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Low threshold linear cavity mode-locked fiber laser using microfiber-based carbon nanotube saturable absorber
Optics and Laser Technology 102 (2018) 240–246
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Low threshold linear cavity mode-locked fiber laser using microfiberbased carbon nanotube saturable absorber K.Y. Lau a, E.K. Ng a, M.H. Abu Bakar a, A.F. Abas b, M.T. Alresheedi b, Z. Yusoff c, M.A. Mahdi a,⇑ a
Wireless and Photonics Networks Research Centre, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Department of Electrical Engineering, College of Engineering, P.O. Box 800, King Saud University, Riyadh 11421, Saudi Arabia c Faculty of Engineering, Multimedia University, 63100 Cyberjaya, Selangor, Malaysia b
a r t i c l e
i n f o
Article history: Received 7 July 2017 Received in revised form 19 December 2017 Accepted 23 December 2017
Keywords: Ultrashort pulse Pulse fiber laser Carbon nanotube
a b s t r a c t In this work, we demonstrate a linear cavity mode-locked erbium-doped fiber laser in C-band wavelength region. The passive mode-locking is achieved using a microfiber-based carbon nanotube saturable absorber. The carbon nanotube saturable absorber has low saturation fluence of 0.98 mJ/cm2. Together with the linear cavity architecture, the fiber laser starts to produce soliton pulses at low pump power of 22.6 mW. The proposed fiber laser generates fundamental soliton pulses with a center wavelength, pulse width, and repetition rate of 1557.1 nm, 820 fs, and 5.41 MHz, respectively. This mode-locked laser scheme presents a viable option in the development of low threshold ultrashort pulse system for deployment as a seed laser. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction In pulse fiber lasers, there are many methods to generate ultrashort pulses through passive mode-locking technique. Nonlinear polarization rotation (NPR) method is one popular method to form passive mode-locked lasers. In this scheme, two polarization controllers (PCs) and a polarization independent isolator (PDI) in the combination of PC-PDI-PC are typically employed to provide minimum loss at the polarizer for the highest possible optical intensity. Ultrashort pulses are then generated with reduced optical loss by adjusting polarization orientation of NPR [1]. Nevertheless, NPR is very sensitive to polarization change and therefore susceptible even to the slightest perturbations. For instance, a minute change in PC plate angles or shifting of optical fiber in the laser cavity could alter the laser properties. Subsequently, a saturable absorber (SA) with ultrafast carrier response, appropriate linear and nonlinear absorption properties is desired for reliable mode-locked operation without generating Q-switched instabilities and multiple pulsing. Semiconductor saturable absorber mirrors (SESAMs) are among the earliest SA discovered to overcome the Q-switched instabilities phenomenon [2]. The advantage of SESAM as passive mode-locker is mainly due to its technological maturity for defect engineering and micro-fabrication growth [3]. However, the limited optical bandwidth and complex fabrication process of SESAMs encouraged researchers to explore other broadband materials such ⇑ Corresponding author. E-mail address: [email protected] (M.A. Mahdi). https://doi.org/10.1016/j.optlastec.2017.12.043 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
as carbon nanotube (CNT). This material shows superior properties such as sub-picosecond (ps) recovery time, mechanical robustness, low saturation intensity, wide tunable bandgap, and easy integration in optical system. The operating wavelength of CNT can also be designed through appropriate engineering of the tube diameter. Lately, microfiber-based 2-D nanomaterials such as graphene and transition metal dichalcogenides (TMD) have been extensively used as passive mode-locker. Graphene has zero bandgap and broadband optical absorption from the ultraviolet to far-infrared region [4]. However, graphene suffers from intrinsic shortcoming due to its gapless behavior which imposes limitation in optoelectronics applications. On the other hand, TMD shows layer dependent transition of the band structure from indirect to direct band gap semiconductor [5]. TMD can be designed with direct bandgap properties especially with monolayer structure but monolayer structure can only be established with precise atomic thickness control technique such as molecular beam epitaxy. This increases the material synthesis and operational complexities. The integration of inline CNT-SA with active gain medium such as erbium-doped fiber (EDF) in optical fiber cavity is capable of generating mode-locked pulses. In ultrashort laser field, ring-cavity laser structure has been employed by research works to generate optical pulses with low pulse repetition rate that is essential in producing high pulse energy [6–9]. On the other hand, a linear cavity laser can be employed as one of alternatives to reduce the repetition rate. In this case, the oscillating light is allowed to travel twice in the gain medium that enhances its total gain and expedite the saturation effect for lasing conditions. However, these bidirectional
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lights interact with SA at higher intensity that can lead to damage induced by thermal effects. Therefore, the CNT material employed for SA fabrication must be able to perform under high intensity and avoid any thermally induced optical damage. The utilization of CNT thin film as SA in fiber ferrule structure has drawn a lot of attention in the past few years. When the CNT material is placed in between two fiber ferrules, this material is in physical contact with the polished fiber ends thus making the device vulnerable to damage [10]. For instance, optical damage to CNT thin film has been reported at continuous wave (CW) optical power of 8.8 mW [11], 30 mW [12], and 50 mW [13]. This condition is worse when CNT/polymer-composite thin film is used since polymer is thermally sensitive to strong laser illumination [13]. This problem is alleviated using CNT/polymer composite in solution form, which could overcome thermal damage induced by physical contact between the thin film and fiber ferrule, as well as avoid oxidation that causes poor time stability [14]. In this work, microfiber-based CNT-SA is employed in a linear fiber laser cavity. The interaction of evanescent light along the microfiber structure aids in distributing the thermal effect transversely. Stable mode-locking operation is observed at pump power of 215.5 mW using the microfiber-based CNT-SA. The linear cavity mode-locked erbium-doped fiber laser (EDFL) yielded pulse width of 820 fs. To the best of the authors’ knowledge, this work presents the lowest passive mode-locking threshold achieved using microfiber-based CNT-SA at 22.6 mW. It is also able to generate stable fs pulses comparable to a typical ring cavity configuration. 