Stimulated Emission. Electrons can absorb photons from medium. Accelerated electrons emit light to return their ground state

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1 Lecture 15 Stimulated Emission Devices- Lasers Stimulated emission and light amplification Einstein coefficients Optical fiber amplifiers Gas laser and He-Ne Laser The output spectrum of a gas laser Laser oscillation conditions Semiconductor lasers, (laser diodes) Rate equation* Light emitters for optical fiber communications

2 Stimulated Emission Electrons can absorb photons from medium Accelerated electrons emit light to return their ground state Spontaneous emission occurs through a relaxation process A photon can stimulate an electron to radiate in phase (coherent) Two levels is not enough to create population inversion!!

3 Einstein Coefficients B 12 = B 21 A 21 B 21 = 8πhυ 3 c 3

4 Two Level Population Inversion?? B 12 = B 21 ( ) R 12 = N 1 Bρ v R 21 = N 2 ( ) Bρ v

5 4-Level Laser System a Pulsed A three energy level laser system Ruby Laser Pulsed or CW Mode A four energy level laser system Nd 3+ :YAG laser (Yttrium Aluminate Garnate) YAG

6 Light Amplification B 12 = B 21 A 21 B 21 = 8πhυ 3 c 3

7 Lecture 16 Stimulated Emission Devices- Lasers Stimulated emission and light amplification Einstein coefficients Optical fiber amplifiers Gas laser and He-Ne Laser The output spectrum of a gas laser Laser oscillation conditions Semiconductor lasers, (laser diodes) Rate equation* Light emitters for optical fiber communications

8 Absorption Cross Section Total Power Absorbed AΔI = -( Iσ ab )N 1 AΔx ΔI IΔx ab N1 = σ = α

9 Emission Cross Section ΔI IΔx = σ em N 2

10 Optical Gain Coefficient Definition g = ΔI IΔx net g = 0 No Gain!!

11 Two Level Population Inversion?? ( ) R 12 = N 1 Bρ v!# " $# σ abs R 21 = N 2 ( ) Bρ v!# " $# σ em σ abs = σ em

12 Optical Gain Coefficient Optical Gain G = exp( gl) Discrete energies turn into manifolds. Absorption can occur from any energy level. Emission occurs from the bottom of the E 2 manifold and top of E 1 manifold Non-linear processes cause frequency dependent absorption and emission cross-sections

13 Erbium Doped Fiber Amplifier (EDFA)

14 Erbium Doped Fiber Amplifier (EDFA) Optical amplifier is based on erbium (Er +3 ion)- doped fiber amplifier. Medium causes Stark Effect in silica-aluminate(sio 3 -Al 2 O 3 ) or silica-germania (SiO 3 -GeO 2 ) Energy bands occurs E 1 -E 0 (30-40meV) population ratio 1 to 4. E 3 is narrow. E 2 is long-lived energy level (10ms)

15 Erbium Doped Fiber Amplifier (EDFA) Definition g = ΔI IΔx net Optical Gain G = exp( gl)

16 Erbium Doped Fiber Amplifier (EDFA)

17 EDFA Configurations

18 EDFA N 0 = N 1 + N 2 Typical characteristics of EDFA small signal gain in db vs launched pump power for two different types of fibers pumped at 980 nm. The fibers have different core compositions and core diameter, and different lengths (L 1 = 19.9 m, and L 2 = 13.6 m) Typical dependence of small signal gain G on the fiber length L at different launched pump powers. There is an optimum fiber length L p.,

19 EDFA Configurations

20 EDFA Saturation Typical dependence of gain on the output signal strength for different launched pump powers. At high output powers, the output signal saturates, i.e. the gain drops. P dbm = 10log P mw 1 mw

21 P pin P sin EDFA P sout Saturated P sout P sout EDFA Saturation Small signal gain P sin G Saturation P sin Maximum P sin Maximum P sin Power Conversion Efficiency (PCE) GAIN η PCE = P sout P sin P pin P sout P pin G = P P sout = 1+η pin PCE P sin P sin η PCE Φ sout Φ pin λ p λ s P Φ photon hc λ η PCE max λ p λ s

22 EDFA Pump Length when N 1 is very small Absorbed Pump Power Absorbed Energy per Unit Time ( AL ) p N 2 ( ) hν p ( ) τ sp N 2 = N 0

23 EDFA Pump Equalization Definition g = ΔI IΔx net

24 FBG: Fiber Bragg Grating Fiber Bragg grating has a Bragg grating written in the core of a single mode fiber over a certain length of the fiber. The Bragg grating reflects any light that has the Bragg wavelength l B, which depends on the refractive index and the periodicity. The transmitted spectrum has the Bragg wavelength missing.

25 EDFA Pump Equalization FBG: Fiber Bragg Grating

26 EDFA Noise Amplified spontaneous emission (ASE) noise in the output spectrum and the amplified signal. The dependence of NF and gain (G) on the input signal power level (P sin ) for an EDFA under forward (codirectional) pumping. NF = SNR in NF(dB) = 10log SNR in SNR out SNR out

27 He-Ne Laser

28 He-Ne Laser Principles He + e He * + e He * + Ne He + Ne * DC or RF High Voltage is used to create electrical discharge Collisions of energetic electrons with He results in He* ions. Metastable (long lasting) He* ions resonantly transfer their energy to Ne atoms Excited Ne atom (2p 5 5s 1 ) create a population inversion 4 states at 2p 5 5s 1 and 10 states at 2p 5 3p 1

29 He-Ne Laser Principles From 2p 5 3p 1 state Ne atom spontaneously decay to 2p 5 3s 1 state. 2p 5 3s 1 state is a metastable state. Excited Ne atoms at 2p 5 3s 1 state can relax through collisions with the walls. Tube diameter limits output powers. Optical cavity with a flat and concave mirror is used. Beam diameter mm, divergence of few milliradians, power few milliwatts. Brewster windows for polarization selection.

30 He-Ne Laser Modes Δυ 1/2 = 2υ o 2k B T ln(2) Mc 2 λ 1/2 Δυ 1/2 λ ϑ Axial or longitudinal modes m λ 2 = L Longitudinal mode number

31 Doppler Effect (Source in Motion) Source at rest f = constant = c λ λ Doppler Effect: λ λ ' = λ v s T = λ v s λ v = λ 1 v s v ( ) f ' = v λ ' f ' = f v v v s

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