An on-chip Fourier transform spectrometer based on mode-multiplexing MZI

Abstract

The present invention discloses an on-chip Fourier transform spectrometer based on a mode-multiplexing MZI, comprising an MZI, a detector, and a Fourier transform module. Light to be measured is input through the MZI input, and the MZI output is sequentially connected to the detector and the Fourier transform module. The mode-multiplexing MZI of the present invention modulates the transmission modes TE0, TE1, and TE2 in the two arms, respectively. Simultaneously, a push-pull modulation electrode causes the modulated modes in the upper and lower arms to produce opposite, larger effective refractive index changes, thereby greatly increasing the effective optical path difference between the two arms. This results in a high-resolution, low-power, simple, and compact on-chip Fourier transform spectrometer. Furthermore, the MZI of the present invention, with its two-arm folding design, is simple and compact.

Description

Technical Field

[0001] The present invention belongs to the technical field of on-chip spectrometers, and in particular relates to an on-chip Fourier transform spectrometer based on a mode multiplexing MZI. Background Art

[0002] A spectrometer is an instrument that detects the wavelength and intensity distribution of electromagnetic radiation. It plays a vital role in a variety of fields, including chemical sensing, biomedical engineering, environmental monitoring, and astronomy. Existing high-performance spectrometers typically consist of dispersive elements, long optical path length arms, and movable components. These components are bulky, complex, and expensive, limiting their application. With the advancement of integrated optics and nanofabrication technologies, on-chip spectrometers have attracted widespread attention over the past few decades. Among them, Fourier transform-based on-chip spectrometers have attracted extensive attention and research due to their simple structure. On-chip Fourier transform spectrometers obtain the interference spectrum of input light at different optical path length differences by varying the optical path difference. These interference spectra are then Fourier transformed to reconstruct the spectrum and obtain the measured spectral information. The resolution of this type of spectrometer depends on the maximum optical path difference between the interfering beams. Modifying the optical path difference can be achieved by modulating the refractive index of the two arms using thermo-optical or electro-optical effects, or by directly changing the arm length difference. However, achieving high resolution requires a larger device size and moving parts, which hinders device miniaturization and rapid measurement applications. However, the current spectrometers that change the optical path difference based on thermo-optical or electro-optical effects require relatively high power consumption or voltage, have relatively low resolution, and have limited working bandwidth. Summary of the Invention

[0003] In order to solve the above technical problems, the present invention proposes an on-chip Fourier transform spectrometer based on mode multiplexing MZI.

[0004] The technical solution adopted by the present invention is: an on-chip Fourier transform spectrometer based on a mode-multiplexing MZI, comprising: an MZI, a detector, and a Fourier transform module. The light to be measured is input through the MZI input terminal, and the MZI output terminal is connected to the detector and the Fourier transform module in sequence.

[0005] The MZI structure includes: an input end 1×2 coupler A, a coupling region 1, a coupling region 2, a coupling region 3, a coupling region 4, an output end 2×1 coupler B, and a metal modulation electrode.

[0006] The upper and lower arms of the MZI are modulation arms and are completely symmetrical, that is, the coupling region 1 is symmetrical with the coupling region 2, and the coupling region 3 is symmetrical with the coupling region 4.

[0007] The MZI upper arm includes: a coupling region 1 and a coupling region 3; the MZI lower arm includes: a coupling region 2 and a coupling region 4.

[0008] Furthermore, the metal modulation electrodes are distributed on both sides of the transmission waveguide, and are used to generate an external electric field to modulate the transmission light in the upper and lower arms of the MZI. The metal modulation electrodes are a push-pull structure, so that the light in the upper and lower arms achieves opposite effective refractive index changes during the modulation process.

[0009] Furthermore, the coupling region 1 includes a directional coupler 11 and a coupler 12; the directional coupler 11 implements coupling from mode TE1 to mode TE0, and the directional coupler 12 implements coupling from mode TE0 to mode TE2. The coupling region 3 includes a directional coupler 31 and a coupler 32; the directional coupler 31 implements coupling from mode TE0 to mode TE1, and the directional coupler 32 implements coupling from mode TE2 to mode TE0.

[0010] Furthermore, the directional coupler is a tapered asymmetric structure, which realizes high-efficiency coupling between TE0 and TE1 and between TE0 and TE2.

[0011] Furthermore, the transmission modes TE0, TE1, and TE2 in the upper and lower arms of the MZI are modulated respectively, that is, the waveguide widths of the upper and lower arms need to support TE0, TE1, and TE2 mode transmission.

