JP2013186423A - Optical waveguide device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide device capable of preventing deterioration of optical characteristics or reliability, while suppressing a radiation loss by a slit.SOLUTION: An optical waveguide device includes: a cladding portion; an optical waveguide arranged inside the cladding portion and having a higher refraction index than the cladding portion; a plurality of slits formed so as to divide the optical waveguide in the cladding portion. In a case where a radiation loss by the slits periodically takes a minimum value if the alienation interval is incremented from 0 μm, the alienation interval between the plurality of slits is set to be the alienation interval having the second smallest value or larger, among alienation intervals corresponding to the minimum value.

Description

  The present invention relates to an optical waveguide device.

  In an optical waveguide element constituted by a planar lightwave circuit (PLC) made of quartz glass, a configuration is disclosed in which a slit for dividing the optical waveguide is formed and an optical material is inserted therein. For example, Patent Document 1 discloses an element in which two slits are formed and a half-wave plate is inserted therein. Such an element is used as a demodulation element for demodulating a D (Q) PSK optical signal, for example, in differential quadrature phase shift keying (DQPSK) or differential phase modulation (DPSK) communication system. The Patent Document 2 describes a technique for filling a plurality of slits with a resin having a thermal expansion coefficient that cancels the thermal expansion of quartz glass, thereby reducing the temperature dependence of AWG (Arrayed Waveguide Grating). ing.

International Publication WO2008 / 084707 Japanese Patent Application No. 2011-198818

Shin Kamei, et al., “Low-Loss and Compact Silica-Based Athermal Arrayed Waveguide Grating Using Resin-Filled Groove” Journal of Lightwave Technology, Vol. 27, Issue 17, pp. 3790-3799 (2009).

  By the way, when the slit which divides | segments an optical waveguide is formed, since a part of light which guided the optical waveguide is radiated | emitted to a clad in a slit, a radiation loss generate | occur | produces. Non-Patent Document 1 discloses that the radiation loss due to the formation of the slits can be suppressed by setting the interval between the slits to about 20 μm.

  However, if the distance between the slits is small, the thickness of the glass portion between the slits becomes thin, so that the mechanical strength of the portion decreases, and the reliability of the optical waveguide element may decrease. When the optical element inserted into the slit protrudes from the slit, if the two slits are brought close to each other, the inserted optical element may interfere with the protruding portion. Such interference may reduce the positional accuracy of the optical element, thereby reducing the optical characteristics of the optical waveguide element, or reducing the reliability of the optical waveguide element.

  The present invention has been made in view of the above, and an object of the present invention is to provide an optical waveguide element in which a loss of radiation due to a slit is suppressed and a decrease in optical characteristics and reliability is suppressed.

  In order to solve the above-described problems and achieve the object, an optical waveguide device according to the present invention includes a clad part, an optical waveguide disposed in the clad part and having a higher refractive index than the clad part, and the clad part A plurality of slits formed so as to divide the optical waveguide, and the spacing between the plurality of slits periodically minimizes radiation loss due to the slits when the spacing interval is increased from 0 μm. In the case of taking a value, it is characterized in that, among the separation intervals corresponding to the minimum value, the second and subsequent separation intervals from the smallest value are set.

  Moreover, the optical waveguide device according to the present invention is characterized in that in the above invention, the optical waveguide device further comprises an optical element inserted into the slit.

  The optical waveguide element according to the present invention is characterized in that, in the above invention, the optical element is a half-wave plate.

  The optical waveguide device according to the present invention is characterized in that, in the above invention, the separation interval is set to a second separation interval from the smallest value.

  The optical waveguide device according to the present invention is characterized in that, in the above invention, the optical waveguide has at least one radiation loss portion formed between the plurality of slits.

  In the optical waveguide device according to the present invention as set forth in the invention described above, the radiation loss portion is formed at a position that substantially divides the spacing between the plurality of slits.

  Moreover, the optical waveguide device according to the present invention is characterized in that, in the above invention, the radiation loss portion is a gap formed in the optical waveguide.

  Moreover, the optical waveguide device according to the present invention is characterized in that, in the above invention, the radiation loss portion is an intersection where the optical waveguide intersects with another optical waveguide.

  The optical waveguide device according to the present invention is characterized in that, in the above invention, the radiation loss portion has a width of 15 μm to 30 μm.

  Further, in the optical waveguide device according to the present invention, in the above invention, the separation interval may be a radiation by the slit when the number of the radiation loss parts is n (n is an integer of 1 or more) and the radiation loss part is not present. The loss is set to a value that is (n + 1) times the separation interval at which the loss is minimized.

  The optical waveguide device according to the present invention is characterized in that, in the above invention, the number of the slits is two.

  The optical waveguide element according to the present invention is a Mach-Zehnder optical interferometer element in the above invention.

  According to the present invention, there is an effect that it is possible to realize an optical waveguide device in which a decrease in optical characteristics and reliability is suppressed while radiation loss due to a slit is suppressed.

