Optical integrated device

Abstract

In the present invention, a second harmonic generator (103) generates pump light and local oscillator light from input light that is input to a second input section (102). A first demultiplexer (104) demultiplexes the pump light and the local oscillator light generated by the second harmonic generator (103). A combiner (105) combines the pump light demultiplexed by the first demultiplexer (104) with measurement light, and an amplifier (106) inputs the pump light and the measurement light combined by the combiner (105) and amplifies the measurement light through optical phase-sensitive amplification. An interferometer (108) causes 50 / 50 interference between the local oscillator light demultiplexed by the first demultiplexer (104) and the measurement light demultiplexed by a second demultiplexer (107).

Description

Optical Integrated Devices

[0001] The present invention relates to integrated optical devices.

[0002] Conventionally, techniques for measuring the amplitude and phase of light in a quantum state have been essential in continuous quantum information technology. An example of light in a quantum state is the quadrature amplitude squeezed state (squeezed light). Squeezed light is an important resource state used as an auxiliary input state necessary for realizing various quantum gates in continuous quantum information technology. Squeezed light is light in which the quantum fluctuations of two non-commutative physical quantities are controlled, and the quantum fluctuation of one of the non-commutative physical quantities is smaller than the quantum fluctuation in the coherent state. Using this squeezed light, for example, quantum entanglement generation, which is the most important operation in quantum information processing, becomes possible.

[0003] Squeezed light is generated using nonlinear optical phenomena. When second-order nonlinear optical effects are used, squeezed light is generated by spontaneous parametric down-conversion (SPDC). SPDC is a phenomenon in which a high-frequency photon incident on a second-order nonlinear optical medium is converted into two lower-frequency photon pairs. When third-order nonlinear optical effects are used, squeezed light is generated by four-wave mixing. In the case of four-wave mixing, a new correlated photon pair is generated from two photons of the same frequency. Both SPDC and four-wave mixing require relatively strong pump light, and some of the pump light components are converted into squeezed light within the nonlinear optical medium.

[0004] Currently, research is being conducted to generate squeezed states using various structures and materials in order to realize quantum information processing, and evaluating the squeezing performance of these structures is also an important technology. Balanced homodyne detection using a spatial optical system is usually used to measure the amplitude and phase of squeezed light. Balanced homodyne detection enables highly sensitive measurement of the quadrature amplitude and phase of light using a relatively simple measurement system, and is therefore widely used in the field of photonics.

[0005] In balanced homodyne detection, squeezed light (measurement light) and local oscillator light (LOL) are interfered with each other using a 50% beam splitter, and the two output lights are detected by another photodetector to obtain amplitude and phase information of the measurement light from the difference signal. In this measurement technology, the squeezed light is subjected to phase-sensitive amplification in the electrical stage after photoelectric conversion to obtain amplitude and phase information of the signal light. For this reason, phase synchronization between the local oscillator light and the squeezed light is essential.

[0006] Balanced homodyne detection has a problem in that the squeezed light returns to a coherent state due to optical loss before the signal is amplified in the electrical stage. Conventionally, balanced homodyne detection used to detect squeezed light has used a configuration including a highly efficient photodetector and a low-loss optical path to reduce loss. However, such a configuration has a problem in that the detection bandwidth of squeezed light is limited.

[0007] To solve the above-mentioned conventional problems, a technique has been proposed to improve the tolerance to optical loss by amplifying the squeezed light signal in the optical stage. As a method for improving the tolerance to optical loss, a technique using optical phase-sensitive amplification as a pre-amplification in the optical stage is known (Non-Patent Document 1).

[0008] Phase-sensitive optical amplification is an amplification method that generates a nonlinear optical effect between pump light and signal light (measurement light), amplifying the in-phase component (Re component) of the signal light and attenuating the quadrature component (Im component) of the signal light based on the relative phase difference with the pump light.

