JP2004132704A - Wavelength monitor and its reference value setting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength monitor capable of monitoring accurately the wavelength of a light source, and its reference value setting method. <P>SOLUTION: Light used for wavelength division multiplexing from a wavelength variable light source 1 is branched into three light by a half mirror. The branched light is received as it is by photodiodes PD1-PD3 through a slope filter 7 and an etalon 8. The wavelength used as a grid standard is determined by the slope filter 7, and a fine change thereof is detected by the etalon 8. Hereby, the wavelength can be monitored accurately. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a wavelength monitoring device for monitoring a signal light wavelength in wavelength division multiplexing optical communication using a laser light source, and a reference value setting method therefor.
[0002]
[Prior art]
2. Description of the Related Art In recent years, in the field of optical communication, a wavelength division multiplexing communication system for simultaneously transmitting laser beams having a plurality of wavelengths to one optical fiber has been used. According to such a wavelength division multiplexing communication system, in order to increase the number of channels within a limited wavelength band, the wavelength of each channel is accurately defined, and the wavelength interval between channels is made as narrow as possible. You need to do multiplexing. In the wavelength division multiplexing communication system, tens to hundreds of signal wavelengths are arranged in a predetermined wavelength band. Noise interference (crosstalk) between signal wavelength channels and deviation of the signal itself from the grid greatly affect transmission quality. Therefore, a laser for transmission is required to have high stability. A distributed feedback laser diode (BFD laser) used as a normal transmission laser is required to have a wavelength stability of ± 3 GHz in a system having a transmission wavelength interval of 100 GHz. BFD lasers usually have a wavelength drift characteristic of 250 pm / ° C. (30 GHz). Therefore, in order to achieve the above stability, it is necessary to control the temperature with an accuracy of 0.1 ° C.
[0003]
However, in the wavelength control in such a conventional laser light source device, in a light source using a distributed feedback semiconductor laser, a relationship between a control amount such as a current and an ambient temperature and an emission wavelength are recognized in advance, and a desired wavelength is controlled. An open-loop control system in which the control amount is changed according to the emission wavelength is used. In addition, even in the external cavity type laser light source device, the relationship between the control amount of the resonance wavelength and the emission wavelength is recognized in advance, and the open-loop control in which the control amount is changed according to a desired emission wavelength is performed. The method is used.
[0004]
However, in practice, there are changes in the external environment temperature and variations among the individual elements, and it is difficult to stabilize the wavelength with high accuracy by temperature control. Therefore, it is necessary to stabilize the wavelength by externally monitoring the wavelength and performing field-back control.
[0005]
As a wavelength monitoring device for monitoring the wavelength of a laser beam, a wavelength monitor that uses a slope filter whose transmission wavelength continuously changes and measures the wavelength based on a level transmitted through the filter is used. .
[0006]
[Problems to be solved by the invention]
However, the conventional wavelength monitor using a slope filter has a disadvantage that sufficient accuracy and resolution cannot be obtained. Therefore, in order to control the wavelength of the laser light source with high accuracy, a wavelength monitor device capable of checking the emission wavelength of the wavelength multiplexed light with high accuracy is required.
[0007]
The present invention has been made in view of such a conventional problem, and the inventions of claims 1 to 5 use a periodic etalon or a Mach-Zehnder filter having a high resolution in a narrow wavelength range to obtain a wavelength. An object of the present invention is to provide a wavelength monitor capable of accurately measuring the wavelength of a multiplexed light source. It is another object of the present invention to provide a reference value setting method which facilitates setting of a reference value at a wavelength of a grid in such a wavelength monitor.
