JPH11287973A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve characteristic deterioration which was caused by a component having reflection and excessive loss as to the optical modulator which modulates light with a high-frequency electric signal as a modulating signal such as a millimeter wave. SOLUTION: For an electrode line 2 for making light operate on a traveling wave electric field, an excessive-length area 15 for propagating the light even after an electric signal has operated on the light while propagated along the electrode line is prepared. The need for a terminating resistance and a low-frequency block element accompanying it is eliminated by giving attenuation of >=5 db in the excessive-length area. Further, the need for an element for bias application is eliminated by applying a bias voltage from an excessive-length area end. This constitution simplifies the constitution of the device and makes it possible to omit various elements, so there is neither reflection nor loss due to their incompleteness, so that characteristics are greatly improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system, and more particularly to an improvement of an external optical modulator for use in transmitting an optical fiber of a high frequency signal such as a millimeter wave.

[0002]

2. Description of the Related Art Millimeter waves (6
Since radio waves in the 0 [GHz]) band are greatly attenuated by air, transmission through an optical fiber is expected as a method for expanding the communication area.

[0003] That is, an optical fiber network is set up, a millimeter wave is modulated on an optical carrier, the optical carrier is sent to the optical fiber, and the optical carrier is transmitted to a radio base station connected to a communication network by the optical fiber, and then converted into a radio wave to transmit the radio wave. It is sent from the base station to a wireless terminal within the service area. The reception wave from the wireless terminal is the reverse of the above, and the base station receives the radio wave wirelessly transmitted by the millimeter wave, and the base station modulates the optical carrier with the millimeter wave before sending it out to the optical fiber.

[0004] In this way, by transmitting the data via the optical fiber and transmitting / receiving the data wirelessly from the radio base station in the service area at the end, it is less affected by the attenuation by air, and the communication area is expanded. .

By the way, an optical modulator is used to modulate an optical carrier with a millimeter wave, and an external modulator is often used for the modulation. As the external modulator, a type in which a metal electrode is formed on an optical waveguide formed of lithium niobate (LN) and modulation is performed using an electro-optic effect is often used.

An external modulator of this type is called a lithium niobate (LN) modulator. In an LN modulator for high-speed modulation, a traveling-wave type electrode is generally employed. This traveling wave electrode will be described with reference to FIG.

In FIG. 16, 1 is an optical waveguide, 2 is an electrode line, and 3 is an input terminal. Then, a high-frequency electric field signal of the traveling wave is applied to the optical waveguide 1 by applying a high-frequency voltage signal of the traveling wave to the electrode line 2 from the input terminal 3. Now, it is assumed that the traveling direction of the traveling wave is from the left side of the figure to the right side.
It is assumed that light is incident in the traveling direction of the traveling wave.

When the light incident on the optical waveguide 1 from the left propagates, it is affected by the high-frequency electric field generated by the voltage signal applied to the electrode line 2 on the optical waveguide 1, and A change in the refractive index occurs inside, and the phase is modulated according to the magnitude of the applied high-frequency voltage signal.

However, the electrode line 2 is a line such as a microstrip line or a coplanar line. in this way,
When an electric signal is input from the input terminal 3 to the electrode line 2,
This electric signal propagates through the electrode line 2 from left to right. At this time, if the speed at which the light propagates through the optical waveguide 1 and the speed at which the electric signal propagates through the electrode line 2 are almost the same, the light is always modulated from the same point of the electric signal as it propagates.

[0010] The change in the refractive index due to the applied voltage is proportional to the magnitude of the applied voltage.
Therefore, a large voltage is required to perform sufficient modulation. However, it is generally difficult to obtain a large voltage with a high-frequency signal.

[0011] Traveling-wave electrodes are the best solution to such problems, and even at low voltages, the traveling light modulates from the same signal throughout the length of the electrode. Upon reception, the signal is sufficiently modulated.
In the case of modulating light with a millimeter wave signal, the application of a traveling wave type electrode is also conceivable.

Many optical modulators, including the LN intensity modulator, operate around an electrical DC operating point. A bias is applied to determine the DC operating point, and the bias is added to the modulation signal and applied to the optical modulator to perform optical modulation. In order to apply a bias voltage (or current) to the modulation signal, a circuit called a bias T having a circuit configuration as shown in FIG. 17 is often used.

The bias circuit T includes a low-frequency block element 6 composed of a capacitor and a high-frequency block element 7 composed of an inductor.
When a modulation signal is applied to terminal 5-1 and a bias voltage (or current) is applied to terminal 5-2, they are output as a combined signal to terminal 5-3. .

