A dynamic-single-mode bundle-integrated-guide distributed-bragg-reflector (BIG-DBR-DSM) laser with short active region was experimentally demonstrated for low threshold current and high sub-mode suppression. CW threshold current of 22 mA was obtained for the case of active region length of 50 µm. Sub-modes were suppressed more than 35 dB.

The spontaneous emission factor in the rate equation of the injection laser is derived with help of the classical electromagnetic theory. The spontaneous emission is treated as the radiations from dipoles located in the active region. The value of the spontaneous emission factor given theoretically is in good agreement with the measured results. The magnitude of the spontaneous emission factor is inversely proportional to the volume of the active region and the spectral width of the spontaneous emission, and is proportional to the energy confinement factor and the fourth power of the wavelength. The possibility of the suppression of the relaxation oscillation by reducing the volume of the active region is predicted theoretically.

Transfer function of mode-coupled graded-index multimode optical fibers is given theoretically with an algebraic method, which can be applied to arbitrary index profile and distance. Using the transfer function thus obtained, the relation between the transmission characteristics and the mode coupling due to the random bending of the fiber axis is investigated. As a result it is shown that the length-dependent transmission characteristics such as frequency characteristics, coupling-induced loss and bandwidth can be represented in a unified manner using the coupling length defined in this paper. The frequency characteristics is determined dominantly by the ratio of the fiber length to the coupling length. Further, the length dependencies of both the bandwidth and the coupling-induced loss are represented by a normalized form which does not depend upon the index profile. For the distance which is far from the coupling length, the bandwidth is inversely proportional to the square root of the product of the fiber length and the coupling length, and the coupling-induced loss is proportional to the ratio of the fiber length to the coupling length.

This paper gives an experimental result on the measurement of the maximum coupling coefficient and the coupling length between the active waveguide and the passive output waveguide in GaAs/AlGaAs integrated twin-guide (ITG) injection lasers prepared by LPE. Very high coupling efficiency between the active and the passive waveguides was experimentally confirmed. The maximum coupling coefficient and the coupling length were measured to be about 90% and 250 µm, respectively. These values were in agreement with theory. At properly chosen parameters, the threshold current density on an ITG-type" laser was as low as 3 kA/cm2 at room temperature.

The harmonic distortion and the intermodulation distortion of a laser diode are calculated to be severely influenced by the coherent interaction with reflected waves and are given to be about 2030 dB larger than intrinsic values of the laser diode without reflected waves.

The effect of the intraband electronic relaxation on the relaxation oscillation of the light output from the injection laser modulated directly is examined theoretically. Assuming the k-selection rule, carriers are devided into two parts; that is, carriers which contribute to the laser gain directly, and the rest. The intraband electronic relaxation is taken into account as the coupling of the two parts of the carriers in rate equations. As the result of the small signal analysis of rate equations, the height of the resonance-like" peak in the frequency response of the direct modulation depends on the electronic relaxation time remarkably in the range from 10-13 to 10-12 s. The intraband electronic relaxation possibly concerns with the suppression of the relaxation oscillation of the light output from the GaAs injection lasers.

A method to estimae the coupling length of the fiber in which modes couple each other due to the random bending of fiber axis is given. The coupling length is estimated from the comparison of the measured transfer function to the theoretical one which is derived previously. The coupling length of a graded-index fiber is estimated to be about 0.8 km using this method. Further, the coupling length of recent graded-index fibers is presumed to be about several Km. For step-index fibers, it is estimated to be in the range between 0.5-5 Km.

Effects of the taper-joining of two single-mode fibers, where the fibers are first spliced by fusion, and are then heated and stretched to form the taper-joint, are analyzed and discussed. Due to reductions both in the lateral and angular offsets in the process of forming the taper, the loss can be reduced by one tenth, which is much smaller than that in the case where fibers are spliced after the formation of the taper. It is also pointed out that the joint must be kept straight to avoid the radiation loss due to curving of the joint. Finally, it is ascertained experimentally that the taper joint can be obtained by the arc-discharge method in both the splicing and the stretching processes.

Lasing action in GaInAs/GaInAsP quantum-wire structure, fabricated by two-step OMVPE growth, electron beam lithography, and wet chemical etching techniques, was obtained for the first time at 77 K with pulsed current injection. GaInAs quantum-wires with size of 10 nm thick and about 30 nm wide were completely separated and embedded in GaInAsP optical confinement layers so as to form a separate-confinement-heterostructure quantum-wire (SCH-QW) laser. The evidence of quantum-wire levels was confirmed by comparing its emission spectrum with that of quantum-film (QF) structure both experimentally and theoretically. The results indicate that there is no serious defects or damages in the laser operation of quantum-wires fabricated by the combination of electron beam lithography, wet chemical etching, and regrowth techniques.

A method of exact analysis for the cylindrical fiber with an arbitrary but azimuthally symmetric refractive-index distribution is given. The index distribution is simulated by a multilayer structure and the analysis is formulated exactly by using iterative multiplication of four by four matrices. Then the four by four matrices are approximately separated into two systems of two by two matrices with a good accuracy for the fiber with small index difference. In this way, the computing time is reduced almost 1/6 times compared with the former method. As an example of the application, the propagation modes of a focusing fiber are identified which previously were not exactly known. It is revealed that the group-velocity differences between the modes which are degenerate in the scalar analysis can be larger than those of the non-degenerate modes.

A new type of a dynamic-single-mode laser, which has distributed reflector (DR) both in active and passive regions, is proposed and analyzed to attain high output efficiency with the superior single mode property without increase of threshold current density.

