Orthogonally polarized RF optical single sideband generation with integrated ring resonators

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本站小编 Free考研考试/2022-01-01

1.
Introduction

Photonic microwave and radio frequency (RF) signal processing^[1-4] has attracted substantial attention for a wide range of applications because of the broad RF bandwidths that it can achieve, its high overall performance, its intrinsically lower loss, its very high reconfigurability and versatility, and finally its extremely high level of insensitivity to electro-magnetic interference. Its main applications lie in the areas of radar, communications and sensing. Many different capabilities have been achieved, such as advanced encoding format modulators^[5-10], devices for RF beam shaping^[11], RF signal processors based on transverse filtering^[12-14], RF channelizers^[15] and spectrum analysis^[16-18], and much more^[19-37].

For RF photonic-based systems, the optical RF signal modulation format directly impacts its transmission capacity as well as its spectral efficiency. Hence, this is a key factor in the design of state-of-the-art photonic RF transmitters^[38]. One advanced modulation format, optical single sideband (OSSB) modulation, has attracted significant attention because of its high spectral efficiency as well as its immunity to distortion induced by dispersion. Recently, a variation on this, termed orthogonally polarized OSSB modulation, has been proposed to increase the system flexibility as well as spectral efficiency. In this approach, the optical sideband and the carrier are polarized in orthogonal directions. Another big advantage of orthogonally polarized OSSB modulation is that it allows the individual control of both the optical sideband and the carrier by using polarization selective components. Hence, it has been employed in many applications from photonic microwave signal processing to antenna beamforming systems^[38-42]. Many methods have been proposed to realize orthogonally polarized OSSB modulation, such as a differential group delay module^[41], acousto-optic modulation^[42], modulators based on Sagnac-loops^[43], quadrature phase shift keying dual-polarization modulation^[44], and polarization-based modulation^{[45, 46]}. Stimulated Brillouin scattering in optical fibers has been reported, in order to control the optical signal polarization^{[47, 48]}. Many of these methods, however, are limited in some respect. RF hybrid couplers create a bottleneck in the system bandwidth, while fiber devices are bulky and limited in size reduction and stability for practical real-world applications.

Other key components for RF systems are RF equalizers, which compensate any imbalance in passive component frequency responses or variations in the gain profile of RF amplifiers^{[49, 50]}. Significant work has been done on photonic-based RF equalizers including devices based on substrate-integrated waveguides^{[51, 52]}. However, these methods have some limitations, including, again, a limited electronic bandwidth, and equalization resolution (minimum equalizing RF bandwidth), lack of RF frequency tunability, and limited transmission spectrum dynamic range. Thus, for advanced RF systems that need high-resolution, wideband RF equalization, and a large dynamic-range, more work is needed. A promising technique that can address these challenges is through the use of integrated microwave photonics^[53]. Our approach also offers all of the attractions of photonic integrated devices, which include a very small size, mass producibility, low power consumption, and high reliability^[54].

We review recent work on orthogonally polarized OSSB generation and a dual-channel RF equalizer^{[19, 20]} achieved through the use of integrated dual-polarization micro-ring resonators (DP-MRR). This work involves both fixed^[19] RF frequency and tuneable RF frequency devices^[20]. For fixed devices, a spectral interval of ~16.6 and ~32.2 GHz between the TE and TM polarized resonances of the DP-MRR is achieved. Lithographic control of the DP-MRR orthogonally polarized resonant mode refractive indices can realize waveguides that have a virtually symmetric cross section, supporting both TM and TE polarizations, yet at the same time displaying quite wide TE to TM polarized spectral intervals of ~16.6 or ~32.2 GHz in the C-band. Hence, the optical sideband and carrier can be separated by looking at the drop-port of the orthogonally polarized resonances, which achieves orthogonally polarized OSSB modulation. Further, we convert the signal automatically from phase modulation to intensity modulation by using the notches at the resonator through-port to suppress one of the sidebands, thus enabling dual-channel RF filtering and equalization. Finally, a large dynamic tuning range of over 55 dB for both the optical carrier-to-sideband ratio of the OSSB signal can be achieved by controlling the polarization angle and extinction ratio of the dual-channel RF equalizer. Subsequently, a continuously RF frequency tunable OP-OSSB generator was reported^[20], based on two different cascaded dual integrated MRRs. The operation RF frequency of the tuneable OP-OSSB generator was determined by the TE to TM resonance spectral interval, which could be tuned dynamically using individually controlled thermo-optical elements for each of the MRRs. This resulted in an operation bandwidth with a very wide RF tuning range. Further, by controlling the input light polarization angle, we achieved a large optical carrier-to-sideband ratio (OCSR) dynamic tuning range of 57.3 dB. These results emphasize the attractiveness of using polarization selective integrated micro-ring resonators to realize orthogonally polarized optical single sideband modulators for RF signals at frequencies as high as 100 GHz.

2.
Integrated microring resonators

The integrated micro-ring resonators (Fig. 1) that formed the core components of the system were based on Hydex glass, a high-index doped silica platform that features CMOS compatible processes^[55–72]. First, a high refractive index (n = ~1.70 at 1550 nm) Hydex core was grown via PECVD (plasma-enhanced chemical vapour deposition), followed by patterning via a UV stepper mask aligner-based lithographically. Waveguides were then formed by dry etching (RIE - reactive ion etching) that achieved very low surface roughness. Finally, an upper cladding consisting of silica glass (n = ~1.44 at 1550 nm) was grown. The radius of the ring with the lower FSR of 49 GHz (or 0.4 nm), used for the fixed RF frequency device, was 592 μm. The insertion loss of the through-port was 1.5 dB, via butt coupling with fiber pigtails and on-chip mode converters. The waveguide was almost symmetric in cross-section (1.5 × 2 μm²) so that the MRRs could exhibit TM and TE polarization modes. The theoretical TM and TE modes [Figs. 1(b) and 1(c)] had effective indices n_{eff_TM} = 1.624 and n_{eff_TE} = 1.627, which produced marginally different FSRs for the two polarizations, and with a comparatively wide TE/TM offset of δ_TE//TM = ~16.6 GHz (Fig. 2) at 1550 nm. The two polarizations had comparable linewidths of ~ 140 MHz, or Q factors of about 1.2 million^[61-65], and with a –20 dB bandwidth of ~ 1 GHz [Fig. 2(b)]. This narrow optical linewidth resulted in a very narrowband RF operation and provided a very sharp optical filtering slope. This enabled operation as low as 500 MHz, with a suppression of better than –20 dB for unwanted sidebands. The RF frequency can be varied by changing the TE/TM mode separation during fabrication by the lithographic design of the waveguide ring radius and cross-section. Hence, the frequency that was demonstrated here at 16.6-GHz is not a limitation of the orthogonally polarized OSSB generator or of the RF equalizer either. Moreover, by using the complementary resonance, a second frequency of operation at 32.4 GHz (= FSR ? δ_TE//TM) can also be realized. In general, ring resonators having FSRs of 200 GHz or even larger^[73] can be achieved, thus yielding operation at even higher RF frequency.

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