JP5059060B2 - Optical spectrum analyzer - Google Patents

Description

  The present invention is an optical spectrum analyzer that is used for optical communication or the like and measures characteristics (for example, wavelength, level (light output), S / N, wavelength dispersion, etc.) of light in the near-infrared to visible light region, which is a type of electromagnetic wave. About.

  In the research and development and manufacturing of optical transmitters for optical communications, or when observing modulated optical signals in actual optical fiber lines, optical spectrum analyzers using diffraction gratings have been generally used. ing. However, an optical spectrum analyzer using this type of conventional diffraction grating has a minimum wavelength resolution of about 50 pm (6.2 GHz in terms of an optical frequency in the 1.55 μm band).

  Therefore, when a signal whose intensity is modulated at a clock frequency of 10 GHz frequently used in trunk optical communication is measured by an optical spectrum analyzer using this type of conventional diffraction grating, as shown by the solid line in FIG. Only a simple unimodal spectrum was displayed. For this reason, detailed information such as the intensity of the sideband generated by the modulation cannot be obtained. In addition, this type of conventional optical spectrum analyzer has only a simple wavelength marker and level marker as an analysis function of the displayed spectrum. Further, it is impossible to determine whether the obtained spectrum waveform has a characteristic suitable for the purpose only by an analysis function using this simple wavelength marker or level marker.

  Incidentally, as a technique for measuring an OSNR (Optical Signal Noise Ratio) in optical communication, an optical measurement device disclosed in Patent Document 1 below is known.

  In the optical measurement device of Patent Document 1, as described in paragraph [0041] of the publication, it is difficult to accurately measure the noise level due to insufficient resolution of the optical spectrum analyzer. A method for improving this resolution is disclosed.

  The OSNR is the ratio between the total power of signal light and the noise level, as described in paragraph [0005] of Patent Document 1. At this time, the main component of the noise is the spontaneous emission light from the normal optical amplifier, and the noise level here has the same meaning as the optical noise level (noise floor level) in the present invention.

JP 2006-29884 A

  However, in the actual development and manufacture of an optical communication device, the α parameter itself of the optical modulator itself or the deviation from the optimum value of the drive voltage or bias voltage has a great influence on the optical signal quality. It was not an indicator to give information on these.

  Since the α parameter itself of the optical modulator itself or the deviation from the drive voltage or bias voltage changes the entire shape of the optical spectrum, the object to be measured can be obtained only by the ratio of the level between two points as in OSNR. It was difficult to express the quality of

  By the way, in recent years, an optical spectrum analyzer having a high wavelength resolution realizing a wavelength resolution of 5 pm (converted to an optical frequency of 1.55 μm band) or less using a heterodyne method has been proposed and put into practical use. When a modulated signal is measured using this optical spectrum analyzer with high wavelength resolution, there is an advantage that the carrier wave and the sideband can be clearly separated and observed as shown by the broken line in FIG. .

  Accordingly, the present invention has been made in view of the above problems, and an optical spectrum that can easily and quickly determine the quality of the optical spectrum of the light to be measured by utilizing the above-described measurement of the modulation signal with high wavelength resolution. The purpose is to provide an analyzer.

In order to achieve the above object, an optical spectrum analyzer according to claim 1 of the present invention is an optical spectrum analyzer 1 that measures an optical spectrum of light to be measured from an object to be measured.
An input unit 12 for setting each determination criterion with the optical noise level for the reference signal light spectrum as the first determination criterion and the allowable range in the level direction and the wavelength direction with respect to the signal light spectrum as the second determination criterion. When,
The optical spectrum of the measured light is compared with the first determination criterion and the second determination criterion, respectively, and the optical spectrum of the measured light with respect to the first determination criterion and the second determination criterion, respectively. And a discriminator 15 for determining whether or not the product is acceptable.

The optical spectrum analyzer according to claim 2 is the optical spectrum analyzer according to claim 1,
A calculation processing unit 14 is provided that calculates the reference signal light spectrum based on the modulation method, bit rate, and signal light center wavelength information input from the input unit 12.

