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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 5 — Mar. 5, 2007
  • pp: 2535–2540
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Ultra broadband UV generation by stimulated Raman scattering of two-color KrF laser in deuterium confined in a hollow fiber

Eiichi Takahashi, Susumu Kato, Yuji Matsumoto, and Leonid L. Losev  »View Author Affiliations


Optics Express, Vol. 15, Issue 5, pp. 2535-2540 (2007)
http://dx.doi.org/10.1364/OE.15.002535


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Abstract

Broad Raman-multi-frequency spectra were generated from the resonant two-color excitation of the deuterium molecule rotational Raman transition (J=0→2), using ultraviolet bi-harmonic lasers with a quartz hollow fiber. Fifty pure rotational Raman spectral lines (34 lines that have intensity within 10% of the strongest spectral line) from 230 to 290 nm were generated at a gas pressure of 30 kPa. Furthermore, vibrational-rotational Raman spectral lines of almost 300 lines from 220 to 600 nm were also generated by increasing the gas pressure to 60 kPa.

© 2007 Optical Society of America

1. Introduction

Recently, the progress of the technology behind ultra-short laser pulse generation, based on stimulated Raman scattering, has shown remarkable progress. Not only has the generation of broad Raman spectral lines been reported, using resonant [1–3

1. H. Kawano, C.H. Lin, and T. Imasaka, “Generation of high-order rotational lines by four-wave Raman mixing using a high-power picosecond Ti:Sapphire laser,” Appl. Phys. B 63, 121–124 (1996). [CrossRef]

] or off-resonant [4–7

4. D.D. Yavuz, D.R. Walker, G.Y. Yin, and S.E. Harris, “Rotational Raman generation with near-unity conversion efficiency,” Opt. Lett. 27, 769–771 (2002). [CrossRef]

,11

11. S. Sensarn, S.N. Goda, G.Y. Yin, and S.E. Harris, “Molecular modulation in a hollow fiber,” Opt. Lett. 31, 2836–2838 (2006). [CrossRef] [PubMed]

,12

12. A. M. Burzo, A. V. Chugreev, and A. V. Sokolov, “Optimized control of generation of few cycle pulses by molecular modulation,” Opt. Commun. 264, 454–462 (2006). [CrossRef]

] bi-harmonic laser excitation, but also the formation of several femto-second to subfemto-second pulses were demonstrated [5–7

5. D.D. Yavuz, D.R. Walker, M.Y. Shverdin, G.Y. Yin, and S.E. Harris, “Quasiperiodic Raman technique for ultrashort pulse generation,” Phys. Rev. Lett. 91, 233602 (2003). [CrossRef] [PubMed]

]. In particular, Yavuz and colleagues reported the generation of a train of pulses, with a pulse width of 5 fs and a pulse separation of 57 fs, from the Fourier synthesis of 15 spectral lines from both the rotational H2 and vibrational D2 Raman transitions [5

5. D.D. Yavuz, D.R. Walker, M.Y. Shverdin, G.Y. Yin, and S.E. Harris, “Quasiperiodic Raman technique for ultrashort pulse generation,” Phys. Rev. Lett. 91, 233602 (2003). [CrossRef] [PubMed]

]. Furthermore, Shverdin and colleagues successfully generated single-cycle pulses using the Fourier synthesis of 7 D2 vibrational Raman spectral lines [6

6. M.Y. Shverdin, D.R. Walker, D.D. Yavuz, G.Y. Yin, and S.E. Harris, “Generation of a Single-Cycle Optical Pulse,” Phys. Rev. Lett. 94, 033904 (2005). [CrossRef] [PubMed]

].

These ultrashort pulses, however, exhibited poor contrast between maxima and minima, and small separation times, because the pulses were formed from the sum of several spectral lines with a large Stokes shift. Thus, in order to obtain ultrashort pulses with deep temporal intensity modulation and large separation, it is necessary to increase the number of Raman spectral lines, where each spectra line has the same order of magnitude in intensity (in other words, within 10% of the strongest spectral line) with a small Stokes shift.

