CN110658661B - A phase calibration method and system for an optical phased array - Google Patents

Abstract

The invention discloses a phase calibration method and system for an optical phased array. The method comprises the following steps: 1) placing a spatial filter in an observation system of the optical phased array, and adjusting the position of the spatial filter to allow only light emitted by two adjacent antennas of the optical phased array to pass through, and simultaneously block light emitted by the other antennas of the optical phased array; 2) after observing interference fringes generated by light emitted by two adjacent antennas from a far field by using an observation system, adjusting the phases of the two adjacent antennas selected currently until the interference fringes generated by the light emitted by the two adjacent antennas are aligned to the optimal emission direction of the phased array antenna array; 3) moving the spatial filter so that light emitted from a new adjacent antenna formed by one antenna of the two selected adjacent antennas and the adjacent antenna passes through the spatial filter; 4) and (5) repeating the steps 2) to 3), and completing the phase distribution calibration of the whole optical phased array after the phase calibration of any two adjacent antennas in the antenna array.

Description

A phase calibration method and system for an optical phased array

technical field

The invention relates to a phase calibration method and system for an optical phased array, and belongs to the field of optical communication.

Background technique

Optical communication is a hot topic in the field of contemporary communication technology. On the basis of the traditional microwave phased array research, an important new optical communication system—optical phased array has been invented. Compared with the traditional phased array, the working wavelength of the optical phased array is shifted from the microwave band to the near-infrared band and even the visible light band, which makes it have the characteristics of no inertia, fast beam scanning, small unit module size and high integration. Advantage. Therefore, optical phased array technology has broad prospects in the fields of lidar, laser communication, free space optical communication and so on.

Of course, while optical phased arrays bring advantages, they also bring new technical challenges. Since the optical phased array radiation beam shape is strictly constrained by the radiation phase of each antenna, in order to achieve a sufficiently clear and distinguishable beam pointing, the output unit requires its initial phase distribution to have high accuracy. However, due to factors such as device layout and process error, it is difficult for the phase distribution of the optical phased array to achieve the uniformity required by the system. Therefore, with the increase of the integration degree and the expansion of the scale of the optical phased array, the phase calibration technology for the optical phased array becomes more and more indispensable.

The traditional phase calibration technology mostly uses the gradient descent algorithm to calibrate the phase of the optical phased array in software, that is, firstly, the obtained far-field pattern of the optical phased array is used as feedback, and then the gradient descent algorithm is used to continuously adjust and iterate. The control voltmeter applied to each unit of the optical phased array finally obtains the calibrated phase distribution. The advantage of this scheme is that the calibration of the entire optical phased array can be achieved without knowing the exact phase of each unit through the calibration process; the disadvantages are: 1. It is necessary to build a complex feedback system to iteratively update the voltmeter, the effect of the algorithm and the feedback system The quality is closely related, and it is difficult to accurately calibrate the small signal emitted by the optical phased array (when the signal-to-noise ratio is low); 2. In principle, the phase of each unit and the output of the optical phased array are nonlinear. The gradient descent method under the condition of large-scale two-dimensional phased array may converge the result to a non-optimal solution, and the calibration effect is not ideal.

That is to say, the existing well-known phase calibration technologies mostly use software methods to realize the calibration of optical phased arrays. The specific implementation needs to build a feedback system, which is cumbersome to operate, and requires high calibration accuracy for the feedback system. For large-scale and complex optical phased arrays Array calibration is not ideal.

SUMMARY OF THE INVENTION

Aiming at the specific requirements of the optical phased array and the technical problems existing in the existing solutions, the purpose of the present invention is to provide a phase calibration method and system for an optical phased array with convenient operation, short iteration time and high alignment accuracy.

