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Novel type of optical fiber sensor was proposed and demonstrated. The print-like fabrication technique fabricates multiple distributed feedback solid state dye lasers on a polymeric optical fiber (POF) with tapered coupling. This multi-active-sidecore structure was easily fabricated and provides multiple functions. Mounting the lasers on the same point of a multimode POF demonstrated a bending radius sensitivity of 20 m without any supports. Two axis directional sensing without cross talk was also confirmed. A more complicated mounting formation can demonstrate a twisted POF. The temperature property of the sensor was also studied, and elimination of the temperature influence was experimentally attained.
©2012 Optical Society of America
1. Introduction
Fiber sensors based on the Bragg reflection are well known electric-wire-free temperature and strain [1,2], bending [3] and multi-axis-bending [4] sensors. Generally, the bending sensitivity requires a supporting structure that converts the bending into strain [4], so the size of the measurement point is relatively large and the dynamic range is limited. Recently, a tilted and chirped grating was reported as a directional bending sensor without any supporting structure, which involved complicated fabrication [5]. Another development in multi-axis and compact sensors was not based on fiber Bragg gratings (FBG), but techniques such as multi-axis active sub-clad (intermediate layer between core and clad) [6, 7]. This detection principle was far from single-mode Bragg gratings. On the other hand, “solid state dye lasers” and its print-like fabrication have been studied for a long time by many research groups [8–10]. On-demand fabrication techniques are effective in integrating lasers with other applications, such as optofluidic chips, micro wells, optical fiber applications, and so on. We suggested a “pen-drawing technique” as an on-demand fabrication technique for a “thick waveguide” such as a polymeric laser [11]. It easily mounts the waveguide laser cavities on any surface such as a curved, pitted and narrow surface.
In this study, we have tried to fabricate a multi-axis “sub-core” (not sub-clad) fiber sensor structure on multiple-mode polymeric optical fibers (POFs). The pen-drawing method fabricates with high accuracy the polymeric waveguides several microns thick, and the waveguides were followed by a dry-recording process to make a distributed feedback (DFB) structure. It can also form tapered couplings on the both ends of the drawn waveguide, and a relatively high laser output coupling can be expected even though the sub-core structure. Since drawing waveguide and rotating POF can be controlled separately, multi-axis or twisted drawing can be obtained easily.
Figure 1 shows the schematic cross section (above), the fabrication process (below, left), and a microscopic image of the fabricated sample (below, right). The vertical single mode DFB dye laser waveguide (sub-core), with a thickness of 3~4μm was directly fabricated on the core of a multiple-mode POF, and the refractive index of the sub-core was tuned to be slightly higher than that of the core. The first-order distributed feedback structure based on the refractive index modification was recorded. The pumping beam from the Nd:YAG DPSS (diode pumped solid state) laser SHG, propagated in the POF core in multiple mode, and it can be partially absorbed by the DFB laser waveguide via the leaky mode. Single vertical-mode DFB lasing can be obtained. Since the laser waveguide has long-tapered couplers on both ends, the DFB laser outputs can be coupled to the core, so the outputs can be collected from both ends of the POF. Since whole matrices are flexible, the DFB output wavelength can be tuned simply by bending the POF. The temperature can also affect the Bragg wavelength.
Fig. 1 , schematic cross section (above) and fabrication process of DFB laser on polymeric optical fiber.(below, left) Microscopic image of drawn DFB laser on a POF(below, right)
2. Fabricating lasers on the POF
The multimode POF (Mitsubishi Rayon, CK-10/20, core diameter d = 0.23 and 0.48mm, PMMA) was mounted in the free-space rotational fiber holder in Fig. 1 (below, left). At first, the clad polymer coating was partially removed, and the laser waveguides were drawn by a highly-accurate dispenser (Musashi Engineering, ML-808FXcom). Though the POF core material was PMMA (n = 1.486), the prepolymer drawing method can fabricate laser waveguides of co-polymer; methylmethacrylate and 2-hydroxyethylmethacrylate in a mixing ratio of 9: 1 (p(MMA0.9: HEMA0.1), n = 1.489) on the curved surface.
The dispenser also could fabricate 3-5mm-long tapered couplers by controlling the speeds on dispensing and drawing. As shown in the right inset in Fig. 1 (below, right), the width was approximately 100μm and the length could be varied between 1mm and 30mm. After drawing, it was annealed at 70°C to complete solidification on the copolymer. Finally, an index type distributed feedback structure was recorded by an interfered UV exposing of (Spectra physics, Beamlok 2060 and Wavetrain, 244nm, 50mW). The taper angle was controlled to less than 0.4 degree. Since the laser threshold energy was measured as just 6% of the pumping pulse energy, as many as 10 DFB lasers can be driven simultaneously. Figure 1 (below, right) shows the waveguide dye laser on the POF (CK-10, 0.23mm core.) and its cross section in the right inset. The typical waveguide size was 100 μm (width) × 10mm (length) × 6μm (thickness).
