JP2013080011A - Optical frequency comb signal generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical frequency comb signal generator which can be made compact and can provide a large modulation depth without using an impedance matching circuit such as a stub.SOLUTION: An optical frequency comb signal generator includes a Mach-Zehnder optical waveguide 2 formed on a ferroelectric substrate 1 and a modulation electrode 31, 32, for modulating light wave propagating through the branch waveguides, provided for each branch waveguide such that the Mach-Zehnder optical waveguide 2 outputs a light wave which simultaneously generates a plurality of optical frequency components having a predetermined frequency difference. Each modulation electrode includes a resonant electrode 31, 32 having different electrode length, and a feeder line 40 is split into two to feed a same modulation signal to each modulation electrode and laid out such that the distance from a branching point 41 to a contact point 44, 45 with each resonant electrode is the same. At the contact points, the standing wave W1, W2 formed on each resonant electrode is in phase with each other, and each feeder line is impedance-matched with corresponding resonant electrode at the contact point 44, 45.

Description

  The present invention relates to an optical frequency comb signal generator, and more particularly, a plurality of optical frequency components having a predetermined frequency difference, each provided with a modulation electrode that modulates a light wave propagating through two branch waveguides of a Mach-Zehnder type optical waveguide. The present invention relates to an optical frequency comb signal generator that obtains a light wave that is simultaneously generated.

  In optical communication technology and optical measurement technology, an optical frequency comb signal generator having a function of simultaneously generating a plurality of optical frequency components having a frequency difference of equal intervals is used. For example, the optical frequency comb signal can be applied as a short pulse light source for optical measurement.

  As a method for generating an optical frequency comb signal, a flat optical frequency comb signal using a two-electrode Mach-Zehnder (MZ) modulator and two types of modulation signals as in Patent Document 1 and Non-Patent Document 1 is used. There is a way to generate.

  Further, as in Patent Document 2, there is a method of generating a flat optical frequency comb signal using a single MZ type modulator and one type of modulation signal.

  In a method of generating a flat optical frequency comb signal using a two-electrode type MZ modulator as in Patent Document 1, a large degree of modulation is required to obtain a wide spectrum band. For example, in order to obtain an ultrashort pulse of about 2 to 3 ps, a large high frequency input of several W level has been required. Further, it is necessary to accurately adjust the amplitude and phase of the two types of modulation signals so that the optical frequency comb signal is generated.

  Similarly to Patent Document 2, a method of generating a flat optical frequency comb signal using one type of modulation signal also has a problem that a large degree of modulation is required. To solve this problem, the modulation electrode can be expected to improve the modulation efficiency. However, it is necessary to provide a stub for impedance matching, and the circuit of the feeder line becomes complicated. There is a problem that the size of the substrate constituting the substrate increases.

JP 2007-248660 A JP 2009-175576 A

Takahide Sakamoto et al., “Conditions for generating an ultra-flat optical frequency comb using a Mach-Zehnder optical modulator”, IEICE Technical Report, vol. 105, no. 172, pp. 49-53, Nov. 2005

  The problem to be solved by the present invention is to generate an optical frequency comb signal that solves the above-described problems and can be miniaturized without using an impedance matching circuit such as a stub and can obtain a large modulation degree. Is to provide a vessel.

  In order to solve the above-mentioned problem, the invention according to claim 1 is a modulation electrode for modulating a light wave propagating in a branching waveguide, wherein a Mach-Zehnder type optical waveguide having two branching waveguides is formed on a ferroelectric substrate. Is provided corresponding to each branching waveguide, and an optical frequency comb signal generator in which an optical wave output from the Mach-Zehnder type optical waveguide becomes an optical wave simultaneously generating a plurality of optical frequency components having a predetermined frequency difference , Each modulation electrode includes resonance electrodes having different electrode lengths, and a feed line for applying one type of modulation signal to each modulation electrode branches one feed line into two, and from the branch point to each resonance electrode And the connection point has the same phase of the standing wave formed at each resonance electrode, and the feed line and the resonance electrode are impedance-matched at the connection point. Have The features.

