JP4766885B2 - Wavelength fluctuation measuring device - Google Patents

Description

  The present invention is used for a laser wavelength control device. In particular, it is suitable for use in excitation current control for setting the wavelength of a semiconductor laser with high accuracy or performing wavelength sweeping at high speed and with high accuracy.

  In a large-capacity WDM transmission system, it is necessary to arrange a large number of optical wavelengths output from a transmitter evenly on the optical frequency. For this reason, it is important to stabilize the wavelength by the control circuit and suppress the fluctuation of the output wavelength.

  In addition, the optical component used in the transmission path, the group velocity dispersion of the optical fiber, and the wavelength dependence of PMD are also important in system design. In order to measure these optical characteristics, it is necessary not only to stabilize the wavelength of the probe light output from the measuring instrument with high accuracy, but also to change the wavelength during the measurement process.

As a conventional technique for monitoring the fluctuation amount of the wavelength of the laser beam, the following method can be cited.
1) Configuration using a medium that absorbs a specific wavelength such as etalon or gas A part of the output of the laser is branched and incident on the medium. By monitoring the output intensity of the medium, information on wavelength fluctuation is obtained. In order to cancel the influence due to the intensity fluctuation of the laser output, the light intensity before the medium input is also monitored (see, for example, Patent Document 1 and Non-Patent Document 1).
2) Configuration for monitoring the beat with the reference laser beam A part of the output of the laser is branched, combined with the output of the reference laser beam having a sufficiently stable wavelength, and then received, and the beat due to interference between the two Monitor ingredients. Information on the wavelength fluctuation of the laser output is obtained from the frequency of the beat component (see, for example, Patent Document 2).
3) Configuration using an interferometer whose optical path length changes periodically. Part of the laser output is branched and input to the interferometer. The interferometer is of a type that splits into two light beams, and its optical path length difference is modulated. The output of the interferometer depends on the wavelength and the optical path length difference, but if the fluctuation amount of the optical path length difference and its period are known, the fluctuation amount of the laser wavelength can be monitored (for example, see Patent Document 3).

Elec. Lett. Vol. 16P. 709 JP-A-8-316559 JP-A-7-58397 JP 2001-27512 A

  However, these configurations have the following problems. In the first method, depending on the wavelength of the laser, the laser light is almost absorbed by the medium. On the other hand, since the optical receiver itself has noise, the accuracy of the monitor is lowered in the wavelength region where the absorption is large. Further, since it is not linear with respect to the magnitude of absorption (or extinction) and the wavelength, it is difficult to accurately determine the amount of wavelength variation.

  In the second method, when the wavelength interval between the laser to be stabilized and the reference laser is widened, the frequency of the beat component of both becomes high, and a broadband detection means is required. Further, the interference intensity of the two lasers strongly depends on the polarization state.

  In the third method, it is necessary to modulate the optical path length accurately and at high speed. Patent Document 3 describes a configuration in which the optical path length is periodically changed by a phase modulator. However, when the wavelength fluctuation of the laser is faster than this modulation period, the fluctuation cannot be detected. In addition, if the reproducibility of the periodic variation of the optical path length is low, an error also occurs in the detection result.

  The present invention has been made under such a background, and can accurately monitor the fluctuation amount of the output wavelength of the laser without depending on the absolute wavelength or the polarization of the laser, and can perform phase modulation. An object of the present invention is to provide a wavelength fluctuation measuring apparatus capable of monitoring the fluctuation amount of the output wavelength of the laser at high speed without being limited by the modulation speed of the optical device.

The present invention is a wavelength fluctuation measuring apparatus for measuring the wavelength fluctuation of a laser beam. The feature of the present invention is that a reference light source having a stable wavelength, a laser beam to be measured, and an output of the reference light source are provided. Selects only the laser beam to be measured from the combining means for combining, the two-beam interferometer connected to the output of the combining means and having a wavelength shifter in one optical path, and the output of the two-beam interferometer First wavelength selection means for taking out the output, second wavelength selection means for selectively taking out only the output of the reference light source from the output of the interferometer of the two light fluxes, and the output of the first wavelength selection means as light First photoelectric conversion means for electrical conversion, second photoelectric conversion means for photoelectric conversion of the output of the second wavelength selection means, output of the first photoelectric conversion means and the second A phase comparator connected to the output of the photoelectric conversion means. The reference light, interference between the measured laser light is set to a wavelength apart to substantially negligible, the wavelength shifter is to be measured laser beam (carrier frequency f ₀₎ and the reference beam ( The carrier frequency f _ref ) is changed by Δf, and the interferometer causes the carrier frequencies f ₀ and f ₀ + Δf to interfere with each other, and causes the carrier frequencies f _ref and f _ref + Δf to interfere with each other. The phase comparator includes means for comparing the phases of the Δf frequency components of the two input signals respectively input from the first and second photoelectric conversion means and outputting a signal corresponding to the phase difference. There is.

Alternatively, the present invention is a wavelength variation measuring apparatus for measuring wavelength variation of a laser beam, and the present invention is characterized by a reference light source having a stable wavelength, a laser beam to be measured, and an output of the reference light source A two-beam interferometer connected to the multiplexing means, having a first wavelength shifter in one optical path, and having a second wavelength shifter in the other optical path; A first wavelength selecting means for selectively extracting only the laser beam to be measured from the output of the two-beam interferometer; and a second for selectively extracting only the output of the reference light source from the output of the two-beam interferometer. A wavelength selection means, a first photoelectric conversion means for photoelectrically converting the output of the first wavelength selection means, and a second photoelectric conversion means for photoelectrically converting the output of the second wavelength selection means; , The output of the first photoelectric conversion means and the second photoelectric A phase comparator connected to the output of the conversion means, and the reference light is set to a wavelength separated so that mutual interference with the laser light to be measured is substantially negligible, The wavelength shifter changes the carrier frequency of both the laser beam to be measured (carrier frequency f ₀ ) and the reference beam (carrier frequency f _ref ) by Δf ₁ , and the second wavelength shifter The carrier frequencies of both the carrier frequency f ₀ ) and the reference light (carrier frequency f _ref ) are changed by Δf ₂ , and the interferometer causes the light of the carrier frequencies f ₀ + Δf ₁ and f ₀ + Δf ₂ to interfere with each other and the carrier frequency The light of f _ref + Δf ₁ and f _ref + Δf ₂ is caused to interfere, and the phase comparator performs phase comparison on the frequency components of Δf = Δf ₁ −Δf ₂ of the two input signals, and outputs a signal corresponding to the phase difference. There is a means to output the number.

