Ultrafast Optics: Vector Cavity Lasers — Physics and Technology

Investigating the operation of fiber lasers where the vector nature of light propagation in the laser cavity is considered.

September 1, 2019

Air Force Research Laboratory, Arlington, Virginia

Fiber lasers have found widespread applications in industrial material processing, scientific research and military systems due to their advantages of easy maintenance, excellent stability, compact size and low cost. One characteristic of fiber lasers, compared to other types of lasers, is that strong light is confined to propagating a long distance in a fiber core that has a very small cross-sectional area. This has the consequence that the nonlinear light interaction with the matter has a very long length. This results in the strength of all nonlinear optical processes being strongly amplified. This means that a conventionally weak, nonlinear optical effect can become significant. Therefore, apart from the practical applications, fiber lasers also constitute an ideal platform for the exploration of various complex nonlinear dynamics.

A schematic diagram of the fiber laser experimental setup. PC: Polarization controller; EDF: Erbium doped fiber; WDM: Wavelength-division multiplexer; OC: Optical coupler; ISO: Isolator; OSA: Optical spectrum analyser.

Previous studies on the operation of fiber lasers have revealed many interesting nonlinear optical phenomena. Studying these nonlinear optical effects has not only led to a better understanding of the operation of fiber lasers, but also has driven the performance of the fiber lasers to the extreme through exploiting features of the nonlinear optical effects for better laser operation. However, previous studies of fiber lasers have mainly focused on the simple scalar cavity lasers, which ignore the vector nature of light propagation in fibers. For many practical cases, e.g. a fiber laser with a polarization sensitive component in the cavity, this is justified as the existence of the polarization sensitive component fixes the polarization of light in the laser cavity. However, for the comprehensive understanding of the features of fiber lasers, the vector nature of light propagation in the cavity fibers has to be considered. Theoretical studies have shown that the vector light propagation in single mode fibers involves the nonlinear coupling between the two orthogonal polarization components of light, which could introduce a number of new nonlinear dynamics, such as various types of vector soliton formation, polarization domains and domain walls, and black-white vector solitons. The aim of this research project is to investigate the operation of fiber lasers with a quasi-vector cavity both numerically and experimentally. It is expected that through this study, a deep and comprehensive understanding of the operation of fiber lasers could be gained, and fiber lasers with novel features could be further developed.

This research mainly focused on the study of two types of quasi-vector cavity fiber lasers. One is with a saturable absorber in the cavity where, due to the effect of the saturable absorber, the laser will be automatically mode locked. Mode locking generates a high peak power optical pulse in the laser cavity. Depending on the concrete laser operation conditions, the mode locked pulse would then be shaped into various forms of dissipative solitons through the nonlinear propagation in the laser cavity. Experimental studies based on the atomic layer graphene mode locked fiber lasers have revealed a number of interesting dissipative soliton operation features.

Another type is the case of the vector cavity fiber lasers without any saturable absorber in the cavity. For this type of fiber laser, it is shown that due to the cavity detuning, strong CW emission is intrinsically unstable as a result of the cavity induced modulation instability. It automatically becomes a periodic pulse train. Under strong pumping, the peak power of the pulses could become sufficiently strong that the nonlinear pulse shaping drives the pulses into dissipative solitons. As there is no saturable absorber in the cavity, the dynamics and features of the dissipative solitons are different from those with a saturable absorber in the cavity. Following convention, the dissipative solitons are called “temporal cavity solitons”.

This work was done by Dingyuan Tang of the Nanyang Technological University for the Air Force Research Laboratory.For more information, download the Technical Support Package (free white paper) here under the Photonics category. AFRL-0274