A variable mechanical optical attenuator

A variable mechanical optical attenuator

Omar Shehab

Department of Computer Science and Electrical EngineeringUniversity of Maryland, Baltimore County

Baltimore, Maryland 21250

[email protected]

April 23, 2012

Optical attenuators

Important component of an optical communication system.

Typically used to preserve optical power for further tuning.

Can be mechanical, optical, photonic, hybrid, electrical,MEMS, polymeric etc.

Electronic and MEMS attenuators are very popular.Mechanical attenuators are extensively used in low priceapplications and for training purpose.

Omar Shehab (UMBC) A variable mechanical optical attenuator April 23, 2012 2 / 22

Mechanical optical attenuator

Light is typically absorbed by semi-reflecting die-electricsubstance.

May have distributed variable refractive index.

The substance may also work as reflector or absorber.

Mirrors or shutters serve this purpose.


Electronic optical attenuator

Requires additional input power.


Related works

Mechanical attenuators

Marxer, Griss, and de Rooij [1999].Dai, Zhao, Cai, and Li [2002].Yamashita, Kawada, , and Takeuchi [1985].

MEMS

Sun, Noell, Zickar, Mughal, Perez, Riza, and de Rooij [2006].

Photonic crystal

Stevenson, Martelli, Canning, Ashton, and Lyytikainen [2005].Mathews, Farrell, and Semenova [2011].Kerbage, Ging, Steinvurzel, Hale, Yablon, Windeler, andEggleton [2002].Wang and Heab [2006].Ian, Steven, Xiaole, and Jun [2009].

FPGA

Li, Jin, Zhang, and Zou [2006].


Proposed schematic


A new design

A new design for a mechanically operated variable opticalattenuator:

Uses the idea of a splitter-combiner (reverse coupler) set up.

The design contains two wheels (one outer and the other isinner), GrIn lenses, a fiber splitter and a combiner.

Reduce the incident optical power level at two phases.

Can be implemented at low cost and may be fabricated atsmall scale.

Traditionally used refraction principal is not used in the firstphase of attenuation.

The internal spaces are filled with properly chosen indexmatching fluid.


How does it work?


Step 1

Light enters into the input channel of the fiber beam splitter. Thedesign proposes industry standard fiber coupling with the input forleast possible insertion loss.


Step 2

The outer windowed rotary wheel is moved one step anticlockwise.So the potential light path from the top most channel of thesplitter to the top most channel of the combiner is blocked. Lightincident along this path will be reflected and eventually absorbedinside the system.


Step 3

Light splits among and propagate through the splitter channels.The GrIn lenses at the windows of the outer wheel guide the lightto pass into the input channels of the combiner. The inputsurfaces of the combiner channels should be cut with appropriatetilt so that back propagation doesn’t occur.


Step 4

Light is combined at the output channel of the combiner and itthen pass through the transparent part of the inner wheel. Theinner wheel is rotated clockwise or anticlockwise to achieveappropriate second level attenuation according to the scale drawnon it. There is a little amount of insertion loss at this stage. As thetransparent wheel is concave the reflected light doesn’t interferewith the incoming signal which is along a straight optical path.


Step 5

A particular amount of light is reflected from the wheel and areduced amount of light comes out of the attenuator. This is theexpected output light.


Interesting features

The only part of the device which should meet the presentindustry standard is the connector.

Doesn’t require any additional power input or regular supplyof optical fluid.

Not wavelength dependent.


Characterization I

If c is the number of closed splitter channels and t is the totalnumber of splitter channels, the attenuation due to themovement of the rotary wheel, Astepwise , is:

AstepwisedB = 10Log10c

t

There will be an insertion loss, Alenses , when the light iscoupled onto the combiner channels through the lenses. Thisloss is a function of the number of open splitter channels, (t -c).


Characterization II

The amount of attenuation at the final stage is determined bythe transmission coefficient, T , of the transparent part of theinner wheel with graded refractive index.

T =4n1n2

2(n1 + n2)

Here n1 and n2 are the refractive indices of two differentmediums.

So, the continuous loss due to the second inner wheel,Acontinuous , is:

AcontinuousdB = 10Log10T = 10Log102n1n2

n1 + n2


Characterization III

So, the total attenuation, Atotal , will be:

AtotaldB = Astepwise + Alenses + Acontinuous

This will also be the dynamic attenuation. So, the dynamicrange of the proposed design, Adynamic , is:

AdynamicdB = Astepwise + Alenses + Acontinuous


Limitations

It needs to be operated manually.


Future plan

Implement the whole design using 2D photonic crystals.

Using MEMS to automate it.


Acknowledgments

O. S. likes to thank Professor Muhammed Zafar Iqbal, Dr.Muztaba Fuad and Professor Samuel J. Lomonaco Jr. for theirinsights and encouragement.


Bibliography I

Xuhan Dai, Xiaolin Zhao, Bingchu Cai, and Wenjun Li. Characterization and development of micromachinedvariable optical attenuator. In Optical Communication, 2002. ECOC 2002. 28th European Conference on,pages 1–2, 2002.

Lapsley Michael Ian, Lin Sz-Chin Steven, Mao Xiaole, and Huang Tony Jun. An in-plane, variable opticalattenuator using a fluid-based tunable reflective interface. Applied Physics Letters, 95:083507–083507–3, 2009.

C. Kerbage, J. Ging, P. Steinvurzel, A. Hale, A. Yablon, R.S. Windeler, and B.J. Eggleton. Air-silica microstructurefiber based variable optical attenuator device. In Optical Fiber Communication Conference and Exhibit, 2002.OFC 2002, pages 468–469, 2002.

Sailu Li, Xiaofeng Jin, Xianmin Zhang, and Yingyin Kevin Zou. Digitally controlled programmable high-speedvariable optical attenuator. Microwave and Optical Technology Letters, 48:10191021, 2006.

Cornel Marxer, Patrick Griss, and Nicolaas F. de Rooij. A variable optical attenuator based on siliconmicromechanics. Photonics Technology Letters, IEEE, 11:233–235, 1999.

Sunish Mathews, Gerald Farrell, and Yuliya Semenova. Experimental demonstration of an all-fiber variable opticalattenuator based on liquid crystal infiltrated photonic crystal fiber. Microwave and Optical Technology Letters,53:539543, 2011.

M. Stevenson, C. Martelli, J. Canning, B. Ashton, and K. Lyytikainen. Photonic crystal fibre optical attenuators.Electronics Letters, 41:1167–1169, 2005.

Winston Sun, Wilfried Noell, Michael Zickar, M. Junaid Mughal, Frank Perez, Nabeel A. Riza, and Nicolaas F.de Rooij. Design, simulation, fabrication, and characterization of a digital variable optical attenuator.Microelectromechanical Systems, Journal of, 15:1190–1200, 2006.

Qian Wang and Sailing Heab. Analysis and design of variable optical attenuators based on nematic liquid-crystalcells. Journal of Modern Optics, 53:481–493, 2006.

Mikio Yamashita, Yasushi Kawada, , and Satoshi Takeuchi. Experimental demonstration of an all-fiber variableoptical attenuator based on liquid crystal infiltrated photonic crystal fiber. Review of Scientific Instruments, 56:478479, 1985.


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A variable mechanical optical attenuator

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