DE102006022197B3 - Coherent radiation source e.g. laser, for optical coherence tomography, has luminescence material layer for radiation conversion arranged in cross section area of focused output radiation - Google Patents

Abstract

The radiation source e.g. laser (1) has a focusing optics (2) that is arranged in an optical path in a light propagation direction, and a luminescence material layer (3) for radiation conversion arranged in a cross section area of a focused output radiation. A spectral filter (5) and an object (6), which is to be illuminated, are arranged in the optical path of the converted radiation. The luminescent material layer is arranged on a transparent carrier (7) and is bordered on a heat conducting material for heat dissipation.

Description

The The invention relates to a radiation source, which radiation of a Lasers as output radiation for used a frequency conversion.

In US 6294800 B1 a frequency conversion from UV light to visible light of different colors is described. For fluorescence excitation, a UV LED or a UV laser diode is provided. In principle, the light of an LED can not be focused efficiently on a spot with a diameter of about 20 μm, since it does not emit spatially coherent light and its light conductance is thus considerably too large for OCT applications.

The spatial coherence the alternative proposed laser diode is not used. The Laser radiation leaves the laser diode always with a large divergence angle, resulting in the diffraction theory follows. There are no optical elements for focusing are provided, the converted light can not be high spatial coherence and is therefore for OCT applications with point scan unsuitable.

According to US Pat. No. 6,765,220 B2 One or more (collimated) lasers write a pattern into a frequency-converting material using mirror scanners. The aim is to test IR sensors for the wavelength range 3-5 μm. No optical element for focusing the laser radiation into the frequency-converting material is provided between laser source (s) and frequency-converting material (see FIG. 1 - 2 ). Therefore, even here, the converted radiation can not have high spatial coherence.

In DE 690 32 477 T2 A device for manufacturing a color cathode ray tube is described, which uses a laser, a focusing optics for structuring a phosphor layer by an exposure process. In DE 691 21 195 T2 For example, to increase the efficiency of a light source, energy outside a desired spectrum is converted by fluorescers into radiation in the desired range. The coating is applied to at least one surface which converts the beam direction or cross-sectional shape of the light beam.

In US 488383 A Cross-section converters are described which form a light-emitting area of a light source into a line-shaped illuminated area. In WO 91/03703 A1 An interferometer is described which includes a waveguide (optical fiber) which frequency converts injected light. The disadvantage of this method is that it is complicated and thus expensive to incorporate frequency-converting substances in sufficient concentrations in the waveguide. Furthermore, the choice of incorporable substances is severely limited because they must be compatible with the waveguide material.

Other Frequency conversion methods such as SHG, SMF and OPO are based on nonlinear multiphoton processes, for the expensive nonlinear crystals required are. These methods only convert the centroid wavelength, but broaden the wavelength spectrum Not. They are therefore unsuitable for generating short-coherent radiation.

• a) gut fokussieren lassen (vorzugsweise auf weniger als ca. 100 μm²),
• b) eine geringe longitudinale Kohärenzlänge aufweisen (ca. 10... 100 μm),
• c) eine hinreichend hohe Strahlungsleistung aufweisen (mindestens einige mW). Diese Anforderungen werden von Superluminiszenzdioden (SLD) grundsätzlich erfüllt. Allerdings sind derartige SLD aufwendig in der Herstellung und daher relativ teuer.

Optical coherence tomography (OCT) requires radiation sources that are:

• a) focus well (preferably less than about 100 μm ² ),
• b) have a low longitudinal coherence length (about 10... 100 μm),
• c) have a sufficiently high radiation power (at least a few mW). These requirements are basically met by superluminescent diodes (SLD). However, such SLD are expensive to manufacture and therefore relatively expensive.

The Invention sets itself the task of a new radiation source create that in particular for the optical coherence tomography suitable is. It should be cheaper as superluminescent diodes and allow a lower coherence length, what to a higher one OCT resolution leads.

The solution The object succeeds according to the invention with the features of the claim 1.

The under claims 2 to 10 are advantageous embodiments of the main claim. The Generation of short coherence Radiation takes place by means of laser light, which is on a cooled phosphor layer is focused. As laser sources are preferably laser diodes in Wavelength range between 400 nm and 800 nm are used. A conventional laser diode (basically any type of laser is possible here) radiates coherent Laser radiation from. The coherence length is typically in the meter range. The laser radiation is on the Fluorescent layer focused on a transparent substrate is applied. The phosphor converts the laser radiation into short-coherent radiation according to its emission spectrum around. Phosphors with high conversion efficiency (up to 90%) are known to the skilled person. The thickness of the phosphor layer is chosen so that the laser light almost completely absorbed and thus exploited to the maximum.

A significant advantage of the invention is the spatial and thus thermal decoupling of the light-generating laser from the location of the short-coherent light generation, which is not ge, for example, in LEDs give is.

Of the Focus diameter of the laser radiation determines the luminous area of the short coherent Radiation. In order to achieve high luminance, the phosphor is effectively cooled. Therefore, the transparent support material one possible high heat conduction on. The of the carrier material absorbed heat will then over good heat conducting copper blocks and / or Peltier elements dissipated. Should this type of cooling not sufficient, the phosphor can by means of an air flow additionally cooled.

