JPH07263786A - Mode synchronism laser device - Google Patents

Abstract

PURPOSE:To improve stability of operation to change of resonator optical path length in a mode synchronism laser device. CONSTITUTION:In a mode synchronism laser device wherein a light modulator 2 and a light amplifier 3 are provided inside a resonator, an optical delay unit 8 is provided inside a resonator and an optical path length of the optical delay unit 8 is controlled to minimize electric power of a side band frequency element by converting a part of resonance output light to an electric signal by a light electric converter 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to generation of ultrashort optical pulses used for optical communication or optical measurement. In particular, it relates to stabilization of the operation of a mode-locked laser device that generates such ultrashort light pulses.

[0002]

2. Description of the Related Art A mode-locked laser can generate an optical pulse train with a high repetition period of 10 GHz or more, can generate an ultrashort optical pulse having a pulse width of several picoseconds or less, can oscillate a wavelength in a wide band, and can generate a long wavelength. It has advantages such as easy generation of transform limit optical pulse (optical pulse with minimum time bandwidth) suitable for distance transmission, and has been used in fields such as large capacity and long distance optical communication and optical measurement. R & D is actively progressing toward the application of

FIG. 17 is a block diagram showing a conventional ring resonator type mode-locked laser device. In this conventional example, a driving power supply 1, an optical modulator 2 driven by the driving power supply 1 to modulate the loss or phase of light at a predetermined frequency, an optical amplifier 3 for amplifying the modulated optical pulse, and a progress of the optical pulse. An optical isolator 4 that defines the direction and blocks reflected return light, an optical coupler 5 that extracts the amplified optical pulse to the outside, and an optical waveguide 7 that optically couples each of these components. Further, the wavelength tunable optical filter 6 may be arranged in the resonator.

The optical modulator 2 is mainly composed of LiNbO ₃
The one using the electro-optical effect such as is used. As the optical amplifier 3, a rare earth-doped optical fiber amplifier doped with a rare earth element such as Er or Nd or a semiconductor laser amplifier is mainly used. Wavelength tunable optical filter 6
For this purpose, a dielectric multi-layer filter is mainly used to change the transmission center wavelength depending on the incident angle of light.
An optical fiber is mainly used as the optical waveguide 7.

18 to 20 show examples of the structure of a rare earth-doped optical fiber amplifier. The rare-earth-doped optical fiber amplifier includes a rare-earth-doped optical fiber 31, a pumping light source 32 that pumps the rare-earth-doped optical fiber 31, and a wavelength division multiplexer that multiplexes or splits the pumping light from the pumping light source 32 and an optical pulse. It is provided with the wave device 33. FIG. 18 shows the configuration in the case of backward pumping, in which pumping light is incident on the rare earth-doped optical fiber 31 in the direction opposite to the optical pulse. FIG. 19 shows the configuration in the case of forward pumping, in which the pumping light is combined with the optical pulse and is incident on the rare earth-doped optical fiber 31. FIG. 20 shows a configuration in the case of bidirectional pumping, in which a rare earth-doped optical fiber 31
Excitation light is incident from both sides of.

FIG. 21 shows an example of the configuration when a semiconductor laser amplifier is used as the optical amplifier 3. The semiconductor laser amplifier 34 is excited by the excitation current source 35 and amplifies the optical pulse.

Next, the operation principle of the conventional mode-locked laser device will be described with reference to FIGS. 22 and 23.
FIG. 22 shows a typical spectrum characteristic obtained by mode locking, and FIG. 23 shows its time characteristic. As shown in FIG. 17, the optical modulator 2, the optical amplifier 3, and the optical coupler 5 are coupled in a ring shape via the optical waveguide 7 to form a ring resonator. The optical path length of each component is the physical length h of that component.
_It is a value obtained by multiplying _i by the refractive index n _i, and the optical path length L of the ring resonator is the sum thereof, that is, L = Σh _i n _i (1) Now, in the ring resonator, the reference resonance frequency f _r
There are a number of longitudinal modes with a frequency spacing given by = c / L (c is the speed of light). Here, the optical modulator 2 in the ring _{resonator, f m = N · f r} (N is an integer of 1 or more) was added to the light modulation ... (2) comprising repetition frequency, as shown in FIG. 22, becomes phase aligned mode-locked oscillation state of all the longitudinal modes of the frequency interval N · f _r, it is an optical pulse train of repetition period 1 / (N · f _r) as shown in FIG. 23 is obtained. This equation (2) is the mode synchronization condition.

