MEMS technologies are widely employed in optical fiber communication system. The combination of MEMS and optical technologies is usually named MOEMS (Micro-Opto-Electro-Mechanical Systems). The most widely applied MOMES devices include VOA (Variable Optical Attenuator), OS (Optical Switch), TOF (Tunable Optical Filter), DGE (Dynamic Gain Equalizer), WSS (Wavelength Selective Switch) and OXC (Optical Cross Connect).
VOA is widely employed in optical fiber communication system for optical power equalization. Among the variable approaches, MEMS VOAs are characterized by small size, low cost and easy fabrication. There are mainly two types of MEMS VOAs in applications, MEMS shutter and MEMS mirror. The first is usually thermally actuated and the second is usually actuated by electrostatic force.
MEMS Shutter VOA
The structure of VOA based on a MEMS shutter is shown in Fig.1 . The MEMS shutter is inserted between two optical fibers. The attenuation depends on the shuttered beam section. In real production, the VOA can be designed as reflection type.
MEMS Mirror VOA
Fig.2 shows the structure of VOA based on a MEMS torsion mirror. The pigtail fibers of a dual-fiber collimator are employed as the input/output ports. The collimated beam is reflected by the MEMS mirror and thus the input/output ports are connected. Torsion of the mirror deflects the beam and results in attenuation.
There are two types of MEMS torsion mirror, i.e. parallel-plate and combdrive, as shown in Fig.3 . The first type of MEMS VOA usually needs driving voltage of more than 10V for attenuation range of 0~20dB. The combdrive helps to reduce the driving voltage to <5V. However, a micro particle can jam the combdrive and thus the yield rate is lower. The combdrive MEMS mirror should be assembled in an ultra-clean environment.
WDL of MEMS Mirror VOA
Both the VOAs based on MEMS shutter and MEMS mirror are widely used. The former is characterized by good performance but relatively complicated assembly. The latter is characterized by easy fabrication but relatively higher WDL (Wavelength Dependent Loss). In broadband applications, the VOA generates different attenuation for different wavelength, which is defined as WDL. Broadband applications require to minimize the WDL.
The WDL results from dispersion of the optical mode field in a single mode fiber (SMF). As we know, different wavelengths have different mode field diameters. The longer wavelength has a larger mode field diameter. Fig.4 shows the dispersion of the optical mode field.
As shown in Fig.4, the optical beam is deflected by the MEMS mirror and thus the spots (corresponding to different wavelengths) deviate from the output fiber core. For the VOA before optimization, all the spots have the same deviation. The attenuation A depends on the deviation X and mode field radius ω, as shown in Eq. (1).
In a relatively limited spectrum range such as C-band (1.53~1.57μm), the dispersion of the mode field in a SMF can be linearly approximated as Eq. (2) .
For the commonly used SMF-28 by Corning Inc., the coefficients are a=5.2μm and b=3.11 @λc=1.55μm. Given a required attenuation Ac for central wavelength λc, the deviation of the spots Xc is obtained as Eq. (3).
The combination of Eq. (1-3) reaches the WDL over wavelength range λs~λl as Eq. (4), where the subscripts s, c, l indicate short, central, long wavelength, respectively.
According to Eq. (4), the higher is the required attenuation Ac, the larger is the spot deviation Xc. Thus a higher attenuation results in a larger WDL, as shown in Fig.5-6. The maximum WDL reaches 0.96dB over attenuation range of 0~20dB and wavelength range of 1.53~1.57μm, according to Fig.6. The measured maximum WDL of a commercialized MEMS VOA may be up to 1.5dB. This is because of dispersion of the optical system, which results in different deviations for the spots of different wavelengths. The circumstance is different from what is shown in Fig.4, where all the spots have the same deviation.
WDL Optimization of MEMS Mirror VOA
The WDL results from two factors, the mode field dispersion and the dispersion of the optical system. The two factors accumulate and result in maximum WDL of nearly 1.5dB. Can the two factors counteract and help to reduce WDL? The answer is yes, while elaborate analysis and design are required.
According to Eq. (1), the longer wavelength has larger mode field diameter and thus has smaller attenuation. As shown in Fig.7, if the optical system can generate more spot deviation for the longer wavelength, it will add to the attenuation of longer wavelength and thus equalize the spectral attenuation.
However, WDL resulting from the two factors can totally counteract only for a specific attenuation level Ac according to Eq. (4). When the attenuation is set to an attenuation different from Ac, residual WDL remains, as shown in Fig.8 .
As we can see in Fig.8, the maximum WDL happens at attenuation of 20dB before optimization. When the attenuation Ac for total counteraction is set at 20dB, the maximum WDL happens at attenuation of about 4dB. When the attenuation Ac for total counteraction is set at 13dB, the maximum WDL is minimized to <0.2dB.
There are different approaches to generate an opposite optical system dispersion. In Fig.9, a prism is inserted between the collimating lens and the MEMS mirror and thus dispersion of the optical system is adjusted to counteract the dispersion of mode field . However, the additional prism adds to the cost and complexity of the device. Fig.10 shows another solution. The collimating lens is fabricated with highly dispersive glass and the angle of the front endface is added to be >10° (it is usually 8° for the traditional devices) .
Based on thorough analysis on the dispersion of the optical system, Zhujun Wan et.al. from Huazhong University of Science and Technology provided a third solution. The material of the collimating lens is the commonly used N-SF11 glass. The curvature radius of the lens is R=1.419. A curve relating the other parameters of the collimating lens for WDL optimization is obtained as Fig.11. Any point on the curve gives the endface angle φ and length L of the collimating lens. When the collimating lens is fabricated with these parameters, the WDL of the VOA can be optimized. Note that the endface angle φ is always negative in Fig.11. Thus the dual-fiber pigtail and the collimating lens need to be aligned as Fig.12(d), while not as Fig.12(c) for the traditional MEMS VOA. The final assembly of their MEMS mirror VOA is shown in Fig.13. The maximum WDL was reported as <0.4dB over attenuation range of 0~20dB and wavelength range of 1.53~1.57μm .
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Written by Zhujun Wan, HYC Co., Ltd
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