GB2209210A

GB2209210A - Apparatus for measuring spectral characteristics of laser radiation

Info

Publication number: GB2209210A
Application number: GB8720438A
Authority: GB
Inventors: Fedor Vladimirovich Karpushko; Alexandr Kimovich Kulpanovich; Sergei Jurievich Volkov; Valery Vasilievich Smirnov; Vladimir Petrovich Shidlovsky
Original assignee: Institut Obschei Fiziki Akademii Nauk Sssr; Institut Fiziki Akademii Nauk Belorusskoi; INST OBSCHEI FIZ AKADEMII NAUK
Current assignee: Institut Obschei Fiziki Akademii Nauk Sssr; Institut Fiziki Akademii Nauk Belorusskoi; INST OBSCHEI FIZ AKADEMII NAUK
Priority date: 1987-09-09
Filing date: 1987-08-28
Publication date: 1989-05-04
Also published as: FR2620224A1; FR2620224B1; GB8720438D0; DE3730257A1

Description

APPARATUS FOR MEASURING SPECTRAL CHARACTERISTICS OF LASER RADIATION The

invention relates to apparatus for measuring spectral characteristiesq and more particularly it relates to an apparatus for measuring the spectral characteristics of laser radiation.

The apparatus for measuring spectral characteristics of laser radiation is specifically intended for automatically monitoring the wavelength and the width of the ra- diation line or band of pulsed and continuous laser beam sources either incorporated in automated industrial or research laser-equipped units, or operated as an independent instrument or device.

There is known an apparatus for measuring spectral characteristics of laser radiation, comprising a Fizeau reflection interferometer (an optical wedge) defined by two planar glass plates, a collimator shaping a planar wavefront of the radiation being measured, having an aperture defined by the linear photodetector used for registe- ring the-spatial distribution of the intensity of the interference field produced by the interferometer in the reflected light, and a system for processing the optical data supplied by the linear photodetector and computing the wavelength of the radiation In a computer (US, A, No. 4173442). The radiation wavelength is measured in this apparatus in two stages,viz. from the spatial period of the interference pattern and from the position of the interference maximums registered by the photodetector.

This known apparatus offers a limited accuracy of measurements cn accomt of its incorporating but one inter ferometer; moreover, it is incapable of determining the spectral structure of the radiation on account of the resolution of the twin-beam interferometer it incorpora tes being inadequately high. Furthermore. this apparatus puts exceptionally stringent requirements on the quality of its optical elements and photodetector, as even the minutest changes in the uniformity of the wavefront incident upon the interferometer and distortions in the photo registration channel, same as non-linearity af the interferometer itself are intolerable.

The relatively high energy threshold of the response of this apparatus is caused by the smallness of the ref- lection factor of unsilvered surfaces of the glass plates serving as the mirrors of the Fizeau interferometers and also by the fact that the input radiation has to be thoroughly spatially filtered in order to obtain a high-quality input front.

There is further known an apparatus for measuring the spectral characteristics of laser radiationy compri sing, arranged in succession one after another in the.

path of the radiation, a beam-shaping telescope, a light -dividing unit for splitting the radiation beam shaped by the telescope into a series of individual split beams, a series of interferometers (e.g. Fizeau interferometers) equalling the number of the individual beams, arranged each in the path of the respective individual split beams.

Z 1 1.

The bases of these interferometers are different and matched by the condition of maintaining substantial equality of the field of dispersion of an interferometer with a greater base to the confidence interval of determination of the wavelength by the preceding interferometer with a smaller base. This apparatus further comprises a series of photodetectors arranged each after the respective interferometer in the radiation pathy and a data processing unit having its inputs connected to the outputs of the respective interferometers (Appl. Physics, 25, 1981, N.Konishi et al., High precision wavelength meter with Fabry-Perot optics"). As in the previously described apparatus of the prior art, measurements are performed by way of analysis of interference patterns or fringes registered by the photodetectors.

The last-described known apparatus incorporates four interferometers. each interferometer being an independent unit independently mounted with the aid of its own mounting and adjusting system, whereby the overall dimensions of the apparatus are increased and the apparatus itself is susceptible to becoming ill-adjusted in operation, which adversely affects the accuracy and stability of measurements (the stability of measurements is understood here as the probability of absence of failures In the adopted algorithm of computing the radiation wavelength by the successive refinement technique, using the data supplied by the interferometers with the successively greater bases). Furthermorel the problem of eva- cuation of the interferometers and their thermostatic control (to say nothing of the desirability of keeping the entire optical unit of the apparatus under thermostatic control) is associated with considerable difficulties when the last-described apparatus of the prior art is operated.

