Tsuyoshi Konishi

All-optical analog-to-digital converter by use of self-frequency shifting in fiber and a pulse-shaping technique

We propose a new, to our knowledge, method for an all-optical analog-to-digital converter by using self-frequency shifting in a fiber and a pulse-shaping technique. Preliminary experimental results show that various digitized temporal bit signals can be generated by the variation of the power of an ultrashort analog input pulse of less than 1 ps.

Resolution Improvement of All-Optical Analog-to-Digital Conversion Employing Self-frequency Shift and Self-Phase-Modulation-Induced Spectral Compression

We demonstrate and investigate resolution improvement of optical quantization using soliton self-frequency shift (SSFS) and optical coding using optical interconnection for an all-optical analog-to-digital conversion (ADC). Incorporating spectral compression into the optical quantization allows us to improve the resolution bit according to the spectral compression ratio with keeping its throughput. The proposed scheme consists of optical quantization using SSFS and self-phase modulation (SPM) induced spectral compression and optical coding using optical interconnection based on a binary conversion table. In optical quantization, the powers of input signals are discriminated by referring to the center wavelengths after the SSFS. The compression of the spectral width allows us to emphasize the differences of their center wavelengths, and improve the number of resolution bits. Optical interconnection generates a bit-parallel binary code by appropriate allocation of a level identification signal, which is provided as a result of optical quantization. Experimental results show the eight periods transfer function, that means, the four read-out bit operation of the proposed scheme in binary code. Simulation results indicate that the proposed optical quantization has the potential of 100 GS/s and 4-b resolution, which could surpass the electrical bandwidth limitations.

Polarization-multiplexed diffractive optical elements fabricated by subwavelength structures.

Polarization-multiplexed phase-only diffractive optical elements with subwavelength structures are proposed and fabricated. The differences among the phase modulations result from the differences among the effective indices exhibited in the subwavelength structures with various filling factors and surface profiles, and the phase retardations are obtained by the relief depth of the structures. The polarization-selective property is achieved by the polarization dependence of the effective indices exhibited in the one-dimensional subwavelength structures and the polarization independence exhibited in the two-dimensional structures. Additionally, the polarization contrast of our polarization-multiplexed elements, defined as the cross talk between the two polarization incidences, is independent of the relief depth. The principle of the polarization multiplexing by use of the subwavelength structures is described, and the fabrication results for the polarization-multiplexed computer-generated holograms are demonstrated.

Optical coding scheme using optical interconnection for high sampling rate and high resolution photonic analog-to-digital conversion

We propose and demonstrate an optical coding scheme using optical interconnection for a photonic analog-to-digital conversion. It allows us to convert a multi-power level signal into a multiple-bit binary code so as to detect it in a bit-parallel format by binary photodiode array. The proposed optical coding is executed after optical quantization using self-frequency shift. Optical interconnection based on a binary conversion table generates a multiple-bit binary code by appropriate allocation of a level identification signal which is provided as a result of optical quantization. Experimental results show that 8-levels analog pulses are converted into 3-bit parallel binary codes.

Ultrafast image transmission by optical time-to-two-dimensional-space-to-time-to-two-dimensional-space conversion

We propose an optical time-to-two-dimensional (2-D)-space-to-time-to-2-D-space conversion technique for ultrafast image transmission with ultrashort-pulse lasers. The proposed technique is based on the concepts of the time–space transform, the spatial time–frequency transform, and their inverses. We describe and analyze the proposed technique for ultrafast all-optical processors that can convert an input 2-D spatial object into a modulated ultrafast optical pulse sequence and can retrieve the original 2-D spatial image from the temporal signals transmitted through the optical fiber channel with ultrahigh bandwidth. To verify the proposed technique, we report preliminary experimental results.

Five-bit parallel operation of optical quantization and coding for photonic analog-to-digital conversion

We report the attempt of optical quantization and coding in 5-bit parallel format for photonic A/D conversion. The proposed system is designed to realize generation of 32 different optical codes in proportion to the corresponding signal levels when fed a certain range of amplitude-varied input pulses to the setup. Optical coding in a bit-parallel format made it possible, that provides 5 bit optical codes from 32 optical quantized pulses. The 5-bit parallel operation of an optical quantization and coding module with 5 multi-ports was tested in our experimental setup.

Fabrication of multilevel phase computer-generated hologram elements based on effective medium theory

A conventional method to synthesize diffractive optical elements and computer-generated holograms (CGHs) with high diffraction efficiency relies on an increase of phase levels. To fabricate such a device, one should perform electron-beam (e-beam) lithography with multiple-dose exposures or multiple-step photolithography. Here we describe a one-step method, which is based on the effective medium theory, for the fabrication of a multilevel phase CGH. The phase modulations required in cells of a CGH are constructed by means of dividing these cells into fine (subwavelength) structures. The surface features of these fine structures control their corresponding indices, and their values can be calculated according to the effective medium theory. By proper selection of the fine structures, based on the requirements of the phase modulation of the cells, a CGH with multilevel phases is synthesized when a binary structure is relieved on the dielectric material. Then the CGH can be fabricated by direct e-beam lithography or one-step photolithography through an amplitude mask followed by an ion-etching treatment. The experimental results showed that the reconstructed wave field is in good agreement with that simulated by a computer, indicating the effectiveness of the proposed method.

10-GS/s 5-bit Real-Time Optical Quantization for Photonic Analog-to-Digital Conversion

We report the experimental demonstration of a 10-GS/s 5-bit real-time optical quantization using the soliton self-frequency shift (SSFS) and multistage spectral compression. We prove the feature of sampling rate transparency at 10 GS/s by transmitting 10-Gb/s multi-intensity level quasi analog signals through our 5-bit optical quantizer. Furthermore, to confirm the performance of our quantizer, we measure the bit-error-rate and calculate the effective number of bits, the integral nonlinearity error, and the differential nonlinearity error.

Resolution upgrade toward 6-bit optical quantization using power-to-wavelength conversion for photonic analog-to-digital conversion

We demonstrate a resolution upgrade toward 6 bit optical quantization using a power-to-wavelength conversion without an increment of system parallelism. Expansion of a full-scale input range is employed in conjunction with reduction of a quantization step size with keeping a sampling-rate transparency characteristic over several 100 sGS/s. The effective number of bits is estimated to 5.74 bit, and the integral nonlinearity error and differential nonlinearity error are estimated to less than 1 least significant bit.

Wavelength- and time-selective reconfigurable optical add/drop multiplexer using time-frequency domain processing

We propose and demonstrate a wavelength- and time-selective reconfigurable optical add/drop multiplexer (ROADM) using time-frequency domain processing. The proposed ROADM is realized by allocating wavelength channels and time slots to corresponding 2D spatial channels on a MEMS optical switch. Experimental results show the wavelength- and time-selective drop operation for a signal with equivalent 3.2 Tb/s (0.64  channels), and the reconfigurability by the switching operation of the MEMS optical switch.