JP5484619B2 - Optical fiber laser - Google Patents

Description

  The present invention relates to a pulse modulation method that repeatedly generates optical pulses having a predetermined period as seed light for obtaining high-power output light, and an optical fiber laser to which the pulse modulation method is applied.

  Currently, processing technology using laser light is attracting attention, and the demand for high-power laser light sources is increasing in fields such as processing and medical use. Among various laser light sources, an optical fiber laser is given as a laser light source that has attracted particular attention. This optical fiber laser employs an optical fiber for amplification in which a rare earth element such as Yb (yttrium), Er (erbium), or Tm (thulium) is added to the core as an optical amplification medium. When excitation light is supplied into the amplification optical fiber, seed light propagating in the amplification optical fiber is amplified. As a result, the amplification optical fiber outputs high-power amplified light, or laser light is output by laser oscillation using the resonator structure. Advantages of optical fiber lasers include that laser light is confined within the optical fiber and that it is easy to handle, and that thermal radiation is good, so large-scale cooling equipment is not required. Can be mentioned.

  As described above, rare earth element-doped fibers are applied to optical fiber lasers. Among such rare earth element-doped fibers, Yb has high conversion efficiency and is widely used as an amplification optical fiber for high power output. Yes. Yb is also excited using excitation light, like other rare earth elements. On the other hand, excitation light that could not be absorbed in the amplification optical fiber is emitted from the other end of the amplification optical fiber.

As a configuration of the optical fiber laser, for example, when a resonator structure using a fiber Bragg grating (FBG) or a reflection mirror is adopted at both ends, an optical switch or an acousto-optic modulator is provided in the resonator. Pulse modulation is performed by arranging (AOM: Acoustic Optical Modulator). Further, a MOPA (Master Oscillator Power Amplifier) type optical fiber laser as described in Patent Document 1 is obtained by performing pulse modulation by directly modulating or externally modulating a seed light source that outputs amplified light. High power output light is obtained by amplifying the light pulse. In any configuration, the output obtained by pulsing the seed light is very high compared to the output during continuous wave operation (CW operation), and stimulated Raman scattering (SRS: Stimulated Raman Scattering) or stimulated Brillouin scattering ( Non-linear phenomena such as SBS (Stimulated Brillouin Scattering) are developed.
JP 2007-029881 A

  As a result of studying a conventional optical fiber laser using an optical pulse as a seed light, the inventors have found the following problems.

  That is, a modulation voltage signal applied to the seed light source with the intention of increasing the pulse energy in a laser system that pulses the output light by directly modulating a seed light source such as an LD (Laser Diode) that outputs the seed light. When the width (the width of the drive signal pulse for generating the optical pulse and the time width of the modulation voltage that defines the drive time of the LD) is increased, SBS is generated in the process of amplifying the seed light power. Therefore, there is a high possibility that the optical components and the excitation light source (LD) constituting the optical fiber laser are destroyed.

  In other words, paying attention to the half-value width of the output light spectrum from the LD serving as the seed light source, if the signal width of the modulation voltage for pulsing the seed light is increased in order to obtain sufficient light pulse energy, the spectrum half-value width becomes smaller. It becomes narrower. Since the gain of SBS increases as the spectrum width becomes narrower, there is a problem that the SBS gain is large and easily generated when the pulse signal width of the modulation voltage applied to the seed light source is long.

  The present invention has been made in order to solve the above-described problems. When a light pulse having a predetermined period is amplified as seed light, a nonlinear scholarly phenomenon such as SBS that increases as the pulse width of the light pulse increases. An object of the present invention is to provide a pulse modulation method and an optical fiber laser having a structure for effectively suppressing the above.

  In the pulse modulation method according to the present invention, the light output from or output from the seed light source is modulated by the modulator into optical pulses having a predetermined cycle. The optical fiber laser according to the present invention is a laser light source that realizes the pulse modulation method.

  Specifically, the modulation pattern, which is a modulation voltage pattern input to the modulator, is composed of a plurality of pulse components corresponding to optical pulses within a predetermined period, and individual pulse widths of the plurality of pulse components are predetermined. It is set to be smaller than the pulse width of the entire optical pulse within the period.

  In the pulse modulation method according to the present invention, it is preferable that the plurality of pulse components have individual pulse widths set to be smaller than ½ of the entire pulse width of the optical pulse within a predetermined period. The plurality of pulse components may have an individual pulse width longer than an interval between adjacent pulse components. In the plurality of pulse components, it is preferable that the interval between the pulses of the adjacent pulse components is equal to or less than one of the rise time and the fall time of the adjacent pulse components. Furthermore, the individual pulse peak values of the plurality of pulse components may be different from each other.

