• Document: Lecture 9: Laser oscillators
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Lecture 9: Laser oscillators  Theory of laser oscillation  Laser output characteristics  Pulsed lasers References: This lecture follows the materials from Fundamentals of Photonics, 2nd ed., Saleh & Teich, Chapter 15. Also from Photonic Devices, Jia-Ming Liu, Chapter 11. 1 Intro  There are a wide variety of lasers, covering a spectral range from the soft X-ray (few nm) to the far infrared (hundreds of m), delivering output powers from microwatts (or lower) to terawatts, operating from continuous wave (CW) to femtosecond (even attosecond) pulses, and having spectral linewidths from just a few hertz to many terahertz.  The gain media utilized include plasma, free electrons, ions, atoms, molecules, gases, liquids, solids, etc.  The sizes range from microscopic, of the order of 10 m3 (recently down to the order of sub m3 for so-called nanolasers), to gigantic, of an entire building, to stellar, of astronomical dimensions.  An optical gain medium can amplify an optical field through stimulated emission. 2 Intro  The laser is an optical oscillator.  It comprises a resonant optical amplifier whose output is fed back to the input with matching phase.  The oscillation process can be initiated by the presence at the amplifier input of even a small amount of noise that contains frequency components lying within the bandwidth of the amplifier.  This input is amplified and the output is fed back to the input, where it undergoes further amplification.  The process continues until a large output is produced.  The increase of the signal is ultimately limited by saturation of the amplifier gain, and the system reaches a steady state in which an output signal is created at the frequency of the 3 resonant amplifier. Laser oscillators  In a practical laser device, it is generally necessary to have certain positive optical feedback in addition to optical amplification provided by a gain medium.  This requirement can be met by placing the gain medium in an optical resonator. The optical resonator provides selective feedback to the amplified optical field.  In many lasers the optical feedback is provided by placing the gain medium inside a “Fabry-Perot” cavity, formed by using two mirrors or highly reflecting surfaces Light output (laser) Gain medium reflectivity (R1 ~ 100 %) R2 < 100 % 4 4 Intro  Two conditions must be satisfied for oscillation to occur:  The amplifier gain must be greater than the loss in the feedback system s.t. net gain is incurred in a round trip through the feedback loop.  The total phase shift in a single round trip must be a multiple of 2 s.t. the feedback input phase matches the phase of the original input. If these conditions are satisfied, the system becomes unstable and oscillation begins. 5 Intro  As the power in the oscillator grows, the amplifier gain saturates and decreases below its initial value.  A stable condition is reached when the reduced gain is equal to the loss.  The gain then just compensates the loss s.t. the cycle of amplification and feedback is repeated without change and steady-state oscillation prevails. gain loss Steady-state power Oscillator 6 power Intro  Because the gain and phase shift are functions of frequency, the two oscillation conditions are satisfied only at one or several frequencies, which are the resonance frequencies of the oscillator.  The useful output is extracted by coupling a portion of the power out of the oscillator.  An oscillator comprises:  An amplifier with a gain-saturation mechanism  A feedback system  A frequency-selection mechanism  An output coupling scheme 7 Intro  The laser is an oscillator in which the amplifier is the pumped active medium.  Gain saturation is a basic property of laser amplifiers.  Feedback is enabled by placing the active medium in an optical resonator, which in its simplest form reflects the light back and forth between its mirrors.  Frequency selection is jointly attained by the resonant amplifier and the resonator, which admits only certain modes.  Output coupl

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