Stochastic resonance in single-mode semiconductor lasers

Owing to the considerable virtues of semiconductor lasers for applications, they have become the main optical source for fiber communication systems recently. The behavior of stochastic resonance (SR) in direct-modulated semiconductor laser systems is investigated in this article. Considering the carrier and photon noises and the cross-correlation between the two noises, the power spectrum of the photon density and the signal-to-noise ratio (SNR) of the modulated laser system were calculated using the linear approximation method. We found that the SR always appears in the dependence of the SNR upon the bias current density, and is strongly affected by the cross-correlation coefficient of the carrier and photon noises, the frequency of modulation signal and the photon lifetime in the laser cavity. Hence, it is promising to use the SR mechanism to enhance the SNR of direct-modulated semiconductor laser systems and improve the quality of optical communication.     

Coupling of concentric semiconductor microring lasers

Coupling between whispering-gallery modes of concentric microcavity lasers was analyzed, and a closed-form three-dimensional vectorial solution for the coupled fields was obtained. Experiments performed with concentric gain-guided vertical cavity ring lasers resulted in regimes of resonant coupling in good agreement with the theory.     

Coupling of large aperture semiconductor lasers by gain profiling

We present results from a theoretical study of two large aperture semiconductor lasers transversely coupled through their gain profiles. We show that we can produce both in- and out-of-phase locking between the lasers and observe a period and bistability in our phase-locking with respect to both design and operating parameters. We discuss the implications of our design for high power laser arrays and compare the performance of our coupled lasers to single devices.     

Analytical evaluation of the line shape of single-mode semiconductor lasers

A simple analytical expression for the line shape of single-mode semiconductor lasers is derived accounting for the presence of the well-known satellite peaks which cause deviation from the classical lorentzian form. An expression for the variance of the difference between the values the instantaneous phase of the field assumes over different time intervals is also obtained which is in good agreement with previously reported experimental results.     

Multimode modeling of digital modulation in nearly single-mode semiconductor lasers

We present multimode modeling of the digitalmodulation characteristics of nearly single-mode semiconductor lasers. The model takes into account the mechanisms of spectral suppression of modal gain; namely, symmetric and asymmetric gain suppressions. The digital modulation performance of the laser is quantitatively examined in terms of the turn-on jitter, on-off ratio and a Q-factor. We study the effect of the modulation parameters on the modulation characteristics of the laser. We also clarify the effect of the asymmetric gain suppression on the modulated signals of the oscillating modes. In addition, we examine the validity of modeling the digital modulation of the laser via a single-mode rate equation model. We show that the modulation characteristics improve with an increase in the bias and/or modulation current or with a decrease in the bit rate due to reduction in the bit-pattern effect. The asymmetric gain suppression does not affect the characteristics of the total laser signal, however it enhances the modulation of the nearest modes on the long-wavelength side of the gain spectrum.     

Coupling of single-mode Ti-diffused LiNbO3 waveguides to single-mode fibers

We report on measurements of the near-field intensity distribution of single-mode T-diffused LiNbO₃ waveguides and on the alignment tolerances when coupled to a single mode fiber. Coupling efficiencies of typically 77 to 87% are calculated and a value as high as 80% has been measured experimentally for a y-cut substrate and TM-like polarisation. The total insertion loss of 1 cm long single-mode Ti-diffused waveguides (c-cut) inserted between single-mode fibers was typically 10 dB.     

A volume holographic optical element for optical coupling to single-mode fibers

We propose and demonstrate a volume holographic optical element (VHOE) for signal-mode fiber coupling and the sending of conjugate waves of reference light for the construction of holographic elements. Given the inherent Bragg conditions of the VHOE, the angular tolerances of the conjugate reading light in both the horizontal and vertical directions are studied under different conditions.     

Suppression of beating noise of narrow-linewidth erbium-doped fiber ring lasers by use of a semiconductor optical amplifier

Beating noise in narrow-linewidth erbium-doped fiber ring lasers puts severe limitations on applications of the lasers. We demonstrate the suppression of beating noise in fiber ring lasers by using a semiconductor optical amplifier in the laser cavity, which acts as a high-pass filter. Two different ring structures are presented as examples to demonstrate this beating noise suppression.     

