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We report the fabrication of a Tellurite photonic crystal fiber, and demonstrate its waveguiding properties. The measured minimum loss is 2.3 dB/m at a wavelength of 1055 nm. The fiber supports several modes, but in practice just the fundamental mode can be used. We have observed strong stimulated Raman scattering in a fiber with an effective area Aeff=21.2µm2, using sub-ns, ~1 µJ pump pulses at 1064 nm.
©2003 Optical Society of America
1. Introduction
In recent years photonic crystal fibers (PCFs) have found applications in diverse areas of science and technology [1,2]. A PCF is a fine strand of glass with air holes running along its length, which give it the ability to confine and guide light. PCF’s range from highly periodic structures within which light is trapped in a large central air hole by a photonic bandgap, through to strongly-confining waveguides where a fine strand of glass is supported only by a web of far finer strands within a protective jacket: effectively an encapsulated glass core surrounded by air. The types of fibers are defined by the arrangement and number of the air holes, their shape and size, and the material and size of the guiding core. PCF’s are drawn from structured preforms. Most reports of PCF’s have described fibers made of silica, and the preforms have been fabricated using the stack and draw process [1–3]. Fibers drawn from other glasses [4–6] and formed using other process [5,6] have also been reported. Tellurite glasses offer a range of useful properties not possessed by silica, such as high refractive index, good infrared transmittance, high optical nonlinearity and relatively low phonon energy among oxide glasses [7]. At the same time, tellurite glasses are more stable than fluoride glasses and have higher rare earth solubilities than chalcogenide glasses [7]. These properties make tellurite glass an excellent candidate for PCF fabrication, and calculations of their waveguiding properties have already been reported [8]. In addition to all above characteristics, the Raman gain peak for Tellurite glasses is roughly ten times greater and has almost twice the bandwidth when compared to fused silica [9]. In this paper we describe the fabrication and transmission characteristics of the first low-loss tellurite-based PCF and report the observed stimulated Raman scattering spectra.
2. Fabrication
Cylindrical glass billets of composition 5Na2CO3-20ZnO-75TeO2 were cast with diameters 20mm and lengths 50mm. We then used these in a forward extrusion process to produce preforms and jacketing tubes. The extruder was mounted vertically on an existing soft-glass fiber-drawing tower, with the drawing furnace used as a heat source. We used a pneumatic actuator attached to a punch to force the glass billets through the steel die. We simultaneously drew the extruded preforms down to a diameter of 1mm. Figs. 1(a) and (b) shows a view through the die used for extruding the preforms, and a micrograph of the preform respectively. Preforms were jacketed with extruded tubes of 4mm OD and 1mm ID and drawn to fiber. We drew fiber lengths up to 50m without difficulty.
Fabricating soft-glass fibers is more difficult than silica fibers due to the smaller working temperature range. Also, the viscosity in soft glasses changes far more sharply with temperature. Nonetheless, the similarity between the shape of the extrusion die and that of the extruded preform (Figs. 1(a) and (b)) demonstrates that the temperature and pressure used were close to optimal. Fig. 1(c) shows an electron micrograph of a fiber cross-section. This fiber has an outer diameter of 190 µm, and a core diameter of 7 µm. The core is suspended by strands of just 100nm thickness and 70 µm length, very effectively isolating the core optically from the outer jacket. We have drawn similar fibers with core sizes as small as 2.6 µm.
3. Basic wave-guiding properties
The optical micrograph (Fig. 1(d)) shows bright guided light transmitted through the core. We measured the loss in the fiber by the cutback method, using a tungsten-halogen lamp and an optical spectrum analyser. The results are shown in Fig 2. A typical minimum loss measured in our fiber is 2.3 dB/m at 1055 nm. The loss in the fiber is partly due that of the bulk glass used for PCF fabrication, and this loss may be significantly reduced by using optical-grade chemicals for forming the glass billets. Another component of the overall loss is the presence of discrete scattering centers in the fiber, which are due to crystallization against the die walls during the extrusion process. In future work we plan to use different glass compositions which are less susceptible to crystallization. The increasing loss on the long-wavelength side of the measured spectrum is a feature of Na-containing tellurite glass compositions [10], and can be reduced through a different choice of glass former. We do not believe that confinement loss is significant on the length scales involved here, due to the length and small width of the strands supporting the core.
Fig. 1. (a) Photograph showing the cross-section of the die used for extrusion, (b) Electron micrograph of an extruded tellurite preform, of outer diameter 1mm, (c) Electron micrograph of tellurite PCF and (d) Transmission view of a tellurite PCF observed under microscope.
Although the fiber in Fig. 1(c) supports many modes, we have easily excited primarily the fundamental mode using laser sources. The properties of this mode can be well modeled as a circular strand of glass surrounded by air, enabling us to readily approximate the propagation characteristics. We have computed the effective area [11] of the fiber shown in Fig. 1(c) to be Aeff=21.2 µm2. Using this Aeff value and n2=2.5×10-19 m2/W [7,12] we calculated the nonlinear coefficient [11], γ=47.8 km-1-W-1 at 1550nm.
4. Raman Spectra
Stimulated Raman scattering (SRS) spectra were recorded using a passively Q-switched microchip Nd:YAG laser as a pump source. We used 600ps pulses at a wavelength of 1064nm and with a repetition rate of 7.24 kHz. We recorded SRS spectra for various incident pump energies from 7 nJ to 1.7 µJ. The coupling efficiency was approximately 30%, and the fiber length was 1.02m. Fig. 3 shows the SRS spectra for some of the incident pump energies. The endface of the fibre suffered irreparable damage if we go above 1.7 µJ. The spectral feature at 1117.1 nm corresponds to the Te-O-Te chain unit’s symmetric stretching mode, while the peaks at 1145.8 nm and 1156.3 nm are associated with the TeO4 bipyramidal structural arrangement and a TeO3+1 (or distorted TeO4) unit respectively. The peaks around 1250 nm can be attributed to second-order SRS, and those observed around 990 nm are anti-Stokes scattering corresponding to the peaks around 1150 nm. The Raman peaks and bandwidth observed in our fiber are very similar to the gain spectra recently reported by Stegeman et..al. [9].
Fig. 2. Measured spectral attenuation in a Tellurite Photonic Crystal Fiber. The size of the core is 7 µm. The minimum measured loss in this fiber is 2.3 dB/m. The cutback measurement was done on a fibre length of 2.5 m.
Fig. 3. Stimulated RamaStimulated Raman spectra from a Tellurite PCF of 1.02m length, using an pump laser wavelength of 1064 nm. All the incident pulse energy values given in the right are in µJ. The coupling efficiency is about 30%.
5. Conclusions
We have demonstrated that tellurite glass can be used to fabricate low-loss photonic crystal fibers. The minimum measured loss was 2.3dB/m, and this should be readily reduced by use of higher-purity starting materials, improved processing conditions, and choice of slightly different glass composition. We have observed first and second order Stokes SRS as well as an anti-Stokes peak using a microchip laser and just a meter of fiber. In future work we plan to extend this work to enable the fabrication of soft-glass PCFs with transmission extending further into the infra-red.
Acknowledgments
This work was funded by Dr. Jas S. Sanghera and Dr. Ishwar D. Aggarwal of the Naval Research Laboratory, through the Office of Naval Research International Field Office (ONRIFO) and by the UK EPSRC. The authors would like to thank Dr. Animesh Jha, University of Leeds for his advice and assistance during the early stages of this work.
References and links
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