OPTICAL MODULE

An optical module includes: a light-receiving element configured to receive an optical signal and convert the optical signal to an electrical signal; a transmission line configured to transmit an electrical signal; and an amplifier element configured to amplify an electrical signal that is output from the light-receiving element and transmitted through the transmission line, wherein characteristic impedance of the transmission line connecting the light-receiving element and the amplifier element is higher than input impedance of the amplifier element.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of this optical module are described in detail with reference to the attached drawings. Like reference numerals refer like constituting elements throughout the following description of the embodiments, and overlapping descriptions thereof are omitted.

Example of Optical Module

FIG. 1is a diagram illustrating an equivalent circuit for one example of the optical module according to an embodiment. As illustrated inFIG. 1, the optical module includes a light-receiving element1, a transmission line2, and an amplifier element3.

The light-receiving element1receives an optical signal and converts it into an electrical signal. An example of the light-receiving element1may be, for example, a photodiode. The photodiode may be represented by a model in which a resistance5and a coupling capacitance6are connected in parallel to a current source4that supplies a current in response to an intensity of an input optical signal. An internal resistance of the photodiode may be, for example, about 100 MΩ, and an output impedance of the photodiode is high.

The transmission line2is connected to the light-receiving element1. The transmission line2transmits an electrical signal output from the light-receiving element1. An example of the transmission line2may be, for example, a micro-strip type transmission line. One example of the micro-strip type transmission line is such that a signal electrode composed of an electrically conductive material is formed on a upper surface of a dielectric substrate such as, for example, polyimide, glass epoxy, or the like, and a ground electrode composed of an electrically conductive material is formed on an undersurface of the dielectric substrate.

The amplifier element3is connected to the transmission line2. The amplifier element3amplifies an electrical signal that is output from the light-receiving element1and transmitted through the transmission line2. An example of the amplifier element3may be, for example, a transimpedance amplifier. The transimpedance amplifier converts a current signal output from the photodiode to a voltage signal. A resistance7inside the transimpedance amplifier represents the input impedance of the transimpedance amplifier. This input impedance is composite impedance when viewed from the input side of the transimpedance amplifier.

As described above, the light-receiving element such as, for example, the photodiode is of high impedance. For example, let Z0 be the characteristic impedance of the transmission line2such as, for example, a micro-strip type transmission line or the like, and Zin be the input impedance of the amplifier element3such as, for example, the transimpedance amplifier or the like. In the optical module according to the embodiment, the characteristic impedance Z0 of the transmission line2is higher than the input impedance Zin of the amplifier element3.

In this way, because the characteristic impedance Z0 of the transmission line2is higher than the input impedance Zin of the amplifier element3, the frequency characteristic of transmission characteristic for the transmission line2may be improved. The transmission characteristic of the transmission line2may be represented by, for example, an incident wave transmission coefficient of the transmission line2that is S21 of S parameter. The incident wave transmission coefficient of the transmission line2may be represented by a ratio of the power of an electrical signal output from the transmission line2to the amplifier element3to the power of an electrical signal input to the transmission line2from the light-receiving element1.

The inventors of the present disclosure have carried out researches and obtained the following findings that allow to improve the frequency characteristic of transmission characteristic for the transmission line2. Below, the findings will be described.

Description of Findings Obtained by the Inventors of the Present Disclosure

FIG. 2is a diagram illustrating a modeled equivalent circuit of the optical module illustrated inFIG. 1. InFIG. 2, for sake of simplicity of description, a system is illustrated in which the coupling capacitance6(seeFIG. 1) of the light-receiving element1is omitted and a simple high impedance circuit11is connected to a simple low impedance circuit13via a transmission line12. The characteristic impedance of the low impedance circuit13is lower than the characteristic impedance of the high impedance circuit11.

The high impedance circuit11is a circuit that models the light-receiving element1. The high impedance circuit11is represented by a model in which a resistance15is connected in parallel to a current source14. The low impedance circuit13is a circuit that models the amplifier element3. The low impedance circuit13is modeled by a resistance17. The characteristic impedance of the low impedance circuit13is expressed as Zin. The characteristic impedance of the transmission line12is expressed as Z0.

In high frequency circuits, it is typical to design the characteristic impedance Z0 of the transmission line12so as that the characteristic impedance Z0 of the transmission line12becomes equal to the characteristic impedance Zin of the low impedance circuit13that serves as a load. In this case, the impedance matching is achieved. Thus, the high impedance circuit11sees only the characteristic impedance Zin of the low impedance circuit13. Accordingly, a high-frequency electrical signal may be transmitted from the high impedance circuit11to the low impedance circuit13without reflection or loss.

FIG. 3is a diagram illustrating an example of Smith chart for an incident wave reflection coefficient S11 when Z0 is larger than Zin in the circuit illustrated inFIG. 2. The incident wave reflection coefficient S11 is represented by a ratio of the power of an electrical signal reflected back to the high impedance circuit11to the power of an electrical signal input to the transmission line12from the high impedance circuit11.

The Smith chart illustrated inFIG. 3is a result obtained by calculating the incident wave reflection coefficient S11 viewed from the high impedance circuit11with a circuit simulator based on the simulation program with integrated circuit emphasis (SPICE). Here, it is assumed that the high impedance circuit11has an impedance of 500Ω, the characteristic impedance Zin of the low impedance circuit13is 50Ω, the characteristic impedance Z0 of the transmission line12is 100Ω, and the transmission line12has a length of 1.5 mm.

