Direct probing of modulated coherent light signals with structures exhibiting fano resonance

Abstract

A waveguide coupled cavity structure configured to exhibit Fano resonance, such as asymmetric Fano resonance or symmetric Fano resonance (inverse Lorentz resonance), is used in an optical receiver or method for directly detecting a coherent optical signal by converting phase modulation of the coherent optical signal into intensity modulation of the optical signal. The waveguide coupled cavity structure is designed such that a transmission spectrum of the Fano resonance overlaps a spectrum of the modulated coherent optical signal to suppress transmission of at least one sideband of the modulated coherent optical signal through the structure, the sideband suppression being asymmetric with respect to a carrier frequency of the modulated coherent optical signal. The invention can be used to directly detect more advanced coherent modulation formats, such as quadrature phase shift keying (QPSK) signals and high order quadrature amplitude modulation (n-QAM) signals.

Description

Technical Field

[0001] This invention relates to an optical receiver and the direct detection of modulated coherent optical signals, particularly to the conversion of phase modulation of coherent optical signals into intensity modulation of optical signals. Background Technology

[0002] Optical receivers are key components of optical communication systems. Their function is to convert optical signals into electrical signals, allowing for the extraction and processing of the data being transmitted. To increase bandwidth, modern optical signals are modulated by encoding information into the amplitude and / or phase of a carrier signal.

[0003] In direct detection, a photodetector responds to changes in the optical power of the received optical signal. Direct detection is typically at the end of any optical receiver and is where the optical-to-electrical domain conversion occurs. Photodetectors are generally speed-limited, and they cannot extract any phase or frequency information from optical signals with constant power. Therefore, the received optical signal typically undergoes different stages of demultiplexing and conversion before being detected by direct detection, as described in, for example, WO 2019 / 199650, US 10,164,765, or US2018 / 091232.

[0004] In coherent detection, both amplitude and phase information are extracted from the received optical signal. In a typical optical coherent receiver, a narrow-linewidth tunable laser, acting as a local oscillator (LO), is combined with the received optical signal, and the resulting hybrid product (superimposed wave) is detected by a balanced detector. The balanced detector consists of two photodetectors connected back-to-back. Under appropriate polarization orientation, frequency detuning, and phase matching between the LO and the received signal, the resulting photocurrent contains both amplitude and phase information of the received optical signal, both of which can be extracted using a digital signal processor (DSP).

[0005] To achieve ultrafast optical transmission rates, optical signals are modulated using advanced coherent modulation formats, such as quadrature phase shift keying (QPSK) and higher-order quadrature amplitude modulation (QAM). Optical receivers used to detect this signal typically fall into one of two categories: coherent optical receivers (such as US 8,406,638) and filter-based optical receivers (such as US10,075,245). Summary of the Invention

[0006] In one embodiment, this disclosure provides an optical receiver for detecting modulated coherent optical signals, the optical receiver comprising:

[0007] - A waveguide-coupled cavity structure, comprising an optical waveguide having an input port for receiving optical signals and an output port for providing output optical signals, wherein,

[0008] The waveguide-coupled cavity structure is configured to exhibit Fano resonance, wherein the waveguide-coupled cavity structure is designed such that the transmission spectrum of the Fano resonance overlaps with the spectrum of the received modulated coherent optical signal to suppress the transmission of at least one sideband of the modulated coherent optical signal through the structure, the sideband suppression being asymmetric with respect to the carrier frequency of the modulated coherent optical signal; and

[0009] - The photodetector is configured to receive the output optical signal from the output port.

[0010] In this disclosure, a coherent optical signal is any signal originating from a coherent light source (e.g., a laser). A coherent optical signal is modulated, meaning the information has been encoded using one or more of phase, frequency, or amplitude modulation techniques employed by any existing modulation method. The degree of coherence of an optical signal is typically quantified by the coherence time or coherence length of the electromagnetic (EM) wave. Generally, the degree of coherence depends on the light source and the "transmission history," such as the medium the signal has traversed and the path length.

[0011] In optics, Fano resonance arises from interference between discrete optical modes and the mode continuum, resulting in a characteristic lineshape in the transmission or reflection spectrum. Device structures that realize Fano resonance can be categorized into in-plane and out-of-plane structures, corresponding to the propagation of light relative to the device plane. In both cases, Fano interference occurs when light propagates within a structure or medium that supports the mode continuum, at a certain optical distance from the optical cavity supporting the discrete optical mode. This allows for the excitation of discrete optical modes from the mode continuum. The optical distance, also known as the optical path length, is the product of the geometric length of the path light travels through a given system and the refractive index of the medium through which the light propagates. Such combinations of structures are said to exhibit Fano resonance. The characteristic lineshape will be summarized later, but it is important to note that modifications to the physical structure design can cause modifications to the resulting lineshape, deviating it from the characteristic lineshape. As long as the physical principle behind such a modified lineshape is interference between discrete and continuous optical modes, these are referred to as Fano resonances. In this disclosure, the term "Fano resonance" is therefore interpreted to encompass both asymmetric and symmetric Fano resonances, the latter also known as inverse Lorentzian resonances. These resonances occur when a coherent optical signal is coupled into a waveguide-coupled cavity structure designed to generate Fano interference at the wavelength of the optical signal. Such a structure is configured to exhibit Fano resonances.

