SE514276C2 - A pulsed coherent laser radar system - Google Patents

Abstract

The present invention relates to a pulsed coherent laser radar system comprising a single-frequency transmitter laser (1) which generates laser pulses. The laser pulses pass a beam splitter (2) which splits the energy of the pulses and conducts a first part to the output unit (12, 13) of the system for transmitting laser radar pulses. A second part is conducted to a first optical fibre (4) and further to an input of a first fibre-optic coupler (5), which connects part of the energy of each pulse to a fibre-optic ring, which comprises a second fibre-optic coupler (6). An output (A3) of the second coupler conducts part of the energy of each pulse remaining in the fibre-optic ring while a second output (A4) discharges pulses in a pulse train with a pulse frequency dependent on the time of a pulse passing round in the fibre-optic ring. The pulse train is combined in a detection device with return pulses from targets, frequency information about the return pulses being recovered.

Description

The invention solves these problems by giving it the design set forth in the appended independent claim. Suitable embodiments of the invention appear from other patent claims.

In the following, the invention will be described in more detail with reference to the accompanying drawing, where fi g. 1 shows a first embodiment of a CLR system with a professional pulse train generator according to the invention, a. 2 shows examples of generated pulse trains in the arm A4 in fi g. 1, the left diagrams showing individual Gaussian pulses and the right ones showing the sum of the pulses, which in this example generate a quasi-constant LO effect, fi g. 3 shows the equivalent as fi g. 2 for a fiber length that is three times longer, which results in a larger time separation between the pulses and fig. 4 shows a second embodiment of a CLR system with a professional pulse train generator according to the invention.

The basic idea of the invention is that a certain part of the pulsed transmitter radiation is deflected and directed to a opt professional ring, from which pulses are diverted turn after turn and form a pulse train. The reflected radiation from a certain distance can thereby be made to mix on the detector with a copy of itself. Signal processing then takes place in the normal way.

There is no specific LO laser and problems with frequency chirp and time coherence limitations can be eliminated or greatly reduced compared to the prior art. For applications involving hard targets, which som deny that the outer totalt section totally dominates the reflection of the target, frequency sharpness can be completely eliminated when the L length L is matched to the distance. When the distance is not matched, the frequency shear will give a signal width (spectral), albeit to a significantly lower extent compared to the prior art. For distributed targets, such as particulate matter in the atmosphere, there will be restrictions due to frequency chirp to remain to approximately the same extent as with prior art. However, problems with frequency instabilities in the LO laser disappear.

Figure 1 shows a CLR system with a professional pulse train generator according to an embodiment of the invention. A laser 1 emits laser radiation pulses. A beam splitter 2 divides the energy of the pulses and leads a first part to the output unit 12,13 of the system for transmitting laser radar pulses. The outdoor unit is of known type and may in the usual way comprise an M4 plate 12 and a beam expander 13.

The beam splitter 2 conducts a second part of the energy of the pulses into a first optical fiber 4, normally via a focusing lens 3. From the first optical beam, the radiation is directed to one input of a first optical switch 5 with two inputs and two outputs. This coupler connects, via its one output A1, a part of each pulse energy into a fi beroptical ring, which comprises a second fi beroptical coupler 6 of the same type as the first and a second optical fi ber L.

One output A3 of the second coupler conducts a portion of each pulse energy remaining in the fi beroptic ring while the other output A4 discharges pulses in a pulse train with a pulse frequency depending on the time for a pulse to pass around the fi beroptic ring. The pulse train is led to a detection device where it is combined with return pulses from the target.

According to fi gur 1 and 4, the detection device may comprise a third fi beroptical coupler 8, wherein the pulse train can be guided via a third optical fi ber 7 to one input of the third fi beroptiska coupler and return pulses from targets via a fourth optical fi ber 9 to a second input of the same coupler . In the example in the figure, the radiation is linked there by a beam splitter 11 and focused into the fourth optical beam by a lens 10. In the third coupler the two radiations are combined and the result is detected with detectors D1, D2 at one or both outputs of the third coupler 8.

