JPH0835910A - Dynamic-characteristic measuring apparatus for turnable element - Google Patents

Abstract

PURPOSE:To provide a dynami-characteristic measuring apparatus whose wavelength measuring range is wide, which can measure a wavelength dynamic characteristic in a counparatively high-speed time region and which can measure a wavelength with high accuracy. CONSTITUTION:The wavelength of a tunable light source 402 as a criterion light source is calibrated sequentially by a high-accuracy wavelength measuring means 412, and the absolute wavelength accuracy of the tunable light source 402 becomes high. In addition, when the wavelength of the tunable light sourace 402 is changed sequentially, a beat signal which corresponds to a difference frequency between the output light of the tunable light source 402 and the output light of a tunable element 401 to be measured can be detected. The waveform of the detected signal is stored in a waveform storage means 411 in a time-series manner, and a wavelength dynamic characteristic is output by a control device 410 on the basis of the stored waveform.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to transient characteristics that occur when wavelengths of wavelength tunable elements such as wavelength tunable lasers and wavelength tunable filters that have wavelength tunable functions integrated, and wavelength dynamic characteristics such as long-term wavelength changes. The present invention relates to a device for measuring a dynamic characteristic of a wavelength tunable element for measuring the.

[0002]

2. Description of the Related Art A wavelength tunable element such as a wavelength tunable laser such as a DBR semiconductor laser or a wavelength tunable filter is used in a coherent optical communication, a wavelength multiplex communication, and an optical transmission / reception device and an optical cross used in an optical communication network using them. Used for connect devices, optical switches, optical frequency standards, optical frequency synthesizers, etc. Hereinafter, a conventional technique related to measurement of dynamic characteristics of a light emitting type wavelength tunable element will be described.

As a first conventional technique, a measurement method using a swept Fabry-Pwrot interferometer is known as a dynamic characteristic measurement technique for a wavelength tunable semiconductor laser having a wavelength tunable function integrated therein. Here, an outline of the first conventional technique will be described with reference to FIG. 4 (for details of the first conventional technique, refer to Reference 1; H. Kobrin, for example.
ski et al., 'Fast wavelength-switching of laser tr
ansmitters and amplifier's.IEEEJ.Selected Areas in
See Commun., Vol.8, No.6, pp.1190-1201, 1990. ).

FIG. 4 is a diagram showing a configuration example (first conventional example) of a wavelength tunable dynamic characteristic measuring device for an emission type wavelength tunable element according to a first conventional technique. In this figure, 101 is a wavelength tunable element to be measured, and here, as an example,
A two-electrode DFB laser is used. 102 is a polarization controller, 103 is a modulator, 104 is a Fabry-Perot filter (Fabry-Perot interferometer), 105 is a photodetector, 1
Reference numeral 06 is a PZT drive circuit for driving an electrostrictive element (PZT) built in the Fabry-Perot interferometer. Also, 1
Reference numeral 07 is a signal generator, 108 is a variable wavelength current setting device, 1
Reference numerals 09 and 110 respectively indicate bias currents having current values of I _r and I _f .

The operation of the first conventional example having the above structure will be described below. In advance, a certain constant bias (bias current 10
9, 110) is applied to switch the oscillation wavelength of the two-electrode DFB laser 101 to a wavelength corresponding to the current value set by the wavelength variable current setting device 108 such as a rectangular wave generator. As a result, the carrier wavelength that changes with time is supplied to the modulator 103 via the polarization controller 102, and the data signal is superimposed here.

On the other hand, the PZT drive circuit 1 includes an electrostrictive element built in the Fabry-Perot filter 104 having the characteristics of a bandpass filter for a specific optical frequency (wavelength).
The center frequency of the Fabry-Perot interferometer is swept by sweeping with the ramp wave generated by 06. Then, the wavelength is measured by comparing the applied voltage value and the peak of the transmitted light intensity detected by the photodetector 105. The configuration and operation of the first conventional example are as described above.

