Electrical mode-hopping noise in external-cavity semiconductor lasers and mode-hopping elimination by a nonoptical control loop

The longitudinal mode-hopping and the associated terminal electrical noise in AlGaAs buried heterostructure lasers in an external-cavity configuration are investigated. It is found that the electrical mode-hopping noise has a1/f-dependence when two external-cavity modes are involved in the mode-hopping. Furthermore, it is found that the optical feedback can suppress the electrical noise significantly below the noise level of the free-running laser. This corresponds to a reduction of the emission line width which is in agreement with the reduction of the electrical noise level. A novel, nonoptical, scheme for active elimination of mode-hopping attributable to thermal drift or mechanical disturbances in CW-operated external-cavity lasers is also proposed and demonstrated. Using this method, where the electrical noise is used as an error signal in a control loop, long-term single external-cavity mode operation and low electrical noise and optical intensity noise levels were maintained.     

Low-frequency mode-hopping optical noise in AlGaAs channeled substrate lasers induced by optical feedback

Experiments on mode-hopping noise induced by optical feedback are presented for AlGaAs CSP lasers. The influence of the amount of feedback and of the output power have been investigated. It turns out that the amount of excess noise associated with mode hopping strongly depends on the occurrence of hysteresis associated with the mode-hopping transition. The results indicate that the gain suppression near the lasing mode is asymmetric.     

Polarization-resolved low-frequency noise in GaAlAs lasers

Low-frequency intensity noise displaying 1/fdependency of the light polarized parallel and perpendicular to the junction plane and the correlation between these noise components is investigated in GaAlAs diode lasers. The absolute noise level for the component polarized parallel to the junction (lasing component) rises about 40 dB as the current passes through lasing threshold, while the noise for the orthogonal polarization drops by about 5 dB. The two noise components are well correlated slightly below threshold (coherence function ∼ 0.9), but the coherence function drops rapidly to near zero above threshold. An exception to this behavior occurs in the lasing mode when mode hopping takes place, when the two noise components are well correlated. A qualitative explanation for these phenomena is based on the relation between spontaneous and stimulated emission rates and fluctuations in the intracavity carrier density.     

Suppressing mode hopping in semiconductor lasers by orthogonal-polarization optical feedback

Mode hopping is characterized by a stochastic exchange of power between two longitudinal modes of a laser, inducing a high-level intensity noise in the laser's output. This investigation proposes an orthogonal-polarization optical feedback (OPF) method to suppress mode hopping in semiconductor lasers. Experimental results indicated that, under some operating conditions, mode hopping could be completely suppressed by an OPF of around -29.3 dB in feedback ratio while the laser was conducted into a single-mode state. Moreover, the mode-hopping region was significantly reduced, enabling the laser's wavelength to be tuned continuously over a wider range. These results can be used to greatly improve the performance of semiconductor lasers.     

Interferometric method for preventing mode-hopping in tunable external-cavity semiconductor lasers

A problem in frequency-tuning of external-cavity lasers is mode-hopping between neighbouring external-cavity modes. We demonstrate a new interferometric method for monitoring mode-hoping and an automatic control circuit for a 1.3 ?m grating external-cavity laser that maintains single-mode operation when the lasing frequency is tuned.     

Effects of nonlinear gain on mode-hopping in semiconductor laserdiodes

A systematic and comprehensive analysis of longitudinal mode-hopping, due to nonlinear gain, and its influence on the design criteria of transverse-mode-controlled semiconductor laser diodes are presented. An existing nonlinear model, which was derived using a density matrix formalism, has been extended in this paper to generate the nonlinear gain coefficient matrix. Properties of the nonlinear gain coefficient matrix, which describes the interaction among cavity modes, are discussed. Using the new nonlinear gain in the steady-state multimode rate equations, conventional Fabry-Perot (FP) and short cavity Fabry-Perot (SFP) semiconductor laser diodes have been numerically simulated. Design issues such as cavity length, cavity volume, facet reflectivity, spontaneous emission factor, mode wavelength, intraband relaxation time, linewidth enhancement factor, and laser structure are also discussed. It is shown that increasing the injection current causes the lasing mode to jump to longer wavelengths. Furthermore, increasing the spontaneous emission factor reduces the dynamic range of laser operation without mode-hopping, and vice versa for short cavity. It has been also shown that the carrier density in the active region shifts to higher values (i.e., experiences a kink) at the onset of mode-hopping. Finally, the total modal gain (linear and nonlinear) competes as the injection current increases     

Intensity fluctuation in semiconductor lasers coupled to external cavity

Broad peaks are observed in the intensity noise spectrum for external cavity AlGaAs lasers. Oscillating longitudinal mode spectrum measured with a high resolution Fabry-Perot interferometer indicates that the intensity noise peaks result from the lasing of several closely spaced external cavity modes. A new mechanism is proposed to explain the observed noise spectrum; the intensity fluctuation is induced by modulation of the dielectric constant in the semiconductor cavity due to the beating of the electric fields for lasing modes. Qualitative agreements between the theory and experiments are obtained.     

