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光纤激光器与其它激光器相比，具有结构简单、成本低、容易实现窄线宽的优点，被广泛应用于光电检测、光纤传感、光通信等领域[1-4]，是近年来研究的热点。双波长单纵模光纤激光器可以通过拍频产生微波信号[5-6]。此方法不需要参考微波源，具有结构简单、产生的微波相位噪声低等优点，被许多研究者重视[7-9]。
获得双波长振荡很容易，而解决稳定性问题绝非易事。有两个问题不可回避：(1)掺铒光纤本身的均匀展宽特性，使振荡的模式具有不稳定性，研究者提出很多办法解决这个问题，比如应用掺铒光纤和其它放大器相结合[10]，或在原谐振腔的基础上加入未抽运的掺铒光纤作为可饱和吸收介质的滤波器来抑制不需要的模式[11]；(2)激光器结构中需要加入纵模选择机制来限制多纵模振荡，比如加入一个萨哥纳克(Sagnac)光纤环镜[12]，或者由光纤布喇格光栅构成的法布里-珀罗(F-P)滤波器[13]。但是这些结构中，由于双波长之间的模式竞争，实现单频振荡依然是很困难的问题。
本文中提出了基于半导体光放大器(semiconductor optical amplifier, SOA)环形滤波器的双波长单频激光器，应用偏振分束和保偏光纤的非线性偏振旋转效应(nonlinear polarization rotation, NPR)实现多波长振荡，应用非均匀展宽增益介质SOA弱化模式竞争，实现了双波长振荡，实现了激光的单频振荡。
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图 1为实验中设计的基于NPR和掺铒环形滤波器的双波长单纵模激光器的结构。掺铒光纤放大器加上驱动电流产生自发辐射的光，使激光器起振，并作为激光器的增益介质，经过一个带增益的环形滤波器，限制激光器的模式，使得双波长振荡处于单纵模状态，偏振控制器用来调整激光谐振腔的偏振态，保偏光纤和隔离器和偏振分束器共同构成在线型周期滤波器，当激光腔内的功率满足一定条件时，便可有多波长输出[14-16]，偏振分束的一端作为输出，对激光器进行监测。图 1中虚线框内是激光器结构中的环形滤波器，光纤环的长度约是1.46m，50:50耦合器4个端口的输入输出光强分别用E1, E2, E3和E4来表示，它们之间的关系如下所示[15]：
$ \left[ \begin{array}{l} {E_3}\\ {E_4} \end{array} \right] = \left[ {\begin{array}{*{20}{c}} {\sqrt {1 - \gamma } }&{{\rm{i}}\sqrt \gamma }\\ {{\rm{i}}\sqrt \gamma }&{\sqrt {1 - \gamma } } \end{array}} \right]\left[ \begin{array}{l} {E_1}\\ {E_2} \end{array} \right] $
(1) 
Figure 1. Schematic of dual-wavelength fiber laser based on SOA ring filter
式中, γ为耦合因子, E4可以表示为：
$ {E_4} = {\rm{i}}\sqrt \gamma {E_1} + \sqrt {1 - \gamma } {E_2} $
(2) 在光纤环中，E4经过掺铒光纤获得了增益(或者损耗)及时间延迟τ，然后送到E2，E4和E2的关系如下：
$ {E_2} = g{{\rm{e}}^{{\rm{i}}\omega \tau }}{E_4} $
(3) 式中，ω是光强的角频率，g为环路增益。将(2)式代入(3)式得:
$ {E_2} = {\rm{i}}g{{\rm{e}}^{{\rm{i}}\omega \tau }}\sqrt \gamma {E_1}/(1 - g{{\rm{e}}^{{\rm{i}}\omega \tau }}\sqrt {1 - \gamma } ) $
(4) 然后将(4)式代回(1)式中得:
$ {E_3} = \frac{{\sqrt {1 - \gamma } - g{{\rm{e}}^{{\rm{i}}\omega \tau }}}}{{1 - g{{\rm{e}}^{{\rm{i}}\omega \tau }}\sqrt {1 - \gamma } }}{E_1} $
(5) $ {E_4} = {\rm{i}}\sqrt \gamma {E_1}/\left( {1 - g{{\rm{e}}^{{\rm{i}}\omega \tau }}\sqrt {1 - \gamma } } \right) $
(6) 两个谱线间的间隔Δω为：
$ \Delta \omega = \frac{1}{\tau }\arccos \left[ {\frac{{4g\sqrt {1 - \gamma } - 1 - {g^2}\left( {1 - \gamma } \right)}}{{2g\sqrt {1 - \gamma } }}} \right] $
(7) 则相应的传输函数T为：
$ \begin{array}{*{20}{c}} {T = {{\left| {{E_4}} \right|}^2}/{{\left| {{E_1}} \right|}^2} = }\\ {\frac{\gamma }{{1 + {g^2}\left( {1 - \gamma } \right) - 2g\sqrt {1 - \gamma } \cos \left( {\omega \tau } \right)}}} \end{array} $
