S. Piazzolla

Phase noise and spectral line shape in semiconductor lasers

Experimental results concerning the study of phase noise in single-mode semiconductor lasers are reported, which show a strict connection between phase and intensity noise. In particular, phase-noise spectrum is found to present a sharp peak at the same peak frequency of intensity-noise spectrum, a fact which is proven to be responsible for the appearance of satellite peaks in the emission line shape. Direct measurements of the line shape, performed by means of a Fabry-Perot interferometer, are in agreement with the line shape evaluated by using phase-noise spectrum measurements.

Theory of noise in semiconductor lasers in the presence of optical feedback

We derive analytical expressions for the power spectral densities of intensity and frequency noise of single mode semiconductor lasers in the presence of an optical feedback. By explicitly taking into account the spontaneous emission processes into the laser mode, we obtain a behavior which, at high frequencies significantly differs from the usual one, and is in good agreement with recent experimental results. In particular, we are able to show in which way the intensity and frequency noise, besides being influenced by the external cavity length, are affected by the presence of the well known resonant peaks in the noise spectra of the solitary laser and how a substantial lowering and flattening in the noise spectra can be obtained with external cavity round trip times shorter than the inverse of the peak resonant frequency. We also present experimental results in good agreement with the theoretical ones.

Frequency and intensity noise in injection-locked semiconductor lasers: Theory and experiments

Analytical expressions for the power spectral densities of intensity and frequency noise of single-mode semiconductor lasers operating in a regime of injection locking are derived by appropriately taking into account the spontaneous emission processes into the lasing modes of both the master and slave lasers. They show how the noise spectra of the slave are influenced by the value of the injected power, by the difference between the emission frequencies of the master and slave optical cavities, and how they are correlated to the noise properties of both the master and the free-running slave. In particular, the very low frequency part of the frequency noise of the slave turns out to coincide with that of the master within a certain frequency region whose range increases as the values of the injected signal does, too. We also present measurements of the power spectral densities obtained by means of an experimental apparatus similar to that described in [1] and show how the experimental results are accounted for by the present theory.

Phase noise in semiconductor lasers: A theoretical approach

Noise phenomena in semiconductor lasers have been extensively studied from an experimental point of view because of their importance in designing optical communication systems. Recently, a semiclassical theory of laser noise has been presented [1]. In this paper, following a different and simpler approach, we obtain analytical expressions for the intensity and phase noise power spectra which agree with experimental results.

Small signal analysis of frequency chirping in injection-locked semiconductor lasers

A theoretical and experimental investigation of frequency chirping in injection-locked semiconductor lasers under a condition of small signal modulation is presented. It shows that a substantial reduction of frequency chirping can be obtained which, however, is effective only up to modulation frequencies of the order of the locking bandwidth and is strongly dependent on the frequency detuning between master and slave.

Experimental measurements and theory of first passage time in pulse-modulated semiconductor lasers

The statistical properties of the first passage time T, the time delay between the electrical switch on instant and the attainment of a fixed value of the output optical power in pulse-modulated semiconductor lasers, are measured by three different techniques. The measurements show that for lasers initially biased under threshold, the amount of time jitter defined as the standard deviation of T, is systematically larger in single-mode than in multimode lasers and is essentially determined by the value of the output optical power corresponding to the on state. An analytical theory which gives the reason for the phenomenon is also developed. Computer simulations of the process in agreement with the experimental and theoretical results are presented. >

Direct measurement of the nonlinear phase shift between the orthogonally polarized states of a single-mode fiber

We measure the phase shift induced by the optical Kerr effect between the two orthogonally polarized states of a birefringent single-mode fiber. The associated noise, which can arise whenever amplitude fluctuations of the source are present, is discussed.

Influence of semiconductor-laser phase noise on coherent optical communication systems

Several authors have recently investigated phase noise in semiconductor lasers and the related problems that arise when such lasers are employed in coherent optical communication systems. We report accurate measurements of high-frequency phase noise in single-mode injection lasers that show the presence of a peak in the phase-noise spectrum at the same frequency as that of the amplitude-noise peak. This peculiar phenomenon must be taken into account when one studies the characteristics of coherent optical communication systems.

Analysis of transients in pulse modulated semiconductor lasers biased near threshold

A theory of time jitter in semiconductor lasers initially biased just below threshold is developed. Simulations of the buildup of emitted optical pulses in lasers starting from below or above threshold are also presented which, for initial biasing up to threshold, are in good agreement with the theory. In particular, we show that time jitter does not vary for biasing currents up to about 0.95 of the threshold value, while it suffers a small increase at threshold and a steep decrease for higher biasing currents.

Influence of an external cavity on semiconductor laser phase noise

Investigation of phase noise in semiconductor lasers allows one to understand the physical processes that influence both the width and the shape of the emission line. Furthermore, the limitations associated with phase noise have to be taken into account when semiconductor lasers are employed in coherent optical communication systems. In this letter we report phase‐noise measurements on single‐mode semiconductor lasers in the presence of optical feedback. We are able to observe, besides the expected linewidth reduction, a peculiar behavior in the high‐frequency region not yet reported in the literature, strongly dependent on the external cavity length.