图 1  光学频率梳简介^[5,6]　(a) 光学频率梳时域波形图, 相邻脉冲之间的时间间隔与相位偏移分别对应光梳重频频率$ {f}_{{\mathrm{r}}} $与载波偏移频率$ {f}_{{\mathrm{o}}} $; (b)光频梳频谱图与f - 2f自参考示意图, 第n根梳齿经过二倍频后与第2n根梳齿拍频即可得到载波偏移频率$ {f}_{{\mathrm{o}}} $; (c)—(e)锁模光纤激光器、电光频梳与微腔光梳示意图

Fig. 1.  An introduction to optical frequency comb^[5,6]: (a) Temporal waveform of optical frequency comb, the time interval and phase shift between adjacent pulses correspond to the repetition rate $ {f}_{{\mathrm{r}}} $ and the carrier frequency offset $ {f}_{{\mathrm{o}}} $ of the optical frequency comb; (b) optical spectra of the optical frequency comb and schematic diagram of f - 2f self-reference, the carrier offset frequency $ {f}_{{\mathrm{o}}} $ can be obtained by doubling the frequency of the nth comb line and then beating with the 2n-th comb tooth; (c) schematic diagram of mode-locked fiber laser, electro-optical comb and microcombs.

图 2  微腔光梳产生的装置和原理图^[21,22,24]　(a)产生微腔光梳的实验装置图, 连续可调激光经过放大器放大后泵浦微腔产生光梳, 同时用光电探测器探测拍频信号; (b)微腔光梳产生的频域原理图, 泵浦激光经过简并和非简并四波混频产生一系列等间隔的边带; (c)孤子锁模原理图, 孤子脉冲由于增益与损耗, 色散和非线性之间的双重平衡而保持稳态; (d) 亮孤子(左)和暗脉冲(右)的时域波形图

Fig. 2.  The device and schematic diagram of microcombs generation^[21,22,24]: (a) Experimental setup for generating microcombs, tunable continuously laser pumps the microresonators to generate microcombs, at the same time, a photodetector is used to detect the beat frequency signal; (b) schematic diagram for microcombs generation in frequency domain, the pump laser produces a series of equally spaced sidebands through degenerate and non-degenerate four-wave mixing process; (c) schematic diagram of soliton mode-locking, the soliton pulse remains stable due to a double balance between gain and loss, dispersion and nonlinearity; (d) temporal waveform of bright soliton (left) and dark pulse (right).

图 3  微腔光梳的产生过程^[23,33,40]　(a) 泵浦激光由腔模蓝失谐向红失谐扫频过程中腔内光场总功率演化过程, 不同颜色区域代表腔内光场处于不同的态, 其中绿色区域为孤子存在区域, 黄色为呼吸子区域, 红色区域孤子不能存在; (b) 图(a)中标注的不同状态区域腔内光场分布及对应光谱图; (c)“功率踢”方法产生孤子光梳激光器频率与功率、光梳功率以及激光与腔模失谐时序变化图; (d)热辅助激光稳定微腔温度原理图; (e)反向扫频方法产生单孤子光梳原理示意图

Fig. 3.  Generation process of microcombs^[23,33,40]: (a) The intracavity power’s evolution process during pump laser frequency scanning from the blue to red detuning of the cavity mode, various color regions represent different states of the optical field within the cavity, the green region is the solitons-exiting region, the yellow region is the breathers’ region, and the red region is the region where solitons cannot exist; (b) temporal intracavity power and optical power spectra of different state regions marked in Fig. (a); (c) timing series of the pump laser frequency and power, optical frequency comb power and the detuning between the pump laser and cavity mode in power kick method; (d) schematic diagram of thermal assisted laser stabilizing temperature of the microresonator; (e) schematic diagram of generating single-soliton optical frequency comb by backward tuning method.

图 4  自注入锁定方法产生微腔光梳^[41,43]　(a)自注入锁定原理示意图, 激光器与微腔之间没有光隔离器, 微腔散射的光可以原路返回激光器腔中反馈腔内光场; (b)自注入锁定过程相图以及动力学曲线, 红色为孤子光梳存在区域, 绿色为调制不稳定态区域, 黑色的线为自注入锁定过程态的演化轨迹; (c)自注入锁定过程光梳功率与拍频信号演化过程; (d), (e)自注入锁定产生的亮孤子光梳和暗脉冲光梳频谱图

Fig. 4.  Generate microcombs via self-injection locking^[41,43]: (a) Schematic diagram of self-injection locking, there is no optical isolator between the laser and the microresonator, the light scattered by the microresonator can return to the laser cavity in the original path to feedback the light field in the laser cavity; (b) phase diagram and dynamic curve of self-injection locking, the red region is the soliton-exiting region, and the green region is modulation instability region, the black curve is the evolution trajectory of self-injection locking; (c) the evolution of comb line power and beat note signal in self-injection locking process; (d), (e) the optical spectra of bright soliton and dark pulse optical frequency comb generated by self-injection locking method.

图 5  产生微腔光梳的各种材料平台^{[46,49–54,58–60]}

Fig. 5.  Various material platforms to generate micocombs^{[46,49–54,58–60]}.

