Analyses of transmission properties and measurement techniques of innovative fiber-optic cables

We are studying innovative fiber-optic cables, such as multi core fiber (MCF) and few mode fiber (FMF), using the space division multiplexing (SDM) system to exceed the transmission capacity limit of single-mode optical fibers (SMF). In designing these fiber-optic cables, understanding of transmission properties and a measuring technique are crucial. We proposed a method to easily measure the structural parameters of MCF and FMF using OTDR and an evaluation method of dispersing properties by interferometry. Figure 1 shows the measurement system to simultaneously measure group delay differences between LP01 and LP11 of each core of the MCF with two cores and two modes. Figure 2 shows the interference waveforms produced between LP01 and LP11 modes of both cores when light enters the two cores of the MCF. Figure 3 shows the group delay differences obtained by performing Fourier transform on the interference waveforms, extracting interference Fourier spectral compositions generated at each core and performing an inverse Fourier transform. The object of this study is to contribute to international standardization by considering and specifying fiber parameters, standard values and test methods necessary for creating the international standard of an SDM fiber for larger capacity transmission.

Figure 1 . Schematic of measurement setup of the MCF’s group delay differences

Figure 2 Interferometric spectrum

Figure 3 Results of differential group delay

Ultrafast all-optical signal generation and detection technique with optical signal processing

While the digital signal processing technique used for optical communications can process complex signals, its limit to the response speed of electronic circuits and an increase in energy consumption with increasing computational efforts are the challenges. To solve this challenge, we have devised a signal detection method using an optical correlator receiver that does not require a high-speed electronic circuit and calculation. Furthermore, we have been studying the large capacity transmission with Nyquist optical time division multiplexing (N-OTDM) system utilizing the solution. Figure 4 shows the external appearance of an optical correlator receiver, and Figure 5 shows the configuration and processing system of the optical correlator receiver for detecting N-OTDM signals. The optical correlator receiver performs correlation calculation of reference light and signal light using the impulse response of a photo receiver as an integration circuit, serving as a matched filter removing demultiplexed N-OTDM signals and out-of-band noises. Also, the receiver can change the filter property by changing reference light’s forms, allowing for the application to wavelength dispersion compensation. Furthermore, we are also studying waveform shaping and high-speed signal generation techniques by utilizing nonlinear optical effects. The purpose of this study is to realize an ultrafast all-optical signal generation and detection technology using optical signal processing and high-speed communication technology with a large capacity consuming less energy.

Figure 4 Optical correlation receiver

Figure 5 N-OTDM signal detection with an optical correlation receiver

Communication and measurement techniques utilizing spatial mode

The capacity of an optical communications system has become larger with multiplexing of time division multiplexing and wavelength division multiplexing, and the multilevel modulation using phase and amplitude of signal lights has been commercialized, and the remaining multiplexing axis is spatial freedom (spatial mode). In mode division multiplexing (MDM) communication, modes are separated mainly with digital signal processing (DSP), but such techniques as freely using a spatial mode without DSP and generating a pure higher-order mode are also required. For that, we are carrying out research on mode converters, mode filters, and mode multiplexers using an optical waveguide. Figure 6 shows the property of a fundamental mode filter by the long-period fiber grating (LPFG) using a two-mode fiber. We have improved the power ratio in the higher-order mode. On the other hand, as the measurement technique for the spatial mode, we have proposed measurement methods for mode excitation ratio and mode coupling coefficient.  Figure 7 shows the relationship between set values of excitation ratio (x-axis) and measured values of excitation ratio (y-axis) measured by the inline type evaluation method using the bend loss’s mode dependency. Also, we have started studying the FMF application to sensors such as a distortion sensor with the fiber interferometer using mode converting technique and FMF.

Figure 6 Tunable fundamental mode filter by LPFG

Figure 7 Measurement of mode excitation ratio by the bending technique