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DBR型光纤激光器由厘米长度的高掺杂光纤和布拉格光栅对构成,这个长度的有效腔长可以增大纵模间隔,结合光栅的滤波效应,实现单纵模输出。当对谐振腔整体施加温度影响时,谐振腔内纵模的位置和布拉格光栅的透过谱位置会发生改变,由于纵模的温漂速率
${S_{\rm{f}}}$ 和增益谱的温漂速率${S_{\rm{q}}}$ 不一致,两者在光谱上的相对位置会发生改变,因此,当温度连续变化时,DBR型光纤激光器可能会出现一个由单纵模变为多纵模后再变回单纵模的过程,如图1所示。图 1 温度连续变化下DBR光纤激光器的输出纵模变化示意图。 (a)初始单纵模状态;(b)即将进入多纵模的状态; (c)多纵模状态; (d)即将变回单纵模的状态
Figure 1. Output longitudinal mode variation of DBR fiber laser under continuous temperature change. (a) Initial single longitudinal mode state; (b) About to enter the state of multi-longitudinal mode; (c) Multi-longitudinal mode state; (d) About to change back to the state of single longitudinal mode
在上述过程中,DBR光纤激光器输出光保持单纵模调谐的温度范围大小取决
${S_{\rm{q}}}$ 和${S_{\rm{f}}}$ 的差值$\Delta S$ ,$\Delta S$ 的值越小,DBR光纤激光器在单纵模调谐范围就越大。此外,在$\Delta S$ 的值和布拉格光栅的反射光谱确定的情况下,减小谐振腔的腔长以增大纵模间隔同样可以增加激光器单纵模调谐的范围。但腔长的减小往往也意味着掺杂光纤的长度变短,这会导致激光器输出功率的下降。因此,为了在保证较大的单纵模温度调谐范围的同时获得一定功率的激光输出,需要合理选择掺杂光纤的长度。DBR光纤激光器纵模间隔可表示为:
$$ \Delta {\nu _{\rm{q}}} = \frac{c}{{2n{L_{eff}}}} $$ (1) 式中:c为真空中光速;n表示光纤纤芯折射率;
${L_{eff}}$ 表示谐振腔的有效腔长,由两部分组成。$$ {L_{eff}} = L + L{'} $$ (2) 式中:
$L$ 为掺杂光纤的长度;$L{'} $ 为根据文献[11]换算后光栅对有效腔长贡献的部分。根据文献[7],为保证激光器的单纵模输出,$\Delta {\nu _{\rm{q}}}$ 应满足:$$ \Delta {\nu _{\rm{q}}} > \frac{1}{2}\Delta {\nu _{\rm{f}}} $$ (3) 式中:
$\Delta {\nu _{\rm{f}}}$ 为由窄带光栅所决定的增益谱宽度。当公式(3)成立时,增益谱内只存在一个纵模,可以轻易实现单纵模输出,但此时纵模间隔太大,限制了谐振腔内掺杂光纤的长度,会导致腔内增益太弱,输出功率极低。而实验研究表明,当纵模温漂速率和增益谱温漂速率之差$\Delta S$ 很小时,为保证较大的单纵模调谐范围,$\Delta {\nu _{\rm{q}}}$ 只需满足:$$ \Delta {\nu _{\rm{q}}} > \frac{1}{3}\Delta {\nu _{\rm{f}}} $$ (4) 当公式(4)成立时,增益谱内虽然存在2个纵模,但通过模式竞争仍然可以较为容易地实现单纵模输出;而当公式(4)不成立时,增益谱内存在2个以上的纵模,即使存在模式竞争也很难保证单纵模输出。联立公式(1)、(2)和(4),可知掺杂光纤长度应满足:
$$ L < \frac{{3c}}{{2n\Delta {\nu _{\rm{f}}}}} - L{'} $$ (5) -
根据DBR激光器单模调谐的原理,结合实验中所用光栅的参数,由公式(4)可以计算得出:只有当掺杂光纤的长度低于11.4 mm时才可以实现稳定的单纵模调谐。考虑到DBR制作误差,实验中分别采用掺杂光纤为8 mm和掺杂光纤为15 mm的DBR光纤激光器在温度连续变化时的纵模变化情况进行了测试,结果如表1所示。由表1可知,15 mm掺杂光纤DBR激光器在温度变化时出现了由单纵模(Single Longitudinal Model, SLM)变化到多纵模(Multi-longItudinal Mode, MLM)再变回单纵模的现象,而8 mm掺杂光纤DBR光纤激光器则可以维持稳定的单纵模输出,图3为温控20 ℃的条件下8 mm掺杂光纤和15 mm掺杂光纤DBR光纤激光器的纵模测试结果。由表1可知,纵模测试测试结果与理论计算预测相符合。
表 1 不同温度下激光器的纵模特性
Table 1. Single longitudinal model characteristics of lasers at different temperatures
Cavity
temperature/℃DBR with 15 mm
Yb3+ doped fiberDBR with 8 mm
Yb3+ doped fiber0 SLM SLM 5 MLM SLM 10 SLM SLM 15 MLM SLM 20 MLM SLM 25 SLM SLM 30 MLM SLM 35 MLM SLM 40 MLM SLM 45 MLM SLM 50 SLM SLM 图 3 20 ℃温控下DBR光纤激光器纵模特性。 (a) 掺杂光纤15 mm DBR;(b) 掺杂光纤8 mm DBR
Figure 3. Longitudinal model characteristics of DBR fiber laser at 20 ℃. (a) Doped fiber 15 mm DBR; (b) Doped fiber 8 mm DBR
将泵浦光功率设置为300 mW,利用TEC温控系统对激光器的整个谐振腔施加温度影响,温度缓慢从0 ℃ 变化至100 ℃,记录下激光器调谐情况如图4所示。可以看出,当谐振腔温度由0 ℃缓慢变化到100 ℃的过程中,激光的中心波长由1063.8 nm变化为1 064.624 nm,即实现了0.824 nm的调谐,在调谐过程中激光始终保持单纵模输出状态。
观测不同调谐温度下激光器输出光中心波长稳定性,将谐振腔所附着的黄铜片温度分别设置为10 ℃、30 ℃和60 ℃,记录其30 min内中心波长的变化情况。 结果表明在10 ℃、30 ℃和60 ℃时信号光的中心波长分别为1063.908 nm、1064.076 nm和1064.328 nm,在30 min内三者的最大中心波长漂移均不超过20 pm,如图5所示。
