JPH08101411A - Laser pulse oscillator - Google Patents

Abstract

(57) [Summary] [Object] To provide a laser pulse oscillator capable of easily generating a short pulse having a specific repetition frequency in harmonic forced mode locking. A loop-shaped rare-earth-doped optical fiber 1 in which an optical coupler 3 for coupling pumping light and an optical splitter 4 for outputting an optical pulse are interposed, and rare-earth-doped optical fiber 1 is provided by harmonic mode locking. In a laser pulse oscillator that outputs an optical pulse having a repetition frequency that is an integral multiple of the fundamental frequency corresponding to the loop length of the doped optical fiber 1 from the optical branching device 4, the modulation frequency is an integer of the fundamental frequency on the rare earth-doped optical fiber 1. The optical modulator 6 set to double the number and the optical modulator 10 set to have a modulation frequency that is an integral multiple of the modulation frequency of the optical modulator 6 are provided, and match the modulation frequency of the optical modulator 6. An optical pulse having a repetitive frequency is output via the optical branching device 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser for generating a high repetition short pulse, which is a laser pulse having a high repetition frequency and a short pulse width, which is necessary for constructing an ultrahigh speed optical communication system, for example. Related to pulse oscillator.

[0002]

2. Description of the Related Art In recent years, much research has been conducted on the generation of highly repetitive optical pulses by an optical fiber laser utilizing a mode-locking technique. Hereinafter, a conventional laser pulse oscillator will be described with reference to FIG. FIG. 5 is a diagram showing a configuration of an example of a conventional laser pulse oscillator, in which reference numeral 1 is an optical fiber doped with a rare earth element (hereinafter referred to as a rare earth doped optical fiber), which is provided in a loop shape. ing. Reference numeral 2 is a pumping light source that generates pumping light for pumping the rare earth-doped optical fiber 1.

Reference numeral 3 is an optical coupler for coupling the pumping light to the rare earth-doped optical fiber 1, 4 is an optical branching device for extracting an output light pulse from the rare earth-doped optical fiber 1, and 5 is a traveling direction of light in the rare earth-doped optical fiber 1. Is an optical isolator for limiting the direction to one direction (clockwise in the figure), 6 is an optical intensity modulator (hereinafter referred to as an optical modulator), and 7 is an optical filter, which are provided on the rare earth-doped optical fiber 1 in order. ing. Also, 8
Is a synthesizer, and 9 is an electric amplifier that amplifies an electric signal output from the synthesizer 8 and supplies the amplified electric signal to the optical modulator 6.

In such a structure, the light pulse is generated as follows. When the rare-earth-doped optical fiber 1 is excited by the excitation light source 2 through the optical coupler 3, the rare-earth-doped optical fiber 1 within the transmission band of the optical filter 7 is
Oscillation of continuous light occurs in the forward direction of the optical isolator 5. Next, the electric signal output from the synthesizer 8 is applied to the optical modulator 6 via the electric amplifier 9.

Generally, assuming that the resonator length is L, the refractive index of the optical fiber is n, and the speed of light is c, when a modulation is applied at a frequency f ₀ = c / (nL) determined by the resonator length, a fundamental wave is generated. The mode synchronization of is realized and a stable pulse train can be generated. Also, the modulation frequency is multiplied by q (q is an integer) times the fundamental frequency determined by the resonator length of the laser, that is, qf ₀ = q
When set to c / (nL), forced mode locking of harmonics oscillating at a frequency q times the fundamental wave can be realized. That is, q light pulses are generated at equal intervals in the resonator of the laser, and a pulse train having a repetition frequency matching the higher-order modulation frequency is generated.

That is, in the laser pulse oscillator having the configuration shown in FIG. 1, the optical branching device 4 outputs a pulse train having a repetition frequency that matches a higher-order modulation frequency. For example, when the resonator length of the laser is 200 m, the fundamental frequency determined by the resonator length is 1 MHz, but q = 100.
00 and the modulation frequency is set to 10 GHz, 10
An optical pulse train having a repetition frequency of GHz is output.

