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Using a home-made seed at 1053 nm from a Yb3+-doped passively mode-locked fiber laser of 1.5 nJ/pulse, 362 ps pulse duration with a repetition rate of 3.842 MHz, a compact, low cost, stable and excellent beam quality non-collinear chirped pulse optical parametric amplifier omitting the bulky pulse stretcher has been demonstrated. A gain higher than 4.0×106, single pulse energy exceeding 6 mJ with fluctuations less than 2% rms, 14 nm amplified signal spectrum and recompressed pulse duration of 525 fs are achieved. This provides a novel and simple amplification scheme.
©2007 Optical Society of America
1. Introduction
Chirped pulse optical parametric amplification (CPOPA) is an attractive technique for the generation of ultrashort laser pulses with high-peak powers [1–8]. The idea of CPOPA, proposed by Dubietis et al. [9], combines the optical parametric amplification (OPA) with the chirped pulse amplification (CPA). The CPOPA [10–15] technique offers significant advantages over other schemes, such as high gain per single pass, a broad gain bandwidth, high output beam quality because of low heat deposition and a small B integral, energy combining of uncorrelated and incoherent pump beams, phase-conjugated output signal and idler pulses and a high contrast ratio owing to a low level of pre-pulses, amplified spontaneous emissions. In CPA, the amplified spontaneous emission generates a long pedestal, spectral clipping and phase distortions lead to an additional background near the main pulse. In CPOPA, since there is no gain outside this time window defined by the pump pulse, only the pedestal caused by parametric superfluorescence which is within the gain time window will be amplified.
Parametric amplification [16–19] is an instantaneous nonlinear optical frequency conversion process, efficient amplification can only be achieved by matching the duration and the phase of pump and seed pulses. One method of matching pulse durations uses a nanosecond (ns) pump. In this scheme the seed pulse is stretched to a sub-nanosecond range in a grating stretcher, which makes entire system complicated and bulky.
To overcome this limitation above, seeded by a home-made Yb3+-doped self-starting passively mode-locked fiber laser omitting dispersion compensation outside the cavity, a compact, low cost, stable and excellent beam quality (M2=1.2) non-collinear chirped pulse optical parametric amplification (NCPOPA) omitting the bulky pulse stretcher and pumped by ns pulse has been demonstrated in this paper. The integrated fiber laser without dispersion compensation is capable of delivering several hundreds picoseconds chirped pulse, as a result, the pulse stretcher has been omitted. This scheme is useful to the production of a relatively compact and low cost NCPOPA system. Overall net gain of higher than 4.0×106 and single pulse energy exceeding 6 mJ with fluctuations less than 2% rms have been achieved. Amplified spontaneous fluorescence constitutes less than 1% of the total output pulse energy. The 14 nm full width at half maximum (FWHM) amplified signal spectrum and the 14 nm (FWHM) idler pulse spectrum have been observed. We also demonstrate that the output pulse can be compressed to 525 femtosecond (fs).
2. Experimental setup
The schematic diagram of NCPOPA based on Yb3+-doped fiber laser is shown in Fig. 1.
Fig. 1. Experimental setup of the CPOPA system HWP: Half-wave-plate, TFP: Thin-film-polarizer, PC: Pockels cell
The signal pulse was generated by a home-made Yb3+-doped self-starting passively mode-locked fiber laser operating at 3.8 MHz and using pairs of diffraction gratings for dispersion compensation outside the laser cavity, which centered at 1053 nm with 10 nm (FWHM) spectrum bandwidth. Figure 2 shows the details on the fiber laser. It consists of a mode-locked oscillator and a power amplifier. The pump source is a fiber-coupled single mode laser diode (LD) operating at the wavelength of 976 nm. Pump light is coupled into the optical loop through a 976 nm/1053 nm wavelength division multiplexer (WDM). The polarization insensitive isolator (ISO) ensures a unidirectional operation cavity. Coupler 1 is the output coupler of the laser. In our fiber laser, the combination of coupler 2 (coupling ratio is 50:50) and coupler 3 (coupling ratio is 97:3) acts as the asymmetric couple. Power of 97% after the coupler 3 goes into the cavity. The operation of the mode-locked figure-eight fiber laser relies on the different phase shifts between counter-propagating pulses in the loop. A single input is split into two counter-propagating pulses with different intensities. When the two pulses propagate in the loop, they have different phase shifts induced by self-phase modulation (SPM). Finally, they recombine at the coupler 2 synchronously. If we select a most suitable coupling ratio of coupler 3 and the positions of polarization controllers (PC1 and PC2), the laser will be much easy self-starting. The Yb3+-doped fiber laser showed the excellent long term stability under the temperature stabilized ultra-clean environmental condition, so the mode-locking operation could be hardly influenced by the external environment.
As a seed source, we used the 362 picoseconds (ps) (FWHM) [see Fig. 3] chirped pulse by omitting dispersion compensation outside the cavity.
