An ultranarrow linewidth littman tunable laser

Abstract

The application discloses an ultra-narrow linewidth Littman tunable laser, comprising a semiconductor laser diode, a collimating lens, a blazed grating and a tunable mirror which is arranged opposite to the grating plane of the blazed grating and is angle-adjustable, wherein the semiconductor laser diode, the collimating lens and the blazed grating are located on the same optical axis, and the grating plane of the blazed grating is distributed in intersection with the optical axis; and the application further comprises a cavity length-adjustable F-P etalon, which is located on the output light path of the zero-order diffraction light of the blazed grating, and the zero-order diffraction light is output as laser after passing through the F-P etalon. In this way, the semiconductor laser diode, the collimating lens, the blazed grating and the tunable mirror form a standard Littman tunable laser, and the cavity length-adjustable F-P etalon is additionally arranged, so that the cavity distance of the F-P etalon is adjusted to match the narrow-band filtering characteristics of the F-P etalon with the output wavelength of the semiconductor laser diode.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, and in particular relates to an ultra-narrow linewidth Littman tunable laser. Background Technology

[0002] Littman structure external cavity semiconductor lasers are widely used in precision spectroscopy, atomic physics, and coherent optical communication due to their compact structure, wide tuning range, and good stability. In this structure, the laser linewidth is typically limited by the external cavity quality factor (Q value) and the performance of the dispersive elements within the cavity. Traditional Littman lasers mainly use a diffraction grating and a tuned mirror to form an external cavity frequency selection mechanism, achieving narrow linewidth output in the range of 100 kHz to 300 kHz.

[0003] However, with the development of cutting-edge fields such as quantum precision measurement, optical atomic clocks, and high-resolution spectroscopy, higher requirements have been placed on the linewidth, frequency noise, and long-term stability of laser sources. Existing Littman lasers can no longer meet the linewidth requirements below 100 kHz or even tens of kHz. The key factors causing this bottleneck are: on the one hand, limited by the current level of micro-nano fabrication technology, the active region structure, waveguide design, and the line density and surface accuracy of diffraction gratings in semiconductor laser chips cannot be further improved, limiting the Q value and frequency selectivity of the external cavity; on the other hand, narrowing the linewidth by extending the physical length of the external cavity or increasing the grating resolution often leads to a reduction in the external cavity free spectral range (FSR), a decrease in tuning sensitivity, and a deterioration in mechanical stability, thereby affecting the tuning range and long-term operational stability of the laser. Therefore, how to further narrow the output linewidth of Littman lasers while maintaining their existing external cavity tuning range and mechanical stability has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0004] To address the aforementioned technical problems, the present invention aims to provide an ultra-narrow linewidth Littman tunable laser with a simple structure, which further narrows the output linewidth of existing Littman tunable lasers.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: an ultra-narrow linewidth Littman tunable laser includes a semiconductor laser diode, a collimating lens, a blazed grating, and a tuning mirror arranged sequentially opposite to the grating plane of the blazed grating and with an adjustable angle. The semiconductor laser diode, the collimating lens, and the blazed grating are located on the same optical axis, and the grating plane of the blazed grating intersects the optical axis. The laser also includes a cavity-length adjustable FP etalon, which is located in the output optical path of the zero-order diffracted light of the blazed grating. The zero-order diffracted light is output as laser light after passing through the FP etalon.

[0006] The beneficial effects of the above-mentioned technical solution of the present invention are as follows: This allows the semiconductor laser diode, collimating lens, blazed grating, and tuning mirror to form a standard Littman tunable laser. The laser emitted by the semiconductor laser diode is typically in the kHz range and diffracts on the grating surface of the blazed grating. The first-order diffracted light serves as feedback, and the zero-order diffracted light serves as output, exhibiting a narrow linewidth laser. The laser wavelength of the semiconductor laser diode can be adjusted by regulating the tuning mirror. Furthermore, by adding an adjustable-cavity FP etalon, the FP etalon possesses narrowband filtering characteristics, which can narrow the zero-order diffracted light output from the blazed grating to the Hz range, or even sub-Hz, and ultimately output as laser light. By adjusting the cavity pitch of the FP etalon, its narrowband filtering characteristics are matched to the output wavelength of the semiconductor laser diode.

