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A simple refractive index sensor based on a small section of fiber damaged by the fiber fuse is proposed and demonstrated with a sensitivity of 350.58 nm/refractive index unit (RIU). For comparison, a hetero-core structure fiber sensor composed of a short no-core fiber (NCF) sandwiched between two pieces of single-mode fibers is demonstrated with a sensitivity of 157.29 nm/RIU. The fiber fuse technique can allow mass production of sensors by incorporating small sections of the damaged fiber of any type into each device. We believe this is the first application of the periodic damage tracks in optical fibers formed by the fiber fuse.
©2014 Optical Society of America
1. Introduction
Optical-fiber sensors have become research topics for the wide range of applications due to their intrinsic properties of a wide-band frequency response, immunity from electromagnetic interference, and compact size. They have the potential for measuring different a number of physical parameters including refractive indices. There are several techniques available for measuring refractive index; for instance, an air-gap LPG been demonstrated by combining a side polished fiber with lithography. The experimental results show a maximum sensitivity of 620 nm/RIU in the sensing of various concentrations of sugar in solution [1]. Combining femtosecond laser micromachining techniques and arc fusion splicing to make two micro air-cavities as a refractive index fiber sensor has been proposed with a sensitivity of 172.4 nm/RIU within a RI range of 1.333 to 1.365 RIU [2]. A fiber Bragg grating (FBG) cascaded with a long period grating (LPG) to create induced wavelength channels for detecting refractive index variation is another example which has been demonstrated [3]. A hetero-core fiber sensor refractometer in which the reflected cladding modes can be excited by connecting a multi-mode fiber to a chirped fiber Bragg grating (CFBG) [4] is also an effective technique for sensing. A core-diameter-mismatch sensor which consists of a short section of thin-core fiber (TCF) with a multimode fiber (MMF) tip inscribed with a fiber Bragg grating (FBG) [5] has also been demonstrated. Multimode interference (MMI) is widely discussed and applied in many fields [6–8], especially for refractive index sensing. A single-mode-multimode (SM) fiber structure has been demonstrated with a RI sensitivity of 94.58 dB/RIU [9]. A RI sensor based on a single-mode–multimode–single-mode (SMS) and FBG fiber structure has achieved a maximum experimental sensitivity of 7.33 nm/RIU in the RI range from 1.324 to 1.439 RIU [10]. A SMS fiber-based refractometer is discussed and an estimated resolution of 4.05 × 10−5 is achieved in a limited RI range of 1.333 to 1.382 RIU [11], and the author also proposes single-mode–tapered-multimode–single-mode (STMS) fiber structures, with an estimated average resolution over the refractive index range of 1.362-1.416 RIU of 2.59 × 10−5. Based on a leaky-guided multimode fiber interferometer (MMFI) operated under refractive-index-matched coupling, the experimental results show a RI sensitivity of about 113,500 nm/RIU within a limited range of 1.454 to 1.456 RIU [12]. All these sensors have good responses but on the whole suffer from complexity, which should ideally be reduced for low-cost practical sensor for widespread use.
The “fiber fuse” (FF) is a well known effect which causes optical damage in and around the core and leaves behind a trail of cavities in the glass [13]. It has been generally assumed that the fiber is no longer usable after damage because light propagation terminates in the damaged fiber, by strong scattering out of the core as a result of the cavities left behind. In this paper, we demonstrate that the light scattered out of the fiber couples to propagating cladding modes which can be used effectively to make a FF based refractive index sensor, amongst others devices. The sensor construction is simple and potentially very cheap, since it can be fabricated in any fiber. We also make a comparison with a no core fiber (NCF) structure based on the MMI principle in near identical structure. We present both, experimental and simulated results to show the performance of these sensors.