It is expected that the number of Raman spectral lines can be increased by increasing the pump laser frequency and decreasing the Stokes shift of the Raman interaction. This is the case because a simple analytical estimate and numerical simulations predicted that the maximum number of Raman spectral lines is represented as N ≈ 1.5 ω0/Ω, where ω0 is pump laser frequency and Ω is the Raman Stokes shift assuming a one dimensional interaction [8

8. L.L. Losev and A.P. Lutsenko “Parametric Raman laser with a discrete output spectrum equal in width to the pump frequency,” Quantum. Electron. 23, 919–926 (1993). [CrossRef]

,9

9. G.S. McDonald, G.H.C New, L.L. Losev, A.P. Lutsenko, and M. Shaw, “Ultrabroad-bandwidth multifrequency Raman generation,” Opt. Lett. 19, 1400–1402 (1994). [CrossRef] [PubMed]

].

2. Experimental setup

In this section a brief description of the 2-color laser system, consisting of the KrF laser and D2 gas, is outlined; a detailed description was presented in the reference 3. Figure 1 is a schematic diagram of the experimental setup. From the KrF pump laser a pulse, with an energy, wavelength and spectral bandwidth of 5 mJ, 248.0 nm and 13.5 GHz, respectively, was generated using a KrF oscillator with injection seeding. The oscillating wavelength was shifted from the wavelength of the KrF maximum gain (248.5 nm) to a shorter wavelength to take into account the Stokes shift and simultaneous amplification of the pump and Stokes radiation. The polarization of the pump laser, initially linear, was changed to rotational polarization using a first lambda quarter plate A to provide selective generation of the first Stokes radiation by maximizing the gain of the rotational transition J=0→2.

Fig. 1. A schematic diagram of the experimental setup of a simple two-color laser system, using a KrF excimer laser; A: first lambda quarter plate, B: second lambda quarter plate, C: Polarizer, Convex lenses: L1,L2:f = 0.9 m; L3: f = 0.25m. The KrF laser oscillator was operated at 248.0nm. The wavelength of the first Stokes light (ortho-D2, J=0 → 2) was 249.1nm. The KrF laser amplified both light pulses simultaneously. The D2 gas was cooled using liquid Nitrogen to maximize the molecular density in the J=0 state. The inner diameter and length of the hollow fiber was 124μm and 30cm, respectively

The pump laser pulse, with rotational polarization, was focused into a Raman cell filled with D2 at a pressure of 30 kPa and temperature 77 K, maintained using liquid nitrogen cooling. The first Stokes radiation (wavelength 249.1 nm) and the original pulses were transmitted through the Raman cell without the complete depletion of the pump laser. Both the first Stokes radiation and the pump laser pulse were within the amplification bandwidth of the KrF laser.

The first Stokes radiation has a counter rotational polarization. It is in phase and in collinear propagation with the pump laser pulse. Before the second Raman cell, the circular polarization of the two-color radiation was converted to a perpendicular linear polarization using the second lambda quarter plate B. Note that the relative angles to the polarizer C are adjustable by rotating the orientation angle of B while keeping the perpendicular polarization of both the pump and the first Stokes radiation. The following polarizer C produces the perpendicular polarization of the 2-color laser parallel with equal intensity by projecting in the polarization direction. The 2-color laser pulse was passed though an aperture and then further amplified using a secondary KrF laser amplifier.

The amplified 2-color KrF laser was then focused into the quartz hollow fiber, through the focusing lens L3, where f = 250 mm. The inner diameter of fiber was 124 μm and 30 cm in length. The Raman cells were made up of a vacuum heat insulation layer and a liquid nitrogen layer surrounding the D2 gas cell, with an inner and outer window made of fused silica with no anti-reflection coating. The distance between the inner window and the entrance of the fiber tip was 6 cm.

