JP2015015337A - Mode-locked laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mode-locked laser which is capable of generating high pulse energy by suppressing the increase in size of a device and increasing an optical path length of a resonator.SOLUTION: A resonator has a configuration in which a plurality of branch optical paths into which an optical path between mirrors is branched by a polarization beam splitter 1 allowing one of two polarized light beams different in polarization direction by 90° of laser light to pass therethrough and reflecting the other are provided and a gain medium 6 is provided in one branch optical path of the plurality of branch optical paths and a polarization rotation mechanism 5 in which a polarization direction of laser light is rotated by 90° when the laser light travels back and forth is provided in the other branch optical path. One polarized light beam which passes the polarization beam splitter 1 from the branch optical path in which the gain medium 6 is provided, and is propagated to the branch optical path in which the polarization rotation mechanism 5 is provided, to have the polarization direction equalized to the same polarization direction as that of the one polarized light beam by travelling back and forth twice in the polarization rotation mechanism 5 and then is made to pass the polarization beam splitter 1 and is propagated in the branch optical path in which the gain medium 6 is provided, to obtain a resonance state.

Description

  The present invention relates to a mode-locked laser that generates a short pulse laser beam by mode-locking a pulse laser beam in a resonator.

Mode-locked lasers use a modulator called a mode locker that periodically loses pulsed laser light, and mode-locks laser light of different wavelengths to generate short-pulse laser light with a pulse width of nanoseconds or less. This is a laser device to be generated.
The short pulse laser beam generated in this way has a high peak power (= pulse energy / pulse width) reaching megawatts, and is therefore used in various medical measuring devices using a non-linear effect.
For example, medical measurement apparatuses using a mode-locked laser include stimulated Raman scattering, two-photon absorption, fluorescence imaging using double and triple harmonics generated in the body, and Coherent Anti-Stokes Raman Scattering.
In these medical measurement devices, the higher the peak power of the pulsed laser light, the stronger the nonlinear effect appears. Therefore, a mode-locked laser with high pulse energy is preferred.
A mode-locked laser with a large pulse energy includes a solid-state mode-locked laser in which a resonator is composed of a plurality of mirrors. The solid-state mode-locked laser includes a gain medium that provides gain and a mode locker that modulates laser light inside a resonator composed of a plurality of mirrors.
The mode locker may be a modulator that is synchronized with the period (round trip period) in which the pulsed laser light makes a round circuit around the resonator, and is an electro-optic element (EO element), an acousto-optic element (AO element), or high intensity light. It is comprised by the saturable absorber which permeate | transmits selectively.
Further, the refractive index change (Kerr lens effect) of the gain medium when the laser beam is transmitted can be used as a mode locker (Kerr lens mode synchronization). In that case, the gain medium doubles as a mode locker.
In addition, a mechanism such as a slit for limiting the transmission of laser light may be included in the resonator in order to enhance the function of car lens mode synchronization. Regardless of which mode locker is used, the mode locker causes a loss to the laser light at the same period as the round trip period or a period that is an integer multiple thereof.
The pulse laser light modulated by the mode locker in this way is a short pulse with a pulse width of 1 ns or less with the phases of the light constituting the pulse laser aligned (mode synchronization).
Laser crystal and laser glass can be used as the gain medium of the solid mode-locked laser.
A light source used for a medical measurement apparatus often uses a titanium sapphire crystal or an alexandrite crystal having a gain in a light transmission window (wavelength 700 nm to 900 nm) of a living body.

Non-Patent Document 1 describes a method of increasing the optical path length of a resonator and increasing the round trip period as a technique for further increasing the energy of pulsed laser light output from a solid mode-locked laser.
FIG. 11 is a schematic diagram showing the configuration of the mode-locked laser described in Non-Patent Document 1.
The laser apparatus of FIG. 11 includes a titanium sapphire crystal, an excitation light source, a high reflection mirror constituting a resonator, a partial reflection mirror that extracts a part of pulsed laser light as an output, and a multiple reflection mirror for extending the optical path length of the resonator. It is composed of pairs.
The pulse laser beam is mode-locked by the Kerr lens effect of the titanium sapphire crystal.
The period of modulation by car lens mode synchronization is equal to the round trip period of the resonator.
Therefore, the pulse laser beam is output at a repetition frequency f (= 1 / T) equal to the reciprocal of the round trip period T.
The pulse energy E (unit: joule) of the mode-locked laser is connected to the time average output W (unit: watts) of the laser device and the repetition frequency f (unit: hertz) of the pulse laser beam in the following relationship.

E = W / f Equation (1)

The repetition frequency f is the reciprocal of the round trip period T and is equal to the speed of light c divided by the optical path length L of the resonator.

f = 1 / T = c / L (2)

That is, even in the mode-locked laser having the same time average output W, when the optical path length L of the resonator is long, the repetition frequency f becomes small and high pulse energy can be generated.
In Non-Patent Document 1, the round trip period T is changed by extending the optical path length of the resonator by a multiple reflection mirror pair, and the repetition frequency of the pulsed laser light is controlled in the range of 3.8-15 MHz.
Non-Patent Document 1 describes that high pulse energy and pulse peak power can be obtained by reducing the repetition frequency of pulsed laser light.

