As we know, optical fiber communication is one of the enabling technologies for Internet and changed the world. The advantage of optical fiber communication is to transmit dozens of wavelengths in a single optical fiber, which is called wavelength-division multiplexing (WDM). The basic devices for WDM transmission are optical filters, which can be realized by fused biconical taper (FBT), thin film filter (TFF), arrayed waveguide grating (AWG) and optical interleaver. TFF and AWG are most commonly used in the WDM system. This paper discusses TFF-based WDM devices.
Thin Film Filter
Fabry-Perot interferometer (FPI) is a commonly used interferometer for spectrum filtering. The structure of a FPI is shown in Fig.1, which consists of two glass plates spaced by a spacer with precise thickness. The inner surfaces of the plates are coated for partial reflection and the outer surfaces are usually anti-reflection (AR) coated.
The FPI can be implemented with dielectric thin films in Fig.2 instead of the bulk optics in Fig.3. The multi-layer films are deposited on a glass substrate. The periodic structure of dielectrics with high/low refractive index (RI) functions as a mirror with partial reflection. The cavity separates the two mirrors.
As the traditional FPI implemented by bulk optics, the Thin-film FPI can also function as spectrum filter. As shown in Fig.3, the transmission peaks are periodic. The finesse of the spectrum increases with the increment of mirror reflectivity. There is only one transmission peak within the free spectrum range (FSR), as shown in Fig.4. When the mirror reflectivity is high, the spectrum linewidth is very narrow, which enables its application in narrow-band filtering.
However, in some special applications such as DWDM transmission, the filters are required to have a flat-top and narrow passband. Such filters require multi-cavity thin films, as shown in Fig.5. The effect of multi-cavity is shown in Fig.6. A filter with more cavities has a wider flat-top and a sharper edge, which is beneficial for DWDM transmission. However, more cavities are accompanied by more ‘mirrors’, which means that the count of thin film layers is multiplied. All the thin films need to be deposited on the glass substrate with perfect uniformity and precise thickness, which decreases the yield rate and adds to the cost.
The design of thin-film filters is very flexible. Besides the narrow band filter with flat-top, many other filters can be implemented by thin films, such as long-pass (LP) filter in Fig.7, gain flattening filter (GFF) in Fig.8. The LP filter can be used for WDM bi-directional transmission, such as transmitting by 1310nm and receiving by 1550nm. The GFF filter is used to flatten the gain spectrum of an erbium doped fiber amplifier (EDFA).
The TFF filters are employed in WDM devices. Fig.9 shows the structure of a three-port WDM device, which consists of a dual-fiber collimator, a single fiber collimator and a TFF filter. The TFF filter is attached on the end-face of the collimating lens of the dual-fiber collimator. WDM signals composing wavelength λ1, λ2,…λn are launched from the common port. The TFF filter transmits only one wavelength λn and reflects the other wavelength. Thus λn exits from the transmission port and the others exit from the reflection port.
In order to de-multiplex all the wavelengths, n three-port devices are cascaded to construct a WDM module, as shown in Fig.10. The TFF filters in each device transmit different wavelengths. The WDM module can function as a de-multiplexer or multiplexer, depending on the signal direction.
The WDM module based on cascaded three-port WDM devices has a relatively large size (typical size for an 8-channel WDM module is 130×90×13mm3), which does not meet the requirement by some special applications. Compact WDM module is developed for such applications, as shown in Fig.11. The TFF filters are fixed on a glass bench and the input/output fiber collimators are aligned one by one. The size of the compact module is typically 50×30×6mm3, which is much smaller. The compact DWDM and CWDM modules are usually called CDWDM and CCWDM, respectively.
As we can see in Fig.10, different wavelengths travel a different number of devices in the module and result in different power loss. The loss uniformity degrades with an increment of the port number. Meanwhile, the maximum loss at the last port is another limitation on the port number. We can see the same problems in the compact WDM module as shown in Fig.11. Thus the TFF-based WDM modules are usually limited to be ≤16 channels.
HYC can provides customized WDM products with a variety of package sizes and wavelength options. For more information about WDM products, please refer to HYC Website: http://www.hyc-system.com/Product/index_208
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 Zhang Jun-Chao, et. al., Design and fabrication of broadband rugate filter, Chinese Physics B, 21(5): 054219, 2012
 Pierre G. Verly, Design of a robust thin-film interference filter for erbium-doped fiber amplifier gain equalization, Applied Optics, 41(16): 3092-3096, 2002
Written by Zhujun Wan, HYC Co., Ltd