MEDICAL LASER SYSTEM

Abstract

A medical laser system for ophthalmological surgery, such as photocoagulation or another method of photo-thermal stimulation. The laser system includes a laser device configured to emit laser radiation at a visible wavelength and a laser controller configured to control the laser system. The laser device includes a laser source configured to emit a source radiation, and a frequency converter configured to receive the source radiation and to output said emitted laser radiation as a frequency-converted output of a frequency conversion process, which has an efficiency dependent on a wavelength of the source radiation. The laser controller is configured to adjust the optical output power of the emitted laser radiation to a selected target value by adjusting an operating temperature of the laser source and/or an operating temperature of the frequency converter such that the frequency converter is operated at an efficiency smaller than the maximum of the efficiency.

Claims

1. A medical laser system for ophthalmological surgery, such as photocoagulation or another method of photo-thermal stimulation, the laser system comprising: a laser device configured to emit laser radiation at a visible wavelength; and a laser controller configured to control the laser system; wherein the laser device comprises: a laser source configured to emit a source radiation, and a frequency converter configured to receive the source radiation and to output said emitted laser radiation as a frequency-converted output of a frequency conversion process, the frequency conversion process having an efficiency dependent on a wavelength of the source radiation, the efficiency having a maximum, the emitted laser radiation having an optical output power; wherein the laser controller is configured to adjust the optical output power of the emitted laser radiation to a selected target value by adjusting an operating temperature of the laser source and/or an operating temperature of the frequency converter such that the frequency converter is operated at an efficiency smaller than the maximum of the efficiency.

2. A medical laser system according to claim 1; wherein the laser device is configured to emit radiation at a visible wavelength and having a beam quality factor M.sup.2 of less than 3.

3. A medical laser system according to claim 2, further comprising a beam delivery system configured to receive the emitted laser radiation and to direct an output radiation to a target area; where the output radiation has a beam quality factor M.sup.2 of less than 3.

4. A medical laser system according to claim 1; further comprising: one or more focussing actuators for adjusting a focal point of the emitted laser radiation or of the output radiation; one or more direction actuators for adjusting a light-emitting part of the laser system so as to selectively direct the emitted radiation or the output radiation to different target areas at different distances from the light-emitting part; and a controller configured to control the medical laser system to a) emit a focussed beam towards a first target area; wherein the one or more focussing actuators are set so as to focus the focussed beam a first focal distance; b) adjust the one or more direction actuators so as to direct the focussed beam to a second target area, and to c) emit the focussed beam towards the second target area without refocusing the emitted radiation or the output radiation.

5. A medical laser system according to claim 1; wherein the laser controller is configured to: determine an optical output power emitted by the laser system, compare the determined optical output power emitted by the laser system to a selected target value, adjust the operating temperature of the frequency converter based on the comparison of the determined optical output power emitted by the laser system with the selected target value so as to reduce the magnitude of the difference between the determined optical output power and the selected target value.

6. A medical laser system according to claim 1; wherein the laser controller is further configured to control a performance parameter of the emitted laser radiation by controlling an injection current to the laser source.

7. A medical laser system according to claim 1; wherein the laser device comprises a tapered diode laser.

8. A medical laser system according to claim 1; wherein the laser device comprises: a substrate; a master oscillator mounted on the substrate and configured to emit a master radiation; an optical amplifier mounted on the substrate and configured to receive and amplify the master radiation and to output the amplified radiation as the source radiation; an optical isolator mounted on the substrate in a radiation path of the master radiation between the master oscillator and the optical amplifier; one or more optical elements mounted on the substrate and configured to direct the master radiation from the master oscillator into the optical isolator and/or to direct the master radiation from the optical isolator towards the optical amplifier; wherein the one or more optical elements are mounted on the substrate by one or more deformable mountings.

9. A medical laser system according to claim 1; wherein at least the laser source is packaged in a housing in an atmosphere containing oxygen.

10. A medical laser system according to claim; comprising an anamorphic lens configured to direct the source radiation into the frequency converter.

11. A medical laser system according to claim 10; wherein the anamorphic lens is mounted on a substrate by a deformable mount.

12. A medical laser system according to claim 1 configured to emit laser radiation at a wavelength in the range from 480 nm to 632 nm.

