⇦Chinese Physics Letters, 2019, Vol. 36, No. 7, Article code 074203 Nanosecond Pulses Generation with Samarium Oxide Film Saturable Absorber * N. F. Zulkipli1, M. Batumalay2, F. S. M. Samsamnun1, M. B. H. Mahyuddin1, E. Hanafi1, T. F. T. M. N. Izam1, M. I. M. A. Khudus3, S. W. Harun1,4** Affiliations 1Photonics Engineering Laboratory, Department of Electrical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia 2Faculty of Information Technology & Sciences, Inti International University, Perdana BBN, Putra Nilai, Nilai 71800, Negeri Sembilan 3Department of Physics, University of Malaya, Kuala Lumpur 50603, Malaysia 4Department of Physics, Faculty of Science and Technology, Airlangga University, Surabaya, Indonesia Received 10 April 2019, online 20 June 2019 *Supported by the INTI Research Grant Scheme 2018 under Grant No INTI-FITS-01-06-2018.
**Corresponding author. Email: swharun@um.edu.my Citation Text: Zulkipli N F, Batumalay M, Samsamnun F S M, Mahyuddin M B H and Hanafi E et al 2019 Chin. Phys. Lett. 36 074203    Abstract Nanosecond pulse generation is demonstrated in a mode-locked erbium-doped fiber laser (EDFL) utilizing a samarium oxide (Sm$_{2}$O$_{3}$) film. The Sm$_{2}$O$_{3}$ film exhibits a modulation depth of 33%, which is suitable for mode-locking operation. The passively pulsed EDFL operates stably at 1569.8 nm within a pumping power from 109 to 146 mW. The train of generated output pulses has a pulse width of 356 nm repeated at a fundamental frequency of 0.97 MHz. The average output power of 3.91 mW is obtained at a pump power of 146 mW, corresponding to 4.0 nJ pulse energy. The experimental result indicates that the proposed Sm$_{2}$O$_{3}$ saturable absorber is viable for the construction of a flexible and reliably stable mode-locked pulsed fiber laser operating in the 1.5 μm region. DOI:10.1088/0256-307X/36/7/074203 PACS:42.55.Wd, 42.60.Gd, 42.70.Nq © 2019 Chinese Physics Society Article Text Nanosecond-duration pulses produced by mode-locked fiber lasers have attracted tremendous interest for applications in various fields including optical communication, mechanical processing and scientific research.[1–3] The lasers have typical repetition rates of few MHz and thus their pulse energies can be simply amplified to microjoule levels using an external optical amplifier. Previously, nanosecond pulses were actively generated using electro-optic and acousto-optic modulators. However, this active modulation approach requires a bulky component and is costly as well. The nanosecond pulses can also be achieved by various passive approaches using either Q-switching or mode-locking schemes. In passive Q-switching, a crystal based saturable absorber (SA) such as Cr:YAG was normally used as a Q-switcher. It is difficult to produce a stable pulse train using this approach. However, stable nanosecond pulses can be achieved in an extended cavity fiber laser, which is mode-locked based on a nonlinear polarization rotation (NPR) technique[4] or by employment of semiconductor saturable absorber mirrors (SESAMs).[5] However, NPR based lasers require the adjustment of the polarization state of the oscillating light in the cavity to achieve mode-locking. Thus, this approach is relatively disadvantageous in terms of reliability and environmental stability. SESAMs provide a much more stable pulses train and thus they are widely used. However, they are still quite costly for purposes of commercial production. Therefore, the passive mode-locking was also largely explored using various types of nano-materials such as graphene[6] and single-walled carbon nanotubes (SWCNTs).[7] SWCNTs are advantageous due to the easier material preparation, but its absorption efficiency, which determines the operation bandwidth, is dependent on its tube sizes. Graphene has the advantage of a wider absorption range and thus, it was widely demonstrated for pulsed laser generation. However, optoelectronics applications of the material have been limited due to its zero-bandgap structure. Recently, other 2D nanomaterials such as transition metal dichalcogenides,[8,9] black phosphorus[10,11] and topological insulators[12,13] have also attracted great interest for SA applications. However, there are still many efforts exploring other new SA materials as well as to design new schemes for generating Q-switched pulses train especially in erbium-doped fiber laser (EDFL) cavities. Despite many new SA materials being proposed and demonstrated, transition metal oxide (TMO) based materials are rarely investigated for Q-switching applications. Previously, titanium dioxide (TiO$_{2})$ and nickel oxide (NiO), which belong to the TMO family, were successfully used as functional SA in promoting pulsed lasers.