CN204086699U - Free space 90-degree optical mixer - Google Patents

Abstract

The utility model relates to a free space 90° optical mixer. It includes a beam combining unit for combining signal beams and local oscillator beams, an in-phase balanced receiving channel that generates two combined beams with relative phase shifts of 0° and 180°, and a positive channel that generates two combined beams with relative phase shifts of 90° and 270° Cross-balanced receiving channel, receiving unit for receiving 0°, 90°, 180°, 270° relative phase shift combined beams. The utility model provides a free-space 90° optical mixer with improved precision and convenient adjustment and assembly.

Description

A Free Space 90° Optical Mixer

technical field

The utility model belongs to the field of space laser communication and relates to an optical mixer, in particular to a free space 90° optical mixer. the

Background technique

The free-space 90° optical mixer is the optical core device of the free-space coherent optical communication terminal. It combines the signal beam and the local oscillator beam and decomposes them into 4 combined beams. °, 180°, 270° relative phase shift, or produce an in-phase balanced receiving channel (0° and 180° two-way) and a quadrature balanced receiving channel (90° and 270° two-way). the

The photosensitive surface of the balanced receiver of the free space coherent optical communication terminal is generally only tens of microns, and the distance between the receiving photosensitive surfaces of the two paths of light (I ⁺ and I ^- or Q ⁺ and Q ^- ) of the balanced receiver fixed. Therefore, the distance between the two beams of the balanced receiver must be precisely controlled, and its accuracy often requires several microns. Existing free-space optical mixers have the following three disadvantages:

1. It is difficult to accurately control the distance between the two beams of the balanced receiver;

2. The size of the device is limited by the distance between the two photosensitive surfaces of the balance receiver, which makes the device too small and difficult to install and adjust;

3. The four branches are separated from each other, it is difficult to control the optical paths of the four branches to be equal, and it is easy to cause time deviation. the

4. It is difficult to control the focusing quality of ordinary lenses, and it is difficult to process and assemble. the

Utility model content

In order to solve the technical problems existing in the background technology, the utility model proposes a free-space optical mixer, which uses an improved transverse shear interferometer to solve the problem of the two-way light of the balanced receiver in the background technology. It is difficult to precisely control the distance between them, the volume is limited, and the optical path difference of the four outputs is difficult to control. the

The mixer consists of λ/2 wave plate HWP1, λ/2 wave plate HWP2, λ/4 wave plate QWP, polarization beam splitter PBS, λ/2 wave plate HWP3, λ/2 wave plate HWP4, transverse shear interferometer SAG1, transverse shearing interferometer SAG2, self-focusing lens AFL1, self-focusing lens AFL2, self-focusing lens AFL3, self-focusing lens AFL4. the

Both the signal light and the local oscillator light are polarized light, and the two paths of light pass through the λ/2 wave plate HWP1 and the λ/2 wave plate HWP2 respectively, and become linearly polarized light at 45° to the XZ plane. Then, the local oscillator light passes through the λ/4 wave plate QWP, and the fast axis of the λ/4 wave plate is 45° to the polarization direction of the local oscillator light, and the outgoing local oscillator light becomes circularly polarized light, and its P wave (parallel component) is the same as The S waves (vertical components) are out of phase by 90°. the

The function of the polarization beam splitter PBS is to reflect the S wave and transmit the P wave. After the signal light S wave and the local oscillator light P wave entering the I branch pass through the 1/2 wave plate HWP3, the polarization direction is rotated by 45°, the signal light is at -45° with the XZ plane, and the local oscillator light is at 45° with the XZ plane. °. In this way, the P-wave component of the signal light is in the same direction as the P-wave component of the local oscillator light, and the S-wave component of the signal light is opposite to the S-wave component of the local oscillator light. the

The transverse shearing interferometer is made of two half-pentagonal prisms with different thicknesses glued together. Part of the glued surface is coated with a polarization splitting film, and the other part is coated with an anti-reflection film. After the transverse shearing interferometer, the P wave component of the signal light and the P wave component of the local oscillator light enter the I ⁺ branch and add, and the S wave component of the signal light and the S wave component of the local oscillator light enter the I ^- branch and subtract, so that It is equivalent to a phase difference of 180° between the I ⁺ branch and the ^I- branch. At the same time, the transverse shearing interferometer cuts the two beams of light from the I ⁺ branch and the ^I- branch to a certain distance, and this distance can be precisely controlled when the interferometer is installed and adjusted. Due to the nature of transverse shearing interferometers, the I ⁺ and ^I- branches have equal optical paths within the interferometer. Similarly, the Q ⁺ branch and the ^Q- branch also meet the above properties.

The outgoing light beams of the I ⁺ branch, ^I- branch, Q ⁺ branch and ^Q- branch are respectively focused on the detector by the self-focusing lens. The aperture of the self-focusing lens is generally only a few millimeters, and the diverging spot of the focused beam is equivalent to the size of the photosensitive surface of the detector.

