Chinese Physics Letters, 2020, Vol. 37, No. 8, Article code 084201Express Letter Electro-Optically Switchable Optical True Delay Lines of Meter-Scale Lengths Fabricated on Lithium Niobate on Insulator Using Photolithography Assisted Chemo-Mechanical Etching Jun-xia Zhou (周俊霞)1,2, Ren-hong Gao (高仁宏)3,4, Jintian Lin (林锦添)3,5*, Min Wang (汪旻)1,2, Wei Chu (储蔚)1,2, Wen-bo Li (李文博)3,4, Di-feng Yin (尹狄峰)3,4, Li Deng (邓莉)1,2, Zhi-wei Fang (方致伟)1,2, Jian-hao Zhang (张健皓)3,4, Rong-bo Wu (伍荣波)3,4, and Ya Cheng (程亚)1,2,3,4,5,6,7* Affiliations 1State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China 2XXL—The Extreme Optoelectromechanics Laboratory, School of Physics and Electronics Science, East China Normal University, Shanghai 200241, China 3State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 4University of Chinese Academy of Sciences, Beijing 100049, China 5Collaborate CAS Center for Excellence in Ultra-intense Laser Science, Shanghai 201800, China 6Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China 7Shanghai Research Center for Quantum Sciences, Shanghai 201315, China Received 28 June 2020; accepted 4 July 2020; published online 9 July 2020 Supported by the National Key R&D Program of China (Grant No. 2019YFA0705000), the National Natural Science Foundation of China (Grant Nos. 11734009, 61590934, and 11874375), the Strategic Priority Research Program of CAS (Grant No. XDB16030300), and the Key Project of the Shanghai Science and Technology Committee (Grant No. 17JC1400400).
*Corresponding author. Email: jintianlin@siom.ac.cn; ya.cheng@siom.ac.cn
Citation Text: Zhou J X, Gao R H, Lin J T, Wang M and Chu W et al. 2020 Chin. Phys. Lett. 37 084201    Abstract Optical true delay lines (OTDLs) of low propagation losses, small footprints and high tuning speeds and efficiencies are of critical importance for various photonic applications. Here, we report fabrication of electro-optically switchable OTDLs on lithium niobate on insulator using photolithography assisted chemo-mechanical etching. Our device consists of several low-loss optical waveguides of different lengths which are consecutively connected by electro-optical switches to generate different amounts of time delay. The fabricated OTLDs show an ultra-low propagation loss of $\sim 0.03$ dB/cm for waveguide lengths well above 100 cm. DOI:10.1088/0256-307X/37/8/084201 PACS:42.82.-m, 42.79.-e © 2020 Chinese Physics Society Article Text Photonic integrated circuits (PICs) have attracted tremendous interest recently as a promising technological platform of various applications ranging from data communication and quantum information processing to sensing and national security.[1–6] The major challenge in the establishment of a universal PIC technology is that the ideal PIC devices should be of low on-chip losses, high integration density and high tunability, which are difficult to be simultaneously achieved based on a single material platform. Recently, lithium niobate on insulator (LNOI) has emerged as an attractive substrate material owing to its excellent optical and electro-optic properties, opening up the avenue for low loss, dense and highly tunable PIC technology. Various photonic structures such as waveguides,[7–14] microresonators,[15–24] Mach-Zehnder interferometers,[25–29] photonic crystal cavities,[30,31] and mode converters[32,33] have been demonstrated on LNOI for low loss and highly tunable PIC applications. Although optical true delay line (OTDL), which is an essential photonic component for generating a certain amount of time delay for an optical signal, is generally required in many PIC applications, it has not been demonstrated on the LNOI material platform using existing fabrication techniques. Typically, to generate time delays on the nanosecond level, optical waveguides of meter-scale lengths should be fabricated with extreme low surface roughness. Although low loss optical waveguides have been fabricated on LNOI with electron beam lithography (EBL) and ion dry etching,[14] the EBL technique is not suitable for fabrication of ultra-large-scale photonic structures due to the inevitable stitching errors. To overcome the issue, we recently have developed a fabrication technique based on photolithography assisted chemo-mechanical etching (PLACE).[25] Here, we report fabrication of the OTDL using PLACE technique, as the OTDLs are recognized as a representative dense PIC device. Ideally, the OTDLs consist of low-loss optical waveguides with small bend radii to enable large delay ranges and small footprints. We show that the PLACE fabrication technique is particularly suitable for such an application which allows for producing meter-long waveguides on the LNOI substrate which can be integrated with micro-electrodes in a straightforward manner for electro-optic tuning. In our experiment, the on-chip lithium niobate (LN) OTDL was fabricated on a commercially available X-cut LNOI wafer (NANOLN, Jinan Jingzheng Electronics Co. Ltd.). The X-cut configuration was chosen for utilizing the largest electro-optical coefficient of LN. The LN thin film with a thickness of 700 nm was bonded onto a silica layer with a thickness of $\sim 2$ µm, and the silica layer was grown on a 0.5 mm-thick LN substrate. The fabrication process of the waveguide-based PIC devices includes five major steps as illustrated in Fig. 1. Firstly, a 600 nm-thick layer of chromium (Cr) film was coated on the surface of the LNOI by magnetron sputtering. Secondly, the Cr film on the LNOI sample was patterned into the waveguide mask using space-selective femtosecond laser (PHAROS, LIGHT CONVERSION Inc.). Thirdly, chemo-mechanical polishing (CMP) process was performed to selectively etch the LN thin film using a wafer polishing machine (UNIPOL802, Kejing Inc.). In this step, the LN thin film protected by the Cr mask was preserved after the CMP process which serves as the waveguides. The smooth sidewalls produced by CMP give rise to low propagation loss.[7,34] Fourthly, the fabricated structure was immersed in the Cr etching solution to remove the Cr mask. Fifthly, a 2 µm-thick layer of Ta$_{2}$O$_{5}$ film was deposited on the top of the LN waveguide to form the cladding which ensures the single-mode propagation in the fabricated waveguides.
