EP1287591A1

EP1287591A1 - Laser with a fiber optical waveguide

Info

Publication number: EP1287591A1
Application number: EP00942026A
Authority: EP
Inventors: Andreas TÜNNERMANN; Holger Zellmer
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 2000-06-07
Filing date: 2000-06-07
Publication date: 2003-03-05
Also published as: WO2001095440A1

Abstract

The invention relates to a laser (100) comprising a fiber optical waveguide (107; 204, 209) inside a resonator, the core of said fiber optical waveguide being suitably doped for exiting laser light (111); and an element provided for polarizing the laser light (111). The invention provides that the polarizing element is a section (104, 109; 207) of the fiber optical waveguide (107; 204, 209) and/or that a device with which the fiber optical waveguide (107; 204, 209) itself or a section (104, 109; 207) thereof can be polarized is provided.

Description

Laser with an optical fiber

The invention relates to a laser with an optical fiber within a resonator, the core of which is suitably doped for excitation of laser light, and with a polarizing element provided for polarizing the laser light.

Such lasers are suitable for applications for which a polarized laser beam is required. In addition to the technical / physical area of application, there are also applications in the consumer area, such as video projection with laser beams, in which the intensity control is not very complex using a simple polarizer in the presence of a polarized laser beam, or in which two images in different polarization states with corresponding polarization states Glasses with polarization-filtering glasses make a stereo image visible to the viewer.

Such lasers for polarizing laser beams usually have a polarizing element in the resonator, in particular with different losses for different polarization devices. Because of the polarizing element provided within the resonator, only light of one polarization direction is amplified, so that almost all of it is used for the laser process Available pump power is converted into the laser beam with the polarization state given by the polarizing element.

Fibers with an elliptical active core can be used as the polarizing element in fiber fibers. However, an anarmophotic and therefore non-circular beam profile is obtained. In addition, the currently possible polarization ratio for linear polarization is 5 to 1, which is too low for many applications.

Furthermore, in order to achieve a polarizing effect, it has been proposed to use fibers with stress birefringence in the active core, the stress birefringence being generated by using materials with different thermal expansion. Until now, this technology was only successfully used with quartz fibers. This means that in many technical applications, in particular in the case of materials with low phonon energies, such as ZBLAN fibers, which can be used in the visible range, such a procedure is not possible.

The object of the invention is therefore to provide a fiber laser of the type mentioned, in which the polarizing element is of particularly simple construction and which can be implemented almost independently of the choice of the glass material suitable for the spectral range.

The object is achieved in that the polarizing element is a section of the optical fiber and / or a device is provided with which the optical fiber itself or a section of it can be polarized.

In contrast to the aforementioned polarizer according to the prior art, the optical fiber itself is used here. In contrast to the example with quartz fibers mentioned above, however, according to the invention it is not the entire fiber which acts as a polarizer, but only a section which is birefringent, for example. According to the invention, a special device can also be provided which can produce the polarization effect in special fibers.

How this happens in detail will become clearer in the following. The most important thing here is that the effort is reduced compared to the prior art. This applies not only because a special polarizer is not required, but also because additional modules for coupling in and out can be saved in or from a known Polarisafor. This is immediately obvious, because even if only a section of the glass fiber were polarized, for example by inserting a section, the desired polarization effect arises. In this case, the cores of the section and the rest of the fiber for reading can be brought so close to one another that the light can pass from the section to the optical fiber provided for reading and back with little loss, so that additional coupling and decoupling optics, as in the case of conventional polarizers are quite common.

In particular with regard to the design of the polarizing element as a section of the optical fiber, it is provided according to a preferred development of the invention that the polarizing element is a special polarizing fiber. For this purpose, birefringent fibers are particularly suitable, such as the quartz fiber exemplified in the introduction, while the rest of the optical fiber is designed as a lasing fiber with regard to the requirements, in particular for the stimulated emission of light of the desired wavelength.

In principle, one then has a section of the optical fiber that is provided and designed for reading and another section that is used to generate the desired direction of polarization. It is completely irrelevant how the special polarizing fiber is arranged relative to the optical fiber provided for reading. It is Z. B. possible to insert the special polarizing fiber in the middle of the optical fiber provided for reading. Another possibility is given by an arrangement in which the optical fiber provided for reading is arranged between polarizing fiber parts. In addition, however, any periodic and / or non-periodic structures with alternating polarizing fiber parts and optical fiber parts provided for reading can also be used. There is no restriction with regard to the arrangement if only the total lengths of the polarizing fiber parts for polarizing and the fiber parts intended for reading are selected to be large enough for the required output power. To simplify production, however, it is particularly advantageous if only a single section is an optical fiber provided for reading and another section is a polarizing fiber.

The following further developments relate above all to the connection between the special polarizing fiber and the optical fiber provided for reading.

