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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 3 — Feb. 5, 2007
  • pp: 829–833
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High nonlinearity glass photonic crystal nanowires

N. A. Wolchover, F. Luan, A. K. George, J. C. Knight, and F. G. Omenetto  »View Author Affiliations


Optics Express, Vol. 15, Issue 3, pp. 829-833 (2007)
http://dx.doi.org/10.1364/OE.15.000829


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Abstract

Soft glass photonic crystal fibers (PCFs) have been fabricated for the first time with the stack and draw process. The same SF6-PCFs have been successfully tapered using a brush flame method. The transverse structure of the PCF does not collapse in the tapering process and core dimensions of the fabricated photonic nanowire has been measured to be 400 nm in diameter. Supercontinuum radiation in excess of one octave has been generated in both the untapered and tapered PCF and, in the latter case, pulse energy thresholds of 65 picojoules at a pump wavelength of 1550 nm were observed.

© 2007 Optical Society of America

Photonic crystal fibers (PCF) have garnered widespread interest for their efficient way of enhancing the nonlinear optical interaction between pulses of light and their bulk constituents. These processes, particularly in relation to SC generation in PCFs, have been the subject of intense research since its inception [for a complete recent review on the subject see Ref. 1

1. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. ,78,1135–1184 (2006). [CrossRef]

].

PCFs made of soft glasses are particularly interesting since their bulk nonlinearity is significantly higher than the more conventionally used fused silica and many efforts have been directed towards their fabrication [2

2. M. D. V. L. Kalashnikov, E. Sorokin, S. Naumov, I. T. Sorokina, V. V. R. Kumar, and A. K. George, “Low-threshold supercontinuum generation from an extruded SF6- PCF using a compact Cr4+: YAG laser,” App. Phys. B 79,591–596 (2004). [CrossRef]

6

6. V. Kumar, A. K. George, W. H. Reeves, J. C. Knight, P. S. Russell, F. G. Omenetto, and A. J. Taylor, “Extruded soft glass photonic crystal fiber for ultrabroad supercontinuum generation,” Opt. Express 10,1520–1525 (2002). [PubMed]

]. Recently, Schott SF6 photonic crystal fiber has been shown to be a convenient source for very broad short-pulse supercontinuum (SC) covering a bandwidth approaching 3000 nm [7

7. F. G. Omenetto and N. A. Wolchover, et al., Opt. Express, “Spectrally smooth supercontinuum from 350 nm to 3 μm in sub-centimeter lengths of soft-glass photonic crystal fibers,” Opt. Express 14,4928–4934 (2006). [CrossRef] [PubMed]

]. Such soft-glass PCFs have been difficult to fabricate because of their low melting temperature and, to date, they have been manufactured by extrusion [5

5. T. Hori, J. Takayanagi, N. Nishizawa, and T. Goto, “Flatly broadened, wideband and low noise supercontinuum generation in highly nonlinear hybrid fiber,” Opt. Express 12,317–324 (2004). [CrossRef] [PubMed]

] or by a die-cast method [8

8. Z. Guiyao, H. Zhiyun, L. Shuguang, and H. Lantian, “Fabrication of glass photonic crystal fibers with a die-cast process,” Appl. Optics 45,4433–4436 (2006). [CrossRef]

]. We present here results on the fabrication of SF6 PCF, SF6 PCF tapers and the performance of both these fibers when ultrafast pulses in the near infrared (λ=1550 nm) are coupled into them.

Fig. 1. Microscope image of the cross section of stacked and drawn high-Δ SF6 PCF used in the experiments.

For these experiments, Schott SF6 photonic crystal fibers were manufactured by using the more traditional stack and draw process for the first time. This method has been repeatedly applied to silica glass with success however it is more challenging to carry out with soft glasses such as SF6 which has a softening temperature of 811 K [6

6. V. Kumar, A. K. George, W. H. Reeves, J. C. Knight, P. S. Russell, F. G. Omenetto, and A. J. Taylor, “Extruded soft glass photonic crystal fiber for ultrabroad supercontinuum generation,” Opt. Express 10,1520–1525 (2002). [PubMed]

] and where the microstructure can collapse during the draw. The fiber perform is made by stacking several capillaries and solid rods in a desired geometry after which it is carefully and slowly heated in order to reach the melting temperature without causing any stress fracture in the glass. As the preform softens, the PCF is drawn.

