We evaluated the energy efficiency per 1-bit transmission of an optical light source on InP substrate to achieve optical interconnection. A semiconductor optical amplifier (SOA) assisted extended reach EADFB laser (AXEL) was utilized as the optical light source to enhance the energy efficiency compared to the conventional electro-absorption modulator integrated with a DFB laser (EML). The AXEL has frequency bandwidth extendibility for operation of over 100Gbit/s, which is difficult when using a vertical cavity surface emitting laser (VCSEL) without an equalizer. By designing the AXEL for low power consumption, we were able to achieve 64-Gbit/s, 1.0pJ/bit and 128-Gbit/s, 1.5pJ/bit operation at 50°C with the transmitter dispersion and eye closure quaternary of 1.1dB.

We developed a high-frequency and integrated design based on a flip-chip interconnection technique (Hi-FIT) as a wire-free interconnection technique that provides a high modulation bandwidth. The Hi-FIT can be applied to various high-speed (>100 Gbaud) optical devices. The Hi-FIT EA-DFB laser module has a 3-dB bandwidth of 59 GHz. And with a 4-intensity-level pulse amplitude modulation (PAM) operation at 107 Gbaud, we obtained a bit-error rate (BER) of less than 3.8×10-3, which is an error-free condition, by using a 7%-overhead (OH) hard-decision forward error correction (HD-FEC) code, even after a 10-km SMF transmission. The 3-dB bandwidth of the Hi-FIT employing an InP-MZM sub-assembly was more than 67 GHz, which was the limit of our measuring instrument. We also demonstrated a 120-Gbaud rate IQ modulation.

We propose a novel structure that can reduce the power consumption and extend the transmission distance of an electro-absorption modulator integrated with a DFB (EADFB) laser. To overcome the trade-off relationship of the optical loss and chirp parameter of the EA modulator, we integrate a semiconductor optical amplifier (SOA) with an EADFB laser. With the proposed SOA assisted extended reach EADFB laser (AXEL) structure, the LD and SOA sections are operated by an electrically connected input port. We describe a design for AXEL that optimizes the LD and SOA length ratio when their total operation current is 80mA. By using the designed AXEL, the power consumption of a 10-Gbit/s, 1.55-µm EADFB laser is reduced by 1/2 and at the same time the transmission distance is extended from 80 to 100km.

Radio over Fiber (RoF) is a promising technology that is suitable for broadband wireless access systems to cover in-building areas and outdoor dead-spots. However, one issue in RoF transmission that should be considered is the nonlinear distortion caused by Electrical/Optical (E/O) converters. Multicarrier RF (Radio Frequency) signal formats such as Orthogonal Frequency Division Multiplexing (OFDM), which are commonly employed in broadband wireless communications, are weak against nonlinearities. To enable the linear transmission of OFDM signal in RoF channel, we propose to employ the Envelop Pulse Width Modulation (EPWM) transmission scheme for RoF channel. Two commonly used E/O converters, Mach-Zehnder modulator and direct-modulation of Distributed Feedback Laser Diode (DFB LD), are employed to validate the proposal. Based on the measured nonlinearities of the E/O converters, they are mathematically modeled and their transmission performance are analyzed. A modified Rapp model is developed for the modeling of the DFB LD. Through simulations and experiments, the proposed scheme is shown to be effective in dealing with the nonlinearities of the E/O converters.

We have developed a high-speed electroabsorption modulator integrated distributed feedback (EA/DFB) lasers. Transmission performance over 10 km was investigated under 25 Gbps and 43 Gbps modulation. In addition, the feasibility of wide temperature range operation was also investigated. An uncooled EA/DFB laser can contribute to the realization of low-power-consumption, small-footprint and cost-effective transceiver module. In this study, we used the temperature-tolerant InGaAlAs materials in an EA modulator. A wide temperature ranged 12 km transmission with over 9.6 dB dynamic extinction ratio was demonstrated under 25 Gbps modulation. A 43 Gbps 10 km transmission was also demonstrated. The laser achieved a clear, opened eye diagram with a dynamic extinction ratio over 7 dB from 25 to 85. The modulated output power was more than +2.9 dBm even at 85. These devices are suitable for next-generation, high-speed network systems, such as 40 Gbps and 100 Gbps Ethernet.

We have fabricated a polymer solid-state microstructure for optical application by two-photon-induced polymerization technique. The photopolymerization resin contains conventional laser-dye and dendrimer. A dendrimer can encapsulate the laser-dyes, limiting cluster formation and intermolecular energy transfer, and promising a high level of optical gain. The effect can be extended to prepare an optically active microstructure using the two-photon-induced polymerization technique. We fabricated a polymeric microcavity, which consisted of < 400 nm-linewidth strips arranged in layer-by-layer structure. The periodic variation in the refractive index gave rise to Bragg reflection. A laser emission was measured in the microcavity under optical excitation. The spectral linewidth was about 0.1 nm above the lasing threshold. We investigate both the material functions in the molecular scale and controlling the device structure for desired applications such as a polymer distributed feedback structure.

This paper describes 40-Gbit/s operation of 1.55-µm electro-absorption (EA) modulators applicable to compact and low-cost transmitters for very-short-reach (VSR) applications. We start by identifying factors that make a multi-quantum-well (MQW) design suitable for high levels of output power and for uncooled operation. From the basic experimental results, we determine that a valence-band discontinuity ΔEv at around 80 meV is optimal in terms of combining high-output-power operation and a good extinction ratio. We then apply the above findings in an InGaAsP-MQW EA modulator that is monolithically integrated with a distributed feedback (DFB) laser, and thus obtain operation with high output power (+1.2 dBm), a high ER (10.5 dB), and a low power penalty (0.4 dB after transmission over 2.6 km of single-mode-fiber). These results confirm the applicability of our EA modulator/DFB laser to VSR applications. After that, we theoretically demonstrate the superiority in terms of ER characteristics of the InGaAlAs-MQW over the conventional InGaAsP-MQW. InGaAlAs-MQW EA modulators are fabricated and demonstrate, for the first time, 40-Gbit/s operation over a wide temperature range (0 to 85).

