The development of the information age requires the establishment of communication network systems with broader coverage, faster transmission speeds, and larger transmission capacities. As a widely promising next-generation space communication technology, space laser communication offers numerous advantages that traditional microwave communication cannot match:

（1）Large communication capacity: The frequency of laser is 3-4 orders of magnitude higher than that of microwaves, with a wider frequency band, enabling the transmission of massive data in a short time.

（2）High transmission rate and low power consumption: Laser communication can achieve speeds of 10 Gbit/s or even higher. During transmission, energy is concentrated and not easily dispersed, resulting in lower power consumption than microwaves.

（3）Strong anti-interference capability: Lasers have an extremely narrow beam divergence angle, making them less susceptible to interception and interference.

Against this backdrop, space communication systems using lasers as information carriers have garnered increasing attention. Space communication technologies have gradually evolved, triggering research booms in various countries and achieving significant progress.

Figure 1. Schematic Diagram of Satellite Laser Communication

Space laser communication consists of transmitting and receiving systems. The transmitting end amplifies the laser to sufficient power and emits modulated laser pulse signals (sound or data) toward the counterpart. The receiving end uses an optical antenna to amplify the collected optical signals and then detects the useful signals, as shown in the principle of Figure 1. Since it is free-space optical communication and relay amplification cannot be achieved in the communication link, in addition to ensuring a high modulation rate, a large transmitted optical power is also required. This creates an insurmountable bottleneck for semiconductor lasers in space communication systems. However, 1550nm fiber lasers, operating in the band of existing communication systems, can be amplified by erbium-doped fibers (EDF) and erbium-ytterbium co-doped fibers (EYDF), offering high output power and strong anti-interference capabilities. As a result, they have become the preferred laser light sources for space laser communication systems.

Space communication distances are generally very long, so laser signals require sufficient amplification by amplifiers to transmit optical signals over longer distances. By the time they reach the receiving end, the long-distance transmission weakens the signals significantly, requiring them to be amplified again for demodulation. Therefore, the emergence of erbium-doped fiber amplifiers (EDFAs) has made 1550nm space laser communication possible. With the continuous development of space communication technologies, erbium-doped and erbium-ytterbium co-doped fiber amplifiers have garnered widespread attention for their optical properties of wide bandwidth and low loss, as well as advantages like light weight. They have been successfully deployed on satellites to undertake tasks such as space communication and data transmission, currently achieving data transmission rates on the order of TB/s, making them critical core components for 1550nm space laser communication.

Additionally, satellite space communication systems mostly operate in space, where massive radiation particles severely affect the lifespan of components. Thus, erbium-doped and erbium-ytterbium co-doped amplifiers must possess radiation resistance, which heavily relies on radiation-resistant erbium-doped fibers and erbium-ytterbium co-doped fibers. Domestic dependence on imported radiation-resistant erbium-doped fibers and erbium-ytterbium co-doped fibers has severely restricted the development of China's space laser communication industry.

Radiation-Resistant Erbium-Doped Fiber

Compared with passive optical fibers, erbium-doped fibers (EDF) are more sensitive to irradiation. In radiation environments such as space and high-energy physics facilities, they are easily affected by irradiation, causing degradation of optical performance indicators and ultimately impacting the normal operation of optical devices. Traditional physical shielding methods can isolate high-energy rays, but they suffer from issues of excessive weight and large volume, making them unsuitable for modern space precision equipment. Therefore, enhancing the intrinsic radiation resistance of active optical fibers is an effective approach to solving the challenges of using active fibers in space.

Current research indicates that there are two main mechanisms for defect generation in fibers under irradiation: ionization damage and displacement damage. Ionization damage occurs when electrons in the fiber absorb energy from incident particles during irradiation and transition to the conduction band, simultaneously generating corresponding holes in the valence band (electron-hole pairs). Displacement damage happens when particles introduced into the fiber matrix by irradiation have sufficiently high energy to cause Si or O atoms to displace, forming related defects. Since displacement damage requires higher energy than ionization damage, ionization damage is the primary mechanism causing irradiation damage in active fibers. After irradiation, fibers experience irradiation-induced loss, which reduces system gain and increases background loss. These effects can render optical systems unstable or even inoperable.

Radiation-Resistant Erbium-Doped Fiber Technology

Changjin Laser employs cerium ion doping technology to eliminate color centers generated by irradiation, enhances the radiation resistance of optical fibers by regulating the doping concentrations and ratios of aluminum and cerium, and uses lanthanum ions to reduce erbium ion clustering effects and improve optical fiber efficiency. Finally, erbium-doped fibers with excellent radiation resistance are prepared, in which the doping concentration of Er³⁺ is approximately 1.9×10²⁵ (Ions/m³). The core and cladding of the prepared optical fiber have diameters of 9μm and 125μm, respectively. Considering that the irradiation dose in the real space environment is in the order of 10²–10⁵ Gy, an irradiation dose of 1500 Gy and a dose rate of 0.2 Gy/s were selected to test the radiation-induced loss and radiation-induced gain changes of the radiation-resistant erbium-doped fiber.

