JP2013098457A - Laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser device which is capable of suppressing generation of ASE light which is generated between optical pulses.SOLUTION: A laser device comprises a laser oscillator which is oscillated in a first wavelength band, a laser driver which pulses the laser oscillator in a predetermined frequency, an optical fiber amplifier which amplifies optical output of the laser oscillator, an exciting light source which supplies exciting light in a second wavelength band to an optical fiber for amplification of the optical fiber amplifier, and an energy consumption light source which supplies energy consumption light in a third wavelength band for consuming energy at an excitation level to the optical fiber for amplification synchronously to the predetermined frequency.

Description

  Embodiments described herein relate generally to a laser apparatus having a fiber amplifier.

  A laser device that amplifies an optical output output from a laser oscillator with an optical fiber amplifier can increase the output, and is used in many fields such as communication, medical treatment, optical measurement, and machining. Such a laser device is called a MOPA (Master Oscillator Power Amplifier) light source.

  As shown in FIG. 10, a conventional MOPA light source includes a laser oscillator (MO) 100 that generates seed light and an optical amplifier (PA) 101 that amplifies the seed light. The laser oscillator 100 includes a laser 102 and a drive circuit 103 that drives the laser 102 in a pulsed manner.

  When an optical fiber amplifier is used as the optical amplifier 101, an optical amplification fiber 104, an excitation light source 105 for exciting carriers to the excitation level of rare earth ions doped in the optical amplification fiber 104, and an excitation light are used. It comprises a coupler 106 coupled to the optical amplification fiber 104, and the seed light is amplified to output a high output light 107.

  For example, assuming an optical fiber amplifier 101 in which Tm (thulium) is doped in the optical amplification fiber 104, a laser diode having a wavelength of 793 nm and capable of CW operation is used as the excitation light source 105, and the laser oscillator (MO) 100 For output, seed light having a wavelength of 1800 nm to 2100 nm is used.

  The operation of such a MOPA light source will be described with reference to FIG. 11A shows the optical output waveform 111 of the seed light output from the laser oscillator 100, and FIG. 11B shows the optical output waveform 112 of the pumping light source 105 (the subscript p is a pulse, c is Represents continuity). FIG. 11C shows an optical output waveform 113 of the laser amplifier 101.

  Normally, when the laser oscillator 100 is pulsed and the output of the excitation light source 105 is CW (Continues Wave) light 112c, the output from the optical fiber amplifier 101 is added to the output light 113c obtained by amplifying the seed light pulse 111. ASE (Amplified Spontaneous Emission) light 114 obtained by amplifying spontaneous emission light is output.

  The ASE light 114 gives a thermal burden when the output of the optical fiber amplifier 101 is incident on a wavelength conversion crystal such as SHG (Second harmonic generation). In addition, it causes noise in optical measurement applications such as sensing.

  In order to prevent this, it is considered that the optical output 112p of the excitation light source 105 is turned ON / OFF in accordance with the timing of the optical pulse of the optical output waveform 111 of the laser oscillator 100 as indicated by the arrow in FIG. . If the optical pulse frequency is a low frequency of about 10 Hz to 100 Hz, the ASE light 114 is effectively removed by adjusting the pulse width and timing of the excitation light source 105 in synchronization with the pulse operation frequency of the seed light output waveform 111. It is possible. However, when the pulse operating frequency is 200 Hz or more, the time (pulse width) for sufficiently pumping carriers cannot be ensured at the pumping level of the optical amplification fiber, so that the optical output 113p from the optical fiber amplifier 101 decreases. Arise. For this reason, this method is effective only for low frequencies of approximately 100 Hz or less. Here, for the sake of explanation, this method will be referred to as an “excitation light source ON / OFF method”.

  In addition, in order to suppress excessive generation of ASE light, there is one that supplies noise suppression light having a constant output to an optical amplification fiber (see Patent Document 1).

JP 2003-273428 A

  The problem to be solved by the present invention is to provide a laser device that can solve the above problems and suppress the generation of ASE light generated between optical pulses.

