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We experimentally demonstrate an operation switchable Erbium-doped fiber laser by employing graphene saturable absorber (GSA) on microfiber. With the introducing of a polydimethylsiloxane layer, a graphene can be considered as a parallel plate on microfiber and induces different propagation losses to TE and TM modes. By the use of such polarization sensitive GSA on microfiber, Erbium doped fiber laser with switchable operation states such as continuous wave, stable Q-switching, Q-switched mode-locking, and continuous-wave mode-locking, can be achieved by simply tuning the polarization states in the laser cavity. Our results show that covering graphene on microfibers could be a promising method for fabricating all fiber SA, and may have high potential in wide applications.
©2013 Optical Society of America
1. Introduction
Passive pulsed fiber lasers have attracted much attention, owning to their unique advantages, such as their simple and compact design and widespread applications on optical communication, medicine, micro-processing and material processing. There are several operation states of short-pulse lasers, such as Q-switched [1], continuous-wave mode-locking (CML) [2], and Q-switched mode-locking (QML) [3]. Q-switched fiber lasers can generate pulse with high energy, CML fiber lasers generate pulse with ultra-short duration, while QML fiber lasers can provide higher peak power than CML. Therefore, operation switchable fiber lasers have high potential in wide applications. Recently, such a kind of fiber laser has been proposed by employing hybrid mode-locking mechanism [4], including semiconductor saturable absorber mirror (SESAM) and nonlinear polarization rotation (NPR) techniques. However, the dual mode-locking mechanism is complex, and SESAMs are usually considered as expensive and complicated-fabrication devices [5]. Another QML fiber laser has been built up based on NPR, but the employed polarization plates and polarization beamsplitters are too bulky for in-fiber integration [6].
As a novel saturable absorber, graphene has been intensively investigated because of the gapless linear dispersion of Dirac electrons and Pauli blocking, which provides an ultra-broad operation bandwidth and an ultrafast recovery time [7, 8]. Moreover, the graphene-based saturable absorber (SA) is superior to SESAM and SWCNTs as it does not require band-gap design and diameter control to improve its performance [9]. Since Bao et al. demonstrated the first mode-locked fiber laser with graphene [10], various passive pulsed fiber lasers by using graphene saturable absorbers (GSAs) have been investigated [11–15]. More recently, a QML fiber laser using a graphene oxide-deposited side-polished fiber as the saturable absorber is demonstrated [16]. Owing to the D shape of the side-polished fiber, a GSA has a high polarization dependent loss and can lead to passively QML by intra-cavity polarization control. Nevertheless, a side-polished fiber still suffers from the drawback that its preparation usually requires a high precision fabrication technology.
Microfiber provides another promising method of integrating GSAs in fiber laser cavities, since it can provide large evanescent field, tight optical confinement, strong field enhancement, and large waveguide dispersion. It can be easily coupled to each other or to other optical material with low loss [17], for example, in microfiber based GSAs [18]. High quality passively pulsed fiber lasers and Broadband all-optical modulator have been achieved with these GSAs [19–21]. However, to the best of our knowledge, the polarization characteristics of microfiber based GSAs have been rarely studied because of the cylindrical symmetry of microfiber, although the polarization characteristics are very important for fabrication and application of optical devices.
In this letter, the polarization dependent saturable absorption of microfiber based GSA is measured experimentally and investigated numerically. With the consideration that the graphene is an infinitesimally thin, local two-sided surface characterized by a surface conductivity [22], a theoretical model describing light propagation characteristics in GSA is established. By the use of microfiber based GSA, Erbium-doped fiber laser with different operation states such as continuous wave (CW), stable Q-switching, QML, CML are obtained by simply tuning the polarization states in the laser cavity.
2. Fabrication and polarization characterization of microfiber based GSA
A schematic of the microfiber based GSA is shown in Fig. 1, where the microfiber is sandwiched between a MgF2 substrate with a low refractive index of 1.376, and a polydimethylsiloxane (PDMS) with a refractive index of 1.413 that supports a graphene film. The microfibers are drawn from standard single mode fibers (SMF) by use of flame-brushing technique, with a diameter down to ~6 μm, a length up to ~5 mm and a low insertion loss down to 0.1dB.
