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Abstract
Although sophisticated novel saturable absorber materials are available for the development of ultrafast lasers, innovative approaches and devices play an increasingly important role in continuously adjusting mode-locked lasers with electrical gating. In this study, electrically switched operational regimes of an Nd:YVO4 all-solid-state mode-locked laser with a high modulation ratio (from 900 ns to 15 ps) are demonstrated for the first time. The laser can automatically switch multiple operation regimes with the assistance of electrical signals using techniques such as Q-switching, Q-switched mode-locking (QML), and continuous-wave mode-locking (CWML). The device is operated at an ultralow electrical modulation power (0.1 nW) to generate sub 15 ps pulses with a high average output power (as much as 800 mW) from a mode-locked laser operating at 1064 nm. The results verify the reversible switching of the operational regimes from QML to CWML and provide a basis for exploring their applications in electro-optical devices.
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1. Introduction
All-solid-state short-pulse lasers have attracted great interest in numerous fields, such as precision measurement, laser medical treatment, and the military [1,2]. Passive Q-switching (QS) and mode locking (ML) are widely used techniques for obtaining ultrafast laser pulses ranging from microseconds to femtoseconds [3,4]. Q-switched solid-state lasers are widely used in material processing, medical treatment, and scientific research because they can provide a high pulse energy [5]. Mode-locked lasers are more attractive for applicability in frequency combs, laser detection, and signal processing owing to their high repetition rates and ultrashort pulse width [3]. The key device in a passively modulated laser is a nonlinear element, which has an intensity-dependent response that favors optical pulse formation over continuous-wave (CW) lasing. The key device is usually a saturable absorber (SA) with distinct nonlinear saturable absorption, an ultrafast recovery rate, and low cost, such as semiconductor SA mirrors, carbon nanotubes, molybdenum disulfide, and black phosphorus [6–8]. To satisfy the demands of diverse laser applications, SA-based pulsed lasers can produce ML [1,4,9] with high repetition rates, QS [5,10] with high pulse energies, and Q-switched mode-locking (QML) [11] with high pulse peak powers by changing the laser cavity structure. The capability of SA-based lasers to switch automatically to any possible operation regime in a fixed laser cavity, such as ML, QS, or QML, undoubtedly enhances their practicality. Therefore, researchers are driven to improve laser flexibility. Some recently published researches have reported and demonstrated switchable operation regimes in a fixed mode-locked laser system through external direct control [12–14]. However, it is difficult to realize this in a passively mode-locked regime because the corresponding absorption of the SAs is fixed.
Graphene is a novel 2D material that has attracted attention for its applicability in ultrafast laser systems owing to its extraordinary electrical and optical properties [15]. One of the unique electro-optic properties of graphene is its interband transition rate and linear absorption can be substantially modified by electrical signals. In particular, when the electron energy is close to the Dirac point of a single atomic graphene layer, slight variations in the carrier density can cause significant shifts in the Fermi level and result in tunable absorption characteristic [16]. Consequently, graphene electro-optic modulators have been developed as upgraded SAs, thereby providing a possible solution to overcome the aforementioned issues of conventional SAs for passive ML. These unique electro-optic properties of graphene have led to research on advanced candidates for SA applications [17,18]. In 2015, Lee et al. first reported a field-effect transistor that incorporated graphene for tunable all-fiber mode-locked lasers; this could electrically control fiber laser operation under different regimes. Subsequently, graphene-based electro-optic modulators have attracted particular interest for potential application as the kernel of a laser system [19]. In 2020, graphene-capacitor-based electrical switching of ML lasers was reported. The reversible ML operation from the femtosecond pulse regime to CW regime of the ring laser was demonstrated [20]. Nevertheless, because all-solid-state mode-locked lasers have a much larger laser spot and lower power density than fiber lasers, their requirements for device modulation efficiency are higher (than those of fiber lasers). Therefore, realizing switchable all-solid-state mode-locked lasers under low modulation power is still a challenge. This has inspired efforts to optimize integrated device structures and fabrication processes.