Figure 2. Cross-Section of Radiation-Resistant Erbium-Doped Fiber

Figure 3. Absorption Spectra and Corresponding Radiation-Induced Loss Spectra of C-Band Radiation-Resistant Erbium-Doped Fiber Before and After 1500Gy Irradiation。

Figure 3(a) shows the pump band, and Figure 3(b) shows the signal band. The absorption spectra were tested using the segment method with a PHOTON KINETICS 2500 absorption spectrometer. The radiation-induced loss spectra were obtained by subtracting the pre-irradiation absorption spectrum from the post-irradiation spectrum. The radiation-induced loss is approximately 1.4 dB/m at the common pump absorption band of 980 nm and 0.8 dB/m at the C-band signal wavelength of 1550 nm.

Figure 3. Original Fiber Absorption Spectrum, Irradiated Fiber Absorption Spectrum, and Corresponding Radiation-Induced Loss of Radiation-Resistant Erbium-Doped Fiber
(a) Pump band; (b) Signal band

Figure 4. Gain Test Structure Diagram for C-Band Radiation-Resistant Erbium-Doped Fiber.The test setup employs a typical forward-pumped EDFA configuration, where WDM (Wavelength Division Multiplexing) denotes the wavelength division multiplexer and ISO (Isolator) represents the optical isolator. The signal source is a tunable C-band light source, while the pump source is a high-stability 980nm laser. Both the original and irradiated erbium-doped fibers were tested at a fixed length of 2.7 meters.During the experiment, the original and irradiated fibers were separately integrated into the gain test system. A 1550nm signal with a power of -20dBm was injected, and the pump power was gradually increased from 50mW to 550mW. The gain performance of the erbium-doped fiber was recorded using a Yokogawa AQ6370D optical spectrum analyzer.This setup allows for direct comparison of the fiber's amplification capabilities before and after irradiation, providing critical insights into its radiation resistance characteristics under realistic operating conditions.

Figure 4. Gain Test System and Fiber Cross-Section of C-Band Radiation-Resistant Erbium-Doped Fiber

Figure 5. Gain vs. Pump Power Curves of C-Band Radiation-Resistant Erbium-Doped Fiber Before and After Irradiation.The original fiber exhibits a gain of 27 dB at a pump power of 100 mW, while the irradiated fiber (1500 Gy) shows a gain of 26.2 dB under the same conditions. The resulting radiation-induced gain change is only 0.8 dB. As the pump power increases, both fibers approach gain saturation. Due to pump bleaching effects, at a pump power of 500 mW, the original fiber achieves a gain of 36.8 dB and the irradiated fiber 36.6 dB, reducing the radiation-induced gain change to 0.2 dB.These results demonstrate the fiber's exceptional radiation resistance, with minimal degradation in amplification performance even after high-dose irradiation, making it well-suited for space-based optical communication systems. 

Figure 5. Gain vs. Pump Power Curves at 1550nm for Radiation-Resistant Erbium-Doped Fiber Before and After Irradiation

After being irradiated with a dose of 1500 Gy and an average dose rate of 0.2 Gy/s, the radiation-induced attenuation (RIA) of the radiation-resistant erbium-doped fiber prepared by MCVD technology is 1.4 dB/m at 980 nm and 0.8 dB/m at 1550 nm. Through EDFA-based tests on radiation-induced gain variation (RIGV), the results show that at 1550 nm, the RIGV values are 0.8 dB at 100 mW pump power and 0.2 dB at 500 mW pump power. This erbium-doped fiber exhibits superior radiation resistance and has broad application prospects in fields such as satellite communication, data acquisition, and space exploration.

With the increasingly widespread use of active fibers in irradiated environments, many domestic optical fiber manufacturers are now conducting in-depth research on radiation-resistant fibers. Through years of technical accumulation and innovative R&D, Wuhan Changjin Laser Technology Co., Ltd. (Changjin Laser) has successfully developed high-performance radiation-resistant erbium-doped fibers and radiation-resistant erbium-ytterbium co-doped fibers by altering fiber doping elements and structures during fiber preparation, winning recognition from customers. As science and technology continue to advance, the applications of radiation-resistant fibers will surely become more extensive. Changjin Laser will continue to increase R&D efforts in the field of radiation-resistant fibers to provide high-performance radiation-resistant active fibers for more potential customers.