[0007] Based on the above technical solution, the present invention can be further improved as follows: Furthermore, the FP etalon includes an FP etalon body and a cavity pitch adjustment component, wherein the cavity pitch adjustment component is disposed on the FP etalon body and is used to adjust the cavity pitch of the FP etalon body; The tuning reflector has a reflector body and an angle adjustment member. The reflector body is disposed on the angle adjustment member, which is used to adjust the tilt angle of the reflector body relative to the blazing grating.

[0008] The beneficial effects of the above technical solution are as follows: the angle adjustment component can adjust the tilt angle of the reflector body relative to the blazed grating to change the output wavelength of the semiconductor laser diode, and the cavity pitch adjustment component can be linked with the angle adjustment component and adaptively adjust the cavity pitch of the FP standard etalon body so that the linewidth of the final output diffraction light has the characteristics of ultra-narrow linewidth and can always maintain a stable ultra-narrow linewidth output.

[0009] Furthermore, it also includes a controller; The driving part of the cavity pitch adjustment component is a piezoelectric ceramic driving component or a MEMS electrostatic driving component; The driving part of the angle adjustment component is a piezoelectric ceramic driving component or a MEMS electrostatic driving component; Both the angle adjustment component and the cavity distance adjustment component are electrically connected to the controller, and the controller adjusts the operation of the angle adjustment component and the cavity distance adjustment component according to a preset mapping relationship.

[0010] The beneficial effect of the above technical solution is that the angle adjustment component and the cavity distance adjustment component can maintain linkage under the control of the controller, and the adjustment sensitivity is high.

[0011] Furthermore, the tuning method of the ultra-narrow linewidth Littman tunable laser is as follows: When the controller receives a tuning command, the angle adjustment component executes the tuning command to adjust the tilt angle of the tuning reflector; The controller synchronously generates a cavity pitch adjustment command according to a preset mapping relationship; The cavity pitch adjustment component executes the cavity pitch adjustment command to adjust the cavity pitch of the FP etalon.

[0012] The beneficial effect of the above technical solution is that the cavity pitch adjustment component and the angle adjustment component are linked according to a preset mapping relationship, that is, when the tuning mirror is at an angle, the corresponding FP standard is at a corresponding cavity pitch.

[0013] Furthermore, the preset mapping relationship is obtained through calibration, and the specific calibration method is as follows: Step 1: With the overall temperature of the ultra-narrow linewidth Littman tunable laser stable, adjust the voltage V1 of the angle adjustment component and the voltage V2 of the cavity pitch adjustment component to zero. Step 2: Set the voltage step V1-i of the angle adjustment component, and record the wavelength of the ultra-narrow linewidth Littman tunable laser output in real time as λ-i; Step 3: Scan the voltage of the cavity pitch adjustment device near the wavelength λ-i, collect the output power P of the ultra-narrow linewidth Littman tunable laser, and find the maximum power value Pmax-i. When the output power is Pmax-i, record the voltage value V2-i of the cavity pitch adjustment device to obtain the voltage pair V1-i, V2-i. Step 4: Repeat steps 2 and 3 until the wavelength λ of the ultra-narrow linewidth Littman tunable laser output in real time traverses the full range of tuning, where i = 1, 2, 3...; Step 5: Construct the preset mapping relationship based on the obtained voltage pairs.

[0014] The beneficial effect of the above technical solution is that the preset mapping relationship between the angle of the tuning mirror and the cavity distance of the FP etalon can be obtained through a limited number of experiments, and then implanted into the controller to realize the linkage adjustment between the angle of the tuning mirror and the cavity distance of the FP etalon.

[0015] Furthermore, it also includes an optical isolator disposed in the optical path between the blazed grating and the FP etalon, the optical isolator being used to prevent the backscattering of diffracted light directed toward the FP etalon.

[0016] The beneficial effect of the above technical solution is that the optical isolator can block external backlight and prevent external measurement optical path from interfering with the output linewidth narrowing process.

