【正文】 fiber (EDF) lasers can provide large output power, high slope efficiency, and a wide tunable wavelength range. As a kind of tunable EDF lasers, multiwavelength fiber lasers with single high birefringence (HiBi) fiber Sagnac loop have been highlighted [13–15]. The output wavelength can be tuned by adjusting the polarization controller (PC), and the wavelength spacing can be tuned by changing the length of polarization maintaining fiber (PMF). However, neither the wavelength spacing nor the linewidth can be tuned independently for single Sagnac loop fiber lasers [16]. DWDM systems require that tuning of the laser wavelength be more flexible, or the applications of these lasers will be limited. However, a multiwavelength fiber laser with double Sagnac loops can provides better tenability and controllability. For this structure, wavelength spacing can be tuned while keeping the linewidth unchanged, and linewidth can be tuned while keeping wavelength spacing unchanged. In this paper, a tunable multiwavelength ring EDF laser with double Sagnac loops is proposed and demonstrated. The multiwavelength selection is performed by two Sagnac loops, and each loop is posed of a 3 dB coupler, a PC, and a segment of HiBi PMF. The b filter characteristics of single and double Sagnac loops are simulated and analyzed. In experiment, theFWHM of the output laser is measured as nm, and the sidemode suppression ratio (SMSR) is 50 dB. By adjusting two PCs, the multiwavelength laser can be widely tuned. By changing the length of the PMF, the wavelength spacing and the linewidth can be tuned independently, pared with a single loop structure. The doubleSagnacloop fiber laser proposed in this work is an extension of previous works on multiwavelength fiber lasers with multiple sections of PMFs in the single Sagnac loop, and it has potential applications in DWDM systems, sensing, and instrument testing. 2. Experimental Setup and Operation Principle The experimental setup of the proposed multiwavelength fiber laser is shown in Fig. 1(a). A 980 nm pump laser diode (LD) is coupled into a segment of EDF through a 980∕1550 nm wavelength division multiplexer (WDM). The laser output is coupled out with a 90/10 fiber coupler, which provides 10% power for feedback. The multiwavelength fiber laser is tuned by the double Sagnac loops. As shown in Fig. 1(b), the HiBi Sagnac loop is posed of a 3 dB coupler, a segment of PMF, and a PC. Port 3 and port 4 are connected together through a PC and a segment of PMF. The beam enters into the coupler from port 1, and is divided into two beams by the coupler averager. These two counterpropagating beams rebine in the loop. Due toHiBi effect of the PMF, there is a phase difference of the lights on two axes (fast axis and slow axis). Hence, when the light passes through the PMF, there is an angle deflection, and there is another angle deflection when the light passes through the PC. After traveling through a fiber loop oppositely, the two beams interfere in the coupler. The output characteristics of the Sagnac loop can be analyzed by a Jones matrix. The Jones transmission matrix of the PMF can be described as （ 1） where ? ?/nL? ? ?? is the phase difference between the light on the fast axis and that on the slow axis in the same transmission distance, L is the effective length of the PMF, ? is the wavelength, and ()n nf ns?? is the effective refractive difference between two axes. Then, nf and ns are the effective refractive indices of the fast axis and the slow axis. Since the polarization angle deflection of the light is ? when the light transmits through a PC, the Jones matrix of the positive transmitting light beam through the PC can be described as （ 2） The electric vector of the incident beam is 1E at port 1, and the electric vector of the incident beam is 2E at port 2 ( 2E =0). 1E is split into 3E and 4E by the coupler. Let 3E? be the optical vector of the 3E through the PC and the PMF, and 4E? be the optical vector of the 4E through the PC and the PMF. Thus, 3E? and 4E? are reflected at port 1 and transmitted at port 2 after coherent superimposition: （ 3） The incident power at port 1 is ? ?211IE, and the reflective power is ? ?211IE??. The reflective power at port 2 is ? ?222IE??. The transmissivity is （ 4） This shows that singleSagnacloop transmissivity is related to the polarization deflection angle and the phase difference between two axes. As shown in the 0= 0jPM FjeJ e?????????c o s sin= sin c o sPCJ ?????????413212 12 EE j EE j ?? ???? ??? ???? ?? ?????? ??2221/ s in c o sT I I ????? simulated results in Fig. 2, the filter period is shorter and the filter bandwidth is narrower when the length of the PMF is increased. However, the period and bandwidth cannot be tuned independently. In addition, with the PMF birefringences higher, the filter period is shorter and the filter bandwidth is narrower. For the double Sagnac loops, a fiber isolator is mounted between two Sagnac loops for eliminating the reflective light. So, the transmissivity of double loops can be described as （ 5） Obviously, the transmissivity of double Sagnac loopsis related to the length or the refraction difference of two segments of PMFs based on Eq. (5). Then, the filter period and bandwidth are determined by a shorter PMF and a longer PMF, respectively, and the output laser can be tuned by adjusting the PCs and the lengths of the PMFs. In a doubleSagnac