Alternatively, the present invention is a wavelength variation measuring apparatus for measuring the wavelength variation of a laser beam. The feature of the present invention is that a laser beam to be measured is input to a first port, and this laser beam is A first coupler that branches and outputs to each of the third and fourth ports, and an optical path connected to the third and fourth ports of the first coupler, and interference of two light beams having a wavelength shifter on one of the optical paths A reference light source having a stable wavelength, and a second coupler for inputting the reference light of the reference light source to the sixth port and branching and outputting the reference light to the seventh and eighth ports, respectively. The seventh and eighth ports are respectively connected to two optical paths of the interferometer, and the laser light to be measured and the reference light travel in opposite directions in the interferometer, The fourth of the coupler and The reference beams input from the three ports are combined and output from the second port, and the laser beams to be measured input from the seventh and eighth ports of the second coupler are A first photoelectric conversion means that is combined and output from the fifth port and photoelectrically converts the output of the fifth port; and a second photoelectric converter that photoelectrically converts the output of the second port And a phase comparator connected to the output of the first photoelectric conversion means and the output of the second photoelectric conversion means, and the wavelength shifter is a laser beam (carrier frequency) to be measured. f ₀ ) and the reference light (carrier frequency f _ref ) are changed by Δf, and the interferometer causes light of carrier frequencies f ₀ and f ₀ + Δf to interfere with each other, and the carrier frequencies f _ref and f _ref + Δf The phase comparator performs phase comparison on the frequency components of Δf of the two input signals respectively input from the first and second photoelectric conversion means, and outputs a signal corresponding to the phase difference There is a means to do.

Alternatively, the present invention is a wavelength variation measuring apparatus for measuring the wavelength variation of a laser beam. The feature of the present invention is that a laser beam to be measured is input to a first port, and this laser beam is A first coupler that branches and outputs to each of the third and fourth ports; an optical path connected to the third and fourth ports of the first coupler; and a first wavelength shifter on one of the optical paths. , A two-beam interferometer having a second wavelength shifter in the other optical path, a reference light source having a stable wavelength, and the reference light of this reference light source are input to the sixth port, and the reference light is supplied to the seventh and seventh light sources. A second coupler that branches and outputs to each of the eight ports, and the seventh and eighth ports are respectively connected to two optical paths of the interferometer, and the laser light to be measured in the interferometer And the reference light in the opposite direction The reference lights input from the fourth and third ports of the first coupler are combined and output from the second port, and the seventh and eighth of the second coupler are combined. A first photoelectric conversion means for combining the laser beams to be measured, which are respectively input from the first port, and output from the fifth port, and photoelectrically converting the output of the fifth port; A second photoelectric conversion means for photoelectrically converting the output of the first port; and a phase comparator connected to the output of the first photoelectric conversion means and the output of the second photoelectric conversion means. The first wavelength shifter changes the carrier frequencies of both the laser light (carrier frequency f ₀ ) and the reference light (carrier frequency f _ref ) to be measured by Δf ₁ , and the second wavelength shifter measures Target laser beam (carrier frequency ₀₎ and the carrier frequency of both the reference light (carrier frequency f _ref) is changed by Delta] f _2, wherein the interferometer causes interference light of the carrier frequency f ₀ + Delta] f ₁ and f ₀ + Delta] f _2, the carrier frequency f _ref The light of + Δf ₁ and f _ref + Δf ₂ is caused to interfere, and the phase comparator performs phase comparison on the frequency components of Δf = Δf ₁ −Δf ₂ of the two input signals and outputs a signal corresponding to the phase difference. There is a means.

  The interferometer has a variable delay in at least one of the optical paths, and the variable delay can include means for setting a difference Δτ between two optical path lengths of the interferometer to an arbitrary value other than 0. .

  The first and second photoelectric conversion units have first and second limiter circuits, respectively, and the first and second limiter circuits are laser light to be measured and light of a reference light source. Regardless of the intensity, means for making the amplitude of the frequency component of Δf constant can be provided.

  Further, in the present invention, a plurality of the wavelength fluctuation measuring apparatuses of the present invention described above are arranged in parallel, the plurality of wavelength fluctuation measuring apparatuses share one reference light source, and each of the wavelength fluctuation measuring apparatuses has the Δf. Are the multi-stage wavelength variation measuring apparatuses having the same Δτ but different Δτ.

  Alternatively, in the present invention, a plurality of the wavelength variation measuring devices of the present invention described above are arranged in parallel, the plurality of wavelength variation measuring devices share one reference light source and a wavelength shifter, and each of the wavelength variation measuring devices is The multi-stage wavelength variation measuring apparatus is the same Δf but different Δτ.

Next, the operation of the present invention will be described with reference to FIG. FIG. 1 is a diagram showing a basic configuration of a wavelength variation measuring apparatus according to the present invention. Light to be measured is input to the optical input terminal 1. Hereinafter, the carrier frequency of the light to be measured is expressed as f ₀ . The Ref light source 2 uses a light source whose wavelength is kept constant regardless of time. Hereinafter, the carrier frequency of the output of the Ref light source 2 is expressed as f _ref . f ₀ and f _ref are set to values sufficiently separated so as not to interfere with each other.

Both are input to the first port # 1 and the second port # 2 of the first coupler C1, are combined, and then branched to the third port # 3 and the fourth port # 4, respectively. Are respectively input to the seventh port # 7 and the eighth port # 8 and multiplexed again. At this time, one branch output from the fourth port # 4 passes through the wavelength shifter 3. The second output of coupler C2 of is branched to two fifth port ♯5 and sixth port # 6, the output of the fifth port ♯5 of one light BRF (Band for blocking f _ref
The light enters the first O / E converter 5 through the Rejection Filter 4, and the output of the other sixth port # 6 enters the second O / E converter 7 through the optical BPF 6 that passes only f _ref .