[0017] Furthermore, it also includes a cavity shell and a first temperature control device. The semiconductor laser diode, collimating lens, blazed grating, tuning mirror and FP standard are all disposed inside the cavity shell. The cavity shell is provided with a light-emitting hole aligned with the FP standard. The first temperature control device is disposed on the cavity shell and is used to adjust the temperature inside the cavity shell.

[0018] The beneficial effect of the above technical solution is that it enables the temperature of the entire ultra-narrow linewidth Littman tunable laser to remain relatively stable during operation, thereby avoiding the impact of temperature fluctuations on the accuracy of optical components.

[0019] Furthermore, the cavity shell is made of Invar alloy.

[0020] The beneficial effect of the above technical solution is that it makes the cavity shell have a low coefficient of thermal expansion and the thermal expansion and contraction is not obvious, thus improving the accuracy of the ultra-narrow linewidth Littman tunable laser.

[0021] Furthermore, it also includes two second temperature control devices, both of which are disposed inside the cavity. One of the second temperature control devices is disposed on the semiconductor laser diode, and the other is disposed on the FP etalon. The two second temperature control devices are used to control the temperature of the semiconductor laser diode and the FP etalon, respectively.

[0022] The beneficial effect of the above technical solution is that by providing a second temperature control device to the optical components that generate a large amount of heat independently, the temperature stability of the entire ultra-narrow linewidth Littman tunable laser during operation can be further improved.

[0023] Furthermore, both the first and second temperature control devices are TEC temperature control components.

[0024] The beneficial effects of the above technical solution are that the first and second temperature control devices do not generate noise or vibration during operation, and they can both heat and cool, enabling more precise temperature control, thereby improving the stability and reliability of optical components. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of the ultra-narrow linewidth Littman tunable laser described in an embodiment of the present invention; Figure 2 This is a schematic diagram of an optical fiber disposed on the cavity shell according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the electrical connections of the controller described in an embodiment of the present invention; Figure 4 This is a schematic diagram of the tuning process of the ultra-narrow linewidth Littman tunable laser described in an embodiment of the present invention; Figure 5 This is a schematic diagram of the calibration process for the preset mapping relationship described in an embodiment of the present invention.

[0026] In the diagram: 1. Semiconductor laser diode; 2. Collimating lens; 3. Blazed grating; 301. Grating plane; 4. Tuning mirror; 401. Mirror body; 402. Angle adjustment device; 5. FP etalon; 501. FP etalon body; 502. Cavity pitch adjustment device; 6. Optical fiber; 7. Optical isolator; 8. Controller; 9. Cavity shell; 91. Light exit aperture; 10. First temperature control device; 11. Second temperature control device. Detailed Implementation

[0027] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings, which illustrate embodiments of the present application. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that the disclosure of this application will be thorough and complete.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0029] It is understood that spatial relation terms such as “below,” “under,” “below,” “below,” “under,” “above,” “above,” etc., can be used here to describe the relationship between one element or feature shown in the figure and other elements or features. It should be understood that, in addition to the orientation shown in the figure, spatial relation terms also include different orientations of the device in use and operation. For example, if the device in the figure is flipped, the element or feature described as “below,” “below,” or “below” will be oriented “above” the other element or feature. Therefore, the exemplary terms “below” and “under” can include both upper and lower orientations. Furthermore, the device may also include other orientations (e.g., rotated 90 degrees or other orientations), and the spatial descriptive terms used herein will be interpreted accordingly.

[0030] It should be noted that the terms "installation," "connection," and "linking" have the same meaning. When one component is considered to be "connected" to another component, it can be directly connected to the other component or connected to the other component through an intermediary component. In the following embodiments, "connection" should be understood as "electrical connection," "communication connection," etc., if the connected circuits, modules, units, etc., have the transmission of electrical signals or data between them.

[0031] It should be understood that the term "and / or" used in this article is merely a description of the relationship between related objects, indicating that there can be three relationships. For example, A and / or B can represent three situations: A exists alone, A and B exist simultaneously, and B exists alone.

[0032] When used herein, the singular forms of “a” and “the” may also include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprising” or “having” specify the presence of the stated feature, whole, step, operation, component, part, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, wholes, steps, operations, components, parts, or combinations thereof.

