Ordinary electrical cable, even coax and twisted pair, is unable to carry current above a certain frequency. At one gigahertz, capacitive and inductive losses become unacceptable for most applications. This attenuation also depends upon the length of the line, a fact that dismayed early telegraphers.
As mentioned in a previous article, high-frequency signals can travel without great loss by means of a waveguide. The transmission mode must be transverse electric (TE) or transverse magnetic (TM), where only one of the fields is transverse to the direction of travel. This contrasts with transverse electric and magnetic (TEM), where two wires, one returning back to the power source, are required to complete a circuit. In TE and TM waveguide transmission, including optical fiber, it is a one-way street from the source to the load.
There’s a parallel situation in regard to resonant circuits. The conventional electronic scenario consists of capacitive and inductive elements either in series or in parallel. The circuit will ring, passing or blocking current at its resonant frequency, which is set by the capacitance and inductance. Obviously there is a limit beyond which this L-C resonance cannot happen. It is between two and three gigahertz. L-C resonators in this range can be made from a one-half-turn coil and incidental circuit capacitance. (Any coil has some capacitance and any capacitor has some inductance.) Such a configuration is difficult to tune and capable of handling only a small amount of current.
Just as a waveguide can convey electromagnetic energy at a higher frequency than conductive cable, so can a resonant cavity oscillate at a higher frequency than a conventional resonant circuit. In the context of electromagnetic energy, including light, a cavity resonator is one in which waves bounce back and forth inside a container, which takes the form of a cylinder or rectangular box with polished inner surfaces. Because there is some finite amount of loss in this arrangement, there must be a continuous input of electromagnetic energy from an outside source, unless what is desired is a damped wave.
The cavity’s lowest resonant frequency is known as the fundamental. The width of the cavity is equal to one-half the resonant wavelength. Such cavity resonators are practical only at microwave frequencies and above, including light. Otherwise, the size becomes prohibitive.
A laser makes use of a cavity resonator, its size determining the frequency of the coherent light emitted. The laser pump comprises the energy input, and the energy source can range from a AAA dry cell to a nuclear reactor. At one end of the resonant cavity is a fully reflective mirror. At the other end is a partially reflective mirror so part of the light reflects back into the cavity to sustain the resonating light energy while the other portion of the light is emitted in the form of a beam that is polarized, collimated and coherent.
After leaving the laser’s resonant cavity, it does not spread in a widening cone, becoming for practical purposes spatially dispersed and lost. Instead, limited by the accuracy of the optical system, it remains confined within a narrow beam, capable of traveling a great distance without appreciable loss.
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