Optical Communication Tutorial Notes (optical devices)

IEG4030-Tutorial 7 Optical Components (1)

1. Outline

Generally, optical components can be categorized into two types: passive optical components and active optical components. Firstly let’s take a look at passive components.

Most commonly used passive components:

Lens; fiber coupler (splitter, combiner); WDM multiplexer/demultiplexer; optical filter; polarization controller; optical isolator; fiber connector; optical circulator; etc.

2. Lens

A common GRIN lens (stands for Graded-Index lens) has a structure showing in the following picture.

The refractive index of GRIN lens is radially-varying, as shown in the x-y coordinate above, which means the function of conventional lens can be realized by a slab-shaped lens. It is commonly used in multiplexer/demultiplexer.

3. Optical coupler

Here is a detailed look on an optical coupler.

Factors that affect the optical coupling include the axial length of the coupling region, size of the reduced radius and the difference in the radii of the two fibers.

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Suppose the signals of two inputs are at about the same wavelength. We introduce two parameters to calculate the power in each output port. coupling ratio (splitting ratio) α

Coupling ratio describes the ratio between two output port. excess loss factor β

=

Then P =P ∙β∙(1−α)+P ∙β∙α P =P ∙β∙α+P ∙β∙(1−α) For a 2x1 coupler, if the coupling ratio is 50/50, P =P ∙β=0.5∙P ∙β , where P =P +P When P =P =P , we can get P =P ∙β

Excess loss = -10log10(β)

For this reason, we can see that the output always has 3-dB loss, unlike linear combination of input power in lots of cases. The lost power goes to other output port which does not appear in a 2x1 coupler.

However, in wavelength selective couplers (WSC) or splitters, the case turns out to be different.

For a combiner:

Characteristics of fiber coupler: see lecture notes pp.13

for a splitter:

4. WDM Multiplexer/ Demultiplexer

Definition: Devices which are used to combine or split two light beams of different wavelengths. Methods of construction: 1) Microoptics

2) Hybrid microoptics/ Integrate optics

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3) Fused fiber coupler(as shown in section 3) 4) Arrayed-waveguide grating (AWG)

5. Optical Filter

Basic characteristics of Fabry-Perot filters include: - Free-spectral range: FSR=c/(2nL)

- Passband: fFP = the FWHM of the transmission function. - Finesse: F=FSR/f =

π√

π )

- Minimum transmission: Tmin =(

Center frequency of FP can be tuned by changing the cavity length. ->piezoelectric material with variable voltage

FSR Passband

6. Polarization controller

Working principle: polarization state can be controlled or varied by applying stress to the fiber, which can introduce fiber birefringence.

7. Optical isolator

8. Fiber connectors

Example:

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(F-P filter) A Fabry-Perot filter is used to select one over 50 channels with 0.3-nm channel spacing. Assume that the refractive index of the filter is 1.5 and the operation wavelength is 1550nm,decide the cavity length and the reflectivity of the mirror in the filter. Solution:

For 50 channels with 0.3-nm channel spacing, we can get:

FSR=0.3x50=15nm=c/(2nL) => L=c/(2n x FSR) =53.3 um

Generally, we consider B≤fFP≤(FSR/3N) to guarantee sufficient suppression of the crosstalk coming from adjacent channels, where B is the signal bandwidth and N the number of channels.

So we could get fFP, then finesse F =FSR/ fFP = 3N = 150, finally we can get R.

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9. Optical Circulator

An optical circulator is a directional device. It is commonly used to control the propagating direction of lights. The working principle of optical circulator has some similarities compared to optical isolator. Key component: Faraday Rotator.

Port 1-> Port 2; Port 2->Port 3. Port 3 cannot direct light to port 1.

10. Optical OADM

Optical OADM is a device performing adding and dropping certain wavelength(s). Before we discuss the structure of OADM, let us take a look at the key component of OADM: Fiber bragg grating (FBG).

An FBG is a kind of grating which reflects light of certain wavelength backward while keeping other wavelength components propagating forward in the same direction.

The wavelength of reflected light is decided by the effective refractive index (the average refractive index in the fiber) and the grating period.

A typical optical OADM can be constructed by optical circulator and FBG. A detailed look:

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It can also be made by optical coupler and FBG if insertion loss is not critical.

Example:

Consider the following optical subsystem, which consist of two 4-port optical circulators and five FBGs. The working principle of the 4-port optical circulator is illustrated in the inset.

The reflection wavelength of each FBG is labeled next to the respective symbol in the figure. Assume we have a set of eight wavelength channels,

, spaced by 100-GHz.

a) If we inject this set of wavelength channels into Port A, what will be the outputs at port B,

C and D?

b) If they are injected to Port D, what will be the outputs at port A, B, C?

c) We have another set of eight wavelength channels, . What

will be the output at Port C and Port D if we inject the previous channels into Port A and the present ones into Port B, respectively, at the same time?

~END~

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