Cadmium Selenide Polymer Microfiber Saturable Absorber for Q-Switched Fiber Laser Applications

Fiber-optic technology has rapidly progressed whereby an optical fiber has been developed not only as a medium for communication purposes between two different locations but also for other applications such as sensing and lasers. Fiber lasers have gained a great deal of interest in recent years because of their proven potential and benefits compared with other types of lasers such as gas, chemical, dye, solid-state and semiconductor lasers. For instance, passively Q-switched fiber lasers have been widely demonstrated compared with the active one due to their many advantages such as compactness, simplicity, and flexibility of design. These fiber lasers have been explored and reported in both erbium (Er) and ytterbium (Yb) gain media using various artificial or real types of saturable absorbers (SAs).^[1,2] The artificial SA is normally generated by a nonlinear polarization rotation effect (NPR) and nonlinear optical loop mirror (NOLM), and it is necessary to utilize a nonlinear fiber property similar to saturable absorption, and it is difficult to optimize.^[3,4] Dye color glass was the first real SA, which was then followed by the semiconductor SA mirror (SESAM).^[5] SESAM requires complex fabrication and needs to be prepared using an expensive packaging.

Lately, various nanomaterials such as carbon nanotube (CNT) and graphene were also demonstrated for generating stable Q-switched fiber laser. CNT was simple and low cost, but it has a limited wavelength range of saturable absorption.^[6] CNT also has poor stability, low damage threshold, and less reliability. Bao et al.^[7] reported that graphene could provide an outstanding saturable absorption that could counter the drawback of CNT. However, graphene has a small saturable absorption modulation depth.^[8] Other two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs)^[9] and black phosphorus (BP)^[10] have also gained a tremendous amount of interest in recent years for optoelectronics applications. For instance, BP has a great deal of attraction since it appears with a narrow direct band gap which can fill the gap between graphene and the wide band gap of TMDs. Both TMDs and graphene SAs do not fit the band gap of the optical communication region in the range of 0.8 eV, while BP has a band gap between 0.3 eV and 1.5 eV.^[11] However, BP is a hydrophilic material whose performance is easily degraded as it is exposed to oxygen and water.^[12] Thus the exploration of BP in the future may have a limitation.

More recently, quantum dot (QD) semiconductor crystal, the 0D material was also joining the group of nanocluster materials for various applications including SA,^[13] synthesis of the solar cell,^[14] as energy filtered electron microscopy^[15] and also for biological application.^[16] It was also reported that QDs grown on a semiconductor substrate could emit laser in the wavelength range of $1.0\,{\rm{\mu }}{\rm{m}}$–$1.9\,{\rm{\mu }}{\rm{m}}$. Cadmium selenide (CdSe) is one of the promising materials in the class of QDs, which has unique properties such as great photoelectrical characteristic and direct band gap, which make it a promising material for photovoltaics and photodetectors.^[17] The CdSe also has a strong fluorescence in the visible region. The band gap energy of CdSe QDs depends on the size of the crystal, which makes it a beneficial material for biological and chemical sensors,^[18] and high-density optical memories.

In this Letter, a Q-switched erbium-doped fiber laser (EDFL) is demonstrated using a newly developed CdSe based SA. The SA is obtained by embedding CdSe into a polymethyl methacrylate (PMMA) microfiber, thus oscillating photons can have better interaction with CdSe nanomaterials compared with the conventional film based SA. To the best of our knowledge, this is the first demonstration of the Q-switching generation using a polymer microfiber based SA.

First, a CdSe solution was obtained based on a similar process as previously reported by Hamizi et al.^[19] In the process, cadmium oxide (CdO) (99.99% purity), selenium (Se) (99.99% purity), and manganese (Mn) acetate (98%) powders were used as a precursor. The solvent was prepared by mixing oleic acid and paraffin oil in the ratio of 3:5. Then, CdO and Mn acetate powders were mixed with the prepared solvent under argon gas flow at a temperature of 160 °C using a three-neck flask. The mixture was then stirred constantly until all the powders were completely dissolved before the solution was distilled under a vacuum to eradicate any remaining acetone. Subsequently, the Se powder was dissolved in paraffin oil at 220 °C. Finally, the 5 ml Mn-Cd solution was swiftly added into the Se-paraffin oil solution, thus it exhibited rapid nucleation and allowed a gradual growth of the CdSe in the QD atomic structure. The CdSe solution then underwent a centrifugation process and was washed in methanol to eradicate unreacted chemicals. Figure 1(a) shows the transmission electron microscopy (TEM) image of the CdSe solution, which was diluted by methanol. As shown in Fig. 1(a), the CdSe suspension size is about 50 nm, which confirms that the fabricated CdSe is in the QD size.

