Design of delay system based on optical fiber transmission

Delay systems or delay lines are widely used in radar, navigation, communication and other fields. This paper introduces the development of a delay system based on fiber transmission, which overcomes the bottleneck of traditional delay systems in terms of implementation methods and meets the long delay requirements of electrical signals in electronic equipment such as radar, navigation, and communication.

In the design of radar and communication electronic equipment, it is often necessary to delay the electrical signal for a long time. It is difficult to achieve long delay with electrical delay lines due to material size limitations. Although, in recent years, surface acoustic wave delay lines have been able to replace cable delay lines in radar, communication and other electronic systems due to their simple structure and small size, they cannot meet the bandwidth requirements of complex modulated signals in radar and communication equipment due to their narrow frequency band and large temperature influence, and it is difficult to achieve stable long delay. Fiber optic transmission technology is a signal transmission delay technology that has been developed and widely used since the 1980s, and its application is now very mature. Because of its characteristics that signal transmission is not affected by the electromagnetic environment, wide frequency band, large delay range, and small temperature change rate, it has gradually become a more ideal choice for delaying signals in the RF and medium frequency bands.

1 Principle of fiber optic delay

The basic principle of fiber optic delay technology is to use the time delay caused by the transmission of optical signals through a certain length of optical fiber. The speed of light signals transmitted in quartz medium is relatively lower than the speed in vacuum. The refractive index of light in vacuum is 1, while the refractive index of light in optical fiber is about 1.47 (for the commonly used G.652 single-mode optical fiber, at a wavelength of 1.550 nm, n=1.467 is often used). The transmission delay formula of light signals in optical fiber is as follows: t=Lxn/v (1)

where t is the transmission time, L is the optical fiber length, n is the medium refractive index, and v is the speed of light in vacuum.

Fiber delay technology utilizes the characteristics of optical transmission and has high anti-interference ability; high bandwidth, the maximum bandwidth can reach 10 Gb/s; large delay range; small rate of change with temperature, the commonly used G.522 optical fiber delay temperature coefficient is about 0.05 ns (km.℃), which basically has no impact on the application.

2 Delay system design

A complete delay system includes the delay and power control functions of the electrical signal. The delay system based on optical fiber transmission includes an input attenuator, a delay optical path, an output attenuator, and a control module. The system block diagram is shown in Figure 1.

2.1 Design of power control module

The power control module of the delay system consists of an input attenuator and an output attenuator. In the delay optical path, the input power range of the direct-modulated laser for electro-optical conversion is small, not exceeding 15 dBm at most, and the performance is optimal at 0 dBm power. In radar and communication applications, the power of RF electrical signals is generally large, and a front coaxial attenuator is required to reduce the input power and control it within the input range of the direct-modulated laser and close to 0 dBm. The effect of adding an input attenuator at the front end of the delay system can also reduce the impact of input power on the delay accuracy of the optical fiber. Because the refractive index of all materials increases with the increase of light intensity, and the input attenuator is used to keep the power of the incident signal at 0 dBm, the impact of the nonlinear refractive index effect of the optical fiber on the delay accuracy can be avoided.

In the design of radar systems, the delay system is often required to simulate the propagation loss of electromagnetic wave signals in space. The formula is: W=30log (1/R) dB. W is the propagation loss, R is the electromagnetic wave propagation distance, and the unit is m. The loss of optical fiber transmission is about 0.2 dB/km. Therefore, at the output end of the delay system, an adjustable coaxial attenuator is required to realize the power control function. Since the coaxial attenuator is made of the same medium, the signal has the same distance after different attenuation, so the consistency of signal delay is better.

2.2 Control module design

The control module of the delay system mainly realizes the change of delay amount, the control of programmable attenuator, status indication and human-computer interaction (or upper computer communication). The typical circuit adopts LM89C51 single-chip microcomputer chip design to realize the above functions. The control module circuit principle block diagram and control program flow chart are shown in Figure 2.

