JP2005091417A - Signal transmission line for optical modulator and optical module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform impedance matching between a signal source for a modulating signal and an optical modulator in a wide frequency band. <P>SOLUTION: A signal transmission line 25 comprises a signal conductor 27, a couple of coplanar ground conductors 28, and a coplanar waveguide (CPWG) with a bottom surface gorund conductor including the bottom surface ground conductor 29, and the modulating signal is supplied from the signal source whose output impedance is 50 Ω to the optical modulator of a light absorbing modulation type laser module 11. The characteristic impedance of the CPWG is 35 to 47 Ω, the resistance value of an inserted series resistance 30 is 5 Ω, and a terminating resistance 31 connected to an end is 50 to 150 Ω. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a signal transmission line for an optical modulator and an optical module. More specifically, the present invention relates to light such as an optical absorption modulator (EA modulator) suitably used for an optical absorption modulation optical laser module (EAML module). The present invention relates to a signal transmission line for transmitting a high-frequency signal to a modulator, and an optical module using the signal transmission line.

  The EAML module includes a distributed feedback (DFB) laser diode and an EA modulator that modulates laser light emitted from the DFB laser diode, which are formed on a common semiconductor substrate. The EAML module has attracted particular attention in the field of optical communications because of its small footprint and low manufacturing cost.

  FIG. 17 shows a conventional EAML module described in Patent Document 1. The EAML module 10 </ b> A is configured as an optical integrated device (chip) 11 housed in an airtight package 12 having a plurality of external pins 13. The high frequency modulation signal input from the external signal pin 13 is transmitted to an EA modulator (not shown) in the optical integrated device 11 via a signal transmission line (unit) 14 in the package. The signal transmission line unit 14 includes a microstrip line 15 and an aluminum nitride line carrier on which the microstrip line 15 is mounted. The line carrier is used to mount the optical integrated device 11 and the signal transmission line unit 14. Supported on a chip carrier 16.

  The microstrip line 15 is connected to the external signal pin 13 via the bonding wire 19, and is connected to the EA modulator in the optical integrated device 11 via the bonding wire 20. A termination resistor 21 is connected to the end of the microstrip line 15. The optical output end of the optical integrated device 11 is coupled to the optical input end of the optical fiber 22, and a modulated optical output signal is supplied to the optical fiber 22. A photodiode 23 is disposed at the rear end of the DFB laser of the optical integrated device 11 to measure the light output level of the laser diode.

  FIG. 18 shows an equivalent circuit of a signal transmission line for transmitting a high-frequency signal from the signal source to the EA modulator. The output impedance of the signal source 40 is designed to be 50Ω, and the microstrip line 15 has a characteristic impedance of 50Ω. The EA modulator 24 is equivalent to a capacitor having a capacitance of 0.7 pF, for example. The termination resistor 21 has a termination resistance (Rt) of 50Ω.

  The combined impedance of the EA modulator 24 and the terminating resistor 21 is about 50Ω in the low frequency band, which is impedance matched to the output impedance (50Ω) of the signal source 40. On the other hand, in the high frequency band of about 10 GHz or more, the capacitive impedance of the EA modulator 40 is lowered, so that the combined impedance is much lower than 50Ω. When the combined impedance is lowered, a mismatch occurs between the impedance of the entire EA modulator including the signal transmission line and the output impedance (50Ω) of the signal source 40. This impedance mismatch causes a signal reflection in which part of the signal power supplied from the signal source 40 toward the EAML module 10A returns to the signal source 40 side.

  The degree of signal reflection is generally evaluated by the magnitude of input return loss. The input return loss is defined by the ratio (decibel) of the signal power returning from the EAML module 10A side toward the signal source 40 side to the entire signal power supplied from the signal source 40 toward the EAML module 10A. Since the input return loss causes a reduction in conversion efficiency when converting the input electric signal to the optical output signal in the optical modulator, it is desirable to suppress the input return loss with a desired frequency bandwidth. The specifications of the optical communication system currently in use stipulate that the input return loss be -10 dB or less in the signal frequency band.

  For example, the input return loss of −10 dB or less is preferably achieved with a 3 dB modulation response frequency exceeding the vicinity of 10 GHz which is the upper limit of a desired frequency band. The 3 dB modulation response frequency is a limit frequency at which the modulated light output is reduced by 3 dB or more. In other words, in the EAML module, it is desirable that the modulation output does not decrease by 3 dB or more and the input return loss is −10 dB or less at the same time around 10 GHz which is the upper limit of the desired frequency band.

