The most popular method to improve the precision o

2022-07-31
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Methods to improve the precision of variable optical fiber delay line technology

1 introduction

nowadays, delay lines have been widely used in signal processing, radar, electronic countermeasures and other fields. For example, the unit delay equipment required for signal processing is a kind of memory, and there must be a delay line - another kind of memory in the moving target display (MIT). In electronic countermeasures, signals need to be stored for a certain period of time before processing, If the receiving radar signal is delayed for a certain time before being sent out, deceptive jamming is completed. In terms of communication and military applications that we are most concerned about, the traditional metal waveguide and coaxial cable can no longer meet the needs of practical applications because of many deficiencies in volume, weight, anti electromagnetic interference ability, crosstalk and loss. However, using optical fiber as the transmission medium has the advantages of light weight, small physical size, good mechanical flexibility It has the inherent advantages of strong anti electromagnetic interference (EMI) and electromagnetic pulse interference (EMP) capability and almost no loss. Therefore, the optical fiber delay line composed of optical fiber and waveguide has a wider application prospect in radar and electronic countermeasures [1]. However, in the practical application of delay lines, there are often different delay time requirements for signals according to different situations. Thus, a single, fixed delay line cannot meet this requirement. Based on this, a variable fiber delay line can be obtained by using an optical switch to select the length of the delay fiber

2 principle of optical fiber delay line

Figure 1 is the schematic diagram of optical fiber delay line unit. The RF electrical signal is input to a laser diode (LD), which converts the input RF electrical signal into an optical signal modulated by the signal, and is coupled into the optical fiber through an optical connector. The photoelectric detector (PD) converts the RF modulated optical signal into the original RF electrical signal. The frequency spectrum of the output RF electrical signal is completely the same as that of the input RF electrical signal, except that the optical fiber is used as the medium to delay for a period of time, that is, the RF signal is instantaneously stored in the optical fiber delay line unit, and the length of the storage time is proportional to the length of the optical fiber, which is the principle of the optical fiber delay line

when the light wave propagates at the speed V in the optical fiber, the length of the delay is proportional to the length L of the optical fiber, so the delay generated by the optical fiber with a specific length l can be expressed as:

in the above formula, n is the wavelength of λ The refractive index of light wave in optical fiber, C is the propagation velocity of light wave in free space. From the expression of △ T, we can see that the length of the delay time is in direct proportion to the length of the optical fiber L. as long as the length of the optical fiber can be changed, or different lengths of optical fibers can be selected through the optical waveguide switch, different delay times can be achieved [2]

3 6-bit variable bidirectional delay unit design

the 6-bit optical waveguide delay unit simulated in this paper selects different delay paths by controlling the switching state of the waveguide directional coupler on the LiNbO3 substrate, thus forming a 6-bit 0~63 τ There are 64 variable bidirectional delay units with different delay. The system is shown in Figure 2. The delay unit is composed of two LiNbO3 substrates, each with two 4 × 4 directional coupler optical switch, each directional coupler is controlled by a bias electrode and a switch electrode, and the purpose of switching "on" or "off" is achieved by controlling the voltage of the switch electrode

the system operates at the wavelength of 1.3 μ m, and uses TM polarization maintaining fiber to × 4 optical switches are connected, and the optical fiber length is accurately cut to produce τ= The fiber length error is controlled within the theoretical value ± 0.8 mm. The extinction ratio and length of the fiber are measured before coupling the fiber with the switch waveguide on the substrate. Achieve 0~63 τ For any kind of delay, the optical signal can only pass through the only selectable path, which needs to be configured with "on" or "off" for 8 of the 16 directional coupler optical switches to achieve the required delay [3 ~ 7]. The corresponding relationship among delay path, fiber length and theoretical delay is shown in Table 1

when the optical signal is transmitted in the 6-bit delay unit, the crosstalk generated by each directional coupler switch will continue to transmit in the delay unit, and may be coupled to the main channel in the later switch. These crosstalk signals pass through a series of different waveguides or optical fibers, so different delay will be generated, which will cause the delay to enter the fast-growing track, and the extinction ratio at the output end of the unit will be reduced; At the same time, when crosstalk signals with different delay are coupled to the main channel, the pulse of the main signal will be broadened in the time domain, and the pulse apex will shift, thus reducing the accuracy of the delay. We can select the "on" or "off" of the eight remaining optical switches that the optical signal has not passed through, so that the crosstalk signal does not pass through the main channel and is output from other paths to the remaining unused output ports. In this way, the extinction ratio can be effectively increased to reach 12.1 billion euros in 2015, and the delay accuracy of the delay unit can be improved

