WO2011066983A2

WO2011066983A2 - Fiber-integrated optical isolator

Info

Publication number: WO2011066983A2
Application number: PCT/EP2010/007353
Authority: WO
Inventors: Jens Geiger; Oliver Fitzau
Original assignee: Fraunhofer-Gesellschaft Zur Förderung Der Angewandten Forschung E.V. Angewandten Forschung E.V.
Priority date: 2009-12-05
Filing date: 2010-12-02
Publication date: 2011-06-09
Also published as: WO2011066983A3; DE102009057186A1; DE102009057186B4

Abstract

The invention relates to an optical isolator which has at least one Faraday rotator (6) between two polarisers or a polariser and a quarter-wave plate (7), wherein one or both polarisers are formed by polarisation-maintaining optical fibres (4, 5) which are wound into a spiral, wherein a radius and/or a pitch of the spiral changes over a length of the polarisation-maintaining fibres. The optical isolator permits the transmission of high power in continuous operation with low insertion loss.

Description

Fiber-integrated optical isolator

Technical application

The present invention relates to an optical isolator formed by a Faraday rotator between two polarizers or by a polarizer and a quarter wave plate.

An optical isolator is characterized by an asymmetric transmission behavior. In one beam direction (forward direction), the optical isolator passes the light with low losses, in the other beam direction (reverse direction) it is blocked. Optical isolators are therefore particularly suitable for coupling optical systems asymmetrically. Although the first system can act on the second system in the transmission direction, the second system in the reverse direction does not react to the first. The effect of optical isolation can be achieved using a Faraday rotator and two polarizers for linearly polarized light. The light of a linearly polarized source passes through the first polarizer in its direction of transmission and is then transmitted through the Faraday

Rotator turned 45 °. The second polarizer stands for this polarization direction in transmission.

Returning, unpolarized light is half blocked by the second polarizer. The now linearly polarized beam passes through the Faraday rotator and is rotated by 45 ° so that it on the first

Polarizer is blocked. Such an insulator blocks back light from both polarizations. An unpolarized source is first split into its two polarizations and then through two

sent separate insulators.

A simplified version of the optical isolator can be realized with a polarizer and a quarter-wave plate. The light from a linearly polarized source passes through the polarizer in its transmission direction and is then converted by the quarter wave plate into circularly polarized light. Will this light without changing the

Polarization state reflected back, for example. From a workpiece in the material processing, it is converted by the quarter wave plate back into linearly polarized light. Its polarization axis is now perpendicular to the polarization of the source and the light is blocked by the polarizer. Such an optical isolator completely blocks its own polarization-conserving reflected light, but only half of the unpolarized returned radiation. An unpolarized source is first split into its two polarizations and then through two

sent separate insulators.

State of the art

Known optical isolators are based on the

Use of a Faraday rotator in conjunction with thin-film or crystal polarizers as separate optical elements (free-jet elements). By Using a collimating and a focusing lens, such optical isolators can be performed fiber-coupled, ie be used between two fibers. However, upon reincorporation of the beam into an optical fiber, comparatively high losses of typically 20 to 50% occur, particularly in the case of a high beam density to be transmitted (eg, less than 2.5 mm at 1065 nm or fundamental mode radiation). When transmitting high average

Benefits can be due to this power loss

Fiber input to be destroyed. In any case, expensive technical precautions must be taken to dissipate the heat generated by the power loss and to prevent misalignment of the coupling. By residual absorption in the Faraday crystal, a thermal lens can form in the crystal at high average powers, which leads to a reduction in the beam quality and causes a shift in the focus position behind the coupling lens. This effect can lead to a drastic deterioration of the

Coupling efficiency lead. Furthermore, the inhomogeneous thermal load causes stresses in the crystal material and thus stress birefringence. This reduces the degree of isolation of the insulator.

