CN105784334A - Fiber laser beam quality measurement method base on photoelectric detector and CCD camera - Google Patents

Abstract

The invention relates to a fiber laser beam quality measurement method base on a photoelectric detector and a CCD camera. The method comprises: an optical axis of a measured light beam is adjusted to be in a horizontal state, and a beam splitter is arranged at an optical path and thus the measured light beam is divided into two parts that are used for measuring a near field and a far field of the light beam. On the basis of measurement of the near field and the far field of the measured light beam as well as a related data analysis, a near field width and a far field width of the light beam are calculated, so that light beam quality M <2> x and light beam quality M <2>y at a corresponding direction are obtained by calculation. An M<2> factor expression based on near field and far field light intensity distribution of a beam is deduced in theory, so that a defect that the existing M<2> factor measuring instrument can not evaluate the large-size fiber laser beam quality easily can be overcome. The provided fiber laser beam quality measurement method is suitable for fiber laser beam quality measurement including a few of high order modes and some cut fiber laser beam quality measurement.

Description

Optical-fiber laser beam quality measuring method based on photodetector and CCD camera

Technical field

The present invention relates to the measuring method of a kind of optical-fiber laser beam quality, especially a kind of method measuring optical-fiber laser beam quality based on photodetector and CCD camera.

Background technology

Beam quality is highly important index in laser practical application, it is generally recognized that it is from the aspect of matter to evaluate laser beam characteristic.The method at present laser beam quality being evaluated is a lot, such as M²The factor, the β factor, Si Telieer ratio etc. [Feng Guoying, Zhou Shouhuan, " discussion of laser beam quality overall merit ", Chinese laser, 36,1643-1653(2009)].In the nineties in 20th century, A.E.Siegman proposition M²The factor evaluates beam quality.M²The factor is defined as the waist width of actual light beam and the product of its far-field divergence angle ratio with the corresponding product of ideal beam, also it is current scientific research personnel and most commonly used evaluation methodology [A.E.Siegman that high-capacity optical fiber laser production firm adopts for optical-fiber laser beam quality, " Newdevelopmentinlaserresonator ", Proc.ofSPIE, 1990,2:1224].M²Factor value, be more than or equal to 1, works as M²The factor is closer to 1, and research worker thinks that the optical field distribution of output beam is closer to Gauss basic mode light beam, and beam quality is more good.

To laser beam M²The method of factor measurement generally includes following three classes: line-of-sight course, two-point method and hyperbolic fitting method.Line-of-sight course is measured more convenient with two-point method, but precision is not high；Hyperbolic fitting method is carried out hyperbolic fit by then passing through measurement multiple spot (5 available points should be at least respectively measured in usual far field, near field), thus precision is higher, but implementation is complicated, generally need to by relevant precision instrument.To optical-fiber laser M²Factor measurement generally adopts the M that Ophir-Spiricon company of the U.S. produces²Factor measuring instrument, its fundamental method of measurement adopted is hyperbolic fitting method, higher [the PengfeiMa of precision, RumaoTao, XiaolinWang, etal, " Coherentpolarizationbeamcombinationoffourmode-lockedfibe rMOPAsinpicosecondregime ", Opt.Express, 2014,22 (4): 4123-4130].But this M²Factor measuring instrument immobilizes because of its CCD limited resolution and convertible lens focal length, causes its scope of application critical constraints, and for the measurement of fundamental-mode gaussian beam, it is generally only available for precise measuring bundle waist width light beam [" M less than 5mm²-200/200s-FWUsersGuide”,2009,Ophir-SpiriconInc.DocumentNo:11382].Therefore, a kind of new method that can be used for measuring the beam quality of large-scale optical fiber laser is found, it appears particularly important.

Summary of the invention

The technical problem to be solved in the present invention is: provide the measuring method of a kind of optical-fiber laser beam quality based on photodetector and CCD camera.The method measures M²Factor principle is correct, and operation is simple, it is not necessary to special precision instrument, and the method is applicable to the optical-fiber laser that waist width is bigger.The laser beam quality of large-sized optical-fiber laser is significant.

The technical scheme is that

M²The definition of the factor is:

(1)

In above formula, ω is the waist width of actual light beam, and θ is the far-field divergence angle of actual light beam, ω₀、θ₀The respectively waist width of ideal beam (taking fundamental-mode gaussian beam in Siegman theory is ideal beam) and far-field divergence angle, and meet:

(2)

Wherein λ is optical wavelength.Bring (2) formula into M²The definition of the factor can obtain:

(3)

(4)

ω_x(θ_x)、ω_y(θ_y) respectively actual light beam x, waist width (far-field divergence angle) on y direction.Therefore the M to light beam²The factor is measured and is just converted to the near field size (ω to light beam_x、ω_y) and far-field divergence angle (θ_x、θ_y) measurement.