[0007]

In the above-mentioned conventional laser pulse oscillator, the pulse width of the optical pulse generated depends on the modulation frequency. That is, when the modulation frequency is determined, an optical pulse having a pulse width limited by the modulation frequency is generated. Generally, the pulse width τ of the optical pulse generated in the forced mode-locked laser is expressed by the following equation (HA Hause, "Waves and fields in optoelectroni
cs ", Prentice-hall series in solid state physical
electronics, p.267).

[Equation 1]

However, in equation (1), α_g l_g Is
Gain of laser medium, α_m l_m Is the depth of modulation, ω_M Is the modulation frequency
Wave number, ω_g Represents the gain band. It is clear from equation (1)
The pulse width is 1 / (ω_M )^1/2 Proportional to. But
Therefore, to shorten the pulse width of the generated optical pulse,
_M Need to be higher. For example, the modulation frequency is 10G
Let τ be the pulse width of the optical pulse generated at Hz.
Then, when the modulation frequency is doubled to 20 GHz,
The pulse width of the optical pulse generated at ^{1 / 2} Double
Become.

Here, the relationship between the pulse width of the output optical pulse and the modulation frequency will be described with reference to FIGS. 6 and 7. FIG. 6 is a diagram showing the modulation waveform and the appearance of the generated optical pulse when the modulation frequency is 10 GHz. In the example of this figure, by applying modulation of 10 GHz, an optical pulse having a repetition frequency of 10 GHz and a pulse width of τ is generated. In general, the modulation waveform is expressed in the form of cos (ω _M t), and when expanded in the vicinity of t = 0,

[Equation 2] Becomes

In order to shorten the pulse width of the generated optical pulse, it is necessary to reduce the curvature where the intensity of the modulation waveform represented by the sine wave is maximum (that is, near t = 0). Therefore, it is necessary to increase the modulation frequency ω _M as shown in FIG. In FIG. 7, the modulation frequency is 20
It is a figure which shows the mode of a modulation | alteration waveform and the optical pulse which generate | occur | produce at GHz. As shown in this figure, the modulation frequency is 2
When doubled, the curvature at which the intensity of the modulation waveform becomes maximum becomes smaller, and the pulse width of the generated optical pulse is 10G.
It becomes 1/2 ^1/2 times the pulse width at Hz. That is, when the modulation frequency is 20 GHz, an optical pulse having a pulse width of τ / 2 ^1/2 is generated.

As described above, when the conventional laser pulse oscillator performs forced mode locking, an optical pulse having a pulse width limited by the modulation frequency for driving the optical modulator is generated, but the pulse width should be shortened. Then, it is necessary to increase the modulation frequency, and inevitably the repetition frequency of the generated optical pulse becomes large. Therefore, it is difficult to generate an optical pulse having a shorter pulse width (that is, an optical pulse having a large duty ratio) at a certain low repetition frequency. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a laser pulse oscillator that can easily generate a short pulse having a specific repetition frequency in harmonic forced mode locking. To do.

[0012]

In order to solve the above problems, a laser pulse oscillator according to a first aspect of the present invention comprises an optical coupler for coupling pumping light and an optical branching device for outputting an optical pulse. In the laser pulse oscillator, which is composed of an inserted optical loop, outputs a light pulse having a repetition frequency of an integral multiple of the fundamental frequency corresponding to the loop length of the light loop from the optical branching device by the harmonic mode locking, A first modulator provided on the loop and having a modulation frequency set to be an integral multiple of the fundamental frequency, and a modulation frequency provided on the optical loop and having a modulation frequency of the first modulator And a second modulator set to be an integral multiple of the optical modulator, and outputting an optical pulse having a repetition frequency matching the modulation frequency of the first modulator via the optical branching device. Features and To have.

