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We present for the first time to the best of our knowledge an all-fiber thulium (Tm) and erbium (Er) doped fiber laser simultaneously mode-locked by a common graphene saturable absorber. The laser consists of two ring resonators combined with a common saturable absorber (SA). The generated optical solitons have a full width at half maximum (FWHM) of 3.9 nm and 4.2 nm for Tm- and Er-doped laser, respectively. The used graphene layers were grown on copper foils by chemical vapor deposition (CVD) and transferred onto the fiber connector end. Broadband and flat absorption spectrum of used SA supports mode-locked operation at 1565 nm and 1944 nm. The repetition frequency of the resonator with Er-doped fiber was 20.19 MHz while the Tm-doped resonator was around 1 m longer and resulted with repetition rate of 18.43 MHz. The reported experiment unambiguously confirms one of the biggest advantage of the carbon nanomaterial (in this case graphene) SAs over semiconductor saturable absorption mirrors (SESAM), which is broadband operation range, allowing to mode-lock two lasers spectrally separated by almost 400 nm.
©2013 Optical Society of America
1. Introduction
Mode-locked lasers based on saturable absorbers are well developed sources widely used in commercially available systems. Practical application of ultrafast lasers with passive mode-locking was made possible by the invention of the semiconductor saturable absorbers (SESAMs) [1]. Now, after 20 years this technology is now highly developed and allows to control all the optical parameters of the SA, like reflection bandwidth, modulation depth, saturation intensity and non-saturable losses [2–4]. Up till now, SESAMs have been used to obtain mode-locked operation of fiber [5,6], bulk [1,2,7] and semiconductor [8,9] lasers. However SESAMs have also some limitations. The technology is based on semiconductor materials with characteristic band gap structures. Consequently, this results in relatively narrowband wavelength operation range, which limits the achievable pulse duration. Moreover, lasers operating at different central wavelengths need different, dedicated SAs. SESAM technology based on quantum well structures involves also expensive and complicated molecular beam epitaxy (MBE). Those imperfections cause that the new class of materials like carbon nanotubes [10–18], graphene [18–44] and topological insulators [44,45] have been recently considered as broadband and cost effective SAs. Chronologically first, single-walled carbon nanotubes (SWCNT) have been used as an effective SA [10]. The saturable absorption effect is observed in semiconductor type SWCNTs. It means that their optical bandwidth is limited, because the energy band gap depends on SWCNTs diameter [46]. However, the SWCNT growth process is not selective (except CoMoCAT method) and SWNTs have a wide diameter distribution and hence, the operation bandwidth is extended at least to 300 nm. Application of polymer-free SA containing SWCNTs with diameters ranging from 1.2 nm to 1.8 nm was successfully used to obtain mode-locking at 1 µm, 1.55 µm and 2 µm [11]. Usage of E11 and E22 transitions is an effective method to obtain mode-locked operation at 1 µm and 1.55 µm with the same SA based on SWCNT [13]. Such a SA can also support simultaneous mode-locked operation in resonator configuration with a common SA for Yb- and Er-doped fiber lasers [14]. Moreover, SWCNT SAs manufacturing technology is simpler and cost-effective in comparison to MBE. Nevertheless, the absorption characteristic of SWNT is not flat in the whole near IR range (1-2 µm) and strongly depends on the nanotubes’ diameter.
Among the Dirac-materials (type of materials where the conduction and valence bands touch at an isolated set of points, called Dirac points), graphene is the most promising candidate as a SA. Its extremely broad, flat and wavelength independent absorption characteristic [19,20] cause that graphene seems to be an universal SA for lasers operating in the near infrared. Since the first demonstrations of graphene mode-locked laser [18,21] a lot of works on graphene based fiber [18,21–37] and bulk [38–41] lasers have been presented. The Q-switched operation of graphene-based lasers has been also presented [42,43]. The graphene-based SAs can be obtained using several methods, commonly available than MBE, like CVD [21–35,38–41], chemical exfoliation (graphene-polymer composites [18,26–31] or graphene optically deposited directly onto fiber core [32,33]) and mechanical exfoliation [34–37]. Thanks to wavelength independent absorption characteristic of graphene, high quality monolayers manufactured by CVD have been used to achieve mode-locked operation in lasers operating from 800 nm [41] to 2500 nm [40]. However up to date there is no demonstration of simultaneous mode-locking of two lasers by a common graphene-based SA.
In this paper we present for the first, time to the best of our knowledge, an all-fiber Tm- and Er-doped fiber laser simultaneously mode-locked by a common graphene saturable absorber. The graphene layers were fabricated by CVD method and transferred onto the fiber connector end. Such prepared SA supported mode locked operation at 1565 nm and 1944 nm. The generated optical solitons have the FWHM of 3.9 nm and 4.2 nm for Tm- and Er-doped, respectively. The repetition frequency of resonator with Er-doped fiber was 20.19 MHz while the Tm-doped resonator was a 1 m longer with the repetition rate of 18.43 MHz. Presented experiment is the first step to synchronization (passive or active) and phase locking of two lasers simultaneously mode-locked by a common graphene SA.
