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在非线性光学领域,FWM是一种常见的三阶非线性效应。当至少两束不同频率(
$v_{1},v_{2},v_{1} < v_{2}$ )的光一同在非线性介质中传播时,由于存在差频的折射率调制,会产生如图5所示的两个新的频率分量,分别为:$$ v_{3}=v_{1}-\left(v_{2}-v_{1}\right) $$ $$ v_{4}=v_{2}+\left(v_{2}-v_{1}\right) $$ 通过持续控制非线性介质长度与输入光功率,则可以将FWM效应逐级传递,进而产生更多的新频率成分。
笔者课题组采用双波长单频光纤激光器输出的1552.2 nm与1552.43 nm两个单频激光作为起始波长,借助高非线性光纤中的级联FWM效应实现多波长单频产生。为了达到FWM效应所需的功率,采用多级光纤放大器对其进行放大。由于放大过程中产生的自发辐射放大(amplified spontaneous emission, ASE)会在后期FWM过程中作为噪声严重影响输出光谱的信噪比以及新光谱成分的生成。为此在放大器最后需要添加一个滤波器来滤除多余的ASE成分。由于所采用的滤波器的损伤阈值仅为300 mW,在放大器中添加了一个AOM对激光的强度进行调制,最终实现高峰值功率、低平均功率的激光输出,这样既可以保证FWM过程所需的功率,同时还可以避免损伤滤波器。
光纤放大器的实验装置如图6所示。由保偏DBR光纤激光器输出的双波长单频激光通过保偏隔离器后,通过一级纤芯放大器进行功率放大。纤芯放大器采用的增益光纤为2 m的PM-ESF-7/125,其吸收系数为~55 dB/m@1530 nm。作为泵浦的974 nm单模半导体激光器最大输出功率为1 W。继而,利用AOM进行斩波,将原本的连续光调制为重复频率100 kHz,脉宽100 ns 的脉冲光。在AOM后再经过一级纤芯放大器与包层放大器。纤芯放大器的基本参数与前一级保持一致。而包层放大器则使用了4 m长的PM-EYDF-6/125作为增益光纤,吸收系数为40±10 dB/m@1535 nm,泵浦源则采用中心波长976 nm,输出功率9 W的多模半导体激光器。放大器输出的激光经过滤波器滤除额外的ASE之后进入高非线性光纤,文中采用的高非线性光纤为长飞公司的NL-1550-0,长度为100 m,光纤的零色散点在1550 nm处。
为了验证由高非线性光纤产生的新光谱成分的确是单频激光,在包层放大器泵浦功率较低时,利用光谱仪和扫描干涉仪对其光谱、纵模情况进行了测量,所得结果如图7所示。图7 (a)为输出光谱图,凭借效应FWM,输出光谱由原先的1552.2 nm和1552.43 nm处的两个波长扩展出了等间隔分布的两对新光谱成分。种子激光器产生的1552.2 nm与1552.43 nm处的光谱信噪比约为50 dB,FWM效应产生的第一对新光谱分量的信噪比约40 dB,产生的第二对新光谱分量的信噪比约20 dB。图7 (b)为输出光谱的扫描干涉仪图像,在一个扫描周期内同时出现四个纵模。对比输出光谱的情况可知,由于扫描干涉仪图像显示的是线性坐标系下的光谱成分,因此图中四个纵模分别对应光谱中的种子激光以及FWM产生的第一对光谱成分。FWM产生的第二对光谱成分与种子相差30 dB,在线性坐标系下的扫描干涉仪图像中无法观察到这组光谱。因此,通过对比光谱图和扫描干涉仪图像可知,通过FWM产生的新光谱成分依然是单频状态,并且间隔与种子光的间隔一致。随着泵浦功率的提升,基于泵浦激光产生的第一级光谱成分的轻度也会随之升高。第二级光谱成分的产生则是依赖于泵浦激光以及第一级光谱成分的一部分,例如,短波长处的第二级光谱是与
$ {\nu }_{2} $ 形成配对光子。随着泵浦光谱$ {\nu }_{1},{\nu }_{2} $ 能量的提升,第一级光谱成分会不断提高,同时$ {\nu }_{1},{\nu }_{2} $ 光谱成分分别与新产生的第二级光谱成分形成配对光子。这一系列过程都会不断消耗泵浦激光以及前级光谱的能量,对这些能量较强的光谱成分进行钳制。通过不断级联过程,最终使得产生的梳状光谱呈现平坦的趋势。图 7 激光器运行状态测试。(a) 光谱图;(b) 扫描干涉仪图像
Figure 7. State test of laser operating. (a) Spectrum; (b) Scanning inter-ferometer image
为了实现更多新光谱成分的产生,继续增加放大器的泵浦功率,最终获得了平均功率130 mW的双波长单频激光输出,对应的峰值功率为13 W。同时对比了在进入高非线性光纤之前是否经过滤波器对于输出的梳状光谱的影响,如图8 (a)所示。通过对比可知,通过滤波器将泵浦激光器的ASE过滤之后,可以大幅提高梳状光谱光谱信噪比,同时产生更多的新光谱成分。经过滤波器之后的整体光的信噪比达到了35 dB。在20 dB的光谱范围内,初始光谱ν1,ν2两侧分别产生26阶和20阶新的光谱成分,光谱跨越1.337 THz。造成短波方向的光谱阶数高于长波方向的光谱阶数的主要原因在于所用高非线性光纤的零色散点在1550 nm处,初始激光的光谱在1552 nm附近,随着更高阶数光谱的产生,短波方向的光谱相较于长波方向的光谱更靠近光纤的零色散波长,这使得FWM效应在短波处产生更多的新光谱成分。图8(b)为图8(a)梳状光谱中1549.5 nm处1 nm范围内的放大部分,从该图中可以明显观察到各个波长的峰值处存在明显的缺失,这主要是由于实验所采用的光谱仪在高分辨率情况下的采样点数有限造成的,由此可以证明实际光谱强度的均匀性更好。
