Optical modulator

1. An optical modulator comprising:

an input optical path;

an optical splitter for splitting optical signal light input from said input optical path into N optical paths, wherein is an integer of 2 or more;

one or more of K-level optical phase modulators arranged in N−1 of said N optical paths; and

an optical coupler for interfering output light from said N optical paths with each other in the same polarization state to send out interfered light as an output optical signal to an output optical path,

wherein one of said N optical paths has no optical phase modulator and amplitude modulated light having amplitude values which is K raised to the power of N−1 or less, or multilevel amplitude-and-phase modulated light modulated in both the amplitude and phase and having signal points which is K raised to the power of N−1 or less, is output to said output optical path, by interfering output light from the N−1 optical paths having said optical phase modulator with output light from the optical path having no optical phase modulator at the same phase or any phase angles other than ±π/2.

2. The optical modulator according to claim 1 ,

wherein at least one of said N optical paths has an optical phase adjuster for adjusting a phase difference with respect to the output light of specific one of the other optical paths.

3. The optical modulator according to claim 2 , further comprising

a phase control unit for automatically controlling said optical phase adjuster so as to maximize an amplitude modulated component of said output optical signal.

4. The optical modulator according to claim 3 , wherein

at least two of said N optical paths have said optical phase adjusters in the case where N is larger than 2, and each of the optical phase adjusters is controlled by said phase control unit so that the amplitude modulated component of the output optical signal is maximized.

5. The optical modulator according to claim 4 , wherein

said N optical paths are coupled by a plurality of couplers arranged in a multi-stage in such a manner that optical light output from a pair of said optical paths is interfered with each other by first one of said couplers and further interfered with optical signal output from another optical path by second one of said couplers,

said optical phase adjusters includes a first optical adjuster arranged in one of said pair of optical paths and a second optical adjuster arranged in said another optical path, and

said phase control unit controls said first optical phase adjuster based on output light of said first one of optical couplers and said second optical adjuster based on output light of said second one of optical couplers so that the amplitude modulated component of the output light output from each of the optical couplers is maximized.

6. The optical modulator according to claim 1 , wherein

at least each of said N−1 optical paths having said optical phase modulator includes a variable optical attenuator for adjusting the amplitude of the optical signal light so that

the output light from said N optical paths is interfered at a predetermined amplitude ratio.

7. The optical modulator according to claim 1 , wherein

at least one of said N optical paths includes an optical phase adjuster for adjusting a phase difference from the output light of specific one of the other optical paths, and

each of said N−1 optical paths having said optical phase modulator includes a variable optical attenuator for adjusting the amplitude of the optical signal light.

8. The optical modulator according to claim 1 , further comprising:

a second optical phase modulator connected in tandem to said output optical path so that the number of signal levels or signal constellation of modulated light output from the second optical phase modulator changes in accordance with the amplitude modulated light or the multilevel amplitude-and-phase modulated light output to the output optical path.

9. The optical modulator according to claim 1 ,

wherein said optical phase modulator is composed of a waveform distortion suppressing type binary phase modulator having a function of suppressing inter-symbol interference.

10. The optical modulator according to claim 1 , wherein said optical phase modulator is one of a binary phase modulator of modulation factor π, a binary phase modulator of arbitrary modulation factor and a quadrature quaternary phase modulator, each of which is composed of at least one single drive Mach-Zehnder modulator or a dual drive Mach-Zehnder modulator.

11. The optical modulator according to claim 1 , wherein

at least two of said N optical paths have said optical phase modulator in the case where N is larger than 2, and

binary electrical digital signals are supplied independently as modulation signals to the optical phase modulators at bit timing matched with each other.

12. An optical modulator for generating a modulated optical signal to an output optical path by splitting input light from an input optical path into N optical paths, wherein N is an integer of 2 or more, and by interfering output light from the N optical paths with each other, wherein:

each of N−1 optical path or paths among said N optical paths has a binary optical phase modulator for generating binary phase modulated light having signal phase points at phase 0 and phase π, and one of said N optical paths has no optical phase modulator; and

binary or more levels of multilevel amplitude modulation or ternary or more levels of multilevel amplitude-and-phase modulation modulated in both amplitude and phase is applied to the input light input to said input optical path, by interfering at output light from the N−1 optical paths having said optical phase modulator with output light from the optical path having no optical phase modulator at the same phase or any phase angles other than ±π/2.

13. The optical modulator according to claim 12 ,

wherein at least two of said N optical paths have said binary optical phase modulators, all of which are Mach-Zehnder type optical phase modulators.