JP2012042380A - Optical spectrum analyzer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical spectrum analyzer capable of measuring optical spectrum, electrical spectrum and electrical power for signal light to be measured with only one device without preparing a complicated measuring system.SOLUTION: The optical spectrum analyzer includes an optical coupler 12 for multiplexing laser light of a single longitudinal mode and the signal light to be measured and then outputting by branching into two, and photoelectric conversion sections 13a, 13b for converting the light from the optical coupler 12 to electrical signals. In this case, in an optical spectrum measuring mode, a light shut-off mechanism 18 is turned on and the laser light and the signal light to be measured make incidence on the optical coupler 12, a signal processing section 16 calculates the optical spectrum of the signal light to be measured based on a differential signal between the electrical signals. In the electrical spectrum measuring mode, the light shut-off mechanism 18 is turned off and only the signal light to be measured makes incidence on the optical coupler 12 and the signal processing section 16 calculates the electrical power of the signal light to be measured based on an addition signal of the electrical signals or one of the electrical signals.

Description

  The present invention relates to an optical spectrum analyzer used for optical communication and the like.

Since the bit rate of the optical communication trunk line is as high as 10 Gbps or higher, external modulation by an optical modulator using a lithium niobate (LiNbO ₃ ) crystal (hereinafter referred to as an LN optical modulator) is used as the optical transmitter. Is done. It is known that the LN optical modulator has a so-called drift characteristic in which the optical loss characteristic with respect to the drive voltage changes with temperature fluctuation and time. Therefore, in an intensity-modulated optical transmitter, the bias voltage of the LN optical modulator needs to be constantly adjusted so that a good point is obtained, and an example of a technique for realizing this is disclosed in Patent Document 1. .

  Specifically, in the method disclosed in Patent Document 1, a low-frequency signal that does not affect the communication signal is superimposed on the drive voltage of the LN optical modulator. The output light from the LN optical modulator is branched by an optical coupler, the branched light is converted into an electric signal by a photodiode, a low frequency signal component contained in the light is detected, and a bias voltage is set so that this is minimized. Is adjusted from time to time. This specific low-frequency signal component is detected by measuring the electrical signal from the photodiode with a combination of an electrical filter that passes the low-frequency signal component and an electrical power meter, or extracting only the low-frequency signal component with an electrical spectrum analyzer. Can be realized.

  Further, in recent years, due to a rapid increase in data traffic, a phase modulation method with a larger capacity has been used instead of the conventional intensity modulation method. Among them, the DPSK (Differential Phase Shift Keying) method, the QPSK (Quadrature Phase Shift Keying) method, and the DQPSK (Differential Quadrature Phase Shift Keying) method are attracting attention.

  An example of a NRZ (Non-Return to Zero) DQPSK optical transmitter and its adjustment measurement system is shown in FIG. Here, the LN optical modulator 30 for DQPSK is used, and the quality of this adjustment greatly affects the performance of the optical transmitter 20. The adjustment items of the LN optical modulator 30 in this example are the amplitude and bias voltage of the drive voltage applied to the I (In-phase) port 32, and the amplitude and bias voltage of the drive voltage applied to the Q (Quadrature) port 33. , And the balance voltage (I-Q balance) of the I port 32 and the Q port 33. Non-Patent Document 1 and the like point out that these adjustments are difficult, and it took time to research and develop and manufacture an optical transmitter.

  The operation of the NRZ-DQPSK optical transmitter and measurement system disclosed in Non-Patent Document 1 and the like will be described with reference to FIG. A pulse pattern signal of a pseudo-random bit sequence (hereinafter referred to as PRBS) is supplied from the pulse pattern generator 36 to the I port 32 and the Q port 33 via the voltage application unit 35 of the optical transmitter 20. Supplied.

