JP4897960B2 - Pulse laser equipment - Google Patents

Description

  The present invention relates to a pulse laser device that generates pulsed laser light.

  Conventionally, a rectangular wave signal light pulse train generated from a light source is laser-amplified using a rare-earth element-doped optical fiber doped with a rare-earth element such as Yb, and this amplified light pulse train is used for laser marking or the like. There is a device.

  In the pulse laser apparatus having the above-described configuration, the signal light pulse train repeats the on and off states at a predetermined repetition period. On the other hand, an excitation light source that supplies excitation energy to a rare earth element continues to supply excitation light to the rare earth element-doped optical fiber, but stimulated emission does not occur when the signal light pulse train is off, so the rare earth element is in an extremely high excitation state. It has become. Here, when the signal light pulse train is turned on and the signal light is input to the rare-earth element-doped optical fiber, the excitation energy accumulated by the rare-earth element is suddenly released, and a surge pulse having a very high peak value is generated. Since relaxation oscillation occurs, the intensity of the amplified optical pulse train becomes extremely unstable.

  Note that in the laser marking device having the same configuration as the pulse laser device described above, the marking that outputs the signal light pulse train from the light source by switching the excitation light to one having a low intensity during the marking pause period in which the signal light pulse train is not output from the light source. In some cases, a technique for preventing the rare earth element from being in an extremely high excited state by switching the excitation light to one having a high intensity and stabilizing the intensity of the amplified optical pulse train is disclosed (see Patent Document 1).

Japanese Patent No. 3411852

  However, when the technique disclosed in Patent Document 1 is applied to prevent surge pulses generated in response to the on / off state of the signal light pulse train, it is necessary to perform switching control of pumping light at extremely high speed. There is a problem that the control becomes complicated.

  The present invention has been made in view of the above, and an object of the present invention is to provide a pulse laser device capable of suppressing generation of a surge pulse by simple control and outputting an optical pulse train having a stable intensity. .

  In order to solve the above-described problems and achieve the object, a pulse laser device according to the present invention includes a light source unit that outputs continuous laser light, pulse generation means that outputs an electric pulse signal sequence, and pulse generation means. A pumping light source connected and driven by the electrical pulse signal train to output a pumping light pulse train; and connected to the light source unit and the pumping light source, the pumping light pulse train amplifies the continuous laser light by exciting rare earth elements And an optical amplification section having a rare earth element-doped optical fiber.

  In the pulse laser device according to the present invention as set forth in the invention described above, the rare earth element-doped optical fiber has a double clad structure.

  In the pulse laser device according to the present invention as set forth in the invention described above, the rare earth element includes at least one of yttrium and erbium.

  In the pulse laser device according to the present invention as set forth in the invention described above, the excitation light source inputs the excitation light pulse train from the same direction as the continuous laser light into the rare earth element-doped optical fiber.

  In the pulse laser device according to the present invention as set forth in the invention described above, the pumping light source inputs the pumping light pulse train from the direction opposite to the continuous laser light into the rare earth element-doped optical fiber.

  Further, in the pulse laser device according to the present invention, in the above invention, the light source unit includes a plurality of light sources that output continuous laser beams having different wavelengths, and each of the light sources connected to the light source unit and output from the plurality of light sources. An optical multiplexer is provided that combines a continuous laser light source and outputs the combined laser light to the optical amplifier.

  According to the present invention, the excitation light source is driven by the electric pulse signal train to output the excitation light pulse train, and the excitation light pulse train amplifies the continuous laser light by exciting the rare earth element. There is an effect that it is possible to realize a pulse laser device capable of suppressing generation and outputting an optical pulse train having a stable intensity.

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

(Embodiment 1)
FIG. 1 is a schematic diagram schematically showing the configuration of the pulse laser apparatus according to Embodiment 1 of the present invention. As shown in FIG. 1, the pulse laser device 100 includes a light source 20, a pulse generator 3, and an optical amplifier 40.