A laser pulse oscillator according to a second aspect is the laser pulse oscillator according to the first aspect, which has a light receiving element and a narrow band filter, and receives the branched light of the optical pulse output from the optical branching device. 1 is provided with extraction means for extracting a sine wave signal having a frequency that is an integral multiple of the fundamental frequency,
A closed loop is configured such that the modulation frequency of the second modulator becomes the frequency of the sine wave signal and the modulation frequency of the second modulator becomes an integral multiple of the frequency of the sine wave signal. It has a feature. Further, claim 3
The laser pulse oscillator described in claim 1 or 2 is characterized in that an optical fiber having a negative dispersion value is inserted into the optical loop to generate an optical soliton.

[0014]

According to the laser pulse oscillator of claim 1,
The light in the optical loop is modulated by the first and second modulators. Here, the modulation frequency of the first modulator is set to be an integral multiple of the fundamental frequency,
The modulation frequency of the second modulator is set to be an integral multiple of the modulation frequency of the first modulator.

Therefore, the optical pulse output from the optical branching device provided on the optical loop has the maximum intensity in the modulation waveform of the first modulator, and the first modulator. A pulse that rises at each point at which the maximum intensity of the modulated waveform obtained by combining the modulated waveform of the second modulator and the modulated waveform of the second modulator coincides. Therefore,
The frequency of the optical pulse output from the optical splitter matches the modulation frequency of the first modulator.

Further, the curvature near the point where the intensity is maximum in the combined modulation waveform is smaller than the curvature near the point where the intensity is maximum in the modulation waveform of the first modulator. Therefore, the maximum points of both modulation waveforms are narrower than the corresponding points of the conventional modulation waveform having an integral multiple of the fundamental frequency. Therefore, if the repetition frequency is the same, an optical pulse having a narrower pulse width than the conventional one is output.

According to a second aspect of the laser pulse oscillator, the extraction means receives the branched light of the optical pulse output from the optical branching device and outputs a sine wave signal having a frequency that is an integral multiple of the fundamental frequency. To extract. Further, a closed loop is formed so that the modulation frequency of the first modulator becomes the frequency of the sine wave signal and the modulation frequency of the second modulator becomes an integral multiple of the frequency of the sine wave signal. Since it is configured, similarly to the first aspect, the frequency of the optical pulse output from the optical branching device matches the modulation frequency of the first modulator. Moreover, if the repetition frequency is the same, an optical pulse having a narrower pulse width than the conventional one is output. Further, according to the laser pulse oscillator of the third aspect, an optical fiber having a negative dispersion value is inserted into the optical loop to generate an optical soliton, so that an optical pulse having a narrower pulse width is stably output. To be done.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of a laser pulse oscillator according to a first embodiment of the present invention.
The same parts as those in FIG. 5 are designated by the same reference numerals, and the description thereof will be omitted. In the laser pulse oscillator shown in FIG. 1, as the rare earth-doped optical fiber 1, for example, an erbium-doped optical fiber is used. In this case, the oscillation wavelength of the laser is in the 1.5 μm band. A semiconductor laser can be used as the excitation light source 2.

On the rare earth-doped optical fiber 1, an optical coupler 3 for coupling the pumping light generated by the pumping light source 2 to the rare earth-doped optical fiber 1, an optical modulator (second modulator) 10, and an output optical pulse. An optical branching device 4 for extracting light, an optical isolator 5 for limiting the traveling direction of light to one direction (clockwise in the figure), an optical modulator (first modulator) 6, and an optical filter 7 having a predetermined transmission band. , Respectively, are provided in order clockwise in the figure. As the optical modulator, for example, a Mach-Zehnder type intensity modulator made of lithium niobate (LiNbO ³ ) or the like can be used.

Further, the laser pulse oscillator shown in FIG. 1 has a synthesizer 8, electric amplifiers 9 and 9, and a frequency converter 11, and an electric signal output from the synthesizer 8 passes through one electric amplifier 9. The electric signal is supplied to the optical modulator 6, and the electric signal is supplied to the optical modulator 10 via the frequency converter 11 and the other electric converter 9. The frequency converter 11 is
It has a function of multiplying the frequency of an input signal by an integer (however, the integer in “integer multiple” means an integer of 1 or more) and outputting the result.

