JP3537372B2 - Variable spectral linewidth light source device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source device using a semiconductor laser, and more particularly to a variable spectral line width light source device capable of continuously changing the spectral line width of emitted light.

[0002]

2. Description of the Related Art Various kinds of communication equipment for incorporation into an optical communication system are newly developed, a communication network such as an optical fiber is newly constructed, and maintenance and inspection of optical communication equipment already incorporated in an optical communication system are performed. Or the like, it is necessary to apply a highly accurate optical test signal to these systems to be tested.

For example, when the above-described loss measurement of a system to be measured is performed, as shown in FIG. 8, a single mode light 2 output from a light source device 1 in which a semiconductor laser is incorporated is, for example, an optical fiber. Is input to the system under test 3 via the. The light 4 that has passed through the measured system 3 is incident on the measuring device 5 via, for example, an optical fiber. The measuring device 5 measures the power (intensity) of the light 4 that has passed through the measured system 3.

FIG. 9 shows a wavelength distribution characteristic of single mode light 2 output from a light source device 1 in which a semiconductor laser is incorporated. As shown in the figure, the wavelength distribution 6 of the single-mode light 2 has a sharp peak waveform centered on the center wavelength λ ₀ substantially determined by the laser material. As a scale indicating the steepness of the distribution waveform 6, the width of the distribution waveform 6 at a value (1/2) P of the peak value P of the optical power in the distribution waveform 6 is defined as a spectral line width _BP. I have.

When measuring the optical wavelength characteristic of the system under test 3, it is preferable that the spectral line width _BP of the light 2 (test optical signal) emitted from the light source device 1 and incident on the system under test 3 is narrow.

In general, when light passes through a boundary between materials having different refractive indexes, a part of incident light is reflected to generate reflected light. Therefore, in the measurement system of FIG. 8, a part of the light 2 emitted from the light source device 1
And returns to the light source device 1 as reflected light. As a result, an interference phenomenon occurs between the reflected light and the light 2 emitted from the light source device 1, and the optical power of the light 2 incident on the measured system 3 fluctuates. When the optical power of the light 2 incident on the measured system 3 greatly changes, the optical power (light intensity) of the light 4 incident on the measuring device 5 via the measured system 3 also greatly changes. Therefore, the loss of the measured system 3 cannot be accurately measured by the measuring device 5.

[0007] The interference phenomenon largely occurs between lights having the same wavelength, and the interference phenomenon is unlikely to occur between lights having different wavelengths. That is, a spectral line width _BP wider than the resonance length due to interference is required. In the distribution waveform 6 shown in FIG. 9, the narrow spectral line width _BP
This indicates that the contained wavelength component is small, the probability of causing interference with the reflected light increases, and the light power (light intensity) of the light 2 greatly fluctuates. On the other hand, if the spectral line width _BP is wide, it indicates that the included wavelength is large, the probability of causing interference with the reflected light is low, and the fluctuation of the light power (light intensity) of the light 2 is small.

However, in general, in a semiconductor laser that emits single-mode light, the spectral line width _BP is narrow for the purpose of measuring the optical wavelength characteristics, and therefore, the reflection at the connection point or inside the system to be measured. There is a problem that the measurement device 5 cannot accurately measure the loss of the system to be measured due to the influence of the interference.

Further, when the system under test 3 is a long-distance optical fiber such as a submarine cable laid on the sea floor and maintenance and inspection of the optical fiber are performed, a pulse test device ( OTDR), sends a pulse signal for testing to this optical fiber, measures the backscattering characteristics, and confirms the presence or absence of disconnection or abnormality of the optical fiber and the position of the disconnection or abnormality from the backscattering characteristics. Like that.

In this case, when light having a narrow spectral line width _BP is used as the light source of the light pulse, the power of the light pulse output from the light source device is increased for the purpose of improving the S / N of the detected backscattering characteristic. Even so, it is known that a non-linear phenomenon related to optical transmission in an optical fiber causes a phenomenon in which the dynamic range is reduced without increasing the power of an optical pulse propagating in the optical fiber. Furthermore, in order to improve the S / N of the detected backscattering characteristic, the spectral line width _BP of the light as the light source of the light pulse should be set wider than increasing the power of the light pulse. Has been demonstrated.

As described above, it is desired that the spectral line width _BP of the light output from the light source device 1 can be set to be wide depending on the measurement items for the system 3 to be measured.

