JP5421935B2 - Light modulator - Google Patents

Description

  The present invention belongs to the field of optical modulators that are high-speed, low in driving voltage, low in DC bias voltage, and good in production yield.

An optical waveguide and a traveling wave electrode are provided on a substrate having a so-called electro-optic effect (hereinafter, the lithium niobate substrate is abbreviated as an LN substrate) such as lithium niobate (LiNbO ₃ ) whose refractive index is changed by applying an electric field. The formed traveling wave electrode type lithium niobate optical modulator (hereinafter abbreviated as LN optical modulator) is applied to a 2.5 Gbit / s, 10 Gbit / s large capacity optical transmission system because of its excellent chirping characteristics. Yes. Recently, application to ultra-high-capacity optical transmission systems of 40 Gbit / s or 100 Gbit / s has been studied, and is expected as a key device.

(First prior art)
The LN optical modulator includes a type using a z-cut substrate and a type using an x-cut substrate (or y-cut substrate). Here, as a first prior art, an LN optical modulator in which the concept of Patent Document 1 is applied to a z-cut LN substrate and a two-electrode coplanar waveguide (CPW) traveling wave electrode is shown, and a schematic top view thereof is shown in FIG. Shown in

In the figure, 1 is a z-cut LN substrate, 2 is a 1.3 μm or 1.55 μm transparent SiO ₂ buffer layer having a thickness D in the wavelength region used in optical communications (thickness D ranges from 200 nm to 1 μm). 3) is an optical waveguide formed by thermal diffusion at 1050 ° C. for about 10 hours after Ti is deposited on the z-cut LN substrate 1, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide).

  Note that the interaction optical waveguide in which the refractive index changes when an electric field is applied interacts with the high-frequency electrical signal interaction unit 5 where the high-frequency electrical signal interacts with the light, the DC bias voltage (or bias voltage), and the light. It comprises a DC bias interaction section 6. That is, the high-frequency electrical signal interaction unit 5 and the DC bias interaction unit 6 are constituted by optical waveguides that constitute the two arms of the Mach-Zehnder optical waveguide, and the two high-frequency electrical signal interaction units. The arms (interactive optical waveguides for high-frequency electrical signals) are 3a and 3b, and the two arms (DC bias interactive optical waveguides) of the DC bias interaction unit are 3a 'and 3b'.

  In this two-electrode type LN optical modulator, center conductors (center electrodes) 4a and 4b are formed above the high-frequency electrical signal interaction optical waveguides 3a and 3b, respectively. Reference numerals 4c, 4d, and 4e denote ground conductors (ground electrodes). The CPW traveling wave electrode 4 in the high-frequency electrical signal interaction section 5 is composed of center conductors 4a and 4b and ground conductors 4c, 4d and 4e.

  FIG. 9 is a cross-sectional view taken along the line AA ′ of FIG. In the DC bias interaction section 6, DC bias electrodes 7a and 7b are formed above the optical waveguides 3a ′ and 3b ′, respectively. 8a and 8b are electric lines of force of DC bias.

  The DC bias electrode 7a above the optical waveguide 3a 'is a central conductor (center electrode) in the DC bias interaction unit 6, and the DC bias electrode 7b in the vicinity of the DC bias electrode 7a is a side conductor (side electrode). The DC bias electrode 7b above the optical waveguide 3b 'is the central conductor (center electrode) in the DC bias interaction section 6, and the DC bias electrode 7a in the vicinity of the DC bias electrode 7b is the side conductor (side electrode). is there.

  As can be seen from FIG. 9, the electric force line 8a is directed from the DC bias electrode 7a to the DC bias electrode 7b in the optical waveguide 3a ′, and the downward electric force line 8a is applied to the optical waveguide 3a ′. On the other hand, since the electric force line 8b is directed from the DC bias electrode 7a to the DC bias electrode 7b also in the optical waveguide 3b ′, the upward electric force line 8b is applied to the optical waveguide 3b ′. As a result, the signs of refractive index changes occurring in the optical waveguides 3a ′ and 3b ′ are opposite to each other.

