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Abstract
We demonstrate a tunable and switchable dual-waveband 100 GHz high-repetition-rate (HRR) ultrafast fiber laser based on dissipative four-wave-mixing (DFWM) mode-locked technique. Each waveband maintains HRR operation. The DFWM effect was realized by combining a Fabry-Perot (F-P) filter and a piece of highly nonlinear fiber (HNLF). The tunable and switchable operations were achieved by nonlinear polarization rotation (NPR) technique. Through appropriately controlling the filtering effect induced by NPR, the laser could operate at two kinds of tunable regimes. One is that the spacing between these two wavebands could be tuned while keeping their center at 1559 nm. The other is that the central position of the entire dual-waveband is tunable while with the same separation between these two wavebands of 13.2 nm. Moreover, the laser could switch between these two wavebands. Correspondingly, the center of the single-waveband has a tuning range of 15.2 nm. This versatile ultrafast fiber laser may find applications in fields of optical frequency combs, high speed optical communications, where HRR pulses are necessary.
© 2017 Optical Society of America
1. Introduction
High-repetition-rate (HRR) pulse sources have attracted considerable attention for their various applications ranging from industrial purposes to fundamental research, such as optical spectroscopy, clock generation and nonlinear optics [1–5]. As ideal optical sources of generating HRR pulses, passively mode-locked fiber lasers have been widely investigated in recent years [6–9]. In order to improve the pulse repetition rate, a diverse range of techniques have been developed, such as shortening the cavity length, generating high-order harmonic and utilizing dissipative four-wave-mixing (DFWM) effect [10–13]. Among them, DFWM technique is simple and practicable, which could generate ultrahigh repetition rate pulse even up to one thousand GHz, determined merely by the free spectral range of the incorporated comb filter. Over the past years, GHz HRR fiber lasers based on DFWM have been researched intensively. M. Peccianti et al. realized a stable 200 GHz HRR ultrafast laser using a nonlinear micro-cavity which simultaneously possesses highly-nonlinear and comb filtering effects [12]. D. Mao et al. demonstrated a repetition-rate tunable HRR pulse fiber laser incorporating an intra-cavity tunable Mach-Zehnder interferometer (MZI) at high pump power [13]. Recently, graphene has been regarded as a promising nonlinear optical material which stimulates a variety of novel optical applications in photonics community, such as Y. -L. Qi et al. employed a graphene-deposited microfiber photonic device (GMPD) and a Fabry-Perot (F-P) comb filter in a fiber laser to obtain a 100 GHz pulse train [7]. Nonetheless, the accurate nonlinear coefficient of the GMPD could not be provided since the deposited amount of graphene could not be exactly measured and controlled, which limits its applications in wide fields. Since it normally needs high nonlinear effect in the laser cavity to achieve DFWM mode-locking operation, therefore, employing a comb filter combining a piece of highly nonlinear fiber (HNLF) in a laser would be more credible and simpler way to obtain GHz HRR pulses.
On the other hand, multi-channel/waveband GHz HRR ultrafast fiber laser would open up new applications in such fields as optical frequency combs and optical communications. However, as to our best knowledge, GHz HRR fiber lasers generally operate at single channel/waveband and were well developed as mentioned above. Generally speaking, to achieve multi-channel/waveband operation in an erbium-doped fiber laser (EDFL), it always needs an intra-cavity comb filter to select the multiple lasing lines. At the same time, the strong homogeneous line broadening of erbium-doped fiber (EDF) should be suppressed to attain the stable operation at room temperature. Up to date, the mode-locked multi-channel EDFLs almost operate at repetition-rate of MHz [14–17]. Considering the potential applications of GHz HRR pulse sources, if a versatile multiple channels/wavebands GHz HRR pulses source is explored, it would be great meaningful for the ultrafast laser community.
