Thermo-optically tunable spectral broadening in a nonlinear ultra-silicon-rich nitride Bragg grating
Fig.1 Simulated spectral broadening at different temperatures at a fixed pulse wavelength of 1536 nm.
Optical pulses are essential ingredients in the deployment of imaging systems, precision manufacturing and high-speed communications. The ability to manipulate their properties, including their spectral bandwidth is therefore an important function.
However, most spectral tuning mechanisms today rely on bandpass filtering, which inadvertently leads to losses in pulse energy. Specific examples of such approaches include the use of acousto-optic filters, fiber Bragg gratings and resonant filters, all of which require truncating the pulse spectrum to achieve spectrally narrower and temporally longer pulses. Approaches which allow spectral tuning of optical pulses without pulse energy losses are therefore highly sought after.
The research group led by Prof. Dawn T.H. Tan from Singapore University of Technology and Design developed a new approach which has been demonstrated for tunable pulse spectra without losses in pulse energy typically associated with spectral truncation. This work was published in Photonics Research, Vol. 9, No. 4, 2021(Yanmei Cao, Ezgi Sahin, Ju Won Choi, Peng Xing, George F. R. Chen, D. K. T. Ng, Benjamin J. Eggleton, Dawn T. H. Tan. Thermo-optically tunable spectral broadening in a nonlinear ultra-silicon-rich nitride Bragg grating[J]. Photonics Research, 2021, 9(4): 04000596).
The method combines the Kerr nonlinearity and the thermo-optic effect present in a complementary metal-oxide semiconductor compatible photonic device to achieve the energy efficient spectral tuning function. Based on the ultra-silicon-rich nitride (USRN) platform which possesses a nonlinear refractive index, 100X larger than in stoichiometric silicon nitride, the spectral tuning device consists of a two-stage device: a cladding-modulated Bragg grating concatenated with a highly nonlinear waveguide.
In the first stage, pulses propagating in the cladding-modulated Bragg grating experience strong grating induced dispersion, three orders of magnitude larger than in a waveguide, with the magnitude decreasing away from the stopband.
When situated on the blue side of the grating, these pulses evolve into Bragg solitons, undergoing high-order temporal compression. The extent of compression varies with the detuning between the grating stopband and the pulse wavelength. Stronger compression results in pulses which then undergo more pronounced spectral broadening in the second stage of the device, through the process of self-phase modulation.
Consequently, varying the extent of Bragg soliton effect compression in the first stage can be achieved by tuning the detuning between the pulse wavelength and the grating stopband. This requirement brings us to the second essential ingredient for spectrally tunable pulses. For a fixed pulse wavelength, detuning between the pulse wavelength and the grating stopband can only be achieved by leveraging mechanisms to shift the location of the grating stopband.
Serendipitously, USRN possesses a large thermo-optic coefficient, an order of magnitude larger than that in stoichiometric silicon nitride. Consequently, increasing the temperature from 25oC to 70oC allows the location of the grating stopband to increase by 5 nm. As a result, the detuning between the pulse wavelength and the grating stopband increases. This in turn results in a continuous decrease in the dispersion experienced by the pulse, a reduction in the temporal compression and therefore a smaller extent of spectral broadening in the USRN waveguide.
Thermo-optic tuning leveraging Bragg soliton dynamics allows a wide dynamic range of spectral widths to be continuously accessed without loss in pulse energy. The combination of a strong Kerr nonlinearity and large thermo-optic coefficient in ultra-silicon-rich nitride allows energy efficient, thermo-optic spectral tuning of laser pulses to be achieved at low powers, as shown in Fig.1.
Experiments to confirm this effect show that a variation on the 30 dB bandwidth from 69 nm to 106 nm is possible as the temperature of the device is increased from 25oC to 70oC.
Importantly, the technique is shown to be significantly more energy efficient than other spectral tuning methods. The wide dynamic range of spectral tuning, particularly with the high energy efficiency are important with considering deployment in practical devices.
From a fundamental standpoint, this work's significance further stems its application of a relatively new concept of on-chip Bragg solitons. While previously studied in fiber, Bragg solitons have only recently been demonstrated on a chip. Combining the augmented dispersion with the large nonlinearities available in high nonlinear figure of merit USRN waveguides, Bragg soliton effects can easily be observed at substantially lower powers than that in fibers. Indeed, much more remains to be studied, and applications discovered, in this exciting class of on-chip devices.
"This technique is especially well suited to integration with short pulse lasers, providing a practical avenue for an easily implemented, low loss technique for the tuning of pulse spectra," says Cao Yanmei, the PhD student and first author of the work.
"The tuning of pulse spectra in turn allows the temporal duration of pulses to be likewise tuned, a feature which is most useful for short pulse laser applications within the domains of imaging, precision manufacturing and high-speed communications," explains Prof. Dawn Tan who supervised this work.
