DEVICE FOR CASCADING OPTICAL AMPLIFIERS

Abstract: Generation of optical pulses having very high peak power. In many applications it is desirable to be able to use pulses having a very high peak level (> 1 kW). A simple method to generate them would be to simply connect optical amplifiers in cascade. However, it can not be made, since each amplifier generates noise, which together with possible signal residues between the pulses can saturate successive amplifiers. This problem has been solved by arranging an element having a non linear transmission between the amplifiers. This element can be constituted of a non linear loop mirror, a saturable absorber or a semiconductor laser amplifier. The loop mirror has today the best performance and can be implemented passively and at a low cost.

Description: Device for cascading optical amplifiers . The present application is intended to generate optical pulses having a very high peak power. The method is simple and non costly and in our implementation passive and independent of pulse repetition frequency, pulse width, wave length, polarization, etc. Background Many optical applications require the availability of short optical pulses having very high peak powers (> 1 kW), e.g. optical distance meters and optical radar systems, so called LIDAR. Today the pulses are most often generated in large complicated lasers making the systems space consuming and costly. In addition, it is not possible to obtain, at a reasonable cost, pulses having wave lengths that are safe to the eyes. A very attractive solution of the above problem would be to have the capability of using modem semiconductor lasers which are small and cheap and can produce light within a wavelength range that is less dangerous to the eyes. However, they cannot produce the pulse peak powers which are required in many applications. By optically amplifying the pulses the peak power can be increased but optical amplifiers generate noise, so called amplified spontaneous emission, what makes it impossible to provide a direct cascaded connection of a plurality of amplifiers which is required for achieving very high peak power. All optical power, which does not originate from the pulses, such as e.g. noise and possible other non desired signal residues, will saturate an amplifier, if it constitutes a sufficiently large fraction of the total power, what results in that the pulses will not be any more amplified. The idea described hereinafter is to only transmit, by means of an element having a non linear optical transmission, only the pulses and then to amplify these again. Important characteristics A pulse source generates optical pulses having a long repetition time in relation to the width of the pulses. They are amplified as much as possible in an optical amplifier, which adds noise between the pulses. In order to be able to successfully amplify the pulses more, all power existing between the pulses must be removed. It is made by using an element having a non linear transmission; components having a low power are not transmitted whereas components having a high power are transmitted. The transmission characteristics of such a non linear element is shown in Fig. 1. After the non linear element a signal is obtained, which only contains the desired pulses and which can be further amplified. The configuration is schematically illustrated in Fig. 2. If the desired peak power level of the pulses has not been obtained in spite of further amplification, the process can be repeated. Important characteristics of the non linear element is that broad band noise is to be processed linearly, i.e. it is not to be transmitted, and that weak coherent signals are to be strongly suppressed. Description of the drawings Figure 1 shows the transmission characteristics of the non linear element. Figure 2 shows fhe block schematic of the actual method of generating optical high power pulses. The pulse source (1) can advantageously be a semiconductor laser which is either pulsed in an electrical way or produces or provides constant light which is then modulated externally. Often weak residual light is obtained between the pulses which can saturate successive amplifiers. The optical amplifiers (2) can be semiconductor laser amplifiers or fibre amplifiers. All optical amplifiers generate broad band optical noise which can also saturate a successive amplifier. The non linear element (3) can advantageously be the non linear loop mirror described hereinafter or e.g. a non linear absorber. Fig. 3 shows schematically the non linear loop mirror in the case where it is used as the actual non linear element. Implementation example We have demonstrated the idea described above experimentally, the non linear element being constituted by the so called non linear loop mirror, originally presented by Doran and Wood 1988 . We have also previously demonstrated how it can be used for improving the signal quality of a digital optical pulse train in a telecommunication system 2 ' . However, in principle an arbitrary optical element having non linear transmission can be used. The non linear loop mirror (see Figure 3) consists in the present case of a Sagnac interferometer, here fibre based (8), in which an asymmetrically placed amplifier or attenuator (4). a non reciprocal phase shifter (5) or a polarization controller and an optical non linear material (6), e.g. fibre, are introduced. The coupler (7), which separates the input and output signals, is to split incoming light equally between the two output ports. The signal in the direction around the loop, which has the highest peak power, obtains a larger non linear phase shift, owing to the fact that the refractive index is dependent on intensity in the optically non linear material (6), than the signai which propagates in the opposite direction. When the difference in the non linear phase shift between the two counter propagating signals is equal to 180°, the transmission is changed from minimum to a maximum, provided that the phase shifter (5) in the loop is correctly set. The interferometer now constitutes an element having transmission characteristics according to Figure 1. Description of patent application parts The patent application parts are shown schematically in appendix B. It is desired to obtain a patent of the basic concept comprising an arbitrary element (3) having a non linear optical transmission which suppresses optical noise and light having low power. The element is located in a chain of optical amplifiers (2) for generating pulses having a high peak level. We see three possible implementations of this element, either by means of a saturable absorber 4 , semiconductor laser amplifiers or the non linear loop mirror as described above. The saturable absorbers existing today have the disadvantage that the ratio of maximum and minimum transmission is not particularly large and a semiconductor laser amplifier is active and generates noise, so that the non linear loop mirror is probably to be preferred. The non linear loop mirror can be implemented in a multitude of ways. Usually it is completely fibre based (8), having a coupler (7), an attenuator/amplifier (4) and a phase shifter/polarization controller (5) in the shape of a discrete components. The optically non linear material (6) is usually constituted of an optical fibre but it can in principle be constituted of an arbitrary material having optical non linear characteristics (see the definition below). This design can also be implemented by means of wave guides, etched in a substrate having all components integrated in the same substrate or as a hybrid design. The third possibility is of course to use open radiation paths and discrete components. The discrete components in the system are constituted of: the coupler (1) - splits incident light equally between two outputs the attenuator/ amplifier (A) - accomplishes that the two oppositely travelling signals in the loop will have different intensities the phase shifter/polarization controller (5) - allows an adjustment of interference conditions of the coupler optically non linear material (6) - material providing a refractive index/absorption dependent on power With the term "optically non linear" is to be understood that the dielectric constant of the medium depends on the strengtn of the optical field. Appendices A. Figures 1-3 B. Schematic description of the parts of the patent application C. Technical report which is to be published later References [1] NJ. Doran and D. Wood, "Nonlinear-optical loop mirror", Opt. Lett., Vol. 13, pp. 56-58, 1988. [2] B.E. Olsson and P.A. Andrekson, "Extinction Ratio Improvement Using the Nonlinear Optical Loop Mirror", IEEE Photon. Tech. Lett., Vol. 7, pp. 120- 122, 1995. [3] B.E. Olsson and P.A. Andrekson, "Noise Filtering with the Nonlinear Optical Loop Mirror", J. of Lightwawe Technol., Vol. 13, pp. 213-215, 1995. [5] A. Yariv, "Optical electronics", HRW, 1985.

