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

Abstract

The present application provides a kind of wide-range continuous tunable laser, which is provided with transparent medium with thickness compensation, the cross-sectional shape of the transparent medium with thickness compensation is right trapezoid, or the front surface of the transparent medium is curved surface, and the rear surface is plane.The present application verifies the influence of the two different transparent medium with thickness compensation characteristics on the tunable range of the laser.The laser emitted from the laser passes through narrow-band interference piece, and the output laser is obtained after the light feedback of partial mirror.The present application changes the cavity length of the laser by using the transparent medium with thickness compensation characteristics, and the cavity mode frequency is different from the transmission frequency f IF of the interference piece, so that the maximum tunable range of the laser is observed.

Description

TECHNICAL FIELD

[0001] The present application relates to a laser, in particular to a wide-range continuously tunable laser with an optical medium with thickness compensation. BACKGROUND

[0002] With the continuous development of laser technology, external cavity semiconductor lasers have been widely used in precision manufacturing, laser radar, optical fiber communication and other fields due to their wavelength tunability and frequency stability. However, the existing external cavity semiconductor lasers still have many limitations in realizing wide-range continuous tunability. The traditional tuning method mainly relies on fiber gratings, piezoelectric mirrors or optical thin films, etc. elements, and the wavelength is adjusted by changing the cavity length. However, this method is prone to mode hopping, which limits the continuity of wavelength tuning. In addition, the existing technology is difficult to achieve stable single-mode output in a large tuning range, and the tuning process is complex and easily disturbed by external factors such as temperature drift and mechanical vibration, thereby affecting the frequency stability and precision of the laser. These limitations make it difficult for current external cavity semiconductor lasers to meet the high requirements of frequency stability and continuous tunability in high-resolution spectral measurement, precision interferometric measurement and other applications. Therefore, improving the frequency tunable range of the laser not only requires breakthroughs in device technology, but also requires breakthroughs in tuning methods.

[0003] Chinese patent application CN 119394219 A discloses a high-resolution small-angle measurement device based on laser frequency measurement. The measurement device includes a laser, wherein a compensation medium with a specific thickness and refractive index is placed in the resonant cavity and rotates simultaneously with the interference sheet. The cavity length is changed at the same time of interference frequency selection, so that the center frequency of the interference sheet transmission peak and the cavity mode frequency change at the same time. This technology realizes high-precision continuous tuning of laser frequency. In an IFECDL, the mode-hop-free tuning range can reach 10GHz only by feeding forward tuning the cavity length. Therefore, in order to ensure continuous tuning, the frequency difference between the cavity mode frequency and the transmission frequency of the same angle should not exceed 10GHz. However, since the thickness of the transparent solid medium is constant, its compensation effect on the cavity mode frequency is limited when it rotates simultaneously with the narrow-band interference sheet, thereby limiting the range of simultaneous change of the cavity mode frequency and the center frequency of the interference sheet transmission peak, i.e. limiting the angle measurement range of the device. The compensation effect of the compensation medium with constant thickness on the cavity mode frequency is limited, which makes it difficult to obtain a laser with a larger continuous tunable range. How to improve the physical structure of the compensation medium to improve the tunable range is an important research direction at present. SUMMARY

[0004] The purpose of the present application is to expand the continuous tunable range of the laser, and to provide a transparent medium with thickness compensation to realize a laser with a larger continuous tunable range.

[0005] The idea of the present application is to apply an optical medium (transparent medium) with thickness compensation to a laser, to change the optical path length of the laser passing through the transparent medium at different angles, i.e. the cavity length of the resonator, by virtue of the linear or nonlinear relationship between the thickness of the transparent medium and the included angle of the optical path, and to control the amount of change in the cavity length by selecting the linear or nonlinear parameters of the included angle of the optical path and the thickness of the transparent medium, so as to synchronize the change in the cavity mode frequency caused by the change in the cavity length with the change in the center frequency of the transmission peak of the interference plate caused by the rotation of the interference plate, and to ensure continuous change in the laser frequency and to expand the tuning range without mode hopping.