  In order to achieve the above object, a laser apparatus according to an embodiment includes a laser oscillator that oscillates in a first wavelength band, a laser driving unit that operates the laser oscillator at a predetermined frequency, and an optical output of the laser oscillator. An optical fiber amplifying unit to be amplified, a pumping light source that supplies pumping light of the second wavelength band to the amplifying optical fiber of the optical fiber amplifying unit, and energy of the pumping level consumed to the amplifying optical fiber And an energy consuming light source that supplies energy consuming light in a third wavelength band to be synchronized with the predetermined frequency.

1 is a block configuration diagram of a laser apparatus according to a first embodiment. The block block diagram of the energy consumption light source drive part which concerns on the same embodiment. The figure explaining operation | movement of the laser apparatus which concerns on the same embodiment. (A) is the optical output waveform of the pumping light source for the optical fiber amplifier, (b) is the optical output waveform of the laser oscillator, (c) is the optical output waveform of the energy consuming light source, and (d) is the optical output of the optical fiber amplifier. It is a waveform. The conceptual diagram showing the energy level of a 3 level system laser. The pulse light transmission timing diagram of the energy consumption light source comprised with a single pulse. (A) shows a laser oscillator output, and (b) shows an input light pulse of an energy consuming light source. The pulse light transmission timing diagram of the energy consumption light source comprised with a several pulse. (A) shows a laser oscillator output, and (b) shows an input light pulse of an energy consuming light source. The structural example of the optical amplification fiber which concerns on the same embodiment. The block block diagram of the laser apparatus which concerns on 2nd Embodiment. The figure explaining the optical pulse transmission timing of the energy consumption light source which concerns on the same embodiment. (A) is a case of a single pulse, and (b) is a case of a plurality of pulses. The block diagram of the conventional MOPA light source. The figure explaining the operation | movement of the conventional MOPA light source.

  Hereinafter, embodiments for carrying out the invention will be described in detail with reference to FIGS. 1 to 11.

(First embodiment)
FIG. 1 shows a block configuration of the laser apparatus of the first embodiment. As shown in FIG. 1, the laser apparatus of the present embodiment includes a laser oscillator (MO) 10 that generates seed light and an optical fiber amplifier (PA) 20 that amplifies seed light pulses output from the laser oscillator 10. Yes.

  The laser oscillator 10 includes a fiber laser oscillator 11, a laser oscillator excitation light source 12 for oscillating the fiber laser oscillator 11 by optical excitation, and controlling the laser oscillator excitation light source 12 to cause the laser oscillator 10 to perform a pulse operation. An oscillator excitation light source drive circuit 13 that controls the optical output pulse frequency and an optical isolator 14 that outputs the optical output in one direction and prevents the influence of reflected return light are included.

  The fiber laser oscillator 11 is provided with high-reflectance mirrors 11a and 11b composed of FBG (fiber Bragg grating) or the like at both ends of an oscillation optical fiber 15 to which rare earth such as Yb, Er, Ho, and Tm is added. ing. The mirrors 11a and 11b have a high reflectance characteristic with respect to the oscillation wavelength, but have a characteristic of transmitting the wavelength of the excitation light source 12 for laser oscillator.

  The optical fiber amplifier 20 includes an optical amplification double-clad fiber 21, an optical fiber amplifier excitation light source 22 for generating a gain in the optical amplification fiber 21, and an amplifier excitation light source drive for driving the optical fiber amplifier excitation light source 22. Circuit 23, an energy consuming light source 24 that consumes excess carriers accumulated in excitation energy to reduce ASE light, and energy that drives the energy consuming light source 24 in synchronization with the frequency of the optical pulse output from the laser oscillator 10 Consumption light source drive circuit 25, for propagating the optical outputs of the optical fiber amplifier excitation light source 22 and the energy consumption light source 24 to the inner cladding of the optical amplifying double clad fiber 21 along the direction opposite to the propagation direction of the optical output pulse. Multimode combiner 26 and pump for optical fiber amplifier With a source 22 and a reflection-free termination section 27 for reflection-free termination at the end of the optical amplification double clad fiber 21 the light output of the energy consumption light source 24. With the above configuration, an optical output 28 in which ASE light is suppressed can be obtained.

