Minimizing the linewidth enhancement factor in multiple-quantum-well semiconductor optical amplifiers

Abstract

Semiconductor optical amplifiers (SOAs) often exhibit pronounced phase noise owing to their inherently high linewidth enhancement factor (LWEF). The signal to noise ratio of a semiconductor optical amplifier is often decreased due to refractive index fluctuations in the gain medium causing distorted phase relationship between the generated photons, which is quantified by the LWEF. A simple and precise theoretical model that offers a prescription for minimizing the LWEF in SOAs is unavailable in the literature. In this study, we have developed an inclusive yet simple algorithmic model that aims to both represent the variation and to provide a strategy for minimizing the LWEF in multiple-quantum-well (MQW) based SOAs. The results of the presented model were verified via a reasonable agreement with experimental results. This study provides a theoretical description of how to adjust the linewidth enhancement factor through tuning of the most critical MQW SOA parameters in the design stage.

Influence of the linewidth enhancement factor on the signal pattern of Fourier domain mode-locked lasers

“…In our computational results, the total number of dips is 1.43 × 10 6 for α = 3 and 5.5 × 10 5 for α = 2, which also suggests that the employed SOA in [5] has an LWEF that varies between 2 and 3. It should be reiterated that the time dependency of the LWEF is not considered in this study, as was the case with prior studies [2,[5][6][7][8][9][10][15][16][17][18][19]. Hence, our estimation for the LWEF of the employed SOA involves a range of values (2 < α < 3) over a single roundtrip rather than a constant value.…”

“…To our knowledge, the LWEF is not listed or specified among the SOA product specifications in practice; however, it is a very important parameter that influences the gain dynamics of any integrated photonic device that employs an SOA [5][6][7][10][11][12][13][14][15][16][17][18][19]. As the aim is to quantify the variation in the strength and density of the power-dips against the variation of the LWEF in this study, it is not necessary to know the precise value of the LWEF of the employed SOA in the experiment.…”

“…For SOAs with quantum-dot based active regions, the LWEF is shown to be extremely small (almost zero) [15] due to three-dimensional electron confinement. The LWEF itself is a measure of noise [18,19] as it quantifies the fluctuations in the refractive index, which naturally introduce additional noise in the signal pattern of FDML lasers. Therefore, a high LWEF is expected to yield more dips in the signal pattern of FDML lasers, but the exact contribution of the LWEF on dip formation and the associated fluctuations in the signal pattern of FDML lasers has not been computed to our knowledge.…”

“…The FDML laser dynamics are mainly determined by the dynamics of the SOA; therefore, the power-dips should originate due to the nonlinear SOA active region dynamics [6]. This is easy to predict since a high SOA LWEF signifies large fluctuations in the refractive index of the active region, which naturally cause sharp changes in the signal pattern [12][13][14][15][16][17][18][19]. Hence, we believe that using an SOA with a high LWEF is very likely to exacerbate the power fluctuations in an FDML laser output signal by amplifying the occurrence of power-dips.…”

“…The LWEF is mainly associated with the intra-band collision time [18]. It is suggested that a high intra-band collision time is associated with more intense carrier collisions as the carriers have more time to accelerate between each collision, which results in sharper changes in the refractive index considering the ensemble of carriers within the active region [18,19]. For a given SOA pump current, minimizing the cross-sectional area of the active medium maximizes the carrier density, which decreases the time between each carrier-collision and reduces the LWEF.…”