Optical interference signal extractor

Abstract: In a device which extracts an optical interference signal (SA) for analyzing the spectrum of light to be measured, the light (11) to be measured is incident to an optical interferometer (20) capable of sweeping an optical path difference and interference light output therefrom is converted into an electric signal to obtain an optical in­ terference signal (SA). On the other hand, the optical po­ wer of the light (11) to be measured is extracted to create an optical power signal, which is provided to a divider, wherein the optical interference signal is divided by the optical power signal to obtain a noise-free optical inter­ ference signal.

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical interference signal extractor for use in measuring the spectral distribution of light. Fig. 1 shows a conventional optical interference signal extractor. Reference numeral 10 indicates a light source for emitting light 11 to be measured and 20 an optical interferometer capable of sweeping an optical path difference, that is, capable of continuously changing an optical path difference. In this example the optical interferometer 20 is shown to be a Michelson interferometer but may also be a Fabry-Perot interferometer. The Michelson interferometer 20 comprises a half mirror or semitransparent mirror 21 for splitting and combining light, a fixed mirror 22 for forming a fixed optical path 24, and a movable mirror 23 for forming a variable optical path 25. The light 11 emitted from the light source 10 is split by the half mirror 21 to the fixed optical path 24 and the variable optical path 25, and light from the former and light from the latter are coupled together or combined by the half mirror 21 into a beam of light, which is converted by a photodetector 30 into an electric signal SA. The light having passed through the fixed optical path 24 and the light having passed through the variable optical path 25 have a phase difference owing to the optical path difference, and hence interfere with each other when they are coupled together by the half mirror 21. The interference of light varies as the movable mirror 23 moves. In other words, the intensity of the interference light from the optical interferometer 20 repeatedly varies corresponding to interference fringes as the movable mirror 23 moves. The intensity variation of the interference light corresponding to the distance of movement Δℓ of the movable mirror 23 from the position where the optical path difference is zero is measured by conversion into the level of an electric signal by the photodetector 30, whereby such an optical interference signal SA as shown in Fig. 2 can be obtained. The movement of the movable mirror 23 for the distance Δℓ causes an optical path difference of 2Δℓ, but in the following description, the distance of movement Δℓ may sometimes be referred to as the optical path difference for the sake of simplicity. By a frequency analysis of the interference signal SA through Fourier transformation, the spectral distribution of the light to be measured 11 can be obtained. Where the moving speed of the movable mirror 23 is made constant, time t corresponding to the distance of its movement may be represented on the abscissa in Fig. 2. Fig. 2 shows the waveform of the interference signal SA when the optical power of the light to be measured 11 is stable. Where the optical power (indicated by SB in Fig. 2) of the light 11 is stable, an interference signal of a good SN ratio can be obtained. On the other hand, where the optical power SB of the light 11 fluctuates as depicted in Fig. 3, the SN ratio of the interference signal SA becomes poor, and the results of its frequency analysis contain, as spectral, also high-order modulation components resulting from the fluctuation in the optical power of the light 11, making it impossible to accurately measure the wavelength distribution in the light 11. The optical interference signal extractor shown in Fig. 1 and explained above is disclosed in the document "J.E.E. Journal of Electronic Engineering", Vol. 23, No. 236, August 1986, page 58-60; N. Arakawa. Although not shown in Fig. 1, this prior art further comprises a reference light source for emitting reference light of a narrow spectrum which is applied to said optical interferometer together with the light to be measured. A second photodetector is used for converting reference interference light emitted from the interferometer to an electrical signal, and a multiplier multiplies the frequeny of the output signal of the second photodetector to a desired frequency and applies the latter as a sampling clock to an A-D converter used for converting the output signal from photodetector 30 into a digital signal for further signal processing. The document "Review of Scientific Instruments", Vol. 59, No. 2, February 1988, pages 242-248, New York, US: F.Ozanam et al, discloses a Fourier transform, electromodulated, infrared spectrometer for studies at the electrochemical interface. This known spectrometer uses a Michelson interferometer and discloses a beam splitter for splitting off a portion of the light incident to the interferometer and directs it to a photodiode in order to obtain a signal representing the power of the light source. This signal is used by a stabilization device driving the light source. In the Michelson interferometer, the optical path length is swept by means of a scanning mirror drive, and the exit beam of the interferometer is focussed on a sample to be investigated. The light beam from the sample is focussed on a photodetector, converting it into an electrical signal. The signal is separated into a DC component and an AC component, these components being processed in resepctive separate channels. The