JPS6313017A - Optical amplitude and phase modulator - Google Patents

Abstract

PURPOSE:To improve frequency utilization efficiency greatly by imposing optical amplitude phase modulation wherein amplitude modulation and phase modulation are performed simultaneously. CONSTITUTION:An amplitude modulation part 2 composed of a Y-branch type interference optical modulator imposes binary amplitude modulation upon input light 1 firstly. At this time, the branch ratio of an input-side Y branch 3 is set to a 1:(2-3<1/2>) amplitude ratio and branch angles of output-side Y branches 3 and 4 are adjusted. Then, an electrode 7 is added to a waveguide 5 which is smaller in the quantity of branch light between the 1st and the 2nd waveguides 5 and 6 branched at the Y branch 3, and the light shift in phase by pi with a voltage applied to the electrode 7. The composite wave output at the branch 4 varies with the voltage applied to the electrode 7. Here, a phase modulation part 8 imposes binary phase modulation on output light by applying a phase modulating electrode 9 with a voltage which makes a pi shift.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical phase modulation device used in coherent optical communications and the like.

[Conventional technology]

Coherent optical communication systems that use optical phase shift keying (PSK modulation) use frequency shift keying (FSK modulation) and amplitude shift keying (ASK modulation) among the current direct detection and coherent optical communication systems. It is attracting attention because it has the feature of being able to achieve higher reception accuracy than other methods.

Several experimental reports have been made regarding coherent optical reception using PSK modulation, and the above-mentioned high sensitivity characteristics have also been confirmed. For example, in the magazine [Electronics Letters (EIectron, Lett,)] 22 by R. Linke et al.
Literature published in Volume 1 (1986) [Coherent light unique transmission over 150 km fiber length at 400 Mb]
/s and IMb/s data late using phase modulation (Coherent light
t wave transmission over
150 ts fiber lengths at 4
00 Mbit/s and I Mbit/5dat
a rate using phase a+odu
ration)”.

However, all of the PSK system experiments that have been conducted so far have only been about two-phase PSK systems, and there have been no reports yet on multilayer PSK systems with four or more phases that have high frequency utilization efficiency.

By the way, looking at the current state of microwave communications where high-density frequency multiplexing is progressing, development of a multilevel amplitude phase modulation method is progressing in order to further increase frequency utilization efficiency.

In the future, it is expected that there will be a desire to send large amounts of information in a narrow band in coherent optical communication systems in order to avoid the effects of dispersion of optical fibers, and in this case, multilevel amplitude phase modulation will be used. This is one effective method.

[Problem that the invention seeks to solve]

However, the optical amplitude phase modulator required to realize this optical amplitude phase modulation method has not yet been developed.

An object of the present invention is to provide an optical amplitude phase modulator that has not been developed in the past.

[Means for solving problems]

The configuration of the optical modulation phase modulator of the present invention includes an amplitude modulation section that inputs information by modulating the intensity of light without changing the phase of the light, and a phase modulation section that performs phase modulation on the output light of this amplitude modulation section. It is characterized in that it includes a phase modulation section that inputs optical phase information independently of the light intensity information, and that each of these modulation sections performs multilevel modulation.

(Operation) The operation principle of the present invention will be explained below by taking a four-value amplitude phase modulator as an example.

In the four-value amplitude phase modulation, the amplitude contains binary information in one binary value and one phase. In order to achieve this amplitude phase modulation, unlike conventional cases where only amplitude modulation or phase modulation is performed, the phase information must be maintained when amplitude modulation is performed, and the amplitude information must be maintained when phase modulation is performed. It is thought that a new problem will arise: For example, when amplitude modulation of light is performed using a directional coupling type high modulator (optical switch) or an absorption type optical modulator, the phase is also modulated at the same time, so these modulators cannot be used as is for amplitude phase modulation. It is not possible.

As a modulator that modulates only the amplitude without affecting phase information, an interferometric optical modulator shown in FIG. 2 can be considered. In the figure, 3 and 4 indicate Y branches, and 5 and 6 indicate waveguides. In order to give binary (1, 2) information to the amplitude, it is necessary to adjust the branch ratio of each Y branch 3.4. Here, the amplitude branching ratio of Y branch 3 on the input side is (3 to
2J2):I, the case where the branching ratio of the Y branch 4 on the output side is set to 1:1 will be considered, but the branching ratio of these Y branches can be adjusted by the branch angle or the size of the guide.

Furthermore, an electrode 7 is added to one waveguide, and the phase of the light is shifted by π by applying a voltage. Figure 3 shows electrode 7.
FIG. 3 is a waveform diagram showing how the amplitude and nine phases of each point in FIG. 2 change depending on the voltage applied to the waveform.

As shown in this figure, when the voltage applied to voltage 7 is 0 (■ = 0>, the output is in the maximum transmission state, and when the voltage V = ■π that gives a phase shift π is applied, ,
The output is 1/2 of that when V=O. Since the phase of the output does not change at this time, an optical amplitude modulator that only performs amplitude modulation without affecting phase information is realized. By further combining such an amplitude modulator with an optical phase modulator, an optical amplitude and phase modulator can be realized.

〔Example〕

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a schematic perspective view showing an optical amplitude phase modulator according to a first embodiment of the present invention. The optical amplitude phase modulator of this example realizes four-value amplitude phase modulation, and is made of Ti-diffused Li.
It is composed of NbC)3 waveguide.