The light source device capable of setting the spectral line width _BP of the light output from the light source device 1 to a large value in this manner is configured, for example, as shown in FIG. The single mode light a output from the semiconductor laser 7 is input to the optical modulator 9 via the optical fiber 8. The optical modulator 9 modulates the incident light a based on the input modulation signal b, and emits the modulated light a ₁ to the outside from the output terminal 11 via the optical fiber 10.

For example, a noise generator 12 for generating white noise
Noise b ₁ output from the input to one terminal 13a of the switch 13. The other terminal 13b of the changeover switch 13 is open. The common terminal 13 c of the changeover switch 13 is connected to the modulation signal input terminal of the optical modulator 9.

[0014] In the light source device having such a configuration, in the state in which switches the changeover switch 13 to the noise generator 12 side, the modulation noise b ₁ output from the noise generator 12 of the optical modulator 9 as a modulation signal b Applied to the signal input terminal. Since the noise is theoretically has an infinite frequency components in the noise b _1, the modulated light a having a distribution waveform 6 shown in FIG. 9, a number on each side of the central wavelength lambda ₀ in the distribution waveform 6 spectra caused by the sidebands generated, resulting in spectral line width B _P of the light a ₁ after light modulation output from the optical modulator 9 is widened.

On the other hand, the changeover switch 13 is connected to an open terminal 13b.
In the state switched to the side, since no modulation signal is input to the modulation signal input terminal of the optical modulator 9, the optical modulator 9 does not perform the modulation operation on the incident light a. spectral line width B _P of the light a ₁ output from the optical modulator 9 is not changed.

[0016] Thus, by using the optical modulator 9 it can widely vary the spectral line width B _P of the light a ₁ output from the output terminal 11.

[0017]

However, FIG.
The light source device using the optical modulator 9 shown in (1) has the following problems to be solved.

That is, the wider the spectral line width _BP of the light a output from the output terminal 11 is, the better it is, and there is a certain optimum range. That is, when the measured system 3 is, for example, an optical communication device, even if light a having a wavelength component outside the wavelength range defined in the specifications of the optical communication device is incident on the optical communication device, Performance cannot be evaluated correctly.

Also, when measuring the characteristics of a long-distance optical fiber such as a submarine cable using a pulse test apparatus (OTDR), the spectral line width in the optimum range is determined by the nonlinear characteristics of the optical transmission in the optical fiber. There should be _BP .

However, in the light source device shown in FIG. 10, there are only two types, that is, whether or not the light a emitted from the semiconductor laser 7 is modulated by the optical modulator 9. Modulated signal b is noise b ₁ generated by noise generator 12. Frequency band of the noise b ₁ is theoretically infinite, a fixed value can not be changed. Therefore, the spectral line width B _P of the light a ₁ which is optically modulated by the optical modulator 9 is also a fixed value.

As described above, in the conventional technique, the spectral line width _BP of the light a output from the semiconductor laser 7 cannot be easily changed.

The present invention has been made in view of such circumstances, and has a simple configuration and a simple output by using a signal other than noise and approximating noise as a modulation signal for an optical modulator. Spectral line width B _P
It is an object of the present invention to provide a spectral line width variable light source device capable of freely changing the setting.

[0023]

In order to solve the above-mentioned problems, a variable spectral line width light source device according to the present invention comprises:
A semiconductor laser that emits light in a single mode, an optical modulator that optically modulates light emitted from the semiconductor laser based on an input modulation signal, and outputs the optically modulated light, and a digital pseudo-random signal And a pseudo-random signal generator for transmitting the generated pseudo-random signal to the optical modulator as a modulation signal, and a pseudo-random signal generator for variably setting the spectral line width of light output from the optical modulator. And a pseudo-random signal control unit that variably sets the number of stages of the register .

In the thus configured variable spectral line width light source device, the optical modulator optically modulates the light emitted from the semiconductor laser based on a modulation signal comprising a digital pseudo-random signal. Since the pseudo-random signal has a characteristic close to noise as is well known, by employing this pseudo-random signal as a modulation signal,
As in the case where the noise indicated by 0 is used for the modulation signal, a spectrum originating from a number of sidebands is generated on both sides of the center wavelength in the distribution waveform, and as a result, the optical modulation is output from the optical modulator. , The spectral line width of the light increases.