  The laser light source 31 disposed in the optical transmitter 20 oscillates a single longitudinal mode laser beam, and the laser beam is phase-modulated by the DQPSK LN optical modulator 30 and output from the optical transmitter 20. The phase modulation signal output from the optical transmitter 20 enters the pseudo optical line 37. The pseudo optical line 37 realizes an optical loss value, a chromatic dispersion value, or a polarization mode dispersion value similar to those of an optical fiber having a length used in actual communication without using an optical fiber. It becomes a load on the transmitter 20.

  The phase modulation signal propagated through the pseudo optical line 37 is converted into an electric signal by the DQPSK optical receiver 38. The error rate of the electrical signal is determined by the error rate detector 39, and each adjustment item is adjusted alternately so that the error rate becomes a predetermined value or less.

  However, if the predetermined value is not set appropriately, there is a possibility that the adjustment is finished before each adjustment item is truly optimized. The present inventors have invented a method that enables adjustments other than IQ balance among these adjustment items using the optical spectrum analyzer 40 (see, for example, Patent Document 2).

The _IQ balance is a phase shift amount φ _IQ between the I port 32 and the Q port 33, and it is necessary to set it to π / 2 in the NRZ-DQPSK system. Specifically, the phase shift amount φ _IQ is maintained at π / 2 by adjusting the electrode voltage of the phase shift unit 34 in the drawing.

The optical spectrum when the _IQ balance is the optimum value (φ _IQ = π / 2) and the optical spectrum when the _IQ balance is changed by 10% from the optimum value (φ _IQ = 0.45π) are shown in FIG. (B). As shown in this figure, it is difficult to detect a change in the _IQ balance from the optical spectrum, and therefore the phase shift amount φ _IQ cannot be adjusted based on the optical spectrum. On the other hand, the following two methods have been proposed.

As one of the methods, for example, it is known that the phase shift amount φ _IQ can be easily adjusted by using a sampling type optical waveform observation device capable of displaying an optical constellation as disclosed in Patent Document 3, for example. It has been. However, this apparatus has a problem that it is very expensive because it uses a lot of ultrahigh-speed electronic circuits.

  On the other hand, Non-Patent Document 2 or Non-Patent Document 3 discloses an adjustment method using a photodiode 41 and an electric power meter 42. In this method, the optical output from the LN optical modulator 30 is branched and a part thereof is converted into an electric signal by the photodiode 41, and the total power of this electric signal up to 100 MHz (example of Non-Patent Document 2) is converted. And the electrode voltage of the phase shift unit 34 is adjusted so that this is minimized.

Here, FIG. 6C shows the calculation result of the total power up to 100 MHz with respect to the phase shift amount φ _IQ . The horizontal axis of the graph represents the quotient of φ _IQ by π, and the vertical axis represents the value calculated by the formula [Equation 3] described later. As described in Non-Patent Document 2 and Non-Patent Document 3, φ _IQ = π / 2 when the power is minimum. This method can realize control even with an ultrahigh-speed signal such as a baud rate of 40 Gbps with a signal in a relatively low frequency range up to 100 MHz, and is therefore less expensive than the case of using the sampling optical waveform observation device disclosed in Patent Document 3. In addition, the phase shift amount φ _IQ can be adjusted. In FIG. 6C, the legend represented by a multiple of the half-wave voltage Vπ represents the drive voltage amplitude (peak to peak value) of both the I and Q ports.

Japanese Patent No. 2642499 Japanese Patent Application No. 2009-155648 JP 2007-93515 A

Takeshi Hoshida, 4 others, "Control technology for polarization multiplexed quaternary phase modulated optical transmitter / receiver", IEICE Technical Report, IEICE, February, Vol. 108, No. 423, OCS 2008- 120, pp. 79-83 Yuichi Akiyama and 4 others, “Examination of bias control method of RZ-DQPSK modulator”, IEICE Technical Report, IEICE, October, 108, No.259, OCS2008-80, pp. 167-170 Pak S. Cho, et al., “Closed-Loop Bias Control of Optical Quadrature Modulator”, IEEE Photonics Technology Letters, November 2006, Vol. 2209-2211

  As described above, regardless of the difference in modulation method such as intensity modulation method and phase modulation method, in adjusting an optical transmitter using an LN optical modulator, in addition to an optical spectrum analyzer, a photodiode and an electric power meter, Or an electric spectrum analyzer is often necessary. Therefore, inconveniences such as an increase in cost for arranging them, a time-consuming setup, and an increase in the area occupied by the measuring instruments have occurred.

