The POP-Q system, first used in 2004 in China, is now the most commonly used grading system, with surgery reports and level III hospitals accounting for the largest proportion of POP-Q applications.

Multi-ð-phase-shifted fiber Bragg gratings and ð-phase-shifted sampled fiber Bragg gratings (SFBGs) with dual wavelength filtering properties are compared. Results show that both of these gratings have the dual wavelength reflective spectra. However, the side lobe of the reflective spectrum of ð-phase-shifted SFBGs is lower than that of multi-ð-phaseshifted gratings. By adjusting the duty cycle in the range of 0.66 0.80, the filtering properties of ð-phase-shifted SFBGs are optimized, and the side lobe suppression ratio (SLSR) is the lowest when the duty cycle is 0.75. The application of the ð-phase-shifted SFBGs in the dual-wavelength laser is also demonstrated.Optical phase-shifted fiber Bragg gratings and sampled fiber Bragg gratings (SFBGs) have wide applications in optical communication systems. The phase-shifted fiber Bragg gratings have strong applications in single wavelength fiber lasers [1] and optical demodulation systems [2] due to the narrow transmission window at the reflective peak [3,4] . The sampled fiber Bragg gratings have important applications as comb filters in multi-wavelength lasers and optical wavelength division multiplexing systems due to the multiple reflective peaks [5][6][7][8][9][10] . Recently the dual wavelength filter has been designed with different kinds of fiber gratings, such as phase-shifted gratings [11] and SFBGs [12] . The dual wavelength spectrum can be obtained by phase-shifted gratings. However, the amplitudes of the side peaks in reflective spectra are relatively high. So how to reduce amplitude of side peak is a key issue to study. The dual wavelength filter has been proposed by SFBGs with a special sampled structure [12][13][14] . The side peak in the reflective spectrum of the SFBGs is improved compared with that of phase shifted gratings. And the gratings have been applied successfully in dual wavelength fiber lasers [15,16] .In this paper, we analyze the requirement to obtain the dual wavelength filter with sampled fiber gratings, and find that the ð phase shift and the periodic structure are two important conditions. In order to obtain the dual wavelength filtering with low side lobe, we optimize the structure parameters of SFBGs and find that the ð phase shifted SFBG with duty cycle in the range of 0.66 0.80 has better dual wavelength properties. The lowest side peak is obtained when the duty cycle is 0.75. A dual-wavelength fiber laser based on a pair of SFBGs (SFBGp) is also demonstrated.The multi-ð-phase-shifted gratings and -phase-shifted SFBGs ( the non-sampled section must be equivalent to ð phase shift, where 4 L/n=(2m+1) , m=1, 2, 3 , and L is the length of the non-sampled section) can realize dual wavelength filtering. The two kinds of gratings both have periodic structure and the equivalent shifted phases are both ð. In a ð phase-shifted grating, inserting a ð phase shift between two grating sections can split one reflective peak into two peaks with the same amplitude. The reflective spectrum can be interpretted as two beam interference. When multiple...

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