Single-design-parameter microstructured optical fiber for chromatic dispersion tailoring and evanescent field enhancement

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Single-design-parameter microstructured optical fiber for chromatic dispersion tailoring and evanescent field enhancement
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  Single-design-parameter microstructured opticalfiber for chromatic dispersion tailoring andevanescent field enhancement Cristiano M. B. Cordeiro, 1, *  Marcos A. R. Franco, 2,3 Christiano J. S. Matos, 4 Francisco Sircilli, 2 Valdir A. Serrão, 2 and C. H. Brito Cruz 1 1 Instituto de Fisica “Gleb Wataghin,” UNICAMP, Campinas-São Paulo, Brazil  2 Instituto de Estudos Avançados, São José dos Campos-São Paulo, Brazil  3 Instituto Tecnológico de Aeronáutica, São José dos Campos-São Paulo, Brazil  4 Programa de Pós-Graduação em Engenharia Elétrica, Univsidade Presbiteriana Mackenzie, São Paulo, Brazil  * Corresponding author: cmbc@ifi.unicamp.br  Received July 9, 2007; revised October 3, 2007; accepted October 10, 2007;posted October 15, 2007 (Doc. ID 85002); published November 8, 2007  A microstructured optical fiber with a single design parameter is proposed and demonstrated. In such astructure three thin, long glass webs join in the fiber center, forming its core. By changing the web thicknessit is possible to tune the zero-dispersion wavelength from   0.7 to   2.0    m  while keeping a tiny core areaand single-mode guidance. Supercontinuum generation is shown in a silica fiber with a web thickness of  850 nm . The small core area and the massive hole area also make the structure very attractive for the sens-ing and study of fluids.  © 2007 Optical Society of America OCIS codes:  060.2280, 060.2310, 060.2370, 060.4370, 060.5530 . Microstructured optical fibers, also known as photo-nic crystal fibers (PCFs) [1], revolutionized the opti- cal fiber field as a result of the previously unthink-able control over fiber dispersion and modal area[2–4]. This control, to a large extent, arises from the high index contrast between the core glass and airholes and from the freedom in choosing the air-holedistribution and shape. The result is that, in suchstructures, the chromatic dispersion can be highlyengineered, making it possible to have, e.g., ultraflat-dispersion fibers [2], fibers with anomalous disper-sion in the visible [3], or even fibers with negative dispersion slope in the near infrared [4]. Although fully regular (periodic) PCFs present justtwo free geometric parameters, namely, the distancebetween consecutive holes (pitch) and the hole diam-eter, real structures always present some geometricaldistortion in the hole shape and location, making thewaveguiding characteristics dependent on a highernumber of relevant geometrical variables. Someother PCFs present an even larger number of free pa-rameters [2,5]. A simpler one-parameter fiberlike structure is a tapered conventional fiber that consistssimply of a thin glass strand. In this case, while thedispersion can also be easily tailored (and indeed su-percontinuum, SC, generation has been obtainedeven with Nd:YAG microchip lasers [6]), the lower mechanical robustness reduces, in practice, the maxi-mum fiber length to a few centimeters.In this work we proposed, fabricated, tested, andmodeled a new type of microstructured optical fiber,named the “Y fiber” [Fig. 1(A)]. Unlike other micro-structured optical fibers that present one ring of holes supporting a solid core [7–9], here the core is structurally formed by the joint of these three webswithout an extra piece of material where the bridgesintersect. The only design parameter that affects theguided mode is then the web thickness. It is shownthat tuning the web thickness   w   allows varying the zero-dispersion wavelength (ZDW) from0.7 to 1.8    m while keeping small effective areas (atthe ZDW), from   1.2    m 2 ( w =0.65    m, ZDW=730 nm) to   5.3    m 2 ( w =1    m, ZDW=1860 nm).Here, the effective area was calculated according to[10], which takes into account that part of the field istraveling in air. The proposed Y fiber presents useful Fig. 1. (A) Fiber scanning electron micrograph. Inset, non-circular fiber’s core. (B) Schematic representation of the fi-ber core. (C) Optical power distribution of fundamental op-tical mode at 1.55    m. 3324  OPTICS LETTERS / Vol. 32, No. 22 / November 15, 20070146-9592/07/223324-3/$15.00 © 2007 Optical Society of America  features suitable for nonlinear optics experimentsand evanescent-field-based sensing.Figure 1(A) shows a scanning electron micrographof the fabricated fiber cross section. While the websare   850 nm thick and   34    m long, and thus jointo form a small core, the air holes that surroundit are massive. The structure was