And the wave function of light [psi _a propagating through the optical waveguide 3a, the electric field E _a of electrical power lines 8a toward emitted by DC bias electrode 7b from the DC bias electrode 7a formed above the optical waveguide 3a refractive index change [Delta] n _a of the optical waveguides 3a resulting from the interaction

Δn∝∬ψ _a ² (x, y) E _a (x, y) dxdy (1)

It is expressed.

  In Patent Document 1, the distance between the optical waveguides 3a ′ and 3b ′ as compared with the distance G between the adjacent DC bias electrodes 7a and 7b (in FIG. 9, the distance between the centers of the optical waveguides 3a ′ and 3b ′). S is sufficiently large, and there are almost no lines of electric force from the DC bias electrode 7a above the optical waveguide 3a ′ to the DC bias electrode 7b above the optical waveguide 3b ′. That is, only the electric force line 8a contributes to the change in the refractive index of the optical waveguide 3a ', and only the electric force line 8b contributes to the change in the refractive index of the optical waveguide 3b'. In this sense, it can be said that the DC bias electrodes 7a and 7b in the first prior art shown in FIG. 9 have an asymmetric coplanar strip (ACPS) structure. As a result of detailed simulation of the ACPS structure, we have found that there is a problem that the refractive index change Δn given by equation (1) is small when a DC bias is applied.

(Second prior art)
FIG. 10 shows a schematic top view of the LN optical modulator disclosed in Patent Document 2. As shown in FIG. In the DC bias interaction unit 6, 9a and 9b are DC bias electrodes. A cross-sectional view taken along the line BB ′ of FIG. 10 is shown in FIG.

  In the second prior art, there are DC bias electrodes 9b on both the left and right sides of the DC bias electrode 9a above the optical waveguide 3a ', and the DC bias electrodes 9a on both the left and right sides of the DC bias electrode 9b above the optical waveguide 3b'. Exists. In other words, the DC bias electrode has a CPW structure, and the efficiency of the refractive index change given by the equation (1) is higher than that of the first prior art of the ACPS structure.

  The DC bias electrode 9a above the optical waveguide 3a 'is the central conductor (center electrode) in the DC bias interaction section 6, and the DC bias electrode 9b in the vicinity of both sides of the line is the side conductor (side electrode). The DC bias electrode 9b above the optical waveguide 3b 'is the central conductor (center electrode) in the DC bias interaction section 6, and the DC bias electrode 9a in the vicinity of both sides of the side is the side conductor (side electrode). ).

  As in the first prior art, the distance S between the optical waveguides 3a ′ and 3b ′ is sufficiently larger than the distance G between the adjacent DC bias electrodes 9a and 9b in this prior art. There are almost no lines of electric force from the DC bias electrode 9a above the DC bias electrode 9b to the DC bias electrode 9b above the optical waveguide 3b ′.

  In this second prior art, when the DC bias electrodes 9a and 9b above the optical waveguides 3a 'and 3b' are considered as the central conductors, they are CPWs having side conductors 9b and 9a in the vicinity of both sides thereof. Structure. That is, as can be seen from FIG. 11, DC bias electrodes 9b and 9a always exist between the optical waveguides 3a ′ and 3b ′. Therefore, when the distance S between the optical waveguides 3a ′ and 3b ′ becomes small, the DC bias electrodes 9b and 9a become extremely thin and patterning becomes difficult, short-circuiting between the DC bias electrodes 9a and 9b is likely to occur, or electrical discharge due to ESD occurs. There has been a problem of disruptive destruction.

Japanese Patent Laid-Open No. 62-14627 Japanese Patent No. 3860443

  As described above, the first conventional technique has a problem that the efficiency of the refractive index change in the DC bias portion is poor. In the second conventional technique, when the distance between the DC bias interaction optical waveguides is reduced, There are problems that patterning becomes difficult, short-circuiting easily occurs between DC bias electrodes, or electrical breakdown due to ESD occurs.