2. Graphene SA preparation and characterization
Graphene used in the dual-wavelength fiber laser was deposited by CVD technique using a 12 µm thick copper foil as a substrate and propane gas as a carbon precursor. To detach graphene from copper, graphene was covered with a thin layer of poly(methyl methacrylate) (PMMA) by spin-coating method. Then graphene was removed from the backside of copper foil to avoid impurities between the top and lateral graphene films formed during copper etching. The PMMA/graphene stack was subsequently introduced into an aqueous solution of ammonium persulfate. After etching copper away, the PMMA/graphene stack was cleaned with deionized water. Next, the PMMA/graphene stack was fished by the substrate to which graphene does not stick and finally dried in an air atmosphere. Figure 1(a) shows the Raman spectra of the graphene on PMMA measured using a Renishaw Raman Microscope, with a x100 objective and a 532 nm frequency doubled Nd:YAG laser as an excitation source, with less than 1 mW of optical power on the sample. For that spectrum, the G (1592 cm−1) and 2D (2685 cm−1) bands are well seen, which confirms the presence of a graphene structure in measured sample. The FWHM (about 28 cm−1) of the 2D band for graphene layer presented as well as much higher intensity of the 2D band by comparison with the intensity of the G band (2D/G>2) are characteristic of a single layer of graphene [47,48]. The 2D band position is 2685 cm−1, which is very close to the value for freestanding graphene. It suggests that the graphene layer transferred from Cu foil is averagely relaxed. Very low D bands spectrum is typical of high quality graphene structures. The peaks at ~1450 cm−1 and ~1735 cm−1 come from the background of the sample - PMMA/carrier substrate stack which is confirmed by its Raman spectrum (Fig. 1(a)). A photograph of the fabricated free-standing PMMA/graphene layer is shown inset Fig. 1(a). Two small pieces of such composite are stacked and deposited onto the tip of the FC/APC connector and connected with another one. The measured transmission through a graphene/PMMA layer deposited on the connector is plotted in Fig. 1b. The average transmission of the composite is at the level of 95%. The fabricated graphene SA has a flat absorption spectrum over the range of 1500 – 2000 nm, which makes it useful in simultaneous mode-locking at two different wavelengths.
Fig. 1 Raman spectrum of the PMMA/graphene composite with photograph of an exemplary fabricated free-standing membrane (a). Transmission measurement of the PMMA/graphene composite with indicated operational bands of Er and Tm-doped lasers (b).
3. Experimental setup
The setup of the dual-wavelength fiber laser is shown in Fig. 2. The laser consists of two ring resonators with one common branch, which contains the graphene SA. The 1.55 µm loop is based on a highly Er-doped fiber (nLight Liekki Er80-4/125), pumped by a 980 nm laser diode via a fused 980/1550 nm wavelength division multiplexer (WDM). A fiber-based in-line polarization controller (PC) allows to adjust the intra-cavity polarization and start the mode-locking. The light is coupled out from the 1.55 µm cavity through a 20% coupler. The 2 µm loop is designed analogously to the Er-laser. It is based on a 1.5 m long piece of Tm-doped fiber (Nufern SM-TSF-9/125), pumped by a 1573 nm laser diode (beforehand amplified in an Er/Yb-doped fiber amplifier, EYDFA). A fiber isolator forces the signal to propagate counterclockwise, while the 1.55 µm signal circulates clockwise. Both signals are combined by a 1570/2000 nm WDM and directed to the common saturable absorber. After passing through the SA, the beams are separated using a filter-type WDM, which reflects the 1.55 µm signal and lets the 2 µm signal pass. Both loops have slightly different lengths, which results in repetition frequencies of 20.2 MHz for the Er-laser and 18.4 MHz for the Tm-laser.
The performance of the laser was observed using an optical spectrum analyzer with scanning range up to 2400 nm (Yokogawa AQ6375), 12 GHz digital oscilloscope (Agilent Infiniium DSO91304A), 7 GHz RF spectrum analyzer (Agilent EXA N9010A) coupled with a 12 GHz photodetector (Discovery Semiconductors DSC2-50S), and an optical autocorrelator (Femtochrome FR-103XL).
4. Experimental results
Stable, fundamental mode-locked operation of the Er-doped loop was observed at pump powers in the range from 20 mW to 30 mW. At higher pump levels pulse breaking and harmonic mode-locking appeared. In order to start the mode-locked operation in the Tm-doped resonator the active fiber was pumped at 1573 nm with powers ranging from 160 –205 mW. Similar to Er-doped laser at higher pump powers pulse breaking was also observed. In order to initiate the mode-locking, slight adjustment of the PCs in both resonators was required. Both lasers operate independently. Losing of mode-locking in one resonator does not affect the pulse operation in the second loop. In order to improve separation between the two signals at 1565 nm and 1944 nm, a filter-type WDM was placed directly before output couplers. We have observed, that fused-type WDMs do not provide proper separation and the signal from the Er-laser was observed at the 2 µm output and vice versa. All measurements were done at 30 mW and 190 mW of pumping for Er and Tm lasers, respectively. In order to investigate both lasers spectra simultaneously the external coupler was used to combine the signals. The generated optical spectra are presented in Fig. 3.
