1. Introduction

Thin film lithium niobate (TFLN) photonics has the potential to revolutionize photonic integrated circuits (PICs) technology thanks to its excellent optical and optoelectronic properties such as broad optical transparency window (0.35-5 µm), high nonlinear coefficient (${d_{33}} ={-} 41.7 \pm 7.8\; pm/V@\mathrm{\lambda } = 1.058{\;\ \mathrm{\mu} \mathrm{m}}$), high refractive index (∼2.2), and large electro-optical effect (${r_{33}} = 30.9\textrm{pm}/\textrm{V}@\mathrm{\lambda } = 632.8\textrm{nm})$ [1–9]. In addition, lithium niobate crystals can also be doped with rare earth ions (REI) to generate optical gain which leads to active on-chip micro-devices including laser sources and waveguide amplifiers [10–12]. Various whispering gallery mode (WGM) microresonator lasers have been successfully fabricated on the TFLN with the doping ion species of Yb³⁺, Er³⁺, Nd³⁺, and Tm³⁺, respectively [13–25]. However, in comparison with the WGM lasers, the Fabry-Perot (FP) resonator-based lasers allow for the generation of high output power benefitted from the extended gain length in the resonator [26,27 ]. Combining the efficient optical gain and electro-optic tuning ability in the active TFLN substrate, it is possible to achieve on-chip high-speed electro-optic tunable FP lasers. Here, we demonstrate, for the first time to the best of our knowledge, an on-chip single-mode FP electro-optically tunable (EOT) laser on Erbium-doped TFLN (Er: TFLN) platform. The FP cavity consists of two Sagnac loop reflectors (SLRs) with a loaded quality (Q) factor of 1.3 × 10⁵ and a free spectral range (FSR) of 68 pm. The fabricated Er: TFLN FP laser structure is integrated with driving electrodes designed for low voltage wide tuning. The measured electro-optic efficiency is 1.7 pm/V with good linearity and observable changes in resonance extinction ratio and linewidth. And a continuous laser tuning of 24 pm around 1544 nm is achieved by −6 V to 6 V driving voltage, the electro-optic efficiency is measured as 2 pm/V with good linearity and no observable changes in resonance extinction ratio and linewidth. The demonstrated electro-optically tunable laser has great potential in optical communication, LiDAR, and laser cooling.

2. Device design and fabrication

The schematic view of our laser structure is shown in Fig. 1(a), where a straight waveguide is fabricated between the two SLRs to form a FP cavity, and a pair of monolithically integrated microelectrodes are fabricated along the straight part of FP cavity. The structure is intentionally designed with an x-cut LN configuration, where the microelectrodes can generate electric fields along Z-axis to ensure the maximum electro-optical tuning efficiency. The fabrication of the laser structure is accomplished in two main procedures: fabrication of the FP cavity and preparation of the microelectrodes. The FP cavity structure was fabricated on a 500-nm-thick X-cut Er: TFLN on insulator (TFLNOI) using the photolithography assisted chemo-mechanical etching (PLACE) technique. The erbium ion (Er³⁺) doping concentration in TFLN is 0.5 mol%. The fabrication resolution of PLACE is around 200 nm, allowing for the preparation of high-performance waveguide-based fundamental photonic structures such as directional couplers. More detailed fabrication processes can be found in Ref. 16. The metal electrodes (10 nm Cr/100 nm Au, deposited using a magnetron sputtering system) are formed along the FP cavity straight waveguide using femtosecond laser selective direct writing in which the high-quality ablation removes the metal thin foil coated on the surface of TFLN. The bottom Cr film serves as an adhesion layer between the TFLN layer and the gold foil. Gold is chosen for the micro-electrodes due to its high conductivity. Figure 2(b) shows the top view of the fabricated electro-optically tunable FP cavity laser with the size of 6.5 mm × 1.5 mm. The gain region is a straight waveguide between the two SLRs, with a length of approximately 5.8 mm. The size of SLRs is 1.8 mm × 0.4 mm, each of which consists of a 2 × 2 3-dB optical coupler whose output ports are fused to form a loop waveguide. Figure 1(c) shows the coupling region of SLRs, where the gap width and the coupling length of directional coupler (DC) are 3.2 µm and 300 µm, respectively. As shown is Fig. 1(b) and (d), the size of paired microelectrodes is 2.8 × 0.4 mm with a gap of 15 µm between them.

