A compact pulsed mid-IR source with 10-ns pulsewidth, few hundred-microjoule energy, and repetition rate adjustable from 10 through 500 kHz is reported. In order to reach up to 30-% efficiency of down-conversion from 1064 nm to 1500/3500 nm, a 1064-nm narrow-line master oscillator is followed by a two-stage fiber amplifier. In turn, this amplifier is spliced with a large-mode-area photonic crystal fiber to ensure high quality of the beam that pumps a 5-mm thick periodically poled MgO-doped lithium niobate. The spectrum of the device replicates that of a cw 1500-nm seed laser.
© 2013 OSA
Narrow-line semiconductor lasers have been proven to be a compact reliable source of short picosecond – to nanosecond pulses with peak power sufficient for effective parametric wavelength-tuning applications. These lasers integrate gain crystals with a length of fiber that has a Bragg grating (FBG) written in its core. Although capabilities for the mid-IR conversion have already been demonstrated for these lasers, the demand for highly precise, stable, and compact remote spectroscopy tools prompts towards their further miniaturization along with bandwidth narrowing while gaining peak power and adjusting repetition rate. In this sense, the most promising applications of the PPLN-based devices are, e.g., laser spectroscopy, survey of gases along transportation lines, monitoring of atmospheric pollutions, eye safe mapping of remote objects, testing of telecom lines and equipment, IR counter-measure tools, etc [1–3]. Other interesting novel routes of applications of the PPLN nonlinear optics are, e.g., photon-pairs and THz generation [4–6].
Field applications drive a specific desire for a nanosecond (ns) mid-IR source that is compact, high peak-power scalable with low-jitter performance, easy mass-producing at relatively low-cost. Besides, the field deployment implies robust optical alignment, less optical elements vulnerable to environmental factors, etc. This requirement of simplicity and robustness can be achieved in single-pass geometry of the PPLN-based mid-IR source [7, 8]. Although multi-pass, i.e. optical parametric-oscillator (OPO) architecture provides higher conversion efficiency and requires lower pump power [9–13], a single-pass scheme is free of factors haltering continuous tuning that are due to the longitudinal mode structure of the OPO cavity.
Recently, we have demonstrated a compact near- to mid- IR optical parametric oscillator based on a PPLN crystal pumped with a Nd3+:YAG / Cr4+:YAG (1.064 μm) microchip laser . Apart of limited tunability of the repetition rate (5 – 25 kHz) that is due to the microchip pump at 808 nm, other drawbacks of this device are relatively high timing jitter (up to 5%) and relatively low pulse energy through the near-to mid-IR. At the same time, a master oscillator – fiber amplifier architecture operating at the eye-safe wavelength has demonstrated to be an effective tool to pump a single-pass PPLN based device [15, 16]. In this paper, we describe a small-footprint near- to mid-IR source that is based on optical parametric generation in a periodically-poled MgO-doped lithium niobate (MgO:PPLN) driven with a master oscillator which is boosted up by a set of diode-pumped fiber amplifiers (MOFA).
2. Experimental arrangement
An near- to mid-IR tunable narrow-band ~10-ns laser system, comprising a narrow-line semiconductor laser (NLSL) and a MgO:PPLN converter is composed of four main units: (i) a master oscillator, (ii) two-stage fiber amplifier, (iii) seed laser, and (iv) an optical parametric oscillator (OPO) converter (see Fig. 1). Here, the device output is provided by signal and idler from a temperature-tunable OPO, which is seeded by a fiber-pigtailed single-frequency DFB laser  and pumped by a pigtailed semiconductor source, produced by Oclaro, that operates at repetition rates up to 500 kHz and delivers up to 2 W of peak power with a 10-ns (FWHM) pulse at 1064 nm. These pulses are further boosted up in a two-stage fiber amplifier where the first stage is made of a length of Yb-doped single-mode (6/125 μm) polarization maintaining fiber that is two-side in-core pumped by a pair of 1-W 975-nm diodes from IPG Photonics and so provide 35-40 dB gain. In the second stage, the GTWave-type two-port specialty fiber (both fibers produced by FORC-Photonics) that provides a 30-35 dB gain for the system in the 12/125-μm Yb-doped core. The large-mode area (20 μm core diameter) polarization-maintaining photonic-crystal fiber (LMA-PhCF) is used as a pigtail allows generation of definite polarization mode, which is crucial for stable and effective non-linear wavelength conversion; it is worth to notice that such increase of the mode diameter provides an end-cap type of action. That is, the large difference between the core diameter of the Yb-doped fiber and that of the end cap in the form of the LMA-PhCF suppresses parasitic feedback for the amplified spontaneous emission (ASE) and therefore potential damage of the FA chain. This, in turn, enhances gain available for the pulse train in the FA.