Here, the shift amount of the wavelength shifter 3 is assumed to be Δf. As described above, f ₀ and f _ref do not interfere with each other, but f ₀ and f ₀ + Δf interfere with each other, so that the output I ₀ of the first O / E converter 5 periodically varies in intensity with Δf. . Expressed as a function of time t,
_{I 0 = [exp {2πif 0} (t-τ 1 -τ 3)} + exp {2πi · (f 0 + Δf) (t-τ 2 -τ 3) + iπk 0/2}] × [ phase conjugation section]
= 1 + cos [2π {Δf (t−τ ₂ −τ ₃ ) + f ₀ (τ ₁ −τ ₂ )} + k ₁ π / 2]
Here, ki is an integer determined by a combination of coupler branches, and τi represents a delay time in each branch (see FIG. 1). The coefficient representing the amplitude was omitted. On the other hand, the same argument holds for the output I _ref of the second O / E converter 7.

I _ref = 1 + cos [2π {Δf (t−τ ₂ ^* −τ ₄ ^* ) + f _ref (τ ₁ ^* −τ ₂ ^* )} + k ₂ π / 2]
It is expressed. However, in consideration of group velocity dispersion, the delay time of each branch is expressed as τi ^* . The output of the first O / E converter 5 and the output of the second O / E converter 7 are input to the phase comparator 8.

The phase comparator 8 detects a phase difference between two signals that vibrate at a frequency of Δf, and outputs a DC signal corresponding to the phase difference. The phase difference Δφ between I ₀ and I _ref is calculated from the above equation: Δφ = 2πΔf (τ ₃ −τ ₄ ^* + τ ₂ −τ ₂ ^* ) + 2πf _ref (τ ₁ ^* −τ ₂ ^* ) − 2πf ₀ (τ ₁ − τ ₂ ) + (k ₂ −k ₁ ) π / 2 [rad] (Equation 1)
And does not depend on time t. When chromatic dispersion is negligible, Equation 1 can be expressed as Δφ = 2πΔf (τ ₃ −τ ₄ ) + 2π (f _ref −f ₀ ) (τ ₁ −τ ₂ ) + (k ₂ −k ₁ ) π / 2 [rad ]
And simplified.

If each delay time and the output f _ref of the Ref light source 2 are constant regardless of time, the phase difference is Δφ = −2πf ₀ (τ ₁ −τ ₂ ) + constant (Expression 2)
Therefore, the fluctuation of f ₀ appears as a change in phase difference. By detecting Δφ by the phase comparator 8, it is possible to detect the wavelength fluctuation of the light to be measured. Even when f ₀ is intentionally changed for wavelength sweeping, the amount of change in f ₀ can be accurately detected. The wavelength shift can be easily realized with an acoustooptic device. The values of the optical path length and wavelength shift may be constant, and there is no need to periodically modulate.

FIG. 9 shows the result of verification by experiment. The light source for Ref is set to 1550 nm, and the light source to be measured is set to around 1570 nm. Wavelength shift was performed by an acousto-optic device, and Δf = 55 MHz. (A) shows I ₀ and I _ref at the start of the experiment. Both waveforms show beats with a frequency of 55 MHz and a period of 18 nsec as theoretically.

(B) shows I ₀ and I _ref immediately after fine tuning of the wavelength of the light source to be measured. Since the phase difference Δφ between I ₀ and I _ref changes as the wavelength changes, the position of the I ₀ peak shifts to the left of the screen on the oscilloscope and vibrates for several seconds near the convergence point.

  After a few minutes, the waveform when the wavelength of the light source to be measured is stabilized is shown in (c). Compared with the state of (a), the peak position was 8 nsec, and a change of 3 rads in terms of phase occurred. This corresponds to a fluctuation of 120 MHz with respect to the carrier frequency.

  According to the present invention, the fluctuation amount of the output wavelength of the laser can be accurately monitored without depending on the absolute wavelength and polarization of the laser, and the high speed without being limited by the modulation speed of the phase modulator. In addition, the fluctuation amount of the output wavelength of the laser can be monitored.

(First embodiment)
A first embodiment of the present invention will be described with reference to FIG. FIG. 2 is a configuration diagram of the wavelength variation measuring apparatus according to the first embodiment.

As shown in FIG. 2, the first embodiment is a wavelength fluctuation measuring apparatus for measuring the wavelength fluctuation of laser light. The first embodiment is characterized by a ref light source 2 having a stable wavelength and a ref light source 2 having a stable wavelength. The first coupler C1 that combines the laser beam to be measured output from the semiconductor laser 10 and the output of the Ref light source 2 is connected to the output of the first coupler C1 and is connected to one optical path as a wavelength shifter. A two-beam interferometer having an acousto-optic element 11, a light BRF 4 as first wavelength selection means for selectively extracting only the laser beam to be measured from the output of the two-beam interferometer, and the interference between the two beams The optical BPF 6 as the second wavelength selection means for selectively extracting only the output of the Ref light source 2 from the output of the meter, the first O / E converter 5 for photoelectrically converting the output of the optical BRF 4, and the output of the optical BPF 6 The photoelectric A second O / E converter 7 for conversion, and a phase comparator 8 connected to the output of the O / E converter 5 and the output of the O / E converter 7, and the reference light is to be measured. The wavelength is set so that mutual interference with the laser beam is substantially negligible, and the acoustooptic device 11 is a carrier of both the laser beam (carrier frequency f ₀ ) and the reference beam (carrier frequency f _ref ) to be measured. The frequency is changed by Δf, and the interferometer causes the light of the carrier frequencies f ₀ and f ₀ + Δf to interfere with each other, and causes the lights of the carrier frequencies f _ref and f _ref + Δf to interfere. There is provided means for comparing the phases of the frequency components of Δf of the two input signals respectively input from the converters 5 and 7 and outputting a signal corresponding to the phase difference.

  The interferometer has a variable delay 12 on at least one of the optical paths, and the variable delay 12 includes means for setting a difference Δτ between two optical path lengths of the interferometer to an arbitrary value other than zero.

  The O / E converters 5 and 7 have first and second limiter amplifiers 16 and 17, respectively. The first and second limiter amplifiers 16 and 17 are a laser beam and a reference light source to be measured. Means for making the amplitude of the frequency component of Δf constant regardless of the light intensity.