  The light source 20 includes an LD 21 that is a semiconductor laser diode that outputs continuous laser light, and an optical fiber 22 that is connected to the LD 21 and has an FBG 22a that is a fiber Bragg grating that reflects light in a narrow band. . The FBG 22a acts as an external optical resonator for the LD 21 by reflecting light in a narrow band including the center wavelength of the continuous laser light output from the LD 21. As a result, the light source 20 operates as a light source that outputs continuous laser light L1 having a narrow spectrum width. The characteristics of the continuous laser light L1 output from the light source 20 are, for example, a center wavelength of about 1064 nm, a spectrum width of 5 pm or less, and a light intensity of about 230 mW. A temperature adjusting means such as a heater or a Peltier element is attached in the vicinity of the portion where the FBG 22a of the optical fiber 22 is formed, and the wavelength of the continuous laser light L1 is controlled by changing the reflected wavelength of the FBG 22a by changing the temperature. May be.

  The pulse generating means 3 outputs an electric pulse signal train E1 having a predetermined repetition period and a duty ratio. The optical amplifier 40 is an optical fiber amplifier using a double clad optical fiber capable of realizing an extremely high optical output, and is an optical isolator 41 and an optical multiplexer connected to the optical isolator 41 by a single mode optical fiber. Fiber bundle) 42, pump light sources 43-1 to 43-n (n is an integer), which is a semiconductor laser, and multimode optical fibers 44-1 to 44 that connect TFB 42 and pump light sources 43-1 to 43-n. -N, a rare earth element-doped double clad optical fiber 45 connected to the TFB 42, and an optical output terminal 46. The optical amplifier 40 also includes a current amplifier 47 that amplifies the electric pulse signal train E1 output from the pulse generator 3 and applies the amplified signal to the pumping light sources 43-1 to 43-n.

  The pulse laser device 100 operates as follows. First, the light source 20 outputs continuous laser light L1 to the optical amplifier 40. The TFB 42 couples the continuous laser light L 1 input through the optical isolator 41 to the rare earth element-doped double clad optical fiber 45. On the other hand, the pulse generator 3 outputs an electric pulse signal train E1 having a predetermined repetition period and a duty ratio to the optical amplifier 40. In the optical amplifier 40, the current amplifier 47 amplifies the input electric pulse signal string E1 to generate an amplified electric pulse signal string E2, and outputs the amplified electric pulse signal string E2 to the pumping light sources 43-1 to 43-n. The excitation light sources 43-1 to 43-n are driven by the amplified electric pulse signal train E2 and output excitation light pulse trains L2 having wavelengths of 900 to 980 nm, respectively. Since the excitation light sources 43-1 to 43-n are electrically connected in series, each excitation light pulse train L2 is synchronized in phase. Next, the multimode optical fibers 44-1 to 44-n guide the output pumping light pulse trains L2 to the TFB 42. The TFB 42 transmits the guided pumping light pulse trains L2 from the same direction as the continuous laser light L1 to the rare earth. It couple | bonds with the element addition double clad optical fiber 45. FIG. That is, the optical amplifier 40 is a so-called forward pumping type.

  Each excitation light pulse train L2 coupled to the rare earth element-doped double clad optical fiber 45 optically excites ions of Yb, which is a rare earth element added to the core portion, while propagating through the rare earth element doped double clad optical fiber 45. As a result, the continuous laser beam L1 coupled to the rare earth element-doped double clad optical fiber 45 is laser amplified by Yb ions in an excited state while propagating through the rare earth element doped double clad optical fiber 45. Here, since the Yb ions are periodically optically pumped by the pumping light pulse train L2, the continuous laser light L1 is also amplified at the same cycle as the pumping light pulse train L2 and output from the light output terminal 46 as the amplified light pulse train L3.

  That is, in the pulse laser device 100, the laser light to be amplified is continuously input to the rare earth element-added double clad optical fiber 45 in time, and the excitation light is an optical pulse train. As a result, Yb ions are prevented from suddenly releasing excitation energy from an extremely high excited state and generating a surge pulse and accompanying relaxation oscillation. Therefore, the amplified optical pulse train L3 has a very stable intensity. Further, the excitation light sources 43-1 to 43-n do not need to be controlled to switch the intensity of the excitation light, and only simple control is required.

