JP2012058696A - Waveguide type optical device and dp-qpsk type ln optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a waveguide type optical device which is small-sized, is capable of separating or combining polarized waves, and has low loss.SOLUTION: An optical waveguide including an optical waveguide for input and an optical waveguide for output is formed on a substrate, and a 3 dB coupler for input and a 3 dB coupler for output are provided, and two optical waveguides are provided between the 3 dB coupler for input and the 3 dB coupler for output. The two optical waveguides constitute arms of a Mach-Zehnder waveguide which includes a wide optical waveguide and a narrow optical waveguide having mutually different widths at least partially so that equivalent refractive indexes of propagated light are different due to structural birefringence. TE-mode light and TM-mode light incident from the optical waveguide for input is divided by about 3 dB after passing the 3 dB coupler for input, and light divided by about 3 dB has the phase changed by different amounts due to propagation through the wide optical waveguide and the narrow optical waveguide, so that a wave in mixture of TE and TM modes is propagated through the optical waveguide for output after passage through the 3 dB coupler for output.

Description

  The present invention relates to the field of low-loss optical devices that are small in size and capable of separating or synthesizing polarized waves.

A lithium niobate (LiNbO ₃ ) modulator is taken up as a high-performance optical component including an optical device having a function of combining or separating polarized waves. A traveling wave electrode type lithium niobate in which an optical waveguide and a traveling wave electrode are formed on a substrate having a so-called electro-optical effect (hereinafter referred to as an LN substrate) whose refractive index is changed by applying an electric field, such as lithium niobate. Optical modulators (hereinafter abbreviated as LN optical modulators) are applied to 2.5 Gbit / s, 10 Gbit / s large capacity optical transmission systems because of their excellent chirping characteristics. Recently, application to ultra-high-capacity optical transmission systems of 40 Gbit / s and further 100 Gbit / s has been studied and is expected as a key device.

(Conventional technology)
FIG. 9 shows a top view of the DP-QPSK type LN optical modulator applied to the 100 Gbit / system as a conventional technique disclosed in Patent Document 1. In the optical modulator 1, a waveguide and a modulation electrode (not shown) are formed on the LN substrate 10.

  Mach-Zehnder optical waveguides MB and MC are provided on both arms of the Mach-Zehnder optical waveguide MA. The waveguides 103 and 104 each have a nested structure.

  That is, the input light introduced from the input fiber 105 to the input optical waveguide 106 of the Mach-Zehnder optical waveguide MA is branched and input to the Mach-Zehnder optical waveguides MB and MC on the arm. The input light to the Mach-Zehnder optical waveguide MB is branched and input to the Mach-Zehnder optical waveguides 101 and 102, and the input light to the Mach-Zehnder optical waveguide MC is branched and input to the Mach-Zehnder optical waveguides 103 and 104. Is done.

  Further, the output lights from the Mach-Zehnder optical waveguides 101 and 102 are combined by the Mach-Zehnder optical waveguide MB and introduced into the arm 108 of the Mach-Zehnder optical waveguide MA, and the output lights from the Mach-Zehnder optical waveguides 103 and 104 are The signals are multiplexed by the Zender optical waveguide MC and introduced into the arm 109 of the Mach-Zehnder optical waveguide MA.

  A modulation signal (not shown) of each LN optical modulator 101 to 104 is given a drive signal of 25 Gb / s from a drive circuit (not shown), and each LN optical modulator 101 to 104 is modulated at 25 Gb / s. Outputs modulated light. Here, DQPSK (differential quadrature phase shift keying) is used as the modulation method of the LN optical modulators 101 and 102 of the Mach-Zehnder optical waveguide MB. The same applies to the modulation schemes of the LN optical modulators 103 and 104 of the Mach-Zehnder optical waveguide MC. Accordingly, 50 Gb / s modulated light is input to the arms 108 and 109 of the Mach-Zehnder optical waveguide MA.

  In the middle of the arm 108 of the Mach-Zehnder optical waveguide MA, a polarization rotation means 107 is provided. The polarization rotation means 107 has a function of rotating the polarization plane of light propagating through the arm 108 by 90 °. That is, for example, when the LN substrate 10 is a z-cut substrate, the TM mode propagates to the input optical waveguide 106 (accurately, although it is TM mode light, it is called in this way for simplicity. Hereinafter, the same applies to light in the TE mode), and the TM mode is converted to the TE mode (of course, in the case of an x-cut substrate, the TM mode and the TE mode may be interchanged). As a result, the light propagating through the arm 108 and the arm 109 of the Mach-Zehnder optical waveguide MA is in a state in which the planes of polarization are inclined by 90 ° from each other as in the TE mode and TM mode.