In this work, we address this issue. Herein, we propose a tunable and switchable dual-waveband all-fiber ring laser delivering 100 GHz HRR pulse trains, in which an intra-cavity birefringence-induced comb filter, i.e. NPR-based ring cavity, plays a crucial role in the realization of the tunable and switchable dual-waveband operation. In our laser, a F-P comb filter combining with a piece of HNLF acts as the key element for achieving the DFWM effect. By appropriately rotating the polarization controllers (PCs), the dual-waveband HRR pulse laser could operate at two kinds of tunable regimes. One is that the spacing between these two wavebands could be flexibly tuned from 13.2 nm to 16 nm while the central position of the entire dual-waveband always remained at 1559 nm. The other tunable regime is that the central position of the entire dual-waveband could be tuned from 1552.5 nm to 1559.3 nm but with the same separation between these two wavebands of 13.2 nm. Moreover, the two wavebands could be switched and the center of the single-waveband could be continuously tuned from 1549.7 nm to 1564.9 nm. This kind of fiber laser with a simple and economic construction may provide more applications for GHz HRR pulses.
2. Experimental setup and operation principle
The schematic diagram of the proposed dual-waveband HRR pulse fiber laser is shown in Fig. 1. An 11 m long EDF pumped by a 980 nm laser through a wavelength-division multiplexer (WDM) serves as the gain medium. Two PCs are used to adjust the polarization state of the propagating light. A polarization-dependent isolator (PD-ISO) assures unidirectional operation of the ring cavity. An 85 m long HNLF with zero dispersion at 1550 nm acts as the nonlinear element to induce the DFWM effect. A fiber pigtailed F-P filter with spectral spacing of 0.8 nm is employed to select the multiple lasing lines, which determines the repetition-rate of the pulses. The fineness and the contrast ratio of the filter are 13 and 23.4 dB, respectively. The total length of the cavity is 115 m. Here, the PD-ISO combining with the intra-cavity birefringence forms a tunable coarse comb filter [15-16], where the location of the transmission peak and the spectral spacing can be flexibly tuned by rotating the PCs. In addition, through a 10% output optical coupler (OC), the laser output was measured by an optical spectrum analyzer (OSA, Yokogawa AQ6317C) and a commercial autocorrelator (FR-103XL).
With proper conditions, the DFWM would intrinsically lead to the generation of phase-coherent sidebands with equal spacing. As all the sidebands keep a regular phase relationship, DFWM could induce passive mode-locking with HRR pulse defined by the spectral spacing of the lasing lines [7, 12-13]. In our laser, a F-P filter helps to easily realize DFWM effect by combining the HNLF, where the repetition rate of the pulse is determined by the free spectral range of the filter. In addition, the laser is a typical setup based on NPR technique. As we know, the NPR-based ring cavity can be equivalent to a length of birefringent fiber with two polarizers at both ends, which can be treated as a Lyot birefringence filter [15]. The transmission function of this intra-cavity birefringence-induced comb filter can be described as [18–20]:
Where, and are the angles between the polarization directions of the polarizers and the fast axes of the fiber, which could be adjusted by rotating the PCs. and are the linear and the nonlinear cavity phase delay, respectively. λ is the operating wavelength, L is equal to the length of the laser cavity, n2 is the nonlinear refractive index, P is the instantaneous power of input signal, and Αeff is the effective core area. Therefore, the transmission coefficient varies periodically with respect to the wavelength. The channel spacing of such a comb filter is mainly dependent on the intra-cavity birefringence, which can be determined by ( is the average cavity fiber birefringence). In our laser, the birefringence is caused by the fiber squeezing of the PCs and the fiber winding. From Eq. (1), one knows that once the cavity birefringence and the angles of and change, the spectral spacing and the transmission peak positions of the intra-cavity birefringence-induced comb filter would correspondingly vary. These could be utilized to realize the tunable operations of the laser. It should be noted that such a comb filter is a coarse one. Therefore, by combining the intra-cavity birefringence-induced comb filter and the DFWM mode-locking technique, we could achieve the tunable and switchable dual-waveband HRR pulses in the proposed fiber laser through carefully adjusting the cavity parameters.3. Experimental results
3.1 Dual-waveband HRR pulses operation