The bias circuit T includes a low-frequency block element 6
The high frequency block element 7 provides isolation between the terminal 5-1 and the terminal 5-2. If the signal frequency is relatively low, such a bias circuit can be easily manufactured using capacitors and inductors.

However, for a high frequency band such as a millimeter wave, an element such as a capacitor or an inductor which can secure ideal characteristics for the frequency band cannot be easily manufactured. For example, it is difficult to produce a product having good characteristics.

By the way, when a bias is applied to the traveling wave type electrode, a structure as shown in FIG. 18A or FIG. 18B is usually adopted. That is, in these, 6 is a low-frequency block element, 7 is a high-frequency block element, 9 is a DC source, 8 is a terminating resistor, 10 is a signal source, and the structure shown in FIG. 1 with respect to the electrode line 2 provided in
The bias voltage from the DC source 9 is applied from the end side, which is the input side of the optical signal, through the high-frequency block element 7, and the modulation signal from the signal source 10 is transmitted through the low-frequency block element 6, The signal is supplied from the end portion which is the input side of the optical signal. That is, the modulation signal and the bias voltage are input from the same position of the electrode line 2.

In order to input the modulation signal and the bias voltage from the same position, the modulation signal from the signal source 10 is passed through the low-frequency block element 6 so that the low-frequency component is not influenced by each other. And the bias voltage cuts off the high-frequency component by passing through the high-frequency block element 7. Further, in order to synthesize the modulation signal and the bias voltage, a branching junction 11 is provided and the two are synthesized. In this configuration, the object is finally given to the electrode line. Also,
In order to terminate the modulation signal, the electrode line 2 is grounded at the end opposite to the modulation signal input side via a series circuit including a low-frequency block element 6 and a termination resistor 8.

FIG. 18B shows a configuration in which a bias voltage is applied from a position opposite to a modulation signal application terminal. In order to input a bias voltage from a position opposite to the modulation signal application terminal, a DC source is used. The bias voltage from 9 is connected to the end of the electrode line 2 opposite to the modulation signal input side after blocking the high frequency component through the high frequency block element 7. In order to connect the terminating resistor 8, the terminating resistor 8 is connected via the low-frequency block element 6. Further, a branch junction 11 is provided to branch the modulation signal and the bias voltage.

It is connected to the modulation signal input side of the electrode line 2 via the modulation signal low-frequency block element 6 from the signal source 10. In the configuration shown in FIGS. 18A and 18B, a signal to be transmitted provided from the signal source 10 is transmitted from the input terminal 3 to the electrode line 2 through the low-frequency block element 6 as a millimeter-wave modulated signal. Give to. By this modulation signal, a high-frequency voltage of a traveling wave is generated in the electrode line 2, and a high-frequency electric field by the traveling wave is applied to the optical waveguide 1.

Now, it is assumed that the traveling direction of the traveling wave is from the left side of the figure to the right side, and that light enters the optical waveguide 1 from the left side of the figure. A signal source 10 for generating a modulation signal based on a millimeter wave signal is located on the left side of the drawing, that is, on the light incident side end. In the case of the configuration shown in FIG. And a bias circuit T composed of a high-frequency block element 7 and a branching / joining section 11, and the signal source 1 is connected via a low-frequency block element of this circuit.
A modulation signal from 0 is applied to the electrode line 2, and a bias is applied to the electrode line 2 via the high-frequency block element.

The right end of the electrode line 2 is on the signal output terminal 4 side, and a terminal resistor 8 for signal termination is connected to the output terminal 4 via a low-frequency block element 7.
The electric signal acts on the light while propagating through the traveling wave electrode,
The electric signal itself hardly changes.

Accordingly, the terminating resistor 8 plays a role of terminating the modulated signal of the millimeter wave provided from the signal source 10 and having finished modulating the light so that the signal is not reflected back at the end of the electrode line. The low-frequency block element 6 is provided to prevent the DC voltage from the DC source 9 from flowing to the terminating resistor 8 (the output resistor when the output of the signal source 10 is not DC cut) and wastefully consuming power. .

FIG. 18B shows a configuration in which a bias circuit T is connected to the output terminal 4 side, and a bias voltage (or current) is supplied from the output terminal 4 side. The operation is the same as in the case of FIG. 18A, except that the bias circuit T is inserted into the terminating resistor 8.

In either configuration, the signal output from the signal source 10 passes through the electrode line 2, passes through the low-frequency block element 6 and the branching junction 11 before being terminated by the terminating resistor 8. Become.

However, it is difficult to fabricate an ideal element suitable for a high-frequency band such as a millimeter wave, so that the signal is reflected and reflected by a low-frequency block element, a branching junction, a terminating resistor, and the like. It is inevitable that excessive loss occurs, which degrades the characteristics of the optical modulator.

