Ultra-Fast Tunable Optical Delay Line Based on Cascaded Silicon Microdisk Resonators

Integrated optical delay lines (ODLs) featuring fast and continuous tunability are important building blocks in microwave photonic signal processing systems. In this work, we propose and demonstrate an ultra-fast tunable ODL based on cascaded silicon microdisk resonators (MDRs). In the proposed integrated ODL, a specially designed MDR is used as the basic delay element, which has an additional slab waveguide to surround the disk and the bus waveguide aiming to suppress higher-order whispering gallery modes (WGMs) and to support the incorporation of a lateral PN junction for electrical tunability. By exploiting the free-carrier dispersion (FCD) effect on silicon, the proposed ODL can be tuned with an ultrahigh speed. Multiple MDRs are connected in series to increase the time delay provided by the ODL. As a demonstration, integrated ODLs with 5 and 10 MDRs are designed and fabricated. The experimental results show that the fabricated ODL provides an ultra-short response time of 989 ps, a low power consumption below 0.58 mW, and a small footprint of 0.13 mm2. The proposed integrated ODL holds great advantages including ultra-high tuning speed, low power consumption and ultra-compact footprint, which is potential to be widely used in next-generation phased array radar and high-speed wireless communication systems.


I. INTRODUCTION
O PTICAL delay line (ODL) is one of the key components in microwave photonic processing systems [1], [2], [3], [4], [5]. Especially, ODL-based beamforming networks are widely used to perform beam steering with a wide operational bandwidth and a high scanning speed [6], [7]. Up to date, most of the commercially available ODLs are realized based on discrete free-space or fiber-optic components, which suffer from a high cost, a large size, a high power consumption and a long response time. To address these problems, integrated ODL fabricated on silicon on insulator (SOI) platform is a promising solution [8], [9], [10], [11], which holds unique advantages of high tuning speed, small footprint, and CMOS compatibility.
In the last few years, numerous silicon integrated ODLs have been demonstrated based on different configurations, such as switchable waveguide delay lines (SWDLs) [12], [13], [14], [15], [16], [17], waveguide Bragg gratings [18], [19], [20], [21], [22], [23], microring resonators (MRRs) [24], [25], [26], [27], [28], [29], [30], [31], microdisk resonators (MDRs) [32], [33], [34], and photonic crystal waveguides (PhCW) [35], [36]. Among them, ODLs based on MRRs or MDRs have attracted intensive attention due to their unique advantages including large group delay, high tuning efficiency, and compact footprint. Previously, an ODL based 56 cascaded MRRs has been demonstrated, which provides a large group delay exceeding 500 ps [24], but the time delay provided by the ODL cannot be flexibly tuned since it is fabricated on a passive chip without any tuning mechanism. In order to achieve continuous time delay tuning, ODLs based on thermally tunable MRRs have been proposed, in which a metal heater is placed on the top of the ring waveguides [26], [27], [28]. However, the tuning speed of this kind of ODL is at the level of tens of microseconds, which is limited by the slow response time of the thermal-optic (TO) effect on silicon. Moreover, the power consumption of thermally tunable ODLs is normally as high as a few tens of milliwatts, which hinders their applications in some fields where low power consumption is required. Alternately, electrically tunable ODLs with a high tuning speed and a low power consumption can also be realized based on an