Working Principle of Fiber Amplifier and Its Application in Wireless Optical Communication

0 Introduction

Wireless optical communication uses laser as information carrier and is a communication method that does not require any wired channel as a transmission medium. Compared with microwave communication, the laser used in wireless optical communication has high frequency, strong directionality (good confidentiality), wide available spectrum, and no need to apply for frequency use license; compared with optical fiber communication, wireless optical communication has low cost, simple and fast construction. It combines the advantages of optical fiber communication and microwave communication, and has become an emerging broadband wireless access method, which has attracted widespread attention. However, severe weather conditions will cause attenuation of the propagation signal of the wireless optical communication system. Scattering particles in the air will cause different degrees of deviation of light in space, time and angle. Particles in the atmosphere may also absorb the energy of the laser, causing the power of the signal to attenuate. In the wireless optical communication system, the low-loss propagation path of the optical fiber communication system no longer exists. The objective nature of the changeable atmospheric environment cannot be changed. In order to obtain better and faster transmission effects, higher requirements are put forward for optical signals transmitted in the atmospheric channel. Generally, the use of high-power optical signals can obtain better transmission effects. With the rapid development of optical fiber amplifiers (EDFA), stable and reliable high-power light sources will meet the requirements of wireless optical communication in various applications.

1 Principle and structure of EDFA

Erbium-doped fiber amplifier (EDFA) has the advantages of high gain, low noise, wide bandwidth, high output power, low connection loss and polarization insensitivity. It directly amplifies optical signals without converting them into electrical signals, and can ensure that optical signals are stably amplified with minimal distortion.

1.1 Principle of EDFA

The pumping process of EDFA requires the use of a three-level system, as shown in Figure 1.

By injecting a sufficiently strong pump light into the erbium-doped fiber, most of the Er3+ ions in the ground state can be pumped to the excited state, and the Er3+ ions in the excited state quickly transfer to the metastable state without radiation. Since the Er3+ ions have a long lifetime in the metastable energy level, it is easy to form a population inversion between the metastable state and the ground state. When the signal photon passes through the erbium-doped fiber, it interacts with the Er3+ ions in the metastable state to produce a stimulated radiation effect, generating a large number of photons that are exactly the same as itself. At this time, the signal photons transmitted through the erbium-doped fiber increase rapidly, resulting in a signal amplification effect. When the Er3+ ions are in the metastable state, in addition to stimulated radiation and stimulated absorption, they also generate spontaneous emission (ASE), which causes noise in the EDFA.

1.2 Structure of EDFA

The typical EDFA structure mainly consists of erbium-doped fiber (EDF), pump light source, coupler, isolator, etc.

Erbium-doped fiber is the core component of EDFA. It uses quartz fiber as the matrix, and erbium ions, a solid laser working material, are doped in the fiber core. In the erbium-doped fiber of several meters to tens of meters, light interacts with matter and is amplified and enhanced. The function of the optical isolator is to suppress light reflection to ensure the stable operation of the amplifier. It must have low insertion loss, be independent of polarization, and have an isolation better than 40 dB.

FIG2 shows a unidirectional pumping structure. In addition, there are reverse pumping and bidirectional pumping structures.

1.3 Characteristics and performance indicators of EDFA

The gain characteristic indicates the amplification capability of the amplifier and is defined as the ratio of output power to input power:

Where: Pout, Pin represent the continuous signal power at the output and input of the amplifier respectively. The gain coefficient refers to the gain obtained by the fiber amplifier when 1 mW pump light power is input from the pump light source, and its unit is dB/mW:

Where: g0 is the small signal gain coefficient determined by the pump intensity. Due to the gain saturation phenomenon, the gain coefficient decreases with the increase of signal power; Is and Ps are the saturated light intensity and saturated light power, respectively, which are quantities that indicate the characteristics of the gain material and are related to the doping coefficient, fluorescence time and transition cross section.

The difference between gain and gain coefficient is that gain is mainly for input signal, while gain coefficient is mainly for input pump light. In addition, gain is also related to pump conditions (including pump power and pump wavelength). The main pump wavelengths currently used are 980 nm and 1 480 nm. Since the gain coefficient is different at different locations, and the gain must be integrated over the entire fiber, this characteristic can be used to obtain a relatively flat gain spectrum by selecting the fiber length.

1.4 EDFA Bandwidth

Gain spectrum bandwidth refers to the wavelength region where the signal light can obtain a certain gain amplification. The actual gain-frequency variation relationship of EDFA is much more complicated than the theoretical one, and it is also related to the matrix fiber and its doping. The gain spectrum width of EDFA has reached hundreds of nanometers. And the gain spectrum is relatively flat. The gain spectrum range of ED-FA is between 1525 and 1565 nm.

2 EDFA Cascade Application

2.1 Cascade structure of EDFA

EDFA often uses a cascade method, such as two-stage or three-stage amplification, to amplify the power of optical signals, especially in high-power (watt-level) applications of wireless optical communications. The reason for using the cascade method is that an optical isolator is inserted into the erbium-doped fiber (EDF) of the EDFA to form a two-stage cascade EDFA with an optical isolator. Since the optical isolator effectively suppresses the reverse spontaneous emission (ASE) of the second stage: EDF, it cannot enter the first stage EDF, reducing the consumption of pump power on reverse ASE, making the pump photons more effectively converted into signal light energy, thereby significantly improving the gain, noise figure and output power characteristics of the EDFA. This paper adopts a multi-stage cascade amplification to amplify the 1-2 mW 1550 nm optical signal to about 1 W through EDFA. The cascade structure is shown in Figure 3.

