Optical amplifier, wavelength-tunable laser, and pulse laser

Abstract

An optical amplifier according to the present invention comprises a gas that serves as an optical gain medium and a thermal radiation source that has wavelength selectivity and emits, by thermal radiation, excitation light which excites the gas. In a frequency characteristic of a gain of the gas excited by the excitation light, the ratio of, to the maximum value of the gain, the minimum value adjacent to the maximum value is not less than 0.4 times.

Description

Optical amplifiers, tunable lasers, and pulsed lasers

[0001] This disclosure relates to optical amplifiers, tunable lasers, and pulsed lasers. This application claims priority under Japanese application No. 2025-012346, filed on 28 January 2025, incorporating all the provisions of the said Japanese application.

[0002] Non-patent document 1 discloses a carbon dioxide laser amplifier using carbon dioxide as the optical gain medium. Patent document 1 discloses a gas laser. This gas laser comprises a gas as the laser medium, a wavelength-selective thermal radiation source that emits excitation light to excite the gas by thermal radiation, and an optical resonator that resonates with the emitted light emitted from the gas by the excitation light.

[0003] Japanese Patent Publication No. 2023-170157

[0004] M. N. Polyanskiy, et al. , “Demonstration of a 2 ps, 5 TW peak power, long-wave infrared laser based on chirped-pulse amplification with “mixed-isotope CO2 amplifiers”, OSA Continuum vol. 3 (2020) 459

[0005] An optical amplifier relating to one aspect of this disclosure comprises a gas as an optical gain medium and a wavelength-selective thermal radiation source that emits excitation light to excite the gas by thermal radiation, wherein in the frequency characteristics of the gain of the gas excited by the excitation light, the ratio of the minimum value adjacent to the maximum value to the maximum value of the gain is 0.4 times or more.

[0006] Figure 1 is a schematic cross-sectional view of an optical amplifier according to one embodiment. Figure 2 is a cross-sectional view along the line II-II in Figure 1. Figure 3 is a plan view showing an example of a thermal radiation source. Figure 4 is a plan view showing a part of Figure 3. Figure 5 is a cross-sectional view along the line V-V in Figure 4. Figure 6 is a graph showing an example of the frequency characteristics of the gain of a gas excited by excitation light. Figure 7 is an enlarged view of a part of the frequency characteristics shown in Figure 6. Figure 8 is a graph showing another example of the frequency characteristics of the gain of a gas excited by excitation light. Figure 9 is a graph showing yet another example of the frequency characteristics of the gain of a gas excited by excitation light. Figure 10 is a schematic cross-sectional view of an optical amplifier according to another embodiment. Figure 11 is a cross-sectional view along the line XI-XI in Figure 10. Figure 12 is a schematic diagram showing an example of the optical path in the optical amplifier shown in Figure 10. Figure 13 is a schematic diagram showing another example of the optical path in the optical amplifier shown in Figure 10. Figure 14 is a schematic cross-sectional view of an optical amplifier according to another embodiment. Figure 15 is a cross-sectional view along the line XV-XV in Figure 14. Figure 16 is a schematic diagram showing an example of the optical path in the optical amplifier shown in Figure 14. Figure 17 is a plan view showing an example of a thermal radiation source. Figure 18 is a schematic cross-sectional view of a tunable laser according to one embodiment. Figure 19 is a schematic cross-sectional view of a tunable laser according to another embodiment. Figure 20 is a schematic diagram showing a pulsed laser according to one embodiment.

[0007] This disclosure provides an optical amplifier, a tunable laser, and a pulsed laser that can improve the gain over a wide frequency band in the frequency characteristics of the gain of a gas that serves as an optical gain medium.

[0008] The present disclosure provides an optical amplifier, a tunable laser, and a pulsed laser that can improve the gain over a wide frequency band in the frequency characteristics of the gain of a gas that serves as an optical gain medium.

[0009] [Description of Embodiments of the Disclosure] First, the contents of the embodiments of the disclosure will be listed and described.

[0010] (1) An optical amplifier comprising a gas as an optical gain medium and a wavelength-selective thermal radiation source that emits excitation light to excite the gas by thermal radiation, wherein in the frequency characteristics of the gain of the gas excited by the excitation light, the ratio of the minimum value adjacent to the maximum value of the gain to the maximum value is 0.4 times or more.

[0011] The inventors of this invention conducted research on an optical amplifier capable of improving the gain over a wide frequency band in the frequency characteristics of the gain of a gas, which is an optical gain medium. As a result, the inventors found the following: In the above frequency characteristics, the ratio of the minimum value adjacent to the maximum value of the gain to the maximum value of the gain serves as an indicator of whether or not the gain has been improved over a wide frequency band in that frequency characteristics. When the ratio of the adjacent minimum value to the maximum value of the gain is 0.4 times or more, the gain has been improved over a wide frequency band in that frequency characteristics. In this optical amplifier, in the above frequency characteristics, the ratio of the adjacent minimum value to the maximum value of the gain to the maximum value of the gain is 0.4 times or more. Therefore, this optical amplifier can improve the gain over a wide frequency band in the frequency characteristics of the gain of a gas, which is an optical gain medium.

[0012] (2) In the above (1), the ratio may be 0.8 times or more.

[0013] In this case, the gain of the gas, which is the optical gain medium, can be further improved over a wide frequency band in terms of its frequency characteristics.

[0014] (3) An optical amplifier comprising a gas as an optical gain medium and a thermal radiation source having wavelength selectivity, which emits excitation light to excite the gas by thermal radiation, wherein the pressure of the gas is 300 kPa (kilopascals) or more.

[0015] Based on the above-mentioned research, the inventors have further discovered the following: When the pressure of the gas used as the optical gain medium is 300 kPa or higher, the gain of the gas used as the optical gain medium can be improved over a wide frequency band in its frequency characteristics. Therefore, this optical amplifier can improve the gain over a wide frequency band in the above frequency characteristics.

[0016] (4) In (3) above, the pressure may be 1 MPa (megapascal) or more.

[0017] In this case, the gain of the gas, which is the optical gain medium, can be further improved over a wide frequency band in terms of its frequency characteristics.

[0018] (5) In any one of (1) to (4) above, an insulating region may be provided between the gas and the thermal radiation source.

[0019] In this case, even if the temperature of the heat radiation source is high, it is possible to prevent the gas temperature from rising.

[0020] (6) In any one of (1) to (5) above, the optical amplifier may further include a container for containing the gas, the container may be cylindrical and extending along the axis, and the thermal radiation source may extend along the axis.

[0021] In this case, the thermal radiation source can irradiate the gas with excitation light over a long region along the axis.

[0022] (7) In any one of (1) to (5) above, the optical amplifier may further include a container for containing the gas, and the inner surface of the container may include a reflective surface for reflecting the excitation light.

[0023] In this case, the excitation light that reaches the reflective surface without being absorbed by the gas can be reflected back towards the gas.

[0024] (8) In the above (7), the reflective surface may be arranged to face the thermal radiation source.

[0025] In this case, even if the excitation light reflected by the reflective surface is not absorbed by the gas, the reflected excitation light returns to the thermal radiation source. Therefore, the energy required to heat the thermal radiation source can be reduced.

[0026] (9) In any one of (1) to (8) above, the thermal radiation source may include a resistive heating element, and the optical amplifier may further include a power supply connected to the resistive heating element.

[0027] In this case, the heat radiation source can be heated by applying electricity.