[0012] Preferably, the optical waveguide supports a continuum of modes, while the waveguide-coupled cavity structure includes an optical cavity supporting a discrete mode with a resonant frequency of ω0. The optical waveguide is positioned within an optical distance of the optical cavity to allow evanescent excitation of the discrete cavity mode from the mode continuum.

[0013] In the prior art, structures exhibiting Fano resonances have been used to realize various optical signal processing functions. Bekele et al. provide a good overview of such structures in their paper "In-Plane Photonic Crystal Devices using Fano Resonances" published in Laser Photonics Rev. 2019, 1900054.

[0014] This disclosure also provides an optical communication kit, including:

[0015] - An optical transmitter comprising a coherent light source and at least one optical phase modulator for encoding data into light from the coherent light source to form a modulated coherent optical signal;

[0016] - An optical receiver as described above, used to receive and detect modulated coherent optical signals.

[0017] This kit, consisting of a corresponding optical transmitter and receiver, provides a simple and cost-effective solution for signal transmission.

[0018] This disclosure also provides a method for converting phase modulation on a coherent optical signal into intensity modulation of the optical signal, the method comprising coupling the coherent optical signal to an optical waveguide exhibiting a cavity structure of Fano resonance, wherein the transmission spectrum of the Fano resonance overlaps with the spectrum of the coherent optical signal to suppress the transmission of at least one sideband of the coherent optical signal through the optical waveguide, the sideband suppression being asymmetric with respect to the carrier frequency of the coherent optical signal.

[0019] The present invention also provides a use of a waveguide-coupled cavity structure exhibiting Fano resonance for converting phase modulation of a coherent optical signal into intensity modulation of the optical signal, wherein the coherent optical signal is coupled into the optical waveguide of the cavity structure, and wherein the transmission spectrum of the Fano resonance overlaps with the spectrum of the coherent optical signal to suppress the transmission of at least one sideband of the coherent optical signal through the optical waveguide, the sideband suppression being asymmetric with respect to the carrier frequency of the coherent optical signal.

[0020] In one embodiment, the use is in an optical receiver for detecting phase modulation of a modulated coherent optical signal.

[0021] In another embodiment, this application is for optical format conversion, preferably for converting the phase modulation format of a modulated coherent optical signal into an intensity modulation format. This application can be in an optical format converter.

[0022] Therefore, in another embodiment, this disclosure provides an optical format converter for converting the phase modulation format of a modulated coherent optical signal into an intensity modulation format, and includes:

[0023] - A waveguide-coupled cavity structure comprising an optical waveguide having an input port for receiving an optical signal and an output port for providing a converted output optical signal, wherein the waveguide-coupled cavity structure is configured to exhibit Fano resonance, and wherein the waveguide-coupled cavity structure is designed such that the transmission spectrum of the Fano resonance overlaps with the spectrum of the received modulated coherent optical signal to suppress the transmission of at least one sideband of the modulated coherent optical signal through the structure, the sideband suppression being asymmetric with respect to the carrier frequency of the modulated coherent optical signal.

[0024] The advantages of the above embodiments are that they provide an all-optical method for converting phase changes in an optical signal into changes in optical intensity. Further advantages of these embodiments are that they can directly probe modulated coherent optical signals. A further advantage of these implementations is that they allow the probe of modulated coherent optical signals without requiring a local oscillator (LO) laser and digital signal processing (DSP). These embodiments also have the advantage of being able to directly probe more advanced coherent modulation formats, such as quadrature phase shift keying (QPSK) signals and higher-order quadrature amplitude modulation (n-QAM) signals.

[0025] In the following sections, some preferred and / or optional features, elements, and examples will be summarized. Where applicable, features or elements related to the optical receiver implementation may be combined with or applied to other implementations or aspects. For example, structural and functional features applied to the optical receiver implementation may also serve as features related to use or method, and vice versa. Furthermore, the inventor's explanation of the basic mechanism of the invention is provided for illustrative purposes and should not be used in hindsight analysis to infer the invention.

[0026] In one embodiment, the local or global minimum of the transmission spectrum of the Fano resonance overlaps with a sideband of the modulated coherent optical signal. This is advantageous because precise alignment of the minimum transmission with the sideband results in stronger sideband suppression compared to the case where the minimum transmission lies between two sidebands. Preferably, the minimum transmission overlaps with the sideband having the maximum amplitude. This is advantageous because it produces the strongest sideband suppression.

