OPTICAL POWER METER

Abstract: An optical power meter system comprises a chopper (8) with a semicircle total reflection mirror (6) along an incident light path, two photo-detectors (10,12) of the different characteristics of wavelength sensitivity, and a comparator (26) for comparing the photo-detection signals derived from the two photo-detectors and for outputting an optical detection signal of a higher level. The photo-detectors (10,12) are positioned in a first path of the light passing beside the chopper and a second path of the light reflected by the chopper, respectively. The comparator may be replaced by an adder for adding the photo-detection signals derived from the two photo-detectors.

Description: The present invention relates to an optical power meter and more paticularly, to an optical power meter for selectively switching at least two photo-detectors. Conventionally, in the field of optical fiber communication, an optical power meter is used to measure the optical characteristics of a light emission element and the transmission characteristics of the optical signal in an optical fiber. A photoelectric conversion element such as a photodiode and a photo-transistor is provided as a photo detector of the optical power meter. The photoelectric conversion element has the characteristics of wavelength sensitivity according to which the photoelectric conversion element responds to the wavelength given thereto. Since the characteristics of wavelength sensitivity of the photoelectric conversion element are fixed, the light of which the wavelength is outside the region of the characteristics of wavelength sensitivity cannot be detected. The characteristics of wavelength sensitivity can cover only a narrow region of the wavelength. To measure the power of the optical signals over the wide wavelength region, a plurality of photoelectric conversion elements must be provided of which the respective wavelength sensitivity characteristics can cover only some parts of the wide wavelength range. Depending on these parts of the wavelength region therefore an appropriate one of the plurality of photoelectric conversion elements must be selected and exchanged. Thus, the measurement of the optical power is troublesome. Accordingly, it is an object of the present invention to provide an improved optical power meter for efficiently measuring the power of an optical signal over a wide wavelength range without exchanging photoelectric conversion elements. It is another object of the present invention to provide an improved optical power meter for selectively switching at least two photo-detectors of which wavelength sensitivity characteristics are different from each other in order to output an optical detection signal of a higher level. Other objects of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention as claimed will become apparent to those skilled in the art from this detailed description. To achieve the above objects, pursuant to an embodiment of the present invention, an optical power meter system comprises chopper means with a total reflection mirror, at least two photo-detectors of the different characteristics of wavelength sensitivity, and comparator means for comparing the photo-detectors signals derived from the two photo-detectors and outputting an optical detection signal of a higher level. The chopper is positioned in an incident light path. The photo-detection are positioned in a first path of the light passing beside the chopper and in a second path of the light reflected by the chopper, respectively. In another preferred embodiment of the present invention, the comparator in the first preferred embodiment can be replaced by adder means for adding the photo-detection signals derived from the two photo-detectors. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawing which are given by way of illustration only, and thus are not limitative of the present invention and wherein: FIG.1 is a block diagram of an optical meter system according to one preferred embodiment of the present invention; FIG.2 is a graph of the characteristics of the wavelength sensitivity of a photoelectric conversion element incorporated within a photo-detector used for the optical power meter system of the present invention; FIG.3 is a graph of photo-detection signals outputted by the photo-detector; FIG.4 is a block diagram of an optical power meter system according to another preferred embodiment of the present invention; and FIG.5 is a graph of the characteristics of wavelength sensitivity given by the optical power meter system of FIG.4. FIG.1 is a block diagram of an optical power meter according to one preferred embodiment of the present invention. The optical power meter system 1 receives the optical signal under the measurement through an optical fiber 2. A light chopper 4 is disposed in an incident light path 1. The light chopper 4 includes a semicircular total reflection mirror 6 which is positioned in the incident light path 1 with being tilted at a predetermined angle, preferably, 45° ,smaller than 90° . The total reflection mirror 6 is coupled to the axis of rotation of a motor 8. A first photo-detector 10 is positioned in the path of the light unreflected by the total reflection mirror 6 of the chopper 4. A second photo-detector 12 is positioned in the path of the light reflected by the total reflection mirror 6 of the chopper 4. As shown in FIG. 2. the first photo-detector 10 comprises a first photoelectric conversion element providing a peak value at a wavelength λ ₁ ( lambda 1 ) while the second photo-detector 12 comprises a second photoelectric conversion element providing a peak value at a wavelength λ ₂ ( lambda 2 ). An amplifier 14 is provided for amplifying a photo-detection signal delivered from the first photo-detector 10. A first synchronous rectifier 16 is provided for synchronously rectifying the photo-detection signal outputted by the amplifier 14. A phase-reverse amplifier 18 is provided for amplifying a photo-detection signal delivered from the second photo detector 12 and reversing the phase of the photo-detection signal at 180° . A second synchronous rectifier 20 is connected to synchronously