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We report a novel burst-mode-operated sub-nanosecond fiber Master Oscillator, Power Amplifier (MOPA) system incorporating direct seed-packet shaping without external modulators. A fast digital-to-analog converter with 1 Gsps sampling rate and 16 bit resolution was developed to control the pulse amplitudes and sequences of a distributed feedback semiconductor seed laser to realize packet-shaped burst mode operation. Optical pulses with durations as short as 700 ps and peak power as high as 1 W can be generated from the seed by applying proper reverse voltages after positive electrical pulses to the laser driver to cancel the residual charges at its gate electrode. The average power of the laser can be amplified to nearly 40 W with FWHM spectral linewidth of ~0.12 nm after three stages of polarization maintaining fiber amplifiers. Different packet shapes including ramp-off, Gaussian, square and double rectangle can be produced from the fiber MOPA by finely pre-shaping the seed pulse bursts. It is believed that such a laser has provided a cost-effective solution to the generation of pulse bursts with arbitrary packet shapes for different practical applications including material micromachining and nonlinear frequency conversion.
© 2016 Optical Society of America
1. Introduction
Nowadays, high power pulsed fiber lasers are finding extensive applications in material processing, ranging and nonlinear frequency conversion, to name a few [1,2]. This field experienced rapid development with the advent of pulse shaping technique by which the pulse durations and/or shapes can be adjusted to realize controllable peak powers. Since then, a variety of investigations have been carried out utilizing this technique to enhance the conversion efficiency of an optical parametric oscillator [3–6], generate specific Raman shifted wavelengths [7], increase the material processing rate [8], etc. However, in the early trials, the peak powers of such lasers were constrained by the onset of Stimulated Brillouin Scattering (SBS) in the amplifier. Because directly modulated laser diodes (LDs) featuring few longitudinal modes were employed as the seeds [9,10] leading to low SBS thresholds. This crisis was then resolved by splicing a fiber Bragg grating to the pigtailed seed LD at a distance of ~2m from the fiber facet with reflectivity higher than the diode front facet to reduce the mode spacing [11]. In this scenario, an Electro-Optic Modulator (EOM) should be used to carve the initial square pulse of the LD into required waveforms at the expense of increased loss and complexity. Another possible solution to significantly increase the SBS threshold was adopting an amplified spontaneous emission (ASE) source instead of an LD as the seed [12]. Thanks to the modeless characteristic of the ASE seed, such a fiber laser could support even higher peak power than its LD-seeded counterparts without triggering the detrimental SBS effect. Nevertheless, the poor coherence of such lasers might make them unsuitable for nonlinear frequency conversion.
Different from these, burst-mode-operated fiber laser with pulse durations less than one nanosecond (ns) emerges as the best candidate because the detrimental SBS nearly ceases to occur for such short pulses while each pulse burst can still accumulate enough energy for efficient material processing [13] or nonlinear frequency conversion [14,15]. Especially for material processing, burst-mode-operated lasers with extremely high intra-burst repetition rate working in the ablation-cooled regime can reduce the laser pulse energies needed for ablation and increase the efficiency of the removal process by an order of magnitude over traditional ultrafast lasers with uniform repetition rates [16]. To date, most of the burst-mode-operated fiber lasers either employed a pulse multiplier to interpolate additional pulses [17] or an external modulator to block part of the seed pulses periodically [18–23] to form the pulse bursts. However, in terms of the pulse-multiplier-based lasers, the seed packet shapes can hardly be adjusted and are distorted significantly after amplification [17]. Although packet shaping have been successfully demonstrated by using external Acousto-Optic Modulators (AOM) with tunable drive power at the cost of decreased efficiency [22,23], the temporal resolution of the packet shaping might be limited by the modest modulation frequency (≤200MHz) and the comparatively long rise time (≥10ns) of the AOM. As an alternative, directly modulated LD can also be served as the seed laser for burst mode operation and no external modulators are needed, which makes it a cost-efficient solution [24]. Nonetheless, the peak power could not be tuned from pulse to pulse for the seed LD in the previous report. Therefore, the burst mode operation could only be realized by two means. One was generating stable pulse trains at the seed and then periodically gating the drive current of the pump diodes for the succeeding fiber amplifiers at low repetition rate (tens of kilo-hertz at most). The other is directly modulating the seed laser into bursts with square packet shapes at high repetition rate and then amplifying them with continuous pump leading to inevitable distortion in output signal packet shapes. As a consequence, neither of the two methods can provide fine packet shaping which might restrain them from specific applications. Pump shaping might be a possible technique to tackle this crisis which has been successfully applied to an Nd: YLF-based laser amplifier chain [25]. However, the average signal power was reduced owing to the finite duty cycle of the pump and the temporal resolution and the inter-burst repetition rate of the signal was limited by the slow response of the drive electronics for the pump owing to their large operating current.
