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We report an all-fiber, all-polarization maintaining (PM) ultrafast Tm-doped fiber laser mode-locked by a multilayer graphene-based saturable absorber (SA). The laser emits 603 fs-short pulses centered at 1876 nm wavelength with 6.6 nm of bandwidth and 41 MHz repetition rate. Graphene used as saturable absorber was obtained via chemical vapor deposition (CVD) on copper substrate and immersed in a poly(methylmethacrylate) (PMMA) support, forming a stable, free-standing foil containing 12 graphene layers, suitable for the use in a fiber laser. The generated 603 fs pulses are the shortest reported pulses achieved from a Tm-doped laser mode-locked by graphene saturable absorber so far. Additionally, this is the first demonstration of an all-PM Tm-doped fiber laser incorporating a graphene-based SA. Such cost-effective, compact and stable fiber lasers might be considered as sources usable in nonlinear frequency conversion, mid-infrared spectroscopy and remote sensing.
© 2015 Optical Society of America
1. Introduction
Thulium-doped fiber lasers operating in the 1.8 – 2.0 μm wavelength range are currently one of the most important branches of laser technology and experienced tremendous progress over the last decade [1–3]. Intense development of such constructions is driven by variety of potential applications. The most prominent include: laser spectroscopy, medicine, and nonlinear frequency conversion. The wavelength around 1.9 μm is a part of so called “eye-safe” region, because the radiation is absorbed in the vitreous body of the eye, without focusing on the retina. Thus, it is suitable for many free-space applications, including remote environmental sensing. The spectral range covered by Thulium-doped fiber lasers (TDFLs), overlaps with absorption lines of several molecules, e.g. carbon dioxide (CO2), nitrous oxide (N2O) or hydrogen bromide (HBr), which creates the possibility of constructing cost-effective trace-gas sensing platforms [4]. Strong water absorption in the 1.8 – 2.0 μm range makes Tm-doped fiber lasers extremely desirable in biomedical applications. Due to strong absorption in water (which is the main constituent of the human body), heating of only small areas is achieved. The light penetration into the tissue is at the level of microns, which allows precise cutting of biological tissue. Additionally, bleeding is suppressed by coagulation [5]. It has been shown, that 2 μm laser outperform 1 μm and 1.55 μm sources in dermatology and surgery, serving as precise and efficient optical scalpels [6,7].
The development of mode-locked Tm-doped fiber lasers started almost 20 years ago [8]. Up till now, a number of demonstrations of ultrafast TDFLs has been reported in the literature. The reported lasers were usually mode-locked using nonlinear polarization evolution (NPE) [9–11], semiconductor saturable absorber mirrors (SESAMs) [12,13] and carbon nanotubes (CNTs) [14]. All those mode-locking techniques are well-established and widely used by the laser community. However, SESAMs suffer from relatively narrowband operation – due to the energy band gap in semiconductors, each SESAM needs to be designed for a specific wavelength. The NPE-based lasers tend to be environmentally unstable due to vulnerability to movements of fibers, and do not always provide turn-key self-starting pulsed operation. Carbon nanotubes should have precisely selected diameters, in order to provide absorption at the desired wavelength range [15] The drawbacks of all mentioned techniques forced the laser community to seek for new, more universal and cost-effective saturable absorber materials. In the recent 5 years, so called two-dimensional (2D) nanomaterials revolutionized the field of mode-locked lasers. The most popular example of a 2D material is graphene, which is composed of one atom-thick layer of carbon, forming a 2D honeycomb lattice. Since the first experimental demonstration of fiber lasers incorporating graphene in 2009 [16,17], an enormous number of reports devoted to graphene-based lasers appeared in the literature. The works are focused mainly on constructions operating at 1 μm [18,19] and 1.55 μm wavelengths [20–23]. Other 2D materials usable in laser technology include: topological insulators (Bi2Te3, Bi2Se3, Sb2Te3) [24–27], and transition metal sulfide semiconductors, like molybdenum disulfide (MoS2) [28,29].
