JP5416658B2 - Optical modulator and optical modulator module - Google Patents

Description

  The present invention belongs to the field of optical modulators having a low driving voltage and capable of high-speed modulation.

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 high capacity optical transmission system of 2.5 Gbps and 10 Gbps because of its excellent transmission characteristics. Recently, application to an ultra-high capacity optical transmission system of 40 Gbps has been studied, and it is expected as a key device.

[First prior art]
This LN optical modulator includes a type using a z-cut LN substrate and a type using an x-cut LN substrate (or a y-cut LN substrate). Here, a z-cut LN substrate disclosed in Patent Document 1 as a first prior art and a z-cut LN substrate and two ground conductors, and a coplanar waveguide (CPW) traveling wave electrode advantageous for fundamental mode propagation are used. FIG. 26 shows a perspective view of the cut LN substrate LN optical modulator. 27 is a cross-sectional view taken along line AA ′ of FIG. Note that the following discussion also holds true for x-cut LN substrates and y-cut LN substrates.

In the figure, 1 is a z-cut LN substrate, 2 is a transparent SiO ₂ buffer layer having a thickness of about 200 nm to 1 μm in a wavelength region used in optical communication such as 1.3 μm or 1.55 μm, and 3 is a z-cut LN. This is an optical waveguide formed by thermally diffusing Ti at 1050 ° C. for about 10 hours after depositing Ti on the substrate 1, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). Here, in order to simplify the description, the Si conductive layer that is normally used to suppress the temperature drift due to the pyroelectric effect is omitted.

  Reference numerals 3a and 3b denote optical waveguides (or interactive optical waveguides) in a portion where an electrical signal and light interact (referred to as an interaction portion), that is, two arms of a Mach-Zehnder optical waveguide. In the case of a phase modulator, a straight optical waveguide may be used. The CPW traveling wave electrode 4 includes a central conductor 4a and ground conductors 4b and 4c. In FIG. 27, the width of the center conductor 4a is about 6 to 20 μm, and generally around 10 μm is used. On the other hand, a gap (or CPW gap) is formed between the center conductor 4a and the ground conductors 4b and 4c.

In the first prior art, a bias voltage (usually a DC bias voltage) and a high-frequency electric signal (also referred to as an RF electric signal) are superimposed and applied between the center conductor 4a and the ground conductors 4b and 4c. The argument holds true for the bias separation type. Further, SiO ₂ buffer layer 2 is equivalent refractive index of the high frequency electric signal _{n m} (or microwave equivalent refractive index _{n m)} the optical waveguide 3a, 3b by approximating the effective refractive index _{n o} of the light propagating the light It plays an important role in expanding the modulation band.

  Next, the operation of the LN optical modulator configured as described above will be described. In order to operate this LN optical modulator, it is necessary to apply a DC bias voltage and an RF electrical signal between the center conductor 4a and the ground conductors 4b and 4c.

The voltage-light output characteristic shown in FIG. 28 is the voltage-light output characteristic of the LN optical modulator in a certain state, and _Vb is the DC bias voltage at that time. As shown in FIG. 28, the DC bias voltage _Vb is normally set at the midpoint between the peak and bottom of the light output characteristic.

  When an LN optical modulator is used in an optical transmission system, an optical modulator module in which a chip of an LN optical modulator, an optical fiber, and a microwave connector for electric signals are fixed to a metal casing (package). I have to.

FIG. 29 shows the structure of the optical modulator module. Some numbers in the figure are the same as those in FIGS. For simplicity, the SiO ₂ buffer layer 2 is omitted.

  Here, 5 is a metal casing, and 6a and 6b are core wires of a microwave connector (not shown) fixed to the metal casing 5. 7a and 7b are cavities formed around the core wires 6a and 6b of the microwave connector. After fixing the LN substrate 1 to the metal housing 5, the CPW traveling wave electrode and the core wires 6a and 6b of the microwave connector are electrically connected.

  Although not shown in the drawing, an optical system is set so that light can be input to and output from the LN optical waveguide 3. Next, the metal lid 8 is fixed to the metal housing 5 to complete the LN optical modulator module. Note that the output side of the high-frequency electrical signal (as seen by comparing FIGS. 26 and 29, the output side of the high-frequency electrical signal corresponds to the side of 6b and 7b) is electrically terminated by a terminating resistor. In that case, the core wire 6b and the cavity 7b of the microwave connector on the output side are unnecessary.

  FIG. 30 shows an enlarged view of the core wire 6a of the microwave connector and the cavity 7a formed in the metal housing 5 in FIG. Here, EF is a line of electric force of a high-frequency electric signal generated between the core wire 6 a of the microwave connector and the cavity 7 a formed in the metal housing 5.

  As can be seen from FIG. 30, the electric force lines EF of the high-frequency electric signal have an axially symmetric distribution around the core wire 6a of the microwave connector. On the other hand, FIG. 31 shows BB ′ in the vicinity of a connection portion (referred to as an input feedthrough portion) 50 of the traveling wave electrode formed on the z-cut LN substrate 1 of FIG. 29 with the core wire 6a of the microwave connector. A cross-sectional view in line is shown. Here, DEF represents a line of electric force generated between the center conductor 4a and the ground conductor 4b. As can be seen from FIG. 31, the electric force lines DEF have a planar distribution at B-B ′ in the input feedthrough portion 50.

