A mode-locked method using two different wavelengths selectively, and a laser device using the same

Abstract

The present application provides a mode-locked pulse light generating filter that simply realizes self-starting mode locking, and a laser device that generates a picosecond, femtosecond pulse laser by being provided with the filter, the laser device being provided with an amplification section that amplifies light within a resonator and outputs the light, and a mode-locked pulse light generating filter that has a first filter section that selectively outputs a first wavelength component that is a wavelength component of an oscillation band within the resonator, and a second filter section that selectively outputs a second wavelength component that is a wavelength component different from the oscillation band.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for simply achieving self-starting mode locking by selectively using two different wavelengths and a laser device. BACKGROUND

[0002] In the past, as a laser device that outputs a laser with a pulse width of several tens of picoseconds or less, a mode locking laser light source using an optical fiber or the like is known (for example, refer to Patent Documents 1 and 2).

[0003] Patent Document 1: US8416817

[0004] Patent Document 2: US7940816 SUMMARY

[0005] PROBLEMS TO BE SOLVED BY THE INVENTION

[0006] In a laser device that generates a picosecond, femtosecond pulse laser, it is preferable to perform mode locked laser oscillation in a simple method and stabilize in a short time. Recently, a mode locked fiber laser that is small and low cost and has excellent environmental stability is used in many industrial applications. In particular, a mode locked fiber laser (ANDi MLFL: All Normal Dispersion Mode-Locked Fiber Laser) that is configured from a polarization maintaining fiber (PMF) and that uses only optical components that exhibit normal dispersion in the oscillation wavelength of the laser is capable of outputting a relatively high pulse energy, and thus is suitable for applications such as microfabrication. Generally, in order to achieve self-starting of mode locking, output adjustment, polarization control, temperature control, and the like of a semiconductor laser that serves as a pump light source are required. In an ANDi MLFL configured using a PMF, it is also difficult to achieve self-starting, and in order to achieve self-starting of mode locking, complex control such as output adjustment of a semiconductor laser that serves as a pump light source is required. Furthermore, in an ANDi MLFL in which a saturable absorption section is not configured using a semiconductor saturable absorption mirror or the like, and in which a nonlinear polarization rotation or a nonlinear amplifying ring mirror-like ANDi MLFL is used in the saturable absorption section, it is particularly difficult to achieve self-starting. In addition, in a laser device, it is preferable that the selection range of the laser oscillation wavelength be wide, but in a mode locked laser, the selectivity of the oscillation wavelength is low, and it is difficult to obtain mode locked laser oscillation at a wavelength with a small stimulated emission cross section.

[0007] METHOD FOR SOLVING THE PROBLEM

[0008] To address the aforementioned issues, in a first aspect of the present invention, a laser device is provided that includes a mode-locked pulse light generating filter (filter section) that easily achieves self-starting mode-locking. This filter section generates picosecond and femtosecond pulsed lasers. The laser device may include an amplification section that amplifies and outputs the laser within a resonator. The mode-locked pulse light generating filter may be disposed within the resonator. The wavelength transmission characteristics of the mode-locked pulse light generating filter can have maxima at at least two wavelengths. Based on these wavelength transmission characteristics, the mode-locked pulse light generating filter selectively allows wavelength components of light to pass through.

[0009] The mode-locked pulsed light generating filter can have a first passband that selectively allows a first wavelength component, which is the wavelength component of the laser oscillation wavelength, to pass through, and a second passband that selectively allows a second wavelength component, which is a wavelength component different from the oscillation wavelength, to pass through.

[0010] The filter section can be a single filter positioned in one location within the laser transmission path. Alternatively, the filter section can have two or more filters positioned in different locations. A first passband and a second passband can be configured within a single bandpass filter. The fiber Bragg grating (FBG) for selecting the first passband and the FBG for selecting the second passband can be positioned within the laser transmission path.

[0011] The pass-through wavelength characteristic of the filter section can have a minimum value between two maxima. The minimum value can be a value that attenuates by more than -10 dB compared to the lower maximum. The minimum value can be attenuated by more than -20 dB compared to the maximum. The minimum value can be attenuated by more than -30 dB compared to the maximum. The pass-through wavelength characteristic of the filter section can be connected between the two maxima. The pass-through wavelength characteristic of the filter section can also not be connected between the two maxima. The pass-through wavelength characteristic of the filter section can also have other components between the first passband and the second passband. These other components can be linear components.

[0012] In the laser output from the laser device, the magnitude of the second wavelength component can be less than 10% of the first wavelength component.

[0013] The passband width of the second filter section can be narrower than that of the first filter section.

[0014] The passband width of the second filter section can be above 0.2 nm.

[0015] The passband width of the second filter section can be below 4.6 nm.

[0016] The attenuation rate of the second filter section relative to the second wavelength component can be greater than the attenuation rate of the first filter section relative to the first wavelength component.

[0017] The wavelength difference between the first center wavelength of the passband of the first filter section and the second center wavelength of the passband of the second filter section can be 18 nm or less.

[0018] The wavelength difference can be 9 nm or more.

[0019] The amplification section can include a Yb fiber. The first center wavelength and the second center wavelength can each be 1020 nm or more and 1100 nm or less.

[0020] The amplification section can include an Er fiber. The first center wavelength and the second center wavelength can each be 1530 nm or more and 1555 nm or less, or 1555 nm or more and 1600 nm or less.

[0021] The amplification section can include an Nd fiber. The first center wavelength and the second center wavelength can each be 1060 nm or more and 1080 nm or less, or 888 nm or more and 914 nm or less.

[0022] The amplification section can include a Tm fiber. The first center wavelength and the second center wavelength can each be 1960 nm or more and 2020 nm or less, or 1860 nm or more and 1960 nm or less.

[0023] The first passband and the second passband can be variable. In a case where the wavelength difference between the first center wavelength and the second center wavelength is increased, the width of the first passband can be increased.

[0024] In a case where the wavelength difference between the first center wavelength and the second center wavelength is decreased, the width of the second passband can be decreased. Alternatively, the attenuation rate in the second passband can also be increased.

