DE3108239A1 - Arrangement and method for measuring optical wavelengths - Google Patents

Abstract

The invention relates to a spectrophotometer of fibre-optic design, which is suitable for measuring optical wavelengths by means of an optical transmission filter. Such an arrangement permits a compact, robust design that is not susceptible to faults.

Description

Licentia Patent-Verwaltungs-GmbH Z13-UL / Yes / rß

Theodor-Stern-Kai 1 UL 8l / 10

D-6OOO Frankfurt (Main) 70 Ulm, 03.03.1981

Honor

Arrangement and method of measurement
optical wavelengths __

The invention relates to an arrangement and a method
for measuring optical wavelengths according to the generic terms
of claims 1 and 19.

Such arrangements and methods are known as
Spectrophotometer or as a spectrophotometer. Included

the intensity of the light is measured as a function of the wavelength or frequency. Known arrangements where ZoB. Monochrometors are used, for example optical grating in which to achieve a spectral Long beam paths are necessary for resolution. There are too simple arrangements for measuring the wavelength of light are known O

Such an arrangement, as described in the writing, D.J.

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Times yon et. al., Electronics Letters _16 (1980), pp. 744-746, is described ^ shows FIG. 1-

The light, shown below as a solid arrow, from a light source Q first reaches a beam splitter S and from there to a first photodetector D and via an optical transmission filter F to a second photodetector D. From the photocurrents X ₁ , i _Q or other electrical signals of the two photodetectors D, D becomes

1 ct

then e.g. the logarithm of the ratio i / i is formed in an evaluation unit A, from which then with the help of the

J. Ct

known filter characteristic of the transmission filter F, the wavelength of the light can be determined. Two photodetectors D, D are necessary so that fluctuations in the light output of the light source Q do not distort the Lead measurement result.

This known arrangement has some disadvantages. A disadvantage is that the direct currents i, i _{o of} the photodetectors D,

\ et 1

D ₀ can be used. These direct currents are for example

influenced by the temperature-dependent dark current as well as by scattered light, so that the measurement result is falsified. Another disadvantage of the known arrangement is that the partial light rays between the light source Q and the Pho-. Death detectors D, D ₀ each pass through different optical elements, in particular the beam splitter S, so that, for example, interferences can develop which lead to incorrect light distribution between the photodetectors DD.

X Ct

The invention is therefore based on the object of specifying an arrangement and a method of the type mentioned, which is particularly important for optical communication

■ - "- ■ ■■ '■■" "3108233

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technology are applicable, and which require a high level of operational safety exhibit.

This object is achieved according to the invention by the in the characterizing Parts of claims 1 and 19 specified Features solved. Appropriate embodiments are compiled in the subclaims.

A first advantage of the invention is that the arrangement can be made so small and mechanically stable. that Z ₀ B. are easy to prepare-to-use devices for the control and maintenance of Lichtleitfaserübertragungsstrecken "

A second advantage is that the arrangement in integrated design can be produced and that costly adjustment work is largely eliminated.

A third advantage is that the arrangement is almost maintenance-free «

The invention is explained below on the basis of exemplary embodiments with reference to schematic drawings explained in more detail »Show it

FIG ₀ 2 to FIG, "9 embodiments, the in FIG. The diagram shown in FIG. 10 serves to explain the method.

In the embodiment according to FIG. 2 gets that from one Light source Q, ζ "B. a semiconductor laser, emitted light via an optical waveguide, not shown, as well as via possibly necessary coupling points (splices, plug connections)

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an input 21 of an optical component ST, e.g. or toggle switch, which turns the light in a second partial light path 23 and a first partial light path 22, at one end of which there is an optical Transmission filter F. e.g. an interference filter is located. That from the transmission filter F or from the second partial light path 23 transmitted light is an optical detector arrangement D supplied / which e.g. consist of several photodetectors or an optical switch and a Photodetector exists. The so-called photo currents and / or photo voltages generated by the detector arrangement D an evaluation unit A, e.g. a microprocessor, supplied, taking into account the optical filter character of the selected transmission filter F from the Photocurrents, as described above, calculate the wavelength and / or frequency of the light. This value will displayed or further processed on a display unit (not shown). It is also possible to do the evaluation To carry out a unit in such a way that measurements are possible with the aid of a multiplex operation. For this purpose, e.g. optical Changeover switches are necessary, which are located in the component ST or the detector arrangement D and by means of the evaluation unit A. This is shown in FIG. 2 shown by means of the connections shown in dashed lines. One further connection, shown in dashed lines, leads from the evaluation unit A to the light source Q, for example a Semiconductor laser or a light emitting diode. By this compound is shown to be in accordance with the invention it is possible to modulate the intensity of the light emitted by the light source Q. This modulation is possible, e.g. through a periodic change in the current injected into the light source Q. This change will e.g. caused by the evaluation unit A. Such a one

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Modulation makes the arrangement of possible disturbance variables e.g. incident scattered light and / or temperature-dependent dark current in the detector arrangement D included photodiode. According to FIG. 10 becomes the light intensity I of the light source Q, for example modulated with a periodic, sinusoidal signal of frequency f around the mean value of the light intensity.