3. Experiments and Results
3.1 Lasing performance
Figure 2 shows an example of the spectral profile of the DFB laser output of the DFB laser of rhodamine6G on CK-10. The waveguide length was 10mm. The bandwidth of the fitted
Vought function (solid line) was 0.072 nm (FWHM), even though the laser was fabricated on a curved surface. This number is almost comparable to those from lasers fabricated on a flat substrate, so the pen-drawing technique can fabricate a unique thickness of film on the curved surface. On the other hand, amplified spontaneous emission as background was suppressed to a level of 10−2 peak intensity. ASE suppression was somewhat poorer than that of lasers on a flat substrate.
To estimate the absorption efficiency along the POF axis, a 100mm-long waveguide was also fabricated, and an effective absorption coefficient, α, of approximately 0.1 cm−1 was obtained for CK-10 POF (d = 0.23mm). This number is much smaller than the product of the absorption coefficient of the waveguide (44 cm−1) and the cross-sectional ratio of the core and the waveguide (0.66%).
The laser threshold energy was also measured and 90 nJ was obtained as the absorbed energy. With a simple estimation, more than 100 lasers can be pumped simultaneously if the 20 μJ pumping energy pulse was coupled into an end of the POF.
Finally, the input/output characteristic was investigated, and the slope efficiency of 7.2% was obtained based on the absorbed energy. This is comparable to our previous work [12].
3.2 Bending sensitivity and multi-axis sub-core structure
The bending sensor can be performed by a wavelength shift of the Bragg wavelength. The wavelength shift seems to be due to the expansion or shrinkage of the waveguide, and it is assumed that the expansion or shrinkage caused by geometrical aberration centered the POF axis. Since the diameter of the core is much larger than waveguide thickness, the POF axis was not assumed to be expanded or shrunk. The shifted wavelength, λ, can be given by;
where λ0 is the initial Bragg wavelength without bending, d is the POF core diameter, and κ is the curvature (1/radius). In this expression, “+” and “-” correspond to the laser-mounted side of the outside (expanded) and inside (shrunk) of the bending curve, respectively.Figure 3 shows the bending properties of a DFB laser on POFs. In Fig. 3 (left), the theoretical wavelength shift / curvature ratio of 0.068 and 0.142 nm·m were estimated and plotted as solid lines. The experimental results (shown as circles, filled-circles) from DFB lasers were also plotted as symbols. Their good agreement shows that a simple geometricalmodel can be acceptable for this structure. The length of the laser waveguide was 10 mm, corresponding to the spatial resolution. The curvature sensitivity can be given by (detectable spectral shift)/(slope in Fig. 3 (left)). Experimentally, 0.05m−1 can be estimated from the spectral width of 0.072 nm(FWHM), response curve of 0.142 nm·m, and the detection limit of the spectral shift (assumed as FWHM/10).
Fig. 3 Bending properties (Left) The theoretical bending response curve (as solid lines), and the experimental results from DFB lasers of Rhodamine6G on CK-10 and CK-20. (Right) Two DFB lasers of Rhodamine640 were fabricated on a POF with right angular separation. They have different Bragg wavelength and independently showed shifts for horizontal and vertical bending.
3.3 Multiple directions bending sensor
As shown in Fig. 3 (right), two DFB lasers of Rhodamine640 were fabricated on a POF (CK-10) with right-angular separation. They have different Bragg wavelengths, 638 and 633 nm. The doping concentration was 2mM for both lasers. By coupling the pumping beam to an end of the fiber mounted without bending at a laser-integrated point, the output spectrum shows twin peaks at 633 nm and 638 nm. Due to a poor spectral resolution of the monitoring spectrometer (Ocean Optics, HR-4000, resolution 1.2nm), the spectral bandwidth was observed broadened, but the laser bandwidth was less than 0.1nmFWHM. When the bending was applied at the point along the laser of 633nm (termed x direction), the shorter spectral peak shifted as shown in Fig. 3 (right, above). Slight shifts were also observed in other peaks, which seemed to be due to the inaccuracy of the bending direction. This shows that bending sensitivities were isolated very well. On the other hand, the laser output amplitude was also affected by bending. If the laser was on the outside of the bent curve, the coupling of the pumping beam from the POF core to the DFB laser can be improved. If the laser was on the inside, the coupling efficiency seems to be decreased. A side-bending of DFB laser also decrease the laser output.