  Hereinafter, the first embodiment will be described in more detail. Here, the wavelength variation of the output light of the semiconductor laser 10 is monitored. The output of the semiconductor laser 10 is partially branched by the third coupler C3 and input to the first port # 1 of the first coupler C1.

  The output of the Ref light source 2 is input to the second port # 2 of the first coupler C1. The wavelength of the Ref light source 2 is about 1 μm. If the wavelength of the output of the semiconductor laser 10 is 1.5 μm to 1.6 μm, the interference with the Ref light source 2 can be substantially ignored.

  The output of the semiconductor laser 10 and the output of the Ref light source 2 are respectively input to the first port # 1 and the second port # 2 of the first coupler C1, and then combined, and then the third port # 3 and the fourth port # 4, the output of one of the fourth ports # 4 is subjected to a wavelength shift of Δf by the acoustooptic device 11, and the output of the other third port # 3 is delayed by the variable delay 12, and then the second Are respectively input to the seventh port # 7 and the eighth port of the coupler C2.

The two outputs of the fifth port # 5 and the sixth port # 6 of the second coupler C2 are input to the optical BRF and optical BPF described in the section of operation, respectively, and then the first O / E converter 5 And input to the second O / E converter 7. As described in the section of operation, the outputs of the O / E converters 5 and 7 are sine waves having a frequency Δf, and the phase difference between them is expressed by Equation 2, but the proportional relationship between the phase difference Δφ and f ₀ is variable. The delay 12 can be adjusted by changing the optical path length τ ₁ . As an example, τ ₁ −τ ₂ = 5 nsec
Then, Δφ decreases by π when f ₀ increases by 100 MHz. In order to avoid the variation of τ ₁ −τ ₂ due to the temperature variation, the portion of the interferometer from the first coupler C 1 to the second coupler C 2 is kept at a constant temperature by the heater 13.

  An object of the present invention is to monitor wavelength fluctuations. However, when the semiconductor laser 10 or the Ref light source 2 has irregular intensity fluctuations, an intensity modulation component also appears in Equation 1 and becomes an error factor. . In order to avoid this, the output of each O / E converter is input to the first BPF 14 and the second BPF 15, and only the frequency component of Δf is input to the first limiter amplifier 16 and the second limiter amplifier 17, Input to the phase comparator 8.

  The output of the phase comparator 8 becomes the wavelength variation information of the semiconductor laser 10, and by monitoring this, the wavelength fluctuation of the semiconductor laser 10 can be monitored. As an application example of the present invention, if the excitation current of the semiconductor laser 10 is adjusted by the excitation current control circuit 9 so that the wavelength variation information becomes constant regardless of time, the output wavelength of the semiconductor laser 10 becomes constant. If the excitation current of the semiconductor laser 10 is adjusted by the excitation current control circuit 9 so that the wavelength variation information constantly increases with time, the output wavelength of the semiconductor laser 10 is continuously swept. A signal flow at this time is shown by a thin line in FIG.

(Second Embodiment)
A second embodiment of the present invention will be described with reference to FIG. FIG. 3 is a configuration diagram of the second embodiment. Here, a WDM coupler CW is used instead of the optical BPF 6 and the optical BRF 4 described in the first embodiment. The output light and Ref light of the semiconductor laser 10 to be measured are branched by the WDM coupler CW and input to the first O / E converter 5 and the second O / E converter 7, respectively. Since other operations are the same as those in the first embodiment, description thereof will be omitted.

(Third embodiment)
A third embodiment of the present invention will be described with reference to FIG. FIG. 4 is a configuration diagram of the third embodiment.

As shown in FIG. 4, the third embodiment is a wavelength fluctuation measuring apparatus that measures the wavelength fluctuation of laser light. The third embodiment is characterized by a Ref light source 2 having a stable wavelength and The first coupler C1 that combines the laser beam to be measured output from the semiconductor laser 10 and the output of the Ref light source 2 is connected to the first coupler C1, and the first wavelength shifter is connected to one optical path. A two-beam interferometer having an acousto-optic element 11-1 and an acousto-optic element 11-2 as a second wavelength shifter on the other optical path, and a laser to be measured from the output of the two-beam interferometer A second wavelength for selectively extracting only the output of the Ref light source 2 from the thirteenth port # 13 of the WDM coupler CW as the first wavelength selecting means for selectively extracting only the light and the output of the interferometer of the two beams. W as a selection means The first O / E converter 5 for photoelectrically converting the output of the 14th port # 14, the 13th port # 13 of the M coupler CW, and the second O / E for photoelectrically converting the output of the 14th port # 14. And an E converter 7 and a phase comparator 8 connected to the output of the first O / E converter 5 and the output of the O / E converter 7, and the reference light is a laser light to be measured. The wavelength is set so that mutual interference can be substantially ignored, and the acoustooptic device 11-1 has carrier frequencies of both the laser light (carrier frequency f ₀ ) and the reference light (carrier frequency f _ref ) to be measured. Is changed by Δf ₁ , and the acoustooptic device 11-2 changes the carrier frequencies of both the laser light (carrier frequency f ₀ ) and the reference light (carrier frequency f _ref ) to be measured by Δf ₂ , and the interferometer , Carrier frequency ₀ + Delta] f ₁ and causes interference of light f ₀ + Delta] f _2, the optical carrier frequency f _ref + Δf ₁ and f _ref + Delta] f ₂ is the interference, the phase comparator 8, Delta] f = Delta] f ₁ with the two input signals -Δf There is a means for performing phase comparison on the _two frequency components and outputting a signal corresponding to the phase difference.

  The interferometer has a variable delay 12 on at least one of the optical paths, and the variable delay 12 includes means for setting a difference Δτ between two optical path lengths of the interferometer to an arbitrary value other than zero.

  The O / E converters 5 and 7 have first and second limiter amplifiers 16 and 17, respectively. The first and second limiter amplifiers 16 and 17 are a laser beam and a reference light source to be measured. Means for making the amplitude of the frequency component of Δf constant regardless of the light intensity.

