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In this paper we report the pulse evolution of a simultaneously mode-locked Erbium-doped fiber laser at 1556-nm-band and 1533-nm-band. We explain the dual wavelength laser operation by means of net gain cross section variations caused by the population inversion rate dependence on the pump power. At 1556-nm-band, we observed pulse duration of 370 fs with bandwidth of 8.50 nm and, for pump power higher than 150 mW, we observe the rise of a CW and mode-locked laser, sequentially, at 1533-nm-band. We show that both bands are simultaneously mode-locked and operate at different repetition rates.
© 2014 Optical Society of America
1. Introduction
Ultrafast passively mode-locked fiber lasers operating at different wavelengths have attracted a great deal of research interests in fiber optical sensing, optical instrumentation, optical signal processing and wavelength-division-multiplexing (WDM) transmission systems. Passively mode-locked Erbium-doped fiber lasers (EDFL) have been largely studied due to their important potential applications for optical communications, metrology systems, and capability of generating femtosecond pulses by techniques such as nonlinear polarization rotation (NPR) [1], nonlinear optical loop mirror (NOLM) [2], semiconductor saturable absorber mirrors (SESAM) [3] and carbon-based-elements such as single-walled carbon nanotubes (SWNTs) [4] and graphene [5] saturable absorbers.
Erbium-doped fiber lasers can also be used for simultaneous multi-wavelength emission and ultrashort pulse generation via passive and active mode-locking, generating pulse trains at different center wavelengths to explore a wide variety of optical applications. Actively mode-locked multi-wavelength lasers have been reported at 10 GHz high repetition rate [6] and at different regimes of operation [7].
On the other hand, passively mode-locked multi-wavelength EDFL were demonstrated in various fiber ring cavities. Dual-wavelength mode-locked EDFL using nonlinear polarization rotation technique was demonstrated theoretically [8] and experimentally [9–11], by increasing the pump power and adjusting the fiber birefringence (gain-loss equilibrium) via polarization controller orientations. Also, multi-wavelength soliton operation using SESAM [12] and NOLM [13] saturable absorbers were reported.
Passively mode-locked lasers based on SWNTs and graphene saturable absorbers have been widely reported and investigated due to broadband operation, ease of fabrication and low saturation power and low non-saturable losses [14].
Dual and switchable mode-locked operation in two EDF gain spectral bands was simply obtained via intracavity loss management by adjusting a variable attenuator inside the cavity [15]. Depending on the cavity loss level, single mode-locked operation at 1532 and 1554 nm and the dual wavelength operation could be achieved experimentally. However, the simultaneous dual-band laser operation is not satisfactorily explained only by cavity losses adjustment, and the transition between the different regimes of operation is not shown in detail. Another dual wavelength mode-locked SWCNTs in EDFL cavity was demonstrated experimentally and numerically at 1532 and 1557 nm [16]. Such experiment achieved the switchable mode-locking regime and attributed the variation of EDF homogeneous gain broadening related to the pump power increase as the main cause. Such results are important for the understanding of multiwavelength Erbium-doped fiber laser. However, so far no detailed report has been done in the evolution and interband transition in dual-band mode-locked Erbium-doped fiber lasers.
In this paper, we report in detail the spectral evolution of a dual-band Erbium-doped fiber laser, emitting simultaneously at 1556 and 1533 nm. We analyse the CW and mode-locked operation of both bands and explain the EDFL dual-band operation via net gain cross section variations caused by changes in the population inversion rates in the gain medium as a function of the pump power.
2. Experimental setup
The experimental setup consists of an all-fiber ring cavity Erbium-doped fiber laser, as shown in Fig. 1. As a gain medium, we use a highly Erbium-doped fiber (EDF) 0.26 m long (OFS EDF150-LD, 150 dB/km @ 1530 nm, −16 ps/(km.nm) @ 1550 nm). The EDF is pumped by a 980 nm semiconductor laser through a WDM/isolator integrated component, making laser and pump signals counter-propagating. For the measurements, we extract 30% of the intracavity signal.
Fig. 1 Erbium-doped fiber laser setup. WDM/ISO: pump/signal combiner and signal isolator integrated component; DCF: dispersion compensating fiber; CNT: carbon nanotubes; PC: polarization controller; OC: output coupler.
The WDM/isolator component has two types of fibers: the pump/signal combined port is made with Corning HI1060 (0.25 m, + 5.5 ps/(km.nm) @ 1550 nm), and the isolated signal port with standard single mode fiber (SMF). As a matter of intracavity dispersion management, we use 0.41 m of a dispersion compensating fiber (DCF, −87 ps/(km.nm) @ 1550 nm, MFD = 4.8 μm), and the remaining of the cavity is made with SMF: WDM pigtail connector (0.18 m), CNT sample (0.50 m), polarization controller (1.09 m), and output coupler (0.82 m). The cavity dispersion map is shown in Fig. 2.