  FIG. 2 is a diagram illustrating waveforms of the electric pulse signal train E1, the amplified electrical pulse signal train E2, the continuous laser light L1, the excitation light pulse train L2, and the amplified light pulse train L3 in the pulse laser device 100 shown in FIG. The horizontal axis represents time, and the vertical axis represents power or light intensity. As shown in FIG. 2, the excitation light pulse train L2 has the same repetition period T and pulse width Δt as the electrical pulse signal train E1 and the amplified electrical pulse signal train E2. As a result, the continuous laser light L1 having a constant intensity over time is output as an amplified optical pulse train L3 having a repetition period T and a pulse width Δt. Therefore, by adjusting the repetition period T and the pulse width Δt of the electric pulse signal train E1, an amplified optical pulse train L3 having a desired repetition period T and a pulse width Δt can be obtained. In practice, for example, the phase of each waveform is slightly shifted due to a difference in signal propagation time or the like, but in FIG. 2, the phases of the respective waveforms are shown to coincide with each other.

  FIG. 3 is a diagram showing a schematic cross section of the rare earth element-doped double clad optical fiber 45 shown in FIG. 1 and a corresponding refractive index profile. As shown in FIG. 3, a rare earth element-doped double clad optical fiber 45 includes a core portion 45a doped with Yb ions, and an inner clad portion 45b having a lower refractive index than the core portion 45a formed on the outer periphery of the core portion 45a. And an outer cladding part 45c having a lower refractive index than the inner cladding part 45b formed on the outer periphery of the inner cladding part 45b, and having a refractive index profile 45d. The excitation light pulse train L2 input to the rare earth element-doped double clad optical fiber 45 optically pumps Yb ions added to the core part 45a while propagating in the core part 45a and the inner clad part 45b in multimode, and the continuous laser light L1 is It propagates through the core portion 45a in a single mode and is laser amplified by Yb ions in an excited state.

  Next, the characteristics of the pulse laser device 100 will be specifically described. FIG. 4 is a diagram showing a waveform of the amplified optical pulse train L3 when the electrical pulse signal train E1 having a repetition frequency of 100 Hz and a duty ratio of 50% is used. In FIG. 4, the horizontal axis represents time, the vertical axis represents the measured voltage value when the amplified optical pulse train L3 is observed with an optical oscilloscope, and the voltage level indicated by the broken line corresponds to the light intensity of 5 W. As shown in FIG. 4, the amplified optical pulse train L3 has a stable intensity without generating a surge pulse, and has a repetition frequency of 100 Hz and a duty ratio of 50%, like the electrical pulse signal train E1. It was confirmed to be a pulse train.

  On the other hand, FIG. 5 is a diagram showing a waveform of the amplified optical pulse train L3 when the electrical pulse signal train E1 having a repetition frequency of 1 kHz and a duty ratio of 50% is used. As shown in FIG. 5, the amplified optical pulse train L3 is stable in intensity without generating a surge pulse even when the repetition frequency is 1 kHz, which is higher than the case of FIG. It was confirmed that the optical pulse train had a similar waveform.

  In FIG. 5, the rising and falling portions of the waveform of the amplified optical pulse train L3 have a gentle shape, which is considered to reflect the time response characteristics of the current amplifier 47. Therefore, by appropriately selecting the current amplifier 47 having a time response characteristic corresponding to the repetition frequency of the electric pulse signal train E1, an amplified optical pulse train L3 having a waveform closer to a rectangle can be obtained.

  Next, FIGS. 6 to 8 show that the repetition frequency of the electric pulse signal train E1 is 1 kHz, the duty ratios are 20%, 50%, and 90%, respectively, and the gain of the current amplifier 47 is controlled to control the amplification optical pulse train L3. It is a figure which shows the waveform of the amplification optical pulse train L3 when the intensity | strength of the excitation light pulse train L2 is controlled so that the intensity | strength in an ON state may be 6W, 12W, 17W, and 22W. 6-8, waveforms S1, S5, and S9 have an intensity of 22W, waveforms S2, S6, and S10 have an intensity of 17W, waveforms S3, S7, and S11 have an intensity of 12W, waveforms S4, S8 and S12 show cases where the intensity is 6 W, respectively. As shown in FIGS. 6 to 8, the amplified optical pulse train L3 has a stable intensity with no surge pulse generated even when the duty ratio and the intensity are changed, and has the same waveform as that of the electric pulse signal train E1. It was confirmed that the optical pulse train had. The gain control method of the current amplifier 47 is a well-known control method and is not complicated.

