Optical modulator and optical transmitter

Abstract

To provide an optical modulator that drives an optical modulation element using some of a plurality of outputs of a drive circuit having the plurality of outputs, and that reduces power consumption or its size while avoiding destabilizing operation.SOLUTION: An optical modulator (1) includes: an optical waveguide element that includes an optical waveguide (31) formed on a substrate (30) and a signal electrode (41) for controlling a light wave propagating through the optical waveguide; a drive circuit (17) that outputs two high-frequency signals; and two terminating resistors that terminate the two high-frequency signal outputs from the drive circuit respectively. One high-frequency signal output of the drive circuit propagates through the signal electrode of the optical waveguide element and is terminated by a first termination resistor (15), which is one termination resistor. The output of the other high-frequency signal of the drive circuit is terminated by a second termination resistor (18), which is the other termination resistor. A resistance value R2 of the second termination resistor is greater than a resistance value R1 of the first termination resistor.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an optical modulator and an optical transmitter. [Background technology]

[0002] In high-speed / large-capacity optical fiber communication systems, optical modulators incorporating optical modulation elements as optical waveguide elements, which consist of an optical waveguide formed on a substrate and a control electrode that controls the light waves propagating through the optical waveguide, are widely used.As optical waveguide elements that perform optical modulation, semiconductor optical modulation elements using semiconductor substrates such as InP substrates and LN optical modulation elements using LiNbO3 (hereinafter also referred to as LN) as a substrate have been put to practical use.

[0003] Patent Document 1 discloses a semiconductor optical modulation module equipped with a semiconductor optical modulation element including a Mach-Zehnder optical waveguide. This semiconductor optical modulation module uses a standardized small housing called a High Bandwidth Coherent Driver Modulator (HB-CDM), and performs optical modulation by inputting two high-frequency electrical signals (hereinafter referred to as high-frequency signals) constituting a pair of differential signals with mutually inverted phases to electrodes formed on each of two parallel waveguides constituting the Mach-Zehnder optical waveguide. A driver circuit element (high-frequency driver) that outputs differential signals to drive such a semiconductor optical modulation element has already been put into practical use in the form of an integrated circuit and is commercially available.

[0004] On the other hand, LN optical modulation elements are widely used in high-speed / large-capacity optical fiber communication systems because they have less optical loss than semiconductor optical modulation elements and can achieve wideband optical modulation characteristics, but dedicated driver circuit elements for driving LN optical modulation elements used in the above-mentioned HB-CDM have not yet been commercialized due to low demand, etc. For this reason, it would be convenient if optical modulators equipped with LN optical modulation elements could use driver circuit elements that have been developed for semiconductor optical modulation modules and are already in practical use.

[0005] However, in the case of an LN optical modulation element that uses an X-cut substrate as the LN substrate, as described in Patent Document 2, a differential signal is not necessary, and it is sufficient to input one high-frequency electrical signal to one signal electrode formed between two parallel waveguides to drive each Mach-Zehnder optical waveguide that constitutes the LN optical modulation element.

[0006] In such an optical modulation element that does not use a differential signal, when a drive circuit element that outputs a differential signal as described above is used, it is necessary to connect the output terminal of one of the high-frequency signals that make up the differential signal to the signal electrode, and to treat the output terminal of the other high-frequency signal by appropriate means.

[0007] However, depending on how the other output terminal is treated, the operation of the drive circuit element may become unstable due to reflection of high frequency signals that may occur in the circuit components connected to that output terminal, or the overall power consumption of the drive circuit element may become inefficient due to unnecessary signal power being output and consumed from that output terminal. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-164243 [Patent Document 2] Patent No. 6933287 Summary of the Invention [Problem to be solved by the invention]

[0009] In view of the above background, an object of the present invention is to reduce power consumption or miniaturize the size of an optical modulator that drives an optical modulation element using a signal output from a driving circuit that outputs multiple signals such as differential signals, while avoiding instability in operation. [Means for solving the problem]

[0010] One aspect of the present invention is an optical modulator including an optical waveguide formed on a substrate and a signal electrode for controlling light waves propagating through the optical waveguide, a drive circuit that outputs two high-frequency signals, and two termination resistors that respectively terminate the two high-frequency signal outputs from the drive circuit. One of the high-frequency signal outputs of the drive circuit propagates through the signal electrode of the optical waveguide element and is terminated by a first termination resistor, which is one of the termination resistors. The other high-frequency signal output of the drive circuit is without propagating through the signal electrode of the optical waveguide element, terminated by a second termination resistor, which is the other termination resistor, and the resistance value of the second termination resistor is larger than the resistance value of the first termination resistor Ku , The two high-frequency signals output from the driving circuit are a pair of differential signals whose phases are inverted from each other, and the substrate of the optical waveguide element is made of X-cut LiNbO 3 It is a circuit board. According to another aspect of the present invention, the sum of the resistance value of the first termination resistor and the resistance value of the second termination resistor is equal to the differential impedance between the outputs of the two high-frequency signals that constitute the differential signal of the drive circuit. According to another aspect of the present invention, the resistance value R1 of the first termination resistor that terminates the output of the one high-frequency signal that drives the signal electrode is in the range of 0.4×Zlin < R1 ≤ Zlin with respect to the value Zlin of the output impedance between the output of the one high-frequency signal and the ground potential in the drive circuit. According to another aspect of the present invention, the value Zlin of the output impedance is 50Ω, and the resistance value R1 of the first termination resistor is 45Ω or less 。 According to another aspect of the present invention, the electrical length L1 of the high-frequency transmission line from the output of the drive circuit to the first termination resistor for the one high-frequency signal and the electrical length L2 of the high-frequency transmission line from the output of the drive circuit to the second termination resistor for the other high-frequency signal are different from each other. According to another aspect of the present invention, the electrical lengths L1 and L2 have a relationship of (L1 - L2) > λ / 2 with respect to the average wavelength λ in the high-frequency transmission lines of the two high-frequency signals. Another aspect of the present invention is an optical transmission device including any of the above optical modulators and an electronic circuit that generates a modulation signal, which is a high-frequency signal for causing the optical waveguide element to perform a modulation operation. [[ID=ZI]] [Effects of the Invention]

