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We demonstrate an all-optical Q-switcher based on graphene saturable absorber (GSA). Due to the cross absorption modulation (XAM) effect in graphene, we can change the transmittance of signal light periodically by introducing a train of laser pulses into graphene. This allows controlling the Q-factor of the cavity. This Q-switcher has many advantages such as all-fiber structure, all-optical modulation, broadband applications. With this Q-switcher, we have successfully fabricated an actively Q-switched ytterbium-doped fiber laser. The pulse repetition rate can be tuned from 30.32 kHz to 101.29 kHz. What’s more, synchronization of the Q-switched laser pulses and modulation laser pulses can be realized, which has many potential applications such as nonlinear frequency conversion, multi-color pump probe spectroscopy and Raman scattering spectroscopy.
© 2015 Optical Society of America
1. Introduction
Q-switched fiber lasers have attracted considerable attention because of its compactness, low cost, and wide applications in material processing, environmental sensing, and long-pulse nonlinear experiments [1–3 ]. Q-switching can be classified into passive and active techniques, depending on use of active modulation of the laser cavity Q-factor. In the case of passive Q-switching, a saturable absorber is incorporated into the cavity, which acts as a self-operating Q-switcher. Recent years, many excellent saturable absorbers such as semiconductor saturable absorbers (SESAMs) [4], single-walled carbon nanotubes (SWCNT) [5,6 ], graphene [7–9 ] and topological insulators [10] have been studied and many passive Q-switched lasers based on these saturable absorbers have been reported. All these passively Q-switched lasers have very simple structures, but their repetition rates depend on the pump power, and cannot be modified without changing other operation parameters, which limits the realistic applications.
Active Q-switching uses an externally-modulated Q-switcher, which allows for flexible repetition rate changes and timing synchronization. Traditional active Q-switched lasers modulated by electro-optic [11] or acousto-optic modulators [12], whose bulk components break the all-fiber structure. Moreover, bulk components require fine alignment and good mechanical stability, which makes them difficult to design practical devices. Recent years, a series of fiber Bragg grating (FBG) modulators have been investigated and all-fiber active Q-switched lasers have been reported based on these modulators [13–17 ]. However, these FBG modulators need special design for working wavelengths. Because of the above advantages and disadvantages, research on an all-fiber, all-optical and broadband Q-switcher is required.
It’s well known that graphene has broadband and fast saturable absorption. Graphene has been considered as an ideal saturable absorber to generate laser pulses in the spectral range from 0.8 μm to 2.5 μm in mode-locked and Q-switched regime [18–21 ]. In addition, cross absorption modulation (XAM) effect in graphene attracted much attention in recent years [22–24 ], which attributes to the optical excitation of carriers that leads to Pauli blocking of part of interband transitions. With XAM effect, the attenuation of light with one frequency in graphene can be tuned by introducing light with another frequency. High-speed and broadband all-optical modulators have been demonstrated successfully by employing the XAM effect in graphene [22, 23 ].
In this paper we present an all-optical all fiber Q-switcher by employing the XAM effect in graphene, which does not need special design for working wavelength. With this graphene-based Q-switcher, a 1.06 μm actively Q-switched all-fiber laser is demonstrated by introducing a train of 1.55 μm laser pulses. The repetition rate of the laser can be tuned from 30.32 kHz to 101.29 kHz with independence of other operation parameters. Synchronization of 1550 nm pulses and 1060 nm pulses can be realized as well, which has many potential applications such as nonlinear frequency conversion, multi-color pump probe spectroscopy and Raman scattering spectroscopy.
2. The graphene-based all-optical Q-switcher
The structure of graphene-based all-optical Q-switcher is shown in the inset of Fig. 1 . It consists of a graphene-covered-microfiber structure, where the microfiber is sandwiched between low refractive index MgF2 substrate (with refractive index 1.37 at wavelength 1.55 μm) and polydimethylsiloxane (PDMS) (with refractive index 1.413 at wavelength at 1.55 μm)-supported graphene film. The microfibers are drawn from standard single mode fibers (SMF) by use of flame-brushing technique, with a diameter down to ~7 μm, a length up to ~5 mm. The graphene film is directly synthesized by CVD method on polycrystalline Cu substrate. To avoid the impact of contamination on light guiding, a new dry transfer method was applied to transfer graphene from copper substrate to PDMS [22]. The wavelength division multiplexer 1(WDM 1) is used to couple the modulation light into the GSA, while the WDM 2 is used to separate modulation light from GSA.
Fig. 1 The schematic experimental setup for measurement of characteristics of the graphene-based all-optical Q-switcher microfiber base GSA.
