A narrow linewidth terahertz wave generation chip and a terahertz wave generation method

Abstract

The present application relates to a kind of narrow linewidth terahertz wave generation chip and terahertz wave generation method, including semiconductor material base and integrated on semiconductor material base optical frequency comb generator, wave divider, two tunable lasers, light modulator and combiner;The optical signal output end of optical frequency comb generator is connected with the input end of wave divider, the input end of two tunable lasers is connected with the output end of wave divider respectively, wherein the output end of one tunable laser is connected with the input end of light modulator, the output end of another tunable laser and the output end of light modulator are all connected with the input end of combiner;Optical frequency comb generator, wave divider, two tunable lasers, light modulator and combiner are all connected by optical waveguide structure;Chip in the present application is small in size, low in cost, high in stability, and compared with traditional terahertz emitter, the linewidth quality of terahertz radiation source is improved.

Description

Technical Field

[0001] This invention relates to the field of terahertz wireless communication technology, and in particular to a narrow-linewidth terahertz wave generation chip and a terahertz wave generation method. Background Technology

[0002] Terahertz (THz) waves are electromagnetic waves with frequencies ranging from 0.1 to 10 THz. Their wavelength range lies between millimeter waves and far-infrared light in the electromagnetic spectrum (30 μm-3 mm), hence they are also called submillimeter waves. Terahertz waves have wide applications in many fields, including the study of the properties of semiconductor materials and high-temperature superconducting materials, tomographic imaging technology, label-free gene testing, cellular imaging, chemical and biological examinations, broadband communications, and microwave direction finding.

[0003] In the field of communications, theoretically speaking, the higher the frequency, the greater the communication capacity. Terahertz waves have a frequency that is 1 to 4 orders of magnitude higher than the microwaves currently in use. They can provide wireless transmission rates of over 10 Gbit / s, which is a height that microwaves cannot reach. Therefore, they can solve the problem of information transmission being limited by bandwidth and also meet users' bandwidth requirements.

[0004] Currently, methods for generating terahertz waves using electronics include back-wave tubes (BWO), gyrotrons, free-electron lasers (FEL), and solid-state electronic frequency doubling. However, these methods are bulky, energy-intensive, expensive, and require complex maintenance, which greatly limits their applications. There are also many reports on methods for generating terahertz waves using photonics, such as generating terahertz waves by pumping a gas cell with a high-power CO2 laser, optical difference frequency generators (DFG), terahertz parametric generators / oscillators (TPG / TPO), and optical Cherenkov radiation effects. However, these methods also suffer from fundamental problems such as low electro-optic efficiency, large size and weight, and the inability to achieve miniaturization.

[0005] The lack of efficient, tunable, low-cost, and miniaturized terahertz wave generation methods that can operate at room temperature has resulted in a very limited understanding of the terahertz band; miniaturized, low-cost semiconductor devices that can be mass-produced are key to the large-scale deployment of commercial terahertz communication systems in the next decade.

[0006] In commercial applications, dual-wavelength light sources based on III-V compound semiconductor materials using monolithic photonic integrated circuits (PICs) have significant advantages in terms of size, cost, reliability, and mass production. By integrating multiple active and passive device units onto a single chip through photonic integration, the size and coupling loss can be further reduced, stability can be improved, and packaging and testing complexity can be lowered. Therefore, monolithic photonic integration is currently the most promising light source solution for realizing commercial high-performance terahertz systems.

[0007] Therefore, this invention provides a novel technical solution for realizing miniaturized, high-performance, and low-cost terahertz light sources, which can greatly promote the research and market application of terahertz technology. Summary of the Invention

[0008] This invention provides a narrow-linewidth terahertz wave generating chip and a terahertz wave generating method. The chip can achieve continuous operation at room temperature, low cost, and miniaturization, and can generate frequency-tunable, high-power, narrow-linewidth terahertz waves, which can greatly promote the research and market application of terahertz technology.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A narrow linewidth terahertz wave generating chip includes a semiconductor material substrate and an optical frequency comb generator, a demultiplexer, two tunable lasers, an optical modulator, and a multiplexer integrated on the semiconductor material substrate.

