CN101013061A - Method for measuring focus and equivalent f coefficient using optical grating type wave-front curvature sensing unit - Google Patents

Abstract

The invention discloses a method using grating wave front curvature sensor to measure focus and equivalent f number, the steps being: firstly, using reference beam to calibrate the grating wave front curvature sensor; placing the measured device in the optical path, and through the transmission or reflection way to generate the dispersing focus wave front of the measured focus, and the dispersing focus wave front incidents into the grating wave front curvature sensor and using the grating wave front curvature sensor to detect the focus away spot of two focus away surfaces, and measuring diameter d1 of the front focus away spot and the rear focus away spot d2, and according to the grating wave front curvature sensor design parameters, obtaining focus away grating equivalent focal length fg, and the short focal length lens focal length fl; calculating the normalized difference S of the two defocus spot diameter d1 and d2; judging the two defocus spots position relation, and through various formulas, respectively calculating the focal length and equivalent f number. The invention has advantages of high measurement precision, wide application range, and low cost.

Description

Method with grating type wavefront curvature sensor focal length measurement and equivalent f-number

Technical field

The present invention is mainly concerned with the optical detection field, refers in particular to a kind of method with grating type wavefront curvature sensor focal length measurement and equivalent f-number.

Background technology

In the prior art, the focal length of measuring convex lens or concave mirror has a lot of methods, can measure with the direct imaging method, by measuring the focal length that object distance and image-forming range calculate them, also can measure the focal length of convex mirror or concave mirror with method of astigmatism, can also measure the radius-of-curvature of the face shape of lens or reflecting sphere mirror in addition with interferometric method, obtain their focal length.The method of in the solid state laser gain medium, measuring thermal lens have also have multiple, the simplest method is to make detecting light beam pass through gain media, and measures focal position moving vertically, and this method has comprised mobile device, can't measure the transient heat focal length of lens, system is also unstable.The wavefront measurement method can complete understanding thermal effect.Classical is to use the interferometer method, and this method is very suitable for the major diameter rod of profile pump, but can not be used for the situation of end pumping; Simultaneously can also use phase shift interference method and lateral shearing interference method, these methods are very accurate, but very complicated.Can also use commercial Shack-Hartmann wave front sensor to measure the wavefront of heat distortion in addition, thereby determine the focal length of thermal lens, but this method equipment cost is higher.

Roddier proposed the curvature sensing technology in 1988, by measuring the light intensity on the symmetrical out of focus face, obtained wavefront curvature and distributed.As shown in Figure 1, the curvature sensing technology can be understood so intuitively: when wavefront was propagated forward, light was along the normal direction transmission of wavefront surface, and recessed wavefront part will cause light to converge, and light intensity becomes stronger after the beam Propagation; On the contrary, protruding wavefront partly makes divergence of beam, and it is more weak that light intensity becomes.Therefore the light intensity difference and the wavefront curvature of corresponding point has corresponding relation on two planes, transmission front and back, just can determine the curvature distribution and the wavefront itself of wavefront by two plane light intensity difference signals before and after measuring.Enter object lens L among Fig. 2 ₁Distorted wavefront Be focused in F.Equidistant S before and after focus ₁And S ₂Measure light distribution on the cross section respectively

With

Can find out qualitatively that if incident wavefront is the desirable wavefront of a no aberration, promptly the curvature of each point is constant on the wavefront, does not also consider the aberration of object lens, then burnt starting section S ₁With defocused section S ₂On light distribution be identical, uniformly.If incident wavefront has distortion, promptly the curvature of each point changes on the wavefront, then S ₁Face and S ₂Light distribution on the face is no longer identical, and is no longer even.If the light intensity on cross section increases, then the light intensity of corresponding point must reduce on another cross section.This shows S ₁And S ₂The curvature distribution of the regularity of distribution of the difference of corresponding point light intensity and incident wavefront has inner link on the cross section.Under the approximation in geometric optics condition, can prove S ₁And S ₂The corresponding point light distribution is poor on the cross section

And the available Poisson equation of relation between the normal direction slope of the curvature distribution of incident wavefront and pupil edge wavefront is expressed as:

$S\left(\stackrel{\→}{r}\right)=\frac{I_2(-\stackrel{\→}{r})-I_1\left(\stackrel{\→}{r}\right)}{I_2(-\stackrel{\→}{r})+I_1\left(\stackrel{\→}{r}\right)}=\frac{\mathrm{\Δ}{I}_2(-\stackrel{\→}{r})-\mathrm{\Δ}{I}_1\left(\stackrel{\→}{r}\right)}{2I_1{\left(\stackrel{\→}{r}\right)}_0}$

Wherein,

Be distorted wavefront,

Be the light distribution of front and back out of focus face, l is a defocusing amount, Be normalized curvature sensor signal.

