Multi-channel output wavelength adjustable laser cascaded by Mach-Zehnder interferometer with unequal arms and micro-ring

Abstract

The invention discloses an unequal-arm Mach-Zehnder interferometer and micro-ring cascaded multi-channel output wavelength tunable laser. The unequal-arm Mach-Zehnder interferometer and micro-ring cascaded multi-channel output wavelength tunable laser comprises a chip; the spot size converter is arranged on the chip; the composite resonant cavity is connected with the spot size converter; the composite resonant cavity comprises a phase modulation arm connected with the spot size converter; the first unequal-arm Mach-Zehnder interferometer is connected with the phase modulation arm; a second unequal-arm Mach-Zehnder interferometer and a third unequal-arm Mach-Zehnder interferometer; and the annular resonant cavity is connected with the output end of the second unequal-arm Mach-Zehnder interferometer and the output end of the third unequal-arm Mach-Zehnder interferometer through waveguides. The invention is used for providing a laser with selectable output channel number and tunable output wavelength, the tuning range of the laser can reach 30nm, and the laser can be used for a dense wavelength division multiplexing system and can be used as a frequency modulated continuous wave laser radar light source.

Description

Technical Field

[0001] The invention relates to the field of multi-channel output wavelength tunable lasers, and in particular to a multi-channel output wavelength tunable laser of an unequal-arm Mach-Zehnder interferometer and a micro-ring cascade. Background Art

[0002] With the development of technologies such as 5G communication, big data, artificial intelligence and the Internet of Things, optical communication technology is developing towards multi-terminal, low latency, low power consumption, big data bandwidth and short distance, which also promotes the rapid development of high-performance lasers, dense wavelength division multiplexing technology, advanced modulation formats, etc. In the large-capacity and long-distance transmission of optical communication systems, technologies such as dense wavelength division multiplexing (DWDM) and wavelength division multiplexing require a large number of light sources with different wavelengths. Most DWDM systems rely on separate, distributed feedback (DFB) lasers of different wavelengths, which require precise wavelength locking, resulting in high costs. Therefore, studying how to achieve integrated and stable DWDM multi-wavelength light sources has become a research focus in recent years. In addition, multi-channel wavelength lasers can also be used in the field of laser radar. Multi-channel wavelength lasers can emit multiple lasers of different wavelengths at the same time, which enables laser radar to collect echo signals of different wavelengths at the same time, enhance the measurement accuracy and resolution of the system, and thus improve its measurement capability, data processing speed and multi-dimensional information acquisition capabilities.

[0003] At present, the main methods for realizing multi-wavelength lasers include: using semiconductor laser arrays with multi-path grating frequency selection technology to achieve precise wavelength selection and output, and the wavelength interval is adjustable, but the cost is high, the process is complex, and precise wavelength locking and temperature control are required; combining erbium-doped fiber lasers with comb filters such as Sagnac interferometers. The structure is relatively simple, but a low-temperature environment (such as liquid nitrogen cooling) is required to suppress the uniform broadening of the erbium-doped fiber; introducing a frequency shifter or phase modulator in the ring cavity to achieve stable output at room temperature, but this method is difficult to accurately select the wavelength output and requires complex circuit control; using erbium-doped fibers with special structures such as dual-core fibers to achieve stable multi-wavelength output at room temperature, and it is difficult to control the wavelength interval and the number of output wavelengths; there are also methods of using nonlinear effects such as four-wave mixing to obtain multi-wavelength output, and forming overlapping cavities through different gain media or using gain media but different cavities to produce different wavelength lasing.

[0004] Lithium niobate (LiNbO3) is an attractive electro-optical device material with a wide transparent window from the ultraviolet to mid-infrared band and a large Pockels coefficient of 32pm / V, which enables efficient, low-voltage and high-speed modulation. In addition to the application of electro-optic modulators, the LiNbO3 integrated photonics platform with a large Pockels coefficient and low transmission loss meets all the requirements for realizing ultrafast, linear and continuous mode-hop-free tuning of narrow linewidth and frequency-agile laser sources. Compared with commercial III-V lasers, silicon-based lithium niobate wavelength tunable lasers use silicon-based materials as substrates and are compatible with existing silicon-based integrated optical devices. Summary of the invention

[0005] The purpose of the present invention is to propose a multi-channel output wavelength tunable laser of unequal-arm Mach-Zehnder interferometer and micro-ring cascade, which is used to provide a laser with selectable output channel number and tunable output wavelength, and its tuning range can reach 30nm. It can be used in wavelength division multiplexing system and as a frequency modulated continuous wave laser radar light source.

