Broadband high-power mid-IR femtosecond pulse ... - OSA Publishing

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Letter

Vol. 40, No. 24 / December 15 2015 / Optics Letters

Broadband high-power mid-IR femtosecond pulse generation from an ytterbium-doped fiber laser pumped optical parametric amplifier CHENGZHI HU,1 TAO CHEN,2 PEIPEI JIANG,1,* BO WU,1 JIANJIA SU,1

AND

YONGHANG SHEN1

1

State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China 2 Key Laboratory of Space Active Opto-Electronics Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China *Corresponding author: [email protected] Received 20 October 2015; revised 13 November 2015; accepted 18 November 2015; posted 18 November 2015 (Doc. ID 252299); published 10 December 2015

We report on a high-power periodically poled MgO-doped lithium niobate (MgO:PPLN)-based femtosecond optical parametric amplifier (OPA), featuring a spectral seamless broadband mid-infrared (MIR) output. By modifying the initial chirp and spectrum of the mode-locked seed laser, the Yb fiber pump laser exhibits a final output power of 14 W with sub-200-fs pulse duration after power amplification and compression. When the OPA was seeded with a broadband amplified spontaneous emission (ASE) source, a damage-limited 0.6 W broadband MIR radiation was experimentally obtained under the pump power of 10.15 W at 82 MHz repetition rate, corresponding to an overall OPA conversion efficiency of 32.7%. The 3 dB bandwidth of the mid-IR idler was 291.9 nm, centering at 3.34 μm. © 2015 Optical Society of America OCIS codes: (190.4410) Nonlinear optics, parametric processes; (320.7090) Ultrafast lasers; (140.3070) Infrared and far-infrared lasers. http://dx.doi.org/10.1364/OL.40.005774

The mid-infrared (MIR) spectral window, often defined as the “molecular fingerprint” region, is of high relevance to precise spectroscopy and optical diagnostics as many molecules have their fundamental vibrational bands in this range [1,2]. Nonlinear parametric conversion technique has been proved to be an attractive route to extend the wavelength of standard ultrafast fiber lasers from readily available near-IR region to longer MIR wavelength. In practice, synchronously pumped optical parametric oscillators (OPOs) are commonly adopted as viable frequency converters for ultrafast sources [3]. However, precise control of the cavity length is required, resulting in relatively high complexity in comparison with optical parametric amplifiers (OPAs). Additionally, OPA offers a solution to the generation of high repetition rate, low temporal coherence sources, which meet urgent demands for some high 0146-9592/15/245774-04$15/0$15.00 © 2015 Optical Society of America

signal-to-noise-ratio applications in imaging systems where pulse-to-pulse variation of typical supercontinuum sources at spectral edge is viewed as a limit to usability [4]. Benefiting from the rapid development of high-power ultrafast fiber lasers [5], achievable output powers and energies from such devices are raised to levels suitable for pumping OPA [6,7]. Referring to the broadband MIR generation, there are two major technologies that can be considered to construct such laser sources with quasi-phase-matched crystals. One is employing chirped PPLN as the nonlinear converter which is able to generate broadband MIR even pumped by lasers with narrow linewidths [8], and the other is utilizing the broadband seeds and high-power broadband fiber lasers as the pump sources [9]. Although these approaches are theoretically different, they both require robust high-power femtosecond fiber lasers for measurable output, allowing high single-pass parametric gain to drive the parametric process into saturation [10] and, thus, suppressing intensity fluctuation. Therefore, long nonlinear crystals and tight focusing of the pump are usually adopted to improve the conversion efficiency. In this Letter, we report on a novel broadband femtosecond MIR source covering 3.2 to 3.5 μm, which is generated by a fiber-laser-pumped, single period MgO:PPLN-based OPA. As the seed was a broadband continuous wave (CW) ASE source, we expect that the broadband MIR output will feature a spectral seamless characteristic. The combination of such a spectral seamless broadband MIR source with a tunable F-P will always result in stable comb-like laser output, totally releasing the complicated cavity mode matching requirement [11]. To the best of our knowledge, this is the first demonstration of such a spectral seamless femtosecond source in the MIR region. The spectral features are quite different from those of the previously reported difference-frequency-generation (DFG) sources using a self-seeded supercontinuum or laser diode as the signal source [12,13]. Because the signal seeds applied to the OPAs referenced above contained longitudinal modes, the spectra of the generated MIR idler waves would also possess plenty of longitudinal modes or even could form equally spaced frequency

