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首先研究了没有种子光注入情况下激光器的功率特征。通过D1和D2分别测试正反向功率的功率输出再相加,可以获得激光器的功率-吸收泵浦光功率曲线,如图2所示,黑线为泵浦功率增加时的功率曲线,红线为泵浦功率减小时的功率曲线,两根曲线并不完全重合,产生了一种双稳态现象。
图 2 1342 nm环形腔激光器在无种子注入的情况下输出功率随着吸收泵浦功率变化
Figure 2. Output power of the 1342 nm ring laser versus absorbed pump power without injection
结合图3所示晶体处的激光光束半径和晶体热透镜之间的关系来看,尽管晶体的热焦距在80~220 mm时激光器都是稳定的,但考虑到泵浦光半径为600 µm,实际能够实现激光振荡的区域更小,由于1342 nm波段低的斯托克斯效率和激发态吸收(ESA)效应,产生激光时的晶体泵浦光热转换系数比没有激光产生时的晶体热转换系数还要多[14]。根据文献中所给出的热转换系数:在没有出光情况下,热转换系数为0.22;在存在1342 nm激光输出的情况下,热转换系数为0.4,如图3中的虚线所示,在相同泵浦功率下晶体热透镜焦距在是否产生激光的不同情况下可分别位于稳区两端。因此,在泵浦光功率上升过程中,1342 nm激光产生后,晶体热焦距由于激光的产生迅速变短,此时焦距越短越,谐振腔越稳定,这促使激光功率变高,然而更高的激光功率会进一步增加晶体产热,从而形成正反馈,激光功率迅速增加,直到达到稳区另外一端,此时如果继续增加泵浦会导致激光光斑半径变大及泵浦光的匹配程度变差,因而增加泵浦并不会让输出功率上升。另一方面,当泵浦光功率下降时,由于存在激光继续产热,需要降低相比上升过程时更多的泵浦光功率才能让激光器离开稳区,导致功率上升过程和下降过程产生了双稳态现象。
图 3 激光晶体中光束半径随着热透镜焦距的变化
Figure 3. Beam radius in laser crystal in versus focal length of thermal len
如图2所示,激光器最大输出功率在泵浦功率下降时出现,约为13.7 W,此时被吸收的泵浦功率约为75 W。
将该输出功率的最高点作为实验的实际工作点,并根据该工作点的腔内空间模式对种子激光进行严格优化,使之主激光器的空间模式与从激光器最大程度地匹配。 锁定激光输出功率由功率计D1测量,反向激光功率由D2测量。图4所示为种子功率对注入锁定功率和反向功率的影响。实验中,尝试降低注入光功率时,反向激光功率会增加,正向激光功率由于反向激光背向散射等效应的影响产生了自调制[15],尽管还是单纵模运转,但是产生了大量强度噪声,锁定系统信噪比下降,当种子功率低于4.47 mW时,锁定系统无法正确捕捉误差信号,激光器处于完全失锁状态,此时激光器处于多纵模震荡状态,正向功率和反向功率均为6.9 W左右。当种子光功率高于该值时,激光器进入锁定状态,正向激光单纵模运行,输出功率会随着种子功率增加而逐渐增加,当种子功率达到20.65 mW时,环形腔的输出功率达到最高13.9 W,相比于种子功率,增益达到28.21 dB,提取效率为18.3%,与先前报道的工作相比[12],增益增加约20 dB以上,且功率输出相比于先前报道的结果(8.3 W)也有50%左右的提升。
图 4 当吸收泵浦功率为75 W时,锁频激光输出功率和反向功率随着种子光功率的变化
Figure 4. Output power of injection-locked laser and backward power versus the seed power with absorbed pump power of 75 W
注意到,即便是种子功率最高的情况下,通过D2仍然可以测到约0.15 W的反向输出功率,并不为0,这是由LI技术的特点决定的,LI技术需要正向功率有一定变化来获得误差信号,于是在调制PZT的过程中,正向功率在某些时刻会有些许下降,则一部分反向运行的激光模式会起振,从而产生反向激光输出。
然后利用波长计Highfinesse WS-6测试了锁定激光的调谐性能,如图5所示,当种子功率为20.65 mW时,激光器实现了从1341.6774 nm到1341.8025 nm的调谐,调谐范围为0.12 nm,如果种子波长超过该范围,锁定就会完全丢失,激光器输出功率会变得很低,这个范围相比于传统的行波放大要小得多[16]。输出功率最高时的激光波长为1341.7359 nm ,此时输出功率为 13.94 W。
图 5 当种子光功率为20.65 mW时,输出功率与种子光波长的关系
Figure 5. Dependence of the output power from the seed wavelength with seed power of 20.65 mW
锁定激光输出的单频性能用共焦F-P干涉仪(美国 Thorlabs公司,型号为 SA200-8B)表征。在激光输出功率最大时,测量的F-P透过率曲线如图6所示。由图6可见,激光器为单频运转,线宽41 MHz。可以发现,放大后的激光线宽比种子激光线宽要宽(种子光线宽约1 MHz),产生这样结果的主要原因是注入光功率较弱,且存在反向运转的激光模式的影响,因而展宽了线宽。
图 6 1342 nm锁定激光输出的F-P扫描干涉仪透过率强度(插图:其中一个透射峰的放大图)
Figure 6. Transmitted intensity of the scanned F-P interferometer for the 1342 nm inject-locked laser output (Inset: magnified profile of one transmitted peak)
在上述条件下,利用CINOGY CS200 光束质量分析仪测量了锁定状态下激光输出的光束质量因子$ M^2 $,如图7所示,其中x轴方向上的光束质量因子为$ M_x^2 $= 1.30,y轴方向上的光束质量因子为$ M_y^2 $ = 1.23。图6中的插图为光斑的二维强度分布,表明激光束为高斯基横模。
图 7 最大功率下输出激光的光束质量因子M2测量(插图:二维光强分布图)
Figure 7. Beam quality factor M2 measured at maximum output power (Inset: 2D beam intensity profile)
对于一些重要应用,亮度是关键指标,激光束的亮度由下式定义[17]:
$$ {B}=\frac{{CP}}{\lambda^{2} M^{2}_x M^{2}_y} $$ 式中:P为功率;λ为激光波长;M2为光束质量因子;C为一常数(对于高斯光束C=1)。通过计算,可以得到在该条件下,激光亮度为B= 4.7×1012 W/m2·sr。
图8为激光器最大输出功率下锁定运转时的功率稳定性曲线与波长稳定性测量图,此时平均功率为13.9 W,1 h内功率稳定度为 ±0.4%,对应的平均波长为1341.74203 nm,1 h内波长的波动范围为2.9 pm。
A high-power single-frequency continuous-wave 1 342 nm Nd:YVO4 laser with injection-locking
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摘要: 研究了一种基于注入锁定技术的888 nm 半导体激光器(LD)泵浦的高功率单频可调谐1342 nm Nd:YVO4激光器。采用最大输出功率20 mW分布式反馈单频半导体1342 nm激光器作为注入种子,利用lock-in (LI)技术,对LD端泵的Nd:YVO4环形腔激光器进行种子注入,实现了单频可调谐激光输出。激光器最大平均输出功率为13.9 W,测量的线宽为41 MHz,调谐范围为1341.6774~1341.8025 nm。x轴和y轴的光束质量$ M^{2} $因子分别为$ M_x^2 $= 1.30和$ M_y^2 $= 1.23。实验结果表明:与先前文献报道的注入锁定1342 nm可调谐激光的结果相比,所需种子功率大幅减小,输出功率也有所提升。
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关键词:
- Nd:YVO4激光器 /
- 连续波 /
- 单频 /
- 可调谐 /
- 注入锁定