FIG. 4is a diagram illustrating an exemplary frequency characteristic for an imaginary part of the incident wave reflection coefficient S11 illustrated inFIG. 3. In the characteristic diagram illustrated inFIG. 4, the vertical axis represents the imaginary part of the incident wave reflection coefficient S11, and the horizontal axis represents the frequency. When the imaginary part of the incident wave reflection coefficient S11 is positive, the resistance is inductive reactance that is a coil-like resistance. On the other hand, when the imaginary part of the incident wave reflection coefficient S11 is negative, the resistance is capacitive reactance that is a capacitance-like resistance.

InFIG. 4, when Z0=Zin, the imaginary part of the incident wave reflection coefficient S11 becomes zero. The circuit becomes non-reflective, and neither the inductive nor capacitive component is apparent. On the other hand, when Z0>Zin, it is apparent fromFIG. 4that the inductive feature stands out more clearly around a certain frequency due to impedance miss-matching. According to this feature, there may be an effect in improving the frequency characteristic that is similar to the case where as if a pure coil component such as a wire is inserted.

FIG. 5is a diagram illustrating an example of Smith chart for the incident wave reflection coefficient S11 when Z0 is less than Zin in the circuit illustrated inFIG. 2. The Smith chart illustrated inFIG. 5is a result obtained by calculating the incident wave reflection coefficient S11 viewed from the high impedance circuit11with a circuit simulator based on the simulation program with integrated circuit emphasis (SPICE). Here, it is assumed that the high impedance circuit11has an impedance of 500Ω, the characteristic impedance Zin of the low impedance circuit13is 50Ω, the characteristic impedance Z0 of the transmission line12is 30Ω, and the transmission line12has a length of 1.5 mm.

FIG. 6is a diagram illustrating an exemplary frequency characteristic for an imaginary part of the incident wave reflection coefficient S11 illustrated inFIG. 5. In the characteristic diagram illustrated inFIG. 6, the vertical axis represents the imaginary part of the incident wave reflection coefficient S11, and the horizontal axis represents the frequency. When Z0<Zin, it is apparent fromFIG. 6that the capacitive feature stands out more clearly. This degrades the frequency characteristic.

FIG. 7is a diagram illustrating an exemplary frequency characteristic of transmission characteristic for the transmission line. In the characteristic diagram illustrated inFIG. 7, the vertical axis represents the transmission characteristic, and the horizontal axis represents the frequency. When compared at a frequency where the transmission characteristic is reduced by 3 dB, the frequency characteristic is improved when Z0>Zin compared to when Z0=Zin. On the other hand, the frequency characteristic degrades when Z0<Zin compared to when Z0=Zin.

Relationship Between Line Width and Impedance of Transmission Line

FIG. 8is a diagram illustrating an example of the transmission line. The transmission line illustrated inFIG. 8may be, for example, a micro-strip type transmission line. This micro-strip type transmission line may include a signal electrode22that is composed of an electrically conductive material and formed on a upper surface of a dielectric substrate21such as, for example, polyimide, glass epoxy, or the like, and a ground electrode23that is composed of an electrically conductive material and formed on an undersurface of the dielectric substrate21.

FIG. 9is a diagram illustrating an exemplary relationship between the line width and the impedance of the transmission line. In the characteristic diagram illustrated inFIG. 9, the vertical axis represents the impedance of transmission line, and the horizontal axis represents the line width of the signal electrode. The characteristic diagram illustrated inFIG. 9is a result obtained by calculating the impedance of transmission line while varying a width W of the signal electrode22. Here, it is assumed that the dielectric substrate21has a thickness H of 25 μm, the signal electrode22has a thickness T of 20 μm, and the dielectric substrate21has a dielectric constant of 3.3, which is the dielectric constant of polyimide, for example. The characteristic impedance is obtained by analysis of an electromagnetic distribution using a finite element method.

As illustrated inFIG. 9, the impedance of transmission line increases as the width W of the signal electrode22becomes narrower. On the other hand, varying the thickness T of the signal electrode22provides only a small effect in increasing the impedance of transmission line. Thus, in the present embodiment, no description will be provided with regard to the varying the thickness T of the signal electrode22. A minimum value of the width W of the signal electrode22is determined in response to a limiting value of the line width that may be manufactured. For example, when a flexible printed board is used as the dielectric substrate21, 30 μm is the minimum value for the width W of the signal electrode22that may be reliably manufactured under a current technology.

It is apparent fromFIG. 9that the impedance of transmission line is 60Ω when the width W of the signal electrode22is 30 μm. A typical input impedance of the transimpedance amplifier is 50Ω. Accordingly, when the input impedance of the transimpedance amplifier is 50Ω, the impedance of transmission line may be set to 60Ω by setting the width W of the signal electrode22to 30 μm. Thus, the condition Z0>Zin may be satisfied.

In other words, the reduction of the width W of the signal electrode22is one way to achieve the condition such that the characteristic impedance Z0 of the transmission line2is higher than the input impedance Zin of the amplifier element3in the optical module illustrated inFIG. 1. In the future, advancement in manufacturing technology may allow stable manufacturing of the signal electrode22with the width W less than 30 μm. In that case, the impedance of transmission line may be increased over 60Ω by further reducing the line width of the signal electrode22.

