JP2008258323A - Pulse laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pulse laser device which has a stable light intensity and can output pulse laser beams of a suppressed noise. <P>SOLUTION: The pulse laser device comprises: a light source for outputting continuous laser beams; a prestage optical amplifier connected to the light source, for amplifying the continuous laser beams for outputting; an optical modulator connected to the prestage optical amplifier, for modulating the amplified continuous laser beams to produce pulse laser beams; and an after-step optical amplifier connected to the optical modulator, for amplifying the pulse laser beams for outputting. The latter part optical amplifier amplifies the continuous laser beams to an optical intensity to the extent that a natural emission light is not laser-oscillated in the latter part optical amplifier. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a pulse laser device that modulates continuous laser light using an optical modulator to generate pulsed laser light.

  2. Description of the Related Art Conventionally, a light source that outputs continuous laser light and a pulse laser device that generates a pulse laser light by modulating a continuous laser light by connecting an optical modulator to the light source has been disclosed (Patent Document 1). In this pulse laser device, an optical modulator is connected to a light source that outputs continuous laser light, and the optical intensity of the continuous laser light is modulated by turning on / off the optical modulator at a predetermined period and duty ratio to generate pulse laser light. After the generation, the generated pulsed laser beam is amplified using an optical fiber amplifier to obtain a pulsed laser beam having a desired light intensity.

JP 2004-348068 A

  However, in the conventional pulse laser apparatus, spontaneous emission light generated in the optical fiber amplifier is amplified to become ASE (Amplified Spontaneous Emission) light, and laser oscillation may be unnecessary and unstable. When this unstable laser oscillation occurs, there is a problem that the light intensity of the output pulse laser beam becomes unstable and noise increases.

  The present invention has been made in view of the above, and an object of the present invention is to provide a pulse laser device capable of outputting pulse laser light with stable light intensity and suppressed noise.

  In order to solve the above-described problems and achieve the object, a pulse laser apparatus according to the present invention includes a light source that outputs continuous laser light, and a pre-stage optical amplifier that is connected to the light source and amplifies and outputs the continuous laser light. An optical modulator that is connected to the preceding optical amplifier and modulates the amplified continuous laser light to generate pulsed laser light; and a latter-stage light that is connected to the optical modulator and amplifies and outputs the pulsed laser light An amplifier, wherein the front-stage optical amplifier amplifies the continuous laser light to a light intensity at which spontaneous emission light does not oscillate in the rear-stage optical amplifier.

  Further, the pulse laser apparatus according to the present invention is the optical fiber amplifier according to the above invention, wherein the front-stage amplifier and / or the rear-stage amplifier uses a double-clad optical fiber in which a rare earth element is added to a core portion as an optical fiber for amplification. It is characterized by being.

  In the pulse laser device according to the present invention as set forth in the invention described above, the optical modulator periodically diffracts the amplified continuous laser light by using an acousto-optic element to intensify the amplified continuous laser light. It is an acousto-optic type light modulator for modulation.

  Further, the pulse laser device according to the present invention is the above-described invention, wherein the optical modulator includes an optical path changing unit that changes an optical path of residual light transmitted through the acousto-optic element in the amplified continuous laser light, and And a light receiving heat converting means for receiving residual light whose optical path has been changed and converting it into heat.

  In the pulse laser device according to the present invention as set forth in the invention described above, the optical modulator includes a band-pass filter that selectively transmits light having a wavelength of the pulse laser beam on the output side of the pulse laser beam. It is characterized by.

  According to the present invention, the front-stage optical amplifier amplifies the continuous laser light to a light intensity at which the spontaneous emission light does not oscillate in the rear-stage optical amplifier, so that the pulsed laser light with stable light intensity and suppressed noise. It is possible to realize a pulse laser device that can output

  Embodiments of a pulse laser device according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

(Embodiment)
FIG. 1 is a schematic diagram schematically showing a configuration of a pulse laser apparatus according to an embodiment of the present invention. As shown in FIG. 1, the pulse laser device 1 includes a light source 2 that outputs continuous laser light, a front-stage optical amplifier 3 that is connected to the light source 2, amplifies and outputs continuous laser light, and a front-stage optical amplifier 3. An optical modulator 4 that connects and modulates the amplified continuous laser light to generate pulsed laser light, and a post-stage optical amplifier 5 that is connected to the optical modulator 4 and amplifies and outputs the pulsed laser light. The front-stage optical amplifier 3 amplifies the continuous laser light to a light intensity at which the spontaneous emission light does not oscillate in the rear-stage optical amplifier 5.

