FR3014604A1 - DUAL PASSAGE OPTICAL FIBER AMPLIFIER FOR POLARIZED LUMINOUS BEAM - Google Patents

Abstract

The invention relates to a double-pass optical fiber amplification system for polarized light beam. According to the invention, such an optical fiber amplification system comprises: a polarization separator; an optical modulator connected to a gate of said polarization splitter; an optical fiber unable to maintain the polarization state of a light beam along its length having a first end connected to said optical modulator and a second end connected to an optical beam reflection means capable of allowing a rotation at 90.degree. ° the polarization of a polarized light beam that impacts it.

Description

FIELD OF THE INVENTION The field of the invention is that of optical fiber amplifiers.

A related field of the invention is that of lasers. More specifically, the invention relates to a double-pass optical fiber amplification system, capable of amplifying a signal in the two longitudinal directions of the optical fiber, and a device transmitting a M.O.P.A type pulsed beam. (acronym for "Master Oscillator Power Amplifier" in English) corresponding. The invention finds particular application in the implementation of high power tunable lasers. 2. State of the Art Optical fiber amplifiers based on a "double-pass" configuration are known, which make it possible to amplify light pulses with a large gain by amplifying them in the two longitudinal directions of the optical fiber. In particular, these double-pass amplifiers are encountered in laser sources of the MOPA type, where they make it possible to amplify a low-power optical beam emitted, for example, by a laser diode (most often the peak power of the amplified beam is between a few milliwatts and a few watts) and output an optical beam having a peak power of the order of one kilowatt. Known "double-pass" amplifiers include an optical fiber having one end connected to an optical circulator or a polarization splitter and the other end of which is connected to a single mirror or a Faraday mirror. When the light signal to be amplified is polarized, it is known, for example from the document WO 2008/016579, to use a polarization-maintaining optical fiber having a high birefringence.

However, the implementation of a polarization-maintaining optical fiber has the disadvantage of being expensive. More generally, a disadvantage of known "double-pass" amplifiers is the appearance of a time instability of the output power due to the large gain, often associated with the phenomenon of "self-lasing" resulting from the internal parasitic reflections at the interfaces components of the double-passage arm (mirror, optical welds, ...). Another disadvantage of the known double-pass amplifier techniques is the persistence, in more or less significant proportions, of a residual amplified spontaneous emission (ASE), due to a lack of saturation of the optical fiber. OBJECTIVES OF THE INVENTION The object of the invention is therefore in particular to overcome the disadvantages of the state of the art cited above.

More specifically, the invention aims to provide an optical fiber amplifier technique for polarized beam that is inexpensive. An object of the invention is also to provide a fiber optic amplifier technique that is simple to implement and compact. Another object of the invention is to provide an optical fiber amplifier technique that can transform a continuous light beam into a pulsed light beam. The invention also aims to provide a fiber optic amplifier technique that is reliable. Yet another object of the invention is to provide a fiber optic amplifier technique whose output power is stable over time. Another object of the invention, in a particular embodiment of the invention, is to provide an optical fiber amplifier technique which generates less amplified spontaneous emission than known double-pass optical fiber amplifiers. 4. Objective of the invention These and other objectives which will appear later are achieved by means of a double-pass optical fiber amplification system for polarized light beam. The invention therefore relates to an amplification system adapted to amplify a continuous or pulsed polarized light beam. According to the invention, such an optical fiber amplification system comprises: a polarization separator; an optical modulator connected to a gate of said polarization splitter; an optical fiber that is unable to maintain the polarization state of a light beam along its length, having a first end connected to said optical modulator and a second end connected to an optical beam reflection means capable of allowing a rotation at 90 ° of the polarization of a polarized light beam which impacts it. Thus, the invention proposes, in a novel way, to implement a modulator in a double-pass amplifier configuration, in order to be able to suppress the spontaneous amplified emission between the pulses and in order to be able to use a continuous light beam at the input of the amplifier. amplification system and emit a pulsed beam output. Note that the invention also advantageously proposes to place the modulator between the separator and the optical fiber. Indeed, the inventors have found that in the case where the modulator is placed between the optical fiber and the reflection means, the power losses through the modulator and the reflection means are important, which limits the gain of the optical system. 'amplification. Furthermore, since the modulated signal is then weak after the round-trip within the reflection means, the optical fiber is not well saturated, which results in a large level of spontaneous emission amplified at the output of the optical system. amplification.