Benefiting from the coarse comb filter based on NPR and the DFWM mode-locking, dual-waveband HRR pulse trains were always achieved in our fiber laser by precisely adjusting the cavity parameters. However, the dual-waveband HRR pulses were weak at a pump power of 80 mW. To obtain higher quality HRR pulses, we increased the pump power to 320 mW and fixed it in the following experiments, which was sufficient to induce strong nonlinear effects in the cavity. Figure 2 shows the typical operation of dual-waveband HRR pulses trains. As shown in Fig. 2(a), the optical spectrum exhibits comb profiles with 0.8 nm spectral spacing determined by the F-P comb filter. Note that, the entire comb spectrum was reshaped into two wavebands envelope with central positions of 1552.1 nm and 1563.3 nm respectively, realized by the intra-cavity birefringence-induced comb filter based on NPR effect. In addition, the laser is very stable since all the sidebands keep a regular phase relationship due to the DFWM effect and there exists the intensity-dependent loss mechanism induced by the NPR technique [15, 21–24]. Figure 2(d) exhibits the matching autocorrelation trace of the dual-waveband HRR pulses trains. Obviously, the period of the pulse train is 10 ps, corresponding repetition rate is 100 GHz, which is well consistent with the spacing of 0.8 nm for the lasing lines. Note that the supermode noise can be seen in the background of the autocorrelation traces. Since the laser cavity is as long as 115 m, it could be estimated that there are about 4000 longitudinal modes falling in a single spectral peak, which leads to supermode noise. In order to reduce the supermode noise to improve the quality of the high repetition-rate pulses, one could shorten the length of the laser cavity, adopt higher fineness comb filter or incorporate a subcavity inside the main laser cavity to decrease the amount of cavity modes [3].
Fig. 2 Dual-waveband HRR pulses operation. (a) The total spectrum, (b) and (c) the filtered spectra, (d)-(f) corresponding autocorrelation traces.
In order to confirm both wavebands operate at 100 GHz repetition rate, an optical tunable filter (Santec OTF-350) was employed outside the laser cavity to filter out each waveband. As the bandwidth and central wavelength of the extra-cavity band-pass filter being adjusted precisely, single waveband was obtained by filtering operation. The corresponding filtered optical spectra are depicted in Figs. 2(b) and 2(c). Here, the intensities of each lasing line reduced a little because of the loss induced by the extra-cavity filter. Figures 2(e) and 2(f) illustrate the relevant autocorrelation traces whose pulse separations are also 10 ps. In this case, we only adjusted the polarization of the input pulses to improve the relative intensity of the autocorrelation traces and fixed the other parameters of the autocorrelator in our experiment. These results proved that the proposed fiber laser delivers 100 GHz HRR pulse train at each waveband through DFWM mode-locking technique.
3.2 Waveband spacing tunable operation
Note that the PCs used in our fiber laser are fiber-squeezing ones. Therefore, the cavity birefringence and the polarization state of the propagating light could be flexibly adjusted in coarse and subtle way [15], which contributes to the different regimes in the dual-waveband mode-locked fiber laser. From Eq. (1), we know that the spectral spacing of the intra-cavity birefringence-induced comb filter can be changed through varying the cavity birefringence. The smaller the cavity birefringence corresponds the larger the spectral spacing. Therefore, the spacing between these two mode-locked wavebands of the laser would correspondingly change. Figure 3 shows the waveband spacing tunable operation by carefully rotating the PCs. As can be seen from Fig. 3, initially the central separation between these two wavebands was 13.2 nm. Then these two wavebands shifted towards the opposite direction with slowly adjusting the orientations of the PCs, while the central position of the entire dual-waveband always remained at 1559 nm. The maximum waveband spacing could extend to 16 nm, which was mainly limited by the cavity loss and the bandwidth of the gain medium. Note that the 3-dB bandwidths of the spectra at each waveband have small differences during the tuning process, which might originate from the wavelength-dependent gain. As we know, the NPR-based ring cavity acts as a coarse spectral filter whose transmission coefficient varies periodically with the wavelength [15]. In addition, by taking the intrinsic advantage of the intensity-dependent loss induced by the NPR technique [21–24], the mode competition could be efficiently suppressed and the dual wavebands operated stably at room temperature.
