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pulsed_laser - ECE 455 Lecture 5 Laser Dyamics Pulsed...

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ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characteriza- tion Pulsing Methods Q-Switching Mode Locking Laser Dynamics and Pulsed Lasers ECE 455 Optical Electronics Tom Galvin Gary Eden ECE Illinois
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ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characteriza- tion Pulsing Methods Q-Switching Mode Locking Introduction In this section, the following subjects will be covered: Non-steady state behavior of lasers Motivations for pulsing lasers Methods for pulsing lasers
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ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characteriza- tion Pulsing Methods Q-Switching Mode Locking Starting a Laser I Gain Medium PUMP Output Coupler High Reflector
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ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characteriza- tion Pulsing Methods Q-Switching Mode Locking Starting a Laser II 1 Before the cavity has begun lasing, the cavity has defined discrete opitcal modes which may oscillate. 2 When the laser is started these modes are seeded by photons emitted via spontaneous emission. 3 All modes which are above threshold when the gain is γ 0 grow exponentially, but the mode with the highest net gain will grow more rapidly than all others. 4 The mode with the highest gain will reach a level where it saturates the medium before the other modes. The gain for all modes falls until only one is above threshold. 5 The other modes decay away and a single mode is left lasing.
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ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characteriza- tion Pulsing Methods Q-Switching Mode Locking Starting a Laser Level Diagram N 1 N 0 N 2 R 3 τ 10 Laser τ 21 N 3 τ 32 Throughout the analysis that follows, we’ll analyze a four-level system as shown above
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ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characteriza- tion Pulsing Methods Q-Switching Mode Locking Starting a Laser III Seeding Rate The first question to answer is how long will it take for a cavity mode above thresold to become seeded? The density of states for photons per unit volume and frequency is: ρ ( ν ) d ν = 8 π n 3 ν 2 c 3 d ν (1) Spontaneous emission can emit into any of these modes. But only one of these modes is the mode with the highest gain d φ dt = N 2 V c τ 21 g ( ν 21 ν 21 1 V c ρ ( ν ν 21 (2) = N 2 τ 21 g ( ν 21 ) 1 ρ ( ν ) (3) = η seed N 2 τ 21 (4)
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ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characteriza- tion Pulsing Methods Q-Switching Mode Locking Starting a Laser IV Buildup Time How long will it take for a single photon to stabilize into laser output? The small signal gain form of the gain equation may be used because saturation effects can be ignored before laser oscillation has begun. I sat = h ν A τ 21 e ( γ 0 - γ th ) ct / n (5) Therefore the buildup time is τ buildup = n ( γ 0 - γ th ) c ln I sat A τ 21 h ν (6)
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ECE 455 Lecture 5 Laser Dyamics Pulsed Lasers Pulsed Laser Characteriza- tion Pulsing Methods Q-Switching Mode Locking Starting a LaserV Buildup Time II There are many ways to estimate the buildup time. The net gain per round trip is G RT = R 1 R 2 exp [2 γ 0 L g ] (7)
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