[0009] Optical phase-sensitive amplification is a low-noise optical amplification mechanism that utilizes nonlinear optical effects and, in principle, has a noise figure of 0 dB. Therefore, it is an effective technology for improving the tolerance to optical loss in the optical stage. In fact, there have been research reports on overcoming conventional bandwidth limitations by using an optical phase-sensitive amplifier with a high amplification factor and a high-speed balanced homodyne detector for optical communications.

[0010] M. Manceau et al., "Detection Loss Tolerant Supersensitive Phase Measurement with an SU(1,1) Interferometer", Physical Review Letters, vol. 119, no. 22, 223604, 2017.

[0011] However, the above-described conventional technology also has a problem. This is the difficulty of phase synchronization. Figure 2 shows the conventional technology. In the conventional technology, first, the phase of the measurement axis of the signal light and the phase of the optical phase-sensitive amplification axis (= the phase of the pump light) are synchronized by a modulator 301. The synchronized pump light and signal light are then multiplexed by a multiplexer 302, amplified by an optical parametric amplifier 303, and demultiplexed by a demultiplexer 304.

[0012] The pump light demultiplexed by the demultiplexing unit 304 is monitored by a first detector 307. The signal light demultiplexed by the demultiplexing unit 304 is interfered with by a 50 / 50 coupler 306 and a local light for balanced homodyne detection, and then detected by a second detector 308 and a third detector 309. Balanced homodyne detection is performed based on the detection results of the second detector 308 and the third detector 309. The multiplexing unit, demultiplexing unit, modulator, amplifier, and interferometer are connected by optical fibers.

[0013] Here, before interference occurs in the 50 / 50 coupler 306, the phases of the signal light amplified by the optical parametric amplifier 303 and the local light must be synchronized in the modulator 305, requiring two phase synchronization lines for one measurement light. In fiber systems that are highly compatible with high-speed optical communication devices, optical path length fluctuations due to fiber vibration and temperature changes exist, making it extremely difficult to control two phase modulators at high speed to achieve phase synchronization. Furthermore, it is difficult to simultaneously control two phase modulators at high speed. This problem of difficulty in phase synchronization becomes more serious as the number of measurement targets increases.

[0014] To improve the stability of the fiber optics of the above-described conventional technology, an integrated system may be considered, as shown in Figure 3, in which a modulator 301, a multiplexer 302a, an optical parametric amplifier 303, a demultiplexer 304a, a modulator 305, and a multimode interferometer such as a 50 / 50 coupler 306 are integrated on a substrate 300, and each of these components is connected by an optical waveguide formed on the substrate 300. Furthermore, the modulators 301 and 305 are configured from electro-optical elements (EO elements) capable of high-speed phase modulation. While this integrated structure can improve the stability of the system by fixing the optical path length, it cannot solve the problem of the need to control two modulators configured from EO elements at high speed and achieve phase synchronization.

[0015] The present invention has been made to solve the above problems, and has as its object to eliminate the difficulty of phase synchronization in measuring the amplitude and phase of squeezed light.

[0016] The optical integrated device according to the present invention includes a first input section formed on a substrate and inputting measurement light, a second input section formed on the substrate and inputting input light of the same optical intensity as the pump light, a second harmonic generator formed on the substrate and generating pump light and local light from the input light, a first demultiplexer formed on the substrate and demultiplexing the pump light and local light generated by the second harmonic generator, a multiplexer formed on the substrate and multiplexing the pump light and measurement light demultiplexed by the first demultiplexer, an amplifier formed on the substrate and inputting the pump light and measurement light multiplexed by the multiplexer and amplifying the measurement light by optical phase sensitive amplification, and an amplifier formed on the substrate and multiplexing the pump light and measurement light output from the amplifier. a second demultiplexer that demultiplexes the local light beam split by the first demultiplexer and the measurement light beam split by the second demultiplexer; an interferometer formed on the substrate and causing 50 / 50 interference between the local light beam split by the first demultiplexer and the measurement light beam split by the second demultiplexer; a first output section formed on the substrate and outputting the pump light demultiplexed by the second demultiplexer; a second output section formed on the substrate and outputting the measurement light beam output by the interferometer; a third output section formed on the substrate and outputting the local light beam output by the interferometer; a modulator that synchronizes the phase of the input light input from the second input section with the measurement light; a first optical waveguide formed on the substrate and guiding the measurement light input from the first input section to the multiplexer; a second optical waveguide that guides the pump light demultiplexed by the first demultiplexer to the multiplexer; a third optical waveguide formed on the substrate and guiding the pump light demultiplexed by the first demultiplexer to the multiplexer; a fourth optical waveguide formed on the substrate and guiding the local light demultiplexed by the first demultiplexer to the interferometer; a fifth optical waveguide formed on the substrate and guiding the measurement light demultiplexed by the second demultiplexer to the interferometer; a sixth optical waveguide formed on the substrate and guiding the pump light demultiplexed by the second demultiplexer to the first output section; a seventh optical waveguide formed on the substrate and guiding the measurement light output from the interferometer to the second output section; and an eighth optical waveguide formed on the substrate and guiding the local light output from the interferometer to the third output section.