2. Optical characterization of microfiber-based CNT-SA The microfiber-based CNT-SA employed in this work is prepared by K. Kieu from University of Arizona [15]. Firstly, a microfiber is tapered with a dimension of 30–50 mm long with taper diameters between 3 and 7 mm. With this dimension, the transmitted light leaks into the surrounding environment instead of being confined inside the fiber core. The evanescent field around the tapered region will interact with the surrounding material, in this case the CNT-polymer composite. This composite is prepared by mixing low refractive index silicone elastomer (polydimethylsiloxane) with commercially available CNT using standard magnetic stirrer for 24 hour. Meanwhile, the fabricated tapered fiber is fixed into the groove of polymer substrate using epoxy. The stirred CNTpolymer composite is then poured into the groove to cover the tapered fiber. This composite is then cured for 12 hour until it is completely hardened. Finally, the fragile microfiber SA is sealed inside a rugged and robust metallic protection sleeve which provides adequate mechanical and environmental protection. Before the CNT-SA is introduced inside the linear cavity EDFL, its optical
241
transmission loss and nonlinear saturable absorption properties at C-band region are characterized. A broadband amplified spontaneous emission (ASE) source (Amonics model ALS-CL-17-B-SC) and Yokogawa AQ6370B optical spectrum analyzer (OSA) is used for optical transmission measurement. From the measurement setup, the reference spectrum is obtained with a bandwidth resolution of 0.02 nm from 1525 nm to 1565 nm by connecting the ASE source directly to the OSA. Next, the SA is spliced between ASE source and OSA. The difference between both measured spectra is then calculated, resulting in the transmission loss spectrum observed in Fig. 1. Based on this figure, the transmission loss for the CNT-SA is found to be around 2.46 dB at 1550 nm. The nonlinear saturable absorption properties of the CNT-SA are then characterized using a twin-detector scheme as shown in Fig. 2 (a) with its schematic diagram portrayed in Fig. 2(b). The M-Fiber Menlosystems femtosecond (fs) seed pulse laser with pulse repetition rate of 250 MHz and pulse duration of 120 fs at 1550 nm central wavelength is employed to boost the nonlinear saturated absorption of the CNT-SA. A variable optical attenuator (VOA) is introduced to control the pulse laser power and a polarizationindependent isolator is spliced at the output port of the VOA to avoid the signal from propagating into the seed pulse laser. A 50/50 coupler is then introduced after the isolator to divide the launched power into two portions; linear absorption measurement from optical power meter-1 (OPM-1) and nonlinear absorption measurement from optical power meter-2 (OPM-2). The nonlinear saturable absorption curve of this CNT-SA is depicted in Fig. 3. Based on the measurement, the SA modulation depth (MD) is identified from the maximum absorption value at 45.3% until the absorption of this SA is halted at 42.8%. Therefore, the MD of this CNT-SA is measured at 2.5%. This value is comparable to other reported CNT-SA with 2.6% MD [16]. According to Fig. 1, the transmission loss of this CNT-SA is 2.46 dB at 1550 nm. Therefore, the light transmission of CNT-SA is approximately at 55.0% which would not affect the laser efficiency significantly. For instance, a SA with even lower transmission at 24.0% shows mode-locking operation in [17]. Contrarily, the absorption value at this point should be nearly 45.0% for absorption versus peak intensity curve. According to Fig. 3, this value is found at 45.3% which is in good agreement with the power transmission loss measurement (see Fig. 1). The saturation fluence (Fsat) is the value whereby its initial value is reduced to 1/e (37.0%) and starts to reflect light at high intensities. Based on Fig. 3, the Fsat is measured at 0.98 mJ/cm2. In general, low Fsat significantly reduces the pump power threshold for mode-locking operation. Therefore, better Fsat performance is obtained in comparison to previous works with 6.8 mJ/cm2 [18] and 10.0 mJ/cm2 [19]. Apart from that, the nonsaturable loss (ans) is in the order of 42.8%, which is lower than ans of 47.0% and 59.2% reported in [20,21], respectively.
3. Linear-cavity mode-locked fiber laser
Fig. 1. Transmission loss of the CNT-SA at C-band region.
Fig. 4(a) illustrates the experimental setup of a linear cavity mode-locked EDFL with its schematic diagram illustrated in Fig. 4(b). A section of 5 m Lucent HP980 EDF with signal absorption coefficient of 3.5 dB/m at 1530 nm and dispersion parameter of 18 ps/nm/km is pumped by a 980 nm laser diode (LD) through a 980/1550 nm wavelength division multiplexer (WDM). The CNT-SA is spliced in between the EDF and PC. The PC is employed to tune the intra-cavity birefringence of the laser resonator. The optical circulator and optical mirror are employed as standing wave reflectors to sustain light oscillation within the laser cavity. Meanwhile, 30.0% of the laser signal is extracted from the cavity for further analysis through the 70/30 coupler while the remaining 70.0% light stays in the cavity.
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Fig. 2. (a) Nonlinear saturable absorption measurement setup of CNT-SA and (b) schematic diagram of the experimental setup.
Fig. 3. Nonlinear saturable absorption curve of CNT-SA.