图 8 多波长单频激光的输出光谱。(a) 完整光谱;(b) 局部光谱分析
Figure 8. Output spectrum of multi-wavelength single-frequency laser. (a) Complete spectrum; (b) Partial spectrum analysis
由于该多波长单频光纤激光器的多波长单频是基于四波混频效应产生的,同时受到泵浦激光低相位噪声特性的影响,各个波长之间具有较低的相位噪声,因此该激光器在时域上是会形成类似于锁模激光器的输出特性,既产生脉冲输出,重频与纵模间隔一致。通过自相关仪对该激光器的输出脉冲进行测量,输出结果如图9所示。
1.5 μm multi-wavelength single-frequency fiber laser based on four-wave mixing effect (Invited)
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摘要: 利用笔者自主搭建的双波长单频光纤激光器作为种子,通过声光调制器以及多级光纤放大后,将激光注入至100 m长的高非线性光纤中,该光纤的零色散点在1 550 nm处。借助高非线性光纤的四波混频效应,最终在峰值功率13 W的泵浦下获得了一系列新的光谱成分,20 dB范围内共产生了46条新光谱。这些光谱跨越了1.337 THz,并且每条光谱中只包含一个纵模。由于新光谱基于四波混频效应产生,不同光谱之间不存在增益竞争等问题,因此,该激光器的多波长单频可以稳定存在,并且光谱强度接近。该多波长单频光纤激光器不仅具有线宽窄、相干性高、噪声低等优势,由于其还可以在全光纤结构下同时输出多个波长的单频激光,这使得其在波分复用光通信、光频率转换、激光雷达、微波光子学等领域具有十分重要的应用。Abstract: Our self-built dual-wavelength single-frequency fiber laser was used as a seed, and after being amplified by an acousto-optic modulator and multi-stage fiber, the laser was injected into a 100-meter long high nonlinear fiber with the zero-dispersion point of at 1550 nm. With the help of the four-wave mixing effect of the highly nonlinear fiber, a series of new spectral components were finally obtained under the pumping of the peak power of 13 W, and a total of 46 new spectra were generated in the range of 20 dB. These spectra spanned 1.337 THz and contained only one longitudinal mode in each spectrum. Since the new spectrum was generated based on the four-wave mixing effect, there was no gain competition between different spectra, so the multi-wavelength single- frequency of the laser can exist stably, and the spectral intensity was close to each other. The multi-wavelength single-frequency fiber laser not only has the advantages of a single-frequency fiber laser such as narrow linewidth, high coherence, and low noise, but also can simultaneously output multiple wavelengths of single-frequency lasers in an all-fiber structure, which makes it possible to have very important applications in the fields of multiplexing optical communication, optical frequency conversion, lidar, microwave photonics and so on.
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Key words:
- fiber laser /
- single-frequency /
- multiple wavelengths /
- four wave mixing
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