A Novel Nonlinear Optical Limiter Based on Stimulated Brillouin Scattering in Highly-Nonlinear Fiber

Abstract

A novel nonlinear optical limiter (NOL) based on stimulated Brillouin scattering (SBS) in highly nonlinear fiber was proposed and experimentally demonstrated at 1550 nm wavelength. The nonlinear optical limiting effects of HNLF were characterized and demonstrated theoretically and experimentally. In a proof-of-concept experiment, we verified that the NOL based on a 50 m HNLF has excellent limiting performance due to its small effective area and high Brillouin gain coefficient. The linear transmittance and lowest nonlinear transmittance of the NOL were 87.5% and 11.9%, respectively.

1. Introduction

Silica glass itself is a low non-linearity material, but silica-based highly nonlinear fibers (HNLFs) have been developed to for use in applications which require high-fiber nonlinearities [1]. The HNLFs have the advantages of high nonlinear coefficients, low splice loss, as well as relative low loss [2]. In earlier years, HNLFs were mostly used in optical wavelength conversion [3], optical signal processing [4], and optical demultiplexers [5]. The application fields can also be extended to supercontinuum light sources [6] and fiber optic sensors [7,8,9,10,11].

A nonlinear optical limiter (NOL) is a device that allows weak light to pass through with high transmittance, while effectively suppressing the high-intensity light [12]. With the development of high-power laser technology, nonlinear optical limiters are in great demand in optical fiber transmission systems, such as a fiber laser-based optical source system [13,14] or a quantum key distribution communication system [15]. The NOL also has great potential applications in all-fiber imaging systems [16] and in low-loss high-power combiners [17] for protecting detectors at the near-infrared (NIR) window. However, existing NOLs are losing their advantages regarding their complex fabrication processes, relatively large connection loss, and sometimes expensive costs. Furthermore, there is no all-fiber NOL in fiber-transmission systems with the wavelength of 1550 nm, which is the commonly used wavelength for optical fiber communication and sensing systems.

In this paper, we proposed a novel all-fiber NOL at 1550 nm wavelength based on stimulated Brillouin scattering in a HNLF. Due to the small fiber effective area and the relative low absorption coefficient of the HNLF, the proposed HNLF-based NOL has excellent performance with high transmittance in weak incident light and low transmittance in strong incident light. As the limiter is a passive device only using HNLF, it also has the advantages of simple structure, light weight, small size, low price, and good stability. In a proof-of-concept experiment, a 50 m HNLF was used as the all-fiber NOL, and the limiting capacity coefficient of the new NOL reached 0.76 and the minimum transmittance reached 11.9%. We also measured the transmittances of 50 m and 300 m G655 fibers under the same conditions and compared the results with those of the 50 m HNLF. We found that the optical limiting effect of 50 m HNLF was better than that of 50 m and 300 m G655 fibers. These experimental results showed that the HNLF has relatively excellent optical limiting ability at the wavelength of 1550 nm, and can be used as a new kind of NOL and optical protection device in fiber optic communication and sensing systems at the wavelength of 1550 nm.

2. Principles

When the incident laser is weak, most of the input power passes through the fiber. The transmittance of the fiber can be named as linear transmittance, which is mainly determined by the material absorption and Rayleigh scattering in the fiber. As the intensity of the incident laser gradually increases beyond the threshold of stimulated Brillouin scattering (SBS), most of the input power will be transferred to the backward propagating Stokes wave [18], resulting in the low transmittance of the fiber. Therefore, the fiber can be used as nonlinear optical limiting medium to allow the weak light passing through with high transmittance, while effectively suppressing the high-intensity incident light by the SBS effect.

The SBS reflectivity R is defined as the ratio of the Stokes output intensity to the input laser intensity, which can be expressed as [19]

$$\frac{G}{G_{th}}=\frac{G_{th}^{-1}\ln R+1}{1-R}$$

(1)

This reflectivity occurs for the specific value G_th of the gain parameter $G=g_{B}I{L}_{eff}$, where $g_B$ is the Brillouin gain coefficient of the fiber and I is the incident laser intensity defined as $I=P_{in}/A_{eff}$, where P_in denotes the incident power and A_eff is the optical effective area of the fiber. The L_eff is the effective interaction length and can be expressed as $L_{eff}=\left(1-e^{-\alpha L}\right)/\alpha$ and L is the length of optical fiber and α is the loss coefficient of the fiber.

