WO2012062622A1

WO2012062622A1 - Plasma shutter

Info

Publication number: WO2012062622A1
Application number: PCT/EP2011/069189
Authority: WO
Inventors: Fergal O'reilly; Tony Donnelly; Thomas Cummins; Marie Mazoyer
Original assignee: University College Dublin, National University Of Ireland, Dublin
Priority date: 2010-11-10
Filing date: 2011-11-01
Publication date: 2012-05-18

Abstract

A plasma shutter (406) for modifying a laser pulse comprises optics (407) arranged to focus an incident laser beam pulse with a given laser power density on a surface (409) which reflects light at a wavelength of the laser. The power density is above a threshold for forming plasma on the surface in a time less than the duration of the incident laser beam pulse, so that a laser pulse reflected from the surface has a shorter duration than the incident laser beam pulse. The reflective surface may be a solid, e.g. metal, or comprise a liquid metal, whereby plasma formation results in absorption of the remainder of the laser pulse.

Description

Plasma shutter

Field of the Invention

The present invention relates to a plasma shutter for modifying a laser pulse. Background Commercially available TEA CO₂ laser pulses have relatively long pulse durations and an irregu lar pu lse shape. For exam ple, Fig . 1 shows a pulse from an Optosystems Infralight SP10 TEA C0₂ laser.

Various schemes have been proposed to control such laser pulses and to provide a better-defined pulse profile. Fig. 2 shows schematically a pinhole plasma shutter where a focused laser pulse 801 is directed at a pinhole 803 in a target disk 802. The laser causes plasma to form around the periphery of the pinhole and eventually lateral expansion of the plasma occludes the pinhole to the laser, so shortening the pulse length of the output laser beam 804. The pinhole plasma shutter has reported pulse width values of as low as 25-60 ns. However, any further reduction in pulse width is limited by the pinhole size and erosion of the pinhole is also a problem.

Fig. 3 shows schematically shuttering with aid of a secondary pulse. The laser pulse that is to be shortened 901 propagates and is focused parallel to a solid target surface. A secondary focused laser beam 903 is incident on a solid target surface 902. The second pulse 903 can be from an external Qswitched laser (Nd:YAGs have been used) but also can be a portion of the main CO₂ pulse which was taken from the main pulse 901 before being incident on target 902. The secondary pulse 903 causes a rapid generation of plasma whose fast expansion away from the target surface 902 shutters the latter part of the pulse 901 producing a shortened pulse 904. However, in the case of using a second external laser the control of the pulse length of the C0₂ beam is severely limited by timing problems (jitter) between the two lasers. Also, the additional cost of an Nd:YAG laser by comparison to the pinhole shuttering approach of Figure 2 makes it significantly more expensive. When using a portion of the main pulse 901 there is a reduction in the main pulse energy which will severely limit the amount of resultant reflected power. It may also be necessary to temporally delay the laser pulse 901 to allow for plasma formation and sufficient expansion at the target 402. This incurs additional cost and uncertainty in the pulse shortening procedure. Fig. 4 shows schematically shuttering with electrode discharge. Here a focused CO2 laser beam 1001 passes between a high voltage electrode 1002 and a ground electrode 1003. A suitably timed discharge between the electrodes modifies the pulse width of the output laser beam 1004.

As will be appreciated, this process requires very precise timing, a high-voltage electric pulse generator and in some cases a second laser is used to assist pulse shortening, again making the system more costly than either of the pinhole or Nd:YAG pulse shuttering approaches above. Also, pulse durations of only 100-1 15 ns have been reported for CO2 lasers.

Fig. 5 shows schematically shuttering a focused input CO2 laser beam 1 101 with a gas chamber 1 102 to provide an output laser beam output 1 103 with a modified pulse length.

This process requires the pressure chamber 1 102 again adding cost and the best- reported pulse duration is 30 ns for a CO2 laser. Also, the repetition rate is limited to about 1 Hz technique, due to gas renewal time.