electrically tunable MRR, in which a PIN diode is incorporated in the MRR [30]. By exploiting the carrier injection effect, the ODL based on an electrically tunable MRR can achieve a high tuning speed and a low power consumption. Compared with the PIN diode, the PN junction structure is potential to achieve a much higher tuning speed by exploiting the carrier depletion effect on silicon, and high-speed optical devices such as silicon MRR modulators with a bandwidth as wide as a few tens of GHz have already been demonstrated [37]. However, ultra-fast tunable ODLs based on electrically tunable MRRs or MDRs with PN junctions are yet to be developed.
In this paper, we propose and demonstrate an ultra-fast tunable ODL based on cascaded silicon MDRs. Different from MRRs, MDRs experience scattering loss only from the outer sidewall of the waveguide, which leads to a better performance in terms of footprint and light-confining capacity. In the proposed integrated ODL, a specially designed MDR with a small radius of 3.7 µm is used as the basic delay element, which has an additional slab waveguide to surround the disk and the bus waveguide aiming to suppress higher-order whispering gallery modes (WGMs) and to support the incorporation of a lateral PN junction for electrical tunability. By exploiting the free-carrier dispersion (FCD) effect on silicon, the proposed ODL can be tuned with an ultrahigh speed. Multiple MDRs are connected in series to increase the time delay provided by the proposed ODL. As a demonstration, integrated ODLs with 5 and 10 MDRs are designed and fabricated. The experimental results show that the fabricated ODL provides an ultra-short response time of 989 ps, a low power consumption of 0.58 mW, and a small footprint of 0.13 mm 2 . The proposed integrated ODL holds great advantages including ultra-high tuning speed, low power consumption and ultra-compact footprint, which is promising to be widely used in next-generation phased array radar and high-speed wireless communication systems. Fig. 1(a) shows the perspective view of the proposed ultrafast tunable ODL. It consists of multiple identical MDRs which are connected serially to form an MDR array. Fig. 1(b) shows the zoom-in view of the MDR which has an all-pass configuration consisting a bus waveguide and a disk waveguide. The proposed MDR has an additional slab waveguide employed to wrap the disk and the lateral sides of bus waveguide, which is used to suppress the higher-order WGMs of the MDR and to support the incorporation of the lateral PN junction. The lateral PN junction of the disk is formed along the edge of the disk to achieve ultrafast tuning of the proposed ODL, which is composed of N, N++, P and P++ doping regions. In order to minimize the absorption loss induced by heavy doping, the P++ and N++ implantation regions are placed 1 µm away from the center of the PN junction. To avoid the optical coupling region between the bus waveguide and the disk to be deteriorated, there is no doping region along the arc with an angle of 120°near the coupling region of the MDR. Fig. 1(c) shows the cross-section view of the MDR. The disk is designed to have an ultra-small radius of 3.7 µm and the slab waveguide has a radius of 9.5 µm. In order to meet the phase matching condition between the disk and the bus waveguide, the effective refractive index of the fundamental mode existed in the bus waveguide should be equal to that of the fundamental WGM existed in the disk waveguide. The bus waveguide is optimized to have a width of 470 nm. The coupling gap between the bus waveguide and the disk has a width of 200 nm. The disk and the slab waveguide of the MDR has a height of 220 nm and 90 nm, respectively.