  In the following, the third embodiment will be described in more detail. Here, two acoustooptic elements 11-1 and 11-2 are used, and each of the two outputs of the third port # 3 and the fourth port # 4 of the first coupler C1 described in the second embodiment is used. It is arranged.

The first and second acoustooptic elements 11-1 and 11-2 are different in the amount of frequency shift. Frequency shift Delta] f ₁ of the first acoustooptic device 11-1, when the frequency shift of the second acoustooptic device 11-2 is assumed to be Delta] f _2, waveform outputted from each O / E converter 5 and 7 Is the absolute value of the frequency Δf ₁ −Δf ₂ .

  In general, the frequency shift amount of the acoustooptic elements 11-1 and 11-2 is about several tens to several hundreds of MHz, and it is difficult to adjust the shift amount. By combining 1 and 11-2, the output frequency of the O / E converters 5 and 7 can be lowered to about several hundred kHz, and wavelength fluctuation information can be obtained even with a low-speed phase comparator or lock-in amplifier. It becomes possible.

(Fourth embodiment)
A fourth embodiment of the present invention will be described with reference to FIG. FIG. 5 is a configuration diagram of the fourth embodiment.

As shown in FIG. 5, the fourth embodiment is a wavelength variation measuring apparatus that measures the wavelength variation of laser light. The fourth embodiment is characterized by a measurement output from the semiconductor laser 10. A target laser beam is input to the first port # 1, and the laser beam is branched and output to the third and fourth ports # 3 and # 4, respectively, and a third coupler C1 of the first coupler C1 is provided. In addition, an optical path is connected to each of the fourth ports # 3 and # 4, and a two-beam interferometer having an acoustooptic element 11 as a wavelength shifter on one optical path, a Ref light source 2 having a stable wavelength, and the Ref light source 2 And the second coupler C2 for branching and outputting the reference light to the seventh and eighth ports # 7 and # 8, respectively, and the seventh and eighth ports # 6 and # 6. 7 and # 8 are the two optical paths of the interferometer In the interferometer, the laser light to be measured and the reference light travel in opposite directions and are respectively input from the third and fourth ports # 3 and # 4 of the first coupler C1. The reference light is combined and output from the second port # 2, and the laser light to be measured input from the seventh and eighth ports # 7 and # 8 of the second coupler C2 is combined. The first O / E converter 5 that outputs from the fifth port # 5 and photoelectrically converts the output of the fifth port # 5, and the second O / E converter that photoelectrically converts the output of the second port # 2 7 and a phase comparator 8 connected to the output of the O / E converter 5 and the output of the O / E converter 7, and the acoustooptic device 11 refers to the laser beam (carrier frequency f ₀ ) to be measured. Both carriers of light (carrier frequency f _ref ) The frequency is changed by Δf, and the interferometer causes the light of the carrier frequencies f ₀ and f ₀ + Δf to interfere with each other, and causes the lights of the carrier frequencies f _ref and f _ref + Δf to interfere. There is provided means for comparing the phases of the frequency components of Δf of the two input signals respectively input from the converters 5 and 7 and outputting a signal corresponding to the phase difference.

  The interferometer has a variable delay 12 in at least one of the optical paths, and the variable delay 12 includes means for setting a difference Δτ between two optical path lengths of the interferometer to an arbitrary value other than zero.

  The O / E converters 5 and 7 have first and second limiter amplifiers 16 and 17, respectively. The first and second limiter amplifiers 16 and 17 are a laser beam and a reference light source to be measured. Means for making the amplitude of the frequency component of Δf constant regardless of the light intensity.

  Hereinafter, the fourth embodiment will be described in more detail. Here, the wavelength variation of the output light of the semiconductor laser 10 is monitored. The output of the semiconductor laser 10 is partially branched by the third coupler C3 and input to the first port # 1 of the first coupler C1.

  The output of the semiconductor laser 10 is branched by the first coupler C1, one of which is output from the fourth port # 4 and subjected to a wavelength shift of Δf by the acoustooptic device 11, and the other is output from the third port # 3 and is variable. After being delayed by the delay 12, the signals are input to the seventh port # 7 and the eighth port # 8 of the second coupler C2 and multiplexed, and output from the fifth port # 5 to be output to the first O / E. Input to the converter 5.

  On the other hand, the reference light output from the Ref light source 2 is input to the sixth port # 6 of the second coupler C2. The reference light is branched by the second coupler C2, one of which is output from the eighth port # 8 and subjected to a wavelength shift of Δf by the acousto-optic element 11, and the other is output from the seventh port # 7 by the variable delay 12. After receiving the delay, the signals are input to the third port # 3 and the fourth port # 4 of the first coupler C1 and multiplexed, output from the second port # 2, and supplied to the second O / E converter 7. Entered. Inflow of light into each light source is blocked by the first and second isolators 18 and 19.

  In this configuration, since the light to be measured and the reference light travel backward in the interferometer, mutual interference can be almost ignored even when the wavelengths of both are relatively close. Further, the wavelength separation means for separating the two can be omitted. Thereby, the structure equivalent to 1st or 2nd embodiment is realizable. Since the operation of the fourth embodiment can be described in the same manner as the first or second embodiment, the description of the operation is omitted here.

(Fifth embodiment)
A fifth embodiment of the present invention will be described with reference to FIG. FIG. 6 is a block diagram of the fifth embodiment.

