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Optical devices made of Si nanowire waveguides

Technology

This page describes:

1) directional coupler

2) polarizer and polarization beam splitter

3) ring resonator

4) Mach–Zehnder interferometer

 

 


Merits of Si nanowire waveguides:

The integration of different optical components on one substrate has many benefits. Similar to electronic devices, an integrated optical circuit may have a lower cost and better functionality. The size of MOSFET transistors are very small and millions of the transistors can be integrated into one electrical circuit. In contrast, the size of optical components is not as small. The typical length of optical components is about a millimeter and only a few optical components can be integrated into one cheap. The length of optical components is limited by the wavelength of light. The size of optical elements can be reduced when Si nanowire waveguides or/and plasmonic waveguides are used.

The optical confinement in a Si nanowire waveguide is strong, because of a high refractive index contrast. Due to strong optical confinement, the Si nanowire waveguides are very narrow with width of 450 nm and they can sharply bend with bending radius as small as 1 mm. Even though the length of Si-nanowire devices has to be still relatively long, by bending the optical device can be packed into small area of a few um2.

The small size is main of optical devices made of Si nanowire waveguides. A compact design is always important for this technology


 

directional coupler

It is important component of many devices made of Si-nanowire waveguide. Depending on device, a different coupling ratio is required:

1) polarizer and polarization beam splitter

100 % for TM mode, small for TE mode

2) Mach–Zehnder interferometer

50 %

3) ring resonator

small


 

The numerical modeling is here.

Note: The numerical modeling is rather well matched to the experiment.


j01 for JEOL EB is here

 


polarizer and polarization beam splitter

Polarizer and Polarization beam splitter

click on image to enlarge it
top view, optical image transmission: input 1 -> output 1
very small size. The size could be as small as ~ 7 um x 3 um or even smaller. high polarization ratio ~ 25-16 dB in 100 nm wavelength range

The polarizer and the polarization beam splitter are rather easy to fabricate with the technology of Si nanowire waveguides. It is one merit of this technology.

A compact polarizer with a high polarization ratio and a broad wavelength-operation range can be fabricated with minimum efforts and cost.

It is used the property of the directional coupler that the distance for 100% coupling is significantly shorter for TM mode that for TE mode.

See here the operation principle of the polarizer

Common parameters:

gap: 400 nm

interaction length: 8 um

-bending radius: 10 um

-distance between outputs 1 and 2 (outputs 1 and 2) 20 um

 

Trade-off:

The larger the gap between coupling waveguides, the larger the polarization ratio, but the longer the interaction length of coupler and size of polarizer

See Fig 5 here

 

If a high polarization ratio is required, it is better to make several polarizers connected in serial rather than one polarizer with a longer length

j01 files of polarizers for JEOL EB is here

polarizer.zip

 


ring resonator

Ring resonator

click on image to enlarge it
top view, optical image transmission from input to output oval shape with optimized directional coupler  
rings of different diameters, high polarization ratio ~ 25 dB plasmonic waveguide is integrated into the ring  

 

 

The width of absorption peak is very small for a ring resonator.

Measured FWHM: ~0.02 nm

A very low coupling between strait waveguide and

 

A ring in the vicinity of a straight waveguide is working as a ring resonator. However, for a better performance the interaction length of resonator and waveguide should be optimized. In this case an oval shape of the ring resonator is better.

 

j01 files of ring resonators for JEOL EB is here

rings.zip

It is important!!! The coupling efficiency between a ring and a straight waveguide should be small

Otherwise, after the ring resonator a single pulse transforms into a train of several pulse.

 

Ring resonator. Dependence on the ring coupling efficiency.

click on image to see video
The yellow line shows the output pulses. The red line shows the reflected pulses as the coupling efficiency between the ring and the straight waveguide varies from 0 % to 100%

Explanation of the video

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

Mach–Zehnder (MZ) interferometer

 

Mach–Zehnder interferometer

click on image to enlarge it
symmetrical asymmetrical
 
top view of symmetrical MZ interferometer top view of asymmetrical MZ interferometer
 
transmission from input 1 to output 1 and 2 for asymmetrical MZ interferometer transmission from input 1 to output 1 and 2 for asymmetrical MZ interferometer. The extension ratio is 20-25 dB

 

 

 

 

 

 

 

j01 for MZ interferometers JEOL EB is here

MZ.zip

 

Light is coupled into input 1 and it is divided into two paths by directional coupler 1. A half of light passes through the arm 1 and another half passes arm 2. Next, both light beams combines at the directional coupler 2.

In the case when both beams are in phase, they both are coupled into output 1. As result, all light passes to output 1, there is no light at output 2

 

In the case when both beams are in phase, they both are coupled into output 1. As result, all light passes to output 1, there is no light at output 2.

In the case when both beams are in antiphase, they both are coupled into output 2. As result, all light passes to output 2, there is no light at output 1.

The output of MZ interferometer is very to phase difference for light between arm 1 and arm 2

 

Both the directional coupler 1 and 2 should have 50% x 50% coupling efficiency. For a larger extension ratio of the interferometer the coupling efficiency should be as closer to 50% x 50% as possible.

 

Asymmetrical Mach–Zehnder interferometer

In this case the lengths of arm 1 and arm 2 are substantially different. The phase difference for light in arms 1 and 2 is very sensitivity to wavelength of light (See left figure). Therefore, a slight change of wavelength of light switches light between output 1 and 2

From wavelength difference between the effective refractive index of waveguide can be easily evaluated using asymmetrical MZ interferometer.

When environment changes (temperature and so on) , the positions of peaks changes as well. Therefore, asymmetrical MZ interferometer can be used as a sensor. (because of the complexity of measurement setup, I do not think it is practical sensor)

 

Application of asymmetrical MZ interferometer:

-wavelength filters

- measurements of effective refractive index of waveguides

-sensors

 

Symmetrical Mach–Zehnder interferometer

In this case the lengths of arm 1 and arm 2 are the same. A very small change of refractive index in one of arms causes switching of light between outputs 1 and 2.

 

Since the electro-optical and magneto-optical effects are weak effects, even a strong applied electric or magnetic field causes only a very slight change of refractive index of a material, which is often hard to detect.

However, if an electric or magnetic field is applied to one arm of the symmetrical MZ interferometer this slight change of refractive index changes substantially the output of the interferometer.

Because of this property, the symmetrical MZ interferometer is used a light modulator, switcher and isolator.

 

Application of symmetrical MZ interferometer:

-electro-optical modulator

-optical isolator

 

 

 

 

 

 

 


Snake-shape waveguides

Snake-shape waveguides

click on image to enlarge it

 

They used to measure the propagation loss and bending loss in Si nanowire waveguides

 

j01 of snake-form waveguides for JEOL EB is here

snake.zip

 

 

 

 

 

 

 

 


 

O switch

O switch. Dependence on the coupling efficiency.

click on image to see video

The yellow line shows the output pulses. The red line shows the reflected pulses as the coupling efficiency varies from 0 % to 100%

Note: a low-frequency detector is used to detect the reflected pulses.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I truly appreciate your comments, feedbacks and questions

I will try to answer your questions as soon as possible

 

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