JP4797733B2 - Polarization-independent optical isolator for high-power lasers - Google Patents

Description

  The present invention relates to a polarization-independent optical isolator for high-power lasers used for countermeasures against return light of high-power lasers used for optical communication and processing, and particularly for high-power lasers whose optical isolation function is unlikely to deteriorate. The present invention relates to an improvement of a polarization-independent optical isolator.

  Semiconductor lasers used for optical communications and solid-state lasers used for laser processing, etc., generate laser oscillation when the light reflected from the optical surface or processing surface outside the laser resonator returns to the laser element. It becomes unstable. If the oscillation becomes unstable, signal noise may occur in the case of optical communication, and the laser element may be destroyed in the case of a processing laser. For this reason, an optical isolator is used to block such reflected return light from returning to the laser element.

  In recent years, polarization-independent in-line optical isolators are used for fiber lasers that are attracting attention as an alternative to conventional YAG lasers. Conventionally, as a Faraday rotator used for an optical isolator for a high-power laser, a terbium gallium garnet crystal film (hereinafter referred to as TGG) or a terbium aluminum garnet crystal film (hereinafter referred to as TAG) has been used. Has been used.

  However, TGG and TAG have a small Faraday rotation coefficient per unit length, and it is necessary to increase the optical path length to obtain a 45 ° polarization rotation angle in order to function as an optical isolator. Another large crystal had to be used. Moreover, in order to obtain high optical isolation, it is necessary to apply a uniform and large magnetic field to the crystal, so a strong and large magnet was used. For this reason, the size of the optical isolator is large. In addition, since the optical path length is long, the laser beam shape may be distorted in the crystal, and an optical system for correcting the distortion may be required. Furthermore, since TGG is expensive, a small and inexpensive Faraday rotator has been desired.

  On the other hand, by using a bismuth-substituted rare earth iron garnet crystal film (hereinafter referred to as RIG) exclusively used in the optical communication field for this type of isolator, the size can be greatly reduced. . However, it is known that RIG increases the light absorption by iron ions when the wavelength of light used is shortened to near 1.1 μm used for processing lasers, and the temperature rises due to this light absorption, causing performance deterioration. .

Therefore, a method for improving the problem of RIG temperature rise has been proposed. The methods described in Patent Documents 1 and 2 leave a gadolinium gallium garnet substrate (hereinafter referred to as a GGG substrate), which is a substrate for RIG growth, which is usually removed by polishing, leaving RIG. It is easy to release the heat generated in In addition, a method using a high thermal conductivity substrate such as sapphire that has been used as a heat dissipation substrate has been proposed (Patent Document 3).
JP 2000-66160 A JP 7-281129 A JP 2005-43853 A

  However, even if the method described in Patent Document 1 is used, if a laser beam with a wavelength of 1.1 μm or less and an output exceeding 1 W is incident, the temperature rises remarkably, so that the performance as a Faraday rotator, that is, the rotation angle is greatly reduced or input There have been problems such as an increase in loss and deterioration in extinction ratio due to birefringence caused by distortion due to a difference in thermal expansion between the GGG substrate and the RIG film. Further, in the method described in Patent Document 1, reflection from the GGG and RIG boundary is incident when an isolator is assembled unless the thickness is controlled so that the Faraday rotation angle is accurately 45 degrees. The polarizer on the side cannot be completely removed, causing deterioration of the extinction ratio. For this reason, there are problems such as difficulty in obtaining a high extinction ratio with a high yield.

  Patent Document 2 proposes a method of arranging GGG or glass on one side or both sides of the RIG substrate. However, any of these methods causes the performance degradation of the Faraday rotator for lasers exceeding 1 W. End up.

  On the other hand, the method described in Patent Document 3 has limited performance degradation even for high-power lasers. However, since sapphire, a heat dissipation substrate, has birefringence, it is extinguished when incorporated into a polarization-independent optical isolator using wedge-shaped birefringent crystals under the same conditions as a normal isolator. It has a problem that the ratio is deteriorated and the required specifications are not satisfied.

  The present invention has been made paying attention to such problems, and the problem is that the present invention is used for a high-power laser having a wavelength of 1.1 μm or less and an output of 2 W or more without causing the above-described problems. Another object of the present invention is to provide a polarization-independent optical isolator in which the optical isolation function is difficult to deteriorate.