The pulse width corresponds to the reciprocal of the oscillation spectrum width Δν determined by the envelope of many longitudinal mode spectra, and the center of this spectrum envelope is the central wavelength (frequency ν
₀ ). In addition, the wavelength tunable optical filter 6 is provided in the resonator.
Is provided, and the oscillation center wavelength can be changed by changing the transmission wavelength.

[0009]

However, in the mode-locked laser device, when the cavity optical path length changes due to temperature change or the like, the longitudinal mode frequency interval _fr changes. Therefore, (2) the type of mode locking condition is satisfied, it is necessary to change the repetition frequency f _m of the mode-locked in accordance with a change in the longitudinal mode frequency interval f _r. Further, when changing the oscillation center wavelength by the wavelength tunable optical filter 6, the refractive index n _i of each component by the wavelength dispersion changes, (1) the optical path length L and the fundamental resonant frequency f _r changes in, There is a problem that the mode-locking condition of the equation (2) does not hold.

As one method for solving such a problem, the modulation frequency f _m is changed according to the change of the optical path length in order to achieve the mode-locking condition with respect to the temperature change and the change of the oscillation wavelength. Can be considered. However, in the above-described application example, the modulation frequency f _m is generally fixed, which is not practical.

An object of the present invention is to solve the above problems and to provide a mode-locked laser device in which the operation stability with respect to the change of the optical path length of the resonator is enhanced.

[0012]

In the mode-locked laser device of the present invention, an optical delay device having an electrically variable optical path length is provided in the resonator, and a part of the output light extracted from the resonator is branched. And a bandpass for extracting at least one frequency component (sideband frequency) component of an integer multiple of the cavity longitudinal mode frequency interval other than an integer multiple of the pulse repetition frequency of the output light from the converted electrical signal. It is characterized by comprising a filter and a processing means for controlling the optical path length of the optical delay device so that the power of the frequency component extracted by the bandpass filter is minimized.

As the resonator, a ring resonator or a Fabry-Perot resonator may be used. An optical filter having a variable transmission wavelength may be provided in the resonator.

[0014]

[Operation] In order to minimize the power of the sideband frequency component of the electric signal converted from the mode-locked laser output light,
It feeds back to the optical delay device in the resonator. This makes it possible to prevent a change in the optical path length of the laser resonator and realize stable mode-locking operation.

It is also possible to change the oscillation wavelength by providing an optical filter having a variable transmission wavelength in the resonator. In this case, a deviation from the mode-locking condition occurs when the oscillation wavelength changes, but by adjusting the resonator length by detecting the deviation of the mode-locking condition from the power of the sideband frequency component, the optical path length of the entire resonator can be adjusted. Can be corrected to set a new mode synchronization condition.

[0016]

1 is a block diagram showing a mode-locked laser device according to a first embodiment of the present invention. In this embodiment, the present invention is applied to a ring-resonance mode-locked laser, and an optical modulator 2 for modulating the loss or phase of light at a predetermined frequency and an optical modulator 2 An optical amplifier 3 that amplifies the optical amplifier, an optical waveguide 7 as an optical unit that optically couples the optical amplifier 3 and the optical modulator 2 to form a resonator, and an output unit that extracts output light from the resonator. The optical coupler 5 of FIG. The optical waveguide 7 forms a ring resonator by connecting the optical modulator 2, the optical amplifier 3, and the optical coupler 5 in a ring shape. The drive power source 1 is connected to the optical modulator 2. The feature of this embodiment is that an optical delay device 8 having an electrically variable optical path length is provided in the resonator, and a part of the output light extracted by the optical coupler 5 is branched. An optical branching device 9 and an opto-electrical converter 10 are provided as means for converting into an electric signal, and the frequency component of an integer multiple of the resonator longitudinal mode frequency interval other than the integer multiple of the pulse repetition frequency of the output light from the converted electric signal, that is, A bandpass filter 12 for extracting at least one of the sideband frequency components is provided, and electric as a processing means for controlling the optical path length of the optical delay device 8 so that the power of the frequency component extracted by the bandpass filter 12 is minimized. It is provided with the signal processor 13.