  FIG. 2 shows a block configuration of the energy consuming light source driving unit 25. The energy consuming light source driving unit 25 is a driving condition setting unit 251 for synchronizing the energy consuming light source 24 with the optical output pulse frequency of the laser oscillator 100, and an energy consuming light source driving circuit 252 that drives the energy consuming light source 24 under this driving condition. Have

  The drive condition setting unit 251 acquires synchronization information for synchronizing with the optical output pulse frequency of the laser oscillator 10, adjusts the transmission timing of the energy consumption light pulse, and adjusts the pulse width of the energy consumption light. A pulse width adjusting unit 254 for adjusting the light output, and a light output adjusting unit 255 for adjusting the light output of the energy consuming light.

  With the above configuration, the operation of the present embodiment will be described with reference to FIG. 3A is a light output waveform of the pumping light source 22 for the optical fiber amplifier, FIG. 3B is a light output waveform of the laser oscillator 10, FIG. 3C is a light output waveform of the consumed energy light source 24, and FIG. ) Is an optical output waveform of the optical fiber amplifier 20.

  First, in this embodiment, as shown in FIG. 3A, the optical output waveform 31 of the optical fiber amplifier pumping light source 22 is set to a constant output. At this time, as shown in FIG. 3B, the optical output waveform of the laser oscillator 10 oscillates at a period (1 / fp) (fp: optical pulse repetition frequency) and outputs an optical output pulse 32. And

  Here, as shown by the arrow in FIG. 3C, the optical output pulse 33 of the energy consuming light source 24 is inserted between the optical output pulses 32 of the laser oscillator 10 and coupled to the optically amplifying double-clad fiber 21. As a result, excessively accumulated carriers in the excitation level of the optically amplifying double-clad fiber 21 are consumed by stimulated emission, and generation of ASE light from the optical fiber amplifier 20 is suppressed as shown in FIG. An optical output pulse 34 can be obtained.

  FIG. 4 is a conceptual diagram showing energy levels of a three-level fiber laser. A three-level system is assumed as an excitation level of the optically amplifying double-clad fiber 21 whose core is doped with Tm (thulium). Carriers (electrons) at the ground level E0 are excited by the optical fiber amplifier pumping light source 22 to the first pumping level E1, and the second pumping level lower than the first energy E1 with a predetermined carrier relaxation time τ. Transition to position E2. The carriers that have transitioned to the second excitation level E2 amplify the seed light input from the laser oscillator 100 through a stimulated emission process.

  Therefore, in the “pumping light source ON / OFF method” in which the optical output of the optical fiber amplifier pumping light source 22 (105) is supplied in synchronization with the optical pulse frequency of the seed light, the optical fiber amplifier 101 increases the optical pulse frequency. The output level of the output optical pulse 113c is remarkably lowered. This is because the carrier relaxation from the first excitation level E1 to the second excitation level E2 cannot be sufficiently performed, and this carrier relaxation time τ It is considered that the operating frequency is rate-controlled.

  In order to prevent a decrease in optical output, it is necessary to supply the optical output of the optical fiber amplifier excitation light source 22 (105) as CW to the optical amplification double-clad fiber 21 (104). When the frequency is low, ASE light is remarkably generated. This is because the seed light is not input even after the carrier relaxation time τ elapses, so that excessive carriers accumulate in the second excitation level E2. For this reason, the level of spontaneous emission light generated by transitioning from the second excitation level E2 to the ground level E0 increases, and the spontaneous emission light is further amplified inside the optical fiber amplifier 20, whereby the output level of the ASE light is increased. Will increase.

  In the present embodiment, energy consuming light is used to suppress this ASE light. The wavelength band of the energy consuming light is set within the wavelength band having the amplification gain of the optical amplification double clad fiber.