Among them, the phase calibration process is mainly realized by the principle of spatial filtering and coherent fringe adjustment. First, the spatial filter element is placed in the observation system of the optical phased array, and its position is adjusted so that it can only allow the light emitted by the two adjacent antennas a and b to pass through, while blocking the light emitted by the remaining antennas. Then, obvious interference fringes can be observed from the pattern observed in the far field by the observation system, and then the phase of the antennas a and b is adjusted by applying a voltage to the antennas a and b until the light emitted by the antennas a and b produces The interference fringes are aligned to the optimal emission direction of the antenna array. Then, move the spatial filter element away from one of the antennas, pass the other two antennas (make sure one is calibrated and the other is not) and repeat the above process. Finally, repeat this calibration process, from antenna to antenna, and then row to row to column. When the phases of any two adjacent antennas in the antenna array are aligned by the above process, the phase distribution of the entire optical phased array is finally completed. calibration. The present invention first proposes a phase calibration method for integrated on-chip optical phased arrays that utilizes the combination of a spatial filter device and an observation system to adjust the antenna interference fringes. This method can also be extended to optical phased arrays of any form. The observation system needs to have the function of switching between the far and near fields or simultaneously observe the results of the far and near fields; if the observation system has the function of switching the far and near fields, it is necessary to adjust the function of the observation system to switch from the near field observation to the far field observation, so as to obtain the interference fringes. The specific implementation method depends on the structure of the observation system, and there are various forms; if the observation system has the function of simultaneously observing the results of the far and near fields, the far-field interference fringes can be directly observed.

Further, the coherent light source can be generated using a monochromatic laser for providing an input light source for the optical phased array. The optical phased array is connected with a monochromatic laser for receiving the input laser, and then inputting the input laser to each antenna respectively. The optical phased array can control the phase of the laser input to each antenna. In some cases, the coherent light source can also be a wavelength-tunable laser, and various methods such as external modulation of a common laser or various types of light sources can be used to change the laser wavelength.

Further, the spatial filter used in the calibration process is composed of a diaphragm, which can also be replaced by slits, small holes and other devices that can block the near-field beam output by the optical phased array.

The invention provides a phase calibration system, through which the far-field observation and the near-field observation of the output beam of the optical phased array can be realized. The field observation can observe the movement of the coherence fringes of adjacent cells to adjust the cell phase. For example, in the near-field observation process, adjust the position of the spatial filtering device so that it can only allow the light from two adjacent antennas a and b to pass through. Interference fringes produced by light. The phase calibration system is composed of N optical elements such as lenses and polarizers. The number of optical elements in the phase calibration system is determined by the required magnification performance of the observation system. Other mature devices such as microscopes can also be used to achieve optical phase control. Observation of the output beam of the array. As shown in Fig. 3, the phase calibration system of this scheme is composed of a spatial filtering device, an observation system and a receiving device, wherein the observation system needs to have the ability to switch between the far and near field observations or to perform both the far and near field observations; the receiving device is used for receiving The signal determines that the interference fringes generated by the light emitted by the currently selected two adjacent antennas are aligned with the optimal emission direction of the phased array antenna array. The present invention proposes for the first time that the optical phased array phase calibration is performed by combining spatial filtering with far and near field switching.

Further, in the calibration measurement step, the calibration may be performed sequentially for each pair of adjacent units, or each adjacent N units may be sequentially calibrated as a group, or the calibration may be performed after grouping in a two-dimensional space.

Further, the optical phased array may be an optical waveguide phased array, a micro-electromechanical system (MEMS) optical phased array, or an optical phased array of liquid crystal and electro-optic crystal materials. Since the phase calibration technology of the present invention is universal in principle, its macroscopic scheme is not necessarily related to the specific structure of the optical phased array, so the interference technology can be applied to optical phased arrays of various materials and structures.

Further, the phase calibration technique can be used to calibrate the phase of a one-dimensional array, or a two-dimensional or multi-dimensional array optical phased array.

Further, the above-mentioned optical phased array can be formed by connecting discrete devices through optical waveguides or spaces, and can also use an integrated process to integrate multiple discrete devices and optical waveguides on one or several substrates, and can also be partially integrated into one. or several substrates.

Compared with the prior art, the positive effects of the present invention are:

1. Directly using the coherent fringe observation method avoids the analysis process of the chaotic far-field pattern caused by the complex random phase distribution, and optimizes the operability of the phase calibration technology in the actual measurement. For large-scale arrays, especially two-dimensional arrays, the measurement time required by this scheme will be greatly reduced.

2. The local optimal solution problem caused by the use of phase calibration techniques based on various algorithms is avoided. The adjustment of the coherent fringes makes the phase of each unit in a strictly uniform state in principle, and the accuracy of the phase of each unit ensures the final phase. The adjustment result is a globally optimal solution for optical phased array calibration. If combined with the relevant calibration algorithm, the algorithm can be more convergent.

3. The calibration error is small and the interference fringes are easy to observe, which reduces the calibration error caused by the complex alignment process. In addition, since the coherence fringes are only determined by the phase difference of each adjacent unit and have nothing to do with the array size and phased array structure, the scheme has good general applicability.