FIG. 1 is a schematic diagram of an optical waveguide device according to the first embodiment. FIG. 2 is a schematic plan view of an experimental waveguide element for examining the relationship between the slit pitch and the slit loss. FIG. 3 is a diagram showing the relationship between the slit pitch and the slit loss. FIG. 4 is a schematic plan view of the optical waveguide device according to the second embodiment. FIG. 5 is a diagram showing the relationship between the gap width and the slit loss. FIG. 6 is a schematic plan view of an experimental waveguide device for investigating the influence of gap misalignment. FIG. 7 is a diagram showing the influence of gap misalignment in the relationship between gap width and slit loss. FIG. 8 is a diagram showing the relationship between the slit pitch and the slit loss when the gap width is changed when the number of slits is 2 and the number of gaps is 2. FIG. FIG. 9 is a diagram showing the relationship between the slit pitch and the slit loss when the gap width is changed when the number of slits is 2 and the number of gaps is 1. FIG. FIG. 10 is a diagram showing the relationship between the slit pitch and the slit loss when the gap width is changed when the number of slits is 2 and the number of gaps is 3. FIG. FIG. 11 is a diagram showing the relationship between the slit pitch and the slit loss when the gap width is changed when the number of slits is 3 and the number of gaps is 1. FIG. FIG. 12 is a diagram showing the relationship between the slit pitch and the slit loss when the gap width is changed when the number of slits is 3 and the number of gaps is 2. FIG. FIG. 13 is a diagram showing the relationship between the slit pitch and the slit loss when the gap width is changed when the number of slits is 3 and the number of gaps is 3. In FIG. FIG. 14 is a schematic plan view of the MZI element according to the third embodiment. FIG. 15 is a diagram showing an example of the relationship between the slit pitch and the slit loss in the MZI element having the configuration shown in FIG. FIG. 16 is a schematic plan view of an MZI element according to the fourth embodiment. FIG. 17 is a plan view showing a schematic configuration of the demodulation delay circuit according to the fifth embodiment. FIG. 18 is a schematic plan view of an optical waveguide device according to the sixth embodiment.

  Embodiments of an optical waveguide device according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. Moreover, in each drawing, the same code | symbol is attached | subjected suitably to the same or corresponding element. Furthermore, it should be noted that the drawings are schematic, and the relationship between the dimensions of each element, the ratio of each element, and the like may differ from the actual ones. Even between the drawings, there are cases in which portions having different dimensional relationships and ratios are included.

(Embodiment 1)
FIG. 1 is a schematic diagram of an optical waveguide device according to the first embodiment. 1A is a plan view and FIG. 1B is a side view. As shown in FIG. 1, an optical waveguide element 10 includes a clad portion 11 formed on a substrate B made of quartz glass or silicon, an optical waveguide 12 as a core portion disposed inside the clad portion, and a clad portion. 11 includes two slits 13 formed in parallel to each other so as to divide the optical waveguide 12 and two half-wave plates 14 a and 14 b as optical elements inserted into the respective slits 13. .

The clad portion 11 is made of quartz glass. The refractive index of the cladding part 11 is 1.465 at a wavelength of 1.55 μm, for example. The optical waveguide 12 is made of quartz glass to which a dopant such as germania (GeO ₂ ) is added to increase the refractive index so that the refractive index is higher than that of the cladding portion 11. The relative refractive index difference Δ of the optical waveguide 12 with respect to the clad portion 11 is 1.2%. The size of the cross section in the plane perpendicular to the longitudinal direction of the optical waveguide 12 is 6 μm × 6 μm. In the region S where the slits 13 are formed, the waveguide width is expanded to 19.5 μm. However, the relative refractive index difference Δ and the cross-sectional size of the optical waveguide 12 are not limited to the above, and are preferably set so that light to be input to the optical waveguide element 10 is guided in a single mode. For example, as shown in FIG. 1, light is input from the left side of the paper and output to the right side of the paper.

  In the slit 13, the slit width W <b> 1 in the longitudinal direction of the optical waveguide 12 is set to such a width that the half-wave plates 14 a and 14 b can be inserted. The slit widths of the two slits 13 are equal, and the slit width W1 is, for example, 15 μm to 30 μm. The half-wave plates 14a and 14b are inserted into the slit 13 and fixed with an optical adhesive.

  Here, the separation interval (pitch) P <b> 1 between the two slits 13 is a distance between the center lines of the slit widths of the two slits 13. In this optical waveguide device 10, the slit pitch P1 is set to a large value of about 170 μm, which is a predetermined value. As a result, the slit pitch P1 can be increased while suppressing radiation loss (slit loss) due to the slits 13.

  By increasing the slit pitch P1, the thickness of the glass portion between the slits 13 is increased, and the mechanical strength of the portion can be increased. Therefore, the reliability of the optical waveguide element 10 can be increased. Further, for example, even if mechanical means such as dicing is employed to form the slit 13 having a depth sufficient to insert the half-wave plates 14a and 14b, the glass portion between the slits 13 is broken. Is less likely to occur. Further, by increasing the slit pitch P1, it is easy to install the half-wave plates 14a and 14b so as not to interfere with each other.

  In addition, when the slit pitch P1 is small when the half-wave plates 14a and 14b are inserted and then fixed in the slit 13 with an optical adhesive, the adhesive applied in each slit 13 is mixed, The two-wave plates 14a and 14b may be inclined so as to approach each other, and the position accuracy of the half-wave plates 14a and 14b in the slit 13 may be lowered. However, it is possible to prevent such a problem from occurring by increasing the slit pitch P1.

  The setting of the slit pitch P1 will be specifically described. In order to investigate the relationship between the slit pitch and the slit loss, the present inventors manufactured the following experimental waveguide elements as Experiment 1, and investigated the characteristics thereof.

