An external-cavity generation of powerful ultrashort pulses in an all-fiber scheme by using a new type of phosphosilicate polarization maintaining fiber is investigated. The phosphorus-related Stokes shifted Raman pulse near 1.3 microns is observed. Optimization of Stokes output spectrum depending on pump pulse duration (chirp), energy and output coupling ratio of the cavity is performed. As result, the output energy of highly-chirped pulses compressible to 570 fs reaches 1.6 nJ.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Pulsed Raman lasers keep great attention over decades mainly due to their ability to generate intensive laser radiation beyond the emission spectrum of typical active media. Starting from the single-mode optical fibers placed into the synchronously-pumped external linear cavity , Raman lasers were gradually evolving with the development of fiber optics technologies [2–5]. The interest has also been boosted by great success of the continuum wave (CW) Raman fiber lasers [6–9] that encourages researchers to try applying similar principles for the pulsed lasers. A novel approach to generate high-energy ultrashort pulses by stimulated Raman scattering (SRS) was demonstrated recently both in the intra- and extra-cavity configurations [10,11]. In the first case, a small feedback inside the cavity of dissipative soliton (DS) Yb-doped fiber laser is introduced at the Stokes wavelength, and a precise time delay for compensation of the group velocity difference between the Stokes and main pulses is provided. The pulse near 1.1 µm generated by SRS process was proved to be a Raman dissipative soliton (RDS) [10, 12] that can be combined coherently with the main DS thus resulting in interference pattern of ~75 fs period . In the second case, a high-energy picosecond laser is used to pump an external passive cavity . At that, only exact matching between the repetition rate of pump pulses and the roundtrip frequency in the external Raman cavity is necessary for a successful RDS generation. Moreover, a much higher RDS energy could be achieved in comparison with the first case, as far as pump and Raman lasers can be optimized independently .
Spectral region around 1.3 µm is one of the first candidates that has not been covered by the active dopants until recently . It is interesting region for the biomedical applications such as multiphoton fluorescence microscopy as the one of so-called water transparency windows exists here and effective attenuation length in biomedical tissue is quite long . So, a significantly greater penetration depth can be obtained without damaging. One of widely used method to obtain femtosecond pulses at new wavelengths is the soliton’s Raman self-frequency shift . The necessary conditions for self-frequency shifting is anomalous dispersion of the fiber, which is easy to provide at 1.5 µm [15,17], where relatively high pump pulse energy and short duration are required. The dispersion in the 1.3 µm region is normal for the most of materials. To attain anomalous dispersion, a special fiber which supports high-order modes could be used . Though about 50-fs pulses have been obtained with the energy up to 30 nJ, the cost of such device is dramatically high that inhibits practical applications. Another way to reach 1.3 µm region is a nonlinear parametric conversion in optical fibers, i. e. fiber optical parametric oscillator (FOPO). A novel concept for it was proposed in , where tunable highly chirped signal and idler pulses compressible down to 600 fs were generated in the external cavity. Optimization of the feedback ratio resulted in pulse energies of up to 250 nJ at wavelength of 1245 nm. This approach also requires an expensive pump source (µJ-level pulses with 60 ps duration), special photonic crystal fiber and bulk optical elements. A direct generation near 1.3 µm has been obtained very recently in the mode-locked cavity with the unique bismuth-doped phosphosilicate fiber . The pulse duration was about 530 fs but the energy did not exceed 9 nJ even after amplification.
Phosphosilicate (P2O5) fiber gives a great benefit for a far-detuned Raman generation due to the large Stokes frequency shift of 39 THz . In particular, it is possible to obtain generation near 1.3 µm in one cascade by using well-developed pulsed Ytterbium-doped fiber sources near 1.1 µm as a pump . A fine adjustment of the pulse wavelength can be done by varying the pump laser parameters and external cavity design respectively. The first realization was reported in  in the linear external P2O5 non-polarization-maintaining (non-PM) fiber cavity. As the pump pulse was quite long (about one microsecond), the resulted pulse was also long (∼3 µs). Critical role of the pump pulse quality was confirmed later in . A similar external P2O5-fiber cavity was pumped by the dissipative solitons stretched up to 350 ps. 270-ps pulses at 1270 nm have been obtained with about 30% efficiency but their optical spectrum did not reproduce the pump’s one. Another attempt to generate coherent pulses near 1.3 µm has been done in  by using a ring synchronously pumped cavity with a translation stage inside for a precise length tuning. Energy and duration of the generated pulses were not specified, but the last one can be estimated from the picture as 20 ps.