[0012] Furthermore, in the on-chip Fourier transform spectrometer, the light to be measured is input from the MZI input end and then achieves 3dB splitting through the 1×2 coupler A. The two beams of light enter the upper arm and lower arm of the MZI respectively for transmission. After the mode TE0 entering the two arms is modulated for the first time by the electrode, it is converted into mode TE1 by the coupler 31 at the end of the modulation arm. Mode TE1 is transmitted in reverse in the modulation arm and modulated for the second time. Thereafter, it is converted into mode TE0 by the coupler 11 at the head end of the modulation arm, and then converted into mode TE2 by the coupler 12. Mode TE2 continues to be transmitted in the modulation arm and modulated for the third time. Finally, it is converted into mode TE0 by the coupler 32 at the end of the modulation arm, and then interferes with the mode TE0 of the lower arm that has also been modulated three times and is output. That is, the interference beam combining of the two arms of the MZI is achieved through the 2×1 coupler B, and is output to the detector and the Fourier transform module in sequence through the MZI output end. Finally, the spectrum to be measured is reconstructed by the Fourier transform module to obtain the spectrum information to be measured.

[0013] Beneficial effects of the present invention: The on-chip Fourier transform spectrometer of the present invention comprises: an MZI, a detector, and a Fourier transform module. The light to be measured is input through the MZI input end, and the MZI output end is connected to the detector and the Fourier transform module in sequence. The present invention uses an MZI based on mode multiplexing to modulate the transmission modes TE0, TE1, and TE2 in the two arms respectively, that is, the coupling regions 1-4 cause the modes TE0, TE1, and TE2 to be modulated in the upper and lower arms of the Mach-Zehnder interferometer, respectively, and a single arm achieves triple modulation. At the same time, the modulation electrodes of the push-pull structure cause the modulated modes in the upper and lower arms to produce opposite and larger effective refractive index changes, thereby greatly increasing the effective optical path difference between the two arms, thereby realizing a high-resolution, low-power, simple and compact on-chip Fourier transform spectrometer. The MZI of the present invention with a two-arm folding design has a simple and compact structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a structural diagram of an on-chip Fourier transform spectrometer based on mode multiplexing MZI of the present invention.

[0015] Figure 2 A cross-sectional view of a thin-film lithium niobate waveguide according to an embodiment of the present invention

[0016] Figure 3 Schematic diagram of the structure of the coupling region 3 in an embodiment of the present invention.

[0017] Figure 4 Schematic diagram of the structure of the coupling region 1 in an embodiment of the present invention.

[0018] Figure 5 FIG. 4 is a dispersion curve diagram of a thin film lithium niobate waveguide in an embodiment of the present invention.

[0019] Figure 6 1 is an optical transmission diagram of the coupling between mode TE1 and mode TE0 in an embodiment of the present invention.

[0020] Figure 7 1 is an optical transmission diagram of the coupling between mode TE2 and mode TE0 in an embodiment of the present invention. DETAILED DESCRIPTION

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] like Figure 1 As shown in the figure, the on-chip Fourier transform spectrometer based on a mode-multiplexing MZI of the present invention includes an MZI (Mach-Zehnder interferometer), a detector, and a Fourier transform module. The light to be measured is input through the MZI input, and the MZI output is connected to the detector and the Fourier transform module in sequence.

[0023] The MZI structure includes: an input end 1×2 coupler A, a coupling region 1, a coupling region 2, a coupling region 3, a coupling region 4, an output end 2×1 coupler B, and a metal modulation electrode.

[0024] The upper and lower arms of the MZI are modulation arms and are completely symmetrical, that is, the coupling region 1 is symmetrical with the coupling region 2, and the coupling region 3 is symmetrical with the coupling region 4.

[0025] The MZI upper arm includes: a coupling region 1 and a coupling region 3; the MZI lower arm includes: a coupling region 2 and a coupling region 4.

[0026] In this embodiment, the coupling region 1 includes a directional coupler 11 and a coupler 12; the directional coupler 11 implements coupling from mode TE1 to mode TE0, and the directional coupler 12 implements coupling from mode TE0 to mode TE2. The coupling region 3 includes a directional coupler 31 and a coupler 32; the directional coupler 31 implements coupling from mode TE0 to mode TE1, and the directional coupler 32 implements coupling from mode TE2 to mode TE0.

[0027] In this embodiment, the directional coupler is a tapered asymmetric structure, which realizes high-efficiency coupling between TE0 and TE1 and between TE0 and TE2.

