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1、4. Stimulated emission laser 4.1 Stimulated emission and amplification 4.2 Stimulated emission rate and Einstein coefficients 4.3 Fiber Laser 4.4 Gas Laser 4.5 Output spectrum of gas laser 4.6 Lasing condition 4.7 Principle of laser diode 4.8 Hetero-junction laser 4.9 Characteristics of laser diode
2、4.10 Steady state rate equation of LD 4.11 Design of gain medium of a laser 4.12 LD for optical fiber communication 4.13 Single frequency solid state laser 4.14 Quantum-well Laser 4.15 Vertical cavity surface-emitted LD 4.16 Laser amplifier 4.1 4.1 Stimulated emission and photon amplification Absorp
3、tion, spontaneous emission, and stimulated emission. E 1 h 1 3 E 2 Metastable state E 1 E 3 E 2 h 3 2 E 1 E 3 E 2 E 1 E 3 E 2 h 2 1 h 2 1 Coherent photons OUT ( a ) ( b ) ( c ) ( d ) E 3 . IN The principle of the laser. (a) Atoms in the ground state are pumped up to the energy level E3 by incoming p
4、hotons of energy 13=E3-E1. (b) Atoms at E3 rapidly decay to the long-lived state at energy level E2 by emitting photons or emitting lattice vibrations; 32=E3-E2. (c) As the state at E2 are long-lived, they quickly become populated and there is a population inversion between E2 and E1. (d) A random p
5、hoton (from a spontaneous decay) of energy 21=E2-E1 can initiate stimulated emission. Photon from the stimulated emission can themselves further stimulate emission leading to an avalanche of stimulated emission and coherent photon being emitted. 4.2 Stimulated emission rate and Einstein coefficients
6、 hNBR 11212 hNBNAR 22122121 2112 RR Absorption rate： Emission rate： In stable condition, absorption rate is equal to emission rates: Tk EE N N B 12 1 2 exp 1exp 8 )( 3 3 Tk hv c h hv B Boltzmann distribution： Relation between intensity of black radiation and frequency: 2112 BB 33 2121 /8/chvBA 21 21
7、 221 221 ,21 ,21 A B NA NB R R sp st 3 3 ,21 ,21 8 hv c R R sp st 1 2 ,21 ,21 N N R R ab st Einstein coefficients： Spontaneous emission coefficient and stimulated emission coefficient： Stimulated emission and spontaneous emission： 4.3 Optical fiber amplifiers 12 NNlGop Gain, stimulated emission cros
8、s section and population inversion： Where , , are stimulated emission cross section population inversion, and length of gain medium： l 12 NN is determined by material properties of gain medium Energy of the Er 3 + ion in the glass fiber E 1 0 1.54 eV 1.27 eV 0.80 eV E 2 E 3 E 3 1550 nm 1550 nm In Ou
9、t 980 nm Non-radiative decay Pump Photoluminescence: light absorption, excitation, nonradiative decay and light emission, and return to the ground state E1. The energy levels have been displaced horizontally for clarity. Energy diagram for the Er3+ ion in the glass fiber medium and light amplificati
10、on by stimulated emission from E2 to E1.Dashed arrows indicate radiationless transitions (energy emission by lattice vibrations). Signal in Signal out Splice Er 3 + -doped fiber (10 - 20 m) Wavelength-selective coupler Pump laser diode Splice = 1550 nm = 1550 nm = 980 nm Termination Optical isolator
11、 Optical isolator A simplified schematic illustration of an EDFA (optical amplifier). The erbium-ion doped fiber is pumped by feeding the light from a laser pump diode, through a coupler, into the erbium ion doped fiber. 4.4 Gas laser eHeeH e * * NeHeNeHe Current regulated HV power supply Flat mirro
12、r (Reflectivity = 0.999) Concave mirror (Reflectivity = 0.985) He-Ne gas mixture Laser beam Very thin tube A schematic illustration of the He-Ne laser (1 s 2 ) (1 s 1 2 s 1 ) 0 2 0 . 6 1 e V He (2 p 6 ) G r o u n d s t a t e s (2 p 5 5 s 1 ) Ne (2 p 5 3 p 1 ) (2 p 5 3 s 1 ) C o l l i s i o n s L a s
13、 i n g e m i s s i o n 6 3 2 . 8 n m 6 0 0 n m C o l l i s i o n s w i t h t h e w a l l s F a s t s p o n t a n e o u s d e c a y 2 0 . 6 6 e V E l e c t r o n i m p a c t The principle of operation of the HeNe laser. Important HeNe laser energy levels (for 632.8 nm emission). Laser tube Laser radi
14、ation r L 4.5 Output of gas laser )1 ( 01 c V vv x 2 02/1 )2ln(2 2 Mc Tk vv B Lm 2 Doppler broadening： When gas molecular move closely to observer, observed radiation frequency： )1 ( 01 c V vv x Condition for lasing： When gas molecular move far from observer, observed radiation frequency： . ( c ) Re
15、lative intensity m Optical Gain Allowed Oscillations (Cavity Modes) m ( b ) L Stationary EM oscillations Mirror Mirror Doppler broadening m ( /2) = L ( a ) (a) Optical gain vs. wavelength characteristics (called the optical gain curves) of the lasing medium. (b) Allowed modes and their wavelengths due to stationary EM waves within the optical cavity. (c) The output spectrum (relative intensity vs. wavelength) is determined by satisfying (a) and (b) simultaneously, assuming