As the optical waveguide 7, in addition to the optical fiber described as the conventional example, a channel type waveguide formed in a planar substrate shape, for example, M. Kawachi et al., "Silica waveg."
uideon silicon and their application to integrated
-optical components ", Opt. & Quantum Electron., 209
The planar lightwave circuit shown in 0,23, pp.391-417 can also be used.

2 to 4 show power spectra of the output light of the mode-locked laser device. FIG. 2 shows the case where the mode-locking condition shown in the equation (2) is completely satisfied, and FIG. 3 shows the case where the resonator length changes and deviates from the mode-locking condition.
Denote sideband frequency components cut out by the bandpass filter 12. The principle of stabilizing the operation will be described with reference to these drawings.

A part of the optical pulse taken out of the resonator by the optical coupler 5 is branched by the optical branching device 9, and the photoelectric converter 1
It is converted into an electric signal by 0. At this time, in the power spectrum of the electric signal, when the driving condition completely satisfies the mode-locking condition, that is, the formula (2), the emission line spectrum of the mode-locking frequency f _m (= modulation frequency) as shown in FIG. Is obtained. Although omitted in this figure, an integral multiple of the frequency components of the mode-locked frequency f _m (2f _m, 3
f _m, ...) are also present.

However, when the cavity length changes and the operation destabilizes due to deviation from the mode condition, the power spectrum shows the longitudinal direction of the laser cavity around the mode-locking frequency f _m as shown in FIG. side band is generated in the period, which corresponds to the mode frequency interval f _r. At this time, the sideband frequency component power P _s changes sensitively to the deviation from the mode-locking condition (change in resonator optical path length). For example, the permissible range of the optical path length change in stabilizing the operation of the mode-locked laser having a cavity optical path length of several tens of meters is about ± 10 μm, but the sideband frequency component power P _s for the optical path length change of several μm Change by 30 dB or more. Therefore,
At least one sideband frequency component S _S is cut out by the bandpass filter 12, and its change amount ΔP _s
By using as an error signal, it is possible to stabilize the mode-locking operation by controlling with high sensitivity.

The sideband frequency component extracted by the bandpass filter 12 can be selected arbitrarily. Also, the sideband frequency component S _S to be selected is not limited to one, and the bandpass filter 12
It is also possible to extract a plurality of subband frequency components by using a wide band. In this case, the mode-locking frequency f
_{If m} and a high-frequency component having a wide band are sufficiently removed, the sideband frequency component power P _s
Can be increased, and highly sensitive detection can be achieved.

FIG. 5 and FIG. 6 are diagrams showing a specific example of the optical delay device 8 and show a structural example utilizing coupling of two optical waveguides by a lens system. Between the two optical waveguides 7, two optical lenses 81 and 82 for optically coupling the two are provided. In the example shown in FIG. 5, the optical waveguide 7 on the incident side, the optical lenses 81 and 82, and the optical waveguide 7 on the outgoing side are arranged on a straight line, and either the optical lens 81 or 82 (82 in this example) for entering or exiting. The corresponding optical waveguide 7 is fixed on the same electric stage 83, and the electric stage 83 is moved along the light beam direction by a drive signal from the drive power source 84. Thereby, the optical path length between the two optical waveguides 7 can be changed. In the example shown in FIG. 6, the optical waveguide 7 and the optical lens 81 on the incident side are arranged in parallel with the optical waveguide 7 and the optical lens 82 on the outgoing side, and are optically coupled by a right-angle mirror 85 using a triangular prism or the like. To do. The right-angle mirror 85 is fixed on the electric stage 83 and moved along the light ray direction to adjust the optical path length.
As the electric stage 83, a pulse stage whose traveling direction changes depending on the polarity of the input electric pulse and whose movement amount is determined by the number of input electric pulses can be used. FIG. 7 shows an operation example when a pulse stage is used.

FIG. 8 is a perspective view showing another example of the optical delay device 8, and shows an example of a structure in which the optical waveguide is extended by tension to change the optical path length. That is, a relatively long optical waveguide 7 such as an optical fiber is formed on the drum 86 made of a piezo element.
And the diameter of the drum 86 is changed by the voltage from the driving power supply 84, thereby changing the tension to the optical waveguide 7 and changing the optical path length.

