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Abstract
We experimentally demonstrated an all-fiber mode-locked erbium-doped laser based on Co2+:ZnSe (Cobalt, Zinc Selenide) thin film as the saturable absorber (SA), which was grown on the fiber taper by electron beam (e-beam) evaporation technology. The modulation depth, non-saturable loss and saturation intensity were found to be ∼12%, 76% and 1.89 MW/cm2, respectively. The X ray diffraction (XRD) results showed that the Co2+:ZnSe thin film was provided with the cubic zinc blende structure. The scanning electron microscope (SEM) and atomic force microscopy (AFM) images showed the prepared thin film has a smooth and uniform surface. When the fiber-taper Co2+:ZnSe SA was inserted into an erbium-doped fiber laser cavity, a stable mode-locked pulse was obtained. The mode-locked pulses had a pulse repetition rate of 17.83 MHz and pulse duration of ∼4 ns throughout the mode-locked operation range of 200-600 mW. The recorded maximum average output power was ∼4.88 mW. The signal to noise ratio (SNR) was obtained to be 64.9 dB, which indicates the favorable stability of the mode-locked pulse. These results demonstrate the Co2+:ZnSe thin film grown on a fiber taper has high optical quality and implementation of stable pulse operation on passively mode-locked Er-doped fiber lasers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Pulse fiber lasers are the focus of extensive research efforts due to share more benefits, such as simple structure, compact, low cost, better stability and freedom from misalignment [1–3]. Mode-locked fiber laser is one of the effective ways to generate high peak power laser pulses. Generally, mode-locked fiber lasers can be divided into actively mode-locked technology and passively mode-locked technology. Compared with active schemes, passively mode-locked method has the superiorities of simplicity and compact structure, low-cost, without modulator requirements [4]. In recent years, passively mode-locked fiber lasers have attracted great attention and are widely used in numerous fields, such as medicine, nonlinear optics, biomedical diagnostics, sensing, optical communication, laser spectroscopy, material processing, frequency metrology, terahertz generation [1, 5–13]. There are several approaches to achieve passively mode-locked lasers, such as nonlinear polarization rotation (NPR) [14], nonlinear amplifying loop mirror (NALMs) [15], and real SAs. In fiber lasers, real SAs are an effective and simple method to achieve passively mode locked due to its no requirement of the fine tuning of polarization states in fiber cavity, and the saturable absorption materials used for SAs have fast recovery time, easy-preparation, wide absorption band, and low saturation intensity properties [16–18].
For ∼1.5 μm Er-doped fiber lasers, there are many choices of real saturable absorbers, including transition semiconductor saturable absorption mirror (SESAM) [19], carbon nanotube [20], graphene [21], metal dichalcogenides [4, 17, 22, 23], graphene oxide [24, 25], and topological insulators [16, 18]. In addition, transition metal (TM) ions doped ZnSe semiconductors are also valid saturable absorbers for passively Q-switched and mode-locked Er-doped fiber lasers. The film of this material has the significant potential to construct all-fiber pulse laser, which can make the structure of the fiber laser become more compact and simpler. A variety of physical or chemical methods can be used to prepare TM2+-doped ZnSe thin films, for instance, radiofrequency (RF) magnetron co-sputtering [26], molecular beam epitaxy (MBE) [27], solution-liquid-solid [28], chemical vapor deposition (CVD) [29], sol-gel [30], pulsed laser deposition [31], thermal evaporation [32], and electro-deposition [33]. However, these methods have their own features. Compared with these methods, the cost of electron beam evaporation technology is lower and more appropriate for large-scale preparation of thin films, and the prepared thin films are uniform and flat with a good quality [34]. Nevertheless, TM2+-doped ZnSe films prepared by electron beam evaporation were rarely reported.
In this work, we prepared Co2+:ZnSe thin film SA for Er-doped fiber laser. For Co2+:ZnSe film used as a saturable absorber of the Er-doped fiber laser, the ground state 4F of the free ion Co2+ is divided into three energy levels 4T1, 4T2 and 4A2 by the ZnSe crystal field, and the transition from 4A2 to 4T1 ensures 1.3-2 μm wide absorption band nearby. Tzong-Yow et al. utilized Co2+:ZnS and Co2+:ZnSe two materials to achieve Q-switched operation of a Er3+:glass laser at 1.54 μm. And depending on the doping concentration and crystal quality, the output pulses were typically 15–65 ns [35]. Yu V Terekhov et al. used Co2+:ZnS and Cr2+:ZnSe saturable absorbers to realize the passive Q-switching of an Er-fiber–Er:YAG hybrid laser at 1645 nm and 1617 nm respectively [36]. Co2+:ZnSe SA has gained growing attention for using in pulse lasers. Up to now, however, there is no report about a Co2+:ZnSe SA used for constructing all-fiber mode-locked laser, which limits its application scope.