simulated withcommercial full-vector finite-element software(COMSOL), keeping the web lengths fixed andchanging their thickness   w  . The manufactured fiberpresented a small deviation from a purely three-webstructure, as the joint of each two webs are smooth-ened in the fabrication process [see inset in Fig. 1 A].Figure 1(B) shows a scheme of the core area: whilethe real fiber is presented in gray (having an in-scribed circle of radius  R ), the purely three-webstructure is indicated with the dotted line (having aninscribed circle of radius  r ). For  w =0.85    m,  R =1.27  r , where  r = w  /   2  * sin     /3  . All simulationswere based on the smoothened profile.The fabricated fiber is numerically found to besingle mode at least from 400 nm. Figure 1(C) pre-sents a plot of the intensity (longitudinal Poynting  vector component) profile of the mode at 1.55    m on alinear scale, where the triangularlike mode shapecan be seen. Contour lines in 10 dB steps are alsoshown. The intensity drops to −60 dB from its peak value at a distance of approximately 10    m from thecore. This distance is increased to 12–13    m for a fi-ber with  w =2.0    m. A set of simulations was then carried out to inves-tigate the dispersion changes as  w  was varied from0.5 to 2.0    m. As can be observed in Fig. 2, the dis- persion spectrum shifts from purely normal   w =0.5    m   to a profile that crosses the zero twice,forming a region of anomalous dispersion, the widthof which increases with  w , between regions of normaldispersion. Thus, the only available free parameter isable to tune the ZDW and the general dispersionshape. The inset in Fig. 2 shows, in black, the two ob-served ZDW values as functions of   w . It is seen thatthe dispersion can be made null at anywhere from  0.7 to   2.0    m. To put this result in perspective,the same analysis is done to a single strand of silicalying in air (gray plot in the inset). Here the only freeparameter is its diameter. The same general trendcan be observed, but with a given ZDW obtained in Y fibers with webs that are thinner than the corre-sponding taper diameter. Note that while the flexibil-ity in controlling the dispersion is somewhat lowerthan that of more complex structured fibers, it iscomparable with that of tapers, with the clear advan-tage of improved robustness and longer lengths.Due to the small fiber core area and its triangularshape, the propagating mode spreads toward the low-refractive-index regions (the three air holes), making the structure guidance sensitive to the materialwithin the holes. To quantify the fiber efficiency as afluid, gas, or liquid sensor, one important parameteris the sensitivity coefficient  S =  n r  /  n  e   f  , where  n r  isthe refractive index of the material within the fiberholes,  n  e  is the modal effective index, and  f   is thefraction of energy in the holes.For the fabricated fiber the sensitivity coefficient isaround 7% for 1.55    m and holes filled with air. This value can, however, be as high as 22% for a fiber with w =0.5    m (at the same wavelength). In Fig. 3 the  S coefficient is plotted as a function of wavelength forfibers with  w  varying from 0.5 to 1.5    m.This same order of sensitivity coefficient can bereached with more standard hexagonallike PCFs[11]. In that case, however, the air holes would be ex-tremely small. By considering a microstructured op-tical fiber with high air filling fraction (air holediameter/hole pitch  d  /   =0.95), the core diameter  2  − d   should be around 1.35    m for a sensitivitycoefficient of    7% at 1.55    m. This implies a hole di-ameter and a hole area of    1.2    m and 1.1    m 2 , re-spectively. On the other hand, the area of each hole inthe Y-like fiber is   1200    m 2 . As the filling average velocity of cylindrical tubes is proportional to the holearea [12], the time to fill a length of Y fiber is ex- pected to be   1000 times faster than that to fill the Fig. 2. Chromatic dispersion of Y-like fibers with differentweb thicknesses. Inset, ZDW as a function of the web thick-ness of the Y-like fiber (black symbols) or taper diameter(gray symbols).Fig. 3. Sensitivity coefficient for fibers with different webthicknesses: from top to bottom,  w =0.5,0.6,0.7,0.85,1.01.5    m. Inset, transmittance spectrum through an 80 psi  4100 Torr   acetylene-filled 30 cm long fiber.November 15, 2007 / Vol. 32, No. 22 / OPTICS LETTERS  3325  same length of a standard PCF presenting the samesensitivity coefficient.The large sensitivity coefficient suggests the useof Y fibers for chemical sensing. A 30 cm samplewas filled with acetylene [C 2 H 2 , pressure 80 psi  4100 Torr  ] and light from a supercontinuum (SC)source composed by a microchip Nd:YAG laser and asuitable PCF was coupled into its core. The