[0006] The invention discloses an unequal-arm Mach-Zehnder interferometer based on a thin-film lithium niobate platform and a micro-ring cascade multi-channel output wavelength tunable laser, comprising: a lithium niobate thin film chip, including a silicon substrate, a silicon dioxide buried oxide layer on the silicon substrate, a lithium niobate thin film on the silicon dioxide buried oxide layer, a silicon dioxide cladding covering the lithium niobate thin film, and a metal electrode for electrical adjustment.

[0007] A pattern spot converter is arranged on the lithium niobate thin film chip, wherein the pattern spot converter is an evanescent wave guide with increased width and is made of lithium niobate material;

[0008] A composite resonant cavity connected to the pattern spot converter, wherein the composite resonant cavity is made of lithium niobate material;

[0009] The composite resonant cavity comprises:

[0010] A phase modulation arm connected to the pattern spot converter;

[0011] a first unequal-arm Mach-Zehnder interferometer connected to the phase modulation arm;

[0012] a first cascade waveguide connected to the first unequal-arm Mach-Zehnder interferometer;

[0013] a second cascade waveguide connected to the first unequal-arm Mach-Zehnder interferometer;

[0014] a second unequal-arm Mach-Zehnder interferometer connected to the first cascade waveguide;

[0015] a third cascade waveguide connected to the second unequal-arm Mach-Zehnder interferometer;

[0016] a fourth cascade waveguide connected to the second unequal-arm Mach-Zehnder interferometer;

[0017] a third unequal-arm Mach-Zehnder interferometer connected to the second cascade waveguide;

[0018] a fifth cascade waveguide connected to the third unequal-arm Mach-Zehnder interferometer;

[0019] a sixth cascade waveguide connected to the third unequal-arm Mach-Zehnder interferometer;

[0020] a ring resonant cavity connected to the third cascade waveguide, the fourth cascade waveguide, the fifth cascade waveguide and the sixth cascade waveguide;

[0021] a cascade waveguide connected to the ring resonator;

[0022] a loop mirror connected to the cascade waveguide;

[0023] An output-end waveguide connected to the third cascade waveguide, the fourth cascade waveguide, the fifth cascade waveguide and the sixth cascade waveguide;

[0024] Each of the two arms of the first unequal-arm Mach-Zehnder interferometer is provided with a set of parallel metal electrodes;

[0025] Each of the two arms of the second unequal-arm Mach-Zehnder interferometer is provided with a set of parallel metal electrodes;

[0026] Two arms of the third unequal-arm Mach-Zehnder interferometer are each provided with a group of parallel metal electrodes.

[0027] The first Mach-Zehnder interferometer is an unequal-arm structure, the waveguide is made of lithium niobate, and the cross section of the waveguide is a convex shape.

[0028] The second Mach-Zehnder interferometer is an unequal-arm structure, the waveguide is made of lithium niobate, and the cross section of the waveguide is a convex shape.

[0029] The third Mach-Zehnder interferometer is an unequal-arm structure, the waveguide is made of lithium niobate, and the cross section of the waveguide is a convex shape.

[0030] The micro-ring resonant cavity is a regular arc structure, the waveguide is made of lithium niobate, and the cross section of the waveguide is a convex shape.

[0031] The cross sections of the input end waveguide, the cascade waveguide and the output end waveguide are all convex-shaped.

[0032] The light inputted by the semiconductor optical amplifier is inputted into the composite resonant cavity through the mode spot converter. The multimode interference coupler of the first unequal-arm Mach-Zehnder interferometer, the second unequal-arm Mach-Zehnder interferometer and the third unequal-arm Mach-Zehnder interferometer is designed to have a splitting ratio of 50:50, so that the input light is divided into two light outputs, and the transmission functions of the two output lights are complementary, thereby avoiding multi-channel output of light of the same wavelength. The resonant spectrum lines of the first unequal-arm Mach-Zehnder interferometer, the second unequal-arm Mach-Zehnder interferometer and the third unequal-arm Mach-Zehnder interferometer are adjusted by applying an electric field, and the resonant wavelength of each unequal-arm Mach-Zehnder interferometer is changed, so that the peak of the transmission spectrum line of the light inputted into the ring resonant cavity coincides with a certain resonant wavelength point of the ring resonant cavity, and the high reflection and loop mirror on the left side of the semiconductor optical amplifier are used to enable the light to oscillate back and forth in the overall structure and achieve final lasing.

