Use of a thickness-compensated optical medium in a continuously tunable laser

Abstract

The application provides the application of transparent medium with thickness compensation in continuous tunable laser, wherein the cross section shape of the transparent medium with thickness compensation is right trapezoid, or the front surface of the transparent medium is curved surface and the back surface is flat surface. The application applies two different transparent mediums with thickness compensation to external cavity semiconductor laser. The laser emitted from the laser passes through narrow band interference piece, and realizes laser output under the light feedback of partial mirror. The application changes the cavity length by using the transparent medium with thickness compensation, and makes the cavity mode frequency different from the transmission frequency f IF of the interference piece, so as to observe the change of the maximum tunable range of the laser.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a laser with a thickness-compensated optical medium and a larger continuously tunable range. Background Technology

[0002] With the continuous development of laser technology, external cavity semiconductor lasers have been widely used in precision manufacturing, lidar, and fiber optic communication due to their wavelength tunability and frequency stability. However, existing external cavity semiconductor lasers still face many limitations in achieving wide-range continuous tunability. Traditional tuning methods mainly rely on components such as fiber gratings, piezoelectric mirrors, or optical thin films to adjust the wavelength by changing the cavity length. However, this method is prone to mode hopping, which limits the continuity of wavelength tuning. Furthermore, existing technologies struggle to achieve stable single-mode output over a large tuning range, and the tuning process is complex and susceptible to interference from external factors such as temperature drift and mechanical vibration, thus affecting the laser's frequency stability and accuracy. These limitations make it difficult for current external cavity semiconductor lasers to meet the extremely high requirements for frequency stability and continuous tunability in applications such as high-resolution spectral measurement and precision interferometry. Therefore, improving the frequency tunability range of lasers requires breakthroughs not only in equipment technology but also in tuning methods.

[0003] Chinese invention patent application CN 119394219 A discloses a high-resolution small-angle measurement device based on laser frequency measurement. This device includes a laser, in which a compensation medium (transparent solid medium) of specific thickness and refractive index is placed in a resonant cavity and rotated simultaneously with a narrowband interferometer. By changing the cavity length while selecting the interference frequency, the center frequency of the interferometer's transmission peak changes simultaneously with the cavity mode frequency, allowing for high-precision continuous tuning of the laser frequency and thus achieving ultra-high-precision continuous measurement of the rotation angle. However, since the thickness of the transparent solid medium is constant, its compensation effect on the cavity mode frequency is limited when rotating simultaneously with the narrowband interferometer, thus limiting the range of simultaneous changes in the cavity mode frequency and the center frequency of the interferometer's transmission peak, i.e., limiting the angle measurement range of the device. Therefore, obtaining a laser with a larger continuously adjustable range remains a current technical challenge. Improving the physical structure of the compensation medium to enhance the tunable range is an important research direction.

[0004] Therefore, breaking through the limitations of existing technologies and further expanding the range of continuously tunable quantities of lasers while maintaining the original laser structure is of great research significance. It will bring significant technological advancements to fields such as optical systems, automation control, navigation and positioning, and satellite communications, and has broad application prospects. Summary of the Invention

[0005] The purpose of this invention is to expand the continuously tunable range of lasers by proposing a transparent medium with thickness compensation to achieve a laser with a larger continuously tunable range.

[0006] The idea behind this invention is to improve the cross-sectional shape of the optical medium (transparent medium) so that its thickness has a linear or nonlinear relationship with the angle of the optical path, thereby changing the optical path length of the laser passing through the transparent medium at different angles, i.e., the cavity length of the resonant cavity. By selecting the linear or nonlinear parameter of the thickness of the transparent medium and the angle of the optical path, the change in the cavity length of the resonant cavity is controlled, so that the change in cavity mode frequency caused by the change in cavity length is synchronized with the change in the center frequency of the transmission peak of the interferometer caused by the rotation of the interferometer, ensuring continuous change of laser frequency and expanding the mode-skip-free tuning range.

[0007] Therefore, the present invention provides a transparent medium with thickness compensation, wherein the cross-sectional shape of the transparent medium with thickness compensation is a right trapezoid, the face containing the hypotenuse of the right trapezoid is the front surface of the transparent medium, the face containing the right-angled side of the right trapezoid is the rear surface of the transparent medium, and the angle between the hypotenuse and the right-angled side of the right trapezoid is β. When the transparent medium is in its initial position, the width within the right trapezoid parallel to the median line and intersecting with the laser beam is the initial thickness D of the transparent medium. g0 When the rotation angle of the transparent medium changes by θ, the width within the right trapezoid parallel to the midline and intersecting the laser beam is the thickness D′ of the transparent medium, where D′ = D g0 +Aθ, where A=tanβ, θ is the change in rotation angle of the transparent medium; or,

[0008] The transparent medium with thickness compensation has a curved front surface and a flat rear surface. Its cross-sectional shape has a curved front side. When the rotation angle of the transparent medium changes by θ, the width within the cross-sectional shape that is parallel to the midline and intersects the laser beam is the thickness D″ of the transparent medium. D″ satisfies the relationship θ = aD″. 2 +b, Where θ is the change in rotation angle of the transparent medium, and a and b are both constants, such as Figure 1 As shown in A and 1B.