Alternatively, the condition Z0>Zin may be satisfied by forming the ground electrode23in such a way that no part of the ground electrode23is present below the signal electrode22. When no part of the ground electrode23is present below the signal electrode22, the distance between the signal electrode22and the ground electrode23becomes larger in comparison with the case where the ground electrode23is present below the signal electrode22. In this way, the impedance of transmission line may be increased, and the condition Z0>Zin may be satisfied.

By making the characteristic impedance Z0 of the transmission line2higher than the input impedance Zin of the amplifier element3, the optical module illustrated inFIG. 1may be able to provide the effect in improving the frequency characteristic similar to that of the case where a coil component is inserted in a signal channel between the light-receiving element1and the amplifier element3. Accordingly, an inexpensive optical module may be provided in comparison with the case where a wire is inserted as the coil component and the length of the wire is adjusted with high precision.

In order to make the characteristic impedance Z0 of the transmission line2higher than the input impedance Zin of the amplifier element3, the width of the signal electrode22of the transmission line2may be made narrower than the width at which the impedance matching may be achieved, or the signal electrode22and the ground electrode23may be arranged so as that no part of the ground electrode23is present below the signal electrode22. Both configurations may be achieved by changing mask patterns to be used in manufacturing the transmission line2to patterns with which the signal electrode22is formed narrower or patterns with which no part of the ground electrode23is formed below the signal electrode22. Accordingly, an inexpensive optical module may be provided.

Another Example of Optical Module

FIG. 10illustrates another example of the optical module according to an embodiment.FIG. 11is a diagram illustrating a cross-section along a cutting line XI-XI ofFIG. 10. The optical module illustrated inFIG. 10andFIG. 11is an example of the optical module in which no ground electrode is present below the signal electrode. As illustrated inFIG. 10andFIG. 11, the optical module includes a substrate31, a photodiode array32, signal electrodes33, a ground electrode34, and a transimpedance amplifier35.

An example of the substrate31may be, for example, a flexible printed board that includes a core material such as polyimide or the like. A dielectric layer of the substrate31is an example of a dielectric layer of the transmission line.

The photodiode array32is an example of the light-receiving element. The photodiode array32is mounted on an upper surface of the substrate31. In the example illustrated inFIG. 10, the photodiode array32is provided with, for example, four photodiodes which are not illustrated in the drawing. Each photodiode is mounted so as that its light-receiving plane faces down, namely the light-receiving plane faces the substrate31. The substrate31is formed in such a way that regions of the substrate31corresponding to the light-receiving planes of the respective photodiodes are transparent or have their respective openings that go through the substrate31.

The signal electrode33is composed of an electrically conductive material and formed on the upper surface of the substrate31from the photodiode array32to the transimpedance amplifier35. In this way, the photodiode array32and the transimpedance amplifier35are connected via the signal electrode33. For example, in the example illustrated inFIG. 10, four lines of the signal electrodes33are provided in response to the four photodiodes of the photodiode array32, respectively. The numbers of the photodiodes and the signal electrodes33may be three or less, or five or more, or may be set in response to the number of channels. The signal electrode33is an example of a first electrically conductive layer of the transmission line.

The ground electrode34is composed of an electrically conductive material and formed, for example, on an undersurface of the substrate31except in part below the signal electrode33. InFIG. 10, a region surrounded with a dotted line in a center portion of the substrate31illustrates part (region)36where the ground electrode34is not present. In the example illustrated inFIG. 10, the part36where the ground electrode34is not present has a rectangle shape. However, the shape of the part36is not limited to the rectangle shape. The ground electrode34is an example of a second electrically conductive layer of the transmission line. The signal electrodes33, the dielectric layer of the substrate31, and the ground electrode34form a micro-strip type transmission line.

InFIG. 11, part surrounded by a dashed-two dotted line is the part36where the ground electrode34is not present below the signal electrode33on the undersurface of the substrate31. The part36where the ground electrode34is not present below the signal electrode33may form an air gap layer or may be filled with a dielectric material. An example of the dielectric material, with which the part36where the ground electrode34is not present below the signal electrode33is filled, may be an epoxy adhesive or other polymer material, for example. On the undersurface of the substrate31, regions corresponding to the light-receiving planes of the photodiodes of the photodiode array32may be formed so as to allow light to pass through. Thus, the region has no part of the ground electrode34, and may form an air gap layer or may be formed from a transparent material.

In the dielectric layer of the substrate31and the ground electrode34, the region corresponding to the light-receiving plane of each photodiode is formed so as to allow light to pass through. In this way, each photodiode is allowed to receive an optical signal emitted from an optical waveguide formed, for example, on an undersurface side of the substrate31. On a back surface of the substrate31, the optical waveguide, which is not illustrated in the figure, is attached to the ground electrode34with an adhesive sheet in between them. The optical waveguide may extend in a direction opposite to the signal electrode33from a position directly below the photodiode array32.

As illustrated inFIG. 11, let ds be the distance between an outer edge of the outmost signal electrode33and an inner edge of the ground electrode34. It is desirable to have less variation in impedance among channels of the signal electrodes33. Thus, it is desirable to have a larger value for the distance ds.

