Black phosphorus crystal as a saturable absorber for both a Q-switched and mode-locked erbium-doped fiber laser

Abstract

We report a simple way to generate Q-switched and mode-locked pulses by incorporating black phosphorus (BP) as a saturable absorber (SA) in an erbium-doped fiber laser (EDFL) cavity. The preparation of BP-based SA is facilitated by a mechanical exfoliation method. Stable mode-locked pulses with a repetition rate of 1 MHz and energy of 7.35 nJ energy per pulse are generated under a maximum pump power of 250 mW in the ring cavity.

---

Introduction

The growth of ultrafast pulsed laser technology has been driven by a high demand from the medical and optoelectronics fields that capitalize on their promising advantages.¹ The use of a saturable absorber (SA) to generate pulsed fiber lasers has been the method of choice for increasingly many researchers nowadays.^2–4 These SAs are semiconductor SA mirrors (SESAM),^5,6 single-walled carbon nanotubes (SWCNTs),^7–11 and carbon materials (graphene nano-sheets, nano-scale graphite, charcoal powder).^12–15 The success of graphene has led to the discovery of new 2D materials such as topological insulators (TIs) and transition-metal di-chalcogenides (TMDs), which are capable in shaping the future direction of fiber laser. Furthermore, graphene suffers from the absence of band-gap and low absorption co-efficiency. Recently, TIs materials such as bismuth telluride (Bi₂Te₃),^16,17 and TMDs materials such as tungsten disulphide (WS₂) and molybdenum disulphide (MoS₂) have captivated plenty of interest due to their unique absorption property in ultrafast laser application.^18–21 TMDs has thickness dependent band-gap, while TIs have indirect band-gap in insulating bulk state. But, those materials compound consist of two different elements which lead to complicated preparation process. In this work, we use another intriguing 2D material which is black phosphorus (BP). Black phosphorus possess a controllable bandgap size that can be fine-tuned by adjusting the number of layers in the material.²² Similar to graphene, van der Waals forces attract the individual atomic layers of BP making it the most stable phosphorus allotrope in the group.^23–25 Lu et al. fabricated multi-layers BP film embedded polymethyl-methacrylate (PMMA) which has modulation depth and saturable intensity of 12.4% and 334.6 GW cm⁻², respectively.²⁶ In meantime, Luo et al. deposited the few-layers BP solution onto microfiber.²⁷ This BP SA has 1–3 layers with 9% modulation depth and 25 MW cm⁻² saturable intensity. Both Lu et al. and Luo et al. prepared their BP solution through liquid phase exfoliation (LPE) method, where involves with complex chemical procedures. Thin (15-layers) and thick (25-layers) BP flakes as SA were mechanically exfoliated from commercial BP bulk.²³ The modulation depth and saturable intensity of thin BP was 6.55 MW cm⁻² and 8.1%, respectively. Its optical properties trend relatively close to our fabricated BP SA. However, the number of layer is high.

In this paper, we present a new and simple approach to generate Q-switched and mode-locked pulses from EDFL cavity. The approach employs a BP based SA, which acts as a Q-switcher or mode-locker. The BP bulk crystal is mechanically exfoliated to obtain few-layers of BP. The material could be attached onto the end-facet of a fiber ferrule, making it an SA device. By placing such optical SA into a laser cavity operating at 1550 nm spectral region, Q-switched and mode locking operation could be obtained. Our research results show that BP is the best SA candidate for EDFL.