  As described above, in the pulse laser device 100 according to the first embodiment, the excitation light sources 43-1 to 43-n are driven by the amplified electrical pulse signal sequence E2 obtained by amplifying the electrical pulse signal sequence E1. Since the pumping light pulse train L2 is output and the pumping light pulse train L2 amplifies the continuous laser light L1 by exciting the Yb ions added to the rare earth element-doped double clad optical fiber 45, the generation of surge pulses can be suppressed with simple control. Thus, an amplified optical pulse train L3 with a stable intensity can be output.

(Embodiment 2)
Next, a pulse laser apparatus according to Embodiment 2 of the present invention will be described. In the pulse laser device according to the second embodiment, in the optical amplifying unit, the pump light source inputs the pump light pulse train from the direction opposite to the continuous laser light into the rare earth element-doped optical fiber. It is different from the laser device.

  FIG. 9 is a schematic diagram schematically showing the configuration of the pulse laser apparatus according to the second embodiment. As shown in FIG. 9, the pulse laser device 200 includes a light source 20 and a pulse generator 3 similar to the pulse laser device 100 according to the first embodiment, and an optical amplifier 50.

  The optical amplifier 50 is an optical fiber amplifier using a double clad optical fiber as in the optical amplifier 40, and includes an optical isolator 51, a rare earth element-doped double clad optical fiber 55 connected to the optical isolator 51, and a rare earth element doped double fiber. A TFB 52 connected to the clad optical fiber 55, pumping light sources 53-1 to 53-m (m is an integer) that is a semiconductor laser, and a multimode optical fiber 54 that connects the TFB 52 and the pumping light sources 53-1 to 53-m. -1 to 54-m, an optical output terminal 56, and a current amplifier 57.

  The operation of the pulse laser device 200 is the same as that of the above-described pulse laser device 100. However, in the pulse laser device 200, each excitation light pulse train output from the excitation light sources 53-1 to 53-m by the TFB 52 is a continuous laser beam. Are coupled to the rare earth element-doped double clad optical fiber 55 from the opposite direction. That is, the optical amplifier 50 is a so-called backward pumping type. As a result, the optical amplifier 50 can make Yb ions added to the rare-earth-doped double clad optical fiber 55 relatively excited on the output side of the amplified optical pulse train, so that the intensity is high and stable amplification. An optical pulse train can be output from the optical output terminal 56.

(Embodiment 3)
Next, a pulse laser apparatus according to Embodiment 3 of the present invention will be described. The pulse laser apparatus according to the third embodiment is implemented in that the pumping light source inputs the pumping light pulse train from the same direction and in the opposite direction to the continuous laser light, that is, from both directions, in the rare earth element-doped optical fiber in the optical amplification unit. This is different from the pulse laser apparatus according to the first and second embodiments.

  FIG. 10 is a schematic diagram schematically showing the configuration of the pulse laser apparatus according to the third embodiment. As shown in FIG. 10, this pulse laser apparatus 300 includes the light source 20 and the pulse generation means 3 similar to the pulse laser apparatuses 100 and 200 according to the first and second embodiments, and the optical amplifier 60.

  The optical amplifier 60 is an optical fiber amplifier using a double clad optical fiber as in the optical amplifiers 40 and 50, and is an optical isolator 61, a TFB 62a connected to the optical isolator 61 by a single mode optical fiber, and a semiconductor laser. Excitation light sources 63a-1 to 63a-n, multimode optical fibers 64a-1 to 64a-n connecting the TFB 62a and the excitation light sources 63a-1 to 63a-n, and a rare earth element-doped double clad optical fiber connected to the TFB 62a 65, a TFB 62b connected to the rare earth element-doped double clad optical fiber 65, pumping light sources 63b-1 to 63b-m, which are semiconductor lasers, and multimode light connecting the TFB 62b and the pumping light sources 63b-1 to 63b-m Fibers 64b-1 to 64b-m, optical output terminal 66, electrical And an amplifier 67.

  The operation of the pulse laser device 300 is the same as that of the pulse laser devices 100 and 200 described above, but in the pulse laser device 300, the excitation light pulse train output from the excitation light sources 63a-1 to 63a-n by the TFB 62a is a continuous laser beam. Are coupled to the rare earth element-doped double clad optical fiber 65, and the excitation light pulse train output from the pump light sources 63b-1 to 63b-m by the TFB 62b is rare earth element doped double clad optical fiber 65 from the direction opposite to the continuous laser light. To join. That is, the optical amplifier 60 is a so-called bidirectional excitation type. As a result, the sum of the intensities of the excitation light pulse trains input to the rare earth element-doped double clad optical fiber 65 can be made extremely high, so that an amplified light pulse train having higher intensity and stability can be output from the optical output terminal 66. it can.