The second conventional technique relates to measurement of dynamic characteristics of a non-emissive type wavelength tunable element, in which a plurality of reference wavelength lasers are prepared and light when the center frequency of the non-emissive element matches the wavelength of the laser. A measurement method is known in which the wavelength is determined by using a swept Fabry-Perot interferometer for the output. Here, an outline of the second conventional technique will be described with reference to FIG. 5 (for details of the second conventional technique, refer to Reference 2; P. Gambini et al., 'Characterizatio, for example.
n of the tuning speed of atrained MQW DFB filter / a
mplifiers by time resolved spectral analysis'.ECO
See C'92, Paper TuPl.12, Berlin, Germany. ).

FIG. 5 is a view for explaining the second conventional technique. In this figure, 223 is a wavelength tunable filter (wavelength tunable element to be measured) 221 and 22 to be measured.
2 is a DFB laser that emits light at different wavelengths, 203 is an isolator, 204 is a polarization controller, 205 is a coupler, 206 is a variable optical attenuator, 207 is an optical amplifier (optical amplifier) such as an erbium-doped fiber amplifier, 208
Is a swept Fabry-Perot interferometer, 209 is an electrostrictive element (P
ZT), 210 is a photoelectric converter, 211 is a current source, 212
Is a temperature control circuit, 213 is a ramp wave generator, 214 is a digitizing scope, 215 is a sampling scope, 216 is a rectangular wave generator, 217 is a variable attenuator, 21
Reference numeral 8 is a control computer, and reference numerals 231 and 232 are bias currents having current values of I ₂ and I ₁ + I ₃ , respectively, which are applied to the measured wavelength variable element 223.

According to this structure, the output laser beams of the DFB lasers 221 and 222, which have been previously emitted at different wavelengths by the temperature control circuit 212 and the current source 211, are emitted by the isolator 203 and the polarization controller 2.
After being multiplexed by the coupler 205 via 04, it is input to the wavelength tunable filter having a certain band pass filter characteristic in the wavelength tunable element 223 to be measured.

Then, light having a wavelength matching the center frequency of the wavelength tunable filter passes through the wavelength tunable filter,
It is input to the swept Fabry-Perot interferometer 208 via the optical amplifier 207. Swept Fabry-Perot interferometer 20
8 to the electrostrictive element 209, the ramp wave generator 21
The center frequency of the Fabry-Perot interferometer 208 is swept by sweeping with the ramp wave generated at 3.
On the other hand, as for the light input to the Fabry-Perot interferometer 208, only light having a wavelength that matches the center frequency of the Fabry-Perot interferometer 208 is output. The output light is converted into an intensity signal by the photoelectric converter 210, and the sampling scope 215 and the digitizing scope 214 compare the voltage value of the ramp wave and the transmitted light intensity in the time domain to determine the wavelength.

A third prior art is a method of measuring the dynamic characteristics of a light emitting type wavelength tunable element (wavelength tunable semiconductor laser) by using a plurality of reference wavelength lasers and using heterodyne detection with a laser beam to be measured. Are known. here,
An outline of the third conventional technique will be described with reference to FIG. 6 (for details of the third conventional technique, refer to Reference 3; Ishida et al., Thermal drift correction during LD frequency switching by current waveform optimization, for example. , IEICE Spring Conference B-957,19
See 93. ).

FIG. 6 is a diagram for explaining the third prior art, in which 301 is a narrow line width L with an external mirror.
D, 302, for example, a measured wavelength variable element such as DBRLD, 303 a polarization controller, 304 a coupler,
305 is a photoelectric converter, 306 is a frequency counter, 307
Is a rectangular wave generator, and 308 is a bias current. According to such a configuration, the polarization axis of the output light of the measured wavelength tunable element 302 and the polarization axis of the output light of the narrow linewidth LD 301 with an external mirror are aligned by the polarization controller 303, and the coupler 304 is used.
Combine at. In the photoelectric converter 305, the combined light detects a beat frequency corresponding to the difference frequency between the wavelength of the wavelength tunable element and the wavelength of the narrow linewidth LD with an external mirror, and the rectangular wave generator 307 generates the beat frequency. The beat frequency is counted by the frequency counter 306 using a rectangular wave as a trigger.