Due to the gapless linear dispersion of Dirac electrons in graphene, the material exhibits a flat and extremely broad, wavelength-independent absorption, ranging from the visible to the mid-infrared [30]. Despite its excellent nonlinear optical properties, also maintained in the > 1.9 μm region, there were only few reports of mode-locked Tm-doped fiber lasers using a graphene-based saturable absorber. The first demonstration was reported by M. Zhang et al. [31]. Mode-locking at 1.94 μm with 3.6 ps pulse duration using chemically exfoliated graphene was achieved. Later, Q. Wang et al. demonstrated TDFL emitting 2.1 ps pulses also using liquid-phase graphene [32]. In 2013 our group reported a TDFL which generated 1.2 ps pulses at 1884 nm, using a CVD-graphene/PMMA composite [33].
Despite a great number of mode-locked Tm-doped fiber lasers in the literature, there were very few reports on all-PM lasers, emitting linearly polarized pulses. As an example, B. Zhang et al. reported a TDFL mode-locked with a SESAM, emitting 20 ps-long output pulses [34]. In a non-PM cavity, a polarization controller is required to adjust the polarization state inside the resonator and start the mode-locking. Such lasers might be sensitive to external perturbations (like vibrations, movement of the fibers, temperature changes, etc.). Additionally, in an non-PM laser, the output solitons are in fact vector solitons, containing two different, freely evolving polarizations. The energy exchange between those two vector solitons may cause formation of sub-sidebands in the optical spectrum, which was extensively studied by H. Zhang et al. [35,36]. The only way to provide self-starting, turn-key and environmentally stable operation, is to develop the cavity in all-fiber technology, using PM fibers and components. Such lasers emit scalar solitons with one polarization state, which is crucial for many applications.
Here, we demonstrate an all-fiber, all-PM Tm-doped ultrafast fiber laser mode-locked with CVD-graphene/PMMA saturable absorber. The laser delivers linearly polarized, 603 fs-short pulses at 1876 nm wavelength with 41 MHz repetition frequency. Such laser might be considered as a source for nonlinear frequency conversion, chirped pulse amplification or trace-gas sensing systems.
2. Graphene fabrication and characterization
The graphene layer was grown by CVD method on copper foil using the Aixtron Black Magic Pro deposition system. To prepare twelve graphene layers on PMMA foil, the graphene layer was detached from copper using the electrochemical delamination method [37]. At first, graphene on Cu foil was covered with a thin layer of PMMA by a spin-coating method. Then using a custom-made mechanism the Cu/graphene/PMMA was placed in low-concentration potassium chloride electrolyte at a rate of 1 mm/s. When the graphene/copper cathode was negatively polarized, hydrogen bubbles appeared at the graphene/copper interface due to the reduction of water molecules, thus allowing gentle detachment of graphene from the substrate. After the delamination process, graphene/PMMA was rinsed with deionized water and transferred onto graphene on copper. The PMMA layer was subsequently removed using acetone and a copper/graphene/graphene stack was created. The graphene transfer process was performed to achieve twelve layers so that the final structure was as follows: copper/graphene x12/PMMA. All these graphene layers on copper were delaminated, cleaned and the PMMA/graphene stack was fished by the substrate to which graphene did not stick and afterwards dried in an air atmosphere.
The 12-layer graphene on PMMA was characterized by Raman spectroscopy using a Renishaw system with a 532 nm Nd:YAG laser as an excitation source. Figure 1(a) shows the Raman spectra of twelve graphene layers on the PMMA substrate. The spectrum contains pronounced G (1587 cm−1) and 2D (2702 cm−1) bands, which is typical of the sp2 hybridization of carbon. Figure 1(b) shows a photograph of a piece of 12-layer graphene foil. The number of layers in the structure was confirmed with optical transmittance measurement, which is a reliable method of defining the number of layers in multilayer graphene stacks [38].
Fig. 1 Raman spectrum of 12-layer graphene on PMMA foil (a) and photograph of the 12-layer graphene/PMMA foil used as saturable absorber (b).