[0011] According to the present invention, in an optical modulator that drives an optical modulation element using a signal output from a part of a drive circuit that outputs multiple signals such as differential signals, it is possible to reduce power consumption or reduce the size while avoiding instability in operation. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a diagram showing a configuration of an optical modulator according to a first embodiment of the present invention. [Figure 2] 2 is a diagram showing the configuration of an optical modulation element used in the optical modulator shown in FIG. [Figure 3] FIG. 2 is a circuit diagram of an electric circuit formed by a driving circuit element, a signal electrode, and a termination resistor. [Figure 4] FIG. 10 is a diagram for explaining a modified example of the first embodiment. [Figure 5] FIG. 10 is a diagram illustrating a configuration of an optical transmitting device according to a second embodiment of the present invention. [Figure 6] FIG. 10 is a diagram showing an example in which a termination resistor is provided in a drive circuit element. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [1. First embodiment] First, a first embodiment of the present invention will be described. Fig. 1 is a diagram showing the configuration of an optical modulator 1 using an optical modulation element that is an optical waveguide element, according to the first embodiment of the present invention. The optical modulator 1 includes a housing 2 and an optical modulation element 3 housed within the housing 2. The optical modulation element 3 is configured as, for example, a DP-QPSK modulator. The housing 2 complies with, for example, the HB-CDM standard, an industry standard ("Implementation Agreement for the High Bandwidth Coherent Driver Modulator (HB-CDM) OIF-HB-CDM-02.0" (published by OIF on July 15, 2021)). Finally, a plate-shaped cover (not shown) is fixed to the opening of the housing 2, and the interior is hermetically sealed.

[0014] The housing 2 is provided with a signal pin 4 for inputting a high-frequency electrical signal used to modulate the optical modulation element 3, and a signal pin 5 for inputting an electrical signal used for adjusting the operating point of the optical modulation element 3. In the following, the impedance and resistance value refer to the impedance and resistance value at the frequency of the high-frequency electrical signal used to modulate the optical modulation element 3, respectively.

[0015] The optical modulator 1 also has an input optical fiber 6 for inputting light into the housing 2 and an output optical fiber 7 for guiding the light modulated by the optical modulation element 3 to the outside of the housing 2, both on the same surface of the housing 2.

[0016] The input optical fiber 6 and the output optical fiber 7 are fixed to the housing 2 via supports 8 and 9, which are fixing members, respectively. Light input from the input optical fiber 6 is collimated by a lens 10 arranged in the support 8, and then input to the light modulation element 3 via a lens 11. However, this is just one example, and light can also be input to the light modulation element 3 in accordance with conventional technology, for example, by introducing the input optical fiber 6 into the housing 2 via the support 8 and connecting the end face of the introduced input optical fiber 6 to the end face of a substrate 30 (described later) of the light modulation element 3.

[0017] The optical modulator 1 also has an optical unit 12 that polarization-combines two modulated lights output from the optical modulation elements 3. The polarization-combined light output from the optical unit 12 is collected by a lens 13 disposed within the support 9 and coupled to the output optical fiber 7.

[0018] Also disposed within the housing 2 of the optical modulator 1 are a relay board 14 and a terminator 16 having four terminator resistors 15a, 15b, 15c, and 15d, each having a predetermined impedance. Hereinafter, the terminator resistors 15a, 15b, 15c, and 15d will be collectively referred to as terminator resistors 15.

[0019] Four drive circuit elements 17a, 17b, 17c, and 17d are mounted on the relay substrate 14 to respectively drive four signal electrodes 41 (described later) provided on the four Mach-Zehnder optical waveguides of the optical modulation element 3. Hereinafter, the drive circuit elements 17a, 17b, 17c, and 17d will also be collectively referred to as drive circuit elements 17. The drive circuit elements 17 are mounted on the relay substrate 14, for example, in the form of an integrated circuit.

[0020] The drive circuit elements 17 are two-signal input, two-signal output amplifier circuits that amplify differential signals, which are two high-frequency electrical signals (hereinafter referred to as "high-frequency signals") that are out of phase with each other (i.e., 180 degrees out of phase), and output the two amplified differential signals. The four (i.e., four pairs of) differential signals input to each of the four drive circuit elements 17 are provided from an external device via signal pins 4, for example. The high-frequency signals that make up the differential signals are, for example, microwave electrical signals that contain signal components at or above the G-band frequency specified by the IEEE standard, specifically 0.2 GHz or above.

[0021] Four termination resistors 18a, 18b, 18c, and 18d are also mounted on the relay board 14. Hereinafter, the termination resistors 18a, 18b, 18c, and 18d will be collectively referred to as termination resistors 18.