Fig. 2 Average dispersion map of the laser fiber components. The average dispersion of the laser cavity is 2.73 ps/(km.nm).
The total cavity length is 3.78 m, yielding a fundamental round-trip repetition rate of 52.973 MHz at 1556 nm. The average cavity dispersion is + 2.73 ps/(km.nm) and the total accumulated dispersion is + 10.36 fs/nm.
CNT sample used in the experiments was prepared as in [17]. A suspension of 40 mg/ml of 1.0 nm mean diameter CNT in UV-curable, commercial optical polymeric adhesive based on urethane (NOA73TM, n = 1.56 and 90% transparency at 1.55 µm) was ultrasonicated, and a droplet of that suspension was placed on top of the fiber ferrule face to form a film.
The characterization of the sample was made with a simple single-pass transmittance experiment at 1550 nm, using a CW laser with power level of 1.0 mW. The overall insertion loss of the sample (including connector losses, film transmittance and CNT non-saturated absorbance) is 2.73 dB, resulting in a total transmittance of 53.3% and an αL = 0.63.
3. Results and discussion
3.1 1556-nm-band operation
Due to an increased overall loss in the Erbium-doped fiber laser cavity, mainly caused by the highly Erbium-doped fiber itself and the presence of a DCF in the setup, lasing at 1558.4 nm only starts for pump power of 50 mW. Soon after, mode-locking operation starts at 1558.4 nm for pump power of 60 mW. Mode-locking operation is self-starting and does not depend on the orientation of the polarization controller. However, for each pump power, the laser bandwidth and pulse duration can be optimized and fine-tuned by adjusting the polarization controller orientation.
As we increase the pump power, laser bandwidth increases, up to a maximum of 8.50 nm, as shown in Fig. 3.
Fig. 3 Laser bandwidth evolution as a function of pump power. The different regions show different laser evolutions, since pulse breaks from 1 to 2 and 4 pulses per round-trip in regions I, II and III, respectively.
Laser bandwidth increases for higher pump powers, and regions I, II and III have different laser bandwidth tendencies. Respectively, regions I, II and III have 1, 2 and 4 pulses per round-trip, which is due to pulse shaping related to the high intracavity peak power.
Since the laser cavity is dispersion-managed, with an average dispersion of + 2.73 ps/(km.nm) and a total accumulated dispersion of + 10.36 fs/nm, the output pulses are almost chirp-free. The shortest pulse was obtained with 164 mW of pump power. The output bandwidth is 8.50 nm, and the autocorrelation trace has a full-width at half maximum duration of 571 fs, corresponding to an actual pulsewidth of 370 fs (assuming sech2 shape), as shown in Fig. 4. The average output power is 0.6 mW.
Fig. 4 a) Laser output bandwidth of 8.5 nm and b) autocorrelation trace for the shortest achieved pulse of 370 fs.
The time-bandwidth product of the pulse is 0.390, which is slightly higher than the transform-limited value of 0.315 expected for soliton-like pulses. However, the pulses propagate through 0.50 m of SMF fiber at laser output and we can estimate that the pulses are chirp-free. Since the soliton period for the intracavity pulses is Z0 = 19.80 m, and Z0 >> Lcavity, no side bands are observed [18,19].
3.2 Dual band operation: 1556-nm and 1533-nm bands
In Fig. 5, we show the laser bands evolution as a function of pump power. Initially, for pump powers from 120 to 164 mW (pink, green, brown, orange and blue curves) 1556-nm-band is mode-locked. Its bandwidth increases from 6.38 to 8.50 nm as pump power increases, even when 1533-nm-band starts operating at continuous-wave regime at pump power of 160 mW (orange and blue curves), as shown in the inset of Fig. 5. At pump power of 166 mW, 1533-nm-band changes from continuous-wave into mode-locking, whereas 1556-nm-band stills mode-locked. For pump powers from 166 to 171 mW both bands are mode-locked, and laser bandwidth at 1533-nm increases from 1.02 to 1.58 nm as laser bandwidth at 1556-nm decreases from 8.50 to 7.89 nm (red and black curves). It suggests that bands compete for the Erbium-doped fiber gain and there is an energy transfer from 1556-nm-band to 1533-nm-band when both bands are simultaneously mode-locked.
Fig. 5 Dual-band laser evolution. For pump powers from 120 to 171 mW, 1556-nm-band is mode-locked, and 1533-nm-band evolves from CW to mode-locking.
Simultaneous multi-wavelength laser emission was enabled because the net gain cross section of the fiber gain medium is the same for each emission band [20], for pump power from 160 to 171 mW. The profile of net gain cross section per wavelength of an Erbium-doped fiber depends on the population inversion rate, which makes the net gain also dependent on the pump power [21]. In this laser system, for pump power level below 160 mW, the net gain cross section of the Erbium-doped fiber has a single peak at the 1556-nm-band, resulting in single emission peak. However, when the pump power is increased for values higher than 160 mW, the net gain cross section of the Erbium-doped fiber varies, giving rise to another emission peak at 1533-nm band. Under these conditions, for pump powers from 160 to 171 mW, simultaneous EDFL dual-band operation is observed.
