Suppression of stimulated Brillouin scattering by

2022-07-25
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Tunable laser source can suppress stimulated Brillouin scattering

in order to achieve greater distance and higher rate transmission, the optical emission power of modern transmission systems is increasing. Therefore, people have to consider nonlinear effects, especially stimulated Brillouin scattering (SBS), and system designers also need to balance the power distribution requirements with the signal loss caused by SBS and other nonlinear effects

in addition to suppressing the SBS and other requirements of the transmitter, we also need a test light source with large output power and eliminate the backscatter caused by SBS. In this paper, the mechanism and conditions of SBS effect will be discussed, and the latest research results of a new adjustable laser source for testing and measurement will be described in detail, so as to explain how to use a new method that can not only eliminate SBS, but also keep the relative intensity noise (RIN) of the signal at a low level, so as to effectively couple the high-power laser to the longer optical fiber

SBS in a long fiber

if the maximum critical power is exceeded when emitting laser into a long fiber, strong reflection may occur due to the linewidth and fiber type, resulting in the power observed at the other end of the fiber reaching the maximum limit. Obviously, this phenomenon will limit the transmission power and cause signal noise. This phenomenon originates from the backscattering of signal light by sound waves in optical fibers. This phenomenon can also occur in shorter fibers, but to a much lesser extent. The scattered light will produce a shift equal to the Brillouin scattering drift frequency and become a lower optical frequency (longer wavelength), which is an inherent characteristic of optical fiber materials. The drift frequency of common single-mode quartz fiber is about 11GHz (wavelength 0.09nm). If the frequency difference between the forward and backward transmitted light in the fiber is exactly equal to the Brillouin scattering drift frequency, the back scattering will cause more forward transmitted light signals to be backscattered. Therefore, if the signal power is large enough, the backscattered light power caused by the stimulated backscattering may exceed the power lost due to fiber attenuation

with the device shown in Figure 1, SBS of a certain system (specific fiber type, fiber length and laser linewidth) can be detected. The laser output end is connected to the optical fiber through a coupler. Due to connection, Rayleigh scattering, SBS scattering and other reasons, there will be backscattered light, and the coupler will distribute 1% of which to the power meter. The output end of the long optical fiber is connected to another power meter used to measure the transmission power. The measurement results of both power meters depend on the transmission power

when the input power is greater than the SBS critical value, the scattering power increases nonlinearly and the transmission power tends to saturation. At the same time, the measured power value also fluctuates strongly. In Figure 2, the transmission power and scattering power of a single-mode fiber with a length of 25km are shown. The length of optical fiber with SBS is usually at least several kilometers, but its value also varies according to the type of optical fiber

a new method to eliminate SBS

in the test and measurement, the power of the adjustable laser has reached +13dbm, which has exceeded the SBS critical value of many optical fibers. If the effective linewidth increases relative to the SBS linewidth (about 20MHz), the critical power value can be increased to suppress SBS. Recently, a SBS elimination method for external cavity tunable lasers has been found. This new feature improves the effective linewidth by modulating the laser wavelength, thus raising the SBS critical power to a higher level. Because this method allows signal input at maximum power at the optical fiber connection, it has all the advantages of high output power, even in long optical fibers such as transmission systems for testing. Other examples include power distribution in the case of transmission distance amplification or testing the Raman amplifier configuration

this new SBS elimination method increases the effective wavelength of the laser by piezoelectric modulation of the length of the resonator, and its frequency is several kilohertz. The modulation frequency is very high, so that in the optical fiber with a length greater than 10km, the wavelength transmitted by different parts of the optical fiber can be guaranteed to be different, the SBS with the same wavelength can be prevented from being generated in the whole optical fiber, and the composite stimulated effect (as if the optical fiber is shorter than the actual length) can be reduced, so as to improve the critical power value of SBS

reduce the relative intensity noise

this new SBS elimination method keeps the power at a constant value so that no additional Rin is generated. The residual am is also very small, and its value is far less than 1%, so that time-domain measurements such as eye diagram test and bit error rate test can be carried out without being affected by SBS elimination work a energy. At the output end of the optical fiber, the value of RIN can be measured with a signal analyzer. Figure 4 shows the signal Rin value measured at the 25km long optical fiber output terminal when the signal input power is +13dbm and SBS is not eliminated; After the new SBS elimination function is connected to the system, the signal quality is improved, and its Rin value is shown in Figure 5

in a word, SBS is a physical effect. If the optical fiber is long and the input power is greater than a few DBM, it may occur. It will cause the sound wave in the optical fiber to scatter the photons, which will limit the transmission power and cause additional Rin, thus affecting the performance of the optical fiber network. Applying some new technologies to the tunable laser source can eliminate SBS under high-power conditions, and its residual amplitude modulation is small, so the rin of the signal is relatively small - thus the wavelength related high-power test can be carried out on the optical fiber link. This is helpful for network equipment manufacturers to design their transmission systems and power material manufacturers to pay special attention to the optimization of distribution, so as to develop transmission networks with high cost performance. (end)

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