From US 7,336,858 Bl an optical isolator is known which has a fiber-integrated Faraday rotator between two fiber-integrated polarizers. The Faraday rotator is characterized by an array

formed alternating magnetic fields surrounding a low birefringent fiber section. At this fiber section close on both sides highly birefringent fiber sections, which as Polarizers over a length of 1 cm oblique

have registered Bragg gratings. In the

Radiation propagating direction is scattered out of the fiber at the first polarizer or Bragg grating in the forward direction. In the transmission of higher average power such a structure, however, can be problematic because of the

Polarizer is coupled out a high power over a relatively small area.

The object of the present invention is to provide an optical isolator having a high power-resistance, so that the transmittable average power in continuous operation in the

Range over 100 W up to multi-kW. The optical isolator should furthermore have a low insertion loss and, in particular when used with fiber lasers, obtain the high beam quality of such lasers.

Presentation of the invention

The object is achieved with the optical isolator according to claim 1. Advantageous embodiments of this optical isolator are the subject of

dependent claims or can be the

refer to the following description and the exemplary embodiment. An alternative embodiment of an optical isolator is specified in claim 9.

The proposed optical isolator has as components at least: a Faraday rotator between two polarizers or a polarizer and a quarter wave plate. The optical isolator

is characterized in that the one or both polarizers are each formed by polarization-maintaining optical fibers which are wound into a spiral, wherein a radius and / or a pitch of the spiral over a length of

polarization-maintaining fibers changes. The wound polarization-maintaining fibers constitute polarizing optical fibers due to their winding.

The design of the polarizers or the

Polarizers as polarizing optical fibers has the advantage that the reverse-light is coupled over a greater length, for example. 1 to 10 m, as is the case when using other polarizers. As a result, the overall performance that can be transmitted or coupled out with such an optical isolator can be increased significantly. The optical isolator is thus for transmission

Medium power in continuous operation up to the multi-kW range can be used. Preferably, all components of the optical isolator are embodied here as fiber-optic elements and integrated in an optical fiber or form a section of an optical fiber which can be obtained, for example, by splicing fibers having different properties. Examples of fiber integrated Faraday rotators or quarter wave plates are known in the art. For this purpose, reference is made by way of example to the already mentioned US Pat. No. 7,336,858 Bl or to E. Voges et al. :

Optical Communication Technology - Manual for

Science and Industry, 1st edition, Berlin: Springer 2002, pages 420-421. The latter publication shows various possibilities of a fiber-integrated quarter wave plate. By integrating all the components of the optical isolator into an optical fiber eliminates the need for coupling and decoupling of the light to be transmitted in fiber-based arrangements, so that no adverse coupling losses occur. The proposed optical isolator allows the use of fibers with small fiber cross sections also for the higher average powers, so that the high-brilliance input radiation produced by a fiber laser as a source is obtained in the beam quality.

In general, the use of the proposed optical isolator, especially in the field of fiber lasers, offers itself, since these system-related the light already lead in an optical waveguide. An example of application are optical power amplifiers, in which the output signal of an optical signal source, for example a laser diode, through the optical isolator into an optical amplifier, for example an ytterbium or

Erbium-doped, optically pumped fiber, is coupled. The amplifier amplifies the signal, but also generates optical interference, eg. B. amplified spontaneous emission (ASE) or stimulated Brillouin scattering, which could act without optical isolator in the signal source and disturb or destroy them. The proposed optical isolator prevents them

Repercussions. Another application example are lasers in material processing, in which the laser radiation is guided via optical fibers. For example, when

Cutting or welding with a laser does not account for a large proportion of the incident laser power

Workpiece absorbs, but reflects. These

Reflections can run back in the worst case directly into the laser and disturb or destroy it. The proposed optical isolator between laser and workpiece prevents this.

