Open Access
Add to BibSonomyAbstract
We report supercontinuum generation in a 16cm long integrated liquid-core optical fiber (LCOF) filled with neat carbon disulfide (CS2). The LCOF was pumped with compact mode-locked fiber lasers operating near 1560nm and 1910nm. The supercontinuum spanned from 1460nm to 2100nm and from 1790nm to 2400nm at 20 dB level for 1560nm and 1910nm pump wavelengths, respectively. The spectral broadening extended mostly to longer wavelengths due to the high Raman contribution to the nonlinear optical response of CS2. The experimental results are in good agreement with numerical simulations.
©2013 Optical Society of America
1. Introduction
Mid-IR light sources have a number of important applications in spectroscopy [1], chemical detection [2, 3], remote sensing [4] and medicine [5]. There are several techniques for building a light source in the mid-IR spectral region [6] such as direct generation from solid state and fiber lasers or indirect generation using difference frequency generation, optical parametric interaction, or supercontinuum generation (SC). Supercontinuum generation seems to be the best approach for generating mid-IR light with extremely large continuous spectral bandwidth since conventional near-IR lasers can be used as the pump source. The technology of near-IR laser sources is quite mature due to years of research, development and commercialization from various groups worldwide.
For SC generation, an appropriate fiber dispersion profile, high nonlinear coefficient and low propagation loss are essential. Light propagating in a fiber does not experience diffraction, which provides for high intensity over a long interaction length. For that reason optical fibers are commonly used to generate SC. For example, Qin et al. have demonstrated that ZBLAN fluoride fiber can generate broad SC up to 6.28µm with a femtosecond pump laser at 1450nm (180fs pulse width, 400kW peak power) [7]. Highly nonlinear tellurite photonic crystal fiber (PCF) has been used to provide SC in the range between 789 to 4870nm using 100fs pulses at 1550nm with 19kW peak power [8]. Unfortunately, highly nonlinear solid-core fibers cannot generate light efficiently at wavelengths longer than 5-6µm due to high material absorption. Another drawback of these fibers is that they are typically very weak mechanically and it is difficult to splice them to conventional silica fibers. For that reason, most of the experimental work on SC generation in specialty optical fibers has used free space coupling which requires careful optical alignment and adjustment.
Recently, liquid-core optical fibers [9–11] and liquid-core photonic crystal fibers (LC-PCF) [12–15] have attracted attention as good candidates for SC generation in the mid-IR region due to the availability of mid-IR transparent and highly nonlinear liquids (e.g., CS2). It has been demonstrated that non-organic liquids such as CCl4 and CS2 have high transmission up to 12µm with few absorption peaks in the mid-IR region [16]. Bromine, an elemental liquid halogen, is transparent even up to 16µm [17]. These liquids have very high nonlinear coefficients that are up to two orders of magnitude higher than that of silica [18]. The potential for using these liquid fibers for nonlinear photonics has been pointed out in theoretical works [11–13]. Nevertheless, so far only a few researchers have demonstrated the use of the liquid core fibers for mid-IR generation. Bethge et al. have demonstrated SC generation ranging from 410 to 1640nm in a LC-PCF filled with water by pumping with 45fs pulses at 1200nm and 150MW peak power [14]. Another SC spanning from 700 to 1300nm has been demonstrated by Vieweg et al. with the use of 210fs pulses at 1030nm and ~10kW peak power in a LC-PCF filled with CCl4 [15]. The SC in these works however spanned only from the visible to the near-IR region. In their pioneering experiment, Bridges et al. used a LCOF filled with bromine to generate 7 Stokes Raman lines from 2.11 to 3.9µm [17]. A similar experiment with CBrCl3 performed by Chraplyvy and Bridges showed 12 discrete Stokes lines covering from 1.06 to 2.3µm [19]. In these experiments only distinct wavelengths were generated. To our knowledge, there is no experimental demonstration of SC generation in the mid-IR region using LCOF or LC-PCF to date.
In this communication, we report for the first time mid-IR SC generation spanning from 1460nm to 2100nm and from 1790nm to 2400nm by using an integrated CS2-filled LCOF and compact mode-locked (ML) pump fiber lasers operating at 1560nm and 1910nm, respectively. Our all-fiber system does not require free space alignment, which makes it practical for building very compact mid-IR light sources.
