Table of contents

In this paper we review recent advances in photonic measurement technologies for high-speed electronics covering the frequency range from gigahertz to terahertz. In the first part we describe the basic technologies for photonic measurement, i.e. the generation and detection of high-frequency electrical signals. In the second part, we discuss recent practical applications, including high-speed integrated circuit probers, sampling oscilloscopes, network analysers, and imaging systems.

In this paper, we report the generation of a high-repetition-rate optical pulse train with a mode-locked laser diode (MLLD) and its control techniques. We discuss key technologies to increase the mode-locking frequency and reveal that passive mode-locking based on an intracavity semiconductor saturable absorber is a promising way to achieve mode-locking at a very high repetition rate exceeding hundreds of GHz. Generation of a transform-limited 480 GHz optical pulse train was successfully achieved using a short-cavity (174 µm) MLLD. We also propose and demonstrate two novel techniques, subharmonic optical synchronous mode-locking and an all-optical down-converting method, to stabilize such an ultrafast MLLD and to evaluate the jitter characteristics accurately by all-optical methods without limitations imposed by an electronic device's speed. Stabilization of ultrafast MLLDs (160 and 480 GHz passive MLLDs) and the jitter measurements were successfully demonstrated.

Transform-limited visible pulses with as short as sub-4 fs duration have been generated for the first time by several improvements made to our previous noncollinear optical parametric amplifier (NOPA). All of the signal–idler group-velocity matching and pump–signal pulse-front matching and angular dispersion matching of the parametric amplification processes and the final fine tuning of dispersion compensation using deformable mirror after a pair of chirped mirrors are essentially important to generate such short pulses. We demonstrated the generation of a continuous, simultaneously phase-matched 250 THz parametrically amplified spectrum. Resulting visible–near-IR signal-wave pulses have been compressed to a 4 fs duration using a micro-machined flexible mirror. Feedback for the iterative computer-controlled dispersion compensation algorithm is based on pulse characterization by a second-harmonic generation frequency-resolved optical gating.

Tunable operation with bandwidth-limited sub-10 fs pulses in the visible (550–700 nm) and near infrared (900–1300 nm) was also performed by changing the seed delay with respect to the pump after increasing the seed chirp. The NOPA is one of the most useful light sources for ultrafast spectroscopy at the present stage on an extremely short timescale.

Application to the vibrational wavepacket dynamics in terms of dynamic mode coupling is discussed. The future prospect of the NOPA is described at the end.

This paper presents a theory and simulation of quantum-dot semiconductor optical amplifiers (SOAs) for high-bit-rate optical signal processing. The theory includes spatial isolation of quantum dots, carrier relaxation and excitation among the discrete energy states and the wetting layer, grouping of dots by their optical resonant frequency under the inhomogeneous broadening, and the homogeneous broadening of the single-dot gain, which are all essential to the amplifier performance. We show that high-speed gain saturation occurs due to spectral hole burning under the optical pulse trains up to at least 160 Gb s^-1 with negligible pattern effect, and that the self-assembled InGaAs/GaAs quantum-dot SOAs have about two to three orders faster response speed than bulk InGaAsP SOAs, with one order larger gain saturation for the 160 Gb s^-1 signals. We also show that switching functions can be realized by the cross gain modulation between the two wavelength channels when the channel separation is within the homogeneous broadening. These results indicate great potential of quantum-dot SOAs for all-optical high-speed switches. As one of their possible applications, we propose a new signal-processing scheme of a `quantum-dot 3R regenerator'.

