JP2009052941A - Optical spectrum monitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure a spectrum in a wide wavelength range at high speed with a high wavelength resolution, at easily-realizable data sampling speed. <P>SOLUTION: A passing center frequency Fi of a filter part 24 is constituted variably, and the passing center frequency Fi of the filter part 24 and a sampling period Ts of an A/D conversion part 26 are set based on the width W of a measured wavelength range whose state is designated by a measuring condition designation means 31, a sweeping time T required for one sweeping of local light L, a measured wavelength resolution R and a spectrum line width of the local light L, and the passing center frequency Fi of the filter part 24 is set corresponding to a measuring condition, to thereby cope with various measuring conditions at the realizable sampling speed. An output from the filter part 24 is subjected to envelope detection by a detection part 25, and a time constant of the envelope detection is set at a value similar to the sampling speed, to thereby improve a spectrum waveform. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technique for realizing high-speed sweep, wide dynamic range, high sensitivity, and high wavelength resolution in an optical spectrum monitor for observing the spectrum of an optical signal.

  A heterodyne type optical spectrum monitor that displays the spectrum is used to evaluate the quality of the optical signal.

  The heterodyne type optical spectrum monitor combines local light that is wavelength-swept and input light to be monitored and enters the photoelectric converter.

  From the photoelectric converter, in addition to a signal that depends only on the amplitudes of the local light and the input light, a signal that depends on the difference frequency component between the two lights is output.

  Therefore, the output signal of the photoelectric converter is input to a bandpass filter having a predetermined pass center frequency Fi, and a component having a frequency difference Fi with respect to the local light whose wavelength is swept is extracted from the measured light. By detecting the signal, the level of the wavelength component having a frequency difference Fi with respect to the wavelength of the local light can be detected.

Here, when the frequency of the local light is swept in the range of F1 to F2, among the wavelength components included in the input light,
| F1-Fi | ~ | F2-Fi |
The level of the frequency component is detected by sweeping the local light.

  However, since the pass center frequency Fi of the bandpass filter is actually much smaller than the light frequencies F1 and F2, the frequency of the local light is swept across the light to be measured. For this reason, a signal is detected at a position of ± Fi (a position separated by 2 · Fi) with respect to one measured light, and an accurate spectrum waveform cannot be obtained.

  For example, as shown in FIG. 8A, in the state where the spectrum of the frequency Fx exists in the light S to be measured, the frequency of the local light L approaches from the lower side (left side in the figure), for example. When the frequency coincides with Fx−Fi), the level of the output signal of the bandpass filter reaches a peak as shown in FIG. 8C, and the frequency of the local light L is swept to the higher side, and the frequency shown in FIG. Thus, when the frequency coincides with the frequency (Fx + Fi), the level of the output signal of the bandpass filter becomes a peak again as shown in FIG. 8C.

  That is, the measurement is performed so that two spectra exist for one spectrum of the light S to be measured.

  As one method for solving this problem, in Patent Document 1, the width of two detected spectra depends on the spectral line width of the local light, and the pass center frequency of the bandpass filter is set as the spectral line of the local light. A technique has been disclosed in which a normal spectrum and an image spectrum overlap each other by being substantially equal to the width so that the spectrum is observed as one spectrum.

Japanese Patent No. 2664255

  According to the technique disclosed in Patent Document 1, the light to be measured can be measured with the wavelength resolution determined by the spectral line width of the local light, and measurement with an extremely high wavelength resolution is possible.

  However, as described above, when the local light is swept at a high speed with a conventional apparatus using a bandpass filter with a relatively narrow passband having a pass center frequency corresponding to the spectral line width of the local light, the local light and the measurement target The frequency at which the intensity of the interference light generated as a result of the light interference is higher than the frequency difference between the local light and the light to be measured.

  For this reason, the amount of signal passing through the band-pass filter is reduced, the sensitivity is lowered, and the spectrum may be lost.

  Further, when performing high-speed sweeping with high wavelength resolution measurement, it is necessary to perform sampling at an interval narrower than the wavelength width of the spectrum line width of the local light, and ultrahigh-speed data sampling is required.