3. Results

3.1 Pure rotational Raman spectral line generation

The spectra of the rotational Raman spectral lines were observed from the interaction of the 2-color laser in the hollow fiber; see Fig. 2. The injected pump energy was 5 mJ. The exit radiation energy was 1 mJ. The fiber transmission was 50% without interaction and 30% with interaction. The reflection losses at the surfaces of the lenses and 4 windows were taken into account in the calculations.

Fig. 2(a) is the complete spectrum of the rotational Raman spectral lines. The 2-color pump wavelengths were 248.0 and 249.1 nm. Thirty-four Raman spectral lines were obtained, where each of these spectral lines exhibits an intensity magnitude within 10% of the strongest line. These lines were determined to be purly rotational from observations at higher resolutions; see Fig. 2(b) and 2(c).

Fig. 2. The rotational spectrum of D2 (at P = 30kPa); (a) the complete spectrum, solid and dashed lines represent different shot measurements to cover the wide spectral region; (b) and (c) are the same spectrum for a higher resolution measurement. No additional lines originating from vibrational Raman were observed.

Figure 3 is the exit laser beam profiles, where the image of without and with interaction are (a) and (b), respectively. The images in (c) and (d) are for the wavelengths of 254 and 268nm, respectively, and were obtained using interference filters. The Raman spectral lines were generated collinearly; however, the low intensity wings are broader for the longer wavelengths.

Fig. 3. The exit laser beam profiles; (a) oscillator image, (b) output profile including all wavelength components of Fig. 2; (c) profile image at 254nm (+/- 2.5nm), (d) at 268nm (+/-2.0nm). The image was taken 120 mm away from the fiber exit. The image size is 6.4 mm horizontally, 4.8 mm vertically.

3.2 Vibrational and rotational Raman spectral line generation

A significant development in the spectral lines was observed when the gas pressure was increased to 60 kPa; see Fig. 4 where the complete spectrum is presented in (a). The merged Raman lines are from 220 to 600 nm. From the detailed measurements of the lines, after calibration using line emissions from a discharge mercury tube, the lines were found to originate from a combination of both rotational and vibrational transitions of the D2 gas; see Fig. 4(b), (c) and (d). Assuming the spectrum (Fig. 4(a)) consists of 8 vibrations with 40 rotational lines, the total number of Raman lines should be approximately 320.

Fig. 4. The generated vibrational-rotational spectrum of D2 (at P = 60kPa). The detail of the spectrum is given as insets to the right of the main plot. The highlighted spectral lines are, A: Q(0) + 20S(0), B: Q(0) + 37S(0), C: 2Q(0) + 4S(0), D: 3Q(0) + 22S(0), E: 2Q(0) + 39S(0), F: 4Q(0) + 6S(0) (Q(0):2991cm-1, S(0):179cm-1). Assuming this spectrum consists of 8 vibrations with 40 rotational lines, the total number of Raman spectral lines was approximately 320.

3. Discussion

During the Raman interaction there was a significant increase in the Stokes components compared with the anti-Stokes components, and a large decrease, 40 %, in the fiber transmittance. The rough estimate of the energy spent in a molecular rotation is only 5 % (assuming 22 Raman spectral lines which have an equal number of photons and for each Ω=179 cm-1). These results can be explained using the theoretical prediction reported in the references 10, that is, the different wave-front surfaces of the Raman components (different fiber modes) resulting from the nonlinear interaction of the waves with bell-like intensity distributions in the fiber. This produces an additional wave-vector mismatch that narrows the output spectrum and results in a shift to the red side. Thus, there is an enhancement of the losses in the fiber. Such components were observed in Fig. 3(b)–(d) as surrounding low intensity light. The waveguide effect of the attenuation and dispersion in the fiber is negligible for a UV laser [13

13. M. Nisoli, S. Stagira, S. De Silvestri, O. Svelto, S. Sartania, Z. Cheng, M. Lenzner, Ch. Spielmann, and F. Krausz, “A novel-high energy pulse compression system: generation of multigigawatt 5-fs pulses,” Appl. Phys. B 65, 189–196 (1997). [CrossRef]

].