[0026]

As described above, in a conventional optical modulator having a traveling-wave-type electrode line, a low-frequency block element is used to apply a modulation signal to the electrode line, and a bias component is cut. Also, use a high-frequency block element to apply a bias, cut high-frequency components,
In addition, a terminating resistor is provided via a low-frequency block element to terminate the modulation signal, and a branching junction for synthesizing and distributing a low-frequency component and a high-frequency component is required.

However, these have insufficient operating characteristics at high frequencies such as millimeter waves, and give attenuation to high frequency signals. As described above, when an optical modulator having a traveling wave type electrode is used at a high frequency such as a millimeter wave band, a low-frequency block element for applying a bias, a branching junction, or a signal for terminating a signal. Excessive loss and reflection were caused by the resistance, which affected the modulation of light, and the modulation characteristics of the optical modulator were deteriorated.

In general, an element is difficult to follow a high frequency, and even if it has sufficient tracking characteristics, it is often expensive. Therefore, it is necessary to adopt a configuration in which elements requiring characteristics for high frequencies are not used as much as possible.

Therefore, the object of the present invention is to
To minimize the use of elements that require characteristics at high frequencies and to provide an optical modulator with good modulation characteristics and high practicality when modulating light with high-frequency modulation signals such as millimeter waves. It is in.

[0030]

In order to achieve the above object, the present invention is configured as follows. [1] In the first invention of the present application, in an optical modulator in which an electric signal propagates through a traveling-wave-type electrode and acts on an optical signal, the traveling-wave-type electrode terminates the operation of the electric signal on the optical signal. It has a surplus area that propagates later, the surplus area has the same impedance as an action area where the electric signal acts on the optical signal, and the surplus area is a predetermined value or more with respect to the electric signal. Specifically, for example, an optical modulator characterized by having a loss of 5 [dB] or more is provided.

The length of the traveling-wave electrode is such that the shift in the modulation timing caused by the shift in the speed of the light traveling in the optical waveguide and the speed of the electric signal traveling in the traveling-wave electrode does not matter. .

On the other hand, at a very high frequency such as a millimeter wave band, the loss of a transmission line such as a microstrip line is large. Therefore, when a sufficiently long line is propagated, the signal is attenuated and disappears in the line. When a millimeter wave propagates through a sufficiently long line in an optical modulator as shown in FIG. 4, the light that propagates in the optical waveguide is effectively acted on for a relatively short length LO for the above-described reason. is there.

Although the length of the LO is sufficient for the original function of the optical modulator, the present invention uses a longer line 2 and propagates the line even after the action on the light is completed. As a result, the millimeter wave signal attenuates in the line and disappears depending on the line length. Since no terminating resistor is used, there is no reflection due to the fact that the terminating resistor does not have ideal characteristics.

According to the present invention, an extra length region is required which is long enough to attenuate the signal. Actually for the following reasons
It is considered that if the attenuation is more than 5 [dB], the length is sufficient. When designing a high-frequency component, a criterion determined by the amount of reflection called a voltage standing wave ratio (VSWR) is often used.

In general high-frequency devices, VSWR = about 2: 1 is often required. This is a reflection amount of -10.
[DB]. When this is applied to the optical modulator of the present invention, if there is a loss of 5 [dB] in one way in the extra length area as shown in FIG. The signal returning to the area is subject to 10 round-trip losses
[DB] is lost and the reflection is -10 [dB].

Since there is a loss in the operating region, the reflection amount is -10 [dB] or less when viewed from the signal input terminal of the optical modulator, which satisfies the general required performance as a high-frequency device. , Can withstand use.

[2] Further, in the second invention of the present application, a DC voltage or a bias voltage comprising a DC voltage and a low frequency signal is applied to the traveling wave type electrode at the end of the traveling wave type electrode on the extra length region side. The optical modulator according to the first invention of the present application is provided with a terminal for applying a voltage.

Since a DC voltage or a low-frequency signal is hardly attenuated in the line, it is uniformly applied to the electrode line 2.
Therefore, as shown in FIG. 2, an RF input terminal 1 for inputting a signal is provided.
3, a bias input terminal 12 is provided. At one end of the bias input terminal 12, the signal input from the RF input terminal 13 is sufficiently attenuated, and even if the reflection is large at the end, there is almost no signal reflected and returned to the region where light interacts with light. .

This means that the electrode line 2 has a function 16 for selectively terminating an RF signal, and equivalently, the electrode line 2 has an RF signal as shown in FIG. It has a termination function 16 for selectively terminating only the signal.