[0033] like Figure 1As shown, this embodiment provides an ultra-narrow linewidth Littman tunable laser, including a semiconductor laser diode 1, a collimating lens 2, a blazed grating 3, and a tuning mirror 4 arranged sequentially and angle-adjustable opposite to the grating plane 301 of the blazed grating 3. The semiconductor laser diode 1, the collimating lens 2, and the blazed grating 3 are located on the same optical axis, and the grating plane 301 of the blazed grating 3 intersects the optical axis. It also includes a cavity-length adjustable FP etalon 5, which is located in the output optical path of the zero-order diffracted light of the blazed grating 3. The zero-order diffracted light is output as laser light after passing through the FP etalon 5. This allows the semiconductor laser diode 1, collimating lens 2, blazed grating 3, and tuning mirror 4 to form a standard Littman tunable laser (its structure and operating principle are existing technologies and will not be described in detail here). The laser emitted by the semiconductor laser diode 1 is on the order of kHz and diffracts on the grating surface of the blazed grating 3. The first-order diffracted light serves as feedback, and the zero-order diffracted light serves as output, which is a narrow-linewidth laser. The laser wavelength of the semiconductor laser diode 1 can be adjusted by adjusting the tuning mirror 4. By adding an adjustable cavity length FP etalon 5, the FP etalon 5 has narrowband filtering characteristics, which can narrow the zero-order diffracted light output from the blazed grating 3 to the order of Hz or even sub-Hz, and ultimately output it as laser light. By adjusting the cavity pitch of the FP etalon 5, the narrowband filtering characteristics of the FP etalon 5 can be matched with the output wavelength of the semiconductor laser diode 1.

[0034] In this embodiment, the zero-order diffracted light output from the blazed grating 3 undergoes multiple reflections and transmissions between the two highly reflective surfaces of the FP etalon 5, forming multiple coherent beams. According to the interference resonance condition of the FP etalon 5, only laser wavelengths that satisfy the formula 2d=mλ (where d is the length of the etalon air cavity, m is a positive integer, and λ is the laser wavelength) can undergo constructive interference, forming a high-intensity transmission peak; while wavelength components deviating from this resonance condition will undergo destructive interference, being significantly suppressed or completely blocked.

[0035] In this embodiment, the transmission peak of the FP etalon 5 has an extremely narrow bandwidth, which acts as a dynamically adjustable narrowband bandpass filter. The zero-order diffracted light output from the blazed grating 3 contains the principal longitudinal mode, side modes, phase noise, and stray frequency components caused by frequency drift. After being filtered by the FP etalon 5, only the resonant wavelength corresponding to the principal longitudinal mode can be efficiently transmitted, while the side modes and stray frequency components are significantly suppressed, thereby achieving a significant narrowing of the laser linewidth.

[0036] In this embodiment, the FP etalon 5 must have cavity length adjustment function. Since the Littman tunable laser composed of semiconductor laser diode 1, collimating lens 2, blazed grating 3 and tuning mirror 4 has the characteristic of continuously adjustable output wavelength, when the angle of tuning mirror 4 is changed (i.e. external cavity wavelength tuning), its matching FP etalon 5 must dynamically change its cavity length to ensure that its transmission peak is always precisely matched with the main longitudinal mode wavelength of the laser, forcing semiconductor laser diode 1 to always operate in single longitudinal mode, thereby achieving linewidth narrowing across the entire wavelength band.

[0037] In this embodiment, the FP etalon 5 includes an FP etalon body 501 and a cavity pitch adjustment member 502. The cavity pitch adjustment member 502 is disposed on the FP etalon body 501 and is used to adjust the cavity pitch of the FP etalon body 501. The tuning mirror 4 has a mirror body 401 and an angle adjustment member 402. The mirror body 401 is disposed on the angle adjustment member 402, and the angle adjustment member 402 is used to adjust the tilt angle of the mirror body 401 relative to the blazed grating 3. This allows the angle adjustment member 402 to adjust the tilt angle of the mirror body 401 relative to the blazed grating 3 to change the output wavelength of the semiconductor laser diode 1. The cavity pitch adjustment member 502 can be linked with the angle adjustment member 402 and adaptively adjust the cavity pitch of the FP etalon body 501 so that the linewidth of the final output zero-order diffraction light has the characteristics of an ultra-narrow linewidth and can always maintain a stable ultra-narrow linewidth output.