In order to transmit the maximum signal power toward the load in a desired frequency band, the impedance of the EA modulator including the signal transmission line 14 and the output impedance of the signal source are matched in the frequency band. That is, it is necessary that the complex impedances are conjugate to each other. Thus, although impedance matching has been conventionally achieved for this purpose, it is difficult to achieve such impedance matching in the entire modulation signal frequency band for an optical modulator that is a capacitive load. It was.
JP-A-9-252164

  In view of the above problems in the prior art, the present invention is a signal transmission for transmitting a modulation signal to an optical modulator that can achieve an input return loss of -10 dB or less in a desired frequency band of 10 GHz or higher. The purpose is to provide a track.

  Another object of the present invention is to provide a signal transmission line having desired modulation response characteristics in the desired frequency band.

  The present invention further aims to provide an EA modulator having such a signal transmission line.

  In order to achieve the above object, a signal transmission line according to the first aspect of the present invention is a signal transmission line for transmitting a modulation signal from a signal source having an output impedance of 50Ω to an optical modulator. A signal conductor and at least one ground conductor combined to have a terminal resistance, and a termination resistor having a resistance value of 50 to 150Ω.

  In a preferred embodiment of the signal transmission line for an optical modulator according to the first aspect of the present invention, the signal transmission line further has a series resistance having a resistance value of 10Ω or less. In this case, more effective impedance matching is obtained.

  In the signal transmission line for an optical modulator according to the first aspect of the present invention, a microstrip line can be constituted by the signal conductor and the ground conductor. Adopting a microstrip line enables good signal transmission. In this case, in order to obtain the characteristic resistance, the width of the signal conductor can be made smaller than the width of a normal signal conductor of a microstrip line having a characteristic impedance of 50Ω. The characteristic impedance can be adjusted with a simple structure.

  Further, the signal conductor and the ground conductor can be configured as a coplanar waveguide. In this case, the ground conductor includes a pair of ground conductors formed on the same plane as the signal conductor and extending across the signal conductor. The ground conductor may further include a planar ground conductor disposed opposite to the signal conductor and the pair of ground conductors.

  In order to obtain the coplanar waveguide (CPW) having the characteristic impedance, the gap between the signal conductor and each of the pair of parallel ground conductors is smaller than the corresponding gap of the CPW having a characteristic impedance of 50Ω. Can be adopted. In this case, the characteristic impedance can be adjusted with a simple structure.

  An optical module according to a second aspect of the present invention includes a laser diode, an optical modulator that modulates laser light emitted from the laser diode, and a signal transmission line that transmits a modulation signal to the optical modulator. In the module, the signal transmission line has a signal conductor and at least one ground conductor combined so as to have a characteristic impedance of 35 to 47Ω, a series resistance having a resistance value of 10Ω or less, and a resistance value of 50 to 150Ω. And a terminating resistor. The series resistance having a resistance value of 10Ω or less includes a series resistance having a resistance value of zero Ω, that is, a case where no series resistance is provided.

An optical modulator signal transmission line according to a third aspect of the present invention is a signal transmission line that transmits a modulation signal from a signal source having an output impedance of R ₁ Ω to an optical modulator. 0.7R _{1 to} 0.94R ₁ Omega signal conductors and at least one ground conductor is combined so as to have a characteristic impedance of, characterized in that it comprises a termination resistor having a resistance value of R ₁ ~3R _{1 Ω.}

  According to the signal transmission line according to the first aspect of the present invention, by setting the characteristic impedance of the signal transmission line and the resistance value of the termination resistor to a predetermined value, between the signal source having the output impedance of 50Ω, For example, in the desired frequency band of 10 GHz or higher, effective impedance matching can be obtained without substantially degrading the modulation response characteristic, so that the input return between the signal source having the output impedance of 50Ω and the optical modulator is possible. Loss can be kept low.

  The optical module according to the second aspect of the present invention has the same advantages as the signal transmission line according to the first aspect of the present invention.

  The signal transmission line according to the third aspect of the present invention has the same advantages as the signal transmission line according to the first aspect of the present invention when receiving a modulated signal from a signal source having an arbitrary output impedance.