Fig. 6 shows schematically a femtosecond laser shutter. Here a femtosecond laser pulse 1201 is focused on a low-reflectivity vacuum dielectric interface/target 1202, so that most of the pedestal and prepulse energy in the femtosecond pulse is transmitted through the target. As the intensity increases in time, a layer of electrons 1203 are produced in a thin layer at the dielectric surface by nonlinear mechanisms such as multiphoton absorption. The medium therefore acquires a "metallic" character. If the electron density exceeds the critical density at the laser wavelength, the reflectivity suddenly increases: the shutter is "triggered," and a reflected laser pulse 1204 is produced. For a proper choice of the incident power density on the target, triggering only occurs at the very beginning of the main pulse, and thus the reflected beam has an improved contrast ratio. The layer 1203 only reflects for a short period of time (sub - ps) due to the lifetime of an over critical dense electron layer. Fig. 7 shows schematically the internals of a Q-Switched lasers optical resonator 1306 and demonstrates pulse shortening via optical Q-switching. The laser system comprises a laser cavity rear reflector 1301 , a laser gain medium 1307, Pockels Cell 1303, polariser 1304 and output coupler 1305 for modifying a laser beam 1302. Q- switching is achieved by putting a variable Pockels cell 1303 inside the laser's optical resonator 1307. The Pockels cell 1303 acts as a Q switch, that is a switch to control the populating of the upper transition in the laser process. When saturation occurs, the Q switch is activated and allows optical amplification and stimulated emission to begin. The energy stored in the laser gain medium 1307 is depleted very quickly giving rise to a short pulse of light, which may have very high peak intensity. Crystals within the Pockels Cell for Q-switching absorb large amounts of the input energy. In the case of CO₂ laser, Q switching the crystal can easily become damaged due to high peak powers, and only works on low energy oscillators. However, pulse durations of less than 500 ps have been reported and these are shorter than achievable with plasma shutter.

Summary of the Invention

According to the present invention there is provided a plasma shutter for modifying a laser pulse, the shutter comprising optics arranged to focus an incident laser beam pulse with a given laser power density on a surface which reflects light at a wavelength of said laser, said power density being above a threshold for forming plasma on said surface in a time less than the duration of said incident laser beam pulse, so that a laser pulse reflected from said surface has a shorter duration than said incident laser beam pulse.

Preferably, said optics are adjustable to vary the power density of said incident laser beam on said surface. Preferably, said optics adjust the distance of said surface from said optics.

Preferably, said reflective surface is metallic. In some embodiments, said reflective surface is solid and in other embodiments, said reflective surface comprises liquid metal.

Preferably, said shutter includes optics arranged to collimate said reflected laser pulse.

The plasma shutter of the invention when coupled to a laser can provide:

• a well defined short pulse of moderate intensity which can be subsequently amplified to realise high intensity laser produced plasmas; or

• a short, well defined laser pulse for lower intensity laser processes such as surface machining/engineering and medical surgeries.

Embodiments of this invention provide a plasma shutter that allows for shortening of the laser pulse length from gas discharge lasers. Embodiments of the invention also provide the ability to remove unwanted features in the laser temporal profile to give a smooth pulse profile. In embodiments of the invention, a CO₂ laser pulse is focused onto a solid target of high reflectivity. The initial portion of the laser pulse is reflected before the heating of the target induces plasma formation. The remainder of the laser pulse is absorbed in the plasma produced. This allows for pulse shaping as the latter part of the laser pulse is removed. It has been shown that the amount of the laser pulse removed is determined by the time it takes for the plasma to form which in turn depends on the laser power density.

Brief Description of the Drawings Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows a pulse from an Optosystems Infralight SP10 TEA CO2 laser;

Fig. 2 shows a pinhole plasma shutter; Fig. 3 illustrates shuttering with a Nd:YAG pulse;

Fig. 4 illustrates electrode discharge shuttering;

Fig. 5 illustrates gas chamber shuttering;

Fig. 6 illustrates Femtosecond laser shuttering;

Fig. 7 illustrates Q-Switched pulse shortening; Fig. 8 shows schematically a liquid metal target plasma shutter according to a first embodiment of the present invention;

Fig. 9 shows schematically a metal/metal coated tape plasma shutter target according to a second embodiment of the present invention;

Fig. 10 shows schematically a rotating/translating metal plasma shutter target according to a third embodiment of the present invention;

Fig. 1 1 shows schematically a laser system arrangement including a plasma shutter according to an embodiment of the present invention;

Fig. 12(a) shows laser pulse profiles for reflected pulses for long time scales. The FWHM is given in each case. Increasing the position from reference position 0 to 120 brings the lens - target distance closer to the focal length of the lens;

Fig. 12(b) shows reflected laser pulse profiles as in Fig. 12(a) but zoomed in on main reflected peak;

Fig. 13 shows reflected pulse duration as a function of lens position; Fig. 14 shows reflected pulse energy as a function of lens position; and Fig. 15 shows reflected power as a function of lens position. Description of the Preferred Embodiments

Referring now to Figure 8, there is shown a liquid metal target plasma shutter according to a first embodiment of the present invention.

