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A multiplexed multi-longitudinal mode fiber laser sensor system is proposed and demonstrated. By incorporating two matched wavelength division multiplexers (WDMs) and a semiconductor optical amplifier (SOA) into a fiber laser cavity, multiwavelength oscillation is established. Each wavelength corresponding to one channel of WDMs contains multi-longitudinal modes. The multiwavelength output of the laser is directed to another WDM which functions as a demultiplexer. By monitoring the longitudinal mode beat frequency generated at photodetectors following the WDM, the sensing information can be demodulated. Preliminary results for multiplexing two sensors measuring strain and temperature are presented to verify the principle of the system.
© 2014 Optical Society of America
1. Introduction
Optical fiber sensors are well developed for their small size, electromagnetic interference (EMI) immunity, high sensitivity, and multiplexing capability. Conventionally, most of multiplexing schemes are based on fiber Bragg grating (FBG) [1–6]. Since the FBG is wavelength-encoded, the demodulation has to detect the wavelength shift with bulky optical instruments or complex demodulation configuration. On the other hand, the demodulation can operate in radio frequency (RF) domain. Compared with the wavelength demodulation, this method is simple, compact, and high-resolution. This kind of sensors can be further classified into two types generally. One of them can be recognized as passive type which is based on microwave photonic filter (MPF) [7–10]. Usually, a vector network analyzer (VNA) is used to acquire the frequency response of the sensor. However, a VNA is costly and the response acquired by the VNA is not very fast. The multiplexing system based on this type is also still not reported to the best of our knowledge. The other one can be recognized as active type. The sensor is implemented by the fiber laser. The output of the laser is sent to a photodetector (PD) to generate beat frequency signal (BFS). As a requirement, there should be at least two modes in the fiber laser. Traditionally, the two modes come from polarization modes [11–16]. However, polarization modes are difficult to establish and the repetition may be not stable. We proposed a novel multi-longitudinal mode fiber laser sensor which has measured the strain, temperature and vibration successfully by detecting the BFS between longitudinal modes [17–19]. This fiber laser sensor is promising with the merits of simple setup and high stability. Up to now, the multiplexing ability of the sensor is still not investigated.
In this paper, a multiwavelength fiber laser sensor system multiplexing multi-longitudinal mode fiber laser sensors is proposed and experimentally demonstrated for the first time. By incorporating two matched wavelength division multiplexers (WDMs) and a semiconductor optical amplifier (SOA) into the laser cavity, stable multiwavelength oscillation can be established. The measurand is applied to the fiber between the WDMs. Each wavelength carries the sensing information corresponding to one channel of the WDMs. The output of the laser is simply sent to another WDM which demultiplex the multiwavelength output, such that each channel can be demodulated independently. A proof-of-concept experiment is carried out. Two channels measuring strain and temperature respectively are demodulated successfully. Compared with other multiplexed fiber laser sensor system [1,3,5,16], the configuration is very simple and the demodulation system is relatively cheap and portable.
2. Principle
The schematic diagram of the proposed multiplexed fiber laser sensor system is illustrated in Fig. 1. The gain of the fiber laser is provided by a SOA. Compared with erbium-doped fiber amplifier (EDFA), SOA is an inhomogeneous medium, which is easy to support stable multiwavelength oscillation [20, 21].
WDM1 and WDM2 are matched to operate as the multiplexer. The oscillation wavelength is determined by the central wavelength of each channel of the WDMs. When the bandwidth of each channel of WDMs is far greater than free spectral range (FSR) of the laser cavity, there exist many longitudinal modes in the oscillation wavelength. The schematic diagram of the principle is shown in the inset of Fig. 1. It should be noted that in conventional multiplexing system based on FBG which is wavelength-encoded, every FBG sensor has to be assigned a specified wavelength range thus limiting the number of sensors to be multiplexed [20]. On the contrary, the oscillation wavelengths determined by the matched WDMs are fixed. Under the gain distribution of the SOA, this system is expected to multiplex more sensors.