Fig. 3 Measured output spectra of the laser: a) 600 nm wide span confirming simultaneous mode locked operation at 1565 nm and 1944 nm, b) optical soliton generated in Er-doped loop. The CW peak at 1573 nm depicted in the inset graph is originated from the unabsorbed pump power from thulium-doped laser, c) optical soliton generated at 1944 nm in Tm-doped loop with characteristic dips resulting from water absorption in air.
The optical spectrum recorded with 0.2 nm resolution in the 600 nm wide span directly illustrates the simultaneous mode-locked operation at 1565 nm and 1944 nm (Fig. 3(a)). The dispersion of both loops was all-anomalous; hence the generated pulses have soliton-like shape with characteristics Kelly’s sidebands. The optical spectra of both generated solitons were measured with the highest available resolution (0.02 nm) and are depicted in Fig. 3(b) and Fig. 3(c) for Er and Tm based resonators, respectively. The FWHM of the soliton generated at 1565 nm was 4.2 nm (Fig. 3(b)). The graph inset Fig. 3(b) shows beside the typical for soliton mode-locking sidebands also the CW peak at 1573 nm originating from unabsorbed pump power from thulium-doped resonator. The 1573 nm wavelength fits the reflection bandwidth of the filter-type WDM. The 1573 nm wavelength was chosen in order to detune from the soliton peak of the Er-laser and avoid destabilization of the mode-locking. Such wavelength provides stable mode-locked operation at 1565 nm and efficient pumping of thulium active fiber. The FWHM of the soliton generated at 1944 nm was 3.9 nm (Fig. 3(c)). The dips at 1944.4 nm, 1945.5 nm and 1946.6 nm observed in high resolution spectrum originate from water absorption lines around 2 µm region. The output powers measured at 2 µm and 1.55 µm outputs were 1.3 mW and 0.5 mW, respectively.
The radio frequency (RF) spectrum measured at 1.55 µm and 2 µm outputs with 500 kHz frequency span and 20 Hz resolution bandwidth (RBW) are depicted in Fig. 4(a) and Fig. 4(b), respectively. The repetition rate of the Er-doped loop was 20.19 MHz, which corresponds to an approx. 9.5 m long cavity. The Tm-doped resonator was around 1 m longer which results in 18.43 MHz repetition frequency. The RF spectra measured with 6 GHz span are depicted in inset graphs of Fig. 4(a) and Fig. 4(b) confirm fundamental mode-locking without any spectral filtering. The signal to noise ratio (S/N) measured at both outputs were better than 65 dB.
Fig. 4 RF spectra measured at: a) 1.55 µm output, b) 2µm output, c) external coupler combining 1.55 µm and 2µm outputs.
Figure 4(c) presents the RF spectrum taken at external coupler combining both laser outputs. The two frequency combs with different repetition frequencies are clearly seen. The corresponding pulse trains are depicted in Fig. 5. The pulses are separated of 54.25 ns and 49.5 ns which correspond to 18.43 MHz and 20.19 MHz repetition frequencies measured at 2 µm and 1.55 µm outputs, respectively.
The autocorrelation traces measured at 1.55 µm and 2 µm laser outputs together with a sech2 fitting are depicted in Fig. 6. The pulse duration after deconvolution and time bandwidth product (TBP) are 933 fs and 0.479 for the soliton pulse centered at 1565 nm and 1.03 ps and 0.318 for pulse centered at 1944 nm.
5. Summary
Concluding, the paper presents for the first time to the best of our knowledge an all-fiber Tm- and Er-doped fiber laser simultaneously mode-locked by a common graphene saturable absorber. The lasers based on common graphene layers prepared by CVD method SA supported mode locked operation at 1565 nm and 1944 nm. The generated optical solitons have FWHM of 3.9 nm and 4.2 nm for Tm- and Er-doped loops, respectively. The repetition frequencies of Er and Tm resonators were 20.19 MHz and 18.43 MHz, respectively. Developed laser configuration provides stable mode-locked operation at wavelength ranges separated of around 400 nm and will be further investigated in terms of passive synchronization and phase locking of both laser loops.
Acknowledgments
The work presented in this paper was supported by the National Science Centre (NCN, Poland) under the project “Saturable absorption in atomic-layer graphene for ultrashort pulse generation in fiber lasers” (decision no. DEC-2011/03/B/ST7/00208)” and by the Polish Ministry of Science and Higher Education under the project no. POIG.01.01.02-00-015/09-00. Research fellowship of two authors (G.S. and J.S.) is co-financed by the European Union as part of the European Social Fund.
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