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Abstract
A photonic microwave frequency divider that is capable to realise tunable high order frequency division, is presented. It is based on injecting an RF phase modulated optical signal into an off-the-shelf DFB laser operating at period-N state. Optical frequency components with a frequency separation of 1/N times the input RF signal frequency are generated by the DFB laser. An optical bandpass filter can be employed to select two optical frequency components to be detected by a photodetector to obtain a divide-by-N RF signal without harmonic components. The proposed frequency divider can be operated over a wide frequency range and has high reconfigurability as it is free of electrical components. Experimental results demonstrate the realisation of frequency division operation with a tunable 1/2 to 1/5 division ratio for different input RF signal frequencies of 8 to 20 GHz by adjusting the DFB laser forward bias current. Over 35 dB harmonic component suppression is demonstrated. A proof-of-concept experiment is also set up to show the frequency divider based on an optically injected semiconductor laser is capable to operate at a high input RF signal frequency of 50 GHz and has a tunable high order division ratio of 1/2 to 1/8.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
A frequency divider is a signal processing device that generates an output signal with a frequency 1/N times the input signal frequency, where N is an integer. It is a building block in frequency synthesisers and phase locked loops for wireless communication applications [1]. Frequency dividers implemented using flip flops and logic gates are classified as digital frequency dividers [2]. Multiple digital frequency dividers can be connected in series to obtain a high division ratio, but they have limited operating frequency ranges. An analog frequency divider can be implemented using an injection locked technique [3] or a regenerative technique [4]. Both techniques require a mixer, a filter, an amplifier and a coupler connected in the way to form a feedback loop. Commercial electronic frequency dividers can be designed to have a high division ratio or a wide operating frequency range. For example, Fairview Microwave FMFD40000 frequency divider has a high division ratio of 1/40 but the operating frequency range is limited to 0.1-12 GHz, and while Keysight N4984A-040 frequency divider has 0.2-40 GHz operating range, it can only generate divide-by-2/4/8 frequency divided signals. Furthermore, these electronic frequency dividers have fixed frequency division ratios. Future systems require a tunable and arbitrary frequency division ratio and a wide operating frequency range beyond 40 GHz. Microwave photonics provides a solution to satisfy these requirements as various microwave photonic signal processors with high reconfigurability and wide bandwidth have been demonstrated [5,6].
Until now, there are only few reports on photonic microwave frequency divider (PMFD) with a tunable frequency division ratio [7–12]. The PMFD presented in [7] is a regenerative PMFD. 1/2 and 1/3 frequency division can be realised by controlling the phase of a signal inside an optoelectronic oscillator (OEO) loop via an electrical phase shifter. The PMFDs presented in [8,9] are also based on the regenerative technique. A tunable division ratio of 1/2 to 1/6 can be obtained by controlling an OEO loop length via a variable optical delay line. A PMFD implemented using the injection locked technique can generate an output RF signal with a tunable division ratio of 1/2 to 1/5 [10]. 2/3, 2/5, 1/2 and 1/3 frequency division can be realised by cascading two dual-parallel Mach Zehnder modulators (DPMZMs) where one of the two DPMZMs is inside an OEO loop [11]. The frequency division ratio can be tuned by controlling the DPMZM bias voltages. All the aforementioned PMFDs are formed by an OEO loop which consists of a number of electrical and optical components. The later involves two DPMZMs and hence six DC bias voltages need to be controlled. By injecting an RF modulated optical signal into a Fabry Perot (FP) laser oscillating in period-one (P1) state, a 1/2, 1/3 and 1/4 frequency divided signal can be generated after photodetection [12]. While this technique has a simple structure compared to those use an OEO loop, it has poor signal-to-noise ratio (SNR) performance, high harmonic components and limited tunable division ratios.
This paper presents an optically injected semiconductor laser based PMFD. It is based on a distributed feedback (DFB) laser operating in a period-N state when it is subjected to optical injection. This produces optical frequency components with fRF/N separation between adjacent components where fRF is the input RF signal frequency. Beating of these frequency components at a photodetector generates a frequency divided signal at fRF/N. Frequency division ratio can be tuned by simply changing the DFB laser forward bias current. Using an optical filter to select only two optical frequency components can largely suppress the harmonics generated after photodetection. Experimental results demonstrate tunable 1/2 to 1/5 frequency division operation at 20 GHz input RF signal frequency with an over 50 dB SNR. A proof-of-concept experiment is also carried out with results showing the proposed structure has the ability to operate at a high RF signal frequency of 50 GHz and generate a tunable 1/2 to 1/8 frequency divided signal.