rectify the photo-detection signal outputted by the phase-reverse amplifier 18. A synchronous-signal generator 23 with a light source and a photo-detector is coupled to the optical chopper 4 and an output signal of the generator 23 is applied to the first synchronous rectifier 16 and the second synchronous rectifier 20 through the synchronous signal circuit 22. A selection circuit 24 is provided for selectively outputting the photo-detection signal of a higher level by comparing the photo-detection signals delivered from the first and the second photo-detectors 10 and 12. In this preferred embodiment, the selection circuit 24 comprises a comparison circuit 26 and a switch 28. The comparison circuit 26 is responsive to the photo-detection signals from the first and the second photo-detectors 10 and 12 to compare the levels of the photo-detection signals in order to output a comparison signal representing which photo detection signal is great. The switch 28 is responsive to the comparison signal outputted by the comparison circuit 26 to switch the photo-detection signals outputted by the first and the second synchronous rectifiers 16 and 20. The switch 28 has three terminals 28a, 28b, and 28c. An optical power meter circuit 29 is coupled to the terminal 28a. The light signal from the optical fiber 2 is incident on the chopper 4 to measure the power of the optical signals with the above-arranged optical power meter. The light from the optical fiber 2 is split to the detectors 10 and 12 by the optical chopper 4, alternatively. Therefore, each output of the detector 10 or 12 has an alternative signal which has 180° phase difference each other, as shown in FIG. 3. The output signal S₁ of the detector 10 is amplified by the amplifier 14, so that the amplified signal is rectified by the first synchronous rectifier 16 with the synchronous signal outputted by the synchronous-signal circuit 22. The synchronously rectified output is applied to the terminal 28b of the switch 28 and to a first input terminal of the comparison circuit 26. The output signal S₂ of the second detector 12 is amplified and phase-reversed at 180° by the phase-reverse amplifier 18 in order to produce a signal S₂' in phase of signal S₁. This rectifier signal S₂' is applied to the second synchronous rectifier 20, so that it is synchronously rectified with the synchronous signal outputted by the synchronous signal circuit 22. The synchronously rectified output is applied to the terminal 28c of the switch 28 and to a second input terminal of the comparison circuit 26. The comparison circuit 26 compares the output signal levels of the first and second synchronous rectifiers 16 and 20. It outputs the comparison signal representing which of the signals S₁ and S₂ has a larger level. The comparison signal is applied to the switch 28. When the comparison signal indicates that the level of the signal S₁ derived from the first photo-detector 10 is larger than the level of the signal S₂ from the second photo-detector 12, the switch 28 is responsive to the comparison signal to connect the common terminal 28a to the terminal 28b. On the contrary, when the comparison signal represents that the level of the signal S₂ is larger than that of the signal S₁ , the switch 28 is responsive to the comparison signal to connect the common terminal 28a to the terminal 28c. Thus, the switch 28 is switched so as to receive the photo-dectection signal of a higher level in response to the comparison of the levels of the photo-detection signals S₁ and S₂. The selection circuit 24, therefore, can continue to output the photo-detection signal derived from the photo-detector 10 or 12 which can appropriately respond to the wavelength of the optical signal under measurement. The power of the optical signal over the added wavelength zones of the first and second photo-detectors 10 and 12 can be measured by the above-arranged optical power meter system 1. In the above preferred embodiment, the total reflection mirror 6 is rotated by the motor 8, but it should not be limited to this. It may be possible that the total reflection mirror 6 is merely slided into the incident light path I with remaining tilted. Instead of the first and the second synchronous rectifiers 16 and 20, a tuning amplifier and a rectifier circuit may be combined in which case the phase-reverse amplifier 18 and the synchronous signal circuit 22 can be omitted. Further, instead of the comparison circuit 24, an adder 30 as shown in FIG. 4 can be provided for adding the synchronously detected output signals from the first and the second synchronous rectifiers 16 and 20. The output of the adder 30 is shown in the chain line and the solid line of FIG. 5, so that the additive characteristics of the photo-sensitivity of the first and the second synchronous rectifiers 16 and 20 can be obtained. As described above, in accordance with the optical power meter system, the light signal from the optical fiber 2 is chopped by the chopper 4 to alternatively switch the light path for the straightly incident light and the reflected light. The straightly incident light and the reflected light are introduced into the photo-detectors respectively having the different characteristics of the wavelength sensitivity. Each of the photo-detectors outputs the photo-detection signal which is inputted into the comparison circuit. The comparison circuit compares the photo-detection signals so as to produce the comparison signal indicating either of the photo-detection signals of a higher level. In other words, the comparison circuit outputs the comparison signal indicating which of the photo-detectors has sensitivity suitable for the wavelength of the light under the measurement. In place of the comparison circuit, the adder may be provided for adding the photo-detection signal levels obtained by the photo-detectors. Thus, measurement of the power over the wide range of the whole wavelength ranges of the every photo-detector is possible. In accordance with the present invention, the power of the optical signal can be efficiently and effectively measured over the wide rangeof the wavelength without having to change photo-electric conversion elements.

