JP5355991B2 - Pulse light source and pulse compression method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pulse light source which has a MOPA structure using a directly-modulated semiconductor laser as a seed light source and is capable of easily outputting pulse light with a sub-nanosecond pulse width. <P>SOLUTION: The pulse light source 1 comprises a seed light source 10, a YbDF (Yb-Doped Fiber) 20, a band-pass filter 30, a YbDF 40, a YbDF 50, etc., and has the MOPA structure. The band-pass filter 30 inputs pulse light which is output from the seed light source 10 and amplified by the first stage YbDF 20, and attenuates the optical power on one of the shorter wavelength side and the longer wavelength side of the input pulse light more than that on the other and then outputs the attenuated pulse light. The YbDF 40 and the YbDF 50 amplify the pulse light output from the band-pass filter 30 and output the amplified pulse light. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a pulse light source and a pulse compression method.

  The pulsed light source is used for industrial applications typified by processing and the like, and there is a tendency that a high output and a short pulse are desired. In particular, a pulse light source used in a laser processing machine for performing fine processing is desired to have a high peak value as much as possible and to have a narrow pulse width for the purpose of reducing the thermal influence on the processing target.

  As a mechanism for generating pulsed light, a Q switch, a mode lock, and the like have been proposed for gas laser light sources and solid laser light sources (see Non-Patent Document 1). A gain switching method using a semiconductor laser is also attracting attention as a more convenient method. In short, since gain switching is realized by direct modulation of a semiconductor laser, the period between pulses is not restricted by the laser's hardware structure as in mode lock, and it does not occur as in an acousto-optic switch like a Q switch. It does not require expensive parts that consume large amounts of power.

However, semiconductor lasers are generally used for communication and measurement because their output light power is lower than conventional laser light sources such as gas laser light sources and solid-state laser light sources, so pulse peak power is required. (See Non-Patent Documents 1 and 2).
Ultrafast optical technology, Chapter 2, Maruzen, issued on March 15, 1990 M. Kakui, et al., Optical Fiber Technology, vol.1, pp.312-317, 1995. FDTeodoro, et al, PhotonicWest2005. J. Limpert, et al., Optics Express, vol.11, p.3332, 2003.

  Recently, a pulse light source with a MOPA (Master Oscillator Power Amplifier) structure that combines a directly modulated semiconductor laser and an optical amplifier (especially an optical fiber amplifier) is used for applications that require high power of 1 kW or more, such as laser processing. (See Non-Patent Document 3). In such a case, it is desirable that the pulse peak of the output light of the semiconductor laser is high in order to reduce the required gain of the optical fiber amplification part as much as possible. That is, it is desired that the amplitude of the modulation current is large.

  However, it is not easy to modulate a current of several hundred mA, and the rise time and the fall time are limited to about several ns (see Non-Patent Document 3). On the other hand, there is a request for a pulse width of less than 1 ns depending on the application, and for example, the pulse width may be required to be in the femtosecond order (see Non-Patent Document 4). However, in order to generate femtosecond pulsed light, a special optical amplification technique such as CPA must be used. Further, since the pulse energy is small, there is a problem of low throughput during laser processing.

  From the above, the inventors found that it was difficult to reduce the width on the time axis in a state where the width on the frequency axis was not widened by a simple method with the current fiber laser technology. thinking.

  The present invention has been made to solve the above problems, and is a pulse light source having a MOPA structure using a directly modulated semiconductor laser having a modulation amplitude exceeding 200 mA as a seed light source. It is an object of the present invention to provide a pulse light source and a pulse compression method that can easily output pulsed light having a pulse width.

The pulsed light source according to the present invention includes (1) a semiconductor laser that is directly modulated and outputs pulsed light, and (2) pulsed light that is output from the semiconductor laser is input, out of the wavelength band of the input pulsed light A first optical filter for selectively outputting a chirped component of the pulsed light by attenuating one of the shorter wavelength side and the longer wavelength side of the peak wavelength of the pulsed light from the other, and (3) outputting from the first optical filter And an optical amplifier that amplifies and outputs the pulsed light. This pulse light source has a MOPA structure, and pulse light output from a directly modulated semiconductor laser attenuates one of the short wavelength side and long wavelength side of the wavelength band from the other by the first optical filter. Is amplified and output by an optical amplifier.

