A Littrow type cavity length compensation jumpless mode continuous tunable semiconductor laser

Abstract

This invention discloses a Littrow-type cavity length-compensated mode-hopping-free continuously tunable semiconductor laser, comprising a laser diode (1), a collimating lens (2), and a diffraction grating (3). Mode-hopping-free continuous tunability is achieved through a micro-elastic material base (4). The micro-elastic material base (4) is hollowed out to create a movable block (401) that can be driven by a piezoelectric ceramic (5), an isosceles triangular groove (404), and a pair of waist arms (406), enabling micro-displacement and micro-angle changes in the waist arms (406), thereby achieving continuous tunability of the output laser. This invention achieves mode-hopping-free continuous tunability of the laser output through an innovative combination of a special mechanical structure and the frequency selection characteristics of the grating. The structure is compact and highly stable, ultimately resulting in a mode-hopping-free continuously tunable external cavity semiconductor laser that meets the requirements of high-end precision measurement.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor laser technology, specifically relating to a Littrow type cavity length compensated mode-skipping continuously tunable semiconductor laser. Background Technology

[0002] With the continuous development of laser technology, external cavity semiconductor lasers have irreplaceable application value in high-end fields such as precision sensing, spectral analysis, laser communication and quantum optics due to their advantages such as narrow linewidth, high side-mode suppression ratio and wide wavelength tuning range.

[0003] In existing related technologies, there are external cavity laser solutions with interferometers as the core components (such as Chinese invention patent applications CN119394219A and CN114899704A, which use a compensation medium and an interferometer to rotate together). However, such solutions all have defects such as limited tuning range and easy mode skipping.

[0004] The inventors of this invention previously proposed a Littrow-shaped external cavity semiconductor laser scheme using a diffraction grating as the wavelength selection element. However, existing Littrow-shaped tunable lasers using diffraction gratings still commonly suffer from "mode hopping". Traditional tuning mechanisms often adjust the diffraction grating angle or cavity length in a single dimension, or although they attempt to adjust in multiple dimensions, they cannot achieve precise synchronization. This results in a relative shift between the external cavity mode wavelength and the first-order diffraction wavelength of the grating during tuning. Once the shift exceeds the longitudinal mode interval, mode hopping occurs, making it impossible to output a stable and continuous wavelength, which is difficult to meet the stringent requirements of high-precision sensing fields.

[0005] Therefore, designing a mechanical control structure that can achieve real-time and precise synchronous matching between the cavity mode wavelength and the grating diffraction wavelength, and constructing a mode-skipping, continuously tunable external cavity semiconductor laser, is a key problem that urgently needs to be solved in this field. Summary of the Invention

[0006] This invention addresses the "mode hopping" phenomenon still present in Littrow-type tunable lasers using diffraction gratings. It aims to propose a solution based on a synchronous displacement and rotation control base, while still using an external cavity structure with a diffraction grating as the wavelength selection element. By synchronously driving the displacement and rotation of the diffraction grating through an integrated structure, the cavity mode wavelength and the grating diffraction wavelength are synchronously matched, thus realizing a new mode-hopping-free continuously tunable semiconductor laser.

[0007] The design concept of this invention is to construct a new synchronous displacement and rotation drive mechanism, and integrate the laser diode and the diffraction grating on the mechanism. Through the controllable elastic deformation of the mechanical structure within the synchronous displacement and rotation drive mechanism, the synchronous matching of the external cavity mode wavelength and the first-order diffraction wavelength of the diffraction grating is achieved, so that the laser output frequency change is continuously tunable, and finally a mode-skipping, continuously tunable external cavity semiconductor laser is realized.