[0008]
[Means for Solving the Problems]
The invention of claim 1 of the present application is directed to an optical branching unit that branches incident light into first to third branched light, and a first photoelectric conversion that directly receives the first branched light branched from the optical branching unit. And a slope filter having a characteristic in which a transmittance characteristic continuously changes with respect to an incident wavelength, and a second branch light branched from the optical branching section is incident thereon, and is obtained through the slope filter. A second photoelectric conversion unit that receives the second branch light, and a characteristic that varies at a period corresponding to a grid interval of the wavelength division multiplexed light, and a third branch light branched from the optical branch unit enters Calculating an etalon, a third photoelectric conversion unit that receives third branch light obtained through the etalon, and a value obtained by normalizing the second photoelectric conversion output with a first photoelectric conversion output; Value and normalized output of pre-stored slope filter , The wavelength of the grid of the incident light is selected, the third photoelectric conversion output is normalized by the first photoelectric conversion output, and the normalized reference value of the selected grid wavelength in the etalon is obtained. By calculating the deviation from the grid, the wavelength monitor unit that detects the wavelength of the incident light based on this is calculated, and the output of the slope filter is normalized for each wavelength of the light used for the wavelength multiplexing communication. And a memory for holding a standardized value and a reference value obtained by normalizing the output of the etalon.
[0009]
According to a second aspect of the present invention, in the wavelength monitor according to the first aspect, a constant temperature layer that holds at least the optical branching unit, the etalon and the slope filter, and a temperature adjustment unit that keeps the constant temperature layer at a temperature within a predetermined range. Is further provided.
[0010]
According to a third aspect of the present invention, in the wavelength monitor of the first aspect, the grid of the wavelength multiplexed light is adjusted so as to be located at an intermediate position between a peak and a valley of the transmission characteristic of the etalon. is there.
[0011]
According to a fourth aspect of the present invention, in the wavelength monitor of the first aspect, the slope filter is a bandpass filter having a center wavelength outside a wavelength change range of incident light.
[0012]
According to a fifth aspect of the present invention, there is provided an optical branching unit for branching incident light into first to third branched light, and a first photoelectric conversion unit for directly receiving the first branched light branched from the optical branching unit. And a slope filter having a characteristic in which a transmittance characteristic continuously changes with respect to an incident wavelength, and a second branch light branched from the optical branching section is incident thereon, and is obtained through the slope filter. A second photoelectric conversion unit that receives the second branch light, and a Mach that has a characteristic that varies at a cycle corresponding to a grid of wavelength multiplexed light and transmits the third branch light that is branched from the optical branch unit. A zender-type filter, a third photoelectric conversion unit that receives the third branch light obtained through the Mach-zender-type filter, and a value obtained by normalizing the second photoelectric conversion output with a first photoelectric conversion output Is calculated, and the value is held in advance. Selecting the wavelength of the grid of the incident light by comparing the normalized output of the rope filter, normalizing the third photoelectric conversion output with the first photoelectric conversion output, and selecting the wavelength of the selected grid. By comparing with a normalized reference value of a Mach-Zehnder type filter, a deviation from the grid is calculated, and based on the calculated value, a wavelength monitor unit for detecting a wavelength of incident light, and a light used for wavelength multiplexing communication. And a memory for holding a value obtained by normalizing the output of the slope filter and a reference value obtained by normalizing the output of the Mach-Zehnder filter for each wavelength.
[0013]
According to a sixth aspect of the present invention, there is provided a method for setting a reference value of an etalon in the wavelength monitor according to the first aspect, wherein the etalon is obtained when light having a wavelength of one grid in a change range of the wavelength multiplexed light is incident. Calculate the normalized output from the etalon, calculate the normalized reference value from the etalon obtained when light of another grid in the change range of the wavelength multiplexed light is incident, and calculate the normalized output value of the wavelength multiplexed light. The reference value at the wavelength of each grid is calculated by linear interpolation.
[0014]
The invention according to claim 6 of the present application is a method for setting a reference value of a Mach-Zehnder filter in the wavelength monitor according to claim 5, wherein light having a wavelength of one grid in a change range of the wavelength multiplexed light is incident. Calculates the normalized output from the Mach-Zehnder filter obtained when the normalized reference value from the Mach-Zehnder filter obtained when light from another grid within the change range of the wavelength multiplexed light is incident , And a reference value at the wavelength of each grid of the wavelength multiplexed light is calculated by linear interpolation.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
(Embodiment 1)
Next, a laser light source device including the wavelength monitoring device according to the first embodiment of the present invention will be described. This laser light source device has a wavelength variable light source 1. The wavelength tunable light source 1 is a light source that oscillates any wavelength of the wavelength division multiplexed light, and is configured by a distributed feedback laser diode (LD). Since the wavelength multiplexed light multiplexes light of many wavelengths at a constant wavelength interval, each predetermined wavelength is called a grid wavelength. The output of the tunable light source 1 is provided to an attenuator (ATT) 3 via a modulator (M) 2. The attenuator 3 controls its intensity level based on a control signal from the outside, and its output is given to the optical splitter 4. The optical splitter 4 splits the incident light into two, supplies one of the split light to the optical unit of the wavelength monitor device, and uses the other light as a light source of the wavelength multiplexed light.