It is an object of the present invention to provide an apparatus for measuring spectral characteristics of laser radiation, which should have its optical unit structured so as to preclude relative maladjustment of the incorporated interferometers in operation, thus yielding a higher accuracy and stability of measurements, permitting to reduce the dimensions of the apparatus and simplifying the task of evacuating the interferometers and keeping them under thermostatic control.

With this object in view, the present invention resides in an apparatus for measuring spectral characteristics of laser radiation, comprising, arranged in succession one after anothe r in the path of the radiation, a beam- -shaping telescope, a light-dividing unit for splitting the beam of radiation shaped by the telescope into a series of individual split beams, a series of Fizeau interferometers equalling the number of the individual split beams of radiatiod, arranged in the paths of the respective individual split beams, of which the bases are selected to be different and matched to one another to satisfy the condition of maintaining substantial equality of the field of dispersion of the interferometer

1 with a greater base to the cDnfidence interval of determination of the wavelength by the preceding interferometer with a smaller base, a series of photodetectors equalling the number of Fizeau interferometers arranged each after the respective interferometer in the path of radiation,, and a data processing unit having its inputs connected to the outputs of the respective photodetectors, In which apparatus, in accordance with the invention, all the Fizeau interlerometers are united in a monoblock unit cf a material transparent for the radiation, having its components interconnected by optical contact, each component including two plates mounted parallel to each other and secured to each other with a gap left therebetween. one of them being the baseplate of the monoblock unit and the other one having a mirror coating applied onto its surface facing the gap, serving as the common mirror for the whole series of the Fizeau interferometers, and a series of wedge-shaped plates of different thicknesses, equalling the number of the Fizeau interferometers, each wedge-shaped plate having the surface defining the optical wedge coated with a mirror coating,,the wedge-shaped plates being mounted in the gap on the baseplate of the monoblock unit so that the edges of the wedges of the wedge-shaped plates are parallel to one another, their surfaces with the mirror coating facing the common mirror and serving as the other mirrors of the respective Fizeau interferometers, the mean thicknesses of the wedge-shaped plates being selected to satisfy the condition of matching the bases of the respective interferometers.

It is expedient that in the apparatus where the shaping- telescope is in the form of a telescope with cylindrical mirrors, the light-dividing unit should include rotatable reflecting mirrors equalling the number of the Fizeau interferometers. mounted parallel to one another and displaced each relative to the preceding one in the direction perpendicular to the axis of the beam of radia- 1() tion by a portion of the diameter of the initial beam of radiation inversely proportional to the number of the Fizeau interferometers, providing for splitting the beam of radiation across the field, the working edges of the rotatable mirrors defined by their relative displacement being perpendicular to the generatrices of the cylindrical surfaces of the mirrors of the telescopes and to the edges af the optical wedges of the wedge-shaped plates.

An apparatus for"measuring spectral characteristics of-laser radiation, constructed in accordance with the present invention. with its optical unit of Fizeau interferometers integrated into a single compact monoblock unit having its common backing plate - the baseplate and its other plate the common mirror, provides for enhancing the long-term stability of the working para- meters of the interferometers, such as the optical length at a given point of the interferometer, the angle of the optical wedge of the interferometer and the incidence angle of the radiation upon the interferometer, and thus c 1 permits to minimize the long-term error in measurement of the radiation wavelength and to enhance the performance stability of the entire instrument, or in other words, steps up the probability of measuring the wavelength without failures by using a computation algorithm based of successive refinement of the wavelength from the data supplied by the Fizeau interferometers with the successively increasing bases. In the preferred embodiment of the invention where the telescope is of the cylindric al-mirror type, the incorporation of the light-dividing unit split- ting the initial radiation beam across the field provides for appreciably enhancing the threshold level of measure ment, be it the power of continuous radiation or the energy of pulsed radiation.

The invention will be further described in connection with its embodiment, with reference being made to the ac companying drawings wherein:

FIG. 1 schematically illustrates an apparatus for mea suring spectral -characteristics of laser radiation; FIG. 2 is a view taken along arrow line A at the monoblock unit of Fizeau interferometerp; FIG. 3 is a sectional view taken on line III-III of FIG. 1.

