Y. Yoshikuni

Optical orthogonal frequency division multiplexing using frequency/time domain filtering for high spectral efficiency up to 1 bit/s/Hz

Summary form only given. We have proposed a novel optical orthogonal frequency division multiplexing technique that can overcome the spectral efficiency limitation of the conventional WDM system. This scheme permits substantial overlapping of the spectrum and can achieve the spectral efficiency up to 1 bit/s/Hz in principle. For demultiplexing, we used a newly developed optical discrete Fourier transformer (DFT) instead of electrical digital processing, which is impossible to apply in the optical frequency range. The optical DFT was realized by using a set of delay lines, a phase shifter and a coupler in the frequency domain and bit synchronization and an optical gate in the time domain. In experimental demonstration of this scheme, error-free operation was obtained with a 0.8 bit/s/Hz of spectral efficiency.

Multiple-phase shift super structure grating DBR lasers for broad wavelength tuning

Broad range wavelength tuning in distributed Bragg reflector (DBR) lasers using a super structure grating with multiple phase shifts is discussed. Experimental results indicate that the laser achieves a maximum tuning range of 105 nm with stable single-mode continuous wave (CW) operation. >

Multiwavelength light source with precise frequency spacing using a mode-locked semiconductor laser and an arrayed waveguide grating filter

A multiwavelength light source with precise optical frequency spacing can be created by extracting an individual mode of a semiconductor mode-locked laser using an arrayed waveguide grating filter. The outputs of the light source are locked to wavelength division multiplexing (WDM) optical frequency grids using an optical frequency standard light source. They can be successfully modulated by 5 Gb/s nonreturn-to-zero (NRZ) signals and can be transmitted through 100-km-long optical fiber.

Theoretical study on enhanced differential gain and extremely reduced linewidth enhancement factor in quantum-well lasers

Low-chirp lasing operation in semiconductor lasers is addressed in a theoretical investigation of the possibility of reducing the linewidth enhancement factor ( alpha factor) in quantum-well (QW) lasers to zero. It is shown that in reducing the alpha factor it is essential that lasing oscillation be around the peak of the differential gain spectrum, not in the vicinity of the gain peak. The condition for such lasing oscillation is analytically derived. The wavelength dependence of the material gain, the differential gain, and the alpha factor are calculated in detail taking into account the effects of compressive strain and band mixing on the valence subband structure. The effect of p-type modulation doping in compressively strained QWs is discussed. It is shown that the alpha factor, the anomalous dispersion part in the spectrum, crosses zero in the region of positive material gain, which makes is possible to attain virtual chirpless operation by detuning. >

Temperature insensitive arrayed waveguide gratings on InP substrates

A temperature insensitive arrayed waveguide grating (TI-AWG) with an InGaAsP-InP material system is proposed. Less than 0.1 /spl Aring///spl deg/C temperature dependence for all eight channels was achieved for the first time even though we used the InGaAsP-InP material system. This novel characteristic is in good agreement with the design principle of the TI-AWG.

Semiconductor arrayed waveguide gratings for photonic integrated devices

This paper reviews recent progress of the semiconductor arrayed waveguide gratings (AWGs) and the integrated semiconductor optical devices including the semiconductor AWGs. Recent research enables us to make semiconductor AWGs with sufficiently low insertion loss and polarization dependence. Using the semiconductor AWGs as core components, the integrated semiconductor optical devices are developing by integrating with the other semiconductor devices including photodetectors, optical amplifiers, and electroabsorption modulators.

200-GHz FSR periodic multi/demultiplexer with flattened transmission and rejection band by using a Mach-Zehnder interferometer with a ring resonator

A periodic multi/demultiplexer that has a square spectrum response with a 200-GHz free spectrum range was fabricated using a modified Mach-Zehnder interferometer with MMI couplers, a delay line, and a ring resonator. A large enhancement of the FSR is obtained by using a deep ridge waveguide. The transmission bandwidths of 1 dB down and -15-dB rejection are about 75 GHz and over 50 GHz, respectively.

Broad-range wavelength tuning in DBR lasers with super structure grating (SSG)

Broad-range wavelength tuning is demonstrated in tunable distributed Bragg reflector (DBR) lasers with the super structure grating (SSG) proposed. The SSG is fabricated by electron beam exposure lithography. The grating performance is measured by transmittance characteristics of the SSG reflector. A maximum tuning range of 63 nm is obtained under the CW condition while keeping a single longitudinal mode. In a 50-nm tuning range, the threshold current variation is as low as 5.0-9.5 mA.<<ETX>>

Mode stabilization method for superstructure-grating DBR lasers

A wavelength stabilization method for widely tunable superstructure-grating (SSG) distributed Bragg reflector (DBR) laser is described. The output characteristics under tuning are studied theoretically and experimentally. It is found that peak reflectivity states, in which the lasing mode is just aligned with the reflection peaks of both DBRs, are obtained at saddle points in the output characteristics while changing the two SSG-DBR currents. Based on these results, a method for the Bragg frequency control of the two SSG-DBRs is proposed. The feedback control circuit keeps the lasing mode at a peak reflectivity state, and it suppresses mode hopping. Additionally, the oscillation mode is locked to arbitrary reference wavelengths of an optical filter. Stabilization at 200 GHz (1.6 nm)-spaced 16 wavelengths was achieved within the wide tuning range of the SSG-DBR laser. Control was maintained under a laser temperature variation of /spl plusmn/5/spl deg/C as a result of the Bragg frequency control of the two DBRs.

Inherently mode-hop-free distributed Bragg reflector (DBR) laser array

An inherently mode-hop-free tuning of 4.5 nm was achieved with a short-active-region distributed Bragg reflector (DBR) laser. The DBR laser contains no phase-adjustment electrode, which greatly simplifies the tuning. Fast tuning of several nanoseconds and error-free operation for 2.5-Gbit/s direct modulation were also observed. A 4-ch array of short-active-region DBR lasers achieved wavelength tuning of 12.5 nm.