We present an all-fiber all-polarization-maintaining (PM) single mode (SM) fiber pulse nonlinear amplification system. The seed laser with a repetition rate of 200 MHz is amplified by two-section erbium-doped PM gain fibers with different peak-absorption rate. The amplified pulse duration can be compressed into 34-fs with 320-mW output power, which corresponds to 1.6-nJ pulse energy and approximate 23.5-kW peak power. In addition, the amplified and compressed pulse is further coupled into the high nonlinear fiber and an octave-spanning supercontinuum generation can be obtained. To the best of our knowledge, it is the highest peak power and the shortest pulse duration obtained in the field of all-fiber all-PM SM pulse-amplification systems.
© 2016 Optical Society of America
In the past decades, significant progresses have been achieved both in mode-locked all-fiber pulse laser systems and all-fiber pulse amplification systems [1–5]. The ultrashort pulsed lasers have already been applied in many fields such as industrial processing, transient optics, supercontinuum (SC) generation, and optical frequency comb [6–8]. Therefore, some researches about how to improve peak power and the pulse energy in an all-fiber system have also been investigated. One of topics is chirped pulse amplification (CPA) technology which is usually used to amplify the pulse power without obviously nonlinear-effect [9, 10]. Recently, a kind of photonic crystal fiber (PCF) named air-guiding photonic bandgap fiber was used to recompress the positively chirped pulses. This PCF based fiber compressor presents much better result than the traditional grating pairs . Beside the CPA technology, the nonlinear pulse amplification (NPA) technology have also been reported in recent years [12–14]. The conventional methods of NPA are named as cubicon amplification and self-similarity amplification. Utilizing cubicon amplification, the pulse with strong chirp has asymmetric temporal and spectral profile. Therefore, self-phase modulation (SPM) during the amplification leads to a negative dispersion slope which can be used for the compensation of third-order dispersion mismatch between the fiber stretcher and bulk grating compressor [15–17]. On the other hand, in the self-similar amplification systems, the spectrum of the pulse is stretched by SPM while the energy of the pulse is amplified [18–20]. It is possible to compress the pulse into a very short duration (sub-100 fs) with a broadband spectrum. The duration of the compressed pulse, in this case, can be much shorter than the duration of seed laser. Consequently, the pulse amplified by these two methods of NPA usually has much shorter pulse duration and higher peak power compared with CPA technology. All these pulses with short duration and good quality are very suitable for some nonlinear processes, SC generation for example, and this process has been abundantly researched .
Although, CPA system is the most common technology in the field of all-fiber pulse amplification, it usually includes many complex stages such as stretcher, pre-amplifier, main amplifier, and spatial compressor. The duration of pulse generated from an all-fiber CPA system which compressed by traditional grating pairs is usually approximate 1 ps. Though the bandgap fiber can compress the pulse duration effectively into about 100-fs, it cannot be connected directly with amplifier. Compared with the traditional CPA system, the pulse generated from NPA system with several-ten-femtosecond duration, is more suitable for the ultrafast process and nonlinear effect investigations. But the NPA system usually needs a pulse pre-shape structure to manage the third-order dispersion or to provide adaptive nonlinearity. For the cubicon amplification systems, most of them have more than 100-m long single-mode fiber (SMF) in order to provide the strong chirp. This is not suitable for some tiny all-fiber system due to the adoption of the long SMF which is used as stretcher. For self-similarity amplification systems, the pulse is also needed to be pre-shaped by the grating pair and the SMF in order to obtain a good pulse profile when it is compressed and the all-fiber based structure cannot be realized.
In this works, a polarization-maintaining (PM) all-fiber pulse amplification system with a NPA technology is presented. This system has two amplification stages and a fiber compressor. The erbium-doped gain fibers of the two amplification stages have different peak-absorption rate (PAR). The first stage is used as the pre-shaper and the second one is used as the main amplifier. After the amplification, the pulse is compressed by using a standard PANDA fiber. Finally, the compressed pulse is applied in a nonlinear process which an octave-spanning SC is generated by the high nonlinearity fiber (HNLF).
The amplification system delivers the maximum output power of 340 mW and 1.7-nJ pulse energy with the repetition rate of 200 MHz. The 34-fs pulse duration can be obtained at the output power of 320 mW. The compressed pulse has a conspicuous broad base and about 50% of the total energy is concentrated in the main part of the pulse. Therefore, the pulse peak power is approximate 23.5 kW. The generated octave-spanning SC ranges from 1.01 to 2.02 μm. This is the first time for us to obtain the shortest pulse duration in a PM all-fiber structure systems. Some detailed results such as autocorrelation traces, spectral curve, and the variation of the output power are presented and discussed. Compared with the existing high-repetition rate, pulse amplification systems based on all PM single-mode (SM) fiber, the highest peak power and the shortest duration are obtained to the best of our knowledge. In addition, this system is very tiny compared with the traditional nonlinear pulse amplification system due to the all-fiber based structures.