For example, let the substrate31be a flexible printed board having a thickness of 25 μm, the signal electrode33has a width of 47 μm, and the distance between the signal electrodes33is 250 μm. The characteristic impedance of a transmission line formed of each signal electrode33is calculated for the case where ds=50 μm. Starting from the left inFIG. 11, calculated characteristic impedances of the signal electrodes33are 70Ω, 130Ω, 130Ω, and 70Ω, respectively.

The characteristic impedance of a transmission line formed of each signal electrode33is calculated also for the case where ds=75 μm. Starting from the left inFIG. 11, calculated characteristic impedances of the signal electrodes33are 85 Ω, 130Ω, 130Ω, and 85Ω, respectively. According to those results, it is apparent that the characteristic of transmission line becomes more even when ds=75 μm in comparison with the case when ds=50 μm. This is because a margin for the impedance increase is smaller as a gap between the signal electrode33and the ground electrode34becomes wider than a certain level.

In view of the foregoing analysis, it is desirable that the distance ds is equal to 75 μm or larger. In general, it is desirable that the distance ds is three times the thickness of the substrate31or larger, for example.

In high frequency circuits, inductivity may be seen as a resistance component represented by ωL. Thus, a transmission band of transmission line starts to degrade when the inductivity of transmission line becomes too strong. For example, simulation results of the incident wave transmission coefficient S21 of an electrical signal are obtained while varying the characteristic impedance of transmission line. In the simulation, it is assumed that the input impedance Zin of the transimpedance amplifier is 50Ω, and photodiodes in which a 3 dB band is about 12 GHz are used.

FIG. 12is a diagram illustrating simulation results of a relationship between the incident wave transmission coefficient S21 and Z0 when the length of transmission line is 1 mm. The vertical axis represents the incident wave transmission coefficient S21, and the horizontal axis represents the frequency. Here, an effect in band expanding is measured by the frequency at which the incident wave transmission coefficient S21 is reduced by 3 dB. FromFIG. 12, it is confirmed that the effect in band expanding is present when Z0 is in the range of about 60Ω to 250Ω. When Z0 increases beyond 250Ω, the inductivity of transmission line becomes too strong. Thus, the transmission band starts to degrade. When Z0 is in the range of about 80Ω to 160Ω, the band reaches substantially the maximum value.

FIG. 13is a diagram illustrating simulation results of a relationship between the incident wave transmission coefficient S21 and Z0 when the length of transmission line is 1.5 mm. The vertical axis represents the incident wave transmission coefficient S21, and the horizontal axis represents the frequency. Here, the effect in band expanding is measured by the frequency at which the incident wave transmission coefficient S21 is reduced by 3 dB. FromFIG. 13, it is confirmed that the effect in band expanding is present when Z0 is in the range of about 60Ω to 160Ω. When Z0 is in the range of about 70Ω to 120Ω, the band reaches substantially the maximum value.

WhenFIG. 12andFIG. 13are compared, it becomes apparent that the range of impedance in which the effect in band expanding may be obtained is wider and the frequency at which the maximum band is achieved is higher in the case where the length of transmission line is 1 mm. Accordingly, it is apparent that the band may be expanded more effectively when the characteristic impedance of transmission line is increased by shortening the length of transmission line.

For example, when the photodiodes and the transimpedance amplifier are flip-chip mounted on the substrate, the minimum value for the length of transmission line is determined by the minimum gap between the photodiodes and the transimpedance amplifier that allows to carry out the flip-chip mounting. When the photodiodes and the transimpedance amplifier are being flip-chip mounted on the substrate, underfill is provided at corresponding locations. The use of underfill increases reliability.

The underfill has a certain length of fillet. Thus, under a current technology, 1 mm is the minimum gap between the photodiodes and the transimpedance amplifier for stable manufacturing. In the future, advancement in manufacturing technology may allow stable flip-chip mounting even when the gap between the photodiodes and the transimpedance amplifier is less 1 mm. In that case, the length of transmission line may be further reduced.

FIG. 14is a diagram illustrating an exemplary relationship between the transmission characteristic of transmission line and the length of part where no ground electrode is present below the signal electrode. In the characteristic diagram illustrated inFIG. 14, the vertical axis represents the transmission characteristic, and the horizontal axis represents the frequency. InFIG. 10, L is the distance of part36where the ground electrode34is not present below the signal electrodes33. Furthermore, the characteristic impedance of transmission line is set to 85Ω, for example.

When compared at the frequency where the transmission characteristic is reduced by 3 dB, it is apparent fromFIG. 14that the higher inductivity may be obtained as L becomes longer, and thus an effect in widening the band may be obtained. L may be freely changed when designing of the optical module. This provides higher flexibility in designing and allows to use photodiodes having various frequency characteristics.

FIG. 15is a diagram in which an equivalent circuit of photodiode is added to the optical module illustrated inFIG. 10. InFIG. 15, the equivalent circuit of photodiode is added only to one of the channels. However, the same applies to the other channels as well.

As illustrated inFIG. 15, in each photodiode37of the photodiode array32, an anode line is connected to the signal electrode33, and a reverse bias voltage Vc is applied to a cathode line. A capacitance38of, for example, about 100 pF connects the cathode line and the ground electrode34. In the example illustrated inFIG. 15, the cathode line of the photodiode is routed to a side opposite to the signal electrode33. However, the cathode line may alternatively be routed to the same side as the signal electrode33.

