
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13807-8
10.1016/j.heliyon.2024.e37776
e37776
Research Article
Black phosphorus film as Q-switcher in neodymium-doped fiber laser
Alghamdi Turki Ali taghamdi@uqu.edu.sa
a⁎
Harun Sulaiman Wadi swharun@um.edu.my
b⁎⁎
a Department of Computer Science and Artificial Intelligence, College of Computing, Umm Al-Qura University, Makkah, Saudi Arabia
b Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia
⁎ Corresponding author. taghamdi@uqu.edu.sa
⁎⁎ Corresponding author. swharun@um.edu.my
10 9 2024
30 9 2024
10 9 2024
10 18 e3777631 7 2024
30 8 2024
10 9 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
We present the successful development of a 1089.4 nm Q-switched laser employing a neodymium-doped fiber (NDF) as the active fiber and black phosphorus (BP) as the saturable absorber (SA). The BP SA was fabricated by inserting BP compound into a polyvinyl alcohol (PVA) host polymer, exhibiting a saturable absorption of 2.8 %. Integrated into an NDFL ring cavity, the SA modified the cavity loss, enabling the production of Q-switched pulses. With an increase in the 808 nm pumping power from 108.6 to 155.9 mW, the laser output pulse duration decreased from 3.74 to 3.54 μs, while the repetition rate improved from 40.6 to 51.0 kHz. The laser demonstrated a stable pulse train output, with a fundamental frequency signal to background noise ratio of 45.78 dB. The highest pulse energy of 1.3 nJ was recorded at 155.9 mW pump power. To the best of our knowledge, this represents the first utilization of BP as a SA or Q-switcher within an NDFL cavity.

Keywords

Black phosphorus
Q-switching
Thin film
Fiber laser
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pmc1 Introduction

The generation of optical pulse trains in fiber laser systems can be achieved through either mode-locking or Q-switching mechanisms. Mode-locked lasers excel in producing short pulses by intricately synchronizing longitudinal modes within the cavity. In contrast, Q-switched lasers generate short pulses, typically in the micro- and nano-second range, by effectively modulating intracavity loss [1]. Q-switched lasers are favoured for applications requiring high pulse energy, such as material processing and LIDAR [[2], [3], [4]]. Q-switching can be implemented through either active or passive technologies [5,6]. Passive techniques, extensively studied for their simplicity in cavity design, offer a self-contained solution compared to active methods, which require external drivers for gain-loss modulation.

While passively Q-switched fiber lasers have been reported using several active media including Ytterbium-doped fiber (YDF) [7], Erbium-doped fiber (EDF) [8], and Thulium-doped fiber (TDF) [9] for operations in the 1.0, 1.5, and 2.0 μm regions, respectively, Neodymium-doped fiber (NDF) have received relatively little attention due to immature manufacturing technology for the gain medium [10]. Only a few reports have explored NDF as an active medium for Q-switched fiber laser applications [11,12]. Therefore, developing effective methods for passive Q-switching using Nd-doped fiber lasers (NDFLs) presents an attractive research opportunity.

One of the most effective passive methods for developing Q-switched fiber lasers is by employing a saturable absorber (SA) transducer. In a laser system, the SA functions to modify the loss within the laser cavity. At low light intensities, the SA absorbs light weakly. As the intensity of the incident light increases, the SA saturates, leading to a decrease in its absorption, effectively lowering the cavity loss. This reduction in loss enables the buildup of optical gain within the cavity, leading to the generation of short and intense pulses of laser light. To date, many SAs transducers have been reported in the literature, including semiconductor SA mirrors (SESAM) [13], graphene [14], topological insulators (TIs) [15], and transition metal chalcogenides (TMDs) [16]. 2D conjugated Ni3(HITP)2 metal-organic frameworks and graphene π–π stacked vertical heterostructures have been shown to serve as excellent SAs. These materials can achieve fundamental mode-locking with a pulse width of 451 fs, harmonic mode-locking with repetition frequencies up to 1.205 GHz, and tunable dual-wavelength mode-locking [17]. Additionally, Ti3C2Tx/graphene vertical heterostructures have demonstrated effectiveness as SAs, particularly for generating femtosecond laser pulses through passive mode-locking [18]. Song et al. proposed that doping Bi2Te3-based materials with Sb enhances their nonlinearity, enabling stable self-starting mode-locking at low gain and pump power, thus offering a novel approach to improving material nonlinearity [19]. Furthermore, Wang et al. utilized a lateral heterostructure based on Sb2Te3– Bi2Te3 in an EDF laser to achieve ultrafast pulse output with a peak repetition rate of 2.29 GHz [20]. Their results confirmed that the lateral heterostructure material outperforms Bi2Te3 alone in terms of nonlinear performance. However, each of these materials comes with its own set of limitations, such as limited damage threshold, high fabrication cost, low modulation depth, slow response times, and challenging production of high-quality materials.