  That is, the pulse laser apparatus 1 includes a front-stage optical amplifier 3 and a rear-stage optical amplifier 5, and the front-stage optical amplifier 3 amplifies continuous laser light having a high time-averaged light intensity. Laser oscillation can be prevented. Further, the pre-stage optical amplifier 3 naturally converts continuous laser light into the post-stage optical amplifier 5 in consideration of the insertion loss of the optical modulator 4 and the time-averaged light intensity of the pulse laser light generated by the optical modulator 4. The emitted light is amplified to a light intensity that does not cause laser oscillation. As a result, the spontaneous emission light does not oscillate in any of the front-stage optical amplifier 3 and the rear-stage optical amplifier 5, so that the pulse laser device 1 can output pulse laser light with stable light intensity and suppressed noise. it can.

  In the conventional pulse laser device, a continuous laser beam is modulated by an optical modulator to generate a pulse laser beam, and then the generated pulse laser beam is amplified by a single optical amplifier to obtain a pulse laser having a desired light intensity. Since light was obtained, a single optical amplifier having a relatively high gain was used. As a result, spontaneous emission light generated in the optical amplifier is also amplified with high gain, and there is a risk that the spontaneous emission light easily oscillates. In addition, when the duty ratio of the generated pulse laser light is high, the time average of the light intensity of the pulse laser light is low, so that the light intensity of spontaneous emission relative to the light intensity of the pulse laser light is relatively high, and the laser Oscillation was more likely to occur.

  Hereinafter, the pulse laser device 1 will be specifically described. FIG. 2 is a schematic diagram schematically showing the configuration of the light source 2 shown in FIG. As shown in FIG. 2, the light source 2 forms an LD 21 that is a semiconductor laser diode that outputs continuous laser light, and an FBG 22 a that is connected to the LD 21 and reflects a narrow band of light and that is a fiber Bragg grating (Fiber Bragg Grating). The optical fiber 22 is provided. The FBG 21 functions as an external optical resonator for the LD 21 by reflecting light in a narrow band including the center wavelength of the continuous laser light output from the LD 21. As a result, the light source 2 operates as a light source that outputs continuous laser light having a narrow spectrum width. The characteristics of the continuous laser light output from the light source 2 are, for example, a center wavelength of about 1053 nm, a spectrum width of 5 pm or less, and a light intensity of about 230 mW. A temperature adjusting means such as a heater or a Peltier element is attached in the vicinity of the portion where the FBG 22a of the optical fiber 22 is formed, and the wavelength of the continuous laser beam is controlled by changing the reflection wavelength of the FBG 22a by changing the temperature. Also good.

  Next, the pre-stage optical amplifier 3 will be described. FIG. 3 is a schematic diagram schematically showing the configuration of the front-stage optical amplifier 3 shown in FIG. As shown in FIG. 3, the pre-stage optical amplifier 3 is a double-clad optical fiber amplifier that can realize an extremely high optical output, and is an optical isolator 31 and an optical multiplexer connected to the optical isolator by a single mode optical fiber. A multimode optical fiber 34 that connects a TFB (Tapered Fiber Bundle) 32, pumping light sources 33-1 to 33-n (n is an integer) such as a semiconductor laser, and TFB 32 and pumping light sources 33-1 to 33-n. -1 to 34-n, and an amplification double clad optical fiber 35 connected to the TFB 32. FIG. 4 is a diagram showing a schematic cross section of the amplifying double clad optical fiber 35 shown in FIG. 3 and a corresponding refractive index profile. As shown in FIG. 4, the amplifying double-clad optical fiber 35 has a refractive index higher than that of a core part 35a to which ions of a rare earth element ytterbium (Yb) are added and a core part 35a formed on the outer periphery of the core part 35a. A low inner clad part 35b and an outer clad part 35c having a lower refractive index than the inner clad part 35b formed on the outer periphery of the inner clad part 35b are provided, and has a refractive index profile 36.

