Abstract:
The invention relates to a system for ophthalmology or dermatology, having a laser unit for generating pulsed laser radiation, the pulse length of the laser pulses lying in the femtosecond range, a hand unit for emitting laser radiation generated by the laser unit, and a flexible light waveguide for delivering the laser radiation generated by the laser unit to the hand unit. The invention furthermore relates to a corresponding hand unit for emitting pulsed laser radiation generated by a laser unit, the laser radiation having pulse lengths in the femtosecond range.

Description:
TECHNOLOGICAL FIELD 
     The invention relates to a system for ophthalmology or dermatology, as well as to a and unit for emitting pulsed laser radiation. 
     BACKGROUND 
     In refractive opthalmological surgery, the refractive properties of a patient&#39;s eye are modified by interventions on the eye in order to correct defective vision. In particular the so-called LASIK method (LASer In Situ Keratomileusis) is known, in which the patient&#39;s cornea is reshaped. According to the conventional LASIK method, a flat corneal incision is made in a first microsurgical operation step with a mechanical instrument, usually a microkeratome. This creates a so-called “flap”, which can be folded away so that underlying corneal tissue (stroma) is exposed. In the subsequent part of the LASIK operation, a particular ablation pattern is removed from the stroma. The flap is then folded back into place and heals relatively rapidly again with the remaining stroma. The conventional mechanical microkeratome uses a sharp, rapidly oscillating blade. 
     Recently, the microkeratome has been replaced by a laser, in particular a femtosecond laser, which makes the aforementioned flat incision in the cornea. The laser is focused onto a plane below the surface of the cornea, and is guided on a path which produces the flap in the same way as the microkeratome does. The extremely short laser pulses used for this, in the femtosecond range, have such high powers that, with a suitable focusing, it is possible to cut without causing internal heating effects or the like by utilising the so-called photodisruptive effect. Compared with the conventional mechanical microkeratome, higher accuracy and better reproducibility of the LASIK cuts are generally obtained. 
     In order to produce the flap by means of a laser, the patient is first positioned below the femtosecond laser. As for the conventional production of a flap by means of a microkeratome, a fixation ring, which is usually fixed by means of suction, is placed onto the eye. A contact glass, which touches the surface of the cornea and flattens it for maximally accurate production of the corneal incision by exerting a certain pressure, is then placed into the suction ring. A usually conical contact device is then connected to the actual femtosecond laser instrument. This is usually done so that the part of the laser instrument, which emits the laser radiation, is moved with the aid of a motor into the conical contact device. At this time, the patient&#39;s eye is already fixed and stressed by the suction ring and the contact glass in a way which is uncomfortable for the patient. The process of making contact between the suction ring and the laser instrument further increases the stress for the patient, since the process is usually not accomplished entirely smoothly. An additional pressure is therefore exerted at least briefly on the patient&#39;s eye, which can be physiologically and psychologically detrimental for the patient. 
     It is therefore an object of the present invention to provide a remedy in respect of this stress. 
     SUMMARY 
     To this end, the invention provides a system having
         a laser unit for generating pulsed laser radiation, the pulse length of the laser pulses lying in the femtosecond range,   a hand unit for emitting laser radiation generated by the laser unit, and   a flexible light waveguide for delivering the laser radiation generated by the laser unit to the hand unit.       

     According to the invention, the laser radiation generated by the laser unit is therefore guided not by means of a rigid free-beam optical connection to the site to be treated; rather, it is guided by means of a flexible light waveguide to a hand unit and is emitted there. The hand unit, connected to the laser unit only via the flexible light waveguide, may be contacted for example with the suction ring or with its coupling device. The hand unit can be configured with a compact size and with a low weight, which on the one hand reduces the psychological stress owing to a substantially smaller unit which is placed onto the eye. At the same time, the actual stress is reduced by facilitated handling and concomitant lower pressure, which is exerted on to the eye. The term “hand unit” is intended here to mean a unit which is configured in respect of its dimensions and its weight so that, for example, it can be guided manually by an operator and for example placed on an eye. 
