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We report an all-fiber passively mode-locked laser based on a saturable absorber fabricated by filling short single-wall carbon nanotubes into cladding holes of grapefruit-type photonic crystal fiber. The single-wall carbon nanotube is insensitive to polarization of light for its one-dimensional structure, which suppresses the polarization dependence loss. Carbon nanotubes interact with photonic crystal fiber with ultra-weak evanescent field, which enhances the damage threshold of the saturable absorber and improves the operating stability. In our experiment, conventional soliton with a pulse duration of 1.003 ps and center wavelength of 1566.36 nm under a pump power of 240 mW is generated in a compact erbium-doped fiber laser cavity with net anomalous dispersion of −0.4102 ps2. The signal to noise ratio of the fundamental frequency component is ~80 dB. The maximum average output power of the mode-locked laser reaches 9.56 mW under a pump power of 360 mW. The output power can be further improved by a higher pump power.
© 2016 Optical Society of America
1. Introduction
All-fiber passively mode-locked lasers (MLs) have tremendous potential applications in ultrahigh speed optical communication, optical fiber sensing, material processing, precision metrology and micromachining, owing to the high output power with excellent efficiency [1,2]. The core of MLs is the saturable absorber (SA) which narrows the propagating pulse circularly. To achieve MLs with higher output power and narrower duration, various mode-locking methods have been proposed. Nonlinear effects of fiber are utilized for artificial saturable absorption effect, for instance, nonlinear optical loop mirrors [3–5] and nonlinear polarization rotations [6–10]. However, it is very difficult to design desired cavities due to the nondeterministic parameters, such as random birefringence and polarization. In recent years, several novel two-dimensional micro/nano-materials with superior optical characteristics are frequently used to fabricate SAs, such as graphene [11–15], graphene oxide [16,17], and topological insulators [18–22]. They have merits of large modulation depth, wide bandwidth, and wavelength-independent saturable absorption characteristics. However, those materials are sensitive to polarization for their two-dimensional structures, which makes them unreliable for non-polarization-maintaining laser cavity. Besides, the charcoal nano-paticle is also a nice choice for the fabrication of SA for the low-cost production technology [23]. But the use of the charcoal nano-particle is limited for its distorted carbon structure.
As a contrast, it has been shown that single-wall carbon nanotube (SWCNT) is ideal for passive mode-locking, due to the insensitivity to polarization of light for its one-dimension structure [24–33]. At present, there are mainly two methods for SWCNTs to interact with the propagating optical field, including the direct-transmission-structure (DTS) and the evanescent-wave-interaction-structure (EWIS). The usual scheme of SAs with DTS is the fiber ferule, and for those EWIS based SAs, the output power of MLs is restricted due to a pump power below ~100 mW [25–28]. Due to light passing through graphene film directly, DTS based SAs suffer the drawback of low thermal damage threshold. To enhance the damage threshold, SAs based on EWIS are frequently investigated. SAs based on tapered fiber and side-polished fiber interacting with SWCNTs have been proposed [30–34]. It employs the evanescent wave to interact with saturable absorption materials. However, they suffer from disturbance from outside airflow because of the fact that saturable absorption materials directly expose in the external environment. So special packages for those EWIS-based SAs are required. Moreover, relatively high polarization dependent loss (PDL) also cannot be ignored in side-polished fiber. For example, the fabricated SA in the Ref [34]. has a PDF of 5.56 dB for its asymmetric structure. Photonic crystal fiber (PCF) can provide saturable absorption materials with a closed environment and longer evanescent wave interaction length [11,18,35]. The SA with ultra-weak evanescent wave interaction can operate well even with a pump power of 7 W [35]. Compared with fiber tapers and side-polished fibers, PCF-based SAs have better mode-locking performance for its weak PDL and excellent operating stability.
In this article, a SA is fabricated by filling short SWCNT solution into cladding holes of grapefruit-type PCF with easiness. After being dried in a dry oven for hours, the liquid-inside PCF is evaporated. SWCNT particles are embedded into the cladding holes of PCF. Only a very weak evanescent wave of PCF interacts with SWCNT particles, which promises a high thermal damage threshold of the SA. The PCF filled with SWCNTs is inserted into a compact erbium-doped fiber ring laser cavity with anomalous group velocity dispersion of −0.4102 ps2. By a balance of nonlinear effects and group velocity dispersion, conventional soliton is generated in the laser cavity.