Input light 1 is first subjected to binary amplitude modulation in an amplitude modulation section 2 composed of a Y-branch type interferometric optical modulator. At this time, the Y branch 3. The branch angle has been adjusted. The first . Second waveguide 5
, 6, an electrode 7 is added to the first waveguide 5 which has a smaller amount of branched light, and the phase of the light is changed by π by applying a voltage to the electrode 7.

The combined output at the seven branches 4 on the output side changes depending on the voltage applied to the electrode 7, and when the applied voltage is O, an output that is 1 dB smaller than the input light is obtained, but if the phase of the light is shifted by π. When the voltage Vπ=8V was applied to the electrode 7, the output was 1/3 of that when the applied voltage was O.

Here, this output light is phase modulated by a phase modulation section 8. This phase modulation is realized by applying a voltage to the phase modulation electrode 9. Here, binary phase modulation is realized by applying a voltage 8■ that gives a π shift to the phase modulation electrode 9 according to the signal.

Actually, it is not possible to realize 10 Mb7/s four-level amplitude phase modulation by applying appropriate voltages to the electrode 7 and the phase modulation electrode 9 according to the signal.

The signal arrangement diagram at this time is shown in FIG.

This modulated signal light is heterodyne detected and demodulated to actually produce 4
It was confirmed that the value signal was demodulated. At this time, as mentioned above, the branch ratio of the Y branch 3 on the input side was adjusted so that the amplitude level ratio during amplitude modulation was 1:3, so the interval between the signal levels of the input signal was as shown in Figure 4. As is clear from the signal arrangement, they are now equally spaced. In this way, since four-value transmission was performed in this embodiment, the modulation speed could be suppressed to half that of binary transmission.

FIG. 5 is a schematic perspective view showing an optical amplitude phase modulator according to a second embodiment of the present invention. The optical amplitude phase modulator of this embodiment is for realizing 16-value amplitude phase modulation (6QAM). Input light 1 is branched at a ratio of 1:1 by branch circuit 20 . A phase shifter 21 is applied to one of the branched lights.
A phase shift of 0° is given. Thereafter, each branched signal light includes an amplitude modulation section 2 having a configuration similar to that of the first embodiment;
A phase modulation section 8 applies four-value amplitude and phase modulation. Since the two four-level amplitude phase modulated lights obtained here are orthogonal to each other, by combining them in the multiplexer 22 while maintaining the orthogonal relationship, 16-level amplitude phase modulated light can be obtained.

By using the optical amplitude phase modulator of this example, the first
It was possible to transmit signals in an even narrower band than in the embodiment described above.

The present invention may be modified in various ways in addition to the above-described embodiments. For example, as an interferometric modulator that does not affect phase information during amplitude modulation, a 3 dB coupling type interferometric optical modulator can be considered in addition to the Y-branch type. At this time, in order to arbitrarily change the ratio of amplitude levels during binary amplitude modulation, the coupling ratio of the coupling section may be shifted from 1:1.

FIG. 6 is a schematic perspective view of a third embodiment of the present invention, in which a control section 15 including a control electrode 14 is added to an optically coupled modulator that simultaneously modulates the phase during amplitude modulation. This is an example of an optical amplitude phase modulator that compensates for the phase change of .

In this example, in the amplitude modulation section 2, a voltage is applied to the electrode 7 in order to change the degree of coupling of light, but at this time the phase of the light also changes. Therefore, the control section 15 compensates for this phase change so that the phase of the light input to the phase modulation section 8 is always constant. Furthermore, in order to perform multi-value conversion, it is necessary to increase the amplitude level, but if the amplitude modulation sections 2 of the present invention are connected in cascade, it is possible to perform multi-value conversion of the amplitude level without changing the phase. At this time, the amplitude modulation section and the phase modulation section may be arranged in any order.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to realize an optical amplitude phase modulator that can perform amplitude modulation and phase modulation simultaneously. This makes it possible to increase the optical frequency utilization efficiency by several orders of magnitude compared to conventional methods, and is expected to be used in future ultra-high-density optical fiber communications.

[Brief explanation of the drawing]

FIG. 1 is a schematic perspective view of a first embodiment of the present invention, FIG. 2 is a schematic perspective view of an interferometric optical modulator for explaining the present invention in detail, and FIG. 3 is a schematic perspective view of each part of FIG. 2. FIG. 4 is a waveform diagram showing one phase of amplitude in FIG. 1, and FIG. 5 is a phase relationship diagram showing the arrangement of signals in FIG. FIG. 6 is a schematic perspective view of the second and third embodiments of the present invention. 1... Input light, 2... Amplitude modulation section, 3.4... Y
Branch, 7... Electrode, 8... Phase modulation section, 9... Phase modulation electrode, 10... Output light, 11... Branch circuit,
12... Phase shifter, 13... Multiplexer, 14...
Control electrode, 15...control unit. Masu

Claims (1)

[Claims] There is an amplitude modulation section that inputs information by modulating the intensity of light without changing the phase of the light, and an amplitude modulation section that performs phase modulation on the output light of this amplitude modulation section to change the phase of the light independently of the information on the light intensity. 1. An optical amplitude phase modulator, comprising a phase modulation section into which information is input, and each of these modulation sections performs multilevel modulation.