  According to a second aspect of the present invention, in the optical frequency comb signal generator according to the first aspect, the electrode end of the resonance electrode uses any combination of open-open, short-circuit-short, short-circuit-open. Features.

  The invention according to claim 3 is the optical frequency comb signal generator according to claim 1 or 2, wherein the length of each resonance electrode arranged along the branching waveguide is such that the optical frequency comb signal is generated. Each length is set so as to satisfy the above.

The invention according to claim 4 is the optical frequency comb signal generator according to any one of claims 1 to 3, wherein the length (L ₁ , L ₂ ) of each resonance electrode is set to L ₁ = n ₁ · c / (2 · f · n _m ), L ₂ = n ₂ · c / (2 · f · n _m ) (where n ₁ and n ₂ are natural numbers, and f is a resonance frequency), and the modulation applied to each branch waveguide When the degrees are A ₁ and A ₂ , the resonance electrode lengths L ₁ and L ₂ are as follows: A ₁ / A ₂ = A ₁ / (A ₁ + π / 2) = (n ₁ / n ₂ ) ^1/2 Each length is set so as to satisfy the above.

  According to the first aspect of the present invention, a Mach-Zehnder type optical waveguide having two branch waveguides is formed on a ferroelectric substrate, and a modulation electrode for modulating a light wave propagating through the branch waveguide corresponds to each branch waveguide. In the optical frequency comb signal generator in which the light wave output from the Mach-Zehnder type optical waveguide is a light wave that simultaneously generates a plurality of optical frequency components having a predetermined frequency difference, each modulation electrode is A feed line having resonance electrodes with different electrode lengths and applying one type of modulation signal to each modulation electrode branches one feed line into two, and the distance from the branch point to the connection point to each resonance electrode is In addition, the connection point has the same phase of the standing wave formed in each resonance electrode, and the feed line and the resonance electrode are impedance matched at the connection point, so that the size can be reduced. Possible, It is possible to provide an optical frequency comb signal generator which can obtain a deal of modulation.

  In other words, when a conventional two-electrode optical modulator is used, two types of modulation signals are required, and the amplitude and phase are adjusted so that the optical signal is flattened by the optical frequency comb. There was a need to do. In the optical frequency comb signal generator of the present invention, only one type of modulation signal may be used, the feed line is branched into two, and the distance (length) from the branch point to each modulation electrode is equal, so the optical frequency comb is flat. There is no need to separately adjust the amplitude and phase so as to satisfy the conditions.

  Further, by using a resonance electrode as the modulation electrode, the modulation efficiency is improved, a large degree of modulation can be obtained, and a low drive voltage can be achieved. In particular, impedance matching is performed by adjusting the feeding position without using a stub or a matching circuit, so that the device can be easily manufactured and the device size can be reduced.

  According to the invention of claim 2, since the electrode end of the resonance electrode can use any combination of open-open, short-circuit-short-circuit, and short-circuit-open, an appropriate resonance electrode can be used depending on the arrangement of the modulation electrode. The structure can be adopted, and the degree of freedom in design is increased.

  According to the invention of claim 3, since the length of each resonance electrode arranged along the branching waveguide is set so as to satisfy the condition for generating the optical frequency comb signal, the optical frequency comb The signal can be easily obtained with a simple configuration.

According to the invention of claim 4, the length (L ₁ , L ₂ ) of each resonance electrode is set to L ₁ = n ₁ · c / (2 · f · n _m ), L ₂ = n ₂ · c / (2 F · n _m ) (where n ₁ and n ₂ are natural numbers, and f is the resonance frequency), and the modulation degree applied to each branch waveguide is A ₁ and A ₂ , the resonant electrode lengths L ₁ and L ₂ Are set to satisfy the condition of A ₁ / A ₂ = A ₁ / (A ₁ + π / 2) = (n ₁ / n ₂ ) ^1/2 and satisfy the condition Thus, an optical frequency comb signal can be easily obtained simply by adjusting the resonant electrode length.