[0013]

In the first and second prior arts described above, the swept Fabry-Perot interferometer used as the optical frequency discriminator is constructed by varying the resonator spacing of the interferometer by an electrostrictive element. , The principle is to sweep the center frequency. The circuits according to the first and second prior arts have an advantage that they can be configured by a simple optical circuit using a swept Fabry-Perot interferometer, and an advantage that they can measure a wide range of wavelength change although their accuracy is relatively low. Have and.

However, as is clear from the operating principle of the swept Fabry-Perot interferometer, since the length accuracy or position accuracy of the electrostrictive element determines the accuracy of the center optical frequency of the interferometer, It is difficult to make highly accurate measurements. Also, although relative wavelength changes can be measured, it is difficult to measure absolute wavelengths. This is because the displacement amount of the electrostrictive element with respect to the voltage generally has hysteresis and the positional accuracy with respect to the same voltage is poor. Further, if it is attempted to compensate the position accuracy and the like, correction processing by external means is indispensable, and the circuit configuration becomes complicated.

Regarding the stability, the Fabry-Perot interferometer has the following characteristics because the resonator length determines the center frequency.
There is a drawback in that the resonator length fluctuates due to the environmental temperature fluctuation, that is, the center frequency fluctuates. In order to compensate for this drawback, it is necessary to stabilize the temperature by, for example, storing the whole Fabry-Perot interferometer in a thermostatic chamber or the like, and there is a possibility that the entire apparatus becomes large-scale.
Furthermore, in order to measure the absolute wavelength, it is necessary to have a configuration using a stabilized laser beam or the like on a stable absorption line of atoms or molecules. Therefore, in order to realize the configuration, there is a possibility that the device scale may be complicated or large.

By the way, in the third prior art, the frequency difference (difference frequency) between the output light of the narrow line width LD 301 with the external mirror and the output light of the wavelength tunable element 302 to be measured, which is the reference, is calculated by the frequency counter 306. Since it is determined according to the counting result, the gate time of the frequency counter 306 determines the time resolution. Therefore, it is advantageous for moderate fluctuations of sub-μs (10 ⁻⁶ to 10 ⁻⁷ s) or more, but cannot be measured for wavelength fluctuations of less than that.

Therefore, the reference line width LD with an external mirror is used.
It is necessary to use 301 which is extremely highly stable. In addition, the measurable wavelength range is determined by the band of the photoelectric converter 305 that detects the heterodyne signal, and therefore is about 0.1.
Although it is possible to measure nm (10 GHz or less in terms of frequency), it is difficult to measure further wavelength change.

The present invention has been made in view of the above circumstances, and it is difficult to measure with high accuracy by the conventional method or apparatus for measuring the dynamic characteristics of a wavelength tunable element. An object of the present invention is to provide a dynamic characteristic measuring device for a wavelength tunable element, which has a wide range and is capable of measuring wavelength dynamic characteristics in a relatively high-speed time domain and capable of highly accurate wavelength measurement. I am trying.

[0019]

In order to solve the above-mentioned problems, the invention described in claim 1 is a wavelength tunable element to be measured which outputs a light to be measured, and a wavelength tunable element to be output from the wavelength tunable element to be measured. A signal source that generates an electric signal for changing the center frequency of the measurement light, a wavelength tunable light source that outputs a wavelength tunable reference light, a wavelength measuring unit that measures the wavelength of the reference light with high accuracy, and the wavelength. A control device that controls the wavelength tunable light source so that the absolute wavelength accuracy of the reference light is highly accurate based on the measurement wavelength of the measuring means, and a multiplexing device that multiplexes and interferes the measured light and the reference light. And a photodetector which receives the output light of the multiplexer as an input and converts a beat signal having a waveform corresponding to the difference frequency between the measured light and the reference light into an electric signal and outputs the electric signal, and the photodetector And a filter that limits the band of the output electrical signal ; And a waveform storage means for storing a time series waveform of the output electrical signal of said filter, said control device, based on the accumulated waveform with said waveform storage means,
It is characterized in that data representing dynamic characteristics is output in a predetermined format.

