Signal regeneration - optical amplifiers In any atom or solid, the state of the electrons can change by: 1) Stimulated absorption - in the presence of a light wave, a photon is absorbed, the electron is excited to a higher energy level. E R light E 1 2) Stimulated emission - in the presence of a light wave, a photon is emitted, the electron drops too a lower energy level. light 3) Spontaneous emission - in the absence of light, a photon is emitted and the electron drops to a lower energy level. light NOTE: For stimulated processes, the absorbed or emitted photon has exactly the same phase φ and frequency ω (or wavelength λ ) as the stimulating light.
Optical amplifiers... continued Stimulated emission rate: r 21(stim) = B I(ω) Stimulated absorption rate: r 12(stim) = B I(ω) E 2 E 2 Spontaneous emission rate: r 12(spon) = A Energy ω ω A B = ω c π 3 3 2 E 1 E 1 A and B are the Einstein coefficients
Optical amplifiers... continued How does the intensity I(ω ) of a light wave change when passing through a medium with n 1 atoms in the ground state and n 2 atoms in the excited state? I( ω) ~ ω ( n2r21 n1r 12 ) = ω ( n2 n1 ) B I ( ω ) x I( ω) x = α I( ω) The absorption/gain α depends on the number of atoms in the excited ( n 2 ) and ground states (n 1 ) α ~ ω ( n n ) B 1 2 Equilibrium (n 1 >n 2 absorption (α positive) Population Inversion (n 2 >n 1 ) ( Gain) ( α negative) Need to pump electrons to N 2 from N 1.
Erbium doped optical amplifier (EDFA) Pump in Er doped glass fibre Amplified signal out Signal in Residual pump out Pump: high power light beam at λ= 980 nm or 1480 nm Signal: Telecomm signal on a λ = 1530-1565 nm light beam Gain ~ 40 db for 50 meters of EDFA fibre Gain > 0 for λ = 1530-1565 nm
Optical signal source Source must have: narrow frequency (wavelength ) linewidth - to avoid dispersion effects since speed of light depends on wavelength coherence (phase and wavelength are constant) - for efficient coupling into a single mode optical fibre, and well defined signal modulation Signal source should be a laser for high speed long distance communications.
Lasers - add mirrors to amplifier L Light in Gain Medium Light out Mirror 1 Mirror 2 Oscillation Threshold: 2 α L i 2 n k 0 L 1 r e e = 0 1) Round trip gain equals round trip loss: α L = 2 ln(r) 2) An integral number (m) wavelengths fit in the cavity: 2L = mλ Produces monochromatic, collimated light out. ω= Ε
Laser Operation Inverted electron population coherent output beam L full mirror 100% partial mirror Final wavelength of output beam determined by need to maintain integral number of half-wavelengths in cavity - can be used for fine-tuning. Creation of population inversion is called pumping. e.g. original ruby laser was pumped with intense white light from xenon flash tube It is extremely difficult to invert a simple 2-level system: - need exactly hv = E 2 - E 1 to pump up electron, which is difficult to do. E 2 pump photon E 1
Laser Pumping However pumping can be made to work in a 3-level system: We pump up to a broad band (easier to do). Electrons decay to E3 and then fall to E1 producing a photon. electron energy E 2 pump states - short lived pump photon E 3 (long-lived excited state) hv = E 2 - E 1 E 1 (ground state) Lasers have horrible energy efficiency: most of pump power is wasted.
Lasers for telecommunications Semiconductor lasers: small efficient electrically driven and can be electrically modulated can be designed to lase at single specified wavelength A semiconductor laser operating between λ = 1530 and 1565 nm is required (i.e. the EDFA band!) Band gap energy ~ 0.8 ev InGaAsP lasers grown on InP substrates
Semiconductor Lasers- pn diode N-doped electrons ω P-doped Conduction Band holes Valence Band Gain - stimulated electron and hole recombination at a p- n or p-i-n junction Wavelength - photon energy approximately equal to the semiconductor band gap energy Mirrors - provided by the cleaved facets of the semiconductor chip, or fabricated gratings Vertical optical confinement - a semiconductor laser is always a waveguiding structure
Semiconductor Lasers First semiconductor laser made from GaAs diode: (1962) p + roughed face n + cleaved face To achieve population inversion, must create the following situation with lots of n and p (out of equilibrium). E C E V E fn fermi energy for electrons filled empty E fp fermi energy for holes Need E Fn - E Fp > E G to achieve this ωe must very heavily dope both sides of junction
Semiconductor Lasers continued Equilibrium energy band diagram: E C E V E F n + -type p + -type apply high forward bias: energy barrier is flattened E Fn active region Inverted region LIGHT E C E V recombination E Fp n + -type p + -type There are a number of problems: - 1) light is not confined to active region (lose power) 2) region of population inversion is not confined. need very high current density to achieve lasing The original semiconductor laser (1962) operated in this way and power dissipation is high Therefore it got hot and can only run in a pulsed mode
Semiconductor lasers continued I (drive current) p - InGaAsP n - InGaAsP Light out n - InP InGaAsP quantum wells