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Abstract
In this manuscript, a novel narrow-bandwidth rectangular optical filter based on multi-phase-shifted fiber Bragg grating (MPSFBG) is proposed. Using the local temperature control technology, the precise controllable phase shifts are introduced at different positions of the fiber Bragg grating (FBG). Therefore, the bandwidth of the MPSFBG-based filter with good shape factor can be reconfigured from 70 MHz to 1050 MHz by flexibly controlling the numbers and the positions of the phase shifts introduced in the MPSFBG. In addition, the center wavelength of the MPSFBG-based filter can be tuned through controlling the MPSFBG’s environment temperature, and the tuning range of 22 GHz is realized. This is one of the best results for the narrow-bandwidth rectangular optical tunable filter with reconfigurable bandwidth. It can be widely used in the processing of reconfigurable signals in the optical communication networks and microwave photonics.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical filter as an important component in the optical signal processing can not only ensure the normal operation of the optical communication system, but is also applied to the microwave signal processing due to their low transmission loss, high working frequency band and immune to electromagnetic interference capabilities, which promotes the development of the electrical signal processing technology [1,2]. With the increasing demand for high-resolution optical information processing and the vigorous development of the microwave photonics, the narrow-bandwidth optical filter becomes one of the indispensable components, and higher requirements are put forward for its performance. In order to suppress the crosstalk with other channels and signal distortion, a rectangular filtering response with steep edges and flat-top is required for the narrow-bandwidth optical filter [3]. Furthermore, in some optical signal processing applications that require flexible switching, such as dense wavelength division multiplexing networks and microwave photonic channelization technology, it is necessary to reasonably configure the bandwidth and center wavelength of the optical filter according to the channel requirements, so as to achieve the reasonable allocation and full utilization of the limited available bandwidth of the optical fiber transmission [4–6]. Therefore, it is of practical significance to investigate the bandwidth-reconfigurable and center wavelength-tunable narrow-bandwidth rectangular optical filter.
Several schemes have been proposed to implement such optical filter: diffraction grating [7,8], stimulated Brillouin scattering (SBS) [9–12], silicon-based micro-ring resonator [13–16] and specially designed FBG [17–21], etc. As a commonly used dispersive element, the diffraction grating can diffract the lights and separate the lights according to its frequencies. This kind of scheme can realize the flexible filtering of optical signals by combining auxiliary mechanisms, such as slit filtering and liquid crystal on silicon. However, the rectangular bandpass filtering response can only be obtained for the bandwidth larger than 10 GHz in this scheme. The bandwidth-reconfigurable and center wavelength-tunable rectangular optical filter with the bandwidth lower than 10 GHz can be achieved by SBS. The bandwidth and shape of the SBS-based filter can be flexibly changed by controlling the pump spectrum. The filter wavelength can be also easily tuned by tuning the wavelength of the pump. However, it is difficult to precisely control the pump spectrum, therefore the exact sharp edges and flat-top as the ideal rectangular filter can be hardly realized. Moreover, the structure of the SBS-based filter is difficult to integrate and costly. With the increasing demand for the integration of the optical component, the optical filters based on silicon-based micro-ring resonators and FBG become attractive. Silicon-based micro-ring filter can realize narrow-bandwidth rectangular filtering response by cascading micro-ring resonators, and the rectangularity of the filtering response becomes higher as the numbers of micro-ring resonators increases. Although the tuning of its center wavelength can be achieved by heating the entire micro-ring structure, the tuning range is limited by its free spectral range. In addition, the waveguide needs to be coupled with the optical fiber in the practical application of the silicon-based micro-ring filter, which will introduce additional loss. Recently, Xihua Zou et al. [17] obtained the narrow-bandwidth rectangular filtering response using a MPSFBG fabricate by the ultraviolet exposure post-processing method. Due to its advantages of low insertion loss, low cost, and compatibility with other optical fiber devices, MPSFBG makes it an ideal choice for implementing narrow-bandwidth rectangular optical filter. However, the bandwidth and center wavelength of this MPSFBG-based filter cannot be changed due to the permanent change of the optical fiber properties caused by the inscribing method.
