Orthogonal oscillator laser gain self-balancing system based on intracavity polarization conversion device

Abstract

The present invention discloses an orthogonal oscillation laser gain self-balancing system based on intracavity polarization conversion, comprising a semiconductor pump source, a coupling lens system, a folding mirror, a gain medium unit, and an output coupler arranged in sequence, wherein a polarization conversion unit is arranged on the side of the folding mirror. The present invention adopts the orthogonal oscillation laser gain self-balancing system based on intracavity polarization conversion of the above structure, and in the process of forming the first linear polarized laser and the second linear polarized laser, the small signal is alternately amplified multiple times by the first gain medium and the second gain medium, respectively, to achieve the effect of simultaneously satisfying high conversion efficiency and power balance.

Description

Technical Field

[0001] The invention relates to the technical field of solid lasers, in particular to an orthogonal oscillation laser gain self-balancing system based on an intra-cavity polarization conversion device. Background Art

[0002] Polarization is one of the most important physical properties of light. Orthogonal polarization lasers have extremely broad application scenarios in the fields of differential interference imaging, laser display, continuous terahertz wave generation, and precision measurement. Therefore, it is of great significance to develop orthogonal polarization laser sources with high conversion efficiency and high polarization degree.

[0003] In the prior art, solid orthogonal polarization laser sources are often realized through the transition of different Stark energy level splitting of solid gain media. The output characteristics of this method are not stable enough, and due to the limitation of the principle, only dual-frequency orthogonal polarization light can be obtained. However, single-frequency orthogonal polarization laser has unique advantages in some application scenarios. Taking precision measurement as an example, it has the advantages of small nonlinear error and measurement range not limited by dual-frequency frequency difference. In view of these shortcomings of the single gain medium method, it is considered to use semiconductor lasers to coaxially pump two blocks of c-axis orthogonally placed gain media to achieve oscillation in the same resonant cavity, and orthogonal polarization lasers can be obtained. This method has a simple structure, no gain competition, can produce single-wavelength orthogonal polarization light, and through frequency selection measures, such as adding standard tools, etc., it can achieve the output of single-frequency narrow-linewidth orthogonal polarization lasers, which has unique advantages.

[0004] When the double gain medium orthogonal placement method is actually applied to generate single-wavelength orthogonal polarization lasers, there is a contradiction between conversion efficiency and orthogonal polarization balance position, that is, after the pump light is absorbed by the first gain medium, the pump light incident on the second gain medium is less, so the polarization oscillation light power output corresponding to the first gain medium is much higher than the polarization oscillation light power corresponding to the second gain medium, resulting in polarization ratio imbalance. To avoid this phenomenon, methods of reducing the absorption of the first gain medium and increasing the absorption of the second gain medium are often used, such as adjusting the output wavelength of the semiconductor laser, reducing the absorption coefficient of the first gain medium, and placing the pump light focus position inside the second gain medium. These methods often lead to a decrease in conversion efficiency and cannot give full play to the advantages of solid-state lasers. Therefore, it is necessary to develop a coaxially pumped dual-gain medium laser that takes into account both conversion efficiency and polarization component balance to better give full play to the advantages of all-solid-state lasers. Summary of the invention

[0005] The purpose of the present invention is to provide an orthogonal oscillation laser gain self-balancing system based on an intracavity polarization conversion device, by inserting a polarization device into the end coaxial pump laser cavity to achieve intracavity polarization conversion, and then based on the polarization characteristics of the laser gain medium, achieve orthogonal polarization gain self-balancing.