  In this way, the electric force lines EF having an axially symmetric distribution (referred to as the eigenmode of the microwave connector) around the core wire 6a of the microwave connector, and between the central conductor 4a of the traveling wave electrode 4 and the ground conductors 4b and 4c. The electric field lines DEF having a planar distribution (referred to as the eigenmode of the CPW traveling wave electrode) generated in FIG. 2 have mismatched distribution shapes, and high-frequency electric signals are transmitted from the core wire 6a of the microwave connector to the traveling wave electrode 4. When propagating to the center conductor 4a and the ground conductor 4b, a component that leaks to the high-frequency electric signal is generated. This is shown in FIG. FIG. 32 is a top view of the z-cut LN substrate 1, and 11 represents a high-frequency electrical signal leaking into the z-cut LN substrate. In general, the leaked high-frequency electric signal is also referred to as a leaky high-frequency electric signal.

FIG. 33 shows the shape and dimensions of the z-cut LN substrate 1 used in the LN optical modulator. In the drawing, the x direction corresponds to the width direction, the z direction corresponds to the thickness direction, and the y direction corresponds to the length direction, and the respective dimensions are L _x , L _z , and L _y . For the leaked high-frequency electrical signal component 11, the z-cut LN substrate 1 functions as a dielectric resonator. That is, it is considered that the leaked high frequency electric signal component excites a resonance mode in the z-cut LN substrate 1 and resonates (referred to as dielectric resonance) therein.

  Once dielectric resonance occurs at that frequency, much of the energy of the high-frequency electrical signal that should propagate from the core wire 6a of the microwave connector to the central conductor 4a and the ground conductors 4b and 4c of the traveling wave electrode is reduced to the z-cut LN substrate 1. It is consumed to excite the resonance mode inside the dielectric resonator composed of the above, and is not effectively used for the modulation of light in the interaction optical waveguides 3a and 3b shown in FIGS. Therefore, a drastic drop (referred to as frequency dip) occurs in the modulation index of light with respect to the modulation frequency.

  For example, when the modulation rate is 10 Gbps, this frequency dip occurs in the frequency region around 10 GHz or lower, or when the modulation rate is 40 Gbps, this frequency dip occurs in the frequency region near 30 GHz or lower. This is a very serious problem in practical use.

[Second prior art]
Proposed in Patent Document 2 to improve the influence of the frequency dip on the modulation index of light by removing the resonance frequency as a dielectric resonator in the first prior art to a higher frequency side than the frequency region necessary for light modulation This technique will be described as a second conventional technique. Here, since the same numbers as those in the first prior art shown in FIGS. 26 to 33 correspond to the same function units, the description of the function units having the same numbers is omitted here.

FIG. 34 shows a cross-sectional view of the z-cut LN substrate 1 of FIG. According to Patent Document 2, the frequency dips f _c of the modulation index

f _c = c ₀ / (2n · d) (1)

It is said that it is given in. Here, c ₀ is the speed of light in vacuum, n is the equivalent refractive index of the high-frequency electrical signal, and d is an important physical quantity, which is the longest length in the cross-sectional view shown in FIG. ).

  Then, the product n · d of the equivalent refractive index n of the high-frequency electrical signal and the length d of the diagonal line is made larger than 0.8 mm and smaller than 11 mm, so that the frequency dip is a high frequency that does not interfere with optical transmission at 10 Gbps. It can be shifted to.

  We actually designed and manufactured an optical modulator using the z-cut LN substrate 1 according to the equation (1). In designing, the equivalent refractive index of the high-frequency electrical signal is 6.56 in the x direction and 5.29 in the z direction in FIG.

  However, when the LN optical modulator is designed and manufactured based on the requirement (1) of the second prior art, it is found that it is difficult to make the LN optical modulator with a high yield from the viewpoint of mechanical strength. It was.

That is, when the 40GHz frequency dip _{f c} as an example, (1) the length of a diagonal line in the cross-sectional view of the z- cut LN substrate 1 from the equation d becomes 0.63 mm. Therefore, assuming that the thickness (L _z ) of the z-cut LN substrate 1 is 0.5 mm, the width (L _x ) of the z-cut LN substrate 1 is 0.39 mm.

Usually, since the length (L _y ) of the z-cut LN substrate 1 is several tens of mm, if the dimension of the cross section of the z-cut LN substrate 1 becomes too small, the rigidity of the z-cut LN substrate 1 is reduced. It becomes extremely weak. For this reason, it has been found that there is a problem in mechanical strength due to an increase in the number of elements that are destroyed in handling in the cutting / polishing process after completion of the wafer process and in the process of mounting the metal housing 5 as an LN optical modulator.

[Third prior art]
FIG. 35 shows a third conventional technique shown for the DQPSK type transmission apparatus proposed in Patent Document 3. In FIG. 35, in addition to the LN optical modulator, a DQPSK signal source 700 that generates a high-frequency electrical signal, an LD 500 that emits light, an ABC circuit 100 that stabilizes a bias, and a phase difference of light between two Child Mach-Zehnder interference systems A bias supply unit 120-2 for supplying a bias voltage for π shift, a phase shift control unit 110 for issuing a phase shift control command, or a bias supply unit 120-3 for supplying a bias voltage for π / 2 shift In other words, FIG. 35 is a DQPSK transmitter.

