Igarashi Koji, Souma Daiki, Takeshima Koki, Tsuritani Takehiro
Opt Express. 2015 Jan 12;23(1):183-94. doi: 10.1364/OE.23.000183.
We propose a novel mode multiplexer based on phase plates followed by a Mach-Zehnder interferometer (MZI) with image inversion. After the higher-order modes are selectively converted from fundamental linear-polarized (LP) modes by the phase plates, the converted modes are coupled without fundamental loss using MZI with image inversion, in which the original spatial pattern and inverted pattern of the optical signal are interfered. Our scheme is also applicable to the coupling of degenerated LP modes such as LP(11a) and LP(11b). First, we numerically and experimentally evaluate the performance of the mode converter based on phase plates. The mode converter is suitable as long as the five LP modes such as LP(01), LP(11ab) and LP(21ab) are sustained in a few-mode fiber (FMF), although the crosstalk due to excitation of undesirable modes is unavoidable when the higher-order modes over LP(02) are sustained in FMF. Next, we develop and characterize the proposed mode multiplexers based on phase plates and MZIs with image inversion. The insertion loss is suppressed to around 3 dB for mode multiplexing of LP(11a) and LP(11b). Using a fabricated mode multiplexer for LP(31a) and LP(31b), we measure the bit-error rate performance of single-polarization mode-multiplexed quadrature-phase shift keying optical signals.
我们提出了一种基于相位板的新型模式复用器，其后跟随一个具有图像反转功能的马赫-曾德尔干涉仪（MZI）。在高阶模式通过相位板从基模线性偏振（LP）模式被选择性转换之后，利用具有图像反转功能的MZI将转换后的模式进行耦合且无基模损耗，在该MZI中光信号的原始空间图案和反转图案相互干涉。我们的方案也适用于简并LP模式（如LP(11a)和LP(11b)）的耦合。首先，我们对基于相位板的模式转换器的性能进行了数值和实验评估。只要诸如LP(01)、LP(11ab)和LP(21ab)等五种LP模式能在少模光纤（FMF）中保持，该模式转换器就是适用的，不过当LP(02)以上的高阶模式在FMF中保持时，由于不期望模式的激发而产生的串扰是不可避免的。接下来，我们开发并表征了基于相位板和具有图像反转功能的MZIs的所提出的模式复用器。对于LP(11a)和LP(11b)的模式复用，插入损耗被抑制到约3 dB。使用为LP(31a)和LP(31b)制作的模式复用器，我们测量了单偏振模式复用正交相移键控光信号的误码率性能。