[0006] To achieve the above object, the present invention provides an orthogonal oscillation laser gain self-balancing system based on an intracavity polarization conversion device, comprising a semiconductor pump source, a coupling lens system, a folding mirror, a gain medium unit, and an output coupler arranged in sequence, wherein a polarization conversion unit is arranged on the side of the folding mirror:

[0007] The coupling lens system is connected to the semiconductor pump source through an energy transmission optical fiber, and the coupling lens system focuses the pump light generated by the semiconductor pump source to obtain focused light;

[0008] The folding mirror is placed at an angle of 45°, one side of the surface of the folding mirror is coated with a high-reflection film and an anti-reflection film, the coated side of the folding mirror faces the gain medium unit, and the folding mirror is used to transmit the focused light and reflect the oscillating light in the resonant cavity to the polarization conversion unit;

[0009] The polarization conversion unit includes a quarter wave plate and a total reflection mirror. The surface of the quarter wave plate is coated with an anti-reflection film. The total reflection mirror is a plane mirror and the surface is coated with a high reflection film. The coating surface of the total reflection mirror faces the gain medium unit. The polarization conversion unit is used to convert the input oscillating light into a 90° polarization and then input it into the gain medium unit for gain. The total reflection mirror serves as a total reflection device of the laser.

[0010] The gain medium unit comprises a first gain medium and a second gain medium, the first gain medium and the second gain medium are coaxially arranged, the crystal optical axis directions of the first gain medium and the second gain medium are perpendicular to the light transmission direction of the laser, the first gain medium and the second gain medium are used to absorb the focused light to generate a first linearly polarized laser and a second linearly polarized laser, the wavelength of the first linearly polarized laser is the same as the wavelength of the second linearly polarized laser, and the polarization direction of the first linearly polarized laser is perpendicular to the polarization direction of the second linearly polarized laser;

[0011] The output coupler is a plane mirror, which is used to form the resonant cavity with the total reflection device. The first linearly polarized laser and the second linearly polarized laser are respectively output through the output coupler after circular reflection in the resonant cavity, and the first output linearly polarized laser and the second output linearly polarized laser are balanced in power.

[0012] Preferably, the first gain medium is used to absorb part of the focused light to achieve continuous gain in a first gain direction perpendicular to the optical platform; the second gain medium is used to absorb another part of the focused light to achieve continuous gain in a second gain direction parallel to the optical platform.

[0013] Preferably, the first gain medium and the second gain medium are a-axis cut crystals, the a-axis of the first gain medium and the a-axis of the second gain medium coincide with the optical axis of the resonant cavity, and the optical axis direction of the first gain medium is perpendicular to the optical axis direction of the second gain medium.

[0014] Preferably, the first gain medium and the second gain medium are doped with Nd 3+ Birefringent laser crystals.

[0015] Preferably, the first gain medium and the second gain medium are made of Nd:YVO 4 , Nd:YLF or Nd:GdVO 4 .

[0016] Therefore, the present invention adopts the orthogonal oscillation laser gain self-balancing system based on the intracavity polarization conversion device of the above structure, and utilizes the end coaxial pumping structure to pump the dual-block gain medium. The oscillation light corresponding to the dual gain medium resonates in the same resonant cavity, and the single-wavelength orthogonal polarization laser is output through the output coupling mirror. In order to improve the conversion efficiency, a folded cavity structure is introduced, and a quarter-wave plate is inserted between the front cavity mirror and the 90° reflecting mirror, so that the polarization direction of the oscillation light changes during the resonance process. The polarization absorption characteristics of the gain medium are further utilized to achieve the common gain of the dual gain media in a single polarization direction, thereby obtaining a gain self-balancing orthogonal polarization laser with high conversion efficiency.

[0017] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic structural diagram of an embodiment of an orthogonal oscillation laser gain self-balancing system based on an intracavity polarization conversion device of the present invention;

[0019] Figure 2 A schematic diagram of the direction of a polarization rotation device according to an embodiment of the present invention;

[0020] Figure 3 A schematic diagram of the gain medium crystal direction and the oscillating light polarization direction according to an embodiment of the present invention;

[0021] Figure 4 Schematic diagram of the gain process of the s component in the small signal P according to an embodiment of the present invention.

[0022] Reference numerals:

[0023] 1. Semiconductor pump source; 2. Coupling lens system; 3. Reflecting mirror; 4. Gain medium unit; 41. First gain medium; 42. Second gain medium; 5. Polarization conversion unit; 51. Quarter wave plate; 52. Total reflection mirror; 6. Output coupler.