A cross section image of the fabricated fiber is shown in Fig.1 which shows a nicely formed regular microstructure. The fiber used in these experiments has a core diameter of 4.5 microns. These fibers typically have dispersion curves with zero dispersion crossing and anomalous dispersion in the near infrared at longer wavelengths (λ>1300 nm) than the similarly structured silica PCFs, which makes them suitable for pumping at (longer) fiber laser wavelengths. For the untapered PCF, the zero dispersion wavelength is estimated to be in the vicinity of 1300 nm at these dimensions.

As expected, the SC radiation generated (Fig. 2) from this short piece of PCF is lacking significant spectral structure from higher order nonlinear effects. The possibility of generating SC at low pump pulse energy is particularly interesting and the combination of the high bulk nonlinearity of soft glasses and the pump wavelength range makes these PCFs compelling for the realization of a compact fiber-based, low-coherence pulsed supercontinuum source.

Fig. 2. Supercontinuum trace from coupling 35 mW in a 6 mm segment of the SF6-PCF shown in Fig. 1. The supercontinuum is generated in a self-phase modulation dominated regime and is lacking typical spectral structure associated to high order nonlinear effects.

While the physical parameters of the bulk material are certainly important, increased mode confinement significantly contributes to the enhancement of the nonlinearity and to the lowering of the SC generation threshold. In recent years there has been considerable interest in the tapering of regular [9

9. W. J. Wadsworth, A. Ortigosa-Blanch, and J. C. Knight, et al., “Supercontinuum generation in photonic crystal fibers and optical fiber tapers: a novel light source,” JOSA B 19,2148–2155 (2002). [CrossRef]

, 10

10. D. A. Akimov, A. A. Ivanov, and M. V. Alfimov, et al., “Two-octave spectral broadening of subnanojoule Cr : forsterite femtosecond laser pulses in tapered fibers,” Appl. Phys. B ,74307–311 (2002). [CrossRef]

] and photonic crystal fibers [11

11. L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Zaharieva Maxwell, and E. Mazur, “Subwavelength-diameter silica wires for low-loss optical wave guiding,” Nature 426,816–819 (2003). [CrossRef] [PubMed]

, 12

12. M. A. Foster, J. Dudley, and B. Kibler, Cao et al., “Nonlinear pulse propagation and supercontinuum generation in photonic nanowires: experiment and simulation,” Appl. Phys. B ,81,363–367 (2005). [CrossRef]

] giving rise to what is sometimes now referred to as a photonic nanowire.

Whereas a solid photonic nanowire poses mechanical challenges in terms of its support, a nanowire based on photonic crystal fiber affords more stability given the larger supporting structure around the sub-wavelength guiding core.

The opportunity of tapering a high nonlinearity glass to further confine the guided mode is clearly auspicious but riddled with the complication of dealing with the significantly lower melting temperatures (compared to >2000 K for silica) which would also potentially compromise the core’s supporting microstructure.

In an attempt to deliver the minimal thermal shock possible to the SF6-PCF, 15 cm segments of SF6-PCF were mounted vertically securing one end of the fiber by means of a fiber chuck while an adjustable constant pulling force was exerted at the other end. A “distance” brush flame approach was used by employing a butane torch where the flame is kept in close proximity but never directly applied onto the PCF.

Fig. 3. Scanning electron microscope image of the cross section of the tapered SF6-PCF. The supporting structure is still present and the core measures to be 400 nanometers across.

When gradual heating of the central portion of the fiber is achieved, the constant pull tapers the fiber. Depending on the separation distance between the flame and the PCF, tapers as long as 2 cm were obtained. Certainly, only a few millimeters of taper are needed for SC generation [7

7. F. G. Omenetto and N. A. Wolchover, et al., Opt. Express, “Spectrally smooth supercontinuum from 350 nm to 3 μm in sub-centimeter lengths of soft-glass photonic crystal fibers,” Opt. Express 14,4928–4934 (2006). [CrossRef] [PubMed]

]. Scanning electron microscope images of the tapered cross section used in these measurements is shown in Fig. 3.

The core is now found to have a transverse dimension around 400 nanometers. It is remarkable to see that the support structure around the core is still present in spite of some collapse noticeable in some of the air holes (the holes themselves have dimensions between 120 and 200 nm).

Losses in the taper are estimated by taking measurements of the output power under identical coupling conditions both for the tapered and untapered fibers. The detected power in the two cases is found to be comparable (difference <2%) provided that the tapering has been executed appropriately.

Fig. 4. Supercontinuum trace generated in (a) an untapered 4 cm segment of PCF and (b) in an equally long piece of PCF with a tapered tip. The length of the tapered fiber is Z=z1+z2=4 cm with the length of the untapered part z1=3.5 cm and where z2 represents the length of the tapered segment of the PCF z2=5 mm. Both traces shown are measured for average powers in the fiber equal to 15 mW. Supercontinuum is convincingly generated in the tapered PCF as opposed to the slight broadening occurring in the untapered piece.