[0028] (10) In any one of (1) to (9) above, the thermal radiation source may include a conductor, and the optical amplifier may further include a coil for inductively heating the conductor and an AC power supply for supplying AC power to the coil.

[0029] In this case, the heat radiation source can be heated without contact.

[0030] (11) A tunable laser comprising one of the optical amplifiers described in (1) to (10) above, and an optical element that feeds back light of a specific wavelength from the emitted light emitted from the gas by the excitation light and emitted from the optical amplifier to the optical amplifier.

[0031] (12) In (11) above, the optical element may change the wavelength of the light that is fed back to the optical amplifier.

[0032] In this case, since the tunable laser is equipped with the optical amplifier described above, laser light of any wavelength in a wide frequency band can be obtained in the frequency characteristics of the gain of the gas, which is the optical gain medium, and the laser oscillation wavelength can be continuously swept.

[0033] (13) A pulse laser comprising: a pulse generator that generates pulsed laser light; a pulse extender that extends the time width of the pulsed laser light; an optical amplifier according to any one of (1) to (10) above, the optical amplifier that amplifies the intensity of the pulsed laser light whose time width has been extended by the pulse extender; and a pulse compressor that compresses the time width of the pulsed laser light whose intensity has been amplified by the optical amplifier.

[0034] In this case, since the pulsed laser is equipped with the optical amplifier described above, it can output pulsed laser light with a short duration that corresponds to a wide frequency band in the frequency characteristics of the gain of the gas, which is the optical gain medium.

[0035] [Details of Embodiments of the Disclosure] Embodiments of the Disclosure will be described in detail below with reference to the attached drawings. In the description of the drawings, the same reference numerals are used for identical or equivalent elements, and redundant descriptions are omitted.

[0036] FIG. 1 is a cross-sectional view schematically showing an optical amplifier according to an embodiment. FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1. The optical amplifier 10 shown in FIG. 1 includes a gas as an optical gain medium and a thermal radiation source 14 having wavelength selectivity. The optical amplifier 10 may further include a container 12, a container 16, a sealing member 20, an optical window W1, an optical window W2, a protrusion 22, and a power source 30. The gas as the optical gain medium may be housed in the container 12. The thermal radiation source 14 emits excitation light TR for exciting the gas by thermal radiation. The optical amplifier 10 can amplify the intensity of the incident light Lin by the emitted light emitted from the gas by the excitation light TR. As a result, the amplified output light Lout is emitted from the optical amplifier 10.

[0037] The container 12 may be cylindrical and extend along the axis Ax. In one example, the container 12 is a cylinder having an outer diameter of 28 mm (millimeters), an inner diameter of 26 mm, and a length of 114 mm.

[0038] The container 16 is disposed outside the container 12. The container 16 is separated from the container 12. The container 16 may be cylindrical and extend along the axis Ax. In one example, the container 16 is a cylinder having an outer diameter of 50 mm, an inner diameter of 46 mm, and a length of 120 mm. The container 12 and the container 16 may form a double tube.

[0039] The sealing member 20 seals the space between the container 12 and the container 16. At the first end of the container 12 on the axis Ax, one sealing member 20 is disposed between the container 12 and the container 16. In one example, the sealing member 20 is an annular member. At the second end of the container 12 on the axis Ax, another sealing member 20 is disposed between the container 12 and the container 16. The container 12, the container 16, and the sealing member 20 may be formed of a single member. The space between the container 12 and the container 16 may be depressurized. An inert gas may be enclosed in the space between the container 12 and the container 16. Examples of the inert gas include nitrogen, argon, and krypton. The pressure of the inert gas may be 1 Pa or less.

[0040] The gas may flow in the space between container 12 and container 16 without using the sealing member 20. The gas flows along axis Ax from the first end to the second end of container 12. The gas may be a gas that does not easily absorb the excitation light TR. This makes it easier for the excitation light TR to reach the gas inside container 12.

[0041] The thermal radiation source 14 may be located outside the container 12. If the thermal radiation source 14 is located outside the container 12, the container 12 may contain a material that transmits excitation light TR. The container 12 may contain at least one of aluminum oxide, zinc oxide, zinc sulfide, zinc selenide, silicon, calcium fluoride, magnesium fluoride, sodium chloride, polyethylene, polypropylene, and polystyrene. An example of aluminum oxide is sapphire. The reflectance of the outer surface of the container 12 to the excitation light TR may be 1% or less. The inner surface of the container 12 may contain a reflective surface that reflects the excitation light TR. The reflectance of the inner surface of the container 12 to the excitation light TR may be 90% or more. The reflective surface may be positioned to face the thermal radiation source 14.

[0042] The thermal radiation source 14 may be placed between the container 12 and the container 16. An insulating region TIR may be provided between the container 12 and the thermal radiation source 14. The insulating region TIR may be a reduced-pressure space between the container 12 and the container 16. In one example, the insulating region TIR may be a nitrogen atmosphere at 1 mPa (millipascal). The thermal radiation source 14 may be connected to the container 16 by at least one projection 22. If the contact area between the thermal radiation source 14 and the projection 22 is small, heat dissipation from the thermal radiation source 14 to the projection 22 can be prevented. Therefore, the thermal radiation source 14 can be heated efficiently. By reducing the number of projections 22, the contact area between the thermal radiation source 14 and the projection 22 can be reduced. By reducing the cross-sectional area of ​​the projection 22 perpendicular to the projection direction of the projection 22, the contact area between the thermal radiation source 14 and the projection 22 can be reduced.

[0043] The optical amplifier 10 may include a plurality of thermal radiation sources 14. In the example shown in FIG. 2, the optical amplifier 10 includes eight thermal radiation sources 14. Each thermal radiation source 14 may extend along the axis Ax. Each thermal radiation source 14 may be a plate-like member having a first surface 14a and a second surface 14b. When each thermal radiation source 14 is a plate-like member, each of the thermal radiation sources 14 may have a thickness of 1 mm, a width of 15 mm, and a length of 100 mm. The first surface 14a is a surface that emits the excitation light TR. The first surface 14a may face the axis Ax and the outer surface of the container 12. The second surface 14b is a surface opposite to the first surface 14a. The second surface 14b may face the inner surface of the container 16. The plurality of thermal radiation sources 14 may be provided so as to surround the axis Ax and the container 12 in a cross section orthogonal to the axis Ax. A plurality of adjacent thermal radiation sources 14 may be connected to form a single thermal radiation source 14. In this case, the thermal radiation source 14 may be cylindrical.

[0044] The optical windows W1 and W2 are provided in the container 12. The optical window W1 and the optical window W2 are arranged to face each other on the axis Ax. The optical window W1 may close the first opening of the cylindrical container 12. The first opening is located at the first end of the container 12 on the axis Ax. The optical window W2 may close the second opening of the cylindrical container 12. The second opening is located at the second end of the container 12 on the axis Ax. The gas in the container 12 may be sealed by the container 12, the optical window W1, and the optical window W2. In this case, a device for performing gas exchange becomes unnecessary. Therefore, the optical amplifier 10 can be miniaturized. The degree of freedom in installing the optical amplifier 10 is increased. In the example shown in FIG. 1, the incident light Lin enters the optical window W1, and the output light Lout is emitted from the optical window W2.

