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
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, 2], intermodally phasematched third harmonic generation [3, 4], large soliton self-frequency shifts , 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 . 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.
There have been only scarce reports dealing with the rich modal nature of microstructured optical fibers, especially cobweb fibers . In fact, such fibers even with core diameters down to ~1 µm can support a large number of guided modes in the visible and a few in the near-infrared wavelength regions. A striking example of the difference in the modal properties of a 2.5 µm core cobweb fiber can be seen in Fig. 1 where SC generation is made to occur either in the fundamental or in a two-lobe higherorder spatial mode by varying the coupling of the λ=810 nm light into the fiber. It must be noted that with the same input power and pulse duration, the SC generated in the higher-order mode extends much further into the blue compared to the the SC radiation generated in the fundamental mode.
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.
We observed that HOUVMs can be generated most efficiently when the input light is launched and propagates in the fiber in one of the higher-order modes, rather than the fundamental mode. A robust technique for efficient selective mode excitation in optical fibers is still lacking so we resorted to one of the two methods. The first is the “offset pumping”, where a focal spot of the input beam comparable in size to the fiber core diameter, or smaller, is strategically positioned on the input cross section of the fiber, sometimes at an angle to the fiber axis. The second approach is the use of the induced morphology change of the fiber input cleave through exposure to a high input average power, Fig. 2(b). The latter method is uncontrolled, but so far yielded the best results overall. The induced change in the surface profile of the initially flat input cleave of the fiber acts as a phase mask for the flat phase front of the incoming light. This phase mask changes significantly the relative efficiencies of modal excitation, clearly diminishing the power fraction carried by the fundamental mode and shifting this power to certain higher-order modes. Not surprisingly, after the melting of the fiber tip, the fundamental mode SC output is greatly suppressed, often not extending at all into the visible portion of the spectrum at an input average power levels of hundredth of milliwatts. On the other hand the HOUVMs are most effectively generated in this case, Fig. 3. The figure shows the far-field intensity distribution through fluorescence of a white card caused by the UV light emerging from a segment of 2.5 µm core diameter cobweb fiber, 15 cm in length. The overall complexity of the field distribution and the very high numerical aperture (greater than 0.6) of the UV light exiting the fiber are striking.
Similar radiation patterns were observed at the output of cobweb fibers with different core diameters. Experiments were performed in 15 cm segments of cobweb fibers having core diameters of 1.6, 2.5, 3, and 4 µm. Some of the far-field intensity distributions are shown in Fig. 4. We note that in all of these cases HOUVMs display a quite large exit numerical aperture of 0.6 and higher which creates some difficulties in working with this light, collimating and near-field imaging in particular. The complexity of the far-field patterns indicates that these are very high-order guided modes of the fiber. Numerical modeling of the modal structure and computed far field intensity distributions for 1.6 µm core MOF were also performed. Indeed, similarly complex and structured high order modes at 300 nm were found, Fig 5. There are, nevertheless, some exceptions, (e.g. 2.5 µm case, lower panel in Fig. 4), where a simple 6-lobe mode is observed. Near-field imaging of this mode is possible and shows also a 6-lobe pattern consistent with the orientation of the fiber, Fig. 6.
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.
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).
At this point we can only speculate on the nature of the nonlinear mechanism responsible for the observed generation of HOUVMs. Based on the previous data obtained with 1550 nm pumping [4, 8], the intermodally phase-matched third harmonic generation (IPM-THG) process seems to be the probable mechanism to explain the experimental data obtained. In this case, certain frequency components from the infrared portion of the SC generated in the fiber become phasematched to certain HOUVMs and the efficiency of the THG-conversion process depends on the corresponding intermodal overlap integral and the interaction length limited by the group velocity mismatch of the interacting modes. It is not clear, however, why no THG generation is observed with pumping into the fundamental mode of the fiber.
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.
One argument in favor of the surface nonlinear mechanism is the fact that a Cherenkovtype  ring is sometimes observed at the fiber output along with the modal field profile. The presence of a ring suggests that UV light escapes the fiber core and reaches the uniform cladding. UV components originating inside the fiber core should, therefore, have their wavevectors directed at an angle exceeding critical angle for total internal reflection, i.e. larger than ~50° with respect to fiber axis, which is highly unlikely. On the other hand, if the UV light is generated at the air-silica interface boundary then this limitation is not imposed since the surface can equally radiate both inside the core and outside into the air of the holes and further into the cladding.
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.
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
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