A laser multi-wavelength measuring device and a measuring method

Abstract

The application discloses a kind of laser multi-wavelength measuring device and measuring method, measuring device includes for the spatial filter for the filtering processing of laser beam to be measured to remove noise, for the collimating mirror group of collimating laser beam after spatial filter filtering processing to parallel light beam, for receiving parallel light beam to carry out interference processing and form interference fringe interference cavity, for the dispersion element of dispersion processing to interference fringe, and for the focusing element of focusing dispersion light beam of dispersion element to photodetector, by photodetector receive the image of laser beam to be measured after focusing and obtain the wavelength component of laser beam to be measured according to image;The application is measured to laser wavelength, based on interference-diffraction combination, can carry out high-precision measurement to multi-wavelength input in wide band.

Description

Technical Field

[0001] This invention relates to the field of laser measurement technology, specifically to a laser multi-wavelength measurement device and method. Background Technology

[0002] CN112857592.A A Compact Laser Wavelength Measurement Device and Measurement Method Thereof proposes a single-wavelength laser measurement device for testing WDM systems and devices. However, multiple wavelengths of laser light are often transmitted simultaneously in WDM systems, and multi-wavelength outputs also occur during the production and testing of related lasers.

[0003] The Fizeau interferometer wavelength meter has advantages such as no moving structure, fast measurement speed, and compact structure. However, its working principle for generating interference fringes is similar to that of the FP etalon. Therefore, when the wavelength difference of multiple laser beams incident on the interferometer cavity is close to the free spectral range of the interferometer cavity, it is unable to distinguish multiple wavelengths due to the spatial resolution limitations of the optical system and detector. Patents "CN109489837.A A Multiwavelength Meter Based on Optical Interferometer" and "CN115077728.B A Multiwavelength Detection Method, Device, and System" discuss similar methods for measuring multiple wavelengths, but neither solves the above problem. They will pseudo-resolve multiple wavelengths with intervals equal to the free spectral range of the interferometer cavity as a single wavelength. The patent "CN114942081.B A method and system for measuring optical wavelength" theoretically achieves multi-wavelength measurement by combining an interferometer with an interferometer cavity. However, in order to achieve a wide spectral range measurement, the system needs to include a scanning motion structure, which compromises the stability of the Fizeau interferometer wavelength meter. Furthermore, the beam directionality at the output slit during wavelength scanning affects the beam angle of the incident interferometer cavity, resulting in a lack of one-to-one correspondence between the phase diagram obtained by the detector and the wavelength. Therefore, this measurement method is not practical for multi-wavelength laser measurement.

[0004] Another configuration used in commercial laser wavelength meters is the multiwavelength meter based on the Michaelson interferometer. This type of multiwavelength meter can measure multiple wavelength input beams through time-resolved Fourier transform, and within the measurement range, it does not exhibit the periodic pseudo-resolution introduced by the free spectral range of the components. However, limited by its scanning time resolution, it cannot distinguish between two laser beams with very small wavelength differences. Taking the most advanced Yokogawa AQ6151B from Japan as an example, there is a blind zone within 5 GHz (approximately 40 pm at 1550 nm) where multiwavelength resolution is not possible. Summary of the Invention

[0005] Technical objective: To address the shortcomings of existing laser wavelength measurement methods that cannot accurately measure the wavelength of lasers with similar wavelength differences, this invention discloses a laser multi-wavelength measurement device and method based on a combination of interference and diffraction, which can perform high-precision measurement of multiple wavelength inputs over a wide wavelength range.

[0006] Technical solution: To achieve the above technical objectives, the present invention adopts the following technical solution:

[0007] A laser multi-wavelength measurement device includes a spatial filter for filtering and removing noise from a laser beam under test, a collimating lens group for collimating the laser beam filtered by the spatial filter into a parallel beam, an interference cavity for receiving the parallel beam and performing interference processing to form interference fringes, a dispersive element for dispersing the interference fringes, and a focusing element for focusing the dispersive beam from the dispersive element onto a photodetector. The photodetector receives an image of the focused laser beam under test and obtains the wavelength components of the laser beam under test based on the image.