Letter combs. Different from those seeds, the ASE seed we adopted is a modeless source, which can effectively bridge the mode gaps of the idler waves of the OPA, in principle, forming the seamless MIR radiation. However, due to the lack of camera and virtually imaged phased array working in the MIR for the time being, the spectral characteristics of such a seamless MIR source were not verified. The experiment setup is schematically illustrated in Fig. 1. A home-built all-normal dispersion (ANDi) [14] Yb fiber oscillator based on nonlinear polarization rotation served as the seed source of the nonlinear amplifier with an average power of 100 mW and a repetition rate of 82 MHz. The chirped output pulse with duration of 2.8 ps can be compressed to below 180 fs, close to the Fourier-limited duration with a spectral bandwidth of 15 nm centered at 1037 nm. It was then pre-amplified by a Yb fiber amplifier with counter pumping configuration consisting of a segment of Yb-doped double clad fiber (DCF) (2 m, Coractive Yb 7/125 DCF) and a polarization controller to improve the polarization extinction ratio to >10 dB. The spectral width and average power after the preamplifier were increased to 35 nm and 1.75 W, respectively, though some oscillations emerged at the edges of the spectrum due to self-phase modulation. The preamplified laser was coupled into the final power amplifier, consisting of a Yb-doped large mode area (LMA) polarization-maintaining (PM) DCF (7 m, Nufern PLMA-YDF-25/ 250-VIII) and two 25 W pump diodes working at 915 nm through a high-power 2 1 × 1 PM combiner. The LMA fiber was coiled around an air-cooled aluminum spool with a diameter of 100 mm to ensure good beam quality. A collimator with half-meter-long matched passive single clad PM fiber was spliced to the gain fiber to eliminate the residual pump similar to [15].The amplified pulses were then compressed by a pair of gratings (1000 lines/mm) in a double-pass configuration with a total efficiency of 77%. Group velocity dispersion of this dispersion delay line is calculated to be −4923 fs2 ∕mm. To optimize the output pulse quality in terms of minimal residual wing structure, another grating pair was employed to adjust the chirp prior to power amplification. A free-space spectral filter inserted into the optical path was applied to extract a smooth spectral window and to block oscillations around the edge of the spectrum, which was considered to impose nonlinear chirp. The optimal output was experimentally obtained by changing the spectral pass band of the filter and the pre-chirp value of the first grating pair precisely and then adjusting the second grating pair to compress the amplified pulses to its shortest FWHM duration at the desired pumping power [16].

Fig. 1. Schematic diagram of the fiber amplifier and OPA; ISO, isolator; HPC, high-power combiner; PC, polarization controller; HWP, half-wave plate; BE, beam expander; DM, dichroic mirror.


