JP5306684B2 - Optical fiber type mode-locked laser and method for controlling mode lock of optical fiber type mode-locked laser - Google Patents

Description

  The present invention relates to an optical fiber type mode-locked laser and a mode lock control method for an optical fiber type mode-locked laser.

  Conventionally, as a measurement light source (for example, a light source for multiphoton excitation of a microscope, a light source for length measurement), an optical fiber type mode-locked laser that outputs light of a short pulse of picosecond and femtosecond is used.

Optical fiber mode-locked lasers are required to generate short pulses stably. As a technique for stably generating a short pulse, an active mode lock method and a passive mode lock method are known. The active mode-locking method is a technique for generating a short pulse by growing a seed pulse that becomes a seed of laser oscillation using a modulator. The passive mode-lock method is a technique for generating a short pulse by using a nonlinear response of light without using a modulator.
As a technique for stably generating a short pulse by the above-described mode lock method, for example, techniques of Patent Documents 1 to 3 are known.
Japanese Patent Laid-Open No. 5-283377 Japanese National Patent Publication No. 10-504659 JP-T-2001-506016

However, the conventional mode-lock method requires a high-precision electric signal generator for driving the modulator and its synchronizing circuit.
For example, in the active mode lock method, since the optical pulse is grown by the modulator, it is necessary to synchronize the repetition frequency of the modulator with the laser resonator frequency with high accuracy. In the active mode lock method, it is necessary to maintain the repetition frequency of the modulator with high accuracy even after the mode lock is started.
For this reason, a technique is used in which the resonator length of the laser is finely adjusted using a piezo element or the like to synchronize the resonator frequency with the repetition frequency of the modulator. Further, a technique is used in which the resonator frequency is monitored by a frequency monitor and fed back to the modulator by a PLL (Phase Locked Loop) to synchronize the repetition frequency of the modulator with the resonator frequency. When these techniques are used, an electric signal generator having a high-accuracy frequency resolution for driving the modulator and a synchronizing circuit (frequency monitor, PLL, etc.) are required.
On the other hand, the passive mode lock method uses optical nonlinearity such as optical Kerr effect and nonlinear polarization rotation. This method often requires a mechanism for generating a seed pulse such as an intensity modulator or a phase modulator in order to start mode locking. In particular, in a mode-locked laser that uses nonlinear polarization rotation in a fiber type, it is necessary to continue to generate seed pulses actively using an intensity modulator or a phase modulator in order to perform a mode-locking operation stably. . For this reason, a highly accurate synchronization circuit was required.
That is, there has been a demand for realizing mode lock without requiring a high-precision electric signal generator or a synchronization circuit.

  An object of the present invention is to provide a technique for initiating mode-locking without requiring a high-precision electric signal generator or a synchronization circuit.

In order to achieve the above object, an optical fiber type mode-locked laser according to the present invention comprises:
In the optical fiber type mode-locked laser that constitutes the laser resonator,
A modulator that modulates continuous wave light generated inside the laser resonator;
A generator for generating a seed pulse from the light modulated by the modulator;
A polarization-maintaining optical fiber that passes the continuous wave light and seed pulse;
An optical amplifier for amplifying the continuous wave light and seed pulse;
A dispersion controller for controlling dispersion of the continuous wave light and the seed pulse;
A saturable absorber that attenuates the light component of the continuous wave and the light intensity of the seed pulse; and
A detector for detecting the light intensity of the continuous wave light and the seed pulse;
A VCO that generates a high-frequency signal to be applied to the modulator;
Based on the light intensity of the seed pulse detected by the detector, the high-frequency signal is controlled, the repetition frequency of the modulator and the resonance frequency of the laser resonator are synchronized, and an optical pulse is generated. A control unit for confirming the generated optical pulse and stopping the operation of the modulator;
It is configured with.

Preferably, the control unit confirms the generation of the light pulse based on the light intensity detected by the detection unit.
Preferably, the control unit performs FM modulation on the high-frequency signal .