  Hereinafter, the present invention will be described in more detail based on embodiments of the present invention with reference to the drawings. FIG. 1 is a plan view of an EAML module having an EA modulator according to the first embodiment of the present invention. The EAML module 10 according to the present embodiment is the same as the conventional EAML module shown in FIG. 17 except for the configuration of the signal transmission line. The signal transmission line 25 of the EAML module 10 of this embodiment is configured as a coplanar waveguide (CPWG) with a planar ground conductor (ground plate).

  Specifically, the EAML module 10 includes an airtight package 12 having a plurality of external pins 13, an optical integrated device 11 configured as an EAML chip housed in the airtight package 12, and a signal transmission line (unit) 25. . A high frequency modulation signal input from a signal source (not shown) via the external signal pin 13 is transmitted to the EA modulator in the optical integrated device 11 via the signal transmission line 25. The signal transmission line 25 is supported on the chip carrier 16 for mounting the optical integrated device 11 and the signal transmission line 25.

  The signal input side of the signal transmission line 25 is connected to the external signal pin 13 via the bonding wire 19, and the signal output side of the signal transmission line 25 is connected to the EA modulator (see FIG. (Not shown). The optical output end of the optical integrated device 11 is optically coupled to the input end of the optical fiber 22, and modulated laser light is transmitted to the outside. The rear end face of the DFB laser diode is optically coupled to the photodiode 23, and the level of light output emitted from the DFB laser diode is measured by the photodiode 23.

  FIG. 2 shows the structure of the signal transmission line unit 25 and the EAML chip 11 shown in FIG. The signal transmission line unit 25 includes a line carrier 26 made of aluminum nitride and a CPWG mounted thereon. The CPWG includes a CPW having a signal conductor 27 formed on the upper surface of the line carrier 26 and a pair of coplanar ground conductors extending in parallel across the signal conductor 27, and a bottom ground conductor (on the bottom surface of the line carrier 26). Ground plate) 29.

  A series resistor 30 is inserted into the signal conductor 27 near the end of the signal conductor 27 on the optical integrated device 11 side. The terminal portion of the signal conductor 27 constitutes an electrode pad 27a. The termination resistor 31 is formed at the end of the signal conductor 27 adjacent to the electrode pad 27a. The electrode pad 27 a is connected to the EA modulator in the EAML module 11 through the bonding wire 20. The pair of coplanar ground conductors 28 and the bottom ground conductor 29 of the CPWG are connected through a plug 32 that penetrates the line carrier 26. The thickness of the line carrier 26 is, for example, 0.9 mm.

  The terminating resistor 31 has a resistance value of 80Ω ± 5%, and the series resistor 30 has a resistance value of 5Ω ± 5%. The signal conductor 27 has a length of 3.2 mm (design dimension, the same applies hereinafter) up to the position of the series resistor 30, and the electrode pad 27a has a length of 0.2 mm. The signal conductor 27 and the electrode pad 27a have a width of 0.3 mm and a thickness of 2 μm, and their characteristic impedance is 41.8Ω ± 1.9Ω. The gap between the signal conductor 27 and each of the pair of coplanar ground conductors 28 is 0.06 mm ± 0.01 mm. This gap is sufficiently narrow compared to a similar gap in CPWG having the same characteristic impedance as the signal source output impedance (50Ω). The combination of the series resistor 30 and the terminating resistor 31 applies about 94% of the input modulation voltage to the EA modulator.

  The signal transmission line 25 of the EA modulator of the above embodiment was evaluated by simulation while comparing with the conventional signal transmission line constituting the comparative example.

  FIG. 3 is an equivalent circuit of the signal transmission line of the EAML module of this embodiment used in the above simulation. In this example, the signal source 40 has an output impedance of 50Ω, the external signal transmission line 33 has a characteristic impedance of 50Ω, the signal transmission line 27 has a characteristic impedance of 41.8Ω, and the series resistor 30 has a resistance of 5Ω. The termination resistor 31 has a resistance value of 80Ω, the bonding wires 19 and 20 have inductances of 0.3 nH and 0.4 nH, respectively, the EA modulator 24 has a series resistance of 5Ω, a parallel resistance of 90Ω and a resistance of 0.4 pF. Each has a capacity.

