基于 Whispering Gallery 模式的可调超窄带光学滤波器

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"基于微 Whispering Gallery 模式的可调谐超窄带光学滤波器" 在光学领域，实现高精度的滤波对于多种应用至关重要，如光通信、光谱分析和激光技术等。本文报告了一种创新的光学滤波器设计，即基于 Whispering Gallery 模式（WGM）的混合微球结构。这种滤波器的特点在于其极高的品质因数（Quality Factor，Q 因数）大于108，以及小于1pm的传输光谱带宽，这显著优于传统的二氧化硅（SiO2）微球。此外，它还具有增强的边模抑制比，能够更有效地抑制非中心模式，提高信号纯度。 Whispering Gallery 模式是微球或微盘等微结构中的一种光学现象，当光在这些结构的表面以近乎全反射的方式传播时，会在内部形成一系列紧密的环形模式。这些模式具有非常高的 Q 因数，使得微球能够在特定频率上存储并保持光能，从而实现对光谱的精细控制。在本研究中，通过在 SiO2 微球上添加涂层，研究人员成功地优化了这种模式，进一步提高了滤波性能。实验中，研究人员详细测量了微球表面的粗糙度和 WGM 传输光谱，以确保其光学特性。他们利用这种混合微球结构实现了0.01nm带宽的单波长光纤激光输出，且该激光器的波长可调谐。这里，SiO2 微球被用作模式选择器，帮助选取特定的 WGM，从而实现精确的滤波效果。混合微球的光学场分布和不同涂层参数下的 WGM 传输光谱也进行了分析。通过调整涂层的厚度和材料属性，可以控制微球的光学响应，进而调整滤波器的工作波长。这一成果不仅为光学滤波技术提供了新的解决方案，也为未来的光学器件设计，特别是高精度激光源和光谱仪的发展，开辟了新的可能。这项工作展示了 Whispering Gallery 模式的混合微球在构建超窄带光学滤波器方面的潜力，其优越的性能指标和可调谐性将有望应用于对光谱分辨率要求极高的科学和工业应用中。未来的研究可能会进一步探索如何扩大这种滤波器的波长覆盖范围，以及如何将其集成到现有的光子学系统中，以满足不断增长的光学通信和量子信息处理需求。

Tunable, ultra-narrow-band optical filter based on a

whispering gallery mode hybrid-microsphere

Hongdan Wan (万洪丹)*, Hongye Li (李宏业), Haohan Zhu (朱浩瀚), Ji Xu (许吉),

Yunqing Lu (陆云清), and Jin Wang (王瑾)**

Nanjing University of Posts and Telecommunications, Nanjing 210023, China

*Corresponding author: hdwan@njupt.edu.cn; **corresponding author: jinwang@njupt.edu.cn

Received June 11, 2016; accepted September 29, 2016; posted online October 27, 2016

We demonstrate an ultra-narrow-band optical filter based on a hybrid-microsphere consisting of a coated SiO

microsphere. As compared to the SiO

microsphere, the hybrid-microsphere produces a quality factor of >10

a transmission spectrum bandwidth of <1 pm, and an increased side-mode suppression ratio. The microsphere

surface roughness and whispering gallery mode (WGM) transmission spectra are measured experimentally.

A 0.01 nm bandwidth, single-wavelength fiber laser output is achieved with a tunable wavelength, using the

SiO

microsphere as the mode selector. The optical field distribution and WGM transmission spectrum of

the hybrid-microsphere with different coating parameters are theoretically investigated by the finite-difference

time-domain method.

OCIS codes: 230.5750, 060.3510, 140.3945, 140.4780.

doi: 10.3788/COL201614.112302.

As applied in optical sensing and communication systems,

narrow-linewidth fiber lasers are of great interest for

development because of their high coherence, flexible

wavelength, and compact structure

[1,2]

. Fiber filters for

narrow-bandwidth laser mode selection are usually real-

ized with fiber gratings, saturable absorbers, Fabry–Pérot

(FP) cavities, etc

[3–5]

, where their bandwidth is usually

broader than 1 GHz. Various cavities including the FP

cavity, the whispering gallery mode (WGM) cavity, and

the polarization controller (PC) cavity are presented

[6–8]

especially since a novel scheme of the integrated FP cavity

using graphene has been recently reporte d

[9,10].

The major

advantages of WGM resonators compared with the FP are

a small size, a large wavelength range where WGM reso-

nators have high quality (Q) factors, and low sensitivity to

mechanical noise

[11]

. Recently, an ultra-narrow-band mode

selection method based on a high Q (Q ≈ 10

) WGM CaF

cavity microsphere is proposed

[12]

. A single-mode laser out-

put with a 650 Hz bandwidth is obtained at 1550 nm.

However, manufacturing such a CaF

microsphere out

of a crystal is time and labor consuming. Compared to

the CaF

microsphere, the SiO

microsphere can be easily

fabricated by utilizing the surface tension effect. However,

the Q factor of SiO

microsphere is prone to be redu ced

due to the ellipse asymmetry and the surface contamina-

tion

[13]

. Therefore, new methods are needed to improve the

cavity Q of SiO

microspheres.

This Letter proposes a new type of WGM hybrid-

microsphere (HM), which consists of a SiO

microsphere

coated with a high refractive index layer on the surface.

Based on the total light reflection, the WGM mode is con-

fined inside the cavity, where the scattering loss and radi-

ation loss can be reduced. Thus, the side-mode suppression

ratio is increased, the bandwidth of the WGM transmis-

sion spectrum is compressed, and a high Q factor is

achieved. The mode distribution in the HM is also

investigated based on the finite-difference time-domain

(FDTD) method. The WGM transmission spectra with

different coating parameters, namely the refractive index

and the layer thickness, are analyzed. Also, a single-

wavelength fiber ring laser based on the WGM micro-

sphere is demonstrated.

Figure

1 shows the theoretical model and the photo

of the tapered fiber-microsphere coupling system. The

tapered fiber is made from a single-mode SiO

fiber,

heated by a Hydroxide flame. Its diameter is controlled

to 3–5 μm. The arc discharging and CO

laser heating

methods are used to fabricate the microsphere, while

the latter has better circular symmetry and surface homo-

geneity, being preferred for achieving a higher Q factor.

As it has been validated by previous reports, the Q fac-

tor of a microsphere mainly depends on the surface rough-

ness

[14,15]

, while the scattering loss and absorption loss of

the SiO

microsphere can be ignored, because the diameter

D ¼ 2R is larger than 300 μm, and the absorption coeffi-

cient is small near the infrared wavelength

[14]

. Figure 2

shows the surface topography of the SiO

microsphere

Fig. 1. Tapered fiber-microsphere coupling system: (a) photo of

the coupling system; (b) microspheres fabricated by using the

laser heating method (right, diameter of 300 μm) and by

using the arc discharging method (left, diameter of 200 μm);

COL 14(11), 112302(2016) CHINESE OPTICS LETTERS November 10, 2016

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