Further, in the mode lock control method of the optical fiber type mode lock laser constituting the laser resonator,
Modulating a continuous wave light generated inside the laser resonator with a modulator;
Generating a seed pulse from the light modulated by the modulator by a generator;
Passing the continuous wave light and seed pulse through a polarization maintaining optical fiber;
Amplifying the continuous wave light and seed pulse by an optical amplifier;
Dispersion control of the continuous wave light and seed pulse by a dispersion controller;
Generating by the saturable absorber that attenuates the weak component of the light of the continuous wave and the seed pulse; and
Detecting the continuous wave light and the light intensity of the seed pulse with a detector;
Generating a high frequency signal to be applied to the modulator by a VCO ;
Based on the light intensity of the seed pulse detected by the detector, the high-frequency signal is controlled, the repetition frequency of the modulator and the resonance frequency of the laser resonator are synchronized, and an optical pulse is generated. A control step of confirming the generated light pulse and stopping the operation of the modulator;
including.

Preferably, the control step confirms the generation of the light pulse based on the light intensity detected by the detection unit.
Preferably, the control step performs FM modulation on the high-frequency signal .

  ADVANTAGE OF THE INVENTION According to this invention, the technique which implement | achieves a mode lock | rock can be provided, without requiring a highly accurate electrical signal generator and a synchronous circuit.

  Embodiments according to the present invention will be described below in detail with reference to the accompanying drawings. However, the scope of the invention is not limited to the illustrated examples.

Embodiments according to the present invention will be described with reference to FIGS. First, the configuration of the optical fiber type mode-locked laser 1 according to the present embodiment will be described with reference to FIG. The optical fiber mode-locked laser 1 includes an optical amplifying unit 11, an optical isolator 12, an optical attenuator 13, an optical filter 14, a dispersion control unit 15, a supersaturated absorption unit 16, a modulator 17, and a tap coupler 18. And an output coupler 19, a polarization maintaining optical fiber 20, and a control circuit 21. The supersaturated absorber 16 and the modulator 17 constitute a mode locker 161. Further, a laser resonator of the optical fiber type mode-locked laser 1 is configured by the configuration of each part shown in FIG. In addition, the order of the units in FIG. 1 can be arbitrarily changed.

The optical amplifying unit 11 includes a polarization maintaining erbium doped fiber 11A.
The polarization-maintaining erbium-doped fiber 11A amplifies continuous wave light and seed pulses. Further, the polarization maintaining erbium-doped fiber 11A amplifies the optical pulse after the mode lock is started. The continuous wave light means a light spectrum (for example, a spectrum having a peak structure at 1558.2 nm) generated inside the laser resonator before the start of mode-lock oscillation. The seed pulse refers to an optical pulse that serves as a seed for generating short pulse light. An optical pulse refers to short pulse light generated from a seed pulse. Mode locking refers to generating an optical pulse that is a short pulse light. Hereinafter, a pulse until mode locking is started is defined as a seed pulse, and a pulse after mode locking starts is defined as an optical pulse.

  The optical isolator 12 passes the seed pulse only in a specific direction. Further, the optical isolator 12 passes the optical pulse only in a specific direction after the mode lock is started.

  The optical attenuator 13 attenuates the seed pulse to an appropriate signal level. The optical attenuator 13 passes an optical pulse after the mode lock is started.

  The optical filter 14 extracts only light having a polarization plane in a specific direction with a seed pulse. In addition, after the mode lock is started, the optical filter 14 extracts only light having a polarization plane in a specific direction with an optical pulse.

  The dispersion controller 15 disperses the continuous wave light and the seed pulse, and compresses the pulse width of the seed pulse. Further, the dispersion control unit 15 generates a seed pulse by compensating dispersion from phase-modulated continuous wave light to generate a seed pulse. In addition, after the mode lock is started, the dispersion control unit 15 applies dispersion to the optical pulse and compresses the pulse width of the optical pulse.

  The supersaturated absorber 16 transmits a portion of continuous wave light or a high intensity of the seed pulse, and attenuates a portion of low intensity. Therefore, when the continuous wave light or the seed pulse passes through the saturable absorber 16 while repeating amplification through the optical amplifier, only the portion where the intensity of the seed pulse is high grows to become an optical pulse. The supersaturated absorber 16 maintains the mode lock by repeating the action of generating the optical pulse after the mode lock is started. That is, the supersaturated absorber 16 corresponds to a mode lock mechanism that generates an optical pulse.

