Microwave and millimeter-wave measurements using photonics

Measuring micro- and millimeter waves with large instantaneous bandwidth and wide frequency coverage (e.g., from several MHz to a hundred GHz or wider) is necessary for applications including medicine, wireless communications, radar, and electronic warfare. (Electronic warfare includes controlling the electromagnetic spectrum by communications jamming, measures to protect against guided missiles, or interception of enemy signals.) However, the so-called electronic bottleneck—the inability of electronic circuits to process data at sufficiently high speeds—hinders purely electronic measurements to meet these stringent requirements.

Fortunately, photonics offer an intrinsic large instantaneous bandwidth that enables nearly real-time operation over a large frequency range for wideband microwave and millimeter-wave measurements.¹In addition, photonic methods also have advantages such as immunity to electromagnetic interference, low loss, and compact size. Consequently, we have proposed several photonic approaches for microwave and millimeter-wave measurements of frequency, angle-of-arrival (AOA), and Doppler frequency shift (DFS).^2–4

For instantaneous frequency measurement, we have proposed a photonic approach using a complementary optical filter pair.² As shown in Figure 1, a microwave/millimeter-wave signal is applied to an electro-optic modulator (EOM, a key device for transferring electrical signals to optical waves). The EOM is biased at the minimum transmission point to suppress the optical carrier, and the carrier-suppressed optical signal is then sent to the complementary optical filter pair with two complementary transmission responses. The microwave frequency is subsequently measured by monitoring the filtered optical powers with two optical power meters. Measurement errors are less than ±0.2GHz across the entire measurement range from 1 to 26GHz.

Figure 1. Photonic approach to instantaneous frequency measurement. A, B: Filters. LD: Laser diode. EOM: Electro-optic modulator.

For AOA measurement, we have proposed a photonic approach using two EOMs, both of which are biased at the minimum transmission point to suppress the optical carrier.³ As shown in Figure 2, two optical components at the carrier wavelength are generated at the output of the second EOM. Their total power is a function of the phase shift induced by the AOA. We simply measure the optical power to obtain the phase shift and hence calculate the AOA (i.e., θ). In the experiment, the phase shift of −160^° to 40^° was measured for a microwave signal at 18GHz with measurement errors less than ±2.5^°.

Figure 2. Photonic approach to angle-of-arrival (AOA) measurement. θ: Phase shift induced by the AOA.

For DFS measurement, we developed a photonic approach able to provide fine resolution and wide frequency coverage.⁴As shown in Figure 3, the DFS (i.e., f_D) between the transmitted signal and the received echo signal is mapped into a doubled spacing between two generated optical sidebands. Subsequently, f_D is extracted by analyzing the spectrum of a low-frequency electrical signal generated from the frequency beating between the two optical sidebands. The measurement errors in f_Dare less than ±5×10⁻¹⁰Hz between −9 and 9kHz for a millimeter signal at 30GHz. Correspondingly, the estimated f_D corresponds to a measured radial velocity of 0–450m/s with a resolution of 5×10⁻¹²m/s. This photonic approach is totally independent of the carrier frequency, allowing a wide-frequency-range operation from the L to W bands (i.e., 1–110GHz, according to the IEEE standard). This frequency range covers most applications, including wireless Internet, radio-frequency identification, cell phones, the global positioning system, satellite television and communications, military/civil radar, and radio astronomy.

Figure 3. (a) Schematic diagram and (b) experimental setup for measuring f_D, the Doppler frequency shift (DFS). EDFA: Erbium-doped fiber amplifier. ESA: Electrical spectrum analyzer. GPIB: General-purpose interface bus. HER-Pol: High-extinction-ratio polarizer. MSG: Microwave signal generator. PC: Polarization controller. PD: Photodetector. PM: Phase modulator. PolM: Polarization modulator.)

In summary, we have proposed and demonstrated photonic approaches to measurements of frequency, AOA, and DFS for micro- and millimeter waves. These approaches can find versatile applications in civil and defense systems using high-frequency radiation, wide frequency coverage, and large instantaneous bandwidth, such as 5G wireless communications, radar, and electronic warfare. In future work, we will develop photonic integrated chips and multiple-dimension detection for micro- and millimeter-wave measurements, in terms of high stability and diverse functionality.

This work was supported in part by the National High Technology Research and Development Program of China (SS2015AA012303), the National Basic Research Program of China (2012CB315704), the Program for New Century Excellent Talents in University of China (NCET-12-0940), and the Sichuan Youth Science and Technology Foundation of China (2015JQ0032). X. Zou was also supported by a fellowship from the Alexander von Humboldt Foundation, Germany.