In the spatial domain, the definition of width of light beam generally has three kinds, i.e. 1/n definition, encircled power and second-order moments.Encircled power and second-order moments mode are both needed to accurately obtain near-field beam hot spot full figure to be measured, and then draw near-field beam width by correlation computations.For obtaining light field whole audience information accurately, generally need to utilize the CCD that target surface is completely covered hot spot that the light intensity of tested light beam is measured, but CCD target surface size is generally only several millimeter, it is impossible to measure large scale hot spot.And with 1/n definition, the measurement of near field light beam width is only needed to measure and obtains the curve of light distribution through beam center largest light intensity point.1/n definition refers on curve of light distribution I (r), maximum I_maxThe half of place's distance between two points be defined as width of light beam, common n value has e², e and 2 etc..For desirable fundamental-mode gaussian beam, 1/e²The width of light beam that definition calculates with 86.5% encircled power, second-order moments is completely the same, thus we select 1/e²Define and near field light beam width is measured, ω can be obtained_xAnd ω_y。

Owing to tested light beam is generally collimated beam, collimator distance, generally at about hundred meters, is basically unchanged in collimator distance inner light beam size, therefore in laboratory, the measurement of the far field beam angle of divergence can not be passed through two-point method or line-of-sight course measurement draws.Condenser lens must be utilized, tested light beam is focused on lens focus position namely the desirable far field of tested light beam, by the measurement of far-field spot being drawn the width of light beam ω on x, y direction of far-field spot_x-farAnd ω_y-farHerein owing to utilizing CCD that far-field measurement can obtain the whole audience light distribution of focal beam spot, thus the width of light beam in far field is calculated the defined formula adopting second moment, as shown in (5), (6) two formulas, this definition make use of the whole audience intensity signal of light beam, calculates more accurate.

(5)

(6)

(7)

I in two formulas above_far(x, y) far-field intensity distribution detected for CCD camera.Calculate far-field spot width of light beam ω on x, y direction_x-farAnd ω_y-farAfterwards, available following (8), (9) two formulas, the far-field divergence angle θ of light beam can be calculated_xAnd θ_y, in formula, f is the focal length of condenser lens.

(8)

(9)

(8), (9) two formulas are substituted into (3), (4) two formulas respectively, obtains the M of tested light beam²The factor is:

(10)

(11)

Utilize (5) ~ (7) and (10) ~ (11) formula can calculate the M of tested light beam²The factor, calculated M²The factor may be used for evaluating the beam quality of tested light beam.

Concrete technical scheme is as follows:

This method utilizes M²Large-scale optical fiber laser beam quality evaluated by factor measuring instrument, tested light beam first carries out beam quality measurement, builds optical path.First by the optical axis level-off of tested light beam 1, put into spectroscope 2 in the optical path, tested light beam is divided into reflection light beam 3 and transmitted light beam 4 two parts, is respectively used near field and the far field of measuring beam.

For the measurement of near-field beam, we utilize the photodetector 6 on the three-dimensional regulation platform 5 being positioned over micrometer caliper to measure.Photodetector 6 can convert the intensity signal detected to information of voltage, is shown in real time by oscillograph 7, and the response of the light of Same Wavelength is linear by photodetector 6, and therefore the magnitude of voltage of oscillograph 7 display can be equivalent to light intensity value.Concrete method of testing is as shown in Figure 2.By regulating with the micrometer caliper in the X-direction of the three-dimensional regulation platform 5 of micrometer caliper, photodetector can be positioned over tested light beam edge by the diverse location of the X-direction of beam center, and then the magnitude of voltage that the light intensity obtaining diverse location is corresponding can be measured.Take an interval and keep certain, take a number and ensure that minimum light intensity values is less than the 10% of largest light intensity value, to ensure the reliability that follow-up data processes.The magnitude of voltage that multimetering is obtained does Gaussian function fitting, draws near-field beam width ω according to fitting function_x.Measuring principle and the step of Y-direction are same as described above.

For the measurement of far field beam, we adopt condenser lens 8 as shown in Figure 1 that transmitted light beam 4 is focused on lens focus position, i.e. far field beam, and by CCD camera 9 to focal beam spot imaging, by computer 10, imaging facula are displayed and data acquisition.CCD camera 9 resolution is sufficiently high, and condenser lens 8 focal distance f answers long enough, with coordinating of focal length, resolution should ensure that far-field spot at least occupies 20 pixels.

