EP2038678A1

EP2038678A1 - Method and system for a passive optical pulse multiplier for producing a pulse sequence for a scanning laser distance meter

Info

Publication number: EP2038678A1
Application number: EP05802564A
Authority: EP
Inventors: Uwe Illmann
Original assignee: Callidus Precision Systems GmbH
Current assignee: Callidus Precision Systems GmbH
Priority date: 2004-10-15
Filing date: 2005-09-28
Publication date: 2009-03-25
Also published as: DE102004051147A1; WO2006042496A1

Abstract

The invention relates to a method and a system for a passive optical pulse multiplier for producing a pulse sequence for a laser distance meter. A pulsed laser, especially a microchip laser (1), provides pulsed laser radiation that is modulated via a plurality of beam splitters T1 (3) or beam splitters/beam combiners T2 (4) and T3 (7) and by using differently long optical paths S1... S5 in such a manner as to convert the initial pulse (6) into, e.g., a pulse group. (5). According to the invention, the different optical paths that inevitably have to be produced can be realized by differentiated windings on fiber coils, thereby allowing to adjust the interpulse period within one pulse group very precisely and to a low value. The frequency of the pulse groups (5) remains unchanged in relation to the initial pulse (6). The inventive system requires only few technical elements that can be mounted one behind the other, such as, e.g., a pulsed laser, especially a microchip laser (1), the beam splitters T1 (3) and T3 (7) and a beam combiner T2 (4) and fiber optic cables/optical fibers (2) that are wound onto corresponding fiber coils. The invention can be used for optical distance meters and in the field of communications engineering.

Description

Method and device for a passive optical pulse multiplier for Er¬ generation of a pulse train for a scanning laser rangefinder

The invention relates to a method and a device for an optical pulse multiplier for generating a pulse train for a scanning laser rangefinder.

From the specialist literature, the following possibilities of generating pulse sequences on the basis of laser measuring optical measuring instruments are known:

Diode lasers or Q-switched solid-state lasers themselves provide pulse sequences of defined frequency. In this case, the pulses of such a pulsed laser (pw laser) with respect to the pulse shape by active or passive optical elements ver¬ changes. Such elements are z. As fiber amplifier or saturable Absor¬ ber.

- In a continuously radiating - z. As cw laser - an optical switch inte¬ grated, such as a mechanical chutter or an electro-optical modulator.

If high pulse repetition frequencies are to be realized with a small space requirement of the laser apparatus and at low operating costs, pulsed diode lasers are usually used. However, if a pulse power in the range of several watts is required at the same time, usually only pulse repetition frequencies in the kilohertz range can be realized with the semiconductor lasers used. To generate pulse repetition frequencies in the megahertz to gigahertz range according to known prior art z. At present it is possible to use a fast-pulsed laser of low power and then to boost the pulses, for B. by means of fiber amplifier.

In known measuring devices, for example for land surveying, electro-optical elements are predominantly used. For devices of high accuracy, modulated laser beams are used to measure the objects. In the case of the phase difference method used, a laser beam is modulated in its intensity. To ensure the uniqueness of the measurement, the modulation takes place with different frequencies. In order to determine the distance, the laser beam is directed at the object to be measured and the radiation reflected or backscattered by the object is received by the receiver of the measuring device. From the phase senverschiebung between the modulation signal of the emitted laser radiation and the received signal can be¬ calculate with the help of the speed of light, the object distance be¬.

For the distance measurement to be accurate, the modulation frequency must be very stable, among other things, since it has a direct effect on the measurement result. If the short-term stability can not be guaranteed, there is also the possibility of averaging or filtering the signals, which, however, requires a longer measuring time. This is usually not a problem with simple distance measurement, because only one point is measured and even a measurement time of e.g. one second is acceptable.

However, if it is a scanning measuring system, the possibility is limited. Therefore, it is all the more important that the emitted laser beam is modulated with a stable frequency, pulse shape and intensity.

If, as with this type of portable measuring devices, costs, space requirements, mass and energy requirement play an important role in the selection of the beam source, then almost exclusively semiconductor lasers or diode-pumped solid-state perlaser or microchip lasers can be used in the higher price segment.

Known methods for generating a stable modulated signal from a continuous radiating laser (cw laser) and a downstream electro-optical modulator, as it is usually used in telecommunication technology at transmission distances with data rates from 10 Gbit / s, are very costly.

As can be seen from the above-described prior art, no solutions are known with the help of which, with little technical effort, pulse sequences z. B. for Laser¬ measuring devices can be provided for which a high frequency of pulse trains and a high pulse power are required.

The invention is therefore based on the object to search for a method and a suitable device for a passive optical pulse multiplier, which under the conditions of a demanding stable frequency of pulse trains up to 2500 MHz and an achievable average power up to 100 mW a simple mechanical , With can be designed with little technical effort device that is suitable for retrofitting already delivered equipment technology or from the outset in laser to be manufactured measuring devices.

According to the invention, the object is achieved as follows, reference being made to the basic inventive idea to claim 1 and 5.

