Regular ArticlesRecent progress in optical dark pulses generation based on saturable absorber materials
Introduction
It is a well-known phenomenon that bright pulses can be produced in fiber laser devices. For example, either in mode-locking regime [1], [2], [3], [4], [5], or Q-switching [6], [7], [8], [9], [10]. With high power, robustness, simple construction process, broadband spectral bandwidth, and ultrafast temporal resolution, bright pulses are extensively applied in telecommunications operations, signal processing, biomedical analysis, fiber sensing, spectroscopy, metrology, and so on [11], [12], [13], [14], [15]. Nevertheless, dark pulses structures are also formed in all-optical fiber systems usually by very similar architectures.
Specifically, the generation of pulses with SA materials has several advantages when compared to other approaches that also enables the generation of diverse pulse regimes. Some examples of passively mode-locking techniques that do not require a physical SA are based on nonlinear phase shifts experienced by the beam at its high-intensity peaks, such as Kerr-lens mode locking [16], [17], [18], nonlinear polarization rotation [19], [20], [21], and also based on intensity-dependent frequency conversion as in the case of nonlinear mirror mode locking [22], [23], [24]. All of them rely on the fact that the nonlinearity which is induced by high-intensity light makes the peak of the pulses to experience either more gain or less loss during a round trip, making the whole mechanism work as if a virtual SA. Although very short pulses can be achieved from these techniques [25], Kerr-lens mode-locking and nonlinear mirror mode locking are usually associated just with bulk lasers, and nonlinear polarization rotation, or other techniques based on it such as mode-locking through a nonlinear loop mirror [26], suffer from instabilities due to fluctuations in ambient temperature, and often exhibit poor self-starting performance. These drawbacks can be avoided by the usage of real SA materials for the generation not just dark pulses, but other pulse regimes as well.
Since the earliest demonstration of a dark pulse in optical fibers in the 80s [27], [28], [29] many efforts have been constantly applied in the generation of these optical structures. For example, we can mention the generation of dark pulses employing waveguide electro-optical modulators [30], by propagating for long distances [31], through induced modulational instability in a highly birefringent fiber [32], utilizing cross-phase modulation in a nonlinear optical loop mirror [33], by the cross-coupling between two different wavelength laser beams in a fiber laser [34], employing a passively mode-locked quantum dot diode laser [35], through a passive mode-locking mechanism that relies on a dissipative four-wave mixing process [36], generated by T-flip-flop circuits [37], by means of a fiber Bragg grating used as a passive filtering element at the output of a mode-locked laser [38], through a mode-locked laser with in-cavity pulse-shaper [39], among other methods [40], [41], [42], [43], [44], [45], [46], [47], [48], [49]. This massive amount of approaches using different experimental configurations and techniques aimed to develop a methodology that would allow the generation of dark pulses in a simple, stable, and repeatable way.
However, the generation of stable dark pulses is a big challenge, requiring specific experimental setups and a precise adjustment in all parameters of each configuration. Recently, the use of saturable absorber (SA) materials incorporated into fiber laser cavities has become an efficient method for generating dark pulses with high stability. In this perspective, we present in this paper a comprehensive literature review covering the usage of SA materials for dark pulses generation, with their main optical properties, as well as the parameters of the experimental setups used.
Dark pulses are optical structures identified by holes of power in a continuous wave (CW) background [50], [51]. A dark pulse can be mathematically described by the following expression [52]:in which is the intensity hole, is the dark pulse width, B governs the hole depth, and is the phase described by:For , the amplitude at the hole center drops to zero, and the pulse is described as a black pulse. Dark pulses with are denominated gray pulses to highlight this characteristic, as displayed in Fig. 1.
Whereas the phase of bright pulses remains constant across the entire pulse, the dark pulse phase changes with a total phase shift of 2 B, i.e., dark pulses are chirped. For the black pulse, the chirp is such that the phase varies abruptly by in the center. This phase variation becomes more gradual and reduced for smaller values of , as evidenced in Fig. 2. The time-dependent phase of dark pulses constitutes a major distinction between the bright and dark pulses.