3.3 Central position tunable operation
Interestingly, there is another tunable regime in our laser, namely, the central position of the entire dual-waveband is tunable while keeping the same spacing between these two wavebands. The central position tunable evolution of the dual-waveband is shown in Fig. 4. Here, the initial central position of the entire dual-waveband is at 1552.5 nm. During the whole tuning process, these two wavebands acted as a whole and shifted towards longer wavelength by changing the orientations of the PCs precisely. The continuously tunable central position could reach up to 1559.3 nm. Note that the spacing between these two wavebands always remained at 13.2 nm. We believe that this tunable operation was also realized by the intra-cavity birefringence-induced comb filter based on NPR technique. However, in this case, the rotating of the PCs mainly changed the angles of and , resulting in the variation of the transmission peak positions Therefore, these two wavebands could keep the same separation and be tuned as a whole. Certainly, the tuning range was confined not only by the bandwidth of the gain medium, but also by the adjusting range of the PCs which had to maintain the cavity birefringence unchanged.
3.4 Waveband switchable and single-waveband tunable operations
As we rotated the PCs, the spectral spacing and the transmission peak positions of the coarse comb filter based on NPR effect can be tuned, resulting that the cavity loss varies with the waveband. Supposing that the cavity loss for one waveband is sufficiently large, the lasing at this waveband is suppressed. Thus, the switchable operation between the dual wavebands of the HRR pulse laser could be achieved, as shown in Fig. 5. Figures 5(a) and 5(d) show the spectrum and the corresponding autocorrelation trace of the dual-waveband HRR pulse trains, whose wavebands spacing is 15.2 nm and the central positions of each waveband are 1549.7 nm and 1564.9 nm, respectively. The spectra of waveband switchable operation are illustrated in Figs. 5(b) and 5(c). The intensity of the entire spectral lines decreases from center to edges significantly, which is the typical characteristic of DFWM mode-locking. Noting that the central positions of the single waveband spectra had a small drift, which is due to the changing of the transmission peak position of the intra-cavity birefringence-induced comb filter by rotating the PCs. Furthermore, since the laser emitted only one waveband, the number of the lasing lines also increased at the same pump power. Correspondingly, the autocorrelation traces with period of 10 ps are depicted in Figs. 5(e) and 5(f), indicating the laser operating at single waveband still maintains HRR pulse regime.
Fig. 5 Switchable operation of the dual-waveband HRR pulses. (a) The entire spectrum, (b) and (c) the switched spectra, (d)-(f) corresponding autocorrelation traces.
For better investigating the performance of the single-waveband HRR pulse train, we further adjusted the PCs. Figure 6(a) illustrates a typical spectrum for single-waveband HRR pulse with the central position of 1553.3 nm. The matching autocorrelation trace is shown in Fig. 6(b), whose pulse separation is 10 ps, which indicates that the fiber laser still delivers 100 GHz HRR pulse train. In this case, we repeatedly scanned the laser output at 5-min interval in 75 minutes, as shown in Fig. 6(c). There are no evident intensity fluctuations and wavelength drifts, verifying that the laser is pretty reliable. Furthermore, benefiting from the transmission peak position of the intra-cavity birefringence-induced comb filter can be tuned by rotating the PCs, the single-waveband tunable operation could also be achieved in the proposed fiber laser. Figure 6(d) shows that the center of the single-waveband could be continuously tuned from 1549.7 nm to 1564.9 nm by carefully manipulating the PCs.
Fig. 6 Single-waveband HRR pulse operation. (a) Spectrum, (b) corresponding autocorrelation trace, (c) stability measurements, (d) tunable operation.