[0009] The present invention also provides the application of the above-described optical medium with thickness compensation in a continuously tunable laser.

[0010] Furthermore, the present invention also provides a continuously tunable laser with a thickness-compensated optical medium. The laser includes a laser generator and a mirror assembly arranged sequentially in the optical path. A high-precision horizontal rotating stage 105 is disposed between the laser generator and the mirror assembly. A narrowband interferometer 103 and a transparent medium 104 are fixed on the high-precision horizontal rotating stage 105. The light output from the laser generator is filtered out of the band by the narrowband interferometer 103 to obtain narrowband light. The narrowband light returns to the laser diode after passing through the transparent medium 104 and the mirror assembly. When the intracavity oscillation reaches a threshold, the laser is output by the mirror assembly. The transparent medium 104 is the thickness-compensated optical medium.

[0011] When the cross-sectional shape of the transparent medium is a right trapezoid, the fixed position of the transparent medium in the laser is such that the height passing through the midpoint of the median line of the right trapezoid is symmetrical to and not parallel to the narrowband interferometer, with the diameter of the rotating stage perpendicular to the optical path as the axis. When the front surface of the transparent medium with thickness compensation is curved and the rear surface is flat, the fixed position of the transparent medium in the laser is such that the height passing through the midpoint of the median line of the cross-sectional shape is symmetrical to and not parallel to the narrowband interferometer, with the diameter of the rotating stage perpendicular to the optical path as the axis.

[0012] At this time, in the laser, when the high-precision horizontal rotating stage rotates, the narrow-band interferometer rotates synchronously with the transparent medium, and the change in the cavity mode frequency Δf of the resonant cavity... g The following relationship must be satisfied:

[0013]

[0014] The resonant cavity mode frequency f g The following relationship must be satisfied:

[0015]

[0016] Where f0 is the cavity mode frequency of the transparent medium at an initial angle α, α is the initial angle between the midline of the cross-sectional shape of the transparent medium and the laser propagation direction, and n eff Let λ be the refractive index of the transparent medium, L be the initial cavity length, λ0 be the cavity mode wavelength of the transparent medium at the initial angle α (its value is equal to the transmission wavelength λ′0 when the interferometer is at the angle (π-α), and D = D′ or D = D′′.

[0017] Taking clear glass as a transparent medium as an example, combined with Figure 1-2 The working principle is explained in detail below with reference to the laser section of CN 119394219A:

[0018] When the initial angle of the white glass is α (i.e., the initial angle between the midline of the cross-sectional shape of the white glass and the laser propagation direction is α): Initial cavity length Therefore, when the white glass rotates clockwise from α to α-θ, the resulting change in cavity length is:

[0019] Where, n eff Let L be the refractive index of the white glass, D be the thickness of the white glass, L1 be the length from the laser output surface to the front surface of the white glass, L2 be the length from the rear surface of the white glass to the front surface of the partial reflector, and θ be the rotation angle of the white glass.

[0020] The relationship between the frequency change c / 2L and the change of half wavelength when the cavity length is changed is as follows:

[0021] The resonant cavity jitter ΔL and the cavity mode frequency change Δf can be obtained. g The relationship is:

[0022]

[0023] Known The change in cavity mode wavelength Δλ and the rotation angle θ of the white glass satisfy the following relationship:

[0024]

[0025] Where λ0 is the cavity mode wavelength of the white glass at an initial angle of α.

[0026] When the cross-sectional shape of the clear glass is a right trapezoid, let the thickness of the clear glass D′=D g0 +Aθ,D g0 Let θ be the initial thickness of the white glass, θ be the change in the rotation angle of the white glass, A = tanβ be the slope of the linear thickness compensation, and β be the angle between the hypotenuse and the right-angled side of the trapezoid; that is, the change in the cavity mode frequency with the rotation of the white glass is:

[0027]

[0028] Therefore, the placement of clear glass has the following characteristics: Figure 1 The initial angle shown is α, and the rotation angle θ of the white glass after clockwise rotation is related to the cavity mode frequency f. g The relationship is:

[0029]

[0030] Where f0 is the cavity mode frequency of the white glass at an initial angle of α.

