Microwave photonic filter with shape-adjustable and switchable passbands based on a multi-wavelength fiber laser

Abstract

In this article, we have proposed and demonstrated a new approach to implementing microwave photonic filter (MPF) with switchable, tunable and reconfigurable passbands based on a multi-wavelength fiber laser (MWFL) cooperating with a fiber Mach–Zehnder interferometer (FMZI). An EDFA and an SOA have been employed as a hybrid gain medium in the laser cavity, to suppress the homogeneous broadening of EDF. By inserting the FMZI in the fiber laser cavity, MWFL with tunable and switchable wavelength spacing has been achieved, which can be used as the multi-wavelength optical source to implement switchable, tunable and reconfigurable MPF together with a dispersive medium. By tuning the length of variable optical delay line (VODL), the wavelength spacing of the MWFL can be changed, and thus the passband’s central frequency of the MPF. By adjusting the polarization states, gain of EDFA or SOA of the laser cavity, the shape of the laser output can be adjustable, and as a result, shape-adjustable passbands of the MPF have been realized. By introducing the reflective FMZI structure as the wavelength selective component and carefully adjusting the polarization states, half number of the passband can be suppressed, which means the MPF can be switchable. The proposed MPF exhibits shape-adjustable and switchable passbands and enjoys more flexibility compared to previous methods.

1 Introduction

Microwave photonic filter (MPF) is an essential technique in radiofrequency system and the processing of microwave signals. It enjoys the advantages of low loss, wide bandwidth, and immunity to electromagnetic interference [1,2,3,4]. Current implementations of the MPF mainly use a single light source with various spectrum sliced technique among which the design and implementation of adjustment on the passbands have been studied intensively [5,6,7,8,9]. Multiple optical source MPF is also a hot spot in recent research. Since the stability, tunability and configurability of the multiple optical source are very important for the MPF’s performance, finding a suitable light source has been a challenge. The structure of using independent lasers operating at different wavelengths is proposed but it is expensive in practice [10]. The output spectrum of the Raman fiber laser has been demonstrated; however, the system noise is relatively high [11]. The tunable laser is achieved by employing dynamic Brillouin gratings in fibers; nevertheless, the structure is too complex [12]. MWFL based on an FMZI and hybrid gain medium have been proposed, whose output exhibits good tunability and stability [13].

In this paper, we have proposed and experimentally demonstrated a switchable multi-passband MPF with shape-adjustable passbands based on an MWFL and dispersive medium. The MWFL is taken as the multi-wavelength light source, and together with the dispersive medium, multi-passband MPF can be achieved. An FMZI is inserted in the fiber laser cavity as the wavelength selective component, and hybrid gain medium of EDFA and SOA are employed to suppress the homogeneous broadening for stable multi-wavelength laser operation. The shape of the passband of the MPF can be changed by carefully adjusting PCs and pump currents of EDFA and SOA. By employing the reflective FMZI as the wavelength selective component in the fiber laser, and adjusting the PCs in the FMZI, the wavelength spacing of the output laser can be switched from Δλ to Δλ/2. As a result, the FSR of the multi-passband MPF can be switched from one time of FSR to twice of that. In our experiment, stable multi-wavelength laser output with the switchable wavelength spacing of Δλ (0.60 nm) and Δλ/2 (0.30 nm) has been achieved by carefully adjusting the PCs, and thus switchable multi-passband MPF can be realized and passbands at 1.95 GHz and 3.90 GHz have been switched off by adjusting PCs, while the amplitude of the first passband at 1.95 GHz can be suppressed over 10 dB.

2 MPF with tunable and shape-adjustable passbands

Figure 1 depicts a typical MPF with MWFL as the optical source. The multi-wavelength light from the MWFL (green dash box) is transmitted into a Mach–Zehnder modulator (MZM) after a polarization controller (PC). Microwave signal from port 1 of the vector network analyzer (VNA) is modulated onto the light by the MZM. The modulated light is amplified by an EDFA, and then pass through a 50 km standard single-mode fiber (SMF) which acts as a dispersive medium. After that, the light is recovered by the PD, and sent to the VNA for measurement.

Fig. 1

The schematic diagram of a typical MPF with MWFL as the optical source

The transfer function of the MPF based on MWFL and a dispersive medium can be expressed as [11],

$$\left| {H_{{{\text{RF}}}} (f)} \right| \propto \cos \left( {\frac{{\pi \lambda _{0}^{2} D}}{c}\frac{{f^{2} }}{2}} \right)\left| {\sum\limits_{{k = 1}}^{N} {P_{k} e^{{ - j\left[ {2\pi f(k - 1)D\Delta \lambda } \right]}} } } \right|,$$

(1)

where f is the frequency of the RF signal, D is the dispersion coefficient of the dispersive medium; $\lambda _{0}$, $P_{k}$, $\Delta \lambda$ and N are central wavelength, the kth harmonic optical power, the wavelength spacing and the number of the wavelength of the MWFL, respectively. The free spectrum range (FSR) of the MPF can be written as

$${\text{FSR}} = \frac{1}{{D\Delta \lambda }}.$$

(2)