In this paper, we report, to the best of our knowledge, the first burst-mode-operated, sub-ns fiber master oscillator-power amplifier (MOPA) system incorporating direct seed-packet shaping. A distributed feedback (DFB) LD controlled by a self-developed high speed, 16 bit digit-to-analog converter (DAC) was employed as the seed laser enabling burst mode operation with fine packet-shaping capability. By applying proper reverse voltages after positive electrical pulses to the seed driver, the seed laser can provide pulse bursts with single pulse durations as short as 700 ps and intra-burst repetition rate as high as 333 MHz. The average power of the laser can be amplified to ~40 W with spectral linewidth of ~0.12 nm measured in full width at half maximum (FWHM) after three stages of fiber amplifiers. More importantly, different output packet shapes can be generated by pre-shaping the seed bursts accordingly with high resolutions in both time and amplitude domain. It is believed that such a burst-mode-operated fiber laser with direct seed-packet shaping capability is cost-effective and suitable for various applications including material micro-machining and nonlinear frequency conversion.
2. Experimental setups
The schematic diagram of the burst-mode-operated, sub-ns fiber MOPA system comprising one seed laser and three stages of fiber amplifiers was depicted in Fig. 1. The seed laser was a DFB LD (LC96A10xx-DFB, II-VI) working around 1064 nm with the highest peak power of 1W in pulsed mode. The DFB laser was driven by a fast analog circuit board (developed by LaserSpectrum Inc., Hangzhou) with peak current higher than 2 A at 1 GHz. An additional circuit board based on a fast DAC chip (DAC5681, Texas Instruments) with 16 bit resolution ranging from −1V to 1V at 1Gsps was developed to control the drive current of the DFB laser by its output voltage to form shaped pulse bursts. The burst settings are sent from a personal computer (PC) to a Field Programmable Gate Array (FPGA) chip (Artix-7, Xilinx) on the same board, by which the data are translated and redirected to the DAC chip periodically. Therefore, by simply setting the pulse parameters for one cycle via PC, the seed laser can generate pulse bursts with desired packet shapes and inter-/intra-burst repetition rates.
Fig. 1 Scheme of the burst-mode-operated, sub-ns fiber MOPA system incorporating direct seed-packet shaping.
The seed laser was then connected to three stages of polarization maintaining (PM) fiber amplifiers to elevate its average power to meet the requirements for practical applications. The first stage was a single mode (SM) fiber amplifier with counter-pumping configuration containing a 1-meter-long Ytterbium (Yb)-doped PM fiber (Yb 401-PM, CorActive), a filter type wavelength division multiplexer (FWDM) and an SM pump at 976 nm. The second stage was also constructed in counter-pumping scheme. A 3-meter-long Yb-doped PM double clad fiber (DCF) (Yb1200 10/125-PM DCF, nLIGHT) and a wavelength-stabilized multimode (MM) laser diode (K976A02RN-9W, BWT) with the highest power of 9W at 976 nm were employed as the gain medium and the pump, respectively. The third stage was developed in co-pumping structure which was composed of two MM pumps (BMU30-915-01-R, II-VI) each with the highest power of 30W at 915 nm, a piece of Yb-doped PM DCF (Yb1200 20/125-PM DCF, nLIGHT) with the length of 2.5 m and a (2 + 1) × 1 pump-signal combiner. A specially designed isolator using passive 10/125 PM single clad fiber (SCF) at the input and passive 20/125 PM SCF at the output, also served as a mode field adaptor, was employed to excite the fundamental mode exclusively in the third amplifier. Besides, the gain fiber was coiled on a forced air cooled aluminum spool with a diameter of 10 cm to further guarantee the excellent beam quality and efficient heat dissipation. The output end of the gain fiber was cleaved at 8° to protect the amplifier from possible damage by the back reflected signal. Finally, the amplified signal and residual pump were collimated by an aspherical lens and then separated by a dichroic mirror for power measurement.