[0017] As described above, according to the present invention, the pump light and the local light are generated from the input light by the second harmonic generator, and an optical waveguide is used, so that the difficulty of phase synchronization in measuring the amplitude and phase of the squeezed light can be resolved.

[0018] Fig. 1 is a plan view showing the configuration of an optical integrated device according to an embodiment of the present invention, Fig. 2 is a plan view showing the configuration of a conventional optical integrated device, and Fig. 3 is a plan view showing the configuration of a conventional optical integrated device.

[0019] An optical integrated device according to an embodiment of the present invention will now be described with reference to Fig. 1. This optical integrated device includes a first input section 101, a second input section 102, a second harmonic generator 103, a first demultiplexer 104, a multiplexer 105, an amplifier 106, a second demultiplexer 107, an interferometer 108, a first output section 109, a second output section 110, and a third output section 111, which are formed (integrated) on a substrate 100.

[0020] Measurement light is input to the first input section 101, and input light having the same optical intensity as the pump light is input to the second input section 102.

[0021] The second harmonic generator 103 generates pump light and local light from the input light input to the second input section 102. The second harmonic generator 103 can be configured from a nonlinear optical crystal in which the polarization of the crystal is inverted periodically between adjacent regions, and the length of each region in the arrangement direction corresponds to the generation of a second harmonic of the input light.

[0022] As the nonlinear optical crystal, it is preferable to use lithium niobate (LN) or lithium tantalate (LT) crystal, which has a high second-order nonlinear optical effect and a wide transmission band. For example, LiNbO3 (LN), KNbO3, LiTaO3 (LT), LiNb x Ta 1-x O3 (0≦x≦1), or KTiOPO4, or a material containing at least one additive selected from the group consisting of Mg, Zn, Sc, and In, can be used.

[0023] The first demultiplexer 104 demultiplexes the pump light generated by the second harmonic generator 103 from the local light. The multiplexer 105 multiplexes the pump light and measurement light demultiplexed by the first demultiplexer 104. The multiplexer 105 can be, for example, a two-input, one-output coupler. The multiplexer 105 can also be, for example, a two-input, two-output multimode interference coupler. In this case, for example, the measurement light passes through a straight port and the pump light passes through a cross port. The multiplexer 105 can also be configured as a directional coupler-type multiplexer / demultiplexer. Note that, taking into consideration processing accuracy and manufacturing tolerances, a multimode interference coupler is considered to be suitable for the multiplexer 105.

[0024] The amplifier 106 receives the pump light and measurement light multiplexed by the multiplexer 105 and amplifies the measurement light by optical phase-sensitive amplification. The amplifier 106 can be configured from a nonlinear optical crystal in which the polarization of the crystal is inverted periodically between adjacent regions, and the length of each region in the arrangement direction corresponds to the generation of the second harmonic of the measurement light.