In order to prepare a saturable absorber device, suitable to use in an all-fiber laser, two small pieces (about 1x1 mm) of the graphene/PMMA composite were sandwiched between two angle-polished fiber connectors. Such SA is characterized by a relatively flat absorption spectrum over the range of 1400 – 2200 nm. The measured transmission through the 24-layer graphene/PMMA composite deposited on the connector is plotted in Fig. 2a. The transmission of the composite is at the level of 50% at around 1550 nm. wavelength and at the level of 40-45% in the 1800-2000 nm range. The transmission drops at wavelengths above 2100-2200 nm, because of increasing absorption of the used polymer material.
Fig. 2 Absorption spectrum of the 24-layer graphene/PMMA composite (a), and the measured nonlinear transmission curve with indicated saturable absorption parameters (b).
In order to characterize the nonlinear optical parameters of the fabricated SA, the device was tested in a power-dependent transmission measurement setup, similar to that presented previously in [17], typically used for characterization of fiber-based saturable absorbers. However, due to lack of a proper pumping source and enough sensitive photodetectors at 1.9 μm wavelength, we have tested the SA using 1560 nm wavelength laser. At this wavelength, the graphene-based SA exhibits about 3.7% of modulation depth (ΔT) with 45% of non-saturable losses (αNS). The saturation fluence (Fsat) of the SA is at the level of 110 μJ/cm2. The power-dependent transmission curve, together with a theoretical fit is plotted in Fig. 2b. The measured 51% transmittance of the unsaturated SA confirms the number of 24 layers in the structure, including the insertion loss of the PMMA layer (approx. 7%). The maximum available fluence in our setup was around 500 μJ/cm2. At this values, we did not observe any damage of the graphene SA during measurement.
3. Experimental setup
The experimental setup of the all-PM femtosecond Tm-doped fiber laser is depicted in Fig. 3. The resonator consists of a 90 cm-long segment of PM Tm-doped fiber with 9 μm core diameter (Nufern PM-TSF-9/125), a 1570/2000 nm PM filter-type wavelength division multiplexer (FWDM), a polarization sensitive isolator, a 10% PM output coupler and the graphene-based saturable absorber. The laser was pumped by a 1568 nm laser diode, amplified in an Erbium-Ytterbium co-doped fiber amplifier (EYDFA) to the maximum level of about 340 mW. The total length of the cavity was around 4.9 meters. The laser consisted of only two types of fibers with negative group delay dispersion (GVD): the PM Tm-doped fiber (GVD equal to 0.076 ps2/m, 90 cm fiber length used), and passive PM Panda-type fiber (GVD equal to 0.068 ps2/m, total length of 4 meters). Thus, the calculated net group delay dispersion (GDD) of the cavity is −0.34 ps2.
All fibers and components used in the laser were polarization maintaining and were spliced using a fusion splicer with precise θ-axis rotation (Fujikura FSM-45PM), in order to match the appropriate polarization axis. The all-fiber, fully-PM design ensures stable and self-starting mode-locked operation, invulnerable to external disturbances. Additionally, there is no need of using polarization controllers in the cavity, which simplifies the whole setup.
4. Experimental results
The performance of the laser was observed using an optical spectrum analyzer (Yokogawa AQ6375), 350 MHz digital oscilloscope (Agilent DSO-X 3034A), 7 GHz radio frequency (RF) spectrum analyzer (Agilent EXA N9010A) coupled with a 12 GHz photodetector (Discovery Semiconductors DSC2-50S), and an autocorrelator (Femtochrome FR-103XL).