Once realized the wavefront curvature sensing technology with spectroscopic method and vibration film modulate emission mirror method in the past, these two kinds of methods exist manufacture craft and require high, defectives such as puppet is measured simultaneously, system's instability.Humans such as Paul M.Blanchard in 2000 defocus grating (coming down to the fresnel's zone plate from axle) and have realized the grating type wavefront curvature sensor.Defocusing grating is a kind of grating of raster graphic generation secondary distortion.According to displacement phase shift theorem (detour phase effect), compare with ordinary straight lines grating, the secondary of figure twist to grating ± 1 order diffraction light belt defocuses and differs, and has equal and opposite in direction, the equivalent focal length of opposite in sign.In the grating type wavefront curvature sensor that people such as Paul M.Blanchard propose, defocusing grating and short focal length lens connects airtight use, is illustrated in figure 3 as grating type wavefront curvature sensor light path synoptic diagram.Defocus grating and in fact also be fresnel's zone plate, have a plurality of diffraction optical axises of common grating, the characteristics such as a plurality of focal lengths of fresnel's zone plate simultaneously, make that defocusing grating has different focal lengths on the different orders of diffraction from axle.Wherein on ± 1 order of diffraction, defocus focal length ± f that grating has the equal and opposite in direction opposite in sign _gWhen defocusing grating and short focal length lens (or image-forming component such as lens combination, camera lens, the Fresnel lens) use of combining closely, the focal distance f of lens _lMain focusing power is provided, and grating is finely tuned focusing power respectively on ± 1 order of diffraction.Therefore, grating lens be combined in ± equivalent focal length on 1 order of diffraction is shorter than and is longer than the focal length of independent lens respectively.If with photodetector, be placed on as ccd detector on the focal plane of lens, the zero order diffracted light of grating lens combination is a focal spot on CCD, this combination ± 1 order diffraction light is the defocused spot of symmetry on CCD.These two defocused spot are corresponding to the light distribution on the needed symmetrical out of focus face of wavefront curvature sensor.Therefore the grating type wavefront curvature sensor can be realized entirely optically by defocusing grating.

Summary of the invention

The technical problem to be solved in the present invention just is: at the technical matters that prior art exists, the invention provides the method with grating type wavefront curvature sensor focal length measurement and equivalent f-number that a kind of measuring method is directly perceived, simple and easy, measuring accuracy is high, applied widely, with low cost.

For solving the problems of the technologies described above, the solution that the present invention proposes is: a kind of method with grating type wavefront curvature sensor focal length measurement and equivalent f-number is characterized in that step is:

1.. at first demarcate the grating type wavefront curvature sensor with reference beam;

2.. device under test is placed light path, produce the wavefront that defocuses by transmission or reflection way with focal length to be measured, this defocuses wavefront goes into to inject in the grating type wavefront curvature sensor and uses the grating type wavefront curvature sensor to survey the defocused spot of two out of focus faces, the diameter d of defocused spot before measuring ₁With back defocused spot d ₂, obtain defocusing grating equivalent focal length f according to the design parameter of grating type wavefront curvature sensor _g, the short focal length lens focal distance f _l

3.. by following formula (5) two defocus spot diameter d ₁And d ₂Normalized poor S be

$S=\frac{d_1-d_2}{d_1+d_2}=\frac{f_g}{f}---\left(5\right)$

4.. when one of two defocused spot that obtain are preceding out of focus, another be afterwards during defocused spot, as can be known | f|＞f _g, then calculate the focal length at wavefront curvature sensor entrance pupil place by following formula (7).Because the focal length that (7) formula measures is relevant with the measuring position, can utilize formula (8) to calculate and the irrelevant equivalent f-number in measuring position, utilize the known emergent pupil bore φ of equivalent f-number and device under test simultaneously, can further calculate the focal distance f at device under test emergent pupil place according to (11) formula _o