  Hereinafter, the configuration of the conventional technology will be described. Reference numerals 200 and 300 denote child MZs, reference numerals 200b and 300b denote modulation electrodes (traveling wave electrodes), reference numerals 200a and 300a denote Y branch optical waveguides of the child MZ, and reference numerals 200c and 300c denote bias electrodes of the π shift unit. 500 is a light source composed of an LD (laser diode), 400 and 600 are parent MZ Y-branch optical waveguides, 700 is a DQPSK signal source, 800-1 and 800-2 are drivers, 900 is a PD (photodiode), 100 is ABC (Auto-bias control) circuit, 110 is a phase shift control unit, 120-1 and 120-2 are so-called π shift bias supplies for setting the phase shift between optical waveguides constituting each child MZ to π. Reference numeral 120-3 denotes a bias supply unit for π / 2 (or 2π / 4) shift between the child MZs (or between the parent Mach-Zehnder interference systems).

  Thus, in the third prior art, four optical waveguides 200a and 300a constituting two Mach-Zehnder interference systems are arranged, and the lateral width of the z-cut LN substrate is several mm. That is, in such a nest type structure, the length of the diagonal line of the substrate cross section is on the order of several millimeters, so the resonance frequency is set to a region higher than a high use frequency such as 40 GHz by the method of Patent Document 2. Was difficult.

JP-A-2-51123 JP-A-3-253814 JP 2008-122786 A

As described above, as no harm in performing the optical transmission, a second prior art proposed to shift the frequency dips f _c of the modulation index to a high frequency range, i.e. the longest in cross-section of the LN substrate becomes long, that is, the length of a diagonal line d, the equivalent refractive index of the high frequency electric signal is n, obtains a frequency dip _{f c} from _{f c = c 0 / (2n} · d), sufficiently high frequency dips _{f c} is In the second prior art in which the diagonal length d is set so that the frequency dip f _c is difficult to remove from the operating frequency region while maintaining the mechanical strength as the optical modulator, the mechanical length is more mechanical. Development of technology that can ensure strength has been desired. In particular, in the nested structure, such as DQPSK, inevitably the width of the LN substrate is wider (the length of a diagonal line in the cross section of the LN substrate in other words becomes longer), the frequency dips f _c is the operating frequency It was difficult to set it to be sufficiently higher than that.

In order to solve the above problems, an optical modulator according to claim 1 of the present invention includes a substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, A traveling wave electrode formed on one surface side and made up of a central conductor and a ground conductor for applying a high frequency electrical signal for modulating the phase of the light, and the traveling wave electrode applies the high frequency electrical signal In the optical modulator, the substrate includes: an interaction unit for modulating the phase of the light; and an input feedthrough unit for applying the high-frequency electrical signal from an external circuit to the interaction unit. Only at a lower part of the input feedthrough part, the thickness part of at least a part is formed thinner than the thickness of the other part excluding the part, the thin part, the interaction part and Intersect The groove portion is formed with a width of the center conductor constituting the input feedthrough portion, and a gap between the center conductor and the ground conductor located on both sides thereof. And a width of a region where a predetermined amount of leaked high-frequency signal that propagates through the substrate leaks from a part of the high-frequency electrical signal applied to the input feedthrough portion , The substrate including the thin portion forms a dielectric resonator, and the thickness of the thin portion and the thickness of the thin portion are such that the resonance frequency determined by the thickness of the thin portion is higher than the frequency of the high-frequency electrical signal. The product of the equivalent refractive index of the high frequency electric signal in the thickness direction is larger than 0.4 mm and smaller than 15 mm.

In order to solve the above-mentioned problem, an optical modulator module according to claim 2 of the present invention is an optical device comprising the optical modulator according to claim 1 and a housing in which the optical modulator is fixed. In the modulator module, a protrusion is formed on the housing, and a part of the pedestal to which the optical modulator is fixed is formed, and the protrusion is formed on a back surface of the thin portion of the substrate. It is characterized by fitting through clearance.

In order to solve the above problems, an optical modulator module according to claim 3 of the present invention, light with a light modulator according to claim 1, and a housing in which the light modulator is fixed to the inside In the modulator module, a pedestal portion of the housing to which the optical modulator is fixed is substantially flat, and a relative dielectric constant is smaller than that of the pedestal portion between the back surface of the thin portion of the substrate and the pedestal portion. According to the thickness of the thin portion so that the resonance frequency determined from the thickness of the thin portion and the thickness of the low dielectric layer is higher than the high frequency electric signal. The thickness of the low dielectric constant layer is set.

In order to solve the above-mentioned problem, an optical modulator module according to a fourth aspect of the present invention is the optical modulator module according to the third aspect , wherein the low dielectric constant layer is air.

In order to solve the above problems, an optical modulator module according to claim 5 of the present invention is the optical modulator module according to claim 3 , wherein the low dielectric constant layer has a relative dielectric constant larger than that of air. The substrate is made of a material smaller than the substrate.