Fig. 1. Schematic of fabrication flow. (a) Depositing a Cr thin film on the top surface of LNOI. (b) Patterning the Cr thin film into the waveguide mask. (c) Transferring the mask pattern into LNOI by chemo-mechanical polish. (d) Removing the Cr thin film by wet chemical etching. (e) Coating the LN waveguide with a cladding layer of Ta$_{2}$O$_{5}$ with a thickness of 2 µm.
To measure the propagation losses in the OTDLs, we used a high-precision loss measurement method based on the comparison of propagation losses in two waveguide arms of unbalanced lengths of a 50% : 50% beamsplitter.[7] This scheme avoids the uncertainty inherently associated with the coupling efficiency in the traditional cut-back loss measurement method. A wavelength tunable laser (New Focus Inc., Model: TLB 6728) was used as the light source. The laser beam was tuned to 1550 nm wavelength and coupled into the waveguide through a fiber taper, and the polarization state of the input light was controlled by an online polarization controller. The light transmitted from the waveguide was first collected by an objective lens (Mitutoyo Inc., Model: M Plan Apo NIR HR, $50\times$) with a numerical aperture (NA) of 0.35 and then recorded with either a power meter (Coherent Inc., Model: OP-2 IR) or an infrared CCD (HAMAMATSU Inc., InGaAs camera). The near field spatial distributions of the transverse electric (TE) and transverse magnetic (TM) modes at 1550 nm wavelength were captured by the infrared CCD. A polarizer was placed between the objective lens and the infrared CCD to allow only the TM or TE mode to enter the CCD. To produce the micro-electrodes, a layer of 600 nm-thick Cr film was first deposited on the Ta$_{2}$O$_{5}$ cladding layer. The Cr layer was then patterned into the micro-electrodes by space-selective ablation with the focused femtosecond laser pulses. The scanning speed was set at 40 mm/s when the repetition rate of laser pulses was at 250 kHz. The laser power was adjusted to only remove the Cr film without damaging the underneath Ta$_{2}$O$_{5}$ cladding layer and LN waveguide. A $100\times$ objective lens with a high ${\rm NA}=0.7$ was used to focus the laser pulses to obtain a tightly focused spot with a diameter of $\sim 1$ µm on the sample. Femtosecond laser ablation was performed by translating the LNOI sample with a computer-controlled XY motion stage with a translation resolution of 100 nm. To demonstrate the generation of variable time delays, we sent 500 fs laser pulses (Laser source: OYSL Inc., Model: FemtoYL-Vary) into a beamsplitter of which the two output ports were connected to two OTDLs of different lengths [see Fig. 2(a)]. Thus, the pulse traveling through different OTDLs will arrive at an optical detector at different times, which can be read out on an oscilloscope. The laser pulses were coupled into the beamsplitter using an objective lens with ${\rm NA}=0.8$ in free space, and the output signals from the two OTDLs were collected with the objective lens with ${\rm NA}=0.35$, and focused into a high-speed optical detector with a bandwidth of 5 GHz. Finally, the signals were simultaneously recorded by an oscilloscope (Tektronix Inc., Model: MDO3104) with a bandwidth of 1 GHz and a sampling rate of 5 GS/s. From the waveform on the oscilloscope, the time delay can be directly measured. Figure 2(a) presents the schematic design of the beamsplitter of a splitting ratio of $\sim 50$% : 50%. Based on our previous result, the precision of the splitting ratio can be controlled within $\sim 2$% as limited by the fabrication resolution, as the splitting ratio is determined by the length of the coupling region.