In the pulse light source according to the present invention, it is preferable that the first optical filter attenuates and outputs the shorter wavelength side of the peak wavelength of the pulsed light in the wavelength band of the input pulsed light from the longer wavelength side. The transmission characteristic of the first optical filter is preferably variable. It is preferable to provide a second optical filter disposed so as to sandwich the optical amplification medium included in the optical amplifier together with the first optical filter. The first optical filter is preferably a band pass filter. It is preferable that at least one of the first optical filter and the second optical filter is a band pass filter. Preferably, both the first optical filter and the second optical filter are band pass filters, and the full width at half maximum of the transmission spectrum of the second optical filter is wider than the full width at half maximum of the transmission spectrum of the first optical filter. Both the first optical filter and the second optical filter are bandpass filters, and the center wavelength of the second optical filter is set between the peak wavelength of the pulsed light and the center wavelength of the transmission spectrum of the first optical filter. Is preferred. The semiconductor laser is preferably a Fabry-Perot type. It is preferable to provide temperature adjusting means for adjusting the temperature of the semiconductor laser. It is preferable that the pulse width of the pulsed light output from the optical amplifier is less than 1 ns. The peak power of the pulsed light output from the optical amplifier is preferably over 1 kW. When the repetition frequency is 1 MHz, the peak power of the pulsed light is preferably more than 10 kW.

The pulse compression method according to the present invention includes (1) a semiconductor laser that directly modulates and outputs pulsed light, and (2) pulsed light output from the semiconductor laser is input, and part of the input pulsed light is input. A first optical filter that transmits the wavelength band and (3) an optical amplifier that amplifies and outputs the pulsed light output from the first optical filter are used. The pulse compression method according to the present invention is shorter than the peak wavelength of the pulsed light in the semiconductor laser output spectrum by adjusting the relative positional relationship between the transmission wavelength range of the first optical filter and the output spectrum of the semiconductor laser. One of the wavelength side and the long wavelength side is attenuated from the other, and the chirping component of the pulsed light is selectively output from the first optical filter .

In the pulse compression method according to the present invention, it is preferable that the first optical filter attenuates the short wavelength side of the peak wavelength of the pulsed light in the wavelength band of the input pulsed light from the long wavelength side and outputs it. The transmission characteristic of the first optical filter is preferably variable. It is preferable to arrange the second optical filter so as to sandwich the optical amplification medium included in the optical amplifier together with the first optical filter. The first optical filter is preferably a band pass filter. It is preferable that at least one of the first optical filter and the second optical filter is a band pass filter. Preferably, both the first optical filter and the second optical filter are band pass filters, and the full width at half maximum of the transmission spectrum of the second optical filter is wider than the full width at half maximum of the transmission spectrum of the first optical filter. The semiconductor laser is preferably a Fabry-Perot type. Both the first optical filter and the second optical filter are bandpass filters, and the center wavelength of the second optical filter is set between the peak wavelength of the pulsed light and the center wavelength of the transmission spectrum of the first optical filter. Is preferred. In addition, it is preferable to provide temperature adjusting means for adjusting the temperature of the semiconductor laser.

  The pulse light source according to the present invention is a pulse light source having a MOPA structure using a directly modulated semiconductor laser having a modulation amplitude exceeding 200 mA as a seed light source, and easily outputs pulse light having a sub-nanosecond pulse width. can do.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a configuration diagram of a pulse light source 1 according to the present embodiment. The pulse light source 1 shown in this figure includes a seed light source 10, a YbDF (Yb-Doped Fiber) 20, a band pass filter 30, a YbDF 40, a YbDF 50, and the like, and has a MOPA structure. This pulsed light source 1 outputs pulsed light having a wavelength near 1060 nm, which is suitable for laser processing.

  The seed light source 10 includes a semiconductor laser that is directly modulated and outputs pulsed light. This semiconductor laser is preferably of the Fabry-Perot type from the viewpoint of increasing power and from the viewpoint of avoiding nonlinear effects such as stimulated Brillouin scattering (SBS). In addition, this semiconductor laser outputs pulsed light having a wavelength of about 1060 nm at which the amplification optical fibers YbDF 20, 40, and 50 have a gain. YbDF 20, 40, and 50 are obtained by adding a Yb element as an active material to the core of an optical fiber mainly composed of quartz glass, and the pumping light wavelength and the amplified light wavelength are close to each other, so that the power conversion efficiency is high. It is advantageous in that it has a high gain in the vicinity of a wavelength of 1060 nm. These YbDF 20, 40, 50 constitute a three-stage optical fiber amplifier.

  The first-stage YbDF 20 is supplied with pumping light output from the pumping light source 22 and passed through the optical coupler 21 in the forward direction. The YbDF 20 receives the pulsed light output from the seed light source 10 and passed through the optical isolator 23 and the optical coupler 21, amplifies the pulsed light, and outputs the pulsed light through the optical isolator 24.

