The pulse evolution in Bi-doped soliton fiber laser with slow and fast saturable absorber has been studied both experimentally and numerically. Semiconductor saturable absorbers with balanced slow and fast absorption recovery mechanisms exhibit a bi-temporal recovery dynamics which permits both reliable start-up of passive mode-locking and short pulse generation and stabilization. The pulse dynamics within the Bi fiber laser cavity have been investigated.
©2010 Optical Society of America
Recently developed Bi-doped fibers have the ability to provide gain within the 1100-1550 nm wavelength range [1,2] and thus leverage the well known advantages of fiber lasers and fiber amplifiers to new applications. Continuous wave (CW) Bi-doped fiber lasers have already been reported to deliver output powers up to several Watts [3,4]. However, there are several challenges in the development of mode-locked Bi-fiber lasers. Due to the relatively low gain coefficient of Bi-doped fiber, the lasers typically have a long cavity length to achieve sufficient gain. The first Bi-fiber lasers mode-locked by semiconductor saturable absorber mirror (SESAM) exhibited relative long ~50 ps pulses with large pulse pedestal . This was attributed to the combined effect of the narrow bandwidth of the fiber Bragg grating reflector, large normal dispersion of the long-length cavity and the non-optimized SESAM. In order to generate subpicosecond high-quality pulses from Bi-fiber lasers, dispersion compensation and a SESAM with picosecond or subpicosecond recovery time is required.
Semiconductor saturable absorber mirrors are widely used to passively mode-lock a large variety of lasers operating in different wavelength regimes [6–8]. Development of high-quality SESAMs operating at 1150-1400 nm requires corresponding absorbing regions grown on top of broadband reflectivity GaAs/AlAs distributed Bragg reflectors (DBR). InGaAsN/GaAs based SESAM used in this study meets these requirements . It was shown that its absorption recovery time can be conveniently controlled by N-ion content generated during the plasma process [10,11].
In this paper, we investigate both experimentally and by numerical simulations the peculiarity of pulse dynamics in a mode-locked Bi-doped soliton fiber laser depending on SESAMs parameters.
The fiber laser cavity is shown schematically in Fig. 1 . The laser cavity comprised ~12 meters of Bi-doped glass fiber, a 1062/1165 pump coupler, and a fiber loop mirror acting as an output coupler with ~97% reflectivity at the lasing wavelength. The fiber was core-pumped with a cw Yb-doped single-mode fiber laser system. A monitoring 5% tap coupler was spliced close to the location of dispersion compensator as seen from Fig. 1. The Bi-doped aluminosilicate fiber with core glass composition of 97 mol:% of SiO2 and 3 mol:% of Al2O3, a core diameter of 8.4 μm, a core/cladding refractive index difference of Δn = 5.5∙10−3, and a cutoff wavelength of ~1100 nm has been described in detail in . The length of the doped fiber was optimized in order to achieve efficient pump absorption while minimizing the length and dispersion of the fiber section of the cavity. The splice loss of Bi-fiber to a standard single mode fiber was ~0.3 dB. An additional dichroic fiber coupler was used at the output of the laser to separate the signal light from residual pump. The normal group velocity dispersion(GVD) of the optical fiber at 1.16 μm wavelength range was compensated by a transmission grating pair with 1250 lines/mm. A polarization controller was used to maintain the optimal functioning of the grating pair.
InGaAsN-based semiconductor saturable absorber mirrors with different absorption recovery times were grown by molecular beam epitaxy to explore the effect of the absorber recovery time on the mode-locking performance. The Absorber #1 comprises 12 InGaAsN quantum wells (QWs) while the Absorbers #2 and #3 used in this study include 4 QWs grown on top of 24.5 pair GaAs/AlAs DBR. The DBR stopband had a center wavelength of ~1140 nm with a 150-nm bandwidth.
In order to decrease the absorber recovery time, the absorbers #2 and #3 were in situ irradiated with N-ions for 2 and 3 minutes, respectively ; the quantum wells were individually exposed to a flux of N-ions after they were grown. The absorption recovery time was measured at ~1160 nm by using a degenerate pump-probe measurement set-up. The results shown in Fig. 2(a) and (b) reveal that the absorption recovery time decreases from ~680 ps for the non-irradiated sample down to 4 ps and 2 ps for the samples irradiated for 2 min and 3 min, respectively. It can be observed that absorption recovery has clear two-exponential decay behavior with fast components of 4 and 2 ps and the slow components of ~40 ps and ~20 ps, respectively. We should mention here that a slow component of the absorption recovery is useful for ensuring a self-starting mode-locking, while the fast component performs efficiently the pulse shaping.
It was found that the laser enabled self-starting mode-locking at a repetition rate of 7.3 MHz for each absorber with steady-state pulses shown in Fig. 3 . The threshold pump powers were ~400 mW, ~600 mW, and ~700 mW, respectively, for the laser cavities with absorbers #1, #2, and #3. The pump powers for the autocorrelations and pulse spectra shown in Fig. 3 (a) and (b) were ~1 W. As it can be seen, the absorber #1 with absorption recovery time of~680 ps supports a pulse with a large pedestal (black trace) that exceeds the ~210-ps scanning range of the autocorrelator. For the absorber #2 with the recovery time of 4 ps, the pulse pedestal is significantly suppressed (red trace). By using the absorber with recovery time of 2 ps, the pulse pedestal is completely avoided providing clean and stable soliton pulses. Figure 3(b) displays the optical spectra illustrating that the spectral width increases with a decrease of the absorption recovery time.