Cho et al., Low-repetition-rate high-peak-power Kerr-lens mode-locked Ti: Al2O3 laser with a multiple-pass cavity, Opt. Lett. Vol. 24, pp. 417-419 (1999)

However, the above conventional example has the following problems.
That is, when the optical path length is extended by using the multiple reflection mirror pair, the area occupied by the multiple reflection mirror pair in the laser device increases, and the apparatus becomes large.
For example, when a resonator with an optical path length of 1 meter is used, the repetition frequency of the pulsed laser light is about 330 MHz. In order to increase the pulse energy of this laser device by 10 times, the optical path length must be extended by about 9 meters.
Assuming that the laser light is incident on the multiple reflection mirror at an interval of 5 millimeters, the area occupied by the multiple reflection mirror pair is about 45 square centimeters.
In order to increase the optical path length and further increase the pulse energy, a multiple reflection mirror pair occupying a larger area is required. Such a pair of multiple reflection mirrors with a large occupation area makes the laser device large and inconvenient for use in a medical field.

  In view of the above problems, an object of the present invention is to provide a mode-locked laser capable of suppressing the enlargement of the device, extending the optical path length of the resonator, and generating high pulse energy.

The mode-locked laser according to the present invention includes a resonator composed of a plurality of mirrors, pumps a gain medium arranged in the resonator with a pumping light source, and emits a pulsed laser beam in the pumped resonator. A mode-locked laser that is mode-locked and generates a short pulse laser beam,
The resonator has a polarization beam splitter that passes one polarized light of two polarized lights different in polarization direction in the pulsed laser light by 90 ° and reflects the other, and the optical path between the mirrors by the plurality of mirrors is branched into a plurality of The gain medium is disposed in one of the plurality of branch optical paths, and the pulse laser light is polarized when the pulse laser light is reciprocated in the other branch optical path. It has a configuration in which a polarization rotation mechanism whose direction rotates by 90 ° is arranged,
The one polarized light that has passed through the polarization beam splitter from the branch optical path in which the gain medium is disposed and is propagated to the branch optical path in which the polarization rotation mechanism is disposed,
The polarization rotation mechanism is reciprocated twice, so that the polarization direction is the same as that of the one polarized light, and the polarization beam splitter is passed in a direction opposite to the first direction,
The branching optical path in which the gain medium is disposed is propagated to be in a resonance state.
The medical measurement device of the present invention is characterized in that the above-described mode-locked laser is included in a light source for medical measurement.
The ablation processing apparatus of the present invention is characterized in that the above-described mode-locked laser is provided in the light source for ablation processing.

  According to the present invention, it is possible to realize a mode-locked laser that can suppress the increase in size of the device, extend the optical path length of the resonator, and generate high pulse energy.

It is a figure explaining the structural example of the mode lock laser in Embodiment 1 of this invention, (a) is a schematic diagram showing the structure of the laser apparatus of Embodiment 1 and Embodiment 7, (b) is the conventional example used as a comparative example. The schematic diagram showing the structure of the laser apparatus. The schematic diagram for demonstrating the resonance state and polarization state of the pulse laser beam of the mode-locked laser in Embodiment 1 of this invention. It is a figure explaining the structural example of the mode-locked laser in Embodiment 2 of this invention, (a) is a schematic diagram showing the structure of the laser apparatus of this embodiment, (b) is the conventional laser apparatus used as a comparative example. The schematic diagram showing a structure. The schematic diagram explaining the structural example of the mode-locked laser in Embodiment 3 of this invention. The schematic diagram explaining the structural example of the mode-locked laser in Embodiment 4 of this invention. It is a schematic diagram for demonstrating the function of the quarter wavelength plate in Embodiment 4 of this invention, (a) is a figure which shows the refraction ellipse of a birefringent crystal, (b) is the light of the light which comprises pulsed laser light The figure which shows spectrum intensity, (c) is a figure which shows the light from which the wavelength which comprises the birefringent crystal plate and the pulsed laser beam which injects into it differs. It is a figure explaining the structural example of the mode-locked laser in Embodiment 5 of this invention, (a) is a schematic diagram showing the structure of the laser apparatus of this embodiment, (b) is the spectrum of the light which comprises pulsed laser light The figure which shows intensity | strength, (c) is a figure which shows the light from which the wavelength which comprises the birefringent crystal plate with a wedge angle | corner, and the pulse laser beam which injects into it differs. FIG. 10 is a schematic diagram illustrating a configuration example of a mode-locked laser according to a sixth embodiment of the present invention. It is a figure which shows the structural example of Embodiment 6 of this invention, (a) is a figure which shows the spectral intensity of the light which comprises a pulse laser beam, (b) is a wavelength which comprises the wedge board and the pulse laser beam which injects into it The figure which shows the light from which (1) differs, (c) is a figure which shows a refractive ellipsoid, (d) is a figure which shows a refractive ellipse. It is a figure which shows the structural example of Embodiment 7 of this invention, (a) is a figure which shows the optical element used as a polarization rotation mechanism with the laser apparatus of this embodiment, (b) is a pulse laser beam which permeate | transmits an optical element. FIG. 5 is a diagram showing the phase difference of the light and the spectral intensity of the light constituting the pulse laser beam. The schematic diagram explaining the structure of the laser apparatus of a prior art example.

