Linewidth of monolithic external cavity DFB lasers

An expression for the linewidth of DFB lasers with lossy external cavities is derived. The influence of facet reflectivity and high feedback levels is discussed     

Linewidth rebroadening in DFB lasers due to weak side modes

A new mechanism for linewidth rebroadening in distributed feedback (DFB) lasers is proposed. It is shown that mode partitioning between the main mode and a weak side mode in conjunction with the difference in longitudinal distributions can lead to strong rebroadening even for side mode suppression ratios (SMRs) larger than 30 dB. An analytical expression for the new linewidth contribution is derived. The contribution increases with the third power of the photon number in the dominant side mode. This agrees with experiments on DFB lasers when one generally observes a strong increase in linewidth where the SMSR decreases below 30 dB. This work and the reported experimental results clearly show that linewidths increase if the side mode suppression goes down. However, it seems that in some DFB lasers rebroadening can occur without any decrease in SMSR, so that other mechanisms need to be considered.<>     

Linewidth enhancement in quantum-well lasers

We have developed explicit approximations for the Hnewidth enhancement factor in quantum-well lasers. These simple expressions represent a quick and easy means of calculating the linewidth enhancement factor under a variety of operating conditions, and help us to understand the physics influencing this important parameter.     

Linewidth enhancement factor for quantum-well lasers

The linewidth enhancement factor ? is calculated for an idealised GaInAs quantum-well laser. Values between 1 and 2 are found for a range of possible lasing energies. Somewhat larger values are expected for GaInAs quantum wells with InP barriers. It is concluded that dramatic reductions in ? are not to be expected for quantum wells.     

Linewidth enhancement factor of 1.3 mu m InGaAsP/InP strained-layer multiple-quantum-well DFB lasers

The linewidth enhancement factor alpha in a 1.3 mu m InGaAsP/InP strained-layer multiple-quantum-well (SL-MQW) distributed feedback (DFB) laser has been evaluated from the relation between the frequency and intensity modulation indexes, and the spontaneous emission spectra below threshold current. It is demonstrated that the measured alpha -parameter of a 1.3 mu m SL-MQW DFB laser is about two, and is much smaller than that in a conventional bulk DFB laser. From the resonance frequency dependence on the output power, it is concluded that this reduction of the alpha -parameter originates in the increased differential gain. The reduction of wavelength chirping, as a result the low alpha -parameter, was experimentally confirmed for the SL-MQW DFB laser.<>     

Linewidth enhancement in distributed-feedback semiconductor lasers

It is demonstrated that the linewidth enhancement in distributed-feedback (DFB) laser diodes can differ significantly from (1+ alpha /sup 2/), where alpha is Henry's linewidth enhancement factor. The influence of the relevant laser parameters, the detuning of the gain peak wavelength, a pi /2 phase shift and a reflecting end facet on the linewidth enhancement is discussed with respect to an improved DFB laser design.<>     

Linewidth reduction in DFB laser by detuning effect

The detuning effect on the spectral linewidth in the DFB laser has been investigated. When we set the lasing wavelength of the DFB laser at the shorter side of the gain peak by 10 nm, a reduction in spectral linewidth of 50% was obtained experimentally. This result agreed with the theoretical prediction.     

Linewidth modifications due to nonorthogonality, inhomogeneity, anddirectionality of longitudinal laser modes

In a laser cavity, deviations from uniformity, whether explicitly present in index and gain or implicitly due to outcoupling, influence the linewidth. An explicit expression for the longitudinal modification factor is derived in the WKB approximation using a nonorthogonal quasi-mode decomposition. Both enhancement and reduction can occur, depending on the specific inhomogeneous distributions of index and gain. Dramatic noise-enhancement figures of 100 or more are predicted for a single-mode laser with strongly localized gain. The author emphasizes the classical rather than the quantum mechanical nature of the modifications, even when the laser operates in the quantum regime     

Linewidth enhancement factor in strained quantum well lasers

The linewidth enhancement factor α of strained quantum-well lasers is analyzed by the k-p perturbation method using the effective-mass approximation. It is found that the α factor in a strained In_0.80Ga_0.20As/InP quantum-well (QW) laser with 1.9% biaxial compression is less than 1.5. For a strained QW laser with p-type modulation doping (MD) of 5×10 ¹⁸ cm^-3, the α factor is as small as 0.8. It is also demonstrated that the spectral linewidth and wavelength chirping in the strained MD QW laser are significantly less than those in conventional bulk and QW lasers     

Linewidth broadening in semiconductor lasers due to mode competition noise

Measurements have shown a correlation between linewidth and amplitude noise in a single longitudinal mode of a semiconductor laser under stable DC operating conditions. Calculations have indicated that the noise produces a broadened linewidth and the apparent discrepancy between experiment and theory can in part be resolved.     