[0022] Of the two high-frequency signals that are differential signals output by the drive circuit element 17, one high-frequency signal is input to one signal electrode 41 of the optical modulation element 3, propagates through the signal electrode 41, and is then terminated by one termination resistor 15. Furthermore, of the two high-frequency signals that are differential signals that are output by the drive circuit element 17, the other high-frequency signal is terminated by one termination resistor 18 mounted on the relay substrate 14.

[0023] The optical modulation element 3 is electrically connected to the relay substrate 14 and the terminator 16 by, for example, wire bonding.

[0024] 2 is a diagram showing an example of the configuration of the optical modulation element 3, which is a DP-QPSK modulator. In FIG. 2, a part of the relay substrate 14 and the terminator 16 are also shown. The optical modulation element 3 is composed of an optical waveguide 31 (the entire area indicated by the thick dotted lines in the figure) formed on a substrate 30, and performs, for example, 200G DP-QPSK modulation. The substrate 30 is, for example, an X-cut LN substrate with an electro-optic effect that has been processed and thinned to a thickness of 20 μm or less (e.g., 2 μm). The optical waveguide 31 is a convex optical waveguide (e.g., a rib-type optical waveguide or a ridge-type optical waveguide) formed on the surface of the thinned substrate 30 and consisting of a strip-like extending convex portion. Here, the refractive index of the LN substrate can locally change due to the photoelastic effect when stress is applied, so in order to reinforce the mechanical strength of the entire substrate, it is generally bonded to a support plate such as a Si (silicon) substrate, glass substrate, or LN substrate.

[0025] The substrate 30 is, for example, rectangular and has two opposing left and right sides 32a and 32b extending in the vertical direction in the figure, and two opposing upper and lower sides 32c and 32d extending in the horizontal direction in the figure.

[0026] The optical waveguide 31 includes an input waveguide 33 that receives input light (indicated by an arrow pointing rightward in the figure) from the input optical fiber 6 on the upper side of the right side 32b of the substrate 30 in the figure, and a branching waveguide 34 that branches the input light into two light beams having the same light intensity. The optical waveguide 31 also includes so-called nested Mach-Zehnder optical waveguides 35a and 35b that are two modulation sections that modulate the respective light beams branched by the branching waveguide 34.

[0027] In nested Mach-Zehnder optical waveguides 35a and 35b, the light propagation direction is turned back 180 degrees at the left part of substrate 30 in the figure, and light is output from side 32b of substrate 30 to the right in the figure by output waveguides 36a and 36b.

[0028] The nested Mach-Zehnder optical waveguides 35a and 35b each include two Mach-Zehnder optical waveguides 37a and 38a, and 37b and 38b, respectively, provided in two waveguide portions that form a pair of parallel waveguides.

[0029] The Mach-Zehnder optical waveguide 37a includes two parallel waveguides 37a1 and 37a2, the Mach-Zehnder optical waveguide 38a includes two parallel waveguides 38a1 and 38a2, the Mach-Zehnder optical waveguide 37b includes two parallel waveguides 37b1 and 37b2, and the Mach-Zehnder optical waveguide 38b includes two parallel waveguides 38b1 and 38b2.

[0030] Bias electrodes 40a, 40b, and 40c for adjusting the operating points by compensating for fluctuations in the bias points due to so-called DC drift of the nested Mach-Zehnder optical waveguides 35a and 35b and the four Mach-Zehnder optical waveguides 37a, 38a, 37b, and 38b are provided on the upper portions of the nested Mach-Zehnder optical waveguides 35a and 35b folded back at the left portion of the substrate 30. These bias electrodes 40a, 40b, and 40c are connected to signal pins 5 of the housing 2 via wire bonding and a conductor pattern (not shown) of the relay substrate 14.

[0031] Furthermore, signal electrodes 41a, 41b, 41c, and 41d for modulating the four Mach-Zehnder optical waveguides 37a, 38a, 37b, and 38b that make up the nested Mach-Zehnder optical waveguides 35a and 35b are provided in the lower portion of the substrate 30. The signal electrodes 41a, 41b, 41c, and 41d are collectively referred to as signal electrodes 41.

[0032] The left sides of the signal electrodes 41a, 41b, 41c, and 41d in the figure extend to the left side 32a of the substrate 30 in the figure and are connected to pads 42a, 42b, 42c, and 42d, respectively. The right sides of the signal electrodes 41a, 41b, 41c, and 41d in the figure bend downward in the figure, extend to the side 32d of the substrate 30, and are connected to pads 43a, 43b, 43c, and 43d.

[0033] The signal electrodes 41a, 41b, 41c, and 41d, together with a ground conductor pattern (not shown) formed on the substrate 30 according to conventional technology, form, for example, a coplanar transmission line having a predetermined impedance Zm. The ground conductor pattern is provided, for example, so as not to be formed on the optical waveguide 31, and the multiple regions of the ground conductor pattern that are divided by the optical waveguide 31 can be connected to each other, for example, by wire bonding or the like.

[0034] Pads 42a, 42b, 42c, and 42d on the left side of signal electrodes 41a, 41b, 41c, and 41d in the figure are connected to output terminals 50a, 50b, 50c, and 50d of one high-frequency signal constituting a differential signal of drive circuit elements 17a, 17b, 17c, and 17d, respectively, via wires bonded to these pads and conductor patterns on relay substrate 14. Pads 43a, 43b, 43c, and 43d on the lower right side of signal electrodes 41a, 41b, 41c, and 41d in the figure are connected to one end of termination resistors 15a, 15b, 15c, and 15d in terminator 16 via bonding wires. The other ends of termination resistors 15a, 15b, 15c, and 15d are each connected to, for example, a ground line (not shown) provided on the substrate constituting terminator 16.