     In a preferred embodiment, the laser unit comprises a fibre laser. A fibre laser provides a high beam quality (typically a beam parameter product ≦1.3) with a very compact design. 
     According to another embodiment, the laser unit and hand unit for the system are connected to one another only by one or more flexible cables, at least one of which comprises the flexible light waveguide. Besides the flexible waveguide, the one or more flexible cables may also comprise for example an electricity supply, a vacuum feed and one or more data lines. 
     According to another embodiment of the system according to the invention, the hand unit can be positioned independently of the laser unit within the range of mobility of the cable. If the hand unit is comparable in its size to a conventional microkeratome, or is dimensioned only slightly larger, the flap can be produced with comparable stress for the patient with simultaneously increased precision of the cut. The stresses described above, which are associated with a conventional femtosecond laser for the flap production, are therefore reduced. 
     In this context, in another embodiment according to the invention, the hand unit comprises a coupling instrument which allows mechanical connection to a human eye. The hand unit is either placed directly onto the eye in combination with the suction ring, or is coupled to a suction ring previously fastened on the eye. 
     In another preferred embodiment of the invention, the laser unit comprises a pulse stretcher for temporal stretching of the laser pulses to pulse lengths of more than 1 picosecond. The pulse stretching makes it possible to reduce the intensity of the laser pulses. Inter alia, reducing the pulse power leads to a lower load for the flexible waveguide. 
     In this context, it is likewise advantageous for the flexible light waveguide to be a photonic transmission fibre having a large mode area. These light waveguides, also referred to as large mode area fibres (LMA fibres), have core diameters of from 20 μm to more than 40 μm. Owing to the distribution of the light power over a larger area and simultaneous delivery in a low mode order, or in the fundamental mode, LMA fibres make it possible to transmit the laser radiation emitted by the laser unit without degrading the beam parameters of the laser unit, or destroying the LMA fibre by excessively high intensities. 
     In this regard, it may furthermore be advantageous for at least a part of the flexible light waveguide to cause pulse compression of the pulsed laser radiation generated by the laser unit. This makes it possible for the hand unit to be configured much more compactly. The compression of the laser pulses, which otherwise needs to be carried out in the hand unit, can therefore be relocated to the flexible waveguide and corresponding components in the hand unit can be obviated. 
     According to an embodiment which is particularly preferred in this regard, the flexible light waveguide is a photonic hollow core fibre. These microstructured optical fibres, also referred to as “photonic crystal fibres” (PCF fibres), contain typically fine capillary structures, filled with air or a gas, in the core or in the cladding region. These structures are so small that the guided light “sees” modified effective material properties of the glass. By varying the spacings of the hole centres and the diameters of the capillary structures, it is possible to control the optical parameters of the fibres and the properties of the light guiding. In particular, the aforementioned pulse compression of the pulsed laser radiation generated by the laser unit can be achieved in this way. 
     As an alternative, in one embodiment according to the invention the hand unit may comprise compression means for temporal compression of the pulses of the laser radiation. Such compression means may for example comprise an optical grating, preferably a transmission grating. 
     In one embodiment according to the invention, the hand unit may furthermore have an electro-optical scanner for beam deflection. Such electro-optical crystals for spatially controlling a light beam are usually based on the Pockels or Kerr effect, in which the optical properties of the medium, for instance the refractive index, are modified by applying an electric field to it. In this way, spatial displacement of the light beam can be produced without moving parts. Acousto-optical modulators can also cause rapid and controllable beam deflections by an induced Bragg grating. As an alternative, for example, it is also possible to employ an electro-optical hologram which is produced by recording a volume phase hologram in a liquid crystal monomer mixture, and which generates efficient and controllable beam deflections using external electrical voltages. 