According to the optical module illustrated inFIG. 10, since the part36where the ground electrode34is not present below the signal electrode33is formed as an air gap layer or filled with a dielectric material, the distance between the signal electrode33and the ground electrode34becomes larger in comparison with the case where the ground electrode34is present below the signal electrode33. In this way, the characteristic impedance of transmission line may be made higher than the input impedance of the transimpedance amplifier35. The ground electrode34may be manufactured in such a way that no part of the ground electrode34is present below the signal electrode33with a mask whose pattern does not allow to form the ground electrode34below the signal electrode33. Accordingly, an inexpensive optical module may be provided.

Another Example of Optical Module

FIG. 16illustrates another example of the optical module according to an embodiment.FIG. 17is a diagram illustrating a cross-section along a cutting line XVII-XVII ofFIG. 16. The optical module illustrated inFIG. 16andFIG. 17is an example of the optical module in which the cathode line of the photodiode is routed to the same side as the signal electrode33. InFIG. 16, the equivalent circuit of photodiode is added only to one of the channels. However, the same applies to the other channels as well.

As illustrated inFIG. 16andFIG. 17, in each photodiode37of the photodiode array32, an anode line is connected to the signal electrode33, and a cathode line is connected to a cathode line path39that is formed separately from but along with the signal electrode33between the photodiode array32and the transimpedance amplifier35. The cathode line path39is composed of an electrically conductive material, and may be formed together with the signal electrodes33by patterning when the signal electrodes33are manufactured. InFIG. 16andFIG. 17, hatching is added to the cathode line path39.

A reverse bias voltage Vc is applied to the cathode line of each photodiode. Here, the cathode line of the photodiode extends to the transimpedance amplifier35via the cathode line path39. Thus, the capacitance38to be connected to the cathode line and a source of the reverse bias voltage Vc to be applied to the cathode line may be integrated in the transimpedance amplifier35.

FIG. 18is a diagram illustrating an exemplary relationship between the impedance of transmission line and a gap between the signal electrode and the cathode line path. In the characteristic diagram illustrated inFIG. 18, the vertical axis represents the impedance of transmission line, and the horizontal axis represents the line width of signal electrode. The characteristic diagram illustrated inFIG. 18is a result obtained by calculating the impedance of transmission line while varying a gap G between the signal electrode33and the cathode line path39. Here, a flexible printed board with a thickness of 25 μm is used as the substrate31, and it is assumed that the distance ds between an outer edge of the outmost cathode line path39and an inner edge of the ground electrode34is sufficiently large.

In the characteristic drawing illustrated inFIG. 18, “WITHOUT BACK SURFACE CUTOUT” corresponds to the case where the ground electrode34is present below the signal electrode33. “G=50 μm”, “G=100 μm”, and “WITHOUT CATHODE LINE PATH” correspond to the cases where the ground electrode34is not present below the signal electrode33. In the cases corresponding to “G=50 μm” and “G=100 μm”, the cathode line path39is present. In the case corresponding to “WITHOUT CATHODE LINE PATH”, the cathode line path39is not present.

It is apparent fromFIG. 18that the characteristic impedance of transmission line surpasses 100Ω when the cathode line path39is not present whereas the characteristic impedance of transmission line becomes lower when the cathode line path39is present by the side of the signal electrode33. The lowering of the characteristic impedance of transmission line occurs because an electromagnetic field of the signal electrode33is being pulled by the cathode line path39. Furthermore, the characteristic impedance of transmission line becomes lower as the gap G between the signal electrode33and the cathode line path39decreases. This is because the cathode line path39pulls the electromagnetic field of the signal electrode33more strongly.

According to the optical module illustrated inFIG. 16, the characteristic impedance of transmission line may be controlled by providing the cathode line path39by the side of the signal electrode33and adjusting the gap G between the signal electrode33and the cathode line path39. Accordingly, higher flexibility in designing may be provided.

Another Example of Optical Module

FIG. 19illustrates another example of the optical module according to an embodiment.FIG. 20is a diagram illustrating a cross-section along a cutting line XX-XX ofFIG. 19.FIG. 21is a diagram illustrating an enlarged view of optical coupling part ofFIG. 20. The optical module illustrated inFIG. 19toFIG. 21is an example of a mounted optical module, in which the optical module illustrated inFIG. 10orFIG. 16may be used, for example.

As illustrated inFIG. 19toFIG. 21, the optical module includes a board41, an electrical connector42, a flexible printed board43, a light-receiving element44, a transimpedance amplifier45, a light-emitting element46, a light-emitting element driver integrated circuit (IC)47, and an optical waveguide48.

Part of the optical module including the light-receiving element44and the transimpedance amplifier45may be, for example, an optical module similar to the one illustrated inFIG. 10. InFIG. 19andFIG. 20, signal electrodes that connect the light-receiving element44and the transimpedance amplifier45are omitted.

Alternatively, the part of the optical module including the light-receiving element44and the transimpedance amplifier45may be, for example, an optical module similar to the one illustrated inFIG. 16. In that case, cathode line paths and signal electrodes that connect the light-receiving element44and the transimpedance amplifier45are omitted fromFIG. 19andFIG. 20.

