JP2003295139A - Optical modulator module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cost-wise and space-wise satisfactory optical modulator module making the use of BIAS-T specially arranged outside a case like the conventional practice unnecessary by miniaturizing the optical modulator and obviating peripheral members for driving the optical modulator in the optical modulator module to which both RF voltage and DC voltage are applied. <P>SOLUTION: The optical modulator module is constituted of a substrate 1 consisting of a material having the electro-optic effect; optical wave guides 2 to 7 formed on the substrate; a pair of electrodes consisting of a signal electrode 8 for modulating the light passing through the inside of the optical wave guides and grounding electrodes 9, 10; and an RF terminating device connected to the pair of electrodes. The RF terminating device is provided with a terminal resistor 27, high frequency bypass capacitors 28, 29 and a low frequency bypass capacitor 30. The signal electrode and the grounding electrodes are connected by arranging the terminal resistor and the high frequency bypass capacitors in series between these electrodes. The signal electrode is grounded by connecting the terminal resistor and the low frequency bypass capacitor in series. Further, a bias resistor 42 for applying DC voltage is connected to a common connecting point 45 of the terminal resistor, the high frequency bypass capacitors and the low frequency bypass capacitor. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator module in which an RF voltage and a DC voltage are mainly applied to signal electrodes of an optical modulator, and more particularly to a DC for applying a DC voltage to the optical modulator. -It relates to an optical modulator module with a BIAS port.

[0002]

2. Description of the Related Art In recent years, with the development of high-speed, large-capacity optical fiber communication systems, there has been a demand for an optical pulse generator which stably generates a high-frequency optical pulse, and is represented by, for example, an external modulator. , An optical modulator for generating an optical pulse capable of high-speed operation using a material having an electro-optical effect such as lithium niobate as a substrate has been put into practical use. Normal,
A high-speed pulse voltage equal to the drive voltage Vπ of the optical modulator is RF
The voltage is applied as a voltage, but the middle point (Qu
A DC voltage is superimposed on the RF voltage and applied so that the RF voltage is applied around the ad point, which is an intermediate point between the maximum output and the minimum output.

1 and 2 show a method of applying a DC bias voltage to a conventional optical modulator module. Here, Z
An optical modulator using a -CUT type substrate (a substrate having a crystal axis direction capable of changing the refractive index most efficiently by the electro-optical effect in a direction perpendicular to the surface of the substrate) is described as an example. FIG. 1 is an optical modulator module in the case where an RF voltage and a DC voltage are superimposed and applied to a signal electrode, and FIG.
It is an optical modulator module in which electrodes for applying an RF voltage and a DC voltage are separated.

In FIG. 1, reference numeral 1 denotes a ferroelectric crystal such as lithium niobate (hereinafter referred to as LN), which forms optical waveguides 2 to 7 by diffusing Ti or the like on the surface. A signal electrode 8 and common electrodes 9 and 10 which are ground electrodes are provided in the vicinity of the optical waveguide.
The RF signal from the RF signal source 11 is applied between the electrodes, and the DC voltage 14 is applied via the coil side end 13 of the BIAS-T 12 of the LC circuit. 20
Is an RF connector, which connects the center pin to the signal electrode 8, and the ends of the common electrodes 9 and 10 on the RF input side are a part 21 of a case 17 in which the substrate 1 forming the optical modulator is housed. 22 is connected. The other end of the signal electrode 8 is R
The F connector 22 is connected to the case 17 or the like via the center pin of the F connector 22 and the DC cut capacitor 15 and the terminating resistor 16 to be grounded. Further, the other ends of the common electrodes 9 and 10 are connected to the parts 23 and 24 of the case 17.

The operation of the optical modulator module shown in FIG. 1 will be described. The CW laser light from the light source is input to the input fiber 1.
8 is incident. The incident light is guided through the input waveguide 2 and is branched by the branch 6 into two branch optical waveguides 3 and 4 substantially evenly. In the branched optical waveguides 3 and 4, phase modulation is performed by the electric field between the signal electrode 8 and the common electrode 9 and the electric field between the signal electrode 8 and the common electrode 10, respectively, and at the multiplexing unit 7, The two demultiplexed lights are combined and interfere with each other, emitted from the output waveguide 5 as intensity-modulated light according to the phase modulation, and emitted through the output fiber 19 to the outside of the optical modulator module. The optical waveguide having such a shape is called a Mach-Zehnder type optical waveguide. The RF signal input from the RF connector 20 and propagating through the signal electrode 8 and the light propagating through the branch optical waveguides 3 and 4 are
The propagation velocities are matched and efficient phase modulation is performed. External signal source 11, DC power source 14 and RF
The ground potential of the terminal 16 is a potential common to the case 17. In the case of such a circuit configuration, BIAS for high frequency applications
The -T12, the DC cut capacitor 15, and the terminating resistor 16 are expensive and mechanically large in size.

