Design and Analysis of an all-fiber MZI Interleaver Based on Fiber Ring Resonator

An all-fiber Mach-Zehnder interferometer (MZI) interleaver using one planar 3×3 fiber coupler, one 2×2 fiber coupler and one 8-shaped fiber ring resonator is developed by the new configuration. Based on its structure, the output spectrum expression is established and described by using the principle of fiber transmission and the matrix transfer function. The results of numerical simulation indicate that when the length difference of interference arms and the coupling coefficients of the couplers are some certain values, it obtains a uniform flat-top passband and similar to rectangular output spectrum. Compared with the traditional MZI interleaver, the isolation in stopband and the rolloff in transition band are strengthen, the 25dB stopband bandwidth and 0.5dB passband bandwidth are simultaneously remarkably improved. Compared with the asymmetrical ring resonator MZI interleaver, the influence of transmission loss on extinction ratio can be effectively reduced. The device has a certain ability to resist the deviation, which reduces the difficulties in fabricating it. The experiment results agree with the theoretical analysis well. The interleaver designed by the proposed approach has favorable performance, which has the potential application value in optical fiber communication system.


INTRODUCTION
In optical fiber communication system, with the demand of communication capacity increasingly growing, using optical fiber bandwidth efficiently is becoming more and more important. As the key device of dense wavelength division multiplexing (DWDM) system multiplexer/de-multiplexer is not only faced with the improvement of technical difficulty, device cost also increases accordingly. The optical interleaver has been proven to be an effective way to increase the capacity by doubling the number of channels [1][2] . The interleaver can not only increase the DWDM system multiplexing channel number, but also has solved the problem of device manufacturing technology.
However, the conventional single-stage MZI interleaver of output spectrum is almost cosine, and their passband width and peak cannot satisfy the actual needs, therefore, the interleaver designed by cascading many MZI stages [11][12] and a ring resonator with MZI(RRMZI) [13][14] are presented. Theoretical analysis shows that the two plans may improve the transmission performance of MZI interleavers. But the cascaded MZI transmission performance is still not ideal. The upper and the lower interference arms of RRMZI are asymmetrical because the ring resonator is coupled to one arm of an MZI, and according to the interference theory, compensating for the transmission loss is needed in practice [13] .
In this paper, a novel all-fiber MZI interleaver is designed with one planar 3×3 fiber coupler, one 2×2 fiber coupler and an 8-shaped fiber ring resonator. Theoretical analysis and experimental verification indicate that designing structure parameters of the device reasonably can realize equivalent bandwidth transmission. The device of output spectrum is almost rectangular, common-mode rejection being high, crosstalk being low, the 25dB stopband and 0.5dB passband are increased remarkably. In addition, the influence of transmission loss on extinction ratio can also be effectively reduced. The device has a certain ability to resist the deviation, which reduces the difficulties in fabricating it. ABSTRACT: An all-fiber Mach-Zehnder interferometer (MZI) interleaver using one planar 3×3 fiber coupler, one 2×2 fiber coupler and one 8-shaped fiber ring resonator is developed by the new configuration. Based on its structure, the output spectrum expression is established and described by using the principle of fiber transmission and the matrix transfer function. The results of numerical simulation indicate that when the length difference of interference arms and the coupling coefficients of the couplers are some certain values, it obtains a uniform flat-top passband and similar to rectangular output spectrum. Compared with the traditional MZI interleaver, the isolation in stopband and the rolloff in transition band are strengthen, the 25dB stopband bandwidth and 0.5dB passband bandwidth are simultaneously remarkably improved. Compared with the asymmetrical ring resonator MZI interleaver, the influence of transmission loss on extinction ratio can be effectively reduced. The device has a certain ability to resist the deviation, which reduces the difficulties in fabricating it. The experiment results agree with the theoretical analysis well. The interleaver designed by the proposed approach has favorable performance, which has the potential application value in optical fiber communication system.

Structure of interleaver based on 8-shaped fiber ring resonator
A novel all-fiber MZI interleaver based on 8-shaped fiber ring resonator is shown in Figure 2, which consists of one planar 3×3 fiber coupler DC0, one 2×2 fiber coupler DC3 and an 8-shaped fiber ring resonator, among which the DC0 is the fused planar 3×3 single mode fiber coupler in which three fibers in the same planar are weakly coupled, and it is used as input device where the function is distribution of the input light. When the coupling coefficient of DC0 is equal to 2 / π and the signal is input from the middle input port, the power coupling ratio is 0.5:0.0:0.5 at the three output ports. Fiber couplers DC0 and DC3, DC3 and 8-shaped fiber ring resonator are connected in series by optical fiber 3 l and 4 l , 5 l and 6 l respectively. T in In Figure 2, the interleaver based on 8-shaped fiber ring resonator is symmetry structure. Analysis of Eq. (6) shows that when the power coupling ratio of DC0 is 0.5:0.0:0.5, and DC3 is 3dB fiber coupler, DC1 and DC2 have the same coupling coefficient, which will obtain symmetric and complementary wavelength response at the both output ports. When It can be seen from Eq.(6) that A P and B P contain transmission phase delay of interference arm and coupling coefficients of fiber couplers. The former decides cycle of the output spectrum, and the later determines waveform of the output spectrum. When the interference arm length and the coupling coefficients select appropriate values, the device can realize comb and equivalent bandwidth of output spectrum. In addition, through numerical calculation Eq. (7), extreme value point and maximum channel segregation can determine 25 . 2 / π k .

