In an optical heterodyne communication system, an optical sensor such as an optical fiber gyro, and the like, a polarization controller which transforms a light having an arbitrary polarization to a light having a constant polarization, or to a light having another arbitrary polarization is necessary to be provided.  In an optical heterodyne receiving system using a single mode optical fiber, especially, it is considered that a fluctuation of a polarization such as a rotation of a polarized light angle to the same direction, which is caused in the transmitting of a signal light through the single mode optical fiber, is produced.  Therefore, the signal light is required to be transformed to a light having an arbitrary polarization.  In this point, an endless polarization controller having no limitation in a polarization controlling operation is required to be developed, to suppress such a fluctuation of a polarization. 
A conventional endless polarization controller, in which an arbitrary polarization is transformed to another arbitrary polarization, is described on pages 290 to 292 of "Electronics Letters, 12th March 1987 Vol.  23, No.  6."  by N. G. Walker, et al.  This conventional endless polarization controller comprises four optical phase modulators which are in series connected to generate birefringences having alternately different directions by 45.degree.,  wherein the optical phase modulators operate in limited ranges to control a polarization of light.  In operation, where one of the four optical phase modulators reaches a limitation of an operating voltage, it is then reset as the polarization control is continued by the other optical phase modulators.  At this reset operation, two optical phase modulators which are next to the reset optical phase modulator on one side thereof are varied in voltage to compensate the fluctuation of an output light polarization due to the voltage resetting.  This compensation is possible to be carried out in only a case where a radius of a circular arc defined on the Poincare sphere in accordance with the variation of a polarization by the reset optical phase modulator is smaller than that of a circular arc defined on the Poincare sphere in accordance with the variation of a polarization by the optical phase modulator which is farther than the other in the aforementioned next two modulators from the reset optical phase modulator. 
It is possible to adjust a largeness and smallness relation between the two circular arcs by changing driving voltages of two optical phase modulators which are positioned on the both sides of the reset optical phase modulator.  However, the conventional polarization controller has a disadvantage in that the aforementioned adjustment can not be carried out dependent on polarization states of input and output lights.  That is, an optical phase modulator is difficult to be reset without the dependency on polarization states of input and output lights. 
Furthermore, even if the resetting operation starts in an allowable condition, there is a possibility in which the condition is no longer met during the resetting period due to the fluctuation of polarizations of the input and output lights.  In this case, the resetting operation is required to be interrupted, and, if the resetting operation is continued, an output light of a predetermined polarization is not obtained to result in a power penalty, so that a stable resetting operation is not carried out. 
Accordingly, it is an object of this invention to provide a method for controlling a polarization of light in which a resetting of a voltage is stably carried out without the dependency on polarizations of input and output lights. 
According to this invention, a method for controlling a polarization of light, comprises steps of:
generating first to fifth birefringences for a propagating light in series having main axes of 0.degree.,  45.degree.,  0.degree.,  45.degree.  and 0.degree.  relative to an arbitrary direction on a surface orthogonal to a propagating direction of said light;
changing magnitude of said first to fifth birefringences to generate first to fifth phase differences .phi..sub.1, .phi..sub.2, .phi..sub.3, .phi..sub.4 and .phi..sub.5 corresponding thereto between each of polarizations corresponding to said main axes and each of polarizations orthogonal to said main axes; and
controlling said second to fourth phase differences .phi..sub.2, .phi..sub.3 and .phi..sub.4 among said first to fifth phase differences .phi..sub.1, .phi..sub.2, .phi..sub.3, .phi..sub.4 and .phi..sub.5 to be appropriate values, thereby transforming said propagating light having an arbitrary polarization at an input terminal to said propagating light having another arbitrary polarization at an output terminal;
wherein, in a case where one of said second to fourth phase differences .phi..sub.2, .phi..sub.3 and .phi..sub.4 reaches an operating limitation, one or both of said first and fifth phase differences .phi..sub.1 and .phi..sub.5 are controlled to be appropriate values along with remaining two phase differences among said second to fourth phase differences .phi..sub.2, .phi..sub.3 and .phi..sub.4, and said one of said second to fourth phase difference .phi..sub.2, .phi..sub.3 and .phi..sub.4 is then restored to be inside an operating range by an arbitrary value, so that said controlling of said second to fourth phase differences .phi..sub.2, .phi..sub.3 and .phi..sub.4 restarts to provide said propagating light having said another arbitrary polarization at said output terminal.