The beam outcoming from such a master oscillator-fiber amplifier (MOFA) system is then focused inside an AR-coated 5-mm thick MgO:PPLN crystal supplied by Labfer and having the poling period of 29.9 μm. The 35-mm length of the crystal is optimized to achieve both high parametric conversion efficiency and quality of the output beams without using external cavities; the crystal is mounted in a compact oven allowing controlled variation of temperature from the room level up to 250 °C with long-term stability of 0.1 °C. The maximum power of the 1540-nm cw seed (10 mW) appeared to be sufficient to initiate narrow-band generation not only at the spectral maximum of parametric superluminescence but also far at the wings of the spectrum. The spectral tuning of the seed laser within 10-nm range can be achieved either by a built-in temperature controller or by variation of the injection current and is an extra option for wavelength adjustment at the idler wavelength.
Although the high gain that along with an excellent thermal management capabilities are main advantages of the Yb-FA, this turns into its main drawback at high intensity reached inside the fiber core. The whole set of optical nonlinearities such as self-phase modulation and self-Raman scattering (see Figs. 2 and 3; obtained with Yokogawa AQ 6370 optical spectrum analyzer) degrade the pulse spectrum, restrict the gain, and, subsequently, set an upper bound for the conversion efficiency; an optical breakdown of the doped-fiber facet becomes a reality, too. In order to avoid these detrimental effects and improve the converter operation, we insert a circulator after the first stage of the MOFA which cleans up the pulse spectrum by means of the in-house made FBG adjusted accordingly, that is having 99.99% reflectivity at 1064 ± 0.1 nm, and built-in inside its arm.
In order to set up and to stabilize the OPO wavelength within the signal-wave spectral range (1.5 – 1.7 μm) and simultaneously to ease the threshold of the MgO:PPLN down-conversion, we incorporate outside the MOFA, as an additional unit, a distributed feedback (DFB) semiconductor cw laser (wavelength, 1538 nm) where the line narrowing is also provided by the FBG also written inside the core of the PM single-mode fiber. That is, this laser plays the role of a signal-wavelength seeder. Its polarization adjustment is achieved by rotating the output connector of the seed laser. The maximal power of cw seeding (10 mW) appeared to be sufficient to initiate narrow-band generation not only at the spectral maximum of parametric super-luminescence but also far in the wings of the spectrum. Spectral tuning of the DFB laser can be achieved by a build-in temperature controller.
An additional optics utilized in the optical scheme were a polarizer aligning the pump beam polarization to the proper PPLN crystal axis (not shown here); a dichroic (T = 100% at 1.06 μm and R ≈100% at the signal wavelength, 1.5 – 1.6 μm) mirror for matching the pump and seed beams inside the MgO:PPLN crystal; and a coupling set of lenses providing optimal spatial matching of all the beams (the pump, seed, and signal / idler ones) inside the PPLN converter.