[0045] Anti-reflective (AR) films may be formed on both surfaces of each of the optical windows W1 and W2. Thereby, in the optical amplifier 10, it is possible to prevent the incident light Lin incident on the container 12 from resonating between the optical window W1 and the optical window W2. When anti-reflective films are formed on both surfaces of each of the optical windows W1 and W2, the anti-reflective films may have a reflectivity of 0.1% or less in the wavelength band of the incident light Lin.

[0046] The gas in the container 12 may contain at least one gas selected from carbon dioxide (CO ２ ), nitrogen oxides (N ２ O, NO ２ , etc.), sulfur oxides (SO ２ , etc.), ozone (O ３ ), ammonia (NH ３ ), methane (CH ４ ), and halogen derivatives of methane as a photogain medium. The halogen derivative of methane refers to a compound in which at least one of the hydrogens of methane is halogenated. The gas in the container 12 may be a mixed gas containing a first gas as a photogain medium and a second gas different from the first gas. The second gas may contain at least one gas selected from nitrogen (N ２ ), oxygen (O ２ ), hydrogen (H ２ ), helium (He), argon (Ar), neon (Ne), hydrogen bromide (HBr), and hydrogen chloride (HCl). The second gas can cause the energy level of the first gas to transition to a low ground state. The second gas can also cool the first gas. The gas in the container 12 may not contain nitrogen.

[0047] In one example, the gas in the container 12 is a mixed gas containing carbon dioxide and helium. In this example, the partial pressure ratio of carbon dioxide to helium in the mixed gas containing carbon dioxide and helium is 1:3. When the gas in the container 12 is the above mixed gas containing carbon dioxide and helium, the container 12 may contain sapphire that transmits the excitation light TR for exciting carbon dioxide.

[0048] In another example, the gas in the container 12 may be a mixed gas containing carbon dioxide, nitrous oxide, and helium. In this example, the partial pressure ratio of carbon dioxide, nitrous oxide, and helium in the mixed gas may be 3:2:15. Alternatively, the partial pressure ratio of carbon dioxide, nitrous oxide, and helium in the mixed gas may be 3:2:45. Alternatively, the partial pressure ratio of carbon dioxide, nitrous oxide, and helium in the mixed gas may be 8:2:90.

[0049] In yet another example, the gas in container 12 may be a mixed gas containing carbon dioxide, hydrogen bromide, and helium. In this example, the partial pressure ratio of carbon dioxide, hydrogen bromide, and helium in the mixed gas may be 1:1:48. In yet another example, the gas in container 12 may be a mixed gas containing carbon dioxide, nitrogen, and helium. In this example, the partial pressure ratio of carbon dioxide, nitrogen, and helium in the mixed gas may be 1:74:25. In yet another example, the gas in container 12 may be a mixed gas containing nitrous oxide and helium. In this example, the partial pressure ratio of nitrous oxide and helium in the mixed gas may be 1:3.

[0050] The thermal radiation source 14 may include at least one of a photonic crystal, a microcavity resonator, and a plasmonic metasurface. The spectrum of the excitation light TR emitted from the thermal radiation source 14 has a peak at the excitation wavelength of the gas used as the optical gain medium described above.

[0051] The thermal radiation source 14 may be a diamond with sufficient thickness, or zinc sulfide or zinc selenide doped with a transition metal such as iron or chromium. For example, a diamond with a thickness of about 1 mm can operate as a wavelength-selective radiation source with wavelengths from 4 μm (micrometers) to 6 μm. The thermal radiation source 14 may also be a photonic crystal containing these materials.

[0052] The power supply 30 is located outside the container 16. The power supply 30 may be connected in parallel to the multiple heat radiation sources 14. The power supply 30 may be a DC power supply. The conductors between the heat radiation sources 14 and the power supply 30 may extend along the projection 22.

[0053] When power is supplied from the power supply 30 to each thermal radiation source 14, excitation light TR is emitted from each heated thermal radiation source 14 into the gas inside the container 12. The gas is excited by the excitation light TR, the intensity of the incident light Lin is amplified, and the output light Lout is emitted from the optical amplifier 10. Excitation light TR emitted from one thermal radiation source 14 that is not absorbed by the gas may be incident on another thermal radiation source 14. In this case, a decrease in the temperature of the thermal radiation source 14 into which the excitation light TR was incident is prevented.

[0054] Figure 3 is a plan view showing an example of a thermal radiation source. Figure 4 is a plan view showing a part of Figure 3. Figure 5 is a cross-sectional view along the line V-V in Figure 4. As shown in Figures 3 to 5, the thermal radiation source 14 may include a substrate SB and a conductive pattern region CP provided on the substrate SB. The substrate SB is, for example, a glass substrate or a polymer substrate. In one example, the substrate SB has a main surface having a long side of 100 mm and a short side of 15 mm, and a thickness of 2 mm.

[0055] The conductor pattern region CP is provided on the first surface 14a of the thermal radiation source 14. The conductor pattern region CP may be a meandering band-shaped region on the first surface 14a, as shown in Figure 3. In one example, the conductor pattern region CP is a band-shaped region with a width of 2.3 mm that meanders at intervals of 0.2 mm.

[0056] The conductor pattern region CP includes a first layer F1, a second layer F2, and a third layer F3. The first layer F1, the second layer F2, and the third layer F3 are provided sequentially on the substrate SB. The first layer F1 and the second layer F2 may extend across the entire conductor pattern region CP. The first layer F1 may be a metal layer. For example, the first layer F1 is an aluminum layer. In one example, the first layer F1 has a thickness of 100 nm. The second layer F2 may be a dielectric layer. For example, the second layer F2 is an aluminum oxide layer. In one example, the second layer F2 has a thickness of 50 nm. The third layer F3 may be a metal layer. For example, the third layer F3 is an aluminum layer. The third layer F3 may be a plurality of island patterns arranged in an array, spaced apart from each other. In one example, the plurality of island patterns are provided at a pitch of 1500 nm. In one example, each island pattern has a square main surface with sides of 980 nm and a thickness of 100 nm.

[0057] The power supply 30 shown in Figure 1 may be connected to the first layer F1, which is a resistive heating element of the thermal radiation source 14. The positive terminal of the power supply 30 may be connected to the first end of the conductor pattern region CP. The negative terminal of the power supply 30 may be connected to the second end of the conductor pattern region CP. The conductor pattern region CP is heated when current flows through the first layer F1.

[0058] Next, the gain in the optical amplifier 10 will be explained with reference to Figure 6. Figure 6 is a graph showing an example of the frequency characteristics of the gain of a gas excited by excitation light. In Figure 6, the horizontal axis of the graph represents the frequency of the incident light Lin [THz (terahertz)]. In Figure 6, the vertical axis of the graph represents the gain normalized by the maximum gain, and its unit is arbitrary unit (Arbitrary Unit: arb.unit). Figure 6 shows the frequency characteristics of the gain when the gas in the container 12 is a mixed gas containing carbon dioxide and helium, and the total pressure of the mixed gas is 750 kPa. In Figure 6, the partial pressure ratio of carbon dioxide and helium in the mixed gas containing carbon dioxide and helium is 1:3.

[0059] The gain of a gas excited by excitation light TR may be expressed, for example, by the ratio of the intensity of the output light Lout to the intensity of the incident light Lin. Hereinafter, the gain of a gas excited by excitation light TR may be simply referred to as gain.