As an experimental evidence of the net gain cross section variation in the laser gain medium, the peak emission wavelength at 1556-nm-band should be blue-shifted as the pump power increases [21]. We measured the shift of the peak emission wavelength for pump powers from 50 to 171 mW and observed a blue-shift of 3.25 nm, as shown in Fig. 6.
Fig. 6 Peak emission wavelength at 1556-nm-band as a function of pump power. From 50 to 171 mW, the blue-shift is 3.25 nm.
The CNT saturable absorber plays a fundamental role on the simultaneous mode-locking EDFL dual-band emission due to its broadband absorption spectrum that acts independently for each band [15]. At the low pump power regime, CNT absorption is low enough to allows the mode-locking at 1556-nm. However, by increasing the pump power there is a variation in the Erbium gain cross-section profile which leads to a change in the gain-loss ratio at 1533-nm that enables mode-locking in this band. Also the recovery time of CNT, in the sub-picosecond scale, is shorter than the temporal delay between 1533-nm and 1556-nm bands (~20 ps).
We used a tunable 1.5-nm-wide bandpass filter positioned at the output in order to analyze separately the emission bands of the EDFL. After the bandpass filter, the signal is splitted in a 50:50 percent ratio for spectral and time analysis, as shown in Fig. 7.
Fig. 7 a) EDFL output spectrum, with 1.5 nm wide filtered 1533-nm and 1556-nm bands; b) output pulse trains at 1533 nm, with a single pulse per round-trip, and 1556 nm, with four pulses per round-trip.
As demonstrated in Fig. 7, laser is mode-locked at both wavelengths simultaneously. For pump power of 170 mW, 1533-nm-band laser is mode-locked with a repetition rate of 52.904 MHz, corresponding to the fundamental repetition rate (1 pulse per round-trip) at 1533 nm. Nevertheless, 1556-nm-band is mode-locked with 4 pulses per round-trip in a repetition rate of 211.890 MHz, corresponding to the 4th harmonic state of the fundamental repetition rate at 1556 nm. These repetition rates of 1 and 4 pulses per round-trip in the 1533-nm and 1556-nm bands, respectively, were observed for all pump powers beyond 166 mW. The difference in the number of pulses per round-trip can be explained by means of intracavity peak power and soliton stabilization. Since the pulses at 1556 nm have duration in the order of 300-500 fs, intracavity peak powers are high, in the order of 40 W. With high peak powers inside laser cavity, nonlinearities become important and the soliton-like pulses break into multiple-pulses to maintain the equilibrium between laser cavity dispersion and nonlinearities. As we observed in Fig. 3, depending on the pump power level, pulse train at 1556-nm band evolves from 1 pulse per round-trip to 2 and 4 pulses per round-trip.
Due to group velocity dispersion, both bands have slightly different repetition rates. Measuring the output pulse train without the bandpass filter, and triggering the signal with the 1533-nm-band repetition rate, we observed that the 1556-nm-band pulse train drifts, as shown in Fig. 8.
Fig. 8 Laser pulse train without bandpass filter at laser’s output. The signal is triggered by the 1533-nm pulse train, and we can observe that 1556-nm pulse train drifts due to unsynchronized repetition rates.
4. Conclusion
We demonstrated in this paper a simultaneous dual-band carbon nanotubes mode-locked Erbium-doped fiber laser. At 1556-nm-band, we observed output pulse trains with 1, 2 and 4 pulses per round-trip, with the shortest pulse of 370 fs and 8.50 nm bandwidth obtained with 164 mW of pump power and a repetition rate of 211.890 MHz.
With increasing pump power, laser gain is transferred from 1556-nm-band to 1533-nm-band, and the wavelength peak of emission is shifted to lower wavelengths. We explained this effect via net gain cross section changes caused by population inversion rates variation dependent on the pump power. Population inversion rate increases as pump power increases, making the net gain cross section peak at the 1556-nm-band to move from longer to shorter wavelengths, and increases the net gain cross section in the 1533-nm-band, thus resulting in simultaneous dual-band EDFL emission.
For pump powers of 160 mW and above, mode-locked laser operation occurs at 1533 and 1556 nm simultaneously. Also, from 166 mW of pump power, both bands are mode-locked, with different repetition rates: thus 1533-nm-band is mode-locked in the fundamental repetition rate, 52.904 MHz, 1556-nm-band is mode-locked in the 4th harmonic of the fundamental repetition rate.
Acknowledgments
We thank Prof. Lúcia A. M. Saito for fruitful discussions. Henrique G. Rosa acknowledges FAPESP for financial support through grant 2010/19085-8. All authors acknowledge MackPesquisa and CAPES for financial support.
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