The technical scheme of the utility model is: a free space 90° optical mixer, including a beam combining unit for combining the signal beam and the local oscillator beam, and generating the in-phase phase of the two combined beams with relative phase shifts of 0° and 180° Balanced receiving channel, an orthogonal balanced receiving channel that produces two combined beams with relative phase shifts of 90° and 270°, and a receiving unit that receives combined beams with relative phase shifts of 0°, 90°, 180°, and 270°;

Its special features are:

The in-phase balanced receiving channel and the orthogonally balanced receiving channel are respectively provided with a transverse shearing interferometer; the transverse shearing interferometer is composed of two half-pentagonal prisms with different thicknesses glued together, and the beams combined by the beam combining unit Then it is cut into two beams by a transverse shearing interferometer, and the distance between the outgoing light of the two beams can be controlled by moving two half-pentagonal prisms in the glued surface during assembly and adjustment;

The incidence area of the glued surface of the above-mentioned transverse shearing interferometer is coated with a polarization beam splitter film, and the transmission area is coated with an anti-reflection film;

The structure of the above-mentioned in-phase balanced receiving channel and quadrature balanced receiving channel is exactly the same;

The above-mentioned receiving unit is respectively focused and received by the second self-focus lens, the third self-focus lens, the first self-focus lens, and the fourth self-focus lens to receive 0°, 90°, 180°, 270° relative phase-shifted combined beams;

The beam splitting unit includes a second wave plate, a fifth wave plate, and a third wave plate arranged sequentially on the local oscillator optical path, and a first wave plate and a fourth wave plate sequentially arranged on the signal optical path; the local oscillator optical path A polarization beam splitter prism is also arranged at the intersection with the signal light path. the

The utility model has the advantages of:

1. The utility model adopts an improved transverse shearing interferometer to realize accurate control of the distance between the two beams of the balance receiver. the

2. The size of the optical path structure of the utility model is no longer limited by the distance between the two channels of the balance receiver, and the volume of the device can be appropriately increased to facilitate installation and adjustment. the

3. The two paths of light of the same balanced receiving channel pass through the transverse shearing interferometer to cut the optical path to be equal, so that the control of the equal optical path of the four paths is simplified to the control of the equal optical path between the two balanced receiving channels. That is, the utility model makes it easier to make the optical paths of the four branches equal. The higher the transmission rate of the space laser communication system, the higher the requirements on the optical path difference of each path, and the more important this advantage of the utility model is. the

4. Both sides of the self-focusing lens are flat, which is easy to install and has excellent focusing quality. the

Description of drawings

Fig. 1 is the schematic diagram of the principle of the utility model;

Fig. 2 is the optical system diagram of the present utility model;

Fig. 3 is the schematic diagram of polarization transverse shearing interferometer in the utility model;

1-first wave plate, 2-second wave plate, 3-third wave plate, 4-fourth wave plate, 5-polarization beam splitter, 6-first self-focusing lens, 7-second self-focusing lens , 8-third self-focusing lens, 9-fourth self-focusing lens, 10-transverse shear interferometer, 11-fifth wave plate. the

Detailed ways

See Figure 1-2, a free-space 90° optical mixer, including a beam combining unit that combines the signal beam and the local oscillator beam, and generates two combined beams with 0° and 180° relative phase shifts for in-phase balanced reception Channel, the quadrature balanced receiving channel that produces 90° and 270° relative phase-shifted two-way combined beams, the receiving unit that receives 0°, 90°, 180°, 270° relative phase-shifted combined beams; in-phase balanced receiving channel and quadrature A transverse shearing interferometer 10 is arranged on the balanced receiving channel; the transverse shearing interferometer 10 is composed of two half-pentagonal prisms with different thicknesses glued together, and the beams combined by the beam combining unit pass through the transverse shearing interferometer 10 is cut into two beams, and the distance between the outgoing light of the two beams can be controlled by the translation of two half-pentagonal prisms in the glued surface during installation and adjustment; the incident area of the glued surface of the transverse shear interferometer 10 is coated with a polarization beam splitter film , the transmission area is coated with an anti-reflection film; the structure of the in-phase balanced receiving channel and the orthogonal balanced receiving channel are exactly the same; the receiving unit is composed of the second self-focusing lens 7, the third self-focusing lens 8, the first self-focusing lens 6, the second Four self-focusing lenses 9 focus and receive 0°, 90°, 180°, 270° relative phase-shift combined beams; the beam splitting unit includes the second wave plate 2, the fifth wave plate 11, the third wave plate arranged in sequence on the optical path of the local oscillator The wave plate 3 and the first wave plate 1 and the fourth wave plate 4 arranged sequentially on the signal optical path; a polarization beam splitter prism 5 is also arranged at the intersection of the local oscillator optical path and the signal optical path. the

When the local oscillator laser enters the system, its polarization direction should be parallel to the XZ plane, and a certain angle deviation is allowed. By adjusting the 1/2 wave plate HWP2, the fast axis is 22.5° to the polarization direction of the local oscillator laser, and the polarization direction XZ of the local oscillator light is at an angle of 45° after passing through HWP2. The direction of the fast axis of the 1/4 wave plate QWP is parallel to the XZ plane, that is, the direction of the fast axis of the QWP is 45° to the polarization direction of the local oscillator. The local oscillator light becomes circularly polarized light after passing through the 1/4 wave plate QWP, and the phase difference between the P wave and the S wave is 90°. Then the local oscillator light passes through the polarization beam splitter, the P wave is transmitted into the I branch, and the S wave is reflected into the Q branch. In this way, a phase difference of 90 degrees is generated in the local oscillator light between the I branch and the Q branch. the

