We demonstrate a stable, picosecond fiber laser mode-locked by cesium lead halide perovskite quantum dots (CsPbBr3-QDs). The saturable absorber is produced by depositing the CsPbBr3-QDs nanocrystals onto the endface of a fiber ferrule through light pressure. A balanced two-detector measurement shows that it has a modulation depth of 2.5% and a saturation power of 17.29 MW/cm2. After incorporating the fabricated device into an Er3+-doped fiber ring cavity with a net normal dispersion of 0.238 ps2, we obtain stable dissipative soliton with a pulse duration of 14.4 ps and a center wavelength at 1600 nm together with an edge-to-dege bandwidth of 4.5 nm. The linear chirped phase can be compensated by 25 m single mode fiber, resulting into a compressed pulse duration of 1.046 ps. This experimental works proves that such CsPbBr3-QDs materials are effective choice for ultrafast laser operating with devious mode-locking states.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Ultrafast fiber lasers without wave-breaking have been under intense investigation upon, due to potential ability of high energy [1–3]. Several mode-locking mechanisms have been proposed, including dissipative soliton (DS) , gain-guided mode-locking , soliton-similariton , according to various dispersion conditions. Among which, the dissipative soliton that formed in net-normal or all- normal dispersion laser cavities has been proved to be an efficient candidate for the convenience of manipulating the pulse duration through dispersion-compensating devices, thanks to its linear phase relation among various longitudinal modes [7–10]. Physical, dissipative soliton are formed under a balance of nonlinearity, dispersion, and gain filtering effect. As an excellent pulsed laser source for high power ultrafast laser through post-amplification, dissipative soliton has been found in laser cavities operating at 1 μm , 1.5 μm , and 2 μm , when working with different rare earth-doped gain mediums.
Meanwhile, there are several nonlinear micro/nano-materials have been utilized to fabricate the saturable absorber (SA) for dissipative mode-locking, including carbon nanotubes, graphene, topological insulators, black phosphorus, and transitional metallic dichalcogenides [13–24]. Together with traditional nonlinear polarization rotation method, those mode-locking techniques are proved to be proper choices for different circumstances. Recently, we also prove that the cesium lead halide perovskite nanocrystals (CsPbX3, X = Cl, Br, I) have shown efficient optical nonlinearity for optical and optoelectronic devices . Due to the edge states and crystallographic defects of the CsPbBr3-QDs saturable absorption has been detected, and stabile soliton laser at 1064 nm with fundamental frequency has been obtained. Those materials with distinct merits have found appreciable application in conventional soliton laser systems with anomalous dispersion . Yet, the application in DS has not been proved in experiment, of which makes the CsPbBr3-QDs suspect in fiber laser systems with more complex dispersion distributions.
In this work, we propose a stable, picosecond DS fiber laser centered at 1600 nm. The SA is prepared by adsorbing CsPbBr3-QDs onto the endface of fiber ferrule, and its nonlinear response is measured with a home-made balanced two-detector. After inserting the fabricated SA into a fiber ring cavity, stable DS laser with fundamental frequency is observed with pump power larger than 70 mW. Our results prove that the CsPbBr3-QDs are desirable choices for ultrafast photonics.
2. Experimental setup
To prepare the CsPbBr3-QDs, quantum dots, we add 69 mg lead bromide (Xi'an Polymer Light Technology Corp.), 0.5 mL oleic acid (Aladdin-reagent), 0.5 mL oleylamine (Aladdin-reagent), and 5 mL 1-octadecene (Aladdin-reagent) to a 25-mL 3-neck round bottomed flask and degassed under vacuum at room temperature for 30 minutes, and then heated the solution under N2 at 120 °C for 30 minutes . Then, the temperature was increased to 150 °C. After complete solubilisation of lead bromide alt, 0.4 mL 120 °C Cs−oleate solution was quickly injected. 5 s later, the reaction mixture was cooled by a water bath. Geen emission was observed under 365 nm ultraviolet light for the resulting CsPbBr3-QDs, as shown in Fig. 1 (a). And its X-ray diffraction spectrum (XRD) in Fig. 1(b) proves the well-defined structures of the crystals . The transmission electron microscopy (TEM) image of the fabricated CsPbBr3-QDs is shown in Figs. 1(c) and 1(d), where the typical edge length of the nanoparticals is about 9 nm.
The fiber ring laser cavity is schematically shown in Fig. 2(a), mainly incorporating 20 m erbium-doped fiber (EDF, Nufern EDFC-980-HP) with a dispersion parameter of −12.2 ps/nm/km, which is pumped by a 980 nm lasers through wavelength division multiplexer (WDM). Besides, unidirectional operation is enforced by a polarization-independent isolator, and a polarization controller (PC) is employed to optimize the net birefringence. 10% port of an optical coupler (OC) is extracted for output. The dispersion parameter of 3.97 m single mode fiber (SMF) in this cavity is 18 ps/nm/km. Therefore, the 23.97 m ring cavity corresponds to a net normal dispersion of 0.238ps2. The temporal performance is monitored by a detector with 1 GHz bandwidth connecting to an oscilloscope (Lecroy, SDA 8600A), and an autocorrelator (APE, Pulse check), while the frequency information is detected by an optical spectrum analyzer (Yokogawa, AQ6370), and a frequency analyzer (Agilent, PSA E4447A).