[0006] To this end, the present application provides a large-range continuously tunable laser, which comprises a laser generating device and a mirror (208) arranged in sequence on an optical path, an exit light end surface of the laser generating device is provided with a second medium pool (202) filled with a second medium, the front wall and the rear wall of the second medium pool (202) are transparent and flat walls and are perpendicular to the optical path, the refractive index of the second medium is greater than the refractive index of vacuum, a narrow-band interference plate (203) and a transparent medium (204) are fixedly connected by a high-precision horizontal rotating table (205) and are arranged in parallel in the second medium pool (202), a partial mirror (207) is arranged outside the rear wall of the second medium pool (202), and the narrow-band interference plate (203), the transparent medium (204) and the partial mirror (207) are all on the optical path; the light output by the laser generating device enters the second medium pool (202) from the front wall, the out-of-band light is filtered out after the light passes through the narrow-band interference plate (203), and narrow-band light is obtained, the narrow-band light exits the second medium pool (202) from the rear wall after passing through the transparent medium (204), and the laser is output by the partial mirror (207) after being reflected by the partial mirror (207) and returning to the laser generating device when the in-cavity oscillation reaches a threshold value.

[0007] The transparent medium (204) is a transparent medium with thickness compensation, the cross-sectional shape of the transparent medium with thickness compensation is a right trapezoid, the face where the oblique waist of the right trapezoid is located is the front surface of the transparent medium, the face where the right angle side of the right trapezoid is located is the rear surface of the transparent medium, the included angle of the oblique waist and the right angle side of the right trapezoid is β, when the transparent medium is in an initial position, the width parallel to the median line and intersecting the laser beam in the right trapezoid is the initial thickness D of the transparent medium s0 When the change amount of the rotation angle of the transparent medium is θ, the width parallel to the median line and intersecting the laser beam in the right trapezoid is the thickness D' of the transparent medium, D' = D s0 +Aθ, wherein A = tanβ, θ is the change amount of the rotation angle of the transparent medium; or,

[0008] The front surface of the transparent medium with thickness compensation is curved, the back surface is flat, the cross-sectional shape of the transparent medium is a trapezoid with curved sides, when the rotation angle change of the transparent medium is θ, the width of the trapezoid parallel to the midline and intersecting with the laser beam is the thickness D" of the transparent medium, and D" satisfies the relationship θ=aD" 2 +b, Wherein, θ is the rotation angle change of the transparent medium, a and b are constants;

[0009] When the cross-sectional shape of the transparent medium is a straight trapezoid, the fixed position of the transparent medium in the laser is that the narrow-band interference piece and the high passing through the midpoint of the midline of the straight trapezoid are arranged in parallel in the second medium pool (202), and in the horizontal projection, the narrow-band interference piece and the high passing through the midpoint of the midline of the straight trapezoid are each equal to the axial distance of the rotating table;

[0010] 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 that the narrow-band interference piece and the high passing through the midpoint of the midline of the cross-sectional shape are arranged in parallel in the second medium pool (202), and in the horizontal projection, the narrow-band interference piece and the high passing through the midpoint of the midline of the straight trapezoid are each equal to the axial distance of the rotating table.

[0011] In the present application, the transparent medium is a sheet-shaped cavity, for example, a closed wall made of glass and filled with air inside.

[0012] In the present application, when the high-precision horizontal rotating table rotates, the narrow-band interference piece rotates synchronously with the transparent medium, and the rotation angle θ of the transparent medium and the cavity mode frequency change Δf s Satisfy the following relationship:

[0013]

[0014] The cavity mode frequency f s Satisfy the following relationship:

[0015]

[0016] Wherein, f0 is the cavity mode frequency of the transparent medium at the initial angle α, L is the length from the output light end face of the laser generating device to the front surface of the partial mirror, c is the speed of light, λ0 is the cavity mode wavelength of the transparent medium at the initial angle α, D is the thickness of the transparent medium sheet, n eff水 Is the refractive index of the second medium, and D=D' or D=D".

[0017] Taking the glass sheet-shaped cavity filled with air inside as the transparent medium, the working principle is explained in detail as follows in combination with FIGS. 1-3:

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

[0019] Wherein, n eff水 is the refractive index of the second medium, D is the thickness of the flaky cavity, L1 is the length from the output light surface of the laser to the front surface of the flaky cavity, L2 is the length from the rear surface of the flaky cavity to the front surface of the partial mirror, and θ is the rotation angle of the flaky cavity.