[0025] The laser device can be provided with a polarizer that makes the laser light linearly polarized. The laser device can be provided with a polarization-maintaining optical fiber that transmits the laser light. The laser device can be provided with a NALM that functions as a saturable absorber.

[0026] The amplification section can be a planar waveguide that includes a rare earth such as Yb, Er, or the like.

[0027] The laser device can be provided with a laser input section that couples the laser light transmitted in the light transmission section and the excitation laser light. The laser input section can be a WDM (Wavelength Division Multiplexing) coupler.

[0028] The laser device can be provided with a laser output section that outputs a predetermined proportion of the laser light transmitted in the light transmission section. A light isolator that specifies the direction of circulation of the laser light can be provided between the laser input section and the laser output section.

[0029] The laser device can have a coupling section that couples the light transmission section and the saturable absorption section. The coupling section can separate laser light input to the loop of the NALM into a component that is transmitted clockwise in the loop and a component that is transmitted counterclockwise in the loop.

[0030] The laser device can have a reflection section that reflects laser light. The second filter section, the first filter section, and the reflection section can be arranged in this order in accordance with a structure that is close to the amplification section. The reflection section can reflect laser light of the first wavelength component that has passed through the first filter section toward the first filter section.

[0031] In a second aspect of the present invention, a mode-locked method of mode-locking laser light is provided. The method selectively passes a wavelength component of light in a path of laser light transmission in accordance with a pass wavelength characteristic that has a maximum value at two or more wavelengths to mode-lock the laser light.

[0032] In addition, the above summary of the invention does not list all the essential features of the invention. In addition, sub-combinations of these feature groups can also be inventions. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 FIG. 1 is a diagram showing a structure example of a laser device 100 according to an embodiment of the present invention.

[0034] Figure 2 FIG. 3 is a diagram showing an example of a pass wavelength characteristic of the filter section 10.

[0035] Figure 3 FIG. 6 is a diagram showing energy levels of electrons in the light transmission section 101.

[0036] Figure 4 FIG. 9 is a diagram showing a stimulated emission cross-sectional area of a Yb fiber used in the amplification section 20.

[0037] Figure 5 FIG. 12 is a conceptual diagram showing that the oscillation at the first wavelength can be stabilized in a short time by having the second filter section 10-2.

[0038] Figure 6 FIG. 15 is a diagram showing temporal waveforms and wavelength distributions of the excitation laser light and the laser light in a case where the second filter section 10-2 is not provided.

[0039] Figure 7 FIG. 18 is a diagram showing temporal waveforms and wavelength distributions of the excitation laser light and the laser light in a case where the second filter section 10-2 is provided.

[0040] Figure 8 FIG. 21 is a diagram showing a structure example of the light transmission section 101 and the saturable absorption section 102.

[0041] Figure 9AIt means in Figure 8 A diagram showing an example of the first passband 301 and the second passband 302 set in the filter section 10.

[0042] Figure 9B It means that it is in use Figure 9A The diagram shows the wavelength distribution of the laser output by the laser device 100 in the case of the first passband 301 and the second passband 302.

[0043] Figure 10A This is a diagram showing another example of the first passband 301 and the second passband 302.

[0044] Figure 10B It means that it is in use Figure 10A The diagram shows the wavelength distribution of the laser output by the laser device 100 in the case of the first passband 301 and the second passband 302.

[0045] Figure 11A This is a diagram showing another example of the first passband 301 and the second passband 302.

[0046] Figure 11B It means that it is in use Figure 11A The diagram shows the wavelength distribution of the laser output by the laser device 100 in the case of the first passband 301 and the second passband 302.

[0047] Figure 12A This is a diagram showing another example of the first passband 301 and the second passband 302.

[0048] Figure 12B It means that it is in use Figure 12A The diagram shows the wavelength distribution of the laser output by the laser device 100 in the case of the first passband 301 and the second passband 302.

[0049] Figure 13A This is a diagram showing another example of the first passband 301 and the second passband 302.

[0050] Figure 13B It means that it is in use Figure 13A The diagram shows the wavelength distribution of the laser output by the laser device 100 in the case of the first passband 301 and the second passband 302.

[0051] Figure 14A This is a diagram showing another example of the first passband 301 and the second passband 302.

[0052] Figure 14B It means that it is in use Figure 14A The diagram shows the wavelength distribution of the laser output by the laser device 100 in the case of the first passband 301 and the second passband 302.

[0053] Figure 15 FIG. 2 is a view showing another configuration example of the filter section 10.

[0054] Figure 16 FIG. 3 is a view showing another configuration example of the light transmission section 101. DETAILED DESCRIPTION

[0055] Hereinafter, the present application will be described through embodiments of the application, but the following embodiments do not limit the application involved in the claims. In addition, the combination of features described in the embodiments is not necessarily all of the means for solving the application.

[0056] Figure 1 FIG. 1 is a view showing a configuration example of a laser device 100 according to one embodiment of the present application. The laser device 100 is a device that generates laser light having a wavelength component of a predetermined oscillation band. The laser device 100 can generate laser light having a pulse width of picoseconds (for example, 1 picosecond to 1000 picoseconds) or femtoseconds (for example, 1 femtosecond to 1000 femtoseconds). In the present specification, a wavelength having the largest intensity among the wavelengths of the laser light output from the laser device 100 is referred to as a first wavelength (or an oscillation wavelength). In addition, a component of the first wavelength among the light included in the laser light output from the laser device 100 is referred to as a first wavelength component. The oscillation band can be a band centered on the first wavelength.

[0057] The laser device 100 includes a filter section 10, an amplification section 20, and a saturable absorption section 102 provided in a path through which laser light is transmitted. The saturable absorption section 102 absorbs a wavelength of a temporal component having a relatively low intensity among the incident laser light. In addition, the saturable absorption section 102 transmits a temporal component having a relatively high intensity without being absorbed. That is, the saturable absorption section 102 absorbs a low-intensity dip portion in a temporal waveform of the laser light and transmits a high-intensity peak portion, thereby narrowing the pulse of the laser light in the time axis. By providing the saturable absorption section 102, it is possible to make the laser light short-pulsed.

