On-chip Fourier transform spectrometer based on mode multiplexing MZI

Abstract

The invention discloses a mode multiplexing MZI-based on-chip Fourier transform spectrometer, which comprises an MZI, a detector and a Fourier transform module, and is characterized in that to-be-detected light is input through the input end of the MZI, and the output end of the MZI is sequentially connected with the detector and the Fourier transform module. The MZI based on mode multiplexing enables the transmission modes TE0, TE1 and TE2 in the two arms to be modulated respectively, and meanwhile, the modulation electrode of the push-pull structure enables the modulated modes in the upper arm and the lower arm to generate opposite and large effective refractive index changes, so that the effective optical path difference of the two arms is greatly increased, and the transmission efficiency of the two arms is improved. Therefore, the on-chip Fourier transform spectrometer which is high in resolution, low in power consumption and simple and compact in structure is achieved, and the MZI structure with the two folded arms 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 can detect the wavelength and intensity distribution of electromagnetic radiation. It plays a pivotal role in many fields such as chemical sensing, biomedical engineering, environmental monitoring, and astronomical science. Existing high-performance spectrometers are usually composed of dispersive elements, long optical path arms, and movable parts. They are large in size, complex in structure, and expensive, which limits their application areas. With the development of integrated optics and nanofabrication technology, on-chip spectrometers have received widespread attention in the past few decades. Among them, on-chip spectrometers based on Fourier transform have received widespread attention and research due to their simple structure. The on-chip Fourier transform spectrometer obtains the interference spectrum of the input light at different optical path differences by changing the optical path difference, and then performs Fourier transform on the interference spectrum to achieve spectral reconstruction and obtain the spectral information to be measured. For this type of spectrometer, its resolution depends on the maximum optical path difference between the interfering lights. The change in optical path difference can be achieved by modulating the refractive index of the two arms based on thermo-optical or electro-optical effects, or directly by changing the arm length difference. Changing the arm length difference means that a larger device size is required to achieve high resolution, and moving parts are also required, which is not conducive to the miniaturization of the device and the scenario of rapid measurement. 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 mode multiplexing MZI, comprising: MZI, detector, 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.

[0005] The specific structure of the MZI 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 to generate an external electric field to modulate the transmission 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.

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

[0010] Furthermore, the directional coupler is a conical asymmetric structure, which realizes efficient coupling between TE0 and TE1 and between TE0 and TE2 respectively.

[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 transmissions.

[0012] Furthermore, in the on-chip Fourier transform spectrometer, after the light to be measured is input from the input end of the MZI, 3dB splitting is achieved through the 1×2 coupler A, and 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 through 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, and then converted into mode TE0 through the coupler 11 at the head end of the modulation arm, and then converted into mode TE2 through the coupler 21. Mode TE2 continues to be transmitted in the modulation arm and modulated for the third time, and finally converted into mode TE0 through the coupler 32 at the end of the modulation arm, and then interferes with 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 output end of the MZI, and finally the spectrum to be measured is reconstructed through 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: MZI, detector, 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 is based on the MZI of mode multiplexing so that the transmission modes TE0, TE1, and TE2 in the two arms are modulated respectively, that is, the coupling regions 1-4 make the modes TE0, TE1, and TE2 modulated in the upper and lower arms of the Mach-Zehnder interferometer respectively, and a single arm realizes three-time modulation; at the same time, the modulation electrode of the push-pull structure makes the modulated modes in the upper and lower arms 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, and 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 The structure 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 the thin film lithium niobate waveguide in an embodiment of the present invention.

[0019] Figure 6 1 is a light 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 in conjunction with the accompanying drawings and embodiments.

[0022] like Figure 1 As shown, an on-chip Fourier transform spectrometer based on mode multiplexing MZI of the present invention comprises: MZI (Mach-Zehnder interferometer), detector, 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.

[0023] The specific structure of the MZI 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 realizes coupling from mode TE1 to mode TE0, and the directional coupler 12 realizes coupling from mode TE0 to mode TE2. The coupling region 3 includes: a directional coupler 31 and a coupler 32; the directional coupler 31 realizes coupling from mode TE0 to mode TE1, and the directional coupler 32 realizes coupling from mode TE2 to mode TE0.

[0027] In this embodiment, the directional coupler is a conical 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 material refractive index of the thin-film lithium niobate, thereby changing the effective refractive index of the transmission mode. A polymer cladding phenylcyclobutene (BCB) is made on the top of the waveguide, and metal modulation electrodes are provided 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, after the light to be measured is input from the input end of the MZI, 3dB splitting is achieved 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 by the electrode for the first time, it is converted into mode TE1 through 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 through the coupler 11 at the head end of the modulation arm, and then converted into mode TE2 through the coupler 21. Mode TE2 continues to be transmitted in the modulation arm and modulated for the third time. Finally, it is converted into mode TE0 through the coupler 32 at the end of the modulation arm, and then interferes with 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 output end of the MZI. Finally, the spectrum to be measured is reconstructed through 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 of the upper arm is converted into mode TE1 through the coupler 31 in the coupling region 3, and the mode TE1 is transmitted in reverse in the modulation arm and undergoes a second modulation. The coupling region 3 is symmetrical with the coupling region 4 above and below, and the light of the lower arm is also converted into mode TE1 through the coupling region 4 and transmitted in reverse in the modulation arm, and undergoes a second modulation.