It is important to highlight here that optical dark soliton formation in optical fibers is a result of mutual interactions between cavity dispersion, fiber nonlinearities, laser gain saturation, and gain bandwidth filtering. In this way, dark pulses are not always necessarily dark solitons, but can be only pulses with a reversed shape, which is the case for most of the dark pulses described in this review. Thus, both the Ginzburg - Landau equation and the nonlinear Schrödinger equation that are often used to describe the generation of solitons in optical fibers cannot be employed to describe all dark pulse regimes.
This review is organized as follows: section II presents the main optical properties of an SA material. In the following, section III outlines the main applications of dark pulses. A typical laser arrangement widely adopted for generating dark pulses is displayed in section IV. Subsequently, a general discussion covering all publications that report the use of materials as SA for the dark pulses generation is reported in section V. Finally, the conclusion and future perspectives are covered in section VI of this paper.
Section snippets
Nonlinear optical properties of a saturable absorber material
The main optical properties of an SA for generating short pulses are the saturation intensity , the modulation depth , and the nonsaturable loss . The relationship between these quantities is expressed by the following equation:in which T is the transmission (in percentage) through the SA material and I is the input light intensity incident on this material. One method that can be applied to measure these parameters of an SA material is to use a power-dependent
Dark pulses applications
Dark pulses have been the topic of intense study nowadays. Several investigations constantly describe fascinating insights into the underlying physics of these structures frequently observed in all-optical fiber schemes, and with wide possibilities for applications in diverse areas of photonics. Studies have already shown that dark pulses suffer distortions resulting from the combination of various effects such as dispersion [61], [62], [63], self-phase modulation [64], and loss [65]. Moreover,
Fiber laser cavity setup for the dark pulses generation
Fig. 4 displays a typical laser arrangement for generating dark pulses. The key component of this configuration is the SA material. When properly incorporated into the laser cavity, it acts as a modulator device for the losses. Moreover, this experimental apparatus employs an optical fiber doped with active rare-earth ions as a gain medium for generating dark pulses. The most used rare-earth ion for this purpose is erbium (Er3+), generating pulses in the region. Another two of those are
Discussion
The development of new SA materials has recently grown exponentially [98], [99], [100], [101], [102], [103], [104], [105], [106], [107], [108], [109], [110], [111], [112], [113]. Part of this growth is due to the use of these materials as passive devices for the generation of different pulse regimes in laser cavities through the dynamics of the interaction of light with these materials inside these optical cavities. Several materials such as single or multiple graphene layers [114], thin layers
Conclusion and future perspectives
In summary, from this comprehensive review presented in this paper, 37 publications reported the application of materials as an SA for the dark pulses generation in optical fibers. In all, 19 different materials have already been used for this purpose. All of these materials have unique optical properties that emerge from their inherent structures with innovative technological potential and therefore promise within the next few years to further speed up the growth of the ultrafast photonics
CRediT authorship contribution statement
Luís C.B. Silva: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. Carlos E.S. Castellani: Methodology, Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge the Fundação de Amparo à Pesquisa e Inovação do Espírito Santo – FAPES/Brazil (project 66/2017) by funding this work.
References (189)
- et al.
Switchable operation of multiple solitons and dissipative soliton resonance in a C-and L-band mode-locked fiber laser
Laser Phys. Lett.
(2020) - et al.
Picosecond steps and dark pulses through nonlinear single mode fibers
Opt. Commun.
(Jun. 1987) - et al.
The generation of quasi-continuous trains of dark soliton-like pulses
Opt. Commun.
(1994) - et al.
Generation of multigigahertz bright and dark soliton pulse trains
Opt. Commun.
(1997) - et al.
Dark pulses with tunable repetition rate emission from fiber ring laser
Opt. Commun.
(2012) - et al.
Dark pulse emission of a fiber laser
Phys. Rev. A
(Oct. 2009) - et al.
Improved laser damage threshold of In 2Se 3 saturable absorber by PVD for high-power mode-locked Er-doped fiber laser
Nanomaterials
(Aug. 2019) - et al.
Improved optical damage threshold graphene Oxide/SiO 2 absorber fabricated by sol-gel technique for mode-locked erbium-doped fiber lasers
Carbon
(Apr. 2019) - et al.
Ultrafast recovery time and broadband saturable absorption properties of black phosphorus suspension
Appl. Phys. Lett.
(Apr. 2015) - et al.
High energy mode-locked Yb-doped fiber laser with Bi 2Te 3 deposited on tapered-fiber
Optik
(Aug. 2017)