The light-receiving element44is mounted on an upper surface of the flexible printed board43through electrodes50in such a way that a light-receiving plane49faces down. The transimpedance amplifier45is mounted on the upper surface of the flexible printed board43.

The light-emitting element46is mounted on the upper surface of the flexible printed board43so as that light may be emitted downward. An example of the light-emitting element46may be, for example, a laser diode. The light-emitting element driver IC47is mounted on the upper surface of the flexible printed board43. The light-emitting element driver IC47outputs a signal to drive the light-emitting element46. InFIG. 19, a signal electrode that connects the light-emitting element driver IC47and the light-emitting element46is omitted.

The board41may be, for example, a printed board to be built in a computer such as, for example, a server apparatus or a peripheral apparatus to be connected to a computer. The electrical connector42is attached to the board41and connected to an end of the flexible printed board43.

The optical waveguide48is attached to a ground electrode51with an adhesive sheet in between them. Here, the adhesive sheet is not illustrated in the drawing. The ground electrode51is formed on a back surface of the flexible printed board43. The optical waveguide48includes a core52and a cladding53. An optical signal travels inside the core52while being reflected at a boundary between the core52and the cladding53. A mirror surface54is formed at an end of the optical waveguide48. By reflecting light at the mirror surface54, the travel direction of light is turned, for example, 90 degrees.

The optical signal traveling inside the core52of the optical waveguide48is reflected at the mirror surface54, penetrates through the flexible printed board43, and enters the light-receiving plane49of the light-receiving element44. An optical signal emitted from the light-emitting element46penetrates through the flexible printed board43, reflects at the mirror surface54of the optical waveguide48, and travels onward through the core52of the optical waveguide48. Parts of the flexible printed board43corresponding to the light-receiving element44and the light-emitting element46may be, for example, made transparent or formed so as to have through-holes to allow light to pass through.

According to the optical module illustrated inFIG. 19, the transmitting and receiving of optical signal may be performed since the light-receiving element44and the light-emitting element46are included. Accordingly, the optical signal may be transmitted and received between server apparatuses or between a server apparatus and a peripheral apparatus, in each of which the optical module illustrated inFIG. 16is included.

Another Example of Optical Module

FIG. 22illustrates another example of the optical module according to an embodiment.FIG. 23is a diagram illustrating a cross-section along a cutting line XXIII-XXIII ofFIG. 22. In the optical module illustrated inFIG. 22andFIG. 23, the optical waveguide48extends along the back surface of the substrate31in the same direction as the signal electrode33from a position directly below the photodiode array32, passing through the part36where the ground electrode34is not present below the signal electrode33, reaches a transimpedance amplifier35side.

In the optical module illustrated inFIG. 22, the optical waveguide48is arranged below the signal electrode33provided between the photodiode array32and the transimpedance amplifier35. In this case, as is the case with the optical module illustrated inFIG. 10, the characteristic impedance of transmission line may be made higher than the input impedance of the transimpedance amplifier35. Accordingly, an inexpensive optical module may be provided.

The inventors of the present disclosure performed simulation on the optical module illustrated inFIG. 22. As a result, it is found that the waveguide loss of the optical waveguide48degrades in that optical module. The following may be a reason of the degradation in waveguide loss of the optical waveguide48.

In the optical module illustrated inFIG. 22, the part36where the ground electrode34is not present below the signal electrode33may form an air gap layer or may be filled with a dielectric material. Accordingly, the part36where the ground electrode34is not present below the signal electrode33is softer than the surrounding ground electrode34. Thus, when the optical waveguide48is attached to the flexible printed board by using a manufacturing method that includes press processing such as, for example, lamination, the optical waveguide48may sometimes bend at the part36where the ground electrode34is not present below the signal electrode33as illustrated inFIG. 23.

It is known that the optical waveguide loss degrades when part of the optical waveguide has a radius of curvature. For example, in typical polymer optical waveguides, the loss rapidly degrades when the radius of curvature decreases to about 2 mm or less.

In view of the above, the inventors of the present disclosure analyzed a deformation amount of the optical waveguide48with using a model illustrated inFIG. 24.FIG. 24is a diagram illustrating the model used in analyzing the deformation amount of optical waveguide, and illustrates a cross-section along the cutting line XXIII-XXIII ofFIG. 22. As illustrated inFIG. 24, it is assumed that a gap between the photodiode array32and the transimpedance amplifier35is 1000 μm, the length of the part36where the ground electrode34is not present below the signal electrode33is LGAPμm, and a combined thickness of the ground electrode34and an adhesive layer55is “d” μm. The adhesive layer55may be an adhesive sheet or the like. In the analysis, the maximum radius of curvature that the optical waveguide48may have is calculated.FIG. 25illustrates a calculation result of the maximum radius of curvature that the optical waveguide48may have.

As calculation conditions, it is assumed that stress6at the time of attaching the optical waveguide48is 1 kgf/mm, Young's modulus E of the optical waveguide48is 308 kgf/mm2, a total thickness of the optical waveguide48is 100 μm, and “d” is 60 μm. With regard to the total thickness of the optical waveguide48and the value of “d”, the values are selected for ease of manufacturing. Furthermore, it is also assumed that the optical waveguide48is a beam with both ends fixed, and that the optical waveguide48is fixed to the adhesive sheet in a state where the deformation of the optical waveguide48is maximized. Under these assumptions, a one-dimensional deformation simulation of the optical waveguide48is performed.

