22 W,240 fs高功率中红外飞秒光学频率梳封面文章
The femtosecond optical frequency comb (FOFC) comprises a series of ultra-short laser pulses with the same temporal separation in the time domain and discrete, equidistant, and stable phase-related frequency components in the frequency domain. The FOFC can accurately measure the absolute frequency of an atomic clock and serve as a natural time-frequency reference. Currently, the most stable and compact light source is the mode-locked erbium-doped fiber laser with a central wavelength of 1.55 μm, typically employing highly nonlinear fibers to broaden the spectrum across the entire transparent range of silica fiber (350?2400 nm). However, the output power of the erbium-doped fiber FOFC is generally in the range of a few hundred milliwatts. Therefore, increasing the output power of the FOFC remains a crucial challenge. The mid-infrared FOFC holds significant application value in next-generation spectroscopy, as it can be used to detect gases such as carbon dioxide and ammonia and extend the FOFC wavelength to the molecular fingerprint spectrum range (3?20 μm) through nonlinear crystals. This spectrum range is vital for chemical composition analysis, making the development of high-power mid-infrared FOFCs a pressing need.
This system comprises an erbium-doped fiber FOFC, a super-continuum converter, a double-cladding thulium-doped fiber amplifier system, and a transmission diffraction grating pulse compressor. Initially, the erbium-doped fiber FOFC utilizes a highly nonlinear fiber with normal dispersion for frequency broadening. Additionally, a self-pump amplifier composed of thulium-doped fiber generates a femtosecond seed with a central wavelength of 1925 nm. This seed is injected into a chirped pulse amplification system comprising a 55 m long highly nonlinear fiber with normal dispersion, a three-stage thulium-doped fiber amplifier, and a transmission diffraction grating pulse compressor. To characterize the noise of the high-power mid-infrared FOFC, we analyze the relative intensity noise and the phase noise of the pulse train using a signal source analyzer. Moreover, we co-couple the super-continuum laser generated by the high-power mid-infrared FOFC in the fluorotellurite fiber with a 1064 nm iodine-stabilized Nd∶YAG laser to detect the beat signal and verify the performance of the high-power mid-infrared FOFC.
The 1.55 μm femtosecond laser output from the erbium-doped fiber femtosecond optical frequency comb is symmetrically broadened to the spectral range of 1100?2200 nm by the highly nonlinear fiber with normal dispersion (Fig.2). The resultant super-continuum laser is injected into the self-pump pre-amplifier to obtain a femtosecond seed with a central wavelength of 1925 nm and an average power of 50 mW [as indicated by the dashed line in Fig.3(a)]. This seed is then broadened to hundreds of picoseconds through the normal dispersion fiber and amplified by the three-stage double-cladding thulium-doped fiber amplifier to yield a picosecond pulse with a central wavelength of 2000 nm and an average power of 36.07 W. After compression, a femtosecond pulse with an average power of 22.72 W and a pulse width of 240 fs is obtained [Fig.3(b)]. The integral values of relative intensity noise and timing jitter are 1.16% and 472 fs, respectively (integral range of 10 Hz?1 MHz) (Figs.4 and 5). The super-continuum laser (Fig.6) generated by the high-power mid-infrared FOFC and the 1064 nm laser produce a beat signal with a signal-to-noise ratio of 40 dB, meeting the counting requirements of the counter (Fig.8).
We demonstrate a high-power FOFC based on an erbium-doped FOFC, generating a 2 μm femtosecond seed through a highly nonlinear fiber with normal dispersion and self-pump pre-amplifier. The highly nonlinear optical fiber with normal dispersion effectively overcomes noise sensitivity issues associated with nonlinear dynamics of abnormal dispersion, such as soliton self-frequency shift and Raman soliton, during super-continuum generation. The femtosecond pulse, obtained with an average power of 22.72 W and a pulse width of 240 fs, marks a significant advancement in developing high-power mid-infrared FOFCs. This development contributes to the spectroscopic analysis of molecular structures and dynamics and facilitates the expansion of optical frequency combs into the molecular fingerprint spectrum range (3?20 μm).