The mode-locked laser of the present invention has been found based on the following idea in order to extend the optical path length of the resonator and generate high pulse energy without increasing the size of the device.
That is, the mode-locked laser according to the present embodiment includes a resonator composed of a plurality of mirrors, and a gain medium disposed in the resonator is excited by an excitation light source, and pulses in the excited resonator are excited. The laser beam is mode-locked to generate a short pulse laser beam.
The resonator of the mode-locked laser includes a polarization beam splitter that transmits one polarized light of two polarized lights different in polarization direction by 90 ° in the pulsed laser light and reflects the other, and between the mirrors by the plurality of mirrors. The optical path is provided with a branched optical path that is branched into a plurality of parts. Further, the gain medium is arranged in one of the plurality of branch optical paths, and when the pulse laser beam reciprocates (passes twice) in the other branch optical path, A polarization rotation mechanism that rotates the polarization direction by 90 ° is disposed.
Then, the one polarized light that has passed through the polarization beam splitter from the branch optical path in which the gain medium is disposed and is propagated to the branch optical path in which the polarization rotation mechanism is disposed,
The polarization rotation mechanism is reciprocated twice, and the polarization direction is the same as that of the one of the polarized light beams. The polarization beam splitter passes the polarization beam splitter in the opposite direction, and propagates through the branched optical path on which the gain medium is arranged. The resonance state is established.
Specifically, the one polarized light that has passed through the polarization beam splitter from one mirror side in the branch optical path in which the gain medium is disposed between the mirrors and has propagated to the branch optical path in which the polarization rotation mechanism is disposed. Is made to reciprocate through the other mirror in the branched optical path in which the polarization rotation mechanism between the mirrors is arranged to have a polarization direction different from that of the one polarization light by 90 °.
Then, the polarization rotation mechanism is moved back and forth again through the other mirror, and the same polarization direction as that of the one polarized light is passed to the one mirror side in the direction opposite to the direction that first passed through the polarization beam splitter. And is configured to propagate through the branch optical path in which the gain medium is disposed to be in a resonance state.

According to the above configuration, by allowing the pulse laser beam to pass through the same optical path twice in different polarization states, the optical path length of the resonator can be extended without increasing the size of the device, and high pulse energy can be generated. Is possible.
Moreover, the above-described mode-locked laser according to the present invention can be applied to a light source for medical measurement to constitute a suitable medical measurement device.
In addition, a suitable ablation processing apparatus can be configured by applying to a light source for ablation processing.
Hereinafter, a configuration example of the mode-locked laser according to the embodiment of the present invention will be described more specifically with reference to the drawings.
[Embodiment 1]
A configuration example of the mode-locked laser according to Embodiment 1 of the present invention will be described with reference to FIG.
As shown in FIG. 1A, the mode-locked laser in the present embodiment is
The resonator includes a polarization beam splitter PBS1 that branches the pulse laser beam, two high-reflection mirrors 2 that reflect the branched pulse laser beam, one partial reflection mirror 3, and one folding mirror 4. It is configured.
Hereinafter, for the sake of explanation, the optical path between the PBS 1 and the high reflection mirror 2 and between the PBS 1 and the partial reflection mirror 3 is referred to as a branched optical path.
For the sake of explanation, the branch optical path between the high reflection mirror 2 including the folding mirror 4 and the PBS 1 is the branch optical path 1, the branch optical path parallel to the branch optical path 2 is the branch optical path 2, and the branch optical path perpendicular to the branch optical path 3 is the branch optical path 3. Call.
P-polarized pulsed laser light (the electric field is parallel to the paper surface) incident on the PBS from the branching optical path 1 or the branching optical path 2 is transmitted without being reflected by the PBS and propagates to the branching optical path 2 or the branching optical path 1. .

On the other hand, S-polarized pulsed laser light (the electric field is perpendicular to the paper surface) incident on the PBS from the branch optical path 2 or the branch optical path 3 is reflected by the PBS and propagates to the branch optical path 3 or the branch optical path 2.
That is, the P-polarized pulse light propagates between the branched optical paths 1 and 2, and the S-polarized pulse laser light propagates between the branched optical paths 2 and 3.
A quarter-wave plate is inserted in the branch optical path 2 as a polarization rotation mechanism 5 that rotates the polarization direction.
This quarter-wave plate rotates the polarization direction by approximately 90 ° every time the pulse laser beam passes twice with respect to the light constituting the pulse laser beam.
The gain medium 6 is a titanium sapphire crystal, and is inserted on the branch optical path 1 so that the optical axis and its optical surface form a Brewster angle.
The titanium sapphire crystal inserted at the Brewster angle has almost zero reflection on the optical surface with respect to the P-polarized laser light.
Therefore, in this embodiment, the pulsed laser light is in an oscillation state so that the polarization state in the branch optical path 1 is P-polarized light. The titanium sapphire crystal is excited by an excitation light source 7 having a wavelength of 532 nm and a condenser lens 8 placed outside the resonator. In the present embodiment, the mode synchronization of the pulse laser beam is performed by the Kerr lens effect of the titanium sapphire crystal.

