A study on wavelength flattened couplers with low crosstalk for CWDM

Abstract

Conventional CWDM(Coarse Wavelength Division Multiplexer) device using a filter and FBT(Fused Biconical Tapered) fiber type has not good performances. So we will study and fabricate CWDM device which has good performances such as low IL(lower than 0.5dB) and high isolation(higher than 20dB). The FBT method is the most common technology making for fiber-type coupler. But it has few limitations when we fabricate CWDM. Incidentally, the optical fiber becomes longer and thinner during the pulling process and is easily broken off nearby 30mm pulling. So It is hard to make it satisfy the desirable wavelength interval because the channel spacing is inversely proportional to the pulling length. Besides, the EL(Excess Loss) arises during the pulling process. Therefore, the obtained maximum isolation is less than 10 dB. In this thesis, we will study and fabricate the CWDM using MZI (Mach-Zehnder Interleaver) to solve these limitations. The MZI does not have the pulling process which brings about the EL. So MZI which is connected two WFC can be achieved the good performance such as low IL and high isolation. Especially, the variation of channel spacing in the output is explicitly simulated as a function of optical path difference. Finally, the IL and isolation of the proposed MZI-model CWDM using WFC are improved 0.368 dB and 20.569 dB, respectively. Its bandwidth is $\pm$3 nm at 0.5 dB point and the tuning length($\delta$ L) is 40.5$\mum$. But, these performances are stable within 0$^\circC$~60$^\circC$ temperature. This temperature limits is the most demerit of MZI model. Therefore, it is necessary to find a packaging method to reduce thermal-effect for future study. For typical application, temperature operation range of the MZI-model CWDM using WFC must be improved into -40~80$^\circC$. Furthermore, as a future work, it is necessary to make the output flattened wavelength of optical multiplexer. This technique can be applied by inserting one extra MZI between th...