  The band pass filter 30 receives the pulsed light output from the seed light source 10 and amplified by the first-stage YbDF 20, and has a shorter wavelength side and a longer wavelength than the peak wavelength of the pulsed light in the wavelength band of the input pulsed light. One of the wavelengths is attenuated from the other and output. A high-pass filter or a low-pass filter may be used instead of the band-pass filter, but the high-pass filter can cut out only the long wavelength side of the seed light source spectrum, and the low-pass filter cut out only the short wavelength side of the seed light source spectrum. I can't. A band-pass filter is suitable because it has both functions.

  The second stage YbDF 40 is supplied with pumping light output from the pumping light source 42 and passed through the optical coupler 41 in the forward direction. The YbDF 40 receives the pulsed light output from the bandpass filter 30 and passed through the optical isolator 43 and the optical coupler 41, amplifies the pulsed light, and outputs the pulsed light through the optical isolator 44. The third stage YbDF 50 is supplied with pumping light output from the pumping light sources 52 to 55 and passing through the combiner 51 in the forward direction. The YbDF 50 receives the pulse light amplified by the second stage YbDF 40, further amplifies the light, and outputs the pulse light to the outside through the end cap 60.

  A more preferable configuration example is as follows. The first stage YbDF 20 is a core pumping system, and pumping light with a pumping wavelength of 975 nm and a constant power of 200 mW is injected in the forward direction. As YbDF20, one having an unsaturated absorption coefficient at a wavelength of 975 nm of 240 dB / m and a length of 5 m is used. The core diameter of YbDF20 is 6 μm, and the NA is about 0.12. The second stage YbDF 40 is a core pumping system, and pumping light having a pumping wavelength of 975 nm and a constant power of 200 mW is injected in the forward direction. As the YbDF 40, one having a unsaturated absorption coefficient at a wavelength of 975 nm of 240 dB / m and a length of 7 m is used. The core diameter of YbDF40 is 6 μm, and the NA is about 0.12. The third-stage YbDF 50 is a cladding excitation method, and a power of 20 W (four 5 W class excitation LDs) is injected in the forward direction at an excitation wavelength of 975 nm. As the YbDF50, a core portion having an unsaturated absorption coefficient of 1200 dB / m and a length of 5 m is used. The core of YbDF50 has a diameter of 10 μm and an NA of about 0.06. The inner cladding of YbDF50 has a diameter of 125 μm and an NA of about 0.46.

  FIG. 2 is a diagram showing a modulation voltage waveform at the time of direct modulation with respect to the semiconductor laser. FIG. 3 is a diagram showing a time waveform of pulsed light output from the semiconductor laser. In FIG. 2, the repetition frequency is set to 100 kHz. The maximum current amplitude that can be modulated by the rise time and fall time of the modulation voltage waveform shown in FIG. 2 is 240 mA. In the time waveform of the pulsed light output from the semiconductor laser to which this modulation signal is applied, as shown in FIG. 3, the half-value width of the overshoot is less than 1 ns, but there are pulse components exceeding 10 ns thereafter. This causes thermal effects during laser processing. On the other hand, it is difficult to maintain a current amplitude of 240 mA by setting the width of the modulation signal to 1 ns or less.

  If the semiconductor laser is of a single wavelength oscillation such as DFB, the relationship between the optical output P and the optical frequency change Δν (so-called chirping) is given by the following equation. Where α is the line width enhancement factor, Γ is the active layer confinement factor, ε is the nonlinear gain coefficient, V is the active layer volume, h is the Planck constant, and ν is the seed light source output. Is the optical frequency.

  FIG. 4 is a diagram illustrating a temporal waveform of the pulsed light output from the semiconductor laser and a temporal change in chirping. As can be seen from the above equation (1), the chirping Δν depends on the time differentiation of the optical output power P. Therefore, as shown in this figure, the substantially maximum chirping occurs when the change in the optical output power P is the steepest. Since the most significant change in the optical output is at the time of overshoot at the rising edge, chirping generally occurs at a high frequency (that is, on the short wavelength side), and even with a Fabry-Perot type semiconductor laser, the short wavelength The spectrum shape has a large tailing to the side.