[0025] As the nonlinear optical crystal, it is preferable to use lithium niobate (LN) or lithium tantalate (LT) crystal, which has a high second-order nonlinear optical effect and a wide transmission band. For example, LiNbO3 (LN), KNbO3, LiTaO3 (LT), LiNb x Ta 1-x O3 (0≦x≦1), or KTiOPO4, or a material containing at least one additive selected from the group consisting of Mg, Zn, Sc, and In, can be used.

[0026] The second demultiplexer 107 demultiplexes the pump light and measurement light output from the amplifier 106. The second demultiplexer 107 can be, for example, a two-input, two-output multimode interference coupler. In this case, for example, the measurement light passes through a straight port and the pump light passes through a cross port. The second demultiplexer 107 can also be configured as a directional coupler-type multiplexer / demultiplexer. Note that, in consideration of processing accuracy and manufacturing tolerance, a multimode interference coupler is considered to be suitable for the second demultiplexer 107.

[0027] The interferometer 108 causes the local light separated by the first demultiplexer 104 and the measurement light separated by the second demultiplexer 107 to interfere at a 50 / 50 ratio. The interferometer 108 can be configured, for example, as a two-input, two-output multimode interference coupler. When measurement light is input to one input port, 50% of its optical power is transmitted to the straight port and the remaining 50% is transmitted to the cross port. Similarly, when measurement light is input to the other input port, 50% of its optical power is transmitted to the straight port and the remaining 50% is transmitted to the cross port. Note that the interferometer 108 can be made of other elements, such as a directional coupler-type multiplexer / demultiplexer, as long as they can divide optical power 1:1. However, considering processing accuracy, manufacturing tolerances, and the like, a multimode interference coupler is suitable.

[0028] The first output port 109 outputs the pump light demultiplexed by the second demultiplexer 107. The second output port 110 and the third output port 111 output the interference light of the local light and the amplified measurement light output from the interferometer 108. The second output port 110 and the third output port 111 are output ports connected to a balanced homodyne detector.

[0029] This optical integrated device also includes a modulator 112 that synchronizes the phase of the input light input from the first input unit 102 with the measurement light. The second harmonic generator 103 generates pump light and local light from the input light whose phase has been synchronized with the measurement light by the modulator 112. The modulator 112 can be configured with an EO element (electro-optic element) that changes the refractive index of the optical waveguide by the electro-optic effect caused by applying an electric field, thereby controlling the phase of the light propagating through the optical waveguide. The EO element is capable of high-speed (> GHz) phase modulation.

[0030] When integrating an EO element modulator 112 on the substrate 100, electrodes of the EO element are formed along the optical waveguide formed on the substrate 100. An electric field is applied to this electrode, causing an electro-optic effect that changes the refractive index of the optical waveguide and controls the phase of light propagating through the optical waveguide. However, because the generation of drift current can make phase control difficult, in such cases the modulator 112 can be provided outside the substrate 100.

[0031] This optical integrated device also includes a first optical waveguide 131, a second optical waveguide 132, a third optical waveguide 133, a fourth optical waveguide 134, a fifth optical waveguide 135, a sixth optical waveguide 136, a seventh optical waveguide 137, and an eighth optical waveguide 138 formed on the substrate 100. Each optical waveguide may be, for example, an optical waveguide with a Si core.

[0032] The first optical waveguide 131 guides the measurement light input from the first input port 101 to the multiplexer 105. The second optical waveguide 132 guides the input light input from the second input port 102 to the second harmonic generator 103. The third optical waveguide 133 guides the pump light demultiplexed by the first demultiplexer 104 to the multiplexer 105. The fourth optical waveguide 134 guides the local light demultiplexed by the first demultiplexer 104 to the interferometer 108.

[0033] The fifth optical waveguide 135 guides the measurement light separated by the second demultiplexer 107 to the interferometer 108. The sixth optical waveguide 136 guides the pump light separated by the second demultiplexer 107 to the first output 109. The seventh optical waveguide 137 guides the interference light between the local light and the amplified measurement light output from the interferometer 108 to the second output 110. Similarly, the eighth optical waveguide 137 guides the interference light between the local light and the amplified measurement light output from the interferometer 108 to the third output 111.