[0033] By applying an electric field to adjust the resonant spectral lines of the first unequal-arm Mach-Zehnder interferometer, the second unequal-arm Mach-Zehnder interferometer, and the third unequal-arm Mach-Zehnder interferometer, the single-channel frequency-selective spectrum and the mode gain difference of the frequency-selective spectrum between different channels can be changed. When the mode gain difference of the single-channel frequency-selective spectrum is large enough, the single-mode lasing of the channel can be achieved, and when the mode gain difference of the frequency-selective spectrum between different channels is similar, the simultaneous output of multi-channel wavelengths can be achieved. Similarly, the structure can change the mode gain difference by adjusting the electric field, thereby selecting whether a certain channel can achieve the lasing output of the wavelength.

[0034] By designing unequal-arm Mach-Zehnder interferometers with different arm length differences, a wide range of wavelength tuning can be achieved by using the vernier effect. By applying an electric field to adjust the resonant spectrum lines of the first unequal-arm Mach-Zehnder interferometer, the second unequal-arm Mach-Zehnder interferometer, and the third unequal-arm Mach-Zehnder interferometer, the central wavelength of the channel can be shifted under the premise that a certain channel can be single-mode lased, thereby achieving adjustable output wavelength.

[0035] Since the reduction of the radius of the microring will lead to an increase in the transmission loss of light, thus limiting the free spectral range of the microring, compared with the cascade of multiple microrings, the use of unequal-arm Mach-Zehnder interferometers and microring cascades can take advantage of the increase in the free spectral range of the unequal-arm Mach-Zehnder interferometer but the loss is basically unaffected, and achieve a wider range of tuning. The microrings at the later stage can effectively increase the mode gain difference based on a smaller 3dB bandwidth, achieving higher quality laser wavelength output.

[0036] Compared with the prior art, the present invention has the following advantages:

[0037] 1. Simultaneous output of multiple wavelengths. The multi-channel wavelength output is achieved by using a cascaded two-stage unequal-arm Mach-Zehnder interferometer and a micro-ring external cavity filter structure.

[0038] 2. Adjustable wavelength. The excellent electro-optical effect of lithium niobate material and the fact that LN material does not produce additional loss due to carrier absorption, as well as the advantage that the electrical tuning speed is faster than thermal tuning, can be modulated at high speed through the electrodes set on both sides of the unequal-arm Mach-Zehnder interferometer, and the push-pull modulation method can be used to achieve the adjustable output wavelength of each channel with a lower voltage.

[0039] 3. The number of channels can be selected, and the wavelength interval of each channel is fixed. The number of output wavelength channels and the output wavelength can be selected by changing the voltage loaded on the two arms of the unequal-arm Mach-Zehnder interferometer.

[0040] 4. Simple structure and easy to integrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Schematic diagram of the structure of a multi-channel wavelength tunable laser realized by using cascaded unequal-arm Mach-Zehnder interferometers and microrings;

[0042] Figure 2 This is a schematic diagram of the simulation of the laser single-channel external cavity filter spectrum line;

[0043] Figure 3 This is the simulation result diagram of the lasing spectrum line of the external cavity laser with variable number of channels;

[0044] Figure 4 This is the simulation result of the lasing spectrum line of the external cavity laser with adjustable wavelength. DETAILED DESCRIPTION

[0045] like Figure 1 As shown, a multi-channel output wavelength tunable laser based on a thin film lithium niobate platform, the structure includes: a lithium niobate thin film chip; a spot size converter 1 (SSC) located on the chip; and a composite resonant cavity connected to the spot size converter 1.

