COMPACT X-BAND SYNTHESIZER

The intermodal oscillation frequency of a MML can be further stabilized for a frequency synthesizer by combining self-forced-oscillation and SML. The phase noise comparison is shown in Figure 9, where the self-mode-locked modes are frequency tuned and employ a SILTPLL to reduce the phase noise by an additional 10 dB at offset frequencies below 100 kHz.

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This improvement is attractive to realize highly stable synthesizers. The U.S. Defense Advanced Research Project Agency (DARPA) has recently challenged the RF synthesizer community with the following performance requirements under the GRYPHON Program:19 stable frequency synthesis from 1 to 40 GHz packaged in under 10 cm3, with phase noise no greater than -150 dBc/Hz at 10 kHz offset from a 10 GHz carrier with a 6 dB/octave roll-off, and the capability to operate in a harsh military environment (-40°C to +85°C temperature range and 40 g vibration). DARPA’s design challenge can be met using the unique features of the MML: 1) the intermodal oscillation frequency can be modified by adjusting the common DBR section length while optimizing the multiple quantum well structure by increasing SOA gain, phase modulation sensitivity and effective cavity length; 2) the optical fiber delay elements can be replaced with cascaded, high-quality factor resonators.20-23 Increasing the cavity length and DBR bandwidth accommodates a large number of modes for the MML. Figure 10 compares the close-in phase noise of the SML as the mode number increases from 5 to 201, showing a significant reduction in phase noise, which achieves the performance requirements of the GRYPHON program.

Figure 10

Figure 10 Phase noise vs. MML mode number. With 201 modes, the phase noise is –156 dBc/Hz at 10 kHz offset from a 10 GHz carrier.

Figure 11

Figure 11 Simulated performance of a 61-mode MML SML, SML with ±0.5 degree random phase error and corrected using SEILDPLL (delays of 5 ms for SEIL and 5 μs and 15 μs for SDPLL with ±0.5 degree phase error SML).

 

As the number of modes increases, phase-locking using SML becomes more sensitive to any internal phase degradation. Referring to Figure 11, note the simulated performance comparison of 61 modes of SML with no phase error and the case with ±0.5 degree of random phase error, where the phase noise degrades by 10 dB. However, when the self-electrical-injection locking and dual self-phase-locked loops (SEILDPLL) is introduced with random phase error using fiber delays of 1 and 3 km, phase noise of -140 dBc/Hz at 10 kHz offset from the carrier is achieved. Similar performance is predicted for cases of a self-forced MML with mode numbers larger than 61. These cases indicate the utility of SML and SEILDPLL to improve the stability of free-running intermodal oscillators to 40 GHz.

To meet DARPA’s 10 cm3 package size, the suggested approach incorporates a heterogeneously integrated InP MML with optical delay elements fabricated with silicon photonics (SiP). A conceptual block diagram is shown in Figure 12, where SiP is used for the passive optical couplers and delay elements, combined with integrated electronics using a low phase noise SiGe RF amplifier, phase detector and loop-filter amplifier as part of the SEILPLL. The green dotted line encloses the InP material heterogeneously mounted on a Si substrate. The black rectangle includes the SiP chip and the overall assembly is outlined in purple. The optical delay elements are realized using high-quality micro-disk resonators.

Figure 12

Figure 12 Conceptual block diagram of a forced oscillation, mode-locked, multi-mode laser synthesizer on a ~10 mm x ~4.2 mm silicon chip.

CONCLUSION

Optoelectronic techniques are quite viable for implementing high stability microwave frequency synthesizers. Significant improvement in frequency stability is attained using custom designed modular implementation of OEO based on the self-forced oscillation technique of SILPLL.3 To reduce size, integrated solutions of SILPLL24 are considered by integrating low noise RFIC using SiGe technology with SiP based optical modulators.25 Compact design of the frequency synthesizer is demonstrated by employing InP-based MML and intermodal oscillation frequency stabilization using concept of SML26 combined with SILPLL using high Q compact resonators.27 A combined design following the intellectual properties described here could potentially meet the stringent requirements of the DARPA’s GRYPHON Program, which has challenged the technical community, a compact RF synthesizer using a heterogeneously integrated InP MML chip on SiP is proposed that will provide 1) low phase noise (-150 dBc/Hz at 10 kHz offset from a 10 GHz carrier), 2) broadband tuning (1 to 40 GHz), 3) small size (≤ 10 cm3) and 4) operation in a rugged environment (-40°C to +85°C temperature range and 40 g vibration). A compact RF synthesizer with a very low phase noise in microwave frequency range could be envisioned by relying on the self-mode locking of a large number of modes of the presented MML with a tunable intermodal RF. Addition of a self-forced oscillation technique assures low phase noise of RF signal over the challenging environments.

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