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设计合理的激光器谐振腔腔型,不仅获得较大的激光输出能量,也使得输出激光光束参数满足光纤的耦合条件。激光器谐振腔输出高斯光束远场发散角
$\theta $ 的关系式为[11]:$$\frac{1}{{{\rho _2}}} = (n - 1)\left(\frac{1}{{{R_1}}} - \frac{1}{{{R_2}}}\right) + \frac{1}{{{R_1}}}$$ (1) $$\theta = \mathop {\lim }\limits_{{\textit{z}} \to \infty } \frac{{\omega ({\textit{z}})}}{{\textit{z}}} = {\left[ {{{\left(\frac{{{\omega _2}}}{{{\rho _2}}}\right)}^2} + {{\left(\frac{\lambda }{{\pi {\omega _2}}}\right)}^2}} \right]^{1/2}}$$ (2) 式中:
$n$ 为谐振腔输出镜的折射率;${R_1}$ 和${R_2}$ 分别为输出镜的内外曲率半径;${\;\rho _2}$ 为出射光束等相面曲率半径;${\omega _2}$ 为出射高斯光束半径;$\omega ({\textit{z}})$ 为z处光斑半径;$\lambda $ 为激光波长。从公式(1)和(2)可以看出,谐振腔输出镜的内外曲率半径以及腔镜的折射率对输出高斯光束远场发散角有较大影响。在不考虑传输系统的像差时,从激光器谐振腔出射后的激光光束在传输过程中其光束参数乘积BPP值保持不变,即激光光束束腰半径和发散角成反比例关系,发散角增大时,束腰光斑直径就增大。从激光器谐振腔出射的高斯光束必须满足光纤的耦合条件,才能经过耦合透镜变换后与光纤参数相匹配。光纤耦合条件为激光光束束腰半径与远场发散角乘积的BPP值必须满足[12]:
$$BPP < \frac{1}{2}{d_{core}} \cdot \arcsin NA$$ (3) 式中:
${d_{core}}$ 为光纤芯径;$NA$ 为光纤的数值孔径。谐振腔腔型为平凸型,为非稳定腔。通过理论计算前腔镜焦距,但217 mm<R1<412 mm和279 mm<R2<453 mm时,输出光束参数乘积满足ZBLAN 光纤的耦合条件。通过对比不同焦距的透镜作为输出镜时的激光输出功率的大小,能够从中选择最优参数的透镜作为谐振腔输出镜。采用焦距分别为R1=250 mm,R2=300 mm和R1=300 mm,R2=350 mm以及R1=350 mm,R2=400 mm的氟化钙透镜作为耦合透镜,测量它们的输出功率的对比图,如图1所示。
从实验结果可以看出,在进行耦合实验时,在满足光纤耦合的条件上,尽可能地选择使激光器输出功率较大的透镜作为谐振腔的输出镜。最终选择了参数为R1=250 mm,R2=300 mm的弯月形透镜作为谐振腔输出镜,并通过实验测量了不同参数的透镜作为耦合透镜时其输出激光的BPP值,来验证理论计算的准确性。
在激光器输出光束直径一定的情况下,若激光束远场发散角较大,其光束参数乘积不满足光纤耦合条件,就无法耦合进光纤。实验中的ZBLAN光纤数值孔径NA=0.29、光纤芯径为400 μm。由公式(3)计算得到耦合的激光光束参数BPP值需要小于59.2 mm·mrad才能满足光纤耦合条件。从公式(1)、(2)可以看出,采用不同曲率半径的透镜替代平面镜作为谐振腔输出镜,可以改变输出光束的发散角。为此,笔者设计了弯月型输出镜的激光器谐振腔,以减小激光光束发散角。实验中激光器采用Er, Cr:YSGG晶体圆棒的尺寸为Ф4 mm×70 mm,
$ {\rm{Er}}^{3+} $ 的浓度为20at.%,$ {\mathrm{C}\mathrm{r}}^{3+} $ 的浓度为3at.%,晶体棒的两端镀有对2.79 μm的增透膜,后腔镜为平面全反镜,在2.79 μm处反射率>99%,用不同曲率半径的弯月型氟化钙透镜作为输出镜,其折射率$n$ =1.4349,凹面面向腔内,两边镀有增透膜,对2.79 μm波长的激光透过率为T=20%。由于激光从激光器输出后其光束参数乘积是保持不变的,可测量激光器输出端后任意位置处的光束参数乘积来表示激光器输出光束参数。实验采用距离激光输出镜16 cm处作为测量点,测量不同曲率半径的弯月型透镜作为输出镜的激光光束发散角和光斑直径,并计算它们的BPP值如表1所示。表 1 不同参数输出镜对应的发散角
Table 1. Divergence angle of the output mirror with different parameters