(Experiment 1)
FIG. 2 is a schematic plan view of an experimental waveguide element for examining the relationship between the slit pitch and the slit loss. In the experimental optical waveguide element 20 made of quartz glass, a plurality of optical waveguides 22 are arranged in parallel in a clad portion formed on a substrate (not shown) made of quartz glass or silicon. Are formed such that two slits 23 are formed and half-wave plates 24 a and 24 b are inserted into the slits 23.

  In addition, the relative refractive index difference Δ of the optical waveguide 22 with respect to the cladding portion 21 is 1.2%. The size of the cross section in the plane perpendicular to the longitudinal direction of the optical waveguide 22 is 6 μm × 6 μm, and in the region where the slit 23 is formed, the waveguide width is expanded to 19.5 μm. The slit widths of the two slits 23 are equal and the slit width is 24 μm. The two slits 23 are formed so as to be inclined by 2.5 degrees in directions opposite to each other in the longitudinal direction of the optical waveguide 22 with respect to a plane perpendicular to the longitudinal direction of the optical waveguide 22. The relative angle between the two slits 23 is 5 degrees. Thereby, the pitch of the two slits 23 in each optical waveguide 22 is different.

  From the left side of each optical waveguide 22 of the experimental optical waveguide device 20, TM polarized light having a wavelength of 1.55 μm (polarized light perpendicular to the substrate surface of the experimental optical waveguide device 20) or TE polarized light (parallel to the substrate surface). Direction polarization), the intensity of the light output from the right side of the paper was measured, and the slit loss in each optical waveguide 22 was examined.

  FIG. 3 is a diagram showing the relationship between the slit pitch and the slit loss. In the legend, “TE” and “TM” indicate values for TE polarized light and TM polarized light, respectively.

  As shown in FIG. 3, the present inventors have found that the slit loss periodically changes as the slit pitch is increased. The arrows Ar1, Ar2, Ar3, Ar4, and Ar5 indicate positions where the slit pitch is about 65 μm, about 170 μm, about 275 μm, about 380 μm, and about 480 μm, respectively, and the slit loss is minimal at these slit pitches. It is a value.

  Therefore, in the optical waveguide device 10 according to the first embodiment, when the slit pitch P1 is increased from 0 μm, when the slit loss periodically takes the minimum value, the separation interval corresponding to the minimum value (arrow Ar1) , Ar2, Ar3, Ar4, Ar5), the second slit pitch from the smallest value is set to about 170 μm. As a result, in the optical waveguide element 10, the radiation loss due to the slits 13 is suppressed, and the pitch P <b> 1 is increased, so that a decrease in optical characteristics and reliability is suppressed.

  The slit pitch P1 may be the second and subsequent minimum values. For example, as shown in FIG. 3, the third minimum value may be about 275 μm, the fourth minimum value may be about 380 μm, and the fifth minimum value may be about 480 μm. However, it is preferable to use the fifth or less from the viewpoint of miniaturization. In FIG. 1, the slit 13 is formed so that its depth direction is perpendicular to the longitudinal direction of the optical waveguide 12, but parallel to the surface and the substrate with respect to the surface perpendicular to the longitudinal direction of the optical waveguide 12. A straight line may be inclined about 8 degrees. As a result, the end surface reflected light in the slit 13 of the optical waveguide 12 is suppressed from returning to the optical waveguide 12. It should be noted that even if the slits are formed in such an inclined manner, the preferred value of the slit pitch P1 is not affected.

(Embodiment 2)
FIG. 4 is a schematic plan view of the optical waveguide device according to the second embodiment. As shown in FIG. 4, the optical waveguide element 30 has a configuration in which the slit pitch is set to the slit pitch P <b> 2 and the optical waveguide 12 is replaced with the optical waveguide 32 in the optical waveguide element 10 shown in FIG. 1.

  The optical waveguide 32 is obtained by providing two gaps 35 as radiation loss portions in the optical waveguide 12. The gap 35 is configured by dividing the optical waveguide 32 and filling the gap with the same material as that of the cladding portion 11. The widths (gap widths) W2 of the two gaps 35 are set to be approximately equal.

  In this optical waveguide element 30, since the radiation mode in the slit 13 is coupled with the radiation mode in the gap 35, the slit loss is suppressed as compared with the case without the gap 35. Further, due to the presence of the gap 35, the slit loss can be suppressed even if the slit pitch P2 is not set to a value at which the slit loss becomes a minimum value as shown in FIG. Therefore, since the slit pitch P2 can be increased while suppressing the slit loss, the optical characteristics and reliability of the optical waveguide element 30 are prevented from being lowered. Further, the degree of freedom in designing the slit pitch P2 is high.

  A preferred value of the gap width W2 will be specifically described. In order to investigate the relationship between the gap width and the slit loss, the present inventors manufactured the following experimental waveguide device as Experiment 2, and examined the characteristics thereof.

(Experiment 2)
The experimental waveguide element has the configuration shown in FIG. However, the two slits were inclined by 8 degrees with respect to a plane perpendicular to the longitudinal direction of the optical waveguide. When the slit is provided with an inclination, the slit pitch is the distance between the center line of the slit width of the two slits at the position where the center of the waveguide and the slit intersect. The relative refractive index difference Δ of the optical waveguide with respect to the cladding is 1.2%. The size of the cross section in the plane perpendicular to the longitudinal direction of the optical waveguide is 6 μm × 6 μm, and the waveguide width is increased to 19.5 μm in the region where the slit is formed. The slit pitch is 200 μm. The slit widths of the two slits are equal, and the slit width is 24 μm. Further, the half-wave plate corresponding to the half-wave plate 14a in FIG. 4 is inserted so that its main axis forms 45 degrees with respect to both the TM polarization direction and the TE polarization direction. The half-wave plate corresponding to the half-wave plate 14b is inserted so that its main axis is parallel to the direction of TE polarization. The gap width was set to 0 μm (no gap) 15 μm, 20 μm, and 25 μm. Further, the gap was formed with the position obtained by dividing the slit pitch into three equal parts as the center position of the gap width.

