Abstract
1. Introduction
Thin film lithium niobate on insulator (LNOI) is emerging as a promising platform for integrated photonic technologies because of its small footprint, broadband ultra-low propagation loss, high optical nonlinear coefficient, and large electro-optical effect[
In this Letter, we demonstrate an EO tunable microlaser based on an -doped high-quality () LN microdisk resonator fabricated by photolithography assisted chemo-mechanical etching (PLACE). By applying voltage on the integrated Cr thin film microelectrodes beside the -doped LN microdisk resonator, the electro-optic modulation with an effective resonance-frequency tuning rate of 2.6 GHz/100 V was achieved. Furthermore, the lasing wavelength of -doped LN microdisk laser can be tuned by 45 pm when the voltage is raised from to 200 V.
2. Device Characterization
In our experiment, the on-chip LN microdisk resonator integrated with Cr film electrodes was fabricated on a 600-nm-thick Z-cut -doped LNOI with a doping concentration of 1% (molar fraction). The -doped LN thin film is bonded by a 2-µm-thick isolation layer on a 0.5-mm-thick undoped LN substrate, which was fabricated by the smart-cut method[
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Figure 1.(a) Schematic of the on-chip Er3+-doped LN microdisk resonator integrated with Cr film electrodes. (b) The top view of the 200-µm-diameter Er3+-doped LN microdisk from the optical microscope. (c) The enlarged image of the rim of the Er3+-doped LN microdisk by a 100× microscope objective.
To characterize the electro-optical tunability of the -doped LN microdisk laser, we used an experimental setup, as shown in Fig. 2(a). Here, a continuous-wave C-band tunable laser (CTL 1550, TOPTICA Photonics Inc.) was used for characterizing the Q factor of the -doped microdisk. Alternatively, a diode laser (CM97-1000-76PM, Wuhan Freelink Opto-electronics Co., Ltd.) operating at the wavelength was chosen to pump the -doped LN microdisk. The polarization states of the tunable laser and pump laser are adjusted using the in-line fiber polarization controller (FPC561, Thorlabs Inc.). The light into and out of the fabricated -doped LN microdisk was coupled by a tapered fiber with a waist of 1 µm. A photodetector (New Focus 1811-FC-AC, Newport Inc.) was directed in the fiber path to measure the transmission spectrum and Q factor of resonant modes of the microdisk. The signal in the output of the fiber was captured by an optical spectrum analyzer (OSA, AQ6370D, Yokogawa Inc.). A direct current (DC) stabilized power source (CE1500002T, Rainworm Co., Ltd.) was used as the voltage generator for Cr electrodes, which provided a variable voltage ranging from 0 V to 500 V. Two probes (ST-20-0.5, GGB Industries Inc.) were used to apply DC voltage on Cr electrodes, respectively. Figure 2(b) illustrates the measured transmission spectrum for the wavelength range from 1540 nm to 1550 nm. Both the fundamental mode and higher-order modes of the -doped LN microdisk are excited, which are labeled with different markers (star, triangle, and square), and the free spectrum range (FSR) of the 200-µm-diameter -doped LN microdisk is measured to be about 1.6 nm. Figure 2(c) is the experimental setup photographed by a cell phone, and the strong green upconversion fluorescence in the -doped LN microdisk pumped by a 976 nm laser can be easily noticed.
Figure 2.(a) Schematic of the experimental setup for tunable Er3+-doped LN microdisk laser. (WG, waveform generator; CTL, C-band tunable laser; PL, pump laser; PC, polarization controller; PD, photodetector; Osc, oscilloscope; OSA, optical spectrum analyzer; VG, voltage generator; OF, optical fiber; EC, electric cable.) (b) The measured transmission spectrum for the wavelength of the Er3+-doped LN microdisk laser. (c) The experimental setup photographed by a cell phone.
The intrinsic Q factors of 80 resonant modes on the -doped LN microdisk produced in a batch were plotted statistically in Fig. 3(a), which displays the distribution of Q factors with different resonant modes. The loaded Q factor was measured at low laser power to avoid thermal broadening effects. The highest intrinsic Q factor of our -doped LN microdisk was measured to be through a double Lorentzian fitting at the wavelength of 1542.39 nm, as shown in Fig. 3(b).
Figure 3.(a) Histogram showing the statistic results of 80 resonant modes in Er3+-doped LN microdisk. (b) The double Lorentzian fitting showing a mode splitting, indicating both intrinsic Q factors of 2.13 × 106 as measured at λ = 1544 nm.
Figure 4(a) shows that the resonant wavelength continuously shifts with the increase of the applied DC voltage; the measurement was performed around the resonant wavelength of 1551.12 nm. Benefiting from the large electro-optical coefficient of LN crystal and a high Q factor of our microdisk resonators, we observe that by changing the electric voltage from −200 V to 200 V, a linear dependence of the resonant wavelength on the pump power is observed, showing that the resonant wavelength shifts with , as shown in Fig. 4(a). The linear fitting in Fig. 4(b) confirms that the resonant wavelengths move linearly with the applied negative and positive voltages across the -doped LN microdisk, and the tuning rates of the applied negative and positive voltages are 2.6 GHz/100 V and 1.5 GHz/100 V.
Figure 4.Electro-optic modulation in Er3+-doped LN microdisk resonator. (a) Normalized transmission measured when −200 V, −150 V, −100 V, −50 V, 0 V, +50 V, +100 V, +150 V, and +200 V voltages were applied on the electrodes. (b) The linear fitting of resonance wavelength shift in the Er3+-doped LN microdisk resonator with the applied negative and positive voltages.
The lasing mode of the -doped LN microdisk shows a strong dependence on the applied voltage. As shown in Fig. 5(a), at the pump laser power of 18 mW, the laser is a single-frequency lasing emission at the wavelength around 1544.658 nm, and with a side mode suppression ratio (SMSR) of 29.12 dB. This should be a result of the strong competition between the lasing modes of different gain efficiencies. Benefiting from the large electro-optical coefficient of the LN crystal, we are able to continuously red-shift the resonant wavelength by by increasing the electric voltage from to 200 V, as shown in Fig. 5(b). This observation indicates that the -doped LN microdisk laser provides an efficient and convenient method for all optical tuning of the on-chip laser wavelength.
Figure 5.(a) Spectrum of the Er3+-doped LN microdisk laser with the pump power at 18 mW. (b) Recorded lasing spectra of the microdisk with the increasing voltage applied on electrodes.
3. Conclusions
To conclude, we have demonstrated an EO tunable microlaser based on an -doped high-quality () LN microdisk resonator. The lasing wavelength of the -doped LN microdisk laser can be tuned by 45 pm when the voltage is changed from to 200 V. This device can find interesting applications in emerging fields including photonic chip, high-speed optical communication, precision metrology, and artificial intelligence. Future investigations will focus on the physical mechanism of single-mode lasing and improving the lasing wavelength electro-optical tuning range by systematical optimizations of the geometries of the microdisk and the microelectrodes.
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