Optical transverse-mode properties of the GaN-based semiconductor laser-diode is characterized by effective refractive index method. In order to stabilize a transverse-mode in the conventional ridge-waveguide structure, very precise control of ridge-height is found to be necessary. On the contrary, a novel 2-step grown structure with 2-dimensional index guiding has wide feasibility for device parameter, excellent stability of large confinement factor in transverse-mode, and small aspect ratio of beam divergence, under the condition that AlN molar fraction of 0.08 in AlGaN current blocking layer and stripe width of 1.5 µm are used.

Compact intra-cavity spectroscopic measurements may be obtained with any material that has an absorption signature under the gain bandwidth of a fiber laser. Experiments have demonstrated that compared with a regular absorption scheme, an increase in sensitivity is achieved when using the intra-cavity configuration. The practical limit for this enhancement is given by the fiber laser noise. Since intra-cavity spectroscopy is essentially a single beam technique, the application of dual-beam noise reduction techniques is not possible. However, considering that a single-mode fiber can support two modes of polarization, we have used a polarization beam splitter to create two independent cavities (x and y polarization) with the same noise, one cavity of which contains the absorber. For the first time, this permits the convenient use of Balanced Ratiometric Detection in conjunction with an intra-cavity absorption arrangement.

A reliable and automatic parameter extraction technique for DFB lasers is developed. The parameter extraction program which is named "LAPAREX" is able to determine many device parameters from a measured sub-threshold spectrum only, including gain- and index-coupling coefficients, and spatial phases of the grating at front and rear facets. Injection current dependence of coupling coefficients in a gain-coupled DFBlaser is observed, for the first time, by making use of it.

Long-period fiber gratings (LPGs) using a high-silica core fiber are presented. A high-silica core fiber has a residual stress in the core, and the grating structure is formed by stress releasing of the core using a focused CO2 laser beam. The dependence of the transmission spectrum on temperature and tensile strength is measured, and low dependence compared with conventional LPGs is observed. These unique characteristics are caused by the difference of temperature and tensile strength changes of the effective indices for the fundamental propagation mode and the cladding mode in the high-silica core fiber.

An improvement of the fiber-optic transceiver having both transmitter and receiver functions of optical time-domain reflectometers is examined. The improvement is achieved by introducing an external optical amplifier without changing the previously reported configuration. The characteristics of the transmitted Q-switched pulse and the receiver gain is studied theoretically and experimentally to estimate the performance improvement. It is found that the introduction of the external optical amplifier is a simple and effective way to the performance improvement.

Generating conditions of the optical feedback noise in self-pulsing lasers were experimentally examined. The noise charcteristics were determined by changing the operating power, the feedback distance and the feedback ratio for several types of self-pulsing lasers. The idea of the effective modulation index was introduced to evaluate the generating conditions in an uniform manner based on the mode competition theory. Validity of the idea was experimentally confirmed for generation of noise.

Both frequency- and time-domain analyses of glide signals from a PZT glide-slider flying over a laser zone-textured (LZT) thin film disk medium were used to determine the slider vibration at a small disk-slider clearance. Slider vibration was found to be particularly dependent on the uniformly placed laser bump and the effects due to the air-bearing stiffness over the LZT medium. We found that a high density of small, pointed laser bumps (10X) has a more distinct impact on airflow than large, jagged-rim craterlike laser bumps (1X) on the slider. We therefore investigated the effect of laser bump density on the slider vibration, and found that marginally higher laser bump density (3X versus 2X) results in higher slider vibration. While resonant vibration has been a major glide problem, the effects of laser bump density have also recently become important in the face of ultralow glide height, 0.5 µ" (12 nm). Its influence can be clearly observed when the disk-slider clearance becomes very small. At such an ultrasmall disk-slider clearance, even minimal slider vibration can be detrimental to the head-disk interface. Taking into account the various contributions of slider vibration and considering possible damage to the head-disk interface, it is clear that the optimization of laser bump design should go beyond just the glide height and coefficient of stiction. It should take into account the effects of laser bump height, density and spatial distribution on vibration-induced flying height variation while maintaining a low glide height and coefficient of stiction. An ideal LZT medium should therefore have low bump height to enable low glide height, i. e. , 0.5 µ" (12 nm), but specific bump shapes and sufficient density to achieve low stiction. Laser bump density should, however, be controlled to moderate its effect on slider vibration and possibly disk-slider collision (297 words).

