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We demonstrate a passively harmonic mode-locked (PHML) fiber laser operating at the L-band using carbon nanotubes polyvinyl alcohol (CNTs-PVA) film. Under suitable pump power and an appropriate setting of the polarization controller (PC), the 54th harmonic pulses at the L-band are generated with the side mode suppression ratio (SMSR) better than 44 dB and a repetition frequency of 503.37 MHz. Further increasing the pump power leads to a higher frequency of 550 MHz with compromised stability of 38.5 dB SMSR. To the best of our knowledge, this is the first demonstration on the generation of L-band PHML pulses from an Er-doped fiber laser based on CNTs.
With the widespread application of optical communication systems, the conventional C-band (1530–1565 nm) is unable to meet the ever-increasing requirements for transmission capacity. To alleviate this problem, the L-band (1565–1625 nm) as an extended wavelength range has been proven to be able to enlarge the optical communication capacity, where the silica fibers also feature low loss.[1] Moreover, it is worth noting that some potential applications such as high speed optical sampling, frequency comb generation, and optical communication have a high demand for repetition rate.[2–4] In consequence, exploring the mode locked fiber laser which combines two features of L-band operation and high repetition rate is of great importance.
In general, mode locked operation can be realized either actively or passively. Compared to active mode locking, passive mode locking is more desirable since it offers great merits of compactness, simplicity, stability, and better pulses quality.[5] Nowadays, various technologies have emerged to realize passive mode locking, for instance, the semiconductor saturable absorber mirror (SESAM),[6] nonlinear polarization rotation (NPR),[7] carbon nanotubes (CNTs),[8] and some other novel materials.[9,10] Especially, CNTs are identified as effective mode lockers since they exhibit distinct merits of ultra-short recovery time, easy fabrication, and wide operation wavelength range.[11] Previous studies have been successfully conducted at the L-band with fiber lasers based on CNTs. Sun et al. reported an L-band fiber laser firstly by using a carbon nanotubes-polyvinyl alcohol (CNTs-PVA) saturable absorber (SA) with a broad absorption at 1.6 μm.[12] In 2013, a passively mode-locked L-band fiber laser using a 180 cm highly bismuth–erbium-codoped fiber as the gain medium was presented, with a given pulse energy of 440 pJ and a pulse duration of 460 fs.[13] In order to achieve ultra-short pulses in this region, Kwon et al. designed a stretched-pulse L-band laser generating 110 fs pulse with a 70 dB signal-to-noise ratio (SNR).[14] However, all of the reports do not concentrate on the repetition rate of the lasers, where the frequency is limited at tens of MHz. Ordinarily, passively harmonic mode-locked (PHML) technology is deemed as one of the most efficient methods to achieve a high repetition rate, where the frequency of the laser can be multiplied when the pump power exceeds a certain value.[15] Nevertheless, there is no report on the generation of PHML pulses operating at L-band from an Er-doped fiber laser using CNTs.
In general, there are two dominant types of CNTs SAs, namely, CNTs film and evanescent-field interaction CNTs. Compared to the evanescent-field interaction CNTs, the CNTs film possesses inherent advantages of easier fabrication procedure, higher flexibility, and compactness, which can be integrated to the cavity just by being inserted between two fiber connectors.[16] Recently, we have demonstrated that the CNTs film can serve as a practical SA to realize a high repetition rate in C band via HML.[17] In this paper, we demonstrate a PHML fiber laser performing in the L-band based on a CNTs-PVA film. Under advisable pump power and polarization state, 503.37 MHz pulses with a 44.7 dB side mode suppression ratio (SMSR) centered at 1595.54 nm are obtained, which corresponds to the 54th harmonic order. It should be emphasized that the level of SMSR reveals the excellent stability of our laser. When the pump power increases up to 212.8 mW, 550 MHz pulses with 38.5 dB SMSR are further achieved. To the best of our knowledge, such high repetition rate pulses centered at the L-band are first realized from a PHML Er-doped fiber laser using CNTs.
Single wall carbon nanotube fabricated by the high-pressure CO (HiPCO) method is used in the experiment. The detailed fabrication procedure of CNTs-PVA film is summarized in Ref. [18]. The resultant CNTs-PVA film is characterized by the absorption spectrum as illustrated in Fig.
![]() | Fig. 1. (color online) The characteristics of CNTs-PVA film: (a) the linear absorption spectrum, (b) the measured Raman spectrum, and (c) the nonlinear transmission. |
The experimental configuration of the proposed PHML fiber laser based on the CNTs-PVA film is illustrated schematically in Fig.
The pulse signals can be visualized on an 8 GHz oscilloscope (OSC, KEYSIGHT DSO90804A) together with a 12.5 GHz photo-detector (PD, Newport 818-BB-51F). The radio frequency spectrum is recorded by a radio frequency (RF) spectrum analyzer (SIGLENT, SSA 3032X). The pulse spectrum is characterized by an optical spectrum analyzer (OSA, Yokogawa AQ6370C). Also, an autocorrelator (FEMTOCHROME, FR-103WS) is utilized to measure the pulse duration.
The mode-locked behavior occurs when the pump laser delivers up to 80 mW. However, the stable single pulse operation is hard to realize, no matter how to adjust the polarization state under the weak pump power. We conjecture that the phenomenon comes from the highly nonlinear effect introduced from the longer EDF, which features a high doping concentration and a relatively smaller mode field diameter as mentioned above. When the pump power reaches 92 mW, the 11st HML is implemented eventually with the proper setting of PC. Further increasing of the pump power will lead to a higher harmonic order. When the pump power is set to 202 mW, the laser operates at the harmonic of 54th with 5.35 mW output power, corresponding to a 503.37 MHz repetition rate, as shown in Fig
Additionally, the repetition rate can scale up to 550 MHz at 59th harmonic under 213 mW pump power. From Fig.
A PHML fiber laser operating at the L-band based on CNTs-PVA film is experimentally demonstrated. The 54th harmonic pulses at a repetition rate of 503.74 MHz centered at 1595.54 nm with 44.7 dB SMSR are obtained under appropriate polarization status and suitable pump power. The highest recorded repetition rate in our laser is 550 MHz with compromised stability when the pump power increases to 213 mW. To the best of our knowledge, this is the first report about the achievement of HML operation at the L-band in an Er-doped fiber laser using CNTs. The noteworthy stability of the laser is demonstrated by the high level of SMSR. The laser we proposed here features favorable performances for some applications such as L-band optical communication systems, spectroscopy, etc.
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