FR2786869A1 - Testing and adjusting optical transmission circuits by sending reference signal through couplers and optical fibre circuits to reflector, detecting reflected signal and using it to correct transmitters - Google Patents

Abstract

A test transmitter (E) and test receiver (R) are coupled (C) through an optical fibre (F1) to a rotating optical joint (JT) and to a reflector (Fr). A standard signal (Sr) modulates the test transmitter (E) and the reflected signal (Sd) is received (R) and detected (CT). When the rotating joint (JT) is turning at a constant speed the detected reflected signal may be used to correct other transmitters (En) for the effect of the rotating joint.

Description

DEVICE FOR TESTING AND REGULATING A CIRCUIT OF

TRANSMISSION

  The device of the invention makes it possible to test and regulate an optical transmission circuit. It makes it possible to measure in real time the amplitude fluctuations of optical transmissions, in particular in the case of

  transmission through a rotating joint.

  The optical transmission test means traditionally used call on two categories of devices: - power meters using a continuous signal and measuring insertion losses between a reference transmitter and receiver; - reflectometers using a pulse signal and measuring

  and locating one-off or distributed losses (by backscatter).

  These devices are not designed to make measurements on transmission lines in operation and, in particular for the second, have temporal resolutions unsuitable for short transmissions (a few meters) comprising close connection points, such as those envisaged in radar applications (especially in the air). The device of the invention has the advantage of being easily

  implemented in transmission systems.

  The transmission line test can be permanent and can be

  unroll during the operation of the equipment.

  The device essentially consists of an optical transmission at a dedicated wavelength, optically multiplexed to the channel to be measured, provided at one end with a specific emitting / receiving optical head and at the other end with a reflecting element. It can be used simultaneously for measuring and regulating signal levels

  analog passing through an optical rotating joint.

  This device applies in particular in radars using optical links multiplexed in wavelength and more generally

  in the field of telemetry in rotating machines.

  The invention therefore relates to a device for testing and regulating a transmission circuit comprising at least one transmitter and / or receiver of information, at least one controlled circuit, and a rotary joint with n inputs and m outputs connecting the circuit. transmitter connected to one of the n inputs to the controlled circuit connected to one of the m outputs, characterized in that it also comprises: - a test transmitter connected to one of the n inputs of the rotary joint; - a first reflection or retro-reflection device connected to one of the m outputs of the rotary joint; - a test receiver connected to one of the n inputs of the rotary joint. The various objects and characteristics of the invention will appear

  more clearly in the description given by way of example and in the figures

  attached which represent: - Figure 1, a general embodiment of the device of the invention; - Figure 2, a measurement signal from an optical transmission 1 5 comprising a rotating joint; - Figure 3, a device according to the invention comprising a calibration system; - Figure 4, a known type of transmission circuit; FIG. 5, a transmission circuit comprising a device according to the invention; - Figure 6, a transmitter control circuit in function

of a detection signal.

  Referring to Figure 1, we will first describe a

  general example of embodiment of the device of the invention.

  A test transmitter E and a test receiver R are coupled by a coupler C to a bidirectional optical link F1 such as an optical fiber. This optical link F1 is connected to an input of an optical rotating joint coupler JT. Generally an optical connector C1 is provided

  to connect the optical link (the fiber).

  The coupler with optical rotating joint JT is also connected to circuits controlled Cx by an optical link F2, to optical fiber by

  example, and by a connector C'2. A reflection device Fr (or retro-

  reflection) is also connected by a Tf fiber and a C2 connector to the

JT rotary joint coupler.

  A reference electrical signal Sr modulates the light emitter E, the radiation of which is coupled in an optical fiber leading to one of the inputs b1 of the optical coupler 2x1, C. The modulated optical signal, propagating in the fiber F1, passes respectively through the optical connector Cl, the optical rotating joint JT and the second optical connector C1. The optical signal is then reflected by the reflecting element Fr located at the end of the optical fiber section Tf and propagates in the opposite direction to the optical coupler C. The part of the optical signal coupled in the branch b2 is detected by the photodetector R and generates a signal

electric lo Sd.

  Insofar as all the passive components have transfer functions independent of the direction of propagation (case of the single-mode components), the signal Sd is indeed the image of the variation as a function of time of the transmission line situated for example between the points A and B. In practice, it can be seen that the rotation of the rotary joint JT is negligible for a time equal to twice the propagation time

light between JT and Fr.

  If Pe and Pd are respectively the optical powers emitted and detected expressed in Watts and coupled in the branches b1 and b2 of the coupler C, we will have: log (Pe / Pd) = 2 (pc + 3dB + Pc, + PJT + Pc2 +. PF) (1) o: Pc, PC1, PJT, PC2 are the respective insertion losses, expressed in dB, of coupler C (assumed to be symmetrical), of connector C1, of

  rotating joint JT and connector C2.

t is the transmission length

  PF is the attenuation of the fiber expressed in dB / unit of length.

