We present a novel measurement scheme using a double-clad fiber coupler (DCFC) and a fiber Bragg grating (FBG) to resolve cladding modes. Direct measurement of the optical spectra and power in the cladding modes is obtained through the use of a specially designed DCFC spliced to a highly reflective FBG written into slightly etched standard photosensitive single mode fiber to match the inner cladding diameter of the DCFC. The DCFC is made by tapering and fusing two double-clad fibers (DCF) together. The device is capable of capturing backward propagating low and high order cladding modes simply and efficiently. Also, we demonstrate the capability of such a device to measure the surrounding refractive index (SRI) with an extremely high sensitivity of 69.769 ± 0.035 μW/RIU and a resolution of 1.433 × 10−5 ± 8 × 10−9 RIU between 1.37 and 1.45 RIU. The device provides a large SRI operating range from 1.30 to 1.45 RIU with sufficient discrimination for all individual captured cladding modes. The proposed scheme can be adapted to many different types of bend, temperature, refractive index and other evanescent wave based sensors.
©2013 Optical Society of America
Cladding modes are guided modes which can be excited easily by scattering light from the core of an optical fiber into the cladding . FBG excited cladding modes are traditionally observed as a series of resonance peaks on the short wavelength side of the transmission spectrum of an FBG , due to the limited overlap of the field of the cladding modes with the guided mode field and the refractive index change across the core. These modes can also be excited with high efficiency as seen in the spectrum of a tilted fiber Bragg grating (TFBG) . These are generally not observed in the reflection spectrum since they may be dissipated through propagation along the high-index polymer jacket of the fiber . Recently, many schemes have been proposed to re-couple the reflected cladding modes back into the core. For example, a sensor has been proposed to re-couple cladding modes into the core of a short fiber excited by tilted fiber Bragg grating (TFBG) in which the sensor fiber was spliced to misaligned fiber [3,4]. The sensor was used to measure the ambient refractive index, as a bend and vibration sensor [3,4]. Such SRI sensor provides a dynamic range from 1.33 to 1.45 with sensitivity of 1100 nW/RIU at 1.33.The re-coupled cladding modes are within the wavelength band of 1538 nm and 1551 nm as merged band with no discrimination between individual cladding modes . Another scheme to re-couple low order cladding modes excited by the TFBG in the core used an abrupt bi-conical fused taper .
Cladding modes have also been re-coupled into the core with a TFBG in which a short multimode fiber or a thin-core fiber section with a mismatched core diameter section was spliced between a single mode fiber and the TFBG . The recoupled cladding modes in the best-case scenario are within the wavelength band of 1541 nm and 1549 nm. The sensor was used to measure the different refractive indices of glycerine-water solutions surrounding the TFBG providing a dynamic range from 1.33 to 1.45 RIU.
A scheme to couple the cladding modes into the core using a hybrid long period grating (LPG) and an FBG device has also been demonstrated. Here, the forward propagating core mode is partially coupled by the LPG into a cladding mode, which in turn is reflected with the core mode by a subsequent FBG, and partially re-coupled into the core by the same LPG [7,8].
One of the problems in all these devices is that, generally when a large number of cladding modes are involved, it is difficult to capture all the modes within a large wavelength band. An efficient discrimination between individual cladding modes is another problem to consider.
In this paper, we present a novel and simple device, which is capable of measuring cladding modes efficiently including both low-order, as well as higher order cladding modes propagating at steeper angles, which are more sensitive to the surrounding medium. We use the reflection from a single FBG in a standard photosensitive single mode fiber to couple the cladding modes into the inner cladding of a fusion spliced and diameter-matched DCF. The signal is viewed on the return port of a double-clad fibre coupler (DCFC).
2. Experimental procedure
The DCF has a single mode core, a lower index first cladding and a depressed outer cladding. Our device consists of a DCFC, whose single mode core is excited by a standard single mode fiber (SMF) spliced to one of its two branches and connected to a swept wavelength system (SWS) in a wavelength range between 1500 and 1560 nm (branch 1), and is schematically shown in Fig. 1 . A single mode photosensitive fiber with an imprinted FBG is fusion spliced to the DCF a short distance away (~5 mm). The device is characterized by measuring the reflected core and cladding light through the DCFC (branch 2) on an oscilloscope.