High speed pulse modulation of the injection lasers at non-bias condition has been studied to reduce the pattern effect. Rate equations involving the effect of spontaneous emission enable us to analyze the pulse response of carriers and photons both starting from zero in the cavity. The increase of threshold current and the decrease of delay time are derived as a function of the width of driving pulse current at the non-bias condition. Theoretical results were in good agreement with the experiments. The effect of accumulation carriers in the active region is analyzed in details. It is pointed out that the effect of accumulation carriers in the active region limits the rate of pulse repetition when the pattern effect is required to be small. To overcome this effect a new method which utilizes the application of reverse pulse to a laser diode immediately after the emission of light pulse is proposed. This method has been proved experimentally to be effective. A series capacitance to a laser diode was used to produce reverse pulse, and in this way two pulses have been applied at the interval of 2 nsec. Further investigation on the pulse repetition rate together with the consideration of series resistance in the driving circuit has lead to the conclusion that a few Gbit/sec is possible with AlGaAs DH lasers at the non-bias condition. In these lasers, however, very thin active layers with low impurity concentration are required.

In order to find the optimum index profile of the single-mode optical fiber with respect to the cabling loss due to the random bends with the Gaussian correlation function, the relation between the random-bend loss and the index profile is investigated. The formula of the random-bend loss is derived for arbitrary index profile. In the derivation, the loss is calculated as the power of dipole radiation due to the equivalent refractive index perturbation in contrast with the conventional mode coupling analysis. Using this formula, random-bend losses are calculated for four types of index profiles; the convex, parabolic, step, and concave type of index profiles, under the condition that connection losses are same for all types of profiles. As results of the comparison, the random-bend loss is minimum in the case of the convex type of profile and it is decreased down to 40% of that in the case of the step profile.

It was shown experimentally that the noise, which is estimated to be due to mode hopping among modes composed of a laser mirror and an external reflection point, was suppressed in distributed-Bragg-reflector(DBR) single mode lasers biased above a certain level. This tendency agree with our theoretical prediction.

Mass transport was first employed in an OMVPE system for 1.55 µm GaInAsP/InP laser. The wafers grown by OMVPE were treated at 700 under cracked PH3 and H2+N2 atmosphere for 1 hr, resulting in buried structure and the BH laser showed low threshold current of 50 mA (pulsed) without optimization.

The problems for the controllability of the quantum state are considered for application to optical communications. The new optimization problems arise in the quantum communication theory in addition to problems considered up to now. The concrete example of the controllability problems is Two-photon coherent state (TCS) generated by Two-photon laser. The optimum conditions for the parameters of TCS, which minimize the error probability, are given when TCS is employed as the transmission state in the quantum optimum receiver of the binary system, and its numerical behavior is shown. Furthermore the noise behavior associated with the TCS in the several optical receivers are shown. As the result, the ultimate noise limit hitherto can be reduced by using TCS in the optical communication systems. The saving power, when we employ the TCS as the transmission state in the binary communication system, is about 3 dB in the direct photo detector, and it is 5 dB in the optical homodyne receiver. Furthermore the saving power in the quantum optimum receiver is 4 dB. The SNR of the direct photo detector is improved by more than 6 dB. The information performance of the homo dyne is superior to the heterodyne when TCS is employed, while the heterodyne is superior to the homodyne in general when an ordinary coherent state is employed. Thus, it is suggested for reduction of the noise problems that the Two-photon laser which will be realized in the future will convey the significant benefits to the optical communications.

A new way to reduce the linewidth enhancement factor of Distributed Reflector dynamic-single-mode (DR-DSM) lasers is proposed, which is detuning of lasing wavelength from the Bragg wavelength. The linewidth enhancement factor αof DR-DSM laser was found to be reduced to half of medium defined value αmedium when the phase shift value between the active and the passive distributed reflectors is 0.25π.

For the dynamic single mode (DSM) laser, the condition for suppression of the noise caused by the mode hopping among composite resonator modes induced by an external reflection is derived theoretically. The gain suppression effect in the laser media suppresses the mode hopping noise under the appropriate laser parameters and the operating conditions. Theoretical results were verified by experiment of noise measurement of BIG-DBR type DSM lasers.

We propose a new type light amplifier composed of an integrated twin-guide (ITG) structure and discuss its amplification characteristics. It has two coupled guides: one is an active guide for the light amplification and the other is an output guide which is a loss-less passive guide for input and output of the light. Using a matrix method we derive two expressions of gain for transmission and reflection types of amplification. We obtain the regions of parameter in which the amplifier operates stably. The reflection type gain is usually larger than the transmission type one, because the path of the reflected light is longer than that of the transmitted light in the active guide. Although the amplifier has a resonator, the transmission type gain becomes insensitive to the wavelength of the amplified light under a certain condition. This feature is useful for the stable and wide-band amplification. We also discuss its noise characteristics by regarding only spontaneous emission as a noise sourse and express the signal to noise ratio and the noise temperature. The noise temperature is considered to be the temperature of a black body which emits the same amount of light as the spontaneous emission of the amplifier. As a result of numerical calculation, the temperature is estimated to be over a few 1000 K of a AlGaAs/GaAs laser amplifier, but the signal to noise ratio is expected to be 10-30 dB.

A new intersectional switch of small size using total internal reflection generated by an electric-field-induced refractive-index-variation in the multiquantum well (MQW) structure is proposed. The intersectional angle is expected theoretically to be more than 10 for the applied field of 32.8 V/µm in GaInAsP/InP MQW waveguide. This switch is expected to be of small size with high speed response, and is integrable monolithically together with integrated lasers.