The number density of molecules in the fiber should be sufficient for Raman interaction. The total number of photons from the bi-harmonic pump lasers per unit area was 2.5×1019 photons/cm2 in the fiber assuming a 2 mJ pulse. However, the number density of the D2 molecule for 30 kPa at liquid nitrogen temperatures is 2.6×1019 molecle/cm3. The conversion length L for the generation of the initial Raman lines, the second Stokes and first anti-Stokes lines was L = 2/gI = 1 cm, where I is the intensity of the bi-harmonic laser (1 GW/cm2) and g is the Raman gain (2 cm/GW). Thus, the number of molecules in the conversion length of a 1 cm fiber was almost equal to the photon density. This suggests that there low levels of the ground state molecule, resulting in a decrease in the Raman gain and a narrowing of the output spectrum.

By increasing the D2 gas pressure, vibrational-rotational Raman spectral lines from 220 to 600 nm were obtained, since the vibrational Raman gain is larger than the rotational Raman gain at high pressures [14–17

14. K.D.Van Den Hout, P.W. Hermans, E. Mazur, and H.F.P. Knaap, “The broadening and shift of the rotational Raman lines for hydrogen isotopes at low temperatures,” Physica 104A, 509–547 (1980).

].

3. Conclusion

Using a hollow fiber, 34 pure rotational Raman lines that have intensity within 10% of the strongest spectral line were obtained from the resonant two-color KrF laser excitation of rotational transitions of the D2 molecule at a pressure of 30 kPa. By increasing the pressure to 60 kPa, 320 vibrational-rotational Raman lines over a spectral range from 220 to 600 nm were generated. The energy conversion efficiency of the pump laser pulse to the Raman lines reached 30%. The results of the vibrational-rotational Raman spectral lines generation will be applied to the quasi-periodic Raman technique of reference 5; the spectrum corresponds to a 1 fs pulse generation for a proper phase control of Raman spectral lines.

Acknowledgments

This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 17540378, 2005.

References and links

1.

H. Kawano, C.H. Lin, and T. Imasaka, “Generation of high-order rotational lines by four-wave Raman mixing using a high-power picosecond Ti:Sapphire laser,” Appl. Phys. B 63, 121–124 (1996). [CrossRef]

2.

A. V. Konyashchenko and L.L. Losev, “Multifrequency Raman generation in liquid carbon tetrachloride with two-color pumping,” Opt. Commun. 260, 712–715 (2006). [CrossRef]

3.

E. Takahashi, L.L. Losev, T. Tabuchi, Y. Matsumoto, S. Kato, I. Okuda, T. Aota, and Y. Owadano, “Generation of 30 pure rotational Raman sidebands using two-color pumping of D2 gas by KrF laser,” Opt. Commun. 257, 133–138 (2006). [CrossRef]

4.

D.D. Yavuz, D.R. Walker, G.Y. Yin, and S.E. Harris, “Rotational Raman generation with near-unity conversion efficiency,” Opt. Lett. 27, 769–771 (2002). [CrossRef]

5.

D.D. Yavuz, D.R. Walker, M.Y. Shverdin, G.Y. Yin, and S.E. Harris, “Quasiperiodic Raman technique for ultrashort pulse generation,” Phys. Rev. Lett. 91, 233602 (2003). [CrossRef] [PubMed]

6.

M.Y. Shverdin, D.R. Walker, D.D. Yavuz, G.Y. Yin, and S.E. Harris, “Generation of a Single-Cycle Optical Pulse,” Phys. Rev. Lett. 94, 033904 (2005). [CrossRef] [PubMed]

7.

AV. Sokolov, M.Y. Shverdin, D.R. Walker, D.D. Yavuz, A.M. Burzo, G.Y. Yin, and S.E. Harris, “Generation and control of femtosecond pulses by molecular modulation,” J. Mod. Opt. 52, 285–304 (2005). [CrossRef]

8.

