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Small angle filter for tilt TO Can package

Hisense combo OLT has a detector with tilt TO can.



This TO can is obliquely posted because two of the four wavelengths are 1270 nm and 1310 nm, which are very close. A filter with a small angle is required to effectively separate the two wave fields.



The two wavelengths of the small-angle filter are partially amplified, and one wavelength is designed to be transmitted and the other wavelength to be reflected.



Why design a small angle?


Because at 0 degree, the transmission and emission light paths of the two overlap completely, and cannot be separated on the space path. If it is 45 degrees, the transmitted light and the reflected light path just present a 90-degree right angle, which is also a common angle. But at this angle, for the blue light, there will be a large dispersion in the wavelength transition band of transmission and reflection.


Using small-angle filters can not only realize the separation of optical paths, but also reduce the degree of dispersion. For example, the wavelength of 1577 and 1490 can be 45 degrees. The wavelength distance is very wide. However, for 1270 ±10nm and 1310 ±20nm, the two wavelength divisions require small angles.




The filter transmission and reflection spectra of these angles are as follows



Why is there a dispersion in the transflectance spectrum?

For light with a wavelength of 1310nm, there are actually two polarization states, P light and S light, and TE/TM O light and E light, just like the distinction between men and women is called differently on different occasions.



The transmission and reflection spectrum of the filter we understand is as follows



However, for the same 1310nm light, the two polarization states of P light and S light actually have different transmission and reflection curves for the same filter. This is the "transition zone". We expect both polarization states to emit at 1270nm and transmit both polarization states at 1310nm.


Then it is necessary to avoid letting the wavelength be in the transition band of the filter, which may cause 1310nm light, one polarization to be transmitted and the other to be reflected, which is different from the expected target of 1310nm with both polarizations transmitted.


For the wavelength interval to be separated, it is very clear. Then the interval is very wide, of course it is possible to use a 45-degree filter. If the wavelength interval is short, small angle slices should be considered.




Explain, the light of the same wavelength, such as 1310nm, deviates from the incident angle of the filter, why there is a difference between the two polarization states? As follows, the light is incident on a plane, vertical incidence (that is, 0 ° filter), oblique incidence (filter at a certain angle), and only the transmission is considered


The 1310nm light, these two polarization states, are incident in the perpendicular direction. The two polarization states, both transmitted, appear to be indistinguishable, not even distinguishing between p-light and s-light.


But if I turn the filter at an angle, from green to gray cotton, the polarization state parallel to the plane of this rotation axis is p-light, and the polarization state perpendicular to this rotation plane is s-light.

Let's take a look again, for s light (blue polarization state at 1310nm), whether it is rotated or not, this polarization looks no different. But for p light (1310nm brown-red polarization state), its incident plane has changed.


This is the separation of the equivalent refractive index of s-light and p-light as the angle of the wave plate changes. The 0° filter is the same for both polarization states at the same wavelength.

However, the filter is rotated at an angle, and the 1310nm light is still the transmitted light path. In fact, there is a "slight" difference between the two polarizations.

The greater the angle, the greater the difference.



From another perspective, transmission is transmission, but the equivalent interface between the polarization state and the s-polarization state.

There are also some factories that do not have TO slopes on the outside, as shown below




There is a matching wave plate, which cooperates with the small angle plate to form a complete 90-degree optical path turning. If the small-angle sheet is 13°, the angle of the matching sheet is 32°, forming a macroscopic 45° reflective sheet, forming a 90° horizontal, flat and vertical structure.


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