  Note that when performing communication or measurement using the output wavelength of the laser device of the present embodiment, it may be preferable from the viewpoint of noise to set the energy consuming light and the seed light to different wavelengths within the amplification gain.

  Next, the pulse light transmission timing of the energy consuming light source inserted between the seed light pulses output from the laser oscillator 10 will be described with reference to FIG. As shown in FIG. 5 (a), for one period between optical pulses output from the laser oscillator 10, a seed light pulse repeated for one period (1 / fp) defined by the optical pulse frequency fp is forward light. A pulse 32s and a backward light pulse 32e are used. As shown in FIG. 5B, the transmission timing of the energy consuming light pulse 33p having the pulse width W is set after T1 time has elapsed from the seed light forward pulse 32s, and the energy consuming light pulse 33p and the next seed light backward pulse are set. The time interval of 32e is set to T2.

  When the time T1 is longer than the carrier relaxation time τ in which carriers transition from the first excitation level E1 to the second excitation level E2, the spontaneous emission light is amplified in the optical fiber amplifier 20, and the output level of the ASE light is increased. Therefore, the time T1 is set to be substantially shorter than the carrier relaxation time τ, and is set so that the carrier of the second excitation level E2 is consumed by the energy consuming light pulse 33p.

  Regarding the time T2, since it is necessary to increase the carrier density up to the number of carriers that cause an inversion distribution with respect to the backward seed light pulse 32e, the carrier of the second excitation level E2 is caused by the falling of the energy consuming light pulse 33p. After consumption stops, it is preferable to set a time of about the carrier relaxation time τ.

  The above explanation is a qualitative explanation, and the number of carriers accumulated or consumed in the excitation level E2 differs depending on conditions such as the light output levels and pulse widths of the seed light pulses 32s and 32e and the energy consumption pulse 33p. These times T1 and T2 need to be set to optimum times through experiments or the like.

  In FIG. 5, a single pulse is assumed for the energy consuming light pulse 33p. However, as shown in FIG. 6, the energy consuming light pulse may be divided into a plurality of pulses and supplied. FIG. 6B shows the case of two energy consuming light pulses 33p1 and 33p2. The pulse widths of the energy consuming light pulses 33p1 and 33p2 are W1 and W2, respectively, and the pulse interval is time T3.

  Similar to the above discussion, the time T3 is preferably set to a time shorter than the carrier relaxation time τ. In this case as well, the light output level, pulse width, and transmission timing of the energy consuming light are sacrificed for the light amplification characteristics by experimenting with the light output characteristics such as the pulse width, light output, and repetition frequency of the seed light pulse. The driving condition setting unit 25 is set with a driving condition that can suppress the generation of ASE light.

  When the seed light pulse is driven at a frequency sufficiently higher than the reciprocal (1 / τ) of the carrier relaxation time τ, the carriers present in the excitation level E2 are consumed by stimulated emission of the continuous seed light pulse. The In this case, since excessive ASE light is not generated, it is not particularly necessary to supply an energy consuming light pulse.

  FIG. 7 is a configuration example of an optical amplification fiber. In the optical amplifying double clad fiber 21 of the present embodiment, a rare earth (Tm) for amplifying light is added to the core 71. And it has a two-layer clad structure of an inner clad 72 surrounding the core 71 and an outer clad 73 further covering the outer periphery thereof. Since the refractive index of the inner cladding 72 is higher than the refractive index of the outer cladding 73, light can be propagated using the outer cladding 73 as a cladding and the inner cladding 72 as a core. Light propagating using the inner cladding 72 as a core is multimode. In addition, the optically amplifying double-clad fiber 21 of this embodiment includes a multimode combiner 26 for supplying light from an optical fiber amplifier pumping light source 22 and an energy consuming light source 24 to the inner cladding. A light supply port 26p for supplying light and a light supply port 26c for supplying energy consumption light are provided on the output side of the optical amplifying double clad fiber 21. Thereby, the pumping light for optical fiber amplifier and the energy consuming light are supplied along the direction opposite to the amplification and propagation direction of the seed light.

