Method for Improving the precision of Variable Optical Fiber Delay Line

1 Introduction

Nowadays, delay lines have been widely used in signal processing, radar, electronic countermeasure, and other fields. For example, the Unit delay device required for signal processing is a kind of memory, and the dynamic target display (MIT) there must be a delay line-another kind of memory. In electronic confrontation, the signal must be stored for a certain period of time before processing. For example, the receiver's radar signal must be delayed for a certain period of time before sending it, then, the disruptive interference is completed. In terms of communication and military applications, the traditional metal waveguide and coaxial cable have many shortcomings in terms of volume, weight, anti-electromagnetic interference capability, crosstalk and loss, however, optical fiber as the transmission medium has the advantages of light quality, small physical size, good mechanical flexibility, anti-electromagnetic interference (EMI) and electromagnetic pulse interference (EMP) because of its strong capability and almost no loss, fiber delay lines composed of optical fiber and waveguide have a wider application prospect in radar and Electronic Confrontation [1]. In the actual application of the delay line, there are often different latency requirements for signals based on different situations. In this way, a single and fixed delay line cannot meet this requirement. Based on this, you can use the optical switch to select the length of the delayed optical fiber to obtain a variable fiber delay line.

2 optical fiber delay line principle

It is an optical fiber delay line unit. RF electrical signals are input into the laser diode (LD), and The LD converts the input RF electrical signals into the optical signals modulated by the signals, and is coupled into the optical fiber through the optical connectors. The photoelectric detector (PD) converts the RF-modulated optical signals to the original RF electrical signals. The frequency spectrum of the output RF electrical signal is exactly the same as that of the input RF electrical signal, but the fiber is used as the medium to delay for a period of time. That is to say, the RF signal is instantly stored in the fiber delay line unit, the storage duration is proportional to the optical fiber length, which is the principle of the optical fiber delay line.

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When a light wave transmits at the speed v in the optical fiber, the delay length is proportional to the length L of the optical fiber. The latency produced by the optical fiber with a specific length L can be expressed:

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In the above formula, the refractive index of a light wave whose wavelength is λ and c is the propagation speed of the light wave in free space. From the △t expression, we can see that the length of the delay is proportional to the length of the optical fiber L, as long as the length of the optical fiber can be changed, alternatively, you can use the Optical Waveguide Switch to select an optical fiber with different lengths to achieve different latency [2].

3-6-bit variable bidirectional delay Unit Design

In this paper, the 6-bit optical waveguide delay unit is calculated by controlling the switching status of the waveguide oriented Coupler on the LiNbO substrate to select different delay paths, thus forming a 6-Bit 0 ~ 63. A total of 64 variable bidirectional delay units with different latencies. As shown in System 2, the delay unit is composed of two LiNbO3 substrates, each of which has two 4x4 oriented Coupler Optical Switches, each of which is controlled by one bias electrode and one switch electrode, the switch electrode voltage is controlled to enable or disable the switch.

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The system works at a wavelength of 1.3 micron and connects a 4x4 Optical Switch Using TM-biased optical fiber. The optical fiber length can be precisely cut to an integer multiple of the 240 ps delayed optical fiber length, the optical fiber length error is controlled at the theoretical value (± 0. within 8mm, the extinction ratio and length of the optical fiber and the substrate are measured before the switched waveguide is coupled. Implement 0 ~ Any type of delay in the 63 s optical signal can only go through a unique path, this path requires "on" or "Off" configuration for 8 of 16 oriented Coupler Optical Switches to achieve the required latency [3 ~ 7]. The relationship between the delay path, the length of the optical fiber and the theoretical delay is shown in table 1.

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When the optical signal is transmitted in a 6-bit delay unit, the crosstalk produced by each directional coupler switch will continue to be transmitted in this delay unit, and may be coupled to the main channel in the subsequent switch, these crosstalk signals pass through a series of different waveguide or optical fiber cables, so different latencies are generated, which will lead to a reduction in the extinction ratio at the output end of the delayed unit. At the same time, the crosstalk signal with different latencies is coupled to the primary channel, which also expands the main signal pulse in the time domain and offsets the pulse vertex, thus reducing the latency accuracy. We can select "on" or "off" for the eight remaining optical switches without passing through the optical signal so that the crosstalk signal does not pass through the main channel, output from other paths to the remaining unused output ports, which can effectively increase the extinction ratio and improve the delay precision of the delay unit.

