⇦Chinese Physics Letters, 2017, Vol. 34, No. 1, Article code 014203 Tunable Single-Passband Microwave Photonic Filter Based on Sagnac Loop and Fabry–Perot Laser Diode * En-Ming Xu(徐恩明)**, Zu-Xing Zhang(张祖兴), Pei-Li Li(李培丽) Affiliations School of Opto-Electronic Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023 Received 6 September 2016 *Supported by the National Natural Science Foundation of China under Grant Nos 61302026, 61275067 and 61575034, and the Jiangsu Natural Science Foundation under Grant No BK2012432.
**Corresponding author. Email: enmingxu@njupt.edu.cn Citation Text: Xu E M, Zhang Z X and Li P L 2017 Chin. Phys. Lett. 34 014203 Abstract A tunable single-passband microwave photonic filter is proposed and demonstrated, based on a laser diode (LD) array with multiple optical carriers and a Fabry–Perot (F-P) laser diode. Multiple optical carriers in conjunction with the F-P LD will realize a filter with multiple passbands. By adjusting the wavelengths of the multiple optical carriers, multiple passbands are merged into a single passband with a broadened bandwidth. By varying the number of the optical carrier, the bandwidth can be adjusted. The central frequency can be tuned by adjusting the wavelength of the multiple optical carriers simultaneously. A single-passband filter implemented by two optical carriers is experimentally demonstrated. DOI:10.1088/0256-307X/34/1/014203 PACS:42.79.Hp, 42.30.Lr, 42.79.Ci © 2017 Chinese Physics Society Article Text Using photonic technology to process microwave and millimeter-wave processing has attracted considerable attention in recent years.[1-4] Microwave photonic processing provides the possibility of processing microwave and millimeter wave signals directly in optical domain without the need of electro-optical and opto-electrical conversions, and it can break through the so-called electronic bottleneck and can be inherently compatible with the fiber-based transmission system. Microwave photonic processing has advantages such as low loss, light weight, broad bandwidth, good tunability and immunity to electromagnetic interference, and it can find many potential applications in ultra-bandwidth wireless mobile communication, array phase radar, sensors of microwave and millimeter-wave, microwave and millimeter-wave signal processing. Although various delay-line-based microwave photonic filters have been reported,[5-8] the frequency response is periodic due to the discrete nature of the sampling process in the time domain. If the time delay difference between two adjacent taps is large, which is true for most implementation, especially an implementation using fiber-delay lines, the free spectral range is small, which may make the filter have multiple passbands within the spectral range of interest. Thus it is highly demanded to implement a microwave photonic filter with only a single passband. Many techniques have been proposed to achieve a single-passband filter.[9-21] A technique based on a sliced broadband source and a long fiber is largely used to achieve a single-passband filter,[9-12] and a technique based on a phase modulation incorporated with an optical notch filter,[13,14] a stimulated Brillouin scattering (SBS),[15,16] or a distributed-feedback semiconductor optical amplifier assisted optical carrier recovery[17] is also employed to implement a single-passband filter. However, the bandwidth is not adjustable. A technique based on the SBS incorporated with variation of the number of pump spectral lines can achieve a bandwidth adjustment.[18-20] However, an extra arbitrary waveform generator and a dual parallel Mach–Zehnder modulator are needed to control the number of pump spectral lines, which is complex. Recently a technique to achieve the bandwidth adjustment by tuning the wavelength of the laser source has been proposed.[21] The bandwidth is tuned by varying the wavelength of the tunable laser while the central frequency is tuned by varying the bandwidth of an optical bandpass filter, and it is complex to achieve both the bandwidth and the central frequency adjustment. In this Letter, a tunable single-passband microwave photonic filter is proposed and experimentally demonstrated. It is based on a laser diode (LD) array with multiple optical carriers and a Fabry–Perot (F-P) laser diode with multiple longitudinal modes. Multiple optical carriers in conjunction with multiple longitudinal modes of the F-P LD will realize a filter with multiple passbands. By adjusting the wavelengths of multiple optical carriers to make the central frequencies of the multiple passbands be close to each other, the multiple passbands will merge into a single passband with a broadened bandwidth. By varying the number of the optical