Optical modulator module

A high frequency substrate, on which a high frequency substrate transmission line for connecting a chip carrier transmission line and a package substrate transmission line is formed, is mounted while being inclined with respect to a package, so that each distance between the transmission lines can be reduced. Thereby, the lengths of wires for connecting the transmission lines can be reduced so as to improve frequency characteristics of an optical modulator module.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent application serial no. 2005-030249, filed on Feb. 7, 2005, the content of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to an optical modulator module, and particularly to the optical modulator module to be used for a transmitter for optical communications.

An optical modulator module has been required to be downsized. JP-A-2003-318601 provides an optical module having a bended transmission line, in which good frequency characteristics can be obtained when being driven at a high frequency, so as to obtain a degree of freedom of the internal arrangement for realization of a high-density package.

A chip carrier24, a high frequency substrate29, and a lens22are mounted in a package28as shown in FIG. 6 in JP-A-2003-318601. An EA-DFB (Electro-Absorption Modulator Integrated Distributed Feedback) laser diode21is mounted on the chip carrier24. The EA-DFB laser diode21is optically coupled to an optical fiber23via the lens22. The chip carrier24and the high frequency substrate29have a chip carrier transmission line25and a high frequency substrate transmission line27, respectively, which are connected to each other via a wire31. In addition, a package substrate also has a package substrate transmission line26which is connected to the high frequency substrate transmission line27via a wire32. A photodiode mounted on a photodiode mount is generally disposed at the rear (opposite to the fiber) of the chip carrier24.

In the optical modulator module disclosed in JP-A-2003-318601, in the case where the chip carrier transmission line25and the package substrate transmission line26are not linearly-arranged, the high frequency substrate transmission line27is bent without deteriorating the frequency characteristics so as to realize a high-density package while obtaining a good optical output waveform.

US 2003/0202800 A1 is the U.S. counterpart application of JP-A-2003-318601.

In order to downsize the optical modulator module, the reduction in dimension of components while securing a degree of freedom of the arrangement is of importance. There has been a problem in that the more the dimension of components is reduced, the more the inductance components of a wire are increased due to the following reason. In the case where the width of a signal transmission line is 200 μm or more, there is provided means by which the inductance of a wire is decreased by connecting the transmission lines via a ribbon wire or plural wires. However, in the case where the substrate is made thinner and the width of the transmission line is reduced for the sake of further downsizing, it is difficult to use the ribbon wire as well as to arrange the plural wires, which results in no choice but to connect the transmission lines via one wire. Consequently, the inductance components are increased to cause reduction in transmission band of the transmission line, and as a result, the optical output waveform of the optical modulator module is distorted.

In order to solve the problem, it is effective to reduce a distance of the wire connection by improving the dimension accuracy of components and the assembly accuracy of components. However, this improvement causes a sharp rise in cost of components.

SUMMARY OF THE INVENTION

The present invention has been achieved in view of solving the problem, and provides an optical modulator module which does not require to tighten the dimension tolerance of components in order to secure a good optical output waveform and which is suitable for reducing the cost.

The present invention can be achieved in such a manner that the high frequency substrate is arranged at an angle with respect to the optical axis in a gap between the chip carrier and the package substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, several embodiments of the present invention will be described with reference to the drawings. The same numeral is given to the equivalent constituent element in each embodiment, and the explanation thereof will be omitted in the subsequent embodiments.

First Embodiment

An optical modulator module according to a first embodiment of the present invention will be described with reference toFIG. 1.FIG. 1is a plan view of a DFB laser diode module.

A DFB laser diode module100inFIG. 1has an outer shape in which an optical fiber holder7connected to an optical fiber6is attached to a package4having nine input pins25. In the package4, there is formed a package substrate5on which a package substrate transmission line12connected to one of the input pins25is formed.

In assembling the DFB laser diode module100, a chip carrier1having a DFB laser diode2mounted and a lens8are mounted on the package4, and the optical fiber6is adjusted to be fixed to the package4so that light emitted from the DFB laser diode2enters to the optical fiber6at maximum. There are formed a via hole and a chip carrier transmission line10in the chip carrier1, and the via hole is connected to the backside of the DFB laser diode2. The chip carrier1is a common ground for the optical modulator and the laser diode. A signal input unit (not shown) of the modulator of the DFB laser diode2is connected to the chip carrier transmission line10via a bonding wire (not shown). A photodiode27mounted on a photodiode mount26monitors light emitted backward from the DFB laser diode2.

