JP3837499B2 - Pulse laser time synchronizer and arbitrary waveform generator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a femtosecond pulse laser generator widely used as an ultrashort pulse light source in the fields of communication, medicine, and science and technology, and more particularly, to a synchronous control device thereof and an arbitrary waveform generator using the same.
[0002]
[Prior art]
Conventionally, there are the following two typical techniques for synchronous control of femtosecond pulse lasers.
The first is synchronous control of independent ultrashort pulse lasers of different wavelengths, the pulse interval of each laser is detected by an electric circuit, such as a counter, and the deviation is a piezoelectric that drives a mirror of one resonator length. This is a method performed by feeding back to the element. In this method, the timing jitter is determined by the resolution of the electric circuit such as the counter and the resolution of the piezoelectric element, and this timing jitter is in the order of picoseconds (1 picosecond = 10 ^ -12 seconds) at the product level (reference [1] : Spectra Physics General Catalog '00 -'01, p18, lok-to-clock), on the order of 10 to 20 femtoseconds at the research level (reference [2]: RK Shelton, et Al., International Conference CLEO2001, CTuG2).
[0003]
The second method is to make two resonators for one laser medium. As for the laser medium to be used, a titanium sapphire laser can be used to extract two-wavelength pulses synchronized in time (reference document [3]: S. J. White, et. Al., International Conference CLEO2000, CThB5, ref. [4]: J. M. Evans, et. Al., Opt. Lett. 18, 1074 (1993), ref. [5]: A. Leitentorfer, et.al., Opt. , 916 (1995), reference [6]: MR X. de Barros et. Al., Opt. Lett. 18, 631 (1993), reference [7]: DR Dykaar, et. al., Opt Lett. 18, 634 (1993)).
[0004]
In the above case, it is considered that two laser pulses cause four-wave mixing in the laser crystal to cause pulse timing synchronization.
[0005]
[Problems to be solved by the invention]
However, the two-wavelength timing-synchronized laser using the second titanium sapphire laser has a single laser medium, and is therefore limited to two adjacent wavelengths because of the characteristics of the laser medium. Moreover, since the titanium sapphire crystal is the gain medium for both pulses, gain competition occurs and the pulse is rather unstable in a situation where the pulses overlap well.
For this reason, it has become difficult to synchronize two pulse lasers having different wavelengths with a two-wavelength timing synchronous laser using a titanium sapphire laser.
[0006]
Further, in the method using the first electric circuit or the like, the influence of the timing jitter is too great, and the synchronous control of the femtosecond pulse laser cannot be performed accurately.
In order to solve the above-described problems, the present invention provides a pulse laser time synchronization apparatus capable of synchronizing timing between two ultrashort pulse lasers having different wavelengths and an arbitrary waveform generation apparatus using the same. For the purpose.
[0007]
[Means for Solving the Problems]
The present invention employs the following means to achieve the above object.
(1) In a time-synchronizing device for a mode-locked pulse laser, the group velocity dispersion of the entire laser resonator is set to a negative value, a first laser medium having a first gain band made of a third-order nonlinear optical medium, and 3 A second laser medium comprising a second nonlinear optical medium and having a second gain band not included in the first gain band; and a first and a second laser medium separately provided with the first and second laser media, respectively. The two resonators are arranged so that both laser pulses are spatially overlapped in one laser medium so as to influence each other's light propagation through the third-order nonlinear effect, The oscillation wavelength of the second laser medium is a wavelength that cannot be oscillated by the crystal of the first laser medium.
[0008]
(2) In the time-synchronized apparatus for mode-locked pulse laser as described in (1) above, the refractive index characteristic of one laser medium arranged so that both laser pulses overlap each other is optical nonlinearity in the time domain of the preceding laser pulse. Propagation of the other laser pulse following the preceding one laser pulse by changing the red displacement or the blue displacement characteristic by the nonlinear refraction effect which is the effect, transmitting the other laser pulse following to the laser medium. It is characterized in that the speed is changed and the timing is synchronized.