  As described above, the energy consuming light propagates in the inner clad 72 in the direction opposite to the seed light, and thus only consumes the carriers accumulated in the second excitation level E2 and does not amplify in the reverse direction. However, when energy consuming light propagating in the reverse direction is input to the laser oscillator 10, it affects the laser oscillation characteristics and needs to be removed. In particular, when a double clad fiber is used in the laser oscillator 10, the isolator 14 is effective only in a single mode propagating through the core 71, so that an optical termination for light propagating through the inner clad is required.

  Accordingly, the optical amplifying double-clad fiber 21 is formed with a non-reflective terminal 27 at the opposite end of the multimode combiner 26, and the pumping light and energy for amplifier supplied from the light supply ports 26p and 26c. The consumed light is terminated at this portion, and can be prevented from being input to the laser oscillator 10 or being subjected to multiple reflections within the optically amplifying double clad fiber 21.

  For example, the non-reflection terminal portion 27 removes a part of the outer clad 73 of the amplifying double clad fiber 21 and coats the outer circumference of the inner clad 72 with a coating agent 74 having a refractive index equal to or higher than the refractive index of the inner clad 72. Can be formed.

  The light propagating through the inner cladding indicated by the arrow is changed from the propagation mode to the radiation mode by the non-reflection end portion 27, so that the light is lost and absorbed by the coating agent 74.

  Further, as described above, the energy consuming light supplied from the light supply port 26c is amplified when passing through the core 71. However, when propagating through the inner clad 72, the energy consuming light is lost without being amplified. Depending on the length of the clad fiber 21, loss conditions, etc., the non-reflection termination part 27 may not be necessarily provided. Further, the non-reflective terminal portion 27 does not necessarily have to be at the end portion.

  As described above, according to the first embodiment, in order to suppress the generation of ASE light, the energy consumption light is supplied between the seed light pulses, thereby consuming excessively accumulated excitation level carriers. Therefore, ASE light can be greatly reduced.

  Also, in principle, this embodiment can effectively reduce ASE light for any repetition frequency of light pulses from low frequency to high frequency. In particular, unlike the conventional “excitation light source ON / OFF method”, there is an effect that the optical output is not reduced at the repetition frequency of the optical pulse of 200 Hz or more.

  In the present embodiment, the laser device in which Tm is doped in the core has been described. However, the same effect can be obtained in an optical fiber doped with other rare earth elements. Further, although energy consuming light is supplied in the direction opposite to the propagation direction of the seed light, a certain effect can be obtained even if supplied in the forward direction.

(Second Embodiment)
In the present embodiment, an embodiment having the same effect as that of the first embodiment will be described. FIG. 8 is a block diagram of a laser apparatus according to the second embodiment.

  As in the first embodiment, the laser apparatus of this embodiment includes a laser oscillator (MO) 10 that generates seed light and an optical fiber amplifier (PA) 80 that amplifies seed light pulses output from the laser oscillator 10. Has been. Since the laser oscillator 10 is the same as that of the first embodiment, description thereof is omitted.

  The optical fiber amplifier 80 eliminates the energy consuming light source 24 and the energy consuming light source driving circuit 25 from the configuration of the first embodiment, and instead uses part of the seed light as energy consuming light.

  That is, as in the first embodiment, the optical amplifying double clad fiber 21, the optical fiber amplifier pumping light source 22 for generating a gain in the optical amplifying fiber 21, and the optical fiber amplifier pumping light source 22 are driven. An amplifier pumping light source drive circuit 23 supplies a light output from the optical fiber amplifier pumping light source 22 and the energy consuming light source to the inner cladding of the optical amplifying double-clad fiber 21 along the direction opposite to the propagation direction of the optical output pulse. The optical output of the mode combiner 26, the optical fiber amplifier pumping light source 22 and the energy consuming light source 24 is added to the non-reflective termination 27 that performs the non-reflective termination at the end of the optical amplifying double-clad fiber 21, and the seed light is predetermined. Branch coupler 81 for branching at a branching ratio, and an optical delay for delaying the branched seed light by a predetermined time. Providing a circuit 82.

