Noise In Semiconductor Lasers Research Articles

Numerical modeling of intensity and phase noise in semiconductor lasers

A self-consistent numerical approach is demonstrated to analyze intensity and phase noise in semiconductor lasers. The approach takes into account the intrinsic fluctuations of the photon number, carrier number, and phase. A new systematic technique is proposed to generate the Langevin noise sources that derive the laser rate equations keeping their cross-correlations satisfied. The simulation is applied to AlGaAs lasers operating in a single mode. The time-varying profiles of the fluctuating photon and carrier numbers and the instantaneous shift of the oscillating frequency are presented. Statistical analysis of the intensity and phase fluctuations is given. The frequency spectra of intensity and phase noise are calculated with help of the fast Fourier transform. The importance of taking into account the carrier number noise source and its cross-correlation with the noise source on the phase is examined by comparing our results with those by conventional methods.

Polarization modal noise and dichroism in vertical-cavity semiconductor lasers

We show quantitatively how the limited polarization stability of vertical-cavity semiconductor lasers is related to the small dichroism and large quantum noise in these lasers. We introduce several techniques to measure this dichroism and noise. Suitable polarization projections are shown to yield either the heterodyne beat or the partition noise of the two polarization modes.

Stabilization of Semiconductor Lasers by Suppressing the Optical Feedback Noise.

Generating mechanism of the optical feedback noise in semiconductor lasers and whose suppressing manner by utilizing the self-pulsation laser or the high frequency superposition method are theoretically reviewed. Those mechanisms are characterized in terms of the mode competition phenomena among the internal cavity modes of the solitary laser and the external cavity modes which are built up by the optical feedback. Suppression of the feedback noise is understood as an effect to shift the operating point to more stable state by help of the modulated photons and electron densities in the laser.

The analysis of noise power spectrum induced by the semiconductor laser phase noise in fibre-optic interferometers considering optical fibre dispersion

In a fibre-optic interferometer, when the frequency linewidth of the light source is larger than a certain value, the influence of optical fibre dispersion should be considered. In this paper, considering the dispersion effect, we present a theoretical analysis of the output noise power spectrum of the fibre-optic interferometer induced by the random phase noise of semiconductor lasers. The analysis focuses on the noise power spectrum influenced by the first-order dispersion of the single-mode optical fibre. The closed expressions of output noise characteristics are derived. The formulations obtained in this paper can be applied to both a dispersion and a non-dispersion fibre-optic interferometer or optical interferometer.

Longitudinal-mode-partition noise and amplitude squeezing in semiconductor lasers

We have measured the longitudinal-mode-partition noise of free-running, grating-feedback external-cavity and injection-locked semiconductor lasers, using a nonsymmetric Mach–Zehnder interferometer. For an external-cavity laser operated at a high pumping level, the contribution of the longitudinal-mode-partition noise to the total intensity noise was negligible, and 0.9 dB of amplitude squeezing was observed. On the other hand, injection locking suppressed the longitudinal-mode-partition noise by 34 dB, and 1.9 dB of amplitude squeezing was observed when the locking bandwidth was 5 GHz. A further increase in the locking bandwidth reduced the suppression of the longitudinal-mode-partition noise and degraded the amplitude squeezing.

Semiclassical model of semiconductor laser noise and amplitude noise squeezing. II. Application to complex laser structures

For pt.I see ibid., vol.33, no.11, p.2097-2104 (1997). We present noise studies of distributed feedback (DFB) laser structures, where spatial hole burning (SHB) plays a key role performed using the model described in part I of this paper with particular emphasis on the influence of SHB, on the coupling coefficient /spl kappa/L, and on the laser facet reflectivities. These structures exhibit high amplitude noise and the possible noise reduction is strongly reduced compared to Fabry-Perot structures. Distributed Bragg reflector (DBR) lasers are better candidates even if their performances are also strictly determined by SHB and the loss in the Bragg reflector. Finally, limitations due to gain suppression are demonstrated for such complex lasers structures. We conclude on the optimum laser structure for amplitude squeezed states generation.

Semiclassical model of semiconductor laser noise and amplitude noise squeezing. I. Description and application to Fabry-Perot laser

A semiclassical model of semiconductor laser noise, based on the Green's function method, is used to derive analytical formulas for the amplitude and frequency noise spectra taking into account incoming vacuum fluctuations and noise due to internal loss. This formalism also takes into account phenomena such as gain suppression as well as spatial hole burning (SHB). The amplitude noise squeezing is studied for Fabry-Perot structures pointing out the influence of the laser structural parameters. A complete agreement with already existing quantum mechanical models is found. However, extension of the model to SHB induces limitations in the squeezing performances, which are very important for more complex structures as will be pointed out in detail, in Part II.