  In order to solve such problems, the present inventors prepared a Faraday rotator having a structure in which a c-plane sapphire single crystal is bonded to each optical surface of the Faraday rotator so that its c-axis is vertical. Then, using this Faraday rotator, an ordinary polarization-independent optical isolator was created.

  That is, this polarization-independent optical isolator includes a wedge-shaped birefringent crystal (rutile) 4 and a Faraday rotator 5 using a Bi-substituted RIG film and a wedge-shaped birefringent crystal as shown in FIG. (Rutile) 4 are arranged in order along the optical axis, and the fiber collimator 3 is arranged on the outer optical axis of each birefringent crystal 4 with the Faraday rotator 5 at the center, and the Faraday rotator. The c-plane sapphire single crystal 5a is bonded to each other so that the c-axis of the sapphire single crystal is perpendicular to each optical surface through which light 5 passes. In FIG. 6, 1 is a light source (semiconductor laser) having a wavelength of 1064 nm, 2 is a fiber, and 61 is a power meter.

  When this polarization-independent optical isolator is evaluated, it is difficult to obtain an isolation of 25 dB or more which is necessary for use in a normal fiber laser because the isolation is distributed between 15 dB and 25 dB. Was confirmed.

  When this cause was investigated, linearly polarized light from the fiber collimator separated into ordinary light and extraordinary light by the wedge-shaped birefringent crystal became elliptically polarized light by the birefringence of the c-plane sapphire single crystal, and the extinction ratio deteriorated. It was confirmed that

  Therefore, a Faraday rotator (hereinafter referred to as a composite film 50) bonded with a c-plane sapphire single crystal is rotated around its central axis 100 as shown in FIG. 7, and the Faraday rotator is rotated with respect to the optical axis. Were sequentially arranged obliquely, and the relationship between the incident angle θ and the extinction ratio was measured. The result is shown in the graph of FIG. From the graph of FIG. 8, unless the incident angle is within 6 degrees, linearly polarized light incident on the c-plane sapphire single crystal becomes elliptically polarized light due to birefringence of sapphire, and the extinction ratio may be less than 25 dB. I understood. In FIG. 7, 7 is a cylindrical magnet, 41 is a polarizer, 42 is an analyzer, and 63 is a detector.

  On the other hand, a polarization-independent optical isolator as shown in FIG. 9 was prepared and evaluated by adjusting the angles so that the incident angles of both ordinary light and extraordinary light were approximately 0 degrees with respect to the c-plane sapphire single crystal. It was found that although the non-reflective coating was applied to the end face of the surface sapphire single crystal, there was a slight amount of reflected return light, and the return loss was about 25 dB. Therefore, when the ordinary light and the extraordinary light were both adjusted to have an incident angle of 1 degree or more, the return loss exceeded 25 dB, and desired characteristics were obtained. In FIG. 9, 62 indicates a 3 dB coupler, and 70 indicates a power meter.

  From such a confirmation test, the angle of the light transmitted through the wedge-shaped birefringent crystal and incident on the c-plane sapphire single crystal of the Faraday rotator is within 1 degree to 6 degrees with respect to the c-axis of the sapphire single crystal. As a result, it has been found that a polarization-independent optical isolator in which the optical isolation function hardly deteriorates can be provided. The present invention has been completed through such technical studies.

That is, the invention according to claim 1
A birefringent crystal having a wedge shape along the optical axis, a Faraday rotator using a Bi-substituted RIG film, and a birefringent crystal having a wedge shape are arranged in this order, and each birefringence is centered on the Faraday rotator. A fiber collimator is disposed on each outer optical axis of the crystal, and a c-plane sapphire single crystal is attached so that the c-axis of the sapphire single crystal is perpendicular to each optical surface through which the light of the Faraday rotator passes. Assuming a polarization-independent optical isolator for high power lasers with a combined structure,
The wedge-shaped birefringent crystal is made of rutile and the wedge angle of the birefringent crystal is set in the range of 0.65 degrees to 3.5 degrees, so that the birefringent crystal is transmitted and the c-plane sapphire single unit is formed. The angle of light incident on the crystal is 1 to 6 degrees with respect to the c-axis of the sapphire single crystal .