  For example, if the spectral line width of local light is 1 MHz (wavelength width of about 10 fm: femtometer), data sampling is required at a wavelength interval of 10 fm or less. Therefore, it is unavoidable to perform ultra-high-speed data sampling or reduce the wavelength sweep speed of local light.

The width of the measurement wavelength sweep range is 100 nm, the sweep time is 2 ms, and the spectral line width of the local light (corresponding to the wavelength resolution) is 1 MHz (wavelength width is about 10 fm), which is 1 / 10th of 100 KHz (wavelength width is about 1 fm). Sampling data at a wavelength interval of
Ts = 2 (ms) × 1 (fm) / 100 (nm)
= 2 × 10 ⁻¹¹ (sec)
As a result, data sampling at 50 GHz is required, which is difficult to realize.

  An object of the present invention is to solve this problem and provide an optical spectrum monitor capable of measuring a spectrum in a wide wavelength range at a high speed with a high wavelength resolution at a data sampling rate that is easy to realize.

In order to achieve the object, an optical spectrum monitor according to claim 1 of the present invention comprises:
A wavelength tunable light source (21) that emits local light that is swept within a specified wavelength range;
A multiplexing unit (22) for multiplexing the local light and the measured light;
A photoelectric conversion unit (23) for receiving light emitted from the multiplexing unit;
A bandpass filter unit (24) having a variable pass center frequency for extracting a difference frequency component between the local light and the light to be measured from an output signal of the photoelectric conversion unit;
A detector (25) for detecting an envelope of the output signal of the filter unit;
An A / D conversion section (26) for sampling the output signal of the detection section at a predetermined sampling period and converting it into digital data;
Measurement condition designating means (31) for designating measurement conditions including a measurement wavelength range (λc, W) and a measurement wavelength resolution (R);
The wavelength of the local light is swept in accordance with the designated measurement wavelength range, the width of the measurement wavelength range (W), the sweep time required for one sweep of the local light (T), and the measurement wavelength resolution (R) Based on the spectral line width of the local light, the center frequency of the filter section and the sampling period of the A / D converter section are set, and the data to be measured is obtained from the data obtained under the set measurement conditions. And an arithmetic processing unit (30) for obtaining a spectral waveform of light.

An optical spectrum monitor according to claim 2 of the present invention is the optical spectrum monitor according to claim 1,
The multiplexing unit is configured to divide and emit the combined light of the light to be measured and the local light,
The photoelectric conversion unit receives the light branched into two at the multiplexing unit by the first photoelectric converter (23a) and the second photoelectric converter (23b), and subtracts the output signals from each other (23c). And subtracting the signal to output a signal from which the DC component has been removed.

The optical spectrum monitor according to claim 3 of the present invention is the optical spectrum monitor according to claim 1 or 2,
A polarization scrambler (40) that stirs the polarization of the local light at a higher speed than the sampling speed of the A / D conversion unit and applies the stirring to the multiplexing unit is provided.

The optical spectrum monitor according to claim 4 of the present invention is the optical spectrum monitor according to claim 1,
The multiplexing unit divides the local light and the measured light into orthogonal polarization components, multiplexes one polarization component of the local light and the measured light, and multiplexes the other polarization components. Configured to emit,
The photoelectric conversion unit includes a first photoelectric converter (23a) that receives the combined light of one polarization component emitted from the combining unit, and a second photoelectric converter (23b) that receives the combined light of the other polarization component. Have
The filter unit includes a first bandpass filter (24a) that receives an output signal of the first photoelectric converter, and a second bandpass filter (24b) that receives an output signal of the second photoelectric converter,
The detector includes a first detector (25a) that detects an envelope of the output signal of the first bandpass filter, and a second detector (25b) that detects an envelope of the output signal of the second handpass filter. Have
The A / D converter includes a first A / D converter (26a) that samples the output signal of the first detector at the predetermined sampling period, and the output signal of the second detector as the first A / D. A second A / D converter (26b) for sampling at a timing synchronized with the converter,
The arithmetic processing unit is characterized in that a spectrum waveform of the measured light is obtained from data obtained by the first A / D converter and the second A / D converter.