It is possible to reduce the fluctuation amplitude due to the modulation to be chosen to be small in comparison to the average light output emitted by the light source Q. By a Such modulation is achieved that optical and / or electrical operating points of the arrangement essentially remain unchanged, so that disturbances e.g. non-linearities and / or glitches can be avoided. In the evaluation facility A (FIG. 2) are then only the signal components the detector array D evaluated, which include the frequency f. By forming the quotient of the see below the signal component belonging to the partial light paths 22, 23 the evaluation is essentially independent of the modulation aid.

In arrangements in which the light source Q and evaluation unit A are spatially far away, e.g. in the case of optical Communication links, it is alternatively possible to modulate the intensity of the light source Q in such a way that that the modulation is not controlled by the evaluation unit. In this case it is advisable to use the To choose modulation frequency f. In a frequency range, for which the evaluation unit A is sensitive enough.

For an interference-insensitive structure of the arrangement, it is advantageous if the arrangement consists of as few as possible

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optical and / or electrical components, such as optical switches, exists. Such embodiments are shown in FIGS. 3 to FIG. 9 shown. In these exemplary embodiments, the detector arrangement D (FIG. 2) consists of photodetectors D and D, which are each located at one end of the partial light paths 22 and 23, respectively. The transmission filter F is located in the first partial light path 22. The _{electrical signals generated by the photodetectors D, D 0} , for example photodiodes either intensity-modulated or unmodulated light is used.

According to FIG. 3 that emitted by the light source Q falls Light directly on the transmission filter F, whose characteristic, the characteristic curves, is chosen such that the wavelength of the light in the transition area of the filter the so-called filter flank. The light is transmitted or reflected and over the first partial light path 22 and the second partial light path 23 are fed to the photodetectors D and D ″, respectively. A change in the wavelength of light therefore causes a change in the intensities of the transmitted or reflected light. By a Corresponding calibration of the arrangement, it is therefore possible to determine the absolute value of the light wavelength.

The spectral resolution of the arrangement is essentially dependent on the steepness of the filter flank of the transmission filter F and the spectral sensitivity of the photodetectors D, D. For a high resolution it is expedient to design the transmission filter F, for example, as an interference filter, since such a filter

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has a large slope "

In the arrangement described, the normal of the filter is at the angle α with respect to the direction of the light beam inclined. If the angle α is too large, the signals at the photodetectors D, D become too polarization-dependent and thus the wavelength determination is uncertain. To avoid such an effect, it is advisable to to choose the angle (X smaller than 15.

A further improvement is achieved if the light from the light source Q is unpolarized, or if there are depolarizing optical elements _{in the beam path between the light source Q and the photodetectors D, D 2.}

For a spatially small and mechanically insensitive structure of the arrangement according to the invention, it is also advantageous to implement the transmission filter F fiber-optically. Such an arrangement is shown in FIG. 4 shown. The light from the light source Q is coupled into one end 21 of an optical fiber , the other end face of which is provided with an optical transmission filter F. The transmitted light reaches the photodetector D. while light for the photodetector D is taken from at least one of the outputs 41, 42 of an optical splitter T. If, for example, an interference filter is used as the transmission filter F, an arrangement of a photodetector D_ at the output 4l has the advantage that the filter effect of the reflected light is also used.

In the embodiment according to FIG. 5 is an optical fiber 51 with the light-guiding fiber core 511 and the shell 512, with an end face 513 of the

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Optical fiber 51 is ground at an angle, and a transmission filter F is applied to this end face 513. The partial light beam passing through the transmission filter F reaches the photodetector D and the reflecting partial light beam reaches the photodetector D. It is useful for the best possible detection. to arrange the photodetector D ₂ directly on the optical fiber 51 or on an optical capillary (not shown) immediately surrounding the fiber, in order to avoid any disturbing total reflections.

