RU2638095C1 - Monostastic optical transceiver - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: monostatic optical transceiver comprises a transmitting optical fibre coupled to the transmitter, a receiving optical fibre coupled to the receiver combined through a fibre optic duplexer, the end face of the output fibre of which is placed near the focal plane of the monostatic optical system. The transmitting optical fibre is in the form of a single-clad light guide having a numerical aperture NA1, a core diameter D1, and a refractive index of the core n1. The receiving and output optical fibre is made in the form of a single fibre with one shell having a numerical aperture NA2, a core diameter D2, and a refractive index of the core n2, with the proviso that NA1/n1<NA2/n2 and D1<D2. The duplexer is made in the form of an angular optical connection between the transmitting and receiving fibres, with the end face of the output fibre of the duplexer ground at an angle (90°-β) to the geometric axis of the fibre.

EFFECT: ensuring the possibility of increasing the level of isolation of counter channels, reducing the losses of received radiation and using conventional, multimode and single-mode optical fibres with one shell.

4 cl, 2 dwg

Description

The invention relates to optical instrumentation and can be used in photovoltaic devices for converting optical information into an electrical signal and vice versa, in particular in wireless optical communication systems, optical locators and other devices.

In many optical devices, to implement their functions as intended, the system must include an optical radiation source and receiver. Such devices include, for example, optical communication systems, laser rangefinders, lidars, etc. In order to reduce the dimensions and improve the stability of the optical axes, such devices are trying to make monostatic. This means that, at least in part of the optical system, the opposing beams of the transmit and receive radiation propagate along the same optical path. In this case, the effects of the radiation of the transmitter on its own receiver arise, as well as additional light losses on devices that provide the combination of oncoming beams, optical duplexers. These issues are especially acute in optical communication systems, where the transmitter and receiver must work simultaneously to obtain full-duplex operation of equipment. The present invention is directed to solving problems of improving the separation of transmission and reception channels in a monostatic optical system and reducing losses in the optical path.

From the prior art it is known a transceiver device of an optical atmospheric communication line, made in the form of external and internal blocks and containing an interface, an optical radiation source, an optical radiation receiver, a focusing system, an optical fiber channel combiner and a fiber light guide, one end of which is fixed to the external unit, that its end is located in the focus of the focusing system, and the other end of the fiber is connected optically to the source through an optical fiber channel combiner by an optical radiation receiver and an optical radiation receiver, wherein the external unit is weatherproof and has a focusing system in it (see RF patent No. 2239285, priority date 10/31/2001, IPC 7 Н04В 10/10, published on 10/27/2004 " Optical atmospheric communication line transceiver "). The fiber channel combiner in the device is made in the form of an optical spectral multiplexer / demultiplexer. Obviously, other well-known solutions can be used as a fiber combiner: optical circulators based on the Faraday effect, various types of fiber splitters widely used in fiber-optic networks (see Listvin VN, Treshchikov VN “DWDM systems ": Scientific publication. - M.: Publishing House" Science ", 2013. - S. 143-144).

A disadvantage of the known transceiver is the low separation factor of the transmission and reception channels. For fiber-optic multiplexers, the channel separation coefficient is 15 dB for adjacent bands and 30-35 dB for non-adjacent channels. The use of fiber splitters for solving the problem introduces large losses into the optical path (for a 1 × 2 splitter, the losses exceed 7 dB). The disadvantages of the device when using the circulator include the ability to work only with the same fibers and a small level of isolation (back reflection), which does not exceed 36-38 dB, as well as a limited wavelength range for one device.

The prior art also knows the transceiver for an optical communication device, which is made in the form of external and internal blocks that are interconnected by a fiber waveguide (RF patent No. 2311738, with priority dated 13.03.2006, IPC Н04В 10/10, published on 27.11. 2007 "Transceiver for an optical communication device"). The external unit of the transceiver is made in an all-weather design and contains a focusing system, and the internal unit is made for indoor conditions, and it contains emitters located on the same substrate adjacent to each other, which are laser diodes or LEDs that form the source of optical radiation, and photocells, which form an optical radiation receiver. Emitters and photocells, located on the same substrate close to each other, are a matrix that is docked with the end of the fiber and proportional to its end. The matrix, consisting of multiple emitters and photocells, receives and converts the optical signal, which eliminates fiber-optic combiners-splitters.

The disadvantage of this device is the low level of separation of the transmission channels and a high level of light energy loss. When light is emitted by emitters, part of it will be reflected both from the near and far ends of the fiber and get on photodetectors located close to the emitters. Light losses increase due to the fact that the area occupied by photodetectors is much less than 50% of the core area of the light guide through which the light propagates, both due to the presence of emitters located on the same site and due to the presence of mandatory protective barriers between the receivers and emitters.

