28-09-2012, 11:44 AM
Hydrofluoric acid flow etching of low-loss subwavelength-diameter biconical fiber tapers
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Abstract:
An etch method based on surface tension driven flows of hydrofluoric acid microdroplets for the fabrication of low-loss, subwavelength-diameter biconical fiber tapers is presented. Tapers with losses less than 0.1 dB/mm are demonstrated, corresponding to an order of magnitude increase in the optical transmission over previous acid-etch techniques. The etch method produces adiabatic taper transitions with minimal surface corrugations. A biconical fiber taper fabricated using this method is used to demonstrate an erbium doped silica microsphere laser.
Introduction
Subwavelength-diameter biconical fiber tapers (SBFTs) have diverse applications ranging
from evanescent coupling [1–4] to optical sensing [5–7] and nonlinear optics [8,9]. SBFTs are
generally produced from single-mode-fibers (SMFs) and consist of three contiguous regions: a
waist region of minimum diameter connected to transition regions on both sides that taper into
the nominal fiber diameter (Fig. 1(d)). Controlled heating and pulling of a SMF is the most
widespread method of fabricating low-loss SBFTs, with optical losses of about 0.1 dB/mm for
waist diameters around 500 nm [10–12]. Alternatively, fiber tapers can also be formed by
chemical etching using hydrofluoric acid (HF). However, a main challenge with existing acidetch
techniques is the resultant high optical losses. To date, demonstrated chemically etched
biconical fiber tapers have exhibited insertion losses in excess of 10 dB even at micron-scale
waist diameters [2,13]. The high losses have been attributed to the formation of surface
corrugations during the etch [2].
Fabrication method
Experimental setup
Figure 1(a) shows a schematic of our etch setup. To fabricate a taper, we began by
mechanically stripping a standard SMF-28 (Corning) fiber and cleansing it with isopropyl
alcohol. The fiber was then held by clamps across an HF-resistant Nalgene petri dish. We then
injected 100 μL of 49% HF (by weight) onto the dish. Due to the hydrophobicity of the
Nalgene dish, an HF droplet formed so that the dish could be raised by a translation stage to
immerse the fiber in the droplet to initiate the etch. The droplet shape allowed the length of
the waist region to be controlled by the immersion depth; short SBFTs required a shallower
immersion compared to long SBFTs. The apparatus was situated on an optical table for
mechanical stability, and the experiments were performed at room temperatures of 22°C ±
1°C. Each etch lasted approximately 75 minutes and only required occasional monitoring. We
terminated an etch by extracting the HF using a pipette and flushing the fiber with deionized
water. Figure 1(b) shows a typical etch progression.
Insertion loss measurements
Figure 3(a) summarizes the optical losses of a series of SBFTs with varying diameters obtained using our setup. The lengths of the waist regions for these tapers were approximately 5.5 mm. An insertion loss of 0.37 dB (0.07 dB/mm) was observed for a waist diameter of 480 nm, and micron-scale diameter tapers had losses slightly above 0.1 dB (0.02 dB/mm). Figure 3(b)-© are scanning electron microscope (SEM) images of the gold-sputtered waist region showing the top and cross-section views of the 480 nm diameter SBFT respectively. The waist exhibited excellent surface smoothness. The cross-section of Fig. 3© is not at the thinnest portion of the waist, but illustrates the production of cross-sections with good circularity at a diameter of about 1 μm. The slight edge visible at the top of the fiber in Fig. 3© was due to the etch asymmetry between the top and bottom of the fiber, and was more pronounced for shallower immersion depths (and therefore shorter waists) which could result in elliptical fiber cross-sections. When viewed under an optical microscope with halogen lamp illumination, the SBFT waist appeared blue-green as in Fig. 3(d). The color was due to interference effects and provided a means to roughly estimate waist diameters [20].
Application:
fiber taper coupled erbium microsphere laser
As a demonstration of a potential application of our SBFTs, we fabricated a fiber taper coupled microsphere laser similar to that in Ref [3]. The waist diameter of the taper was approximately 1 μm. The taper coupled 980 nm pump light into an Er3+ doped silica microsphere and out-coupled the laser light. The Er3+ microsphere was 30 μm in diameter and was formed using a fusion splicer by arc-melting an HF-etched highly doped Er3+ fiber taper (Leikki ER110-4/125). The etching removed the cladding from the doped fiber to preserve the Er3+ concentration during the arc-melting process. Figure 4(a) shows the output laser power as a function of launched pump power for the dominant lasing mode at 1534 nm measured using an optical spectrum analyzer. The laser threshold occurred at a launched pump power of about 2.2 mW. The insets show the microsphere coupled to the fiber waist region and the up-conversion fluorescence. Figure 4(b) shows the multi-mode laser spectrum at a launched pump power of 6 mW.
Conclusion
In summary, we have demonstrated the fabrication of low-loss, subwavelength-diameter biconical fiber tapers (SBFTs) using HF etching. Low surface roughness and high taper adiabaticity are general characteristics of the SBFTs produced using this method. Millimeter-scale waist length control could be achieved by changes in immersion depth within an HF microdroplet; however, production of centimeter-scale waist lengths requires the deliberate control of droplet geometry. In contrast to previous chemical etching methods, our present method produces SBFTs with optical losses under 0.1dB/mm, which is comparable to heat-pulled SBFTs [12]. An added benefit of our technique is that the composition of the core is preserved; thus our approach applies equally well to doped, index-graded, or nonlinear fibers. Finally, by controlling the post-etch surface charge density, our SBFTs can either be passivated or further functionalized with select particles or molecules.