Q-Switched Laser System, in Particular for Laser Lithotripsy

The invention relates to a Q-switched laser system of the type which includes a laser-active medium in a resonator construction, an arrangement for optical pumping, and a passive Q-switch.

Solid-body laser systems for laser lithotripsy are known from the publications WO 90/12544, WO 90/04358, WO 91/05332 and AT-B-380634. In these systems, which operate with short, single pulses, the fiber which transmits the energy to the location of application is destroyed very quickly in practical use because of the short pulse lengths used in the systems. Moreover, the technical structures of these laser systems are very cost- and maintenance-intensive.

In addition, experimental investigations have revealed that it is favorable to operate with a so-called double pulse in laser lithotripsy, which involves the transmission in rapid succession of two pulses to the location of application, with the one pulse being shifted toward the actual energy-carrying pulse toward the short-wave range ("blue") typically representing the harmonic of the carrier wave length. This combination permits premature ignition of the optical breakdown (plasma) at the rock surface to be broken up, and thus allows for a greater effectiveness in rock breaking. However, the disadvantage associated herewith is rapid fiber burnup in practical use.

It is therefore the object of the invention to disclose a Q-switched laser system of the generic type mentioned at the outset which is particularly suited for laser lithotripsy, generates laser pulses that do not lead to premature wear of the transmission fibers, operates efficiently and can be produced cost-effectively.

This object is accomplished by a laser system in which a resonator extension is provided for increasing the laser pulse length, the resonator extension including an optical waveguide.

With the known state of the art as a point of departure, it has been seen that, surprisingly, the efficiency of rock breaking in laser lithotripsy not only depends on the earliest possible ignition of a plasma and the resulting shock wave and the production of cavitation bubbles, but is primarily determined by the fulfillment of an inclusion condition for the plasma with a subsequent compressional shock.

Surprisingly, the theoretical and experimental investigations by the inventors were able to corroborate that optimal inclusion conditions are given when one takes into consideration the delay time of the acoustical shock waves with the retention of certain relationships between the pulse duration and the geometry of the end surface of the fiber that transmits the energy to the location of application. The rule of thumb was found to be that the pulse duration for the optimal inclusion condition corresponds in nanoseconds to 1.5 times the diameter of the energy-applying fibers in micrometers. This means that, for example, in a Q/Q fiber having a 360 .mu.m core and--depending on the manufacturer--having a diameter at the optical cladding of approximately 400 to 420 .mu.m, the optimum pulse duration for the lithotripsy is approximately 600 to 650 ns.

The invention therefore incorporates the concept of configuring the generic laser system such that it delivers corresponding pulses whose length--depending on the application fiber that is used--is particularly between 200 ns and 1 .mu.s, possibly up to a few .mu.s. However, conventional laser-active media and media which are preferred for cost reasons, such as neodymium-doped laser crystals, especially Nd:YAG and Nd:YAlO.sub.3, only have pulse durations between 7 and 15 ns in normal Q-switched operation.

According to the invention, this limitation is overcome by an optical extension of the resonator by means of a long optical waveguide. (A resonator extension using an optical waveguide approximately 18 m in length is typically required for a pulse duration of approximately 650 ns.)

The effect achieved with this measure is that, not only can the pulse duration of the laser system be altered cost-effectively in broad ranges as a function of the length of the waveguide--particularly between 200 ns and a few .mu.s without the emitted pulse peak output being significantly changed (variations <10% were determined), but that simultaneously achieved are a smoothing of the disadvantageous, statistical intensity oscillations (so-called "spikes") which occur during a laser pulse and lead to increased fiber wear, and homogenization of the spatial intensity profile with suppression of excessive intensity elevations in the beam cross-section (so-called "hot spots"). The consequential creation of spatially and temporally smoothed pulse profiles is of considerable advantage for the transmission of the pulses by means of coupled-on, external light waveguides. In particular, it extends their service life significantly.

The invention also includes the concept of stimulating the laser-active medium with a pumping energy which is higher than the pump energy necessary for generating single pulses; with this higher energy, pulse cascades ("bursts") can be produced which--corresponding to the above-described operational conditions of laser pulse radiation in laser lithotripsy--have advantages in terms of effectiveness vis-a-vis single pulses.

FIG. 1a is a schematic representation of a laser system according to an embodiment of the invention,

FIG. 1b is a schematic representation of the primary optical components of a laser system according to an embodiment of the invention which is modified with respect to FIG. 1a,

FIG. 2a is the schematic representation of the path of the beam in a multi-pass waveguide as a component of a further embodiment of the invention, and

FIG. 2b is a schematic representation of the embodiment which includes the multi-pass waveguide.

FIG. 1a schematically shows a preferred optical arrangement of a long-pulse, solid-body lithotripsy laser as an embodiment of the invention. In this instance, an assembly 1 comprises a focussing lens 1.3, a fiber-optic resonator extension 1.2 and an arched resonator end mirror 1.1, whose arch and spacing from the exit of the fiber of the resonator extension (1.2) are dimensioned such that, taking into consideration the numerical aperture of the fiber, the radius of curvature corresponds approximately to half the distance between the fiber end surface and the mirror surface.

An assembly 2 includes a conventional laser cavity. Here 2.1 indicates a laser crystal, preferably Nd:YAG or Nd:YAlo3, 2.2 indicates a flash lamp for optical pumping, and 2.3 a unit for energy supply and system control.