[0024] A, fast axis direction of the quarter wave plate;

[0025] s, the optical axis direction of the first gain medium, i.e., the first gain direction;

[0026] p, the optical axis direction of the second gain medium, i.e., the second gain direction;

[0027] s`, first linear polarization laser;

[0028] p`, second linearly polarized laser. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0030] Example

[0031] As shown in the figure, the orthogonal oscillation laser gain self-balancing system based on the intracavity polarization conversion device includes a semiconductor pump source 1, a coupling lens system 2, a folding mirror 3, a gain medium unit 4, and an output coupler 6 arranged in sequence, and a polarization conversion unit 5 is arranged on the side of the folding mirror 3.

[0032] The central wavelength of the laser output from semiconductor pump source 1 is 808 nm, which is coupled to coupling lens system 2 through a power transmission fiber with a core diameter of 400 μm and NA=0.22. The beam expansion ratio is 1:1, and the output beam waist radius is 178 μm. The beam quality factor M 2 The coupling lens system 2 focuses the pump light generated by the semiconductor pump source 1 to obtain focused light.

[0033] The folding mirror 3 is placed at an angle of 45°, and its surface is coated with an 808nm anti-reflection film and a 1064nm high-reflection film at a 45° angle. The coated side of the folding mirror 3 faces the gain medium unit 4. The folding mirror 3 transmits the focused light and reflects the oscillating light in the resonant cavity to the polarization conversion unit 5.

[0034] The gain medium unit 4 includes a first gain medium 41 and a second gain medium 42 arranged coaxially, wherein the first gain medium 41 and the second gain medium 42 are Nd-doped 3+ Birefringent laser crystals such as Nd:YVO 4 , Nd:YLF or Nd:GdVO 4 The two gain media in this embodiment are Nd:YVO cut along the a-axis with the same size and doping concentration. 4 , whose size and doping concentration are 3×3×5mm 3 and 0.2-at.%, respectively, with absorption coefficients of 213m -1 and 230m -1The crystal end faces are coated with anti-reflection films of 808nm and 1064nm bands, and the light transmission direction is a-axis, that is Figure 3 The first gain medium 41 is placed 90° rotated relative to the second gain medium 42 with the light transmission direction as the axis. Nd:YVO 4 The crystal is a uniaxial crystal with natural birefringence. Since the stimulated emission cross section of π-polarized light (parallel to the c-axis) is four times that of σ-polarized light (perpendicular to the c-axis), its pump absorption is also polarization-dependent. 4 F 3 / 2 → 4 I 11 / 2 ), so both gain media can obtain effective π-polarized output.

[0035] The optical axis direction of the first gain medium 41 coincides with the x-axis, which is the first gain direction s, that is, the first gain medium 41 amplifies the oscillating light with a polarization direction of the first gain direction to generate a first linearly polarized laser s`; the optical axis direction of the second gain medium 42 coincides with the y-axis, which is the second gain direction p, that is, the second gain medium 42 amplifies the oscillating light with a polarization direction of the second gain direction to generate a second linearly polarized laser p`.

[0036] The oscillation gain process of polarized laser is as follows:

[0037] Assume that the initial small signal light is P, the first gain direction is s direction, and the second gain direction is p direction.

