Introduction

The traditional methods for soliton generation in optical fibers use laser sources which generate stable transform-limited ultra-short light pulses, the pulse shape and spectrum of which coincide with those of soliton pulses in fibers. For a long time only color-center lasers satisfied these conditions, and these lasers were used in the majority of soliton experiments (see, for example, Mollenauer et al., 1980; Mollenauer and Smith, 1988). The soliton laser (Mollenauer and Stolen, 1984) is also based on a color-center laser. The successes in semiconductor laser technology has made it possible to use laser diodes in recent soliton experiments (see, for example, Iwatsuki et al., 1988).

In this chapter we shall discuss alternative methods for soliton generation in fibers. In these methods the laser radiation coupled into the fiber is not a fundamental soliton at the fiber input, but the fundamental solitons are formed from the radiation due to the non-linear and dispersive effects in the fiber. Methods for the generation of a single fundamental soliton as well as high-repetition rate (up to THz range) trains of fundamental solitons, which are practically non-interacting with each other, will be described. High-quality adiabatic fundamental soliton compression and the effect of stabilisation of the femtosecond soliton pulse width in fibers with a slowly decreasing second-order dispersion will also be discussed. We shall discuss the problem of adiabatic soliton compression up to a duration of less than 20 fs, so we shall also consider a theoretical approach for the description of ultrashort pulse (USP) propagation through the fiber.

Theoretical properties of light wave envelope propagation in optical fibers are presented. Generation of bright and dark optical solitons, excitation of modulational instabilities and their applications to optical transmission systems are discussed together with other non-linear effects such as the stimulated Raman process.

Introduction

The envelope of a light wave guided in an optical fiber is deformed by the dispersive (variation of the group velocity as a function of the wavelength) and non-linear (variation of the phase velocity as a function of the wave intensity) properties of the fiber. The dispersive property of the light wave envelope is decided by the group velocity dispersion (GVD) which may be described by the second derivative of the axial wavenumber k (= 2π/λ) with respect to the angular frequency ω of the light wave, ∂2k/∂ω2 (= k″). k″ is related to the coefficient the group velocity delay D in ps per deviation of wavelength in nm and per distance of propagation in km, through k″ = Dλ2/(2πc) where λ is the wavelength of the light and c is the speed of light. For a standard fiber, D has a value of approximately –10 ps/nm · km for the wavelength of approximately 1.5 μm. D becomes zero near λ = 1.3 μm for a standard fiber and near λ = 1.5 μm for a dispersion-shifted fiber.

The non-linear properties of the light wave envelope are determined by a combination of the Kerr effect (an effect of the increase in refractive index n in proportion to the light intensity) and stimulated Brillouin and Raman scatterings.