Abstract

Recently, we have shown that a well-engineered cw mode-locked Nd:YLF laser with intracavity compensation of the thermal lens astigmatism generates nearly diffraction- and bandwidth-limited pulses about two times shorter than Nd:YAG, although with the same average power and increased stability.¹,² Additional advantages of YLF include a naturally polarized output (no thermal depolarization), a better pointing stability, and less pulse jitter when mode locked. In this paper we demonstrate further improvements in system performance, particularly in output power. For reasons which will be discussed, we decided to build a dual-rod system. Both rods are oriented such that the c-axes form an angle of 90° with each other. A halfwave plate located between the rods ensures a linearly polarized output (at 1.053 µm). In this way, the astigmatic thermal lens of the first rod¹ is compensated by the second one. Thus, a dynamic compensation of the thermal lensing is achieved for this structure. The resonator consists of a flat high reflector and a – 120-cm output coupler. The cavity length is 1.5 m (to allow mode locking at 100 MHz). A spherical lens (f = 38 cm), located at 47 cm from the high reflector, ensures a good filling of both rods in TEM₀₀ operation. Both rods, which are as close to each other as is practically possible, are located between the intracavity lens and the output coupler. The AO mode locker is placed near the output coupler. No intracavity polarizer is needed since the cavity is unstable for operation at 1.047 µm. The output beam has an excellent Gaussian profile without ellipticity. The beam characteristics are quasi-independent of pump power as a result of the dynamic compensation of the overall thermal lensing effects. The mode-locked output power is more than twice that of our single rod cavity² without noticeable pulse broadening. The stability of the system is also superior compared to the single rod design.

© 1989 Optical Society of America