Polarization combiner and optical modulation device

A polarization combiner includes: a base member that includes a body portion, an arm portion extending from the body portion, and a notch portion surrounded with the body portion and the arm portion; a polarization rotating element that is fixed to the arm portion of the base member and that rotates a polarization direction of a first polarized wave; and a polarization combining element that is fixed to the base member so as to face the notch portion of the base member and the polarization rotating element, the polarization combining element combining two polarized waves entering from a surface facing the notch portion and the polarization rotating element, the two polarized waves including the first polarized wave whose polarization direction is rotated by the polarization rotating element and a second polarized wave passing the notch portion.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-044604, filed on Mar. 7, 2014, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a polarization combiner and an optical modulation device.

BACKGROUND

Generally, optical transmission systems may use optical modulators which perform optical modulation by using, for example, a dual polarization differential quadrature phase shift keying (DP-QPSK) scheme. In the DP-QPSK scheme, a light beam input to the optical modulator is split into two light beams. After an electrical signal is superimposed on each of these two light beams, the two light beams are combined.

To superimpose electrical signals on two split light beams, ferroelectric crystal such as lithium niobate (LiNbO3) may be used. When ferroelectric crystal is used, the electrical signals are superimposed on the light beams in waveguides inside the crystal. In order to do so, crystal having a prescribed size needs to be placed. This poses a certain limit to downsizing of the optical modulators. Accordingly, the optical modulators formed by using a semiconductor chip have been examined in recent years to implement downsized and high-efficiency optical modulators.

To combine two light beams, a polarization coupler including a polarization rotating element and a polarization combining element may be used. The polarization coupler rotates the polarization direction of one light beam, out of two light beams which travel side by side, by using a polarization rotating element such as a wave plate, and combines these two light beams whose polarization directions are perpendicular to each other by using a polarization combining element such as a polarization beam combiner (PBC) prism. Specifically, when a light beam31passes through a wave plate20, the polarization direction of the light beam31becomes perpendicular to the polarization direction of a light beam32as illustrated inFIG. 11for example. Since a PBC prism10has a polarized light separating film provided to reflect the polarized light beam31and to transmit the polarized light beam32, the light beam31is reflected on the polarized light separating film and is combined with the light beam32which passes through the polarized light separating film.

The aforementioned polarization coupler is generally configured so that the wave plate20is bonded to the PBC prism10as illustrated inFIG. 11. More specifically, the polarization rotating element is fixed to the polarization combining element with fixatives such as adhesives. In this case, the fixatives applied to a bonding surface between the polarization rotating element and the polarization combining element may overflow to the periphery of the bonding surface, and this may lead to formation of a region called a fillet.

The fillet formed of the fixatives hinders passage of light beams. Accordingly, when two light beams are combined in the aforementioned polarization coupler, incident positions of these two light beams are adjusted so that the light beams pass along the routes that circumvent the fillet. Specifically, in the configuration of the polarization coupler illustrated inFIG. 11for example, the light beam32is made incident on the PBC prism10at a position away from the periphery of the bonding surface between the PBC prism10and the wave plate20. Since the incident position of the light beam32is adjusted in this way, the fillet formed around the bonding surface between the PBC prism10and the wave plate20does not hinder passage of the light beam32.

However, in the case of inputting two light beams into such a polarization coupler, it is difficult to reduce a distance between two light beams to a fixed value or less. More specifically, for example inFIG. 11, the light beam32is made incident on the PBC prism10at a position away from the periphery of the bonding surface between the PBC prism10and the wave plate20. Accordingly, a fixed interval is provided between the light beam31and the light beam32. As a result, when the above-described polarization coupler is applied to, for example, optical modulators, two light beams with electrical signals superimposed thereon are distanced from each other. This makes it difficult to achieve sufficient downsizing of the optical modulators. This also applies to devices other than the optical modulators. In the devices including the aforementioned polarization coupler to combine two light beams, these two light beams are placed at a certain interval. As a result, downsizing is disadvantageously limited.

