DE10302785A1 - Fiber optic laser resonator for optical coherence tomography has a polarization control unit with which the polarization direction in the resonator can be actively adjusted - Google Patents

Abstract

Laser assembly, especially for optical coherence tomography, has a fiber optic laser resonator (24) with at least a polarization control unit (26, 27) for altering the polarization direction if the resonator and an output coupler (29) for coupling out of broadband laser radiation. The polarization control unit actively adjusts the polarization direction in the resonator.

Description

The invention relates to a laser arrangement, in particular as a light source for optical coherence tomography, according to the generic term of claim 1.

Optical coherence tomography (English: OCT - Optical Coherence Tomography) is an imaging test that for example from HUANG, D .: "Optical Coherence Tomography", Science Vol. 254, Page 1178 ff .; POVAZAY, B .: "Submicrometer axial resolution optical coherence tomography ", Optics letters, Vol. 27, No. 20, page 1800 ff. And from HARTL, I. et. a1: "Ultrahigh-resolution optical coherence tomography using continuum generation in an air-silica microstructure optical fiber ", Optics letters, Vol. 26, No. 9, page 608 ff.

In order to achieve the greatest possible depth resolution, it is important that the light irradiated into the examination object has the widest possible bandwidth and a correspondingly short coherence length. It is therefore known from the publications listed above to use short-pulse lasers as the light source for optical coherence tomography. A special embodiment of such short-pulse lasers are fiber lasers which are known per se and, for example, in MATSAS, VJ et al .: "Selfstarting passively mode-locked fiber ring soliton laser exploiting nonlinear polarization rotation", Electronics Letters, Vol. 28, No. 15, page 1391 ff .; FERMANN, ME: "Ultrashort-Pulse Sources based on Single-Mode Rare-Earth-Doped Fibers", Applied Physics B, 58, 197-209 (1994) and in US 5,513,194 are described. Such fiber lasers usually have an optical fiber, the direction of polarization of the pulses carried in the optical fiber being set by passive polarization control devices.

A disadvantage of the known setting the direction of polarization is the great sensitivity to environmental influences, such as for example temperature fluctuations, vibrations and mechanical Tensions.

From the publications mentioned at the beginning it is also known as a light source for optical coherence tomography Using luminescent diodes that are inexpensive and easy Allow handling.

A disadvantage of such luminescent diodes is however, the poor optical resolution of more than 20 μm.

Ti sapphire lasers can also be used Central wavelengths between 780 and 850 nm or Cr-YAG laser with central wavelengths at 1.3 μm as a light source for the optical coherence tomography used become.

A disadvantage of these types of lasers however, the highly complicated technical structure, what an operation made necessary by trained specialist personnel.

The invention is therefore the object based on a laser arrangement with the shortest possible coherence length for the optical Coherence tomography too create that as possible insensitive to environmental influences, such as for example vibrations, temperature fluctuations and mechanical Tensions, and one is possible has simple technical structure, so that no operation by qualified specialist staff is required.

This task is based on the known fiber laser described above according to the preamble of claim 1, by the characterizing features of the claim 1 solved.

The invention encompasses the general technical teaching, the direction of polarization in the optical fiber of the fiber laser to actively adjust so that the disruptive influence of vibrations and temperature fluctuations can be compensated.

The laser arrangement according to the invention therefore has a polarization control device which determines the direction of polarization actively adjusts in the optical fiber of the fiber laser. The term an active setting of the polarization direction means in Framework of the invention that influencing the direction of polarization to changing environmental conditions, for example due to vibrations or temperature fluctuations can be adjusted.

In a variant of the invention for active adjustment of the polarization direction in the optical Fiber used a shaft plate that can be rotated by a motor is to the one you want Setting the polarization direction in the optical fiber. such Wave plates are known per se and are, for example, in YOUNG, M .: "Optics and Lasers ", page 188 ff. (Springer-Verlag), so that in the following a detailed description of wave plates can be omitted can and the content of this publication is fully attributable to the present description. The shaft plate can be rotated, for example, by an electric motor take place, but the wave plate can also within the scope of the invention can be rotated by other actuators to the desired direction of polarization adjust in the optical fiber. Here the shaft plate rotates preferably about an axis of rotation to the direction of propagation of the light is coaxial.

In another variant of the invention, on the other hand, at least one piezo actuator is used for actively setting the direction of polarization in the optical fiber of the fiber laser, which, depending on its electrical control, presses on the optical fiber of the fiber laser from the outside and thereby influencing the polarization direction of the optical fiber due to the change in birefringence. In this variant of the invention, a plurality of piezo actuators can also be used, each exerting pressure on the optical fiber from the outside in the radial direction, the direction of pressure of the individual piezo actuators being rotated relative to one another with respect to the central axis of the optical fiber. The individual piezo actuators press on the optical fiber from different directions.