Description of drawings

FIG. 1 is a schematic diagram of the principle of the optical phased array of the present invention.

FIG. 2 is a schematic diagram of the phase calibration system of the present invention.

FIG. 3 is a specific implementation form of the optical phased array phase calibration system scheme in the present invention.

4 is a result demonstration and analysis diagram of the phase calibration effect in the present invention;

(a) is the far-field distribution before phase calibration, (b) is the far-field distribution after phase calibration,

(c) is the effect of calibration error at 8 × 8 scale, (d) is the effect of calibration error at 32 × 32 scale.

Detailed ways

The solution of the present invention will be described in further detail below.

The far-field electric field distribution U(θ _x ,θ _y ) of the optical phased array is affected by the phase and electric field of the phased array radiation element, and can be expressed as the Fourier transform of its near-field electric field distribution E(x,y), Its far-near field conversion formula can be expressed as

Among them, for a uniform optical phased array with a scale of M×N, its near-field electric field distribution E(x,y) can be expressed as

项满足如下关系(

与

为固定值)时，Among them, only when the

items satisfy the following relation (

and

is a fixed value),

The integral result of the far-field light intensity of the entire optical phased array can be simplified to the following form

From the above derivation, it can be concluded that as long as the initial radiated electric fields of adjacent row cells and adjacent column cells maintain the same phase difference, clearly distinguishable radiation beams can be observed in the far-field image. However, due to the influence of the phased array structure and process errors, the phase of the unit radiation beam is often a random and non-uniform fixed value, which requires an external phase shift to calibrate the phase to the above requirements. Therefore, the present invention only needs to calibrate the phase difference of each adjacent unit, and can completely calibrate the optical phased array that satisfies the above-mentioned principle regardless of its specific material and structure.

Figure 2 shows a specific embodiment of the present invention: a monochromatic laser is used as a coherent light source, and the generated coherent light is coupled into an optical phased array, and the optical phased array radiates a random but stable near-field beam from the antenna. The observation system can be switched to the far-field (Fourier) plane for observation. An adjustable spatial filtering device is inserted into the observation system so that only the near-field beams of two adjacent antennas can pass through the observation system and produce coherent fringes in the far-field receiving plane. The coherent fringes at this time are only determined by the phase difference between two adjacent antennas, so a phase shift can be added to move the fringes to specific positions. At this time, the phase calibration of the two antennas is completed, and the other two antennas of the mobile spatial filtering device pass (ensure that one antenna has been calibrated and the other is not) and the above process is performed again. By repeating the above process continuously, any two adjacent antennas in the array antenna (regardless of the row and column) are calibrated, and the phases of all the antennas are calibrated. The above is a particularly specific implementation manner, wherein under the condition that the basic functions of each module and step are guaranteed to remain unchanged, there may be various specific implementation manners. details as follows:

For the coherent light source part, an external monochromatic laser can be used to generate a coherent light source coupled into the optical phased array device, or the coherent light source can be integrated in the optical phased array device to avoid the loss caused by the coupling of the external light source.

For the observation system, the standard 4f system can be used to easily switch between the far field and the near field of the observation beam, or a simple system composed of a movable single lens can be used for the far and near field observations. For special needs, more lens groups or complex equipment can also be added to meet the design and construction of the observation optical path.

For spatial filtering, apertures, apertures, or slits can be used to pass radiation beams from adjacent antennas while filtering radiation from other antennas. Of course, for some special requirements, such as antenna grouping filtering, more complex spatial filtering devices can also be used. The way of grouping filtering can further shorten the time of the whole calibration process, but it will reduce its calibration accuracy.

Next, the calibration effect of this scheme is introduced. In the test example of the present invention, a silicon-based waveguide optical phased array (the antenna spacing is 40 μm×10 μm) is used as the calibration object, and a series of calibration processes are carried out. The chaotic but fixed far-field pattern generated by the optical phased array before calibration is shown in Fig. 4(a), and the far-field planar receiving pattern after calibration is shown in Fig. 4(b). It can be seen that the calibration process has significantly improved the shaping of the main lobe beam. After careful measurement, the calibrated side lobe suppression ratio (defined here as the ratio of the highest peak of the main lobe to the secondary peak of the side lobe in the same maximum field of view) ) increased by 8.1dB compared to before calibration. Similar results can be obtained after repeated tests on multiple silicon-based waveguide optical phased arrays with the same structure. The silicon-based waveguide optical phased array is only a specific implementation example of this solution. Since the calibration process of the present invention is not constrained by the structure and material of the phased array in principle, this solution can be applied to various types of optical phased arrays. In the initial phase calibration, it has strong versatility.