[0060] In the graph of Figure 6, characteristic C1 shows the frequency characteristics of the gain of the gas excited by the excitation light TR. More specifically, characteristic C1 shows the raw frequency characteristics without the filter described later being applied. As described above, in this embodiment, in the mixed gas containing carbon dioxide and helium in the container 12, the partial pressure ratio of carbon dioxide to helium is 1:3, so characteristic C1 shown in Figure 6 shows the frequency characteristics of the gain of the mixed gas.

[0061] As shown in Figure 6, in characteristic C1, local maxima and local maxima appear alternately depending on the frequency, and the gains corresponding to each frequency differ significantly from one another. For this reason, a filter is applied to characteristic C1 to flatten its shape. In the graph of Figure 6, characteristic C2 shows the frequency characteristics of the gas gain obtained by applying the filter to characteristic C1. In one example, the filter flattens the shape of characteristic C1 by a predetermined value. Here, flattening the shape of characteristic C1 includes, for example, setting the gains in characteristic C1 that are greater than or equal to the predetermined value to that predetermined value, and converging the gains smaller than the predetermined value to zero. The predetermined value may be, for example, 20% of the maximum value (maximum gain) of characteristic C1, or 50% of the maximum gain.

[0062] In the optical amplifier 10, it is required to improve the gain over a wide frequency band. In the above frequency characteristics, the ratio of the minimum value adjacent to the maximum value of the gain to the maximum value of the gain serves as an indicator of whether or not the gain has been improved over a wide frequency band in the frequency characteristics. Here, the relationship between the maximum value and the minimum value adjacent to the maximum value will be explained in more detail with reference to Figure 7. Figure 7 is an enlarged view of a part of the frequency characteristics shown in Figure 6. In Figure 7, the horizontal axis of the graph represents the frequency of the incident light Lin [THz]. In Figure 7, the vertical axis of the graph represents the gain normalized by the maximum value of the gain [arb. unit].

[0063] In the example shown in Figure 7, the maximum value of characteristic C1 is shown as gain G1, and the local minimum values ​​adjacent to gain G1 are shown as gains G2 and G3. In this example, gains G2 and G3 are the gains shown by the frequencies (frequencies f2 and f3) adjacent to the frequency (frequency f1) that shows gain G1 in characteristic C1. In the optical amplifier 10, the ratio of gains G2 and G3 to gain G1 is 0.4 times or more. This makes it possible to improve the gain over a wide frequency band in the optical amplifier 10. In the optical amplifier 10, the ratio of gains G2 and G3 to gain G1 may also be 0.8 times or more.

[0064] In one example, an improvement in gain over a wide frequency band may occur if the frequency band showing a gain above a predetermined value in characteristic C1 exceeds a predetermined frequency width. The predetermined frequency width may be, for example, 0.5 THz, 1.5 THz, or 2.0 THz.

[0065] In the optical amplifier 10, the gain of at least one of the gains G2 and G3 may be 0.4 times or more relative to the gain G1, or 0.8 times or more. That is, in the optical amplifier 10, the gain of only one of the gains G2 and G3 may be 0.4 times or more relative to the gain G1, or the gains of both the gains G2 and G3 may be 0.4 times or more relative to the gain G1.

[0066] The gas pressure inside container 12 may be 300 kPa or more, 500 kPa or more, or 1 MPa or more. The gas pressure inside container 12 may be 2 MPa or less, or 5 MPa or less. If the gas inside container 12 is a mixed gas containing the first gas and the second gas, the gas pressure inside container 12 is the total pressure of the mixed gas. By setting the gas pressure inside container 12 to 300 kPa or more, it becomes possible to easily and reliably set the ratio of gains G2 and G3 to gain G1 to 0.4 times or more. In this embodiment, the gas inside container 12 is a mixed gas containing carbon dioxide and helium, and the gas pressure (total pressure) inside container 12 is 750 kPa. In a mixed gas containing carbon dioxide and helium, the partial pressure ratio of carbon dioxide to helium is 1:3.

[0067] Here, referring to Figures 8 and 9, the relationship between the gain when the ratio of gains G2 and G3 to gain G1 is 0.4 times or more, and the gain when the ratio of gains G2 and G3 to gain G1 is less than 0.4 times, will be explained. Figure 8 is a graph showing another example of the frequency characteristics of the gain of a gas excited by excitation light. Figure 9 is a graph showing yet another example of the frequency characteristics of the gain of a gas excited by excitation light. In Figures 8 and 9, the horizontal axis of the graph shows the frequency of the incident light Lin [THz], and the vertical axis of the graph shows the gain normalized by the maximum gain [arb. unit].

[0068] Figure 8 shows the frequency characteristics of the gain as characteristic C3 when the gas in container 12 is a mixed gas containing carbon dioxide, helium, and nitrogen, and the total pressure of the mixed gas is 1 MPa. In Figure 8, the partial pressure ratio of carbon dioxide, helium, and nitrogen in the mixed gas is 1:74:25. In the example shown in Figure 8, the maximum value of characteristic C3 is shown as gain G4, and the local minimums adjacent to gain G4 are shown as gains G5 and G6.

[0069] Figure 9 shows the frequency characteristics of the gain as characteristic C4 when the gas in container 12 is a mixed gas containing carbon dioxide, helium, and nitrogen, and the total pressure of the mixed gas is 100 kPa. In Figure 9, the partial pressure ratio of carbon dioxide, helium, and nitrogen in the mixed gas is 2:5:13. In the example shown in Figure 9, the maximum value of characteristic C4 is shown as gain G7, and the local minimums adjacent to gain G7 are shown as gains G8 and G9.

[0070] In characteristic C3, the ratio of gains G5 and G6 to gain G4 is 0.4 times or greater. More specifically, the ratio of gains G5 and G6 to gain G4 is greater than 0.8 times. When the above-described filter is applied to such characteristic C3, the frequency band in which the gain shown in characteristic C3 is greater than or equal to a predetermined value is wide, making it possible to improve the gain over a wide frequency band.

[0071] In contrast, in characteristic C4, the ratio of gains G8 and G9 to gain G7 is less than 0.4. More specifically, the ratio of gains G8 and G9 to gain G7 is less than 0.1. Therefore, even if the same filter applied to characteristic C3 is applied to characteristic C4, characteristic C4 does not have a frequency at which the gain exceeds the predetermined value, making it difficult to improve the gain over a wide frequency band.

[0072] From the above, with the optical amplifier 10, in characteristic C1, the ratio of gains G2 and G3 to gain G1 is 0.4 times or more, so the gain can be improved over a wide frequency band in the frequency characteristics of the gain of the gas, which is the optical gain medium. With the optical amplifier 10, there is no need to excite the gas by discharge, so a high voltage power supply for discharge is not required. Therefore, the optical amplifier 10 can be made smaller and lighter.

[0073] When the ratio of gains G2 and G3 to gain G1 is 0.8 times or more, the gain of the gas, which is the optical gain medium, can be further improved over a wide frequency band in its frequency characteristics.

[0074] In the optical amplifier 10, since the pressure of the gas used as the optical gain medium is 300 kPa or higher, the ratio of gains G2 and G3 to gain G1 can be easily and reliably set to 0.4 times or more. Therefore, the gain can be improved over a wide frequency band in the frequency characteristics of the gain of the gas used as the optical gain medium.

[0075] When the pressure of the gas used as the optical gain medium is 1 MPa or higher, the ratio of gains G2 and G3 to gain G1 can be easily and reliably set to 0.8 times or more. Therefore, the gain of the gas used as the optical gain medium can be further improved over a wide frequency band in its frequency characteristics.