     The invention furthermore provides a hand unit for emitting pulsed laser radiation generated by a laser unit, having
         a fibre input, through which the laser radiation generated by the laser unit enters the hand unit,   optics for guiding the laser radiation, and   a compression instrument for temporal pulse compression of the laser radiation entering through the fibre input and, as an alternative or in addition to the compression instrument, having   a scanner or an electro-optical crystal for beam deflection of the laser radiation.       

     The concept according to the invention—i.e. providing a unit for ophthalmology or dermatology which can be moved independently of the laser radiation source, the use of which leads to less stress for the patient—is therefore implemented in a hand unit. The same applies for the hand unit, likewise according to the invention, for emitting pulsed laser radiation generated by a laser unit, wherein the laser radiation has pulse lengths in the femtosecond range. 
     One of the basic concepts of the present invention is to provide a compact femtosecond laser source, essentially based on fibres, as an easily handleable laser instrument for opthalmological applications. A femtosecond fibre laser is employed as the laser source. It has been found that even pulse energies of approximately 400 nanojoules at 200 kHz are sufficient in order to carry out a very smooth, easily openable flap cut in less than 15 seconds. The fundamental elimination of large-volume power supply units and elaborate active cooling apparatus by using a fibre laser makes it possible to provide a femtosecond LASIK microkeratome with comparable dimensions to a conventional mechanical blade-based microkeratome. It is furthermore proposed that the low pulse energies of less than 1 microjoule, previously stretched to pulse lengths of &gt;1 picosecond, are to be transmitted through transmission fibres with a large mode core without degrading the good beam parameters of the femtosecond laser source or destroying the fibres with large mode diameters by excessively high intensities. The subsequent pulse compression to &lt;500 femtoseconds can then be carried out by miniaturised optical elements in a handpiece. Possible miniature components are a transmission grating for the femtosecond pulse compression and an electro-optical scanner, which functions without moving parts. This beam deflection principle, which is based on the use of an electro-optical crystal, allows entirely sufficient deflection angles of up to 5° in extremely short times of about 1 microsecond. For a wavelength range of from 400 nm to more than 2500 nm, laser beam deflections are therefore possible which, with focused radiation, achieve results that are comparable to a conventional X-Y galvanometer scanner. As an alternative, it is possible to use a so-called photonic fibre with a hollow core, which, with suitable dimensioning, can not only transmit but also compress the picosecond or femtosecond laser pulse. The grating compression in the handpiece can therefore be obviated, and the latter can be made even more compact. This invention provides a femtosecond microkeratome which is suitable as a tool for refractive surgery, in a similarly compact way as the widespread blade-based mechanical microkeratomes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further explained below with the aid of the appended drawings, in which: 
         FIG. 1  represents an exemplary embodiment of an opthalmological laser system, 
         FIG. 2  represents an alternative laser system according to the invention with a simple focusing hand unit for medical application and 
         FIG. 3  represents another exemplary embodiment according to the invention of a laser system with a simple, replaceable glass tip for the contact treatment of opthalmological or other tissues. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an exemplary embodiment according to the invention of an opthalmological laser system  10 . The system has a laser unit  12  and a hand unit  16 , which are connected to one another by a cable  14  having a light waveguide. An eye  100  to be treated is furthermore represented schematically in  FIG. 1 . 
     The laser unit  12  is a femtosecond laser unit configured as a tabletop instrument, which comprises a femtosecond fibre laser oscillator  18 , an amplifier  20  and a pulse stretcher  22 . The components of the laser unit may be structurally combined in a single housing as represented in  FIG. 1 , or formed by two or more separate units which are connected through suitable light waveguides. The fibre laser oscillator  18  operates in a wavelength range of from 1020 nm to 1070 nm. As an alternative, a fibre laser oscillator for the wavelength range of from 1500 nm to 1600 nm could also be used. After the amplifier  20 , pulse energies of between 10 nanojoules and &gt;100 nanojoules are provided, typically 300 nanojoules. The pulse length is from 100 femtoseconds to 800 femtoseconds, typically 300 femtoseconds. The repetition frequency for the pulses is from 0.5 MHz to 100 MHz, typically 5 MHz. The pulse stretcher  22  increases the pulse length to values of between 1 picosecond and more than 10 picoseconds. 