FIG. 2 is a schematic diagram for explaining the resonance state and the polarization state of the pulse laser beam in the present embodiment.
In the figure, the black arrow represents the pulse laser beam propagating on the optical path, and the direction thereof indicates the propagation direction of the pulse laser beam.
Further, the direction P and the direction S indicate the directions of the electric field in the P-polarized and S-polarized states.
Hereinafter, the resonance of the pulse laser beam in the resonator will be described with reference to FIGS. (A): A P-polarized pulse laser propagating through the branch optical path 1 enters the PBS. Since it is P-polarized light, it passes through the PBS and propagates to the branch optical path 2.
(B): The pulse laser beam that has passed through the quarter-wave plate twice has a polarization state of S-polarized light. The S-polarized pulse laser light is reflected by the PBS and propagates to the branch optical path 3.
(C): The pulse laser beam reflected and returned by the partial reflection mirror is incident on the PBS in the S-polarized state and propagates to the branch optical path 2. The laser beam light that has passed through the partial reflection mirror is output.
(D): The pulsed laser light that has passed through the quarter wavelength twice changes its polarization state from S-polarized light to P-polarized light. The pulsed laser light in the P-polarized state passes through the PBS and propagates to the branch optical path 1.
(E): The pulse laser beam transmitted through the titanium sapphire crystal and reflected by the partial mirror passes through the titanium sapphire crystal again, and then returns to the state shown in (A).
The pulse laser beam resonates in the resonator through the states shown in the above (A) to (E).
Therefore, when the lengths of the branched optical paths 1-3 are expressed as L1, L2, and L3, respectively, the optical path length of the resonator is expressed as (L1 + 2 × L2 + L3) × 2≈ (L1 + L2 × 2) × 2.

FIG. 1B shows a configuration of a conventional laser device shown as a comparative example.
In the conventional configuration, the PBS and the polarization rotation mechanism are not inserted in the resonator, and there is no branch optical path 3 through which the S-polarized pulse laser light propagates.
The optical path length of the conventional laser device shown in FIG. 1B is (L1 + L2) × 2.
Compared to the configuration of the conventional laser device of FIG. 1B, the optical path length of the mode-locked laser of this embodiment is approximately 2 × L2.

[Embodiment 2]
A configuration example of the mode-locked laser according to Embodiment 2 of the present invention will be described with reference to FIG.
In the mode-locked laser according to the present embodiment, a pair of multiple reflection mirrors that reflect the pulse laser beam a plurality of times is inserted in the branch optical path between the polarization beam splitter and the polarization rotation mechanism. The other configuration is basically the same as that of the first embodiment.
In the present embodiment, as shown in FIG. 3A, the optical path is divided into the branched optical paths 1-3 by the polarization beam splitter, as in the first embodiment.
The arrangement of the high reflection mirror, the partial reflection mirror, the folding mirror, the titanium sapphire crystal, the excitation light source, and the condenser lens in the laser device is the same as that in the first embodiment.
In the configuration of the second embodiment, a multiple reflection mirror pair 9 for extending the optical path length is inserted into the branched optical path 2.
In the present embodiment, the optical path length when passing through the multiple reflection mirror pair can be considered to be sufficiently longer than the optical path lengths of other portions. Therefore, if the optical path length when passing through the multiple reflection mirror pair once is represented by L, the optical path length wave of the resonator can be represented by approximately L × 4.

FIG. 3B shows a configuration of a conventional laser apparatus shown as a comparative example.
In the configuration of the conventional example, the PBS and the polarization rotation mechanism are not inserted, and there is no branch optical path 3 through which the S-polarized pulse laser light propagates.
The optical path length of the laser device shown in FIG. 3B is approximately represented by L × 2.
Compared with the configuration of the conventional laser device of FIG. 3B, the optical path length of the mode-locked laser of this embodiment is approximately twice as long.

[Embodiment 3]
A configuration example of the mode-locked laser according to Embodiment 3 of the present invention will be described with reference to FIG.
The mode-locked laser according to the present embodiment has a configuration in which the polarization rotation mechanism is arranged in each of the branched optical paths branched by two or more light beam splitters.
A multi-reflection mirror pair for reflecting the pulse laser beam a plurality of times is inserted in each branch optical path between the polarization beam splitter and the polarization rotation mechanism. The other configuration is basically the same as that of the first embodiment.
Specifically, as shown in FIG. 4, the optical path is branched by two PBSs 12 and 13. The branched optical path shown in FIG. 4 is referred to as a branched optical path 1-5.
A titanium sapphire crystal is inserted as a gain medium 6 in the branched optical path 1 sandwiched between two PBSs.
The optical surface of the titanium sapphire crystal and the optical axis form a Brewster angle θB, and the pulsed laser light propagating in the P-polarized light in the branch optical path 1 enters the oscillation state as in the first embodiment.
A common multiple reflection mirror pair 9 is inserted into the branch optical path 2 and the branch optical path 4.
A quarter-wave plate is inserted as a polarization rotation mechanism 5 in each of the branch optical path 2 and the branch optical path 4.