The invention described in claim 2 is the same as claim 1.
In the invention described in (3), it is provided between the wavelength tunable light source and the multiplexer, further comprising a frequency transition means for transitioning the frequency of the reference light by a predetermined amount and entering the multiplexer. I am trying. Further, the invention according to claim 3 is the invention according to claim 1 or 2, wherein the control device outputs a three-dimensional graph based on the dynamic characteristics accumulated by the waveform accumulating means. It has a feature.

The invention according to claim 4 is the same as claim 1.
The invention according to any one of items 1 to 3 is characterized in that the filter is a low-pass filter. Further, the invention according to claim 5 is characterized in that, in the invention according to claim 4, the cutoff frequency of the low-pass filter is equal to or higher than a frequency which is a reciprocal of a rising time of the dynamic characteristic.

[0022]

According to the above construction, the absolute wavelength accuracy of the variable wavelength light source becomes highly accurate by successively calibrating the wavelength of the variable wavelength light source as the reference light source by the highly accurate wavelength measuring means. That is, it is possible to obtain a highly accurate wavelength.
Further, by sequentially changing the wavelength of the variable wavelength light source, a beat signal corresponding to the difference frequency between the output light of the variable wavelength light source and the output light of the variable wavelength element to be measured is detected. The waveform of the detected signal is accumulated in the waveform accumulating means in time series, and the wavelength dynamic characteristic is output by the controller based on the accumulated waveform. Therefore, highly accurate wavelength measurement and wavelength dynamic characteristic measurement in a wide wavelength range are possible.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of a dynamic characteristic measuring apparatus for a wavelength tunable element according to a first embodiment of the present invention. The apparatus having the configuration shown in this figure shows the wavelength dynamic characteristic of a light emitting type wavelength tunable element. It is for measuring. Specifically, it is an apparatus for measuring the wavelength dynamic characteristics of a DBR laser which is a typical laser in a semiconductor laser integrated with a wavelength variable function.

In FIG. 1, reference numeral 401 denotes a wavelength tunable element to be measured, in which a DBR laser chip is used. Reference numeral 402 denotes a wavelength variable light source serving as a wavelength reference, 403 is a polarization controller, 404 is a coupler, 405 is a switch or coupler, 406 is a photoelectric converter, 407 is a low-pass filter, and 410 is a control device including a CPU, ROM,
It is composed of a computer equipped with a RAM and various I / O interfaces. Further, 411 is a waveform accumulating means such as a digital sampling oscilloscope, 412 is a highly accurate wavelength measuring means such as a Michelson interferometer, 414.
Is a signal generator, and 420, 421 and 422 are respectively
The active region bias current (current value is _Ia ), the phase adjustment region bias current (current value is _Ip ), and the DBR region bias current (current value is _Id ) are shown.

The operating principle of the dynamic characteristic measuring apparatus for the wavelength variable element according to the first embodiment will be described below. First, bias currents 420, 421, and 422 are applied to the active region, the phase adjustment region, and the DBR region of the multi-electrode DBR laser used as the light emitting type wavelength tunable element 401 to be measured. Then, an electric signal for wavelength switching (hereinafter referred to as a wavelength switching signal) is generated by the signal generator 414, and the wavelength switching signal is superimposed on the bias currents 421 and 422.

The light of the wavelength tunable element 401 to be measured, which changes with time, and the wavelength tunable light source 402 serving as a reference light source whose absolute wavelength has been accurately measured in advance by the wavelength measuring means 412.
Of the polarization controllers 403 and 403, respectively.
Via a coupler 404. From the combined light, the photoelectric converter 406 detects a beat signal corresponding to the difference frequency between the light of the wavelength tunable element 401 to be measured and the light of the wavelength tunable light source 402, and the low pass filter 407 limits the band. Add.

