Optical modulator module

An optical modulator module includes: a semiconductor modulator that includes a plurality of output waveguides; a first cylindrical lens that has a longitudinal direction in a direction in which the plurality of output waveguides are aligned, and through which lights output from the plurality of output waveguides penetrate; and a plurality of second cylindrical lenses each having a longitudinal direction that intersects with the longitudinal direction of the first cylindrical lens and allowing a corresponding light of the lights output from the plurality of output waveguides to penetrate therethrough.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-106703, filed on May 26, 2015, the entire contents of which are incorporated herein by reference.

FIELD

A certain aspect of embodiments described herein relates to an optical modulator module.

BACKGROUND

LiNbO3external modulators (hereinafter, referred to as an LN modulator) are widely used in the high-speed optical communication system (see Japanese Patent Application Publication No. 2010-156842, for example). However, the material property of the LN modulator makes the reduction in size of the LN modulator difficult. Thus, semiconductor modulators have been developed as a small external modulator.

SUMMARY

According to an aspect of the present invention, there is provided an optical modulator module including: a semiconductor modulator that includes a plurality of output waveguides; a first cylindrical lens that has a longitudinal direction in a direction in which the plurality of output waveguides are aligned, and through which lights output from the plurality of output waveguides penetrate; and a plurality of second cylindrical lenses each having a longitudinal direction that intersects with the longitudinal direction of the first cylindrical lens and allowing a corresponding light of the lights output from the plurality of output waveguides to penetrate therethrough.

DESCRIPTION OF EMBODIMENTS

As previously described, semiconductor modulators have been developed as a small external modulator. However, since a light output from the semiconductor modulator has an elliptical shape, optical coupling loss may be caused by a mode mismatch when the semiconductor modulator is optically coupled to an optical fiber.

Hereinafter, embodiments will be described with reference to accompanying drawings.

First Embodiment

FIG. 1Ais a schematic diagram of an optical modulator module100in accordance with a first embodiment. As illustrated inFIG. 1A, the optical modulator module100has a structure designed so that an input coupling optical system10, a semiconductor modulator20, a polarization-rotating coupling optical system30, an output coupling optical system40, and monitoring photodetectors50aand50bare arranged in a package60. The optical modulator module100is coupled to an input fiber200and an output fiber300.

FIG. 1Bis an enlarged view of the semiconductor modulator20. As illustrated inFIG. 1B, the semiconductor modulator20includes one input waveguide21, a 3 dB coupler22that splits a light entering from the input waveguide21, IQ modulators23aand23beach modulating the corresponding split light, and output waveguides24athrough24dthat output signal lights and monitoring lights corresponding to the signal lights from the IQ modulators23aand23b. For example, the output waveguides24band24care waveguides through which a signal light propagates, and the output waveguides24aand24dare waveguides through which a monitor light propagates.

The input coupling optical system10causes a light from the input fiber200to enter the input waveguide21. The polarization-rotating coupling optical system30polarization-rotates one of two identical linearly polarized signal lights output from the output waveguides24band24c, and then polarization-multiplexes them. The output coupling optical system40couples the polarization-multiplexed signal light to the output fiber300. Each of the monitoring photodetectors50aand50breceives the corresponding one of two monitor lights output from the output waveguides24aand24d.

The output waveguides24athrough24dconfine lights with a semiconductor such as InP. In this structure, the light is strongly confined. Thus, the mode field of the waveguided light is very small, approximately submicron to several micrometers. The spread angle θ of a light emitted to the air from the semiconductor waveguide is expressed by the following equation (1) where λ represents a wavelength and ω represents the radius of the 1/e2diameter of the mode field of a waveguided light. Thus, the output light from the semiconductor waveguide has a large spread angle. For example, θ is 28 degrees when ω is 1 μm and λ is 1550 nm.
θ=λ/(πω)  (1)

On the other hand, in the LN modulator used as an external modulator, the mode field of the LN waveguide is close to the mode field of a fiber. A typical fiber has a mode field diameter of 10 μm and a spread angle of 5.7 degrees. The mode field of the LN waveguide has a shape relatively close to a circle. Thus, the LN waveguide is easily coupled to an optical fiber having a circular mode field.

