JPH0814664B2 - Light modulator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical modulator that supplies a microwave electric signal to a traveling wave electrode to modulate DC light into an optical signal having a predetermined wavelength.

(Conventional Technology) Conventionally, as a technology in such a field, Tadashi Sueda, "Optical Electronics", first edition (1985-4-15), Shokoido P.182.
There was one described in -185. The configuration will be described below with reference to the drawings.

FIG. 2 is a diagram showing a configuration example of a conventional optical modulator using a traveling wave electrode.

This optical modulator includes a crystal substrate 1 having an electro-optic effect such as a LiNbO ₃ -Z plate, and the crystal substrate 1 is provided with an optical input port 2 _a and an optical signal output port 2 _b , and Between the input / output ports 2 _a , 2 _b , two branched arms 3 _a ,
A Mach-Zehnder interferometer 3 having 3 _b is formed. These input / output ports 2 _a and 2 _b and the interferometer 3 are formed by waveguides. Traveling wave electrodes 4 are provided on both sides of the interferometer 3.
And the ground electrode 5 are formed. Further, the input terminal 6 for inputting the microwave electric signal is the coaxial cable 7
Is connected to one end of the traveling wave electrode 4 via the center conductor 7 _a of
The other end of the traveling wave electrode 4 is connected to one end of an earth electrode 5 via a terminating resistor 8 for impedance matching,
And the other end of the ground electrode 5 is connected to the ground via the outer conductor 7 _b of the coaxial cable 7 further.

In the above configuration, when the microwave electric signal is supplied to the input terminal 6 and the DC light is supplied to the input port _2a , an electric field is generated between the traveling wave electrode 4 and the earth electrode 5, and the electric field causes the interferometer 3 to operate. There is _a phase difference between the light waves traveling in the two arms 3 _a and 3 _b, and the optical signal 3 of a predetermined wavelength is output port.
_Sent from 2b.

In this type of optical modulator, since the light wave and the microwave propagate in the same direction, the interferometer 3 becomes a kind of transmission line for the microwave, and the bandwidth limitation due to the electric capacity is eliminated unlike the centralized type. Has a characteristic that a wide modulation band can be obtained.

(Problems to be Solved by the Invention) However, the optical modulator configured as described above has the following problems.

If the length of the traveling wave electrode 4, that is, the element length is L and the voltage of the supplied microwave electric signal is V _in , then L × V _in = constant value, and in order to reduce the micro voltage V _in It is necessary to increase the element length L. However, since the traveling speed of light and the traveling speed of microwaves are different, if the element length L is increased, the phase difference between the two becomes large and the modulation band falls, so that an optical modulator with a low voltage and a wide band can be obtained. Was difficult.

SUMMARY OF THE INVENTION The present invention provides an optical modulator that solves the problem that the above-described conventional technique has, in that it is difficult to obtain an optical modulator with a low voltage and a wide band.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides an optical modulator that supplies a microwave electric signal to a traveling wave electrode to modulate DC light into an optical signal of a predetermined wavelength. N (where N ≧ 2) traveling-wave electrodes arranged in the traveling direction of the light wave for modulation into an optical signal of a predetermined wavelength, and power for dividing the power of the microwave electric signal for modulation into N equal parts. Dividing means,
The power dividing means delays the power divided into N equal parts by a predetermined time and sequentially supplies the power to each of the traveling wave electrodes.

(Operation) According to the present invention, since the optical modulator is configured as described above, after the microwave electric signal is divided into a plurality of powers by the power dividing means, the respective powers are sequentially delayed by the delaying means. And is sequentially supplied to the N traveling wave electrodes. Therefore, by setting the power delay time to the optimum value,
It is possible to equalize the traveling speeds of light and microwaves, thereby increasing the modulation band while suppressing an increase in operating voltage. Therefore, the above problem can be solved.

(Embodiment) FIG. 1 is a block diagram of an optical modulator showing a first embodiment of the present invention.