The optical spectrum of the linear cavity mode-locked EDFL shown in Fig. 5 is measured using a Yokogawa AQ6370B OSA with resolution bandwidth of 0.02 nm. The evolution of fiber laser is observed from CW laser at 14.8 mW lasing threshold to modelocked lasers at 222.7 mW maximum pump power. The central wavelength (kc) of MLFL is measured at 1557.1 nm and 1557.7 nm at 22.6 mW and 222.7 mW pump powers, respectively. Low mode-locked laser pump power threshold of 22.6 mW is measured in Fig. 5, which is substantially better in contrast to 39 mW [22],
92 mW [18] and 60 mW [19]. This is in conjunction to the lower Fsat described in Fig. 3. Moreover, symmetrical multiple Kelly’s sidebands distributed on both sides of the spectrum denotes the soliton features with net anomalous dispersion regime [23]. The Kelly’s sidebands are generated due to the enhanced resonance effect between soliton and its respective shedding wave seeded from periodic perturbation. In addition, the sharpened peaks of the Kelly’s sidebands are generated by the phase-matching condition between perturbed soliton and radiated dispersive waves occurring during the multiple round-trip circulation with a multiple of 2p, which results in constructive interferences at specific wavelengths [24]. Fig. 6 depicts the optical spectrum at pump power of 22.6 mW (single pulse laser threshold), 108.3 mW (multiple pulse laser threshold), and 215.5 mW (maximum pump power). Based on Fig. 6, broader spectral bandwidth (Dk) is observed at higher pump power. For instance, Dk1, Dk2 and Dk3 are measured at 4.9 nm, 6.1 nm, and 6.6 nm, respectively. The group velocity dispersion (GVD) of the entire laser cavity is calculated and the measured fiber length is tabulated in Table 1. In this table, L denotes the fiber length and b2 indicates the dispersion parameters of the optical fibers at 1550 nm. Single-mode fiber (SMF-28) is typically used in fiber laser applications with low attenuation of around 0.02 dB/km at 1550 nm. Corning Hi-1060 is used at the common port of WDM for efficient fiber coupling from 980 nm to 1550 nm. EDF plays a role as active gain medium
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Fig. 4. (a) Experimental setup and (b) schematic diagram of linear cavity mode-locked EDFL.
for spontaneous emission and generating laser through stimulated emission with signal circulation in laser resonator. The total GVD of laser cavity is calculated at 0.178 ps2 which denotes that it operates in the net anomalous dispersion regime. The impact of net anomalous dispersion is the elimination of positive chirping induced by self-phase modulation. This forms soliton-based mode-locked laser. Soliton evolves in such a way that a pulse can propagate inside optical fiber without distortion in time domain across long propagation distance. This is very crucial for pulse stability during the operation. The evidence of soliton-based modelocking is observed by the appearance of Kelly’s sidebands as shown in Fig. 5. The pulse train measurement is taken from a 10 GHz Tektronix TDS 3012C digital phosphor oscilloscope and a 50 GHz Picometrix D-8IR photodetector. Since the proposed linear cavity mode-locked fiber laser has low threshold, the characterization of single pulse mode-locked laser is performed at pump power of 22.6 mW as shown in Fig. 7. Under this situation, a stable pulse is obtained due to the absence of low intensity multiple pulses at the noise floor. Therefore, this scheme is more suitable to be used as a seed laser. The pulse repetition rate is achieved at 5.41 MHz (184.8 ns response time). Fig. 8 illustrates the measured radio frequency (RF) spectrum using Gw Instek GSP-830 electrical spectrum analyzer. The measurement is taken with resolution bandwidth and video bandwidth of 300 Hz. The RF spectrum is measured in order to obtain the
peak-to-pedestal extinction ratio (PER). In this case, PER is defined as the peak to background contrast ratio measured from the peak level of desired signal to the highest level of background noise. This parameter is measured according to the first order frequencydomain pulse of the mode-locked laser. The RF spectrum of fundamental pulse presents PER of 60.92 dB at 22.6 mW pump power as shown in Fig. 8. The autocorrelation trace of the linear cavity mode-locked EDFL is portrayed in Fig. 9 using Alnair Labs HAC-200 autocorrelator. Fig. 9 shows the pulse width measurement of fundamental pulse laser at 820 fs for 22.6 mW pump power. The pulse durations are presented in full-width at half maximum (sFWHM) value after deconvolution factor of 0.648 for sech2 curve fitted profile. This profile has strong temporal wings and is typically applied for soliton pulses. The temporal shape of the soliton pulses is expressed by;
PðtÞ ¼ Pp sech
2
t
ss
¼
Pp ; 2 cosh sts
ð1Þ
where P(t) is the optical pulse power, Pp is peak power, t is the sech2 curve fitted profile and ss is pulse duration. The pulse duration in this work is shorter than the approximately 4 ps measurement acquired in [25]. A more comparable work is in [26], which reported 838 fs pulse duration for their linear-cavity mode-locked scheme utilizing single-walled nanotube-SA. Similar type of SA is employed
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Fig. 7. Pulse train of the proposed linear cavity mode-locked EDFL.
Fig. 5. (a) Perspective view and (b) top view of optical spectrum for linear cavity mode-locked EDFL at different pump powers. Fig. 8. RF spectrum from the proposed linear cavity mode-locked EDFL.
Fig. 6. Optical spectrum with spectral bandwidth measurement.
Table 1 Net GVD of the entire laser cavity. Fiber type
L (m)
b2 (ps2/km)
Net GVD (ps2)
SMF-28 Hi-1060 EDF
13 1 5
22 7 23
0.286 0.007 0.115
in [27] with distributed ultrafast laser scheme which generates pulse duration of 4.1 ps due to its narrow spectral bandwidth of 0.68 nm. A shorter pulse duration is reported at 318 fs using the same linear cavity setup in [28]. Nevertheless, the GVD value of the linear laser cavity in [28] is predicted with more positive value than our work due to the absence of multiple Kelly’s sidebands from their optical spectrum. In addition, the pulse is fitted using Gaussian fitting rather than sech2 fitting, which indicates that the result is more attuned towards the stretched pulse profile. The time bandwidth product is deduced at 0.50 that proves that the ultrashort pulses are slightly chirped from an ideal sech2 pulse profile at 0.315.
