On-chip arrayed waveguide grating fabricated on thin film lithium niobate

We design an on-chip 8-channel TFLN AWG and fabricate the device using photolithography assisted chemo-mechanical etching (PLACE) technique. We experimentally measure the transmission of the fabricated TFLN AWG near the central wavelength of 1550 nm. We obtain an on-chip loss as low as 3.32 dB, a single-channel bandwidth of 1.6 nm and a total-channel bandwidth of 12.8 nm. The crosstalk between adjacent channels was measured to be below -7.01 dB within the wavelength range from 1543 nm to 1558 nm, and the crosstalk between non-adjacent channels was below -15 dB.

one waveguide channel or vice versa.In the optical communications industry, AWGs are used as wavelength division multiplexed (WDM) data transmitters for expanding the capacity and improving the transmission rate of optical networks [1][2][3].In addition, AWG can also be used in micro-spectrometer and on-chip optical coherence tomography due to their low insertion loss, high spectral resolution, low cost, small footprints and monolithic integration [4,5].Therefore, significant endeavors have been undertaken to create high-performance AWGs utilizing diverse materials such as silica, silicon, silicon nitride, and polymers [6][7][8][9][10][11][12].Silica-based AWG has been successfully commercialized due to its ultra-low absorption loss (~0.1 dB/km) and mature processing technique, whereas its large footprint (cm 2 ) caused by the low refractive index of silica (~1.44@1550 nm) impedes high-density integration [13].In contrast, silicon has much higher refractive index (~3.48@1550nm), enabling significant reduction of the footprint of AWG.Unfortunately, silicon has high absorption loss (~0.5 dB/cm) in the infrared communication band and it cannot operate in visible-band [14][15][16].Silicon nitride has a moderately high refractive index (~1.98@1550nm) as well as low absorption loss (~0.0013 dB/cm) [17,18].For applications requiring high-speed reconfiguration, the absence of electro-optic effect inhibits silicon-nitride based AWG from fast tuning of its operation wavelength.Polymer-based AWG has the advantages of low cost, flexibility, and ease of fabrication, but it typically has relatively low thermal stability and damage threshold [11,12,19].
Here, we demonstrate an AWG fabricated on the thin film lithium niobate (TFLN) platform.Featured with the high electro-optic coefficient ( 33 = 30.9pm/V) , wide transmission window (0.35-5 μm), low optical loss (~0.002 dB/cm), high refractive index (~2.2),and high thermal stability, and benefitted from the advances of wafer processing technologies, the TFLN becomes an ideal PIC platform for a wide range of applications, such as optical communication, quantum technology, and optical computation [20][21][22][23][24][25][26][27][28][29].We design a low-loss 8-channel AWG on TFLN and fabricate the device using the photolithography assisted chemo-mechanical etching (PLACE) technique.Compared with conventional lithography and reactive ion etching (RIE) process, the PLACE technique not only has large exposure field and high writing speed but also has ultra-low scattering loss [21].The fabricated TFLN AWG has an on-chip insertion loss as low as 3.32 dB, whereas the insertion loss of the 8-channel TFLN AWG fabricated by lithography and RIE process is 25 dB [30].The single-channel bandwidth and a total-channel bandwidth are measured as 1.6 nm and 12.8 nm, respectively.The crosstalk between adjacent channels was measured to be less than -7.01 dB within the wavelength range of 1543 nm-1558 nm.Besides, the crosstalk between non-adjacent channels was measured below -15 dB.The fabricated TFLN AWG holds great promise for various applications demanding on-chip wavelength multiplexing/demultiplexing, such as spectral routing of PIC based supercontinuum source and integrated pulse shaper for ultrashort pulses due to fast and efficient electro-optic tuning.The TFLN AWG can also be combined with other optical devices, such as high-speed optical modulators, micro-lasers and waveguide amplifiers, to further improve the performance and application range of TFLN photonic chips [31][32][33][34][35][36].