By to tested near-field beam, the measurement in far field and Correlative data analysis, near-field beam width (ω can be calculated_xAnd ω_y) and far field width (ω_x-farAnd ω_y-far), and then utilize formula (10) ~ (11) that the beam quality M in respective direction can be calculated_x ²And M_y ²。

Adopt the present invention can reach techniques below effect:

1. it is deduced the M in the near field based on light beam and far-field intensity distribution theoretically²Factor expression, solves M²Factor measuring instrument is difficult to evaluate the defect of large-scale optical fiber laser beam quality.

2. simply correct based on the optical-fiber laser beam quality measuring method principle of photodetector and CCD camera, and owing to measuring method being discrete photoelectric device, it is not necessary to other precision instruments, easy to operate, light path can easily be accommodated.

3. it is applicable to the optical-fiber laser beam quality containing a small amount of higher order mode based on the optical-fiber laser beam quality measuring method of photodetector and CCD camera measure.

4. it is applicable to, based on the optical-fiber laser beam quality measuring method of photodetector and CCD camera, the optical-fiber laser beam quality measurement that part is blocked.

Accompanying drawing explanation

Fig. 1 is the optical-fiber laser beam quality new method of measuring principle schematic based on photodetector and CCD camera,

The tested light beam of 1-；2 spectroscopes；3-reflects light beam；4-transmitted light beam；5-is with the three-dimensional regulation platform of micrometer caliper；6-photodetector；7-oscillograph；8-condenser lens；9-CCD camera；10-computer.

Fig. 2 is the near-field beam measuring principle schematic diagram based on photodetector,

Fig. 3 is the near field intensity distribution figure in the X-direction of tested light beam,

Fig. 4 is the near field intensity distribution figure in the Y-direction of tested light beam,

Fig. 5 is the tested far field beam surface of intensity distribution after lens focus,

Fig. 6 is for utilizing M²The beam quality result figure that-200s-FW measuring instrument is measured.

Detailed description of the invention

As shown in Figure 1, first by the optical axis level-off of tested light beam 1, put into spectroscope 2 in the optical path, tested light beam is divided into reflection light beam 3 and transmitted light beam 4 two parts, is respectively used near field and the far field of measuring beam.For the measurement of near-field beam, we utilize the photodetector 6 on the three-dimensional regulation platform 5 being positioned over micrometer caliper to measure.Photodetector 6 can convert the intensity signal detected to information of voltage, is shown in real time by oscillograph 7, and the magnitude of voltage of oscillograph 7 display can be equivalent to light intensity value.Concrete method of testing is as shown in Figure 2.By the micrometer caliper in the X-direction of adjustment 5, photodetector can be positioned over tested light beam edge by the diverse location of the X-direction of beam center, and then the magnitude of voltage that the light intensity obtaining diverse location is corresponding can be measured.The magnitude of voltage that multimetering is obtained does Gaussian function fitting, draws near-field beam width ω according to fitting function_x.Measuring principle and the step of Y-direction are same as described above.For the measurement of far field beam, we adopt condenser lens 8 as shown in Figure 1 that transmitted light beam 4 is focused on lens focus position, i.e. far field beam, and by CCD camera 9 to focal beam spot imaging, by computer 10, imaging facula are displayed and data acquisition.By to tested near-field beam, the measurement in far field and Correlative data analysis, near-field beam width (ω can be calculated_xAnd ω_y) and far field width (ω_x-farAnd ω_y-far), and then utilize formula (10) ~ (11) that the beam quality M in respective direction can be calculated_x ²And M_y ²。

This laboratory has built experiment porch as shown in Figure 1 according to design above, measures near field and the far field of tested light beam.Near field light intensity values is accompanying drawing 3(X direction such as) and accompanying drawing 4(Y direction) shown in, data point is photodetector measured data value, and curve is Gauss curve fitting curve.X and light path in Y-direction test process have certain adjustment, thus X is different with Y-direction light intensity value, but unidirectional test carries out under identical conditions, therefore measurement data can reflect the light distribution of tested near-field beam.X and the Intensity of Gaussian fitting function respectively (12) of Y-direction, (13) formula.Utilize foregoing 1/e²Near field light beam width is calculated by definition, can obtain ω_xAnd ω_yRespectively 7.20mm and 7.16mm.