Between the package substrate transmission line12and the chip carrier transmission line10, there is provided a high frequency substrate3in an oblong shape on which a high frequency substrate transmission line11having two bended portions is formed and whose length is inclined in the counterclockwise direction by about four degrees from the optical axis direction (x direction). The package substrate transmission line12is connected to the high frequency substrate transmission line11by a gold wire14. The chip carrier transmission line10is connected to the high frequency substrate transmission line11by a gold wire13. Note that the backsides of the package substrate transmission line12, the high frequency substrate transmission line11, and the chip carrier transmission line10are grounded. The characteristic impedance of each transmission line is 50Ω, and the pattern width is 100 μm. The numeral24denotes a gold ball on the first bonding side of the gold wire. The diameter of the gold wire is 25 μm and the diameter of the gold ball after bonding is 100 μm which is the same as the pattern width.

The optical modulator modulates output light that is continuously emitted from the laser diode on the basis of an electrical signal (modulated signal) being output from a signal source (not shown) so that the DFB laser diode module100transmits an optical signal. The electrical signal is transmitted to the optical modulator via each of the above-described transmission lines.

The high frequency substrate transmission line11formed on the high frequency substrate3runs upward from the vicinity of one of the corners of the high frequency substrate3while being in parallel with the width of the high frequency substrate3, bends at a right angle so as to be in parallel with the length of the high frequency substrate3, and then bends again so as to be in parallel with the width of the high frequency substrate3. Accordingly, the high frequency substrate transmission line11reaches the vicinity of the opposing corner. The high frequency substrate3may be of a square shape. A rectangular shape is utilized as the broader concept of a square shape and an oblong shape. Further, the high frequency substrate transmission line11may include a portion that is inclined with respect to the outer shape of the high frequency substrate3. A connecting portion of the high frequency substrate transmission line11with the package substrate transmission line12is referred to as an output terminal, and a connecting portion of the high frequency substrate transmission line11with the chip carrier transmission line10is referred to as an input terminal. As similar thereto, a connecting portion of the package substrate transmission line12with the high frequency substrate transmission line11is referred to as an input terminal, and a connecting portion of the chip carrier transmission line10with the high frequency substrate transmission line11is referred to as an output terminal. In consideration of the tolerance of the mounting position for the package substrate and the tolerance of the assembly of the chip carrier, the high frequency substrate3is required to have the width shorter than the minimum distance between the package substrate and the chip carrier. Therefore, the distances between the high frequency substrate3and the chip carrier and between the high frequency substrate3and the package carrier are 200 μm, respectively, in nominal dimension. In order to shorten the distances, the high frequency substrate3is inclined with respect to the optical axis, so that the high frequency substrate3becomes closer to the package substrate and the chip carrier in the y-coordinate direction. In the meantime, the high frequency substrate3is deviated with respect to the package substrate and the chip carrier in the x direction. However, the deviation is small enough to disregard. Inclining the high frequency substrate3with respect to the optical axis means that the high frequency substrate3is arranged at an angle with respect to the optical axis. In this case, the angle does not include 0 degree (that is, in parallel with the optical axis).

The high frequency substrate3is, if being represented in general, a substrate on which each x-y coordinate of an input terminal11aof the high frequency substrate transmission line11differs from each x-y coordinate of an output terminal lib of the high frequency substrate transmission line11. In other words, each x-y coordinate of an output terminal12bof the package substrate transmission line12differs from each x-y coordinate of an input terminal10aof the chip carrier transmission line10.

The high frequency substrate3abuts against the chip carrier1while being rotated in the counterclockwise direction, and is fixed without abutting against the package substrate5. The high frequency substrate3does not abut against the package substrate5because they are not affected by thermal expansion of the package4. Note that the abutment in this specification means that the minimum distance between the two transmission substrates is 50 μm or less by measuring the distance on the surface of the transmission substrate. On the contrary, no abutment means that the minimum distance between the two transmission substrates is more than 50 μm. The minimum distance between the two transmission substrates is measured by a micrometer mounted on a microscope.