[0038] like Figure 3 As shown, the ultra-narrow linewidth Littman tunable laser in this embodiment also includes a controller 8; the driving part of the cavity pitch adjustment component 502 is a piezoelectric ceramic driver or a MEMS electrostatic driver; the driving part of the angle adjustment component 402 is a piezoelectric ceramic driver or a MEMS electrostatic driver (which are commonly used driving devices in the optical field, characterized by sensitive control and high operating accuracy); both the angle adjustment component 402 and the cavity pitch adjustment component 502 are electrically connected to the controller 8, and the controller 8 adjusts the operation of the angle adjustment component 402 and the cavity pitch adjustment component 502 according to a preset mapping relationship. This allows the angle adjustment component 402 and the cavity pitch adjustment component 502 to maintain linkage under the control of the controller 8, and the adjustment sensitivity is high.

[0039] The controller 8 described in this embodiment can be an MCU controller or an FPGA controller, but it is not limited to these.

[0040] like Figure 4As shown, the tuning method of the ultra-narrow linewidth Littman tunable laser in this embodiment is as follows: When the controller 8 receives a tuning command, the angle adjustment component 402 executes the tuning command to adjust the tilt angle of the tuning mirror 4; the controller 8 synchronously generates a cavity pitch adjustment command according to a preset mapping relationship; the cavity pitch adjustment component 502 executes the cavity pitch adjustment command to adjust the cavity pitch of the FP etalon 5. This ensures that the cavity pitch adjustment component 502 and the angle adjustment component 402 are linked according to a preset mapping relationship, that is, when the tuning mirror 4 is at a certain angle, the corresponding FP etalon 5 is at a corresponding cavity pitch.

[0041] like Figure 5 As shown, the preset mapping relationship in this embodiment is obtained through calibration. The specific calibration method is as follows: Step 1: With the overall temperature of the ultra-narrow linewidth Littman tunable laser stable, adjust the voltage V1 of the angle adjustment component 402 and the voltage V2 of the cavity pitch adjustment component 502 to zero. Step 2: Set the voltage step V1-i of the angle adjustment component 402, and record the wavelength of the ultra-narrow linewidth Littman tunable laser output in real time as λ-i; Step 3: Scan the voltage of the cavity pitch adjustment device 502 near the wavelength λ-i, collect the output power P of the ultra-narrow linewidth Littman tunable laser, and find the maximum power value Pmax-i. When the output power is Pmax-i, record the voltage value V2-i of the cavity pitch adjustment device 502 to obtain the voltage pair V1-i, V2-i (similar to two-dimensional coordinate values). Step 4: Repeat steps 2 and 3 until the wavelength λ of the ultra-narrow linewidth Littman tunable laser output in real time traverses the full range of tuning, where i = 1, 2, 3...; Step 5: Construct the preset mapping relationship (i.e., functional relationship, similar to the standard curve production process) based on the obtained voltage pairs. In this way, the preset mapping relationship between the angle of the tuning mirror 4 and the cavity distance of the FP etalon 5 can be obtained through a limited number of experiments, and then implanted into the controller 8 to realize the linkage adjustment between the angle of the tuning mirror 4 and the cavity distance of the FP etalon 5.

[0042] like Figure 1 As shown, the ultra-narrow linewidth Littman tunable laser in this embodiment also includes an optical isolator 7. The optical isolator 7 is disposed in the optical path between the blazed grating 3 and the FP etalon 5. The optical isolator 7 is used to prevent the backscattering of diffracted light directed at the FP etalon 5. The optical isolator can block external backscattering and prevent external measurement optical paths from interfering with the output linewidth narrowing process.