3. Results and discussion
A typical snapshot of a pulse launched into the second cascade of the FA is shown in Fig. 4; it does not experience any noticeable deviation from the original shape and pulsewidth during this amplification mainly because the short fiber length secures an adiabatic regime of the amplification even though the gain factor is not moderate; the ASE noise amplification is also ruled out owing to the end-cap action of the LMA-PhCF. Notice that its short (about 10 ns) duration together with an opportunity to vary pulse repetition rate (10 – 500 kHz) is a remarkable feature of the pump (MOFA) source, that allows, at one hand, to reach large intensities inside the PPLN nonlinear crystal (~1 GW/cm2) and thus to provide a high conversion efficiency of the parametric process. On the other hand, it allows to have potentially fulfilled the demands of high and controllable repetition rates of pulsing that is crucial in the majority of applications. We also have to admit that the average power show practically no changes as the repetition rate is increased. Naturally, it follows the energy dropoff at higher train rates. It is also worth noticing that the transverse distribution of the pump beam (1064 nm) is virtually Gaussian that is as well an important demand to adjust an efficient OPO process.
Properly focusing the output beam inside the 5-mm thick MgO:PPLN crystal (using a Tydex lens set with a focal length 15-20 cm, providing up to a 600 μm waist diameter) and a suitable length of the PPLN (3 – 5 cm), we are able to get a rather high (25 – 30%) efficiency (see Fig. 5) of the fundamental-to-signal (1.064 μm → 1.5 – 1.6 μm) conversion; notice that at these conditions the conversion efficiency to the idler wave, 3.4 – 3.2 μm, is about 5% what is lower than one may expect. The corresponding pulse energies at the OPO output are 220 μJ (1.6 μm, signal wave) and 30 μJ (@3.2 μm) (at maximal pump pulse energy delivered to the PPLN sample of ~700 μJ). Probably, the 35-40% conversion efficiency is a limit for this typeof a single-pass OPO. The rest of the pump is either non-convertible, or wasted out due to the generation of higher out-of-phase harmonics; the indication for the latter is an intensive red through violet emission coming out from the crystal (notice here that apart of non-phase matched second-harmonic generation for both the pump and signal waves, respectively, green and red, we also observe the fourth harmonic of the signal wave). This observation suggest that further increase of the pump energy is unlikely to be favorable, as it might saturate the parametric conversion process via non-phase matched second- and fourth-harmonic generation for the pump and signal wave instead of enhancing the idler energy.
As it is mentioned above, a set of MgO:PPLN samples is available that enables us to study dependence of the OPO efficiency at the signal (1.5 – 1.6 μm) wavelength on the crystal length. The results of the measurements are summarized in Fig. 6, from where one can see that the OPO efficiency is increasing and, then, slightly saturating function of the pump pulse energy. It is also readily seen that the PPLN length ≥4 cm provides generation efficiency of the order of 20%. Therefore, one can restrict himself to the 3 – 5 cm of the MgO:PPLN working length at fulfillment of the commercial prototype.
Spectral characterization of the device output is given in Figs. 7 and 8. It is readily (see Fig. 7) seen the mid-IR idler wavelength to be tuned as much as 250 nm, whereas the near-IR signal wavelength within ~70 nm (in the temperature range provided by our experimental arrangement). These ranges can be further extended by heating the crystal up to 350°C and by tunable seed within the parametric super- luminescence spectrum (see below). The spectral width of the OPO output (see Fig. 8) increases with the pump pulse energy. The fine structure observed in the spectra previously  that were attributed to generation of several longitudinal modes by the master oscillator there, is not seen here. Also notice that the OPO system are found to deliver nearly diffraction-limited beams both at the near- and mid- IR tunable wavelengths (M2 ≤ 2.65 at 1.6 μm); pulse-to-pulse energy instability did not exceed 0.1%.