2. Principle and design of amplification
The detailed schematic of the pulse amplification system is shown in Fig. 1. This system is very compact and constituted by an all-fiber mode-locked seed laser, two PM erbium-doped gain fibers and a fiber compressor. In addition, this system can be applied for the SC generation by using a HNLF and light is finally outputted through a collimator. The types of the fiber with different color in Fig. 1 are shown with a brief name and will be discussed in detail below.
One can find the principle and the detailed structure of the seed laser in our previous work . The output of the seed laser is 1.8 mW and the spectral bandwidth is around 5 nm with the center wavelength of 1560 nm. The duration of the pulse formed by the oscillator is around 230 fs with a repetition rate of 200 MHz. Before amplification, few negative chirp is accumulated due to the abnormal dispersion of the pigtail fiber.
Firstly, the seed laser is coupled into an Erbium-doped PM fiber (EDF25-PM, D = −27.99 ps/nm/km, PAR = 25 dB/m@1530 nm, represented by shallow green line in Fig. 1 and named Er25-doped PM fiber) through a compact combined component which consists of a PM isolator and a wavelength division multiplexer (PMIWDM). As the first stage, the gain fiber provides the functions both of pulse shaper and gain medium. The 3-m long gain fiber with a low-gain factor is chosen in order to pre-stretch the pulse with a small and nearly linear chirp, As a result, the dispersion mainly affects the pulse duration and the nonlinear effect can be regardless. On the other hand, the nonlinearity in this stage is also investigated carefully in order to keep a good quality of the spectrum. Only 50-mW power of forward pump laser diode (LD) is chosen. This pump power is high enough for the main amplification. The output power of this stage is about only 10 mW, and the duration of the pulse with a small positive chirp is stretched into about 1 ps.
Secondly, the pre-shaped pulse is coupled into another kind of Erbium-doped PM fiber (Er80-4/125-HD-PM, D = −12~-18 ps/nm/km, PAR = 98 dB/m@ 1530 nm with dark green line in Fig. 1 and named Er80-doped PM fiber) also through a PMIWDM. In this stage, the gain fiber must provide the functions both of spectral stretcher and gain medium. The purpose of this stage is just opposite of the first stage. High average power is one condition of high peak power when pulse duration is maintained. The high peak power ensures that the spectral width can be effectively broadened. The broad spectral width is one necessary in order to obtain an ultrashort pulse duration. Therefore, to amplify the average power into a high level and stretch the spectrum into a broad width, the pulse duration should be remained. So the length of the gain fiber is around 1 m. And the power of only one forward pump LD is not enough for the amplification purpose, so that another backward pump LD is adopted in this stage. After the amplification, the width of the spectrum is broadened into around 50 nm. The maximum output power of this stage is 340 mW corresponding to the maximum pump power (750 mW). However, the power of the pump should keep in a certain range in order to obtain good shape when the pulse is compressed. The best shape of the compressed pulse is obtained when the output power is around 320 mW. In this situation, 600-mW forward pump power and 750-mW backward pump power are adopted.
Thirdly, the amplified pulse with a small positive chirp and broad spectral width is coupled into a normal PANDA transmitted fiber (SM15-PS-U25A, D = 18 ps/nm/km, shown as red line in Fig. 1 and named PM-1550). The pulse is recompressed by the negative dispersion of the transmitted fiber. After optimizing the length, around 1-m transmitted fiber is appropriate for the dispersive compensation. The shortest duration of the compressed pulse is 34 fs with an average output power of 320-mW. Considering the 200-MHz repetition rate, the pulse energy is 1.6 nJ which leads to a peak power of approximate 23.5 kW. Because the system is PM all-fiber-based structure, it is very tiny and the linear polarization is maintained. Meanwhile, there have few losses in each part.
Finally, the compressed pulse with the high peak power is coupled directly into a normal 0.65-m PM HNLF (PM-HNLF, D = −1.5 to ± 2.0 ps/nm/km, γ = 10.7 W-1/km, shown as iridescent line in the Fig. 1 and named PM-HNLF) for the nonlinear process of the SC generation. Because of the high peak power (>10 kW), the enough pulse energy (nJ-level) and the short pulse duration (sub-50 fs), an octave-spanning SC is generated. The SC light is outputted through a collimator.
In addition, the pump LD, adopted for each amplified stage, works at 980-nm wavelength and the maximum output is 750 mW. The light of the pump LD is coupled into the amplifier by PM-980 fiber (PM980-XP, shown as violet line in Fig. 1 and named PM-980).
3. Result and discussions
The cavity length of the seed laser is changed in this work and the repetition rate is changed into 200 MHz. The symmetrical pulse and spectrum shape indicate a good pulse quality. The spectral curve, the autocorrelation trace and mode-locked pulsed train of the seed laser are shown in Figs. 2(a), 2(b), and 2(c) respectively.