  The present invention has been made to solve such a conventional problem, and without preparing a complicated measurement system, an optical spectrum measurement function, an electric spectrum measurement function, and an electric power measurement can be performed by a single unit. An object of the present invention is to provide an optical spectrum analyzer that realizes functions.

  In order to solve the above problems, an optical spectrum analyzer according to claim 1 of the present invention includes a wavelength tunable laser light source that emits a laser beam of a single longitudinal mode in a measurement wavelength range, and a laser emitted from the wavelength tunable laser light source. An optical coupler that multiplexes the light and the signal light to be measured, branches the combined light into two, and outputs the two lights output from the optical coupler and converts them into an electrical signal An optical spectrum analyzer comprising two photoelectric conversion units, a measurement mode control unit that outputs a control signal for designating one of a plurality of measurement modes, and the wavelength variable according to the control signal A light blocking mechanism for turning on / off the incidence of the laser light emitted from a laser light source to the optical coupler, and a signal for performing signal processing corresponding to each measurement mode according to the control signal And when the measurement mode control unit outputs a control signal designating a first measurement mode from the plurality of measurement modes, the light blocking mechanism is turned on and the laser is turned on. The light and the signal light under measurement enter the optical coupler, and the signal processing unit calculates an optical spectrum of the signal light under measurement based on a difference signal between the electric signals from the two photoelectric conversion units. When the measurement mode control unit outputs a control signal designating the second measurement mode from among the plurality of measurement modes, the light blocking mechanism is turned off and only the signal light to be measured is the light. The signal processing unit enters the coupler, and the signal processing unit generates an electric signal of the signal light under measurement based on an addition signal of the electric signals from the two photoelectric conversion units or an electric signal from one of the photoelectric conversion units. It has a configuration and calculates the power.

  With this configuration, an optical spectrum measurement function and an electric power measurement function can be realized by a single unit without preparing a complicated measurement system.

  Further, in the optical spectrum analyzer according to claim 2 of the present invention, when the measurement mode control unit outputs a control signal designating a third measurement mode from the plurality of measurement modes, the light blocking mechanism includes The signal light to be measured is turned off and only the signal light to be measured enters the optical coupler, and the signal processing unit adds the electrical signal from the two photoelectric conversion units, or the electrical signal from one photoelectric conversion unit. The electrical spectrum of the signal light to be measured is calculated by performing a fast Fourier transform on the signal.

  With this configuration, an optical spectrum measurement function, an electric spectrum measurement function, and an electric power measurement function can be realized by a single unit without preparing a complicated measurement system.

  The optical spectrum analyzer according to claim 3 of the present invention has a configuration characterized in that the light blocking mechanism includes an optical switch disposed between the wavelength tunable laser light source and the optical coupler. May be.

  According to a fourth aspect of the present invention, the wavelength tunable laser light source includes a semiconductor laser that oscillates a single longitudinal mode laser beam, and the light blocking mechanism includes a single longitudinal laser It may have a configuration including an electric switch for turning on / off the supply of a drive current for oscillating the mode laser beam.

The optical spectrum analyzer according to claim 5 of the present invention may have a configuration in which a branching ratio of the optical coupler is 1: 1.
With this configuration, the amplitude noise component of the laser light from the wavelength tunable laser light source can be suppressed, and the measurement accuracy of the signal light to be measured can be increased.

  The present invention provides an optical spectrum analyzer that realizes an optical spectrum measurement function, an electric spectrum measurement function, and an electric power measurement function by a single unit without preparing a complicated measurement system.

