rtc a l axis = Voltage j (iO O m V per division)

Horizontal axis « time

-[ (1 0 ,0msper division)

I Vertical axis » Vol&ge

; 1 (2,00 V per division)

: Horizontal axis = time ■ (50. Ops per division)

Figure 3.8 The photo diode trace at the cavity exit with the beam transverse mode locked and actively longitudinally mode locked.

Figure 3.9: The longitudinally mode locked and transversely mode selected beam at the exit to the cavity, when the oscilloscope is on the IMÜ termination setting.

Dye concentrate is added to the solvent in the pumping reservoir in 1ml doses and the

oscilloscope trace of the photodiode output is monitored after each addition. As the dye

concentration increased the number of mode locked trains decreased. The dye

concentration is increased until there is only one mode locked train left, shown as a

single step in Figure 3.9.

Once the passive and active mode locking has been achieved the laser must be

checked to see under what conditions double pulsing will occur. Although the rod is

water cooled, there is a temperature differential between the outer part of the rod and the

core. If the differential is great enough the rod can act as a thermal lens due to the

difference in refractive index of the laser medium over the cross section of the rod. The

result can be that the focusing of the laser beam is such that two or more transverse

modes oscillate within the cavity. Figure 3.10 shows an oscilloscope trace from the

photodiode at the exit of the laser cavity of double pulsing of the beam. The two peaks

are due to two transverse modes oscillating in the laser cavity.

I V « rticjl axis « Voltage 12.00 V per division) Horiaontal axis = Time tIO Ous per division)

Figure 3.10: Double pulsing of the laser beam.

The temperature differential occurs not only when the laser has been running for a long

period of time but also when the voltage to the flashlamps is increased. The difference

between the normal operating voltage and that required for the production of double

pulsing should be greater than 40 V according to the manufacturer’s specifications. If

the voltage difference were less than 40 V then the energy of the beam must be

decreased to prevent thermal lensing occurring after prolonged running of the laser.

More dye concentrate should be added to the solvent so that more photons are

absorbed and the beam energy decreased.

Under stable mode locked conditions, the energy per pulse of the beam at the exit of

the well-aligned laser cavity should be 3.5 - 4 mJ. The laser manufacturer states that 4

mJ is the safe limit for the optics in the cavity. Any increase beyond this energy and

there is a risk that the optic coatings will be damaged by the laser light. Figure 3.11

shows the photodiode trace of the mode locked train at the cavity exit and viewed on

the oscilloscope with a 50 Q terminator.

V*rUcË *xti F VolüLg#

(1.00 ^ per dhrislon)

Figure 3.11: The oscilloscope trace of a mode locked train of puises viewed with a 50Ü terminator.

3.2.3 Beam Alignment of the Non-Cavity Optics, Pulse Selection and

Second Harmonic Generation in the Nd-YAG Picosecond Laser

Alignment of the optics through the étalon, E, to the laser exit depends primarily on the angles of the three mirrors, M2, M3 and M4.

= direction of the laser light O = circularly polarized light

^ = horizontally polarized light ( • ) = vertically polarized light

Pos 1 550 mJ CC3 CV3 CC2 CV2 HW P GTP M3 M2 Pos6 37 mJ DM2 P3 Q W P2 QWP1 1.26 kV

-►o

Figure 3.12: Schematic of the outer cavity components in the Nd-YAG laser. (The notation is as for Figure 3.1).

The procedure of alignment is carried out with the power to the amplifier rod off. Thus, the beam profiles are very weak and any gross misalignment does not lead to damage of the optics. The alignment procedure begins by adjusting the angle of the first mirror in this section of the laser, M2 in Figure 3.12. In order to align M2 the beam profile in front of MS is monitored (shown in Figure 3.12 as Pos.1). Since the beam profile has been expanded by two telescopes, CC2 and CV2, and CCS and CVS it is easier to observe aberrations in the beam profile. Once the gross mis­ alignment has been eliminated by aligning M2, MS can be adjusted while the beam profile in front of M4 is monitored (shown in Figure S.12 as Pos.2). Careful examination of the diffraction rings within the profile caused by the beams’ passage through the optics provides information about the degree of alignment. Care must be taken, though, since dust on the optics can also create diffraction rings resulting in misinterpretation of the reason for the shape of the beam profile. Once a satisfactory beam profile has been produced M4 can be aligned. The beam profile in front of QWP2 is monitored (shown in Figure 3.12 as Pos.S). The beam profile at

the exit of the laser is observed and if the diffraction rings are not concentric, adjustment of M4 is necessary. It is always necessary to return to M2, MS and M4 since the adjustment of the optics is an iterative procedure. Figure 3.1 S shows the results of good and bad alignment of the laser optics.

W e ll aligned

Badly aligned