[0009]
(3) In the mode-locked pulse laser time synchronization apparatus according to (1), the interval between adjacent frequency components of the first laser pulse based on the first laser medium and the second based on the second laser medium. The interval between adjacent frequency components of the laser pulse is synchronized by four-wave mixing, which is an optical nonlinear effect in the frequency domain, and the master laser pulse causes a temporal refractive index change in the laser medium, and the slave laser is delayed. In this case, the refractive index is changed so that the group speed is increased when the slave laser is advanced, and the group speed is decreased when the slave laser is advanced.
(4) The mode-locked pulse laser time synchronization apparatus according to the above (1) is characterized in that the first laser medium is titanium sapphire and the second laser medium is chromium forsterite.
(5) The mode-locked pulse laser time synchronization apparatus according to the above (1) is characterized in that the first laser medium is titanium sapphire and the second laser medium is chromium tetravalent doped yag.
[0010]
(6) The arbitrary waveform generation device includes the time synchronization device of the mode-locked pulse laser according to any one of (1) to (5), at least a nonlinear optical crystal, and a half mirror, and the time synchronization device outputs sum frequency light and the optical frequency ω1 of the optical frequency ω3 plus an optical frequency ω2 to the optical frequency ω1 and the laser pulses of different the optical frequency ω1 wavelength 1 and the wavelength 2 to be an optical frequency ω2 through the nonlinear optical crystal, respectively to Output as the difference frequency light of the optical frequency ω4 obtained by subtracting the optical frequency ω2 from the light , and output the light of the wavelength 1 as the light of the optical frequency 2ω1 having the wavelength 5 which is half of the wavelength 1 through the nonlinear optical crystal, output as the light of the light frequency 2ω2 to be half the wavelength 6 of the wavelength 2 light wavelength 2 through the nonlinear optical crystal, the light of the wavelength 1 and the wavelength 2 is output as it Is characterized in that the [0011]
(7) above (6) in the arbitrary waveform generator according, 850 nm wavelength 1 of the laser pulse to be the optical frequency .omega.1, the wavelength 2 of the laser pulse to be the optical frequency .omega.2 1275 nm, the wavelength of the sum frequency light 3 is 510 nm.
(8) The arbitrary waveform generation device according to (6), wherein laser pulses having an arbitrary number of different wavelengths among the laser pulses having wavelengths 1 to 6 are synthesized and output.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a comparative characteristic diagram of a two-wavelength synchronous laser.
[0013]
The left figure of FIG. 1 shows the characteristics of a conventional two-wavelength synchronous laser. In this figure, the vertical axis is the light energy value, and the horizontal axis is dimensionless. That is, ω1 and ω2 are optical frequency components separated by a repetition frequency of a master laser having a wavelength of, for example, 800 nm in titanium sapphire, and ω3 and ω4 are laser light having another wavelength in titanium sapphire, for example, a wavelength of 850 nm. Optical frequency components separated by a repetition frequency of. Since the laser medium is a titanium sapphire crystal, the pulses of 800 nm and 850 nm have energy values in the gain band in the nonlinear medium (titanium sapphire crystal), and the energy difference between ω1 and ω2 and ω3 and ω4 is Δω. As a constant value.
[0014]
For this reason, in the conventional two-wavelength synchronous laser, each pulse needs to be a laser beam having a wavelength having an energy value in the gain band in the nonlinear medium (titanium sapphire crystal). Will be restrained.
On the other hand, the present invention is characterized in that the wavelength of the laser light used by the two-wavelength synchronous laser is set to a remote wavelength without being restricted by the crystal characteristics of one laser medium.
