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Abstract
In this study, W-band (75–110 GHz) antenna-coupled optical modulators with a cyclo-olefin polymer (COP) lower cladding, an electro-optic (EO) polymer waveguide, and a gap-embedded patch antenna array were fabricated using a transfer and bonding method of a poled EO polymer film. A carrier-to-sideband ratio (CSR) of 56 dB corresponding to an optical phase shift of 3 mrad was obtained under irradiation with 100 GHz electromagnetic waves with a power of 12.8 W/m2. Our devices have the potential to achieve highly efficient wireless–optical signal conversion in radio-over-fiber (RoF) systems.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The use of high-frequency electromagnetic waves in the W-band (75–110 GHz) and above or terahertz waves (0.1–10 THz) is attracting attention for accelerating and increasing the capacity of wireless communication. In a wireless communication system that uses high-frequency electromagnetic waves, baseband signal processing is expected to be centralized in the base station, and the radio-over-fiber (RoF) technology, in which the base station and many remote antenna units (RAUs) are connected by optical fibers, is considered important [1,2]. To support such many RAUs, small and low-cost devices that mutually convert optical and wireless signals must be developed.
Regarding the wireless transmitters for RAUs of RoF, technologies such as uni-traveling-carrier photodiodes (UTC-PDs) that can directly convert an optical signal into a wireless signal have been proposed [3]. Moreover, as the wireless receiver for the RAUs, technologies using radio frequency (RF) mixers or RF detectors based on compound semiconductor technologies are also available [3–5]. In the former case, the received 100–300 GHz band wireless signal and a local oscillator (LO) signal are mixed and down-converted into an intermediate frequency (IF) signal using an RF mixer, and that is further converted into an optical signal using an optical modulator. In the latter case, the received wireless signal is converted into an electric signal using an RF detector such as a Schottky barrier diode. After amplification using an RF amplifier, the electric signal is converted into an optical signal using an optical modulator. However, these receivers, composed of the LO, RF amplifier, and optical modulator, have complicated structures and limitations in terms of cost and size. Other methods in which the received wireless signals are amplified using RF amplifiers and converted into optical signals by directly driving optical modulators have also been proposed [6,7]. Although the mechanism is relatively simple, cost and size are a few existing issues. Research and development are also underway on 100–300 GHz band receivers based on Si CMOS and SiGe technologies [8,9]. However, these technologies also require a separate optical modulator for converting an electric signal into an optical signal, and power consumption increases as the frequency increases.
To address such problems, antenna-coupled optical modulators that directly convert wireless signals into optical signals have been developed. Modulators using plasmonic waveguides with electro-optic (EO) polymers in the 300 GHz [10] and 60 GHz [11] bands, EO polymer waveguides in the 36 GHz band [12,13], silicon/EO polymer hybrid waveguides in the 8.4 GHz band [14], lithium niobate (LN) waveguides in the 90 GHz [15] and 60 GHz bands [16], and semiconductor waveguides in the 57.5 GHz band [17] have been reported. These antenna-coupled optical modulators have small and simple device structures and are power efficient, as they do not require an external power supply. In addition, because delay does not occur during wireless–optical signal conversion, there is an advantage in realizing an ultra-low-delay wireless communication system. However, it is difficult to develop devices that feature wireless–optical signal conversion efficiency, high-frequency operability, and productivity.
The EO polymers [18,19] can have large EO coefficients (r33 > ∼100 pm/V) compared to inorganic EO materials such as LN (r33 = 32 pm/V), and the figure of merit (FOM) for optical phase modulation (n3r (n: refractive index)) can exceed the value of the inorganic materials (LN: FOM = 340, EO polymer: FOM > ∼420). In addition, they are characterized by relatively low loss over a wide terahertz region [20] and capable of ultrafast responses above hundreds of gigahertz. In antenna-coupled optical modulators, the antenna size can be increased using materials with low dielectric constants [21]. Since the EO polymers have lower dielectric constants (ε = ∼3) compared to inorganic EO materials, the wireless reception efficiency is expected to be improved using EO polymers and polymer substrates with low dielectric constants.