2. Principle
The first report of low power optical damage in optical fibers, also known as the fiber fuse, was reported by one of the authors 26 years ago [14]. The damage self-propagates, with the energy being provided by the laser power in the fiber. The most prominent tell-tale signs of damage are the sudden drop in the transmitted power and the bluish-white plasma-like emission at the damage point as it propagates towards the light source [15]. Closer examination of the core region reveals a continuous set of periodic cavities, a result of the damage process. These cavities are typically a few microns in diameter and periodically spaced by 10’s of microns. Till now, efforts have concentrated on avoiding the occurrence of this effect, since it is devastating for devices and for communication systems, or in arresting it as soon as it begins [16] to prevent propagation. The fiber fuse made for our sensor [17] is caused by heating the end of an optical fiber while propagating a few watts of optical power in the core. The absorption in glass increases dramatically at a temperature above ~1420 Kelvin, at which point the optical power in the core begins to provide the energy required to heat the fiber. Temperatures can rise rapidly to >10,000 Kelvin forming a plasma, damaging the core region of the fiber, and starting the fiber fuse. Unless the optical power is turned off, the periodic damage trail is created throughout the length of the fiber. The process of cavitation is poorly understood, however, optical propagation becomes lossy throughout the entire length of fiber as a result of very strong optical scattering out of the core by the cavities. The period of the cavities formed varies as a function of the intensity of the light in the core during the damage process. The refractive index is changed periodically in fiber core with the cavities, with a typical period of several 10’s of microns.
In the fused-fiber (FF: we use the term interchangeably with the fiber fuse) based sensor, single mode light transmitted in the core is scattered by the periodic cavities to cause the light to couple into the fiber cladding. The interference between the scattered cladding modes (generally >3) induces multimode interference (MMI), which can be detected at the output.
For the hetero-core structure made with the no-core fiber (NCF), single mode light beam traveling in the SMF can excite guided modes of the NCF and lead to MMI as well. According to previously studied theory of the MMI, the multimode-interference effect and the length of the MMI section can be calculated by using [18]
where m corresponds to the self-image number and Lπ is the beat length,where correspond to the effective refractive index of SMF-28 and NCF with the free-space wavelength of λm. acladding is the radius of the cladding both of SMF-28 and NCF. By combining Eq. (1) and Eq. (2), the free-space wavelength λm can be expressed as:We employed a beam propagation method to calculate the field along the length within the proposed fiber structures of the damaged fiber and the NCF. We use the following parameters for our numerical simulation: a standard single-mode fiber (SMF28) is chosen as basic fiber for the FF with the respective index for the core and cladding of 1.4509 and 1.444 RIU at a wavelength of input light of 1550 nm. The radius of SMF core is 4.1 μm. The damage cavity is assumed to have a radius of 2.5μm and the refractive index in the cavity is 1. The period of cavities is 14 μm. The parameters of the NCF fiber chosen are the refractive index is 1.444 RIU and the radius of 62.5 μm. The length both of two fibers is set with 7 mm.
With the damage centers in the FF based sensor we use an equivalent “average” length, and the NCF based sensor also has a length L. The corresponding loss-dip wavelength of the transmission due to MMI is denoted as λm. Referring to Eq. (3), when the effective refractive index of modes in the FF or NCF is changed by the surrounding refractive index, the loss-dip wavelength λm changes proportionally as MMI is strongly influenced by these changes.
3. Experimental procedure
For the FF based sensor, standard single mode fiber (SMF-28) was used. Figure 1 shows the process of fabricating the damaged grating using the fiber fuse. A high power single-mode 1550 nm wavelength IPG laser was used as the source. 12.5 W of radiation was coupled into a 3 m long piece of SMF-28. The output end of the fiber is brought into contact with the metal surface of the optical table to start the fuse. Examination of the fiber reveals the periodic damage cavities throughout its length with very good periodicity easily visible under the microscope, as shown in Fig. 2. This damaged fiber had cavities with periods of 13.96 μm.
Fig. 1 The fabrication of damaged fiber by the fiber fuse. By using IPG laser as a input source, the output fiber is touched to the metal surface to start the fiber fuse.
Fig. 2 The photographs of the damaged fiber under a microscope, shows periodic cavities in the fiber core region with the period of 13.96 μm.
A small, 7 mm section of the damaged fiber is cut and coupled on both sides to SMF-28 fiber to form the FF based sensor. The periodic refractive index modulation of the cavities/glass interfaces formed in the core is ~0.5. In our sensor, we do not rely on the periodicity of the damage to cause synchronous coupling to the cladding modes, but on the strong scattering into cladding modes from the cavities, and therefore consider it as a micro-structured fiber. Figure 3 shows the schematic diagram of the FF, connected to an offset output fiber to sample the cladding modes. This is due to fact that the light in the core of the damaged fiber is strongly scattered into the cladding to cause the enormous attenuation due to the large scattering coefficient.