[0015]
The right figure of FIG. 1 is a characteristic diagram of the dual wavelength synchronous laser of the present invention. In this figure, the vertical axis is the light energy value, and the horizontal axis is dimensionless. That is, ω1 and ω2 are optical frequency components separated from each other by a repetition frequency of laser light having a wavelength in titanium sapphire, for example, 850 nm, and ω3 and ω4 are laser light having another wavelength in titanium sapphire, for example, wavelength of 1275 nm. These are optical frequency components separated by a repetition frequency. Since the laser medium is a titanium sapphire crystal, the 850 nm pulse has an energy value in the gain band in the nonlinear medium (titanium sapphire crystal), but the 1275 nm pulse is in the nonlinear medium (titanium sapphire crystal). It has no energy value in the gain band. As a result, the laser beam including ω3 and ω4 can be obtained so that the energy values of ω3 and ω4 of the slave laser can be taken outside the gain band in the nonlinear medium (titanium sapphire crystal) including the energy values of ω1 and ω2 on the master laser side. The wavelength can be selected, that is, a medium different from the above nonlinear medium (titanium sapphire crystal) can be used. The energy difference between ω1 and ω2 and between ω3 and ω4 takes a constant value as Δω.
[0016]
As a result, the present invention eliminates the disadvantage that the conventional two-wavelength synchronous laser needs to make each pulse a laser beam having an energy value in the gain band in the nonlinear medium (titanium sapphire crystal). Two wavelengths can be selected without being restricted by the crystal characteristics of the two laser media. Therefore, in the conventional two-wavelength synchronous laser, four-wave mixing uses an actual level transition, but in the present invention, a non-resonant transition can be used for the wavelength on the slave laser side. Therefore, no gain competition occurs.
[0017]
FIG. 2 shows a two-wavelength pulsed laser time synchronizer. This device consists of two laser media and their respective excitation light, prism and mirror. The first laser medium, titanium sapphire crystal, is excited by the second harmonic green light of an argon laser or an Nd-doped laser, and includes a mirror (M1, M2, M8), an output coupler (OC1), a prism (P1, P2). ) Constitutes a resonator. From this resonator, a pulse having a center wavelength of 850 nm and a pulse width of 20 fs is repeatedly generated at about 75 MHz. The mirror (M8) is fixed to the piezo element so that the resonator length can be adjusted by an external voltage.
[0018]
The chrome forsterite crystal, which is the second laser medium, is excited by an Nd: YVO ₄ laser (wavelength 1064 nm), a mirror (M3, M4, M5, M6, M7), an output coupler (OC2), a prism (P3, P4), a resonator is constituted by a semiconductor saturable absorption mirror (SESAM). This resonator generates a pulse having a center wavelength of 1275 nm and a pulse width of 40 fs.
[0019]
The oscillating light of the chrome forsterite laser passes through the titanium sapphire crystal in its own resonator, and overlaps with the oscillating light of the titanium sapphire laser to cause interaction. Within the titanium sapphire crystal, four-wave mixing of the optical frequency of the titanium sapphire laser and the optical frequency of the chromium forsterite laser occurs, and the two pulses work to cause time synchronization.
[0020]
As shown in the right diagram of FIG. 1, the present invention differs from the conventional two-wavelength titanium sapphire laser in that the energy value of the laser light of the chrome forsterite laser does not fall within the gain band of the titanium sapphire crystal. For a laser, the titanium sapphire laser is no longer a gain medium, that is, a four-wave mixing occurs in a titanium sapphire crystal, which is a transparent material, and passive time timing synchronization occurs. If this method is possible, synchronization between pulses of wavelengths that are far apart can be achieved, and there is no competition for gain.
[0021]
(The above time synchronization mechanism)
Each explanation in the frequency domain and the time domain will be given.
The titanium sapphire laser is the master laser and the chrome forsterite laser is the slave laser. The master laser generates ultrashort pulses by synchronizing with the car lens mode. When a chromium forsterite laser is incident on a titanium sapphire crystal, four-wave mixing as shown in FIG. 3 occurs. An optical frequency of ω4 is newly generated from the optical frequency components ω1 and ω2 of the master laser and the optical frequency component ω3 of the slave laser by four-wave mixing.
[0022]
At this time, ω1−ω2 = ω4−ω3. Since ω1-ω2 corresponds to the repetition frequency of the master laser and ω4-ω3 corresponds to the repetition frequency of the slave laser, the repetition frequency of the slave laser is drawn into the repetition frequency of the master laser. The generation of ω4 by four-wave mixing can be interpreted as the transfer of energy from the master laser to the slave laser. In other words, the gain increases when the repetition frequency of the titanium sapphire crystal, which is a transparent material for the slave laser, matches that of the master laser.