[0008] Based on the above ideas, this invention provides a Littrow-type cavity length compensated mode-skipping continuous tunable semiconductor laser. The laser includes a laser diode 1, a collimating lens 2, and a diffraction grating 3 arranged sequentially in the optical path. The laser includes a micro-elastic material base 4. The micro-elastic material base 4 is hollowed out to form a movable block 401. The movement range of the movable block 401 is limited by a movable block groove 403. A connected piezoelectric ceramic groove 405 is provided at the rear end of the movable block groove 403. An isosceles triangular groove 404 is provided at the front end of the movable block groove 403. Two waist arms 406 are machined along the waist of the isosceles triangle in the isosceles triangular groove 404. Each waist arm 406 is connected to the micro-elastic material base 4 through its respective waist arm fulcrum 407. The waist arm fulcrum 407 is located at the base angle of the isosceles triangle. The rear ends of the waist arms 406 are fixedly connected to the front ends of the movable block 401.

[0009] The diffraction grating 3 is mounted on the rear end of a waist arm 406 via a grating base 6, and the horizontal projection of the diffraction grating 3 and the waist arm 406 are on the same plane. A piezoelectric ceramic 5 is installed in a piezoelectric ceramic groove 405. The front end of the piezoelectric ceramic 5 abuts against the moving block 401 and the rear end abuts against the micro-elastic material base 4. The laser diode 1 is installed at the midpoint O of the bottom edge of the isosceles triangular groove 404, and the diffraction grating 3 is located in the optical path of the emitted light of the laser diode 1.

[0010] The light emitted from laser diode 1 is collimated into parallel light by collimating lens 2 and then directed towards diffraction grating 3, resulting in 0th-order and 1st-order diffraction light. The diffraction angle of the 1st-order diffraction light is equal to the incident angle. The 1st-order diffraction light returns along the original optical path to laser diode 1 and oscillates. When the 1st-order diffraction wavelength of diffraction grating 3... cavity mode wavelength When the two parts are matched and the intracavity oscillation reaches the threshold, an outer cavity is formed, at which point the 0th order diffracted light is output as laser light.

[0011] In this invention, for ease of explanation and not limitation, the direction toward the laser diode in the microelastic material base is referred to as "front" and the direction toward the piezoelectric ceramic is referred to as "back".

[0012] In this invention, the first-order diffraction wavelength of the diffraction grating The Littrow condition is always satisfied: ,in The grating constant of the diffraction grating is . It is the angle at which the laser enters the diffraction grating. Cavity mode wavelength. Always satisfied , where L is the length of the external cavity (the distance from the laser emission end to the surface of the diffraction grating), and q is the longitudinal mode number.

[0013] In this invention, the mode-skipping-free continuous tunability of the laser refers to the ability of the first-order diffraction wavelength of the grating and the cavity mode wavelength to remain synchronously matched when the piezoelectric ceramic 5 deforms and pushes the moving block 401 to move a micro-distance within the moving block slot 403. Specifically, when the moving block 401 moves a micro-distance under the forward force of the piezoelectric ceramic 5 (in this invention, the micro-distance movement is typically on the order of tens of micrometers to millimeters), a pair of waist arms 406 connected to the moving block 401 undergo a slight positional change around the waist arm fulcrum 407. Consequently, the diffraction grating 3 fixed on the waist arm 406 undergoes translation along the optical path and angular deflection around the incident point, causing the external cavity length L and the grating incident angle to change synchronously and continuously. This ensures that the first-order diffraction wavelength of the diffraction grating and the cavity mode wavelength remain equal and change synchronously, achieving mode-skipping-free continuous tunability of the output laser.

[0014] In this invention, to maximize the continuously tunable range, the diffraction grating 3 needs to be positioned on the laser emission beam, and the position of the laser incident point on the diffraction grating remains unchanged during the movement of the diffraction grating 3 driven by the micro-elastic material base 4. The laser emission endpoint is defined as point O, and the point on the surface of the diffraction grating is defined as point A.