  Then, TM polarized light or TE polarized light with a wavelength of 1.55 μm was input from the left side of the experimental waveguide element, the intensity of the light output from the right side of the paper was measured, and the slit loss was examined.

  FIG. 5 is a diagram showing the relationship between the gap width and the slit loss. In the legend, “TE” and “TM” indicate values for TE polarized light and TM polarized light, respectively.

  As shown in FIG. 5, it was confirmed that a slit loss smaller than about 1.2 dB, which is a slit loss when there is no gap (gap width 0 μm), can be realized by forming a gap. In particular, when the gap width is 20 μm, it was confirmed that the slit loss can be reduced to about 0.6 dB, which is about 0.8 dB lower than about 1.2 dB which is a value when there is no gap.

  Next, in order to confirm the influence of the gap misalignment, as shown in FIG. 6, the position of the gap 45 is set to the distance d from the experimental optical waveguide 40 used in Experiment 2 while keeping the gap width W2 the same. An experimental optical waveguide 50 shifted in the longitudinal direction was manufactured, and the gap loss was measured. The distance d was set to +15 μm (shift to the right side of the paper), 0 μm, and −15 μm (shift to the left side of the paper).

  FIG. 7 is a diagram showing the influence of gap misalignment in the relationship between gap width and slit loss. In the legend, for example, “TE_−15” indicates a value for TE polarized light when the distance d is −15 μm.

  As shown in FIG. 7, it is preferable that there is no gap misalignment. However, even if the gap misalignment is about ± 15 μm, the increase in slit loss due to this is about 0.2 dB to 0.3 dB. Therefore, even if there is a gap misalignment, the slit loss can be made smaller than when there is no gap. Further, the gap displacement can be considered as a relative displacement between the gap and the slit. Accordingly, the result of FIG. 7 also shows that the slit loss can be reduced even when the slit forming position is shifted by about ± 15 μm with respect to the gap position as compared with the case without the gap.

  Next, the present invention will be further described using calculation results obtained by a beam propagation method (BPM method). In the following, an optical waveguide having the configuration shown in FIG. 4 was used as the optical waveguide element for calculation. However, the relative refractive index difference Δ of the optical waveguide with respect to the cladding is 1.2%. The size of the cross section in the plane perpendicular to the longitudinal direction of the optical waveguide is 6 μm × 6 μm. As the radiation loss, only the radiation loss in the direction perpendicular to the substrate surface of the optical waveguide element is calculated. This is because, in the direction parallel to the substrate surface (the width direction of the optical waveguide), the radiation loss can be made substantially zero by widening the waveguide width as in the configuration shown in FIG.

  As calculation conditions, the number of gaps is 0, 1, 2, and 3. The gap is defined as the center position of the gap width, where n is the number of gaps and the slit pitch is divided into (n + 1) equal parts. The gap width is changed between 15 μm and 30 μm. The number of slits is 2 or 3. The slit width of each slit is equal, and the slit width is 24 μm. The slit pitch is changed in the range of 40 μm to 200 μm. Further, it is assumed that the slit is filled with a material having a refractive index of 1.464. This refractive index is a value set so that the calculation result and the experimental result of Experiment 1 substantially coincide with each other when the number of gaps is zero.

  FIG. 8 is a diagram showing the relationship between the slit pitch and the slit loss when the gap width is changed when the number of slits is 2 and the number of gaps is 2. FIG. In the legend, “No Gap” indicates that there is no gap (the number of gaps is 0), and “GapW = 15” indicates that the gap width is 15 μm.

  As shown in FIG. 8, the slit loss when there is no gap is in good agreement with the result shown in FIG. Further, the fact that the slit loss when the gap is present and the slit pitch is 200 μm is reduced by about 0.6 dB compared with the case where there is no gap is in good agreement with the result shown in FIG. When the gap width is 15 μm to 30 μm, especially 20 μm to 30 μm, and the slit pitch takes the first minimum value (minimum value) when the slit pitch is increased from 0 μm, there is no gap. It was confirmed that the minimum value of the slit loss was smaller than that without the gap.

  Furthermore, it has been confirmed that the slit loss can be minimized when the slit pitch is set to a value about three times the slit pitch at which the slit loss is minimized when there is no gap, without depending on the gap width.

  Next, FIG. 9 is a diagram showing the relationship between the slit pitch and the slit loss when the gap width is changed when the number of slits is 2 and the number of gaps is 1. FIG. FIG. 10 is a diagram showing the relationship between the slit pitch and the slit loss when the gap width is changed when the number of slits is 2 and the number of gaps is 3. FIG. From FIG. 8, FIG. 9, and FIG. 10, it does not depend much on the gap width, and is about (n + 1) times the slit pitch (n is an integer equal to or greater than 1) that makes the slit loss minimum when there is no gap. When the slit pitch is set to a value, it was confirmed that the slit loss can be minimized and the minimum value of the slit loss is smaller than that in the case where there is no gap.