BP-SA preparation and characterization

The mechanical exfoliation method is used to convert the bulk layer of BP into atomically thinner layer. The preparation of the BP as an SA is illustrated in Fig. 1. This technique has been applied previously in the fabrication of a graphene based SA in ultra-fast fiber laser applications.^28,29 As shown in Fig. 1(a), relatively thin flake of BP is peeled off from a BP crystal with a purity of 99.995% using clear scotch tape. The flake is then pressed repeatedly to flatten it so that it become thin enough to transmit light with a high efficiency. Then, a small portion of the tape is cropped and attached to a standard FC/PC fiber ferrule end surface after applying index matching gel. Inset of Fig. 1(a) shows close-up of BP SA attached to the end-facet of a fiber ferrule. Fig. 1(b) shows the field emission scanning electron microscopy (FESEM) of the BP SA. The image indicates the uniform BP layers with nonappearance of >1 μm voids in the composite SA. The voids may contribute to non-saturable scattering misfortunes. Fig. 1(c) shows the high peak of phosphorus in the spectroscopy data which confirms the presence of BP material on the surface of the scotch tape.

Fig. 1 (a) SA preparation image. (b) FESAM. (c) EDS.

Fig. 2 shows the BP optical properties examined by Raman spectrum and non-linear absorption. Fig. 2(a) is the Raman spectrum, which is recorded by a spectrometer when a 514 nm argon laser radiated on the flakes for 10 ms with an exposure power of 50 mW. Three unique Raman peaks are evident at 359 cm, 434 cm⁻¹ and 462 cm⁻¹, related to the A¹_g, B_2g and A²_g vibration modes of layered BP. Both A²_g and B_2g vibration modes correspond to the in-plane oscillation of phosphorus atoms in the BP layer while the A¹_g mode is related to the out-of-plane vibration. The ratio of the A¹_g and silicon (Si) peaks provides an estimate of the thickness of the BP layer which is around 4 to 5 nm.³⁰ Since the thickness of single-layer BP is approximately 0. 6–0. 8 nm,^1,27 we expect the SA to have 5– 8 layers of BP. Guo et al. reported the peak A²_g at 462 cm⁻¹ indicates this BP SA has more than 4 layers.³¹ BP has a direct bandgap from 0.3 eV (bulk) to 1.51 eV (monolayer), while 0.8 eV and 0.59 eV for 3-layers and 5-layers, respectively.^27,31 The bandgap for 1561 nm is perfectly match at 0.8 eV or between 3-layers. However, in our condition the saturable absorption occurs after photon (1561 nm) possesses bandgap energy (0.8 eV) above the respective SA bandgap (0.59 eV). This generates electron–hole pairs with excess photon energy. Then, excess photon energy will induces kinetic energy to the electron and may dissipated in form of heat (phonon). The balance twin-detector measurement technique is used to confirm the saturable absorption of the multilayer BP SA to prove the nonlinear optical characteristics of BP. The polarization state in the experiment is controlled by a polarization controller (PC). Input pulse laser source is constructed using nonlinear polarization (NPR) technique. The input laser power is varied as a function of incident intensity on the BP tape. The reference power for normalization and transmitted power are recorded. Fig. 2(b) shows the saturable absorption property of the SA as the material absorption decreases with increasing peak intensity. The simple two-level SA model²⁹ were matched for the absorption experimental data where α(I) is the absorption, is equivalent to the total value of α_s is the modulation depth, over 1 + I/I_sat as the I is the input intensity, and I_sat is the saturation intensity with addition of α_ns which represents the non-saturable absorption. As demonstrated to Fig. 2(b), the non-saturable intensity and saturation intensity acquired are 6.9% and 0. 25 MW cm⁻², individually. The absorption saturation was obtained at relatively low fluence due to the leading of nonlinear optical response, as the mechanically exfoliated BP meets essential criteria of indifferent SA for fiber laser. Linear absorption of BP is sensitive to light due to its characteristics as an anisotropic crystal. Further examination was carried out by varying the light polarization state using polarization controller (PC). It is found that the output intensity of the laser demonstrates main two states; one for low yield force (almost close to zero) or with yield control pattern comparable to Fig. 2(b) which proves that the polarization-dependent happened because of anisotropic layered of BP based SA material characteristics.²⁵ Subsequently, to match the SA transmission axis with the polarization of oscillating light, the PC must be utilized in the ring cavity.