In the present invention, by propagating fundamental mode to the traveling wave electrode, the frequency dips f _c of the optical modulation index is to be determined by the resonance frequency of the substrate as a dielectric resonator. Then, utilizing the fact that the influence of the size of the substrate sides becomes smaller in inverse proportion to their square for the frequency dips f _c determined by the resonance frequency of the substrate as a dielectric resonator. That is, by smaller than the thickness of other portions of the thickness of the neighboring region of the input full Idosuru unit leakage high-frequency electrical signal is present that most affect the substrate resonance as a dielectric resonator, a frequency dip f _c Shift to higher frequency range. The thickness of the other regions in the substrate is kept thicker, but because originally the leakage high-frequency electrical signals in this region is not present the thickness of the substrate in this region does not affect the frequency dip f _c. As a result, since only a part of the substrate is thin and the thickness of many other regions is thick, the present invention ensures the mechanical strength as an optical modulator, which is difficult with the second prior art, while maintaining the frequency. it is possible to shift to a sufficiently high frequency region dip f _c. Further, since there are four or more optical waveguides, the width of the substrate is widened, and as a result, even in a nest type structure such as DQPSK in which the length of the diagonal line in the cross section of the substrate is long, in the present invention, leakage high-frequency electric signals are not generated. since it is the existing area by thinning the portion of the thickness of the substrate, while securing the mechanical strength of the LN optical modulator chip, efficiently frequency dips f _c be set higher than the desired frequency Is possible.

1 is a perspective view of an LN optical modulator according to a first embodiment of the present invention. 1 is a perspective view of a z-cut LN substrate according to a first embodiment of the present invention. Sectional view along BB 'in FIG. The figure explaining the principle of the present invention using the top view of FIG. Measurement result of optical modulation of the LN optical modulator according to the first embodiment of the present invention The perspective view of the z-cut LN board | substrate which concerns on the 2nd Embodiment of this invention. Sectional drawing in CC 'of FIG. Sectional drawing in EE 'of FIG. Top view of FIG. The perspective view of the z-cut LN board | substrate which concerns on the 3rd Embodiment of this invention. Sectional drawing in FF 'of FIG. Top view of FIG. The perspective view of the z-cut LN board | substrate which concerns on the 4th Embodiment of this invention. Sectional drawing in GG 'of FIG. Sectional drawing in HH 'of FIG. Top view of FIG. The perspective view of the z-cut LN board | substrate which concerns on the 5th Embodiment of this invention. Sectional drawing in JJ 'of FIG. Sectional drawing in II 'and KK' of FIG. Top view of FIG. Top view of a z-cut LN substrate according to a sixth embodiment of the present invention 21 is a cross-sectional view taken along line MM ′ of FIG. Sectional drawing in LL 'and NN' of FIG. The perspective view of the LN optical modulator module which concerns on the 7th Embodiment of this invention The perspective view of the LN optical modulator module which concerns on the 8th Embodiment of this invention 1 is a perspective view of an optical modulator according to a first conventional technique. Sectional drawing in the A-A 'line | wire of the optical modulator based on 1st prior art The figure explaining operation | movement of the optical modulator based on 1st prior art. 1 is a perspective view of an optical modulator module according to a first conventional technique. The figure explaining distribution of the electric line of force of the high frequency electric signal in the microwave connector part of the optical modulator module concerning the 1st prior art The figure explaining the distribution of the electric force line | wire of the high frequency electric signal in the feedthrough part for input shown by the B-B 'line of the module of the optical modulator which concerns on 1st prior art. Diagram explaining propagation of leaky high frequency electrical signal Perspective view of z-cut LN substrate according to second prior art Cross-sectional view of z-cut LN substrate according to second prior art DQPSK transmitter according to third prior art

  Hereinafter, embodiments of the present invention will be described. However, since the same reference numerals as those in the conventional embodiment shown in FIGS. 26 to 35 correspond to the same functional units, description of the functional units having the same numbers is omitted here. To do. Further, the optical waveguide, traveling wave electrode, interaction portion, and input feedthrough portion are described as being formed in the same manner as in the conventional embodiment, but are not limited thereto.

[First Embodiment]
The perspective view about only the 1st Embodiment and a board | substrate in this invention is shown in FIG. 1 and FIG. 2, respectively. Here, 9 is a z-cut LN substrate. A high-frequency electric signal is input from an external circuit to the traveling wave electrode via a connector, an input feed-through portion, or the like. In the present invention, the thickness of the substrate in the region where the leakage high-frequency signal exists is reduced. Reference numeral 10 denotes a region where the thickness is reduced (or a thin portion; a thin portion). Moreover, FIG. 3 shows a cross-sectional view taken along line BB ′ of FIG. If the thickness of the z-cut LN substrate 9 is D, the width of the thin portion 10 is W, and the thickness of the substrate there is T, then D> T. 10 'is a counterbore part.