  The pre-stage optical amplifier 3 operates as follows. First, the excitation light sources 33-1 to 33-n output excitation light having a wavelength of 900 to 980 nm, the multimode optical fibers 34-1 to 34-n guide the output excitation light to the TFB 32, and the TFB 32 guides. The excited pump light is coupled to the amplification double clad optical fiber 35. Here, the excitation light coupled to the amplifying double-clad optical fiber 35 optically excites Yb ions added to the core part 35a while propagating through the core part 35a and the inner clad 35b in multimode. At the same time, the TFB 32 couples the continuous laser light input through the optical isolator 31 to the amplifying double clad optical fiber 35. Here, the continuous laser beam coupled to the amplifying double-clad optical fiber 35 interacts with Yb ions in the excited state while propagating through the core portion 35a in a single mode, and is optically amplified by the stimulated emission action. On the other hand, among the Yb ions in the excited state, those that do not interact with the continuous laser light generate spontaneous emission light having a wide band with a predetermined probability. This spontaneously emitted light interacts with Yb ions in an excited state and is optically amplified to become ASE light. However, since the light intensity of continuous laser light is sufficiently high, it is not amplified until laser oscillation occurs. If the front-stage optical amplifier 3 operates in a gain saturation region where the gain is saturated with respect to the light intensity of the input continuous laser light, the light intensity of the spontaneous emission light generated in the front-stage optical amplifier 3 decreases. Therefore, laser oscillation of spontaneous emission light can be prevented more reliably.

  In addition, as described above, the pre-stage optical amplifier 3 takes the continuous laser light into consideration after taking into account the insertion loss of the optical modulator 4, which will be described later, and the time-averaged light intensity of the pulsed laser light generated by the optical modulator 4. Then, the latter-stage optical amplifier 5 amplifies the spontaneous emission light to a light intensity that does not cause laser oscillation, for example, 7 W.

  Next, the optical modulator 4 will be described. FIG. 5 is a schematic cross-sectional view schematically showing the configuration of the optical modulator 4 shown in FIG. As shown in FIG. 5, the optical modulator 4 includes an optical input unit 42 a and a light output unit 42 b provided with a collimator lens, an acoustooptic element 43 made of tellurium dioxide, and an ultrasonic wave such as a piezoelectric element. An application unit 44, an optical prism 46 that is an optical path changing unit, and a light receiving heat conversion unit 47 are provided. Further, a signal generator 45 connected to the ultrasonic wave application means 44 is provided outside the housing 41. The light receiving heat conversion means 47 includes a heat conversion part 47c having a spherical cavity 47ca, a lens 47a, and a lens holder 47b that holds the lens 47a and fixes it to the heat conversion part 47c.

  Next, the operation of the optical modulator 4 will be described. 6 and 7 are explanatory diagrams for explaining the operation of the optical modulator 4 shown in FIG. First, as shown in FIG. 6, the signal generator 45 outputs a pulse voltage having a desired repetition cycle and pulse width to the ultrasonic wave application unit 44, and the ultrasonic wave application unit 44 supplies the acoustooptic device 43 to a desired repetition cycle and An ultrasonic wave is applied with a time width to generate an acoustic wave in the acoustooptic device 43. As a result, a diffraction grating 43 a having a high refractive index is formed in the acoustooptic device 43 with a desired period and time width. The continuous laser beam L1 input to the optical input unit 42a is periodically diffracted by the diffraction grating 43a to become a pulsed laser beam L2 which is the primary light, and a pulse having a desired repetition period and pulse width is output from the optical output unit 42b. Output as laser light L3. The pulse laser beam L3 has a period of, for example, 100 kHz, a pulse width of 10 nsec, and a duty ratio of 0.1%. The time-averaged insertion loss of the optical modulator 4 is, for example, 14 dB. When the light intensity of the continuous laser light L1 is 7 W, the peak light intensity of the pulsed laser light L3 is about 0.7 W.

  On the other hand, as shown in FIG. 7, the residual light L4 that is not diffracted by the diffraction grating 43a and transmitted through the acoustooptic device 43 out of the continuous laser light L1 input to the light input section 42a is subjected to HR coating on the reflecting surface. The optical path is changed by an angle of 90 degrees by the optical prism 46 and is input to the light receiving heat converting means 47. In the light receiving heat converting means 47, the lens 47a held by the lens holder 47b receives and diffuses the residual light L4 to be diffused light L5, and the heat converting portion 47c is diffused light on the inner surface formed by the cavity 47ca. The light intensity is attenuated and lost while repeatedly reflecting L5 multiple times. That is, the energy of the diffused light L5 is converted into heat while the diffused light L5 is being reflected multiple times, and the converted heat is diffused throughout the heat conversion part 47c and disappears by air cooling. As a result, the residual light L4 is applied to the housing 41, the temperature of the housing 41 rises, and adverse effects such as thermal deformation and melting of the adhesive are prevented. In particular, when the duty ratio of the pulsed laser light generated in the optical modulator 4 is as small as 0.1%, for example, most of the light intensity of the continuous laser light L1 becomes the residual light L4. In this case, when the light intensity of the continuous laser light L1 input to the light input unit 42a is high, the possibility that the housing 41 is adversely affected when the residual light L4 is irradiated onto the housing 41 is particularly strong. The effect of erasing the energy of the residual light L4 in the heat conversion part 47c as in the embodiment is particularly remarkable.