[0011] The optical signal output terminal of the aforementioned optical frequency comb generator is connected to the input terminal of the aforementioned wave splitter. The two output terminals of the aforementioned wave splitter are respectively connected to the input terminals of the two aforementioned tunable lasers. The output terminal of one of the aforementioned tunable lasers is connected to the input terminal of the aforementioned optical modulator. The output terminal of the other aforementioned tunable laser and the output terminal of the aforementioned optical modulator are both connected to the input terminal of the aforementioned wave combiner.

[0012] The optical frequency comb generator, wave splitter, two tunable lasers, optical modulator and wave combiner are all connected by an optical waveguide structure.

[0013] Preferably, the optical frequency comb generator described above is either a semiconductor mode-locked laser or an integrated nonlinear optical frequency comb device.

[0014] Preferably, the above-mentioned wave demultiplexer adopts either an arrayed waveguide grating or a 1×2 multimode interference coupler.

[0015] Preferably, the tunable laser described above is any one of a distributed feedback semiconductor laser, a distributed Bragg reflector semiconductor laser, or a sampled grating distributed Bragg reflector laser.

[0016] Preferably, the optical modulator includes a phase modulation region and an amplitude modulation region. The phase modulation region includes a phase n-type InP layer, a phase quantum well region, and a vertical pip layer arranged from bottom to top. The amplitude modulation region includes an n-type InP layer, a modulation quantum well region, a modulation grating region, and an amplitude p-type InP layer arranged from bottom to top.

[0017] Preferably, the optical modulator, the wave demultiplexer, the wave combiner, and the optical waveguide are all passive structures, while the optical frequency comb generator and the two tunable lasers are active structures. The multiple active and passive structures are monolithically integrated through photonic integration.

[0018] A method for generating narrow-linewidth terahertz waves, using the aforementioned generation chip, comprises the following steps:

[0019] An optical frequency comb with equal spacing is generated using an optical frequency comb generator.

[0020] The optical signal output from the optical frequency comb generator is divided into two optical signals by a wave splitter. The signals from the two output ends are injected into two tunable lasers respectively for optical injection locking.

[0021] The light wave output from one of the tunable lasers enters the optical modulator and is modulated at high speed.

[0022] The light wave modulated by the optical modulator and the light wave output from another tunable laser enter the combiner, and then the two-mode laser signal is output from a single waveguide.

[0023] A dual-mode laser signal in a single waveguide is coupled to a photoconductive switch mixer or photodetector with an integrated antenna, ultimately radiating a terahertz wave.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] 1. The optical frequency comb generator, demultiplexer, two tunable lasers, optical modulator and multiplexer are integrated on a semiconductor substrate. It is small in size, low in cost, and highly stable. It can operate at room temperature and is conducive to mass production. It can meet the commercial product requirements of high performance, low cost and mass production in the wireless communication market.

[0026] 2. By generating a series of equally spaced optical frequency combs through an optical frequency comb generator, the optical signal has good coherence, which easily leads to extremely narrow and strong pulses in the time domain of the generated optical signal, resulting in good mode-locking effect after injection into the laser.

[0027] 3. A series of equally spaced optical frequency combs are generated by an optical frequency comb generator, and then two tunable lasers are locked by optical injection, thereby optimizing the optical frequency response and providing a stable dual-wavelength, high-power, narrow-linewidth terahertz source. Compared with traditional terahertz transmitters, the linewidth quality of the terahertz radiation source is improved, providing a solution for reducing the bit error rate of high-speed terahertz communication systems. The integrated chip frequency is flexibly and continuously adjustable and covers a long transmission frequency window, thereby significantly reducing the complexity of terahertz communication systems. Attached Figure Description

[0028] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0029] Figure 1 This is a structural diagram of the chip of the present invention;

[0030] Figure 2 A cross-sectional view of a tunable laser and an optical modulator assembly on a substrate.