  In the pulse modulation method according to the present invention, the peak current of the plurality of pulse components may be adjusted by modulating the drive current of the seed light source. In this case, voltage modulation and current modulation are performed separately, but the respective modulation timings are the same.

  An optical fiber laser according to the present invention includes a seed light source, a modulator, and an optical fiber amplifier. The modulator is for modulating the amplified light output from or output from the seed light source into an optical pulse having a predetermined repetition period, and is electrically connected to the seed light source or the previous seed It is provided on the optical path of the light output from the light source. The optical fiber amplifier amplifies and outputs the light to be amplified modulated into the optical pulse.

  In particular, in the optical fiber laser according to the present invention, the modulation pattern that is a pattern of the modulation voltage input to the modulator is composed of a plurality of pulse components corresponding to the optical pulses within a predetermined period, and the plurality of pulse components. The individual pulse width is set to be smaller than the pulse width of the entire optical pulse within a predetermined period.

  The embodiments according to the present invention can be more fully understood from the following detailed description and the accompanying drawings. These examples are given solely for the purpose of illustration and should not be considered as limiting the invention.

  Further scope of applicability of the present invention will become apparent from the detailed description given below. However, while the detailed description and specific examples, while indicating the preferred embodiment of the invention, are presented for purposes of illustration only, various modifications and improvements within the scope of the invention It will be apparent to those skilled in the art from the detailed description.

  According to the pulse modulation method of the present invention, when generating a seed light pulse in the optical fiber laser, the modulation pattern of the driving electrical signal for instructing direct modulation or external modulation of the seed light source is adjusted. That is, an electric signal pattern for generating one optical pulse is composed of a plurality of pulse components, and the pulse width of the generated optical pulse is adjusted by adjusting the signal interval, signal width, intensity, etc. of the plurality of pulse components. Enlargement and effective suppression of nonlinear optical phenomena are realized simultaneously.

  Hereinafter, embodiments of a pulse modulation method and an optical fiber laser according to the present invention will be described in detail with reference to FIGS. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

(First Embodiment of Optical Fiber Laser)
FIG. 1 is a diagram showing a configuration of a first embodiment of an optical fiber laser according to the present invention, specifically, a MOPA type optical fiber laser. In FIG. 1, an optical fiber laser 100 according to the first embodiment includes an amplification optical fiber 10, an optical coupler 20, an excitation light source 31, an optical fiber 32, a seed light source 41, an optical fiber 42, a modulator 51, a modulation voltage. The generator 55, the electric signal line 52, the optical isolator 61, the transmission optical fiber 11, and the light emitting end 70 are provided. In the optical fiber laser 100 according to the first embodiment, the modulation voltage generator 55 is a function generator that generates a modulation pattern for pulse modulation, and the modulation voltage is also generated from the modulation voltage generator 55. The modulator 51 has a function of modulating the pulse drive of the seed light source 41 itself, and is composed of electronic components inside the substrate. The modulation voltage output from the modulation voltage generator 55 is input to the modulator 51, and the seed light pulse L is repeatedly generated in the seed light source 41.

  In the optical fiber laser 100 according to the first embodiment, the pumping light from the pumping light source 31 that has passed through the optical fiber 32 and the seed light pulse (amplified light) from the seed light source 41 that has passed through the optical fiber 42 and the optical isolator 61. Are combined by the optical coupler 20. The combined light from the optical coupler 20 is incident on one end of the amplification optical fiber 10. In the amplification optical fiber 10 through which the combined excitation light and seed light propagate, rare earth elements (Yb, Er, Tm, Ho, Nd, Pr, Tb, etc.) added to the amplification optical fiber 10 are excitation light. The seed light pulse is amplified by being excited by. Then, the seed light pulse amplified in the amplification optical fiber 10 passes through the transmission optical fiber 11 fusion-connected at the other end A of the amplification optical fiber 10 and then is output from the light emitting end 70 to the outside. Is done.

  For example, the amplification optical fiber 10 has a cross-sectional structure and a refractive index profile as shown in FIG. That is, the amplification optical fiber 10 is provided on the outer periphery of the core 10a and the core 10a having a predetermined refractive index extending along a predetermined axis, as shown in the region (a) of FIG. A first cladding 10b having a refractive index lower than that of the core 10a and a second cladding 10c having a refractive index lower than that of the first cladding 10b provided on the outer periphery of the first cladding 10b are provided. In FIG. 2, a region (b) is a refractive index profile 150 along the longitudinal direction L1 of the amplification optical fiber 10 (a direction orthogonal to the optical axis of the amplification optical fiber 10), and the region 151 is a region of the core 10a. The refractive index along the radial direction L1, the region 152 indicates the refractive index along the radial direction L1 of the first cladding 10b, and the region 153 indicates the refractive index along the radial direction L1 of the second cladding 153. The core 10a, the first cladding 10b, and the second cladding 10c constitute a double cladding structure. The core 10a propagates the seed light pulse in a single mode, and the first cladding 10b propagates the excitation light in a multimode. Yb is added as a rare earth element to the core 10a, and the seed light pulse is amplified in the core 10a.

