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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 11, Iss. 8 — Apr. 21, 2003
  • pp: 910–918
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Nonlinear generation of very high-order UV modes in microstructured fibers

Anatoly Efimov, Antoinette J. Taylor, Fiorenzo G. Omenetto, Jonathan C. Knight, William J. Wadsworth, and Philip St. J. Russell  »View Author Affiliations


Optics Express, Vol. 11, Issue 8, pp. 910-918 (2003)
http://dx.doi.org/10.1364/OE.11.000910


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Abstract

Cobweb microstructured optical fibers are often strongly multimode in the visible and near infrared regions. This may lead to a number of intermodally phase-matched nonlinear processes. Here we describe a process of nonlinear generation of very high-order UV modes by pumping such fibers with 100 fs Ti:sapphire pulses. Wavelengths as short as 260 nm are generated through a mechanism distinct from supercontinuum generation.

© 2003 Optical Society of America

1. Introduction

It is common knowledge today that various microstructured optical fibers (MOF) display enhanced nonlinear properties as compared to regular glass fibers. This is due to the strong confinement of guided modes in the fiber core which enhances peak intensity, and unusual dispersion properties which determine various phase-matching and group velocity-matching conditions needed for efficient nonlinear processes.

A number of spectacular experiments were performed in the past to demonstrate nonlinear behavior of such fibers with femtosecond pulse pumping at various central wavelengths. Examples include supercontinuum (SC) generation [1

1. J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,” Opt. Lett. 25, 25 (2000). [CrossRef]

, 2

2. A. V. Husakou and J. Herrmann, “Supercontinuum generation, four-wave mixing, and fission of higherorder solitons in photonic-crystal fibers,” J. Opt. Soc. Am. B 19, 2171 (2002). [CrossRef]

], intermodally phasematched third harmonic generation [3

3. F. G. Omenetto, A. J. Taylor, M. D. Moores, J. Arriaga, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, “Simultaneous generation of spectrally distinct third harmonics in a photonic crystal fiber,” Opt. Lett. 26, 1158 (2001). [CrossRef]

, 4

4. L. Tartara, I. Cristiani, V. Degiorgio, F. Carbone, D. Faccio, M. Romagnoli, and W. Belardi “Phasematched nonlinear interactions in a holey fiber induced by infrared super-continuum generation,” Opt. Commun. 215, 191 (2003). [CrossRef]

], large soliton self-frequency shifts [5

5. X. Liu, C. Xu, W. H. Knox, J. K. Chandalia, B. J. Eggleton, S. G. Kosinski, and R. S. Windeler, “Soliton self-frequency shift in a short taperedair-silicamicrostructure fiber,” Opt. Lett. 26, 358 (2001). [CrossRef]

], and others.

In spite of the centrosymmetric nature of silica glass material, which conventionally can exhibit only odd-order nonlinearities, reports of second harmonic generation in these fibers have been previously published [6

6. J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Optical properties of high-delta air-silica microstructure optical fiber,” Opt. Lett. 25, 796 (2000). [CrossRef]

]. In such a case, it appears that even-order nonlinear terms in such structures provide a non-zero nonlinear contribution, which could be explained, for example, by surface-nonlinear effects. In this regard microstructured optical fibers may, in fact, represent a good test bed for various guided surface-nonlinear processes. Solid verification of this conjecture is still lacking.

In this paper we describe a series of experimental observations on the generation of very-high order fiber modes in the ultraviolet region of the spectrum in the proximity of λ~300 nm and shorter. The pumping conditions for the generation of these modes are similar to the ones used for SC generation with the only difference being the variation of the launch conditions of the 800 nm radiation into the fiber, which will be detailed in the following section.