For an optical phased array with a scale of M×N, for example, if the phase of each antenna has 10 random states, the number of iterations required by the traditional gradient descent iterative algorithm is 10 ^MN times, and the number of iterations of this scheme is ( NM-1) times, the number of iterations is greatly reduced, and the operability is strong. In addition, the maximum value of the calibration error of a single antenna observed in the process of testing the present invention is 0.3π, and the error of this level has little impact on the performance of the entire optical phased array system and is within an acceptable range. As shown in Figure 4(c) and Figure 4(d), if the array scale continues to increase, the impact of this error on the system performance will become smaller. Therefore, the calibration error of the present invention is small, and the scope of application is wide.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. Thus, provided that these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims (10)

1. A method of phase calibration for an optical phased array, comprising the steps of:

1) placing a spatial filter in an observation system of the optical phased array, and adjusting the position of the spatial filter to allow only light emitted by two adjacent antennas of the optical phased array to pass through, and simultaneously block light emitted by the other antennas of the optical phased array;

2) after observing interference fringes generated by light emitted by two adjacent antennas from a far field by using an observation system, adjusting the phases of the two adjacent antennas selected currently until the interference fringes generated by the light emitted by the two adjacent antennas are aligned to the optimal emission direction of the antenna array;

3) moving the spatial filter so that light emitted by a new adjacent antenna formed by one of the two selected adjacent antennas and the adjacent antenna passes through the spatial filter while light emitted by the rest of the antennas of the optical phased array is blocked;

4) and (5) repeating the steps 2) to 3), and completing the phase distribution calibration of the whole optical phased array after the phase calibration of any two adjacent antennas in the antenna array of the optical phased array.

2. The method of phase calibration for an optical phased array as claimed in claim 1, wherein the observation system is an observation system having near-field and far-field switching, and the functions of the observation system are adjusted as necessary to switch between near-field observation and far-field observation.

3. The method of claim 1, wherein the observation system is an observation system having a function of simultaneously observing far and near field results.

4. A phase calibration method for an optical phased array as claimed in claim 1, 2 or 3, wherein the phases of two adjacent antennas are adjusted by applying a voltage to the two selected adjacent antennas until interference fringes produced by the light emitted therefrom are aligned with the optimum direction of emission of the antenna array.

5. The method of phase calibration for an optical phased array of claim 1, wherein the optical phased array is an optical waveguide phased array, a micro-electromechanical systems optical phased array, an optical phased array of liquid crystal material, or an optical phased array of electro-optic crystal material.

6. A phase calibration system for an optical phased array comprising a spatial filter, an observation system and receiving means; wherein,

the spatial filter is arranged in the observation system and is used for allowing the light emitted by two adjacent antennas of the optical phased array to pass through and blocking the light emitted by the other antennas of the optical phased array;

the observation system is arranged between the optical phased array and the receiving device and is used for near-field observation and far-field observation; wherein adjusting the position of the spatial filter to allow only light emitted from two adjacent antennas to pass is done by near field observation; observing interference fringes generated by light emitted by two adjacent antennas through far field observation, and then adjusting the phase of the two currently selected adjacent antennas according to the coherent fringes observed by the far field to ensure that the interference fringes generated by the light emitted by the two currently selected adjacent antennas are aligned to the optimal emission direction of the antenna array;

and the receiving device is used for judging whether interference fringes generated by the light emitted by the two currently selected adjacent antennas are aligned to the optimal emitting direction of the antenna array or not according to the received signals.

7. A phase calibration system for an optical phased array as claimed in claim 6, wherein said optical phased array is connected to a monochromatic laser for receiving laser light outputted from the monochromatic laser and then inputting the received laser light to each antenna separately.

8. The phase calibration system for optical phased arrays as claimed in claim 6, wherein said observation system is an observation system with far-near field switching or an observation system with the function of simultaneously observing far-near field results.

9. A phase calibration system for optical phased arrays according to claim 6, wherein the optical phased array is formed by discrete devices connected by optical waveguides or space, or by integrating multiple discrete devices and optical waveguides on one or more substrates using an integration process.

10. The phase calibration system for an optical phased array of claim 6, wherein the optical phased array is an optical waveguide phased array, a micro-electromechanical systems optical phased array, an optical phased array of liquid crystal material, or an optical phased array of electro-optic crystal material.

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