document FR-A-2 621 753 discloses a transmitter-receiver system designed such as to compensate for any attenuation the transmitted signal is subjected to in the transmission path between the transmitter and the receiver. To that end, on the transmitting side, any uncertain DC component of the signal to be transmitted is removed and a known DC component added instead. The sum of the AC component and the known DC component is transmitted to the receiver. In the receiver, the AC component and the DC component, each attenuated by the same factor in the transmission path, are separated from each other and the AC component is amplified by an amplifier whose gain is controlled by the DC component. Provided the gain of the amplifier is inversely proportional to the DC component, the attenuation factor in the AC component will be removed. SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical interference signal extractor which permits the removal of noise resulting from a variation in the optical power of light to be measured. This object is achieved with an optical interference signal extractor as claimed. In the solution according to the present invention, the power signal may be obtained either by applying the optical interference signal to a low-pass filter or by directly converting the light to be measured into an electric signal through use of a second photodetector. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram for explaining the prior art; Fig. 2 is a waveform diagram for explaining the waveform of an optical interference signal; Fig. 3 is a waveform diagram showing an optical interference signal affected by optical power variations; Fig. 4 is a block diagram illustrating an embodiment of the present invention; Fig. 5 is a block diagram illustrating another embodiment of the present invention; Fig. 6 is a block diagram illustrating another embodiment of the present invention; and Fig. 7 is a block diagram illustrating still another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 4 illustrates in block form an embodiment of the present invention. In Fig. 4 the parts corresponding to those in Fig. 1 are identified by the same reference numerals, and since the arrangement of the optical interferometer 20, which is an optical system for measuring the light 11 from the light source 10, is exactly the same as shown in Fig. 1, no description will be given of the optical interferometer 20. In this embodiment, the optical interference signal SA from the photodetector 30 is branched to a low-pass filter 40, wherein a low-frequency component SB of the optical interference signal SA is extracted. The low-frequency component SB of the optical interference signal SA is a DC-like optical power signal SB of the light 11 containing a low-frequency variation component. That is, where the light source 10 is being excited by the commercial power line, a noise of 50 or 60 Hz, or 100 or 120 Hz gets mixed in the optical interference signal SA. Also in the case of DC lighting, a fluctuation in the DC power source voltage bring about a low-frequency optical power variation. The optical power signal SB containing this optical power variation as a noise component is extracted by the low-pass filter 40 and is applied to an arithmetic unit 50. The optical interference signal SA from the photodetector 30 is also provided to the arithmetic unit 50, wherein it is divided by the optical power signal SB, obtaining an optical interference signal SC with no noise component. Letting the power level of the light 11 from the light source 10 be represented by I B and the wavelength of arbitrary light in the frequency band in which an analysis is made, for example, the center frequency, be represented by λ₀, the waveform of the optical interference signal SA at the power level I A shown in Fig. 2 can be given by the following expression: I A = I B + I B γcos 2π 2Δℓ λ₀ where γ is referred to as coherency, which represents interference light components at wavelengths except λ₀ and varies with the distance of movement Δℓ of the movable mirror 23. Further, 2Δℓ/λ₀ is called a fringe frequency. In Fig. 4, letting the moving speed of the movable mirror 23 be represented by V, then Δℓ = Vt, so that Expression (1) can be expressed as follows: I A = I B + I B γcos 2π 2Δℓ λ₀ = I B + I B γcos 2πf F ·t where f F represents the fundamental frequency of the optical interference signal SA, such as shown in Fig. 2, detected by the photodetector 30 while moving the movable mirror 23 at the speed V in Fig. 4, and this fundamental frequency may sometimes be called the fringe frequency. In practice, the moving speed of the movable mirror 23 is selected such that the fringe frequency f F is about 50 kHz, for instance. The power I B of the light 11 emitted from the light source 10 fluctuates as mentioned previously in respect of Fig. 3, and letting its fluctuation component and DC component be represented by ΔI(t) and I₀, respectively, the power I B of the light 11 is expressed as follows: I B = I₀ + ΔI(t) The fluctuation component ΔI(t) modulates the amplitude of the optical interference signal as is evident from Expression (3), so that even if the frequency of the fluctuation component ΔI(t) (taken as f n ) is sufficiently lower than the fringe frequency f F , spectra of f F ± f n and f F ± 2f n , for example, appear as noise in the vicinity of the fringe frequency f F in the frequency band to be measured. Consequently, the waveform of the optical interference signal SA, which is actually detected by the photodetector 30, contains noise components as depicted in Fig. 3. In the embodiment shown in Fig. 4, the light power signal SB which is the input light 11, given by Expression (3), is extracted by the low-pass filter 40 from the optical interference signal