[0031] Explanation of reference numerals in the attached figures:

[0032] 1. Semiconductor substrate; 2. Optical frequency comb generator; 3. Wavelength divider; 4. Tunable laser; 5. Optical modulator; 6. Wavelength combiner; 7. Laser n-type InP layer; 8. Lower confinement layer; 9. Gain quantum well region; 10. Upper confinement layer; 11. Gain grating region; 12. Laser p-type InP layer; 13. Phase region n-type InP layer; 14. Phase quantum well region; 15. Vertical pip layer; 16. Amplitude region n-type InP layer; 17. Modulation quantum well region; 18. Modulation grating region; 19. Amplitude region p-type InP layer. Detailed Implementation

[0033] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0035] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0036] This invention provides a narrow-linewidth terahertz wave generation chip, such as... Figure 1 As shown, the device includes a semiconductor substrate 1 and an optical frequency comb generator 2, a demultiplexer 3, two tunable lasers 4, an optical modulator 5, and a multiplexer 6 integrated on the semiconductor substrate 1. By integrating the optical frequency comb generator 2, demultiplexer 3, two tunable lasers 4, optical modulator 5, and multiplexer 6 onto the semiconductor substrate 1, the resulting chip is small in size, low in cost, and highly stable. It can operate at room temperature, is easy to use, and is conducive to mass production, meeting the commercial product demands of the wireless communication market for high-performance, low-cost, and high-volume production. Specifically, the semiconductor substrate 1 uses III-V compound semiconductor materials, such as indium phosphide (InP) and gallium arsenide (GaAs). All are natural direct bandgap materials, with their valence band top and conduction band bottom located at the same position in wave vector k-space. Electron-hole recombination does not require momentum exchange, thus exhibiting high luminous efficiency. Furthermore, InP, GaAs, and their solid solutions InGaAsP and InGaAlAs can cover the commonly used O-band (1260-1360nm, center wavelength 1310nm) and C+L-band (C-band: 1530-1565nm, center wavelength 1550nm; L-band: 1565-1625nm) in optical communication. At the same time, since III-V compound semiconductor materials have high electron mobility in both low and high fields, they can improve the operating speed of lasers and reduce losses.

[0037] Specifically, the optical signal output of the optical frequency comb generator 2 is connected to the input of the demultiplexer 3. The two outputs of the demultiplexer 3 are connected to the inputs of two tunable lasers 4, respectively. The output of one tunable laser 4 is connected to the input of the optical modulator 5. The outputs of the other tunable laser 4 and the optical modulator 5 are both connected to the input of the combiner 6. The optical frequency comb generator 2, the demultiplexer 3, the two tunable lasers 4, the optical modulator 5, and the combiner 6 are all connected by an optical waveguide structure. Thus, the optical signal generated by the optical frequency comb generator 2 is injected into the two tunable lasers 4 to achieve mode locking. Subsequently, the optical signal output from one of the tunable lasers 4 enters the optical modulator 5 to achieve high-speed modulation of the output light wave. The optical signal output from the other tunable laser 4 and the optical signal output from the optical modulator 5 enter the combiner 6 together for output. Finally, the output is coupled to a photoconductive switch mixer or photodetector with an integrated antenna, which can ultimately radiate terahertz waves.

[0038] Specifically, the optical frequency comb generator 2 uses either a semiconductor mode-locked laser or an integrated nonlinear optical frequency comb device to generate a series of equally spaced optical frequency combs. An optical frequency comb is a broadband spectrum composed of numerous spectral comb teeth with stable frequencies and strictly equal spacing. In the time domain, it manifests as a series of equally spaced ultrashort pulse outputs. The light at different frequencies has a relatively stable phase relationship. With a definite phase relationship, the pulses in the time domain become extremely narrow and strong. The narrower the pulses in the time domain, the better the mode-locking effect. Thus, the optical signal generated by the optical frequency comb generator 2 is injected into the tunable laser 4. Through injection locking technology, the optical signals output by the two tunable lasers 4 are high-power and narrow-linewidth, thereby providing a stable dual-wavelength, high-power, and narrow-linewidth terahertz source and improving the linewidth quality of the terahertz wave.

