A Q-switched, 1.89 µm fiber laser using an Fe3O4-based saturable absorber

Journal of Luminescence 195 (2018) 181–186

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A Q-switched, 1.89 µm fiber laser using an Fe3O4-based saturable absorber ⁎

T

Joonhoi Koo, Junsu Lee, Jihwan Kim, Ju Han Lee

School of Electrical and Computer Engineering, Faculty of Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Nonlinear optical materials Saturable absorption Optical fiber lasers Q-switched lasers

The potential of Fe3O4 nanoparticles as a base material for saturable absorbers (SAs) that can operate in the 2 μm-wavelength regime, was investigated. An SA was fabricated on a sandwich-structured fiber-ferrule platform through the deposition of an Fe3O4 mixture with polyvinyl alcohol (PVA) onto the end surface of the fiber ferrule. Using the prepared Fe3O4/PVA-composite-based SA within a Tm-Ho codoped fiber laser, stable Qswitched pulses were readily produced at 1894 nm. The temporal characteristics of the output pulses were investigated as a function of the pump power, and the maximum energy of the output pulses was 322 nJ. The temporal width of the output pulses was measured as 2.6 µs at the maximum pulse-repetition rate of 37.2 kHz. The output performance of the passively Q-switched Tm-Ho-codoped fiber laser in this work was compared with those of recently demonstrated, passively Q-switched Tm-doped fiber lasers for which the other types of saturable-absorption materials.

1. Introduction Q-switched fiber lasers are a useful light source for various applications such as optical communication, optical sensing, bio surgery, and material processing [1–3]. Through Q-switching, a laser cavity can produce giant pulses of high energy with a submicrosecond temporal width. Q-switching is induced in a laser cavity by alternating the cavity Q-factor in either active or passive ways. Active Q-switching uses an electrical-signal-driven optical modulator within a cavity, while passive Q-switching is based on a passive-modulation device that does not require any external electrical-signal sources [3,4]. Pulsed lasers that are based on passive Q-switching have advantages over the ones that are based on active Q-switching, such as a simple configuration and a lowcost implementation, even with the issue of external-pulse synchronization. To achieve passive Q-switching in a laser cavity, one of the commonly used techniques is the incorporation of a saturable absorber (SA) into the cavity. Since the SA is a passive optical device, the optical absorption of which varies depending on the incident-beam intensity, it can induce a self-started, periodic beam modulation within a laser cavity [5–8]. Saturable absorption is known as a nonlinear optical property of semiconducting materials, which possess an energy-band structure with the conduction and the valence band, and this property occurs due to the Pauli's blocking principle [7,8]. Until now, the commonly used SAs have been based on the III-V-compound semiconductors, and based on these materials, SAs are now commercially available [9–11]. Even if the performance of the III-V-compound semiconductor-based SAs has been ⁎

proven as excellent so far, their fundamental limitations such as a narrow operating bandwidth and the need of a complicated and expensive fabrication facility have triggered intensive investigations of alternative and new materials that can overcome the drawbacks that are inherent to the III-V-compound semiconductors over the past decade. In 2004, Set et al. reported that carbon nanotubes (CNTs) could be a low-cost and high-performance alternative due to an ultra-fast recovery time and a broad operation bandwidth [12,13]. Broadband saturableabsorption properties were also found in graphene, which is a two-dimensional (2D) hexagonal structure composed of carbon atoms [14–17]. Recently, topological insulators (TIs) [18–26], gold nanoparticles [27–31], black phosphorous [32–35], and transition-metal dichalcogenides (TMDs) [36–44] have also been identified as efficient saturable-absorption materials. Further, it has been recently reported that another material group called the transition metal oxides (TMOs) that includes TiO2, ZnO, and Fe3O4 possesses nonlinear saturable-absorption properties [45–47]. Among the TMOs, Fe3O4 is an attractive material since it features a high third-order nonlinearity and a nonlinear optical absorption [47–49]. It was recently demonstrated that Fe3O4-nanoparticle-based SAs can be used for the implementation of passively Q-switched fiber lasers at a wavelength of 1.5 µm [47,50]. It should be noted that Fe3O4 is a semi-metallic semiconductor with a narrow band gap of 0.14 eV [51]; this indicates that Fe3O4 can exhibit light absorption at the wavelengths of the mid-infrared wavelength regime. In this paper, our recent investigation results on the potential of the

Corresponding author. E-mail address: [email protected] (J.H. Lee).

https://doi.org/10.1016/j.jlumin.2017.11.013 Received 5 June 2017; Received in revised form 10 November 2017; Accepted 11 November 2017 Available online 14 November 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.

