Class Notes (1_21)

Class Notes (1_21) - EE540 CLASS NOTES Introduction Before...

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1 EE540 CLASS NOTES Introduction Before the invention of the maser and laser in the 1950’s, electronics concerned itself with interactions between “free” electrons (electrons not bound to a particular ionic core) and electromagnetic fields, both self-fields and external fields. With the maser came the realization that “bound” (not free) electrons could provide solutions to many problems in communications, frequency standards and the generation and detection of electromagnetic radiation, and even in the transmission of power. Especially motivating for laser development was the theoretical promise of optical beams whose spectral widths might be microhertz or less. This promise has been realized. Bound electrons are governed in their motions entirely by the quantum mechanics that had been developed a quarter century before the maser. This new subject matter was given the name “Quantum Electronics”. The classical idea of electrical current plays only a supporting role in quantum electronics (QE). Full recognition of this new area was formalized by the founding of the IEEE Journal of Quantum Electronics (JQE) in1965. These Notes will be limited to that part of quantum electronics that is concerned with the description, analysis and design of masers and lasers (acronyms for “light, and microwave, amplification by stimulated emission of radiation”). As is common practice, the words “laser” and “maser” here refer both to oscillators and to amplifiers; context is required if either is being referred to explicitly.
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2 Since laser oscillators are all feedback oscillators, they can only be analyzed once the properties of laser amplifiers are understood. Therefore, these Notes first develop, in Chapters 1,2 and 3, a simple rate-equation theory of laser amplifiers, and then use it to design diode-pumped solid-state ( DPSS) laser oscillators and semiconductor diode laser oscillators in Chapters 4,5 and 6. We will concentrate on questions, first, of amplifier gain, bandwidth, power and efficiency, and then of oscillator threshold, power and efficiency. This concentration allows us to limit our descriptive variables to ‘photon flux’, ‘flux density’ and ‘photon number density’ in a simpler mathematical framework. As a result, we will not need to use Schrodinger’s equation, or wave functions, or matrix elements, or even Maxwell’s equations. We will only need to use the quantum rate equations that govern the “creation” and “annihilation” of photons, while conserving energy via concomitant “quantum jumps” of atoms from one quantized energy level to another. A brief review of the experimental framework needed for this approach to quantum electronics follows. When light of a certain color or wavelength range falls on a photomultiplier, a photodiode, or on another type of photodetector (PD) or quantum counter , the light causes observable pulses of output electric current. Each current pulse is said to signal
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