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Abstract
We present a novel broadband divide-by-2 microwave photonic injection locking frequency divider (ILFD) based on a dual-loop optoelectronic oscillator (OEO). In the proposed scheme, a tunable microwave photonic filter is used to replace the traditional electrical filter, which makes sure a large tuning range of the ILFD. The microwave photonic ILFD whose center frequency tracks the tunable frequency of the free-running OEO, links up with every single locking range together. Thus the frequency range is only determined by the tunable OEO. In the experiment, a wide operating frequency range from 4.51 GHz to 34.88 GHz is realized. Furthermore, a divide-by-3 ILFD is experimentally demonstrated with the help of a frequency mixer.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Phase-locked loop frequency synthesizers are extensively used in modern radar, sensors, high-rate data communication systems and beyond. In the frequency synthesizer systems, the frequency divider is one of the most critical building blocks whose characteristics directly affect the performance of the whole system [1]. Working in the highest band of frequency synthesizers, high speed and wideband frequency dividers are expected to connect different bands.
In contrast to several typical structures of the frequency dividers, for example, dynamic logic frequency divider, current logic frequency divider, and Miller frequency divider, the electrical injection locking frequency divider (ILFD) generally have high operating band and low power consumption, and usually serves as the first stage of high-frequency dividing [2]. However, because of the bandwidth restrictions of electric filters, most of the traditional ILFDs have limited bandwidth. Conflicts are also existed mutually in previous designs for the optimization between the high operating band and wide locking range. So the design difficulty of the ILFDs mainly lies in how to expand the locking range in the high operating frequency.
Various high-frequency or broad locking range ILFDs have been reported, for example, based on a self-frequency-tracking transformer [3], a time-interleaved dual injection locking scheme [4], fast calibration algorithm [5], or special process or material like InGaAs/InP HBT technology [6], etc. Frequency divider designs in microwave photonics ways are another ideal solution, thanks to the advantages of microwave photonics technology including large accessible bandwidth, high data-transfer capacity, immunity to electromagnetic interference and so on. Several microwave photonics ways have been proposed recently. For example, ultra-low phase-noise frequency divisions are realized by photomixing of highly coherent laser sources, with the help of frequency combs [7,8] or cascaded Brillouin oscillation [9].
Microwave photonic frequency dividers using optoelectronic oscillator (OEO) have also been reported in [10–13]. The OEO has been mainly considered previously as a promising solution to generate microwave signals with ultra-low phase noise and high-frequency stability. In [10], they divided a 20-GHz signal to 10 GHz with the help of an EBPF whose center frequency and 3-dB bandwidth are 10 GHz and 50 MHz, respectively. In [11], a large division ratio from 2 to 5 using the optical comb was reported. A wideband microwave frequency divider without electrical filter was realized based on carrier-suppressed optical double sideband modulation in [12]. In [13], a divided-by-n frequency division mainly using a dual-parallel Mach–Zehnder modulator was proposed. An optoelectronic parametric oscillator [14] based on parametric conversion can also realize frequency division in the condition of degenerate oscillation stable. The phase evolution in [14] is not linear, leading to steady multimode oscillation that is not bounded by the cavity delay. At present, the conventional frequency dividers based on electronics [15] can work at 60 GHz. But the output waveforms are distorted in low frequencies. It may lead to limitations on some applications. Such a wide bandwidth or even more can be achieved if wideband optoelectronic devices can be used. However, most of the systems are restricted by the electrical filter, which limit the extension of the operating frequency range.
In this paper, a novel broadband divide-by-2 microwave photonic ILFD based on an OEO is presented and experimentally demonstrated. Using a tunable microwave photonic filter (MPF) to replace the electric filter, a large tunable range of the ILFD is realized. The microwave photonic ILFD whose center frequency tracks the tunable frequency of free-running OEO, links up with every single locking range together. Thus the frequency range of this ILFD is no longer limited by the narrow locking range, only determined by the frequency range of the tunable OEO. A wide operating frequency range from 4.51 GHz to 34.88 GHz is realized in the experiment and the phase noise performance is also investigated. Furthermore, variable division ratios can also be realized in small modified design. A divide-by-3 ILFD is experimentally demonstrated with the help of a frequency mixer.