In order to solve the above problems, an optical modulator according to claim 1 of the present invention is a substrate having an electro-optic effect, at least two optical waveguides for guiding light formed on the substrate, A traveling wave electrode formed on one side of the substrate and having a center electrode and a ground electrode for a high frequency electrical signal through which the high frequency electrical signal for modulating the light propagates; and a bias electrode for applying a bias voltage to the light A high-frequency electrical signal interaction unit for modulating the phase of the light by applying the high-frequency electrical signal to the traveling-wave electrode, and applying a bias voltage to the bias electrode. And a bias interaction unit for adjusting the phase of the light, wherein the bias electrode is located above one of the two optical waveguides of the bias interaction unit. A first bias electrode formed; a second bias electrode formed over the other of the two optical waveguides of the bias interaction portion; and a part of the second bias electrode, A side electrode formed on the opposite side of the second bias electrode with one bias electrode in between, and a part of the first bias electrode, the second bias electrode on the other optical waveguide And another side electrode formed on the opposite side of the first bias electrode on the one optical waveguide, with the edge of the first bias electrode at the bias interaction portion the distance between the edge of the second bias electrode (G ₂₎ is the distance (G ₃₎ of the edge of the edge and the side location electrode of said first bias _electrode, and said another edge of the second bias electrode With the edge of the side electrode Is less than or equal to three times of the release _{(G 1),}
Said first bias electrode, wherein the second bias electrode, and the side location electrode to forming the CPW structure lines of electric force emitted from the first bias electrode is coupled to said second bias electrode and the side location electrode In addition, the second bias electrode on the other optical waveguide, the first bias electrode on the one optical waveguide, and the other side electrode are connected to the first bias electrode and the other side electrode, respectively. the emitted electrical field lines is characterized in that it formed a CPW structure that binds to the second bias electrode.

In order to solve the above-described problem, an optical modulator according to a second aspect of the present invention is the optical modulator according to the first aspect , wherein the distance between the two optical waveguides in the bias interaction unit is It is characterized by being narrower than the distance between the two optical waveguides in the high-frequency electrical signal interaction part.

In order to solve the above-described problems, an optical modulator according to a third aspect of the present invention is the optical modulator according to the first or second aspect , wherein the optical waveguide is a child Mach-Zehnder on a branch optical waveguide of a parent Mach-Zehnder optical waveguide. becomes a nested optical waveguide having an optical waveguide, respectively, together comprise the child Mach said the dust optical waveguide to the high frequency electric signal interaction portion by a scan for interaction portion, the bi for a scan to the parent Mach-Zehnder optical waveguide It is characterized by having an interaction part.

In order to solve the above-mentioned problems, an optical modulator according to a fourth aspect of the present invention is the optical modulator according to the third aspect , wherein two optical components in the parent Mach-Zehnder optical waveguide in which the bias interaction section is formed. The distance between the waveguides is an intermediate line between two optical waveguides in the first child Mach-Zehnder optical waveguide, and the second child Mach-Zehnder optical waveguide adjacent to the first child Mach-Zehnder optical waveguide. It is characterized by being narrower than the distance from the intermediate line of the two optical waveguides.

  According to the present invention, in the DC bias interaction section, the DC bias electrode required in the second prior art is eliminated, and the DC bias electrodes formed above the optical waveguide are electrically coupled to each other. By making the optical waveguide closer, it is possible to realize a refractive index change with higher efficiency than the first prior art, and the difficulty in patterning, which has been a problem in the second prior art, between the DC bias electrodes 9a and 9b. It is possible to solve the problem of short circuiting or electrical breakdown due to ESD.