SA which is the output of the photodetector 30, given by Expression (1), and the ratio SA/SB between the levels of the signals SA and SB is computed by the arithmetic unit 50. From Expressions (2) and (3) the level ratio SA/SB is expressed as follows: SA/SB = I A /I B = 1 + γcos 2πf F ·t Since the output signal SC of the computing element 50 is free from the influence of the power fluctuation of the input light 11 as is evident from Expression (4), its waveform is substantially the same as that of the optical interference signal shown in Fig. 2 which is provided when the power of the input light 11 undergoes no fluctuation. Consequently, an accurate spectrum of the input light 11 can be obtained by a frequency analysis of the output signal SC of the arithmetic unit 50 through the fast Fourier transformation, for instance. In this embodiment, by selecting the cut-off frequency of the low-pass filter 40 to be lower than the band of the fringe frequency corresponding to the spectrum measuring band, all noises attributable to fluctuation components lower than the above-mentioned cut-off frequency can be removed from the optical interference signal SA. Although in the Fig. 4 embodiment the optical power signal SB corresponding to the power of the incident light 11 is extracted by the low-pass filter 40 from the optical interference signal SA which is the output of the photodetector 30, the power of the light 11 from the light source 10 may also be detected directly as the optical power signal SB. An embodiment therefor is depicted in Fig. 5, in which the light 11 emitted from the light source 10 is split by a half mirror 12 into light which passes through the half mirror 12 and is then incident to the optical interferometer 20 and light which is reflected by the half mirror 12 and is then incident to a second photodetector 41. The power signal SB corresponding to the power of the light 11 is detected by the second photodetector 41 and is provided to the arithmetic unit 50 for the division SA/SB. The other parts of this embodiment are the same as those in the Fig. 4 embodiment and no description will be given of them for the sake of simplicity. According to the Fig. 5 embodiment, all noises attributable to fluctuation components in the entire frequency band contained in the light 11 can be removed from the optical interference signal SA. Fig. 6 illustrates a modified form of the embodiment shown in Fig. 4. As described previously, the optical interference signal SA is obtained at the output of the photodetector 30 as the movable mirror 23 of the optical interferometer 20 moves at a constant speed, but if the moving speed of the movable mirror 23 varies, the fringe frequency of the optical interference signal SA also fluctuates accordingly and no accurate results of spectrum analysis can be obtained. In the embodiment shown in Fig. 6 reference light which has a line spectrum of a stable wavelength, such as laser light, is applied to the optical interferometer 20, together with the light to be measured, and a reference optical interference signal is derived from reference interference light which is emitted from the optical interferometer 20. The optical interference signal of the light to be measured is sampled by a clock of the fringe frequency of the reference optical interference signal or a frequency N times higher than the fringe frequency, by which samples of the optical interference signal corresponding to the distance of movement of the movable mirror 23 are successively generated without being affected by the variation in the moving speed of the movable mirror 23. In this embodiment a reference light source 60 is provided in addition to the light source 10 and reference light 61 emitted from the reference light source 60 is applied to the optical interferometer 20, together with the light to be measured 11, as shown in Fig. 6. The reference light source 60 is one that is stable in wavelength and has a narrow spectrum characteristic, such as a He-Ne laser. In the optical interferometer 20 the reference light 61 is also split to the fixed optical path 24 and the variable optical path 25 and both rays of light therefrom are combined by the half mirror 21 into reference interference light, which is received by a photodetector 42 for conversion into an electric signal. By using, as the reference light source 60, a light source which emits light of a stable wavelength and a narrow spectrum characteristic, the amplitude of a reference optical interference signal SD which is provided from the photodetector 42 varies sinusoidally with a constant period, i.e. a constant fringe frequency, with respect to the distance of movement Δℓ of the movable mirror 23. In other words, the reference optical interference signal SD has a peak for each fixed distance of movement of the movable mirror 23. Where the moving speed of the movable mirror 23 changes, the position of the peak also changes correspondingly on the time axis. The reference optical interference signal SD is provided to an N-multiplier 43, from which an N-multiplied signal is applied as a sampling clock CL to an A-D converter 31, wherein the level of the optical interference signal SA of the light to be measured 11 from the photodetector 30 is converted to a digital value for each sampling clock CL. The value of N of the N-multiplier 43 is selected such that the frequency of the sampling clock CL is more than twice higher than the fringe frequency of the optical interference signal SA. The sample data available from the A-D converter 31 is provided to a memory 32, in which sample data of one sweep of the movable mirror 23 is written in association with the sweep. The sample data from the A-D converter 31 is provided also to a digital low-pass filter 44, by which a