To the best of our knowledge, this is the first report about the passively mode-locked Er-doped fiber laser based on the Co2+:ZnSe SA. The Co2+:ZnSe thin film was deposited on the fiber taper by e-beam evaporation technology. XRD results showed the prepared thin film was provided with a cubic zinc blende structure. The SEM and AFM characterization indicated the film surface was smooth and flat with a good quality. Finally, we used the fiber-taper Co2+:ZnSe thin film as saturable absorber to realize passively mode-locked Er-doped fiber laser. The mode-locked pulses had a pulse duration of ∼4 ns and a maximum output power of 4.88 mW. And at the central wavelength of 1562 nm with a 3 dB bandwidth of ∼4 nm was demonstrated under the pump power of 500 mW. The SNR was 64.9 dB with the repetition rate of 17.83 MHz. This work shows the Co2+:ZnSe thin film can be valid saturable absorber for constructing passively mode-locked Er-doped fiber lasers.
2. Fabrication and characterization of the fiber-taper Co2+:ZnSe SA
The Co2+:ZnSe thin film coated fiber taper with waist diameter (∼28 μm) and waist length (∼1.2 cm) was prepared by e-beam evaporation technology. First, we placed cobalt targets, ZnSe targets, and fiber taper stuck on a slide glass into a vacuum chamber with 6.4×10−4 pa. Next, the electron beam was used to focus the light spot on the targets and evaporated the targets onto the substrate. In this experiment, according to the parameters we set, the thickness of the Co2+:ZnSe thin film was approximately 500 nm and the deposition time was ∼2 hours at the growth temperature of 250 ℃. When the vacuum chamber was gradually cooled down to the room temperature, the preparation process of a fiber taper Co2+:ZnSe SA was completed.
Meanwhile, to characterize the optical performance of the Co2+:ZnSe thin film, we prepared it on the sheet of silicon and sapphire substrates with fiber taper together at the same electron beam evaporation process. We conducted the scanning electron microscope of the Co2+:ZnSe thin film as shown in Fig. 1(a), which confirms the surface of the Co2+:ZnSe thin film prepared by e-beam evaporation under vacuum is smooth and flat without voids, cracks and obvious defects are observed. All the phenomena indicate a good quality of the thin film. The structure of the film is shown in the inset in Fig. 1(a). The bottom was the substrate, and then a layer of ZnSe was coated on the substrate, the doped layer (Co) was deposited in the middle, and the last layer was ZnSe thin film. A typic energy dispersive spectrometer (EDS) pattern is presented in Fig. 1(b). The EDS spectrum of the prepared film verifies the presence of target elements (Zinc, Selenium, and Cobalt species). The relative atomic percentage is 44.74, 54.53 and 0.73, respectively, which clearly indicates the prepared film without contain any foreign elements. In addition, we used atomic force microscopy (AFM, MFP-3D-BIO) to evaluate the surface roughness of the Co2+:ZnSe thin film with scanning area of 5×5 µm. The surface image of the Co2+:ZnSe thin film is shown in Fig. 1(c) and the corresponding marked red height curve is shown in Fig. 1(d). The root mean square (RMS) roughness of the fabricated film was 2.84 nm, which also demonstrates a very good smooth surface morphology of our sample.
Fig. 1. (a) The SEM images of Co2+:ZnSe film, Inset: The structure of the layers. (b) EDS spectrogram of the Co2+:ZnSe film. Insert of (b) the atom ratio, (c) The AFM surface images of Co2+:ZnSe film, (d) Corresponding the marked height curve.