Claims: An optical power meter for measuring the power of an optical signal of an incident light, comprising: synchronous-signal generation means (22) for generating a synchronization signal; light path switching means (4) for alternatively switching a first light path and a second light path of the incident light at a predetermined cycle in response to the synchronization signal; first photo-detector means (10) disposed in the first light path, said first photo-detector means having the light sensitivity of a first wavelength of the incident light; second photo-detector means (12) disposed in the second light path, said second photo-detector means having the light sensitivity of a second wavelength of the incident light different from the first wavelength; phase-reverse means (18) for reversing the phase of a second photo-detection signal generated by said second photo-detector means (12); first detection means (16) for detecting the level of a first photo-detection signal generated by the first photo-detector means in synchronization with the synchronization signal; second detection means (20) for detecting the level of the second photo-detection signal, after reversed by said phase-reverse means, in synchronization with the synchronization signal; and selection means (24) responsive to the outputs of said first and said second detection means for comparing the levels of the outputs and for selectively outputting either of the first or the second photo-detection signals with a higher level. The optical power meter of claim 1, wherein said selection means (24) comprises a comparison circuit (26) and a switch (28), said comparison circuit (26) comparing the levels of the first photo-detection signal and the second photo-detection signal derived from said first detection means (16) and said second detection means (20) and producing a comparison signal depending on the comparison result, and said switch (28) comprising a first contact (28b) connected to said first detection means and a second contact (28c) connected to said second detection means, and a movable contact (28a) moved in response to the comparison signal. The optical power meter of claim 1, wherein said light path switching means (4) comprises a chopper (4) having a total reflection mirror (6), said total reflection mirror (6) being tilted at an angle smaller than 90° in relation to the passing path of the incident light, said chopper moving the position of the total reflection mirror so as to obstruct or not to obstruct the path of the incident light, in which, in case where the total reflection mirror is moved outside the incident light path, the incident light passes through the first light path and, in case where the total reflection mirror is moved into the incident light path, the incident light is reflected to pass through the second light path. An optical power meter for measuring the power of an optical signal of an incident light, comprising: synchronous-signal generation means (22) for generating a synchronization signal; light path switching means (4) for alternatively switching a first light path and a second light path of the incident light at a predetermined cycle in response to the synchronization signal; first photo-detector means (10) disposed in the first light path, said first photo-detector means having the light sensitivity of a first wavelength of the incident light; second photo-detector means (12) disposed in the second light path, said second photo-detector means having the light sensitivity of a second wavelength of the incident light different from the first wavelength; phase-reverse means (18) for reversing the phase of a second photo-detection signal generated by said second photo-detector means; first detection means (16) for detecting the level of a first photo-detection signal generated by the first photo-detector means in synchronization with the synchronization signal; second detection means (20) for detecting the level of the second photo-detection signal, after reversed by said phase-reverse means, in synchronization with the synchronization signal; and adder means (24) responsive to said first detection means and said second detection means for adding the outputs of said first and said second detection means. The optical power meter of claim 4, wherein said light path switching means (4) comprises a chopper (4) having a total reflection mirror (6), said total reflection mirror (6) being tilted at an angle smaller than 90° in relation to the passing path of the incident light, said chopper moving the position of the total reflection mirror so as to obstruct or not to obstruct the path of the incident light in response to the synchronization signal, in which, in case where the total reflection mirror is moved outside the incident light path, the incident light passes through the first light path and, in case where the total reflection mirror is moved into the incident light path, the incident light is reflected to pass through the second light path.

Inventors: ISHIKAWA, KUNIO, SHIMOMURA, TEIICHI
Applicants: SHIMADZU CORPORATION
Application year: 1986
Main classification: G01J1
IPC codes: G01J1, G01J3