Application of thickness-compensated optical medium in wide-range continuous tunable laser

Abstract

The invention provides a wide-range continuous tunable laser, a transparent medium with thickness compensation is arranged in the laser, the cross section of the transparent medium with thickness compensation is in the shape of a right trapezoid, or the front surface of the transparent medium is a curved surface, and the rear surface of the transparent medium is a plane. The influence of two different transparent media with thickness compensation characteristics on the tunable range of the laser is verified. And laser emitted from the laser passes through the narrow-band interference sheet, is fed back by the partial reflecting mirror and then is output. According to the invention, the cavity length of the laser is changed by adopting the transparent medium with the thickness compensation characteristic, the difference between the cavity mode frequency and the transmission frequency fIF is obtained, and the change of the maximum tunable range of the laser is observed.

Description

Technical Field

[0001] The present invention relates to a laser, and particularly to a widely tunable continuous laser with an optical medium having thickness compensation. Background Art

[0002] With the continuous development of laser technology, external cavity semiconductor lasers have been widely used in fields such as precision manufacturing, lidar, and optical fiber communication due to their wavelength tunability and frequency stability. However, there are still many limitations in achieving wide-range continuous tunability in existing external cavity semiconductor lasers. 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, resulting in limited continuity of wavelength tuning. In addition, it is difficult to achieve stable single-mode output in a large tuning range in the prior art, and the tuning process is complex and vulnerable to external factors such as temperature drift and mechanical vibration, thus affecting the frequency stability and accuracy of the laser. These limitations make it difficult for current external cavity semiconductor lasers to meet the application requirements with extremely high demands for frequency stability and continuous tunability, such as high-resolution spectral measurement and precision interferometric measurement. Therefore, improving the frequency tuning range of lasers requires not only breakthroughs in equipment technology but also 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. Among them, a compensation medium with a specific thickness and refractive index is placed in the resonant cavity, and it rotates simultaneously with the interference film. While performing interference frequency selection, the cavity length is changed, so that the center frequency of the transmission peak of the interference film and the cavity mode frequency change simultaneously. This technology realizes high-precision continuous tuning of the laser frequency. In an IF ECDL, only by feed-forward tuning the cavity length, the mode-hop-free tuning range can reach 10 GHz. Therefore, to ensure continuous tuning, the frequency difference between the cavity mode frequency and the transmission frequency at the same angle should not exceed 10 GHz. 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 narrowband interference film, thus restricting the range of simultaneous change of the cavity mode frequency and the center frequency of the transmission peak of the interference film, that is, restricting the angle measurement range of the device. The limited compensation effect of the compensation medium with a constant thickness on the cavity mode frequency makes it a current technical difficulty to obtain a laser with a larger continuously adjustable range. How to improve the physical structure of the compensation medium to increase the tunable range is an important research direction at present. Summary of the Invention

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

[0005] The idea of the present invention is to apply an optical medium (transparent medium) with thickness compensation to a laser. By utilizing the linear or non-linear relationship between the thickness of the transparent medium and the angle of the optical path, the optical path length of the laser passing through the transparent medium at different angles is changed, that is, the cavity length of the resonant cavity. By selecting the linear or non-linear parameters of the angle between the thickness of the transparent medium and the optical path to control the change amount of the resonant cavity length, the change in the cavity mode frequency caused by the change in the resonant cavity length is synchronized with the change in the center frequency of the transmission peak of the interference filter caused by the rotation of the interference filter, ensuring that the laser obtains a continuously varying laser frequency and expanding the mode-hop-free tuning range.