The spectral data for the SC exhibits some spectral structure. This is most likely due the combination of the shortened nonlinear lengths caused by the smaller mode and the mechanical difficulties of holding the taper fixed and consistently pointing towards the optical spectrum analyzer. Under these conditions, however, it is established that broad supercontinuum (from 650 nm to >1750 nm) is generated for pump pulse energies of 125 pJ at 1550nm. Significant broadening (Δλ between 900nm and >1750 nm) is still found to occur for in-fiber average powers of 5 mW, which correspond to pulse energies of 65 picoJoules. The result is very encouraging for generation of broadband sources pumped by fiber-based femtosecond oscillators and could provide a very practical route for a convenient broadband source.

Summarizing, a soft-glass, high nonlinearity SF6-PCF was realized by stack-and-draw fabrication. This implies that added versatility and control in PCF design are made possible and realization of tailored dispersion curves, polarization control and other designs can be possible in this material. The nonlinear quality of this fiber has been verified by measuring the transformation of 1550 nm femtosecond pulses which has proven consistent with the previously studied extruded SF6 fiber and has generated smoother and broad SC radiation. The soft glass fiber was successfully tapered and a photonic nanowire having a 400 nm core diameter was realized. SC generation in this structure has been shown to have remarkably low threshold, down to below 10 mW or sub 100 pJ. While issues still need to be overcome to make the nanowires reliably mechanically stable, the low threshold SC generation, coupled with 1550 nm pumping, promises to introduce considerable opportunity in ultrabroadband, portable sources dramatically reducing the requirements placed on fiber-based ultrafast pump sources.

Acknowledgments

The authors gratefully acknowledge the help of Brian Lawrence for the SEM imaging and Hannah Perry with the manufacturing. This work has been supported under the NSF STTR Program.

References and links

1.

J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. ,78,1135–1184 (2006). [CrossRef]

2.

M. D. V. L. Kalashnikov, E. Sorokin, S. Naumov, I. T. Sorokina, V. V. R. Kumar, and A. K. George, “Low-threshold supercontinuum generation from an extruded SF6- PCF using a compact Cr4+: YAG laser,” App. Phys. B 79,591–596 (2004). [CrossRef]

3.

H. Hundertmark, D. Kracht, D. Wandt, C. Fallnich, V. Kumar, A. K. George, J. C. Knight, and P. S. Russell, “Supercontinuum generation with 200 pJ laser pulses in an extruded SF6 fiber at 1560 nm,” Opt. Express 11,3196–3201 (2003). [CrossRef] [PubMed]

4.

P. Petropoulos, H. Ebendorff-Heidepriem, V. Finazzi, R. Moore, K. Frampton, D. J. Richardson, and T. M. Monro, “Highly nonlinear and anomalously dispersive lead silicate glass holey fibers,” Opt. Express ,11,3568–3573 (2003). [CrossRef] [PubMed]

5.

T. Hori, J. Takayanagi, N. Nishizawa, and T. Goto, “Flatly broadened, wideband and low noise supercontinuum generation in highly nonlinear hybrid fiber,” Opt. Express 12,317–324 (2004). [CrossRef] [PubMed]

6.

V. Kumar, A. K. George, W. H. Reeves, J. C. Knight, P. S. Russell, F. G. Omenetto, and A. J. Taylor, “Extruded soft glass photonic crystal fiber for ultrabroad supercontinuum generation,” Opt. Express 10,1520–1525 (2002). [PubMed]

7.

F. G. Omenetto and N. A. Wolchover, et al., Opt. Express, “Spectrally smooth supercontinuum from 350 nm to 3 μm in sub-centimeter lengths of soft-glass photonic crystal fibers,” Opt. Express 14,4928–4934 (2006). [CrossRef] [PubMed]

8.

Z. Guiyao, H. Zhiyun, L. Shuguang, and H. Lantian, “Fabrication of glass photonic crystal fibers with a die-cast process,” Appl. Optics 45,4433–4436 (2006). [CrossRef]

9.

W. J. Wadsworth, A. Ortigosa-Blanch, and J. C. Knight, et al., “Supercontinuum generation in photonic crystal fibers and optical fiber tapers: a novel light source,” JOSA B 19,2148–2155 (2002). [CrossRef]

10.