When Y μm is the maximum displacement amount of the optical waveguide48, Y may be approximated by the following equation (1), where 1 mm4is the second moment of area. In this analysis, the calculation is carried out with a less preferable condition. That is, an optical waveguide sheet having a thickness of 0.1 mm and a width of 0.3 mm is assumed to be used.

The maximum radius of curvature R mm that the optical waveguide48may have is expressed by the following equations (2), (3), and (4).

FIG. 25is a diagram illustrating the calculation result for the maximum radius of curvature that the optical waveguide may have. In the characteristic diagram illustrated inFIG. 25, the vertical axis represents the maximum radius of curvature R of the optical waveguide, and the horizontal axis represents the length LGAPof the part where no ground electrode is present below the signal electrode. As illustrated inFIG. 25, when LGAPis larger than 1400 μm, the deformation of the optical waveguide48stops at the value of the combined thickness “d” of the ground electrode34and the adhesive layer55. From that point, the maximum radius of curvature of the optical waveguide48continues to increase.

However, when LGAPsurpasses 1900 μm, bump portions of the photodiode array32and the transimpedance amplifier35overlap with the part36where the ground electrode34is not present below the signal electrode33. Thus, in such a case, flip-chip mounting of the photodiode array32or the transimpedance amplifier35on the flexible printed board does not have to be performed.

On the other hand, when LGAPis equal to 200 μm or less, the deformation amount of the optical waveguide48decreases. Thus, the maximum radius of curvature that the optical waveguide48may have increases. For example, in polymer optical waveguides, it is known that no excessive loss occurs when the bending radius with respect to a stacking direction of the core52and the cladding53is equal to 2 mm or larger. Accordingly, it is apparent fromFIG. 25that when LGAPis 200 μm or less, the bending radius of the optical waveguide48becomes 2 mm or larger, and no excessive loss may occur.

Another Example of Optical Module

FIG. 26illustrates another example of the optical module according to an embodiment.FIG. 27is a diagram illustrating a cross-section along a cutting line XXVII-XXVII ofFIG. 26. The optical module illustrated inFIG. 26andFIG. 27is capable of reducing the degradation in waveguide loss of the optical waveguide48in view of the foregoing findings obtained from the analysis of the deformation amount of the optical waveguide48.

As illustrated inFIG. 26andFIG. 27, the part36where the ground electrode34is not present below the signal electrode33is divided into a plurality of divisions. Each division of the part36where the ground electrode34is not present below the signal electrodes33may form an air gap layer or may be filled with a dielectric material. According to the analysis result of the deformation amount of the optical waveguide48, it is desirable that each division of the part36where the ground electrode34is not present below the signal electrodes33has a length of 200 μm or less.

InFIG. 27, hatching is added to the ground electrode34for easy understanding of the drawing. In an example illustrated inFIG. 26andFIG. 27, nine divisions of the part36where the ground electrode34is not present below the signal electrodes33are illustrated for each channel. Alternatively, the part36may include eight divisions or less, or ten divisions or more for each channel. Furthermore, the part36where the ground electrode34is not present below the signal electrode33does not have to be divided into the same number of divisions for all the channels.

According to the optical module illustrated inFIG. 26, the radius of curvature that the optical waveguide may have increases since the part36where the ground electrode34is not present below the signal electrode33is finely divided into a plurality of divisions. Accordingly, the degradation in waveguide loss of the optical waveguide48may be reduced. Furthermore, in each channel, the plurality of divisions are provided in the part36where the ground electrode34is not present below the signal electrode33. Thus, similar inductivity may be provided as is the case with the part36where the ground electrode34is not present below the signal electrode33is not divided. Accordingly, the frequency characteristic of transmission characteristic for the transmission line may be improved. A shape of each division of the part36where the ground electrode34is not present below the signal electrode33does not have to be limited to rectangular.

Another Example of Optical Module

FIG. 28illustrates another example of the optical module according to an embodiment. In the optical module illustrated inFIG. 28, an anode line of each photodiode of the photodiode array32is connected to the signal electrode33, a cathode line is connected to the cathode line paths39, and the part36where the ground electrode34is not present below the signal electrode33is divided into a plurality of divisions. The cathode line path39is formed along the signal electrode33but separately from the signal electrode33.

InFIG. 28, the signal electrode33and the cathode line paths39are illustrated only for one channel. However, the same applies to the other channels as well. The optical waveguide that is not illustrated in the drawing may extend in the same direction as the signal electrode33from a position directly below the photodiode array32, or may extend in a direction opposite to the signal electrode33.

In an example of the optical module illustrated inFIG. 28, it is assumed that the length of the transmission line between the photodiode array32and the transimpedance amplifier35, namely the length of the signal electrode33and the cathode line path39is 1 mm. Furthermore, it is assumed that each division of the part36where the ground electrode34is not present below the signal electrode33has a length of 0.05 mm. The divisions of the part36where the ground electrode34is not present below the signal electrode33are placed at 0.05 mm interval in a 0.8 mm long block arranged in a center portion between the photodiode array32and the transimpedance amplifier35. Furthermore, in this example, it is assumed that the photodiode array32and the transimpedance amplifier35are mounted on a substrate that is not illustrated in the drawing, and this substrate has a thickness of 25 μm. The line width of the signal electrode33is 40 μm, and the distance between the cathode line path39and the signal electrode33is 50 μm.