Further, the optical modulator module shown in FIG.
RF electrode for applying RF voltage and DC for applying DC voltage
It is separated from the electrodes. In FIG. 2, the substrate and the case are not shown for simplicity. Also, RF
The signal source 11, the DC power supply 14, and the terminating resistor 16 are provided outside the case 17. For the branched optical waveguides 3 and 4, an RF electrode pair formed by the signal electrode 8 and the common electrodes 9 and 10 that are ground electrodes, and a DC electrode pair formed by the electrodes 25 and 26 to which a DC voltage is applied are They are provided independently of each other. The structure for separating the RF electrode and the DC electrode as shown in FIG. 2 has an advantage that the BIAS-T and the DC cut capacitor are unnecessary as compared with the optical modulator module shown in FIG. Since the electrode pair is provided, the length of the branched optical waveguides 3 and 4 becomes longer than that of FIG. 1, and as a result, the case 17 of the optical modulator module is obtained.
Has the drawback of increasing the dimensions of.

Further, FIG. 3 shows an optical modulator in which an RF electrode and a DC electrode are integrated, and at an end portion of an electrode pair formed by a signal electrode 8 and common electrodes 9 and 10 which are ground electrodes,
This is an optical modulator module in which an RF terminator 31 is connected and the RF terminator is housed in a case 17. RF terminator 31
Is composed of a terminating resistor 27, high frequency bypass capacitors 28 and 29 to the common electrodes 9 and 10, and a low frequency bypass capacitor 30 grounded to the case 17.
The RF signal applied from the RF signal source 11 to the RF input connector 20 through the BIAS-T 12 propagates through the coplanar electrode including the signal electrode 8 and the common electrodes 9 and 10, and is RF-terminated by the terminator 31 at the other end. It That is, the terminating resistor 27 having a resistance value close to the impedance of the electrode pair is connected to the end 32 of the signal electrode 8 and the terminating resistor 2 is connected.
7, the other end 33 of the high frequency bypass capacitors 28 and 29 connected to the common electrodes 9 and 10 respectively, and the low frequency bypass capacitor 30 connected to the case 17
Is connected to the other end.

In the optical modulator module as shown in FIG. 3, by providing the high frequency bypass capacitors 28 and 29, the high frequency potential between the common electrodes 9 and 10 can be kept equal, and the harmful slot mode is not generated. It becomes possible to suppress. However, similar to the optical modulator module shown in FIG. 1, since a DC bias from the DC power source 14 is superimposed on the signal voltage of the RF signal source 11, a BIAS-T12 for high frequency application is required, and the BIAS is used.
Since it is necessary to dispose the -T12 outside the case 17, connect the RF signal source 11 to the capacitor side terminal, and connect the DC power source 14 to the coil side terminal 13, respectively, it is costly and expensive. There is also a space problem.

Further, as shown in FIG. 4, the RF signal source 11 normally comprises a pulse generator source 34 and a driver 35 which receives a pseudo random (PRBS) pulse from the pulse generator source 34 and generates an amplified signal. The output portion 41 has an open drain 36 and the drain 36 and the power source Vd.
An LC circuit 40 having a coil 37 in between and a capacitor 39 between the drain 36 and the output end 38 is formed. The LC circuit 40 is usually provided outside the driver.

[0010]

The problem to be solved by the present invention is to solve the above-mentioned problems in an optical modulator module to which both an RF voltage and a DC voltage are applied, and to downsize the optical modulator. A peripheral member for driving the optical modulator is omitted, and in particular, there is no need to use a BIAS-T arranged outside the case as in the conventional case, in terms of cost,
And to provide an optical modulator module excellent in space. Moreover, due to such miniaturization, it is possible to form a DWDM transmission system in which optical modulators related to a plurality of wavelengths are densely arranged.

