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Abstract
In this study, a double-ring erbium-doped fiber (EDF) laser with an optical switch and a fiber Bragg grating (FBG)-referenced interrogating system was developed and demonstrated. This double-ring configuration can achieve high power amplified spontaneous emission, enabling laser oscillation even within the L-band. The output range and signal-to-noise ratio (SNR) of the double-ring EDF laser were measured to be 1512–1610 nm and 55 dB. In addition, the interrogating system using FBGs for reference resulted in precision improvement of ∼25 pm over those achieved in previous studies, reaching a precision of about 7 pm. The high power, broad tuning range, and sufficiently high precision of the proposed interrogating system make it promising for use in FBG-based sensing applications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber Bragg grating sensors have the advantages of being able to detect various physical quantities and immunity to electro-magnetic interference [1–4]. Consequently, FBG sensors have become strong competitors for existing pre/magnetic operating sensors in areas such as health monitoring, fuel level monitoring, and strain sensing [5]. To read the sensor values of FBG sensors, various methods of measuring the variations of the reflected wavelengths resulting from strain, temperature, etc. have been presented. Typical methods include the use of passive edge filters [6,7], array waveguide gratings [8,9], interferometric methods [10,11], and fuse couplers [12,13]. Broadband sources such as light-emitting diodes, superluminescent diodes, and amplified spontaneous emission (ASE) sources have been used to implement these various measurement methods. However, these sources have low signal power and a narrow tuning rang [14]. To overcome these shortcomings, research has been conducted using wavelength-swept lasers (WSLs), which emit light at different wavelengths according to sweeping time. WSLs sweep the output wavelength continuously and periodically within a certain period of time, and the corresponding wavelengths are amplified to form a peak of high power light. This output peak is reflected by the FBG sensor or array and is detected by a photodetector simultaneously. It can be obtained by synchronizing the amount of light reflected at a specific time with the specific wavelength data of the measured light. The measurement device applied to these light signal for use in additional processes or analyses is called an interrogator and is used in various sensing applications [1–4]. Interrogators provide high measurement accuracy, high repeatability, and multiplexing capacity which are desirable characteristics in sensing application field. These features are mainly determined by the range of the wavelength spectrum of the WSL, signal intensity, and linearity, and to improve these characteristics, high power, a broadband tuning range, and high tuning linearity are required. Various tunable optical filters have been used to produce WSLs, such as a rotating polygon mirror [15,16] and a piezo-electrical transducer (PZT)-driven fiber Fabry-Perot (FFP) filter [17,18]. Among the WSLs produced through these techniques, WSLs using erbium doped fiber (EDF) have been studied extensively due to the properties of EDF, which has high power and a broad tuning range [19–21].
EDF laser light sources have the advantages of high wavelength and power repeatability, low pumping threshold, and high signal-to-noise ratio (SNR), among others. However, their bandwidths correspond to fast nonradiative decay from the ${}^4I13/2$ energy level to the ${}^4I15/2$ energy level of Er3+ ions [22,23], which is associated with wavelength ranges of about 1520–1560 nm, corresponding to the C band. Three types of studies have been conducted on the use of high-dopant EDF, different host material glass fibers, and different lasing structures to produce a C + L band EDF laser light source for high resolution and a wider tuning range.
The first involves the use of a high concentration EDF, which shows high efficiency. A short, high-concentration EDF can reduce the entire cavity loss due to the decreased EDF length, and deep saturation with high power and low noise figure (NF) is possible even with low pump power. However, shorter distances between each Er3+ ions result in performance degradation such as homogeneous up-conversion (HUC) and inhomogeneous interaction (PIQ) [24]. The second type of approach involves the use of other host material glass fibers with ions such as aluminum, germanium, and ytterbium [25,26]. When lanthanum ions such as ytterbium and erbium ions are co-doped, the absorption efficiency of the erbium ions increases, facilitating deep saturation. In an aluminum co-doped fiber, broader fluorescence emission and a flatter gain spectrum can be obtained compared to those achieved using the original silica-based fiber. Further, by using a fiber such as a germane-silica or bismuth-silica EDF, the emission wavelength range can be changed. However, to amplify the broad wavelength range (especially over a wavelength of 1580nm in L-band), a relatively long EDF is required, leading to the performance disadvantages of gain and increased noise figure [27]. Finally, an amplifier and laser structure that achieves a broad tuning range through structural changes of two or more fibers with different amplification ranges was proposed [28–31]. This method can produce a broader bandwidth than would be possible by amplifying a single EDF. However, the corresponding structures have disadvantages such as the presence of C/L-band wavelength division multiplexer (WDM) coupling band gap loss, which reduces the amplification efficiency between bands, and the use of band pass filters (BPFs) to eliminate the ASE of other EDFs.
