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Abstract
Small-sized, ultra-low phase noise photonic microwave oscillators at 10, 20, 30, and 40 GHz are demonstrated using electro-optical frequency division. At 40 GHz, a record-low, to our knowledge, phase noise of ${-}{153}\;{\rm dBc/Hz}$ is achieved (10 kHz offset).
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Low phase noise microwave oscillators are ubiquitous across a wide range of application areas including modern communications, test and measurement, aerospace and defense, advanced digital signal processing, and radio astronomy. During the last decade, photonic generation of microwave signals based on optical frequency division (OFD) has become a paradigm shift in microwave signal generation [1]. OFD has led to the generation of microwave signals with the lowest phase noise at 10 GHz [1,2], and 10 GHz clocks with ${10^{- 18}}$ stability [3]. In OFD, the absolute frequency stability of an ultra-stable reference laser is transferred to the microwave domain via an octave-spanning mode-locked-laser frequency comb. OFD microwave systems also typically include a pulse interleaver and a highly linear photodetector (to minimize amplitude noise (AM) to phase noise (PM) conversion in the optical-to-electrical conversion process [1]). A new variation of OFD is electro-OFD (eOFD) [4,5] (schematic in Fig. 1), which uses an electro-optical (EO) comb as opposed to a mode-locked frequency comb. eOFD transfers the relative (as opposed to absolute) frequency stability of a dual-frequency reference to a microwave-rate voltage-controlled oscillator (VCO) that drives the EO modulators used to produce the comb. By extracting microwave power directly from the stabilized VCO, it avoids the optical pulse detection step and also results in higher power microwave signals.
Fig. 1. Architecture of electro-optical frequency division (eOFD) based on dual laser reference, electro-optical frequency comb, and a microwave rate VCO.
Other important considerations for oscillators include low amplitude noise and, for field deployment, small size, light weight, low power consumption, and robustness against temperature variation. Here, we report small, field-deployable, ultra-low phase noise (ULPN) eOFD oscillators operating across a range of microwave frequencies (10, 20, 30, and 40 GHz). The phase noise at 40 GHz is a record low compared to all photonic and electronic oscillators, and the oscillator has size ${25\, \rm {cm}} \times \;{23 \,\rm{cm}} \times \; {5 \,\rm{cm}}$.
The architecture of the oscillators at all frequencies is nominally the same as in [4]. However, the greatly enhanced performance and small size result from an improved reference described in patents [6–8] and development of compact electronic control circuits. To achieve 10 GHz eOFD output frequency, a 10 GHz VCO is used to generate dual EO combs by sending the dual laser reference through a fast EO modulator. The 10 GHz VCO is then servo-controlled in a feedback loop via detection of the amplified VCO-phase error from the dual EO combs [4]. Representative single-side-band (SSB) phase noise of a 10 GHz eOFD oscillator is shown in Fig. 2(a) (blue curve). The measured SSB phase noise is ${-}{154}\;{\rm dBc/Hz}$ at 10 kHz offset, and ${-}{160}\;{\rm dBc/Hz}$ at 100 kHz offset (10 GHz carrier). The measured AM noise of the oscillator is also shown in Fig. 2(a) (green curve) with a level of ${-}{158}\;{\rm dBc/Hz}$ at 10 kHz and ${-}{162}\;{\rm dBc/Hz}$ at 100 kHz offset. The phase noise and AM noise are measured using a signal source analyzer. RF power is ${\gt}{23}\;{\rm dBm}$, and the units use about 50 Watts wall-plug power provided from a standard 12 V DC power supply.
The eOFD oscillator center frequency is varied by selection of the VCO frequency (which results in EO tuning of the EO-comb line spacing). Figure 2(b) shows the SSB phase noise and AM noise for a ULPN 20 GHz eOFD oscillator. Although not shown, a 30 GHz eOFD oscillator with similar performance was also built and tested.