Output mirror parameters/mm Divergence angle/mrad Spot diameter/mm BPP value/mm·mrad 1 R1=∞,R2=∞ 44.63 5.91 131.90 2 R1=150,R2=200 31.10 6.03 93.77 3 R1=200,R2=250 27.62 5.37 74.10 4 R1=250,R2=300 20.60 3.26 33.57 通过实验数据计算得到第四组激光束的参数乘积BPP值为33.57 mm·mrad,满足光纤耦合条件。因而选用曲率为
${{R}}_{1}$ =250 mm、${{R}}_{2}$ =300 mm的弯月型透镜作为激光器谐振腔的输出镜。 -
激光光束经过透镜后其光束参数会发生变化。透镜可作为激光光束与光纤之间的耦合元件,用于改变激光光束参数,使得传输到光纤端面的激光光斑小于光纤芯径,同时激光光束发散角小于光纤的数值孔径,以保证光束能量全部耦合进入光纤。耦合到光纤端面的光束参数是由耦合透镜焦距、物方高斯光束共焦参数、像方光束束腰半径以及其距离透镜的物距共同决定[13]。在短焦距透镜且入射光束束腰距前焦点足够远时,像方光斑可表示为:
$${\omega _1} = \dfrac{{f\lambda }}{{\pi {\omega _0}\sqrt {1 + \left(\dfrac{{\lambda r}}{{\pi \omega _0^2}} \right)^2}}} = \dfrac{{f\lambda }}{{\pi \omega (l)}}$$ (4) 经过透镜变换后激光光束的发散角为:
${\theta _1} = 2\dfrac{\lambda }{{\pi {\omega _1}}}$ ,像方数值孔径为:$${R_{NA}} = n\sin \frac{\lambda }{{\pi {\omega _1}}}$$ (5) 式中:
${\omega _0}$ 和${\omega _1}$ 分别为物方光束束腰半径和像方光束束腰半径;$r$ 为高斯光束束腰距透镜的距离;$\omega (l)$ 为高斯光束入射至透镜表面处的束腰半径;n为激光所在介质的折射率,空间激光到光纤端面之间充满空气,空气折射率n=1。激光光束在空气中传播时,单透镜的焦距越小,像方束腰数值越小,聚焦效果越好。但在光纤能量传输时,不能一味追求小的光斑,光斑越小发散角也就越大。耦合透镜的选择不仅要考虑像方光斑小于光纤芯径同时还要考虑光纤端面的发散角也要小于光纤的数值孔径。针对文中实验中的ZBLAN玻璃光纤,光纤端面的激光光斑大小以及发散角需满足的条件为:
${\omega _1} \leqslant 400$ μm,${R_{NA}} \leqslant 0.29$ 。实验测得透镜表面光束束腰半径$\omega (l)$ 为$1.5\;\rm mm$ ,代入公式(4)绘制的像方腰斑${\omega _1}$ 与透镜焦距f 的关系图(图2)可以看出,当透镜焦距 f < 675 mm时,像方光斑小于光纤芯径400 μm。从公式(5)绘制的像方激光光束数值孔径${R_{NA}}$ 与像方光斑半径${\omega _1}$ 的关系图(图3)可以看出,当像方激光光束腰斑大于3 μm时,像方光束数值孔径均小于光纤的数值孔径0.29,此时对应的透镜焦距为f =5.17 mm,只有当f > 5.17 mm时,像方激光光束数值孔径才始终小于光纤的数值孔径。通过以上计算对比发现,使得像方激光光斑直径小于400 μm,同时光束发散角小于0.29的透镜焦距范围为:5.17 mm<f<675 mm。根据市面上以及实验室已有的透镜参数,最终选择焦距为20 mm、50 mm、75 mm、100 mm和300 mm氟化钙凸透镜作为耦合透镜,两面镀上对波长为2.79 μm激光传输的增透膜,实验对比了不同参数的耦合透镜对耦合效率的影响。图 2 像方腰斑
${\omega _1}$ 与透镜焦距f的关系Figure 2. Relationship between waist spot ω1 of image side and focal length f of lens
图 3 像方激光数值孔径
${R_{NA}}$ 与光斑半径${\omega _1}$ 的关系Figure 3. Relationship between numerical aperture RNA and spot radius ω1 of image side laser
图4为采用在焦距范围内的不同焦距的透镜对耦合效率的影响对比图。从实验结果可以看出,随着透镜焦距的增大,输出激光能量在降低。尽管它们之间相差不大,但是为了得到最高的耦合效率,笔者在实验中采用了焦距为20 mm的氟化钙透镜作为耦合透镜。