Wavelength-swept EDF lasers have been studied for use as light sources in FBG interrogating systems [32–36]. The high SNRs and broad tuning ranges of wavelength-swept EDF lasers enable multi-channel and broad range interrogating system. However, the use of tunable filters reduces the linearity between wavelength and time, resulting in poor measurement modality. Various studies have been conducted to increase this linearity, which have yielded high linearity but a narrow bandwidth about 11.8 nm [37,38].
In this paper, we propose a WSL using two EDFs, a tunable filter and an optical switch, and an FBG interrogating system to overcome the non-linearity between time and wavelength in a broad wavelength range. A broad tuning range and high power were achieved using EDFs with different host materials, and the optical switch was used to achieve a bandwidth of about 98 nm above 0 dBm in the overall gain spectrum so that there was no band gap loss between the C and L band. We also sought to overcome the nonlinearity of WSL by utilizing certain FBGs for reference, which enabled us to develop an FBG interrogating system with an accuracy of about 7 pm in a WSL system with linearity of 0.9986 at a band width of 80 nm.
2. Theoretical basis
2.1 Complex effects in EDF lasers
Figure 1 provides schematics of the energy level substructures of EDFs connected in series with different host glass material, including the absorption and fluorescence transitions. The material and composition of the host glass affect the solubility and doping environment of the rare-earth dopant. These characteristics also affects the emission life, absorption, emission, Stark effect, and excited state absorption (ESA) which occur in energy transfer from ${}^4I13/2$ to ${}^4I9/2$ by absorbing light with a wavelength of approximately about 1590 nm. If lanthanum ions such as erbium are placed inside the crystalline host, the spherical symmetry of the rare-earth ion will be broken. Thus, the 4F atomic states of erbium become nondegeneracy. Stark levels with different energies at the same energy level, which will change the emission wavelength [39]. This broadening mechanism can be referred to as Stark splitting and is the cause of the broad absorption and emission ranges of EDFs.
Fig. 1. Schematics of the energy level substructures for EDFs connected in series, with the Stark components for absorption and fluorescence transitions represented (C-band EDFs typically use germanium/aluminum doped fiber, and L-band EDF generally use bismuth based fiber)
The ASE of the C-band EDF with an emission spectrum around a wavelength of 1530 nm is absorbed by the L-band EDF. At this time, various energy transfers (resonant energy transfer, stepwise up-conversion, cooperative luminescence, cooperative energy transfer and simultaneous photon absorption, PIQ, and HUC) between Er3+ ions will occur, affecting the efficiency of the optical amplifier. Stark splitting also occurs during the energy transition of Er3+ and affects the absorption and emission spectrum, as well as between EDFs with different host materials. L-band EDF absorbs the ASE light of the C-band EDF and has ASE around a wavelength of 1560 nm due to the host material of the L-band EDF. The ASE of the L-band EDF around 1560 nm has a wavelength range different from that of the C-band EDF emission. Therefore, by using two EDFs with different emission wavelength ranges as a laser source in conjunction with an optical switch, a broader wavelength range can be obtained than with a single EDF laser source.