[0027] In one embodiment, the resonant frequency of the optical cavity can be tuned to adjust the transmission spectrum of the Fano resonance to match the spectrum of the modulated coherent optical signal. This is advantageous because it allows for precise tuning of the transmission minimum of the transmission spectrum to overlap with the desired sidebands of the modulated coherent optical signal.

[0028] In a preferred embodiment, the optical receiver is part of a balanced receiver setup. This is advantageous because it suppresses noise. Attached Figure Description

[0029] Figure 1 This is a diagram showing the characteristic line shape of the Fano resonance and the line shape of the Lorentz resonance.

[0030] Figures 2A to 2C Waveguide-coupled cavity structures exhibiting different types of Fano resonances are shown.

[0031] Figures 3A to 3E A figure is shown illustrating numerical simulation results of embodiments of the present disclosure.

[0032] Figures 4A to 4E Different physical implementations of the optical receiver (4A) and waveguide-coupled cavity structure (4B-4E) according to this disclosure are shown.

[0033] Figures 5A to 5C These are scanning electron microscope images showing photonic crystal structures exhibiting different types of Fano resonances.

[0034] Figures 6A to 6C It shows Figures 5A to 5C The corresponding Fano line shape of the photonic crystal structure.

[0035] Figures 7A to 7C The present disclosure illustrates a setup for directly probing different coherent modulation formats according to an embodiment of the present disclosure.

[0036] Figures 8A to 8D A graph is shown illustrating the digital simulation results of converting a QPSK signal into an intensity-modulated signal using Fano resonance according to an embodiment of the present disclosure.

[0037] Figure 9 An optical communication kit according to an embodiment of the present disclosure is shown. Detailed Implementation

[0038] The resonant spectrum of an optical cavity is characterized by a symmetrical line shape known as the Lorentzian resonance, peaking at the resonant frequency ω0, and having a linewidth Δω. The ratio ω0 / Δω is a dimensionless measure of the cavity's time storage capacity and is expressed as the quality factor Q. A typical Fano transmission spectrum can be written as:

[0039]

[0040] Where t B It is the non-resonant amplitude transmission coefficient of the system, q is the Fano asymmetry parameter or q parameter, and the normalized frequency detuning is δ=(ω-ω0) / γ, where γ is the cavity field attenuation rate, given by γ=ω0 / 2Q. It can be seen from equation (1) that T max =|t B | 2 (q 2 The maximum transmission value of +1) occurs when δ = 1 / q, while the minimum transmission value T min =0 occurs when δ = -q.

[0041] The Fano asymmetry parameter determines the linear profile of the resonance. Figure 1 The power transmission linetypes as a function of wavelength detuning are shown for four cases with q values ​​of -1, 0, 1, and 100. In the case of q = 0 (black dashed line), a symmetrical dip at the resonant frequency is shown, indicating destructive interference between the discrete mode and the mode continuum. Here, this is called symmetrical Fano resonance, while in some literature it is referred to as inverse Lorentz or anti-resonance. In the cases of q = -1 and q = 1 (shown by gray and black solid lines respectively), corresponding to the asymmetric linetypes, constructive and destructive interferences exist between the discrete mode and the mode continuum. The spectral position of the transmission minimum is blue-shifted (red-shifted) compared to the transmission maximum of q = -1 (1). Therefore, for negative and positive values ​​of the Fano asymmetry parameter, the linetype is often referred to as blue-parity or red-parity Fano resonance. In both cases, the resonant frequency is located midway between the transmission maximum and transmission minimum in the spectrum. By carefully selecting the Fano q parameter, an asymmetric Fano resonance can be designed, where the cavity's resonant frequency lies anywhere in the spectrum between the transmission maximum and minimum values. The Fano transmission linetypes corresponding to q values ​​of -1, 0, and 1 are the aforementioned characteristic linetypes. In this disclosure, when a waveguide-coupled cavity structure is referred to as being "configured to exhibit a Fano resonance," it means that when a coherent optical signal of a predetermined wavelength is coupled into the waveguide-coupled cavity structure, an asymmetric Fano resonance (corresponding to q≈-1 or 1) or a symmetric Fano resonance (corresponding to q≈0, also known as an inverse Lorentz resonance) will occur due to interference between discrete modes and the mode continuum.

[0042] Figure 2A A waveguide-coupled cavity structure 2 is shown, comprising a continuous optical waveguide 4 supporting modes and an optical cavity 6 supporting discrete modes. The optical waveguide 4 has an input port 8 for receiving optical signals and an output port 10 for providing output optical signals. This structure uses a frequency ω at the input port... sThe cavity is excited by light, and the transmitted light is collected from the output port. The cavity resonant frequency is ω0, and its field amplitude is denoted by a(t). The cavity modes decay to the input and output ports at rates γ1 and γ2, respectively. The out-of-plane attenuation rate is γ in The rate of change of the cavity field can be written as:

[0043]

[0044] The first term in equation (2) represents the detuning dependence of the cavity field envelope amplitude and its attenuation, while the second term represents the coupling between the input field and the cavity field.

