Photonic Microwire and Nanowire Devices: Fabrication and Applications

Nan-Kuang Chen1, Junjie Zhang2, and Chinlon Lin3

`1Department of Electro-Optical Engineering, National United University, Miaoli, Taiwan 360, R.O.C. 2Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai 201800, China

3Bell Lab and Bellcore, Retired. nankuang@gmail.com

Abstract- Photonic microwire and nanowire devices made of silica or multicomponent glasses have opened a new era for fiber photonics. In order to significantly enhance various optical functions like nonlinearity, photosensitivity, fluorescence, birefringence, special dispersion and so forth in the micro- or nano-fiber devices, different kinds of ions or host glass are employed. In this paper, we report the fabrication and applications of several kinds of photonic microwire and nanowire devices including the multicolor fluorescence emission, polarizers, and filters.

I. INTRODUCTION

In contrast to bulk optical devices, all-fiber active and passive components are featured with singlemode, low optical losses, easy alignment/coupling, good environment stability, compact size, flexibility, long interaction length, strong power confinement, low thermal load, and high laser beam quality. In order to achieve various optical functions like nonlinearity, photosensitivity, optical gain, special dispersion and so forth, different kinds of ions are usually doped into fiber core or different matrix glasses are employed as the host medium to attain the above targets. For core dopants, the lead [1,2] and germanium [3,4] can be used to significantly improve the nonlinearity, germanium [5] and boron [6] can respectively be used to introduce and enhance photosensitivity, cerium together with silver can be used to initiate the photo-thermal refractive properties [7], silver anisotropic nanoparticles can be used to generate strong birefringence, erbium or ytterbium can be used to give optical gain [8], sodium can be used to enlarge poling efficiency [9], neodymium can be used to increase the Brillouin spectral width [10], samarium can be used to provide saturable absorption [11], while the boron and fluorine can be respectively used to make fiber more dispersive and less dispersive [12]. On the other hand, the fluoride and chalcogenide matrix glasses can respectively expand the gain bandwidth due to its low phonon energy [13] and extend the transmission widow to mid-infrared region [14] while the phosphate is also renowned as a matrix glass with a very high doping solubility for compact waveguide amplifiers [15]. In contrast, fused silica is a well-known high phonon- and high bandgap-energy glass due to the stringent glass networks it has. The standard silica fiber is therefore robust and with low loss over visible/near infrared region, but is dispersive and with low heavy ions solubility, low quantum efficiency of doped ions, short energy level lifetime for

upconversion emission, and narrow gain bandwidth [16]. Consequently, the silica fiber is good for transmission but is disadvantageous for serving as functional devices like nonlinear or amplifying components, which usually comprise giant atoms or molecules inside the host glass. More recently, the nonlinear or amplifying photonic micro-/nanowires using multicomponent glasses like fluoride, phosphate, bismuthate, chalcogenide, and tellurite glasses are extensively studied to investigate the optical characteristics in the sub-wavelength scale [17-20] as well as enhance the nonlinearity and gain efficiency, engineer the dispersion, reduce the free carrier lifetime of semiconductor, and minimize the footprint of integrated devices [21], ascribing to the very strong power confinement. However, the power delivering through evanescent coupling between the silica fiber and the nanowire could be inefficient, unstable, and mechanically weak [18]. Besides, a high evanescent coupling efficiency only stringently occurs when the two fibers are identical but the wavelength dependent coupling effect makes the transmission bandwidth limited [22]. A bridging wire, of course made of multicomponent glass for special optical functions, capable of connecting the standard silica fibers could be an important evolution for achieving in-line novel and compact fiber active, passive and functional devices. In this work, we demonstrate the power delivering between two silica fiber tapers and the multicolor upconversion emissions of blue-violet, green, and red lights of Tm3+ and Er3+ ions [23,24] simultaneously, using a cw 1064 nm Nd:YAG laser as the pump light, through a 21-mm-long non-silica photonic wire made of Tm3+/Er3+ codoped tellurite glass [25,26]. The angled-cleaved fiber tapers are made by a twist-and-cut method and the achieved angle is 8.3�. Accordingly, the preliminary result of the insertion loss can be less than 8.85 dB. The longest tapered length (LW) and the thinnest tapered diameter (DW) of the tellurite bridging wire is respectively 21 mm and 5.3 �m. Besides the suppression of cavity resonances, the insertion loss can be further reduced by alleviating the excitations of higher order modes using silica fiber tapers with a proper tapered diameter (DT) of around the wavelength scales and a cleaved angle at taper tip. The 21-mm-long tellurite microwire can afford the maximal tensile strength under a counterpoise with the weight of 3.72 grams. Consequently, a stronger power confinement and field overlap for achieving high nonlinearity or high optical gain is highly promising and different kinds of optical host glasses like germanate or

germinate/tellurite/floride/chalcogenide can be used for inscribing fiber gratings [27], generating supercontinuum [4,28,29], and nanowire laser [30], respectively, based on this novel, simple, cost-effective, non-silica micro-wire-bridging technique for silica fibers.