2. Topology and operation principle
Figure 1 shows the structure of the proposed 1/N PMFD. A laser source generates a continuous wave (CW) light, which is modulated by an input RF signal in an optical phase modulator (OPM). The electric field of the phase modulated optical signal is given by
Fig. 2. (a) Phase modulator output optical spectrum (blue lines) and free running SL output optical spectrum (green dashed lines). Output optical spectra of the SL oscillating in (b) P2, (c) P3 and (d) P4 states. The red dashed lines are the new optical frequency components generated by periodic oscillation in the SL.
The DFB laser, which is subject to optical injection, is referred to as a slave laser (SL). The phase modulated optical signal, which is injected into the SL, is referred to as an injection light wave from a master laser (ML) [13]. The frequency of the light generated by the SL without optical injection is called the SL free running frequency and is designed to be between the frequency of the optical carrier and the lower 1st order sideband of the ML as shown in Fig. 2(a). Since the optical carrier and the lower 1st order sideband have high amplitudes and they are close to the light generated by the free running SL, the SL is mainly interacted with these two optical frequency components. This is equivalent to injecting two different-frequency CW light into a semiconductor laser. The behaviour of a semiconductor laser subject to dual-beam injection has been investigated [14,15]. The optically injected semiconductor laser can be oscillated in period-one (P1), period-two (P2), … or period-N (PN) state depending on the injection strengths and the detuning frequencies. The injection strengths ξ1(2) are defined as the square root of the power ratio of the ML beam 1(2) and the free running SL. The detuning frequencies fi1(2) are defined as the SL free running frequency offset from the ML beam 1(2) frequency. Simulation results presented in [14] and [15] show the output of a dual-beam injected semiconductor laser oscillating in PN state consists of frequency components at the two injection light beam frequencies (fML1 and fML2) plus frequency components at (fML2-fML1)m/N away from the two injection light beams where m is an integer.
In the proposed structure, for a given input RF signal frequency fRF, the SL forward bias current can be adjusted to alter the injection strengths and the detuning frequencies. This results in the SL oscillating in PN state, which generates frequency components at fc-mfRF/N. Figure 2(b)–2(d) show the output spectra of the SL oscillating in P2, P3 and P4 states respectively. It can be seen from the figure that a number of equally spaced frequency components are generated by the SL. The separation between two adjacent frequency components is fRF/N for the SL oscillating in PN state. Note that different SL output frequency components have different amplitudes. The frequency components that are close to the SL free running frequency have higher amplitudes than those away from the SL free running frequency. Experimental results presented in the following section show the two optical frequency components that have the highest amplitudes are located at the ML lower 1st order sideband frequency fc-fRF and at fc-(N-1)fRF/N. An optical bandpass filter (OBPF) can be connected to the optical circulator Port 3 to pass these two frequency components while suppressing other frequency components. The electric field at the output of the optical bandpass filter can be written as
Note that the proposed 1/N PMFD has a similar structure as the reported 1/2 PMFD [18]. However, in the proposed structure, the SL is operated at a high order periodic oscillation state, which to our knowledge, very little research on experimental investigation of the behaviour of an optically injected semiconductor laser operating at this state has been conducted. Furthermore, the proposed 1/N PMFD has the ability to suppress the harmonic components and has a tunable high frequency division ratio. Compared to the PMFDs based on the regenerative or injection locked technique [7–11], the optically injected semiconductor laser based 1/N PMFD does not require any electrical component. The proposed 1/N PMFD only uses a single bias-free OPM for RF signal modulation whereas multiple Mach Zehnder modulators (MZMs) are needed in the 1/N PMFDs based on the regenerative technique [7–9,11].
