APPLIED PHYSICS LETTERS 93, 261107 共2008兲

The observation of comblike transmission spectrum from a tapered single mode fiber tip Y. Li and X. Baoa兲 Department of Physics, University of Ottawa, 150 Louis Pasteur Street, Ottawa, Ontario K1N 6N5, Canada

共Received 3 November 2008; accepted 9 December 2008; published online 29 December 2008兲 Tapered fiber tips are fabricated by the fast stretching of a heated single mode fiber until it breaks. As the fiber tip diameter decreases, the propagation mode converts from the single core mode to a few cladding modes. The cladding modes are then emitted from the fiber tip and interfere with each other in the near field. Along the propagation direction, the optical field is found to be modulated due to multibeam interferences. Interestingly, broadband transmission spectrums show equidistant comblike peaks. A tunable spatial filter based on a fiber tip is realized to reshape a broadband light source. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3059574兴 Optical interference provides an effective way to reveal and study the coherence properties of light. A simple method to produce optical interference fringes is to split one single light beam into two or several beams using narrow parallel slits, and let the beams interfere with each other in free space. In this letter, instead of the parallel slits, a tapered fiber tip was used to split the fundamental propagation mode in a single mode fiber 共SMF兲 into several cladding modes. Our tapered fiber tips were fabricated using the heating and stretching technique.1,2 By adjusting the heating area and the stretching speed, fiber tips with different profiles can be fabricated. It is known that by tapering a SMF, propagation modes other than the fundamental mode could be generated due to the decrease in the fiber diameter.3,4 In this study, guided cladding modes were created in tapered fiber tips. These cladding modes then interfered with each other in the near field of the tips. One free end of a transversely cleaved SMF was located in front of the fiber tip for probing the interferences. Along the propagation direction, optical field intensities were found to be modulated as the probe fiber moved away from the fiber tip. Furthermore, when an erbium doped fiber amplifier 共EDFA兲 was used as a broadband light source, equidistant comblike peaks were observed in multiwavelength transmission spectra. The peak separations and signal to noise ratio 共SNR兲 were discovered to be related to the shape of the fiber tip. These observed interferences in the spatial and the wavelength domains are mainly caused by different optical path lengths of the emitted cladding modes. Tapered fiber tips are applied in many fields, such as near-field scanning optical microscopy,5 optical coherence tomography,6 optical tweezers,7 and nanoprobes.8 Benefiting from their micro- or nanoscale size, the fiber tip provides a near-field, easy-handling, and high resolution detection. The realization of multibeam interferences based on tapered fiber tips has great potential in photonic areas, such as optical filters and optical cantilevers. In this study, a tunable spatial filter based on a tapered fiber tip has been realized in a fiber loop ring to reshape the output spectrum of a broadband EDFA. The fabrication of tapered fiber tips is similar to that of fiber tapers.1,2 The basic apparatus is composed of two linear a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0003-6951/2008/93共26兲/261107/3/$23.00

translation stages, a gas flame for heating the fiber, and a computer for control. The fiber was held by two translation stages, while a microtorch moved back and forth along the fiber to heat it. When the heated section became soft, the two stages started to stretch the fiber in opposite directions. Contrary to the “slow stretching” for making adiabatic fiber tapers, a “fast stretching” is needed for tapered fiber.7 In this experiment, the heated fiber was drawn at a speed of 2 mm/s until it broke at the waist point. Finally, the fiber tip with a paraboliclike profile was formed due to the surface tension of the fused silica. A representative tapered fiber tip is shown in Fig. 1. As shown in Fig. 1, a cleaved SMF is located in front of the fiber tip as a probe to measure its near-field optical properties. A diode laser at a wavelength of 1470 nm was utilized as the light source. The probe fiber was held by a five-axis translation stage with a spatial resolution of 40 nm on the X-Y-Z axis 共MTS50, Thorlabs, Inc.兲. A photodiode was connected to the probe fiber to record optical field intensities. The position of the probe fiber was adjusted to be parallel to the fiber tip and to optimize the measured field intensity. The light intensities were recorded every 100 nm as the probe fiber was moved away from the fiber tip along the propagation direction. Figure 2 shows the field distribution over a distance of 30 ␮m with 6 dB power variation. Interestingly, besides the attenuation of the light intensities, periodic peaks

FIG. 1. The photograph of experimental setup for measuring the optical near field of tapered fiber tips. The left is one free end of a SMF held by a five-axis translation stage, which is used as a probe. The scale bar is 10 ␮m and the inset at the right corner is showing the taper cladding radius of the fiber tip in this photograph.

93, 261107-1

© 2008 American Institute of Physics

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Appl. Phys. Lett. 93, 261107 共2008兲

Y. Li and X. Bao

FIG. 2. The optical field distribution along the fiber propagation direction. The black dots represent the experimental data every 100 nm.

with a pitch of 1 ␮m were also observed. The modulation of light intensities was resulted from the interference of cladding modes emitting outside of the fiber tip. As mentioned above, when tapering a SMF, several propagation modes other than the fundamental mode can be excited. Originally, the fundamental mode LP01 is still the only propagation mode in the fiber. This mode is also called the core mode as its effective index 共neff兲 is between the cladding and core indices 共ncladding and ncore兲. When the core diameter is reduced during tapering, neff of the core mode decreases continually. When it reaches the cladding index, the core mode LP01 is not guided anymore and cladding modes are generated. Intermodal beating among these cladding modes has been observed in many previous studies.9–11 This is also referred to as the core mode “cutoff.”3,4 The transition occurs at the point where the V number of the core-cladding interface is given by12 Vcore/cladding ⬵

冑

再

0.26 2 1+ ln S ln S

冎

−1/2

.