Designing high spectral purity microwave oscillators at higher Ka band frequencies (26.5–40 GHz) is quite challenging due to the frequency limitations in active devices [9,10]. Also, due to multiplicative/additive phase noise, low phase noise performance cannot be readily transferred to higher frequencies in the Ka band from lower frequencies using multipliers, mixers, and amplifiers [9,10]. Other types of ULPN photonic microwave oscillators, such as OFD photonic microwave oscillators [1,2], opto-electronic oscillators [11], and monolithic 1 GHz mode locked lasers (with pulse interleavers) [12] have so far operated in the X band (8–12 GHz).
A 40 GHz ULPN eOFD oscillator having a size of ${25\, \rm {cm}} \times \;{23 \,\rm{cm}} \times \; {5 \,\rm{cm}}$ is shown in Fig. 3 and includes all electrical control and power conditioning circuits. To characterize the phase noise performance, two similar 40 GHz eOFD oscillators were built and their joint phase noise was measured. The measured phase noise for each 40 GHz oscillator is ${-}{135}\;{\rm dBc/Hz}$ at 1 kHz offset and ${-}{153}\;{\rm dBc/Hz}$ at 10 kHz and 100 kHz offset (40 GHz carrier). The 10 kHz offset phase noise (${-}{153}\;{\rm dBc/Hz}$ for 40 GHz carrier) is a record-low phase noise level for all oscillators at 40 GHz (electrical or photonic based), to the best of our knowledge. For comparison, the phase noise values of a 40 GHz Keysight PSG signal generator (red markers) and a research-grade 40 GHz air–dielectric aluminum-cavity-stabilized oscillator (black markers) [9] are also shown in Fig. 3. As with the other oscillators, this 40 GHz unit is powered by a single 12 V DC supply with simple on/off control (and remote USB control).
Fig. 3. SSB phase noise (blue) of a 40 GHz ULPN eOFD oscillator. For comparison, the phase noises of a 40 GHz Keysight PSG generator (red) and 40 GHz cavity stabilized oscillator (black) [9] are shown. Inset: photograph of the 40 GHz unit (${25\, \rm {cm}} \times \;{23 \,\rm{cm}} \times \; {5 \,\rm{cm}}$). Right axis: SSB phase noise in ${{\rm rad}^2}/{\rm Hz}$.
All of the e-OFD oscillators can be synchronized to an external clock. Figure 4(a) shows the measured frequency of the 40 GHz ULPN eOFD oscillator when it is locked to an external 1.25 GHz clock source for a period of 500 h. Figure 4(b) shows the measured ambient room temperature while performing the frequency counter measurement in Fig. 4(a). The 40 GHz eOFD oscillator maintained stable locked operation to the external clock, even while the room temperature cycled many times with temperature swings of 9°C.
Fig. 4. (a) Frequency counter measurement over 500 h of operation of the 40 GHz ULPN eOFD oscillator while it is synchronized to an external 1.25 GHz clock. (b) Ambient room temperature for measurement in (a).
We report small-sized, ULPN photonic microwave oscillators at X to Ka band output frequencies (SSB phase noise at 10 kHz offset of ${-}{154}$, ${-}{150}$, and ${-}{153}\;{\rm dBc/Hz}$ for 10, 20, and 40 GHz carrier frequencies, respectively). The units also feature ultra-low AM noise. The oscillators have high RF output power (${\gt}{23}\;{\rm dBm}$), are robust against ambient temperature variations, and operate with a single 12 V power supply (50 W wall-plug power typical). With advanced EO-comb generation, wall-plug power approaching 20 W is possible. These oscillators significantly advance field-deployable, ULPN photonic microwave oscillators from 10 to 40 GHz.
Funding
Defense Advanced Research Projects Agency (W912CG-20-C-0034).
Acknowledgment
The views, opinions, and/or findings expressed are those of the author(s) and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government. Distribution Statement “A” (Approved for Public Release, Distribution Unlimited).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results in this paper may be obtained from the authors upon reasonable request.
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