[0058] The filter section 10 and the amplification section 20 can be a structure including an optical fiber or a structure connected to an optical fiber. The filter section 10 and the amplification section 20 can be arranged in a ring in which laser light circulates or in a path in which laser light travels back and forth. In addition, the filter section 10, the amplification section 20, and the saturable absorber section 102 can each be a separate component or circuit arranged at a specific position in the laser device 100 or can be composed of a plurality of components or circuits arranged dispersedly in the laser device 100. In addition, the laser device 100 as a whole can function as the filter section 10, the amplification section 20, or the saturable absorber section 102. That is, the respective components such as optical fibers of the laser device 100 can be combined to function as the filter section 10, the amplification section 20, or the saturable absorber section 102. At least a part of the optical fiber through which laser light is transmitted in the laser device 100 can be a polarization maintaining optical fiber (PMF). All the optical fibers constituting the laser device 100 can also be polarization maintaining optical fibers.

[0059] In addition, a common component can function as at least two of the filter section 10, the amplification section 20, and the saturable absorber section 102. For example, the amplification section 20 can function as at least a part of the filter section 10, and the saturable absorber section 102 can function as at least a part of the filter section 10.

[0060] The amplification section 20 amplifies the intensity of laser light passing therethrough. The amplification section 20 can have, for example, an optical fiber to which an impurity such as a rare earth element is added. The impurity is, for example, ytterbium (Yb), but is not limited thereto. In addition, the material of the optical fiber is, for example, quartz glass, but is not limited thereto. The amplification section 20 can also have a planar waveguide including a rare earth element such as Yb or Er (erbium).

[0061] The filter section 10 has a prescribed pass wavelength characteristic according to which a wavelength component of light (in this example, spontaneous emission amplified light or laser light) is selectively passed. The pass wavelength characteristic refers to a characteristic indicating the proportion of the intensity of light passing at each wavelength with respect to the intensity of incident light. As an example, the filter section 10 is a band-pass filter that attenuates a wavelength component outside a prescribed passband. The pass wavelength characteristic of the filter section 10 has a maximum value at at least two or more wavelengths. The filter section 10 of this example functions as a filter through which a mode-locked pulse used to start oscillation of laser light is passed.

[0062] In addition, the laser device 100 can have a polarizer that makes laser light transmitted in an optical fiber linearly polarized light. By adjusting the polarization axis of a polarization maintaining optical fiber between the polarizer and the filter section 10, the intensity of laser light transmitted from the filter section 10 to the polarizer can also be adjusted.

[0063] Figure 2is a graph showing an example of the pass wavelength characteristic of the filter section 10. Figure 2 In the graph, the horizontal axis indicates the wavelength of light transmitted in the filter section 10, and the vertical axis indicates the transmittance of each wavelength in the filter section 10. The transmittance is the ratio of the intensity of light after passing through the filter section 10 to the intensity of light before passing through the filter section 10.

[0064] The pass wavelength characteristic of the filter section 10 can be the pass wavelength characteristic of the laser device 100 as a whole. As an example, in a case where the laser circulates around a prescribed loop path, the pass wavelength characteristic of the filter section 10 can be the pass wavelength characteristic when the laser circulates one round on the loop path. Also, in a case where the laser goes back and forth on a prescribed path, the pass wavelength characteristic of the filter section 10 can be the pass wavelength characteristic when the laser goes back and forth one time on the path. In a case where the laser device 100 is provided with an explicit filter, the pass wavelength characteristic of the filter section 10 can also be the characteristic of the filter. The explicit filter is a component having a configuration known as a filter, such as an FBG.

[0065] As described above, the pass wavelength characteristic has at least two local maxima (in this example, local maximum 201 and local maximum 202). In this example, the wavelength of the local maximum 201 corresponds to the oscillation wavelength (referred to as first wavelength λ1) of the laser device 100. However, the wavelength of the local maximum 201 can not strictly coincide with the oscillation wavelength. Also, the wavelength of the local maximum 202 (referred to as second wavelength λ2) is a wavelength different from the oscillation wavelength. Also, the pass wavelength characteristic of this example has a mountain-shaped characteristic with each local maximum as a peak, but can also have a flat characteristic continuously showing local maxima with a prescribed wavelength width.

[0066] The filter section 10 has a first passband 301 including the first wavelength λ1 and a second passband 302 including the second wavelength λ2. In the graph, Figure 2 In the graph, an example is shown in which the first wavelength λ1 is larger than the second wavelength λ2, but the first wavelength λ1 can also be smaller than the second wavelength λ2. In this example, each passband is a frequency band in which the transmittance is one-half or more of the maximum. As described above, the first passband 301 is a frequency band including the first wavelength λ1 (oscillation wavelength). That is, the first passband 301 selectively passes the first wavelength component, which is a wavelength component of the oscillation frequency band, among the natural radiation amplification light or laser light incident. The second passband 302 is a frequency band including the second wavelength λ2 different from the first wavelength λ1. In this specification, the component of the second wavelength λ2 included in the natural radiation amplification light or laser light transmitted in the path is referred to as the second wavelength component. The second passband 302 selectively passes the second wavelength component, which is a wavelength component different from the oscillation frequency band, among the natural radiation amplification light or laser light incident.

[0067] The filter section 10 can be one filter provided in one place in the transmission path of the laser light (i.e., inside the resonator of the laser light), or can have two or more filters provided in different places. As one example, both the first passband 301 and the second passband 302 can be set in one bandpass filter. In other examples, a fiber Bragg grating (referred to as an FBG) that selects the first passband 301 and an FBG that selects the second passband 302 can also be provided in the transmission path of the laser light.