Optical fibers for light transmission, in particular glass fibers, consist of a high-refractive core and a low-refractive sheath, both typically of quartz glass. This structure is surrounded by a plastic layer. This protects on the one hand the quartz glass, on the other hand has also optical functions. A low - profile plastic layer ensures that light is also guided in the sheath of the fiber (double sheath fiber). An index-adapted or high refractive coating causes the light not in the

Fiber cladding is guided and decoupled over the length of the fiber. Will polarized light in a glass fiber

coupled, the polarization state is initially maintained. However, the state of polarization may change over the length of the fiber, for example, due to stress birefringence in bending the fiber (linear to circularly polarized). On short pieces, specific polarization states can be set in a targeted manner. On long stretches is the Polarization state of the light guided in a normal glass fiber but not stable.

Polarization preserving fibers, as used in

proposed optical isolator, have a strong internal birefringence and lead linearly polarized light along one of the

Main axles is coupled, under receipt of the

Polarization over long distances. Due to birefringence, one of the two major axes has a slightly larger numerical aperture than the other (fast axis - slow axis). This leads to a different bending damping for the two

Polarization axes. This bending attenuation is exploited in the proposed optical isolator to form a polarization-maintaining fiber, a polarizing optical fiber. The polarization-maintaining optical fiber is wound for this purpose, wherein the winding radius is selected so that the better-guided polarization (slow axis, parallel to the line connecting the tension bars) experiences low bending losses in the fiber and the poorer guided axis (fast axis, perpendicular to the connecting line between the voltage bars) high bending losses. The fiber has a polarizing effect.

Preferably, a polarization-maintaining fiber having a small numerical aperture NA of the core (NA <0.1) is used here. Through this little one

Numerical aperture, the fiber is generally more flexible bending and the two polarization modes can be well separated by the bending damping. When winding the fiber onto a winding body, for example a cylinder, with a constant bending radius and a constant pitch of the resulting spiral, a constant bending damping over the wound-up length is achieved. The attenuation of the unwanted polarization takes place exponentially in this case, so that a comparatively high power and

Heat fraction occurs directly at the beginning of the wound fiber.

However, in the proposed insulator, the polarization-maintaining fibers are wound so that the radius and / or pitch of the resulting spiral changes along the length of the polarization-maintaining fiber. Preferably, the winding or bending radius decreases over the length. This results in a more even decoupling of the unwanted

Polarization achieved over the length of the fiber.

For example, the fiber may be in an Archimedean (linear increase or decrease in radius) or

logarithmic (exponential increase or decrease in radius) spiral wound. By varying the pitch (thread pitch) can also be a

desired distribution of the decoupled power over the length can be achieved. Goal of the variation of the

Winding or bending radius and / or the pitch is to use the dependence of the bending loss of the bending radius to adjust the relative damping (eg. Percent per meter) so that the absolute damping (eg. In watts per meter) constant becomes low. To this

In this way, damage caused by an excessive local thermal load can be very cleverly avoided. Preferably, a polarization-maintaining fiber with high refractive plastic coating

(Refractive index greater than that of the fiber cladding) used, so that the light that has leaked from the fiber core can also escape from the jacket. Such a fiber is then preferably wound on a winding body, which consists of a

Exiting radiation absorbing and good heat conductive material. The bobbin then serves both to absorb the exiting radiation and to dissipate the heat resulting from the absorption quickly.

In a further embodiment is as

polarizing optical fiber used a double cladding fiber, in which the emerging from the core

Radiation in the jacket is dissipated. This light can then be decoupled from the jacket at a suitable location and a further targeted recycling

fed or absorbed at a suitable location. The blocked or leaked into the jacket light is not locally converted in the region of the polarizer into heat and can be, for example, distributed over a large length from the jacket and absorbed.