The closest in technical essence to the proposed solution is a compact monostatic transceiver device (US patent US20120154783 A1 dated November 9, 2010, IPC G02B 6/32, "COMROST MONOSTATIC OPTICAL RECEIVER AND TRANSMITTER"), which is selected as a prototype.

A compact monostatic transceiver consists of a transmitting fiber connected to a transmitter, a receiving fiber connected to a receiver, connected through a fiber optic duplexer, the end of the output fiber of which is in the focus of the optical monostatic system. The duplexer is made on the basis of fiber with a double sheath (VDO), while the radiation of the transmitter propagates through the core of the VDO, and the received radiation is concentrated in the inner shell of the VDO. The separation of the channels is carried out due to the fact that the VDO is fused with a multimode fiber so as to provide optical contact for the transition of the received radiation from the inner sheath of the fiber with a double sheath into a multimode fiber, which is receiving and connected to the receiver of optical radiation.

The disadvantages of this device include the complexity of the design of the fiber with two shells, a small level of isolation between the receiver and the transmitter (back reflection), losses for input radiation (on serial samples, this value reaches 3 dB).

The technical result of the invention is to expand the functionality of the device monostatic optical transceiver, namely increasing the level of isolation, reducing the loss of received radiation and the possibility of using conventional, multimode and single-mode optical fibers with a single sheath.

The technical result is achieved in that a monostatic optical transceiver containing a transmitting optical fiber connected to a transmitter, a receiving optical fiber connected to a receiver, combined through a fiber optic duplexer, the end of the output fiber of which is located near the focal plane of the monostatic optical system, characterized in that :

the transmitting optical fiber is made in the form of a fiber with one cladding having a numerical aperture NA1, the diameter of the core D1 and the refractive index of the core n1,

the receiving and output optical fiber is made in the form of a single fiber with one cladding having a numerical aperture NA2, core diameter D2 and core refractive index n2,

the duplexer is made in the form of an angular optical connection of the transmitting and receiving fibers so that the ratio

,

,

where α is the angle between the geometric axes of the receiving and transmitting fibers, counted from the end of the fiber connected to the photodetector,

the end face of the output fiber of the duplexer is ground at an angle (90 ° -β) to the geometric axis of the output fiber, provided that the angle β lies in the range

,

,

and also provided that NA2 / n2> NA1 / n1 and D2> D1.

The angular optical connection of the transmitting and receiving fibers is carried out by alloying the manufacture of an X-splitter of two fibers, the remaining free end of the fiber connected to the transmitter being processed at an angle β, or in the form of a spherical surface with a radius significantly larger than the diameter of the core, or wound on a coil with a bending radius less than critical.

The operation of the proposed device is illustrated in FIG. 1, which shows a device of a monostatic optical transceiver. The numbers in the drawing indicate:

1 - photodetector;

2 - optical fiber attached to a photodetector;

3 - radiation source;

4 - optical fiber connected to a radiation source;

5 - duplexer;

6 - output optical fiber;

7 - transceiver monostatic optical system;

8 - transmitted radiation;

9 - received radiation.

The device monostatic optical transceiver contains: a photodetector 1 connected to an optical fiber 2; a radiation source 3 connected to an optical fiber 4; a duplexer 5 connected by optical fibers 2 and 4 with a photodetector 1 and a radiation source 3, respectively, and the end face of the output fiber 6 of the duplexer 5 is placed in the focus of the monostatic optical system 7.

Duplexer 5 provides separation of the transmitted 8 and received 9 emissions by welding the receiving fiber 2 having a core diameter D2, a numerical aperture NA2 and a refractive index of the core n2, and an optical fiber 4 connected to the transmitter and having a core diameter D1, a numerical aperture NA1 and core refractive index n1, with NA2 / n2> NA1 / n1 and D2> D1. The optical fiber 4 is optically connected to the side surface of the optical fiber 2 at an angle α counted from the side of the end of the optical fiber 2 connected to the photodetector 1, so that the condition

.

.

The end face of the output end 6 of the duplexer 5, optically connected to the transceiver monostatic optical system 7, is ground at an angle (90-β) to the axis of the output fiber, provided that

.

.

Technical solutions that match the totality of the essential features of the claimed invention have not been identified, which allows us to conclude that the claimed invention meets the condition of patentability "novelty".

The claimed essential features that predetermine the receipt of the specified technical result, do not explicitly follow from the prior art, which allows us to conclude that the claimed invention meets such a patentability condition as "inventive step".

Monostatic optical transceiver operates as follows.