The degree of spread of the spectral line width of the light after the light modulation largely depends on the frequency component of the modulated signal. In other words, when the frequency component increases, each spectrum caused by a number of sidebands on both sides of the center wavelength in the distribution waveform occurs at a position away from the center wavelength, and as a result, the light after light modulation Becomes wider.

Therefore, the register of the pseudo-random signal generator characterizing the frequency characteristics of the pseudo-random signal
By variably controlling the number of stages, the spectral line width of the light after light modulation can be variably set.

[0027]

[0028] Generally, the pseudo random signal generator for generating a digital pseudo-random signal is composed of a plurality of registers and one or more exclusive OR circuits connected in series. The period of this pseudo random signal is determined by the number of registers n . Therefore, the number of stages of this register is
By changing, the spectral line width of the light after the light modulation can be variably set.

According to another aspect of the present invention, the optical modulator in the above-described variable spectrum line width light source device comprises an optical intensity modulator or an optical phase modulator.

[0030]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is a block diagram showing a schematic configuration of a variable spectral linewidth light source device according to a first embodiment of the present invention. The same portions as those of the conventional light source device shown in FIG. 10 are denoted by the same reference numerals, and detailed description of the overlapping portions will be omitted.

The semiconductor laser 7 outputs a single mode light a having a wavelength distribution characteristic indicated by a wavelength distribution 6 having a spectral line width B _S1 centered on the center wavelength λ ₀ shown in FIG. . The light a emitted from the semiconductor laser 7 is input to the light intensity modulator 14 via the optical fiber 8. The light intensity modulator 14 modulates the incident light a based on the modulation signal b ₁ applied to the modulation signal input terminal, and emits the modulated light a ₂ from the output terminal 11 to the outside via the optical fiber 10. I do.

The PN signal generator 15 as a pseudo-random signal generator is provided with an M-sequence PN as a pseudo-random signal.
And generating a signal d, and sends the PN signal d as a modulation signal b ₁ to the modulating signal input terminal of the optical intensity modulator 14. The PN signal generator 15 is connected to a PN signal controller 16 that variably sets parameters characterizing the PN signal d output from the PN signal generator 15. This PN
The signal control unit 16 includes a clock frequency setting unit 17 and a PN stage number setting unit 18.

FIG. 2 is a schematic diagram showing a schematic configuration of the light intensity modulator 14. Light a incident from the optical fiber 8 via the input connector 19 is transmitted through two optical paths 20 having the same configuration.
a. After being branched into 20b, each optical path 20a. 2
The light is incident on the electro-optical elements 21a and 21b interposed in Ob. Electrodes 22 for applying an electric field in a direction orthogonal to the transmission direction of light a are provided on each of the electro-optical elements 21a and 21b.
a, 22b, 23a and 23b are attached.

On the other hand, the modulation signal b ₁ input from the modulation signal input terminal is converted by the signal dividing circuit 26 into two signals c ₁ and c ₂ whose phases are separated from each other by, for example, 90 degrees. 21b of electrodes 22a, 22b, 23a,
23b. Each electro-optical element 21a, 21b
When an electric field is applied to the electrodes 22a, 22b, 23a, and 23b, the phase of the waveform of the light a passing through the inside is shifted by an amount corresponding to the applied electric field. That is, each of the electro-optical elements 21a, 21b and the electrodes 22a, 22b, 2
Reference numerals 3a and 23b each constitute an optical phase modulator.

Each light phase-modulated by each of the electro-optical elements 21a and 21b passes through a corresponding one of optical paths 24a and 24b, is subjected to waveform synthesis, and is subjected to a light intensity-modulated light a through an output connector 25. _{The signal 2} is sent to the output terminal 11 via the optical fiber 10.

As described above, in the light intensity modulator 14, the incident light a is divided into two optical paths, subjected to phase modulation with signals having different phases, and then input by synthesizing the two. it is possible to obtain light a ₂ that is strongly light modulated according to the modulation signal b _1.

FIG. 3 is a block diagram showing a schematic configuration of the PN signal generator 15. As shown in FIG. N registers 30 (R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , R ₅ ) are provided between an input terminal 28 for inputting an initial value and an output terminal 29 for outputting a PN signal d.
, R _N-1 , R _N ) are connected in series. The output signal of the output terminal 29 and the input signal of the final stage register 30 (R _N ) are input to each input terminal of an exclusive OR circuit (EXOR) 31. An output signal output from the output terminal of the exclusive OR circuit 31 is input to the input terminal 28.

