 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

17ControlNumber 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 nm possible –Can use external cavity with diffraction grating on one mirror Linewidth of 10 MHz

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19ControlNumber Glitches in real lasers Line broadening Turn-on delay Mode hopping Chirp Relaxation oscillations Relative Intensity Noise (RIN) Phase noise Intercavity noise Drift

20ControlNumber 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

21ControlNumber Turn on delay Delay from application of power to production of coherent light Spectrum sharpens as full power reached Source: Dutton

22ControlNumber 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

23ControlNumber 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)

24ControlNumber 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

25ControlNumber 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

26ControlNumber 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

27ControlNumber 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

28ControlNumber 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

29ControlNumber 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

30ControlNumber Simple laser (continued) Source: Dutton

31ControlNumber 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

32ControlNumber 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 nm Source: Dutton

33ControlNumber 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

34ControlNumber 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

35ControlNumber 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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38ControlNumber 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

39ControlNumber 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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42ControlNumber Ericsson PGT

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44ControlNumber 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

45ControlNumber 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

46ControlNumber Tunable lasers (continued) Source: Alcatel

47ControlNumber 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 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

48ControlNumber VCSEL (continued) Source: Dutton

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