[0035] On the other hand, output terminals 51a, 51b, 51c, and 51d of the other high-frequency signal constituting the differential signal of each of drive circuit elements 17a, 17b, 17c, and 17d are connected, via conductor patterns on relay board 14, to one end of termination resistors 18a, 18b, 18c, and 18d mounted on relay board 14. The other ends of termination resistors 18a, 18b, 18c, and 18d are connected to a ground pattern (not shown) provided on relay board 14.

[0036] Hereinafter, the output terminals 50a, 50b, 50c, and 50d of the drive circuit elements 17a, 17b, 17c, and 17d will be collectively referred to as output terminals 50, and the output terminals 51a, 51b, 51c, and 51d will be collectively referred to as output terminals 51.

[0037] Here, the drive circuit element 17 has line impedances Zlin, which are output impedances measured between the output terminals 50 and 51 of the two high-frequency signals constituting the differential signal and the ground potential, that are equal to each other. This line impedance Zlin is, for example, 50 Ω. Furthermore, the differential impedance Zdif, which is the output impedance of the drive circuit element 17 measured between the output terminals 50 and 51 of the differential signal, is 100 Ω.

[0038] In the optical modulator 1 having the above configuration, the high-frequency signal output from one output terminal 50 of the drive circuit element 17, which outputs the amplified differential signal, propagates through the corresponding signal electrode 41 and is then terminated by the termination resistor 15. The high-frequency signal output from the other output terminal 51 of the drive circuit element 17 is terminated by the corresponding termination resistor 18 mounted on the relay substrate 14. Here, the termination resistors 15 and 18 correspond to the first termination resistor and the second termination resistor in this disclosure, respectively.

[0039] Fig. 3 shows, using circuit symbols, the driver circuit element 17 and its loads, the signal electrode 41 and the termination resistors 15 and 18. In Fig. 3, the resistance of the termination resistor 15, which is the first termination resistor, is R1, the resistance of the termination resistor 18, which is the second termination resistor, is R2, and the impedance of the signal electrode 41 at the signal frequency of the high-frequency signal output by the driver circuit element 17 is Zm. Fig. 3 also shows the output equivalent circuit of the driver circuit element 17 using the line impedance Zlin. Note that the differential impedance Zdif of the output of the driver circuit element 17 measured between output terminals 50 and 51 is 2 × Zlin.

[0040] In this embodiment, in particular, in the optical modulator 1, the resistance value R2 of the second termination resistor 18 is set to a value larger than the resistance value R1 of the first termination resistor 15. That is, R1 and R2 have the following relationship: R2>R1 (1)

[0041] Conventionally, in a two-input, two-output high-frequency amplifier circuit such as the drive circuit element 17, the two outputs are terminated by loads having impedances that are approximately the same as the output impedance of the high-frequency amplifier circuit and that are the same as each other, and are used to output approximately the same power. In contrast to this, in this embodiment, the resistance values ​​R1 and R2 of the termination resistors 15 and 18 connected to the two high-frequency signal outputs of the drive circuit element 17, respectively, are different from each other.

[0042] Generally, electrical energy such as current and high frequency waves tends to pass easily through areas with low resistance and has difficulty passing through areas with high resistance. For this reason, by setting the resistance values ​​of the termination resistors connected to the two outputs of a two-output amplifier circuit such as drive circuit element 17 to different values ​​(i.e., by making them asymmetric), the output power of the two outputs of the amplifier circuit can be made asymmetric.

[0043] In this embodiment, as described above, one of the two high-frequency signal outputs of the drive circuit element 17 is terminated by the termination resistor 15 having a resistance value R1 after propagating through the signal electrode 41 of the optical modulation element 3, and the other high-frequency signal output is terminated by the termination resistor 18 having a resistance value R2 greater than R1. This allows the output power of one of the two high-frequency signal outputs of the drive circuit element 17 propagating through the signal electrode 41 to be reduced relative to the output power of the other high-frequency signal output, thereby reducing the power consumption of the drive circuit element 17. Furthermore, it is also possible to reduce the power supplied to the drive circuit element 17 by the amount corresponding to the increase in the output power of the high-frequency signal propagating through the signal electrode 41, thereby reducing power consumption. Furthermore, this reduction in power consumption can be achieved simply by changing the resistance values ​​R1 and R2 of the termination resistors 15 and 18 connected to the differential signal outputs of the drive circuit element 17, and therefore can be achieved very simply without requiring major design changes.

[0044] As a result, in the optical modulator 1 of this embodiment, the overall power consumption of the optical modulator 1 can be easily reduced compared to the conventional configuration described above in which the two high-frequency signal outputs of the drive circuit element 17 are terminated with the same resistance value.

[0045] For the same reason, when the drive circuit element 17 is driven with the same power consumption as in the conventional configuration, the power of one of the high-frequency signals output to the signal electrode 41 can be made greater than the power of the other high-frequency signal, compared to the conventional configuration, so the length of the signal electrode 41 can be shortened, making it easier to broaden the bandwidth of the modulation operation of the optical modulator 1 and reduce its size.