According to a first preferred development of the invention, the special polarizing fiber is spliced onto the optical fiber provided for reading. It is advantageous to support polarization if the spliced ends are joined together at an angle, in particular the Brewster angle.

According to another preferred development of the invention, the special polarizing fiber is connected to the optical fiber provided for reading via a fiber coupler. Such fiber couplers are known, for example, from telecommunications, in which standard fiber connections are made, in particular via special fiber couplers, into which the fiber ends to be connected are inserted on both sides on a defined axis. A liquid medium within this fiber coupler ensures a corresponding refractive index between the cores of the two fibers in order to enable the light to pass from one end of the fiber part into the other without loss.

In particular, if there is a need to make a special adjustment between the optical fiber provided for reading and the special polarizing fiber, a particularly preferred development of the invention is recommended in which the special polarizing fiber is separated from the optical fiber provided for reading, whereby an optical system, in particular a lens system, is provided between it and the polarizing fiber for coupling and decoupling the laser light into and out of the special polarizing fiber.

However, it is particularly advantageous if the polarization is adjustable. For this reason, a controllable device for the polarization is provided in accordance with the following preferred developments of the invention. It is particularly preferred that a device is provided with which the optical fiber and the special fiber can be pressurized radially and rotated in the circumferential direction.

Such devices are available, for example, as so-called PoLaRITE ™ polarization controllers from General Photonics Corporation, in which the fiber is pressurized with the aid of a special fiber holder via an adjusting screw. This type of polarization setting can be provided, for example, on the optical fiber provided for reading. However, it is also possible to connect a special fiber, as already shown, with the optical fiber provided for reading, the pressure then being applied only to the special fiber. Furthermore, with a special polarizing fiber which is connected to the optical fiber provided for reading, a certain proportion of polarization can be brought about and a further polarization component can be set via the optical fiber provided for reading or another fiber part. These examples show that in principle it is completely irrelevant at which point or in which way the fiber is pressurized. The selection of the optimal positions and the corresponding design of the fiber parts is well within the skill of a specialist, so that no specific teaching has to be given for the special application.

In a similar, albeit differently working, development of the invention it is provided that at least one loop is formed in the fiber or in the special fiber, the angle of which can be changed to other loops.

Devices in which loops are arranged to be adjustable relative to one another are generally available. For the mode of operation and special structure of such adjusting devices, reference is made to US 4,389,090.

Such devices are manufactured, for example, in the USA by the company "Fiber ControJ Industries". They have three so-called paddles, into each of which several turns of an optical fiber are inserted. The three paddles can be tilted towards one another. By adjusting the angle of the paddles to one another and to the desired one The direction of polarization of the output beam is sufficient with three paddles to allow adjustment of the full Poincare sphere of polarization. In addition to the manual setting mentioned above, it is also possible to set the polarization electrically. For this purpose, in the examples mentioned above, motor controls could be used to pressurize a fiber part or to change the angle between the loops. In addition to the pressurization, it is also possible to provide special polarizing fibers that are magnetically or electro-optically sensitive.

In particular, for example, the output power of the laser can be controlled via such a setting option via the polarization control if, according to a preferred development of the invention, a setting device for the polarization is provided and a control device is connected to this setting device, with which the power of the polarized output beam in particular is kept constant becomes. For this purpose, in principle only the power of the laser beam has to be measured during operation, which is supplied to the control device for regulation in a known manner as the actual value after comparison with a target value. The power of the output beam for obtaining the actual value can be determined by branching off a small partial beam, for example with the aid of a partially transparent mirror.

The laser is constructed in a particularly simple manner according to a preferred development of the invention, in which at least one end face of the optical fiber or the special polarizing fiber is mirrored as a resonator mirror. This saves a special mirror and a corresponding coupling / decoupling optics between the optical fiber and the mirror. The effort is then significantly reduced compared to other laser structures. The resonator mirror here does not only mean metallized surfaces or dielectric layers, for example, Bragg structures incorporated in fibers can also be used for mirroring (fiber Bragg gratings).

It is particularly preferred according to an advantageous development of the invention that the laser light is coupled out of the laser on the pump side, a dichroic mirror in particular being provided for separating pump and laser radiation. You then only need a single, highly coated resonator mirror, which is used to couple the pump radiation or to couple out the laser radiation. This also reduces the effort. Instead of a dichroic mirror and due to the polarization of the laser light, pump light and laser light can also be separated by a polarization beam splitter if the pump light is also appropriately polarized.

As will also become clear from the above, the invention, with its developments, in particular because practically innumerable types of fibers with the most varied properties can be used to realize them, is largely independent of the type of laser process. For example, the invention can be used not only in the excitation of simple laser states but also, for example, in so-called upconversion lasers, in which the lasing atoms are brought into a very high state by a multiphoton excitation process, which then falls back to the ground state due to stimulated emissions. Then the achievable wavelength is significantly less than the wavelength of the pump radiation. Such lasers are particularly interesting for the generation of visible light, in which infrared radiation from commercially available laser diodes can be used as the pump structure.