Schematic explaining the operating principle of the optical spectrum analyzer according to the present invention 1 is a block diagram showing a configuration of an optical spectrum analyzer of a first embodiment Block diagram showing the processing contents of the signal processor The block diagram which shows the structure of the optical spectrum analyzer of 2nd Embodiment Schematic diagram showing a configuration example of a conventional optical transmitter and measurement system The graph which shows the optical spectrum of a phase modulation signal when IQ balance changes

Hereinafter, embodiments of an optical spectrum analyzer according to the present invention will be described with reference to the drawings.
In the present invention, a heterodyne optical spectrum analyzer is used as a basic technique. This basic technique is disclosed in the following document.
[Patent Document 3] Japanese Patent Laid-Open No. 3-1515938 [Non-Patent Document 4] DM Baney, B. Szafraniec and A. Motamedi, “Coherent Optical Spectrum Analyzer”, IEEE Photonics Technology Letters, March 2002, Vol. 14, No. 3, pp. 355-357

  In this specification, a state where the optical spectrum analyzer according to the present invention is set for optical spectrum measurement is referred to as an “optical spectrum measurement mode”. In addition, in this specification, a spectrum waveform obtained by displaying an electric signal received by a photodiode with an electric spectrum analyzer is defined as “electric spectrum”, and the optical spectrum analyzer according to the present invention is set for this measurement. This is called “electric spectrum measurement mode”. The power up to a predetermined band of the received light signal is defined as “electric power”, and the state in which the optical spectrum analyzer according to the present invention is set for this measurement is called “electric power measurement mode”.

  First, the operation principle of the optical spectrum measurement mode will be described with reference to FIG. The wavelength tunable laser light source 11 oscillates a single longitudinal mode laser beam in a phase continuous state over a measurement wavelength range. In this specification, the light emitted from the wavelength tunable laser light source 11 is referred to as “laser light”.

  Laser light emitted from the wavelength tunable laser light source 11 enters the optical coupler 12. The signal light to be measured such as the phase modulation signal output from the optical transmitter and the laser light from the wavelength tunable laser light source 11 are combined by the optical coupler 12 and are incident on the photodiodes 13c and 13e. Here, the combined light output from the two output ports of the optical coupler 12 has beat frequency components (interference components of the signal light to be measured and the laser light) having opposite phases. Note that the branching ratio of the optical coupler 12 is adjusted to 1: 1 (50%), and the propagation time and light amount of light from the optical coupler 12 to the photodiodes 13c and 13e are equal.

  The output light from the optical coupler 12 is converted into an electric signal by the photodiodes 13c and 13e and amplified by the amplifiers 13d and 13f. Signals from the amplifiers 13d and 13f are converted into digital signals by the analog-digital (AD) converters 15a and 15b.

  Next, the subtractor 26a takes the difference between the digital signals output from the two AD converters 15a and 15b. Here, when the branching ratio of the optical coupler 12 is 1: 1 as described above, if the difference between the two digital signals is taken, only the beat frequency component can be extracted, and the amplitude noise of the laser light can be obtained. The component can be suppressed. Such a configuration is based on a principle similar to that of a generally known balanced optical detector such as Non-Patent Document 3, and is a technique that is often employed in an optical heterodyne optical spectrum analyzer. . In the present invention, such a function as a balanced optical detector is realized by software processing.

  Further, the power calculator 26b calculates an integral value of power in the bandwidth set as the wavelength resolution width from the difference signal of the digital signal. From this integrated value, the optical power of the signal light under measurement at the oscillation wavelength of the wavelength tunable laser light source 11 is obtained. Therefore, the wavelength of the wavelength tunable laser light source 11 is sequentially swept to sequentially display the optical power at each oscillation wavelength. An optical spectrum can be obtained by plotting on the display screen.

  Next, the operating principle of the electric spectrum measurement mode will be described with reference to FIG. In the electrical spectrum measurement mode, the laser light from the wavelength tunable laser light source 11 is not incident on the optical coupler 12, but the photodiodes 13c and 13e perform direct detection instead of heterodyne detection.

