Patent Application
20110122903
The present invention relates to a wide-band wavelength-variable or tunable laser device or apparatus.
In recent years, by the use of technique of wavelength conversion using nonlinear optical crystal etc., it becomes possible to obtain laser from the UV light region to the infrared region.
For example, it is known that a laser light having a wavelength of 193 nm equaling the wavelength of ArF laser can be obtained by generating a fourth harmonic wave of Ti sapphire laser by laser oscillation at a wavelength of 772 nm (patent document 1). Moreover, it is known that difference frequency mixing of Nd:YAG laser and Ti:Al2O6 (Ti sapphire) laser makes it possible to obtain a difference frequency light of 3.2 μm˜20 μm (patent document 2). In the case of generation of a laser light of a desired wavelength by the wavelength conversion technique as mentioned above, in order to realize a wavelength variable or tunable light source capable of wideband oscillation, it is desirable to achieve a capability of varying an oscillation wavelength of a fundamental wave laser as wide as possible. To this end, it is required to select a laser medium having a wide band gain and to form a resonator of a lower loss.
However, the laser medium generally used, having a crystal structure, is not suitable for wide band oscillation, and an optimum laser medium is not found yet.
In the case of a laser medium having a glass structure, by contrast, each of constituent atoms can assume a more random ligand field, and hence light generation of wider band is feasible. Moreover, as compared to silica glass or quartz glass of covalent bonding, fluoride glass of ionic bonding is greater in stark splitting, and has a capability of generating light in a wider range. For example, laser oscillation using Pr3+-doped fluoride glass fiber is confirmed at wavelength at which the gain forms a peak (491 nm, 520 nm, 605 nm, 635 nm) (non-patent document 1, patent document 3). Moreover, by the use of Pr3+-doped fluoride glass fiber excited with Ar+ laser having a wavelength of 476.5 nm, laser oscillation is confirmed discretely in ranges of 601˜618 nm, 631˜641 nm, 690˜703 nm and 707˜725 nm as confirmed by J. Y. Allain et al. (non-patent document 2). However, since the excitation (3H4→3P0) with light having a wavelength of 476.5 nm is direct excitation to emission upper level, the stimulated emission of (3P0→3H4) is generated by the excitation light itself, and hence the population inversion is difficult to attain.
Furthermore, laser oscillation is achieved around 491 nm, 521 nm and 635 nm by direct excitation of (3H4→3P1, 3P2) by the use of Pr3+-doped fluoride glass fiber excited by blue semiconductor laser (as reported by non-patent document 3, patent document 4). However, a wide oscillation characteristic over the blue region and green region is not shown.
In a wide range wavelength-variable or tunable light source, discretely distributed variable wavelength ranges are reported in wavelength ranges of 601˜618 nm, 631˜641 nm, 690˜703 nm, 707˜725 nm, as mentioned above. However, there is not yet found a laser apparatus capable of continuously varying the laser oscillation wavelength over a still wider range.
It is an object of the present invention to provide a wide range wavelength-variable or tunable laser apparatus in a wide range including regions of blue, green, orange˜deep red and near-infrared.
According to the present invention there is provided, in a laser apparatus comprising: an excitation light source comprising a GaN based laser diode having a wavelength range of 420˜475 nm; and a resonator comprising a laser medium configured to emit light by excitation light from the excitation light source and disposed in the optical path between a first mirror allowing transmission of the excitation light from the excitation light source and reflecting light in a desired wavelength band excluding the excitation light and a second mirror reflecting light in a or the desired wavelength band, and a first wavelength selecting means disposed in the optical path between the laser medium and the second mirror;
a wavelength-variable laser apparatus (first apparatus) which is characterized in that the wavelength-variable laser apparatus can vary a laser oscillation wavelength continuously by using at least a Pr3+-doped fluoride glass as the laser medium.
The first apparatus may be a wavelength-variable laser apparatus (second apparatus) wherein the first wavelength selecting means includes one of a prism, an etalon, a birefringent filter, a dielectric multilayer film filter and a grating.
The first apparatus may be a wavelength-variable laser apparatus (third apparatus) wherein the laser medium forms a core portion of an optical waveguide.
The third apparatus may be a wavelength-variable laser apparatus (fourth apparatus) wherein both ends of the laser medium are connected, respectively, with optical waveguides of silica-based glass.
