An on-chip optical isolator based on singular point gain-loss modulated grating

Abstract

This invention belongs to the field of integrated photonics and non-reciprocal optical devices, specifically relating to an on-chip optical isolator based on a singularity gain-loss modulation grating. This invention embeds the gain-loss modulation grating into a microcavity, utilizing the asymmetric reflection characteristics at the singularity to establish a direction-dependent intracavity field distribution. Combined with the spatially selective dissipation of a nonlinear periodic loss structure, it successfully achieves high-isolation direction-selective transmission. Compared to existing technologies, this invention does not rely on magneto-optical materials or an external magnetic field. The singularity operating state can be maintained through static structural design, avoiding the complexity and instability caused by dynamic tuning. It also possesses advantages such as high isolation and CMOS process compatibility, providing a scalable all-optical solution for on-chip optical isolation.

Description

Technical Field

[0001] This invention belongs to the field of integrated photonics and non-reciprocal optical device technology, specifically relating to an on-chip optical isolator based on a singular point gain loss modulation grating. Background Technology

[0002] Optical isolators are core passive devices in modern optical communication systems. Their core function is to allow optical signals to pass through unidirectionally while blocking reverse transmission, thereby protecting sensitive components such as lasers from reflected light interference. In modern dense wavelength division multiplexing (DWDM) systems, multiple wavelength channels transmit simultaneously in optical fibers. Even minute reflections at any connector, filter, or amplifier interface can generate coherent noise, causing laser frequency jitter and linewidth broadening, and in severe cases, even leading to increased system bit error rate and communication interruption. This is particularly true in high-speed direct-modulation laser applications, which are highly sensitive to back-reflected light. Reflected light can cause laser operating wavelength drift, increased intensity noise, and linewidth broadening. By integrating an optical isolator at the laser output, the effects of reflected light can be effectively suppressed, significantly improving the laser's operating stability and signal-to-noise ratio, thus ensuring the transmission quality and reliability of the entire optical communication system.

[0003] The core technology of traditional optical isolators is based on the Faraday magneto-optical effect. By applying an external magnetic field to modulate the optical rotation properties of magneto-optical materials, the polarization state of light is non-reciprocally rotated, thereby isolating reverse light. However, the materials such as iron garnet used in these magneto-optical devices are incompatible with mainstream silicon-based photonic integrated circuit processes, making it difficult to achieve miniaturization and low-cost monolithic integration. This has become one of the main bottlenecks in the development of large-scale integrated photonic chips.

[0004] To address this problem, researchers have begun exploring non-reciprocal optical schemes that do not require magneto-optical materials. Among these, non-Hermitian optical systems based on the parity-time (PT) symmetry principle have shown great potential. In optical systems, PT symmetry requires the complex refractive index distribution of the system to satisfy the condition n(r) = n * (-r), where r represents the spatial coordinates. This condition implies that the real part of the refractive index is even-symmetric about space, while the imaginary part, characterizing gain or loss, is odd-symmetric. When gain and loss are precisely balanced, the system is in a state called the singularity point (EP). In this state, the system's eigenvalues ​​and eigenstates undergo degeneracy and fusion, exhibiting non-reciprocal transport characteristics that are highly sensitive to the direction of incident light. Currently, studies have experimentally verified the EP state by introducing gain and loss into coupled microring resonant cavities. However, most existing solutions rely on dynamically tuning the coupling distance between microcavities or the position of internal scatterers to accurately reach and maintain the EP. This introduces complex active control mechanisms, reducing the system's stability and practicality, and making it difficult to meet the needs of practical chip-level applications.

[0005] Therefore, there is an urgent need in this field for a high-performance on-chip optical isolator solution that is simple in structure, stable and reliable, requires no external dynamic tuning, and is compatible with standard photonic integration processes. Summary of the Invention

[0006] To address the aforementioned problems and shortcomings, and to resolve the issue that existing on-chip optical isolators cannot simultaneously satisfy the requirements of simple structure, stable reliability, no need for external dynamic tuning, and compatibility with standard photonic integration processes, this invention provides an on-chip optical isolator based on a singular point gain loss modulation grating. This invention's device achieves high-performance non-reciprocal transmission without the need for magneto-optical materials or dynamic tuning; through precise geometric design, it utilizes the offset of periodic losses to control losses in the optical field, ensuring the stability and reliability of the device's operation; and it is fully compatible with CMOS processes.