2. Principle
A traditional electrical ILFD model [16] typically contains a nonlinear block $f(e)$ and a filtering block $H(\omega )$. The function $f(e)$ expresses a series of harmonic and inter-modulation terms, and the function $H(\omega )$ expresses a band-pass filter with a central wavelength of ${\omega _0}$. Without an incident signal, it can be considered as an LC oscillator model which outputs a sine wave at ${\omega _0}$. When the incident signal ${\omega _i}$ is injected in the loop, the mixed frequencies ${\omega _i} + {\omega _0}$ and ${\omega _i} - {\omega _0}$ are produced by an ideal mixer. Then after filter $H(\omega )$, only the frequencies near the ${\omega _0}$ are retained. If the difference between ${\omega _0}$ and ${\omega _i} - {\omega _0}$ are less than the locking range, ${\omega _i} - {\omega _0}$ would be pulling to ${\omega _0}$ with the help of injection locking effect, so there will be only one frequency exist. Therefore, ${\omega _i} - {\omega _0} = {\omega _0}$, ${\omega _0}$ precisely equal to ${\omega _i}/2$. So the frequency of the incident signal is divided in half and oscillating in steady-state when it is located around half of the free-running signal that within its locking range.
Figure 1 represents a schematic of the proposed microwave photonic ILFD and shows how the frequency division is realized in the OEO system. The main body of the ILFD is a tunable OEO based on an MPF. The microwave photonic ILFD consists of a tunable laser source (TLS), a wide-band phase modulator (PM), a polarization controller (PC), an optical circulator, a phase shifted fiber Brag grating (PS-FBG), an erbium-doped fiber amplifier (EDFA), an optical coupler, an optical tunable delay line (OTDL), two photodetectors (PDs), a long single-mode optical fiber (SMF), a phase shifter (PS), an electronic amplifier (EA), and three electronic power dividers. The TLS’s light wave is modulated by the electrical signal in the PM, and then adjusted by the PC. The polarization-sensitive PS-FBG is used for converting the phase modulation to the intensity modulation. One of the first-order sidebands matches with the notch of PS-FBG and gets a phase shift of $\pi $ in the reflection spectrum. The modulating signal with the optical carrier is amplified by the EDFA and split into two parts, then sending to two PDs, respectively.
Fig. 1. A schematic of a divide-by-2 microwave photonic ILFD. A divide-by-3 ILFD can be realized if replaces the gray box. The inset shows the process of injection locking, and the locking ranges of different free-running oscillating frequency cover a wide band. TLS: tunable laser source; PM: phase modulator; PC: polarization controller; PS-FBG: phase shift fiber brag grating; EDFA: erbium doped fiber amplifier; SMF: single-mode optical fiber; PD: photodetector; PS: phase shift; OTDL: optical tunable delay line; EA: electronic amplifier; ESA: electrical spectrum analyzer.
The configuration of the dual-loop OEO has excellent properties such as single-mode, high side-mode suppression ratio, and stability. In the long loop, there is a long SMF to obtain a high quality (Q) factor. The OTDL and the PS in the short loop is used for adjusting the oscillating frequency with a fine resolution. When we adjust the wavelength of the TLS, the microwave photonic BPF can be easily tuned. So the oscillation frequency of the free-running OEO is determined by the wavelength of the laser, and the length of the loop together, and limited by the bandwidth of optoelectronics devices such as FBG, modulator and PD. The signal outputting from PD is monitored by ESA. After amplifying by the EA, the microwave signal is combined with the incident signal from the microwave source and feedback to the PM.
Assuming the free-running frequency of the OEO is ${\omega _0}$ and the incident signal frequency is ${\omega _{inj}}$. After modulated by PM and reflected by PS-FBG, the beating signal can be expressed as ${\omega _b} = {\omega _{inj}} - {\omega _0}$. With the free-running frequency ${\omega _0}$ is moving close to the beating signal ${\omega _b}$, we calculate and describe the state of the spectrum. When ${\omega _0}$ is far from ${\omega _b}$, two different frequency components oscillating in the loop cause inter-modulation distortion (IMD). The input signal can be expressed as
So a series of inter-modulation terms appear at the frequency spectrum. When ${\omega _0}$ nearly approaches but lies out of the locking range of ${\omega _b}$, we explain the model in quasi-lock and fast beat conditions. There are a large number of sidebands around ${\omega _0}$, which are not arise from the IMD [17]. When ${\omega _b}$ is located in the overall locking range around ${\omega _0}$, the free-running frequency is pulled in the beating signal and finally locked in it, this is called injection locking status. At this moment, ${\omega _0}$ is equal to ${\omega _b}$. In another word, ${\omega _0} = {\omega _{inj}}/2$, a 2:1 microwave photonic ILFD is realized. Different from [14], the scheme proposed here is based on an injection locked oscillator, where the oscillation frequency is pulled to the harmonic frequency. With the help of injection locking, the desired frequency division is realized.