carrier, the bandwidth of single passband can be adjusted. The central frequency can be tuned by adjusting the wavelengths of the multiple optical carriers simultaneously. A single-passband filter using two LDs is experimentally demonstrated. The schematic diagram of the proposed tunable single-passband microwave photonic filter is shown in Fig. 1, and the illustration of the principle of the filter is shown in Fig. 2. In the proposed filter, $N$ light waves from an LD array are multiplexed by an optical combiner. A polarizer controller (PC) connected after each light wave is used to mitigate the polarization loss. The $N$ optical carriers are then fed into a Sagnac loop via a circulator. The $N$ optical carriers are divided into two parts by an optical coupler (OC), thus one part travels in the clockwise (CW) direction and the other part travels in the counterclockwise (CCW) direction. A phase modulator (PM) driven by an rf signal is incorporated in the Sagnac loop to perform phase modulation, and the PM is located close to the loop center. In the Sagnac loop, the light wave along the CW direction is phase modulated by the rf signal, while the light wave along the CCW direction is negligible due to the velocity mismatch.[22] Since the light wave coupled from one waveguide to another waveguide in OC$_{1}$ has 90$^{\circ}$ optical phase shift, the CW and CCW propagating phase modulated optical signals at the Sagnac-loop output are 180$^{\circ}$ out of phase. Therefore, each optical carrier is largely suppressed at the Sagnac-loop output. The majority portion of each optical carrier is reflected back to the Sagnac-loop input and then travels along the lower path via the circulator.[23] The carrier-suppressed phase-modulated optical signal is injected into the F-P LD. The lower and the upper sidebands of the rf signal at the Sagnac-loop output have a phase difference ($\Delta \varphi$) of 180$^{\circ}$ while equal amplitudes due to the phase modulation. When one sideband is amplified by one longitudinal mode of the F-P LD, the amplitude balance is destroyed. The phase-modulated signal is converted into an intensity-modulated signal. At OC$_{2}$, the amplified sideband and the corresponding optical carrier reflected from the Sagnac loop are combined, and a single-passband microwave photonic filter is realized. The bandwidth of the passband corresponds to the bandwidth of the effective gain at one longitudinal mode of the F-P LD, and the bandwidth can be adjusted by increasing the drive current. However, to ensure a stable filter response, the drive current of the F-P LD should be controlled to avoid lasing of the F-P LD, and the drive current has an upper limit for a given injection optical power. The central frequency is determined by wavelength difference between the longitudinal mode of the F-P LD and the optical carrier. The $N$ optical carriers in conjunction with the $N$ longitudinal modes of the F-P LD will generate a filter with $N$ passbands. Adjusting the wavelengths of the $N$ optical carriers to make the central frequencies of the $N$ passbands close to each other, the $N$ passbands will merge into a single passband with a broadened bandwidth. In this case, the bandwidth of the passband is determined mainly by the number of the optical carrier which can be tuned by varying the number of the optical carrier. In addition, the LD array can be replaced by a single-sideband modulator-based recirculating frequency shifter if a high frequency rf signal is provided.[24] To demonstrate the operation principle, an experimental setup with two LDs is performed. Two light waves from LD$_{1}$ and LD$_{2}$ are combined by an OC and then fed into a Sagnac loop via a circulator. The Sagnac loop consists of an OC, two PCs and a PM, as shown in Fig. 1. A PM (JDS Uniphase, 20Gb/s) is located close to the center of the Sagnac loop to achieve the phase modulation. The carrier-suppressed phase-modulated signals are then injected into the F-P LD (Thorlabs S1FC1550). At the output of the F-P LD, the two carrier-suppressed single-sideband phased-modulated signals combine with the two corresponding optical carriers reflected back from the Sagnac loop to realize a dual-passband filter. Adjusting the wavelengths of the two LDs, the dual-passband filter can become a single-passband filter. A photodetector (PD, New Focus 10058B, 20 GHz) is used to perform the optical-to-electrical conversion. Finally, the filter frequency response is analyzed by a vector network analyzer (VNA; Agilent E8364A).
cpl-34-1-014203-fig1.png
Fig. 1. Schematic diagram of the proposed single-passband microwave photonic filter.
cpl-34-1-014203-fig2.png
Fig. 2. Illustration of principle of the proposed single-passband filter.
