JP2010238865A - Mode synchronized laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mode synchronized laser device wherein stable mode-synchronized laser light can be oscillated even if any environmental change or a parameter change occurs. <P>SOLUTION: A mode synchronized laser device 10 comprises a mode synchronization control apparatus 20 for sampling a part of mode synchronized laser light and controlling mode synchronization on the basis of the sampled laser light. The mode synchronization control apparatus 20 includes a polarization-maintaining tap coupler 22 for splitting laser light in a predetermined splitting ratio, a band pass filter 23 for selectively passing laser light of a predetermined wavelength domain in the laser light split by the polarization-maintaining tap coupler 22, a light-receiving element 24 for receiving laser light passed through the band pass filter 23, and a control circuit 25 connected to the light-receiving element 24 for monitoring and controlling mode synchronization on the basis of the quantity of light received by the light-receiving element. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a mode-locked laser device, and more particularly to a mode-locked laser device that generates pulsed light used in the fields of optical communication, optical measurement, material processing, THz wave generation, physics, and the like.

  The pulsed light from the ultrashort pulse light source has features such as a small pulse width, a large peak power, and a wide band. Taking advantage of these features, the application of ultra-short pulse light to the fields of optical communication, optical measurement, material processing, THz (terahertz) wave generation, physics and the like is being studied. Among them, the fiber-type ultrashort pulse light source is particularly attracting attention because it is smaller than a solid-state laser and its operation is stable.

  As a method for generating pulsed light from an ultrashort pulse light source, a mode-locked laser is generally well known (see Non-Patent Document 1). In the resonator, frequency components that are integral multiples of the frequency corresponding to the resonator length reinforce each other. For this reason, an equally spaced mode occurs in the spectrum. If the phases of these modes can be matched, short pulsed light can be generated.

  Mode-locked lasers are roughly classified into active mode-locked lasers and passive mode-locked lasers. The active mode-locked laser generates ultrashort pulse light by modulating the intensity or phase of light propagating in the resonator. In general, an intensity modulator or a phase modulator is often used for modulation in accordance with the repetition frequency of pulsed light.

  On the other hand, a passive mode-locked laser performs mode locking using a saturable absorber whose loss varies with the intensity of light. The saturable absorber has a feature that the loss is small when the light intensity is high, and the loss is large when the light intensity is weak. Therefore, when the pulsed light is passed through the saturable absorber, the loss of the skirt component is larger than the component near the peak of the pulsed light (the portion where the light intensity is high). That is, the saturable absorber has an effect of narrowing the pulsed light. As the pulsed light passes through the saturable absorber in the resonator many times, its pulse width gradually decreases. Thereafter, the pulsed light is in a steady state, and stable pulsed light is generated.

  In general, there are many methods using a supersaturated absorber utilizing the supersaturated absorption characteristics of semiconductors and carbon nanotubes, but there is also a method of applying a nonlinear effect in an optical fiber. The method of applying the nonlinear effect utilizes the feature that the magnitude of the nonlinear effect depends on the light intensity.

  For example, non-linear polarization rotation and non-linear loop mirror (NOLM) have been reported. Nonlinear polarization rotation uses the fact that the magnitude of polarization rotation changes according to the intensity of light. By transmitting the light to the polarizer after the nonlinear polarization rotation, a loss corresponding to the intensity of the light can be given. The NOLM has a configuration for propagating light to each other through a loop composed of an optical fiber and a tap coupler. The transmittance of the NOLM changes according to the difference in nonlinear phase shift of light propagating in the opposite directions. Since the magnitude of the nonlinear phase shift is proportional to the light intensity, the transmittance of the NOLM depends on the intensity of the incident light.

  In recent years, an 8-shaped (Figure-8) polarization-maintaining mode-locked laser that combines a modulator and a nonlinear optical amplification loop mirror (NALM) has been proposed (see Non-Patent Document 2). NALM is a kind of NOLM and is characterized in that an optical amplifier is arranged in the loop. Since the incident light is amplified by the optical amplifier, the nonlinear phase shift increases. Therefore, NALM can draw a transmission function with lower power than NOLM. In this polarization mode-locked laser, mode-locked oscillation is performed by using a NALM and a modulator, and once mode-locked, mode-locked oscillation is continued even if the modulation by the modulator is stopped. Therefore, it operates as passive mode synchronization.