[0028] In this embodiment, the modes TE0, TE1, and TE2 in the upper and lower arms of the MZI are modulated respectively, and the waveguide widths of the upper and lower arms need to support transmission of the TE0, TE1, and TE2 modes.

[0029] In this embodiment, the metal modulation electrodes are distributed on both sides of the transmission waveguide to generate an external electric field to modulate the light in the upper and lower arms of the MZI, and the metal modulation electrodes are a push-pull structure, so that the light in the upper and lower arms achieves opposite effective refractive index changes during the modulation process.

[0030] In this embodiment, the waveguide core material used is thin-film lithium niobate. Those skilled in the art should know that thin-film lithium niobate has an electro-optical effect. An external electric field can change the refractive index of the thin-film lithium niobate material, thereby changing the effective refractive index of the transmission mode. A polymer cladding layer of phenylcyclobutene (BCB) is made on the top of the waveguide, and metal modulation electrodes are on both sides of the waveguide. The schematic diagram of the waveguide cross section of the MZI is shown in FIG. Figure 2 shown.

[0031] In this embodiment, in the on-chip Fourier transform spectrometer, the light to be measured is input from the MZI input end and then achieves 3dB splitting through the 1×2 coupler A. The two beams of light enter the upper arm and lower arm of the MZI respectively for transmission. After the mode TE0 entering the two arms is modulated for the first time by the electrode, it is converted into mode TE1 by the coupler 31 at the end of the modulation arm. Mode TE1 is transmitted in reverse in the modulation arm and modulated for the second time. Thereafter, it is converted into mode TE0 by the coupler 11 at the head end of the modulation arm, and then converted into mode TE2 by the coupler 12. Mode TE2 continues to be transmitted in the modulation arm and modulated for the third time. Finally, it is converted into mode TE0 by the coupler 32 at the end of the modulation arm, and then interferes with the mode TE0 of the lower arm that has also been modulated three times and is output. That is, the interference beam combining of the two arms of the MZI is achieved through the 2×1 coupler B, and is output to the detector and the Fourier transform module in sequence through the MZI output end. Finally, the spectrum to be measured is reconstructed by the Fourier transform module to obtain its spectral information.

[0032] In this embodiment, the structural diagram of the coupling region 3 is as follows: Figure 3 As shown by Figure 3 It can be seen that when the light is transmitted to the end of the modulation arm, the light in the upper arm is converted into mode TE1 by the coupler 31 in the coupling region 3. Mode TE1 is transmitted in the modulation arm in the reverse direction and undergoes a second modulation. The coupling region 3 and the coupling region 4 are symmetrical in the upper and lower directions. The light in the lower arm is also converted into mode TE1 by the coupling region 4 and transmitted in the modulation arm in the reverse direction and undergoes a second modulation.

[0033] The structural diagram of coupling area 1 is as follows Figure 4 As shown by Figure 4 It can be seen that the light is transmitted to the head end of the modulation arm again. The mode TE1 light of the upper arm is first converted to mode TE0 by the coupler 11 in the coupling region 1, and then converted to mode TE2 by the coupler 12. Mode TE2 continues to be transmitted forward in the upper arm and undergoes a third modulation. The coupling region 1 and the coupling region 2 are symmetrical in the upper and lower directions. The light of the lower arm is also converted to mode TE2 by the coupling region 2 and transmitted forward in the modulation arm and undergoes a third modulation.

[0034] The upper arm mode TE2 is transmitted to the end of the modulation arm and converted into mode TE0 through the coupler 32. The lower arm mode TE2 is transmitted to the end of the modulation arm and is also converted into mode TE0 through the coupling region 4. The upper and lower arm lights are interfered and combined through the output end 2×1 coupler B.

[0035] Based on the above mode multiplexing, the light on the MZI transmission arm undergoes three modulations. The modulated light is in modes TE0, TE1, and TE2 respectively. That is, the optical path difference (OPD) between the upper and lower arms of the MZI is expressed as follows:

[0036] OPD=2·(Δneff-0 +Δn eff-1 +Δn eff-2 )·L

[0037] Among them, Δn eff-i (i=0,1,2) indicates mode TE i The effective refractive index change under the applied electric field V, L represents the length of the modulation region on a single arm of the MZI; therefore, the half-wave voltage Vpi(λ) is expressed as follows:

[0038]

[0039] Where λ represents the wavelength. Combining the expressions of optical path difference (OPD) and half-wave voltage Vpi(λ), the phase difference between the upper and lower arms of the MZI is expressed as follows:

[0040]

[0041] In this embodiment, the MZI output is connected to the detector and the Fourier transform module in sequence. The interference light passes through the MZI output end and is detected by the detector. The signal I(V) is expressed as follows:

[0042]

[0043] Where T(λ) represents the MZI transmission spectrum, A in (λ) represents the input spectrum;

[0044] Finally, the detector signal is Fourier transformed to reconstruct the input spectrum, which is expressed as follows:

[0045]

[0046] Further simulation verification is carried out. In this embodiment, the operating wavelength is 1550nm, and the thickness of the X-cut lithium niobate thin film is h LN =600nm, etching depth h=200nm.