[0046] Furthermore, in a configuration such as that described in Patent Document 1, in which both high-frequency signals of the differential signal output from the drive circuit element are used for modulation operation, it is necessary to match the line impedance of each high-frequency signal output with the termination resistor (i.e., to ensure symmetry in the resistance values ​​of the two termination resistors), as well as to match the differential impedance of the two high-frequency signal outputs with the combined resistance value of the termination resistors.If this matching is imperfect, the optical modulation operation of the optical modulation element may become unstable.

[0047] In contrast, in the optical modulator 1 of this embodiment, only one of the two high-frequency signals that make up the differential signal is used for modulation operation, so compared to the conventional configuration, instability is less likely to occur in the optical modulation operation or amplification operation even if there is an impedance mismatch between the line impedance and the termination resistor or if the symmetry of the resistance values ​​of the two termination resistors is lost.

[0048] Among the drive circuit elements that amplify differential signals, there may be some that include an equivalent circuit that equalizes the output power of the two high-frequency signals that make up the differential signal output (i.e., controls these output powers to be equal to each other).However, since the configuration of this embodiment makes the magnitude of output power asymmetric between the differential signal outputs of drive circuit element 17 by asymmetrically setting the resistance values ​​of the termination resistors connected to those outputs, it is desirable that drive circuit element 17 not include an equivalent circuit for output power such as the one described above.

[0049] Below, several aspects will be described regarding the relationship between the resistance values ​​R1 and R2 of the termination resistors 15 and 18 connected to each of the drive circuit elements 17 in the optical modulator 1 of this embodiment. Note that in the optical modulator 1, any one of the following aspects may be commonly applied to the four drive circuit elements 17, or different aspects may be applied to each of the four drive circuit elements 17.

[0050] [1.1 First Aspect] In a first aspect of this embodiment, the resistance value R2 of the termination resistor 18 is set to the same value as the line impedance Zlin of the drive circuit element 17 so as to suppress reflection of high-frequency signals at the termination resistor 18, and the resistance value R1 of the termination resistor 15 is set to be smaller than the resistance value R2 of the termination resistor 18. That is, R1, R2, and Zlin are set to have the following relationship: R2=Zlin>R1 (2)

[0051] Specifically, for example, the line impedance Zlin of the drive circuit element 17, the impedance Zm of the signal electrode 41, and the resistance value R2 of the termination resistor 18 are set to 50Ω, and the resistance value R1 of the termination resistor 15 is set to 40Ω.

[0052] In equation (2) representing the first aspect described above, if the resistance value R1 of the first termination resistor 15 is made smaller, the signal power of the high-frequency signal output from the drive circuit element 17 to the signal electrode 41 increases accordingly. Therefore, from the perspective of increasing the signal power output to the signal electrode 41, if the line impedance of the drive circuit element 17 is 50 Ω, for example, it is desirable that the resistance value R1 of the termination resistor 15 be less than 50 Ω, for example, 45 Ω or less, and it is even more effective if it is 40 Ω or less.

[0053] However, if the resistance value R1 of the termination resistor 15 is set to be significantly lower than the line impedance Zlin of the drive circuit element 17, it may increase the reflection of the high-frequency signal due to impedance mismatch at the termination resistor 15. In this case, the high-frequency signal reflected at the termination resistor 15 (hereinafter referred to as the reflected high-frequency signal) may reach the output terminal 50 of the drive circuit element 17, causing instability in the operation of the drive circuit element 17. Therefore, from the perspective of suppressing the reflection of the high-frequency signal at the termination resistor 15 and ensuring stability in the operation of the drive circuit element 17, it is desirable that the line impedance Zlin of the drive circuit element 17 and the resistance value R1 of the termination resistor 15 have the following relationship: 0.4Zlin <R1<Zlin (3)

[0054] [1.2 Second Aspect] As a second aspect of the resistance values ​​R1 and R2 of the termination resistors 15 and 18 in the optical modulator 1 of this embodiment, the resistance value R1 of the termination resistor 15 is set to the same value as the line impedance Zlin of the drive circuit element 17 so as to suppress reflection of high-frequency signals at the termination resistor 15, and the resistance value R2 of the termination resistor 18 is set to be larger than the resistance value R1 of the termination resistor 15. In other words, R1, R2, and Zlin are set to have the following relationship: R2>R1=Zlin (4)

[0055] Specifically, for example, the line impedance Zlin of the drive circuit element 17, the impedance Zm of the signal electrode 41, and the resistance value R1 of the termination resistor 15 can be set to 50Ω, and the resistance value R2 of the termination resistor 18 can be set to 60Ω.

[0056] According to the aspect shown in formula (4), the resistance value R1 of the termination resistor 15 matches the line impedance Zlin of the drive circuit element 17, thereby suppressing the generation of reflected high-frequency signals in the termination resistor 15 and therefore preventing these reflected high-frequency signals from destabilizing the optical modulation operation of the optical modulation element 3. In other words, this aspect is suitable for cases where low power consumption of the optical modulator 1 is desired while suppressing adverse effects on the optical modulation operation due to impedance mismatch.

