LASER-DRIVEN LIGHT SOURCE DEVICE

Abstract

A laser-driven light source device includes a laser oscillation unit configured to emit laser light, and a plasma vessel configured to contain and seal a discharge medium therein. The laser-driven light source device also includes an optical system configured to condense the laser light emitted from the laser oscillation unit, and direct the laser light to an inside of the plasma vessel to generate a plasma. The laser oscillation unit includes a control unit configured to perform an on/off control on the generation of the laser light to modulate an output of the laser light such that the laser light is generated during an on-time of several sec to several msec and the laser light is not generated during an off-time. The off-time is decided such that the plasma in the plasma vessel does not disappear.

Claims

1. A laser-driven light source device comprising: a laser oscillation unit configured to emit laser light; a plasma vessel configured to contain and seal a discharge medium therein; and, an optical system configured to condense the laser light emitted from the laser oscillation unit, and direct the laser light to an inside of the plasma vessel to generate a plasma, the laser oscillation unit including a control unit configured to perform an on/off control on generation of the laser light to modulate an output of the laser light such that the laser light is generated during an on-time of several sec to several msec and the laser light is not generated during an off-time, the off-time being decided to avoid disappearing of the plasma in the plasma vessel.

2. The laser-driven light source device according to claim 1, wherein the laser oscillation unit includes: a laser resonator that contains a laser medium therein; a pumping device configured to excite the laser medium; and an electricity feeding device configured to feed a power to the pumping device, and the control unit is configured to perform the on/off control on the electricity feeding device such that the on-time becomes several sec to several msec and the off-time becomes the period that is sufficiently short to avoid the disappearing of the plasma.

3. The laser-driven light source device according to claim 1, wherein the plasma vessel includes a body having a front opening and a rear opening, a light incident window disposed at the rear opening of the body, and a light emission window disposed at the front opening of the body, the body has a reflecting surface, and the reflecting surface is a concave surface, a closed and sealed space is formed by the body, the incident window, and the emission window, the sealed space contains and seals the discharge medium therein, and the laser light from the laser oscillation unit is introduced to the inside of the plasma vessel from the incident window.

4. The laser-driven light source device according to claim 2, wherein the plasma vessel includes a body having a front opening and a rear opening, a light incident window disposed at the rear opening of the body, and a light emission window disposed at the front opening of the body, the body has a reflecting surface, and the reflecting surface is a concave surface, a closed and sealed space is formed by the body, the incident window, and the emission window, the sealed space contains and seals the discharge medium therein, and the laser light from the laser oscillation unit is introduced to the inside of the plasma vessel from the incident window.

5. The laser-driven light source device according to claim 3, wherein the emission window is transparent to ultraviolet light, and the incident window is transparent to the laser light.

6. The laser-driven light source device according to claim 4, wherein the emission window is transparent to ultraviolet light, and the incident window is transparent to the laser light.

7. The laser-driven light source device according to claim 3, wherein the optical system includes a condensing lens to condense the laser light emitted from the laser oscillation unit such that the condensed laser light is condensed to a focal point of the reflecting surface of the body.

8. The laser-driven light source device according to claim 4, wherein the optical system includes a condensing lens to condense the laser light emitted from the laser oscillation unit such that the condensed laser light is condensed to a focal point of the reflecting surface of the body.

9. The laser-driven light source device according to claim 1, wherein the control unit performs the on/off control after a start-up period of the light source device.

10. The laser-driven light source device according to claim 2, wherein the control unit performs the on/off control after a start-up period of the light source device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 illustrates a laser-driven light source device according to an embodiment of the present invention.

[0029] FIG. 2 is a cross-sectional view of an exemplary plasma vessel used in the light source device shown in FIG. 1.

[0030] FIG. 3 illustrates an intensity distribution of laser light emitted from the light source device shown in FIG. 1.

[0031] FIG. 4A shows the relationship between laser power and time according to a conventional arrangement.

[0032] FIG. 4B shows the relationship between the laser power and time according to the light source device of the embodiment of the invention.

[0033] FIG. 4C shows the relationship between spectral intensity and wavelength according to the conventional arrangement.

[0034] FIG. 4D shows the relationship between the spectral intensity and wavelength according to the embodiment of the invention.

[0035] FIG. 5 shows a conventional laser-driven light source device.

[0036] FIG. 6A and FIG. 6B are views useful to describe incident laser light and a plasma according the light source device shown in FIG. 5.

[0037] FIG. 7A to FIG. 7D are views useful to describe intensity and a spectral distribution of the laser light according to a conventional arrangement.

DETAILED DESCRIPTION

[0038] An embodiment of the present invention will be described with reference to the accompanying drawings. Referring to FIG. 1, illustrated is a laser-driven light source device according to one embodiment of the present invention. A plasma vessel 1 contains and seals an ionizable discharge medium such as noble gas and mercury therein.

[0039] Laser light emitted from a laser oscillation unit 12 is condensed by a condensing lens 11 and enters the plasma vessel 1.