2.2 Fiber Bragg grating
An FBG is an optical fiber that reflects a particular wavelength of light via periodic grating patterns carved inside the core. The wavelength reflected by the FBG can be expressed as
where $\lambda_B$ is the Bragg wavelength, which is reflected by the periodic grating pattern; ${n_{eff}}$ is the effective refractive index of the fiber; and Λ is the pitch of the periodic grating pattern. The reflected wavelength changes with temperature, strain, and polarization [40]. The wavelength shift with variations in temperature and strain can be expressed as follows [41]. where $\Delta \varepsilon $ is the change in strain; $\Delta T$ is the change in temperature; $K_\varepsilon $ and $K_T$ are the strain and temperature coefficients, respectively. Polarized light has a different effective refractive index depending on the polarization state, causing wavelength shifting in the FBG. An FBG, which can measure the wavelength shift due to these factors, can provide a high multiplexing capacity of more than 10,000 FBGs in a single fiber [42]. Due to their high multiplexing capacities, FBGs are mainly used in sensors and communication but are also utilized for interrogation.An APD is a highly sensitive solid state quantum optical detector with an avalanche amplification zone. Because of their avalanche amplification zones, APDs are typically used for low light detection such as single photon detection. In this experiment, the APD employed for FBG interrogation was an InGaAs APD. An InGaAs APD has an absorption window at wavelengths of 900–1700 nm wavelength ranges which are widely used in optical fiber sensors. APDs, which are advantageous for low power light detection, can improve the shortcomings of low reflection FBGs and are also advantageous for implementing multichannel systems [43,44]. However, there are inherent noise such as shot noise and dark current in APDs. Due to the inherent noise in an APD, the input signal should have a higher power than the noise equivalent power (NEP) which is the minimum detectable signal power. The NEP can be expressed as
where In is the noise current, $\Re $ is the APD responsivity. Section 3 describes the light source and FBG interrogating system, as well as a WSL with more than 0 dBm of power available even in an APD with a low responsivity of 0.8 A/W.3. Laser source
3.1 Double-ring EDF laser configuration
Figure 2 shows a schematic diagram of the proposed double ring EDF laser configuration. The C-band EDF (EDFC-980-HP, Nufern) is forward pumping through the 980/1550nm WDM coupler and 980 nm pump laser diode at 205 mW. The 15-m-long C-band EDF has a concentration of about 1500 ppm and is fusion-spliced with an optical isolator to prevent feedback such as noise from the output port. A 1×2 optical switch (Crystal latch 3-stage bi-directional, Agiltron) optionally switches the direction of light propagation. When the optical switch receives a +5 V DC signal to propagate light to the C-band direction, the ASE power of the C-band EDF is connected to a 3 dB coupler. A PZT-driven FFP tunable filter (FFP Filter, Lambda Quest) with an insertion loss of 3 dB transmit the wavelength corresponding to the driving voltage in the ASE of the C-band. The filtered ASE of the C-band EDF is connected to the 980/1550 WDM coupler, forming a ring structure in the direction of the blue line. Since seed signal injection is possible through this ring structure, a single high power wavelength peak of the C-band can be obtained through stimulated emission. This peak propagates outward through the 90:10 coupler, and the output may vary depending on the tap ratio of the coupler [45].
When the optical switch receives a -5 V DC signal to propagate light in the L-band direction, the ASE power of the C-band EDF is connected to another 980/1550 WDM coupler and L-band EDF (EDFL-980-HP, Nufern). The 18-m-long L-band EDF has a concentration of about 4000 ppm and is fusion-spliced with an optical isolator. The FFP tunable filter transmit the wavelength corresponding to the driving voltage in the ASE of the L-band EDF. The filtered ASE of the L-band EDF is connected to the 980/1550 WDM coupler, forming a ring structure in the direction of the orange line. Since seed signal injection is possible through this ring structure, a single high power wavelength peak of the L-band can be obtained thorough stimulated emission. The absorption coefficients of the C and L-band EDFs are 9 dB/m and 16.5 dB/m, respectively, at 980 nm [30].
In a fiber ring laser structure, there are two possible methods of obtaining laser output in ranges in which the emission–absorption coefficient is relatively low (especially at both tails of the erbium gain band). The first is to achieve a high ASE power to maintain laser oscillation [46], which usually regulates the reflectivity of the output coupler. The second is to reduce the entire cavity loss which can increase the lasing efficiency [47]. We focused on reaching a high ASE power to obtain laser output in ranges in which the emission-absorption coefficient is relatively low.
Figure 3 shows the ASE spectral output power in the C and L-band EDFs according to the pump powers of the two laser diodes. Figure 3(a) indicates that the range of the ASE in the C-band EDF pumped to a 980 nm laser diode of 205 mW is approximately 60.6 nm, from 1514.8 nm to 1575.4 nm, with power above -40 dBm. The ASE of the L-band EDF is filtered by a tunable filter, and no ASE amplification from the double-ring laser structure is obtained from the C-band EDF. Figure 3(b) demonstrates that the range of the ASE in the L-band EDF pumped to a 980nm laser diode of 370 mW is approximately 31.4 nm from 1553.5 nm to 1584.9 nm with power above -40 dBm. However, the ASE range above -40 dBm with the L-band EDF pumped to a 980 nm laser diode of 205 mW and with the C-band EDF pumped to a 980nm laser diode of 80 mW is approximately 53.7 nm, from 1549.1 nm to 1602.8 nm. Co-pumping through the C-band EDF ASE and 980nm laser diode can yield a broader and higher ASE power than simply increasing the output of a single pump power. Therefore, the C-band ASE source raises the ASE output power and pump efficiency of the L-band EDF [29].