transmit-tance spectrum was recorded with an optical spec-trum analyzer with a resolution of 0.05 nm and isshown in the inset of Fig. 3. It is possible to clearly see the several absorption lines of the gas. Biologicalspecies detection and characterization is anothertype of experiment likely to benefit from the Y fiber.The presence of large holes near wavelength-scalecores allows large samples, such as bacteria, to beput into close contact with the guided mode.To demonstrate SC generation, short pieces   10 cm   of the fabricatedY fiber (ZDWs of    740 and1500 nm) were illuminated with 640 or 740 nmbeams from an optical parametric amplifier laser sys-tem delivering 50 fs pulses with 1 kHz repetition rateand average power from 5 to 250    W (peak powerfrom 0.1 to 5 MW). The spectra generated were col-lected with a standard multimode fiber and mea-sured with an optical spectrum analyzer with 10 nmresolution. Figure 4( A) shows the evolution of the broadening with the input average power for a fiberpumped in the normal dispersion regime    p =640 nm  , while in Fig. 4(B) the same evolution is shown for pumping close to the ZDW     p =740 nm  .In both cases the observed spectral trends arerather similar to those described in [13] for a highly nonlinear, conventional solid-core PCF, indicating that the same nonlinear processes are involved. Forpumping at 640 nm, the spectral broadening is rea-sonably symmetric for low powers and is likely to becaused by self-phase modulation. As the power is in-creased, the spectrum reaches the ZDW [gray line inFig. 4(A)] and becomes asymmetric. This trend sug-gests soliton formation above the ZDW followed bysoliton self-frequency shift due to intrapulse Ramanscattering [13].For pumping closer to the ZDW [Fig. 4(B)] a moresymmetric spectral broadening is obtained for allused powers. At 200    W, a 730 nm wide SC (at a−20 dB level from the peak) was generated in lessthan 10 cm of Y fiber. In a 45 cm long sample a spec-trum (not shown) as wide as 1000 nm was achieved.The observed trend is expected to derive from a domi-nant solitonic dynamics, with a high-order soliton be-ing generated and subsequently fissioned into severalfundamental solitons [13].Concluding, we have numerically analyzed and ex-perimentally demonstrated a new type of microstruc-tured optical fiber that has just one free design pa-rameter. Chromatic dispersion tailoring and a highevanescent field within massive internal holes makethe structure suitable for nonlinear optical experi-ments, including SC generation, and fluid-basedsolid-core fiber devices.The authors acknowledge Fundação de Amparo àPesquisa do Estado de São Paulo and Conselho Na-cional de Desenvolvimento Científico e Tecnológicofor financial support. The electron microscopy imageof Fig. 1(A) was performed at Laboratório Nacionalde Luz Síncrotron, Campinas. References 1. P. Russell, Science  299 , 358 (2003).2. K. Saitoh, M. Koshiba, T. Hasegawa, and E. Sasaoka,Opt. Express  11 , 843 (2003).3. J. C. Knight, J. Arriaga, T. A. Birks, A. Ortigosa-Blanch, W. J. Wadsworth, and P. St. J. Russell, IEEEPhoton. Technol. Lett.  12 , 807 (2000).4. D. V. Skryabin, F. Luan, J. C. Knight, and P. St. J.Russell, Science  301 , 1705 (2003).5. C. M. B. Cordeiro, M. A. R. Franco, G. Chesini, E. C. S.Barretto, R. Lwin, C. H. Brito Cruz, and M. C. J.Large, Opt. Express  14 , 13056 (2006).6. S. G. Leon-Saval, T. A. Birks, W. J. Wadsworth, P. St. J.Russell, and M. W. Mason, Opt. Express  12 , 2864(2004).7. V. V. R. K. Kumar, A. K. George, J. C. Knight, and P.St. J. Russell, Opt. Express  11 , 2641 (2003).8. H. Ebendorff-Heidepriem, P. Petropoulos, S. Asimakis, V. Finazzi, R. C. Moore, K. Frampton, F. Koizumi, D. J.Richardson, and T. M. Monro, Opt. Express  12 , 5082(2004).9. M. C. P. Huy, G. Laffont, V. Dewynter, P. Ferdinand, P.Roy, J.-L. Auguste, D. Pagnoux, W. Blanc, and B.Dussardier, Opt. Lett.  32 , 2390 (2007).10. V. Finazzi, T. M. Monro, and D. J. Richardson, IEEEPhoton. Technol. Lett.  15 , 1246 (2003).11. T. M. Monro, D. J. Richardson, and P. J. Bennett,Electron. Lett.  35 , 1188 (1999).12. R. H. Sabersky, A. J. Acosta, E. G. Hauptmann, and E.M. Gates,  Fluid Flow: A First Course in Fluid Mechanics , 4th. ed. (Prentice Hall, 1999).13. J. M. Dudley, G. Genty, and S. Coen, Rev. Mod. Phys. 78 , 1135 (2006).Fig. 4. SC generation in 98 mm long Y fibers (spectralwidth increases with input power). Dashed lines, pumpwavelength. (A) Pump at 640 nm and input average powersof 5, 15, 22, 30, and 50    W; gray line, ZDW. (B) Pump at740 nm and input average powers of 15, 40, 70, 110, and210    W. The width of the highest pump power spectrum(gray horizontal arrow) is more than 730 nm. 3326  OPTICS LETTERS / Vol. 32, No. 22 / November 15, 2007
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