[0040] The laser oscillation unit 12 includes a laser resonator 13, a pumping device 14 connected to the laser resonator 13, an electricity feeding device 15, which is connected to the pumping device 14 and feeds power to the pumping device 14, and a control unit 16, which is connected to the electricity feeding device 15 and controls the electricity feeding device 15.

[0041] The laser resonator 13 has a pair of reflecting mirrors, namely, a partial reflection mirror and a total reflection mirror. A laser medium is arranged in an optical path inside the laser resonator 13.

[0042] The pumping device 14 configured to excite the laser medium is coupled to the laser resonator 13. The pumping device 14 may supply light to excite the laser medium, and may have a plurality of laser diodes (LDs) and/or a lamp.

[0043] The pumping device 14 is coupled to the electricity feeding device 15 that is controlled by the control unit 16.

[0044] The control unit 16 adjusts the power feeding from the electricity feeding device 15 to the pumping device 14 in response to a signal from a function generator (e.g., a sign signal or a rectangular wave signal) at a constant cycle. The pumping device 14 excites the laser resonator 13 by energy that corresponds to the power received from the electricity feeding device 15.

[0045] FIG. 2 illustrates the detail of the exemplary plasma vessel 1. The plasma vessel 1 includes a pillar-shaped body 2 made of a ceramics material or a similar material, a light emission window 3 disposed at a front surface of the body 2 (right surface of the body 2 in FIG. 2), and a light incident window 4 disposed at a rear surface of the body 2.

[0046] A reflecting surface 5 is formed on the front surface side of the body 2. The reflecting surface 5 is a concave surface in this embodiment. A laser light passing hole 6 that penetrates the concave reflecting surface 5 in an optical axis direction is made at the center of the concave reflecting surface 5. The rear end of the laser light passing hole 6, i.e., the light incident portion is chamfered, forming a tapered surface 6a. The tapered surface 6a is configured so as to avoid that the condensed laser light be cut off at the entrance of the laser light passing hole 6 when the laser light is introduced to the laser light passing hole 6 through the light incident window 4.

[0047] The concave reflecting surface 5 may be formed into a parabolic shape or an elliptical shape. In this embodiment, the concave reflecting surface 5 is a reflecting surface with the parabolic shape. On the concave reflecting surface 5, a metal deposition film is provided. Specifically, aluminum or the like is deposited on the reflecting surface 5. Alternatively, a dielectric multilayer film may be provided on the reflecting surface 5.

[0048] The light emission window or the light exit window 3 disposed in front of the reflecting surface 5 has a transparency to ultraviolet light and the light incident window 4 at the rear of the reflecting surface 5 has a transparency to laser light. Both of the windows 3 and 4 are made of a vitreous material such as crystal, sapphire, and quartz glass.

[0049] The light emission window 3 whose outer peripheral surface is metalized is bonded with an elastic ring member 7 by brazing with a silver solder and the like. On the other hand, a metallic tubular body 8 is bonded to the metalized front end portion of the body 2 by brazing. The ring member 7 and the metallic tubular body 8 are welded and bonded together by, for example, TIG welding or laser beam welding. Thus, the light emission window 3 is mounted to the front opening of the body 2.

[0050] Similarly, the light incident window 4 with the metalized outer peripheral surface is bonded to a metal block 9 by brazing, and a metallic tubular body 10 is bonded to the metalized rear end portion of the body 2 by brazing. The metal block 9 is welded to the metallic tubular body 10. Accordingly, the light incident window 4 is mounted to the rear opening of the body 2.

[0051] The body 2, the light emission window 3, and the light incident window 4 thus assembled constitute the plasma vessel 1, and the inside of the plasma vessel 1 defines a sealed (closed) space S. The sealed space S contains and seals a noble gas such as xenon gas, krypton gas, and argon gas and a light emission gas such as mercury gas, depending upon the emission wavelength. The noble gas and the light emission gas are examples of the discharge medium.

[0052] The laser light from the laser oscillation unit 12 enters the light incident window 4 of the plasma vessel 1 while being condensed by the condensing lens 11. Then, the laser light is condensed at a focal point (focus position) F of the concave reflecting surface 5. Thus, the plasma is generated at and around the focal point F, and excitation light generated by excitation of the discharge medium is reflected by the concave reflecting surface 5 and exits from the light emission window 3 to the outside.

[0053] As illustrated in FIG. 3, the laser oscillation unit 12 emits the CW laser light and also emits the pulse laser light in a superimposing manner during the start-up period (ignition period), thus generating the plasma (firing source) by the irradiation with the pulse laser light. The plasma (firing source) is maintained by energy from the CW laser light emitted simultaneously. During the start-up period, the output of the CW laser light is not modulated.

[0054] After the start-up period, the illustrated embodiment controls the output of the CW laser light that is used to maintain the plasma. The control unit 16 performs on/off control on the electricity feeding device 15 such that the electricity feeding device 15 is activated during a predetermined on-time and deactivated in a predetermined off-time to modulate the output of the CW laser light emitted from the laser oscillation unit 12.

[0055] The on-time is several sec to several msec in this embodiment. The control unit 16 controls the off-time such that the plasma does not disappear due to the off-time.