Fig. 1. Fabrication procedures of a tellurite glass wire between two silica fiber tapers under a 50x CCD microscope. (a) The melting tellurite glass is initially clustered into a sphere at one end of fiber taper. (b) Another fiber taper is attached to the tellurite glass sphere. (c) The tellurite glass is heated and stretched. (d) The tellurite wire is keeping stretching to a smaller desired diameter (DW) and the nodes formed with tellurite glasses at the splicing junctions can thus be blurred. The inset picture with a pink frame between (a) and (b) is the 5-mm-thick tellurite glass.

II. FABRICATION AND EXPERIMENTSA high nonlinear or high gain multicomponent glass

usually has a quite large index when compared with the silica. A high index difference at the interfaces is known to introduce strong cavity resonances like in a Fabry-Perot resonator [31]. In order to deliver a broadband white light from one silica fiber taper to another, the catastrophe optical losses coming from cavity resonances must be carefully handled. It is known that an 8� angled-cleaved fiber end is usually employed as the APC (Angled Physical Contact) connector to avoid the reflection from the fiber end. Accordingly, the tapered standard singlemode fiber (Corning: SMF-28) is twist and cut to obtain an angled tip for suppressing any possible resonance. There could be also a minor benefit that the angled tip provides a larger contact area to improve the binding strength between silica fibers and photonic wire. In addition to the cavity resonances, the excitations of higher order modes due to the abrupt change of core diameter and NA can also lead to huge optical losses. The use of tapered fiber is to expand the mode field outside the original core until its evanescent field is thoroughly exposed to occupy the shrunken cladding so that the mode field can be transferred to the photonic bridging wire smoothly and the higher order modes excitations are substantially suppressed. Moreover, the DW of bridging wire can be controlled by choosing a proper DT of silica fiber taper as a seed during tapering and a high index Tm3+/Er3+ codoped tellurite wire with a diameter of around the wavelength scale is helpful to enhance the field overlaps to convert the cw 1064 nm Nd:YAG laser light into blue-violet, green, and red lights. Also, this micro-wire-bridging technique, compared with the fiber drawing technique using perform, is advantageous to draw the glass into fiber at a temperature of much lower than melting point. Though the melting point of the tellurite glass is typically

less than 570�C and is much lower than the melting temperature above 1500�C of silica fiber, the wire drawing for glasses can be working well at the temperature of softening point (~350�C for tellurite) which is somewhat higher than glass transition temperature (~300�C for tellurite [25]) but is much lower than the melting point. Hence, this micro-wire-bridging technique is an efficient low temperature fabrication method. In contrast to bulk glass, the tapered fibers also have a merit to provide stronger field confinement in micro/nanowire to enhance the nonlinearity of nonlinear devices and improve the quantum efficiency of lasers and amplifiers. For lasers and amplifiers, the gain bandwidth as well as the upconversion energy level lifetime can be both improved, if a low phonon energy glass like tellurite or fluorozirconate is employed as the host for active ions [16,32].

1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65

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tellurite wire DW

= 15.7 �m and silica tapers DT = 30 �m

tellurite wire DW = 13 �m and silica tapers DT = 30 �m

Tran

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sion

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)

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(a)

1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65-35

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tellurite wire DW = 34.2 �m and silica tapers DT = 70 �m tellurite wire DW = 25.4 �m and silica tapers DT = 50 �m tellurite wire D

W = 17.1 �m and silica tapers D

T = 30 �m

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Fig. 2. Spectral responses of the tellurite wire with (a) DW, DT, and LW of (15.7 �m, 30 �m, 20.2 mm) and (13 �m, 30 �m, 21.3 mm), respectively, when bridging two perpendicular cleaved and (b) DW, DT, and LW of (34.2 �m, 70 �m, 17.2 mm), (25.4 �m, 50 �m, 19.5 mm) and (17.1 �m, 30 �m, 21 mm), respectively, when bridging the two angled-cleaved silica fiber tapers. (RES: 1nm)