Hereinafter, the resonance state and polarization state of the pulsed laser light in the resonator will be described.
(A): A P-polarized pulse laser propagating through the branch optical path 1 enters the PBS 12. Since it is P-polarized light, it passes through the PBS and propagates to the branch optical path 2.
(B): The pulse laser beam that has passed through the quarter-wave plate twice has a polarization state of S-polarized light. The S-polarized pulse laser light is reflected by the PBS and propagates to the branch optical path 3. At this time, it passes through the multiple reflection mirror pair 9 twice.
(C): The pulse laser beam reflected and returned by the partial reflection mirror 3 is incident on the PBS 12 in the S-polarized state, reflected and propagated to the branch optical path 2. At that time, the laser beam transmitted through the partial reflection mirror 3 becomes an output.
(D): The pulsed laser light that has passed through the quarter-wave plate twice changes its polarization state from S-polarized light to P-polarized light. The pulsed laser light in the P-polarized state passes through the PBS and propagates to the branch optical path 1. At this time, the light passes through the multiple reflection mirror 9 twice.
(E): A P-polarized pulse laser that passes through the titanium sapphire crystal and propagates through the branch optical path 1 enters the PBS 13. Since it is P-polarized light, it passes through the PBS and propagates to the branch optical path 4.
(F): The pulse laser beam that has passed through the quarter-wave plate twice has a polarization state of S-polarized light. The S-polarized pulsed laser light is reflected by the PBS and propagates to the branch optical path 5. At this time, it passes through the multiple reflection mirror pair 9 twice.
(G): The pulse laser beam reflected and returned by the reflecting mirror is incident on the PBS 13 in the S-polarized state, reflected and propagated to the branch optical path 4.
(H): The pulsed laser light that has passed through the quarter-wave plate twice changes its polarization state from S-polarized light to P-polarized light. The pulsed laser light in the P-polarized state passes through the PBS and propagates to the branch optical path 1.
At this time, it passes through the multiple reflection mirror pair 9 twice. The pulse laser beam propagating to the branch optical path 1 passes through the titanium sapphire crystal and returns to the state (A).

In the present embodiment, the optical path length when passing through the multiple reflection mirror pair 9 can be considered to be sufficiently longer than the optical path lengths of other portions.
Therefore, if the optical path length when passing through the multiple reflection mirror pair once is represented by L, the optical path length wave of the resonator is represented by L × 8.
Compared with the configuration of the conventional laser device of FIG. 3B, the optical path length of the mode-locked laser of this embodiment is approximately four times longer.

[Embodiment 4]
A configuration example of the mode-locked laser according to Embodiment 4 of the present invention will be described with reference to FIG.
The mode-locked laser according to the present embodiment is composed of a birefringent crystal plate and a diffraction grating pair installed so as to sandwich the birefringent crystal plate. The other configuration is basically the same as that of the first embodiment.
Specifically, as shown in FIG. 5, the optical path is branched to the branched optical path 1-3 by the PBS 1 as in the first embodiment, and a titanium sapphire crystal is inserted into the branched optical path 1 as the gain medium 6.
The optical surface of the titanium sapphire crystal and the optical axis form a Brewster angle, and the beam light that becomes P-polarized light enters the oscillation state in the branch optical path 1.
P-polarized laser light propagates between the branched optical path 1 and the branched optical path 2 via the PBS 1, and S-polarized laser light propagates between the optical branched optical path 2 and the branched optical path 3 via the PBS 1.
A birefringent crystal plate 52 and a diffraction grating pair 51 sandwiching the birefringent crystal plate 52 are inserted in the branch optical path 2 as a polarization rotation mechanism.
Since the pulsed laser light is composed of light of different wavelengths, the laser light incident on the diffraction grating from the PBS is diffracted at different angles for each wavelength, and enters the birefringent crystal plate 52 at an incident angle depending on the wavelength. Incident.

FIG. 6 is a schematic diagram for explaining the function of the quarter-wave plate in the present embodiment.
The short axis and long axis of the ellipse shown in FIG. 6A represent the cross section of the refractive ellipsoid of the birefringent crystal constituting the birefringent crystal plate 52 (hereinafter referred to as a refractive ellipse).
In the figure, the laser light is incident at a substantially right angle with respect to the paper surface, the electric field of P-polarized light is lateral to the paper surface (axis X direction), and the electric field of S-polarized light is upward (paper Y direction).
The major and minor axes of the refractive ellipse are located on the bisector of the axis X and the axis Y. The major axis and minor axis directions are referred to as axis A and axis B, respectively.
When S-polarized light and P-polarized laser light are transmitted through the birefringent crystal, the electric field component divided into the axis A and the axis B of the ellipse has different refractive indexes n1 expressed by the major axis and the minor axis of the ellipse. And n2.
Therefore, when the thickness d of the birefringent crystal plate 52 and the wavelength λ of the transmitted light satisfy the relationship of the following equation (3), the axis A and the axis A with respect to the pulsed laser light that has passed through the wavelength plate twice. The phase of the electric field component parallel to B is rotated by 180 °, and the direction of polarization is rotated by 90 °.

(N1−n2) × d = λ / 4 Formula (3)

This means that when the light having the wavelength λ passes through the birefringent crystal plate twice, the polarization direction is switched between P-polarized light and S-polarized light.