Figure 2 shows the schematic diagram of the MWFL to implement MPF with tunable and shape-adjustable passbands. The ring laser cavity consists of an optical isolator (ISO), optical couplers (OC), SOA, Erbium-doped fiber amplifier (EDFA), variable optical delay line (VODL) and polarization controller (PC). The hybrid gain medium of SOA and EDF in this scheme can significantly reduce the mode competition effect to realize stable laser output. The fiber Mach–Zehnder interferometer (FMZI) in the dash box, which is composed of a PC and a VODL in each arm between two 3 dB couplers is used as a wavelength-selective component in the laser cavity. The transmission of the FMZI has a comb-like profile and the wavelength spacing of the laser can be expressed as,

$$\Delta \lambda = \lambda ^{2} /n\Delta L,$$

(3)

where n and $\Delta L$ are refractive index of the propagation medium and the length difference of the two arms of the FMZI, respectively. From Eq. (3), we can see that the wavelength spacing $\Delta \lambda$ is determined by $\Delta L$. So that by adjusting the VODL we can achieve tunable wavelength spacing of output laser, and thus the FSR of the MPF. Meanwhile, one can see from Eq. (1) that the frequency response of the MPF relies on the weight of each tap, and by carefully adjusting the polarization state, gain of EDFA or SOA in the laser cavity, the output power of each wavelength can be tailored, which means that the passband of the MPF can be configurable.

Fig. 2

The schematic diagram of the MWFL based on an FMZI and hybrid gain medium

In the experiment, MPFs with different FSRs have been achieved by varying the length of VODL, and the MPF’s frequency response and corresponding laser output of the MWFL are shown in Fig. 3. The currents applied to EDFA and SOA are 120 mA and 130 mA, respectively. One can see that when the length of the VODL is set to different values, the wavelength spacing of the laser can be tuned to be 0.851 nm, 1.226 nm and 1.667 nm, and thus the FSR of the MPF can be 1.32 GHz (up), 0.92 GHz (middle) and 0.67 GHz (below), respectively.

Fig. 3

The frequency response of the MPF (left) and the corresponding spectrum of the MWFL (right) with different lengths of the VODL

In our experiment, we have also investigated the configurability characteristics of the MPF by carefully adjusting PC in the FMZI when the wavelength spacing of the output laser is fixed at 1.03 nm. The frequency response of the MPF and corresponding laser output are shown in Fig. 4, from which one can see that the 3 dB bandwidth and the suppression ratio of the passband can be changed by adjusting the polarization state of the PC in the FMZI. Under the condition that the gains of SOA and EDFA are fixed, different shapes of the MPF’s passbands can be obtained by adjusting the PCs. As shown in Fig. 4, different passbands’ shape (regarded as sawtooth, Gaussian and Sinc shape) can be obtained by carefully adjusted the PCs at state 1 to 3. The PCs are adjusted independently, to achieve the desirable MPF’s frequency responses for specific applications. Different polarization states result in different envelopes of the output laser of the MWFL, which influence the weight of each sampling tap of the MPF so that reconfigurable passbands can be achieved.

Fig. 4

The frequency response of the MPF (left) and the corresponding spectrum of the MWFL (right) under different polarization state

We have also investigated the MPF’s performance with different gain of hybrid gain mediums, and the results are exhibited in Figs. 5 and 6. One can see from Fig. 5 that, when the current of EDFA increases from 60 to 120 mA, while the gain of SOA is a constant, the suppression ratio of the corresponding MPF increases from 15.98 to 18.79 dB, which is shown in Fig. 5. The suppression ratio of the MPF gets the best result of 19.71 dB when the current of EDFA is 100 mA. When the current of EDFA keeps increasing to 120 mA, the larger gain will amplify the noise in the cavity and reduce the suppression ratio of MPF. When the current of SOA raises, the laser output spectrum shifts to longer wavelength with different envelopes, and then the shape of MPF’s passbands change from triangle to taper. By carefully adjusting the gain of the hybrid gain mediums, passbands with configurability and adjustable shape can be achieved, which entitles the proposed MPF to more flexibility and configurability. Programmable waveform shaper [14] can be used to realize full control of the power of each tap individually, and thus MPF with arbitrary positive tap value.

Fig. 5

The frequency response of the MPF (left) and corresponding spectrum of MWFL (right) with different currents on EDFA

Fig. 6

The frequency response of the MPF and corresponding spectrum of the MWFL with different current on SOA

3 MPF with switchable passbands

Figure 7 shows the experimental setup of the proposed switchable multi-passband MPF. In our experiment, we have adopted the structure of reflective FMZI in the laser cavity of the MWFL, which is taken as the optical source of the MPF and whose schematic diagram is portrayed in the dashed box of Fig. 7, to achieve MPF with switchable passbands.