II. DEVICE DESIGN AND FABRICATION
The proposed device is fabricated using standard CMOS fabrication processes on a SOI wafer in IME. ODLs consisting of 5 MDRs and 10 MDRs are designed and fabricated. Fig. 2

A. Delay Performance
The magnitude response and the delay performance of the fabricated ODLs are characterized using an optical vector analyzer (OVA). The OVA is realized based on a polarizationdiverse swept-wavelength interferometry, which mainly consists of a frequency-swept laser, two Mach-Zehnder interferometers (MZIs), and a parameter extraction unit. The group delay value of the ODL can be obtained directly from the OVA [38]. Firstly, the performance of a single MDR which is used as the basic delay element is evaluated. Fig. 3(a) illustrates the normalized transmission spectrum of the MDR delay element. As can be seen, the disk supports only the fundamental WGM while the higher-order WGMs are suppressed effectively. The fiber-tofiber I/O coupling loss is 18 dB, which can be further reduced by optimizing the structure of the grating coupler. The free spectrum range (FSR) of the MDR is measured to be 30.03 nm, which is determined by the radius of the MDR. Fig. 3(b) gives a zoom-in view of the resonance spectrum around the wavelength of 1548.326 nm, and the notch has a 3-dB bandwidth of 0.14 nm,  which corresponds to a Q-factor of 11060. The extinction ratio of the notch is 9.45 dB. The measured group delay of the MDR is shown in Fig. 3(c). As can be seen, a group delay of 40 ps near the resonance wavelength can be obtained. The specially designed MDR has an ultra-compact footprint and a strong group delay, which can be used as a basic delay element in an integrated ODL.
Then, the fabricated ODL composed of 5 MDRs is characterized using the OVA. As depicted in Fig. 4(a), by applying different forward-biased voltages to the MDR, the normalized transmission spectrum of the ODL is blue shifted, which is resulted from the decrease of the effective refractive index of the waveguide induced by the free-carrier injection of the PN junction. It should be noted that the optical loss of the fabricated ODL is increased due to the additional absorption loss caused by the injected free-carriers in the PN junctions of the disks. Fig. 4(c) shows the group delay responses of the ODL under forward biased. At the wavelength detuning of 0.225 nm, the measured delay of ODL varies from 53.91 ps to 3.295 ps as the bias voltage increases from 0 V to 1 V. When the ODL is under reverse biased, the depletion regions of the PN junctions are widened due to the extraction of the free carriers. As a result, when the driven voltage is increasing, the effective refractive index of the waveguide is increased, which leads to a red-shift of the transmission spectrum of the ODL. Fig. 4(b) and (d) illustrate the transmission spectra and the group delays of the ODL under reverse biased. As the reverse-biased voltage increasing, the optical loss of ODL is reduced slightly due to the decrease of the number of free carriers in PN junctions of the disks. At the wavelength detuning of 0.225 nm, the group delay of the ODL varies from 53.91 ps to 65.08 ps when the reverse-biased voltage increases from 0 V to 12 V. The delay of the ODL is continuously tunable, which is limited only by the smallest voltage change that can be applied. The resolution of the voltage source (Keithley 2614B) used in the experiment is 500 µV within a voltage range of 20 V. The average delay tuning precision of the ODL composed of 5 MDRs is calculate to be 1.27e-05 ps and 2.33e-07 ps under forward biased and reversed biased, respectively. The loss per ps delay increment of the ODL composed of 5 MDRs is calculated to be 0.35 dB/ps and 0.36 dB/ps under forward biased and reversed biased, respectively.
The performance of the ODL composed of 10 MDRs is also evaluated. Fig. 5(a) and (c) illustrate the transmission spectra and group delay responses of the ODL under forward biased. Since the dynamic range of the OVA is only 60 dB, the deteriorated transmission of the ODL composed of 10 MDRs caused by free-carrier injection cannot be measured accurately. As the forward-biased voltage increases from 0 V to 1 V, the normalized transmission spectrum of the ODL is blue shifted and the group delay changes from 70.81 ps to 16.58 ps at the wavelength detuning of 0.33 nm. Fig. 5(b) and (d) illustrate the transmission spectra and group delay responses of the fabricated ODL under reverse biased. As the reverse-biased voltage increases from 0 V to 12 V, the normalized transmission spectrum of the ODL is red shifted and the group delay of the ODL varies from 70.81 ps to 92.673 ps at the wavelength detuning of 0.33 nm. The average delay tuning precision of the ODL composed of 10 MDRs is 1.36e-05 ps and 4.55e-07 ps under forward biased and reversed biased, respectively. The loss per ps delay increment of the ODL composed of 10 MDRs is calculated to be 0.44 dB/ps under forward biased and reversed biased. To reduce the insertion loss, a balanced side-coupled integrated spaced sequence of resonator (SCISSOR) structure may be introduced into the ultra-fast tunable ODL [26], [27]. Fig. 6 shows the measured power consumption of the ODLs. As shown in the Fig. 6(a), when the ODL composed of 5 MDRs is under forward biased, the power consumption is below 0.065 mW for a group delay ranging from 53.91 ps to 3.295 ps. When the ODL composed of 10 MDRs is under forward biased, the power consumption is below 0.58 mW for a group delay ranging from 70.81 ps to 16.58 ps. As illustrated in Fig. 6(b), when the ODL composed of 5 MDRs is under reverse biased, the power consumption is below 1.02 µW for a group delay ranging from 53.91 ps to 65.08 ps. When the ODL composed of 10 MDRs is under reverse biased, the power consumption is below 1 µW for a group delay ranging from 70.81 ps to 92.673 ps.