In order to obtain good upconversion emission efficiency in the Tm3+/Er3+ codoped tellurite microwire, a long interaction length and a low loss power transmission between silica taper and tellurite wire are crucial. To check the insertion losses of this tellurite wire, a white-light source comprising multiple superluminescent diodes spanning 1250-1650 nm is launched into the silica fiber taper. The transmitted output power from the

silica fiber taper

tellurite sphere

(a) (b) (c)

tellurite wire

silica fiber taper

tellurite wire 12 mm (d)

nodes

other end of fiber taper is recorded by an optical spectrum analyzer under the optical resolution (RES) of 1nm. The normalized transmission loss is shown in Figs. 2(a) and 2(b). In Fig. 2(a), the cavity resonances can be clearly observed when the two silica fiber tapers for bridging the tellurite wire are perpendicularly cleaved. The DW, DT, and LW of the tellurite wire for sample 1 and 2 are (15.7 �m, 30 �m, 20.2 mm) and (13 �m, 30 �m, 21.3 mm), respectively. Since the LW can be as long as 21.3 mm, there should be a lot of cavity resonances existed in the microwire. The measured spectra with much less resonances are because the two reflection planes in wires are not strictly parallel due to the wire is slightly bent during tapering processes. From Fig. 2(a), the extinction ratio of resonant loss peak can even go above 22.5 dB to cause huge optical losses. As mentioned above, the angled-cleaved fiber tapers are advantageous to avoid the cavity resonances in such a high index photonic bridging wire and the corresponding spectra are shown in Fig. 2(b). DW, DT, and LW of the tellurite wire for sample 1, 2, and 3 are (34.2 �m, 70 �m, 17.2 mm), (25.4 �m, 50 �m, 19.5 mm) and (17.1 �m, 30 �m, 21 mm), respectively. The cleaved angles are among 8� to 10�. The lowest transmission loss is about 8.85 dB at 1557.8 nm. The losses go beyond 20 dB at the wavelengths shorter than 1330 nm and no significant resonances are observed. The insertion loss is higher for the shorter wavelengths since the scattering losses or the Fresnel reflection losses from the junction as well as from the tellurite nodes are both higher for the shorter wavelengths. Though the lowest insertion loss is still as high as above 8.85 dB at this stage of experiments, the high index tellurite photonic bridging microwire with LW of 21 mm can still successfully deliver the broadband optical power between silica fiber tapers and efficiently generate the multicolor upconversion emission lights as discussed later. The DW is proportional to DT since a smaller DT can help generate a smaller DW. The average losses decrease when the DW gradually decreases. This is because a smaller DTcan not only enlarge the guiding core to smoothly transfer the fundamental mode power into the tellurite wire but also avoid the big clustered tellurite nodes to scatter the guiding lights. Accordingly, the insertion loss could be significantly reduced by using silica tapers and bridging wire with diameters of around a few tens of micrometers. Besides, the remained clustered tellurite nodes at the junction should be further blurred or removed to significantly reduce the insertion loss by a scanning flame. Since the photonic microwire can provide a much longer interaction length as well as a better field confinement than that in the bulk glass, the upconversion efficiency is expected to be enhanced in the microwire. To compare the upconversion emission spectra of photonic microwire and bulk glass, a 975 nm laser light is respectively launched into the sample 3 in Fig. 2(b) with pump power of 103 mW and a 5-mm-thick bulk glass with pump power of 308 mW to excite the Tm3+ and Er3+ ions and the upconversion lights were measured using a spectrometer (Stellarnet: EPP2000-VIS-10). In Fig. 3, the 975 nm pump light makes the gain medium to give upconversion emissions over the red wavelength band. The intensity of upconversion from photonic microwire is stronger than that from bulk glass.

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nsity

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.)

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Fig. 3. The red upconversion emissions of the Tm3+/Er3+ codoped photonic microwire and bulk glass under 975 nm laser pumping.

In conclusion, we have demonstrated a Tm3+/Er3+ codoped tellurite photonic bridging microwire that can connect two angled-cleaved silica fiber tapers for power delivery and simultaneously generate multicolor upconversion emissions including the blue-violet, green, and red lights using a 270 mW 1064 nm Nd:YAG pump laser light. The upconversion efficiency is significantly improved in microwire than in bulk glass due to a longer interaction length and a better field confinement. The length of photonic bridging microwire can be as long as 21.3 mm. The uses of angled-cleaved silica fiber tapers are shown to be helpful in avoiding cavity resonances and delivering the broadband white light toward the other fiber. These photonic bridging microwires made of non-silica glass materials have silica lead-in and lead-out fibers to be compatible to standard fibers. They are highly promising to be employed for novel fiber components like blue-violet lasers for biophotonic applications. The fabrication and applications of photonic microwire and nanowire devices including the polarizers, and filters will be presented in the conference.

ACKNOWLEDGMENT

This work was supported in part by the R.O.C. National Science Council under Grants NSC 98-2221-E-239-001-MY2.

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