The modulator 17 modulates continuous wave light, a seed pulse and an optical pulse generated in the laser resonator. When the modulator 17 is an intensity modulator, the intensity modulator modulates continuous wave light generated inside the resonator to generate a seed pulse. In this case, the modulator 17 corresponds to a generation unit that generates a seed pulse.
Further, when the modulator 17 is a phase modulator, it modulates continuous wave light, seed pulses and optical pulses generated in the laser resonator. The phase-modulated continuous wave light passes through the dispersion medium of the dispersion controller 15 to generate a seed pulse. In this case, the modulator 17 and the dispersion control unit 15 correspond to a generation unit that generates a seed pulse. The modulator 17 corresponds to a mode lock starting mechanism for starting mode lock.

  The tap coupler 18 branches a seed pulse for detection by a PD (photo diode) 22. The output coupler 19 branches the optical pulse output from the laser.

  The polarization-maintaining optical fiber 20 allows continuous wave light and seed pulses to pass therethrough. Further, after the mode lock operation is started, the optical pulse is allowed to pass.

  The control circuit 21 includes a PD 22 as a detector, a control unit 23, a VCO (Voltage Controlled Oscillator) 24 as an electric signal generator, and an electric amplifier 25.

  The PD 22 branches and photoelectrically converts part of the continuous wave light and seed pulse in the laser resonator, and detects the light intensity (power). The PD 22 detects the power of the optical pulse after the mode lock is started.

The control unit 23 controls the electrical signal ( high frequency signal ) of the VCO 24 based on the power detected by the PD. Specifically, the control unit 23 controls the high frequency signal of the VCO 24 so that the optical pulse is generated in synchronization with the repetition frequency of the modulator 17 and the repetition frequency of the laser resonator.

The VCO 24 outputs a high frequency signal to be applied to the modulator 17 based on an instruction from the control unit 23.

The electric amplifier 25 amplifies the high frequency signal output from the VCO 24 and outputs the amplified high frequency signal to the modulator 17. When a high frequency signal is input, the modulator 17 modulates the optical signal based on the input high frequency signal .

Next, the operation will be described with reference to FIG.
As a precondition, it is assumed that continuous wave light is generated inside the laser resonator. The resonance frequency of the laser resonator is assumed to be 30 MHz.

  First, the continuous wave light generated inside the laser resonator is modulated by the modulator 17. Here, when the modulator 17 is an intensity modulator, a seed pulse is generated from continuous wave light by the intensity modulator. When the modulator 17 is a phase modulator, the continuous wave light is phase-modulated by the phase modulator, and a seed pulse is generated by passing the phase-modulated light through the dispersion medium of the dispersion controller. Then, the generated seed pulse passes through the supersaturated absorber 16 so that the seed pulse becomes steep.

  Specifically, the seed pulse repeatedly passes through the loop of the polarization maintaining optical fiber 20 shown in FIG. That is, the seed pulse passes through the saturable absorber 16 while repeating amplification through the optical amplifying unit 11, so that a portion with high intensity grows and becomes an optical pulse.

  At this time, the seed pulse output from the modulator 17 is branched by the tap coupler 18. The branched seed pulse is input to the PD 22, and the power of the seed pulse is detected by the PD 22. Then, the power detected by the PD 22 is output to the control unit 23.

Based on the input power, the control unit 23 controls the electrical signal ( high frequency signal ) of the VCO 24. Here, when the resonance frequency of the laser resonator is different from the repetition frequency of the modulator, the seed pulse propagating many times in the laser resonator receives modulation from the modulator 17 at different timings. Will not grow (ie the seed pulse will not be steep). Therefore, it is necessary to synchronize the resonance frequency of the laser resonator and the repetition frequency of the modulator 17 with high accuracy. For this reason, the high frequency signal that is the output of the VCO 24 is controlled by the control unit 23 so that an optical pulse is generated in synchronization with the repetition frequency of the modulator 17 and the resonance frequency of the laser resonator. For example, when the resonance frequency of the laser resonator is 30 MHz , the high-frequency signal that is the output of the VCO 24 is controlled so that the repetition frequency of the modulator 17 is synchronized with 30 MHz.

  Then, when the repetition frequency of the modulator 17 reaches 30 MHz (that is, when the repetition frequency of the modulator 17 and the resonance frequency of the laser resonator are synchronized), mode locking is started. At this time, the power detected by the PD 22 increases.

