A millimeter wave / terahertz beamforming chip and method based on a wideband continuous adjustable optical delay line

Abstract

A millimeter wave / terahertz beam forming chip and method based on a wideband continuous adjustable optical delay line, relates to the field of millimeter wave / terahertz wireless communication. The traditional adjustable optical delay line using full cascade switch realizes millimeter wave / terahertz phased array beam forming, which can greatly increase the complexity and system loss, and the adjustable optical delay line using micro-ring structure is limited by bandwidth. Therefore, the present application provides a method for realizing wideband beam forming by using a wideband high-precision continuous optical adjustable delay line, and the chip comprises an optical beam splitter, a continuous adjustable true delay line array and a photodetector array. Among them, the delay line uses a cascade of discrete switch type delay line and continuous adjustable delay line to realize large range and high precision delay. The present application uses a Mach-Zehnder interferometer as a tunable optical power coupler, and realizes accurate continuous delay control by configuring the phase difference of the waveguide arm; the present application provides a control mechanism for the carrier and the sideband across the free spectral range (FSR), maps the optical carrier and optical sideband of the terahertz single sideband signal into different FSR periods respectively, and makes the center frequency of the wideband optical sideband accurately fall in the flat area near the anti-resonance point of the delay spectrum, so as to realize large bandwidth, dispersion-free high-precision delay in the terahertz band, and fundamentally eliminate the wideband beam squint effect.

Description

Technical Field

[0001] This invention relates to the fields of microwave photonics, millimeter-wave / terahertz wireless communication and integrated optoelectronics, specifically to a photonic millimeter-wave / terahertz beamforming chip and control method based on an optical broadband continuously tunable true delay line architecture. Background Technology

[0002] As 6G communication technology evolves towards "Internet of Everything," the exponential growth in single-device traffic places stringent demands on communication speed, latency, and reliability, driving the need for wider spectrum usage. It is projected that around 2030, millimeter-wave / terahertz wireless communication technology will support 6G networks achieving peak speeds of Tbps and experience speeds of 10–100 Gbps, becoming a core support for overcoming future communication capacity bottlenecks. Millimeter-wave / terahertz waves, with their abundant spectrum resources, ultra-wideband transmission potential, and high-precision positioning and sensing capabilities, possess irreplaceable advantages in 6G mobile communication, high-resolution radar, and ultra-high-speed short-range transmission. Millimeter-wave / terahertz waves have extremely rich spectrum resources and are the core carrier supporting ultra-high-speed data transmission in next-generation mobile communication networks. However, terahertz waves are highly susceptible to atmospheric absorption and dust particle scattering during transmission in free space, resulting in severe signal attenuation. Therefore, phased array technology is needed for beamforming, using the signal superposition of multiple antenna elements to significantly improve the transmission power and spatial directivity of terahertz signals.

[0003] Traditional millimeter-wave / terahertz beamforming primarily relies on electronic phased array technology. However, electronic phase shifters face significant insertion loss issues at high frequencies. Furthermore, electronic phased arrays suffer from severe beam squint: because they calculate a constant phase difference based on a single frequency, when transmitting broadband millimeter-wave / terahertz signals, signal components deviating from the center frequency will point at different spatial angles, causing the signal to deviate from the target direction.

[0004] The use of true optical delay mechanisms, which introduce a real physical time delay in the optical domain that does not change with the radio frequency, is currently one solution to fundamentally eliminate the broadband beam-squinting effect. However, the center frequency of millimeter-wave / terahertz signals is extremely high, and to achieve large-scale beam deflection in an array antenna, the system requires high precision in delay control between adjacent array elements. Existing technologies mainly include two types of optical delay implementation schemes, but both have insurmountable technical bottlenecks. First, the traditional cascaded switch-type optical delay line with discrete delay control requires the integration of a large number of optical switches in a multi-stage cascaded architecture design to achieve high-precision delay stepping. This approach not only significantly increases chip size and control complexity, but the additional optical switches also introduce significant optical transmission losses, degrading the overall signal-to-noise ratio of the system. Second, delay lines built based on the slow light effect of micro-ring resonators are constrained by the inherent limitations of the delay-bandwidth product, resulting in severely limited operating bandwidth, making it difficult to adapt to the high-speed transmission and communication requirements of millimeter-wave / terahertz broadband signals.