[0046] The composite resonant cavity comprises: a phase modulator arm 2 (phase shifter) connected to a pattern spot converter 1; a first unequal-arm Mach-Zehnder interferometer 3 (AMZI) connected to the phase modulator arm 2; a first cascade waveguide 4 connected to the first unequal-arm Mach-Zehnder interferometer 3; a second cascade waveguide 5 connected to the first unequal-arm Mach-Zehnder interferometer 3; a second cascade waveguide 6 connected to the first cascade waveguide 4; a third cascade waveguide 7 connected to the second unequal-arm Mach-Zehnder interferometer 6; a fourth cascade waveguide 8 connected to the second unequal-arm Mach-Zehnder interferometer 6; a third unequal-arm Mach-Zehnder interferometer 9 connected to the second cascade waveguide 5; and a fifth cascade waveguide 10 connected to the third unequal-arm Mach-Zehnder interferometer 9; a sixth cascade waveguide 11 connected to the third unequal-arm Mach-Zehnder interferometer 9; an output waveguide 13 connected to the cascade waveguide 7; an output waveguide 12 connected to the cascade waveguide 8; an output waveguide 14 connected to the cascade waveguide 10; an output waveguide 15 connected to the cascade waveguide 11; a ring resonator 16 connected to the cascade waveguide 7, the cascade waveguide 8, the cascade waveguide 10 and the cascade waveguide 11; a cascade waveguide 17 connected to the ring resonator 16; and a loop mirror 18 connected to the cascade waveguide 17.

[0047] The waveguide of the composite resonant cavity is made of lithium niobate material, and the cross section of the waveguide is convex-shaped. The waveguide height is 400nm, of which the top convex height is 200nm and the top convex width is 900nm. The sum of the arm length difference between the upper and lower arms of the first unequal-arm Mach-Zehnder interferometer 3 is 204μm, the sum of the arm length difference between the upper and lower arms of the second unequal-arm Mach-Zehnder interferometer 6 is 265μm, and the sum of the arm length difference between the upper and lower arms of the third unequal-arm Mach-Zehnder interferometer 11 is 265μm. The gap between two adjacent metal electrodes on the modulation arm of the unequal-arm Mach-Zehnder interferometer is 4.5μm, and the power-on mode of the upper and lower arms is push-pull modulation. The waveguide gap between the cascade waveguide 7 and the annular resonator 12 is 850 nm, the waveguide gap between the cascade waveguide 8 and the annular resonator 12 is 840 nm, the waveguide gap between the cascade waveguide 10 and the annular resonator 12 is 840 nm, and the waveguide gap between the cascade waveguide 11 and the annular resonator 12 is 850 nm. The annular resonator is a pure circular arc with a radius of 198 μm.

[0048] The first unequal-arm Mach-Zehnder interferometer 3, the second unequal-arm Mach-Zehnder interferometer 6, the third unequal-arm Mach-Zehnder interferometer 9, and the ring resonator 12 constitute a filter adjustable structure. Free spectral range FSR of unequal-arm Mach-Zehnder interferometer AMzI It can be expressed as

[0049]

[0050] Where λ is the center wavelength of the device, n g is the group refractive index of the waveguide, and ΔL is the sum of the length differences of the two arms of AMZI.

[0051] The transfer function T of a certain output of a single unequal-arm Mach-Zehnder interferometer AMZI It can be expressed as

[0052]

[0053] Wherein, FSR is the free spectral range of the unequal-arm Mach-Zehnder interferometer, and λ is the output wavelength. The transmission functions of the two outputs of the unequal-arm Mach-Zehnder interferometer are complementary.

[0054] The drop-end transfer function T of a single ring resonator MRR It can be expressed as

[0055]

[0056] Among them, α is the self-loss of the ring resonator, k is the coupling coefficient between the ring resonator and the straight waveguide, λ is the output wavelength, and FSR is the free spectral range of the ring resonator.

[0057] The output function T of any channel of the laser after passing through the ring resonator total It can be expressed as

[0058] T total =T AMZI1 *T AMZI2 *T MRR ……(4)

[0059] Among them, T AMZI1 is the transmission function of the first-stage unequal-arm Mach-Zehnder interferometer, T AMZI2 is the transmission function of the second-stage unequal-arm Mach-Zehnder interferometer, T NER is the transfer function of the ring resonator.

[0060] Each of the three unequal-arm Mach-Zehnder interferometers has two sets of electrodes, and the resonant spectrum lines are adjusted through push-pull modulation, thereby realizing the tuning of the laser output wavelength and the selection of the number of output channels.

[0061] After coupling the SOA with the SSC, the light is selected through composite resonant cavity filtering and the high reflection and loop mirror on the left side of the SOA are used to enable the light to propagate back and forth in the overall structure and enhance the optical signal through the SOA, ultimately achieving lasing.