13. A medical laser system according to claim 1; wherein at least the laser source is packaged in a housing in an atmosphere containing oxygen.

14. A medical laser system according to claim 1; comprising a sensor for detecting the optical output power of the emitted laser radiation; and wherein the laser controller is configured to adjust the operating temperature of the frequency converter and/or of the laser source until the detected optical output power matches the selected target value.

15. A medical laser system according to claim 1; wherein the laser source is a continuous-wave laser source.

16. A medical laser system according to claim 1; comprising a user-controllable shutter for selectively blocking the emitted laser radiation; and wherein the laser controller is configured to prevent opening of the shutter during adjustment of the optical output power.

17. A medical laser system according to claim 1; wherein the selected target value is a user-selectable target value selectable between an uppermost selectable target value is larger than between 500 mW and 2 W; and wherein the lowermost selectable target value is between larger than 50 mW and smaller than 500 mW.

18. A medical laser system according to claim 1; wherein the laser controller is configured to adjust the operating temperature of the frequency converter and/or of the laser source such that the frequency converter is operated in a regime in which a change of the efficiency of the conversion process as a function of the operating temperature of the frequency converter is at least 10% per Kelvin.

19. A medical laser system according to claim 1; wherein the laser controller is configured to adjust the operating temperature of the frequency converter and/or of the laser source such that the frequency converter is operated in a regime in which the efficiency of the conversion process is greater than 2% and less than 95% of a maximum conversion efficiency as a function of the operating temperature of the frequency converter and/or of the laser source.

20. A medical laser system according to claim; wherein the laser controller is configured to adjust the operating temperature of the frequency converter and/or of the laser source until the wavelength of the source radiation is displaced from a wavelength at which maximum conversion efficiency is obtained by more than 10 pm and less than 100 pm, such as by more than 20 pm and less than 100 pm.

21. A medical laser system according to claim 1; wherein the laser controller is configured to adjust the operating temperature of the frequency converter and/or of the laser source until the operating temperature of the frequency converter deviates from a temperature at which maximum conversion efficiency is obtained by more than 0.1 K and less than 1 K.

22. A method for adjusting the optical output power of a laser system in a medical laser system to a selected target value, such as a system for ophthalmological surgery, such as a photocoagulation laser system or another photo-thermal stimulation laser system, the laser system comprising: a laser source emitting source radiation in a first wavelength interval, such as an infrared laser source emitting infrared light; a frequency converter having an input and an output, the frequency converter being configured to receive the source radiation from the laser source at the input and to convert the source radiation to converted radiation in a second wavelength interval, e.g. visible light, and the output being configured to output the converted radiation, and an optical output power of the converted radiation being dependent on a operating temperature of the frequency converter, a control element, e.g. a heater and/or a cooling element, operable to adjustment and stabilize the operating temperature of the frequency converter, an output configured to allow the converted radiation to exit the laser system, wherein the method comprises actively controlling the temperature of the frequency converter so as to adjust or stabilize the optical output power emitted by the laser system by performing the following steps of: a. determining the optical output power emitted by the laser system, b. comparing the determined optical output power emitted by the laser system to the selected target value, c. adjusting the operating temperature of the frequency converter based on the comparison in step (b) so as to reduce the magnitude of the difference between the determined optical output power and the selected target value.

23. A method according to claim 22; wherein the target value is user-selected.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0077] FIGS. 1-2 show schematic block diagrams of examples of a laser device.

[0078] FIG. 3 illustrates the adjustment of the optical output power by adjusting the operating temperature of the frequency converter.

[0079] FIG. 4 shows a schematic block diagram of a medical laser system.

[0080] FIG. 5 shows a schematic block diagram of a scanning medical laser system.

DETAILED DESCRIPTION

[0081] Various aspects and embodiments of a medical laser apparatus disclosed herein will now be described with reference to the drawings.