(Embodiment 4)
Next, a pulse laser apparatus according to Embodiment 4 of the present invention will be described. The pulse laser device according to the fourth embodiment includes a plurality of light sources each having a light source unit that outputs continuous laser beams having different wavelengths, and is connected to the light source unit to combine the continuous laser light sources output from the plurality of light sources. The pulse laser apparatus according to the first to third embodiments is different from the pulse laser apparatus according to the first to third embodiments in that an optical multiplexer that outputs a wave and outputs it to the optical amplification unit is provided.

  FIG. 11 is a schematic diagram schematically showing the configuration of the pulse laser apparatus according to the fourth embodiment. As shown in FIG. 11, the pulse laser device 400 includes light sources 20a and 20b similar to the light source 20 of the pulse laser device 100 according to the first embodiment, a pulse generator 3, and an optical amplifier 40. And a 3 dB coupler C which is an optical multiplexer.

  The light sources 20a and 20b respectively output continuous laser beams L4a and L4b having different wavelengths. The 3 dB coupler C combines the continuous laser beams L4a and L4b with an intensity ratio of approximately 1: 1 and outputs the combined laser beams to the optical amplifier 40. The pulse generator 3 and the optical amplifier 40 operate in the same manner as in the first embodiment to laser-amplify the combined continuous laser beams L4a and L4b and output an amplified optical pulse train L5.

  The amplified optical pulse signal train L5 is a combined optical pulse train obtained by combining two synchronized optical pulse trains having different wavelengths. That is, the pulse laser apparatus 400 according to the fourth embodiment stabilizes the combined optical pulse train with a simpler configuration than when outputting optical pulse trains from individual light sources and synthesizing these optical pulse trains in synchronization. Can output in intensity. Instead of the 3 dB coupler C, a WDM coupler that can multiplex light of different wavelengths with low optical loss may be used.

  In the first to fourth embodiments, Yb is used as the rare earth element, but erbium (Er) may be used. The wavelength of the continuous laser light output from the light source can be selected as appropriate within the amplification band of the rare earth element. For example, Yb has a wavelength in the range of 1000 to 1150 nm, and Er has a wavelength of 1520. The wavelength is in the range of 1610 nm.

  The pulse laser device according to the present invention can be suitably used as a green light source of a laser display in combination with a second harmonic generation (SHG) element made of a nonlinear optical medium such as PPLN. In particular, the pulse laser device according to Embodiment 4 sets the wavelength of continuous laser light to 1060 nm and 1064 nm, respectively, and generates a synchronized composite optical pulse train having wavelengths of 530 nm and 532 nm using an SHG element. A green light source capable of realizing a high-quality laser display.

  Further, the pulse laser device according to the present invention can be suitably used as a light source for a flow cytometer used for fluorescence observation of cells in combination with an SHG element.

  By the way, the SHG element is a means for generating a second harmonic using a nonlinear optical crystal or the like. As described above, when light having a wavelength of 1064 nm is input, green light having a wavelength of 532 nm, which is the second harmonic light, is generated. generate. That is, by using the SHG element, the wavelength of the input light can be halved and the energy can be doubled.

  In addition, SHG elements include a bulk type that is used at a high optical input of about 1 W or more and a waveguide type that is used at a relatively low optical input level below that. Therefore, when the optical pulse train output from the pulse laser device according to the present invention has a light intensity of about 1 W or more in the on state, it is preferable to combine with a bulk type SHG element, and about 1 W or less in the on state. For example, when it has a light intensity of about 100 mW, it is preferable to combine it with a waveguide furnace type SHG element.

