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Abstract
We demonstrate the dual-wavelength quasi mode-locked operation of a passively mode-locked Er-doped fiber ring laser with twisted fiber in the cavity and a double-pass amplifier with a Faraday mirror. The configuration allows very strict control of the polarization. We observed synchronous dual-wavelength generation of noise-like pulses (NLP) with spectra centered at 1534 nm and 1562 nm. Rotation of the azimuth of the radiation propagating in the cavity caused a switching between the generation of NLPs and packets of pulses with typical solitons spectrum. In the latter regime, we observed partially synchronous sub-pulses at two different wavelengths.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Passively mode-locked fiber lasers (PMLFL) allow the generation of coherent and stable ultrashort optical pulses, including conventional (conservative) solitons, dispersion-managed solitons [1], and different types of dissipative solitons [2], which include soliton molecules [3] and dissipative soliton resonance (DSR) solitons [4]. Besides the generation of stable waveforms, PMLFL can operate in regimes such as soliton rain [5] or noise-like pulses (NLP) [6], as well as a symbiotic regime [7,8]. The regime of generation depends on many factors, such as cavity dispersion, amplification of the active fiber, spectral selectivity, attenuation caused by all elements inserted in the cavity, birefringence ,and nonlinearities [7,9]. In spite of numerous demonstrations of the different regimes of PMLFL operation, it is not clear yet which experimental conditions lead to a determined regime of operation of PMLFLs. One of the reasons of this situation is the difficulty to perform a precise polarization control in fibers. The common way to control polarization is to use polarization-maintaining (PM) fibers. However, these fibers cannot be used in lasers exploiting nonlinear polarization rotation for mode-locking. We have to note that an attempt was made to use PM fibers in lasers exploiting nonlinear polarization evolution in PM fiber (which is different from nonlinear polarization rotation) [10]. However, this configuration has no flexibility to change the operation from one regime to another in the same cavity, which is an important advantage of PMLFLs. Another method to control polarization is the use of twisted fibers [11]. Fiber twist cancels linear birefringence and makes stable the polarization ellipticity along the fiber.
Generally, single-wavelength pulses have been generated by PMLFLs. However, several dual- or multi-wavelength PMLFLs have also been reported [12–20]. Stable, compact, and flexible optical pulse sources simultaneously operating at multiple wavelengths have various potential applications. In most works published on the subject, asynchronous pulses were observed, i.e. pulses with different wavelengths traveling at different velocities in the laser cavity. Lasers producing asynchronous pulses were shown to be effective pulse sources for fast and long-range pump-probe techniques [21], for measurements of absolute frequency of continuous-wave terahertz radiation in real time [22], and for picometer-resolution dual-comb spectroscopy [23]. Synchronous pulses can also be used for applications such as Raman spectroscopy [24], terahertz generation [25], and timing distribution [26]. Several approaches using the synchronization of lasers with different cavities were discussed [19,20]. It is much more challenging to obtain synchronous generation of pulses with different wavelengths in a single cavity using the same path for both pulses [15]. In this paper, synchronous generation was demonstrated in a cavity with low anomalous dispersion.
In fiber ring lasers based on nonlinear polarization rotation (NPR), both ellipticity and azimuth have a primordial role in determining the mode locking operation, as they shape the nonlinear transmission characteristic through the polarizer. In [11], a NPR-based ring cavity with twisted fiber in the cavity, which allows strict polarization control, was designed. However, in a twisted fiber the azimuth depends on wavelength because of twist-induced circular birefringence. Therefore, the regime of generation depends on wavelength, which is undesirable for multi-wavelength mode-locked lasers.
In this paper, we report the investigation of a NPR-based ring laser, whose cavity includes two sections of the fiber twisted in opposite directions (right hand and left hand). This procedure cancels the effect of circular birefringence. We observed synchronous dual-wavelength generation in two regimes: noise-like pulses and quasi noise-like pulses with wavelengths centered at 1532 nm and 1564 nm. The quasi-noise-like pulses present Kelly sidebands in the spectra, which are typical of soliton generation, whereas the autocorrelation traces have a pedestal with an amplitude of about 10% of the peak amplitude. The spectra of the two pulses were well separated. The average dispersion of the ring cavity was measured as 13.4 ps/nm/km, close to the dispersion of a standard single-mode fiber. The switching between both regimes was achieved by fine adjustment of polarization azimuth of the pulses traveling in the cavity and occurs nearly at the same azimuth for both wavelengths.