[0005] Therefore, there is an urgent need in this field for a novel photonic millimeter-wave / terahertz beamforming architecture that balances large operating bandwidth, high latency accuracy, and low transmission loss. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a millimeter-wave / terahertz beamforming chip and method based on broadband continuously tunable optical delay lines. In existing terahertz beamforming schemes, traditional electrically controlled phased arrays suffer from severe broadband "beam squinting" problems. When using optically cascaded switch delay lines with discrete delay adjustment (such as multi-bit N-bit structures), achieving high-precision delay steps to meet system requirements requires numerous cascaded optical switches, leading to complex chip structures and introducing high transmission losses. Furthermore, micro-ring-based delay lines are limited by the delay-bandwidth product, making it difficult to meet large bandwidth requirements. Existing systems struggle to simultaneously achieve large operating bandwidth, high delay control accuracy, and low insertion loss. To achieve the above objectives, this invention provides a millimeter-wave / terahertz beamforming chip based on a broadband continuously tunable optical delay line, comprising an optical beamsplitter, a continuously tunable true delay line array, and a photodetector array connected in sequence. The optical beamsplitter is a 1×N tree-like beam splitting structure, composed of multiple cascaded 3dB couplers or Y-branch waveguides, used to uniformly split the input single-sideband optical signal carrying digital information into multiple identical optical signals. The continuously tunable true delay line array contains multiple independent optical delay channels, used to provide continuous broadband optical delay modulation for each optical signal. The photodetector array consists of N high-speed photodetectors, whose input ends are optically coupled to the output end of the continuously tunable true delay line array, used to convert the delayed optical signals into millimeter-wave / terahertz signals to drive the phased array antenna at the back end.

[0007] Furthermore, each optical delay channel in the continuously tunable true delay line array consists of an N-bit cascaded switch-type delay line and a continuously tunable delay line. The N-bit switch-type delay line contains N+1 MZI-based optical switches for switching delay paths of different lengths on the chip to achieve discrete delay adjustment. The N-bit switch-type delay line can provide discrete delay adjustment from 0 to (2^N–1). The continuously tunable delay line consists of two Mach-Zehnder interferometers and a pair of delay waveguides of different lengths cascaded together to provide continuous delay control within the discrete basic delay step range. Cascading the N-bit switch-type delay line and the continuously tunable delay line enables seamless continuous control across the entire delay range. The discrete delay step size of the discrete switch-type delay line is matched with the maximum continuous delay control amount of the continuously tunable delay line. The maximum total delay required by the phased array elements and the number of bits N of the discrete switch-type delay line are determined based on the antenna array layout of the phased array and the target beam deflection angle requirements.

[0008] Furthermore, the continuously adjustable delay line is composed of two Mach-Zehnder optical switches and a cascaded delay waveguide, used to provide continuous delay control within the basic delay step range, thereby achieving the total delay difference required by the system. The Mach-Zehnder optical switch consists of two couplers and a pair of waveguide arms, each waveguide arm integrating a phase shifter to control its respective optical phase; the optical phase difference between the two pairs of waveguide arms in the two Mach-Zehnder optical switches is defined as Δ... 1 and Δ 2. The corresponding power coupling coefficients are K1 and K2, respectively. When the power coupling coefficients are set to satisfy the complementary condition, that is, K1 = 1 - K2, the continuously adjustable delay line achieves continuous optical delay response at its cross input and output ports. When the equal condition is satisfied, K1 = K2, continuous optical delay response is achieved at the through input and output ports.

[0009] Furthermore, the photodetector array employs a unidirectional carrier transport photodetector (UTC-PD) that directly outputs a terahertz signal using heterodyne beat frequency.

[0010] This invention also provides a millimeter-wave / terahertz beamforming control method based on the aforementioned chip. The method is characterized by using the maximum delay of a continuously adjustable delay line to determine the free spectral range (FSR) of its periodic delay spectrum; employing a carrier and sideband cross-FSR modulation mechanism to map the optical carrier and the broadband optical sideband carrying radio frequency information in the single-sideband signal to different FSR periods; and simultaneously accurately mapping the center frequency of the optical sideband to a flat region at the anti-resonance point of two adjacent FSR centers in the delay spectrum. Utilizing the extremely low group delay dispersion in this region, high-precision delay modulation is achieved within a broadband range in the millimeter-wave / terahertz frequency band.

