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Abstract
A magneto-optical switch responding to signal with 200 ps rise time was demonstrated. The switch uses current-induced magnetic field to modulate the magneto-optical effect. Impedance-matching electrodes were designed to apply high-frequency current and accommodate the high-speed switching. A static magnetic field generated by a permanent magnet was applied orthogonal to the current-induced ones and acts as a torque and helps the magnetic moment reverse its direction which assist the high-speed magnetization reversal.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, internet traffic has increased as a result of the widespread use of information and communication tools like smartphones and tablets. Internet traffic is anticipated to rise even more as the Internet of Things and 5 G communication systems develop. Due to its potential to offer high-capacity communication, optical communication is widely adopted in this field. Furthermore, photonics has been used for application specific computation such as neural network accelerators [1–4] and quantum computing accelerators [5–7].
Optical switches are important components in optical cross-connect (OXC) which is an optical communication system that changes the path taken by the optical signals being transferred. Optical switches are expected to offer a quick switching speed, low power consumption, and small footprint to handle the growth in traffic demand. Current OXCs used in reconfigurable optical add/drop multiplexers (ROADMs) operate at a switching time in the millisecond scale [8], and have a small number of path reconfigurations compared to packet switching methods. Therefore, volatile switches such as thermo-optic OXCs [9–11] and electro-optic OXCs [11–13] suffer a large power consumption since the switches always have to be powered on. Micro electro mechanical systems (MEMS) based OXCs has microsecond switching times and low power consumption, however it has a large footprint [14]. Phase change material (PCM) based OXCs [15–18] can realize a nonvolatility with switching time of several hundred nanoseconds. However, the number of switching cycles is limited due to it requiring high temperature to change its phase.
Magneto-optics has unique features like nonreciprocity, nonvolatility and magnetic field driving. MO effect provides effective index change larger than electro-optic Pockels effect and can respond faster than thermo-optic effect. Using a magneto-optical (MO) material $\textrm{Ce}{\textrm{Y}_2}\textrm{F}{\textrm{e}_5}{\textrm{O}_{12}}$(Ce:YIG) and a current inducting electrode, a current driven MO switch was realized [19]. However, potential response speed related to magnetization reversal has not been fully investigated.
In this paper, a high frequency transient response measurement on the fabricated MO switch was carried out. To do that, impedance-matching electrodes to apply high-frequency current was designed because its time constant would hinder actual performance. In addition to the coplanar electrode measured in [19], a single line electrode was measured. Additionally, heat and magnetic field profiles were simulated. Then, the devices were fabricated using a-Si:H waveguide on Ce:YIG. It was observed that the static magnetic field applied orthogonally to magnetization could speed up the magnetization reversal. Such a current-driven MO device is useful for not only OXC but also cryogenic applications such as quantum computing and superconducting microprocessor because semiconductors suffer from free-carrier freeze-out and voltage-driven optical devices do not match impedance with superconducting circuits at low temperature [20].
2. Device structure
In this study, two types of impedance-matching electrode with coplanar electrode and looped single line electrode with three laps were investigated. Figure 1 (a) and (c) show the cross section of the MO switch. A 500-nm-thick Ce:YIG layer is epitaxially grown on a (Ca, Mg, Zr)-substituted Gd3Ga5O12 (SGGG) substrate with sputtering. Then a 240-nm-thick hydrogenated amorphous silicon (a-Si:H) is deposited with plasma-enhanced chemical vapor deposition (PE-CVD) and patterned waveguides with electron beam lithography and reactive ion etching. To prevent the increase in propagation loss caused by the diffusion of Fe ions from Ce:YIG into the a-Si:H, a 20-nm-thick SiO2 interlayer is deposited between each layers with PE-CVD. A 700-nm-thick SiO2 overcladding is formed on the a-Si:H waveguide with PE-CVD. A 400-nm-thick Ag electrode is deposited on the SiO2 overcladding with physical vapor deposition and patterned with liftoff process.
Fig. 1. (a) Cross section and (b) Optical microscope image of the MO switch with the coplanar line-loaded MO switch. (c) and (d) show those with the looped single line electrode with three laps.
