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Saturable absorber based on the fiber coupler coated by CNTs

https://doi.org/10.1016/j.yofte.2021.102524Get rights and content

Highlights

Abstract

We report on a new method for fabrication of saturable absorbers for operation in ring fiber lasers as a mode-locking element taking advantages of the manufacturing technologies of fiber couplers and microfiber tapers coated with carbon nanotubes. It is demonstrated in the experiment that the transmittance of the fabricated saturable absorber increases with an increase of the input radiation power. Also, we have shown that the fabricated saturable absorber enables generation of picosecond soliton pulses in ring fiber laser configuration.

Introduction

Currently, ultrashort pulse lasers are effectively used in various fields of everyday life. They are exploited both in research and services delivered to mass consumers, e.g., medicine, cosmetology, etc. Owing to a multiple consumer appeal, fiber lasers definitely benefit over all mode-locked lasers. Small size, relatively low cost, convenient fiber output, and high beam quality make them attractive for a large number of applications [1], [2], [3], [4], [5], [6]. To deliver ultrashort pulses, a fiber laser configuration comprises either active [7] or passive mode–locking element with an incorporated saturable absorber. Low-cost and reliable passive mode-locking, mainly exploited in ultrashort pulse laser sources, enables generation of the pulses with a pulse duration much shorter than the modulator switching time [8].

Saturable absorbers (SA) are the key elements of passively mode-locked lasers. They conventionally could be divided in two types: artificial and natural SAs. Saturable absorbers based on Kerr lensing [9], nonlinear mirror [10], and nonlinear polarization rotation (NPR) belong to the first group [11], [12]. NPR saturable absorbers are mainly used with fiber laser configurations. Although the lasers based on such SAs are easy-to-use, they are environmentally sensitive and require adjustment to initiate lasing that limits their practical application. Semiconductor mirrors (SESAM) [13], [14] or SAs based on topological insulators [15], graphene [16], and carbon nanotubes (CNT) [17], [18] belong to the group of natural SAs. Besides low cost and ease of use, the CNT technology is advantageous over the SESAM technology in ability to modulate transmittance thus simplifying laser circuits based on ring fiber cavities. As a result, CNT-based saturable absorbers have replaced SESAMs in a number of fiber laser applications [19], [20], [21].

The principle methods of CNT-based SAs fabrication are deposition of CNTs on a film, fiber end face [22], [23], chemically etched d-shape fiber [24] or micro-taper obtained by thermal drawing of a standard fiber [25]. Each of the methods has specific limitations and drawbacks. In particular, the film method significantly limits the radiation power in the cavity [26], the d-shape technology requires special fiber structure and rather complicated [24]. In this work, we focus on the microfiber method of SA fabrication, in which CNTs are deposited on a fiber thermally drawn up to the diameters compared with the radiation mode area [25], [27]. The evanescent field based CNT SA, manufactured by this method, has an advantage over the schemes using CNTs deposited on a film or fiber end face enabling generation of higher pulse energy and peak power. Such absorbers are simple to manufacture and easy-to-use, but suffer specific drawbacks. Due to a small diameter of taper waist region (~10 μm), when coated with CNTs at a high concentration or from a liquid phase, SA experiences high losses induced by the fiber deformation. The same restrictions impede the uniform distribution of CNTs over the microfiber surface. In addition, fiber tapers are sensitive to environment that can deteriorate their performance characteristics. To overcome these problems, the use of polymer composites has been proposed [28]. Along with additional protection of the microfiber, a polymer with a low refractive index (e.g., 2,2,2-trifluoroethyl methacrylate (PTFEMA), n = 1.42) provides the effective distribution of nanomaterial on its surface and prevents formation of agglomerations, thereby reducing SA losses from estimated 50% down to 15% [29], [30].

In this work, we push forward development of the microfiber method for fabricating CNTs-based SAs. For this purpose, the fiber couplers commonly used in fiber circuits are employed. The fabrication process includes simultaneous thermal drawing of two parallel fibers down to the diameters close to the mode field size followed by fusion [31]. The fibers fused into a coupler are elongated enough to enable interaction of radiation with the CNT film on their surface and saturable absorption. Since fiber coupler is a mechanically stronger structure than a standard microfiber, it is able to provide a simplified technology of CNT-film deposition in a polymer composition while maintaining a high saturation absorption coefficient. In particular, simplification of the technology within this approach allows using less precise (e.g., aerosol) methods of nanomaterial coating, which, in its turn, enables deposition of larger CNT-volumes and optimization of SA properties.