The power detected by the PD 22 increases by about 3 dB depending on the oscillation condition and output. In the case of a mode-locked laser resonator composed of a polarization-maintaining optical fiber having this configuration, mode-locked oscillation is maintained even if the drive of the modulator is stopped after mode-locked oscillation. When a power increase (3 dB increase) is detected by the PD 22 at the start of mode-lock oscillation in this configuration, the control unit 23 confirms the generation of an optical pulse (that is, confirms the start of mode lock), and the VCO 24 Then, the operation of the modulator 17 is stopped. Specifically, it is controlled so that the high-frequency signal output by the control unit 23 from the VCO 24 becomes OFF (as the high-frequency signal is not output from the VCO 24).

When mode locking is started, the optical pulse still passes through the saturable absorber 16 and the optical amplifier 11 repeatedly. At this time, the mode lock of the optical pulse is maintained by the supersaturated absorption characteristic of the supersaturated absorption unit 16 (characteristic that transmits a part with a high intensity of the optical pulse and attenuates a part with a low intensity more).
An example of a series of mode-lock oscillation start processes will be described with reference to FIGS. 2A and 2B show the RF spectrum measured when the optical output before the mode-lock oscillation is electrically converted by the PD, and FIG. 3 shows the optical spectrum. In FIG. 2, the repetition frequency of the electrical signal that drives the phase modulator is 36.00 MHz. At this time, in FIG. 2A, the harmonic component fluctuates randomly. In FIG. 3, the optical spectrum indicates continuous wave oscillation having a sharp peak structure at 1558.2 nm. Subsequently, when the repetition frequency of the electric signal for driving the phase modulator is matched with the laser resonator frequency of 38.62 MHz, mode-lock oscillation is started. 4A and 4B show the RF spectrum, and FIG. 5 shows the optical spectrum. In FIG. 4A, harmonic components that fluctuated randomly before the start of mode-lock oscillation become stable after the start of mode-lock. Further, as shown in FIG. 5, the spectrum band of the optical spectrum is expanded. As the optical spectrum expands, the output increases. In order to maintain stable mode-locked oscillation, the output of the excitation light may be adjusted. In this example, the output of the excitation light is reduced to about half. Stable mode-locked oscillation is determined by the RF spectrum envelope approaching flat. Subsequently, the RF spectrum and the optical spectrum when the electric signal for driving the phase modulator is stopped are shown in FIGS. In the case of stable mode-lock oscillation, the envelope of the RF spectrum becomes flat as shown in FIG. 6 and 7, stable oscillation was confirmed even when the electric signal for driving the phase modulator was stopped.

Next, the relationship between the voltage input to the VCO 24 and the repetition frequency of the modulator 17 will be described with reference to FIG. The vertical axis in FIG. 8 represents the repetition frequency of the modulator 17. The horizontal axis in FIG. 8 represents the voltage input to the VCO 24. As shown in FIG. 8, as the voltage input to the VCO 24 increases, the repetition frequency of the modulator 17 also increases. Thereby, for example, when synchronizing the repetition frequency of the modulator 17 with the resonance frequency 30 MHz of the laser resonator, the voltage input to the VCO 24 is controlled by the control unit 23, and the repetition frequency of the modulator 17 becomes 30 MHz.

As described above, according to the present embodiment, the electric signal is controlled based on the light intensity of the seed pulse, the repetition frequency of the modulator 17 and the resonance frequency of the laser resonator are synchronized, and an optical pulse is generated. After confirming the generation of the pulse, the operation of the modulator 17 is stopped. Thereby, after confirming the generation of the optical pulse (after confirming the start of the mode lock), the modulator 17 does not operate, so a high-precision electric signal generator for driving the modulator 17 and its synchronizing circuit ( It is possible to provide a technique for realizing mode lock without requiring a PLL or frequency monitor. Therefore, the optical fiber type mode-locked laser 1 can be reduced in size and cost. Further, since the polarization maintaining optical fiber 20 is used, it is possible to prevent the mode lock condition from being changed due to the change of the polarization.

  Moreover, generation of an optical pulse can be confirmed based on the light intensity (power) detected by the PD 22.

(Modification)
A modification of the embodiment according to the present invention will be described. In the following, the same parts as those of the optical fiber type mode-locked laser 1 are denoted by the same reference numerals, and the detailed description thereof is used to explain the different parts.

When the high-frequency signal output from the control by the control unit 23 VCO 24 is FM modulated, the spectrum width of the high-frequency signal is wider than if it is not FM-modulated. Thereby, when synchronizing the repetition frequency of the modulator 17 with the resonance frequency (30 MHz) of the laser resonator, the spectrum width of the high-frequency signal is wide, so that the resonance frequency of the laser resonator can be easily found. Therefore, the sweep resolution of the repetition frequency of the modulator 17 can be lowered.

