Ultrafast Optical Signal Research Articles

Switching to optical for a faster tomorrow

Describes some initial studies of a promising new technology for ultrafast optical switching and signal processing. These initial demonstrations of the potential of these devices should lead to further work on the development and integration of these devices into optoelectronic chips with greatly enhanced functionality.

Ultra-fast optoelectronic circuit usingresonant tunnelling diodes and uni-travelling-carrier photodiode

An ultra-fast optoelectronic circuit, using resonant tunnelling diodes (RTDs) and a uni-travelling-carrier photodiode (UTC-PD) is proposed. At extremely low power consumption, the circuit can demulitiplex an ultra-fast optical data signal into an electrical data signal with a lower bit rate. The monolithically fabricated circuit demultiplexed an 80 Gbit/s optical signal into a 40 Gbit/s electrical signal at 7.75 mW.

Ultrafast optical signal measurement using optoelectronic techniques

A novel ultrafast optical signal measurement technique is proposed that uses a fast photodetector and electrooptic (EO) sampling. The technique can satisfy three requirements for the measurement: large bandwidth, high sensitivity, and polarization independence. The measurement systems are demonstrated using two kinds of EO sampling configurations. One system uses direct EO sampling. It affords a larger bandwidth. The system can discern 100-Gbits−1 return-to-zero (RZ) optical pulse-pattern signals with an on–off ratio of 22dB. The other system uses a waveguide intensity modulator in EO sampling. It affords higher sesitivity and can measure an 80-Gbits−1 RZ optical eye diagram. It is theoretically expected that the sensitivity of our system is higher than that of the all-optical technique.

Demultiplexed detection of ultrafast optical signal using interferometric cross-correlation technique

A novel demultiplexing method using the interferometric cross-correlative technique is described and experimentally demonstrated for detecting ultrahigh-speed optical signals. In the proposed technique, cross correlation between the local pulse is observed through the first-order mixing of lightwaves to detect the existence of signal pulses. The only factor limiting the bit rate is the pulse width of the local oscillator's light: the responses of all other phenomena including detector's electrical circuits do not affect the maximum signal detection speed. Several characteristic of the detection method are clarified through the theoretical investigation. In an experiment, the demultiplexed detection of a 25 Gb/s pulse train is successfully demonstrated, for the first time to the author's knowledge, using the proposed method.

Low-noise, pulsewidth tunable picosecond tofemtosecond pulse generation by spectral filtering ofwideband supercontinuum with variable bandwidtharrayed-waveguide grating filters

Pulsewidth tunable short optical pulses (0.37 ps ~ 11.3 ps) are generated with an average time-bandwidth product of 0.46 by spectral filtering of a 100 nm-broadened supercontinuum with variable bandwidth arrayed-waveguide grating filters. The BER measurements show that the generated pulses are low-noise and promising for use in ultrafast optical signal processing.

Ultrafast optical signal processing using semiconductor laser amplifiers

Optical fibres exhibit a bandwidth of about 50 THz, which enables data transmission in excess of 100 G bits -1 . This potential for ultrafast optical signal transmission stimulates the research on appropriate functional devices for optical switching and optical signal processing in general. A promising device is the semiconductor laser amplifier (SLA). It enables optical signal processing in the femtosecond range when it is operated under conditions where intraband dynamics with subpicosecond time constants are dominating. A suitable experimental technique for operating the SLA under these conditions is four-wave mixing. Based on this technique, various applications for optical signal processing have been investigated, such as optical switches, demultiplexers, wavelength converters and phase conjugators. In this article we describe these experiments and discuss the advantages and disadvantages of optical signal processing using SLAs.

Ultrafast optical pulse-pattern signal measurementusing optoelectronic techniques

A novel ultrafast optical pulse-pattern signal measurement technique using a fast photodetector and electro-optic sampling is demonstrated. The technique can measure 100 Gbit/s optical pulse-pattern signals with high sensitivity and polarisation independence.

Construction of 2n prime-sequence codes for optical code division multiple access

The construction of a special family of 2n codes, so-called '2n prime-sequence' codes, for optical code division multiple access (CDMA) has recently been investigated. Since the codes pose the algebraic properties of the prime-sequence and 2n codes, those optical coding architectures, which are especially designed for either of the two codes, can be jointly employed to optimally cope with various constraints (e.g. power budget and cost) imposed by the systems using optical CDMA. As a result, an optical coding architecture, which is particularly attractive for ultrafast optical signal processing and waveguide implementation for the future, high capacity all-optical CDMA networks with potentially 3 dB ideal power ideal, has been proposed. However, the cardinality of the codes is very limited and shown to approach zero when the code weight is sufficiently large. A generalised family of 2n prime-sequence codes, which has the same cardinality as the original prime-sequence codes, is constructed and supported with complete mathematical proofs.

Femtosecond signal processing by second-order spectral holograpy

We report femtosecond optical signal processing by using second-order spectral holography. Enhanced second-order diffraction from a thermoplastic plate makes possible new signal-processing operations not possible with first-order holography. Experiments resulting in output waveforms equal to the autoconvolution function of the input waveform are described, and the possibility of implementing a simple associative memory for ultrafast optical signals is discussed.