3.4 Twisting sensor
To detect twisting with this system, the mounted POF was slightly rotated during the laser drawing as shown in Fig. 4 . The rotation speed of the free-rotational fiber mount was π/3 rad/s and the drawing speed was 10mm/s. For DFB recording the fiber was twisted to cancel the rotation and record DFB under the condition that the waveguide is parallel to the fiber axes. Figure 4 (right) shows the twisting detection experiment. The wavelength shift can be given by
where L is drawing length, θ0 is the rotational angle in drawing and λ0 is the Bragg wavelength.Fig. 4 (left) Rotation drawing fabrication of DFB laser for twisting detection and conformation of the twisting test . (right) Result of spectral shift on twisting detection.
Though ∂λ/∂λ can be increased by decreasing L and increasing θ0, this preliminary experiment adopted θ0~π/3 due to the limitation of the fabrication process. Since ∂λ/∂λ of 1.42nm/rad can be estimated from Eq. (2), the wavelength shift around 0.5nm in Fig. 4 (right) corresponds to 0.35 rad. Though this sensitivity is low, it can easily be improved by decreasing L and increasing θ0. 25.1 nm/rad can be expected under L = 5 mm and θ0 = 2π. The improvement of the fabrication process will attain this in the near future.
3.5 Temperature calibration and discussion
Even so this fiber sensor also suffers from the influence of temperature due to the refractive index dependency and the linear expansion of the POF. However, we found that it was possible to eliminate the temperature effect. Under the assumption that the change in the propagation mode can be negligible, the wavelength change can be given by,
where T is temperature shift from initial temperature, neff is effective refractive index, Λ0 is initial pitch of the DFB, dn/dT and α are the differential of the refractive index with respect to temperature and the coefficient of linear expansion, respectively. In the case of PMMA material, dn/dT and α are −1.1 × 10−4 and 5 × 10−5, respectively, so the term in T2 is negligible, and from the opposite sign of the dn/dT and α, we can expect an elimination of the temperature dependency. In the experiment, the core waveguide of (p(MMA0.9:HEMA0.1) showed good cancellation as shown in Fig. 5 . It contains theoretical dependencies of the indexof dispersion and linear expansion (solid lines) and experimental measurements (circles). It shows the DFB lasing wavelength was almost stable against the fiber temperature over a range 25 to 85° C. The estimated linear fitting slope was as low as 4.6 pm/K.Fig. 5 Cancellation of the temperature influence of DFB laser on POF. Calculated Bragg wavelength functions of temperature based on linear expansion (solid line) and index change (dashed line). Circle symbols are experimental result.
Furthermore, we theoretically discussed about a third individual DFB laser to improve the temperature correction. If the three DFB wavelengths of λx, λy, λT were used as shown in Fig. 6 , the bending radius Rx, Ry and ΔT can be obtained by
where θ = π/2 and φ = 5π/4, the wavelength shift ratio σi = (λi-λOi)/λOi, (i = x, y and T), and λOi is the initial wavelength of λi. This formula can improve the temperature calibration.3.6 Discussion on sensitivities and applications
Finally, sensitivity was discussed. Since the detection scheme of “sub-core” fiber sensor is similar to that of FBG sensor, a similar sensitivity can be expected if the bandwidth of Bragg structure are comparable. In this work, DFB laser bandwidth (72pm in FWHM) seems comparable to general FBGs (0.05~0.1nm in FWHM). From Fig. 3, our “sub-core” fiber shows curvature sensitivity of 0.05 m−1. On the other hand, the detectable curvature about 1m−1 can be estimated for the sub-cladding fiber sensor [5], if we assumed that sensitivity is 10% of lowest measured experimental result.
Due to the printable fabrication scheme of this organic material and “sub-core” fiber structure, very low cost bending sensors like disposable usage can be expected. Disposable optical bending sensor with soft material sensor can realize embedded monitor in a cloth or medical system, for instance, patient activity monitor or an endoscope (fiberscope) positioning in body. Subsequently, short duration (periodically maintained) monitor such as small and instant architecture, energy-power cables, plants, and so on. We also study about durability extension and replacement of solid dye medium using PDMS matrix [13]. It will expand applicable region of this work in future.
4. Conclusion
A novel type of fiber sensor for bending, strain and twisting was proposed and demonstrated. Pen-drawing fabrication techniques can a mount polymeric DFB laser waveguide on a core of polymeric optical multimode fiber. The waveguide on the POF core with tapered couplings on both sides can realize an active sub-core structure for multifunctional measurement points on the POF, and it can detect 0.05m−1 curvature without any supporting jig. The multi-axis sub-core structure can detect multi-axis bending, and twisting drawing of waveguide provided twisting sensor. The temperature influence can be almost eliminated by using a PMMA POF and a p(MMA:HEMA) laser waveguide and a low temperature effect of 4.5pm/ °C was confirmed.
Acknowledgment
This study was supported by JST Feasibility Study (FS) Seed Excavation -A Program, Japan.
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