2. Experimental Setup
In our experiment, short SWCNTs (XFS05, XFNANO) with a length of 1~3 μm are in the form of black agglomerate powder. Therefore, it is impossible to fill SWCNTs directly into the holes of grapefruit-type PCFs, and SWCNT solution must be prepared. On account of the nanoscale size of SWCNTs, nanotubes are agglomerated by strong Van-der Waals force. We sonicate about 20 mg SWCNTs dispersed in 50 ml dimethyl formamide for two hours. After being centrifuged for 5 minutes, separated SWCNT particles suspend in the upper liquid, while massive particles precipitate on the bottom. As shown in the inset of Fig. 1(a), the upper liquid containing separated SWCNTs is transparent. Figure 1(a) depicts the Raman spectrum of SWCNTs. The intensity ratio of D band respect to G band is smaller enough (~10−3) to qualitatively manifest extremely high purity of SWCNTs. Furthermore, the radial breathing mode appears at ~220 cm−1, indicating diameters of SWCNTs mainly to be 1~2 nm. As the diameter of the SWCNT is almost three orders smaller than its length, it can be treated as a kind of one-dimensional material.
Fig. 1 (a) Raman spectrum of SWCNTs, and the inset is SWCNT solution. (b) PDL of the SA, and the inset is the PCF filled with SWCNT solution.
The fabricated SWCNT solution can be siphoned into the cladding holes of PCF with easiness. The PCF filled by SWCNT solution is shown in the inset of Fig. 1(b), where we can clearly observe columns of liquid inside the PCF. SWCNTs suspended in liquid will flow in the cladding holes and deteriorate the stability of the operating state. Moreover, when the liquid-filled-PCF is spliced between two single mode fibers (SMFs), larger splice loss will be induced. Therefore, we have to dehydrate the PCF in a dry oven for hours until the liquid column in PCF has been evaporated. As seen in Fig. 1(b), PDL of PCF after dehydrated is only 1~1.8 dB, which is detected by a polarization measurement system (PSGA-101-A, General Photonic Polawise). This value includes a PDL of 0.6~0.8 dB formed due to manual fusion of PCF and SMF. The circularly symmetric structure of PCF is destroyed for the collapse of holes caused by electrical discharge. In our recent work [35], PDL of the SA, which is fabricated by filling the reduced graphene oxide into PCF, is about 3 dB. This loss contains a splice loss of 0.5 dB. So compared with other SAs filled in two-dimension materials, the PDL is relatively low for the polarization-insensitivity of SWCNTs [24]. We notice that small PDL of the SA is still inevitable for the incomplete uniform distribution of SWCNTs in the 6 cladding holes of PCF. The uniformity of distribution can be further improved by enhancing the concentration of SWCNT solution.
Figure 2(a) shows the photograph of the cross section of the grapefruit-type PCF. To calculate the relative intensity of evanescent wave contacting with SWCNTs on the inner wall of holes, we calculate the fundamental mode distribution of PCF. As shown in Fig. 2(b), the simulation results indicate that energy is mainly concentrated in the center region of PCF. As we know, the structure of PCF is symmetric, and only the distribution of light intensity on the red cutting line passing through the center of PCF are needed to be taken for consideration. The ratio ξ defined as the light intensity of inner walls of cladding holes to that in the center is 7.7 × 10−7, indicating an ultra-weak interaction between light and the saturable absorption material. The distribution of SWCNTs has no effect on the light mode inside PCF for so weak energy of the evanescent wave experienced by SWCNTs. The saturable absorption material makes a decisive contribution to the nonlinearity of the SA [36], and the nonlinearity is positively correlated to the interaction. Therefore, ξ is treated as a reference parameter of nonlinearity. So small value of ξ implies diminished nonlinearity of SA and ensures that the SA can be robust even under a very high pump power.
Fig. 2 (a) Cross section of PCF. Diameters of the center region and inner cladding are about 3 μm and 18 μm, respectively. (b) Fundamental mode distribution of the PCF, and the distribution of light intensity on the red cutting line, where Ictr and Ih are the intensity in the center of PCF and the inner wall of holes respectively, and ξ is defined as the ratio of Ih and Ictr.
