高功率高峰值重复率Yb光纤放大器的二次谐波生成研究

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"这篇研究论文详细探讨了一种基于镱(Yb)掺杂光纤放大器的高峰值功率、高重复频率主振荡功率放大器(MOPA)系统及其二次谐波生成(SHG)现象。该系统中的振荡器采用主动Q开关微芯片激光器，工作在50kHz的重复频率下，脉宽为2.8ns。放大器利用了镱掺杂的保偏光纤，具有大的模式面积，并由17W的激光二极管激发。实验结果显示，放大后的平均输出功率达到10W，光-光转换效率为59%。在MOPA系统中，研究人员分别使用了KTP和LBO晶体进行SHG实验，获得了21%和40%的转换效率，以及0.92W和3.3W的最大SHG功率，分别对应KTP和LBO晶体。" 这篇论文的核心知识点包括： 1. **MOPA系统**：Master Oscillator Power Amplifier（MOPA）是一种常见的激光系统架构，它由一个主振荡器（Master Oscillator）和一个功率放大器组成。主振荡器产生初始的低功率激光脉冲，然后通过功率放大器提高其功率。 2. **镱掺杂光纤放大器**：镱是一种常用的光纤激光器活性介质，因为它可以吸收和发射近红外光，适合用于高功率激光系统。镱掺杂光纤具有高的增益系数和良好的热管理性能。 3. **主动Q开关**：Q开关是控制激光器输出脉冲能量和脉宽的一种技术，通过改变激光腔的Q值（即谐振腔的品质因数）来实现。主动Q开关通常使用电光效应或声光效应，在特定时间瞬间打开，使激光快速释放能量，形成高峰值功率的短脉冲。 4. **二次谐波生成(SHG)**：SHG是非线性光学效应的一种，它可以将单个激光波长的光转换为其两倍频率的光。在这个实验中，SHG用于将1064nm的近红外光转换为532nm的绿光。 5. **KTP和LBO晶体**：KTP（磷酸钛酸钾）和LBO（硼酸锂）是非线性光学晶体，常用于SHG过程。KTP适用于较宽的波长范围，而LBO则具有较高的非线性系数和更快的响应速度。 6. **转换效率和最大SHG功率**：转换效率是指输入光功率转换为二次谐波输出光功率的比例，而SHG最大功率是实验中能够达到的二次谐波输出的最大功率。在这次实验中，KTP和LBO的转换效率分别为21%和40%，对应的SHG最大功率为0.92W和3.3W。这些知识点展示了高功率光纤激光器的设计、优化和非线性光学转换的实践，对于理解和改进激光技术，尤其是在工业加工、精密测量、生物医学和科学研究等领域有着重要的应用价值。

May 31, 2007 / Vol. 5, Supplement / CHINESE OPTICS LETTERS S111

Second harmonic generation of high peak power,

high repetition rate from Yb-doped ﬁber ampliﬁer

Hiroaki Sunaga, Ryusuke Horiuchi, Kazuya Jyosui, Kazuyoku Tei,

Shigeru Yamaguchi, Kenzo Nanri, and Tomoo Fujioka

Department of Physics, Tokai University, 1117 Kitaka name Hiratuka-shi Kanagawa 259-1292, Japan

A high peak power, high repetition rate master oscillator power ampliﬁer (MOPA) system incorporating

an Yb-doped ﬁber ampliﬁer and its second harmonic generation (SHG) were investigated in detail. The

oscillator is actively Q-switched microchip laser at repetition rate of 50 kHz with a pulse width of 2.8

ns. The ampliﬁer employing Yb-doped polarization maintaining ﬁber and having a large mode area was

excited by a laser diode with an optical power of 17 W. As results, the ampliﬁed average output power of 10

W and optical-optical conversion eﬃciency of 59% were achieved. In this MOPA system, experiments were

performed by using KTP and LBO crystals. The conversion eﬃciency of 21% and 40%, SHG maximum

power of 0.92 and 3.3 W were obtained for KTP and LBO crystals respectively.

OCIS codes: 190.2620, 060.2320, 140.4480.