As discussed in FIGS. 29 to 31 of the first prior art, it is a fact that all the power of the high-frequency electric signal input from the outside to the traveling wave electrode 4 of the LN optical modulator propagates to the traveling wave electrode 4. an upper difficult, leakage to the substrate at the input feed-through portion 50 of the traveling-wave electrode, causing a dielectric resonator having a frequency dip f _c. This also applies to the present embodiment shown in FIGS. Reference numeral 60 denotes an interaction region (interaction part) where a high-frequency electric signal and light propagating through the interaction optical waveguides 3a and 3b interact. FIG. 4 shows how the leaked high-frequency electrical signal 11 propagates. The leaked high frequency electric signal 11 generates substrate resonance.

Hereinafter, the concept of the present invention will be described. Returning to the dielectric resonator shown in FIG. In other words, the resonance frequency of the dielectric resonator consisting z- cut LN substrate 1 shown in FIG. 33 considered to correspond to a frequency dip f _c. The frequency dip f _c is on the upper surface (substrate surface opposite to the substrate surface on which the traveling wave electrode is formed) (traveling wave substrate surface on which the electrode is formed) and the lower surface of the z- cut LN substrate 1 in FIG. 33 If there is metal

_{f c = (c 0/2} ) · {(m x / (n x · L x)) 2 + (m y / (n y · L y)) 2 + (m z / (n z · L z) ² } ^1/2 (2)

Also, suppose that there is a metal on the upper surface of the z-cut LN substrate 1 and there is a sufficiently deep gap below the lower surface.

_{f c = (c 0/2} ) · {(m x / (n x · L x)) 2 + (m y / (n y · L y)) 2 + ((m z -1/2) / ( _nz · L _z )) ² } ^1/2 (3)

It becomes. Of course, the present invention can be realized even if the gap below the z-cut LN substrate 1 has a finite thickness. However, in order to simplify the explanation, it is assumed that the thickness of the gap is sufficiently thick. explain.

Here, m _{x, m y,} and m _z are natural numbers representing the order of the resonance, The problem is the resonant frequency of the lowest order (i.e., 1). 2 or more _m x, _{m y,} and _{m z} corresponds to the second- or higher-order modes. n _{x, n y,} and _{n z} is the equivalent refractive index of the high frequency electric signals in each direction of x, y, and z, _{often, n x = n y = 6.56} , n z = 5.29 You may think.

Further, as can be seen from the comparison between the equations (2) and (3), in the width direction (x direction) of the z-cut LN substrate 1, there is a metal on one end surface, and the end surface on the opposite side has a wide space. If you are in contact, the _{m x} may be replaced by _m x -1/2.

Considering the form of the formula in which the width L _x , the length L _y , and the thickness L _z of the z-cut LN substrate 1 are used in the formulas (2) and (3), 2 is not the product of “the longest length in the cross section (usually the length of the diagonal line) and the equivalent refractive index of the high-frequency electrical signal in that direction”, The key point is the product of the thickness of the part and the equivalent refractive index of the high-frequency electrical signal in that direction.

That is, in the present invention shifts to a sufficiently higher frequency region than the band used in the optical transmission frequency dip f _c determined by the product of the equivalent refractive index of the high frequency electric signal in the thickness of the substrate and the thickness direction of the substrate.

The above will be described specifically using equations. In the equations (2) and (3),

L _z << L _y , L _x (4)

Assuming that (2) and (3) are

f _c = (c _0/2 ) · m _z / (n _z · L _z ) (5)
_{f c = (c 0/2} ) · (m z -1/2) / (n z · L z) (6)

And can be expressed, the frequency dips _{f c} is determined by the z- cut LN substrate thickness _{L z} of the substrate 9. In the present invention, L _z in the equation (5) or (6) is approximately determined by the thickness dip f _c of the thinnest portion of the z-cut LN substrate 1 (T in FIG. 3). The

As described above, in the present invention, among the high-frequency electrical signals propagated from the external circuit (not shown) to the traveling wave electrode 4 through the microwave connector (not shown) of the LN optical modulator, the z− in the region where the leaked high-frequency electrical signal propagates. cut by LN to reduce the thickness of the substrate of the substrate 9, substantially (5) and (6) and its thin substrate having a thickness T and the thickness L _z of the substrate of formula. Thus it is made higher than the frequency domain using a frequency dip f _c generated by the dielectric resonator.

Here, the relationship between L _z and L _x will be mentioned. Since L _x also contains a form which is inversely proportional to the square of the L _x in the expression of frequency dips f _c, L _x is the L _x with respect to the frequency dips f _c When more than twice the size of L _z The influence is sufficiently small to be 1/4 of the influence of L _z , and it can be considered that L _z << L _x holds.

Since the thickness of the z-cut LN substrate 9 other than the thin portion 10 is thicker than the thin portion 10, it is needless to say that the mechanical strength can be increased, but the diagonal length in the cross section of the substrate is shortened. Unlike the second conventional technique to be defined, the lateral width L _{x of the} substrate can be increased. Therefore, by using the present invention, there is an advantage that sufficient mechanical strength as an optical modulator can be secured.

Note that when there is a finite gap under the lower surface of the z-cut LN substrate 1, the relative permittivity of the z-cut LN substrate 1 is different from the equation (3) or (6). Since the square root (which can be approximated to the equivalent refractive index) is high, the influence of the z-cut LN substrate 1 is larger than the gap. Therefore, z-idea thickness cut LN substrate 9 can be higher than the use frequency domain a frequency dip f _c by providing the thin portion are the same.