[0006] To this end, the present invention provides a widely tunable continuous laser. The laser includes a laser generating device and a reflector (208) sequentially arranged on the optical path. A second medium cell (202) filled with a second medium is arranged on the output light end face of the laser generating device. The front wall and the rear wall of the second medium cell (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 narrowband interference filter (203) and a transparent medium (204) are fixedly connected through a high-precision horizontal rotating table (205). The narrowband interference filter (203) and the transparent medium (204) are arranged in parallel in the second medium cell (202). A partial reflector (207) is arranged on the outer side surface of the rear wall of the second medium cell (202). The narrowband interference filter (203), the transparent medium (204), and the partial reflector (207) are all on the optical path. The light output by the laser generating device enters the second medium cell (202) from the front wall, and the out-of-band light is filtered out after passing through the narrowband interference filter (203) to obtain narrowband light. The narrowband light leaves the second medium cell (202) from the rear wall after passing through the transparent medium (204), and is reflected by the partial reflector (207) and then returns to the laser generating device. When the oscillation in the cavity reaches the threshold, laser light is output by the partial reflector (207).

[0007] Wherein, 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 surface where the oblique waist of the right trapezoid is located is the front surface of the transparent medium, and the surface where the right-angled side of the right trapezoid is located is the rear surface of the transparent medium. The included angle between the oblique waist and the right-angled side of the right trapezoid is β. When the transparent medium is in the initial position, the width parallel to the median line and intersecting with 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 with the laser beam in the right trapezoid is the thickness D' of the transparent medium, D' = D s0 + Aθ, where A = tanβ and 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 a curved surface, and the rear surface is a plane. The cross-sectional shape of the transparent medium has a curved front side. When the rotation angle change of the transparent medium is θ, the width parallel to the median line and intersecting with the laser beam within the cross-sectional shape is the thickness D″ of the transparent medium, and D″ satisfies the relationship θ = aD″ 2 +b, where θ is the rotation angle change of the transparent medium, and both a and b are constants;

[0009] 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 narrowband interference filter and the height passing through the midpoint of the median line of the right trapezoid are arranged in parallel in the second medium cell (202), and in the horizontal projection, the distances of the narrowband interference filter and the height passing through the midpoint of the median line of the right trapezoid from the axis of the rotating table are equal;

[0010] When the front surface of the transparent medium with thickness compensation is a curved surface and the rear surface is a plane, the fixed position of the transparent medium in the laser is such that the narrowband interference filter and the height passing through the midpoint of the median line of the cross-sectional shape are arranged in parallel in the second medium cell (202), and in the horizontal projection, the distances of the narrowband interference filter and the height passing through the midpoint of the median line of the right trapezoid from the axis of the rotating table are equal.

[0011] In the present invention, the transparent medium is a sheet-shaped cavity, such as a sealed wall made of glass and filled with air inside.

[0012] In the present invention, when the high-precision horizontal rotating table rotates, the narrowband interference filter 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 satisfy the following relationship:

[0013]

[0014] The cavity mode frequency f of the resonant cavity s satisfies the following relationship:

[0015]

[0016] where 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, and 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 as the transparent medium as an example, the working principle is specifically explained as follows with reference to FIGS. 1-3:

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

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

[0020] According to the relationship that when the cavity length changes by half a wavelength, the frequency changes by c / 2L:

[0021] The relationship between the resonator jitter ΔL and the change in cavity mode frequency Δf s is:

[0022]

[0023] Given The relationship between the change in cavity mode wavelength Δλ and the rotation angle θ of the sheet-shaped cavity satisfies the following:

[0024]

[0025] where, λ0 is the cavity mode wavelength of the sheet-shaped cavity when the initial angle is α, and its value is equal to the transmission wavelength λ′0 of the interference sheet at angle α.

[0026] When the cross-sectional shape of the sheet-shaped cavity is a right trapezoid, let the thickness D′ of the sheet-shaped cavity = D s0 + Aθ, D s0 is the initial thickness of the sheet-shaped cavity, θ is the change in the rotation angle of the sheet-shaped cavity, A = tanβ is the slope of the linear thickness compensation, and β is the included angle between the inclined waist and the right-angled side of the right trapezoid; that is, the change in cavity mode frequency as the sheet-shaped cavity rotates is:

[0027]

[0028] Therefore, when the sheet-shaped cavity is placed at the initial angle α as shown in Figure 1, the relationship between the rotation angle θ after the sheet-shaped cavity rotates clockwise and the cavity mode frequency f s is:

[0029]

[0030] where, f0 is the cavity mode frequency of the sheet-shaped cavity when the initial angle is α.

