1. Introduction

Featured with its broad optical transparency, high refractive index, large electro-optical and nonlinear optical coefficients, the integrated thin film lithium niobate (TFLN) photonics have emerged as a promising platform for realization of high-performance photonic integrated circuit (PIC) both for classical and quantum applications [1–7]. Moreover, since the lithium niobate (LN) crystal proves to be an attractive host material for rare earth ions (REIs), various on-chip microlasers and waveguide amplifiers have been successfully fabricated on the REI-doped TFLN recently [8–17]. The on-chip microlasers are first demonstrated with multi-frequency lasing [8–12]. Single-frequency microlaser is further realized through the Vernier effect with two evanescently coupled microresonators, though the delicate resonant mode matching between the coupled microresonators imposes stringent requirements on the fabrication precision [18–21]. Recently, the single-frequency output in a single Er³⁺-doped thin film lithium niobate (Er: TFLN) microring resonator by introducing mode-dependent loss and gain competition has been reported, and such on-chip single-frequency microlaser was fabricated by electron beam lithography (EBL) and inductively coupled plasma (ICP) dry etching [22]. In this work, we demonstrate an integrated single-frequency microring laser operating around 1531 nm wavelength fabricated by the photolithography assisted chemo-mechanical etching (PLACE) technique. This technique has the large writing field (∼8 inch) and high writing speed (∼1 cm²/min), but this technique cannot fabricate the coupling devices which need small gap less than 5 µm before. Here, we overcome the difficulty in realizing monolithic integration of waveguide-coupled microring laser by the PLACE technique [23,24]. Benefited from the large writing field and high writing speed, the PLACE technique provide superior agility and scalability concerning the footprint and power capacity of the on-chip microlaser based on the microring resonators, promising in low cost on-chip microlasers, with broad applications ranging from optical communication and photonic integrated circuit (PIC) to precision metrology and large-scale sensing.

2. Fabrication method

In our experiment, the microring resonator was fabricated on an Er: TFLN wafer by the PLACE technique. As shown in Fig. 1(a), the Er: TFLN wafer was prepared by bonding a Z-cut Er³⁺-doped 500-nm-thick TFLN onto a holder wafer with 2-µm-thick SiO₂ and 500-µm-thick silicon. The concentration of Er³⁺ ions in the TFLN is 1 mol%. The specific fabrication process includes four major steps as shown in Fig. 1(b)-(e). Firstly, a 200 nm-thick layer of chromium (Cr) film is coated on the surface of the Er: TFLN wafer by magnetron sputtering. Secondly, the Cr film on the Er: TFLN sample is patterned into the microring resonator mask using space-selective femtosecond laser direct-writing with the center wavelength∼1030 nm (PHAROS, LIGHT CONVERSION Inc.). The radius of curvature of the microring is designed to be 200 µm with a width of 1 µm, the width of the bus waveguide and the coupling gap between microring and bus waveguide are set to be 1 and 4.8 µm, respectively. Thirdly, chemo-mechanical polishing (CMP) process is performed to selectively etch the TFLN using a wafer polishing machine (UNIPOL802, Kejing Inc.). In order to achieve the coupling between the bus waveguide and the microring, a ribbed waveguide with weak ability to limit the mold field is used. In this configuration, the etching rate of the TFLN is related to the distance between the Cr masks. Under the same polishing time, the longer distance between the Cr masks, the easier it is for the polishing slurry to enter the groove between the Cr masks and the faster for the etching rate of the TFLN. The smaller gap between two Cr mask cannot be used to polish the TFLN, because the polishing slurry cannot be injected into the Cr mask gap and touches the surface of TFLN. As shown in the Fig. 1(d), after the CMP process, the thickness of the TFLN left in the area 2 (h₂) is significantly higher than that of other areas (h₃). Fourthly, the fabricated structure was immersed in the Cr etching solution to remove the Cr mask and a secondary CMP was also carried out for thinning the microring resonator to eliminate the sharp part of the top of the microring resonator after the first CMP (Fig. 1(e)).