$f=\frac{f_g}{S}---\left(7\right)$

$f/\#=\frac{f}{d}=\frac{2f_l}{d_1-d_2}---\left(8\right)$

5.. when two defocused spot that obtain all be preceding defocused spot or after defocused spot, as can be known | f|≤f _gCalculate the focal length at wavefront curvature sensor entrance pupil place by following formula (9), or through type (10) calculates and the irrelevant equivalent f-number in measuring position, utilize the known emergent pupil bore φ of equivalent f-number and device under test simultaneously, can further calculate the focal distance f at device under test emergent pupil place according to (11) formula _o

f＝f _gS (9)

$f/\#=\frac{f}{d}=\frac{2f_l}{d_1+d_2}---\left(10\right)$

f _o＝φf/# (11)

Described step 1. in, at first adjust the incident angle of reference beam, make three diffraction patterns that detect on the detection plane in the grating type wavefront curvature sensor be positioned at the detection plane center, adjust the distance between grating lens combination and the detector plane, make timing signal two defocused spot equal and opposite in directions.

Compared with prior art, advantage of the present invention just is:

1, the present invention's method of grating type wavefront curvature sensor focal length measurement and equivalent f-number, the light path of total system is aimed at easily.In the grating type wavefront curvature sensor, the incident beam process defocuses grating and short focal length lens is imaged on the detector plane.The light-sensitive surface size w of the field angle θ of grating type wavefront curvature sensor and CCD, the focal distance f of short focal length lens _lRelevant: $\mathrm{\θ}=2\mathrm{arctg}\frac{w}{2f_l},$ This field angle reaches more than 20 °.Therefore as long as incident beam can just can detect on the CCD test surface in the field angle scope of grating type wavefront curvature sensor.Further adjust the incident angle of incident beam, will have the light beam that defocuses wavefront roughly is imaged onto ccd detector by the grating type wavefront curvature sensor center.By adjusting the distance between grating lens combination and the ccd detector, make the focal plane of detection plane and short focal length lens overlap.During two planes overlapping, the equal and opposite in direction of two defocused spot that detect.Just finished the light path aligning that grating type wavefront curvature sensor measurements defocuses the measuring system of wavefront focal length this moment;

2, the present invention is fairly simple with the demarcation of the method measuring system of grating type wavefront curvature sensor focal length measurement and equivalent f-number.Usually earlier demarcate the grating type wavefront curvature sensor before measuring with the reference beam of beam-expanding collimation, as long as the focal plane adjustment of detection plane and short focal length lens is overlapped, be that two spot sizes that ccd detector detects equate, the variation of two spot sizes that after this detect, the focal length that defocuses wavefront that just corresponding lens, spherical reflector or the thermal lens that inserts causes.Even before inserting the optical element of focal length to be measured, detecting light beam itself has defocusing or focal length to a certain degree because of a variety of causes, at this moment also only need to adjust the distance between detection plane and the short focal length lens, make two defocused spot equal and opposite in directions that ccd detector detects, the variation of two spot sizes that after this detect, the focal length that defocuses wavefront that just corresponding the lens that inserted, spherical reflector or thermal lens cause;

3, the measuring method of measurement thermal lensing effect proposed by the invention is simpler and easy, only needs to use photodetector, detects the size of two hot spots as ccd detector, and lower to the requirement of ccd detector, common CCD can satisfy the measurement needs.The computing method of measurement thermal lensing effect proposed by the invention are more directly perceived, and computing method are simple algebraic operation, do not need complicated Processing Algorithm;

4, interferometer mode and Shack-Hartmann are very sensitive to factors such as vibrations, need vibration isolation measurement environment preferably.Grating type wavefront curvature sensor used in the present invention system antijamming capability is strong, and is insensitive to vibrating, and measurement environment required low, and factory or common lab condition all can;

5, use the with low cost of grating type wavefront curvature sensor, critical component is to defocus grating, and remaining part all is camera lens (or image-forming components such as lens combination, camera lens, Fresnel lens), photodetectors of industrialization, as ccd detector etc.

Description of drawings

Fig. 1 is the synoptic diagram of curvature sensing technology;

Fig. 2 is a curvature sensing technology optical principle synoptic diagram;

Fig. 3 is a grating type wavefront curvature sensor light path synoptic diagram;

Fig. 4 is | f|＞f _gThe time grating type wavefront curvature sensor measure the geometric relationship synoptic diagram of thermal lensing effect;

Fig. 5 is | f|≤f _gThe time grating type wavefront curvature sensor measure the geometric relationship synoptic diagram of thermal lensing effect;

Fig. 6 is the experimental layout synoptic diagram of the present invention with grating type wavefront curvature sensor focal length measurement;

Fig. 7 is a grating type wavefront curvature sensor focal length measurement schematic flow sheet of the present invention;

Fig. 8 is the dielectric rod reference hot spot synoptic diagram that the grating type wavefront curvature sensor does not measure during pumping;

Fig. 9 is the hot spot synoptic diagram that measures when thermal lensing effect is the most serious after 10,20,30,40 pulses of pumping.