  Further, FIGS. 11, 12, and 13 are diagrams showing the relationship between the slit pitch and the slit loss when the gap width is changed when the number of slits is 3 and the number of gaps is 1, 2, and 3, respectively. . 11, 12, and 13, even when the number of slits is 3, the slit pitch does not depend much on the gap width, and the slit pitch is about (n + 1) times the slit pitch at which the slit loss is minimized when there is no gap. It was confirmed that the slit loss can be minimized and the minimum value of the slit loss is smaller than when there is no gap.

  As described above, the slit pitch is set to a slit pitch having a value approximately (n + 1) times (n is an integer of 1 or more) the slit pitch at which the slit loss is minimized when there is no gap. Thus, the slit loss can be minimized and smaller than when there is no gap.

  Furthermore, as in the first embodiment, the slit pitch may be set to a slit pitch at which the slit loss takes the second or third minimum value when the slit pitch is increased from 0 μm. For example, as shown in FIGS. 9 and 11, the slit loss that is the second or third minimum value is also smaller than the slit loss that is the second or third minimum value when there is no gap.

(Embodiment 3)
FIG. 14 is a schematic plan view of a Mach-Zehnder interferometer (MZI) element that is an optical waveguide element according to the third embodiment. As shown in FIG. 14, the MZI element 60 made of quartz glass includes a clad portion 61 formed on a substrate made of quartz glass or silicon, and an optical waveguide 62 as a core portion arranged inside the clad portion 61. And two slits 63a and 63b formed in parallel to each other so as to divide the optical waveguide 62 in the cladding portion 61, and two half-wave plates 64a and 64b inserted into the slits 63a and 63b, respectively. And.

  The optical waveguide 62 includes two input optical waveguides 62a on the light input side, a 50% optical coupler 62b including a directional coupler, two arm optical waveguides 62c, and 50% including a directional coupler. The optical coupler 62d and two output optical waveguides 62e on the light output side are sequentially connected. Each of the optical waveguides constituting the optical waveguide 62 has a relative refractive index difference Δ with respect to the cladding portion 61 of 1.2%. The size of the cross section of each optical waveguide in the plane perpendicular to the longitudinal direction is 6 μm × 6 μm. In the arm optical waveguide 62c, the waveguide width is 19.5 μm in the region where the slits 63a and 63b are formed. It has been expanded. However, the relative refractive index difference Δ and the cross-sectional size of the optical waveguide 62 are not limited to the above, and are preferably set so that light to be input to the MZI element 60 is guided in a single mode. Further, an optical path difference is provided between the two arm optical waveguides 62c so that an FSR (Free Spectral Range) is about 23 GHz.

  The slit 63a is formed at the center in the longitudinal direction of the arm optical waveguide 62c, and the slit 63b is formed on the output optical waveguide 62e side of the slit 63b, both of which divide the arm optical waveguide 62c. The slits 63a and 63b are formed to be inclined by 8 degrees with respect to a plane perpendicular to the longitudinal direction of the optical waveguide 62, with a line parallel to the plane and the substrate as an axis. The slit widths of the slits 63a and 63b are equal, for example, 15 μm to 30 μm.

  The half-wave plate 64a inserted into the slit 63a is inserted so that its main axis forms 45 degrees with respect to both the TM polarization direction and the TE polarization direction, and is fixed with an optical adhesive. The half-wave plate 64b inserted into the slit 63b is inserted and fixed with an optical adhesive so that its principal axis is parallel to the direction of TE polarization.

  In this MZI element 60, by using the half-wave plates 64a and 64b, as described in Patent Document 1, even when polarization conversion occurs in the 50% optical coupler 62b, Since the interference condition is the same as the interference condition of normal light that is not polarization-converted, deterioration of PDF (Polarization Dependent Frequency shift) can be suppressed. Note that PDF is a phenomenon in which the frequency peak of transmission characteristics generated by an optical interferometer causes a difference between two polarization states (TM polarization and TE polarization) of light propagating through an optical waveguide. It is.

  Further, in the MZI element 60, the slit pitch P3 between the two slits 63a and 63b is set to a large value of about 170 μm, like the slit pitch P1 in the first embodiment. Thereby, in the MZI element 60, the optical loss and the deterioration of the reliability are suppressed while the slit loss due to the slits 63a and 63b is suppressed.

  The setting of the slit pitch P3 will be specifically described. In order to investigate the relationship between the slit pitch and the slit loss in the MZI element according to Embodiment 3, the present inventors manufactured an MZI element having the configuration shown in FIG. 14 and variously set the slit pitch. . Then, TM polarized light or TE polarized light with a wavelength of 1.55 μm is input from one of the input optical waveguides of each manufactured MZI element, and the intensity of the light output from one of the output optical waveguides is measured. I examined the loss.

  FIG. 15 is a diagram showing an example of the relationship between the slit pitch and the slit loss in the MZI element having the configuration shown in FIG. As shown in FIG. 15, similarly to the result of FIG. 3, the slit loss changed periodically as the slit pitch was increased. Arrows Ar6 and Ar7 indicate the positions at which the slit pitches are about 170 μm and about 275 μm, respectively, and the slit loss has a minimum value at these slit pitches, as in FIG. It was confirmed that the slit loss can be reduced to about 0.8 dB by setting the slit pitch to about 170 μm and about 275 μm.

(Embodiment 4)
FIG. 16 is a schematic plan view of an MZI element according to the fourth embodiment. As shown in FIG. 16, the MZI element 70 has a configuration in which the slit pitch is set to the slit pitch P4 and the optical waveguide 62 is replaced with the optical waveguide 72 in the MZI element 60 shown in FIG. 14.

