A nonreciprocal optical element and method for operating it to effect separate control of intensity and phase of counter propagating beams over a very wide range. The balancing pipe is provided with a first end to supply fuel to the first tank and a second end to suck fuel from the second tan, wherein a check valve is arranged upstream of the return pipe in the balancing pipe between the at least two tanks allowing a flow direction from the second tank to the first tank and blocking a flow in the reverse direction. The controller changes the deceleration control pattern in accordance with the state of the image-taking optical system detected by the state detector. The frame pattern encloses the second pattern.