In a further variant of the invention becomes the active setting of the polarization direction in the optical Fiber of the fiber laser uses an integrated optical component, that depending from its electrical control due to electro-optical effects affects the direction of polarization. The one in the optical fiber of the fiber laser broadband pulses are thus through the integrated optical Component led and are in accordance with the electrical control of the integrated optical component aligned according to a predetermined direction of polarization. The integrated optical component can be, for example is a lithium niobate component.

In a preferred alignment form the invention is a regulation, the active polarization control device, to achieve an optimal output pulse. There is a controller for this provided, the input side a parameter of the coupled out of the fiber laser Laser radiation detects and the motor, the piezo actuator or the lithium niobate component regulates accordingly. For example, the performance of the Fiber laser decoupled laser radiation determined as a parameter and can be optimized within the framework of the regulation.

The fiber laser is preferably on the output side with an optical amplifier connected, the laser radiation decoupled from the fiber laser strengthened being the optical amplifier for example, an erbium-doped fiber amplifier (EDFA amplifier).

However, the problem with such optical amplifiers can be the influence of the amplification on the properties of the light pulses, which may be changed to the disadvantage of a subsequent nonlinear distribution process, the broadening process being described in detail later. In a variant of the invention, therefore, a further active or passive polarization control device is arranged between the fiber laser and the amplifier and / or after the amplifier. This polarization control device can be implemented in a conventional manner, for example in US 5,513,194 is described, so that the content of this publication is fully attributable to the present description. However, it is alternatively also possible for the polarization control device arranged upstream or downstream of the optical amplifier to actively set the polarization direction in the manner according to the invention described above.

You can also continue at this point other pulse characteristics changed what is also known as "pulse shaping".

In addition, the fiber laser preferably connected to a device which the frequency spectrum which broadens the laser radiation extracted from the fiber laser, this device preferably arranged behind the amplifier is.

The facility to disseminate the Frequency spectrum of the outcoupled laser radiation can, for example have an optical fiber which the coupled laser radiation widened by nonlinear optical effects, such as in TAMURA, K.R. et al: "Fundamentals of stable continuum generation at high repetition rates ", IEEE Journal of Quantum Electronics, Vol. 36, No. 7 page 773 ff. Described in detail is, so the content of this publication also with regard to the spectral broadening of the coupled laser radiation this description is attributable to unnecessary repetitions to avoid.

With the optical fiber for expansion of the frequency spectrum of the outcoupled laser radiation are, for example, a photonic crystal fiber.

However, it is alternatively also possible for Broadening the frequency spectrum an optical fiber is used the glass fiber preferably having a relatively thin core to make nonlinear To exploit effects in the glass fiber.

When using a glass fiber for Broadening of the frequency spectrum of the coupled laser radiation it is advantageous if the fiber laser has a central wavelength, where the group velocity dispersion (GVD - Group Velocity Dispersion) of the glass fiber is essentially zero.

It also takes place in the frame the invention preferably a wavelength-dependent filtering of the Fiber laser emitted laser radiation through an optical filter, which is preferably arranged after the device for spectral broadening is. Such a wavelength-dependent filtering laser radiation is particularly useful in optical coherence tomography advantageous because the frequency spectrum of the laser radiation to the wavelength-dependent absorption and scatter behavior of the measurement object can be adjusted. Furthermore allows the filtering also an adaptation of the frequency spectrum to the special Requirements of the optical coherence tomograph, especially with regard to the optical transfer function and evaluation algorithms used.

The fiber laser used in the context of the invention preferably has as the laser medium an optical fiber doped with rare earths such as erbium.

Furthermore, the optical fiber has of the laser preferably a first fiber section with positive Dispersion and a second fiber section with negative dispersion on, the length and the dispersion properties of the two fiber sections preferably are adapted to each other in such a way that the dispersion effects in mutually compensate for the two fiber sections.

It should also be mentioned that the optical fiber of the fiber laser is preferably arranged in a ring, as in FIG US 5,513,194 is described in detail.

However, it is alternatively also possible for the optical fiber of the fiber laser to be in the form of an eight or as in the case of a Sagnac reflector, as also in FIG US 5,513,194 is described.

Furthermore, the optical fiber can also guided linearly , as for example in FERMANN, M.E .: "Ultrashort-Pulse Sources based on Single-Mode Rare-Earth-Doped Fibers ", Applied Physics B, 58, 197-209 (1994).

