4.3 Principles of Laser Diodes
175
Active medium
Front mirror
80%
Rear mirror
R = 100%
Cavity length
Example of allowed modes in the laser cavity
(a)
(b)
Fig. 4.18 a Two parallel light-reflecting mirrored surfaces define a Fabry-Perot resonator cavity;
b schematic of a simple laser design and some allowed lasing modes
semiconductor crystal. The purpose of the mirrors is to establish a strong optical
feedback in the longitudinal direction. This feedback mechanism converts the device
into an oscillator (and hence a light emitter) with a gain mechanism that compensates
for optical losses in the cavity at certain resonant optical frequencies. The sides of
the cavity are simply formed by roughing the edges of the device to reduce unwanted
emissions in the lateral directions.
As the light reflects back and forth within the Fabry-Perot cavity, the electric
fields of the light interfere on successive round trips. Figure 4.18b shows that those
wavelengths which are integer multiples of the cavity length interfere constructively.
Thus their amplitudes add when they exit the device through the right-hand facet.
All other wavelengths interfere destructively and therefore cancel themselves out.
The optical frequencies at which constructive interference occurs are the resonant
frequencies of the cavity. Consequently, spontaneously emitted photons that have
wavelengths at these resonant frequencies reinforce themselves after multiple trips
through the cavity so that their optical field becomes very strong. The resonant
wavelengths are called the longitudinal modes of the cavity because they resonate
along the length of the cavity.
Figure 4.19 illustrates the behavior of the resonant wavelengths for three values
of the mirror reflectivity (R = 0.4, 0.7, and 0.9; see Sect. 2.11). The plots give the
relative intensity as a function of the wavelength relative to the cavity length. As
can be seen from Fig. 4.19, the width of the resonances depends on the value of
the reflectivity. The result is that the resonances become sharper as the reflectivity
increases. Figure 4.19 also illustrates the free spectral range (FSR), which is the
spacing in either optical frequency or wavelength between two successive reflected or
transmitted optical intensity maxima or minima. Chapter 10 provides further details
on the operational theory of Fabry-Perot cavities or etalons.
175
Active medium
Front mirror
80%
Rear mirror
R = 100%
Cavity length
Example of allowed modes in the laser cavity
(a)
(b)
Fig. 4.18 a Two parallel light-reflecting mirrored surfaces define a Fabry-Perot resonator cavity;
b schematic of a simple laser design and some allowed lasing modes
semiconductor crystal. The purpose of the mirrors is to establish a strong optical
feedback in the longitudinal direction. This feedback mechanism converts the device
into an oscillator (and hence a light emitter) with a gain mechanism that compensates
for optical losses in the cavity at certain resonant optical frequencies. The sides of
the cavity are simply formed by roughing the edges of the device to reduce unwanted
emissions in the lateral directions.
As the light reflects back and forth within the Fabry-Perot cavity, the electric
fields of the light interfere on successive round trips. Figure 4.18b shows that those
wavelengths which are integer multiples of the cavity length interfere constructively.
Thus their amplitudes add when they exit the device through the right-hand facet.
All other wavelengths interfere destructively and therefore cancel themselves out.
The optical frequencies at which constructive interference occurs are the resonant
frequencies of the cavity. Consequently, spontaneously emitted photons that have
wavelengths at these resonant frequencies reinforce themselves after multiple trips
through the cavity so that their optical field becomes very strong. The resonant
wavelengths are called the longitudinal modes of the cavity because they resonate
along the length of the cavity.
Figure 4.19 illustrates the behavior of the resonant wavelengths for three values
of the mirror reflectivity (R = 0.4, 0.7, and 0.9; see Sect. 2.11). The plots give the
relative intensity as a function of the wavelength relative to the cavity length. As
can be seen from Fig. 4.19, the width of the resonances depends on the value of
the reflectivity. The result is that the resonances become sharper as the reflectivity
increases. Figure 4.19 also illustrates the free spectral range (FSR), which is the
spacing in either optical frequency or wavelength between two successive reflected or
transmitted optical intensity maxima or minima. Chapter 10 provides further details
on the operational theory of Fabry-Perot cavities or etalons.