Nanosecond high peak power sources of near-infrared,

visible and ultraviolet radiation are expected for indus-

try, and medical applications

[1]

. Especially high rep-

etition, short pulse laser is requested from industrial.

The Yb-doped ﬁber master oscillator power ampliﬁer

(MOPA) system, which can generate high peak power

and high repetition rate pulses, and has many other

advantages, such as vibration-proof, compactness and

water free for cooling

[2,3]

, has been attractive for in-

dustrial micro-machining

[4]

. In our system, a compact

actively Q-switched microchip laser is ampliﬁed in an

Yb-doped ﬁber. The gain material of the microchip laser

is Nd:YVO

which ensures high gain and short pulse

width at higher repetition rate. The seed pulse of wave-

length 1064 nm is ampliﬁed in the Yb-doped double-clad

ﬁber, which has the emission of Yb-ion from 1000 to 1100

nm. The upper limit of peak power in a ﬁber ampliﬁer is

determined by the ﬁber nonlinearities, such as stimulated

Brillouin scattering (SBS), stimulated Raman scattering

(SRS) and self-focusing

[5−7]

. The increased mode area

of double-clad ﬁber pushes up the limit of peak power

limited by the above ﬁber nonlinearities.

In this paper, the ampliﬁcation characteristic of Yb-

doped ﬁber MOPA system is studied, and the second

harmonic generation (SHG) (532 nm) eﬃciencies by us-

ing KTP and LBO crystal are compared. The mismatch-

ing inﬂuence of the self phase modulation (SPM) in ﬁber

on SHG is also discussed.

Figure 1 shows the Yb-doped ﬁber MOPA system and

Fig. 1. Schematic diagram of the optical system.

SHG. For the present experiment, the ﬁber ampliﬁer

was seeded with the 1064-nm output of the Q-switched

Nd:YVO

microchip laser, which was pumped by an 808-

nm diode laser. The seed light produced with a nearly

Gaussian temporal proﬁle (2.8 ns, full width at half max-

imum (FWHM)) and an pulse energy of 9.4 μJatarep-

etition rate of 50 kHz.

The Yb-doped polarization maintaining double-clad

ﬁber is used as the gain material. The high pulse energy

and peak power were enabled by the use of large mode

area core ﬁber. The Yb-doped ﬁber had a core diameter

of 30 μm, numerical aperture of 0.06. The Yb-doped ﬁber

was pumped by a laser diode operated at wavelength of

978 nm and output of 17 W. The seed pulse and pump

light were counter-propagating

[8]

The second harmonic (532 nm) of the ampliﬁer out-

put was generated by KTP and LBO crystals. The KTP

crystal was arranged at the position with a beam waist

radius of 162.4 μm. The critical phase matching of type

II (θ =90

◦

, ϕ =23.5

◦

) was selected for KTP, and its size

was 5×5×10 (mm). The LBO crystal was arranged at the

position with a beam waist radius of 66.2 μmandthe

temperature was maintained at 148

◦

C. The non-critical

phase matching of type I (θ =90

◦

, ϕ =0

◦

) was selected

for LBO. The size was 3×3×10 (mm). The proper po-

larization orientation was selected by λ/2 plate.

Figure 2 shows the ampliﬁcation characteristic of Yb-

doped ﬁber MOPA system of 1064 nm. When the ﬁber

was pumped by the maximum LD power of 17 W, the

ampliﬁed output power of 10 W was obtained with the

optical-optical eﬃciency of 59%. The corresponding peak

power was 71.4 kW. In this case, the average input power

of seed pulses was 0.33 W. The limit of maximum peak

power is decided by nonlinear eﬀects in ﬁber rather than

optical damage in our case. SRS limits the peak power

as well.

The threshold power in relation with SRS that occurs

in ﬁbers can be approximated as

SRS

≈

16A

eﬀ

, (1)

where g

is Raman gain, A

eﬀ

is the eﬀective mode ﬁeld

area, L

eﬀ

is the eﬀective ﬁber length and K is the po-

larization dependence factor

[9,10]

. It is assumed that

1671-7694/2007/S1S111-04

 2007 Chinese Optics Letters

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