Finally, it should be mentioned that the laser arrangement according to the invention described above in particular is suitable as a light source in optical coherence tomography. Preferably the laser arrangement according to the invention therefore with an optical coherence tomograph connected.

When used as a light source for an optical coherence tomograph, the direction of polarization in the optical fiber can also be controlled passively, as in US 5,513,194 is described. The invention therefore also includes the combination nation in US 5,513,194 described laser arrangement with an optical coherence tomograph.

Other advantageous developments the invention are characterized in the dependent claims or are Below, along with the description of the preferred embodiment of FIG Invention with reference to the figures explained. Show it:

1 1 shows a schematic representation of an arrangement for optical coherence tomography with a laser arrangement according to the invention,

2 the structure of the laser arrangement 1 .

3 a schematic representation of the in the laser arrangement 2 used fiber laser as well

4a - 4b different variants of polarization control devices for setting the polarization direction in the fiber laser according to 3 ,

The schematic representation in 1 shows a largely conventional arrangement for optical coherence tomography, with a laser arrangement as the light source 1 is used, which is detailed in 2 is shown and will be described in detail later.

Because of the largely conventional structure of the in 1 The arrangement shown is only briefly described below, in addition to the publications HUANG, D .: "Optical Coherence Tomography", POVAZAY, B .: "Submicrometer axial resolution optical coherence tomography" and HARTL, I .: "Ultrahigh." -resolution optical cohernce tomography using continuum generation in an air-silica microstructure optical fiber ". The content of these publications is therefore fully attributable to the present description with regard to the structure and mode of operation of arrangements for optical coherence tomography.

The laser arrangement 1 outputs ultra-short, broadband laser pulses with a short coherence length and guides them to a beam splitter 2 to, which can be designed, for example, as a Michelson interferometer.

The beam splitter 2 shares that from the laser array 1 supplied laser radiation in two partial beams 3 . 4 on, the partial beam 3 of two optical lenses 5.1 . 5.2 with the focal length f on a measurement object 6 is focused, the measurement object 6 for example, can be a human tissue.

The other beam 4 is contrasted with an optical lens 7 on a mirror 8th directed, the mirror 8th is displaceable in the direction of the arrow.

The on the measurement object 6 or on the mirror 8th reflected partial beams 3 , respectively. 4 are then from the beam splitter 2 merged, with that of the measurement object 6 backscattered laser pulses with those from the mirror 8th reflected laser pulses interfere.

The interfering partial beams 3 and 4 are then from the beam splitter 2 a detector 9 fed.

Through a lateral shift of the partial beam 3 relative to the measurement object 6 can then create a sectional view of the measurement object 6 are recorded, the depth of the sectional image being the result of an axial displacement of the mirror 8th can be adjusted.

The detector 9 is in the conventional way on the output side with an amplifier 10 , a bandpass filter 11 , an analog / digital converter 12 and a computer 13 connected to the evaluation of the measurement data.

The following is now based on 2 the structure of the laser arrangement according to the invention 1 described.

The laser arrangement has the light source 1 a fiber laser 14 on that detailed in 3 is shown and will be described in detail later.

The laser arrangement 1 generates ultra-short, broadband laser pulses with a short coherence length, the short coherence length being that of the laser arrangement 1 generated laser pulses enables a large depth resolution in optical coherence tomography. The laser arrangement works in detail 1 as follows.

The one from the fiber laser 14 generated laser pulses are a polarization control device 15 (PC - Polarization Controller) which determines the direction of polarization of the fiber laser 14 emitted laser pulses controls.

There is also the possibility to influence other pulse properties at this point, which is also known as "pulse shaping".

The polarization control device is on the output side 15 with an erbium-doped fiber amplifier 16 connected, which optimally amplifies the laser pulses for the subsequent spectral broadening.

The one from the fiber amplifier 16 amplified laser pulses are then a further polarization control device 17 leads, which sets the polarization direction of the laser pulses.

The two polarization control devices 15 . 17 make it possible to optimally adjust the polarization for amplification and broadening.

The polarization control device is on the output side 17 with one facility 18 connected, which widens the frequency spectrum of the laser pulses to increase the depth resolution in optical coherence tomography. The facility 18 consists of a glass fiber with a small core with a diameter of less than 4 µm and a group velocity dispersion close to zero at 1.5 µm. With regard to the description of the broadening of the frequency spectrum in the device 18 To avoid repetition, the publication TAMURA, KR et al: "Fundamentals of stable continuum generation at high repetition rates", IEEE Journal of Quantum Electronics, Vol. 36, No. 7 page 773 ff. And TAMURA, K. et al "Pulse dynamics in stretched-pulse fiber lasers", Appl. Physics letters 67 (2), page 158 ff. The content of these publications is therefore fully attributable to the present description with regard to the techniques for broadening the frequency spectrum of the laser pulses.