[0068] In addition, the passband characteristic of the filter section 10 can have a minimum value 203 between the two maximum values. The minimum value 203 can be a value that attenuates by -10 dB or more compared to the lower maximum value. The minimum value 203 can attenuate by -20 dB or more, or -30 dB or more compared to the maximum value. In addition, the passband characteristic of the filter section 10 can be connected between the two maximum values, or can not be connected. By connected between the two maximum values, it is meant, for example, a case where the minimum value 203 is 10% or more of the lower maximum value. In addition, the passband characteristic of the filter section 10 can have other components 204 between the first passband 301 and the second passband 302. The components 204 can be, for example, linear components.

[0069] By having the passband characteristic of the filter section 10 have two maximum values, in at least a portion of the transmission path of the laser light, the laser light contains the first wavelength component and the second wavelength component. By the laser light containing the second wavelength component that is different from the first wavelength component (oscillation wavelength), it is possible to induce oscillation at the first wavelength λ1, and it is possible to stabilize the oscillation at the first wavelength λ1 in a short time.

[0070] Figure 3 is a diagram that illustrates the energy levels of the electrons of the Yb-doped optical fiber in the amplification section 20. In Figure 3 In the example of, an example of an Yb-doped optical fiber is shown, but the laser medium is not limited thereto, and can be an optical fiber doped with another rare earth, or a planar waveguide of LINBO3, phosphoric glass, or quartz glass doped with a rare earth. The excitation energy level and the upper laser level can be the same, and the energy levels are not limited thereto. The electrons of the amplification section 20 are in a state of inverted distribution in which the number of electrons in the upper laser level is greater than the number of electrons in the lower laser levels 1 and 2.

[0071] The electrons of the excitation energy level move to a lower energy level, and thereby emit light of a wavelength corresponding to the energy level difference. By causing the electrons of the upper laser level to move to a plurality of energy levels, the laser light contains a plurality of wavelength components. In this example, the lower laser level corresponding to the first wavelength λ1 is set as the lower laser level 1, and the lower laser level corresponding to the second wavelength λ2 is set as the lower laser level 2.

[0072] Figure 4is a graph showing the stimulated emission cross section area of the Yb fiber used in the amplification section 20. In Figure 4 the horizontal axis is the wavelength, and the vertical axis is the cross section area. In Figure 4 the example, an example in which the fiber contains Yb is shown, but the material of the fiber is not limited to this.

[0073] The first wavelength λ1 in the filter section 10 is set to be a wavelength in which the stimulated emission cross section area is a certain value or more in the distribution characteristic of the wavelength component shown in Figure 4 In the distribution characteristic of the wavelength component shown in

[0074] Figure 5 is a conceptual diagram showing that by having the second passband 302, it is possible to stabilize the oscillation at the first wavelength λ1 in a short time. In Figure 5 In

[0075] In the amplification section 20 in Figure 1 when a large induced emission occurs in the second wavelength component by the Q-switching operation, Figure 3 the number of electrons in the lower laser level 2 increases. Due to this, the inversion distribution between the upper laser level and the lower laser level 2 becomes small, and the second wavelength component becomes small with time. On the other hand, in the first wavelength component, a large induced emission does not occur, and thus the inversion distribution is maintained between the upper laser level and the lower laser level 1. Due to this, the first wavelength component is amplified moderately, and the oscillation at the first wavelength easily occurs in a short time.

[0076] In a general mode-locked laser, in the case where the mode-locked pulse oscillation stops once, in order to obtain the mode-locked pulse again, it is necessary to perform readjustment of the driving current of the semiconductor laser for laser excitation. Usually, it takes several tens of seconds to several minutes or so. In the present method, even in the case where the oscillation at the first wavelength stops due to some reason, by the presence of the second wavelength component, it is possible to automatically and rapidly start the oscillation at the first wavelength again.

[0077] Figure 6 is a graph showing the time waveform and the wavelength distribution of the excitation laser and the laser in the case where the first passband 301 is provided and the second passband 302 is not provided. The laser is the laser output from the laser device 100.

[0078] In stage 501, the intensity of the excitation laser is increased. If the intensity of the excitation laser is increased, an oscillation component of high intensity is generated in the laser (stage S502). The state of stage S502 continues for several seconds to several minutes. While the intensity of the excitation laser is maintained, a plurality of mode-locked pulses are generated in the temporal waveform (stage S503). In this state, if the intensity of the excitation laser is reduced, a mode-locked pulse of a prescribed oscillation wavelength remains (stage S504).

[0079] Thus, without providing the second passband 302, in order to start oscillation of the laser, the intensity of the excitation laser is increased, causing Q-switching oscillation in which a pulse of very high intensity is generated (stage S502). Thereafter, the pulse of high intensity is divided into a plurality of pulses, becoming a state in which one or more pulses exist within the oscillator, which is called multi-pulse oscillation (stage S503). Finally, by reducing the intensity of the excitation laser, stable single-pulse oscillation is achieved (stage S504). Therefore, in order to generate laser of a prescribed oscillation wavelength, several minutes or so are sometimes required.

[0080] Figure 7 is a graph showing the temporal waveform and the wavelength distribution of the excitation laser and the laser in a case in which the first passband 301 and the second passband 302 are provided. As described above, by providing the second passband 302, oscillation at the first wavelength λ1 can be performed. In this example, the intensity of the excitation laser can also not be increased in stage S601, and the pulse generated in the Q-switching oscillation is relatively small. As a result, oscillation at the first wavelength λ1 is started without passing through the state of multi-pulse oscillation (S603). In one example, using a Yb fiber, in a case in which the first wavelength λ1 is set to 1040 nm and the second wavelength λ2 is set to 1030 nm, laser can be generated in 2 seconds or less on average.

[0081] Figure 8 is a graph showing a configuration example of the laser device 100. The laser device 100 of this example has an optical transmission section 101 and a saturable absorption section 102. The optical transmission section 101 has a filter section 10, a long-length fiber section 23 that functions as an amplification section 20, an amplification section 21, a laser input section 30, a laser output section 40, an optical fiber 50, an optical isolator 60, and a coupling section 70. The respective constituent elements of the optical transmission section 101 are connected by the optical fiber 50. In the optical transmission section 101 of this example, laser circulates in the optical transmission section 101. In addition, the optical transmission section 101 can also be an all-fiber device in which the respective constituent elements are formed by optical fibers. The optical transmission section 101 of this example is connected to the saturable absorption section 102 by the optical fiber 50, and laser reciprocates between the optical transmission section 101 and the saturable absorption section 102. In this example, as the saturable absorption section 102, a nonlinear amplifying loop mirror (NALM) is used.

