Abstract

Recently there has been a lot of theoretical and experimental works reporting that group velocity can exceed the speed of light in vacuum. This so-called superluminal (faster-than-light) propagation of an optical pulse has been realized in passive linear media such as absorbers, waveguides, and dielectric mirrors.¹ This seemingly nonsensical behavior of the group velocity provoked a controversy over the meaning of group velocity, and it can be now understood as a pulse-reshaping phenomenon, where Einstein causality is still not violated.² On the other hand, group velocity can be very small at the gain center of an active media due to the same physical reason. In this work, though not complete, we report some evidences for the first time for slow pulse propagation through a transparent solid state crystal with Raman gain due to 1.06 μm pumping. This effect may possibly be used as an effective optical pulse delay generator which can be used for signals in the 1.5 μm communication band when pumped with 1.3 μm, since no resonance is involved in this phenomenon.

© 1999 Optical Society of America