Schematic top view of the first embodiment of the optical modulator of the present invention. Sectional drawing in the CC 'line of FIG. 1 showing 1st Embodiment of this invention Schematic top view of the second embodiment of the optical modulator of the present invention. Schematic top view of the third embodiment of the optical modulator of the present invention. Schematic top view of the fourth embodiment of the optical modulator of the present invention. Sectional drawing in the FF 'line | wire of FIG. 5 showing 4th Embodiment of this invention Schematic top view of the fifth embodiment of the optical modulator of the present invention. Schematic top view of the first prior art Sectional drawing in the AA 'line of FIG. 8 showing 1st prior art Schematic top view of the second prior art Sectional drawing in the BB 'line of FIG. 10 showing the 2nd prior art

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the conventional embodiments shown in FIGS. 8 to 11 correspond to the same functional units, description of the functional units having the same numbers is omitted here. To do.

(First embodiment)
FIG. 1 shows a schematic top view of the first embodiment of the present invention. This embodiment is composed of a high-frequency electrical signal interaction unit 5 and a DC bias interaction unit 6 ″. The high-frequency electrical signal interaction unit 5 includes the two-electrode traveling wave electrode 4 as in the first and second prior arts shown in FIGS. FIG. 2 is a cross-sectional view taken along the line CC ′ of FIG. Here, a DC bias electrode 11b is formed above the optical waveguide 3a ', and a DC bias electrode 11a is formed above the optical waveguide 3b'.

  As can be seen from FIG. 2, in the first embodiment, the distance in the DC bias interaction unit 6 ″ is narrower than the distance between the two arms in the high frequency electrical signal interaction unit 5. Yes. In this configuration, the distance S between the optical waveguides 3a ′ and 3b ′ in the DC bias interaction portion 6 ″ is sufficiently small as compared with the first conventional technology and the second conventional technology. As a result, the electric lines of force 12a and 12c emitted from the DC bias electrode 11b reach the two (both adjacent) DC bias electrodes 11a, respectively. Accordingly, the DC bias electrode 11b above the optical waveguide 3a ′ has a CPW structure with the DC bias electrode 11b as a central conductor, and the DC bias electrode 11a above the optical waveguide 3b ′. A CPW structure having a central conductor as a center.

  The DC bias electrode 11b above the optical waveguide 3a 'is the central conductor (center electrode) in the DC bias interaction section 6, and the DC bias electrode 11a near the left side of the line is the side conductor (side electrode). It is. There is no side conductor near the right side of the line. On the other hand, the DC bias electrode 11a above the optical waveguide 3b ′ is the central conductor (center electrode) in the DC bias interaction section 6, and the DC bias electrode 11b in the vicinity of the right side of the line is the side conductor (side electrode). It is. There is no side conductor near the left side of the line.

  That is, in the present invention, unlike the second prior art, there is no side conductor between them even if the distance S between the optical waveguides 3a ′ and 3b ′ is reduced. There is an advantage that the problem of patterning of the DC bias electrode, the electrical short circuit, or the electrode breakdown due to ESD can be solved.

Here, the distance between _{G 2,} the side location and DC bias electrode 11b above the optical waveguide 3a' between upper 11a above the DC bias electrode 11b and the optical waveguide 3b' optical waveguide 3a' the present invention the ratio of the distance between the distance _{G 1} between the electrodes 11a, or the distance _{G 2,} the ratio of the distance DC bias electrode 11a above the optical waveguide 3b' and distance _{G 3} between its side location electrodes 11b is important. That is, it is desirable that these ratios are about 5 times or less, preferably 3 times or less. Alternatively, if it is twice or less, particularly the same level, there is a remarkable effect.

  The important point in the present invention is that the DC bias electrodes 11a and 11b located above the DC bias interaction optical waveguides 3a ′ and 3b ′ can be electrically coupled to each other.

(Second Embodiment)
FIG. 3 is a schematic top view of a second embodiment in which the present invention is applied to a single electrode type LN optical modulator. The present embodiment is composed of a high-frequency electrical signal interaction unit 5 ′ and a DC bias interaction unit 6 ″. The configuration of the high-frequency electrical signal interaction unit 5 ′ is different from that of the first embodiment.

  The high-frequency electrical signal interaction section 5 ′ is composed of a central conductor 4a ′ formed above the high-frequency electrical signal interaction optical waveguide 3a and ground conductors 4c ′ and 4d ′ formed side by side on both sides thereof. It has a single electrode structure. Also in the present embodiment, the present invention can exhibit the effect effectively.