 

Fig. 1. (a) Schematic of the on-chip electro-optically tunable Er: TFLN FP laser structure. (b) Fabricated Er: TFLN FP laser and electrical contacts. (c) The coupler region of SLRs, inset: cross-section (x-z plan) of the coupler region of SLRs. (d) Close-up optical micrograph of the metal electrodes and the optical waveguide, inset: cross-section (x-z plan) of the electrode region.

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Fig. 2. (a) Experimental setup for quality measurment and laser wavelength tuning demonstration of the fabricated electro-optically tunable Er: TFLN FP laser (CTL: continuously tunable laser; FPC: fiber polarization controller; LD: laser diode; WDM: wavelength division multiplexers; EOT TFLN FP laser: electro-optically tunable Er: TFLN FP laser; DCPS: direct current power supply; OSA: optical spectrum analyzer; PD: photodetector). (b) Photograph of a direct voltage-driven Er: TFLN FP laser by two DC probes, scattered the green upconversion fluorescence of Er: TFLN FP resonator pumped by the 1480-nm LD. (c) Cross-section view of the simulated optical TE mode profile and electrical field.

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3. Results and discussions

3.1 Experimental setup for optical and electro-optical characterization of the fabricated Er: TFLN FP laser

We measured the quality (Q) factor, electro-optic tuning resonance wavelength, and electro-optic tuning laser wavelength of our Er: TFLN FP laser device using the experimental setup shown in Fig. 2(a). Light from a C-band continuously tunable laser (CTL 1550, TOPTICA Photonics Inc.) is coupled into and collected from the waveguide facets of our device using lensed fibers, with the polarization state controlled by a 3-paddle fiber polarization controller (FPC561, Thorlabs Inc.). The 1480 nm pump laser beam from a continue wave laser diode (LP-PUMP-1480-B-500, Micro photons (shanghai)Technology Co. LTD) is coupled into the Er: TFLN FP cavity using fiber-based 1480 nm/1550 nm wavelength division multiplexer (WDM). The transmitted light from the output of the Er: TFLN cavity was collected by a lensed fiber and directed into a photodetector (New focus 1811, Newport Inc.). An optical spectrum analyzer (OSA: AQ6375B, YOKOGAWA Inc.) is also used to analyze the spectra of the laser signal. A direct current (DC) voltage supply (IPMP250-1 L, INTERLOCK) is used as the voltage generator for microelectrodes of our device, and a pair of DC probes (ST-20-0.5, GGB) is used to connect the on-chip metal contacts. Figure 2(b) shows the upconverted green fluorescence emission of the Er: TFLN FP cavity pumped by the 1480-nm LD. Figure 2(c) shows the numerically simulated overlap between the corresponding optical and electric fields in the waveguide.

3.2 Resonance and electro-optically resonance tunable characterization of fabricated Er: TFLN FP cavity

Figure 3(a) shows a typical transmission spectrum of a FP cavity with a free spectral range (FSR) of 68 pm and a loaded quality (Q) factor of 1.3 × 10⁵. Figure 4(a) displays the transmission spectra of the resonant mode under electro-optic tuning for the fabricated Er: TFLN FP cavity. When the voltage increases from −25 to 25 V with a step of 5 V, the resonant wavelength of Er: TFLN FP cavity has a red-shift due to the reduced effective index caused by electro-optical effect of LN. The resonant wavelength shift is about 80 pm, which exceeds a FSR (∼68 pm) of this Er: TFLN FP cavity. As shown in Fig. 4(b), the resonant wavelength has a linear relation with the voltage. A linear fit is performed to analyze the relationship between the wavelength shift of the resonant peak and the applied voltage, and the EO tuning rates are 1.7 pm/V.