[0082] FIG. 1 shows an example of a laser device comprising a laser source 100 and a frequency converter 101, such as a frequency doubling crystal. The laser source is provided in a configuration where the radiation 103 from a laser 102 is coupled through an isolator 105 to a semiconductor optical amplifier 106. This configuration is referred to as a hybrid master oscillator power amplifier (MOPA) with isolation stage between the laser oscillator and the power amplifier. The laser 102 is also referred to as the master oscillator (MO); it may e.g. be a distributed feedback (DFB) laser or another suitable single-frequency laser. The power amplifier (PA) 106 may e.g. be a tapered optical amplifier. The amplified beam 111 is then fed into the frequency converter 101 which outputs the output beam 112 of the laser device as a frequency-converted beam. The laser device comprises a temperature control device 144, e.g. a Peltier element, a resistor or the like, operable to control the temperature of the frequency converter 101. To this end, the laser device comprises a laser control unit 122 operable to control the temperature control device. It will be appreciated that the laser control unit may also control other control parameters of the laser device.

[0083] The isolator is a bulk optic component utilizing a free space propagating collimated optical beam. To this end, the laser source comprises a collimating lens 107 arranged in the beam path between the laser 102 and the isolator 105 and configured to collimate the radiation from the laser and direct it into the isolator 105. The laser source further comprises an optical lens 108, e.g. a collimating lens, arranged in the beam path between the isolator and the optical amplifier 106 and configured to focus the collimated beam from the isolator into the optical amplifier. Advantages of this configuration are that the isolator loss can be compensated by gain in the power amplifier. Thus in the 1000 nm wavelength region low cost miniature thin film garnet isolators can be used. This reduces overall cost, improves performance, and reduces overall size relative to an integrated MOPA or high powered tapered laser followed by a larger-aperture low-loss isolator while maintaining or improving insensitivity to reflection.

[0084] In this hybrid configuration a proper alignment between the laser and the collimating lens and between the collimating lens and the power amplifier is critical. In one embodiment, this alignment is achieved by deformable lens holder alignment structures. In particular all components are either directly or indirectly mounted on a substrate. The lenses 107 and 108 may be mounted on the substrate via the deformable lens holder alignment structures, also referred to as deformable lens mounts. The deformable lens holder alignment structures may e.g. be structures as described in U.S. Ser. No. 06/559,464.

[0085] The laser device further comprises a photodetector 110 or other suitable sensor operable to monitor the optical output power of the emitted, frequency-converted laser beam 112. To this end, the laser device comprises a collimating lens 104 collimating the emitted laser beam 112 and a beam splitter 109 or other suitable optical element for redirecting a minor portion of the collimated emitted laser beam 112 towards the photodetector 110. It will be appreciated that the laser device may further comprise one or more additional detectors for monitoring performance characteristics of the master oscillator, e.g. for monitoring the wavelength or output power. The output of the photodetector is fed into the laser control unit 122 so as to allow the laser control unit to control the temperature control device 144 based on the optical power monitored by the photodetector and based on a selected target value of the output power which may be user-selected. The laser device further comprises a shutter 138 for selectively blocking the emitted laser beam 112. The shutter may be user-controlled but the laser control unit may prevent the shutter from opening when the power (or other performance parameters of the laser device) is/are adjusted.

[0086] The laser device further comprises a lens assembly 113 positioned in the beam path between the optical amplifier of the laser source and the frequency converter. The lens assembly may consist of a single anamorphic lens or of a set of lenses configured so as to compensate for astigmatism in the output radiation from the tapered amplifier. The lens assembly may also be mounted on one or more deformable mounts as described in connection with lenses 107 and 108.

[0087] It is known that high power semiconductor lasers in the 1000 nm wavelength range packaged in hermetic environments can suffer sudden degradation due to breakdown of residual contaminates at the facet component. In the hybrid configuration shown in the above examples not only could his happen at the laser facet but at both facets of the optical amplifier. This degradation can be avoided by packaging the laser in an atmosphere containing a slight amount of oxygen.

[0088] FIG. 2 shows another example of a laser device comprising a laser source 200 and a frequency converter 101, such as a frequency doubling crystal. The laser device of FIG. 2 is similar to the one of FIG. 1 except that the laser source 200 of the laser device of FIG. 2 is a tapered laser diode 202.