FIG. 6B shows the spectral intensity of the light constituting the pulse laser beam.
Thus, the pulsed laser light includes light of different wavelengths.
Therefore, the thickness d of the birefringent crystal that is optimal as a quarter-wave plate differs for each wavelength. For example, when the short pulse includes light of λ1 and λ2 (λ1 <λ2) centering on the wavelength λ0, the optimum thickness d is d1 and d2 (d1 <d2).
As shown in FIG. 6 (c), according to the present embodiment, the light constituting the pulsed laser light is incident on the birefringent crystal plate 52 at an angle depending on the wavelength. Therefore, the transmission distances d1 and d2 for each wavelength. And can be changed.
Therefore, if the angle of the birefringent crystal plate 52 is adjusted so as to have an optimum thickness for each wavelength, the phase rotation angle can be compensated for the wavelength. In the present embodiment, the wavelength band of the light that forms the pulsed laser light is 795-805 nm.
Therefore, in order for the polarization rotation mechanism 5 to function as a quarter-wave plate for light in this wavelength band, the separation angle of 795 nm and 805 nm light is θ = cos−1 (795/805) = 9 °. I just need it. The diffraction angle β of the diffraction grating can be expressed as the following equation (4), where α is the incident angle of light, N is the number of gratings per unit length, and n is the order of diffraction.

sin α ± sin β = nNλ (4)

When laser light is incident on a diffraction grating of N = 2300 lines / mm at an angle of α = 60 °, the above-mentioned separation angle can be almost obtained.

[Embodiment 5]
A configuration example of the mode-locked laser according to Embodiment 5 of the present invention will be described with reference to FIG.
In the mode-locked laser according to the present embodiment, the polarization rotation mechanism includes a diffraction grating pair and a wedge plate made of a birefringent crystal,
The diffraction grating pair is provided closer to the polarizing beam splitter than the wedge plate. The other configuration is basically the same as that of the first embodiment.
Specifically, as shown in FIG. 7A, the optical path is branched to the branched optical path 1-3 by the PBS 1 as in the first embodiment.
P-polarized laser light propagates between the branched optical path 1 and the branched optical path 2 via the PBS 1, and S-polarized laser light propagates between the optical branched optical path 2 and the branched optical path 3 via the PBS 1.
A titanium sapphire crystal is inserted as a gain medium 6 in the branch optical path 1.
The optical surface of the titanium sapphire crystal and the optical axis form a Brewster angle, and the beam light that becomes P-polarized light enters the oscillation state in the branch optical path 1.
The oscillated pulse laser beam has the spectral intensity shown in FIG.
A diffraction grating pair 51 is inserted in the branch optical path 2. The light transmitted through the diffraction grating pair 51 is separated for each wavelength, and becomes parallel light in which the wavelength is spatially distributed between the diffraction grating pair 51 and the mirror 2. A birefringent crystal plate 53 with a wedge angle is inserted into the parallel light.
FIG. 7C shows a birefringent crystal plate 53 with a wedge angle ψ and light of different wavelengths λ1 and λ2 incident thereon.

According to this embodiment, the laser light diffracted by the diffraction grating 51 at different angles for each wavelength is transmitted through different locations of the birefringent crystal plate 53 with the wedge angle ψ.
Therefore, by adjusting the wavelength separation distance D and the wedge angle ψ, it is possible to find a condition satisfying the expression (3) for the light of the wavelengths λ1 and λ2.
In this embodiment, the wavelength band of the light constituting the pulse laser beam is 790 to 810 nm, and a quartz plate is used as the birefringent crystal plate.
At this time, a difference in thickness of the quartz plate of about 1.1 μm is required between 790 nm and 810 nm. When the separation distance D between the wavelengths 790 nm and 810 nm is 5 mm, the required wedge angle ψ is about 0.013 °.

[Embodiment 6]
A configuration example of the mode-locked laser according to Embodiment 6 of the present invention will be described with reference to FIG.
In the mode-locked laser according to the present embodiment, the polarization rotation mechanism includes a diffraction grating pair, a quarter-wave plate, and a wedge plate made of a birefringent crystal,
From the side close to the polarizing beam splitter, the diffraction grating pair, the quarter-wave plate, and the wedge plate made of the birefringent crystal are arranged in this order. The other configuration is basically the same as that of the first embodiment.
Specifically, as shown in FIG. 8, the optical path is branched to the branched optical path 1-3 by the PBS 1 as in the first embodiment.
P-polarized laser light propagates between the branched optical path 1 and the branched optical path 2 via the PBS 1, and S-polarized laser light propagates between the optical branched optical path 2 and the branched optical path 3 via the PBS 1.
A titanium sapphire crystal is inserted in the branch optical path 1.
The optical surface of the titanium sapphire and the optical axis form a Brewster angle, and the beam light that becomes P-polarized light in the branch optical path 1 enters an oscillation state. The oscillated pulse laser beam has the spectral intensity shown in FIG.
A diffraction grating pair 51 is inserted in the branch optical path 2. The light transmitted through the diffraction grating pair is separated by the wavelength, and becomes parallel light in which the wavelength is spatially distributed between the diffraction grating pair 51 and the mirror 2.
A wave plate 54 and a wedge plate 55 made of a birefringent crystal are inserted in the parallel light. These diffraction grating 51, wave plate 54 and wedge plate 55 function as the polarization rotation mechanism 5.