As shown in FIG. 6, the fifth embodiment is a wavelength variation measuring apparatus that measures the wavelength variation of laser light. The feature of the fifth embodiment is a measurement output from the semiconductor laser 10. A target laser beam is input to the first port # 1, and the laser beam is branched and output to the third and fourth ports # 3 and # 4, respectively, and a third coupler C1 of the first coupler C1 is provided. The optical paths are respectively connected to the fourth ports # 3 and # 4, the acoustooptic element 11-1 as the first wavelength shifter is provided in one optical path, and the acoustooptics as the second wavelength shifter is provided in the other optical path. A two-beam interferometer having an element 11-2, a Ref light source 2 having a stable wavelength, and reference light from the Ref light source 2 are input to the sixth port # 6, and the reference light is input to the seventh and eighth ports # 6. 7 and # 8 are branched to the second output. The seventh and eighth ports # 7 and # 8 are connected to two optical paths of the interferometer, respectively, and the laser light to be measured and the reference light are reversed in the interferometer. The reference lights traveling in the direction and inputted from the third and fourth ports # 3 and # 4 of the first coupler C1 are combined and outputted from the second port # 2, and the second coupler C2 The laser beams to be measured input from the seventh and eighth ports # 7 and # 8 are combined and output from the fifth port # 5, and the first output for photoelectric conversion of the output of the fifth port # 5 is performed. O / E converter 5, second O / E converter 7 for photoelectrically converting the output of second port # 2, and the output of O / E converter 5 and the output of O / E converter 7. A phase comparator 8 and an acoustooptic device 11-1 is a measurement object. Referring to the carrier frequency of both the laser light (the carrier frequency f ₀₎ and the reference beam (carrier frequency f _ref) is changed by Delta] f _1, acousto-optic device 11-2 of the measurement target laser light (carrier frequency f ₀₎ the carrier frequency of both the light (the carrier frequency f _ref) is changed by Delta] f _2, wherein the interferometer causes interference light of the carrier frequency f ₀ + Delta] f ₁ and f ₀ + Delta] f _2, the carrier frequency f _ref + Delta] f ₁ and f The light of _ref + Δf ₂ is caused to interfere, and the phase comparator 8 includes means for performing phase comparison on the frequency components of Δf = Δf ₁ −Δf ₂ of the two input signals and outputting a signal corresponding to the phase difference. By the way.

  The interferometer has a variable delay 12 on at least one of the optical paths, and the variable delay 12 includes means for setting a difference Δτ between two optical path lengths of the interferometer to an arbitrary value other than zero.

  The O / E converters 5 and 7 have first and second limiter amplifiers 16 and 17, respectively. The first and second limiter amplifiers 16 and 17 are a laser beam and a reference light source to be measured. Means for making the amplitude of the frequency component of Δf constant regardless of the light intensity.

  Hereinafter, the fifth embodiment will be described in more detail. Here, the wavelength variation of the output light of the semiconductor laser 10 is monitored. The output of the semiconductor laser 10 is partially branched by the third coupler C3 and input to the first port # 1 of the first coupler C1.

The output of the semiconductor laser 10 is branched by the first coupler C1, one of which is output from the fourth port # 4 and subjected to a wavelength shift of Δf ₂ by the acoustooptic device 11-2, and the other is output from the third port # 3. Then, after being subjected to a wavelength shift of Δf1 by the acoustooptic device 11-1 and being delayed by the variable delay 12, they are respectively input to the seventh port # 7 and the eighth port # 8 of the second coupler C2. Is output from the fifth port # 5 and input to the first O / E converter 5.

On the other hand, the reference light output from the Ref light source 2 is input to the sixth port # 6 of the second coupler C2. The reference light is branched by the second coupler C2, one of which is output from the eighth port # 8 and subjected to a wavelength shift of Δf ₂ by the acoustooptic device 11-2, and the other is output from the seventh port # 7 and is variable. After receiving a delay by the delay 12 and receiving a wavelength shift of Δf ₁ by the acoustooptic device 11-1, it is inputted to the third port # 3 and the fourth port # 4 of the first coupler C 1, respectively. Is output from the second port # 2 and input to the second O / E converter 7. Inflow of light into each light source is blocked by the first and second isolators 18 and 19.

  In this configuration, since the light to be measured and the reference light travel backward in the interferometer, mutual interference can be almost ignored even when the wavelengths of both are relatively close. Further, the wavelength separation means for separating the two can be omitted. Thereby, a configuration equivalent to the third embodiment can be realized. Since the operation of the fourth embodiment can be described in the same manner as the third embodiment, the description of the operation is omitted here.

(Sixth embodiment)
A sixth embodiment of the present invention will be described with reference to FIG. FIG. 7 is a configuration diagram of the sixth embodiment.

  As shown in FIG. 7, a plurality of wavelength fluctuation measuring apparatuses described in the second or third embodiment are arranged in parallel, each of the plurality of wavelength fluctuation measuring apparatuses has a two-beam interferometer, and these The plurality of wavelength variation measuring apparatuses share one Ref light source 2.

  All interferometers have acousto-optic elements 11-1, 11-2, 11-3 which are wavelength shifters in at least one optical path, and all interferometers have variable delays 12-1, 12-2. In at least one optical path.

In the sixth embodiment, the difference frequency Δf between the two optical paths in the interferometer caused by the wavelength shifter must be equal for all interferometers. One interferometer may include acoustooptic elements 11-1 (Δf ₁ ) and 11-2 (Δf ₂ ) as a plurality of wavelength shifts, but at the stage of being combined, the difference frequency (Δf ₁₋ Δf ₂ ) must be equal to the wavelength shift Δf ₃ caused by the acousto-optic element 11-3.

Further, in the sixth embodiment, the delay time differences between the two optical paths generated in each interferometer must be different values. That is, the delay time from # 17 to # 9 in FIG. 7 is τ ₁ , the delay time from # 18 to # 10 is τ ₂ , the delay time from # 19 to # 21 is τ ₁ ′, and # 20 to # 22. Δτa and Δτb are set so that τ ₁ −τ ₂ and τ ₁ ′ −τ ₂ ′ have different values when the delay time to reach τ ₂ ′.

  Hereinafter, the sixth embodiment will be described in more detail. Here, two interferometer portions described in the second or third embodiment are prepared. One is a two-beam interferometer that is branched by the 6-1 coupler C6-1 and combined by the 7-1 coupler C7-1, and the other is the 6-2th This is a two-beam interferometer that is branched by a coupler C6-2 and combined by a seventh-second coupler C7-2.

These interferometers have acoustooptic elements 11-1, 11-2, 11-3, and also have a first variable delay 12-1 and a second variable delay 12-2. As described in the section of operation, the outputs of the respective O / E converters 5-1, 7-1 and 5-2, 7-2 are sine waves having a frequency Δf, and their phase difference is expressed by Equation 2. If the phase difference exceeds 2π, the variation amount cannot be determined. That is, if the fluctuation of f ₀ exceeds 1 / (τ ₁ −τ ₂ ), measurement becomes impossible. When the delay time difference (τ ₁ −τ ₂ ) of the interferometer is reduced, the measurable range is expanded. However, in order to precisely measure the fluctuation of f ₀ is must take a large proportional coefficient of Δφ and _{f 0 (τ 1 -τ 2)} .