[0023]
(Light wave mixing)
ω1 and ω2 are longitudinal mode frequencies of the titanium sapphire laser. ω3 and ω4 are longitudinal mode frequencies of the chrome forsterite laser. In practice, each includes a number of longitudinal modes. The longitudinal mode interval ω1-ω2 of the titanium sapphire laser corresponds to Δω, and a longitudinal mode ω4 that is separated from the longitudinal mode ω3 of the chromium forsterite laser by Δω is generated. As a result, the longitudinal mode interval of the chrome forsterite laser becomes the same value as that of the titanium sapphire laser. That is, the repetition frequency f = 2πΔω is synchronized.
[0024]
(Time domain)
FIG. 4 is a diagram for explaining the timing synchronization operation in the time domain of the present invention.
Assume that the master laser pulse is ahead of the slave laser pulse. The refractive index changes in the titanium sapphire crystal depending on the intensity of the master laser pulse. The refractive index in the crystal increases as the pulse peak value increases. This temporal refractive index change causes a red displacement of the spectrum at the rising edge of the master laser pulse and a blue displacement at the falling edge. Since the slave laser pulse is located at the trailing edge of the master pulse, it receives a blue displacement of the spectrum. Since the group velocity dispersion of the entire laser resonator is set to a minus value, a pulse that has received a blue displacement shortens the time required to go around the resonator. Therefore, the slave laser pulse works to catch up with the master laser pulse.
[0025]
When the master laser pulse is delayed compared to the slave laser pulse, the opposite effect occurs, so that the circulation time of the slave laser becomes longer, and the laser pulse also works so as to approach. Thus, the two laser pulses are synchronized.
(Timing synchronization in the time domain)
Both the master laser pulse and the slave laser pulse are moving to the left in the titanium sapphire crystal. At this time, it is assumed that the master laser pulse has advanced first.
[0026]
The slave laser pulse overlaps the falling edge of the master laser pulse. At this time, the spectrum of the slave laser pulse undergoes blue displacement. When the entire spectrum is subjected to a blue displacement, the time taken to make one round of the resonator is shortened, and the overlap becomes larger in time when it is next spatially overlapped again. If the slave laser pulse overtakes the master laser pulse, the slave laser pulse is now subjected to a red displacement, and the time taken to make a round becomes longer. In this way, pulses continue to overlap.
[0027]
When the resonator lengths are far away from each other, the laser operates at a repetition frequency according to each resonator length. When the resonator length is changed by piezo and enters the range where the pull-in occurs, the repetition frequency of the slave laser is pulled into the master laser and perfectly matches. 5 and 6 show photographs of an oscilloscope in which the master laser pulse train and the slave laser pulse train were taken simultaneously. FIG. 5 shows the case where the resonator lengths are greatly separated. When the trigger is applied with the pulse train of the master laser, it can be seen that the pulse train of the slave laser flows because it is not synchronized. When the resonator length is made closer by piezo, the pulses are synchronized as shown in FIG. FIG. 7 is a plot of the difference of each repetition frequency with respect to the change in the resonator length. It can be seen that the difference in the repetition frequency is zero within a certain range.
[0028]
FIG. 6 is a measurement diagram showing the synchronization state of the present invention, which is the result of observing the master laser pulse train and the slave laser pulse train simultaneously with an oscilloscope. FIG. 5 shows an example in which the deviation of the resonator length is large. In FIG. 5, the upper stage is a master laser pulse, and the lower stage is a slave laser pulse. When triggered by the master laser pulse, the slave laser pulse flows because it is not synchronized. FIG. 6 shows an example in which synchronization occurs as a result of adjusting the resonator length.
[0029]
FIG. 7 is an explanatory diagram of the synchronization characteristics of the present invention.