[0015] In this invention, a micro-elastic material base 4 enables continuous and tunable output laser without mode skipping. The base can be made of rigid metal (such as readily available metals like copper, iron, and stainless steel) and utilizes the inherent elasticity of the metal. Through hollowing out, components such as a moving block 401, a moving block groove 403, an isosceles triangular groove 404, and a pair of waist arms 406 are constructed. These components can undergo micro-distance movement under force within the base. A piezoelectric ceramic 5 is installed in the piezoelectric ceramic groove 405. When the piezoelectric ceramic 5 is energized, it pushes the moving block 401 forward, causing the pair of waist arms 406 to change position around their fulcrum. Figure 3 As shown. The laser emission point is always set at point O. To ensure that the laser is incident on point A, the grating base 6 can be installed and fixed above the rear end of the waist arm 406, and the diffraction grating 3 can be fixed on the grating base 6, ensuring that the diffraction grating 3 and the emitted light are at the same horizontal height.

[0016] In this invention, point A of the isosceles triangular groove 404 is the vertex of the isosceles triangle, point O is the foot of the perpendicular from point A to the base, and point B is an endpoint of the base. Line segments AB, AO, and OB constitute the waist, height, and base of the triangle, respectively. In the following description, these will be used to refer to the waist arm, the optical axis reference, and the base.

[0017] In this invention, in order to better control the position of the diffraction grating, the waist arm where AB is located is the core part for controlling the diffraction grating 3. The waist arm fulcrum 407 can be processed into a shape that facilitates the realization of the fulcrum function (for example, the waist arm fulcrum can be processed into a style with a cross-sectional area smaller than that of the waist arm), ensuring that the waist arm 406 only undergoes a small positional change when subjected to force and always keeps the waist arm 406 in a straight state, avoiding bending.

[0018] In this invention, when the piezoelectric ceramic 5 is energized, the fulcrum 407 of the waist arm undergoes slight elastic deformation under force, while the waist arm 406 maintains its shape, thus changing the position of the diffraction grating. Specifically, the deformation of the fulcrum 407 causes point A to move along the AO direction to point A', corresponding to a displacement of the diffraction grating, achieving continuous adjustment of the resonant cavity length L; simultaneously, the waist arm 406 undergoes a micro-angular position change, with B moving to point B' (due to the small deformation amplitude, typically on the order of micrometers, it is approximately a linear movement), causing the diffraction grating fixed on the AB arm to deflect at an angle, achieving an incident angle... Continuous changes, such as Figure 3 As shown.

[0019] In this invention, the frequency selection parameters of the diffraction grating 3 must match the operating frequency of the laser diode 1. After the laser diode 1 emits a coherent beam, it is collimated into a parallel beam by the collimating lens 2. This parallel beam is incident on the diffraction grating 3, and through the dispersion and frequency selection of the grating, laser light of a specific wavelength is filtered out and reflected back into the laser diode 1, forming optical resonance and laser oscillation. By driving the piezoelectric ceramic 5, the waist arm 406 is subjected to force and undergoes a micro-position change around the fulcrum, and the diffraction grating 3 undergoes synchronous displacement and angular rotation accordingly. During this process, the first-order diffraction wavelength of the grating and the cavity mode wavelength of the resonant cavity are synchronously adjusted, thereby completing continuous wavelength tuning. When the optical oscillation in the resonant cavity reaches the lasing threshold, a stable laser with continuously tunable wavelength is finally reflected out by the diffraction grating 3.

[0020] In a preferred embodiment, the movable block 401 is connected to the micro-elastic material base 4 via at least one swing fulcrum 402. Particularly preferably, a pair of symmetrically arranged swing fulcrums 402 are used to connect the movable block 401 to the micro-elastic material base 4.

[0021] In this invention, the micro-elastic material is a metal, such as copper, iron, stainless steel, or other commercially available and easily processed metal materials.

[0022] In this invention, when the moving block 401 moves back and forth within the moving block slot 403, causing a change in the incident angle, the first-order diffraction wavelength... With cavity mode wavelength It changes synchronously with the angle and achieves continuous, mode-free tuning.