The high frequency substrate3in an oblong shape is mounted while being inclined with respect to the side faces of the package in the first embodiment, so that the distance between transmission lines can be made shorter, the bonding wire can be made shorter, and the modulator module with excellent characteristics can be obtained. Note that the content of improvement will be described in greater detail in a second embodiment.

The first embodiment is described by using the DFB laser diode module. However, the same effect can be obtained even by using the optical modulator module with no laser diode, which is applicable to the following embodiments. Note that the DFB laser diode module is the optical modulator module because the DFB laser diode module includes the modulator.

Further, the high frequency substrate3is made to abut against the chip carrier1in the above-described embodiment. However, the high frequency substrate3may abut against the package substrate5without abutting against the chip carrier1. Fixing the high frequency substrate3in such a manner allows the bonding wire, which is provided on the side where the high frequency substrate3does not abut, to absorb the extension and contraction caused by temperature change of the package, and as a result, the modulator module having good characteristics and high reliability can be realized. This is commonly applicable to the following embodiments.

Second embodiment

An optical modulator module according to the second embodiment of the present invention will be described with reference toFIGS. 2 to 6.FIG. 2is a plan view of a DFB laser diode module.FIG. 3is a plan view of a high frequency substrate.FIG. 4is an equivalent circuit of transmission lines of the DFB laser diode module.FIG. 5is a diagram explaining deviation in band of the transmission lines of the DFB laser diode module.FIG. 6is a plan view of main parts of an EA-DFB laser diode module obtained by modifying the second embodiment.

InFIG. 3, a high frequency substrate9according to the second embodiment is provided with chamfered portions19at the corners in the vicinity of the ends of the high frequency substrate transmission line11. The provision of the chamfered portions19allows the wire length between the chip carrier transmission line10and the high frequency substrate transmission line11and the wire length between the high frequency substrate transmission line11and the package substrate transmission line12to be shorter in the case where the high frequency substrate9is arranged while being inclined with respect to the side faces of the chip carrier1and the package substrate5.

InFIG. 2, the high frequency substrate9is arranged while being inclined in the counterclockwise direction in such a manner that the side face of the high frequency substrate9abuts against the side face of the chip carrier1in the vicinity of a chamfered portion19-1and the other side face of the high frequency substrate9does not abut against the side face of the package substrate5. The end portions of the chip carrier transmission line10and the package substrate transmission line12can be made closer to the corresponding end portions of the high frequency substrate transmission line11of the high frequency substrate9by chamfering the corners of the high frequency substrate9. Accordingly, the lengths of wires15and16are made shorter as compared to the case where the corners of the high frequency substrate9are not chamfered. Shortening the lengths of the wires15and16improves a decrease in transmission band.

Effects of the first and second embodiments will be described with reference toFIGS. 4 and 5.FIG. 4is an equivalent circuit from the chip carrier transmission line10to the package substrate transmission line12of the EA-DFB laser diode module as shown inFIG. 1or2. This model is configured by a chip carrier substrate transmission line model20, a high frequency substrate transmission line model21, a package substrate transmission line model22, and a wire model23in which wires are represented by inductances.

FIG. 5shows results in which transmission bands of the transmission lines of the EA-DFB laser diode module where arranging methods of the high frequency substrate are parameterized are simulated by using the equivalent circuit shown inFIG. 4

The horizontal axis of the graph represents a frequency, and the vertical axis represents deviation in band. The deviation in band is calculated by using a value at a frequency of 1 GHz as a standard. The EA-DFB laser diode module is assumed to operate at 10 Gbits/s, and therefore the deviation in band is simulated up to 20 GHz. c inFIG. 5represents a case where the length of the high frequency substrate is arranged while being aligned to the optical axis direction. B inFIG. 5represents a case where the high frequency substrate with no chamfered portions is arranged while being rotated in the counterclockwise direction so as to abut against the chip carrier. Further, A inFIG. 5represents a case where the high frequency substrate with the chamfered portions is arranged while being rotated in the counterclockwise direction so as to abut against the chip carrier. The inductance of the wire model23becomes small in the case where the high frequency substrate abuts against the chip carrier.