[0076] If an insulating region TIR is provided between the container 12 and the thermal radiation source 14, the temperature of the gas inside the container 12 can be prevented from rising even if the temperature of the thermal radiation source 14 is high.

[0077] When the thermal radiation source 14 extends along axis Ax, the thermal radiation source 14 can irradiate the gas with excitation light TR over a long region along axis Ax. When the thermal radiation source 14 is arranged to surround axis Ax in a cross-section perpendicular to axis Ax, the thermal radiation source 14 can irradiate the gas with excitation light TR from many directions.

[0078] If the inner surface of the container 12 includes a reflective surface that reflects the excitation light TR, the excitation light TR that reaches the reflective surface without being absorbed by the gas can be reflected back towards the gas.

[0079] When the reflective surface on the inner surface of the container 12 is positioned to face the thermal radiation source 14, the reflected excitation light TR returns to the thermal radiation source 14 even if it is not absorbed by the gas. Therefore, the energy required to heat the thermal radiation source 14 can be reduced.

[0080] When the thermal radiation source 14 is equipped with a first layer F1 and the power supply 30 is connected to the first layer F1, the thermal radiation source 14 can be heated by applying power.

[0081] If the thermal radiation source 14 is located outside the container 12 and the container 12 contains a material that transmits excitation light TR, the degree of freedom in the placement of the thermal radiation source 14 is improved.

[0082] Figure 10 is a schematic cross-sectional view showing an optical amplifier according to another embodiment. Figure 11 is a cross-sectional view along the line XI-XI in Figure 10. The optical amplifier 10A shown in Figures 10 and 11 may be the same as the optical amplifier 10, except as follows: The optical amplifier 10A may include optical members OC1 and OC2 instead of optical windows W1 and W2.

[0083] Optical members OC1 and OC2 are provided in the container 12. Optical members OC1 and OC2 are arranged facing each other along axis Ax. Optical member OC1 may close the first opening of the cylindrical container 12. Optical member OC2 may close the second opening of the cylindrical container 12. The gas inside the container 12 may be sealed by the container 12, optical members OC1 and OC2. In this case as well, a gas exchange device is not required. Therefore, the optical amplifier 10A can be miniaturized. The degree of freedom in installing the optical amplifier 10A is increased.

[0084] Optical component OC1 includes an optical window W3 and an optical mirror M1, each having a semicircular shape. Optical component OC2 includes an optical window W4 and an optical mirror M2, each having a semicircular shape. Optical window W3 and optical mirror M2 are arranged facing each other along axis Ax. Optical mirror M1 and optical window W4 are arranged facing each other along axis Ax.

[0085] In one example, when optical members OC1 and OC2 are viewed from a direction along axis Ax, optical windows W3 and W4 do not necessarily have to overlap. When optical members OC1 and OC2 are viewed from a direction along axis Ax, optical mirrors M1 and M2 do not necessarily have to overlap.

[0086] The optical amplifier 10A, like the optical amplifier 10, may also be equipped with multiple thermal radiation sources 14. In the example shown in Figures 10 and 11, the optical amplifier 10A is equipped with 12 thermal radiation sources 14.

[0087] Figure 12 is a schematic diagram showing an example of the optical path in the optical amplifier shown in Figure 10. As shown in Figure 12, for example, incident light Lin entering from optical window W3 is reflected by optical mirror M2 and guided to optical mirror M1. Next, this light is reflected by optical mirror M1 and emitted as output light Loout from optical window W4. By arranging the optical windows W3, W4 and optical mirrors M1, M2 as described above, the optical path length in the optical amplifier 10A is increased, and the gain can be further improved.

[0088] Anti-reflective coatings may be formed on both sides of optical windows W3 and W4. This prevents the incident light Lin incident on the container 12 from resonating between optical windows W3 and W4 in the optical amplifier 10A. When anti-reflective coatings are formed on both sides of optical windows W3 and W4, the anti-reflective coatings may have a reflectivity of 0.1% or less in the wavelength band of the incident light Lin. The surfaces of optical mirrors M1 and M2 may be coated with, for example, gold. The focal lengths of optical mirrors M1 and M2 may be, for example, 500 mm.

[0089] Figure 13 schematically shows another example of the light path in the optical amplifier shown in Figure 10. The optical amplifier 10A may further include a roof prism RP.

[0090] For example, the roof prism RP is positioned facing the optical window on the opposite side of optical window W3 and optical window W4 from the optical window through which the incident light Lin enters. In the example shown in Figure 13, the roof prism RP is positioned facing optical window W4.

[0091] In the example shown in Figure 13, incident light Lin entering from the optical window W3 is reflected by the optical mirror M2 and guided to the optical mirror M1. Next, the light is reflected by the optical mirror M1 and guided through the optical window W4 to the roof prism RP. Next, the light reflected by the roof prism RP returns into the container 12, and the light that returns into the container 12 is guided to the optical mirror M1. Next, the light is reflected by the optical mirror M1 and guided to the optical mirror M2, and that light is reflected by the optical mirror M2 and emitted from the optical window W3 as output light Lout. In this way, when the optical amplifier 10A is equipped with a roof prism RP, the optical path length in the optical amplifier 10A is further increased, and the gain can be improved even more reliably.

[0092] The optical amplifier 10A provides the same effect as the optical amplifier 10. Furthermore, as described above, the optical path length in the optical amplifier 10A is increased, which allows for an even greater improvement in gain.

[0093] Figure 14 is a schematic cross-sectional view showing an optical amplifier according to another embodiment. Figure 15 is a cross-sectional view along the line XV-XV in Figure 14. The optical amplifier 10B shown in Figures 14 and 15 may be the same as the optical amplifier 10, except for the following points. The optical amplifier 10B shown in Figures 14 and 15 may be equipped with a thermal radiation source 114 instead of a thermal radiation source 14. The optical amplifier 10B may be equipped with a heating device 130 instead of a power supply 30. The optical amplifier 10B may be equipped with a container 112 instead of a container 12. The optical amplifier 10B may be equipped with a dielectric member 116 instead of a container 16. The optical amplifier 10B may be equipped with a sealing member 120 instead of a sealing member 20. The optical amplifier 10B may be equipped with optical members OC3 and OC4 instead of optical windows W1 and W2.

[0094] The container 112 may be cylindrical, extending along axis Ax. In one example, the container 112 is a rectangular tube. The container 112 comprises an optical window W5, a heat sink HS, and a sealing member 112a. In one example, the optical window W5 is a plate-shaped member having a length of 920 mm and a width of 260 mm. An example of the material for the optical window W5 is the same as the example of the material for the container 12. An anti-reflective coating may be formed on both sides of the optical window W5. If an anti-reflective coating is formed on both sides of the optical window W5, the anti-reflective coating may have a reflectance of 1% or less in the wavelength band of the excitation light TR (for example, from wavelength 4 μm to wavelength 5 μm). Since calcium fluoride has a reflectance of 1% or less in the wavelength band of the excitation light TR, for example, if the material of the optical window W5 is calcium fluoride, the above-mentioned anti-reflective coating does not need to be formed on both sides of the optical window W5.