测试了激光器的泵浦效率以及泵浦功率变化时的单纵模情况,在谐振腔温度分别为10 ℃、30 ℃和60 ℃的条件下进行测试,测试结果如图6所示。测试结果表明:当谐振腔温度为10 ℃、30 ℃和60 ℃时激光器的起振阈值均为25 mW左右,当泵浦功率为380 mW时激光器输出功率约为7.4 mW,接近饱和输出状态,此时的泵浦效率为2%。不同温度下激光器的泵浦效率曲线基本一致,且从起振到饱和输出的过程中输出光始终保持单纵模状态。
在保持谐振腔温度为20 ℃,情况下测试了30 min内激光器输出功率的变化情况,结果表明30 min内信号光的平均功率为6.95 mW,功率最大波动小于1%。在相同的条件下令激光通过一偏振片后再通过一偏振分光棱镜后接入功率计中,通过旋转偏振片测试输出光的偏振特性。测试结果表明激光器输出光为线偏振光,偏振消光比大于24 dB。
搭建了基于自外差拍频检测原理的线宽检测系统[13],其结构如图7所示。在延时臂采用单端损耗低于0.2 dB的环形器(Circulator)和1064 nm反射镜将18 km延迟光纤(Hi1060)倍增至36 km,以提升最小测量线宽。
为验证此线宽检测系统的有效性,将窄线宽种子源(NKT Photonics, Koheras Adjustik Y10)接入此系统中,测得其线宽为3.85 kHz,与其标称线宽相吻合,测试结果如图8(a)所示。将信号光接入线宽检测系统,测试结果如图8(b)所示。由图可知,拍频信号峰值20 dB处的线宽为87.8 kHz,根据文献[13],该数值约为实际线宽的20倍,因此,激光器输出光的实际线宽为4.4 kHz。
图 8 线宽测试结果。(a)标准种子源线宽;(b) DBR光纤激光器线宽
Figure 8. Result of linewidth test. (a) Linewidth of standard seed source; (b) Linewidth of DBR fiber laser
最后,测试了激光器输出光的相对强度噪声,为了更加准确地描述激光器输出光的相对强度噪声,测试了未接收信号光时光电探测器的噪声基底以及未接光电探测器时的频谱仪的噪声基底,测试结果如图9所示。可以看出激光器输出光的弛豫震荡峰值位于900 kHz处,且其峰值幅度为−110 dB/Hz,在频率大于1.5 MHz以后,激光器的相对强度噪声会稳定在−145 dB/Hz附近。
Wavelength tuning with temperature in single longitudinal mode DBR fiber laser
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摘要: 报道了采用DBR方式,利用8 mm的高浓度掺Yb3+单模光纤,实现了波长为1064 nm的单纵模调谐激光稳定输出的实验结果。该DBR谐振腔有效腔长为16 mm,输出最大功率为7.4 mW,通过半导体制冷器温控改变谐振腔的温度,实现了0.824 nm的单纵模无跳模调谐。采用光纤外差法,并利用低损耗环形器和光纤反射镜倍增延迟线长度提升测量精度的方式,测量得到激光最大线宽为4.4 kHz。单纵模激光的弛豫震荡峰位于900 kHz处,其相对强度噪声为−110 dB/Hz,当频率大于1.5 MHz时相对强度噪声为−145 dB/Hz。Abstract:
Objective Distributed Bragg reflection (DBR) single longitudinal mode fiber lasers have been widely studied and applied due to their simple resonant cavity structure and good stability. However, the narrow tuning range of current DBR lasers limit their applications in many important fields such as spectral synthesis, laser frequency locking, and coherent detection, etc. Therefore, how to improve its tuning range is of greater research value. And improving the mode-free hopping tuning range of DBR fiber lasers has become the research objective in this study. Methods First of all, according to the principle that the center wavelength of fiber grating drifts is caused by resonant cavity temperature change, the equivalent length theory of the fiber grating, the longitudinal mode spacing theory, and the relationship between the gain spectrum of the doped fiber and the intracavity mode competition, the mechanism of DBR single longitudinal mode fiber laser to achieve single longitudinal mode output and the variation of the longitudinal mode in the resonant cavity during the temperature tuning process are theoretically analyzed. Secondly, DBR single longitudinal mode fiber lasers were built based on the theoretical analysis, and two DBR lasers with different equivalent cavity lengths were constructed by using two different lengths of doped fibers. A temperature controller built with a Thermoelectric Cooler (TEC) and a brass sheet was used to control the temperature of the resonant cavity, and the variation of the center wavelength of the output laser and the longitudinal mode of the output laser during the change of the resonant cavity temperature from 0 ℃ to 100 ℃ were tested. Results and Discussions As a result, the conditions that the cavity length of the resonant cavity of DBR single longitudinal mode fiber laser needs to meet in order to realize the temperature tuning without hopping mode are deduced. Besides, different single and multiple longitudinal mode output results during temperature tuning of lasers with different equivalent cavity lengths verify the correctness of the analytical result of cavity length constraint condition for DBR single longitudinal mode laser (Fig.3, Tab.1). And then, 8 mm high-concentration Yb3+ doped single mode fiber is used to achieve a stable single-longitudinal-mode laser at the wavelength of 1064 nm with the distributed Bragg reflection method. The effective cavity length of the DBR resonator is 16 mm and the maximum laser output power is 7.4 mW. The single longitudinal mode tuning of 0.824 nm without mode hopping is achieved by varying the resonant cavity temperature (Fig.4). With the low loss circulator and the fiber mirror to multiply delay fiber length of the heterodyne method to improve measurement accuracy (Fig.7), the measured maximum linewidth of the laser is 4.4 kHz. The relative intensity noise of the laser was tested using a photodetector. The relaxation oscillation peak of the output laser is located at 900 kHz with a relative intensity noise of −110 dB/Hz. The relative intensity noise is −145 dB/Hz when the frequency is greater than 1.5 MHz. Conclusions In summary, through theoretical and experimental studies, a 1064 nm, mode-hopping-free tuning range of 0.8 nm distributed Bragg reflective single longitudinal mode fiber laser was proposed, and a series of its key parameters were tested, which has certain application value. -
Key words:
- fiber laser /
- single frequency /
- temperature tuning /
- distributed Bragg reflection
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图 1 温度连续变化下DBR光纤激光器的输出纵模变化示意图。 (a)初始单纵模状态;(b)即将进入多纵模的状态; (c)多纵模状态; (d)即将变回单纵模的状态
Figure 1. Output longitudinal mode variation of DBR fiber laser under continuous temperature change. (a) Initial single longitudinal mode state; (b) About to enter the state of multi-longitudinal mode; (c) Multi-longitudinal mode state; (d) About to change back to the state of single longitudinal mode
表 1 不同温度下激光器的纵模特性
Table 1. Single longitudinal model characteristics of lasers at different temperatures
Cavity
temperature/℃DBR with 15 mm
Yb3+ doped fiberDBR with 8 mm
Yb3+ doped fiber0 SLM SLM 5 MLM SLM 10 SLM SLM 15 MLM SLM 20 MLM SLM 25 SLM SLM 30 MLM SLM 35 MLM SLM 40 MLM SLM 45 MLM SLM 50 SLM SLM -
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