Tuning of fiber lasers by use of a single-mode biconic fiber taper

We report a novel scheme to build a compact, tunable fiber laser. The tuning mechanism is based on the transmission property of a single-mode biconic fiber taper. While pulling the taper, we observe oscillations in the transmitted optical power that are due primarily to interference between a pair of excited modes within the tapered region, which are eventually coupled into the unstretched single-mode fiber at the end of the taper. A similar mechanism causes the modulation of the transmitted optical spectrum after the taper has been pulled and stabilized. It is this spectral modulation by the taper that is exploited here to control the wavelength of a fiber laser. The modulation can be adjusted by stretching the taper, thus enabling the tuning of the laser wavelength. We have built a 32 mW Er-doped tunable ring fiber laser with a continuous tuning range of over 20 nm and a signal-to-noise ratio of better than 45 dB over the entire tuning range; our output power is limited only by the available pump power.     

Operation of Brillouin dynamic grating in single-mode optical fibers

The first (to our knowledge) observation of Brillouin dynamic grating in conventional single-mode fibers is reported, and the characterization is demonstrated with respect to the external parameters for the grating generation. When a 100 m single-mode fiber is used, a reflectance of 8% with a spectral bandwidth as low as 2.4 MHz is achieved, which is less than 10% of ordinary Brillouin gain bandwidth.     

Reflectometric measurement of birefringence rotation in single-mode optical fibers

We present a novel reflectometric technique for the measurement of orientation and modulus of the linear birefringence vector in single-mode optical fibers. The technique provides information also on circular birefringence, although this component, if present, appears as a rotation of the linear birefringence. A detailed theoretical analysis is reported and validated by experimental results.     

Sensitivity of quantum-dot semiconductor lasers to optical feedback

The sensitivity of quantum-dot semiconductor lasers to optical feedback is analyzed with a Lang-Kobayashi approach applied to a standard quantum-dot laser model. The carriers are injected into a quantum well and are captured by, or escape from, the quantum dots through either carrier-carrier or phonon-carrier interaction. Because of Pauli blocking, the capture rate into the dots depends on the carrier occupancy level in the dots. Here we show that different carrier capture dynamics lead to a strong modification of the damping of the relaxation oscillations. Regions of increased damping display reduced sensitivity to optical feedback even for a relatively large alpha factor.     

Mode hopping in single-mode microchip Nd：YAG lasers induced by optical feedback

The mode hopping phenomenon induced by optical feedback in single-mode microchip Nd：YAG lasers is presented. With optical feedback, mode hopping strongly depends on two factors： the ratio of external cavity length to intra-cavity length, and initial gains of the two hopping modes, When external cavity length equals an integral multiple of intracavity length, there is almost no mode hopping. However, if the external cavity length does not equal an integral multiple of intra-cavity length, mode hopping occurs. The ratio of external cavity length to intra-cavity length determines the position of two-mode hopping, The initial gains of the two hopping modes determine the corresponding peak values and oscillating periods of them in the intensity modulation curves.     

Gaussian-like tapered grating quarter wave-shifted DFB semiconductor lasers for high-power single-mode operation

Received: 27 July 1998/Revised version: 25 September 1998     

Self-organization in semiconductor lasers with ultrashort optical feedback

The dynamical behavior of a single-mode laser subject to optical feedback is investigated in the limit, when the delay time is much shorter than the period of the relaxation oscillations. Use of an integrated distributed feedback device allows us to control the feedback phase. We observe two kinds of Hopf bifurcations associated with regular self-pulsations of different frequencies.     

Numerical study of converting beat-note signals of dual-frequency lasers to optical frequency combs by optical injection locking of semiconductor lasers

The optical injection locking of semiconductor lasers to dual-frequency lasers is studied by numerical simulations. The beat-note signals can be effectively transformed to optical frequency combs due to the effective four wave-mixing in the active semiconductor gain medium. The low-noise Gaussian-like pulse can be obtained by locking the relaxation oscillation and compensating the gain asymmetry. The simulations suggest that pulse trains of width below 30 ps and repetition rate in GHz frequency can be generated simply by the optical injection locking of semiconductor lasers. Since the optical injection locking can broaden the spectrum and amplify the optical power simultaneously, it can be a good initial stage for generating optical frequency combs from dual-frequency lasers by multi-stage of spectral broadening in nonlinear waveguides.     

Synchronization of chaotic semiconductor lasers by optical injection with random phase modulation

It is shown that identical synchronization of two chaotic semiconductor lasers can be achieved by injection of a common optical signal with randomly varying phase. An optical signal with randomly modulated phase is injected into two semiconductor lasers which have chaotic oscillations due to optical feedback. Strong correlation between complex intensity oscillations of the two lasers is observed even though the intensity of the common injection signal is constant. Characteristic properties of this type of synchronization are shown, in particular, the dependence of the synchronization threshold on the injection strength and the rate of phase modulation, and the dependence of the intensity correlation on the difference in phase of optical feedback.