[0045] The output field amplitude of the structure is given by equation (3):

[0046]

[0047] The first term in equation (3) represents the direct transmission of the input signal to the output port. The amplitude of this path is determined by the amplitude transmission coefficient of the partial transmitting element (PTE) 13. The second term in equation (3) represents the contribution of the empty field to the output field. The interference of these two terms produces Fano resonance.

[0048] Figures 2A to 2C Three different exemplary designs are shown, with different values ​​of the Fano asymmetric parameter or q parameter q. Figure 2A This shows that the result corresponds to q = 1 and in equation (1) Figure 1 The design of the red parity Fano resonance with the black solid line in the middle. Figure 2B This shows that the result corresponds to q = -1 and in equation (1) Figure 1 The design of the blue parity Fano resonance with gray solid lines in the image. Finally, Figure 2C This shows that the result corresponds to q = 0 and in equation (1) Figure 1 The design of the symmetrical Fano resonance (also known as the inverse Lorentz resonance) with the black dashed line in the figure.

[0049] If the optical waveguide does not allow direct transmission from the input port to the output port, then transmission can only be achieved via tunneling through the cavity. When the resonance is isolated from other modes, this will result in a traditional Fabry-Perot resonance with a characteristic Lorentz line shape. This is a special case of q = 100 in equation (1), and... Figure 1 The line is shown as a gray dashed line. This line type is the conventional Lorentz line type at the cavity resonance, with no interference features between the discrete mode and the mode continuum. Therefore, this line type is not a Fano resonance, nor is it a result of a structure exhibiting a Fano resonance.

[0050] Therefore, preferably, the optical waveguide of the waveguide-coupled cavity structure allows light to be transmitted directly from the input port to the output port.

[0051] According to the present invention, by using a Fano resonance structure to change the amplitude equality and phase relationship between the optical carrier and the optical sideband, a phase-modulated coherent optical signal can be converted into an intensity-modulated signal. (Refer to the following...) Figures 3A to 3E This principle is illustrated through numerical simulation.

[0052] Figure 3A This is a graph (gray solid line) showing the normalized spectrum of a 40 GHz phase-modulated (PM) input signal. The PM spectrum is centered at 1550 nm and consists of equidistant sidebands shifted k times from the optical carrier. m , where f m This is the phase modulation frequency. The figure also shows the transmission spectrum of a waveguide-coupled cavity structure according to an embodiment of this disclosure, exhibiting red parity Fano resonances (solid black line) and the phase response (right ordinal number) of the Fano resonances (dashed black line). In order to... Figure 1 Compared to the line types in the text, it should be noted that... Figure 1 The horizontal axis in the graph represents frequency, while... Figure 3A The horizontal axis in the graph represents the wavelength.

[0053] Figure 3B The spectrum of a PM signal transmitted through a waveguide-coupled cavity structure according to an embodiment of this disclosure is shown. Changes in sideband amplitude and phase are proportional to the transmission line shape of the Fano resonance. In particular, transmission in the k=2 sideband is suppressed.

[0054] The minimum transmission frequency of the Fano resonance preferably overlaps with the sidebands of the modulated coherent optical signal, such as... Figure 3A This is advantageous because precise alignment of the minimum transmission with the sideband results in stronger sideband suppression compared to the case where the minimum transmission is located between two sidebands. Preferably, the minimum transmission overlaps with the sideband having the maximum amplitude. This is advantageous because it produces the strongest sideband suppression, such as... Figure 3B In this case, to ensure ideal overlap, the resonant frequency of the optical cavity can be tuned to adjust the relationship between the transmission spectrum of the Fano resonance and the spectrum of the modulated coherent optical signal or the carrier signal.

[0055] Figure 3C and Figure 3D The strength of the input PM signal and the converted signal, as well as the power and phase of the phase-modulated signal, were compared over time. The input PM signal had a constant power of 1mW (black line). To simulate signal impairment caused by propagation, a signal-to-noise ratio (SNR) of 20dB was assumed for additive white Gaussian noise. The phase of the input PM signal (gray line) oscillated sinusoidally within the range of [0, 2π] at 40GHz, with a corresponding period of 25ps. On the other hand, Figure 3DThe converted signal in the figure shows the oscillation power (black line) between 0mW and 1.2mW, with an oscillation period identical to the oscillation period of the phase of the input PM signal. A higher peak power is observed in the PM-to-IM converted signal compared to the average power of the input PM signal. Figure 3D The phase (gray line) of the PM to IM conversion signal shown indicates the oscillation corresponding to the oscillation period of the PM input signal. Figure 3E The power spectral density (PSD) of the input PM signal (black line) and the converted signal (gray line) is compared. A modulated radio frequency (RF) signal at 40 GHz is clearly visible in the converted signal's spectrum, while this modulated RF signal is not visible in the input PM signal's spectrum. This demonstrates that an amplitude imbalance has been achieved between the sidebands, indicating intensity modulation in the converted signal. Note that the PM signal's spectrum has been attenuated by 10 dB for visual clarity.