The coupling out of the jacket can be done, for example, with an inverted multimode pump coupler. In a multimode pump coupler is the actual

Signal fiber surrounded by a ring of multimode fibers and the whole fiber bundle is fused. By suitable design, this component can be constructed so that the leaked into the jacket Light is coupled into the multimode fibers and the signal light is passed through the signal fiber in the middle with little loss. A part of the jacket light can be detected, for example. With a photodiode to possibly a

Shutdown of the laser when exceeding one

certain intensity of this coat of light. Furthermore, a part of the jacket light to

Determination of, for example, the ASE level in an amplifier stage, for the detection of stimulated Brillouin scattering (SBS) or also for a spectral analysis of the light. When using a quarter-wave plate, this may, for example, be designed in the form of one or more fiber loops whose winding levels can be adjusted to one another. Due to the bending of the fiber, the input-side linear polarization is converted into a circular polarization. Furthermore, the quarter-wave plate can also be designed in the form of mechanical or piezoelectric clamping plate whose clamping axes are rotated with each other. By the

induced voltages, the fiber is birefringent and converts the input-side linear polarization in a circular polarization. Examples of the basic structure of such fiber-intergrated quarter-wave plates from one or more fiber loops or with mechanical or piezoelectric clamp actuators are known from the above-mentioned publication by E. Voges et al. visible light. The use of a polarizer and quarter wave plate construction has the advantage that, unlike the other Embodiment that uses the Faraday effect, no fiber component with a large Verdet constant is required. Finally, an alternative embodiment of an optical isolator is proposed which comprises at least one Faraday rotator between two

Polarizers or a polarizer and a

Quarter wave plate has. The one or both polarizers are formed in this case by fiber-optic elements having a double-cladding structure with a fiber core and a fiber cladding provided with a coating or cladding having a lower refractive index than the fiber cladding, so that the Fiber core entering the fiber cladding optical radiation in

Fiber coat is performed. In this embodiment, regardless of the formation of the polarizers, the light coupled into the fiber cladding can be dissipated and separated from the core light at a suitable point. The blocked light is thereby not locally converted in the polarizer into heat, but is first removed in the fiber cladding. At a suitable location, it can then be decoupled and absorbed over a long distance from the jacket. In addition, in a fiber-integrated design of the components, the signal light does not leave the fiber, so that the arrangement has very low transmission losses.

The proposed optical isolator can be used in fiber amplifiers, fiber lasers, and in measurement and communication technology or material processing deploy. In particular, allows the optical

Isolator Isolation of single stages in low amplification (up to 10 W) fiber amplifiers and high average power amplifiers (> 10 W). It protects, for example, fiber lasers against repercussions from the application, for example during laser welding, cutting or ablation, especially for high-power lasers (> 10 W).

Brief description of the drawings

The proposed optical isolator becomes

below with reference to an embodiment in

Connection with the figures briefly explained again. Hereby show:

Fig. 1 shows an example of the structure of a

polarization maintaining fiber in the

Cross-section ;

Fig. 2 is a first example of the schematic structure of the proposed optical isolator;

Fig. 3 is a second example of the schematic structure of the proposed optical isolator; and

4 shows an example of the decoupling of

Coat light.

Ways to carry out the invention

Figure 1 shows schematically in cross section the

Structure of a polarization-maintaining fiber, as used to form the polarizing optical fiber in proposed optical isolator can be used. The polarization-maintaining fiber has a strong internal birefringence, with the two major axes (fast axis, slow axis) shown in the figure. The along the respective

Axis occurring refractive index jumps between the core 1 and the sheath 2 of this fiber are in the

Diagrams of the figure also indicated. to

Generation of the birefringence, this fiber in a known manner voltage rods 3, which are embedded in the shell 2. By bending the fiber, the polarization mode is decoupled from the core 1 in the fast axis, enters the shell 2 and is guided there. The polarization along the slow axis, however, undergoes low bending losses and is thus performed with only small transmission losses in the core 1 of the fiber. The fiber used in this example has μπι a cladding diameter of 400 μπι a core diameter of 20, a numerical aperture of the core of 0.06 and a birefringence of 3.5 x 10 ^"4. In a winding or bending radius of 35 mm In the case of such a fiber, a very good polarization separation occurs

unwanted polarization is conducted in the jacket and can be decoupled from the jacket at a suitable location.