This optical modulator includes a crystal substrate 11 having an electro-optic effect such as a LiNbO ₃ -Z plate, and the crystal substrate 11 has an optical input port.
12 _a and an optical signal output port 12 _b are formed, and a Mach-Zehnder interferometer 13 having two branched arms 13 _a and 13 _b is formed between the input / output ports 12 _a and 12 _b. Has been formed. These input / output ports 12 _a and 12 _b and the interferometer 1
3 is composed of a waveguide. On both sides of the interferometer 13,
N (where N ≧ 2) traveling wave electrodes 14 _{a to} 14 _d and a ground electrode 15 are formed, and the impedance between the traveling wave electrodes 14 _{a to} 14 _d and the ground electrode 15 is, for example, 50Ω. They are connected by the terminating resistor 16 _a ~ 16 _d for alignment.

Also, an input terminal for inputting a microwave electric signal
17 includes the outer conductor 1 connected to the center conductor _18a and ground.
Via a coaxial cable 18 having a 8 _b, the (N-1) pieces of 1-input 2-output type microwave power splitter 19 _a ~ 19 _c of being connected, the power divider 19 _a ~ 19 _c The power of the input microwave is divided into N. One output side of the power divider 19 _a is connected to the traveling-wave electrode 14 _a and the ground electrode 11 by a coaxial cable 18, further power splitter 19 _a and the other output side, and two output side of the power splitter 19 _b of Are connected to the traveling wave electrodes 14 _{b to} 14 _d and the ground electrode 15 via the coaxial cable 18 and the microwave delay devices 20 _{a to} 20 _c , respectively. The delay time of the delay devices 20 _a , 20 _b , 20 _c is 20 _a <20 _b.
It is set to <20 _c .

Here, the power divider 19 _a ~ 19 _c, for example, Yokogawa Hewlett Packard (YHP) manufactured product number 11667
A, 11667B or the like may be used. However, a structure in which the microwave stripline is simply branched cannot be used because of problems such as large reflection loss due to impedance mismatch at the branch or a small electric field after branching. .

 Next, the operation of FIG. 1 will be described.

While supplying a microwave electric signal to the input terminal 17,
When supplied to the input port 12 _a direct current light, power of the microwave electric signal is divided into two parts by the power divider 19 _c, further the two divided power is bisected respectively by a power divider 19 _a, 19 _b after, one output of the power divider 19 _a is supplied between the traveling wave electrode 14 _a and the ground electrode 15, an electric field is generated between the traveling wave electrode 14 _a and the ground electrode 15. The output of the other end of the power divider 19 _a is supplied between the traveling wave electrode 14 _b and the ground electrode 15 is a fixed time delay in the delay circuit 20 _a, a power divider 19 further while the output delay device 20 _b of the _b It is delayed and supplied between the traveling wave electrode 14 _c and the ground electrode 15, and the other output of the power divider 19 _b is further delayed by the delay device 20 _c and supplied between the traveling wave electrode 14 _d and the ground electrode 15. To go. Thus, when the microwave power sequentially delayed by the delay devices 20 _a , 20 _b , and 20 _c is sequentially supplied between the traveling wave electrodes 14 _{b to} 14 _d and the ground electrode 15, an electric field is generated between them. It will occur. Corresponding to this, the DC light supplied to the input port 12 _a travels while branching through the two arms 13 _a and 13 _b of the interferometer 13, and is modulated between them to generate an optical signal of a predetermined wavelength. Output from output port _12b .

Here, the input port 12 _a is input from the DC light two arms 13 _a, 13 is divided by _b each traveling wave electrode 14 _a to 14
_The time T _a for reaching _d and the microwave input from the input terminal 17 are delayed by the delay devices 20 _{a to} 20 _c and
14 and time T _b to reach _a to 14 _d is set to be the same. Therefore, the DC light inputted in each of the traveling wave electrode 14 _a to 14 _d locations, two arms 13 of the interferometer 13
It accumulates the phase difference between _a, 13 _b. At this time, the microwave power is equally divided into 1 / N by the power dividers 19 _{a to} 19 _c and supplied to the traveling wave electrodes 14 _{a to} 14 _d , but (electric field) ² is proportional to the microwave power. , The electric field generated between each traveling wave electrode 14 _{a to} 14 _d and the ground electrode 15 is However, it does not become so small. The phase difference required to switch the interferometer 13 is π. Required electric field E
Is the total length of the electrode 13, that is, the element length is L, then E = constant × π / L. When divided into N, the required electric field E ₀ is Is. On the other hand, the 3 dB band suitable for evaluating the modulation frequency is N / L
Is proportional to, so that the 3 dB band / E ₀ is Is proportional to Thus the more the division number N, said can match precisely the arrival time T _b of the arrival time T _a microwave of light, i.e. the progression speed and the microwave traveling speed of the light with high precision They can be matched, which allows wideband modulation at low voltage.