The seed beam at 10 Hz repetition rate leaving the Pockels cell was collimated by the 1.8:1 lens telescope and injected into the first OPA stage. The diameter of the horizontally polarized seed beams after the telescope was roughly 2.5 mm in the first OPA stage. The energy of the seed pulse injected into the first class OPA stage was 1.5 nJ. The output of the first OPA stage was resized by a 1:1.1 beam expanding telescope to ∼2.8 mm diameter. The resulting signal pulse was injected into the second OPA stage.
The pump pulse was generated by a commercial Q-switched single-longitudinal mode seeded by Nd:YAG laser operating at 532 nm (Continuum Powerlite 8010), which produces a 10Hz pulse train of 800 mJ, 6 ns (FWHM) pulse. The pump laser output was split into two beams and the maximum pump pulse energy of each stage was 150 mJ. The energy of both pump pulses can be continuously adjusted using a combination of a half-wave plate and a thin film polarizer, which allows gradual increase of pump power and reduced probability of optical damage. The vertically polarized pump beam of each stage was collimated by 2.7:1 telescope to the diameter of ~3 mm and then sent to the nonlinear crystal.
The timing system utilizes a Stanford Research Systems DG-535 signal generator and a 5046E (MEDOX Inc.). The measured jitter of our timing system is less than +/-0.5 ns.
BBO crystal was chosen as the nonlinear medium because of its high effective nonlinear coefficient. The first and second stage crystals had a cross-section of 6 mm×6 mm and their lengths were 16 mm. Both antireflection-coated crystals were cut at 22.86° to provide the largest possible gain bandwidth and to facilitate type I non-collinear angular phase matching with an external non-collinear angular between the seed and the pump of 1.1°. Each crystal was mounted on precision polarizer holder, where there was a precision translation stage and precision rotary stage below it to optimize the phase matching angle and the non-collinear angle between the pump and the seed beams.
3. Results and discussion
We first examined the amplification gain of this NCPOPA system based on Yb3+-doped fiber laser by using a laser energy meter. The gain is defined as the ratio of amplified energy to the input signal energy. Figure 4 shows the amplification gain versus different pump intensities dependence for each individual stage. Each point of the curve represents the average value over 600 shots.
The energy of the input seed pulse was 1.5 nJ, corresponding to a 84 W/cm2 intensity. As can be seen from the Fig. 4(a), a maximum gain of ∼3740 was obtained for the first stage with a 350 MW/cm2 pump intensity. The output signal from the first stage served as the input signal to be amplified in the second stage. A saturated gain of ∼1100 was achieved at this stage with a 350 MW/cm2 maximum pump intensity [Fig. 4(b)]. The overall net gain from the two-stage OPA reached 4.0×106, corresponding to the final output energy of 6 mJ with fluctuations less than 2% rms.
Fig. 4. Experimental gain of the amplifier versus the pump pulse intensity:(a) the first CPOPA stage and (b) the second CPOPA stage.
In combination with a measurement of the NCPOPA output power with and without seeding, the remaining amount of superfluorescence in the presence of seed light can be estimated to be less than ∼1% of the total output power.
The spectrum of the input seed and final amplified output was measured with HR2000 spectrometer (Ocean Optics). Figure 5(a) shows the measured seed and final amplified pulse spectrum with a bandwidth of 10 nm and 14 nm (FWHM), respectively. At the same time, we examined the spectrum of the output idler pulse with a bandwidth of 14 nm (FWHM) centered at 1075 nm as shown in Fig. 5(b).
Fig. 5. Measured spectrum of (a) input seed pulse and amplified signal pulse, (b) idler pulse obtained by the NCPOPA
The spatial intensity profile for the amplified output signal is shown in Fig. 6.
The temporal intensity profile of the recompressed signal pulse is shown in Fig. 7. The measured pulse duration is 525 fs (FWHM).
4. Conclusion
In conclusion, we have demonstrated a novel CPOPA system that utilizes a home-built integrated fiber laser omitting dispersion compensation and delivering several hundreds picoseconds chirped pulse as a seed source, which omits the bulky pulse stretcher. This scheme does not introduce additional complexity before the amplification. This system is remarkably compact, economical and stable. And we have also achieved an excellent beam quality after amplification. The overall net gain of the two-stage OPA higher than 4.0×106 and single pulse energy exceeding 6 mJ with fluctuations less than 2% rms are reached. The 14 nm (FWHM) amplified signal spectrum, the 14 nm (FWHM) idler pulse spectrum and recompressed pulse duration of 525 fs have been observed. Since the broadband mode-locked Yb3+-fiber laser [20–21] has been demonstrated, broadband NCPOPA based on the fiber laser will be obtained in the future.
Acknowledgments
This project is financially supported by the National Natural Science Foundation of China (under Grant<Project>No. 60408002) and the Natural Science Foundation of Shannxi Province in China (under Grant<Project> No. 2004F02).
References and links
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