FR2639433A1 - METHOD FOR MEASURING THE POWER OF A RADIATION AND ASSEMBLY FOR APPLYING THE METHOD - Google Patents

Abstract

The subject of the invention is a method for measuring the wavelength and the power of radiation, in particular at a narrow range of wavelengths, as well as an assembly for the application of this method. This assembly comprises at least two radiation detection devices 4, 4 'and 5 having different spectral sensitivity characteristics, which receive the radiation to be measured. The initial electrical signals 7, 8 delivered by the radiation detection devices are used and compared with each other by a processing circuit which supplies an electrical signal proportional to the power and an electrical signal proportional to the length. These signals can in particular be used for the servo-control of laser diodes. The method and the assembly, objects of the invention, are particularly suitable for analyzing a range of wavelengths, centered on a given value.

Description

The subject of the invention is a method for measuring the power of a

  radiation, in particular at a narrow range of wavelengths, using a radiation detection device which, exposed to this radiation, emits at least one electric signal proportional to the power. The simplest measure of the power of radiation is by means of a transducer. of an electro-optical transducer in the case of optical radiation, which delivers an initial signal, proportional

  to the power of incident radiation. However, the usual methods

  it consists in measuring separately the wavelength and the power

of a radiation.

  Conventional methods for measuring the wavelength of a radiation are based on the appearance of diffraction or interference effects. Figure la presents a mounting for applying such a method according to a known principle. This assembly uses a prism 2 which, because of its refractive index higher than that of the medium in

  which propagates the radiation 1 in the form of a parallel beam.

  refracts this radiation. The part of the radiation with shorter wavelength is more strongly refracted than that with longer wavelength. Therefore, in the plane 4, it is possible to detect the part of shorter wavelength (for example blue light in the case of optical radiation) in the direction 5, while the part of greater length d wave (red light) can be detected

following the direction 6.

  FIG. 1b shows another known method which consists of using a diffraction grating 3 which, in the simplest case, can be constituted by a slot. Through this grating, an incident beam 1 is diffracted so that, in a plane 4, the part of the radiation having the longest wavelength can be detected in the direction 6

  and the part having the lowest wavelength in the direction 5.

  In these two processes the different wavelengths of a radiation

are spread out linearly.

  FIG. 2 also briefly mentions the known method of determining the wavelength via the

  interference phenomenon using the so-called Michelson interferometer.

  By means of a separating plate or a divider cube 2 a parallel light beam 1 is divided into at least two partial beams. These partial beams are reflected in their direction of incidence by two mirrors 3 and 4 and returned to the separating surface of 2. Part of the resulting radiation propagates along the incident beam 1 but in the opposite direction, while the other part of the radiation can be detected by the detector 5. When one of the mirrors, for example the mirror 4, moves parallel to the direction of propagation of the beam, interference fringes, alternately light and dark, will appear on the detector 5 , provided that the radiation is coherent. The wavelength is then proportional to the displacement of the mirror and inversely proportional to the number of periods of light and dark fringes appearing on the detector. This method only applies to the analysis of monochromatic radiation and can not therefore be used for radiation consisting of a mixture of

different wavelengths.

  The object of the invention is to define a method for measuring the wavelength and the power of a radiation of the type indicated and to design an assembly for applying this method and for producing with relatively simple technical means, a simultaneous measurement of the power of a radiation and its preponderant wavelength, magnitudes which are

  for example necessary for the precise adjustment of a laser diode.

  Starting from the process as described above, the object sought by the invention is achieved by the use of at least two radiation detection devices, having different spectral sensitivity characteristics, which receive the radiation at measuring and delivering initial electrical signals which are compared with each other to obtain an electric signal proportional to the power and a wavelength dependent electrical signal. The process, consistent

  according to the invention, allows a simultaneous measurement of the power of a

  and its preponderant wavelength with special means

simply.

  The operation of the initial signals delivered by the radiation detection devices is preferably done by forming the difference between the two initial signals, the resulting signal being able to be used for a feedback on a servo circuit ensuring the stabilization of the wavelength, in particular for

a laser diode.

  The processed signal, proportional to the wavelength can be obtained by dividing the difference between the two initial signals delivered by

  the detectors by one of these signals.

  The electric signal proportional to Ja power as that proportion-

  the wavelength can be used as feedback signals.

  action in a servo circuit ensuring the stabilization of the wavelength. For a whole series of applications, such as for example the regulation of a laser diode, it is sufficient to determine the centering of the range of wavelengths which. anyway, is

a narrow band range.

  The feedback signal can usefully be used to implement an actuator, the initial signal of the detector being amplified or attenuated under the effect of this aclioneur so that the feedback signal

become null.

  The invention also encompasses an assembly designed for the application of the method, object of the invention. This arrangement essentially comprises two radiation detection devices having different spectral sensitivity characteristics. These radiation detection devices are arranged to receive the radiation emitted by the same source; a processing circuit, connected to the outputs of

  radiation detection devices, transforms the initial signals.

  delivered by these devices, in an electrical signal proportional to the

  power and an electrical signal proportional to the wavelength.