[0031] Similarly, when the front surface of the clear glass is curved and the back surface is flat, and its cross-sectional shape is such that the front side is curved, the thickness D″ of the clear glass satisfies the relationship Both a and b are constants; that is, the change in frequency with respect to the rotating cavity mode frequency of the white glass is:

[0032]

[0033] In this case, when the initial angle of the white glass is α, the rotation angle θ of the white glass is related to the cavity mode frequency f. g The relationship is:

[0034]

[0035] On the other hand, in this invention, the narrowband interferometer rotation angle With transmission wavelength λ IF The relational expression is as follows:

[0036]

[0037] λ′0 is the transmission wavelength when the interferometer is at an angle of π-α, n eff2 Let be the refractive index of the narrowband interferometer. Since the positions of the narrowband interferometer and the transparent medium are symmetrical about the diameter of the rotating stage perpendicular to the optical path and are not parallel, when the interferometer rotates clockwise by θ from the initial angle π-α, the angle of the narrowband interferometer will change.

[0038] According to the formula The rotation angle of the narrowband interferometer can be obtained. With transmission frequency f IF The relationship is:

[0039]

[0040] In this invention, the laser generating device is a conventional setup in the art, typically comprising a laser diode and a first collimating lens arranged sequentially in the optical path. The laser diode can operate in any wavelength band; preferably, for ease of acquisition or economic reasons, a 780nm laser diode can be used. Those skilled in the art can also select laser diodes with other operating wavelengths, such as 420nm, 850nm, or 1550nm laser diodes.

[0041] Because narrowband filter external cavity feedback semiconductor lasers can achieve a mode-hopping-free tuning range of approximately ±10 GHz by adjusting the length of the feedforward tuning cavity, the cavity mode frequency f is crucial to ensure continuous tuning. g Transmission frequency f at the same angle IF The frequency difference between them should not exceed 10 GHz.

[0042] Compared to existing technologies, this invention utilizes two different transparent media with thickness compensation characteristics in a narrowband filter external cavity feedback semiconductor laser. The laser emitted from the laser is filtered through a narrowband interferometer to obtain narrowband light, which in turn filters the broadband light emitted from the laser diode into a narrowband beam. After optical feedback through a partial reflector, the measured laser light is output by the reflector group when the intracavity oscillation reaches a threshold. This invention has been verified to use a transparent medium with thickness compensation characteristics to change the cavity length while satisfying the cavity mode frequency f. g With transmission frequency f IF Under the condition that the frequency difference between them should not exceed 10 GHz, the corresponding angle variation range is larger, that is, a larger continuous tunable range is obtained. Attached Figure Description

[0043] Figure 1 A and Figure 1 B is a transparent medium with thickness compensation;

[0044] Figure 2 This is a schematic diagram of the laser in Example 1;

[0045] Among them: 101, laser diode; 102, first collimating lens; 103, narrowband interference plate; 104, transparent medium; 105, high-precision horizontal rotary stage; 106, rotary stage mounting base; 107, first collimating lens; 108, partial reflector; 109, second collimating lens; 110, reflector.

[0046] Figure 3 The relationship between the cavity mode frequency change caused by the rotation angle of white glass with different thickness compensation and the transmission frequency change caused by the simultaneous rotation of the interferometer when the initial angles are 10° and 15° is given.

[0047] Figure 4 The rotation angle of white glass with different thicknesses D and f g The relationship between different rotation angles of the interferometer and f IF The relationship. Detailed Implementation

[0048] The following examples are used to explain the technical solutions of the present invention in a non-limiting manner.

[0049] Example 1 examines the effect of the thickness of a transparent solid medium with different parameters on the continuously tunable range.

[0050] like Figure 2 The laser shown operates at a wavelength of 780nm. The device mainly comprises, sequentially arranged in the optical path, a laser diode 101, a first collimating lens 102, a narrowband interferometer 103, and a transparent medium 104 (made of white glass with a refractive index of n). eff=1.5) Focusing lens 107, partial reflector 108, second collimating lens 109, and reflector 110 are fixed on a high-precision horizontal rotating stage 105, which is mounted on a rotating stage mounting base 106. Narrow-band interferometer 103 and transparent medium 104 are fixed on the precision rotating platform 105. Their placement is symmetrical about the diameter of the rotating stage perpendicular to the optical path, ensuring that the narrow-band interferometer 103 is highly symmetrical and not parallel to the midpoint of the median line passing through the cross-sectional shape of the transparent medium 104.