This typical circuit can receive commands through the MAX232 serial communication interface or through the panel buttons. It uses the I/O port of the LM89C51 chip and the corresponding drive circuit to control the state of the optical switch to complete the switching of different analog distances. It changes the attenuation of the delay system by controlling the programmable attenuator, and uses a 0802A LCD to indicate the state of delay and attenuation.

2.3 Delay optical path design

The delay optical path is the core part of the delay system based on optical fiber transmission. The design must consider the impact of attenuation, dispersion, and temperature on system performance. In order to make the delay system adaptable to different applications, it is designed as a system with variable delay. As shown in Figure 3.

Taking the design requirements of 50 m (delay of 0.167 μs) as a step and a maximum delay distance of 16,500 m (delay of 55 μs) as an example, the development of a delay system in radar applications is as follows: To achieve a simulated distance with a step size of 50 m, the length of the minimum fiber ring corresponding to formula (1) is 68.2 m, as shown in Figure 3. The lengths of the fiber rings connected to each subsequent optical switch are 68.2x21 m, 68.2x22 m, 68.2x23 m, 68.2x24 m, 68.2x25 m, 68.2x26 m, 68.2x27 m and 68.2x28 m respectively; such a design can achieve a delay distance with a step size of 50 m and a range of 0-16,500 m, with a total of 512 delay distances to choose from. [page]

We use a 2x2 optical switch and a differential structure design to achieve switching of different delays. The average insertion loss of this type of optical switch is about 0.8 dB. In addition, due to the use of a differential structure, each delay path passes through the optical switch the same number of times, and the loss consistency is good. The commonly used G.652 optical fiber loss is about 0.2 dB/km. According to the above parameters, the loss of the entire optical path is determined by four parts: the electro-optical conversion efficiency of the direct-modulated laser, the optical fiber path loss, the electro-optical conversion efficiency of the photodetector, and the input-output impedance ratio; it can be expressed by formula (2):

ηTX is the electro-optical conversion efficiency of the direct-modulated laser, and the ηTX obtained according to the test is 0.075.ηRX is the photoelectric conversion efficiency of the photodetector, and the ηRX obtained according to the test is 0.65.Lop is the loss of the optical fiber path, including the following parts: the transmission loss of the optical fiber itself, the insertion loss of the optical switch and the loss of each optical connector; calculated based on the longest optical fiber length of 22,495 m, the maximum optical fiber transmission loss is 4.5 dB (the loss coefficient of the standard single-mode optical fiber is 0.2 dB/km); the insertion loss of each optical switch is 0.8 dB, and there are 10 optical switches in total, so the total insertion loss of the optical switch is 8 dB; the insertion loss of each optical connector is 0.2 dB; there are 20 optical connectors that the optical link needs to pass through, so the loss introduced by the optical connector is 4 dB; thus, the loss Lopt of the entire optical link is 4.5+8+4=16.5 dB.Rin and Rout are the input matching impedance and the output matching impedance, respectively, both of which are 50Ω. According to the above analysis, the gain GdB of the RF signal passing through the optical path calculated by formula (2) is -42.7 dB. The relationship between the delay attenuation LdB of the simulated radar echo signal and the simulated distance H satisfies formula (3):

Thus, for the minimum simulation distance of 50 m, the delay attenuation is -51 dB; for the maximum simulation distance of 16,500 m, the delay attenuation is -126.5 dB; the delay attenuation required for the simulation of radar echoes ranges from -51 to -126.5 dB. This system can meet the requirement of the maximum attenuation of -51 dB, and the final output RF signal attenuation can be adjusted between -48 and -129.5 dB through the coaxial adjustable attenuator.

For the delay system requiring smaller loss, we can add an optical amplifier in front of the photodetector. The input power of the optical amplifier is generally selected to be -25 to -10 dBm, and the optical path loss is 16.5 dB, which can fully meet the requirements and has a certain margin. In order to reduce the noise coefficient of the optical amplifier, an ASE filter can be added inside the amplifier to lock the output wavelength to the wavelength of the laser. When the system transmits analog signals, the output optical power of the optical amplifier should be kept above 0 dBm so that the optical receiver has a better demodulation effect. The output of the amplifier can be connected to an optical receiver or cascaded with the next-level device. In order to achieve smaller losses, an RF amplifier can be connected in series after the photodetector.