Proposal and finite-difference time-domain simulation of whisperinggallery mode microgear cavity

The existence of many high Q whispering gallery modes in microdisk and microcylinder lasers seriously affects the internal efficiency in lasing operation and disturbs the enhancement of the spontaneous emission factor. To suppress these modes except for one lasing mode, we propose the microgear cavity having a grating with the same period as that of the mode standing wave. A finite-difference time-domain simulation theoretically demonstrates that the microgear selects one resonant mode that satisfies the unique condition regarding the mode order and the phase. This paper describes the dependence of the mode behavior on some structural parameters, and concludes that a deep grating, which seems to be possible in an experiment, allows for sufficient suppression of any types of nonlasing modes and the enhancement of the Q factor of the lasing mode     

Dual-wavelength DBR fiber laser

Stable continuous-wave lasing of two longitudinal modes of a distributed Bragg reflector fiber laser is reported. Intensity noise and coupling between the modes was characterized for both 0.8- and 0.2-nm mode separations.     

Theory of mode competition noise in semiconductor injection lasers

A theoretical analysis of the mode-competition noise in semiconductor injection lasers is given. The source of the noise is supposed to be fluctuations of the number of photons and electrons on optical emission, and is amplified by optical gain in which the mode-competition phenomena are taken into account. When a lasing mode jumps to another mode, noise becomes largest and shows1/f^{alpha}characteristics on its frequency spectrum, where the index ofalpha = 2is theoretically obtained based on a perturbation analysis of the noise. The noise of an individual mode is in general larger than that of the overall lasing modes. The noise reduces to the level of the quantum noise when the laser operates either in single mode or in stable multimodes.     

Partition fluctuations in nearly single-longitudinal-mode lasers

A simple model of mode partition noise is developed, which is valid for semiconductor lasers operating CW, having a single active region and oscillating primarily in one dominant mode. It is shown that the intensity fluctuations in the nonlasing modes occur with characteristic times of a few nanoseconds and have exponential probability distributions. We show that the lasing mode is able to follow adiabatically fluctuations in the nonlasing modes in such a way that the total intensity remains constant. Our model quantitatively accounts for the observations of Linke et al. that the frequency of "dropouts" in the lasing-mode intensity decreases exponentially with the ratio of average intensities of the lasing and nonlasing modes, and that in digital communication applications a ratio of about 50 is required to reduce the resulting bit-error probability tolsim 10^{-9}. We also account for the probability distribution of the lasing-mode intensity, which was observed by Liu et al to be exponential at low intensity and Gaussian at high intensity. The exponential fluctuations constitute partition noise induced in the lasing mode by large Gaussian fluctuations in the field amplitude of the nonlasing mode. Additional noise sources associated with the carrier density and the lasing-mode field produce Gaussian behavior at high intensities plus a negligible broadening of the low-intensity exponential tail.     

Mechanism of the noise reduction method by superposition ofhigh-frequency current for semiconductor injection lasers

Semiconductor injection lasers sometimes reveal excess noise called the mode hopping noise, which is associated with the mode hopping phenomena among longitudinal modes. The mode hopping phenomena are caused by coupling effects among lasing modes. To reduce this kind of noise superposition of high-frequency (HF) current on the injection current is frequently used. This reduction method is theoretically analyzed based on the mode competition theory and is compared with experimental measurements. It is confirmed that the coupling effect among the longitudinal modes is released with the HF superposition because of enhanced vibration of the injected electron, resulting in reduction of the mode hopping noise     

Mode-switching in semiconductor lasers

In this paper, we experimentally analyze the modal dynamics of quantum-well semiconductor lasers. Modal switching is the dominant feature for semiconductor lasers that exhibit two or several active longitudinal modes in their time-averaged optical spectrum. In quantum-well lasers, these dynamics involve a periodic switching among several longitudinal modes, which follows a well-determined sequence from the bluest to the reddest mode in the optical spectrum. This feature is radically different from the well-known noise-driven mode-hopping occurring in bulk lasers which involves only two main modes. We analyze the differences in modal dynamics for these two kinds of laser by comparing the modal switching statistics and by studying the effects of noise and modulation in the pumping current.     