The cutoff frequency of the low-pass filter 407 is set in advance to a value equal to or higher than the frequency which is the reciprocal of the rising time of the dynamic characteristic, and twice the cutoff frequency becomes a wavelength measurement error. Further, the waveform accumulating means 411 accumulates the waveform output from the low-pass filter 407 in order in time series. The waveform accumulated by the waveform accumulator 411 is the control line 4
The data is transferred to the control device 410 via 15, and is subjected to predetermined data processing, and converted into a three-dimensional graph.

The wavelength tunable element 4 to be measured, which is obtained by the dynamic characteristic measuring apparatus of the wavelength tunable element having the above-mentioned structure and operating principle
An example of a three-dimensional graph representing the wavelength dynamic characteristic of 01 is shown in FIG. The graph shown in this figure is for measurement with a wavelength accuracy of 0.05 nm.

FIG. 2 is a view showing the schematic arrangement of a dynamic characteristic measuring apparatus for a wavelength tunable element according to the second embodiment of the present invention,
The device having the configuration shown in this figure is for measuring the wavelength dynamic characteristics of a non-emissive type wavelength tunable element. In particular,
For example, it is a device for measuring the wavelength dynamic characteristics of a multi-electrode DFB filter as shown in Reference 2.

In FIG. 2, reference numeral 501 denotes a variable wavelength element to be measured, which uses a multi-electrode DFB filter. 5
Reference numeral 02 denotes a variable wavelength light source as a reference light source, 503 a polarization controller, 504 a coupler, 505 a coupler or switch, 506 a photoelectric converter, 507 a low-pass filter, 508 a frequency shifter such as an acousto-optic device, 51
Reference numeral 0 is a controller having a ROM, RAM, various I / O interfaces, etc., 511 is a waveform accumulating means such as a digital sampling oscilloscope, 512 is a highly accurate wavelength measuring means such as a Michelson interferometer, 514 is a signal generator, 515 control lines, 521 and 522, respectively, current value I _f, indicating the bias current of I _r.

The operating principle of the dynamic characteristic measuring apparatus for a wavelength variable element according to the second embodiment will be described below. First, bias currents 521 and 522 below the threshold value are applied to each region of the multi-electrode DFB filter used as the non-emission type wavelength tunable element 501 to be measured. Then, the signal generator 514 generates a wavelength switching signal and superimposes the wavelength switching signal on the bias currents 521 and 522.

Then, the transmission center wavelength of the light of the wavelength tunable light source 502, which is the reference light source whose absolute wavelength has been measured with high accuracy by the wavelength measuring means 512 in advance, via the polarization controller 503, changes with time. Measuring variable element 50
Apply to 1. Therefore, the light passing through the measured variable element 501 is only the light having the center wavelength of the same filter, and the light having other wavelengths does not pass. Variable device under test 50
The transmitted light of No. 1 and the light of the variable wavelength light source 502 that has been frequency-shifted by the frequency shifter 508 are combined by the coupler 504 via the polarization controllers 503 and 503.

The photoelectric converter 506 detects a beat signal corresponding to the difference frequency between the frequency-shifted light and the transmitted light of the variable element under test 501, and the detection signal is band-limited by the low-pass filter 507. . The cutoff frequency of the low-pass filter 507 is set in advance to a value equal to or higher than the frequency that is the reciprocal of the rising time of the dynamic characteristic, and twice the cutoff frequency becomes a wavelength measurement error. In addition, the waveform accumulating means 511 accumulates the waveforms output from the low pass filter 507 in chronological order. The waveform accumulated by the waveform accumulating means 511 is transmitted to the control device 510 via the control line 515.
And is subjected to predetermined data processing and converted into a three-dimensional graph.

As described above, according to the first and second embodiments of the present invention, the wavelength tunable elements 401, 5 to be measured are measured.
01 makes it possible to measure the wavelength dynamic characteristics in a wide wavelength range with high accuracy. In addition, the low pass filter 40
By appropriately changing the cutoff frequencies of 7, 507, the wavelength dynamic characteristics in the narrow wavelength range which can be measured by the conventional technique can be made equal to the performance by the conventional technique. In addition, it is a format (3
It can be output as a dimensional graph.