In this paper we present the first results on highly-chirped Raman dissipative solitons generation near 1.3 µm in an all-fiber scheme by using a new type of P2O5 polarization maintaining (PM) fiber forming an external ring cavity. In comparison with the described approaches, the generation of Raman dissipative solitons is a principally new and perspective method of shifting the carrier frequency of a powerful ultrashort pulse. We perform the complex investigation of the Raman pulse optical spectra as a function of output coupling ratio and pump pulse duration. As a result, we are able to compress output pulses down to 570 fs by the external diffraction grating compressor. For today, it is the shortest duration that has ever been achieved near 1.3 µm for the pulses generated through stimulated Raman scattering effect in P2O5-fiber.
Scheme of the experimental setup is presented in Fig. 1. We started from an all-fiber highly-chirped DS oscillator built of the 6 µm-core diameter fiber . Output pulses have duration of ~10 ps and repetition rate of 15.35 MHz. A fiber stretcher (FS) is inserted to increase the duration of pump pulses up to 250 ps. An amplifier built up of double-clad Yb3+ active fiber, pump-combiner and multi-mode laser diode (LD), is added after FS. So, the pulse energy amounts to ~20 nJ. We have made the length of the silica fiber between amplifier and P2O5 fiber as short as possible (only 3 meters) to prevent from Raman pulse which could appear just after amplification. All together these blocks form a pump source for the external Raman cavity. This cavity consists of a PM wavelength division multiplexer (WDM, 1064/1120 nm), 40 m-long PM P2O5 fiber (P708PM MID2, FORC, Moscow), and PM output coupler with different coupling ratio (R is a fraction of power removed from the cavity). The P2O5 fiber is a PANDA-type polarization-maintaining fiber with the polarization extinction ratio >20 dB, estimated group velocity dispersion (GVD) β2 ∼ 12 ps2/km and Raman gain coefficient at 1320 cm−1 >5 dB/(km W). The splice losses between standard and P2O5 fibers do not exceed 0.03 dB. As zero-dispersion wavelength of standard fibers is approximately 1.3 µm the total cavity dispersion value could be estimated at 0.48 ps2. There is no optical isolator as the roundtrip direction is defined by the pump radiation. The external Raman cavity is four times longer than the cavity of the pump laser. The main reason for that is increasing pulse interaction distance. Length of the cavity is precisely adjusted by the variable delay line (VDL) manufactured by OZ Optics Ltd. All the scheme consists of PM components only. Generated RDSs are compressed externally by a diffraction gratings pair (Spectrogon Ltd., 1500 grooves/mm). Transmission coefficient at the working wavelength is only 20%, that does not matter for our proof of principle experiment. The interferometric autocorrelation measurements of the dechirped RDSs are made with the Avesta AA-20DD autocorrelator. For the chirped pulses we use Mesaphotonics Ltd. frequency-resolved optical gating (FROG) system with additional translation stage that gives extended working range. The optical spectra are characterized by a conventional optical spectrum analyzer (OSA, Yokogawa 6370).
3. Results and discussion
First of all, we performed an optimization of the pump pulse duration, and therefore the value of optical chirp. Duration of 10 ps (without FS), 50 ps and 90 ps with different lengths of FS were applied. Corresponding optical spectra are shown in Fig. 2(a). The peak around 1070 nm corresponds to the pump pulse and the P2O5 Stokes (~39 THz) shifted Raman pulse could be seen at the wavelength of ∼1.25 µm. Only for the 50-ps pump pulses its spectral shape has steep edges, which are typical for highly-chirped dissipative soliton operation. It could be explained by the GVD difference that resulted in some walk-off of the generated pulse. Interaction distance is higher for the longer pump pulse but the efficiency is also higher with higher peak power (shorter pump durations). So, there is an optimum between the pulse peak power and duration which in our case corresponds to 50-ps pump pulses. It is this pulse duration was used further.
During the experiment with different pump pulse duration the output coupling (OC) was 1%. As the next step, coefficient of the OC was optimized by using PM couplers with different coupling ratio and readjusting the cavity length each time. Fiber couplers with 20% and 60% of the OC were applied additionally to the 1% coupler. Obtained optical spectra are presented in Fig. 2(b). At the maximum output coupling the RDS spectrum degrades dramatically. Moreover, the peak around 1120 nm is observed that corresponds to GeO2 Stokes (~13 THz) shifted stochastic Raman pulse in the P2O5 fiber. The fact is that a higher intracavity energy of the generated pulse stimulates a Raman conversion and reduces the energy of the pump pulse down to Raman threshold at the first meters of P2O5 fiber. Otherwise, when we have higher OC and lower intracavity energy the pump pulse has a high energy at longer distance and generation of stochastic Raman pulse appears at GeO2 peak. So, the 20% output coupling ratio seems to be the most acceptable for further investigations.