The graphene film is directly synthesized by CVD method on the polycrystalline Cu substrate. A polymer clad resin (EFiRON, PC-373, refractive index of 1.376) is utilized to support the graphene, because of its flexibility, high adsorption capacity, and low refractive index of 1.413. To avoid the impact of contamination on light guiding, a new dry transfer method by using a two-layer-composite-structure of polyethylene terephthalate and silica gel was applied to transfer the single-layer graphene with a size of 5 × 5 mm2 from the copper substrate to PDMS [21]. With this method, the transferred graphene possesses a cleaner and more continuous surface, a lower doping level, and a higher optical transmittance and conductivity than that transferred by thermal release tape and PMMA [23]. Therefore, there is little effect of contaminants on the experiment, and the reproducibility of the experiment is satisfactory. PDMS has a larger size of 20 × 20 mm2 in order to completely cover the tapered area and to decrease the influence of refractive index mutation on the evanescent fields of the microfiber. To ensure that there is no gap in the contact surface between the microfiber and graphene, a pressure is applied on PDMS.
The polarization dependent saturable absorption of GSA is measured by using a home-made 1.5 ps mode-locked fiber pulse laser operating at 1.55 μm. Figure 2 schematically shows the experimental configuration of the measurement. The output of the fiber laser is spliced by an in-line polarization controller (PC) and a fiber polarizer, to realiz a linearly polarized light. Then the polarization state of the laser pulse injected into GSA is tuned by the subsequent in-line PC. Within the output power range of the pulsed fiber laser, the polarization dependent saturable absorption of GSA is measured, as shown in Fig. 3. For TE mode, the transmission increases by 3.4% due to absorption saturation, when the incident peak power density is raised from 1.2 to 1154 MW/cm2. The nonsaturable absorption for TE mode is 2.45 dB, which is lower than the insertion loss of GO-deposited D-shaped fiber [16]. For TM mode, the change of transmission is 3.5% when the incident peak power density is increased from 1.2 to 1145 MW/cm2, and the nonsaturable absorption is 5.65 dB. Evidently, the TM mode has a relatively higher nonsaturable absorption than the TE Mode.
Fig. 2 The schematic experimental setup for measurement of polarization dependent absorption of microfiber base GSA.
Fig. 3 The measured nonlinear absorption of the microfiber based GSA for TE mode (red line) and TM mode (black line).
To obtain further insight, a theoretical model is built to study the light propagation characteristics in GSA. Because of the small diameter of the microfiber (~6 μm) and the relatively large thickness of PDMS (~1 mm), the cross-section of the GSA can be simplified as a silica cylinder sandwiched between the MgF2 and graphene/PDMS parallel plates, as shown in Fig. 4(a). The graphene can be considered as an embedded conductive interface between two dielectrics (silica microfiber/graphene/PDMS) [22]. With finite element method, the modal fields of TE and TM modes in GSA are calculated. From Fig. 4(b) we can see that the magnitude of TE mode is much larger than that of TM mode at the interface of microfiber/graphene, because the electric field of TE mode is parallel to the graphene sheet. It means that TM modes suffer higher propagation loss than these of TE modes, i.e. a polarization dependent absorption is realized.
Fig. 4 (a) The schematic diagram of cross-section of microfiber based GSA. (b) Calculated modal fields of TE (green line) and TM (blue line) modes in microfiber based GSA. The region of microfiber is in 10 < x< 16 μm. The graphene locates at x = 16μm.
3. Experimental setup
With this polarization sensitive GSA, an Erbium-doped fiber laser (EDFL) is constructed, whose operation state can be easily controlled by tuning the polarization in the cavity. The experimental configuration of EDFL is shown in Fig. 5. A 1 m heavily EDF (Thorlabs EDF-80) is used as the gain medium, which is pumped by a 980 nm laser diode (LD) through a fused wavelength division multiplexer (WDM). A PC is used to tune the polarization state in the cavity while a polarization independent isolator (PI-ISO) maintains the unidirectional laser pulse propagation. The laser output is directed through the 10% port of a coupler. The group velocity dispersion (GVD) is one of the key factors to maintain the fiber laser operation stability. The GVD of the EDF is about 25 ps/nm/km. The total cavity length is ~6.9 m. The rest of the cavity consists of SMF with a GVD of ~17 ps/nm/km. Thus, the net cavity dispersion obtained is ~-0.096 ps2. The operation performance is evaluated by a 200 MHz bandwidth photodetector and an oscilloscope. An optical spectrum analyzer (OSA) with a 0.1 nm resolution is utilized to measure the output spectrum.