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

[0032]

[0033] In this case, when the initial angle of the sheet-shaped cavity is α, the relationship between the rotational angle θ of the sheet-shaped cavity and the cavity mode frequency f s is:

[0034]

[0035] On the other hand, in the present invention, the rotational angle of the narrowband interference filter and the transmitted wavelength λ IF are related as follows:

[0036]

[0037] where λ′0 is the transmitted wavelength when the narrowband interference filter is at the initial angle α, and n eff2 is the refractive index of the narrowband interference filter, where

[0038] According to the formula the rotational angle of the narrowband interference filter and the transmitted frequency f IF are related as

[0039]

[0040] In the present invention, the laser generating device is a conventional setting in the art, and generally includes a laser diode and a first collimating lens sequentially arranged on the optical path. Among them, the working band of the laser diode can be any band. Preferably, from the perspective of facilitating acquisition or saving economy, a 780 nm band laser diode can be used. Those skilled in the art can also select laser diodes with other working wavelengths, such as 420 nm, 850 nm, 1550 nm laser diodes, etc.

[0041] Since the tunable range without mode hopping that can be achieved by the external cavity feedback semiconductor laser with a narrowband filter by tuning the cavity length through feedforward is about 10 GHz. Therefore, in order to achieve continuous tunability, the frequency difference between the cavity mode frequency f s and the transmitted frequency f IF at the same angle should not exceed ±10 GHz.

[0042] Compared with the prior art, the present invention applies two different transparent media with thickness compensation characteristics to an external cavity feedback semiconductor laser of a narrowband filter. The laser emitted from the laser passes through a narrowband interference film to obtain narrowband light, filtering the broadband light emitted by the laser diode into a narrowband light beam. After optical feedback by a partial mirror, when the oscillation in the cavity reaches the threshold, the laser to be measured is output by the mirror group. The present invention uses a transparent medium 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 in the maximum tunable range. Description of the Drawings

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

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

[0045] Wherein: 201, laser diode, 202, second medium cell, 203, narrowband interference film, 204, sheet-shaped 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-shaped cavity in water;

[0047] Figure 4 is the difference relationship between the change in the cavity mode frequency caused by the rotation angle of the sheet-shaped cavity with different thickness compensations and the change in the transmission frequency caused by the simultaneous rotation of the interference film when the initial angles are 10° and 15°;

[0048] Figure 5 is the relationship between the rotation angle of the sheet-shaped cavity with different thicknesses D and f s and the relationship between the different rotation angles of the interference film and f IF ; Detailed Embodiment

[0049] The following embodiments are used to explain the technical solution of the present invention non-restrictively.

[0050] Embodiment 1

[0051] As Figure 2The laser shown has a working wavelength band of 780 nm. It mainly includes a laser diode 201 and a second medium cell 202. The second medium cell 202 is filled with saturated sodium chloride solution. A narrowband interference filter 203 and a sheet-shaped cavity 204 (made of glass and filled with air) are fixedly arranged in parallel on a high-precision horizontal rotating stage 205. The high-precision horizontal rotating stage 205 is inverted above the second medium cell 202 (the inversion relationship is not shown in the figure for the sake of simplicity), so that the narrowband interference filter 203 and the sheet-shaped cavity 204 are inserted into the second medium cell 202. In the horizontal projection position relationship, the rotation axes of the narrowband interference filter 203, the sheet-shaped cavity 204, and the high-precision horizontal rotating stage 205 are all located on the optical path, and the distances from the center of the narrowband interference filter 203 and the center of the sheet-shaped cavity 204 to the axis of the high-precision horizontal rotating stage 205 are equal. In the height position relationship, the narrowband interference filter 203 and the sheet-shaped cavity 204 in the second medium can both be penetrated by the laser.