  FIG. 4 is a diagram schematically showing a configuration of a conventional mode-locked laser device.

  As shown in FIG. 4, the mode-locked laser device 40 includes a polarization maintaining intensity modulator 41 that modulates the intensity of the laser light, and a polarization that defines the direction of rotation of the laser light and the polarization axis. Dependent isolator 42, polarization maintaining tap coupler 43 that branches a part of the circulating laser beam (or mode-locked laser), nonlinear optical amplification loop mirror 44, and polarization maintaining tap coupler that connects the two loops 45 and a polarization dependent isolator 51 connected to the branch path of the polarization maintaining tap coupler 43. The nonlinear optical amplification loop mirror 44 includes a PM-EDF (Polarization-maintaining Erbium-doped Fiber) 44a which is an amplification medium. The mode-locked laser device 40 includes an excitation light source 46 and a polarization maintaining WDM coupler 47 that is connected to the excitation light source 46 and guides laser light from the excitation light source to the PM-EDF 44a. The length of the PM-EDF 44a is 1 to 2 m, the absorption at 1530 nm is 27.5 dB / m, and the dispersion at 1530 nm is −34.4 ps / nm / km. A function generator 48 for controlling the modulation is connected to the intensity modulator 41. Further, in this mode-locked laser device, all of the portions where the laser light propagates are connected by the polarization maintaining optical fiber 49, and it is possible to oscillate stable mode-locked pulse light (mode-locked laser light). is there.

E. P. Ipen, "Principles of Passive Mode Locking", Applied Physics B58, 159-170, 1994. J. et al. W. Nicholson and M.M. Andrewjco, “A polarization maintaining, disperse managed, femtosecond figure-eight fiber laser”, Optics Express, Vol. 14, no. 18, 8160-8167, 2006

  However, since the mode-locked laser apparatus configured as described above is very delicate, stable mode-locked laser light may not be able to oscillate due to the influence of environmental changes or parameter changes. For example, the resonator length may change due to a temperature change, the loss of each optical component may change, or the gain characteristics of the EDF may change. In addition, the light power and wavelength from the excitation light source may slightly change. In such a case, stable mode-locked laser light may not be oscillated.

  An object of the present invention is to provide a mode-locked laser apparatus that can oscillate a stable mode-locked laser beam even when environmental changes or parameter changes occur while confirming mode-locked oscillation.

  To achieve the above object, a mode-locked laser device according to a first aspect of the present invention is a mode-locked laser device that generates a mode-locked laser beam by modulating a laser beam in a predetermined path. Branch means for branching the laser light, selection means for selecting a part of the laser light branched by the branch means, light receiving means for receiving the laser light selected by the selection means, and amount of light received by the light receiving means And monitoring means for monitoring mode synchronization based on the above.

  The monitoring means preferably monitors mode synchronization of the laser beam based on the optical power received by the light receiving means.

  Further, it is more preferable that the monitoring unit determines that the laser beam is not mode-synchronized when the optical power received by the light receiving unit is equal to or less than a predetermined threshold value.

  The mode-locked laser apparatus further includes control means for executing a recovery operation so that the laser light is mode-locked when the monitoring means determines that the laser light is not mode-locked. preferable.

  More preferably, the control means temporarily increases the drive current of the light source that generates the laser light and then decreases the drive current to a steady-state drive current in the recovery operation.

  The branching unit is preferably a tap coupler that branches the laser light in the predetermined path at a predetermined branching ratio.

  The selection means is preferably a filter that transmits laser light in a predetermined wavelength region.

  Further, the filter is a laser beam in a wavelength region in which a difference between the optical power received by the light receiving unit when the mode is synchronized and the optical power received by the light receiving unit when the mode is not synchronized is large. More preferably.

  A mode-locked laser device according to a second aspect of the present invention is a method for controlling a mode-locked laser device that generates a mode-locked laser beam by modulating a laser beam in a predetermined path, the laser beam in the predetermined path being Based on the branching step for branching, the selection step for selecting a part of the laser light branched by the branching step, the light receiving step for receiving the laser light selected by the selection step, and the amount of light received in the light receiving step And a monitoring step of monitoring mode synchronization of the laser beam.

