I. Mito

Input power limits of single-mode optical fibers due to stimulated Brillouin scattering in optical communication systems

Stimulated Brillouin scattering (SBS) limits the optical power that can be transmitted through a single-mode fiber in long-distance optical communication systems, the authors have investigated SBS gain and threshold characteristics with amplitude-shift-keying (ASK), frequency-shift-keying (FSK), and phase-shift-keying (PSK) modulated lights to estimate the input power limitation set by SBS. It was shown that maximum fiber-input powers or the SBS thresholds for fixed-pattern (1010 . . .) ASK, FSK, and PSK modulated lights are 2, 4, and 2.5 times higher, respectively, than the threshold for unmodulated light. Theoretical predictions were experimentally verified by SBS gain measurements with FSK and PSK modulated lights. The first direct observation of SBS with FSK modulated light pumping is also described. >

Selective metalorganic vapor phase epitaxial growth of InGaAsP / InP layers with bandgap energy control in InGaAs / InGaAsP multiple-quantum well structures

Abstract Selective metalorganic vapor phase epitaxial growth on a mask-patterned planar InP substrate is studied. For direct applications to active layers and passive waveguide layers in photonic devices, InGaAsP/InP layers are selectively grown on typically 2 μm wide open stripe regions between pairs of SiO 2 mask stripes. For such relatively narrow growth regions, layer thicknesses are enhanced not only by group III species that are laterally diffused in the gas phase, but also by the species that migrate from surfaces of side facets to the top surface. A flat grown surface is obtained by decreasing the amounts of the surface migrated species, which results in an increase of migration lengths on the top surfaces. Selectively grown InGaAs/InGaAsP multiple-quantum well (MQW) ridge structures with crystallographically smooth side facets and flat interfacesare successfully obtained. By changing the mask stripe width, the bandgap energy of the MQW structure is modified. Group III composition change of selectively grown InGaAs layers, which occurs in both of the gas phase diffusion and the surface migration processes, results in an additional bandgap energy shift in thickness-modulated InGaAs quantum wells. A peak wavelength shift as large as 200 nm is obtained from photoluminescence spectra of simultaneously grown MQW ridge structures, while maintaining flat interfaces.

InGaAsP double-channel- planar-buried-heterostructure laser diode (DC-PBH LD) with effective current confinement

A new high-performance 1.3-μm InGaAsP semiconductor laser is described, in which effective current confinement into the active region has been realized. A p-n-p-n current blocking structure is made by liquid-phase epitaxy (LPE) on both sides of the active-stripe mesa which is defined by a pair of channels in the double-heterostructure wafer. The double-channel-planar-buried-heterostructure laser diodes (DC-PBH LDs) exhibit high-laser performances, such as a high differential quantum efficiency of 78-percent maximum, which results in high electrical to optical power conversion efficiency 43 percent, and high light output power of over 50 mW, as a result of the improvement in the current blocking structure. The threshold current temperature sensitivity is found experimentally to be reduced remarkably by increasing the doping concentration in the p-cladding layer. Characteristic temperature as high as 100 K has been obtained. CW operation is possible up to 130°C.

Single frequency and tunable laser diodes

The state-of-the-art technologies for single-frequency and frequency-tunable laser diodes are reviewed. Spectral linewidth characteristics for distributed-feedback laser diodes are discussed, based on experimentally observed discrepancies from the theory and improvements obtained with quantum-well active regions. Frequency-tunable mechanisms are reviewed mainly for monolithic tunable laser diodes with distributed-Bragg-reflector (DBR) or distributed-feedback configurations. >

High light output-power single-longitudinal-mode semiconductor laser diodes

Issues related to realizing high light output-power single-longitudinal-mode InGaAsP/InP DFB LDs have been studied experimentally and theoretically. Asymmetry in the reflections as the front and rear facets of DFB cavity has been found to result in enhanced single-longitudinal-mode selectivity and higher light output power from the front facet. Single-longitudinal-mode CW light output power as high as 103 mW has been obtained in a 1.3-μm DFB-DC-PBH LD with facet reflectivities of 2 and 80-90 percent. Low internal quantum efficiency is pointed out to be an important problem for 1.5-μm LDs.

Spectral characteristics for 1.5 µm DBR laser with frequency-tuning region

A 1.5 μm frequency-tunable distributed Bragg reflector (DBR) laser was developed. Frequency tuning was performed by injecting current into the DBR region where the refractive index was reduced to result in the Bragg frequency change. Threshold current and external quantum efficiency were 16 mA and 26 percent, respectively. The maximum continuous wavelength tuning range was 1.03 nm ( = 136 GHz). Spectral linewidth was maintained at an almost constant value when the wavelength was tuned. Large frequency modulation (FM) efficiency of 1-3 GHz/mA and flat FM response up to a few hundred MHz were obtained.

1.5 μm tunable wavelength filter using a phase-shift-controlled distributed feedback laser diode with a wide tuning range and a high constant gain

We demonstrate a 1.5 μm tunable wavelength filter which uses a newly developed phase‐shift‐controlled distributed feedback laser diode. A tuning range as wide as 120 GHz (9.5 A) with 24.5 dB constant gain has been achieved. An 18‐channel wavelength selection with less than −10 dB crosstalk is expected with this filter.

Lasing mode and spectral linewidth control by phase tunable distributed feedback laser diodes with double channel planar buried heterostructure (DFB-DC-PBH LD's)

Lasing mode selection between ±1 modes across the stop-band and spectral linewidth tunability have been experimentally observed with newly developed phase tunable distributed feedback laser diodes with double channel planar buried hetetostructure (DFB-DC-PBH LDs) for the first time.

High-performance single-longitudinal-mode operation of InGaAsP/InP DFB-DC-PBH LD's

The lasing performance of InGaAsP/InP distributed feedback laser diodes with double-channel planar buried heterostructure (DFB-DC-PBH LDs) is reported for end-titled and antireflection (AR) coated configuration. High-power CW single-longitudinal-mode (SLM) operation over 55-mW light output at room temperature, high-temperature CW SLM operation over 105°C, as well as stable SLM operation under 2-Gb/s high-speed direct modulation, have been attained for 1.3-μm band DFB-DC-PBH LDs. 1.5-μm band DFB-DC-PBH LDs have also exhibited excellent DFB lasing characteristics, such as high power over 20 mW and high temperature over 75°C CW SLM operation. DFB SLM yield in the laboratory was also examined for 1.3-μm DFB-DC-PBH LDs, giving rise to a good prospect for practical use in optical-fiber communication systems.

Tunable wavelength filters using λ/4-shifted waveguide grating resonators

We report on wide tuning range 1.5 μm wavelength filters using λ/4‐shifted waveguide grating resonators. A tuning range of a transmission resonance wavelength as wide as 42 A by carrier injection and two‐channel wavelength signal switching was achieved.