JP2014187079A - Fiber laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fiber laser device in which spontaneous emission light is suppressed and efficiency in pulse amplification is improved.SOLUTION: A fiber laser device includes a seed light source, a polarizing holding fiber, an excitation light source, an optical coupler, a polarizer, and a spontaneous emission light control part. The polarizing holding fiber includes a first end part and a second end part on the side opposite to the first end part, with rare earth element added, and emits seed light as pulse amplification light while holding polarizing state of the seed light introduced from the first end part. The optical coupler allows the pulse amplification light to pass, and introduces excitation light into the second end part of the polarizing holding fiber. The polarizer is arranged in such a manner as a transmission axis is parallel to a polarizing axis of the pulse amplification light, and capable of reflecting an orthogonal component to the transmission axis in the light emitted from the second end part of the polarizing holding fiber. In spontaneous emission light, an output of excitation light is controlled by detecting inclination of light intensity of the reflection light of the polarizer relative to the light intensity of the seed light.

Description

  Embodiments described herein relate generally to a fiber laser apparatus.

  Laser processing devices and optical response devices require high-power pulsed laser light. For this purpose, pulse seed light from a solid-state laser or semiconductor laser is often amplified by a fiber laser amplifier to obtain high-power pulse laser light.

  In the fiber laser amplifier, the pumping light is absorbed by the rare earth element added to the core, and gain is generated by stimulated emission to amplify the pulse seed light. However, since fiber laser amplifiers generally have a wide wavelength range that produces gain, it is difficult to efficiently extract the energy pumped to a high output level and stored.

  For example, an optical fiber to which thulium (Tm) or the like is added emits spontaneous emission light (ASE: Amplified Spontaneous Emission) by energy stored in the optical fiber. As the spontaneous emission increases, the efficiency of pulse amplification decreases.

JP 2006-93235 A

  Provided is a fiber laser device in which spontaneous emission light is suppressed and the efficiency of pulse amplification is improved.

  The fiber laser device of the embodiment includes a seed light source, a first polarization maintaining fiber, a first excitation light source, a first optical coupler, a first polarizer, and a first spontaneous emission light control unit. Have. The seed light source emits linearly polarized seed light. The first polarization-maintaining fiber has a first end and a second end opposite to the first end, and a first rare earth element is added to the first end. The seed light introduced from the light source is emitted as first pulse amplified light while maintaining the polarization state of the seed light. The first excitation light source emits first excitation light for exciting the first rare earth element. The first optical coupler passes the first pulse amplified light and introduces the first excitation light into the second end of the first polarization maintaining fiber. The first polarizer is disposed so that a transmission axis is parallel to a polarization axis of the first pulse amplified light, and is out of light emitted from the second end of the first polarization maintaining fiber. A component orthogonal to the transmission axis can be reflected. The first spontaneous emission light control unit controls the output of the first excitation light by detecting a gradient of the light intensity of the reflected light from the first polarizer with respect to the light intensity of the seed light.

FIG. 1A is a configuration diagram of the fiber laser device according to the first embodiment, and FIG. 1B is a graph showing the amplifier output dependency on the input and the spontaneous emission light output dependency on the input. . 2A is a waveform diagram of a pulse train of seed light, and FIG. 2B is a waveform diagram of a pulse width. It is a schematic cross section of a polarization maintaining fiber. It is a graph which shows the light absorption spectrum or emission spectrum of thulium. 5A is a schematic diagram showing the configuration of a fiber laser device according to a comparative example, FIG. 5B is a waveform diagram of seed light at point A, and FIG. 5C is a waveform diagram of pulse amplified light at point B. . 6A is a graph for explaining the spread of the emission spectrum with respect to the wavelength in the comparative example, and FIG. 6B is a graph for explaining the fiber amplifier characteristics. It is a block diagram of the fiber laser apparatus concerning 2nd Embodiment. It is a block diagram of the fiber laser apparatus concerning 3rd Embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1A is a configuration diagram of the fiber laser device according to the first embodiment, and FIG. 1B is a graph showing the amplifier output characteristics.
The fiber laser device includes a seed light source 10, a first polarization maintaining fiber 12, a first polarizer 14, a first optical coupler 16, a first excitation light source 18, and a spontaneous emission light control unit 20. Have.