  As described above, the present invention is not the “product of the longest length in the cross section (usually a diagonal line) and the equivalent refractive index of the high-frequency electrical signal in that direction” in the second prior art, but the opposite “ The product of the thickness of the substrate and the equivalent refractive index of the high-frequency electrical signal in that direction in the cross section is considered to be a key point, and the idea is completely different from the conventional second technique.

Specifically, according to the second prior art, that in order to shift the frequency dips f _c higher frequency range, dont not specifically narrow the z- cut LN width of the substrate 1 (L _x) In the present invention, on the contrary, even if the width (L _x ) is increased, in the z-cut LN substrate 9, the frequency dip f is reduced by reducing the thickness of the input feedthrough portion and the substrate region 10 in the vicinity thereof. _The structure is such that _c is sufficiently high.

Generally, in the nested type LN optical modulator as described in Patent Document 3, since inevitably the width because the number of the optical waveguide is often becomes wide as several mm, the frequency dips f _c in the second prior art It was difficult to make it high enough. However, the present invention, in which the resonance frequency is set higher by the thickness of the thinned portion of the substrate rather than the length of the diagonal line in the cross section of the substrate, can also be applied to a modulator having a nested structure such as DQPSK or DP-QPSK. It is.

As can be seen from FIG. 1, the input feedthrough 50 and the interaction 60 are at a large angle with each other, so that the high-frequency electrical signal leaked to the substrate in the vicinity of the input feedthrough 50 (see FIG. 1). Does not affect the interaction section 60. Therefore, there is no problem even if the substrate in the region including the interaction portion 60 is thick. Therefore, according to the present invention, the thickness of the substrate is reduced only in the region where the high-frequency electrical signal leaked to the substrate exists, and the thickness of the substrate in the other regions is increased. Therefore, while ensuring the strength of the substrate, accurate harmful frequency dip f _c for optical transmission, there is a great advantage that ensures and can be shifted to a higher frequency than readily used frequency domain.

  Next, further discussion will be made on “the product of the thickness of the substrate and the equivalent refractive index of the high-frequency electrical signal in that direction”.

In general, (in other words, not high Q value of the resonance of a dielectric resonator) having a frequency band which is not a sharp relative frequency dip f _c is the frequency, so when considering the optical transmission of 10Gbps, the frequency want to set the order of 13GHz about 30% higher if possible as dip f _c, wherein when a 10GHz to be minimum secured, the product of the equivalent refractive index of the high frequency electric signal in that direction and the thickness of the "substrate ((5) The larger value of “n _z · L _z )” in equation (6) is 15 mm.

On the other hand, with respect to the smaller value of n _z · L _z , the present invention in which only a part of the substrate is thinned as the thickness L _z of the thin portion of the z-cut LN substrate 1 in the process that we are currently performing. Then, if the thin part is about 0.2 mm, the yield can be secured. Although the equivalent refractive index _nz in resonance in the thickness direction has been discussed as 5.29, it can be considered up to about 2. Therefore, as a result, “the thickness of the substrate is the largest in the region where the leakage high-frequency electric signal exists in the cross section. The smaller value of the product of the thickness of the thinned portion and the equivalent refractive index of the high-frequency electrical signal in that direction (n _z · L _{z in the} equations (5) and (6)) is 0.4.

FIG. 5 shows the frequency response characteristics of the modulation index of the LN optical modulator manufactured using the z-cut LN substrate 9 according to the present invention. Here, in order to clarify the effects of the present invention, the width L _x the z- cut LN substrate 9 was adopted dare to 6mm that can not be tolerated in the second prior art. The substrate thickness L _z (T in FIG. 3) was 0.3 mm, and the substrate length L _y was 50 mm. The thick part (D in FIG. 3) of the z-cut LN substrate 9 is 1 mm.

In FIG. 5 is a characteristic of the LN optical modulator according to the present invention, the frequency dips f _c is not observed until 40 GHz. Therefore, without the very small elements as in the second prior art, it can be seen that it is possible to shift the frequency dips f _c of the modulation index to a frequency higher than 40 GHz.

  In addition, even if T in FIG. 3 was 0.5 mm, there was no significant difference in characteristics. However, T is the same 1 mm as D, that is, a large frequency dip occurs unless the present invention is used. If possible, it is considered that T should be 0.6 mm as much as possible, and 0.7 mm at the maximum.

As described above, according to the present embodiment, the thickness of the substrate in the region where no leakage high-frequency electrical signal exists, that is, the region away from the input feedthrough portion can be increased, and the width of the z-cut LN substrate 1 is sufficiently large. since it widely, while shifting to higher frequencies a frequency dip f _c of the modulation index, can ensure the mechanical strength with no problem in assembling the LN optical modulator module for rigid by the thick portion and a wide width of the substrate There is an advantage.