SUMMARY

According to an aspect of an embodiment, a polarization combiner includes: a base member that includes a body portion, an arm portion extending from the body portion, and a notch portion surrounded with the body portion and the arm portion; a polarization rotating element that is fixed to the arm portion of the base member and that rotates a polarization direction of a first polarized wave; and a polarization combining element that is fixed to the base member so as to face the notch portion of the base member and the polarization rotating element, the polarization combining element combining two polarized waves entering from a surface facing the notch portion and the polarization rotating element, the two polarized waves including the first polarized wave whose polarization direction is rotated by the polarization rotating element and a second polarized wave passing the notch portion.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be explained with reference to accompanying drawings. Note that the embodiments are not intended to limit the present invention.

[a] First Embodiment

FIG. 1is a diagram illustrating the configuration of an optical modulator100according to the first embodiment. The optical modulator100illustrated inFIG. 1is connected to optical fibers110aand110b. The optical modulator100includes ferrules120aand120b, a lens130, an optical modulation chip140, a microlens array (hereinbelow abbreviated as MLA)150, a polarization coupler160, and a lens170.

The ferrules120aand120baccommodate ends of the optical fibers110aand110b, respectively, and fix the positions of the optical fibers110aand110b. In the optical modulator100illustrated inFIG. 1, signal light is input from the optical fiber110aand the ferrule120a, and is output from the ferrule120band the optical fiber110b.

The lens130condenses the signal light emitted from the end of the optical fiber110aaccommodated in the ferrule120a, and inputs the obtained light beam to the optical modulation chip140.

The optical modulation chip140, which is formed of a semiconducting material, splits the light beam input from the lens130into two light beams, and superimposes electrical signals on the respective light beams. The optical modulation chip140then outputs the two light beams to the polarization coupler160via the MLA150. The optical modulation chip140may output a monitoring light beam for monitoring the operation of the optical modulation chip140besides the two light beams having the electrical signals superimposed thereon.

The MLA150outputs the light beams on which the electrical signals are superimposed by the optical modulation chip140, toward the polarization coupler160. That is, the MLA150outputs two light beams which travel side by side to the polarization coupler160. The two light beams output by the MLA150have an identical polarization direction.

The polarization coupler160combines two light beams output from the MLA150, and outputs a light beam including two polarized waves whose polarization directions are perpendicular to each other. More specifically, the polarization coupler160rotates the polarization direction of one light beam output from the MLA150, and then combines this light beam with the other light beam. The polarization coupler160outputs the obtained light beam. In the present embodiment, since the fillet formed from fixatives is not present in between two light beams incident on the polarization coupler160, the two light beams can be positioned close to each other. As a result, even when the optical modulation chip140is downsized to a maximum extent, the two light beams output from the optical modulation chip140can be combined by the polarization coupler160. A specific configuration of the polarization coupler160will be described in detail later.

The lens170projects the light beam output from the polarization coupler160to the end of the optical fiber110baccommodated in the ferrule120b.

FIG. 2is a perspective view illustrating the configuration of the polarization coupler160according to the first embodiment. As illustrated inFIG. 2, the polarization coupler160includes a base member210, a wave plate220, and a PBC prism230.

The base member210is formed from, for example, a glass material, and serves as a base material to which the wave plate220and the PBC prism230are bonded. That is, the wave plate220is bonded to one surface of the base member210, while the PBC prism230is bonded to another surface. The base member210may be formed from a material whose thermal expansion coefficient is close to the thermal expansion coefficients of the materials forming the wave plate220and the PBC prism230. The base member210may be formed from, for example, a metal material as long as they fulfill this condition.

The wave plate220is a half-wave plate which is formed from, for example, a Quartz crystal and which rotates the polarization direction of one light beam, which is output from the optical modulation chip140, 90 degrees. That is, the wave plate220functions as a polarization rotating element that rotates the polarization direction of one light beam out of two light beams output from the optical modulation chip140.

