Ultra-high Bit Research Articles

Modeling of mode control and noise in self-pulsating phaseCOMB lasers

Self-pulsations (SPs) in phase-controlled mode beating lasers (PhaseCOMB) are very attractive for all-optical clock recovery at ultra-high bit rates. In this paper, we apply the comprehensive simulation tool Longitudinal Dynamics in Semiconductor Lasers, developed by us, for studying the SP features of PhaseCOMB lasers, considering the effects of spontaneous emission noise, longitudinal spatial hole burning, and gain dispersion. In particular, the importance of mode control for adjusting the PhaseCOMB operating conditions is pointed out. The simulation results are confirmed by measurements on fabricated devices.

Voltage control of a magnetization easy axis in piezoelectric/ferromagnetic hybrid films

We have established a spontaneous magnetization-axis switching in ferromagnetic films by applying a low voltage to a piezoelectric layer in a newly developed hybrid system comprised of the ferromagnetic and piezoelectric films. The magnetization easy axis along which a spontaneous magnetization is oriented, is readily switchable by a voltage without applying an external magnetic field through both the inverse magnetostrictive and piezoelectric effects of CoPd and lead–zirconate–titanate alloy films, respectively. This challenging work provides a new way into the memory writing as well as storage means of ultrahigh bit densities in nonvolatile magnetic random access memory.

Single domain observation for synthetic antiferromagnetically coupled bits with low aspect ratios

Arrays of the synthetic antiferromagnetic (SyAF) patterned bits were successfully fabricated with micron to submicron sizes. Magnetic domain structure was investigated using magnetic force microscopy (MFM) for the SyAF bits as well as monolayer bits. MFM image of Co90Fe10(10 nm)/Ru(0.6 nm)/Co90Fe10(6 nm) SyAF bits demonstrates single domain structure for even a small aspect ratio of 1, while the 10 nm Co90Fe10 monolayer bits showed multi-domain structure for the aspect ratios below 2. This result indicates the predominance of the SyAF for ultrahigh bit density magnetic random access memory devices, because the bits with aspect ratio of 1 possessing single domain structure can provide size-independent switching field.

Modeling of Short Optical Pulse Generation Using Electroabsorption Modulators

Ultra-short optical pulses synchronized to an electrical clock are required in ultra-high bit rate soliton transmission and optical signal processing systems. Use of sinusoidally-driven electroabsorption (EA) modulators is described to generate transform-limited optical pulses. We model pulse generation in sinusoidally-driven lumped-electorode EA modulators, defining a new device figure of merit. Generated optical pulse width is calculated as a function of extinction ratio slope for applied voltages 3dB bandwidth and sample length, and calculated results are compared with those reported for various modulator structures that include bulk modulators, InGaAs/InAlAs, and InGaAsP/InGaAsP multiple quantum well (MQW) modulators. This model is useful for designing and using EA modulators in optical short pulse applications.

Mux-driver-EAM in single module – a solution for ultra-high bit rate applications

The integration of the final three higher-speed functions of an electro-optical emitter in a single module – the multiplexer, the driver and the electroabsorption modulator (EAM) – is demonstrated. The module has operated up to a bit rate of 40 Gbit/s. The Mux-driver was made using in-house InP-double heterojunction bipolar transistor technology. The proposed solution has the following features: compactness, low bit rate inputs, no need of a bandpass capacitor or bias tee between driver and EAM, and possibility to adjust the driving offset voltage, electrical control output, low power consumption, etc.

Co alloy longitudinal thin film media with ultrahigh coercivity

The effect of CoCrTa intermediate layer and postannealing on the magnetic properties and microstructure of CoCrPt/Cr thin film media has been investigated. Through a postdeposition annealing in a vacuum oven, a tremendous increase of coercivity up to 6000 Oe was observed for these CoCrPt/CoCrTa/Cr films, in which the thickness of CoCrTa intermediate layer is about 2.6 mm. X-ray diffraction results showed that the hexagonal-close-packed crystallographic structure for the cobalt alloy did not show any change for the sample after postannealing. In-plane transmission electron microscope images for the CoCrPt/CoCrTa/Cr films showed no change of the morphology before and after annealing and about 15% increase of the grain size for the films after annealing. Delta M curves and remanent squareness results showed that intergrain diffusion may result in this improvement of magnetic properties. Atomic force microscope images showed that the surface roughness for the sample after annealing was almost the same as that before annealing. After tailoring the thickness of the magnetic layer to 10 nm, the coercivity as high as 5200 Oe and Mrt value as low as 0.24 memu/cm2 were obtained, which may support ultrahigh bit density than current media.

Transmission of 2-ps optical pulses at 1550 nm over 40-km standard fiber using midspan optical phase conjugation in semiconductor optical amplifiers

We demonstrate transmission of 2-ps optical pulses at 1550 nm over 40 km of standard fiber by employing midspan optical phase conjugation in semiconductor optical amplifiers (SOAs). The second-order group-velocity dispersion of the fiber is completely compensated and the third-order dispersion becomes a major transmission limitation. This experiment shows that the midspan optical phase conjugation system using SOAs is applicable to ultrahigh bit rates greater than 100 Gb/s.