Moreover, the optical spectrum monitor of Claim 5 of this invention is the optical spectrum monitor in any one of Claims 1-4,
The arithmetic processing unit is configured to obtain a sampling frequency (1 / (T · R / R) obtained from a width (W) of the measurement wavelength range, a sweep time (T) required for one sweep of the local light, and the measurement wavelength resolution (R). A frequency that is approximately equal to the larger of 1/2 of W)) and the spectral line width of the local light is set as the passing center frequency of the filter unit.

Moreover, the optical spectrum monitor of Claim 6 of this invention is the optical spectrum monitor in any one of Claims 1-5,
The detection unit is configured so that the time constant of envelope detection can be varied,
The arithmetic processing unit sets an envelope detection time constant of the detection unit to a value corresponding to the measurement condition.

  As described above, the optical spectrum monitor of the present invention is configured so that the pass center frequency of the filter unit can be varied, and the specified measurement wavelength range width (W), the sweep time required for one sweep of local light (T ), The measurement wavelength resolution (R), and the spectral line width of the local light, the pass center frequency of the filter unit and the sampling period of the A / D conversion unit are set, and the filter unit according to the measurement conditions By setting the pass center frequency, it is possible to cope with various measurement conditions at a feasible sampling rate.

  In addition, since envelope detection with a relatively large time constant is adopted as the detection unit, a waveform improvement effect that fills the gap between the normal spectrum and the image spectrum during high-speed sweeping or high-resolution measurement can be expected.

  In the optical spectrum monitor having the configuration according to claim 2, a signal from which the DC component has been removed can be output from the photoelectric conversion unit, and the influence of the DC drift of the photoelectric converter can be eliminated.

  Further, in the optical spectrum monitor having the configurations of claims 3 and 4, it is possible to eliminate the influence on the measurement due to the polarization state of the light to be measured and the local light.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows the configuration of an optical spectrum monitor 20 to which the present invention is applied.

  The wavelength variable light source 21 of the optical spectrum monitor 20 is, for example, a MEMS type external resonance variable wavelength light source, and emits local light L swept in a predetermined wavelength range to the multiplexing unit 22.

  The multiplexing unit 22 combines the local light L and the measured light S, and enters the combined light Sa into the photoelectric conversion unit 23.

  The photoelectric conversion unit 23 receives the combined light Sa, and outputs an electric signal E depending on the local light L, the measured light S, and the difference frequency component included in the combined light Sa.

  The filter unit 24 is configured as a bandpass type whose pass center frequency Fi can be varied, extracts a signal Ea centered on the frequency Fi from the output signal E of the photoelectric conversion unit 23, and outputs the signal Ea to the envelope detector 25. .

  The filter unit 24 may be, for example, either a method of selecting a plurality of band pass filters having different pass center frequencies Fi by a switch or a method of changing a circuit constant for determining the pass center frequency of one band pass filter, but depending on an image. In order to reduce the drop in the spectrum waveform, it is desirable that the low-frequency characteristics extend to near the frequency zero (direct current).

  The detection unit 25 detects the envelope of the output signal Ea of the filter unit 24 with a predetermined time constant Ct, and outputs the detection output Eb to the A / D conversion unit 26. The envelope detection time constant Ct of the detection unit 25 may be fixed, but is configured to be variable, and can be variably set from the arithmetic processing unit 30 as indicated by the dotted line in FIG. If so, a wider variety of measurement conditions can be handled.

  The A / D conversion unit 26 samples the detection output Eb at a predetermined sampling period Ts, converts it to digital data D, and outputs it to the arithmetic processing unit 30.

  The arithmetic processing unit 30 sets the wavelength sweep range (for example, depending on the center wavelength λc and the sweep width W) and the sweep time T (this sweep time T may be fixed) designated by the measurement condition designating unit 31 as a wavelength variable light source. 21 and the local light L is swept within a specified wavelength sweep range (λc−W / 2 to λc + W / 2) at T time per sweep.