The PBC prism230is formed from, for example, silica glass, and combines two light beams output from the optical modulation chip140. Specifically, a light beam incoming without passing through the wave plate220passes and travels straight through the PBC prism230, while a light beam incoming through the wave plate220is reflected inside the PBC prism230. The PBC prism230combines these light beams and outputs a combined light beam. In other words, the PBC prism230functions as a polarization combining element that combines two light beams output from the optical modulation chip140.

FIG. 3is a front view illustrating the configuration of the polarization coupler160according to the first embodiment. InFIG. 3, two light beams output from the optical modulation chip140enter the polarization coupler160from the near side to the back side.

As illustrated inFIG. 3, the base member210is substantially U-shaped. More specifically, the base member210has two arm portions212extending from a body portion211, with a notch portion213being formed between the two arm portions212. The wave plate220is bonded to an end face212aof the arm portion212. The PBC prism230illustrated with a broken line inFIG. 3is bonded to back-side surfaces of the body portion211and the arm portion212at a position facing the notch portion213and the wave plate220. More specifically, the wave plate220and the PBC prism230are each positioned when they are bonded to the base member210. Therefore, the wave plate220and the PBC prism230are not directly bonded to each other.

In such a configuration, out of two light beams output from the optical modulation chip140, one light beam passes through the wave plate220and then enters the PBC prism230. The other light beam passes through the notch portion213, and directly enters the PBC prism230. In this case, since the base member210has the notch portion213formed between two arm portions212, any portion that bonds two members is not present in between the light beam incident on the wave plate220and the light beam passing through the notch portion213. More specifically, the fillet formed by an adhesive or the like overflowing from the bonding surface is not present in between the light beam incident on the wave plate220and the light beam passing through the notch portion213. As a result, even when a distance between the light beam incident on the wave plate220and the light beam passing through the notch portion213is reduced, passage of the light beams is not hindered by the fillet. Therefore, even when the optical modulation chip140is downsized and a pitch between two light beams is reduced, these two light beams can still be combined by the polarization coupler160.

FIG. 4is a plan view illustrating the configuration of the polarization coupler160according to the first embodiment. As illustrated inFIG. 4, a light beam301output from the optical modulation chip140passes through the wave plate220and then enters the PBC prism230, while a light beam302passes through the notch portion213of the base member210and directly enters the PBC prism230.

In this case, an optically invalid region221is provided in a peripheral edge portion of the wave plate220as a region which is optically not valid due to minute cracks and the like generated during processing such as cutting and polishing. Similarly, reflection films231and232having polarization selectivity are formed on the PBC prism230, and an optically invalid region233is provided in the vicinity of the reflection film232. Accordingly, the light beam301enters the wave plate220without passing the optically invalid region221and the optically invalid region233.

In the present embodiment, the position where the PBC prism230is bonded to the base member210is adjusted, so that the optically invalid region221and the optically invalid region233overlap with each other as viewed along the traveling direction of the light beams301and302. Accordingly, a substantial width of the optically invalid region between the light beam301and the light beam302is minimized, and so the pitch between the light beam301and the light beam302can further be reduced.

The reflection films231and232of the PBC prism230are formed from, for example, a dielectric multilayer or the like. The reflection films231and232reflect a light beam of a specified polarization direction while transmitting a light beam whose polarization direction is perpendicular to the aforementioned light beam. In the present embodiment, the polarization direction of the light beam301is rotated 90 degrees by the wave plate220. Accordingly, at the time when the light beam301enters the PBC prism230, the polarization directions of the light beam301and the light beam302are perpendicular to each other. Therefore, the light beam301is reflected on the reflection films231and232, while the light beam302passes through the reflection film232. As a result, the light beams301and302whose polarization directions are perpendicular to each other are combined, and the obtained light beam is output to the lens170.