Fig. 3 The schematic diagram of FF sensor made by a 7 mm-long damaged fiber with offset output SMF for sampling the cladding modes. The single-mode light is scattered by the cavities into the cladding to cause the enormous attenuation of the core mode.
In order to make a comparison with an MMI device, an equivalent sensor based on an NCF fiber is used. A schematic perspective of the hetero-core-structure NCF is shown in Fig. 4, where a similar length section of 7 mm of the NCF (as for the FF) is sandwiched between two SMFs with the output end SMF again offset to form the sensor. The NCF is a high-precision pure silica fiber with an un-doped core, which was fabricated by the Prime Optical Fiber Corporation.
Fig. 4 The schematic diagram of the hetero-core-structure fiber, including the NCF of 7-mm length with offset output SMF. The NCF can be viewed as an MMF with lower refractive index surrounding medium.
When incident light propagates in the NCF, the outer medium of the fiber with lower refractive index cladding layer facilitates total internal reflection due to Fresnel reflection. Thus, the section of NCF can be viewed as a MMF with a core of 125 μm which is surrounded by a lower-refractive-index outer environment.
4. Experiment results and discussions
The experimental set-up for sensing RI is shown in Fig. 5, in which a broadband light source (BBS) and an optical spectrum analyzer (OSA) are utilized for monitoring the transmission spectrum of the FF and as well as the NCF sensor.
Fig. 5 Experimental setup for measuring different liquids of different refractive indices. Drops of the refractive index oils are used on the sensing area of the damaged fiber and NCF, respectively.
Figure 6(a) shows the transmission spectrum of the FF based sensor with air as the surrounding medium. As the single mode light transmitted in the core is scattered by the periodic cavities into the fiber cladding, the interference between the scattered cladding modes induces a loss-dip in the transmission spectrum. Near field photographs at the corresponding resonance wavelengths of 1535, 1548, 1570 and 1587 nm indicate that cladding modes are indeed excited. The mode field distribution at the wavelength 1535 nm shows strong cladding mode coupling, and relatively weaker cladding mode coupling at 1570 nm. The original transmission spectrum is measured by collecting the cladding mode power from the damaged fiber as shown in Fig. 3, in which the highest peak corresponds with the strongest coupled core mode. After inverting the original spectrum, the core mode becomes the deepest loss-dip. The inversion is performed by using the data at the peak resonant wavelengths exciting the interference of cladding modes, which is located at 1535 nm as shown in Fig. 6(b). This is value of power in the decibel scale (dB but for this conversion is considered as dBm) is converted to a linear scale (in watts) of the original transmission spectrum of Fig. 6(a). Then the maximum value is subtracted from all of the other values in the spectrum. This result is plotted on a decibel scale (dB), where all values except the loss-dip become flattened with the power level of about −30 dB. Note that to avoid a -∞ value, the difference is arbitrarily made close to zero. There is a slight error bar as a result of the inversion, which is shown in Fig. 8.
Fig. 6 (a) The original spectrum and the near field photograph of the cladding modes of the FF. (A: 1535 nm with LP05, B: 1548 nm with LP05, C: 1570 nm with LP05, and D: 1587 nm with LP04). (b) The inverted transmission spectrum of (a) with resonant wavelength at 1535 nm.
At room temperature, when the surrounding liquid refractive index is increased from 1.32 to 1.45 RIU, the relevant loss-dips shift, as shown in Fig. 7. This figure also demonstrates that the wavelength of loss-dip shifts toward longer wavelengths as the refractive index increases.
Fig. 7 The measured inverted transmission spectrum for different index oils with the FF sensor with period of 13.96 μm. The loss-dip shifts to longer wavelengths as the refractive index is increased.
Figure 8 shows the relationship between the wavelength of the loss-dip and the refractive index varying from 1.32 to 1.45 RIU, although the measurements can be made beyond this refractive index. Within this range, the wavelength of the loss-dip shifts from 1536.6 nm to 1587.8 nm, which corresponds to a sensitivity of 350.58 nm/RIU. We estimate the resolution of our measurement to be ~2.85 × 10−5 RIU−1. Figure 9 shows the near fields simulation results of FF sensor with n = 1 and n = 1.4 at free-space wavelength of 1535 nm.