Fig. 3. Spectral output power according to pump laser power for the (a) C-band EDF ASE and (b) L-band EDF ASE.
3.2 Applied voltages and control system
It is well known that the relationship between the driving voltage of the piezo actuator and the displacement is nonlinear and path dependent in a PZT driven FFP tunable filter because of intrinsic hysteresis [48,49]. These nonlinear properties can cause large errors and instability in wavelength-measurement devices using FFPs. Furthermore, such devices using FFPs require additional feedback loop systems or additional demodulation systems due to these nonlinearities [50,51]. Although FFP filters are sufficiently linear at short bandwidths of about 10 nm [38], they have low linearity across broad bandwidths.
The microcontroller employed in this study based on an ARM Cortex-M3 CPU (SAM3X8E, Atmel) controls the voltage with DC/DC converters (XL6019, XLSEMI), which can supply high voltage to the APD and tunable filter. Two voltage regulators supply stable voltage to the optical elements. One voltage regulator (LM2596, Texas Instruments) supplies the 980 nm laser diode, and the other (KIA7805A, KEC) supplies the switching voltage. The tunable filter driving ramp like voltage has a peak-to-peak value of about 18 V, which is broad enough to cover a range of 1510–1610 nm, and to ignore noise caused by rapid voltage changes. Figure 4(a) shows the tunable filter driving voltage according to time within a sweeping cycle. When the voltage is increased from the lowest to the highest value in the previous cycle, the driving voltage exhibits a transient response caused by the switch characteristics of the DC-DC converter. Thus, time is required to initialize the voltage a steady-state response. Then, an L-band sweep from 21 V to 11 V is performed for about 6.1 ms while keeping the sweep slope as constant as possible. When the FFP filter has a voltage corresponding to 1545 nm, the optical switch uses a switching time to change the C-band. This time can prevent unwanted noise from occurring when the optical switch changes the optical path. Then, a C-band sweep from 11 V to 5 V is performed for about 3.6 ms while keeping the sweep slope as constant as possible. After the C-band sweeping time is the ramp stabilization time, which consists of the last driving voltage cycle. The actual time to sweep the wavelength is based on the operating speed minus the voltage initialization, stabilization and switching times, which yields a sweeping time of 9.78 ms. This time is negligible relative to the overall operating time. Figure 4(b) shows the driving voltage according to the filtered wavelength. Due to the intrinsic nonlinearity of the FFP filter, the relationship between the driving voltage and wavelength has a coefficient of determination at 0.9986. Thus, the relationship seems sufficiently linear, but it is difficult to use in an interrogator system that requires high precision. In fact, the relationship between driving voltage and wavelength has a standard deviation of 4.12 nm, which makes it difficult to achieve high precision. Therefore, Section 4 proposes an interrogating system that uses a nonlinear FFP filter but can have sufficiently high precision.
Fig. 4. Tunable filter driving voltage according to (a) time within a sweeping cycle and (b) filtering wavelength
4. Interrogating system
4.1 FBG-referenced interrogating system
Figure 5 provides a schematic diagram of the interrogating system. For FBG interrogation with high precision even when a nonlinear FFP filter is utilized, we employed a single fiber with serially engraved FBGs as a reference. The interrogating system was implemented with C# and Arduino programming, using an ARM Cortex-M3 CPU (SAM3X8E, Atmel) with a 84Mhz clock as the microcontroller. The APD (GPD-PSA1-50AR, Shenzhen Yigudian Technology) utilized in this study has a response time of 0.3 ns, and the analog-digital converter (ADC) chip (AD7606, Analog Devices) has a 16bit ADC with 200 Ksps (kilo samples per second). At the operating speed of the ADC, approximately 2000 raw data can be collected per channel at 100 Hz, but the actual number of raw data per channel is approximately 1000 due to the transmission and storage of data between the ADC and microcontroller unit.