  The arms 108 and 109 of the Mach-Zehnder optical waveguide MA are arranged so as to have the same interval as the interval between the two holes of the two-hole ferrule collimator 111 on the output side end surface (the end surface on the right side in FIG. 9) of the LN substrate 10. Yes. The polarization beam combiner 20 functioning as an optical device for combining TM mode and TE mode light is a spatial collimating optical system in which optical elements of a two-hole ferrule collimator 111, a polarization beam combiner 110, and a one-hole ferrule collimator 112 are arranged. It is configured as a system.

  As the polarization beam combiner 110, for example, a material made of a bulk material such as rutile or calcite can be used. In particular, two plates (polarization separation plates 110A and 110B) of the same thickness such as rutile are used. A Savart plate is used that is laminated so that its optical axes are orthogonal.

  The 1-hole ferrule collimator 112 has a configuration in which a fiber collimator using a single mode fiber is built in the hole portion of the 1-hole ferrule, and is disposed to face the 2-hole ferrule collimator 111. Reference numeral 113 denotes an output optical fiber.

JP 2010-156842 A

  As described above, the conventional technology uses a polarization beam combining element made of a bulk material such as rutile or calcite as an optical device having a function of combining polarized waves of the TM mode and the TE mode, and the manufacturing cost is high. However, there is a problem that the structure becomes large. Furthermore, this prior art configuration also requires components such as a two-hole ferrule collimator and a one-hole ferrule collimator, leading to a further increase in cost and size. In other words, the high cost, complicated structure, and large dimensions of optical devices that have the function of synthesizing polarization in the prior art are problems that must be solved urgently, and there is an urgent need to develop optical devices that can solve these problems. there were.

  In order to solve the above-mentioned problem, the waveguide type optical device according to claim 1 of the present invention has an optical waveguide having an input optical waveguide and an output optical waveguide formed on a substrate, and the input type is 3 dB. Two 3 dB couplers, a coupler and an output 3 dB coupler, are provided with two optical waveguides between the input 3 dB coupler and the output 3 dB coupler, and the two optical waveguides Mach-Zehnder waveguide arm comprising a wide optical waveguide and a narrow optical waveguide at least partially different from each other so that the equivalent refractive index differs due to structural birefringence, The TE mode and TM mode light incident from the input optical waveguide is divided into approximately 3 dB after passing through the input 3 dB coupler, and the divided light is divided into approximately 3 dB each of the wide optical waveguide and the narrow width. Light Receiving each different amounts of phase change by propagating the road, after the output 3dB coupler passage, mixed wave of the TE mode and the TM mode to the output optical waveguide is characterized by the propagated. By configuring in this way, it functions as a polarization beam combiner.

  In order to solve the above-described problem, the waveguide type optical device according to claim 2 of the present invention includes an optical waveguide having an input optical waveguide and two output optical waveguides formed on a substrate. There are two 3 dB couplers, an input 3 dB coupler and an output 3 dB coupler, and two optical waveguides are provided between the input 3 dB coupler and the output 3 dB coupler, and the two optical waveguides are propagated. The arm of the Mach-Zehnder optical waveguide comprising a wide optical waveguide and a narrow optical waveguide at least partially different from each other is configured so that the equivalent refractive index of light is different due to structural birefringence. A TE mode and TM mode mixed wave incident from the input optical waveguide is divided into about 3 dB after passing through the input 3 dB coupler, and the light divided into about 3 dB is divided into the wide optical waveguide and Above Each of the two optical waveguides propagates through each of the two output optical waveguides after passing through the output 3 dB coupler. It is characterized by doing. With this configuration, it functions as a polarization separator.

  In order to solve the above-mentioned problems, a waveguide type optical device according to a third aspect of the present invention is the waveguide type optical device according to the first or second aspect, wherein the narrow optical waveguide has a structure complex. It has refraction and is characterized in that the TE mode and TM mode have different refractive indexes.

  In order to solve the above problems, a waveguide type optical device according to a fourth aspect of the present invention is the waveguide type optical device according to any one of the first to third aspects, wherein the narrow optical waveguide is used. Has a ridge structure.