The schematic diagram of the ring laser cavity is shown in Fig. 3(a). We use 1 m erbium-doped fiber (E08-A352A-01-1B22) with a dispersion coefficient of 15.7 ps/nm/km as the gain medium. To ensure enough anomalous dispersion of the cavity, ~17 m SMF with dispersion coefficient of 18 ps/nm/km is used in the laser cavity. The net cavity dispersion τ2 is −0.4102 ps2 calculated by the Eq. (1):
where λ = 1550 nm is the operating wavelength of the ring cavity. De and Ds are the dispersion coefficient of PCF and SMF, respectively. Le and Ls are defined as the length of the erbium-doped fiber and SMF. The 980 nm continuous wave pump laser is connected with the cavity by a 980/1550 nm wavelength division multiplexer (WDM). A polarization controller (PC) is used to adjust the PDL and stochastic dispersion in the cavity. The feedback and output of the ML are achieved by a 90/10 coupler inserted between the PC and the isolator. A polarization-independence isolator is employed to ensure the unidirectional operation of the ML and protects the pump laser from the reflected light induced by other optic devices. The SA, which is fabricated by filling SWCNTs into the PCF, has a modulation depth of about 2% and is spliced into the cavity with an insertion loss of ~3.5 dB. The modulation depth of the fabricated SA is measured by the two-balanced detection method [14]. Figure 3(b) shows the relatively flat transmission spectrum of the SA, and no interference phenomenon is observed. The insertion loss is measured by an optical spectrum analyzer (OSA, SI720, Micron Optics) with a broadband light source ranging from 1520 nm to 1570 nm. As seen in the insert inside the dashed box, the length of PCF with deformed holes is 224 μm, which indicates a very short length of deformed PCF, compared with the whole 3.7 cm PCF. When PCF is spliced with SMF, only the distribution of SWCNTs nearby the end face of PCF will be influenced. Therefore, the overwhelming majority of SWCNTs are firmly embedded in the PCF and have a stable distribution before and after the PCF spliced. We use an OSA (Q8384, Advantest) and an autocorrelator (Pulse Check, APE) to observe the output optical spectrum and measure the duration of pulses, respectively. Besides, the radio frequency (RF) spectrum and pulse train of the ML are measured by a frequency analyzer (DSA815, Rirol Tech) and a real time oscilloscope (Infiniium MSO 9404A, Agilent Tech).Fig. 3 (a) The schematic diagram of the ML. As seen in the dashed box, only the evanescent wave escaping from the center region of PCF interact with SWCNTs embedded in holes. (b) The insertion loss of PCF monitored by the OSA (SI720).
3. Experimental results and discussion
Figure 4(a) shows the average output power of the ML with different pump powers from 40 mW to 360 mW with a step size of 40 mW. The maximum average output power obtained in the experiment is 9.56 mW. The fitting curve indicates that the average output power of the ML has a linear relationship with the pump power, and thus the pump efficiency ηoutput is ~2.3%, which is restricted by the total loss of the laser cavity. The efficiency can be enhanced by optimizing the splice parameter of PCF and selecting EDF with higher doping concentration. Figure 4(b) shows output spectra for different pump powers, where shapes of spectra almost remain the same. However, the full width at half maximum (FWHM) of the spectrum broadens with an increased pump power, owing to the enhanced net optical gain and increased synchronous oscillating modes [11]. The center wavelength almost keeps unchanged, which is shown as the black line with rectangular dots in Fig. 4(c), indicating a stable mode-locking state of the laser. When the pump power is higher than 200 mW, the FWHM remains the same, due to the limit of the gain bandwidth. The maximum value measured in our experiment is 5.16 nm, as marked by the blue line with dots in Fig. 4(c). Figure 4(d) shows the spectrum of the output pulse with a pump power of 240 mW, where the center wavelength is 1566.36 nm, and the FWHM of the spectrum is 5.16 nm. The symmetrical Kelly sidebands in the spectrum prove that the conventional soliton is generated in the ring cavity with anomalous dispersion. Sidebands are caused by interference between the pulse and the dispersive wave radiated by the pulse which are periodically perturbed by gain stimulated by the erbium-doped fiber and loss from the laser output [37].
Fig. 4 (a) Output powers Vs. pump powers. Pout and Ppump are the output power and the pump power respectively. ηoutput is the slope of the fitting curve. (b) The output spectra for different pump powers. (c) The FWHM and center wavelengths of spectra for different pump powers. (d) The output spectrum for a pump power of 240 mW.