3. Results and discussions
In order to evaluate the temporal behaviors of the seed DFB laser, the electrical drive pulses with different settings and the corresponding optical pulses were measured in the first place. Figures 2(a) and 2(b) show the electrical and optical bursts of the seed laser with intra-burst repetition rate of 100 MHz and inter-burst repetition rate of 2.5 MHz, respectively. It can be seen that although the electrical pulses were generated precisely with square packet shapes, the measured optical pulses were not with the same shapes as the electrical signal. The pulse bursts saw an obvious offset in their intensities and this offset varied periodically in phase with the inter-burst repetition rate. We attribute this to the residual charges at the gate electrode of the drive transistor controlling the drive current. In order to cancel the residual charges, reverse voltages were applied to the laser driver after each positive pulse, as was depicted in Fig. 2(c). Consequently, the optical pulses became stable with negligible offset, as was shown in Fig. 2(d). In addition, the optical pulse duration was measured to be ~700 ps, albeit driven by slightly longer electrical pulses with width of ~1.1 ns, which were demonstrated in Figs. 2(e) and 2(f), respectively. The applied reverse voltages, which could reduce the effective electrical pulse width, might be the reason. Therefore, in order to get offset-free pulse bursts with single pulse duration well below 1 ns, proper reverse voltages were selected and applied to the laser driver to generate pulse bursts with different packet shapes and inter-/intra-repetition rates in the succeeding experiments.
Fig. 2 The electrical drive pulses and the corresponding optical pulses of the seed: (a), (c), (e): drive pulses from the DAC board; (b), (d), (f): measured corresponding optical pulses
As the temporal behaviors of the seed laser, namely the master oscillator, had been studied, the power scaling was carried out thereafter to investigate the performance of the amplifier chain. Clearly, seed bursts with different settings will result in different average powers. In order to saturate the main amplifier with enough input power for different burst parameters, the equivalent overall duty cycle of the modulated seed was kept higher than 0.5%, i.e. with average power higher than 0.3 mW. The average power was then amplified to >10 mW, depending on the seed power, after the first amplifier. This value was then increased to ~500 mW by the second amplifier. This modest average power was selected because it was large enough to saturate the gain in the main amplifier whilst not too large to reduce the threshold of Stimulated Raman Scattering (SRS). Under the highest pump power of 56 W, an output power of 39.4W was recorded from the main amplifier with calculated conversion efficiency of ~70%, as was illustrated in Fig. 3. The measured ASE was more than 30 dB lower than the laser signal peak and the FWHM spectral linewidth of the signal was ~0.12 nm at the highest output power, which should be favorable to nonlinear frequency conversion and laser ranging. No evident SRS signal and roll-over in conversion efficiency can be found even at the highest output power indicating that this laser system is only pump power limited and higher output power is expected when higher pump power is available. It is also worth mentioning that the spectral characteristics of the MOPA system remained almost unchanged for different seed bursts as long as the overall duty cycle was larger than 0.5% showing its adaptability.
Fig. 3 Amplified laser power as a function of the launched pump power for the main amplifier; the insets show the measured spectra of the seed laser and after three stages of amplification with large wavelength scale (up) and around the peak wavelength of the signal (bottom).