2. UV modes generation and characterization

The experimental setup employed to study high-order UV modes (HOUVM) consists of a 82 MHz repetition rate, 100 fs Ti:sapphire laser (Spectra Physics Tsunami), phase compensating optics, polarization and power control optics followed by a 3-axis launch stage into a cobweb MOF. All experiments described in this paper utilize cobweb MOFs with different core diameters. Scanning electron microscope (SEM) image of a typical fiber is shown in the inset of Fig. 2(b). We used several high-numerical aperture (NA) Geltech aspheric lenses to couple pump light into the fiber providing focal spots down to 1 µm in diameter. To visualize the orientation of the fiber structure, its transverse scale and morphology in situ we developed a technique of transverse guidance scans. By rastering the input tip of the fiber relative to the fixed focal spot of the focusing lens and monitoring the transmitted power we can build up the transverse profile of the fiber. After acquiring such a profile we can precisely place the focal spot of the input beam at any desired point on the fiber cross section with a click of a mouse. Two examples of experimental transverse fiber profiles can be seen in Fig. 2. A freshly cleaved 2.5 µm core diameter MOF tip is imaged in Fig. 2(a), where the hexagonally symmetric structure of the fiber core is clearly seen along with the supporting structure wall crossings where the light can also be guided. On the other hand, Fig. 2(b) shows the same fiber tip after it was damaged by the high-power input radiation. In the latter case the six-fold symmetry of the core is lost, whereas the supporting structure is virtually unchanged. We use high-power damaged input tips to more efficiently excite higher-order modes of the MOF.

Fig. 1. Supercontinuum generated in two different spatial modes of the 2.5 µm core cobweb fiber: a) fundamental mode, and b) two-lobe higher-order mode. A blue flare on the right overlapping the red portion of the spectrum is due to the second-order diffraction off the grating.

Fig. 2. Transverse fiber guidance scans for a) freshly cleaved fiber, and b) same fiber, but damaged by the input radiation clearly showing the altered morphology of the core tip. Inset: SEM image of a typical cobweb fiber used.

Nonlinear HOUVM excitation proved to be very sensitive to the input light coupling and polarization. Varying either of these parameters different HOUVMs could be produced with different efficiencies, often with SC generation strongly suppressed. This observation suggests that the nonlinear generation mechanism involves intermodal phase-matched process between one of the higher-order mode at the fundamental wavelength and a different HOUVM in the UV part of the spectrum. This mechanism appears to be distinct from the one responsible for the SC generation.

Dependence of the UV spectrum on the input pump power at a fixed pump wavelength of λ~790 nm was measured for two different fibers, and is shown in Fig. 7. The UV spectrum for the 2.5µm core diameter case, Fig. 7(b), is more reach, which can be understood by recognizing that a large diameter core fiber supports substantially larger number of guided modes available for phase-matching. Surprisingly, however, in both cases the UV spectra show almost no dependence on input power in that the major peaks and spectral bands are stationary as the pump power is varied.

Fig. 3. Far field image of one of the HOUVMs generated in 2.5 µm core MOF.
Fig. 4. Examples of different far field mode profiles of ultraviolet radiation detected at the output of different core diameter microstructured fibers. Green colored images are obtained with white cards painted with the fluorescent marker.
Fig. 5. Numerical modeling results for the 1.6 µm fiber. a) left, model of the MOF where the star-shaped silica guiding region (dark gray) is surrounded by the air region (light gray); a) right, far field calculation geometry; b), c), d) three modes of certain similarity to the observed ones with near-field intensity distributions shown on the left and far-field profiles—on the right. Modal effective indices are indicated as well.
Fig. 6. Near field intensity distribution of 305 nm HOUVM generated in 2.5 µm core MOF. This image corresponds to the far-field distribution for 2.5 µm fiber shown in Fig. 4, lower panel.
Fig. 7. Dependence of the UV spectra on the input average power for the case of a) 1.6 µm and b) 2.5µm core diameter MOFs. Intensity-color scaling is logarithmic.
Fig. 8. Pump wavelength dependence of the UV spectra (left) and corresponding visible-infrared spectra (right) for 1.6µm core fiber and 660 mW average pump power.