In this all-optical Q-switcher, the signal light is centered at 1060 nm and the modulation light is centered at 1550 nm. First, we measured the 1060 nm optical transmission of this device as a function of the power of 1550 nm light. Both the 1060 nm and 1550nm light were continuous wave lasers, while the power of 1060 nm light was 1 mW. Fig. 2(a) shows the transmission of 1060 nm light with different modulation light power. It is clear that the 1060 nm transmission increased with the power of 1550 nm light. The transmission changed by 5% when the power of 1550 nm light varied from 0 mW to 930 mW. This phenomenon was due to XAM effect in graphene. The electrons of graphene excited by modulation light will crossover the energy bands, and form Fermi-Dirac distribution which will take up the position of electrons excited by signal light. Considering the Pauli blocking principle, the absorption of graphene to signal light will be decreased. Because of graphene’s linearly dispersive conduction and valence bands and broadband optical response [21–25 ], this Q-switcher does not need special design for working wavelength.
Fig. 2 The properties of the graphene-based all-optical Q-switcher. (a) The 1060 nm optical transmission as a function of the power of 1550 nm light. (b) The transient response of Q-switcher used in this experiment.
Second, we measured the transient response of this device to estimate the maximum switching speed by modulating the 1550 nm light with a square pulse possessing rise and fall times of ~50 ns. The operation performance was evaluated by a 200 MHz bandwidth photodetector. The measured temporal shapes of the applied 1550 nm pulse and the 1060 nm pulse corresponding pulse from the all-optical Q-switcher are shown in Fig. 2(b). A delay of 1.4 μs between modulation light and the signal light is shorter than that of FBG-based acousto-optic Q-switcher [15], which indicates a faster response of the all-optical graphene-based Q-switcher. The rise and fall times, which were defined as the temporal duration between 10% and 90% of the maximum transmission, were found to be ~20.2 μs and ~21.4 μs in Fig. 2(b) respectively. Considering the fast relaxation of optical excited carriers, the relatively long rise and fall times of the Q-switcher is mainly due to the relatively large diameter and length of the microfiber. With a narrower and shorter microfiber, the repetition rate of the Q-switcher will easily exceed 1 MHz [23].
3. All-optical, actively Q-switching of the ytterbium-doped fiber laser
With this graphene-based all-optical Q-switcher, an ytterbium-doped fiber laser (YDFL) was constructed, which employed a passively Q-switched erbium-doped fiber laser as modulation light. The structure of YDFL is shown in Fig. 3 . A 50 cm ytterbium-doped Fiber (OFS 1100) was used as the gain medium, which was pumped by a 980 nm laser diode (LD) through a WDM. A 1065 nm fiber filter with bandwidth of 10 nm was used to limit the output wavelength of the YDFL. The laser output is directed through the 20% port of a coupler. A homemade 1550 nm Q-switched fiber laser served as the modulation setup. The total length of YDFL was 17.2 m. The output characteristics were evaluated by a 200 MHz bandwidth photodetector, oscilloscope and optical spectrum analyzer (OSA) with a 0.02 nm resolution.
4. Results and discussion
The continuous wave laser at 1060 nm was observed firstly with the pump power of 138 mW. Then the Q-switched erbium-doped fiber laser centered at 1550 nm was turned on as modulation light. The output power, pulse duration and repetition rate of the modulation light were fixed at 55.7 mW, 3.15 μs and 40.19 kHz respectively. When the pump power of YDFL was increased to 177.5 mW, the stable Q-switched pulse of 1060 nm can be obtained by adjusting the PC state appropriately. Keeping the PC state fixed, the maximum output power of 0.99 mW can be obtained when the pump power reaches the maximum of 382.4 mW.
The output pulse train and spectrum of the Q-switched fiber laser are shown in Figs. 4(a) and 4(b) . It can be seen that the repetition rate of the laser is in accordance with that of the modulation light. It should also be noted that there is a delay between 1060 nm pulse and 1550nm pulse of about 5 μs, which is larger than the delay shown in Fig. 2. This effect mainly results from the build-up process of the Q-switched laser pulses. When 1550 nm pulse was incident into the Q-switcher and the cavity loss began to be switched to a low level, the photon density in the cavity is extremely small. Period of time is needed for the photon density to be accumulated to a high level to build up laser pulses. The build-up time Tb can be roughly estimate by [26]
where δc is fractional loss per round-trip due to cavity losses and output coupling, T = L/c is the round-trip time inside the laser cavity and r is initial inversion ratio. In our experimental setup, the cavity length is 17.2 m, so that T≈84 ns, the output coupling coefficient is 20% corresponding to δc = ln(1/0.8)≈0.22. With the consideration of the laser bing initially pumped to three times of the threshold (r = 3), the build-up time will be about 4.5 μs, which agrees with our experimental result.Fig. 4 Output characteristics of the Q-switched YDFL with modulation repetition rate of 40.19 kHz. (a) Output pulse trains under pump power of 382.4 mW. (b) Output optical spectrum of YDFL. (c) Output power and pulse width as a function of pump power. (d) Pulse energy and peak power as a function of pump power.