Due to the gain saturation effect in the fiber MOPA, packet shape distortion usually occurs during power amplification. Hence, the study on the packet shaping of the burst-mode-operated fiber MOPA was started at investigating the packet shape distortion in the amplifier for two typical seed pulse bursts with square and Gaussian packet shapes. The intra-burst repetition rate was set to be 100 MHz and two different inter-burst repetition rates of 250 kHz and 500 kHz were selected for comparison. Figures 4(a)-4(c) plot the drive voltage of the seed, measured single burst and burst train after amplified to the maximum power, respectively, at inter-burst repetition rate of 250 kHz. Figures 4(d)-4(f) plot these values for pulse bursts with 500 kHz inter-burst repetition rate. Besides, numerical algorithm adapted from the single-pulse-shaping model proposed in [26] were also performed and the computed packet shapes for each case were plotted and compared in Figs. 4(b) and 4(e), respectively. It can be seen from Figs. 4 (a) and (d) that slightly higher reverse voltages were applied to the seed driver for square-shaped bursts with higher repetition rate to annihilate the residual charges. Because in the preliminary experiment, we found that larger reverse voltages should be used in pulse bursts with higher duty cycle. We attributed this to the higher power consumption on the transistor which would slightly affect its electrical performances. Therefore, the reverse drive voltage witnessed a steeper drop for pulse bursts with 500 kHz inter-burst repetition rate than their counterparts with 250 kHz inter-burst repetition rate in order to produce stable square shaped pulse bursts for both cases. Although square-shaped seed bursts were generated in both repetition rates, the amplified optical bursts saw a gentler decline with higher inter-repetition rate owing to less energy storage for each burst. In addition, the average deviations between the measured and calculated pulse bursts were computed to be 2.75% and 2.15% for pulse bursts with inter-burst repetition rates of 250 kHz and 500 kHz, respectively, indicating good agreement between the measured and calculated results.
Fig. 4 The drive voltages of the seeds, measured single bursts together with calculated packet shapes and burst trains after amplification for square-shaped seed bursts with intra-burst repetition rate of 100 MHz; (a)-(c) pulse bursts with inter-burst repetition rate of 250 kHz; (d)-(f) pulse bursts with inter-burst repetition rate of 500 kHz.
Figure 5 presents the pulse bursts with Gaussian packet shape with the same intra-burst and inter-burst repetition rate as those in the previous cases. The drive voltages for each case were plotted in Fig. 5(a) and 5(d) while the corresponding optical bursts after amplification together with the calculated packet shapes were depicted in Fig. 5(b) and 5(e), respectively. The pulse trains showing the inter-burst repetition rates were illustrated in Fig. 5(c) and 5(f), respectively. Indeed, seed bursts with Gaussian packet shape gain much less distortion than their counterparts with square packet shapes. Because the packet shape distortion in fiber amplifier resulted from gain saturation is dominated by the input packet shape itself. However, slight yet distinguishable distortion still occurred when the inter-burst repetition rate was 250 kHz, which was shown in Fig. 5(b). As the inter-burst repetition rate increased up to 500 kHz, the distortion became negligible owing to the reduced energy storage before each burst. The computed average deviation between the measured and calculated pulse bursts were 7.46% and 9.25%, respectively, for pulse bursts with inter-burst repetition rates of 250 kHz and 500 kHz. The large deviations were contributed by the significant relative error occurred in the front and rear ends of the burst packets. We attribute this to the poor response of the photodiode on small signal intensities and we believe the actual pulse bursts should be better than the measured results. Except for this, the calculated packet shapes for the four cases were all in good agreement with the measured data implying the feasibility of this model.
Fig. 5 The drive voltages of the seeds, measured single bursts together with calculated packet shapes and burst trains after amplification for Gaussian shaped seed bursts with intra-burst repetition rate of 100 MHz; (a)-(c) pulse bursts with inter-burst repetition rate of 250 kHz; (d)-(f) pulse bursts with inter-burst repetition rate of 500 kHz.
As one of the most important applications, nonlinear frequency conversion usually requires the burst shapes to be square or double rectangle [3–6]. Therefore, pre-shaping of the seed burst is usually needed to compensate the gain saturation and thus obtain the desired packet shapes after amplification. In order to get the required packet shapes of the seed, numerical calculations were carried out at first. The calculation was started at an initial guess of the seed burst. Then the amplified burst of the supposed seed burst was computed by solving the rate equations for each amplification stage [26]. Based on the differences between the target and calculated results, modifications were made to each pulse of the seed burst successively. Afterwards, another calculation was performed to check the effectiveness of the corrections. Usually, this process would iterate twice at most until reaching the final results.