  The optical waveguide 72 has a configuration in which two arm optical waveguides 62c are replaced with two arm optical waveguides 72c in the optical waveguide 62. The two arm optical waveguides 72c are obtained by providing the two arm optical waveguides 62c with two gaps 75 as radiation loss portions, respectively. The gap 75 is configured by dividing the arm optical waveguide 72 c and filling the gap with the same material as that of the cladding portion 61. The gap widths of the four gaps 75 are set to be approximately equal.

  In this MZI element 70, as in the second embodiment, the radiation mode in the slits 63a and 63b is coupled to the radiation mode in the gap 75, so that the slit loss is suppressed as compared with the case without the gap 75. Further, due to the existence of the gap 75, the slit loss can be suppressed even if the slit pitch is not set to a value at which the slit loss becomes a minimum value as shown in FIG. Therefore, the slit pitch can be increased while suppressing the slit loss, so that the optical characteristics and reliability of the MZI element 70 are prevented from being lowered. Also, the degree of freedom in designing the slit pitch is high.

  The slit pitch and the gap width can be set to suitable values shown in the above-described Experiment 2 and the calculation result using the BPM method. For example, the slit pitch can be 200 μm and the gap width can be 20 μm.

(Embodiment 5)
FIG. 17 is a plan view showing a schematic configuration of the demodulation delay circuit according to the fifth embodiment. The demodulation delay circuit 101 shown in FIG. 17 is, for example, a 40 Gbps DQPSK delay demodulation device used in an optical transmission system using a DQPSK system with a transmission rate of 40 Gbps (see also Patent Document 2 by the present inventor).

  The demodulation delay circuit 101 monitors 5% of the optical signal propagated through the input optical waveguide 102 in order to monitor the input optical waveguide 102 to which the DQPSK signal is input and the optical power of the input DQPSK signal by the monitor PD. A tap coupler 180 that branches to the output waveguide 181, a Y branch waveguide 103 as an optical branching device that branches the remaining optical signal that has not been branched by the tap coupler 180, and a branch by the Y branch waveguide 103. MZI elements 104 and 105 that respectively delay the DQPSK signal thus generated by 1 bit. A monitor PD (not shown) is connected to the monitor output waveguide 181. In the present embodiment, the branching ratio of the tap coupler 180 is 5%, preferably 20% or less, and more preferably 5% to 10%.

  The demodulation delay circuit 101 further includes half-wave plates 147 and 170, and has waveguide intersections 162 and 164. The half-wave plates 147 and 170 will be described later.

  The MZI element 104 has an input side coupler 106 connected to the waveguide 114 connected to one output side of the Y branch waveguide 103, and two output ends connected to the two output optical waveguides 121 and 122, respectively. An output-side coupler 107 and two arm optical waveguides 108 and 109 which are delay waveguides having different lengths connected between the couplers 106 and 107 are provided. Similarly, the MZI element 105 has two output ends on the input-side coupler 110 connected to the waveguide 115 connected to the other output side of the Y-branch waveguide 103 and the two output optical waveguides 123 and 124, respectively. It has a connected output side coupler 111 and two arm optical waveguides 112 and 113 having different lengths, which are delay waveguides connected between the two couplers 110 and 111. The MZI elements 104 and 105 are set in the same manner as the MZI element of the fourth embodiment in terms of relative refractive index difference, optical waveguide size, FSR, and the like.

  The input-side couplers 106 and 110 and the output-side couplers 107 and 111 are 2-input × 2-output type 50% couplers, respectively. One of the two input ends of the input side coupler 106 of the MZI element 104 is connected to the waveguide 114. One of the two input ends of the input side coupler 110 of the MZI element 105 is connected to the waveguide 115.

  Further, the MZI element 104 is formed by bending the arm optical waveguides 108 and 109 so that the light propagation direction in the input-side coupler 106 and the light propagation direction in the output-side coupler 107 differ by approximately 180 degrees. Similarly, the MZI element 105 is formed by bending the arm optical waveguides 112 and 113 so that the light propagation direction in the input-side coupler 110 and the light propagation direction in the output-side coupler 111 differ by approximately 180 degrees.

  The two output ends (through port and cross port) of the output side coupler 107 of the MZI element 104 are connected to the output optical waveguides 121 and 122, respectively. Similarly, two output terminals (through port and cross port) of the output side coupler 111 of the MZI element 105 are connected to output optical waveguides 123 and 124, respectively. The output optical waveguides 121, 122, 123, and 124 are connected to output ports Pout1, Pout2, Pout3, and Pout4, respectively.

  Further, in the MZI element 104, the phase of the DQPSK signal propagating through the longer arm optical waveguide 108 is set to the symbol rate of the phase of the DQPSK signal propagating through the shorter arm optical waveguide 109. The optical path length difference is delayed by a delay amount corresponding to 1 bit (1 bit time slot: 1 time slot). For example, when the symbol rate is 40 Gbps, the symbol rate of each of the I channel and the Q channel may be 20 Gbps, which is half, so the delay amount is 50 ps (picosecond). Similarly, in the MZI element 105, the phase of the DQPSK signal propagating through the longer arm optical waveguide 112 is set to a symbol rate with respect to the phase of the DQPSK signal propagating through the shorter arm optical waveguide 113. The optical path length difference is delayed by a delay amount corresponding to 1 bit (for example, when the symbol rate is 40 Gbps, the delay amount is 50 ps). Note that the delay amount is not limited to an amount corresponding to exactly one bit. For example, depending on the system configuration, each bit may be set to interfere with an adjacent bit as a delay amount that is substantially 1 bit but slightly shifted from 1 bit.