The result shows that the deviation in band at 10 GHz is improved from −1.59 dB to −0.99 dB by arranging the high frequency substrate while inclining the same with respect to the wall surfaces of the package, and further the deviation in band at 10 GHz is improved to −0.60 dB by using the high frequency substrate with the chamfered portions. This simulation clarifies that the transmission band of the EA-DFB laser diode module is improved.

According to the second embodiment, the chamfer of the corners of the high frequency substrate can suppress an increase in wire inductance, a decrease in transmission band of the EA-DFB laser diode module, and distortion of an optical output waveform, in the case where it is difficult to arrange plural wires in parallel or to use a ribbon wire because width of a signal electrode of the transmission line is narrow.

Note that a chamfer angle may not be 45 degrees (45 degrees chamfer), but may be about 4 degrees. The angle is preferably about 10 degrees. Further, the chamfer may be a round chamfer.

A modified embodiment of the second embodiment will be described with reference toFIG. 6. A unique point whereFIG. 2differs fromFIG. 6is that the chip carrier transmission line10is connected to the high frequency substrate transmission line11via the bonding wire15or the gold ball24.

The high frequency substrate9is chamfered up to the vicinity of the high frequency substrate transmission line11, and accordingly if the high frequency substrate9abuts against the chip carrier, the distance between the high frequency substrate transmission line11and the chip carrier transmission line10becomes in the order of 50 μm. If the gold ball that is the first bonding of the wire bonding is provided so that the gold ball bridges the high frequency substrate transmission line11and the chip carrier transmission line10and if the wire is cut right after that, the gold ball24can be realized. The connection by the gold ball is the connection by the bonding wire.

The gold ball24that is the first bonding of the wire bonding is 100 μm in diameter as described above, and it is essential to separate the first bonding portion from the second bonding portion by 50 μm or more, even though the outer shape of the bonding capillary is disregarded. It is a matter of course that the gold wire needs to make a loop and has inductance. On the other hand, the gold ball does not need to make a loop and has an effect of decreasing inductance.

In this embodiment, the connection of the chip carrier transmission line10to the high frequency substrate transmission line11is made by welding the gold ball. However, brazing, soldering, and the like are applicable. This modified embodiment is applicable to the third embodiment.

Third Embodiment

An optical modulator module according to a third embodiment of the present invention will be described with reference toFIGS. 7 to 9.FIG. 7is a plan view of main parts of a DFB laser diode module.FIG. 8is a plan view of a pack-age substrate.FIG. 9is a plan view of a chip carrier substrate.

A package substrate18shown inFIG. 8has the chamfered portion19formed at the left corner (opposite to the optical axis direction) of the package substrate transmission line12. Further, a chip carrier substrate17shown inFIG. 9has the chamfered portion19formed at the right corner (the optical axis direction) of the chip carrier transmission line10. These chamfered portions serve as a relief with respect to aforementioned high frequency substrate. Therefore, the relief is not limited to be in a chamfered shape, but may be in a dent shape.

InFIG. 7, the high frequency substrate3, on which there is formed the high frequency substrate transmission line11that bridges and connects the chip carrier transmission line10and the package substrate transmission line12, is fixed in such a manner that the high frequency substrate3is rotated in the counterclockwise direction between the chip carrier17and the package substrate18so as to abut against the chip carrier17without abutting against the package substrate18. The wire length between the chip carrier transmission line10and the high frequency substrate transmission line11can be made shorter by providing the chamfer portions to not the high frequency substrate3but the chip carrier17and the package substrate18, as compared to the case where no chamfered portions are provided. As similar thereto, the wire length between the high frequency substrate transmission line11and the package substrate transmission line12can be made shorter.

According to the third embodiment, shortening the lengths of the wires15and16can minimize a decrease in transmission band due to wire inductance, and can suppress distortion of optical output characteristics of the DFB laser diode module.

According to the third embodiment, it is possible to obtain a good optical output waveform without tightening the dimension tolerance of components of the transmission lines in the optical modulator module of a high-density package.