[0008] Preferably, the interference cavity of the present invention includes a front surface perpendicular to the parallel beam and a rear surface forming a wedge angle with the front surface. Both the front and rear surfaces are coated with a beam-splitting film. After the parallel beam enters the interference cavity from the front surface, it is reflected and transmitted between the front and rear surfaces. The beam emitted from the rear surface forms interference fringes along the wedge angle direction of the interference cavity.

[0009] Preferably, the dispersive element of the present invention is a diffraction grating, and the grating's scribe line direction is parallel to the wedge angle direction of the interference cavity.

[0010] Preferably, the focusing element of the present invention is a one-dimensional focusing element, which adopts a cylindrical mirror, the focusing direction is perpendicular to the wedge angle direction of the interference cavity, and the photodetector is disposed on the back focal plane of the focusing element.

[0011] Preferably, the photodetector of the present invention is an area array detector, wherein the free spectral range of the area array detector is: FSR Inf ≥Δλ, Where Δλ is the wavelength range corresponding to the pixels of the area array detector along the direction perpendicular to the diffraction grating, f is the focal length of the focusing element, L is the number of line pairs of the diffraction grating, θ is the angle between the diffracted light of the diffraction grating and the direction perpendicular to the diffraction grating, and m is the grating order used.

[0012] Preferably, the dispersion direction resolution of the dispersive element and focusing element of the present invention on the photodetector is smaller than the free spectral range of the interferometer cavity.

[0013] Preferably, the spatial filter of the present invention includes an objective lens for focusing and coupling the laser beam under test, and a filter aperture located at the exit end of the objective lens, so that the transverse mode of the beam exiting through the filter aperture is the Gaussian transverse mode.

[0014] Preferably, the collimating lens group of the present invention is an achromatic and spherical lens, employing an off-axis parabolic mirror with wavefront aberration less than λ, where λ is the minimum wavelength value within the wavelength measurement range of the device.

[0015] The present invention also provides a laser multi-wavelength measurement method using the above-mentioned laser multi-wavelength measurement device, comprising the steps of: filtering the laser beam to be measured to remove noise and collimating it into a parallel beam; performing interference processing on the parallel beam to form interference fringes; focusing the interference fringes onto a photodetector after dispersion by a dispersive element; and calculating the wavelength value by detecting the distribution of the interference fringes by the photodetector.

[0016] Beneficial effects: The laser multi-wavelength measurement device and method provided by the present invention have the following beneficial effects:

[0017] 1. This invention separates the light to be measured into multiple signals smaller than the free spectral range of the interferometer cavity within a broadband spectral range using a dispersive element, and measures multi-wavelength laser input. This avoids the indistinguishability of wavelengths due to periodic overlap in the interferometer cavity, and allows the light to be focused onto a photodetector after dispersion by a focusing element, thereby achieving accurate measurement of the laser wavelength.

[0018] 2. This invention measures multi-wavelength laser input in a narrow-band spectral range using an interference cavity. The minimum resolvable wavelength interval is very small, and the measurement resolution is tens of times higher than that of traditional monochromatic dispersion devices.

[0019] 3. The detection device of the present invention does not contain any moving structure, all components are fixedly installed, the overall structure is compact and the measurement stability is high.

[0020] 4. The photodetector of the present invention uses an area array detector, and the free light distribution range of the area array detector is larger than the wavelength range corresponding to the direction perpendicular to the diffraction grating lines, so that the stripes generated by different wavelengths will not overlap on the area array detector, thereby detecting multi-wavelength laser beams. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0022] Figure 1 This is a schematic diagram of the measuring device of the present invention;

[0023] Figure 2 This is a schematic diagram illustrating the laser beam wavelength resolution of this invention;

[0024] Figure 3 This is a schematic diagram of the interference fringes generated by the interference cavity of the present invention in the XZ plane of the photodetector;

[0025] Figure 4 This is a schematic diagram of the dispersion fringes generated by the diffraction grating of the present invention on the YZ plane of the photodetector;

[0026] Figure 5 This is a schematic diagram of the wavelength (spectral) intensity distribution obtained on the photodetector in an embodiment of the present invention;

[0027] Figure 6 This is a schematic diagram showing in-phase interference fringes on multiple rows of pixels of a photodetector within a broadband spectral range when measuring multi-wavelength laser input in an embodiment of the present invention.

