2.1

Operating principle and device under test

To build modulators with operation frequencies reaching the sub-THz range we use structures capable of guiding and processing surface plasmon polaritons (SPP). SPPs are electromagnetic surface waves existing at the interface between a metal and an insulator[15]. The guiding structure consists of a metallic slot waveguide, having a width of a few tens of nanometres, created between two metal electrodes, as visible in Fig. 1(b2). Light is fed to the slot where it excites the SPP using a tapered silicon waveguide. When the slot is filled with an organic material displaying second-order optical nonlinearity[16], an electric field applied between the electrodes will induce a linear variation of the material refractive index by means of Pockels effect. This structure therefore implements an electro-optic phase modulator. High modulation efficiency is achieved thanks to the strong light confinement in the slot as well as the excellent overlap between the optical and the electrical field profiles. This, in turn, allows to keep the modulator short (few tens of micrometres) which limits the SPP propagation losses. In addition, such modulator displays very high speeds thanks to its very low RC constant; the high conductivity of the metal electrodes yields sub-1 Ω resistances, and the small slot height (approx. 150 nm) and length (few tens of micrometres) keeps capacitance to the femtofarad range, yielding cut-off in the THz range. To keep parasitic capacitances low, the metallic electrical pads of the modulator are kept as small as possible (while still allowing electrical contact to the device using a probe with 50 μm pitch) and a ground-signal-ground configuration. Further details on the electrical modelling of the modulator are available in[17].

An intensity modulator is created by placing two phase modulators in an interferometer structure. In detail, the device under tests in Fig. 1(b1) includes one 20 μm–long POH phase modulator in each arm, Fig. 1(b2). Each phase modulator uses a 120 nm-wide slot and the metal-insulator-metal (MIM) waveguide is filled with the organic electro-optic material HD-BB-OH/YLD124[18]. The silicon linear tapers coupling light to and from the slot[19] are 1 micron-long and display a loss of approximately 1.1 dB each. Grating coupler losses are in the order of 5 dB/coupler. By cut-back measurement, losses in the plasmonic waveguide amount to approximately 0.5 dB/μm.

2.2

Frequency response and linearity

In view of application to analog radio-over-fiber links, we performed experiments to characterize relevant characteristics such as frequency response, nonlinear distortions and power handling of the POH-MZM.

The frequency response was tested over the range from 75 MHz up to 500 GHz, limited by the available test equipment. A continuous-wave (CW) laser at a wavelength of 1547.5 nm with 0 dBm optical power was fed into the POH-MZM.

We used two different setups to generate modulating carriers in the frequency ranges 15-70 GHz and 200-500 GHz. In the lower frequency range, the signal was generated by electrical means. In the frequency range between 200 and 500 GHz, the test signal was generated optically by heterodyning two tuneable laser sources on a UTC-PD to produce a THz signal at a frequency equal to the difference between the ones of the two lasers. An optical spectrum analyser (OSA) is used to measure the amplitude ratio between the optical carrier and the first sidelobe. The resulting frequency response is displayed in Fig. 2(a) and shows a flat response over the complete 75 MHz to 500 GHz range, and the response never crosses the -3 dB line compared to the low-frequency value. For comparison, the grey lines in the same figure display the responses measured in a previous experiment by Hoessbacher et al. in[20], who obtained them with a similar POH-MZM device. Complete details on the experimental setups and the procedure used to measure the modulator response are reported in[14].

Figure 2.

(a) Measured response of the POH-MZM over the frequency range 75 MHz-500 GHz. The figure is obtained normalizing the measured power of the optical modulation sidebands, to the electrical power available at the input of the POH-MZM. The device features a flat response up to 500 GHz. (b-c) Second-order (IMD2) and third-order (IMD3) intermodulation distortions for the POH-MZM (a) and a commercial GaAs modulator (b). The horizontal axis indicates the available RF power at the modulator input generated by the source. The third-order input intercept point for the POH-MZM is at 18.9 dBm, very close to the 19.0 dBm obtained with a commercial high-performance GaAs modulator with the same half-wave voltage. Image edited from[14].

Modulator linearity is a crucial parameter in all analog applications because it affects the system spurious-free dynamic range (SFDR) directly[10, 21]. The SFDR is a quantitative indication of the realistic power range over which a two-port network is capable of operating in a linear regime without being limited by noise[10]. While noise poses a lower bound to this power range, nonlinear distortions (e.g. generation of spurious tones), determine the upper bound. A typical way of probing nonlinear distortions is the method known as two-tone test[21]. With this technique we have characterized the second- and third-order intermodulation distortions (IMD2 and IMD3), which limit the SFDR in sub-octave and multi-octave conditions, respectively[21]. In detail, we have compared the input intercept point of the POH-MZM with the one of a commercial modulator with the same half-wave voltage (U²T model MZMO2120: V_π = 3 V, electro optical response of 33 GHz at 3 dB, typical insertion loss of 5 dB, typical extinction ratio of 23 dB), see Fig. 2(b-c). We obtained a value of 18.9 dBm, versus a value of 19 dBm for the commercial device. This result proves that the the intermodulation distortions from the POH-MZM are as low as those obtained from a commercial modulator optimized for high-performance analog applications, with the same half-wave voltage.