  The seed light source 10 is linearly polarized and emits pulse seed light 11 having a wavelength of λ1. The seed light source 10 may include, for example, an optical fiber oscillator, an output side polarizer, and the like, and emit linearly polarized pulse seed light. Alternatively, the seed light source 10 may be configured to amplify linearly polarized laser light emitted from the pumping semiconductor laser with an optical fiber amplifier. The wavelength λ1 of the pulse seed light 11 can be set to 1.95 to 2.0 μm, for example.

  The first polarization maintaining fiber 12 has a first end 12a on which the pulse seed light 11 is incident and a second end 12b on which the pulse amplified light is emitted. The first excitation light Ge <b> 1 is introduced to the second end portion 12 b of the first polarization maintaining fiber 12 through the first optical coupler 16. That is, the first polarization maintaining fiber 12, the first optical coupler 16, the first excitation light source 18 that emits the first excitation light Ge1, and the spontaneous emission light control unit 20 include the first The fiber amplifier A1 is configured.

  The wavelength λ2 of the first excitation light Ge1 may be 0.75 to 0.8 μm or the like and may be a continuous wave. The semiconductor laser usually has a linear polarization axis parallel to the surface of the light emitting layer and the light exit surface. The first excitation light source 18 includes a light source 18a and a drive circuit 18b thereof.

  In the first embodiment, the first polarizer 14 is provided between the first polarization maintaining fiber 12 and the first optical coupler 16. The transmission axis (perpendicular to the paper surface) 14a of the first polarizer 14 is arranged to be parallel to the polarization axis (perpendicular to the paper surface) 12c of the pulse amplified light amplified from the pulse seed light 11 and to transmit the pulse amplified light. The In this specification, parallel means that the crossing angle is 2 degrees or less.

  The first polarizer 14 can reflect an orthogonal component with respect to the transmission axis 14 c in the light emitted from the second end 12 b of the first polarization maintaining fiber 12.

  Further, the polarization axis of the first excitation light Ge1 emitted from the first optical coupler 16 is parallel to the transmission axis 14a of the first polarizer 14. For this reason, the first excitation light Ge <b> 1 can pass through the first polarizer 14 and enter the second end portion 12 b of the first polarization maintaining fiber 12.

  The spontaneous emission light control unit 20 outputs a control signal S2 toward the first excitation light source 18 based on the detector 20a on which the reflected light from the first polarizer 14 enters and the output signal S1 of the detector 20a. Circuit 20b. For this reason, the spontaneous emission light control unit 20 detects the gradient b of the light intensity Pd of the incident light Gd to the detector 20a out of the reflected light by the first polarizer 14 with respect to the light intensity Ps of the seed light 11. Thus, the output of the first excitation light Ge1 is controlled.

  Note that the gradient b of the light intensity Pd of the incident light Gd to the detector 20a with respect to the light intensity Ps of the seed light 11 is represented by ΔPd / ΔPs as shown in FIG.

  In FIG. 1B, the horizontal axis represents the light intensity Ps of the pulse seed light 11 input to the fiber amplifier A1, and the vertical axis represents the amplifier output (dashed line) P1 and the light intensity Pd of the detector incident light (solid line). The horizontal axis and the vertical axis are both logarithmic scales. That is, the straight line represents the constant gain of the amplifier. When the pulse seed light intensity Ps is low, most of the pulse amplified light emitted from the first polarization maintaining fiber 12 is transmitted through the first polarizer 14.

  On the other hand, the remaining small portion is reflected by the first polarizer 14, enters the detector 20a, and looks like a region with a gentle slope a. That is, the region of the gradient a is a part of the pulse amplified light, and includes the light reflected by the first polarizer 14 and the spontaneous emission light.

  As the input Ps of the seed light 11 increases, the output P1 of the pulse amplified light increases. For this reason, the incident light Gd to the detector starts to increase rapidly as the spontaneous emission light ASE1 which is non-polarized light. The spontaneous emission light control unit 20 detects the change in the gradient and controls the output of the first excitation light source 18 so as to suppress the increase of the spontaneous emission light ASE1.