One of the first to fourth apparatus may be a wavelength-variable laser apparatus (fifth apparatus) wherein pulsed light is generated by disposing a dispersion compensating device for compensating for wavelength dispersion of the resonator as a whole, between of the laser medium and the optical path of the second mirror.
The fifth apparatus may be a wavelength-variable laser apparatus (sixth apparatus), wherein the dispersion compensating device is a second wavelength selecting means paired with the first wavelength selecting means.
A wideband wavelength-variable laser capable of continuously scanning the wavelength can be provided by the present invention.
Condensing lens 102 is preferably provided with an antireflection film to the excitation laser wavelength, and the collimator lens 105 and prism 106 are preferably provided with an antireflection film to the laser oscillation wavelength. Prism 106 need not be a triangular prism. Prism 106 may be in the form in which an incidence plane and an exit plane are cut to form a Brewster's angle with respect to the optical axis. Although the prism is used as a first wavelength selecting means, it is optional to use etalon, birefringent filter, grating or dielectric multilayer film filter.
In the case of obtaining an output of laser, it is optional to enable partial transmission by lowering the reflectivity at the laser wavelength, of the excitation light transmission/laser light reflection filter 103 or movable mirror 107, or to interpose a tap filter between optical paths in the resonator.
In the case of varying the oscillation wavelength, the object can be achieved by scanning the incident angle or the reflection angle by rotation of prism 106 or movable mirror 107, and feeding back only light of a desired wavelength to the fiber 104.
Furthermore, a laser oscillation characteristic of a wider band is obtained when a plurality of upper levels or lower levels are close to one another.
In order to obtain a widest band laser oscillation characteristic, use is made of fluoride glass doped with Pr3+, as laser medium.
To excite Pr3+ efficiently, the range of excitation wavelength is required to be contained in the range of 420˜475 nm. A more desirable range is 430˜473 nm though it is not possible to determine or stipulate the range totally unconditionally since the desirable range is dependent on the type of glass. A still more desirable range is 435˜455 nm in which the absorption coefficient is high.
Thus, according to the invention, the wavelength variation range or variable wavelength range is expanded since the population inversion is easier by excitation of Pr3+ with a characteristic wavelength.
For example, in the case of obtaining a wideband wavelength varying characteristic with an amplification medium doped with Pr3+, it is possible to vary the oscillation wavelength continuously around the wavelength range of 470˜500 nm, by using transition of (3P1, 3P0)→3H4.
Moreover, it is possible to vary the oscillation wavelength continuously around the wavelength range of 590˜750 nm, by using transition of 3P1→3H6, 3P0→(3H6, 3F2), 3P1→3F3, 3P0→(3F3, 3F4), and/or 3P1→3F4.
Moreover, it is possible to vary the oscillation wavelength continuously around the wavelength range of 510˜550 nm, by using transition of (3P1, 3P0)→3H5.
Moreover, it is possible to vary the oscillation wavelength continuously around the wavelength range of 750˜960 nm, by using transition of (3P1, 3P0)→1G4.
In the vicinity of a wavelength of 590 nm, there exists the ground level absorption (3H4→1D2). Therefore, laser oscillation is difficult in the vicinity of this wavelength.
In the vicinity of a wavelength of 800 nm, there exists an excited state absorption (1G4→(3P0, 3P1, 3P2, 1I6)). Therefore, in order to obtain laser oscillation in these wavelength bands, it is required to reduce the resonator loss to a very small value.
For example, it is possible to reduce the resonator loss due to absorption and thereby to make the band of the oscillation wavelength wider by using a resonator having a loss lower than or equal to 1%, or by using auxiliary excitation light having a wavelength included in the absorption wavelength (Ar+ laser having a wavelength of 467 nm, a wavelength conversion laser having a wavelength of about 580 nm, or a semiconductor laser having a wavelength of about 850 nm, for example) to suppress the ground level absorption (3H4→(1D2, 3P0, 3P1, 3P1, 1I6) for example) and the excited state absorption (1G4→(3P0, 3P1, 3P2, 1I6), for example). By using the above-mentioned techniques, it is possible to constitute a variable wavelength light source capable of scanning the wavelength continuously over the entirety of the wavelength range of about 460 nm˜about 1000 nm with the Pr3+-doped laser medium.