[0011] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention employs a composite architecture combining a switch-type delay line and a continuously adjustable delay line, replacing the traditional, highly complex, fully cascaded switch structure. This design reduces the number of optical switches used, significantly lowers the optical signal insertion loss of a single-channel delay line, effectively simplifies the system's control complexity, and improves overall energy efficiency, enabling high-precision delay control at the sub-picosecond level (better than 0.1 ps).

[0012] 2. This invention utilizes the mechanism of a Mach-Zehnder optical switch, which can change the optical power coupling coefficient by adjusting the phase difference between its two arms, and combines this with sideband-carrier modulation across the free spectral range to precisely map the center frequency of the broadband signal to be transmitted to a flat region near the anti-resonance point of the delay spectral line. This mechanism controls the delay error of each delay state to a low level across a wide frequency range covering millimeter-wave / terahertz communication, overcoming the bottleneck of the traditional delay line's tradeoff between high bandwidth and high precision, and eliminating the beam-squinting problem in high-capacity broadband wireless communication. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the general architecture for millimeter-wave / terahertz beamforming based on a broadband continuously tunable optical delay line proposed in this invention. Figure 2 This is a schematic diagram of the overall architecture of the millimeter-wave / terahertz beamforming network according to the first embodiment of the present invention; Figure 3 The following is a structural diagram and simulation results of the switch-cascaded continuously adjustable true delay line in the second embodiment of the present invention: (a) the switch-cascaded continuously adjustable delay structure and its operating state; (b) different Δ... (c) Working principle of cross-FSR delay control; Figure 4 The above are simulation results of the delay response of the continuously adjustable delay line in the second embodiment of the present invention. Detailed Implementation The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. These embodiments are implemented based on the technical solutions of the present invention and provide detailed implementation methods and operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0014] Example 1: Terahertz Beamforming Network To better illustrate the technical solution of the present invention, this embodiment first introduces a general architecture for millimeter-wave / terahertz beamforming, and then provides specific chip implementation examples in combination with the requirements of the 220 GHz frequency band.

[0015] like Figure 1 As shown, this invention proposes a general architecture for millimeter-wave / terahertz beamforming based on broadband continuously tunable optical delay lines. At the transmitting end, the optical carrier and radio frequency signal are modulated by an optical single-sideband modulation module to generate a single-sideband optical signal. This signal is then uniformly divided into M paths by a 1×M optical power beamsplitter. Each signal enters an optical delay line array, where each delay unit is powered by a source providing 0 to (2^3) optical delay lines. N –1) Discrete delay-adjustable N-bit switching delay line with a 0 to 1000 N-bit switching delay line. It is composed of cascaded continuously adjustable delay lines with continuous time adjustment; the combination of the two can achieve a range of 0 to 2. N Continuous delayed regulation.

[0016] After time delay modulation, the M-channel optical signals are injected into the photodetector array and converted into radio frequency electrical signals with precise delay differences through an opto-heterodyne beat frequency process. Finally, these electrical signals drive the phased array antenna at the back end to radiate millimeter-wave / terahertz waves into free space. By adjusting the delay of each channel, beam pointing angle can be precisely controlled without beam squint.

[0017] This embodiment provides a specific terahertz beamforming chip designed based on the 220GHz frequency band, such as... Figure 2 As shown, its system architecture, starting from the optical input end, includes, in sequence: a 1×16 optical power beam splitter 1, a 16-channel continuously adjustable true delay line array 2, and a unidirectional carrier transport photodetector (UTC-PD) array 3.

[0018] In the workflow, the input terahertz single-sideband optical signal carrying digital information (including the optical carrier and the broadband optical sideband carrying radio frequency information) is first evenly split into 16 paths by beam splitter 1. Each signal independently enters the delay line array 2, achieving precise and continuous control of the optical signal group delay. The optical signal that has completed time delay reconstruction is then injected into the photodetector array 3, where it is down-converted through an opto-heterodyne beat frequency process to generate a terahertz radio frequency electrical signal. The output of the photodetector is directly electrically coupled to the 4×4 terahertz phased array antenna at the back end, driving the beam to be transmitted into space.