[0056] exist Figure 4A In one embodiment shown, the present invention provides an optical receiver 1 for detecting modulated coherent optical signals, the optical receiver comprising... Figures 2A to 2C The waveguide-coupled cavity structure 2 and photodetector 12 are related, the photodetector being configured to receive the output optical signal from the output port 10. The waveguide-coupled cavity structure 2 is configured to exhibit Fano resonance, such as... Figure 1 and Figures 2A to 2C Furthermore, the waveguide-coupled cavity structure 2 is designed such that the transmission spectrum of the Fano resonance overlaps with the spectrum of the modulated coherent optical signal to suppress the transmission of at least one sideband of the modulated coherent optical signal through the structure. This sideband suppression is asymmetric with respect to the carrier frequency of the modulated coherent optical signal. Figures 3A to 3B This example is shown in the image.

[0057] When the modulated coherent optical signal (example shown in) Figure 3C When the optical signal is input to the optical receiver 1 via the input port 8, the waveguide-coupled cavity structure 2 provides the converted output optical signal via the output port 10 (an example is shown in...). Figure 3D The photodetector 12 (photodiode in this paper) detects the power of the output optical signal from the output port 10. Figure 3D (The black line in the image) and the output photocurrent from the photodiode contains modulated information on the modulated coherent optical signal. This photocurrent can then be processed by electrical signal processing.

[0058] In one example, the waveguide-coupled cavity structure can be implemented as an in-plane photonic crystal device, such as... Figure 4B As shown. Figure 4BThis is a scanning electron microscope (SEM) image of an exemplary Fano structure implemented using hexagonally arranged photonic crystal pores on a 340 nm thick indium phosphide substrate. Waveguide 14 is a line defect type formed by removing a row of pores, while cavity 16 is an H0 nanocavity formed by removing the pores from the cavity center. The typical radius of the pores is approximately 120 nm, while the lattice constant of the photonic crystal is approximately 450 nm. Partial transmitting elements are implemented via a single pore placed in the middle of the waveguide beneath the cavity. In a preferred embodiment, the resonant frequency of the optical cavity is tuned via thermo-optical effects and by heating the material of the photonic crystal structure in which the cavity is formed. For this purpose, the optical receiver may include a heating element configured to tune the resonant frequency of the optical cavity by heating the waveguide-coupled cavity structure.

[0059] In another example, the waveguide-coupled cavity structure 2 can be implemented as a ring resonator 18, including a racetrack-shaped resonator or other shaped ring, laterally coupled to the bus waveguide 20, such as... Figure 4C and Figure 4E As shown. Similar to photonic crystal devices, the ring resonator 18 and the accompanying bus waveguide 20 can be implemented on silicon, glass, or other III-V semiconductor platforms on an insulating substrate. In exemplary embodiments of the invention, the ring resonator typically has a diameter of tens of micrometers. The ring resonator can be designed, through its size and material composition, to have its resonant frequency at or near the carrier frequency of the received optical signal. The design parameters of the ring resonator are well known in the art. The resonant frequency of the ring resonator can be tuned using thermo-optical effects and by heating the material in which the ring resonator is formed.

[0060] In an exemplary embodiment, such as Figure 4D and Figure 4E As shown, the ring resonator can be coupled to two waveguides 20 and 22 on opposite sides of the ring. In this configuration, the additional waveguide 22 has the technical effect of radiating a loss channel from the ring resonator, reducing the transmission at the resonance point to zero, thereby providing the advantage of achieving a higher extinction ratio.

[0061] The present invention also provides a use and method, utilizing Figures 2A to 2C The waveguide-coupled cavity structure 2 is described, and includes an optical receiver 1. This use and method derives from the utilization of the waveguide-coupled cavity structure 2 already described, and also covers the uses and method steps corresponding to the embodiments of the optical receiver described below.

[0062] According to the embodiments, with Figures 4A to 4CThe waveguide-coupled cavity structure 2 also includes an optical format converter. This optical format converter is equivalent to the receiver 1 omitting the photodetector 12. Therefore, the use of the waveguide-coupled cavity structure 2 for optical format conversion, preferably for converting the phase modulation format of a modulated coherent optical signal into an intensity modulation format, is also disclosed in the description of the receiver 1.

[0063] As previously mentioned, modifications to the design of the optical waveguide and / or waveguide-coupled cavity structure can lead to modifications in the resulting transmission spectrum. In one embodiment, the optical waveguide includes a partial transmitting element (PTE) that influences the Fano asymmetry parameters of the waveguide-coupled cavity structure, as also discussed previously with respect to equation (1). This is advantageous because it can alter the shape of the transmission spectrum and / or the location of the transmission minimum to improve overlap with the sidebands of the modulated coherent optical signal. The coupling phase between the waveguide and the cavity can be varied depending on the location of the PTE, thereby determining the spectral locations of constructive and destructive interference, referred to as the maximum and minimum transmission points in the transmission spectrum, respectively. Furthermore, placing the PTE on the left or right side of the mirror plane passing through the middle of the cavity ( Figure 2A and Figure 2B The dashed line in the diagram will disrupt the symmetry of the structure, resulting in different attenuation rates of the cavity field at the input and output ports. Furthermore, the waveguide's transmission coefficient can be controlled by the size of the PTE aperture. A large PTE radius leads to low light transmission through the waveguide.