FIG. 29is a diagram illustrating simulation results of the frequency characteristic of the incident wave transmission coefficient S21 for the optical module illustrated inFIG. 28. The vertical axis represents the incident wave transmission coefficient S21, and the horizontal axis represents the frequency. In the optical module illustrated inFIG. 28, the characteristic impedance Z0 of transmission line is higher than the input impedance of the transimpedance amplifier35because of providing the part36where the ground electrode34is not present below the signal electrode33. In contrast, Z0 becomes equal to Zin when there is no part36where the ground electrode34is not present below the signal electrode33.

It is apparent fromFIG. 29that, judging from the effect in band expanding at the frequency at which the incident wave transmission coefficient S21 is reduced by 3 dB, the frequency characteristic is improved when Z0>Zin in comparison with the case when Z0=Zin. In other words, the frequency characteristic of transmission characteristic for the transmission line may also be improved by finely dividing the part36where the ground electrode34is not present below the signal electrode33. Thus, according to the optical module illustrated inFIG. 28, the degradation of optical waveguide loss may be reduced and the band may be widened since the part36where the ground electrode34is not present below the signal electrode33is finely divided.

Another Example of Optical Module

FIG. 30illustrates another example of the optical module according to an embodiment.FIG. 31is a diagram illustrating a cross-section along a cutting line XXXI-XXXI ofFIG. 30. The optical module illustrated inFIG. 30andFIG. 31is an example of a mounted optical module, in which the optical module illustrated inFIG. 22,FIG. 26, orFIG. 28may be used, for example.

As illustrated inFIG. 30andFIG. 31, the optical waveguide48extends along the back surface of the flexible printed board43from a position directly below the light-receiving element44to a transimpedance amplifier45side and beyond. The light-receiving element44and the light-emitting element46may be arranged side by side in a direction that crosses an extending direction of the optical waveguide48at 90 degrees, for example. Furthermore, the light-emitting element driver IC47and the transimpedance amplifier45may be arranged on opposing sides of a line along which the light-emitting element46and the light-receiving element44are arranged side by side.

Part of the optical module including the light-receiving element44and the transimpedance amplifier45may be, for example, an optical module similar to the one illustrated inFIG. 22orFIG. 26. InFIG. 30andFIG. 31, signal electrodes that connect the light-receiving element44and the transimpedance amplifier45are omitted.

Alternatively, part of the optical module including the light-receiving element44and the transimpedance amplifier45may be, for example, an optical module similar to the one illustrated inFIG. 28. In that case, cathode line paths and signal electrodes that connect the light-receiver element44and the transimpedance amplifier45are omitted fromFIG. 30andFIG. 31.

According to the optical module illustrated inFIG. 30, cross-talks between transmission and reception may be reduced by separating the light-emitting element driver IC47and the transimpedance amplifier45. Furthermore, compared with the other case where the light-emitting element46and the light-receiving element44are arranged side by side, the distance between the light-emitting element driver IC47and the transimpedance amplifier45may be reduced in the extending direction of the optical waveguide48. This arrangement may further reduce the size of optical module.

Manufacturing Method of Optical Module

FIG. 32toFIG. 34are diagrams illustrating an exemplary manufacturing method of the optical module according to an embodiment. As illustrated inFIG. 32, a substrate is prepared. In the substrate, a copper foil is attached on both sides of a dielectric layer composed of, for example, polyimide or the like. Next, a pattern of transmission line such as the signal electrodes33and the cathode line paths and a pattern of the ground electrode34are formed by, for example, photolithographic resist transfer and copper foil etching applied on the both sides of the substrate.

Next, as illustrated inFIG. 33, bumps for flip-chip mounting are formed on each of the photodiode array32and the transimpedance amplifier35. Subsequently, devices such as the photodiode array32, the transimpedance amplifier35, and the like are mounted on the substrate31by using a flip-chip mounting technique such as a technique utilizing ultrasonic waves, a C4 technique, or the like. After the mounting of each device, underfill or sidefill is provided under each device. InFIG. 33, the electrodes50are junction portions formed of the bumps for flip-chip mounting.

Next, as illustrated inFIG. 34, the optical waveguide48is attached to the back surface of the substrate31with the adhesive layer55in between. The adhesive layer55may be an adhesive sheet or an adhesive agent. When the adhesive sheet is used, part of the adhesive sheet may be cut in advance so as to match a shape of the ground electrode34, or part of the adhesive sheet does not have to be cut in advance.

According to the foregoing steps, an optical module such as is illustrated in, for example,FIG. 10,FIG. 16,FIG. 22,FIG. 26, orFIG. 28may be manufactured. Furthermore, an optical module such as is illustrated in, for example,FIG. 19orFIG. 30may be manufactured by connecting an optical module such as is illustrated inFIG. 10,FIG. 16,FIG. 22,FIG. 26, orFIG. 28to an electrical connector of a board in a server apparatus or the like.