low-frequency component containing the DC component and the fluctuation component of the power of the input light 11 is extracted as the optical power signal SB, and data about the optical power signal SB of one sweep of the movable mirror 23 is also written into a memory 45. After completion of the write of the data of one sweep the data SA and SB are read out of the memories 32 and 45 and provided to an arithmetic unit 51, wherein the division SA/SB described above with regard to Fig. 4 is performed to obtain the optical interference signal data SC from which the fluctuation component has been removed. The data SC thus obtained may be subjected to, for example, a spectrum analysis by an FFT analyzer 52, by which the spectrum of the input light 11 can be obtained. The optical interference signal SC obtained by the division SA/SB does not include the term of the power I B of the input light 11 as seen from Expression (4), and consequently, the spectral component obtained through frequency analysis of the result of the division SA/SB has been normalized by the power I B of the input light 11 and is unrelated to the power level of the input light 11. That is, if the spectrum distribution of the input light 11 remains unchanged, spectra of the same amplitudes are obtained irrespective of the magnitude of the power of the input light 11, and the results of analysis do not contain information about the power level of the input light 11. In the Fig. 6 embodiment, the arithmetic unit 51 multiplies the result of the division SA/SB by the power level P₀ of the input light 11 as a coefficient so as to obtain a spectrum of an amplitude proportional to the power of the input light 11. Consequently, the output data SC of the arithmetic unit 51 assumes a value proportional to the power level P₀ of the input light 11, and the amplitude of the spectrum obtained by the frequency analysis of the data SC becomes also proportional to the power level P₀. The coefficient P₀ that is used here is one of the data written in the memory 45, preferably, that data SB of a series of data stored in the memory 45 for one sweep which corresponds to the center position of the sweep. More preferably, the movable mirror 23 of the optical interferometer 20 is moved for a distance from -Δℓ to +Δℓ in terms of the optical path difference between the fixed and movable optical paths 24 and 25, and the optical power data SB at an address corresponding to the vicinity of the position where the optical path difference is zero is read out of the memory 45 and is used as the coefficient P₀. Still more preferably, the optical power data SB in a predetermined number of periods before and after the position of zero optical path difference are read out of the memory 45 and averaged by an averager 47 indicated by the broken line in Fig. 6, and the resulting average is used as the coefficient P₀. In this instance, the coefficient P₀ is less affected by the fluctuation component contained in the optical power. The computing element 51 outputs the optical interference signal SC from which the noise component has been removed. By a frequency analysis of the optical interference signal SC with the frequency analyzer 52, the optical spectrum of the light 11 to be measured can be displayed on a display 53. Moreover, an envelope detector 54 is provided, by which a variation in the amplitude of the optical interference signal SC, that is, the coherency γ in Expression (4), is detected. By displaying the coherency γ on a display 55 with the optical path difference Δℓ represented on the abscissa, the coherency of the light 11 being measured can be displayed. Besides, fluctuations of the light 11 can be observed by displaying on a display 56 the optical power signal data SB read out of the memory 45. Fig. 7 illustrates another modification of the Fig. 4 embodiment, in which the method of employing the reference light 61 is applied to the Fig. 5 embodiment as is the case with the Fig. 6 embodiment. In Fig. 7 the parts corresponding to those in Figs. 5 and 6 are identified by the same reference numerals and characters. As depicted in Fig. 7, a portion of the light 11 emitted from the light source 10 is reflected by the half mirror 12 to the photodetector 41, wherein the power of the light 11 is converted to the electric signal SB, which is applied to an A-D converter 46. The A-D converter 46 converts the level of the optical power signal SB into a digital value upon each application thereto of the sampling clock CL derived by the N-multiplier 43 from the optical interference signal SD of the reference light 61. The output data of the A-D converter 46 is written into the memory 45 in association with the sweep of the movable mirror 23. The construction and the operation of this embodiment are identical with the Fig. 6 embodiment, and hence no further description will be repeated. As described above, according to the present invention, even if the optical power of the light to be measured 11 fluctuates, an optical interference signal free from the fluctuation component can be obtained. It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

