Analysis of the second- and third-order intermodulation distortion in DFB lasers including dynamic spatial hole burning effect

Distributed feedback (DFB) lasers with narrow linewidths are vital in high-coherence photonics. However, the quarter-wave phase shift (QWPS) that enforces single-mode emission drives a non-uniform photon density distribution and longitudinal spatial hole burning (LSHB) effect, broadening the linewidth. Here, we propose a step apodized coupling surface grating DFB (SAC-DFB) laser that divides the cavity into two segments with distinct coupling coefficients and evaluates it using a time-domain traveling wave model within a linewidth analysis framework.Co-optimizing the coupling coefficient distribution and segment length ratio flattens the photon density distribution under different facet coatings.To relate the grating structure, optical field, and LSHB effect induced linewidth (Δν NN ), we define three physics-based parameters: the photon density distribution uniformity factor f, carrier dynamics parameter U K (w) ,its optical field intensity weighted counterpart U H (w) . The ordinary least squares fit identify U H (w) as the dominant correlate of Δν NN . Relative to QWPS-DFB lasers matched in cavity length and facet coatings, optimized SAC-DFB lasers reduce Δν NN by up to approximately 68% and decrease the total linewidth by more than 35%; extending the cavity to 1500 μm yields a total linewidth of 85.85 kHz at 150 mA with high-reflection/anti-reflection facet coating. A neural network predicts f from grating parameters with a coefficient of determination exceeding 99.5%, thereby enabling manufacturing-oriented screening. Collectively, this study establishes a general, physics-guided framework that relates grating structure and optical field modulation to linewidth, both qualitatively and quantitatively, and demonstrates a fabrication tolerant SAC architecture that suppresses the LSHB effect and achieves narrow linewidths.

This paper presents a systematic investigation of the third-order intermodulation distortion characteristics in distributed feedback (DFB) semiconductor lasers. The influence of several nonlinearities, such as longitudinal spatial hole burning, gain compression, and relaxation oscillation, is considered. Detailed analysis shows that it is possible to make different nonlinearities cancel one another to give a low intermodulation distortion by choosing the appropriate DFB structure and beat conditions. Specifically, conditions for cancellation between spatial hole burning and gain compression nonlinearities are introduced.

Injection locking of vertical-cavity surface-emitting laser arrays is analyzed at steady state, including the effect of spatial hole burning. A free-running laser array (i.e., without injection), that operates well-above threshold, can exhibit multimode oscillations. Consequently, each of the free-running modes (at different frequencies) needs a different locking injection power. For low pump levels, just above threshold, the array is single mode and, as expected, the results bear a close resemblance to those of the average gain analysis, which ignores spatial hole burning.

A computer model called CLADISS is presented for the analysis of multisection diode lasers. The model allows for the analysis of a wide variety of multisection devices with discrete or distributed internal reflections. The simulator can carry out a threshold, DC, AC, and a noise analysis. The threshold analysis determines the threshold of the various longitudinal modes of the laser. The power versus current and the wavelength versus current characteristics are found with the self-consistent DC analysis. CLADISS includes all of the longitudinal variations by dividing each laser section in many short segments. Both the optical field and carrier density are discretized according to this segmentation. To demonstrate the capabilities of CLADISS some nonlinear effects in DFB lasers are treated. Instabilities induced in the side-mode suppression ratio by spatial hole burning are considered. The effects of spatial hole burning and side modes on the FM response on the linewidth are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Frequency-modulated (FM) frequency combs constitute an exciting alternative to generate equidistant spectra. The full set of Maxwell-Bloch equations is reduced to a single master equation for lasers with fast gain dynamics to provide insight into the governing mechanisms behind phase locking. It reveals that the recently observed linear frequency chirp is caused by the combined effects of spatial hole burning, group velocity dispersion, and Kerr nonlinearity due to asymmetric gain. The comparison to observations in various semiconductor lasers suggests that the linear chirp is general to self-starting FM combs.

We report on an experimental study of 888 nm pumped, passively mode-locked, high-power Nd:YVO4 lasers with an enhanced cavity design, involving spatial hole burning (SHB) in the active medium. We observed a significant pulse shortening due to the concept of “Gain-at-the end,” despite using long gain length up to 30 mm. A 31.6 W average output power TEM00 Nd:YVO4 oscillator, providing 16.2 ps pulses at an repetition rate of 96 MHz is presented. The pulse duration turns out to be primarily a function of the effective gain length, which can be explained by means of SHB. A further pulse shortening with decreasing gain length down to 9.5 ps at 11.1 W average output power is demonstrated.