[0035]

As described above, according to the present invention,
By sequentially calibrating the wavelength of the variable wavelength light source serving as the reference light source by the highly accurate wavelength measuring means, the absolute wavelength accuracy of the variable wavelength light source becomes highly accurate. That is, it is possible to obtain a highly accurate wavelength. Further, by sequentially changing the wavelength of the variable wavelength light source, a beat signal corresponding to the difference frequency between the output light of the variable wavelength light source and the output light of the variable wavelength element to be measured is detected. The waveform of the detected signal is accumulated in the waveform accumulating means in time series, and the wavelength dynamic characteristic is output by the controller based on the accumulated waveform. Therefore, it is possible to perform highly accurate wavelength measurement and wavelength dynamic characteristic measurement in a wide wavelength range. Further, by appropriately changing the cutoff frequency of the low-pass filter used, there is an effect that it is possible to achieve the same performance as that of the related art with respect to the wavelength dynamic characteristics of the narrow wavelength range realized by the related art.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of a dynamic characteristic measuring apparatus for a wavelength tunable element according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a schematic configuration of a dynamic characteristic measuring device for a wavelength tunable element according to a second embodiment of the present invention.

FIG. 3 is a diagram showing an example of a three-dimensional graph obtained by the first embodiment of the present invention.

FIG. 4 is a diagram showing a schematic configuration of a wavelength dynamic characteristic measuring device according to a first conventional technique.

FIG. 5 is a diagram showing a schematic configuration of a wavelength dynamic characteristic measuring device according to a second conventional technique.

FIG. 6 is a diagram showing a schematic configuration of a wavelength dynamic characteristic measuring device according to a third conventional technique.

[Explanation of symbols]

 401, 501 wavelength tunable element under measurement 402, 502 wavelength tunable light source 403, 503 polarization controller 404, 504 coupler (combiner) 405, 505 switch or coupler 406, 506 photoelectric converter (photodetector) 407, 507 low Band-pass filter (filter) 410,510 Control device 411,511 Waveform storage means 412,512 Wavelength measurement means 414,514 Signal generator (signal source) 508 Frequency shifter (frequency transition means)

Claims (5)

[Claims] 1. A wavelength tunable element to be measured which outputs a light to be measured, a signal source which generates an electric signal for changing the center frequency of the light to be measured output from the wavelength tunable element to be measured, and a wavelength tunable element. A wavelength tunable light source that outputs the reference light, wavelength measuring means that measures the wavelength of the reference light with high accuracy, and absolute wavelength accuracy of the reference light with high accuracy based on the measurement wavelength of the wavelength measuring means. A control device for controlling the variable wavelength light source, a multiplexer for multiplexing and interfering the measured light and the reference light, and an output light of the multiplexer as an input, and the measured light and the reference light A photodetector that converts a beat signal having a waveform corresponding to the difference frequency with light into an electric signal and outputs the electric signal, a filter that limits the band of the electric signal output from the photodetector, and outputs the electric signal. Waveform accumulation hands that accumulate signal waveforms in time series DOO comprising a said control device, based on the accumulated waveform with said waveform storage means, dynamic characteristic measuring device of the tunable element and outputs data representing the dynamic characteristics in a predetermined format. 2. A frequency transition means inserted between the wavelength tunable light source and the multiplexer, for transitioning the frequency of the reference light by a predetermined amount and entering the multiplexer. The dynamic characteristic measuring device for a wavelength tunable element according to claim 1. 3. The dynamic characteristic of the wavelength variable element according to claim 1, wherein the control device outputs a three-dimensional graph based on the dynamic characteristic accumulated by the waveform accumulating means. measuring device. 4. The dynamic characteristic measuring device for a wavelength tunable element according to claim 1, wherein the filter is a low-pass filter. 5. The cutoff frequency of the low pass filter is
5. The dynamic characteristic measuring device for a wavelength tunable element according to claim 4, wherein the frequency is equal to or higher than the frequency that is the reciprocal of the rise time of the dynamic characteristic.