Optically controlled in-line graphene saturable absorber for the manipulation of pulsed fiber laser operation

We demonstrate an optically tunable graphene saturable absorber to manipulate the laser operation in pulsed fiber laser system. Owing to the strongly enhanced evanescent field interaction with monolayer graphene, we could realize an efficient control of modulation depth in the graphene saturable absorber by optical means through cross absorption modulation method. By integrating the tunable graphene saturable absorber into the fiber laser system, we could switch the laser operation from Q-switching through Q-switched mode-locking to continuous wave mode-locking by adjusting only the optical power of the control beam. In addition, we realized a hybrid Q-switching of fiber laser by periodical modulation of the absorption of the graphene saturable absorber, where we observed that the repetition rate of the Q-switched laser could be continuously tuned according to the modulation frequency of the applied external signal. ©2016 Optical Society of America OCIS codes: (060.2310) Fiber optics; (060.3510) Lasers, fiber; (140.4050) Mode-locked lasers; (160.4330) Nonlinear optical materials. References and links 1. F. Bonaccorso, Z. Sun, T. Hasan, and A. C. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics 4(9), 611–622 (2010). 2. P. Avouris and M. Freitag, “Graphene Photonics, Plasmonics, and Optoelectronics,” IEEE J. Sel. Top. Quantum Electron. 20(1), 6000112 (2014). 3. E. Hendry, P. J. Hale, J. Moger, A. K. Savchenko, and S. A. Mikhailov, “Coherent nonlinear optical response of graphene,” Phys. Rev. Lett. 105(9), 097401 (2010). 4. T. Gu, N. Petrone, J. F. McMillan, A. van der Zande, M. 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Introduction
Graphene, a single sheet of carbon atoms arranged in a hexagonal lattice, has been intensively studied for the applications to future photonic and optoelectronic devices because of its exceptional electrical and optical properties including high carrier mobility, gapless band structure, and optical transparency over wide spectral window [1,2].The material also exhibited large optical third-order nonlinearity with ultrafast response, which facilitated its applications in the field of nonlinear optics such as coherent four wave-mixing [3], regenerative nonlinear signal oscillation [4] and saturable absorbers (SAs) [5].In particular, the application of graphene as SAs have attracted great attention because graphene possesses several merits.For example, it does not require bandgap engineering contrary to the conventional SAs such as semiconductor saturable absorber mirrors (SESAMs) [6] or carbon nanotubes [7][8][9].The operation range of the graphene SAs can also cover from near infrared (IR) to far-IR with uniform optical absorption and relatively high optical damage threshold.Utilizing these advantages, various pulsed fiber lasers have been demonstrated by employing layered graphene [5,[10][11][12][13] or graphene-related materials including reduced graphene oxide and graphene flake composite [14][15][16].
One of the interesting features of graphene material is that its optical properties can be controlled by stacking graphene layers [17] or shifting the Fermi level through electrical or chemical means [18,19].With this method, a graphene-based optical modulator with broad operation bandwidth has been reported by electrical gating [20].Electrical control of all-fiber graphene device was also demonstrated to report electrically tunable nonlinear saturable absorbers [21].In addition, it has been revealed that the optically excited carriers can alternatively tune the linear optical absorption in graphene by Pauli blocking principle [22,23], which enabled all-optical modulation of the light in a graphene-based device with large bandwidth [23].In nonlinear regime, this can control nonlinear absorption properties of graphene SA (GSA), such as modulation depth and non-saturable loss.By applying this scheme to the fiber laser system, optical control of pulse duration was reported in a passively mode-locked femto-second (fs) fiber laser with GSA [24].However, previous study of tunable GSA report small variation (< 1%) of modulation depth through the optical control [24].Although, in principle, the nonlinear absorption in the GSA can be significantly manipulated through cross absorption modulation (XAM) via evanescent field interaction, optical control of fiber laser operation for different pulsed laser states has not been demonstrated to the best of our knowledge.This is due to the limited nonlinear interaction of light with graphene.
In this paper, we demonstrate optically controlled in-line GSA which has large tunability in modulation depth through the enhancement of optical interaction with graphene.The inline GSA was fabricated by transferring a large area and uniform monolayer graphene onto the side-polished fiber (SPF).By employing additional overcladding on the SPF, we could realize strongly enhanced evanescent field interaction with monolayer graphene with reduced non-saturable scattering loss [25].We describe the details of the optical tuning behavior of the GSA by applying control beam at 980 nm to the SA.Experimental demonstration of different pulsed laser modes is also described when the GSA was applied to a fiber ring laser with the control beam power as the only variable parameter.