  In the sixth embodiment, in the interferometer branched by the 6-1 coupler C6-1 and combined by the 7-1 coupler C7-1, the time difference between the two optical paths is made small, and conversely, In the interferometer branched by the 6-2 coupler C6-2 and combined by the 7-2 coupler C7-2, the time difference between the two optical paths is set large.

The outputs of the interferometers are input to the first WDM coupler CW-1 and the second WDM coupler CW-2, respectively, and the first wavelength variation information and the second WDM coupler CW-2 are input in the same manner as in the second or third embodiment. Wavelength fluctuation information. By combining these two wavelength variation information, it is possible to widen the wavelength measurable range without losing precision.
Ref light is incident from ports # 14-1 and # 14-2, and second O / E converter 7-1 and fourth O / E converter 7-2 are connected to ports # 15 and # 16, respectively. By doing so, the laser beam to be measured and the reference beam may be configured to travel in opposite directions within the interferometer.

(Seventh embodiment)
A seventh embodiment of the present invention will be described with reference to FIG. FIG. 8 is a configuration diagram of the seventh embodiment.

  In the seventh embodiment, as shown in FIG. 8, a plurality of wavelength variation measuring devices described in the second or third embodiment are arranged in parallel, and the plurality of wavelength variation measuring devices include one Ref light source 2 and The acousto-optic element 11-2 which is a wavelength shifter is shared. In the seventh embodiment, the delay time differences between the two optical paths generated in each interferometer must all be different values.

That is, the delay time from # 26 to # 9 in FIG. 8 is τ ₁ , the delay time from # 27 to # 10 is τ ₂ , the delay time from # 28 to # 21 is τ ₁ ′, and # 27 to # 20. Δτa and Δτb are set so that τ ₁ −τ ₂ and τ ₁ ′ −τ ₂ ′ have different values when the delay time to reach τ ₂ ′.

  That is, in the seventh embodiment, by providing one acousto-optic element 11-2 between the eighth coupler C8 and the ninth coupler C9, two interferometers share one wavelength shifter. Can do.

Thus, as described in the sixth embodiment, in all interferometers, the condition that the difference frequency between the two optical paths must be Δf is shared by the two interferometers. By doing, there is an advantage that can be easily satisfied.
In the sixth and seventh embodiments, the example in which two pieces of wavelength variation information are combined by arranging two interferometers in the second or third embodiment in parallel has been described. A plurality of the above-described plurality of wavelength variation information may be combined in parallel.

  According to the present invention, it is possible to detect the wavelength fluctuation of the laser light source at high speed and with high accuracy, and thereby, in a large-capacity WDM transmission system, a large number of optical wavelengths output from the transmitter are evenly arranged on the optical frequency. Therefore, the wavelength can be stabilized, and the fluctuation of the output wavelength can be suppressed. In addition, it is possible to accurately measure optical characteristics such as optical velocity and group velocity dispersion of optical components and optical fibers used in the transmission path, and PMD wavelength dependency.

The figure which shows the wavelength fluctuation measuring apparatus of the basic composition of this invention. The figure which shows the wavelength variation measuring apparatus of 1st Embodiment. The figure which shows the wavelength variation measuring apparatus of 2nd Embodiment. The figure which shows the wavelength variation measuring apparatus of 3rd Embodiment. The figure which shows the wavelength variation measuring apparatus of 4th Embodiment. The figure which shows the wavelength variation measuring apparatus of 5th Embodiment. The figure which shows the wavelength variation measuring apparatus of 6th Embodiment. The figure which shows the wavelength variation measuring apparatus of 7th Embodiment. The figure which shows the verification result of the effect of this invention.

Explanation of symbols

1 Optical input terminal 2 Ref light source 3 Wavelength shifter 4 Optical BRF
5 First O / E converter 6 Optical BPF
7 Second O / E converter 8 Phase comparator 9 Excitation current control circuit 10 Semiconductor laser 11, 11-1 to 3 Acousto-optic element 12, 12-1, 12-2 Variable delay 13 Heater 14, 14-1, 14 -2, 15, 15-1, 15-2 BPF
16, 16-1, 16-2, 17, 17-1, 17-2 Limiter amplifiers 18, 19 Isolators C1-C9 couplers CW, CW-1, CW-2 WDM couplers # 1- # 26 ports

Claims (8)