Further special features of the invention also result from the following description of exemplary embodiments with reference to the attached drawing. Show it:

Figure 1 shows a first embodiment of the invention.

Fig. 2 shows another embodiment as Fig. 1, but with a

Polarization controller and a special coupling optics;

Fig. 3 shows another embodiment in which pump radiation and laser radiation are coupled out or coupled out from the same side of the resonator.

In Fig. 1 the basic structure of a fiber laser is shown, on which most of the essential features can be made clear. The pump radiation 102 is coupled into a fiber laser 100 from a pump source 101, which is preferably a laser diode, via coupling optics 103. The fiber laser 100 has a resonator formed from mirrors 105 and 110. The active material for the laser process excited by the pump radiation 102 is formed by a suitably doped core of a fiber 107 provided between the resonator mirrors 105 and 110,

The pump radiation 102 is coupled directly into the fiber via the coupling optics 103 and stimulates the laser process. The laser radiation 111 then generated then leaves the fiber laser 100 through the resonator mirror 110. In order to simplify the construction, in particular the mirrors 105 and 110, in contrast to the example from FIG. 1, can also be provided as reflecting on the optical fiber 107. In addition to mirroring with metallic and / or dielectric layers, so-called “fiber bragy gratings” can also be used, that is, changes in the refractive index introduced into the fiber 107, which likewise have a reflective effect if the wavelength is tuned appropriately.

The fiber laser shown in FIG. 1 has a special feature compared to conventional fiber lasers. The active optical fiber 107 does not extend continuously from the mirror 105 to 110. At coupling points 106 and 108, respectively, it is coupled to special fiber pieces 104 and 109, which guide the light to the mirrors 105 and 110. The fiber pieces 104 and / or 109 can be used with a suitable design for the polarization of the laser light.

In the exemplary embodiment of FIG. 1, the fiber 109 belonged to the type of fibers which have different losses for different polarization states. For this purpose, it can be designed, for example, as a so-called single-mode polarizing fiber or as a polarization-maintaining fiber with additional polarizing properties. As a result, a polarization direction for the laser excitation and thus for the stimulated emission of the corresponding polarization state is preferred in the resonator, which also affects the polarization of the laser beam 111 in such a way that a polarization state occurs much more frequently than the one orthogonal to it.

A fiber with the same property can also be provided in the fiber section 104. In general, however, a single strand 104 or 109 with the corresponding properties is sufficient to set the polarization, so that the desired polarization of the output beam 111 takes place. The arrangement shown in the starting example with the lasing fiber between two polarizing fiber pieces 104 and -109 is therefore to be understood as purely exemplary. Any arrangement of any pieces of laser-active optical fibers 107 and polarizing fiber pieces 104 and 109 will cause the same effect, but it should be noted that the degree of polarization essentially depends on their design. Such a structure, as shown in Fig. 1, was experimentally realized with PrYb fiber fibers. A linear polarization of greater than 10: 1 was found in the laboratory.

The coupling points 106 and 108 mentioned above can be designed in various ways. In the embodiment of Fig. 1, the fibers were spliced together. Another possibility of joining is by gluing. It may also be advisable to use the fiber couplers known from telecommunications technology, in which the fiber ends to be coupled are brought together on an optical axis and a liquid ensures the refractive index adjustment between the two fiber ends.

In particular, it is advantageous to bevel the joined fiber ends at the coupling points 106 and 108, since then the passage of light from a polarization device is also preferred at the coupling point. The Brewster angle is particularly suitable as an angle for the inclination.

Further special features of the lasers shown here can be seen in FIG. 2. The same reference numerals always mean the same function in all figures.

In contrast to FIG. 1, the coupling point 108 is now realized with the aid of an optical system 211, which couples the light from one fiber part 207 into the other fiber part 209. This optical system will generally be a lens group.

In contrast to FIG. 1, the fiber pieces 204 and 209 shown in FIG. 2 also take over the generation of the laser process. The fiber piece 207 in between, on the other hand, serves for polarization.

For this purpose, the fiber 207 provided for polarization is wound in a plurality of loops 212, which are pressure-operated by means of a so-called fiber polarization controller be charged. The loops 212 extend radially from the optical axis of the fiber and are mutually adjustable in angle, as is known in particular from US 4,389,090. By changing the angles between the loops 212 relative to one another, a complete setting for all degrees of freedom of the polarization is possible.