Since the short-coherent light is emitted from a small volume of typically less than 1000 μm ³ , it can be relatively well collimated with a second optic. The short-coherent radiation generated in the phosphor generally has too great a spectral width (ie, too short a coherence length). Therefore, a spectral filter is located downstream, which is located in the collimated beam path. This spectral filter is preferably a bandpass with the desired spectral width and characteristic. Subsequently, the short-coherent radiation is used for illumination. For example, it is focused for OCT measurement in the anterior chamber or on the retina of the human eye.

in the Contrary to the limitations of LEDs, far higher surface luminance can be achieved here the short-coherent one Radiation can be achieved because in principle any number of laser beams Focus on a small illuminated area without significant losses to let. The laser intensity is essentially limited only by the thermal stability of the Phosphor.

The Invention is particularly suitable, the limited luminance of LEDs in far-field fluorescence microscopy (especially at 540-580 nm). Furthermore, it is also used in scanning OCT microscopy intended. For OCT devices with a fiber coupling of the short-coherent light becomes a low-cost laser diode used. The coupling of the laser radiation takes place in an optical fiber with a core diameter of about the lateral resolution (focus diameter) corresponds to the OCT application. The fiber end of the optical fiber is coated with the phosphor, the laser radiation largely absorbed and light the desired wavelength emitted. The phosphor is made by means of a UV-curing Adhesive applied to the fiber end.

Conveniently, After a collimation of the radiation, a spectral filtering takes place of the emitted short-coherent Light with an absorption and / or interference filter, so that the desired coherence function corresponding wavelength spectrum (e.g., Gaussian function with FWHM = 20-40 nm). In the further the fiber end is imaged on the sample.

to heat dissipation the fiber end should be in a hole of a copper block. additionally Here is an air flow cooling meaningful.

By the use of a phosphor turns the coherent laser radiation into incoherent (fluorescence) radiation converted with a bandwidth of typically 100 nm. Therefore, mostly a spectral post-filtering necessary.

It Permanently stable phosphors with conversion efficiencies exist up to 90%. The short-coherent Radiation source is particularly in eye diagnostic devices Used OCT basis. in principle But the radiation source can also be used for all other types of OCT measurements be used.

One Another field of application is fluorescence microscopy, in which it There are fluorophores for that do not have efficient semiconductor radiation sources available.

Especially It is difficult to find powerful LEDs or laser diodes with focal wavelengths in the Range 540-580 nm. This spectral gap is due to the radiation source filled.

The The invention will be described below with reference to exemplary embodiments. Show it:

1 : Short-coherent radiation source with an excitation source

2 : Fluorescence measurement of rhodamine 800, dissolved in ethanol

3 : Fluorescence measurement of rhodamine 800, embedded in optical putty

4 : Short-coherent radiation source with multiple excitation sources

5 : Short-coherent radiation source with a moving fluorescent layer in reflection

6 Short coherent radiation source with a moving fluorescent layer in transmission

7 Short-coherent radiation source with an optical fiber

8th Short coherent radiation source accordingly 5 with the representation of an OCT application on the eye

According to 1 A coherent radiation source sends a laser 1 , a semiconductor laser diode preferably emits radiation of wavelength λ ₁ . With the help of a first optic 2 This radiation is applied to a phosphor layer 3 focused, wherein the focus diameter is chosen so that it is adapted to the subsequent use of the fluorescent light. For an OCT application on the eye, a focus diameter of 20 μm is typically used.

The phosphor layer converts the coherent excitation light via luminescence (in the narrower sense generally via fluorescence) into short-coherent radiation with the centroid wavelength λ ₂ > λ ₁ . It is considered here only 1-photon excitation, since the cross-sections for multi-photon excitations are too small. Thus, λ ₂ -λ _{1 represents} the Stokes shift in luminescence. The coherence length of the luminescent radiation is given to a good approximation by: I c = λ 2 2 / (Δλ) (1)

Δλ is the spectral half-width of the luminescent light. λ ₁ can be in the range 300-2000 nm, λ ₂ in the range 350-2500 nm. For OCT applications on the eye or on the skin, preference is given to: λ ₁ = 600-1300 nm, λ ₂ = 650-1400 nm, particularly preferably: λ ₁ = 750-900 nm, λ ₂ = 800-950 nm.

Because the luminescent light of a small volume, defined by the focus diameter of the laser 1 and the thickness of the phosphor layer 3 is emitted, the luminescent light has a high spatial coherence. This is the crucial prerequisite for efficiently refocusing the luminescent light on a small spot in a subsequent application. The good focusability is a mandatory requirement for scanning OCT methods. The luminescent light by means of an optic 4 collimated to it through a spectral filter 5 (eg interference filter, absorption filter) which limits the bandwidth Δλ. Thus, under the condition of the equation (1), the coherence length becomes adjustable.