  According to the present invention, it is possible to quickly and easily determine the quality of the optical spectrum of the light to be measured.

  Further, the quality of the optical spectrum of the light to be measured can be comprehensively determined by combining the determination of the optical noise level (noise floor level) and the determination of the spectrum shape.

It is a block block diagram of the optical spectrum analyzer which concerns on this invention. (A)-(c) It is explanatory drawing of each determination line in the optical spectrum analyzer which concerns on this invention. (A)-(d) It is a figure which shows the determination result of the optical spectrum by the comparison with each determination line in the optical spectrum analyzer which concerns on this invention. It is a figure for demonstrating the difference in the optical spectrum by the difference in the wavelength resolution of an optical spectrum analyzer.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings.

Embodiment of the present invention
<System configuration of the device>
First, the overall configuration of the optical spectrum analyzer according to the present invention will be described with reference to FIGS.

The optical spectrum analyzer 1 of this example has a constant driving voltage to an optical communication semiconductor laser diode module 21 (hereinafter referred to as “LD module 21”) as an optical communication light source, and a lithium niobate (LiNbO ₃ ) crystal. , An optical modulator driver 23 for the optical modulator 22, a desired pulse pattern (for example, pseudo-random bit system Pseudo-Random Bit Sequence: An optical transmitter 20 having a pulse pattern generator 24 for generating (PRBS) is connected as an object to be measured via an optical fiber 30, and the optical spectrum of the light to be measured modulated by the optical modulator 22 which is the object to be measured. In addition to the conventional OSNR evaluation described above, a plurality of preset judgment lines And to determine the quality of the optical spectrum of the light to be measured on the basis of.

  Therefore, as shown in FIG. 1, the optical spectrum analyzer 1 of this example includes an optical conversion unit 11, an input unit 12, a storage unit 13, a calculation processing unit 14, a determination unit 15, a display control unit 16, and a display unit 17. In general, it is configured.

  The optical conversion unit 11 has functions of a local optical oscillator, an optical coupler, a photoelectric conversion unit, and an A / D conversion unit in a conventionally known heterodyne type optical spectrum analyzer (see, for example, Japanese Patent Laid-Open No. 2001-249053). Light to be measured from the optical transmitter 20 is converted into an electric signal by photoelectric conversion processing, and this electric signal is converted into a digital signal by A / D conversion processing and output to the calculation processing unit 14.

  The input unit 12 includes input devices such as various operation buttons, a numeric keypad, and a keyboard provided in the main body. The input unit 12 inputs an instruction to start or stop the measurement of the optical spectrum of the light to be measured. Information input of modulation conditions (modulation method, bit rate, signal light center wavelength, etc.) of the optical transmitter 20 necessary for calculating a signal light spectrum, information input for a plurality of types of determination lines to be described later, etc. Is going.

  The storage unit 13 stores a processing program for calculating a reference signal light spectrum (a program including equations 1 to 9 described later). In addition, the storage unit 13 stores the power level of the reference signal light spectrum calculated by the calculation processing unit 14 in a rearranged state according to the power level of the measured optical spectrum of the measured light. To match the power levels of the signal light spectrum and the measured light spectrum, rearrange the signal light spectrum so that the least square error between them is minimized, or rearrange the signal light spectrum so that the peak power levels match easily. You may do it. Furthermore, the storage unit 13 stores information on a plurality of types of determination criteria.

  Here, the plurality of types of determination criteria indicate an allowable range with respect to a signal light spectrum at an optimum time as a reference. In this example, an allowable optical noise level (hereinafter referred to as a noise floor level) C [dBm] with respect to an optimal signal light spectrum as a reference is set as a first determination criterion, and an allowable range ± A [ dB] and the allowable range ± B [nm] in the wavelength direction are used as the second determination criterion. These determination criteria are set in advance as determination lines by information input from the input unit 12, and setting information regarding these determination lines is stored in the storage unit 13.