[0039] Specifically, Figure 1 The solid lines connecting the various components indicate passive optical waveguide structures, which are used for optical signal transmission. The arrows point in the direction of optical signal transmission.

[0040] Preferably, the wave splitter 3 is used to divide the one optical signal output by the optical frequency comb generator 2 into two optical signals. The signals at the two output ends are respectively injected into the tunable laser 4. It can be any one of an arrayed waveguide grating or a 1×2 multimode interference coupler.

[0041] Preferably, the tunable laser 4 is an active device, such as... Figure 2As shown, its epitaxial layers, from the insulating substrate along the growth direction, sequentially include: an n-type InP layer, referred to as the laser n-type InP layer 7, which connects to the laser driving negative electrode; a lower confinement layer 8 in the gain region, referred to as the lower confinement layer 8, with a material bandgap wavelength of approximately 1100 nm; a quantum well region in the gain region, referred to as the gain quantum well region 9, which is generally formed by stacking potential wells and barriers sequentially, with a material bandgap wavelength of approximately 1550 nm, used to confine charge carriers to the designed energy level through the quantum well structure, thereby generating photons and providing gain for the entire laser structure; and an upper confinement layer 10 in the gain region, referred to as the upper confinement layer 10, which, along with the lower confinement layers 8, is used to confine the light field to... Between these two epitaxial layers, the material bandgap wavelength is around 1100nm; the grating region of the laser, referred to as the gain grating region 11, selects a single specific laser lasing wavelength by designing the Γ value of the grating, and the material bandgap wavelength is around 1200nm; the p-type InP layer, referred to as the laser p-type InP layer 12, is mainly used to form the laser ridge waveguide structure, to form the material refractive index difference, to suppress transverse higher-order modes, and at the same time, the epitaxial layer is connected to the laser driving positive electrode; specifically, the tunable laser 4 can be one of a distributed feedback semiconductor laser, a distributed Bragg reflection semiconductor laser, or a sampling grating distributed Bragg reflection laser.

[0042] Preferably, such as Figure 2 As shown, the optical modulator 5 is an electro-absorption modulator, which includes a phase modulation region and an amplitude modulation region. The epitaxial layer of the phase region, from the insulating substrate along the growth direction, sequentially includes: an n-type InP layer, referred to as the phase region n-type InP layer 13; a phase quantum well region 14, with a material band gap of about 1400 nm, which changes the effective optical path and thus the phase of light through the quantum confinement Stark effect—the change in the exciton absorption peak under the electric field leads to a change in refractive index; and a longitudinal pip layer 15, which is used to connect the electrodes of the phase modulator. The epitaxial layer of the amplitude modulation region, from the insulating substrate along the growth direction, sequentially includes: an n-type InP layer, referred to as the amplitude region n-type InP layer 16. Furthermore, the epitaxial layer is used to connect the ground (G) electrode of the modulator's RF electrode; the modulation quantum well region 17 has a material bandgap wavelength of about 1400nm. Through the quantum confinement Stark effect—the change in the exciton absorption peak under the electric field leads to a change in the refractive index—it changes the effective optical path and thus changes the phase of the light. The modulator is divided into upper and lower arms, and amplitude modulation can be finally achieved by combining the upper and lower optical paths; the modulation grating region 18 precisely controls the wavelength of the modulated light passing through the modulator; the p-type InP layer, abbreviated as amplitude region p-type InP layer 19, is used to form the modulator ridge waveguide structure and to form the material refractive index difference. At the same time, the epitaxial layer is connected to the signal (S) electrode of the modulator driving RF electrode.

[0043] Specifically, the prefixes in the names of the laser n-type InP layer 7, gain quantum well region 9, gain grating region 11, laser p-type InP layer 12, phase region n-type InP layer 13, phase quantum well region 14, amplitude region n-type InP layer 16, modulation quantum well region 17, modulation grating region 18, and amplitude region p-type InP layer 19 are for convenience. Figure 2 The winning number.