The grazing incidence X ray diffraction (XRD) was used to characterize the crystal properties of the grown Co2+:ZnSe thin film. The grazing angle is 2° and the 2θ angles is 20∼60°. The corresponding XRD patterns along with the standard data of ZnSe (PDF#37-1463) are shown in Fig. 2(a). The good match between the diffraction peaks in Fig. 2(a) and the standard ZnSe suggested that the obtained the Co2+:ZnSe thin film shows zinc blende cubic crystal structure with a preferred orientation along (111). The results show that the diffraction peaks of the doped film are in good agreement with the pure ZnSe film, which also indicates that the prepared Co2+:ZnSe film has fine crystallinity. The Raman spectrum of the Co2+:ZnSe thin film grown on the sapphire substrate is shown in Fig. 2(b). The Raman results show that the film has good crystal quality, which is consistent with XRD analysis. The obvious peaks located at ∼203.26 cm-1 and ∼249.55 cm-1 are attributed to transversal optical (TO) and longitudinal optical (LO) phonon modes of ZnSe matrix, respectively. In addition, a peak located at 374.18 cm-1 is attributed to the signal from the sapphire substrate. Compared with ZnSe bulk polycrystalline, ZnSe films have wider LO Phonon mode, which shows the deposited films are partially amorphous [37].
Fig. 2. (a) The XRD spectrum of Co2+:ZnSe film (b) The Raman spectrum of Co2+:ZnSe film, (c) Non-linear saturable absorption of the Co2+:ZnSe SA.
In this work, we utilized a typical balanced twin-detector measurement technique for characterization the nonlinear absorption of the SA. The detailed description of the balanced twin-detector measurement technique can be found in the Ref. [38]. In our experiment, the Co2+:ZnSe film was placed between two fiber connectors to form a fiber-integrated device. And a 1550 nm mode-locked laser with the repetition frequency of 25.7 MHz and the pulse width of 52.5 ps was employed to measure the relationship between the transmittance of the device and the pump power density. The variety of incident average power was used by a variable optical attenuator (VOA). The output from VOA was divided into two parts through a 80/20 optical coupler. One output (20%) was directed into the first power meter was used to monitor the input signal, while another section (80%) entered the Co2+:ZnSe film, and then exited into the second power meter. Figure 2(c) shows measured transmittance as a function of peak intensity, which clearly proves its saturable absorption characteristics. The experimental data of the transmission is fitted according to the following formula:
3. Results and discussion
In order to investigate the performance of the Co2+:ZnSe SA, we inserted the fiber-taper Co2+:ZnSe SA into the all-fiber ring cavity with erbium-doped gain fiber, as shown in Fig. 3. The ring cavity is composed of wavelength division multiplexer (WDM), erbium-doped fiber (EDF), optical coupler (OC), polarization dependent isolator (ISO), polarization controller (PC) and Co2+:ZnSe SA. A 976 nm laser diode (LD) pump light was coupled to the EDF through a WDM. We employed a piece of 1 m long Er-doped fiber (Er-80, 8/125) as the laser gain medium with a dispersion parameter of ∼15.7 ps/(nm·km) and a 80:20 optical coupler was used to output the mode-locked pulse. A polarization independent isolator and a polarization controller were employed to realize unidirectional operation of the light in the ring cavity and adjust the polarization state of the propagation light, respectively. The total length of the cavity was about 11.6 m [10.6 m long single-mode fiber (SMF-28) with a dispersion value of 18 ps/(nm·km) was used to adjust the dispersion value of the cavity]. Thus, the net cavity dispersion of the Er-doped fiber laser was calculated to be -0.382 ps2. The average output power was measured with a power meter (Thorlabs, S132 C) and we monitored the time characteristics of the output pulse by using an oscilloscope (Rohde & Schwarz, RTO1022, 2 GHz bandwidth) with a photodetector (KG-PR-200 M-A, 200 MHz). Meanwhile, the laser spectrum was recorded by an optical spectrum analyzer (Yokogawa, AQ6375B).
Fig. 3. Schematic diagram of all-fiber mode-locked laser based on the fiber-taper Co2+:ZnSe SA. LD is the laser diode; WDM is the wavelength division multiplexer; EDF is the erbium-doped fiber; OC is the optical coupler; PC is the polarization controller; ISO is the isolator.