FIG. 9B shows a wedge plate and incident light of wavelengths λ1 and λ2.
FIG. 9C shows a refractive ellipsoid of a birefringent crystal used for the wedge plate. In the figure, an arrow K roughly indicates the propagation direction of the laser light in the crystal. The crystal main axis P is perpendicular to one optical surface of the wedge plate.
FIG. 9D is a cross section (refractive ellipse) when the refractive ellipsoid shown in FIG. 9C is cut along a plane perpendicular to the propagation direction K of the laser beam.
9B and 9D, the axis X and the axis Y substantially coincide with the directions of the electric fields of S-polarized light and P-polarized light, respectively.
An axis A and an axis B are bisectors of the axis X and the axis Y. The solid line in FIG. 9 is a refraction ellipse when the propagation direction K and the main axis P are parallel, and the broken line is a refraction ellipse when the propagation direction K is inclined in the direction of the axis A.
At this time, the major axis and the minor axis of the ellipse are parallel to the axis A and the axis B.
The major axis and minor axis represent the refractive index perceived by the electric field parallel to it. When the propagation direction K coincides with the principal axis P of the crystal, the refractive ellipse is circular (broken line), and the refractive index does not depend on the polarization.

However, when the birefringent crystal plate is tilted in the axis A direction, a difference n1′−n2 ′ is generated in the refractive index between the axis A direction and the axis B direction. This refractive index difference n1′−n2 ′ increases as the inclination of the birefringent crystal plate increases.
Since the birefringent crystal plate has a wedge shape, the distances d1 and d2 through which light of wavelengths λ1 and λ2 passes are different.
Therefore, when the wavelength plate 54 is made of a birefringent crystal having a thickness d having refractive indexes n1 and n2, the following equations (5) and (6) are satisfied. The wedge plate 55 made of a refractive crystal functions as a quarter wavelength plate compensated for the wavelength.

(N1−n2) × d + (n1′−n2 ′) × d1 = λ1 / 4 (5)
(N1−n2) × d + (n1′−n2 ′) × d2 = λ2 / 4 Formula (6)

The conditions of the above equations (5) and (6) can be satisfied by adjusting the thickness of the wedge plate and the wedge angle ψ, and the separation distance D between the wavelengths λ1 and λ2 incident on the wedge plate.

[Embodiment 7]
A configuration example according to the seventh embodiment of the present invention will be described with reference to FIG.
FIG. 10 shows an optical element used as the polarization rotation mechanism of this embodiment.
The polarization rotation mechanism of the present embodiment is composed of an optical element in which a plurality of periodic structure layers having a period shorter than the wavelength of light constituting the pulse laser beam are stacked. The other configuration is basically the same as that of the first embodiment.
Specifically, as in the first embodiment, the optical path is branched to the branched optical path 1-3 by the PBS, and the P-polarized laser light propagates between the branched optical path 1 and the branched optical path 2 through the PBS, The S-polarized laser light propagates between the optical branch optical path 2 and the optical path 3 via the PBS.
A titanium sapphire crystal is inserted in the branch optical path 1. The optical surface of the titanium sapphire crystal and the optical axis form a Brewster angle, and the laser beam that becomes P-polarized light in the branch optical path 1 enters an oscillation state.
The polarization rotation mechanism inserted into the branch optical path 2 is an optical element having a periodic structure shorter than the wavelength of the short pulse laser beam. The optical element used in this embodiment includes a substrate and a plurality of periodic structure layers.
FIG. 10A is a schematic diagram showing the form.
FIG. 10B shows the phase difference φA−φB sensed by the electric fields in the directions of the axis A and the axis B when the pulse laser beam is transmitted through the optical element shown in FIG.
In FIG. 10B, the spectral intensity of the pulsed laser beam is also plotted. The optical element configured by stacking the plurality of periodic structure layers of the present embodiment gives a phase difference of π / 2 almost accurately in a wavelength band wider than the spectral band of the pulsed laser beam. That is, it functions as a quarter-wave plate almost accurately with respect to light having a wavelength included in the pulsed laser light.
Therefore, by using a wave plate formed by laminating periodic structure layers, the polarization state is converted almost accurately between P-polarized light and S-polarized light for light of different wavelengths constituting the pulse laser beam. be able to.

As mentioned above, although the said embodiment demonstrated the case where a waveplate and a wedge board were comprised with one birefringent crystal plate, a waveplate may be comprised with several birefringent crystal plates.
In that case, the left side of Equation (3) is expressed as the sum of a plurality of birefringent crystal plates.
For example, when a wavelength plate is composed of a birefringent crystal plate having a refractive index (n1, n2) and having a length d and a birefringent crystal plate having a refractive index (n1 ″, n2 ″) and having a length L2. In order for the wave plate to function as a ¼ wave plate, the following equation should be satisfied.

(N1−n2) × 2 × d + (n1 ″ −n2 ″) × 2 × d ″ = λ / 4

The effect of the present invention can also be obtained by using a wave plate composed of a plurality of such birefringent crystal plates instead of the wave plate of the above embodiment.

The excitation light source used in the above embodiment is desirably a small laser diode light source.
When a laser diode light source is used as an excitation light source, the area occupied by the resonator in the laser device is almost the same, so the effect of the present invention that can suppress the size of the resonator becomes more remarkable.
For example, when a titanium sapphire crystal is used as the gain medium, it is desirable to use a semiconductor laser diode having an oscillation wavelength of 400 to 600 nm based on a GaN compound semiconductor and other semiconductors. In addition, when an alexandrite crystal is used as a gain medium, it is desirable to use a semiconductor laser diode having an oscillation wavelength of 350 to 680 nm based on a GaN compound semiconductor, a GaAs compound semiconductor, and other semiconductors.