[0007] An on-chip optical isolator based on a singular point gain-loss modulation grating comprises four parts: an input / output bus waveguide, an optical resonant cavity, a gain-loss modulation grating, and a nonlinear periodic loss structure.

[0008] The input / output bus waveguide is used for inputting and outputting optical signals. It is coupled with the optical resonant cavity through evanescent wave to form a coupling region. By adjusting the gap and coupling length of the coupling region, the system is made to be in an overcoupled condition at the target operating point.

[0009] The optical resonant cavity is a ring cavity, serving as the core carrier for optical signal transmission and interference. When the device is working, the signal light incident from the front (from the input end to the output end) enters the optical resonant cavity through the coupling region and propagates in a clockwise direction; the signal light incident from the back (from the output end to the input end) enters the optical resonant cavity through the coupling region and propagates in a counterclockwise direction.

[0010] The geometry of the optical resonator is designed according to the operating wavelength so that at least one resonant mode falls within the operating band.

[0011] The gain-loss modulation grating, as the core functional unit for generating the EP state and non-reciprocal reflection, is integrated and embedded in the straight section of the optical resonant cavity. This grating is composed of multiple spatially periodic units arranged along the propagation direction, and the total length of the grating is... This is the product of the length of a single period and the number of periods. Within each periodic unit, the complex refractive index is precisely modulated along the direction of light propagation, satisfying the parity-time symmetry condition, i.e., the complex refractive index distribution... satisfy ;in, z represents the coordinate along the direction of light propagation, and the real part of the refractive index. The phase constant is determined and exhibits an even function distribution in space, while the imaginary part of the refractive index... The refractive index corresponds to gain or loss and exhibits an odd function distribution. The positive and negative values ​​of the imaginary part of the refractive index correspond to periodically alternating loss and gain regions. Specifically, within a complete period Λ, the complex refractive index distribution is described by the following four continuous regions:

[0012] (1)

[0013] Where n0 represents the background refractive index, Δn r For the transformation of the real part of the refractive index, correspondingly, -iΔn g Indicates gain, iΔn L This represents the loss. When Δn is satisfied... r =Δn g =Δn L When the three equilibrium conditions are met, the system enters EP and exhibits significant non-reciprocal optical properties.

[0014] The nonlinear periodic loss structure is disposed on another straight section of the optical resonant cavity. This structure consists of multiple periodic loss strips with a certain duty cycle arranged along the light propagation direction. The strip period matches the period of the local interference fringes formed by the superposition of opposing traveling waves within the cavity. Each loss strip has intensity-dependent additional absorption characteristics; that is, the local loss it introduces increases monotonically with increasing light intensity at that location and gradually approaches saturation.

[0015] The coverage length of the nonlinear periodic loss structure along the straight segment is: ,in The key to achieving optical isolation lies not only in generating a non-reciprocal response, but also in converting this response into effective blocking of forward light while maintaining low-loss conduction of reverse light. This invention achieves this goal through ingenious geometric design: by controlling the offset distance Δx between the vertical central axis of the periodic loss structure within the optical resonant cavity and the vertical central axis of the straight section of the embedded racetrack cavity, the value of which ranges from [value missing]. .

[0016] Δx is a key geometric parameter controlling the relative spatial position between the interfering standing wave and the nonlinear periodic loss structure within the cavity. By precisely designing Δx, it can be ensured that during forward transmission, the coherent constructive point caused by the interference of the two beams within the cavity is located on the periodic material loss, increasing the transmission light loss within the cavity. This satisfies the coherent destructive phase with the through light in the input / output bus waveguide, effectively blocking the forward-transmitted optical signal. For the reverse-transmitted light, due to the non-reciprocal reflection characteristics of the gain-loss modulation grating operating at EP, the reflection coefficient is close to zero, and there is no reflected light. Therefore, there is no interference effect within the cavity. Although the loss structure causes loss to the transmitted light within the cavity, compared to the forward incident phase, the loss is reduced, causing the cavity to be in an overcoupled state at this time. Thus, the optical signal can pass through the device, achieving conduction.

[0017] Furthermore, the optical resonant cavity is a racetrack-shaped resonant cavity, consisting of two straight sections of the same size and a semi-circular curved section connected end-to-end to form a closed optical path. The length of the straight section is... The bending radius is The perimeter of the optical resonant cavity is .