  FIG. 4 is an equivalent circuit of a conventional EA modulator 10A used as a comparative example. The signal transmission line 38 has a signal conductor having a length of 3.4 mm, a width of 0.3 mm, and a thickness of 0.2 μm, and a pair of coplanar ground conductors, and a gap between the signal conductor and the ground conductor is formed. A characteristic impedance of 50Ω was provided as 0.12 mm. Other configurations are the same as those of the EAML module of the embodiment shown in FIG.

  FIGS. 5A and 5B show the frequency dependence of the real part and the imaginary part of the complex input impedance of each EAML module obtained by the above simulation, respectively. The broken line indicates the input impedance of the EAML module of the present embodiment, and the solid line indicates the input impedance of the EAML module of the comparative example. As shown in FIG. 5A, in the EAML module of this embodiment, the real part of the input impedance at a frequency of 10 GHz or less is about 40Ω to 80Ω, and the real part of the input impedance in the comparative example is about 30Ω to 140Ω. Therefore, the frequency dependence of the real part of the input impedance is improved compared to the comparative example. Similarly, as shown in FIG. 5B, in the EAML module of the present embodiment, the imaginary part of the input impedance when the frequency is 10 GHz or less is j25Ω to −j17Ω, and the imaginary part of the impedance in the comparative example is j53Ω. Since it is ˜−j64Ω, the frequency dependence of the imaginary part of the input impedance is greatly improved as compared with the comparative example.

  FIGS. 6A and 6B show the input return loss and the modulation response characteristic of each EAML module obtained by the simulation. The modulation response characteristic is indicated by the ratio (decibel) of the optical output power to the total power of the input modulation signal. As shown in FIG. 6A, in the EAML module of the present embodiment, the input return loss in the vicinity of a frequency of 3 GHz, for example, falls within −20 dB or less due to the improvement of the frequency dependence of the input impedance. In the EAML module, it has been found that the input return loss is remarkably improved from the comparative example, considering that the input return loss in the frequency band is −10 dB. The improvement of the input return loss extends to the frequency band of 12 GHz.

  As shown in FIG. 6B, in consideration of the fact that the 3 dB modulation response frequency of the present embodiment is around 14 GHz and the 3 dB modulation response frequency of the comparative example is 15 GHz, the present embodiment is 3 dB. The modulation response characteristic is slightly inferior to the comparative example. However, considering that the conventional EAML module cannot satisfy the specification of the input return loss in the high frequency band, and that the specification can be satisfied for the first time by the EAML module of the present embodiment example, The EAML module of the present embodiment is sufficient to compensate for the slight decrease in the 3 dB modulation response frequency.

  FIG. 7 shows an EA modulator according to this embodiment. The series resistance is fixed to 5Ω, the termination resistance is fixed to 80Ω, and the characteristic impedance of the signal transmission line composed of CPWG is changed variously to change the frequency of the input return loss. The simulation result which investigated the dependence is shown. In this simulation, the characteristic impedance of CPWG was changed from 50 Ω as in the past to 47 Ω, 45 Ω, 40 Ω, 37 Ω, and 35 Ω according to this embodiment. The conventional signal transmission line has a characteristic impedance of 50Ω, a termination resistance of 50Ω, and no series resistance. As is clear from FIG. 7, in the EA modulator of this embodiment, a significant improvement in input return loss was observed particularly in the frequency band between 6 GHz and 10 GHz.

  Next, an EA modulator according to a second embodiment of the present invention will be described. The EA modulator according to the present embodiment differs from the EA modulator of the first embodiment in that a coplanar waveguide (CPW) having no bottom ground conductor is used as a signal transmission line. It is the same.

  According to the simulation, the real part and imaginary part of the complex input impedance, the input return loss, and the improvement of the modulation response characteristic in the EA modulator of the second embodiment are the same as those of the first embodiment. . In order to show the improvement of the comparative example of the EA modulator according to the present embodiment, the real part and the imaginary part of the input impedances of both EA modulators are shown in FIGS. 8A and 8B, respectively. Also, the input return loss and the modulation response characteristic are shown in FIGS. 9A and 9B, respectively. The EA modulator of the comparative example used is the same as the modulator of the comparative example shown in FIG. 4 except that the signal transmission line is composed of CPW.