  In addition, the absorption of pumping light in the amplification optical fiber 10 is determined by the characteristics of the amplification optical fiber 10, and mainly includes the mode field diameter (MFD), the outer diameter of the first cladding 10b, and the addition of rare earth elements in the core 10a. It changes by adjusting the density. For example, in a Yb-doped optical fiber having an additive concentration of about 10,000 ppm, MFD of about 7 μm, the first cladding 10b having an outer diameter of 130 μm, and a length of 5 m, about 2.4 dB of pump light is absorbed in the pump wavelength 915 nm band. In FIG. 3, a graph G310 indicates an absorption cross section, and a graph G320 indicates a discharge cross section. In the case of this Yb-doped optical fiber (corresponding to the amplification optical fiber 10), about 2.4 dB of excitation light is absorbed in the 915 nm wavelength band. The wavelength band of the excitation light may be a 975 nm band, and the excitation wavelength band varies depending on the type of rare earth element added.

  The excitation light source 31 includes, for example, an LD. The wavelength of the excitation light output from the excitation light source 31 is a 915 nm band, a 940 nm band, or a 975 nm band. The seed light source 41 includes, for example, an LD. The modulator 51 directly modulates the seed light source 41 by applying a modulation voltage E to the seed light source 41 via the electric signal line 52 (pulse modulation). In the first embodiment, the wavelength of the seed light pulse output from the seed light source 41 is in the wavelength range of 1030 to 1130 nm, for example, 1060 nm.

  Each of the optical fiber 32 and the transmission optical fiber 11 provided between the excitation light source 31 and the optical coupler 20 has a cross-sectional structure and a refractive index profile as shown in FIG. That is, as shown in the region (a) of FIG. 4, the optical fibers 32 and 11 include a core 32 a extending along a predetermined axis and having a predetermined refractive index, and a core provided on the outer periphery of the core 32 a. A clad 32b having a refractive index lower than 32a is provided. 4 is a refractive index profile 320 along the longitudinal direction L2 of the optical fiber 32 (the direction perpendicular to the optical axis of the optical fiber 32), and the region 321 is the radial direction L2 of the core 32a. And the region 322 indicate the refractive index along the radial direction L2 of the cladding 32b. The core 32a propagates the excitation light output from the excitation light source 31 in multimode.

FIG. 5 is a diagram illustrating a configuration of the optical coupler 20. The optical coupler 20 shown in FIG. 5 has a plurality of (seven in the example shown in FIG. 5) optical input / output ports P _{1 to} P ₇ on one side, and a common port on the other side. with a P _0. The optical coupler 20 combines the lights input to the optical input / output ports P _{1 to} P ₇ and outputs them from the common port P ₀ . Further, the optical coupler 20 branches the light input to the common port P _0, outputs the respective branched light from the optical output port P ₁ to P _7.

The optical fiber on the common port P ₀ side of the optical coupler 20 has a double clad structure similar to that of the amplification optical fiber 10 and is connected to the amplification optical fiber 10. The light input / output port P ₁ is optically connected to the seed light source 41 via the optical fiber 42. The optical input / output port P ₂ is optically connected to the excitation light source 31 via the optical fiber 32. The other light input / output ports P _{3 to} P ₇ may also be optically connected to other pumping light sources via other optical fibers.

  Next, embodiments of the pulse modulation method according to the present invention will be described. In the following description, after describing a comparative example to be compared with each embodiment, each embodiment will be described with comparison with the comparative example.

(Comparison example of pulse modulation method)
The pulse modulation method according to the comparative example performs pulse modulation by directly modulating the seed light source 41 in the optical fiber laser 100 shown in FIG.

  FIG. 6 shows a modulation pattern of the modulation voltage E applied from the modulator 51 to the seed light source 41 in the pulse modulation method according to this comparative example. The modulation pattern is a modulation voltage input to the modulator 51, is input from a function generator or a voltage generator, and is used for generating an optical pulse. The modulation period in the modulation pattern corresponds to one period of an optical pulse to be generated, and is composed of a signal ON period T1 and a signal OFF period T2. The signal ON period T1 is a period in which a pulse component P having a signal width W is substantially applied to the seed light source 41. Specifically, in the pulse modulation method according to this comparative example, the drive voltage applied to the seed light source 41 is modulated at a repetition frequency of 50 kHz and a pulse signal width of the pulse component P of 30 ns. It is possible to apply a modulation voltage having a signal width of 30 ns to the seed light source 41, but this signal width depends on the response (falling / falling coefficient) of the light source and the electronic circuit mounted for pulse modulation. Therefore, in the modulation pattern, the time width of the signal ON period T1 (pulse component P) and the signal OFF period T2 may be different.