Fig. 2 BP optical characteristics. (a) Raman spectrum. (b) Nonlinear absorption behaviour.

Experimental setup

The schematic configuration of the suggested mode-locked laser is shown in Fig. 3. The mode-locked EDFL has a total cavity length of 204 m which enables it to operate in anomalous dispersion region of −4.44 ps². The cavity length consist of 2.4 m long erbium-doped fiber (EDF) and 6.6 m long standard single mode fiber (SMF), with group velocity dispersion (GVD) of 27.6, and −21.7 ps² km⁻¹, respectively. Additional 195 m long standard SMF was added into the cavity to realize mode-locking operation. The fiber laser was pumped by a 980 nm laser diode via a 980/1550 nm wavelength division multiplexer (WDM). The gain medium used in the ring cavity is a 2.4 m length erbium-doped fiber (EDF) with an erbium concentration of 2000 ppm. The EDF has a numerical aperture of 0.24 and absorption of 24 dB m⁻¹ at 1550 nm. A polarization free isolator is employed to guarantee unidirectional light propagation in the cavity and hence encourage self-starting laser.³² The BP-SA is sandwiched between two fiber connectors with index matching gel as adhesive. The laser yield is obtained through a 90/10 output coupler, which allows 10% of the laser to be channelled out and analysed by an optical spectrum analyzer (Yokogawa, AQ6370B) and a 350 MHz digital oscilloscope (GWINSTEK: GDS-3352) with 1.3 GHz quick InGaAs photo-detector (Thorlabs, DET10D/M). The pulse width of the output laser is measured using a femtosecond autocorrelator.

Fig. 3 Schematic configuration of mode-locked EDFL incorporating BP-SA.

Results and discussion

Stable Q-switched EDFL self-started at 56 mW pump power and was maintained up to the maximum pump power of 250 mW. The output pulse trains under different pump power was detected using a 350 MHz digital oscilloscope and are shown in Fig. 4(a). At 56 mW pump power, the repetition rate is 31.53 kHz and it increases to 62.74 kHz at 153 mW pump power. At the maximum pump power of 250 mW, the repetition rate is 82.85 kHz. Increment in repetition rate in tandem with pump power is a characteristic of Q-switched laser.¹⁹ Noting that the output pulses trains were still stable with relatively uniform intensity distribution during the entire process of tuning the pump power, we can reasonably presume that the fiber laser works in highly stable Q-switching regime. Fig. 4(b) displays the output spectrum of the suggested Q-switched EDFL constructed utilizing the BP-SA. The output spectrum has a span setting of 100 nm and a resolution setting of 0.02 nm acquired from a Yokogawa AQ6370B optical spectrum analyzer (OSA) at the end of 10% port of the EDFL setup. Under these conditions, an absolute lasing wavelength toward 1550 nm is examined. The Fig. 4(c) indicates the pulse energy and output power as a function of the pump power for the suggested EDFL with BP. As shown in the figure, the pulse energy is increasing as the pump power raising. The maximum pulse energy is 51 nJ at 250 mW pump power. The slope efficiency obtained is 1.89%. The pulse repetition rate and pulse duration of the Q-switched fiber laser as functions of the occurrence pump power was indicated in Fig. 4(d).

Fig. 4 Q-switched EDFL performances. (a) Pulse train under different pump power. (b) Output spectrum at 250 mW pump power. (c) Output power and calculated pulse energy. (d) Repetition rate and pulse width. (e) RF spectrum at 250 mW pump power with 400 kHz span.