  The heat conversion part 47c is preferably made of a material having a high melting point and a high temperature resistance and a high thermal conductivity, such as a metal such as aluminum or ceramics. Further, the inner surface of the heat conversion part 47c may be subjected to a plating process such as a radiant process, NiP electroless plating, or a black alumite to increase the light absorption coefficient of the inner surface. Further, it is preferable that a heat insulating material is interposed between the heat conversion means 47 and the housing 41 to prevent heat conduction from the heat conversion means 47 to the housing 41. As the heat insulating material, a heat insulating resin that has high temperature resistance and high heat insulating properties such as Delrin (registered trademark), PPS (polyphenylene sulfide), and thermoplastic polyimide, and has little outgas is preferable. Furthermore, cooling means such as a cooling fin, a cooling fan, a heat pipe, and a Peltier element may be attached to the heat conversion unit 47c to increase the efficiency of dissipation of the converted heat.

  Next, the post-stage optical amplifier will be described. FIG. 8 is a schematic diagram schematically showing the configuration of the post-stage optical amplifier 5 shown in FIG. As shown in FIG. 8, the post-stage optical amplifier 5 is a double-clad type optical fiber amplifier having the same configuration as the pre-stage optical amplifier 3 and capable of realizing an extremely high optical output, and includes an optical isolator 51, a TFB 52, , Excitation light sources 53-1 to 53-n, multimode optical fibers 54-1 to 54-n, and amplifying double-clad optical fiber 55. The amplification double-clad optical fiber 55 has the same cross section and refractive index profile as the amplification double-clad optical fiber 35 shown in FIG.

  The post-stage optical amplifier 5 operates in the same manner as the pre-stage optical amplifier 3 described above. The pre-stage optical amplifier 3 amplifies the continuous laser light to an appropriate light intensity after taking into account the insertion loss of the light modulator 4 and the time-averaged light intensity of the pulse laser light generated by the light modulator 4. Therefore, the post-stage optical amplifier 5 operates without spontaneous emission of laser oscillation. As a result, the post-stage optical amplifier 5 outputs pulsed laser light with stable light intensity and suppressed noise.

  Next, specific output characteristics of the pulse laser device 1 according to the present embodiment will be described. In the pulse laser device 1, a continuous laser beam having a center wavelength of 1053.106 nm, a line width of 5 pm, and a light intensity of 231 mW is output from the light source 2, and this continuous laser is amplified to a light intensity of 7 W by the pre-stage optical amplifier 3. The continuous laser beam is modulated by the optical modulator 4 to generate a pulse laser beam having a repetition period of 100 kHz, a pulse width of 10 nsec, a duty ratio of 0.1%, and a peak light intensity of 0.7 W. The amplified light was amplified by the amplifier 5 until the light intensity at the peak of the pulsed light became 402 W and output. FIG. 9 is a diagram schematically showing the time-light intensity characteristics of the light output from the pulse laser device 1, and FIG. 10 is a diagram schematically showing the spectrum of the light output from the pulse laser device 1. In the time-light intensity characteristic shown in FIG. 9, reference symbol S1 indicates a pulse laser beam component, and reference symbol S2 indicates an ASE light component. Here, the light intensity of the pulse laser beam component S1 was stable, and no peak indicating laser oscillation was observed in the ASE light component S2. Further, in the spectrum of FIG. 10, reference numeral S3 indicates a pulse laser beam component, and reference numeral S4 indicates an ASE light component. Here, the light intensity and spectrum shape of the pulsed laser beam component S3 were stable. Further, the light intensity of the ASE light component S4 is a high intensity that accounts for about 25% of the total intensity of the light output from the pulse laser device 1, but no peak indicating laser oscillation is observed in the ASE light component S4. The spectrum shape was also stable.