  In this comparative example, simply applying a modulation voltage having a signal width of 30 ns to the seed light source 41 does not cause a problem that causes damage to the optical components constituting the optical fiber laser 100, but amplifies the modulated seed light pulse. It is known that SBS develops in the amplification optical fiber 10 in the process of doing so. If the amplification operation is continued while the SBS expression state is maintained, the optical components and the excitation light source (LD) constituting the optical fiber laser 100 shown in FIG. 1 are damaged. As the SBS avoidance method as described above, for example, a method of lowering the nonlinear threshold of the amplification fiber 10 itself such as a method of increasing the core diameter of the amplification optical fiber 10 or a method of shortening the amplification optical fiber 10 is considered. It was done.

Here, the spectral width of incident light (fiber excitation component) is known as one of the parameters depending on the threshold and gain of the SBS, and the relationship is shown in the following equation (1). In the configuration of FIG. 1, the laser light propagating through the core of the amplification optical fiber 10 becomes a fiber pumping component, so the threshold value and gain of the SBS depend on the spectrum width of this laser light.

As can be seen from the above equation (1), when Δν _P >> Δν _B , the Brillouin gain is reduced by Δν _P / Δν _B times. The seed light (laser light) of the LD, which is the seed light source 41, has a relationship of Δν _P >> Δν _B , and the Brillouin gain depends on the spectral width of this seed light (the spectral width of the fiber excitation component). That is, the wider the spectrum width, the smaller the Brillouin gain.

  For example, when a modulation voltage having a signal width of 30 ns is applied from the modulator 51 to the seed light source 41 as in this comparative example, the actual measured value of the half width of the output seed light pulse is about 0.6 nm. On the other hand, the spectrum half width at the time of CW operation without pulse modulation is about 1.1 nm. The spectrum half-width of the seed light that has been pulse-modulated in this way is approximately half the spectrum half-width during CW operation. It can also be seen from the above equation (1) that the SBS gain value itself increases.

  FIG. 7 shows the normalized spectrum of the optical pulse (graph G720) pulse-modulated (modulated with a signal width of 30 ns) by the pulse modulation method according to the comparative example as the seed light and the continuous light (graph G710) applied as the seed light. is there. As can be seen from FIG. 7, the spectrum half width becomes narrower as the pulse width of the seed light pulse is wider. Inevitably, in the pulse modulation method according to this comparative example, simply expanding the pulse width of the seed light pulse inevitably damages each part of the optical fiber laser 100 due to an increase in SBS.

(First Embodiment of Pulse Modulation Method)
Next, a first embodiment of the pulse modulation method according to the present invention will be described. In the following description, an operation when the pulse modulation method is applied to the optical fiber laser 100 shown in FIG. 1 will be described. Therefore, the pulse modulation method is realized by the modulator 51 directly modulating the seed light source 41.
FIG. 8 is a diagram for explaining the first embodiment of the pulse modulation method according to the present invention. That is, the modulation voltage E applied from the modulator 51 to the seed light source 41 is adjusted according to the modulation pattern shown in FIG. The modulation period in the modulation pattern of the modulation voltage E corresponds to one period of an optical pulse to be generated, and is composed of a signal ON period T1 and a signal OFF period T2. In the pulse modulation method according to the first embodiment, in the signal ON period T1, the optical pulse generation pattern P has a plurality of pulse components P ₁ to P ₁ each having a signal width W shorter than the pulse width of one optical pulse to be generated. P ₃ is constituted by, and is arranged only in a distant state signal interval D of the plurality of pulse components P ₁ to P _3. In the first embodiment, the optical pulse generation pattern P is configured by the _three pulse components P _{1 to} P ₃ under the condition that the signal width W is 10 ns and the signal interval D of the pulse components is 10 ns. . At this time, each of the modulation voltage value pulse component P ₁ to P ₃ (peak voltage value) is consistent, also consistent pulse components P ₁ to P ₃ each drive current value applied to the seed light source 41 ing. Note that the signal interval D of each of the pulse components P _{1 to} P ₃ can be set to 5 ns, 2 ns, or 1 ns or less.

FIG. 9 shows normalized spectra of an optical pulse pulse-modulated by the pulse modulation method according to the first embodiment as seed light and an optical pulse pulse-modulated by the pulse modulation method according to the comparative example as seed light. In FIG. 9, a graph G920 is a seed light pulse obtained when the signal width W of the optical pulse generation pattern P is set to 30 ns (comparative example) and pulse modulation is performed. A graph G910 is a seed light pulse obtained when pulse modulation is performed according to a light generation pattern P configured by _three pulse components P _{1 to} P ₃ each having a signal width W of 10 ns.