As described in the foregoing, according to the present embodiment, the wave plate and the PBC prism are bonded to the base member having the arm portions and the notch portion, so that the wave plate and the PBC prism are not directly bonded. Out of two light beams which are output from the optical modulation chip and travel side by side, one light beam passes through the wave plate bonded to the top ends of the arm portions of the base member, while the other light beam passes through the notch portion. Both the light beams then enter the PBC prism. Accordingly, any portion that bonds two members is not present in between the two light beams, and so the fillet formed with an adhesive or the like is not present either. As a result, even when the pitch between the two light beams is reduced, passage of the light beams is not hindered and thereby downsizing of the devices can be promoted.

[b] Second Embodiment

A second embodiment is characterized by the point that a phase correction plate is bonded to the vicinity of the notch portion of the base member so as to allow the phases of two light beams traveling side by side to coincide with each other.

Since an optical modulator according to the second embodiment is similar in configuration to the optical modulator100according to the first embodiment, the description thereof will be omitted. In the second embodiment, the configuration of the polarization coupler160is different from that in the first embodiment.

FIG. 5is a front view illustrating the configuration of the polarization coupler160according to the second embodiment. InFIG. 5, portions identical to those inFIG. 3are denoted by identical reference numerals and the description thereof will be omitted. InFIG. 5, two light beams output from the optical modulation chip140enter the polarization coupler160from the near side to the back side.

As illustrated inFIG. 5, in the present embodiment, a phase correction plate240is placed so as to cover the notch portion213of the base member210. That is, the phase correction plate240is bonded to the periphery of the notch portion213of the base member210. The light beam that passes the wave plate220receives a phase delay. The phase correction plate240imparts a phase delay identical to the above-stated phase delay to a light beam that passes through the phase correction plate240. However, the phase correction plate240does not change the polarization direction of the light beam that passes through the phase correction plate240.

In such a configuration, out of two light beams output from the optical modulation chip140, one light beam passes through the wave plate220and then enters the PBC prism230. The other light beam passes through the phase correction plate240and the notch portion213, and then enters the PBC prism230. In this case, the phase correction plate240is bonded to three sides of the base member210while surrounding the notch portion213. However, any portion that bonds the members is not present in between the light beam incident on the wave plate220and the light beam incident on the phase correction plate240. More specifically, the fillet formed by an adhesive or the like overflowing from the bonding surface is not present in between the light beam incident on the wave plate220and the light beam incident on the phase correction plate240. As a result, even when a distance between the light beam incident on the wave plate220and the light beam incident on the phase correction plate240is reduced, the passage of the light beams is not hindered by the fillet. Therefore, even when the optical modulation chip140is downsized and the pitch between two light beams is reduced, these two light beams can still be combined by the polarization coupler160.

FIG. 6is a plan view illustrating the configuration of the polarization coupler160according to the second embodiment. InFIG. 6, portions identical to those inFIG. 4are denoted by identical reference numerals and the description thereof will be omitted. As illustrated inFIG. 6, a light beam301output from the optical modulation chip140passes through the wave plate220and then enters the PBC prism230, while a light beam302passes through the phase correction plate240and the notch portion213of the base member210and then enters the PBC prism230.

In this case, an optically invalid region241similar to those in the wave plate220and the PBC prism230is provided in a peripheral edge portion of the phase correction plate240. Accordingly, the light beams301and302respectively enter the wave plate220and the phase correction plate240without passing through the optically invalid regions221,233, and241.

In the present embodiment, the positions where the PBC prism230and the phase correction plate240are bonded to the base member210are adjusted, so that the optically invalid regions221,233, and241overlap with each other as viewed along the traveling direction of the light beams301and302. Accordingly, a substantial width of the optically invalid region in between the light beam301and the light beam302is minimized, and so the pitch between the light beam301and the light beam302can further be reduced.

