Foundation of optical transmitter
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Basic characteristics of LASER and LED
![Optical Transmitter
converts electrical signal into optical signal
Optical
Transmitter
I hν
Types: LASER
[Light Amplification by Stimulated Emission of Radiation]
LED
[Light Emitting Diode]
9/17/2020 2Arpan Deyasi, RCCIIT, India](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Foundation of optical transmitter
- 1. Course: Electronic Devices paper code: EC301 Course Coordinator: Arpan Deyasi Department of Electronics and Communication Engineering RCC Institute of Information Technology Kolkata, India 9/17/2020 1Arpan Deyasi, RCCIIT, India Topic: Optical Transmitter
- 2. Optical Transmitter converts electrical signal into optical signal Optical Transmitter I hν Types: LASER [Light Amplification by Stimulated Emission of Radiation] LED [Light Emitting Diode] 9/17/2020 2Arpan Deyasi, RCCIIT, India
- 3. Bulk LASER Semiconductor LASER Quantum LASER Different types of LASER 9/17/2020 3Arpan Deyasi, RCCIIT, India
- 4. BulkLASER(3-level) N0 N1 N2 E0 E1 E2 sp. emi st. emi N0>>N1, N2 hν1=E2-E0 st. abs hν2=E2-E1 hν3=E1-E0 9/17/2020 4Arpan Deyasi, RCCIIT, India
- 5. BulkLASER(4-level) N0 N2 N3 E0 E2 E3 sp. emi st. emi N0>>N1, N2, N3 hν1=E3-E0 st. abs hν2=E3-E2 hν3=E2-E0 N1 st. emi hν4=E2-E1 E1 9/17/2020 5Arpan Deyasi, RCCIIT, India
- 6. SemiconductorLASER Vbi 9/17/2020 6Arpan Deyasi, RCCIIT, India EC EFI EV Vbi -Vf hν=EC-EV
- 7. Semiconductor LASER For operation, both electrical and optical biases are required g f E V q ≈ 9/17/2020 7Arpan Deyasi, RCCIIT, India
- 8. Semiconductor LASER: Drawbacks Improper depletion width ---- ill-defined active region ---- carrier confinement is not possible Improper modal volume ---- mode confinement is not possible Larger active region --- higher threshold current density ------- continuous room temperature operation is not possible 9/17/2020 8Arpan Deyasi, RCCIIT, India
- 9. Quantum Well LASER E1 E2 E3 hν1=E3-E2 hν3=E2-E1 hν2=E3-E1 32 32 hc E λ = ∆ 31 31 hc E λ = ∆ 21 21 hc E λ = ∆ 9/17/2020 9Arpan Deyasi, RCCIIT, India
- 10. SemiconductorLED Vbi 9/17/2020 10Arpan Deyasi, RCCIIT, India EC EFI EV Vbi -Vf hν=EC-EV
- 11. 9/17/2020 Arpan Deyasi, RCCIIT, India 11 Steady state electron density in LASER & LED ( ) p d J n n dt qd τ ∆ ∆ = − Rate equation for recombination Under equilibrium 0 d dt =
- 12. 9/17/2020 Arpan Deyasi, RCCIIT, India 12 Steady state electron density in LASER & LED 0 p J n qd τ ∆ − = pJ n qd τ ∆ =
- 13. 9/17/2020 Arpan Deyasi, RCCIIT, India 13 Recombination rate in LASER & LED t r nr J r r r qd = + = Total recombination rate Total number of recombination in the device under forward bias fi R q =
- 14. 9/17/2020 Arpan Deyasi, RCCIIT, India 14 Internal Quantum Efficiency in LASER & LED Internal Quantum Efficiency = Number of photons generated inside the device Number of electrons injected
- 15. 9/17/2020 Arpan Deyasi, RCCIIT, India 15 External Quantum Efficiency in LASER & LED External Quantum Efficiency = Number of photons emitted Number of EHPs responsible to produce photocurrent r r ext t r nr r r r r r η = = +
- 16. 9/17/2020 Arpan Deyasi, RCCIIT, India 16 Recombination rate in LASER & LED r r ext t t r R r R η = = Rr: number of radiative recombination in the whole volume of active region Rt: total number of recombination in the whole volume of active region
- 17. 9/17/2020 Arpan Deyasi, RCCIIT, India 17 Recombination rate in LASER & LED r ext tR Rη= f r ext i R q η=
- 18. Generated optical power o rP R hν= 9/17/2020 18Arpan Deyasi, RCCIIT, India Generated optical power in LASER & LED f o ext i P h q η ν= f o ext i c P h q η λ =
- 19. Output power 0 1.24 f exti P η λ = 9/17/2020 19Arpan Deyasi, RCCIIT, India Generated optical power in LASER & LED
- 20. Responsivity --- ratio of emitted optical power to injection current 1.24 ext R η λ = 9/17/2020 20Arpan Deyasi, RCCIIT, India Responsivity in LASER & LED 0 f P R i =
- 21. Difference between LASER and LED Parameter LASER LED Working Principle Stimulated Emission Spontaneous emission Driving Current Lower (~ 5 -40 mA) Higher (~ 50-100 mA) Nature of emitted light Coherent, monochromatic Incoherent, polychromatic Power to light Conversion Efficiency Approx 80 % Approx 20 % Numerical Aperture Extremely low large 9/17/2020 21Arpan Deyasi, RCCIIT, India
- 22. 9/17/2020 Arpan Deyasi, RCCIIT, India 22 Difference between LASER and LED Parameter LASER LED Response faster slower Junction Area Narrower wider Lifetime longer smaller Spectrum width narrower larger Cost high economical