Embodiment

Below with reference to the drawings and specific embodiments the present invention is described in further details.

Lens, reflecting sphere mirror, thermal lens etc. can produce the spherical wave front with certain curvature radius or focal length, i.e. quadravalence zernike polynomial distorted wavefront---defocus wavefront.Incident wavefront is when defocusing wavefront, and two defocus spot diameter that the grating type wavefront curvature sensor is collected change.Therefore, can determine the focal length of incident wavefront by simple spot diameter measurement, i.e. the focal length of institute's survey lens, reflecting sphere mirror, thermal lens.When the incident wavefront of grating type wavefront curvature sensor is a plane wave, the diameter d of the back defocused spot that the grating type wavefront curvature sensor measures ₁Diameter d with preceding defocused spot ₂Equate.When incident wavefront is to defocus wavefront, the diameter d of two defocused spot that the grating type wavefront curvature sensor measures ₁And d ₂Unequal.

The equivalent focal length that grating lens is combined on ± 1 order of diffraction is

$f_{\mathrm{eff}}=\frac{1}{\frac{1}{f_l}\±\frac{1}{f_g}}.---\left(2\right)$

Be combined in by defocusing wavefront focal distance f, grating lens ± equivalent focal length f on 1 order of diffraction _EffThe total equivalent focal length f that determines _WholeFor

$f_{\mathrm{whole}}=\frac{1}{\frac{1}{f_l}\±\frac{1}{f_g}+\frac{1}{f}}.---\left(3\right)$

When | f|＞f _gThe time, one of two defocused spot are preceding out of focus, another is the back defocused spot.As shown in Figure 4, be | f|＞f _gThe time grating type wavefront curvature sensor measurement defocus the geometric relationship figure of wavefront.The diameter d of two defocused spot on the detection plane relevant with total effective focal length ₁And d ₂For

$\left\{\begin{array}{ccc}{d}_1=(\frac{1}{f_g}+\frac{1}{f})f_{l}d& \mathrm{for}+1& \mathrm{order}\\ d_2=(\frac{1}{f_g}-\frac{1}{f})f_{l}d& \mathrm{for}-1& \mathrm{order}\end{array}\right.---\left(4\right)$

Wherein, d is the spot diameter that the entrance pupil place defocuses wavefront, f _lBe the short focal length lens focal length, f _gBe to defocus the grating equivalent focal length, two defocus spot diameter d ₁And d ₂Normalized poor S be

$S=\frac{d_1-d_2}{d_1+d_2}=\frac{f_g}{f}---\left(5\right)$

Two defocus spot diameter d ₁And d ₂Absolute difference be

$d_1-d_2=\frac{2f_{l}d}{f}---\left(6\right)$

Therefore, as | f|＞f _gThe time, the focal distance f that defocuses wavefront can be determined by the normalized difference of two defocus spot diameter that measure

$f=\frac{f_g}{S},\mathrm{if}\left|f\right|>f_g---\left(7\right)$

Because the focal length that (7) formula measures is relevant with the measuring position, the focal length of device under test is apart from sum between the measured focal length of (7) formula and curvature sensor and the device under test.Measure for long-focus, can ignore the influence of measuring position measurement result; But, then can not ignore for short focometry.Available formula (8) calculates and the irrelevant equivalent f-number in measuring position.Utilize the known emergent pupil bore φ of equivalent f-number and device under test, can further calculate the focal distance f at device under test emergent pupil place according to (11) formula _o

The equivalent f-number f/# that defocuses wavefront is

$f/\#=\frac{f}{d}=\frac{2f_l}{d_1-d_2}---\left(8\right)$

When | f|≤f _gThe time, two defocused spot all are preceding defocused spot or back defocused spot, as shown in Figure 5.In like manner, the focal distance f that defocuses wavefront is

f＝f _gS， if|f|≤f _g (9)

The equivalent f-number f/# that defocuses wavefront is

$f/\#=\frac{f}{d}=\frac{2f_l}{d_1+d_2}---\left(10\right)$

Device under test emergent pupil place focal distance f _oAnd the pass between the equivalent f-number, emergent pupil bore is

f _o＝φf/# (11)

From (7) and (9) formula as can be seen, the grating type wavefront curvature sensor almost can be measured the gamut of the focal length that defocuses wavefront theoretically.But ccd detector is subject to factors such as resolution in the reality, if it is excessive to defocus the focal length of wavefront, then the variation of two defocus spot diameter is very little, and ccd detector can't detect this variation.