The process of generating an optical pulse in the laser pulse oscillator having the above configuration will be described below. When the rare earth-doped optical fiber 1 is excited by the excitation light source 2 via the optical coupler 3, continuous light oscillation occurs in the forward direction of the optical isolator 5 within the transmission band of the optical filter 7. Next, the electric signal output from the synthesizer 8 is sent to the optical modulator 6 via the electric amplifier 9 on one side and to the frequency converter 11 as well.
And to the optical modulator 10 via the other electric amplifier 9. At this time, the frequency (modulation frequency) of the signal that drives the optical modulator 10 is set to an integral multiple of the modulation frequency of the optical modulator 6 by the frequency converter 11. Here, the modulation frequency of the optical modulator 6 is ω ₁ , and the modulation frequency of the optical modulator 10 is ω ₂ . However, ω ₂ is an integer multiple of ω ₁ (ω ₂ = p
ω ₁ and p are integers of 1 or more).

The modulation waveform of the optical modulator 6 is COS (ω ₁
t), since the modulation waveform of the optical modulator 10 is represented by COS (ω ₂ t), when both the optical modulators 6 and 10 are driven,
The modulation waveform is represented by the product of the modulation waveforms of the optical modulators 6 and 10 as shown in the following equation.

(Equation 3)

In this case, since two modulations are applied in the resonator, the transmittance of the entire system becomes maximum when the points at which the intensity of each modulation waveform becomes maximum overlap, and an optical pulse is generated at that point. To do. This generation frequency is ω ₁ , and the repetition of the generated optical pulse is frequency ω ₁ .
Here, when the expression (3) is expanded in the vicinity of t = 0, ω ₂ =
Since it is pω ₁ , the following equation is obtained.

[Equation 4]

Comparing equation (4) with equation (2), the modulation frequency ω _M has the same form as (p ² +1) ^1/2 ω ₁ . That is, the intensity of the combined modulation waveform should be maximized by increasing p (near t = 0).
Curvature becomes smaller than that when one optical modulator is driven at the modulation frequency ω ₁ , and the pulse width of the generated optical pulse becomes shorter. The pulse width τ at this time is given by the following equation.

(Equation 5)

Therefore, the pulse widths of the optical pulses generated when the optical modulators 6 and 10 are driven at the modulation frequencies ω ₁ and ω ₂ , respectively, are as follows when one optical modulator is driven at the modulation frequency ω _1. It is 1 / (p ² +1) ^1/4 times the pulse width of the generated optical pulse. As described above, according to the laser pulse oscillator described above, while the repetition frequency of the generated optical pulse is fixed at ω ₁ , the short pulse that cannot be obtained by the conventional technique unless the repetition frequency is set to be larger than ω _1. Light pulses of width can be generated.

Here, for example, ω ₁ = ω ₂ = 10 GHz
Then, the pulse width of the optical pulse generated when both of the optical modulators 6 and 10 are driven will be considered with reference to FIGS. 1 and 2 (in this case, since p = 1, frequency conversion is performed). The container 11 is unnecessary). Note that FIG. 2 is a diagram showing various waveforms generated under the above conditions. As is clear from FIG. 2, the repetition frequency of the generated optical pulse is determined by the cycle in which the points at which the intensity of each modulation waveform is maximized overlap. Therefore, here, it becomes 10 GHz. What is developed by combining the modulation waveform of the optical modulator 6 (first modulator) and the modulation waveform of the optical modulator 10 (second modulator) in the vicinity of t = 0 is: By setting p = 1 in the equation (4), the following equation is obtained.