[7,35] After the beamsplitter, we fabricated several OTDLs of various lengths of 25.06 cm, 54.86 cm, and 111.26 cm, which are schematically illustrated as the upper waveguide in Fig. 2(a). The lower waveguide in Fig. 2(a), however, has a fixed length of only 20 mm, which is negligible in comparison with the lengths of upper waveguides. The powers of the output beams from the upper and lower waveguides were measured with the power meter for characterizing the propagation loss in the OTDLs. Figure 2(b) shows the picture of a fabricated meter-scale long OTDL taken by a digital camera, which is placed near a 1-Yuan Renminbi (RMB) coin with a diameter of 25 mm. Figure 2(c) shows the scanning electron microscope (SEM) image of the area indicated by the yellow box in Fig. 2(b). The waveguides appear uniform and smooth. The smooth surface of the waveguides is further confirmed with the optical micrograph in Fig. 2(d), in which the waveguides and the beamsplitters are free from visible defects such as scratches and pits. The beamsplitter can be clearly seen with a 130 µm-long coupling region. Figure 2(e) exhibits the SEM image of the cross section of the waveguide fabricated using the PLACE technique, showing a top width of 1.44 µm and a wedge angle of 61$^{\circ}$. The TE and TM mode fields simulated using COMSOL are presented as the left and right insets in Fig. 2(e), respectively. Here, the effective refractive indices for the TE and TM modes in the waveguides are 2.107 and 2.094, respectively. And the second-order dispersions for the TE and TM modes are 5.95$\times 10^{-25}$ ${\rm s}^{2}\cdot$m$^{-1}$ and 7.45$\times 10^{-25}$ ${\rm s}^{2}\cdot$m$^{-1}$, respectively. For the TE mode, the propagation loss of the OTDL with a length of 109.26 cm is measured to be $0.0286\pm 0.001$ dB/cm at 1550 nm wavelength, which is calculated by comparing the output powers from the ports 1 and 2. For the TM mode, the measured propagation loss was $0.030\pm 0.001$ dB/cm at 1550 nm wavelength. The measured propagation losses in the meter-long OTDLs are slightly lower than the propagation loss of 0.042 dB/cm reported in our previous investigation.[7] This should be attributed to improvement on the fabrication process. For instance, we refined our laser ablation process in the fabrication of the Cr mask which leads to less debris generation and in turn more efficient suppression of the generation of scratches during the CMP process. In the same investigation, we also fabricated two OTDLs with shorter lengths of 23.06 cm and 52.86 cm. The corresponding propagation losses in the two shorter OTDLs were measured to be $0.0285\pm 0.001$ dB/cm and $0.0287\pm 0.001$ dB/cm for the TE mode, and $0.031\pm 0.001$ dB/cm and $0.032\pm 0.001$ dB/cm for the TM mode, respectively. The data presented in Fig. 2(f) also show that the losses measured with the OTDLs of very different lengths are close to each other, which is reasonable as the same fabrication conditions have been employed in the fabrication of all the OTDL devices.
Fig. 2. (a) Schematic design of a beamsplitter connected with two waveguides of different lengths. The upper waveguide serves as the OTDL. (b) Digital camera picture of an OTDL with a total length of 109.26 cm. (c) SEM image of the bend section of the OTDL in panel (b). (d) Optical micrograph of the fabricated beamsplitter in the OTDL device. (e) Cross-sectional scanning electron microscope (SEM) image of the ridge waveguide. Inset: simulated mode field distributions of TE and TM modes. (f) Propagation loss as a function of the length of the OTDL. Inset: near field distributions of the TE modes at the output port 1 and 2.
