Apparatus for Measuring Spectral Power of a Light Beam

This invention relates to an optical measuring apparatus for measuring an absolute intensity of a light beam and, in particular, a spectrometer for measuring a spectral distribution.

A spectrometer is known as an optical measuring apparatus in which case a light beam is incident on a diffraction grating to permit it to be separated into respective light beams. Upon the rotation of the grating, the light beam components having the respective wavelengths are successively sent to photodetectors where the beam intensity of the respective wavelengths is measured in which case the photodetector is located in a specified direction to the grating. As a result, measurement is made of the spectral distribution of the light beams which has been measured by the spectrometer.

In this known spectrometer, the spectral distribution of the light beam can exactly be measured since the light beam components which have been diffracted by the diffraction grating are introduced into the photodetector. That is, a spectral element, such as a diffraction grating, has the characteristic of which the spectral efficiency differs depending upon the polarized components of the light beam. Now suppose that, for example, a light beam of a certain wavelength .lambda. having a relative output power of 10 is splitted at the diffraction grating. Even in this case, the photodetector detects a relative output power of 5 if the light beam contains more S polarization components than P polarization components and detects a relative output power of 3 if the light beam contains more P polarization components than S polarization components. This phenomenon is observed as the special characteristic of a diffraction grating shown in FIG. 1. In FIG. 1, Pa denotes a spectral characteristic for a P polarization component having a deflection plane parallel to the grooves of the diffraction grating, which Sa denotes a spectral characteristic for a P polarization component having a deflection plane perpendicular to the grooves of the diffraction grating. As evident from the characteristic graph of FIG. 1, since the conventional optical measuring apparatus is of such a type that light beam components simply diffracted at the diffraction grating are introduced directly into the photodetectors, the absolute power of the light beam cannot be precisely measured since that spectral efficiency differs depending upon the ratio between the wavelength and the deflection components of the light means.

It is accordingly the object of this invention to provide an optical measuring apparatus which can accurately measure the absolute power of a light beam without being adversely affected by the deflection characteristic inherent in a spectrally analyzing optical element.

According to this invention there is provided an optical measuring apparatus for measuring the spectral distribution energy of a light beam, comprising:

a first separating means having a specific spectral efficiency characteristic for spectrally separating a to-be-measured light beam into respective wavelength components;

a second separating means having a specific percent loss characteristic for each of the wavelength components, for sequentially separating the separated wavelength components into at least two polarized beam components at which time the separated wavelength components are attenuated due to the specific percent loss characteristic;

detecting means having a specific photoelectro conversion characteristic with respect to each of the wavelength components, for detecting the separated polarized beam components to convert the separated polarized beam components into electric signals; and

means for correcting the electric signals with the spectral efficiency characteristic, percent loss characteristic and photoelectro conversion characteristic and for analyzing an absolute value of output light power for the respective separated wavelength component.

FIG. 1 is a graph showing a relation between spectral efficiency for polarized components of a spectrally analyzing optical element with the wavelength;

FIG. 2 shows a diffraction optical system for an optical measuring apparatus according to one embodiment of this invention;

FIG. 3 is a plan view showing a diffraction optical system for the optical measuring apparatus of FIG. 2;

FIG. 4 is a plan view showing a measuring optical unit for the optical measuring apparatus shown in FIG. 2;

FIG. 5 is a block diagram showing a measuring circuit for the optical measuring apparatus according to said one embodiment of this invention;

FIG. 6 is a graph showing a relation between a photoelectric conversion efficiency of photodetector shown in FIG. 5 and the wavelength; and

FIG. 7 is a block showing an optical measuring apparatus according to another embodiment of this invention.