D. A. Akimov, A. A. Ivanov, and M. V. Alfimov, et al., “Two-octave spectral broadening of subnanojoule Cr : forsterite femtosecond laser pulses in tapered fibers,” Appl. Phys. B ,74307–311 (2002). [CrossRef]

11.

L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Zaharieva Maxwell, and E. Mazur, “Subwavelength-diameter silica wires for low-loss optical wave guiding,” Nature 426,816–819 (2003). [CrossRef] [PubMed]

12.

M. A. Foster, J. Dudley, and B. Kibler, Cao et al., “Nonlinear pulse propagation and supercontinuum generation in photonic nanowires: experiment and simulation,” Appl. Phys. B ,81,363–367 (2005). [CrossRef]

OCIS Codes
(060.2280) Fiber optics and optical communications : Fiber design and fabrication
(060.7140) Fiber optics and optical communications : Ultrafast processes in fibers
(160.2290) Materials : Fiber materials
(190.4370) Nonlinear optics : Nonlinear optics, fibers
(320.5550) Ultrafast optics : Pulses
(320.7140) Ultrafast optics : Ultrafast processes in fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: December 20, 2006
Revised Manuscript: January 25, 2007
Manuscript Accepted: January 26, 2007
Published: February 5, 2007

Citation
N. A. Wolchover, F. Luan, F. G. Omenetto, A. K. George, and J. C. Knight, "High nonlinearity glass photonic crystal nanowires," Opt. Express 15, 829-833 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-3-829


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References

  1. J. M. Dudley, G. Genty, S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys., 78,1135–1184 (2006). [CrossRef]
  2. M. D. V. L. Kalashnikov, E. Sorokin, S. Naumov, I. T. Sorokina, V. V. R. Kumar, A. K. George, “Low-threshold supercontinuum generation from an extruded SF6- PCF using a compact Cr4+: YAG laser,” App. Phys. B 79,591–596 (2004). [CrossRef]
  3. H. Hundertmark, D. Kracht, D. Wandt, C. Fallnich, V. Kumar, A. K. George, J. C. Knight, P. S. Russell, “Supercontinuum generation with 200 pJ laser pulses in an extruded SF6 fiber at 1560 nm,” Opt. Express 11,3196–3201 (2003). [CrossRef] [PubMed]
  4. P. Petropoulos, H. Ebendorff-Heidepriem, V. Finazzi, R. Moore, K. Frampton, D. J. Richardson, T. M. Monro, “Highly nonlinear and anomalously dispersive lead silicate glass holey fibers,” Opt. Express, 11,3568–3573 (2003). [CrossRef] [PubMed]
  5. T. Hori, J. Takayanagi, N. Nishizawa, T. Goto, “Flatly broadened, wideband and low noise supercontinuum generation in highly nonlinear hybrid fiber,” Opt. Express 12,317–324 (2004). [CrossRef] [PubMed]
  6. V. Kumar, A. K. George, W. H. Reeves, J. C. Knight, P. S. Russell, F. G. Omenetto, A. J. Taylor, “Extruded soft glass photonic crystal fiber for ultrabroad supercontinuum generation,” Opt. Express 10,1520–1525 (2002). [PubMed]
  7. F. G. Omenetto, N. A. Wolchover, et al., Opt. Express, “Spectrally smooth supercontinuum from 350 nm to 3 μm in sub-centimeter lengths of soft-glass photonic crystal fibers,” Opt. Express 14,4928–4934 (2006). [CrossRef] [PubMed]
  8. Z. Guiyao, H. Zhiyun, L. Shuguang, H. Lantian, “Fabrication of glass photonic crystal fibers with a die-cast process,” Appl. Optics 45,4433–4436 (2006). [CrossRef]
  9. W. J. Wadsworth, A. Ortigosa-Blanch, J. C. Knight, et al., “Supercontinuum generation in photonic crystal fibers and optical fiber tapers: a novel light source,” JOSA B 19,2148–2155 (2002). [CrossRef]
  10. D. A. Akimov, A. A. Ivanov, M. V. Alfimov, et al., “Two-octave spectral broadening of subnanojoule Cr : forsterite femtosecond laser pulses in tapered fibers,” Appl. Phys. B, 74307–311 (2002). [CrossRef]
  11. L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Zaharieva Maxwell, E. Mazur, “Subwavelength-diameter silica wires for low-loss optical wave guiding,” Nature 426,816–819 (2003). [CrossRef] [PubMed]
  12. M. A. Foster, J. Dudley, B. Kibler, Cao et al., “Nonlinear pulse propagation and supercontinuum generation in photonic nanowires: experiment and simulation,” Appl. Phys. B, 81,363–367 (2005). [CrossRef]

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