Lateral mode discrimination in surface emitting DBR lasers withcylindrical symmetry

The effective reflectivity and the threshold gain of all lateral modes in a surface emitting DBR laser with cylindrical symmetry are analyzed using the coupled-mode approach. Our results indicate that when a cavity is designed for operation in the odd modes, all even modes are effectively eliminated. A small perturbation is introduced into the complex dielectric constant of the active region to suppress the unwanted lateral modes of odd symmetry. The model predicts that lasing will occur in a single mode with a narrow far-field pattern     

Analysis on state of polarization of stimulated Brillouinscattering in an optical fiber ring-resonator

As a basic study of a Brillouin fiber-optic gyro, the state of polarization of stimulated Brillouin scattering in an optical fiber ring-resonator is analyzed. A high-birefringent fiber ring-resonator with 90° polarization axis rotation at the splice is considered as a candidate to have a stable lasing. It has two special states of polarization as the lasing modes, namely the polarization lateral modes. The lasing frequencies, namely the longitudinal modes for one state are fixed just in the middle point between the two neighboring lasing frequencies for the other in this resonator. This means that the stable single mode lasing may be obtained. The lasing threshold, the alternation of the lasing modes are discussed both by the simplified theory and the strict theory     

Characteristics of mode-hopping noise and its suppression with the help of electric negative feedback in semiconductor lasers

Some detailed properties of mode-hopping noise in semiconductor injection lasers are reported, and the validity of electric negative feedback as a method of suppressing the noise is demonstrated. Mode-hopping noise was found to be an intrinsic property of the laser, independent of optical feedback (reinjection of the output light) reflected at the surface of an external device. The optical feedback works to shift the operating point of the laser from a stable condition having single-mode operation to an unstable condition revealing the modehopping phenomena. Characteristics of the mode-hopping noise support well the theoretical analysis in which the source of the noise is assumed to be fluctuations of the emission and the electron wave accompanied by spontaneous emission and in which the noise is amplified through mechanism of the mode competition phenomena. Mode-hopping noise was suppressed with the help of the electric negative feedback from the optical detector to the injection current source more effectively than by another reducing method of superposition of the high-frequency current. The suppressed noise level was lower than that at stable single-mode operation.     

Multicolor single-wavelength sources generated by a monolithic colliding pulse mode-locked quantum well laser

The authors report on the separation of single longitudinal modes from the mode-locked program spectrum of a 300-GHz monolithic colliding pulse mode-locked (CPM) semiconductor quantum-well laser. Experimentally, the selected longitudinal mode shows a 10-dB reduction of low-frequency relative intensity noise compared to that of the selected mode from the same laser in continuous-wave (CW) lasing conditions. The strong phase coherence among the passively mode-locked longitudinal modes reduces the partition noise of the unlocked CW laser.<>     

Tunable grating-coupled laser oscillation and spectral hole burningin an InAs quantum-dot laser diode

Emission spectra are investigated of a low-threshold InAs quantum-dot laser of the “dots-in-a-well” (DWELL) type operating near 1230 nm. An external dispersion cavity with a diffraction grating is coupled to the laser diode to suppress the subsidiary modes and to tune the central wavelength. A wavelength-dependent competition between the grating-coupled mode and the internal Fabry-Perot modes of the laser suggests that a hole burning in the spectral density of a DWELL laser occurs with a characteristic spectral half width of ~13 nm (10.5 meV). Simple models of spectral “flattening” and spectral hole burning are presented to explain the broad free-running and grating-coupled lasing spectra of the DWELL device     

Tunable dual-wavelength fiber laser

A narrow-linewidth, tunable, dual-wavelength fiber laser operating at room temperature with each lasing wavelength in single-longitudinal-mode operation is demonstrated. A commercially available tunable fiber Bragg grating was used to tune one of the lasing lines. An unpumped elliptical-core erbium-doped fiber was used as a saturable absorber to suppress mode hopping. Wavelength switching was achieved using a polarization controller. The linewidth (FWHM, full width at half maximum) of the laser line was 6.7 MHz and the OSNR (optical signal to noise ratio) was more than 40 dB.     

高功率横流CO2激光横模随时间变化的测量

用激光光束分析仪实时在线测量了一台连续运转的高功率横流CO2激光器的激光横模,获得了较为完整的激光横模随时间的变化关系.测量结果表明,激光输出由两个分离的、非对称的高阶横模叠加组成("双模"),两横模中心相距约8 mm.形成"双模"的原因是串接式谐振腔的前腔和后腔两者的中心轴线不重合以及阴极铜管形变.横模随激光器运行时间的延长而逐步蜕化,呈更高阶和复杂化趋向.初步分析表明,横模的时间变化主要与激光器连续运转过程中腔内气体组分发生的变化有关.