A light beam to be measured is introduced into diffraction optical system 2 as shown in FIG. 2. This diffraction optical system has an optical arrangement as shown, for example, in FIG. 3. That light beam from a light source, for example, a semiconductor laser, is conducted through optical fiber 4 to optical connector 6 and thence to concave mirror 8 where it is reflected. The reflected laser beam converges at diffraction grating 10 where it is diffracted to allow it to be separated into various beam wavelength components. The various wavelength components irradiated from grating 10 are directed in their respective specific directions. Some of these wavelength components are reflected on concave mirror 12 where it is directed at concave mirror 14. Here, when diffraction grating 10 is rotated by rotating unit 14 at low speed as indicated by an arrow I in FIG. 3, then the various wavelength components directed in their respective directions from grating 10 are sequentially transferred through concave mirror 12 to concave mirror 14. The wavelength component reflected on concave mirror 14 is introduced into a polarized wave separation optical system. The reflected wavelength component is converted into a substantially parallel light rays by a combination lens system, which is comprised of convex and concave lenses 16 in the polarized light wave separation optical system, and then is incident onto one end of a polarizing optical element, such as calcite 18. While the light beam transmits through calcite 18, it is directed in the different directions with some of it as ordinary light beams (for example, P polarized light beam components) and some as extraordinary light beam (for example, S polarized light beam components). In this case these ordinary and extraordinary light beams are completely separated, as such, at the other end of calcite 18. As shown in FIG. 4, at the other end of calcite 18, total reflection surfaces 20A and 20B are formed in an intersecting fashion with one located above and the other below, noting that the P polarized light component is reflected on total reflection surface 20A and that the S polarized light component is reflected on total reflection surface 20B. The P and S polarized light beams emerging from total reflection surfaces 20A and 20B are directed toward half-mirrors 22A and 22B, respectively, where each is splitted into first and second light components. The first light components reflected on half-mirrors 22A and 22B converge, respectively by convex lenses 26, at their photodetectors 24A and 24B where each first light beam is detected. Second light rays reflected on half-mirrors 22A and 22B pass through corresponding filters 28 in which case only long waveform components are filtered. The second light rays of long waveform, after passing through corresponding convex lenses 32, converge at photodetectors 30A and 30B, respectively, where the long wavelength beam is detected. Here, photodetectors 24A and 24B for shor wavelength are adapted to detect a short wavelength component of, for example, 0.6 to 1.0 .mu.m, whereas photodetectors 30A and 30B for long wavelength are adapted to detect a long wavelength component of, for example, 0.9 to 1.8 .mu.m, noting that filters 28 filters a long wavelength component of, for example, 0.9 .mu.m or over. Lenses 16, 26 and 32, calcite 18, half-mirrors 22A and 22B and photodetectors 24A, 24B and 30A and 30B are adjustably mounted on base 34 as shown in FIG. 4.

As shown in FIG. 5, photodetectors 24A, 30A and 24B, 30B are connected respectively through corresponding preamplifiers to first and second switches 36A and 36B in switching unit 36. Switching unit 36 is operated responsive to a switching signal from signal generator 40 so that, at the time of detecting short wavelength light beams, photodetectors 24A and 24B are coupled respectively through first and second switches 36A and 36B to first and second A/D converters 38A and 38B and that, at the time of detecting long wavelength light beams, photodetectors 30A and 30B are coupled respectively through first and second switches 36A and 36B to first and second A/D converters 38A and 38B. That is, at the time of detecting the short wavelength light beams, the short wavelength light beams are converted by photodetectors 24A and 24B into photoelectric signals which are in turn supplied through first and second switches 36A and 36B to first and second A/D converters 38A and 38B where they are converted to digital signals. The digital signals are supplied to CPU 44 in signal processing unit 42. Similarly, at the time of detecting the long wavelength light beams, the long wavelength light beams are converted by photodetectors 30A and 30B into electric signals which are in turn supplied through first and second switches 36A and 36B to first and second A/D converters 38A and 38B where they are converted to digital signals. The digital signals are supplied to CPU 44 in signal processing unit 42. Data processed by CPU 44 is stored in RAM 48 in memory unit 46 and data stored in RAM 48 is corrected by CPU 44 with the use of the aforementioned processed data thus stored in ROM 50. By so doing, the light beam generated from the light source is spectrally analyzed so that it is displayed on display section 52 of a CRT, etc.