[0018] Furthermore, the number of periods N of the gain-loss modulation grating ranges from 40 to 60, and the refractive index modulation amplitude Δn ranges from 1 × 10⁻⁶. -3 Up to 5×10 -4 Within this parameter range, by balancing the grating period number and the refractive index modulation amplitude, the reflected light reflected back by the grating during forward transmission can be made nearly equal in amplitude to the transmitted light directly transmitted. The closer the amplitudes of the two beams are, the higher the contrast of the interference standing wave formed when they meet in the lower straight section. When the high contrast of the interference standing wave and the spatial position of the periodic loss structure are precisely aligned by Δx, the local light intensity at the coherent constructive point is significantly enhanced, causing the saturable loss strip to generate greater additional loss. This results in closer critical coupling of the forward-transmitting light in the coupling region, ultimately improving the isolation of the device.

[0019] In summary, compared with the prior art, the present invention has the following significant advantages: First, by integrating the real part modulation and imaginary part modulation of the refractive index into the same gain-loss modulation grating, a single structure can simultaneously provide a Bragg backscattering channel and a non-Hermitian asymmetric coupling channel, significantly reducing the system complexity and parameter matching difficulty required for EP construction; Second, by combining the direction-dependent reflection asymmetry of the EP grating with the spatially selective intensity-dependent dissipation of the nonlinear periodic loss structure, a complete conversion link from "intracavity direction-dependent field distribution" to "port direction-selective transmission" is established, achieving high isolation; Third, the device as a whole does not depend on external magnetic fields and magneto-optical materials, and the entire structure can be realized based on mainstream integrated photonic platforms such as silicon-based, silicon nitride, or thin-film lithium niobate, exhibiting excellent on-chip integration compatibility and process feasibility. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the device structure in the embodiment;

[0021] Figure 2 The transmission spectra and corresponding isolation levels at different ports are shown in the embodiments.

[0022] Figure 3 The effect of the positional offset of the nonlinear periodic loss structure on isolation is illustrated in the embodiment. Detailed Implementation

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0024] In this embodiment, as Figure 1 The structure is shown below: The optical resonant cavity is a racetrack-shaped optical resonant cavity, consisting of two semicircles of radius R and a length of L. s It consists of two straight segments. The specific parameters of the device are as follows: the length of the straight segment of the resonant cavity is L. s With a diameter of 80 μm and a radius of R = 10 μm, the total perimeter of the resonant cavity is approximately 2π × 10 + 2 × 80 ≈ 223 μm, and the loss of the racetrack-shaped optical resonant cavity is 1 dB / cm. The core purpose of this size is to match its free spectral range with the optical mode of the operating band, i.e., to ensure that the operating wavelength is at the intrinsic resonant mode of the resonant cavity. A semi-circular waveguide segment of the racetrack-shaped resonant cavity is coupled to the bus waveguide via evanescent wave coupling to form a coupling region. The coupling coefficient is designed to be 0.07 to ensure that the system operates in an overcoupled state, achieving efficient transmission of optical power between the bus waveguide and the resonant cavity. (Reference) Figure 1 In this embodiment, the direction from port 1 to port 2 is defined as the positive direction, and the reverse direction is defined as the direction from port 2 to port 1. In this device structure, the resonant cavity mainly excites a clockwise (CW) traveling wave mode when incident in the positive direction, and mainly excites a counterclockwise (CCW) traveling wave mode when incident in the negative direction. The terms "positive incidence", "reverse incidence", "CW", and "CCW" used thereafter shall be based on this.

[0025] A PT-symmetric grating (gain-loss modulation grating) is integrated into the straight section of a racetrack-shaped resonant cavity. This gain-loss modulation grating has a length of 30 μm, and the period of each unit is approximately Λ≈352 nm. The refractive index distribution within each period strictly follows formula (1), where the background refractive index is set to 2.2, and the refractive index parameter is precisely adjusted to satisfy Δn. r =Δn g =Δn L The condition = 0.0007 makes the system work in EP state.