Also by adjusting the 1/2 wave plate HWP1, the fast axis of HWP1 is 22.5° to the polarization direction of the signal light. After the signal light passes through HWP1, its polarization direction XZ forms an angle of 45°, that is, the P wave component and the S wave component are equal. Then the signal light passes through the polarization beam splitter, the P wave is transmitted into the Q branch, and the S wave is reflected into the I branch. the

After the signal light S wave and the local oscillator light P wave entering the I branch pass through the 1/2 wave plate HWP3, the polarization direction is rotated by 45°, the signal light is at -45° with the XZ plane, and the local oscillator light is at 45° with the XZ plane. °. In this way, the P-wave component of the signal light is in the same direction as the P-wave component of the local oscillator light, and the S-wave component of the signal light is opposite to the S-wave component of the local oscillator light. After the transverse shearing interferometer, the P wave component of the signal light and the P wave component of the local oscillator light enter the I ⁺ branch to add, and the S wave component of the signal light and the S wave component of the local oscillator light enter the I ^- branch phase This is equivalent to a 180° phase difference between the difference frequency component between the I ⁺ branch and the I ^- branch. Similarly, there is also a phase difference of 180° between the Q ⁺ branch and the Q ⁻ branch.

The distance between the two beams of the I ⁺ branch and the ^I- branch, that is, the shearing amount of the interferometer, is precisely controlled by the interferometer. The two prisms of the interferometer are glued together with photosensitive glue. When installing and adjusting, a laser beam with a polarization direction of 45° is used to enter the transverse shearing interferometer, and an analyzer is added to the output end of the transverse shearing interferometer (at 45° to the XZ plane), and a digital camera is placed at a certain distance behind the analyzer . Then the two beams of light cut by the transverse shearing interferometer, that is, the two beams of light I ⁺ and I ^- will interfere on the focal plane of the digital camera. Use the computer to display its interference fringe pattern in real time. Theoretical calculation N ₀ interference fringes account for the number of CCD pixels M ₀ of the digital camera:

${\mathrm{<span\; class="notranslate">\; m</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; d</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}$

in,

λ is the laser wavelength output by the laser,

f is the focal length of the digital camera,

d is the CCD pixel size,

Δ is the transverse shear amount required by the design. the

The two prisms that have been glued and not yet solidified are moved relatively slightly, and N ₀ interference fringes are read, accounting for the number M of CCD pixels of the digital camera. When the number of CCD pixels M is M ₀ , stop moving, and light-cure the glued surface to position the two prisms. The interferometric method installs and adjusts the transverse shearing interferometer, and the theoretical accuracy of the distance between the I ⁺ branch and the ^I- branch can reach 1 wavelength of the laser used.

The output beams of the I ⁺ branch, I ^- branch, Q ⁺ branch, and Q ^- branch are respectively focused on the detector by the self-focusing lens, and the size of the focused beam spot is equivalent to that of the photosensitive surface.

Claims (5)

1. A free-space 90° optical mixer, including a beam combining unit that combines the signal beam and the local oscillator beam, and generates an in-phase balanced receiving channel of two combined beams with relative phase shifts of 0° and 180°, generating 90° ° and 270° relative phase shift two-way combined beams with orthogonal balance receiving channel, receiving unit for receiving 0°, 90°, 180°, 270° relative phase shifted combined beams; It is characterized by: The in-phase balanced receiving channel and the orthogonally balanced receiving channel are respectively provided with a transverse shearing interferometer; the transverse shearing interferometer is composed of two half-pentagonal prisms with different thicknesses glued together, after being combined by the beam combining unit The light beam is then cut into two beams by a transverse shearing interferometer, and the distance between the outgoing light of the two beams can be controlled by moving two half-pentagonal prisms in the glued surface during assembly. 2. A kind of free-space 90° optical mixer according to claim 1: it is characterized in that: the incidence area of the glued surface of the transverse shear interferometer is coated with a polarization beam splitter film, and the transmission area is coated with an increasing Permeable membrane. 3. A free-space 90° optical mixer according to claim 2, characterized in that: the structure of the in-phase balanced receiving channel and the quadrature balanced receiving channel are exactly the same. 4. A kind of free-space 90° optical mixer according to claim 3: it is characterized in that: the receiving unit consists of a second self-focus lens, a third self-focus lens, a first self-focus lens, a fourth The self-focusing lens focuses and receives 0°, 90°, 180°, 270° relative phase shift combined beams. 5. A free-space 90° optical mixer according to claim 4: it is characterized in that: the beam splitting unit includes a second wave plate, a fifth wave plate, a third A wave plate and a first wave plate and a fourth wave plate arranged sequentially on the signal light path; a polarization beam splitter prism is also set at the intersection of the local oscillator light path and the signal light path.