[0089] FIG. 3 illustrates the adjustment of the optical output power by adjusting the operating temperature of the frequency converter. In particular, FIG. 3 illustrates the dependence of the frequency conversion efficiency on the operating temperature of the frequency converter and on the wavelength. FIG. 3 shows the efficiency of the frequency conversion process as a function of wavelength for two different operating temperatures. The conversion efficiency as a function of wavelength for a first operating temperature is schematically illustrated by curve 340, while the conversion efficiency as a function of wavelength for a second, different operating temperature is schematically illustrated by curve 341. As can be seen from FIG. 3, a change in operating temperature of the frequency converter results in a shift of the maximum efficiency to a different wavelength. The frequency of the source radiation emitted by the laser source, i.e. the wavelength before frequency conversion, is illustrated by the dashed line 339. It will be appreciated that the source radiation in practice will be a narrow peak of finite width, i.e. the line 339 may be considered as representing the center wavelength of the source radiation.

[0090] As can easily be seen from FIG. 3, for different operating temperatures of the frequency converter, the conversion efficiency at the actual source wavelength differs considerably, as illustrated by the dotted lines 342 and 343, respectively.

[0091] Accordingly, by varying the operating temperature of the frequency converter, the conversion efficiency at which the frequency converter converts the source radiation can be controlled. Since, for a given optical power of the source radiation, the optical power of the frequency-converted output beam depends on the conversion efficiency, the optical output power can be controlled by adjusting the operating temperature of the frequency converter.

[0092] It will further be noted that the wavelength of the source radiation depends on the operating temperature of the laser source. Therefore, the power control may also be performed by adjusting the operating temperature of the laser source, which causes the dashed line to be moved along the horizontal axis relative to the conversion efficiency curve. However, controlling of the output power by adjusting the operating temperature of the laser source thus also changes the wavelength of the emitted, frequency-converted output beam which, in some embodiments may be less desirable.

[0093] Generally, the laser controller adjusts the operating temperature of the frequency converter and/or the operating temperature of the laser source such that the frequency converter operates at a wavelength on an upward or downward slope of the conversion efficiency as a function of wavelength and, in particular at a wavelength displaced from an optimum wavelength where that conversion efficiency has a maximum and/or displaced from a flat portion where the conversion efficiency substantially does not change in dependence of the wavelength. Considering that thermal adjustment of either a frequency converting crystal or of the laser diode itself is often no faster than a time scale of 1 s, a fast response in this stabilization and control scheme may be achieved when operating in a regime in which the slope of the phase matching curve is 10% of the maximum conversion efficiency per kelvin or more, such as 15% of the maximum conversion efficiency per kelvin or more.

[0094] The control process implemented by the laser control unit may utilise any suitable control loop, e.g. based on a monitored optical output power. An example of an algorithm for such a control loop may be based on the PID (proportional-integral-derivative) control loop scheme. In such an implementation, the desired output power (target value) as a function of time t is denoted P.sub.s(t) and the measured output power is denoted P(t). The error e(t)=P.sub.s(t)P(t) is the difference of the two. The temperature set point T.sub.s(t) of the frequency converter is the control variable. It can calculated as

[00001]$T_s\left(t\right)=T_s\left(0\right)+K_P.Math.e\left(t\right)+K_I.Math.{}_0^t.Math.e\left(\right).Math.d.Math.+K_D.Math.\frac{d.Math.e\left(t\right)}{d.Math.t}$

where K.sub.P, K.sub.I and K.sub.D are constants that determine the temporal behavior of the control loop and T.sub.s(0) is an initial guess of the desired temperature set point, e.g. a predetermined constant. As time passes, the temperature set point is continually adjusted to be closer and closer to the value that results in the desired output power. If the desired output power is changed to a new value, e.g. due to a user input, the above control loop algorithm will subsequently change the temperature set point to achieve the new desired output power.

[0095] FIG. 4 shows a schematic block diagram of a medical laser system. The laser system comprises a laser unit 220, a foot switch 231 and a slit lamp adapter 232.

[0096] The laser unit comprises a laser device 221, a control unit 222 comprising control electronics and a user interface, and a fiber focus assembly 223.

[0097] The laser device 221 may be or comprise a laser device as described in FIG. 1 or 2. The laser device 221 is communicatively connected to the control unit 222, and the laser device outputs a laser beam 112 that is fed into the fiber focus assembly 223.