Laboratory experience has shown that the reflectivity R rises rapidly for incident light power slightly above the SBS threshold power and that essentially $G_{th}\approx 21$ in optical fibers [20]. When the incident light power further increases, the value of $G_{th}{}^{-1}\ln R$ is gradually approaches 0, and thus Equation (1) can be approximated as

$$\frac{G}{G_{th}}\simeq \frac{1}{1-R}$$

(2)

Because the intensity of Rayleigh scattering compared with that of SBS can be ignored when SBS occurs, the transmittance of fiber T can be approximately expressed as $T=1-\alpha -R$, where $\alpha$ is the absorption coefficient of the fiber. Thus the Equation (2) can be rewritten as

$$T=\frac{G_{th}}{P_{in}{g}_B{L}_{eff}}·A_{eff}-\alpha$$

(3)

From Equation (3), we can see that, for optical fibers of the same length, the smaller the effective area A_eff and the larger Brillouin gain coefficient $g_B$, the better the limiting effect of the fiber on the strong incident laser and the lower its light transmittance T. Due to its small effective area and large Brillouin gain coefficient, the HNLF can be one of the best candidates for nonlinear limiting media. Please note that this SBS-based NOL have no effect on the spectrum of the incident light propagating forward along the fiber.

As it shown in

Table 1. Main parameters of three fibers.

	HNLF	G655 [21]	G652 [22]
$\alpha$ (dB/km)	1.5	0.22	0.2
$A_{eff}$ @1550 nm (μm2)	11 [1]	80	60
$g_B$ (m/W)	7.19 × 10−11	6.5 × 10−11	2.0 × 10−11
$v_B$ (GHz)	9.4 [1]	10.64	10.8

, different from the single-mode fibers (SMFs) such as G655 fibers and G652 fibers, the optical effective area of HNLF is as small as 8.5~11 μm² [2], which is about 1/6 of those SMFs at the wavelength of 1550 nm. Although the HNLF has higher loss than other SMFs, due to the limited length of the optical fiber used as the limiter, the HNLF can have a high light transmittance T when weak light is incident. Therefore, the HNLF is expected to be used as an excellent nonlinear limiting medium.

Assuming the fiber length is 50 m and using parameters in Table 1, we obtain the numerical results of transmittances of HNLF and G655 and G652 fibers by utilizing Equation (3). When the incident power injected into the fibers increased gradually to 3 W, the corresponding transmittances decreased gradually, and the results are shown in Figure 1. In order to accurately evaluate the nonlinear optical limiting ability of materials, we defined the optical limiting capacity coefficient δ as [23]

$$\delta =T_{max}-T_{min}$$

(4)

From Figure 1, when the incident power was 3 W, the coefficient δ for HNLF and G655 and G652 fibers were 0.99, 0.83, and 0.60, respectively. Therefore, the optical limiting ability of HNLF was much better than that of the two SMFs.

In Figure 2, we show the transmittances of HNLFs with three different lengths. As shown in Figure 2, the minimum transmittance T_min of HNLF with lengths of 10 m, 50 m, and 100 m were respectively 10.6%, 2.2%, and 1.1%, which were decreasing with the increase of fiber length. The linear transmittance T_max of the three different length HNLFs were 99.6%, 98.3%, and 96.6%, respectively. The values of T_min for the 50 m HNLF and 100 m HNLF were not significantly different, but the limiting capacity coefficient δ for the 50 m HNLF was better than that of the 10 m and 100 m HNLFs. Therefore, we used a NOL based on a 50 m HNLF in our proof-of-concept experiment to evaluate its performance.

3. Experiment and Discussion

In a proof-of-concept experiment, a 50 m HNLF was used as the NOL. To evaluate the performance of the NOL based on the 50 m HNLF, the output power and transmittance of the NOL were measured by the experimental setup shown in Figure 3. For comparison, we also measured those of NOLs based on a 50 m and a 300 m G655 fiber.