LN is well known for its large electro-optic coefficient, thus it is straightforward to fabricate optical switches using LN waveguides.[25] By combinations of OTDLs of different lengths with the optical switches, we can obtain a higher number of OTLDs for generating various amounts of time delay. Figure 3(a) shows the design of such an electro-optically switchable OTDL device. It consists of two OTDLs (OTDL I and II) of lengths of 11.22 cm and 23.06 cm, three Mach-Zehnder interferometers (MZIs) which serve as the optical switches, and a set of microelectrodes fabricated along the two arms of each MZI. The whole device has a footprint of $4.5{\,\rm cm} \times 0.7{\,\rm cm}$. Each MZI can be individually controlled by adjusting the voltages applied on the microelectrodes for choosing different delay paths. In our design, all the short delay arms, which are shown as the upper waveguide arms after the optical switches, have a same length of $\sim 8$ mm. Due to the fact that the lengths of OTDL I and II are 11.22 cm and 23.06 cm, respectively, a delay step of $\Delta t = 0.789$ ns (i.e., determined by the length difference of $\Delta L = 11.84$ cm between the OTDL I and II) can be obtained. The maximum time delay provided by the switchable OTDL in Fig. 3(a) is 2.285 ns when both the OTDLs I and II are chosen for constructing the delay path as calculated based on the refractive index of LN. Figure 3(b) shows the top-view optical micrograph of the fabricated switchable OTDL device, which is placed near a 1-Yuan RMB coin for size comparison. To examine the influence of the optical switches on the propagation losses of the OTDLs, the continuous-wave TE polarized laser at 1550 nm wavelength was coupled into the OTDL device. By adjusting the voltage applied to the MZIs, the beam coupling into the port 1 of the device was routed either to travel through only OTDL I, only OTDL II or both OTDLs I and II. The losses measured with only OTDL I, only OTDL II, and both OTDLs I and II are 0.308 dB, 0.645 dB, and 0.960 dB, respectively, as shown in Fig. 3(c). The perfect linear dependence of the total loss as a function of the length of OTDL indicates that the influence of the optical switches is negligible. Again, the propagation loss can be determined as $\sim 0.02927$ dB/cm from the slope of the fitting line in Fig. 3(b), which is in excellent agreement with the measured propagation loss presented in Fig. 2(f). In our experiment, the half-wave voltage of all the MZIs was measured to be $\sim 5$ V. At last, we directly measured the time delay generated in the combination of OTDL I and II of a length of 32.68 cm using a femtosecond laser (OYSL Inc., Model: FemtoYL-Vary). The pulse duration measured before entering the OTDL was $\sim 500$ fs, which can be broadened to $\sim 1.2$ ns (i.e., full width at half maximum, FWHM, as measured from the waveform recorded on oscilloscope) after traveling through the OTDL device due to the material and geometric dispersions. In the measurement, the 500 fs pulses of TE polarization were first sent into the first MZI of the OTDL device and split into two beams. One beam travels through the two upper waveguides in Fig. 3(a) and the other travels through the OTDLs I and II consecutively, giving rise to the largest relative time delay between the two pulses. As we have described above, the relative optical path length of 32.68 cm corresponds to a relative time delay of 2.285 ns in theory. Experimentally, the two output signals were measured to arrive at the photodetector with a time difference of $\sim 2.2$ ns. The waveform recorded on the oscilloscope is shown in Fig. 3(d), in which the two peaks (i.e., corresponding to the waveforms of the two stretched pulses after passing through the two delay paths in the OTDL device) are separated from each other by a time difference of $\sim 2.2$ ns. The result agrees well with the theoretically calculated time delay of 2.285 ns. The difference in the experimental measurement and theoretical simulation can be attributed to the deviation of the theoretically calculated effective refractive index from the real effective index in the fabricated waveguides. In addition, the different amplitudes of the two peaks in Fig. 3(d) is caused by the additional propagation loss in the longer delay line. Here we calculated the propagation loss based on the difference in the amplitudes of the two peaks as shown in Fig. 3(d). The calculated loss of 0.0286 dB/cm again nicely agrees with our measurements in Fig. 2(f). Thus, the optical loss of $\sim 0.03$ dB/cm measured in the OTDLs is reliable, indicating a $\sim 3$ dB loss for an OTDL of one meter length.
Fig. 3. (a) Schematic of the MZI switchable OTDL. (b) The digital camera photo of the fabricated OTDL device. (c) The measured losses of OTDL I, II and both, showing a linear dependence on the length of OTDL. (d) The waveform recorded on the oscilloscope, showing two peaks separated by a time delay of 2.2 ns. Negative time means the pulses arrive first at the photodetector.
To summarize, we have fabricated the single-mode OTDLs of meter-scale lengths on the LNOI platform using PLACE technique. The measured time delay of $\sim 2.2$ ns in the OTDL of 32.68 cm agrees well with theoretical prediction. The loss in the fabricated OTDLs is measured to be $\sim 0.03$ dB/cm. The capability of fabricating meter-scale-length waveguides of low propagation loss is not only useful for OTDL application, but also beneficial for large-scale PIC applications. For many years, the PIC community has been suffering from the lack of high-quality optical waveguides which can possess all the desirable properties including low propagation loss, high refractive index contrast, and high electro-optic tuning efficiency at once. Our results suggest that the LNOI waveguides fabricated by the PLACE technique can fulfill all of these requirements, making the LNOI a promising material platform for large-scale dense PIC applications.
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