  According to the present invention, the laser beam in the predetermined path is branched by the branching unit, a part of the branched laser beam is selected, and the mode synchronization is monitored based on the received light amount of the selected laser beam. Thus, mode-locked oscillation can be confirmed, and stable mode-locked laser light can be oscillated even when environmental changes or parameter changes occur. In addition, since laser light of a predetermined wavelength region is selected and transmitted and mode synchronization is monitored based on the amount of received laser light, mode synchronization can be monitored with high accuracy.

1 is a diagram schematically showing a configuration of a mode-locked laser device according to an embodiment of the present invention. It is a figure which shows the spectrum of the mode synchronous laser synchronized with the mode synchronous laser apparatus of FIG. It is a figure which shows the modification of the tap coupler in FIG. It is a figure which shows schematically the structure of the conventional mode-locked laser apparatus.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram schematically showing a configuration of a mode-locked laser apparatus according to an embodiment of the present invention.

  In FIG. 1, a mode-locked laser device 10 includes a polarization maintaining modulator 11 that modulates the intensity of a laser beam, and a polarization-dependent isolator that defines the direction in which the laser beam circulates and defines the polarization axis thereof. 12, a polarization maintaining tap coupler 13 that branches a part of the circulating laser light (or mode-locked laser), a non-linear amplifying loop mirror (NALM) 14, and a polarization that connects the two loops. And a wave holding tap coupler 15. The nonlinear optical amplification loop mirror 14 includes a PM-EDF (Polarization-maintaining Erbium-doped Fiber) 14a that is an amplification medium. The mode-locked laser device 10 includes a pumping light source 16 that generates laser light, a polarization-maintaining WDM coupler 17 that is connected to the pumping light source 16 and guides the laser light from the pumping light source to the nonlinear optical amplification loop mirror 14; have. A function generator 18 for controlling the modulation is connected to the modulator 11. Further, in this mode-locked laser device, all of the portions through which the laser light (pulse light) propagates are connected by the polarization maintaining optical fiber 19, and stable mode-locked laser light can be oscillated. In the present embodiment, in the mode-locked laser device 10, the optical fiber through which the laser light propagates is a figure 8 (Figure-8). However, the optical fiber is not limited to this and may be a linear type. However, it may be a sigma type.

  Further, the mode-locked laser device 10 includes a mode-locking control device (control means) 20 that samples a part of the mode-locked laser light and controls mode-locking based on the sampled laser light.

  The mode synchronization control device 20 includes a polarization-dependent isolator 21 connected to the branch path of the polarization-maintaining tap coupler 13 and a polarization that branches the laser light output from the polarization-dependent isolator 21 at a predetermined branch ratio. A wave-holding tap coupler (branching means) 22, a band-pass filter (selecting means) 23 that selectively transmits laser light in a predetermined wavelength region out of the laser light branched by the polarization-maintaining tap coupler 22, and a band A light receiving element (light receiving means) 24 that receives the laser light transmitted through the pass filter 23, and a control circuit (monitoring means) 25 that is connected to the light receiving element 24 and monitors and controls mode synchronization based on the amount of light received by the light receiving element. And have. As the light receiving element 24, for example, a photodiode, a photomultiplier tube, or the like can be used.

  In this mode-locked laser device, the left loop and the right loop in the figure are connected by a tap coupler 15. In the left loop, the light propagating counterclockwise is isolated by the polarization dependent isolator 12. Therefore, only clockwise light propagates in the left loop. The light propagating clockwise in the left loop is modulated by the modulator 11, and part of the light is extracted out of the resonator by the tap coupler 13. Then, the tap coupler 15 branches the light propagating clockwise in the right loop and the light propagating counterclockwise. Of the light propagating in the right loop, both the light propagating clockwise and the light propagating counterclockwise are amplified by the PM-EDF 14a. However, the intensity of the clockwise and counterclockwise light pulses changes depending on the branching ratio and path of the tap coupler 15, and a difference occurs in the nonlinear phase shift. When these two laser beams are coupled again by the tap coupler 15, interference depending on the phase shift difference occurs. When interference occurs such that the light pulses are strengthened at the peak of the light pulse and weakened at the bottom of the light pulse, the light pulse becomes thinner as it circulates, and a steady state is reached after a predetermined period.

  Next, a mode synchronization control method performed by the mode synchronization controller 20 will be described.