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2. Preparation of the Fe3O4 saturable absorber

(a)

10 kV

x50,000

To form the composite of the Fe3O4 and the PVA, 100 mg of the Fe3O4 nanoparticles in 10 ml of deionized (DI) water were mixed with 50 mg of PVA. The Fe3O4 nanoparticle used in this work is commercially available. The PVA was used to ensure a more uniform distribution of Fe2O4 particles when the particles were deposited on the flat-end surface of an FC/APC fiber ferrule using a drop-cast method. The material properties of the used Fe3O4 nanoparticles were characterized by using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The measured SEM image is shown in Fig. 1(a). The size of the Fe3O4 nanoparticles varied from 50 nm to 300 nm, indicating that the Fe3O4 nanoparticles are not composed of fewlayered structures. Fig. 1(b) shows the measured EDS spectrum of the Fe3O4 nanoparticles. Two strong intensity peaks, which represent Fe and O, are clearly observable. The atomic ratio between the Fe and the O is nearly 3–4, as was expected. Fig. 2(a) shows a real photo of the Fe3O4/PVA solution. The linear optical absorption of the prepared Fe3O4/PVA solution was measured over a wavelength range of 400–2000 nm, as shown in Fig. 2(b). The broadband linear absorption is also clearly shown. To fabricate the SA, the prepared Fe3O4/PVA-composite solution was deposited onto the flat-end surface of an FC/APC fiber ferrule using a drop-cast method [26,41,44]. Another FC/APC fiber ferrule was then connected to the Fe3O4/PVA-composite-deposited ferrule. The insertion loss and the polarization-dependent loss of the prepared ferrule-based SA were ~ 6 dB and ~ 0.2 dB, respectively. In order to measure the PDL of our prepared SA we measured the beam propagation loss of the SA while changing the polarization state of an input continuous-wave (CW) beam with a polarization controller. We have not optimized our prepared SA in terms of insertion loss for this particular demonstration since it is not straightforward to obtain a thin and uniform layer using the drop cast method. Due to a lack of a high power Tm-doped fiber amplifier in our laboratory we could not measure the optical damage threshold. Our prepared SA was not damaged when a 100-mW CW beam at 1950 nm was launched into the SA. This indicates that our prepared SA has an optical damage threshold larger than 100 mW. However, it should be noticed that a SA based on a ferrule-based sandwich structure would not be suitable for high-energy Q-switching of a fiber laser since it usually has a relatively low damage threshold. The nonlinear optical transmission of the prepared SA was measured using a mode-locked Tm-Ho-codoped fiber laser with an outputpulse width of ~ 1.5 ps, operating at a wavelength of 1.9 µm. Fig. 3 shows the measured nonlinear transmission curve together with a fitted curve based on the following well-known equation [52]:

100 nm

(b)

Fig. 1. Measured (a) SEM image and (b) EDS spectrum of the Fe3O4 nanoparticles.

Fe3O4 nanoparticles as a base material for SAs that can operate in the 2 μm-wavelength regime, are reported. More specifically, the use of a composite of the Fe3O4 nanoparticles and polyvinyl alcohol (PVA) for the implementation of a fiberized SA that can operate at the wavelengths near 2 µm is experimentally demonstrated. An Fe3O4-based SA is fabricated on a sandwich-structured fiber-ferrule platform through the deposition of the Fe3O4/PVA mixture onto the end of a fiber ferrule. By using the Fe3O4/PVA-based SA within a thulium-holmium (Tm-Ho)codoped fiber-laser ring cavity, the stable Q-switched pulses can readily be generated at a wavelength of 1.89 µm. The temporal width of the output pulses is measured at 2.6 µs at the maximum pulse-repetition rate of 37.2 kHz.

Fig. 2. (a) Real photo of the prepared Fe3O4/PVA-composite solution. (b) Measured linear optical-absorption spectrum.

(a)

Absorbance (arb. unit)

0.30 0.25

(b)

0.20 0.15 0.10 0.05 0.00 400

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1800

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Transmission (%)

1.8% and ~ 6.4 W, respectively. The saturation intensity is estimated at ~ 8.2 MW/cm2.