     Connected to the laser unit  12 , there is a passive transmission fibre which is fitted in a cable  14  and is configured in the present exemplary embodiment as a so-called LMA fibre (LMA=“Large Mode Area”). The core diameter of such fibres is typically between 10 and 50 μm, and the length of the transmission fibre may be between 0.5 m and 2 m. Shorter or much longer versions may, however, also be envisaged. As an alternative, the transmission fibre could also be configured as an active fibre, i.e. the fibre itself functions as an amplification medium for the laser radiation emitted by the laser unit  12 . For better handling, the transmission fibre is embedded in a cable which may also comprise electricity, vacuum and/or data lines in addition to the optical waveguide. 
     The cable  14  establishes a connection between the femtosecond laser unit  12  and the hand unit  16  configured as a treatment handpiece. The housing of the hand unit  16  has a handle  38  and a fibre input  24 , through which the pulsed laser radiation generated by the femtosecond laser unit  12  enters the handpiece  16 . There, the divergent light beam leaving the transmission fibre is collimated along a first optical axis A by means of a collimator lens  26  and directed onto a transmission grating  28 . The transmission grating  28  compresses the laser pulses, which have been stretched by the pulse stretcher  22  in the femtosecond laser unit  12 , to the pulse duration of typically 500 femtoseconds or less which is suitable for the opthalmological intervention. The light beam leaving the transmission grating  28  is deviated by a dichroic reflection mirror  30 . This serves as a beam splitter: it has a high reflectivity for the wavelength of the femtosecond pulses, whereas it is highly transmissive for the visible spectral range. 
     The light beam is aligned with an electro-optical deflector  32  by the reflection mirror  30 . The electro-optical deflector  32 , which is also referred to as a scanner, deflects the incident light beam by up to ±5° as a function of the voltage applied to the deflector, with a response time of approximately 1 microsecond. The electro-optical deflector  32  may on the one hand comprise an electro-optical crystal which operates according to the Kerr principle. As an alternative, it is also possible to use an electro-optical holographic grating which can be generated by recording a volume phase hologram in a liquid crystal monomer mixture. By this holographic technology, switching times of 50-5000 microseconds can be achieved with an angular accuracy of ±3°. In both cases, the electro-optical deflector  32  is transmissive in a wavelength range of from 400 nm to 1600 nm. 
     The pulsed light beam deflected by the electro-optical deflector  32  is focused by an f-theta objective  34  onto the working plane  36 , which is symbolized by a double arrow. By means of the f-theta objective  34 , the light beam focus is held in the overall scan field independently of the incidence angle in the working plane  36 . 
     The handpiece  16  has two principal optical axes A and B. The aforementioned first optical axis A is defined by the collimation lens  26  in conjunction with the fibre input  24 , and the second is established by the reflection mirror  30  together with the subsequent components detector  32  and f-theta objective  34 . In a preferred embodiment, the f-theta objective  34  can be displaced in the direction of the optical axis B in order to allow depth adjustment of the working plane  36  and therefore also three-dimensional shaping of the flap cut. 
     A CCD camera  40  is furthermore fitted in the handpiece  16 . It is arranged along the optical axis B on the same side as the reflection mirror  30 , which lies away from the eye  100  being treated. By means of the CCD camera, owing to the transmissivity of all the optical elements along the optical axis B in the visible range, the flap production by means of femtosecond laser pulses can be monitored and optionally controlled in real-time. The housing of the handpiece  16  is provided with a spacer cone  44 , which can be coupled to a suction ring  42  fastened on the eye  100 . The space cone  44  furthermore comprises an applanation window  46 , the function of which will be explained below. 