[Second Embodiment]
A perspective view of the z-cut LN substrate 12 used in the second embodiment of the present invention is shown in FIG. Here, a region where the thickness is reduced in the z-cut LN substrate 12 is 13. Moreover, sectional views taken along lines CC ′ and EE ′ of FIG. 6 are shown in FIGS. 7 and 8, respectively. A top view of FIG. 6 is shown in FIG.

  As described in the first embodiment shown in FIG. 1, the input feed-through unit 50 and the interaction unit 60 form a large angle with each other, so that the leaked high-frequency electric signal affects the interaction unit 60. do not do. This also applies to the second embodiment shown as FIGS. The leaked high-frequency electrical signal exists only in the vicinity of the input feed-through section (not shown in FIGS. 6 to 9, but see 50 in FIG. 1 shown as the first embodiment). Therefore, in the second embodiment, the region 13 whose thickness is reduced to the middle of the lateral width of the z-cut LN substrate 12 is provided. In other words, the thickness of the substrate is reduced only in the region where the leaked high-frequency electrical signal exists, and the thickness of the other regions is increased, so that there is an advantage that the strength of the substrate is higher than in the first embodiment. Reference numeral 13 'denotes a counterbore part, which is processed using a dicer.

[Third Embodiment]
FIG. 10 shows a perspective view of the z-cut LN substrate 14 used in the third embodiment as a reference embodiment in the present invention. Here, the thinned regions in the z-cut LN substrate 14 are 15a, 15b, and 15c. FIG. 11 is a cross-sectional view taken along the line FF ′ of FIG. A top view of FIG. 10 is shown in FIG.

In the third embodiment, the thinned area is divided and provided as 15a, 15b, 15c. While maintaining the strength of the substrate in this structure, it is possible to achieve the effect of the present invention that shifts the frequency dips f _c to the high frequency side.

[Fourth Embodiment]
FIG. 13 shows a perspective view of the z-cut LN substrate 16 used in the fourth embodiment as a reference embodiment in the present invention. Here, the thinned regions in the z-cut LN substrate 16 are designated as 17a, 17b, and 17c. Further, cross-sectional views taken along lines GG ′ and HH ′ in FIG. 13 are shown in FIGS. 14 and 15, respectively. A top view of FIG. 13 is shown in FIG.

In the fourth embodiment, a thin portion is divided and formed partway in the horizontal direction of the substrate, and has both the characteristics of the second embodiment and the third embodiment. the f _c can be shifted to a higher frequency side, and the substrate strength is very high.

[Fifth Embodiment]
A perspective view of the z-cut LN substrate 18 used in the fifth embodiment of the present invention is shown in FIG. Here, a region where the thickness is reduced in the z-cut LN substrate 18 is 19. FIG. 18 is a cross-sectional view taken along line JJ ′ of FIG. 17, and FIG. 19 is a cross-sectional view taken along lines II ′ and KK ′ of FIG. A top view of the z-cut LN substrate 18 is shown in FIG. In the present embodiment, since there is no thin portion of the substrate at the longitudinal edge of the z-cut LN substrate 18, the mechanical strength as the z-cut LN substrate 18 is very high.

[Sixth Embodiment]
FIG. 21 shows a top view of the z-cut LN substrate 20 used in the sixth embodiment as a reference embodiment of the present invention. Here, the thinned regions in the z-cut LN substrate 20 are designated as 21a, 21b, and 21c. FIG. 22 is a cross-sectional view taken along line MM ′ in FIG. 21, and FIG. 23 is a cross-sectional view taken along lines LL ′ and NN ′ in FIG. Also in this embodiment, since there is no thin portion of the substrate at the longitudinal edge of the z-cut LN substrate 20 and the thin portion is divided, the mechanical strength as the z-cut LN substrate 20 is remarkably high. .

[Seventh Embodiment]
As a seventh embodiment of the present invention, FIG. 24 shows an LN optical modulator module in which the LN optical modulator of the first embodiment described in FIGS. As can be seen from FIG. 24, in the seventh embodiment, the surface of the pedestal 23 'of the housing 5 to which the z-cut LN substrate 1 is fixed by an adhesive such as a conductive paste is substantially flat. Needless to say, the counterbore part 10 'of the z-cut LN substrate 1 may be filled with a conductive paste or applied. Alternatively, a conductive film such as Si may be coated and then fixed to the pedestal 23 ′ of the housing 5 with a conductive paste or a non-conductive adhesive.

  A low dielectric constant layer (air, or a material having a relative dielectric constant larger than air and smaller than that of the substrate) having a relative dielectric constant lower than that of the substrate 9 is provided between the back surface of the counterbore 10 'and the pedestal 23. Depending on the thickness of the thin portion 10, the low dielectric constant is set so that the resonance frequency determined from the thickness of the portion 10 where the thickness of the substrate 9 is thin and the thickness of the low dielectric constant layer is higher than that of the high frequency electrical signal. The thickness of the rate layer may be set.

[Eighth Embodiment]
As an eighth embodiment of the present invention, FIG. 25 shows an LN optical modulator module in which the LN optical modulator of the first embodiment described in FIGS. As can be seen from FIG. 25, in the eighth embodiment, the surface of the housing 22 on which the z-cut LN substrate 1 is fixed by an adhesive such as a conductive paste is mounted on the surface 23 ′ of the surface of the metal housing 22. The protrusion 23 provided in the is formed. It is preferable that the protrusion 23 is in close contact with the thin portion 10 or is fitted into the counterbore 10 ′ with an appropriate predetermined clearance, and both are fixed by a conductive paste or the like.