Engineering new organic crystals for nonlinear optics: from molecules to oscillator

The quest for more efficient nonlinear optical materials of increased optical quality is being spurred by the development of optical communication systems that require ultrafast broad band optical signal processing functions. Conversely, nonlinear optical phenomena enlarge traditional spectroscopic approaches to encompass more complex and informative multiphotonic pathways. In this stimulating context, organic nonlinear materials have been recognized as forefront candidates for fundamental and applied investigations involving, in a joint effort, chemists, material scientists and optical physicists. After reviewing the molecular engineering foundations of the domain, specifically with respect to an efficiency-transparency trade-off, the author concentrates on a more specific case that embodies, from the definition of the molecule all the way down to applied physics endgoals, the key concepts and methodological tools in the field. A crucial part of such study is the definition and implementation of crystal growth methods leading to the high optical quality demanded by parametric phenomena. This approach is best exemplified in the paradigmatic case of N-4-nitrophenyl-(L)-prolinol (NPP) which, after almost a decade of effort involving crystal growth and ultrafast time resolved spectroscopy, has recently reached sufficient quality to lead to optical parametric oscillation. Stable coherent tunable oscillation in the near IR from 0.9 to 1.7 mu m has thus been demonstrated for the first time in an organic molecular crystal, with specific advantages over mineral candidates, and room for further performance improvement. The author recalls in conclusion that new molecular and crystalline pathways are still opening up, such as those based on molecular nonlinearities of octupolar origin or organomineral crystals.

Special Issue on Nonlinear-Optic Waveguide Materials and Devices. Potential of Nonlinear Optical Waveguides.

Owing to the high density photon confinement nature of optical waveguide, it is expected that the nonlinear-optical waveguide devices gain improved performance over the bulk coun-terparts. The potential and the problem of useful nonlinear devices are discussed with special focus on (1) nonlinear-optical materials, (2) all optical switching devices, and (3) nonlinear optical signal conversion systems. It is concluded that these devices will open a profound way of ultrafast optical signal processing, if a good combination of material, device and system technologies is established.

Picosecond time-space holographic interferograms stored by persistent spectral hole burning

A novel approach to interferometric processing of ultrafast optical signals is demonstrated by using hologram storage in persistent spectral hole burning media. Multiple time-space holographic images of ultrafast scenes are interferometrically compared by using read-out with a frequency-tunable narrow band laser. The feasibility of simultaneous detection of spatial and spectral phase distortions of picosecond wavefronts is demonstrated.

4.65 Gbit/s optical four-bit pattern matching using silica-based waveguide circuit

An ultra-fast coherent optical pattern matching circuit fabricated with silica-based waveguides on a silicon substrate is presented. This circuit can detect any specified pattern in an ultra-fast optical signal sequence. Experimental four bit pattern matching is successfully demonstrated at a bit-rate of 4.65 Gbit/s.

Ultrafast carrier dynamics in modified semiconductor-doped glass

The recombination lifetime in CdSx Se1−x semiconductor-doped glass was measured by time-resolved photoluminescence. It was found that long-term exposure of the glass to high-intensity laser pulses produced a permanent reduction of the free-carrier lifetime from 80 ps to less than 10 ps. This change could be reversed by thermal annealing and should have applications to ultrafast optical signal processing.

Storage and reproduction of an ultrafast optical signal with arbitrarily time dependent wavefront and polarization

Making use of photochemically accumulated stimulated photon echo the storage and reconstruction of time- and space-domain variations of light fields with arbitrarily variable polarization states and their full phase conjugation were accomplished.

Noncollinear second-harmonic-generation-based ultrafast optical signal processing for optical digital computing

The use of second harmonic generation (SHG) effect for ultrafast digital optical computing is proposed. A number of SHG-based optical architectures are presented. Experimental results of an ultrafast optical digital multiplication are included.

Intensity Dependent Index Of Refraction In Organic Materials

Because of the large variety of materials and material forms available, organic materials are attractive candidates for ultrafast nonlinear optical signal processing systems based on the intensity-dependent index of refraction, i.e., the change of index of refraction with a change in applied optical intensity. As an example, we consider an organic polymeric system, the polydi-acetylenes. Through molecular engineering (organic synthesis and Langmuir-Blodgett film growth combined with monolayer deposition techniques), the polydiacetylene was prepared in the technologically important format of a planar waveguide on a grating. By measuring the change in the angle that the incident laser radiation couples into the guided-wave mode (via the grating) with the change in intensity of the laser radiation, we determined the intensity-dependent index of refraction to be 10-6 (MW/cm2)-1 in the transparent region of the material. Implications of this reseach for future work related to signal processing applications are discussed.

Controlled picosecond gating and amplification of ultrafast optical signals

A positively gated ultrafast amplifying shutter is described. The population inversion created by the action of an intense picosecond pulse in a saturable absorbing dye provides the means for high gain amplification of optical signals. The introduction of a second pulse at a wavelength within the emission band of the dye rapidly depletes the inversion and thus terminates the gain. Gated amplifier gains from 50 to 100 are reported and control of delays between the pulses allows shutter aperture times to be reproducibly varied from ∼12 to ≳60 psec.