(Equation 6)

From the above equation (6), it can be seen that the curvature becomes small where the intensity of the combined modulation waveform becomes maximum (near t = 0). Further, the pulse width of the generated optical pulse becomes shorter as the curvature becomes smaller. Therefore,
The pulse width of the optical pulse is 1/2 as compared with the pulse width generated when one optical modulator is driven at 10 GHz.
It becomes ^1/4 times. That is, it is possible to generate an optical pulse having a short pulse width which could not be obtained at a repetition frequency of 10 GHz.

Next, a second embodiment of the present invention will be described. FIG. 3 is a diagram showing an example of the configuration of the laser pulse oscillator according to the second embodiment. In this figure, the same parts as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. In FIG. 3, reference numeral 12 denotes an optical branching device, and an optical branching device 4
The laser output (light pulse) output from is branched. 1
Reference numeral 3 denotes a clock extractor (extracting means) composed of a light receiving element, a narrow band electric filter, and an electric amplifier, which is a sine wave having a specific frequency from one of the two optical pulses output from the optical branching device 12. Extract and output the signal.

Reference numerals 14 and 14 respectively denote optical modulators 6 and 1 respectively.
Is a phase shifter for adjusting the phase of the electric signal supplied to 0,
One phase shifter 14 adjusts the phase of the clock signal extracted by the clock extractor 13 and supplies it to the optical modulator 6 via one electrical amplifier 9. Also, the other phase shifter 14
Adjusts the phase of the electric signal obtained by frequency-converting the clock signal extracted by the clock extractor 13 by the frequency converter 11, and the optical modulator 10 via the other electric amplifier 9.
Supply to

Next, a process in which the laser pulse oscillator having the above-mentioned configuration outputs an optical pulse will be described. First,
The output of the laser is taken out through the optical branching device 4, and this output (optical pulse) is further branched by the optical branching device 12.
Then, one output of the optical branching device 12 is connected to the clock extractor 1
3 is input, where a clock signal that is a sine wave of a specific frequency is extracted.

This clock signal is phase-adjusted by one phase shifter 14 and then amplified by one electric amplifier 9 and applied to the optical modulator 6. The frequency of the clock signal extracted by the clock extractor 13 is multiplied by an integer by the frequency converter 11 and applied to the optical modulator 10 via the other phase shifter 14 and the other electric amplifier 9. That is, as shown by the dotted line in FIG. 3, a closed loop is formed from the reception of the laser output to the application of the clock signal to each of the optical modulators 6 and 10. In the resonator, the light is intensity-modulated at a frequency synchronized with the clock signal, but since this light is originally a signal emitted from the laser, it is always optimally modulated.

Here, let us consider a case where a 10 GHz clock extractor is used as the clock extractor 13 and both optical modulators 6 and 10 are driven at 10 GHz. A clock signal with a frequency near 10 GHz that does not match an integral multiple of the fundamental frequency cannot generate a stable pulse train, and thus disappears in the clock extraction process.
In the clock signal that matches an integral multiple of the fundamental frequency, the modulation frequency and the repetition frequency of the optical pulse completely match, so that stable pulse oscillation is gradually strengthened.

When such an operation is repeated, it is noisy at first, and a certain 1 corresponding to an integral multiple of the fundamental frequency is obtained.
Only one clock signal (frequency around 10 GHz) remains. That is, each optical modulator 6 and 10 is driven by only one clock signal, and 10 GHz harmonic mode locking is achieved. The repetition frequency of the optical pulse obtained in this case is 10 GHz, which means that an optical pulse having a shorter pulse width than the conventional one can be generated.

The pulse width of the optical pulse obtained at this time is
It is the same as the pulse width of the optical pulse obtained by the first embodiment described above. Similar to the first embodiment, the pulse width of the optical pulse generated when the optical modulator 6 is driven at the modulation frequency ω ₁ and the optical modulator 10 is driven at the modulation frequency ω ₂ is given by the equation (5). Therefore, an optical pulse having a shorter pulse width than that of the conventional one can be obtained.