Further, the MZI elements 104 and 105 have interference characteristics that are 90 degrees out of phase. For this reason, the optical path length difference between the arm optical waveguides 108 and 109 of the MZI element 104 is set longer than the delay amount corresponding to 1 bit by a length corresponding to 1 / 4π in the phase of the optical signal. On the other hand, the optical path length difference between the arm optical waveguides 112 and 113 of the MZI element 105 is set shorter than the delay amount corresponding to 1 bit by a length corresponding to 1 / 4π in the phase of the optical signal.
As a result, the phase of the light in the adjacent time slot that interferes with the MZI element 104 and the phase of the light in the adjacent time slot that interferes with the MZI element 105 are shifted by 90 degrees.

  The half-wave plates 147 and 170 are inserted into slits 148 and 171 provided so as to divide the arm optical waveguides 108, 109, 112 and 113 of the MZI elements 104 and 105. The half-wave plate 147 is inserted such that its main axis forms 45 degrees with respect to both the TM polarized wave direction and the TE polarized wave direction. The half-wave plate 170 is inserted so that its principal axis is parallel to the direction of TE polarization. The slit 148 is located at the center in the longitudinal direction of each arm optical waveguide 108, 109, 112, 113, and the slit 171 is formed at a slit pitch of about 275 μm on the output side. The slits 148, 171 are inclined by 8 degrees with respect to a plane perpendicular to the longitudinal direction of the portion where the arm optical waveguides 108, 109, 112, 113 are parallel to each other, with a line parallel to the plane and the substrate as an axis. Is formed.

  Note that, at the intersections 162 and 164, the arm optical waveguide 109 of the MZI element 104 and the arm optical waveguide 112 of the MZI element 105 intersect, but light (DQPSK signal) propagating through each arm waveguide is transmitted at each intersection. After passing through 162, 164, it propagates through the same arm waveguide as it is.

  Further, the optical path length of the arm optical waveguide 109, which is the shorter arm waveguide of the MZI element 104, is different from the optical path length of the arm optical waveguide 113, which is the shorter arm waveguide of the MZI element 105, and Y Each optical path length from the branching waveguide 103 to the output side of the MZI element 104 (output ports Pout1 and Pout2 of the output optical waveguides 121 and 122) through the arm optical waveguide 109 of the MZI element 104, and from the Y branching waveguide 103 The optical path lengths from the MZI element 105 through the arm optical waveguide 113 to the output side of the MZI element 105 (output ports Pout3 and Pout4 of the output optical waveguides 123 and 124) are all substantially equal.

  Further, heaters A, B, C, D, E, F, G, and H for trimming are formed on the arm optical waveguides 108, 109, 112, and 113 of the MZI elements 104 and 105, respectively. Each of the heaters A to H is a Ta-based thin film heater formed above the corresponding arm optical waveguide and formed on the clad portion by sputtering.

  In the demodulation delay circuit 101, in the MZI element 104, the input DQPSK signal is branched by the Y branch waveguide 103, and the branched DQPSK signal is an arm optical waveguide 108 having a different length of the MZI element 104. , 109 is propagated. The MZI element 104 delays the phase of the DQPSK signal propagating through the arm optical waveguide 108 by a delay amount + 1 / 4π corresponding to one bit of the symbol rate with respect to the phase of the optical signal propagating through the arm optical waveguide 109. It has become. Similarly, the MZI element 105 causes the phase of the DQPSK signal propagating through the arm optical waveguide 112 to be a delay amount −1 / 4π corresponding to one bit of the symbol rate with respect to the phase of the optical signal propagating through the arm optical waveguide 113. It is supposed to be delayed.

  Also in the demodulation delay circuit 101, the degradation of the PDF can be suppressed to, for example, 250 MHz or less by using the ½ wavelength plates 147 and 170 as in the third embodiment. Further, the slit pitch between the two slits 148 and 171 is set to a large value of about 275 μm, similarly to the slit pitch P1 of the first embodiment. Thereby, in the MZI elements 104 and 105, the slit loss due to the slits 148 and 171 is suppressed, and the deterioration of the optical characteristics and reliability is suppressed. For example, the slit loss can be reduced by about 0.5 dB as compared with the case where the slit pitch is set to 200 μm.

(Embodiment 6)
FIG. 18 is a schematic plan view of an optical waveguide device according to the sixth embodiment. As shown in FIG. 18, the optical waveguide device 80 includes a clad portion 81 formed on a substrate made of quartz glass or silicon, an optical waveguide 82 as a core portion disposed inside the clad portion, and a clad portion 81. 3, three slits 83 a, 83 b, 83 c formed in parallel to each other so as to divide the optical waveguide 82, and a polarizer 84 a, a half-wave plate as an optical element inserted in each slit 83 a, 83 b, 83 c 84b, the polarizer 84c, and the optical waveguide 82 are provided with gaps 85 as two radiation loss portions provided between the slits.

  The transmission axis of the polarizer 84a is inserted so as to be parallel to one polarization (for example, TM polarization). The transmission axis of the polarizer 84c is inserted so as to be parallel to the polarization orthogonal to the transmission axis of the polarizer 84a (for example, TE polarization). The half-wave plate 84b is inserted so that its main axis forms 45 degrees with respect to both the TM polarization direction and the TE polarization direction.

