Phase modulator and optical modulation device
View Patent ↗An optical modulator 100 includes an input unit 110 , a splitter unit 120 , a first waveguide 131 , a second waveguide 132 , a directional coupler unit 140 , a first output unit 151 , a second output unit 152 , a first liquid-crystal cell 161 , and a second liquid-crystal cell 162 . The splitter unit 120 , the first waveguide 131 , the second waveguide 132 , and the directional coupler unit 140 form an optical interferometer that branches an input light and causes the branched lights to interfere. The first liquid-crystal cell 161 and the second liquid-crystal cell 162 transmit the lights branched in the optical interferometer and delay the lights according to applied signals, respectively. The second liquid-crystal cell 162 is thinner than the first liquid-crystal cell 161.
1. An optical modulator comprising:
a first liquid-crystal cell that includes a first liquid-crystal layer between a pair of substrates;
a second liquid-crystal cell that includes a second liquid-crystal layer between a pair of substrates; and
a drive circuit that drives the first liquid-crystal cell and the second liquid-crystal cell, wherein
the second liquid-crystal layer is thinner than the first liquid-crystal layer, and
the drive circuit includes
a phase-modulation unit that delays a light passing through the first liquid-crystal cell and applies a phase-modulation signal to control the phase of the light; and
a small-modulation unit that delays a light passing through the second liquid-crystal cell and applies a small-phase-modulation signal to finely control the phase of the light.
2. The phase modulator according to claim 1 , wherein the drive circuit detects a variation component that is caused by the small-phase-modulation signal and included in the light that has passed through the first liquid-crystal cell and the second liquid-crystal cell, and controls the phase-modulation signal based on a result of detection.
3. The phase modulator according to claim 1 , wherein the thickness of the liquid-crystal layer of the second liquid-crystal cell is ¼ or less of the thickness of the first liquid-crystal cell.
4. The phase modulator according to claim 1 , wherein the first liquid-crystal cell and the second liquid-crystal cell are layered on each other.
5. The phase modulator according to claim 4 , wherein one of the substrates of the first liquid-crystal cell and one of the substrates of the second liquid-crystal cell are a common substrate.
6. The phase modulator according to claim 1 , wherein the polarization direction of an input light to the first liquid-crystal cell or the second liquid-crystal cell is parallel to the slow axis of the first liquid-crystal cell or the second liquid-crystal cell.
7. The phase modulator according to claim 1 , wherein a polarizer or a polarization beam splitter is arranged at a position where an input light input to the first liquid-crystal cell or the second liquid-crystal cell passes through.
8. The phase modulator according to claim 1 , further comprising a first correction liquid-crystal cell and a second correction liquid-crystal cell that are layered on each other, wherein
the first correction liquid-crystal cell has the same configuration as the first liquid-crystal cell except that the slow axis thereof is shifted by 90 degrees, and
the second correction liquid-crystal cell has the same configuration as the second liquid-crystal cell except that the slow axis thereof is shifted by 90 degrees.
9. An optical modulation device comprising:
an optical interferometer that branches an input light and causes branched lights to interfere; and
a phase modulator according to claim 1 , wherein
the first liquid-crystal cell and the second liquid-crystal cell transmit the branched lights, respectively.
10. The optical modulation device according to claim 9 , wherein
the optical interferometer includes an input unit, a splitter unit, a first waveguide, a second waveguide, and an directional coupler unit, and
the first liquid-crystal cell and the second liquid-crystal cell are arranged on the first waveguide or the second waveguide.
11. The optical modulation device according to claim 9 , wherein
the optical interferometer includes a splitter, a plurality of mirrors, and a combiner, and
the first liquid-crystal cell and the second liquid-crystal cell are arranged between any two of the mirrors.
12. The optical modulation device according to claim 9 , wherein
the optical interferometer includes a splitter and a combiner,
the first liquid-crystal cell and the second liquid-crystal cell are arranged between the splitter and the combiner, and
a light is multiple-reflected between the splitter and the combiner.
13. The optical modulation device according to claim 9 , wherein
the optical interferometer includes a directional coupler and a ring waveguide, and
the first liquid-crystal cell and the second liquid-crystal cell are arranged in the ring waveguide.
14. The optical modulation device according to claim 9 , wherein
the optical interferometer includes a directional coupler and a plurality of mirrors, and
the first liquid-crystal cell and the second liquid-crystal cell are arranged between any two of the mirrors.
15. An optical modulation device comprising:
an optical interferometer that branches an input light and causes branched lights to interfere; and
a phase modulator according to claim 1 , wherein
the first liquid-crystal cell or the second liquid-crystal cell are reflective liquid-crystal elements that reflect the branched lights, respectively.
16. The optical modulation device according to claim 15 , wherein
the optical interferometer includes an input unit, a circulator, an optical coupler, a first waveguide, and a second waveguide, and
the first liquid-crystal cell and the second liquid-crystal cell are arranged at a position radiated by a light output from the first waveguide or the second waveguide.
17. The optical modulation device according to claim 15 , wherein
the optical interferometer includes an input unit, a circulator, a collimating lens, a half mirror, and a reflection unit,
the half mirror reflects, and outputs a part of a light from the collimating lens to the reflection unit, and
the first liquid-crystal cell and the second liquid-crystal cell are arranged at a position radiated by the rest of the light from the collimating lens.
18. The optical modulation device according to claim 15 , wherein
the optical interferometer includes an input unit, a circulator, a half mirror, a reflection unit, and a polarization beam splitter,
the half mirror reflects, and outputs a part of a light from the circulator to the reflection unit, and outputs the rest of the light from the circulator to the polarization beam splitter, and
the first liquid-crystal cell and the second liquid-crystal cell are arranged at a position radiated by a light output from the polarization beam splitter.
19. The optical modulation device according to claim 15 , wherein
the optical interferometer includes a collimating lens, a reflection unit, a spectroscope, a half mirror, and a polarization beam splitter,
the half mirror reflects, and outputs a part of an input light to the reflection unit, and outputs the rest of the light to the collimating lens,
the spectroscope separates a light from the collimating lens according to wavelength, and outputs the wavelength-separated lights to the polarization beam splitter, and
the first liquid-crystal cell and the second liquid-crystal cell are arranged at a position radiated by wavelength-separated lights output from the polarization beam splitter.
20. The optical modulation device according to claim 15 , wherein
the optical interferometer includes an input unit, a circulator, a collimator, and a half mirror,
a light output from the collimator is radiated on the half mirror, and
the first liquid-crystal cell and the second liquid-crystal cell are arranged at a position radiated by a light that has passed through the half mirror.