[0047] Based on COMSOL simulation, the dispersion curve of the corresponding waveguide can be obtained, such as Figure 5 As shown in Figure 1, different waveguide widths w support different waveguide modes. For the mode-multiplexed MZI, the waveguide widths of its two arms need to support the transmission of modes TE0, TE1, and TE2. Therefore, according to the dispersion curve, the waveguide width of the two arms is designed to be w = 4 μm. For the tapered asymmetric directional coupler in the coupling region, the structure is as follows: Figure 3 and 4 As shown, where w a1 、w a2 、w b1 、w b2 、w c1 、w c2is the waveguide width at different positions in the coupling region, g1 and g2 are the spacing between the two waveguides of the directional coupler in the coupling region, L1 and L2 are the coupling lengths of the directional coupler in the coupling region. The waveguide width and spacing in the coupling region must meet the requirements for high-efficiency coupling of the corresponding modes. The coupling region includes two types of coupling, namely, the coupling between TE0 and TE1 and the coupling between TE0 and TE2. Combined with the dispersion curve and Lumerical Mode transmission simulation, the specific dimensions of the waveguide in the coupling region can be designed as shown in Tables 1 and 2, namely the structural parameters of the coupling between TE0 and TE1 and the structural parameters of the coupling between TE0 and TE2.

[0048] Table 1

[0049] <![CDATA[w a1 ]]> <![CDATA[w a2 ]]> <![CDATA[w b1 ]]> <![CDATA[w b2 ]]> <![CDATA[g1]]> <![CDATA[L1]]> 1.1μm 1.3μm 3.4μm 2.9μm 0.8μm 660μm

[0050] Table 2

[0051] <![CDATA[w a1 ]]> <![CDATA[w a2 ]]> <![CDATA[w c1 ]]> <![CDATA[w c2 ]]> <![CDATA[g2]]> <![CDATA[L2]]> 1.1μm 1.3μm 5.5μm 4.8μm 0.8μm 705μm

[0052] According to the calculation results of Lumerical Mode, the insertion loss of the mode coupling between TE0 and TE1 is 0.21dB, the insertion loss of the mode coupling between TE0 and TE2 is 0.29dB, and the optical transmission diagrams of the coupling between TE0 and TE1 and between TE0 and TE2 are as follows: Figure 6 and 7 It should be noted that the waveguides of different widths on the MZI are connected by adiabatic tapered transition waveguides.

[0053] The modulation electrodes are distributed on both sides of the waveguide, such as Figure 2 As shown, the electrode structure size can be designed according to COMSOL simulation. The electrode material is gold and the electrode thickness is h. e =500nm; to ensure that the electrode has extremely low absorption loss of light transmitted in the waveguide, the distance from the electrode to the waveguide edge is designed to be 2μm, that is, the electrode spacing g = 8μm; in this embodiment, the modulation length L of each arm of the MZI is 10cm, and the half-wave voltage Vpi(1550nm) = 0.09V can be obtained according to COMSOL calculation.

[0054] In this embodiment, for the reconstruction of the spectrum to be measured, when a broadband optical signal (1520-1610nm) is input to the MZI input end, its transmission spectrum T(λ) can be obtained by connecting the MZI output end through a spectrometer; by testing Vpi at different wavelengths, the wavelength-related half-wave voltage function Vpi(λ) can be obtained; the obtained transmission spectrum T(λ) and Vpi(λ) are input into the Fourier transform module. When the light source to be measured is input to the MZI input end, the MZI output end is connected to a detector to obtain a detection signal I(V). The detector is connected to the Fourier transform module to perform calculations according to the spectrum reconstruction formula to realize the reconstruction of the spectrum to be measured.