[0062] The specific steps are as follows:

[0063] 1. Use the MODE of Lumerical simulation software to simulate the relevant parameters of LN ridge waveguide and import Interconnect.

[0064] 2. Use Comsol simulation software to simulate the electro-optical effect of lithium niobate and import the relationship between voltage and waveguide refractive index into Interconnect.

[0065] 3. Use Matlab software to simulate the relationship between the arm length difference of the two-stage cascade unequal-arm Mach-Zehnder interferometer, the radius of the ring resonant cavity and the mode gain difference of the final output filter spectrum line, and obtain the device parameter value that satisfies the gain difference to meet the design requirements. The simulation results of the external cavity filter spectrum line are as follows: Figure 2 As shown, the tuning range is 30nm.

[0066] 4. Use the Interconnect of Lumerical simulation software to simulate the overall structure of the laser. By changing the voltage value loaded on the unequal-arm Mach-Zehnder interferometer, the simulation results of the lasing spectrum with variable output channels are as follows: Figure 3 By changing the voltage value loaded on the unequal-arm Mach-Zehnder interferometer, the simulation results of the single-channel wavelength-tunable lasing spectrum are shown in (a), (b), and (c). Figure 4 As shown in (a) and (b).

Claims

1. A multi-channel output wavelength tunable laser of unequal-arm Mach-Zehnder interferometer and micro-ring cascade, characterized in that: include: chip; A pattern spot converter disposed on the chip; A composite resonant cavity connected to the pattern spot converter; The composite resonant cavity comprises: A phase modulation arm connected to the pattern spot converter; a first unequal-arm Mach-Zehnder interferometer connected to the phase modulation arm; a second unequal-arm Mach-Zehnder interferometer and a third unequal-arm Mach-Zehnder interferometer connected to the output end of the first unequal-arm Mach-Zehnder interferometer via a waveguide; A ring resonant cavity connected to the output ends of the second unequal-arm Mach-Zehnder interferometer and the third unequal-arm Mach-Zehnder interferometer through a waveguide.

2. The multi-channel output wavelength tunable laser of unequal-arm Mach-Zehnder interferometer and micro-ring cascade according to claim 1, characterized in that: The chip is a lithium niobate film chip, comprising a silicon substrate, a silicon dioxide buried oxide layer on the silicon substrate, a lithium niobate film on the silicon dioxide buried oxide layer, a silicon dioxide cladding covering the lithium niobate film, and a metal electrode for electrical adjustment.

3. The multi-channel output wavelength tunable laser of the unequal-arm Mach-Zehnder interferometer and micro-ring cascade according to claim 1, characterized in that: The pattern spot converter is an evanescent wave guide with increased width, and is made of lithium niobate material.

4. The multi-channel output wavelength tunable laser of unequal-arm Mach-Zehnder interferometer and micro-ring cascade according to claim 1, characterized in that: The phase modulation arm is made of lithium niobate material.

5. The multi-channel output wavelength tunable laser of unequal-arm Mach-Zehnder interferometer and micro-ring cascade according to claim 1, characterized in that: The output end of the ring resonant cavity is connected with a cascade waveguide and a loop mirror in sequence.

6. The multi-channel output wavelength tunable laser of unequal-arm Mach-Zehnder interferometer and micro-ring cascade according to claim 1, characterized in that: Each of the two arms of the first unequal-arm Mach-Zehnder interferometer is provided with a set of parallel metal electrodes; Each of the two arms of the second unequal-arm Mach-Zehnder interferometer is provided with a set of parallel metal electrodes; Two arms of the third unequal-arm Mach-Zehnder interferometer are each provided with a group of parallel metal electrodes.

7. The multi-channel output wavelength tunable laser of unequal-arm Mach-Zehnder interferometer and micro-ring cascade according to claim 1, characterized in that: The waveguide of the first Mach-Zehnder interferometer is made of lithium niobate and has a convex cross section; The waveguide of the second Mach-Zehnder interferometer is made of lithium niobate and the cross section of the waveguide is a convex shape; The waveguide of the third Mach-Zehnder interferometer is made of lithium niobate and the cross section of the waveguide is in a convex shape.

8. The multi-channel output wavelength tunable laser of unequal-arm Mach-Zehnder interferometer and micro-ring cascade according to claim 1, characterized in that: The waveguide of the annular resonant cavity is a regular arc structure, the waveguide is made of lithium niobate and the cross section of the waveguide is a convex shape.

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