  Here, when the center wavelength of the bandpass filter 30 included in the pulse light source 1 shown in FIG. 1 is intentionally shifted from the maximum intensity wavelength of the output light spectrum of the seed light source 10 to the short wavelength side or the long wavelength side, FIG. Only the chirping component shown in FIG. 3 can be cut out, and the pulse exceeding 10 ns after the overshoot shown in FIG. 3 can be removed. However, this method naturally also blocks most components of the pulse output of the seed light source 10 by the band pass filter 30, so that YbDFs 20, 40, and 50 are provided as optical amplifying units to compensate for this. . The optical fiber amplifying unit including these YbDFs 20, 40, and 50 has a sufficient gain and can exhibit a gain of several tens dB.

  FIG. 5 is a diagram illustrating a time waveform of the pulsed light output from the pulsed light source 1 according to the present embodiment. Here, the transmission wavelength band is adjusted by using a band-pass filter 20 having a variable transmission wavelength band. In the figure, a time waveform A1 is obtained when the transmission wavelength band of the bandpass filter 20 is set to the longest wavelength side as much as possible and the pulse peak is maximized. The time waveform A2 is obtained when the transmission wavelength band of the bandpass filter 20 is set as short as possible and the pulse peak is maximized. The time waveform A3 is obtained when the transmission wavelength band of the bandpass filter 20 is superimposed on the output spectrum of the seed light source 10 as much as possible.

  As shown in this figure, the pulse peak rather increases due to the effect of the YbDF 20, 40, 50, which is an optical fiber amplifier. However, if the center wavelength of the bandpass filter 30 is shifted too much with respect to the seed light source spectrum, it is a matter of course that light does not pass through the bandpass filter 30. The time waveforms A1 and A2 in FIG. 5 are results when the center wavelength of the transmission wavelength band of the bandpass filter 30 is adjusted to the long wavelength side or the short wavelength side so that the pulse peak becomes maximum. The pulse peak is about 10% when the center wavelength of the transmission wavelength band of the bandpass filter 30 is matched with the long wavelength side of the seed light source spectrum compared with when the center wavelength is matched with the short wavelength side. It is expected to be high and have a steep fall of the pulse, so that the thermal effect during laser processing can be suppressed.

  Although the transient response characteristics of the semiconductor laser vary widely from sample to sample, even when a semiconductor laser that does not cause overshoot at all is used as the seed light source 10, the results shown in FIG. 6 are obtained. A better result was obtained when the center wavelength of the filter 30 was matched with the longer wavelength side of the seed light source spectrum. FIG. 6 is a diagram showing a time waveform of pulsed light output from the pulsed light source 1 when a semiconductor laser that does not cause overshoot at all is used as the seed light source 10. In the figure, a time waveform B1 is obtained when the transmission wavelength band of the band-pass filter 20 is set as long as possible and the pulse peak is maximized. Time waveforms B2 to B6 are obtained by gradually shifting the transmission wavelength band of the bandpass filter 20 to the short wavelength side with respect to the case of the time waveform B1. As can be seen from this figure, a better result was obtained when the center wavelength of the band pal filter 30 was matched with the long wavelength side of the seed light source spectrum.

  In the above description, the method of changing the transmission spectrum of the bandpass filter 30 to adjust the relative positional relationship between the seed light source spectrum and the transmission spectrum of the bandpass filter 30 has been described. However, almost the same effect can be expected. While adjustment of the band pal filter 30 requires mechanical adjustment such as angle adjustment of the dielectric multilayer film, the temperature adjustment of the seed light source 10 can be performed only by electronic control, so that it is excellent in terms of reproducibility and controllability. Yes.

  As described above, the pulse light source 1 according to the present embodiment is a pulse light source having a MOPA structure using a directly modulated semiconductor laser having a modulation amplitude exceeding 200 mA as a seed light source, and has a sub-nanosecond pulse width. The pulsed light can be easily output. In particular, the pulse light source 1 according to the present embodiment can output pulsed light having a peak power exceeding 1 kW, and can output pulsed light having a pulse width of less than 1 ns. It is suitable for. Further, since the pulsed light source 1 according to the present embodiment directly modulates the semiconductor laser included in the seed light source 10, the degree of freedom of modulation is higher than that of the mode lock.

  Incidentally, in the pulse light source 1 having the configuration shown in FIG. 1, a Fabry-Perot type semiconductor laser is used as the seed light source 10. For shortening the pulse, as shown in FIGS. 7A and 7B, the center wavelength of the bandpass filter 30 provided at the rear stage of the seed light source 10 is in the state of the plot C2 or C3 in the figure. The full width at half maximum of the pulse can be compressed from about 5 ns to 0.5 ns.