[0055] In summary, the present invention adopts a folded structure in which the two arms of the MZI based on mode multiplexing are completely symmetrical. The coupling regions 1-4 enable the modes TE0, TE1, and TE2 to be modulated in the upper and lower arms of the Mach-Zehnder interferometer, respectively, that is, a single arm realizes three-fold modulation. At the same time, the modulation electrodes of the push-pull structure cause the modulated modes in the upper and lower arms to produce opposite and larger effective refractive index changes, thereby greatly increasing the optical path difference between the upper and lower arms, thereby realizing a high-resolution, low-power, simple and compact on-chip Fourier transform spectrometer.

[0056] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and it should be understood that the scope of protection of the present invention is not limited to such specific descriptions and embodiments. For those skilled in the art, the present invention can be modified and varied in various ways, such as using a thermo-optic effect instead of an electro-optic effect to change the optical path difference. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included within the scope of the claims of the present invention.

Claims

1. An on-chip Fourier transform spectrometer based on a mode-multiplexing MZI, comprising: MZI, detector, Fourier transform module; The light to be measured is input through the MZI input terminal, and the MZI output terminal is connected to the detector and the Fourier transform module in sequence; The MZI structure includes: an input end 1×2 coupler A, a first coupling region, a second coupling region, a third coupling region, a fourth coupling region, an output end 2×1 coupler B, and a metal modulation electrode; The first coupling region is used to convert the mode TE1 into the mode TE0, and then convert the mode TE0 into the mode TE2 and guide the forward transmission; the third coupling region is used to convert the mode TE0 into the mode TE1 and guide the reverse transmission, and convert the mode TE2 into the mode TE0 and guide the forward transmission; The upper and lower arms of the MZI are modulation arms and are completely symmetrical, that is, the first coupling region is symmetrical with the second coupling region, and the third coupling region is symmetrical with the fourth coupling region. The MZI upper arm includes a first coupling region and a third coupling region; the MZI lower arm includes a second coupling region and a fourth coupling region.

2. The on-chip Fourier transform spectrometer based on mode multiplexing MZI according to claim 1, characterized in that: The metal modulation electrodes are distributed on both sides of the transmission waveguide and are used to generate an external electric field to modulate the transmitted light in the upper and lower arms of the MZI. The metal modulation electrodes are a push-pull structure, so that the light in the upper and lower arms achieves opposite effective refractive index changes during the modulation process.

3. The on-chip Fourier transform spectrometer based on mode multiplexing MZI according to claim 2, characterized in that: The first coupling region includes: a first directional coupler and a second directional coupler; the first directional coupler realizes coupling from mode TE1 to mode TE0, and the second directional coupler realizes coupling from mode TE0 to mode TE2; the third coupling region includes: a third directional coupler and a fourth directional coupler; the third directional coupler realizes coupling from mode TE0 to mode TE1, and the fourth directional coupler realizes coupling from mode TE2 to mode TE0.

4. The on-chip Fourier transform spectrometer based on mode multiplexing MZI according to claim 3, characterized in that: The directional couplers are all tapered asymmetric structures, which respectively realize high-efficiency coupling between TE0 and TE1 and between TE0 and TE2.

5. The on-chip Fourier transform spectrometer based on mode multiplexing MZI according to claim 4, characterized in that: The transmission modes TE0, TE1, and TE2 in the upper and lower arms of the MZI are modulated respectively, that is, the waveguide widths of the upper and lower arms need to support TE0, TE1, and TE2 mode transmission.

6. The on-chip Fourier transform spectrometer based on mode multiplexing MZI according to claim 5, characterized in that: In the on-chip Fourier transform spectrometer, after the light to be measured is input from the MZI input end, it is subjected to 3dB splitting through the 1×2 coupler A. The two beams of light enter the upper arm and the lower arm of the MZI respectively for transmission. After the mode TE0 entering the two arms is modulated for the first time by the electrode, it is converted into mode TE1 by the third directional coupler at the end of the modulation arm. Mode TE1 is transmitted in reverse in the modulation arm and modulated for the second time. Thereafter, it is converted into mode TE0 by the first directional coupler at the head end of the modulation arm, and then converted into mode TE2 by the second directional coupler. Mode TE2 continues to be transmitted in the modulation arm and modulated for the third time. Finally, it is converted into mode TE0 by the fourth directional coupler at the end of the modulation arm. It then interferes with mode TE0 of the lower arm that has also been modulated three times and is output. That is, the interference beam combination of the two arms of the MZI is realized by the 2×1 coupler B, and is output to the detector and the Fourier transform module in sequence through the MZI output end. Finally, the spectrum to be measured is reconstructed by the Fourier transform module to obtain the spectrum information to be measured.

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