Fig. 9. Autocorrelation trace with sech2 profile fitted curve of the linear cavity mode-locked EDFL.
Fig. 10 illustrates the evolution of average output power and pulse energy as a function of pump power. The average output power is measured using an optical power meter while the pulse energy is calculated by dividing the average output power to the repetition rate. Based on this figure; the CW laser, fundamental pulse laser, and multiple pulse laser pump power thresholds are measured at 14.8 mW, 22.6 mW and 108.3 mW, respectively. With pump power variation from 22.6 mW to 215.5 mW, the average output power evolves from 0.69 mW to 12.11 mW while the pulse energy grows from 0.13 nJ to 2.36 nJ. This corresponds to output power development and pulse energy evolution efficiencies of 5.9% and 11.56 pJ/mW, respectively. The maximum output power and pulse energy in this work is approximately 4 and 27 times larger than the measurement of 3.0 mW reported in [16] and 85.9 pJ
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K.Y. Lau et al. / Optics and Laser Technology 102 (2018) 240–246 Table 2 Comparison of linear cavity mode-locked EDFL incorporating CNT-SA.
Fig. 10. Power and pulse energy evolution as a function of pump power.
SA type
Mode-locking threshold (mW)
Central wavelength (nm)
Repetition rate (MHz)
Pulse width (ps)
Ref.
Sandwich Sandwich Sandwich Sandwich Sandwich Reflector fiber ferrule Reflector fiber ferrule Reflector fiber ferrule Microfiber Microfiber
96.0 92.0 20.0 33.0 10.6 74.0
1563.3 1560.1 1557.9 1560.0 1560.0 1558.0
211.80 62.20 3.07 5.51 5.73 98.35
0.68 0.76 3.90 0.84 4.70 0.17
[16] [18] [25] [26] [27] [22]
25.0
1560.0
9.85
0.32
[28]
30.0
1563.0
15.00
0.44
[30]
85.0 22.6
1558.0 1557.1
10.00 5.41
0.69 0.82
[31] This work
tion. The only work utilizing a microfiber-based CNT-SA is reported in Ref. [31]. However, the mode-locking pump power threshold is achieved at 85.0 mW. In this work, the mode-locked EDFL has lower mode-locking pump power threshold at 22.6 mW using microfiber-based SA. This raises the feasibility of developing a thermally stable seed laser with low mode-locked pump power threshold to meet various practical applications. 4. Conclusion
Fig. 11. (a) Perspective view and (b) top view of stability test for linear cavity mode-locked EDFL over an observation period of 60 min.
reported in [28]. In addition, the CNT-SA performs mode-locking operation in linear cavity at 215.5 mW pump power, which shows improvement compared to [11–13] with lower damage threshold value. This high output power seed laser is achieved without employing any power amplifiers. The stability test of linear cavity mode-locked EDFL is observed over an observation time of 60 min as presented in Fig. 11. The measurement is recorded every 10 minute time intervals at maximum pump power of 215.5 mW. Based on this figure, the operation of mode-locked EDFL is well maintained with no sign of central wavelength drift and CW component. This confirms that the mode-locking operation is stable owing to the robustness of microfiber-based CNT-SA [29]. Table 2 summarizes recent development of linear cavity modelocked EDFL employing CNT-SA. Most CNT-SA incorporates fiber ferrule coupling method such as sandwich and reflector types to perform the mode-locking operation. The disadvantage of fiber ferrule coupling method is as aforementioned in the Introduction sec-
We have successfully demonstrated a linear cavity mode-locked EDFL with microfiber-based CNT-SA. This work presents ultrashort optical pulse of 820 fs at 1557.1 nm central wavelength. The slower pulse repetition rate of 5.41 MHz is achieved as the linear cavity compels light to travel two times the fiber length during each roundtrip, which also leads to double amplification effect. This scenario leads to low threshold power of 22.6 mW for mode-locking operation. The employment of microfiber-based CNT-SA allows high light intensity to propagate while maintaining its nonlinear properties essential for mode-locking. However, the need for a polarization controller to maintain the polarizationdependent stability of the laser introduces excess bulk and complexity to the configuration. Therefore, an all polarization maintaining setup could be considered to further improve this system. In conclusion, the proposed linear cavity configuration is capable of ultrashort pulse generation at low pump power. The proposed scheme is attractive towards the development of low cost ultrashort pulse laser to be utilized as a seed laser. Acknowledgement We would like to express our sincere appreciation to Dr. Khanh Kieu, University of Arizona for fabricating the CNT-SA sample in this work. The authors also extend our appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP#0106. References [1] J. Sotor, I. Pasternak, A. Krajewska, W. Strupinski, G. Sobon, Sub-90 fs a stretched-pulse mode-locked fiber laser based on a graphene saturable absorber, Opt. Express 23 (21) (2015) 27503–27508. [2] M. Haiml, R. Grange, U. Keller, Optical characterization of semiconductor saturable absorbers, Appl. Phys. B 79 (3) (2004) 331–339. [3] D.H. Sutter, L. Gallmann, N. Matuschek, F. Morier-Genoud, V. Scheuer, G. Angelow, T. Tschudi, G. Steinmeyer, U. Keller, Sub-6-fs pulses from a SESAMassisted Kerr-lens modelocked Ti:sapphire laser: at the frontiers of ultrashort pulse generation, Appl. Phys. B: Lasers Opt. 70 (1) (2000) 5–12.