Experimental investigation of 2.79 μm Cr, Er: YSGG laser fiber coupling
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摘要: 采用传能光纤替代导光臂传输激光能量能够极大地改善医用手柄的灵活性、降低系统复杂程度、提高激光传输效率。设计研制了2.79 μm Er, Cr: YSGG激光器及其光纤耦合系统。分析了激光器谐振腔输出镜对输出高斯光束参数的影响,设计弯月型透镜作为激光器谐振腔输出镜减小激光光束发散角,并选择合适的耦合单透镜,满足了数值孔径为0.29、芯径为400 μm的ZBLAN玻璃光纤耦合条件。实验结果表明,在弯月型透镜作为激光输出镜,耦合透镜焦距为20 mm时,可实现激光传输耦合效率高达83%,最大传输功率6 W,满足了激光医疗仪器的临床应用需求。
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关键词:
- Cr, Er: YSGG激光 /
- 单透镜 /
- 谐振腔 /
- 光纤 /
- 空间耦合
Abstract: Using the energy transmitting fiber to replace the optical guide arm can greatly improve the flexibility of the medical handle, reduce the complexity of the system and improve the efficiency of laser transmission. A 2.79 μm Er, Cr: YSGG laser and its fiber coupling system were designed and developed. The influence of the output mirror of the laser resonator on the parameters of the output Gaussian beam was analyzed. A meniscus type lens was designed as the output mirror of the laser resonator to reduce the divergence angle of the laser beam, and a suitable coupling single lens was selected to meet the coupling conditions of the ZBLAN glass fiber with a numerical aperture of 0.29 and a core diameter of 400 μm. The experimental results show that when the meniscus type lens is used as the laser output mirror and the focal length of the coupling lens is 20 mm, the coupling efficiency of the laser transmission can reach up to 83%, and the maximum transmission power is 6 W, which meets the clinical application requirements of the laser medical instrument.-
Key words:
- Cr, Er: YSGG laser /
- single lens /
- resonant cavity /
- fiber /
- space coupling
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表 1 不同参数输出镜对应的发散角
Table 1. Divergence angle of the output mirror with different parameters
Output mirror parameters/mm Divergence angle/mrad Spot diameter/mm BPP value/mm·mrad 1 R1=∞,R2=∞ 44.63 5.91 131.90 2 R1=150,R2=200 31.10 6.03 93.77 3 R1=200,R2=250 27.62 5.37 74.10 4 R1=250,R2=300 20.60 3.26 33.57 -
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