FIG. 3 is a block diagram of an optical modulator showing a second embodiment of the present invention, and the same elements as those in FIG. 1 are designated by the same reference numerals.

In this optical modulator, as in the first embodiment, the crystal substrate
An input port 12 _a and an output port 12 _b are formed on 11 and two branched arms 1 are provided between the input / output ports 12 _a and 12 _b.
3 _a, 13 a Mach-Zehnder type interferometer 13 having a _b are formed. N traveling wave electrodes 24 _{a to} 24 _d having _a length L ₀ and N earth electrodes 25 corresponding to the traveling wave electrodes 24 _{a to} 24 _d are provided on both sides of the interferometer 13.
_{a to} 25 _d are formed, and the traveling wave electrodes 24 _{a to} 24 _d and the ground electrodes 25 to 25 _d are respectively connected by terminating resistors 26 _{a to} 26 _d for impedance matching of, for example, 50Ω. ing. An input terminal 17 for inputting a microwave electric signal has a center conductor _18a and an outer conductor 1 as in the first embodiment.
Via a coaxial cable 18 having a 8 _b (N-1) pieces of 1-input 2-output type microwave power splitter 19 _a ~ 19 _c of being connected, the input micro by the power divider 19 _a ~ 19 _c The power of the wave is divided into N.

In this embodiment, instead of the delay unit 20 _a to 20 _c of the first embodiment, on a crystal substrate 11 (N-1) to form a delay line 30 _a to 30 _c of the microwave, of The propagation distance is changed by the delay lines 30 _a to 30 _c to change the amount of delay. That is, the traveling wave electrode 24 _a and the ground electrode 25 _a of the input port 12 _a side is connected to one output side of the direct power splitter 19 via a coaxial cable 18, further traveling wave electrode 24 _b and the ground electrode 25 _b the other output of the power divider 19 _a via the delay line 30 _a coaxial cable 18 of length L _m, traveling-wave electrodes 24 _c and the ground electrode
25 _c is a delay line 30 _b having a length of 2 L _m and one output side of the power divider 19 _b via the coaxial cable 18, and the traveling wave electrode 24 _d and the ground electrode 25 _d on the output port 12 _b side have a length. via the delay line 30 _c and the coaxial cable 18 of 3L _m to the other output side of the power splitter 19 _b, it is connected.

The lengths L _m , 2L _m and 3L _{m of the} delay lines 30 _a , 30 _b and 30 _c and their arrangement angles θ shown in FIG. 3 may be determined as follows.

The length L _m of the delay line 30 _a is equal to each of the divided traveling wave electrodes 24 _a
It is determined by the length L _{0 of} 24 _d and the ratio of the propagation velocities between light and microwave. The speed of light is C, the refractive index of light is n ₀ , and this n ₀
As the value of n _m of the corresponding microwave, the propagation velocity of light
C / n ₀ and microwave propagation velocity are C / n _m . Time light reaches the starting end of the N-th traveling wave electrodes from the input port 12 _a tau
₀ is Similarly, the time τ _m for the microwave to reach the beginning of the Nth traveling wave electrode is However, ξ _m ; Phase difference between light and microwave at input.

Becomes (2) In the equation, the input light to the input port 12 _a is because it is a direct current light, it is possible to put the phase difference ξ _{m =} 0. If the phase difference τ ₀ = τ _m , then from equations (1) and (2)
L ₀ n ₀ = L _m n _m . Therefore, L _m = L ₀ n ₀ / n _m may be set.
For example, when a LiNbO ₃ -Z plate is used as the crystal substrate 11, n ₀ / n _m ≈1 / 2 is usually satisfied in the LiNbO ₃ -Z plate, so that L _m / L ₀ = 1/2 Therefore, if the arrangement angle .theta..apprxeq.30.degree. In FIG. 3 is set, the same action and effect as those of the first embodiment can be obtained.

FIG. 4 is a characteristic diagram showing the performance when a LiNbO ₃ —Z plate is used as the crystal substrate 11 in FIGS. 1 and 3. Here, the distance between the arms 13 _a and 13 _b is 15 μm, and the light wavelength is 1.3 μm.
m, impedance of each terminating resistor 16 _{a to} 16 _d , 26 _{a to} 26 _d
As Ω, the performance curve A of the 3 dB band and the performance curve B of the element length when a 5 V microwave electric signal is supplied to the input terminal 17 are shown.