[0082] The excitation laser is input to the laser input section 30. The laser input section 30 couples the excitation laser with the laser transmitted in the optical transmission section 101 and transmits it in the optical fiber 50. The laser input section 30 is, for example, a WDM (Wavelength Division Multiplexing) coupler.

[0083] The amplification section 21 of this example is provided between the laser input section 30 and the laser output section 40. In addition, between the structure A and the structure B in the ring-shaped optical transmission section 101 means a region from the structure A to the structure B in the circumferential direction of the laser. The amplification section 21 can have a Yb-added optical fiber (YDF). The long optical fiber section 23 is provided between the laser input section 30 and the laser output section 40. The optical fiber section 23 of this example is provided between the amplification section 21 and the laser output section 40. The optical fiber section 23 can have a non-polarization maintaining optical fiber (Non-PMF). Either of the optical fiber section 23 and the amplification section 21 can be provided alone. The optical fiber section and the amplification section 21 amplify the intensity of the laser transmitted in the optical transmission section 101 with the excitation laser. The configuration of the long optical fiber section 23 and the amplification section 21 is not limited to the example of FIG. 2. Figure 8

[0084] Between the laser input section 30 and the laser output section 40, an optical isolator 60 that specifies the circumferential direction of the laser can be provided. The optical isolator 60 of this example is provided between the amplification section 21 and the long optical fiber section 23.

[0085] The laser output section 40 of this example is configured between the long optical fiber section 23 and the filter section 10. The laser output section 40 outputs a predetermined proportion of the laser transmitted in the optical transmission section 101. For example, the laser output section 40 outputs 10% to about 80% of the laser that passes therethrough as output laser to the outside. The lower limit of the ratio of the output laser to the laser that passes through the laser output section 40 can also be less than 10% (for example, 1%). In addition, the upper limit of the ratio can also be about 90%. The remaining laser is transmitted in the optical transmission section 101. The laser output section 40 is, for example, an OC (Output Coupler).

[0086] The filter section 10 passes the wavelength components of the set passband of the laser transmitted in the optical transmission section 101 and attenuates the wavelength components outside the passband. The filter section 10 of this example is an optical bandpass filter that sets the first passband 301 and the second passband 302 described in the first embodiment. An optical isolator 60 can be provided between the filter section 10 and the laser output section 40. Figures 1 to 7

[0087] ​​The coupling section 70 couples the light transmission section 101 and the saturable absorption section 102. The coupling section 70 of the present example separates the laser light input to the loop of the NALM into a component that is transmitted clockwise in the loop and a component that is transmitted counterclockwise in the loop. The coupling section 70 of the present example is disposed between the optical fiber section 23 and the laser output section 40, but the disposition of the coupling section 70 is not limited to this.

[0088] The saturable absorption section 102 receives the laser light that has passed through the laser input section 30, and absorbs the wavelength component of the temporal component that constitutes a pulse below a prescribed intensity. The saturable absorption section 102 inputs the wavelength component of the laser light received from the light transmission section 101 that is higher than the prescribed intensity to the light transmission section 101.

[0089] The saturable absorption section 102 of the present example generates a phase difference in accordance with the intensity difference with respect to the clockwise transmitted component and the counterclockwise transmitted component. In the coupling section 70, the laser light is transmitted from the saturable absorption section 102 to the light transmission section 101 with a transmission characteristic that corresponds to the phase difference of the two components. Therefore, the saturable absorption section 102 attenuates the temporal component of which the intensity is low, and transmits the temporal component of which the intensity is high in the clockwise direction of the light transmission section 101.

[0090] The saturable absorption section 102 of the present example has an amplification section 103, an optical fiber 106, and a laser input section 104. The respective constituent elements of the saturable absorption section 102 are connected in a ring shape by the optical fiber 106. The laser input section 104 couples the excitation laser light and the laser light that is transmitted counterclockwise in the saturable absorption section 102.

[0091] The amplification section 103 is disposed on a path that goes clockwise from the coupling section 70 toward the laser input section 104, and amplifies the laser light. The amplification section 103 is, for example, a Yb-doped optical fiber.

[0092] Figure 9A is a graph that shows an example of the first passband 301 and the second passband 302 that are set in the filter section 10. Figure 8 The vertical axis of the graph of Figure 9A represents the ratio of the intensity of the laser light output from the filter section 10 to the intensity of the laser light input to the filter section 10. That is, in the case where the intensity is 1, the attenuation in the filter section 10 is 0 dB.

[0093] The center wavelength (first wavelength) of the first passband 301 of the present example is 1040 nm, and the bandwidth is 1.8 nm. In addition, the center wavelength (second wavelength) of the second passband 302 is 1030 nm, and the bandwidth is 1.5 nm. In the example of Figure 9A , the first passband 301 is a Gaussian shape, and the second passband 302 is a rectangular shape, but the shapes of the first passband 301 and the second passband 302 can each be any of a Gaussian shape and a rectangular shape.

[0094] Figure 9B is a graph showing the wavelength distribution of the laser light output from the laser device 100 in the case where the first passband 301 and the second passband 302 shown in Figure 9A are used. In this example, the laser light having the wavelength distribution shown in Figure 9B is obtained instantaneously (within 5 seconds) after the excitation laser light is input. In contrast, in the case where only the first passband 301 is set to the filter section 10, the laser light having the wavelength distribution shown in Figure 9B is obtained after 20 minutes or so elapses from the input of the excitation laser light. That is, it is known that by setting the second passband 302, the laser light oscillated at the first wavelength can be obtained instantaneously.

[0095] In addition, the size P2 of the second wavelength component in the laser light output from the laser device 100 can be 10% or less of the size PI of the first wavelength component. The P2 can be 1% or less of the PI, or 0.1% or less.

