Method of driving a polarization mode selective semiconductor laser with a wide modulation band, and an optical communication system

Claim 2

A method according to claim 1, characterized in that the step of injecting the sub-modulation current is executed when a frequency of the switching is lower than a predetermined frequency.

Claim 3

A method according to claim 2, characterized in that the predetermined frequency is approximately 1 MHz.

Claim 4

A method according to claim 1, characterized in that the phase and/or a modulation amplitude of the sub-modulation current is so adjusted as to decrease a phase difference between the main modulation current and an output optical signal from said semiconductor laser.

Claim 5

A method according to claim 1, characterized in that the phase of the sub-modulation current is so adjusted as to be the same phase as or an opposite phase to a phase of the main modulation current.

Claim 6

A method according to claim 5, characterized in that whether the phase of the sub-modulation current is to be the same phase as or an opposite phase to the phase of the main modulation current is determined in accordance with whether a sign of a slope of a boundary line, which is determined by an orthogonal coordinate system of the sub-modulation current and the main modulation current, and on which the two polarization modes of said semiconductor laser are switched, is positive or negative.

Claim 7

A method according to claim 4, characterized in that a ratio of modulation amplitudes of the sub-modulation current and the main modulation current is so adjusted that a slope of a boundary line, which is determined by an orthogonal coordinate system of the sub-modulation current and the main modulation current, and on which the two polarization modes of said semiconductor laser are switched, is substantially parallel to a synthetic amplitude of the sub-modulation current and the main modulation current on the orthogonal coordinate system.

Claim 8

An optical communication method in an optical communication system which transmits information in the form of an optical signal between nodes, comprising the steps of: switching two polarization modes of an output from a semiconductor laser having a plurality of electrodes in accordance with information to be transmitted in a node for transmitting information; outputting light having one of the two polarization modes to an optical transmission line; and receiving the optical signal on said optical transmission line in a node for receiving information, characterized in that the step of switching the two polarization modes is the step defined in any one of claims 1 to 7.

Claim 9

A method according to claim 8, characterized in that output optical signals from a plurality of nodes for simultaneously transmitting information in said optical communication system have different wavelengths.

Claim 10

A semiconductor laser device for outputting two polarization modes with different planes of polarization by switching the two polarization modes, comprising: a semiconductor laser having not less than two electrodes, characterized by further comprising first injecting means for injecting a main modulation current into one of said electrodes of said semiconductor laser; and second injecting means for injecting a sub-modulation current whose phase is adjusted with respect to the main modulation current into at least one of said electrodes except for the electrode into which the main modulation current is injected.

Claim 11

A device according to claim 10, characterized in that the sub-modulation current is injected when a frequency of the switching is lower than a predetermined frequency.

Claim 12

A device according to claim 11, characterized in that the predetermined frequency is approximately 1 MHz.

Claim 13

A device according to claim 10, characterized by further comprising: a modulation power supply; and means for supplying an output from said modulation power supply to said means for injecting the main modulation current and said means for injecting the sub-modulation current.

Claim 14

A device according to claim 13, characterized in that said second injecting means has a frequency characteristic and does not inject the sub-modulation current if a frequency of the output from said modulation power supply exceeds a predetermined frequency.

Claim 15

A device according to claim 14, characterized in that the predetermined frequency is approximately 1 MHz.

Claim 16

A device according to claim 10, characterized in that said semiconductor laser is a distributed feedback laser including a diffraction grating near an optical waveguide containing a light-emitting layer, so that said light-emitting layer contains a quantum well structure, and a wavelength corresponding to an energy band gap between an energy level of a light hole and a ground level of an electron in a quantum well of said quantum well structure is close to a Bragg wavelength of said diffraction grating.

Claim 17

A device according to claim 16, characterized in that threshold gains at the Bragg wavelength in the two polarization modes are substantially equal.

Claim 18

A device according to claim 16, characterized in that said quantum well structure is a strained quantum well structure into which a strain is introduced, so that in a quantum well an energy level of a light hole and an energy level of a heavy hole are equal or the energy level of a light hole is closer to a ground level of an electron.

Claim 19

A device according to claim 18, characterized in that the strain is a tensile strain introduced into an internal quantum well of said quantum well structure.

Claim 20

A device according to claim 16, characterized in that said quantum well structure is a multiple quantum well structure.

Claim 21

A device according to claim 13, characterized in that said first injecting means passes the output from said modulation power supply through a first amplifier and injects the output as a first modulation current into one of said electrodes of said semiconductor laser, and said second injecting means passes the output from said modulation power supply through a second amplifier and injects the output as a second modulation current into at least one of said electrodes except for the electrode into which said first injecting means injects the first modulation current, so that a cutoff frequency of said second amplifier is a predetermined frequency, and the second modulation current is not injected if a frequency of the output from said modulation power supply exceeds the predetermined frequency.

Claim 22

A device according to claim 21, characterized in that the outputs from said first and second amplifiers have the same phase.

Claim 23

A device according to claim 21, characterized in that the outputs from said first and second amplifiers have opposite phases.

Claim 24

A device according to claim 21, characterized in that a ratio of gains of said first and second amplifiers is so adjusted as to decrease a difference between a phase of the main modulation current and a phase of an output optical signal from said semiconductor laser.

Claim 25

A device according to claim 13, characterized in that said first injecting means passes the output from said modulation power supply through a first voltage-current converter, DC-couples an output from said first voltage-current converter and a first DC current, and injects the resultant current as a first modulation current into one of said electrodes of said semiconductor laser, and said second injecting means passes the output from said modulation power supply through a second voltage-current converter, DC-couples an output from said second voltage-current converter and a second DC current, injects the resultant current as a second modulation current into at least one of said electrodes except for the electrode into which said first injecting means injects the first modulation current, and has a low-pass filter with a predetermined cutoff frequency for filtering the output from said second voltage-current converter, so that the second modulation current is not injected if a frequency of the output from said modulation power supply exceeds the predetermined frequency.

Claim 26

A device according to claim 25, characterized in that said first and second voltage-current converters DC-couple the output from said modulation power supply at the same phase.

Claim 27

A device according to claim 25, characterized in that said first and second voltage-current converters DC-couple the output from said modulation power supply at opposite phases.

Claim 28

A device according to claim 25, characterized in that a ratio of gains of said first and second voltage-current converters is so adjusted as to decrease a difference between a phase of the main modulation current and a phase of an output optical signal from said semiconductor laser.

Claim 29

A node used in an optical communication system for transmitting information in the form of an optical signal between nodes, comprising: a semiconductor laser device for outputting two polarization modes with different planes of polarization by switching the polarization modes in accordance with information to be transmitted; and polarization mode selecting means for selectively outputting one of the two polarization modes of the output from said semiconductor laser device, characterized in that said semiconductor laser device is a device defined in any one of claims 10 to 28.

Claim 30

An optical communication system for transmitting information in the form of an optical signal between nodes, comprising: a node for transmitting information and a node for receiving information, characterized in that said node for transmitting information is a node defined in claim 29.