Various optical nonlinearities in semiconductors are investigated in reference to ultrafast all-optical gating. It is shown that a state-filling-type nonlinearity, such as the band-filling effect (BFE) in a semiconductor, is suitable for many ultrafast gating applications: that is, in conjunction with a recently developed differential phase-modulation (DPM) technique, ultrafast and clean gating with a considerably lower optical excitation than in conventional techniques with ferroelectric materials, for example, is possible. Two methods to use the BFE in the DPM regime were experimentally evaluated. One is to optically pump a semiconductor in order to generate photocarriers, and the other is to use a semiconductor optical amplifier (SOA) to enhance the former BFE by means of stimulated emission. With optical pumping, a gating speed faster than a few hundred femtoseconds is possible, but a pulse energy of a few picojoules is required. This pulse energy can be significantly reduced with the use of an SOA, but in this case the gating speed is also reduced. The gating characteristics, including gating speed, gating energy, wavelength, noise, and linearity in signal intensity, were compared between the two methods for various applications.

We present an imaging technique to visualize instantaneous intensity distributions of propagating femtosecond optical pulses by making use of the optical Kerr effect (OKE), which is called femtosecond time-resolved optical polarigraphy (FTOP). By modifying the apparatus from that previously reported, we have succeeded in taking instantaneous profiles of femtosecond optical pulses under loosely focused propagation with a satisfactorily large dynamic range, even in a single-shot observation. In particular, we show pulse propagation behaviours in 2, 3 and 4 atm nitrogen gas when pulses with a 5.0 mm diameter are focused by an f = 350 mm lens. FTOP images reveal the dependence of the propagation behaviour on the gas pressure. They also show alterations in the distribution widths and the pulse energy as the pulse progresses, which is considered to result from nonlinear interactions. Shot-to-shot fluctuations of pulse propagations are directly observed from the FTOP images taken under the same experimental parameters. We also discuss the possibility of applying the OKE echo to at-a-stretch measurements at several temporal points.

Ultrabroadband optical pulses generated through self-phase and induced-phase modulation effects and ultrashort optical pulses whose phases were compensated for using a 4f pulse shaper with a spatial phase modulator were generated. Interferometric autocorrelation, frequency-resolved optical gating and spectral phase interferometry for direct electric-field reconstruction (SPIDER) measurements were made to characterize these pulses, and the results were compared. The generation of 5.0 fs (2.4 cycle) or shorter optical pulses was confirmed. For much shorter pulses, below-two-cycle or monocycle optical pulses, single-shot characterization excluding the errors due to the pulse-to-pulse fluctuation is essential. The sensitivity of SPIDER, which is the most advantageous characterization technique apart from its low sensitivity, was improved by a factor of about a hundred (~1 nJ/THz-bandwidth). Instead of a chirped reference pulse split from the pulse to be characterized, a powerful external pulse from a Ti:sapphire laser amplifier as a highly intensive chirped pulse was employed. By use of this modified SPIDER, the characterization of an over-one-octave ultrabroadband optical pulse was performed. This modified-SPIDER method is the most promising for characterization of monocycle optical pulses.

We demonstrate single-photon detection with a GaAs/AlGaAs modulation-doped field effect transistor. High sensitivity of the transistor to light is obtained by incorporating in its structure a layer of self-organized InAs quantum dots in close proximity to the conducting channel but separated by a thin AlGaAs barrier. We show that capture of a single photoexcited carrier by a quantum dot results in a sizeable change in the source–drain current of the transistor, allowing the detection of a single photon.

Spectroscopies using terahertz (THz) radiation excited by ultrashort laser pulses have been rapidly developing recently. In this paper, the principles of various types of THz time domain spectroscopies (THz TDSs), i.e. transmission-, reflection-and ellipsometry-type THz TDSs, and their applications to the characterization of semiconductors are described. In addition to the THz TDS using a femtosecond laser, a sub-THz TDS system using a cheap and compact continuous multimode laser diode is also described.