The further embodiment of the invention can be found in claims 2 to 5 and 6 to 8.

For the solution according to the invention further explanations should follow.

According to the method, a pulsed laser, in particular a microchip laser as the radiation source, which can deliver a pulse frequency of only a few kilohertz, but has a high pulse power. However, these different conditions are not sufficient for rapid measurement by means of the phase difference method.

Therefore, in the beam path of the laser beam, which is already present as a pulsed, in one or more numbers, 50% beam splitter and 50% beam unifying are given. These optically redirect the incoming laser pulse and split it into 2 beams with the same pulses, but always with the following half pulse power. The pulses which are expelled from the first beam splitter then subsequently run through different optical path lengths in order to be brought together again with a time offset into a beam combiner following in the beam path.

The latter beam combiner behaves analogously, but in inverse proportion, like a 50% beam splitter.

As a result of the optical path length difference generated as described above, 2 pulse groups then produce 25% power at the same frequency, measured at the original pulse, and at the same amplitude. This can in turn be followed by a beam splitter, in which case the pulse groups reaching it again pass through different optical path lengths. The laser beams emerging from the latter have a further doubled pulse sequence, with further decreasing intensity of the power. This can be recognize that the pulse spacing can be influenced within a pulse group only over the optical path length difference of the two beam paths. The pulse spacing can be set very small and precise, whereby a high stable repetition frequency with almost identi¬'s pulses (frequency, amplitude, phase) is achieved.

The sequence of the division and combination of a pulse and pulse groups resulting therefrom can be continued until a point at which a desired pulse sequence is reached, taking into account a beam path which is always of different path length difference and has the same pulse groups.

The device to be designed for carrying out the method of a passive optical pulse multiplier uses the aforementioned pulsed laser, in particular the microchip laser for generating a laser beam. In its beam path, 50% beam splitters with optical deflection function (by 90 °) to be designated as such are arranged below, which act alternately as beam splitters and beam combiners. The optical path length differences to be realized as a fundamental method condition after a beam splitter before entering a beam combiner can be produced via differently long windings of optical fibers / cables used.

The invention will be explained in more detail with reference to an embodiment. It should be used on subsequent figures.

Figure 1: Schematic representation of the procedure

FIG. 2: Comparison of the original laser pulse for the first beam combination as a function of I = f (t)

The reference signs used are:

1 - Pulsed laser, in particular Microchiplaser I = intensity

2 - fiber optic cable t p = pulse width

3 - beam splitter T ₁ S ₁ ... S ₅ = optical path 4 - beam combiner / beam splitter T ₂ C _M = speed of light in the medium

5 - pulse groups T ₁ = period of the pulse repetition frequency

6 - pulse f = pulse repetition frequency

7 - beam combiner / beam splitter T ₃

A pulsed laser, in particular a microchip laser 1 with an output of, for example, 100 mW, emits a pulsed laser beam with the frequency of the pulse 6, this being a defined optical path by means of z. B. a / a light guide / fiber 2 durch¬ runs. This pulse 6 enters a first beam splitter Ti ₁ 3, as. optical element can be a beam splitter cube and fulfills a deflection function of 90 °.

The laser pulses emerging therefrom now have half the power of the original pulse and are forcedly delayed in time - through the different optical path lengths S ₄ and s ₃ - to the following beam combiner / beam splitter T ₂ 4. For this combination, instead of the original pulse, pulse groups result with a further halving of the original laser power. They are again passed over the different lengths optical paths S ₄ and S ₅ in a further beam combiner / beam splitter T ₃ 7, after which a further doubling of the pulse groups with the same frequency, measured at the original pulse, is achieved.

The division and combination can be continued by the juxtaposition of beam splitters T ₁ 3 and beam combiner / beam splitters T ₂ 4 and T ₃ 7 until the desired, stable producible sequence of pulse groups in the frequency of the original pulse is reached.

The schematic sequence of the method is characterized according to FIG. 1 and explained with the illustration of the physical / mathematical relationships according to FIG.

Below is a concrete calculation example:

There is a pulsed laser, in particular a microchip laser 1 with a pulse power of 300 watts, a frequency of 20 kHz and a pulse width tp of 1 ns before. If pulse groups consisting of 4 individual pulses with a pulse repetition frequency of 500 MHz are to be generated, then the laser radiation is coupled into the optical fiber / guide cable 2 and traverses the path S ₁ to the first beam splitter T ₁ 3. Einkoppel¬ losses at this point still require an available pulse power of 160 watts. In this variant, a fiber-optic component is to be used as the beam splitter T ₁ 3, referred to as Y-coupler [modification of an X-coupler], which has a ratio of 1: 1. The partial beams emerging from it now have the pulse power of 80 watts and pass through the different distances s ₂ and S ₃ until they are combined in the beam combiner T ₂ 4. The latter also has the properties of Ti 3. So that a pulse repetition frequency f of 500 MHz can be achieved, the pulse along the path S ₃ must be delayed relative to S ₂ by the period of the pulse repetition frequency T i. It therefore carries T i = 1 / f = 1/500 MHz = 2x 10 ^'9 s = 2 ns.