[0043] like Figure 1 As shown, the ultra-narrow linewidth Littman tunable laser in this embodiment further includes a cavity housing 9 and a first temperature control device 10. The semiconductor laser diode 1, collimating lens 2, blazed grating 3, tuning mirror 4, and FP etalon 5 are all disposed within the cavity housing 9. The cavity housing 9 has an exit aperture 91 aligned with the FP etalon 5. The first temperature control device 10 is disposed on the cavity housing 9 and is used to regulate the temperature within the cavity housing 9. This ensures that the temperature of the entire ultra-narrow linewidth Littman tunable laser remains relatively stable during operation, thereby preventing temperature fluctuations from affecting the accuracy of the optical components.

[0044] like Figure 1 As shown, in this embodiment, the linewidth of the zero-order diffraction light output by the blazed grating 3 is narrowed by the FP etalon 5 and then emitted through the light-emitting aperture 91 to the outside of the cavity shell 9.

[0045] like Figure 2 As shown, in this embodiment, an optical fiber 6 can be connected to the light output hole 91. At this time, the linewidth of the zero-order diffraction light output by the blazed grating 3 is narrowed by the FP standard etalon 5 and then output through the optical fiber 6. Specifically, in this embodiment, an optical fiber coupling head can be set at the light output hole 91 for the optical fiber 6 to be connected. This is a conventional technical means in the field and will not be described in detail here.

[0046] In this embodiment, the semiconductor laser diode 1, collimating lens 2, blazed grating 3, tuning mirror 4, FP etalon 5, optical fiber 6, and optical isolator 7 are all optical components.

[0047] like Figure 1 As shown, the cavity housing 9 in this embodiment is made of Invar alloy. This gives the cavity housing 9 a low coefficient of thermal expansion, resulting in minimal thermal expansion and contraction, thus improving the accuracy of the ultra-narrow linewidth Littman tunable laser.

[0048] In this embodiment, the controller 8 can be disposed outside the cavity shell 9, and the cavity shell 9 can also be provided with a wire hole for the conductive wire to pass through (which is a conventional technical means in the art and will not be described in detail here).

[0049] like Figure 1 As shown, in this embodiment, the cavity shell 9 can be a cuboid shell with a hollow interior, and multiple first temperature control devices 10 can be provided, that is, at least one can be provided on each side of the cavity shell 9, which makes its temperature control more sensitive and faster.

[0050] like Figure 1As shown, the ultra-narrow linewidth Littman tunable laser in this embodiment also includes two second temperature control devices 11. Both second temperature control devices 11 are disposed within the cavity housing 9. One second temperature control device 11 is disposed on the semiconductor laser diode 1, and the other second temperature control device 11 is disposed on the FP etalon 5. The two second temperature control devices 11 are used to control the temperature of the semiconductor laser diode 1 and the FP etalon 5, respectively. By independently matching the second temperature control device 11 to the optical components with large heat generation, the temperature stability of the entire ultra-narrow linewidth Littman tunable laser during operation can be further improved.

[0051] In this embodiment, among the optical components, only the semiconductor laser diode 1 and the FP standard etalon 5 are temperature-sensitive and generate heat during operation. Therefore, a second temperature control device 11 is independently set for temperature regulation.

[0052] like Figure 3 As shown, in this embodiment, the first temperature control device 10 and the second temperature control device 11 can both be electrically connected to the controller 8.

[0053] In this embodiment, both the first temperature control device 10 and the second temperature control device 11 are TEC temperature control components [which consist of a TEC temperature regulating chip (also known as a thermoelectric cooler or semiconductor cooler) and a temperature sensor, with the temperature sensor integrated on the TEC temperature regulating chip, and both electrically connected to the controller. The TEC temperature regulating chip can heat or cool according to the controller's instructions, while the temperature sensor monitors the output temperature of the TEC temperature regulating chip in real time and provides feedback to the controller, thus enabling the TEC temperature control component to precisely adjust the output temperature]. This ensures that the first temperature control device 10 and the second temperature control device 11 operate without noise or vibration, and since they can both heat and cool, they can more accurately regulate the temperature, thereby improving the stability and reliability of the optical components.

[0054] In the preset mapping relationship calibration method described in this embodiment, step 1 is under the condition that the entire ultra-narrow linewidth Littman tunable laser is started and running. First, the temperature of the ultra-narrow linewidth Littman tunable laser is kept in a stable state (without frequent fluctuations) by the first temperature control device 10 and the second temperature control device 11.