[0011]

In order to solve the above problems, the invention according to claim 1 provides a substrate made of a material having an electro-optical effect, an optical waveguide formed on the substrate, and the optical waveguide. In an optical modulator module in which an electrode pair including a signal electrode and a ground electrode for modulating light passing therethrough and an RF terminator are connected to the electrode pair, the RF terminator is a terminating resistor and a high frequency bypass. And a low-frequency bypass capacitor, and the terminal resistance and the high-frequency bypass capacitor are arranged in series between the signal electrode and the ground electrode, and the signal electrode is connected to the terminal resistance and the low-frequency bypass capacitor. A frequency bypass capacitor is connected in series and grounded, and at a common connection point between the terminating resistor and the high frequency bypass capacitor and the low frequency bypass capacitor,
A feature is that a bias resistor for applying a DC voltage is connected.

According to the first aspect of the present invention, it is possible not only to suppress the generation of the slot mode by the RF terminator as in the conventional case, but also to effectively apply the DC voltage to the signal electrode through the bias resistor. Therefore, without using BIAS-T or separating the RF electrode from the DC electrode, the RF
An optical modulator module can be formed in which both the voltage and the DC voltage are applied.

According to a second aspect of the invention, in the optical modulator module according to the first aspect, the bypass resistance is substantially the same as the impedance value of the electrode pair, and the conductance of the electrode pair. It is characterized by being 1/10 or less of the resistance.

According to the invention of claim 2, the applied D
Since 90% or more of the C voltage can be applied to the signal electrode, the DC bias can be more efficiently input to the optical modulator.

According to a third aspect of the invention, in the optical modulator module according to the first or second aspect, the RF terminator and the bias resistor are also accommodated in a case that accommodates the substrate. Characterize.

According to the invention of claim 3, an optical modulator module in which an RF terminator and a bias resistor are housed in a case together with an optical modulator, and peripheral members necessary for each optical modulator are integrated with the optical modulator. Thus, even when a plurality of optical modulators are used in combination, they can be easily handled, and the arrangement of the modules, circuit wiring, etc. can be simplified and made compact.

The invention according to claim 4 is the optical modulator module according to any one of claims 1 to 3, characterized in that the optical waveguide is of a Mach-Zehnder type.

According to the invention of claim 4, even in the optical modulator having the Mach-Zehnder type optical waveguide, the BIA
Since the DC voltage can be applied without using ST or separating the RF electrode and the DC electrode, it is possible to provide an optical modulator module that is more cost-effective and space-saving.

According to a fifth aspect of the invention, in the optical modulator module according to the fourth aspect, signal electrodes are respectively provided to two branched optical waveguides on the optical waveguide forming a Mach-Zehnder type. Is formed.

According to the invention of claim 5, an optical modulator in which signal electrodes are respectively formed for two branched optical waveguides on the optical waveguide forming the Mach-Zehnder type (hereinafter referred to as Dual type optical modulation). (Referred to as “Bias-T”), by arranging an RF terminator and a bias resistor for each signal electrode, without using the BIAS-T,
It is possible to easily apply a DC voltage to each signal electrode, and it is possible to provide a dual-type optical modulator module that is more cost-effective and space-saving.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to preferred examples. As a substrate forming the optical modulator, a material having an electro-optical effect, for example, lithium niobate (L
iNbO ₃ ; hereinafter referred to as LN), lithium tantalate (LiTaO ₃ ), PLZT (lead lanthanum zirconate titanate), and a silica-based material, and particularly easy to configure as an optical waveguide device and anisotropic. It is preferable to use a LiNbO ₃ crystal, a LiTaO ₃ crystal, or a solid solution crystal composed of LiNbO ₃ and LiTaO ₃ for the reason that it is large. In this embodiment, an example using lithium niobate (LN) will be mainly described. In the following, a substrate having a crystal axis direction capable of changing the refractive index most efficiently by an electro-optical effect in a direction perpendicular to the surface (so-called “Z-CUT type substrate”) will be mainly described, but the present invention is It is not limited to Z-cut substrates. Therefore, it goes without saying that the positional relationship between the branched optical waveguide, the signal electrode, the ground electrode, etc. may be changed according to the type of the substrate.