[0020] According to the relationship between the change in the cavity length and the half wavelength, the relationship between the frequency change c / 2L and the change in the cavity length is:

[0021] The relationship between the fluctuation ΔL of the resonant cavity and the amount of change Δf of the cavity mode frequency is: s

[0022]

[0023] Given The relationship between the amount of change Δλ of the cavity mode wavelength and the rotation angle θ of the flaky cavity is:

[0024]

[0025] Wherein, λ0 is the cavity mode wavelength of the flaky cavity when the initial angle is α, and the value thereof is equal to the transmission wavelength λ'0 of the interference film at the angle α.

[0026] When the cross-sectional shape of the flaky cavity is a right trapezoid, let the thickness D' of the flaky cavity be D s0 +Aθ, D s0 is the initial thickness of the flaky cavity, θ is the amount of change in the rotation angle of the flaky cavity, A=tanβ is the slope of the linear thickness compensation, and β is the included angle between the oblique side and the right angle side of the right trapezoid; that is, the amount of change in the cavity mode frequency is:

[0027]

[0028] Therefore, the relationship between the rotation angle θ of the flaky cavity after the clockwise rotation and the cavity mode frequency f s when the initial angle of the flaky cavity is α is as shown in FIG. 1.

[0029]

[0030] Wherein, f0 is the cavity mode frequency of the flaky cavity when the initial angle is α. ​

[0031] Similarly, when the front surface of the sheet-like cavity is curved and the rear surface is flat, and its cross-sectional shape is such that the front side is curved, the thickness D″ of the sheet-like cavity satisfies the relationship Both a and b are constants; that is, the change in the frequency of the rotating cavity mode of the plate-shaped cavity is:

[0032]

[0033] In this case, when the initial angle of the sheet-like cavity is α, the rotation angle θ of the sheet-like cavity is related to the cavity mode frequency f. s 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 narrowband interferometer is at an initial angle α, and n eff2 Let be the refractive index of the narrowband interferometer, where

[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] Since the mode-hopping-free tuning range achievable by narrowband filter external cavity feedback semiconductor lasers through feedforward tuning of the cavity length is approximately 10 GHz, the cavity mode frequency f is required to achieve continuous tunability. s Transmission frequency f at the same angle IF The frequency difference between them should not exceed ±10GHz.

[0042] Compared with the prior art, the present application applies two different transparent media with thickness compensation characteristics to a narrow-band filter external cavity feedback semiconductor laser. The laser emitted from the laser passes through the narrow-band interference piece to obtain narrow-band light, the broadband light emitted by the laser diode is filtered into a narrow-band beam, and after the light feedback of the partial mirror, the measured laser is output by the mirror group when the in-cavity oscillation reaches the threshold. The present application uses transparent media with thickness compensation characteristics to change the cavity length, and the cavity mode frequency f s is subtracted from the transmission frequency f IF to observe the change of the maximum tunable range. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1A and Figure 1B transparent media with thickness compensation;

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

[0045] Wherein: 201, laser diode, 202, second medium pool, 203, narrow-band interference piece, 204, sheet cavity, 205, high-precision horizontal rotating table, 206, rotating table fixing seat, 207, partial mirror, 208, mirror, 209, first half-wave plate;

[0046] Figure 3 is a schematic diagram of the angle change of the sheet cavity in water;

[0047] Figure 4 is the difference between the change of the cavity mode frequency caused by the rotation angle of the sheet cavity with different thickness compensation and the change of the transmission frequency caused by the simultaneous rotation of the interference piece when the initial angle is 10° and 15°;

[0048] Figure 5 is the relationship between the rotation angle of the sheet cavity with different thickness D and f s and the relationship between the different rotation angles of the interference piece and f IF . DETAILED DESCRIPTION

[0049] The following examples are used to non-limitingly explain the technical solutions of the present application.

[0050] Example 1

[0051] As Figure 2The laser shown, working wave band is 780nm. Mainly includes laser diode 201 and second medium pool 202, the second medium pool 202 is filled with saturated sodium chloride solution, narrow band interference sheet 203 and sheet cavity 204 (made of glass, filled with air inside) are fixed in parallel on high-precision horizontal rotating table 205, high-precision horizontal rotating table 205 is inverted above second medium pool 202 (inverted relationship is not drawn in the figure for easy to show), so that narrow band interference sheet 203 and sheet cavity 204 are inserted into second medium pool 202. In horizontal projection position relationship, the rotation axis of narrow band interference sheet 203, sheet cavity 204 and high-precision horizontal rotating table 205 are all located on the light path, the center of narrow band interference sheet 203 and the center of sheet cavity 204 are equidistant from the axis of high-precision horizontal rotating table 205. In height position relationship, narrow band interference sheet 203 and sheet cavity 204 in the second medium can be passed by laser.