We describe the basic properties of photoconductive (PC) antennas as emitters and detectors of terahertz (THz) pulsed radiation. The efficiency of PC antennas is discussed, taking into account the saturation effect due to field screening by photo-generated carriers. We show that maximum emission efficiency under a constant pump power condition is achieved by adjusting the PC gap size so that the pump intensity (power divided by PC area) is equal to the characteristic saturation intensity. We also discuss the bandwidth of PC antennas and conclude that PC antennas are capable of generating and detecting ultra-broad THz radiation beyond 10 THz when short enough laser pulses are used. Our conclusion is demonstrated experimentally by detecting ultra-broadband THz radiation, whose spectrum distributed up to 40 THz, with a PC antenna. As an example of applications of THz pulsed radiation based on a PC emitter-detector system, results for the imaging of objects in powders are presented and discussed.

We have proposed and developed a technique to synthesize an optical coherence function into arbitrary shapes. By using this technique, which we call the `synthesis of an optical coherence function', various distributed optical sensing schemes have been developed, which have no mechanical moving parts nor data calculation. In these schemes, we do not use a pulsed lightwave but instead use a continuous wave, whose correlation is controlled by frequency modulation or phase modulation. We have proposed a reflectometry system to diagnose fibre optic subscriber networks. Fibre optic distributed force sensing systems have also been developed, which are applicable to smart structures and security systems. In a similar way, we have proposed a system to measure strain distribution along an optical fibre through the Brillouin scattering caused in the fibre. Spatial resolution of just 1 cm has been demonstrated by this system, which is 100 times higher than the practical limitation of conventional pulsed-lightwave techniques. Such a high spatial resolution is suitable for smart material applications. Two- or three-dimensional distributed sensing has also been developed by this technique. An optical tomography system has been proposed, which has fewer mechanical moving parts. A system for surface shape measurement for a multi-layered object has also been developed.

With the progress of information technology, the need for an accurate personal identification system based on biological characteristics is increasing the demand for this type of security technology instead of conventional systems using ID cards or pin numbers. Among other features, the face is the most familiar element and is less subject to psychological resistance. As a simple and compact recognition system satisfying the required performance, we implemented a hybrid system based on the optical recognition principle using a multi-level zone plate as a Fourier-transform lens and we report the preliminary results of their application to face recognition.

In this paper, we present the design procedure and fabrication process for an improved version of a second-generation compact parallel correlator (named COPaC II), the size of which is 20 × 24 × 43 cm³ and weight 6 kg. As a result, we obtained a low error rate of 0% as the false match rate and 0.3% as the false non-match rate, thus the COPaC II significant identification security level is sufficiently stable. With the aim of further enhancing the throughput and robustness, we conducted performance tests where the system is used as a computer log-in device and as a pre-screening device for crime investigation. In both experiments, a high rate of successful recognition, such as 90% recognition and 94% rejection rate for log-in, was obtained. Experiments on twins to check the disguise recognition, and on the effects of changes in brightness and arbitrary size of images to test its robustness are also included.

In this paper a new high-speed vision device and its application to a grasp system is proposed, and we discuss a processing architecture for grasping based on visual and tactile feedback designed with real-time control in mind. First, we describe a high-speed vision chip that serves as a robotic eye that includes a general-purpose parallel processing array along with a photo-detector all on a single silicon chip. Next, we present a grasping algorithm based on real-time visual and tactile feedback, and a high-speed sensory–motor fusion system for robotic grasping. We then describe a grasping experiment using high-speed vision, and finally, based on these results, the effectiveness of high-speed sensory–motor fusion for robotic grasping is discussed.

Easy and inexpensive single nucleotide polymorphisms (SNPs) typing systems are required for the practice of genetic testing as well as genetic medicine. Most of the SNPs typing systems use laser-induced fluorescence detection coupled with fluorophore tagging on DNA, which are expensive. A new simple and inexpensive SNPs typing system is presented. It uses a bioluminometric assay coupled with modified primer extension reactions and an inexpensive photodiode array for the luminometric detection. The reagents consumed in the assay are also inexpensive. Although the system is very small, simple and inexpensive, it gives enough sensitivity for detecting target DNAs as small as 10 fmol, which is good enough for SNPs typing.