In the case of an optical pulse generation pattern P composed of _three pulse components P _{1 to} P ₃ with a signal interval D of 10 ns and a signal width W of 10 ns, the half width of the seed light pulse obtained is 1.03 nm, In the case of an optical pulse generation pattern P composed of _three pulse components P _{1 to} P ₃ with an interval D of 5 ns and a signal width W of 10 ns, the spectrum half width of the seed light pulse obtained is 0.8 nm. From the comparison results of the spectral widths shown in FIG. 9, the half-value width is larger than that in the case where the pulse width is simply modulated with the signal width W of the optical pulse generation pattern P being 30 ns (comparative example). I understand.

  Referring to the difference between the optical pulse generation pattern formed by arranging three pulse components with a signal interval D and the optical pulse generation pattern P having a long signal width W, the response of the Yb-doped fiber itself is within one period. If the input power at is the same, the same pulse energy can be obtained. Therefore, the peak of one pulse component having a signal width W of 10 ns is formed between the optical pulse generation pattern having a signal width W of 30 ns and the optical pulse generation pattern including three pulse components having a signal width W of 10 ns. The value is about 1/3. This principle does not depend on the signal interval D between the pulse components.

  FIG. 10 is a diagram for explaining the relationship between the signal width W and the standardized half-value width of the seed light pulse for one pulse component in the modulation pattern. In FIG. 10, as reference data, the half-value width (first embodiment) of the seed light pulse pulse-modulated by three pulse components each having a signal width W set to 10 ns is plotted.

  As can be seen from FIG. 10, the half width of the seed light pulse obtained by the pulse modulation becomes narrower as the signal width W of one pulse component is increased. Further, the half width (about 1.03 nm from FIG. 9) of the seed light pulse obtained when the optical pulse generation pattern P is configured by three pulse components each having a signal width W of 10 ns has a signal width W of about 1.03 nm. It can be seen that the half-value width of the seed light pulse obtained when the optical pulse generation pattern P is configured with one pulse component of 30 ns (comparative example) is clearly wider. From this result, by applying the modulation voltage E of the optical pulse generation pattern P in which a plurality of pulse components having a narrow signal width W are arranged from the modulator 51 to the seed light source 41, the SBS that is expressed in the amplification optical fiber 10 is obtained. Reduction effect can be expected. Although it has been known that an optical pulse with a short pulse width is less likely to be affected by SBS, as far as the inventor knows, by adjusting the modulation pattern for pulse modulation, without shortening the pulse width of the optical pulse, No attempt has been made to reduce the effects of SBS.

(Second Embodiment of Pulse Modulation Method)
In the pulse modulation method according to the present invention, by making the signal interval D of the pulse components P _{1 to} P ₃ in the modulation pattern of the modulation voltage E shorter than each signal width W, the effect of overlapping of the pulse components is used. Can do. This depends on the rise / fall time (hereinafter referred to as Tr / Tf) of the seed light pulse when one pulse component is applied to the photoelectric conversion element. The higher the Tr / Tf of the seed light pulse, the more difficult it is to overlap. Tr and Tf here refer to the time until the intensity reaches 10% and 90%, respectively, with respect to the peak voltage value. FIG. 11 shows a response result when one pulse component having a signal width W of 10 ns is given to the seed light source 41. In FIG. 11, a graph G1110 shows the modulation pattern of the modulation voltage E, and a graph G1120 shows the waveform of the seed light pulse when the modulation pattern is given. As shown in FIG. 11, the response of the LD that is the seed light source 41 is relatively slow, with a rise time of about 4.6 ns and a fall time of about 9.4 ns. Accordingly, the modulation pattern of the modulation voltage E applied to the seed light source 41 that generates the seed light pulse is a case where the modulation pattern and the optical pulse response deviate if the signal interval D is narrowed between the pulse components even if the signal width W is 10 ns. There is. The second embodiment of the pulse modulation method according to the present invention has been completed based on the above-mentioned considerations. Hereinafter, the pulse modulation method according to the second embodiment will be described with reference to FIGS. However, it explains in detail.

  First, FIG. 12 is a diagram for explaining a second embodiment of the pulse modulation method according to the present invention. FIG. 13 is a graph showing the light receiving level of an optical pulse pulse-modulated by the pulse modulation method according to the second embodiment.