The following is the luminescent light of the OCT application 6 supplied, which serves for example for the examination of the anterior chamber of the eye or the fundus. In particular, the length of the anterior chamber of the eye or retinal detachments can thus be spatially measured. Compared to superluminescent diodes (SLD), the radiation source according to the invention has the advantage that higher depth resolutions can be realized. This results from the fact that the depth resolution can not be better than I _c / 2. SLDs have a spectral width of at most a few 10 nm, while with a phosphor easily more 100 nm can be achieved.

When Phosphors come in all available phosphors with suitable properties (excitation and emission spectra, Fading resistance, temperature resistance) in question. As inorganic phosphors suitable are, for example, so-called "phosphors", such as silicate germanate, europium (II) activated alkaline earth ortho silicate phosphors, or quantum dots on a semiconductor basis.

When organic phosphors come in particular all laser dyes, such as the rhodamine or styryl dyes in question.

The 2 and 3 show measurements with a structure according to 1 ,

at 2 there is the phosphor layer 3 from liquid ethanol in which the laser dye Rhodamine 800 was dissolved. For excitation, a 633 nm HeNe laser was used.

3 shows a slightly different fluorescence spectrum since the same dye was embedded in an optic putty OK2008. For excitation, a 633 nm HeNe laser was used.

A Spectral filtering has not yet been done here, but would be for one OCT application useful to suppress the secondary maximum at about 790 nm.

In the descriptions to the 4 to 8th Only the changes that are compared to the 1 be made. Like reference numerals designate the same parts.

4 shows that instead of a laser source in 1 several laser sources 1 which are already each the first focusing optics 2 contain. Because of the low light conductance of laser radiation, in principle very many laser sources can be used to increase the intensity in the focus.

The arrangement according to 5 uses a phosphor layer 3 resting on a rotatably mounted disc 10 is applied ring-like. Because the surface 13 is mirrored, on which the phosphor layer 3 is applied, the excitation light is used more efficiently and increases the yield of usable fluorescent light.

Through the rotating disc 10 the life of the phosphor is considerably extended (more than a factor of 100) and the heat is distributed more spatially by the absorption of light. Unabsorbed laser radiation does not get directly into the useful beam path.

The arrangement according to 6 corresponds to the in 5 shown, with the difference that the rotatably mounted disc 10 containing the annular phosphor layer 3 carries, is operated in Transmisssion.

In the arrangement according to 7 becomes the light of the laser 1 by means of the first focusing optics 2 in a light guide 12 coupled, wherein the phosphor layer 3 on the end of the light guide 12 is applied, but no phosphor in the light guide 12 located. The light guide may be formed as a rod, as a waveguide or as an optical fiber.

The representation in 8th corresponds to the in 5 showing the use of the short coherent light source in an OCT on the eye. The short-coherent radiation is through a second focusing optics 11 in an optical fiber 14 coupled and guided via optical elements not shown to the eye. The OCT application 6 For example, it can be integrated in a fundus camera.

1

laser

2

first focusing optics

3

Phosphor layer

4

collimating optics

5

spectral

6

OCT application

7

transparent carrier

8th

heat dissipation

9

engine

10

rotating disc

11

second focusing optics

12

optical fiber

13

mirrored surface

14

optical fiber

R

radius

λ ₁

wavelength

λ ₂

wavelength

Δλ

Wavelength range

Claims (10)

Radiation source, which radiation of a laser ( 1 ) used as the output radiation, wherein in the beam path in the light propagation direction, a first focusing optics ( 2 ) and subsequently in the cross-sectional area of the focused output radiation a phosphor layer ( 3 ) are arranged for beam conversion, further in the beam path of the converted radiation, a spectral filter ( 5 ) and subsequently an object to be illuminated ( 6 ) are arranged. Radiation source according to Claim 1, characterized in that the phosphor layer ( 3 ) on a transparent support ( 7 ) is arranged. Radiation source according to Claim 1, characterized in that the phosphor layer ( 3 ) on a mirrored surface ( 13 ) of a carrier is arranged. Radiation source according to Claim 1, characterized in that the phosphor layer ( 3 ) on a light exit surface of an optical fiber ( 12 ) is arranged. Radiation source according to Claim 1, characterized in that radiation from more than one laser ( 1 ) on the phosphor layer ( 3 ). Radiation source according to Claim 1, characterized in that the phosphor layer ( 3 ) ring-like on a rotating disk ( 10 ) is applied. Radiation source according to Claim 1, characterized in that the phosphor layer ( 1 ) to a good heat-conducting material for heat dissipation ( 8th ) adjoins. Radiation source according to Claim 1, characterized in that the phosphor layer ( 3 ) with a cooling air flow for heat dissipation ( 8th ) is overflowed. Radiation source according to claim 1, characterized in that further in the light propagation direction in front of the spectral filter ( 5 ) a collimating optic ( 4 ) is arranged. Radiation source according to Claim 1, characterized in that, furthermore, in the light propagation direction downstream of the spectral filter ( 5 ) a second focusing optics ( 11 ) and an optical fiber ( 14 ) are arranged.