Claims: Claims 1. A device for cascading optical amplifiers intended to amplify optical pulses, characterized in that an element having non linear optical transmission is arranged between the optical amplifiers in such a way that optical pulses are transmitted whereas light having a low intensity is strongly attenuated. 2. A device according to claim 1, characterized in that the element having a non linear optical transmission is constituted of a saturable absorber. 3. A device according to claim 1, characterized in that the element having a non linear optical transmission is constituted of a semiconductor laser amplifier. 4. A device according to claim 1, characterized in that the element having a non linear optical transmission is constituted of a non linear loop mirror (Fig. 3). 5. A device according to claim 4, characterized in that the non linear loop mirror is implemented by means of optical fibres and fibre components. 6. A device according to claim 4, characterized in that the non linear loop mirror is implemented as a hybrid circuit or otherwise by means of wave guides. 7. A device according to claim 4, characterized in that the non linear loop mirror is implemented by means of discrete components in open radiation paths.

Inventors: ANDREKSON PETER, OLSSON BENGT-ERIK, ANDREKSON, PETER, OLSSON, BENGT-ERIK
Applicants: ANDREKSON PETER, OLSSON BENGT ERIK, ANDREKSON, PETER, OLSSON, BENGT-ERIK
Application year: 1996
Main classification: G02F1
IPC codes: H01S3, H04B10