The optimal pump pulses have been characterized by OSA and FROG technique [Fig. 3]. The pulse spectra are presented in Fig. 3(a) for a disabled amplifier (Osc), RDS generation threshold (11 nJ) and the highest (18 nJ) energy level at which GeO2 related stochastic peak appears beyond 1.3 µm [Fig. 4(a)]. They are typical for a highly-chirped DS and observed at all energy levels. Maximum RDS energy was limited by the available amplifier pump power. Radio-frequency spectrum presented at the inset in Fig. 3 gives more that 60 dB signal-to-noise ratio (SNR). FROG measurements [Fig. 3(b)] also confirm a high quality of developed pump source.
The output spectrum and temporal shape of RDS before and after dechirping are also characterized in details [Fig. 4]. Spectral width gradually increases from ~9 nm to ~17 nm (by −10 dB) with increasing the pump energy. The output average power in such configuration is 25 mW that corresponds to energy of 1.6 nJ outside the external cavity and ∼8 nJ inside the cavity that nearly corresponds to the threshold of GeO2-related Stokes pulse [Fig. 4(a)]. Pulse compressing has been performed for the 1.2 nJ energy level. We were able to achieve 570 fs pulse duration from the 72 ps chirped RDS [Fig. 1(b)]. So, the compression factor is more than 120. The spectrally limited duration estimated by the Fourier transformation of the output spectrum is about two times lower (280 fs). Such difference could be explained by a high value of third order dispersion appearing in the compressor as the diffraction gratings are optimized for 1 µm region.
We have demonstrated a possibility of dissipative soliton generation at 1.245 µm wavelength in the external P2O5 fiber cavity via stimulated Raman scattering effect of pump pulses at 1.07 µm. The optical spectrum had sharp edges at 50 ps pump pulse duration and at 20% of cavity output coupling. The output energy of the generated pulses reached 1.6 nJ. Since the pulses were generated via Raman effect, the laser may be called as a Raman dissipative soliton (RDS) oscillator. The obtained pulses may be further amplified, e. g. Raman amplification in the same P2O5-fiber or by the bismuth-doped amplifier . We believe that the developed laser source is interesting for applications in nonlinear optical bioimaging - multiplex coherent anti-Stokes Raman scattering microscopy , multiphoton fluorescence microscopy , and optical coherence tomography . In comparison with the soliton self-frequency shift technique, our method does not require neither expensive femtosecond pump source nor special large mode area fibers with anomalous dispersion in this spectral region [26, 27]. Thus, it may burst the progress in the mentioned applications and significantly extend their scope of use.
Russian Science Foundation (RSF) (17-72-10129).
The authors thank M. E. Likhachev, V. V. Velmiskin, M. Yu. Salgansky, and A. Yu. Laptev for development and fabrication of the P2O5 fiber and A. V. Ivanenko for offered references and discussions. The authors also acknowledge the multiple-access center “High-resolution spectroscopy of gases and condensed matters” at IAE SB RAS for the use of their equipment.
References and links
1. K. Smith, P. N. Kean, D. W. Crust, and W. Sibbett, “An experimental study of a synchronously pumped fibre raman oscillator,” J. Mod. Opt. 34, 1227–1233 (1987). [CrossRef]
2. P. N. Kean, B. D. Sinclair, K. Smith, W. Sibbett, C. J. Rowe, and D. C. Reid, “Experimental evaluation of a fibre raman oscillator having fibre grating reflectors,” J. Mod. Opt. 35, 397–406 (1988). [CrossRef]
3. A. S. Kurkov, V. V. Dvoyrin, V. M. Paramonov, O. I. Medvedkov, and E. M. Dianov, “All-fiber pulsed Raman source based on Yb:Bi fiber laser,” Laser Phys. Lett. 4, 449–451 (2007). [CrossRef]
4. D. Lin, S.-u. Alam, P. S. Teh, K. K. Chen, and D. J. Richardson, “Tunable synchronously-pumped fiber Raman laser in the visible and near-infrared exploiting MOPA-generated rectangular pump pulses,” Opt. Lett. 36, 2050–2052 (2011). [CrossRef] [PubMed]