In this study, an electrically active controllable all-solid-state mode-locked laser with a graphene-based on-chip modulator that can function as an upgraded SA was demonstrated. The graphene modulator exhibits adjustable absorption characteristics at ultralow electrical modulation power owing to the optimized electrode structure and dielectric layer. In particular, the modulator is suitable for free-space optical systems owing to its vertical device structure and unique array electrodes. To the best of our knowledge, this is the first report of an electrically switched Nd:YVO4 all-solid-state mode-locked laser with a high modulation ratio (900 ns to 15 ps). The device operates at an ultralow electrical modulation power (0.1 nW) to generate sub 15 ps pulses with a high average output power (as much as 800 mW) from a mode-locked laser operating at 1064 nm. In addition, the operational regimes of the mode-locked laser can be reversibly switched by the gate voltage from QS, QML, and continuous-wave mode-locked locking (CWML) operations.
2. Preparation and characterization of graphene electro-optic modulator
The schematic of the electro-optic modulator is presented in Figs. 1(a) and (b). The preparation steps are as follows. (i) A 185 nm thick indium-tin oxide (ITO) layer on a quartz substrate is carved into a ring shape using a direct laser etching machine (PEROVS ZN-18). The ring-shaped ITO functions as both a space light positioning window and drain/source of the transistor to be produced. (ii) A 10 × 10 mm monolayer graphene sheet grown by chemical vapor deposition (CVD) is mechanically transferred onto the ring-shaped ITO substrate. Monolayer graphene is grown on 30 µm thick copper foils in a typical quartz tube CVD reactor at 1035 °C using CH4/H2 as precursors. Subsequently, the prepared graphene is transferred onto the ITO substrate by spin-coating polymethyl methacrylate (PMMA) as a support layer on top of the graphene/copper sheet. The copper foil is etched away in an ammonium persulfate solution bath. To protect the graphene layer from humidity and oxygen in the atmosphere, the PMMA layer is retained and employed as an insulating layer, as shown in Fig. 1(c). (iii) The Au gate electrodes are fabricated at the center of the ITO circular ring in a square mesh shape by magnetron sputtering, as shown in Fig. 1(d), both to increase the electric field intensity across the entire photoelectric interaction area and optically calibrate the light propagating through the center of the ring electrode. The thickness and width of the Au electrodes are 50 nm and 25 µm, respectively. The Au electrodes have good stability and high transparency at 1064 nm, which effectively reduces device loss. According to the size of the graphene applied to the ITO electrode, the actual photoelectric interaction region of the modulator can be estimated as 1 × 1 cm.
Fig. 1. Schematic of the graphene modulator structure fabrication: (a) optical image of the fabricated device; (b) 3D schematic of the gate-variable graphene modulator device; (c) preparation process of the graphene device; and (d) optical microscopy image of the device (scale bar: 200 µm).
A scanning electron microscopy (SEM) image of this graphene modulator is shown in Fig. 2(a) to demonstrate the SiO2 substrate/ITO electrode/graphene modulation layer/PMMA insulator layer/Au gate, where the total thickness of the ITO/graphene/PMMA layers is 500 nm. Here, Fig. 2(b) shows the Raman spectrum of the graphene film, including the 2D and G peaks at 1590 and 2694 cm−1, respectively. Furthermore, the ID/IG (intensity ratio of D peak over G peak) and I2D/IG (intensity ratio of 2D peak over G peak) are 0.09 and 3.1, respectively. The Raman spectrum indicates that the quality of large-scale CVD-synthesized graphene, as shown in the inset, is comparable to that of the pure graphene monolayer [18]
Fig. 2. Schematic of the graphene electro-optic modulator structure fabrication: (a) SEM image of the cross-sectional structure of this modulator; and (b) Raman spectrum of the graphene layer after device fabrication (inset: microscope image of the graphene).
When the device operates, it must be perpendicular to the light path of the laser resonant cavity. One challenge for a direct graphene modulator is the limited electro-optical function region, which is the size of the laser beam radius (approximately 160 µm). Therefore, in this study, to further improve their electro-absorption modulation efficiency, the gate electrodes were designed in a dense grid shape at the top surfaces, wherein the interband transitions of graphene within the limited electro-optical modulation region could be maximized. Ab initio numerical simulations were performed to investigate the charge distribution intensity (c, C/m2) within the device using the finite-element method, as shown in Fig. 3.