For example, N registers 30 (R ₁ , R ₂ ,
R ₃ , R ₄ , R ₅ ,..., R _N−1 , R _N )
Registers 30 (R ₁ , R ₂ , R
_{3, R 4, R 5,} ..., the R _n), the bypass switch 32 for bypassing the corresponding register 30 is attached. Each bypass switch 32 is arbitrarily opened and closed by a PN stage number setting unit 18 provided in the PN signal control unit 16. When the number of closing the bypass switch 32 is increased, PN
As the number of stages decreases and the number of open bypass switches 32 decreases, the number of PN stages increases.

Further, the clock signal k of the frequency f _S outputted from the clock signal generator 33 is divided by a frequency divider 34 into 1 /
Is divided into M is applied to each register 30 as a new clock signal k _1. Each time the clock of the clock signal k ₁ is input, each of the registers 30
Fetch the data (bit data) stored in
The data (bit data) stored therein is sent to the register 30 at the subsequent stage.

The frequency division ratio (1 / M) of the frequency divider 34 can be changed and set by the clock frequency setting unit 17 in the PN signal control unit 16. PN stage number setting unit 18 and clock frequency setting unit 1
7 can be easily operated by an operator using a keyboard or the like.
That is, the operator sets the PN in the PN signal generator 15 to PN.
The number of stages and the frequency f _C of the clock signal k ₁ can be arbitrarily set.

In the PN signal generator 15 having such a configuration, when the bypass switch 32 is open and an initial value is set at the input terminal 28 and the clock generator 33 is started, the output terminal 29 A PN signal d having a period (bit period) of (2 ^N −1) and a bit rate of (f _S / M) is output. The period (2 ^N −1) and the bit rate (f _S / M) can be arbitrarily set. The period (2 ^N −1) and the bit rate (f _S / M) are PN
This constitutes the frequency component of the signal d.

In the thus configured light source device with variable spectral line width, the light a
Has a distribution waveform 6 with an extremely narrow spectral line width B _S1 shown in FIG. When the light a having this spectrum line width B _S1 is modulated by the light intensity modulator 14 with the modulation signal b ₁ composed of the PN signal d, the light intensity modulated light a ₂ becomes FIG.
By increasing the number of PN stages in the PN stage number setting unit 18 in the PN signal control unit 16 having the distribution waveform 6a having a wide spectral line width B _S2 shown in FIG. 2B, the spectrum of the light intensity modulated light a ₂ is increased. The line width B _S2 can be expanded to an arbitrary value within a predetermined range. When the number K of PN stages of the PN signal d is increased, the above-described period (2 ^K −1) becomes longer, and the bandwidth of the included frequency becomes wider. When the frequency bandwidth is wide, the PN signal d is close to noise, and a large number of spectra attributable to the sidebands on both sides of the center wavelength in the distribution waveform are generated at positions away from the center wavelength. Spectral line width B of light a ₂ after intensity modulation
_S2 becomes wider.

Conversely, setting the number of PN stages in the PN signal control unit 16
By reducing the number of PN stages in the
Light a after intensity modulation_TwoSpectral line width B_S2Is narrow
Become. Therefore, it is not necessary to change the number of PN stages of the PN signal d.
And the spectral line width B _S2Can be freely set
You.

Next, the clock frequency setting unit 17 sets the PN
When the frequency f _C (= f _S / M) of the clock signal of the signal d is increased, as shown in FIG.
_{In the second} distribution waveform 6a, the interval between the spectra caused by a large number of sidebands appearing on both sides of the center wavelength λ ₀ is widened. As a result, the spectral line width of the light a ₂ after light intensity modulation is obtained. B _S2 becomes wider.

Therefore, by changing the frequency f _C of the clock signal of the PN signal d, the spectral line width B _S2
Can be freely set.

As described above, in the variable spectral line width light source device of the first embodiment, the output is provided from the output terminal 11 by changing the number of PN stages in the PN signal d or the frequency f _C of the clock signal of the PN signal d. The spectral line width B _S2 of the light a ₂ can be variably set.

(Second Embodiment) FIG. 6 is a block diagram showing a schematic configuration of a variable spectral linewidth light source device according to a second embodiment of the present invention. The same parts as those of the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and detailed description of the overlapping parts will be omitted.

