We report on an integrated-optic 10 x 10 filter available for an add/drop multiplexer of a variable-capacity optical orthogonal frequency division multiplexing (OFDM) signal. The filter was composed of ten switches and ten delay lines sandwiched by two slab star couplers and was fabricated by use of a silica waveguide. The filter functions as both optical discrete Fourier transform and inverse discrete Fourier transform circuits for demultiplexing and multiplexing the optical OFDM signal, respectively. We show measured one input by multi-output and multi-input by one output transmittance regarding the filter. We achieved the filter that could process the optical OFDM signal ranging from 10 to 50 Gbaud per channel. The filter is helpful to the prospective adaptive optical network where the guard bands are decreased, and the channel number, symbol rate per channel and modulation formats are flexibly changed corresponding to the traffic and transmission distance.
We report on a chromatic dispersion slope compensator fabricated with a silica waveguide. The compensator was composed of two arrayed-waveguide grating wavelength filters and an array of delay lines. Output ports of the first filter were jointed to input ports of the second filter through the delay lines. Each wavelength component demultiplexed at the first filter was assigned with delay, which was needed for realizing the dispersion slope compensation characteristics. The obtained dispersion slope and bandwidth were -3.03 ps/nm2 and 19.8 nm, respectively, at 1.55 μm. The compensator has the ability to equalize the dispersion slope of about a 43 km-long dispersion-shifted fiber.
We report on a waveguide-type chromatic dispersion compensator comprising a pair of arrayed-waveguide gratings and an array of 100 fixed delay lines. The compensator was fabricated with a silica waveguide with the relative index difference of 2.3%. We connected the first arrayed-waveguide grating output to the second arrayed-waveguide grating input through the delay lines. Each demultiplexed wavelength component at the first arrayed-waveguide grating experiences assigned delay for the dispersion compensation and is again multiplexed at the second arrayed-waveguide grating. The compensator can trim the dispersion of ten channels with spacing of 2.0 nm. The obtained dispersion compensation value and bandwidth at each channel were about -53 ps/nm and 1.6 nm, respectively. The compensator is helpful to the dispersion compensation for high-speed intensity-modulated signals including on-off keying and pulse amplitude modulation signals.
We propose and report on a method that demultiplexes multiple carrier channels directly in the terahertz (THz)-domain by utilizing a THz-wave asymmetric Mach-Zehnder interferometer (AMZI). The frequency division multiplexing (FDM) channels can be demultiplexed with the AMZI. As preliminary investigation, we fabricated the AMZI by employing THz-wave bulk components including beam splitters, mirrors, and lenses. We show demultiplexed results of a two channel FDM on-off keying signal in the 300 GHz band, which was produced by utilizing a high-speed photo-mixing, with the AMZI.
We report on pulse amplitude modulation (PAM) communication in the terahertz (THz)-band using an integrated-optic PAM signal emulator, which consists of asymmetric Mach-Zehnder interferometer (MZI). The asymmetric MZI, whose edge couplers are composed of symmetric MZIs (coupling ratio tunable couplers), produces an optical PAM signals from an input optical on-off keying signal by adjusting each phase shift at each interferometer arm. We generated an optical 25 Gsymbol/s three-level PAM (PAM3) signal with this emulator and converted the generated optical signal into a PAM3 signal in the 300 GHz band with high-speed photo-mixing. We show obtained results on the THz-wave 25 Gsymbol/s PAM3 communication.
We report on Nyquist wavelength division multiplexing (WDM) communication in the terahertz (THz)-band utilizing an integrated-optic spectrum synthesizer. The synthesizer is composed of two arrayed-waveguide gratings, and an array of phase shifters and variable optical attenuators, and it can individually control amplitude and phases of up to 64 optical discrete frequency components with 40 GHz spacing. Three optical frequency combs are produced by combining the synthesizer with a mode-locked laser diode. The produced combs are used to generate a THz-wave Nyquist WDM signal with photo-mixing. We demonstrate 2 x 40 Gbit/s Nyquist WDM communication in the 300 GHz band.
We report on an integrated-optic spectrum synthesis circuit that can individually control amplitude and phase of 40 GHzspaced and 64 frequency components. This synthesis circuit is fabricated with silica waveguide technology, and is composed of two arrayed-waveguide gratings (AWGs), and an array of variable optical attenuators (VOAs) and phase shifters. The AWGs function as frequency multi/demultiplexers, and the VOA and phase shifter are used to adjust amplitude and phase of each spectral component, respectively. The size of the circuit is 80 mm x 80 mm, and the manipulation accuracy relating to the VOA and phase shifter is in the order of 0.1 dB and 0.01, respectively. The VOA extinction ratio, namely, the dynamic range is about 30 dB.