Moreover, the composition of the fluoride glass used in the present invention may be ZBLAN type, Al—Zr type, or Al type. The ZBLAN type or ZBLAN based fluoride glass is a glass composed mainly of ZrF4—BaF2—LaF3—YF3—AlF3—NaF—REF3 (where RE represents a rare earth element). The Al—Zr type or Al—Zr based glass is a glass composed of AlF3—ZrF4—MgF2—CaF2—SrF2—BaF2—NaF2—REF3 (where RE represents a rare earth element), etc. The Al type or based glass is a glass composed mainly of AlF3—BaF2—CaF2—MgF2—SrF2—YF3—REF3 (where RE represents a rare earth element). ZBLAN type is suitable for highly efficient laser oscillation. The use of Al—Zr type or Al type is desirable when the laser medium is needed to be disposed in the environment in which importance is given to the durability of the glass.
Furthermore, it is possible to generate pulsed light by disposing a dispersion compensation or compensating element in the optical path between the above-mentioned laser medium and the second mirror. The dispersion compensation element used in this invention is a device for providing unequal, delays to different wavelengths, and the dispersion compensation device is composed of wavelength selecting means, etc. As the dispersion compensation device, it is possible to employ prism, grating, fiber grating (called chirped fiber grating especially when used for the dispersion compensation), dispersion compensating mirror, and/or photonic crystalline fiber. For example, it is possible to generate pulsed light by providing a second wavelength selecting means as the dispersion compensation element, in addition to the first wavelength selecting means included in the example of
In general, when light waves of different wavelengths propagate in fiber, the propagation speeds of waves of different wavelengths differ (called wavelength dispersion) in dependence on the waveguide structure and material. Therefore, when the first wavelength selecting means is disposed in the resonator, it is possible to produce short pulses in time by setting, in the resonator, a dispersion compensation device for correcting wavelength dispersion in the whole of the resonator. In the example shown in
In the case of obtaining an output of laser, it is optional to enable partial transmission by lowering the reflectivity at the laser wavelength, of the excitation light transmission/laser light reflection filter 3103 or movable mirror 3107, or to interpose a tap filter in optical paths in the resonator.
As the method for varying the oscillation wavelength of the pulsed light, there is a following example. The light reaching the movable mirror 3107 is dispersed by the prism. Therefore, by using the movable mirror smaller in size than the beam size of the dispersed light as shown in
However, the size of movable mirror 3107 is not limited. For example, even in the case using the movable mirror 3107 having the size larger than the beam size of the dispersed light, it is possible to vary the oscillation wavelength of the pulsed light by interposing a slit between the prism 3106-b and the movable mirror 3107, and allowing transmission of only light of a desired wavelength band of the dispersed light. As another option, it is possible to vary the oscillation wavelength of the pulsed light by interposing a spatial modulating device such as a liquid crystal spatial modulator), and by limiting light in a wavelength region or by varying the transmittance of each wavelength. As still another option, it is possible to vary the oscillation wavelength of the pulsed light by interposing the above-mentioned slit or the spatial modulating device between the prism 3106-a and the prism 3106-b.
Furthermore, when there is no need for varying the oscillation wavelength of the pulsed light, it is possible to set the size of movable mirror 3107 to an arbitrary value.
Instead of adding a prism as the dispersion compensation element, it is optional, for example, to use a mirror serving also as a dispersion compensating mirror for the excitation light transmission/laser light reflection filter 3103 or movable mirror 3107. Alternatively, the excitation light transmission/laser light reflection filter 3103 or movable mirror 3107 may be replaced by a chirped fiber grating. Moreover, photonic crystalline fiber may be included somewhere in the resonator.
When high quality short pulsed light is desired, it is optional to dispose a saturable absorber in the resonator. When the light power density at the saturable absorber is adjusted, it is optional to interpose a lens and to condense light on the saturable absorber.
Moreover, it is optional to interpose a modulating device such as acousto-optic device when modulation of the repetition frequency of the pulsed light is desired.
The following is detailed examination with reference to practical examples. However, the present invention is not limited to the practical examples.
The wavelength-variable laser apparatus shown in
When the thus-constructed wavelength-variable light source and nonlinear optical crystal are combined to obtain a second harmonic generation, it is possible to obtain an UV wavelength-variable light source of the wavelength range of 299.1˜321.13 nm (the wavelength width 22 nm) in the UV region. Furthermore, by sum frequency mixing of the UV wavelength variable light source and a laser of 488 nm (for example, argon ion laser or DPSS laser), it is possible to obtain a wavelength range of 185.5˜193.7 nm (wavelength width). The thus-obtained light source can provide a wavelength (193.3 nm) of ArF laser.