We have also considered the dependence of the UV spectra on pump wavelength and the results are shown in Fig. 8 for 1.6µm core fiber pumped with average power of 660 mW. We observe that spectral features around 260 nm and 330 nm are virtually unchanged as the pump wavelength is tuned over 60 nm from 770 nm to 830 nm. The shape of the infrared portion of the supercontinuum, extending from 800 nm to 1400 nm, is, on the other hand, quite sensitive to the pump wavelength, Fig. 8(right).

The fact that higher-order mode launching at the fundamental wavelength is essential for efficient HOUVM excitation can be brought about by two reasons: First, higherorder modes carry larger portion of their power outside the fiber core and therefore have stronger field amplitudes at the glass-air interface. This makes potential contribution from guided nonlinear surface effects more pronounced. Second, simulations show that dispersion properties of higher-order modes are vastly different from those of the fundamental mode, Fig. 9, and this could play an important role in any phasematching process.

Fig. 9. Calculated dispersion profiles of the first 10 modes of a 1.6 µm MOF. Solid curves: lowest order modes having only one zero-dispersion point in the visible. Dashed curves: higher-order modes which have another zero-dispersion point in the infrared.

3. Conclusion

In this work we described the experimental observation of very high order modes nonlinearly generated through conversion of 100 fs Ti:sapphire pump pulses in a number of cobweb fibers with different core diameters but similar spatial structure. These modes are observed only if the pumping radiation of several hundred milliwatts of average power centered near 800 nm is coupled in higher-order modes of the MOF. This can be achieved by either “offset pumping” or by changing the morphology of the input fiber tip. The efficiency of supercontinuum generation in this case can be strongly suppressed in the visible region of the spectrum, but that of HOUVMs generation is strongly increased. Most of the generated UV modes exit the fiber at very high effective numerical apertures. The analysis of the UV radiation shows almost no dependence of the spectral peaks on the pump wavelength and input power. Varying the input coupling conditions and polarization, however, affects the observed spectra considerably in that, although separate spectral peaks remain stationary, new peaks may emerge and other diminish or disappear entirely from the spectrum.

4. Acknowledgements

This research is supported by the Los Alamos Directed Research and Development (LDRD) program by the Department of Energy. W. J. Wadsworth is a Royal Society University Research Fellow.

References and links

1.

J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,” Opt. Lett. 25, 25 (2000). [CrossRef]

2.

A. V. Husakou and J. Herrmann, “Supercontinuum generation, four-wave mixing, and fission of higherorder solitons in photonic-crystal fibers,” J. Opt. Soc. Am. B 19, 2171 (2002). [CrossRef]

3.

F. G. Omenetto, A. J. Taylor, M. D. Moores, J. Arriaga, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, “Simultaneous generation of spectrally distinct third harmonics in a photonic crystal fiber,” Opt. Lett. 26, 1158 (2001). [CrossRef]

4.

L. Tartara, I. Cristiani, V. Degiorgio, F. Carbone, D. Faccio, M. Romagnoli, and W. Belardi “Phasematched nonlinear interactions in a holey fiber induced by infrared super-continuum generation,” Opt. Commun. 215, 191 (2003). [CrossRef]

5.

X. Liu, C. Xu, W. H. Knox, J. K. Chandalia, B. J. Eggleton, S. G. Kosinski, and R. S. Windeler, “Soliton self-frequency shift in a short taperedair-silicamicrostructure fiber,” Opt. Lett. 26, 358 (2001). [CrossRef]

6.

J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Optical properties of high-delta air-silica microstructure optical fiber,” Opt. Lett. 25, 796 (2000). [CrossRef]

7.

C. Kerbage, B. J. Eggleton, P. Westbrook, and R. S. Windeler, “Experimental and scalar beam propagation analysis of an air-silica microstructure fiber,” Opt. Express 7, 113 (2000). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-7-3-113 [CrossRef] [PubMed]

8.