  The arithmetic processing unit 30 sets the pass center frequency Fi of the filter unit 24 and the sampling period Ts of the A / D conversion unit 26 according to the sweep width W, the sweep time T, and the wavelength resolution R, and is obtained under the conditions. The obtained data D is stored, the waveform data of the spectrum is generated, and the waveform is displayed on the display 32.

  Specifically, with respect to the wavelength sweep width W, the sweep time T, and the measurement wavelength resolution R, a sampling frequency 1 / (T × R / W) half the value U required for calculation, that is, U = 1 / (2 × T × R / W) and the spectral line width of the local light L, a pass center frequency Fi equal to the larger frequency is set, and the sampling period Ts is, for example, the pass center frequency Fi. Set to at least twice the value. The maximum value of the measurement wavelength resolution R is the spectral line width (for example, 10 fm) of the local light L, but here, it indicates a value (1 pm, 10 pm) or the like that is actually desired.

For example, when the wavelength sweep width W is 100 nm, the sweep time T is 1 ms, the spectral line width of the local light L is 1 MHz (wavelength width of about 10 fm), and the wavelength resolution R is specified as 0.1 nm, the value U ( Hz) is
U = 1 / (2 × T × R / W) = 500 KHz
Therefore, since it is smaller than the spectral line width of the local light L, the pass center frequency Fi of the filter unit 24 is set to 1 MHz, which is the spectral line width of the local light L, as shown in FIG.

When the wavelength sweep width W is 100 nm, the sweep time T is 1 ms, the spectral line width of the local light L is 1 MHz (wavelength width of about 10 fm), and the wavelength resolution R is specified as 0.01 nm, the value U (Hz )
U = 1 / (2 × T × R / W) = 5 MHz
Since the spectral line width of the local light L is smaller, the pass center frequency Fi of the filter unit 24 is set to 5 MHz as shown in FIG.

  In this way, by variably controlling the pass center frequency Fi of the filter unit 24 according to the wavelength sweep width W, the sweep time T, and the measurement wavelength resolution R as measurement conditions, high-speed measurement with high sensitivity and high wavelength resolution is possible. It becomes.

  Further, if the pass band width of the filter unit 24 can be varied according to the pass center frequency Fi, it is possible to cope with more various measurement conditions.

  In the measurement with the low-speed wavelength sweep or the low wavelength resolution, the value U is equal to or smaller than the spectrum line width of the local light L. Therefore, the pass center frequency Fi of the filter unit 24 is the spectrum line width of the local light L. The normal spectrum and the image spectrum of the light S to be measured are not significantly separated and observed.

  However, in the case of measurement with high-speed wavelength sweep or high wavelength resolution, the value U increases, and the frequency difference between the pass center frequency Fi of the filter unit 24 and the spectral line width of the local light L increases, and ( Although the normal spectrum Sr and the image spectrum Si of the light to be measured can be observed separately as in a), even in such a case, the optical spectrum monitor 20 of the embodiment has a filter unit. Since the output Ea of 24 is envelope-detected by the detector 25, as shown by the dotted line in FIG. 1, the time constant Ct of envelope detection is set by the arithmetic processing unit 30 to, for example, the sampling period Ts (= T By setting the value close to (× R / W), it is possible to prevent the waveform from falling between the normal spectrum Sr and the image spectrum Si as shown in FIG. It can be prevented over the di spectrum Si is observed.

  The configuration shown in FIG. 1 is the basic configuration of the optical spectrum monitor of the present invention, and a specific configuration example will be described below.

  FIG. 4 shows a configuration example of the multiplexing unit 22 and the photoelectric conversion unit 23. The multiplexing unit 22 is an optical coupler type, and combines the measured light S and the local light L, and the combined light. The light is split into two light beams Sa1 and Sa2 and enters the photoelectric conversion unit 23. Note that the phases of the light Sa1 and Sa2 are inverted from each other.