[0026] A nonlinear periodic loss structure is positioned in the lower straight section of the racetrack-shaped resonant cavity, with a coverage length of 75 μm. The loss structure consists of multiple periodically arranged loss strips, with a strip period of [missing information]. The period is matched with the local interference fringes propagating in opposite directions within the cavity. The duty cycle is 0.5, meaning that the loss stripe and the non-loss stripe each occupy half of each period. The loss stripe exhibits intensity-dependent saturable characteristics: it provides a linear substrate loss of 6.3 dB / cm under low light intensity conditions, and as the light intensity approaches the characteristic saturation intensity of 5 mW, the loss increases with increasing light intensity and gradually approaches the saturation value.

[0027] By precisely controlling the offset distance Δx of the periodic loss structure, and through system parameter optimization, it was determined that when Δx = 0 μm, the forward incident coupling reaches critical coupling, forward transmission is greatly suppressed, while reverse light remains conductive, resulting in high transmittance. Combined, the device exhibits an isolation of 47 dB in this state.

[0028] Figure 2 The spectral characteristics of the device are shown. A 1550nm signal light with an input power of 0.05mW enters the bus waveguide from the input end, and is partially coupled into the racetrack-shaped resonant cavity through the evanescent wave coupling region, propagating in a clockwise (CW) direction within the cavity. When the light wave passes through the gain-loss modulation grating in the upper straight section, due to the asymmetric backscattering characteristics of the grating at the singular point, part of the CW component is coupled to the counterclockwise (CCW) direction, forming a strong CCW component. Thus, two opposing traveling wave components, CW and CCW, exist simultaneously within the cavity. When these two components meet in the lower straight section, they interfere and form a spatially periodic standing wave intensity distribution. Due to the design of the Δx position, the coherent constructive point of the standing wave is located precisely on the periodic loss strip. At this point, the local light intensity is high, and it is in the steep response region of saturable absorption, with the strip introducing a large additional loss. This additional loss significantly increases the effective attenuation within the cavity for forward incidence, achieving critical coupling at the coupling point. Forward transmission is suppressed, resulting in extremely low transmittance at the output. A 1550nm signal light enters the bus waveguide from the reverse port and, after coupling into the resonant cavity via the coupling region, primarily excites the CCW component. When the light wave passes through the gain-loss modulation grating in the upper straight section, due to the asymmetric reflection characteristics of the singularity, the CCW component is efficiently transmitted, with almost no reverse CW coupling. Therefore, under reverse incidence conditions, almost only the CCW traveling wave component propagates within the cavity, and no significant interference standing wave is formed in the lower straight section, only a uniform light intensity distribution is produced. Although periodic loss stripes exist, their local intensity is lower than the coherent constructive peak value under forward incidence. The additional loss introduced by the stripes is small, and the cavity remains in an overcoupled state. Reverse transmission is not significantly suppressed, resulting in high transmittance at the output, allowing for efficient signal conduction.

[0029] Figure 3The relationship between device isolation and offset distance Δx is further illustrated. Throughout the scan range, the isolation exhibits an oscillating variation with a period of approximately 0.352 μm, which is highly consistent with the period of the interference fringes of the opposing propagating light field within the cavity. At Δx = 0 μm, the isolation reaches a maximum peak of 47.55 dB; as Δx increases, the isolation rapidly decreases, reaching a trough of approximately 8.81 dB at Δx ≈ 0.094 μm. Subsequently, the isolation rises again, reaching a second peak of 47.55 dB at Δx ≈ 0.352 μm. This oscillating characteristic repeats stably within the scan range, with another isolation peak of 47.55 dB appearing at Δx ≈ 0.705 μm. The offset distances corresponding to the three peaks are 0 μm, 0.35 μm, and 0.705 μm, respectively, approximately integer multiples of 0.352 μm, indicating that the location of the local extrema of the isolation is determined by the periodic characteristics of the interference fringes within the cavity.

[0030] As can be seen from the above embodiments, this invention embeds a gain-loss modulation grating into a microcavity, utilizes the asymmetric reflection characteristics at the singularity point to establish a direction-dependent intracavity field distribution, and combines this with the spatially selective dissipation of a nonlinear periodic loss structure to successfully achieve high-isolation direction-selective transmission. Compared with existing technologies, this invention does not rely on magneto-optical materials and external magnetic fields, and can maintain the singularity point's operating state through static structural design, avoiding the complexity and instability caused by dynamic tuning. It also possesses advantages such as high isolation and CMOS process compatibility, providing a scalable all-optical solution for on-chip optical isolation.