[0055] In this embodiment, the semiconductor laser diode 1 can also be electrically connected to the controller 8, which makes the entire ultra-narrow linewidth Littman tunable laser highly automated.

[0056] Example The laser output from blazed grating 3 (zero-order diffraction light) has a center wavelength of 1550 nm, a linewidth of 250 kHz (typical for Littman lasers), a side-mode rejection ratio of 35 dB, and a mode-free tuning range Δν of 20 GHz. Therefore, the linewidth needs to be narrowed to within 100 kHz. (1) Calculation of the free spectral range (FSR) of the FP standard etalon. The Free Spectral Range (FSR) is the frequency difference between two adjacent resonance transmission peaks of the FP etalon 5, and its calculation formula is as follows: To ensure that the FP etalon 5 has only one resonant transmission peak within the full laser tuning range and to avoid multimode interference, the following condition must be met: FSR greater than or equal to Δν (i.e., FSR ≥ 20 GHz). Here, FSR = 25 GHz is chosen (for redundancy and to improve stability). Substituting these values ​​into the formula, the initial cavity length d0 of the FP etalon 5 is calculated as follows:

[0057] (2) Calculation of resonance order m: From the resonance formula We can obtain:

[0058] The resonance order m must be a positive integer. We take m=8 (closest to the calculated value to ensure resonance matching). After correction, the initial cavity length d of the FP etalon is 5. 0修正 :

[0059] Corrected FSR 修正 : It still meets the requirement of FSR ≥ 20 GHz and is compatible with the mode-hopping tuning range.

[0060] (3) Calculation of FP standard etalon 5 fineness (F) and transmission peak bandwidth: The fineness F of the FP etalon determines the transmission peak bandwidth, and the calculation formula is as follows: (R is the reflectivity of the reflecting surface), transmission peak bandwidth Δν FP =FSR / F.

[0061] Objective: To narrow the laser linewidth from 250 kHz to ≤100 kHz, therefore Δν_FP ≤ 100 kHz must be satisfied. Substituting Δν... FP =100 kHz and FSR 修正 ≈24.19 GHz, required precision F for computation:

[0062] Selecting a reflectivity R of 99.8% for the reflective surface, and substituting it into the precision formula for verification:

[0063] At this time, the transmission peak bandwidth Δν FP =24.19×10 9 / 1570≈15.41 kHz, which is much smaller than 100 kHz, fully meeting the target of narrowing the linewidth to ≤100 kHz.

[0064] (4) Calculation of the 5-cavity length adjustment range of the FP standard etalon: The laser mode-hopping-free tuning range Δν = 20 GHz, and the corresponding wavelength tuning range Δλ can be derived from the relationship between frequency and wavelength. calculate:

[0065] From the resonance formula The relationship between the cavity length adjustment range Δd and the wavelength tuning range Δλ is as follows:

[0066] Substituting m=8 and Δλ≈0.160 nm, we get:

[0067] That is, the cavity length of the FP standard etalon 5 needs to be dynamically adjusted within the range of 6.2μm±0.32 nm to adapt to the mode-hopping-free tuning range of the laser at 20 GHz and ensure resonant matching throughout the entire range.

[0068] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. An ultra-narrow linewidth Littman tunable laser, comprising a semiconductor laser diode (1), a collimating lens (2), a blazed grating (3), and a tuning mirror (4) arranged sequentially and angle-adjustable opposite to the grating plane (301) of the blazed grating (3), wherein the semiconductor laser diode (1), the collimating lens (2), and the blazed grating (3) are located on the same optical axis, and the grating plane (301) of the blazed grating (3) intersects with the optical axis, characterized in that, It also includes a cavity length adjustable FP etalon (5), which is located on the output optical path of the zero-order diffraction light of the blazed grating (3), and the zero-order diffraction light is output as laser light after passing through the FP etalon (5).