Based on the finite-difference method, the multi-transverse-mode behaviours of vertical-cavity surface-emitting lasers (VCSELs) are investigated numerically. For two types of injection current with different contact radii, the transverse-mode competition, the spatial distribution of the carriers inside the active region of VCSELs and the carrier diffusion effects are investigated thoroughly. Results show that the spatial dependences of the optical modes and the current injection profile play important roles in determining the transverse-mode behaviours of VCSELs. It is found that a narrow disc contact or a centre-vicinity ring contact is favourable for the excitation of the fundamental mode, while the higher-order modes are benefited for a wide disc contact or a ring contact with larger contact radius; the carrier distribution shows nonuniformities in both the radial and the azimuthal directions when the spatial nonuniformities of the optical mode are considered. It further shows that the carrier diffusion also affects the laser modal competitions and the spatial hole burning (SHB) effects greatly by influencing the carrier spatial distribution.

The near-field emission profiles of oxide confined GaAs vertical cavity surface emitting lasers (VCSELs) with 20 m aperture have been investigated at different operating temperature and different driving current. The subthreshold emission profile provided the information of carrier distributions. At 20oC, a uniform plateau profile was observed at subthreshold emission, which then transferred to a fundamental mode at just above the threshold current. At higher driving current, the fundamental mode evolved into higher order modes due to the spatial hole burning effect. However, at 90oC the subthreshold emission was no longer a uniform plateau profile but showed some locally high gain regions off the aperture center. The subsequent lasing mode profiles showed high order mode in coincidence with these locally high gain regions at 90oC. The higher order mode profiles remained nearly unchanged under different temperature conditions when driving at constant current above the threshold. These locally high gain regions probably caused by the non-uniformity of the open aperture and the current crowding effect. In addition to the spatial hole burning and thermal lensing effect, these locally high gain regions appeared at elevated operation temperature also affected the higher order mode transitions of oxide confined VCSELs.

An approximate method of analysis of nonlinear operation of Fabry-Perot lasers including longitudinal electric field dependence and the spatial hole burning effect is presented. The method is based on a semiclassical approach and provides an approximate expression relating the normalized small signal gain to the normalized output power and laser parameters. The influence of the spatial hole burning effect on the laser characteristics is investigated. It is shown that this effect determines the value of the optimal coupling strength, as well as causing changes in the laser characteristics.

By tailoring the complex index modulations at an exceptional point, we demonstrate a microring laser without spatial hole burning effect and producing a robust single-mode emission with enhanced laser slop efficiency and stability.

Using the spatially-dependent multimode rate equations, this paper presents a detailed study of both the complex coupling effect and the spatial hole burning effect on side mode suppression in DFB lasers.

In this paper, we demonstrate the laser characterization of Cr:ZnS/Se polycrystalline gain media in non-selective unpolarized, linearly polarized, and twisted mode cavities. Lasers were based on post-growth diffusion-doped, commercially available antireflective-coated Cr:ZnSe and Cr:ZnS polycrystals with a length of 9 mm. The spectral output of lasers based on these gain elements in non-selective unpolarized and linearly polarized cavities was measured to be broadened to ∼20-50 nm due to the spatial hole burning (SHB) effect. SHB alleviation in the same crystals was realized in the "twisted mode" cavity, with linewidth narrowing to ∼80-90 pm. Both broadened and narrow-line oscillations were captured by adjusting the orientation of intracavity waveplates with respect to facilitated polarization.

Photonic generation of microwave signal using a dual-wavelength single-longitudinal-mode distributed Bragg reflector fiber laser

The spectral linewidth of a long distributed feedback (DFB) laser above threshold at high output power is investigated theoretically. Longitudinal spatial hole burning (LSHB) is shown to have important effects on the lasing characteristics and the broadening of linewidth. A four-wave coupled mode equation is developed to describe the LSHB phenomenon. The threshold gain and the carrier distribution are solved self-consistently above threshold at various output powers. The simulation results show that the power-linewidth product can increase by a factor greater than 2 due to LSHB for long DFB lasers. This theory has important implications on the limits of linewidth reduction by increasing the length of DFB lasers and on possible future device design for coherent optical system.

The competition characteristics of different orders of whispering-gallery lasing modes in a single ZnO microwire with hexagonal cavity are investigated comprehensively. As the scale of the cavity increases, the mode competition would be emerging strongly. The high-order whispering-gallery mode (WGM) lasing is indicated to be transverse electric modes with polarization dependence of about 86%. In addition, the competition of whispering-gallery lasing modes depends strongly on the diameter of cavities and the excitation intensities due to the effects of spatial hole burning. The Power Fourier transfer gives the distinct path length of fundamental and high-order optical field. Moreover, the finite difference time domain presents the optical field of high-order WGM lasing that tends to be closer to the center of hexagonal cavities. Remarkably, the Purcell factors of WGM are enhanced strongly with the reduction in diameter of hexagonal cavities, and the maximum value of F p approaches 20.