The invention according to claim 2
A birefringent crystal having a wedge shape along the optical axis, a Faraday rotator using a Bi-substituted RIG film, and a birefringent crystal having a wedge shape are arranged in this order, and each birefringence is centered on the Faraday rotator. A fiber collimator is disposed on each outer optical axis of the crystal, and a c-plane sapphire single crystal is attached so that the c-axis of the sapphire single crystal is perpendicular to each optical surface through which the light of the Faraday rotator passes. Assuming a polarization-independent optical isolator for high power lasers with a combined structure,
The wedge-shaped birefringent crystal is made of YVO ₄ and the wedge angle of the birefringent crystal is set in the range of 1.1 degrees to 5.2 degrees, so that the birefringent crystal is transmitted and c-plane sapphire is transmitted. The angle of light incident on the single crystal is 1 to 6 degrees with respect to the c-axis of the sapphire single crystal .

Next, the invention according to claim 3 is
A birefringent crystal having a wedge shape along the optical axis, a Faraday rotator using a Bi-substituted RIG film, and a birefringent crystal having a wedge shape are arranged in this order, and each birefringence is centered on the Faraday rotator. A fiber collimator is disposed on each outer optical axis of the crystal, and a c-plane sapphire single crystal is attached so that the c-axis of the sapphire single crystal is perpendicular to each optical surface through which the light of the Faraday rotator passes. Assuming a polarization-independent optical isolator for high power lasers with a combined structure,
By forming the wedge-shaped birefringent crystal from calcite and setting the wedge angle of the birefringent crystal in the range of 1.6 degrees to 9.3 degrees, the birefringent crystal is transmitted through the c-plane sapphire single unit. The angle of light incident on the crystal is 1 to 6 degrees with respect to the c-axis of the sapphire single crystal .

According to the polarization-independent optical isolator for a high-power laser according to any one of claims 1 to 3 , the light transmitted through the wedge-shaped birefringent crystal and incident on the c-plane sapphire single crystal of the Faraday rotator. Since the angle is set within the range of 1 to 6 degrees with respect to the c-axis of the sapphire single crystal, a high isolation effect can be obtained even when used for a high-power laser with a wavelength of 1.1 μm or less and an output of 2 W or more. Can be maintained.

  Hereinafter, embodiments of the present invention will be described in detail.

  First, the Faraday rotator used in the present invention sandwiches a bismuth-substituted RIG film having a rotation angle of 45 degrees as described above with a c-plane sapphire single crystal that is a transparent heat dissipation substrate having high thermal conductivity, The completed composite film is cut into chips of a desired size using a dicing saw or the like. At least one end face of this chip is to thermally contact the four corners of the portion 1.5 times or more from the laser incident diameter with a heat sink having high thermal conductivity.

  The c-plane sapphire single crystal having a thickness of 0.5 mm was used. In addition, an antireflection film is provided in advance on the side of the c-plane sapphire single crystal in contact with air, and the antireflection film is also provided on the side of the surface in contact with the adhesive.

Then, the c-plane sapphire single crystal is bonded to the optical surface of the bismuth-substituted RIG film using an epoxy resin having an absorption coefficient of 0.1 cm ⁻¹ or less in a wavelength range of 750 nm to 1100 nm. When the c-plane sapphire single crystal, the bismuth-substituted RIG film, and the c-plane sapphire single crystal are bonded together with an epoxy resin, the thickness of the adhesive is prevented from becoming larger than 10 μm by pressing from above. When the c-plane sapphire single crystal is a parallel plane, it is bonded to a bismuth substitution RIG film and then cut into a required size with a dicing saw.

The composite film (bismuth-substituted RIG film in which c-plane sapphire single crystal is bonded to each optical surface) thus fabricated is used, and the angle between the fiber collimator 3 and the wedge is 5 degrees as shown in FIGS. 2 and the composite film (ie, the Faraday rotator 5 in which the c-plane sapphire single crystal 5a is bonded) with respect to the optical axis as shown in FIG. Is tilted in the direction in which the incident angle of the light coming from the birefringent crystal (rutile) 4 to the c-plane sapphire single crystal 5a is reduced by 3 degrees or more and 6.5 degrees or as shown in FIG. tilt angle of incidence decreases direction to the c-plane sapphire single crystal 5a of the light flowing out from the birefringent crystal (rutile) 4 within 6.5 degrees 3 degrees to the fiber collimator 3, according to a reference example for Polarization-independent light for high-power lasers Isolator can be obtained.