  In order to observe the change in the amplitude of the signal light under measurement, an adder 26c performs addition processing on the output signals of the photodiodes 13c and 13e. An electrical spectrum is obtained by performing fast Fourier transform on the signal subjected to the addition processing (or may be one of the output signals) by FFT 26d. The display unit 17 displays the electrical spectrum thus obtained.

  Next, the operation principle of the electric power measurement mode will be described with reference to FIG. Also in the electric power measurement mode, the laser light from the wavelength tunable laser light source 11 is not incident on the optical coupler 12, and the photodiodes 13c and 13e are directly detected instead of heterodyne detection.

  In order to observe the change in the amplitude of the signal light under measurement, an adder 26c performs addition processing on the output signals of the photodiodes 13c and 13e. Electric power can be obtained by performing integration over a predetermined bandwidth after performing fast Fourier transform on the added signal (or one of the output signals) in the power calculation unit 26e.

  Alternatively, as will be described in the first embodiment below, the power calculation unit 26e regards the added signal (or one of the output signals) as a time-series signal, obtains a time dispersion value, and calculates a time average value of 2 Normalization processing may be performed by multiplication. Accordingly, it can be seen from Non-Patent Documents 2 and 3 that the detection of the minimum power point is facilitated.

  As described above, the heterodyne optical spectrum analyzer (optical spectrum measurement mode in this specification) and the low-frequency signal measurement configuration (electric spectrum measurement mode or electric power in this specification) required for adjustment of the LN optical modulator. In the power measurement mode), there are common components, and in the present invention, these are combined into one device by a unique device. This will be described in detail in the first and second embodiments.

(First embodiment)
A first embodiment of an optical spectrum analyzer according to the present invention will be described with reference to FIGS. FIG. 2 is a block diagram showing an example of the configuration of the optical spectrum analyzer 1 of the present embodiment.

  As shown in FIG. 2, the optical spectrum analyzer 1 includes a wavelength tunable laser light source 11 that emits a laser beam in a single longitudinal mode in a measurement wavelength range, a laser beam emitted from the wavelength tunable laser light source 11, and an optical transmitter 20. The signal light to be measured transmitted from the optical coupler 12, the optical coupler 12 for branching the combined light into two, and the two lights output from the optical coupler 12 to receive the electrical signal And two photoelectric conversion units 13a and 13b for conversion into

  Here, the wavelength tunable laser light source 11 includes a semiconductor laser (not shown) that oscillates a single longitudinal mode laser beam, and outputs an oscillation wavelength signal including information on the oscillation wavelength of the single longitudinal mode. It is like that.

  Anti-aliasing low-pass filters (LPF) 14a and 14b are arranged downstream of the photoelectric conversion units 13a and 13b, and AD converters 15a and 15b are arranged downstream of the low-pass filters 14a and 14b. Is done.

  Furthermore, the optical spectrum analyzer 1 performs AD conversion on the oscillation wavelength signal from the wavelength tunable laser light source 11, and processes the digital signals output from the AD converters 15a to 15c to obtain an optical spectrum, an electrical spectrum, Alternatively, the signal processing unit 16 that calculates the electric power, the display unit 17 that displays the calculation result of the signal processing unit 16, and the wavelength tunable laser light source 11 and the optical coupler 12 are arranged. An optical switch 18 serving as a light blocking mechanism for turning on / off incidence of the emitted laser light to the optical coupler 12, and a measurement mode control unit for outputting a control signal for designating one of a plurality of measurement modes 19.

  The optical coupler 12 is, for example, a 3 dB optical coupler having a branching ratio of 1: 1, and has two input ports and two output ports. The photoelectric conversion units 13a and 13b include photodiodes 13c and 13e, and amplifiers 13d and 13f that amplify output signals from the photodiodes 13c and 13e.

  The measurement mode control unit 19 includes, for example, an operation unit (not shown) for an operator to specify a measurement mode. The measurement mode control unit 19 sends an optical switch control signal for switching on / off of the optical switch 18 according to the measurement mode selected by the operator, and corresponds to each measurement mode to the signal processing unit 16. A signal processing control signal for performing signal processing is transmitted.