 

Fig. 3. Measured transmission spectrum of a high-Q (1.3 × 10⁵) Er: TFLN FP cavity.

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Fig. 4. (a) Measured transmission spectra of a high-Q FP cavity exhibit resonant wavelength shift with applied DC voltage. (b) Linear resonant wavelength shift as a function of DC voltage, the measured tuning efficiency is 1.7 pm/V.

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3.3 Lasing emission and electro-optically wavelength tunable characterization of the fabricated Er: TFLN FP cavity laser

As shown in Fig. 5(a), we demonstrated the wavelength tunability. Here, A 1480 nm laser diode was used as the pump source to excite erbium ions from the ground state I_11/2 to the excited state I_13/2, resulting in a population inversion, which ultimately produces laser emissions around 1544 nm through stimulated radiation process. Compared to 980 nm pump laser, 1480 nm pump laser has a higher quantum efficiency in Er³⁺-doped laser system. An optical spectrum analyzer (OSA) was used to receive the output signal from the output waveguide of the fabricated Er: TFLN FP cavity. Similarly, the laser wavelength was tuned by applying different voltages on the microelectrodes through a pair of probes. The green light observed in the FP laser cavity shown by Fig. 2(b) indicates the up-conversion fluorescence is generated during the pumping process. Figure 5(a) depicts the laser wavelength under different voltage loads on microelectrodes, the laser wavelength on voltages of −6 V, −3 V, 0 V, + 3 V, and +6 V were displayed here. Figure 5(b) shows the laser wavelength as a function of the applied voltage, it can be observed that the laser wavelengths have a linear relation with the applied voltage. The laser wavelength increased by approximately 24 pm with an applied voltage of 12 V, and the EO tuning rate is about 2 pm/V. This is consistent with the EO tuning efficiency of the previous resonance wavelength in the Er: TFLN FP cavity.

 

Fig. 5. (a) Measured optical spectra of lasing tuned over 20.4 pm wavelength range. The resolution of OSA is set to 0.003 nm. (b) The laser wavelength as a function of the applied voltage. (c) Measured optical spectra of mode-hopping phenomena when the DC voltage exceeds 10 V.

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However, it was found in the experiment that the laser wavelength exhibits mode-hopping when the applied voltage exceeds about 10 V. As shown in Fig. 4(c), the two dashed lines respectively represent the laser wavelengths theoretically corresponding to the linear changes when the microelectrode was loaded with +10 V and +20 V. Actually, the laser wavelengths were measured at these two voltages are shown by the two solid lines on the left side of Fig. 4(c). The blue solid line represents the voltage of +20 V, the red solid line represents the voltage of +10 V. It can be seen that the laser wavelengths are significantly different from those calculated theoretically. This is because the original mode does not have an advantage over other modes in terms of mode competition for the gain when the applied voltage is too large, and thus cannot effectively suppress the other modes of comparable gain, leading to generation of different mode or even multiple modes.

3.3 Conclusions and outlook

In conclusion, we have demonstrated an on-chip electro-optically tunable FP cavity laser on Er³⁺-doped thin film lithium niobate (Er: TFLN). The FP cavity consists of two SLRs with a loaded quality (Q) factor of 1.3 × 10⁵ and a FSR of 68 pm. The fabricated Er: TFLN FP laser structure is integrated with driving electrodes designed for high-speed EO tuning, and a continuous laser tuning of 24 pm around 1544 nm is achieved within −6 V to 6 V voltage range. Further improvement on the single-mode operation stability and wavelength tuning range of the laser can be achieved by incorporating mode filters into the FP laser cavity which is now under investigation.