  FIG. 10 shows the configuration of the signal transmission line of the EA modulator according to the third embodiment of the present invention. The signal transmission line 35 is configured as a microstrip line, and includes a signal conductor 36 disposed on the top surface of the line carrier 26 and a bottom ground conductor 29 disposed on the bottom surface of the line carrier 26. The signal conductor 36 has a length of 3.2 mm and a width of 0.46 mm. This width is larger than the width of a conventional signal conductor having a characteristic impedance of 50Ω, which is 0.32 mm. In this example, the distance between the signal conductor 36 and the bottom ground conductor 29 is 0.3 mm. The performance of the EA modulator of the present embodiment and that of the comparative example were compared by simulation. The microstrip line of the comparative example has a characteristic impedance of 50Ω and a termination resistance of 50Ω, and the circuit configuration is the same as that shown in FIG.

  The simulation results are shown in FIGS. 11A and 11B and FIGS. 12A and 12B, as in the previous embodiment. As is apparent from these drawings, the EA modulator of this embodiment example also has the same improvement in characteristics as the first and second embodiment examples.

  In order to investigate the characteristic impedance of the signal transmission line, the optimum values of the series resistance and the termination resistance, the optimum insertion position of the series resistance, etc., the EA modulator of the present invention was further subjected to various simulations. Referring to the above simulation etc., finally, the optimum resistance value of the series resistance is 5Ω, the optimum characteristic impedance is 41.8Ω, and the optimum insertion position of the series resistance is 3. Each 4 mm position was obtained.

  Further, the following first and second simulations were performed on the EA modulator of the present invention in order to investigate each preferable range of series resistance, termination resistance, characteristic impedance, and preferable circuit configuration. These simulations were performed using the EA modulator of the third embodiment.

  FIG. 13 shows each circuit configuration of the EA modulators of the prior art and Examples 1 to 3 used in the first simulation. The conventional EA modulator has a microstrip line characteristic impedance of 50Ω, a termination resistance of 50Ω, a signal conductor width of 0.28 mm, and a length of 3.2 mm. The characteristic impedance of the microstrip line of each embodiment is 40Ω, and the termination resistance is 80Ω. In Example 1 and 2, the signal conductor has a width of 0.425 mm and a length of 3.2 mm. The series resistance of Examples 2 and 3 is 5Ω. In Example 3, the termination resistor was connected between the node connecting the signal conductor and the series resistor and the ground. The signal conductor of Example 3 has a width of 0.425 mm and a length of 3.2 mm.

  FIGS. 14A and 14B show the input return loss and the modulation response characteristic obtained in the first simulation, respectively. As is clear from these figures, Examples 1 to 3 show improvement in input return loss in the frequency band of 5 GHz to 11 GHz, and no deterioration in modulation response characteristics in the vicinity of 10 GHz. In this simulation, it was found that better results were obtained when a series resistance of 5Ω was inserted than when no series resistance was inserted.

  FIG. 15 shows a circuit configuration of the conventional EA modulator used in the second simulation using the microstrip line and the EA modulators of Examples 1 to 7. The conventional EA modulator of FIG. 15 is the same as the conventional EA modulator used in the first simulation. The EA modulators of Examples 1 to 7 had 50, 60, 70, 80, 90, 100, and 150Ω termination resistors, respectively, and the series resistance and characteristic impedance were fixed to 5Ω and 35Ω, respectively. The width of the signal conductor in the microstrip line of each embodiment is 0.53 mm and the length is 3.2 mm.

  FIG. 16 shows the input return loss obtained by each EA modulator. The EA modulators of Examples 1 to 7 have a lower input return loss than the conventional EA modulator in the frequency band of 5 GHz to 11 GHz, but at 10 GHz or more, the -10 dB specified in the specifications is slightly decreased. Beyond. This simulation shows that the input return loss can be reduced by reducing the characteristic impedance of the signal transmission line from the conventional 50Ω to 35Ω, and that the termination resistance can be appropriately selected between 50Ω and 150Ω. Furthermore, it was confirmed that it is preferable to insert a series resistance of 5Ω.

  Although the present invention has been described based on the preferred embodiment thereof, the signal transmission line and the optical module for the EA modulator of the present invention are not limited to the configuration of the above embodiment example. Various modifications and changes from the configuration of the examples are also included in the scope of the present invention. For example, although the present specification has described an EA modulator within an EAML module, the present invention is applicable to any type of optical modulator disposed alone or integrated within an optical module. it can.