4. Discussions
In the laser, with appropriate settings of the parameters, i.e. the nonlinear effect and the filter, the DFWM would result in the generation of phase-coherent sidebands with equal spacing. As all the sidebands have a regular phase relationship, DFWM could induce passive mode-locking with pulse repetition-rate defined by the spectral spacing of the lasing lines. In our experiments, we adopted a F-P filter and a piece of HNLF to help realize DFWM effect, where the repetition-rate of the pulse is determined by the F-P filter. In addition, the intra-cavity birefringence-induced spectral filter based on NPR effect exerts a coarse comb filtering effect on the lasing lines, which makes the dual-waveband operation possible. However, the main challenge of stable multi-waveband EDFLs at room temperature is the strong homogeneous line broadening. Here, our laser efficiently suppressed the mode competition and obtained the stable dual-waveband passively mode-locked operation at room temperature by taking the intensity-dependent loss mechanism induced by the NPR technique [15]. Furthermore, because the transmission peak location and the spectral spacing of the coarse comb filter vary with the rotations of the PCs, the central position and the spacing of the lasing wavebands of HRR pulses can be continuously tuned by carefully adjusting the PCs, as well as flexibly switched. In order to confirm that the tunable and switchable operations of the dual-waveband HRR pulse fiber laser were really caused by the intra-cavity birefringence-induced coarse comb filter based on NPR effect, we removed the F-P filter and measured the laser output. The laser operated at mode-locked dual-wavelength regime, which could be tuned and switched by adjusting the PCs. These comparative results demonstrated that the coarse comb filter based on NPR effect indeed contributed to the tunable and switchable operation of the dual-waveband HRR pulse laser. It should be noted that the stability of the NPR mode-locking technique is not better than the real saturable absorber, such as carbon nanotube (CNT) [25–28]. Here, we use the filtering effect induced by the NPR to achieve dual-waveband and tunable/switchable operations in our fiber laser [15].
To extend the applications of HRR pulses, the tunable repetition-rate, more wavebands and lower pump power in the HRR ultrafast fiber laser should be explored in the future research. Since the repetition-rate for pulses is merely determined by the incorporated comb filter. Therefore, exploring the comb filter with higher fineness and flexible tuning spectral spacing is worthwhile for generation of tunable repetition-rate pulses with high performance. Note that, the number of wavebands is related with the intra-cavity birefringence-induced spectral filter in our NPR-based cavity. So that, more wavebands could be obtained through precisely optimizing the cavity parameters such as the fiber length, the fiber squeezing of the PCs and the fiber winding. In addition, the HRR pulse fiber laser could operate at a lower pump power if the cavity loss induced by the elements in the laser was efficiently reduced.
5. Conclusion
We demonstrate the generation of tunable and switchable dual-waveband 100 GHz HRR pulses in a fiber laser by utilizing DFWM mode-locking technique and NPR effect. Each waveband operates at HRR mode-locking regime. Benefiting from the birefringence-induced spectral filtering effect based on NPR, the spacing between these two wavebands could be extended continuously from 13.2 nm to 16 nm and the central position of the entire dual-waveband could be tuned flexibly from 1552.5 nm to 1559.3 nm through precisely adjusting the PCs. Moreover, a switching operation between these two wavebands could be easily realized and the center of the single-waveband could shift continuously from 1549.7 nm to 1564.9 nm. Our research provides a simple and economic method for generating tunable and switchable dual-waveband HRR pulses, which might extend its applications where multi-waveband HRR pulses are necessary.
Funding
National Natural Science Foundation of China (Grant Nos. 61378036, 61307058, 11304101, 11474108); Science and Technology Program of Guangzhou (Grant No. 201607010245); Key Program of Natural Science Foundation of Guangdong Province (Grant No. 2014A030311037); Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No. 2014A030306019); Program for the Outstanding Innovative Young Talents of Guangdong Province (Grant No. 2014TQ01X220); Program for Outstanding Young Teachers in Guangdong Higher Education Institutes (Grant No. YQ2015051); Top-Notch Graduate Foundation of South China Normal University.
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