For the EO polymer to exhibit a second-order nonlinear optical effect, a poling process is required to orient the EO molecules contained in the EO polymers. In the device manufacturing processes using EO polymers, a device structure consisting of an EO polymer layer, conductive clad layers, and poling electrodes is first manufactured, and thereafter a voltage is applied between the electrodes to pole the EO polymer through the clad layers [22,23]. Conductive clads with a higher conductivity than that of the EO polymer layer, such as sol-gel silica, are required to efficiently apply a voltage to the EO polymer layer. However, because conductive clads have the property of strongly absorbing terahertz waves, highly efficient terahertz devices are difficult to produce using this method. Against this background, we proposed a method for transferring and bonding an EO polymer film that has been poled in advance onto a substrate [24]. We also fabricated devices that combine the EO polymer film with the terahertz wave low absorption loss materials made of cyclo-olefin polymers (COP).
In this study, we fabricated and evaluated W-band antenna-coupled optical modulators with a ground electrode, a COP lower cladding, an EO polymer waveguide, and a gap-embedded patch antenna array by transferring and bonding the poled EO polymer film. With this method, it is possible to fabricate devices using the low-loss COP of an arbitrary thickness as a lower cladding with the ground electrode. Moreover, highly efficient antenna-coupled optical modulators can be realized. In addition, our EO polymer waveguide-type devices fabricated by the transfer and bonding method are easy to manufacture and are advantageous in terms of industrial production.
2. Experimental
2.1 Device structure and antenna design
Figures 1(a) and 1(b) depict the schematic of W-band antenna-coupled optical modulators using the EO polymer waveguide. The device consists of a gold ground electrode on a silicon substrate, a lower cladding made of COP (ZEONOR, ZEON), an EO polymer waveguide, an upper clad made of UV-curable resin (FE4048, NTT Advanced Technology), gap-embedded gold patch antennas, and a protection layer made of the UV-curable resin. When W-band electromagnetic waves with a polarization direction orthogonal to the waveguide are irradiated from the upper surface of the device, a large electric field in the z-direction is obtained at the waveguide position located just below the edge of the gap of the patch antennas [25]. In the EO polymer waveguide produced by the transfer method, the EO molecules are oriented vertically (z-direction), and the phase of light with transverse magnetic (TM) polarization propagating in the waveguide is shifted by the EO effect due to the electric field Ez. Using a patch antenna array in which patch antennas are arranged with a period (LA) corresponding to the distance traveled by the light in the waveguide while the RF electromagnetic wave travels one wavelength, the optical phase shifts by the patch antennas can be accumulated. Furthermore, in the optical modulators used in this study, the gap-embedded patch antennas whose positions are shifted in the plus or minus y-direction with respect to the waveguide are arranged in the LA/2 period so that positive and negative peaks of RF electromagnetic waves can be used [25]. In addition, the density of the antennas can be doubled. Figure 1 (c) depicts the optical mode profile of the EO polymer waveguide obtained by electromagnetic wave simulation using the finite element method (COMSOL Multiphysics, COMSOL). Using a small waveguide with a size of approximately 1.6 μm, a small mode field size (∼2 μm) was obtained, and the distance between the waveguide and the upper gold patch antennas could be reduced. Therefore, an enhanced electric field in the vicinity of the antenna gap could be efficiently used. From the simulation result, we obtained a value of 1.57 as an effective refractive index (ne) and a value of 0.78 as a field interaction factor (Γ), which is the ratio of the optical power existing in the EO polymer waveguide core [26].
Fig. 1. Schematics of (a) complete view, and (b) cross-sectional view of W-band antenna-coupled optical modulators using an electro-optic (EO) polymer waveguide and a gap-embedded gold patch antenna array. (c) Simulated fundamental optical mode with transverse magnetic (TM) polarization. (d) Structure of a side-chain type EO polymer used for device fabrication.
A side-chain-type EO polymer with a dicyclopentanyl methacrylate (DCPMA) backbone with a glass transition temperature (Tg) of approximately 160 °C (Fig. 1(d)) was used for the device fabrication. We assumed the refractive index of the EO polymer in the TM direction as 1.63 by considering the increase in the refractive index due to poling. Figures 2(a) and 2(b) depict the refractive indices and absorption coefficients of the COP film in the terahertz region, respectively. The measurement was performed using a commercial THz-TDS spectrometer [24] that uses a COP sample laminated to a thickness of approximately 1.1 mm. The refractive indices were flat with respect to the frequency up to 3.5 THz, and the refractive index at 100 GHz was approximately 1.54. The absorption coefficients were less than 1 cm-1 in the measurement region, and the coefficient at 100 GHz was less than approximately 0.1 cm-1, which further indicated a small absorption loss (tanδ < ∼0.003).
Fig. 2. (a) Refractive indices, and (b) absorption coefficients of the cyclo-olefin polymer (COP) (ZEONOR, ZEON) in the terahertz region.