Recognizing this limitation, a novel narrow-bandwidth rectangular optical filter based on MPSFBG with reconfigurable bandwidth and tunable center wavelength is proposed and experimentally demonstrated in this manuscript. In this novel MPSFBG-based filter, the phase shifts of the MPSFBG are achieved by performing the local temperature control technique on the uniform FBG. The phase shifts introduced by this technique do not cause permanent changes to the structure of the FBG, that is to say, the phase shifts can be erased. Therefore, the numbers and the positions of the phase shifts in this MPSFBG can be flexibly changed and a narrow-bandwidth rectangular optical filter with reconfigurable bandwidth is realized. By changing the numbers and the positions of the phase shifts in the MPSFBG, the 3-dB bandwidth of the MPSFBG-based filter can be adjusted from 70 MHz to 1050 MHz and the shape factor can be maintained at about 0.5. In this manuscript, the shape factor is defined as the ratio between the 3 dB and the 20 dB bandwidths to characterize the rectangularity of the transmission peak. Moreover, the center wavelength tuning of the MPSFBG-based filter can be achieved by using thermoelectric cooler (TEC) to control the temperature of the packaged FBG. And a tuning range of 180 pm (∼22 GHz) is experimentally realized with a tuning coefficient of 9.10 pm/K. This is one of the best results for the narrow-bandwidth rectangular optical tunable filter with reconfigurable bandwidth and can be widely used in the processing of reconfigurable signals in the optical communication networks and microwave photonics.
2. Principle
Figure 1 shows the schematic of microwave photonic channelization based on optical filter [22,23]. At the transmitter, the optical carriers with different frequencies are multiplexed at a wavelength division multiplexer and then split into two paths by a coupler. For the upper path, the optical carriers are modulated by the microwave signals that contain information and the sub-carriers are generated. Then, an optical filter is used to filter out a sub-carrier. For the lower path, the optical carriers are frequency shifted and the frequency offset between the optical carriers and the sub-carrier filtered out by the upper path can be controlled. With the rapid increase in communication transmission rate and wavelength division multiplexing density, the rectangular optical filter with bandwidth ranging from MHz to several GHz and several tens of GHz tuning range becomes an indispensable component to realize this microwave photonic channelization link.
In this manuscript, a narrow-bandwidth rectangular optical filter with reconfigurable bandwidth and tunable center wavelength is proposed and its principle is shown in Fig. 2. According to the previous research [17–21], for the MPSFBG with ${\theta _i}\textrm{ = }\pi$, a narrow-bandwidth transmission peak with low insertion loss and high rectangularity can be obtained in the stopband of its transmission spectrum by optimizing the positions of the phase shifts. This narrow-bandwidth transmission peak can be used as a bandpass filter. Taking a MPSFBG with the length of 60 mm as an example, the transmission spectrum is numerically simulated by the transmission matrix method [17]. Table 1 shows the length of each sub-FBGs for the 60 mm-long MPSFBG to obtain a narrow-bandwidth filtering response with low insertion loss and high rectangularity. The same results can also be achieved for the MPSFBG with different lengths by designing the length of the sub-FBGs according to the ratio of the sub-FBGs lengths in Table 1 [24].
Table 1. The length of each sub-FBGs for MPSFBG with 60mm length to obtain a narrow-bandwidth flat-top filtering response
Then, the bandwidth-reconfigurable characteristic and center wavelength-tunable characteristic of the MPSFBG-based filter are analyzed. Figure 3 depicts the bandwidth reconfiguration capability of the MPSFBG-based filter. The different color curves shown in Fig. 3(a) represent the transmission spectrum of the MPSFBG with different phase shift numbers. Figure 3(b) is a partial enlarged view of the transmission peak in Fig. 3(a). It can be clearly seen from Fig. 3(c) that the bandwidth of the transmission peak can be reconfigured from MHz to several GHz with different phase shift numbers. As shown in Fig. 3(d), the shape factor of the MPSFBG transmission peak increases with the increase of the phase shift numbers, that is, the better the rectangularity of the transmission peak is as the numbers of the phase shifts increases.