5. H. Chen, S.-P. Chen, Z.-F. Jiang, K. Yin, and J. Hou, “All-fiberized synchronously pumped 1120 nm picosecond Raman laser with flexible output dynamics,” Opt. Express 23, 24088 (2015). [CrossRef] [PubMed]
6. E. M. Dianov and A. M. Prokhorov, “Medium-power CW Raman fiber lasers,” IEEE J. Sel. Top. Quantum Electron. 6, 1022–1028 (2000). [CrossRef]
7. Y. Feng, L. R. Taylor, and D. B. Calia, “150 W highly-efficient Raman fiber laser,” Opt. Express 17, 23678 (2009). [CrossRef]
8. S. K. Turitsyn, S. A. Babin, A. E. El-Taher, P. Harper, D. V. Churkin, S. I. Kablukov, J. D. Ania-Castañón, V. Karalekas, and E. V. Podivilov, “Random distributed feedback fibre laser,” Nat. Photonics 4, 231–235 (2010). [CrossRef]
9. E. A. Zlobina, S. I. Kablukov, A. A. Wolf, A. V. Dostovalov, and S. A. Babin, “Nearly single-mode Raman lasing at 954 nm in a graded-index fiber directly pumped by a multimode laser diode,” Opt. Lett. 42, 9 (2017). [CrossRef] [PubMed]
10. S. A. Babin, E. V. Podivilov, D. S. Kharenko, A. E. Bednyakova, M. P. Fedoruk, V. L. Kalashnikov, and A. A. Apolonski, “Multicolour nonlinearly bound chirped dissipative solitons,” Nat. Commun. 5, 4653 (2014). [CrossRef] [PubMed]
13. D. S. Kharenko, A. E. Bednyakova, E. V. Podivilov, M. P. Fedoruk, A. Apolonski, and S. A. Babin, “Feedback-controlled Raman dissipative solitons in a fiber laser,” Opt. Express 23, 1857 (2015). [CrossRef] [PubMed]
14. A. M. Khegai, F. V. Afanas’ev, K. E. Riumkin, S. V. Firstov, V. F. Khopin, D. V. Myasnikov, M. A. Mel’kumov, and E. M. Dianov, “Picosecond 1.3-µm bismuth fibre laser mode-locked by a nonlinear loop mirror,” Quantum Electron. 46, 1077–1081 (2016). [CrossRef]
15. C. Xu and F. W. Wise, “Recent advances in fiber lasers for nonlinear microscopy,” Nat. Photonics 7, 875–882 (2013). [CrossRef]
17. P. Cadroas, L. Abdeladim, L. Kotov, M. Likhachev, D. Lipatov, D. Gaponov, A. Hideur, M. Tang, J. Livet, W. Supatto, E. Beaurepaire, and S. Février, “All-fiber femtosecond laser providing 9 nJ, 50 MHz pulses at 1650 nm for three-photon microscopy,” J. Opt. 19, 065506 (2017). [CrossRef]
18. L. Rishoj, G. Prabhakar, J. Demas, and S. Ramachandran, “30 nJ, ~50 fs All-Fiber Source at 1300 nm Using Soliton Shifting in LMA HOM Fiber,” in “Conf. Lasers Electro-Optics,” (OSA, Washington, D.C., 2016), c, p. STh3O.3.
19. T. Gottschall, T. Meyer, M. Schmitt, J. Popp, J. Limpert, and A. Tünnermann, “Four-wave-mixing-based optical parametric oscillator delivering energetic, tunable, chirped femtosecond pulses for non-linear biomedical applications,” Opt. Express 23, 23968 (2015). [CrossRef] [PubMed]
20. D. S. Kharenko, V. A. Gonta, and S. A. Babin, “50 nJ 250 fs all-fibre Raman-free dissipative soliton oscillator,” Laser Phys. Lett. 13, 025107 (2016). [CrossRef]
22. P. Elahi, G. Makey, A. Turnali, O. Tokel, and F. O. Ilday, “1.06µm–1.35µm Coherent pulse generation by a synchronously-pumped phosphosilicate Raman fiber laser,” in “2017 Conf. Lasers Electro-Optics Eur. Eur. Quantum Electron. Conf.”, (Munich, 2017).
23. H. Kano and H.-O. Hamaguchi, “Vibrationally resonant imaging of a single living cell by supercontinuum-based multiplex coherent anti-Stokes Raman scattering microspectroscopy,” Opt. Express 13, 1322–1327 (2005). [CrossRef] [PubMed]
25. K. M. Ratheesh, P. Prabhathan, L. K. Seah, and V. M. Murukeshan, “Gold nanorods with higher aspect ratio as potential contrast agent in optical coherence tomography and for photothermal applications around 1300 nm imaging window,” Biomed. Phys. Eng. Express 2, 055005 (2016). [CrossRef]
26. M. E. Pedersen, J. Cheng, K. Charan, K. Wang, C. Xu, L. Grüner-Nielsen, and D. Jakobsen, “Higher-order-mode fiber optimized for energetic soliton propagation,” Opt. Lett. 37, 3459 (2012). [CrossRef]
27. K. Wang, N. G. Horton, K. Charan, and C. Xu, “Advanced fiber soliton sources for nonlinear deep tissue imaging in biophotonics,” IEEE J. Sel. Top. Quantum Electron. 20, 50–60 (2014).