Low-frequency intensity noise in semiconductor lasers

The low-frequency intensity noise at 25 MHz of a Fabry-Perot semiconductor laser is measured as a function of injection current. All the measurements are taken at room temperature and the laser is operated with a commercial current source (the conditions under which laser diodes are often used). At the highest injection current of twice threshold, the intensity noise is 5.5 dB above the shot-noise limit. When the longitudinal side mode suppression of the laser is 20 dB or larger, the intensity noise is modeled adequately by an expression derived from the single-mode, small-signal, linearized, semiclassical rate equations. All the parameters used in the theory are derived or referenced.

QUANTUM NOISE REDUCTION IN LASERS

The intensity noise of semiconductor lasers was brought below shot noise by about 30% using constant current supply together with line narrowing techniques. This result is shown to be in good agreement with theoretical predictions of a laser theory including regulated pumping. The latter theory is applied to the case of Nd:YVO 4 microlasers pumped by the sub-shot noise semiconductor lasers.

Theoretical analysis of noise-reduction effect in semiconductor lasers with help of self-sustained pulsation phenomena

The excess noise of semiconductor lasers has been reduced mostly with the help of the self-sustained pulsation in experiments. A theoretical analysis is given for mechanisms of the noise generation and the reduction method basing on the mode competition theory. Basic equations of these dynamics are given in the form of rate equations, where the nonlinear gain terms and Langevin noise sources are included. Effect of the optical feedback is introduced to give changes in the threshold gain level and by building of external cavity modes formed by the facet of the laser and the reflecting surface. The self-sustained pulsation phenomena are examined in the narrow stripe structure geometries having the saturable absorbing region beside the active region. The amount of noise is expressed relating to time averaged values of the photons and the carrier densities in laser, whose levels are modified by the pulsation. It is shown that the excess noise due to mode competition among the internal cavity modes is well suppressed by the self-sustained pulsation, while noise due to mode competition among the external cavity modes is mostly suppressed but sometimes not suppressed. More general discussions for the mechanisms are also given qualitatively by help of topological charts for dynamics. Three types of mode competitions are indicated. The first one is with a bistable state due to the symmetric profile of gain suppression for wavelength distribution. The second one is with an elliptic eddy motion of dynamics due to weak asymmetric property of the gain suppression. The third one is with a circular eddy motion of dynamics due to the strong asymmetric property of the gain suppression. Some difficulty in the noise reduction in the case of the strong asymmetric property of the gain suppression is mentioned.

Two-photon spectroscopy: A technique for characterizing diode-laser noise.

Using Doppler-free Rb two-photon excitation measurements, we present support for the hypothesis that semiconductor-laser noise is characterized by pure phase-diffusion noise. We measured near-Lorentzian shapes for both the laser and the two-photon excitation spectra and a slope of 3.7\ifmmode\pm\else\textpm\fi{}0.3 for the dependence of the two-photon excitation width on the laser width. These represent measurements of the second-order field statistics of a naturally operating laser where the noise is dominated by spontaneous emission. The measured spectral shape and slope are in excellent agreement with the Mollow model, which predicts a Lorentzian spectral shape and a slope of 4 for weak-field, two-photon excitation with a pure phase-diffusion field (Lorentzian spectral density). The dominance of phase-diffusion noise is further corroborated by an analytically solvable microscopic noise model that includes phase and amplitude noise.

Analysis of relative intensity noise in semiconductor lasers and its effect on subcarrier multiplexed lightwave systems

It is found that the presence of the asymmetric nonlinear gain causes the longitudinal modes of a Fabry-Perot laser as well as a nearly single-mode laser to couple such that the low-frequency relative intensity noise is greatly enhanced. For the first time we have included this asymmetric mode coupling to adequately model the translation of the enhanced low-frequency noise to the signal band of a subcarrier multiplexed transmission system in the presence of both modulation and fiber dispersion. This effect, which is crucial in determining the system's signal-to-noise ratio, is also verified experimentally. Theoretical and experimental investigations of system impairment caused by noise translation and fiber dispersion are also performed. Excellent agreement between the theoretical predictions and the experimental results is obtained. >

Intensity noise and linewidth characteristics of laser diodes with phase conjugate optical feedback

The intensity noise and linewidth characteristics of semiconductor lasers with phase conjugate optical feedback are investigated numerically. The results are compared to those from conventional mirror optical feedback. The dynamics of semiconductor lasers with phase conjugate optical feedback are also considered. The relevance to phase conjugate optical feedback of the feedback regimes used to describe conventional mirror optical feedback is discussed.