(Third embodiment)
FIG. 4 is a schematic top view of a third embodiment in which the present invention is applied to a DQPSK type LN optical modulator. The present embodiment is composed of a high-frequency electrical signal interaction section 5 ″, a DC bias interaction section 6 ″ ″, and a DC bias interaction section 6 ″, on the branch optical waveguide of the parent Mach-Zehnder optical waveguide. Nest type optical waveguides having two child Mach-Zehnder optical waveguides (combinations of 30a and 30a 'and 30b and 30b').

  6 ″ ″ is a DC bias interaction unit for a child Mach-Zehnder, and 13a, 13b, 13c, and 13d are DC formed above the high-frequency electrical signal interaction optical waveguides 30a, 30a ′, 30b, and 30b ′. Bias electrode. Reference numeral 6 ″ denotes a DC bias interaction unit for the parent Mach-Zehnder, which is the same as in the first embodiment. In order to simplify the description, the domain-inverted structure and traveling wave electrode necessary for the single electrode type are omitted.

  In this embodiment, the distance between the two optical waveguides 3a ′ and 3b ′ in the parent Mach-Zehnder optical waveguide in which the bias interaction section 6 ″ is formed is equal to the intermediate line between the child Mach-Zehnder optical waveguides 30a and 30a ′. The distance between the adjacent child Mach-Zehnder optical waveguides 30b and 30b ′ is smaller than the distance L. Of course, the present invention is not limited to this configuration.

  Needless to say, the traveling wave electrode formed on the child Mach-Zehnder in the DQPSK type LN modulator may be a two-electrode type. Also in the present embodiment, the present invention can exhibit the effect effectively.

(Fourth embodiment)
FIG. 5 is a schematic top view of a fourth embodiment in which the present invention is applied to a single electrode type LN optical modulator. The present embodiment is composed of a high-frequency electrical signal interaction unit 5 ′ and a DC bias interaction unit 6 ″ ″. The configuration of the DC bias interaction unit 6 ″ ″ is different from that of the second embodiment.

  In the present embodiment, as the configuration of the DC bias electrodes 14a and 14b of the DC bias interaction unit 6 ″, the optical waveguide 3a ′ has a CPW structure, but the optical waveguide 3b ′ has an ACPS structure. 6 is a cross-sectional view taken along line FF ′ of FIG. Reference numerals 15a and 15b denote electric lines of force. Although the reduction effect of the DC bias voltage is slightly deteriorated as compared with the first embodiment having the CPW structure for both of the optical waveguides 3a ′ and 3b ′, the effect of the present invention that the DC bias can be reduced can be realized.

(Fifth embodiment)
FIG. 7 is a schematic top view of the fifth embodiment of the present invention. The present embodiment is composed of a high-frequency electrical signal interaction unit 5 ″ ″ and a DC bias interaction unit 6 ″. In the present embodiment, the distance between the two optical waveguides 3a and 3b in the DC bias interaction section 6 "is narrower than the two optical waveguides 3a and 3b in the high-frequency electrical signal interaction section 5"". ing. Thereby, since the high frequency electrical signal interaction unit 5 '''' and the DC bias interaction unit 6 '' can be designed independently, there is an advantage that the DC bias voltage can be further reduced.

(About each embodiment)
Although the planar structure has been described in the above embodiment, it goes without saying that a ridge structure may be used.

  Further, the distance between the two optical waveguides in the high-frequency electrical signal interaction unit and the distance between the two optical waveguides in the DC bias interaction unit may be the same or different. .

  Further, although the CPW electrode has been described as an example of the traveling wave electrode, it goes without saying that various traveling wave electrodes such as an asymmetric coplanar strip (ACPS) and a symmetric coplanar strip (CPS), or a lumped constant electrode may be used. Further, the case where there are one and two Mach-Zehnder interference systems has been described, but the present invention can also be applied to a complicated nest structure having more Mach-Zehnder interference systems including DP-QPSK.