[0023] In a preferred embodiment, the laser diode 1 has an anti-reflection coating on its emitting light end face.

[0024] This invention also provides a method for measuring minute displacements based on a Littrow-type cavity length-compensated, mode-skipping-free, continuously tunable semiconductor laser. The principle behind its implementation is as follows:

[0025] (1) When the laser is in its initial state, the micro-elastic material base 4 does not deform, and the angle formed by OB and AB is the initial angle. The incident angle corresponding to the diffraction grating is also The length of OA is the initial cavity length. The length of AB can be determined using trigonometric functions. At this time, the first-order diffraction wavelength of the diffraction grating cavity mode wavelength They are equal, as can be seen from the grating equation. From the cavity mode wavelength formula It can be known that the ordinal number of the vertical module Due to the length of AB It is fixed; the initial cavity length L can be determined by the initial laser wavelength. Length of AB and grating constant The calculation shows that the length of AB is... ,Depend on ,in available Through trigonometric functions, we know The initial cavity length can be obtained. .

[0026] (2) When point A is displaced by x to At point OA The length of the curve becomes Lx, at which point B is displaced to B. Point, the angle formed by the original OB and AB. Change from OB With A B The angle formed That is, the incident angle of the diffraction grating becomes Using trigonometric function formulas, we can know From the grating equation, the first-order diffraction wavelength at this point is:

[0027]

[0028] (3) After the grating is displaced, the length of the outer cavity becomes The corresponding cavity mode wavelength is Substituting this into the formula for q to the cavity mode wavelength, we can obtain... From step (1), we can know the initial cavity length. ,Right now:

[0029]

[0030] From formulas (1) and (2), we know that the first-order diffraction wavelength is... cavity mode wavelength Equally, the laser output can achieve mode-free laser hopping.

[0031] Will Substituting them in, we get:

[0032]

[0033] According to formula (3), under the condition of known initial wavelength, the correspondence between output laser wavelength and displacement x can be established.

[0034] This invention designs a micro-elastic material base 4, which enables the piezoelectric ceramic 5 to drive the base to deform, thereby achieving precise control over the position change of the diffraction grating 3. This allows the output laser to have mode-free and continuously tunable characteristics, ultimately constructing a mode-free and continuously tunable semiconductor laser. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the laser's structure;

[0036] Figure 2 Schematic diagram of the synchronous displacement and rotation control base;

[0037] Figure 3 This is a schematic diagram of the movement of the diffraction grating;

[0038] Figure 4 The range of continuously tunable frequencies achievable at different initial wavelengths;

[0039] Figure 5 The frequency range that can be continuously tuned with a displacement of 15µm;

[0040] Among them, 1. laser diode, 2. collimating lens, 3. diffraction grating, 4. micro-elastic material base, 5. piezoelectric ceramic, 6. grating base;

[0041] 401. Moving block; 402. Swinging fulcrum; 403. Moving block slot; 404. Isosceles triangular slot; 405. Piezoelectric ceramic slot; 406. Waist arm; 407. Waist arm fulcrum. Detailed Implementation

[0042] The following examples are used to explain the technical solutions of the present invention in a non-limiting manner.

[0043] like Figure 1 The laser shown includes a laser diode 1, a collimating lens 2, a diffraction grating 3, and a micro-elastic material base 4.

[0044] Micro-elastic material base 4 Figure 2 As shown, a stainless steel block is hollowed out using a CNC lathe. This includes machining a movable block 401 located within a movable block groove 403. The rear end of the movable block groove 403 is machined into a piezoelectric ceramic groove 405, and the front end into an isosceles triangular groove 404. A pair of arm-shaped sections 406 are machined within the isosceles triangular groove 404 along the sides of the isosceles triangle, and connected to a micro-elastic material base 4 via their front arm fulcrums 407. The rear ends (point A) of the arm-shaped sections 406 are fixedly connected to the front ends of the movable block 401.