The apparatus for measuring spectral characteristics of laser radiation, embodying the invention and schematically illustrated in FIG. 1 comprises, successively arranged one after another in the path of radiation, an inlet diaphragm 1. a beam-shaping telescope 2 with cylindrical mirrors 3, 4, a light-dividing unit 5 for splitting the beam 6 of radiation shaped by the telescope 2 into a series of individual split beams 7 of smaller cross-section, a series of Fizeau interferometers equalling the number of the individual split beams 7 of radiation, arranged each in the path of the respective split beam 7. the Fizeau interferometers being united into a monoblock unit 8, a series of photodetectors 9 equalling, the number of the Fizeau interferometers, mounted each after the respective interferometer, and a data processing unit 10 having its lc) inputs connected to the outputs of the respective photo- detectors 9.

In the embodiment being described, the series Is a series of four, i.e. the apparatus includes four photo detectors 9, four Fizeau interferometers united in the mo noblock unit, and so on.

The cylindrical mirrors 3 and 4 (FIGS 1, 3) serve as the objective lens and collimator of the telescope 2.

The light-dividing unit 5 includes four rotatable fully-reflecting mirrors 11 mounted parallel to one another at a 450 angle to the optical line 12 defined as the line including the centre of the aperture of.the diaphragm 1 and the matching foci of the cylindrical mirrors 3, 41 per pendicularly to the generatrices of these mirrors. The working faces 13 of the mirrors 11, of a width "kll, are limited by the edges 14, the mirrors 11 being so positioned that their edges 14 are perpendicular to the generatrices of the cylindrical mirrors 3, 4 and are relatively displaced in a direction perpendicular to the line 12 of the beam 6.

1 1 Each successive edge 14 is thus displaced relative to the edge 14 of the preceding mirror 11 along the path 6 by a portion of the diameter D of the initial beam 15, inversely proportional to the number of the beamsplitting mirrors 11, i.e. by D/4 in the presently described embodiment, providing for splitting the radiation beam 6 downstream of the telescope 2 across the field.

The monoblock unit 8 (FIGS 1, 2) of Rizeau interferometers includes two parallel optically planar plates 16 1() and 17 of which the first one serves as the baseplate of the monoblock unit 8 and the second one serves as the common mirror of the Fizeau interferometers, with a partly transmitting mirror coating 18 applied onto the inner surface of this other plate 17. The plates 16 and 17 are set with a gap "all left therebetween with the aid of spacer posts 19, the planar surfaces of the plates 16 and 17 facing the gap "all being- strictly parallel. The baseplate 16 of the monoblock unit 8 supports four wedge-shaped plates 20 with two optically planar sides and different mean distan- ces al, a 29 a31 a 4 between these sides (see FIG. 2). One side of each plate 20 is secured to the-baseplate 16 of the monoblock unit 8, and its opposite side facing the mirror coating 18 is coated with a partly transmitting mirror coating 21 which defines jo intly with the mirror coating 18 a Fizeau interferometer of the optical wedge type. The edges 22 of the wedges of the Fizeau interferometers, which are the conditional lines of intersection of the planes of the mirror coatings 18 and 219 are substantially parallel to one another.

The spacer posts 19 and the mean sizes al, a29 c131 a4 of the wedge-shaped plates 20 are governed by the con dition of matching the mean bases hIl h 2 9 h 39 h 4 of the Fizeau interferometers.

The posts 19 and wedge-shaped plates 20 are connec ted to the plates 16 and 17 with provisions for deep optical contact. All the components of the monoblock unit 8 of the presently described embodiment are made of a material transparent for the radiation, preferably mol ten quartz.

The monoblock unit 8 is so positioned and secured relative to the light-dividing unit 5 that the direction of the optical line 12 of the beam 6 of radiation upon its reflection by the rotatable mirrors 11 should be nor mal to the plane of the mirror coating 18, and the edges 22 of the wed-es of the respective interferometers should be normal to the edges 14 of the rotatable mirrors 11.

The f our photodetectors 9 (equalling the number of the Fizeau interferometers) are linear silicon light detec- tors of the charge-coupled device type - CCD-photodetectors. To step up the efficiency of light registration, cylindrical focusing leases 23 are mounted in front of the photodetectors 9. which, however, can be omitted if photodetectors with sufficiently great widths of their photosensitivity range are used.