(8) 自由频谱范围F为:
$ F = 2{\rm{ \mathsf{ π} /}}\tau $
(9) 对环形滤波器进行仿真，其中γ=0.5，g=1.2，F=140MHz, 得到如图 2所示的传输函数。
Figure 2. Simulation spectrum of ring filter
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当掺铒光纤放大器的驱动电流调整到180mA时，在不加滤波器之前，调整偏振控制器的位置使激光腔内的偏振态发生改变，当腔内的相位和增益等条件达到自洽时，开始出现激光。图 3a为多波长振荡时的光谱(未加滤波器)。理论上多波长间隔是均匀的，但实验结果表明多波长的间隔不一样，这是由于掺铒光纤放大器的增益不平坦，且掺铒光纤放大器是均匀展宽介质，模式竞争比较激烈，有一部分波长被抑制掉了。图 3a中共有4个功率比较高的波长，1544.4nm, 1545.2nm, 1546.5nm, 1546.9nm, 3dB带宽分别为0.12nm, 0.13nm, 0.12nm, 0.15nm。加上滤波器，并调整半导体光放大器的驱动电流为100mA时，只剩下1544.4nm, 1545.2nm两个波长，3dB带宽都是0.05nm。激光器环形腔、环形滤波器以及非线性偏转旋转作用，三者都有选频作用，在不加环形滤波器时，有多个波长振荡，加上环形滤波后，有一部分波长被抑制掉了，故只剩下了双波长。
Figure 3. a—spectrum of multi-wavelength laser b—spectrum of dual-wavelength laser with filter
作者用可调谐滤波器分别滤除其中一个波长, 并进行了稳定性和激光器线宽的测试。图 4是对波长为1544.4nm的单纵模振荡1h内的输出功率进行的测试，输出功率波动为0.6dB。用延时干涉的方法对其进行了测试，选20km单模光纤作为延时，图 5为频谱仪获取结果，洛伦兹拟合后线宽值为5.7kHz。
Figure 4. Laser power fluctuation within 1h
Figure 5. Linewidth measurement of laser
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设计了一种基于SOA和和掺铒光纤滤波器的环形腔激光器，利用NPR效应获得多频率同时振荡，掺铒光纤环形滤波器起到滤波作用，通过调整SOA的驱动电流的大小和偏振控制器，可以获得双波长激光振荡。仿真表明：环形滤波器具有梳状滤波作用；光谱仪上显示，由不加滤波器的0.12nm~0.13nm被压缩到0.05nm，并测量出激光器的线宽为5.7kHz，说明引入的环形滤波器滤波效果十分明显，输出激光的线宽得到了有效的压缩。
基于环形滤波器的双波长单频光纤激光器
Dual-wavelength single frequency fiber laser based on ring filter
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摘要: 为了解决双波长激光器的稳定性问题、达到压窄双波长激光器线宽的目的，采用在激光器结构中加入环形滤波器的方法，抑制了不需要的振荡模式。对滤波器进行了计算和仿真，得到了梳状谱；通过实验得到了线宽为5.7kHz的单纵模双波长激光。经过输出功率稳定性测试，1h内功率波动为0.6dB。结果表明，环形滤波器的作用是十分明显的。Abstract: In order to solve the problem of dual wavelength laser stability and achieve the purpose of narrowing the line width, the unwanted oscillation modes was limited by adding ring filter into the laser structure. After simulation and calculation of filter, the dressing spectrum was obtained. Single frequency dual-wavelength laser with linewidth of 5.7kHz was obtained by experiments. After output power stability test, the power fluctuation within 1h was 0.6dB. The result shows that the effect of ring filter is very obvious.
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Key words:
- lasers /
- dual-wavelength fiber laser /
- nonlinear polarization rotation /
- fiber filter
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Figure 3. a—spectrum of multi-wavelength laser b—spectrum of dual-wavelength laser with filter
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