Takeshi Fujisawa

All-optical logic gates based on nonlinear slot-waveguide couplers

A novel design of all-optical logic gates based on nonlinear slot-waveguide couplers is proposed. NOT, OR, and AND logic gates can be realized by a simple single optical-directional coupler configuration. Strong polarization dependencies of slot waveguides are effectively utilized for realizing polarization-independent optical-directional couplers in the linear regime and polarization-dependent all-optical switches in the nonlinear regime. All the simulations performed in this paper were performed for three-dimensional nonlinear channel waveguide structures by using rigorous numerical schemes based on the full-vector finite-element method specially formulated for nonlinear optical waveguides.

Chromatic dispersion profile optimization of dual-concentric-core photonic crystal fibers for broadband dispersion compensation

Chromatic dispersion profile of dual-concentric-core photonic crystal fibers is optimized for broadband dispersion compensation of single mode fibers (SMFs) by using genetic algorithm incorporated with full-vector finite-element method. From the numerical results presented here, it is found that by increasing the distance between central core and outer ring core, larger negative dispersion coefficient and better dispersion slope compensation are possible. There is a tradeoff between the magnitude of negative dispersion coefficient and dispersion slope compensation due to the concave dispersion profile of dual-concentric-core photonic crystal fibers. In spite of the tradeoff, dual-concentric-core photonic crystal fibers having larger negative dispersion coefficient as well as compensating for dispersion slope of SMFs in the entire C band with large effective area can be designed.

Polarization-independent optical directional coupler based on slot waveguides

A polarization-independent optical directional coupler based on slot waveguides is proposed and analyzed by using rigorous full-vectorial analysis methods based on a finite-element scheme. Properly choosing materials and structural parameters makes the coupling length for quasi-TE modes become equal to that for quasi-TM modes. Tolerances to operating wavelength and structural parameters are also discussed. The proposed coupler can be used for highly integrated optical circuits without polarization diversity schemes.

Theoretical investigation of ultrasmall polarization-insensitive 1/spl times/2 multimode interference waveguides based on sandwiched structures

A polarization-insensitive 1times2 multimode interference beam splitter based on sandwiched structures is proposed and investigated theoretically for the components of ultrasmall optical integrated circuits. Strong polarization dependencies arising in Si-wire waveguide systems are compensated by introducing sandwiched structures

Design and Fabrication of 10-/40-Gb/s, Uncooled Electroabsorption Modulator Integrated DFB Laser With Butt-Joint Structure

This paper describes the wide temperature range operation of an electroabsorption modulator (EAM) integrated with DFB laser diodes (LDs) (EML) designed to reduce the power consumption and size of optical transmitters. We optimized the multiquantum wells (MQWs) for LD and EAM separately to realize uncooled operation. We employed a conduction band offset (¿Ec) of around 250 meV for the LD and 150 meV for the EAM. The number of well layers was set at 6 for the LD and 12 for the EAM, respectively. We fabricated the EML using a butt-joint (BJ) process to allow us to design the LD and the EAM independently. We introduced a ridge waveguide structure for the LD and EAM waveguides, and designed the width of the LD and EAM mesa to achieve a high optical coupling efficiency between the LD and the EAM. We then used the 200-¿ m-long EAM for 10-Gb/s operation and the 150- ¿m-long EAM for 40-Gb/s operation, and thus obtained a dynamic extinction ratio of over 9 dB at 10 Gb/s from -25 °C to 100 °C and of 8.2 dB at 40 Gb/s from -15 to 80°C. We achieved a power penalty of less than 2 dB after an 80-km single-mode fiber (SMF) transmission at 10 Gb/s and a 2-km SMF transmission at 40 Gb/s over a wide temperature range. These results confirm the suitability of this EML with a BJ structure for use as a 10-Gb/s or 40-Gb/s uncooled light source.

Time-domain beam propagation method for nonlinear optical propagation analysis and its application to photonic Crystal circuits

A time-domain beam propagation method (BPM) based on a finite-element scheme is newly formulated for nonlinear optical propagation analysis. In order to obtain steady-state solutions, a way of continuous-wave (CW) excitation is also described. The validity of this method is verified by numerical examples: self-focusing guiding phenomena and nonlinear gratings. Furthermore, this approach is also applied to characterizing nonlinear photonic crystal circuits. Specifically, a grating structure designed to modify the characteristics of light propagating within a photonic crystal waveguide and a stub-like structure including nonlinear rods are proposed, and the potential for use as optical limiting and switching devices is investigated.

1.3-μm, 4 × 25-Gbit/s, EADFB laser array module with large-output-power and low-driving-voltage for energy-efficient 100GbE transmitter

A 1.3-μm, 4 × 25-Gbit/s, EADFB laser array module with large output power and low driving voltage is developed for 100GbE. A novel rear grating DFB laser is introduced to increase the output power of the laser while keeping the single mode lasing, which is desirable for a monolithic integration. Also, InGaAlAs-based electroabsorption modulators make very-low-driving-voltage operation possible due to their steep extinction curves. With the module, very clear 25-Gbit/s eye openings are obtained for four wavelengths with the driving voltage of only 0.5 V while securing the dynamic extinction ratio required by the system. These results indicate that the presented module is a promising candidate for energy-efficient future 100GbE transmitter.

1.3-

A 1.3- monolithically integrated light source for metro area 100-Gb/s Ethernet is developed. Four 25-Gb/s electroabsorption modulators integrated with distributed-feedback lasers and their multiplexer are monolithically integrated on one chip. A shallow ridge waveguide is used for lasers and modulators for large modulation bandwidth, and a deep ridge waveguide is used for the multiplexer region due to its low bending and radiation losses. The integration of hetero-waveguide devices enables the very small chip size of 22.6 . With the device, 100-Gb/s (4 25-Gb/s simultaneous) operation is demonstrated with the clear eye-openings, and error-free 40-km single-mode fiber transmission is achieved for the first time.

\mu

We demonstrate 50-Gb/s direct modulation by using 1.3-μm distributed-feedback lasers with a ridge waveguide structure. We employed InGaAlAs material for a multiple-quantum well to obtain a low damping factor K, and fabricated a ridge waveguide structure buried in benzocyclobutene to realize a structure with a low parasitic capacitance. In addition, to obtain high maximum frequency relaxation oscillations fr, we designed the cavity length L), and achieved a 3-dB-down frequency bandwidth of 34 GHz. We realized 50-Gb/s clear eye openings with a back-to-back configuration, and achieved a mean output power of over 5.0 dBm, and a dynamic extinction ratio of 4.5 dB. We measured the 50-Gb/s transmission characteristics, and obtained clear eye openings for transmissions over 20-, 40-, and 60-km single-mode fibers (SMF). We also measured the bit-error-rate performance, and obtained an error-free operation and a power penalty of less than 0.5 dB after a 10-km SMF transmission.

m 4

We demonstrate 2.33-mum-wavelength InP-based distributed feedback (DFB) lasers with InAs-In0.53Ga0.47 multiple-quantum wells as the active region. The maximum output power is 20 mW at 25degC and the maximum operating temperature is as high as 95degC. Stable single-mode operation with a sidemode suppression ratio of 30 dB is obtained, and the emission wavelength of the laser is finely controlled from 2.335 to 2.348 mum by adjusting the injection current and the operating temperature. The current-tuning and temperature-tuning rates of the DFB wavelength are +0.007 nm/mA and +0.148 nm/K, respectively.