ROM 50 stores, as processed data, measured data which are initially obtained with respect to analytical equation data as will be set out below and optical property data of diffraction grating 10, polarizing optical element 18, and photodetectors 24A, 24B, 30A and 30B, that is, with respect to the diffraction efficiencies Ap and 4B.sub.S for the P and S polarizing components of various wavelengths as shown in FIG. 1, percent losses C.sub.P D.sub.S of the P and S polarizing components of various wavelengths in polarizing element 18, and photoelectrical conversion efficiencies L1 and L2 of photodetectors 24A, 24B, 30A and 30B for P and S polarizing components of various wavelengths as shown in FIG. 6.

The measuring operation of the aforementioned optical measuring apparatus will be explained below.

When a light beam to be measured is introduced into a diffraction optical system as shown in FIG. 3, then a light beam Q is separated by the diffraction grating into respective wavelength components. Upon receipt of a command from CPU 44, signal generator 40 delivers a rotating signal to rotating unit 14 to permit the grating to rotate at low speed. In this way, the light beam components ranging from a short to a long wavelength are sequentially introduced into the measuring optical apparatus as shown in FIG. 4. In this measuring optical apparatus, the light rays are separated into P and S components by the polarizing optical element. These components are detected by photodetectors 24A, 24B, 30A and 30B. In the short wavelength detection mode, signal generator 40 delivers a first switching signal upon receipt of a command from CPU 44. Switching unit 36 is actuated responsive to the switching signal whereby photodetectors 24A and 24B are coupled to A/D converters 38A and 38B, respectively. As a result, first and second electric signals e.sub.1 and e.sub.2 corresponding to P and S polarized light beams are supplied to A/D converters 38A and 38B where they are converted to digital data. The digital data are sequentially stored in RAM 48. When diffraction grating 10 are rotated through a certain angle, switching is made from a short to a long wavelength mode. That is, when a rotating signal is generated from signal generator 40 for a predetermined time interval, then a mode switching signal is sent fom CPU 44 to the signal generator. Upon the issuance of a command from CPU 44, signal generator 40 delivers a second switching signal to switching unit 36 and thus switching unit 36 is actuated responsive to the switching signal to cause photodetectors 30A and 30B to be coupled to A/D converters 38A and 38B. Thus, third and fourth electric signals e.sub.3 and e.sub.4 corresponding to P and S polarized light beams of longer wavelength are supplied to A/D converters 39A and 38B where they are converted to digital data. The digital data of longer waveform corresponding to third and fourth electric signals are sequentially stored in RAM 48. The digital data of short and long wavelengths are processed based on the optical property data of the grating etc. and analytical equation data stored in ROM 50 and the absolute output power of the measured light beams is computed, the result of which is displayed on CRT 52. The aforementioned analytical equation data can be expressed by an equation (1).

or

Equation (1) will now be explained below.

If Gin represents the input energy level of a given wavelength component of a light beam to be measured,

since the input power level Gin is separated into P and S polarized beam components having levels Pin and Sin.

If G out represents the output energy level of the wavelength components emerging from grating 10,

The respective wavelength components diffracted at diffraction optical system are sent to polarizing optical element 18 where each is separated into P and S polarized beam components. Here the respective output energies of the respective polarized beam components are given below:

The electric signals e.sub.1 and e.sub.2 generated from the photodetectors having the photoelectrical conversion efficiencies L1 and L2 are given below:

From Equations (1) and (2), Gin in Equation (1) can be reduced to:

The optical measuring apparatus may be so configured so to have arrangement shown in FIG. 7. In the apparatus shown in FIG. 7, P and S polarized beam components separated at a polarized element are introduced into optical switch 54 and they are alternatively introduced into photodetector 24. That is, the P polarized beam component is incident onto photodetector 24 when optical switch 54 is turned OFF and the S polarized beam component is incident onto photodetector 24. In this way, electric signals e.sub.1 and e.sub.2 corresonding to the P and S polarized beam components are alternately generated from photodetector 24 in a time-division fashion and processed to obtain an absolute output energy of the light beam.

Although, in the aforementioned embodiment, grating 10 has been explained as being used as a spectrally analyzing optical element, it will be evident that an acoustooptical element may be used instead.