In contrast, the light output from the semiconductor waveguide typically has different mode fields in the width direction (the x direction) and the thickness direction (the y direction) of the waveguide. For example, the mode field diameter in the thickness direction (the y direction) is less than the mode field diameter in the width direction (the x direction) in the output waveguides24athrough24d. In this case, as illustrated inFIG. 2, the spread angle of the emitted light in the y direction is greater than that in the x direction. In this case, the emitted light has an elliptical shape of which the major axis corresponds to the y direction. Thus, when the semiconductor waveguide is coupled to an optical fiber having a circular mode field, optical coupling loss is caused by the mode mismatch. Therefore, the emitted lights from the output waveguides24athrough24dare preferably shaped into a circle.

For example, the mode field in the x direction may be made smaller and circular by narrowing, for example, ridging, the widths of the outputting end faces of the output waveguides24athrough24dto enhance the confinement of the light in the x direction. However, in this case, a slight manufacturing error greatly changes the mode field. Thus, the control of the mode field is difficult. Accordingly, as illustrated inFIG. 3AandFIG. 3B, a beam shaping optical system in which a spot size conversion system combining anamorphic prisms is implemented may be considered.

FIG. 3AandFIG. 3Billustrate a beam shaping system between the semiconductor modulator20and the output fiber300.FIG. 3Ais a top view of the beam shaping system, andFIG. 3Bis a side view of the beam shaping system. As illustrated inFIG. 3AandFIG. 3B, spherical lenses61aand61b, an anamorphic prism62, and an anamorphic prism63are arranged between the output waveguides24band24cand the polarization-rotating coupling optical system30. Furthermore, a condenser lens64is arranged between the polarization-rotating coupling optical system30and the output fiber300. However, this structure increases the number of components. In addition, this structure makes the adjustment of the optical axes of optical devices more complicated. Therefore, the cost increases.

A light is also shaped into a circle by kicking a part of the end of the light having an elliptical shape by a circular aperture. However, the aspect ratio of the light of the semiconductor waveguide becomes approximately two to three times. Thus, the kicking of the light reduces the power, resulting in the increase in coupling loss to the light fiber eventually. The use of a micro lens array (MLA) with an aspherical shape (e.g., an elliptical shape) enables to inhibit the light from being kicked and shape the beam. However, complex shapes are difficult to manufacture in micro lens arrays fabricated by photolithography process, such as silicon micro lens arrays.

When two or more semiconductor waveguides are arranged in an array as the semiconductor modulator20, the manufacturing error in the positions of the centers of the lenses in the MLA causes the optical axis deviations between the semiconductor waveguide and each lens of the MLA. This optical axis deviations cause the optical axis deviation in the position of the optical fiber and greatly affect the optical coupling efficiency and PDL (polarization-dependent loss). In the optical system that couples a semiconductor waveguide to an optical fiber, when the mode field diameter oil of the semiconductor waveguide and the mode field diameter ω2 of the optical fiber meet the condition of magnification m=ω2/ω1, the optical axis deviation at the semiconductor waveguide side increases by m times. For example, in the case of ω1=2 μm and ω2=5 μm, when the optical axis deviates by 1 μm at the semiconductor waveguide side, the optical axis deviates by 2.5 μm at the optical fiber side (at this time, the coupling efficiency decreases by approximately 2.2 dB). The variation in angles can be corrected by adding an optical element. However, the addition of the optical element increases the number of components and makes the adjustment of the optical axes more complicated, leading to the increase in costs.

Thus, to form a collimated light and couple the collimated light to an optical fiber with high efficiency without increasing the number of components, the present embodiment uses two cylindrical lens arrays having shapes relatively easily manufactured from a high refractive index material and of which the longitudinal directions intersect each other.

FIG. 4AandFIG. 4Billustrate a beam shaping system in accordance with the present embodiment.FIG. 4Ais a top view of the beam shaping system, andFIG. 4Bis a side view of the beam shaping system. As illustrated inFIG. 4AandFIG. 4B, an MLA70and an MLA80are arranged between the output waveguides24band24cand the polarization-rotating coupling optical system30in this order from the semiconductor modulator20side in the light propagation direction.