As mentioned above, the most important point of the first stage is how to obtain a pulse without serious distortion and a high-quality spectra without split. Both 1-m and 3-m Er25-doped PM fibers are considered in the first amplification stage. However, comparing the autocorrelation trace of the pulse as shown in the inset of Fig. 3, 3-m-length Er25-doped PM fibers is more suitable for the main amplification, which enables the pulse stretch into 1 ps. The different nonlinear strength is also investigated in order to obtain a good spectral quality. The evolution of the spectrum at different pump power are shown in Fig. 3 (the ordinate is linear unit). As shown in Fig. 3, the spectrum at 50-mW pump power is already good enough for the next amplified stage. Therefore, 3-m-length gain fiber and the 50-mW pump power are chosen in the first amplification stage. The jitter on the top of the spectrum is caused by the seed laser, and the variation of spectral envelope is caused by SPM effect.
In the second-stage amplification, the most important assignment is to amplify the output power and broaden the spectral width. The length of Er80-doped PM fiber adopted in this stage is around only 1 meter to avoid additional stretching of the pulse. The maximum output power is 340 mW and the slope efficiency is about 23%. The output power versus pump power and the output spectrum at an output power of 340 mW is presented in Fig. 4. The spectral width is broadened into 54 nm at the 340-mW output power. Although the shape of the spectrum is modulate by SPM and is different from the seed laser, the quality of the spectrum is still acceptable for the compressing.
Because the small chirp provided by the first stage is fixed, suitable length of the PM-1550 fiber can be chosen to compress the pulse into the shortest duration through dispersion compensation. Therefore, approximate 1 m PM-1550 fiber with negative dispersion is used as the compressor. The evolutions of spectrum and pulse autocorrelation trace at different pump power are shown in Fig. 5. The spectrum is broadened and split into multi-peak structure due to SPM with the increasing of pump power till to 1350 mW (600-mW forward pump and 750 mW-backward pump). However, the pulse envelope shows no serious distortion and the duration of the pulse is shortened with the increase of the pump power. With further increasing the pump power, more fringes are generated on the spectrum and the spectral width is shorten due to the high-order nonlinearity. Meanwhile, the pulse duration becomes wider and the pulse is split. Although the further variation of the spectrum and the pulse shape cannot be obtained due to the limitation of the maximum pump power, the best one have already been obtained around 1350-mW pump power, corresponding to 320-mW output power. The spectral curve and the autocorrelation trace in this case are presented in Fig. 6. As shown in the figure, the shortest pulse duration is 34 fs if a Lorentz pulse is assumed.
Finally, the amplified and compressed pulse is coupled into the HNLF to verify the quality of the pulse. The octave-spanning SC can be generated if the pulse duration is shorter than 50 fs with nanojoule pulse energy. In addition, the length of the HNLF can be shortened if the pulse has a high peak power . In the best situation mentioned above, an octave-spanning SC is generated by injecting the 34-fs pulse into 0.65-cm HNLF. The SC begin with wavelength shorter than 1.01 μm and end with the wavelength longer than 2.02 μm. Therefore, the pulse obtained from this system indeed has very short pulse duration and high peak power. The spectrum of the SC is shown in the Fig. 7.
In conclusion, an all-fiber PM SM ultrashort pulse laser system with high repetition rate and high average power is presented. The maximum output power can reach 340 mW. Furthermore, 320-mW output power with the repetition rate of 200 MHz is obtained corresponding to the pulse energy of 1.6 nJ. The pulse duration is compressed into 34 fs resulting to the peak power of approximate 23.5 kW. In addition, the third-order dispersion between the pulse per-shaper (first amplification stage) and the compressor (standard PANDA fiber) is not perfect matched. Therefore, the pulse generated by this system has a relatively broad pedestal and may has a double-scale structure. If third-order dispersion of the fiber compressor can be precisely designed, this double-scale structure and the pedestal could avoid. In other word, an ultrashort pulse without the pedestal could be realized. This is a further work that will be researched in the future. The autocorrelator used to measure the pulse shape can only give the duration value of the fitting curve, so that the pulse duration of 34 fs is based on the assumption of a Lorentz pulse shape. Compared with similar all-fiber systems, this fitted pulse duration is indeed the shortest one to the best of our knowledge. By utilizing this system, the SC generated from the HNLF is an octave-spanning. Taking SC generation as an example, this system is proved suitable for the application of some ultrafast and nonlinear processes.
Project supported by the CAS/SAFEA International Partnership Program for Creative Research Teams and Natural Science Foundation of China (61275164, 11573058). This project was also supported by the Open Research Fund of State Key Laboratory of Transient Optics and Photonics, Chinese Academy of Sciences, No. SKLST201401. This work was also supported by National Major Scientific Instrumentation Development Program of China (2011YQ120022).
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