[0028] Figure 7 This is a schematic diagram showing multiple sets of interference fringes on a single row of pixels of the detector within a narrow band spectrum when measuring multi-wavelength laser input in an embodiment of the present invention.

[0029] Among them, 1-spatial filter, 2-collimating lens group, 3-interference cavity, 4-dispersive element, 5-photodetector, 6-focusing element, 7-objective lens, and 8-filter aperture. Detailed Implementation

[0030] The present invention will now be described more clearly and completely by way of a preferred embodiment in conjunction with the accompanying drawings, but this does not limit the invention to the scope of the described embodiment.

[0031] like Figure 1 The image shown is a laser multi-wavelength measurement device disclosed in this invention, comprising a spatial filter 1 for filtering and removing noise from the laser beam under test, a collimating lens group 2 for collimating the laser beam filtered by the spatial filter 1 into a parallel beam, an interference cavity 3 for receiving the parallel beam and performing interference processing to form interference fringes, a dispersive element 4 for dispersing the interference fringes, and a focusing element 6 for focusing the dispersive beam of the dispersive element 4 onto a photodetector 5. The photodetector 5 receives the image of the focused laser beam under test and obtains the wavelength components of the laser beam under test based on the image.

[0032] Specifically, the spatial filter of the present invention includes an objective lens 7 for focusing and coupling the laser beam under test, and a filter aperture 8 located at the exit end of the objective lens 7, so that the transverse mode of the beam exiting through the filter aperture 8 is the Gaussian transverse mode, ensuring that the wavefront of the beam exiting after passing through the subsequent collimating lens group is a plane wave. The collimating lens group is an achromatic and spherical aberration-correcting lens, using an off-axis parabolic mirror, with a wavefront aberration less than λ, where λ is the minimum wavelength value within the wavelength measurement range of the device. The laser beam under test is focused and coupled by the objective lens 7 and incident on the filter aperture 8, which is located at the focal point of the collimating lens group, collimating the divergent light into parallel light, and then the parallel light is incident into the interference cavity 3 for interference processing.

[0033] The interference cavity 3 of the present invention includes a front surface perpendicular to the parallel beam and a rear surface forming a wedge angle with the front surface. Both the front and rear surfaces are coated with a beam-splitting film. After the parallel beam enters the interference cavity 3 from the front surface, it is reflected and transmitted between the front and rear surfaces. The beam emitted from the rear surface forms interference fringes along the wedge angle direction of the interference cavity 3.

[0034] The laser beam is processed by the interference cavity 3, the dispersive element 4 and the focusing element 6 for multi-wavelength identification, and then projected onto the photodetector 5.

[0035] The parallel beam exiting the collimating lens group is incident at an approximately perpendicular angle from the front surface of the interference cavity 3. The interference cavity of this invention uses a wedge plate with a beam-splitting film coated on both the front and back surfaces. One side of the wedge plate's inclined surface is the rear surface, and the side opposite the inclined surface is the front surface. The wedge angle is the angle of the wedge block, denoted as α. The center thickness of the wedge block is t. If we consider that the refractive index of the medium sandwiched between the front and back surfaces of the wedge plate is n, then the free spectral range (FSR) at the center of the interference cavity is:

[0036]

[0037] Where λ is the minimum wavelength value within the wavelength measurement range of the device.