4. Results and discussion
In the experiment, the pump power is fixed at 280 mW. The fiber laser is firstly tuned to operate in the CML state. The measured oscilloscope trace of the output pulses is presented in Fig. 6(a). It can be seen that the repetition rate is 31.3 MHz, which is in agreement with the cavity length of the fiber laser. With the measurement of optical spectrum and autocorrelation trace displayed in Figs. 6(b) and 6(c), a 3 dB spectral bandwidth of 7.2 nm and a temporal width of 430 fs are obtained. It is clear that the pulse duration is much shorter than that obtained by using graphene oxide-deposited side-polished fiber [16], due to the low insertion loss of the microfiber based GSA.
Fig. 6 Output characteristics of EDFL operated in CML state. (a) Output pulse trains measured by oscilloscope. (b) Autocorrelation trace of laser output. (c) Output optical spectrum centered at 1568nm.
By changing the polarization state, the CML operation could be switched to Q-switched. The typical oscilloscope trace and optical spectrum of the laser are shown in Fig. 7. It is clear that the 3 dB spectral bandwidth of Q-switched operation is about 0.53 nm, which is much narrower than that of the CML operation, implying a lack of fixed phase relationship. By adjusting PC, various repetition rate and pulse width can be obtained. During the tuning process, the repetition rate changed in the range 34.9-53.0 kHz and the pulse width changed in the range 2.78-2.90 μs.
Fig. 7 Output characteristics of EDFL operated in Q-switched state. (a) Output pulse trains measured by oscilloscope. (b) Output optical spectrum with the FWHM of 0.53nm.
Between the CML operation and the Q-switched operation, QML operation state can be achieved by carefully adjusting the polarization state. The results for QML EDFL are shown in Fig. 8. The QML envelope repetition rate is about 83 kHz. The QML pulse spacing is about 32 ns, which is corresponding to the repetition rate of 31.3 MHz, similar to the repetition rate of CML operation as shown in Fig. 6 (a).
In addition, CW operation can also be observed. The output spectrum is reduced to a very narrow linewidth, whose 3 dB bandwidth is about 0.06 nm, as shown in Fig. 9.
The switching of the operation states of EDFL could be attributed to the polarization dependent saturable absorption of GSA. According to the theory given by Okhotnikov et al. [4, 5], the operation state of the passively pulsed fiber laser can be determined by the critical intracavity pulse energy Ec:
where Esat, G is the gain saturation energy of the EDFL, Esat,A is the saturation energy of SA, and ΔR is the saturable absorption of SA. For a stable CML operation, the intracavity pulse energy must be larger than Ec. In our case, the different polarization modes suffer different nonsaturable absorption and saturable absorption. For TE mode, the nonsaturable and saturable absorptions are much lower than these of TM mode. Therefore, with a fixed pump power, the intracavity pulse energy of TE mode is much higher than that of the TM mode, and may reach Ec and support CML operation according to Eq. (1). When the laser pulse polarization state in GSA is tuned from TE mode to TM mode, the cavity loss increases gradually. As a result, the intracavity pulse energy decrease continuously, which makes the saturable absorption unobvious and induces the repetition reducing and pulse widening. Finally, when the pulse energy is very low, the saturable absorption disappears, and then the laser operates with CW state.In addition, during the operation states switching of EDFL, the output wavelength is also changed. It could be attributed to the change of loss spectrum of the laser cavity with the polarization state in cavity. It means that when the polarization state is different, the loss distribution along the wavelength also changes, which results in different lasing wavelengths.
5. Conclusion
In conclusion, a polarization sensitive GSA based on microfiber is demonstrated. By introducing a thick PDMS layer, the microfiber based GSA can be described by a cylinder sandwiched between two parallel plates. With numerical calculation, the polarization dependent saturable absorption of GSA is attributed to different attenuations of the TE and TM modes. By using the microfiber based GSA, fiber lasers with different operation states including CW, stable Q-switching, QML, and CML can be obtained by simply tuning the polarization states in the laser cavity. The fiber lasers operated in several states may have potential applications in related areas.
Acknowledgment
This work was supported by the Chinese National Key Basic Research Special Fund (grant 2011CB922003), the Natural Science Foundation of China (grant 11174159) and the National Science Fund for Talent Training in Basic Sciences (grant J1103208). We acknowledge the support of Prof. Jing Chen (School of Physics, Nankai University, Tianjin, China) for helpful discussions.
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