Theory of a frequency-shifted feedback laser L.P. Yatsenko a , B.W. Shore b, * , K. Bergmann b a Institute of Physics, Ukrainian Academy of Sciences, prospect Nauki 46, 03650, Kiev-39, Ukraine b Universit€ at Kaiserslautern, 67653 Kaiserslautern, Germany Received 11 November 2003; received in revised form 10 February 2004; accepted 11 March 2004 Abstract We present a new approach to the description of the output from a frequency-shifted feedback (FSF) laser seeded by a phase-fluctuating but stationary continuous-wave (CW) laser. We illustrate the new analysis by showing how short frequency-chirped pulses arise for appropriate operating conditions. We show the equivalence of two common view- points of the FSF laser output as either a moving comb of equidistant frequencies or as a fixed set of discrete fre- quencies. We also consider operation of a FSF laser when there is no external seeding laser, and instead the cavity radiation originates with spontaneous emission. Ó 2004 Elsevier B.V. All rights reserved. PACS: 42.55.)f; 42.60.Da; 42.55.Ah Keywords: Optics; Lasers; Frequency-shifted feedback 1. Introduction Conventional lasers rely on multiple passes through a gain medium to reinforce a preselected frequency, thereby obtaining near-monochromatic output. For many purposes, such as excitation of Doppler-broadened atomic transitions, it is desir- able to have light spread over a broader range of frequencies. One means of fulfilling that objective is to introduce a frequency shifting element into the laser feedback loop such that successive passes of a wavepacket take place with different carrier frequencies. The operation of such a frequency shifting feedback (FSF) laser has been considered by several research groups [1–46] and it has been used for a number of practical applications [12,21,36,41,47,48]. Although much is known about the behavior of FSF lasers and some of their surprising properties (see e.g. [20,24]), there remain some fundamental questions. Even the precise nature of the field emerging from such a laser re- mains a matter for discussion and confusion [33]. Although all authors agree on the elements that comprise a FSF device, there are conflicting ap- proaches concerning detailed description of the electric field EðtÞ that emerges from the device. In part this is because a FSF laser can operate in a variety of regimes: the output may range from * Corresponding author. E-mail address: shore@physik.uni-kl.de (B.W. Shore). 0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.03.049 Optics Communications 236 (2004) 183–202 www.elsevier.com/locate/optcom