  FIG. 4A shows a pulse waveform when the output pulse of the seed light source 10 is deformed by adjusting the center wavelength of the bandpass filter 30 provided at the subsequent stage of the seed light source 10. FIG. 5B shows the spectrum in that case. FIG. 2C is an enlarged view of a part of FIG. The plot C1 in the figure shows the case where there is no bandpass filter. Plots C2 to C7 show the case where the center wavelength of the bandpass filter is gradually changed from the long wavelength side to the short wavelength side.

  If the center wavelength of the band-pass filter 30 is significantly detuned from the center wavelength of the spectrum of the seed light source 10 as in plots C2 and C3, the ASE generated in the downstream YbDF increases. In order to suppress such an ASE component, as shown in FIG. 8, it is desirable to insert a plurality of bandpass filters inside an optical amplifier connected downstream of the seed light source.

  FIG. 8 is a configuration diagram of a pulse light source 2 according to another embodiment. The pulse light source 2 shown in this figure includes a seed light source 10, a YbDF 110, a band pass filter 120, a YbDF 130, a band pass filter 140, a YbDF 150, a YbDF 160, and the like, and has a MOPA structure. The pulsed light source 2 outputs pulsed light having a wavelength of about 1060 nm that is suitable for laser processing.

  YbDFs 110, 130, 150, and 160 amplify pulse light having a wavelength of around 1060 nm output from the seed light source 10, and Yb element is added as an active substance to the core of an optical fiber made of glass. YbDFs 110, 130, 150, and 160 are advantageous in that the pumping light wavelength and the amplified light wavelength are close to each other and are advantageous in terms of power conversion efficiency, and are advantageous in that they have a high gain in the vicinity of a wavelength of 1060 nm. These YbDFs 110, 130, 150, and 160 constitute a four-stage optical fiber amplifier.

  The first-stage YbDF 110 is supplied with pumping light output from the pumping light source 112 and passing through the optical coupler 113 and the optical coupler 111 in the forward direction. The YbDF 110 receives the pulsed light output from the seed light source 10 and passed through the optical isolator 114 and the optical coupler 111, amplifies the pulsed light, and outputs the pulsed light through the optical isolator 115.

  The band pass filter 120 receives the pulse light amplified by the first stage YbDF 110 and passed through the optical isolator 115, and selects one of the short wavelength side and the long wavelength side of the wavelength band of the input pulse light from the other. Attenuate and output.

  The second stage YbDF 130 is supplied with pumping light output from the pumping light source 112 and passing through the optical coupler 113 and the optical coupler 131 in the forward direction. The YbDF 130 receives the pulsed light output from the bandpass filter 120 and passed through the optical isolator 131, amplifies the pulsed light, and outputs the pulsed light.

  The bandpass filter 140 receives the pulsed light amplified by the second stage YbDF 130 and attenuates one of the short wavelength side and the long wavelength side of the wavelength band of the inputted pulsed light from the other and outputs the attenuated one. .

  The third stage YbDF 150 is supplied with pumping light output from the pumping light source 152 and passed through the optical coupler 151 in the forward direction. The YbDF 150 receives the pulsed light output from the bandpass filter 140 and passed through the optical isolator 153, amplifies the pulsed light, and outputs the pulsed light.

  The fourth stage YbDF 160 is supplied with pumping light output from the pumping light sources 162 to 166 and passing through the combiner 161 in the forward direction. The YbDF 160 receives the pulse light amplified by the third stage YbDF 150 and passed through the optical isolator 167 and the combiner 161, further amplifies it, and outputs the pulse light to the outside through the end cap 60.

  A more preferable configuration example is as follows. The first stage YbDF110 is a single clad Al co-doped silica-based YbDF, the Al concentration is 5 wt%, the core diameter is 10 μm, the clad diameter is 125 μm, and the 915 nm band excitation light unsaturated absorption is 70 dB. / M, the excitation light unsaturated absorption peak at 975 nm band is 240 dB / m, and the length is 7 m. The second stage YbDF 130 is a single clad Al co-doped silica-based YbDF, the Al concentration is 5 wt%, the core diameter is 10 μm, the clad diameter is 125 μm, and the 915 nm band excitation light unsaturated absorption is 70 dB. / M, the excitation light unsaturated absorption peak at 975 nm band is 240 dB / m, and the length is 7 m.