Linewidth rebroadening in semiconductor lasers due to lateral spatial holeburning

The linewidth of semiconductor lasers is calculated taking into account a transversely inhomogeneous carrier distribution within the active region which gives an additional contribution to laser linewidth if the material linewidth enhancement factor is carrier density-dependent. For the first time it is theoretically shown that lateral spatial holeburning causes linewidth rebroadening in BH DFB lasers at high output power.<>     

Linewidth enhancement factor and nonlinear gain in ZnSesemiconductor lasers

We have investigated the effects of the Coulomb interaction on the optical gain and the refractive index of ZnSe semiconductor lasers. The Coulomb interaction increases the differential gain, leading to a decrease of the threshold carrier density. Its influence on the linewidth enhancement factor and the nonlinear gain coefficient is relatively small because it increases both the gain and the refractive index simultaneously. We have compared the linewidth enhancement factor α and the nonlinear gain coefficient ϵ for ZnSe and GaAs lasers with the effects of the Coulomb interaction taken into account. For typical values of total cavity losses, the values of α and ϵ are higher for ZnSe lasers compared with GaAs lasers     

Theoretical calculation of the linewidth enhancement factor of DFB lasers

The linewidth enhancement factor ? of a single-longitudinal-mode DFB laser is theoretically analysed. It is shown that the oscillating wavelength should be selected at the wavelength 100?200 ? shorter than that of the gain peak to obtain a smaller ? value. It is also shown that the lower threshold gain gives a smaller linewidth enhancement factor.     

Amplified-spontaneous-emission spectrum of the radiation field in surface-emitting DFB lasers

In this paper, the authors calculate the amplified-spontaneous-emission spectrum of the radiation field in surface-emitting distributed feedback (DFB) lasers. The response of the laser cavity to the Langevin noise source in the frequency domain is obtained using the newly developed Green's functions for the slowly varying amplitudes of the guided waves. The authors show that the power spectra from the surface and the edge are different, and this discrepancy is due to excitation of the radiation field by the interference between the counter-propagating waves inside the cavity. This feature can be properly exploited in the design of surface-emitting DFB lasers for optical communications.     

Power enhancement in argon II narrow-tube lasers due to zeeman effect by a transverse magnetic field

The role of the Zeeman effect for the power enhancement of a small-bore argon ion laser (488.0 nm) by applying the transverse magnetic field has been investigated both experimentally and theoretically. The σ-polarized laser has a larger output power than the π-polarized laser in the wide range of magnetic field strength. Specifically, at weak magnetic fields, enhancement occurred only for the σ-polarized laser. The experimental results are interpreted in terms of the semiclassical theory of the Zeeman laser. A narrow spectrum envelope is observed in the σ-polarized laser. In the case of internal mirrors without Brewster window, it is demonstrated experimentally that π and σ polarization coexist and compete with each other.     

Large-signal dynamic model for gain-coupled DFB lasers based on the transmission-line laser model

New scattering matrices are developed for the transmission-line laser model (TLLM) to enable the dynamics of gain-coupled DFB lasers to be studied. The resulting model is able to simulate large-signal transient responses and spectral characteristics of gain-coupled DFB semiconductor lasers, and gives threshold gains in excellent agreement with analytical formulations.<>     

Direct cavity assessment and mode optimisation technique for DFB fibre laser

A technique for direct real-time assessment of distributed feedback fibre laser cavity conditions during operation is demonstrated and used to provide a cavity mode conditioning feedback mechanism to optimise output performance. Negligible wavelength drift is demonstrated over a 52 mW pump power range.     

Linewidth broadening and mode splitting due to weak feedback in single-frequency 1.5 ?m lasers

Measurements of the effect of weak feedback on single-frequency 1.5 ?m lasers are reported. The results are found to agree well with published theoretical analyses. Sensitivity to feedback is found to be the same for DFB, C3 and Fabry-Perot lasers. These results indicate that coherent systems using DFB lasers may require greater than 50 dB isolation.     

Linewidth rebroadening due to nonlinear gain and index induced by carrier heating in strained quantum-well lasers

The carrier heating has been recently recognized as one of the main origins of nonlinear gain, in particular in strained quantum-well lasers. The asymmetry of this effect introduces a nonlinear refractive index. The joint effects of nonlinear gain and nonlinear refractive index that are both due to carrier heating together with the spatial-hole-burning give rise to an increase in the carrier density and in the linewidth enhancement factor. These effects can explain the linewidth rebroadening at high power in phase-shifted single-mode DFB lasers.     

Thermal frequency drift suppression in tunable DFB lasers by plasmainduced frequency shift enhancement

The authors propose enhanced-plasma-effect (EPE) lasers for coherent optical frequency division multiplexed networks. In the EPE lasers, the blue frequency shift due to the plasma effect is enhanced by incorporating a very thick p-side optical guide layer as a carrier reservoir for multiple quantum-well distributed feedback laser and it surpasses the red frequency shift due to the thermal effect. The results of demonstrating a frequency step response maintained in the blue shift region and an enhanced blue shift frequency modulation response over the whole modulation frequency range from DC are presented for the first time