[14,15] In another work, Das et al. demonstrated a passively Q-switched ytterbium-doped fiber laser using samarium-doped fiber as an SA.[16] In this Letter, we report the utilization of samarium oxide (Sm$_{2}$O$_{3})$ film-based SA in generating stable mode-locking pulses from an EDFL. To obtain the SA, commercial Sm$_{2}$O$_{3}$ powder particles are embedded into a polyvinyl alcohol (PVA) film. This is performed so that the SA can be easily integrated into an EDFL cavity. The results show that a mode-locking output pulse train has a pulse width of 356 ns with repetition at fundamental frequency of 0.97 MHz. The average output power of 3.91 mW was obtained at a pump power of 146 mW, corresponding to 4.0 nJ pulse energy which was successfully realized, and the proposed Q-switched laser operates at 1569.8 nm. This is a desirable wavelength especially for optical communications and sensing applications. We fabricated the Sm$_{2}$O$_{3}$ polymer film by mixing the Sm$_{2}$O$_{3}$ powder and polyvinyl alcohol (PVA) solution. In this work, 1 g of PVA powder was dissolved into 120 ml of distilled water to produce the PVA solution. A magnetic stirrer was used to stir the mixture for about 24 h, at room temperature. Then, 50 mg of Sm$_{2}$O$_{3}$ powder was mixed into 50 ml of the prepared PVA solution. Sm$_{2}$O$_{3}$ PVA solution was further stirred for at least 1 h with a magnetic stirrer. The mixed solution was then sonicated for at least 1 h in an ultra-sonic bath. This process completely dispersed the powder by breaking the bond between the molecules that were bound by the van der Waals force. Finally, the homogenous solution obtained was dispensed onto a small glass Petri dish. It was allowed to dry for approximately 48 h at room temperature to form a thin film.
cpl-36-7-074203-fig1.png
Fig. 1. Sm$_{2}$O$_{3}$ PVA film: (a) physical image, (b) FESEM image, (c) linear absorption profile, and (d) nonlinear optical curve of the SA film.
The image of the fabricated Sm$_{2}$O$_{3}$ PVA film, which has a thickness of about 50 µm, is given in Fig. 1(a). It can be observed that the white film is partially transparent and the Sm$_{2}$O$_{3}$ seems to be evenly distributed across the film. The existence of Sm$_{2}$O$_{3}$ particles in the PVA film was confirmed by the high-resolution image in Fig. 1(b), which was provided by the field-emission scanning electron microscope (FESEM) measurement. It shows a homogeneous distribution of Sm$_{2}$O$_{3}$ particles in the film. The linear optical absorption profile of the Sm$_{2}$O$_{3}$ PVA film is shown in Fig. 1(c), which indicates that the absorption loss of around 3.9 dB at 1569 nm. For comparison, the absorption profile of the pristine PVA was also measured, as shown in Fig. 1(c). The pristine PVA has a significantly lower absorption of 0.5 dB at 1569 nm, which indicates that the mode-locking operation is mainly attributed to the Sm$_{2}$O$_{3}$. It is worth noting that the absorption rate is too low to show the peak profile of PVA film absorption. On the other hand, nonlinear optical response is also an important parameter for SA. To confirm the saturable absorption ability of the fabricated Sm$_{2}$O$_{3}$ PVA film, we employed the amplified mode-locked fiber laser to measure nonlinear optical response based on a balanced twin-detector measurement technique. In the proposed technique, a 1560 nm mode-locked laser source operating at 1.0 MHz repetition rate with 3 ps pulse width was used to perform the measurement. As the absorption power is recorded as a function of launched photon intensity on the Sm$_{2}$O$_{3}$ film by varying the input laser power as shown in Fig. 1(d), it is observed that the absorption decreases with optical intensity, which verifies the saturable absorption property of the Sm$_{2}$O$_{3}$ film. The SA film has a modulation depth of 33%, saturable intensity 25 MW/cm$^{2}$ and nonsaturable absorption of 65%. The modulation depth of the film is relatively high, thus the mode-locked laser operating in picosecond regime is difficult to be realized. A smaller modulation depth may lead to strong pulse shaping by the SA, which can lead to a shorter pulse duration and reliable self-starting, but also to Q-switching instabilities.
cpl-36-7-074203-fig2.png
Fig. 2. Configuration of the Sm$_{2}$O$_{3}$ based nanosecond mode-locked EDFL. Inset: a tiny piece of the Sm$_{2}$O$_{3}$ PVA film, which was placed onto the ferrule tip with the aid of index matching gel.