As a method of manufacturing an optical modulator, Ti is thermally diffused on an LN substrate to form an optical waveguide, and then an electrode is formed on the LN substrate without providing a buffer layer over a part or the whole of the substrate. In order to reduce the propagation loss of light in the optical waveguide by directly forming
A buffer layer of ₂ or the like is provided, and a Ti / Au electrode pattern is formed on the buffer layer and a gold plating method is applied to make it several tens μ
There is a method of forming a signal electrode and a ground electrode having a height of m and forming the electrodes indirectly. It is also possible to have a multilayer structure in which a film body of SiN, Si, or the like is provided on the buffer layer. In general, an optical modulator is manufactured by forming a plurality of optical modulators on a single LN wafer and finally separating the individual optical modulator chips into chips.

Next, the constitution that characterizes the present invention will be described. FIG. 5 shows an embodiment of the present invention, and FIG. 6 shows a conceptual diagram showing its function. In FIG. 5, reference numeral 44 denotes the case of the optical modulator module, and the LN element 1 is the same as that in FIG. Further, in FIG. 5, the RF connector and the ground connection of the common electrode to the case are the same as in FIG. 1, and are omitted here for the sake of easy understanding of the characteristic points. The RF signal from the signal source 34 is output by the driver 35 and the output BIAS-T.
It is input to the RF input pin 20 of the module via 40.

The RF signal is transmitted to the coplanar electrode pairs 8, 9,
Phase modulation is applied to the light propagating in 10 and propagating in the branch optical waveguides 3 and 4. The RF signal propagating through the signal electrode 8 receives the resistance 27 connected to the electrode end 32 at the electrode end 32.
And the common electrodes 9 and 10 connected in series with the resistor 27.
Is terminated by an RF terminator composed of high-frequency bypass capacitors 28 and 29 to the module and a low-frequency bypass capacitor 30 connected from the connection point 45 of the resistor 27 to the case 44 of the module.

A bias resistor 42 is connected to the connection point 45, and the other end of the bias resistor 42 is connected to the DC power source D.
The C voltage can be applied. The DC resistance of the RF signal electrode seen from the connection point 45 is ideally because the signal electrode 8 is connected to the common electrode potential (case potential) through the capacitors 39, 28, 29 and 30. It is infinite in terms of DC. However, in reality, as shown in the equivalent circuit of FIG.
As described above, it is equivalent to the distributed constant circuit including the inductance 48, the resistance 49, the capacitor 51, and the inter-electrode conductance 50. Therefore, the resistance of the RF electrode viewed from the connection point 45 is determined by the conductance 50 and is about 10
The resistance value is 0 kΩ or more. Therefore, the bias resistor 42 whose resistance value is one-tenth or less of the conductance value is connected to the connection point 45, and the bias resistor 4 is connected.
When a DC voltage is applied from the other end 43 of 2
% Or more is applied between the electrodes (between the signal electrode 8 and the common electrodes 9 and 10).

The RF signal from the signal source 46 is the above-mentioned RF signal.
Since it is absorbed by the terminator, the presence of the biasing resistor 42 for applying the DC voltage does not affect the signal waveform or the magnitude of the signal. In order to keep the operating point stable, the DC bias voltage superimposes a weak low-frequency signal having a frequency outside the used frequency band on the DC voltage, observes the intensity change of the light-modulated light, and superimposes it. In general, so-called auto bias control is performed in which a feedback signal for stabilizing the operating point is generated from the intensity change component related to the signal and is fed back to the DC voltage. here,
When about f (Hz) is used as the low frequency signal, the impedance of the electrode 8 viewed from the connection point 45 is 1 / (2
πfC1), 1 / (2πfC2), 1 / (2πfC3)
In parallel (C1 and C2 are the capacitances of capacitors 28 and 29, C3 is the capacitance of capacitor 30), and 1 / (2πf
C4), 1 / (2πfC5) (C4 is the electrode capacitance, C5
Is the capacitance of the capacitor 39 on the signal source side) and the resistors 45, 4
9 and 50 and the resistance of the signal source of 50Ω. C
1, C2 is about 560 pF, C3 and C5 are about 0.1 μF, and C4 is about 10 pF. Also, the resistance 50 is 1
If the resistance is 00 kΩ or more, the resistance 49 is an ohmic resistance of the electrode 8 and is several ohms. Since the superposition frequency f is about 1 kHz, the impedance of the electrode viewed from the connection point 45 is mainly controlled by C3 and is about 1.6 kΩ. By providing the bias resistor 42 with a resistance value equal to or lower than the resistance value, a voltage that is equal to or more than half the superimposed signal voltage applied to the terminal 43 can be applied to the electrode 8.