[0044] Specifically, the optical modulator 5 in this embodiment is based on the quantum confinement Stark effect (QCSE). An external vertical electric field tilts the quantum well bandgap, causing a redshift of the absorption peak. The specific working principle is as follows: In semiconductor quantum well materials, the movement of electrons and holes is confined by the quantum well barrier, and excitons are quasi-two-dimensional excitons. Excitons can exist at room temperature, thus forming a sharp exciton absorption peak on the absorption curve. Due to the presence of the exciton absorption peak, the absorption curve of multi-quantum well materials has a steep edge. When an electric field is applied in a direction perpendicular to the quantum well wall, the quantum well bandgap tilts, the quantum energy levels of electrons and holes decrease, and the absorption bandgap redshifts. Furthermore, due to the Kramers-Kronig relationship, the change in the absorption coefficient leads to a change in the effective refractive index of light, thereby enabling high-speed phase modulation of the laser in each path of the optical modulator 5. After being combined by the optical modulator 5, high-speed amplitude modulation greater than 60 GHz can be achieved, better serving the next generation of high-speed terahertz communication systems supporting 100 Gb / s and higher.

[0045] Specifically, the optical modulator 5 requires a fabrication process combining wet and dry etching, with an etching accuracy of 20%. Furthermore, the optical modulator 5 employs deep etching, which must penetrate the quantum well region. Additionally, the optical modulator 5 requires the use of the BCB process. The ridge width of the optical modulator 5 is approximately 2-3 μm, with an accuracy requirement as high as 0.15 μm, which is an order of magnitude higher than the accuracy requirement of a laser. The amplitude region of the optical modulator 5 is a push-pull electrode that can receive RF signals.

[0046] Preferably, the optical modulator 5, the demultiplexer 3, the multiplexer 6, and the optical waveguide are all passive structures, while the optical frequency comb generator 2 and the two tunable lasers 4 are active structures. Multiple active and passive structures are monolithically integrated through photonic integration. The optical frequency comb generator 2, the tunable lasers 4, the optical modulator 5, the multiplexer 6, and the optical waveguide structure are all integrated on a monolithic semiconductor material substrate 1. The resulting monolithic photonic integrated chip is on the order of millimeters, achieving miniaturization, integration, high performance, and low cost of terahertz light sources. It is a true system-on-a-chip (SoC) and a chip-level solution for realizing terahertz light sources.

[0047] Preferably, an electro-optical isolation insulation process is added between the tunable laser 4 and the optical modulator 5 to ensure that when the chip is working, the charge carriers injected into the tunable laser 4 and the optical modulator 5 are controlled within their respective regions and will not enter each other's devices. At the same time, it is necessary to ensure that there is no light reflection between the tunable laser 4 and the optical modulator 5, thereby ensuring the independent operation of the devices.

[0048] A method for generating narrow-linewidth terahertz waves, using the aforementioned generation chip, comprises the following steps:

[0049] An optical frequency comb with equal spacing is generated using an optical frequency comb generator 2.

[0050] The optical signal output from the optical frequency comb generator 2 is divided into two optical signals by the wave splitter 3. The signals from the two output ends are injected into two tunable lasers 4 respectively for optical injection locking.

[0051] The light wave output from one of the tunable lasers 4 enters the optical modulator 5 and is modulated at high speed.

[0052] The light wave modulated by the optical modulator 5 and the light wave output by another tunable laser 4 enter the combiner 6, and then the waves are combined, and the dual-mode laser signal is output from a single waveguide.

[0053] A dual-mode laser signal in a single waveguide is coupled to a photoconductive switch mixer or photodetector with an integrated antenna, ultimately radiating a terahertz wave.

