Erbium-doped fiber ring cavity assisted by an FBG and PS-FBG reflector for refractive-index measurements - INVITED

. This work presents an interrogator system based on an erbium-doped fiber ring cavity for refractive-index measurements. This fiber ring cavity is assisted by a fiber Bragg grating and a phase-shift fiber Bragg grating, both with a similar central emission wavelength to increase the output power levels.


Introduction
One of the most relevant physical parameters in many different biological and biochemical applications is the refractive index (RI) [1].Optical fiber-based sensors have a series of very outstanding characteristics such as immunity to electromagnetic interference, resistance to corrosion, high bandwidth or small dimensions, making them ideal elements for making RI sensors [2].The use of fiber tips as intensity modulated fiber optic sensors as a sensing element overcomes one of the most frequent problems of this method of measurement, since only the end of the fiber needs to be immersed in the medium to be characterized.Thus, if the sensor is damaged, it is more easily replaceable, thus reducing the maintenance costs of the interrogation system [3][4].Besides the variety of different fiber tip configurations for RI detection, such as cleaved multimode fiber tips or microspheres [5], several fiber ring laser (FRL) configurations have been implemented to minimize source fluctuations and to efficiently use the gain provided by gain media, such as doped fibers [4], [6,7].In this experimental study, an interrogation system based on an erbium-doped fiber (EDF) ring cavity assisted by fiber Bragg reflectors for RI measurements is presented.The capacity to detect different refractive indices using this system has been verified by immersing the sensor head in different liquids as oxygenated water, ultrapure water, ethanol or ISOpropyl alcohol.

Experimental configuration
Figure 1 shows the experimental configuration of the proposed erbium doped ring cavity fiber.All experimental measurements were conducted at ambient temperature and did not use vibration isolation or temperature compensation techniques.As shown, a wavelength division multiplexer 980/1550 nm (WDM) injects 20 mW of pump power at 976 nm into the fiber ring cavity.The pump laser diode is controlled by temperature to ensure that the laser output is as stable as possible.The gain medium is connected to the WDM common port, consisting of 3.4 m highly doped erbium-doped fiber (EDF) M-12 (980/125, Fibercore Inc.).Thereafter, a 90% coupler is used in order to extract 10% of the signal reflected from the ring to an optical spectrum analyzer (OSA) or a power meter and a phase-shift fiber Bragg grating (PS-FBG), connected to the output port of this optical coupler.This signal is later divided into two branches through a 3dB optical coupler.The first output port of this 50% optical coupler is connected to a fiber Bragg grating (FBG) at the end of which a transversal cut is made.The other output port is a cross-cut SMF and used as a sensing head [7].This presented configuration does not ensure unidirectional operation and therefore spatial hole-burning (SHB) is expected to be generated.Figure 2 illustrates the overlap of the reflected output spectra of the PS-FBG (red line) and the FBG (black line), with central wavelengths emission of 1549.7 nm and 1549.8 nm in that order.

Experimental results
The EDF ring cavity output spectrum when pumped by a 976-nm laser at 100 mW and the sensing head is in contact with air can be observed in Figure 3.The measured output spectrum shows an output peak power, centered on the Bragg wavelength of the reflectors, at about 1549.75 nm, of around −20 dBm, with an optical signal to noise ratio (OSNR) of 55 dB.Hence, a refractive index characterization is performed by immersing the coupler output port acting as sensing head in different liquids, and evaluating the reflected optical power attained in each one of these measurements, with the use of a power meter.In addition to this, and in order to justify the use of the PS-FBG into the fiber ring cavity, a comparison between the results obtained when using only one FBG (black line) or both gratings (blue line) is carried out.The reflected power when the sensor is in contact with the air is 122.3 µW when using both the PS-FBG and the FBG, however this power drops drastically to 15.8 µW when the PS-FBG is removed.Figure 4 illustrates the sensing characterization with (blue line) and without (black line) the PS-FBG, when the sensing head is immersed in oxygenated water, ultrapure water, ethanol or ISO-propyl alcohol.In view of the results presented in Figure 4, the use of this PS-FBG results in a significant increase in the detected power levels for the refractive indices tested.

Conclusions
This work experimentally demonstrated an interrogation system based on an erbium-doped fiber ring cavity for refractive index measurements.By means of a 3dB optical coupler and being assisted by the overlapped optical spectra of an FBG and a PS-FBG reflectors, refractive indices can be easily discriminated.This interrogation system does not need to guarantee unidirectional operation, so it can be employed even when spatial holeburning effect is observed.An experimental demonstration of the capacity to detect different refractive indices using this system has been carried out by immersing the sensor head in different liquids as oxygenated water, ultrapure water, ethanol or ISO-propyl alcohol.Finally, improved performance has also been experimentally demonstrated by including a PS-FBG reflector, resulting in significantly higher reflected power output levels.

Fig. 1 .
Fig. 1.Experimental setup of the erbium-doped fiber ring cavity implemented for refractive-index measurements.

Fig. 3 .
Fig. 3. Ring cavity EDF output spectrum pumped by a 976-nm laser at 100mW when the sensing head is in contact with air.

Fig. 4 .
Fig. 4. Sensor characterization with (blue line) and without (black line) PS-FBG when the sensing head is in contact with