  The two detection devices can be constituted by

  diodes, one of them being preceded by an optical filter which eliminates

  certain ranges of wavelengths of the radiation to be measured.

  The optical filter used is designed or chosen so that the range of wavelengths that it passes (measurement range) corresponds to the

  rising edge or falling edge of the filter characteristic.

  As a result, the sensitivity characteristic of one of the detection devices has a decrease for the measurement range considered.

  or a growth of almost linear pace. The optical filter used

  It can also be a filter that blocks daylight.

  To better define the center of the range of wavelengths to be measured, it is finally also possible to provide at least one device

  additional radiation detection, having a characteristic

  that of different slope sensitivity and a decrease or a

  virtually linear growth for the measurement range considered.

  The division of the radiation emitted by the source in several parts can

  to be usefully done by means of a diffuser.

  The assembly according to the invention is in particular designed to serve as

  servo system of a laser diode.

  However, the invention is not limited to the optical field, the process and the assembly, objects of the invention, which can be used in a wide range of electromagnetic radiation, provided to provide elements adapted to the range of length of wave that. we try to treat. The diagrams attached in the appendix illustrate in greater detail, by way of example, the object of the invention; these figures show: FIG. la, lb and 2: known assemblies for measuring the wavelength and the power of a radiation; Fig. 3: a diagram of the sensitivity characteristics of the two radiation detection devices; Fig. 4: an assembly for the application of the method according to the invention; Fig. 5: another example of assembly for the application of the method

according to the invention.

  The two assemblies shown in FIGS. 4 and 5 make it possible to obtain, with very limited technical means, an electrical signal proportional to the power and an electrical signal proportional to the wavelength of a radiation. Such signals are necessary, for example, for servoing laser diodes or for correcting measurements made using laser diodes. The assemblies presented also make it possible, in a simple manner, to compare the wavelengths of different laser diodes. By means of a simple calibration device it is also possible to measure in absolute value of

  The wavelength. The assembly shown in FIG.

  two radiation detection devices 4, 4 ', consisting of ordinary photodiodes which normally serve only for measuring the power of a radiation. However, in the shown arrangement, one of the photodiodes 4 'is preceded by an optical filter 5 which eliminates a certain range of wavelength, for example not interested in the servo process. The optical filter is chosen so that the range of wavelengths that it passes (measurement range) corresponds to the rising edge or the falling edge of the filter characteristic. As a result, the spectral sensitivity characteristics of the detection devices become different for the measurement range under consideration. This is illustrated in Figure 3 which gives a presentation

  idealized of two curves of sensitivity. On the ordinate 1 we have the sensitivity

  spectral head and on the abscissa 2 the wavelength. For the range of wavelengths considered 5-6, the sensitivity of the first detector 4 remains substantially constant, while that of the second detector 4 'decreases, in the example shown, in a linear manner for increasing wavelengths. The ratio of the signals delivered by the two detectors, relative to a given radiation power which can for example be directly supplied by the first detector 4, is directly proportional to the wavelength. For a feedback signal for servocontrolling the wavelength of laser diodes, for example, it is sufficient to differentiate between the two signals delivered by the detectors. The presented assembly and the method, objects of the invention, thus provide information on the centering of a range of wavelengths, it being understood that a set of wavelengths spread over a wide band will not be disjointed. ,

  as this occurs in the well-known diffraction-based method.

  In the example of application presented, especially intended for the servocontrol of laser diodes, one is only interested in the central part of the wavelength range that

way is narrow.

  In the assembly presented in FIG. 4, the radiation 1 to be measured passes first into a beam splitter, for example by

  a diffuser 2, for obtaining two partial beams 3 and 3 '.

  In the case of a strongly divergent radiation or during the presence

  diffuse light, the division of the beam already exists beforehand.

  Just make sure that the partial beams 3 and 3 'have the same wavelength and that the ratio of their powers remains constant

  during the duration of the measurement. The partial beam 3 is sent directly

  on the detector 4, while the partial beam 3 'passes through a filter 5 before reaching the detector 4'. The initial signals 7 and 8, issued at the output of the detectors 4 and 4 ', are exploited or treated

  in a circuit 9. The use of these signals can be made from

  rentes ways, for example as indicated above by forming the difference between the two signals, followed by a division by power, to obtain at the output of the operating circuit 9 at 10 an electrical signal proportional to the power and in 11 an electric signal proportional to the wavelength. These signals can either be displayed or undergo another treatment to serve for example to enslavement

of laser diodes.

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  Figure 5 shows another example of mounting, in which all the elements that are already in the diagram in Figure 4, are designated by the same references. In the version of FIG. 5, the radiation detection device 4 "constitutes a unit comprising a photodiode and a filter, for example a filter removing daylight. The reference 4 designates an ordinary photodiode In the version presented, the filter 5 fulfills the following function: It is necessary that in the range of wavelengths to be considered, the variation of the ratio of the spectral sensitivities of the two detectors is monotone,

  to avoid any ambiguity in the assignment of the wavelength.

