Light amplifiers in which light signals are directly amplified without conversion into electric signals have been extensively studied in various research and investigation facilities and expected as a key device of future light transmission systems. This is because the light amplifier is, in fact, free of any bit rate with ease in great capacitance and information from a multichannel can be amplified as a whole. One of such a light amplifier is a light amplifier which makes use of an optical fiber whose cores is doped with a rare earth element (or ion) such as Er, Nd, Yb or the like (which may be hereinafter referred to simply as "doped fiber") and wherein signal light and pumping light are introduced into the doped fiber in the same or opposite directions. The optical fiber amplifier using the doped fiber has the advantages that the gain is not of the polarization dependence with low noises and that the combination loss with a transmission path is small. This type of optical fiber amplifier may be particularly applied in the following manner.
(a) At the transmission side, the optical fiber amplifier is employed as an optical power booster to compensate for a possible loss or to increase the transmission power.
(b) At the receiving side, the optical fiber amplifier is used as an optical pre-amplifier to improve the receiving sensitivity.
(c) The optical fiber amplifier is used as a repeater to make a small-sized and reliable repeater.
With light whose wavelength is in the range of 0.8-1.6 .mu.m, production and application techniques of optical fibers using quartz glass adapted for long-range transmission have been established. The optical fiber is obtained by drawing an optical fiber preform in the form of a thick rod. The optical fiber preform should have a compositional gradient along the section thereof which is properly formulated as designed. A typical and known method of producing an optical fiber preform is one wherein a glass composition formed by chemical conversion of reactant gases is deposited on a quartz reaction tube such as by MCVD (modified chemical vapour deposition). In the MCVD process, reactant gases such as, for example, SiCl.sub.4 and O.sub.2 have been charged into the quartz reaction tube and heated to a level necessary for the reaction. While the heating zone or section is moved along the length of the quartz reaction tube, a fresh glass layer is deposited on the inner wall surface of the tube. A number of layers, e.g. 20-30 layers, are repeatedly deposited. The respective layers are individually controlled in composition, so that the composition of an optical fiber along the section obtained from the preform can thus be controlled. After deposition of the layers to a satisfactory extent, the quartz reaction tube is collapsed to obtain a rod-shaped optical fiber preform. This optical fiber preform is drawn to obtain an optical fiber.
In the MCVD process, it is usual to employ reactant substances which are able to be gasified at room temperature. For instance, there are used SiCl.sub.4 for obtaining SiO.sub.2 which is a main constituent of the optical fiber and GeCl.sub.4 for obtaining GeO.sub.2 used to control a refractive index.
For the production of doped fibers, it is not possible to obtain reactant substances of rare earth elements which are gasified at room temperature, like SiCl.sub.4 and GeCl.sub.4. For this reason, a practically satisfactory concentration of doped rare earth elements cannot be obtained only by the MCVD technique. Accordingly, there have been proposed methods of attaining a practically satisfactory concentration of rare earth elements in the following manner.
One of the hitherto proposed methods for fabricating optical fiber preformes adapted for producing doped fibers is a method wherein when a core glass is formed on the inner surface of a quartz reaction tube according to the MCVD technique, a compound of a rare earth element accommodated in a chamber formed at one end portion of the quartz reaction tube is heated with a burner and gasified. The gas is introduced into the quartz reaction tube along with a reactant substance which has been gasified at room temperature and is used as a core material, thereby depositing a core glass doped with the rare earth element. Another hitherto proposed method for producing a doped fiber is one which comprises the steps of (a) depositing a core glass in the form of a soot on the inner surface of a quartz reaction tube so that the core glass is not vitrified, (b) immersing the quartz reaction tube, wherein the soot-like glass has been deposited, in a solution containing a compound of a rare earth element as a solute, thereby impregnating the solution in the soot-like core glass, and (c) drying the solution and collapsing the tube.
With the former method, the vapor pressure of the rare earth element compound is so low that the compound is liable to settle out and the quartz reaction tube has an inevitable temperature distribution along the length thereof. Accordingly, the concentration of the doped rare earth element is apt to become non-uniform especially along the length of the optical fiber preform. In addition, it is difficult to accurately control the concentration of the doped rare earth element.
With the latter method, it is necessary that after deposition of the soot-like core glass on the inner surface of the quartz reaction tube mounted on a lathe, the quartz reaction tube have been once removed from the lathe and immersed in the solution. Subsequently, the quartz reaction tube is again set on the lathe and subjected to a collapsing step and the like, with attendant disadvantages that the method requires complicated operations with defects being likely to be produced. If the distribution in concentration of a rare earth element is caused to be produced along the radial direction of the optical fiber preform, it is necessary to immerse the quartz reaction tube in the solution a plurality of times. The above disadvantages are vital.
In the latter method, the amount of the impregnated solution in the soot-like core glass may vary depending on the conditions of the soot-like core glass including a grain size, leading to the problem that highly accurate control of the concentration of the doped rare earth element is difficult. If the doping concentration of the rare earth element is scattered, the gain and an optimum pumping light wavelength are also scattered in the optical fiber amplifier constituted of the doped fiber. This is why the concentration of the doped rare earth element has to be controlled in high accuracy.
The mode field of propagating light of an optical fiber has a so-called Gaussian distribution where the amplitude of the electric field is made high at the central portion of the core. Accordingly, the best amplification characteristic is not necessarily obtained when the distribution of the doping concentration of the rare earth element is constant along the radial direction of the core. Proper control of the distribution in concentration of the rare earth element may enable one to obtain optical amplification in an efficient manner. In this sense, the distribution in concentration of the rare earth element along the radial direction of the core should preferably be set in an arbitrary way.