Black phosphorus (BP) has emerged as a compelling 2D material for SA applications, given its established applications in electronics and optoelectronic devices [21]. BP offers advantages such as high conductivity, a distinctive direct bandgap that can be adaptively altered by adjusting the number of layers, and mechanical exfoliability due to its elemental structure derived from phosphorus [22]. BP features a broad bandgap range, from 0.3 eV in its bulk state to 2 eV in its monolayer state [23]. This range effectively spans the gap between graphene and TMDs, bridging the saturable absorption spectrum from visible to mid-infrared wavelengths. Additionally, BP exhibits remarkable anisotropic properties in its electrical, optical, and vibrational behaviors [24]. These attributes position BP as a highly promising candidate for use in pulsed lasers. Previous study has demonstrated stable Q-switched operation in YDF laser using BP as a Q-switcher [25]. We also demonstrated the use of BP-coated onto a D-shaped fiber as a mode-locker for generating picosecond soliton pulses in EDF laser (EDFL) cavity [26]. However, as far as we are aware, there are no reports addressing the application of BP in NDFL cavities.

In general, YDF is more commonly used than NDF for high-power fiber laser and amplifier applications due to its higher efficiency. However, for some low-power applications, NDF can be preferable because it operates as a pure four-level laser system, offering several advantages. These include greater wavelength flexibility and a broader spectral bandwidth. NDF can emit at various wavelengths, such as 1064 nm, 1342 nm, and 1550 nm, while YDF typically emits in the range of 1030–1070 nm, offering less wavelength flexibility. Additionally, NDF's broader emission spectrum is beneficial for applications requiring tunable or multi-wavelength sources. In this paper, we present the development of a passively Q-switched fiber laser utilizing a BP thin film as the SA for the first time. The BP is embedded within a polyvinyl alcohol (PVA) thin film and integrated into an NDFL cavity to generate a stable train of Q-switched pulses. It exhibits a modulation depth of 2.8 %, a saturation intensity of 2 MW/cm2, and a non-saturable loss of 5.6 %. We observed a stable output pulse train operating at 1089 nm with a repetition rate of 51 kHz, which is higher than the previous YDFL.

2 SA preparation and experimental setup

The BP-based SA device was fabricated by incorporating BP powder into Polyvinyl alcohol (PVA) to create a composite thin film. Initially, 1 g of PVA powder was dissolved in 120 ml of distilled water through continuous stirring for approximately 3 h at room temperature using a magnetic stirrer, resulting in a homogeneous PVA solution. The BP solution was prepared by dispersing 5 mg of BP powder in a solution of isopropanol-water cosolvents. Isopropanol was chosen as the solvent due to its compatibility with the oxidation affinity of the BP flakes [27]. Subsequently, the BP solution was mixed with 10 ml of the previously prepared PVA solution. The BP and PVA compound underwent thorough mixing through agitation for three days at room temperature, with the magnetic stirrer set at a mixing rate of 350 rpm. After mixing, the saturated compound underwent sonication in a dry ice bath at 22 °C for 6 h to ensure complete dissolution. Following sonication, the solution was allowed to rest for 1 h before being centrifuged for 15 min at 4000 rpm. The resulting solution was then transferred onto a 3.0 cm diameter Petri dish and left to dry at room temperature for 72 h, resulting in the formation of a thin film comprising BP-PVA. The dried thin layer was carefully removed from the Petri dish and attached to a fiber ferrule measuring 1 mm × 1 mm. This fiber ferrule attachment was subsequently coupled with another FC/PC fiber ferrule to assemble the SA device.

Fig. 1(a) presents the image of the fabricated film, exhibiting a thickness of approximately 50 μm. To examine the physical characteristic and elemental composition of the BP-PVA thin film, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) were utilized, respectively. In Fig. 1(b), the SEM image illustrates a uniform surface morphology of the BP-PVA thin film, highlighting a well-dispersed distribution of BP particles within the PVA matrix. This uniform distribution significantly enhances the film's efficacy as a SA material. The EDX analysis (depicted in Fig. 1(c)) reveals the presence of phosphorus (P), carbon (C), and oxygen (O), with the carbon and oxygen components mainly originating from the PVA matrix. Fig. 1(d) displays the nonlinear curve of the BP-PVA film, obtained using the power-dependent transmission technique. It specifies that the BP film exhibits a saturable absorption of approximately 2.8 %, sufficient for modulating the cavity loss for Q-switching. The saturation intensity and non-saturable loss are 2 MW/cm2 and 5.6 %, respectively.Fig. 1 (a) The image of BP-PVA film, (b) The SEM image at 1000 times magnification, (c) EDX analysis result, and (d) nonlinear absorption curve.

