We experimentally demonstrate a robust Si-photonic waveguide architecture that realizes dynamically encircling an exceptional point (EP) in the optical domain and broadband asymmetric modal transmission as an essential consequence. The structure consists of a pair of coupled channel waveguides and an adjacent slab-waveguide patch that enable precise lithographic controls on the phase velocities and radiation rates of the guided photonic modes. Complex modal index and inter-mode coupling constant profiles required for the encircling-an-EP parametric control are precisely coded in the geometry of those elements. The device created on this basis induces the symmetry-exchanging adiabatic state flip for one transmission direction and symmetry-preserving anti-adiabatic state-jump for the transmission in the opposite direction. In fabrication, we use a state-of-the-art electron-beam lithography for creating mm-long devices with nm-scale transversal precision. A comprehensive spectral measurement for the intensity and phase distributions of the transmitted optical states is obtained with a specially designed phase-sensitive infrared microscopy integrated with a tunable diode-laser system and spectrum analyzer. On this basis, we confirm in the experiment the highly asymmetric modal transmission persisting over a broad spectral band exceeding 100 nm in the telecommunications window around 1,550 nm. Hence, we establish a substantive experimental step toward broadband non-reciprocal photonic devices based on the unique non-Hermitian dynamics.
This study analyzes phase mismatching values of guided-modes in a multimode waveguide with a weak-guiding configuration. In addition, a previously undefined self-imaging phenomenon, referred to herein as the extraneous self-imaging (Ex_SI) phenomenon, was found to result from the modal phase mismatching effect. The evolution principle of the Ex_SI phenomenon is theoretically verified, while the results from the numerical method are compared with graphical and analytical forms of the newly suggested method. Moreover, under a specific condition, the numerical results and the analytical results are compared to analyze the evolution of the Ex_SI phenomenon.
This paper would like to discuss a self-imaging phenomenon in a multimode interference (MMI) coupler. From experiment, different self-images, which are undefined in MMI theory, are observed. These undefined self-images are named 'extraneous self-images' (Ex_SI) out of convenience. In order to estimate the applicability of the Ex_SI, the characteristics of both the 0-dB self-images (SI), which is defined in MMI theory, and the Ex_SIs are compared and analyzed through simulation and experiment. The results show that the Ex_SI has an imaging period that is the same as the 0-dB SI and that the excess loss and the extinction ratio of the Ex_SI improve more than that of the 0-dB SI, as the imaging period increases. Also, this paper introduces the wavelength multiplexer (MUX) for the wavelengths of 1310 nm and 1550 nm using the Ex_SI phenomenon. The optimum length of the multimode waveguide, with a width of 18 microns, is confirmed as a 3670 um wavelength MUX. For wavelengths of 1310 nm and 1550 nm, the excess losses are measured as -0.4 dB and -0.45 dB, respectively, while the extinction ratios are measured as 16.9 dB and 19.7 dB, respectively.
This paper introduces the novel 1 × 3 wavelength multiplexer (MUX) using the extraneous self-imaging (Ex_SI)
phenomenon, which is not mentioned in multimode interference (MMI) theory. The Ex_SI phenomenon in silica-based
multimode waveguides is experimentally studied, and then with data for the Ex_SI phenomenon, the wavelength MUX
for the wavelengths of 1310 nm, 1490 nm and 1550 nm is developed. The novel 1 × 3 wavelength MUX is consisted of
cascaded two multimode waveguides which have the same width of 18 &mgr;m. For wavelengths of 1310 nm, 1490 nm and
1550 nm, the excess losses are measured as -1.37 dB, -0.63 dB and -1.10 dB, respectively, while the extinction ratios are
measured as 15.93 dB, 24.28 dB and 15.66 dB, respectively.
The self-image profiles in a multimode waveguide can be estimated from the phase relations of the guided-modes.
However, in MMI theory, the propagation constants of each guided-mode are defined as a perfectly confined condition.
Because of the effect of the Goos-Haenchen shift, there are large phase differences for each guided-mode between the
predictions of MMI theory and the actual phenomena. Because the relative phases of the guided-modes are not the same
as the theoretical ones in a multimode waveguide, it is possible that an undefined self-image can be produced. In this
paper, such a phenomenon is called the 'pseudo-self-imaging phenomenon'. This phenomenon can be observed in both
the simulation and the experimental results. The excess loss has been measured, and the results are the same as -0.5dB
for both the 1st 0-dB self-image and the 1st pseudo-self-image. These results may have been occurred by the phase
mismatch of the guided-modes. The extinction ratios are also measured, and the results are 15.5dB for the 0-dB self image
and 16.2dB for the pseudo-self-image. From the result, it is shown that the pseudo-self-image is formed after the
0-dB self-image and the pseudo-self-images' intensity can be higher then the 0-dB self-images.
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