A narrow-line-width laser (linewidth < 15 kHz) with a wavelength of 1549.9 nm was used as the pump source. It was amplified by an Erbium-doped fiber amplifier (EDFA). The amplified signal was then filtered by an optical band-pass filter (OBPF) with a 3.5 GHz bandwidth, and launched into the coupler with a ratio of 99:1. Then the light was injected into the NOLs via a circulator to generate the Brillouin backscattered signal. The incident power was measured from the 1% port of a 99:1 coupler. The output power of the NOLs was measured at the point A. These powers were measured by using an optical spectrum analyzer (AQ6370D, Yokogawa, Japan).

The NOL experimental results are shown in Figure 4 and Figure 5. The output power of the NOL increasing with the incident optical power is shown in Figure 4. We defined the Brillouin threshold power as the input power for which the backscattered power reaches 1% of the input pump power [20,24], which was measured by monitoring the power in the forward and backward direction at points A and B. The SBS thresholds of 50 m HNLF, 50 m G655 fiber and 300 m G655 fiber are 65.1 mW, 517.6 mW and 71.8 mW, respectively, which are in good agreement with the theoretical results. As that shown in Figure 4, when the incident power is well above the SBS threshold of the 50 m HNLF (P_in ≥ 3 P_th, where P_th is the SBS threshold), the output power of the NOL based on the 50 m HNLF increases more and more slowly.

As shown in Figure 5, the values of T_min and δ for NOL based on the 50 m HNLF were 0.12 and 0.76, respectively. Additionally, the corresponding values for 300 m G655 fiber were 0.108 and 0.84, respectively, and those for the 50 m G655 fiber were 0.60 and 0.28, respectively. As shown in Figure 5, when a nonlinear coefficient of the fiber was high, the transmittance of the fiber decreased faster with a smaller minimum transmittance and a larger coefficient δ with the same optical length. Thus, the nonlinear optical limiting ability of HNLF is better than that of the G655 fiber.

As shown in

Table 2. Comparison of different infrared broadband NOL technologies.

Materials	Limiting Principle	[Tmax, Tmin] (at Wavelength)	Wavelength Range
50 m HNLF (this work)	SBS	[87.5%, 11.2%] (@1550 nm)	1530~1565 nm
GO in NMP [25]	RSA	[81%, 42%] (@1750 nm)	400–1800 nm
GST phase change material [26]	Phase change	[80%, 0.02%] (@1500 nm)	1250–2000 nm
GO Ormosil glasses [27]	--	[40%, 18%] (@532 nm)	532–1570 nm

, we listed the performances of the NOL based on the 50 m HNLF and compared several NOLs based on other principles in NIR wavelength range. In contrast, the NOL technology we proposed in this paper has the advantages of excellent NOL performance.

The advantage of SBS-based NOL is that most of the energy is transferred to backscattering, avoiding thermal effects on the material and thus avoiding the adverse effect on NOL performance. In this paper, we experimentally demonstrated the performance of NOL when the incident light is a continuous-wave (CW) laser. If the incident light is a laser pulse, the recent progress of the probe pulse design can also be combined with this limiter to improve the performance of spatial resolution, signal-to-noise ratio (SNR), frequency measurement accuracy, sensing range, and measurement speed in the BOTDR system [25]. In addition, the limiter can be used in some special structures to improve the performance of SNR in the transducer [26].

4. Conclusions

In conclusion, we demonstrated a novel nonlinear optical limiter based on stimulated Brillouin scattering in highly-nonlinear fiber, which has the advantages of simple structure, light weight, small size, low price, and good stability. For the same launched pump power, the power of Brillouin scattering in HNLF was higher than that in G655 and G652 fibers. In a proof-of-concept experiment, the NOL based on a 50 m HNLF provided excellent limiting performance at the wavelength of 1550 nm. The linear transmission of 50 m HNLF was as high as 87.5%, and the lowest nonlinear transmissions were reduced to 11.9%. The proposed method provides a new promising approach to design excellent optical limiters for fiber optic communications and sensing systems at the wavelength of 1550 nm.