Next, instead of the above-described method in which the composite film (Faraday rotator 5 with the c-plane sapphire single crystal 5a bonded) or the fiber collimator 3 is arranged obliquely with respect to the optical axis, a wedge-shaped birefringent crystal wedge Isolation of about 25 dB can also be obtained by adjusting the angle to a predetermined range. That is, as shown in FIG. 4, the composite film or the fiber collimator 3 composed of the Faraday rotator 5 and the c-plane sapphire single crystal 5a is not arranged obliquely with respect to the optical axis, and the wedge of the birefringent crystal 4 By adjusting the angle to a predetermined range, the angle of light transmitted through the birefringent crystal 4 and incident on the c-plane sapphire single crystal 5a of the Faraday rotator 5 is 1 degree with respect to the c-axis of the sapphire single crystal 5a. The polarization-independent optical isolator for a high-power laser of the present invention set within 6 degrees can be obtained.

Examples of the birefringent crystal include rutile, YVO ₄ , and calcite. When a birefringent crystal having a wedge shape is formed of these materials, the angle of the wedge is shown in Table 1 below. It becomes such an angle.

従って、複屈折結晶をルチルで構成した場合には、楔の角度を０．６５度から３．５度の範囲に設定することにより、ｃ面サファイア単結晶に入射する光の角度をサファイア単結晶のｃ軸に対して１度以上６度以内に設定できる。また、上記複屈折結晶をＹＶＯ₄で構成した場合には、楔の角度を１．１度から５．２度の範囲に、また、複屈折結晶を方解石で構成した場合には、楔の角度を１．６度から９．３度の範囲に設定することにより、上記ｃ面サファイア単結晶に入射する光の角度をサファイア単結晶のｃ軸に対して１度以上６度以内に設定することが可能となる。

Therefore, when the birefringent crystal is made of rutile, the angle of light incident on the c-plane sapphire single crystal is set by setting the wedge angle in the range of 0.65 degrees to 3.5 degrees. It can be set within the range of 1 to 6 degrees with respect to the c axis. When the birefringent crystal is made of YVO ₄ , the wedge angle is in the range of 1.1 to 5.2 degrees, and when the birefringent crystal is made of calcite, the wedge angle is set. By setting the angle from 1.6 degrees to 9.3 degrees, the angle of the light incident on the c-plane sapphire single crystal is set to 1 degree or more and 6 degrees or less with respect to the c-axis of the sapphire single crystal. Is possible.

Hereinafter, reference examples of the present invention will be described.
[ Reference example ]

  As shown in FIG. 1, this polarization-independent optical isolator includes a wedge-shaped birefringent crystal (rutile) 4, a Faraday rotator 5 using a Bi-substituted RIG film, and a wedge-shaped birefringent crystal (rutile). ) 4 are arranged in order along the optical axis, and a fiber collimator (collimator lens) 3 is arranged on the outer optical axis of each birefringent crystal 4 with the Faraday rotator 5 at the center. The c-plane sapphire single crystal 5a is bonded to each other so that the c-axis of the sapphire single crystal is perpendicular to each optical surface through which the light of the rotor 5 passes. Further, the composite film 50 composed of the Faraday rotator 5 and the c-plane sapphire single crystal 5a is covered with a heat sink 15 made of brass as shown in FIG. 1, and is integrally held by a holder. In FIG. 1, 1 is a light source (semiconductor laser) having a wavelength of 1064 nm, 2 is a fiber, 7 is a magnet, and 80 is a power meter. The thickness of the Faraday rotator 5 at the wavelength of 1064 nm is 140 μm, and the insertion loss is about 0.6 dB.

  First, both sides of the Faraday rotator 5 were polished to a desired thickness, and an antireflection film for an adhesive was applied to both sides. A C-plane sapphire single crystal 5a substrate having a thickness of 500 μm, with one side coated with an antireflection film for adhesive and the other side coated with an antireflection film for air, on both sides of the Faraday rotator 5, It bonded together through the epoxy adhesive with a small absorption coefficient in 1064 nm. The composite film 50 thus obtained was cut with a dicing saw so as to be 2.0 mm square.

  The composite film 50 is covered with the heat sink 15 made of brass, and is inclined between the heat sink 15 and the optical axis by 4 degrees, and is inserted between the birefringent crystals (rutile) 4 in which the wedge angle is set to 5 degrees. It was.