[0095] As shown in Figure 15, the sealing member 112a seals the space between the optical window W5 and the heat sink HS. At the first end of the container 112 in the short direction, one sealing member 112a is positioned between the optical window W5 and the heat sink HS. In one example, the sealing member 112a is a plate-shaped member. At the second end of the container 112 in the short direction, another sealing member 112a is positioned between the optical window W5 and the heat sink HS.

[0096] The heat sink HS is a plate-shaped member extending along axis Ax. In one example, the heat sink HS is a plate-shaped member having a length of 920 mm and a width of 260 mm. In one example, the heat sink HS is made of aluminum. The heat sink HS may also be made of copper. The heat sink HS may be provided with a channel for flowing a cooling fluid. The cooling fluid may contain water. In one example, the temperature of the cooling fluid is room temperature (25°C). Alternatively, the heat sink HS may comprise a main body and a plurality of fins provided on the main body.

[0097] The dielectric member 116 is positioned on the outside of the container 112. Specifically, the dielectric member 116 is positioned on the outside of the optical window W5 of the container 112. The dielectric member 116 is separated from the optical window W5. In one example, the dielectric member 116 is a plate-shaped member having a length of 920 mm and a width of 260 mm. An example of the material for the dielectric member 116 is the same as the example of the material for the container 16.

[0098] As shown in Figures 14 and 15, the sealing member 120 seals the space between the optical window W5 and the dielectric member 116. At the first end in the short direction of the optical window W5, one sealing member 120 is positioned between the optical window W5 and the dielectric member 116. In one example, the sealing member 120 is a plate-shaped member. At the second end of the optical window W5, another sealing member 120 is positioned between the optical window W5 and the dielectric member 116. The space between the optical window W5 (container 112) and the dielectric member 116 may be depressurized. An inert gas may be sealed in the space between the optical window W5 and the dielectric member 116.

[0099] The thermal radiation source 114 may be positioned between the optical window W5 and the dielectric member 116. A thermal insulation region TIR may also be provided between the optical window W5 and the dielectric member 116. The thermal radiation source 114 may be connected to the dielectric member 116 by at least one projection 22.

[0100] The thermal radiation source 114 may be a plate-shaped member having a first surface 114a and a second surface 114b. The first surface 114a is the surface from which the excitation light TR is emitted. The first surface 114a faces the axis Ax and the outer surface of the optical window W5. The second surface 114b is the surface opposite to the first surface 114a. The second surface 114b faces the inner surface of the dielectric member 116.

[0101] Optical members OC3 and OC4 are provided in the container 112. Optical members OC3 and OC4 are arranged facing each other along the axis Ax. Optical member OC3 may close the first opening of the cylindrical container 112. The first opening is located at the first end of the container 112 along the axis Ax. Optical member OC4 may close the second opening of the cylindrical container 112. The second opening is located at the second end of the container 112 along the axis Ax. The gas inside the container 112 may be sealed by the container 112, optical members OC3 and OC4. In this case as well, a gas exchange device is not required. Therefore, the optical amplifier 10B can be miniaturized. The degree of freedom in installing the optical amplifier 10B is increased.

[0102] Optical component OC3 may include an optical window W6 and an optical mirror M3. Optical component OC4 may include an optical mirror M4, an optical window W7, and an optical mirror M5. The optical window W6 and the optical mirror M4 are arranged facing each other along axis Ax. The optical mirror M3, the optical window W7, and the optical mirror M5 are arranged facing each other along axis Ax.

[0103] Figure 16 is a schematic diagram showing an example of the optical path in the optical amplifier shown in Figure 14. As shown in Figure 16, for example, incident light Lin entering from the optical window W6 is reflected by the optical mirror M4 and guided to the optical mirror M3. Next, the light is reflected by the optical mirror M3 and guided to the optical mirror M5. The light guided to the optical mirror M5 is reflected by the optical mirror M5 and guided again to the optical mirror M3. The light guided to the optical mirror M3 is reflected by the optical mirror M3 and guided to the optical mirror M4. The light guided to the optical mirror M4 is reflected by the optical mirror M4 and guided again to the optical mirror M3. The light guided to the optical mirror M3 is reflected by the optical mirror M3 and emitted as output light Loout from the optical window W7. In this way, when the optical amplifier 10C is equipped with optical members OC3 and OC4, the optical path length in the optical amplifier 10C is increased, and the gain can be further improved.

[0104] In one example, in the optical amplifier 10B, the gas in the container 112 is a mixed gas containing carbon dioxide, hydrogen bromide, and helium. In this example, the partial pressure ratio of carbon dioxide, hydrogen bromide, and helium in the mixed gas is 1:1:48. When the partial pressure ratio of carbon dioxide, hydrogen bromide, and helium in the mixed gas is 1:1:48, the total pressure of the mixed gas in the container 112 is 5 MPa.

[0105] Figure 17 is a plan view showing an example of a thermal radiation source. The thermal radiation source 114 shown in Figure 17 may comprise a substrate SB and a conductive pattern region CP1 provided on the substrate SB. The conductive pattern region CP1 is provided on the first surface 114a of the thermal radiation source 114. The conductive pattern region CP1 is the same as the conductive pattern region CP except that it is provided in a spiral shape on the first surface 114a.

[0106] As shown in Figures 14 and 15, the heating device 130 includes a coil 132 for inductively heating the conductors of the heat radiation source 114 (for example, the first layer F1 and the third layer F3 of the conductor pattern region CP1), and an AC power supply 134 for supplying AC power to the coil 132. The coil 132 may be a spiral coil. The spiral coil has a pattern corresponding to the conductor pattern region CP1. The coil 132 faces the conductor pattern region CP1 of the heat radiation source 114. In one example, the frequency of the AC power supply 134 is 50 Hz.

[0107] With the optical amplifier 10B, the same effects as with the optical amplifier 10 can be obtained. Furthermore, with the optical amplifier 10B, the heat radiation source 114 can be heated non-contact by the heating device 130. Therefore, the heat emitted from the heat radiation source 114 to the outside through the conductor can be reduced. Thus, the heat radiation source 114 can be heated efficiently. If the coil 132 is a spiral coil having a pattern corresponding to the conductor pattern region CP1, the energy conversion efficiency from electrical energy to thermal energy can be increased.

[0108] Furthermore, according to the optical amplifier 10B, the heat sink HS can cool the gas inside the container 112. If the heat sink HS is equipped with a channel for flowing a cooling fluid, the cooling effect of the gas is further enhanced.

[0109] In other optical amplifiers 10 and 10A, a thermal radiation source 114 and a heating device 130 may be used. This allows the thermal radiation source 114 to be heated without contact.

[0110] In other optical amplifiers 10 and 10A, a container 112 may be used. This allows the gas inside the container 112 to be cooled using the heat sink HS.

[0111] Figure 18 is a schematic diagram showing a tunable laser according to one embodiment. The tunable laser 100 shown in Figure 18 comprises an optical amplifier 10C and an optical element 40. In the tunable laser 100, laser light Lb is emitted when light based on emitted light La emitted from a gas excited by excitation light TR resonates. In one example, the tunable laser 100 is a so-called Littrow-type tunable laser.

[0112] The optical amplifier 10C shown in Figure 18 may be the same as the optical amplifier 10, except for the following point. The optical amplifier 10C may be equipped with an optical lens L1 instead of an optical window W1. In one example, the optical lens L1 is a convex lens. The focal length of the optical lens L1 is, for example, 115 mm.