Numerical Simulation
Application of optimization algorithm, without regard to transmission loss ( 1 W ), the simulation result is shown in Figure 3 Figure 3 shows, interleaver output spectrum of the port A and B are the same two groups of bandwidth periodic spectral lines, and their frequency interval is 100 GHz. Flat-top can offset the negative influence of channel wavelength drift.
It is clear in Figure 3 that the proposed interleaver is attractive in flat stopband/passband and big isolation, and the 25dB stopband and 0.5dB passband are 26.2GHz and 38.4GHz. In Ref. [14], conventional MZI interleaver's output spectrum is similar to cosine wave, ICETA 2015 02022-p.3 and the 25dB stopband and 0.5dB passband are 3.6GHz and 21.3GHz. In Ref. [11], MZI interleaver is cascaded, and the 25dB stopband and 0.5dB passband are 15.8GHz and 30.2GHz. By comparison, the novel interleaver through 8-shaped fiber ring resonator phase adjustment and the output spectrum produces curve steep edge and is more close to the rectangular wave, and the 25dB stopband and 0.5dB passband are wider than others.

Influence of Coupling Coefficient on the Interleaver Response
Eq. (7) shows that when the coupling coefficients of DC0 and DC3 are constant, the length differences of interference arms are constant, and the output spectrum mainly depends on coupling coefficients of DC1 and DC2. They will determine the size of the channel segregation and shape of output spectrum. In the process of experiment, the experimental environment and production process will influence ideal value of coupling coefficient and there is a deviation. Set as the deviation of coupling coefficient, the numerical simulation results show the deviation in range. If the coupling coefficients of DC1 and DC2 increase simultaneously, the 25dB stopband will change the range of 21.1~26.2GHz, and the 0.5dB passband will change the range of 35.5~38.4GHz. Figure 4(1) is the output spectrum of interleaver with , its stopband and passband compared with the best value decrease slightly, and its side-lobe level decreases. If the coupling coefficients of DC1 and DC2 reduce simultaneously, the 25dB stopband will change the range of 26.2~29.4GHz, and the 0.5dB passband will change the range of 38.4~40.9GHz. Figure 4(2) is output spectrum of interleaver with k k k k ' 2 1 , and its stopband and passband compare with the best value increase slightly, but the side-lobe level also increases. If the coupling coefficient of DC1 reduces and the coupling coefficient of DC2 increases, the output spectrum will basically remain unchanged, while side-lobe will change slightly. Figure 4(3) is output spectrum of interleaver with , the 25dB stopband is 26.24GHz, and the 0.5dB passband is 38.27GHz. Because of the structure symmetry, when the coupling coefficient of DC1 increases and the coupling coefficient of DC2 reduce, the simulated results are consistent with Figure 4(3).
By analyzing the interleaver output spectrum performance, it can be concluded that when slight deviation of the coupling coefficients of DC1 and DC2 exists, the 25dB stopband and 0.5dB passband will show deviation, but the change is not obvious, and the channel segregation can be above 30dB, which can satisfy the actual needs. It also indicates that the performance can be controlled in 5% deviations during the fabrication, which reduces the difficulties in fabricating the all-fiber MZI interleaver.

Influence of Trasmission Loss on the Interleaver Response
In the discussion about transmission performance of interleaver, the transmission loss of optical signal in the 8-shaped fiber ring resonator is ignored. But the fiber must be curved by the fiber ring so that fiber transmission loss will certainly be introduced. In order to analyze influence of transmission loss on interleaver response, choose different normalized loss value metric optical fiber auxiliary ring structure in Ref. [13], the interleaver based on 8-shaped fiber ring resonator is proposed in this paper, and there are no amplitude differences about the two interference beams, and the influence of transmission loss on extinction ratio can be effectively reduced.

EXPERIMENTAL APPARATUS
The basic principle of interleaver is the light interference. Because the optical fiber welding will introduce waveguide discontinuity, which will affect insertion loss and polarization of optical fiber, the continuous melting method should be used. In the process of melt control, the compute test system will monitor the change of couplers ratio and interference arm's length. First, according to the conventional method to melt 3dB fiber coupler DC3, from one end of the DC3 insert another optical fiber continuous melting pull DC0, and then from the other end of the DC3 fusion DC1 of 8-shaped fiber ring resonator, to the two synthetic fibers drawn from the DC3 insert another optical fiber. Inserted fiber and DC3 draw one fiber to fuse coupler DC1, and the output port of inserted fiber and DC3 draws another optical fiber to fuse coupler DC2. When couplers coupling ratio and interference arm's length meet the requirement, inserted fiber will weld in DC2 out end with DC1 insert end. The method of melting drawn by fiber interference arms and fiber couplers can refer to Ref. [15][16].
The output performance of the experimental interleaver is tested, light source is Santec company TSL2210 wavelength tunable laser, and wavelength range is 1520~1580nm. The linear polarized light is input through middle port of DC0, its power is 1mW, and port A and B of ring resonator measure the output power. Figure 6 is tested output of spectrum of the experimental sample. Compared with Figure 3, the influence of experimental environment makes micro variable exists in the experimental sample output spectrum , and the peak also declines slightly because of transmission loss. Overall experimental sample spectrum agrees with theoretical analysis well.

CONCLUSION
The all-fiber MZI interleaver based on 8-shaped fiber ring resonator is proposed in this paper, and its structure parameters are obtained and described by theoretical analysis and numerical simulation, which indicates that when the length difference of interference arms and the coupling coefficient of the couplers are some certain values, the 25dB stopband and 0.5 passband are improved remarkably, which can achieve almost rectangular spectrum response, and the influence of transmission loss on extinction ratio can reduce remarkably. The experimental results agree with the theoretical analysis. The interleaver designed by the proposed approach has favorable performance, which has the potential application value in optical fiber communication system. The study may provide reference for the facture of the apparatus later.