2. Experimental setup
The experimental setup is shown in Fig. 1
Fig. 1 Schematic diagram of: (a) the passively mode-locked EDFL, (b) the birefringent Fiber optical loop mirror (FOLM).
After the PBS a variable wave retarder (VWR) was inserted. The VWR uses a mechanical system that applies pressure to the fiber and converts the linear polarization from the PBS port 3 into any desired ellipticity. The mechanical system of the VWR produces adjustable linear birefringence using a travel millimeter screw that compresses the fiber sandwiched between two plates. The input polarization at the VWR was adjusted to form an angle of 45° with respect to the direction of applied pressure so that the ellipticity can be changed without rotation of the azimuth. The total length of the cavity is 42 m with a total dispersion of ~0.6 ps/nm. At the 50/50 coupler output, a 70/30 coupler was spliced. The 30% port is used to monitor the polarization state inside the cavity at the output of the 50/50 coupler by a polarimeter (PAX5710R3-T, Thorlabs). Generally, the polarization at the output of the 50/50 coupler is not the same as at the polarimeter input due to some residual birefringence of fibers and couplers. However, this birefringence can be compensated by the polarization controller (PC) included before the polarimeter input to ensure that the polarization measured by the polarimeter is the same as the polarization at the 50/50 coupler output. The 70% port of the coupler was connected to the spectrum analyzer and the autocorrelator (Femtochrome 103-XL).
A birefringent fiber optical loop mirror (FOLM) shown in Fig. 1(b) was used as a spectral filter to separate pulses with different wavelengths. The FOLM filter consists of a 50/50 coupler, two azimuth rotators and a 7-cm segment of Hi-Bi fiber. The FOLM transmission presents a sinusoidal dependence on wavelength. The length of the Hi-Bi fiber was chosen to have a period of 60 nm. The position of the maxima of transmission/reflection can be shifted by changing the Hi-Bi fiber temperature. We adjusted the FOLM to have maximal transmission at 1562 nm and maximal reflection at 1534 nm; in this way, the pulses at these wavelengths can be observed simultaneously at Output-1 and Output-2, respectively.
3. Results and discussions
To begin we rotated the AR and determined the azimuth of the polarization (by using the polarimeter) for maximal and minimal transmission of radiation through the PBS. In the following results, the angle for maximal transmission is referred to as 0°. To start mode-locking we adjusted the ellipticity at 23° through VWR, set the azimuth to 15°, the pump power to ~200 mW and observed continuous-wave (CW) generation at two wavelengths, centered with several lines at 1534 nm and at 1562 nm.
The lasing wavelength is defined by the wavelength of the gain maxima of the EDF and the wavelength-dependent loss of the cavity. Some authors use a spectral filter to obtain dual-wavelength operation [15,16]. However dual-wavelength operation was also observed without any special filter inserted in the cavity. This is possible because the gain spectrum profile is determined by the population inversion between the energy levels, and can thus be modified by changing pump power, signal power, or both to have two equal gain maxima. In [14] switching of the lasing wavelength from 1558 nm to 1532 nm was observed when the cavity loss was increased, and thus the signal power was decreased. The dependence of the gain spectrum profile on pump power is discussed with some details in [29]. Even if no filter is inserted in the cavity, some filtering effect may exist because of the fiber birefringence, so that the lasing wavelength can be tuned or switched by PC adjustments [30,31]. In our configuration we tried to cancel both linear and circular birefringence, so that we may consider that the wavelength-depended cavity loss does not play an important role, and that the principal mechanism of dual-wavelength generation is through obtaining two equal gain maxima of the EDF by adjusting pump power. At low pump power we observed a single line of CW generation at 1532 nm, whereas at pump powers higher than the one used in the experiments presented here, the laser generated single line at 1558 nm.
After obtaining dual-wavelength generation, a mechanical stimulation yielded stable mode-locking operation. Figure 2(a)
Fig. 2 (a) Optical spectra, NLP (red), CW (green); (b) autocorrelation trace (inset shows a close-up on the central spur).