  In this example, the optimum signal light spectrum as a reference is a theoretical optical spectrum calculated using a calculation formula based on a design value described later, or a code error rate of the optical transmitter 20 is measured. The optical spectrum in a state in which the drive error and the bias voltage are adjusted while the code error rate is minimized.

  In the method using the calculation formula described later, for example, an allowable deviation amount of the drive voltage and bias voltage of the optical modulator 22 in design is obtained, and the optical spectrum at each maximum value is calculated, and these are calculated. Is set as a criterion for determination. Further, in the method of obtaining the optimum optical spectrum while actually measuring the code error rate, the code error rate increases and deteriorates if the drive voltage or bias voltage of the optical modulator 22 is shifted from this optimum condition, for example. The optical spectrum when the code error rate is increased to an allowable code error is measured, and a region including this is set as a criterion.

  In any of the above methods, the center wavelength error (variation) allowed in the design in the wavelength direction and the output power error (variation) allowed in the design in the level direction are added. You may do it.

  The calculation processing unit 14 calculates a reference signal light spectrum according to the processing program stored in the storage unit 13. The calculated signal light spectrum as a reference is stored in the storage unit 13 in a state where the power level is rearranged in accordance with the power level of the optical spectrum of the light to be measured. A method for calculating the signal light spectrum calculated according to the processing program will be described in detail later.

  The discriminating unit 15 discriminates the quality of the optical spectrum of the light to be measured based on a plurality of types of judgment criteria (judgment lines) with respect to the reference signal light spectrum stored in the storage unit 13, and a noise floor discriminating means 15a. And spectral shape discrimination means 15b.

  The noise floor discriminating means 15a determines whether or not the optical spectrum of the light to be measured exceeds the noise floor level C [dBm] that is the first criterion in the preset noise floor evaluation region. Whether the noise floor is good or bad is determined.

More specifically, the noise floor discriminating means 15a uses the area below the wavelength λ ₁ or the area above the wavelength λ ₂ as the noise floor evaluation area, and the optical spectrum level of the light to be measured is the noise floor evaluation area and the noise floor level C [ If it does not exceed dBm], the optical spectrum of the light to be measured is determined as “pass”. On the other hand, if the level of the light spectrum of the light to be measured exceeds the noise floor level C [dBm] in the noise floor evaluation region, the light spectrum of the light to be measured is determined as “fail”.

  The spectrum shape discriminating means 15b has a level-direction tolerance ± A that is a second determination criterion in the spectrum shape evaluation region where the optical spectrum of the light to be measured whose power level matches the reference signal light spectrum. The pass / fail of the spectrum shape is determined based on [dB] and whether it is within the allowable range ± B [nm] in the wavelength direction.

More specifically, the spectrum discriminating means 15b uses a region having a wavelength of λ ₁ or more and a wavelength of λ ₂ or less as a spectrum shape evaluation region, and the optical spectrum of the light to be measured is within the spectrum shape evaluation region within an allowable range ± A [dB], ± B If it falls within [nm], the optical spectrum of the light to be measured is determined as “pass”. On the other hand, if the optical spectrum of the measured light does not fall within the allowable ranges ± A [dB] and ± B [nm] in the spectrum shape evaluation region, the optical spectrum of the measured light is determined as “fail”. ing.

  The display control unit 16 controls the display of the display unit 17 so as to display a setting input screen when setting a plurality of types of determination lines, display a quality result of the light spectrum of the light under measurement, and the like.

  The display unit 17 is configured by a display device such as a liquid crystal display (LCD), for example, and displays a setting input screen (not shown) under the control of the display control unit 16 and the optical spectrum of the light to be measured in the display form shown in FIG. Multiple types of judgment lines are displayed.

  The display unit 17 may display the light spectrum of the light under measurement in a color-coded manner according to the determination result of pass or fail by the control of the display control unit 16 based on the determination result of the determination unit 15. .

  For example, if the pass spectrum is judged to be acceptable for all of the plurality of types of judgment criteria (first judgment criteria, second judgment criteria) with respect to the optical spectrum of the measured light, the optical spectrum of the measured light is black. If any one of a plurality of types of determination criteria is determined to be unacceptable, the light spectrum of the measured light is highlighted with a color other than black (for example, red).