In our experiment, we adjusted the PC in the cavity, the mode-locked laser operation was achieved with the 200 mW pump power. The emission spectrum is shown in Fig. 4(a), based on the fiber-taper Co2+:ZnSe SA with evanescent wave interaction, the Er-doped fiber laser can be mode-locked at 1562 nm. The mode-locked state has a spectral width of ∼4 nm at 3 dB, and we can clearly observe the obvious Kelly sidebands in the spectrum, which indicates the typical soliton feature of the mode-locked laser operation. The distance between the center wavelength and the first-order Kelly sideband (Δd) is depicted in Fig. 4(a), which was measured to be ∼8 nm. The time domain of the uniform output pulse trains shape at the pump power of 500 mW is shown in Fig. 4(b), which depicts less fluctuation with the pulse repetition rate of 17.83 MHz. The pulse repetition rate coincides well with the calculated value by using 11.6 m long laser cavity. The single pulse width was measured to be ∼4 ns with the pump power of 500 mW, as shown in Fig. 4(c). This has reached the limit of our detector. In order to survey the stability of the single-wavelength soliton pulse. We measured the radio frequency spectrum of the on the fiber-taper Co2+:ZnSe SA based mode-locked pulse laser as shown in Fig. 4(d). When the fundamental pulse repetition rate was 17.83 MHz, a strong signal peak with a signal to noise ratio (SNR) of 64.9 dB was obtained. The inset in Fig. 4(d) shows the span RF spectrum of 200 MHz, which indicates that mode-locked pulses with a good stability were obtained in our work.
Fig. 4. (a) Optical spectrum of the generated pulses. The 3 dB spectral width is ∼4 nm at 1652 nm. (b) Mode locked pulse sequence. (c) Single pulse with a pulse width of ∼4 ns, (d) The RF spectrum of the mode-locked laser. Inset: RF spectrum for a span of 200 MHz.
The average output power as a function of the pump power is shown in Fig. 5(a). The output power increased linearly from 0.48 mW to 4.88 mW with the increase of the pump power from 150 mW to 600 mW. The tendency of the single pulse trace with the increase of the pump power is shown in Fig. 5(b). The width of the mode-locked pulses fluctuate slightly with the variety of the pump power and the whole remains stable. The narrowest single pulse width was measured to be ∼4 ns, which has reached the limit of our detector. Typical laser spectrum under different pump power is depicted in Fig. 5(c). The evident Kelly sidebands on the spectrum can be clearly observed, which was the typical feature of the soliton mode-locked laser operation. The center wavelengths of these spectra are all at ∼1562 nm without obvious shift. The results indicate mode-locked laser operation have a good and stable quality.
Fig. 5. (a) The average output power versus the pump power, (b) The pulse width as a function of pump power, (c) optical spectra of mode-locked laser operation under different pump power.
Table 1 exhibits a summary the reported optical performance of the passively mode-locked of Er-doped fiber lasers based on real saturable absorbers of different materials. The results demonstrate the maximum average output power of 1.2 mW to 185.3 mW, the pulse width of ∼220 fs to 19 ns, and the signal-to-noise ratio is 40 to 64.9 dB [4, 16–18, 22–25, 39–44]. Through this comparison, we also found that the SNR of 64.9 dB obtained in our work was the largest among all SNRs, which indicates that Co2+:ZnSe thin film is a good candidate material of highly-SNR for achieving passively mode-locked of Erbium-doped fiber laser.
Table 1. Comparison of passively mode-locked Er-doped fiber lasers based on various SAs
4. Conclusions
In conclusion, Co2+:ZnSe thin film on the fiber taper was successfully fabricated by electron beam evaporation technology. The XRD patterns showed the peaks of the sample can be indexed as the cubic zinc blende structure. The characterization results of SEM and AFM indicated the thin film have a good quality with the smooth and flat surface. We experimentally demonstrated passively mode-locked Er-doped fiber laser at 1.5 μm based on fiber-taper Co2+:ZnSe SA. The nonlinear absorption characteristics of the Co2+:ZnSe thin film with the ∼12% modulation depth, the 76% non-saturable loss, and the approximately 1.89 MW/cm2 saturation intensity were measured. Passively mode-locked pulse laser operation with a repetition frequency of 17.83 MHz and a SNR of ∼64.9 dB was obtained. The mode-locked pulses possess narrowest pulse width of ∼4 ns and a recorded maximum average output power of ∼4.88 mW. At the central wavelength of 1562 nm with a 3 dB bandwidth of ∼4 nm was exhibited with the pump power of 500 mW. This work proved that the Co2+:ZnSe thin film could be used an excellent saturable absorber material for passively mode-locked implementations of Er-doped fiber lasers.
Funding
National Natural Science Foundation of China (61905169).
Disclosures
The authors declare no conflicts of interest.s
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