The DC voltage can be expected to have the same effect by providing a resistor having the above resistance value between the capacitor 39 and the input terminal 20 or between the terminal 20 and the signal electrode end 52 on the RF input side. However, in this case, the impedance changes depending on the capacitance and inductance of the resistor and the pattern for connecting the resistor, resulting in RF
It will affect the signal.

FIG. 7 shows an example in which the present invention is applied to a dual type optical modulator. Independent coplanar electrode pairs 53 and 54, 55 and 56, 57 and 55 are provided in the branched optical waveguides 3 and 4, respectively, and RF signals are applied to the signal electrodes 53 and 56 from RF signal sources 58 and 59, respectively. At the other end of the signal electrode, terminal resistors 60, 61, high frequency bypass capacitors 62, 63, 64, 65, low frequency bypass capacitors 66, 67, and DC bias resistor 68,
The RF terminators 72 and 73 composed of 69 are used to terminate the RF signal in the same manner as described above. When each constant is as described above, D applied from terminals 70 and 71
It is possible to apply 90% or more of the C voltage, or a half value voltage or more of the superimposed 1 kHz signal voltage to the signal electrodes 53 and 56 without affecting the applied RF signal.

[0029]

As described above, according to the present invention, an expensive and large volume BIAS-type external to the optical modulator module is provided.
It is possible to apply a DC bias voltage to a signal electrode that integrates the RF electrode and the DC electrode without using T and without affecting the applied RF signal. It is possible to provide an optical modulator module that is superior in view.

[Brief description of drawings]

FIG. 1 is a conventional example of an optical modulator module in which an RF electrode and a DC electrode are integrated.

FIG. 2 is a conventional example of an optical modulator module in which an RF electrode and a DC electrode are separated.

FIG. 3 is a conventional example of an optical modulator module including an RF terminator.

FIG. 4 is a schematic diagram of an RF signal source.

FIG. 5 is an embodiment of an optical modulator module according to the present invention.

6 is a diagram showing an equivalent circuit of FIG.

FIG. 7 is an embodiment of the module of the dual type optical modulator according to the present invention.

[Explanation of symbols]

1 substrate 2 Optical waveguide 3, 4 branch optical waveguide 6 branches 7 Multiplexing section 8 signal electrodes 9,10 Ground electrode (common electrode) 11 RF signal source 12 BIAS-T 14 DC power supply 17 cases 31 RF terminator 42 Bias resistor 42

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shinsuke Kanno             28 Sumitomo Osaka, 6-6 Rokubancho, Chiyoda-ku, Tokyo             Inside Cement Co., Ltd. F-term (reference) 2H079 AA02 AA12 BA03 CA05 DA03                       DA22 EA05 FA03 HA14 HA15

Claims (5)

[Claims] 1. An electrode pair including a substrate made of a material having an electro-optical effect, an optical waveguide formed on the substrate, a signal electrode for modulating light passing through the optical waveguide, and a ground electrode. And an optical modulator module in which an RF terminator is connected to the electrode pair, the RF terminator having a terminating resistor, a high frequency bypass capacitor, and a low frequency bypass capacitor, and between the signal electrode and the ground electrode. Is connected in series by arranging the terminating resistor and the high frequency bypass capacitor in series, and the signal electrode is grounded by connecting the terminating resistor and the low frequency bypass capacitor in series. An optical modulator module, wherein a bias resistor for applying a DC voltage is connected to a common connection point of the high frequency bypass capacitor and the low frequency bypass capacitor. 2. The optical modulator module according to claim 1, wherein the bypass resistance is substantially equal to or higher than the impedance value of the electrode pair, and is 1/10 or less of the conductance resistance of the electrode pair. An optical modulator module characterized by: 3. The optical modulator module according to claim 1 or 2, wherein the RF terminator and the bias resistor are also accommodated in a case that accommodates the substrate. . 4. The optical modulator module according to claim 1, wherein the optical waveguide is a Mach-Zehnder type. 5. The optical modulator module according to claim 4, wherein a signal electrode is formed for each of two branched optical waveguides on the optical waveguide forming a Mach-Zehnder type. And an optical modulator module.