A laser pulse 101 is incident on an optical system 102 including a focusing lens and/or mirror which is located a distance d from the surface of a liquid metal 104 contained within a bath 103. The beam reflects 105 from the surface of the liquid metal where it is incident on collimating optics 106 to provide an output collimated reflected laser beam 107 with modified pulse length. Examples of the laser and reflective material will be given below, but it is sufficient to say that adjusting the optical system 102, for example, by varying the distance d, focuses the incident laser on the liquid metal with varying degrees of power density. This in turn affects the time taken for plasma to form on the surface of the metal 103 and to prevent the reflection of the beam 105, so modifying the length of the reflected pulse.

As an alternative to adjusting the optics 102, the position of the surface could be adjusted to vary the power density of the laser incident on the surface.

Turning now to fig 9, there is shown a coated tape plasma shutter target according to a second embodiment of the invention. As in the first embodiment, a laser pulse 201 is incident on an optical system 202 located a distance d from a tape 203 coated with a reflective material. The tape could for example be formed of Mylar. The tape is fed from a feed reel 204 to a take up reel 205 as the reflective surface is consumed through laser ablation. As in the first embodiment a reflected laser beam 206 is picked up by collimating optics 207 to provide an output collimated reflected laser beam 208 with modified pulse length.

Fig. 10 shows a third embodiment of the invention where the liquid metal bath of the first embodiment is replaced with a rotating/translating plasma shutter target 1037104'. Here, a reflective target 104' is preferably removably mounted on a stage 103' which is arranged to rotate and translate the target to refresh the reflective surface as it is consumed. In one example, the target could comprise a metal-coated silicon wafer. Indeed silicon itself could for certain lasers provide a suitable reflective surface. Fig. 1 1 shows a plasma shutter 406 according to an embodiment of the invention within a laser system. A laser oscillator unit 401 includes a rear reflector 402 and a laser window 403. Within the unit 401 a gain medium (laser gas mixture) 405 is located between a pair of Brewster windows 404 and this produces the oscillator pulse shown. The emitted laser pulse is picked up by the shutter 406 through a window 403 and focussed by adjustable optics 407 (as in the first, second and third embodiments), at a highly reflective target 409. Again the reflected pulse is collimated by an adjustable lens 408 and is emitted through a second window 403. As shown, the shutter output, although potentially more discrete than the oscillator pulse, has a lower peak power. The shutter output is therefore fed through a multipass amplifier unit 410, again comprising Brewster windows 404 and a gain medium 405, to produce an amplified pulse as shown.

For any of the embodiments above, focusing the incident laser pulse onto metal targets at intensities of approximately 10⁸Wcm^"2 induces plasma formation due to vaporisation and ionisation of a thin layer at the metal surface. When the target is highly reflective, a portion of the laser pulse is reflected away from the target surface in the time it takes for plasma to form. This allows for the pulse duration to be modified and adjusted, by controlling plasma formation time according to the applied laser power density. Moving the incident focussing optics with respect to target position (or vice versa) changes the laser power density.

Fig 12 (a) shows a series of graphs of irradiation of a gold (Au) coated silicon wafer and it can be seen that for different lens positions, and thus laser power density, that the pulse duration of the reflected pulse can be varied. Fig 12 (b) shows the same but with the x-axis scale reduced. As the lens-target distance (d) approaches the lens focal length, the laser spot area is reduced, the power density is increased and the reflected laser pulse duration decreases. The variation of pulse duration with lens position can be seen in Fig. 13.

As the latter part of the laser pulse is absorbed by the plasma produced, a significant amount energy is lost in the reflected laser pulse. This is quantified in Fig. 14 for the same conditions as were present for the measurements made in Fig. 13. Combining Fig. 13 and 14, the reflected power is plotted in Fig 15. From Fig. 15, it can be seen that the reflected power is of the order of 10MW and considering that focused laser spot areas greater than 10^"3 cm² are attainable, the laser power density of the shortened pulse can be as high as 10¹⁰ W cm^"2. This is sufficient for generation of hot x-ray emitting laser produced plasma as well as a host of other applications, some of which will be described later.

In the example above, gold is used as a reflective metal layer; however, other examples of reflective material include silver, silicon or Gallinstan or indeed any highly reflective (at the laser wavelength) surface.