In the pulse modulation method according to the second embodiment, the bias voltage E applied from the modulator 51 to the seed light source 41 is adjusted according to the modulation pattern shown in FIG. The modulation period in the modulation pattern corresponds to one period of an optical pulse to be generated, and is composed of a signal ON period T1 and a signal OFF period T2. In the pulse modulation method according to the second embodiment, in the signal ON period T1, the optical pulse generation pattern P has a plurality of pulse components P ₁ each having a signal width W shorter than the pulse width of one optical pulse to be generated, P ₂ is constituted by these two pulse components P _1, signal interval D of P ₂ is set to be shorter than the signal width W. Thus, in the pulse modulation method according to the second embodiment, the generation is performed by narrowing the signal interval D between the pulse components P ₁ and P ₂ so that the response delay of the LD that is the seed light source 41 can be used. Seed light pulses can be superimposed.

In the second embodiment, the signal width W of each of the _two pulse components P ₁ and P ₂ in the modulation pattern is set to 20 ns and the signal interval D is set to 2 ns in consideration of the overlap of the seed light pulses. As a result, the full width at half maximum of the optical pulse finally output from the optical fiber laser 100 is about 35 ns. That is, according to the pulse modulation method according to the second embodiment, it is possible to realize generation of an optical pulse having a full width at half maximum of 30 ns or more that could not be achieved by the generation of SBS in the comparative example. Incidentally, the amplification gain at the time the amplification optical fiber 10. Since the large, first modulation voltage peak value of the pulse component P ₁ is 0.6 times of the second modulation voltage peak value of the pulse component P ₂ Is set. Thereby, it becomes possible to moderate the transient response in optical amplification.

(Third Embodiment of Pulse Modulation Method)
FIG. 14 is a diagram for explaining a third embodiment of the pulse modulation method according to the present invention. In the pulse modulation method according to the third embodiment, when the amplification gain in the amplification optical fiber 10 is large, a difference occurs in the light reception level of the optical pulse output from the optical fiber laser 100. Therefore, in the pulse modulation method according to the third embodiment, the modulation voltage E applied to the seed light source 41 for generating the seed light pulse is changed between the pulse components.
That is, in the pulse modulation method according to the third embodiment, the modulation voltage E applied from the modulator 51 to the seed light source 41 is adjusted according to the modulation pattern shown in FIG. The modulation period in the modulation pattern of the modulation voltage E corresponds to one period of an optical pulse to be generated, and is composed of a signal ON period T1 and a signal OFF period T2. In the pulse modulation method according to the third embodiment, in the signal ON period T1, the optical pulse generation pattern P has a plurality of pulse components P ₁ to P ₁ each having a signal width W shorter than the pulse width of one optical pulse to be generated. P ₃ is constituted by, signal interval D of these three pulse components P ₁ to P ₃ is set to be shorter than the signal width W. Note that the signal widths W1 to W3 may be different or the same. Further, the signal intervals D1 and D2 may be different or the same.

  This configuration also makes it possible to expand the pulse width of the seed light pulse while effectively suppressing the occurrence of SBS in the amplification optical fiber 10.

(Fourth Embodiment of Pulse Modulation Method)
FIG. 15 is a view for explaining a fourth embodiment of the pulse modulation method according to the present invention. In the pulse modulation method according to the fourth embodiment, the drive current value supplied to the seed light source 41 is modulated so as to coincide with the modulation timing of the modulation voltage E.

That is, in the pulse modulation method according to the fourth embodiment, the drive current supplied from the modulator 51 to the seed light source 41 is modulated according to the drive current modulation pattern shown in FIG. The modulation period in the modulation pattern corresponds to one period of an optical pulse to be generated, and is composed of a signal ON period T1 and a signal OFF period T2. In the pulse modulation method according to the third embodiment, in the signal ON period T1, the optical pulse generation pattern P has a plurality of pulse components P ₁ to P ₁ each having a signal width W shorter than the pulse width of one optical pulse to be generated. P ₃ is constituted by, signal interval D of these three pulse components P ₁ to P ₃ is set to be shorter than the signal width W. In the fourth embodiment, the modulation voltage peak values of the pulse components P _{1 to} P ₃ are different in order to reduce the influence of the transient response during optical amplification, but the signal width W and the signal interval D are respectively different. They may match or be different.

  This configuration also makes it possible to expand the pulse width of the seed light pulse while effectively suppressing the occurrence of SBS in the amplification optical fiber 10.

  As described above, the direct modulation method in the optical fiber laser 100 according to the first embodiment shown in FIG. 1 has been described as each embodiment of the pulse modulation method according to the present invention, but each implementation of the pulse modulation method according to the present invention has been described. Needless to say, the embodiment can be applied to optical fiber lasers having various configurations. Hereinafter, typical configurations of various optical fiber lasers to which the pulse modulation method according to the present invention can be applied will be described.