 

Fig. 1. (a)The configuration of the Er: TFLN wafer. (b) The Cr film was coated on the surface of Er: TFLN wafer. (c) The Cr film on the Er: TFLN sample was patterned into the waveguide and microring resonator mask using space-selective femtosecond laser. (d) The first CMP process. (e) The Cr wet etching and second CMP process.

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3. Results and discussion

The fabricated microring laser is shown in Fig. 2(a), the Er: TFLN microring resonator is design with improved quality factors using quarter Bezier curves due to mode match in coupling region and lower bending loss than the conventional circular ring resonator [25]. The length of the perimeter is about 1 mm. Figure 2(b) shows the close-up optical micrograph of the LN waveguide. The gap in both sides of LN waveguide is about 4 µm. Figure 2(c) shows the close-up optical micrograph of the coupling region of the microring resonator where the mode match between the straight waveguide and the microring resonator with quarter Bezier curves. The different thicknesses of the gap bottom and the waveguide outer edge are clearly visible by the color changes in the optical micrograph of the coupling region.The scanning electron microscope (SEM) image of the cross section of the designed waveguide-ring coupling region is shown in Fig. 2(d), and the viewing angle of the SEM image is about 52°. The Er: TFLN is coated with gold thin film for the sake of focused ion beam (FIB) cutting and clear imaging in the SEM. The Er: TFLN is shown in purple and the silica layer is shown in gray. The gap between the straight waveguide and the ring resonator was set to be 4.8 µm. The Er: TFLN waveguides with a top width of 935 nm and a thickness of 465 nm (h₁) are located on the top of a 2-µm-thick SiO₂ layer. The fabricated LN waveguide is slightly thinner than the original thickness of the TFLN due to the second CMP process. Besides, due to the anisotropic polishing rate of the CMP process, the thickness at the bottom of the waveguide-ring gap is 300 nm (h₂) while the thickness of the outer edge of waveguide is 230 nm. So the tilt angle of waveguide is

 

Fig. 2. (a) Optical micrographs of the fabricated optical devices. (b) Close-up optical micrograph of the LN waveguide. (c) Close-up optical micrograph of the coupling region of the microring resonator. (d)The false color scanning electron microscope (SEM) images of the cross section of the gap between waveguide and microring, the Er: TFLN is shown in purple and the silica layer is shown in gray, the gold thin film is coated on the surface to benefit focused ion beam cutting and clear imaging the cross section of the coupling area in the SEM. The simulated electric field profile of the (e) symmetry and (f) anti-symmetry mode

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The simulated electric field profiles of the symmetry and anti-symmetry mode of the coupling region are shown in Fig. 2(e) and (f). The coupling efficiency K in the coupling area can be estimated as 0.04 dB at 1550 nm using equation

The L = 50 µm is the coupling length, n₁and n₂ are the effective refractive index of the symmetry and anti-symmetry mode, respectively, The required coupling efficiency K for critical coupling between the waveguide and the microring is calculated as 0.03 dB using

The $\alpha \approx 0.3$ dB/cm is the waveguide propagation loss, L_r= 1 mm is the perimeter of the microring resonator.

The measuring experimental setup of the Er:TFLN microring is shown in Fig. 3(a). Light from a laser diode (CM97-1000-76PM, II-VI Laser Inc.) and a tunable telecom laser (CTL 1550, TOPTICA Photonics Inc.) are coupled into and collected from the waveguide facets using lensed fibers with edge coupling loss of 8.5 dB per facet. The polarization of the pump light was adjusted by a 3-paddle fiber polarization controller (FPC561, Thorlabs Inc.). The power of the input pump laser was monitored by a power meter (PM100D, Thorlabs Inc.). A photodetector (New focus 1811, Newport Inc.) was placed in the fiber path for the quality (Q) factor measurements of resonant modes in the microring resonator. The spectra of the output beam were measured by an optical spectrum analyzer (OSA: 75B, YOKOGAWA Inc.). Figure 3(b) shows the green upconversion fluorescence micrograph when the 976 nm laser diode is input into the Er:TFLN microring, and the bus waveguide shows waviness because the high-order multimode of pump laser is stimulated in the waveguide. Figure 3(c) shows a transmission spectrum of a microring resonator with a loaded Q factor of 3.2×10⁵ at the wavelength 1530.9 nm. The Q factor is lower than previous work using PLACE technique mainly due to higher absorption loss of Er:TFLN and higher coupling loss of the monolithic waveguide-coupled microring structure [23]. As shown in Fig. 3(d), only one lasing emission peak is collected by OSA in the wavelength range of 1500 -1600 nm probably due to mode-dependent loss and gain competition. Figure 3(e) shows the enlarged spectrum around lasing emission, featuring a linewidth of 45 pm at 1530.9 nm which is limited by the resolution of the OSA (∼0.01 nm).

