Lab 4 Laser - Institutt for elektronikk og telekommunikasjon - NTNU

1.2 Laser fundamentals 8
1.2.2 Laser resonators and standing waves
The optical feedback and delay
functions are served by an optical
resonator, such as the one shown
in Fig. 1.1a constructed with two
mirrors that face each other. The
mirrors serve to recirculate, or feed
back, the light with efficiency R =
R1R2, the product of the reflectivities
R1 and R2 of the two mirrors.
The delay time t = 2L/c is
the time it takes light to complete
a round trip between the mirrors.
The net gain for a complete round
trip is GR and must be equal to
unity to sustain steady oscillation.
The condition GR = 1 means that
the gain compensates for the loss
and the oscillator will operate with
steady power. (What if GR < 1 or
GR > 1?).
There is an additional criterion
that after each round trip the light
wave crests line up with the crests
from the previous round trip. This
is called resonance and results in
a standing wave as shown in Fig.
1.1b. This condition is similar
to the resonances associated with
standing sound waves in an organ
pipe or on a guitar string. The condition
on the mirror separation is
that one round trip contains an integral
number of wavelengths 2L =
mλ, where m is an integer. The
corresponding resonant frequencies
are f m = c/λ = mf fsr , where the
free spectral range f fsr = c/2L is
the separation between resonance
frequencies. Fig. 1.1b has harmonic
number m = 4, but a typical
visible wavelength laser resonator
might have length L = 30 cm operating
at wavelength l = 600 nm
so that m = 1 million.
Figure 1.2a shows resonant fre-
Figure 1.1a: Mirror resonator
Figure 1.1b: Standing wave in a mirror
resonator
Figure 1.2: Typical laser output spectrum