Fig. 3. (a) The transmission spectrum of the MPSFBG changes with the numbers of the phase shifts; (b) the partial enlarged view of the transmission peak in the MPSFBG transmission spectrum; (c) the bandwidth of the MPSFBG transmission peak changes with the numbers of the phase shifts; (d) the shape factor of the MPSFBG transmission peak changes with the numbers of the phase shifts.
Figure 4 illustrates the center wavelength tuning capability of the MPSFBG-based filter. The center wavelength of the MPSFBG-based filter can be tuned by adjusting the temperature of the MPSFBG due to the thermal-expansion effect (thermal-expansion coefficient:$\sim 5.6 \times {10^{ - 7}}/K$) and thermo-optical effect (thermo-optical coefficient: $\sim 8.5 \times {10^{ - 6}}/K$) of the silica optical fiber [25]. As shown in Fig. 4, when the temperature changes by 20 K, a 282.6 pm (∼35 GHz) center wavelength tuning range of the MPSFBG-based filter can be achieved with a tuning coefficient of 14.13 pm/K. In addition, the performance of the filter will not deteriorate with tuning the center wavelength by uniform temperature controlling along FBG. Thus, it can be concluded that the bandwidth reconfiguration of the MPSFBG-based filter can be achieved by changing the numbers and the positions of the phase shifts, and the center wavelength tuning can be achieved by changing the temperature of the MPSFBG.
Fig. 4. (a) The MPSFBG transmission peak changes with environment temperature; (b) the center wavelength of the MPSFBG transmission peak changes as a function of MPSFBG’s temperature. ΔT represents the temperature change of the MPSFBG, $\Delta {\lambda _\textrm{c}}$ represents the center wavelength change of the MPSFBG transmission peak.
3. Experiments
Based on the above principle, a structure shown in Fig. 5 is designed to realize such bandwidth-reconfigurable and center wavelength-tunable MPSFBG-based filter. In this structure, the MPSFBG are realized by introducing the phase shifts at different positions of the uniform FBG through the local temperature control technique which has been utilised to introduce the phase shifts in chirped FBG [26,27]. The thermistors are fixed in the base of each copper sheet, and the copper sheets pass through the insulation shell made of polyimide to contact the FBG. The mini-TEC heats the small section of the FBG placed in the optical fiber groove of the copper sheet through the copper sheet base. Because of the thermo-optical effect and thermal-expansion effect, the effective refractive index and period of the heated small section FBG will change with the temperature, forming an accumulated phase shift in the heated area. The value of the phase shift introduced by this technique can be determined by
where ${n_{eff}}$ is the effective refractive index, ${L_{PS}}$ is the length of the heated region, ${\lambda _B}$ is the Bragg wavelength, $\Delta T$ is the temperature change, $\alpha$ is the thermal-expansion coefficient, and $\beta$ is the thermo-optical coefficient. Considering the influence of the FBG heating length on the MPSFBG spectrum [28] and the rigidity of metal copper, the width of the copper sheet is designed to 1 mm in our experiment. The temperature control module (ILX Lightwave, LDC-3724C) used in the experiment has the precision of 0.01 K. Therefore, for the FBG of 1550 nm band, the control precision of this specially designed structure on the phase shift is calculated to be 0.00017π, which fully satisfies the requirement of the phase shift precision (0.001π) for the MPSFBG to obtain the narrow-bandwidth rectangular filtering response [24].Especially, the heating process is nondestructive for the FBG, that is, the introduction of the phase shift is not due to the permanent change in the property of the FBG and the phase shift will disappear as the heating stopped. Therefore, the transmission peak of the MPSFBG can be reconfigured by switching and heating the mini-TECs with different marked number in Fig. 5(c). The positions of the nine-independent mini-TECs in Fig. 5(c) is designed according to Table 1 (two mini-TECs numbered ② represent the phase shift positions of the MPSFBG with two phase shifts, three mini-TECs numbered ③ represent the phase shift positions of the MPSFBG with three phase shifts, and four mini-TECs numbered ④ represent the phase shift positions of the MPSFBG with four phase shifts). By switching and heating the different numbered mini-TECs, the numbers and the positions of the phase shifts in the MPSFBG are accordingly changed, resulting in the bandwidth of the MPSFBG-based filter reconfigured. In addition, the insulation shell separates the base of the copper sheet from the space where the FBG is packaged. On the one hand, the insulation shell prevents the copper sheet from being exposed to the air, which reduces the heat exchange rate between the copper sheet and the surrounding environment and makes it easy to realize the temperature control of the copper sheet. On the other hand, the insulation shell prevents the environment temperature where the FBG is packaged from being affected by the heat dissipation of the copper sheet and makes it easy to realize independent control of the environment temperature where the FBG is packaged. Thus, the center wavelength tuning of the MPSFBG-based filter can be realized simply through controlling the FBG’s environment temperature by the ordinary-TEC.
