SE469452B - FIBER OPTICAL SYSTEM FOR DISPOSAL OF RADAR ACCOUNT - Google Patents

Description

469 452 2 6. The system dynamics will be insufficient if the input switches have high losses. There are mechanical switches (BT&D and JDC, etc.) with only 1 dB losses and <15 ms in switching time for monomod fibers. In the present application, mono mode fibers must be used to obtain the least possible dispersion. The optical isolation of the switches is between 50 and 60 dB.

There are also LiNb03 switches (Ericsson, Pilkington, Crystal Technology, etc.) with 4 to 8 dB losses in the transmission from fiber to fiber. They are much faster and have switching times of <10 ns and optical isolation of 25 to 30 dB. They are commercially available in the 1: 2, 2: 2 and 4: 4 configurations. On request, even more complicated designs can be obtained.

For the actual simulation of aircraft movements, even very extreme ones, slow (mechanical) switches can be used. Requirements for extremely fast switching (tsg 1 us) exist only so that virtually all radar pulses can be captured by the system. The highest possible pulse repetition frequency (PRF) for a radar of the current type is 100 kHz.

The inventive solution to the present problem with the need for low attenuation and fast switching involves using a system which comprises immediate or very fast connection of signals to two or more parallel delay lines, each comprising slow mechanical switches with low attenuation.

The fiber optic delay system works by coupling optical fibers of suitable lengths into such a combination that the desired total length is obtained. The number of switches in series (ie switch depth) can be selected by selecting the switch width, ie by selecting how many switch-off options can be selected for each switch-on option.

The optimal choice depends partly on the size of the losses at each switch and partly on the technical difficulty of manufacturing switches with many outputs. In a system where you have low attenuation in each switch, it is advantageous to use the switch depth.

BNSDOCID: 4-69 4522 It can be mathematically shown that a cascade of fibers interconnected by switches with as many inputs and outputs, g, will function as a mathematical position system. In such, taiet p is written as a polynomial in base g, _ .ij ~ 1 NJ 1 p - rj-g + rj_1-g ... and j are the assumed positions. A common position system is the binary where g = 2 and the only digits are 0 and 1. The use of such systems is usually called binary. In the decimal system, as is well known, the base = 10 and the numbers 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. The transition from one position system to another can take place with: r> = 9-q + r The equation is used in a successive heitais division where taïet p is divided by the base g and gives a quotient q and a remainder r. The quotient q is replaced in the next iteration whereby a new quotient and a new residue are obtained11. When q siutiigen biivit iika with noii, the antaïet expresses iterations, j, how many positions are needed to extract the number in the new position system. The remnants, r, taiar about viika figures included in the taiet.

The method can also be used to calculate how many switches are needed to, with the aid of fiber lengths, increasing with a given increment zxi, and with the aid of switches, with the same number of inputs and outputs, g, achieve an optical signal delay with a certain maximum fiber length. The fiber lengths ir inserted into the system have different lengths, which increase in equidistant steps of 131 in a geometric series. The total interconnected length 1 is the sum of the connected lengths with j switches, J li .ii 1 = Åïl ii = 22. lå 1-r¿'g = zli ¿2_ ri-g i = oi = oi = o BNSDOCID; eee 452 4 The number of switches required is calculated for a system with ¿xi = 10 m and 5 m, respectively, where the largest total connected fiber length is set to 2000 m. The table below indicates the number of switches required for different switch widths, gzg.

¿Si = 10 m switch width 2 3 4 6 12 number of switches 8 5 4 3 2 zíi = 5 m switch width 2 3 4 5 20 number of switches 9 5 4 3 2 In the current application for distance hooking of radar echoes, use equipment which includes a receiver of the radar signal.

The radar signal received in this way is converted into an optical signal, which takes place in the first means 1. Before one after said first means there is one or more second means Za, 2b for immediate transmission or very fast coupling of the signal to two or more parts. covers including delay leads. These delay wires consist of monomode fibers 5 in ascending lengths and electromechanically controlled switches 6, designed with, in the normal way for these, very low optical attenuation. Furthermore, there are third means 3 for interconnecting the outputs of the delay conductors. Before one after said third means there is one fourth fourth means 4 which converts the optical radiation into a radio frequency signal. In addition, there is a transmitter for this signal as well as control electronics which control the various switches so that, in a known manner, incoming successive radar signals from the same channel are copied through the optical fibers so that they are successively delayed aiit more before they are retransmitted.