[0034] The optical integrated device may further include an optical path length adjuster 113 formed midway along the fourth optical waveguide 134 on the substrate 100. The optical path length adjuster 113 adjusts the phase difference between the pump light and the local light, which is generated by the optical path length difference between the first optical path length between the second harmonic generator 103 and the multiplexer 105 and the second optical path length between the second harmonic generator 103 and the interferometer 108, to nπ radians (n ​​is an integer). The optical path length adjuster 113 adjusts the optical path length difference between the first optical path length and the second optical path length so that the signal intensity of balanced homodyne detection of the interference light between the local light and the amplified measurement light output from the second output port 110 and the interference light between the local light and the amplified measurement light output from the third output port 111 is maximized. The optical path length adjuster 113 may be configured as a thermo-optical element that changes the refractive index by a thermo-optical effect caused by thermal changes caused by the flow of current.

[0035] According to the embodiment, by generating pump light and local light using the second harmonic generator 103 and fixing the optical path length using an optical waveguide formed on the substrate 100, improved stability and simplified phase synchronization can be achieved. The greater the refractive index difference between the core and cladding of the optical waveguide, the smaller the device can be, and the shorter the optical path length of the optical waveguide can be, resulting in more stable operation. Furthermore, since the amplitude and phase of the measurement light amplified by the optical integrated device are measured using a high-speed balanced homodyne detector for high-speed optical communications, it is desirable that the wavelengths of the measurement light and local light are, for example, 1550 nm, which is in the communications wavelength band, and the pump lightwave is 775 nm. The optical integrated device according to the embodiment is applicable regardless of wavelength, and devices can be designed according to wavelength by appropriately setting the wavelength.

[0036] According to the embodiment, the pump light and the local light are generated using the second harmonic generator 103. Therefore, if the phase difference between the pump light and the local light, which is caused by the optical path length difference between the first optical path length between the second harmonic generator 103 and the multiplexer 105 and the second optical path length between the second harmonic generator 103 and the interferometer 108, is nπ radians, only one optical path between the phase of the pump light (= the phase of the local light) and the phase of the signal light is required for phase synchronization, which can halve the number of phase synchronizations compared to the conventional technology and eliminate the difficulty of phase synchronization.

[0037] The optical path length adjuster uses a thermo-optic (TO) element, which changes the refractive index by the thermo-optic effect caused by thermal changes caused by the flow of current. Unlike the phase control mechanism described above, which requires high speed, there is no need for high-speed operation in order to guarantee the optical path length difference due to thermal fluctuations. Since this is usually on the order of MHz or less, low-speed phase control is sufficient, and it is thought that even a thermo-optic element with a relatively slow response speed can be sufficiently responsive.

[0038] It is desirable to provide a spot size converter (SSC) for efficient light coupling at each of the first input port 101, the second input port 102, the first output port 109, the second output port 110, and the third output port 111. In particular, it is desirable to provide a structure for low-loss coupling at the first input port 101, which is a port to which signal light, which is quantum-state light with low optical loss tolerance, is input.

[0039] As described above, according to the embodiment of the present invention, the pump light and the local light are generated from the input light by the second harmonic generator, and an optical waveguide is used, so that the difficulty of phase synchronization in measuring the amplitude and phase of the squeezed light can be resolved.

[0040] The conventional high-speed quantum-state light detection system described in the Background Art section requires phase synchronization during the phase-sensitive amplification described above and high-speed phase synchronization of a balanced homodyne detector. Configurations using optical fiber systems or simple integrated structures require two high-speed phase synchronization mechanisms, making phase synchronization difficult. This problem becomes more severe as the number of detection targets increases, as the number of phase synchronization mechanisms also increases. To solve this problem, the above-described embodiment uses a circuit configuration including an amplifier and optical waveguide that amplifies using optical phase-sensitive amplification. By using an optical integrated circuit, the optical path length is fixed, improving the stability of the entire balanced homodyne detection system while suppressing phase fluctuations and improving performance. In addition, by generating pump light and local light from a single second-harmonic generator, systems that previously required high-speed phase synchronization using two high-speed modulators can now be achieved using a single modulator, significantly simplifying phase synchronization.