Fabrication of the SAs and laser experiment
Figure 1(a) schematically depicts our in-line GSA.We cut a groove at a quartz block with a radius of curvature of 25 cm and embedded a standard single-mode fiber (SMF) in the groove.After fixing the SMF using a ultra-violet (UV) curing epoxy, we side-polished the SMF until the distance between core boundary and polished surface becomes about 1 μm.A cross-sectional and side-view of the side-polished fiber (SPF) embedded in the quartz block is shown in the Fig. 1(b).A monolayer graphene sheet fabricated by the chemical vapor deposition (CVD) method [26] was then transferred onto polished surface to make grapheneevanescent field interaction of the core mode.The interaction length in this device at the wavelength of 1550 nm was estimated to be 1.6 mm for the radius of curvature of 25 cm [27].After transferring the graphene, the optical absorption of the SPF was increased to 0.58 dB with slight polarization dependent loss (PDL) of 0.03 dB.We then applied an overcladding with matched refractive index to increase the graphene-light interaction that also significantly increased the PDL to 13dB [25].With this structure, the graphene SA demonstrate strong small signal absorption and large modulation depth at the wavelength of the signal beam at 1550 nm.
Optical control of the nonlinear absorption properties of GSA was realized through XAM in a graphene sheet [22] where the light at a shorter wavelength (980 nm) was used to adjust the absorption of the signal beam at longer wavelength.When the control beam is absorbed in a graphene layer, the absorption of the signal beam is reduced by XAM [24,28].Nonlinear transmission properties of the fabricated GSA was experimentally studied for various powers of continuous wave (CW) control beam and the signal beam, and Fig. 2 summarizes the results.The GSA initially exhibited optical transmission of about 50% at low power level of signal beam at the wavelength of 1550 nm for TM mode whose polarization direction is normal to the graphene layer.The optical transmission increases up to 75% for the applied signal beam power of ~800 mW, resulting in modulation depth of 25% in the absence of control beam.When the control beam at 980 nm was applied to the GSA, the transmission of the device significantly changed particularly at low power level, leading to the reduction of the modulation depth as shown in the Fig. 2(a).The modulation depth of the GSA changed from 25% to 3.1% by adjusting the control beam power from 0 to 84 mW.In case of TE mode having parallel polarization direction along the graphene layer, the incident signal beam experiences only a small transmission of 2.5% in the monolayer graphene at low power level as shown in Fig. 2(b).The transmission increases to 50.2% at high power level in the absence of the applied control beam, resulting in the modulation depth of 47.7% over the signal power range of about 800 mW.This value decreased to 28.5% when 84 mW of control beam power is applied.In case of TE mode, the transmission curve of the SA could not reach full saturation level over the available power range of the signal beam.Therefore, we expect the actual modulation depth of the GSA may be larger than that we experimentally measured in this case.For both TM and TE modes, it can be seen that the major effect of the control beam is on the modulation depth while other optical parameters such as non-saturable loss and saturation power did not change significantly.We integrated the GSA device into an Er-doped fiber ring-laser system, as depicted in Fig. 3. Total cavity length of the laser was 40 m including a 30-cm-long Er-doped fiber (EDF, Liekki Er-80).An output coupler (OC) and an isolator were employed to partially extract the light from the laser cavity and to make uni-directional operation in the ring laser, respectively.The laser cavity includes two wavelength division multiplexing (WDM) couplers and two 980-nm laser diodes (LDs) for pumping the EDF and for controlling the GSA.The control beam from LD2 was operated in CW or modulated mode, as shown in the inset of the Fig. 3 to manipulate the laser operation state.A pulsed laser can operate in different modes such as CW mode-locking, Q-switched mode-locking (QML), and Q-switching.A number of system parameters such as small signal gain, gain saturation power, gain relaxation time, modulation depth and saturation power of the SA determines that operating mode of a laser [29][30][31].In our experiment, we varied only the control beam power from LD2 that controls mainly the modulation depth of the SA as discussed earlier.Figure 4 schematically illustrates the switching behavior of the laser operation as the modulation depth of the SA varies.The condition needed for the modelocking of a laser is well described in references 29 and 30.While operating in CW modelocking state, the laser will switch to QML regimes if Q-switching condition is satisfied as [31], , .
Here E P , q p , T R and τ L are pulse energy, loss of the light at the SA, cavity round trip time, and gain recovery time in a mode-locked laser system, respectively.E sat,L is the saturation energy of the gain fiber.As can be seen in Eq. ( 1), a greater modulation depth will increase the left side of the inequality and therefore can lead to QML.The required condition for the Q-switching operation without mode-locking is similar to expression (1), but with E p and E sat,L replaced by the average power P cw and the gain saturation power P sat,L,cw of a CW laser [31].Figure 5 shows experimental results for the switching behavior between different laser operating modes in a pulsed fiber laser system through optical control of the GSA.Here the laser output was measured in time domain by using a photodetector and an oscilloscope for several control beam power levels.Initially the fiber laser including fabricated GSA operated at Q-switching state (Fig. 5(a)) without control beam (Pc = 0 mW) where the repetition rate and pulse duration of the Q-switched laser output were 8.0 kHz and 20µs, respectively.When the power of the control beam was increased to 34 mW, we observed that the laser state was switched to QML.  Figure 6 shows some details of the CW mode-locked fiber laser output pulses.The FWHM of the pulse measured by using an intensity auto-correlator was 980 fs as shown in Fig. 6(a) and the spectral bandwidth in optical spectrum was 2.6 nm as shown in Fig. 6(b).As a result, the time-bandwidth product was about 0.31, which is close to the transform-limited value for a secant hyperbolic pulse.It should be noted that there might be enhancement by the nonlinear polarization rotation (NPR) effect with the presence of the large PDL in the GSA.Although we could not isolate the contribution from the NPR, we think the dominant effect was from the GSA based on the observation that the laser operation was stable against the polarization controller (PC) setting changes.Figure 6(c) and its inset show the radio frequency (RF) spectrum of the laser output pulse train, indicating stable operation of the CW mode-locked fiber laser.We further investigated the laser properties while applying sinusoidally modulated control beam to the GSA where the peak power and minimum power of the modulated signal was 30