[0033] The structural diagram of coupling area 1 is shown in 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, and the mode TE1 light of the upper arm is first converted into mode TE0 by the coupler 11 in the coupling area 1, and then converted into mode TE2 by the coupler 12. Mode TE2 continues to be transmitted forward in the upper arm and undergoes the third modulation. The coupling area 1 and the coupling area 2 are symmetrical up and down. The light of the lower arm is also converted into mode TE2 by the coupling area 2 and transmitted forward in the modulation arm and undergoes the 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 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, and the modulated light is mode 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 the MZI single arm; 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 expression of the upper and lower arms of MZI is as follows:

[0040]

[0041] In this embodiment, the MZI output is connected to the detector and the Fourier transform module in sequence, and the interference light is output through the MZI output end and the signal I (V) detected by the detector 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 can be Fourier transformed to reconstruct the input spectrum, which is expressed as follows:

[0045]

[0046] Further simulation verification is performed. In this embodiment, the working wavelength is 1550nm, and the thickness of the X-cut lithium niobate 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 the figure, different waveguide widths w support different waveguide modes. For the mode-multiplexing MZI, the waveguide widths of the 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 spacings 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 high-efficiency coupling of the corresponding modes, and the coupling region includes two types of coupling, 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 mode coupling between TE0 and TE1 is 0.21dB, the insertion loss of mode coupling between TE0 and TE2 is 0.29dB, and the optical transmission diagrams of coupling between TE0 and TE1 and between TE0 and TE2 are shown in Figure 2. 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 selected is gold and the electrode thickness is h. e =500nm; in order to ensure that the electrode has extremely low absorption loss to the transmitted light in the waveguide, the distance from the electrode to the edge of the waveguide 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 input end of the MZI, its transmission spectrum T(λ) can be obtained by connecting the output end of the MZI through a spectrometer; by testing Vpi at different wavelengths, the half-wave voltage function Vpi(λ) related to the wavelength can be obtained; the obtained transmission spectrum T(λ) and Vpi(λ) are input to the Fourier transform module, and when the light source to be measured is input to the input end of the MZI, the output end of the MZI is connected to the detector to obtain the detection signal I(V), and the detector is connected to the Fourier transform module to perform calculations according to the reconstruction formula of the spectrum to be measured, thereby realizing the reconstruction of the spectrum to be measured.

[0055] In summary, the two arms of the MZI based on mode multiplexing in the present invention adopt a folded structure, and the upper and lower arms are completely symmetrical, wherein 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, and 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 should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. For those skilled in the art, the present invention may have various modifications and variations, such as using thermo-optical effect instead of electro-optical 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 in the scope of the claims of the present invention.

Claims

1. An on-chip Fourier transform spectrometer based on 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 specific 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 includes a metal modulation electrode; 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) in the upper and lower directions, and the coupling region (3) is symmetrical with the coupling region (4) in the upper and lower directions; The MZI upper arm comprises: a coupling region (1) and a coupling region (3); and the MZI lower arm comprises: a coupling region (2) and a coupling region (4).

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 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.

3. The on-chip Fourier transform spectrometer based on mode multiplexing MZI according to claim 2, characterized in that: The coupling region (1) comprises: a directional coupler (11) and a coupler (12); the directional coupler (11) realizes coupling from mode TE1 to mode TE0, and the directional coupler (12) realizes coupling from mode TE0 to mode TE2; the coupling region (3) comprises: a directional coupler (31) and a coupler (32); the directional coupler (31) realizes coupling from mode TE0 to mode TE1, and the directional coupler (32) 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 conical 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 input end of the MZI, 3dB splitting is achieved through the 1×2 coupler A. 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 by the electrode for the first time, it is converted into the mode TE1 through the coupler (31) at the end of the modulation arm. The mode TE1 is reversely transmitted in the modulation arm and modulated for the second time. After that, it is converted into the mode TE0 through the coupler (11) at the head end of the modulation arm, and then converted into the mode TE2 through the coupler (21). The mode TE2 continues to be transmitted in the modulation arm and modulated for the third time. Finally, it is converted into the mode TE0 through 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 then output. That is, the interference beam combination of the two arms of the MZI is achieved through the 2×1 coupler B, and is sequentially output to the detector and the Fourier transform module through the output end of the MZI. Finally, the spectrum to be measured is reconstructed through the Fourier transform module to obtain the spectrum information to be measured.

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