Further, in the laser pulse oscillator according to the second embodiment, even if the resonator length changes due to temperature fluctuation and the repetition of the optical pulse changes, modulation is performed with the clock signal synchronized with the repetition frequency of the optical pulse. Therefore, there is no deviation between the modulation frequency and the optical pulse repetition frequency. That is, the waveform of the optical pulse is not deteriorated by the temperature fluctuation. Therefore, pulse oscillation continues stably for a long time. Further, since a high-precision synthesizer and an active negative feedback circuit for stabilizing the resonator are not required, the economical advantage is great.

In addition, in the laser pulse oscillator according to the second embodiment, for example, as shown in FIG.
By inserting the optical pulse compression optical fiber 15 between 0 and the optical branching device 4 for extracting the laser output, the pulse width of the generated optical pulse can be further shortened by using the effect of the optical soliton. . Here, the optical soliton will be described. An optical soliton is a stable optical pulse generated by balancing the pulse width expansion due to the negative dispersion of the optical fiber and the pulse width compression due to the self-phase modulation effect. It has the characteristic of propagating.

The peak intensity P _{N = 1} required to produce a standard soliton with _{N = 1} is given by:

(Equation 7) In the equation (7), D is the group velocity dispersion at the wavelength λ of the optical fiber, c is the speed of light, n ₂ is the nonlinear refractive index, and τ
Is the pulse width, and w is the spot size of the optical fiber.

That is, the optical pulse compression optical fiber 15
By making the group velocity dispersion of N to be negative, an optical soliton can be generated and a short pulse whose pulse width does not spread can be obtained. For example, group velocity dispersion D = -1 ps / km / nm, pulse width τ = 2 ps, spot size w = 3 μ
m and the wavelength λ = 1.55 μm, the peak intensity required to generate a standard soliton is about 216 from equation (7).
It becomes mW. When the repetition frequency of the optical pulse is 10 GHz, the average intensity in the resonator is about 4.3 mW. This level of intensity can be easily generated in the laser pulse oscillator according to the first and second embodiments.

That is, the optical fiber 15 for optical pulse compression
, The dispersion value in the 1.55 μm wavelength band is -1 ps
/ Km / nm dispersion-shifted fiber is used, and the modulation frequency of each of the optical modulators 6 and 10 is 10 GH.
By setting z, a short pulse having a repetition frequency of 10 GHz and a pulse width of 2 ps can be stably generated. Under the above conditions, the modulation frequency is 10
The pulse width of the optical pulse obtained when only one GHz optical modulator is used is about 3.0 to 2.7 ps. That is, by using the two optical modulators 6 and 10,
Repetition frequency is 1 which could not be generated by conventional technology
It is possible to easily generate an optical pulse of 0 GHz and a pulse width of 2 ps.

Further, there is a feature that the noise of the generated optical pulse can be removed or reduced. The noise of the optical pulse includes spontaneous emission optical noise generated from the rare earth-doped optical fiber 1 and a dissipative wave (non-soliton component) that does not form an optical soliton. The spontaneous emission noise and the non-soliton component are continuous lights each having a temporally uniform intensity.

These noises are first pulse-modulated by passing through the optical modulator 6 in FIG. At this time, the light intensity in the valley portion of the modulated signal is weakened, so that noise is reduced. The thus pulse-modulated noise spreads due to the group velocity dispersion of the optical fiber while propagating through the optical fiber in the resonator, and becomes continuous again. The noise that becomes continuous in this way is the optical modulator 1
By passing 0, the pulse shape is formed again, and the light intensity of noise becomes weak as in the case described above.

By repeating such processing, the noise of the optical pulse is removed. As described above, by using the two optical modulators 6 and 10, not only an optical pulse with a short pulse width can be generated, but also an optical pulse with a high S / N ratio from which noise is removed is generated. There is an advantage that can be done. In the above-described first and second embodiments, the operation of each laser pulse oscillator has been described centering around the case of generating an optical pulse having a repetition frequency of 10 GHz. It can also be used to obtain optical pulses of repetition frequencies other than.