Finally, we are going to demonstrate the result of a low-power cw near-IR seeded into the OPO converter when the wavelength of this seed matches the signal-wavelength of the converter. While assembling the whole unit of the OPO source, the output of the seed laser, which is a diode whose linewidth is narrowed by means of the FBG, is successfully projected into the working area of the MgO:PPLN crystal. The yield of this assembling is demonstrated in Fig. 9 where the OPO spectra at the signal wavelength in the 1520-1560 nm range are compared for the cases where there is no seed (see Fig. 9(a)) and where a 10-mW seed signal at the wavelength 1538 nm (see Fig. 9(b)) is launched into the sample. It is readily seen the drastic narrowing, even at the highest level of 1064-nm pumping, of the output spectrum down to that of 0.2 nm that is the DFB seed-laser spectral width (compare the OPO output spectrum without seeding, whose bandwidth is more than 10 nm) that set almost 90% of the signal energy within the 1538-nm peak. Notice here that the pulsed OPO demonstrated with a narrow emission line that stems from low-power cw seeding can find its application for the spectroscopic purposes and environmental monitoring.
The call for a hybrid, bulk-discrete, laser listened though the few recent years, seems to be very much meaningful: laser systems like the one described in this paper is able to combine advantages of both bulk- and fiber-laser systems. This device has advantages of the dc-modulated narrow-line semiconductor laser, simple, highly-efficient and wavelength-stable fiber amplifier, and of the robust, single-pass, large-aperture OPO. For this hybrid design to become a superior alternative to all-bulk or all-fiber tunable laser systems the new quality brought by the optimized design of the FA is not enough. The peak-power constraint set by the laser damage of the silica fiber must be lifted out too. This can be reached by further lowering the gain in the final FA that, however, is followed by, e.g., two-pass diode-pumped bulk amplifier working with a collimated FA output.
We report a compact and robust source capable of generating almost diffraction-limited light ranged from near- to mid- IR (1.5 – 1.7-μm / 3.4 – 3.2-μm) and comprising a narrow-line 1.064-µm 15-ns MOFA-type system as a pump source, a cw DFB (1.5 – 1.6-μm) seed laser, and a MgO:PPLN-based OPO converter. The laser system delivers nearly diffraction-limited beams both at the near- and mid-IR tunable wavelengths. The pulse-to-pulse energy instability does not exceed 0.1%. Optical elements of the scheme can be easily incorporated into a housing with the dimensions of 500 × 250 × 160 mm3. A variety of the undertaken measures (e.g. optimization of PPLN length; temperature tuning; spectrum narrowing using the seeder; dimensions minimization, etc.) and technical treatment of the resultant device parameters allows us to reveal the presented source to be ideal for many applications as high-precision spectroscopy, photo-medicine, environmental control, scanning of remote objects in the eye-safe spectral domain, IR-countermeasures, telecommunications, etc.
We are grateful to P.E. Powers and J.W. Haus for numerous helpful discussions, A. Ya. Shur for supplying us with PPLN samples, and E. G. Gerasimov, A.I. Moshkunov and K. E. Gordeev for technical assistance. We also thank anonymous referees whose comments considerably improved the manuscript. This work was partially supported by the Federal Task Program “Human Resources in Innovations,” grant number 14.B37.21.1237.
References and links
2. D. Richter, A. Fried, B. P. Wert, J. G. Walega, and F. K. Tittel, “Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection,” Appl. Phys. B 75(2-3), 281–288 (2002). [CrossRef] [PubMed]
3. D. Richter, B. P. Wert, A. Fried, P. Weibring, J. G. Walega, J. W. C. White, B. H. Vaughn, and F. K. Tittel, “High-precision CO2 isotopologue spectrometer with a difference-frequency-generation laser source,” Opt. Lett. 34(2), 172–174 (2009). [CrossRef] [PubMed]
4. G. Fujii, N. Namekata, M. Motoya, S. Kurimura, and S. Inoue, “Broadband source of photon pairs at optical telecommunications wavelengths using a type-II periodically poled lithium niobate,” Opt. Express 15, 12769–12776 (2007). [CrossRef] [PubMed]
5. J. A. L’Huillier, G. Torosyan, M. Theuer, Y. Avetisyan, and R. Beigang, “Generation of THz radiation using bulk, periodically and aperiodically poled lithium niobate – Part 1: Theory,” Appl. Phys. B 86, 185–196 (2007).