[0098] The fiber focus assembly 223 feeds the laser beam into an optical fiber 224 which is operable to guide the laser beam into the slit lamp adapter. The fiber focus assembly is also communicatively connected to the control unit 222. The fiber focus assembly comprises optics operable to focus the laser beam 112 into the optical fiber 224. The laser beam 112 is for use as a treatment beam. The fiber focus assembly may further be operable to couple a red (e.g. 635 nm) diode aiming beam down the same optical path as the treatment laser. The fiber focus assembly may further house a photodiode pickoff to monitor the laser output for calibration and safety.

[0099] The slit lamp adapter 232 directs the laser beam as treatment beam 233 into a subject's eye 234. The slit lamp adapter comprises an optical train operable to focus the laser output to a user-selectable spot sizes at a target tissue. The slit lamp adapter may further comprise a dichroic mirror operable to reflect the aiming and treatment laser beams towards the target, but to allow other visible wavelengths to pass. The slit lamp adapter may further comprise one or more eye-safety filters operable to protect a user from laser reflections through the visual axis. The slit lamp adapter may further comprise a mechanical mount operable to adapt to pre-existing diagnostic slit lamps. The slit lamp adapter may further comprise circuitry, e.g. a PCB, operable to communicate with the control unit 222, e.g. for facilitating spot size selection

[0100] The foot switch 231 may be a wired or a wireless device that may comprise redundant contacts for safety. The foot switch may be operable to be pressed by the user so as to cause the system to deliver laser light to the target tissue. To this end, the foot switch may be operable to control a shutter for selectively blocking the laser light.

[0101] The foot switch 231 and the slit lamp adapter are communicatively connected to the control unit 222.

[0102] The control unit 222 may comprise one or more of the following: [0103] power input and AC/DC conversion circuitry, e.g. implemented as one or more PCBs, [0104] control circuitry, e.g. implemented as one or more control PCBs, which may include on-board firmware to provide commands to all sub systems on laser control, sub-system control, safety monitoring, and laser pulse generation, temperature control. [0105] a user Interface which may be include knobs and a display, and/or a touchscreen interface. The user interface allows the user to select the desired parameters and provides the user with system status.

[0106] FIG. 5 shows a schematic block diagram of a scanning medical laser system, e.g. a laser scanning photocoagulator. The scanning medical laser system comprises a laser device 221 and a laser control unit 222, e.g. as described in connection with

[0107] FIG. 4, the laser device 221 is operable to feed a laser beam into an optical fiber 224, e.g. via a fiber focus assembly (not shown) as described in connection with FIG. 4. The optical fiber feeds the laser beam into a focussing unit 335 and further into a scanning unit 336. The focussing unit comprises adjustable focussing optics adjustable by one or focussing more actuators. The adjustable focussing optics of the focussing unit are controlled by a scanning control unit 337 which may be a separate control unit or integrated into laser control unit 222. For example, the focussing unit may be implemented as a slit lamp adapter, e.g. as described in connection with FIG. 4. Similarly, the scanning unit is controlled by the scanning control unit to direct the laser beam to desired target locations. The scanning unit 336 may e.g. comprise an X & Y axis galvanometer or other types of direction actuators which receives instructions from the control electronics 337 on the beam position and the pattern to be displayed. The blanking between each spot in a pattern may be provided by an electronic shutter, a mechanical shutter or by another galvanometer.

[0108] Generally, there are three axes in ophthalmic laser delivery that should be parfocal to get consistent and safe results. These are the visual, the illumination and laser axes must all be in focus at the same point. The illumination and visual axes are provided by the slit lamp and the laser is adapted to focus to their plane. Many lasers have a very narrow depth of focus which makes it difficult for the user to get consistent laser uptake because the laser is moving in and out of focus even though the visual and illumination fields are OK. The longer the laser depth of focus, the more forgiving the laser is to out of focus use and it makes it much easier on the Doctor to perform the treatment. The excellent beam quality of embodiments of the laser device described herein provides a longer depth of focus and causes the laser device to be much easier to use than prior art systems.

[0109] In the claims enumerating several means, several of these means can be embodied by one and the same element, component or item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

[0110] It should be emphasized that the term comprises/comprising when used in this specification is taken to specify the presence of stated features, elements, steps or components but does not preclude the presence or addition of one or more other features, elements, steps, components or groups thereof.