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Full length article
Low threshold linear cavity mode-locked fiber laser using microfiberbased carbon nanotube saturable absorber K.Y. Lau a, E.K. Ng a, M.H. Abu Bakar a, A.F. Abas b, M.T. Alresheedi b, Z. Yusoff c, M.A. Mahdi a,⇑ a
Wireless and Photonics Networks Research Centre, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Department of Electrical Engineering, College of Engineering, P.O. Box 800, King Saud University, Riyadh 11421, Saudi Arabia c Faculty of Engineering, Multimedia University, 63100 Cyberjaya, Selangor, Malaysia b
a r t i c l e
i n f o
Article history: Received 7 July 2017 Received in revised form 19 December 2017 Accepted 23 December 2017
Keywords: Ultrashort pulse Pulse fiber laser Carbon nanotube
a b s t r a c t In this work, we demonstrate a linear cavity mode-locked erbium-doped fiber laser in C-band wavelength region. The passive mode-locking is achieved using a microfiber-based carbon nanotube saturable absorber. The carbon nanotube saturable absorber has low saturation fluence of 0.98 mJ/cm2. Together with the linear cavity architecture, the fiber laser starts to produce soliton pulses at low pump power of 22.6 mW. The proposed fiber laser generates fundamental soliton pulses with a center wavelength, pulse width, and repetition rate of 1557.1 nm, 820 fs, and 5.41 MHz, respectively. This mode-locked laser scheme presents a viable option in the development of low threshold ultrashort pulse system for deployment as a seed laser. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction In pulse fiber lasers, there are many methods to generate ultrashort pulses through passive mode-locking technique. Nonlinear polarization rotation (NPR) method is one popular method to form passive mode-locked lasers. In this scheme, two polarization controllers (PCs) and a polarization independent isolator (PDI) in the combination of PC-PDI-PC are typically employed to provide minimum loss at the polarizer for the highest possible optical intensity. Ultrashort pulses are then generated with reduced optical loss by adjusting polarization orientation of NPR [1]. Nevertheless, NPR is very sensitive to polarization change and therefore susceptible even to the slightest perturbations. For instance, a minute change in PC plate angles or shifting of optical fiber in the laser cavity could alter the laser properties. Subsequently, a saturable absorber (SA) with ultrafast carrier response, appropriate linear and nonlinear absorption properties is desired for reliable mode-locked operation without generating Q-switched instabilities and multiple pulsing. Semiconductor saturable absorber mirrors (SESAMs) are among the earliest SA discovered to overcome the Q-switched instabilities phenomenon [2]. The advantage of SESAM as passive mode-locker is mainly due to its technological maturity for defect engineering and micro-fabrication growth [3]. However, the limited optical bandwidth and complex fabrication process of SESAMs encouraged researchers to explore other broadband materials such ⇑ Corresponding author. E-mail address: [email protected] (M.A. Mahdi). https://doi.org/10.1016/j.optlastec.2017.12.043 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
as carbon nanotube (CNT). This material shows superior properties such as sub-picosecond (ps) recovery time, mechanical robustness, low saturation intensity, wide tunable bandgap, and easy integration in optical system. The operating wavelength of CNT can also be designed through appropriate engineering of the tube diameter. Lately, microfiber-based 2-D nanomaterials such as graphene and transition metal dichalcogenides (TMD) have been extensively used as passive mode-locker. Graphene has zero bandgap and broadband optical absorption from the ultraviolet to far-infrared region [4]. However, graphene suffers from intrinsic shortcoming due to its gapless behavior which imposes limitation in optoelectronics applications. On the other hand, TMD shows layer dependent transition of the band structure from indirect to direct band gap semiconductor [5]. TMD can be designed with direct bandgap properties especially with monolayer structure but monolayer structure can only be established with precise atomic thickness control technique such as molecular beam epitaxy. This increases the material synthesis and operational complexities. The integration of inline CNT-SA with active gain medium such as erbium-doped fiber (EDF) in optical fiber cavity is capable of generating mode-locked pulses. In ultrashort laser field, ring-cavity laser structure has been employed by research works to generate optical pulses with low pulse repetition rate that is essential in producing high pulse energy [6–9]. On the other hand, a linear cavity laser can be employed as one of alternatives to reduce the repetition rate. In this case, the oscillating light is allowed to travel twice in the gain medium that enhances its total gain and expedite the saturation effect for lasing conditions. However, these bidirectional
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lights interact with SA at higher intensity that can lead to damage induced by thermal effects. Therefore, the CNT material employed for SA fabrication must be able to perform under high intensity and avoid any thermally induced optical damage. The utilization of CNT thin film as SA in fiber ferrule structure has drawn a lot of attention in the past few years. When the CNT material is placed in between two fiber ferrules, this material is in physical contact with the polished fiber ends thus making the device vulnerable to damage [10]. For instance, optical damage to CNT thin film has been reported at continuous wave (CW) optical power of 8.8 mW [11], 30 mW [12], and 50 mW [13]. This condition is worse when CNT/polymer-composite thin film is used since polymer is thermally sensitive to strong laser illumination [13]. This problem is alleviated using CNT/polymer composite in solution form, which could overcome thermal damage induced by physical contact between the thin film and fiber ferrule, as well as avoid oxidation that causes poor time stability [14]. In this work, microfiber-based CNT-SA is employed in a linear fiber laser cavity. The interaction of evanescent light along the microfiber structure aids in distributing the thermal effect transversely. Stable mode-locking operation is observed at pump power of 215.5 mW using the microfiber-based CNT-SA. The linear cavity mode-locked erbium-doped fiber laser (EDFL) yielded pulse width of 820 fs. To the best of the authors’ knowledge, this work presents the lowest passive mode-locking threshold achieved using microfiber-based CNT-SA at 22.6 mW. It is also able to generate stable fs pulses comparable to a typical ring cavity configuration. 