Fig. 3. charge distribution intensity within the vertical structures by the FEM. (a) two-dimensional electrode layouts, (b) Three dimensional vertical multilayer heterostructure with insulator layer; (c) Tuning the Fermi level of graphene by electrical biasing; (d) Electric field model under different aperture size (inset: electric field model of the aperture);(e) Optical absorption characteristic of the modulator.
The charge distribution density of the grid electrodes is better than that of the blank area, as shown in Fig. 3(a), owing to the field enhancement effect. To meet the design criteria better, a spatial electrode structure of the device, including an intermediate insulating layer, was simulated, as shown in Fig. 3(b). The optical transitions from the valence band to conduction band depend primarily on the Fermi level of graphene, as shown in Fig. 3(c). Owing to the electrostatic doping of electrons, the Fermi level of the graphene shifts up or down according to the bias polarities, thereby resulting in a decrease in the photon absorption of graphene because of the Pauli blocking principle [21,22]. Moreover, the graphene Fermi level (EF) can be defined by the electron density (n) using the following expression.
where ${\upsilon _F}$ is the Fermi velocity (1.1 × 108 cm/s), and $\hbar$ is Planck’ s constant. Note that smaller aperture size results in a higher electric field model as well as higher charge density concentrations, as shown in Fig. 3(d). Consequently, the field enhancement is derived from the modulation voltage. However, dense electrodes produce greater optical losses and a higher ML threshold. Considering the process conditions and the principles mentioned above, see Fig. 3(d), an optimized aperture size of 55 µm can be confirmed. The optical absorption intensity of the graphene modulator is measured as shown in Fig. 3(d). The absorption of the device is 8.12% at 1064 nm and the optical losses caused by graphene was 2.5%.Modulation depth is an important parameter for the saturable absorption coefficient, which affects the output performance of a mode-locked laser. To prove the reliability of the gate-switched laser system further, the nonlinear transmission effects of the graphene modulator were measured at different biases. The nonlinear transmittance of the device was measured by a balanced synchronous twin-detector measurement system, which adopted a Q-switched pulsed laser (center wavelength: 1064 nm, pulse duration: 4 ns) as the excitation light source. Here, Fig. 4(a) and (b) show the experimental data and curves fitted using the following formula.
where the transmission, saturable intensity, input intensity, and nonsaturable absorbance coefficient are substituted with $T(I)$, ${I_{sat}}$, I, and ${T_{ns}}$, respectively. The results prove that the modulator exhibits a significant modulation depth change from 7.3% to 4.1% with the assistance of the gate voltage that is derived from the increase in the carrier concentration of graphene. If the modulation depth is too small, the saturable absorption effect of the SA is not obvious and ML may take a long time. In contrast, the saturated loss increased with the modulation depth. If the modulation depth is too large, the laser cavity loss increases, thereby resulting in a QS condition. Therefore, the graphene modulator makes it possible to control the operational regimes electrically.Fig. 4. Measuring modulation depth points and their corresponding fitting curve of the graphene modulator at various voltages: (a) 0 V; and (b) 10 V.