F. G. Omenetto, A. Efimov, A. J. Taylor, J. C. Knight, W. J. Wadsworth, and P. St. J. Russell, “Polarization dependent harmonic generation in microstructured fibers,” Opt. Express 11, 61 (2003). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-1-61 [CrossRef] [PubMed]

9.

J. Thogersen and J. Mark, “Third harmonic generation in standard and erbium-doped fibers,” Opt. Commun. 110, 435 (1994). [CrossRef]

OCIS Codes
(060.2270) Fiber optics and optical communications : Fiber characterization
(060.4370) Fiber optics and optical communications : Nonlinear optics, fibers
(190.2620) Nonlinear optics : Harmonic generation and mixing
(190.4160) Nonlinear optics : Multiharmonic generation
(190.4370) Nonlinear optics : Nonlinear optics, fibers
(190.7110) Nonlinear optics : Ultrafast nonlinear optics
(320.7140) Ultrafast optics : Ultrafast processes in fibers

ToC Category:
Research Papers

History
Original Manuscript: February 25, 2003
Revised Manuscript: March 31, 2003
Published: April 21, 2003

Citation
Anatoly Efimov, Antoinette Taylor, Fiorenzo Omenetto, Jonathan Knight, William Wadsworth, and P. Russell, "Nonlinear generation of very high-order UV modes in microstructured fibers," Opt. Express 11, 910-918 (2003)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-11-8-910


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References

  1. J. K. Ranka, R. S. Windeler, A. J. Stentz, �??Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,�?? Opt. Lett. 25, 25 (2000). [CrossRef]
  2. A. V. Husakou, J. Herrmann, �??Supercontinuum generation, four-wave mixing, and fission of higherorder solitons in photonic-crystal fibers,�?? J. Opt. Soc. Am. B 19, 2171 (2002). [CrossRef]
  3. F. G. Omenetto, A. J. Taylor, M. D. Moores, J. Arriaga, J. C. Knight, W. J. Wadsworth, P. St. J. Russell, �??Simultaneous generation of spectrally distinct third harmonics in a photonic crystal fiber,�?? Opt. Lett. 26, 1158 (2001). [CrossRef]
  4. L. Tartara, I. Cristiani, V. Degiorgio, F. Carbone, D. Faccio, M. Romagnoli, W. Belardi �??Phasematched nonlinear interactions in a holey fiber induced by infrared super-continuum generation,�?? Opt. Commun. 215, 191 (2003). [CrossRef]
  5. X. Liu, C. Xu, W. H. Knox, J. K. Chandalia, B. J. Eggleton, S. G. Kosinski, R. S. Windeler, �??Soliton self-frequency shift in a short taperedair-silicamicrostructure fiber,�?? Opt. Lett. 26, 358 (2001). [CrossRef]
  6. J. K. Ranka, R. S. Windeler, A. J. Stentz, �??Optical properties of high-delta air-silica microstructure optical fiber,�?? Opt. Lett. 25, 796 (2000). [CrossRef]
  7. C. Kerbage, B. J. Eggleton, P. Westbrook, R. S. Windeler, �??Experimental and scalar beam propagation analysis of an air-silica microstructure fiber,�?? Opt. Express 7, 113 (2000). <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-7-3-113">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-7-3-113</a> [CrossRef] [PubMed]
  8. F. G. Omenetto, A. E.mov, A. J. Taylor, J. C. Knight, W. J. Wadsworth, P. St. J. Russell, �??Polarization dependent harmonic generation in microstructured fibers,�?? Opt. Express 11, 61 (2003). <a href="http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-1-61">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-1-61</a> [CrossRef] [PubMed]
  9. J. Thogersen, J. Mark, �??Third harmonic generation in standard and erbium-doped fibers,�?? Opt. Commun. 110, 435 (1994). [CrossRef]

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