It is a figure explaining the outline of the optical frequency comb signal generator of this invention. It is a figure explaining the combination in case the edge part of a resonant electrode becomes (a) open-open, (b) open-short circuit, (c) short circuit-short circuit. It is a figure explaining the outline of the optical frequency comb signal generator which used PLC (planar optical circuit) for a part of board | substrate.

Hereinafter, the optical frequency comb signal generator of the present invention will be described in detail.
As shown in FIG. 1, in the light control element of the present invention, a Mach-Zehnder type optical waveguide 2 having two branch waveguides (21, 22) is formed on a ferroelectric substrate 1, and propagates through the branch waveguide. A modulation electrode (31, 32) for modulating a light wave to be provided is provided corresponding to each branch waveguide, and the light wave output from the Mach-Zehnder optical waveguide simultaneously generates a plurality of optical frequency components having a predetermined frequency difference. In the optical frequency comb signal generator to be a light wave, each modulation electrode has resonance electrodes (31, 32) having different electrode lengths, and a feed line for applying one type of modulation signal to each modulation electrode is 1 One feed line 40 is branched into two, the distance from the branch point 41 to the connection point (44, 45) to each resonance electrode is equal, and the connection point is a standing wave formed at each resonance electrode. (Waveforms indicated by dotted lines. W1, W2) Phases are in phase, and the power feed line and the resonance electrode, characterized in that in the impedance matching at the connection point (44, 45).

  As the ferroelectric substrate 1, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof can be used. In particular, lithium niobate (LN) or lithium tantalate (LT) crystals having a high electro-optic effect are preferably used. In the light control element of the present invention, as shown in FIG. 1, a configuration in which a resonance electrode is disposed on an optical waveguide can expect the most effective modulation. Therefore, a Z-cut substrate is preferable.

  The optical waveguide 2 can be formed by a method of forming a ridge on the substrate, a method of adjusting the refractive index of a part of the substrate, or a method of combining both. In the ridge type waveguide, other portions are removed by mechanical cutting or chemical etching so as to leave a substrate portion to be an optical waveguide. It is also possible to form grooves on both sides of the optical waveguide. In the method of adjusting the refractive index, the refractive index of a part of the substrate surface corresponding to the optical waveguide becomes higher than the refractive index of the substrate itself by using a thermal diffusion method such as Ti or a proton exchange method. Configure as follows.

The modulation electrode includes a signal electrode such as a resonance electrode (31, 32), a feed line (wiring for supplying a modulation signal to the resonance electrode, 40, 42, 43), a ground electrode (not shown), and the like. The modulation electrode can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like. Also, each electrode is optionally arranged via a buffer layer such as SiO ₂ film between a substrate. The buffer layer has an effect of preventing light waves propagating through the optical waveguide from being absorbed or scattered by the modulation electrode. Moreover, as a structure of a buffer layer, in order to relieve the pyroelectric effect of a thin plate, a Si film or the like can be incorporated as necessary.

Next, conditions for the optical modulator shown in FIG. 1 to function as an optical frequency comb signal generator will be described.
In the following, the modulation electrode is a resonance type and both ends are open. The modulation signal supplied to the feed line is branched into two and each connected to the resonance electrode. The distance from the branch point to the connection point to the resonance line is adjusted to be equal. The connection point to the resonance electrode is a position where the impedance is matched and the standing wave standing on each electrode is in phase.