Accordingly, even after the modulation of the light is completed, the extra length area in which the electric signal propagates can completely change the impedance of the transmission line because the termination function 16 selectively terminates only the signal. Therefore, the impedance matching at the branching junction 11 is provided as in the conventional example in which the bias circuit T including the low-frequency block element 6, the high-frequency blocking element 7, and the branching junction 11 is provided. And the number of low-frequency block elements 6 can be reduced, so that the characteristic deterioration due to reflection and loss caused by those elements is small.

The low-frequency block element 6 on the side of the RF input terminal 13 is not required if the same low-frequency block element is provided in the output section inside the signal source 10. In such a case, there is no element that easily causes impedance mismatch in the signal propagation path, and the performance of the optical modulator is improved. In addition, in this configuration, unlike the related art, there is no resistor for consuming the power of the bias voltage in the portion to which the bias voltage is applied despite the absence of the branch junction and the low-frequency block element. Is small and economical.

[3] In the third aspect of the present invention, the traveling wave type electrode is a coplanar type line, and the width of the signal line is gradually narrowed in the extra length region.
An optical modulator according to the first or second invention is provided.

In an electric signal line, when the impedance is specified, there is an optimum signal line width that minimizes the propagation loss. If it is wider than that, the loss increases due to radiation loss, and if it is smaller than that, the loss increases due to ohmic loss.
In the present invention, the ohmic loss is increased by narrowing the signal line width in the surplus region compared to the signal line width in the operation region, and the signal is lost at a shorter distance. With ohmic losses, the signal is converted to heat in the line and has no other effect.

Further, if the signal line width changes abruptly at the point where the signal moves from the action area to the extra length area, reflection may be caused. The thickness of the substrate of the optical modulator is substantially uniform in the plane. Microstrip lines and coplanar lines are often used for high-frequency lines manufactured on a substrate. To change the width of the signal line without changing the impedance in the microstrip line, it is necessary to change the substrate thickness, which is not practical. On the other hand, the coplanar line is suitable for the present invention only by changing the distance between the signal line and the ground on the same plane.

[0045]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the drawings. (First Embodiment) FIG. 1 is a configuration diagram of an optical modulator as an example in the first embodiment. In the figure, only the signal lines of the electrode lines are shown, and the ground is omitted. As the electrode line 2, a microstrip line, a coplanar line, or the like is used.

In FIG. 1, reference numeral 1 denotes an optical waveguide through which light to be modulated passes, and reference numeral 2 denotes a traveling-wave-type electrode line, which is an electrode line on the signal line side. Reference numeral 6 denotes a low-frequency block element, 10 denotes a signal source for outputting a high-frequency band modulation signal, 12 denotes a bias input terminal serving as a bias voltage input terminal, 13 denotes an RF input terminal for inputting a modulation signal, and 17 denotes a bias. This is a bias source that outputs a voltage.

As shown in FIG. 1, the optical modulator of the present invention
The electrode line 2 is divided into a working region 14 and a surplus region 15. The first half of the electrode line 2 in the light propagation direction with respect to the optical waveguide 1 is the working region 14, and the second half thereof is a surplus region.

Of these areas, the action area 14 is an area for modulating light by applying an electric field by a modulation signal to light, and the extra length area 15 is an area provided for attenuating the modulation signal. is there. Therefore, the arrangement of the optical waveguide 1 in the electrode line 2 is devised so that the high-frequency electric field of the traveling wave due to the modulation signal acts on the light in the optical waveguide 1 only in the operation region 14. That is, the optical waveguide 1 is configured to pass through the working region 14 of the electrode line 2 and to be drawn out to the outside in the surplus region 15 so as not to be affected by the operation in the surplus region 15.

In the present invention, the electrode line 2 has no curvature and is straight, and the optical waveguide 1 is mainly arranged in the working area 14 of the electrode line 2, and the extra length area 15 is provided.
In the figure, the electrodes are arranged so as to be drawn out of the electrode line 2.
Therefore, the optical waveguide 1 is bent out of the electrode line 2 after passing through the working region 14.

The length of the traveling-wave-type electrode is such that the deviation of the modulation timing caused by the deviation of the traveling speed of the light traveling in the optical waveguide and the electric signal traveling in the traveling-wave-type electrode does not matter. ing.

The working area 14 of the electrode line 2 is provided with an RF input terminal 13 on one end thereof, and the end on the extra length area 15 side is provided with a bias input terminal 12. In this device, the output of the signal source 10 blocks the bias voltage, so that the electrode line 2 is transmitted from the RF input terminal 13 through the low-frequency block element 6.
, And a bias is applied to the electrode line 2 from the bias source 17 via the bias input terminal 12 at the end of the extra length region 15 of the electrode line 2. The bias voltage given from the bias input terminal 12 is a DC voltage,
Or, it consists of a DC voltage and a low frequency signal.

