An optical transmitter comprising: a bit converter configured to process input bits in units of four bits, and to generate an output bit sequence having eight bits based on one of a first conversion table, a second conversion table, and a third conversion table, wherein the first, second and third conversion tables are different from each other; and a symbol mapper configured to allocate the output bit sequence to an X-polarization and to a Y-polarization of each of two consecutive time slots, wherein the bit converter is further configured to: perform, at a first ratio, a process of generating the output bit sequence based on the first conversion table, wherein the first ratio corresponds to a first repetition rate of performing the process of generating the output bit sequence based on the first conversion table; perform, each at a second ratio lower than the first ratio, a process of generating the output bit sequence based on the second conversion table and a process of generating the output bit sequence based on the third conversion table, wherein the second ratio corresponds to a second repetition rate of performing the process of generating the output bit sequence based on each of the second and third conversion tables; and perform the process of generating the output bit sequence based on the second conversion table and the process of generating the output bit sequence based on the third conversion table such that a sequence of two most significant bits and a sequence of two least significant bits of the output bit sequence are respectively associated with values that are allocated respectively to two constellation points having phases different by 180 degrees on a complex plane. a bit converter configured to process input bits in units of four bits, and to generate an output bit sequence having eight bits based on one of a first conversion table, a second conversion table, and a third conversion table, wherein the first, second and third conversion tables are different from each other; and a symbol mapper configured to allocate the output bit sequence to an X-polarization and to a Y-polarization of each of two consecutive time slots, wherein the bit converter is further configured to: perform, at a first ratio, a process of generating the output bit sequence based on the first conversion table, wherein the first ratio corresponds to a first repetition rate of performing the process of generating the output bit sequence based on the first conversion table; perform, each at a second ratio lower than the first ratio, a process of generating the output bit sequence based on the second conversion table and a process of generating the output bit sequence based on the third conversion table, wherein the second ratio corresponds to a second repetition rate of performing the process of generating the output bit sequence based on each of the second and third conversion tables; and perform the process of generating the output bit sequence based on the second conversion table and the process of generating the output bit sequence based on the third conversion table such that a sequence of two most significant bits and a sequence of two least significant bits of the output bit sequence are respectively associated with values that are allocated respectively to two constellation points having phases different by 180 degrees on a complex plane. perform, at a first ratio, a process of generating the output bit sequence based on the first conversion table, wherein the first ratio corresponds to a first repetition rate of performing the process of generating the output bit sequence based on the first conversion table; perform, each at a second ratio lower than the first ratio, a process of generating the output bit sequence based on the second conversion table and a process of generating the output bit sequence based on the third conversion table, wherein the second ratio corresponds to a second repetition rate of performing the process of generating the output bit sequence based on each of the second and third conversion tables; and perform the process of generating the output bit sequence based on the second conversion table and the process of generating the output bit sequence based on the third conversion table such that a sequence of two most significant bits and a sequence of two least significant bits of the output bit sequence are respectively associated with values that are allocated respectively to two constellation points having phases different by 180 degrees on a complex plane.