[0043]

As described above, according to the present invention,
The light in the optical loop is modulated by the first and second modulators. The modulation frequency of the first modulator is set to be an integral multiple of the fundamental frequency, and the modulation frequency of the second modulator is an integral multiple of the modulation frequency of the first modulator. Therefore, the frequency of the optical pulse output from the optical branching device matches the modulation frequency of the first modulator.

The curvature near the point where the intensity is maximum in the combined modulation waveform is smaller than the curvature near the point where the intensity is maximum in the modulation waveform of the first modulator. Therefore, the maximum points of both modulation waveforms are narrower than the corresponding points of the conventional modulation waveform having an integral multiple of the fundamental frequency. Therefore, if the repetition frequency is the same, it is possible to output an optical pulse having a narrower pulse width than the conventional one.

The frequency of the sine wave signal extracted from the optical pulse output from the optical branching device (frequency that is an integral multiple of the fundamental frequency) becomes the modulation frequency of the first modulator, and Since the closed loop is configured such that the frequency that is an integral multiple of the frequency of the sine wave signal becomes the modulation frequency of the second modulator, there is an effect that an optical pulse with a narrow pulse width can be stably output. There is (claim 2). Furthermore, since an optical fiber having a negative dispersion value is inserted in the optical loop to generate an optical soliton, there is an effect that an optical pulse having a narrower pulse width can be stably output (claim 3). ).

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example of a laser pulse oscillator according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a modulation waveform and a state of an optical pulse in the laser pulse oscillator.

FIG. 3 is a diagram showing a configuration example of a laser pulse oscillator according to a second embodiment of the present invention.

FIG. 4 is a diagram showing another configuration example of the same laser pulse oscillator.

FIG. 5 is a diagram showing a configuration example of a conventional laser pulse oscillator.

FIG. 6 is a diagram showing a modulation waveform and a state of an optical pulse in the laser pulse oscillator.

FIG. 7 is a diagram showing a modulated waveform and a state of an optical pulse in the laser pulse oscillator.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Rare-earth-doped optical fiber, 2 ... Excitation light source, 3 ... Optical coupler, 4 ... Optical splitter, 5 ... Optical isolator, 6 ... Optical modulator (first modulator), 7 ... Optical filter, 8 ... Synthesizer, 9 ... Electric amplifier, 10 ... Optical modulator (second modulator), 11 ... Frequency converter, 12 ... Optical splitter, 13 ... Clock extractor (extracting means), 14 ... Phaser, 15 ... Optical fiber for optical pulse compression.

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Claims (3)

[Claims] 1. An optical loop comprising an optical coupler for coupling pumping light and an optical branching device for outputting an optical pulse, wherein the loop length of the optical loop is reduced by harmonic mode locking. A laser pulse oscillator for outputting an optical pulse having a repetition frequency that is an integral multiple of the corresponding fundamental frequency from the optical branching device, is provided on the optical loop, and the modulation frequency is set to be an integral multiple of the fundamental frequency. A second modulator provided on the optical loop and having a modulation frequency set to be an integral multiple of the modulation frequency of the first modulator; A laser pulse oscillator, wherein an optical pulse having a repetition frequency that matches a modulation frequency of a third modulator is output through the optical branching device. 2. An extracting unit having a light receiving element and a narrow band filter, which receives the branched light of the optical pulse output from the optical branching device and extracts a sine wave signal having a frequency that is an integral multiple of the fundamental frequency. A closed loop such that the modulation frequency of the first modulator becomes the frequency of the sine wave signal and the modulation frequency of the second modulator becomes a frequency that is an integral multiple of the frequency of the sine wave signal. The laser pulse oscillator according to claim 1, wherein the laser pulse oscillator comprises: 3. The laser pulse oscillator according to claim 1, wherein an optical fiber having a negative dispersion value is inserted into the optical loop to generate an optical soliton.