  Thereby, the user can recognize whether the light spectrum of the light to be measured is acceptable or not only by looking at the color of the waveform of the light spectrum of the light to be measured displayed on the display unit 17.

<Processing operation of the device>
Next, a series of processing operations until the above-described optical spectrum analyzer 1 determines whether the optical spectrum of the light under measurement is acceptable or not will be described.

  First, a reference signal light spectrum is calculated. Here, a case will be described in which pseudo-random bit PRBS is used as the signal supplied from the pulse pattern generator 24 and NRZ-OOK is used as the modulation method of the measured light from the optical transmitter 20.

  First, data signals are set to a [n], n = 0, 1,..., N−1. The value of a [n] is 0 or 1, and N represents the number of data. Here, a PRBS data string is inserted. Next, a number R for dividing one bit in time is appropriately determined. This value corresponds to the time resolution. NR memory areas b [m], m = 0, 1,... (NR−1) are prepared, and a value determined by the following equation 1 is stored.

Resemble the optical modulator driver 23 and the filter bandwidth B _e, a signal obtained by the signal b [m] is passed through the filter is denoted by c [m]. Specifically, discrete Fourier transform of b [m] (hereinafter, referred to as "DFT") after, functional form corresponding to B _e and the bit rate B, and the frequency characteristics of the filter (for example, fifth-order Bessel filter) C [m] is obtained by multiplying the numerical value determined from, and inverse DFT of the result.

  A signal e [m] = x [m] + jy [m] representing an output electric field from the optical modulator 22 is calculated using a series of equations (2) to (6) below. The electric field e [m] is a complex signal, x [m] and y [m] represent its real part and imaginary part, and j represents an imaginary unit.

In Equation 5, ε _A is an offset value when the driving voltage giving the maximum amplitude in the optical modulator 22 is 1, and ε _B is an offset value when the driving voltage at 50% transmission is a bias voltage. Where α in Equation 6 represents the α parameter value of the optical modulator 22.

  This completes the calculation up to e [m] in the case of NRZ-OOK. In the case of an RZ-OOK signal, the method of determining b [•] in Equation 1 may be changed, and in the case of a modulation method other than OOK, Equations 3 to 6 may be changed according to the modulation method.

  Equation 7 below is a method for obtaining the optical spectrum I [m] from the obtained e [m], and is common to each modulation scheme. The signal e [m] is subjected to DFT to obtain the frequency component E [m] of the electric field, and I [m] is calculated by Equation 7.

  Then, a rounding process corresponding to the wavelength resolution of the optical spectrum analyzer 1 is performed on the optical spectrum obtained in Equation 7. An embodiment of the rounding process will be described. The number of data K is appropriately determined, and the filter characteristic G [k] is calculated by the following formula 8.

However, C is a normalization constant and W is a number determined according to λ _RES . K is determined by Equation 8 so that G [K] is sufficiently small.

  Next, the rounded optical spectrum I ′ [m] is calculated by the following equation (9).

  On the right side of Equation 9, the argument of G [•] is the absolute value | k | of the subscript k. If the subscript m + k of I [•] in the sum is negative, add NR, and if it is greater than or equal to NR By subtracting NR, it becomes 0 or more and NR-1 or less.

  The peak of the optical spectrum I ′ [m] calculated as described above is rearranged in accordance with the measured peak of the optical spectrum and stored in the storage unit 13 as a reference signal light spectrum.

  In the configuration described above, the optical spectrum I ′ is calculated based on the processing program stored in the storage unit 13 of the optical spectrum analyzer 1. Then, the optical spectrum I ′ may be calculated, and the calculation result may be transferred from the terminal device to the optical spectrum analyzer 1. In this case, the calculation result is stored in the storage unit 13 of the optical spectrum analyzer 1 as a reference signal light spectrum.