2. Experimental setup and results
A schematic diagram of our experimental setup is shown in Fig. 1(a). It includes three main components: the pump laser source, the integrated LCOF filled with CS2 and a monochromator for spectral domain characterization. We built and used pump laser sources operating at 1560nm and 1910nm for mid-IR SC generation. The first pump source was an amplified compact mode-locked fiber laser operating at 1560nm based on Er3+-doped gain fiber technology. The seed oscillator was mode-locked using a fiber taper embedded in carbon nanotube/polymer composite saturable absorber (SA). A detailed description of the SA can be found in Ref [20]. The oscillator emitted femtosecond pulses at 42MHz repetition rate and ~1mW average output power. The pulse train from the seed oscillator was first amplified using an Er3+-doped fiber amplifier before launching into the LCOF. The spectral bandwidth and the pulse duration after the amplifier at full width at half maximum (FWHM) were measured to be 10nm and 400fs, respectively. A variable attenuator was spliced after the amplifier to control the injected power to the LCOF.
Fig. 1 (a) – Schematic diagram of the experimental setup. (b) –Transmission spectrum of neat liquid CS2 measured using a spectrophotometer with a 1cm long cuvette.
We used a commercial (Polymicro Inc.) hollow core silica fiber with 1.8µm core diameter to build the integrated LCOF. This small core fiber will support single mode operation for λ>1550nm after filling it with CS2 whose refractive index is 1.587 at 1560nm as determined using the Cauchy dispersion parameters for CS2 [21]. The integrated LCOF was prepared following the procedure reported in [9]. CS2 was filled by capillary force, so no special pressurized equipment was needed. In this work, we have optimized the gap-splicing technique reported in [9] by using an ultrahigh NA fiber as the bridge-fiber for better mode matching. As the result, the average transmission of the integrated LCOF (SMF28-LCOF-SMF28) was measured to be around 64% at 1560nm. This is about 1dB of loss for each splice between the LCOF and the SMF28 fiber. Here we neglected the absorption loss of CS2, which is very small in the near-IR region. Several integrated LCOF samples were prepared for the supercontinuum generation experiments. The length of LCOF in each sample was 16 +/− 0.5cm. A typical transmission spectrum of the neat liquid CS2 used in our experiments, which was measured using a 1cm long cuvette with a UV-VIS-NIR Cary spectrophotometer (Model number 5000), is presented in Fig. 1(b). We notice nearly 100% transmission for a very broad wavelength range except an absorption dip of about 30% at 2.22µm. The origin of this absorption dip is most likely the overtone of the vibrational band ν3 of CS2 at ~1500cm−1 (3ν3 = 4500cm−1 = 2.22μm) [22].
We spliced one end of the integrated LCOF directly to the output of a variable attenuator for controlling the injected power. The other end of the integrated LCOF was used to deliver light into a monochromator (SPEX M270). The grating in the spectrometer had 300 grooves/mm with a blaze angle optimized for 2µm. The monochromator was equipped with a cooled InAs detector which could measure wavelengths from 1µm to about 4µm.
We recorded the output spectrum of the supercontinuum at 8mW and 50mW input power levels. The spectra are shown in Fig. 2. We found that the spectrum at the output of the LCOF was extended primarily to longer wavelengths. The shorter wavelength edge of the SC is about 1460nm (at −20 dB level). The longer wavelength edge reached 2100nm at ~50mW average pump power (corresponding to 3kW peak power). For higher peak powers of the pump laser, the transmission through the LCOF reduced significantly after a few minutes of operation. We repeated the experiment with other integrated LCOF samples and observed the same behavior, so this is a repeatable feature which is currently a limitation of our approach. Furthermore, we observed bubbles emerging from the splice between the LCOF and the standard single mode fiber when the pump peak power was >3kW. A photograph of the splice, where the bubble formation is visible, is shown in Fig. 3. Green light generation and a noticeable acoustic signal were also present during the bubble formation process. Previous works have pointed out that multi-photon absorption (MPA) can occur in CS2 for near-IR excitation wavelengths, due to strong linear absorption for wavelengths in near-UV region [23], and this absorption can result in heating [24–26]. We hence assume that the mechanism of bubble formation may come from this MPA facilitated heating, which can lead to a significant temperature increase and create bubbles in the LCOF (the boiling point of neat liquid CS2 is only 46°C at ambient pressure). To verify this hypothesis, we launched a CW laser at 1560nm with ~200mW average power and then a CW laser at 980nm with >100mW power into another integrated LCOF sample; no bubble formation or transmission decrease was observed over a long period of time because MPA is ineffective with a CW laser.
Fig. 2 Normalized pump laser spectrum and supercontinuum spectra at the output of the LCOF using 1560nm femtosecond laser pumping at 8mW and 50mW average powers. Spectra are separated by 10 dB for clarity.