As expected, the repetition rate increased and pulse width decreased as the input pump was raised from 56 mW to 250 mW. The repetition rate expanded from 31. 53 kHz to 82. 85 kHz while the pulse width diminished starting from 9. 36 μs to 5. 52 μs and at the same time the incident pump power shifted starting with 56 mW to 250 mW. Toward each pump energy and pulse repetition rate, no amplitude modulations in these pulse trains were observed and the Q-switched pulse output was stable. This demonstrates that there might have been no self-mode locking impact throughout the Q-switching operation. Fig. 5 indicates the vicinity of repetition rate in radio frequency range for 400 kHz span toward the maximum pump power of 250 mW. The signal-to-noise ratio (SNR) of 37 dB confirmed that the generated Q-switch EDFL has a good stability at repetition rate of 82. 85 kHz. Within 400 kHz span, up to 4th order harmonics of fundamental repetition rate were recorded. The 82.85 kHz repetition rate is corresponded to pulse period of 9. 8 μs.

Fig. 5 Mode-locked EDFL performances. (a) Output spectrum at 250 mW pump power. (b) Output power and calculated pulse energy. (c) Repetition rate under different pump power. (d) Pulse train at 250 mW pump power. (e) Autocorrelation trace. (f) RF spectrum with 10 MHz span.

With the addition of 195 m of SMF in our ring cavity fiber laser will increase the non linearity effect which induced spectral broadening and allows the generation of a stable mode-locking pulse. In the experiment, the self-starting mode locking pulse was obtained at 52 mW pump power. Fig. 5(a) shows the optical spectrum of mode-locked pulses. The spectrum centered at around 1561 nm with the 3 dB spectral width of 0.985 nm. A weak Kelly side-band is also observed in the spectrum, which indicates that the generated pulse is at edge of entering a soliton regime.³³ This corresponds to the strong anomalous dispersion (−4.44 ps²) in the cavity. Further reduce the additional SMF length may decreased the cavity dispersion and compress the pulse width.³⁴ However, stable soliton mode-locking regime can be generated only with this appropriate cavity length (additional 195 m long SMF). Fig. 5(b) shows the average output power and single pulse energy of the mode-locked laser against the input pump power. As shown in the figure, the output power increases from 1.224 mW to 7.38 mW as the pump power is raised from the threshold value of 52 mW to 250 mW. The slope efficiency is calculated to be around 3.14%, which is relatively high due to the low insertion reduction from the SA. The pulse energy is also linearly increased with the pump power where the maximum pulse energy of 7.35 nJ was obtained at pump power of 250 mW. Fig. 5(c) shows the repetition rate against the pump power. As the pump power increased from 52 mW to 250 mW, the repetition rate is continuously stable and there is no fluctuation in the line.

The temporal characteristics of the mode-locked EDFL after the 10 dB coupler are also investigated by using an autocorelator and oscilloscope. Fig. 5(d) shows a stable mode-locked pulse train with a peak to peak spacing of 1 ns, which matches with cavity length of 204 m. The peak power of pulse train taken was 53 mV. The oscilloscope trace shows a pulse width of 88 ps, but the actual pulse width is so much smaller due to the resolution limitation of the oscilloscope. The pulse width can be measured utilizing an auto-correlator or mathematically calculated based on time bandwidth product (TBP).

However, for EDFL the autocorrelator (Alnair Labs, HAC-200) is used to ascertained the pulse width. Its pulse width can be measured between 0.3 ps and 15 ps by applying a two-photon absorption method. Then, the measured optical pulses are instantaneously displayed in real-time with temporal resolution of 25 fs.

The function of assumed pulse shape can be converted from the FWHM auto-correlator trace width to the FWHM pulse width. Fig. 5(e) indicates the measured auto-correlator pulse trace with FWHM of 4.13 ps and sech² pulse profile. The actual FWHM of the pulse is about 2.66 ps for the assumed sech² pulse shape. The broad pulse width can be further compress through cavity dispersion management and also by improving the modulation depth of the SA. The output picosecond pulses was observed at the room temperature and the output is very stable which corresponding to the radio frequency spectrum as indicated in Fig. 5(f). Our laser cavity demonstrate a clear continuous mode-locked operation at the stable regime toward the maximum power at 250 mW which provided fundamental frequency of 1 MHz and no other radio frequency component is observed which has a very high SNR up to 70 dB. No pulse breakup effect be measured since the maximum pump power operation is limited to 250 mW. Overall, the long term stability is good since the pulse is stable for at least 24 hours. The obtained 70 dB SNR confirming the stability of the fabricated device. By further enhancement, our developed device has a potential to be commercial.

