Analysis and Experimental Study on Transmission Characteristics of Double Sagnac Loop Filter

Objective An optical fiber Sagnac loop has been widely used to manufacture optical fiber sensors and optical fiber Sagnac loop filters because of its excellent characteristics such as simple structure, low insertion loss, and low cost. However, the single Sagnac comb filter cannot meet the requirements of some practical laser systems due to its fixed channel spacing. Recently, some research groups have proposed multi-wavelength erbium-doped fiber lasers based on a double Sagnac loop filter, however the transmission characteristics of this filter have not been investigated, which restricts its application in the fields of multi-wavelength fiber lasers and optical fiber sensing. Therefore, in this study, the transmission characteristics of a double Sagnac loop filter constructed by two segments of polarization-maintaining fiber (PMF) in parallel are analyzed and discussed. The measurement results of transmission characteristics are consistent with those obtained by theoretical analysis. Moreover, a channel-spacing tunable multi-wavelength erbium-doped fiber laser utilizing a double Sagnac loop filter with the assistance of four wave mixing (FWM) effect is experimentally demonstrated. Therefore, it is significantly important to analyze and experimentally investigate the transmission characteristics of a double Sagnac loop filter, which is helpful for the design of future multi-wavelength fiber lasers. Methods In this study, the transmission function of a double Sagnac loop filter is first calculated in detail by the transmission matrix theory, and the incident light is assumed to be in an arbitrary polarization state. Then, the simulated transmission spectra are carried out by the Matlab software. The theoretical analysis and simulation results indicate that the double Sagnac loop filter has the polarization-independent characteristic and the tunable channel spacing. To demonstrate the transmission characteristics of the double Sagnac loop filter, the experimental apparatus for measuring the transmission spectra of the filter is designed. Note that we insert a polarization controller (PC) between the erbium-doped fiber amplifier and the double Sagnac loop filter to control the polarization state of the light entering into the filter. Finally, in order to verify the tunable channel spacing of the double Sagnac loop filter in the laser system, we implement the double Sagnac loop filter in the erbium-doped fiber laser while using a highly nonlinear fiber to provide the FWM effect. Results and Discussions The transmission function of a double Sagnac loop filter is obtained by adopting the transmission matrix theory. Based on the results of theoretical analysis, the transmission spectra of the double Sagnac loop filter are simulated. In this study, the lengths of PMF 1 and PMF 2 are 6. 6 m and 13. 51 m, respectively. The birefringence differences of PMF 1 and PMF 2 are 4.0 x 10(-4) and 5.1 x 10(-4), respectively. When the polarization angle of PC 1 (theta(1)) is pi/6 and the polarization angle of PC 2 (theta(2)) is 0, the channel spacing of the filter is 0.9 nm [Fig. 2(a)]. When theta(1) = 0 and theta(2) = pi/6, the channel spacing of the filter is 0.35 nm [Fig. 2(b)]. In the transmission characteristic test of the double Sagnac loop filter, by adjusting PC 1 and PC 2, the measured transmission spectra with channel spacings of 0.9 nm and 0.35 nm are obtained [Figs. 4(a) and 4(b)], which are consistent with the simulation results [Figs. 2 (a) and 2 (b)]. Furthermore, a irregular spectrum [Fig. 4 (c)], corresponding to the situation that theta(1) not equal 0 and theta(2) not equal 0 in Eq. (11), can be observed by appropriately tuning the orientations of PC 1 and PC 2. When the channel spacing of the spectrum is fixed (0.35 nm, 0.9 nm, or irregular channel spacing) , the spectral shape remains unchanged (Fig. 5) in spite of arbitrarily adjustment of the polarization angle of PC-1. Therefore, adjusting the polarization state of the incident light cannot influence the transmission spectra of the double Sagnac loop filter, indicating that it is a polarization-independent filter. Based on the theoretical analysis and measurement results of the double Sagnac loop filter, a channel spacing tunable multi-wavelength fiber laser utilizing a double Sagnac loop filter with the assistance of the FWM effect is proposed. A highly nonlinear fiber with a length of 105 m is used to effectively alleviate the mode competition. When the pump power is set to be 512 mW, the triple-wavelength lasing with a channel spacing of 0.9 nm [Fig. 7(a)] and a sextuple-wavelength lasing with a channel spacing of 0.35 nm [Fig. 7(b)] can be achieved by tuning PC 1 and PC 2, which are accordance with the simulation results [Figs. 2(a) and 2(b)] and measurement results [Figs. 4(a) and 4(b)]. It is worth noting that the channel spacing tuning process is reversible and has good repeatability. Conclusions In this study, the transmission characteristics of a double Sagnac loop filter, composed of two segments of PMF in parallel, are analyzed, simulated and measured in detail, which proves that the proposed double Sagnac loop filter has the polarization-independent characteristic and the tunable channel spacing. According to the results of simulation and measurement, a channel spacing tunable multi-wavelength erbium-doped fiber laser utilizing a double Sagnac loop filter with the assistance of the FWM effect is designed. The laser can output multi-wavelength lasing lines with channel spacings of 0.35 nm and 0.9 nm. Owing to the polarization-independent characteristic of the double Sagnac loop filter, the polarization-dependent devices are not interfere with the transmission spectra of the filter. Thus, the double Sagnac loop filter exhibits the simple structure, polarization-independence, the tunability of channel spacing, and a great potential application in the field of multi-wavelength fiber lasers.