Various methods have been proposed for detecting peak optical power in time domain multiplexing [52]. The neural network and polynomial fitting methods show the highest precision but have long execution times. Meanwhile, it is difficult to measure multiple peaks with the maximum method and the precision increases rapidly at low SNRs. Thus, we selected the weighted centroid of gravity (wCOG) method using a threshold, which enables high precision and rapid execution even at low SNRs. To increase the stability of the peak data, cross-correlation of a normal distribution with a standard deviation value of 2 and interpolation were used. Section 4.2 describes the algorithm in detail.
4.2 Wavelength calculation algorithm
Figure 6 depicts a schematic diagram of the proposed FBG-referenced wavelength interrogating algorithm. Although prior studies have been conducted using FBG sensors as references for the wavelength interrogation [53–55], the reference FBG should be inserted into a single serial FBG sensor in this method, and the further the wavelength of the FBG being measured from the reference wavelength, the higher the standard deviation (approximately 35.37 pm) [55]. Therefore, we chose to interrogate the wavelength by using a single fiber with serially engraved FBGs for reference. Each of the 1000 ADC data described in Section 4.1 has an index number from 1 to 1000. The index number of a peak detected by the wCOG method can be expressed as
5. Experimental results
Figures 7(a) and (b) present the wavelength according to the driving voltage of the FFP tunable filter and the peak power at that wavelength for the C- and L-bands, respectively. As mentioned in Section 3.1, a high ASE power directly affects the amplification range. The C-band tuning range with significant output power (more than 0 dBm) is from 1512 nm to 1569 nm (approximately 57 nm), while the L-band tuning range with significant output power is from 1545 nm to 1610 nm (approximately 65 nm). Thus, switching the C-band EDF ASE to change the amplification direction according to the output wavelength band would result in a wavelength range of about 1512–1610 nm, representing a bandwidth increase of about 41 nm. Figure 7(c) shows the measured output powers of single-ring and double-ring EDF lasers in the L-band. Figure 7(d) depicts the measured C- and L-band SNRs of the double-ring EDF laser. The average of SNR from 1510 nm to 1545 nm is about 54.87 dB. In C-direct propagation, the average SNR from 1545 nm to 1569 nm is 29 dB, which is decreased by the L-band ASE connected using a coupler. However, in L-direct propagation, the average SNR from 1544 nm to 1610 nm is 56.55dB and includes the entire range with relatively low SNRs in C-direct propagation. Thus, the overall SNR is about 55 dB. This SNR does not differ much from that of existing EDF laser and is sufficiently high to be used. The entire output spectrum was measured with an optical spectrum analyzer (MS9740B, Anritsu) at a resolution of 0.05 nm.
Fig. 7. Measured spectral output of (a) proposed double-ring C-band EDF laser and (b) proposed double-ring L-band EDF laser. (c) Measured output power of single-ring and double-ring EDF lasers. (d) Measured C- and L-band SNRs of double-ring EDF laser
The comparison results of the proposed double-ring laser are shown in Table 1.
Table 1. The comparison analysis of the proposed double-ring laser.
Figure 8(a) provides visual descriptions of the index noise, accuracy, and precision. The error of the ADC and peak detection method results in index noise (peak to peak) of about 0.1 index numbers. This error of ∼5 pm affects the accuracy of the measured wavelength. As shown in Fig. 8(b), the accuracy is ∼100 pm due to the reference index noise. The FBG-referenced method shows no significant improvement in accuracy. However, the precision has a similar amount of index noise for various wavelength spacings. In addition, Accuracy and precision have higher standard deviation values than index noise and have a minimum of 5.04 pm to maximum of 11.9 pm.
Fig. 8. (a) Description of index noise, accuracy, and precision. (b) Precision and accuracy of proposed interrogating system according to wavelength spacing between references
6. Conclusion
This work presented an interrogating system using a double-ring WSL with an EDF, which includes an optical switch and a tunable filter to amplify the ranges of the C-band (from 1512 nm to 1569 nm) and L-band (from 1545 nm to 1610 nm). Using this structure, we obtained a bandwidth of about 98 nm, with significant power over 0 dBm. An FBG-referenced interrogating algorithm, with high precision despite the nonlinearity of the tunable filter was also developed. The FBG-referenced interrogating system achieved 7 pm precision, which is about 25 pm better than those obtained in previous research using a single reference. This work has important implications for the use of WSLs in optical coherence tomography [57]. However, further work is needed to improve the relatively low accuracy.
Funding
Korea Institute of Science and Technology (2E30090, 2V08590).
Disclosures
The authors declare that there are no conflicts of interest.
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