  The signal processing unit 16 is configured to execute a program stored in a CPU (Central Processing Unit) and a RAM (Random Access Memory) by the CPU, but the processing in the optical spectrum measurement mode and the electric spectrum measurement mode to be described later is common. Since there are many, it can also be comprised by FPGA (Field Programmable Gate Array).

Hereinafter, the operation of the optical spectrum analyzer 1 in the optical spectrum measurement mode (first measurement mode) will be described.
First, the operator selects the optical spectrum measurement mode by the measurement mode control unit 19. The measurement mode control unit 19 sends an optical switch control signal for turning on the optical switch 18 (conduction state). As a result, both the laser light from the wavelength tunable laser light source 11 and the signal light to be measured enter the optical coupler 12. Further, the measurement mode control unit 19 sends out a signal processing control signal for causing the signal processing unit 16 to perform signal processing corresponding to the optical spectrum measurement mode.

Next, processing contents of the signal processing unit 16 in the optical spectrum measurement mode will be described with reference to FIG.
First, the signal processing unit 16 applies the photodiodes 13c and 13e to the signals of the photoelectric conversion units 13a and 13b converted into digital signals by the AD converters 15a and 15b (PD1 signal and PD2 signal in the drawing, respectively). A balance correction process is performed to correct variations in sensitivity. This process is performed based on the wavelength versus sensitivity characteristics of the photodiodes 13c and 13e and the wavelength versus loss characteristics of the optical coupler 12 at each wavelength stored in the RAM.

  Next, the signal processing unit 16 performs a subtraction process for obtaining a difference between the balance-corrected PD1 signal and the PD2 signal. As a result, since only the beat frequency component of the signal can be extracted as described above, it is possible to suppress noise due to fluctuations in the amplitude of the laser beam.

  Next, the signal processing unit 16 allows only a predetermined band component of the signal subjected to the subtraction process to pass through a band pass filter (BPF) by software. This bandwidth determines the wavelength resolution as an optical spectrum analyzer.

  Next, the signal processing unit 16 calculates an integral value of power in a predetermined bandwidth of the signal transmitted through the BPF. Since this integrated value becomes the optical power per wavelength resolution width at the oscillation wavelength of the wavelength tunable laser light source 11, the signal processing unit 16 performs the wavelength value adding process based on the oscillation wavelength signal output from the wavelength tunable laser light source 11. Do.

  Accordingly, the optical spectrum can be obtained by sequentially sweeping the wavelength of the wavelength tunable laser light source 11 and plotting each optical power at the oscillation wavelength on the display screen of the display unit 17.

Hereinafter, the operation of the optical spectrum analyzer 1 in the electric spectrum measurement mode (third measurement mode) will be described.
First, the electric spectrum measurement mode is selected by the measurement mode control unit 19 by the operation of the operator. The measurement mode control unit 19 sends an optical switch control signal for turning off the optical switch 18 (blocking state). As a result, the laser light from the wavelength tunable laser light source 11 does not enter the optical coupler 12, and only the signal light to be measured enters the optical coupler 12. Further, the measurement mode control unit 19 sends out a signal processing control signal for causing the signal processing unit 16 to perform signal processing corresponding to the electric spectrum measurement mode.

Next, processing contents of the signal processing unit 16 in the electric spectrum measurement mode will be described with reference to FIG.
First, as in the optical spectrum measurement mode, the signal processing unit 16 varies the sensitivity of the photodiodes 13c and 13e with respect to the PD1 signal and the PD2 signal converted into digital signals by the AD converters 15a and 15b. Balance correction processing for correction is performed.

  Next, the signal processing unit 16 performs an addition process of adding the PD1 signal and the PD2 signal that have been subjected to balance correction, and then performs only a predetermined band component of the signal subjected to the addition process by a bandpass filter (BPF) by software. Pass through. Note that, in the above addition processing, the addition processing of the PD1 signal and the PD2 signal may not be performed, and only one of the signals may be output to the BPF.