2. The ultra-narrow linewidth Littman tunable laser according to claim 1, characterized in that, The FP etalon (5) includes an FP etalon body (501) and a cavity pitch adjustment member (502). The cavity pitch adjustment member (502) is disposed on the FP etalon body (501) and is used to adjust the cavity pitch of the FP etalon body (501). The tuning mirror (4) has a mirror body (401) and an angle adjustment member (402). The mirror body (401) is disposed on the angle adjustment member (402), which is used to adjust the tilt angle of the mirror body (401) relative to the blazing grating (3).

3. The ultra-narrow linewidth Littman tunable laser according to claim 2, characterized in that, It also includes the controller (8); The driving part of the cavity pitch adjustment component (502) is a piezoelectric ceramic driving component or a MEMS electrostatic driving component; The driving part of the angle adjustment component (402) is a piezoelectric ceramic driving component or a MEMS electrostatic driving component; The angle adjustment component (402) and the cavity distance adjustment component (502) are both electrically connected to the controller (8). The controller (8) adjusts the operation of the angle adjustment component (402) and the cavity distance adjustment component (502) according to a preset mapping relationship.

4. The ultra-narrow linewidth Littman tunable laser according to claim 3, characterized in that, The tuning method for the ultra-narrow linewidth Littman tunable laser is as follows: When the controller (8) receives a tuning command, the angle adjustment member (402) executes the tuning command to adjust the tilt angle of the tuning reflector (4); The controller (8) synchronously generates a cavity pitch adjustment command according to a preset mapping relationship; The cavity pitch adjustment member (502) executes the cavity pitch adjustment command to adjust the cavity pitch of the FP etalon (5).

5. The ultra-narrow linewidth Littman tunable laser according to claim 3 or 4, characterized in that, The preset mapping relationship is obtained through calibration, and the specific calibration method is as follows: Step 1: Under the condition that the overall temperature of the ultra-narrow linewidth Littman tunable laser is stable, adjust the voltage V1 of the angle adjustment component (402) and the voltage V2 of the cavity pitch adjustment component (502) to zero. Step 2: Set the voltage step V1-i of the angle adjustment component (402), and record the wavelength of the ultra-narrow linewidth Littman tunable laser output in real time as λ-i; Step 3: Scan the voltage of the cavity pitch adjustment device (502) near the wavelength λ-i, collect the output power P of the ultra-narrow linewidth Littman tunable laser, and find the maximum power value Pmax-i. When the output power is Pmax-i, record the voltage value V2-i of the cavity pitch adjustment device (502) to obtain the voltage pair V1-i, V2-i. Step 4: Repeat steps 2 and 3 until the wavelength λ of the ultra-narrow linewidth Littman tunable laser output in real time traverses the full range of tuning, where i = 1, 2, 3...; Step 5: Construct the preset mapping relationship based on the obtained voltage pairs.

6. The ultra-narrow linewidth Littman tunable laser according to any one of claims 1-4, characterized in that, It also includes an optical isolator (7), which is disposed in the optical path between the blazed grating (3) and the FP etalon (5), and is used to prevent the backscattering of diffracted light directed toward the FP etalon (5).

7. The ultra-narrow linewidth Littman tunable laser according to any one of claims 1-4, characterized in that, It also includes a cavity shell (9) and a first temperature control device (10). The semiconductor laser diode (1), collimating lens (2), blazed grating (3), tuning mirror (4) and FP etalon (5) are all disposed in the cavity shell (9). The cavity shell (9) is provided with a light-emitting hole (91) aligned with the FP etalon (5). The first temperature control device (10) is disposed on the cavity shell (9) and is used to regulate the temperature inside the cavity shell (9).

8. The ultra-narrow linewidth Littman tunable laser according to claim 7, characterized in that, The cavity shell (9) is made of Invar alloy.

9. The ultra-narrow linewidth Littman tunable laser according to claim 7, characterized in that, It also includes two second temperature control devices (11), both of which are disposed inside the cavity shell (9). One of the second temperature control devices (11) is disposed on the semiconductor laser diode (1), and the other second temperature control device (11) is disposed on the FP etalon (5). The two second temperature control devices (11) are used to control the temperature of the semiconductor laser diode (1) and the FP etalon (5), respectively.

10. The ultra-narrow linewidth Littman tunable laser according to claim 9, characterized in that, The first temperature control device (10) and the second temperature control device (11) are both TEC temperature control components.

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