Page 164 - An Introduction to Microelectromechanical Systems Engineering

Fiber-Optic Communication Devices 143

A basic laser consists of an optical amplification medium (a gain medium) posi-
tioned inside a resonant cavity [6, 7]. The amplification medium can be a gas (e.g.,
helium-neon or argon), a crystal (e.g., a ruby or neodymium), or, most commonly, a
semiconductor material (e.g., GaAs, AlGaAs, or InP, depending on the wavelength
of interest). The resonant cavity, in its simplest form, consists of two partially
reflecting surfaces with an optical separation equal to an integral number of half
wavelengths. Its role is to provide positive optical feedback by circulating light
within its geometrical boundaries (see Figure 5.7).
In an optical amplifier, an electrical current or a high-intensity light excites
(pumps) electrons from a low-energy (ground) state to a high-energy (excited) state.
When the population of electrons in the excited state exceeds that in the ground
state, the material reaches population inversion and becomes capable of a physical
process known as stimulated amplification [8]. In this process, an incoming photon
whose energy is equal to the energy difference between the excited and ground states
stimulates the relaxation of an electron to its ground state, thus releasing a photon
that is coherent (i.e., preserving the phase) and chromatic (i.e., preserving the wave-
length) with the incident photon. When an optical amplifier is placed within an opti-
cally resonant cavity, light reflects back and forth inside the resonator with coherent
amplification at every pass within the gain medium—it is this positive feedback that
gives rise to the high intensity of the laser beam. However, resonance occurs only at
certain specific wavelengths or frequencies—these are called the cavity longitudinal
modes and are separated by a frequency equal to c/2L, where c is the speed of light
within the medium and L is the optical cavity length [9]. At these frequencies, the
optical length of the cavity is an integral number of half wavelengths. Light at other
wavelengths rapidly decays. For relatively long cavities (>0.5 mm), multiple dis-
crete modes coexist within the available spectrum of the gain medium, and the light

λ
L = m
2
Partially reflecting mirrors Gain Filter
medium
Output light

Resonant cavity

Transmission

Filter function
Cavity modes

c Frequency
2L Lasing mode

Figure 5.7 Illustration of the building blocks of a laser. A gain medium amplifies light as it
oscillates inside a resonant cavity. Only select wavelengths called longitudinal cavity modes that
are separated by a frequency equal to c/2L may exist within the cavity. A wavelength filter with a
narrow transmission function selects one lasing mode and ensures that the output light is
monochromatic.