At a speed of light in the fiber (CM) of approximately 2 × 10 m / s, the required path difference (delta s = S ₃ -S ₂ ) results for the required path difference: delta s = CM × Ti = 2 × 10 ⁸ m / sx 2 x 10 ^"9 s = 0.4 m.

If one chooses as a distance S ₂ = 100 mm, the required distance s ₃ for the second partial beam is: s ₃ = 0.1 m + 0.4 m = 0.5 m.

It follows that the optical fiber for the distance S ₂ = 100 mm and the other fiber for the distance s ₃ = 500 mm long must be.

Behind the beam combiner / beam splitter T ₂ 4 there are thus 2 partial beams, each with identical double pulses. However, the pulses only have half the power of 40 watts here.

In order for a pulse group 5 of 4 pulses to occur, the step of path length differentiation between S ₅ and s _{4 is again} carried out. However, since each sub-beam already contains double pulses here, the partial beam must be delayed by 2 periods on the s ₅ wer¬.

If a 100 mm long fiber is also used for S ₄ , a fiber length follows for the length S ₅ : S ₅ = s ₄ + c _M × 2 × Ti = S ₄ + 2 × delta s = 100 mm + 2 × 400 mm = 900 mm. The two partial beams pass through S ₄ and S ₅ in the beam combiner / beam splitter T ₃ 7. Da¬ then again arise two equal partial beams, which now have pulse groups of 4 Impulses sen, with a pulse power of 20 watts is realized.

To reduce the required fiber lengths, nesting of the pulse trains can also be used.

If one varies the path length difference delta s between the individual steps, which has a direct effect on the time interval of the pulses, then a coded pulse sequence can also be made available.

Finally, another special feature of the method lies in the fact that the pulse sequence consists of individual pulses that are coherent with one another. As a result, this method can also be used for the coherence radar or similar measuring methods which use the property of the coherence of laser radiation.

The advantage of the method is the knowledge that the pulse spacing within a pulse group can only be determined by an optical path length difference (s ₂ to S ₃ and S ₄ to S ₅ ) of the two beam paths and the distances can be set very small and accurate. Thus, a high and stable repetition frequency of desired pulse groups can be generated. The different optical paths are expediently realized by fiber spools.

As a beam splitter, a fiber / x coupler can be used as an alternative to the beam splitter cube.

With the method and a device for carrying out the method are extremely short Impulsfol¬ without auf¬ manoeuvrable, electronic components for the generation of pulse groups gen in the range of up to 10 ^{~ 15} s to produce.

The design of different optical path lengths si ... ^' s ₅ can also be achieved on atmospheric ways, without the use of optical fibers / fibers 2.

Claims

claims

1. A method for a passive optical pulse multiplier for generating a pulse train for a scanning laser rangefinder, characterized da¬ by a pulse (6) of a pulsed laser, in particular a Microchi¬ plasers (1) in a beam splitter T ₁ (3) by means of a Lichtleitkabels- / fiber (2) gelei¬ tet, divided there and subsequently on different lengths of optical paths S ₂ and S ₃ in a beam combiner / beam splitter T ₂ (4) is performed and outgoing from it ing pulse groups (5) over again different lengths optical paths S ₄ and S ₅ in the beam combiner / beam splitter T ₃ (7) get, after each beam association the frequency of the generatable pulse groups (5) in comparison ^' to the pulse (6) is maintained and their pulse train is increased.

2. A method for a passive optical pulse multiplier according to claim 1, characterized ge indicates that by means of different optical paths S ₁ ... S ₅ and the resulting path length differences, time delays of the pulses (6) and pulse groups (5) can be adjusted. ^■

3. A method for a passive optical pulse multiplier according to claim 1, characterized ge characterized in that a series circuit in alternation zwi¬ rule beam splitter Ti (3) and beam combiner / beam splitter T ₂ (4) and T ₃ (7) er¬ follows.

4. A method for a passive optical pulse multiplier according to claim 1, characterized ge characterized in that extremely short pulse sequences are generated up in the range of 10 ^" ¹⁵ s.

5. A method for a passive optical pulse multiplier according to claim 1, characterized ge indicates that the individual pulses within the Impuls¬ groups are mutually coherent.

6. Device for a passive optical pulse multiplier for generating a pulse train for a scanning laser rangefinder, characterized da¬ by that a pulsed laser beam via optical fiber / fibers (2) with the ab¬ alternating sequence successively arranged radiation dividers T ₁ (3) and T ₃ (7) and the Beamvereingern T ₂ (4) is in operative connection.

7. Device for a passive optical pulse multiplier claim 6, ge characterized in that the different long optical paths S ₂ and S ₃ or S ₄ and S _{5 are produced} by means of windings on fiber spools.

8. Use of the device of a passive optical pulse multiplier for generating a pulse train for devices of laser metrology or Nachricht¬ entechnik.