We present a theoretical analysis of the errors of a Zeeman laser interferometer due to the nonorthogonally elliptic polarizations of laser modes. The polarizations make the measured Doppler frequency-shift signal become a modulated and nonharmonic signal and make the phase shift unstable and nonrepeatable when the signal is measured by a phasemeter. If the orthogonality error between two major axes of the laser polarizations is 6 degrees , the relative errors of the phasemeter output will exceed 200% when the measured phase shift is changed from -16.2 degrees to +3.5 degrees . We also introduce a way to eliminate the errors.

This paper gives both theoretical and experimental proof of elliptical polarization and nonorthogonality of Zeeman laser output. It verifies that the polarized modes of the output have 4-7 degrees orthogonal error which is induced by anisotropy in the laser cavity. A new formula for calculation of this orthogonal error has been derived which also provides a way to measure the anisotropy of the laser cavity. We also discuss the influence of nonorthogonality on the spectrum of the laser.

The noise spectra and frequency chirping of semiconductor lasers in the presence of arbitrary amounts of optical feedback are analyzed. Short external cavities with strong optical feedback are found to reduce the noise dramatically in semiconductor lasers, especially in the high-frequency regime. Frequency chirping is shown to be closely related to the nonlinear gain effect.Coherent light-wave transmission and high-bit-rate single-mode fiber communication systems require stable lasers with narrow linewidths. External-cavity semiconductor lasers are a feasible means of meeting this requirement. The intensity and phase-noise power spectral densities of semiconductor lasers in the presence of weak optical feedback have been studied by Spano et al. 1 However, recent experimental and theoretical research indicates that strong externalcavity optical feedback is preferred. 2 4 Tkach and Chraplyvy experimentally distinguished the various operating reigmes according to the level of feedback in long external-cavity lasers. 4 In the strong-feedback regime (regime V in Ref. 4) the laser operates stably with narrower linewidths for all phases of the feedback. The linewidth dependence for and dynamic stability of semiconductor lasers with weak and strong optical feedback are known.5 ' 6 However, a unified study of the intensity and phase-noise spectra and frequency chirping in lasers with arbitrary amounts of optical feedback has not been developed. This Letter presents a theoretical derivation of simple formulas that describe these effects. This analysis also incorporates the effect of nonlinear gain. We show that short external cavities with strong optical feedback dramatically reduce the intensity and phase noises, especially in the high-frequency regime, and that nonlinear gain has a significant impact on frequency chirping but not on the Lorentzian linewidth. The external-cavity semiconductor laser investigated here is a three-mirror-cavity model (Fig. 1). By taking into account multiple random reflections in the external cavity, the rate equations of semiconductor lasers in the presence of arbitrary amounts of optical N(t) = -GI(t) _ N(t) + M(t) + FN(t),where I(t) is the number of photons, N(t) is the number of minority carriers, G is the gain coefficient, AG is the net gain coefficient, 4n(t) is the optical phase noise, Q and wo are the optical frequency with and without feedback, respectively, a is the linewidth enhancement factor, and F,(t) (x = I, X, N) represent the Langevin noises. r = noLic and ri = ni/c are the round-trip time delays in the passive and active cavities, respectively, R, is the rate of spontaneous emission, re is the electron lifetime, M(t) is the injection current, and T dwoHere r 2 is the amplitude reflectivity of the diode facet facing the external cavity and R 3 is the effective amplitude reflectivity, including coupling loss, of the external mirror.Equations (1)- (3) are now linearized, and thus limited to the case of relatively short external-cavity lasers. Linearized versions of 1(t), N(t...

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In this letter, an improved all‐fiber 90° optical hybrid is reported. The experimental unit has the following characteristics: low insertion loss, high stability, wide operation bandwidth, and simple fabrication process of all functional components. The unit will find applications in phase diversity coherent optical communication systems and interferometric sensing systems.