In the present embodiment, when the light beam301passes through the wave plate220, the polarization direction of the light beam301is rotated 90 degrees and also a specified phase delay occurs in the light beam301. However, a phase delay identical to this phase delay is also imparted to the light beam302by the phase correction plate240. Accordingly, at the time when the light beams301and302enter the PBC prism230, their polarization directions are perpendicular to each other but their phases are allowed to coincide with each other.

According to the present embodiment as described in the foregoing, out of two light beams which are output from the optical modulation chip and travel side by side, one light beam passes through the wave plate, and thereby a phase delay is imparted thereto. In this case, an identical phase delay is also imparted to the other light beam by the phase correction plate. Accordingly, the phases of two light beams incident on the PBC prism can be allowed to coincide with each other.

Although the phase correction plate240is bonded to the periphery of the notch portion213in the present embodiment, the phase correction plate240may be contained in the notch portion213. In this case, respective sides of the phase correction plate240are bonded to the inner surfaces of the body portion211and the arm portions212which form the notch portion213.

In the present embodiment, the phase correction plate240is bonded to the periphery of the notch portion213. However, an optical element having a function other than the phase correction function may be bonded to the periphery of the notch portion213. For example, a wave plate for rotating the polarization direction of light beams, like the wave plate220, may be bonded to the periphery of the notch portion213. In short, when an optical element that optically changes a light beam is bonded to the periphery of the notch portion213, the light beam that passes through the notch portion213may optically be changed. If the wave plate is bonded to the periphery of the notch portion213, the wave plate and the wave plate220may rotate the polarization directions of light beams 45 degrees in directions opposite to each other for example. This makes it possible to set the polarization directions of these two light beams perpendicular to each other and to allow their phase delays to coincide with each other.

A third embodiment is characterized by the point that the arm portion of the base member is further extended to widen the bonding surface between the base member and the wave plate, so that the wave plate is reliably fixed.

Since an optical modulator according to the third embodiment is similar in configuration to the optical modulator100according to the first embodiment, the description thereof will be omitted. In the third embodiment, the configuration of the polarization coupler160is different from that in the first embodiment.

FIG. 7is a front view illustrating the configuration of the polarization coupler160according to the third embodiment. InFIG. 7, portions identical to those inFIG. 3are denoted by identical reference numerals and the description thereof will be omitted. InFIG. 7, two light beams output from the optical modulation chip140enter the polarization coupler160from the near side to the back side.

As illustrated inFIG. 7, support arm portions214further extend from the top ends of the arm portions212of the base member210in the present embodiment. Both ends of the wave plate220are bonded to surfaces of the two support arm portions214which are vertical to a thickness direction of the support arm portions214.

According to such a configuration, as in the first embodiment, even when a distance between the light beam incident on the wave plate220and the light beam passing through the notch portion213is reduced, the passage of the light beams is not hindered by the fillet. Therefore, even when the optical modulation chip140is downsized and the pitch between two light beams is reduced, these two light beams can still be combined by the polarization coupler160. Since the wave plate220is bonded to relatively wide surfaces of the support arm portions214, the wave plate220can firmly be fixed.

FIG. 8is a plan view illustrating the configuration of the polarization coupler160according to the third embodiment. As illustrated inFIG. 8, a light beam301output from the optical modulation chip140passes through the wave plate220and then enters the PBC prism230, while a light beam302passes through the notch portion213of the base member210and directly enters the PBC prism230.

The support arm portions214of the base member210are thinner than the body portion211and the arm portions212. Accordingly, even when the wave plate220is bonded to the support arm portions214, the combined thickness of the wave plate220and the support arm portion214does not surpass the thickness of the body portion211and the arm portion212. Therefore, the wave plate220does not project from the surface of the base member210.