1 引言
飞秒光学频率梳在时域上是由相同间距的超短激光脉冲串构成的,在频域上是由一系列离散、等间距且具有稳定相位关系的频率分量组成的,可以实现原子钟精度的绝对频率计量,是天然的时频基准[1-2]。飞秒光学频率梳在精密测量[3-4]、光谱学[5-6]、冷原子[7]等相关领域中有着重要的应用意义。同时,下一代应用场景对飞秒光学频率梳的要求也越来越高,包括宽光谱带宽、低相位噪声、高梳齿功率以及高稳定性、紧凑性和环境鲁棒性等。目前,飞秒光学频率梳中最突出、最稳定和最紧凑的光源是锁模掺铒光纤激光器,中心波长一般为1.55 μm[8],通常使用高非线性光纤将掺铒光纤锁模激光器的光谱展宽到石英光纤的整个透明范围(350~2400 nm)内,但是掺铒光纤飞秒光学频率梳的输出功率通常在百毫瓦量级,梳齿功率较低。所以提升光学频率梳的输出功率一直是重要的挑战[9],目前比较成熟的方案是使用掺镱光纤搭建高功率飞秒光学频率梳[10-11],掺镱光纤的吸收波长和发射波长更接近,具有更高的量子效率,可以有效削弱放大过程中的发热影响,是高功率飞秒光学频率梳的首要选择。
目前,中红外波段飞秒光学频率梳在下一代光谱学中有着非常重要的应用价值,可用于检测二氧化碳、氨气等特殊气体[12],且通过非线性晶体可将光学频率梳的波长进一步扩展到分子指纹谱范围(3~20 μm)内[13-16],这个波段是化学成分分析的重要光谱区间。正因如此,迫切需要开发中红外飞秒光学频率梳。为了产生中红外飞秒光学频率梳,通常是直接搭建基于掺铥光纤的高重复频率锁模激光器,通过啁啾脉冲放大(CPA)技术提升脉冲的平均功率[17],锁定后可以获得高功率飞秒光学频率梳。但是受限于中红外波段的色散补偿光纤等商用光纤的发展,锁模光纤激光器的搭建成本较高,并且掺铥光纤锁模激光器通常以高功率的1.55 μm连续光光源作为泵浦源,高功率1.55 μm连续光光源会带来更多噪声。
对掺铒光纤飞秒光学频率梳进行非线性频率变换可获得2 μm飞秒种子源,再进行高功率放大,可产生中红外飞秒光学频率梳。2012年Adler等[18]使用高非线性光纤,通过非线性频率变换将输出波长转移到1.93 μm,通过双包层掺铥光纤产生了平均功率为2.9 W、脉冲宽度为141 fs、重复频率为100 MHz的中红外光学频率梳。2020年Heidt等[19]通过正色散高非线性光纤抑制了非线性频率变换过程中的噪声,获得了输出脉冲宽度为66 fs、平均功率为0.5 W、重复频率为80 MHz的低噪声中红外光学频率梳。同年,Xing等[20]通过高非线性光纤将1.55 μm飞秒激光频移到2 μm,放大之后使用高非线性光纤进行脉冲自压缩,获得了脉冲宽度为9.5 fs、中心波长为2000 nm的少周期光学频率梳。
为了进一步获得具有更高平均功率的中红外飞秒光学频率梳,本文基于掺铒光纤飞秒光学频率梳,搭建了高功率中红外飞秒光学频率梳。首先使用正色散高非线性光纤进行频率扩谱,消除光谱展宽过程中反常色散带来的噪声敏感性;再利用一段掺铥光纤构成自泵浦的预放大器,产生中心波长为1925 nm、平均功率为50 mW的飞秒激光;然后将该飞秒激光作为种子源并进行展宽,通过三级双包层掺铥光纤放大器放大,输出平均功率为36.07 W、中心波长为2000 nm的皮秒激光;最后经过光栅对压缩,输出平均功率为22.72 W、脉冲宽度为240 fs、压缩效率为62.9%的飞秒脉冲。
2 实验装置
高功率中红外飞秒光学频率梳的实验装置如
图 1. 高功率中红外飞秒光学频率梳的实验装置图
Fig. 1. Experimental setup of high-power mid-infrared femtosecond optical frequency comb
将超连续光中滤出的2 μm飞秒脉冲作为双包层掺铥光纤放大器的种子源,该放大器采用CPA技术。使用长度为55 m的NDHNF 进行时域拉伸展宽,该光纤在2000 nm处的色散为-41.2 ps/(mm·km),用于补偿后续三级光纤放大器带来的反常色散,抑制放大过程中的非线性,避免脉冲分裂等情况的发生。在展宽光纤和放大器之间插入半波片和四分之一波片,对脉冲偏振态进行调节,有效缓解由较长的光纤链路导致的退偏。前两级光纤放大器的泵浦源采用以多模光纤为输出端的793 nm半导体激光器,多模光纤的纤芯直径为105 μm,最大输出功率15 W。前两级光纤放大器的增益光纤均采用长度为3 m的双包层掺铥光纤(DC-TDF),其纤芯和包层直径分别为10 μm和130 μm。通过光纤合束器将泵浦光正向耦合到双包层掺铥光纤的第一包层中,将信号光耦合到双包层掺铥光纤的纤芯中。为了有效隔离反向光和避免各级放大器之间的串扰,在一、二级光纤放大器之间熔接了高功率单向隔离器。