[0051] The cross-sectional shape of the transparent medium 104 is as follows Figure 1 As shown in Figure A, it is a right trapezoid, where the plane containing the right-angled side is its back surface.

[0052] During operation, the light output from the 780nm laser diode is collimated, filtered to remove out-of-band light by a narrow-band interferometer, and then passes through a transparent solid medium. After passing through a focusing lens, it is reflected by a partial reflector. When the oscillation within the resonant cavity reaches a threshold, the laser is output from the partial reflector. By simultaneously rotating the interferometer and the transparent solid medium, the center frequency of the transmission peak of the interferometer changes synchronously with the cavity mode frequency, achieving continuous tunability of the output frequency.

[0053] In this embodiment, the continuously tunable range of laser frequency achieved by setting a transparent medium with thickness compensation is examined:

[0054] The cavity length is set to 10 cm, the initial angle is α = 10°, λ0 = 780 nm, and a transparent solid medium with different included angles β between the front and rear surfaces is selected. The refractive index of the transparent solid medium is n. eff =1.5, the design thickness is D′=0.048+θ×A, A=tanβ, calculate the f caused by different values ​​of A for each transparent solid medium and the rotation angle of the interferometer at θ. g -f IF Curves, such as Figure 3 As shown in (a). Wherein,

[0055]

[0056] in,

[0057] pass Figure 3 (a) It can be seen that when the value of A is chosen to be 0.00417, f satisfies g -f IF The angle variation range is relatively large under conditions not exceeding ±10GHz. For example... Figure 4 As shown in (a), we plot the cavity mode frequencies f when the optical element thicknesses D = D′ = 0.048 + 0.00417θ and D = 0.048 m, respectively. g and transmission frequency f IFObserve the tunability of the laser, where the horizontal axis represents the change in θ and the vertical axis represents the frequency.

[0058] The results show that when the frequency difference between the cavity mode frequency and the transmission frequency at the same angle does not exceed ±10 GHz, i.e., f g -f IF Not exceeding -1.0×10 10 ~1.0×10 10 When using a conventional transparent medium with a thickness of D = 0.048 (meters), the corresponding angle variation range is -0.59° to 0.57°, and the corresponding tunable laser frequency range is 3.858 × 10⁻⁶ Hz. 14 Up to 3.862×10 14 Hz (according to) Figure 4 (a) Green dashed line), the adjustable range is 4×10 11 Hz. When using a transparent medium with an angle between the front and rear surfaces, a wider range can be obtained depending on the value of A. For example, when D′=0.048+0.00417θ, the frequency difference between the cavity mode frequency and the transmission frequency at the same angle (i.e., f) is satisfied. g -f IF When the GHz frequency does not exceed ±10 GHz, the corresponding angle variation range is -3.86° to 1.25°, and the corresponding tunable laser frequency range is 3.851 × 10⁻⁶. 14 Up to 3.864×10 14 Hz (according to) Figure 4 (a) The red dashed line indicates that the adjustable value is 1.3 × 10⁻⁶. 12 Hz. It can be seen that when the transparent medium adopts the original structure, that is, when its front and back surfaces are parallel, the continuous tunable range of the laser is small, while the transparent medium with thickness compensation of the present invention can expand the continuous tunable range of the laser.

[0059] Under the same conditions, with an initial angle of α = 15° and a designed thickness of D′ = 0.04575 + θ × A, calculate the effect of f on the rotation angle of each transparent solid medium and the interferometer when the plate is rotated by θ for different values ​​of A. g -f IF Curves, such as Figure 3 As shown in (b).

[0060] pass Figure 3 (b) It can be seen that when the value of A is chosen to be 0.00278, f is satisfied. g -f IF The angle variation range is relatively large under conditions not exceeding ±10GHz. For example... Figure 4(b) shows the cavity mode frequencies f when the optical element thicknesses D = D′ = 0.04575 + 0.00278θ and D = 0.04575m, respectively. g and transmission frequency f IF Observe the tunability of the laser.

[0061] It can be seen that when f is satisfied g -f IF When the difference does not exceed ±10 GHz, and a conventional transparent medium with a thickness of D = 0.04575 (meters) is used, the corresponding angle variation range is -0.59° to 0.56°, and the corresponding tunable laser frequency range is 3.876 × 10⁻⁶. 14 Up to 3.881×10 14 Hz (according to) Figure 4 (b) The blue dashed line indicates that the adjustable range is 5 × 10. 11 Hz. When using a transparent medium with an angle between the front and rear surfaces, a wider range can be obtained depending on the value of A. For example, when D′=0.04575+0.00278θ, the frequency difference between the cavity mode frequency and the transmission frequency at the same angle (i.e., f) is satisfied. g -f IF When the GHz frequency does not exceed ±10 GHz, the corresponding angle variation range is -4.67° to 1.37°, and the corresponding tunable laser frequency range is 3.861 × 10⁻⁶. 14 Up to 3.884×10 14 Hz (according to) Figure 4 (b) The pink dashed line indicates that the adjustable range is 2.3 × 10⁻⁶. 12 Hz.