Figure 3(a) depicts the electric field of the device, Ez, with the gap-embedded patch antenna calculated by a three-dimensional electromagnetic wave simulation using the finite element method. The amplitude of the y-polarized incident electric field (E0) of the W-band electromagnetic waves is set to 1 V/m. The bandwidth of the patch antenna increases with an increase in the thickness of the dielectric material on the ground electrode; however, there is a trade-off relationship with the obtained electric field strength. Here, the COP thickness is set to 50 μm to obtain a 3-dB bandwidth larger than 6 GHz. In the simulations, a typical loss of an epoxy resin (tanδ = ∼0.03) [27] was assumed for the loss of the UV-curable resin. From the simulation results, we confirmed that a large Ez was obtained in the proximity region just below the edge of the gap-embedded patch antenna. Figure 3(b) depicts the frequency dependence of Ez at the waveguide position. Ez at the waveguide position was obtained by averaging the electric field in the waveguide core region. Using the designed structure, an electric field enhancement factor (Ez/E0) of 81 was obtained at the waveguide position at a frequency of 100 GHz.
Fig. 3. (a) Simulated electric field in the z-direction under irradiation of y-polarized 100 GHz electromagnetic waves with the incident angle (θ) of 0°. (b) Frequency dependence of the electric field in the z-direction at the waveguide position.
2.2 Optical phase modulation with a patch antenna array
To quantitatively consider optical phase modulation using antenna-coupled optical modulators, we analyzed the optical phase modulation using a patch antenna and a patch antenna array, including the position-shifted patch antennas used in this study according to the literature [12]. The change in the refractive index due to the EO effect is expressed as [26],
where nEO is the refractive index of the EO polymer when no voltage is applied, and E is the applied electric field. The small variation in the effective refractive index (ne) of the waveguide can be expressed as follows: where Γ is the field interaction factor [26]. From Eqs. (1) and (2), the change in the refractive index of the waveguide due to the EO effect is expressed by the relation:Here, Ewg is the electric field at the waveguide position. When the device is in the far field region in RF irradiation, it is assumed to be a plane wave at the device position, and the incident electric field at time t and location x on the device surface can be given by:
where kRF = 2π/λRF; ωRF and θ are the angular frequency and incident angle of the RF electromagnetic wave, respectively. λRF is the wavelength of RF electromagnetic waves in a vacuum. When the electric field enhancement factor at the waveguide position with respect to the incident RF electric field is E0wg/E0RF, the electric field at the waveguide position can be expressed as Ewg = E0wgsin(kRFx sinθ-ωRFt).The small optical phase shift that the TM-polarized light propagating in the waveguide undergoes while traveling a small distance dx can be expressed by the following equation:
Here, kop = 2π / λop, where λop is the wavelength of light in a vacuum. When the entrance position where light enters a patch antenna is x0 = 0, the time at which the phase front of the light incident on the entrance of the patch antenna at time t0 reaches position x can be expressed by t’ ≈ (ng/c)x + t0. Here, c is the speed of light in a vacuum, and ng is the group refractive index of the waveguide. It is assumed that the change in group velocity (ng/c) due to the phase shift of light due to the RF electric field is negligible. Assuming δne ≈ δng, the optical phase shift accumulated while the light travels through the patch antenna of length L can be expressed as follows:
Here, it is defined as u ≡ sinθ-ng.
When N patch antennas of length L are arranged with a period LA, the accumulated optical phase shift can be expressed as,
By mathematically transforming Eq. (8) [12], we obtained
Here, BN is expressed as,
In the device used in this study, gap-embedded patch antennas whose positions were shifted in the positive or negative y-direction with respect to the waveguide are arranged in the LA/2 period. When the total number of patch antennas is N' = 2N, the accumulated optical phase shift can be approximated as,
2.3 Device fabrication process
Figure 4 outlines the device fabrication process. An EO polymer film poled at a voltage of 100 V/μm at approximately 150 °C was transferred and bonded [24] onto a silicon substrate with a 200-nm thick Au ground electrode and a 50-μm thick COP layer (Figs. 4(b), 4(c)). To increase the bonding strength, the surface was activated by oxygen plasma and treated with a silane coupling agent, and then the substrates were heat pressed at 100 °C. The bonding process was also used to form the COP layer on the Si substrate with Au ground electrodes, which were coated with a SiO2 layer to increase the bonding strength (Fig. 4(a)). After forming a waveguide of the EO polymer by photolithography and reactive ion etching (RIE) (Fig. 4(d)), an upper clad made of UV-curable resin was formed (Fig. 4(e)). Thereafter, 200-nm thick gold antenna patterns were formed by photolithography and wet etching, and a protective layer made of UV-curable resin was formed on the surface. A device with a length of 1 cm was obtained by dicing.