(Second Embodiment of Optical Fiber Laser)
FIG. 16 is a diagram showing the configuration of the second embodiment of the optical fiber laser according to the present invention. In FIG. 16, the optical fiber laser 200 according to the second embodiment includes an amplification optical fiber 10, an optical coupler 20, an excitation light source 31, an optical fiber 32, a seed light source 41, an optical fiber 42, a modulation voltage generator 55, an electric It has the same configuration as the optical fiber laser 100 according to the first embodiment in that the signal line 52, the optical isolator 61, the transmission optical fiber 11, and the light emitting end 70 are provided. However, in the optical fiber laser 100 according to the first embodiment, the modulator 51 has a configuration of the direct modulation method in which the seed light source 41 is modulated. However, the optical fiber laser 200 according to the second embodiment has an external modulation method. The configuration is provided. Specifically, the optical fiber laser 200 according to the second embodiment includes an acousto-optic modulator (AOM) 50 between the seed light source 41 and the optical isolaser 61. Since the AOM 50 itself is a modulator, when a modulation pattern is input from the modulation voltage generator 55 to the AOM 50, the AOM 50 pulsates the light output from the seed light source 41 according to the modulation pattern. Do. In the second embodiment, the AOM 50 and the modulation voltage generator 55 constitute a pulse modulator 500.

  In the optical fiber laser 200 according to the second embodiment, the modulator 51 can implement any of the pulse modulation methods according to the first to fourth embodiments described above.

(Third embodiment of optical fiber laser)
FIG. 17 is a diagram showing the configuration of the third embodiment of the optical fiber laser according to the present invention. The optical fiber laser 300 according to the third embodiment is similar to the first embodiment with respect to the configuration in which the seed light source 41 is directly modulated by the modulator 51, but the excitation method is different. That is, the structural difference between the optical fiber laser 400 according to the third embodiment and the optical fiber laser 100 according to the first embodiment is a configuration in which the optical fiber laser 100 according to the first embodiment performs forward pumping. On the other hand, the optical fiber laser 300 according to the third embodiment is configured to perform backward pumping.

  Specifically, the optical fiber laser 300 according to the third embodiment shown in FIG. 17 includes an amplification optical fiber 10, an optical branching device 21, an excitation light source 33, an optical fiber 34, a seed light source 41, an optical fiber 42, and modulation. 51, a modulation voltage generator 55, an electric signal line 52, an optical isolator 61, a transmission optical fiber 11, and a light emitting end 70.

In the optical fiber laser 300 according to the third embodiment, the light incident end of the amplification optical fiber 10 is fusion-connected to the light emitting end of the optical isolator 61 at the point B. On the other hand, the optical branching device 1 is disposed on the light emitting end side of the amplification optical fiber 10. The configuration of the optical splitter 21 is the same as that of the optical coupler 20 shown in FIG. 5, but the optical input / output port P ₀ is connected to the light emitting end of the amplification optical fiber 10. On the other hand, any one of the optical input / output ports P _{1 to} P ₇ is optically connected to the excitation light source 33 via the optical fiber 34, and another port is a transmission light at point A. The light incident end of the fiber 11 is fusion-connected.

  With the above configuration, backward pumping is performed in the optical fiber laser 300 according to the third embodiment. In the optical fiber laser 300 according to the third embodiment, the modulator 51 can implement any of the pulse modulation methods according to the first to fourth embodiments described above.

(Fourth Embodiment of Optical Fiber Laser)
Furthermore, FIG. 18 is a figure which shows the structure of 4th Embodiment of the optical fiber laser based on this invention. The optical fiber laser 400 according to the fourth embodiment is the same as the first embodiment regarding the configuration in which the seed light source 41 is directly modulated by the modulator 51, but the excitation method is different. That is, the structural difference between the optical fiber laser 400 according to the fourth embodiment and the optical fiber laser 100 according to the first embodiment is a configuration in which the optical fiber laser 100 according to the first embodiment performs forward pumping. On the other hand, the optical fiber laser 400 according to the fourth embodiment is configured to perform bidirectional pumping.

Specifically, the optical fiber laser 400 according to the fourth embodiment shown in FIG.
Amplifying optical fiber 10, optical coupler 20, optical splitter 21, pumping light sources 31, 33, optical fibers 32, 34, seed light source 41, optical fiber 42, modulator 51, modulation voltage generator 55, electrical signal line 52 , An optical isolator 61, a transmission optical fiber 11, and a light emitting end 70.