(12)

(13)

The measurement of far field beam is realized by condenser lens and CCD camera imaging.Condenser lens focal distance f is 1.04m, CCD target surface resolution is 4.4 μm/pixel.Far-field spot image is as shown in Figure 5.Utilize second-order moments far field beams width recited above, through can be calculated ω_x-farAnd ω_y-farRespectively 66.25 μm and 60.70 μm.And then by ω_x/ω_yAnd ω_x-far/ω_y-farBring formula (10)/(11) into, the beam quality M in respective direction can be calculated_x ²/M_y ²For 1.35/1.24.

In order to verify the result that above-mentioned beam quality measuring method calculates, we utilize the M of Ophir-Spiricon company²The beam quality of tested light beam has been measured by-200s-FW measuring instrument, measurement result as shown in Figure 6, M_x ²/M_y ²For 1.34/1.30.By contrasting it is found that the optical-fiber laser beam quality measuring method based on photodetector and CCD camera can realize optical-fiber laser beam quality is comparatively accurately measured.

Claims (3)

1., based on the optical-fiber laser beam quality measuring method of photodetector and CCD camera, utilize M²Large-scale optical fiber laser beam quality evaluated by factor measuring instrument, tested light beam is carried out beam quality measurement, build optical path, it is characterized in that, first by the optical axis level-off of tested light beam (1), put into spectroscope (2) in the optical path, tested light beam is divided into reflection light beam (3) and transmitted light beam (4) two parts, it is respectively used near field and the far field of measuring beam

For the measurement of near-field beam, utilize photodetector (6) measurement on the three-dimensional regulation platform (5) being positioned over micrometer caliper,

For the measurement of far field beam, adopt condenser lens (8) that transmitted light beam (4) is focused on lens focus position, i.e. far field beam, and by CCD camera (9) to focal beam spot imaging, by computer (10), imaging facula is displayed and data acquisition,

By to tested near-field beam, the measurement in far field and data analysis, namely calculating near-field beam width ω_xAnd ω_yWith far field width ω_x-farAnd ω_y-far, and then calculate the beam quality M in respective direction_x ²And M_y ²。

null2. the optical-fiber laser beam quality measuring method based on photodetector and CCD camera according to claim 1，It is characterized in that，Described photodetector (6) converts the intensity signal detected to information of voltage，Shown in real time by oscillograph (7)，The response of the light of Same Wavelength is linear by photodetector (6)，Therefore the magnitude of voltage that oscillograph (7) shows is equivalent to light intensity value，By regulating with the micrometer caliper in the X-direction of the three-dimensional regulation platform (5) of micrometer caliper，Photodetector is positioned over tested light beam along by the diverse location of the X-direction of beam center，And then measure the magnitude of voltage that the light intensity obtaining diverse location is corresponding，Take an interval and keep fixing，Take a number and ensure that minimum light intensity values is less than the 10% of largest light intensity value，The magnitude of voltage that multimetering is obtained does Gaussian function fitting，Near-field beam width ω is drawn according to fitting function_x, the measuring process of Y-direction is identical with X-direction.

3. the optical-fiber laser beam quality measuring method based on photodetector and CCD camera according to claim 1, it is characterised in that M²The calculating process of the factor is as follows:

M²Factor definition is:

(1)

In above formula, ω is the waist width of actual light beam, and θ is the far-field divergence angle of actual light beam, ω₀、θ₀The respectively waist width of ideal beam and far-field divergence angle, and meet:

(2)

Wherein λ is optical wavelength, brings (2) formula into M²The definition of the factor can obtain:

(3)

(4)

ω_x(θ_x)、ω_y(θ_y) respectively actual light beam x, waist width on y direction, i.e. far-field divergence angle, the therefore M to light beam²The factor is measured and is just converted to the near field size (ω to light beam_x、ω_y) and far-field divergence angle (θ_x、θ_y) measurement,

Use 1/e²Define and near field light beam width is measured, namely obtain near field size ω_xAnd ω_y；

The width of light beam in far field is calculated the defined formula adopting second moment, as shown in (5), (6) two formulas,

(5)

(6)

(7)

I in formula above_far(x, y) far-field intensity distribution detected for CCD camera calculate far-field spot width of light beam ω on x, y direction_x-farAnd ω_y-farAfterwards, utilize formula (8), (9), namely calculate the far-field divergence angle θ of light beam_xAnd θ_y, in formula, f is the focal length of condenser lens,

(8)

(9)

(8), (9) two formulas are substituted into (3), (4) two formulas respectively, obtains the M of tested light beam²The factor is:

(10)

(11)

Utilize (5) ~ (7) and (10) ~ (11) formula can calculate the M of tested light beam²The factor.