Experiment Fitting

Nonsaturable Loss

3. Q-switching of a Tm-Ho-codoped fiber lasers

28

To evaluate the efficacy of the prepared SA, it was inserted into a Tm-Ho-codoped fiber ring cavity. Fig. 4(a) shows the experimental schematic of the built fiber ring laser. For the cavity, a 1 m length of the Tm-Ho-codoped fiber was used as a gain medium (TH512, CorActive). The gain medium was pumped by a 1550 nm laser diode via a 1550/ 2000 nm wavelength division demultiplexer (WDM). An isolator was used for a unidirectional propagation of the oscillated beam within the cavity. A 50/50 coupler was employed to extract the laser output. The prepared Fe3O4-based SA was inserted between the coupler and the isolator. The insertion losses of the coupler, isolator, and WDM were measured at ~ 0.8, ~ 1.2, and ~ 0.5 dB, respectively, at 1.9 µm. The output coupler had a ~ 3-dB loss. The total cavity length is ~ 5.8 m. During this work, a polarization controller was not used because the Qswitching phenomenon occurred irrespective of the polarization status of the oscillating beam within the cavity. With the fiber-laser setup ready, the laser output was observed with the combination of a photodetector and an oscilloscope while the pump power was increased. As the pump power was increased, the laser started lasing in the CW mode at a pump power of ~ 100 mW. Since the insertion loss of our prepared ferrule-based SA was ~ 6 dB, the cavity loss was high. Such a high cavity was the main reason why the pump threshold was quite high. A further increasing of the pump power changed the CW-lasing mode into the Q-switching mode at a pump power of 120 mW. Fig. 4(b) shows the measured optical spectrum with the resolution bandwidth of 0.3 nm at a pump power of 200 mW. The center wavelength was measured as ~ 1894 nm. Multiple peaks appeared in the optical spectrum, indicating that many longitudinal modes oscillated simultaneously because no mode selecting component was used within the cavity [53]. The sorts of multi-peak spectra are previously reported in Tm-doped Q-switched fiber lasers without using bandpass filters [53,54]. For comparison the measured optical spectrum of the output from the laser without our prepared SA is shown in Fig. 4(c). It is clearly evident that the peak wavelength of the output beam spectrum blue-shifted while its spectral width broadened. Fig. 5 shows the measured oscilloscope traces of the output pulses from the laser for the various pump powers of 122 mW, 137 mW, 152 mW, and 200 mW. It is obvious from the figure that the stable Qswitched pulses were generated from the laser. The minimum temporal width of the output pulses of 2.6 µs was obtained at the maximum repetition rate of 37.2 kHz. Above the 200 mW pump power, the Qswitched pulses became unstable and disappeared. During this work, the mode-locking phenomenon did not occur. Then, the variations in the temporal width and the repetition rate, the average output power, and the single pulse energy of the output pulses as functions of the pump power were measured. The measured results are summarized in Fig. 6. Fig. 6(a) shows the temporal width and the repetition rate of the Q-switched pulses as a function of the applied pump power. The pulse width decreased from 13.7 µs to 2.6 µs with the increasing of the pump power due to a strong pump-induced gain compression effect, as reported in Ref. [20,35]. The repetition rate of the Q-switched pulses increased from ~ 19.4 kHz to ~ 37.2 kHz as the pump power was enlarged, which is commonly observed in passively Q-switched lasers. The measured average output power and single pulse energy of the output pulses as a function of the pump power are shown in Fig. 6(b). The maximum values of the average output power and single pulse energy at the maximum pump power of ~ 200 mW were measured to be ~ 12 mW and ~ 322 nJ, respectively. The electrical spectra of the output pulses were also measured at a pump power of 200 mW with both narrow and wide spans, as shown in Fig. 7. The signal-to-noise was measured to be ~ 51 dB, which indicates that stable Q-switched pulses were generated.

27 26

Modulation Depth ~1.8%

25 Saturation Power : ~6.4 W

24 23

10

20

30

40

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Incident Peak Power (W) Fig. 3. Measured nonlinear optical transmission of the SA at a wavelength of 1.9 µm.

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Wavelength (nm) Fig. 4. (a) The experimental laser schematic. (b) Measured optical spectrum of the output pulses at a pump power of 200 mW. (c) Measured optical spectrum of the output from the laser without our prepared SA at 200 mW.