Figure 2 depicts the configuration of the proposed nanosecond mode-locked EDFL using the prepared Sm$_{2}$O$_{3}$ PVA film as a saturable absorber (SA). The EDFL cavity comprises of a 2.8-m-long gain medium, a 90/10 output coupler, an isolator, a wavelength division multiplexer (WDM), a 200-m-long standard single mode fiber (SMF) and the newly developed Sm$_{2}$O$_{3}$ PVA film based SA. A 980-nm pump was used to pump the erbium-doped fiber (EDF) via the WDM based on the forward pumping scheme. At 980 nm, the pump absorption rate was about 14.5 dB/m. The isolator was deployed inside the cavity to prevent any detrimental effects inside the laser ring resonator by ensuring the unidirectional propagation of light. A fiber-compatible SA device was produced by inserting a small segment of the prepared Sm$_{2}$O$_{3}$ PVA film between two fiber ferrules via a fiber adapter. At the connection, the parasitic reflection is minimized by applying index matching gel. The output coupler was used to tap out the 10% of output for observation and retain 90% to oscillate in the ring cavity. An optical spectrum analyzer (OSA) with 0.05 nm spectral resolution was used to observe the optical spectrum of the laser. The output pulse train was analyzed with 500-MHz digital oscilloscope via a photodetector. The optical power meter was then used to replace the OSA to measure the average output power of the laser. The Q-switching repetition rate and the stability of the laser were analyzed by measuring the electrical radio frequency (rf) spectrum signal using a 7.8 GHz rf spectrum analyzer. The laser cavity overall length was about 211 m.
cpl-36-7-074203-fig3.png
Fig. 3. The characteristics of the output pulses train: (a) output spectrum, (b) typical pulses train, (c) enlarged pulse train, and (d) rf spectrum.
Firstly, we investigate the performance of the proposed Sm$_{2}$O$_{3}$ based mode-locked laser by varying the 980 nm pump power launched into the gain medium. Stable and self-starting mode-locked operation is obtained just by adjusting the pump power over a threshold of 109 mW. The optical spectrum obtained at pump power of 109 mW is plotted in Fig. 3(a). It indicates a laser output with a peak power intensity of $-$21.8 dBm and central wavelength of 1569.8 nm. The 3 dB bandwidth of the spectrum is measured to be around 1.1 nm, indicating a slight spectral broadening effect due to self-phase modulation (SPM) effect in the cavity. A weak Kelly sideband-like structure is also observed on the optical spectrum. Since the pulse duration is in the range of nanoseconds, the mode-locked laser is not operating in soliton regime. In this work, PVA film functions to hold the Sm$_{2}$O$_{3}$ so that it can be easily incorporated into a laser setup and it is not involved in the mode-locking operation. However, the saturable absorption effect can also be induced by PVA due to O–H bond absorption in a wide absorption band in the range of 1.5 µm–2.0 µm.[17,18] To verify that the Sm$_{2}$O$_{3}$ material was responsible for the laser's mode-locking pulse generation, the Sm$_{2}$O$_{3}$ PVA film was replaced with the pristine PVA film in the setup. In this case, no mode-locking pulses were visible on the oscilloscope at any pump powers. This confirms that the mode-locking operation was attributed to the Sm$_{2}$O$_{3}$ SA. The mode-locking operation is limited to the maximum pump power of 146 mW. No pulses were observed beyond this pump power. Figure 3(b) shows the oscilloscope trace of the mode-locked pulse train indicating very stable mode-locking without any Q-switching instabilities. An enlarged view of the pulses is shown in Fig. 3(c). The pulse period was found to be 1.03 µs that corresponds to a repetition rate of 0.97 MHz. The fundamental frequency corresponds well to the cavity length of 211 m. The pulse width was measured to be approximately 356 ns at full wave half maximum (FWHM). Based on the rf spectrum obtained at 109 mW pump power as provided in Fig. 3(d), the stability of the pulsed laser was further investigated. The fundamental frequency was found to be 0.97 MHz with at least 9 harmonics. This result is in good agreement with the pulse period obtained via the oscilloscope trace. The harmonic trend indicates that the mode-locking pulses are operating in nanosecond regime. From the rf spectrum, it can also be seen that the SNR is high at 56 dB, which indicates a stable mode-locking operation. To substantiate that the laser's mode-locking pulse generation was due to the Sm$_{2}$O$_{3}$, the Sm$_{2}$O$_{3}$ PVA film was replaced with a pristine PVA film. Using this configuration, no pulses were visible on the oscilloscope at any pump power. This affirms that the prepared Sm$_{2}$O$_{3}$ SA is responsible for the mode-locking operation.
cpl-36-7-074203-fig4.png
Fig. 4. Output power and pulse energy responses at various pump powers for the mode-locked laser.