An AR coating is applied at a wavelength of 448 nm to the optical components (602, 603, 604, 605) through which the excitation light passes. An AR coating is applied at a wavelength of 510˜700 nm to the optical components (611, 612) through which the laser light passes. The connection loss between the silica fiber and fluoride fiber was 0.3 dB for each connection point at a wavelength of 635 nm. An excitation side silica fiber end is connected with the dielectric multilayer coated ferrule 606 (AR: 448 nm, HR: 550˜650 nm) allowing the excitation light to pass through without loss and reflecting the laser light. The opposite silica fiber end is connected with the antireflective coated ferrule 610 (AR: 510˜720 nm) allowing the laser light to pass through without loss. Part of the oscillating laser light is reflected by the surface of the prism. The output is monitored by measuring the reflected light 616 with an optical spectrum analyzer (ANDO AQ6315A).
This practical example is similar in construction to the first practical example except that the filter 406 is replaced by a filter which is high in reflectivity in a region of blue-green. That is, as shown in
An AR coating is applied at a wavelength of 448 nm to the optical components (402, 403, 404, 405, 406) through which the excitation light passes. An AR coating is applied at a wavelength of 470˜960 nm to the optical components (408, 409) through which the laser light passes. The transmission loss of the resonator in the wavelength-variable laser apparatus was 0.8 dB (round trip). A excitation light source side fiber end 407-a on the excitation light source side is polished perpendicularly so as to form a right angle, and attached tightly to the filter 406. A fiber end 407-b on the opposite side opposite to the excitation light source side is polished obliquely at an angle of 8° to suppress Fresnel reflection. Part of the oscillating laser light is reflected by the surface of the prism. The output is monitored by measuring the reflected light 411 with the optical spectrum analyzer (ANDO AQ6315A).
This practical example is similar in construction to the first practical example except that the filter 406 and mirror 410 are replaced by units having wide band property and a high reflectivity. That is, as shown in
An AR coating is applied at a wavelength of 448 nm to the optical components (402, 403, 404, 405, 406) through which the excitation light passes. An AR coating is applied at a wavelength of 470˜960 nm to the optical components (408, 409) through which the laser light passes. The transmission loss of the resonator in the wavelength variable laser apparatus was 0.8 dB (round trip). A excitation light source side fiber end 407-a on the excitation light source side is polished perpendicularly so as to form a right angle, and attached tightly to filter 406. A fiber end 407-b on the opposite side opposite to the excitation light source side is polished at an angle of 8° to suppress Fresnel reflection. Part of the oscillating laser light is reflected by the surface of the prism. The output is monitored by measuring the reflected light 411 with the optical spectrum analyzer (ANDO AQ6315A).
This practical example is similar in construction to the first practical example except that the filter 406 and mirror 410 are replaced by units which are high in reflectivity in the infrared region. That is, as shown in
An AR coating is applied at a wavelength of 448 nm to the optical components (402, 403, 404, 405, 406) through which the excitation light passes. An AR coating is applied at a wavelength of 470˜960 nm to the optical components (408, 409) through which the laser light passes. The transmission loss of the resonator in the wavelength-variable laser apparatus was 0.8 dB (round trip). A excitation light source side fiber end 407-a on the excitation light source side is polished perpendicularly so as to form a right angle, and attached tightly to filter 406. A fiber end 407-b on the opposite side opposite to the excitation light source side is polished at an angle of 8° to suppress Fresnel reflection. Part of the oscillating laser light is reflected by the surface of the prism. The output is monitored by measuring the reflected light 411 with the optical spectrum analyzer (ANDO AQ6315A).
When the thus-obtained wavelength-variable light source and a nonlinear optical crystal are combined to obtain a second harmonic wave, it is possible to obtain UV wavelength-variable light source or light sources of a wavelength range of 239˜249 nm (wavelength width 11 nm), a wavelength range of 257˜274 nm (wavelength width 18 nm), a wavelength range of 298˜368 nm (wavelength width 71 nm), and a wavelength range of 435˜465 nm (wavelength width 31 nm) in the UV region. Thus, it is possible to obtain wavelengths of N2 laser (337 nm), part of the wavelength range of Ce:LiSAF laser (285˜299 nm), XeF laser (351 nm), XeCl laser (308 nm), KrF laser (248 nm) etc.