3. Experimental setup
Figure 5 presents the schematic of the gate-switchable mode-locked Nd:YVO4 laser applying a graphene modulator. Here, the length of the linear cavity is 172.1 cm; the four arm lengths are L1 = 245 mm, L2 = 355 mm, L3 = 1000 mm, and L4 = 121 mm. The laser is forward-pumped by an 808 nm fiber-coupled laser, which is coupled into the cavity through a 1:1 coupling system with a beam radius of 200 µm. The numerical aperture of the fiber pigtail is 0.22; the laser crystal is 3 × 3 × 5 mm with an Nd-doped concentration of 0.5 at.%, which is maintained at a temperature of 25 °C by a water cooling system. In this setup, a W-type folded cavity was designed for the ML operation, consisting of a laser gain medium, three flat folding mirrors (M1, M2, and M3), and an output mirror (M4). The left side (S1) of the Nd:YVO4 crystal acts as the input mirror and underwent an antireflection (AR) coating at 808 nm and a high-reflection (HR) coating at 1064 nm, whereas the right side (S2) of the crystal underwent an AR coating at 1064 nm. The flat folding mirrors M1 and M2 (R1 = 500 mm), and M3 (R2 = 206 mm) were all HR coated at 1064 nm. A plane mirror with a transmission of 5% at 1064 nm was used as output mirror. The purpose of using the output mirror with low transmittance was to enable more photons to oscillate in the resonator so that the laser threshold could be reached with a lower pump power. Thus, the laser gain medium was effectively protected. Simultaneously, to reduce the astigmatism caused by folding, the deflection angle of the folded mirror in the resonator cavity was less than 10°. Using the ABCD matrix propagation theory, the laser mode radii were calculated and found to be approximately 180 µm in the Nd:YVO4 crystal and approximately 35 µm on the graphene modulator.
4. Experimental results
First, in CW operation, the maximum average output was 1.32 W, which was obtained under an absorbed pump power of 9.2 W, as shown in Fig. 6(a). The absorbed pump power refers to the power of the pump laser after passing through the gain medium with a certain loss. The absorption coefficient of Nd:YVO4 crystal in our research is 0.6. Therefore, the absorbed pump power is about 60% of the pump power. Next, the graphene modulator was inserted into the laser cavity, where it was employed as an SA. Using the above-mentioned cavity design, the output power increased linearly with the pump power. With the incremental trend of pumping power, stabilized CWML operation occurred under an absorbed pump power of 6.06 W, corresponding to an average output power of 498 mW. An advantage of a graphene modulator is its ability to switch the operational regimes of mode-locked lasers electrically. This can be achieved by modulating the optical nonlinear properties of graphene in the laser system. The gate voltage was increased from −20 to 20 V at a constant absorbed power and ultrafast ML pulses with gate-variable operation were achieved. Consequently, the output power increased with the gate voltage, as shown in Fig. 6(b), owing to the onset of Pauli blocking [23]. Due to the electrostatic doping of electrons, the Fermi level of the graphene shifts up or down according to the bias polarities, resulting in the decrease of the photon absorption of graphene from Pauli blocking principle. As the amplitude of voltage increased, the shift in the Fermi level reduced the passive losses, and the output power increased. Therefore, there was a dip point in the output power of the laser at the same and fixed input pump power when the voltage magnitude was 0 V. In addition, the graphene in the device was doped with carriers during the preparation process, which showed a slight N-type semiconductor property. therefore, the device exhibited a more effective modulation under a positive voltage, thereby resulting in a difference in the output power for negative and positive gate voltages.
Fig. 6. Output power of the mode-locked laser: (a) average output power versus the absorbed pump power; and (b) average output power versus the voltage.
When the absorbed pump power was 1.81 W, a typical QS pulse train appeared, as shown in Fig. 7(a), where the measured pulse duration was 900 ns because the saturation intensity was too small [24]. To experimentally confirm the feasibility of the electrically controllable nonlinear in-line SA, the absorbed power was maintained at 2.1 W while simultaneously increasing the gate voltage on the device. Thus, the laser could automatically switch multiple operation regimes with the assistance of an electrical signal from QML to ML.
Fig. 7. Electrical switching of pulses from the output of the mode-locked laser: (a) unstable Q-switched pulse trains at a pump power of 1.81 W; (b) QML pulse trains at different voltage; (c) CWML pulse trains at the Vgs of 10 V; and (d) autocorrelation trace of the CWML pulse at Vgs of 10 V.
The detailed performance characterization is illustrated in Fig. 7. The generated laser pulses were received by a fast photodiode (Newport Model 818-BB-21) with a rising time of 250 ps and recorded by a digital oscilloscope with a bandwidth of 500 MHz (Agilent MSO7052B). The laser showed a stabilized QML operation at an applied Vgs of 2 V, as shown in Fig. 7(b). When the applied Vgs reached 10 V, typical CWML operation corresponding to an average output power of 97 mW was achieved. Furthermore, as the amplitude of the voltage signal decreased, the Q-switched envelope began to reappear. With an increase in the pump power and voltage, the maximum average output power of the laser could reach as much as 800 mW. The autocorrelation curve of the mode-locked pulse is shown in Fig. 7(d).