Claims

1. An on-chip optical isolator based on a singular point gain-loss modulation grating, characterized in that: It consists of four parts: input / output bus waveguide, optical resonant cavity, gain-loss modulation grating, and nonlinear periodic loss structure. The input / output bus waveguide is used for inputting and outputting optical signals. It is coupled with the optical resonant cavity through evanescent wave to form a coupling region. By adjusting the gap and coupling length of the coupling region, the system is made to be in an overcoupled condition at the target operating point. The optical resonant cavity is a ring cavity. When the device is working, the signal light incident from the front enters the optical resonant cavity through the coupling region and propagates in a clockwise direction; the signal light incident from the rear enters the optical resonant cavity through the coupling region and propagates in a counterclockwise direction. Here, "front" refers to the direction from the input end to the output end, and "rear" refers to the direction from the output end to the input end. The geometric dimensions of the optical resonant cavity are designed according to the working wavelength so that at least one resonant mode falls within the working wavelength band. The gain-loss modulation grating is integrated and embedded in the straight section of the optical resonant cavity, generating EP state and non-reciprocal reflection. It is composed of multiple spatially periodic units arranged along the propagation direction, and the total length of the grating is... The complex refractive index is the product of the length of a single period and the number of periods; within each periodic unit, the complex refractive index satisfies the parity-time symmetry condition along the direction of light propagation, and the complex refractive index distribution... satisfy ,in, z represents the coordinate along the direction of light propagation, and the real part of the refractive index. The phase constant is determined and exhibits an even function distribution in space, while the imaginary part of the refractive index... The refractive index corresponds to gain or loss and exhibits an odd function distribution; the positive and negative values ​​of the imaginary part of the refractive index correspond to periodically alternating loss and gain regions; within a complete period Λ, the complex refractive index distribution is described by the following four continuous regions: （1） Where n0 represents the background refractive index, Δn r For the transformation of the real part of the refractive index, correspondingly, -iΔn g Indicates gain, iΔn L Indicates loss; when Δn is satisfied r =Δn g =Δn L When the three equilibrium conditions are met, the system enters EP. The nonlinear periodic loss structure is disposed on another straight segment of the optical resonant cavity, the length of which is... It consists of multiple periodic loss strips with a certain duty cycle arranged along the direction of light propagation. The period of the strips matches the period of the local interference fringes formed by the superposition of the traveling waves propagating in opposite directions in the cavity. Each loss strip has intensity-dependent additional absorption characteristics, that is, the local loss it introduces increases monotonically with the increase of the light intensity at that location and gradually tends to saturate. The coverage length of the nonlinear periodic loss structure along the straight segment is: ,in By controlling the vertical central axis of the periodic loss structure within the optical resonant cavity and the vertical central axis of the straight section of the runway cavity into which it is embedded, there exists an offset distance Δx, the value of which ranges from [value missing]. ; Δx ensures that during forward propagation, the coherent constructive point caused by the interference of the two beams in the cavity is located on the periodic material loss; while for the reverse propagation, there is no interference effect, and compared with the forward incident light, the loss is reduced, causing the cavity to be in an overcoupled state at this time.

2. The on-chip optical isolator based on a singular point gain loss modulation grating as described in claim 1, characterized in that: The optical resonant cavity is a racetrack-shaped resonant cavity, consisting of two straight sections of the same size and a semi-circular curved section connected end-to-end to form a closed optical path. The length of the straight section is... The bending radius is The perimeter of the optical resonant cavity is .

3. The on-chip optical isolator based on a singular point gain loss modulation grating as described in claim 2, characterized in that: The nonlinear periodic loss structure is integrated in the lower straight section within the cavity, and its loss increases with increasing strength. =75μm, duty cycle is 0.

5.

4. The on-chip optical isolator based on a singular point gain-loss modulation grating as described in claim 3, characterized in that: The offset distance Δx is 0 μm, 0.35 μm or 0.705 μm.

5. The on-chip optical isolator based on a singular point gain loss modulation grating as described in claim 1, characterized in that: The number of periods N of the gain-loss modulation grating ranges from 40 to 60, and the refractive index modulation amplitude Δn ranges from 1 × 10⁻⁶. -3 Up to 5×10 -4 .

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