Laser radiation from a laser or after an optical amplifier created by a radiation source 3 is directed to an optical fiber 4 (for example, quartz single-mode with NA1 = 0.12, D1 = 8 μm, n1 = 1.485). Then the radiation along this fiber enters the duplexer 5, which is formed by welding fiber 4 and optical fiber 2 (for example, quartz multimode with NA2 = 0.27, D2 = 62.5 μm, n2 = 1.485) at an angle α.

, где n - показатель преломления сердцевины оптического волокна. Поэтому при вводе излучения из одного волокна в другое угол ввода входящего излучения должен создать условия, при которых угловые параметры вводимого излучения обеспечили бы полное внутреннее отражение при распространении данного излучения в другом волокне. Вывод условий для такого согласования показан на Фиг. 2. На Фиг. 2 введены обозначения:In accordance with the laws of optics, optical radiation modes can propagate inside the optical fiber only with angles that provide total internal reflection. This radiation emerging from the fiber is characterized by a numerical aperture NA, which is equal to NA = Sin (γ), where γ is the half angle of divergence of the radiation emerging from the optical fiber into the air. Accordingly, the numerical aperture of radiation inside the fiber is

where n is the refractive index of the core of the optical fiber. Therefore, when introducing radiation from one fiber into another, the angle of input of incoming radiation should create conditions under which the angular parameters of the input radiation would provide complete internal reflection during the propagation of this radiation in another fiber. The derivation of conditions for such matching is shown in FIG. 2. In FIG. 2 designations introduced:

1 - optical axis of the input radiation;

2 - optical axis of a multimode optical fiber;

3 - multimode fiber;

α is the angle between the optical axis of the input radiation and the optical axis of the multimode fiber;

θ1 is the angle of divergence of the input radiation corresponding to the numerical aperture of the optical fiber 4;

θ2 is the angle of divergence of radiation corresponding to the numerical aperture of a multimode optical fiber 2.

The angles θ1 and θ2 are related by the relations with the numerical apertures of the optical fibers by the following expressions:

Since the condition of total internal reflection must be fulfilled, from FIG. 2 it follows that the angle between the optical axis of the input radiation and the optical axis of the multimode fiber must meet the condition that the cone of the optical input radiation is inside the cone of permissible angles of propagation of radiation in the multimode fiber. Based on FIG. 2 it comes down to the condition

.α <θ2-θ1 or

.

For example, for the above parameters of fiber 4 and fiber 2, this condition is satisfied when Sin (α) <0.101 or α <5.8 °. If the angle α does not fulfill this condition, then part of the input radiation will exit the multimode fiber, and this will lead to radiation losses. It should also be noted that another additional condition for the duplexer to work is the ratio NA2 / n2> NA1 / n1. This is due to the fact that when the difference in the values of NA2 / n2-NA1 / n1 decreases, the angle α tends to zero. For the same optical fibers (for example, in fiber splitters), when the indicated values are equal, an optical transfer from one fiber to another occurs using an optical “tunnel” transition of optical radiation, which depends on the radiation wavelength and the optical contact length. This ensures the operation of the splitter for the purpose of separation of the initial radiation in different channels, but leads to large losses both in input and in reception of radiation (a total of more than 7 dB) if it is used as a duplexer, which is essentially an optical circulator.

After the optical radiation is introduced through the duplexer 5 into the multimode fiber 2, this radiation without losses, but with a changed spatial distribution, reaches the end of the output fiber 6, in practice both fibers form a single whole. In order to exclude the back reflection of radiation 8 from the end of the optical fiber 6, the end face is polished at an angle of 90 ° -β to the axis of the optical fiber 6. This angle must correspond to the condition that the radiation 8 propagating inside the fiber with the maximum allowable angle corresponding to the numerical aperture NA2 / n2, at reflection from the polished end to the double angle β did not lie inside the cone with an angle corresponding to the numerical aperture NA2 / n2. This requirement is met provided that

.

.

On the other hand, the angle β should not exceed the angle of total internal reflection for all modes in a multimode fiber, which is satisfied under the condition

.

.

Thus, the condition β is imposed on the angle β

.

.

So, for example, for multimode fiber 6 with the above parameters 0.492> Sin (β)> 0.183 or 29.4 °> β> 10.5 °.

With the exclusion of reflected radiation, scattered light remains. For optically polished surfaces, the fraction of scattered light is 0.1-0.01% of the power of radiation incident on the surface. If we assume that the radiation is scattered in all directions, then only that fraction of the scattered radiation that falls into the solid angle corresponding to the numerical aperture NA2 / n2 returns back. Thus, the fraction P of radiation 8 P ₀ , which can go back, is determined by the expression

,

,

or isolation coefficient I in decibels

,

,

where K is the scattering coefficient. For the above multimode fiber parameters, the calculated insulation coefficient should be minus 51-61 dB.