Fig. 4. Schematic of the device fabrication process: (a) preparation of Si substrate with a Au ground electrode and COP layer, (b, c) transfer and bonding of a poled EO polymer film, (d) formation of an EO polymer waveguide, and (e) formation of an upper clad, gold antenna structures and protective layer. The arrows in the EO polymer layer indicate the direction of the poled EO molecules.
2.4 Experimental setup
Figure 5 depicts the experimental setup for device evaluation by W-band electromagnetic wave irradiation. Microwaves generated by a microwave synthesizer (68337C, Anritsu) were multiplied and amplified six times using an active multiplier (QMC-MX6-10F21HS, Quantum Microwave) and thereafter irradiated using a horn antenna with an antenna gain of 23.6 dBi at 100 GHz. The irradiation power from the horn antenna was 20.5 dBm at 100 GHz, and the distance between the horn antenna and the device was set to 40 cm, which is larger than the distance (2D2/λRF = ∼24 cm (at 100 GHz)) separating the far field region. Here, D is the aperture size (19 mm) of the horn antenna. Light from a narrow linewidth (<15 kHz) continuous-wave laser (RIO ORION, RIO) with a power of approximately 8 mW and a wavelength of 1.535 μm was adjusted to TM polarization using a polarization controller and introduced into the device using a tapered lensed fiber. The output light from the device was measured using an optical spectrum analyzer (AQ6370D, Yokogawa).
Fig. 5. (a) Simulated electric field in the z-direction while irradiating 100 GHz electromagnetic wave from the top of the device. (b) Frequency dependence of the electric field in the z-direction at the waveguide position.
3. Results and discussion
Figures 6(a) and 6(b) depict the appearance and microscopic images of the fabricated device. The gap-embedded patch antennas had a length (L) of 0.48 mm and a width (W) of 1.6 mm, an antenna gap (g) of 5 μm, a half-antenna period (LA/2) of 0.88 mm, and an antenna number (N’) of 10. Figure 6(c) depicts the scanning electron microscope images of the cross-section of the device. From the images, it was confirmed that the EO polymer waveguide existed below the edge of the Au antenna gap. The thickness of the upper cladding on the waveguide is 2.6 μm, which is sufficiently large for the mode size (∼2 μm) so that the enhanced electric field can be used without any light loss due to the Au antennas. To evaluate the EO coefficient of the EO polymer film after the transfer, the poled EO polymer film was transferred onto an ITO substrate, and thereafter an upper transparent electrode was formed, and the EO coefficient was measured using the transmission ellipsometry method [28]. The obtained r33 at a wavelength of 1.55 μm was 36 pm/V. In the present device, the total of the coupling loss and the propagation loss was approximately 15 dB. The relatively large loss can include the effects of the machining conditions on the waveguide surface and the waveguide end faces.
Fig. 6. (a) Appearance, and (b) microscopic images of the fabricated W-band optical modulator. (c) Scanning electron microscope images (reflected electron images) of the device cross-section.
Figure 7 (a) depicts the optical spectrum of the output from the W-band optical modulator when irradiated with y-polarized 100 GHz band electromagnetic waves. The irradiation power density at the device position was estimated to be 12.8 W/m2. Under this condition, sidebands with a carrier-to-sideband ratio (CSR) of 56 dB were clearly observed. The CSR of the first-order sideband can be approximated as [12],
where m is the modulation depth corresponding to the optical phase shift, and m << 1. The modulation depth (m) corresponding to the observed CSR was 3 mrad. We also estimated a figure of merit to be 0.32 W-1/2 according to the literatures [12,13]. Figure 7(b) depicts the CSRs as a function of the frequency of the irradiated electromagnetic waves. In the fabricated device, the minimum CSR value was obtained at an irradiation frequency of 100 GHz. The frequency dependence of the CSRs can depend not only on the frequency characteristics of the patch antenna obtained by the simulation, as depicted in Fig. 3(b) but also on the period LA of the patch antennas, as calculated by Eqs. (9)–(11). The calculated frequency characteristic curve reproduced the tendency of the experimentally obtained frequency characteristics of the CSRs. The experimentally obtained 3-dB operation bandwidth was approximately 6 GHz.Fig. 7. (a) Optical spectrum of output light from the W-band optical modulator under irradiation of y-polarized 100 GHz electromagnetic waves. (b) Carrier-to-sideband ratios (CSRs) as a function of the frequency of the irradiated electromagnetic waves. The dotted line is the calculated results based on Eqs. (9)–(11) and the electric field enhancement factors depicted in Fig. 3(b). The irradiation angle (θ) of the W-band electromagnetic waves was 0°.