[0052] The output light end face of laser diode 201 is close to the front wall of second medium pool 202, and partial mirror 207 is arranged on the rear wall of second medium pool 202. At this time, there is an initial angle α between the center line of sheet cavity 204 and the direction of laser as shown in Figure 3

[0053] The cross-sectional shape of sheet cavity 204 is shown in Figure 1A , which is a right trapezoid, and the plane where the right angle side is located is its rear surface.

[0054] When working, the light output by 780nm laser diode is collimated, filtered by narrow band interference sheet, then passes through sheet cavity, and reaches partial mirror to be reflected. When the oscillation in the resonant cavity reaches the threshold, laser is output by partial mirror. By rotating interference sheet and sheet cavity at the same time, the center frequency of interference sheet transmission peak and the cavity mode frequency are changed synchronously, and the output frequency is continuously tunable in a large range.

[0055] In this embodiment, the cavity length is set to 10cm, the initial angle is α=10°, λ0=780nm, the sheet cavity with different angles between front surface and rear surface is selected, the thickness of sheet cavity is designed as D'=0.0685+θ×A, and the f s -f IF curve caused by the rotation angle θ of sheet cavity under different A values is calculated as shown in Figure 4 (a).

[0056]

[0057] From Figure 4 (a), it can be obtained that when the value of A is selected as 0.00025, f g -f IF ​The angle variation range is relatively large under conditions not exceeding ±10GHz. For example... Figure 5 As shown in (a), we plot the cavity mode frequencies f when the optical element thicknesses D = D′ = 0.0685 + 0.00025θ and D = 0.0685 m, respectively. s and transmission frequency f IF Observe 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, under the condition that the frequency difference between the cavity mode frequency and the transmission frequency at the same angle does not exceed ±10 GHz, when the sheet cavity adopts the original structure (i.e., its front and rear surfaces are parallel), and the sheet cavity thickness is D = 0.0685 m, the corresponding angle variation range is -3.16° to 3.2°, and the corresponding tunable laser frequency range is 3.871 × 10⁻⁶. 14 Up to 3.853×10 14 Hz (according to) Figure 5 (a) The green dashed line indicates that the adjustable range is 1.8 × 10⁻⁶. 12 Hz. When using a sheet-like cavity with an angle between the front and rear surfaces, for example, when the thickness is chosen as D′=0.0685+0.00025θ, when f S -f IF Not exceeding -1.0×10 10 ~1.0×10 10 The laser frequency is Hz, with an angle variation range of -6.87° to 7.51°, corresponding to a tunable laser frequency range of 3.887 × 10⁻⁶ Hz. 14 Up to 3.846×10 14 Hz (according to) Figure 5 (a) The red dashed line indicates that the adjustable value is 4.1 × 10⁻⁶. 12 Hz, significantly greater than the continuously tunable range achievable using conventional sheet-like cavities.

[0059] Similarly, when the initial angle is set to α = 15° and D′ = 0.0655 + θ × A, the effect of f caused by rotating the transparent solid medium at a rotation angle of θ is calculated for different values ​​of A. s -f IF Curves, such as Figure 4 As shown in (b).

[0060] pass Figure 4 (b) It can be seen that when the value of A is chosen to be 0.00023, f is satisfied. g -f IF The angle variation range is relatively large under conditions not exceeding ±10GHz. For example... Figure 5(b) shows the cavity mode frequencies f when the optical element thicknesses D = D′ = 0.0655 + 0.00023θ and D = 0.0655 m, respectively. s and transmission frequency f IF Observe the tunability of the laser.

[0061] The results show that, under the condition that the frequency difference between the cavity mode frequency and the transmission frequency at the same angle does not exceed ±10 GHz, when the sheet cavity adopts the original structure (i.e., its front and rear surfaces are parallel), for a sheet cavity with a thickness of D = 0.0655 (meters), the angle variation range is -1.91° to 2.31°, and the corresponding tunable laser frequency range is 3.887 × 10⁻⁶. 14 Up to 3.869×10 14 Hz (according to) Figure 5 (b) The blue dashed line indicates that the adjustable range is 1.8 × 10⁻⁶. 12 Hz. However, when using a sheet-like cavity with an angle between the front and rear surfaces, for example, with a thickness of D′=0.0655+0.00023θ, the corresponding angle variation under the same conditions is -2.89°~11.73°, and the corresponding tunable laser frequency range is 3.892×10⁻⁶ Hz. 14 Up to 3.847×10 14 Hz (according to) Figure 5 (b) The pink dashed line indicates that the adjustable range is 4.5 × 10⁻⁶. 12 Hz. Significantly greater than the continuously tunable range achievable using conventional sheet-like cavities.