1 is a plan view of an EAML module having an EA modulator according to a first embodiment of the present invention. The perspective view of the signal transmission line shown in FIG. FIG. 2 is an equivalent circuit diagram of an EA modulator in the EAML module shown in FIG. 1. The equivalent circuit diagram of the conventional EA modulator. (A) And (b) is a graph which respectively shows the frequency dependence of the real part and imaginary part of the impedance of the EA modulator of 1st Embodiment, and the EA modulator of a comparative example. (A) And (b) is a graph which respectively shows the frequency dependence of the real part and imaginary part of the impedance of the EA modulator of 1st Embodiment, and the EA modulator of a comparative example. Graph showing the frequency dependence of input return loss when the series resistance is fixed at 5Ω and the characteristic impedance is varied. (A) And (b) is a graph which respectively shows the frequency dependence of the real part and imaginary part of the impedance of the EA modulator which concerns on the 2nd Example of this invention, and the EA modulator of a comparative example. (A) And (b) is a graph which respectively shows the frequency dependence of the input return loss and the modulation response characteristic of the EA modulator which concerns on a 2nd Example, and the EA modulator of a comparative example. The perspective view of the signal transmission line of the EA modulator which concerns on the 3rd Example of this invention. (A) And (b) is a graph which shows the frequency dependence of the real part and imaginary part of the impedance of the EA modulator of 3rd Example, and the EA modulator of a comparative example, respectively. (A) And (b) is a graph which respectively shows the frequency dependence of the input return loss and the modulation response characteristic of the EA modulator of the first embodiment and the EA modulator of the comparative example. FIG. 3 is an equivalent circuit diagram of a plurality of EA modulators used in the first simulation. (A) And (b) is a graph which respectively shows the frequency dependence of the input return loss and modulation response characteristic of an EAML module which were obtained by the 1st simulation. FIG. 6 is an equivalent circuit diagram of a plurality of EA modulators used in the second simulation. The graph which shows the input return loss of the EAML module obtained by the 2nd simulation. The top view of the conventional EAML module. FIG. 18 is an equivalent circuit diagram of the EAML module of FIG. 17.

Explanation of symbols

10: EAML module 10A: EAML module 11: optical integrated device 12: airtight package 13: external pin 14: signal transmission line unit 15: microstrip line 16: chip carrier 19, 20: bonding wire 21: termination resistor 22: optical fiber 23: Photodiode 24: EA modulator 25: Signal transmission line (unit)
26: Line carrier 27: Signal conductor 27a: Electrode pad 28: Coplanar ground conductor 29: Bottom ground conductor 30: Series resistor 31: Terminating resistor 32: Plug 33: External signal transmission line 38: Signal transmission line 40: Signal source

Claims (7)

In a signal transmission line for transmitting a modulation signal from a signal source having an output impedance of 50Ω to an optical modulator,
A signal conductor and at least one ground conductor combined to have a characteristic impedance of 35 to 47Ω;
A signal transmission line for an optical modulator, comprising: a termination resistor having a resistance value of 50 to 150Ω.   The signal transmission line for an optical modulator according to claim 1, further comprising a series resistor having a resistance value of 10Ω or less.   The signal transmission line for an optical modulator according to claim 1, wherein the signal conductor and the ground conductor constitute a microstrip line.   The ground conductor includes a pair of ground conductors formed on the same plane as the signal conductor and extending across the signal conductor, and the signal conductor and the ground conductor constitute a coplanar waveguide (CPW). 3. A signal transmission line for an optical modulator according to 2.   5. The signal transmission line for an optical modulator according to claim 4, wherein the ground conductor further includes a planar ground conductor disposed to face the signal conductor and the pair of ground conductors. In an optical module having a laser diode, an optical modulator that modulates laser light emitted from the laser diode, and a signal transmission line that transmits a modulation signal to the optical modulator, the signal transmission line includes:
A signal conductor and at least one ground conductor combined to have a characteristic impedance of 35 to 47Ω;
A series resistor having a resistance value of 10Ω or less;
An optical module comprising: a terminating resistor having a resistance value of 50 to 150Ω. In a signal transmission line for transmitting a modulation signal from a signal source having an output impedance of R ₁ Ω to an optical modulator,
A signal conductor and at least one ground conductor combined to have a characteristic impedance of 0.7 R _{1 to} 0.94 R ₁ Ω;
A signal transmission line for an optical modulator, comprising: a termination resistor having a resistance value of R _{1 to} 3R ₁ Ω.

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