Fund Project:Project supported by the National Key R&D Program of China (Grant No. 2016YFF0200204)
Received Date:21 September 2020
Accepted Date:09 November 2020
Available Online:24 March 2021
Published Online:05 April 2021
Abstract:Narrow-linewidth femtosecond optical frequency comb plays an important role in the fields, such as optical clock comparison, time frequency transfer, ultrastable microwave generation, absolute distance measurement, high precision spectroscopy, etc. Due to the influence of the lifetime of the upper energy level in the gain medium, the linewidth of Er-fiber combs is generally on the order of several hundred kilohertz. In order to narrow the linewidth of comb teeth, an effective method is to insert a fast response electro-optic modulator (EOM) into the laser cavity, so that the servo bandwidth of fiber comb is extended to several hundred kilohertz, which provides a feedback mechanism for fast servo locking. Among them, a high quality femtosecond laser is the core. Based on this, the influence of the EOM on the parameters of Er-fiber femtosecond laser is studied in this paper. By calculating the refractive index, group velocity dispersion, and phase delay of the electro-optic crystal, the influence of the introduction of the EOM on the laser performance is analyzed. A LiNbO3 (LN) crystal with a length of 3 mm and x-cut is selected as the EOM and inserted into the laser cavity. The influence of the applied voltage of the EOM on the repetition rate and carrier envelope offset frequency of the laser are obtained experimentally. When the voltage on the LN crystal changes from -200 to 200 V, the adjustment of repetition rate is 60 Hz and the carrier envelope offset frequency is 25 MHz. Then the two parameters are phase locked through the EOM. Furthermore, by phase locking the beat note between the fiber comb and a narrow-linewidth continue wavelength laser at 1542 nm, it is verified that the introduction of the EOM can expand the servo bandwidth of the laser to more than 236 kHz, which provides a technical basis for establishing narrow linewidth femtosecond optical frequency combs. The following work will verify the performance of comb line, that is, when the comb is locked to a narrow-linewidth laser (such as 1542 nm), the performance of comb line at wavelength (such as 698, 729 nm, and so on) of distant place will be analyzed in detail. Keywords:Er-doped fiber femtosecond laser/ Er-doped fiber optical frequency comb/ electro-optic modulator/ servo lock
在1550 nm处, ne = 2.1375, no = 2.2111, 晶体长度为3 mm. 当在晶体两端加200 V电压时, 如果光在沿y轴和z轴上都有分量, 那么得到相位差为0.08π. 较短长度LN晶体加电压后带来的相位差不大, 但是需要重新调整腔内波片的角度实现锁模. 在系统中, 为了保证晶体出射激光的偏振态不受电压的影响, 调节晶体入射光的偏振方向使其与晶体的一个感应主轴重合. 对于沿着z轴加电场的情况下, LN晶体的折射率椭球不发生旋转, 因此只需保证晶体入射光的偏振沿水平或者垂直方向即可. -->
4.1.激光器状态
激光器腔内不加EOM时, 激光器的整体结构与文献[21]类似. 激光器中增益光纤长度为35 cm, WDM两端尾纤长度分别为15和12 cm, 两个准直器的尾纤长度分别为20和37 cm. 两个准直器之间的空间距离为5.5 cm. 在650 mW抽运功率下, 激光器在连续光状态下可以输出150 mW, 锁模后平均功率为85 mW. 此时加入EOM, 调节LN晶体放置角度, 使腔内激光垂直入射到LN晶体表面, 激光器输出功率保持不变. 在最佳锁模状态下, EOM的放入或者取出对激光器的自动启动锁模没有影响. 图4给出了激光器腔内有无EOM时的锁模光谱. 从图中可以看出, EOM的引入对激光器锁模影响不大. 这主要源于EOM中LN晶体通光长度较短, 对激光器的色散影响可以忽略不计. 图 4 激光器腔内有无EOM时的锁模光谱 Figure4. Spectra of the Er-fiber femtosecond laser with and without an intra-cavity EOM.