The MLA70includes one cylindrical lens71(a first cylindrical lens). The cylindrical lens71has a shape obtained by cutting out a part of the side surface of a cylinder. The cylindrical lens71is arranged on the output face of the MLA70so that the convex side of the cylindrical lens71faces the polarization-rotating coupling optical system30. The longitudinal direction of the cylindrical lens71corresponds to the central axis of the cylinder. When the cylindrical lens71is viewed from the longitudinal direction, the cross-section is the same at any point. The cylindrical lens71may have a vault shape, and the curvature radius of the curved surface may not be constant.

The cylindrical lens71is arranged so that the longitudinal direction is parallel to the direction in which the output waveguides24band24care aligned in the MLA70. In addition, the cylindrical lens71has a length that allows the output lights from the output waveguides24band24cto penetrate through the cylindrical lens71. As illustrated inFIG. 4B, the curvature of the cylindrical lens71is located in the thickness directions (the y direction) of the output waveguides24band24c. Accordingly, the spread of the emitted light in the y direction is shaped.

On the output face of the MLA80, located are two cylindrical lenses81(second cylindrical lenses) so that the convex sides face the polarization-rotating coupling optical system30. The two cylindrical lenses81are arranged so that each of the lights output from the output waveguides24band24cpenetrates through the corresponding cylindrical lens81. The longitudinal direction of each cylindrical lens81intersects with the longitudinal direction of the cylindrical lens71. For example, the longitudinal direction of each cylindrical lens81intersects with the longitudinal direction of the cylindrical lens71at right angles. As illustrated inFIG. 4A, the curvature of the cylindrical lens81is located in the width directions (the x direction) of the output waveguides24band24c. Accordingly, the spread of the emitted light in the x direction is shaped.

The above configuration shapes the spread in the y direction in the cylindrical lens71, and shapes the spread in the x direction in the cylindrical lens81. Accordingly, the excess loss and the effect of diffraction due to the kicking of the beam is minimized, and a collimated light is allowed to be formed. That is to say, the optical coupling loss is reduced.

The cylindrical lenses71and81preferably have refractive indexes greater than the refractive index of glass (1.4 to 2.1). This configuration increases the change in the refractive index experienced when a light enters the cylindrical lenses71and81, making it easy to collimate a beam. For example, the cylindrical lenses71and81preferably have refractive indexes equal to 3 or greater. For example, the cylindrical lenses71and81are preferably made from silicon.

The shape of the collimated light becomes an ellipse or a circle. When the collimated light has an elliptical shape, the use of the condenser lens64having an anamorphic shape such as a cylindrical lens or an aspherical lens enables to correct the aspect ratio of the collimated light, thereby allowing to form a circular beam at the input end face of the optical fiber. When the collimated light has a circular shape, the use of the condenser lens64having a rotational symmetry with respect to the optical axis direction allows to form a circular beam at the input end face of the optical fiber.

FIG. 5AthroughFIG. 5Fillustrate examples of the arrangement of the cylindrical lens71and the cylindrical lenses81.FIG. 5A,FIG. 5C, andFIG. 5Eare top views.FIG. 5B,FIG. 5D, andFIG. 5Fare side views. As illustrated inFIG. 5AandFIG. 5B, the cylindrical lens71may be arranged on the input face of the MLA70so that the convex side faces the output waveguides24band24c. Alternatively, as illustrated inFIG. 5CandFIG. 5D, the cylindrical lenses81may be arranged on the input face of the MLA80so that the convex sides face the output waveguides24band24c. Alternatively, as illustrated inFIG. 5EandFIG. 5F, the cylindrical lens71may be arranged on the input face of the MLA70so that the convex side faces the output waveguides24band24c, and the cylindrical lenses81may be arranged on the input face of the MLA80so that the convex sides face the output waveguides24band24c.

As described above, the convex sides of the cylindrical lenses71and81may face any of the output waveguides24band24cand the polarization-rotating coupling optical system30. The cylindrical lenses81may be arranged closer to the output waveguides24band24cthan the cylindrical lens71is. However, when the output lights from the output waveguides24band24chave a spread angle in the y direction greater than the spread angle in the x direction, the cylindrical lens71is preferably arranged closer to the output waveguides24band24cthan the cylindrical lens81is. This is because this configuration can reduce the spread in the y direction.