  Then, the signal processing unit 16 performs fast Fourier transform (FFT) processing on the signal that has passed through the BPF, calculates an electrical spectrum, and plots it on the display screen of the display unit 17.

Hereinafter, the operation of the optical spectrum analyzer 1 in the electric power measurement mode (second measurement mode) will be described.
First, the electric power measurement mode is selected by the measurement mode control unit 19 by the operation of the operator. The measurement mode control unit 19 sends an optical switch control signal for turning off the optical switch 18 (blocking state). As a result, the laser light from the wavelength tunable laser light source 11 does not enter the optical coupler 12, and only the signal light to be measured enters the optical coupler 12. Further, the measurement mode control unit 19 sends out a signal processing control signal for causing the signal processing unit 16 to perform signal processing corresponding to the electric power measurement mode.

Next, processing contents of the signal processing unit 16 in the electric power measurement mode will be described with reference to FIG.
First, as in the electrical spectrum measurement mode, the signal processing unit 16 varies the sensitivity of the photodiodes 13c and 13e with respect to the PD1 signal and the PD2 signal converted into digital signals by the AD converters 15a and 15b. Balance correction processing for correction is performed.

  Next, the signal processing unit 16 performs an addition process of adding the PD1 signal and the PD2 signal whose balance has been corrected, and then a predetermined band component of the signal subjected to the addition process by a bandpass filter (BPF) by software. Only pass through. Note that, in the above addition processing, the addition processing of the PD1 signal and the PD2 signal may not be performed, and only one of the signals may be output to the BPF.

And the signal processing part 16 performs the calculation process as shown in the following [Equation 1]-[Equation 3] with respect to the signal which permeate | transmitted BPF. The PD1 signal and the PD2 signal are sampled at a constant time interval, and each sampling data is represented as Q _o (t _n ). The average value of N pieces of sampling data Q _o (t _n ) (n = 0,..., N−1) that are continuous in time is expressed as the following [Equation 1]. In the following formulas, a symbol with a line on “Q” is used as a symbol indicating an average value of Q _o (t _n ) (n = 0,..., N−1). .

Further, the variance V of Q _o (t _n ) (n = 0,..., N−1) is expressed by the following equation.

From the average value of Q _o (t _n ) (n = 0,..., N−1) and the variance V obtained as described above, an amount R corresponding to the electric power represented by the following equation can be obtained. The signal processing unit 16 plots the amount R corresponding to the electric power on the display screen of the display unit 17.

(Second Embodiment)
A second embodiment of the optical spectrum analyzer according to the present invention will be described with reference to FIG. FIG. 4 is a block diagram showing an example of the configuration of the optical spectrum analyzer 2 of the present embodiment. Note that a description of the same configuration as in the first embodiment is omitted.

  The optical spectrum analyzer 2 of the present embodiment has a single longitudinal mode laser as a semiconductor laser inside the wavelength tunable laser light source 11 instead of including an optical switch as the optical spectrum analyzer 1 of the first embodiment as an optical blocking mechanism. An electrical switch (not shown) that switches on / off of supply of a drive current for oscillating light is provided. When this electrical switch is turned off, the laser oscillation itself of the semiconductor laser is stopped.

  In the present embodiment, the measurement mode control unit 19 sends a light source control signal for switching on / off the electrical switch inside the wavelength tunable laser light source 11 according to the measurement mode selected by the operator, A signal processing control signal for causing the signal processing unit 16 to perform signal processing corresponding to each measurement mode is transmitted. Since the electrical switch is less expensive than the optical switch, there is an advantage that the cost of the entire apparatus can be reduced as compared with the optical spectrum analyzer 1 of the first embodiment.

  As described above, the optical spectrum analyzer according to the present invention can realize an optical spectrum measurement function, an electric spectrum measurement function, and an electric power measurement function with one unit without preparing a complicated measurement system. it can.