To avoid the influence of dispersion of the refraction factor, pressure and composition of the gaseous medium filling the apparatus upon the measurement ac- i l 111 1 -11curacy, the monoblock unit 8 is accommodated within a sealed-away chamber 24 which is preferably evacuated. The radiation enters the chamber 24 via a window 25. To avoid the influence of temperature upon the accuracy and stability of measurements, the chamber 24 is preferably thermostabilized. i.e. kept under thermostatic control. Another may to avoid the influence of temperature is to have the chamber 24 heat- insulated, with the temperature of the monoblock unit 8 measured and corrections for.the measured temperature being introduced into the results of measurement of the radiation wavelength.

The power of the telescope 2, the length of the mirrors 11 along the edge 14 and the length of the wedge-shaped plates 20 also equalling 11 ty are selected to satisfy the condition of matching the sizes of the interference patterns in the direction of dispersion of the wedge interferometers to the length of the light-sensitivity range of the photodete ctors 9. The angles 0 of the wedges of the interferometers are selected to satisfy the con- dition of registering the predetermined number of interference bands.

The unit 10 for processing the optical date is a multichannel one, the number of its channels equalling the number of the photodetectors 9. Each channel includes a controller 26 of the respective CCD-photodetector 9, performing the functions of controlling the photodetector, shaping the video signal, A-D conversion and real-time preprocessing of the interferogram with the aid of an 0 incorporated microprocessor. The unit 10 further includes a built-in central processor 27 connected with the controllers 26 and provided with a control keyboard and a display unit 28. This configuration of the unit 10 for processing optical data provides for speeding up the operation and response of the apparatus.

The operation of the apparatus for measuring spectral characteristics of laser radiation, embodying the present j() invention and schematically illustrated in the appended drawings FIGS 1 to 3 is, as follows.

The initial beam 15 of investigated radiation, having passed the inlet diaphragm 1 and cylindrical telescope 2, is sent as the beam 6 into the light-dividing unit 5 where it is split into four individual light beams 7 equalling the number of the Fizeau interferometers employed. The splitting of the beam 6 across the field is preferred in the apparatus being described, as it enhances the energy response of the apparatus due to optimized matching of the spatial configuration of the beam 7 to the size of the photosensitive range of the respective photodetector 9.

The incorporation of the cylindrical telescope 2. of the cylindrical focusing lenses 23 in front of the photodetectors 9 and of the I'I'gh-C,-dividing unit 5 for splitting the radiation beam 6 across the field provides for essentially lowering the response threshold of the apparatus, be it the power of continuous radiation or the energy of pulsed radiation.

N 1^1 Z The flat wavefront of the radiation in the form of a narrow band (the individual beams 7) extended in the direction of dispersion of the respective Fizeau interferometer is incident upon the plate 17 normal ly to the common mirror coating 18 and shapes behind the plate 17 an interference pattern or fringe in the form of narrow bands called the equal-thickness bands, to be registered by the linear photodetectors 9. Signals generated by the photodetectors 9 are representative of Information on the spatial interference pattern of the radiation being 7 analyzed, this information being handled in the respective controllers 26 to determine the position of the interference bands and their width, to be used by the central processor 27 for determining the wavelength of the laser radiation and presenting the output data on the display unit 28.

The algorithms of determining the spectral characteristics of the radiation are based on the results of calibration of the Fizeau interferometers residing in establishing the parameters of the interferometers by analyzing the interference patterns produced by radiation with known in advance spectral characteristics - the dat= line s.

The accuracy of det ermination of the wavelength of laser radiation by the apparatus embodying the invention is essentially defined by the size of the base h. of the last-in-succession interferometer (the one with the grea test base). The bases hl, h -2 9 h 3 of the rest of the Fizeau -14interferometers are selected to satisfy the condition of substantial equality of the field of dispersion of an interierometer with a greater base to the confidence interval of determination of the wavelength (according to a preset confidence level) at the preceding interferometer with a smaller base; and at the stage of refinement of the wavelength with the use of the interferometer with the smallest base h,, the predetermination of the wavelength is carried out from the period of the inter- ference patterns. Thus. it is obvious that by increasing the number of Fizeau interferometers, the same ultimate measurement accuracy can be attained with substantially less strict requirements put before the degree of refi- _nement of the wavelength at each computation stage. This.

in its turn, implies putting less strict requirements before the accuracy of analysis of interference patterns, as well as before the manufacturing quality of the optical components.