Claims: An optical interference signal extractor comprising: an optical interferometer (20) which receives light to be measured and which outputs interference light of said light to be measured, and in which an optical path difference is sweepable; and optical interferometer signal generating means (30) which receives said interference light incident from said optical interferometer and outputs a first optical interference signal which is an electric signal corresponding to the intensity of said interference light; characterized by optical power signal extracting means (40; 41) for generating an optical power signal corresponding to the power of said light to be measured; and arithmetic means (50) for dividing said first optical interference signal from said optical interference signal generating means by said optical power signal from said optical power signal extracting means and for outputting the result of said division as a second optical interference signal. The optical interference signal extractor of claim 1 wherein said optical power signal extracting means includes a low-pass filter (40) connected to said optical interference signal generating means (30) and has a predetermined cut-off frequency. The optical interference signal extractor of claim 1 wherein said optical signal extracting means includes a photodetector (41) which receives a portion of said light to be measured and detects and outputs its power as said optical power signal. The optical interference signal extractor of claim 1 wherein said optical interference signal generating means includes: a first photodetector (30) which receives and converts said interference light to an electric signal; an A-D converte (31) which converts the output of said first photodetector to a digital signal and outputs it as said first optical interference signal; a reference light source (60) for emitting reference light of a narrow spectrum which is applied to said optical interferometer (20), together with said light to be measured; a second photodetector (42) for converting reference interference light emitted from said optical interferometer into an electric signal in accordance with said reference light; and a multiplier (43) which multiplies the frequency of the output signal of said second photodetector to a desired frequency and applies the multiplied output as a sampling clock to said A-D converter; and wherein said optical power signal extracting means includes a digital low-pass filter (44) which is supplied with the output of said A-D converter and has a predetermined cut-off frequency for extracting said optical power signal in digital form. The optical interference signal extractor of claim 4 wherein said arithmetic means includes: a first memory (32) for storing said first optical interference signal output from said A-D converter (31); a second memory (45) for storing said optical power signal output from said digital low-pass filter (44); and dividing means (51) supplied with said first optical interference signal and said optical power signal read out of said first and second memories, for dividing said first optical interference signal by said optical power signal. The optical interference signal extractor of claim 1 wherein said optical interference signal generating means includes: a first photodetector (30) which receives and converts said interference light into an electric signal: a first A-D converter (31) for converting the output of said first photodetector into a digital signal for output as said first optical interference signal; a reference light source (60) for emitting reference light of a narrow spectrum which is applied to said optical interferometer (20), together with said light to be measured; a second photodetector (42) for converting reference interference light emitted from said optical interferometer into an electric signal in accordance with said reference light; and a multiplier (43) which multiplies the frequency of the output signal of said second photodetector to a desired frequency and applies the multiplied output as a sampling clock to said first A-D converter; and wherein said optical power signal extracting means includes: a third photodetector (41) which receives a portion of said light to be measured, detects its power and outputs the detected power as said optical power signal; and a second A-D converter (46) connected to the output of said third photodetector, for converting said optical power signal into a digital optical power signal upon each application of said sampling clock from said multiplier. The optical interference signal extractor of claim 6 wherein said arithmetic means includes: a first memory (32) for storing said first optical interference signal output from said first A-D converter; a second memory (45) for storing said optical power signal output from said second A-D converter; and dividing means (51) supplied with said first optical interference signal and said optical power signal read out of said first and second memories, for dividing said first optical interference signal by said optical power signal. The optical interference signal extractor of claim 5 or 7 wherein said arithmetic means includes multiplying means (51) whereby the divided output of said dividing means (51) is multiplied by one of sample data of said optical power signal read out of said second memory (45), as a constant coefficient, the multiplied output of said multiplying means being output as said second optical interference signal. The optical interference signal extractor of claim 5 or 7 wherein said arithmetic means includes: averaging means (47) for averaging a plurality of sample data of said optical power signal read out of said second memory (45) and providing the averaged output as a constant coefficient; and multiplying means (51) for multiplying said constant coefficient and said divided output of said dividing means and providing the multiplied output as said second optical interference signal. The optical interference signal extractor of claim 2, 4 or 5 wherein the cut-off frequency of said low-pass filter (40;44) is lower than a frequency band for measuring said optical interference signal corresponding to a wavelength region for measuring said light to be measured.

Inventors: NIKI SHOJI, PAWLIK JON R, NIKI, SHOJI, PAWLIK, JON R.
Applicants: ADVANTEST CORP, ADVANTEST CORPORATION
Application year: 1990
Main classification: G01J3
IPC codes: G01J3