Abstract:Objective The single-frequency lasers operating at 1.3 µm have been extensively investigated in a variety of fields, including quantum optics and fiber optical communication. A noteworthy example is the single-frequency tunable continuous-wave 671 nm laser based on frequency doubling of 1 342 nm laser, which is near the D-line atomic transitions (670.97 nm) of lithium vapor. Consequently, it finds important applications in the field of atomic physics related to lithium atoms, such as optical cooling, lithium atom interferometer, and lithium isotope separation. In applications like the lithium atom interferometer, a higher output power of the laser can yeild improved results. At present, the output power of single-frequency continuous-wave 1 342 nm diode lasers is as low as several miliwatts, necessitating application by Raman laser amplifiers and solid-state laser amplifiers. Therefore, the entire system becomes large and heavy. This paper introduces an injection-locked single-frequency tunable 1 342 nm Nd:YVO4 laser with high output power. The injection-locked laser offers the advantages of a small size and high gain, making it suitable for special demands. Methods In this paper, an injection-locked 1 342 nm continuous-wave single-frequency tunable Nd:YVO4 laser is developed. The system employs an end-pumped Nd:YVO4 ring laser as the slave laser, with a distributed feedback laser (DFB) as the seed laser. The seed laser is coupled into the Nd:YVO4 ring laser through output mirror (Fig.1). To achieve cavity length locking, a lock-in (LI) module is employed. The LI module detects laser intensity through a photoelectric detector and provides feedback control by adjusting the voltage on the piezoelectric transducer (PZT). The Nd:YVO4 ring laser operates bidirectionally under free operation. When the PZT on the cavity mirror is adjusted to match the cavity length with the wavelength of the injected seed laser, the laser can operate unidirectionally, resulting in a single-frequency continuous-wave 1 342 nm laser. The laser's tuning capability is achieved by changing the wavelength of the seed laser. Results and Discussions The measured output laser power in free operation is 13.9 W as recorded by power meter (Fig.2). In this state, the influence of the seed power on the injection locking of the ring laser is obtained (Fig.3). An output power of 13.9 W for the injection-locked laser is achieved with an input seed laser power of 20.69 mW. Under this condition, the tuning range of the laser is measured by a wave-meter, and a tuning range from 1 341.677 4 nm to 1 341.802 5 nm is achieved (Fig.4). Simultaneously, the laser line-width is studied using an F-P scanning interferometer (Fig.5). The laser operates in a single frequency with a line-width of approximately 41 MHz. The line-width of the output laser is enhanced compared to the seed laser, a result attributed to low seed power and the reverse-running laser mode in the cavity. The beam quality factors M2 of the injection-locked 1 342 nm laser are determined to be $ M_x^2 $ = 1.3 in the x direction and $ M_y^2 $ = 1.23 in the y direction using a laser beam analyzer (Fig.6). The power fluctuations (RMS) at the 13.9 W of the laser are measured and the stability is better than ± 0.5% (Fig.7). Conclusions A high-power tunable single-frequency 1 342 nm Nd:YVO4 laser based on LI injection-locked technology was successfully designed. The output power of injection-locked 1 342 nm laser reached 13.9 W, with a DFB seed laser power of 20 mW. The tunning range of the laser system was analyzed using a wave-meter, and the measured tuning range spanned from 1 341.677 4 nm to 1 341.802 5 nm. Various characteristics, including beam quality, laser line-width, power stability were comprehensively measured. To achieve the better stability and lower system noise, the methods of employing a seed laser with higher power and implementing methods such as reducing vibration and enclosed the structures have been identified as effective. -
Key words:
- Nd:YVO4 laser /
- continuous-wave /
- single-frequency /
- tunable /
- injection-locked
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