Derivation according to above theory and formula can get measuring method of the present invention:

1.. at first demarcate the grating type wavefront curvature sensor with reference beam;

2.. device under test is placed light path, produce the wavefront that defocuses by transmission or reflection way with focal length to be measured, this defocuses wavefront goes into to inject in the grating type wavefront curvature sensor and uses the grating type wavefront curvature sensor to survey the defocused spot of two out of focus faces, the diameter d of defocused spot before measuring ₁With back defocused spot d ₂, obtain defocusing grating equivalent focal length f according to the design parameter of grating type wavefront curvature sensor _g, the short focal length lens focal distance f _l

3.. by following formula (5) two defocus spot diameter d ₁And d ₂Normalized poor S be

$S=\frac{d_1-d_2}{d_1+d_2}=\frac{f_g}{f}---\left(5\right)$

4.. when one of two defocused spot that obtain are preceding out of focus, another be afterwards during defocused spot, as can be known | f|＞f _g, then calculate focal length or equivalent f-number respectively by following formula (7) and formula (8);

$f=\frac{f_g}{S}---\left(7\right)$

$f/\#=\frac{f}{d}=\frac{2f_l}{d_1-d_2}---\left(8\right)$

5.. when two defocused spot that obtain all be preceding defocused spot or after defocused spot, as can be known | f|≤f _g, calculate focal length or equivalent f-number respectively by following formula (9) and formula (10);

f＝f _gS (9)

$f/\#=\frac{f}{d}=\frac{2f_l}{d_1+d_2}---\left(10\right)$

Wherein, its scaling method is in step (1): the incident angle of at first adjusting reference beam, make three diffraction patterns that detect on the detection plane in the grating type wavefront curvature sensor be positioned at the detection plane center, adjust the distance between grating lens combination and the detector plane, make timing signal two defocused spot equal and opposite in directions.

The experimental layout figure of use grating type wavefront curvature sensor focal length measurement as shown in Figure 6.Generally speaking, demarcate the grating type wavefront curvature sensor with the reference beam of beam-expanding collimation earlier, adjust the distance between the combination of detection plane and grating lens, make two defocused spot equal and opposite in directions that ccd detector detects, the variation of two spot sizes that after this detect, the focal length that defocuses wavefront that just corresponding the lens that inserted, spherical reflector or thermal lens cause.By modes such as transmission or reflections, defocus curvature sensor before the wavefront incident afferent echo by what element such as lens, reflecting sphere mirror, thermal lens or reason produced, survey the hot spot of two out of focus faces by being positioned at ccd detector on the short focal length lens focal plane in the wavefront curvature sensor.Calculate the size of two defocused spot, calculate focal distance f to be measured or equivalent f-number f/# by formula (7), (8), (9), (10).

Even before inserting the optical element of focal length to be measured, detecting light beam itself has defocusing or focal length to a certain degree because of a variety of causes.Can calculate through the principle identical with preceding surface analysis, at this moment only need to adjust the distance between detection plane and the grating lens, make two defocused spot equal and opposite in directions that ccd detector detects, the variation of two spot sizes that after this detect, the focal length that defocuses wavefront that just corresponding the lens that inserted, spherical reflector or thermal lens cause.When two defocused spot equal and opposite in directions, the CCD test surface is positioned at incident wavefront through on the imaging plane of short focal length lens.