An assembly 3 includes a partially-transparent resonator end mirror 3.4, a non-linear crystal 3.3 for frequency doubling, typically a KTP crystal, and a passive Q-switch 3.2, typically Cr 4+:YAG or LiF(F2-), and a relay optics 3.1 for collimating the radiation.

In an advancement of the concept of the invention, a further increase in the efficiency in producing the first harmonic can be achieved by means of the insertion of a polarization-optical assembly, comprising a Brewster angle polarizer and a retroreflector at essentially a right angle thereto, between either the components 3.2 and 3.3 of the assembly 1 or components 3.2 and 3.3 of the assembly 3.

Assuming the use of a neodymium-doped YAG laser crystal having a 5 mm diameter and 5 cm length, and a pumping energy at the flash lamp of approximately 30 J, the following, typical initial values can be achieved for the preferred embodiment: pulse duration, adjustable between 200 ns and 1 .mu.s, depending on the length of the fiber extension in the resonator, with an approximate 20 m fiber length, mJ at 1064 nm base emission and approximately 15 Mj at 532 nm (second harmonic), assuming a base absorption of the passive Q-switch of approximately 25%. With variations in pulse length of 200 ns to 1 .mu.s, the input energy only changes by about 10%. In the use of polarized laser radiation, the initial energy of the second harmonic increases to approximately 22 to 25 mJ either by means of the insertion of the above-mentioned additional elements of the use of a double-refracting laser material, for example Nd:YAlO.sub.3. In this case, the core diameter of the resonator extension is only 280 .mu.m, so that, taking into consideration the divergence stipulated by the numerical aperture, the transmission of the total emitted energy by means of a Q/Q fiber 4 having a 360 .mu.m core diameter is possible for use in lithotripsy on a rock 5.

According to the invention, a further increase in shattering efficiency in lithotripsy is achieved by means of stimulation of the laser crystal with a pumping energy which is higher than the pumping energy necessary to generate single pulses. A consequence of the increase by 30%, i.e., to approximately 40 J, is the emission of a cascade of single pulses whose temporal spacing is approximately 50 .mu.s with a base absorption of the Q-switch of 25%, and whose temporal spacing can be approximately halved when the base absorption is increased by a factor of 2. This pulse cascade (also called burst) leads to a significant increase in the shattering efficiency on the rock 5 in lithotripsy, because the sequential pulses can be used directly to generate sequential compressional shocks by means of the optimized inclusion conditions.

Another possible embodiment of the above-described arrangement is as a solid-body laser having an erbium-doped laser crystal which emits wavelengths between 1.54 and 2.94 .mu.m, depending on the host crystal or matrix. In this instance, the following laser media are particularly suited for use: erbium:YAG, erbium:YSSG and erbium:YAG co-doped with chromium and thulium, so-called CTE lasers. A zirconium fluoride fiber or an aluminum fluoride fiber is preferred as the waveguide.

In a modification of this embodiment, erbium glass lasers based on chalkogenide glasses (e.g. germanium, arsenic or sulfur glasses) are also used; fibers made of the same chalkogenide glass can be used as waveguides.

Optical crystals such as lithium niobate or lithium iodate are used as passive Q-switches. However, other known Q-switches besides these can also be used.

The use of lenses for focussing the beam inside the resonator can be omitted when the optical waveguide is provided at both ends with a taper-coupler (a coupling-in layer having an approximately wedge- or trumpet-shaped cross-section).

FIG. 1b shows such a configuration having two tapers 1.5, which are spliced to the fiber-optical waveguide 1.2 as end pieces; this configuration is further modified with respect to the arrangement according to FIG. 1a, differing in the sequence of laser crystal 2.1 and waveguide 1.2 between mirror 1.1 and Q-switch 3.2 and the partially-transparent mirror 3.4, and in the configuration of the laser crystal as an erbium-doped crystal.

In a similar manner, holmium-doped, solid-body lasers and solid-body lasers having frequency multiplication in the blue and ultraviolet spectral range, and pulsed gas lasers, for example excimer lasers and nitrogen lasers, can also be implemented.

It is also possible to provide only one of the ends of the waveguide with a taper-coupler.

In another embodiment of the concept of the invention, a planar optical waveguide can also be used in place of the optical fiber as a multi-pass reflection plate. An embodiment of such a planar multi-pass waveguide 1.21 is illustrated in FIG. 2a. As can be seen in FIG. 2a, this waveguide essentially has the shape of a flat cuboid or a plate which is provided with a coupling-in/-out segment 1.22, and whose large optical path length is effected by multiple beam passage with reflection at the walls of the cuboid.

FIG. 2b shows an embodiment of the optical part of the laser system that extensively corresponds to the arrangement illustrated in FIG. 1b and uses a multi-pass waveguide 1.21 of the type shown in FIG. 2a in place of the wound fiber waveguide 1.2 shown in FIG. 1b. All other elements correspond to those in FIG. 1b, and are thus not explained again.

Examples of the material used for the planar optical waveguide--depending on the parameters of the generated laser beam--are chalkogenide glasses of suitable composition (e.g. germanium arsenic, sulfur) or crystals such as sapphire or yttrium aluminum garnet, which are highly-transparent in the appropriate spectral range, or other host crystals of the respective laser media or, for example, silicon, germanium or zinc selenide.

The invention is not limited in its embodiment to the above-disclosed, preferred embodiment. Rather, a number of variations are conceivable which use the illustrated solution, even in fundamentally differently configured embodiments.