[0016]
Between these, light from the wavelength variable light source is connected by an optical fiber and transmitted to each unit. This optical fiber is preferably a polarization-maintaining optical fiber. The optical splitter 4 may be a splitter having a structure in which optical fibers are fused, or may be configured using a beam splitter.
[0017]
Next, the optical unit will be described in detail. In the optical unit, half mirrors 5 and 6 for splitting light are arranged in series with the light source. These half mirrors 5 and 6 are light splitters for splitting the light of the light source into three. In the light branching section, the level of each branched light may be equal. In this case, the reflectance of the half mirror 5 is set to 33%, and the reflectance of the half mirror 6 is set to 50%. The light reflected by the half mirror 5 is incident on a photodiode PD1, which is a first light receiving element. The light incident on the half mirror 6 is supplied to a photodiode PD2 as a second light receiving element via a slope filter 7. The slope filter 7 is a filter whose transmittance changes monotonously in the wavelength range of the incident light and whose wavelength-transmittance characteristics are known. The slope filter 7 is used to roughly determine the wavelength of the incident light. A wideband bandpass filter may be used as the slope filter. In this case, the center wavelength is shifted from the change range of the incident light, and an inclined portion is used. The light transmitted through the half mirror 6 enters the etalon 8. The etalon 8 has a FSR (free spectral range) having a period corresponding to a grid of wavelength multiplexed light in which the wavelength variable light source 1 is used, and is a Fabry-Perot type narrow period filter. The etalon 8 is, for example, a solid etalon in which two layers of reflection films of about ８λ of incident light are applied to both surfaces of a glass plate having a thickness of 1 mm, and is a reflection film having a low reflectance of about 15% here. The transmission characteristic of the etalon 8 changes periodically with wavelength, but it is preferable that the output of the etalon has the largest change at the wavelength of each grid of the wavelength multiplexed light. Then, a photodiode PD3, which is a third light receiving element, is disposed at a position where the branched light transmitted through the etalon 8 is received. These optical elements are housed in a thermostat 9 to keep the temperature at a predetermined value. A temperature control unit 10 for maintaining the temperature at a predetermined value is connected to the thermostat 9 holding the optical element.
[0018]
The outputs of the photodiodes PD1 to PD3 are given to light receiving amplifiers 11 to 13 in the signal processing unit. The light receiving amplifiers 11 to 13 convert an optical signal into an electric signal and amplify the electric signal, and constitute first to third photoelectric conversion units together with the photodiodes PD1 to PD3, respectively. The outputs of these light receiving amplifiers are supplied to a microcomputer 15 via an A / D converter 14. The microcomputer 15 has a wavelength monitor 15A for monitoring the wavelength of the light source based on these outputs, a wavelength controller 15B for setting the wavelength to a predetermined value based on the detected wavelength, and a light amount for keeping the light amount level constant. This achieves the function of the control unit 15C. The microcomputer 15 is connected to a memory 16 which stores wavelength data and stores a program. An interface (I / F) 17 for inputting and outputting data from an external device is connected. The output from the wavelength controller 15B of the microcomputer 15 is supplied to the LD driver 19 via the D / A converter 18. The LD driver 19 controls the wavelength by controlling the current and the like of the tunable laser diode in the tunable light source 1 based on the wavelength control signal. The output of the light quantity control unit 15C is provided to the attenuator 3 via the D / A converter 20. Thus, the output level from the attenuator 3 is set to a predetermined value.
[0019]
Here, the normalized transmittance-wavelength characteristics of the slope filter 7 are stored in the memory 16 in advance for each wavelength of the grid of the wavelength multiplexed light. FIG. 2 shows the normalized characteristics of the slope filter 7 and the channels Ch1 to Ch16 of the grid of the wavelength multiplexed light.