In the variable spectral line width light source device of the second embodiment, the light intensity modulator 14 in the first embodiment device shown in FIG. Other configurations are the same as those of the first embodiment shown in FIG.

FIG. 7 is a schematic diagram showing a schematic configuration of the optical phase modulator 35. Light a incident from the optical fiber 8 via the input connector 36 is incident on an electro-optical element 38 inserted in the optical path 37. The electrode 3 for applying an electric field to the electro-optical element 38 in a direction orthogonal to the transmission direction of the light a.
9a and 39b are attached. The modulation signal U ₁ input from the modulation signal input terminal is applied to the electrode 3 of the electro-optical element 38.
9a and 39b. When an electric field is applied to the electrodes 39a and 39b, the electro-optical element 38 shifts the phase of the waveform of the light a passing through the inside by an amount corresponding to the applied electric field. Light a ₃ phase-modulated by the electro-optical element 38
Is an optical phase-modulated light a ₃ through the output connector 40.
Is transmitted to the output terminal 11 via the optical fiber 10.

In the thus configured variable spectral line width light source device of the second embodiment, the light a outputted from the semiconductor laser 7 is phase-modulated by the PN signal d in the optical phase modulator 35. As a result, since the wavelength component contained in the phase modulated light a ₃ is widely dispersed,
The spectral line width B _S2 increases.

The degree of phase modulation can be changed by changing the number of PN stages of the PN signal d or the frequency f _C of the clock signal of the PN signal d. Therefore, it is possible to obtain substantially the same operation and effect as the spectral line width variable light source device of the first embodiment.

The present invention is not limited to the above embodiments. In each embodiment, the clock frequency setting unit 17 and the PN stage number setting unit 18 are provided in the PN signal control unit 16, but only one of them may be used.

[0053]

As described above, in the variable spectral line width light source device of the present invention, a PN signal other than noise and approximated to noise is used as a modulation signal for the optical modulator. The frequency characteristic of the PN signal can be easily changed by changing the PN stage . Therefore, it is possible to freely change the setting of the spectral line width of the light that is easily output with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a variable spectral linewidth light source device according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a schematic configuration of a light intensity modulator incorporated in the variable spectral line width light source device.

FIG. 3 shows P incorporated in the variable spectral line width light source device.
1 is a block diagram showing a schematic configuration of an N signal generator.

FIG. 4 is a diagram showing a comparison between a spectral line width of light output from a semiconductor laser and a spectral line width of light output from an optical modulator in the variable spectral line width light source device.

FIG. 5 is a diagram showing distribution characteristics of light-modulated light when the clock frequency in the variable spectral line width light source device is changed.

FIG. 6 is a block diagram showing a schematic configuration of a variable spectral line width light source device according to a second embodiment of the present invention.

FIG. 7 is a schematic diagram showing a schematic configuration of an optical phase modulator incorporated in the variable spectral line width light source device.

FIG. 8 is a schematic diagram of a measurement system for measuring light loss measurement for a general system to be measured.

FIG. 9 is a diagram showing a distribution waveform of light emitted from the light source device.

FIG. 10 is a block diagram showing a schematic configuration of a conventional light source device.

[Explanation of symbols]

7 Semiconductor lasers 8, 19 Optical fiber 11 Output terminal 14 Light intensity modulator 15 PN signal generator 16 PN signal control unit 17 Clock frequency setting unit 18 PN stage number setting unit 35 Optical phase modulation unit _{a, a 1, a 2,} a 3 ... light b, b ₁ ... modulated signal d ... PN signal

Continuation of the front page (56) References JP-A-5-289125 (JP, A) JP-A-11-225109 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1 / 00-1/125

Claims (2)

(57) [Claims] 1. A semiconductor laser (7) for emitting light of a single mode, and an optical modulator for optically modulating light emitted from the semiconductor laser based on an input modulation signal and outputting the optically modulated light. Modulators (14, 35); a pseudo-random signal generator (15) for generating a digital pseudo-random signal and transmitting the generated pseudo-random signal as a modulation signal to the optical modulator; the spectral line width of the light in order to variably set that, Regis of the pseudo random signal generator
A variable spectral line width light source device comprising: a pseudo-random signal control section (16) for variably setting the number of stages of the data (30) . 2. The light modulator according to claim 1, wherein the light modulator is a light intensity modulator.
The variable spectral line width light source device according to claim 1, wherein the light source device is one of one of an optical phase modulator and an optical phase modulator.