The growth and saturation characteristics of an electromagnetic (EM) wave in a Smith-Purcell free-electron laser (FEL) with a Bragg cavity are investigated in detail with the aid of numerical simulation based upon the fluid model of the electron beam. To analyze the problem, a two-dimensional (2-D) model of the Smith-Purcell FEL is considered. The model consists of a planar relativistic electron beam and a parallel plate metallic waveguide, which has a uniform grating carved on one plate. For confinement and extraction of EM waves, a Bragg cavity is formed by a couple of reflector gratings with proper spatial period and length, which are connected at both ends of the waveguide. The results of numerical simulation show that a compact Smith-Purcell FEL can be realized by using a Bragg cavity composed of metallic gratings.

In order to enhance the energy transfer efficiency in a rectangular Cherenkov laser, we propose to vary properly the permittivity of a loaded dielectric in the transverse direction. With the aid of particle simulation, we investigate the amplification characteristics of the rectangular Cherenkov laser with a dielectric permittivity varied in the transverse direction, demonstrating the effectiveness of our proposal for efficiency enhancement.

We first demonstrate a self-pulsation phenomenon in a semiconductor ring laser(SRL). Not only self-mode-locked optical pulse but self-Q-switched optical pulse can be observed in a SRL. Furthermore, experimental results show that the repetition period of the Q-switched optical pulse train can be controlled by the injection current to a SRL.

Operation of fiber-grating semiconductor laser (FGL) has been stabilized by using the semiconductor optical amplifier which has a simple slant-waveguide structure. The emission wavelength, which depends on a temperature, shows hysteresis. Employing the directly modulated FGL at 2.5 Gb/s, transmission over 400 km in standard optical fiber has been successfully achieved.

Operation of fiber-grating semiconductor laser (FGL) has been stabilized by using the semiconductor optical amplifier which has a simple slant-waveguide structure. The emission wavelength, which depends on a temperature, shows hysteresis. Employing the directly modulated FGL at 2.5 Gb/s, transmission over 400 km in standard optical fiber has been successfully achieved.

In order to enhance the energy transfer efficiency in a Cherenkov laser, we propose to use a tapered waveguide with a dielectric thickness properly varied stepwise in the longitudinal direction. With the aid of particle simulation, we investigate the nonlinear characteristics of the Cherenkov laser with the tapered waveguide, demonstrating the effectiveness of our proposal for efficiency enhancement.

We propose a method of reducing laser-clipping-induced distortion in a subcarrier multiplexed (SCM) optical-cable TV system. This scheme reduces amplitude peaks of the SCM signal by controlling the phases of video carriers to prevent the clipping which occurs when these peaks fall below the threshold of a laser-diode. It is experimentally shown that using this method reduces the bit error rate in an AM-VSB / QAM hybrid optical-transmission system.

Wavelength tunable laser diodes are critical components in a wide variety of WDM and packet switching architectures. And also wavelength-tuned short pulses generated from the semiconductor laser diodes are of great importance for the developments of ultrahigh speed and WDM optical communication systems. Over the past several years, both continuously and discontinuously tunable lasers incorporating periodically sampled and chirped grating have been studied theoretically and experimentally. These laser diodes show the wide tuning range of above 60 nm, stable lasing condition, and large side-mode suppression ratio. Directly modulated semiconductor laser diodes, even those with a single mode, exhibit a dynamic frequency chirp during the on/off modulation. The dynamic linewidth broadening caused by such a large frequency chirp can result in a significant penalty in the performance of high-speed long-haul optical communication systems. The CW laser diodes integrated with an external EA modulator are an breakthrough to realize the high-speed optical systems with low chirp. And also the short pulse generation using the external modulator has been realized experimentally, whose principle of the pulse generation is the optical gating of the electroabsorption modulator. In this paper, widely tunable laser diodes incorporating periodically sampled and chirped gratings and an external modulator are analyzed using an improved time-domain dynamic model. First, it is demonstrated that the improved model is very powerful in simulating the complex laser diodes with active and passive sections. And, the dynamic properties of the sampled grating DBR and chirped grating DBR laser diodes are investigated. Second, the modulation characteristics of the laser diode integrated with the external electroabsorption modulator are studied. It is shown that the external modulation are superior to the direct modulation in the aspect of the lower frequency chirp. And the pulse generation by the optical gating of the external modulator is observed theoretically.

