This paper describes the recent progress made in developing wavelength tunable semiconductor light sources for WDM applications. Wide and quasi-continuous wavelength tunings were investigated for a wavelength-selectable laser and a wavelength tunable distributed Bragg reflector (DBR) laser having a super structure grating (SSG). A wavelength-selectable laser consisting of a DFB laser array, a multi-mode interferometer (MMI), and a semiconductor optical amplifier (SOA) demonstrated a quasi-continuous tuning range of 46.9 nm by using temperature control. A wavelength-tunable DBR laser with SSG exhibited a quasi-continuous tuning range of 62.4 nm by using three tuning current controls. Wavelength stabilization was also demonstrated under the temperature variations of 5.

This paper reports on broad-range wavelength tuning characteristics of DBR lasers which make use of a newly proposed multiple-phase-shift super structure grating (SSG). The reflection characteristics of the SSG reflector are analyzed theoretically. We found that the SSG reflector has periodic sharp reflection peaks each with high reflectivities thus making it a suitable wavelength selective reflector for single-mode lasers. The expected characteristics were evident in multiple-phase-shift SSGs fabricated using a new method which involves multiple-phase-shift insertion. DBR lasers with multiple-phase-shift SSGs were fabricated and their wavelength tuning characteristics were studied. The maximum tuning range is 105 nm in the single longitudinal mode under a CW condition. Dynamic single mode operation was also observed throughout the tuning range.

Apodised chirped gratings based on InGaAsP/InP deep-ridge waveguides with vertical-groove surface gratings were fabricated. Reflectivity ripple and group delay ripple were reduced from around 4 dB to 1 dB and from around 5 ps to 2 ps, respectively, by apodisation over a wavelength range of around 20 nm.

A Fourier domain optical coherence tomography system for obtaining a two-dimensional image is constructed. Imaging characteristics of the OCT system in a transverse direction are experimentally investigated. Angle dependence of reflection intensity from a smooth surface is clearly observed and analyzed with consideration of spatial mode coupling to a fiber.

This paper describes the design of polarization insensitive InP-based arrayed waveguide gratings (AWGs), and the characteristics of fabricated devices. The use of a deep-ridge waveguide structure made the fabrication of compact polarization-insensitive AWGs possible. As a result, a low crosstalk (-30 dB) 8-channel AWG and a large-scale (64 channel) AWG with 50 GHz channel spacing could be fabricated. An integrated circuit containing an 8-channel AWG with photodetectors is also described.

The thermal response of a tunable laser is analyzed by using a mode density method based on a Fourier-Laplace analysis. This method introduces a mode density function for mode distribution of the Fourier-Laplace transform and gives temperature time-dependency in an integral form instead of an infinite weighted summation. When symmetric structures are assumed, the mode density method gives the transient thermal response in a simple form: error functions (spherical-symmetry case) and exponential integral functions (cylindrical-symmetry case). The cylindrical-symmetry analysis was extended to the noncylindrical-symmetry model and the thermal response of the tunable laser was calculated by the mode density method. The result shows good agreement with a Fourier-Laplace analysis (deviation 2%) and experimental results. As a rough estimation, the thermal response of the laser is in proportion to the logarithm of time in some range that depends on the chip and tuning-section size of the laser.

We propose and demonstrate a novel ladder interferometric filter that exhibits flat-topped spectral response for use in wavelength-division-multiplexing (WDM) based photonic networks. We numerically analyze the flattened spectral response in a ladder-type filter by modifying the transfer matrix of ladder interferometer. Conventional parabolic-shaped and flat-topped-designed ladder interferometric filters are fabricated, and characterized. We demonstrate a flat-topped filter response in the fabricated device. The shape factor, which is defined by the ratio of -1 dB bandwidth to -10 dB bandwidth, is improved from 0.32 to 0.54. The tunability and the increase in filter extinction ratio of the proposed device are also discussed.

Optical coherence tomography using a tunable single-mode laser is investigated to clarify the effects of long coherence length and step-wise frequency changes.