[0096] The passband width of the second passband 302 can be smaller than the passband width of the first passband 301. The width of the passband of the filter section 10 can be the width of the wavelength band in which the intensity of the wavelength component of the input laser light is half or less. That is, it can be the width of the wavelength band in which the transmittance of the filter section 10 is 50% or more. The passband width of the second passband 302 can be 90% or less, or 70% or less, or 50% or less of the passband width of the first passband 301.

[0097] Figure 10A is a graph showing another example of the first passband 301 and the second passband 302. The center wavelength (first wavelength) of the first passband 301 of this example is 1048 nm, and the bandwidth is 3.5 nm. In addition, the second passband 302 is the same as that of Figure 9A .

[0098] Figure 10B is a graph showing the wavelength distribution of the laser light output from the laser device 100 in the case where the first passband 301 and the second passband 302 shown in Figure 10A are used. In this example, the laser light having the wavelength distribution shown in Figure 10B is obtained after 5 seconds or so elapses from the input of the excitation laser light. In contrast, in the case where only the first passband 301 is set to the filter section 10, the laser light oscillated at the first wavelength cannot be obtained.

[0099] Figure 11A is a graph showing another example of the first passband 301 and the second passband 302. The first passband 301 of this example is the same as that of Figure 9AThe example of the second passband 302 is the same as that of the first passband 301. The center wavelength (second wavelength) of the second passband 302 is 1030 nm, and the bandwidth is 1.8 nm. However, the second passband 302 attenuates by -1.5 dB at the second wavelength. In contrast, the first passband 301 attenuates by 0 dB at the first wavelength.

[0100] Figure 11B is a graph showing the wavelength distribution of the laser light output from the laser device 100 in the case where the first passband 301 and the second passband 302 shown in Figure 11A are used. In this example, if at least 10 seconds or so elapse from the input of the excitation laser light, laser light having the wavelength distribution shown in Figure 11B is obtained. In this way, the attenuation rate of the second passband 302 with respect to the second wavelength component can be greater than the attenuation rate of the first passband 301 with respect to the first wavelength component. The attenuation rate of the second passband 302 with respect to the second wavelength component can be 90% or less, or 70% or less, or 50% or less of the attenuation rate of the first passband 301 with respect to the first wavelength component. Thus, the second wavelength component is easily suppressed in the laser light output from the laser device 100.

[0101] Figure 12A is a graph showing another example of the first passband 301 and the second passband 302. The first passband 301 of this example is the same as that of Figure 9A . The center wavelength (second wavelength) of the second passband 302 is 1030 nm, and the bandwidth is 4.6 nm.

[0102] Figure 12B is a graph showing the wavelength distribution of the laser light output from the laser device 100 in the case where the first passband 301 and the second passband 302 shown in Figure 12A are used. In this example, if at least 10 seconds or so elapse from the input of the excitation laser light, laser light having the wavelength distribution shown in Figure 12B is obtained. However, by increasing the bandwidth of the second passband 302, a part of the laser light passes through the second passband 302. On the other hand, the laser light that has passed through the first passband 301 is greatly expanded in wavelength component within the laser device 100 due to the self-phase modulation effect. A part of the laser light that has passed through the second passband 302 overlaps with a part of the wavelength expanded by the self-phase modulation effect, and these wavelength components interfere. Thus, the noise component around the second wavelength becomes large. The bandwidth of the second passband 302 is preferably 4.6 nm or less.

[0103] In addition, even if the bandwidth of the second passband 302 is reduced to 0.2 nm, the same as in the example shown in Figure 9B , laser light oscillating at the first wavelength is obtained. However, if the bandwidth of the second passband 302 is made too small, it is sometimes difficult to perform laser light oscillation at the first wavelength. The bandwidth of the second passband 302 is preferably 0.2 nm or more.

[0104] In this example, the bandwidth of the second passband 302 is changed, but even if the bandwidth of the first passband 301 is changed, the laser light of the first wavelength is obtained. However, if the bandwidth of the first passband 301 is too small, it is difficult to obtain the laser light of the first wavelength, and thus the bandwidth of the first passband 301 can be 0.8 nm or more. The bandwidth of the first passband 301 can be 50% or more of the bandwidth of the second passband 302.

[0105] Figure 13A is a graph showing another example of the first passband 301 and the second passband 302. The first passband 301 of this example is the same as that of the example of Figure 9A The center wavelength (second wavelength) of the second passband 302 is 1033 nm, and the bandwidth is 1.5 nm.

[0106] Figure 13B is a graph showing the wavelength distribution of the laser light output from the laser device 100 in a case where the first passband 301 and the second passband 302 shown in Figure 13A In this example, if at least 10 seconds or so elapse from the input of the excitation laser light, the laser light having the wavelength distribution shown in Figure 13B is obtained. However, by reducing the wavelength difference between the second passband 302 and the first passband 301, the spectral component that passes through the second passband 302 easily interferes with the spectral component that is expanded by the self-phase modulation effect after passing through the first passband 301. Therefore, as shown in Figure 13B In the example of Figure 13A the second wavelength of the second passband 302 is changed, but even if the first wavelength of the first passband 301 is changed, the laser light of the first wavelength can be obtained.

[0107] The wavelength difference between the center wavelength (first wavelength) of the first passband 301 and the center (second wavelength) of the second passband 302 is preferably 9 nm or more. The wavelength difference can be 10 nm or more. In addition, the value obtained by subtracting half of the bandwidth of each passband from the difference between the center wavelengths can be 7.35 nm.

[0108] In addition, if the wavelength difference between the first wavelength and the second wavelength is too large, even if the light of the second wavelength is generated, it is sometimes difficult to induce the light of the first wavelength. Therefore, the wavelength difference between the first wavelength and the second wavelength is preferably 18 nm or less. The wavelength difference can be 15 nm or less, or 12 nm or less. In addition, the value obtained by subtracting half of the bandwidth of each passband from the difference between the center wavelengths can be 16.35 nm or less.