Due to the replacement of an electric filter with the tunable MPF, the bandwidth of the loop filter is not the limitation of ILFD’s operating frequency anymore. The operating frequency is only bounded by the tunable range of the OEO. In addition, the PMs are not dc biased, which would eliminate the bias drifting problem existing in an MZM. Even so, increasing the locking range is still significant, because it restricts the bandwidth of incident signal itself. The locking range in a narrow sense is called injected locking range (ILR) that the free-running frequency is invariant. Broadly speaking, if the free-running frequency moves with the phase shift, temperature or something else, the locking range is the superposition of the tunable range of free-running and the ILR that called frequency tuning range (FTR). The conventional approaches to increase the ILR include decrease the power of free-running oscillator, increase the power of the incident signal, or sacrifice the Q factor. These above methods are all suitable for our scheme.
So how to accurately tune the free-running frequency for locking the beating signal is the key to the proposed ILFD system. In this microwave photonic ILFD, a TLS, an OTDL and a PS will be set for the coarse and precision tuning. Limited to the picometer magnitude step of the TLS, the coarse tuning step is about hundreds of MHz. The oscillating frequency of the OEO is also related to the loop delay time [18]. The OTDL and the PS are equivalent to bring in a tunable delay time in the optical loop and electric loop, respectively. Although each of them only controls the oscillating frequency in a single loop directly, the dual-loop OEO oscillating frequency is able to change ultimately because of the Vernier effect. As shown in the inset of Fig. 1, every single-mode of the fine-tunable OEO is lined tightly to ensure the full coverage of ILR. There is always a proper frequency of the OEO which can lock to the beating signal. Therefore, the center frequency of the microwave photonic ILFD tracks the frequency of free-running OEO, expanding the original narrow ILR. Moreover, the PS will change the phase difference between the incident and the free-running signal, which can be used to reduce the power of the inter-modulation terms.
3. Experiment and results
As illustrated in Fig. 1(b), the microwave photonic ILFD for frequency division consists of a TLS (Yenista T100S-HP) with a tunable step of 0.001 nm, a 2-Ch PM (Versawave), an ultra-narrow notch PS-FBG, an EDFA (Keopsys) which can deliver up to 33 dBm of saturated output power, an OTDL, two 18-GHz PDs (Agilent 11982A), an 800-m long SMF, a PS (Connphy), and EA which provide a gain of 15 dB. The frequency spectrum is monitored by electrical spectrum analyzer (ESA R3182).
3.1 Frequency Division Results
Without the incident signal, a tunable OEO is implemented based on the PS-FBG. The total gain of the loop is first tuned to under the threshold of oscillating. Then a 30.2-GHz incident signal is lunched to make the free-running frequency and the beating frequency run in the loop together. Figure 2 shows two frequency spectrums before and after injection locking. The visible peak at 15.08 GHz is the output signal after frequency dividing. The peak at 30.20 GHz is caused by the harmonic wave of OEO and the incident signal. We only used an erbium-doped fiber amplifier and an electronic amplifier to provide enough gain for the OEO loop in our experiment. There was no other power control schemes. Figure 2(c) shows the zoom-in view of the spectrum of the output signal. The side mode suppression ratio is more than 60 dBm. This ILFD system adopting a series of improvement measures such as using dual-loop construction and add an EDFA before the PDs to avoid the mode-hopping and improve phase-noise. That is because the PDs working at saturation condition will restrain the inherent relative intensity noise (RIN) noise from the laser source [19].
Fig. 2. The frequency spectrum of ILFD before (a) and after (b) injection locking. (c) The spectrum zooms in on the frequency after dividing at 15.08 GHz. The RBW is 10 kHz.
In the injection locking states, the oscillator is pulled from its natural frequency ${\mathrm{\omega }_0}$ with the move of the incident frequency ${\omega _{inj}}$, and the output signal is precisely half the frequency of the incident signal. As shown in Fig. 3, with the changing of incident frequency of 140 kHz, the output signal moved 70 kHz. The ILR is about 70 kHz, which is proportional to the power of the incident signal. Two pair of sidebands distance of 8 kHz and 16 kHz from resonant peak appears clearly in the ESA. Replacement of every device in the loop has no influence on these two unknown sidebands. Moreover, the oscillating frequency is sensitive to the environment temperature because of the FBG.
Fig. 3. Pulling phenomenon by 14.23 GHz shows the ILR about 70 kHz. The span is 1 MHz, and the RBW is 1 kHz.
The Fig. 4 illustrates the OEO can continuously tuned in 5 MHz which is much bigger than FSR. Because of the continuously tunability of the OEO, the frequency divider can work at any frequency without the limitation of the injected power.
Fig. 4. The tunability of the OEO. (a) The oscillation frequency switches at adjacent modes at 10.08 GHz. (b) The oscillation frequency tuned continuously in 1 MHz at 7.2 GHz.