The process flow diagram of use grating type wavefront curvature sensor focal length measurement as shown in Figure 7.At first demarcate the grating type wavefront curvature sensor with reference beam.Adjust the incident angle of light beam, make three diffraction patterns that detect on the detection plane in the grating type wavefront curvature sensor be positioned at the detection plane center.Adjust the distance between grating lens combination and the detector plane, make timing signal two defocused spot equal and opposite in directions.Secondly, in light path, insert element or reasons such as lens, reflecting sphere mirror, thermal lens, produce the wavefront that defocuses with focal length to be measured by transmission or reflection way.This defocuses wavefront goes into to inject in the grating type wavefront curvature sensor, uses wavefront curvature sensor to survey the hot spot of two out of focus faces.Calculate the size of two hot spots by Computer Analysis.Judge magnitude relationship between focal length to be measured and the grating focal length by the variation of out-of-focus appearance then.At last, if | f|＞f _g, then use (7), (8) formula to calculate focal length or equivalent f-number; If | f|＜f _g, then use (9), (10) formula to calculate focal length or equivalent f-number.

In a specific embodiment, use the grating type wavefront curvature sensor to measure thermal lensing effect and verify experimentally.Use the grating type wavefront curvature sensor to measure the thermal lensing effect of solid state heat capacity laser gain media rod.This solid state heat capacity laser average power 2kW, wavelength is 1053nm, burst operative mode, high repetition frequency is 10Hz.The parallel He-Ne laser beam of a branch of expansion is passed through dielectric rod as detecting light beam.This dielectric rod is the Nd ion doped glass bar, by six roots of sensation xenon lamp side uniform pumping.Detecting light beam goes into to inject the grating type wavefront curvature sensor after by dielectric rod, is measured the thermal focal length of surveying the dielectric rod that light detected by the grating type wavefront curvature sensor.Experiment is measured under the situation of shoot laser not.The grating focal length that defocuses of grating type wavefront curvature sensor is 3600mm, and the entrance pupil bore is 28mm, and the focal length of the short focal length lens of combining closely with grating is 105mm.Defocus grating by the designed phase type and eliminate of the interference of zero level focal beam spot two defocused spot.Fig. 8 is the dielectric rod reference hot spot that the grating type wavefront curvature sensor does not measure during pumping.Fig. 9 is the hot spot when the most serious by the thermal lensing effect that measures with the grating type wavefront curvature sensor after 10,20,30,40 pulses of pumping respectively for dielectric rod.By simple spot diameter computing method, as area-method, determine the diameter of two defocused spot, the normalization that calculates two spot diameters is poor, utilizes (7) formula to calculate the focal length of thermal lens at last, and result of calculation such as table 1 are listed.

The thermal focal length that table 1. measures with wavefront curvature sensor under different pumping pulse number situations

Pulse number	10	20	30	40
					The thermal focal length f that measures with wavefront curvature sensor (unit: rice)	156	82	45	31

Claims (2)

1, a kind of method with grating type wavefront curvature sensor focal length measurement and equivalent f-number is characterized in that step is:

1.. at first demarcate the grating type wavefront curvature sensor with reference beam;

2.. device under test is placed light path, produce the wavefront that defocuses by transmission or reflection way with focal length to be measured, this defocuses wavefront goes into to inject in the grating type wavefront curvature sensor and uses the grating type wavefront curvature sensor to survey the defocused spot of two out of focus faces, the diameter d of defocused spot before measuring ₁With back defocused spot d ₂, obtain defocusing grating equivalent focal length f according to the design parameter of grating type wavefront curvature sensor _g, the short focal length lens focal distance f _l

3.. by following formula (5) two defocus spot diameter d ₁And d ₂Normalized poor S be

$S=\frac{d_1-d_2}{d_1+d_2}=\frac{f_g}{f}---\left(5\right)$

4.. when one of two defocused spot that obtain are preceding out of focus, another be afterwards during defocused spot, as can be known | f|＞f _g, then calculate focal length or equivalent f-number respectively by following formula (7) and formula (8);

$f=\frac{f_g}{S}---\left(7\right)$

$f/\#=\frac{f}{d}=\frac{2f_1}{d_1-d_2}---\left(8\right)$

5.. when two defocused spot that obtain all be preceding defocused spot or after defocused spot, as can be known | f|≤f _g, calculate focal length or equivalent f-number respectively by following formula (9) and formula (10).

$f=f_{g}S---\left(9\right)$

$f/\#=\frac{f}{d}=\frac{2f_1}{d_1+d_2}---\left(10\right).$

2, the method with grating type wavefront curvature sensor focal length measurement and equivalent f-number according to claim 1, it is characterized in that: described step 1. in, at first adjust the incident angle of reference beam, make three diffraction patterns that detect on the detection plane in the grating type wavefront curvature sensor be positioned at the detection plane center, adjust the distance between grating lens combination and the detector plane, make timing signal two defocused spot equal and opposite in directions.

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