[0038] When passing through the gain medium unit 4, the mode of the small signal P within the range of the gain coefficient curve is amplified and enhanced. As mentioned above, the small signal P contains polarizations in two directions, s and p, which correspond to the first gain medium 41 and the second gain medium 42, respectively. The two small signal components are continuously amplified to achieve orthogonal polarization laser output. Specifically, when the s component in the small signal P oscillates for the first time, it is amplified by the first gain medium 41, and then the s component is reflected by the 45° folding mirror 3 to the polarization conversion unit 5, and reflected by the total reflection mirror 52. After these two times of passing through the 1 / 4 wave plate, it is converted into the p component, and is reflected by the folding mirror 3 to the gain medium unit 4 again, amplified by the second gain medium 42, and then reflected to the polarization conversion unit 5, and the above process is repeated for multiple oscillations. The oscillation process of the p component in the small signal P is the same. In the final output light, the first linearly polarized laser s' and the second linearly polarized laser p' are obtained by multiple alternating amplifications of the first gain medium 41 and the second gain medium 42. After the small signal P reaches gain saturation after multiple amplifications, the gain coefficient g n Approaching the loss factor g tBalance is reached, and the stable output laser is a power-balanced orthogonal polarization laser. Since the two gain media alternately gain the orthogonal polarization components of the small signal P, share the same pump source, and realize feedback in the same resonant cavity, the first linear polarization laser s' and the second linear polarization laser p' have the same wavelength, perpendicular polarization directions, and balanced polarization components.

[0039] The polarization conversion unit 5 includes a quarter wave plate 51 and a total reflection mirror 52. Figure 2 As shown, the quarter wave plate 51 is placed perpendicular to the z-axis, with the fast axis direction and the x-axis at an angle of 45°, generating a 90° phase delay for the 1064nm band laser, and coated with a 1064nm anti-reflection film. The quarter wave plate 51 converts the input oscillating light with the polarization direction of the first gain direction and the second gain direction into left-handed circularly polarized light and right-handed circularly polarized light, respectively, and converts the input left-handed circularly polarized light and right-handed circularly polarized light into the second gain direction and the first gain direction, respectively. The total reflection mirror 52 is a plane mirror coated with a 1064nm high reflection film, and the coating surface is close to the gain medium crystal. The total reflection mirror 52 serves as a total reflection device for the laser.

[0040] The output coupler 6 is a plane mirror with a transmittance of 15% for oscillating light in the 1064nm band. It is used to form a resonant cavity with the total reflection device. The first linearly polarized laser s' and the second linearly polarized laser p' are respectively output through the output coupler 6 after circular reflection in the resonant cavity. The first output linearly polarized laser and the second output linearly polarized laser are balanced in power.

[0041] From the theoretical value of numerical calculation, according to the typical four-level system rate equation, the continuous linear polarization laser power output by the two gain media can be calculated as:

[0042] (1)

[0043] The subscript Indicates Gain medium, is the effective cross-sectional area of ​​the laser beam, is the transmittance of the output coupling mirror system 12, is the total round-trip loss in the resonant cavity, is the saturation power density, is the small signal gain. Both the saturated power density and the small signal gain are determined by the pump parameters The calculation result is:

[0044] (2)

[0045] in, is the quantum efficiency, is the pump power incident on the corresponding gain medium, is the absorption coefficient, is the number of doped particles, is Planck's constant, is the pump light frequency, is the crystal length, The end-face coaxial pump structure is axially Combined with the above technical analysis, any component of the orthogonal polarization laser output is obtained by multiple alternating gains of the two gain media, that is, the gain contribution of any gain medium to the output polarization component is equivalent, that is,

[0046]

[0047] If the pump power is 10W, the pump light is focused on the left end surface of the first gain medium 41, the resonant cavity length is 80mm, and the intrinsic loss is 0.02, it can be calculated that the output powers of the two orthogonal polarization components are both 2.52W, achieving high conversion efficiency and balanced power output.

[0048] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0049] 1. The present invention provides a compact, low-cost single-wavelength orthogonal polarization output laser. It uses end-face coaxial pumping to pump two coaxially placed laser crystals. The folded cavity is "L" shaped, with a simple configuration. The neodymium-doped gain medium used is relatively easy to obtain, achieving maximum simplification, miniaturization and low cost of orthogonal polarization lasers.

[0050] 2. The present invention provides a method for generating orthogonally polarized lasers with high conversion efficiency. The all-solid-state laser itself has the advantage of high conversion efficiency. Nd:YVO with a wide absorption spectrum and a large stimulated emission cross section is selected. 4 The crystal uses end-face pumping to give full play to its advantages, adopts a combined gain medium, avoids the gain competition problem, and the single output coupler 6 setting can also significantly improve the conversion efficiency and achieve high-efficiency and high-performance laser output.