Figure 8 depicts the irradiation angle dependence of the CSRs in 100 GHz electromagnetic wave irradiation. Because the irradiation angle dependence of the optical modulator using the patch antenna array can depend on the period LA of the patch antennas, as calculated using Eqs. (9)–(11), devices with directivity for different angles can be obtained by changing the value of the period LA. In the fabricated device, the minimum CSR value was obtained when the irradiation angle was approximately θ = -2°, and the calculated angle-dependent curve reproduced the tendency of the experimentally obtained CSR values.
Fig. 8. CSRs as a function of the irradiation angles (θ) of the electromagnetic waves with a frequency of 100 GHz. The dotted line is the calculated results based on Eqs. (9)–(11).
In the antenna-coupled optical modulator using the patch antenna array, the length L of the patch antenna is designed by calculating the distance traveled by the light in the waveguide, while the RF electromagnetic waves travel approximately a quarter to half a wavelength. Therefore, a smaller refractive index of the waveguide results in a larger antenna length L. That is, when the electric fields at the waveguide positions are the same, the optical phase shift obtained per patch antenna can be increased using a polymer waveguide with a small dielectric constant. Furthermore, using materials with small dielectric constants, the size (width W) of the patch antenna that resonates at a certain frequency can be increased so that the power that can be received by the antenna increases, so does the electric field obtained in the vicinity of the gap. For this reason, in devices using waveguides made of LN with a relatively large dielectric constant, the effective dielectric constant is lowered by combining low-k materials [21].
The CSR of 56 dB at 20.5 dBm irradiation power in this study is comparable to the efficiency (the CSR of 40 dB at 33 dBm irradiation power) in a previous report using an LN waveguide with a patch antenna array on a low-k substrate in the 90 GHz band [15]. Therefore, our device has sufficient performance compared to the devices with the similar antenna array structures. By contrast, the optical phase shift experimentally obtained in this study was approximately 0.37 times that obtained from the theoretical calculations using the values obtained from the simulations. We estimated that the processing temperature of the polymer in RIE was less than 100 °C and confirmed that the EO polymer with a Tg of 160 °C had a decrease in EO coefficients of less than 6% after being left at 105 °C for 1 h. Therefore, it is unlikely that the cause of the inconsistency with the theoretical calculations is the decrease in the EO coefficients due to the relaxation of the orientation of the EO molecules. As another factor, although we used the COP with a low absorption loss in the terahertz region as the lower clad, we used the UV-curable resin made of epoxy as the upper clad and protective layer. The reason for using the UV-curable resin is that when an upper COP clad is formed on the waveguide by spin-coating, the adhesion between the COP and the waveguide is very low. However, an epoxy resin has large losses in the RF region [27]. We assumed a typical loss value (tanδ = 0.03) for the epoxy resin in the simulation, but the actual loss of the UV-curable resin may be higher. For example, the loss tangent (tanδ) of the epoxy resin with additives can be greater than 0.25 [27]; the simulations using this value showed that the electric field enhancement factor (Ez/E0) was less than 47 at 100 GHz. It is expected that the wireless–optical signal conversion efficiency can be improved by solving the problem of adhesion and using low-loss materials for the upper cladding and the protective layer. Other possible causes include the effects of machine errors and electromagnetic field simulations simplified owing to computational resource issues.
4. Conclusions
We fabricated and evaluated W-band antenna-coupled optical modulators consisting of a lower ground electrode, a lower COP cladding, an EO polymer waveguide, and a gap-embedded patch antenna array using the transfer and bonding method of the poled EO polymer film. The device exhibited a CSR of 56 dB corresponding to an optical phase shift of 3 mrad under irradiation of the 100 GHz electromagnetic waves with a power of 12.8 W/m2 and a 3-dB operation bandwidth of approximately 6 GHz.
Funding
Japan Society for the Promotion of Science (18K04274).
Acknowledgments
We thank Dr. Yoh Ogawa for his advice on the construction of the optical system.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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