The calculated pre-shapes of seed bursts with inter-burst repetition rate of 250 kHz and 500 kHz both saw ramp-on trends to get square packet shapes, as were plotted in Figs. 6(a) and 6(d). The measured pulse bursts and target packet shapes after amplification were compared in Figs. 6(b) and 6(e) while the pulse trains indicating the inter-burst repetition rates were plotted in Fig. 6(c) and 6(f), respectively.
Fig. 6 The drive voltages of the seed, measured single bursts together with target packet shapes and burst trains after amplification with intra-burst repetition rate of 100 MHz; (a)-(c) pulse bursts with inter-burst repetition rate of 250 kHz; (d)-(f) pulse bursts with inter-burst repetition rate of 500 kHz. (The calculation process to get the required pre-shape of the seed can be found in Visualization 1)
Similarly, in order to get double rectangle shaped bursts with intra-burst repetition rate of 100 MHz, the required pre-shapes of the drive voltages were calculated and plotted in Figs. 7(a) and 7(d), respectively. Fig. 7(b) and 7(c) illustrate the measured single burst together with the calculated target packet shape and the burst train with inter-burst repetition rate of 250 kHz, respectively. Fig. 7(e) and 7(f) show these results for pulse bursts with inter-burst repetition rate of 500 kHz.
Fig. 7 The drive voltages of the seed, measured single bursts together with target packet shapes and burst trains after amplification with intra-burst repetition rate of 100 MHz; (a)-(c) pulse bursts with inter-burst repetition rate of 250 kHz; (d)-(f) pulse bursts with inter-burst repetition rate of 500 kHz. (The calculation process to get the required pre-shape of the seed can be found in Visualization 2)
It can be seen from Figs. 6 and 7 that by pre-shaping the seed laser according to our numerical results, the measured packet shapes of the burst-mode-operated fiber MOPA were mostly in good agreement with the target packet shapes. However, there were still some subtle differences between the target and measured pulse bursts in packet shapes. The computed average deviation between the measured and calculated pulse bursts with square shape bursts are 2.34% and 2.52%, respectively, for pulse bursts with inter-burst repetition rates of 250 kHz and 500 kHz. These values for pulse bursts with double rectangle shapes were 4.66% and 3.7%, respectively. We attributed this to the slight nonlinearity of the voltage-to-current relation for the laser driver. More precise pre-shaping of the seed laser can be expected either by calibrating the nonlinearity of the driver beforehand or by constantly monitoring the burst shapes and then modifying the drive voltage of the seed with a closed loop in real time at the cost of dramatic increase in expenses. Although there are slight deviations between the calculated and experimental results without careful calibration or real time feedback, we believe these results are still acceptable and are already good enough for practical applications.
4. Conclusions
In summary, we have experimentally demonstrated a burst-mode-operated, sub-ns fiber MOPA system with direct seed-packet shaping. A fast DAC board with 1 Gsps sampling rate and 16 bit resolution as well as an analog drive circuit with 1 GHz bandwidth at 2 A were employed as the driver of a DFB LD for packet-shaped burst mode operation. With proper adjustment of the drive voltage, pulses with widths as short as 700 ps and peak powers as high as 1 W can be generated from the seed. The average power of the laser can be amplified to nearly 40 W after three stages of PM fiber amplifiers. Pulse bursts with different packet shapes including ramp-off, Gaussian, square and double rectangle can be produced from the fiber MOPA by directly pre-shaping the seed pulse bursts. To the best of knowledge, this is the first burst-mode-operated fiber MOPA system incorporating direct seed-packet shaping without external modulators. Even shorter pulses with durations of ~300 ps are readily available using the same analog LD drive circuit with the help of a commercial arbitrary wave generator with higher bandwidth. We believe our system has offered a cost-effective solution to the generation of pulse bursts with arbitrary packet shapes for many practical applications.
Funding
National Natural Science Foundation of China (NSFC) (61505236), Innovation Program of Shanghai Institute of Technical Physics (CX-2), Program of Shanghai Subject Chief Scientist (14XD1404000).
Acknowledgments
Dr. Tao Chen acknowledges nLIGHT Inc. for their excellent free fiber samples and Mr. Xiangyu Meng from Lasfiberio Co. Ltd. for the customized FWDM.
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