L.L. Losev and A.P. Lutsenko “Parametric Raman laser with a discrete output spectrum equal in width to the pump frequency,” Quantum. Electron. 23, 919–926 (1993). [CrossRef]

9.

G.S. McDonald, G.H.C New, L.L. Losev, A.P. Lutsenko, and M. Shaw, “Ultrabroad-bandwidth multifrequency Raman generation,” Opt. Lett. 19, 1400–1402 (1994). [CrossRef] [PubMed]

10.

K.S. Syed, G.S. McDonald, and G.H.C. New, “Transverse effects in ultrabroadband multifrequency Raman generation,” J. Opt. Soc. Am. B 17, 1366–1375 (2000). [CrossRef]

11.

S. Sensarn, S.N. Goda, G.Y. Yin, and S.E. Harris, “Molecular modulation in a hollow fiber,” Opt. Lett. 31, 2836–2838 (2006). [CrossRef] [PubMed]

12.

A. M. Burzo, A. V. Chugreev, and A. V. Sokolov, “Optimized control of generation of few cycle pulses by molecular modulation,” Opt. Commun. 264, 454–462 (2006). [CrossRef]

13.

M. Nisoli, S. Stagira, S. De Silvestri, O. Svelto, S. Sartania, Z. Cheng, M. Lenzner, Ch. Spielmann, and F. Krausz, “A novel-high energy pulse compression system: generation of multigigawatt 5-fs pulses,” Appl. Phys. B 65, 189–196 (1997). [CrossRef]

14.

K.D.Van Den Hout, P.W. Hermans, E. Mazur, and H.F.P. Knaap, “The broadening and shift of the rotational Raman lines for hydrogen isotopes at low temperatures,” Physica 104A, 509–547 (1980).

15.

R.W. Minck, E.E. Hagenlocker, and W.G. Rado, “Stimulated pure rotational Raman scattering in deuterium,” Phy. Rev. Lett. 17, 229–231 (1966). [CrossRef]

16.

W.K. Bischel and M.J. Dyer, “Temperature dependence of the Raman linewidth and line shift for the Q(1) and Q(0) transitions in normal and para-H2,” Phys. Rev. A 33, 3113–3123 (1986). [CrossRef] [PubMed]

17.

K.C. Smyth, G.J. Rosasco, and W.S. Hurst “Measurement and rate law analysis of D2 Q-branch line broadening coefficients for collisions with D2, He, Ar, H2 and CH4,” J. Chem. Phys. 87, 1001–1011 (1987). [CrossRef]

OCIS Codes
(190.4380) Nonlinear optics : Nonlinear optics, four-wave mixing
(190.5650) Nonlinear optics : Raman effect
(190.5890) Nonlinear optics : Scattering, stimulated
(190.7110) Nonlinear optics : Ultrafast nonlinear optics

ToC Category:
Nonlinear Optics

History
Original Manuscript: November 29, 2006
Revised Manuscript: January 22, 2007
Manuscript Accepted: January 31, 2007
Published: March 5, 2007

Citation
Eiichi Takahashi, Susumu Kato, Yuji Matsumoto, and Leonid L. Losev, "Ultra broadband UV generation by stimulated Raman scattering of two-color KrF laser in deuterium confined in a hollow fiber," Opt. Express 15, 2535-2540 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-5-2535


