Paper Review. Special Topics in Optical Engineering II (15/1) Minkyu Kim. IEEE Journal of Quantum Electronics, Feb 1985

Size: px

Start display at page:

Download "Paper Review. Special Topics in Optical Engineering II (15/1) Minkyu Kim. IEEE Journal of Quantum Electronics, Feb 1985"

Samson Simpson
5 years ago
Views:

1 Paper Review IEEE Journal of Quantum Electronics, Feb 1985

2 Contents Semiconductor laser review High speed semiconductor laser Parasitic elements limitations Intermodulation products Intensity noise Large signal effects Conclusion

3 Interaction between light and matters <Photon interacting with an hydrogen atom> + - hν Three interaction processes are possible Absorption Spontaneous Emission Stimulated Emission R 12 = B 12 N 1 ρ R sp = B sp N 2 R 21 = B 21 N 2 ρ ρ : photon density, N 1,2 : electron density at E 1,2 B 12,21,sp : constants

4 Interaction between light and matters R 12 = R sp + R 21 : Equilibrium condition N 2 = exp E 2 E 1 : Statistical mechanics N 1 kt ρ hν = 8πhν 3 c 3 {exp hν kt 1} : Black-body radiation B 21 = 1, B sp = 8πhν3 B 12 B 12 c 3 Absorption Spontaneous Emission Stimulated Emission R 12 = B 12 N 1 ρ R sp = B sp N 2 R 21 = B 21 N 2 ρ ρ : photon density, N 1,2 : electron density at E 1,2 B 12,21,sp : constants

5 Optical amplifier Amplifier Stimulated emission should be dominant P in P out = G P in Pumping N 2 > N 1 (Population Inversion)

6 LASER LASER : Light Amplification by Stimulated Emission of Radiation LASER = Optical amplifier + Mirrors Use initial photon produced by spontaneous emission Recycle photons produced by stimulated emission Use mirrors for recycling photons Condition for sustaining photons in laser ⑴e gl = 1 R (Gain > mirror loss) ⑵e j2nk 0L = 1 (No loss after one round trip)

7 Conditions for lasing Lasing conditions : ⑴e gl = 1 g R th = 1 ln 1 L R ⑵e j2nk0l = 1 λ = 2L (m = 1,2,3, ) n m

8 Semiconductor laser Absorption Spontaneous Emission Stimulated Emission R 12 hν = B 12 N 1 (E 1 ) P 2 (E 2 ) ρ(hν) R sp hν = B sp N 2 (E 2 ) P 1 (E 1 ) R 21 hν = B 21 N 2 (E 2 ) P 1 (E 1 ) ρ(hν) For population inversion, N 2 P 1 N 1 P 2 > 1 Electron & hole should be injected (Forward bias in PN junction)

9 Semiconductor laser structure

10 High speed semiconductor laser Rate equations ⑴ dn dt = J ed N τ s A N N om P ⑵ dp dt = A N N om P P τ p + β N τ s N: carrier density N om : carrier density for transparency P: photon density J: pump current density d: thickness of active layer τ s : spontaneous recombination lifetime of carriers τ p : photon lifetime A: optical gain coefficient β: fraction of spontaneous emission entering to lasing mode Small signal & linearization f T relaxation oscillation frequency = 1 2π Ap 0 τ p A, p 0, τ p high speed semiconductor laser

11 Parasitic elements limitation Semiconductor laser I Bonding wire I oxide insulator active layer p doped confining layer n doped confining layer Contact resistance Substrate Parasitic capacitance Intrinsic diode Circuit modeling current flowing into the intrinsic diode η = voltage of the signal source 1 = s 2 2 ω + s 0 ω 0 Q + 1 ω 0 = 50 + R LRC(50 + R), Q = LRC L + 50RC Second-order low pass filter type

12 Parasitic elements limitation

13 Parasitic elements limitation Circuit modeling 1+e Z = Z 2kW 0 1 e 2kW, W : width of top contact Z 0 = R dist jωc dist, k = jωr dist C dist Higher frequency higher propagation constant(k) electric field cannot penetrate far beyond laser junction

14 Intermodulation products Third order intermodulation Third order intermodulation can be a problem in multichannel frequency division transmission

15 Intensity noise Relative Intensity Noise RIN = P 0 : average light output power < ΔP > 2 : mean square intensity fluctuation spectral density of the light output < ΔP >2 < P 0 > 2 With rate equations, RIN ~ 1 p 0 ω 3 ω 2 ω R ω2 ω R 4 τr 2

16 Large signal effects Non-linearity problem for large signal Optical modulation depth for high speed below ~70 percent <Effects of increasing optical modulation depth>

17 Conclusion Rate equations ⑴ dn dt = J ed N τ s A N N om P ⑵ dp dt = A N N om P P τ p + β N τ s N: carrier density N om : carrier density for transparency P: photon density J: pump current density d: thickness of active layer τ s : spontaneous recombination lifetime of carriers τ p : photon lifetime A: optical gain coefficient β: fraction of spontaneous emission entering to lasing mode f T = 1 2π Ap 0 τ p A, p 0, τ p high speed semiconductor laser Parasitic elements limitations Intermodulation products Intensity noise Large signal effects

Laser Basics. What happens when light (or photon) interact with a matter? Assume photon energy is compatible with energy transition levels.

Laser Basics. What happens when light (or photon) interact with a matter? Assume photon energy is compatible with energy transition levels. What happens when light (or photon) interact with a matter? Assume photon energy is compatible with energy transition levels. Electron energy levels in an hydrogen atom n=5 n=4 - + n=3 n=2 13.6 = [ev]