共1兲

For the fiber we used 共Corning, SMF-28™兲 ncore = 1.4681, ncladding = 1.4627, and the diameter ratio S 共cladding diameter/ core diameter兲 = 15.06. So we have Vcore/cladding = 0.82, which means the cladding modes replace the core mode when the fiber is tapered down to 23.2 ␮m 共outer radius兲 in this experiment. Figure 1 shows that the tapered fiber tip has a diameter of ⬃3 ␮m, which is much smaller than the transition diameter. It is also noted that the fiber diameter decreases sharply at the end of the tip 共see the slope of the profile curve in Fig. 1 inset兲. Previous theoretical calculations have shown that the number of cladding modes reduces when the fiber is tapered down.4 Thus, it is reasonable to assume that some cladding modes emit outside of the fiber before they propagate to the tip end. Finally, all these modes or beams interfere with each other in the near field of the tip and result to the observed spatial fringes shown in Fig. 2. For a further inspection of multibeam interferences, broadband transmission spectrums were obtained using an L-band EDFA as the light source. The amplified spontaneous emission 共ASE兲 from the EDFA has a spectral coverage of 80 nm with a reasonable intensity 共see Fig. 4 inset兲. The ASE light propagated through the tapered fiber tips and went into the probe fiber as in the above experiments. Connecting the probe fiber to an optical spectrum analyzer 共OSA兲 共Agilent

FIG. 3. 关共a兲 and 共b兲兴 The broadband transmission spectrums of tapered fiber tips with different tip profiles. The insets show photographs of the corresponding fiber tips. The relative intensity ratios are displayed more clearly at the bottom, which are obtained by subtracting background EDFA ASE spectrums.

86142A兲, we measured the broadband transmission spectrums of two tapered fiber tips with different tip profiles. As shown in Figs. 3共a兲 and 3共b兲, the two spectrums present similar comblike peaks, but with different SNR and peak separations. The comblike peaks are equidistant with average separations of 1.70⫾ 0.02 and 1.14⫾ 0.03 nm, respectively. The highest SNR is 16 dB for tip a and only 5 dB for tip b. These spectral differences are believed to be related to their different tip profiles. The inset in Fig. 3 indicates that tip a has a smaller end and a lower slope than tip b. As we discussed above, during the tapering, some cladding modes emit outside of the fiber before they reach the tip end. A larger slope of the tip profile means larger decrease rate in fiber diameters and can lead to more cladding modes leaving the fiber in the unit length of the taper. Therefore, in the case of tip b, the probe fiber receives more beams or radiation modes from the

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Appl. Phys. Lett. 93, 261107 共2008兲

Y. Li and X. Bao

FIG. 4. 共Color online兲 Tapered fiber tips applied as tunable spatial filter to reshape the output spectrum of an L-band EDFA. The left inset is showing the spatial filter setup. The right inset shows the EDFA output spectrums with 共solid兲 and without 共dash兲 the loop ring. The output spectrums of the fiber loop are displayed in the center. The frequencies of central peaks can be tuned from 1561 to 1566 nm by moving the fiber tip or receiving fiber transversely.

tip, giving rise to smaller mean peak separations. Taking advantage of the comblike transmission peaks, tapered fiber tips can be applied as spatial filters to reshape the output spectrum of broadband light sources. As shown in Fig. 4 共left inset兲, a tapered fiber tip, a 50/50 coupler, and an L-band EDFA constructed a fiber loop ring. An OSA was connected to the output port of the coupler to monitor the output spectrum. We found that the output spectrums were greatly changed from the EDFA’s original ASE spectrum 共see the right inset in Fig. 4兲. Narrow peaks with an average linewidth of ⬃0.7 nm appeared in the short wavelength range. The central peaks originated from the comblike peaks as shown in Fig. 3, but were selectively amplified with the help of the fiber loop ring.13 Within the fiber loop, the light filtered by the fiber tip was divided by the 3 dB coupler. Onehalf of the light intensity stayed in the loop ring and then was amplified by the EDFA again. Thus, among the competition of comblike peaks, the highest intensity peak received the most gain from the EDFA and became the dominant spectral component in the loop ring. More interestingly, the central frequencies of these peaks can be tuned from 1561 to 1566 nm, as shown in Fig. 4, by adjusting the positions of the fiber tip or receiving fiber in the transverse direction. This is not surprising since the highest intensity peaks can be slightly varied by changing the relative positions between the fiber tip and the probe fiber. Also, the long wavelength part is found to be suppressed in the output spectrums. This is not only because the EDFA has lower gain at long wavelength

range. Even without the loop ring structure the long wavelength component was remarkably attenuated. The higher loss of long wavelength light could be caused by large taper angles.9 Plus, long wavelength light has a larger diffraction angle when it leaks out of the fiber. The probe fiber is located in front of the fiber and it can only receive the beam within small solid angle. Thus, the long wavelength light with large divergence angle is hard to collect with the probe fiber. Miniature structured fiber tips can be also applied as optical cantilevers for mechanical or biomedical sensing. For instance, a tip’s mechanical vibrations and resonance frequencies can be measured, which are related to the tip’s physical properties, such as mass, density, and shape. By monitoring its resonance frequency shifts, the change in mass, density, or shape can be detected. More importantly, tapered fiber tips are easily and economically fabricated. In conclusion, we have studied the optical properties of tapered SMF tips. Equidistant comblike peaks were observed in broadband transmission spectra. These comblike peaks are mainly due to the multibeam interferences of cladding modes in the near field. Finally, the tapered fiber tip was applied as a tunable spatial filter to reshape broadband light sources. The authors acknowledge the stimulating discussion with Professor Liang Chen and the financial support from the Canadian funding agencies: Discovery Grant of NSERC under Grant No. G33241 共Natural Science and Engineering Research Council of Canada兲 and the Canada Research Chair program. 1

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