Fig. 7

The schematic diagram of the proposed switchable multi-passband MPF

The transfer function of a single FMZI can be written as T₁(ω). After PC2 and the fiber mirror and PC2 again, the light propagates into the FMZI the second time, and its transmission function can be written as T₂(ω). The total transmission function of the reflective FMZI can be approximated to be

$$T_{{{\text{MZ}}}} \left( \omega \right) \propto T_{1} \left( \omega \right) \times T_{2} \left( \omega \right) \propto \frac{1}{4}~\left[ {1 + \left( {V_{1} + ~V_{2} } \right)} \cos \left( {2\pi \frac{\omega }{{\Delta \omega }}} \right) + \frac{1}{2}V_{1} V_{2} \cos 2\left( {2\pi \frac{\omega }{{\Delta \omega }}} \right) + \frac{1}{2}V_{1} V_{2}\right] .$$

(4)

$T_{{{\text{MZ}}}} \left( \omega \right)$ represents the transmittance of the reflective FMZI and $\Delta \omega$ is the free spectrum range of the FMZI. The FMZI contains two wavelength spacing terms that are $~\cos \left( {2\pi \frac{\omega }{{\Delta \omega }}} \right)$ and $\cos 2\left( {2\pi \frac{\omega }{{\Delta \omega }}} \right)$, respectively. By adjusting the polarization state of PCs, the FSR of transmission spectrum of the reflective FMZI can be switched between the two values, and thus the corresponding MPF’s passbands can be switchable.

When the length of the VODL is set at 115.343 mm, the wavelength spacing of the laser output is about 0.60 nm, and the measured MPF’s frequency response and the corresponding optical spectrum of the laser output are illustrated in Fig. 8a, b, respectively. One can see that there are three passbands in the 6.50 GHz frequency range and the FSR is 1.95 GHz. By carefully adjusting the PCs, the wavelength spacing of the MWFL can be switched to 0.30 nm, and the FSR of the MPF is switched to 3.90 GHz, and the MPF’s frequency response and the corresponding optical spectrum of the MWFL are shown as Fig. 8c, d. One can notice that there is only one passband in the 6.50 GHz frequency range. In Fig. 8, we would like to demonstrate the switchable characteristics of the proposed MPF. One can see from Fig. 8a, c that, passbands at 1.95 GHz and 3.90 GHz can be switched off by adjusting PCs, and the amplitude of the first passband at 1.95 GHz can be suppressed over 10 dB. The central frequency of the switchable MPF can also be tuned by varying the length of VODL, and when the length of VODL is set at 114.318 mm, the measured frequency response of the switchable MPF and the corresponding output optical spectrum of the MWFL are shown in Fig. 9.

Fig. 8

The measured frequency response of the switchable MPF with FSR of 1.95 GHz (a) and 3.9 GHz (c), and the corresponding spectra of the MWFL (b) and (d) when the length of the VODL is set at 115.343 mm

Fig. 9

The measured frequency response of the switchable MPF with FSR of 1.4 GHz (a) and 2.8 GHz (c), and the corresponding spectra of the MWFL (b) and (d) when the length of the VODL is set at 114.318 mm

We have also studied the performance of the proposed switchable MPF with different gains of the hybrid gain medium. Figure 10 illustrates the measured frequency response with different currents of SOA (from 70 to 100 mA), when the gain of EDFA is fixed at 8.5 dBm. One can see that, with the increasing current of the SOA, the central frequencies of the MPF’s passbands stay unchanged, while the shape of MPF’s passbands change from triangle to taper with the 3-dB bandwidth getting narrower, shown as the zooming figure of Fig. 10. We also have investigated different MPF frequency responses when changing the output power of EDFA with the current of SOA unchanged, and the measured frequency response is shown in Fig. 11. When the power of EDFA increases from 8 to 10 dBm, the suppression ratio increases from 9.85 to 11.73 dB.

Fig. 10

The frequency response of the switchable MPF with different current of the SOA; insert is the zooming of the passband

Fig. 11

The frequency response of the switchable MPF with different gain of the EDFA

4 Conclusions

In this paper, we have proposed and experimentally demonstrate an MPF with shape-adjustable and switchable passbands based on an MWFL. In the experiment, stable MWFL output has been achieved by introducing an FMZI in the laser cavity cooperating with the hybrid gain medium including an EDFA and an SOA, which acts as the multi-wavelength optical source of the proposed MPF scheme. The central frequencies of the passbands can be tunable by varying the length of VODL in the FMZI, and by carefully adjusting PCs, gain of EDFA or SOA, the shape of the MPF’s passbands can be adjustable, which entitles the MPF with more flexibility and configurability. Furthermore, when the reflective FMZI structure is introduced into the laser cavity as the wavelength selective component, and by changing the polarization states of PCs, MPF with switchable passbands has been implemented. In our experiment, when the length of VODL is set at 115.343 mm, by carefully adjusting the PCs the wavelength spacing of the MWFL can be switched from 0.60 to 0.30 nm, which results in an MPF with switchable passbands with the FSR switching from 1.95 to 3.90 GHz. The proposed MPF exhibits shape-adjustable and switchable passbands and enjoys more flexibility compared to previous methods.