  In order to solve the above-mentioned problem, a waveguide type optical device according to claim 5 of the present invention is the waveguide type optical device according to any one of claims 1 to 4, wherein the substrate is a semiconductor. It is characterized by that.

  In order to solve the above-described problem, a DP-QPSK LN optical modulator according to claim 6 of the present invention is a DP-QPSK LN optical modulator configured to include a polarization beam combiner. The synthesizing part is constituted by the waveguide type optical device according to claim 1.

  According to the present invention, by adjusting the structural birefringence of light propagating through the two optical waveguides by making the widths of the two optical waveguides different, the function of polarization synthesis or polarization separation that is small and low in manufacturing cost It is possible to realize an optical device which is a waveguide type optical device having the following. The present invention has an excellent feature that the function of polarization synthesis or polarization separation can be realized even if the refractive indexes of the TE mode and TM mode of the narrow optical waveguide are not equal.

Top view of an LN optical modulator to which the waveguide type optical device of the present invention is applied FIG. 5 is a partially enlarged view of 50 of FIG. 1, a waveguide type optical device according to the first embodiment of the present invention; Sectional drawing in AA 'of FIG. The figure explaining the principle of operation of the waveguide type optical device of the present invention The figure explaining the principle of operation of the waveguide type optical device of the present invention The figure explaining the principle of operation of the waveguide type optical device of the present invention Waveguide type optical device according to the second embodiment of the present invention Sectional drawing in BB 'of FIG. Top view of a prior art LN optical modulator having a polarization combiner (optical device) using bulk material

  Hereinafter, embodiments of the present invention will be described. Since the same reference numerals as those in the prior art shown in FIG. 9 correspond to the same parts, description of the parts having the same numbers is omitted here.

[First Embodiment]
FIG. 1 shows a DP-QPSK optical modulator incorporating a waveguide type optical device that is an optical device according to the first embodiment of the present invention. Here, a waveguide type optical device having a function of synthesizing TM mode and TE mode polarization is shown as 50. An enlarged view of the waveguide type optical device 50 of the first embodiment is shown in FIG. Moreover, FIG. 3 shows a cross-sectional view taken along the line AA ′ of FIG.

  Hereinafter, the case where the LN substrate 10 is a z-cut substrate will be discussed.

As shown in FIG. 2, the TE mode propagating through the arm 108 and the TM mode propagating through the arm 109 are incident on the 3 dB coupler 211 formed of a multimode interference system (MMI). After the light branched by the input 3 dB coupler (or simply 3 dB coupler) 211 is divided into 3 dB units and propagates through the arms 212 and 213, the arm 114 having a length L ₁ and a width W ₁ and a wide arm 114, length width _{L 2} propagates through the _{W 2} narrower arm 115. Then, after being synthesized as a light in which the TM mode and the TE mode are mixed by a Y branch having arms 116 and 117 (this Y branch may be called an output 3 dB coupler 215 or simply a 3 dB coupler), the output light Propagates to the waveguide 118 (functions as a so-called polarization combiner). Needless to say, the term arm used in this specification means an optical waveguide. A Mach-Zehnder optical waveguide is configured as the entire optical waveguide (from the arms 108 and 109 to the output optical waveguide 118).

  Next, the principle of the present invention will be discussed. In FIG. 4, the width of the arm 114 or the arm 115 is set as W on the horizontal axis (variable). Show. Although the equivalent refractive index is accurate, it is expressed as a refractive index for simplicity. In general, in the case of an LN substrate, the TE mode has a higher refractive index than the TM mode, but the electric vector is parallel to the surface of the LN substrate 10 than the TM mode in which the electric vector is perpendicular to the surface of the LN substrate 10. The mode is more susceptible to the width of the arm 114 and the arm 115.

  In general, when the refractive index of the TE mode and the refractive index of the TM mode are different from each other in terms of material constants, the birefringence is called material birefringence. In the case of an LN substrate, since the strain generated by thermally diffusing Ti to form an optical waveguide is extremely small, there is almost no influence of stress birefringence caused by stress. Therefore, in this specification, structural birefringence that can give a large change in the refractive index is considered.