  When the rotary joint is rotating at constant speed, PJT is a periodic function with period T = 1 / N (N number of rotations per second), Figure 2 shows in this case the shape of the signal Sd as a function of time. Under these conditions, a detection circuit CT connected to the receiver R is capable of supplying each transmitter such as En connected to the rotating joint JT, a correction signal making it possible to compensate in advance for the effect of the transmission through the rotating joint. . Similarly, the detection circuit CT controls in the receivers such as Rec the degradation of

  the transmission due to the JT rotating joint.

  FIG. 3 shows an optical test head incorporating an optical switch CO in order to calibrate the test transmitter / receiver pair E / R. The electric control CE of this switch makes it possible to direct the light energy coming from the coupler C, either towards the line to be measured, or

  towards a reflecting element F'r identical to Fr.

  If in the connection to the reflecting element F'r, P'd is the optical power detected by the receiver R, the attenuation expressed in dB i0 of the line to be measured will be given by: log (Pd / P'd) = -2 (Pci + PJT + PC2 + (.Pf) (2) Pd, Pc1, Pc2, let pf having the meanings given above

by relation (1).

  Future rotary air radars will rely heavily on the use of fiber optic links to transmit both analog and digital signals through an optical rotary joint

  single-channel thanks to wavelength multiplexing.

  French patent no. 74 01800 relates to this type of multiplexing.

  FIG. 4 shows a diagram for a radar application, of multichannel bidirectional multiplexed optical transmissions through

a rotating joint.

  In the upward direction, from the radar base, towards the aerial on the transmitting antenna side, the analog signals modulated by the transmitting frequency Fe, by the local oscillators OL1 and OL2 and by the digital control signals Snc are carried respectively by the lengths

of B waves, X2, X3 and X4.

  In the downward direction, from the air to the base, the digital signals Sn, and Sn2 (limited to 2 in this schematic diagram), are carried

  by the wavelengths X5 and X6 respectively.

  These wavelengths are optically multiplexed / demultiplexed thanks to the components (Mx / Dx), and (Mx / Dx) 2, located respectively in the base and in the aerial so as to propagate between these last two components in a fiber optical and to cross

  simultaneously the JT single-channel optical rotating joint.

  The wavelengths Xi come from light emitters EXi and

  detected by RXi photoreceptors.

  Figure 5 shows the insertion of the test device according to the invention

in the architecture of Figure 4.

  The emission wavelength k7 of the test transmitter E is chosen to be distinct from the wavelengths Xi to Xe so that it can be multiplexed / demultiplexed by the components (Mx / Dx) 1 and (Mx / Dx) 2 which will have in this case an additional channel. The branch from (Mx / Dx) 2, corresponding to the wavelength?, Is terminated by the element

reflecting Fr.

  The test device therefore supervises here the optical transmission chain between the base and the aerial of the radar including the main passive components: - the multiplexers / demultiplexers (Mx / Dx), and (Mx / Dx) 2, - the C1 connectors and C2, - the JT rotary joint,

  - the main transmission optical fibers F1, F2.

  If the equipment requires it, it is possible, thanks to the test device, to keep constant the amplitudes of the analog signals sent to the air regardless of the attenuation fluctuations of the

transmission line.

  The principle is shown diagrammatically in FIG. 6. The signal Sd coming from the test receiver R is sent to the control circuits CC ,, CC2 and CC3 of the analog transmitters EX1, EX2 and EX3 in order to control the amplitude of the control signals of these at the amplitude variation of Sd thus ensuring the levels detected by the photodetectors Rkl, RX2, RX3 independent of the attenuation fluctuations of the main transmission line. Passive components Optical fiber is of the single mode type, commonly used today in optical telecommunications. Linear losses are

  less than 1 dB / km in cable version suitable for radar applications.

  Cl and C2 singlemode connectors are available in many versions and ensure losses for all models

less than 0.5 dB.

  The JT rotary joint will generally consist of two monomode priming fibers provided with collimating lenses, so as to parallelize and widen the beam, and precision mechanical elements allowing rotation between the two lenses. The fiber-to-fiber insertion losses are around 3 dB and the attenuation fluctuation in rotation is

less than 1 dB.

  The 1x2 C coupler will either be of the "all fiber" type or of the planar "integrated optics" type provided with priming fibers. This coupler is symmetrical (50/50 separation) and also has an insertion loss

lo less than 1 dB.

