figure 9


Figure 9 Measured phase noise of the X-Band (a) and K-Band (b) OEOs with SIL = 3 km and DSPLL = 3 and 8 km.

Figure 8 shows the phase noise measurement setup for the synthesizer. A Keysight E3631A is adjusted in constant current mode for tuning the YIG filter, and a Rohde & Schwarz FSWP is used for the phase noise measurement. At X-Band, the single sideband (SSB) phase noise is −110 dBc/Hz at 1 kHz from the carrier and −137 dBc/Hz at 10 kHz from the carrier for carrier frequencies from 8 to 12 GHz. In the time domain, this translates to 4.395 fs measured at side-mode markers of 35 and 200 kHz from the carrier. At K-Band frequencies, SSB phase noise is −103 dBc/Hz at 1 kHz from the carrier and −128 dBc/Hz at 10 kHz from the carrier for frequencies from 16 to 24 GHz. This translates to a time domain response of 6.961 fs measured at side-mode markers of 35 and 200 kHz from the carrier. For demonstration, the overall system is mounted in a 19-in. rack; the size can be reduced for specific applications. Figure 9 shows X- and K-Band phase noise measurements for different fiber lengths.

In terms of size and power, the YIG filter is the dominant component in these opto-electronically driven frequency synthesizers. The main current consumer is the YIG filter, which draws 150 mA at +10 VDC and consumes about 1.5 W power. The amplifier, with two channels, draws 80 to 160 mA at +10 VDC and consumes as much as 1.6 W power. The mixer and LPFA, which use a combination of frequency translation and filtering to extract the RF/microwave signals from the higher frequency optical signals, draw about 110 mA (60 + 5 + 45 mA) from +15, +5 and −5 VDC supplies, respectively. In stark contrast, the photo detector uses very low current and power, with its three cells each drawing about 10 mA (30 mA total) at +5 VDC and using about 0.15 W power. The broadband dual-channel amplifier draws roughly 80 mA per channel from a +10 V supply, or 160 mA and 1.6 W total.

Monolithic OEOs

Recently, Tang et al.24 demonstrated an integrated OEO (see Figure 10), where both the optical and electrical parts are packaged on a 5 × 6 cm2 printed circuit board. The measured phase noise for an oscillation frequency of 7.30 GHz was −91 dBc/Hz at 1 MHz offset, with an injection current of 44 mA. The reported integrated solution is not attractive because of limited tuning and poor phase noise performance due to the high RIN of the directly modulated laser.

figure 10


Figure 10 IOEO block diagram (a), assembly (b) and photonic components (c).

figure 11


Figure 11 Block diagram of RF beat-note generating laser system using DBR multi-mode laser pairs.26

In this work, integrated topologies using monolithic fabrication techniques compatible with Si photonics are explored to reduce size and cost while improving temperature sensitivity. A chip-level multi-mode laser generates beat-notes at RF25 but suffers from poor phase noise characteristics. The concept in Figure 11 shows an alternative laser configuration, consisting of four sections26 consisting of distributed Bragg reflector (DBR), gain, phase tuning and electro-absorption modulator. The DBR is used as a filter to select the laser output frequency.27 The phase tuning section,28 the phase modulator in the DBR laser, performs frequency tuning. Varying the DC bias voltages drives different output frequencies from each multi-mode laser.29 The output Y-junction provides an input to a high speed photodetector for efficient detection of the ultra-stable RF beat note. Gain is provided by an InGaAsP-InAsP multi-quantum well structure for operation at about 1550 nm, where a threshold current of about 30 mA is estimated. Work is in progress for fabricating these designs in monolithically integrated components.30


The described SILDPIL OEO synthesizer in a 19-in. rack-mount enclosure is based on patented techniques18-19 for portability; the size can be reduced for specific applications and requirements. Work is progressing to develop a monolithic integrated solution for hybrid optoelectronic systems that combines the integrated microwave and photonics circuits on-chip. Recent development of photonics integration material platforms, including SOI, InP and Si3N4, opens the way for an OEO on-chip for 5G applications.


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