In the optical modulator having such a configuration, a predetermined bias is applied to the entire electrode line 2 by applying the bias voltage from the bias source 17 to the bias input terminal 12, and the operating point of the modulation is determined. ing. Further, an electrode line 2 is provided on the optical waveguide 1, and a high-frequency modulation signal from a signal source 10 is applied from an RF input terminal 13 to an operation area 14 side of the electrode line 2.

The high frequency modulation signal is applied to the electrode line 2
, A traveling wave electric field is generated from the action region 14 side toward the extra length region 15 side. On the other hand, the optical waveguide 1 has the electrode line 2
The light to be modulated propagates from the end side of the working region 14 on the side of the working line 14, and the light is modulated in the working region 14 of the electrode line 2 by the action of the traveling wave electric field.

Thus, the working area 14 of the electrode line 2
Inside, an electric field by an electric signal propagating on the electrode line 2 acts on the light propagating in the optical waveguide 1. After passing through the action area 14 in the electrode line 2, the electrode line 2
The optical waveguide 1 goes out of the electrode line 2 so as not to be affected by the electric field due to the applied electric signal. As a result, the light is output from the optical modulator as appropriately modulated light.

On the other hand, the high-frequency signal output from the signal source 10 and applied to the electrode line 2 for forming a traveling wave electric field is:
It proceeds to the surplus length area 15 of the electrode line 2 and receives a loss in this area. That is, as the length of the propagating line increases, the attenuation of the high-frequency signal increases. Therefore, the extra length area 15
At the end of the stage, the level is greatly reduced.

At the end of the extra length region 15, there is provided a bias input terminal 12 for applying a bias voltage for determining an operating point of the optical modulator. A DC voltage source 17 is connected to this terminal to apply a bias voltage based on the DC voltage. (Note that a bias voltage composed of a DC voltage and a low-frequency signal is sometimes applied). However, unlike a high-frequency signal, a DC voltage or a low-frequency signal is hardly attenuated in the electrode line 2 even when applied to the electrode line 2 having a long line length. , And fulfills the meaning of the original bias.

Therefore, in the optical modulator of the present invention, the signal source 1
In order to electrically combine the output of 0 and the bias and apply the resultant to the electrode line 2, the low-frequency blocking element and the high-frequency blocking element conventionally provided on the modulation signal input side of the electrode line. The block circuit and the bias circuit including the branching junction and the bias source are eliminated, and as shown in FIG.
That is, the bias input terminal 12 is provided at the opposite end of the F input terminal 13, that is, at the end of the electrode line 2 on the extra length region 15 side.

At the end where the bias input terminal 12 is provided, the signal input from the RF input terminal 13 is sufficiently attenuated. There is almost no modulated signal that returns to the region that interacts with light (that is, the active region of the electrode line 2).

This means that the electrode line 2 has a function 16 for selectively terminating an RF signal, and equivalently, the electrode line 2 has a function as shown in FIG. It has a termination function 16 for selectively terminating only the signal.

Therefore, even after the modulation operation on the light is completed, the extra length area in which the electric signal is propagated can completely change the impedance of the transmission line because the termination function 16 selectively terminates only the signal. This means that only a signal can be terminated without applying a bias, and a bias circuit T including a low-frequency block element 6, a high-frequency block element 7, and a branch junction 11 is provided to apply a bias from a bias source, as in the conventional example. There is no problem of imperfect impedance matching at the branching junction 11, and the number of low-frequency block elements 6 for blocking low-frequency components, which is a bias voltage component, can be reduced. The characteristic deterioration due to loss is small.

The low-frequency block element 6 on the RF input terminal 13 side is not necessary if the same low-frequency block element is provided in the output section inside the signal source 10. In such a case, there is no element that easily causes impedance mismatch in the signal propagation path, and the performance of the optical modulator is improved. In addition, in this configuration, unlike the related art, there is no resistor for consuming the power of the bias voltage in the portion to which the bias voltage is applied despite the absence of the branch junction and the low-frequency block element. Is small and economical.

As described above, according to the present invention shown in the first embodiment, in an optical modulator in which an electric signal propagates through a traveling-wave-type electrode and acts on an optical signal, the traveling-wave-type electrode has the electric signal. In addition to having a surplus area that propagates even after the operation on the optical signal, the surplus area has an impedance equivalent to an operation area where the electric signal operates on the optical signal, The long region is 5 for the electrical signal.
The optical modulator was configured to have a loss of [dB] or more.