The output of the laser is sent to WDM3 and the multiwavelength is demultiplexed. Then, each wavelength impinges on a PD, in which many BFSs are generated between any two different longitudinal modes. One certain BFS is the sum of many beat signals with the same frequency. As a consequence, although there is mode hopping in the laser cavity, it has little effect on the measurement. The BFS can be expressed as
where p and q are mode numbers, c is the light velocity in vacuum, n is the effective refractive index, Li is the length of i-th channel between the matched WDMs, Lm is the main cavity length except the length of i-th channel, and N = p - q is used to denote the BFS.A part of the fiber between the two matched WDMs is set as sensing fiber. If the strain applied to the sensing fiber is changed, the BFS will shift, given as
where Pe is the strain-optic coefficient, li is the length of the sensing fiber, and Δε is the applied strain.If the temperature applied to the sensing fiber is changed, the BFS will also shift, given as
where α and ξ are linear expansion and thermo-optic coefficient, and ΔT is the temperature change.It can be seen from Eq. (2) and Eq. (3) that the BFS changes linearly with strain and temperature. As a result, the strain and temperature can be measured by monitoring the BFS respectively.
3. Experiment results and discussion
A proof-of-concept experiment based on the schematic setup in Fig. 1 is performed. The laser cavity is built in a ring configuration. A SOA acts as gain medium. The polarization controller (PC) before the SOA is adjusted to optimize a best state. The isolator after the SOA ensures a unidirectional operation. The channels between two commercial WDMs (WDM1 and WDM2) are matched. The WDMs include channel 22 (Ch22) to channel 29 (Ch29) anchored on the ITU grid. The isolation between adjacent channels is more than 30 dB. Each channel of the WDMs can be employed as a sensor. In this proof-of-concept experiment, two adjacent channels of Ch22 and Ch23 are selected. The bandwidth of Ch22 and Ch23 is 25GHz. In Ch22, a section of bare single-mode fiber between the matched WDMs with the length of 5 m is rolled tightly between two copper columns. One column is fixed on a stationary stage (SS) and the other one is fixed on a translation stage (TS). The strain is applied to the fiber by moving the translation stage. In Ch23, a section of bare single-mode fiber between the matched WDMs with the length of 5 m is coiled and placed in an oven for temperature sensing. The variable attenuator (ATT) in each channel is adjusted to balance the power of each wavelength. Through the 20% port of the coupler, the output of the laser is coupled out of the cavity and sent to WDM3, which also includes Ch22 and Ch23 functioning as demultiplexer. So the wavelengths are separated such that the sensing information contained in wavelength is demultiplexed. The BFSs are generated at a PD connected to each channel of WDM3 and then monitored by a radio frequency spectrum analyzer (RFSA). By tracking the shift of BFSs, the sensors are demodulated.
When the driving current of the SOA is tuned to be 36mA, the fiber laser starts to oscillate. In this experiment, the operating current is kept at 81 mA. Two wavelengths at 1559.53 nm and 1558.71 nm approximately corresponding to the central wavelength of Ch22 and Ch23 of the WDMs are observed and the output power of the laser is −7dBm. Figure 2(a) illustrates the optical spectrum of the output of the fiber laser. After demultiplexed by WDM3, the optical spectra of Ch22 and Ch23 are shown in Figs. 2(b) and 2(c) and the power is −11.18 dBm and −11.74 dBm respectively. It can be seen that the Ch22 has a small residual power from Ch23. The side mode suppression ratio (SMSR) is measured to be 34.1 dB which is large enough to lead to a low crosstalk based on experiment results given below.
Fig. 2 Optical spectra measured (a) before WDM3, (b) after Ch22 of the WDM3, (c) after Ch23 of the WDM3.