9 and 10 are views showing still another configuration example of the optical delay device 8, FIG. 9 is a perspective view, and FIG. 10 is a sectional view of a plane crossing the ray direction. In this configuration example, the optical path length is changed by electrically changing the refractive index of the optical waveguide. That is, the optical waveguide 88 whose refractive index is electrically changed is formed on the substrate 87, and the electrodes 89 are provided on both sides thereof. When a voltage V is applied between the two electrodes 89, an electric field E is generated in the optical waveguide 88, the refractive index changes, and the optical path length changes. A ferroelectric such as LiNbO ₃ having an electro-optical effect can be used as the optical waveguide 88 whose refractive index electrically changes.

FIG. 11 shows an operation example of the optical delay device 8 shown in FIG. 8 or FIGS. In this case, the movement amount is determined by the absolute value of the input electric signal.

As described above, by using the optical delay device 8 capable of changing the optical path length by the electric signal, the drive signal S _D from the electric signal processor 13 can be fed back to the resonator length of the laser. Becomes

FIG. 12 is a diagram for explaining the operation of the electric signal processor 13 for driving the optical delay device 8. Electric signal processor 1
The operation algorithm of No. 3 differs depending on the optical delay device 8 used. First, the operation when the configuration shown in FIG. 5 or FIG. 6 is used as the optical delay device 8, and then the operation when the configuration shown in FIG. 8 or FIGS.

When the structure shown in FIG. 5 or 6 is used as the optical delay device 8, first, the drive signal S _D (the number of pulse signals) is changed to sweep the resonator optical path length over a certain range, measuring the sideband frequency component power P _s that changes at that time. The sweep range of the optical path length is set sufficiently larger than the maximum value expected as the deviation of the optical path length from the mode-locking condition caused by the temperature change or the like. 12
Indicates the relationship between the sideband frequency component power P _s obtained by this sweep and the optical path length L. If the optical path length L ₀ that minimizes the sideband frequency component power P _s and the drive signal value S _D ′ at that time are detected, this drive signal value S
_D' is input to the optical delay device 8 and the initial value of the optical path length is set to L ₀ . Since the optical delay device 8 having the configuration shown in FIG. 5 or FIG. 6 has a structure that maintains the optical delay amount as it is even when the signal is cut off, after that, the drive signal S _D remains unchanged until the sideband frequency component power P _s changes. The value can be zero.

The optical path length changes due to temperature changes and the like, and the sideband frequency component power P _s is set to a preset power limit value P _limit (a value lower than the upper limit of the power P _s that can be regarded as stable for the operating state). If it exceeds, the optical delay device drive signal S _D = + ΔS _D is first input to the optical delay device 8 to _check the change in the power P _s . If the electric power P _s decreases, + ΔS until the electric power P _s becomes less than or equal to the electric power limit value P _limit.
Repeat _D input. When P _s ≤ P _limit ,
The value of the optical delay device drive signal S _D is returned to zero. On the contrary, the optical delay device drive signal S _D = + ΔS _D is input to the optical delay device 8 and the power P
_{When s} increases, the sign of the optical delay device drive signal is inverted to −ΔS _D , and the power P _s is decreased. By repeating this, the optical delay device drive signal S _D is returned to zero when P _s ≦ P _limit . Thereby, the change in the optical path length of the resonator can be suppressed, and the operation can be stabilized.

When the structure shown in FIG. 8 or 9 or 10 is used as the optical delay device 8, the optical path length of the resonator is first swept like the case where the structure shown in FIG. 5 or 6 is used. , The drive signal value S _D ′ at which the sideband frequency component power P _s has the minimum value is detected, and the initial value L ₀ of the optical path length is set. However, in this case, since the optical delay amount is determined by the absolute value of the drive signal, the drive signal value S _D ′ must be maintained as it is in order to maintain the optical delay amount as it is.

When the optical path length changes due to temperature changes or the like and the sideband frequency component power P _s exceeds the power limit value P _limit , the optical delay device drive signal S _D is changed from S _D ′ to S _D ′.
Change to _D ′ + ΔS _D and see the change in power P _s . When the power P _s decreases, the value of the optical delay device drive signal S _D is further increased, and this is repeated until the power P _s becomes the power limit value P _limit or less. The increase of the optical delay device drive signal S _D is stopped when P _s ≦ P _limit . On the contrary, when the power P _s is increased, the optical delay device drive signal is changed to S _D ′ −ΔS _D , and similarly, the optical delay device drive signal S _D is decreased until P _s ≦ P _limit . Thereby, the change in the optical path length of the resonator can be suppressed, and the operation can be stabilized.