The optical signal is generated by LD laser and is a modulated signal. The first stage of amplification uses a single-clad erbium-doped fiber amplifier and a 980 nm single-mode semiconductor laser as a pump source to amplify the optical power to around 50 mW. The first stage uses a single-mode semiconductor laser pump to stably and reliably amplify the optical signal to a certain power, ensuring the integrity of the entire optical signal and providing a higher optical power foundation for the next stage of optical amplification. The second stage uses a double-clad fiber amplifier and a multi-mode semiconductor laser pump source to amplify the optical power to about 1 W. The core of the double-clad fiber amplifier is larger than that of the single-clad fiber, and the pump power can be effectively coupled into the core, so that the output power of the second-stage optical signal can reach the watt level.

2.2 Gain of EDFA cascade application

2.2.1 Gain calculation

The overall optical power gain of the EDFA cascade is:

Where: Pout represents the output optical power after two-stage amplification of EDFA, and Pin represents the input optical power to be amplified.

In this paper, optical amplification adopts two-stage cascade amplification, and the gain of the first stage is G1:

The output of the first stage is the input of the second stage, P'out = P'in = P, so:

That is, the overall gain is equal to the sum of the two-stage gains. The overall optical power gain of this article is:

The first-stage gain is 17 dB, and the second-stage gain is 13 dB. The 1 W optical power is collimated and focused, and then emitted into the atmospheric channel by an optical lens, which greatly improves the effective transmission distance of the optical signal.

2.2.2 Factors affecting gain

The gain of EDFA is related to many factors, such as the length of erbium-doped fiber. As the length of erbium-doped fiber increases, the gain goes through a process from increasing to decreasing. This is because as the length of the fiber increases, the pump power in the fiber will decrease, causing the number of particle inversions to decrease. Ultimately, the number of erbium ions at the low energy level is greater than the number of erbium ions at the high energy level, and the number of particles returns to normal.

Due to the loss of the erbium-doped fiber itself, the number of photons absorbed in the signal light is greater than the number of photons generated by stimulated radiation, causing the gain to decrease. From the above discussion, it can be seen that for a certain incident pump power, there is an optimal length of erbium-doped fiber that maximizes the gain. The relationship between the gain and the length of the erbium-doped fiber is shown in Figure 4.

The gain of EDFA is also related to the input light level, pump light power and the concentration of erbium ions Er3+ in the optical fiber. For example, the gain coefficient when a small signal is input is greater than the gain coefficient when a large signal is input. When the input light is weak, the consumption of high-energy electrons is reduced and can be fully supplied from the pump, so the stimulated radiation can be maintained to a considerable extent. When the input light becomes stronger, due to the insufficient supply of high-energy electrons, the increase in stimulated radiation light becomes less, so saturation occurs. The greater the pump light power and the longer the erbium-doped fiber, the greater the 3 dB saturated output power. Secondly, when the concentration of Er3+ exceeds a certain value, the gain will decrease, so the erbium ion concentration of the erbium-doped fiber must be controlled.

After using EDFA, the power injected into the optical fiber is increased, but when it reaches a certain value, it will produce optical fiber nonlinear effects and light leakage effects, which affect the transmission distance and transmission quality of the system. In addition, the dispersion problem has become a prominent problem that limits the system. G653 optical fiber (dispersion-shifted fiber DSF) or non-zero dispersion fiber (NZDF) can be used to solve this problem.

2.3 Improvement of EDFA Cascade

The reason for using the EDFA cascade is that, firstly, the optical isolator inserted between the two stages effectively suppresses the reverse spontaneous emission (ASE) of the second EDF, preventing it from entering the first EDF, reducing the consumption of pump power on reverse ASE, and making the pump photons more effectively converted into signal light energy; secondly, after being divided into two stages, the gains of each can be arbitrarily allocated, and the corresponding gains can be changed according to different gain requirements and application environments. However, in order to obtain the best optical power gain while ensuring that the signal is not distorted, some problems need to be solved:

(1) Since the gain is divided into two levels, how to allocate the gain between the two levels can make the realization of optical amplification easier under the existing EDF, pump source power and other conditions. This is closely related to the amplification capacity of the EDF, the size and stability of the pump power, the wavelength of the pump light and its mode, etc.

(2) Under a certain pump power at each stage, find the optimal length of the erbium-doped fiber. When the EDF is too short, the gain is reduced due to insufficient pump absorption; when the EDF is too long, the pump power gradually decreases because the pump light is absorbed by erbium ions in the EDF. When the power drops below the pump threshold, the population inversion cannot be formed. At this time, this part of the EDF not only has no amplification effect on the signal light, but also absorbs part of the amplified signal, resulting in a decrease in gain and an increase in the noise coefficient.

(3) If higher optical power output is required, tens of watts or even hundreds of watts, a higher-level cascade method can be considered. This is because as the gain increases, the power demand of the pump source will be very high due to conversion efficiency issues, and the required single-stage EDF length will also increase significantly. Such working conditions are often difficult to achieve and are not very stable. Using a higher-level cascade can divide the gain into multiple levels, which is easy to implement and control, and the overall gain characteristics of the optical module are also greatly improved.

3 Conclusion

This paper proposes a method of using EDFA cascade to achieve a 30dB gain of optical signals, meeting the requirements of optical power propagation in wireless optical communications, and enabling optical signals to be transmitted over long distances and with high stability in atmospheric channels. At the same time, based on the existing foundation, issues that need to be improved are proposed, which points out the direction for further research in the future.