Fig. 8 The simulated and experimentally demonstrated relationship between the refractive index and the loss-dip wavelength shift of FF damaged fiber sensor with a period of 13.96 μm.
Fig. 9 The simulations of the near fields for the FF fiber, (a) n = 1, (b) n = 1.4 at a wavelength of 1535 nm.
Simulations performed with an approximated diamond shape for the 284 cavities in the 7 mm long FF section, with the period of ~14 microns as used in the FF sensor, shows a very similar transmission spectrum, shown in Fig. 6(a). This data is plotted in Fig. 8 to make a comparison with the experimental results. The simulated slope of 667.6 nm/RIU compares reasonably with the measured response of 350.58 nm/RIU. The difference may be attributed to the detailed scattering mechanism from the assumptions made on the shape of the cavities.
For the RI sensor based on the 7 mm long NCF, when the surrounding index is increased from 1.30 to 1.40 RIU, the loss-dip shifts from 1548.3 nm to 1563.9 nm. In Fig. 10, we can clearly observe that the loss-dip wavelength shifts toward longer wavelengths as the refractive index is increased.
Fig. 10 The measured transmission spectrum for different index oils with the hetero-core structure sensor based on 7-mm long NCF and the inserted spectrum shows the spectral responses of a 40-mm long NCF, but with the output fiber not offset.
This phenomenon of loss-dip shift is attributed to the interference between the few co-propagating modes in the NCF. Also plotted in the inset are the experimental results when the length of the NCF is increased to 40 mm but with no offset of the output fiber. Figure 11 indicates the relationship between the loss-dip wavelengths and the refractive indices varying linearly from 1.30 to 1.40 RIU, with a slope of 157.29 nm/RIU. With a longer length of NCF of 40 mm inserted between two SMFs with no-offset in the output SMF, the measured sensitivity of refractive index is only 110.87 nm/RIU. The sensitivity of 40-mm long NCF is lower than the 7-mm long, this fact is due to the fact that only the central region of the modes is sampled. This changes far more slowly than the wings of the cladding mode, as wavelength and refractive index changes (See Fig. 9). The offset output SMF can be used effectively to improve the sensitivity of the sensor.
Fig. 11 The relationship between the refractive index and the loss-dip wavelength shift of the hetero-core structure sensor. The experimental results are in good agreement with simulation.
7. Conclusion
In this paper we have shown for the first time an MMI structure fabricated with a short piece of fused-fiber (FF) and demonstrated how it may be used as a refractive index sensor. The sensitivity of the 7 mm long sensor is measured to be 350.58 nm/RIU; the simulated value is 667.6 nm/RIU, which is dependent on the exact shape of the cavities used in the simulations. An equivalent NCF sensor of the same length demonstrates a sensitivity of only 157 nm/RIU, which also agrees reasonably well with the simulated response of 125 nm/RIU. The reasons for a higher sensitivity of the FF based sensor we believe is due the greater number of modes excited by the damage cavities and hence the output power changes faster than for the predominantly few cladding modes (<3) [18] in the case of the NCF. A fused-fiber based sensor (FF) made with only a 7 mm long damaged fiber, shows that miniaturisation is possible, and that no other special fiber is required to fabricate this sensor, as it can be made in virtually any SMF. In the fused-fiber based sensor configuration, the technique provides a simple-structure, and of potentially low-cost. The period of the damage in fibers is controllable by the power launched in the fiber to alter the amount of scattering into cladding modes. Since the FF can be generated in very long lengths within a few minutes, it is easy to produce many sensors and replicate them very cheaply. Our preliminary results show the first useful application of the fiber fuse damaged fiber. We believe that other applications will follow with better control and fabricating techniques for assembling the FF based sensor. The performance of different period FF sensors is being currently studied and will be reported elsewhere.
Acknowledgments
The authors would like to specifically thank the National Science Council of Taiwan, for sponsoring this research under Contract No. 102-2917-I-035-005 and NSC 101-2221-E-035-046-MY2. R.K. acknowledges support from the Canada Research Chairs Program of the Govt. of Canada.
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