Moreover, the amplification system proposed by the invention proves inexpensive to implement because of the use of an optical fiber that does not maintain polarization, or in other words that has a low degree of refractive power.

According to a particular aspect of the invention, said optical fiber is a unimodal fiber. According to a particular embodiment of the invention, said fiber is a micro-structured fiber. In a particular embodiment of the invention, said optical modulator is an acousto-optic modulator. In a particular embodiment of the invention, said optical modulator is an electro-optical modulator. In a particular embodiment of the invention, said modulator is a semiconductor modulator (SOA). Advantageously, an optical fiber amplification system as described above comprises spectral filtering means housed between said fiber optical and said reflection means. This substantially reduces the spontaneous emissions amplified for wavelengths different from that of the pulsed signal propagating in the optical fiber amplification system. Advantageously, an optical fiber amplification system as described above comprises means for modulating the energy of pumping means associated with said optical fiber. This eliminates the low frequency spontaneous emissions amplified between two consecutive pulses. According to a particular embodiment of the invention, said reflection means comprise a Faraday rotator mirror. According to a particular embodiment of the invention, said reflection means comprise a quarter-wave plate and a mirror.

This gives a compact amplification system.

In a particular embodiment of the invention, said reflection means comprise a Faraday rotator and a reflective Bragg grating. Advantageously, said optical fiber is a fiber doped with rare earth ions. It may in particular be an optical fiber doped with Erbium ions or doped with Ytterbium ions or co-doped with Erbium and Ytterbium ions. The invention also relates to a device for transmitting a pulsed optical beam comprising a laser source capable of generating a continuous optical beam connected to a gate of the polarization splitter of a double-pass optical fiber amplification system such that described above. This gives a device type M.O.P.A, also called amplified oscillator, able to emit a highly coherent beam and a high power.

Note that the laser source may be, for example, a semiconductor laser diode or a fiber laser. 5. List of Figures Other features and advantages of the invention will appear more clearly on reading the following description of two embodiments of the invention, given as simple illustrative and non-limiting examples, and the accompanying drawings. among which: FIG. 1 represents an exemplary embodiment of a double-pass optical fiber amplification system according to the invention coupled to a laser diode within a device for emitting a pulsed optical beam amplified oscillator type; FIG. 2 illustrates another exemplary embodiment of a double-pass optical fiber amplification system according to the invention. 6. Detailed description of embodiments of the invention 6.1. EXAMPLE OF EMBODIMENT FIG. 1 illustrates an exemplary embodiment of a double-pass optical fiber amplification system 10 according to the invention for amplifying a polarized light signal, integrated into a device of FIG. emission of a pulsed optical beam 100, of the MOPA type

The amplification system 10 is powered by a semiconductor laser diode 11 emitting a continuous polarized light beam 112 which is injected into a first gate 12 of a polarization splitter 13 and emerges by a second gate 14 connected to a terminal end. an electro-optical intensity modulator based on a Mach-Zehnder interferometer. The state (active or off) of the modulator 15 is controlled by a control device (not shown in Figure 1) which controls the duration and shape of the pulses generated by the modulator. The modulator 15 will modulate and filter temporally the polarized continuous beam 112 and thus transform it into a short pulse train 113. It should be noted that in addition the modulator 15 makes it possible to filter the amplified spontaneous emissions between each pulse . The other end of the modulator 15 is connected to an Erbium ion-doped single-modal optical fiber 16 which will amplify the pulses 113. This optical fiber 16 is supplied with energy by a pump diode 17. In this embodiment of the invention, the pumping mode has been optimized so as to avoid the appearance of non-linear couplings when an optical signal propagates in the gain section. In variants of this embodiment of the invention, the optical fiber 16 may be doped, or co-doped, with any other suitable rare earth ion, such as ytterbium, thulium, holmium, or praseodymium for example. In another variant, it may be envisaged to use one or more "double-cald" optical fibers, without departing from the scope of the invention.