[0019] This embodiment is for a center frequency of 220 GHz (spatial wavelength) The antenna array is designed using a half-wavelength array with an element spacing of d = 0.68 mm. To meet the beam deflection requirement of ±45°, a maximum true delay of approximately 1.6 ps is required between adjacent elements. Considering the cascaded arrangement of four elements, the maximum required total delay difference between the first and last elements is 9.6 ps. By controlling 16 continuously adjustable true delay lines, the system can generate terahertz beams corresponding to different spatial pointing angles while maintaining beam direction consistency over the ultra-wideband range.

[0020] Example 2: Broadband Continuously Adjustable Delay Line Structure Design Based on Example 1, this example provides a detailed description of the physical structure and core control mechanism of the continuously adjustable true delay line.

[0021] As shown in Figure 3, each optical delay channel adopts a composite architecture consisting of a single-stage discrete switch-type delay line (i.e., a 1-bit switch) cascaded with a continuously adjustable delay line. To meet the system's total delay requirement of 9.6 ps, this embodiment sets the fixed delay of the 1-bit switch to 4.8 ps and the adjustment range of the continuously adjustable delay line to 0. 4.8ps. This design significantly reduces the number of single-channel optical switches from more than 7 in traditional multi-bit cascade to about 2, keeping the single-channel insertion loss to within about 2dB.

[0022] The key to achieving broadband dispersion-free delay lies in the free spectral range (FSR) modulation mechanism of the continuously tunable portion. As shown in Figure 3(a), this structure consists of two Mach-Zehnder (MZI) optical switches and a pair of unbalanced delay waveguides of different lengths connected in the middle. A TiN thermal heater is integrated on each waveguide arm, changing the phase difference between the two arms (defined as Δ) through a thermo-optical effect. 1 and Δ 2) The MZI is configured to function as a tunable power coupler rather than a simple pass-through switch. When the heater is adjusted so that the power coupling coefficients of the two MZIs satisfy the complementary condition (K1 = 1 – K2), the system outputs at the cross-port; when the power coupling coefficients satisfy the equal condition (K1 = K2), the system outputs at the through port. By precisely locking the operating wavelength at the constructive interference point of the optical transmission spectrum, the group delay can be adjusted from 0 to 1 as the coupling coefficient K1 continuously changes from 0 to 1. Continuous adjustment is achieved between 4.8ps.

[0023] For terahertz signals at 220 GHz, the designed FSR in this embodiment is 208.3 GHz (corresponding to a maximum delay of 4.8 ps). Figure 3 As shown in (c), since the radio frequency is higher than the FSR of the delay line, the present invention utilizes the periodic delay response to map the optical carrier in the single sideband signal to one FSR period, while mapping the broadband optical sideband to another adjacent FSR period, thereby achieving physical isolation between the carrier and the sideband.

[0024] This embodiment uses tuning control to precisely position the center frequency of the 40 GHz optical sideband at the anti-resonance point of the delay spectrum (i.e., the exact center region between two adjacent FSRs). Because the group delay near this point has an extremely low rate of change with frequency, simulation results show that the delay error is strictly controlled within 0.09 ps over a wide bandwidth from 200 GHz to 240 GHz. This cross-FSR anti-resonance point tuning mechanism achieves wideband, continuous, and high-precision delay tuning in the 220 GHz terahertz band.

Claims

1. A millimeter-wave / terahertz beamforming chip based on a broadband continuously tunable optical delay line, characterized in that, The system includes an optical beamsplitter, a continuously tunable true delay line array, and a photodetector array connected in sequence. The optical beamsplitter is a 1×N tree-like beam splitting structure, composed of multiple cascaded 3dB couplers or Y-branch waveguides, used to uniformly split the input single-sideband optical signal carrying digital information into multiple identical optical signals. The continuously tunable true delay line array contains multiple independent optical delay channels, used to provide continuous broadband optical delay modulation for each optical signal. The photodetector array consists of N high-speed photodetectors, whose input end is optically coupled to the output end of the continuously adjustable true delay line array. It is used to convert the delayed optical signals into millimeter-wave / terahertz signals to drive the phased array antenna at the back end.