Fig. 3 Photograph of the bubbles rising up from the splice in the cuvette filled with CS2 when the pump power of the femtosecond fiber laser at 1560nm was > 50mW. Green light generation (green dot) and noticeable acoustic signal were also present during the bubble formation process.
These observations support the hypothesis that LCOF filled with CS2 experiences an MPA heating effect for pump wavelengths shorter than 1560nm. Due to the presumed MPA nature of the process, we expect that shorter near IR pump wavelengths will have lower maximum peak power that can be transmitted through the fiber without degradation. This limits the amount of injected power and consequently limits the broadening of the SC spectrum.
To overcome this MPA issue, we decided to use a longer wavelength pump: a mode-locked Tm-doped fiber laser at 1910nm. A SA based on a taper embedded in carbon nanotube/polymer composite was also used to mode-lock the Tm3+-doped fiber laser. Detailed description of the mode-locked Tm-doped fiber laser can be found in Ref [27]. The ML oscillator emitted a femtosecond pulse train (~900fs) at a repetition rate of 40MHz and ~4mW average output power. The output pulse train from the oscillator was scaled up to ~102mW (limited by our available 1570nm pump power) by using a Tm3+-doped fiber amplifier. The duration of the pulses at the output of the amplifier became 180fs and the spectral bandwidth grew to ~38nm due to nonlinear spectral broadening in the amplifier. The estimated peak power was 14kW for 102mW average power. The total transmission through the integrated LCOF at 1910nm was measured to be ~60%. The LCOF filled with neat CS2 supported single mode operation as well at 1910nm (V number ~1.97). The spectrum from the output of the LCOF at 16mW and 102mW pump powers were recorded and plotted in Fig. 4.
Fig. 4 Normalized pump laser spectrum and the supercontinuum spectra at the output of the LCOF using 1910nm femtosecond laser pumping at 16mW and 102mW average powers. Spectra are separated by 10dB for clarity.
At the maximal 102mW available pump power, we generated a broad SC ranging from 1790nm to 2400nm. A dip appeared at around 2.22µm in the SC spectrum due to the absorption of CS2 in this region (see Fig. 1(b). We did not observe the MPA heating effect in this case even at our highest available pump power level of 102mW. This means that higher pump power may be used to expand the SC spectrum further to the longer wavelength side.
We performed numerical simulation to investigate the behavior of the SC at different pump power levels. The generalized nonlinear Schrodinger equation, which describes the propagation of optical pulses in an optical fiber, was solved numerically using the 4th order Runge–Kutta in the interaction picture method [28].
First, we performed the numerical simulation with similar conditions as used in the experiments. The dispersion of the LCOF was calculated up to fifth order of the Taylor expansion by combining the waveguide dispersion for the 1.8µm diameter core fiber and the material dispersion of CS2. The calculated values of dispersion for both wavelengths 1560nm and 1910nm are summarized in Table 1. The linear loss of the fiber was neglected because of the short fiber length and low absorption loss of CS2 except for the peak absorption at 2.22µm, which was deduced from the absorption spectrum shown in Fig. 1(b) to be 35m−1. The nonlinear coefficient of CS2 was taken from Ref [25]. to be 12·10−19 W/m2. We used the model of Raman response function reported in Ref [12]. The fractional contribution of the Raman response to the total nonlinear polarization was 0.89. The pump pulses were considered transform-limited with Gaussian form. The pulse duration of 400fs and 180fs were used for 1560nm and 1910nm, respectively.
Table 1. Calculated effective area and dispersion values used in our simulation for 1560nm and 1910nm.
The simulation results for different input power levels are presented in Fig. 5. The spectrum in our simulations extends mostly to the longer wavelength side because the Raman contribution to the nonlinear optical response of CS2 plays the dominant role in the SC generation process. These observations match the behavior of SC generation in the experiment. The simulation results are in good agreement with the experimental data for both pump wavelengths (see Figs. 2, 4 and 5).
Fig. 5 Simulated spectra of pump and supercontinuum generation in 16cm of LCOF using 1560nm pumping (a) and 1910nm pumping (b) at different average pump powers. The fiber parameters and pump pulse characteristics used in the calculations were similar to the experimental conditions. Spectra are normalized and separated by 10dB for clarity.