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References

  1. H. Kawano, C. H. Lin, T. Imasaka, "Generation of high-order rotational lines by four-wave Raman mixing using a high-power picosecond Ti:Sapphire laser," Appl. Phys. B 63, 121-124 (1996). [CrossRef]
  2. A. V. Konyashchenko and L. L. Losev, " Multifrequency Raman generation in liquid carbon tetrachloride with two-color pumping," Opt. Commun. 260, 712-715 (2006). [CrossRef]
  3. E. Takahashi, L. L.  Losev, T. Tabuchi, Y. Matsumoto, S. Kato, I. Okuda, T. Aota, Y. Owadano, "Generation of 30 pure rotational Raman sidebands using two-color pumping of D2 gas by KrF laser," Opt. Commun. 257, 133-138 (2006). [CrossRef]
  4. D. D. Yavuz, D. R. Walker, G. Y. Yin, and S. E. Harris, "Rotational Raman generation with near-unity conversion efficiency," Opt. Lett. 27, 769-771 (2002). [CrossRef]
  5. D. D. Yavuz, D. R. Walker, M. Y. Shverdin, G. Y. Yin, and S. E. Harris, "Quasiperiodic Raman technique for ultrashort pulse generation," Phys. Rev. Lett. 91, 233602 (2003). [CrossRef] [PubMed]
  6. M. Y. Shverdin, D. R. Walker, D. D. Yavuz, G. Y. Yin, and S. E. Harris, "Generation of a single-cycle optical pulse," Phys. Rev. Lett. 94, 033904 (2005). [CrossRef] [PubMed]
  7. A. V. Sokolov, M. Y. Shverdin, D. R. Walker, D. D. Yavuz, A. M. Burzo, G. Y. Yin, and S. E. Harris, "Generation and control of femtosecond pulses by molecular modulation," J. Mod. Opt. 52, 285-304 (2005). [CrossRef]
  8. L. L. Losev and A. P. Lutsenko "Parametric Raman laser with a discrete output spectrum equal in width to the pump frequency," Quantum. Electron. 23, 919-926 (1993). [CrossRef]
  9. G. S. McDonald, G. H. C. New, L. L. Losev, A. P. Lutsenko, and M. Shaw, "Ultrabroad-bandwidth multifrequency Raman generation," Opt. Lett. 19, 1400-1402 (1994). [CrossRef] [PubMed]
  10. K. S. Syed, G. S. McDonald, and G. H. C. New, "Transverse effects in ultrabroadband multifrequency Raman generation," J. Opt. Soc. Am. B 17, 1366-1375 (2000). [CrossRef]
  11. S. Sensarn, S. N. Goda, G. Y. Yin, and S. E. Harris, "Molecular modulation in a hollow fiber," Opt. Lett. 31, 2836-2838 (2006). [CrossRef] [PubMed]
  12. A. M. Burzo, A. V. Chugreev, and A. V. Sokolov, " Optimized control of generation of few cycle pulses by molecular modulation," Opt. Commun. 264, 454-462 (2006). [CrossRef]
  13. M. Nisoli, S. Stagira, S. De Silvestri, O. Svelto, S. Sartania, Z. Cheng, M. Lenzner, Ch. Spielmann, F. Krausz, "A novel-high energy pulse compression system: generation of multigigawatt 5-fs pulses," Appl. Phys. B 65, 189-196 (1997). [CrossRef]
  14. K. D. Van Den Hout, P. W. Hermans, E. Mazur, and H. F. P. Knaap, "The broadening and shift of the rotational Raman lines for hydrogen isotopes at low temperatures," Physica 104A, 509-547 (1980).
  15. R. W. Minck, E. E. Hagenlocker, W. G. Rado, "Stimulated pure rotational Raman scattering in deuterium," Phys. Rev. Lett. 17, 229-231 (1966). [CrossRef]
  16. W. K. Bischel and M. J. Dyer, "Temperature dependence of the Raman linewidth and line shift for the Q(1) and Q(0) transitions in normal and para-H2," Phys. Rev. A 33, 3113-3123 (1986). [CrossRef] [PubMed]
  17. K. C. Smyth, G. J. Rosasco, and W. S. Hurst "Measurement and rate law analysis of D2 Q-branch line broadening coefficients for collisions with D2, He, Ar, H2 and CH4," J. Chem. Phys. 87, 1001-1011 (1987). [CrossRef]

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