Fig. 1 .
Fig. 1.Schematic representation of (a) optically tunable GSA and (b) its cross-sectional and side view.

Fig. 2 .
Fig. 2. Experimental data of nonlinear transmission properties of the GSA as a function of applied control beam power.Nonlinear transmission curve of the CW signal beam (1550 nm) at (a) TM mode and (b) TE mode with several CW control beam powers at 980 nm.

Fig. 3 .
Fig. 3. Schematic of Er-doped fiber ring laser integrated with optically tunable in-line monolayer GSA device.Inset: Two types of waveform of the control beam incident to the GSA by the LD2.

Fig. 4 .
Fig. 4. Simplified diagram of the condition for CW mode-locking, QML and Q-switching for different magnitude of the modulation depth in the GSA.

Figure 5 (
Figure5shows experimental results for the switching behavior between different laser operating modes in a pulsed fiber laser system through optical control of the GSA.Here the laser output was measured in time domain by using a photodetector and an oscilloscope for several control beam power levels.Initially the fiber laser including fabricated GSA operated at Q-switching state (Fig.5(a)) without control beam (Pc = 0 mW) where the repetition rate and pulse duration of the Q-switched laser output were 8.0 kHz and 20µs, respectively.When the power of the control beam was increased to 34 mW, we observed that the laser state was switched to QML. Figure 5(b) shows measured output of the QML laser where Q-switched pulses at a repetition rate of 20 kHz includes fine mode-locked pulse train.The inset of Fig. 5(b) show the pulse train at extended time scale where the mode-locked pulse train is clearly seen at a repetition rate of 5.09 MHz.At the control beam power of 42 mW, we observed CW mode-locking state as shown in Fig. 5(c).

Fig. 5 .
Fig. 5. Characteristics of the pulse train at fiber laser output (a) Q-switching state without control beam (P c = 0 mW), (b) QML state with control beam power of 34 mW (inset: extended view of the pulse train in time scale) and (c) CW mode-locking state at the control beam power of 42 mW.

Fig. 6 .
Fig. 6.Laser output properties of the CW mode-locked laser.Measured (a) pulse duration (b) optical spectrum and (c) RF spectrum of the pulse train (inset: RF spectrum at wide time span).