Then, the output radiation 8 leaves through the end face of the fiber 6 into space and enters the optically conjugated monostatic optical system. The optical system may be a lens, mirror or mirror-lens. This system forms the required beam of radiation 8 of the necessary divergence depending on the application and directs it into the open space. In the case of a lidar, this radiation is reflected from the target and returns back, and in the case of wireless optical communication systems, radiation from a remote terminal is received at the input of the monostatic optical system.

The received radiation 9 (Fig. 1) focuses on the fiber end and enters the core of the fiber 6, where it is channelized within its numerical aperture. According to the operating conditions of optical instruments, the spatial, amplitude, and phase distribution of the parameters of light at the reception never corresponds to similar parameters when it is emitted. Therefore, when the received radiation propagates through the duplexer 5, a completely different mode radiation state is created in it. With the passage of this new composition of the spatial distribution of radiation through the radiation input section of the transmitter, part of the radiation can flow into fiber 4, leading to radiation loss. These losses are proportional to the contact area of fibers 2 and 4, which depends on the angle of fiber welding α, on the ratio of the square of the numerical apertures of these fibers, as well as on the mode composition of the radiation excited in the fiber to the received signals. The estimates for the above parameters of the fibers show that the loss from the flow of radiation into the fiber 4 can be from 0.6 to 25%, depending on the angle of welding. A practical measurement of the losses upon excitation of the fiber by the received signal with a convergence angle corresponding to 0.8 * NA2 showed a loss value within 10%.

The patentability condition "industrial applicability" is confirmed by the example of a specific implementation of the inventive transceiver for an optical wireless communication device.

In the practical implementation of the proposed solution, the channel duplexer was made on the basis of an alloyed (welded) X-splitter. For its manufacture, standard quartz fibers were used for telecommunications: single-mode 9/125, conforming to G.652C and IEC 60793-2-50 Ture B 1.3 standards, and multi-mode 62.5 / 125 according to IEC 60793-2-10 Type A 1 .b. The technology of fusion of fibers made it possible to obtain their optical connection with an angle α in the range of 1.5-3 degrees. To exclude back reflection from the free 4th end of a single-mode fiber, its end face was processed into a sphere with a radius of 2 mm by applying a drop of compound with a refractive index close to 1.485. The output fiber of the duplexer was polished into a plane with optical quality at an angle β = 13 degrees. A three-lens diffractive quality lens with a focal length of 200 mm was used as a monostatic optical system for a transceiver. A laser with a wavelength of 1550 nm was used as a transmitter, and a PIN photodiode as a receiver.

Measurement of the parameters of a monostatic optical transceiver showed the following results: transmission radiation loss - up to 10%, reception radiation loss - up to 15%, channel isolation was 62 ÷ 64 dB.

Thus, the total losses in the transceiver (transmission plus reception) are not more than 1 dB, and the measured value of the channel isolation turned out to be even better than the calculated value. This can be explained by the fact that the scattering at the polished fiber end is not omnidirectional in nature, as was accepted in the calculation model, but a more elongated indicatrix, which corresponds to the Mie scattering law.

Claims (12)

1. Monostatic optical transceiver, comprising: a transmitting optical fiber connected to a transmitter, a receiving optical fiber connected to a receiver, combined through a fiber optic duplexer, the end of the output fiber of which is located near the focal plane of the monostatic optical system, characterized in that: the transmitting optical fiber is made in the form of a fiber with one cladding having a numerical aperture NA1, the diameter of the core D1 and the refractive index of the core n1, the receiving and output optical fibers are made in the form of a single-clad fiber with a numerical aperture NA2, core diameter D2 and core refractive index n2, the duplexer is made in the form of an angular optical connection of the transmitting and receiving fibers so that the ratio

where α is the angle between the geometric axes of the receiving and transmitting fibers, counted from the end of the fiber connected to the receiver, the end face of the output fiber of the duplexer is ground at an angle (90 ° -β) to the geometric axis of the fiber, provided that the angle β lies in the range

and also provided that NA2 / n2> NA1 / n1 and D2> D1. 2. The monostatic transceiver according to claim 1, characterized in that the angular optical connection of the transmitting and receiving fibers is performed by welding the X-splitter, and the remaining free end of the fiber connected to the transmitter is processed at an angle β. 3. The monostatic transceiver according to claim 1, characterized in that the angular optical connection of the transmitting and receiving fibers is performed by welding the X-splitter, and the remaining free end of the fiber connected to the transmitter is made in the form of a spherical surface with a radius significantly larger than core diameter. 4. The monostatic transceiver according to claim 1, characterized in that the angular optical connection of the transmitting and receiving fibers is performed by welding the X-splitter, and the remaining free end of the fiber connected to the transmitter is wound on a coil with a bending radius less than critical.

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