[0057] [1.3 Third Aspect] As a third aspect of the resistance values ​​R1 and R2 of the termination resistors 15 and 18 in the optical modulator 1 of this embodiment, the resistance values ​​R1 and R2 of the termination resistors 15 and 18 are set so that the resistance value R2 is greater than the line impedance Zlin of the drive circuit element 17 and the resistance value R1 is smaller than the line impedance Zlin of the drive circuit element 17, within a range in which the sum of these resistance values ​​is the same as the differential impedance Zdif of the drive circuit element 17. In other words, R1, R2, Zlin, and Zdif are set to have the relationship expressed by the following equation. R1+R2=Zdif and R2>Zlin>R1 (5)

[0058] Specifically, for example, the differential impedance Zdif and line impedance Zlin of the drive circuit element 17 are 100Ω and 50Ω, respectively, the impedance Zm of the signal electrode 41 is 50Ω, the resistance value R1 of the termination resistor 15 is 40Ω, and the resistance value R2 of the termination resistor 18 is 60Ω.

[0059] According to the aspect represented by formula (5), the difference between the resistance values ​​R1 and R2 can be set larger because the instability of the drive circuit operation can be suppressed by ensuring a match between the combined resistance value of the termination resistors 15 and 18 and the differential impedance of the drive circuit element 17. Therefore, in this aspect, the signal power of the high-frequency signal output from the drive circuit element 17 to the termination resistor 18 can be further reduced relative to the signal power of the high-frequency signal output from the drive circuit element 17 to the signal electrode 41.

[0060] [1.4 Fourth Aspect] As a fourth aspect of the resistance values ​​R1 and R2 of the termination resistors 15 and 18 in the optical modulator 1 of this embodiment, the resistance values ​​R1 and R2 of the termination resistors 15 and 18 are both set to be smaller than the line impedance Zlin of the drive circuit element 17 and to satisfy the above-mentioned formula (1). That is, R1, R2, and Zlin are set to have the relationship of the following formula. Zlin>R2>R1 (6)

[0061] Specifically, for example, the line impedance Zlin of the drive circuit element 17 and the impedance Zm of the signal electrode 41 are 50Ω, the resistance value R1 of the termination resistor 15 is 30Ω, and the resistance value R2 of the termination resistor 18 is 40Ω.

[0062] In the embodiment shown in equation (6), the resistance value R2 of the termination resistor 18 is set to a value greater than the resistance value R1 of the termination resistor 15, so the signal power of the high-frequency signal output from the drive circuit element 17 to the termination resistor 18 can be reduced compared to the signal power of the high-frequency signal output from the drive circuit element 17 to the signal electrode 41. However, in this embodiment, the resistance values ​​R1 and R2 of the termination resistors 15 and 18 are both different from the line impedance of the drive circuit element 17, so the reflected high-frequency signal generated by the reflection of the high-frequency signal at the termination resistors 15 and 18 can be larger than in other embodiments. Therefore, this embodiment can be applied when the drive circuit element 17 is resistant to the reflected high-frequency signal incident from the output terminals 50 and 51.

[0063] Although the line impedance Zlin is set to 50Ω in this embodiment, it is not limited to this value and may be set to another value such as 40Ω.

[0064] 2. Modification of the First Embodiment Next, a modification of the optical modulator 1 according to this embodiment will be described. In a two-input, two-output amplifier circuit such as the drive circuit element 17 that amplifies and outputs a differential signal, if the phases and intensities of the two reflected high-frequency signals incident on the two output terminals are aligned, the circuit operation may be significantly affected by interference between the reflected high-frequency signals.

[0065] For this reason, when using an amplifier circuit that outputs a differential signal, the resistance values ​​of the termination resistors for each of the two high-frequency signal outputs that make up the differential signal output, as well as the electrical lengths of the two transmission paths leading to those termination resistors, are generally designed to be the same.

[0066] However, if the electrical lengths of the two transmission lines are made approximately the same as in the conventional configuration described above, but the resistance values ​​of the two termination resistors are made different as in this embodiment, the reflected high-frequency signals generated in the two termination resistors will propagate back through the two transmission lines over the same electrical length and may interfere with each other inside the amplifier circuit, adversely affecting the amplification operation.

[0067] 4, in this modification, the electrical length L1 of the high-frequency transmission path, including the signal electrode 41, from the output terminal 50 of the drive circuit element 17 to the termination resistor 15 is made different from the electrical length L2 of the high-frequency transmission path on the relay substrate 14 from the output terminal 51 to the termination resistor 18. This configuration can be applied to any of the first to fourth aspects of the resistance values ​​R1 and R2 described above.

[0068] As a result, the reflected high-frequency signals that may be generated at the termination resistors 15 and 18 will have different phases and intensities when they reach the drive circuit element 17, thereby suppressing interference between these reflected high-frequency signals at the drive circuit element 17 and effectively suppressing the adverse effects of the reflected high-frequency signals (i.e., return electrical signals) on circuit operation.

[0069] In the configuration of the optical modulator 1, since the electrical length L1 includes the electrical length of the signal electrode 41 of the optical modulation element 3, it is preferable to set the electrical lengths L1 and L2 so that the relationship L1>L2 is satisfied.

[0070] Furthermore, from the viewpoint of further suppressing interference of reflected high frequency signals, it is more preferable that the electrical lengths L1 and L2 satisfy the following relationship: (L1-L2)>λ / 2 (7) Here, λ is the wavelength of the differential signal (high frequency signal) output by the drive circuit element 17 in the high frequency transmission line.

[0071] As an example, if the average frequency of the differential high-frequency signal is 20 GHz, the wavelength of microwaves in a vacuum is approximately 15 mm, so λ / 2 is 3.75 mm if the effective refractive index of each high-frequency transmission line is 2. Therefore, in this case, it is preferable to design the high-frequency transmission lines up to the termination resistors 15 and 18 so that the difference in electrical length (L1 - L2) is longer than 3.75 mm.