[0113] In one example, in the optical amplifier 10C, the gas in the container 12 is a mixed gas containing carbon dioxide and helium. In this example, the partial pressure ratio of carbon dioxide to helium in the mixed gas is 1:99. When the partial pressure ratio of carbon dioxide to helium in the mixed gas is 1:99, the total pressure of the gas in the container 12 may be, for example, 2 MPa.

[0114] The optical element 40 returns light of a specific wavelength from the emitted light La that is released from the gas by the excitation light TR and also emitted from the optical amplifier 10C to the optical amplifier 10C. The optical element 40 may change the wavelength of the light that is returned to the optical amplifier 10C. The optical element 40 may also include a diffraction grating 42. The diffraction grating 42 diffracts the emitted light La emitted from the optical amplifier 10C.

[0115] In one example, the diffraction grating 42 is positioned at the focal point of the optical lens L1. The orientation of the diffraction grating 42 is set, for example, so that the first-order diffracted light Lc of the emitted light La is returned into the optical amplifier 10C. When the diffraction grating 42 is positioned as described above, the emitted light La, collimated by the optical lens L1, is incident on the diffraction grating 42. The diffraction grating 42 returns the first-order diffracted light Lc of the incident emitted light La into the optical amplifier 10C. In this case, the zero-order diffracted light Ld of the emitted light La is not returned into the optical amplifier 10C.

[0116] Here, the reflectivity of the diffraction grating 42 with respect to the first diffracted light Lc is greater than the reflectivity of the optical window W2 with respect to the first diffracted light Lc. In one example, the reflectivity of the diffraction grating 42 with respect to the first diffracted light Lc is 80%, and the reflectivity of the optical window W2 with respect to the first diffracted light Lc is 10%. Therefore, in the tunable laser 100, the first diffracted light Lc of the emitted light La is resonated, and laser light Lb is emitted from the optical window W2. The wavelength of the laser light Lb is determined according to the orientation of the diffraction grating 42. By rotating the diffraction grating 42 within the range in which the first diffracted light Lc is returned into the optical amplifier 10C, the wavelength of the laser light Lb can be continuously changed. In this way, in the tunable laser 100, a Littrow-type optical resonator is formed between the diffraction grating 42 and the optical window W2.

[0117] With the tunable laser 100, since the tunable laser 100 is equipped with an optical amplifier 10C, laser light of any wavelength in a wide frequency band can be obtained in the frequency characteristics of the gain of the gas, which is the optical gain medium, and the laser oscillation wavelength can be continuously swept.

[0118] Figure 19 is a schematic diagram showing a tunable laser according to another embodiment. The tunable laser 100A shown in Figure 19 includes an optical amplifier 10D and an optical element 40A instead of an optical amplifier 10C and an optical element 40. In the tunable laser 100A, as in the tunable laser 100, the laser light Lb is emitted when the light based on the emitted light La emitted from the gas by the excitation light TR resonates. In one example, the tunable laser 100A is a so-called Littmann type tunable laser.

[0119] The optical amplifier 10D shown in Figure 19 may be the same as the optical amplifier 10, except for the following: The optical amplifier 10D may be equipped with an optical mirror M6 instead of an optical window W1. In one example, the optical mirror M6 is a spherical mirror. The focal length of the optical mirror M6 is, for example, 240 mm.

[0120] In one example, in the optical amplifier 10D, the gas in the container 12 is a mixed gas containing carbon dioxide and helium. In this example, the partial pressure ratio of carbon dioxide to helium in the mixed gas is 1:99. When the partial pressure ratio of carbon dioxide to helium in the mixed gas is 1:99, the total pressure of the gas in the container 12 may be, for example, 2 MPa.

[0121] Like optical element 40, optical element 40A also returns light of a specific wavelength to the optical amplifier 10D from the emitted light La that is released from the gas by the excitation light TR and also emitted from the optical amplifier 10D. Optical element 40A may change the wavelength of the light that is returned to the optical amplifier 10D. Optical element 40A may also include a diffraction grating 42A and an optical mirror M7. The diffraction grating 42A diffracts the emitted light La emitted from the optical amplifier 10D. The optical mirror M7 reflects the diffracted light of the emitted light La that has been diffracted by the diffraction grating 42A.

[0122] In one example, the diffraction grating 42A is positioned at the focal point of the optical mirror M6. The orientation of the diffraction grating 42A is set so that, for example, the zero-order diffracted light Ld of the emitted light La is not fed back into the optical amplifier 10D. The optical mirror M7 is positioned facing the diffraction grating 42A. In one example, the optical mirror M7 is positioned so that the first-order diffracted light Lc is reflected by the diffraction grating 42A. The orientation of the optical mirror M7 is adjustable so that, for example, the first-order diffracted light Lc of a selected wavelength is fed back into the optical amplifier 10D by the diffraction grating 42A. That is, the first-order diffracted light Lc of the emitted light La is fed back into the optical amplifier 10D by the diffraction grating 42A and the optical mirror M7. In this case, the zero-order diffracted light Ld of the emitted light La is not fed back into the optical amplifier 10D.

[0123] In one example, the light La emitted from the optical amplifier 10D is incident on the diffraction grating 42A and diffracted at an angle corresponding to its wavelength. Of the diffracted light diffracted by the diffraction grating 42A, the first-order diffracted light Lc is reflected by the optical mirror M7 and then incident on the diffraction grating 42A again. The first-order diffracted light Lc that has been re-incident on the diffraction grating 42A is fed back into the optical amplifier 10D. The first-order diffracted light Lc that has been fed back into the optical amplifier 10D is reflected by the optical mirror M6 and then incident on the diffraction grating 42A again. In the tunable laser 100A, the first-order diffracted light Lc of the emitted light La is made to resonate, and the zero-order diffracted light Ld of the emitted light La diffracted by the diffraction grating 42A is output as laser light Lb. In this way, in the tunable laser 100A, a Littmann-type optical resonator is formed between the optical mirror M7 and the optical mirror M6.

[0124] The tunable laser 100A can achieve the same effect as the tunable laser 100.

[0125] Figure 20 is a schematic diagram showing a pulsed laser according to one embodiment. The pulsed laser 200 shown in Figure 20 comprises a pulse generator 210, a pulse extender 220, an optical amplifier 10, and a pulse compressor 230.

[0126] The pulse generator 210 generates pulsed laser light Lp. In one example, the center wavelength of the pulsed laser light Lp may be 10.6 μm, and the pulse width of the pulsed laser light Lp may be 10 ps (picoseconds) or less. The pulse generator 210 may be, for example, an optical parametric oscillator, a carbon dioxide laser, or an optical comb combining the above carbon dioxide laser and an optical resonator.

[0127] The pulse expander 220 expands the time width of the pulsed laser light Lp generated by the pulse generator 210. In one example, the pulse expander 220 expands the time width of the pulsed laser light Lp by dispersing it. The pulse expander 220 is located upstream of the optical amplifier 10, between the pulse generator 210 and the optical amplifier 10. The optical amplifier 10 amplifies the intensity of the pulsed laser light Lp whose time width has been expanded by the pulse expander 220. The pulse compressor 230 compresses the time width of the pulsed laser light Lp whose intensity has been amplified by the optical amplifier 10. In one example, the pulse compressor 230 compresses the time width of the pulsed laser light Lp whose intensity has been amplified by dispersing it. The pulse compressor 230 is located downstream of the optical amplifier 10.