2. Optical characterization of microfiber-based CNT-SA The microfiber-based CNT-SA employed in this work is prepared by K. Kieu from University of Arizona [15]. Firstly, a microfiber is tapered with a dimension of 30–50 mm long with taper diameters between 3 and 7 mm. With this dimension, the transmitted light leaks into the surrounding environment instead of being confined inside the fiber core. The evanescent field around the tapered region will interact with the surrounding material, in this case the CNT-polymer composite. This composite is prepared by mixing low refractive index silicone elastomer (polydimethylsiloxane) with commercially available CNT using standard magnetic stirrer for 24 hour. Meanwhile, the fabricated tapered fiber is fixed into the groove of polymer substrate using epoxy. The stirred CNTpolymer composite is then poured into the groove to cover the tapered fiber. This composite is then cured for 12 hour until it is completely hardened. Finally, the fragile microfiber SA is sealed inside a rugged and robust metallic protection sleeve which provides adequate mechanical and environmental protection. Before the CNT-SA is introduced inside the linear cavity EDFL, its optical
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transmission loss and nonlinear saturable absorption properties at C-band region are characterized. A broadband amplified spontaneous emission (ASE) source (Amonics model ALS-CL-17-B-SC) and Yokogawa AQ6370B optical spectrum analyzer (OSA) is used for optical transmission measurement. From the measurement setup, the reference spectrum is obtained with a bandwidth resolution of 0.02 nm from 1525 nm to 1565 nm by connecting the ASE source directly to the OSA. Next, the SA is spliced between ASE source and OSA. The difference between both measured spectra is then calculated, resulting in the transmission loss spectrum observed in Fig. 1. Based on this figure, the transmission loss for the CNT-SA is found to be around 2.46 dB at 1550 nm. The nonlinear saturable absorption properties of the CNT-SA are then characterized using a twin-detector scheme as shown in Fig. 2 (a) with its schematic diagram portrayed in Fig. 2(b). The M-Fiber Menlosystems femtosecond (fs) seed pulse laser with pulse repetition rate of 250 MHz and pulse duration of 120 fs at 1550 nm central wavelength is employed to boost the nonlinear saturated absorption of the CNT-SA. A variable optical attenuator (VOA) is introduced to control the pulse laser power and a polarizationindependent isolator is spliced at the output port of the VOA to avoid the signal from propagating into the seed pulse laser. A 50/50 coupler is then introduced after the isolator to divide the launched power into two portions; linear absorption measurement from optical power meter-1 (OPM-1) and nonlinear absorption measurement from optical power meter-2 (OPM-2). The nonlinear saturable absorption curve of this CNT-SA is depicted in Fig. 3. Based on the measurement, the SA modulation depth (MD) is identified from the maximum absorption value at 45.3% until the absorption of this SA is halted at 42.8%. Therefore, the MD of this CNT-SA is measured at 2.5%. This value is comparable to other reported CNT-SA with 2.6% MD [16]. According to Fig. 1, the transmission loss of this CNT-SA is 2.46 dB at 1550 nm. Therefore, the light transmission of CNT-SA is approximately at 55.0% which would not affect the laser efficiency significantly. For instance, a SA with even lower transmission at 24.0% shows mode-locking operation in [17]. Contrarily, the absorption value at this point should be nearly 45.0% for absorption versus peak intensity curve. According to Fig. 3, this value is found at 45.3% which is in good agreement with the power transmission loss measurement (see Fig. 1). The saturation fluence (Fsat) is the value whereby its initial value is reduced to 1/e (37.0%) and starts to reflect light at high intensities. Based on Fig. 3, the Fsat is measured at 0.98 mJ/cm2. In general, low Fsat significantly reduces the pump power threshold for mode-locking operation. Therefore, better Fsat performance is obtained in comparison to previous works with 6.8 mJ/cm2 [18] and 10.0 mJ/cm2 [19]. Apart from that, the nonsaturable loss (ans) is in the order of 42.8%, which is lower than ans of 47.0% and 59.2% reported in [20,21], respectively.
3. Linear-cavity mode-locked fiber laser
Fig. 1. Transmission loss of the CNT-SA at C-band region.
Fig. 4(a) illustrates the experimental setup of a linear cavity mode-locked EDFL with its schematic diagram illustrated in Fig. 4(b). A section of 5 m Lucent HP980 EDF with signal absorption coefficient of 3.5 dB/m at 1530 nm and dispersion parameter of 18 ps/nm/km is pumped by a 980 nm laser diode (LD) through a 980/1550 nm wavelength division multiplexer (WDM). The CNT-SA is spliced in between the EDF and PC. The PC is employed to tune the intra-cavity birefringence of the laser resonator. The optical circulator and optical mirror are employed as standing wave reflectors to sustain light oscillation within the laser cavity. Meanwhile, 30.0% of the laser signal is extracted from the cavity for further analysis through the 70/30 coupler while the remaining 70.0% light stays in the cavity.
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Fig. 2. (a) Nonlinear saturable absorption measurement setup of CNT-SA and (b) schematic diagram of the experimental setup.
Fig. 3. Nonlinear saturable absorption curve of CNT-SA.