6. J. A. L’Huillier, G. Torosyan, M. Theuer, C. Rau, Y. Avetisyan, and R. Beigang, “Generation of THz radiation using bulk, periodically and aperiodically poled lithium niobate – Part 2: Experiment,” Appl. Phys. B 86, 197–208 (2007).
7. P. E. Powers, K. W. Aniolek, T. J. Kulp, B. A. Richman, and S. E. Bisson, “Periodically poled lithium niobate optical parametric amplifier seeded with the narrowband filtered output of an optical parametric generator,” Opt. Lett. 23(24), 1886–1888 (1998). [CrossRef] [PubMed]
8. M. J. Missey, V. Dominic, P. E. Powers, and K. L. Schepler, “Periodically poled lithium niobate monolithic nanosecond optical parametric oscillators and generators,” Opt. Lett. 24(17), 1227–1229 (1999). [CrossRef] [PubMed]
9. S. T. Yang and S. P. Velsko, “Frequency-agile kilohertz repetition-rate optical parametric oscillator based on periodically poled lithium niobate,” Opt. Lett. 24(3), 133–135 (1999). [CrossRef] [PubMed]
10. O. Kokabee, A. Esteban-Martin, and M. Ebrahim-Zadeh, “Efficient, high-power, ytterbium-fiber-laser-pumped picosecond optical parametric oscillator,” Opt. Lett. 35(19), 3210–3212 (2010). [CrossRef] [PubMed]
11. A. Gaydardzhiev, D. Chuchumishev, I. Buchvarov, D. Shumov, and S. Samuelson, “High Energy, Sub-nanosecond, 0.5-kHz, Mid-IR OPO based on PPSLT Pumped at 1064 nm,” in CLEO/Europe and EQEC 2011 Conference Digest, OSA Technical Digest (CD) (Opt. Society of America, 2011), paper CD_P19.
12. D. Chuchumishev, A. Gaydardzhiev, T. Fiebig, and I. Buchvarov, “Subnanosecond, mid-IR, 0.5 kHz periodically poled stoichiometric LiTaO3 optical parametric oscillator with over 1 W average power,” Opt. Lett. 38(17), 3347–3349 (2013). [CrossRef] [PubMed]
13. D. V. Chuchumishev, A. G. Gaydardzhiev, D. Shumov, S. Samuelson, T. Fiebig, C. Richter, and I. Buchvarov, “PPSLT KHz OPO/OPA Tunable in 3-3.5 µm Pumped by 1ns 30mJ Nd-laser System,” in CLEO: 2013, OSA Technical Digest (online) (Opt. Society of America, 2013), paper CW1B.6.
14. S. M. Klimentov, A. V. Kiryanov, I. V. Mel’nikov, and P. E. Powers, in: International Conference CLEO / Europe IQEC 2007 (Munich, Germany, 2007), Advanced Program, paper # CA9–3-THU; A. V. Kiryanov, S. M. Klimentov, I. V. Mel’nikov, P. E. Powers, and Yu. N. Korkishko, “IR-tunable narrow-band nanosecond converter with a microchip source and periodically poled lithium niobate,” Las. Phys. Lett. 5, 253–258 (2008).
15. S. Desmoulins and F. Di Teodoro, “Watt-level, high-repetition-rate, mid-infrared pulses generated by wavelength conversion of an eye-safe fiber source,” Opt. Lett. 32(1), 56–58 (2007). [CrossRef] [PubMed]
16. A. Henderson and P. EsquinasiK. Tankala, ed., “23-watt 77% efficient CW OPO pumped by a fiber laser”, in Fiber Lasers VII: Technology, Systems, and Applications, K. Tankala, Editors, Proceedings of SPIE 7580 (SPIE, Bellingham, WA 2010), 75800D. [CrossRef]