In the mode division multiplexing (MDM) system, differential mode delay (DMD) restricts the quality of transmission. Thus, it is necessary to precisely measure DMD for compensation and system design. The DMD measurement method using low-coherence digital holography (LCDH) has been proposed. This method can obtain not only accurate DMD but also spatial mode fields. However, in this method, an SMF as the reference arm is needed and its length should be particularly adjusted to a fiber under test (FUT) for low-coherence interferometric measurement. We propose a DMD measurement method by reference-free low-coherence digital holography (RF-LCDH). In the proposed method, we generate a new optical path from the light emitted from the FUT, which is regard as internal-reference light. The proposed method enables us to obtain DMD and spatial mode fields without the SMF as the reference arm by using internal-reference light. In the experiment, we measured DMD of a 10-mode fiber to confirm the basic operation of the proposed method. As the result, without using additional SMFs for reference arm, the proposed method achieved the measurement accuracy which was in good agreement with that of the conventional method.
The mode-division multiplexing (MDM) technique enables the transmission of multiple signals within a multi-mode
fiber (MMF) or a few-mode fiber (FMF). To construct an efficient and flexible MDM network in the same way as a
wavelength-division multiplexing network, a mode conversion method with low modal crosstalk is required for
switching between arbitrary spatial modes. However, in general, modal crosstalk is strongly dependent on the intensity
pattern before mode conversion, and it is increased particularly for higher order modes. In order to reduce modal
crosstalk, we propose a method using a random diffuser and a spatial light modulator (SLM). In the proposed method,
firstly, the input spatial mode is dispersed uniformly by the random diffuser. Subsequently, the diffused phase
distribution is canceled and converted into the desired spatial mode by the SLM, which displays phase difference
between desired and diffused modes. Consequently, every spatial mode can be evenly converted into a desired mode.
Here, we numerically simulate and confirm that the proposed method can reduce modal crosstalk compared to the
conversion method without the random diffuser.
We propose a spatial mode generation technology using spatial cross modulation (SCM) for mode division multiplexing (MDM). The most well-known method for generating arbitrary complex amplitude fields is to display an off-axis computer-generated hologram (CGH) on a spatial light modulator (SLM). However, in this method, a desired complex amplitude field is obtained with first order diffraction light. This critically lowers the light utilization efficiency. On the other hand, in the SCM, the desired complex field is provided with zeroth order diffraction light. For this reason, our technology can generate spatial modes with large light utilization efficiency in addition to high accuracy. In this study, first, a numerical simulation was performed to verify that the SCM is applicable for spatial mode generation. Next, we made a comparison from two view points of the coupling efficiency and the light utilization between our technology and the technology using an off-axis amplitude hologram as a representative complex amplitude generation method. The simulation results showed that our technology can achieve considerably high light utilization efficiency while maintaining the enough coupling efficiency comparable to the technology using an off-axis amplitude hologram. Finally, we performed an experiment on spatial modes generation using the SCM. Experimental results showed that our technology has the great potential to realize the spatial mode generation with high accuracy.
We proposed a new technology for tomographic imaging based on beam diffusion and wavefront reconstruction through digital phase conjugation (DPC). The principle of this technology is highly unique and completely different from that of well-known optical coherence tomography (OCT) because it does not utilize the coherence property of light. In our experiment, it was shown that the depth resolution of smaller than 5μm is obtained when using the objective lens with NA of 0.42. In addition, we achieved the extraction of the information from a specific specimen among several specimens arranged along optical axis.