  The photoelectric conversion unit 23 receives the two lights Sa1 and Sa2 by the photoelectric converters 23a and 23b, respectively, and gives the output signals to the subtractor 23c, thereby removing the DC component included in the outputs of the photoelectric converters 23a and 23b. To do.

  That is, the output E1 of the photoelectric converter 23a is (DC component + difference frequency component between measured light and local light), and the output E2 of the photoelectric converter 23b is (DC component-difference frequency component between measured light and local light). By taking the difference, the signal E appears twice as much as (difference frequency component between measured light and local light). Note that the sum frequency component of the light to be measured and the local light is too high to be ignored by the photoelectric converter.

  In this configuration, compared with a configuration in which one output of the multiplexing unit 22, for example, the light Sa1 is received by one photoelectric conversion unit 23a, the difference frequency component can be detected with twice the intensity, and the influence of DC drift can be reduced. Since it can be removed, S / N improvement of 3 dB or more can be achieved.

  FIG. 5 shows a configuration example in which the local light L emitted from the wavelength tunable light source 21 is incident on the polarization scrambler 40, and the local light L ′ whose polarization state is randomized is generated and incident on the multiplexing unit 22. Is shown. In this case, the polarization scrambler 40 stirs (scrambles) the polarization state at a cycle shorter than the sampling cycle Ts of the A / D converter 26. By scrambling the polarization state of the local light L in this way, a measurement result that does not depend on the polarization state of the light S to be measured can be obtained.

  Further, FIG. 6 shows that in the multiplexing unit 22, the light S to be measured is divided into P-polarized light and S-polarized light whose polarization directions are orthogonal to each other, and the local light L is similarly divided into two polarized light components. The combined light Sap between them and the combined light Sas between the other polarization components are emitted.

  Then, one combined light Sap is received by one photoelectric converter 23 a of the photoelectric conversion unit 23, and the other combined light Sas is received by the other photoelectric converter 23 b of the photoelectric conversion unit 23.

  Further, the outputs Ep and Es of the respective photoelectric converters 23a and 23b are inputted to the filter unit 24 and given to the band pass filters 24a and 24b in which the pass center frequency Fi is set in the same manner as described above, and the output signals Eap and Eas are given. Envelope detection is performed by the two detectors 25a and 25b of the detector 25, and the detected outputs Ebp and Ebs are converted into digital data Dp and Ds by the two A / D converters 26a and 26b of the A / D converter 26. And input to the arithmetic processing unit 30.

  The arithmetic processing unit 30 obtains the spectrum of the swept light by combining the data Dp and Ds obtained for the two orthogonal polarization components, generates the wavelength versus spectrum waveform data, and displays it on the display 23.

  In addition, the above-described combining unit 22 converts the local light L and the measured light S into P-polarized and S-polarized components Lp, Ls, Sp, and Ss by a beam splitter 22a, for example, as shown in FIG. The combined light Sap of the P-polarized components Lp and Sp is emitted from the 0 ° polarizer 22b, and the combined light Sas of the S-polarized components Ls and Ss is emitted from the 90 ° polarizer 22c.

  Further, as shown in FIG. 7B, the polarization beam splitter 22a divides the local light L and the measured light S into P-polarized and S-polarized components Lp, Ls, Sp, and Ss, respectively, and components Ls orthogonal to each other. , Sp is incident on the 45 ° polarizer 22d, the combined light Sap of the components Ls ′ and Sp ′ whose polarization is inclined by 45 ° is emitted, and the components Lp and Ss orthogonal to each other are incident on the 45 ° polarizer 22e. Combined light Sas of components Lp ′ and Ss ′ whose polarization is inclined by 45 ° is emitted.