[0109] Figure 14A is a graph showing another example of the first passband 301 and the second passband 302. The second passband 302 of this example is the same as that of the example ofFigure 9A The example of the first passband 301 is the same. The center wavelength (first wavelength) of the first passband 301 is 1040 nm, and the bandwidth is 1.8 nm. However, the first passband 301 attenuates by -2.8 dB at the first wavelength.

[0110] Figure 14B is a graph showing the wavelength distribution of the laser light output from the laser device 100 when the first passband 301 and the second passband 302 shown in Figure 14A are used. In this example, if at least 10 seconds or so elapses from the input of the excitation laser light, laser light having the wavelength distribution shown in Figure 14B is obtained. However, by increasing the attenuation rate of the first passband 301, the relative size of the second wavelength component (1030 nm) is larger than in the example of Figure 9B Therefore, the attenuation rate at the first wavelength of the first passband 301 can be 50% or more, 70% or more, or 90% or more of the attenuation rate at the second wavelength of the second passband 302.

[0111] It is preferable that the first passband 301 and the second passband 302 be frequency bands corresponding to the material of the optical fiber of the amplification section 20. That is, as explained in Figure 4 , it is preferable that each passband be set in a wavelength band in which the intensity of the laser light generated by the optical fiber is constant or more.

[0112] As one example, in the case where the amplification section 20 includes a Yb optical fiber, both the first wavelength and the second wavelength are preferably 1020 nm or more and 1050 nm or less. In the case where the amplification section 20 includes an Er optical fiber, both the first wavelength and the second wavelength are preferably 1530 nm or more and 1555 nm or less or 1555 nm or more and 1600 nm or less. It can also be that one of the wavelengths is 1530 nm or more and 1555 nm or less and the other of the wavelengths is 1555 nm or more and 1600 nm or less. In the case where the amplification section 20 includes a Nd optical fiber, both the first wavelength and the second wavelength are preferably 1060 nm or more and 1080 nm or less or 888 nm or more and 914 nm or less. It can also be that one of the wavelengths is 1060 nm or more and 1080 nm or less and the other of the wavelengths is 888 nm or more and 914 nm or less. In the case where the amplification section 20 includes a Tm optical fiber, both the first wavelength and the second wavelength are preferably 1960 nm or more and 2020 nm or less or 1860 nm or more and 1960 nm or less. It can also be that one of the wavelengths is 1960 nm or more and 2020 nm or less and the other of the wavelengths is 1860 nm or more and 1960 nm or less.

[0113] The first passband 301 and the second passband 302 can be variable. That is, the center wavelength and the bandwidth of each passband can be variable. For example, the center wavelength (first wavelength) of the first passband 301 can be changed according to the wavelength of the laser to be generated. The filter section 10 can increase the bandwidth of the first passband 301 in a case where the wavelength difference between the center wavelength (first wavelength) of the first passband 301 and the center wavelength (second wavelength) of the second passband 302 is increased. As the wavelength difference is increased, it is difficult to induce the first wavelength component, but by increasing the bandwidth of the first passband 301, it is possible to promote oscillation at the first wavelength.

[0114] In addition, in a case where the wavelength difference between the first wavelength and the second wavelength is reduced, the bandwidth of the second passband 302 can be reduced. By reducing the wavelength difference, the proportion of the second wavelength component interfering with the first passband 301 increases, but by reducing the bandwidth of the second passband 302, it is possible to suppress this interference. In addition, in a case where the wavelength difference between the first wavelength and the second wavelength is reduced, the attenuation rate at the second wavelength of the second passband 302 can also be increased. Thereby, it is also possible to suppress this interference.

[0115] Figure 15 is a view that shows another example of the configuration of the filter section 10. The filter section 10 of the present example is connected to the ring-shaped optical fiber 50 via a coupling section 80. The coupling section 80 transmits laser light that circulates around the ring-shaped optical fiber 50 to the filter section 10 and transmits light from the filter section 10 to the ring-shaped optical fiber 50.

[0116] The filter section 10 of the present example has a first filter section 10-2 that selects and transmits light of the first passband 301 and a second filter section 10-2 that selects and transmits light of the second passband 302. The first filter section 10-1 and the second filter section 10-2 of the present example are FBGs. The first filter section 10-1 and the second filter section 10-2 are provided in series with respect to the coupling section 80. The first filter section 10-1 and the second filter section 10-2 can both be provided in the vicinity of the coupling section 80.

[0117] Figure 16 is a view that shows another example of the configuration of the light transmission section 101. The light transmission section 101 of the present example differs from the light transmission section 101 described in Figure 8 or Figure 15 in that it does not have the amplification section 20, the amplification section 21, the laser light input section 30, and the optical isolator 60. The other configuration is the same as in the examples of Figure 8 or Figure 15 The optical fiber 50 can function as the amplification section 20 or the amplification section 21. The filter section 10 of the present example is disposed between the laser light output section 40 and the coupling section 70. The filter section 10 can also be connected to the optical fiber 50 via the coupling section 80 as in the example of Figure 15 .

[0118] In addition, in Figures 1 to 16 the example, the saturable absorber 102 is a NALM, but the saturable absorber 102 can use a semiconductor saturable absorber mirror (SESAM) or the like as an absorber. In addition, a saturable absorption mechanism using a Nonlinear Optical Loop Mirror (NOLM), a Nonlinear Polarization Rotation (NPR) can be adopted.

[0119] The present application has been described using embodiments, but the technical scope of the present application is not limited to the scope described in the above embodiments. It will be apparent to those skilled in the art that various changes or modifications can be made to the above-described embodiments. It is therefore understood that the embodiments encompassed within the technical scope of the present application are not limited to the above-described embodiments and include all technical equivalents of the embodiments disclosed along with other embodiments that can be derived by a person skilled in the art using common general knowledge based on the disclosure of the present application.