  First, in the loop on the left side in the figure, the laser beam branched by the tap coupler 13 is filtered by the bandpass filter 23, and only the laser beam having a predetermined wavelength is received by the light receiving element 24. The received laser beam is converted into a current corresponding to the light input by the light receiving element 24 and output to the control circuit 25. In the control circuit 25, the current from the light receiving element 24 is input, and it is determined whether or not the optical power (light intensity) corresponding to the input current value is larger than the threshold value. When the optical power corresponding to the current value input to the control circuit 25 is larger than the threshold value, it is determined that the laser beam is mode-synchronized. If the optical power corresponding to the current value input to the control circuit 25 is less than or equal to the threshold value, it is determined that the mode is not synchronized, and the control circuit 25 performs a recovery operation so that the laser beam is mode synchronized. The

  That is, the control circuit 25 also serves as a monitoring unit that monitors the mode synchronization of the laser beam, and monitors the mode synchronization of the laser beam based on the optical power received by the light receiving element 25. If the optical power received by the light receiving element 25 is greater than the threshold, the control circuit 25 determines that the laser light is not mode-synchronized, while the optical power received by the light receiving element 25 is less than or equal to the threshold. If there is, it is determined that the laser beam is not mode-synchronized.

  The mode synchronization recovery operation executed by the control circuit 25 is executed as follows. For example, the drive current of the excitation light source 16 is temporarily increased to perform mode synchronization, and then the drive current is reduced to a steady-state drive current. Also, after the modulation operation by the modulator 11 and the output of the excitation light from the excitation light source 16 are once turned off, the modulation operation by the modulator 11 and the output of the excitation light are turned on again, and the drive current of the excitation light is set to a steady state. Set to the drive current.

  FIG. 2 is a diagram showing the spectrum of the mode-locked laser synchronized by the mode-locked laser device 10 of FIG. 1, in which the spectrum when the mode is synchronized is indicated by a solid line, and the case where the mode is not synchronized is indicated by a broken line. Show.

  The average optical power when the mode is synchronized and the average optical power when the mode is not synchronized are 6.43 dBm and 6.23 dBm, respectively, and the difference between the average optical powers is very small. Therefore, it is difficult to determine whether or not the mode is synchronized with only the average optical power. Therefore, in the present embodiment, only laser light in a wavelength region having a large optical power difference between when mode synchronization is performed and when mode synchronization is not performed is selected by filtering. Thereby, it is possible to accurately determine whether or not the mode is synchronized.

  Specifically, as shown in FIG. 2, laser light having a wavelength of about 1570 nm is selected and monitored by a bandpass filter having a full width at half maximum of 3 nm. For example, when a laser beam having a wavelength of 1540 to 1590 nm is selected, the optical power difference between when the mode is synchronized and when the mode is not synchronized is about 20 dBm, and it is determined whether or not the mode is synchronized. This is a sufficient optical power difference. Note that the optical power difference necessary for performing the determination has an optimum value according to the light receiving element and the control circuit, and therefore the wavelength of the laser beam to be monitored may be selected according to the light receiving element and the control circuit.

  In the control circuit 25, an intermediate value between the optical power when the mode is synchronized and the optical power when the mode is not synchronized is set as the threshold value. When the optical power of the laser light to be detected is greater than the intermediate value, it is determined that the mode is synchronized. When the optical power of the laser light to be detected is less than the intermediate value, the mode is not synchronized. Determined.

  According to the present embodiment, the laser light is branched by the polarization maintaining tap coupler 22, the laser light in a predetermined wavelength region is selected and transmitted by the band pass filter 23, and the laser light transmitted through the band pass filter 23 is transmitted. Mode synchronization is monitored based on the amount of received light. Thus, mode-locked oscillation can be confirmed, and stable mode-locked laser light can be oscillated even when environmental changes or parameter changes occur. In addition, laser light in a wavelength region with a large optical power difference between a predetermined wavelength region, particularly when mode synchronization is not performed and mode synchronization is selected and transmitted, and mode synchronization is performed based on the received light amount of the laser light Therefore, mode synchronization can be monitored with high accuracy.

  In this embodiment, the polarization maintaining tap coupler 13 is arranged at a position as shown in FIG. 1, but the present invention is not limited to this, and it is arranged at any position as long as a part of the circulating laser beam can be extracted. It may also have any configuration.

  In the present embodiment, the control circuit 25 monitors and controls mode synchronization. However, the control circuit 25 is not limited to this and may perform only mode synchronization monitoring. A monitoring circuit that monitors mode synchronization and a control circuit that controls mode synchronization based on the monitoring result of the monitoring circuit may be provided.