According to the configuration described above, the pulsed laser light passes through the optical path branched by the PBS twice in the polarization state of S-polarized light and P-polarized light and enters a resonance state.
Therefore, the optical path length can be extended without increasing the area occupied by the resonator in the laser device. When the optical path length is q times, the pulse energy is also almost q times.
Therefore, compared with the conventional example, the pulse energy is about (L2 × 2 + L1) / (L2 + L1) times in the configuration of the first embodiment, about twice in the configuration of the second embodiment, and about four times in the configuration of the third embodiment. Become.

By using the polarization rotation mechanism described in the embodiment 4-6 in the configuration of the embodiment 1-3, the p-polarized light and the s-polarized light can be switched almost accurately with respect to light having different wavelengths included in the pulse laser beam. Is possible.
By increasing the accuracy of switching between p-polarized light and s-polarized light, the loss in the PBS that switches the propagation direction depending on the polarization is reduced, and higher pulse energy can be obtained over a wide wavelength change range.
For example, when the laser device is a mode-locked pulse laser using a titanium sapphire crystal or alexandrite crystal as a gain medium, and the pulse laser beam has a center wavelength of 800 nm and a pulse width of 100 femtoseconds or more, the light constituting the pulse laser beam The spectral width can be 10 nm or less.
At this time, the polarization direction of the light constituting the pulse laser beam can be rotated by 90 ° using any of the configurations described in Embodiment 4-7.
In the case of a pulse laser beam having a pulse width of 100 femtoseconds or less, or a laser device capable of switching the oscillation wavelength, the wavelength of the light passing through the polarization rotation mechanism is wider than the wavelength width of 10 nm.
In this case, the polarization direction of the light passing through the polarization rotation mechanism can be rotated by 90 ° by using any of the configurations described in Embodiment 5-7.

1: Polarized beam splitter (PBS)
2: High reflection mirror 3: Partial reflection mirror 4: Folding mirror 5: Polarization rotation mechanism 6: Gain medium 7: Excitation light source 8: Condensing lens

Claims (12)

A mode-locked laser comprising a resonator composed of a plurality of mirrors, pumping a gain medium disposed in the resonator with a pumping light source, and mode-locking laser light in the pumped resonator ,
In the resonator, an optical path between the plurality of mirrors is branched into a plurality of beams by a polarization beam splitter that transmits one polarized light of two polarized lights whose polarization directions are different from each other by 90 ° and reflects the other. With a branching optical path,
A polarization rotation mechanism in which the gain medium is disposed in one of the plurality of branch optical paths, and the polarization direction of the laser light is rotated by 90 ° when the laser light is reciprocated in the other branch optical path. Has a configuration in which
The one polarized light that has passed through the polarization beam splitter from the branch optical path in which the gain medium is disposed and is propagated to the branch optical path in which the polarization rotation mechanism is disposed,
The polarization rotating mechanism is reciprocated twice to set the same polarization direction as that of the one polarized light, and then passes through the polarization beam splitter, and propagates through a branched optical path in which the gain medium is arranged to be in a resonance state. A mode-locked laser.   2. The mode-locked laser according to claim 1, wherein a pair of multiple reflection mirrors that reflect the laser beam a plurality of times are inserted in a branched optical path between the polarization beam splitter and the polarization rotation mechanism. The branch optical path in which the polarization rotation mechanism is arranged has a configuration in which a polarization rotation mechanism is arranged in each branch optical path branched by two or more light beam splitters,
2. The mode-locked laser according to claim 1, wherein a pair of multiple reflection mirrors that reflect the laser beam a plurality of times are inserted in respective branch optical paths between the polarization beam splitter and the polarization rotation mechanism. .   The said polarization rotation mechanism is comprised by the diffraction grating pair installed so that the birefringent crystal plate and the said birefringent crystal plate may be pinched | interposed, The Claim 1 thru | or 3 characterized by the above-mentioned. Mode-locked laser. The polarization rotation mechanism includes a diffraction grating pair and a wedge plate made of a birefringent crystal,
4. The mode-locked laser according to claim 1, wherein the diffraction grating pair is provided at a position closer to the polarization beam splitter than a wedge plate. 5. The polarization rotation mechanism includes a diffraction grating pair, a quarter wave plate and a wedge plate made of a birefringent crystal,
4. The diffraction grating pair, the quarter wavelength plate, and a wedge plate made of the birefringent crystal are arranged in this order from the side close to the polarizing beam splitter. The mode-locked laser according to item.   4. The polarization rotation mechanism is configured by an optical element in which a plurality of periodic structure layers having a period shorter than the wavelength of light constituting laser light are stacked. A mode-locked laser described in 1.   The mode-locked laser according to any one of claims 1 to 7, wherein the polarization rotation mechanism has a configuration that functions as a quarter-wave plate.   9. The mode-locked laser according to claim 1, wherein the gain medium is made of a titanium sapphire crystal.   The mode-locked laser according to any one of claims 1 to 8, wherein the gain medium is composed of alexandrite crystals.   A medical measurement apparatus comprising the mode-locked laser according to any one of claims 1 to 10 in a light source for medical measurement.   An ablation processing apparatus comprising the mode-locked laser according to any one of claims 1 to 10 in a light source for ablation processing.