  Here, how the functions of the light output adjustment unit 255, the pulse width adjustment unit 254, and the transmission timing adjustment unit 253 in the first embodiment are achieved will be described.

  The optical output can be adjusted by the branching ratio of the branch coupler 81. Moreover, you may insert an optical attenuator as needed.

  At the light transmission timing, as shown in FIG. 9A, the delay time is controlled using an optical waveguide 91a such as an optical fiber so that it can be supplied to a predetermined position between the seed light pulses. Since the branched light of the seed light is used as the energy consuming light, there is an advantage that the optical pulse frequency can be automatically synchronized.

  As for the adjustment of the optical pulse width, as shown in FIG. 9B, a plurality of delay circuits (two delay circuits 91a and 91b in FIG. 9) are provided, branched by the coupler 92, and combined by the coupler 93. By making waves, it becomes possible to satisfy the transmission timing conditions shown in FIGS.

  As described above, according to the second embodiment, the energy consuming light source 24 and the energy consuming light source driving circuit 25 are eliminated, and a part of the seed light is used as energy consuming light instead. Simplified. In addition, since the same effect as that of the first embodiment can be obtained only by adding passive components, it greatly contributes to high reliability and cost reduction.

  According to this embodiment, generation of ASE light generated between light output pulses of the laser device can be significantly suppressed.

  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. For example, although a fiber laser has been described as the laser oscillator 10, a semiconductor laser or the like may be used. 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.

10 ... Laser oscillator,
11: Fiber laser oscillator,
11a, 11b ... Fiber Bragg grating,
12 ... Excitation light source for laser oscillator,
13 ... Excitation light source drive circuit for oscillator,
14: optical isolator,
20: optical fiber amplifier,
21 ... Double clad fiber for optical amplification,
22: Excitation light source for optical fiber amplifier,
23 ... Excitation light source drive circuit for amplifier,
24 ... Energy consuming light source,
25. Energy consumption light source driving circuit,
26 ... multimode combiner,
27: Non-reflective termination,
71 ... Core,
72. Inner clad,
73 ... outer cladding,
74 ... coating agent,
82: Optical delay circuit.

Claims (8)

A laser oscillator that oscillates in a first wavelength band;
A laser driving section for pulsing the laser oscillator at a predetermined frequency;
An optical fiber amplifier for amplifying the optical output of the laser oscillator;
A pumping light source that supplies pumping light of a second wavelength band to the amplification optical fiber of the optical fiber amplifier unit;
An energy consuming light source that supplies, to the amplification optical fiber, energy consuming light in a third wavelength band that consumes energy at the excitation level in synchronization with the predetermined frequency;
A laser device.   The energy consuming light is supplied between the optical pulses output from the optical fiber amplifier, and the optical output, pulse width, and transmission timing of the energy consuming light source are set based on the relaxation time of the excitation level of the optical fiber. The laser device according to claim 1.   The laser device according to claim 2, wherein the amplification optical fiber is formed of a double clad fiber, and the energy consuming light is supplied so as to propagate through an inner clad of the double clad fiber.   4. The laser device according to claim 3, wherein a part of the double clad fiber has a non-reflection termination portion for non-reflection termination of light propagating through the inner clad. A coupler that branches a part of the optical output of the laser oscillator at a predetermined branching ratio and the number of branches;
One or more delay circuits for delaying the transmission timing of the branched light branched by the coupler;
The laser device according to claim 4, wherein the branched light is the energy consuming light source.   The laser device according to claim 5, wherein the non-reflective termination part removes a part of the outer clad of the double clad fiber, and the outer circumference of the inner clad is coated with a coating agent having a refractive index equal to or higher than the refractive index of the inner clad.   7. The energy consumption light is supplied along a direction opposite to a propagation direction of an optical output pulse of the amplification optical fiber, and the non-reflection termination is formed on an input side of the amplification optical fiber. Laser equipment.   The laser apparatus according to claim 2, wherein the first wavelength band is included in the third wavelength band.

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