In order to evaluate the actual spectrum response of the MPSFBG-based optical filter obtained by the above theory and structure, we use the experimental setup shown in Fig. 5(a) to conduct the experiment. A uniform FBG with a length of 60 mm is packaged in the structure we designed. During the experiment, a high-precision optical spectrum analyzer (APEX, AP2041B) with a resolution of 0.04 pm is used to monitor the transmission spectrum of this uniform FBG. Figure 6 shows the experimentally measured original transmission spectrum of the uniform FBG at room temperature (296.15 K) and its theoretical simulation.
Fig. 6. The transmission spectrum of the uniform FBG without heating. The flat bottom of the experimental spectrum is a result of the limited measurement range of the optical spectrum analyzer.
Then, the bandwidth reconfigurable characteristic of the proposed MPSFBG-based filter is verified by switching and heating the mini-TECs with different marked number in Fig. 5(c). As shown in Fig. 7, the red solid curve is the experimentally measured transmission peak of the MPSFBG with different phase shift numbers, and the blue dashed curve is the theoretical simulation result obtained by the transfer matrix method. It can be seen that the experimental results are consistent with the theoretical simulation results. With the increase in the numbers of phase shifts introduced, the 3-dB bandwidth of the MPSFBG-based filter is adjusted from 70 MHz to 1050 MHz, the shape factor is increased from 0.36 to 0.63, and the insertion loss is less than 1 dB. To the best of our knowledge, this is one of the best results for the bandwidth-reconfigurable and center wavelength-tunable optical filter. Subsequently, the MPSFBG-based filter with three phase shifts is taken as an example to demonstrate the center wavelength tunable characteristic of the MPSFBG-based filter. As shown in Fig. 8, with the environment temperature changes by 20 K, the center wavelength of the MPSFBG-based filter varies in the range of 180 pm (∼22 GHz) at a coefficient of 9.10 pm/K, and the performance of the filter does not deteriorate with the change of the center wavelength. Compared with the above theoretical results, the tuning range of the central wavelength obtained in the experiment is relatively small under the same temperature changes and this difference can be attributed to the temperature gradient between the environment and the FBG itself. In addition, the center wavelength tuning of the MPSFBG-based filter realized by tuning the environment temperature of the FBG is equivalent to a secondary temperature control of the FBG. Thus, the optical filter obtained by this scheme is not sensitive to the outside temperature changes and has strong robustness.
Fig. 7. Bandwidth-reconfigurable characteristic of the MPSFBG-based filter, and t represents the temperature of the copper sheet for local temperature control of the uniform FBG. (a) Two phase shifts; (b) three phase shifts; (c) four phase shifts.
Fig. 8. Center wavelength-tunable characteristic of the MPSFBG-based filter. (a) The transmission peak changes with environment temperature; (b) the center wavelength changes as a function of environment temperature and the errors from a linear fit. ΔT represents the temperature change of the environment, $\Delta {\lambda _\textrm{c}}$ represents the center wavelength change of the MPSFBG transmission peak.