The output power and single pulse energy performances at varying input pump powers are shown in Fig. 4. The plot shows that the output power escalated linearly from 3.03 mW to 3.91 mW with the increase of the input pump power from 109 mW to 146 mW. This can be translated to a slope efficiency of about 2.41%. This relatively low efficiency was the result of high insertion loss from the SA. Cavity optimization can be used to further improve the efficiency which depends on the intra-cavity loss. It is also observed from the graph that the pulse energy increases linearly when the pump power is increased. At 146 mW pump power, the maximum pulse energy of 4.0 nJ was obtained. These results verify that the Sm$_{2}$O$_{3}$ material could be used as mode-locker in EDFL cavity for C band operation. The pulse performance in the proposed Sm$_{2}$O$_{3}$ based laser was also compared to similar approaches based on other SAs as listed in Table 1. Compared to graphene, SWCNT and GaN materials, the Sm$_{2}$O$_{3}$ provides a higher attainable pulse energy. However, the silver nanoparticle (SNP) based SA provides a higher pulse energy compared to the proposed Sm$_{2}$O$_{3}$ SA. This is attributed to the silver based laser, which was operated at a higher pump power. Furthermore, the modulation depth of the silver SA (19%) is more suitable for mode-locking operation. Nevertheless, the proposed Sm$_{2}$O$_{3}$ has a lower insertion loss of 0.53 dB at 1569 nm and thus the laser operated at a lower threshold power compared to the SNP based laser. The performance of mode-locked pulses including the threshold pump power is expected to be further improved by optimization of both SA parameter and the laser cavity design. The Sm$_{2}$O$_{3}$ SA loss can be improved by reducing the PVA film thickness. The reduction of Sm$_{2}$O$_{3}$ concentration inside the film is also expected to reduce the modulation depth and to enhance the mode-locking performance of the laser. These results verify the mode-locking ability and show that the Sm$_{2}$O$_{3}$ material could be used in optoelectronics and communication devices in the C band region. These nanoseconds pulsed fibre laser based on the Sm$_{2}$O$_{3}$ PVA SA would offer simplicity, compactness and reliability towards the development of portable laser source system. Future work should focus on realizing mode-locking using PVA films. It is expected that the mode-locked EDFL could be achieved using a thicker PVA film because the modulation depth should be increased for mode locking.
Table 1. Performance comparison of the proposed Sm$_{2}$O$_{3}$ SA with other SAs.
SA Materials $\lambda$ (nm) Threshold (mW) Pulsewidth (ns) Repetition rate (kHz) Maximum pulse energy (nJ)
Graphene 1560 27 6 388 1 Ref.  [19]
SWCNT 1570.4 41.3 332 909 0.34 Ref.  [7]
GaN 1562 149.51 485 967 3.1 Ref.  [20]
SNP 1561.5 198 202 1000 52.3 Ref.  [21]
Sm$_{2}$O$_{3}$ 1569.8 109 356 969 4 This work
In conclusion, generation of nanosecond pulses has been successfully demonstrated in an EDFL through a mode-locking process using a homemade Sm$_{2}$O$_{3}$ PVA film as an SA. The fabricated SA film has a modulation depth and saturable optical intensity of 33% and 25 MW/cm$^{2}$, respectively. The EDFL can produce stable mode-locked nanosecond pulses operating at 1569.8 nm with spectral bandwidth of 1.1 nm, fundamental repetition rate of 0.97 MHz and pulse duration of 356 ns. The average output power of 3.91 mW is obtained, which corresponds to a single pulse energy of 4.0 nJ when the pump power is fixed at 146 mW. These results suggest that the proposed Sm$_{2}$O$_{3}$ has a promising application for ultrafast light generation operating in the C-band region. References Recent Advances in Mechanical MicromachiningNext Generation