Conclusions

We have demonstrated a generation of Q-switched, and mode-locked pulses in our EDFL ring cavity. A stable self-starting Q-switched laser was achieved in the laser cavity as the pump power is increased above 52 mW. By varying the pump power from the threshold pump power to the maximum power of 250 mW, the repetition rate can be tuned from 31.53 kHz to 82.85 kHz while the pulse width can be reduced from 9.36 μs down to 5.52 μs. It is shown that adding 195 m SMF and raising the pump power produce a mode-locked pulse with a fundamental frequency of 1 MHz and a pulse width of 2.66 ps. These results show that both Q-switched and mode-locked lasers can be realized by utilising the BP-based SA in an appropriate cavity design.

Acknowledgements

This work is supported by Malaysia Ministry of Science, Technology and Innovation (Grant No: SF014-2014), Malaysia Ministry of Higher Education (Grant No: LRGS(2015)NGOD/UM/KPT), and University of Malaya (Grant No: PG105-2014B).

References

1. F. Xia, H. Wang and Y. Jia, Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics, Nat. Commun., 2014, 5, 4458.
2. M. E. Fermann and I. Hartl, Ultrafast fibre lasers, Nat. Photonics, 2013, 7(11), 868–874.
3. K. Scholle, et al., 2 μm laser sources and their possible applications, INTECH Open Access Publisher, 2010.
4. N. Nishizawa, Ultrashort pulse fiber lasers and their applications, Jpn. J. Appl. Phys., 2014, 53(9), 090101.
5. P. Li, et al., Subpicosecond SESAM and nonlinear polarization evolution hybrid mode-locking ytterbium-doped fiber oscillator, Appl. Phys. B: Lasers Opt., 2015, 118(4), 561–566.
6. J. Liu, J. Xu and P. Wang, High repetition-rate narrow bandwidth SESAM mode-locked Yb-doped fiber lasers, IEEE Photonics Technol. Lett., 2012, 24(7), 539–541.
7. F. Wang, et al., Wideband-tuneable, nanotube mode-locked, fibre laser, Nat. Nanotechnol., 2008, 3(12), 738–742.
8. S. Yamashita, et al., Saturable absorbers incorporating carbon nanotubes directly synthesized onto substrates and fibers and their application to mode-locked fiber lasers, Opt. Lett., 2004, 29(14), 1581–1583.
9. Z. Sun, et al., L-band ultrafast fiber laser mode locked by carbon nanotubes, Appl. Phys. Lett., 2008, 93(6), 61114.
10. K. Kieu and M. Mansuripur, Femtosecond laser pulse generation with a fiber taper embedded in carbon nanotube/polymer composite, Opt. Lett., 2007, 32(15), 2242–2244.
11. S. Y. Set, et al., Laser mode locking using a saturable absorber incorporating carbon nanotubes, J. Lightwave Technol., 2004, 22(1), 51–56.
12. P. L. Huang, et al., Stable mode-locked fiber laser based on CVD fabricated graphene saturable absorber, Opt. Express, 2012, 20(3), 2460–2465.
13. Y.-H. Lin, et al., Using graphene nano-particle embedded in photonic crystal fiber for evanescent wave mode-locking of fiber laser, Opt. Express, 2013, 21(14), 16763–16776.
14. G. Lin and Y. Lin, Directly exfoliated and imprinted graphite nano-particle saturable absorber for passive mode-locking erbium-doped fiber laser, Laser Phys. Lett., 2011, 8(12), 880.
15. Y.-H. Lin, Y.-C. Chi and G.-R. Lin, Nanoscale charcoal powder induced saturable absorption and mode-locking of a low-gain erbium-doped fiber-ring laser, Laser Phys. Lett., 2013, 10(5), 055105.