Figure 5(a) depicts the pulse train in time domain. The period of the pulse train is 110.25 ns, which is determined by the total length of the cavity. This value approaches to the theoretical result calculated by Eq. (2):
where c and n are the speed of light in vacuum and the refraction index of the fiber respectively, and Ltotal is defined as the total length of the laser cavity. It can be seen that the fundamental frequency pulse is obtained in the cavity, and the single pulse energy is estimated to be 661.3 pJ. The repetition frequency of the output pulse train keeps unchanged, when the pump power varies from 80 mW to 360 mW. On this occasion, no harmonic mode-locking pulse appears for the reduced accumulated nonlinear effect of the SA, and single pulse energy is enhanced with an increased pump power. In order to observe the output stability, the corresponding RF spectrum with a range of 100 MHz is measured. As shown in Fig. 5(b), the frequency interval between adjacent frequency components is 9.1 MHz, which is measured under a span resolution of 1 KHz, perfectly consisting with the results in the time domain. The signal to noise ratio (SNR) of the fundamental frequency component reaches ~80 dB, which is higher than that in Ref [34,35], indicating a good stability of output [25,38].Fig. 5 (a) The pulse train in the time domain of ML. (b) The RF spectrum of pulses at the fundamental frequency of 9.1 MHz, and the inset is the RF spectrum of pulses within 100 MHz. (c) The autocorrelation trace of pulse and the sech2 fitting curve. (d) The pulse duration under different pump powers.
As shown in Fig. 5(c), the autocorrelation trace of the soliton pulse is fitted by the sech2-shape curve with a FWHM of 1.548 ps, corresponding to a pulse duration of 1.003 ps. The relatively large net anomalous dispersion of −0.4102 ps2 causes the pulse widening. Sub 200 fs pulse is generated from a graphene mode-locked laser with a net dispersion value of −0.052 ps2 [39]. However, for their laser cavity, the single pulse power is relatively restricted for a pump power of less than 50 mW. The soliton condition claims that the production of pulse duration and energy is determined by the group velocity dispersion and nonlinearity of the laser cavity [40]. In this case, the narrow pulse duration is obtained in the cavity with small net dispersion for low pulse energy. However, in order to satisfy the generation condition of the conventional soliton, additional anomalous dispersion for our cavity is necessary to balance the enhanced self-phase modulation effect caused by the high pulse peak power. The narrower pulse duration can be obtained by improving the nonlinear transmittance of the SA [39]. Thus the time bandwidth production of output pulse is 0.633, which is larger than standard conventional soliton with a sech2 shape. This implies a large phase chirp in the pulse. Chirps in the pulse are accumulated because of large anomalous dispersion of the laser cavity and self-phase modulation excited by the high pulse peak power. Moreover, with the increase of the peak power, the stable balance between self-phase modulation and dispersion is disturbed. Figure 5(d) shows pulse durations with different pump powers, and the changing tendency of the pulse duration is almost opposite to the FWHM of the spectrum. When the pump power is more than 240 mW, the pulse duration slightly broadens due to enhanced spectral sidebands and nonlinear frequency chirps.
4. Conclusions
We have fabricated an all-fiber passively mode-locked laser based on a SWCNT-PCF SA. Weak evanescent-wave-interaction for SWCNTs enhances the thermal damaged threshold of the SA. Besides, the one-dimension structure of SWCNT ensures its polarization-insensitivity to diminish PDL of the SA. Conventional soliton with fundamental frequency is generated in the ring cavity with anomalous dispersion. A maximum power of the ML is 9.56 mW with a pump power of 360 mW. Due to the relatively large dispersion in the cavity and pulse chirps, the narrowest pulse duration of 1.003 ps is obtained. Furthermore, a SNR of 80 dB in the RF spectrum indicates a good operating state of the ML. The fabricated SA has various potential applications in the high power and ultrafast lasers with high stability for a high damage threshold and a reduced PDL.
Funding
Natural Science Foundation of China (NSFC) (61575190, 61405020, 61635004); Science Fund for Distinguished Young Scholars of Chongqing (CSTC2014JCYJJQ40002); Project of Natural Science Foundation of Chongqing (No. CSTC2013JCYJA40029).
Acknowledgement
Thanks for the device support from Prof. Fei Wang of Chongqing University of Technology.
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