Takeo Katayama

Experimental Demonstration of Multi-Bit Optical Buffer Memory Using 1.55-

We demonstrate a novel all-optical buffer memory using 1.55-μm polarization bistable vertical-cavity surface-emitting lasers (VCSELs). A one-bit data is stored as one of two orthogonal polarization states of a VCSEL in this memory. The polarization state is transferred from the VCSEL to another VCSEL which is optically connected in cascade as a shift register. A 4-bit optical buffer memory is constructed using two sets of the shift-register memory connected in parallel. These results show the technical feasibility of multi-bit optical buffer memory.

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Operation conditions of all-optical buffer memories based on polarization bistable vertical-cavity surface-emitting lasers (VCSELs) were investigated. The switching power dependence on the frequency detuning of the input optical signal was measured for a 980-nm polarization bistable VCSEL. The proper operating conditions for 10-Gb/s memory operation were also measured and found to be on the negative detuning side. Calculations based on a two-mode rate-equation model successfully explained the measured characteristics of the switching power dependence on the frequency detuning and the operating conditions. Maximum data rates for the memory operation strongly depended on the Q factor (i.e., photon lifetime) of the VCSELs. A possibility of 40-Gb/s memory operation using a polarization bistable VCSEL with a Q factor as low as 500 was shown.

Polarization Bistable Vertical-Cavity Surface-Emitting Lasers

The fastest known operation of all-optical flip-flop memory was experimentally demonstrated using a 980-nm polarization bistable vertical-cavity surface-emitting laser (VCSEL). Operating conditions of the input signal power and the frequency detuning to achieve the fast optical memory operation were characterized experimentally. At the optimum condition, 1-bit data signals were arbitrarily sampled and memorized from a 2(6)-1 pseudorandom bit sequence return-to-zero signal at 20 Gb/s by using AND gate and memory functionalities obtained from the polarization bistability. In addition, 1-bit memory operation was achieved for a 6-bit non-return-to-zero signal at 40 Gb/s. Both memory operations required 250-microW data signal power and had optical gain. The high potential of all-optical flip-flop memories based on polarization bistable VCSELs for use in ultrafast all-optical future networks was demonstrated.

High Switching-Speed Operation of Optical Memory Based on Polarization Bistable Vertical-Cavity Surface-Emitting Laser

A numerical model for the investigation of the ultrafast gain properties in asymmetrical multiple quantum-well semiconductor optical amplifiers has been developed considering propagation of ultrashort optical pulses with different wavelengths. The dynamics of the number of carriers and carrier temperature are investigated for each quantum well. The results agree with the experimental results of pump probe measurements with different wavelengths. It is shown that gain recovery is slower for higher energy wells for pump signals of all wavelengths.

All-optical memory operation of 980-nm polarization bistable VCSEL for 20-Gb/s PRBS RZ and 40-Gb/s NRZ data signals

We fabricated a VCSEL incorporating a polarization-independent HCG mirror on SOI for a novel polarization-bistable functional device. The VCSEL oscillated under an optical short pulse excitation at 300 K.

Ultrafast gain dynamics in asymmetrical multiple quantum-well semiconductor optical amplifiers

We discuss our demonstration of gigahertz switching of lasing circular polarizations in a (110) GaAs-based vertical-cavity surface-emitting laser (VCSEL) obtained by optical spin injection. The (110)-VCSELs significantly longer spin relaxation time (2.8 ns) than its carrier lifetime (0.42 ns) enabled it to consecutively oscillate in orthogonal circular polarizations with a high degree of circular polarization, when the time delay between two optical pump pulses with orthogonal circular polarizations was longer than 1 ns. Rate equation analysis closely reproduced the measured results.

1.55-μm VCSEL with polarization-independent HCG mirror on SOI

We have demonstrated polarization bistability in 1.55 µm vertical-cavity surface-emitting lasers (VCSELs). InAlGaAs/InP VCSELs with square mesa structures oscillated in the lowest-order transverse mode and in one of the two orthogonal linear polarization states. Highly stable all-optical flip-flop operation was performed at the telecommunication wavelength for the first time by injecting two orthogonally polarized light pulses.

Switching of Lasing Circular Polarizations in a (110)-VCSEL

We investigated detuning and repetition-frequency dependencies of minimum polarization switching optical power for all-optical flip-flop operation using 1.55-mum polarization bistable VCSELs. We demonstrated it up to 3.1-GHz switching frequency by 10-fJ optical pulses.

Polarization Bistable Characteristics of 1.55 µm Vertical-Cavity Surface-Emitting Lasers

Polarization switching between two orthogonal linear polarizations was experimentally demonstrated in newly designed 980-nm vertical-cavity surface-emitting lasers ( VCSELs ). A square-shaped mesa structure was prepared in a p-type distributed Bragg reflector mirror. Four p-side electrodes were placed in contact with the four sides of the square mesa. When the same current was injected into the four electrodes, the VCSEL oscillated with 0 or 90 linear polarizations. When the current was asymmetrically injected (the same current for two electrodes on the opposite sides of the square, but different for neighboring electrodes), the direction of the linear polarization became perpendicular to the direction of the line connecting the two electrodes injected with the larger current. It was confirmed that the VCSELs oscillated with a single frequency and the lowest order transverse mode in all cases.

All-optical flip-flop operation using 1.55 μm polarization bistable VCSELs

We demonstrate an optical header processing system using a 1.55-μm polarization bistable vertical-cavity surface-emitting laser (VCSEL) that operates as an all-optical AND gate and a holding element. Depending on the state of 1 bit in the 4-bit header, the VCSEL held one of two-orthogonal polarization states. The light of the VCSEL output through a polarizer switched and held the output state of an optical switch. This enables the payload to be sent to the destination determined by the header. The characteristics of the polarization bistable VCSEL enable low-power consumption and lower latency operation. The system also has the potential to process multibit optical headers involved in ultrafast optical packets.