Fig. 1

The laser setup schematic is depicted in Fig. 2, illustrating the utilization of the prepared BP PVA film-based SA as the Q-switcher. The active medium of the laser consisted of an 11-m-long NDF with an Nd ion absorption of 8.5 dB/m at 810 nm, specifically chosen for operation in the 1080 nm region. The length of the NDF is optimized to match the available 808 nm pump power. This Nd-doped active fiber featured a numerical aperture of 0.18 and a cutoff wavelength of 990 nm. Pumping of the system was achieved using an 808 nm single-mode laser diode with a maximum power output of approximately 250 mW, with the pump light introduced into the ring NDFL cavity through an 800/1060 wavelength division multiplexer. To ensure unidirectional operation, a polarization-insensitive isolator was added within the NDFL cavity. Measurement of the generated laser was conducted using an 80/20 optical coupler, extracting 20 % of the output pulses. For characterization of the output pulses, high-speed equipment was employed, including an InGaAs photodetector, a 350 MHz digital oscilloscope (GWINSTEK, GDS-3352), a 7.8 GHz spectrum analyzer from Anritsu, a power meter (PM100D-S122C), and an optical spectrum analyzer (OSA, AQ6317). The NDFL system has a cavity length of approximately 14 m.Fig. 2 The Q-switched NDFL cavity with BP film as SA.

Fig. 2

3 Results and discussion

A Q-switched pulse train centered at 1089.4 nm is reliably generated at a threshold pump power of 108.6 mW, with Q-switching operation observable throughout the pump power range of 108.6–155.9 mW. Notably, the NDFL operates in a continuous-wave regime in the absence of the BP SA from the cavity. Fig. 3 (a) illustrates the output optical spectrum of the Q-switched pulse train centered at 1089.4 nm, similar to other studies employing comparable cavity setups [28]. Spectral broadening is observed due to the self-phase modulation effect. Oscilloscope traces at three different pump powers (108.6, 128.9, and 155.9 mW) are presented in Fig. 3 (b), (c), and (d), respectively. The uniform distribution of pulses confirms clean and stable Q-switching with no transient effects. To investigate the cause of the Q-switching phenomenon, we replaced the BP SA in the cavity with a clean ferrule. Even after extensive adjustments to the pump power and manipulation of optical fibers through bending and twisting, no pulsing was obtained during this period. Nevertheless, when the BP film was reintroduced into the cavity, pulsing resumed. These observations affirm that BP was the sole contributor to the pulsing operation, while other components played a minor role.Fig. 3 (a) Output spectrum of the Q-switched NDFL, (b–d) Typical pulse train at different pumping powers (b) 108.6, (c) 128.9, and (d) 155.9 mW.

Fig. 3

The strength of the saturable absorption impacts how quickly it can switch from absorbing to transmitting light. A stronger saturable absorption means the material can absorb more light before reaching saturation, which can lead to shorter pulse durations. When the SA becomes saturated, it transitions to a state where it transmits light more efficiently, leading to the generation of short, intense pulses. While a strong SA can enhance pulse energy and reduce pulse duration, it might also lead to longer intervals between pulses due to the longer time required for the SA to recover from saturation. Therefore, the repetition rate could be lower if the SA's saturation characteristics are not optimized. Fig. 4 (a) illustrates the laser's repetition rate and pulse width as a function of laser diode power. With the pumping power increasing from 108.6 to 155.9 mW, the repetition rate and pulse width change from 40.6 to 51.0 kHz and from 3.74 to 3.54 μs, respectively. Further reduction of the pulse width can be achieved by increasing the saturable absorption of the SA or reducing the inter-cavity loss. This behaviour is characteristic of Q-switched lasers. With an increase in laser diode power, more power is delivered to the gain medium, leading to increased power supplied to the SA. Consequently, the saturation time of the SA decreases, resulting in faster rise and fall times of the pulses. Thus, as the laser diode power rises, the repetition rate escalates while the pulse width shrinkages.Fig. 4 (a) The repetition rate, pulse width (b) Output power, and pulse energy performances with respect to launched 808 nm photon power.