Figure 3 shows the spectra of the optical signals before the F-P LD and after OC$_{2}$, and the transmission response of the F-P LD. As can be seen from Fig. 3, the two optical carriers are effectively suppressed (blue) after the Sagnac loop and recovered at the output of OC$_{2}$ before the PD. The amplitude balance of the phase-modulated signal is destroyed by the F-P LD, as shown in Fig. 3 (red). As a result, the phase-modulated signal is converted into an intensity-modulated signal. To avoid an additional beat signal generated by the two optical carriers or the two longitudinal modes of the F-P LD at the output of the PD, the wavelength difference between LD$_{1}$ and LD$_{2}$ or the two adjacent longitudinal modes of the F-P LD should be sufficiently large. As can be seen from Fig. 3 (black), the two adjacent longitudinal modes of the F-P LD are 1549.736 nm and 1550.875 nm, respectively. The wavelength spacing between the adjacent longitudinal modes is more than 1.15 nm, corresponding to a beat frequency of about 145 GHz, which is too high to be detected by the PD.
cpl-34-1-014203-fig3.png
Fig. 3. Spectra of the optical signals before the F-P LD and after OC$_{2}$, and the transmission response of the F-P LD.
cpl-34-1-014203-fig4.png
Fig. 4. Frequency response of the single-passband filter with center tuning realized by LD$_{1}$.
When one of the two LDs (LD$_{1}$ and LD$_{2}$) is turned on, a single-passband filter can be realized. The central frequency can be tuned by adjusting the LD wavelength to vary the wavelength difference between the optical carrier and the longitudinal mode of the F-P LD. When only LD$_{1}$ is turned on, and the wavelength of the LD$_{1}$ is set at 1549.75 nm, a single-passband filter with a central frequency of 1.7 GHz is obtained, as shown in Fig. 4 (black). When the wavelength of the LD$_{1}$ is increased by a step of 0.01 nm from 1549.75 nm, the central frequency of the passband is shifted to the right from 1.7 GHz by a step of about 1.25 GHz, as shown in Fig. 4. As can be seen, the central frequency of the passband is changed from 1.70 to 8.95 GHz. The 3-dB bandwidth of the passband is around 60 MHz. For the same principle, when only LD$_{2}$ is turned on, and the wavelength of the LD$_{2}$ is increased by a step of 0.01 nm from 1550.889 to 1550.946 nm, the central frequency of the passband is shifted to the right from 1.72 to 8.92 GHz by a step of about 1.25 GHz, as shown in Fig. 5. The 3-dB bandwidth of the passband is around 63 MHz. Note that the noise is large near dc, which is caused by an additional small broad peak on the right side of the longitudinal mode.
cpl-34-1-014203-fig5.png
Fig. 5. Frequency response of the single-passband filter with center tuning realized by LD$_{2}$.
cpl-34-1-014203-fig6.png
Fig. 6. Frequency response of the filter realized by LD$_{1}$ and LD$_{2}$.
When LD$_{1}$ and LD$_{2}$ are simultaneously turned on, a filter with two passbands is realized. The interval between two passbands can be varied by adjusting one wavelength or two wavelengths. When the wavelengths of LD$_{1}$ and LD$_{2}$ are set at 1549.768 and 1550.924 nm, respectively, the measured frequency response of the filter is shown in Fig. 6 (blue). It can be seen that two passbands with the central frequencies of 4.01 and 6.17 GHz are observed. The 3-dB bandwidths of two passbands are about 66 and 62 MHz, respectively. When the wavelength of LD$_{1}$ is increased to 1549.773 nm, while the wavelength of LD$_{2}$ is reduced to 1550.919 nm, the passband generated by LD$_{1}$ is shifted to right, and the one generated by LD$_{2}$ is shifted to the left. Two passbands with the central frequencies of 4.69 and 5.49 GHz are observed (red), and the 3-dB bandwidths of two passbands are about 63 and 61 MHz, respectively. As can be seen, the interval between two passbands is reduced to become closer. When the wavelength of LD$_{1}$ is increased further to 1549.776 nm, and the wavelength of LD$_{2}$ is reduced further to 1550.916 nm, two passbands are merged into a single passband, as shown in Fig. 6 (cyan). The 3-dB bandwidth of the merged single passband is about 135 MHz. As can be seen, the bandwidth of the merged single passband generated by two LDs is broadened, compared with a single-passband filter generated by only one LD.
cpl-34-1-014203-fig7.png
Fig. 7. Frequency response of the merged single-passband filter with center tuning realized by LD$_{1}$ and LD$_{2}$.