Figure 1 (b) and (d) show the overview of the fabricated MO switches with coplanar electrode and looped single line electrode with three laps, respectively. For the simplification of the design and measurement, a 1-input and 1-output Mach Zehnder Interferometer (MZI) was adopted. The MZI uses multi-mode interferometers (MMIs) for the 3-dB splitters/combiners, and has an arm length of 800 µm.
3. Electrode design
Two types of electrodes were designed. The coplanar line has $13.8\,\mathrm{\mu}\textrm{m}$ wide signal line and $50\,\mathrm{\mu}\textrm{m}$ wide ground line with $2\,\mathrm{\mu}\textrm{m}$ gap in between. The single line electrode is $10\,\mathrm{\mu}\textrm{m}$ wide and looped 3 times with a gap of $2\,\mathrm{\mu}\textrm{m}$. The characteristic impedance of the coplanar electrode was designed to be $50\,\mathrm{\Omega }$ and the resistance of the single line electrode was designed to be $50\,\mathrm{\Omega }$ to conform with the characteristic impedance of the measurement equipment.
Next, the high frequency characteristics of the electrodes was considered. In the actual experiment, a pulse pattern generator which can generate a signal up to 6.25 GHz was used. Therefore, simulation in the frequency range from 50 Hz to 10 GHz was executed using a high-frequency electromagnetic simulation software, COMSOL Multiphysics. The transmission coefficients of the electrodes are shown in Fig. 2 (a) and (b), respectively. The single line electrode has a small gap between each loops, thus it acts as an inductor. Therefore, frequency dependent impedance mismatch by the inductive component caused low transmittance at around 1.5 GHz.
Fig. 2. Simulation results of (a),(b) transmission coefficient (c),(d) magnetic field induced by current flow in the electrode (e),(f) temperature rise caused by the joules heating, for the coplanar electrode and the single line electrode, respectively.
Then, the magnetization of the magneto-optical material was calculated using a static electro-magnetic simulation software COMSOL Multiphysics. The pulse pattern generator used in the measurement setup can generate up to 2.0 Vpp, therefore the current flowing in the electrode can be calculated to be 20 mA. The magnetization profile generated by the current in the electrode is shown in Fig. 2 (c) and (d), respectively. The obtained magnetization in Ce:YIG were 12 mT and 60 mT, respectively. This is 8% and 40% of saturation magnetization (150 mT), which will decrease the magnitude of the MO effect. Therefore, the length of the phase shifter was increased to compensate for the low MO effect. The final length of the phase shifter is set to 800 μm to obtain a MO phase shift of π.
Finally, the temperature rise caused by Joule heating was simulated by finite element methods using COMSOL Multiphysics. In the same way as the previous simulation, 20 mA will be used for the maximum current. The temperature profile generated by the current in the electrode are shown in Fig. 2 (e) and (f), respectively. The temperature rise is about 0.8 K and 2.5 K, respectively. Since the temperature increase in the both MZI arms, it does not affect switching operation. In addition, in this order of magnitude of temperature rise, the MO effect will not degrade largely [21].
4. Characterization
A permanent magnet was used to apply a static magnetic field. The static magnetic field applied orthogonal to the current-induced ones acts as a torque and helps the magnetic moment reverse its direction, resulting in speed up of the magnetization reversal [22]. Figure 3 (a) shows the direction of the static magnetic field with relation to the current induced magnetic field. The current induced magnetic fields are oriented anti-parallel with the MZI arms. Figure 3 (b) shows the measurement setup with the permanent magnet. The magnetic field of ∼500 Oe was estimated to be applied into the device plane.
Fig. 3. (a) Orientation of the static magnetic field. (b) Measurement setup for fiber coupling system with high-frequency probe and external permanent magnet
4.1 Wavelength characteristics
Figure 4 shows the measurement setup. A continuous wave (CW) light having broad wavelength from an amplified spontaneous emission (ASE) light source was polarized in TM with a polarizer and launched into the device under test (DUT). The electrode was provided with zero, positive and negative DC currents by a DC voltage source and a RF probe. The output wavelength characteristics was measured with a spectrum analyzer. The static magnetic field was not applied during this measurement. Figure 5 (a) and (b) shows the wavelength characteristics of MO switches loading the coplanar electrode and looped single line electrode, respectively. For the coplanar electrode loaded MO switch, an extinction ratio of 14 dB was observed near 1542 nm with ${\pm} 99\; \textrm{mA}$ current. For the looped single line electrode, an extinction ratio of 17 dB was observed near 1557 nm with ${\pm} 28\; \textrm{mA}$ current.
