With the rise of 5G technologies and massive deployment of 5G base stations, wireless access of terminals with high speed and large capacity is realized. Meanwhile, the traffic in optical fiber network increases rapidly. It is predicted that the current optical fiber network will become the bottleneck of information exchange in the future 12-18 months. The upgrading of optical fiber network is urgent. The representative trend is that the technologies for long-haul network (LHN) will be sunk to metropolitan area network (MAN), including DWDM (Dense Wavelength Division Multiplexing), ROADM (Reconfigurable Optical Add-Drop Multiplexer) and coherent receiving techniques. This paper discusses some of the passive optical devices for the coming 5G applications.
Tunable Optical Filter (TOF) for Coherent Receiving
In DWDM optical network, tunable optical filter (TOF), as one of the most important dynamic optical devices, is used to realize such functions as channel selection, optical performance monitoring (OPM) and optical channel monitoring (OCM) in the wavelength domain. The requirements of optical network for TOF include low loss, wide tuning range and good filtering characteristics.
Wavelength selectivity is a self-contained characteristics of coherent receivers. TOF is not necessary for a coherent receiving system. However, a TOF with a relatively wider linewidth (than that required for OPM and OCM) is usually employed before a coherent receiver. It can filter out the white noise and improve the optical signal-to-noise ratio (OSNR). Thus the ever high sensitivity of the coherent receiver is further increased.
As shown in Fig.1, a TOF before coherent receiving has a simple structure, including a dual-fiber collimator, a phase grating and a MEMS mirror. The wideband optical signals are angularly dispersed by the grating and the MEMS mirror selectively reflects the passband wavelength to the output. Theoretical analysis shows that the more is the collimated beam diameter, the narrower is the filtering linewidth. The coherent receiving system doesn’t need a narrow band TOF. However, cost down is the first consideration. Thus the simplified structure is selected.
Optical Performance Monitoring (OPM) Module
In the DWDM system, tens of wavelengths are transmitted in an optical fiber. The wavelengths are switched at the ROADM nodes in wavelength granularity. Thus the optical fiber links become complicated. The power levels of the wavelengths in a fiber link become rather unbalanced, as shown in Fig.2. In order to ensure the reliable operation of the optical network, it is essential to monitor the optical signal-to-noise ratio (OSNR) in each optical fiber link. Optical performance monitor (OPM) is a functional module that is used to monitor the OSNR in the optical fiber links. OPM is widely employed in the ROADM-based optical network to keep its reliable operation.
At present, the mostly employed OPM module is constructed based on a TOF with narrow passband. The main approach for the narrow-band TOF is designed based on free-space optics with a MEMS mirror for wavelength selection, as shown in Fig.3. The grating dispersed the wideband optical signals and the MEMS mirror selectively reflects the passband wavelength to the output. According to theoretical analysis, in order to obtain a narrow passband, the focal length of the collimating lens in the dual-fiber collimator should be rather long to ensure that the collimated beam diameter is large enough. However, the diameter of the MEMS mirror is limited (usually <Φ1mm). Thus a compression component is required to compress the beam size before its incidence on the MEMS mirror. The compression component is comprised of two lenses with different focal lengths and in confocal alignment.
The TOF for OPM application is required to have an ultra-narrow passband for analysis of the OSNR in a fiber link. Theoretical analysis shows that the passband width of the TOF depends on the beam size incident on the grating. The relation between the FWHM (3dB-bandwidth) and beam size is simulated as Fig.4 . Beam waist 2ω>2.8mm is required to obtain 3dB-bandwidth<0.2nm. Such a large beam spot should be compressed before its incidence on the MEMS mirror.
Optical Channel Monitoring (OCM) Module
The management of the complex fiber links in a ROADM-based optical network requires another functional module OCM, which can monitor the power level of each channel and provide feedback signal for channel equalization. The TOF for OCM application is used to filter out the DWDM channels one by one. Considering the wavelength jittering of DWDM signals, the TOF is required to have a narrow passband with a flat top.
At present, the mostly used TOF solution for OCM is based on free-space optics, as shown in Fig.10 . The key component is a thin-film filter (TFF, labeled 508 in Fig.10) driven by a rotatory motor. The TFF is designed to have a narrow passband with a flat top. The rotation changes the incident angle on the TFF and thus the central wavelength of the passband is tuned. The spectral transmission of the TFF is polarization dependent due to the oblique incidence. Thus polarization diversity is introduced through two beam displacers (BDs, labeled 504 and 514 in Fig.10) and two half-waveplates (HWPs, labeled 506 and 512 in Fig.10). The input optical beam is split as two parallel beams and both pass the TFF as s-wave.
Actually, the structure in Fig.5 is a tunable add/drop multiplexer. The adding/dropping of single channel is via the add/drop port in the right. The other channels are reflected by a rotatory mirror 510 (driven by the same motor as TFF 508) and a static mirror 516. Then the reflected channels emit from the output via an optical circulator 502.
When operating as a TOF for OCM application, some of the elements in Fig.10 can be omitted, including the optical circulator 502, the rotatory mirror 510 and the static mirror 516.
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Bingcheng Mo, Rui Zhong, and Zhujun Wan, Line Width Analysis of a Tunable Optical Filter Based on Free-Space Optics, Optik, 125(21): 6488-6490, 2014
Wen Liu, Kan Yu, and Jin Chang, et. al., Thin film tunable optical filter, United States Patent 20080043311.A1, Accelink Technologies Co., Ltd., 2008
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Written by Zhujun Wan,Jianwei Feng HYC Co., Ltd