[0120] Note that the order of execution of each process of the devices, systems, programs, and methods shown in the claims, the specification, and the drawings is not particularly limited unless otherwise specified as "before," "prior to," and the like, and unless the output of the preceding process is used in the subsequent process. With regard to the flow of actions in the claims, the specification, and the drawings, the actions are described using "first," "next," and the like for convenience, but this does not mean that the actions must be performed in this order.

[0121] Explanation of Reference Signs

[0122] 10, filter section; 10-1, first filter section; 10-2, second filter section; 20, amplification section; 21, amplification section; 23, optical fiber section; 30, laser input section; 40, laser output section; 50, optical fiber; 52, wavelength characteristics; 60, optical isolator; 70, coupling section; 80, coupling section; 100, laser device; 101, light transmission section; 102, saturable absorber section; 103, amplification section; 104, laser input section; 106, optical fiber; 201, maximum value; 202, maximum value; 203, minimum value; 204, component; 301, first passband; 302, second passband.

Claims

1. A mode-locked laser device comprising an amplifier for generating laser light, wherein, The laser device includes a filter section disposed within a resonator. This filter section exhibits maxima at at least two wavelengths due to its wavelength characteristics, and selectively allows wavelength components of light to pass through based on these wavelength characteristics. After the laser oscillates, a first wavelength component surrounds the resonator without obstruction along the loop path of the laser within the resonator. This first wavelength component is the wavelength component of the laser's oscillation wavelength. The oscillation of the laser in the first wavelength component is triggered by the second wavelength component of the laser, which passes through the filter section and is different from the first wavelength component.

2. The mode-locked laser device according to claim 1, wherein, The wavelength transmission characteristics of the filter section include: a first passband containing any wavelength of the maximum value, allowing the first wavelength component to selectively pass; and a second passband containing any wavelength of the maximum value, allowing the second wavelength component to selectively pass.

3. A mode-locked laser device, which generates laser light, comprising: The filter section, which is disposed within the resonator, has a maximum value at at least two wavelengths due to its wavelength characteristics, and selectively allows the wavelength components of light to pass through according to the wavelength characteristics. as well as An amplification section, disposed in the loop path of the laser, includes an optical fiber or an amplification section connected to an optical fiber. After the laser oscillates, a first wavelength component surrounds the resonator without obstruction along the loop path of the laser within the resonator. This first wavelength component is the wavelength component of the laser's oscillation wavelength. The wavelength transmission characteristics of the filter section include: A first passband, which includes wavelengths of any of the said maxima, allows the first wavelength component to pass selectively; And a second passband, which includes any of the wavelengths of the said maxima, allowing selective passage of a second wavelength component, which is a wavelength component different from the said oscillation wavelength. The stimulated emission cross-sectional area of ​​the second wavelength component of the optical fiber is greater than the stimulated emission cross-sectional area of ​​the first wavelength component.

4. The mode-locked laser device according to claim 3, wherein, The second wavelength component of the laser passing through the filter section induces oscillations in the oscillation wavelength.

5. The mode-locked laser device according to any one of claims 2 to 4, wherein, In the laser output by the laser device, the magnitude of the second wavelength component is less than 10% of the magnitude of the first wavelength component.

6. The mode-locked laser device according to any one of claims 2 to 4, wherein, The width of the second passband is narrower than the width of the first passband.

7. The mode-locked laser device according to any one of claims 2 to 4, wherein, The resonator includes an amplification section that amplifies the laser beam. The amplification section includes Yb optical fiber. The center wavelengths of the first passband and the second passband are both above 1020 nm and below 1100 nm.

8. The mode-locked laser device according to any one of claims 2 to 4, wherein, The resonator includes an amplification section that amplifies the laser beam. The amplification section includes Er optical fiber. The center wavelength of the first passband and the second center wavelength of the passband are both above 1530nm and below 1555nm or above 1555nm and below 1600nm.

9. The mode-locked laser device according to any one of claims 2 to 4, wherein, The resonator includes an amplification section that amplifies the laser beam. The amplification section includes Nd optical fiber. The center wavelength of the first passband and the center wavelength of the second passband are both above 1060nm and below 1080nm or above 888nm and below 914nm.

10. The mode-locked laser device according to any one of claims 2 to 4, wherein, The resonator includes an amplification section that amplifies the laser beam. The amplification section includes a Tm optical fiber. The center wavelength of the first passband and the center wavelength of the second passband are both above 1960nm and below 2020nm or above 1860nm and below 1960nm.

11. The mode-locked laser device according to any one of claims 2 to 4, wherein, The first passband and the second passband are variable. The width of the first passband is increased by increasing the wavelength difference between the center wavelength of the first passband and the center wavelength of the second passband.

12. The mode-locked laser device according to any one of claims 2 to 4, wherein, The first passband and the second passband are variable. While reducing the wavelength difference between the center wavelength of the first passband and the center wavelength of the second passband, the width of the second passband is reduced, or the attenuation rate in the second passband is increased.

13. The mode-locked laser device according to any one of claims 1 to 4, wherein, The laser device also includes a polarization-maintaining optical fiber for transmitting the laser.

14. The mode-locked laser device according to any one of claims 1 to 4, wherein, The laser device also has a NALM that functions as a saturable absorber.

15. The mode-locked laser device according to any one of claims 1 to 4, wherein, The filter section is used to allow the mode-locked pulse that initiates the oscillation of the laser to pass through.

16. A mode-locking method that enables laser to perform mode-locking, wherein, Within the resonator of the laser, based on the transmission wavelength characteristics that have maxima at at least two wavelengths, a first wavelength component of the light and a second wavelength component different from the first wavelength component are selectively allowed to pass through, thereby enabling mode-locking of the laser. After the laser oscillation, the first wavelength component propagates throughout the entire loop path of the laser and surrounds the resonator. The first wavelength component is the wavelength component of the laser's oscillation wavelength. The second wavelength component induces oscillations in the laser light of the first wavelength component.

17. The mold-locking method according to claim 16, wherein, The wavelength characteristics include: a first passband containing any of the wavelengths of the said maxima, allowing the first wavelength component to pass selectively; and a second passband containing any of the wavelengths of the said maxima, allowing the second wavelength component to pass selectively.

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