The figure which shows the structure of embodiment of this invention The figure for demonstrating operation | movement of embodiment The figure for demonstrating operation | movement of embodiment The figure which shows the principal part structural example of embodiment of this invention. Diagram showing configuration using polarization scrambler The figure which shows another embodiment of this invention The figure which shows the structural example of the principal part of FIG. Diagram for explaining the operation of a heterodyne type optical spectrum monitor

Explanation of symbols

  20: Optical spectrum monitor, 21: Wavelength variable light source, 22: Multiplexing unit, 23: Photoelectric conversion unit, 23a, 23b ... Photoelectric converter, 23c: Subtractor, 24: Filter unit, 24a , 24b... Band-pass filter, 25... Detector, 25a, 25b... Detector, 26... A / D converter, 26a, 26b ... A / D converter, 30. ... Measurement condition designating means, 32 ... Display, 40 ... Polarization scrambler

Claims (6)

A wavelength tunable light source (21) that emits local light that is swept within a specified wavelength range;
A multiplexing unit (22) for multiplexing the local light and the measured light;
A photoelectric conversion unit (23) for receiving light emitted from the multiplexing unit;
A bandpass filter unit (24) having a variable pass center frequency for extracting a difference frequency component between the local light and the light to be measured from an output signal of the photoelectric conversion unit;
A detector (25) for detecting an envelope of the output signal of the filter unit;
An A / D conversion section (26) for sampling the output signal of the detection section at a predetermined sampling period and converting it into digital data;
Measurement condition designating means (31) for designating measurement conditions including a measurement wavelength range (λc, W) and a measurement wavelength resolution (R);
The wavelength of the local light is swept in accordance with the designated measurement wavelength range, the width of the measurement wavelength range (W), the sweep time required for one sweep of the local light (T), and the measurement wavelength resolution (R) And setting the pass center frequency of the filter unit and the sampling period of the A / D conversion unit based on the spectral line width of the local light, and from the data obtained under the set measurement conditions, The optical spectrum monitor provided with the arithmetic processing part (30) which calculates | requires the spectrum waveform of light. The multiplexing unit is configured to divide and emit the combined light of the light to be measured and the local light,
The photoelectric conversion unit receives the light branched into two at the multiplexing unit by the first photoelectric converter (23a) and the second photoelectric converter (23b), and subtracts the output signals from each other (23c). 2. The optical spectrum monitor according to claim 1, wherein a signal from which a direct current component has been removed is output by subtracting the signal.   The polarization scrambler (40) which agitates the polarization of the local light at a higher speed than the sampling speed of the A / D conversion unit and gives the polarization to the multiplexing unit. The optical spectrum monitor according to claim 2. The multiplexing unit divides the local light and the measured light into orthogonal polarization components, multiplexes one polarization component of the local light and the measured light, and multiplexes the other polarization components. Configured to emit,
The photoelectric conversion unit includes a first photoelectric converter (23a) that receives the combined light of one polarization component emitted from the combining unit, and a second photoelectric converter (23b) that receives the combined light of the other polarization component. Have
The filter unit includes a first bandpass filter (24a) that receives an output signal of the first photoelectric converter, and a second bandpass filter (24b) that receives an output signal of the second photoelectric converter,
The detector includes a first detector (25a) that detects an envelope of the output signal of the first bandpass filter, and a second detector (25b) that detects an envelope of the output signal of the second handpass filter. Have
The A / D converter includes a first A / D converter (26a) that samples the output signal of the first detector at the predetermined sampling period, and the output signal of the second detector as the first A / D. A second A / D converter (26b) for sampling at a timing synchronized with the converter,
The optical spectrum monitor according to claim 1, wherein the arithmetic processing unit obtains a spectrum waveform of the light to be measured from data obtained by the first A / D converter and the second A / D converter.   The arithmetic processing unit is configured to obtain a sampling frequency (1 / (T · R / R) obtained from a width (W) of the measurement wavelength range, a sweep time (T) required for one sweep of the local light, and the measurement wavelength resolution (R). 5. A frequency substantially equal to a larger one of 1/2 of W)) and a spectral line width of the local light is set as a passing center frequency of the filter unit. An optical spectrum monitor according to claim 1. The detection unit is configured so that the time constant of envelope detection can be varied,
The optical spectrum monitor according to claim 1, wherein the arithmetic processing unit sets a time constant of envelope detection of the detection unit to a value corresponding to the measurement condition.

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