[0064] Figures 5A to 5C This is a scanning electron microscope (SEM) image of a waveguide coupled cavity structure 2 implemented using hexagonally arranged photonic crystal vents according to an exemplary embodiment of the present invention. Figures 6A to 6C The corresponding theoretical (Formula (1)) transmission spectrum (black solid line) and measured transmission spectrum (gray solid line) are shown.

[0065] Figure 5A The structure exhibits Figure 6A The diagram shows a symmetrical Fano (or inverse Lorentz) resonance. Cavity 6 is of the quasi-H1 type, formed by reducing the radius of the central vent and moving the vents around the cavity away from the center. Note that this structure does not have PTE vents in the waveguide.

[0066] Figure 5B The structure exhibits Figure 6B The red parity Fano resonance is shown. Here, the optical waveguide 4 includes a PTE 13 located in the center plane of the waveguide relative to the center of cavity 16. Cavity 16 is of type H0 formed by creating vents around the cavity away from the cavity center.

[0067] Figure 5C The structure exhibits Figure 6CThe blue parity Fano resonance is shown. Here, the optical waveguide 4 includes a PTE 13, which is located one lattice constant to the left of the central plane relative to cavity 16, thus breaking the mirror symmetry of the structure. Figure 5B Same H0 type.

[0068] exist Figures 5A to 5C In this structure, the waveguide is of type W1, formed by removing one row of air holes and moving the innermost row of air holes toward the center of the waveguide. Assuming the quality factor and cavity mode symmetry are the same, choosing quasi-H1 or H0 does not affect the lineform. However, the H0 cavity is characterized by greater mode constraint and better overlap between the cavity mode and the material.

[0069] Therefore, in a preferred embodiment, apart from the placement of the PTE, the minimum transmission spectral position through the waveguide-coupled cavity structure is determined by parameters such as the resonant frequency and the cavity's quality factor.

[0070] While waveguide-coupled cavity structures exhibiting symmetric Fano (inverse Lorentz) resonances can be used to convert phase-modulated signals into intensity-modulated signals, structures exhibiting asymmetric Fano resonances are often advantageous because a larger extinction ratio can be achieved between maximum and minimum transmission. This can be used to achieve more effective suppression of phase-modulated sidebands. Therefore, in a preferred embodiment, the waveguide-coupled cavity structure exhibits blue or red parity Fano resonances.

[0071] Figures 7A to 7C A schematic diagram of some setups for detecting coherent optical signals modulated according to different coherent modulation formats is shown.

[0072] Figure 7A A proposed receiver setup 70 for directly probing a binary phase-shift keying (BPSK) signal is shown, wherein the input BPSK signal is transmitted via a red parity Fano device 2'. The output signal is then directly probed by a photodiode 12. The output photocurrent of the photodiode is then converted into a voltage using a transimpedance amplifier (TIA) 22. Different waveguide-coupled cavity structures can be used to convert the BPSK signal into an intensity-modulated signal based on symmetrical or blue or red parity Fano resonances. This setup is similar to... Figure 4A Receiver 1 as described.

[0073] Figure 7BThe receiver setup 72 shown can be used to convert a quadrature phase shift keying (QPSK) signal into an intensity-modulated signal, thereby enabling direct detection of the QPSK signal. The received optical signal is split into two groups by a splitter 24, each containing a Fano resonance structure exhibiting opposite symmetry (red parity structure 2' and blue parity structure 2"). The time-varying optical power of the signal following the blue and red parity Fano is detected using a balanced photodiode setup 21. The balanced photodiode 21 comprises two photodiodes connected in series. It is also referred to as a balanced optical receiver and is designed to compare the photocurrent difference between two correlated optical signals while suppressing any common fluctuations in the input signals. The photocurrent difference is fed to a transimpedance amplifier 22, which produces an output voltage proportional to the photocurrent difference.

[0074] Figures 8A to 8D The demonstration uses Figure 7B The settings convert the QPSK signal into an intensity-modulated signal, as shown in the figure. Figure 8A The normalized optical spectrum (gray line) and the transmission spectrum of the blue parity (black dashed line) and red parity (black solid line) Fano resonances of the QPSK signal are shown. Figure 8B The normalized optical spectra of the signals transmitted via the blue parity (black line) and red parity (gray line) Fano resonances are shown. Figure 8C The constellation diagram of the input QPSK signal at 15dB SNR is shown. Figure 8D An eye diagram is shown, formed by superimposing the output voltage of 22 onto two symbol slots (as shown in Figure 7(g)). It shows seven voltage levels corresponding to the seven unique bit transitions.