In FIG. 7, the horizontal axis represents the change (μm) in the resonator length of the master laser, and the vertical axis represents the difference Δf _RT (Hz) between the repetition frequencies of the master laser and the slave laser. In FIG. 7, when the resonator length of the master laser is changed, the repetition frequency of the slave laser matches the repetition frequency of the master laser in the range of about 1 μm. In other words, synchronize.
[0030]
Next, another embodiment of the present invention will be described.
FIG. 8 is a schematic diagram of a sum frequency light generator according to another embodiment of the present invention.
The sum frequency light generator uses the two-wavelength pulse laser time synchronization device described in the first embodiment, and generates a pulse of wavelength 1 (ω1) and wavelength 2 (ω2) from the time synchronization device of the two-wavelength pulse laser. Then, both pulses are output as sum frequency light (optical frequency: ω3 = ω1 + ω2) through the lens and the nonlinear optical crystal.
[0031]
In this experiment, the sum frequency light generator uses a titanium sapphire laser as the master and a chrome forsterite laser as the slave, but what if the wavelength range of the slave laser is transparent to the gain medium of the master laser? This can be achieved by combining lasers.
Also, as shown in FIG. 8, two-wavelength pulses that are significantly separated from each other are output in synchronization, so that the sum-frequency light mixing of the two-wavelength pulses can be easily realized. When a titanium sapphire laser and a chrome forsterite laser are used, the wavelength of the sum frequency light is around 510 nm, which becomes a green femtosecond light source. The second harmonic of the titanium sapphire laser is blue (wavelength 425 nm), and the second harmonic of the chrome forsterite laser is red (wavelength 637 nm). Therefore, femtosecond red, green, and blue primary colors can be generated simultaneously from this apparatus, and application to lighting apparatuses and drawing apparatuses (projectors, etc.) is expected. In addition, it is highly useful as a stable green femtosecond light source as a light source for physics and chemistry experiments.
[0032]
In FIG. 8, light of wavelength 1 (optical frequency; ω1) and wavelength 2 (optical frequency; ω2) is ½ wavelength through the nonlinear optical crystal, that is, wavelength 5 (optical frequency: ω5 = ω1 × 2). , Output light of wavelength 6 (optical frequency: ω6 = ω2 × 2), and through a nonlinear optical crystal, sum frequency light of wavelength 3 (optical frequency; ω3 = ω1 + ω2), wavelength 4 (optical frequency; ω4 = ω1− By outputting the difference frequency light of ω2) and outputting the light of wavelength 1 (optical frequency; ω1) and wavelength 2 (optical frequency; ω2), light of six types of wavelengths can be produced. When light of these wavelengths is appropriately combined, a photoelectric field having an arbitrary waveform can be created. The apparatus shown in FIG. 8 generates difference frequency light simultaneously with sum frequency light.
[0033]
In the apparatus of FIG. 8, for example, if wavelength 1 is 1275 nm and wavelength 2 is 850 nm, a sum frequency light of 510 nm is generated. At the same time, when the second harmonic of wavelength 1 (wavelength 637 nm) and the second harmonic of wavelength 2 (wavelength 425 nm) are generated, a femtosecond light source synchronized in time with the three primary colors of RGB can be generated from one apparatus.
[0034]
【The invention's effect】
According to the present invention, according to the first to fifth aspects, the oscillation wavelength of the second laser medium is set to a wavelength that cannot be oscillated by the crystal of the first laser medium, and the laser pulses whose oscillation wavelengths are greatly separated can be synchronized. become able to.
[0035]
The two-wavelength pulse laser time synchronization apparatus of the present invention can generate pulses of two different wavelengths with very little timing jitter by generating pulses of each wavelength using two laser media and passively synchronizing them. Further, since the laser medium that causes timing synchronization is a transparent material for one pulse, there is no gain competition. Therefore, more stable timing synchronization occurs.
According to the present invention, an arbitrary waveform generating device for a photoelectric field can be made according to the matters described in the sixth or eighth aspect.
According to the present invention, an RGB signal for image color display can be easily created.
[Brief description of the drawings]
FIG. 1 is a comparative characteristic diagram of a two-wavelength synchronous laser.
FIG. 2 is a diagram illustrating a synchronization device according to an embodiment of the present invention.
