Optical Attenuator: A Key Technology for Optical Signal Control

According to the working principle:
     Displacement optical attenuator: lateral displacement coupler, longitudinal displacement coupler

     Directly coated optical attenuator (absorption film, reflection film)
     Attenuation sheet attenuator
     Liquid crystal optical attenuator

1.Lateral displacement optical attenuator 

Features:
     Only used for fixed attenuators, large return loss, polarization-independent bulk devices

a.Lateral displacement type

The mode field distribution is:

The mode field distribution of the second fiber end face is:

The lateral coupling efficiency can be expressed by the overlapping field integral, with the spacing being negligible:

The loss after lateral misalignment is then

For single-mode graded-index optical fiber,

b.Single-mode and multimode fiber models

     According to the above principle, the lateral displacement parameters of different losses can be achieved by controlling the lateral displacement method, and the required optical attenuator can be obtained by a certain mechanical method.
     The range of displacement determines that it is not a variable optical attenuator, but only a fixed optical attenuator, and is welded or bonded.
     The return loss is very high, greater than 60dB.

2.Longitudinal displacement optical attenuator

Features
Used for fixed attenuators and some small adjustable optical attenuators
Polarization-independent bulk devices

b.Longitudinal displacement type

In the case of single-mode fiber:

 

Where B0 is the correction factor,

S is the distance between the two fiber end faces 

 S-Ls curve

c. S-Ls curve diagram

d. FC type and small type attenuator structure diagram

3.Direct Coating Optical Attenuator

 

e.Direct Coating Optical Attenuator structure diagram

      This is an attenuator that directly coats a metal absorption film or reflection film on the end face of the optical fiber or the glass substrate to attenuate the light energy. Commonly used evaporated metal films include: Al film, Ti film, Cr film, W film, etc. If Al film is used, a layer of SiO2 or MgF2 film is often coated on it as a protective film. 

4.Light attenuation sheet type

     The attenuation plate type optical attenuator directly fixes the attenuation plate with absorption characteristics on the end face of the optical fiber or in the optical path to achieve the purpose of attenuating the optical signal. This method can be used not only to make fixed optical attenuators, but also to make variable optical attenuators. The specific production method is to fix the attenuation plate directly in the collimated optical path through a mechanical device. When the optical signal is collimated by a quarter-pitch self-focusing lens and passes through the attenuation plate, the light energy is attenuated and then focused and raked into the optical fiber by the second self-focusing lens. By using attenuation plates with different attenuation amounts, an optical attenuator with the corresponding attenuation value can be obtained.

      Features: Does not have continuously variable attenuation characteristics

f.Light attenuation sheet type  structure diagram

g. Spectral characteristic index diagram

     Commonly used materials for attenuation include: infrared colored optical glass, crystal, optical film, filter and other inorganic and organic materials. However, because the optical attenuator has many requirements such as optical stability, chemical stability, volume, cost, etc., the materials commonly selected as attenuation elements are colored glass and filter. 

     If the absorption thin film filter method is used to make an optical attenuator, a neutral density filter is often used as an attenuator. The spectral range of this filter is widened, and theoretically, the attenuation intensity of the optical signal of each wavelength is almost the same, so a wide bandwidth attenuator can be obtained.

h. Wavelength-Transmittance diagram

Dual-wheel variable optical attenuator (step-by-step):

i: Dual-wheel variable optical attenuator (step-by-step) diagram

Each attenuation disc—L is equipped with six attenuation plates: 0dB, 5dB, 10dB, 15dB, 20dB, and 25dB. By rotating the two discs, the different attenuation plates on the

two discs are combined to obtain ten levels of attenuation, namely 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50dB. The attenuation plate can be made of coated or absorbing glass.

Continuously variable optical attenuator:

j: Continuously variable optical attenuator

    Its attenuation component has been changed accordingly. It is composed of a step attenuator and a continuously variable attenuator. The attenuation of the step attenuator is 0, 10, 20, 30, 40, 50dB, and the attenuation of the continuously variable attenuator is 0-15dB. Therefore, the total attenuation adjustment range is: 0-65dB. In this way, through the combined effect of coarse and fine gears, the purpose of continuously attenuating light energy can be achieved.

