A radar-type microwave imaging system utilizing a Doppler sensor which contains a Gunn oscillator is described. In the system, the synchronous detector is modified to dual diode detection in order to obtain complex hologram. Experimental results are also described.

Spectral properties of an optically injection-locked semiconductor laser are analytically calculated and are experimentally verified the locking bandwidth is comparable to or smaller than the linewidth of a slave laser. In this case, the spectral lineshape becomes non-Lorentzian and its linewidth strongly depends on the locking bandwidth.

We propose and demonstrate an excellent linearly frequency-swept laser diode (LD) for sensing system utilizing frequency-moduleted continuous-wave (FMCW) technique. In order to linearly sweep the optical frequency, we adopt a reference interferometer and an electric phase comparator. The interference beat signal of the reference interferometer is phase-compared with an external reference rectangular signal having a fixed frequency near the interference beat signal frequency by a lock-in amplifier. The error signal from the lock-in amplifier is fed back to the modulating signal of the injection current of the LD. Thus, a phase-locked loop composed of optical and electric circuits can be established, and the beat signal frequency is locked to the frequency of the reference signal. The optical frequency of the LD is, therefore, excellently linearly swept in time. In order to experimentally confirm the linearlity of the proposed method, we apply this light source to the FMCW reflectometry. Resultingly, the improvement of the linearity is estimated to be about 10 dB. And the theoretically limited spatial resolution of the FMCW reflectometry is achieved.

Spectral properties of an unlocked semiconductor laser under an external light injection are numerically analyzed and experimentally observed. It is found that the unlocked spectrum consists of a shifted carrier and a number of harmonics and that their distributions are varied by the detuning.

The influence of 1/f noise in FM noise on the spectral linewidth of semiconductor lasers in a normal operation has been investigated. The linewidth calculated from the FM noise spectrum considering the 1/f noise agrees with that obtained from the delayed self-heterodyne method.

A novel and simple measuring method for locking bandwidths of injection-locked semiconductor lasers is proposed. The measuring system based on this method can visualize the overall locking bandwidth and quickly measure the boundaries. Some experimental results are also presented.

For the sake of small mechanical vibrational amplitude measurements using a laser diode and an optical fiber interferometer, two stabilized measuring systems based on different methods are described. The system stabilities compensating for environmental perturbations are experimentally confirmed. In both systems, the vibrational amplitude can be measured absolutely.

We have proposed a multiplexed fiber-optic sensor system using an optical loop with a frequency shifter. The measured output power spectrum of the system has shown that the multiprexed signals superimpose upon a noise pedestal which is like a series of hill peaks. In this paper, the output power spectrum is theoretically analyzed from the output intensity autocor-relation function. It displays that the noise pedestal originates from the laser phase-induced intensity noise. The noise level depends on the coherence time of the laser source. The positions of peaks are decided by the working frequency of the frequency shifter in the optical loop. The sensitivity of the system are related to the bandwidth B, the coherence time Tc, the sensor number n to be multiplexed, the loop loss α, and the fiber coupler parameters. Properly choosing these parameters is beneficial to improve the sensitivity of system.

Stability property of an optically injection-locked semiconductor laser taking account of gain saturation is discussed. Numerical analysis shows that stable locking region is broadened due to gain saturation. This is because of rapid damping of relaxation oscillation due to gain saturation. It is also found that stable locking region is also broadened with increasing injection current since damping of relaxation oscillation becomes strong with increasing injection current. Numerical calculations of lasing spectrum show that the magnitude of sidepeaks appeared at harmonics of relaxation oscillation frequency under unstable locking condition are suppressed due to gain saturation.

A novel measuring system on the amplitude and phase of the RF magnetic field is proposed. It is composed of a laser diode (LD) linked with a small loop antenna, an optical fiber transmission line, and phase-sensitive detection instruments. The LD output light is directly intensity-modulated by the RF current induced in the loop antenna by the RF magnetic field under test. The instantaneous amplitude and phase of the RF magnetic field are preserved in the modulated light because of the quick response of LD and are detected by the remote detection system. The characteristics of the calibration, the sensitivity, the minimum detectable field, and the frequency response are analyzed in detail. The system is highly sensitive, nonperturbing, and noninteractive. In addition to these advantages, the excellent property for measuring the phase of the RF magnetic field is experimentally confirmed by many field measurements. Resultantly, the minimum detectable RF magnetic field is obtained as 10 µA/cm (38 nW/cm2 in power density) remarkably lower than others.

The characteristic features of a mutual optical injection-locking system using semiconductor lasers are both analytically and experimentally examined. It is found that in the case of low injection level, locked frequency, locked powers and locking bandwidth of the system are suitably characterized in terms of both a figure of merit specifying unilaterality of optical injection and a modified round-trip phase angle of optical coupling circuit between lasers. The analysis is applicable not only to unilateral optical injection-locking system but also to mutual optical injection-locking system with an arbitrary inverse optical injection. The results of experiment using AlGaAs CSP lasers are found to be in good agreement with the analysis provided that the mutual injection levels are low.

The coherence characteristics of semiconductor laser driven by high-frequency current injection are measured under various operation conditions. The measured results are in agreement with analytical results derived from a simple model, in which all the modes are individually frequency-modulated.