  The third stage YbDF150 is a double clad phosphate glass-based YbDF, the P concentration is 26.4 wt%, the Al concentration is 0.8 wt%, the core diameter is 10 μm, and the first cladding diameter is Is about 125 μm, the cross section of the first cladding is octagonal, the 915 nm band excitation light unsaturated absorption is 1.8 dB / m, and the length is 3 m. The fourth stage YbDF160 is a double clad Al co-doped silica-based YbDF, the Al concentration is 5 wt%, the core diameter is 10 μm, the clad diameter is 125 μm, and the 915 nm band excitation light unsaturated absorption is 80 dB. / M and a length of 3.5 m.

  The wavelengths of the excitation light supplied to the YbDFs 110, 130, 150, and 160 are all in the 0.975 μm band. The excitation light supplied to the YbDF 110 has a power of 200 mW and is a single mode. The excitation light supplied to the YbDF 130 has a power of 200 mW and is a single mode. The excitation light supplied to the YbDF 150 has a power of 2 W and is multimode. The excitation light supplied to the YbDF 160 has a power of 14 W and is multimode.

  The full width at half maximum of the transmission spectrum of each of the bandpass filters 120 and 140 is 3 nm. FIGS. 9 and 10 are diagrams schematically showing how the ASE is removed by the band-pass filters 120 and 140 in the pulse light source 2 according to the present embodiment.

  As shown in FIG. 9, when the center wavelength of the transmission spectrum (D1 in the figure) of the bandpass filter 120 substantially matches the peak wavelength of the spectrum of the output light of the seed light source 10 (D2 in the figure). Can keep the power of light (D3 in the figure) output from the bandpass filter 120 high, and is compared with the ASE component included in the light (D4 in the figure) output from the YbDF 130 in the subsequent stage. The S / N ratio can be kept high.

  On the other hand, as shown in FIG. 10, the center wavelength of the transmission spectrum (E1 in the figure) of the bandpass filter 120 is greatly shifted from the peak wavelength of the spectrum of the output light from the seed light source 10 (E2 in the figure). In this case, the power of light (E3 in the figure) output from the bandpass filter 120 is greatly attenuated with respect to the input time, and light output from the YbDF 130 in the subsequent stage (in the figure) The S / N ratio compared with the ASE component contained in E4) is greatly deteriorated. In order to avoid this, the S / N ratio of the light (E5 in the figure) output from the bandpass filter 120 can be improved by further inserting the bandpass filter 140 downstream thereof. At this time, the center wavelength of the bandpass filter 140 is preferably set closer to the peak wavelength of the output spectrum of the seed light source 10 than the center wavelength of the bandpass filter 120.

  In the pulse light source 2 shown in FIG. 8, when the seed light source 10 is continuously oscillated at an output power of 20 dBm, the power of the pulse light output from the end cap 60 is 10.7 W as a specific configuration as described above. is there. When the center wavelength of the bandpass filter 120 is shifted from the peak wavelength of the output spectrum of the seed light source 10 to the long wavelength side as indicated by plot C2 in FIG. 7, the time average input of the input light to the second stage YbDF 130 The power decreases, and the ASE output from the second stage YbDF 130 increases. As a result, immediately after the second stage YbDF 130, the ratio of the ASE power to the amplified optical output power (hereinafter referred to as the S / N ratio for the sake of convenience) decreases.

  Since the band pass filter 140 is provided at the subsequent stage of the second stage YbDF 130, and the phosphate glass system YbDF having a narrow ASE band is used as the third stage YbDF 150, FIG. 11 and FIG. The S / N ratio can be improved as shown in FIG.

  FIG. 11 is a diagram showing a configuration of an ASE light source 3 using YdDF150 or YbDF160. The ASE light source 3 outputs 7 W of excitation light from each of the six excitation light sources 213 to 218, and supplies the excitation light to the YbDF 210 via the combiner 211. As the YbDF 210, YdDF150 (8m) or YbDF160 (10) is used. The combiner 211 has a pumping light input port composed of six multimode fibers, an amplified light input port composed of one single mode fiber, and an output port composed of one double clad fiber. The isolator 219 reduces the backflow of light at a wavelength of 1060 nm.

  FIG. 12 is a diagram showing a spectrum of output light from the ASE light source 3 using YdDF150 or YbDF160. In this figure, the spectrum when the input optical power to the ASE light source 3 is zero and YbDF150 is used as YbDF210 is shown by plot F1, and the spectrum when YbDF160 is used as YbDF210 is shown by plot F2. ing. As shown in this figure, the S / N ratio can be improved by using phosphate glass YbDF as the third stage YbDF150.