Claims

1. A widely continuously tunable laser, the laser comprising a laser generating device and a mirror (208) arranged in sequence in an optical path, an exit light end surface of the laser generating device being provided with a second medium pool (202) filled with a second medium, front and back walls of the second medium pool (202) being transparent and flat walls and being perpendicular to the optical path, the second medium having a refractive index greater than that of vacuum, a narrow-band interference sheet (203) and a transparent medium (204) being fixedly connected by a high-precision horizontal rotating table (205) and being arranged in parallel in the second medium pool (202), a partial mirror (207) being arranged outside a back wall of the second medium pool (202), the narrow-band interference sheet (203), the transparent medium (204) and the partial mirror (207) all being in the optical path; light output by the laser generating device enters the second medium pool (202) from the front wall, is filtered to remove out-of-band light after passing through the narrow-band interference sheet (203), and narrow-band light is obtained, the narrow-band light exits the second medium pool (202) from the back wall after passing through the transparent medium (204), is reflected by the partial mirror (207) and returns to the laser generating device, and when intracavity oscillation reaches a threshold, measured laser is output by the partial mirror (207); characterized in that The transparent medium (204) is a transparent medium with thickness compensation, a cross-sectional shape of the transparent medium with thickness compensation is a right trapezoid, a face where a sloping leg of the right trapezoid is located is a front surface of the transparent medium, a face where a right angle leg of the right trapezoid is located is a back surface of the transparent medium, an included angle of the sloping leg and the right angle leg of the right trapezoid is β, and a width, which is parallel to a median line and intersects with a laser beam, in the right trapezoid is an initial thickness D of the transparent medium when the transparent medium is in an initial position s0 A width, which is parallel to a median line and intersects with a laser beam, in the right trapezoid is a thickness D' of the transparent medium when a rotation angle change amount of the transparent medium is θ, D' = D s0 +Aθ, wherein A = tanβ, θ is the rotation angle change amount of the transparent medium; or, The front surface of the transparent medium with thickness compensation is curved, the back surface is flat, the cross-sectional shape of the transparent medium is curved on the front side, when the rotation angle variation of the transparent medium is θ, the width parallel to the median line and intersecting with the laser beam in the cross-sectional shape is the thickness D" of the transparent medium, D" satisfies the relationship θ=aD"+b 2 +b, Wherein, θ is the rotation angle variation of the transparent medium, a and b are both constants; when a cross-sectional shape of the transparent medium is a right trapezoid, a fixed position of the transparent medium in the laser is such that the narrow-band interference sheet and a height passing through a midpoint of a median line of the right trapezoid are arranged in parallel in the second medium pool (202), and in a horizontal projection, the narrow-band interference sheet and the height passing through the midpoint of the median line of the right trapezoid each have an equal distance from an axis of the rotating table; when the transparent medium with thickness compensation has a curved front surface and a flat back surface, a fixed position of the transparent medium in the laser is such that the narrow-band interference sheet and a height passing through a midpoint of a median line of the cross-sectional shape are arranged in parallel in the second medium pool (202), and in a horizontal projection, the narrow-band interference sheet and the height passing through the midpoint of the median line each have an equal distance from an axis of the rotating table; When the high-precision horizontal rotating table rotates, the narrow-band interference plate rotates synchronously with the transparent medium, and the rotation angle θ of the transparent medium and the change amount Δf of the cavity mode frequency s satisfies the following relationship: Resonator cavity mode frequency f s satisfies the following relation: where f0is the cavity mode frequency of the transparent medium at the initial angle a, L is the length from the output facet of the laser generating device to the front surface of the partial reflector, c is the speed of light, λ0is the cavity mode wavelength of the transparent medium at the initial angle a, D is the thickness of the transparent medium, n eff水 is the refractive index of the second medium, and D = D' or D = D".

2. The laser of claim 1, wherein the transparent medium is a sheet-shaped cavity.

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