Fig. 5. Wavelength characteristics of (a) coplanar electrode loaded MO switch (b) looped single line electrode loaded MO switch
4.2 Transient response
Figure 6 shows the measurement setup. A CW light of 1550 nm wavelength and TM polarization was launched into the DUT from a tunable laser diode and a polarization maintained single mode fiber. With a pulse pattern generator (PPG), an electrical signal with square wave modulation was generated and applied on the electrode with a high frequency probes having 5 or 3 pins (GSGSG and GSG) for the coplanar and looped single line electrodes, respectively. For the coplanar electrode, the left GSG pins of the GSGSG is used for applying the signal and the right GSG pins was terminated with an external 50 Ω termination resistor. The output of the DUT was coupled to an erbium-doped fiber amplifier (EDFA) and was detected by an avalanche photodiode (APD). The output signal of the APD is observed with a sampling oscilloscope.
The temporal response and its amplitude of the coplanar and single line electrode loaded MO switches are shown in Fig. 7 and Fig. 8, respectively. Figure 7 (a) and (b) shows the temporal response of optical output without and with the presence of static magnetic field respectively. The electric signal’s amplitude was 2.0 Vpp and the switching frequency was varied from 0.125 GHz to 6.250 GHz. Although the input electric signal is square wave, the temporal response over 1 GHz shows a sinusoidal waveform. This is because the magnetization response cannot follow the short rise time of the electric signal.
Fig. 7. Optical response of the coplanar electrode loaded MO switch with varying switching frequencies for (a) without static magnetic field (b) with static magnetic field and (c) Amplitude (difference between maximum and minimum) of the modulated optical power at each switching frequencies
Fig. 8. (a) Optical response of the single line electrode loaded MO switch with varying switching frequencies (b) Amplitude (difference between maximum and minimum) of the modulated optical power at each switching frequencies
As shown in Fig. 7 (c), in the case of the coplanar electrode loaded MO switch, as the switching frequency is increased, the amplitude of modulated optical output power increase at a specific frequency and decreases at frequencies higher than the peak. This is caused by ferromagnetic resonance (FMR) and the magnetization reversal not being able to complete in one cycle at higher frequencies. This results in lower magneto-optic effect, which decreases the amplitude of the modulated output power. In the case where there is no static magnetic field, 1.5 GHz was the highest frequency where a clear waveform was observable. However, when a static magnetic field is applied, the frequency where the amplitude peak occurs increased, and the waveform was observable up to 2.5 GHz. The rise time of this waveform is below 200 ps.
As shown in Fig. 8 (b), in the case of the single line electrode loaded MO switch, the frequency where the amplitude peak occurs increases when the static magnetic field is used, which agrees with the result of the coplanar electrode. However, as shown in Fig. 2 (b), since the transmission coefficient around 2 GHz is low, the amplitude of the modulated optical power decreased.
5. Conclusion
High-speed optical modulation of an MO switch consisting of Ag electrode, a-Si:H waveguide, and Ce:YIG MO material was demonstrated. Impedance matched coplanar electrode and single line electrodes were designed. Furthermore, an external permanent magnet was used to apply a static magnetic field to assist the magnetization reversal. For the wavelength characteristics, 14 dB extinction ratio was observed at ${\pm} $99 mA and 17 dB extinction ratio at ${\pm} $28 mA for the coplanar and single line electrode loaded MO switch respectively. In the transient response measurement, for a 2.0 Vpp signal, a switching signal up to 2.5 GHz and 2.5 GHz respectively were observed. To the best of our knowledge, 200 ps rising time is the shortest response time demonstrated for a current-driven Mach-Zehnder MO switch.
Funding
Japan Society for the Promotion of Science (19H02190, 22K18805); Core Research for Evolutional Science and Technology (JPMJCR18T4); New Energy and Industrial Technology Development Organization (JPNP16007, JPNP20004).
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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