This setting is particularly advantageous if you want to guarantee particularly high degrees of polarization for the laser radiation 111 at the output. In particular, however, it is also possible to control the polarization control and in particular to regulate the power of the laser beam 111 in the output when part of this laser radiation is branched off, converted electrically via a photodetector and this electrical variable is compared as an actual value with a setpoint value, after which the comparison, a controlled variable is formed with which the controller is readjusted by adjusting the angle of the loops 212. The exact implementation of such regulations are known from the prior art and do not require any further explanation here.

From the example of Fig. 2 it was clear that one can also achieve a corresponding polarization and rotation of the pressurized part against the rest of the fiber by pressurization.

This can also be used, for example, by using a continuous fiber 107 pressurized on a section in the example of FIG. 1, the coupling points 106 and 108 being omitted, so that the fiber sections 104 and 109 for this example are only a continuation of the laser-active fiber 107 are to be considered. A pressure on the fiber 107 with known devices then likewise leads to a change in polarization and thus to a polarized output beam of the laser radiation 111.

While in the preceding examples the laser radiation 111 was taken from the opposite side of the coupling of the pump radiation 102, the laser beam 111 can also be taken from the same side from which the pump radiation 102 is also coupled into the fiber 107. This requires a change in the coupling optics 103 in a known manner. A dichroic mirror 313 is also used to separate the pump radiation 102 from the laser radiation 111. Instead of a dichroic mirror you can on the basis of the defined Polarization directions of laser radiation 111 also use a polarization beam splitter to separate pump radiation 102 and laser radiation 111.

The changes given in the previous examples can of course also be carried out in the embodiment of FIG. 3.

In particular, the previous exemplary embodiments are distinguished by a simple generation of polarized laser radiation using conventional i.e. non-polarization-maintaining optical fibers.

Another effect unexpectedly occurred. When using such fibers it was found that there is better protection of the mechanically / thermally sensitive end faces of fibers for lasers in the visible and infrared spectral range. This is due to the fact that special fibers 104, 109 can be selected, the less sensitive to laser radiation and ^• environmental influences than for example provided with laser-active material special optical fiber 107th

In addition to the aforementioned setting of a polarization state via pressure on the fiber, magneto-optically or electro-optically active materials can also be used for special fiber parts and the desired polarization can be generated by corresponding electrical or magnetic fields. Due to the separation of polarization and laser excitation by means of the. The illustrated embodiment of different fiber parts allows all known effects to be exploited independently of the laser excitation, so that there is little restriction for the choice of material with regard to the laser processes used and for the different types of polarization in the design of such lasers. Surprisingly, it is possible to achieve this advantage without having to deviate significantly from the simple structure that is customary for a fiber laser.

Claims

Expectations

1. Laser (100) with an optical fiber (107; 204, 209) within a resonator, the core of which is suitably doped for excitation of laser light (111), and with a polarizing element provided for polarizing the laser light (111), characterized in that that the polarizing element is a section (104, 109;

207) of the optical fiber (107; 204, 209) and / or a device (212) is provided with which the optical fiber (107; 204, 209) itself or a section (04, 09; 207) of which it can be polarized.

2. Laser (100) according to claim 1, characterized in that the polarizing element is a special polarizing fiber (104, 109; 207).

3. Laser (100) according to claim 2, characterized in that the special polarizing fiber (104, 109; 207) is spliced onto the optical fiber provided for reading (107; 204, 209).

4. Laser (100) according to claim 3, characterized in that the spliced ends, in particular at the Brewster angle, are chamfered.

5. Laser (100) according to claim 2, characterized in that the special polarizing fiber (104, 109; 207) is connected via a fiber coupler to the optical fiber provided for reading (107; 204, 209).

6. Laser (100) according to claim 2, characterized in that the special polarizing fiber (104, 109; 207) of the intended for reading

Optical fiber (107; 204, 209) is separated and between them and the polarizing fiber an optical system (211), in particular a

Lens system, for coupling and decoupling the laser light (111) in and out of the special polarizing fiber (104, 109; 207) is provided.

7. Laser (100) according to one of claims 1 to 6, characterized in that a device (212) is provided with which the optical fiber (107; 204, 209) or the special fiber (104, 109; 207) radially with Pressure can be applied and rotated in the circumferential direction.

8. Laser (100) according to one of claims 1 to 7, characterized in that in the fiber (107) or in the special fiber (104, 109; 207) at least one

Loop is formed, the angle to other, loops can be changed.

9. Laser (100) according to one of claims 1 to 8, characterized in that an adjusting device for the polarization and a control device suitable for this purpose are provided, which in particular keep the power of the polarized laser beam (111) constant.

10. Laser (100) according to one of claims 1 to 9, characterized in that at least one end face of the optical fiber (107; 204, 209) or the special polarizing fiber (104, 109; 207) is mirrored as a resonator mirror.

11. Laser (100) according to one of claims 1 to 10, characterized in that the laser light (111) at the Pumpse ^' rte is coupled out of the laser and in particular a dichroic mirror (313) is provided for separating pump and laser radiation ,