In the embodiment according to FIG. 6, an optical fiber 51 is firmly connected to an optical rod 6l with imaging properties. This rod 6l transforms the light coupled into the optical fiber 51 into an essentially parallel light beam. The properties of such an optical rod are, for example, from the article by T. Uchida et. al., "Optical Characteristics of a light focusing fiber guide and its applications", IEEE Journal of Quantum Electronics QE-6 (197O),

Pp. 606-612, known. A transmission filter F is attached to one end 613 of the optical rod 6l. One Such an arrangement has the particular advantage that the light falling on the transmission filter F is essentially is divergence-free (parallel). Such a property increases the interference filters in particular Effectiveness.

In particular, the arrangements according to FIG. 3i 5> 6 have the disadvantage that the transmission filter F is not perpendicular is irradiated. A further development of the invention is therefore as shown in FIG. 7 or 8 therein, a part of the emitted by the light source Q and into a light

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Waveguide 51 coupled light to pass essentially perpendicularly through a transmission filter F to the photodetector D, while another part of the light reaches the photodetector D immediately or after reflection. The transmission filter F is therefore attached to the parts 72 of an end face of the optical waveguide 51 that are perpendicular to the direction of light, while the part 73 of the end face which is inclined to the direction of light is designed in such a way that the partial light path 23 is refracted (FIG. 7) or reflected (FIG. 8 ) will. In the case of an optical waveguide 51 according to FIG. 7 or 8, it is appropriate ₍ - to choose the diameter of the light-guiding fiber core as large as possible so that the largest possible number of light waves can propagate in such a way that

ßlXL

that / at the fiber end surface possibly occurring interference pattern influences the division of the light between the photodetectors D., D ₀ as little as possible. Light-1 d>

Conductive fibers usually have core diameters of up to around% 00, mn, if even larger core diameters are required it is advisable, for example, to switch to light-conducting glass rods with a diameter of a few mm.

In the embodiment according to FIG. 9 becomes the transmission filter F realized by a defined curvature of the optical waveguide 51, the filter effect of a such a curved optical waveguide is described, for example, in patent application P 29 26 977.0. Advantageously the optical waveguide 51 should then be single-wave. That Light from the light source Q is initially located in the fiber core 511 of the optical waveguide 5I and depending on the deformation of the optical waveguide 51, a more or less large portion of light reaches the cladding 512 of the optical waveguide 51 »The light energy in the fiber cladding becomes 512

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_{finally passed to the photodetector D 2} with the aid of an optical fiber 90, while the light energy remaining in the fiber core 511 reaches the photodetector D. The coupling of the light from the core 511 into the cladding 512 is dependent on the wavelength, so that the ratio of the light outputs at the photodetectors D, D is also dependent on the wavelength. This effect can be used to measure the wavelength of light.

If there is a large difference in the refractive index between the core 511 and the cladding 512 of the optical fiber 51 becomes the divergence of the light rays in the optical fiber 51 so large that its filter quality, especially when using an interference filter is affected.

The invention enables at least one photodetector D, D (FIG. 8) directly on the optical waveguide 51 is attached. With such a structure, possible malfunctions, e.g. due to mechanical vibrations and / or clearances, largely avoided. The arrangement according to the invention is still so small and compact buildable that such an arrangement can be designed as a component of a measuring device that of the determination other physically measurable quantities are used. For example, such an arrangement is in a so-called Optical fiber ring interferometer can be used for the mood absolute rotation rates is used.

As described at the outset, the resolving power of an arrangement according to the invention is essentially dependent on the edge steepness of the transmission filter and the sensitivity of the photodetectors. With one currently mogliehen Arrangement, for example, a change in the light wavelength of approximately IA can be measured.

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Claims (1)