Since the width _{W 1} of the arm 114 for example 10μm and wide, [Delta] n in FIG. 4 is relatively large (i.e., towards the refractive index of the TE mode is substantially higher than the refractive index of the TM mode). On the other hand, as the width W _{2 of the} arm 115 is reduced, Δn decreases as shown in FIG. 4 (condition I: W ₂ = 7 μm, condition II: W ₂ = 5.5 μm, condition III: W ₂ = 4 μm). You can also.

Thus, the width of the arm 114 and the arm 115 is set to be as wide as, for example, W ₁ = 10 μm and W ₂ = 5 μm, and the length L ₁ of the arm 114 and the length L ₂ of the arm 115 are appropriately set. By doing so, it is possible to add a phase difference to the light propagating through the arm 114 and the arm 115. As a result, the arm 116 and the arm 117 in FIG. 2 are efficiently combined by the Y branch forming the arm, and can be propagated to the output optical waveguide 118. As described above, in the present invention, the refractive index of the TE mode and the TM mode of the narrow arm 115 may be different (in other words, the narrow arm 115 may have structural birefringence).

  Therefore, the optical device having the polarization combining function can be operated by adopting the configuration shown in FIG. That is, in FIG. 2, arms 108 and 109 serve as input optical waveguides, 118 serves as an output optical waveguide, 3 dB coupler 211 serving as an MMI serves as an input 3 dB coupler, and Y branch arms 116 and 117 serve as an output 3 dB coupler.

  This process can be easily understood by considering the case where the mixed wave of the TM mode and the TE mode is incident from the output optical waveguide 118 (incident from the right in FIG. 2) in FIG. That is, after the light divided into 3 dB propagates through the arms 116 and 117, a phase difference occurs between the lights by the arms 114 and 115, and then propagates through the arms 112 and 113, and then the TE mode by the 3 dB coupler 211. Are demultiplexed into the TM mode and emitted to the arm 108 and the arm 109 (function as a so-called polarization separator). That is, in this case, in FIG. 2, 118 is an input optical waveguide, arms 108 and 109 are output optical waveguides, Y-branch arms 116 and 117 are input 3 dB couplers, and 3 dB coupler 211 that is an MMI is an output 3 dB coupler. Play a role.

Then, consider the case where the length _{L 2} of the length _{L 1} and the arm 115 of the arm 114 for example in order to simplify the explanation below was _L 1 ₌ L 2 = 1.5 _mm, or L 1> _{L 2,} L ₁ <may be _{L 2.}

Here, for simplicity, the width and length of the arm 212 and 213, and 116 and 117 by equal, is described by focusing on the length _{L 2} of the length _{L 1} and the arm 115 of the arm 114 However, in designing when the widths and lengths of the arms 212 and 213 and 116 and 117 are different, it is necessary to consider the total optical path lengths of the arms 212, 114, and 116 and the total optical path lengths of the arms 213, 115, and 117. Needless to say. The total optical path lengths of the arms 212, 114, and 116 and the total optical path lengths of the arms 213, 115, and 117 differ depending on the form of the 3 dB coupler that divides light into 3 dB, such as MMI, directional coupler, and Y branch. .

5 (it is better polarization extinction characteristic as the vertical axis increases) is an example of a polarization extinction ratio in a case where the difference between the width W ₂ of width W ₁ and the arm 115 and the variable arm 114. Here, L ₁ = L ₂ = 1.5 mm and W ₁ = 10 μm. Thus, it can be seen that the difference between the width W ₂ of width W ₁ and the arm 115 of the arm 114 for the structural birefringence is an optimum level. In other words, by utilizing the structural birefringence induced by changing the width of the arm, it is possible to realize a small-sized waveguide type optical device for synthesizing the polarization, which is provided by the arm 114 and the arm 115. There is an optimum value for the phase difference of the obtained light.

FIG. 6 shows the length L ₁ of the arm 114 for polarization synthesis and the optimum length L _{opt of the} length L ₂ of the arm 115 when the width W _{2 of the} arm 115 is a variable (for simplicity, Here, L ₁ = L ₂ , but L ₁ ≠ L ₂ may be used as described above. It can be seen that L _opt can be shortened as conditions I, II, and III correspond to FIG.

[Second Embodiment]
In the present invention, the change of the structural birefringence felt by the electric vectors of the TM mode and the TE mode is used. In the first embodiment, the influence of the width of the optical waveguide (arm) on the structural birefringence is used. Next, a more effective structure will be considered.