  The operating circuit 9 can be designed very simply. In the version presented, for example, the signal proportional to the power can be directly the signal delivered to the output of the detector 4, while the signal 11 proportional to the wavelength is obtained by means of a simple component, assuring the subtraction between the signals 7 and 8 delivered at the output of the detection devices. Signals

  thus obtained are amply sufficient to ensure servocontrol. In division

  Furthermore, the result of the subtraction of the two signals by the signal 7, which is proportional to the power, gives the signal 11 which

  depends only on the wavelength.

  The present invention therefore proposes a method and a scale which make it possible, with limited technical means, to solve problems in an interesting manner for specific applications, such as by

  example of servo or control operations.

  Without departing from the scope of the present invention, the specialist will be able to make modifications and supplements. Thus, it is possible, for example, to better define the center of the range of wavelengths to be measured, to provide at least one additional device for detecting the

8 2639433

  radiation, having a characteristic of sensitivity of different slope and a decrease or growth substantially linear for the measurement range considered. By combining the signal delivered at the output of this additional detector with the signals provided by the other detectors, various problems relating to control or control operations can be solved and it is also possible to proceed with

  more precise measurements of wavelengths. In addition, the presentation

  the invention is not limited to the use of two or three

  radiation detection devices.

Claims (15)

claims   A method for measuring the power of a radiation, in particular at a narrow range of wavelengths, using a radiation detection device which, exposed to this radiation, emits at least one electrical signal proportional to the power characterized in that at least two radiation detection devices (4, 4 'and 5; 4, 4 ") having different spectral sensitivity characteristics receive the same radiation to be measured (1) and the signals 8, 4), are delivered from at least the two radiation power detectors (4, 4 'and 5; 4, 4 "), are compared with one another to obtain the electrical signal proportional to the power and a   electrical signal function of the wavelength.   2. Process according to claim 1, characterized in that   form the difference between the initial signals, delivered by the   radiation detection systems.   3. Method according to claim 2, characterized in that the signal resulting from the difference between the initial signals is used to obtain a feedback effect in a servo circuit, ensuring the stabilization of the wavelength. , in particular from a laser diode.   4. Method according to one of claims 2 or 3, characterized by the   after forming the difference between the initial signals of the detectors, the differential signal thus obtained is divided by the   initial signal of one of the detectors (7 or 8).   Process according to one of Claims 2 to 4, characterized by   that the power proportional electric signal and the wavelength proportional electric signal are used to obtain a feedback effect in a servo circuit, ensuring the   stabilization of the wavelength.   6. Process according to claim 3 or 5, characterized in that   that the feedback signal implements an actuator.   7. Method according to claim 6, characterized in that, through the actuator, the initial signals of the detectors are either amplified or attenuated, so that the signal of   retro-action is reduced to zero.   8. Assembly for the application of the process according to one of the   Claims 1 to 7, characterized in that at least two   radiation detection devices (4, 4 'and 5; 4, 4 "), having different spectral sensitivity characteristics, are arranged to receive the radiation (1), emitted by the same source, and that a circuit processing device (9), connected to the outputs of the radiation detection devices, transforms the initial signals delivered by these devices (4, 4 'and 4, 4 ") into an electrical signal proportional to   power and an electrical signal proportional to the wavelength.   9. Circuit arrangement according to claim 8, characterized in that one of the radiation detection devices (4) is constituted by a normal photodiode, while the other (4 'and 5; 4 ") is constituted by a photodiode comprising an optical filter (5) which eliminates certain ranges of wavelengths.   10. Assembly according to claim 9, characterized in that the optical filter (5) is designed so that the range of wavelengths it passes (measurement range) corresponds to the sidewall   amount or the falling edge of the filter characteristic.   11. Assembly according to one of claims 8 to 10, characterized   in that the sensitivity of one of the radiation detection devices (4) remains constant for the range of wavelengths in which the measurements take place and the sensitivity of the other radiation detection device ( 4 'and 5; 4 ") shows linear growth or decay in the length range wave measurements.   12. Assembly according to one of claims 9 to 11, characterized   in that the other radiation detecting device (4 '; 4 ") is preceded by a daylight blocking filter or the radiation detecting device (4") has a light blocking filter of the day. l   Assembly according to one of Claims 8 to 12, characterized   in that a diffuser (2) is placed between the radiation source   and the detection devices (4, 4 ', 4, 4 ").   - 14. Assembly according to one of claims 8 to 13, characterized   in that at least one additional device for detecting   radiation. having a slope different from its sensitivity characteristic which must have roughly linear growth or decay for the range of wavelengths in which measures.   15. Assembly according to one of claims 8 to 14, characterized   in that it is part of the servo system of a laser diode.