  FIG. 13 is a diagram illustrating a pulse waveform of the output pulse light of the pulse light source 2 according to the present embodiment. In the figure, the repetition frequency has values of 100 kHz, 166.7 kHz, 200 kHz, 312.5 kHz, 500 kHz, 1 MHz, and 2.5 MHz. FIG. 14 is a diagram showing the relationship between the repetition frequency of the output pulsed light and the pulse peak of the pulse light source 2 according to this embodiment. When measuring the pulse waveform, a spatial attenuator with an attenuation of about 65 dB is inserted after the end cap 60 at the output end of the pulse light source 2, and the output light from the end cap 60 is converted into a photoelectric conversion module (SIR5-FC) manufactured by Sorabo. Type) and the electric output waveform from the photoelectric conversion module was observed with an oscilloscope (DL9240) manufactured by Yokogawa Electric Corporation.

  As shown in these figures, a pulse peak of 10 kW or more can be realized even when the repetition frequency of the output pulse light is 1 MHz, and a pulse peak of 56 kW can be realized when the repetition frequency of the output pulse light is 100 kHz. It was. It should be noted that the phosphate glass system YbDF as the third stage YbDF 150 is inferior in excitation efficiency (ratio of the amplified light output obtained at a specific excitation power) as compared with the fourth stage YbDF 160. Thus, in the configuration shown in FIG. 8, an Al co-doped quartz-based YbDF is used as the final stage YbDF 160.

  The center wavelength of the bandpass filter 140 is preferably set closer to the peak wavelength of the output spectrum of the seed light source 10 than the center wavelength of the bandpass filter 120. The full width at half maximum of the transmission spectrum of each of the bandpass filters 120 and 140 is set to 3 nm. However, the spectrum of the amplified light after passing through the bandpass filter 120 is generally the peak wavelength of the output of the seed light source 10 and the bandpass filter 120. As a result of the shift from the center wavelength, the bandpass filter 120 has a wider band than the original transmission spectrum. Therefore, the full width at half maximum of the transmission spectrum of the bandpass filter 140 is preferably wider than the full width at half maximum of the transmission spectrum of the bandpass filter 120. .

  However, as shown in the plot C2 in FIG. 7B, the full width at half maximum of the transmission band of the original bandpass filter is 3 nm, and the full width at half maximum of the spectrum after transmission is 4 nm. It is about. Therefore, the full width at half maximum of the transmission spectrum of the bandpass filter 140 is desirably about 1.5 times the transmission spectrum of the bandpass filter 120.

  Further, the band-pass filter does not necessarily have two stages, and may have three or more stages. Further, the band-pass filter is not necessarily required, and a short wavelength side transmission filter (SWPF) or a long wavelength side transmission filter (LWPF) may be used in combination.

It is a lineblock diagram of pulse light source 1 concerning this embodiment. It is a figure which shows the modulation voltage waveform at the time of the direct modulation with respect to a semiconductor laser. It is a figure which shows the time waveform of the pulsed light output from a semiconductor laser. It is a figure which shows the time waveform of the pulsed light output from a semiconductor laser, and the time change of chirping. It is a figure which shows the time waveform of the pulsed light output from the pulse light source 1 which concerns on this embodiment. It is a figure which shows the time waveform of the pulsed light output from the pulse light source 1 which concerns on this embodiment. It is a figure which shows the pulse waveform and spectrum at the time of deform | transforming the output pulse of the seed light source 10 by adjusting the center wavelength of the band pass filter 30 provided in the back | latter stage of the seed light source 10. FIG. It is a block diagram of the pulse light source 2 which concerns on this embodiment. It is a figure which shows typically the mode of ASE removal by the band pass filters 120 and 140 in the pulse light source 2 which concerns on this embodiment. It is a figure which shows typically the mode of ASE removal by the band pass filters 120 and 140 in the pulse light source 2 which concerns on this embodiment. It is a figure which shows the structure of the ASE light source 3 using YdDF150 or YbDF160. It is a figure which shows the spectrum of the output light of the ASE light source 3 using YdDF150 or YbDF160. It is a figure which shows the pulse waveform of the output pulse light of the pulse light source 2 which concerns on this embodiment. It is a figure which shows the relationship between the repetition frequency and pulse peak of the output pulsed light of the pulse light source 2 which concern on this embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2 ... Pulse light source, 10 ... Seed light source, 20 ... YbDF, 21 ... Optical coupler, 22 ... Excitation light source, 23, 24 ... Optical isolator, 30 ... Band pass filter, 40 ... YbDF, 41 ... Optical coupler, 42 ... Excitation light source, 43, 44 ... optical isolator, 50 ... YbDF, 51 ... combiner, 52-55 ... excitation light source, 60 ... end cap, 110 ... YbDF, 111 ... optical coupler, 112 ... excitation light source, 113 ... optical coupler, 114 , 115 ... optical isolator, 120 ... band pass filter, 130 ... YbDF, 131 ... optical coupler, 140 ... band pass filter, 150 ... YbDF, 151 ... optical coupler, 152 ... excitation light source, 153 ... optical coupler, 160 ... YbDF, 161: Combiner, 162-166: Excitation light source, 167: Optical isolator.