We used our numerical modeling tool to simulate the SC formed in the LCOF at higher pump powers than what we currently have to understand how far the spectrum can extend. The duration of the pulses was still 180fs, the center laser wavelength was at 1910nm and repetition rate was 40MHz. The results of our simulation are shown in Fig. 6 for different pump powers. At 1W average power (corresponding to 138kW peak power) of the injected pump, the spectrum extended to 4.1µm. With 4W average pump power (560kW peak power) we observed a very broad SC that reached beyond 9µm. It is important to note that our calculation did not take into account the absorption loss in the silica cladding of the LCOF or the absorption peaks in CS2 beyond 3µm. At such high peak power levels we might see the MPA effect which may damage the integrated LCOF as we have observed in case of 1560nm pumping. We will also need to consider self-focusing effect which we estimate to happen at ~283kW peak power [29].To realize our simulation results we will need to optimize not only the pump laser system to achieve higher peak power levels but also the material used in the hollow core fiber.
Fig. 6 Simulated spectra of the pump and supercontinuum generation in 16cm of LCOF using 1910nm pumping at average powers of 1W and 4W. Spectra are normalized and separated by 10dB for clarity.
3. Conclusion
We have demonstrated for the first time supercontinuum generation in the mid-IR region using an integrated LCOF filled with CS2. We were able to generate a SC covering from 1460nm to 2100nm using an Er-doped femtosecond fiber laser at 1560nm and from 1790nm to 2400nm using a Tm-doped femtosecond fiber laser at 1910nm. We observed that spectral broadening mostly occurs on the longer wavelength side due to the significant contribution of the slow Raman process to the total nonlinear optical response of liquid CS2. This feature is very interesting for mid-IR generation. Detailed numerical modeling was carried out and the calculation results are in good agreement with our experimental data.
Acknowledgments
The authors would like to acknowledge support from the National Science Foundation through CIAN NSF ERC under grant #EEC-0812072, the AFOSR COMAS MURI under grant #FA9550-10-1-0558 and the State of Arizona TRIF funding.
References and links
1. K. Miyamoto, K. Ishibashi, K. Hiroi, Y. Kimura, H. Ishii, and M. Niwano, “Label-free detection and classification of DNA by surface vibration spectroscopy in conjugation with electrophoresis,” Appl. Phys. Lett. 86(5), 053902 (2005). [CrossRef]
2. M. R. McCurdy, Y. Bakhirkin, G. Wysocki, R. Lewicki, and F. K. Tittel, “Recent advances of laser-spectroscopy-based techniques for applications in breath analysis,” J Breath Res 1(1), 014001 (2007). [CrossRef] [PubMed]
3. H. Li, D. A. Harris, B. Xu, P. J. Wrzesinski, V. V. Lozovoy, and M. Dantus, “Standoff and arms-length detection of chemicals with single-beam coherent anti-Stokes Raman scattering,” Appl. Opt. 48(4), B17–B22 (2009). [CrossRef] [PubMed]
4. D. D. Nelson, M. S. Zahniser, J. B. McManus, C. E. Kolb, and J. L. Jimenez, “A tunable diode laser system for the remote sensing of on-road vehicle emissions,” Appl. Phys. B 67(4), 433–441 (1998). [CrossRef]
5. B. S. Ross and C. A. Puliafito, “Erbium-YAG and Holmium-YAG laser ablation of the lens,” Lasers Surg. Med. 15(1), 74–82 (1994). [CrossRef] [PubMed]