The amplitude of the pulses presents some periodical fluctuations with a period of 9.3 μs. The distance between pulses within the envelope is equal to 211.8 ns, meaning that a single pulse circulates in the ~40-m long cavity. An example of pulse waveform is shown in Fig. 3(b)
Fig. 3 (a) Train of pulses measured at the 10% port of the 90/10 coupler. (b) Waveform of a single pulse in the train.
A small rotation of the azimuth, by approximately 1° - 2°, towards the minimum transmission of the polarizer produces significant changes of the generation regime. The NLP spectrum switches to a dual-wavelength soliton-like spectrum, see Fig. 4(a)
, where spectra measured separately at outputs 1 and 2 of the FOLM are shown. We can also see from this figure that the FOLM separates the spectra quite well. Respective autocorrelation traces are shown in Fig. 4(b). The autocorrelation traces again show a narrow peak riding a pedestal, however the ratio between peak and pedestal amplitudes is larger than that of typical NLPs. From the spectral and temporal properties depicted in Fig. 4, we can say that this regime is a transition between those of solitons and NLPs. Similar autocorrelation traces were observed in [33] where large pedestal with low amplitude was attributed to the averaging of cross-correlations between numerous pulses in rapid relative motion. It must be noted however that the spectrum observed in [33] is closer to that of NLPs than to the spectrum of solitons. It may depend on the number and density of soliton bunches and on the number and density of solitons in the bunch. It is interesting to note finally that we observed a very abrupt switching between the two regimes.To understand the complex structure of the pulses generated in our laser, we separated the two wavelengths using the FOLM as a spectral filter. Figure 5(a)
Fig. 5 Measurements of the NLPs centered at ~1534 nm (red line) and ~1562 nm (green line) by the FOLM. (a) Single shot oscilloscope trace. (b) Waveform of single ns pulses at 1534 nm and 1562 nm.
Fig. 7 Partially synchronized pulses: (a) single shot of short-wavelength (red trace) and long-wavelength pulses (green trace); (b) waveforms of the synchronized pulses.
A sequence of 45 measurements of long-wavelength pulses are shown in Fig. 8
for 45 round-trips, measured when the oscilloscope was triggered by the short-wavelength pulses. The interval between shots is 48.7 μs. We can see that one pulse (the synchronized one) appears at the same time in all roundtrips whereas the other pulses move towards longer times. It means that the long-wavelength pulses (except one) propagate more slowly than the short-wavelength pulse, corroborating anomalous cavity dispersion. The slope of the traces allows estimating a value of dispersion of 13.4 ps/nm/km, which is very close to the average cavity dispersion of ~12.5 ps/nm/km (estimated from the lengths and dispersion values of fibers used in the experiment). This means that both synchronized pulses propagate with the velocity of the short-wavelength pulse.After further rotation of the azimuth towards the minimal transmission of the polarizer, the synchronized pulse disappears. Figure 9
shows a sequence of 50 shots of both short- and long-wavelength pulses. The vertical line shows the short-wavelength pulse. It remains single in the cavity. The slope of the trajectories of long-wavelength pulses is the same as it was in Fig. 8. The spectra and the autocorrelation traces are also very similar to those shown in Fig. 4.Finally, it must be emphasized that, after separating the wavelength components, in all cases of quasi-mode-locked regimes, we found that the inner structure of the temporal envelope is composed of sub-pulses at two different wavelengths, which are synchronized, unsynchronized, or partially synchronized, depending on the transmission of radiation through the PBS.
4. Conclusions
We investigated a NPR-based ring laser with a cavity composed of two oppositely twisted sections of fiber, which allows the cancellation of circular birefringence. We observed the generation of dual-wavelength synchronized NLPs with a wavelength difference of 30 nm in a cavity with large anomalous average dispersion of about 13 ps/nm/km. We were able to observe the abrupt switching between dual-wavelength NLP and a transition regime with a spectrum like that of solitons at each wavelength, however with double-scaled autocorrelation traces with a short peak riding a pedestal. The switching occurred at the same azimuth for both wavelengths. Switching between the generation of NLP and the transition regime occurred when the azimuth was slightly rotated towards the angle of minimum transmission through the polarizer, thus increasing the switching power, a result that corroborates those of numerical calculations [7].
Funding
Consejo Nacional de Ciencia y Tecnología (287315).
Acknowledgments
H. Santiago-Hernandez thanks CONACyT postdoctoral fellowship 298828.
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