[0128] In the example shown in Figure 20, the pulse extender 220 comprises a pair of diffraction gratings 221, 222 and a pair of diffraction gratings 223, 224. In one example, the pair of diffraction gratings 221, 222 and the pair of diffraction gratings 223, 224 may be the same. In the pulse extender 220, dispersion is imparted to the pulsed laser light Lp by the pair of diffraction gratings 221, 222 and the pair of diffraction gratings 223, 224.

[0129] Specifically, when pulsed laser light Lp is diffracted by diffraction gratings 221 to 224, the short-wavelength component of the pulsed laser light Lp passes through a path having a first optical path length. In contrast, the long-wavelength component of the pulsed laser light Lp passes through a path having a second optical path length that is longer than the first optical path length. In this way, in the pulse extender 220, the pulsed laser light Lp is divided into the above-mentioned multiple wavelength components by passing through paths with different optical path lengths. In one example, the above-mentioned multiple wavelength components have the same wavelength width.

[0130] In the example shown in Figure 20, the wavelength components separated from the pulsed laser light Lp are shown as pulsed laser light Lp1, Lp2, and Lp3. In Figure 20, pulsed laser light Lp1 corresponds to the short wavelength component of pulsed laser light Lp. Pulsed laser light Lp3 corresponds to the long wavelength component of pulsed laser light Lp. Pulsed laser light Lp2 corresponds to the wavelength component between the wavelength component of pulsed laser light Lp1 and the wavelength component of pulsed laser light Lp3.

[0131] The pulsed laser beams Lp1, Lp2, and Lp3 are amplified in intensity by the optical amplifier 10. Hereafter, the pulsed laser beams Lp1, Lp2, and Lp3 whose intensity has been amplified by the optical amplifier 10 may be referred to as pulsed laser beams Lp4, Lp5, and Lp6.

[0132] In the example shown in Figure 20, the pulse compressor 230 comprises a pair of diffraction gratings 231, 232 and a pair of diffraction gratings 233, 234. In one example, the pair of diffraction gratings 231, 232 and the pair of diffraction gratings 233, 234 may be the same. In the pulse compressor 230, the pair of diffraction gratings 231, 232 and the pair of diffraction gratings 233, 234 impart inverse dispersion to the pulsed laser beams Lp4, Lp5, and Lp6.

[0133] Specifically, when the pulsed laser beams Lp4, Lp5, and Lp6 are diffracted by the diffraction gratings 231 to 234, the pulsed laser beam Lp4 passes through a path with a third optical path length. In contrast, the pulsed laser beam Lp6 passes through a path with a fourth optical path length shorter than the third optical path length, and the pulsed laser beam Lp5 passes through a path with a fifth optical path length, which is between the third and fourth optical path lengths. Thus, in the pulse compressor 230 and the pulse expander 220, the relationship between the magnitudes of the optical path lengths of the paths through which the short-wavelength and long-wavelength components of the pulsed laser beam Lp pass is reversed. As a result, the pulsed laser beams Lp4, Lp5, and Lp6 are temporally integrated, and the time width of the pulsed laser beam Lp is compressed. From the above, the pulsed laser 200 is able to output pulsed laser beam Lp with amplified intensity.

[0134] According to the pulsed laser 200, since the pulsed laser 200 is equipped with an optical amplifier 10, it can output pulsed laser light with a short time width that corresponds to a wide frequency band in the frequency characteristics of the gain of the gas, which is the optical gain medium.

[0135] The pulsed laser 200 may also include optical amplifiers 10A and 10B instead of optical amplifier 10.

[0136] While preferred embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above embodiments.

[0137] All embodiments disclosed herein should be considered illustrative and not restrictive. The scope of the present invention is indicated by the claims, not in the sense described above, and all modifications within the meaning and scope equivalent to the claims are intended.

[0138] 10...Optical amplifier 10A to 10D...Optical amplifier 12...Container 14...Thermal radiation source 14a...First surface 14b...Second surface 16...Container 20...Sealing member 22...Protrusion 30...Power supply 40...Optical element 40A...Optical element 42...Diffraction grating 42A...Diffraction grating 100...Tunable laser 100A...Tunable laser 112...Container 112a...Sealing member 114...Thermal radiation source 114a...First surface 114b...Second surface 116...Dielectric member 120...Sealing member 130...Heating device 132...Coil 134...AC power supply 200...Pulse laser 210...Pulse generator 220...Pulse extender 221 to 224...Diffraction grating 230...Pulse compressor 231 to 234...Diffraction grating Ax...Axis C1 to C4…Characteristics CP…Conductor pattern region CP1…Conductor pattern region F1…First layer F2…Second layer F3…Third layer G1 to G9…Gain HS…Heat sink L1…Optical lens La…Emitted light Lb…Laser light Lc…First-order diffracted light Ld…North-order diffracted light Lin…Incident light Loot…Output light Lp…Pulsed laser light Lp1 to Lp6…Pulsed laser light M1 to M7…Optical mirror OC1 to OC4…Optical component RP…Roof prism SB…Substrate TIR…Adiabatic region TR…Excitation light W1 to W7…Optical window

Claims

1. An optical amplifier comprising a gas as an optical gain medium, and a wavelength-selective thermal radiation source that emits excitation light to excite the gas by thermal radiation, wherein in the frequency characteristics of the gain of the gas excited by the excitation light, the ratio of the minimum value adjacent to the maximum value of the gain to the maximum value is 0.4 times or more.

2. The optical amplifier according to claim 1, wherein the ratio is 0.8 times or more.

3. An optical amplifier comprising a gas as an optical gain medium and a thermal radiation source having wavelength selectivity, which emits excitation light to excite the gas by thermal radiation, wherein the pressure of the gas is 300 kPa or higher.

4. The optical amplifier according to claim 3, wherein the pressure is 1 MPa or more.

5. The optical amplifier according to any one of claims 1 to 4, wherein an insulating region is provided between the gas and the thermal radiation source.

6. The optical amplifier according to any one of claims 1 to 5, further comprising a container for containing the gas, wherein the container is cylindrical and extends along an axis, and the thermal radiation source extends along the axis.

7. The optical amplifier according to any one of claims 1 to 5, further comprising a container for containing the gas, wherein the inner surface of the container includes a reflective surface for reflecting the excitation light.

8. The optical amplifier according to claim 7, wherein the reflective surface is arranged to face the thermal radiation source.

9. The optical amplifier according to any one of claims 1 to 8, wherein the thermal radiation source further comprises a resistive heating element and a power supply connected to the resistive heating element.

10. The optical amplifier according to any one of claims 1 to 9, wherein the thermal radiation source further comprises a conductor, a coil for inductively heating the conductor, and an AC power supply for supplying AC power to the coil.

11. A tunable laser comprising: an optical amplifier according to any one of claims 1 to 4; and an optical element that feeds back light of a specific wavelength from the emitted light emitted from the gas by the excitation light and emitted from the optical amplifier to the optical amplifier.

12. The tunable laser according to claim 11, wherein the optical element changes the wavelength of the light that is fed back to the optical amplifier.

13. A pulsed laser comprising: a pulse generator for generating pulsed laser light; a pulse extender for extending the time width of the pulsed laser light; an optical amplifier according to any one of claims 1 to 4, wherein the optical amplifier amplifies the intensity of the pulsed laser light whose time width has been extended by the pulse extender; and a pulse compressor for compressing the time width of the pulsed laser light whose intensity has been amplified by the optical amplifier.

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