6. I. T. Sorokina and K. L. Vodopyanov, eds., Solid-State Mid-Infrared Laser Sources (Springer, 2003).
7. G. Qin, X. Yan, C. Kito, M. Liao, C. Chaudhari, T. Suzuki, and Y. Ohishi, “Ultrabroadband supercontinuum generation from ultraviolet to 6.28μm in a fluoride fiber,” Appl. Phys. Lett. 95(16), 161103 (2009). [CrossRef]
8. P. Domachuk, N. A. Wolchover, M. Cronin-Golomb, A. Wang, A. K. George, C. M. B. Cordeiro, J. C. Knight, and F. G. Omenetto, “Over 4000 nm bandwidth of mid-IR supercontinuum generation in sub-centimeter segments of highly nonlinear tellurite PCFs,” Opt. Express 16(10), 7161–7168 (2008). [CrossRef] [PubMed]
9. K. Kieu, L. Schneebeli, R. A. Norwood, and N. Peyghambarian, “Integrated liquid-core optical fibers for ultra-efficient nonlinear liquid photonics,” Opt. Express 20(7), 8148–8154 (2012). [CrossRef] [PubMed]
10. O. D. Herrera, L. Schneebeli, K. Kieu, R. A. Norwood, and N. Peyghambarian, “Slow light based on stimulated Raman scattering in an integrated liquid-core optical fiber filled with CS2.,” Opt. Express 21(7), 8821–8830 (2013). [CrossRef] [PubMed]
11. Y. Xu, X. Chen, and Y. Zhu, “Modeling of micro-diameter-scale liquid core optical fiber filled with various liquids,” Opt. Express 16(12), 9205–9212 (2008). [CrossRef] [PubMed]
12. R. Zhang, J. Teipel, and H. Giessen, “Theoretical design of a liquid-core photonic crystal fiber for supercontinuum generation,” Opt. Express 14(15), 6800–6812 (2006). [CrossRef] [PubMed]
13. H. Zhang, S. Chang, J. Yuan, and D. Huang, “Supercontinuum generation in chloroform-filled photonic crystal fibers,” Optik (Stuttg.) 121(9), 787–789 (2010). [CrossRef]
14. J. Bethge, A. Husakou, F. Mitschke, F. Noack, U. Griebner, G. Steinmeyer, and J. Herrmann, “Two-octave supercontinuum generation in a water-filled photonic crystal fiber,” Opt. Express 18(6), 6230–6240 (2010). [CrossRef] [PubMed]
15. M. Vieweg, T. Gissibl, S. Pricking, B. T. Kuhlmey, D. C. Wu, B. J. Eggleton, and H. Giessen, “Ultrafast nonlinear optofluidics in selectively liquid-filled photonic crystal fibers,” Opt. Express 18(24), 25232–25240 (2010). [CrossRef] [PubMed]
16. K. Nakanishi and P. H. Solomon, Infrared Absorption Spectroscopy (Holden Day, 1977).
17. T. J. Bridges, A. R. Chraplyvy, J. G. Bergman Jr, and R. M. Hart, “Broadband infrared generation in liquid-bromine-core optical fibers,” Opt. Lett. 7(11), 566–568 (1982). [CrossRef] [PubMed]
18. P. P. Ho and R. R. Alfano, “Optical Kerr effect in liquids,” Phys. Rev. A 20(5), 2170–2187 (1979). [CrossRef]
19. A. R. Chraplyvy and T. J. Bridges, “Infrared generation by means of multiple-order stimulated Raman scattering in CCl4- and CBrCl3-filled hollow silica fibers,” Opt. Lett. 6(12), 632–633 (1981). [CrossRef] [PubMed]
20. K. Kieu and M. Mansuripur, “Femtosecond laser pulse generation with a fiber taper embedded in carbon nanotube/polymer composite,” Opt. Lett. 32(15), 2242–2244 (2007). [CrossRef] [PubMed]
21. A. Samoc, “Dispersion of refractive properties of solvents: chloroform, toluene, benzene, and carbon disulfide in ultraviolet, visible, and near-infrared,” J. Appl. Phys. 94(9), 6167–6174 (2003). [CrossRef]
22. G. Blanquet, J. Walrand, J.-F. Blavier, H. Bredohl, and I. Dubois, “Fourier transform infrared spectrum of CS2: analysis of the 3v3 band,” J. Mol. Spectrosc. 152(1), 137–151 (1992). [CrossRef]
23. B. Kleman, “Near-ultraviolet absorption spectrum of CS2,” Can. J. Phys. 41(12), 2034–2063 (1963). [CrossRef]
24. M. Falconieri and G. Salvetti, “Simultaneous measurement of pure-optical and thermo-optical nonlinearities induced by high-repetition-rate, femtosecond laser pulses: application to CS2,” Appl. Phys. B 69(2), 133–136 (1999). [CrossRef]
25. T. Kawazoe, H. Kawaguchi, J. Inoue, O. Haba, and M. Ueda, “Measurement of nonlinear refractive index by time-resolved z-scan technique,” Opt. Commun. 160(1-3), 125–129 (1999). [CrossRef]
26. R. A. Ganeev, A. I. Ryasnyansky, N. Ishizawa, M. Baba, M. Suzuki, M. Turu, S. Sakakibara, and H. Kuroda, “Two- and three-photon absorption in CS2,” Opt. Commun. 231(1-6), 431–436 (2004). [CrossRef]
27. Q. Fang, K. Kieu, and N. Peyghambarian, “An all-fiber 2-µm wavelength-tunable mode-locked laser,” IEEE Photon. Technol. Lett. 22, 1656–1658 (2010).
28. S. Balac and F. Mahé, “Embedded Runge–Kutta scheme for step-size control in the interaction picture method,” Comput. Phys. Commun. 184(4), 1211–1219 (2013). [CrossRef]
29. G. Fibich and A. L. Gaeta, “Critical power for self-focusing in bulk media and in hollow waveguides,” Opt. Lett. 25(5), 335–337 (2000). [CrossRef] [PubMed]