The high frequency signal input to the MZ modulator is a sine wave, and the phase modulation applied to both arms is expressed by the following equation (1). However, i is given to distinguish each resonance electrode, i = 1 means one resonance electrode 1 (reference numeral 31), and i = 2 means the other resonance electrode 2 (reference numeral 32).
θ _i = A _i · sin (ω _m t) + B _i ··· (1)

Here, the amplitude of the phase modulation by the high frequency signal, ω _m is the modulation frequency, B _i is the direct current when the light wave travels through the branched waveguide (hereinafter also referred to as “arm”) including the phase modulation by the DC bias. Phase advance. The high frequency signals input to the arm 1 (reference numeral 21) and the arm 2 (reference numeral 22) make the distances (the lengths of the feed lines 42 and 43) from the branch point 41 to the connection points (44, 45) equal, and the timing Are completely matched.

The output waveform of MZ type light modulation can be expressed by the following equation (2).
E = 1 / 2E _in · e ^{i (ω0 + n · ωm) t} · e ^iB1 · [J _n (A) + J _n (A + ΔA) · e ^iΔθ ]
(2)

In Expression (2), J _n (•) is an n-th order first-type Bessel function, and A, ΔA, and Δθ have the following meanings, respectively.
Modulation efficiency (amplitude value) of phase modulation at the resonance electrode 1 A ₁ = A
Modulation efficiency (amplitude value) of phase modulation at the resonance electrode 2 A ₂ = A + ΔA
DC optical phase difference between two arms Δθ = B ₁ −B ₂
Ω ₀ is the frequency of light incident on the optical modulator. Further, n corresponds to the modulation order of the frequency comb component, and the elements constituting the series respectively correspond to the electric field of the frequency comb component.

The frequency at which each frequency comb component appears is ω ₀ + nω _{m as} can be seen from equation (2). When the power of each frequency comb component is obtained from the equation (2), the following equation (3) is obtained.
P _n = 1/4 · P _in · [J _n ² (A) + J _n ² (A + ΔA) + 2J _n (A) J _n (A + ΔA) · cos (Δθ)] (3)
P _in is the power of the light entering the modulator.

Now when A is big enough,
± ΔA + Δθ = π (4)
P _n is
P _n = P _in · [1-cos (2Δθ)] / [2p (2A ₁ + ΔA)] (5)
And a constant value. In particular,
ΔA = 1 / 2π, Δθ = 1 / 2π or 3 / 2π (6)
In this case, the output light power becomes maximum.

In other words, if the modulation efficiency of the arm 1 and the arm 2 can be adjusted so as to satisfy the condition of the expression (6), it can be seen that one type of modulation signal is sufficient.
As shown in FIG. 1, the lengths of the respective resonance electrodes are L ₁ and L ₂ . In order to resonate at the frequency f, the length of each resonance electrode only needs to satisfy the expressions (7-1, 7-2).
L ₁ = n ₁ · c / (2 · f · n _m ) (7-1)
L ₂ = n ₂ · c / (2 · f · n _m ) (7-2)
Here, f is the modulation frequency, _nm is the effective refractive index of the microwave, c is the speed of light in vacuum, and n ₁ and n ₂ are independent natural numbers.

Now, when the modulation efficiency of the arm 1 is A ₁ , the modulation efficiency A ₂ of the arm ₂ is expressed by Expression (8).
A2 = A1 · (n ₂ / n ₁ ) ^1/2 ... (8)
The electrode length that satisfies the equation (6) may be adjusted so as to satisfy the equation (9).
_{A 1 / A 2 = A 1} / (A 1 + π / 2) = (n 1 / n 2) 1/2 ··· (9)

For example, when the modulation efficiency of each arm is A ₁ = 4.5π and A ₂ = 5.0π so that an optical frequency comb is generated, the resonance electrodes of arms 1 and 2 are respectively L ₁ = 9 / 2Λ _m and L ₂ = 11 / 2Λ _m may be set. Note that Λ _m means the wavelength of the microwave.

As described above, the optical frequency comb signal generator of the present invention improves the modulation efficiency by making the modulation electrode resonant, and generates a flat optical frequency comb only by adjusting the electrode lengths L ₁ and L _2. It is configured to satisfy the conditions.