The optical spectrum of the linear cavity mode-locked EDFL shown in Fig. 5 is measured using a Yokogawa AQ6370B OSA with resolution bandwidth of 0.02 nm. The evolution of fiber laser is observed from CW laser at 14.8 mW lasing threshold to modelocked lasers at 222.7 mW maximum pump power. The central wavelength (kc) of MLFL is measured at 1557.1 nm and 1557.7 nm at 22.6 mW and 222.7 mW pump powers, respectively. Low mode-locked laser pump power threshold of 22.6 mW is measured in Fig. 5, which is substantially better in contrast to 39 mW [22],
92 mW [18] and 60 mW [19]. This is in conjunction to the lower Fsat described in Fig. 3. Moreover, symmetrical multiple Kelly’s sidebands distributed on both sides of the spectrum denotes the soliton features with net anomalous dispersion regime [23]. The Kelly’s sidebands are generated due to the enhanced resonance effect between soliton and its respective shedding wave seeded from periodic perturbation. In addition, the sharpened peaks of the Kelly’s sidebands are generated by the phase-matching condition between perturbed soliton and radiated dispersive waves occurring during the multiple round-trip circulation with a multiple of 2p, which results in constructive interferences at specific wavelengths [24]. Fig. 6 depicts the optical spectrum at pump power of 22.6 mW (single pulse laser threshold), 108.3 mW (multiple pulse laser threshold), and 215.5 mW (maximum pump power). Based on Fig. 6, broader spectral bandwidth (Dk) is observed at higher pump power. For instance, Dk1, Dk2 and Dk3 are measured at 4.9 nm, 6.1 nm, and 6.6 nm, respectively. The group velocity dispersion (GVD) of the entire laser cavity is calculated and the measured fiber length is tabulated in Table 1. In this table, L denotes the fiber length and b2 indicates the dispersion parameters of the optical fibers at 1550 nm. Single-mode fiber (SMF-28) is typically used in fiber laser applications with low attenuation of around 0.02 dB/km at 1550 nm. Corning Hi-1060 is used at the common port of WDM for efficient fiber coupling from 980 nm to 1550 nm. EDF plays a role as active gain medium
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Fig. 4. (a) Experimental setup and (b) schematic diagram of linear cavity mode-locked EDFL.
for spontaneous emission and generating laser through stimulated emission with signal circulation in laser resonator. The total GVD of laser cavity is calculated at 0.178 ps2 which denotes that it operates in the net anomalous dispersion regime. The impact of net anomalous dispersion is the elimination of positive chirping induced by self-phase modulation. This forms soliton-based mode-locked laser. Soliton evolves in such a way that a pulse can propagate inside optical fiber without distortion in time domain across long propagation distance. This is very crucial for pulse stability during the operation. The evidence of soliton-based modelocking is observed by the appearance of Kelly’s sidebands as shown in Fig. 5. The pulse train measurement is taken from a 10 GHz Tektronix TDS 3012C digital phosphor oscilloscope and a 50 GHz Picometrix D-8IR photodetector. Since the proposed linear cavity mode-locked fiber laser has low threshold, the characterization of single pulse mode-locked laser is performed at pump power of 22.6 mW as shown in Fig. 7. Under this situation, a stable pulse is obtained due to the absence of low intensity multiple pulses at the noise floor. Therefore, this scheme is more suitable to be used as a seed laser. The pulse repetition rate is achieved at 5.41 MHz (184.8 ns response time). Fig. 8 illustrates the measured radio frequency (RF) spectrum using Gw Instek GSP-830 electrical spectrum analyzer. The measurement is taken with resolution bandwidth and video bandwidth of 300 Hz. The RF spectrum is measured in order to obtain the
peak-to-pedestal extinction ratio (PER). In this case, PER is defined as the peak to background contrast ratio measured from the peak level of desired signal to the highest level of background noise. This parameter is measured according to the first order frequencydomain pulse of the mode-locked laser. The RF spectrum of fundamental pulse presents PER of 60.92 dB at 22.6 mW pump power as shown in Fig. 8. The autocorrelation trace of the linear cavity mode-locked EDFL is portrayed in Fig. 9 using Alnair Labs HAC-200 autocorrelator. Fig. 9 shows the pulse width measurement of fundamental pulse laser at 820 fs for 22.6 mW pump power. The pulse durations are presented in full-width at half maximum (sFWHM) value after deconvolution factor of 0.648 for sech2 curve fitted profile. This profile has strong temporal wings and is typically applied for soliton pulses. The temporal shape of the soliton pulses is expressed by;
PðtÞ ¼ Pp sech
2
t
ss
¼
Pp ; 2 cosh sts
ð1Þ
where P(t) is the optical pulse power, Pp is peak power, t is the sech2 curve fitted profile and ss is pulse duration. The pulse duration in this work is shorter than the approximately 4 ps measurement acquired in [25]. A more comparable work is in [26], which reported 838 fs pulse duration for their linear-cavity mode-locked scheme utilizing single-walled nanotube-SA. Similar type of SA is employed
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Fig. 7. Pulse train of the proposed linear cavity mode-locked EDFL.
Fig. 5. (a) Perspective view and (b) top view of optical spectrum for linear cavity mode-locked EDFL at different pump powers. Fig. 8. RF spectrum from the proposed linear cavity mode-locked EDFL.
Fig. 6. Optical spectrum with spectral bandwidth measurement.
Table 1 Net GVD of the entire laser cavity. Fiber type
L (m)
b2 (ps2/km)
Net GVD (ps2)
SMF-28 Hi-1060 EDF
13 1 5
22 7 23
0.286 0.007 0.115
in [27] with distributed ultrafast laser scheme which generates pulse duration of 4.1 ps due to its narrow spectral bandwidth of 0.68 nm. A shorter pulse duration is reported at 318 fs using the same linear cavity setup in [28]. Nevertheless, the GVD value of the linear laser cavity in [28] is predicted with more positive value than our work due to the absence of multiple Kelly’s sidebands from their optical spectrum. In addition, the pulse is fitted using Gaussian fitting rather than sech2 fitting, which indicates that the result is more attuned towards the stretched pulse profile. The time bandwidth product is deduced at 0.50 that proves that the ultrashort pulses are slightly chirped from an ideal sech2 pulse profile at 0.315.