In the optical fiber laser 300 according to the fourth embodiment, the optical coupler 20 is disposed on the light incident end side of the amplification optical fiber 10, while the optical splitter is disposed on the light output end side of the amplification optical fiber 10. 21 is arranged. The light input / output port P ₀ of the optical coupler 20 is fusion-bonded to the light incident end of the amplification optical fiber 10 at point B. _One of the optical input / output ports P _{1 to} P ₇ of the optical coupler 20 is optically connected to the light emitting end of the optical isolator 61, while the other port is excited via the optical fiber 32. Optically connected to the light source 31. The optical input / output port P ₀ of the optical branching unit 21 is fusion-connected to the light emitting end of the amplification optical fiber 10 at point C. _One of the optical input / output ports P _{1 to} P ₇ of the optical splitter 21 is fusion-connected to the light incident end of the transmission optical fiber 11 at point A, while the other port is the optical fiber 34. And is optically connected to the excitation light source 33.

  With the above configuration, bidirectional excitation is performed in the optical fiber laser 400 according to the fourth embodiment. In the optical fiber laser 400 according to the fourth embodiment, the modulator 51 can implement any of the pulse modulation methods according to the first to fourth embodiments described above.

  From the above description of the present invention, it is apparent that the present invention can be modified in various ways. Such modifications cannot be construed as departing from the spirit and scope of the invention, and modifications obvious to one skilled in the art are intended to be included within the scope of the following claims.

It is a figure which shows the structure of the optical fiber laser (1st Embodiment of the optical fiber laser which concerns on this invention) which can apply the pulse modulation method which concerns on this invention. It is the figure which shows the cross-section of an optical fiber for amplification, and its refractive index profile. It is a graph which shows the wavelength dependence of each absorption cross section of an optical fiber for amplification, and emission cross section. It is the figure which shows the cross-section of the optical fiber for transmission, and its refractive index profile. It is a figure for demonstrating the structure of an optical coupler. It is a figure for demonstrating the pulse modulation method which concerns on a comparative example. It is the normalized spectrum of the light pulse pulse-modulated by the pulse modulation method according to the comparative example as the seed light and the CW light applied as the seed light. It is a figure for demonstrating 1st Embodiment of the pulse modulation method which concerns on this invention. FIG. 4 is a normalized spectrum of an optical pulse that is pulse-modulated by the pulse modulation method according to the first embodiment as seed light and an optical pulse that is pulse-modulated by the pulse modulation method according to the comparative example as seed light. FIG. It is a figure for demonstrating the relationship between the signal width and the standardized full width at half maximum of a seed light pulse about one pulse component in a modulation pattern. It is a graph which shows the relationship between the waveform of a pulse-modulated optical pulse, and a modulation pattern. It is a figure for demonstrating 2nd Embodiment of the pulse modulation method which concerns on this invention. It is a graph which shows the light reception level of the optical pulse pulse-modulated by the pulse modulation method which concerns on 2nd Embodiment. It is a figure for demonstrating 3rd Embodiment of the pulse modulation method which concerns on this invention. It is a figure for demonstrating 4th Embodiment of the pulse modulation method which concerns on this invention. It is a figure which shows the structure of 2nd Embodiment of the optical fiber laser which concerns on this invention. It is a figure which shows the structure of 3rd Embodiment of the optical fiber laser which concerns on this invention. It is a figure which shows the structure of 4th Embodiment of the optical fiber laser which concerns on this invention.

  DESCRIPTION OF SYMBOLS 100, 200, 300, 400 ... Optical fiber laser, 41 ... Seed light source (LD), 10 ... Optical fiber for amplification, 11 ... Optical fiber for transmission, 31, 33 ... Excitation light source, 50 ... AOM, 51 ... Modulator, 55 ... modulation voltage generator, 500 ... pulse modulation device, E ... modulation voltage.

Claims (6)

A seed light source,
It is for modulating the amplified light output from or output from the seed light source into an optical pulse having a predetermined period, and is electrically connected to the seed light source or output from the seed light source A modulator provided on the optical path of the emitted light;
An optical fiber laser having an optical fiber amplifier that amplifies and outputs the amplified light modulated into an optical pulse,
The modulation pattern, which is a modulation voltage pattern input to the modulator, includes an electric signal pattern that generates one optical pulse within one modulation period , and a plurality of pulse components. optical fiber laser in which the individual pulse width is equal to or smaller than the pulse width of the one light pulse. Wherein the plurality of pulse components, the optical fiber laser according to claim 1, wherein the individual pulse width and wherein less than half that of the pulse width of one optical pulse to be the product. 3. The optical fiber laser according to claim 1, wherein each of the plurality of pulse components has an individual pulse width longer than an interval between adjacent pulse components. Wherein the plurality of pulse components, the spacing between pulses of the pulse components adjacent, claim 1-3, characterized in that at most one of rising time and falling of the pulse component adjacent time The optical fiber laser according to one item . Wherein the plurality of individual pulse peak value of the pulse components, according to claim 1-4 or the optical fiber laser of one claim of, wherein each different. The seed the drive current of the light source is modulated, wherein the plurality of one optical fiber laser of one of claims 1-5, characterized by adjusting the peak values of the pulse component.

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