A 1.3-µm InGaAlAs-MQW RWG DFB laser with low-resistance notch-free grating during operations of up to 12.5 Gb/s at 115 was fully investigated. This performance was achieved by combining the high differential gain of the InGaAlAs MQW active layer, high characteristic temperature of RWG structures, and low-resistance notch-free grating. In addition, transmission over 30-km was achieved with the laser running at up to 115. These results confirm the suitability of this type of laser for use as a cost-effective light source in 10-Gb/s datacom applications.

Various types of wavelength-selectable light sources (WSLs) and wavelength-tunable laser diodes (LDs) have been developed, and the one based on an array of distributed feedback (DFB) laser diodes (LDs) has the advantage of tuning that is both simple and stable tuning. It requires only the selection of a DFB-LD and a temperature control. We report on monolithically integrated WSLs with a DFB-LD array, multimode interference (MMI) coupler, semiconductor optical amplifier (SOA), and electro-absorption (EA) modulator. To make them compact, we introduced microarray structures and to ensure that they were easy to fabricate, we used selective area growth. For the WSL with an integrated EA modulator, we developed a center-temperature-shift method that optimizes the detuning wavelength between the lasing wavelength and the absorption edge wavelength of the EA-modulator. Using this method, we obtained a uniform extinction ratio and were able to demonstrate error-free 2.5-Gb/s transmission over a 600-km fiber span. A CW-WSL without an EA-modulator should provide enough output power to compensate the loss caused by the external modulators, but the high-power operation of a CW-WSL is sensitive to optical feedback from the front facet. We therefore used an angled facet in our WSLs and eliminated a mode hop problem. More than 20 mW of fiber-coupled power was obtained over 23 ITU-T channels on a 50-GHz grid.

An optical detection system using a DFB laser with a very small aperture is theoretically proposed. The threshold gain level in DFB laser is sensitively varied with combined reflections by the facet and the corrugation as well as with the optical injection reflected at the surface of the optical disk. Variation of the threshold gain level is counted as the voltage change on electrodes of the laser. It is found that sensitivity of the optical detection with a well-designed DFB laser becomes six times larger than that with conventional Fabry-Perot ones. Field distribution around the small aperture is analyzed taking into account both the near-field and the radiation field. Numerical data on the voltage change are given as examples of the detection system.

The effect of wavelength detuning on the device performance of identical-epitaxial-layer (IEL) electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers is studied in detail. Based on the lasing behavior of integrated devices with different amount of wavelength detuning and the photocurrent spectra under different reverse biases, the optimal wavelength detuning is experimentally determined to be around 30-40 nm for our IEL integrated devices. By adopting gain-coupled DFB laser section, integrated devices with optimal wavelength detuning have demonstrated excellent single mode performances. The extinction ratio is measured to be greater than 15 dB at -3 V, and the modulation bandwidth is around 8 GHz.

The effect of wavelength detuning on the device performance of identical-epitaxial-layer (IEL) electroabsorption (EA) modulator integrated distributed feedback (DFB) lasers is studied in detail. Based on the lasing behavior of integrated devices with different amount of wavelength detuning and the photocurrent spectra under different reverse biases, the optimal wavelength detuning is experimentally determined to be around 30-40 nm for our IEL integrated devices. By adopting gain-coupled DFB laser section, integrated devices with optimal wavelength detuning have demonstrated excellent single mode performances. The extinction ratio is measured to be greater than 15 dB at -3 V, and the modulation bandwidth is around 8 GHz.

A theoretical study on high slope-efficiency phase-shifted DFB laser diodes is presented. We have proposed a new grating structure called asymmetrically-pitch-modulated (APM) grating, and calculated its slope- efficiency and single-mode-yield. In order to take into account the modulated grating period; we have developed an F-matrix which directly includes a chirped grating structure. APM phase-shifted DFB laser diodes consist of a uniform grating in one half section of the cavity and a chirped grating in the other half. This structure causes asymmetrical field distribution inside the cavity and the optical output power from one facet is larger than that from the other facet. According to the simulation results, when the normalized coupling coefficient κ L is 3.0, the front-to-rear output power ratio is 2.6, while the single-mode-yield remains at 100%, and simultaneously the slope-efficiency improvement becomes 65% better than that of ordinary quarter-wave phase-shifted DFB lasers of the same κ L value.

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.

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.

A detailed numerical solution of the design criteria of in-phase lateral and single-longitudinal-mode operation GaInAsP/InP DFB laser arrays is presented. The analysis, including broad-area pumped and stripe-geometry pumped index-guided arrays, was carried out on the basis of the eigenvalue equation method. It is shown that there exists a cut-off array pitch co, at which all of the higher-order array modes are cut off. For the pitch larger than the cut-off pitch co, the modal discrimination is evaluated by the threshold gain difference between the in-phase lateral and higher-order array modes. As a result, the modal discrimination was found to decrease with the increase of the number of elements and the array pitch which is limited to be smaller than twice the cut-off pitch co to attain a stable in-phase lateral- and single-longitudinal-mode operation.