[0075] Therefore, in a preferred embodiment, the optical receiver 72 is preferably configured to detect quadrature phase shift keying signals, and thus the optical receiver 72 includes:

[0076] - A first waveguide-coupled cavity structure and a second waveguide-coupled cavity structure, each comprising a first optical waveguide and a second optical waveguide, each having an input port for receiving an optical signal and an output port for providing an output optical signal, wherein the first waveguide-coupled cavity structure is configured to exhibit blue parity Fano resonance, and the second waveguide-coupled cavity structure is configured to exhibit red parity Fano resonance, and wherein the first waveguide-coupled cavity structure and the second waveguide-coupled cavity structure are designed such that the transmission spectrum of the Fano resonance overlaps with the spectrum of the modulated coherent optical signal to suppress the transmission of at least one sideband of the modulated coherent optical signal through the structure, the sideband suppression being asymmetric with respect to the carrier frequency of the modulated coherent optical signal;

[0077] - A first separator 24 is used to separate the modulated coherent optical signal between the input ports of the first waveguide-coupled cavity structure and the second waveguide-coupled cavity structure; and

[0078] - A balanced photodiode is set at the output of the first waveguide coupled cavity structure and the second waveguide coupled cavity structure.

[0079] Figure 7C A proposed receiver setup 74 for directly probing high-order quadrature amplitude modulation (n-QAM) signals is shown. Compared to QPSK signals, intensity detection is required in addition to phase detection. Therefore, the received signal is split by splitter 26 into a phase detection branch 75 and an amplitude detection branch 76. The signal in phase detection branch 75 is further split by splitter 24 into two groups containing Fano devices with opposite parity, similar to the setup above. Figure 7B The detection setup for the QPSK signal described herein. The amplitude detection branch 76 employs a direct detection scheme, using photodiode 77 and TIA 78. High-order quadrature amplitude modulation (n-QAM) is widely used in long-distance optical communication links due to its high system capacity and increased spectral efficiency. Therefore, there is a great need for cost-effective receivers capable of detecting n-QAM modulation formats.

[0080] Therefore, in a preferred embodiment, the optical receiver 74 is preferably configured to detect higher-order quadrature amplitude modulation signals because, in addition to including the optical receiver setup 72, the optical receiver 74 also includes:

[0081] - A second splitter 26 is used to separate modulated coherent optical signals between a phase detection branch 75 and an amplitude detection branch 76. The phase detection branch 75 includes a first splitter 24 configured to receive modulated coherent optical signals from the second splitter, and the amplitude detection branch 76 includes a photodetector configured to receive optical signals from the second splitter.

[0082] In another embodiment, the present invention provides Figure 9 The optical communication kit 80 shown includes an optical receiver 1 and an optical transmitter 82 according to an embodiment of the present invention. The optical transmitter includes a coherent light source 86 (typically a laser) and an optical modulator 88 for encoding incoming data into light from the coherent light source to form a modulated coherent optical signal.

[0083] In an exemplary embodiment, the optical modulator 88 of kit 80 includes one or more optical phase modulators, such as only an optical phase modulator without other modulators (e.g., intensity modulators). In the method based on this embodiment, a coherent optical signal is provided by modulating only the phase of the optical carrier signal without modulating the amplitude of the optical carrier signal. This embodiment is suitable for QPSK systems and provides the advantage that, according to an embodiment of the invention, the modulated coherent optical signal is optimized for detection by the optical receiver, thereby improving the quality of the generated intensity-modulated signal. This provides further advantages of system simplicity and lower cost, as fewer components will be involved in the transmitter and receiver.

[0084] In another exemplary embodiment, the optical modulator 88 includes at least one optical phase modulator and at least one amplitude or intensity modulator. This embodiment allows for the transmission and detection of advanced modulation formats, such as 16-QAM signals.

[0085] For advanced modulation formats, such as n-QAM, the optical modulator 88 will also include an amplitude modulator or a more general IQ modulator. An optical network including waveguides, such as an optical fiber link 84, receives the modulated coherent optical signal from the optical transmitter 82 and ultimately delivers the modulated coherent optical signal to the optical receiver 1. The description of the optical receiver 1 relates to... Figure 4A It receives and detects modulated coherent optical signals to provide analog-coded incoming data 'in' and data 'out'.

Claims

1. An optical format converter for converting the phase modulation format of a modulated coherent optical signal into an intensity modulation format, the optical format converter comprising: A waveguide-coupled cavity structure (2, 2', 2") comprising an optical waveguide (4, 14, 20) having an input port (8) for receiving an optical signal and an output port (10) for providing an output optical signal, wherein the waveguide-coupled cavity structure is configured to exhibit Fano resonance, and wherein the transmission spectrum of the Fano resonance overlaps with the spectrum of the received modulated coherent optical signal to suppress the transmission of at least one sideband of the modulated coherent optical signal through the waveguide-coupled cavity structure, the sideband suppression being asymmetric with respect to the carrier frequency of the modulated coherent optical signal.