I report on Nyquist wavelength division multiplexing (WDM) communication in the terahertz (THz)-band for improving the transmission capacity, whose received signal is processed with high-speed optical technology. The Nyquist WDM signal in the THz-band is generated from an optical Nyquist WDM signal by photo-mixing. The received THz-wave Nyquist WDM signal is down-converted into a radio-frequency signal, and is again transferred to an optical signal through an optical intensity modulator. Then, the desired Nyquist WDM channel in the reproduced optical signal is demultiplexed with an optical filter. I present the operating principle of the proposed procedure and its some preliminary experimental results using a 2 × 40 Gbit/s Nyquist WDM signal.
KEYWORDS: Optical filters, Electronic filtering, Single mode fibers, Signal detection, Signal generators, Fiber optics, Radio optics, Antennas, Heterodyning, Modulation
I report a seamless method to convert a high-speed single-channel signal in the terahertz (THz)-band into an optical signal by using heterodyne detection, optical intensity modulation, and optical filtering. A spectral-efficient singlechannel 40 Gbit/s signal in the THz-band, which was generated from a 40 Gbit/s optical signal shaped with a Nyquist filter, was converted into an optical signal. A sideband of the converted optical signal was extracted with another filter, and the extracted signal was transmitted to a fiber-optic link. I explain the characteristics of the optical signal passed through a single-mode fiber. The signal could be transmitted to a 9 km-long fiber without chromatic dispersion compensation.
Optical orthogonal frequency division multiplexing (OFDM) utilizes orthogonal sub-carrier channels whose symbol rates are equal to their frequency spacing. Optical OFDM is effective in increasing the spectral efficiency of optical communication, and is applicable for highly spectral-efficient and adaptive optical networks including an elastic network as well as point-to-point transmission. As transmission capacity is varied depending on the traffic and transmission distance in these adaptive optical networks, we need to develop an OFDM signal demultiplexer with high-speed processing and low-power consumption, which can flexibly deal with various symbol rate and number sub-carrier channels in the optical domain. We previously reported an integrated-optic demultiplexer for variable optical OFDM signals, which is composed of an array of variable optical attenuators (VOAs) before a slab star coupler-type optical discrete Fourier transform (DFT) circuit. However, this demultiplexer showed large loss variation (several dB) when changing its characteristics in response to various symbol rate OFDM sub-carriers.
In this presentation, I propose and report a tunable optical OFDM demultiplexer that consists of an array of VOAs, optical DFT circuit, and an array of Mach-Zehnder interferometer-type tunable couplers. The newly added array of tunable couplers after the optical DFT circuit could efficiently utilize output lightwave and decrease the loss variation (intrinsically zero). I report the operating principle of the proposed tunable demultiplexer, its characteristics evaluation, and preliminary experimental results of the tunable demultiplexer fabricated with silicon waveguide technology. The size, processable channel number and symbol rate of the demultiplexer were 1 mm x 8 mm, 2 to 8 and 10 to 40 Gsymbol/s, respectively.
I propose a real-time demultiplexing method of terahertz-wave OFDM signal, which utilizes the optical technology. The received and amplified electrical OFDM signal is transferred into an optical OFDM signal through an optical modulator. Then, the optical OFDM signal is processed with an integrated-optic DFT circuit, which is mainly composed of a slab star coupler. This optical DFT circuit is compact, and can demultiplex up to a 10 × 10 Gbaud (100 Gbaud) optical OFDM signal. I report the operating principle of the method and some preliminary experimental results.
I report a lattice-form tunable optical dispersion compensator whose number of operable channels is doubled with the use of an interleave filter. In this scheme, interleaved signals are introduced into two different inputs of the lattice-form compensator and are provided with the same dispersion value based on unitary properties of the compensator transfer matrix. Thus this configuration can activate inoperable bands that account for nearly half of the total bandwidth in existing lattice-form compensators. The proposed compensator was realized by connecting the silica waveguide-based interleave filter and dispersion compensator with fibers. The dispersion of -283 to 252 ps/nm was obtained over 40 GHz bandwidth with a period of 50 GHz.
We demonstrate an integrated-optic device for multiplexing an OFDM signal based on optical IFFT. The silica waveguide-based multiplexer is composed of four inputs followed by mutually connected directional couplers, an array of delay lines, and a combiner. Four signals modulated with different sequences of data are fed into the multiplexer, and an optical OFDM signal is generated through the IFFT process directly in the optical domain. We report the configuration, operating principle, and experimental results to indicate that the multiplexer operates properly. We successfully generated 40 and 80 Gbit/s OFDM signals with the multiplexer.
I propose and demonstrate a novel and simple optical exclusive OR circuit for binary signals, which is composed of a balanced photo-detector and a Mach-Zehnder intensity modulator. Two input optical signals into the photo-detector produce an electrical signal to drive the modulator. The modulator outputs an optical exclusive OR signal of the two input optical signals. I report the configuration, operating principle, and primary experimental results using 10 Gbit/s binary signals to demonstrate that the exclusive OR circuit operates properly. The proposed simple circuit is suitable for the future hybrid integration of photonics and electronics.
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