  FIG. 7 is a top view of the second embodiment of the present invention, and FIG. 8 is a sectional view taken along line BB ′ of FIG. Reference numerals 119 and 120 denote grooves formed by etching, which form 121 ridges. In this embodiment, grooves 119 and 120 are further formed in the first embodiment.

In the ridge structure to etch the side of the arm, towards the TE mode by utilizing the fact susceptible than TM modes, it is possible to increase the structural birefringence, properly setting the W _R of FIG. 8 By doing so, L _opt can be further shortened.

[Various embodiments]
In the above embodiment, the 3 dB coupler made of MMI is used on the light input side and the Y branch is used on the light output side, but this may be reversed, or both may use the MMI coupler or Y branch. Alternatively, a directional coupler may be used.

  Further, although the polarization synthesis for synthesizing the TM mode and the TE mode has been discussed, as described above, the polarization propagation for separating the TM mode and the TE mode may be reversed.

  In the above embodiment, the waveguide type optical device 50 is formed on the LN substrate 10. However, the waveguide type optical device 50 may be configured as a separate substrate independent of the LN substrate 10.

It goes without saying that the present invention can be applied not only to a dielectric substrate such as an LN substrate but also to a semiconductor substrate. In particular, in the case of a semiconductor optical waveguide, the magnitude relationship of the refractive index can be switched between the TE mode and the TM mode, so that the length of L _opt in FIG. 6 can be further shortened.

DESCRIPTION OF SYMBOLS 1: Optical modulator 10: LN board | substrate 20: Polarization synthesis part 50: Waveguide type | mold optical device 101-104: LN optical modulator 105: Optical fiber for input 106: Optical waveguide for input 107: Polarization rotation means 108, 109: Arm (or optical waveguide for output)
212, 213, 114, 115, 116, 117: Arm 110: Polarization combining element 110A, 110B: Polarization separator 111: Two-hole ferrule 112: One-hole ferrule 118: Output optical waveguide (or input optical waveguide)
119, 120: groove 121: ridge portion 211: input 3 dB coupler (or output 3 dB coupler)
215: Output 3 dB coupler (or input 3 dB coupler)
MA, MB, MC: Mach-Zehnder optical waveguide

Claims (6)

An optical waveguide having an input optical waveguide and an output optical waveguide is formed on a substrate, and has two 3 dB couplers, an input 3 dB coupler and an output 3 dB coupler, the input 3 dB coupler and the output Two optical waveguides are provided between the 3 dB couplers,
The two optical waveguides include a Mach including a wide optical waveguide and a narrow optical waveguide at least partially different in width so that the equivalent refractive index of propagating light differs due to structural birefringence. It constitutes the arm of the Zender waveguide,
The TE mode and TM mode light incident from the input optical waveguide is divided into approximately 3 dB after passing through the input 3 dB coupler,
The light divided into about 3 dB each undergoes a different amount of phase change by propagating through the wide optical waveguide and the narrow optical waveguide,
A waveguide type optical device, wherein a mixed wave of TE mode and TM mode propagates through the output optical waveguide after passing through the output 3 dB coupler. An optical waveguide having an input optical waveguide and two output optical waveguides is formed on a substrate. The optical waveguide has two 3 dB couplers, an input 3 dB coupler and an output 3 dB coupler. Two optical waveguides are provided between the output 3 dB couplers,
The two optical waveguides include a Mach including a wide optical waveguide and a narrow optical waveguide at least partially different in width so that the equivalent refractive index of propagating light differs due to structural birefringence. It constitutes the arm of the Zender optical waveguide,
A mixed wave of TE mode and TM mode incident from the input optical waveguide is divided into approximately 3 dB after passing through the input 3 dB coupler,
The light divided into about 3 dB each undergoes a different amount of phase change by propagating through the wide optical waveguide and the narrow optical waveguide,
A waveguide type optical device, wherein after passing through the output 3 dB coupler, TE mode light and TM mode light propagate to each of the two output optical waveguides.   3. The waveguide type optical device according to claim 1, wherein the narrow optical waveguide has structural birefringence, and the refractive indexes of TE mode and TM mode are different.   The waveguide optical device according to any one of claims 1 to 3, wherein the narrow optical waveguide has a ridge structure.   The waveguide type optical device according to any one of claims 1 to 4, wherein the substrate is a semiconductor.   2. A DP-QPSK LN optical modulator including a polarization beam combiner, wherein the polarization beam combiner includes the waveguide type optical device according to claim 1. Type LN optical modulator.

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