  The detector 20a can be a photodiode, a phototransistor, or the like. When the wavelength range of spontaneous emission light is 1 μm or less, a Si photodiode can be used. When the wavelength range of spontaneous emission light is longer than 1 μm, a photodiode made of InGaAs, InGaAsP, or the like can be obtained.

2A is a waveform diagram of a pulse train of seed light, and FIG. 2B is a waveform diagram of a pulse width.
As shown in FIG. 2A, the seed light source 10 emits a pulse train of the seed light 11 in the first period T1. The pulse train is emitted during the emission period tp1 and is not emitted during the non-emission period tp2. The period T1 can be, for example, between 50 μs (repetition frequency corresponding to 20 kHz) to 100 μs (repetition frequency corresponding to 10 kHz).

  The modulation operation period TM of the seed light 11 includes an emission period tp1 and a non-emission period tp2. The repetition frequency corresponding to the modulation operation period TM is often a low frequency such as 1 kHz or less. Further, as shown in FIG. 2B, the pulse width W1 can be set to 10 to 100 ns or the like.

FIG. 3 is a schematic cross-sectional view of a polarization maintaining fiber.
If linearly polarized laser light passes through a non-polarization maintaining fiber, it gradually becomes elliptically polarized due to a phase difference induced by heat or mechanical action.

  On the other hand, the polarization maintaining fiber (or polarization plane preserving fiber) 12 has, for example, stress applying portions 12c on both sides of the core 12a. The polarization axis of the seed light 11 is made to substantially coincide with the axis (Slow axis) through which the center of the core 12a, the center of the stress applying portion 12c pass, by adjusting the optical axis. In this way, it is possible to stably maintain the polarization state of the seed light 11, suppress polarization fluctuation, and emit pulse amplified light with high beam quality. Alternatively, the polarization axis of the seed light 11 may be substantially coincident with the Fast axis.

FIG. 4 is a graph showing the light absorption spectrum or emission spectrum of thulium.
When thulium (Tm) is used as the rare earth element, various wavelengths of excitation light for emitting infrared laser light of 1.8 to 2.1 μm can be selected. For example, a semiconductor laser element using an AlGaAs-based material has a light absorption region in a wavelength range of 0.75 to 0.8 μm. preferable. The rare earth element may be erbium, ytterbium, or the like.

5A is a schematic diagram showing the configuration of a fiber laser device according to a comparative example, FIG. 5B is a waveform diagram of seed light at point A, and FIG. 5C is a waveform diagram of pulse amplified light at point B. .
The seed light of the fiber amplifier according to the comparative example has the waveform shown in FIG. The pulse seed light 111 is amplified by the polarization maintaining fiber 112 and becomes a pulse waveform shown in the waveform diagram of FIG. 5C in the emission period tp11.

  In addition, during the seed light non-emission period tp12, the energy from the excitation light source 118 continues to excite the rare earth element. For example, when excitation is continued for a period of 10 times the first period T11, approximately 10 times the energy for exciting one pulse of seed light is accumulated in the polarization maintaining fiber 112.

FIG. 6A is a graph for explaining the spread of the emission spectrum with respect to the wavelength in the comparative example, and FIG. 6B is a graph for explaining the fiber amplifier characteristics.
In FIG. 6A, the vertical axis represents the relative spectral intensity, and the horizontal axis represents the wavelength (μm). The energy stored in the optical fiber is emitted as spontaneous emission light having a broad emission spectrum of, for example, 1.53 (λL) to 1.56 (λH) μm and having no polarization.

  In this case, as shown in FIG. 6B, as the input Ps increases, the average light output including spontaneous emission light and pulse amplification light increases. On the other hand, as the input Ps increases, the optical output pulse peak value becomes saturated. That is, as shown in FIG. 1B, the spontaneous emission light sharply increases with respect to the input Ps, and the pulse amplification efficiency is lowered. For this reason, the pulse peak value is saturated and the power consumption increases.