Second Embodiment

The first embodiment provides a cylindrical lens to each of the MLA70and the MLA80. A cylindrical lens may be located on both faces of one MLA.FIG. 6AandFIG. 6Billustrate a beam shaping system in accordance with a second embodiment.FIG. 6Ais a top view of the beam shaping system, andFIG. 6Bis a side view of the beam shaping system. As illustrated inFIG. 6AandFIG. 6B, the cylindrical lens71may be located on the input face of the MLA70so that the convex side faces the output waveguides24band24c, and the cylindrical lenses81may be located on the output face of the MLA70so that the convex sides face the polarization-rotating coupling optical system30. This configuration reduces the number of components. In addition, the complexity in the optical-axis adjustment is reduced. Therefore, the cost is reduced.

Variation

The output waveguides24band24cmay be tapered. This configuration allows the mode field in the x direction or in the y direction to be adjusted. Thus, the matching of the mode field with the mode field of the output fiber300becomes easy.FIG. 7AandFIG. 7Billustrate the output waveguides24band24cin accordance with a variation.FIG. 7Ais a top view of the output waveguides24band24c, andFIG. 7Bis a side view of the output waveguides24band24c.FIG. 7AandFIG. 7Balso illustrate the beam shaping system.

For example, the output waveguides24band24cmay be tapered so that the width increases at closer distances to the output end. As described above, the change in the width of the waveguide in the x direction allows to change the relationship between the emission distance from the MLA70and the size of the mode field. The same effect is obtained in the y direction by changing the thickness of the waveguide. However, in a semiconductor modulator, the thickness of the waveguide is determined based on the modulation efficiency, and it is technically difficult to change the thickness in a single substrate. Therefore, the waveguide is preferably tapered in the x direction. That is to say, the adjustment of the width of the waveguide enables to match the mode field with the mode field of the optical fiber.

In the above described embodiments and the variation, when the central axis of the cylindrical lens71is adjusted to be parallel to the waveguide array (the output end face of the semiconductor modulator20), the optical axis deviation in the optical fiber due to the manufacturing error of the MLA can be reduced. Accordingly, the coupling loss and PDL are reduced. The following describes the detail of this effect.

FIG. 8is a top view of a beam shaping system. In the example ofFIG. 8, the cylindrical lens71is located on the input face of the MLA70, and the cylindrical lenses81are located on the output face. The cylindrical lens71has a length that allows the lights emitted from the output waveguides24athrough24dpenetrate through the cylindrical lens71. The cylindrical lenses81are located to correspond to the output waveguides24athrough24d. Thus, four cylindrical lenses81are located.

FIG. 9AandFIG. 9Bare diagrams for describing a case where a spherical lens array is used instead of a cylindrical lens.FIG. 9Aillustrates the semiconductor modulator20viewed from the polarization-rotating coupling optical system30side.FIG. 9Bis a side view. As illustrated inFIG. 9A, when four spherical lenses65independent from each other are used, the positions of the four spherical lenses65deviate from each other due to the manufacturing error and the like. In this case, especially the deviation of the position of the outer spherical lens65is large. Accordingly, as illustrated inFIG. 9B, the emission angles vary between signal lights66band66cand monitor lights66aand66din the y direction.

FIG. 10AandFIG. 10Bare diagrams for describing a case where the cylindrical lens71is used.FIG. 10Aillustrates the semiconductor modulator20viewed from the polarization-rotating coupling optical system30side.FIG. 10Bis a side view. When the cylindrical lens71is used, the lens positions with respect to the output waveguides24athrough24bare not independent of each other, and the lens positions thus hardly deviate. Thus, when the cylindrical lens71is used, only the adjustment of the central axis (the longitudinal direction) of the cylindrical lens71with respect to the semiconductor modulator20allows to reduce the variations in the emission angles between the signal lights66band66cand the monitor lights66aand66din the y direction. To make the description of the effect easy, the subjects of the output lights are four signals inFIG. 9AthroughFIG. 10B. However, the same effect is obtained when the subjects of the output lights are two signals as described in each embodiment.