A possible alternative to the third optical coupler 8 is discrete optics. After the ring, you can use a beam splitter that mixes LO radiation and target radiation. which in that case is not connected in fi ber.

The detector signal can generally be written as in, (i) = æ (P ,,, + Ph, + 2, / P ,,, P ,,, sin [(2 fl f ,,, (f) - 2fl fæ, (f)) i + 41.1), where SH is the sensitivity of the detector, Pm is the power of the local oscillator, Pm, is the received power, fw is the frequency of LO laser, f., Is the frequency of radiation reflected from the target and goa is a phase difference between LO radiation and target radiation. It appears from the formula that in the hard target application, any frequency dependencies on the time of the transmitter laser will be compensated by the fact that the frequency dependence of the LO laser is exactly the same.

The length of the optical beam in the optical ring, the second optical beam L, determines the time interval between the pulses, see Figure 1. L can be easily adjusted depending on the application and laser source in question. The degree of coupling of the couplers 5 and 6 determines the pulse effects. Furthermore, the pulse time, fiber length and the degree of coupling of the couplers determine how the power varies with time.

The output powers of the various fi arms A2 and A4 are given by N - n P ,, (f) = P ,,,, s (fr ,,) - c, .10-2 + c, * - 10 * Z / = ,, ,, s (if ° -n%) - (c, -1o- =) '- (c3-1o-2) n = 1 and -4 NL »-z" -2 "P ,,, (f) = c, -c, 10 ZPMS ff., - n¿) - (c, -1o) - (c3-1o), n = 0 where PA; and P44 is the effect in arrays A2 and A4, respectively, n is an integer indicating how many revolutions the radiation has passed through the ring and N indicates how many revolutions the radiation is allowed to be in the ring before it is dumped, which in Figure 4 is marked with D. See further below .

Figures 2 and 3 show examples of generated pulse trains in the arm A4 in Figure 1.

Gaussian pulses have been adopted here. However, the gain is not limited to Gaussian pulses, but other time forms are also possible.

The left diagrams show individual Gaussian pulses and the right sum of the pulses. In the example, the beam splitter 2 of 100 W splits the transmitter radiation and the couplers 5 and 6 are 99: 1 couplers.

By selecting 99: 1 coupler, where 99% of the radiation, which is conducted through the first optical fi bem 4 to the first optical coupler 5, is connected to the output A2 and 1% is connected via the output A1 into the fiber optic ring and of this radiation , to the second optical coupler 6, 99% is connected via A3 left in the fiber optic ring and only 1% is switched off via the output A4, a substantially constant level of the pulses in the pulse train in the third optical fi bem 7. This is done by, at each passage of an optical coupler in the fi professional ring, the main part, 99%, remains in the ring and only 1% is switched off as pulses in the pulse trains in arms A2 and A4. If the pulses in the pulse trains are sufficiently dense, as in Figure 2, a quasi-constant LO effect is generated.

The third optical coupler 8 may in many contexts suitably be a 50:50 coupler which mixes the radiation from the two inputs in equal proportions which are presented with equal strength on the two outputs. In other cases, other proportions may be preferable, for example 80:20.

For efficient utilization of available laser power, it is advisable to use single modes for the current wavelength. If multimode is used, there is a risk that the radiation is distributed between different modes, which can lead to signal loss. Furthermore, control over the polarization is important. A transmitter / receiver switch included in the system is often based on polarization. In Figures 1 and 4, the transmitter / receiver switch consists of a polarization-dependent beam splitter 11 and an H4 plate 12. This configuration results in only a linear component of received radiation being conducted to the detector. The LO radiation must then have the same polarization state for maximum signal. The polarization can be controlled in various known ways. You can, of course, use polarization-preserving fl bridges. However, you can also use a cheaper fi ber and closely monitor fi bems "winding" in the system. Finally, you can use a polarization modulator together with a cheaper fi ber. The polarization modulator can be both a manual mechanical modulator and an electrically controlled one.

It is ideal to use rectangular pulses in time, as any time overlap between the LO pulses can give rise to interference signals at specific frequencies.