Ultra-high bit rate optical phase conjugation/wavelengthconversion in DSF and SOA with novel configuration incorporating inline fibre DFB lasers

The authors report a novel polarisation insensitive optical phase conjugator or wavelength converter using orthogonally polarised fibre distributed feedback lasers as four-wave mixing pumps in both dispersion shifted fibre and semiconductor optical amplifiers. Ultra-high bit rate operation up to 40 Gbit/s and mid-span spectral inversion transmission over 204 km of standard fibre is demonstrated.

Intensity and frequency noise transmission along singlemode fibre at zero-dispersion wavelength

A rigorous small-signal theory for linear and dispersive singlemode fibre communication systems operating at the zero-dispersion wavelength is presented. From this theory, the intensity and frequency noise transmission along the singlemode fibre is assessed. The new theory shows excellent consistency with accurate numerical simulations of intensity and frequency noise spectra after singlemode fibre transmission. Theoretical and simulation results show that fibre transmission at zero-dispersion wavelength leads to intensity and frequency noise spectra at fibre output, similar to intensity and frequency noise spectra, respectively, at the input to the fibre. Accordingly, at zero-dispersion wavelength, second-order dispersion effects on the intensity and frequency noise are irrelevant even for ultra-high bit rates and long-haul fibre links.

ULTRAFAST OPTICAL TIME DEMULTIPLEXERS USING SEMICONDUCTOR OPTICAL AMPLIFIERS

The operating principle of the utilization of resonant optical nonlinearities in recently demonstrated ultra-fast all-optical interferometric switching devices is presented. Optimum switching windows expected from these devices are investigated with a simple theoretical analysis. The experimental results of these devices are presented along with the nonlinear optical properties of a semiconductor optical amplifier. The information from the experimental results of the nonlinear optical properties are used to address a problem associated with a saturation of the semiconductor optical amplifier by an incoming ultra-high bit rate data stream. We also discuss the advantages and disadvantages in different operation modes — same or different wavelengths for the control and data signals. Finally, we discuss the relative advantages and disadvantages among these devices.

Optimum pump pulse selection for demultiplexer application of four-wave mixing in semiconductor laser amplifiers

We report on an analysis of four-wave mixing in semiconductor laser amplifiers used for demultiplexing time-division multiplexed data streams at ultra-high bit rates. The analysis is focused on the optimum control pulse energy and width with respect to the switching efficiency, channel crosstalk, and jitter tolerance. We use a simple model of the time resolved gain saturation and four-wave mixing. It turns out, that an optimum pulse energy exists with respect to the switching efficiency. The requirements on the pulse width are contrary for the channel crosstalk and the jitter tolerance. Results for an optimum system design are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Compositional Separation in Co-Cr Based Alloy Films

Using <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">59</sup> Co nuclear magnetic resonance and a chemical etching technique, we observed that compositional separation which produces highly Co-rich precipitates occurs in (1) a Co-Cr film which exhibited ultra-high bit density in perpendicular magnetic recording and (2) Co-Cr-X (X=Ta, Pt, Ni) films for longitudinal magnetic recording. It is expected that this kind of separation will enable the compositional microstructure of Co-Cr based alloy films to be changed from a homogeneous structure to a particulate one suitable for either perpendicular or longitudinal high-density recording.

Compositional Separation in Co-Cr Based Alloy Films.

Using 59Co nuclear magnetic resonance and a chemical etching technique, we observed that compositional separation which produces highly Co-rich precipitates occurs in (1) a Co-Cr film which exhibited ultra-high bit density in perpendicular magnetic recording and (2) Co-Cr-X (X=Ta, Pt, Ni) films for longitudinal magnetic recording. It is expected that this kind of separation will enable the compositional microstructure of Co-Cr based alloy films to be changed from a homogeneous structure to a particulate one suitable for either perpendicular or longitudinal high-density recording.

Concatenated soliton fibre link

An easy and practical ‘soliton fibre’ link by concatenating uniform fibre sections is proposed. An ultrahigh bit rate losslimited fibre communication system can be achieved.

Diode circuits for pulse regeneration and multiplexing at ultrahigh bit rates

Clocked step-recovery diode (SRD) circuits are investigated for regenerating and multiplexing PCM-type signals in the range from 0.1 to a few gigabits per second. One regenerator type is particularly suited for operating with signals in the 1-V range, whereas a differential version employing a magic T was developed for handling signals of down to about 5 mV. By making use of line transformers as coupling networks, high-level versions have been cascaded. Experiments performed at 0.3 and 1 Gbit/s yielded voltage amplifications (peak amplitudes) of 2.5-5.5 for single stages, and insertion power gains of 7-11 dB for 2-3 stage cascades. Diode stages have also been used for multiplexing 4 and 2 individual bit streams to give a combined output signal at 1 and 2 Gbit/s, respectively. In a preliminary multiplexer experiment an output at 4.5 Gbit/s was obtained. Finally, possibilities are discussed for improving the performance of the regenerators/multiplexers and for their applications.