16. Y.-H. Lin, et al., Using n-and p-type Bi₂Te₃ topological insulator nanoparticles to enable controlled femtosecond mode-locking of fiber lasers, ACS Photonics, 2015, 2(4), 481–490.
17. Y.-H. Lin, et al., Soliton compression of the erbium-doped fiber laser weakly started mode-locking by nanoscale p-type Bi₂Te₃ topological insulator particles, Laser Phys. Lett., 2014, 11(5), 055107.
18. P. Yan, et al., Microfiber-based WS2-film saturable absorber for ultra-fast photonics, Opt. Mater. Express, 2015, 5(3), 479–489.
19. P. Yan, et al., Passively mode-locked fiber laser by a cell-type WS2 nanosheets saturable absorber, Sci. Rep., 2015, 5, 12587.
20. H. Liu, et al., Femtosecond pulse erbium-doped fiber laser by a few-layer MoS₂ saturable absorber, Opt. Lett., 2014, 39(15), 4591–4594.
21. J. Du, et al., Ytterbium-doped fiber laser passively mode locked by few-layer Molybdenum Disulfide (MoS₂) saturable absorber functioned with evanescent field interaction, Sci. Rep., 2014, 4, 6346.
22. H. O. H. Churchill and P. Jarillo-Herrero, Two-dimensional crystals: Phosphorus joins the family, Nat. Nanotechnol., 2014, 9(5), 330–331.
23. Y. Chen, et al., Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and mode-locking laser operation, Opt. Express, 2015, 23(10), 12823–12833.
24. S. P. Koenig, et al., Electric field effect in ultrathin black phosphorus, Appl. Phys. Lett., 2014, 104(10), 103106.
25. L. Li, et al., Black phosphorus field-effect transistors, Nat. Nanotechnol., 2014, 9(5), 372–377.
26. S. Lu, et al., Broadband nonlinear optical response in multi-layer black phosphorus: an emerging infrared and mid-infrared optical material, Opt. Express, 2015, 23(9), 11183–11194.
27. Z.-C. Luo, et al., Microfiber-based few-layer black phosphorus saturable absorber for ultra-fast fiber laser, arXiv preprint arXiv:1505.03035, 2015.
28. A. Martinez, K. Fuse and S. Yamashita, Mechanical exfoliation of graphene for the passive mode-locking of fiber lasers, Appl. Phys. Lett., 2011, 99(12), 121107.
29. K. Novoselov, Nobel lecture: Graphene: Materials in the flatland, Rev. Mod. Phys., 2011, 83(3), 837.
30. A. Castellanos-Gomez, et al., Isolation and characterization of few-layer black phosphorus, 2D Mater., 2014, 1(2), 025001.
31. Z. Guo, et al., From black phosphorus to phosphorene: basic solvent exfoliation, evolution of Raman scattering, and applications to ultrafast photonics, Adv. Funct. Mater., 2015, 25(45), 6996–7002.
32. K. Tamura, et al., Unidirectional ring resonators for self-starting passively mode-locked lasers, Opt. Lett., 1993, 18(3), 220–222.
33. Y.-H. Lin and G.-R. Lin, Kelly sideband variation and self four-wave-mixing in femtosecond fiber soliton laser mode-locked by multiple exfoliated graphite nano-particles, Laser Phys. Lett., 2013, 10(4), 045109.
34. C.-Y. Yang, et al., Pulse-width saturation and Kelly-sideband shift in a graphene-nanosheet mode-locked fiber laser with weak negative dispersion, Phys. Rev. Appl., 2015, 3(4), 044016.

---

Footnote

† These authors contributed equally to this work.

---