Effect of carrier lifetime on mode partition noise in multimode semiconductor lasers

The dependence of mode partition noise (MPN), and its association with the eye diagram and power penalty, on carrier lifetime in 1.3 μm InGaAsP multimode semiconductor lasers for a 1.2 Gbit/s lightwave transmission system was investigated. It was found that lasers with shorter carrier lifetimes showed less MPN, and hence, a better eye opening and a lower power penalty, than the lasers with longer carrier lifetimes. The significant dependence of MPN, eye opening, and power penalty on carrier lifetime in multimode semiconductor lasers suggests that the carrier lifetime of the laser, which depends on the laser design, can be used as an important parameter for characterizing the performance of high-speed lightwave transmission systems.

Volterra functional series expansions for noise in semiconductor lasers

In this paper, we present a full analysis of noise in semiconductor lasers. Time-covariance and cross-covariance functions of all variables governing single-mode semiconductor laser behavior are given including all Langevin noise sources and considering all terms of the laser rate equations, RIN, carrier noise spectrum, frequency noise spectrum, coherence between the FM and AM noise, variance of the phase change and coupling between amplitude and phase are investigated analytically in their more general form. Spectral line shape has been derived showing dissymmetrical satellite lines centered at multiple integers of relaxation frequency around a central line. It is shown that neglecting the shot noise and considering only the carrier contribution in phase dynamics (common procedure) are too general approximations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Phase noise reduction by self-phase locking in semiconductor lasers using phase conjugate feedback

A theoretical analysis of the behavior of the frequency/phase noise of semiconductor lasers with external phase conjugate feedback is presented. It is shown that the frequency noise is drastically reduced even for lasers with butt-coupled phase conjugate mirrors. In this laser system, the phase noise takes a finite-low value corresponding to a state of first-order self-phase locking of the laser. As a result, the spectral shape of the laser signal does not remain Lorentzian but collapses around the carrier to a delta function with a close to carrier noise level of less than -137 dBc/Hz. The total phase variance of this laser signal, in a 20 GHz noise bandwidth, is less than 0.002 rad/sup 2/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Linewidth reduction and frequency stabilization of a semiconductor laser with a combination of FM sideband locking and optical feedback

We describe a novel method for semiconductor laser noise reduction that uses a combination of optical and electronic feedback. A Doppler-free Faraday resonance in Cs vapor provided both optical feedback and discrimination for an electronic feedback scheme incorporating FM sideband spectroscopy. The introduction of electronic feedback further reduced the low-frequency fluctuation noise power by more than 2 orders of magnitude, resulting in a linewidth of 1.4 kHz.

Transfer-matrix theory of the modulation and noise of multielement semiconductor lasers

A general theory of the modulation response and noise of multielement semiconductor lasers is presented based on a transfer-matrix method combined with the Green's function method. An arbitrary laser structure is represented by an assemblage of stacked layers, each of which is assumed to have uniform carrier density and noise sources. A rate equation for the electric field envelope, with which analytical expressions for the small-signal modulation response and the intensity and FM noise are derived in terms of the transfer matrix elements, is derived. The theory can be applied to DFB (distributed feedback) and DBR (distributed Bragg reflector) lasers, coupled-cavity lasers, multielectrode lasers, vertical-cavity stacked-layer lasers, and Fabry-Perot lasers. One of the main advantages of this theory is that the longitudinal cavity effect is incorporated by a simple multiplication and summation of the transfer matrices corresponding to the individual laser segments.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Compared merits of hard/soft decoding for coherent optical communications impaired by phase noise

Phase noise of semiconductor lasers severely degrades the performance of coherent optical fiber communication systems, imposing restrictions on the admissible laser linewidth. The application of a forward error controlling (FEC) scheme is a suitable means to combat the bit-error-rate (BER) degradation due to phase noise and to relax the laser linewidth requirement. In this paper, a heterodyne coherent binary phase-shift keying (BPSK) system and a heterodyne differential phase-shift keying (DPSK) system, both with a Viterbi decoded convolutional coding scheme, are investigated to illustrate the improvements of the system performance. In particular, for the first time, the use of soft decision decoding in such systems is analyzed and its advantage over hard decision decoding is demonstrated. >

On the quantum limit of intensity and phase noise in negative frequency feedback lasers

The quantum limit of intensity and phase noise in negative frequency feedback semiconductor lasers is calculated. It is shown that under some conditions the number-phase uncertainty can be reduced to the ultimate Heisenberg uncertainty limit.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>