图 6  微腔光梳的频谱宽度^[45]　(a) 集成氮化硅微腔扫描电子显微镜及其横截面图像; (b)利用色散波来拓展微腔光梳频谱宽度, 1930 nm处的色散波大大拓宽了频谱范围; (c)图(a)中腔的集成色散

Fig. 6.  The spectra bandwidth of microcombs^[45]: (a) The scanning electron microscopy images and cross section of integrated Si₃N₄ microresonator; (b) expand the bandwidth of microcombs using dispersive waves, the dispersive waves located at 1930 nm greatly broaden the spectrum range; (c) integrated dispersion of the microresonator in Fig. (a).

图 7  微腔光梳的效率^[69–73]　(a) 微腔光梳产生过程中能量流动示意图; (b) 暗脉冲光梳频谱图, 插图为耦合出腔的暗脉冲时域波形图; (c) 脉冲泵浦产生微腔光梳示意图; (d) 激光腔孤子光梳示意图; (e) 使用辅助腔回收泵浦光示意图; (f) 使用耦合腔偏移泵浦模式频率

Fig. 7.  Efficiency of microcombs^[69–73]: (a) Energy flow chart of microcombs generation; (b) optical spectra of dark pulse optical frequency comb, the inset is the temporal waveform of dark pulse emitted out of the microresonator; (c) schematic diagram of pulse pumping microcombs; (d) schematic diagram of laser-cavity soliton; (e) schematic diagram of recycling pump by using auxiliary cavity; (f) shift the frequency of pump mode using auxiliary cavity.

图 8  光谱顶部平坦的微腔光梳^[80,81]　(a) 多频率光子晶体微腔示意图, 微腔内部刻蚀了不同空间周期的光栅结构来调节色散; (b) 多频率光子晶体微腔色散曲线, 图(a)中的光栅结构导致了模式分裂成蓝移和红移的模式; (c), (d) 多频率光子晶体微腔中产生的亮孤子和暗脉冲光梳光谱图; (e) 通过滤波产生奈奎斯特孤子光梳的原理示意图; (f) 不同滤波阶数产生的奈奎斯特孤子光梳光谱图

Fig. 8.  Flat-top micocommbs^[80,81]: (a) Schematic diagram of multi-frequency photonic crystal microresonators, grating structures with different spatial periods are etched inside the microresonator to adjust the dispersion; (b) dispersion curve of multi frequency photonic crystal microcavities, where the grating structures in fig. (a) leads to single mode splitting to blue- and redshifted modes; (c), (d) the optical spectra of bright soliton and dark pulse generated in multi-frequency photonic crystal microresonators; (e) schematic diagram of generating Nyquist soliton by spectral filtering; (f) the optical spectra of the Nyquist soliton optical frequency comb generated by various spectral filtering order.

图 9  微腔光梳的噪声研究^[59,83,84]　(a) 微腔光梳合成微波的原理示意图, 连续激光在微腔中产生孤子脉冲, 随后将脉冲序列耦合到高速光电探测器上, 即可产生频率为光梳重频的微波信号, 这也是常用的光梳重频测量方法; (b)“安静点”操作抑制光梳噪声示意图, 在特定失谐下, 孤子重频对失谐变化敏感度最小, 相应光梳重频噪声最低; (c) 布里渊克尔孤子原理图, 泵浦激光先在微腔中激发布里渊激光, 再用布里渊激光泵浦微腔产生孤子光梳; (d) 注入锁定原理图, 泵浦激光通过电光调制器产生边带, 利用注入锁定效应将光梳梳齿锁定在调制产生的边带上

Fig. 9.  Research on the noise of microcombs^[59,83,84]: (a) Schematic diagram of synthesizing microwave signal using microcombs, a continuous laser generates soliton pulses within a microresonator, and this pulse sequence is subsequently coupled to a high-speed photodetector to generate microwave signals with a frequency equaling to the repetition rate of the microcombs, this process also serves as a conventional method for measuring the repetition rate of optical frequency combs; (b) schematic illustration of noise suppression in microcombs through "quiet point" operation, the microcomb's repetition rate exhibits minimal sensitivity to detuning under a specific detuning condition, which coincides with the microcomb state characterized by the lowest repetition rate noise; (c) schematic diagram of Brillouin Kerr soliton, the pump laser generates Brillouin laser in the microresonator, and then the Brillouin laser pump the microresonator to generate soliton comb; (d) schematic diagram of injection locking, the pump laser is modulated by electro-optic modulator to generate a pair of sidebands, the comb lines are locked to the modulated sidebands via injection locking effects.

图 10  微腔光梳的应用^[92–100]. 微腔光梳被广泛应用于通信、微波合成、激光雷达、光谱学、光计算、光钟、光学频率合成、光学频率分频与量子光源等领域

Fig. 10.  Applications of microcombs^[92–100]: Microcombs can be widely applied to communication, microwave synthesis, Lidar, spectroscopy, optical computing, optical-frequency synthesizer, optical frequency division, quantum light source, and other fields.