The SA in the above laser scheme is fabricated by depositing the CsPbBr3-QDs onto the endface of fiber ferrule. the liquid solution on the gold mirror, and dried in vacuum oven at 35 degrees centigrade. Primarily, a cleaned fiber ferrule is immersed into the prepared CsPbBr3-QDs solution, and it is illuminated by a continuous wave laser at 980 with a power of 40 mW. After 10 minutes, its end fact is covered by the CsPbBr3-QDs nanocrystals, and the transmission is drop to −2.5 dB. We measure the nonlinear optical response of the fabricated SA with a home-made balanced two-detector methods as the same in , where a home-made pulse source with a pulse duration of 285 fs, repetition rate of 7.4 MHz, center wavelength at 1563 nm is used. The nonlinear characteristics with pulse intensity is plotted as in Fig. 2(b). As the CsPbBr3-QDs exhibiting a ultrafast hot-carrier relaxation time of 0.6 ps [29–31], which is much smaller than that the obtained pulse duration, a fast saturable absorber model is suitable in this experiment . The experimental data is fitted by the typical nonlinear transmission formula, assuming a simplified two-level fast saturable absorption model [28, 32]:
3. Results and discussion
The dependence of the output average power on the input pump power is depicted in Fig. 3. Although a linear fitting is performed, we observe continuous wave state (CW), Q-switched mode-locking state (QML), and DS state for various pump powers. As shown, for pump power below 20 mW, the laser system operating at CW, while Q-switched mode-locked pulses are found for pump between 20 mW and 70 mW. When pump is larger than lager than 70 mW, where the intracavity pulse energy is larger than the criterion for stable dissipative mode-locking, we obtain stable DS. Here, the maximum pump power we used is 130 mW, and a maximum average power of 5.16 mW is obtained. We do not observe pulse breaking, as the generated pulse is far below the typical transform-limited due to the large net normal dispersion. It is reasonable to anticipate a much larger output power DS when the SA is fabricated using evanescent field interaction, such as fiber taper or photonic crystal fiber, through which the damage threshold can be increased sufficiently.
Figure 4(a) presents the optical spectra for those operating states, and the corresponding temporal trains are depicted in Fig. 4(b). Increasing the pump power with optimizing the polarization states, the resulting optical spectrum evolves into a rectangular shape. While, we also observe a gradual broadening on the two ends of the spectrum. This is induced by the unbalance of the dispersion and the nonlinear phase shift of the SA. In the next step, the spectrum can be improved via optimizing the dispersion management and the fabrication process of the SA. In the operating transition, the period of the envelope on the corresponding temporal trains diminish gradually. And we obtain neat pulse train with a period of 117.3 ns.
The typical characteristics of stable DS for pump power at 110 mW is shown in Fig. 5, where the average power is 4.34 mW. The stable dissipative mode-locking state can be verified by its RF spectrum with a contras ratio larger than 50 dB at the fundamental frequency of 8.528 MHz, and excellent signal to noise ratio in 1 GHz bandwidth. The full width at half maximum (FWHM) of the optical spectrum is about 4.5 nm. The pulse duration measured by an autocorrelator is about 20.3 ps when assuming a Gauss shape. Therefore, the actual pulse duration of the DS is about 14.4 ps, and the time-bandwidth product (TBP) is 5.17, due to large chirp in the longitudinal modes. This value is much larger than the 0.315 for a standard transform limited pulse. In fact, the linear chirping of the DS pulse can be utilized to compress the pulse duration with adequate anomalous dispersion.
Figure 6 shows the autocorrelation trace of the DS after propagating with 25 m SMF, whose pulse duration is compressed from 20.3 ps to 1.48 ps. Therefore, the actual pulse duration is 1.05 ps. The resulting TBP is reduced to 0.379, and a peak power of 484. 7 W is obtained. Due to the nonlinear phase relation in the spectral broadening in the optical spectrum, a small pedestal on the autocorrelation trance is shown. This deficiency may deteriorate the peak power. Yet, as we have discussed, optimized fabrication process of the SA will bring better performance of the DS. Also, we find that a longer compression SMF will result in pulse breaking due to accumulated high nonlinearity. Thus, an optimized anomalous dispersion for each DS pulse with specific characteristics.
To test the operating performance of the DS system, we record the compressed pulse width and the average power within 8 hours, for an optimized 25 m SMF. The experimental data are averaged using 10 measurements. As shown in Fig. 7, the average power shows a fluctuation less than 0.005 mW, while the corresponding compressed pulse duration exhibits a fluctuation less than 30 fs. Those features indicate that this laser is rather stable for a long time in the lab condition even in the harmonic mode-locking regime.
We have proposed a stable, dissipative soliton fiber laser mode-locked by CsPbBr3-QDs. The saturable absorber produced by depositing the CsPbBr3-QDs nanocrystals onto the endface of fiber ferrule possess a modulation depth of 2.5%. We obtained DS centered at 1600 nm with an edge-to-dege bandwidth of 4.5 nm. Due to linear phase among various longitudinal modes, the pulse duration can be compressed from 14.4 ps to 1.046 ps with additional 25 m single mode fiber. Our experimental results indicate that this CsPbBr3-QD is an effective choice for stable dissipative mode-locked fiber laser that would find potential applications in laser micro-processing, high power laser system, optical communication, sensing, et al.
Key Research and Development Project of Ministry of Science and Technology (2016YFC0801200); National Natural Science Foundation of China (NSFC) (61705023, 61635004, 61405020, 61520106012); National Postdoctoral Program for Innovative Talents (BX201600200); General Financial Grant from the China Postdoctoral Science Foundation (2017M610589); Postdoctoral Science Foundation of Chongqing (Xm2017047); Science Foundation of Chongqing (CSTC2017JCYJA0651); Science Fund for Distinguished Young Scholars of Chongqing (CSTC2014JCYJJQ40002); Science and Technology on Plasma Physics Laboratory (6142A0403050817); Fundamental Research Funds for the Central Universities (106112017CDJXY120004).
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