[About each embodiment]
In the above description, the input feed-through portion has been described. Needless to say, the present invention may also be applied to an output feed-through portion. 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. In addition to the Mach-Zehnder type optical waveguide, other optical waveguides such as directional couplers and straight lines may be used as the optical waveguide. Further, although the embodiment in which the present invention is applied to the input feedthrough portion of the traveling wave electrode has been described, it goes without saying that the present invention can also be applied to the output input feedthrough portion.

  In the above embodiment, the substrate has an x-cut, y-cut or z-cut plane orientation, that is, a crystal x-axis, y-axis or z-axis in a direction perpendicular to the substrate surface (cut plane). The plane orientation in each of the embodiments described above may be the main plane orientation, and other plane orientations may be mixed as the sub-plane orientation. In addition to the LN substrate, Needless to say, other substrates such as lithium tantalate may be used.

  As described above, the optical modulator according to the present invention has an effect that the RF modulation performance can be improved, and is useful as an optical modulator having a high driving speed and a low driving voltage.

1: z-cut LN substrate (substrate, LN substrate)
2: SiO ₂ buffer layer (buffer layer)
3: Optical waveguide 3a, 3b: Optical waveguide of the interaction part (optical waveguide)
4: traveling wave electrode 4a: central conductor 4b, 4c: ground conductor 5, 22: metal housing 6a, 6b: core wire of microwave connector 7a, 7b: cavity around core wire of microwave connector 8: metal lid 9, 12, 14, 16, 18, 20: z-cut LN substrate (region where the thickness is thick)
10, 13, 15a, 15b, 15c, 17a, 17b, 17c, 19, 21a, 21b, 21c: z-cut LN substrate (part where the thickness is thin)
10 ': Counterbore part 23: Projection provided on the base of the surface of the metal casing 23': Base of the surface of the metal casing 11: High-frequency electric signal that leaks from the traveling wave electrode and causes substrate resonance 50: Feed for input Through unit 60: Interaction unit 100: ABC circuit 110: Phase shift control unit 120-1, 120-2, 120-3, 120-41, 120-42: Bias supply unit 200, 300: Child MZ
200a, 300a: Y branch optical waveguide of child MZ 200b, 300b: Modulation electrode (traveling wave electrode)
200c, 300c: bias electrode 500: LD (laser diode)
400, 600: Parent MZ Y-branch optical waveguide 800-1, 800-2: Driver 700: DQPSK signal source 900: Photodiode (PD)

Claims (5)

A substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and a high-frequency electric signal formed on one surface side of the substrate and modulating the phase of the light A traveling wave electrode comprising a central conductor and a ground conductor of
The traveling wave electrode modulates the phase of the light by applying the high frequency electrical signal, and an input feedthrough for applying the high frequency electrical signal from an external circuit to the interaction portion In the optical modulator comprising a unit,
The substrate includes a thin portion formed only at a lower portion of the input feedthrough portion, wherein the thickness of at least a portion of the portion is thinner than the thickness of other portions excluding the portion,
The thin portion is composed of a groove formed in a direction intersecting with the interaction portion,
The width of the groove is greater than the sum of the width of the central conductor constituting the input feed-through portion and the gap between the central conductor and the ground conductor located on both sides thereof, and the input A portion of the high-frequency electrical signal applied to the feedthrough portion for leakage is configured with a width of a region where a predetermined amount of leaked high-frequency signal propagates through the substrate,
The substrate including the thin portion forms a dielectric resonator, and the thickness of the thin portion and the thickness of the thin portion are such that the resonance frequency determined by the thickness of the thin portion is higher than the frequency of the high-frequency electrical signal. An optical modulator characterized in that the product of the equivalent refractive index of the high-frequency electrical signal in the thickness direction is larger than 0.4 mm and smaller than 15 mm. An optical modulator module comprising: the optical modulator according to claim 1; and a housing in which the optical modulator is fixed.
The casing is formed with a protruding portion in which a part of the pedestal portion to which the optical modulator is fixed is raised, and the protruding portion is fitted to the back surface of the thin portion of the substrate via a predetermined clearance. An optical modulator module comprising: An optical modulator module comprising: the optical modulator according to claim 1 ; and a housing in which the optical modulator is fixed.
A pedestal portion of the housing to which the optical modulator is fixed is substantially flat, and a low dielectric constant layer having a lower relative dielectric constant than the substrate is provided between the back surface of the thin portion of the substrate and the pedestal portion. The thickness of the low dielectric constant layer according to the thickness of the thin thickness portion so that the resonance frequency determined from the thickness of the thin thickness portion and the thickness of the low dielectric constant layer is higher than the high frequency electrical signal. An optical modulator module characterized in that is set . The optical modulator module according to claim 3, wherein the low dielectric constant layer is air . 4. The optical modulator module according to claim 3 , wherein the low dielectric constant layer is made of a material having a relative dielectric constant larger than that of air and smaller than that of the substrate .

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