  In the above embodiment, a band-pass filter that selectively transmits the wavelength of the pulse laser beam may be provided on the output side of the pulse laser beam generated by the optical modulator 4. FIG. 11 is a schematic cross-sectional view schematically illustrating a configuration including a band transmission filter that is a modification of the optical modulator 4. The optical modulator 40 includes a bandpass filter 48 formed of a dielectric multilayer filter in the optical modulator 4 shown in FIG. 5, and the bandpass filter 48 emits spontaneously emitted light generated in the pre-stage optical amplifier 3. By shutting off, laser oscillation of spontaneous emission light can be prevented more reliably in the rear-stage optical amplifier 5, and energy conversion efficiency from pumping light to pulsed laser light can be increased in the rear-stage optical amplifier 5. Since the band pass filter 48 is provided on the output side of the pulse laser light L2, the light intensity input to the band pass filter 48 is lower than that provided on the side where the continuous laser light L1 is input. As a result, the band pass filter 48 is less likely to be damaged by light.

It is the schematic which represented typically the structure of the pulse laser apparatus which concerns on embodiment of this invention. It is the schematic which represented the structure of the light source shown in FIG. 1 typically. FIG. 2 is a schematic diagram schematically illustrating a configuration of a front-stage optical amplifier illustrated in FIG. 1. It is a figure which shows the typical cross section of the double clad optical fiber for amplification shown in FIG. 3, and a corresponding refractive index profile. FIG. 2 is a schematic cross-sectional view schematically illustrating the configuration of the optical modulator illustrated in FIG. 1. It is explanatory drawing explaining operation | movement of the optical modulator shown in FIG. It is explanatory drawing explaining operation | movement of the optical modulator shown in FIG. FIG. 2 is a schematic diagram schematically illustrating a configuration of a rear-stage optical amplifier illustrated in FIG. 1. It is a figure which shows typically the time-light intensity characteristic of the light output from the pulse laser apparatus. It is a figure which shows typically the spectrum of the light output from the pulse laser apparatus. FIG. 6 is a schematic cross-sectional view schematically illustrating a configuration including a band transmission filter that is a modification of the optical modulator illustrated in FIG. 5.

Explanation of symbols

1 Pulse laser device 2 Light source 21 LD
22 optical fiber 22a FBG
3 Pre-stage optical amplifier 31, 51 Optical isolator 32, 52 TFB
33-1 to 33-n, 53-1 to 53-n Excitation light source 34-1 to 34-n, 54-1 to 54-n Multimode optical fiber 35, 55 Double-clad optical fiber for amplification 35a Core portion 35b Inside Clad part 35c External clad part 36 Refractive index profile 4, 40 Optical modulator 41 Case 42a Optical input part 42b Optical output part 43 Acoustooptic element 43a Diffraction grating 44 Ultrasonic wave application means 45 Signal generator 46 Optical prism 47 Light receiving heat conversion Means 47a Lens 47b Lens holder 47c Thermal conversion part 47ca Cavity 48 Band pass filter 5 Rear stage optical amplifier

Claims (5)

A light source that outputs continuous laser light;
A pre-stage optical amplifier connected to the light source and amplifying and outputting the continuous laser light;
An optical modulator connected to the pre-stage optical amplifier and modulating the amplified continuous laser light to generate pulsed laser light;
A post-stage optical amplifier connected to the optical modulator and amplifying and outputting the pulsed laser beam;
And the front-stage optical amplifier amplifies the continuous laser light to a light intensity at which the spontaneous emission light does not oscillate in the rear-stage optical amplifier.   2. The pulse laser device according to claim 1, wherein the pre-stage amplifier and / or the post-stage amplifier is an optical fiber amplifier using a double clad optical fiber in which a rare earth element is added to a core portion as an amplification optical fiber. .   The optical modulator is an acousto-optic type optical modulator that modulates the intensity of the amplified continuous laser light by periodically diffracting the amplified continuous laser light using an acousto-optic element. The pulse laser device according to claim 1 or 2.   The optical modulator includes: an optical path changing unit that changes an optical path of residual light that has passed through the acousto-optic element in the amplified continuous laser light; and a light receiving heat that receives the residual light that has changed the optical path and converts it into heat. The pulse laser device according to claim 3, further comprising a conversion unit.   The said optical modulator is provided with the band pass filter which selectively permeate | transmits the light of the wavelength of this pulsed laser beam on the output side of the said pulsed laser beam, The any one of Claims 1-4 characterized by the above-mentioned. Pulse laser equipment.

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