The technique can be advanced further by increasing the dynamic range of the spectral tunability. In addition, mechanisms other than the thermo-optic effect which allow the stopband of the grating to be tuned could also be investigated, perhaps allowing new paradigms of ultrafast spectral tuning of pulses to be discovered.
脉冲频谱新方法，实现高效能可调谐光谱展宽
图1 在1536 nm的固定脉冲波长下模拟不同温度下的光谱展宽
光脉冲是成像系统、精密制造和高速通信中必不可少的组成部分。因此，其特性（包括光谱带宽）具备可操控性是一种重要的功能。但目前大多数光谱调谐机制都依赖于带通滤波，这会导致脉冲能量的损失。研究人员采用相关方法，如声光滤波器、光纤布拉格光栅和谐振滤波器等进行脉冲频谱调谐，但这些方法都需要截断脉冲频谱才能获得频谱更窄且时间更长的脉冲。因此，探索一种可对光脉冲进行光谱调谐而不损失脉冲能量的方法至关重要。
近日，新加坡科技设计大学Dawn T.H. Tan教授课题组在Photonics Research 2021年第4期(Yanmei Cao, Ezgi Sahin, Ju Won Choi, Peng Xing, George F. R. Chen, D. K. T. Ng, Benjamin J. Eggleton, Dawn T. H. Tan. Thermo-optically tunable spectral broadening in a nonlinear ultra-silicon-rich nitride Bragg grating[J]. Photonics Research, 2021, 9(4): 04000596) 发表的文章中提出了一种新的可调谐脉冲频谱的方法, 该方法避免了与频谱截断相关的脉冲能量损失。
该方法将克尔非线性和热光效应（存在于互补金属氧化物半导体兼容光子器件中）结合起来，实现了高能效的光谱调谐功能。这种光谱调谐器件基于具有高非线性折射率（比化学计量氮化硅大100倍）的超富硅氮化物（USRN）平台，它包括两个相互级联的阶段：一个包层调制的布拉格（Bragg）光栅和一个高非线性波导。
在第一阶段，脉冲在包层调制Bragg光栅中传输时会受到强烈的光栅诱导色散，该色散比波导中的色散大三个数量级，且随着与阻带之间失谐量的增大而减小。当脉冲波长位于光栅阻带的蓝色一侧（短波长侧）时，这些脉冲演化成Bragg孤子，经历高阶孤子时域压缩，压缩程度随光栅阻带和脉冲波长之间的失谐量而变化。在较强的时域压缩下产生的脉冲，通过自相位调制过程，在器件的第二阶段表现出更明显的光谱展宽。
因此，可通过调节脉冲波长和光栅阻带之间的失谐量来改变第一阶段的Bragg孤子效应的压缩程度，而这一需求带来了实现可调谐脉冲频谱的第二个重要因素。对于固定的脉冲波长，只有通过改变光栅阻带的位置, 才能调控脉冲波长与光栅阻带之间的失谐。
USRN具有较大的热光系数，比化学计量的氮化硅大一个数量级。因此，将温度从25℃增加到70℃可使光栅阻带的位置增加（红移）5 nm。因此，脉冲波长与光栅阻带之间的失谐量增大。但这将导致脉冲经历的色散持续降低，脉冲时域压缩程度减少，故在USRN波导中光谱展宽减小。
利用Bragg孤子动力学的热光调谐可以在不损失脉冲能量的情况下, 连续获得宽动态范围的光谱宽度。在USRN中，强克尔非线性和大热光系数使得激光脉冲在低功率下可以实现高效的热光光谱调谐，如图1所示。
实验表明，当器件温度从25℃增加至70℃时，频谱的30 dB带宽可以实现从69 nm到106 nm的变化。最重要的是，这项技术已被证明比其他光谱调谐方法更节能。频谱调谐的宽动态范围，特别是高能量效率对于技术的实际应用非常重要。这工作的重要性还在于它应用了一个相对较新的概念——芯片上Bragg孤子。这一概念以前在光纤中曾被研究，但直到最近Bragg孤子才在芯片上得到证实。
将高非线性品质因数USRN波导中的增强色散和高非线性特性相结合，可以很容易地在比光纤更低的功率下观察到Bragg孤子效应。事实上，在这类芯片器件中，还有许多现象及应用有待研究和发现。
“这项技术特别适合与短脉冲激光器集成，为调谐脉冲光谱提供了一种实用的、易于实现的、低损耗的技术。”这项工作的第一作者、博士研究生曹严梅如是说。
指导这项工作的Dawn T.H. Tan教授解释说：“脉冲光谱的调谐可以使脉冲的时间间隔也被调谐，该特性对于短脉冲激光在成像、精密制造和高速通信领域有着广泛的应用。”
通过增加频谱可调谐性的动态范围，可以进一步提高该项技术。此外，还可以通过研究热光效应以外的其他机制来调节光栅的带隙，为发现脉冲超快光谱调谐的新方法提供了可能性。