Licentia Patent-Verwaltungs-GmbH Z13-UL / Yes / rß Theodor-Stern-Kai 1 UL 81 / IO D-6000 Frankfurt (Main) 70 TJIm, 03.03.1981 Patent claims Iy arrangement for measuring optical wavelengths by means of an optical. Filters, in particular for optical communication technology, characterized by a) at least one optical component (ST), the light coupled into its input (21) essentially Divided into at least two partial light paths (22, 23) without loss and / or interference b) at least one optical transmission filter (F) is present in at least a first partial light path (22) such that the wavelength of the transmitted light is essentially in the wavelength range of a The filter edge of the transmission filter (F) falls e) at least one second partial light path (23), which by means of the component (ST) coupled light in the essentially passes on without loss and / or interference or essentially that from the transmission filter (F) transmits reflected light - 2 - UL 81/10 d) at least one at the ends of the partial light paths (22,
23) existing detector arrangement (D), which over
the partial light paths (22, 23) detected light transmitted simultaneously or essentially alternately. 2. Arrangement according to claim I _1, characterized in that the component (ST) and at least one Te, il at least one partial light path (23) in an optical unit (51)
are summarized (FIG. 5) 3. Arrangement according to claim 2, characterized in that the unit (51) is an optical waveguide. 4. Arrangement according to claim 2, characterized in that the unit (51) is an essentially single-wave optical waveguide. 5. Arrangement according to claim 1, characterized in that the transmission filter (F) is an interference filter. 6. Arrangement according to claim 1, characterized in that the transmission filter consists of a helical and / or spiral curved optical waveguide is formed (FIG. 9), 7. Arrangement according to claim 6, characterized in that the curved optical waveguide is a single-wave optical waveguide is. 8. Arrangement according to one of the preceding claims, characterized in that a detector arrangement (D) is present is made up of at least one optical switch and at least one photodetector. - 3 - UL 81/10 9o arrangement according to claim 8, characterized in that that the detector arrangement (D) consists of photodetectors (D, D), which are each arranged at the end of a partial light path (22, 23). 10 "Arrangement according to one of the preceding claims, characterized in that at least one evaluation unit (A) is present, the at least one of at least evaluates the signal generated by a photo detector in such a way that that the intensity of the light from at least two partial light paths (22, 23) can be evaluated. ll _o arrangement according to claim 10, characterized in that in the evaluation unit (A) at least one microprocessor is present. 12., Arrangement according to one of the preceding claims, characterized in that at least one optical waveguide (51) is present, on one end face of which at least one transmission filter (F) is attached and that at least two photodetectors (Dp D ₂ ) dex ~ art are arranged that reflected from the transmission filter (F) or transmitted light can be evaluated (FIG. 5). 13 ο Arrangement according to Claim 12, characterized in that the transmission filter (F) is inclined in such a way with respect to the longitudinal axis of the optical waveguide (51). that the transmitted light has an angle of reflection (ε £) of less than 15. l4 "Arrangement according to claim 12 or 13, characterized in that that the optical waveguide (6l) is an optical rod with light-imaging properties. - 4 - UL 81/10 15. Arrangement according to one of the preceding claims, characterized in that an optical waveguide (71) with a first end face (72) on which a transmission filter (F) is attached, and an inclined end surface (73) is present, which transmits and / or reflects light, and that the transmission filter (F) transmitted or transmitted by the inclined end face (73) and / or reflected light from at least one photodetector (D, D_) can be fed (FIG. 7; FIG. 8). 16. The arrangement according to claim 15, characterized in that at least one photodetector (Dp) directly on the Optical waveguide (51) or on one of the optical waveguides (51) is arranged at least partially surrounding the optical capillary. 17. Arrangement according to one of the preceding claims. characterized in that with a transmission filter 1
(F) that of a curved; consists of a core and cladding area consisting of optical waveguide (9I), on one end face of which a first photodetector (D) is arranged. and at least one second photodetector (Dp) is coupled to its jacket area before and / or after the curvature (FIG. 9). 18. Arrangement according to one of the preceding claims, characterized in that an end face of an optical waveguide is partially covered with a transmission filter and that the light transmitted through the end face can be fed to at least one photodetector. 19. Procedure for measuring optical wavelengths using - 5 - UL δι / ίο an arrangement according to one or more of the preceding Claims, characterized in that intensity-modulated Light is used whose degree of modulation is less than one (FIG. 10). SO »Method according to claim 19, characterized in that the light is essentially modulated with a monofrequency oscillation of the frequency f. and that in the detector arrangement (D) and / or in a downstream evaluation unit (A) essentially those associated with frequency f Signal components are evaluated. 21 _O The method of claim 19 or claim 20, characterized in that ein.Modulationsgrad is selected such that a change caused by the mean value of light intensity is negligible. 22. The method according to any one of claims 19 to 21, characterized characterized in that at least one optical operating point of a measurement object remains essentially unchanged during the measurement remain" 23. The method according to any one of claims 19 to 22, characterized characterized in that at least one semiconductor laser and / or at least one light-emitting laser is used as the light source Diode is used. 24 ο Method according to Claim 23, characterized in that the light intensity is modulated by modulating the injected electric current. 25 ". Method according to one of Claims 19 to 2k, characterized in that unpolarized light and / or less - 6 - UL 81/10 at least one light depolarizing component is used. * 26. The method according to any one of claims 19 to 25, characterized »that in at least one evaluation unit (A) the signals generated by means of a first or second partial light path (22, 23) are essentially divided and that taking into account the filter course of the transmission filter (F) the wavelength and / or frequency of the light can be determined. 27. The method according to any one of claims 19 to 26, characterized in that at least one change at least a wavelength and / or frequency of the light at least one measurable variable causing the change can be determined will. 28. The method according to any one of claims 19 to 271 characterized in that an absolute rotation is determined as the measured variable.