Abstract
The path-averaged model is applied to described soliton characteristics in the anomalous cavity dispersion fiber laser with semiconductor optical amplifier. It is shown that, by off-setting the optical filter relative to the gain spectral maximum, it is possible to control velocity and frequency of both the fundamental optical soliton and chirped dissipative solitons.
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Employment of a semiconductor optical amplifier (SOA) to provide both gain and gain modulation for mode-locking in fiber lasers is a viable alternative to using an active rare-earth-doped fiber (see, e.g., [1–16] and references therein). The SOA, initially developed for optical communication and signal processing applications, has many attractive properties including compact size, broadband gain, and the possibility of direct gain modulation by controlling the injection current. The combination of a fiber resonator with an SOA to generate optical pulses can take advantage of both fiber-optic and semiconductor technologies.
However, the nonlinear properties of the SOA [17] make stability and control of the generated pulses a non-trivial physical and engineering problem. Adding an SOA to the optical fiber cavity where pulse dynamics is determined by the interplay of the Kerr effect and dispersion further increases the total number of parameters and degrees of freedom in the resulting nonlinear system. Management of the complex evolution of light in SOA-based lasers requires understanding of the underlying nonlinear dynamics and study of the corresponding mathematical models. It is well known that under certain conditions, simplified path-average soliton models, including dissipative solitons, can be applied to characterize light evolution with small amplitude and phase changes over one round trip [18]. These models are generic for different types of lasers and, moreover, the underlying Ginzburg–Landau equation describes various phenomena and a number of applications in different areas of physics [19]. Soliton lasers have the advantage of relative simple construction, well-defined transform-limited (un-chirped) pulse shape, well-understood features, and a possibility to use powerful theoretical methods in design (see, e.g., [20–23] and references therein). In more general terms, nonlinear coherent structures in lasers and resonators are the key to optical frequency comb technology [24–29]. Soliton theory [20,21,30,31] includes various developed techniques for soliton control—stabilization of pulse parameters in the presence of perturbations or reduction of soliton jitter due to noise or nonlinear interactions. This can be especially interesting in the case of SOA-based fiber lasers where, without control elements, the generated optical pulses continuously change their central frequency and time position. Soliton theory also might be useful to quantify the dependence of fluctuations of pulse characteristics on system parameters.
In general, an accurate description of the cyclic transit intra-cavity dynamics of radiation in a laser resonator can be provided by Poincaré mapping [32] of the light field over a round trip. The overall modification of the properties of a field $A(t)$ over the round trip is given by the operator that is a product of consecutive transformations of the field by the elements of the laser cavity. These elementary transformations can be both a point action (e.g., filter or coupler) or distributed (e.g., nonlinear evolution in an optical fiber). For instance, in the fiber laser context, these basic elements may include: different fiber spans, amplifier(s), saturable absorber, optical filter, out-coupler, and so on. When resonator components have a nonlinear response, in general, these operators are non-commuting. Here, we introduce a path-average model for the laser system that comprises the following basic elements: optical fiber with the anomalous dispersion, SOA as a gain medium, and spectral filter with the frequency off-set from the central frequency of the SOA. We assume here that the saturable absorber, responsible for the generation of pulses from noise, does not impact the final pulse shaping.
Note that even when it is possible to accurately describe a single nonlinear element, it is not always feasible to define the overall transformation over one round trip in a simple way. For instance, in the laser system considered here, the operator corresponding to the transfer function of the semiconductor optical amplifier reads [17]:
Following [37–39], we introduce the master model describing the evolution of an optical pulse in a fiber cavity with an SOA (with the central frequency at $\omega _c$) path averaged over the one round trip $L$ as
Let us start from the analysis of the stabilization of a fundamental transform limited optical soliton by the filtering. Consider a standard perturbation theory for a single soliton [30,31,38,40]:
with $A$ (assumed to be positive), $T$, $\Omega$, and $\phi$ being functions of $z$. Applying the standard soliton perturbation theory, we can describe the adiabatic changes of the fundamental soliton parameters under the combined action of the filtering and amplification by the SOA. Evolution of the parameters $A$, $\Omega$, and $T$ (dynamics of $\phi$ can be separated) is given by the following set of the ordinary differential equations:Note that the fundamental soliton is not necessarily an attractor for the solution formed from the initial noise. It is just a particular branch of the more general family of steady state waveforms that can be generated in SOA-amplified fiber lasers. As the SOA transfers amplitude modulation into the phase modulation, we can expect in these lasers coherent structures with time-dependent phase—chirp pulses. Indeed, there are exact analytical solutions of Eq. (4) in the form of chirped solitons derived for similar models in [41] and [37] in different contexts:
Energy $E$ of the chirped soliton is $E=\int {|u_{chs}|^2 d\tau }= 2A_0^2 T_0$ and the full pulse width at half maximum is the same as for the fundamental soliton $T_{FWHM}=1.763 T_0$. The spectral power density for chirped soliton reads:
These analytical solutions can be used to determine the conditions of existence of the stationary chirped soliton pulses that are not moving ($\sigma =0$) in the considered frame. This can be achieved by adjusting the off-set filter as
In this case, the soliton frequency $\omega _{01}$ is found as It is seen that by varying the filter strength $\eta$ around the point $\eta _1 = 0.194$ [that corresponds to $q(\eta _1)=1/\alpha _H$], the soliton frequency $\omega _{01}$ changes sign making possible both a red and blue off-set of the pulse from the SOA central frequency.While the path-average model analysis is limited to small changes of the pulse power and phase over a single round trip (see, though, recent work [42]), it provides a useful analytical description of the soliton solutions. In particular, it shows that off-set spectral filtering can be used to manipulate the central frequency of the soliton and its velocity. It has been shown recently in [15,16] that nonlinear dynamics in SOA-based fiber laser can be used to provide for a spectral tunability of the generated pulses. The analytical description above [i.e., Eq. (17)] quantifies this possibility within the limit of validity of the path-average model.
Note that the access gain $\delta >0$ gives rise to the instability of the CW waves outside of the soliton solution. However, this instability is suppressed in the laser by the saturable absorber. The saturable absorber effect is assumed here to be responsible for the self-starting of the pulse generation from noise, but not affecting soliton shaping at higher powers. Contribution of the saturable absorber can be accounted for both in the soliton perturbation analysis and in the description of the chirped solitons (see for details [39]) leading to more cumbersome analytical results. The impact of the saturable absorber on soliton properties in SOA-based fiber lasers will be presented elsewhere.
Funding
Engineering and Physical Sciences Research Council (EP/W002868/1).
Acknowledgments
I would like to thank Daria Khudozhitkova, Anastasia Bednyakova, Natalia Manuylovich, and Elena Turitsyna for help with the preparation of the manuscript.
Disclosures
The author declares no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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