[0051] 3. The present invention provides a method for obtaining orthogonal polarization lasers with high conversion efficiency and power balance. The power balance point is an important issue of concern for orthogonal polarization lasers. At this point, some special application scenarios can be met, such as improving interference efficiency. This laser introduces a folded cavity structure, and inserts a quarter-wave plate 51 between the folding mirror 3 and the 90° total reflection mirror 52, so that the polarization direction of the oscillating light changes during the resonance process. The polarization absorption characteristics of the gain medium are further utilized to achieve the common gain of the dual gain media in a single polarization direction, thus overcoming the contradiction between high conversion efficiency and power balance. At the same time, this insensitive characteristic enhances the adjustability of the laser and makes it easy to obtain the optimal working point.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. An orthogonal oscillation laser gain self-balancing system based on an intracavity polarization conversion device, characterized in that: The invention comprises a semiconductor pump source, a coupling lens system, a folding mirror, a gain medium unit, and an output coupler which are arranged in sequence, and a polarization conversion unit is arranged on the side of the folding mirror: The coupling lens system is connected to the semiconductor pump source through an energy transmission optical fiber, and the coupling lens system focuses the pump light generated by the semiconductor pump source to obtain focused light; The folding mirror is placed at an angle of 45°, one side of the surface of the folding mirror is coated with a high-reflection film and an anti-reflection film, the coated side of the folding mirror faces the gain medium unit, and the folding mirror is used to transmit the focused light and reflect the oscillating light in the resonant cavity to the polarization conversion unit; The polarization conversion unit includes a quarter wave plate and a total reflection mirror, the surface of the quarter wave plate is coated with an anti-reflection film, the total reflection mirror is a plane mirror and the surface is coated with a high reflection film, the coating surface of the total reflection mirror faces the gain medium unit, the polarization conversion unit is used to convert the input orthogonal polarization oscillation light into 90° polarization and then input it into the gain medium unit for gain, and the total reflection mirror serves as a total reflection device of the laser; The gain medium unit comprises a first gain medium and a second gain medium, the first gain medium and the second gain medium are coaxially arranged, the crystal optical axis directions of the first gain medium and the second gain medium are perpendicular to the light transmission direction of the laser, the first gain medium and the second gain medium are used to absorb the focused light to generate a first linearly polarized laser and a second linearly polarized laser, the wavelength of the first linearly polarized laser is the same as the wavelength of the second linearly polarized laser, and the polarization direction of the first linearly polarized laser is perpendicular to the polarization direction of the second linearly polarized laser; The output coupler is a plane mirror, and the output coupler is used to form the resonant cavity with the total reflection device. The first linearly polarized laser and the second linearly polarized laser are respectively output through the output coupler after circular reflection in the resonant cavity, and the first output linearly polarized laser and the second output linearly polarized laser are balanced in power.

2. The orthogonal oscillation laser gain self-balancing system based on an intracavity polarization conversion device according to claim 1, characterized in that: The first gain medium and the second gain medium are a-axis cut crystals, the a-axis of the first gain medium and the a-axis of the second gain medium coincide with the optical axis of the resonant cavity, the optical axis direction of the first gain medium is perpendicular to the optical axis direction of the second gain medium, and the first gain medium is placed with respect to the second gain medium rotated 90° with the light transmission direction as the axis.

3. The orthogonal oscillation laser gain self-balancing system based on an intracavity polarization conversion device according to claim 1, characterized in that: The first gain medium and the second gain medium are doped with Nd 3+ Birefringent laser crystals.

4. The orthogonal oscillation laser gain self-balancing system based on an intracavity polarization conversion device according to claim 1, characterized in that: The first gain medium and the second gain medium are made of Nd:YVO4, Nd:YLF or Nd:GdVO4.

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