While the examples above have been described in terms of a CO2 laser, other examples of laser source include: CO; Excimer lasers including ArF, KrF, KrCI, XeF, XeCI, N₂, F₂; Chemical lasers including HF, DF, 01, I; or Dye lasers.

The invention can potentially find use in at least the following applications:

1) Production of highly charged ions of high Z materials

Current cancer therapy is based on proton bombardment of tumours. Highly charged ions of Carbon are being investigated for increased penetration properties and x-ray production. There is also significant scope for highly charged ions of high Z materials to be used in therapy. A high intensity short pulse laser is a cost effective tabletop source of highly charged ions of any material. The present invention coupled to an existing gas discharge laser and also an amplification stage can provide a route to highly charged ions as opposed to more expensive and more complicated femtosecond laser systems.

2) Production of EUV and X-ray emitting sources

Highly charged ions of high Z materials emit x-rays which are useful for a range of applications including EUV lithography, imaging and surface defect inspection. High intensity lasers utilising short pulses are required for x-ray pulses and our invention allows pulse shortening of gas discharge lasers that may be used for this application.

3) Improving definition, efficiency and reducing peripheral damage in laser machining Gas discharge lasers have been used extensively in surface machining as well as laser cutting due to their ability to ablate matter from virtually any material. Pulsed C0₂ lasers have been used extensively in industry due to their high electrical efficiency and to produce well-defined, clean cuts/features in laser engineered surfaces. UV excimer lasers have also found use due to enhanced material removal capabilities due to an increased absorption depth with respect to C0₂ laser irradiation. Shortened pulses are desirable in each case due to a reduction in the thermal conduction of laser energy to surrounding areas. A shorter pulse will give a highly desirable smaller heat affected zone (HAZ) around laser- engineered features. For reduced pulse duration less energy is required to machine the same feature, giving rise to increased efficiency.

4) Reducing invasiveness during laser based medical procedures

Both C0₂ and excimer lasers are used extensively in laser corrective surgeries. C0₂ lasers are routinely used in otolaryngology, gynecology, neurosurgery, plastic surgery, dermatology and oral & maxillofacial surgery. Excimer lasers are used for laser vision correction, dermatology and angioplasty. In all cases the lasers used for these procedures are configured such that minimal trauma is caused to the surrounding tissue so a reduction in the laser pulse duration can potentially improve the non-invasiveness of these medical procedures. 5) Enhanced target resolution for Light Detection and Ranging (LIDAR)

LIDAR (sometimes referred to as laser radar) utilises a pulsed laser to illuminate a far away object or target. The backscattered light from the target is picked up by an optical collection system and analysed to give detail of the target position and structure. Both excimers and CO₂ lasers are used for LIDAR and any reduction in the laser pulse duration will equate to an improvement in target resolution. LIDAR has many applications such as atmospheric research, meteorology, air pollution measurements, traffic speed enforcement, aerial surveying and geomorphology which may benefit from reduced pulse duration that our pulse shortening technique can avoid. The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims

Claims:

1 . A plasma shutter for modifying a laser pulse, the shutter comprising optics arranged to focus an incident laser beam pulse with a given laser power density on a surface which reflects light at a wavelength of said laser, said power density being above a threshold for forming plasma on said surface in a time less than the duration of said incident laser beam pulse, so that a laser pulse reflected from said surface has a shorter duration than said incident laser beam pulse.

2. A plasma shutter as claimed in claim 1 , wherein said optics are adjustable to vary the power density of said incident laser beam on said surface.

3. A plasma shutter as claimed in claim 2, wherein, said optics adjust the distance of said surface from said optics.

4. A plasma shutter as claimed in claim 1 , wherein said reflective surface is metallic.

5. A plasma shutter as claimed in claim 1 , wherein said reflective surface is either solid or comprises liquid metal.

6. A plasma shutter as claimed in claim 1 , wherein said shutter includes optics arranged to collimate said reflected laser pulse.

7. A plasma shutter as claimed in claim 1 wherein said reflective surface comprises one of: gold, silver, silicon or Gallinstan.

8. A laser system comprising a laser source combined with a plasma shutter according to claim 1 .

9. A laser system according to claim 8 wherein said laser source comprises one of a: CO2 laser, a CO laser, an Excimer laser, a Chemical or a Dye laser.

10. A laser system according to claim 8 wherein said laser source is arranged to focus a laser pulse on to a solid surface of high reflectivity, wherein an initial portion of the laser pulse is reflected by the surface, before the heating of the surface induces plasma formation and the remainder of the laser pulse is absorbed in the induced plasma.