[0045] The diffraction grating 3 is mounted on the rear end (point A) of the waist arm 406 via the grating base 6, and the laser diode 1 is mounted on point O, ensuring that the diffraction grating 3 is located in the optical path of the emitted light from the laser diode 1.

[0046] To visually and clearly demonstrate the dynamic adjustment process of the diffraction grating, Figure 3 A comparative schematic diagram of diffraction grating 3 before and after displacement is given. (From...) Figure 2 As can be seen from the structural relationship, the diffraction grating 3 is fixedly connected to the waist arm 406 via the grating base 6. Therefore, when the piezoelectric ceramic 5 is energized and drives the moving block 401 to press the waist arm 406, the waist arm 406 undergoes micro-displacement and micro-angle deflection around the fulcrum, thereby driving the diffraction grating 3 to move and rotate synchronously. It is worth noting that during the entire displacement and rotation process, the laser incident point O always remains unchanged, and the point of action of the laser incident on the diffraction grating moves synchronously with the grating. This design ensures that the relative position of the optical axis and the frequency selection surface of the grating is constant, effectively avoiding mode jumps caused by incident point offset, thereby ensuring the continuity and stability of wavelength tuning.

[0047] In practical applications, the length of AB is set to The line density of the diffraction grating is 1200 lines per millimeter (corresponding to the grating constant of the diffraction grating). (Approximately 833.33 nm), the variation of the continuous tuning range of the laser under different system bandwidths with the initial wavelength can be analyzed using formulas (1) and (2).

[0048] In physical implementation, the actual continuously tunable range is limited by the finite spectral bandwidth of the semiconductor gain chip. The actual continuously tunable range is determined by the effective spectral width of the semiconductor gain medium, which is referred to as the system bandwidth of the laser.

[0049] In this embodiment, the specific system bandwidth is 40nm. Taking an initial wavelength of 780nm and a system bandwidth of 40nm as an example, its single-sided tuning range is ±20nm, corresponding to an output laser wavelength range of 760nm-800nm. The adjustment range of displacement x can be calculated from formula (3) as -1.2mm-1.2mm. The calculation method remains unchanged when the initial wavelength changes.

[0050] In this embodiment, when the system bandwidth is 40nm and the initial center wavelength is 633nm, 780nm, 852nm, or 1064nm, formula (3) is combined with formula... The frequency range of continuous laser tuning can be obtained, and the result is derived from... Figure 4 The results show that the laser can achieve tuning bandwidths of 30.3THz, 19.7THz, 16.6THz and 10.6THz at the different initial wavelengths mentioned above, all of which have a wide tuning range.

[0051] In this embodiment, the displacement range generated by the piezoelectric ceramic 5 driving the diffraction grating 3 is on the order of micrometers. Taking the Soleb PK44M3B8P2 piezoelectric ceramic as an example, its maximum displacement can reach 15 μm. Figure 5 The continuously tunable frequency range of the system at an initial wavelength of 780 nm is demonstrated. Calculations show that when the piezoelectric ceramic displacement varies within the range of 0-15 μm, the system can achieve a frequency range from 3.84615 × 10⁻⁶ nm. Hz to 3.84739× The continuous frequency tuning is 124 GHz, with a total tuning bandwidth of 124 GHz.

[0052] Compared with existing technologies, this invention effectively overcomes the mode-hopping problem caused by changes in laser frequency with resonant cavity length and cavity mode frequency shift in traditional external cavity semiconductor lasers, significantly improving the continuity and stability of wavelength tuning. This invention innovatively combines the high-precision frequency selection characteristics of a diffraction grating with an integrated precision mechanical structure. Through controllable driving, it achieves synchronous displacement and angular rotation of the diffraction grating, ensuring coordinated and precise matching between the cavity mode wavelength and the first-order diffraction wavelength of the grating. This fundamentally suppresses mode-hopping and achieves stable and reliable mode-hopping-free continuous tuning of the laser output frequency. Based on this, this invention successfully constructs a compact, wide-range, and highly stable mode-hopping-free continuously tunable external cavity semiconductor laser, meeting the requirements of high-end applications such as high-resolution spectral measurement, precision interferometry, and coherent detection for continuously tunable wavelengths and highly stable frequencies. This provides crucial support and technological impetus for the development of high-precision sensing and precision optical measurement technologies.