A wavelength fluctuation measuring device for measuring the wavelength fluctuation of laser light,
A reference light source that outputs a reference light having a stable wavelength;
Multiplexing means for multiplexing the input laser beam to be measured and the reference light ;
A two-beam interferometer connected to the output of the multiplexing means and having a wavelength shifter in one optical path;
A first wavelength selection means for extracting from the output of the interferometer of the two beams selectively only the laser beam of the measurement object,
A second wavelength selection means for extracting from the output of the interferometer of the two beams selectively to only the reference light,
First photoelectric conversion means for photoelectric conversion of the output of the first wavelength selection means;
Second photoelectric conversion means for photoelectrically converting the output of the second wavelength selection means;
A phase comparator connected to the output of the first photoelectric conversion means and the output of the second photoelectric conversion means;
The reference light, interference between the laser beam of the measurement target is set to a wavelength apart to substantially negligible,
Wherein the wavelength shifter, the carrier frequency of both of the measurement target of the laser beam (the carrier frequency f ₀₎ and the reference light (carrier frequency f _ref) is changed by Delta] f,
The interferometer causes light of carrier frequencies f ₀ and f ₀ + Δf to interfere, and causes light of carrier frequencies f _ref and f _ref + Δf to interfere,
The phase comparator includes means for comparing the phases of the Δf frequency components of the two input signals respectively input from the first and second photoelectric conversion means and outputting a signal corresponding to the phase difference. A wavelength fluctuation measuring device characterized by that. A wavelength fluctuation measuring device for measuring the wavelength fluctuation of laser light,
A reference light source that outputs a reference light having a stable wavelength;
Multiplexing means for multiplexing the input laser beam to be measured and the reference light ;
A two-beam interferometer connected to the multiplexing means and having a first wavelength shifter in one optical path and a second wavelength shifter in the other optical path;
A first wavelength selection means for extracting from the output of the interferometer of the two beams selectively only the laser beam of the measurement object,
A second wavelength selection means for extracting from the output of the interferometer of the two beams selectively to only the reference light,
First photoelectric conversion means for photoelectric conversion of the output of the first wavelength selection means;
Second photoelectric conversion means for photoelectrically converting the output of the second wavelength selection means;
A phase comparator connected to the output of the first photoelectric conversion means and the output of the second photoelectric conversion means;
The reference light is set to a wavelength that is far enough that mutual interference with the laser light to be measured is substantially negligible,
Wherein the first wavelength shifter, the carrier frequency of both said measured laser beam (carrier frequency f ₀₎ and the reference light (carrier frequency f _ref) is changed by Delta] f _1,
Said second wavelength shifter, the carrier frequency of both of the measurement target of the laser beam (the carrier frequency f ₀₎ and the reference light (carrier frequency f _ref) is changed by Delta] f _2,
The interferometer causes light of carrier frequencies f ₀ + Δf ₁ and f ₀ + Δf ₂ to interfere, and causes light of carrier frequencies f _ref + Δf ₁ and f _ref + Δf ₂ to interfere,
The phase comparator comprises means for comparing the phases of the frequency components of Δf = Δf ₁ −Δf ₂ of the two input signals and outputting a signal corresponding to the phase difference. . A wavelength fluctuation measuring device for measuring the wavelength fluctuation of laser light,
A first coupler for inputting a laser beam to be measured to the first port, and branching and outputting the laser beam to the third and fourth ports;
A two-beam interferometer having optical paths connected to the third and fourth ports of the first coupler and having a wavelength shifter in one of the optical paths;
A reference light source that outputs a reference light having a stable wavelength;
Enter the participation illumination of this to the sixth port, and a second coupler for branching outputs the reference light to the seventh and eighth ports,
The seventh and eighth ports are respectively connected to two optical paths of the interferometer;
In the interferometer, the laser beam to be measured and the reference light travel in opposite directions,
The reference lights input from the third and fourth ports of the first coupler are combined and output from the second port,
The laser beams to be measured input from the seventh and eighth ports of the second coupler are combined and output from the fifth port,
First photoelectric conversion means for photoelectric conversion of the output of the fifth port;
Second photoelectric conversion means for photoelectrically converting the output of the second port;
A phase comparator connected to the output of the first photoelectric conversion means and the output of the second photoelectric conversion means;
Wherein the wavelength shifter, the carrier frequency of both of the measurement target of the laser beam (the carrier frequency f ₀₎ and the reference light (carrier frequency f _ref) is changed by Delta] f,
The interferometer causes light of carrier frequencies f ₀ and f ₀ + Δf to interfere, and causes light of carrier frequencies f _ref and f _ref + Δf to interfere,
The phase comparator includes means for comparing the phases of the Δf frequency components of the two input signals respectively input from the first and second photoelectric conversion means and outputting a signal corresponding to the phase difference. A wavelength fluctuation measuring device characterized by that. A wavelength fluctuation measuring device for measuring the wavelength fluctuation of laser light,
A first coupler for inputting a laser beam to be measured to the first port, and branching and outputting the laser beam to the third and fourth ports;
A two-beam interferometer having optical paths connected to the third and fourth ports of the first coupler, having a first wavelength shifter in one optical path, and a second wavelength shifter in the other optical path When,
A reference light source that outputs a reference light having a stable wavelength;
Enter the participation illumination this to a sixth port, and a second coupler for branching outputs the reference light to the seventh and eighth ports,
The seventh and eighth ports are respectively connected to two optical paths of the interferometer;
In the interferometer, the laser beam to be measured and the reference light travel in opposite directions,
The reference lights input from the third and fourth ports of the first coupler are combined and output from the second port,
The laser beams to be measured input from the seventh and eighth ports of the second coupler are combined and output from the fifth port,
First photoelectric conversion means for photoelectric conversion of the output of the fifth port;
Second photoelectric conversion means for photoelectrically converting the output of the second port;
A phase comparator connected to the output of the first photoelectric conversion means and the output of the second photoelectric conversion means;
Wherein the first wavelength shifter, the carrier frequency of both said measured laser beam (carrier frequency f ₀₎ and the reference light (carrier frequency f _ref) is changed by Delta] f _1,
Said second wavelength shifter, the carrier frequency of both of the measurement target of the laser beam (the carrier frequency f ₀₎ and the reference light (carrier frequency f _ref) is changed by Delta] f _2,
The interferometer causes interference light of the carrier frequency f ₀ + Δf ₁ and f ₀ + Δf _2, by interference light of carrier frequency f _ref + .DELTA.f1 and f _ref + Δf _2,
The phase comparator comprises means for comparing the phases of the frequency components of Δf = Δf ₁ −Δf ₂ of the two input signals and outputting a signal corresponding to the phase difference. . The interferometer has a variable delay in at least one optical path,
5. The wavelength variation measuring apparatus according to claim 1, wherein the variable delay includes means for setting a difference Δτ between two optical path lengths of the interferometer to an arbitrary value other than 0. 6. The first and second photoelectric conversion means have first and second limiter circuits, respectively.
Wherein said first and second limiter circuits, which regardless of the light intensity of the light intensity and the reference light source of the laser light to be measured which is the input, comprising means for a constant amplitude of the frequency components of the Δf Item 5. The wavelength fluctuation measuring device according to any one of Items 1 to 4. A plurality of wavelength variation measuring devices according to claim 1 are arranged in parallel,
The plurality of wavelength variation measuring devices share one reference light source,
Each of the wavelength fluctuation measuring apparatuses is a multistage wavelength fluctuation measuring apparatus in which Δf is equal, but a difference Δτ between two optical path lengths of the interferometers is different from each other. A plurality of wavelength variation measuring devices according to claim 1 are arranged in parallel,
The plurality of wavelength variation measuring devices share one reference light source and a wavelength shifter,
Each of the wavelength fluctuation measuring apparatuses is a multistage wavelength fluctuation measuring apparatus in which Δf is equal, but a difference Δτ between two optical path lengths of the interferometers is different from each other.

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