[0038] This ensures that the interference cavity can interfere with all incoming laser beams to form interference fringes. The dispersion direction resolution of the dispersive element 4 and the focusing element 6 on the photodetector 5 is slightly higher than the free spectral range of the interference cavity 3. This ensures that the interference cavity will not produce two sets of phase overlapping fringes with wavelength differences approximately equal to the free spectral range within the spectral range corresponding to a single row of pixels on the detector. This avoids the interference cavity becoming indistinguishable due to periodic overlap.

[0039] like Figures 2-4 As shown, the dispersive element 4 of the present invention uses a diffraction grating. The grating's scribe line direction is parallel to the wedge angle direction of the interference cavity. The scribe line direction is denoted as the X direction, and the direction of the laser beam incident on the interference cavity is denoted as the Z direction. The laser beam is perpendicular to both the X and Z directions, and the upward direction is the Y direction as shown in the figure. The interference fringes exit the interference cavity and move at a certain angle into the diffraction grating. After being diffracted by the diffraction grating, diffracted light is formed with a dispersion angle of θ along the Y direction. The wavelength component beam after diffraction is focused in the Y direction onto the photodetector by the subsequent focusing element. Specifically, the focusing element 6 of the present invention is a one-dimensional focusing element, using a cylindrical mirror, with the focusing direction in the Y direction, perpendicular to the wedge angle direction of the interference cavity. The photodetector 5 is disposed on the back focal plane of the focusing element 6.

[0040] The photodetector 5 of this invention employs a planar array detector, wherein the free spectral range of the planar array detector is: FSR Inf ≥Δλ, Where Δλ is the wavelength range corresponding to the pixels of the area array detector along the direction perpendicular to the diffraction grating, f is the focal length of the focusing element, L is the number of line pairs of the diffraction grating, θ is the angle between the diffracted light of the diffraction grating and the direction perpendicular to the diffraction grating, and m is the grating order used; in order to prevent the fringes generated by different wavelengths from overlapping on the area array detector, so that the area array detector can accurately acquire the fringe images of each wavelength of the laser, thereby realizing accurate wavelength measurement of multi-wavelength lasers.

[0041] The present invention also provides a laser multi-wavelength measurement method using the above-mentioned laser multi-wavelength measurement device, comprising the steps of: filtering the laser beam to be measured to remove noise and collimating it into a parallel beam; performing interference processing on the parallel beam to form interference fringes; focusing the interference fringes onto a photodetector after dispersion by a dispersive element; and calculating the wavelength value by detecting the distribution of the interference fringes by the photodetector.

[0042] The following specific embodiments illustrate the laser measurement process of the present invention.

[0043] like Figure 5 The diagram shows a schematic of a photodetector receiving fringe light intensity signals. The pixels along the Y-axis receive light intensity signals of different wavelength ranges. The periodic fringes along the X-axis in each row represent the interference phase distribution of the wavelength to be measured. The specific wavelength value can be calculated based on the fringe phase. The following sections detail two specific scenarios for distinguishing wavelengths during multi-wavelength measurements.

[0044] Figure 6 This is a schematic diagram illustrating the presence of in-phase interference fringes on multiple rows of pixels of the detector within a broadband spectral range when measuring multi-wavelength laser input according to the embodiment of the present invention. At this time, light signals are detected on four consecutive rows of pixels, with the second and fourth rows of fringes having the same X-axis coordinates, meaning the two wavelengths are almost in phase. If the wavelength range corresponding to these four rows of pixels is 800–800.400 nm, the free spectral range of the interference cavity is 0.100 nm, and the wavelength corresponding to the second row of fringes is λ2 = 800.145 nm, then the wavelength value corresponding to the fourth row of in-phase fringes is approximately λ3 = 800.345 nm. However, in a traditional Fizeau laser wavelength meter, the interference cavity with the same free spectral range cannot simultaneously distinguish the wavelengths of these two input signals due to the overlapping of the in-phase fringes.