Assuming a Gaussian pulse shape, the pulse width of the CWML laser was measured to be 15 ps by commercial autocorrelation (Femtochrome Research Inc., FR-103HP) under a gate voltage of 10 V. The calculated time-bandwidth product of the mode-locked laser was about 4.26, which was greater than the transform-limited value of 0.441 for Gaussian pulses, indicating that the CWML pulses were frequency chirped. This switching operation can be explained by the shift in the Fermi energy level of graphene to the polar directions by the gate voltage, which blocks the further interband transition of electrons. Consequently, the graphene SA became sufficiently saturated easily, and the optical loss in the cavity decreased. This significantly improved the instability condition of the resonant cavity by changing the laser operation to ML [19,25]. The performance of the device was sustained well for repeated experiments over several months because PMMA can effectively prevent the oxidation of graphene at room temperature.
Figure 8 shows the variation of round-trip loss of the graphene modulator as a function of the applied bias voltage. By using the fact that the threshold pump power is proportional to the sum of passive losses and the output coupler transmission of the resonator, we estimated the voltage dependent loss of the device as depicted in Fig. 8. As can be seen, the optical insertion loss of the modulator decreased from 9.6% at zero bias to 5.1% at 10 V of bias. For bias voltages between 0 and 10 V, the optical loss of the device could be effectively modulated. The high insertion loss of the graphene resulted in insufficient intracavity power, leading to QML regime. Therefore, the operational regime of the mode-locked laser can be reversibly switched by changing optical loss.
The corresponding characteristics of the mode-locked laser are shown in Fig. 9. The spectra of the QS laser and CWML laser were measured by a fiber optic spectrometer (Seemantech S3000-VIS) with a resolution of 0.25 nm as shown in Fig. 9(a) and (c). The central wavelength of the QS laser was 1064.3 nm with a 0.84-nm spectral bandwidth. The central wavelength and spectral bandwidth of the CWML laser were calculated to be 1064.1 and 0.91 nm, respectively. Radio frequency (RF) spectral measurements are used to characterize the stability of the mode-locked laser. The radio frequency (RF) spectra of QS laser was measured as shown in Fig. 9(b), the signal-to-noise ratio (SNR) of RF signal can reach up to ∼38 dB at 465 kHz. To verify mode-locking stability, radio-frequency (RF) spectra were recorded in different spans (Fig. 9(d)). The signal-to-noise ratio of the RF signal can reach up to approximately 44 dB at approximately 87.2 MHz. The inset of Fig. 9(d) shows a 500 MHz wide-span measurement of the signal, which confirms the mode-locked operation. Moreover, the laser operation was maintained stably for a few hours during the experiment.
Fig. 9. Output characteristics of the mode-locked laser: (a) optical spectrum of the QS laser; (b) RF spectrum of the QS laser; (c) optical spectrum of the CWML laser; (d) RF spectrum of the CWML laser.
5. Conclusion
In summary, a graphene-based novel saturable absorber device is proposed to achieve a short-pulse formation by all-solid-state passively mode-locked laser. Due to the the Fermi level’s shift of graphene in our device, the optical absorption properties of modulator can be electrically manipulated without changing the materials. The operation regimes of the mode-locked laser can be flexibly switched using electrical signal, such as QS, QML, and CWML. The device is operated at an ultralow electrical modulation power (0.1 nW) to generate sub 15 ps pulses with a high average output power (as much as 800 mW) from a mode-locked laser operating at 1064 nm. We expect that such switchable all-solid-state laser operation can be considered as a suitable solution for developing versatile pulse laser seed sources, which can potentially find a number of applications both in Q-switching and mode-locking regimes (for example, material processing, medical treatment, frequency combs, laser detection, or optical frequency metrology).
Funding
Jiangsu Students' Platform for innovation and entrepreneurship training program (202010300040); National Natural Science Foundation of China (62175114); National Natural Science Foundation of China (61875089).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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