[0020]
The memory 16 also holds a value obtained by normalizing the output from the etalon 8. Since the output from the etalon 8 shows a periodic change in output with respect to the wavelength as shown in FIG. 3, if the period of the etalon 8 and the grid of the wavelength-multiplexed light exactly match, for example, FIG. As shown in FIG. 3, all grids have the same value. However, when the period slightly deviates from the grid, a slightly different value is obtained for each grid as shown in FIG. Therefore, the normalized output ratio of each grid is normally stored in the memory 16 as shown in FIG.
[0021]
Next, the operation of the wavelength monitor according to the present embodiment will be described with reference to the flowchart in FIG. Light from the tunable light source 1 enters the optical splitter 4 via the modulator 2 and the attenuator 3. The light further split by the light splitter 4 enters the half mirrors 5 and 6 in the optical unit. The light reflected by the half mirror 5 enters the first photodiode PD1, and the transmitted light enters the half mirror 6. The light reflected by the half mirror 6 passes through the slope filter 7 and enters the photodiode PD2. The light transmitted through the half mirror 6 enters the etalon 8, and the transmitted branched light is received by the photodiode PD3. As described above, since the etalon 8 is provided with a reflective film having a low reflectance of about 15%, the characteristics of the surface reflected light with respect to the wavelength fluctuate periodically, and the period is the free spectrum range (FSR) of the etalon. Is determined by
[0022]
On the other hand, the slope filter 7 converts outputs obtained by the photodiodes PD1, PD2, and PD3 into outputs P1, P2, and P3 by light receiving amplifiers 11, 12, and 13, respectively. These values are input to the microcomputer 15 via the A / D converter 14 (Step S1). Next, in step S2, the characteristics of the slope filter 7 can be normalized by dividing the level P2 of the second split light by the level P1 of the first split light. FIG. 2 shows the overall characteristics of the normalized slope filter 7, and the wavelength of the incident light can be roughly calculated based on the characteristics after the normalization. Therefore, in step S3, the wavelength data λi of the wavelength multiplexed light grid closest to this ratio is read from the memory 16. Next, in step S4, a reference value at the wavelength λi obtained from the etalon 8 is read from the memory 16. The reference value is always the same value as shown in FIG. 3 when the FSR of the etalon 8 exactly matches the grid, and otherwise differs as shown in FIG. 4 for each wavelength as shown in FIG. It becomes. Then, in step S5, the output P3 of the etalon 8 is divided by P1 for normalization. A change Δλ in wavelength from the reference wavelength is calculated from the output obtained in step S5 (step S6). Thus it is possible to calculate the reference wavelength lambda _i and wavelength Δλ which differ therefrom. By adding the reference wavelengths λi and Δλ in step S7, the wavelength λ of the incident light can be accurately calculated and output. Further, the wavelength control unit 15B calculates an error between the emission wavelength and the set wavelength, and controls the emission wavelength of the variable wavelength light source so that the error becomes zero. The control signal is supplied to the LD driver 19, and the temperature and the current of the variable wavelength light source 1 are controlled. The light quantity control unit 15C controls the attenuation ratio of the optical attenuator 3 so as to be at the set level. In this case, the emission wavelength of the laser light source can be accurately controlled using the wavelength monitor described above.
[0023]
(Embodiment 2)
Next, a second embodiment of the present invention will be described. In this embodiment, the normalized value of the etalon of the wavelength monitor is measured for the characteristics of a predetermined channel, for example, the characteristics at both ends without calculating the reference values for all the channels of the wavelength multiplexed light, and interpolation is performed based on the measurement result. The reference value of each channel is calculated based on the reference value. The overall configuration of this embodiment is the same as that of the first embodiment shown in FIG.
[0024]
FIG. 6 is a flowchart showing the operation of the interpolation process. When the linear interpolation process is started, first, in step S11, the wavelength of the light source is set to the shortest wavelength channel, for example, channel 1, and light of that wavelength is input. Next, in step S12, the ratio P3 / P1 is calculated, and the ratio is set to α1. Next, in step S13, the light of the channel m having the longest wavelength, for example, the light of the wavelength of the channel 16 is input, and the ratio P3 / P1 at that time is calculated and set to αm (step S14). Next, in step S15, a reference value at that time is calculated for each channel by linear interpolation. If the number of channels is m (16 in this case) as shown in the following equation, the reference value αi of each channel can be calculated by the following equation.