[0052] The output optical end face of the laser diode 201 is closely attached to the front wall surface of the second medium cell 202, and a partial mirror 207 is arranged on the rear wall surface of the second medium cell 202. At this time, there is an initial angle α between the median line of the sheet-shaped cavity 204 and the laser direction as Figure 3 shown.

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

[0054] During operation, the light output by the 780 nm laser diode is collimated, the out-of-band light is filtered by the narrowband interference filter, and then passes through the sheet-shaped cavity and reaches the partial mirror to be reflected. When the oscillation in the resonant cavity reaches the threshold, the laser is output by the partial mirror. By simultaneously rotating the interference filter and the sheet-shaped cavity, the center frequency of the transmission peak of the interference filter and the cavity mode frequency can be synchronized to change, realizing a wide range of continuously tunable output frequencies.

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

[0056]

[0057] Through Figure 4 (a), it can be obtained that when the value of A is selected as 0.00025, it satisfies f g -f IFThe angular variation range under the condition of not exceeding ±10 GHz is relatively large. For example, Figure 5 As shown in (a), we respectively plot the cavity mode frequency f when the thickness of the optical element D = D' = 0.0685 + 0.00025θ and D = 0.0685 m, s , and the transmission frequency f IF , and observe the laser tunability. The abscissa is the change in θ, and the ordinate is 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, that is, its front surface is parallel to the rear surface, for the sheet cavity with a thickness of D = 0.0685 m, the corresponding angular variation range is -3.16° to 3.2°, and the corresponding laser frequency tunability range is from 3.871×10 14 to 3.853×10 14 Hz (according to the green dashed line in Figure 5 (a)), and the tunability is 1.8×10 12 Hz. When using a sheet cavity with an included angle between the front surface and the rear surface, for example, when the thickness is selected as D' = 0.0685 + 0.00025θ, when f S -f IF does not exceed -1.0×10 10 to 1.0×10 10 Hz, the angular variation range is -6.87° to 7.51°, and the corresponding laser frequency tunability range is from 3.887×10 14 to 3.846×10 14 Hz (according to the red dashed line in Figure 5 (a)), and the tunability is 4.1×10 12 Hz, which is significantly larger than the continuously tunable range achieved when using a conventional sheet cavity.

[0059] Similarly, when the initial angle is set to α = 15° and D' = 0.0655 + θ×A, and different values of A are calculated, the curves of f s -f IF caused by the rotation angle θ of each transparent solid medium are plotted, as shown in Figure 4 (b).

[0060] From Figure 4 (b), it can be obtained that when the value of A is selected as 0.00023, the angular variation range that satisfies the condition of f g -f IF not exceeding ±10 GHz is relatively large. For example, Figure 5(b), we respectively plot the cavity mode frequency \(f\) when the thickness of the optical element \(D = D'=0.0655 + 0.00023\theta\) and \(D = 0.0655m\), s , as well as the transmission frequency \(f\) IF , and observe the laser tunability.

[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 \(\pm10GHz\), when the sheet cavity adopts the original structure, that is, its front surface and back surface are parallel, for a sheet cavity with a thickness \(D = 0.0655m\), the angle change range is \(-1.91^{\circ}\) to \(2.31^{\circ}\), and the corresponding laser frequency tunability range is from \(3.887\times10\) 14 to \(3.869\times10\) 14 Hz (according to the blue dashed line in Figure 5 (b)), and the tunability is \(1.8\times10\) 12 Hz. When using a sheet cavity with an included angle between the front surface and the back surface, for example, when the thickness is selected as \(D'=0.0655 + 0.00023\theta\), under the same conditions, the corresponding angle change amount is \(-2.89^{\circ}\) to \(11.73^{\circ}\), and the corresponding laser frequency tunability range is from \(3.892\times10\) 14 to \(3.847\times10\) 14 Hz (according to the pink dashed line in Figure 5 (b)), and the tunability is \(4.5\times10\) 12 Hz. It is significantly larger than the continuously tunable range achieved when using a conventional sheet cavity.