  FIGS. 3A and 3B are diagrams showing a modification of the tap coupler 13 in FIG.

  In FIG. 3A, the tap coupler 13 has an output port and a monitor port arranged on the opposite side of the output port, and extracts a part of the circulating laser light and externally. The laser beam output to the output port is branched to the output port, and the monitored laser beam is branched to the monitor port. In FIG. 3B, the tap coupler 13 branches a part of the circulating laser light and leads it to the mode synchronization control device 20, and a part of the circulating laser light branches outside. And a tap coupler 13b for guiding. At this time, the propagation direction of the laser beam branched by the tap coupler 13b is opposite to the circulation direction of the circulating laser beam. 3C, which is basically the same as the configuration shown in FIG. 3B, the propagation direction of the laser beam branched by the tap coupler 13b is the same as the circulation direction of the circulating laser beam. It is different.

  In the present embodiment, a bandpass filter is used to select a part of the wavelength components of the laser beam. However, the present invention is not limited to this, and a low-pass filter (LPF) that selects a long wavelength region of the laser beam. ) Or a high-pass filter (HPF) that selects a short wavelength region of the laser light may be used. However, since the optical power in the long wavelength region tends to be small when the mode is not synchronized, when the long wavelength region is selected, the optical power difference between when the mode is synchronized and when the mode is not synchronized increases. Therefore, it is more preferable to use a filter that selects the long wavelength region.

  Further, in the central wavelength region of the laser light, the optical power difference between the case where the mode is synchronized and the case where the mode is not synchronized is small. Therefore, it is preferable to select the center wavelength region while avoiding the central wavelength region. Further, in the case of a spectrum that produces a sharp spike component such as the wavelength 1557 nm in FIG. 2, it is preferable to select the spectrum avoiding the wavelength region.

DESCRIPTION OF SYMBOLS 10 Mode-locked laser apparatus 11 Modulator 12, 21 Isolator 13, 15, 22 Tap coupler 14 Nonlinear optical amplification loop mirror 14a PM-EDF
DESCRIPTION OF SYMBOLS 16 Excitation light source 17 WDM coupler 18 Function generator 19 Optical fiber 20 Mode synchronous control apparatus 23 Band pass filter 24 Light receiving element 25 Control circuit

Claims (9)

In a mode-locked laser device that generates mode-locked laser light by modulating laser light within a predetermined path,
Branching means for branching the laser light in the predetermined path;
Selection means for selecting a part of the laser beam branched by the branching means;
A light receiving means for receiving the laser light selected by the selection means;
A mode-locked laser device comprising: monitoring means for monitoring mode synchronization of the laser beam based on the amount of light received by the light-receiving means.   2. The mode-locked laser device according to claim 1, wherein the monitoring unit monitors mode synchronization of the laser beam based on the optical power received by the light-receiving unit.   2. The mode-locked laser device according to claim 1, wherein the monitoring unit determines that the laser beam is not mode-synchronized when the optical power received by the light-receiving unit is equal to or less than a predetermined threshold value. .   3. The mode according to claim 2, further comprising a control unit configured to execute a recovery operation so that the laser beam is mode-synchronized when the monitoring unit determines that the laser beam is not mode-synchronized. Synchronous laser device.   5. The control unit according to claim 4, wherein, in the restoration operation, the control unit temporarily increases a drive current of the light source that generates the laser light, and then reduces the drive current to a steady-state drive current. Mode-locked laser device.   The mode-locked laser device according to claim 1, wherein the branching unit is a tap coupler that branches the laser light in the predetermined path at a predetermined branching ratio.   The mode-locked laser device according to claim 1, wherein the selection unit is a filter that transmits laser light in a predetermined wavelength region.   The filter transmits laser light in a wavelength region having a large difference between the optical power received by the light receiving unit when the mode is synchronized and the optical power received by the light receiving unit when the mode is not synchronized. The mode-locked laser device according to claim 7. A method for controlling a mode-locked laser device that generates laser by modulating a laser beam within a predetermined path,
A branching step for branching the laser beam in the predetermined path;
A selection step of selecting a part of the laser beam branched by the branching step;
A light receiving step for receiving the laser light selected in the selection step;
And a monitoring step of monitoring mode synchronization of the laser beam based on the amount of light received in the light receiving step.

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