Fig. 4

Fig. 4 (b) illustrates the average output power, and pulse energy of the NDFL as a function of pump power. The pulse power linearly increases from 12.8 to 66.4 μW, exhibiting a slope efficiency of 1.15 % as the pump power escalates from 108.6 to 155.9 mW. Pulse energy is estimated by dividing the average output power by the repetition rate, increasing from 0.3 to 1.3 nJ over the same pump power range. The increase in pump power leads to higher population inversion, thereby enhancing the average output power, and pulse energy of the Q-switched laser. Fig. 5 presents the RF spectrum of the Q-switched NDFL over a 1 MHz span. It exhibits a peak-to-pedestal extinction of approximately 45.8 dB at the fundamental frequency of 51.0 kHz, indicating the stability of the laser. Moreover, the laser operates stably in laboratory conditions for over 24 h without any visible degradation in performance. During the modulation depth measurement, a pump power of over 500 mW was transmitted through the BP film SA. No signs of damage to the SA were observed, as we successfully achieved Q-switched fiber laser operation with the same SA device. This indicates that the SA's damage threshold exceeds 500 mW. We did not achieve mode-locked pulses in the current cavity configuration. Since cavity dispersion and nonlinearity are critical for mode-locking, we anticipate that mode-locking can be realized with modifications to the cavity, such as the insertion of a specialized fiber to balance nonlinearity with dispersion.Fig. 5 RF spectrum from the Q-switched NDFL.

Fig. 5

Table 1 compares our proposed Q-switched laser with previous systems that use various materials such as graphene, topological insulators, TMDs, and MAX phase materials. Our Q-switched laser exhibits performance comparable to these alternatives. However, the pulse energy is relatively low because the gain medium is an NDF, which operates on a four-level laser system. To enhance pulse energy, we could use higher pump power with a longer gain medium or implement a cladding pumping system, especially given the widespread availability of high-power multimode laser diodes operating at 808 nm. The proposed Q-switched laser demonstrated exceptional stability in the laboratory setting. It consistently operated at 1089.4 nm for Q-switched pulses, with its spectral characteristics remaining stable and peak intensity fluctuations confined to a narrow range of ±0.1 dB. Furthermore, extended monitoring with an oscilloscope over two days confirmed the consistent stability of the pulse trains.Table 1 Q-switching performance for various SA materials.

Table 1SA	Max pump power (mW)	Rep rate (kHz)	Pulse width (μs)	Pulse energy (nJ)	Output Power (mW)	λ (nm)	Ref	
Graphene	151.5	67.8	6.02	206	32	1558.3	[29]	
CVD-Bi2Se3	NA	195.3	NA	111.2	21.7	1360.6	[10]	
MoS2	170	38.43	5.02	141.3	5.43	1551.4	[30]	
ZnPc	148	48	3.6	71	3.5	1561.4	[31]	
WSe2	300	49.6	3.1	33.2	1.23	1560	[32]	
Ti3C2Tx	81	96	2.3	125	12	1559.9	[33]	
BP	97.6	32.9	10.8	328	NA	1060	[25]	
Nb2AlC	150	31.4	8.6	66	2.1	1562.3	[34]	
BP	155.9	51.0	3.5	1.3	0.07	1089.4	This work	

4 Conclusion

We have demonstrated Q-switching in the 1-μm wavelength region utilizing a BP film as the SA and an NDF as the active medium. The simplicity and compactness of the laser oscillator stem from employing an NDF pumped by an 808 nm laser diode. The SA device was created by embedding BP compound into a PVA host material, resulting in a thin film SA with a notable modulation depth of 2.8 %. This SA, inserted between fiber ferrules, was incorporated into a resonant NDFL cavity, facilitating the generation of microsecond pulses operating at a wavelength of 1089.4 nm with a tunable frequency ranging from 40.6 to 51.0 kHz. Temporal characterization revealed that at a pump power of 155.9 mW, we achieved the shortest pulse width of 3.54 μs, the highest repetition rate of 51.0 kHz, and the highest pulse energy of 1.3 nJ. These results underscore the significant potential of BP for operation in the 1.0-μm region, offering promising opportunities for further exploration and utilization of BP-based materials in various photonic applications.

CRediT authorship contribution statement

Turki Ali Alghamdi: Writing – review & editing, Project administration, Methodology, Funding acquisition, Formal analysis, Conceptualization. Sulaiman Wadi Harun: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This financial support of the National Science, Technology and Innovation Plan (MAARIFAH), 10.13039/501100004919 King Abdulaziz City for Science and Technology (KACST), Kingdom of Saudi Arabia (Grant no: 14-ELE1454-10 ) is acknowledged.
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