[0041] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0042] 100...substrate, 101...first input section, 102...second input section, 103...second harmonic generator, 104...first demultiplexer, 105...multiplexer, 106...amplifier, 107...second demultiplexer, 108...interferometer, 109...first output section, 110...second output section, 111...third output section, 112...modulator, 113...optical path length adjuster, 131...first optical waveguide, 132...second optical waveguide, 133...third optical waveguide, 134...fourth optical waveguide, 135...fifth optical waveguide, 136...sixth optical waveguide, 137...seventh optical waveguide, 138...eighth optical waveguide.

Claims

1. A first input section formed on a substrate for inputting measurement light; A second input section formed on the substrate for inputting input light of the same optical intensity as the pump light; A second harmonic generator formed on the substrate for generating the pump light and local light from the input light; A first demultiplexer formed on the substrate for demultiplexing the pump light generated by the second harmonic generator from the local light; A multiplexer formed on the substrate for multiplexing the pump light and measurement light demultiplexed by the first demultiplexer; An amplifier formed on the substrate for inputting the pump light and measurement light signaled by the multiplexer and amplifying the measurement light by optical phase sensitive amplification; A second demultiplexer formed on the substrate for demultiplexing the pump light and measurement light output from the amplifier; An interferometer formed on the substrate for causing 50 / 50 interference between the local light demultiplexed by the first demultiplexer and the measurement light demultiplexed by the second demultiplexer. a first output section formed on the substrate and outputting the pump light demultiplexed by the second demultiplexer; a second output section formed on the substrate and outputting the measurement light output from the interferometer; a third output section formed on the substrate and outputting the local light output from the interferometer; a modulator that synchronizes the phase of the input light input from the second input section with the measurement light; a first optical waveguide formed on the substrate and guiding the measurement light input from the first input section to the multiplexer; a second optical waveguide formed on the substrate and guiding the input light input from the second input section to the second harmonic generator; a third optical waveguide formed on the substrate and guiding the pump light demultiplexed by the first demultiplexer to the multiplexer; and a fourth optical waveguide formed on the substrate and guiding the local light demultiplexed by the first demultiplexer to the interferometer. an optical integrated device comprising: a fifth optical waveguide formed on the substrate and guiding the measurement light demultiplexed by the second demultiplexer to the interferometer; a sixth optical waveguide formed on the substrate and guiding the pump light demultiplexed by the second demultiplexer to the first output portion; a seventh optical waveguide formed on the substrate and guiding the measurement light output from the interferometer to the second output portion; and an eighth optical waveguide formed on the substrate and guiding the local light output from the interferometer to the third output portion.

2. An optical integrated device according to claim 1, further comprising an optical path length adjuster formed midway along the fourth optical waveguide on said substrate for adjusting the phase difference between said pump light and said local light, which is generated by the difference in optical path length between a first optical path length between said second harmonic generator and said multiplexer and a second optical path length between said second harmonic generator and said interferometer, to nπ radians (n ​​is an integer).

3. An optical integrated device according to claim 2, wherein the optical path length adjuster adjusts the difference in optical path length between the first optical path length and the second optical path length so as to maximize the signal intensity of balanced homodyne detection of the measurement light output from the second output section and the local light output from the third output section.

4. An optical integrated device according to claim 2 or 3, wherein the optical path length adjuster is composed of a thermo-optical element.

5. An optical integrated device according to claim 1, wherein the second harmonic generator is composed of a nonlinear optical crystal in which the polarization of the crystal is inverted periodically in adjacent regions, and the length of each region in the arrangement direction corresponds to the generation of the second harmonic of the input light.

6. An optical integrated device according to claim 1, wherein the amplifier is constructed from a nonlinear optical crystal in which the polarization of the crystal is inverted periodically in adjacent regions, and the length of each region in the arrangement direction corresponds to the generation of the second harmonic of the measurement light.

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