Furthermore, by sum frequency mixing of the UV wavelength-variable light source obtained by the second harmonic wave of the light source according to the present invention, for example, with a laser of 488 nm (such as argon ion laser or DPSS laser), it is possible to obtain wavelength ranges of 160.4˜164.9 nm, 168.3˜175.5 nm, 185.0˜209.8 nm and 230.0˜238.1 nm. With the thus-obtained light source, it is possible to obtain wavelengths of Xe2 excimer laser (172 nm) and ArF laser (193.3 nm).
Furthermore, by sum frequency mixing of the UV wavelength-variable light source obtained by the second harmonic wave of the light source according to the present invention, for example, with a laser of 405 nm (such as GaN type semiconductor laser), it is possible to obtain wavelength ranges of 150.3˜154.2 nm, 157.2˜163.4 nm, 171.7˜192.8 nm and 209.7˜216.5 nm. With the thus-obtained light source, it is possible to obtain wavelength of F2 laser (158 nm).
Furthermore, with a combination of laser light sources having different wavelengths obtained according to the present invention, it is possible to obtain a laser of a new wavelength band, by sum frequency•difference frequency generation.
In the above practical examples 1˜5, the laser medium is in the form having fiber structure. However, this is not a limiting condition imposing limitation. Instead of the fiber structure, it is optional to employ a configuration including a waveguide structure, or a configuration of a bulk form made of Pr3+-doped fluoride glass. Moreover, needless to say, the laser oscillation wavelength range is further widened when the loss of the resonator is reduced or the excitation power is increased.
Since dye is significant in fading, the movable mirror 1111 was used while being rotated. Among two beams of light reflected by beam splitter 1112, one was inputted to an autocorrelator (Mini produced by APE GmbH) and used for pulse duration measurement, and the other beam was used for power measurement and wavelength measurement. An AR coating is applied at a wavelength of 448 nm to the optical components (1102, 1103, 1104, 1105) through which the excitation light passes. An AR coating is applied at a wavelength of 510˜720 nm to the optical component (1109) through which the laser light passes. The wavelength dispersion of fluoride fiber 1107 used in this practical example is −240 ps/km/nm at a wavelength of 635 nm, and this wavelength dispersion is compensated for by using the pair of triangular prisms 1110.
By measuring one of the reflected beams of beam splitter 1112 with the optical spectrum analyzer (ANDO AQ6315A), laser oscillation was confirmed around a wavelength of 635 nm. By measuring the other reflected beam with the autocorrelator simultaneously, the pulse duration or width was 13 ps.
Then, when the position of movable mirror 1111 was change, laser output was obtained in the vicinity of wavelength of 716 nm, and the pulse duration at that time was 10 ps.
If use is made of a saturable absorber manufactured more accurately and/or a dispersion compensating device, it is possible to obtain short pulsed light (short in terms of time).
Although the prism is used as the dispersion compensating device here, it is possible to use fiber Bragg grating, dispersion compensating mirror, or photonic crystalline fiber.
The fiber used in each of the practical examples 1˜6 is a multi-mode fiber at a laser oscillation wavelength band. However, it is optional to use a single mode fiber.
The present invention can be utilized as a wide band wavelength-variable or tunable light source in the visible light region. Moreover, since the wide band wavelength-variable light source is feasible in the visible light region, in the case of forming a wavelength-variable light source in the UV region using the wavelength conversion technique, the number of wavelength conversions can be reduced compared with a UV region wavelength-variable light source based on wavelength conversion using infrared laser as the fundamental wave, and hence it is possible to improve the efficiency and widen the variable wavelength range.
Therefore, in a wavelength-variable UV light source usable also in the field of lithography, it is possible to utilize as the light source of the fundamental wave. Furthermore, in the field of display•projection, it is possible to use as a light source superior in color reproducibility since the a light source of any wavelength in the blue region and green region can be obtained according to the present invention.
Moreover, the present invention can be utilized as a pulsed light source, and specifically, the present invention is utilizable in the field of processing or working (especially, forming of transparent material such as plastics).
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-177496 | Jul 2008 | JP | national |
| 2008-289340 | Nov 2008 | JP | national |
| 2009-142411 | Jun 2009 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/061681 | 6/26/2009 | WO | 00 | 12/9/2010 |