Claims (23)

A semiconductor laser that is directly modulated and outputs pulsed light;
The pulsed light output from the semiconductor laser is input, one of the shorter wavelength side and the longer wavelength side than the peak wavelength of the pulsed light in the wavelength band of the input pulsed light is attenuated from the other, and the pulse A first optical filter that selectively outputs a chirping component of light;
An optical amplifier that amplifies and outputs the pulsed light output from the first optical filter.   2. The pulse light source according to claim 1, wherein the first optical filter attenuates and outputs a shorter wavelength side than a peak wavelength of the pulsed light in a wavelength band of input pulsed light.   The pulse light source according to claim 1, wherein a transmission characteristic of the first optical filter is variable. The pulse light source according to claim 1, further comprising a second optical filter disposed so as to sandwich an optical amplification medium included in the optical amplifier together with the first optical filter.   The pulse light source according to claim 1, wherein the first optical filter is a band-pass filter.   The pulse light source according to claim 4, wherein at least one of the first optical filter and the second optical filter is a band-pass filter. Both the first optical filter and the second optical filter are bandpass filters,
The pulse light source according to claim 4, wherein the full width at half maximum of the transmission spectrum of the second optical filter is wider than the full width at half maximum of the transmission spectrum of the first optical filter. Both the first optical filter and the second optical filter are bandpass filters,
5. The pulse light source according to claim 4, wherein a center wavelength of the second optical filter is set between a peak wavelength of the pulsed light and a center wavelength of a transmission spectrum of the first optical filter.   2. The pulse light source according to claim 1, wherein the semiconductor laser is of a Fabry-Perot type.   2. The pulse light source according to claim 1, further comprising temperature adjusting means for adjusting the temperature of the semiconductor laser.   The pulse light source according to claim 1, wherein the pulse width of the pulsed light output from the optical amplifier is less than 1 ns.   2. The pulse light source according to claim 1, wherein the peak power of the pulsed light output from the optical amplifier exceeds 1 kW.   13. The pulse light source according to claim 12, wherein the peak power of the pulsed light exceeds 10 kW when the repetition frequency is 1 MHz. A semiconductor laser that is directly modulated and outputs pulsed light;
A first optical filter that receives pulsed light output from the semiconductor laser and transmits a part of the wavelength range of the input pulsed light;
Using an optical amplifier that amplifies and outputs the pulsed light output from the first optical filter,
By adjusting the relative positional relationship between the transmission wavelength region of the first optical filter and the output spectrum of the semiconductor laser, the shorter wavelength side and the longer wavelength side of the peak wavelength of the pulsed light of the semiconductor laser output spectrum A pulse compression method characterized in that either one is attenuated from the other and the chirping component of the pulsed light is selectively output from the first optical filter .   15. The pulse compression method according to claim 14, wherein the first optical filter attenuates and outputs a shorter wavelength side than a peak wavelength of the pulsed light in a wavelength band of input pulsed light from a long wavelength side.   The pulse compression method according to claim 14, wherein the transmission characteristic of the first optical filter is variable. The pulse compression method according to claim 14, wherein a second optical filter is disposed so as to sandwich an optical amplification medium included in the optical amplifier together with the first optical filter.   15. The pulse compression method according to claim 14, wherein the first optical filter is a band pass filter.   The pulse compression method according to claim 17, wherein at least one of the first optical filter and the second optical filter is a band-pass filter. Both the first optical filter and the second optical filter are bandpass filters,
The pulse compression method according to claim 17, wherein the full width at half maximum of the transmission spectrum of the second optical filter is wider than the full width at half maximum of the transmission spectrum of the first optical filter. Both the first optical filter and the second optical filter are bandpass filters,
The pulse compression method according to claim 17, wherein a center wavelength of the second optical filter is set between a peak wavelength of the pulsed light and a center wavelength of a transmission spectrum of the first optical filter.   15. The pulse compression method according to claim 14, wherein the semiconductor laser is of a Fabry-Perot type.   15. The pulse compression method according to claim 14, further comprising temperature adjusting means for adjusting the temperature of the semiconductor laser.

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