A number of different variants of the invention are conceivable, some of which are shown in Figure 4. To determine whether the target is moving towards or away from the CLR radam, an acousto-optical modulator or other component for frequency shifting the radiation can be connected in connection. to the first 4, third 7 or fourth 9 optical fi bem.

Furthermore, a opt professional switch S can be placed in the ringen ring to interrupt the circulation of radiation in the fi professional ring after a certain time. 10 514 276 6 You can also use switches S to connect different lengths of other optical optical L, L .. in parallel in the professional ring. With the switches, you can easily select different ring extensions, which gives an easy choice of the pulse repetition frequency in the pulse trains.

Finally, you can connect fl your fi professional rings of the specified type one after the other. For example, if another ring is connected to another A2, another LO is obtained with almost the same power / time dependence as the first. This can be useful in applications where two measuring directions are interesting and when using fixed optics (not scanning). If mä your measurement directions are interesting, you can achieve as many LO as measurement directions by connecting additional fi professional rings one after the other.

Claims (9)

10 15 20 25 30 35 514 276 7 Patent claims: A pulsed coherent laser radar system, comprising a single frequency transmitting dases (1) emitting laser pulses, a first beam splitter (2) which divides the energy of the pulses and conducts a first part to an output unit (12,13) for the system arranged to transmit laser radar pulses and a second part to a storage device arranged to emit a pulse train based on the energy of the second part, a second beam splitter (11) which conducts a part of the energy of the return pulses from targets entering via the output unit to a detection device, the return pulses being combined with the pulse train. if the return pulses are recovered, characterized in that the storage device comprises a first professional ring with a first (5) and a second (6) professional coupler and a second optical fiber (L, L1), that the laser pulses from the first beam splitter are guided via a first optical fiber (4) to a first input of the first fi professional coupler, that the first fi professional coupler, via an output (A1), connects a part of each pulse energy into the first fi professional ring, that a first output (A3) of the second fi professional switch conducts a part of each pulse energy through the first fi professional ring, that a second output (A4) of the second switch switches out pulses in said pulse train with a pulse frequency depending on the time of a pulse to pass around in the first fi professional ring, that the detection device comprises a third fi professional coupler (8) with detectors (D1, D2) at one or fl era of its outputs that the pulse train is led via a third optical fiber (7) to a first input of the third fi professional coupler and return pulses from targets are conducted via a fourth optical fi ber (9) to a second input of the same coupler, A system according to claim 1, characterized in that the first and second fi professional connectors have two inputs and two outputs and have a ratio of the order of 99: 1 between what it connects to the two outputs and that the first coupler (5) engages about 1% into the first fiber optic ring and the second coupler (6) engages about 1% of the fi professional optical ring. A system according to any one of claims 1-2, characterized in that the optical fibers are polarization preserving. Ll lllllli .liiiii 10 15 20 25 '514' 2'7'6 '8 A system according to any one of claims 1-2, characterized in that the optical beams are wound so that one has control over the direction of polarization. A system according to any one of claims 1-2, characterized in that a polarization modulator is connected to the optical beams so that one has control over the polarization direction. A system according to any one of the preceding claims, characterized in that it comprises, in connection with the first (4), third (7) or fourth (9) optical beam, a component for frequency shifting the radiation, for example a acousto-optical modulator, which component is used when determining the sign of a possible Doppler shift. A system according to any one of claims 1-6, characterized in that it comprises a fi professional switch (S) in the first fi professional ring which interrupts the circulation of the radiation in the ring after a certain time. A system according to any one of claims 1-6, characterized in that at least two different lengths of second optical fibers (L, L ,,) via switches (S) are connected in parallel in the first fiber optic ring and constitute alternative paths with different time delay. A system according to any one of the preceding claims, characterized in that at least one further fi beroptic coupler is connected to an output leading from the first fi beroptic coupler (5) which does not lead to the first fiber optic ring and that each said at least one further fiber optic coupler couples radiation to an fi professional optical ring, from which a pulse train is led to an input of the detection device in the same manner as from the first fiber optic ring.