4 system simulation and result analysis

the 6-bit variable optical fiber delay line adopts the photon transmission components of virtual Photonics (all integrated components are connected to the photon transmission design suite, PTDs) for simulation calculation. The simulation model is shown in Figure 3

in this simulation system, a 1.3 m wavelength, 1MW energy CW laser is used as the light source, a high-speed M-Z lithium niobate modulator, and a 10 GHz Gaussian pulse signal is used as the modulation signal. After a 6-bit delay unit, the output signal is analyzed in frequency domain and time domain respectively. We select 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 Figure 4. All subsequent delays τ~ sixty-three τ Are the relative delays relative to the "0" delay path. In Figure 4, B is the relative delay relative to the "0" delay path after 8 τ、 sixteen τ And 48 τ Simulation graph obtained after delay

it can also be calculated from the simulation figures and parameters that the average insertion loss of the delay unit is -19.9 dB, which is mainly caused by the loss of the optical switch of the directional coupler and the coupling loss between the waveguide and the optical fiber. The average delay error between the simulation value and the expected theoretical value of the delay unit is 12.8 PS. the delay error is mainly caused by the cutting accuracy of the fiber length, the unequal length of the two arms of the directional coupler optical switch and the feedback coupling of the crosstalk signal. The cutting accuracy of the length of the delay fiber and the unequal length of the two arms of the directional coupler switch are related to the device manufacturing process. However, we can control the "on" and "off" states of the eight optical switches that have not passed through the optical signal to output the crosstalk signal to the unused output port to reduce the impact of the crosstalk signal on the main signal

by combining the states of 8 optical switches that the main signal has not passed through for many times, we can get several groups of different simulation results. From the results, we can see that the crosstalk generated by the optical switch that is closer to the output port of the signal in the path that the main signal passes through has the greatest impact on the delay accuracy of the main signal. Therefore, when using this method, We follow the principle of preferentially outputting the crosstalk generated by the optical switch closer to the output port to the unused port, that is, when there is a conflict between the crosstalk output generated by the front and rear optical switches to the unused port path, we give priority to ensure that the rear crosstalk signal is output to the unused port without passing through the main channel (as shown in Table 2, the implementation of 0 τ、 eight τ、 sixteen τ And 48 τ In case of time delay, the optical switch selects the path of main signal and crosstalk signal)

Figure 5 shows the optical signal passing through 8 τ For the delay path, the method of controlling the crosstalk signal output to the unused output port and the two groups of curves with different delay obtained by this method are not adopted. As can be seen from the figure, after 8 τ The expected delay of the delay path is 1920 PS, the error between the improved delay value and the expected delay is 6.1 PS, and the error between the initial delay and the expected delay is 14.3 PS. Therefore, comprehensive τ~ sixty-three τ A total of 64 kinds of delay data can be obtained by controlling the "on" and "off" states of 8 optical switches that have not passed through the optical signal, so that the crosstalk signal can be output to chart 1 and discharged into) for submission; The method to the unused output port reduces the influence of crosstalk signal on the main signal pulse broadening and vertex offset in time domain, and the average delay error is reduced from 12.8 PS to 7.9 PS, thus effectively improving the delay accuracy

5 conclusion

on the basis of introducing the principle of optical fiber delay line and the advantages of optical fiber delay line compared with traditional electric delay line, such as light weight, small physical size, good mechanical flexibility, strong anti electromagnetic interference and electromagnetic pulse interference ability and almost no loss, this paper × The theoretical analysis and system simulation of the 6-bit optical fiber delay line composed of 4 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 by controlling the directional coupler optical switch in the delay unit, with 0 ~ 63 τ There are 64 bidirectional variable delay functions

2) by controlling 8 of the 16 optical switches without optical signal, the crosstalk signal is directly output to the unused output port without passing through the main channel. The purpose of increasing the delay accuracy is achieved, and the average delay error is reduced from 12.8 PS to 7.9 PS

3) the simulation results are in good agreement with the theoretical analysis values, and several main parameters of the 6-bit fiber delay line are obtained, which provides a sufficient theoretical basis for further experimental devices in the future. (end)

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