In operation of the apparatus, such parameters of the interferometers as the optical length at a given point, the angle c- of the optical wedges and the incidence angle of radiation upon the interferometer, same as some other parameters, are susceptible to variation with time, eausing errors in wavelength mesurements and reduced proba- bility of unfailed measurements (the absence of failure is understood as proper determination of the orders of magnitude of the interference maximums by all the interferometers). To minimize measurement errors and enhance Z1 the credibility level of mesurements by accounting for variations of the above parameters, similar apparatuses of the prior art are usually recalibrated with the use of datum lines of laser radiation.

The apparatus for measuring spectral characteristics of laser radiation, wherein, in accordance with the present invention, all the interferometers are united in a compact monoblock unit, provides for enhanced long-term stability of the working parameters which, in its turn, permits to operate it for long periods without recalibration.

1 16

Claims

A 1. Apparatus for measuring spectral characteristics of radiation, comprising, arranged successively in the radiation path, a beam-shaping device; a light-dividing unit for splitting the radiation beam into a plurality of resultant beams; a plurality of Fizeau interferometers, one for each resultant beam; a plurality of photodetectors, one for each interferometer; and a data processing unit having inputs connected to the outputs of the photodetectors; wherein the plurality of Fizeati interferometers are formed in a common block and share a common mirror.
2. An apparatus for measuring spectral characteristics of laser radiation, comprising, arranged in succession one after another in the path of the radiation, a beam-shaping telescope, a light-dividing unit for splitting the radiation beam shaped b.- the telescope into a series of individual split beams, a series of Fizeau interferometers equalling the number of the individual split beams, arranged each in the path of the respective individual split beam and having different bases selected to satisfy the condition of maintaining substantial equality of the field of dispersion of the interferometer with a greater base to the confidence interval of wavelength determination at the preceding interferometer with a smaller base, a series of photodetectors equalling the number of the Fizeau interferometers arranged each after the respective interferometer in the path of radiation, and a data processing unit having its inputs connected to the outputs of the respective photodetectors,'all said Fizeau interferometers being united in a common monoblock unit of a material transparent for the radiation, having its components connected by optical contact and including 0-1 two plates mounted parallel to each other and secured together with a gap left therebetween, one of the plates f= serving as the baseplate of the monoblock unit and the other 1 plate having its surface facing the gap coated with a C= C5 C) mirror coating serving as the common mirror of all said series of Fizeau interferometers, and a series of wedge- shaped plates equalling the number of said Fizeau interferometers. having different thicknesses and each having one surface defining the optical wedge coated with a mirror coating, the wedge-shaped plates being mounted in the gap on the baseplate of the monoblock unit so that the edges of the optical wedges are parallel to one another, the surfaces with the mirror coatings facing the common mirror and serving as the other mirrors of the respective said Fizeau interferometers, the mean thicknesses of the wedgeshaped plates being selected to satisfy the condition if matching the bases of said Fizeau interferometers.
3. An apparatus as claimed in Claim 2, wherein the beamshaping telescope includes cylindrical mirrors, the lightdividing unit including rotatable reflecting mirrors equalling the number of said Fizeau interferometers, mounted parallel to one another and relatively displaced in the direction perpendicular to the line of the beam, each successive mirror relative to the preceding mirror, by a portion of the diameter of the initial beam, of radiation inversely proportional to the number of said Fizeau interferometers, providing for splitting the radiation beam across the field, the working edges of the rotatable mirrors defined by their relative displacement being perpendicular to the generatrices of the cylindrical surfaces of the mirrors-,Ofthe telescope and to the edges of' the optical wedges of the wedge-shaped plates.
4. An apparatus for measuring spectral characteristics of laser radiation as claimed in the preceding Claims, substantially as hereintofore described and illustrated in the appended drawings.

q Pub'is,,e-d 19SEE a. The Paten: Of.,ic!e. Sta:e H:,== 66 71 E... H-::rn London V.'ClR 4TP F'arze7 c---e- may be obtained frc= Ihe Patent Office. Sales Branch, St MaiT Cray. Orpingcr.. Kent BR5 3RD Printed ky Multiplex techniques ltd. & Ma-7 Crky. Kent. Con 1 87