2. The optical format converter according to claim 1, wherein, The minimum transmission spectrum of the Fano resonance overlaps with the sideband of the modulated coherent optical signal.

3. The optical format converter according to claim 2, wherein, The minimum transmission spectrum in the transmission spectrum overlaps with the sideband of the modulated coherent optical signal with the maximum amplitude.

4. The optical format converter according to any one of the preceding claims, wherein, The optical waveguide supports a continuum of modes, and wherein the waveguide-coupled cavity structure includes an optical cavity (6, 16) supporting discrete modes having a resonant frequency ω0, the optical waveguide being positioned within an optical distance of the optical cavity to allow evanescent excitation of the discrete modes from the mode continuum.

5. The optical format converter according to any one of the preceding claims, wherein, The optical waveguide includes a partial transmitting element PTE (12) for controlling the Fano resonance parameters.

6. The optical format converter according to any one of the preceding claims, wherein, The waveguide-coupled cavity structure is a ring resonator (18), and the optical format converter further includes an additional optical waveguide (22) that is evanescently coupled to the waveguide-coupled cavity structure to provide a loss channel for radiating the ring resonator.

7. An optical receiver (1, 70, 72, 74) for detecting modulated coherent optical signals, comprising an optical format converter according to any one of claims 1 to 6 and a photodetector (12) arranged to receive an output optical signal from an output port.

8. The optical receiver according to claim 7, wherein, The waveguide-coupled cavity structure is a first waveguide-coupled cavity structure (2'), which is configured to exhibit red parity Fano resonance. The optical receiver further includes: A second waveguide-coupled cavity structure (2”) includes an optical waveguide (4, 14, 20) having an input port (8) for receiving an optical signal and an output port (10) for providing an output optical signal. The waveguide-coupled cavity structure is configured to exhibit a blue parity Fano resonance, and the transmission spectrum of the blue parity Fano resonance overlaps with the spectrum of the received modulated coherent optical signal to suppress the transmission of at least one sideband of the modulated coherent optical signal through the second waveguide-coupled cavity structure. The sideband suppression is asymmetric with respect to the carrier frequency of the modulated coherent optical signal. A first separator (24) is used to separate the modulated coherent optical signal between the input port of the first waveguide-coupled cavity structure and the input port of the second waveguide-coupled cavity structure; and A balanced photodiode (21) is provided, which includes the photodetector and is disposed at the output of the first waveguide coupled cavity structure and the second waveguide coupled cavity structure.

9. The optical receiver according to claim 8, further comprising: A second splitter (26) is used to separate the modulated coherent optical signal between a phase detection branch and an amplitude detection branch, the phase detection branch including the first splitter configured to receive the modulated coherent optical signal from the second splitter, and the amplitude detection branch including a photodetector (77) configured to receive the optical signal from the second splitter.

10. An optical communication kit (80), comprising: An optical transmitter (82) includes a coherent light source (86) and at least one optical phase modulator (88) for encoding data onto light from the coherent light source to form a modulated coherent optical signal. The optical receiver according to any one of claims 7 to 9 is used to receive and detect the modulated coherent optical signal.

11. The optical communication kit according to claim 10, wherein, The optical transmitter also includes at least one optical intensity modulator for encoding data.

12. The use of a waveguide-coupled cavity structure (2, 2', 2"), which exhibits Fano resonance, for converting phase modulation of a coherent optical signal into intensity modulation of the optical signal, wherein, The coherent optical signal is coupled into an optical waveguide (4, 14, 20) of a waveguide-coupled cavity structure, wherein the transmission spectrum of the Fano resonance overlaps with the spectrum of the coherent optical signal to suppress the transmission of at least one sideband of the coherent optical signal through the optical waveguide, and the sideband suppression is asymmetric with respect to the carrier frequency of the coherent optical signal.

13. The use according to claim 12, wherein, The application is in optical receivers (1, 70, 72, 74) for detecting the phase modulation of modulated coherent optical signals.

14. A method for converting phase modulation on a coherent optical signal into intensity modulation of an optical signal, the method comprising: The coherent optical signal is coupled into an optical waveguide configured to exhibit a Fano resonance waveguide-coupled cavity structure, wherein the transmission spectrum of the Fano resonance overlaps with the spectrum of the coherent optical signal to suppress the transmission of at least one sideband of the coherent optical signal through the optical waveguide, and the sideband suppression is asymmetric with respect to the carrier frequency of the coherent optical signal.

15. A method for detecting a modulated coherent optical signal, the method comprising: The method of claim 14 is used to convert phase modulation on a coherent optical signal into intensity modulation of the optical signal, and the intensity modulation of the output is detected from the waveguide-coupled cavity structure.

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