FIG. 13 is a block diagram showing a concrete configuration example of the electric signal processor 13. This electrical signal processor 13
Includes a low-frequency transmission filter 131 for analog signals, an analog / digital converter 132, and a computer 133 using a microcomputer or the like, and optionally a digital / analog converter 134. The low-frequency transmission filter 131 extracts a change component of the resonator optical path length from the sideband frequency component S _s . Here, the band of the low frequency transmission filter 131 is set to be as narrow as possible, including a fluctuation frequency component due to temperature and other factors that cause a change in the resonator optical path length. This allows
It is possible to remove noise components in the high frequency region. Next, the analog-to-digital converter 132 converts the sideband frequency component S _s, which is an analog signal, into a digital signal. Then, the computer 133 determines the control signal (polarity, pulse number or level) according to the above algorithm. When a digital signal drive as shown in FIG. 5 or 6 is used as the optical delay device 8, the optical delay device drive signal S _D is directly supplied from the computer 133 to the optical delay device 8. On the other hand, in the case of using the analog signal drive shown in FIG. 8 or 9 and 10, the optical delay drive signal S _D is generated by using the digital-analog converter 134.

Here, an example in which the low-frequency transmission filter 131 is arranged before the analog / digital converter 132 is shown, but the digital / analog converter 134 is provided on the output side.
When using, it can be arranged in the output stage.
Further, when a digital filter is used as the low frequency transmission filter 131, it may be arranged at the subsequent stage of the analog / digital converter 132 or the computer 133.

As described above, in this embodiment, the bandpass filter 1 is converted from the electric signal from the photoelectric converter 10.
2, the sideband frequency component S _s is extracted, and the electric signal processor 13 drives the drive signal S to the optical delay device 8 installed in the resonator so that the power thereof, that is, the sideband frequency component power P _s is minimized. _{Give D} feedback. As a result, the optical path length of the resonator is kept constant, and stable mode-locked laser operation can be realized.

FIG. 14 is a block diagram showing the mode-locked laser device according to the second embodiment of the present invention. In this embodiment, the present invention is applied to a Fabry-Perot resonance type mode-locked laser, and an optical modulator 2, an optical amplifier 3 and an optical delay device 8 are provided.
It is different from the first embodiment in that it is provided with two light reflectors 27 which are arranged so as to sandwich them. The light reflector 27 reflects most of the incident light but partially transmits it as output light. The operation of this embodiment is the same as that of the first embodiment.

FIG. 16 is a block diagram showing a mode-locked laser device according to the third embodiment of the present invention. This example
The difference from the first embodiment is that the variable wavelength optical filter 6 is provided in the resonator. The variable wavelength optical filter 6 can change the transmission center wavelength within the gain bandwidth of the optical amplifier 3. As such a wavelength tunable optical filter 6, in addition to a conventional dielectric multilayer filter, for example, Hirabayashi et al., "600 Channel Selectable Liquid Crystal Tunable Wavelength Filter", Proc. It is also possible to use an etalon type wavelength tunable optical filter using a liquid crystal as shown in FIG.

As described above, the variable wavelength optical filter 6
When the oscillation wavelength of the resonator is changed by, the optical path length L = Σh _i n _i of the resonator changes due to wavelength dispersion of the refractive index.
Therefore, the reference resonance frequency _fr = c / L changes, and (2)
Mode locking condition of the equation _{f m = N · f r (} N is an integer of 1 or more) does not hold. However, if the wavelength change by the wavelength tunable optical filter 6 is sufficiently slower than the response time of the feedback system, the shift of the mode-locking condition is detected by the sideband frequency component power P _s , and the optical delay device 8
By adjusting the physical length of the resonator, it is possible to correct the change in the optical path length of the entire resonator. Therefore, even if the oscillation wavelength of the laser changes, the mode-locking condition can be automatically achieved without changing the mode-locking frequency (= modulation frequency).

FIG. 17 is a block diagram showing a mode-locked laser device according to the fourth embodiment of the present invention. This embodiment differs from the second embodiment in that the wavelength tunable optical filter 6 is provided in the resonator, and the operation is the same as that of the third embodiment.

[0039]

As described above, in the mode-locked laser device of the present invention, the electric power of the sideband frequency component of the electric signal converted from the output light is fed back to the optical delay device in the resonator. As a result, the operation can be stabilized against changes in the resonator length caused by temperature changes and the like. Further, since the mode-locking condition can be automatically achieved even when the oscillation wavelength of the laser is changed, the mode-locking frequency is constant and the operation is stable.