3. Experimental results
An experiment was set up based on the structure shown in Fig. 1 to verify the proposed PMFD can realise tunable 1/N frequency division operation. The laser source was a wavelength tunable laser (Keysight N7711A), which generated a 13 dBm CW light into a 20 GHz bandwidth OPM (EOSpace PM-0S5-20). Note that in practice, a DFB laser, which generates a CW light with a frequency close to the SL free running frequency, can be used instead of a tunable laser. This is because the frequency of the injection light wave from a ML does not need to be tuned for realising 1/N frequency division operation. The OPM was driven by a 20 GHz 14.2 dBm RF signal from a microwave signal generator. A polarisation controller (PC) was connected to the phase modulator output port. It was used to maximise the injection efficiency. The RF phase modulated optical signal from the ML was routed from Port 1 to 2 of an optical circulator. It was injected into an off-the-shelf DFB laser. A laser diode driver (Newport 505B) and a temperature controller (Newport 350B) were employed to provide a forward bias current to the DFB laser and to stabilise the DFB laser temperature. Changing the forward bias current alters both the power and the frequency of the light generated by the DFB laser. This consequently alters the injection strengths and the detuning frequencies of the optically injected semiconductor laser. The output of the DFB laser routed from Port 2 to 3 of the optical circulator into an erbium-doped fibre amplifier (EDFA) followed by a tunable optical bandpass filter (TOBPF) (Alnair Lab BVF-300CL). The system output optical signal was detected by a photodetector (Discovery Semiconductor DSC30S), which had a 3-dB bandwidth of around 23 GHz.
The frequency of the light generated by the DFB laser was around 193.510 THz. Hence, the frequency of the tunable laser was set to be 193.511 THz so that the DFB laser free running frequency is between the frequencies of the carrier and the lower 1st order sideband of the RF phase modulated optical signal. The optical spectrum of the injection light, i.e. the RF phase modulated optical signal, was measured on an optical spectrum analyser (OSA) at the optical circulator Port 2. It is shown by the black line in Fig. 3. Note that the optical carrier, which had a frequency of 193.511 THz was normalised to 0 Hz. The 1st and 2nd order sidebands at ±20 GHz and ±40 GHz away from the optical carrier can be seen in the figure. As shown in the figure, the amplitudes of the 1st order sidebands are around 10 dB below the optical carrier. The DFB laser forward bias current and temperature were adjusted to 27.1 mA and 25.4°C respectively, so that optical frequency components at fc±mfRF/2 were generated at the output of the DFB laser when it was subjected to optical injection. The blue line in Fig. 4(a) shows the optically injected semiconductor laser output spectrum measured at the optical circulator Port 3. Frequency components at ±10 GHz, ±20 GHz, ±30 GHz and ±40 GHz away from the injection light carrier frequency can be seen. This indicates that the DFB laser is oscillating in P2 state. The figure also shows the frequency components that have the highest and the second highest amplitudes are located at fc-fRF/2 and fc-fRF respectively. The optical spectrum of the DFB laser operated under the same forward bias current and temperature but without optical injection is shown by the red line in Fig. 3. Two sets of injection strength and detuning frequency can be obtained from the power and frequency of the injection light carrier and lower 1st order sideband relative to that of the SL. They are (ξ1, fi1) = (0.419, 6.491 GHz) and (ξ2, fi2) = (0.162, -13.479 GHz). Similarly, the SL forward bias current was adjusted to 35.4, 38.5 and 40.2 mA while the SL temperature was fixed at 25.4°C, for the SL to generate optical frequency components at fc±mfRF/3, fc±mfRF/4 and fc±mfRF/5, so that the SL was oscillated at P3, P4 and P5 state respectively. The corresponding optical spectra generated by the SL without and with optical injection are shown in Fig. 3 and Fig. 4(b)–4(d) respectively. It can be seen from Fig. 4 that the optical frequency component at fc-(N-1)fRF/N have the highest amplitudes. The passband of the TOBPF was adjusted, so that the TOBPF has a magnitude response as shown by the dashed line in Fig. 4, to pass the optical frequency components at fc-(N-1)fRF/N and fc-fRF. The TOBPF output optical spectra when the SL is oscillating in P2-P5 states are shown by the red line in Fig. 4.
Fig. 3. ML output optical spectrum when the OPM is driven by a 20 GHz RF signal (black line). Optical spectra of the free running SL for realising 1/2 (red line), 1/3 (pink line), 1/4 (green line) and 1/5 (blue line) frequency division. The arrow indicates the ML optical carrier frequency, which is normalised to 0 Hz.
Fig. 4. Optical spectra before (blue line) and after (red line) the TOBPF for realising (a) 1/2, (b) 1/3, (c) 1/4 and (d) 1/5 frequency division. Magnitude response of the TOBPF for selecting two optical frequency components (black dashed line). The arrow indicates the ML optical carrier frequency, which is normalised to 0 Hz.
Figure 5 shows the SL forward bias current required to realise different frequency division ratios. As shown, the higher the frequency division ratio, the higher the SL forward bias current is needed. For a given SL forward bias current shown in Fig. 5, a set of injection parameters (ξ1, fi1, ξ2, fi2) can be obtained from the frequency and power of the injection light wave and the free running SL. Figure 6 shows the injection strengths and the detuning frequencies required for realising 1/2 to 1/5 frequency division at 20 GHz input RF signal frequency. As shown, the injection strengths reduce but the detuning frequencies increase as the frequency division ratio increases.
Fig. 5. SL forward bias current required for realising different frequency division ratios when the input RF signal frequency is 20 GHz.
Fig. 6. (a) Injection strengths and (b) detuning frequencies obtained from the power and frequencies of the injection light carrier (blue dots) and the injection light lower 1st order sideband (red square) relative to that of the free running SL, for different frequency division ratios.
Figure 7 shows the frequency divider output electrical spectra measured on an electrical spectrum analyser (ESA) connected to the photodetector output, when the SL is operating at P2 to P5 states. Since the input RF signal frequency is 20 GHz and the output RF signal frequency of 10 GHz, 6.7 GHz, 5 GHz and 4 GHz can be seen in the figure, 1/N frequency division operation is verified. The results show the division ratio can be tuned by controlling the SL forward bias current. With the use of an TOBPF, harmonic components at the frequency divider output are largely suppressed to more than 35 dB below the frequency divided signal. It can be seen from Fig. 7 that the frequency divider output SNR is around 50 dB for a 510 kHz noise bandwidth, which is comparable to the regenerative based 1/N PMFD [8] that requires two electrical amplifiers with a total gain of 47 dB. It was found that the frequency divider output noise power increases with the linewidth of the ML optical source. Hence a narrow linewidth laser source is needed to obtain a high SNR. Due to low electrical-optical-electrical conversion efficiency, the proposed 1/N PMFD has a high noise figure. Although the regenerative based 1/N PMFD [8] consists of two electrical amplifiers, which results in a higher gain than the proposed 1/N PMFD, the electrical amplifiers also amplify the noise present in the system. Hence the noise figure of the regenerative based 1/N PMFD [8], which can be found from the system gain and output noise power, is similar to the proposed optically injected semiconductor laser based 1/N PMFD noise figure. System parameters need to be optimised in order to improve the PMFD SNR and noise figure performance.
Fig. 7. Output electrical spectra of the proposed 1/N PMFD when the SL forward bias current is adjusted to realise 1/2 (red), 1/3 (pink), 1/4 (green) and 1/5 (blue) frequency division. The input RF signal frequency is 20 GHz.
The ability of the proposed 1/N PMFD to generate a frequency divided signal with different division ratios for different input RF signal frequencies was investigated. Since the OPM half-wave voltage is frequency dependent, the input RF signal power was adjusted as the RF signal frequency changed to have a fixed RF modulation index of 0.86. Figure 8 shows the frequency division ratios that can be obtained by adjusted the SL forward bias current for an RF signal with different frequencies of 8 GHz to 20 GHz into the OPM. Note that divide-by-5 operation cannot be achieved for an input RF signal at 18 GHz and below. The reason can be seen from Fig. 3, which shows when the input RF signal frequency is 20 GHz, the SL free running frequency required to generate a divide-by-5 RF signal is close to the ML lower 1st order sideband frequency at -20 GHz. For an 18 GHz input RF signal, the lower 1st order sideband is -18 GHz away from the optical carrier. This causes the SL free running frequency to be outside the ML carrier and lower 1st order sideband frequency range. Hence P5 oscillation in the SL when the input RF signal frequency is 18 GHz and below, cannot be achieved. On the other hand, increasing the input RF signal frequency can ensure the SL free running frequency to be within the ML carrier and lower 1st order sideband frequency range. However, frequency division operation for an input RF signal with a frequency of above 20 GHz cannot be demonstrated due to the microwave signal generator available for experiment has a maximum output frequency of 20 GHz. The experimental result presented in Fig. 8 shows the SL forward bias current needs to be adjusted as the frequency of the RF signal into the PMFD changes. This indicates that, as in other reported PMFDs which require a specific set of system parameters for realising frequency division operation at a given input RF signal frequency, the proposed 1/N PMFD only works for single-tone modulation.
Fig. 8. SL forward bias current required to realise 1/2 (red), 1/3 (pink), 1/4 (green) and 1/5 (blue) frequency division for different input RF signal frequencies.
The frequency divider output stability was investigated. This was done by measuring the power and frequency of the frequency divided signal when the input RF signal frequency was 20 GHz and the SL forward bias current was set to realise 1/4 frequency division operation. It was found that, for a measurement period of 30 minutes, the output RF signal frequency remained constant at 5 GHz and the output RF signal power had less than 1 dB change. After around 30 minutes, the polarisation state of the injection light wave and the SL forward bias current need to be slightly adjusted to maintain the 1/4 frequency division operation. Integrating the ML, the optical circulator and the SL shown in Fig. 1 on the same substrate can reduce the effect of environmental perturbation that causes change in the frequency divider performance. Experimental results also show 1/4 frequency division operation can be maintained without the need of readjusting the system parameters for around ±2 dB change in the input RF signal power.
In order to verify the proposed 1/N PMFD has the ability to operate over a wide frequency range with a tunable high order division ratio using the available equipment, the ML was implemented by two tunable lasers (Keysight N7711A and Santec WSL-100) followed by two variable optical attenuators (VOAs) and a 50:50 optical coupler (OC), as shown in Fig. 9. The frequency of the Santec tunable laser were fixed at 193.510 THz. The light generated by this laser was used as a reference and is normalised to 0 Hz as shown by an arrow in Fig. 10. The frequency of the Keysight tunable laser was adjusted to be around -20 GHz away from the Santec tunable laser. The Keysight tunable laser power was around 10 dB below the Santec tunable laser. The black line in Fig. 10(a) shows the optical spectrum measured at the 50:50 optical coupler output. As shown, the two optical frequency components generated by the two tunable lasers have a similar frequency separation Δf and power difference as the ML carrier and lower 1st order sideband shown in Fig. 3. They were used as the carrier and the lower 1st order sideband of the phase modulated optical signal in the proposed 1/N PMFD. Changing the Keysight tunable laser frequency alters the separation of the two light wave, which is equivalent to changing the frequency of the RF signal into the 1/N PMFD. This enables the demonstration of frequency division operation at high frequencies without using a high-frequency microwave signal generator. The output of the 50:50 optical coupler was connected to a PC followed by an optical circulator. The rest of the setup, which included a DFB laser, an EDFA, a TOPBPF and a photodetector, was the same as the 1/N PMFD experimental setup. The SL temperature was fixed at 25.4°C and the SL forward bias current was adjusted to 28.5 mA, which is similar to 27.1 mA used for realising the 1/2 frequency division operation in the proposed 1/N PMFD. Frequency components at mΔf/2 away from the ML were generated as can be seen from the SL output optical spectrum shown by the blue line in Fig. 10(b). The TOBPF with a magnitude response shown by the dashed line in Fig. 10(b) allows the two optical frequency components at around -20 GHz and -10 GHz away from the Santec tunable laser frequency to pass through the optical filter. These two frequency components beat at the photodetector generated a stable electrical signal at around 10 GHz. This verifies the SL can be oscillated in P2 state when it is interacted with two optical frequency components in the injection light wave. This shows it is valid to use two optical frequency components as the ML carrier and lower 1st order sideband in the proposed structure, to verify the optically injected semiconductor laser based 1/N PMFD can be operated over a wide frequency range with a tunable high order division ratio.
Fig. 9. Experimental setup of using two tunable lasers to implement the ML for demonstrating the proposed 1/N PMFD has a wide operating frequency range.
Fig. 10. (a) Optical spectra of the ML (black line) and the free running SL (red line). (b) Optical spectra before (blue line) and after (red line) the TOBPF when the optically injected semiconductor laser is oscillated at P2 state. Magnitude response of the TOBPF designed for passing two high-amplitude optical frequency components (black dashed line). The arrow indicates the Santec tunable laser output optical frequency, which is normalised to 0 Hz. The frequency separation between the two frequency components from the ML is around 20 GHz.
The Keysight tunable laser frequency was set at 193.560 THz so that the two light wave into the SL have around 50 GHz separation as shown by the black line in Fig. 11. The SL forward bias current was adjusted to 49.4 mA and 56 mA for the SL to oscillate in P3 and P7 state. The corresponding SL output spectra are shown in Fig. 12(a) and 12(b). As shown, there are 2 and 6 optical frequency components between the injection light wave from the Santec and Keysight tunable lasers. The SL free running spectra when the SL is oscillated in P3 and P7 states, are shown in Fig. 11. As shown, the SL free running frequency is in between the Santec and Keysight tunable laser frequencies. The TOBPF passband was adjusted to allow the two highest-amplitude optical frequency components to pass through the filter. The TOBPF output optical spectra are shown by the red line in Fig. 12. The two optical frequency components at the TOBPF output were detected by the photodetector as in the case of the proposed 1/N PMFD. Figure 13 shows the photodetector output electrical spectra when the SL is oscillating in P2-P8 states. The corresponding injection strengths and the detuning frequencies are shown in Fig. 14. Figure 13 shows, by adjusting the SL forward bias current and the TOBPF passband, a frequency component at Δf/N is generated and the harmonic components are suppressed. The experimental results demonstrate periodic oscillation in an optically injected semiconductor laser can be at 50 GHz and the laser can be operated at P2 to P8 states by controlling the SL forward bias current. Hence the proposed 1/N PMFD based on an optically injected semiconductor laser has the potential of operating over a wide frequency range and having a tunable high order division ratio.
Fig. 11. Optical spectra of the ML (black line) and the free running SL required for the SL to oscillate at P3 (pink line) and P7 (brown line) state when the SL is subjected to optical injection. The arrow indicates the Santec tunable laser output optical frequency, which is normalised to 0 Hz. The frequency separation between the two frequency components from the ML is around 50 GHz.
Fig. 12. Optical spectra before (blue) and after (red) the TOBPF when the optically injected semiconductor laser is oscillated at (a) P3 and (b) P7 state. Magnitude response of the TOBPF designed for passing two high-amplitude optical frequency components (black dashed line). The arrow indicates the Santec tunable laser output optical frequency, which is normalised to 0 Hz. The frequency separation between the two frequency components from the ML is around 50 GHz.
Fig. 13. System output electrical spectra when the SL forward bias current is adjusted so that the SL is oscillating in P2 (red), P3 (pink), P4 (green), P5 (blue), P6 (cyan), P7 (brown) and P8 (black) states. The frequency separation between the two frequency components from the ML is around 50 GHz.
Fig. 14. (a) Injection strengths and (b) detuning frequencies obtained from the power and frequencies of the injection light generated by Keysight tunable laser (blue dots) and Santec tunable laser (red square) relative to that of the free running SL, when the SL is oscillating in PN states.
Finally, a comparison between the performance of the reported and proposed tunable 1/N PMFDs is shown in Table 1. This shows the PMFDs implemented using an optically injected semiconductor laser have the advantage of do not require electrical components. The experimental results presented in this paper demonstrate the optically injected semiconductor laser based PMFD has the ability to realise microwave frequency division with a tunable high order division ratio and a large harmonic suppression.
Table 1. Comparison between the reported and proposed tunable 1/N PMFDs
4. Conclusion
A divide-by-N PMFD based on high order periodic oscillation in an optically injected semiconductor laser has been presented and experimentally demonstrated. The SL can be operated at different periodic oscillation states by adjusting the forward bias current into the SL. Optical frequency components with a frequency separation of fRF/N between two adjacent frequency components are generated when the SL is oscillating in PN state. Beating of these optical frequency components at the photodetector generates an electrical signal at fRF/N. Using an optical bandpass filter to ensure only two optical frequency components are detected by the photodetector can eliminate the harmonic components generated after photodetection. The proposed 1/N PMFD involves only optical components and hence it has a very wide operating frequency range. An experiment has been conducted based on the proposed structure. The results demonstrate tunable 1/2 to 1/5 frequency division with a 50 dB SNR and over 35 dB harmonic suppression for a 20 GHz input RF signal. A proof-of-concept experiment using two laser sources with a 50 GHz frequency separation has been set up. The results demonstrate, for the first time, microwave photonics has the potential to realise tunable high order 1/2 to 1/8 frequency division for a 50 GHz input RF signal.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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