The length of the traveling-wave-type electrode is such that the shift in the modulation timing caused by the shift in the speed of the light traveling in the optical waveguide and the speed of the electric signal traveling in the traveling-wave-type electrode does not matter. .

On the other hand, for an electric signal of a very high frequency such as a millimeter wave band, a loss in a transmission line such as a microstrip line is large. Therefore, when a sufficiently long line is propagated, the signal is attenuated and disappears in the line. When a millimeter-wave signal is propagated in a sufficiently long line in an optical modulator as shown in FIG. 4, the light which propagates in the optical waveguide is effectively acted on for a relatively short length for the above-described reason. Within the range of LO.

Although the length of the LO is sufficient for the original function of the optical modulator, utilizing this fact, in the present invention, the longer electrode line 2 is used, and the effect on light is terminated. After that, the electrode line is propagated.

As a result, the millimeter wave signal is attenuated in the electrode line, or the millimeter wave signal disappears in the electrode line depending on the set electrode line length. As described above, a sufficiently long electrode length is ensured as a traveling-wave type electrode line, the optical transmission line is arranged to pass through a part of the electrode line, and the surplus length of the electrode line is used as an attenuation region for electric signals. As a result, a configuration is adopted in which a terminating resistor is not used. Therefore, there is no reflection due to the termination resistor not having ideal characteristics.

The configuration without bias application as shown in FIG. 4 is used for an LN phase modulator or the like. In the present invention, the electrode line used has, in addition to an active area for modulating light by applying a traveling wave electric field by a high-frequency modulation signal, an extra-long area where the high-frequency modulation signal is sufficiently attenuated. The surplus area is actually 5 [d] for the following reason.
B] If there is a surplus area length that can secure the above attenuation amount,
It is considered long enough.

That is, when designing a high-frequency component,
A criterion determined by the amount of reflection called a voltage standing wave ratio (VSWR) is often used. And, in general high-frequency devices, VSWR = about 2: 1 is often required. This corresponds to a reflection amount of -10 [dB].

When this is applied to the optical modulator of the present invention, as shown in FIG. 5, if the configuration is such that there is a loss of 5 [dB] one way in the extra length region, the signal is totally reflected at the end of the electrode line. Even if the signal returns, the signal returning from the extra length area 15 to the action area 14 suffers a loss of 10 [dB] due to the loss of the round trip, and is reflected at -10 [dB].

Since there is also a loss in the working area 14 in the electrode line 2, the optical modulator provided with the extra length area having a loss of 5 [dB] is -10 when viewed from the modulation signal input terminal (RF input terminal). The reflection amount is [dB] or less, which sufficiently satisfies the general required performance as a high-frequency device, and can withstand practical use.

FIG. 6 is a block diagram showing another embodiment of the optical modulator according to the present invention. In the above example, the electrode line 1 is straight and the optical waveguide 1 side is bent from the middle. In this example, on the contrary, the optical waveguide 1 side is straight and the electrode line 2 side is halfway. I bend it.

That is, the action area 1 in the electrode line 2
In FIG. 4, the optical waveguide 1 and the electrode line 2 are overlapped, but the optical waveguide 1 is made linear and the electrode line 2 is bent so as to gradually separate them. Other parts are the same as those in FIG.

Both the optical waveguide 1 and the electrode line 2 have a large loss against a sharp bend. Therefore, it is necessary to bend the optical waveguide 1 and the electrode line 2 in the action region 14 with a sufficiently large radius of curvature with little loss.

Since high-frequency signals such as millimeter waves are greatly attenuated in the transmission line, only high-frequency electric signals are selectively attenuated while propagating in the extra length region 15. On the other hand, a low-frequency signal such as a bias voltage is applied with a negative voltage across the electrode line.

In such a configuration, elements such as a bias T circuit for applying a bias voltage and a terminating resistor for terminating a millimeter wave signal are not required, and reflection and excessive loss by these elements are eliminated. In addition, the performance of the device in a high frequency range is improved.

Ideally, the larger the amount of attenuation of the electric signal in the extra length region 15, the better, and therefore, the longer the extra length region 1
5 is required. However, the length of the extra length area 15 is required to be minimized due to mounting and package size problems. The amount of attenuation required for the extra length region 15 may be 5 [dB] or more in one way for the reason described above. Therefore, as shown in FIG. 5, the surplus length region 15 may have a length having an attenuation of 5 [dB].