  The material of the clad part 81 and the material and size of the optical waveguide 82 are the same as the corresponding elements in the above embodiments. The slit width of the three slits 83a, 83b, 83c is 24 μm. The slit pitches P5 and P6 are substantially equal, for example, 275 μm. Each gap 85 is formed by setting the position obtained by dividing the slit pitch P5 or P6 into three equal parts as the center position of the gap width. The gap width W3 is 20 μm.

  In this optical waveguide element 80, when light having a majority of linearly polarized components parallel to the transmission axis of the polarizer 84a is input from the left side of the page, the polarizer 84a cuts the crosstalk component included in the light. The half-wave plate 84b polarizes and transmits the light, and the polarizer 84c further cuts and transmits the crosstalk component included in the polarization-converted light, and outputs it to the right side of the page. To do.

  In this optical waveguide element 80, the slit loss of the slits 83a, 83b, and 83c is suppressed, and the deterioration of optical characteristics and reliability is suppressed.

  The optical element inserted into the slit is not limited to a half-wave plate or a polarizer, and other polarization elements or optical filters can be applied. The types of optical elements inserted into the slits may be different from each other. Further, the number of slits and the number of gaps are not particularly limited. The number of slits may be plural, and the number of gaps may be at least one. The slit pitch and gap width can be set so that the slit loss is reduced and minimized or minimized in consideration of the number of slits and the number of gaps. In addition, even when no optical element is inserted into the slit, by setting the slit pitch to an appropriate value and / or providing a gap, light loss in which the optical loss and reliability are suppressed while the radiation loss due to the slit is suppressed is suppressed. A waveguide element can be realized.

  Moreover, in this invention, if it is a radiation loss part which generate | occur | produces radiation loss like a gap, it can replace with a gap and can be provided. As such a radiation loss portion, there is an intersection where the optical waveguides intersect each other.

  The optical waveguide device according to the present invention is not limited to an MZI device, but can be applied to various optical waveguide circuits such as an AWG device, a coherent mixer, and a splitter.

  The present invention is not limited to an optical waveguide element made of quartz glass, but can be applied to optical waveguide elements made of various materials that can form an optical waveguide, such as other glass, polymers, and semiconductors.

  Further, the present invention is not limited by the above embodiment. What was comprised combining each component mentioned above suitably is also contained in this invention. Further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspect of the present invention is not limited to the above-described embodiment, and various modifications can be made.

10, 30, 80 Optical waveguide device 11, 21, 22, 61, 81 Clad portion 12, 22, 32, 62, 72, 82 Optical waveguide 13, 23, 63a, 63b, 83a, 83b, 83c, 148, 171 Slit 14a, 14b, 24a, 24b, 64a, 64b, 84b, 147, 170 1/2 wavelength plate 20 Optical waveguide element for experiment 35, 45, 75, 85 Gap 40, 50 Optical waveguide for experiment 60, 70, 104, 105 MZI element 62a, 102 Input optical waveguide 62b, 62d 50% optical coupler 62c, 72c, 108, 109, 112, 113 Arm optical waveguide 62e, 121, 122, 123, 124 Output optical waveguide 84a, 84c Polarizer 101 Demodulation delay Circuit 103 Branching waveguide 106, 110 Input side coupler 107, 111 Output side Plastic 114, 115 Waveguide 162, 164 Intersection 180 Tap coupler 181 Monitor output waveguide A, B, C, D, E, F, G, H Heater Ar1, Ar2, Ar3, Ar4, Ar5, Ar6 Arrow B Substrate d Distance P1, P2, P3, P4, P5, P6, Ar7 Slit pitch Pout1, Pout2, Pout3, Pout4 Output port S area W1 Slit width W2, W3 Gap width

Claims (12)

A cladding part;
An optical waveguide disposed in the cladding part and having a higher refractive index than the cladding part;
A plurality of slits formed so as to divide the optical waveguide in the cladding portion;
The separation interval between the plurality of slits is a separation interval corresponding to the minimum value when radiation loss due to the slit periodically takes a minimum value when the separation interval is increased from 0 μm. An optical waveguide device characterized in that the second and subsequent spacing intervals are set from the smallest value.   The optical waveguide element according to claim 1, further comprising an optical element inserted into the slit.   The optical waveguide element according to claim 1, wherein the optical element is a half-wave plate.   The optical waveguide device according to any one of claims 1 to 3, wherein the separation interval is set to a second separation interval from a smaller value.   The optical waveguide device according to any one of claims 1 to 4, wherein the optical waveguide has at least one radiation loss portion formed between the plurality of slits.   The optical waveguide element according to claim 5, wherein the radiation loss portion is formed at a position that substantially divides a separation interval between the plurality of slits.   The optical waveguide device according to claim 5, wherein the radiation loss portion is a gap formed in the optical waveguide.   7. The optical waveguide element according to claim 5, wherein the radiation loss portion is an intersection where the optical waveguide intersects with another optical waveguide.   9. The optical waveguide device according to claim 5, wherein a width of the radiation loss portion is 15 μm to 30 μm.   The separation interval is a value (n + 1) times the separation interval where the number of the radiation loss portions is n (n is an integer of 1 or more) and the radiation loss due to the slit becomes the minimum value when there is no radiation loss portion. The optical waveguide device according to claim 5, wherein the optical waveguide device is set as follows.   The optical waveguide device according to claim 1, wherein the number of slits is two.   The optical waveguide element according to claim 1, wherein the optical waveguide element is a Mach-Zehnder optical interferometer element.

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