Abstract

Previous demonstrations of additive-pulse mode-locked lasers have shown that mode-locking occurs over a fraction of the interferometric cycle of the difference in length between the main and external cavities. Consequently, either passively or actively stabilized cavities were necessary for continuous operation. Contrary to these previous demonstrations we present results from a system that mode-locks continuously over many interferometric cycles. Our system consists of a Nd:YLF laser coupled to an external cavity with a multimode fiber as the nonlinear medium. When a combination of the higher-order proper modes is excited in the fiber, the mode-locking behavior is insensitive to slow fluctuations of cavity lengths even if the amplitude of the fluctuation is many micrometers. The underlying locking mechanism is unable to follow fast fluctuations, and we determined that it has a response time in the sub-Hertz and Hertz regions. Our system can be operated for hours without any adjustment. Although higher-order modes are excited in the external cavity, the output beam remains largely in the TEM₀₀ mode.

© 1991 Optical Society of America