However, since the signal is attenuated even in the action area 14, as shown in FIG. 7, it is sufficient that the action area 14 and the extra length area 15 have a total attenuation of 5 [dB]. If there is an attenuation of 5 [dB] or more only in the action region 14, no extra length region 15 is provided at all as shown in FIG.
The terminal 12 for the bias voltage may be drawn out without providing the bias T circuit or the terminating resistor at the end of the electrode line 2. With this configuration, the substrate and the package of the optical modulator can be reduced in size because the configuration is simple and the number of components is small.

Further, in the present invention, another electric element 26, for example, an element that exerts some action on the bias voltage may be formed in the extra length region 15 as shown in FIG.

At this time, the electric signal propagating through the electrode line 2 does not interact with the electric element 26 because the electric signal is sufficiently attenuated in the extra length region 15 or has a different operation band. The bias voltage may be applied to the terminal 27 in the same manner as in the previous examples. Alternatively, depending on the circuit design, a voltage may be applied to the terminal 28 so that the bias voltage is applied to the extra length region 15 after performing some processing. May be.

As described above, this embodiment eliminates the need for a terminating resistor, eliminates the need for a branch junction, and minimizes the use of elements that require characteristics at high frequencies.
In modulating light with high frequency modulation signals such as millimeter waves,
An optical modulator with good modulation characteristics and high practicality can be provided.

(Second Embodiment) Next, an embodiment for shortening the surplus area will be described as a second embodiment with reference to FIG.

The embodiment shown in FIG. 11 has a configuration in which the shape of the electrode line 2 is gradually reduced. That is, the extra length area 15
In this embodiment, the signal line width of the electrode line 2 is gradually narrowed. The line is a coplanar line and the signal line 2
The width between the signal line 21 and the ground 20 in the electrode line 2 is reduced by reducing the distance between the signal line 21 and the ground 20 while keeping the impedance constant.

As the signal line 21 becomes narrower, the ohmic loss increases, and only a high-frequency signal can be selectively attenuated with a shorter length. At this time, the width of the signal line 21 and the signal line 2 are set so that the impedance does not fluctuate due to a sudden change.
It is preferable that the distance between 1 and the ground 20 be gradually reduced.

Similarly, as shown in FIG.
The width of the track may be gradually increased. At this time, the signal loss is mainly due to radiation loss, so the radiated signal will not be picked up by other parts and become noise or cause resonance. Consideration such as disposing an absorber is required.

According to the present invention, if the impedance is kept equal to that of the action region 14, the resistance value increases toward the end of the extra length region 15 as shown in FIG. To create a pure resistance R,
5 may be configured to increase the loss. Thus, the extra length region 15 can be configured to be shorter, which is a technique that contributes to downsizing.

[0086] High-frequency signals such as millimeter waves have a large loss due to bending of the line. The degree of loss depends on the curvature of the bend, and the smaller the radius of curvature, the greater the loss. Therefore, in the present invention, the electric signal may be attenuated by bending the portion of the electrode line 2 on the extra length region 15 side with a small radius of curvature as shown in FIG.

As a result of this configuration, the length of the extra length area 15 can be shorter. However, at this time, since radiation loss increases, it is necessary to consider noise and resonance as in the case described above.

As shown in FIG. 14, when the line width is gradually narrowed and the line is bent with a small curvature,
The electric signal attenuation effect is further increased. As described above, in the second embodiment, the length of the surplus area is shortened by giving a front to the traveling wave type electrode, reducing the width of the signal line, or incorporating a resistor.

When the impedance is specified in the electrode line for the modulated electric signal, there is an optimum signal line width that minimizes the propagation loss. If the width is wider than that, the loss increases due to radiation loss, and if the width is narrower, the loss increases due to ohmic loss.

In the present invention, the signal line width of the action area is
The ohmic loss is increased by narrowing the signal line width in the extra length region, and the signal is lost at a shorter distance. And, in the ohmic loss, the modulation signal by the high frequency is converted into heat in the electrode line, so that it has no influence on the others.

Also, if the signal line width changes abruptly at the point where the signal moves from the action area to the extra length area, it causes reflection, so that the width is gradually narrowed. The thickness of the substrate on which the optical modulator is formed is substantially uniform in the plane. Microstrip lines and coplanar lines are often used for high frequency lines manufactured on a substrate. To change the width of the signal line without changing the impedance in the microstrip line, it is necessary to change the substrate thickness, which is not practical.

On the other hand, the coplanar line only needs to change the distance between the signal line and the ground on the same plane.
Suitable for the present invention. Therefore, according to the present invention, the use of elements that require characteristics for high frequencies is minimized, and when modulating light with a high-frequency modulation signal such as a millimeter wave, the modulation characteristics are good and practical. A high optical modulator can be provided.