For example, if the traveling-wave electrode _{14 a ~14 d, 24 a ~24} d division number N is 4, the curves A, modulatable bandwidth of 3dB band 0-1
4GH _Z , and further from curve B, the total length of the traveling wave electrode, that is, the element length is 4 cm.

Also, in the case of the division number N is 8, modulatable bandwidth of 3dB bandwidth 0～20GH _Z, element length is 5.7 cm.

Thus, it can be understood that by increasing the number of divisions N, the modulation band can be expanded while the operating voltage is maintained at a low voltage such as 5V.

FIG. 5 is a block diagram of an optical modulator showing a third embodiment of the present invention, in which the same elements as those in FIG. 1 are designated by the same reference numerals.

In this optical modulator, a compound semiconductor substrate 41 is used, a multiple quantum well (MQW) type or electroabsorption type optical waveguide 43 is formed on the substrate 41, and an input port is provided on one end face of the optical waveguide 43. 43 _a , and the output port 43 _b is formed on the other end face. N traveling wave electrodes 44 _{a to} 44 _d are formed on the optical waveguide 43, and _a ground electrode 45 is formed on the bottom surface of the substrate 41. The traveling wave electrodes 44 _{a to} 44 _d and the ground electrode 45 are formed on the ground electrode 45. And are connected by terminating resistors 16 _{a to} 16 _d .
Input terminal 17 for inputting the microwave electric signal, like the first embodiment, the coaxial cable 18, each traveling through a power divider 19 _a ~ 19 _c, and the delay unit 20 _a to 20 _c The wave electrodes 43 _a to 43 _d and the ground electrode 45 are connected.

In the above configuration, as well as supplied to the input port 43 _a direct current light, if supplying microwaves electrical signal to the input terminal 17, as in the first embodiment, the traveling speed of light therethrough waveguide 43 , traveling wave electrode 44 _a ~ 44 _d and the traveling velocity of the microwave to be supplied is equal to the bandwidth wide optical modulator is obtained at a low voltage.

The present invention is not limited to the embodiments shown, for example, or replace the delay unit 20 _a to 20 _c of FIG. 5 in the third diagram of the delay line 30 _a to 30 _c or interferometer 13, optical waveguide 43 , Traveling wave electrode
_{14 a ~14 d, 24 a ~24} d, 44 a ~44 d, power divider 19 _a ~ 19 _c,
Delayer 20 _a to 20 _c, it is also possible to modify the shape and structure other than illustrated delay element 30 _a to 30 _c or the like.

(Effect of the Invention) As described in detail above, according to the present invention, the N traveling wave electrodes are driven by the microwave electric signal whose power is divided into N equal parts. It is possible to equalize the speed and the traveling speed of the microwave, whereby the modulation band can be expanded at a low operating voltage.

[Brief description of drawings]

FIG. 1 is a block diagram of an optical modulator showing a first embodiment of the present invention, FIG. 2 is a block diagram of a conventional optical modulator, and FIG. 3 is an optical modulation showing a second embodiment of the present invention. Fig. 4 shows the configuration of the vessel
5 and FIG. 5, and FIG. 5 is a configuration diagram of an optical modulator showing a third embodiment of the present invention. 11 …… Crystal substrate, 12 _a , 43 _a …… Input port, 12 _b , 43 _b ……
Output port, 13 ... Mach-Zehnder type interferometer, 14 _a to 1
4 _d , 24 _a 〜 24 _d , 44 _a 〜 44 _d・ ・ ・ Traveling wave electrode, 15,25 _a 〜 25 _d , 45
...... Grounding electrode, 17 …… Input terminal, 19 _{a to} 19 _c …… Power divider, 20 _{a to} 20 _c …… Delayer, 30 _a to 30 _c …… Delay line.

Claims (1)

[Claims] 1. N pieces (where N ≧ 2) arranged in the traveling direction of the light wave in order to modulate the light wave into an optical signal of a predetermined wavelength.
Traveling power electrode, power dividing means for power-dividing the modulating microwave electric signal into N equal parts, and the power divided into N equal parts by the power dividing means is delayed for a predetermined time, so that each traveling wave electrode An optical modulator, comprising: