Based on an optically injected semiconductor laser (OISL) operating at period-one (P1) nonlinear dynamical state, high-purity millimeter-wave generation at 60 GHz band is experimentally demonstrated via 1/4 and 1/9 subharmonic microwave modulation (the order of subharmonic is with respect to the frequency fc of the acquired 60 GHz band millimeter-wave but not the fundamental frequency f0 of P1 oscillation). Optical injection is firstly used to drive a semiconductor laser into P1 state. For the OISL operates at P1 state with a fundamental frequency f0 = 49.43 GHz, by introducing 1/4 subharmonic modulation with a modulation frequency of fm = 15.32 GHz, a 60 GHz band millimeter-wave with central frequency fc = 61.28 GHz ( = 4fm) is experimentally generated, whose linewidth is below 1.6 kHz and SSB phase noise at offset frequency 10 kHz is about −96 dBc/Hz. For fm is varied between 13.58 GHz and 16.49 GHz, fc can be tuned from 54.32 GHz to 65.96 GHz under matched modulation power Pm. Moreover, for the OISL operates at P1 state with f0 = 45.02 GHz, a higher order subharmonic modulation (1/9) is introduced into the OISL for obtaining high-purity 60 GHz band microwave signal. With (fm, Pm) = (7.23 GHz, 13.00 dBm), a microwave signal at 65.07 GHz ( = 9fm) with a linewidth below 1.6 kHz and a SSB phase noise less than −98 dBc/Hz is experimentally generated. Also, the central frequency fc can be tuned in a certain range through adjusting fm and selecting matched Pm.
© 2016 Optical Society of America
Photonic generation of microwave signals has attracted considerable attention due to its potential applications in radio-over-fiber (RoF) communication, clock frequency division, broadband wireless access networks, satellite communication systems, radar systems, and so on [1–3]. A number of photonic microwave generation techniques have already been proposed and demonstrated, including direct modulation [4,5], external modulation [6,7], optoelectronic oscillators (OEOs) [8,9], dual-mode lasers emitting simultaneously at two different wavelengths [10,11], and optical heterodyne [12–15] etc. Each approach has its own advantages and some room for improvement. Direct modulation is simple and easy to implement, but it is difficult to obtain high frequency microwave signals due to the limitation of the laser modulation bandwidth. Compared with direct modulation, external modulation can generate high frequency microwave signals, but the modulator requires high driving voltage and introduces large insertion loss. Optoelectronic oscillators are capable of generating microwave signals with high stability, but the frequency tunability of microwave signals is limited by the electronic bandwidth of high frequency electronic components. The dual-mode lasers emitting simultaneously at two different wavelengths can produce low phase noise microwave signals, but the generated microwave signals have poor ability of tuning due to fixed frequency spacing between the two lasing modes. Compared with optoelectronic oscillators and dual-mode lasers, the optical heterodyne method can generate widely tunable microwave signals up to terahertz (THz) range by heterodyning two independent semiconductor lasers, but the generated microwave signals exist strong phase noise due to the two phase uncorrelated optical beams, and then extra phase locking techniques is needed to lock the optical phase. At present, several phase locking techniques such as optical phase lock loop , optical injection locking , and optical injection phase lock loop  have been reported. In , heterodyne signals within the range between 85 GHz and 120 GHz with up to −10 dBm output power have been successfully generated based on a monolithically integrated photonic source consisting of two distributed feedback (DFB), eight semiconductor optical amplifiers (SOA), two electro-absorption modulators (EAM), and two uni-travelling carrier photodiodes (UTC-PD). Moreover, high purity and low phase noise signals could be achieved after further phase stabilization based on optical injection locking to two optical tones derived from an optical frequency comb generator .
Compared with these techniques mentioned above, the approach based on period-one (P1) nonlinear dynamics of optically injected semiconductor laser (OISL) offers some unique advantages for photonic microwave generation [2, 16–34]. Firstly, the scheme is an all-optical scheme without suffering from limited electronic bandwidths . Secondly, by simply adjusting the power and frequency of the optical injection, the microwave frequency generated by P1 oscillation can be continuously tuned from a few to tens or even hundreds of gigahertz [17,18]. Thirdly, single sideband (SSB) optical spectra structure can be achieved by adjusting the injection parameters and has the advantage of minimizing the microwave power penalty caused by chromatic dispersion of the optical fiber . Moreover, the OISL system can be used as an AM-to-FM conversion module to reduce the fading effects induced by AM modulation in the RoF system . However, due to intrinsic spontaneous emission noise of the laser, the microwave generated by P1 dynamics usually has a relatively large linewidth on an order of megahertz, which limits its practical applications. In order to improve the spectral purity of generated microwave, some microwave stabilization techniques such as direct current modulation of the injected laser at generated microwave frequency [22–24], introducing optoelectronic feedback [25,26] or optical feedback [27–30], and adopting optical modulation sideband injection locking , have been successively proposed, and the related results demonstrated that these methods are effective for achieving high-purity photonic microwave signal at relatively low frequency.
In order to avoid spectral congestion at low frequency, the millimeter-wave frequency band at 60 GHz is being considered for broad-band wireless access systems, and then high-purity millimeter-wave carrier acquisition will be a key issue. For generating a millimeter-wave at 60 GHz band, these stabilization techniques mentioned above may face some challenges. For example, the stabilization technique through directly current modulating the injected laser at the generated microwave frequency is not suitable for achieving photonic millimeter-wave at 60 GHz band due to limited modulated response frequency of the laser. For the stabilization technique through introducing optoelectronic feedback, a photodetector, an electronic microwave amplifier and attenuator are needed to compose a feedback loop, and obviously according electronic elements used in the feedback will be high cost for generating 60 GHz band millimeter-wave. For the stabilization technique through introducing optical feedback, the electronic bandwidth restriction can be bypassed but the microwave linewidth can only be reduced by two orders of magnitude to the range of tens of kHz. The optical modulation sideband injection locking approach possesses an application potential for generating high-purity 60 GHz band millimeter-wave, but the system complexity and cost will be pending problems to be solved due to these requirements for a nonlinearly distorted microwave reference and an optical phase modulation with lower roll-off in high frequencies. In order to overcome the technical defects mentioned above, a high-order subharmonic double-locking technique is applied for generating high-purity 60 GHz band millimeter-wave in this paper. For n-order subharmonic double-locking technique, a current with a frequency of f0/n is used to modulate the OISL for achieving high spectral purity microwave with the frequency f0. Based on 1/2 subharmonic locking technique, low phase variance microwave signals have been obtained [32,33]. Recently, we have proposed and preliminarily demonstrated a high-purity microwave (~20 GHz) acquisition technique based on an OISL via higher order subharmonic locking technique . In , a 1/3 or 1/4 subharmonic modulation signal with weak power is introduced to an OISL operating at P1 oscillation for locking the P1 oscillation frequency f0 (about 20 GHz), and then a high-quality microwave signal whose frequency is located at a frequency in the vicinity of f0 can be acquired.
In this work, high-purity millimeter-wave generation at 60 GHz band is experimentally demonstrated based on an OISL via 1/4 and 1/9 subharmonic microwave modulation. Here the order of subharmonic is with respect to the frequency fc of the acquired 60 GHz band millimeter-wave due to that the acquired fc is far away from the fundamental frequency f0 of P1 oscillation and is just high-order multiple times of the loaded modulation frequency fm, which is different from [32–34] where the order of subharmonic is defined with respect to f0 since the generated microwave frequency fc is close to f0. Different from our previous work reported in , the applied power Pm of subharmonic modulation signal in this study is higher than that in  and needs to be accurately selected. As a result, the frequency of generated 60 GHz band millimeter-wave signals can be much larger than the P1 frequency f0.
2. Experimental setup
The schematic of experimental setup is shown in Fig. 1. A tunable laser (Santec, TSL-710) with a wide tuning range of 1480-1640 nm and a maximum power of 20 mW is used as the master laser (ML). A high-speed directly modulated commercial 1.55-μm quantum well (QW) distributed feedback (DFB) semiconductor laser packaged with a fiber pigtail is used as the slave laser (SL). The bias current and temperature of the SL are controlled by a high accuracy and low noise current-temperature controller (ILX-Lightwave, LDC-3724C). The optical output of ML firstly passes through an erbium doped fiber amplifier (EDFA), a polarization controller (PC), a variable attenuator (VA), and then is split into two parts by a fiber coupler (FC1). One part is sent to power meter (PM) to monitor the injected power Pi, and the other is injected into the SL through an optical circulator (OC). During experiments, the polarization controller is employed to match the polarization states of the ML and SL, and the variable attenuator is used to adjust the injection power level. The output of the SL passes through OC, FC2, and then is sent to a detection system. The optical spectrum is monitored by an optical spectrum analyzer (OSA, Ando AQ6317C, 0.015 nm resolution), and an electrical spectrum analyzer (ESA, R&S®FSW, 67 GHz bandwidth) is used to measure power spectra and single sideband (SSB) phase noise through a photo-detector (PD, U2T-XPDV3120R, 70 GHz bandwidth). A microwave frequency synthesizer (MFS, Agilent E8257C) is used to supply a modulation signal with modulation frequency fm and modulation power Pm.
In order to characterize some basic properties of the free-sunning SL, Fig. 2(a) shows the experimentally measured power-current curve of the SL. As seen in this diagram, the threshold current Ith of the SL is about 3 mA. Figures 2(b) and 2(c) show the variation of the relaxation oscillation frequency of the free-running SL with the bias current and the square-root of the laser output power, respectively. The relaxation oscillation frequency characterizes the maximum modulation ability of the laser, which can be extracted by directly modulating the SL with low modulation power. Obviously, as shown in Figs. 2(b) and 2(c), within our experimental range, the relaxation resonance frequency nonlinearly increases with the bias current but linearly increases with the square-root of the output power. Such a varied trend is identical to that reported in , and corresponding physical mechanism has also been given in . During following experiments, the SL is biased at 25 mA and is temperature stabilized at 16.95 °C. Under these operating conditions, the free-running SL has an output power of 2.13 mW, a lasing wavelength of 1548.61nm, and a relaxation oscillation frequency of about 7.50 GHz.
3. Results and discussion
3.1 P1 dynamics
Under proper injection power Pi and frequency detuning fi between the frequency of the injection beam and the central frequency of the free-running SL, an OISL can be driven into period-one (P1) state with fundamental frequency f0. Figure 3 displays the optical spectra (left column) centered at the free-running frequency of the SL, the power spectra (middle column), and the detailed power spectra centered at f0 with a resolution bandwidth (RBW) of 100 kHz (right column), of the SL operating at the P1 oscillations under two different injection parameters (Pi, fi) of (0.61 mW, 36.16 GHz) and (0.63 mW, 39.41 GHz). From this diagram, it can be seen that there are two optical components with almost equal amplitudes emerging upon optical spectra, and the beat between the two optical components generates P1 oscillation frequency f0, which is 45.02 GHz for (Pi, fi) = (0.61 mW, 36.16 GHz) and 49.43 GHz for (Pi, fi) = (0.63 mW, 39.41 GHz), respectively. Under the two cases of different injection parameters, the photonic microwave signals originated from the P1 oscillations have a relatively large linewidth of about 2.0 MHz (Fig. 3(a3)) and 2.4 MHz (Fig. 3(b3)), respectively. Here, instead of using traditional definition of 3-dB linewidth, the spectral linewidths in Figs. 3(a3) and 3(b3) are calculated statistically by using standard deviation of the spectral power distribution due to the frequency jitter . In the following, the P1 oscillation with relatively poor frequency stability will be used as an indispensable seed source for generating high quality 60 GHz band millimeter wave.
3.2 1/4 Subharmonic modulation for generating 60 GHz band millimeter wave
A subharmonic microwave modulation is further introduced into the OISL for achieving a high-purity 60 GHz band millimeter-wave. For the case that the OISL operates at the P1 state with f0 = 49.43 GHz as shown in Fig. 3(b), Figs. 4(a1) and 4(a2) record the optical spectrum and power spectrum after adopting 1/4 subharmonic modulation with (fm, Pm) = (15.32 GHz, 20.00 dBm), respectively. As shown in Fig. 4(a1), the strongest peak corresponds to the injection light, and the secondary peak emerges at an offset of about 61.28 GHz with respect to the frequency of the injection light. The power spectrum in Fig. 4(a2) shows that there exists a strong peak at 61.28 GHz, which is equal to 4fm. Generally, direct modulation can also be used to obtained microwave with multiple modulation frequency. For comparison, the corresponding optical spectrum and power spectrum that the SL is only directly modulated are also given in Figs. 4(b1) and 4(b2), respectively. Under this case, the optical spectrum behaves symmetric distribution. Combining Fig. 4(a1) with Fig. 4(b1), it can be seen that the role of the injection light is to make the optical spectrum of the SL under current modulation shift to longer wavelength, and the beating between the shifted optical components and the injection devotes to the peaks emerging at the power spectrum. Therefore, it can be concluded that, when the components in optical spectrum of the SL under current modulation are shifted by the injection light to those frequencies deviating multiple fm from fi, the high-purity microwave with multiple fm can be obtained. In other words, if one of the shifted modulation sidebands locates at a frequency near the injection light, the regeneratively amplified injection signal may be locked by the modulation sideband, and then a high-purity microwave is obtained. The power spectrum in Fig. 4(b2) shows the frequency peaks located at fm and 2fm can be observed but no peaks located at 3fm and 4fm can be observed. Therefore, only adopting direct modulation with fm = 15.32 GHz cannot generate a millimeter-wave with frequency at 60 GHz band.
In order to characterize the detailed features of above generated 60 GHz band millimeter-wave, Fig. 5 shows the detailed power spectrum (Fig. 5(a)) and SSB phase noise (Fig. 5(b)) centered at 61.28 GHz. As presented in Fig. 5(a), the 61.28 GHz millimeter-wave is stably locked with a 3-dB linewidth below 1.6 kHz, and the difference between the central peak and the noise floor is about 60 dB. Figure 5(b) indicates the measured SSB phase noise as a function of the frequency offset to 61.28 GHz, and one can see that the phase noise at the 10 kHz offset frequency is about - 96 dBc/Hz. It should be pointed out that, on the one hand, for this given Pm = 20.00 dBm, there exists a very slight tunable locking range (about 3.5 MHz) around fm = 15.32 GHz, which corresponds to a change of about 14 MHz around 61.28 GHz. Once fm is outside the locking region, the frequency purity will be extremely deteriorated. On the other hand, for the given fm = 15.32 GHz, the adjusted range of modulation power around Pm = 20.00 dBm required for locking is also small and about 0.12 dBm. Therefore, if one of modulation parameters is determined, the other modulation parameter should be adjusted finely to stabilize the millimeter-wave.
Next, we investigate the frequency tunability of generated millimeter-wave. Since the frequency of generated millimeter-wave is always equal to 4fm, the frequency tuning can be implemented through simply adjusting the modulation frequency fm. Generally, for different fm, the modulation power Pm required to generate pure millimeter-wave will be varied. In Fig. 6, we display the dependence of Pm required for locking on fm. From this diagram, it can be seen that, the higher fm is, the stronger Pm is needed. Here, the phase noise variance is estimated by integrating the single sideband power spectrum normalized to the central peak and offset by 3–800 MHz, and the micrwave signal is regarded as well locking if the phase noise variance is less than 0.1 rad2 . As shown in Fig. 6, there is a minimum required fm for generating high purity microwave by using 1/4 subharmonic modulation, and the lowest fm is about 13.58 GHz (the matched Pm is about 16.00 dBm). In order to avoid damage to the SL, Pm is controlled to be not more than 22.00 dBm. Under this condition, the maximum of fm is about 16.49 GHz, and the frequency of generated millimeter-wave can be up to 65.96 GHz ( = 4fm) for the OISL operating at P1 with a fundamental frequency of 49.43 GHz. Therefore, for fm is varied from 13.58 GHz to 16.49 GHz, the frequency fc of generated high-purity 60 GHz band millimeter-wave can be tuned in the region of (54.32 GHz-65.96 GHz) through selecting matched modulation power. Further experiments show that besides the case of f0 = 49.43 GHz, as long as the P1 frequency f0 is located at a reasonable frequency range of about 43.00 GHz-57.00 GHz, high purity 60 GHz band microwave signal can always be generated by adopting 1/4 subharmonic modulation.
To further understand the quality of generated millimeter-wave, Fig. 7 displays the SSB phase noise at 10 kHz offset frequency as a function of modulation frequency fm for the generated 4fm microwave signals showed in Fig. 6. Since there is a matched Pm for obtaining pure 4fm microwave signal for each fm, Fig. 7 can also be regarded the SSB phase noise at 10 kHz offset frequency as a function of Pm. As seen in this diagram, the SSB phase noises for all generated millimetre-waves are below −93 dBc/Hz at the 10 kHz offset frequency, and the lowest SSB phase noise is about −96 dBc/Hz at (fm, Pm) = (15.32 GHz, 20.00 dBm).
3.3 1/9 subharmonic modulation for generating 60 GHz band millimeter wave
Above experimental results have demonstrated that high-purity 60 GHz band microwave signal can be generated via 1/4 subharmonic modulation. In fact, our further experiments have also shown that high-purity 60 GHz band microwave signal can be generated via higher order subharmonic modulation. For simplicity, we take 1/9 subharmonic modulation as an example to present our results. Figures 8(a1) and 8(a2) show the optical spectrum and power spectrum of the SL under both optical injection and current modulation of (fm, Pm) = (7.23 GHz, 13.00 dBm), respectively, where the OISL operates at a P1 state with f0 = 45.02 GHz. The optical spectrum in Fig. 8(a1) shows that there are several components with similar amplitude, in which the strongest peak locates at an offset of 65.07 GHz ( = 9fm) with respect to the frequency of the injection light. As presented in Fig. 8(a2), the 60 GHz band microwave signal with a frequency of 65.07 GHz ( = 9fm) can be obtained. Besides the frequency component 9fm, other frequency components are also found in the power spectrum, and further investigations demonstrate that all of these frequency components can also be locked. For comparison, the corresponding results that the SL is only directly modulated are also given in Figs. 8(b1) and 8(b2). The optical spectrum shows that the SL operates out of the linear modulation area and possesses asymmetric distribution, which is different from that obtained in Fig. 4(b1). The reason is that the actual current amplitude loaded to SL under (fm, Pm) = (7.23 GHz, 13.00 dBm) may be larger than that under (15.32 GHz, 20.00 dBm). The power spectrum in Fig. 8(b2) shows no high-order harmonics at 9fm ( = 65.07 GHz) can be observed. Therefore, it could be deduced that the combination of P1 oscillation and subharmonic modulation under appropriate operation parameters generates pure 60 GHz band microwave signal. The physical reason behind is similar to the case for 1/4 subharmonic modulation.
Figure 9 shows the detailed power spectrum (Fig. 9(a)) and the SSB phase noise (Fig. 9(b)) centered at 65.07 GHz. As presented in Fig. 9(a), the 65.07 GHz millimeter-wave has a 3-dB linewidth below 1.6 kHz, and the difference between the central peak and the noise floor is about 61 dB. Meanwhile, the corresponding SSB phase noise shown in Fig. 9(b) is about −98 dBc/Hz at the 10 kHz offset frequency. Similar to 1/4 subharmonic modulation at f0 = 49.43 GHz, there also exists a small tunable fm (or Pm) range for fixed Pm = 13.00 dBm (or fm = 7.23 GHz). Figure 10 displays phase noise variance as a function of (a) modulation power Pm with fm = 7.23 GHz and (b) modulation frequency fm with Pm = 13.00 dBm. From this diagram, one can see that there exists a range for Pm (or fm) in which the phase noise variances can be decreased to a relative low level (about 0.08 rad2), and the minimum phase noise variances are 0.0642 rad2 emerged at Pm = 12.91 dBm for a fixed fm = 7.23 GHz and 0.0719 rad2 at fm = 7.247 GHz for a fixed Pm = 13.00 dBm, respectively. The reason for that the modulation parameters should be set within a range for well locking is easy to understand. As analyzed above, in order to achieve high-purity microwave, the components in optical spectrum of the SL under current modulation should be shifted to those frequencies deviating multiple fm from fi via the injection light, and therefore the value of Pm (fm) must be chosen under a fixed fm (Pm) for determined injection parameters. Due to higher-order subharmonic modulation and higher microwave frequency, the level (about 0.08 rad2) of the phase noise variances in the locking regions is higher than that under 1/2 or 1/3 subharmonic modulation reported in [32, 34]. Furthermore, the tunable modulation power range is about 0.22 dBm (from 12.85 to 13.07 dBm) around Pm = 13.00 dBm for a fixed fm = 7.23 GHz (see Fig. 10(a)), and the tunable modulation frequency range is about 36 MHz (from 7.22 to 7.256 GHz) around fm = 7.23 GHz for a fixed Pm = 13.00 dBm (see Fig. 10(b)), which corresponds to a change of about 324 MHz around 9fm = 65.07 GHz. The variation trend of the phase variance with Pm (or fm) is similar to that reported in , and its physical mechanism has been given in . As for the case of f0 = 49.43 GHz with 1/4 subharmonic modulation mentioned above, due to extremely small tuning ranges of fm (about 3.5 MHz) and Pm (about 0.12 dBm), we cannot fully show the phase noise variance as a function of Pm with fm = 15.32 GHz and fm with Pm = 20.00 dBm. Furthermore, compared 1/4 subharmonic modulation at f0 = 49.43 GHz with 1/9 subharmonic modulation at f0 = 45.02 GHz, we find that although the former uses stronger modulation power (about 20.00 dBm) to acquire pure 60 GHz band microwave signal, but the microwave quality and phase noise level is not better than that of the latter. It should be noted that, here the value of Pm only characterizes the output signal power from the microwave frequency synthesizer (MFS) and is not the actual current modulation power into the SL. Through comparing Fig. 4(b1) and Fig. 8(b1), it can be found that, for 1/4 subharmonic modulation with fm = 15.32 GHz, the current modulation with Pm = 20 dBm is weak modulation since the SL still operates at linear modulation area. However, for 1/9 subharmonic modulation with fm = 7.23 GHz, the current modulation with Pm = 13 dBm belongs to strong modulation since it has driven the SL to operate out of the linear modulation area. Therefore, it can be predicted that, through improving the response of the system to high frequency, the power of 1/4 subharmonic modulation required for generating high quality microwave can be decreased.
Figure 11 shows the frequency tunability of generated 60 GHz band millimeter-wave under 1/9 subharmonic modulation. From this diagram, one can see that the frequency of generated millimeter-wave is always equal to 9fm, which is similar to that under 1/4 subharmonic modulation. Also, there exists minimum required fm for generating high performance 60 GHz band microwave signal, which is about 7.05 GHz (the matched Pm is about 11.00 dBm). It should be pointed out that due to 67 GHz bandwidth limitation of the electrical spectrum analyzer (ESA) used in this experiment, the maximum of fm is about 7.40 GHz (the matched Pm is about 14.00 dBm). Therefore, by varying fm from 7.05 GHz to 7.40 GHz and matching suitable Pm, the frequency of 60 GHz band millimeter-wave can be tuned from 63.45 GHz to 66.60 GHz (about 3.15 GHz). With respect to 1/9 subharmonic locking, 1/4 subharmonic locking has a wider tunable frequency range (see Fig. 6), and the central frequency fc of generated high-purity 60 GHz band millimeter-wave can be tuned from 54.32 GHz to 65.96 GHz (about 11.64 GHz) through varying fm from 13.58 GHz to 16.49 GHz. Meanwhile as shown in Fig. 12, with the increase of fm, the SSB phase noises for all generated millimetre-waves are below −94 dBc/Hz at the 10 kHz offset frequency, and the lowest SSB phase noise is about −98 dBc/Hz at (fm, Pm) = (7.23 GHz, 13.00 dBm). Further experiments also shows that for other suitable f0, high purity 60 GHz band microwave signals can also be generated by adopting 1/9 subharmonic modulation.
Via 1/4 and 1/9 subharmonic modulation locking technique, the generation of high-purity 60 GHz band millimeter-wave is experimentally demonstrated based on an optically injected semiconductor laser (OISL) operating at P1 state. Under only optical injection with (Pi, fi) = (0.61 mW, 36.16 GHz) and (0.63 mW, 39.41 GHz), the optically injected SL is driven into P1 state with fundamental frequency f0 = 45.02 GHz and 49.43 GHz, and the 3-dB linewidth of generated millimeter-wave is about 2.0 MHz and 2.4 MHz, respectively. For the case of f0 = 49.43 GHz, through further introducing 1/4 subharmonic modulation with (fm, Pm) = (15.32 GHz, 20.00 dBm), a 61.28 GHz millimeter-wave with a linewidth below 1.6 kHz and a phase noise less than - 96 dBc/Hz is obtained. Moreover, through adjusting fm and selecting matched Pm, frequency-tunable millimeter-waves (within 54.32 GHz-65.96 GHz) with phase noises below −93 dBc/Hz can be achieved. For the case of f0 = 45.02 GHz, higher order 1/9 subharmonic modulation is introduced into the OISL for obtaining high-purity 60 GHz band microwave signal. With (fm, Pm) = (7.23 GHz, 13.00 dBm), a microwave signal at 65.07 GHz ( = 9fm) is also experimentally generated, whose linewidth is below 1.6 kHz and SSB phase noise at offset frequency 10 kHz is about −98 dBc/Hz. Meanwhile the 60 GHz band millimeter-wave can be tuned between 63.45 GHz and 66.60 GHz. This presented scheme for generating high-quality 60 GHz level microwave signals is relatively simple and low cost since no high-frequency microwave reference or electronic elements are needed. Though the high purity millimeter-wave generation beyond 100 GHz has not been implemented due to the limitation of test instrument, we believe that this technique possesses a potential to acquire a high quality millimeter-wave beyond 100 GHz. By adopting the scheme proposed by , the OISL under higher-order subharmonic modulation can be used for RoF uplink transmission. Furthermore, since multiple frequencies with equal spacing have been preliminarily observed in this work, this technique may possess a potential for generating microwave frequency comb, which deserves attention.
Natural National Science Foundation of China (NSFC) (61275116, 61475127, 61575163); Natural Science Foundation of Chongqing City (CSTC2016jcyjA0575); Fundamental Research Funds for the Central Universities of China (XDJK2014C120, XDJK2014C079).
The authors would like to thank the reviewers for their professional and valuable suggestions to help us further survey and improve this research work.
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