The output power, pulse width, peak power and pulse energy varied as a function of the pump power are shown in Figs. 4(c) and 4(d). It can be seen that the pulse duration decreased when the pump power was increasing. This can be attributed to incomplete inversion between pulses. When the pump power is higher, the initial inversion ratio will be higher, and a narrower pulse width will be obtained [26]. The narrowest pulse duration of 2.16 μs was obtained with the maximum pump power, which is comparable with that of the pulse output from FBG-based all-optical Q-switched fiber laser [17]. During this process, the repetition rate of Q-switched laser was always equal to the repetition rate of modulation light. In order to prove that the Q-switched pulse of 1060 nm was induced by the modulation of 1550 nm light, we turned off the Q-switched erbium-doped fiber laser. The pulse of 1060 nm disappeared immediately and could not be obtained even with careful adjustment of the PC state and pump power. This indicates that the 1550 nm laser pulses modulated the Q-factor of cavity and helped to form 1060 nm laser pulses. It means that, with this all-optical Q-switcher, not only actively Q-switched pulse of 1060 nm can be acquired, but also synchronized laser pulses of 1060 nm and 1550 nm can be realized.
When the repetition rate of 1550 nm pulse light was tuned from 30.32 kHz to 101.29 kHz, stable Q-switched pulse of 1060 nm was also acquired, whose repetition rate varied with that of modulation light. At each fixed Q-switching frequency, the increasing of the pump power delivered to the cavity increased the output peak power and reduced the pulse duration simultaneously, as shown in Figs. 4(a) and (b). By keeping the pump power fixed at 382 mW, we measured the output characteristics varied as a function of the repetition rate, which are shown in Fig. 5 . The output pulses width was observed to fluctuate between 2.61 μs and 5.21 μs, the output pulse peak power almost decreased with the repetition rate. The variations of pulse duration and peak power with the increasing repetition rate are due to the decrease of initial inversion ratio and energy storage for pulse periods. It can also be seen that the average output power did not vary significantly with the repetition rate when the pump power was fixed.
Fig. 5 Output characteristics of the YDFL with pump power of 382 mW. (a) Pulse energy and pulse width as a function of repetition rate. (b) Average power and peak power as a function of repetition rate.
The pulse duration of this actively Q-switched fiber laser is not as narrow as ns order, because the structures of this Q-switcher and the YDFL cavity were not optimized. The pulse duration is affected by the modulation depth and response rate of the Q-switcher, as well as the laser cavity design. Higher modulation depth and faster response rate will lead to shorter pulse duration. When the diameter of microfiber in Q-switcher is reduced, more energy will be leaked into graphene, which leads to high modulation depth. Meanwhile, shorter microfiber will make the transit time of the light through the switcher shorter and response rate higher. It has been demonstrated that with a microfiber of 1 μm in diameter and 2 mm in length, the modulation depth and response rate of Q-switcher will be up to 8.5% and 200 GHz, respectively [23]. With such Q-switcher and linear cavity of length shorter than 1 m, Q-switched pulse duration of ~200 ns can be expected [27, 28 ].
5. Conclusion
We have reported an actively Q-switched all-fiber laser by a new kind of all-optical Q-switcher. Q-switching is achieved based on XAM effect of graphene. This Q-switcher has many advantages such as all-fiber structure, all-optical modulation, broadband application. The narrowest Q-switched pulse duration is 2.16 μs and its repetition rate can be tuned from 30.32 kHz to 101.29 kHz. With this all-optical Q-switcher, not only actively Q-switched laser pulses can be acquired, but also the synchronization between Q-switched laser and modulation laser can be realized, which has many potential applications, such as nonlinear frequency conversion, multi-color pump probe spectroscopy and Raman scattering spectroscopy.
Acknowledgments
This work was supported by the National Nature Science Foundation of China (grant 61475076, 61405139, 11174159), the Nature of Science Foundation of Tianjin (grant 14JCYBJC16400), International Cooperation Program sponsored by MOE, the Chinese National Key Basic Research Special Fund (grant 2011CB922003) and the National Science Fund for Talent Training in Basic Sciences (grant J1103208).
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