1. May 02002 Chuck DiMarzio, Northeastern University 10100-9-1 ECE-1466 Modern Optics Course Notes Part 9 Prof. Charles A. DiMarzio Northeastern University Spring 2002

2. May 02002 Chuck DiMarzio, Northeastern University 10100-9-2 Lecture Overview Basics of CW Lasers –Gain –Feedback Pulsed Lasers –MOPA –Gain-Switched –Q-Switched –Mode-Locked

3. May 02002 Chuck DiMarzio, Northeastern University 10100-9-3 Some Material Properties Absorption Energy Emission StimulatedSpontaneous Absorption Emission

4. May 02002 Chuck DiMarzio, Northeastern University 10100-9-4 Laser Gain Materials –Solid Insulating Materials Semiconductors –Liquid eg. Dyes –Gas Pump Mechanisms –Electrical Discharge –Electrical Current –Light Flashlamp Laser –Chemical –Thermal –Other

5. May 02002 Chuck DiMarzio, Northeastern University 10100-9-5 Rate Equations for 2 Levels Energy 0 1 2 3 Populations Photons Actual Rate Equations Include Other Levels as Well

6. May 02002 Chuck DiMarzio, Northeastern University 10100-9-6 Typical Laser Materials Energy 0 1 2 3 4-Level Pump Fast Laser Energy 1 2 3 3-Level Pump Fast Laser

7. May 02002 Chuck DiMarzio, Northeastern University 10100-9-7 4-Level Steady State, No Lasing Energy 0 1 2 3 4-Level Pump

8. May 02002 Chuck DiMarzio, Northeastern University 10100-9-8 Gain vs. Pump R 03 g R 13 g 4-Level 3-Level

9. May 02002 Chuck DiMarzio, Northeastern University 10100-9-9 Feedback f Gain Round Trip

10. May 02002 Chuck DiMarzio, Northeastern University 10100-9-10 Threshold Gain f Gain Round Trip Amplitude Equation

11. May 02002 Chuck DiMarzio, Northeastern University 10100-9-11 Laser Frequency f Gain f Cavity Modes Round Trip Phase Equation

12. May 02002 Chuck DiMarzio, Northeastern University 10100-9-12 Steady State f Gain f Cavity Modes Round Trip Amplitude Equation

13. May 02002 Chuck DiMarzio, Northeastern University 10100-9-13 Gain Saturation Mechanism Laser Light Depletes Upper-State Population Lower Level Has a Fast Decay Time –Laser Does Not Pump Upper Level Populations End Nearly Equal Energy 0 1 2 3

14. May 02002 Chuck DiMarzio, Northeastern University 10100-9-14 Gain Saturation Modes ff Homogeneously Broadened Line Inhomogeneously Broadened Line

15. May 02002 Chuck DiMarzio, Northeastern University 10100-9-15 Master Oscillator & Power Amp Master Oscillator (CW Laser) Typically a few Watts Modulator Typically E/O With Pulsed Input Power Amplifier 30 dB? for kilowatts output Faraday Isolator Rejects Reflected Light

16. May 02002 Chuck DiMarzio, Northeastern University 10100-9-16 Gain Switched Laser t Pump Gain Power

17. May 02002 Chuck DiMarzio, Northeastern University 10100-9-17 Q-Switched Laser t Pump Gain Power Cavity Q

18. May 02002 Chuck DiMarzio, Northeastern University 10100-9-18 Mode-Locked Laser Modulator at f=FSR Gain Medium f Gain f Cavity Modes

19. May 02002 Chuck DiMarzio, Northeastern University 10100-9-19 Mode Locking Example “Laser” Frequency 10 GHz. (for illustration only) FSR = Modulation Frequency = 100 MHz. 11 Modes Laser Modes Sum Irradiance

20. May 02002 Chuck DiMarzio, Northeastern University 10100-9-20 Second Harmonic a vx (Electron as a Mass on a Spring) a v x

21. May 02002 Chuck DiMarzio, Northeastern University 10100-9-21 Energy Level Diagrams Fluorescence 2-photon

22. May 02002 Chuck DiMarzio, Northeastern University 10100-9-22 Some Lasers (1) Helium Neon –Gas; Elect. Discharge –633 nm Wavelength –milliwatts CW Argon Ion –Gas; Elect. Discharge –514, 488, and others –Watts CW Nd:YAG –Glass; Flashlamp or Laser Pumped –1064 nm –Watts Average Carbon Dioxide –Gas; Elect. Discharge –Around 10.6  m –Watts to kWatts, either CW or pulsed

23. May 02002 Chuck DiMarzio, Northeastern University 10100-9-23 Some Lasers (2) Diode –Elect. Current Low Voltage –Red to NIR –mW and up –Pulsed, Modulated to GHz, and CW –Small non-circular beam output Dyes –Usually Pumped by Another Laser –Typically Visible Wavelengths Usually Quite Widely Tunable (eg. Grating) –nJ or more –Limited Lifetime (often requires flow)

24. May 02002 Chuck DiMarzio, Northeastern University 10100-9-24 Green “Laser” Pointer Laser Diode Nd:YAG Laser Frequency Doubler Battery 780 nm 1064 nm532 nm

25. May 02002 Chuck DiMarzio, Northeastern University 10100-9-25 Titanium Sapphire Laser Laser Diode Nd:YAG Laser Frequency Doubler Power 780 nm 1064 nm532 nm Titanium Sapphire Red to NIR Very Broad Band and Can Be Mode- Locked