In recent years, we have developed an advanced environmental monitoring system (AEMS) containing an eco-sensor, meaning a sensor for the measurement of environmental pollutants, based on lipid membranes, for continuous monitoring of underground water in industrial areas such as semiconductor factories (Ishimori Y, Tamura H, Kawano K, Aoyama N and Tamiya E 2000 Proc. Photonic East 2000 pp 43-50). The AEMS project is made up of three components as follows: (1) the eco-sensor, (2) prediction of plume propagation using a computer simulation technique, and (3) the environmental protection method. In this paper, we would like to focus on the study of the eco-sensor. We considered modified lipid membranes to serve as good models for cell membranes because they would be ideal hosts for receptor molecules of biological origin or disruptive environmental pollutants. Thus, we selected the lipid membrane as an environmental sensing element. In attempting to improve the applicability and the responsivity of bilayer lipid membranes (BLMs) in the eco-sensor, we have investigated automatic BLM preparation devices. An automatic BLM preparation device was made by use of an inkjet mechanism. The reproducibility of the BLM preparation was remarkably improved. The sensitivity to volatile organic chlorinated compounds such as cis-1, 2-dichloroethylene was of the order of 10 ppb using mono-olein BLMs even in real underground water. We have also been developing a smaller eco-sensor for practical use.

A proposal is made of the use of fibre optic sensors for chemical sensing where the measurements of rate coefficients of time-dependent chemical reactions are performed. When an optical fibre is bent and a part of its cladding is stripped off, the sensitivity of the system improves because of the higher penetration depth achieved in this case. The sensor works on the principle of evanescent wave absorption spectroscopy. The rate constant of a chemical reaction is determined using this principle, and the validity of the result is confirmed by comparing it with that obtained by a spectrophotometer. Comparison is made between the results obtained using bent as well as straight probes, and it is noticed that the system incorporating the former gives results closer to those obtained by spectroscopic measurements, which is essentially because of the improved sensitivity of the system.

The field of measurement science and technology has been enriched by the achievements in both science and technology, either through the introduction of new measurement principles or through the opening of new markets. This is especially true in observing the rapid progress in photonics, or the application of optical technology in information systems. Optical communication systems are exploiting both the spectral diversity and ultra-high speed nature of optical signals. Materials with microscopic structures offer new optical properties, opening a new field: `nanophotonics'. Ultrashort laser pulses enable the generation of intense electromagnetic waves at terahertz frequencies, which have not been applied to advanced measurements. Together with the advancement of electronics, novel sensing techniques have been created to respond to the new demands in the fields of environmental protection, life sciences and security assurance.

The aim of this special issue is to provide the reader with a selection of research on advanced photonic measurement techniques and novel sensors. These are expected to be the key technologies in advanced information technology and to also prove useful in finding solutions to global issues, such as security, health and environmental problems. Specifically, this special issue covers ultra-short pulse and terahertz techniques, optical fibre sensors, functional sensing for recognition, robotic sensors, biosensors and eco-sensors.

The research on ultra-short or ultra-fast optical pulses will open new techniques in the field of high-speed electronics, spectroscopy by terahertz radiation, optical oscilloscopes and optical switching. These are leading to many applications, not only in optical communications systems but also in other fields such as characterization of materials and medical imaging. As an ultimate optical technology, a single-photon detector is also discussed, which can provide secure optical quantum communications in combination with a single-photon emitter.

As for novel sensing, advanced concepts relating to distributed optical sensing for smart materials applications and facial recognition are presented, which are now of widespread interest in providing a secure society. Additionally, real-time sensor fusion is demonstrated in the paper on robotic grasping.

Research work relating to organic compound monitoring, SNP (Single Nucleotide Polymorphism) typing and chemical kinetics is also included. Systems based on these sensing methods are very useful in the fields of environmental protection and human health, which are of great concern nowadays.

We hope this special issue will be of interest to the readers of Measurement Science and Technology and will stimulate new directions in research.