  In the above description, the high-frequency electrical signal interaction unit has been described with reference to a drawing that does not include polarization reversal. However, whether the high-frequency electrical signal interaction unit includes or does not include polarization reversal is discussed with respect to the DC bias interaction unit. Needless to say, polarization inversion may be included.

  It goes without saying that not only the LN substrate but also other substrates such as lithium tantalate and semiconductors may be used.

1: z-cut LN substrate 2: SiO ₂ buffer layer 3: optical waveguide 3a, 3b, 30a, 30a ′, 30b, 30b ′: optical waveguide 3a ′, 3b ′ for high frequency electric signal interaction unit: for DC bias Optical waveguide 4, 4 ': traveling wave electrode 4a, 4a', 4b: central conductor of traveling wave electrode 4c, 4c ', 4d, 4d', 4e: grounding conductor of traveling wave electrode 5, 5 ', 5 ″, 5 ″ ″: high-frequency electrical signal interaction unit 6, 6 ′, 6 ″, 6 ″ ″: DC bias interaction unit 7a, 7b, 9a, 9b, 11a, 11b, 13a, 13b, 13c, 13d, 14a, 14b: DC bias electrodes 8a, 8b, 10a, 10b, 12a, 12b, 12c, 15a, 15b: lines of electric force

Claims (4)

A substrate having an electro-optic effect, at least two optical waveguides for guiding light formed on the substrate, and a high-frequency electric signal that is formed on one surface side of the substrate and modulates the light propagates A traveling wave electrode having a center electrode and a ground electrode for high-frequency electrical signals, and a bias electrode for applying a bias voltage to the light,
The optical waveguide has a high frequency electrical signal interaction unit for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode, and a bias voltage applied to the bias electrode. In an optical modulator comprising a bias interaction unit for adjusting the phase of light,
The bias electrode is
A first bias electrode formed above one of the two optical waveguides of the bias interaction unit;
A second bias electrode formed above the other of the two optical waveguides of the bias interaction unit;
A side electrode that is a part of the second bias electrode and is formed on the opposite side of the second bias electrode with the first bias electrode in between ;
Another part of the first bias electrode formed on the opposite side of the first bias electrode on the one optical waveguide with the second bias electrode on the other optical waveguide in between A side electrode ,
In the bias interaction unit, the distance (G ₂ ) between the edge of the first bias electrode and the edge of the second bias electrode is the distance between the edge of the first bias electrode and the edge of the side electrode ( G ₃ ), and not more than three times the distance (G ₁ ) between the edge of the second bias electrode and the edge of the other side electrode ,
Said first bias electrode, wherein the second bias electrode, and the side location electrode to forming the CPW structure lines of electric force emitted from the first bias electrode is coupled to said second bias electrode and the side location electrode With
The second bias electrode on the other optical waveguide, the first bias electrode on the one optical waveguide, and the other side electrode are emitted from the first bias electrode and the other side electrode. optical modulator, wherein the electric force lines are formed a CPW structure that binds to the second bias electrode. 2. The distance between the two optical waveguides in the bias interaction unit is narrower than the distance between the two optical waveguides in the high-frequency electrical signal interaction unit. Light modulator. The optical waveguide is a nested optical waveguide having a child Mach-Zehnder optical waveguide on a branch optical waveguide of the parent Mach-Zehnder optical waveguide,
3. The child Mach-Zehnder optical waveguide includes the high-frequency electrical signal interaction unit and the bias interaction unit, and the parent Mach-Zehnder optical waveguide includes the bias interaction unit. The light modulator described . The distance between the two optical waveguides in the parent Mach-Zehnder optical waveguide in which the bias interaction portion is formed is an intermediate line between the two optical waveguides in the first Child Mach-Zehnder optical waveguide. The optical modulator according to claim 3, wherein the optical modulator is narrower than a distance between the first child Mach-Zehnder optical waveguide and an intermediate line of two optical waveguides in the adjacent second Child Mach-Zehnder optical waveguide .

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