Figure 5(a) shows the frequency spectrum after dividing in the whole band. It confirms that the operating frequency band can be tuned from 4.51 GHz to 34.88 GHz through the cooperation of 0.12 nm tuning range of TLS. The dotted red line shows an uneven response curve of the ILFD with a power fluctuation of 13.6 dBm. This is possibly because the unflatness of the EA gain curve and the PD response curve. And the cascaded EAs amplify the fluctuation. However, the oscillation frequency is hard to tune sometimes because of the transformation of polarization states. With the help of tuning TLS, PS and OTDL in combination, we can tune almost all frequency in the operating band.
Fig. 5. (a) The frequency spectrum after dividing in the whole band. The RBW is 30 kHz. (b) Phase-noise of the 7 GHz output signal after dividing (red line), the free-running oscillating signal at 14 GHz (blue line), and the signal source at 14 GHz (yellow line).
Phase noise is an important standard of signal quality, which is necessary to pay attention to. Single-side band phase noise spectra of a free-running signal and an injection locked signal after frequency division at 7 GHz are measured by a phase noise analyzer, the results are shown in Fig. 5(b). The measured phase noise after frequency division is −114.2 dBc/Hz@10 kHz, which is much better than the free-running signal. Within the locking range, the trend of the phase noise is coinciding with the input signal. After that, it behaves like the free-running OEO. With an 800-m SMF, the system sacrifices the phase noise for a wider band ILR. The phase noise of the system may come from the relative intensity noise of TLS, shot noise of PD, thermal noise of EDFA and EA and so on. The TLS we used has a stronger RIN noise of −145 dB/Hz measured at 0 dBm. The phase noise can be improved by using some high-performance devices if necessary.
Furthermore, long-term stability is worth considering by removing the electrical filter. In the proposed frequency divider, the noise would not be enhanced because the FBG is also a passive device as the electrical filter. However, since the notch bandwidth of the FBG we used in the experiment is about 100 MHz, which is larger than bandwidth of the EBPF, the stability might be distorted due to the increased mode competition effect. Allan deviation is a typical criteria to evaluate the stability of an oscillator. The measured results are shown in Fig. 6. The Allen deviation of the injection locked signal is around $0.98 \times {10^{ - 12}}$ at 1 s, which improves by three orders of magnitude compared with the free-running signal.
Fig. 6. The comparison of Allan deviation with the free-running signal, injection locked signal, and microwave source.
3.2 Divide-by-3 Ilfd
At last, the system could realize a variable division ratios frequency divider with the help of a frequency mixer or multiplier. For instance, a divide-by-3 ILFD is experimentally demonstrated by using a frequency mixer, as shown in Fig. 1. The oscillation signal ${\omega _0}$ split from the dual-loop OEO is entered to the mixer. So the signal of ${\omega _{inj}} - {\omega _0}$ and ${\omega _0}$ are applied to the PM together. The calculation of frequency division is the same as we mentioned in the principle section. When the signal of ${\omega _{inj}} - 2{\omega _0}$ is located in the locking range, a 3:1 microwave photonic ILFD is realized.
In keeping with the 2:1 ILFD, the operating band and the phase noise are measured and the results are shown in the Fig. 7. Figure 7(a) confirms that the operating frequency band can be tuned from 13.53 GHz to 41.76 GHz. The dotted red line represented the response curve shows a 9.8 dBm power fluctuation. And the phase noise after frequency division is −101.6 dBc/Hz@10 kHz. The divide-by-3 ILFD make a small performance difference than the divide-by-2 ILFD. So it’s reasonable to assume that variable division ratios ILFD is possible with the mixer or multiplier.
Fig. 7. (a) The frequency spectrum after dividing into 3 in the whole band. The RBW is 30 kHz. (b) Phase-noise of the output signal after dividing (red line), the free-running oscillating signal at 10 GHz (blue line), and the signal source at 34 GHz (yellow line).
4. Conclusion
In this work, we present and experimentally demonstrate a novel broad band divide-by-2 microwave photonic ILFD based on a dual-loop OEO. In the proposed scheme, a tunable microwave photonic filter is used to replace the traditional electrical filter, which makes sure a large tuning range from 4.51 GHz to 34.88 GHz of the ILFD. The microwave photonic ILFD whose center frequency tracks the tunable frequency of the free-running OEO, link up with every single locking range together. Thus the frequency range of this ILFD is no longer limited by the narrow locking range, only determined by the frequency range of the tunable OEO. The phase noise of the microwave signal after frequency dividing exhibited a good performance, which is about 6-dB better than that of the free-running signals. The phase noise performance can be improved if high-performance devices such as a high-Q PS-FBG, a low RIN TLS, or other low loss devices are employed. In addition, variable division ratios are possible with the help of a frequency mixer or multiplier.
Funding
National Key Research and Development Program of China (2018YFB2201902); National Natural Science Foundation of China (61925505, 62075212).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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