Abstract
1. Introduction
Nonlinear optical limiting (NOL) technology is to allow weak light to pass through with high transmission while blocking strong light with low transmission and thus can prevent the human eyes and sensitive optical sensors from being irreversibly damaged by high-intensity lasers[
In this Letter, we innovatively propose an infrared broadband NOL technology based on stimulated Brillouin scattering (SBS) in fiber. The fibers have low-loss from 3 to 8 µm (except for 4.5–4.8 µm), which ensures high transmission under weak light in a wide range of wavelengths[
2. Principle
As is similar in other types of optical fibers, the SBS in fiber is a nonlinear optical phenomenon caused by the interaction of optical and acoustic waves in the optical fiber[
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For applications involving SBS, it is hoped that the medium has large Brillouin gain coefficient and long effective interaction length to stimulate the SBS effect easily, which can be clearly seen from Eq. (6). Therefore, in this Letter, the fibers are selected as NOL media due to not only providing a long effective interaction length , but also a high Brillouin gain coefficient , which is reported to be [
Figure 1.Schematic diagram of the experimental setup.
3. Experimental Setup
The experimental setup used to evaluate the NOL performance of fibers is shown in Fig. 1. A solid-state pulsed laser with a wavelength of 3.6 µm was used as the light source for the experiment. The pulse frequency and pulse width of the laser were set to 1 Hz and 10 ns, respectively. An aperture with a diameter of 1 mm was added to ensure that the output laser is the fundamental mode. Two neutral density filters were used to prevent the energy meter and the photodetector from being damaged by the laser, respectively. The laser beam is divided into two beams by the beam splitter, and the energy meter was used to record the pulse energy of the laser. The fiber port was fixed on a three-dimensional (3D) translation stage in order to couple the laser into the fiber as much as possible. The fiber used in our experiment was commercial fiber (IRflex Corporation) with a core diameter and cladding diameter of 100 µm and 170 µm, respectively. According to Eq. (5), combined with the finite element analysis method, the average effective mode area is calculated to be . We evaluated the NOL performance of fibers with lengths of 1 m and 0.5 m, respectively. The intensity of the transmitted laser was detected by a photodetector, and the transmitted pulse was displayed on an oscilloscope. During the experiment, we set the laser current to control the intensity of the output laser. In each measurement, 20 data points were recorded and averaged to reduce measurement uncertainty.
4. Results and Discussion
The NOL experimental results of 1 m and 0.5 m fibers are shown in Fig. 2. The result of the output optical power density increasing with the incident optical power density is shown in Fig. 2(a). It can be seen from the Fig. 2(a) that the output power density increases linearly with the increase of the incident power density when the incident laser is weak. When the incident optical power densities exceed the SBS threshold, which are, respectively, and for 1 m and 0.5 m fibers, the output power density increases more and more slowly until there is no significant change. Finally, the output power density of the fibers with lengths of 1 m and 0.5 m is stable around and , respectively, which shows a characteristic that an ideal NOL technology should possess[
Figure 2.NOL experimental results of As2Se3 fibers with the length of 1 m and 0.5 m. (a) Output power density increases with the incident power density. (b) Normalized transmission decreases with the incident intensity.
As shown in Fig. 2(b), when the incident laser is weak, the transmission remains high and almost unchanged so that the weak signal light can be transmitted with the light power loss of 4.8 dB/m at the wavelength of 3.6 µm. However, the transmission decreases rapidly with increasing incident power density and eventually drops to 0.89% and 1.23% at the lowest level, respectively. The limiting threshold is defined as the incident power density where the transmission falls to 50% of the normalized linear transmission[
As shown in Fig. 3, the output pulses of the 1 m fiber under different incident power densities can be clearly observed on the oscilloscope after the output pulses are detected by the photodetector. As illustrated in Fig. 3(a), the transmitted laser intensity increases linearly when the incident power density is too weak to excite the SBS effect, and thus the shape of the transmitted pulse is similar to that of the incident pulse. As the incident power density continues to increase beyond the SBS threshold (), the incident laser beam will be scattered backward as the result of stimulated scattering processes. Therefore, the back edge of the transmitted pulse starts to drop abruptly, and the tail of pulse forms a “platform”[
Figure 3.Transmitted pulses under low and high incident power densities. (a) Transmitted pulses with the low incident intensity of 3.9 MW/cm2, 6.4 MW/cm2, and 8.3 MW/cm2, respectively. (b) Transmitted pulses after the SBS effect occurs with the high incident intensity of 39.9 MW/cm2, 550.0 MW/cm2, and 1163.1 MW/cm2, respectively.
As shown in Table 1, we list some NOL performances of 1 m and 0.5 m fibers and compare several infrared broadband NOL technologies based on other principles in recent years[
Limiting Principle | [Tmax, Tmin] (at wavelength) | Limiting Threshold ( | Laser Induced Damage Threshold ( | Wavelength Range | |
---|---|---|---|---|---|
1 m | SBS | [90.36%, 0.89%](@3.6 µm) | 22.7 | 3–8 µm | |
0.5 m | SBS | [95.06%, 1.23%](@3.6 µm) | 28.4 | 3–8 µm | |
GO in NMP[ | RSA | [81%, 42%](@1750 nm) | 62.5 | – | 400–1800 nm |
GO Ormosil glasses[ | NA | [40%, 18%](@532 nm) | 22.5 | 532–1570 nm | |
GST phase change material[ | Phase change | [80%, 0.02%](@1500 nm) | – | – | 1250–2000 nm |
Table 1. Comparison of Different Infrared Broadband NOL Technologies
5. Conclusion
In summary, we propose an infrared broadband NOL technology based on the SBS effect in fiber. The experimental results show that this NOL technology has excellent limiting performance for a pulsed laser with a typical infrared wavelength of 3.6 µm. The linear transmissions of 1 m and 0.5 m fibers are higher than 90%, and the lowest nonlinear transmissions are reduced to 0.89% and 1.23%, respectively. The advantage of SBS-based NOL is that most of the energy is transferred to backscattering instead of absorption, avoiding thermal effects on the material and thus avoiding the bad effect on NOL performance. In addition, the SBS-based technology has broadband excellent NOL performance because SBS is not sensitive to wavelength. The disadvantage may be that a relatively large medium length (meter scale) is required to obtain large SBS backscattering. Fortunately, optical fibers can provide a large SBS interaction length within a small volume to achieve excellent NOL performance. This proposed technology exhibits excellent NOL performance in a wide wavelength range, high laser induced damage threshold, and fast response speed, which may have important application prospects in the field of infrared broadband laser protection.
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