Then, as shown in FIG. 1, an optical component composed of a birefringent crystal (rutile) 4, a composite film 50, a magnet 7 and the like is inserted and adjusted between the fiber collimators (collimator lenses) 3, and a reference example of the present invention. When the polarization-independent optical isolator according to the above was constructed, an input loss of 1.5 dB and an isolation of 32 dB were obtained, and these characteristics were almost unchanged even with a 2 W laser.

  The polarization-independent optical isolator according to the present invention maintains a high isolation effect even when used for a high-power laser having a wavelength of 1.1 μm or less and an output of 2 W or more. It has the potential to be widely used as an optical isolator for output lasers.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration explanatory diagram of a polarization-independent optical isolator for a high-power laser according to a reference example of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration explanatory diagram of a polarization-independent optical isolator for a high-power laser according to a reference example of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration explanatory diagram of a polarization-independent optical isolator for a high-power laser according to a reference example of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration explanatory diagram of a polarization-independent optical isolator for a high-power laser according to an embodiment of the present invention. Explanatory drawing of the birefringent crystal | crystallization made into the wedge shape as a component in the polarization independent type optical isolator for high power lasers. FIG. 10 is a configuration explanatory diagram of a polarization-independent optical isolator according to a conventional example. Explanatory drawing which shows the method to measure the relationship between the incident angle to the C-plane sapphire single crystal of a composite film in a polarization independent optical isolator, and an extinction ratio. The graph which shows the relationship between the incident angle to C surface sapphire single crystal, and an extinction ratio. Explanatory drawing which shows the method of measuring the return loss resulting from the reflected return light from the said c surface sapphire single-crystal end surface.

Explanation of symbols

2 Fiber 3 Fiber collimator 4 Wedge-shaped birefringent crystal 5 Faraday rotator 5a c-plane sapphire single crystal

Claims (3)

A birefringent crystal having a wedge shape along the optical axis, a Faraday rotator using a Bi-substituted RIG film, and a birefringent crystal having a wedge shape are arranged in this order, and each birefringence is centered on the Faraday rotator. A fiber collimator is disposed on each outer optical axis of the crystal, and a c-plane sapphire single crystal is attached so that the c-axis of the sapphire single crystal is perpendicular to each optical surface through which the light of the Faraday rotator passes. In a polarization-independent optical isolator for a high-power laser having a combined structure,
The wedge-shaped birefringent crystal is made of rutile and the wedge angle of the birefringent crystal is set in the range of 0.65 degrees to 3.5 degrees, so that the birefringent crystal is transmitted and the c-plane sapphire single unit is formed. A polarization-independent optical isolator for a high-power laser , characterized in that the angle of light incident on the crystal is 1 to 6 degrees with respect to the c-axis of the sapphire single crystal . A birefringent crystal having a wedge shape along the optical axis, a Faraday rotator using a Bi-substituted RIG film, and a birefringent crystal having a wedge shape are arranged in this order, and each birefringence is centered on the Faraday rotator. A fiber collimator is disposed on each outer optical axis of the crystal, and a c-plane sapphire single crystal is attached so that the c-axis of the sapphire single crystal is perpendicular to each optical surface through which the light of the Faraday rotator passes. In a polarization-independent optical isolator for a high-power laser having a combined structure,
The wedge-shaped birefringent crystal is made of YVO ₄ and the wedge angle of the birefringent crystal is set in the range of 1.1 degrees to 5.2 degrees, so that the birefringent crystal is transmitted and c-plane sapphire is transmitted. A polarization-independent optical isolator for a high-power laser , wherein the angle of light incident on the single crystal is 1 to 6 degrees with respect to the c-axis of the sapphire single crystal . A birefringent crystal having a wedge shape along the optical axis, a Faraday rotator using a Bi-substituted RIG film, and a birefringent crystal having a wedge shape are arranged in this order, and each birefringence is centered on the Faraday rotator. A fiber collimator is disposed on each outer optical axis of the crystal, and a c-plane sapphire single crystal is attached so that the c-axis of the sapphire single crystal is perpendicular to each optical surface through which the light of the Faraday rotator passes. In a polarization-independent optical isolator for a high-power laser having a combined structure,
By forming the wedge-shaped birefringent crystal from calcite and setting the wedge angle of the birefringent crystal in the range of 1.6 degrees to 9.3 degrees, the birefringent crystal is transmitted through the c-plane sapphire single unit. A polarization-independent optical isolator for a high-power laser , characterized in that the angle of light incident on the crystal is 1 to 6 degrees with respect to the c-axis of the sapphire single crystal .

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