The DCFC is made by fusing and tapering two DCFs . The fibers are stripped of their protective jacket, cleaned, installed and inspected on a custom-made fiber coupler fabrication station. As a first step, the fibers are fused at high temperature using an oxy-propane micro-torch. The torch, mounted on a three-axis motorized stage, moves back and forth along the fibers to control the fusion length. As a second step, both ends of the structures are pulled apart using two additional motorized stages moving at a speed of 50 µm/s. The optical response is measured in real-time during fabrication. The DCF (Nufern, East Granby, CT, SM-9/105/125-20A) has a core diameter of 9 μm, an inner cladding diameter of 105 μm and fluorine-doped outer cladding diameter of 125μm of lower refractive index. The numerical aperture (NA) of the single mode core of the DCF is 0.12 and the inner cladding NA is 0.20. Our optical spectrum analyzer could accommodate a maximum light guiding diameter of only 62 micron and was therefore insufficient for the 105 micron inner cladding diameter in our experiments. We therefore used an integrating sphere power meter which has an entrance slit of 2.2 mm connected to the DCFC. The back reflected cladding modes are collected at the splice between the photosensitive fiber and the DCF by the inner cladding of the DCF and routed to the power meter by the second branch of the DCFC (see Fig. 1).
The FBG was imprinted in a photosensitive fiber by a commercially available 224 nm Nd:YVO4 laser which produces pulses of 7 ns an energy of 30 μJ at a repetition frequency of between 0.1 and 30 kHz with a maximum average power of 300 mW (Xiton Photonics). The beam is about 1 mm in diameter and is slightly elliptical. This laser has an excellent beam profile and stable laser output. It has a high wall-plug efficiency compared to the Argon laser used in the past for FBG fabrication . A mirror on a linear translation stage was used to scan the fiber, and a 20 cm cylindrical lens is used to focus the light. A Bragg wavelength at 1554.5 nm was imprinted with a length of 12 mm.
3. Results and discussion
Figure 2 shows the transmission and the reflection spectra in the proposed scheme normalized with respect to the transmitted power at 1560 nm. The transmission spectra consist of two bands, a strong Bragg resonance at 1554.5 nm and several narrow resonances, which are the cladding modes on the short wavelength side of the Bragg resonance. The cladding mode resonances begin at a wavelength of 1547 nm and consist of relatively strong low order modes, which are more confined and propagate at low angles. It can be seen that the reflection spectra of the cladding modes consist of modes within a wavelength band from 1543 nm to 1547 nm, which complements a limited sub-set of the cladding mode resonances seen in the transmission spectra.
Characterisation of the coupler response is shown in Fig. 3 . These experiments were conducted by exciting the port where the FBG was situated but without the FBG in place. The DCFC was designed to act as a null coupler for the core mode and as an 50:50 coupler for the inner cladding modes from 1265 to 1325 nm . At 1555 nm, the measured power in the core mode in the second branch where the reflected light from the DCFC is measured reaches ~-11.5 dB including the 10.5 dB coupler loss (see Fig. 3). The characterization of the spectral response of the DCFC core transmission is achieved with a conventional broadband source and an optical spectrum analyzer. In addition, the splice loss between the SMF28 and the DCF is ~1 dB.
The reflected power in the lowest order cladding mode is ~-5.3 dB since the maximum value for transmission by inner cladding modes is 33.5% in the second branch of the DCFC coupler between 1500 nm and 1547 nm (see Fig. 3). The characterization of the spectral response of the DCFC inner cladding transmission is achieved by using a wavelength-swept source and a diffuser. The diffuser is used to excite the high and low order modes in the inner cladding of the DCF .
In order to improve the efficiency of our device to capture greater number of the back reflected cladding modes at the splice between the FBG photosensitive fiber and the inner cladding of the DCF, the cladding diameter of the photosensitive SMF and the inner cladding diameter of the DCF have to be matched by decreasing the SMF diameter by 20 μm. This is achieved by etching the SMF, removing 20 μm of the cladding. In our scheme, the etched transition region (see Fig. 1) consists of the FBG in the photosensitive SMF and a small part of the DCF. To ensure better wet etch uniformity, a buffered oxide etch (BOE) [diluted HF with ammonium fluoride (NH4F)] is used to slow down the etch rate of SiO2 and to promote more uniform etching of the glass surface. The dilution ratio used in our experiment was 7:1 NH4F:HF. The reflection spectra in Fig. 4 show that decreasing the diameter by 20 µm increases the captured higher order modes significantly and increases the power of the captured low order modes as well.
Figure 5 shows both the reflection and transmission spectra of the device after HF wet etching. It can be seen that both low and high order modes, which are excited by the FBG can be recaptured efficiently by the proposed scheme. The captured cladding modes are within the wavelength band of 1500 nm and 1547 nm. Low order modes are captured very effectively, and the weakest high order mode are also well resolved and exceed 8 dB above the noise level, which can therefore be used in sensing. The different individual cladding modes can be easily discriminated and tracked.
Although the coupling loss of the DCFC increases with higher order cladding modes, there is sufficient discrimination even for the 41st cladding mode, to be used for sensing (see Fig. 5). The reflected power in the highest order cladding modes shown in Fig. 5 should ideally reach −12 to −14 dB including the 4.8 dB coupler loss. However, the splice loss between the SMF and the DCF of ~1 dB needs to be optimised, as this affects higher order modes more severely than lower order modes.