The laser starts to operate in the mode-locked regime by itself just after turning the pump on at around 215 mW of power. The pumping power might be further increased to 290 mW without losing the mode-locking, however, the bandwidth does not further increase above 242 mW. Instead, we can observe growing peak power of the Kelly’s sidebands in the spectrum. All measurements have been performed at 242 mW of pump power. The average output power of the laser at this pumping level was 1.5 mW. The obtained efficiency is comparable to other reports on mode-locked TDFLs pumped in the 1.56 μm band [39]. The 1568 nm source could be turned on and off repeatedly, and the mode-locking was always restored with the same spectral properties. The optical spectrum of the laser output measured with high resolution (0.05 nm) is depicted in Fig. 4(a). The graph inset Fig. 4(a) shows the spectrum recorded with 100 nm span. It is centered at 1876 nm and has a typical soliton-like shape with characteristic Kelly’s sidebands, which results from the all-anomalous cavity design. From the location of the Kelly’s sidebands, we can calculate the net GDD of the laser resonator, according to the following formula [40]:
where D2 denotes the second-order dispersion (GDD), n is the sideband order, τp is the pulse duration, Δλ denotes the sideband separation, and λc is the central wavelength. By inserting the measured values (603 fs pulse duration, 1876 nm central wavelength and 20 nm sideband separation at n = 1), we obtain |D2| equal to 0.34467 ps2, which exactly matches the net cavity dispersion calculated from the available fiber dispersion data. The full width at half maximum (FWHM) bandwidth of the emission is 6.6 nm. The absorption dips, that can be seen in the high-resolution spectrum originate from the strong water absorption lines in atmospheric air, which are located in the 1.8 – 1.9 μm spectral region. We have simulated the absorption of light over a 1 m path in air with 1% water content using the high-resolution transmission molecular absorption (HITRAN) database [41]. The simulated absorption lines are plotted in Fig. 4(a) (with red line). The location and amplitude of the dips in the spectrum ideally match the simulated water absorption lines.Fig. 4 Optical spectrum of the laser, together with the water absorption lines taken from the HITRAN database (a), and the measured 603 fs pulse autocorrelation trace (b).
Figure 4(b) shows the measured autocorrelation trace of the pulses generated by the laser. If we assume the sech2 pulse shape, typical for optical solitons, the pulse duration is equal to 603 fs. With 6.6 nm of spectral bandwidth, the calculated time-bandwidth product (TBP) is 0.341. It means, that the pulses are nearly transform-limited, and are only 8% longer than the theoretical limit (555 fs for the transform-limited pulse with TBP equal to 0.315).
Figure 5(a) shows the RF spectrum of the laser measured with 33 Hz resolution bandwidth (RBW). The repetition rate is equal to 41.46 MHz, corresponding to a 4.9 meter long cavity. The signal to noise ratio (SNR) is larger than 70 dB, which is better than in previously reported graphene-based 2 μm lasers [31–33]. The RF spectrum in the full available span (7 GHz) showing a broad spectrum of harmonics is depicted inset Fig. 5(a). The pulses are equally spaced by 24.1 ns, which corresponds to 41.46 MHz mode spacing. The recorded oscilloscope trace of the pulse train is depicted in Fig. 5(b). No signs of multi-pulsed or Q-switched operation were observed.
Fig. 5 The RF spectrum of the mode-locked laser measured with 1.5 MHz frequency span and 33 Hz RBW (inset: spectrum in the full available 7 GHz span) (a), corresponding pulse train with 24.1 ns pulse spacing (b).
5. Summary
In conclusion, we have demonstrated for the first time, to the best of our knowledge an all-fiber, all-polarization maintaining ultrafast Tm-doped fiber laser incorporating graphene as saturable absorber. Pulses as short as 603 fs centered at 1876 nm wavelength with 6.6 nm of bandwidth were generated at a 41.46 MHz repetition frequency. The saturable absorber was based on a free-standing, 24-layer CVD-grown graphene/PMMA composite. The SA exhibits 3.7% of modulation depth with 45% of non-saturable losses, and a saturation threshold of 110 μJ/cm2. The reported 603 fs pulses are the shortest reported pulses achieved from a Tm-doped laser mode-locked by graphene saturable absorber so far, two times shorter than the previous record (achieved from a non-polarization maintaining laser). The presented laser might be considered as a stable and robust seed source for mid-IR spectroscopy and trace-gas sensing applications.
Acknowledgments
The work presented in this paper was supported by the National Science Centre (NCN, Poland) under the project “Passive mode-locking in dispersion-managed ultrafast Thulium-doped fiber lasers” (decision no. DEC-2013/11/D/ST7/03138). The research on graphene fabrication, leading to these results has also received funding from the European Union 7th Framework Programme under grant agreement n°604391 (Graphene Flagship).
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