4. System Simulation and result analysis
The 6-bit variable optical fiber delay line uses the Photonic Transmission Design Suite (PTDS) of Virtual Photonics for simulation calculation. The simulation model is shown in figure 3.

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In this simulation system, a 1.3 μm wavelength, 1 MW Continuous Wave Laser as the light source, high-speed M-Z lithium bromide modulated device, 10 GHz Gaussian pulse signal as the modulation signal, after 6-bit delay units, the output signals are analyzed in the frequency domain and time domain respectively. We choose the time difference between the input and output of the optical signal passing through the "0" delay path as our reference value, as shown in A in 4. All latencies in the end ~ 63 Tau is the relative delay relative to the "0" delay path. In B, it is the simulation image obtained after the delay relative to the "0" delay path after 8 Tau, 16 Tau and 48 Tau.

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It can be calculated from the simulation graphics and parameters that the average insertion loss of the delay unit is-19.9 dB, which is mainly caused by the loss of the oriented Coupler Optical Switch and the coupling loss between the waveguide and the optical fiber. The average latency error between the simulation value of the delay unit and the expected theoretical value is 12.8 ps, the delay error is mainly caused by the cutting precision of the optical fiber length, the feedback coupling of the Two-arm unequal length and crosstalk signal of the oriented Coupler Optical Switch. The cutting precision of the delayed optical fiber length and the unequal length of the two arm of the oriented coupler switch are related to the device manufacturing process, however, we can use the "on" and "off" Status of the eight optical switches that control the non-over optical signals to output the crosstalk signal to the unused output port, to reduce the effect of Crosstalk on the main signal.

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The simulation results of multiple groups of different groups of optical switches are obtained by combining the status of the eight optical switches without the active signal, we can see from the results the crosstalk produced by the optical switch that is closer to the output port of the signal in the main signal path, which has the greatest impact on the delay precision of the main signal, therefore, when using this method, we follow the principle of preferentially outputting crosstalk from optical switches that are closer to the output port to unused ports, that is, when there is a conflict between the current plane and the optical switch that produces crosstalk output to the unused port path, we give priority to ensure that the subsequent crosstalk signals do not pass through the main channel, output to unused ports, such as table 2, lists the latency of 0, 8, 16, and 48, select the path of the main signal and crosstalk signal from the optical switch ).

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It is a delay path of optical signals passing through. The method for outputting the control crosstalk signal to unused output ports is not used, and the two sets of curves with different latencies obtained after this method are used, from the figure, we can see that the expected latency after the 8 s delay path is 1920 ps, and the error between the improved latency value and the expected latency is 6.1 ps, the error between the initial latency and the expected latency is 14.3 ps. Therefore, comprehensive ~ 63 records a total of 64 types of delayed data. The "on" and "off" states of the 8 optical switches that control the non-over optical signals can be obtained, the method of outputting crosstalk signals to unused output ports reduces the influence of crosstalk signals on main signal pulse widening and vertex offset in the time domain, the average latency error is reduced from 12.8 ps to 7.9 ps, which effectively improves the latency accuracy.

5. jieshu

This article introduces the optical fiber delay line principle and the optical fiber delay line is light in quality, small in physical size, good in mechanical flexibility, strong in anti-electromagnetic interference and electromagnetic pulse interference ability, and almost no loss and other advantages, the theoretical analysis and System Simulation of the 6-bit optical fiber delay line composed of four 4x4 optical switches are carried out, and the following conclusions are obtained:

1) The 6-bit optical fiber delay line simulated in this paper can select different delay paths through the control of the oriented Coupler Optical Switch in the delay unit ~ 63. A total of 64 bidirectional variable latency functions.

2) A method is proposed to control 8 optical switches without passing through optical signals in 16 optical switches so that the crosstalk signal can be directly output to unused output ports without passing through the main channel, to increase the latency accuracy, the average latency error is reduced from 12.8 ps to 7.9 ps.

3) the simulation results are in good agreement with the theoretical analysis values. Several major parameters of the 6-bit optical fiber delay line are obtained, which provides a sufficient theoretical basis for future experiments on devices.