[0045] Figure 7This is a schematic diagram illustrating multiple sets of interference fringes on a single row of pixels in a narrow-band spectral range when measuring multi-wavelength laser input in the embodiment of the present invention. At this time, light signals are detected on three consecutive rows of pixels, with the third row exhibiting two sets of interference fringes with similar periodic frequencies and distinguishable phases. If the wavelength range corresponding to this row of pixels is 800.200–800.300 nm, and the free spectral range of the interference cavity is 0.100 nm, the wavelength values ​​obtained through phase calculation for the two sets of fringes are λ3 = 800.245 nm and λ4 = 800.265 nm, respectively. By reducing the included angle α of the wedges to increase the fringe phase resolution, sub-picometer wavelength resolution can be obtained, whereas traditional Fourier transform laser wavelengthmeters cannot resolve multi-wavelength inputs with such small wavelength intervals.

[0046] Accordingly, this invention achieves simultaneous measurement of multiple wavelength laser inputs within a broadband spectral range while maintaining the same resolution level as traditional laser wavelength meters, thus solving the problems of false resolution and resolution blind zone in traditional laser wavelength meters.

[0047] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A laser multi-wavelength measurement device, characterized in that, The system includes a spatial filter (1) for filtering and removing noise from the laser beam under test, a collimating lens group (2) for collimating the laser beam after filtering by the spatial filter (1) into a parallel beam, an interference cavity (3) for receiving the parallel beam and performing interference processing to form interference fringes, a dispersive element (4) for dispersing the interference fringes, and a focusing element (6) for focusing the dispersive beam of the dispersive element (4) onto a photodetector (5). The photodetector (5) receives the image of the laser beam under test after focusing and obtains the wavelength components of the laser beam under test based on the image. The dispersive element (4) is a diffraction grating, and the grating lines are parallel to the wedge angle of the interference cavity. The focusing element (6) is a one-dimensional focusing element, using a cylindrical mirror, with the focusing direction perpendicular to the wedge angle direction of the interference cavity, and the photodetector (5) is set on the back focal plane of the focusing element (6); The photodetector (5) is an area array detector, and the free spectral range of the area array detector is: , ,in Let f be the wavelength range corresponding to the pixels of the area array detector along the direction perpendicular to the diffraction grating lines, f be the focal length of the focusing element, and L be the number of line pairs of the diffraction grating. denoted by , where is the angle between the diffracted light from the diffraction grating and the direction perpendicular to the diffraction grating, and m is the grating order used. The dispersion direction resolution of the dispersive element (4) and the focusing element (6) on the photodetector (5) is greater than the free spectral range of the interferometer cavity (3).

2. The laser multi-wavelength measurement device according to claim 1, characterized in that, The interference cavity (3) includes a front surface perpendicular to the parallel beam and a rear surface forming a wedge angle with the front surface. Both the front and rear surfaces are coated with a beam-splitting film. After the parallel beam enters the interference cavity (3) from the front surface, it is reflected and transmitted between the front and rear surfaces. The beam emitted from the rear surface forms interference fringes along the wedge angle direction of the interference cavity (3).

3. The laser multi-wavelength measurement device according to claim 1, characterized in that, The spatial filter (1) includes an objective lens (7) for focusing and coupling the laser beam to be tested, and a filter aperture (8) located at the exit end of the objective lens (7), so that the transverse mode of the beam emitted through the filter aperture (8) is the Gaussian transverse mode.

4. The laser multi-wavelength measurement device according to claim 1, characterized in that, The collimating lens group (2) is an achromatic and spherical lens, employing an off-axis parabolic mirror with wavefront aberration less than 100°. , This is the minimum wavelength value within the wavelength measurement range of the device.

5. A laser multi-wavelength measurement method, using the laser multi-wavelength measurement device according to any one of claims 1-4, characterized in that, The process includes the following steps: filtering the laser beam to be tested to remove noise and collimating it into a parallel beam; interfering the parallel beam to form interference fringes; focusing the interference fringes onto a photodetector after dispersion by a dispersive element; and calculating the wavelength value based on the distribution of the interference fringes detected by the photodetector.

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