Device Design and Fabrication
In our design, the AWG is to be fabricated on 300-nm thick Z-cut TFLN platform.This not only avoids the generation of higher-order modes in our current waveguide configuration but also maintains the invarient refractive index regardless of the light propagation directions in the plane of TE polarization.Figure 1   For the same wavelength, the output light has the same phase difference, so the light output from the array waveguide interferes in the second FPR, the light of different wavelengths focus on different output waveguide ports and exit from different output waveguides, thus realizing demultiplexing.To ensure the low bending loss of the array waveguide, the overall bending radius is set to be greater than 600 μm.Under the central wavelength of 1550 nm, the effective refractive index of the waveguide is calculated as 1.75158.Based on the working principle of the AWG, we designed the 8channel AWG at the central wavelength of 1550 nm.In our design, the length of FPR is 610.589μm, the length difference (ΔL) between adjacent array waveguides is 85.508 μm, and the wavelength spacing of 1.6 nm between adjacent channels.The loss of different channels was simulated and calculated using the beam propagation method as shown in Figure 2(b).According to the simulation results, the loss of different channels was calculated between -2.8 dB and -5.4 dB, and the operating wavelength range of the The prepared TFLN on insulator wafer is 300-nm thick Z-cut TFLN on 4.7-μmthick SiO2 film on 500-μm-thick silicon substrate (NANOLN, Jinan Jingzheng Electronics Co. Ltd.), the thickness error of the TFLN is within 10 nm.The TFLN AWG was fabricated by the PLACE technique, the specific processing steps are as follows.
Firstly, a 200-nm-thick chromium film was deposited on top of the TFLN using magnetron sputtering.Second, the chromium hard mask pattern for the AWG structure was fabricated by femtosecond laser direct-write lithography.Thirdly, the smooth waveguide edges were etched by chemo-mechanical polish and greatly reduce the scattering loss of waveguide.Finally, to meet the refractive index conditions for singlemode transmission and reduce absorption loss, 1-μm-thick cladding SiO2 film was deposited using inductively coupled plasma chemical vapor deposition (ICPCVD) at low temperature of 80 ℃.More fabrication details of the PLACE technique can be found in our previous work [35,37].Figure 3

Results and Discussions
The optical spectrum measurement device for the TFLN AWG is shown in in Figure 4.
A C-band continuously tunable laser (CTL 1550, TOPTICA Photonics Inc.) was coupled to the TFLN AWG using a lensed fiber.The polarization state of both the signal laser are adjusted using an in-line fiber polarization controller (FPC561, Thorlabs Inc.).
The output signal from the TFLN AWG is measured by an electrical signal by photodetector (New Focus 1811-FC-AC, Newport Inc.) through lensed fiber.The captured signal is then converted to electrical signals and analyzed by an oscilloscope (Tektronix MDO3104).The couple loss between the lensed fiber and AWG was calculated to be 8.67 dB.When calculating the on-chip insertion loss of the TFLN AWG, the coupling loss was removed.
(a) illustrates the typical cross-section of TFLN rib waveguide fabricated by PLACE technique, which was covered with 1-μm-thick SiO2 film.Figure1(b)shows the simulated TE mode at 1550 nm in the fabricated TFLN waveguide where the top width is 1.1 μm, the bottom width is 5.2 μm, the etching depth by the chemo-mechanical polish is 250 nm, and the waveguide was supported by the bottom TFLN slab with a thickness of 50 nm.

Figure
Figure 1．(a) A schematic diagram of cross-section of the fabricated TFLN waveguide.(b) Simulated TE mode profile at λ=1550 nm in z-cut TFLN rib waveguide.

Figure 2 (
Figure 2(a) is a conceptual illustration of the designed 8-chanels TFLN AWG based on the above waveguide configuration.It is composed of input/output waveguides, two free-propagation regions (FRP) and an array of 20 waveguides.The input/output waveguides are distributed on the circumference of the Roland circle of the two FPRs, and the ends of the array waveguide are located on another side of the FPR, with a fixed length difference ΔL between the adjacent array waveguides.The principle of the AWG is that when a light signal containing multiple wavelengths enters the first FPR from the input waveguide, it diffracts and couples into the array waveguides, and the array waveguides introduce a certain phase shift to the transmitted light signal.For the same wavelength, the output light has the same phase difference,
(a) is a micrograph of the fabricated 8channel TFLN AWG with a length of 1.7 cm and a width of 0.7 cm.Figure3(b)is the zoom-in micrograph of the FPR region, where the diffracted light field in the Roland circle is connected to the array waveguide through a taper structure, thereby reducing the insertion loss of the device.Figure3(c)shows the micrograph of the array waveguide at a magnification factor of 100×, the width of waveguide is 1.1 μm.After chemical mechanical polishing, the edge of the waveguide is extremely smooth, which can greatly reduce the propagation loss of the device.

Figure 3 .
Figure 3. (a)The micrographs of the fabricated 8-channel TFLN AWG.(b)The magnified micrograph of the FPR.(c)The micrograph of the array waveguides.

Figure 4 .
Figure 4. Schematic diagram of optical spectrum measurement setup for the AWG.

Figure 5 .
Figure 5. Measurement of the spectrum of the 8-channel AWG.(a) overall spectrum measurement results of the 8 channels.(b) single channel spectrum measurement results.

Figure 6 .
Figure 6.The TFLN AWG chip and the output images captured by the infrared camera when different center wavelengths are injected in the input.