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The output beam of the fiber laser was then aligned to pass through a free-space PM isolator with a clear aperture of 5 mm for parametric conversion. Two half-wave plates were placed before and after the isolator to control the polarization direction of the laser to pass the isolator first and then align to the principal axis of the PPMgLN crystal. The pump laser beam after the first dichroic mirror (DM1) was focused to a beam diameter of about 80 μm (measured at 1∕e 2 ) at the waist to ensure enough peak power density by a plane–convex thin lens with a focal length of 200 mm. A 50 mm long homemade PPMgLN crystal with a fixed period of 31.4 μm was mounted inside an oven to allow temperature tuning in the range of 20°C–100°C with a precision of 0.1°C. The unconverted pump was filtered out by the second dichroic mirror (DM2), while the signal and idler output were separated using a third dichroic mirror (DM3). We varied the pre-chirp and the spatial filter to seek an optimized compressed output as the pump for broadband parametric conversion with relatively long crystal length. Under pump power of 38 W, 20.6 W amplified pulses were obtained with an excellent beam profile. Figure 2 demonstrates the corresponding optical spectra for 3.2, 7, and 14 W output power after pulse compressor, corresponding to pulse energies of 40, 85, and 170 nJ, respectively. The corresponding spectral widths (measured at −3 dB) are 8.9, 12.1 and 19.1 nm centering at 1058 nm. As shown in the inset of Fig. 2, the autocorrelation trace of the compressed pulse has a width of 248 fs at the output power of 20.6 W (the value directly output from the amplifier before compression) with negligible pedestal, in contrast with the autocorrelation trace of the corresponding transform-limited pulse calculated from the output spectrum. To realize spectral seamless MIR output, we employed a commercial unpolarized CW fiber ASE source emitting a wide spectrum ranging from 1.565 to 1.61 μm [shown in Fig. 3(c)] as the seed signal of the OPA. The collimated beam of the ASE source was expanded to about 5 mm and then aligned to get a good spatial overlap with the pump beam after DM1. The working temperature of the OPA was determined by carefully tuning the central signal wavelength of the OPG (when the ASE was blocked) to that of the ASE seed by temperature

Fig. 2. Measured spectrum of the fiber laser after pulse compression at different output powers; inset top: laser output power and pulse duration as functions of the depleted pump power; inset bottom: autocorrelation traces of the amplified pulses with average power of 14 W (black curve) and the corresponding autocorrelation of transform limited pulses (blue curve).

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Fig. 3. (a), (b) Signal and idler output power as functions of pump power in OPA/OPG operation. The red circles and the black rectangles are measured data points, and the solid lines are linear fits. (Right) Total power conversion efficiency versus the pump power. (c), (d) Recorded spectra of signal and (e), (f ) idler output under OPA/OPG configuration at different pump powers.

control. Figure 3(a) presents the power scaling results of the single-pass femotosecond OPA with signal seed power of 75 mW. The measured pump threshold was about 1.5 W. The parametric powers of 2.94 W for signal and 0.60 W for idler were recorded at the maximum pump power of 10.8 W. The signal and idler slope efficiencies are calculated to be 31% and 6.2%, respectively. The 3 dB bandwidth of the signal spectrum, measured by an optical spectrum analyzer (Yokogawa, AQ6375), was recorded to be 5, 15.3, and 16.8 nm. Figure 3(e) illustrates the idler spectrum measured by a Fourier transformed infrared spectrometer (ARCOPTIX, FTIRRocket). The measured 3 dB bandwidths under different pump powers were 95, 240, and 291.9 nm. As a comparison, similar measurements were carried out for OPG operation. Under the same highest pump condition, a signal power of 2.35 W at a slope efficiency of 30.1%, together with an idler power of 0.44 W at a slope efficiency of 6%, was achieved with a threshold of 3.5 W, as were depicted in Fig. 3(b). Figures 3(d) and 3(f ) illustrate the measured signal and idler spectra of the OPG, respectively, under different pump powers. The bandwidth of the signal was 26.2 nm at the maximal pump, with a rippled spectral profile similar to that of the pump spectra and the corresponding idler bandwidth, was 340 nm. Comparing Figs. 3(e) and 3(f ), the measured idler bandwidth in OPG was evidently wider than that of the seeded OPA at the maximal pump, but approximately equal at lower pump. We attributed such a difference of spectral width in the idler output to the bandwidth limited seed. Note that there are two peaks in the spectra of signal under both OPA and OPG operations. The left peak located at 1568 nm may be related to the