     The continuous attenuation film is made by vacuum coating a metal absorption film on a circular optical glass sheet. A special fan-shaped device is used to cover the glass substrate. Since this special covering device can continuously and evenly change its opening angle, the thickness of the film layer coming out of the steamer can be gradually and evenly changed. Therefore, the purpose of continuously changing the attenuator can be achieved.

     The manufacturing method of this filter is similar to that of the sector-shaped gradient filter. By simply changing its covering device accordingly, its optical density can be changed linearly with the direction of the filter translation. In this way, when the filter is translated perpendicular to the optical path, the attenuation of the optical attenuator can be adjusted.

5.Translational Optical Attenuator

k. Translational Optical Attenuator Schematic 

6.Intelligent Mechanical Optical Attenuator 

     The intelligent attenuator controls the electric gear through the circuit to drive the translation filter, and then feeds back the actual attenuation feedback signal detected by the data encoding disk to the circuit for correction, thereby achieving the purpose of automatic driving, automatic detection and display of light attenuation.

l.Intelligent Mechanical Optical Attenuator Schematic diagram

7.Liquid crystal optical attenuator

   The light signal incident from the optical fiber becomes parallel incident light after passing through the self-focusing lens. The parallel light is divided into two polarized light beams with mutually perpendicular polarization planes by the beam splitter P1. When passing through the liquid crystal element without any voltage, the two polarized light beams rotate 90 degrees at the same time. The rotated polarized light is then combined into a beam of parallel light by another beam splitter P2 which is 90 degrees from the optical axis of Pl, and then finely combined into the optical fiber by the second self-focusing lens.
    When a certain voltage is applied to the two electrodes of the liquid crystal, the twisted arrangement of the liquid crystal crystal direction produces a certain angle of deflection, so that the part of the liquid crystal. o light and e light, the polarization plane rotates. Among them, the part of the polarization direction rotated 90 degrees. o light and e light are combined by the beam splitter P1 into a beam of parallel light for emission, and the remaining polarized light cannot be combined and is emitted at a certain angle.

m. Liquid crystal optical attenuator  Schematic 

 Performance and Test of Optical Attenuator

     The development of optical communication technology has put forward the following requirements on the performance of optical attenuators: low insertion loss, high return loss, good resolution linearity and repeatability, large adjustable attenuation range, high attenuation accuracy, small device size, and good environmental performance. Among them, resolution linearity depends on the durability of the attenuation element, the reading display method and the mechanical adjustment structure adopted, and repeatability also depends on the reading display method and the mechanical adjustment mechanism adopted.
Attenuation and insertion loss

    Attenuation and insertion loss are important technical indicators of optical attenuators. The attenuation indicator of fixed optical attenuators is actually its insertion loss, while variable optical attenuators have separate insertion loss indicator requirements in addition to attenuation. The insertion loss of high-quality variable optical attenuators is below 1.0dB. In general, ordinary variable optical attenuators can be used if this indicator is less than 0.3dB.

    The insertion loss of the optical attenuator mainly comes from the insertion loss of the fiber collimator and the transmittance accuracy and coupling process of the attenuation unit. The process focuses on the production of the fiber collimator. If the fiber and the self-focusing lens and the two fiber collimators are well coupled, the insertion loss of the entire optical attenuator can be greatly reduced.
    Coupling of fiber and self-focusing (GRIN) lens 

    Coupling between fiber collimators

Attenuation and insertion loss testing:

n.Pigtail Optical Attenuators and Connector Port Optical Attenuators Schematic 

    The main materials are Si, Ge or InGaAs.
    It is used to test the power of light, expressed in dBm or mW. 

    Provide reference light sources for testing, divided into LED, LD (FP, DFB).
    Generally, the intensity of LD light source is between -4dBm ~ -10dBm.
    The intensity of LED is between -20dBm ~ -30dBm.

