Page 327 - Tunable Lasers Handbook

6 Transition Metal Solid-state Lasers 2

When wavelength control devices are utilized in laser resonators, the resolu-
tion is higher than predicted by using the single-pass approximation. For exan-
ple, in a pulsed laser the pulse propagates through the wavelength control device
several times as it evolves. Theory indicates and experiments have verified that
the resolution increases as the number of passes through the walrelength control
device increases [71]. Ifp is the number of passes through the wavelength con-
trol device that the pulse makes during the pulse evolution time interval, the res-
olution is increased by the factor p-?. Thus. when estimating the spectral band-
width of the laser output. the resolution of the wavelength control devices must
be known as well as the pulse evolution time interval.
Injection wavelength control utilizes a low-power or lowenergy laser.
referred to as a seed oscillator, to control the wavelength of a more energetic oscil-
lator referred to as a power oscillator. Either a pulsed or a cw single-longitudinal-
mode oscillator, that is, B single-wavelength oscillator, may be used to produce the
laser output needed for injection control [72-741. Injection seeding can utilize
length control of the power oscillator for high finesse resonators or length control
may be omitted for low finesse resonators. If length control is not utilized, the seed
laser resonator is not necessarily matched to the resonances of the power oscillator.
However. the output of the power oscillator will tend to occur at a resonance of the
power oscillator resonator nearest to the seed laser. Because this may not corre-
spond exactly to the injected wavelength. some wavelength pulling effects may
occur. In some cases, the injected wavelength will occur almosr exactly between
two adjacent resonances of the power oscillator. In this case, the power oscillator
will tend to oscillate at two wavelengths. On the other hand, if length control is uti-
lized, the resonances of the power oscillator match the resonances of the seed
oscillator. In this case, operation at a single wavelength is more likely. Hom?ever.
the power oscillator must be actively matched to the resonances of the seed oscilla-
tor. complicating the system.
Injection seeding has several advantages over passive wavelength control.
By eliminating or minimizing the wavelength control devices in the power oscil-
lator. losses in this device are decreased. Concomitant with a decrease in the
iosses is the attainment of higher efficiency. In addition, wavelength control of
the low-power or lowenergy seed laser is usually better than that of the wave-
length control of a high-power or high-energy device. Finally. optical devices
that are prone to laser induced damage are eliminated from the high-energy laser
device. therefore higher reliability is possible. However, the system is compli-
cated by the necessity of a separate wavelength-controlled oscillator.
Power o'r energy required from the seed oscillator to injection lock or injec-
tion seed a power oscillator can be estimated [75]. Power requirements for injec-
tion seeding are lower if length control is utilized. However. for low-finesse res-
onators. the difference is not great. The power or energy required for injection
seeding depends on the degree of spectral purity required. In essence. the pulse
evolving from the seed must extract the stored energy before the pulse evolving