  There are a plurality of conditions for generating the optical frequency comb, and the end of the resonance electrode is only an open-open combination (both ends of the resonance electrode 3 are not short-circuited with the ground electrode 5) as shown in FIG. Alternatively, a combination of an open circuit and a short circuit as shown in FIG. 2B and a combination of a short circuit and a short circuit as shown in FIG.

  In order to generate an optical frequency comb at the resonant electrode, it is necessary to satisfy the equation (9). Table 1 shows the length of the resonant electrode where an optical frequency comb is generated in the case of open-open, Table 2 open-short, and Table 3 short-short. When it did not occur, it was marked with ×, and when it occurred, it was marked with ○.

  From Tables 1 to 3, it is understood that even when the end of the resonance electrode is changed to open or short-circuit, the optical frequency comb signal can be generated by the two resonance electrodes under a limited condition.

  The optical frequency comb signal generator of the present invention is not limited to that shown in FIG. 1, but can also be configured using a PLC (planar optical circuit) as shown in FIG. For example, a part of the optical waveguide 2 is formed on a substrate 10 on which an optical waveguide such as quartz or semiconductor can be formed, and bonded to the ferroelectric substrate 1 to constitute an optical frequency comb signal generator.

  Furthermore, in the optical frequency comb signal generator of the present invention, impedance matching between the feed line and the resonance electrode can be easily matched by adjusting the connection point between the two. However, in order to further improve the impedance matching, it is possible to add a stub or a matching circuit to the feed line connected to the resonance electrode as necessary.

  As described above, according to the present invention, it is possible to provide an optical frequency comb signal generator that can be miniaturized and can obtain a large modulation degree without using an impedance matching circuit such as a stub. It becomes.

DESCRIPTION OF SYMBOLS 1 Ferroelectric substrate 2 Optical waveguide 31, 32 Resonant electrode

Claims (4)

A Mach-Zehnder type optical waveguide having two branch waveguides is formed on a ferroelectric substrate, and a modulation electrode for modulating a light wave propagating through the branch waveguide is provided corresponding to each branch waveguide, and the Mach-Zehnder type In the optical frequency comb signal generator in which the light wave output from the optical waveguide becomes a light wave that simultaneously generates a plurality of optical frequency components having a predetermined frequency difference,
Each modulation electrode includes resonant electrodes with different electrode lengths,
A feed line that applies one type of modulation signal to each modulation electrode branches one feed line into two, and the distance from the branch point to the connection point to each resonance electrode is equal,
Further, the connection point has an optical frequency characterized in that the phase of the standing wave formed in each resonance electrode is in phase, and the feed line and the resonance electrode are impedance matched at the connection point. Com signal generator.   2. The optical frequency comb signal generator according to claim 1, wherein an electrode end of the resonant electrode uses any combination of open-open, short-circuit-short-circuit, and short-circuit-open. vessel.   3. The optical frequency comb signal generator according to claim 1 or 2, wherein the length of each resonance electrode arranged along the branch waveguide is such that each length satisfies a condition for generating the optical frequency comb signal. An optical frequency comb signal generator characterized by being set. The optical frequency comb signal generator according to any one of claims 1 to 3, wherein the length (L ₁ , L ₂ ) of each resonance electrode is L ₁ = n ₁ · c / (2 · f · n _m ). , L ₂ = n ₂ · c / (2 · f · n _m ) (where n ₁ and n ₂ are natural numbers, and f is the resonance frequency), and the degree of modulation applied to each branch waveguide is represented by A ₁ and A ₂ . Then, the resonant electrode lengths L ₁ and L ₂ have lengths so as to satisfy the condition of A ₁ / A ₂ = A ₁ / (A ₁ + π / 2) = (n ₁ / n ₂ ) ^1/2 , respectively. An optical frequency comb signal generator characterized by being set.

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