The soliton sidebands seen in the spectra become more pronounced with reduced recovery time indicating a stable soliton operation of the laser with the fast SESAM. The similar tendency was observed in the pulse train monitored at the oscilloscope. Contrary to the slow absorbers, mode-locking with fast absorber #3 revealed temporally stable pulse train even for long-span measurements. The average cavity dispersion in these measurements was estimated to be ~-0.4 ps2.
The cavity dispersion also plays a role in the pulse shaping. When the average cavity dispersion was increased to ~-0.65 ps2 by lengthening the grating pair separation, the use of absorber #2 resulted in nearly pedestal-free pulses with pulse duration of ~1.3 ps. On the other hand, when the laser was operated with average cavity dispersion lower than ~-0.3 ps2, even the use of absorber #3 resulted in a small pedestal in output pulses. The pumping power was also observed to have a contribution to the pulse pedestals. When operated close to the laser threshold, the pulse pedestals were slightly weaker compared to higher pumping powers.
3. Numerical simulations and discussions
To address the specific features characteristic to mode-locked Bi fiber laser, we assume that
- 1. the cavity has long length and consequently high tendency to multiple pulsing;
- 2. the laser generates pulses with low pulse (E), therefore, Eq is very close to Eq + 1; q – number of pulses circulating in the laser cavity.
Figures 4(a) and (b) illustrate the simulated spectra and autocorrelations for different recovery time of saturable absorber and cavity dispersion of −0.4 ps2. When running the simulations it could be observed that for low dispersion and long recovery time, the temporal pulse shape and spectrum change slightly for each consecutive roundtrip passage through the cavity.
The autocorrelations and spectra presented here have been averaged over 8000 roundtrips to emulate measured autocorrelation and spectrum. As it can be seen, the simulation predicts the pedestal formation observed in the experiments. The simulations also show that the pumping rate has an influence on the size of the pulse pedestal. When the gain of the Bi-doped fiber is increased from 0.35 dB/m to 0.45 dB/m, the size of the pulse pedestal increases significantly, as it can be seen in the Fig. 4(a) for the absorbers #1 and #2. The inset to Fig. 4(a) shows an irregular pulse bunching at the large temporal span for Absorber #1. A multiple pulsing appears then as an effective pulse pedestal seen in the autocorrelation trace. The simulation results are in a qualitative agreement with the experimental data shown in Fig. 3(a). Small deviations in the autocorrelations can be explained due to minor variation of pumping levels used in the experiments. The simulated pulse spectra shown in Fig. 4(b) are in very good agreement with the measured spectra given in Fig. 3(b). Difference in the depth of the soliton sidebands might be due to some additional spectral filtering in the experimental fiber cavity. Figure 5 shows simulation results for the laser with total intra-cavity dispersion of −0.65ps2. The pedestal levels have dropped compared to the case with cavity dispersion of −0.4ps2, as it was also noticed in the experiments. This can be understood by the stronger soliton formation and pulse shaping with higher anomalous dispersions. However, the effect of the cavity dispersion on the pulse pedestal is not as strong as the effect of the absorber recovery time or the fiber pumping level. The inset to Fig. 5 shows the pulse shape for the laser cavity with the Absorber #2 initiating the mode-locked operation. The smaller satellite pulse results in the observed small pedestal of the autocorrelation.
The observed pulse pedestals are likely to be due to relatively long absorber recovery times. Indeed, after the multiple passage of the pulse the absorber is saturated, however the pulse sees net gain for relatively long time window (~20-1000 ps depending on the absorber) . Within this time interval, the spontaneous noise can be amplified disturbing the pulse quality or preventing the mode-locked operation completely. The autocorrelation pedestals in the mode-locked Bi-laser experiments are emulated by multiple pulses propagating in the cavity together with the stronger fundamental pulse.
The tendency of the slow absorber for supporting the multiple pulsing operation within its window of low losses can be explained by looking at the absorber saturation in the simulations. As a measure for the SESAM saturation, the simulated losses for a pulse was used, rather than the simple measure of pulse energy divided by saturation energy which is an inaccurate measure for the saturation when the pulse durations are in the same order of magnitude as the recovery time. For the slowest absorber with −0.40 ps2 of dispersion and 0.45 dB/m of gain, the saturable losses in the absorber are 2% for the entire group of pulses. When isolating the strongest pulse in the group, the saturable losses of 25% would occur if this pulse would be a single pulse. When looking at the simulation of the fastest absorber, the whole group of pulses has losses of 34% whereas the strongest pulse in the group would only have losses of 26% (the losses are lower for a single pulse than for the group of pulses because the strongest pulse was only looked at). The slow absorber favors lower energy pulses that saturate the absorber much better when they are spaced closely together and therefore favors multiple pulsing. The fast absorber provides as good saturation for a high energy single pulse as for a group of pulses and, therefore, favors single pulse operation more than the slow absorber. The saturation fluencies of the absorber also have a role in the pulse generation. The low saturation fluence of the Absorber #1 supports the self-starting operation of the laser, but on the other hand generation of multiple noise-like pulses together with the long absorber recovery time . Whereas, the higher saturation fluencies of the Absorber #2 and #3 promotes single or few-pulses operation.