Claims

1. A Littrow-type cavity length compensated, mode-skipping-free, continuously tunable semiconductor laser, the laser comprising a laser diode (1), a collimating lens (2), and a diffraction grating (3) sequentially arranged in the optical path, characterized in that... The laser includes a micro-elastic material base (4), which has a movable block (401) through a hollowed-out process. The movement range of the movable block (401) is limited in the movable block groove (403). A connected piezoelectric ceramic groove (405) is provided at the rear end of the movable block groove (403), and an isosceles triangular groove (404) is provided at the front end of the movable block groove (403). Two waist arms (406) are processed along the waist of the isosceles triangle in the isosceles triangular groove (404). Each waist arm (406) is connected to the micro-elastic material base (4) through its own waist arm fulcrum (407). The waist arm fulcrum (407) is located at the base angle of the isosceles triangle. The rear ends of the waist arms (406) are fixedly connected to the front ends of the movable block (401). The diffraction grating (3) is mounted on the rear end of a waist arm (406) via a grating base (6), and the horizontal projection of the diffraction grating (3) and the waist arm (406) are on the same plane. A piezoelectric ceramic (5) is installed in a piezoelectric ceramic groove (405). The front end of the piezoelectric ceramic (5) abuts against the moving block (401), and the rear end abuts against the micro-elastic material base (4). The laser diode (1) is installed at the midpoint O of the bottom edge of the isosceles triangular groove (404), and the diffraction grating (3) is located on the output light path of the laser diode (1). The light emitted from the laser diode (1) is collimated into parallel light by the collimating lens (2) and then incident on the diffraction grating (3), resulting in 0th-order diffracted light and 1st-order diffracted light. The diffraction angle of the 1st-order diffracted light is equal to the incident angle. The 1st-order diffracted light returns to the laser diode (1) along the original optical path and oscillates. When the 1st-order diffraction wavelength of the diffraction grating (3) is... cavity mode wavelength When the two parts are matched and the intracavity oscillation reaches the threshold, an outer cavity is formed, and the 0th order diffracted light is used as the output laser.

2. The Littrow-type cavity length compensated, mode-skipping-free, continuously tunable semiconductor laser according to claim 1, characterized in that... The movable block (401) is connected to the micro-elastic material base (4) via at least one swing fulcrum (402).

3. The Littrow-type cavity length compensated, mode-skipping-free, continuously tunable semiconductor laser according to claim 1, characterized in that... The microelastic material is a metal.

4. The Littrow-type cavity length compensated, mode-skipping-free, continuously tunable semiconductor laser according to claim 1, characterized in that... When the moving block (401) moves back and forth within the moving block slot (403), causing a change in the incident angle, the first-order diffraction wavelength... With cavity mode wavelength It changes synchronously with the angle and achieves continuous, mode-free tuning.

5. The Littrow-type cavity length compensated mode-skipping-free continuously tunable semiconductor laser according to claim 1, characterized in that... An anti-reflection coating is provided on the output light end face of the laser diode (1).

6. A method for measuring minute displacements based on a Littrow-type cavity length-compensated, mode-skipping-free, continuously tunable semiconductor laser, characterized in that... Using the Littrow-type cavity length-compensated mode-skipping continuous tunable semiconductor laser as described in claim 1, when the diffraction grating (3) moves under the action of the moving block (401), the displacement distance x of point A satisfies the following relationship: in, The initial laser wavelength, Let AB be the length. The grating constant is Let be the first-order diffraction wavelength when point A is displaced by x. The wavelength is the cavity mode wavelength.

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