[0056] Considering that the pulse width of the pulse laser light is in the order of ns, the on-time has a width at least 1000 times or more (i.e., at least 1000 ns).

[0057] Since the light source device of the embodiment intermittently emits the laser light, it might not be said that the laser light is continuous (CW) laser light when considering just the term. However, the embodiment of the present invention provides the on-time and the off-time and supplies the laser light intermittently in the case where the CW (continuous) laser light is suppled; therefore, the description expresses this as the modulation of the output of the (CW) laser light in comparison with the pulse laser light. In other words, the on-time and the off-time in the embodiment do not relate to the pulse laser light, but they only relate to the continuous laser light.

[0058] As described above, the present invention modulates the output of the CW laser light to maintain the plasma. Referring to FIG. 4A-4D, the following will describe the advantages of such modulation.

[0059] Since the plasma is intermittently irradiated with the laser light, the average laser power (intensity) in this embodiment is lower than the case where the laser light is continuously emitted to the plasma. The average laser power is calculated including the period during which the laser light is not emitted to the plasma (off-time).

[0060] If the emitted laser power is represented by P, the period during which the laser light is emitted to the plasma (on-time) is represented by Ton, and the period during which the laser light is not emitted to the plasma (off-time) is represented by Toff, the average energy Pa is given by the following equation:

Pa=PTon/(Ton+Toff).

[0061] Since the laser power P during the on-time is identical (the laser power does not change), the plasma temperature is identical (plasma temperature does not change) and the shape of the emission spectrum from the plasma is identical.

[0062] In FIG. 4C, an area with the wavelength or less in the spectrum is represented by S1, and an area with the wavelength or more is represented by S2 when the laser power of the CW laser light is represented by P (FIG. 4A). FIGS. 4A and 4C are derived from a conventional arrangement. When the CW laser light is modulated, an area with the wavelength or less is represented by S3, and an area with the wavelength or more is represented by S4, as shown in FIG. 4D. FIGS. 4B and 4D are derived from the embodiment of the present invention. Then, the following equation is obtained.

S1:S2=S3:S4

[0063] For example, when is 200 nm, a ratio of the vacuum ultraviolet area S1 (200 nm or less) to the remaining area S2 (200 nm or more) in FIG. 4C is equal to (or similar to) a ratio of the vacuum ultraviolet area S3 (200 nm or less) to the remaining area S4 (200 nm or more) in FIG. 4C.

[0064] Thus, the intermittent input of electricity (i.e., the on/off control by the control unit 16) ensures obtaining the spectrum having a profile similar to that of the spectrum obtained when a large input is continuously input even when the average laser power is decreased. The average laser power in FIG. 4A is P, and the average laser power in FIG. 4B is Pa. Pa is lower than P.

[0065] As described above, decreasing the average laser power allows both of avoiding the enlargement of the entire laser oscillation unit and efficiently increasing emission intensity in the ultraviolet area (for example, the vacuum ultraviolet area) with a predetermined short wavelength.

[0066] In other words, with the use of the similar average laser power for the continuous power supply (conventional arrangement) and the intermittent power supply (the present invention), the present invention ensures obtaining the emission spectrum with the increased ultraviolet area.

[0067] It should be noted that the ignition source used to generate the plasma (firing source) at the start-up (ignition) period is not limited to the pulse laser light. For example, a pair of electrodes may be disposed in the plasma vessel, and the plasma may be generated by applying a high voltage between these electrodes and causing a dielectric breakdown.

EXAMPLES

[0068] The following passages will describe one working example. [0069] Plasma vessel: tubular bulb made of synthetic quartz glass (with electrodes for ignition) [0070] Sealed gas: Xe 10 atm [0071] Laser: fiber laser (M21.1, beam diameter is 14 mm) [0072] Wavelength: 1070 nm [0073] Condensing lens: f=40 mm [0074] Laser output: Modulated CW laser, average output is 211 W (on-time is 80 s, off-time is 80 s, peak value is 419 W)

[0075] The above-described laser-driven light source device of the present invention was compared with a comparative example using a CW laser (no modulation is applied to the CW laser) with the output of 212 W.

[0076] The output of VUV (wavelength: 160 nm to 180 nm) was 9,770 (arbitrary unit: a.u.) in the comparative example if it was expressed by a spectrum integrated value. On the other hand, the output of VUV was 11,864 in the working example of the present invention. Thus, although the average laser output was identical, the VUV output of the present invention was increased to about 1.2 times.

[0077] As described above, the laser-driven light source device according to the present invention irradiates the plasma with the laser light output of which is modulated such that the on-time becomes several sec to several msec and the off-time becomes the period that does not cause the disappearing of the plasma, when the laser light is directed to the inside of the plasma vessel to maintain the plasma in the plasma vessel. Accordingly, although the average laser output is small, the spectrum similar to the spectrum obtained by the large output can be obtained. Thus, while the enlargement of the laser oscillation unit is avoided, and the emission intensity of the ultraviolet region with the predetermined short wavelength or less can be efficiently increased.

[0078] This application is based on Japanese Patent Application No. 2017-136778 filed in Japan on Jul. 13, 2017, and the entire disclosure thereof is incorporated herein by reference.