  For example, when adjusting an NRZ-DQPSK optical transmitter using the optical spectrum analyzer according to the present invention, the photodiode 41 and the electric power meter 42 required in the conventional measurement system (FIG. 5), Furthermore, the optical coupler (not shown) for branching the optical output from the LN optical modulator 30 to the photodiode 41 and the electric power meter 42 can be removed, and the measurement system can be simplified and the cost can be reduced.

  In addition, by using the optical spectrum analyzer according to the present invention, a photodiode and an electric spectrum analyzer should be prepared for adjustment when a low-frequency signal is superimposed on an optical modulator of an intensity modulation type optical transmitter. Measurement can be performed with a simple configuration.

  Further, as another application of the optical spectrum analyzer according to the present invention, it is also effective for measuring the relative intensity noise characteristic of a semiconductor laser measured by a combination of a photodiode and an electric spectrum analyzer.

1, 2, 40 Optical spectrum analyzer 11 Wavelength variable laser light source 12 Optical coupler 13a, 13b Photoelectric converter 13c, 13e Photodiode 13d, 13f Amplifier 14a, 14b Low-pass filter 15a, 15b, 15c AD converter 16 Signal processor 16 17 Display unit 18 Optical switch (light blocking mechanism)
19 Measurement mode control unit 20 Optical transmitter 26a Subtractor 26b, 26e Power calculation unit 26c Adder 26d FFT
30 LN optical modulator 31 Laser light source 32 I port 33 Q port 34 Phase shift unit 35 Voltage application unit 36 Pulse pattern generator 37 Pseudo optical line 38 DQPSK optical receiver 39 Error rate detector 41 Photo diode 42 Electric power meter

Claims (5)

A wavelength tunable laser light source (11) that emits laser light in a single longitudinal mode in the measurement wavelength range;
An optical coupler (12) for combining the laser light emitted from the wavelength tunable laser light source and the signal light to be measured, and branching the combined light into two and outputting it;
In an optical spectrum analyzer comprising two photoelectric conversion units (13a, 13b) that receive two lights output from the optical coupler and convert them into electrical signals,
A measurement mode control unit (19) for outputting a control signal for designating one of a plurality of measurement modes;
A light blocking mechanism (18) for turning on / off incidence of the laser light emitted from the wavelength tunable laser light source to the optical coupler in accordance with the control signal;
A signal processing unit (16) that performs signal processing corresponding to each measurement mode according to the control signal;
When the measurement mode control unit outputs a control signal designating a first measurement mode from the plurality of measurement modes, the light blocking mechanism is turned on and the laser light and the signal light to be measured are While entering the optical coupler, the signal processing unit calculates an optical spectrum of the signal light to be measured based on a difference signal between the electrical signals from the two photoelectric conversion units,
When the measurement mode control unit outputs a control signal designating a second measurement mode from the plurality of measurement modes, the light blocking mechanism is turned off and only the signal light to be measured is the optical coupler. And the signal processing unit calculates the electric power of the signal light under measurement based on the addition signal of the electric signals from the two photoelectric conversion units or the electric signal from one of the photoelectric conversion units. An optical spectrum analyzer characterized by this.   When the measurement mode control unit outputs a control signal designating a third measurement mode from the plurality of measurement modes, the light blocking mechanism is turned off and only the signal light to be measured is the optical coupler. And the signal processing unit performs fast Fourier transform on the addition signal of the electric signals from the two photoelectric conversion units or the electric signal from one of the photoelectric conversion units, so that the measurement target The optical spectrum analyzer according to claim 1, wherein an electrical spectrum of the signal light is calculated.   The optical spectrum analyzer according to claim 1, wherein the light blocking mechanism includes an optical switch disposed between the wavelength tunable laser light source and the optical coupler. The wavelength tunable laser light source includes a semiconductor laser that oscillates a single longitudinal mode laser beam,
The said light shielding mechanism consists of an electric switch which turns on / off the supply of the drive current for oscillating the laser beam of a single longitudinal mode to the said semiconductor laser. Optical spectrum analyzer.   The optical spectrum analyzer according to any one of claims 1 to 4, wherein a branching ratio of the optical coupler is 1: 1.

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