  The reflecting element Fr can adopt several technologies: reflecting treatment on the face of the fiber (negligible loss of insertion), - Bragg grating inscribed in the fiber, adapted to the wavelength. 7 of the transmitter of the optical test head (negligible loss of insertion),

  - looped coupler 1x2 symmetrical, identical to C (loss "go-

  8 dB return ": 2x3 dB separation + 2x1 dB

insertion loss).

  The CO switch can be, for example, optomechanical; the insertion loss is then typically 0.5 dB for a time of

  25 ms switching and .0015 dB repeatability.

  Multiplexers / demultiplexers (Mx / Dx), and (Mx / Dx) 2 are currently being developed to multiply digital bit rates in optical telecommunications arteries. The separation between channels in the 1.5 pm transmission window is normalized to multiples of 0.8 nm (100 GHz). In the example shown in Figure 5, we can use two optical network multiplexers with a separation between channels of 1.6 nm. The insertion losses per component are of the order of 4 dB and

The insulation is> 30 dB.

  Active components Laser diodes will preferably be used as emitters

  GainAsP Distributed Network (DFB) transmitting in the 1.5 pm window.

  The digital channels will use direct modulation (modulation of the control current by the signal to be transmitted) for bit rates up to 2.5 Gb / s with coupled transmitted powers of the order of mW. Depending on the frequency (S, C, X, Ku band, etc.) and the quality of the signals to be transmitted, the analog channels will use either direct modulation or external modulation. In the latter case, the transmitter will consist of a DFB laser diode emitting a continuous signal of a few tens of mW at the desired wavelength, followed by an external modulator

(LiNbO3 or GaAs).

  Finally, the photodetectors are GainAs photodiodes

  o10 adapted to the frequencies to be detected.

  In the case of its application in rotary radars, the invention has the advantage of not requiring any active component in the air and the test is therefore possible in the absence of power to the air

  (checks before start-up).

  In addition, the test takes into account

  optical multiplexers / demultiplexers, and the device can self-calibrate.

  The device measures in real time the fluctuation of the

transmission with rotation.

  The signal from the optical test head can be used to

  slave the amplitude of analog optical transmitters.

  This same signal can be used to monitor the

  transmission line over time.

  In the foregoing, the invention has been described using optical components and devices. However, the invention is applicable in the case of microwave transmissions. The transmitters and receivers transmit and receive in RF, the transmission circuits, reflection and

  rotating joint transmit RF waves.

Claims (8)

  1. Device for testing and regulating a transmission circuit comprising at least one transmitter and / or receiver of information, at least one controlled circuit, and a rotary joint (JT) with n inputs and m outputs connecting the circuit d emission connected to one of the n inputs to the controlled circuit connected to one of the m outputs, characterized in that it also comprises: - a test transmitter (E) connected to one of the n inputs of the rotary joint (JT); - a first reflection (Fr) or retro-reflection device connected to one of the m outputs of the rotary joint; - a test receiver (R) connected to one of the n inputs of the rotary joint. 2. Device according to claim 1, characterized in that said transmission circuit is an optical transmission circuit, said transmitter and / or receiver operates in optics, the rotary joint is a rotary joint optical.   3. Device according to claim 2, characterized in that the test transmitter (E) and the test receiver (R) are connected to the same   entry of the rotating optical joint (JT).   4. Device according to claim 3, characterized in that it comprises an optical switch (CO) located between the test transmitter (E) and the test receiver (R) on the one hand and the coupler on the other hand and allowing to connect the test transmitter and the test receiver either to the rotating joint   (JT), or to a second reflection device (F'r).   5. Device according to claim 3, characterized in that it comprises an optical coupler (C) with 2 inputs-1 output, the test transmitter (E) and the test receiver (R) being each connected to an input and the exit   being connected to the optical rotating joint (JT).   6. Device according to claim 2, characterized in that it comprises a detection circuit (CT) connected to the test receiver (R) receiving the signals received by the receiver and detecting in the variations   of these signals when the optical rotating joint (JT) is rotating.   7. Device according to claim 2, characterized in that each transmitter transmits a particular wavelength or range of wavelengths and each receiver is capable of receiving a particular wavelength or range of wavelengths and in that it comprises a first multiplexer / demultiplexer ((Mx / Dx),) making it possible to connect the transmitters / receivers to the rotary joint (JT), as well as a second multiplexer / demultiplexer ((Mx / Dx) 2) allowing connecting the rotary joint (JT) to the controlled circuits, the first reflection device (Fr) being connected to the rotary joint by the second multiplexer / demultiplexer. 8. Device according to claim 7, characterized in that it comprises an optical switch (CO) located between the test transmitter (E) and the test receiver (R) on the one hand and the first multiplexer / demultiplexer d on the other hand and allowing to connect the test transmitter and the test receiver either to the first multiplexer / demultiplexer or to the second device of reflection (F'r).