[0072] [3. Second Embodiment] Next, a second embodiment of the present invention will be described. This embodiment is an optical transmitting device 60 equipped with the optical modulator 1 according to the first embodiment. FIG. 5 is a diagram showing the configuration of the optical transmitting device 60 according to this embodiment. This optical transmitting device 60 includes the optical modulator 1, a light source 61, and a modulation signal generating unit 62. The modulation signal generating unit 62 is an electronic circuit that generates a high-frequency signal (modulation signal) for causing the optical modulator 1 to perform a modulation operation. The modulation signal generating unit 62 inputs, to the optical modulator 1, four modulation signals corresponding to the four signal electrodes 41 of the optical modulation element 3 included in the optical modulator 1 in the form of differential signals, based on, for example, transmission data provided from an external source. As a result, the optical modulator 1 modulates the light from the light source 61 incident from the input optical fiber 6 and outputs the modulated light via the output optical fiber 7.

[0073] In the optical transmitter 60 having the above configuration, the optical modulator 1 described above is used, so the entire device can be configured inexpensively, while the power consumption can be reduced and the size can be made small.

[0074] 4. Other Embodiments The present invention is not limited to the configurations of the above-described embodiments and their alternative configurations, and can be implemented in various forms without departing from the spirit of the present invention.

[0075] For example, in the above-described embodiment, the termination resistor 18 that terminates the high-frequency signal that is not used to drive the optical modulation element 3 out of the differential signals output by the drive circuit element 17 is mounted on the relay substrate 14, but it may also be provided inside the element, as in the drive circuit element 17-1 shown in Figure 6.

[0076] Furthermore, in the above-described embodiment, the driving circuit element 17 outputs a differential signal composed of two high-frequency signals with opposite phases, but it may also output multiple high-frequency signals with the same phase. Even in this case, when some of the multiple high-frequency signals are used to drive the optical modulation element 3, as in the above-described embodiment, the resistance of the termination resistor for the high-frequency signals not used for driving the optical modulation element 3 can be made larger than the resistance of the termination resistor for the high-frequency signals used to drive the optical modulation element 3, thereby reducing the power consumed by the high-frequency signals not used to drive the optical modulation element 3. This reduces the power consumption of the driving circuit element 17, or increases the power of the signals used for driving, thereby shortening the length of the signal electrode 41, thereby reducing the power consumption of the optical modulator 1 or miniaturizing its size.

[0077] 5. Configurations supported by the above embodiments The above embodiment and modifications support the following configurations.

[0078] (Configuration 1) An optical modulator comprising: an optical waveguide element having an optical waveguide formed on a substrate and a signal electrode that controls light waves propagating through the optical waveguide; a drive circuit that outputs two high-frequency signals; and two termination resistors that terminate the two high-frequency signal outputs from the drive circuit, respectively; wherein one high-frequency signal output from the drive circuit propagates through the signal electrode of the optical waveguide element and is terminated by one of the termination resistors, a first termination resistor, and the other high-frequency signal output from the drive circuit is terminated by the other of the termination resistors, a second termination resistor, and the resistance value of the second termination resistor is greater than the resistance value of the first termination resistor. According to the optical modulator of configuration 1, when an optical modulation element is driven using some of the signal outputs of a drive circuit having multiple signal outputs such as differential signals, it is possible to reduce the power consumption of the signal outputs that are not used to drive the optical modulation element. Therefore, the optical modulator can be inexpensively configured using a drive circuit such as a differential signal amplifier that can be procured inexpensively on the market, while reducing its power consumption and miniaturizing its size.

[0079] (Configuration 2) The two high-frequency signals output by the drive circuit are a pair of differential signals with inverted phases with respect to each other, and the optical modulator according to Configuration 1. According to the optical modulator of Configuration 2, for example, an inexpensive and highly reliable differential signal amplification element that has already been commercialized can be used to configure the optical modulator without causing an increase in power consumption.

[0080] (Configuration 3) The sum of the resistance value of the first termination resistor and the resistance value of the second termination resistor is equal to the differential impedance of the two high-frequency signal outputs that constitute the differential signal of the drive circuit, and the optical modulator according to Configuration 2. According to the optical modulator of Configuration 3, since the differential impedance of the output of the drive circuit and the combined resistance value of the first termination resistor and the second termination resistor are configured to match, the adverse effects on the stability of the drive circuit operation caused by the reflected high-frequency signals that may occur in the first termination resistor and the second termination resistor can be suppressed.

[0081] (Configuration 4) The resistance value R1 of the first termination resistor that terminates the output of one of the high-frequency signals that drives the signal electrode is in the range of 0.4×Zlin < R1 ≤ Zlin with respect to the value Zlin of the output impedance between the output of the one high-frequency signal and the ground potential in the drive circuit, and the optical modulator according to any one of Configurations 1 to 3. According to the optical modulator of Configuration 4, it is possible to avoid the operation of the drive circuit becoming unstable by suppressing the reflection of the high-frequency signal in the first termination resistor.

[0082] (Configuration 5) The value Zlin of the output impedance is 50Ω, and the resistance value R1 of the first termination resistor is 45Ω or less, and the optical modulator according to Configuration 4. According to the optical modulator of Configuration 5, it is possible to suppress the reflection of the high-frequency signal in the first termination resistor and maintain the stability of the operation of the drive circuit.