C. Tuning Speed
The high-speed tunability of ODL consisting of 5 MDRs is demonstrated. Fig. 7 illustrates the experimental setup for the demonstration of the rapid tunability of the ODL. The wavelength of the tunable laser source (TLS) is set to be 1548.95 nm which corresponds to a wavelength detuning of 0.256 nm. A 50 MHz electrical square-wave signal generated by an arbitrary waveform generator (AWG) is applied to the PN junction of the MDR via a high-speed radio-frequency (RF) probe. The optical  signal at the output of the ODL is sent into a photodetector (PD), and the electrical signal generated by the PD is recorded by an oscilloscope (OSC). In the experiment, the high-speed tunability of ODL is only characterized under forward biased due to the limited output voltage range of the AWG. The peak-to-peak voltage and the offset voltage of the electrical square-wave signal are set to be 800 mV and 400 mV, respectively. The response of the fabricated ODL is monitored by an OSC. Fig. 8(a) illustrates the time domain waveform of electrical square-wave signal generated by the AWG. Fig. 8(b) illustrates the measured output response of the ODL. The 10%-90% rise time and fall time of the output waveform are 519 ps and 989 ps respectively, which means that a sub-ns switching time is achieved by the proposed integrated ODL. It is noticed that the 10%-90% fall time is slower than the 10%-90% rise time. The rise time of the optical transmission is much shorter than the rise time of the charge injection and the fall time is determined by the extraction time of the carrier [39]. However, the total drift current to extract the large amount of the injected carriers is limited by the large resistance of the PN junction incorporated in the ODL.

D. Transmission Quality of Optical Signal
The quality of the optical signal transmitted through the ODL consisting of 5 MDRs is evaluated. The experimental setup is shown in Fig. 9(a). A TLS is used to generate a continuous wave optical carrier, which is launched into the Mach-Zehnder modulator (MZM) via a polarization controller (PC). A 1-Gbps NRZ OOK pseudo-random binary sequence (PRBS) signal with a length of using 2 10 -1 is sent into the MZM. The modulated signal at the output of the MZM is launched in to the integrated ODL chip. The output signal from the chip is amplified by erbium-doped fiber amplifier (EDFA). An optical bandpass filter (BPF) is connected after the EDFA to filter out the amplified spontaneous emission (ASE) noise. The eye diagrams of the optical signal can be observed using an OSC. Fig. 9(b) shows the measured B2B eye diagram when the ODL chip is not inserted into the system. Fig. 9(c) shows the eye diagram when the ODL chip is incorporated but the optical carrier is located at the off-resonance wavelength, in which zero group delay is introduced. Fig. 9(d) and (e) show the eye diagrams with group delays of 41 ps and 63 ps. The Q factor of the eye diagrams shown in Fig. 9(c), (d) and (e) is 17.902, 15.489 and 5.455 respectively, which verifies that the proposed ODL can be used in the high-speed communication systems for precise time delay controlling. Table I compares the performance of the proposed integrated ODL with that of the state-of-art integrated ODLs, including SWDLs [12], [13], [14], [15], waveguide Bragg grating [20], [21] and MRRs [26], [27], [28], [30]. As can be seen, the proposed integrated ODL can achieve continuous time delay tuning with an ultrahigh speed by exploiting the FCD effect on silicon. However, the proposed ultra-fast tunable ODL suffers from a large insertion loss due to the incorporation of the PN junction. Previously, a pipin lateral diode structure has been proposed and incorporated into the optical modulator to achieve a lower insertion loss and a wide modulation bandwidth [40], [41]. Therefore, a tunable ODL with a lower insertion loss may be realized by introducing the pipin lateral diode structure into the MDR for electrical tunability.

IV. DISCUSSION AND CONCLUSION
In conclusion, an ultra-fast tunable ODL based on cascaded silicon MDRs was proposed and experimentally demonstrated. In the proposed ODL, a specially designed slab waveguide surrounding the disk was used to suppress higher-order WGMs and to support the incorporation of a lateral PN junction for electrical tunability. By exploiting the FCD effect on silicon, the time delay of the proposed ODL can be tuned with an ultrahigh speed. Multiple MDRs are connected in series to increase the time delay provided by the proposed ODL. A proof-of-concept demonstration is performed. Integrated ODLs composed of 5 and 10 MDRs are designed, fabricated, and characterized. Experimental results demonstrate that the fabricated ODL can achieve an ultra-short response time of 989 ps, a low power consumption of 0.58 mW, and a small footprint of 0.13 mm 2 . The proposed ODL features ultrafast tunability, compact footprint and low power consumption, which holds potential to be widely used in next-generation phased array radar and high-speed wireless communication systems.