   The attenuation accuracy of the optical attenuator is one of the important indicators of the optical attenuator. Usually, the attenuation accuracy of the mechanical optical attenuator is ±0.1 times of its attenuation.
   The attenuation of the attenuation chip optical attenuator depends on the transmittance and uniformity of the metal evaporated film layer. According to the Bragg law, the transmittance depends on the internal transmittance of the absorbing material and its thickness.
Return loss

     Among the parameters of optical devices, return loss (RL) is an important indicator that affects system performance. The impact of returned light on communication systems is well known. It can cause laser relative intensity noise, nonlinear attenuation, and laser drift, which deteriorates the performance of communication systems.
    The return loss of an optical attenuator refers to the ratio of the light energy incident into the optical attenuator to the light energy reflected from the attenuator along the incident light path. The return loss of a high-performance optical attenuator is above 40dB.

     Return loss is caused by reflection caused by mismatch between components and air refraction series. Usually, the return loss caused by planar components is about 14dB. With sufficient anti-reflection film and appropriate bevel polishing and assembly process, the return loss of the entire device can reach more than 50dB. In the case of axisymmetry, if the interface of the component adopts an inclined surface with an angle, the reflected light will be segregated from the incident light path at a certain angle. Therefore, in order to improve the return loss, it is necessary to coat the surface of each component with an anti-reflection film, use a bevel lens, and tilt each optical component or match the refractive index during design.

Return loss - Return loss caused by attenuation components

     By placing the optical attenuation element tilted relative to the optical axis, the return loss at the interface between a single optical element and the air is:

θb is the angle at which the optical attenuation element is tilted to the optical axis:
n0 is the refractive index of air;
n1 is the refractive index of the fiber core;
w is the mode field radius of the fiber;
A is the self-focusing constant.
The mode field radius after the light is emitted from the lens:

     If the refractive index matching method is used, the purpose of reducing reflected light can also be achieved. When the light beam is incident vertically on the optical interface, the return loss at the component interface can be expressed as 

 Where n1, n2 are the refractive indices on both sides of the optical interface

Return loss---Return loss caused by fiber collimator

    For the optical attenuator using a collimator, the main source of its return loss lies in the collimated optical path of the incident light. It mainly comes from reflections from three surfaces: reflections from the end face of the single-mode optical fiber, and reflections from the front and rear faces of the GRIN lens. To improve the return loss of the optical attenuator, the following measures can be taken. First, an anti-reflection film should be plated on the end face, which will be very helpful in improving the RL of the optical attenuator; secondly, a beveled optical fiber collimator should be used. Theoretically, when uncoated and with a tilt angle of 0 degrees, the return loss is about 14dB; and when the end face of the fiber collimator GRIN lens is plated with a 0.1% anti-reflection film and the attenuation element has a tilt angle of 8 degrees, the return loss of the optical attenuator can reach more than 60dB. In addition, when the optical fiber attenuator is bevel-coupled to the GRIN lens, its return loss is also related to the wavelength. The return loss at an operating wavelength of 1310nm is greater than that at 1550nm. However, it can be reduced by increasing the bevel angle.

Return loss---spectral characteristics 

     In some special applications, the attenuator needs to have a higher attenuation accuracy within a certain bandwidth, and its attenuation spectrum line has better flatness. Therefore, the optical attenuator also has requirements for spectrum characteristics. However, only in the use of measurement, calibration and other occasions, there are strict requirements for this. Therefore, this indicator is not used as a regular test indicator for attenuators, and is only measured when necessary. In general, the spectrum loss of a fixed optical attenuator is not greater than 0.5dB in the range of -30 to +30nm.

Return loss---spectral characteristics

Spectral characteristics of scanning LED light source and scanning optical attenuator

 

Application and Development of Optical Attenuator 

    From the perspective of market demand, on the one hand, optical attenuators will develop in the direction of miniaturization, serialization, and low price. On the other hand, since ordinary optical attenuators are already quite mature, future research on optical attenuators will focus on their high performance. Here, it should be pointed out that because the reflection of any device will cause frequency drift and line noise of the light source, this has a particularly large impact on high-speed optical communication systems, CATV networks, and coherent optical communications, and may even cause the entire system to be misused and destroy its normal operation. High return loss optical attenuator is an important direction for the development of attenuators 

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