The facility 18 is finally with a filter 19 connected, which filters the laser pulses depending on the wavelength, in order to match their frequency spectrum to the wavelength-dependent absorption or scattering behavior of the measurement object 7 adapt.

In addition, the Filtering also an adaptation of the frequency spectrum to the special Requirements of the optical coherence tomograph, especially with regard to the optical transfer function and evaluation algorithms used.

The filter 19 is finally with the in 1 shown optical coherence tomograph 20 connected and couples ultrashort, broadband laser pulses with a short coherence length into the beam splitter 2 on.

Furthermore, the laser arrangement 1 a measuring element 21 on the input side with the fiber laser 14 and with the filter 19 is connected and the coherence length L _COHERENCE and / or spectral width of the radiation is determined.

The fiber laser 14 is largely conventional and in a similar form, for example in the publication already mentioned at the beginning US 5,513,194 described. The content of this publication is therefore regarding the structure and the functioning of the fiber laser 14 to the full extent of the present description, so that only a brief description follows below.

The fiber laser consists of a fiber resonator 24 , which is largely realized by fibers. For example, it can have a ring-shaped or linear structure.

The fiber laser 14 has one or more pump lasers designed as diode lasers as the pump source 22 on, but other light sources can be used instead of the diode laser.

The pump laser 22 is with an input coupler 23 connected to that of the pump laser 22 emits laser radiation into a fiber resonator, wherein in the ring-shaped optical fiber resonator 24 an erbium-doped fiber section 25 is arranged as a laser medium.

Furthermore, in the ring-shaped fiber resonator 24 two polarization control devices 26 . 27 arranged which the polarization direction of the laser pulses in the fiber resonator 24 to adjust.

Also located in the ring of the fiber resonator 24 an integrated optical isolator 28 ,

For decoupling ultra-short broadband laser pulses from the fiber laser 14 this has an output coupler 29 on, which couples out part of the circulating laser radiation, for example 10%.

The structure and the functioning of the two polarization control devices are of particular importance here 26 . 27 which, in contrast to the prior art, do not passively but actively adjust the direction of polarization in the optical fiber resonator 24.

This active setting of the polarization direction offers the advantage of being largely insensitive to environmental influences, such as Vibrations, temperature fluctuations and mechanical stresses, since the direction of polarization can be readjusted, as detailed below is described.

For this purpose, the fiber amplifier 14 for each of the two polarization control devices 26 . 27 one controller each 30 . 31 on, with the two controls 30 . 31 the respective polarization control devices assigned to them 26 . 27 with a tax Drive the first signal and thereby determine the influence on the direction of polarization.

The regulation of the control of the two polarization control devices 26 . 27 takes place depending on the coherence length L _COHERENCE, that of the measuring element 21 in 2 is detected, the electrical control of the two polarization control devices 26 . 27 after the starting process takes place in such a way that the coherence length L _{COHERENCE becomes} minimal. During the starting process of the fiber laser 14 the polarization control device is set 26 . 27 against it so that a circulating pulse in the fiber resonator 24 arises.

Furthermore, there is a polarization-dependent loss unit in the fiber resonator 32 arranged as well as a facility 33 for dispersion control.

In the 4a to 4b are different variants of the active polarization control devices according to the invention 26 . 27 described, which actively influence the polarization direction.

So the in 4a shown variant of the polarization control device 26 to influence the direction of polarization in the fiber resonator 24 several piezo actuators 34.1 - 34.3 which, depending on the applied control voltage, exert a pressure from the outside radially on the outer surface of the fiber resonator 24 exercises. Depending on that by the piezo actuators 34.1 - 34.3 The pressure exerted then becomes the direction of polarization in the fiber resonator 24 affected.

At the in 4b shown variant of the polarization control device according to the invention 26 In contrast, the direction of polarization is influenced by a wave plate 35 by an electric motor 36 about one to the central axis of the fiber resonator 24 coaxial axis of rotation 37 is rotatable and thereby an active adjustment of the polarization direction in the fiber resonator 24 allows. The structure and functioning of the wave plate 35 is described for example in YOUNG, M .: "Optics and Lasers", page 188 ff. (Springer Verlag), so that the content of this publication of the present description with regard to the structure of the wave plate 35 is to be expected.