FIG. 2 shows a schematic representation of an example of the structure of the proposed optical isolator with the two polarizing optical fibers 4, 5 and the fiber-integrated Faraday rotator 6 interposed therebetween. The optically polarizing fibers 4, 5 are made of polarizing optical fibers. obtained sustaining fibers which are wound on a winding - body 8. This is indicated schematically in the figure. The Faraday rotator 6 can by using a fiber section with a

suitable Verdet constant and external magnetic field generating devices are formed, as they are known from US 7,336,858 Bl.

Figure 3 shows schematically a structure of an optically polarizing fiber 4 and a fiber-integrated quarter-wave plate 7, which in turn here only schematically indicated and, for example, may be formed from fiber loops or with suitable terminal plates, as described in the publication by E. Voges et al. are known.

For the derivation of the light coupled into the jacket 9 of unwanted polarization can

Multi-mode pump coupler can be used. FIG. 4 shows an exemplary embodiment in which the

Signal fiber 12 is surrounded by a ring of multimode fibers 11, wherein the entire fiber bundle is fused together. The blocked light 9 coupled into the cladding couples into the ring of multimode fibers 11 and is separated therefrom by the signal light 10 of the core, which is guided by the signal fiber 12 in the middle. In this way, the unwanted polarization can be suitably dissipated via the multimode fibers 11 and fed at a suitable point of further utilization or absorption. LIST OF REFERENCE NUMBERS

1 fiber core

2 fiber coat

3 voltage bars

4 polarizing optical fiber

5 polarizing optical fiber

6 fiber-integrated Faraday rotator

7 fiber-integrated quarter wave plate

8 bobbins

9 coat light

10 signal light

11 multimode fiber

12 signal fiber

Claims

claims

Optical isolator having at least:

- a Faraday rotator (6) between two

Polarizers or

a polarizer and a quarter wave plate (7),

wherein the one or both polarizers are formed by polarization-maintaining optical fibers (4, 5) wound into a spiral, wherein a radius and / or a pitch of the spiral changes over a length of the polarization-maintaining fibers.

Optical isolator according to claim 1,

characterized,

that all components of the optical isolator are designed as fiber-optic elements and integrated in an optical fiber or one

Form section of an optical fiber.

Optical isolator according to claim 2,

characterized,

that the components of the optical isolator to form the portion of the optical fiber

are spliced.

Optical isolator according to one of Claims 1 to 3, characterized

that the polarization-maintaining fibers have a numerical aperture of the core of ^ 0.1.

Optical isolator according to one of Claims 1 to 4, characterized

in that the polarization-maintaining optical fibers (4, 5) have a fiber core (1) and a fiber cladding (2) provided with a coating or sheath which has a higher

Has refractive index as the fiber cladding (2).

Optical isolator according to claim 5,

characterized,

in that the polarization-maintaining optical fibers (4, 5) are wound on a winding body which is formed from an optical radiation-absorbing and heat-conducting material.

Optical isolator according to one of Claims 1 to 4, characterized

in that the polarization-maintaining optical fibers (4, 5) have a double-cladding structure with a

Fiber core (1) and a fiber cladding (2) having a coating or sheath

is provided, which has a lower refractive index than the fiber cladding (2), so that from the fiber core (1) in the fiber cladding (2) entering optical radiation in the fiber cladding (2) is guided.

Optical isolator according to claim 7,

characterized,

in that a multimode pump coupler is arranged on at least one of the polarization-maintaining optical fibers (4, 5) through which the fibers in the fiber cladding (2) guided optical radiation is dissipated.

Optical isolator having at least:

a Faraday rotator between two polarizers or

a polarizer and a quarter wave plate, said one or both polarizers being formed by fiber optic elements having a double cladding structure with a fiber core (1) and a cladding (2) provided with a coating or cladding having a lower refractive index as the fiber cladding (2), so that from the fiber core (1) in the fiber cladding (2) entering optical

Radiation in the fiber cladding (2) is performed.