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In the second stage, we prepared a molten CdSe-PMMA solution by mixing 1 ml of the CdSe solution with a gram of PMMA powder as a based polymer. About 7 ml of acetone was then added into the glass beaker that contains the mixture of CdSe solution and PMMA powder. This acetone liquid was used to dissolve the mixture. Then, the acetone liquid was added continuously to overcome the evaporation issues of acetone. The mixture in the glass beaker was successfully dissolved by stirring with speed of 900 rpm and temperature of 30 °C, for about 1 h and 30 min. Finally, the molten CdSe-PMMA solution was obtained. Figure 1(b) presents the weight of elements in the fabricated CdSe-PMMA solution, which was obtained based on energy dispersive spectroscopy (EDS) analysis. As shown in the figure, the total concentration of CdSe was measured to be about 0.56 wt%, which is in the range of normal doping concentration. The higher concentration level of carbon (C) and oxygen (O) were observed due to the presence of PMMA.

To fabricate the CdSe doped microfiber, we use the direct drawing technique. Before that, a standard silica single mode fiber (SMF) with core and cladding diameters of $8\,{\rm{\mu }}{\rm{m}}$ and $125\,{\rm{\mu }}{\rm{m}}$, respectively, was tapered to about 6 cm length with $3\,{\rm{\mu }}{\rm{m}}$ diameter waist. The tapering process was carried out using a flame brushing technique, and thus we obtained a tapered fiber with an adiabatic transition region and uniform waist. Then, the taper waist region was cut in the middle, and both tapered fibers were separated at about 0.25 cm as shown in Fig. 2(a). Figure 2(b) illustrates the CdSe doped microfiber fabrication process by physically drawing from the composite solution. A silica SMF with a perfect fiber-end tip was immersed in the molten CdSe-PMMA solution before it is vertically pulled out with fast speed. The molten CdSe-PMMA droplet quickly quenched in the air, and subsequently obtained the free-standing CdSe doped microfiber. The microfiber diameter can be controlled by the pulling speed and viscosity of the polymer (which depends on the hot plate temperature). The CdSe doped microfiber was then cleaved and transferred to fill the gap between two silica microfibers via van der Waals and electrostatic force as shown in Fig. 2(c).

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The nonlinear optical profile of the fabricated CdSe doped PMMA microfiber device was also measured using a twin-balanced detector measurement technique. The measurement system consists of a self-constructed passively mode-locked fiber laser (wavelength=1550 nm, pulse width=1.5 ps, repetition rate=17.4 MHz), a variable optical attenuator, a 3 dB optical coupler and a two-channel power meter. Figure 3 shows the profile, which indicates that this CdSe based SA has a modulation depth, a saturable intensity, and a nonsaturable absorption of 11 %, 0.25 MW/cm² and 34%, respectively. It is expected that these parameters are capable of converting a CW laser into a pulse regime.

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Figure 4 illustrates the schematic diagram of the proposed Q-switched EDFL using the newly fabricated CdSe doped PMMA microfiber as an SA. A 2.4-m-long erbium-doped fiber (EDF) with an erbium ion concentration of 2000 ppm, and numerical aperture (NA) of 0.24, was used as a gain medium. It was pumped by a 980-nm laser diode (LD) via a wavelength division multiplexer (WDM). The isolator was used to force the unidirectional operation of the laser. The SA device was incorporated into the cavity between the coupler and isolator via a fiber connector. The output from the laser cavity was extracted by a 90/10 optical coupler for optical spectrum and pulse train measurement. The spectral characteristic was measured using an optical spectrum analyzer (OSA) with a spectral resolution of 0.02 nm while the temporal characteristics were measured using a 500 MHz oscilloscope and a 7.8 GHz radio-frequency (RF) spectrum analyzer via a 1.2 GHz photodetector. The total cavity length was measured to be around 9 m.