[Brief description of drawings]

FIG. 1 is a block diagram showing a mode-locked laser device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a power spectrum when the mode-locking condition is completely satisfied.

FIG. 3 is a diagram showing a power spectrum when deviated from a mode-locking condition.

FIG. 4 is a diagram showing a power spectrum of a sideband frequency component cut out by a bandpass filter.

FIG. 5 is a diagram showing a specific example of an optical delay device.

FIG. 6 is a diagram showing another specific example of the optical delay device.

FIG. 7 is a diagram showing an operation example when a pulse stage is used.

FIG. 8 is a perspective view showing another example of the optical delay device.

FIG. 9 is a perspective view showing still another example of the optical delay device.

10 is a sectional view of the optical delay device shown in FIG.

FIG. 11 is a diagram showing an operation example.

FIG. 12: Sideband frequency component power P _s and optical path length L
FIG.

FIG. 13 is a block diagram showing a specific configuration example of an electric signal processing circuit.

FIG. 14 is a block diagram showing a mode-locked laser device according to a second embodiment of the present invention.

FIG. 15 is a block diagram showing a mode-locked laser device according to a third embodiment of the present invention.

FIG. 16 is a block diagram showing a mode-locked laser device according to a fourth embodiment of the present invention.

FIG. 17 is a block diagram showing a conventional ring resonator mode-locked laser device.

FIG. 18 is a block diagram showing a configuration example of a rare earth-doped optical fiber amplifier, showing a configuration in the case of backward pumping.

FIG. 19 is a block diagram showing a configuration example of a rare earth-doped optical fiber amplifier, showing the configuration in the case of forward pumping.

FIG. 20 is a block diagram showing a configuration example of a rare earth-doped optical fiber amplifier, showing a configuration in the case of bidirectional pumping.

FIG. 21 is a block diagram showing a configuration example when a semiconductor laser amplifier is used as an optical amplifier.

FIG. 22 is a diagram for explaining the operating principle of a conventional mode-locked laser device, and is a diagram showing typical spectral characteristics obtained by mode-locking.

FIG. 23 is a diagram for explaining the operation principle of a conventional mode-locked laser device, showing the time characteristic of output.

[Explanation of symbols]

 1 Driving Power Supply 2 Optical Modulator 3 Optical Amplifier 4 Optical Isolator 5 Optical Coupler 6 Wavelength Tunable Optical Filter 7 Optical Waveguide 8 Optical Delay Device 9 Optical Divider 10 Photoelectric Converter 12 Bandpass Filter 13 Electrical Signal Processor 31 Rare Earth Doped Optical fiber 32 Excitation light source 33 Wavelength multiplexer / demultiplexer 34 Semiconductor laser amplifier 35 Excitation current source 81, 82 Optical lens 83 Motorized stage 84 Driving power supply 85 Right angle mirror 86 Drum 87 Substrate 88 Optical waveguide 89 Electrode 131 Low frequency transmission filter 132 Analog / Digital converter 133 Computer 134 Digital-to-analog converter

Claims (4)

[Claims] 1. An optical modulator for modulating a loss or phase of light at a predetermined frequency, an optical amplifier for amplifying light modulated by the optical modulator, and the optical amplifier and the optical modulator. In a mode-locked laser device comprising optical means for optically coupling to form a resonator, and output means for extracting output light from the resonator, in which light having an electrically variable optical path length is provided in the resonator. A delay unit is provided, a unit for branching a part of the output light extracted by the output unit to convert it into an electric signal, and a resonator other than an integer multiple of the pulse repetition frequency of the output light from the converted electric signal. A bandpass filter that extracts at least one frequency component that is an integer multiple of the longitudinal mode frequency interval, and the power of the frequency component that is extracted by this bandpass filter should be minimized. Mode-locked laser device comprising the processing means for controlling the optical path length of the optical delay. 2. The mode-locked laser device according to claim 1, wherein the optical means includes an optical waveguide that connects the optical modulator, the optical amplifier, the output means, and the optical delay device in a ring shape. 3. The mode-locked laser device according to claim 1, wherein the optical means and the output means include two optical reflectors arranged with the optical modulator, the optical amplifier and the optical delay device interposed therebetween. 4. The mode-locked laser device according to claim 1, further comprising an optical filter having a variable transmission wavelength in the resonator.