In the present embodiment, after passing through the wave plate220, the light beam301enters the PBC prism230through between the two support arm portions214. The light beam302passes through the notch portion213between the two arm portions212and enters the PBC prism230. The light beam301is then reflected on the reflection films231and232, while the light beam302passes through the reflection film232. As a result, the light beams301and302whose polarization directions are perpendicular to each other are combined, and the obtained light beam is output to the lens170.

As described in the foregoing, according to the present embodiment, the support arm portions further extend from the arm portions of the base member, and the wave plate is bonded to the surfaces of the support arm portions which are vertical to the thickness direction of the support arm portions. Accordingly, a bonding surface where the wave plate and the base member are bonded can be widened, so that the wave plate can firmly be fixed.

A fourth embodiment is characterized by the point that molding of the base member is facilitated by forming the base member into a substantially L-shape.

Since an optical modulator according to the fourth embodiment is similar in configuration to the optical modulator100according to the first embodiment, the description thereof will be omitted. In the fourth embodiment, the configuration of the polarization coupler160is different from that in the first embodiment.

FIG. 9is a front view illustrating the configuration of the polarization coupler160according to the fourth embodiment. InFIG. 9, portions identical to those inFIG. 3are denoted by identical reference numerals and the description thereof will be omitted. InFIG. 9, two light beams output from the optical modulation chip140enter the polarization coupler160from the near side to the back side.

As illustrated inFIG. 9, in the present embodiment, the base member210includes a body portion211and one arm portion215, and they are substantially L-shaped as a whole. The wave plate220is placed in a notch portion216surrounded with the body portion211and the arm portion215. More specifically, one side of the wave plate220is bonded to an inner surface215aof the arm portion215.

According to such a configuration, as in the first embodiment, even when a distance between the light beam incident on the wave plate220and the light beam passing through the notch portion216is reduced, the passage of the light beams is not hindered by the fillet. Therefore, even when the optical modulation chip140is downsized and the pitch between two light beams is reduced, these two light beams can still be combined by the polarization coupler160. Since the base member210has a substantially L-shaped simple configuration, molding of the base member210can be facilitated.

FIG. 10is a plan view illustrating the configuration of the polarization coupler160according to the fourth embodiment. As illustrated inFIG. 10, a light beam301output from the optical modulation chip140passes through the wave plate220and then enters the PBC prism230, while a light beam302passes through the notch portion216of the base member210and directly enters the PBC prism230.

Although one side of the wave plate220is bonded to the arm portion215of the base member210, the wave plate220is provided at a position distanced from the body portion211of the base member210. Accordingly, a region through which the light beam302passes is formed between the wave plate220in the notch portion216and the body portion211.

In the present embodiment, the light beam301passes through the wave plate220and then enters the PBC prism230. The light beam302passes through a region between the wave plate220in the notch portion216and the body portions211, and then enters the PBC prism230. The light beam301is then reflected on the reflection films231and232, while the light beam302passes through the reflection film232. As a result, the light beams301and302whose polarization directions are perpendicular to each other are combined, and the obtained light beam is output to the lens170.

As described in the foregoing, according to the present embodiment, the base member is formed into a substantially L shape, and the wave plate is bonded to the arm portion of the base member at a position distanced from the body portion. As a result, molding of the base member can be facilitated, and two light beams with a small pitch can be combined with a simple configuration, so that downsizing of the device can be promoted.

In the disclosed respective embodiments, the polarization couplers160provided in the optical modulator100have been described. However, the polarization couplers160in the respective embodiments may be applied to various optical modules different from the optical modulator. That is, the polarization couplers160in the aforementioned respective embodiments may be used for optical modules which are configured to combine two light beams or separate one light beam.

In the aforementioned respective embodiments, the PBC prism230is used as a polarization combining element that combines polarized waves. However, the present invention is not limited thereto. As a polarization combining element, birefringent crystal or the like may also be used.

According to one aspect of the polarization combiner and the optical modulation device disclosed by the present application, the effect of being able to promote downsizing of the devices is implemented.