  The fiber laser device of the comparative example does not have means for suppressing spontaneous emission light. For this reason, at the time of exchanging the excitation light source, it is necessary to measure the pulse peak value and the emission spectrum and perform optical adjustment so that the spontaneous emission light becomes smaller than a predetermined value. Such optical adjustment becomes more complicated as the pulse width of the seed light becomes shorter. Further, since the emission spectrum of laser light and the emission spectrum of spontaneous emission light overlap, it is not easy to detect the generation of spontaneous emission light with high accuracy.

  On the other hand, in the first embodiment, the spontaneous emission light control unit 20 detects the incident light Gd to the detector 20a, controls the driving condition of the first excitation light source 18, and increases the spontaneous emission light ASE1. Can be suppressed. For example, the control circuit 20b detects the generation of the spontaneous emission light ASE1 from the gradient of the incident light Gd to the detector 20a. The control circuit 20b outputs a control signal S2 toward the drive circuit 18a so as to further reduce the output of the first excitation light Ge1. The drive circuit 18a controls the excitation light intensity by reducing the operating current of the light source 18b such as a semiconductor laser. In this way, high pulse amplification output can be emitted with high amplification efficiency while suppressing an increase in spontaneous emission light ASE1.

FIG. 7 is a configuration diagram of a fiber laser device according to the second embodiment.
The fiber laser device has a first fiber amplifier A1 and a second fiber amplifier A2 cascade-connected thereto. By connecting the fiber amplifiers in multiple stages, it becomes easy to increase the gain and output.

  If the first spontaneous emission light ASE1 is generated by the first fiber amplifier A1, it enters the second fiber amplifier A2. The first spontaneous emission light ASE1 is further amplified by the second polarization maintaining fiber 13 and is emitted from the second fiber amplifier A2. The second fiber amplifier A2 generates the second spontaneous emission light ASE2 therein. That is, the second fiber amplifier A2 may generate the first and second spontaneous emission lights ASE1 and ASE2 to further reduce the pulse amplification efficiency.

  On the other hand, in the second embodiment, the first spontaneous emission light control unit 20 of the first fiber amplifier A1 suppresses the spontaneous emission light ASE1 to reduce the amount of light incident on the second fiber amplifier. . The second fiber amplifier A2 can also suppress the increase in spontaneous emission light ASE2. For this reason, an increase in spontaneous emission light is suppressed, and a fiber laser device with higher gain and pulse amplification output can be obtained.

FIG. 8 is a configuration diagram of a fiber laser device according to the third embodiment.
The third embodiment includes a spontaneous emission light control unit 20 that is common to the first fiber amplifier A1 and the second fiber amplifier A2.

  For example, the detector for detecting the spontaneous emission light ASE1 from the first fiber amplifier A1 and the spontaneous emission light ASE2 from the second fiber amplifier A2 and the control circuit connected thereto can be made common. In this case, the first polarizer 14 is provided between the first optical coupler 16 and the second polarization maintaining fiber 13, and the light emitted from the first end of the second polarization maintaining fiber 13. Among them, the component orthogonal to the transmission axis is reflected.

  Further, the first spontaneous emission light control unit 20 detects the gradient of the light intensity of the light emitted from the second polarization maintaining fiber 13 and reflected by the first polarizer 14 with respect to the light intensity Pd of the seed light 11. Thus, the output of the second excitation light Ge2 is controlled. Alternatively, only the detector can be shared. In 3rd Embodiment, a structure becomes simple and an apparatus can be reduced in size.

  With the fiber laser devices according to the first to third embodiments, an increase in spontaneous emission light is suppressed, and the efficiency of pulse amplification is improved. Further, when the fiber laser device of this embodiment is connected to an optical parametric oscillator, the wavelength of the pulse amplified light can be converted to a long wavelength, and can be used for an optical response device or the like.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 10 Seed light source, 11 Seed light, 12 1st polarization maintaining fiber, 13 2nd polarization maintaining fiber, 14 1st polarizer, 15 2nd polarizer, 16 1st optical coupler, 17 2nd optical coupling , 18 first excitation light source, 19 second excitation light source, 20 first spontaneous emission light control unit, 20a detector, 20b control circuit, 21 first spontaneous emission light control unit, light intensity of Ps seed light , P1, P2 Light intensity of pulse amplified light, light intensity of incident light to Pd detector, S2 control signal