FIG. 3 is an explanatory diagram of four-wave mixing according to the present invention.
FIG. 4 is a diagram illustrating a timing synchronization operation in the time domain according to the present invention.
FIG. 5 is a measurement diagram showing an asynchronous state.
FIG. 6 is a measurement diagram showing a synchronization state of the present invention.
FIG. 7 is an explanatory diagram of synchronization characteristics of the present invention.
FIG. 8 is a schematic view of a sum frequency light generator according to another embodiment of the present invention.

Claims (8)

The group velocity dispersion of the entire laser resonator is set to a negative value, and the first gain band is composed of a first laser medium having a first gain band made of a third-order nonlinear optical medium and a third-order nonlinear optical medium. Comprising a second laser medium having a second gain band not included in the first and second resonators each having the first and second laser mediums individually. In the laser medium, the two laser pulses are spatially overlapped so as to influence each other's light propagation via the third-order nonlinear effect, and the oscillation wavelength of the second laser medium is set to the first wavelength. A mode-locked pulse laser time-synchronizing device characterized by a wavelength that cannot be oscillated by a crystal of a laser medium.   2. A time synchronization apparatus for a mode-locked pulse laser according to claim 1, wherein the refractive index characteristic of one laser medium arranged so that both laser pulses overlap each other is a non-linear effect that is an optical nonlinear effect in the time domain of the preceding one laser pulse. By changing the red displacement or blue displacement characteristics due to the refraction effect, the other laser pulse that follows is transmitted through the laser medium, and the propagation speed of the other laser pulse that follows the preceding laser pulse changes. A time synchronization device for a mode-locked pulse laser, wherein the timing is synchronized with each other.   2. The time synchronization apparatus for a mode-locked pulse laser according to claim 1, wherein an interval between adjacent frequency components of the first laser pulse based on the first laser medium and a second laser pulse based on the second laser medium. The interval between adjacent frequency components is synchronized by four-wave mixing, which is an optical nonlinear effect in the frequency domain. The master laser pulse causes a temporal refractive index change in the laser medium, and the group velocity when the slave laser is delayed. A time-synchronizing device for a mode-locked pulse laser, characterized in that the refractive index change is caused so that the group velocity becomes slower when the laser beam is faster and the slave laser is advanced.   2. A time synchronization apparatus for a mode-locked pulse laser according to claim 1, wherein the first laser medium is titanium sapphire and the second laser medium is chromium forsterite.   2. A time synchronization apparatus for a mode-locked pulse laser according to claim 1, wherein the first laser medium is titanium sapphire and the second laser medium is chromium tetravalent doped yag. . 6. A mode-locked pulse laser time synchronizer according to any one of claims 1 to 5, comprising at least a nonlinear optical crystal and a half mirror, and a wavelength 1 and light having different optical frequencies ω1 output from the time synchronizer. difference in optical frequency ω4 laser pulses having a wavelength of 2 to a frequency ω2 minus the optical frequency ω2 from the sum frequency light and the optical frequency ω1 of the optical frequency ω3 plus an optical frequency ω2 to the optical frequency ω1 through the nonlinear optical crystal Output as frequency light, output light of wavelength 1 through the nonlinear optical crystal as light of optical frequency 2ω1, which is half the wavelength 1, and output light of wavelength 2 through the nonlinear optical crystal. arbitrary waveform generator, characterized in that output as the light of the light frequency 2ω2 to be half the wavelength 6 of the wavelength 2, the light of the wavelength 1 and the wavelength 2 is to output as it Location. In arbitrary waveform generator according to claim 6, 850 nm wavelength 1 of the laser pulse to be the optical frequency .omega.1, the wavelength 2 of the laser pulse to be the optical frequency .omega.2 1275 nm, and 510nm the wavelength 3 of the sum frequency light An arbitrary waveform generation device characterized by that.   7. The arbitrary waveform generating apparatus according to claim 6, wherein laser pulses having an arbitrary number of different wavelengths among the laser pulses having wavelengths 1 to 6 are synthesized and output.

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