Fig. 9. Autocorrelation trace with sech2 profile fitted curve of the linear cavity mode-locked EDFL.
Fig. 10 illustrates the evolution of average output power and pulse energy as a function of pump power. The average output power is measured using an optical power meter while the pulse energy is calculated by dividing the average output power to the repetition rate. Based on this figure; the CW laser, fundamental pulse laser, and multiple pulse laser pump power thresholds are measured at 14.8 mW, 22.6 mW and 108.3 mW, respectively. With pump power variation from 22.6 mW to 215.5 mW, the average output power evolves from 0.69 mW to 12.11 mW while the pulse energy grows from 0.13 nJ to 2.36 nJ. This corresponds to output power development and pulse energy evolution efficiencies of 5.9% and 11.56 pJ/mW, respectively. The maximum output power and pulse energy in this work is approximately 4 and 27 times larger than the measurement of 3.0 mW reported in [16] and 85.9 pJ
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K.Y. Lau et al. / Optics and Laser Technology 102 (2018) 240–246 Table 2 Comparison of linear cavity mode-locked EDFL incorporating CNT-SA.
Fig. 10. Power and pulse energy evolution as a function of pump power.
SA type
Mode-locking threshold (mW)
Central wavelength (nm)
Repetition rate (MHz)
Pulse width (ps)
Ref.
Sandwich Sandwich Sandwich Sandwich Sandwich Reflector fiber ferrule Reflector fiber ferrule Reflector fiber ferrule Microfiber Microfiber
96.0 92.0 20.0 33.0 10.6 74.0
1563.3 1560.1 1557.9 1560.0 1560.0 1558.0
211.80 62.20 3.07 5.51 5.73 98.35
0.68 0.76 3.90 0.84 4.70 0.17
[16] [18] [25] [26] [27] [22]
25.0
1560.0
9.85
0.32
[28]
30.0
1563.0
15.00
0.44
[30]
85.0 22.6
1558.0 1557.1
10.00 5.41
0.69 0.82
[31] This work
tion. The only work utilizing a microfiber-based CNT-SA is reported in Ref. [31]. However, the mode-locking pump power threshold is achieved at 85.0 mW. In this work, the mode-locked EDFL has lower mode-locking pump power threshold at 22.6 mW using microfiber-based SA. This raises the feasibility of developing a thermally stable seed laser with low mode-locked pump power threshold to meet various practical applications. 4. Conclusion
Fig. 11. (a) Perspective view and (b) top view of stability test for linear cavity mode-locked EDFL over an observation period of 60 min.
reported in [28]. In addition, the CNT-SA performs mode-locking operation in linear cavity at 215.5 mW pump power, which shows improvement compared to [11–13] with lower damage threshold value. This high output power seed laser is achieved without employing any power amplifiers. The stability test of linear cavity mode-locked EDFL is observed over an observation time of 60 min as presented in Fig. 11. The measurement is recorded every 10 minute time intervals at maximum pump power of 215.5 mW. Based on this figure, the operation of mode-locked EDFL is well maintained with no sign of central wavelength drift and CW component. This confirms that the mode-locking operation is stable owing to the robustness of microfiber-based CNT-SA [29]. Table 2 summarizes recent development of linear cavity modelocked EDFL employing CNT-SA. Most CNT-SA incorporates fiber ferrule coupling method such as sandwich and reflector types to perform the mode-locking operation. The disadvantage of fiber ferrule coupling method is as aforementioned in the Introduction sec-
We have successfully demonstrated a linear cavity mode-locked EDFL with microfiber-based CNT-SA. This work presents ultrashort optical pulse of 820 fs at 1557.1 nm central wavelength. The slower pulse repetition rate of 5.41 MHz is achieved as the linear cavity compels light to travel two times the fiber length during each roundtrip, which also leads to double amplification effect. This scenario leads to low threshold power of 22.6 mW for mode-locking operation. The employment of microfiber-based CNT-SA allows high light intensity to propagate while maintaining its nonlinear properties essential for mode-locking. However, the need for a polarization controller to maintain the polarizationdependent stability of the laser introduces excess bulk and complexity to the configuration. Therefore, an all polarization maintaining setup could be considered to further improve this system. In conclusion, the proposed linear cavity configuration is capable of ultrashort pulse generation at low pump power. The proposed scheme is attractive towards the development of low cost ultrashort pulse laser to be utilized as a seed laser. Acknowledgement We would like to express our sincere appreciation to Dr. Khanh Kieu, University of Arizona for fabricating the CNT-SA sample in this work. The authors also extend our appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP#0106. References [1] J. Sotor, I. Pasternak, A. Krajewska, W. Strupinski, G. Sobon, Sub-90 fs a stretched-pulse mode-locked fiber laser based on a graphene saturable absorber, Opt. Express 23 (21) (2015) 27503–27508. [2] M. Haiml, R. Grange, U. Keller, Optical characterization of semiconductor saturable absorbers, Appl. Phys. B 79 (3) (2004) 331–339. [3] D.H. Sutter, L. Gallmann, N. Matuschek, F. Morier-Genoud, V. Scheuer, G. Angelow, T. Tschudi, G. Steinmeyer, U. Keller, Sub-6-fs pulses from a SESAMassisted Kerr-lens modelocked Ti:sapphire laser: at the frontiers of ultrashort pulse generation, Appl. Phys. B: Lasers Opt. 70 (1) (2000) 5–12.
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