Claims (7)

A seed light source that emits linearly polarized seed light;
A first rare earth element is added, and has a first end and a second end opposite to the first end, and the seed light introduced from the first end A first polarization maintaining fiber that emits as a first pulse amplified light while maintaining a polarization state;
A first excitation light source that emits first excitation light for exciting the first rare earth element;
A first optical coupler that passes the first pulse-amplified light and introduces the first excitation light to the second end of the first polarization-maintaining fiber;
The transmission axis is arranged so as to be parallel to the polarization axis of the first pulse-amplified light, and the orthogonal component with respect to the transmission axis of the light emitted from the second end of the first polarization-maintaining fiber is A reflective first polarizer;
A first spontaneous emission control unit that controls the output of the first excitation light by detecting a gradient of the light intensity of the reflected light by the first polarizer with respect to the light intensity of the seed light;
A fiber laser device.   The first spontaneous emission light control unit calculates a gradient of the reflected light from the first detector that detects the reflected light from the first polarizer, and when the gradient becomes a predetermined value or more. The fiber laser device according to claim 1, further comprising: a first control circuit that outputs a control signal for reducing the light intensity of the first pumping light toward the first pumping light source.   The fiber laser device according to claim 1, wherein the first polarizer is provided between the first polarization maintaining fiber and the first optical coupler. A first end portion to which the second rare earth element is added and the first pulse amplified light is introduced; and a second end portion opposite to the first end portion; A second polarization-maintaining fiber that emits as second pulse-amplified light while maintaining the polarization state of the pulse-amplified light of
A second excitation light source that emits second excitation light for exciting the second rare earth element;
A second optical coupler that passes the second pulse-amplified light and introduces the second excitation light to the second end of the second polarization-maintaining fiber;
The transmission axis is arranged so as to be parallel to the polarization axis of the second pulse-amplified light, and the orthogonal component to the transmission axis of the light emitted from the second end of the second polarization maintaining fiber is A reflective second polarizer;
A second spontaneous emission light control unit for controlling the output of the second excitation light by detecting a gradient of the light intensity of the reflected light by the second polarizer with respect to the light intensity of the seed light;
The fiber laser device according to claim 1, further comprising: The first spontaneous emission light control unit calculates a gradient of the reflected light from the first detector that detects the reflected light from the first polarizer, and when the gradient becomes a predetermined value or more. A first control circuit that outputs a control signal for reducing the light intensity of the first excitation light toward the first excitation light source;
The second spontaneous emission light control unit calculates a gradient of the reflected light from the second detector that detects the reflected light from the second polarizer, and when the gradient becomes a predetermined value or more. The fiber laser device according to claim 4, further comprising: a second control circuit that outputs a control signal for reducing light intensity of the second pumping light toward the second pumping light source. The first polarizer is provided between the first polarization maintaining fiber and the first optical coupler;
The fiber laser device according to claim 4 or 5, wherein the second polarizer is provided between the second polarization maintaining fiber and the second optical coupler. A first end portion to which the second rare earth element is added and the first pulse amplified light is introduced; and a second end portion opposite to the first end portion; A second polarization-maintaining fiber that emits as second pulse-amplified light while maintaining the polarization state of the pulse-amplified light of
A second excitation light source that emits second excitation light for exciting the second rare earth element;
A second optical coupler that passes the second pulse-amplified light and introduces the second excitation light to the second end of the second polarization-maintaining fiber;
Further comprising
The first polarizer is provided between the first optical coupler and the second polarization-maintaining fiber, and out of the light emitted from the first end of the second polarization-maintaining fiber. , Reflecting the orthogonal component to the transmission axis,
The first spontaneous emission light control unit detects a light intensity gradient of light emitted from the second polarization maintaining fiber and reflected by the first polarizer with respect to the light intensity of the seed light, The fiber laser device according to claim 4, wherein the output of the second pumping light is controlled.

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