2. The millimeter-wave / terahertz beamforming chip according to claim 1, characterized in that, Each optical delay channel in the continuously tunable true delay line array consists of an N-bit cascaded switch-type delay line and a continuously tunable delay line; the N-bit switch-type delay line contains N+1 Mach-Zehnder interferometer (MZI)-based optical switches for switching delay paths of different lengths on the chip to achieve discrete delay adjustment; the N-bit switch-type delay line can provide delays from 0 to (2... N –1) Discrete delay adjustment, where the delay step size of the discrete adjustment is... The continuously adjustable delay line is composed of two Mach-Zehnder interferometers and a pair of delay waveguides of different lengths cascaded together, used to provide a delay step within the discrete basis, i.e., from 0 to... Continuous delay control; the delay can be adjusted from 0 to 2 by cascading an N-bit switching delay line and a continuously adjustable delay line. N The continuous delay control is achieved by matching the discrete delay step size of the discrete switching delay line with the maximum continuous delay control amount of the continuously adjustable delay line; and the maximum total delay required by the phased array elements and the number of bits N of the discrete switching delay line are determined according to the antenna array layout of the phased array and the target beam deflection angle requirements.

3. The millimeter-wave / terahertz beamforming chip according to claim 2, characterized in that, The Mach-Zehnder interferometer in the continuously adjustable delay line consists of two 3dB couplers and a pair of waveguide arms, with a phase shifter integrated on each waveguide arm. By adjusting the driving signal of the phase shifter, the optical phase in the corresponding waveguide arm is changed, thereby adjusting the optical power coupling ratio of each output port of the Mach-Zehnder interferometer.

4. The continuously adjustable delay line according to claim 2, characterized in that, The optical power coupling coefficient is adjusted by the optical phase difference between two pairs of waveguide arms in two Mach-Zehnder interferometers. When the power coupling coefficients of the two Mach-Zehnder interferometers satisfy the complementary condition, the continuously adjustable delay line can achieve a continuously tuned optical delay response at its cross-output port. When the power coupling coefficients of the two Mach-Zehnder optical switches satisfy the equal condition, the continuously adjustable delay line can achieve a continuously tuned optical delay response at its through-output port. The continuously adjustable delay line exhibits a periodic delay spectrum in the frequency domain, with a relatively flat delay near the anti-resonance point, allowing signals over a large bandwidth to simultaneously obtain a flat delay.

5. The millimeter-wave / terahertz beamforming chip according to claim 1, characterized in that, The high-speed photodetector can be a unidirectional carrier transport photodetector (UTC-PD) or a PIN structure, made of III-V or germanium materials.

6. A millimeter-wave / terahertz beamforming control method based on the chip according to any one of claims 1 to 5, characterized in that, The system includes the following steps: determining the target beam deflection angle and calculating the required target delay for each array element in real time based on the system's phased array antenna array layout; decomposing the target delay into two parts for allocation, one part being configured as a coarse-tuned delay by a discrete switching delay line, and the other part being configured as a continuously adjustable delay by a continuously adjustable delay line; acquiring the optical carrier frequency and optical sideband frequency of the input single-sideband optical signal; and, during system operation, considering the carrier and sideband separation characteristics of millimeter-wave / terahertz signals in the optical domain, employing a cross-free spectral range delay modulation mechanism based on the continuously adjustable delay line. The free spectral range ensures that the optical carrier and the broadband optical sideband carrying radio frequency information are physically isolated and mapped to adjacent free spectral range periods, while ensuring that the center frequency of the broadband optical sideband is precisely aligned with the broadband flat region near the anti-resonance point. Under this mapping condition, by adjusting the phase shifter drive signal on the Mach-Zehnder interferometer waveguide arm in the continuously adjustable delay line, the optical power coupling coefficient is changed, thereby continuously adjusting the group delay in this flat region. This utilizes the high consistency of the group delay in this region to achieve broadband and high-precision delay control within the target range.

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