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In this experiment, the EDFL achieved CW at a pump power of 21 mW. The fiber laser changes its state from CW to Q-switching mode when the pump power is increased above the threshold pump power of 34 mW. As the pump power is further increased, the repetition rate increases with the pump power up to the pump power of 74 mW. Figure 5 shows the output spectrum of the EDFL at the pump power 74 mW. As seen in the figure, the laser operates at a wavelength of 1533.6 nm with a 3 dB bandwidth of 0.2 nm. Figure 6 shows the typical oscilloscope trace of the Q-switching pulse train at three different pump powers: 34 mW, 52 mW and 74 mW for the EDFL. The pulse generated having a constant shape, frequency and pulse width with no timing jitter presence. As shown in the figure, the pulse characteristics such as repetition rate and pulse width change with the pump power. For comparison, we also prepared a similar SA device based on pure molten PMMA. As the undoped microfiber is incorporated inside the similar EDFL cavity, we observe no presence of Q-switching operation. This result confirms that the Q-switching operation is generated by the use of 0.56 wt% CdSe.

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During the process of increasing the pump power from 34 mW to 74 mW, the pulse duration of stable output pulses reduces from $7.96\,{\rm{\mu }}{\rm{s}}$ to $4.84\,{\rm{\mu }}{\rm{s}}$, while the repetition rate monotonically increases from 37 kHz to 64 kHz, as shown in Fig. 7. It can be found that the pulse duration becomes smaller and the repetition rate becomes larger with increasing the pump power. This also presents a typical feature of passively Q-switched lasers. The pump rate for the upper laser level increases with the pump power and causes the reduction of the pulse width and the increase of the repetition rate. The demonstrated laser shows no obvious degradation under the laboratory condition for 2 h. In our experiment, no stable pulse trains are observed when the pump power goes beyond 74 mW. The laser becomes CW operation when the pump power increases above 80 mW. If the pump power is reduced to 74 mW again, the Q-switching will reappear. This is because the CdSe based SA exhibits a saturable absorption at moderate laser intensities, while optical limiting induced by excited states absorption would occur at higher intensities.

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Pulse energy is found to change with the increase of the pump power and the output power monotonically as shown in Fig. 8. The output power increases from $106\,{\rm{\mu }}{\rm{W}}$ to $207\,{\rm{\mu }}{\rm{W}}$ with a slope efficiency of about 0.25%, while the single pulse energy is varied from 0.94 nJ to 1.16 nJ as the pump power is increased from 34 mW to 74 mW. The relatively low output pump power and pulse energy are attributed to the high cavity loss at the SA device. The high loss is attributed to the PMMA microfiber loss and the coupling loss between the silica and PMMA microfiber. Therefore, the efficiency of the laser is significantly low. Optimization of the SA fabrication is expected to improve the performance of the laser. To verify the effects of the CdSe PMMA microfiber based SA on Q-switching, the SA device in the EDFL cavity was replaced with two optical fiber clean connectors connected by a flange without the microfiber device. The corresponding SMFs' length is not changed. It is observed that only CW laser operation is obtained, and no pulsed operation when the pump power is increased from 0 mW to 800 mW, which confirms that the above Q-switched EDFL has been induced by the CdSebased SA. Figure 9 shows the RF spectrum of the Q-switched EDF at the maximum pump power of 74 mW. The RF spectrum indicates a signal-to-noise ratio (SNR) of 47 dB for the fundamental frequency at 64 kHz. This proves the stability of the laser.

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In conclusion, we have successfully developed a new SA based on CdSe doped PMMA microfiber. As the SA is incorporated into an EDFL cavity, a stable Q-switching pulse train is obtained to operate at 1533.6 nm with a slope efficiency of 0.25%. The CdSe based SA has a modulation depth of 11%. Varying the pump power within 34 mW to 74 mW, the repetition rate of the laser can be tuned from 37 kHz to 64 kHz. The pulse width is decreased from $7.96\,{\rm{\mu }}{\rm{s}}$ to $4.78\,{\rm{\mu }}{\rm{s}}$ with the increase of the pump power. At a pump power of 74 mW, the maximum pulse energy is measured to be about 1.16 nJ.