  Further, in the pulse laser device according to the present invention, since continuous laser light is input from the light source unit to the optical amplification unit, even if the electric pulse signal train is in the off state, light resulting from the continuous laser light is transmitted to the optical amplification unit. May be output from. The light output in the OFF state deteriorates the extinction ratio, which is the light intensity ratio between the ON state and the OFF state of the optical pulse train output from the pulse laser device. Here, when the pulse laser device and the SHG element are combined, the second harmonic light output from the SHG element may require an extinction ratio of about 20 dB. In order to realize the extinction ratio of 20 dB, the pulse laser device is appropriately designed so that the intensity of the light output in the off state is reduced, and the extinction ratio of the optical pulse train output from the pulse laser device is greater than 10 dB. It is necessary to make it.

It is the schematic which represented typically the structure of the pulse laser apparatus which concerns on Embodiment 1 of this invention. It is a figure which shows each waveform of the electric pulse signal sequence in the pulse laser apparatus shown in FIG. It is a figure which shows the typical cross section of the rare earth element addition double clad optical fiber shown in FIG. 1, and a corresponding refractive index profile. It is a figure which shows the waveform of the amplification optical pulse train when the repetition frequency is 100 Hz and the electric pulse signal train having a duty ratio of 50% is used. It is a figure which shows the waveform of an amplified optical pulse train when a repetition frequency is 1 kHz and an electric pulse signal train having a duty ratio of 50% is used. Amplification when the repetition frequency of the electric pulse signal train is 1 kHz, the duty ratio is 20%, and the intensity of the excitation light pulse train is controlled so that the intensity in the ON state of the amplified light pulse train is 6 W, 12 W, 17 W, 22 W It is a figure which shows the waveform of an optical pulse train. Amplification when the repetition frequency of the electric pulse signal train is 1 kHz, the duty ratio is 50%, and the intensity of the excitation light pulse train is controlled so that the intensity in the ON state of the amplified light pulse train is 6 W, 12 W, 17 W, 22 W It is a figure which shows the waveform of an optical pulse train. Amplification when the repetition frequency of the electric pulse signal train is 1 kHz, the duty ratio is 90%, and the intensity of the excitation light pulse train is controlled so that the intensity in the ON state of the amplified light pulse train is 6 W, 12 W, 17 W, 22 W It is a figure which shows the waveform of an optical pulse train. It is the schematic which represented typically the structure of the pulse laser apparatus which concerns on Embodiment 2 of this invention. It is the schematic which represented typically the structure of the pulse laser apparatus which concerns on Embodiment 3 of this invention. It is the schematic which represented typically the structure of the pulse laser apparatus which concerns on Embodiment 4 of this invention.

Explanation of symbols

100-400 pulse laser device 20, 20a, 20b Light source 21 LD
22 optical fiber 22a FBG
3 Pulse generator 40-60 Optical amplifier 41-61 Optical isolator 42, 52, 62a, 62b TFB
43-1 to 43-n, 53-1 to 53-m, 63a-1 to 63a-n, 63b-1 to 63b-m Excitation light source 44-1 to 44-n, 54-1 to 54-m, 64a -1 to 64a-n, 64b-1 to 64b-m Multimode optical fiber 45 to 65 Rare earth element added double clad optical fiber 45a Core portion 45b Inner clad portion 45c Outer clad portion 45d Refractive index profile 46 to 66 Optical output terminal 47 ~ 67 Current amplifier

Claims (5)

A light source unit including a plurality of light sources that output continuous laser light having different wavelengths from each other ;
An optical multiplexer for connecting and outputting each continuous laser light source connected to the light source unit and outputting from the plurality of light sources;
Pulse generating means for outputting an electric pulse signal train;
An excitation light source for outputting pumping light pulse train is driven by said electric pulse signal train is connected to said pulse generating means, said by the excitation light pulse train is connected to said optical multiplexer and said excitation light source for exciting the rare earth element An optical amplifying unit having a rare earth element-doped optical fiber for amplifying the continuous laser light input from an optical multiplexer ;
A pulse laser device comprising:   2. The pulse laser device according to claim 1, wherein the rare earth element-doped optical fiber has a double clad structure.   The pulse laser apparatus according to claim 1, wherein the rare earth element includes at least one of yttrium and erbium.   The pulse laser apparatus according to claim 1, wherein the excitation light source inputs the excitation light pulse train from the same direction as the continuous laser light to the rare earth element-doped optical fiber.   5. The pulse laser device according to claim 1, wherein the pumping light source inputs the pumping light pulse train into the rare earth element-doped optical fiber from a direction opposite to the continuous laser light.

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