Recently, the team of Qiu-Shi Guo from the City University of New York and Alireza Marandi from the California Institute of Technology published an article entitled "Ultrafast mode-locked laser in nanophotonic lithium niobate" in the journal Science. "The team cleverly combined the high laser gain of III-V semiconductors with the excellent electro-optical properties of thin-film lithium niobate and fabricated an electrically pumped mode-locked laser with a high peak pulse power through hybrid integration, which has a repetition frequency of 10 GHz near 1065 nm, an optical pulse width of 4.8 ps, a pulse energy of more than 5 pJ, and a peak power of more than 0.5 pJ. pJ, and peak power greater than 0.5 W. Moreover, its laser output pulse energy and peak power have reached the highest level of mode-locked lasers under the nanophotonics platform.
Lithium niobate crystal material is a rare artificial crystal material that combines piezoelectric, electro-optical, acousto-optic, photoelastic, nonlinear, photorefractive and laser-active effects, etc. Together with the advantages of stable mechanical properties, easy processing, high temperature resistance, corrosion resistance, abundant raw material sources, low price, and easy to grow into large crystals, especially after the implementation of different dopants can show a variety of special properties, it is the most comprehensive and comprehensive photonics properties that have been discovered so far. It is the crystal with the most photonic properties and the best comprehensive indexes discovered by people so far, and has a very broad market application prospect. Therefore, it is also known as the "optical silicon" material in the photonic era, and is widely used in high-performance filters, electro-optical devices, holographic storage, 3D holographic displays, nonlinear optical devices, and optical quantum communication.
Mode-locked lasers are capable of generating intense, coherent, ultrashort optical pulses on picosecond and femtosecond time scales, and thus can realize applications in such cutting-edge fields as extreme nonlinear optics, optical atomic clocks, optical frequency combs, bio-imaging, and photonic computing. However, the current conventional mode-locked lasers have the disadvantages of high price, high power consumption and large size, and the mode-locked lasers based on heterogeneous integration of III-V semiconductors with lithium niobate nanophotonic platforms are expected to achieve higher output power and higher tunability, so the related technologies have attracted extensive attention from researchers.
Aiming at the technological bottleneck in the field of mode-locked lasers, the research team has broken through the size limitation of traditional mode-locked lasers by integrating a III-V gain medium and a lithium niobate phase modulator, and realized mode-locked lasers with excellent performance while shrinking the size to the chip level.
Mode-locked lasers can be divided into two mechanisms: passive mode-locking and active mode-locking. As shown in Figure (A) below, to realize active mode-locking of the laser, the authors added an electro-optic phase modulator based on thin-film lithium niobate inside the laser resonant cavity; since the refractive index of lithium niobate changes periodically under the electro-optic effect, which results in the optical pulses failing to maintain a steady state inside the cavity, the research team also designed a good match between the phase modulation time period and the round-trip time of the optical pulses inside the cavity, and utilized dispersion offsetting to achieve a good match with the phase modulation time period. Therefore, the research also designed to achieve a good match between the phase modulation time period and the round-trip time of the optical pulse in the cavity, and use the dispersion to offset the accumulated chirp, and compensate the optical pulse loss based on the laser gain, and finally realize the phase locking, as shown in the following figures (B-C); the schematic diagram of the realized integrated lithium niobate mode-locked on-chip laser is shown in the following figure (D).
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Operating principle and device schematic of the integrated lithium niobate mode-locked on-chip laser.
According to the research team, with its high output peak power and precise frequency control capability, the mode-locked laser is expected to build an ultrafast nonlinear optical system with full on-chip integration, thus realizing optical frequency combs, supercontinuum light sources and atomic clocks with full frequency locking. This will greatly facilitate the development of optical communications, medical imaging, precision measurement, computing and other fields. "In the longer term, the on-chip mode-locked laser may have irreplaceable applications in the fields of coherent communications, precision timing, and precision measurement."
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Current tuning analysis results of the integrated active mode-locked laser
In addition, conventional solid-state and fiber mode-locked lasers based on active mode-locking mechanisms can only achieve mode-locking within a limited range of external modulation frequencies, and when the external modulation frequency exceeds the relevant range, the laser output optical pulse will lose its fixed phase relationship (i.e., it loses coherence). As shown in the figure above, compared with conventional active mode-locked lasers, the integrated lithium niobate on-chip mode-locked laser realized in this study has a large tunable range of pulse repetition frequency and is capable of generating coherent optical pulses in the 200 MHz modulation frequency range. In addition, the carrier frequency and pulse repetition frequency of the pulsed laser can be significantly changed by adjusting the pump current or modulation frequency of the laser. This means that the mode-locked laser can be manipulated in a variety of ways. By precisely feedback controlling the laser's pump current or modulation frequency, the pulse repetition frequency and carrier frequency of the laser can be precisely controlled, thus realizing an optical frequency comb that can precisely control the frequency, which is of great significance for applications in precise frequency measurement.
Conventional semiconductor mode-locked lasers typically integrate the gain region and saturable absorber (mode-locked element) on the same semiconductor chip. Due to the complex carrier dynamics of the tribo-five semiconductor, the laser can only achieve ultrashort pulse generation in a very narrowly driven pump current operating region, which is not conducive to the realization of high-power laser output. However, this study fully unleashes the high power output capability of the tribo-five semiconductor by utilizing thin-film lithium niobate as an active mode-locking element.
Based on the excellent properties of thin-film lithium niobate, the team has achieved a series of results in the fields of thin-film lithium niobate, integrated optics and nonlinear optics. For example, the second-order nonlinear optical effect of thin-film lithium niobate nanophotonics was utilized to demonstrate the fastest (46 femtoseconds), ultra-low energy (80 femtokinetic) all-optical switching on an integrated optics platform to date. On the thin-film lithium niobate platform, we have also realized an optical parametric amplifier with very high gain (100 dB/cm) and very large gain bandwidth (600 nm), a wide range of frequency-tunable optical parametric oscillator, and the highest quantum compression (4.9 dB) in the field of integrated optics to date.
Ian S. Osborne, editor of Science, praised the research: "Mode-locked lasers are a cutting-edge technology in ultrafast science, enabling frequency combs with ultrashort coherent optical pulses and precise spacing. By integrating a III-V gain medium with a lithium niobate phase modulator, we have broken through the size constraints of conventional mode-locked lasers and realized a mode-locked laser with excellent performance while downsizing to the chip scale. The results of this research are expected to have practical applications in cutting-edge fields such as precision measurement and spectroscopy."
