In early 1930, there have been some scientists purposed to use fiber as a medium of transmission waveguide of the light. However, in that age, the manufacture of purification of glass and technology of semiconductor was not developed as well as what we see today. Besides, there was no reliable light source, and the insertion loss of fiber was very large. The loss of transmission was above 1000dB per kilometer before. Therefore, it didn’t not adapt to fiber optical communication system. Until 1960, Maiman, an America scientist, demonstrated a ruby laser and the first laser was born in the world. In 1962, Hall and Nathan etc la, they invented a GaAs semiconductor laser. It improved the idea of transmission of using fiber as a medium of the waveguide. In 1966, Gao-kun, a scientist of non-Chinese citizen of Chinese origin, proposed this idea. The great contributions of these people open the door of the studies of the fiber optical communication system.
A fiber amplifier can be converted into a laser by placing it inside a cavity designed to provide optical feedback. Such lasers are called fiber lasers. Many kinds of rare-earth ions, such as erbium (Er), neodymium (Nd), and ytterbium (Yb), can be used to make fiber lasers capable of operating over a wide range of wavelength extending from 0.4 to 4µm. The first fiber laser, demonstrated in 1961, was based on an Nd-doped fiber with 300µm core diameter, but it had high insertion loss [1]. In 1973, low-loss silica fibers were used to build diode-pumped fiber lasers and soon after such fibers became available [2].
However, it was not until the late 1980s that fiber lasers began to attract a lot of interests in research. The initial main emphases of the research were on Nd- and Er-doped fiber lasers. Nd-doped fiber lasers are of considerable practical interest since they can be pumped by GaAs semiconductor lasers operating near 0.8µm. On the other hand, Er-doped fiber lasers can operate in several wavelength regions, ranging from visible to far infrared. The wavelength of 1.55µm regions has attracted the most attention because it coincides with the low-loss region of silica fibers for optical communication applications.
The performance of Erbium-doped fiber lasers (EDFLs) improves considerably while they are pumped at the 980 or 1480nm wavelength, because of the absence of the excited-state absorption. The 980nm pump wavelength yields higher gains than a 1480nm pump at high powers. This comes from the fact that 980nm has achieved a higher inversion than 1480nm. But the amplified spontaneous emission (ASE) starts growing as the pump power is increased; the higher inversion created at the beginning of the fiber by the 980 pump creates a better seed for the forward ASE than that created by the 1480nm pump [3]. In public, there are three kinds of pumping mechanism: forward pumping, backward pumping, and bidirectional pumping.
Furthermore the wavelength of pumping power and pumping mechanisms are decided by the applications and demands of the EDFLs.
As early as 1989, a 980nm-pumped EDFL exhibited a slope efficiency of 58%
against the absorbed pump power [4]. They also exhibited good performances when pumped at 1480nm. The choices between the pumping wavelength 980 and 1480nm and structures are not always clear since each pumping way has its own merits.
In a 1989 experiment on active mode-locking, 4ps pulses were generated by using a ring cavity which included 2km of standard fibers with large anomalous GVD [5]. In 1992, a fiber laser provided 3.5 to 10ps pulses with a transform-limited time-bandwidth product of 0.32 at the repetition rate up to 20GHz [6]. The laser was used in a system experiment to demonstrate such a laser source which can be used in soliton communication systems at the bit rate up to 8Gb/s. In the same year, a stabilization scheme for a mode-locked erbium fiber laser which relies on locking the pulse phase with that of the drive source was reported [7]. In 1993, a EDFL produced 6ps pulses at the repetition rate up to 40GHz and with the output wavelength tunable over a wide range of 40 to 50nm [8]. In 1999, the technique of regenerative mode locking with phase-locked loop (PLL) produced a 40GHz pulse train with tuning range from 1530 to 1560nm and pulsewidth as short as 0.9ps by using a soliton effect in the fiber cavity [9]. It was also demonstrated for ultrahigh-speed optical communication in the time [Time-Division multiplexing (TDM)] and frequency domains [Wavelength-Division multiplexing (WDM)]. Using the dispersion-managed soliton technique and soliton effect, the 1 Tb/s (40Gb/s × 25 channels) WDM soliton transmission and ultrahigh speed optical TDM transmission which exceeds 1Tb/s was achieved [10]. Related to the theory of active mode-locked lasers, more details will be discussed in chapter2.
Passive mode locking is an all-optical nonlinear technique capable of producing ultra-short optical pulse, without requiring any active component, such as a modulator, inside the laser cavity. Saturable absorbers have been used for passive mode locking since early 1970s. It is the sole method available for use until the invention of the additive-pulse mode locking techniques emerged. Additive-pulse mode-locking was first demonstrated in a soliton laser by Mollenauer and Stolen (1984) [11]. Subsequent
research showed that this method can be applied to non-soliton systems as well [12].
In 1992, the technique of nonlinear polarization rotation was first used in order to build passive mode-locked fiber lasers [13] and it was quickly demonstrated that stable and self-starting pulse trains of subpicosecond pulses at a 42MHz repetition rate can be generated by using this technique [14]. Moreover, ultra-short pulses less than 100fs at a repetition rate of 48MHz were obtained in a ring cavity configuration in which the net dispersion was positive [15].
As short as femtosecond pulses and stable ultrahigh-speed pulse train can be available respectively from the passive mode-locked fiber laser and the active mode-locked fiber laser. With these advantages and different characteristic, nowadays, fiber lasers have been widely used in several different areas especially for the ultrahigh-speed fiber optical communication and nonlinear optics experiments.