  Next, as an allowable range for the signal light spectrum serving as the reference, an allowable range ± A [dB] in the level direction as shown in FIG. 2A and an allowable range ± in the wavelength direction as shown in FIG. B [nm] and an allowable optical noise level (noise floor level) C [dBm] as shown in FIG. 2C are set by inputting information from the input unit 12, respectively.

Here, FIGS. 3A to 3D are a combination of the above-described plural types of allowable determination lines. Here, the discriminating unit 15 sets a region below the wavelength λ ₁ or a region exceeding the wavelength λ ₂ as a noise floor evaluation region, and sets a region between the wavelength λ ₁ and the wavelength λ ₂ as a spectrum shape evaluation region. As shown in (d) to (d), the quality of the optical spectrum of the light to be measured is determined based on whether or not the actual measurement value of the light to be measured is within each evaluation region.

  That is, the noise floor discriminating means 15a of the discriminating unit 15 determines that the noise floor item is acceptable when the measured value of the measured light in the noise floor evaluation region is equal to or lower than C [dBm] which is the first criterion. Otherwise, it is determined as rejected. The spectrum shape discriminating means 15b of the discriminating unit 15 passes the optical spectrum shape item if the measured values of the measured light in the spectrum shape evaluation area are all within the allowable range ± A [dB] and ± B [nm]. Otherwise, it is determined to be unacceptable. Then, the display control unit 16 outputs and displays the determination result of the determination unit 15 on the display unit 17.

Specifically, in the case of FIG. 3A, the optical spectrum of the light to be measured measured in the region below the wavelength λ _{1 and} the region above the wavelength λ ₂ is below the noise floor determination line. Therefore, the noise floor discriminating means 15a of the discriminating unit 15 determines that the optical spectrum of the light to be measured is “pass” in the noise floor inspection. In addition, the optical spectrum measured in the region of the wavelength λ ₁ or more and the wavelength λ ₂ or less is in the region surrounded by the optical spectrum determination line. Accordingly, the spectrum shape determining means 15b of the determining unit 15 determines that the optical spectrum of the measured light is “pass” even in the optical spectrum shape inspection. Then, the display unit 17 displays that the optical spectrum of the light to be measured is “pass” in both the noise floor inspection and the optical spectrum shape inspection.

In the case of FIG. 3B, the measured light spectrum of the light to be measured is below the noise floor judgment line in the region below the wavelength λ ₁ , but the portion exceeding in the region exceeding the wavelength λ ₂ (A in the figure). There is. Therefore, the noise floor discriminating means 15a of the discriminating unit 15 determines that the optical spectrum of the light to be measured is “failed” by the noise floor inspection. On the other hand, the optical spectrum of the light to be measured measured in the region from the wavelength λ _{1 to the} wavelength λ ₂ is in the region surrounded by the optical spectrum determination line. Accordingly, the spectrum shape determining means 15b of the determining unit 15 determines that the optical spectrum of the measured light is “pass” in the optical spectrum shape inspection. Then, the display unit 17 displays that the optical spectrum of the light to be measured is “failed” in the noise floor inspection and “passed” in the optical spectrum shape inspection.

In the case of FIG. 3C, the optical spectrum of the light to be measured measured in the region below the wavelength λ _{1 and} the region above the wavelength λ ₂ is below the noise floor determination line. Therefore, the noise floor discriminating means 15a of the discriminating unit 15 determines that the optical spectrum of the light to be measured is “pass” in the noise floor inspection. On the other hand, the optical spectrum of the light to be measured measured in the region of the wavelength λ ₁ or more and the wavelength λ ₂ or less partially exceeds the region surrounded by the optical spectrum determination line (B in the figure). Accordingly, the spectrum shape determining means 15b of the determining unit 15 determines that the optical spectrum of the measured light is “failed” in the optical spectrum shape inspection. Then, the display unit 17 displays that the optical spectrum of the light to be measured is “PASS” in the noise floor inspection and “FAIL” in the optical spectrum shape inspection.