[0062] Example 2

[0063] The setup is the same as in Example 1, except that the transparent medium with a right-angled trapezoidal cross-section is replaced with a transparent medium with a curved front surface and a flat rear surface. When its cross-sectional shape is such that the front side is curved, the thickness D′′ satisfies the relationship With initial angles of α = 10° and α = 15° respectively, plot the f values ​​for different values ​​of a and b for each transparent solid medium and the interference plate rotated by an angle θ. g -f IF Curves, such as Figure 3 As shown in (c) and (d). Among them,

[0064]

[0065]

[0066] pass Figure 3 (c) Appropriate When f is satisfied g -f IF The angle variation range is relatively large under conditions not exceeding ±10GHz. For example... Figure 4 As shown in (c), we plot the thickness of the optical element respectively. The cavity mode frequency f when D = 0.048m g and transmission frequency f IF Observe the tunability of the laser.

[0067] The results show that when the initial angle α = 10°, When f is satisfied g -f IF Under conditions not exceeding ±10 GHz, the corresponding angle variation range is -2.11 to 2.27°, and the tunable laser frequency range is 3.855 × 10⁻⁶. 14 Up to 3.867×10 14 Hz (according to) Figure 4 (c) Orange dashed line), the adjustable value is 1.2 × 10 12 Hz. Significantly greater than the continuously tunable range achievable using conventional transparent media.

[0068] And when the initial angle α = 15°, When f is satisfied h -f IF Within ±10 GHz, the corresponding small angle variation is -3.61 to 1.12°, and the tunable laser frequency range is 3.865 × 10⁻⁶. 14 Up to 3.883×10 14 Hz (according to) Figure 4 (d) The tunable value is 1.8 × 10⁻⁶ (medium cyan dashed line). 12 Hz. This is also significantly greater than the continuously tunable laser frequency range achievable using conventional transparent media.

[0069] As can be seen, the transparent medium with thickness compensation of the present invention, when applied in a laser, satisfies f g -f IF Under conditions not exceeding ±10 GHz, the continuous tunable range of the laser can be effectively improved.

Claims

1. A continuous tunable laser with a transparent medium with thickness compensation, the laser comprising a laser generating device and a mirror group arranged in sequence on an optical path, a high-precision horizontal rotating table (105) being arranged between the laser generating device and the mirror group, a narrow-band interference film (103) and a transparent medium (104) being fixed on the high-precision horizontal rotating table (105), the light output by the laser generating device being filtered to remove out-of-band light after passing through the narrow-band interference film (103) to obtain narrow-band light; the narrow-band light returning to the laser diode after passing through the transparent medium (104) and the mirror group, the measured laser being output by the mirror group when the intracavity oscillation reaches a threshold value; characterized in that, When the cross-sectional shape of the transparent medium is a right trapezoid, the fixed position of the transparent medium in the laser is such that the height passing through the midpoint of the median of the right trapezoid is symmetric to and non-parallel to the wide band interference piece with the diameter of the rotation table perpendicular to the light path as the axis, when the variation of the rotation angle of the transparent medium is θ, the width parallel to the median and intersecting the laser beam in the right trapezoid is the thickness of the transparent medium ; When the front surface of the transparent medium with thickness compensation is curved and the back surface is flat, the fixed position of the transparent medium in the laser is such that the height passing through the midpoint of the median line of the cross-sectional shape is symmetrical to the narrow band interference piece and is not parallel with the diameter of the rotating table perpendicular to the light path as the axis, when the variation of the rotation angle of the transparent medium is θ, the width parallel to the median line and intersecting the laser beam in the cross-sectional shape is the thickness of the transparent medium ; When the high-precision horizontal rotating table rotates, the narrow-band interference plate rotates synchronously with the transparent medium, and the variation of the resonant cavity mode frequency is satisfies the following relationship: resonator cavity mode frequencies satisfies the following relation: wherein is the cavity mode frequency of the transparent medium at an initial angle a, a being an initial angle between a median line of a cross-sectional shape of the transparent medium and a direction of propagation of the laser light, is a refractive index of the transparent medium, is a cavity mode wavelength of the transparent medium at an initial angle a, and or .

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