Influences on the semiconductor laser properties of external optical feedback, i.e., return of a portion of the laser output from a reflector external to the laser cavity, have been examined. Experimental observations with a single mode laser is presented with analysis based on a compound cavity laser model, which has been found to explain essential features of the experimental results. In particular, it has been demonstrated that a laser with external feedback can be multistable and show hysteresis phenomena, analogous to those of non-linear Fabry-Perot resonator. It has also been shown that the dynamic properties of injection lasers are significantly affected by external feedback, depending on interference conditions between returned light and the field inside the laser diode.

External optical feedback effects on semiconductor injection laser properties

The phenomena occurring in semiconductor lasers due to weak external optical feedback are reviewed, including mode hopping and related mode-hopping noise, linewidth narrowing and broadening, and the transition to the coherence collapse regime. Guidelines are given for designing semiconductor lasers, both edge emitters and VCSEL's, with high endurance against external optical feedback. >

External optical feedback phenomena in semiconductor lasers

In optical fiber networks, the semiconductor laser source may be subjected to unavoidable optical feedback from fiber pigtails, fiber connectors and other components, unless expensive optical isolators are used. Therefore, a good understanding of external optical feedback phenomena in semiconductor lasers is required. This paper will review these phenomena, including linewidth narrowing and broadening, mode hopping phenomena and the transition to the so-called coherence-collapse regime. In particular, laser designs with high endurance against optical feedback will be discussed.

As new optical devices for increasing further the utility of and to expand the application of fiber-optic communications, grating multiplexers and isolators have been developed for 0.8 μm band employing microoptic approach. The development of these devices is the subject of this paper. The devices have desirable features of small size, compactness, high optical performances, and high reliability. The grating multiplexer consists of a graded-index rod, a blazed reflection grating replicated onto the graded-index rod slanting facet or a wedge facet, and an input-output fiber array. Simple calculations have been done to determine necessary element parameters for a given channel spacing. Experimental results are presented for five-channel multiplexers devised using a SELFOC®lens. Around 3 dB insertion loss and less than -30 dB crosstalk have been obtained for about 35 nm channel spacing in overall device size of 18 \times 13 \times 50 mm. Faraday rotation optical isolators for 0.8 μm band have been miniaturized by employing an efficient paramagnetic glass Faraday rotator, a magnet with a through hole and a folded optics in the Faraday rotator. The path number in the folded optics has been optimized in terms of trading-off between the magnet size and the insertion loss. A 0.9 dB insertion loss including fiber coupling loss and 36 dB isolation have been obtained in overall device size of 24 \times 24.5 \times 42 mm. Results on the temperature and wavelength dependence of the isolation are also presented. In addition, fundamental properties of optical circulators for 0.8 μm band and optical isolators and circulators both for 1.3 μm band, developed as extended modifications of the optical isolators for 0.8 μm band, are briefly described.

Microoptic grating multiplexers and optical isolators for fiber-optic communications

Simple analytical expressions for the oscillation frequency, quantum FM noise spectrum, oscillation power spectrum, spectral linewidth, and direct optical frequency modulation efficiency in semiconductor lasers with external grating feedback are presented. Experimental results for the grating loaded AlGaAs lasers are in good agreement with the theoretical predictions. Remarkable reduction in spectral linewidth to less than 50 kHz is achieved by feeding back only 10-3of the output power. Oscillation frequency stabilization, frequency jump behavior, and reduction in direct optical frequency modulation efficiency are also discussed.

Oscillation center frequency tuning, quantum FM noise, and direct frequency characteristics in external grating loaded semiconductor lasers

Output intensity noise spectra of semiconductor lasers coupled to optical fibers with end-butt joints are measured to show periodic peaks with a fundamental frequency decided only by the fiber length. The frequency is found to coincide with the reciprocal of the roundtrip time of the light beam in the optical fibers.

Self-coupled phenomena of semiconductor lasers caused by an optical fiber

This paper reports that lasing spectra of semiconductor lasers coupled to optical fibers with end-butt joint show nearly random lasing at wavelengths decided only by the fiber length. Wavelength spacing is found to coincide with the resonance wavelength spacing of the optical cavity formed by the optical fiber.

I. Ikushima

Papers

Self-coupled phenomena of semiconductor lasers caused by an optical fiber

Lasing spectra of semiconductor lasers coupled to an optical fiber