Although various embodiments have been described above, the present invention is not limited to the above-described examples, and can be implemented with various modifications. For example, in the examples so far, a configuration in which one electrode line is provided on one optical waveguide has been described, but the present invention is not limited to such an embodiment.

For example, as shown in FIG. 15, the present invention can be applied to a case where the structure acts on a plurality of optical waveguides 30-1 and 30-2. In this case as well, after the operation is completed, the signal propagates through the extra length region 15 to selectively attenuate only the signal.

In each of the above embodiments, a lithium niobate optical modulator has been described as an example of an optical modulator. However, if the type of optical modulator uses a traveling wave type electrode, the material is not limited to this. Not limited to

[0096]

As described above, according to the present invention, a high-frequency electric signal such as a millimeter wave is used as a modulation signal, and a traveling-wave electric field is generated by the modulation signal to act on light propagating in an optical waveguide. In the optical modulator that modulates the light, a configuration is provided in which an extra length region is provided in which the high-frequency electric signal propagates even after finishing acting on the light while propagating through the electrode line. At a high frequency such as a millimeter wave, the loss when propagating through the line is large, and the device of the present invention using such a high frequency signal as a modulation signal has a configuration in which sufficient attenuation is provided in a surplus length region. The need for resistors and associated low frequency blocking elements is eliminated. Further, when the bias voltage is applied from the end of the extra length region, since the high frequency signal is almost attenuated at the end of the extra length region, the isolation between the bias voltage source and the signal source can be maintained. As a result, a circuit composed of a bias applying element such as a high-frequency block element or a branch junction, which has been required in the past, becomes unnecessary, so that the device configuration is simplified, and various elements can be omitted. It is possible to provide an optical modulator in which characteristics are greatly improved without reflection or loss due to completeness.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of an embodiment of the present invention.

FIG. 2 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of an embodiment of the present invention.

FIG. 3 is a view for explaining the present invention, showing an equivalent configuration in the embodiment of the present invention.

FIG. 4 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of an embodiment of the present invention.

FIG. 5 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of an embodiment of the present invention.

FIG. 6 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of the embodiment of the present invention.

FIG. 7 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of the embodiment of the present invention.

FIG. 8 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of an embodiment of the present invention.

FIG. 9 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of an embodiment of the present invention.

FIG. 10 is a diagram illustrating the present invention, and is a schematic diagram illustrating an example of an embodiment of the present invention.

FIG. 11 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of the embodiment of the present invention;

FIG. 12 is a diagram for explaining the present invention, and is one of the embodiments of the present invention.

FIG. 13 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of an embodiment of the present invention.

FIG. 14 is a diagram for explaining the present invention, and is a schematic diagram for explaining an example of an embodiment of the present invention.

FIG. 15 is a view for explaining the present invention, and is a schematic view for explaining an example of the embodiment of the present invention;

FIG. 16 is a view for explaining a conventional technique.

FIG. 17 is a view for explaining a conventional technique.

FIG. 18 is a view for explaining a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical waveguide 2 ... Electrode line 3 ... Input terminal 4 ... Output terminal 5 ... Terminal 6 ... Low frequency block element 7 ... High frequency block element 8 ... Terminal resistance 9 ... DC source 10 ... Signal source 11 ... Branch junction 12 Bias input terminal 13 RF input terminal 14 Action region 15 Extra length region 16 Extra length region 17 Bias source 19 Coplanar line 20 Ground 21 Signal line 25 Line incorporating pure resistance 26 Electric element 27, 28 terminal 30 optical waveguide

Claims (4)

[Claims] 1. A traveling wave type electrode is provided in an optical waveguide,
By applying an electric signal for modulation to the traveling wave type electrode to generate a traveling wave electric field, the optical modulator modulates the optical signal in the optical waveguide by the electric field. The electrode has a working region for applying an electric field to the optical waveguide and a surplus region provided exclusively for attenuating a signal. The surplus region has an impedance equivalent to that of the working region and the electric signal. An optical modulator having an electrode length that gives a loss equal to or more than a predetermined value to the optical modulator. 2. The optical modulator according to claim 1, wherein said predetermined value is 5 [dB] or more. 3. A configuration comprising a bias applying means for applying a DC voltage or a bias voltage comprising a DC voltage and a low frequency signal, wherein the bias voltage is applied from an end of the traveling wave type electrode on the extra length region side. The optical modulator according to claim 1, wherein: 4. The traveling wave type electrode is a coplanar type line, and the signal line width is gradually narrowed in the extra length region. 2. The optical modulator according to claim 1.