Section snippets

Manufacturing of a CNT-coated coupler

Single-walled CNTs (SWCNT) used for coupler coating have been synthesized by the electric arc technology at the Institute of Problems of Chemical Physics, the Russian Academy of Sciences (Chernogolovka, Russia). An average diameter of SWCNTs is 1.5 nm. Inset in Fig. 1 (a) shows the absorption spectrum of the used CNTs in the wavelength range 550–2400 nm. SWCNTs have been subjected to multistage liquid-phase treatment in order to obtain stable and homogeneous suspensions in a solution of

Measurement of the saturable absorption

The key characteristic of a SA is the dependence of losses it introduces on the intensity (or power) of the input radiation. If the losses decrease with the input power, SA can be used for mode-locking and pulse generation through positive optical feedback in the laser cavity it provides [12].

A standard configuration shown in Fig. 3 (a) has been employed to evaluate characteristics of the fabricated CNT-coupler used as a saturable absorber [34], [35]. A soliton laser operating at the wavelength

Study of CNT-coupler in a ring fiber laser scheme

To study the mode-locking provided by the fabricated CNT - coupler, the coupler has been incorporated into a standard Er-doped fiber ring laser. The ring laser configuration (Fig. 4) contains no polarization-sensitive elements, so mode-locking through the nonlinear polarization rotation is prohibited in this scheme.

The laser cavity contains 0.5 m of Er-doped EDF-150 fiber with a normal dispersion of 19 ps2/km at 1550 nm. All other elements are connected by an SMF-28 fiber with a dispersion of

Conclusions

In this study, we propose a new fabrication method of SA for mode-locking of the ring fiber lasers taking advantages of the manufacturing technologies of fiber couplers and tapers coated with carbon nanotubes. The proposed method develops the technology of microfiber tapers coated with CNTs, but simplifying it, in particular, allowing usage of less complicated methods of nanomaterial deposition. The simplicity of the proposed CNT- coupler technology is in the use of widely available materials

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

This work was supported by the Russian Science Foundation (grant no. 19-72-10037) and the Ministry of Higher Education and Science of the Russian Federation (8th Megagrant Program, application # 2020-220-08-1369).

References (37)

  • H. Kataura et al.

    Optical properties of single-wall carbon nanotubes

    Synth. Met.

    (1999)
  • W. Shi et al.

    Fiber lasers and their applications

    Appl. Opt.

    (2014)
  • M.E. Fermann, I. Hartl, Ultrafast fibre lasers.Nature photonics,7 (11) (2013) 868-874....
  • S. Droste et al.

    Optical frequency comb generation based on erbium fiber lasers

    Nanophotonics

    (2016)
  • U. Keller

    Recent developments in compact ultrafast lasers

    Nature

    (2003)
  • R.R. Gattass et al.

    Femtosecond laser micromachining in transparent materials

    Nat. Photonics

    (2008)
  • K. Wang et al.

    Advanced fiber soliton sources for nonlinear deep tissue imaging in biophotonics

    IEEE J. Sel. Top. Quantum Electron.

    (2013)
  • M. Bello-Jiménez et al.

    Actively mode-locked all-fiber laser by 5 MHz transmittance modulation of an acousto-optic tunable bandpass filter

    Laser Phys. Lett.

    (2018)
  • H.A. Haus

    Mode-locking of lasers

    IEEE J. Sel. Top. Quantum Electron.

    (2000)
  • T. Brabec et al.

    Kerr lens mode locking

    Opt. Lett.

    (1992)
  • F.Ö. Ilday et al.

    High-energy femtosecond stretched-pulse fiber laser with a nonlinear optical loop mirror

    Opt. Lett.

    (2002)
  • V.J. Matsas et al.

    Self-starting, passively mode-locked fibre ring soliton laser exploiting non-linear polarisation rotation

    Electron. Lett.

    (1992)
  • C.J. Chen et al.

    Soliton fiber ring laser

    Opt. Lett.

    (1992)
  • A. Isomaki et al.

    Semiconductor mirror for optical noise suppression and dynamic dispersion compensation

    IEEE J. Quantum Electron.

    (2003)
  • O. Okhotnikov et al.

    Ultra-fast fibre laser systems based on SESAM technology: new horizons and applications

    New J. Phys.

    (2004)
  • C. Zhao et al.

    Ultra-short pulse generation by a topological insulator based saturable absorber

    Appl. Phys. Lett.

    (2012)
  • Q. Bao et al.

    Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers

    Adv. Funct. Mater.

    (2009)
  • M. Chernysheva, A. Rozhin, Y. Fedotov, C. Mou, R. Arif, S.M. Kobtsev, S.K. Turitsyn, Carbon nanotubes for ultrafast...
  • Cited by (0)

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