For the development of long-delay systems, the influence of dispersion also needs to be considered. The chromatic dispersion limited bandwidth of optical transmission can be calculated by the following formula (4), where Bc is the chromatic dispersion limited bandwidth, △λ (nm) is the spectral line width, C (λ) (ps/nm.km) is the optical fiber chromatic dispersion coefficient, and L (km) is the optical fiber length.

From formula (4), we can see that in order to reduce the impact, the laser spectrum (FWHM) is required to be as narrow as possible and the chromatic dispersion coefficient of the optical fiber is required to be as small as possible. At present, the FWHM of some lasers on the market reaches 10MHz (8x10-5nm). In terms of optical fiber selection, the commonly used G.652 optical fiber dispersion coefficient is about 20ps/nm.km. Based on this, it can be calculated that the chromatic dispersion limit bandwidth of an optical signal with a wavelength of 1550 nm transmitted on a G.652 optical fiber for 165 km is: Bc=0.44x106/△λ. C (A). L =0.44x106/8x10-5x20x165 (5)

=1.26x106 MHz Therefore, as long as a suitable laser is selected, the fiber dispersion will not affect the performance indicators of the system.

In the actual development of the delay system, we also need to consider the electrical signal delay caused by components such as optical-electrical conversion, electrical-optical conversion, and signal input and output attenuators. The system has a delay zero point H0 (tested to be less than 50 m). In the development of such a delay system, the zero point can be calibrated to 50 m by adjusting the fiber length between the direct-modulated laser and the 1x2 optical switch, and the remaining fiber lengths remain unchanged. After this adjustment, the zero point is used as the first delay distance (i.e. 50 m), and the subsequent simulation distances can meet the technical requirements of accurately simulating each integer distance point. In the specific development, it should also be noted that the delay of the differential structure of the 2x2 optical switch is the difference between the two paths. When cutting the optical fiber, the length of the optical fiber ring is L=L0+68.2 m, and L0 is the fiber length of the short-circuit path.

The transmission mode of the signal in the optical fiber is mainly determined by the difference in the incident angle of the ray, and the incident angle of the ray often changes due to the bending of the optical fiber, thereby changing the transmission mode of the ray. When the optical fiber is severely bent, the ray may even penetrate the optical fiber and cause energy loss. Generally speaking, the smaller the radius of the fiber bend, the greater the loss, and vice versa. When designing the fiber ring and fixing the fiber connector, the fiber bend radius should be increased as much as possible (generally not less than 3 cm).

3 System Verification

The pulse signal delay of this delay system was verified using the oscilloscope method (using the DP070604 oscilloscope from TEK in the United States). The test data is shown in Table 1.

A point of 1 000m was selected and tested 10 times. The repeatability of the system was tested. The data are as follows: 3 333.35 ns, 3 333.34 ns, 3 333.35ns, 3 333.35 ns, 3 333.34 ns, 3 333.35 ns, 3 333.34 ns, 3 333.34 ns, 3 333.34 ns, 3 333.35 ns, 3 333.34 ns. The repeatability of the browsing result can be obtained from the Bessel formula:

the repeatability of the corresponding simulation distance is 5x10-4m. It can be seen from the data in Table 1 that due to process technology reasons we cannot get the desired integer simulation distance every time, but from the repeatability test data, it can be seen that the delay system based on optical fiber transmission has the characteristics of high reliability and high stability.

4 End

In Chinese, the fiber optic transmission delay technology is used to realize the long delay system of complex debugging signals used in radar and communication through reasonable design. This delay system has the characteristics of high anti-interference and high reliability, large delay range, large bandwidth and high stability, which greatly reduces the cost and time of related tests, verification and simulation of radar and communication systems. With the rapid development of fiber optic technology and the gradual maturity of technology, the application of delay systems based on fiber optic transmission will become more and more extensive and practical.