 

Fig. 3. (a) An measuring experimental setup for the Er:TFLN microring(CTL: Continuously Tunable Laser; FPC: fiber polarization controller; LF: lensed fiber; PD: photodetector; OSA: optical spectrum analyzer.). (b)The green upconversion fluorescence of Er: TFLN microring when the 976 nm pump diode laser is injected. (c) A transmission spectrum of the microring resonator with a loaded Q factor of 3.2×10⁵ at the wavelength 1530.9 nm. (d) Output spectrum of the Er: TFLN microring laser shows only one lasing emission peak. (e) The enlarged spectrum around wavelength 1530.9 nm.

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Figure 4(a) shows the spectra of the Er: TFLN microring laser at the increasing pump powers. The dependence of the lasing power of the Er: TFLN microring laser on the inputted pump power is illustrated in Fig. 4(b). The lasing threshold is found to be around 24.5 mW and the slope efficiency is derived as 8.33×10⁻⁶ by linear fitting. The threshold is higher and the slope efficiency is lower than previous work due to the longer perimeter of the Er:TFLN microring which necessitates higher pump power to invert erbium ions [22]. As shown in Fig. 4(c), the lasing wavelength is nearly linear blue shift with the pump power, the slope is about −3.27 pm/mW. It is attributed to the photorefractive effect where the refractive index of LN decreases with the increase of pump power. The photorefractive effect is more dominant than thermal effect in the Er: TFLN microring. As shown in Fig. 4(d), we also measured both the transmission spectrum of the Er:TFLN microring (shown in red) and the amplified spontaneous emission(ASE) spectrum (shown in blue) from a Er:TFLN waveguide. The free spectral range (FSR) of Er:TFLN microring is about 1 nm. The ASE of Er:TFLN waveguide has a sharp gain peak near 1531 nm. It can be seen that there is one resonance of the microring which is coincident with the sharp gain peak of ASE near 1531 nm. Furthermore, the identical Q factors for all the resonances shown in Fig. 4(d) indicate comparable losses of the microring in the investigated spectral range. Therefore, the resonance around the gain peak will laser first and then grow faster when increasing the inputted pump powers, which in turn can suppress other resonances from lasing due to gain competition. Further systematic investigation is needed to clarify the observed single-frequency lasing behavior of the waveguide-coupled microring resonator.

 

Fig. 4. (a) The spectra of the Er: TFLN microring laser at the increasing pump powers. (b) The dependence of the lasing power of the Er: TFLN microring laser on the input pump power. (c) Lasing wavelength shift with increased pump power. (d)ASE of an Er: TFLN straight waveguide (blue) and normalized transmission of Er: TFLN microring.

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4. Conclusion

In conclusion, we demonstrate an integrated single-frequency microring laser operating around 1531 nm wavelength fabricated by PLACE technique. This technique can achieve the higher output power in on-chip microlaser based on the fabricated bigger microring resonators. Moreover, the PLACE technique, in conjunction with integrated low propagation loss lithium niobate photonics, can be used to realize low cost and mass production of tunable on-chip microlasers, with applications ranging from optical communication and PIC to precision measurement. In the future, we will control the particle size of polishing slurry, polishing pressure, polishing speed and the gap of coupling area to improve the thickness uniformity. We will also control the length of coupling area and microring to achieve higher Q factor.