αi = α1− (i−1) (α1−αm) / (m−1)
After calculating the reference value of each channel in this way, it is stored in the memory 16. This eliminates the need to provide light of a grid wavelength for each channel and calculate a reference value for each channel from the ratio, and thus facilitate calibration work. The linearly interpolated values obtained in this manner may be stored in the memory 16 as they are, or only two values, α1 and αm, at which linear interpolation can be performed are stored, and the current wavelength obtained from the slope filter 7 is obtained. The reference value of the wavelength of the grid may be calculated each time by linear interpolation based on the approximate value λ of the above.
[0025]
In this embodiment, a solid etalon having a coating with a reflectance of 15% is used as the etalon 8. If the reflectance of the etalon 8 is increased, the fluctuation range of the transmittance is increased, but it is shifted from a sine waveform. Also, if the reflectance is reduced, the transmission characteristic becomes closer to a sine wave, but the resolution is reduced because the amplitude value is reduced. Therefore, this reflectance is preferably in the range of, for example, 10 to 20%. Here, the reflectance of the etalon is set to 15%. The FSR can be set to 100 GHz by setting the thickness of the etalon to, for example, 1 mm. Further, the etalon may be not only a solid etalon but also a gap etalon formed of a pair of parallel flat plates.
[0026]
In the first and second embodiments, a Fabry-Perot etalon filter is used as a narrow-period filter, but a Mach-Zehnder filter may be used. Further, the FSR of the filter having the periodic characteristic is set to be the same as the transmission wavelength (grid) of the wavelength-division multiplexed light, but may be set to an integer fraction thereof. Further, in order to ensure high resolution, it is preferable that the reference wavelength is set between the peak and the valley. The wavelength monitor may include the optical splitter 4 shown in FIG. The optical splitter 4 may fuse an optical fiber to split a part of the light passing through the optical fiber, or may use a beam splitter.
[0027]
【The invention's effect】
As described above in detail, according to the first to fifth aspects of the present invention, it is possible to accurately monitor the wavelength of a light source device serving as a light source of wavelength multiplexed light over a wide range while utilizing the high resolution of an etalon or a Mach-Zehnder filter. can do. Further, the size of the optical system can be extremely reduced, and the reliability can be improved because there are no movable parts. According to the second aspect of the present invention, since the main components of the optical system are held in a constant temperature bath and kept at a constant temperature, a wavelength monitor which is not affected by a temperature change can be provided. Further, according to the wavelength setting device of claims 6 and 7 of the present application, it is not necessary to calculate the reference value for each grid of the wavelength multiplexed light, and the effect that the adjustment processing can be performed extremely easily can be obtained.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an overall configuration of a laser light source device including a wavelength monitor according to a first embodiment of the present invention.
FIG. 2 is a graph showing a wavelength-transmittance characteristic of a normalized slope filter of the present invention.
FIG. 3 is a graph (part 1) showing a normalized wavelength-transmittance characteristic of the etalon according to the present embodiment and a grid of the wavelength multiplexed light thereof.
FIG. 4 is a graph (part 2) showing a normalized wavelength-transmittance characteristic of the etalon according to the present embodiment and a grid of the wavelength-multiplexed light thereof.
FIG. 5 is a flowchart showing an operation of the wavelength monitor according to the first embodiment.