Well-defined wavelength distributed feedback laser diodes (DFB-LDs) are required in WDM network systems. Since the EDFA gain bands have been expanded, even more wavelengths are needed for large-capacity dense-WDM transmission systems. A precisely pitch-controlled Bragg grating fabricated by electron beam (EB) lithography is very attractive for realizing these DFB-LDs. This paper describes this precise pitch- and phase-controlled grating delineated by a novel method called weighted-dose allocation variable-pitch EB-lithography (WAVE). In this method, an EB-dose profile for the grating is precisely controlled by a combination of the allocation and weighting of multiple exposures. This enables us to fabricate a precise fixed-pitch grating as well as a flexible grating with a continuously chirped structure. The stitching error at the exposure field boundary, the grating pitch, and the phase shift were evaluated by using a moire pattern generated by superimposing the microscope raster scan and the grating on a wafer. We also estimated amounts of the stitching errors from fabricated and calculated lasing characteristics, and clarified that the affect of the errors on the single-mode stability of LDs is negligible. Precise wavelength controlled λ/4 phase shifted DFB-LDs were successfully demonstrated as a result of both the WAVE method and the highly uniform MOVPE crystal growth.

Different-wavelength distributed feedback laser diodes with integrated modulators (DFB/MODs) are fabricated on a single wafer operate at wavelengths from 1. 52 µm to 1. 59 µm, a range comparable to the expanded Er-doped fiber amplifier gain band. A newly developed field-size-variation electron-beam lithography enables grating pitch to be controlled to within 0. 0012 nm, and narrow-stripe selective metal-organic vapor-phase epitaxy is used to control the bandgap wavelength of laser active layers and modulator absorption layers for each channel. The channel spacing of fabricated 40-channel DFB/MODs is 214 GHz in average with a standard deviation of 0. 39 nm. Very uniform lasing and modulating performances are achieved, such as threshold currents about 10 mA and extinction ratios about 20 dB at -2 V in average. These devices have been used to demonstrate 2. 5-Gb/s transmission over 600 km of a normal fiber with a power penalty of less than 1 dB.

Input-wavelength-insensitive tunable wavelength conversion was achieved in the range of 1530 to 1560 nm using cascaded semiconductor laser wavelength converters (a DFB laser and an SSG-DBR laser). The power penalty in the wavelength conversion of input signal between 1530 and 1555 nm, where the wavelength ranged between 1537 and 1557 nm, is less than 1 dB for 5 Gbit/s signals.

A series of transparent photochemical acid-generators (PAGs) has been successfully prepared and investigated to apply ArF excimer-laser lithography. These PAGs were synthesized as new alkylsulfonium salts that have cycloalkyl groups but no aromatic ones. They were almost transparent at 193. 4 nm and have high acid-generation efficiency enough to use for ArF excimer-laser resists. The photochemical reaction of these alkylsulfonium salts occurs mainly due to the S-C bond fission. A resist utilizing the PAGs was capable to resolve a 0. 2µm L/S pattern at a 50-mJ/cm2 dose with an aqueous alkaline developer. These PAGs are promising materials for use in ArF excimer-laser lithography.

The chirp noise effect in a directly modulated semiconductor laser diode (LD) is experimentally studied. A previous theoretical study reported that, when an LD is directly modulated, turn-on jitter caused by spontaneous emission, combined with chromatic dispersion, becomes a source of noise in fiber transmission and restricts system performance. This paper points out that, on the contrary, imperfection in LD driving circuits causes chirp noise and limits transmission performance in actual systems. Experiments regarding dependence of chirp noise on LD modulation conditions are also presented, which show that a high relaxation oscillation frequency and a short turn-on delay time are preferable from the viewpoint of chirp noise.