The central frequency of the merged single-passband filter can be tuned by shifting the wavelengths of the two LDs simultaneously in the same directions by the same step. When the wavelengths of LD$_{1}$ and LD$_{2}$ are set at 1549.776 and 1550.916 nm, respectively, and the wavelengths of LD$_{1}$ and LD$_{2}$ are increased (reduced) simultaneously by a step of 0.01 nm, the central frequency is shifted to the right (left) by a step of about 1.25 GHz. The measured results are shown in Fig. 7. As can be seen, the central frequency of the broadened passband is tuned from 1.50 to 8.77 GHz. To achieve a flat passband of the merged single-passband filter, for a given drive current of the F-P LD, two powers of the two LDs need to be adjusted independently. Note that the two powers of the LDs and the driven current of the F-P LD are kept constant during the tuning process. In conclusion, a tunable single-passband microwave photonic filter has been proposed and demonstrated, which was based on an LD array with multiple optical carriers and an F-P LD with multiple longitudinal modes. Multiple optical carriers in conjunction with multiple longitudinal modes of the F-P LD would realize a filter with multiple passbands. By adjusting the wavelengths of optical carriers to make the central frequencies of the multiple passbands be close to each other, the multiple passbands would merge into a single passband with a broadened bandwidth. By varying the number of the optical carrier, the bandwidth of merged single passband could be adjusted. The central frequency could be tuned by shifting the wavelengths of the optical carriers simultaneously in the same direction. A single-passband filter using two LDs is experimentally demonstrated. A bandwidth adjustment from about 62 to 135 MHz is obtained, and the central frequency of the broadened passband with tuning range of 1.50–8.77 GHz is achieved. References Microwave photonics combines two worldsMicrowave PhotonicsA Widely Tunable Photonic-Assisted Microwave Notch Filter with High Linearity Using a Dual-Parallel Mach—Zehnder ModulatorA Microwave Photonic Notch Filter Using a Microfiber Ring ResonatorMicrowave Photonic Signal ProcessingPhotonic signal processing of microwave signalsHybrid Active-Passive Microwave Photonic Filter with High Quality FactorPhotonic microwave tunable single-bandpass filter based on a Mach-Zehnder interferometerSingle passband microwave photonic filter using continuous-time impulse responseWidely tunable single-bandpass microwave photonic filter employing a non-sliced broadband optical sourceWidely tunable single-bandpass microwave photonic filter based on polarization processing of a nonsliced broadband optical sourceFrequency- and Notch-Depth-Tunable Single-Notch Microwave Photonic FilterA Narrow-Passband and Frequency-Tunable Microwave Photonic Filter Based on Phase-Modulation to Intensity-Modulation Conversion Using a Phase-Shifted Fiber Bragg GratingWidely Tunable Single Bandpass Microwave Photonic Filter Based on Phase Modulation and Stimulated Brillouin ScatteringWidely Tunable Single-Passband Microwave Photonic Filter Based on Stimulated Brillouin ScatteringWidely Tunable Single-Passband Microwave Photonic Filter Based on DFB-SOA-Assisted Optical Carrier RecoveryBandwidth-tunable narrowband rectangular optical filter based on stimulated Brillouin scattering in optical fiberTunable sharp and highly selective microwave-photonic band-pass filters based on stimulated Brillouin scatteringTailoring of the Brillouin gain for on-chip widely tunable and reconfigurable broadband microwave photonic filtersSingle passband microwave photonic filter with wideband tunability and adjustable bandwidthPerformance and modeling of broadband LiNbO/sub 3/ traveling wave optical intensity modulatorsMicrowave Photonic Downconversion Using Phase Modulators in a Sagnac Loop InterferometerAnalysis of the stability and optimizing operation of the single-side-band modulator based on re-circulating frequency shifter used for the T-bit/s optical communication transmission
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