3.1

Sub-mm-wave analog photonic links

A possible application enabled by the modulator in Fig. 1 is the creation of analog Radio-over-Fiber (RoF) links at sub-THz frequencies. We have implemented such link as shown in Fig. 3(a-b), demonstrating direct THz-to-optical-to-THz conversion over >100 GHz bandwidth (220-325 GHz range), only limited by our electrical test equipment.

Figure 3.

(a) Photonic link scenario connecting a remote antenna to a central office. (b) Experimental setup. The POH-MZM modulates an optical carrier f_carrier with a mm-wave signal in the 220-325 GHz frequency range generated by UTC-PD 1. The modulated carrier is transported on a single-mode fiber (SMF), is amplified by an erbium-doped fiber amplifier (EDFA) and is received by UTC-PD 2. The detected mm-wave is analyzed with an electrical spectrum analyzer (ESA) using a mm-wave extension module. (c) Plasmonic organic hybrid Mach-Zehnder modulator; (d) false-color scanning-electron microscope image of a plasmonic phase modulator. Image edited from[14].

The signal to be transmitted along the link is generated using UTC-PD 1, emulating the antenna source in Fig. 3(a), and are used to directly modulate an optical carrier using a POH-MZM, Fig. 1(c). The modulated signal, after propagation through the single-mode fiber link, is amplified using a low-noise erbium-doped fiber amplifier (EDFA). The mm-waves are then detected with another UTC-PD (UTC-PD2) and are analysed using an electrical spectrum analyser (ESA) equipped with a mm-wave extension module. The transmitted (red circles) and received power spectrum (blue triangles) are displayed in Fig. 3(c). The link gain is obtained as the ratio of these two quantities and is plotted in Fig. 3(d). The link features a flat gain response over the complete 220-325 GHz frequency range. A ripple of ±2.1 dB across the band is attributed to ripples in the attenuator loss, which could not be tested in our laboratory at the time of the experiment.

The noise figure of an analog link can be written as[10]

where P_N is the total noise power dissipated on the load. In a basic externally-modulated photonic link, composed by a Mach-Zehnder modulator, a fiber link, and EDFA and a photodetector, the total link noise power is given by the sum of different noise terms:

The NF can be calculated to be link to be approximately 45.8 dB at 300 GHz. Improving the fiber-to-chip coupling loss from the current 5 dB to state-of-the-art values near 2.5 dB, the overall noise figure of the link improves from 45.8 dB to 40.3 dB and the link gain improves from -24 dB to -14 dB.

Based on the noise and linearity characterization described in the previous section, it is possible to determine the spurious-free dynamic range of the analog optical link, as shown in Fig. 4. For the analog link test, we used a modulator with a higher V_π,RF (6.7 V instead of 3 V) and a lower loss compared to the one used in the nonlinear distortion test. The power of the third order intermodulation terms (IMD3) reduces as the sixth power of V_π,RF and as the second power of the loss L, according to the equation[10, 22], yielding a reduction of IMD3 power of 34.5 dB. This results in an IIP3 of 38.1 dBm and an SFDR3 of 105.2 dB/Hz^2/3, as shown in Fig. 3. Similarly, the IMD2 power reduces by the fourth power of V_π,RF and as the second power of the loss L, by 27.6 dB, and the SFDR2 increases to 109.48 dB/Hz^1/2, as shown in Fig. 3. A detailed analysis of the performance metrics of this link (link gain, noise figure and spurious free dynamic range) are reported in[14].

Figure 4.

Estimation of spurious-free dynamic range of the 325 GHz analog photonic link. Edited from[14].

3.2

Plasmonic mixers

Seamless integration of fiber and wireless networks at millimetre and terahertz frequencies demands for high-speed and efficient optical-to-wireless and wireless-to-optical converters. Wireless-to-optical conversion is generally achieved through electrical amplification and mixing followed by modulation. This normally requires separate electrical and optical high-speed components, which are complex and whose interfacing is not trivial at those high speeds. Recent efforts demonstrated Mach-Zehnder modulators directly driven by arrays of patch antennas and polarization-reversed structures, capable of converting the wireless signal to a single-sideband optical signal in the 60 GHz band[23].

Recently, Salamin et al. successfully transmitted 20 and 10 Gbit/s over wireless distances of 1 and 5 m, respectively, at a carrier frequency of 60 GHz, and converted them to an optical signal using a plasmonic phase modulator directly integrated with a resonant four-leaf-clover (4LC) antenna[9]. Such result could be demonstrated without resorting to any high-speed amplification or frequency conversion electronics in the front-end. This result is made possible by the inherent built-in plasmonic field enhancement (up to a factor 90,000) enabled by the plasmonic mixer. In addition, the complete electro-optic device has a footprint of only 0.315 mm². Compared to previous approaches, the plasmonic one allows straightforward extension to the THz frequencies. The same author proposed and demonstrated a similar plasmonic mixer capable of operation in the 300 GHz band[24]. Very recently, it was also shown how the same plasmonic technology can enable new broadband and ultra-efficient THz field detectors for THz time-domain spectroscopy[25]. The demonstrated plasmonic detector showed an electro-optic response up to 2.5 THz, confirming that plasmonics is could be an ideal solution for high-speed applications, especially at THz frequencies where no other technology was shown yet.