In certain embodiments of the invention, see Figures 2 and 3, the second member 2b is a straw divider, positioned after said first member 1.

In other embodiments of the invention, see Figure 1, the second means 2a is a fast electronic switch, located before said first BNSDOCID: 1 'means 466. In this embodiment the third means 3 may be a beam combiner , placed before said fourth means 4. The third means 3 may instead be a fast electronic switch, placed after said fourth means 4. This solution, which is shown in all the figures, is also suitable in the previously mentioned embodiment according to figures 2 and 3 .

The first means 1, which converts the radio frequency signal into an optical signal, comprises a laser. Either a laser which is directly modulated by the radar signal or a laser with a subsequent electro-optical modulator which is controlled by the radar signal is conceivable.

Figure 1 shows an embodiment of the invention with eight optical 2: 2 switches 6 of electromechanical type and optical fibers 5 in ascending lengths. To compensate for the slow update time, several parallel delay links are used. These are quickly connected to the microwave side with an electronic switch 2a at the desired time. When the connections are complete, all incoming signals can be delayed with the set delays, regardless of the waveform. Meanwhile, the next desired delay combination is connected to other delay links. In this way, multiple delays can be achieved. With this configuration, it is possible to quickly switch between several delay lines and in this way achieve delays of radar echoes simultaneously and independently of each other.

Figure 2 shows a cheaper version where only one laser 1 is used and the optical signal is distributed in three delay links simultaneously with an optical beam splitter Zb. This results in 5 dB greater optical loss and thus less dynamics.

The configuration in Figure 3 means a further simplification and only two delay links are used. The optical beam splitter 2b, which divides the radiation into two equally strong beams, provides 3 dB in optical loss.

This embodiment does not allow for redundancy nor for parallel delay, which is why it should often be advantageous instead to choose a solution with three parallel delay links. eNspocio;

Claims (8)

'\ f fi i-ašl 452 é f' Patent claim: A fiber optic remote radar echo tracking system, characterized in that it comprises a receiver for a radar signal, first means (1) converting received radar echoes into optical form, before or after said first means second means (2a, 2b) for coupling of the signal to two or more parallel delay links, said two or more parallel delay links, which consist of delay lines in the form of monomode fibers (5) in ascending lengths and switches (6) which can connect any combination of the delay lines, third means ( 3) for interconnecting the outputs of the delay links, before or after said third means fourth means (4) which converts the optical radiation into radio frequency form, a transmitter for delayed radar echoes and control electronics which control used switches (2a, 3.6) so that incoming successive radar echoes from the same source are directed to the transmitter via different delay links and within each link via the optional k the combination of delay lines, wherein said second and third means (2a, 2b, 3) are selected on the basis that they should be able to connect successive radar echoes of the highest possible pulse repetition frequency for current radar stations to different delay links, preferably fast electronic switches on the radio frequency side or a beam splitter beam combiner on the optical side, and as said switches (6) in each delay link are selected electromagnetically controlled switches, on the premise that they should have very low optical attenuation and a switching frequency sufficient to be able to change the combination of connected delay lines during the time the current delay link is inactive. A fiber optic system according to claim 1, characterized in that the second member (Zb) is a beam splitter, located after said first member (1). A fiber optic system according to claim 1, characterized in that the second means (2a) is a fast electronic switch, placed before said first means (1). A fiber optic system according to claim 3, characterized in that the third means is a beam combiner, located before said fourth means. A fiber optic system according to claim 2 or 3, characterized in that the third means (3) is a fast electronic switch, located after said fourth means (4). A fiber optic system according to any one of claims 1-5, characterized in that the first means (1) is a laser which is directly modulated by the radar signal. A fiber optic system according to any one of claims 1-5, characterized in that the first means (1) consists of an unmodulated laser and a subsequent electro-optical modulator, controlled by the radar signal. A fiber optic system according to any one of claims 1-7, characterized in that the number of widgets is three. BNSDOCIDI