     The human eye  100  to be treated is also represented schematically in  FIG. 1 . The vitreous body  110  is depicted, as well as the sclera  120  lying in the front region of the eye next the cornea  130  to be treated. The lens  140  is furthermore indicated, and the exit of the optic nerve is indicated schematically opposite the lens. 
     In order to produce a flap cut, the suction ring  42  is initially placed onto the cornea  130  of the eye  100  and aligned, and suction is applied to it. The handpiece  16  is subsequently connected via the spacer cone  44  to the suction ring  42 , for example through vacuum suction (not shown). The cornea  130  is thereby pressed against the applanation window  46 , so that the cornea  130  is provided with a planar surface approximating the applanation window  46  in the contact region. Optionally, after the coupling, the depth of the incision plane may be set by adjusting the f-theta objective  34  along the optical axis B. The flap cutting is then carried out by means of the pulsed laser radiation generated by the femtosecond laser instrument  12  and guided through the transmission fibre  14  to the handpiece  16 . The laser beam is deflected in a suitable way in the working plane  36  by the electro-optical deflector  32 , in order to generate the desired cut geometry. Optionally, three-dimensional cut guiding may also be carried out by interaction of the deflector  32  and f-theta objective  34 . 
       FIG. 2  shows an alternative embodiment of the present invention, in the form of a compact femtosecond laser system  200  with a simple focusing handpiece  216  for medical application in ophthalmology or dermatology. The laser source, in the form of a femtosecond laser unit  212 , is constructed similarly as the embodiment shown in  FIG. 1 , i.e. it likewise comprises a laser oscillator  218  for generating laser pulses in the femtosecond range, an amplifier  220  and a pulse stretcher  222 . In contrast to the transmission fibre used in  FIG. 1 , a so-called photonic crystal fibre  214  with a hollow core is used in the embodiment of  FIG. 2 . Inter alia, this causes temporal pulse compression of the pulses generated and stretched by the laser unit  212 . This obviates the need for grating compression in the handpiece  216 . The handpiece  216  becomes even more compact, since the pulse compression already takes place in the hollow core fibre  214  which simultaneously functions as a transmission grating. Since the hollow core fibre guides the femtosecond pulse in a glass-free empty space, the fibre is not destroyed even by compressed pulses in the femtosecond range with a high intensity. Besides a collimator lens  226 , the handpiece  216  consequently only has a focusing objective  234 , which is represented schematically in  FIG. 2  by two lenses, in its housing. The light beam emerging from the focusing objective is directed onto tissue  202  to be treated. This may be a skin region or tissue in the eye. 
       FIG. 3  shows another embodiment of the invention in the form of a femtosecond laser system  300  with a simple, replaceable glass tip for the contact treatment of ophthalmological and other tissue. In respect of the laser source (laser unit  312  with laser oscillator  318 , amplifier  320  and pulse stretcher  322 ) and the transmission fibre  314  (photonic crystal fibre), the embodiment represented in  FIG. 3  is similar to that of  FIG. 2 . In contrast to the embodiment represented in  FIG. 2 , the embodiment of  FIG. 3  has a treatment handpiece  316  equipped with a replaceable “fibre tip”  304 , instead of the handpiece  216  provided with imaging optics. This fibre tip  304  consists, for example, of quartz glass—other similar materials such as sapphire may also be envisaged—and forms the termination of the transmission fibre. Its length is for example between 5 and 10 mm, the tip having a diameter of approximately 100 μM. The fibre tip  304  acts as a light-guiding element for the laser radiation of the laser unit  312 , which leaves the transmission fibre  314  and is intended to be guided to the front end of the fibre tip  304 . The shape of the front end of the fibre tip  304  determines the focal diameter of the laser light on the tissue  302  to be treated. For sterilisation reasons, the fibre tip  304  is replaceable. This embodiment of the invention may for example be used for glaucoma laser treatment, resection of the trabecular tissue in the eye or corneal keratoplasty. The fibre tip  304  may additionally be equipped with a temperature sensor, which signals any unacceptable heating which the treated tissue may experience.