Light Sources for Biomedical Applications1-GHz optical communication system using chaos in erbium-doped fiber lasersGeneration and amplification of high-energy nanosecond pulses in a compact all-fiber laserUltra-low repetition rate linear-cavity erbium-doped fibre laser modelocked with semiconductor saturable absorber mirrorIonization in alkali-halogen atom collisions: Comparison of the Landau-Zener-Stueckelberg approximation with JWKB calculationsNanosecond soliton pulse generation by mode-locked erbium-doped fiber laser using single-walled carbon-nanotube-based saturable absorberS-band Q-switched fiber laser using molybdenum disulfide (MoS 2 ) saturable absorberPassively Q-switched erbium-doped fiber laser at C-band region based on WS_2 saturable absorberBlack phosphorus crystal as a saturable absorber for both a Q-switched and mode-locked erbium-doped fiber laserGeneration of Mode-Locked Ytterbium Doped Fiber Ring Laser Using Few-Layer Black Phosphorus as a Saturable AbsorberMode-locked erbium-doped fiber laser based on evanescent field interaction with Sb2Te3 topological insulatorPassively Q-switched Erbium-doped and Ytterbium-doped fibre lasers with topological insulator bismuth selenide (Bi 2 Se 3 ) as saturable absorberA generation of 2 μm Q-switched thulium-doped fibre laser based on anatase titanium(IV) oxide film saturable absorberNickel oxide nanoparticles as a saturable absorber for an all-fiber passively Q-switched erbium-doped fiber laserPassively Q-switched Yb- and Sm-doped fiber laser at 1064nmPassively Q-switched fiber lasers based on pure water as the saturable absorberSelf-starting ultrafast fiber lasers mode-locked with alcoholNanosecond-pulsed erbium-doped fiber lasers with graphene saturable absorberNanosecond pulse generation with a gallium nitride saturable absorberNanosecond Pulse Generation with Silver Nanoparticle Saturable Absorber
[1] Dornfeld D, Min S and Takeuchi Y 2006 CIRP Ann. 55 745
[2] Clowes J 2008 Opt. Photonik 3 36
[3] Luo L G, Chu P L and Liu H F 2000 IEEE Photon. Technol. Lett. 12 269
[4] Mao D, Liu X, Wang L, Lu H and Feng H 2010 Opt. Express 18 23024
[5] Chen L, Zhang M, Zhou C, Cai Y, Ren L and Zhang Z 2009 Electron. Lett. 45 731
[6] Azooz S M, Harun S W, Ahmad H, Halder A, Paul M C, Pal M and Bhadra S K 1975 Chem. Phys. Lett. 32 1
[7] Ismail M A, Harun S W, Zulkepely N R, Nor R M, Ahmad F and Ahmad H 2012 Appl. Opt. 51 8621
[8] Ahmad H, Ismail M A, Suthaskumar M, Tiu Z C, Harun S W, Zulkifli M Z, Samikannu S and Sivaraj S 2016 Laser Phys. Lett. 13 035103
[9] Ahmad H, Ruslan N, Ismail M, Reduan S, Lee C, Sathiyan S, Sivabalan S and Harun S W 2016 Appl. Opt. 55 1001
[10] Ismail E, Kadir N, Latiff A, Ahmad H and Harun S W 2016 RSC Adv. 6 72692
[11] Hisyam M B, Rusdi M F M, Latiff A A and Harun S W 2017 IEEE J. Sel. Top. Quantum Electron. 23 39
[12] Sotor J, Sobon G, Grodecki K and Abramski K 2014 Appl. Phys. Lett. 104 251112
[13] Haris H, Harun S, Muhammad A, Anyi C, Tan S, Ahmad F, Nor R, Zulkepely N and Arof H 2017 Opt. Laser Technol. 88 121
[14] Latiff A A, Rusdi M F M, Hisyam M B, Ahmad H and Harun S W 2017 J. Mod. Opt. 64 187
[15] Nady A, Ahmed M H M, Latiff A A, Numan A, Ooi C R and Harun S W 2017 Laser Phys. 27 065105
[16] Das G, Chaboyer Z J, Navratil J E and Drainville R A 2015 Opt. Commun. 334 258
[17] Xian T, Zhan L, Gao L, Zhang W and Zhang W 2019 Opt. Lett. 44 863
[18] Wang Z, Zhan L, Wu J, Zou Z, Zhang L, Qian K, He L and Fang X 2015 Opt. Lett. 40 3699
[19] Xu J, Wu S, Liu J, Wang Q, Yang Q H and Wang P 2012 Opt. Commun. 285 4466
[20] Samsamnun F S M, Zulkipli N F, Khudus M I M A, Bakar A S A and Majid W H A and Harun S W 2019 OSA Continuum 2 134
[21] Rosdin R Z R R, Ahmad M T, Muhammad A R, Jusoh Z, Arof H and Harun S W 2019 Chin. Phys. Lett. 36 054202