激光器输出激光经过1∶3的光纤分束器分成三路, 第一路直接进入探测器PD(EOT, 3000 A)用于探测激光器重复频率fr. 图5为激光器中加入EOM锁模后的射频曲线. 从图5中可以看出激光器重复频率为163 MHz. 第二路通过后续放大、扩谱、f-2f干涉仪, 实现40 dB信噪比的f0信号输出, 如图6所示. 图6插图为HNLF扩谱后的倍频程光谱, 光谱覆盖1100到2200 nm. 第三路在本系统中用于与1542 nm窄线宽激光器拍频获取拍频信号fb. 图 5 激光器中加入EOM锁模后的射频曲线, 其中插图为163 MHz处的频谱 Figure5. Radio frequency of the Er-fiber femtosecond laser with an intra-cavity EOM. The insert is the radio frequency at 163 MHz.
图 6 激光器载波包络偏移频率, 插图为扩谱后的倍频程光谱图 Figure6. Signal-to-noise ratio of carrier-envelop offset frequency in 100 kHz resolution bandwidth (RBW). The insert is octave spanning spectrum after HNLF.
EOM中LN晶体施加电压后, 对激光器重复频率和f0信号会产生影响, 因此EOM可以作为伺服器件用于对激光器重复频率和f0的锁定. 当EOM晶体上的电压在–200到200 V之间变化时, 激光器重复频率的变化量约为30 Hz, 如图7(a)所示. 激光器重复频率变化主要是由于EOM中晶体施加电压后引起折射率变化所致. 在激光器重复频率漂移量较小的条件下, 可以利用EOM对激光器重复频率进行锁定. 当EOM晶体上的电压从–200到200 V逐渐增大时, 激光器f0信号的变化量约为25 MHz, 如图7(b)所示. 在电压调节过程中f0信噪比保持不变. 图 7 EOM晶体电压对激光器参数的影响 (a) EOM晶体电压对激光器重复频率的影响; (b) EOM晶体电压对激光器载波包络偏移频率的影响 Figure7. Diagram showing the change in laser parameters at different voltage on EOM: (a) The change in repetition rate; (b) the change in carrier envelope offset frequency.
图 8 EOM晶体电压改变时, 激光器输出光谱变化 Figure8. Evolution of the spectra of the Er-fiber femtosecond laser with the changing of the voltage on EOM.
图 10 重复频率锁定后的频率变化 (a) 采用EOM锁定重复频率; (b) 采用PZT锁定重复频率 Figure10. Residual fluctuations of the repetition rate when it is phase-locked: (a) Phase-locked by EOM; (b) phase-locked by PZT.
为了对比, 重复频率与参考频率混频输出的误差信号通过微波线切换模块切换进入伺服锁定环路PPL4. 由于激光器重复频率漂移, 因此调节频率综合器使其输出频率位于163623411.431 Hz附近, 以匹配此时的重复频率. 其中PPL4中伺服模块控制PZT驱动器输出电压直接作用到PZT上, 通过改变PZT伸缩量实现激光器重复频率的锁定. 图10(b)给出了7.5 h的锁定时间内激光器重复频率的平均值和标准差. 重复频率锁定后的平均值为163623411.43121 Hz, 标准差为0.473 mHz. 图11给出了为分别采用EOM和PZT锁定重复频率时所获得的相对Allan偏差曲线. 在平均时间0—1000 s内, 两种锁定方法得到的相对Allan偏差基本一致. 因此, 当重复频率不需要长时间锁定时, 可以在飞秒激光器中直接加入EOM以简化重复频率锁定时飞秒激光的复杂性. 而先前无论是PZT控制端镜还是PZT拉伸光纤[14,21,25], 都需要在激光器建立过程中把伺服器件加入激光腔内. 后续采用EOM结合与对激光器底板温度控制相结合的方式有望实现激光器重复频率的长时间连续锁定. 图 11 采用EOM和PZT锁定重复频率后, 所获得的重复频率的相对Allan偏差曲线 Figure11. Calculated Allan deviations when the repetition rate was phase-locked by EOM and PZT respectively.