As described above, according to the present modification, the sweep resolution of the repetition frequency of the modulator 17 can be lowered by FM-modulating the electric signal ( high-frequency signal ).

  In addition, the detailed structure and detailed operation of the optical fiber type mode-locked laser 1 in the present embodiment and modifications can be changed as appropriate without departing from the spirit of the present invention.

It is a figure which shows the structure of the optical fiber type mode lock laser of embodiment which concerns on this invention. (A) (B) is a figure which shows the RF spectrum which detected the laser output before mode-lock oscillation start by PD. It is a figure which shows the optical spectrum of the laser output before a mode-lock oscillation start. (A) (B) is a figure which shows the RF spectrum which detected the laser output at the time of the mode-lock oscillation start by PD. It is a figure which shows the optical spectrum of the laser output at the time of mode-lock oscillation start. (A) and (B) are diagrams showing an RF spectrum in which laser output is detected by a PD after mode lock oscillation is started and an electric signal for driving the phase modulator is stopped. It is a figure which shows the optical spectrum of the laser output after mode-lock oscillation is started and the electric signal which drives a phase modulator is stopped. It is a figure which shows the relationship between the voltage input into VCO, and the repetition frequency of a modulator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber type mode lock laser 11 Optical amplifier 11A Polarization maintenance erbium addition fiber 12 Optical isolator 13 Optical attenuator 14 Optical filter 15 Dispersion control part 16 Saturation absorption part 17 Modulator 18 Tap coupler 19 Output coupler 20 Polarization maintenance optical fiber 21 Control circuit 22 PD
23 Control unit 24 VCO
25 Electric amplifier

Claims (6)

In the optical fiber type mode-locked laser that constitutes the laser resonator,
A modulator that modulates continuous wave light generated inside the laser resonator;
A generator for generating a seed pulse from the light modulated by the modulator;
A polarization-maintaining optical fiber that passes the continuous wave light and seed pulse;
An optical amplifier for amplifying the continuous wave light and seed pulse;
A dispersion controller for controlling dispersion of the continuous wave light and the seed pulse;
A saturable absorber that attenuates the light component of the continuous wave and the light intensity of the seed pulse; and
A detector for detecting the light intensity of the continuous wave light and the seed pulse;
A VCO that generates a high-frequency signal to be applied to the modulator;
Based on the light intensity detected by the detector, the high-frequency signal is controlled, the repetition frequency of the modulator and the resonance frequency of the laser resonator are synchronized , an optical pulse is generated, and the generated light A control unit for confirming a pulse and stopping the operation of the modulator;
An optical fiber type mode-locked laser. The controller is
The optical fiber type mode-locked laser according to claim 1, wherein the generation of the light pulse is confirmed based on the light intensity detected by the detection unit. The controller is
The optical fiber mode-locked laser according to claim 1 or 2, wherein the high-frequency signal is FM-modulated. In a mode-locking control method of an optical fiber type mode-locked laser constituting a laser resonator,
Modulating a continuous wave light generated inside the laser resonator with a modulator;
Generating a seed pulse from the light modulated by the modulator by a generator;
Passing the continuous wave light and seed pulse through a polarization maintaining optical fiber;
Amplifying the continuous wave light and seed pulse by an optical amplifier;
Dispersion control of the continuous wave light and seed pulse by a dispersion controller;
Generating by the saturable absorber that attenuates the weak component of the light of the continuous wave and the seed pulse; and
Detecting the continuous wave light and the light intensity of the seed pulse with a detector;
Generating a high frequency signal to be applied to the modulator by a VCO ;
Based on the light intensity of the seed pulse detected by the detector, the high-frequency signal is controlled, the repetition frequency of the modulator and the resonance frequency of the laser resonator are synchronized, and an optical pulse is generated. A control step of confirming the generated light pulse and stopping the operation of the modulator;
A mode-locking control method for an optical fiber type mode-locking laser. The control step includes
5. The mode lock control method for an optical fiber type mode-locked laser according to claim 4, wherein the generation of the light pulse is confirmed based on the light intensity detected by the detection unit. The control step includes
6. The mode lock control method for an optical fiber mode-locked laser according to claim 4, wherein the high-frequency signal is FM-modulated.

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