−I T (I ) = 1 − ΔT ⋅exp ⎛ ⎞ − Tns, ⎝ Isat ⎠ ⎜

⎟

(1)

where T is the transmission, ΔT is the modulation depth, I is the incident-pulse energy, Isat is the saturation energy, and Tns is the nonsaturable loss. The modulation depth and the saturation power are ~ 183

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Fig. 5. Measured oscilloscope traces of the output pulses for various pump powers: (a) 122 mW, (b) 137 mW, (c) 152 mW, and (d) 200 mW.

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Fig. 8. Oscilloscope traces of the output pulses, which were measured every 10 min for 1 h at a pump power of 200 mW.

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Table 1 Performance comparison of the passively Q-switched fiber lasers for which various saturable absorption materials are used in the 2 µm wavelength region. Saturable absorber

Modulation depth (%)

Wavelength (nm)

Max. pulse energy (nJ)

Repetition rate (kHz)

Min. pulse width (µs)

Max. output power (mW)

Pump conversion efficiency (%)

Ref.

Graphene Bi2Se3 MoS2 MoSe2 Gold Nanorods Black Phosphorus Fe3O4

8 – – – 2.3 24 1.8

~ 2000 1980 ~ 2000 1924 1914 1912 1892

85 313 ~ 1000 42 19.85 632.4 322

~ 53 8.4–26.8 33.6–48.1 14–21.8 ~ 40.5 to ~ 47.5 69.4–113.3 19.4–37.2

1.4 4.18 1.76 5.5 2.64 0.71 2.6

4.5 8.4 47.3 ~ 0.915 0.94 71.7 12

~ ~ ~ – ~ ~ ~

[15] [20] [44] [41] [31] [35] This Work

Next, to check the temporal stability of the fiber laser, the oscilloscopetrace variation of the output pulses was observed every 10 min for 1 h at the pump power of 200 mW, as shown in Fig. 8. A stable operation of the laser with an almost-constant pulse width was clearly observed despite a slight drift that is due to the environmental temperature changes. Lastly, the output performance of the passively Q-switched Tm-Hoco-doped fiber laser in this work was compared with that of recently demonstrated, all-fiberized passively Q-switched lasers at 2000 nm using other nanomaterial-based saturable absorption materials, as follows: graphene [15], Bi2Se3 [20], MoS2 [44], MoSe2 [41], gold nanorods [31], and black phosphorus [35]. Note that this comparison was conducted only for fiberized SAs excluding free-space SAs. The results are summarized in Table 1. The fabricated Fe3O4 SA of this study exhibited the smallest modulation depth compared with the graphene, gold nanorods, and black phosphorus. Even if the modulation depth of 1.8% is sufficient to induce the Q-switching from the proposed laser configuration, a further improvement of the modulation depth is essentially needed. In terms of the pump-conversion efficiency, the proposed laser is better than the lasers for which graphene-, Bi2Se3-, Bi2Te3-, MoS2-, and MoSe2-based SAs are used, even if it is inferior to the black-phosphorus-based laser. This comparison indicates that the passive Q-switching performance of the proposed Fe3O4 SA is comparable with that of the SAs that are based on other types of saturableabsorption materials.

0.3 0.3 1.4 0.6 11.9 6

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF2015R1A2A2A11000907, NRF-2015R1A2A2A04006979), Republic of Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

[10] [11] [12] [13] [14] [15] [16]

4. Conclusions

[17] [18]

In conclusion, the use of an Fe3O4/PVA-composite-based SA for the passive Q-switching of a Tm-Ho co-doped fiber laser has been experimentally demonstrated. It has been shown that the stable Q-switched pulses were readily generated at 1894 nm when the prepared SA was incorporated into a Tm-Ho co-doped fiber ring cavity. The temporal characteristics of the output pulses were investigated as a function of the pump power, and the maximum energy of the output pulses is 322 nJ. Through the performance comparison of the proposed laser with the recently demonstrated Q-switched Tm-doped fiber lasers for which graphene, topological insulators, and TMDs are used, it was shown that the proposed Fe3O4-based SA is comparable with the SAs that are based on other kinds of saturable-absorption materials in terms of the passive Q-switching capability. In order to enhance our laser performance, an optimization of our laser cavity in terms of beam propagation loss needs to be conducted, including substantial improvement of the insertion loss of our prepared SA. This experimental demonstration is a confirmation of the broadband saturable-absorption capability of the Fe3O4 SAs. We believe that Fe3O4 has a non-negligible advantage for the implementation of practical SAs compared to 2-D nanomaterials from a perspective of cost-effectiveness since they do not require expensive and complicated fabrication processes.

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