[0054] The working principle is as follows: A narrow linewidth laser signal is injected into the tunable laser 4 to improve the frequency modulation characteristics of the tunable laser 4. The injection mode-locking can increase the modulation bandwidth of the tunable laser 4 and reduce the nonlinear effect of the tunable laser 4. By injecting a relatively coherent optical signal generated by the optical frequency comb into the single-mode tunable laser 4, the tunable laser 4 outputs a narrow linewidth optical signal.

[0055] By independently tuning the output wavelengths λ1 and λ2 of two tunable lasers 4 by changing the injection current of the two tunable lasers 4, the final terahertz wave frequency fTHz is generated according to the beat frequency principle. Determined by the frequencies f1 and f2 of the two tunable lasers 4 (assuming f1 > f2), fTHz = f1 - f2. The relationship between the wavelength interval and the frequency interval satisfies the equation fTHz = c*(λ2 - λ1) / (λ1 * λ2), where c refers to the speed of light in a vacuum, c = 299792458 m / s. Therefore, by changing the continuously tunable dual-mode wavelength difference, the corresponding continuously tunable terahertz wave can be obtained.

[0056] The above-mentioned terahertz wave generation method combines injection-locking technology, tunable dual-wavelength laser beat frequency generation of terahertz source, and monolithic photonic integration technology, providing a novel technical solution for realizing miniaturized, high-performance, and low-cost terahertz light sources.

[0057] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A narrow-linewidth terahertz wave generating chip, characterized in that, It includes a semiconductor material substrate and an optical frequency comb generator, a demultiplexer, two tunable lasers, an optical modulator, and a multiplexer integrated on the semiconductor material substrate. The optical signal output terminal of the optical frequency comb generator is connected to the input terminal of the wave splitter, and the two output terminals of the wave splitter are respectively connected to the input terminals of the two tunable lasers. The output of one of the tunable lasers is connected to the input of the optical modulator, and the output of the other tunable laser and the output of the optical modulator are both connected to the input of the combiner. The optical frequency comb generator, wave splitter, two tunable lasers, optical modulator and wave combiner are all connected by an optical waveguide structure. The work steps are as follows: An optical frequency comb with equal spacing is generated using an optical frequency comb generator. The optical signal output from the optical frequency comb generator is divided into two optical signals by a wave splitter. The signals from the two output ends are injected into two tunable lasers respectively for optical injection locking to obtain high-power narrow-linewidth optical waves. The light wave output from one of the tunable lasers enters the optical modulator and is modulated at high speed. The light wave modulated by the optical modulator and the light wave output from another tunable laser enter the combiner, and then the two-mode laser signal is output from a single waveguide. A dual-mode laser signal in a single waveguide is coupled to a photoconductive switch mixer or photodetector with an integrated antenna, ultimately radiating a high-power, narrow-linewidth terahertz wave.

2. The terahertz wave generating chip according to claim 1, characterized in that, The optical frequency comb generator described herein employs either a semiconductor mode-locked laser or an integrated nonlinear optical frequency comb device.

3. The terahertz wave generating chip according to claim 1, characterized in that, The wave demultiplexer can be either an arrayed waveguide grating or a 1×2 multimode interference coupler.

4. The terahertz wave generating chip according to claim 1, characterized in that, The tunable laser is any one of a distributed feedback semiconductor laser, a distributed Bragg reflector semiconductor laser, or a sampled grating distributed Bragg reflector laser.

5. The terahertz wave generating chip according to claim 1, characterized in that, The optical modulator includes a phase modulation region and an amplitude modulation region. The phase modulation region includes a phase n-type InP layer, a phase quantum well region, and a vertical pip layer arranged from bottom to top. The amplitude modulation region includes an amplitude n-type InP layer, a modulation quantum well region, a modulation grating region, and an amplitude p-type InP layer arranged from bottom to top.

6. The terahertz wave generating chip according to claim 1, characterized in that, The optical modulator, the wave demultiplexer, the wave combiner, and the optical waveguide are all passive structures, while the optical frequency comb generator and the two tunable lasers are active structures. Multiple active and passive structures are monolithically integrated through photonic integration.

Images