As shown in Fig. 3, there are many BFSs in the frequency spectra. If the output of the laser is directly sent to a PD without WDM3, the BFSs are disordered as shown in Fig. 3(a). After the multiwavelength is demultiplexed by WDM3, The BFSs are also demultiplexed as shown in Figs. 3(b) and 3(c). It can be seen that the BFSs are equally spaced at intervals of 7.3 MHz in Ch22 and 4.8 MHz in Ch23. In theory, every BFS can be employed as sensing signal. According to Eq. (1), each BFS is the sum of many beat signals with the same frequency. However, high-frequency BFS is composed of less beat signals, thus the intensities of high-frequency BFSs are lower than low-frequency BFSs and it can be found in Fig. 4 and [22]. On the other hand, based on Eq. (2) and Eq. (3), the frequency shift of BFS is proportional to the BFS under the same strain or temperature. Consequently, the high-frequency BFSs have higher sensitivities than low-frequency BFSs. Hence, there is a tradeoff between the intensity and sensitivity. From the other point of view, many BFSs could provide flexible solutions satisfying different requirements. In this experiment, the BFS of 10038 MHz in Ch22 which has the SNR of 14.5 dB and 3 dB bandwidth of 595 kHz is selected as the sensing signal while the BFS of 10084 MHz which has the SNR of`10.9 dB and 3 dB bandwidth of 442 kHz is selected in Ch23. Both of them shown in the insets of Fig. 4 have relatively high sensitivities and low SNRs.
Fig. 3 Frequency spectra of BFSs from 9950 to 10050 MHz (a) before WDM3, (b) after Ch22 of WDM3, (c) after Ch23 of WDM3.
Fig. 4 Frequency spectra of BFSs from 0 to 6000 MHz (a) after Ch22 of WDM3, (b) after Ch23 of WDM3. The insets show the BFS selected for sensing.
To determine the multiplexing capability, the strain applied on Ch22 is changed from 0 με to 1400 με, while the temperature applied in Ch23 keeps unchanged. The responses of the two channels are shown in Fig. 5(a). The sensitivity of BFS of Ch22 is −1.18 kHz/με, and the BFS of Ch23 keeps nearly unchanged showing a low crosstalk.
Fig. 5 (a) The responses of BFS in Ch22 and Ch23 to strain applied on Ch22. (b) The response of BFS in Ch22 and Ch23 to temperature applied on Ch23.
In a second arrangement, the strain applied on Ch22 keeps unchanged while temperature in Ch23 increases from 25 °C to 130 °C. The results are given in Fig. 5(b). The BFS of Ch23 is linearly proportional to the temperature and the sensitivity is −9.74 kHz/°C. The BFS of Ch22 keeps almost unchanged. Again, the low crosstalk level is revealed. In conclusion, the Ch22 and Ch23 can be independently demodulated.
At last, the stability of the two channels is tested. Keeping the strain and temperature unchanged, the BFSs of the two channels are sampled every 10 minutes for 2 hours. The results are shown in Fig. 6. The root-mean-square deviation of Ch22 and Ch23 is 0.0037 MHz and 0.0055 MHz which correspond to 3.14 με and 0.56 °C.
4. Conclusion
In conclusion, a novel multiplexed multi-longitudinal mode fiber laser sensor system is proposed. By incorporating two matched WDMs and a SOA as the inhomogeneous gain medium in the laser cavity, the multi-longitudinal fiber lasers are multiplexed. The laser output is simply directed to another WDM followed by a PD in each channel, and the multiplexed sensors are demultiplexed and demodulated by the BFS generated at the PD. Compared with conventional multiplexed fiber sensor systems which mainly demodulated by the shift of wavelength, the demodulation scheme is easy-setup, relatively cheap and stable. An experiment multiplexing two multi-longitudinal mode fiber laser sensors is demonstrated. The experimental results show low crosstalk between the adjacent two sensors determining the ability of the system to multiplex more sensors.
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