Finally, the proposed MPSFBG-based filter is applied to the microwave photonic channelization link shown in Fig. 1, and the filtering performance of the MPSFBG-based filter is tested. The output optical signal of the laser with the center wavelength of 1559.352 nm is used as the optical carrier in this application. The microwave signal generated by the signal generator (Keysight, N5183B) is modulated onto the optical carrier by the electro-optic modulator (Photline, MXAN-LN-20) as shown in the black curve in Fig. 9(a), and the generated sub-carriers are sent into the optical filter. The red curve and blue curve in Fig. 9(a) represent the MPSFBG-based filter with rectangular filtering response and the hypothetical traditional Lorentz-type filter with the same 3-dB bandwidth, respectively. The 3-dB bandwidth of MPSFBG-based filter is flexibly controlled according to the frequency of the microwave signals. The sub-carriers filtered by the MPSFBG-based filter and Lorentz-type filter are depicted in Fig. 9(b) and Fig. 9(c). It can be seen from the Fig. 9(b1) and Fig. 9(c1) that the signal-to-noise ratio (SNR) of the signal obtained by the MPSFBG-based filter is about 26.85 dB, which is 14.40 dB higher than the SNR of the signal obtained by the Lorentz-type filter, proving that the filtering performance of the MPSFBG-based filter is better than the Lorentz-type filter. Comparing the three cases in Fig. 9, the Lorentz-type filter has the best filtering performance in this case of MPSFBG-based filter which containing two phase shifts, therefore the higher SNR improvement can be achieved by the MPSFBG-based filter in other two cases in Fig. 9. In addition, since the center wavelength of the MPSFBG-based filter is tunable, the filter can select the appropriate sub-carrier to filter out according to the demodulation needs of the receiver as shown in Fig. 10. Therefore, the narrow-bandwidth rectangular optical filter with reconfigurable bandwidth and tunable center wavelength proposed in this manuscript can be regarded as an ideal tool for high-precision optical filtering in microwave photonic application.
Fig. 9. Application of bandwidth-reconfigurable characteristic of the MPSFBG-based filter in microwave photonic channelization link. (a1), (b1) and (c1) represent case 1: 2 phase shifts, 300 MHz Microwave signal; (a2), (b2) and (c2) represent case 2: 3 phase shifts, 500 MHz Microwave signal; (a3), (b3) and (c3) represent case 3: 4 phase shifts, 1500 MHz Microwave signal.
Fig. 10. Application of center wavelength-tunable characteristic of the MPSFBG-based filter in microwave photonic channelization link.
4. Conclusion
In conclusion, we have proposed and experimentally demonstrated a novel narrow-bandwidth rectangular optical filter based on MPSFBG with reconfigurable bandwidth and tunable center wavelength. The reconfiguration of the proposed MPSFBG-based filter bandwidth from 70 MHz to 1050 MHz is achieved by flexibly controlling the numbers and the positions of the phase shifts introduced in the MPSFBG, maintaining low insertion loss and good shape factor. The center wavelength with 22 GHz tuning range is achieved by tuning the environment temperature where the FBG is packaged. Furthermore, the proposed MPSFBG-based filter is applied to a typical microwave photonic scheme and SNR improvement better than 14.40 dB is achieved compared to the traditional Lorentz-type filter. This narrow-bandwidth rectangular optical filter with reconfigurable bandwidth and tunable center wavelength can be widely used in the microwave photonics and optical fiber sensing fields.
Funding
National Key Research and Development Program of China (2020YFB0408300); Youth Innovation Promotion Association of the Chinese Academy of Sciences (YIPA2021244); National Natural Science Foundation of China (61535014, 61775225, 61805260, 61905262); Shanghai Sailing Program (18YF1426100); Natural Science Foundation of Shanghai (18ZR1444300); State Administration for Science, Technology and Industry for National Defense (HTKJ2020KL504004).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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