Letter self-frequency doubling of idler output at a shorter wavelength side. This could be one of the possible explanations to the discrepancy of signal-to-idler output power ratio from theoretical calculation. In addition to this, the coating on the end surfaces of the PPMgLN crystal may be another factor in deviation of signal-to-idler output power ratio. The transmittance around the signal band is primarily assured, while the transmittance around the idler is not optimized. We found in the experiment that a further increase in the pump power always caused damage in the MgO:PPLN crystal in both OPA/OPG situations, while parasitic visible radiation of OPG was significantly weak under the same pump power. It was calculated that the peak optical intensity exceeded 7.5 GW∕cm2 inside the crystal, close to the 10 GW∕cm2 damage threshold [17]. No beam distortion was observed before the crystal breakdown. By enlarging the pump spot size we could avoid the damage, but this limited the extractable output power as the pump threshold was also increased. The dominating reason for the damage may be related to the self-focusing via the cascaded second-order nonlinearity induced by the high pump intensity [18] and, thus, the broadband modeless ASE injection brought little reduction of the damage threshold in this configuration. Furthermore, the corresponding autocorrelation duration of the signal increased slightly to 446 fs, while the pulse shape remained close match to that of the corresponding pump with an average power of 7 W, indicating OPA operation in the saturable gain regime, as seen in Fig. 4(a). The pulse is believed to exist large positive chirp as the zero dispersion point of PPLN is reported to be near 1700 nm [19]. This is also manifested by the influence of the seed laser power on the parametric gain in the single-pass OPA process. With the pump power fixed at 10 W, we measured the signal output power by seeding the OPA at different power levels. As illustrated in Fig. 4(b), with nearly 85 mW average seed power, saturated maximum extractable power from the OPA could be obtained, and the corresponding gain is ∼14 dB. Note that the seed was CW and the signal was sub-picosecond pulse width with 82 MHz repetition rate; the seed power of 85 mW corresponded to an effective average seed power of only ∼7 μW within the pulse duration and, thus, the 2.3 W output represents a gain as high as 55 dB. The wavelength-dependent power fluctuation and conversion efficiency could be improved with careful synchronization between the pump and the modulated seed, which requires further investigation. In summary, we presented a PPMgLN-based high-power, femtosecond OPA/OPG, which was pumped by a pre-chirp managed Yb-doped fiber amplifier seeded by pulses from an ANDi-type fiber laser. After two sequential amplification

Fig. 4. (a) Autocorrelation of signal pulse in OPG and OPA and the corresponding pump. (b) Signal output power and gain as functions of different seeding power in OPA.

Letter stages, we extracted 14 W sub-200-fs output as the pump source. When the OPA was seeded with a CW ASE around 1.58 μm, 0.6 W of broadband MIR output has been achieved in a single-pass OPA with a total conversion efficiency of 32.7%. Though the pulse duration of the idler was not measured due to the instrument limitation, we believe that the MIR output is still femtosecond pulses since PPMgLN exhibits anomalous dispersion in this regime, while pump pulse was highly positive chirped. The amenable techniques applied in this Letter can be used for further development of compact MIR laser with wider and flatter spectral output, which offers the flexibility for high resolution and sensitivity spectroscopy. Compared with other traditional approaches to achieve similar spectral seamless source like modeless lasers [20], the signal and idler beam in our system will enjoy the benefit of both high peak power, narrow pulse width induced rapid response, and an increase of precision inasmuch as a decrease in standard deviation for the single-shot spectra [21]. Funding. National Natural Science Foundation of China (NSFC) (11304277, 61405174, 61505236). REFERENCES 1. A. Schliesser, N. Picqué, and T. W. Hänsch, Nat. Photonics 6, 440 (2012). 2. S. A. Diddams, L. Hollberg, and V. Mbele, Nature 445, 627 (2007). 3. C. Gu, M. Hu, L. Zhang, J. Fan, Y. Song, C. Wang, and D. T. Reid, Opt. Lett. 38, 1820 (2013). 4. W. J. Brown, S. Kim, and A. Wax, J. Opt. Soc. Am. A 31, 2703 (2014).


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