In the case of FIG. 3D, the measured light spectrum of the light to be measured is below the noise floor determination line in the region below the wavelength λ ₁ , but the portion exceeding in the region exceeding the wavelength λ ₂ (A in the figure). There is. Therefore, the noise floor discriminating means 15a of the discriminating unit 15 determines that the optical spectrum of the light to be measured is “failed” by the noise floor inspection. In addition, the optical spectrum of the measured light measured in the region of wavelength λ ₁ or more and wavelength λ ₂ or less partially exceeds the region surrounded by the optical spectrum determination line (B in the figure). Accordingly, the spectrum shape determining means 15b of the determining unit 15 determines that the optical spectrum of the measured light is “failed” even in the optical spectrum shape inspection. Then, the display unit 17 displays that the optical spectrum of the light to be measured is “failed” in both the noise floor inspection and the optical spectrum shape inspection.

  As described above, the optical spectrum analyzer according to the present invention allows the user to use a plurality of types of determination criteria (first determination criterion, second determination criterion) including the allowable level of stimulated spontaneous emission light generated from the light source or the optical amplifier. Is provided with a determination unit 15 that determines whether or not the optical spectrum of the light to be measured is included within the allowable ranges of the plurality of types of determination criteria. It is. At that time, the reference signal light spectrum is calculated by giving a predetermined modulation condition, and the result is stored in the storage unit 13, or the optical spectrum when the acceptable product is measured is stored in the storage unit 13. It is obtained by doing. As a result, the quality of the optical spectrum of the light to be measured can be determined easily and quickly.

  In the case of optical communication like the optical spectrum analyzer of this example, unlike wireless communication, spontaneous emission light over a wide wavelength band is often superimposed as noise by an optical fiber amplifier. Although it is necessary to judge pass / fail by both shapes, according to the optical spectrum analyzer of this example, the judgment of the optical spectrum of the light to be measured by combining the judgment of the optical noise level (noise floor level) and the judgment of the spectrum shape. Pass / fail can be comprehensively determined.

  By the way, in the above-described embodiment, instead of the optical spectrum theoretical value obtained by calculation, for example, if the device to be measured is an optical transmission device, the bit error rate evaluation is actually performed, and the light of the device determined to be acceptable The spectrum may be measured and stored as a reference signal spectrum in a storage unit inside the optical spectrum analyzer. In that case, the setting of a plurality of types of determination criteria is performed in the same manner as in the above-described embodiment.

  In addition, the method using the measured value of the optical spectrum as a determination criterion is not limited to the optical spectrum by the modulation as described in the above-described embodiment. For example, the transmission loss characteristic of an optical component is measured, and the non-defective product is measured. The optical spectrum may be used as a reference value.

  The optical spectrum analyzer for carrying out the present invention is not limited to the heterodyne type, and is a conventionally known diffraction grating type optical spectrum analyzer (for example, Japanese Patent No. 2892670) as long as the sideband of the measured light is observed. May be.

DESCRIPTION OF SYMBOLS 1 Optical spectrum analyzer 11 Optical conversion part 12 Input part 13 Memory | storage part 14 Calculation process part 15 Discriminating part 16 Display control part 17 Display part 20 Optical transmitter 21 LD module 22 Optical modulator 23 Optical modulator driver 24 Pulse pattern generator 30 Optical fiber

Claims (2)

In the optical spectrum analyzer (1) for measuring the optical spectrum of the light to be measured from the object to be measured,
An input unit for setting each determination criterion with the optical noise level with respect to the signal light spectrum serving as a reference as the first determination criterion and the allowable range in the level direction and the wavelength direction with respect to the signal light spectrum as the second determination criterion ( 12)
The optical spectrum of the measured light is compared with the first determination criterion and the second determination criterion, respectively, and the optical spectrum of the measured light with respect to the first determination criterion and the second determination criterion, respectively. An optical spectrum analyzer comprising: a determination unit (15) for determining whether or not the product is acceptable. A calculation processing unit (14) for calculating the reference signal light spectrum based on information of a modulation method, a bit rate, and a signal light center wavelength input from the input unit (12). Item 7. An optical spectrum analyzer according to Item 1.

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