An important pulse shaping mechanism in this study was also the soliton pulse shaping. The soliton formation allowed self-starting operation with each absorber. In the soliton regime, the laser is able to sustain larger amount of noise arising behind the pulse . However, if the dispersion is reduced to a less anomalous value (in this study ~-0.3 ps2), even the strong soliton shaping mechanism cannot prevent the formation of the pulse pedestal when slow absorbers are used. Actually, the soliton-area theorem determines the pulse energy for a cavity with a certain dispersion and non-linearity for certain pulse duration. When the minimum pulse duration supported by the system increases, the pulse energy has to decrease proportionally. Therefore single pulse operation is favored in a soliton laser with a fast absorber because it favors shorter and therefore pulses with higher energy compared to a soliton laser with a slow absorber.
To obtain here clean pulses with pulse duration in the range of few hundreds of femtoseconds or less, a SESAM with even faster absorption recovery should be used. In conclusion, a very careful and detailed design of the mode-locked Bi-doped fiber laser system (both the absorber and the fiber cavity) is required for ultra-short and high-quality pulses.
4. Pulse evolution in Bi-doped fiber cavity
The Bi-fiber laser was then studied with the fastest absorber initiating the mode-locked operation (cavity dispersion was set to ~-0.4 ps2). The aim was to analyze the pulse evolution in the laser cavity, as it has been shown that the pulses propagating inside the dispersion managed fiber laser cavity encounter strong temporal evolution [15,16]. The same numerical simulations were used as described in previous section.
Figure 6 illustrates the evolution of the spectral width and the time-bandwidth product (TBP) during a one cavity round-trip started from the location of the SESAM (0 mm) and then mapped according to the laser schematic shown in Fig. 1. The pulses at the output 3 have only a small chirp with a time-bandwidth product of ~0.64 and spectral width of 2.0 nm. However, the pulses have a significantly broader spectrum close to the dispersion compensation end of the cavity (output 1 and 2) and the pulses undergo a strong temporal chirp. In particular, the pulses at the output 2 are strongly negatively chirped with a simulated spectral width of 2.7 nm and time-bandwidth product of 1.02. The negative chirp can be then compensated by about 10 m of standard single mode optical fiber, with normal dispersion at the laser wavelength, placed at output 2. The estimated pulse width after compression was ~0.6 ps.
According to simulations an additional 5% tap coupler was spliced close to the grating pair dispersion compensator (see Fig. 1) in order to get the broadest temporal bandwidth and negative chirp of the pulses. The pulse evolution was then investigated experimentally by recording the optical spectrum and the corresponding autocorrelation at the three different outputs (see Fig. 7 ). As it can be seen from Fig. 6 (scatters) the pulses at the output 3 are nearly transform-limited with a time-bandwidth product of 0.46. Close to the dispersion compensation end of the cavity at the output 1 and 2 pulses are chirped and have a significantly broader spectrum, as also expected from the simulation. The pulses at the output 2 have spectral width of 2.8 nm and time-bandwidth product of 1.04, which is in very good agreement with the simulation.
The pulse compression in the fiber output pigtail (output 2) was then experimentally investigated by splicing standard single mode fiber to the output. It was found that the shortest pulses were obtained with the output fiber length of ~13 m. The compression of negatively chirped 1.67-ps pulses resulted in nearly bandwidth limited 0.7-ps pulses, as seen in Fig. 7. We note that these compressed pulses are shorter than the pulses recorded at the output 3 with pulse width of 1.05 ps and spectral width of 2.0 nm. This is the shortest pulse duration reported for a Bi-doped fiber laser.
The pulse dynamics of a mode-locked Bi-doped fiber laser has been studied in detail both numerically and experimentally using semiconductor saturable absorbers with different absorption recovery time. It was shown that the pulse quality was improved significantly when the absorber recovery time was decreased from the initial value of ~680 ps down to 2 ps. The effect of the gain fiber pumping level, absorption saturation, and average cavity dispersion were also shown to have a detrimental effect on the pulse generation and shaping in the Bi-doped fiber laser operating around 1160 nm. In addition, pulse propagation dynamics of the Bi-fiber laser cavity were investigated.
The authors acknowledge the support of the graduate school of Tampere University of Technology, the Finnish Foundation for Economic and Technology Sciences, the Jenny and Antti Wihuri Foundation, the Nokia Foundation, the Emil Aaltonen Foundation, and the Elisa Foundation. A. V. Kholodkov and K. M. Golant are acknowledged for fabricating the Bi-doped fiber, and V. M. Korpijärvi for his help with epitaxial growth.
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