FIG. 6 is a flowchart illustrating wavelength interpolation processing of a wavelength monitor according to the second embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Wavelength variable light source 2 Modulator 3 Attenuator 4 Optical splitter 5, 6 Beam splitter 7 Slope filter 8 Etalon 9 Thermostat 10 Temperature controller 11-13 Light receiving amplifier 14 A / D converter 15 Microcomputer 15A Wavelength monitor 15B Wavelength control Unit 15C light amount control unit 16 memory 17 interface 18, 20 D / A converter 19 LD driver PD1 to PD3 photodiode

Claims (7)

An optical branching unit that branches incident light into first to third branched light,
A first photoelectric conversion unit that directly receives the first branch light branched from the light branch unit;
A slope filter having a characteristic in which transmittance characteristics continuously change with respect to an incident wavelength, and receiving a second branched light branched from the optical branching unit;
A second photoelectric conversion unit that receives a second branch light obtained through the slope filter;
An etalon having a characteristic that fluctuates at a period corresponding to a grid interval of the wavelength multiplexed light, and into which a third branch light branched from the optical branching unit is incident;
A third photoelectric conversion unit that receives third branch light obtained through the etalon,
By calculating a value obtained by normalizing the second photoelectric conversion output with the first photoelectric conversion output, and comparing the calculated value with a normalized output of a slope filter stored in advance, the grid of the incident light is calculated. By selecting a wavelength, normalizing the third photoelectric conversion output with the first photoelectric conversion output, and comparing the selected grid wavelength with a normalized reference value in the etalon, the deviation from the grid. And a wavelength monitor unit for detecting the wavelength of the incident light based on the calculated value.
A wavelength monitor, comprising: a memory for holding a value obtained by normalizing the output of the slope filter and a reference value obtained by normalizing the output of the etalon for each wavelength of light used for wavelength division multiplexing communication. The wavelength according to claim 1, further comprising: a constant temperature layer that holds at least the optical branching unit, the etalon, and the slope filter; and a temperature adjustment unit that keeps the constant temperature layer at a temperature within a predetermined range. monitor. 2. The wavelength monitor according to claim 1, wherein a grid of the wavelength multiplexed light is adjusted to be located at an intermediate position between a peak and a valley of the transmission characteristic of the etalon. The wavelength monitor according to claim 1, wherein the slope filter is a band-pass filter having a center wavelength outside a wavelength change range of incident light. An optical branching unit that branches incident light into first to third branched light,
A first photoelectric conversion unit that directly receives the first branch light branched from the light branch unit;
A slope filter having a characteristic in which transmittance characteristics continuously change with respect to an incident wavelength, and receiving a second branched light branched from the optical branching unit;
A second photoelectric conversion unit that receives a second branch light obtained through the slope filter;
A Mach-Zehnder filter having a characteristic that fluctuates at a cycle corresponding to the grid of the wavelength multiplexed light and transmitting the third branched light branched from the optical branching unit;
A third photoelectric conversion unit that receives third branch light obtained through the Mach-Zehnder filter;
By calculating a value obtained by normalizing the second photoelectric conversion output with the first photoelectric conversion output, and comparing the calculated value with a normalized output of a slope filter stored in advance, the grid of the incident light is calculated. Selecting a wavelength, normalizing the third photoelectric conversion output with the first photoelectric conversion output, and comparing the selected photoelectric conversion output with a normalized reference value of the wavelength of the selected grid in the Mach-Zehnder filter; A wavelength monitor unit that calculates a deviation from the wavelength, and detects a wavelength of the incident light based on the deviation,
A memory for holding a value obtained by normalizing the output of the slope filter and a reference value obtained by normalizing the output of the Mach-Zehnder filter for each wavelength of light used for wavelength division multiplexing communication. Wavelength monitor. A method for setting a reference value of an etalon in the wavelength monitor according to claim 1,
Calculating a normalized output from the etalon obtained when light having a wavelength of one grid of the change range of the wavelength multiplexed light is incident;
Calculate a normalized reference value from an etalon obtained when light of another grid in the change range of the wavelength multiplexed light is incident,
A reference value setting method for a wavelength monitor, wherein a reference value at the wavelength of each grid of the wavelength multiplexed light is calculated by linear interpolation. A method for setting a reference value of a Mach-Zehnder filter in the wavelength monitor according to claim 5,
Calculating a normalized output from the Mach-Zehnder filter obtained when light having a wavelength of one grid in the change range of the wavelength multiplexed light is incident;
Calculate a normalized reference value from a Mach-Zehnder filter obtained when light of another grid in the change range of the wavelength multiplexed light is incident,
A reference value setting method for a wavelength monitor, wherein a reference value at the wavelength of each grid of the wavelength multiplexed light is calculated by linear interpolation.

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