To demonstrate that the proposed scheme can be adapted as an evanescent wave based sensor, we characterize the response of the device to different surrounding refractive index (SRI) liquids from 1.30 to 1.51 (Cargille refractive index matching liquids).
Figure 6 shows the reflection spectra in response to different SRI. It can be seen that for SRI of 1.30, all the higher order modes can be discriminated and few high cladding modes are affected giving a spectra similar to that shown in Fig. 5 where the SRI is air. Increasing the SRI affects the higher order modes which disappear first, followed by the more confined cladding modes until all the cladding modes including the low order modes disappear at an SRI of 1.51. As the SRI gradually approaches and then exceeds the effective refractive indices of the cladding modes, they can no longer exist as bound modes and hence the high order modes are attenuated first. As is obvious, monitoring the Bragg reflection simultaneously allows self-referencing of the ambient temperature during measurements.
The disappearing individual cladding modes can be easily discriminated and tracked by increasing the SRI between 1.37 and 1.45, indicating that the device can be used to determine the change in SRI by tracking the number of the attenuated cladding modes, as can be seen in Fig. 7 . For example, by increasing the RI from 1.37 to 1.39, six cladding modes disappear and by increasing the RI further to 1.41, 10 more cladding modes disappear until almost all the cladding modes vanish when the SRI is ˃1.45. The disappearance of the reflected cladding modes was fitted to a straight line using linear regression between 1.37 and 1.45 showing a step of ~2.15 × 10−3 RIU/ mode lost.
Figure 8 shows another technique for measuring the SRI. Here, the reflected power of the proposed device is averaged over all the cladding modes. The cladding power decreases with increasing refractive index, indicating that the device can be used to determine the change in SRI by measuring the total cladding power reflected, noting that the Bragg reflection power remains unchanged with changing SRI. The power in the reflected Bragg wavelength was measured from 1552.5 nm to 1560 nm, while the cladding power was measured from 1500 nm to 1547.5 nm. Also, the reflected cladding power was fitted to 2 straight lines using linear regression between 1.30 and 1.37 and between 1.37 and 1.45, where it can be seen that there is a larger decrease in the cladding and total power between the SRIs compared with the lower SRIs in the range 1.30 and 1.37. The effective RIs of the high power containing lower order cladding modes lie between SRIs of 1.37 to 1.45. By matching the SRI with their effective refractive indices, the relevant modes disappear and hence the cladding or total power drops significantly. Between 1.45 and 1.51, it can be seen that the cladding power decrease is relatively small since almost all the cladding modes have vanished as they are no longer guided.
With this experimental scheme used in our measurements, the device shows a maximum sensitivity of 69.769 ± 0.035 μW/RIU between SRIs of 1.37 and 1.45 with a maximum standard deviation of 0.035 µW in response to different SRI measurements. The device shows minimum sensitivity of 4.043 μW/RIU between RIs of 1.30 and 1.37 indicating that the device is sensitive to RI changes down to 1.30 and providing a large dynamic range of SRI from 1.30 to 1.45. The device has high resolution for the higher sensitivity regime equivalent to an SRI change of 1.433 × 10−5 RIU ± 8 × 10−9 RIU, assuming an optical power meter with measurement resolution of 1 nW.
Finally, we note that the entire device could be simplified, by being fabricated in DCF fibre alone, including the FBG, so that no splicing of the FBG would be necessary. Only the FBG region would have to be etched so that the inner cladding could be exposed to the SRI for sensing. This would ensure minimum loss of the device. Furthermore, the DCFC fabrication process for the coupler can be optimised to ensure lower coupling loss for the high order modes and to improve the isolation between the cores of the DCFs. Work is under way to make this improvement, and the results will be reported elsewhere. Also, we note that the DCF coupler may also be used as an effective device to capture forward propagating cladding-modes excited by a long period grating (LPG), when used at the output.
In conclusion, an experimental scheme for a simple and novel device to recapture efficiently a large number of discriminated cladding modes in reflection from a fiber Bragg grating with a double clad fiber coupler has been proposed and demonstrated. The coupler is made by fusing and tapering two double clad fibers. Etching the single mode fiber with a Bragg grating inscribed in the core increases the efficiency of capturing the high order cladding modes. Using the scheme, low and high order cladding modes within a wavelength band from 1500 nm and 1547 nm can be captured with sufficient efficiency for the modes to be used in sensing. We have demonstrated this device for SRI sensing. The device shows a maximum sensitivity to SRI of 69.769 ± 0.035 μW/RIU and a resolution of 1.433 × 10−5 ± 8 × 10−9 RIU between 1.37 and 1.45. The device has a large dynamic range to operate from 1.30 and 1.45. In addition, we have demonstrated a new method in which the individual cladding mode peaks can be used to measure the SRI in a stepped fashion with a sensitivity of ~2.15 × 10−3 RIU per mode. We anticipate that this device will have many applications for evanescent wave sensing systems.
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