Finally, the polarization direction is influenced in the fiber resonator 24 in the embodiment according to 4c thanks to an integrated lithium niobate component 38 , the polarization direction in the fiber _resonator depending on the applied control voltage U _CTRL1 24 set accordingly. For this purpose, the lithium niobate component 38 a waveguide structure 39 on.

The invention is not based on the above described preferred embodiments limited. Rather, a variety of variations and modifications are possible also make use of the inventive idea and therefore in fall within the protection zone.

Claims (20)

Laser array ( 1 ), especially for optical coherence tomography, with an optical fiber laser resonator ( 24 ), at least one polarization control device ( 26 . 27 ) to influence the direction of polarization in the fiber laser resonator ( 24 ) and with an output coupler ( 29 ) to extract broadband laser radiation from the fiber laser resonator ( 24 ), characterized in that the polarization control device ( 26 . 27 ) the direction of polarization in the optical fiber laser resonator ( 24 ) actively sets. Laser array ( 1 ) according to claim 1, characterized in that the polarization control device ( 26 ) a wave plate ( 35 ) by a motor ( 36 ) is rotatable about the desired direction of polarization in the optical fiber laser resonator ( 24 ) to set. Laser array ( 1 ) according to claim 1, characterized in that the polarization control device ( 26 ) at least one piezo actuator ( 34.1 - 34.3 ) which, depending on its electrical control, presses on the optical fiber from the outside and thereby the direction of polarization in the optical fiber laser resonator ( 24 ) influenced. Laser array ( 1 ) according to claim 1, characterized in that the polarization control device ( 26 ) an integrated component ( 38 ) that influences the direction of polarization depending on its electrical control. Laser array ( 1 ) according to one of claims 2 to 4, characterized in that for controlling the polarization control device ( 26 . 27 ) a controller ( 30 . 31 ) is provided, which detects a parameter (L _COHERENCE ) of the _outcoupled laser radiation on the input side and the polarization control device ( 26 . 27 ) regulates accordingly. Laser array ( 1 ) according to at least one of the preceding claims, characterized in that the output coupler ( 29 ) with an optical amplifier ( 16 ) is connected, which amplifies the outcoupled laser radiation. Laser array ( 1 ) according to claim 6, characterized in that the amplifier ( 16 ) is an erbium-doped fiber amplifier. Laser array ( 1 ) according to claim 6 or 7, characterized in that before the amplifier ( 16 ) and / or after the amplifier ( 16 ) a polarization control device ( 15 . 17 ) is arranged, which is the polarization direction of the laser radiation affected. Laser array ( 1 ) according to at least one of the preceding claims, characterized in that the output coupler ( 29 ) with one facility ( 18 ) is connected to broaden the frequency spectrum of the coupled laser radiation. Laser array ( 1 ) according to claim 9, characterized in that the device ( 18 ) has an optical fiber which guides the outcoupled laser radiation and widens its frequency spectrum through nonlinear optical effects. Laser array ( 1 ) according to claim 10, characterized in that the optical fiber serving to expand the frequency spectrum is a photonic crystal fiber. Laser array ( 1 ) according to claim 10, characterized in that the optical fiber used to broaden the frequency spectrum is a glass fiber. Laser array ( 1 ) according to claim 12, characterized in that the glass fiber has a core and a jacket, the core being substantially thinner than in conventional glass fibers, in particular less than 4 microns. Laser array ( 1 ) according to claim 12 or 13, characterized in that the fiber laser has a central wavelength at which the dispersion of the fiber used for broadening is substantially equal to zero. Laser array ( 1 ) according to at least one of the preceding claims, characterized in that the output coupler ( 29 ) with an optical filter ( 19 ) which filters the outcoupled laser radiation depending on the wavelength. Laser array ( 1 ) according to at least one of the preceding claims, characterized in that an erbium-doped optical fiber ( 25 ) is provided. Laser array ( 1 ) according to at least one of the preceding claims, characterized in that the optical fiber laser resonator ( 24 ) has a first fiber section with positive dispersion and a second fiber section with negative dispersion. Laser array ( 1 ) according to claim 17, characterized in that the length and the dispersion properties of the two fiber sections are matched to one another such that the dispersion effects in the two fiber sections are mutually compensated. Laser array ( 1 ) according to at least one of the preceding claims, characterized in that the optical fiber laser resonator ( 24 ) of the fiber laser is arranged in a ring. Laser array ( 1 ) according to at least one of the preceding claims, characterized in that the output coupler ( 29 ) with an optical coherence tomograph ( 20 ) is connected and as the light source of the coherence tomograph ( 20 ) serves.