The second end of the optical fiber 16 is connected to an optical filter 18 making it possible to limit the amplified spontaneous emission level for wavelengths different from those of the light beams propagating in the optical fiber 16, followed by a mirror The Faraday rotator mirror 19 allows the amplified beam to propagate again in the gain section. It comprises a Faraday rotator 110 for rotating the 45 ° polarization of the incoming polarized signal and a reflecting mirror 111 on a narrow band centered on the wavelength of the input signal, to attenuate any different emission of the input signal. signal to amplify. When the pulses 114 amplified within the optical fiber 16 and filtered impact the Faraday rotator mirror 19, the Faraday rotator 110 thus makes a first rotation of the polarization of the pulsed beam by 45 °, and then the pulsed beam is reflected on the mirror 111 before its polarization is again turned by 45 ° passing a second time in the Faraday rotator 110. The polarization of the pulses 115 at the output of the Faraday rotator mirror 18 is thus at 90 ° to the polarization pulses 114 at the entrance of the Faraday rotator mirror 19.

The pulses 114 then propagate in the opposite direction in the optical fiber 16 where they are amplified a second time, pass into the modulator 15 and are switched by the polarization separator 13 on its third port 116. 6.2. Another Exemplary Embodiment There is shown in FIG. 2 another exemplary embodiment of a dual-pass amplification system 20 according to the invention. In this embodiment of the invention, the Faraday reflector mirror has been replaced by a Faraday rotator 21 aligned with a reflective Bragg grating 22, the other components of the amplification system being identical to those of the imaging system. amplification 10 previously described.

Claims (12)

REVENDICATIONS1. A double-pass optical fiber amplification system for a polarized light beam, characterized in that it comprises: a polarization splitter; an optical modulator connected to a gate of said polarization splitter; an optical fiber that is unable to maintain the polarization state of a light beam along its length, having a first end connected to said optical modulator and a second end connected to an optical beam reflection means capable of allowing a rotation at 90 ° of the polarization of a polarized light beam which impacts it. 2. Dual-pass optical fiber amplification system according to claim 1, characterized in that said optical fiber is a unimodal optical fiber. A double-pass optical fiber amplification system according to claim 1, characterized in that said optical fiber is a micro-structured optical fiber. 4. double-pass optical fiber amplification system according to any one of claims 1 to 3, characterized in that said optical modulator is an acousto-optic modulator. A double-pass optical fiber amplification system according to any one of claims 1 to 3, characterized in that said optical modulator is a semiconductor modulator (SOA). 6. Double-pass optical fiber amplification system according to any one of claims 1 to 3, characterized in that said optical modulator is an electro-optical modulator. A dual-pass optical fiber amplification system as claimed in any one of claims 1 to 6, characterized in that said reflection means comprise a Faraday rotator mirror. The double-pass optical fiber amplification system according to any one of claims 1 to 6, characterized in that said reflection means comprise a Faraday rotator and a reflective Bragg grating. 9. Dual-pass optical fiber amplification system according to any one of claims 1 to 8, characterized in that it comprises spectral filtering means housed between said optical fiber and said reflection means. 10. Double-pass optical fiber amplification system according to any one of claims 1 to 9, characterized in that it comprises means for modulating the energy of pumping means associated with said optical fiber. 11. Double-pass optical fiber amplification system according to any one of claims 1 to 10, characterized in that said optical fiber is a fiber doped with rare earth ions. 12. A device for emitting a pulsed optical beam, characterized in that it comprises a laser capable of generating a continuous optical beam connected to a gate of the polarization splitter of a double-pass optical fiber amplification system. according to any one of claims 1 to 11, comprising: - a polarization separator; an optical modulator connected to a gate of said polarization splitter; an optical fiber that is unable to maintain the polarization state of a light beam along its length, having a first end connected to said optical modulator and a second end connected to an optical beam reflection means capable of allowing a rotation at 90 ° of the polarization of a polarized light beam which impacts it.