为了进一步提升功率,主放大器的增益光纤选用长度为5 m的大模场双包层掺铥光纤(LMA-TDF),该掺铥光纤的纤芯直径为25 μm,包层直径为400 μm,并使用模场适配器将前两级放大器光纤和大模场双包层掺铥光纤模场进行适配。主放大器的泵浦源是以多模光纤作为输出端的793 nm半导体激光器,其纤芯直径为200 μm,最大输出功率为80 W,通过光纤合束器以正向泵浦的方式进行泵浦,有效避免残余泵浦光对前两级放大器的损伤。同时在每级放大器的掺铥光纤之后均熔接一个泵浦剥离器,用于消除残余泵浦光。为了保证整套系统的正常运转,三级光纤放大器均被放置在水冷板上,以保持工作温度稳定。为了保证主放大器的横模稳定和单模运行,对大模场掺铥光纤进行弯曲盘绕处理,抑制大模场掺铥光纤中高阶模的产生。主放大器的输出端光纤使用8°角切割以防止回光反射,并在二、三级放大器之间熔接了高功率光纤环形器,可以有效地导出主放大器中产生的背向反射光。主放大器的输出光经透镜准直后,用一对透射式衍射光栅进行脉冲压缩,其刻线密度是560 line/mm。
实验中分别使用波长分辨率为0.02 nm的光谱分析仪、自相关仪对中红外光学频率梳的频域和时域进行探测,并使用信号源分析仪对其噪声特性进行测试和分析。
3 实验结果与分析
掺铒光纤飞秒光学频率梳在高非线性光纤中产生的超连续光谱如
图 2. 超连续光谱随入射光功率的演化图
Fig. 2. Evolution of super-continuum spectrum with incident light power
图 3. 掺铥光纤放大器的输出特性。(a)种子光和放大器输出的光谱;(b)压缩脉冲的自相关轨迹
Fig. 3. Output characteristics of thulium-doped fiber amplifier. (a) Output spectra of seed light and amplifier; (b) autocorrelation trace of pulse after compression
输出光经准直后,使用半波片调节偏振态,使得输出光经过光栅对的衍射效率达到最高,压缩光路采用双通式设计,压缩效率约为62.9%,最终获得平均功率为22.72 W的飞秒激光输出。
为了分析高功率中红外飞秒光学频率梳的噪声特性,将种子光脉冲和放大压缩之后的信号光脉冲分别入射到光电二极管上,利用低通滤波器滤除电学信号中的高频部分,只保留低频部分,最后利用信号源分析仪的基带噪声测量功能,测量得到待测光的相对强度噪声(RIN)功率谱。
图 4. 高功率中红外光学频率梳的相对强度噪声特性。(a)种子光和放大压缩后脉冲的RIN;(b)RIN积分曲线
Fig. 4. Relative intensity noise characteristics of high-power mid-infrared optical frequency combs. (a) RIN of seed light and amplified pulse after compression; (b) integrated RIN curves
此外,将高功率中红外光学频率梳的光脉冲入射到高速光电探测器上,用低通滤波器滤出基带重复频率信号,使用射频放大器放大射频信号,利用相位噪声分析功能测量待测脉冲序列的相位噪声功率谱,测量结果如
图 5. 种子光和放大压缩后脉冲的相位噪声功率谱
Fig. 5. Phase noise power spectra of seed light and amplified pulse after compression
为了验证高功率中红外飞秒光学频率梳的性能,我们将高功率中红外飞秒激光注入到纤芯直径为5.5 μm的氟碲酸盐光纤中,在9.3 W的入射功率下,产生
图 8. 高功率中红外光学频率梳与Nd∶YAG激光器的拍频信号
Fig. 8. Beat signal between high power mid-infrared optical frequency comb and Nd∶YAG laser
4 结论
报道了一种基于掺铒光纤飞秒光学频率梳的高功率中红外飞秒光学频率梳。该系统包括掺铒光纤飞秒光学频率梳、中红外超连续扩谱模块、三级双包层掺铥光纤放大器和基于透射式衍射光栅对的压缩器四部分。掺铒光纤飞秒光学频率梳输出的1.55 μm飞秒激光,通过正色散高非线性光纤和由掺铥光纤构成的自泵浦预放大器,产生2 μm飞秒激光,有效克服了反常色散非线性动力学(如孤子自频移、拉曼孤子等)带来的噪声敏感性。种子源的脉冲宽度通过正色散光纤展宽到百皮秒,再通过三级双包层掺铥光纤放大器获得中心波长为2000 nm、平均功率为36.07 W的皮秒脉冲激光,通过光栅对压缩后得到脉冲宽度为240 fs、平均功率为22.72 W的中红外飞秒脉冲输出,其相对强度噪声和时间抖动的积分值分别为1.16%和472 fs(积分范围为10 Hz~1 MHz)。高功率中红外光学频率梳通过氟碲酸盐光纤产生的超连续光谱和1064 nm碘稳频Nd∶YAG激光器激光拍频,获得信噪比为40 dB的信号,该信噪比已经满足计数器的计数要求,验证了高功率中红外光学频率梳可以满足频率测量的需求。高功率中红外飞秒光学频率梳的发展为分子物质结构和动力学的光谱分析提供了强大的工具,有助于光学频率梳进一步向分子指纹谱范围(3~20 μm)扩展。
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