Claims

1. A large-range continuously tunable laser, which comprises a laser generating device and a reflector (208) arranged in sequence on the optical path. An end face of the laser generating device for emitting light is provided with a second medium cell (202) filled with a second medium. The front wall and the rear wall of the second medium cell (202) are transparent and flat walls and are perpendicular to the optical path. The refractive index of the second medium is greater than that of vacuum. A narrow-band interference filter (203) and a transparent medium (204) are fixedly connected through a high-precision horizontal rotating table (205), and the narrow-band interference filter (203) and the transparent medium (204) are arranged in parallel in the second medium cell (202). A partial reflector (207) is arranged on the outer side face of the rear wall of the second medium cell (202). The narrow-band interference filter (203), the transparent medium (204) and the partial reflector (207) are all on the optical path. The light output by the laser generating device enters the second medium cell (202) from the front wall, and the out-of-band light is filtered out after passing through the narrow-band interference filter (203) to obtain narrow-band light. The narrow-band light leaves the second medium cell (202) from the rear wall after passing through the transparent medium (204), is reflected by the partial reflector (207) and then returns to the laser generating device. When the intracavity oscillation reaches the threshold, the measured laser is output by the partial reflector (207). Characterized in that 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 surface where the oblique waist of the right trapezoid is located is the front surface of the transparent medium, and the surface where the right-angled side of the right trapezoid is located is the rear surface of the transparent medium. The included angle between the oblique waist and the right-angled side of the right trapezoid is β. When the transparent medium is in the initial position, the width parallel to the median line and intersecting with the laser beam within the right trapezoid is the initial thickness D of the transparent medium s0 , when the rotation angle change of the transparent medium is θ, the width parallel to the median line and intersecting with the laser beam within the right trapezoid is the thickness D' of the transparent medium, and D' = D s0 +Aθ, where A = tanβ and θ is the rotation angle change of the transparent medium; or, The front surface of the transparent medium with thickness compensation is a curved surface, and the rear surface is a plane. The cross-sectional shape of the transparent medium has a curved front side. When the rotation angle change of the transparent medium is θ, the width parallel to the median line and intersecting with the laser beam within the cross-sectional shape is the thickness D″ of the transparent medium, and D″ satisfies the relationship θ = aD″ 2 +b, where θ is the rotation angle change of the transparent medium, and both a and b are constants; 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 narrow-band interference filter and the height passing through the midpoint of the median line of the right trapezoid are arranged in parallel in the second medium cell (202), and in the horizontal projection, the distances of the narrow-band interference filter and the height passing through the midpoint of the median line of the right trapezoid from the axis of the rotating table are equal to each other. When the front surface of the transparent medium with thickness compensation is a curved surface and the rear surface is a plane, the fixed position of the transparent medium in the laser is such that the narrow-band interference filter and the height passing through the midpoint of the median line of the cross-sectional shape are arranged in parallel in the second medium cell (202), and in the horizontal projection, the distances of the narrow-band interference filter and the height passing through the midpoint of the median line of the right trapezoid from the axis of the rotating table are equal to each other.

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

3. The laser according to claim 1, characterized in that When the high-precision horizontal rotating table rotates, the narrowband interference filter 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 satisfy the following relationship: The resonant cavity mode frequency f s Satisfies the following relationship: Among them, f0 is the cavity mode frequency of the transparent medium at the initial angle α, L is the length from the output optical 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, and n eff水 is the refractive index of the second medium, and D = D′ or D = D″.

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