[0083] (Configuration 6) The substrate of the optical waveguide element is an X-cut LiNbO3 substrate, and the optical modulator according to any one of Configurations 1 to 5. According to the optical modulator of configuration 6, in an optical modulator using an LN substrate, even when the optical modulation element is driven using a portion of the signal output of a drive circuit having signal outputs of multiple high-frequency signals such as differential signals, the power consumption of the optical modulation element can be reduced or its size can be made smaller.

[0084] (Configuration 7) An optical modulator described in any one of configurations 1 to 6, wherein the electrical length L1 of the high-frequency transmission path for one of the high-frequency signals from the output of the drive circuit to the first termination resistor and the electrical length L2 of the high-frequency transmission path for the other high-frequency signal from the output of the drive circuit to the second termination resistor are different from each other. According to the optical modulator of configuration 7, it is possible to further prevent the operation of the driver circuit from becoming unstable due to interference between the two reflected high-frequency signals that may occur at the first and second terminator resistors.

[0085] (Configuration 8) The optical modulator according to configuration 7, wherein the electrical lengths L1 and L2 have a relationship of (L1-L2)>λ / 2, where λ is the average wavelength of the two high-frequency signals in the high-frequency transmission line. According to the optical modulator of configuration 8, it is possible to more effectively prevent the operation of the driver circuit from becoming unstable due to interference between the two reflected high-frequency signals that may occur at the first and second terminator resistors.

[0086] (Configuration 9) An optical transmitter comprising the optical modulator according to any one of configurations 1 to 8, and an electronic circuit that generates a modulation signal, which is a high-frequency signal for causing the optical waveguide element to perform a modulation operation. According to the optical transmitter of configuration 9, since the optical modulator according to any one of configurations 1 to 8 is used, the optical transmitter can be configured inexpensively while reducing its power consumption or size. [Explanation of symbols]

[0087] 1...optical modulator, 2...housing, 3...optical modulation element, 4, 5...signal pin, 6...input optical fiber, 7...output optical fiber, 8, 9...support, 10, 11, 13...lens, 12...optical unit, 14...relay board, 15, 15a, 15b, 15c, 15d, 18, 18a, 18b, 18c, 18d...termination resistor, 16...terminator, 17, 17a, 17b, 17c, 17d...drive circuit element, 30...substrate, 31...optical waveguide, 32a, 32b, 32c, 32d...side, 33...input waveguide, 34...branching waveguide, 35, 35a, 35b...nested Mach-Zehnder type optical waveguide Wavepath, 36a, 36b...Output waveguide, 37a, 37b, 38a, 38b...Mach-Zehnder optical waveguide, 37a1, 37a2, 37b1, 37b2, 38a1, 38a2, 38b1, 38b2...Parallel waveguide, 40a, 40b, 40c...Bias electrode, 41, 41a, 41 b, 41c, 41d...signal electrode, 42a, 42b, 42c, 42d, 43a, 43b, 43c, 43d...pad, 50, 50a, 50b, 50c, 50d, 51, 51a, 51b, 51c, 51d...output terminal, 60...optical transmitter, 61...light source, 62...modulation signal generation unit.

Claims

1. an optical waveguide element including an optical waveguide formed on a substrate and a signal electrode for controlling a light wave propagating through the optical waveguide; a drive circuit that outputs two high frequency signals; two termination resistors for terminating two high frequency signal outputs from the drive circuit, respectively; An optical modulator comprising: one high frequency signal output of the drive circuit propagates through the signal electrode of the optical waveguide element and is terminated by a first termination resistor, which is one of the termination resistors; the other high frequency signal output of the drive circuit is terminated by a second termination resistor, which is the other termination resistor, without propagating through the signal electrode of the optical waveguide element; the resistance value of the second termination resistor is greater than the resistance value of the first termination resistor; the two high-frequency signals output by the drive circuit are a pair of differential signals whose phases are inverted from each other, the substrate of the optical waveguide element is an X-cut LiNbO 3 substrate; Optical modulator.

2. the sum of the resistance value of the first termination resistor and the resistance value of the second termination resistor is equal to the differential impedance of the two high frequency signal outputs of the drive circuit that constitute the pre-differential signal; 2. The optical modulator according to claim 1.

3. The resistance value R1 of the first termination resistor that terminates the output of the one high frequency signal that drives the signal electrode is, relative to the value Zlin of the output impedance between the output of the one high frequency signal and the ground potential in the drive circuit, 0.4 × Zlin < R1 ≦ Zlin is in the range of 3. The optical modulator according to claim 1.

4. The output impedance value Zlin is 50Ω, The resistance value R1 of the first termination resistor is 45Ω or less.

4. The optical modulator according to claim 3.

5. an electrical length L1 of a high-frequency transmission path for one of the high-frequency signals from the output of the drive circuit to the first termination resistor and an electrical length L2 of a high-frequency transmission path for the other high-frequency signal from the output of the drive circuit to the second termination resistor are different from each other; 5. The optical modulator according to claim 1.

6. The electrical lengths L1 and L2 are, with respect to the average wavelength λ of the two high-frequency signals in the high-frequency transmission path, (L1-L2)>λ / 2 have a relationship of 6. The optical modulator according to claim 5.

7. An optical modulator according to any one of claims 1 to 6; an electronic circuit for generating a modulation signal, which is a high-frequency signal for causing the optical waveguide element to perform a modulation operation; An optical transmitting device comprising:

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