JP3699854B2 - Light source frequency stabilization device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for stabilizing the frequency of a light source used for Wavelength Division Multiplex (hereinafter referred to as WDM) optical communication.
[0002]
[Prior art]
Also in the field of optical communication, an increase in transmission amount is required. In order to cope with this increase in transmission amount, conventionally, a spatial multiplexing method that increases the number of pairs of optical fibers and a method of multiplexing signals in the electrical domain (frequency division multiplexing, time division multiplexing) have been used.
[0003]
However, it is difficult to cope with a sudden increase in transmission amount only by these methods. For this reason, in recent years, development of WDM optical communication technology has been promoted. In this WDM optical communication, a plurality of WDM light sources having different wavelengths are used. And the light of these several light sources injects into one optical fiber. As a result, since a plurality of independent transmission paths can be configured in one optical fiber, it is possible to cope with a sudden increase in transmission amount. In this case, however, a technique for increasing the accuracy and stability of the light source frequency of the WDM light source is indispensable.
[0004]
In order to achieve this object, a technique described in, for example, Sakamoto, Oda et al., “Light source frequency stabilization characteristics using a Mach-Zehnder type filter in a field environment”, 1997 IEICE B-10-216 has been proposed. Yes. In this technique, a plurality of light source frequencies of a WDM light source are fixed at peak points of a plurality of transmission frequencies of a Mach-Zehnder type filter.
[0005]
[Problems to be solved by the invention]
By the way, when using a Mach-Zehnder type filter to stabilize the frequency of the WDM light source, it is necessary to pay attention to the following points.
1. The Mach-Zehnder type filter theoretically has a characteristic that a transmission frequency region and a stop frequency region are repeated in a comb-like manner at equal frequency intervals. However, in practice, the filter characteristics may deviate from the target frequency due to manufacturing errors, polarization dependency, and the like. Therefore, the peak points of the plurality of transmission frequencies and the plurality of light source frequencies of the WDM light source may be shifted.
2. The light source frequency cannot be freely selected because of the restriction due to the comb-like characteristics of the Mach-Zehnder type filter.
3. It is necessary to introduce a plurality of light source frequencies of the WDM light source to the peak points of the plurality of transmission frequencies exhibited by the Mach-Zehnder type filter. For this reason, it is necessary to apply a sine wave voltage to the heater power supply of the filter for dithering. Therefore, in some cases, it may be necessary to consider the influence of temperature changes on peripheral devices. The dithering referred to here is to swing the comb-like characteristic of the Mach-Zehnder filter according to a certain period.
[0006]
[Means for Solving the Problems]
Therefore, the present invention includes a light source that outputs modulated light, and a superimposed signal generation circuit that modulates the intensity of the output light of the light source at a predetermined angular velocity. The optical signal intensity-modulated by the superimposition signal generation circuit is input to the dispersion generator. The dispersion generator generates a delay corresponding to the wavelength of the input optical signal. The phase difference detector detects a phase difference between the optical signal before being input to the dispersion generator and the optical signal output from the dispersion generator. Based on the detected phase difference, the wavelength of the output light of the light source is controlled so that the phase difference becomes a predetermined reference value.
[0007]
Another invention includes a plurality of light sources that output their own modulated light, and a superimposed signal generation circuit that modulates the intensity of the output light of these light sources at an angular velocity that is arbitrarily set uniquely. . The optical signals that have been intensity-modulated by the superimposed signal generation circuit are combined and input to the dispersion generator. The dispersion generator generates a delay corresponding to the wavelength of the input optical signal. The phase difference detector detects a phase difference between the optical signal before being input to the dispersion generator and the optical signal output from the dispersion generator. Based on the detected phase difference, the wavelength of the output light of the light source is controlled so that the phase difference becomes a predetermined reference value.
[0008]
Here, the phase difference detector includes a mixer that mixes an optical signal input to the dispersion generator and a signal output from the dispersion generator, and a filter that extracts a phase difference component from the output of the mixer. The output of this filter is compared with a reference value preset for each frequency of each light source. The comparison result is output as a control signal for controlling the wavelength of the modulated light output from the light source.
[0009]
Still another invention includes a switch for selecting a plurality of light sources that output unique modulated light. In addition, a superimposition signal generation circuit is provided that modulates the intensity of the optical signal selected by the switch at a preset angular velocity. The optical signal intensity-modulated by the superimposition signal generation circuit is input to the dispersion generator. The dispersion generator generates a delay corresponding to the wavelength of the input optical signal. The phase difference detector detects a phase difference between the optical signal before being input to the dispersion generator and the optical signal output from the dispersion generator. Based on the detected phase difference, the wavelength of the output light of the light source is controlled so that the phase difference becomes a predetermined reference value.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail. In the following embodiments, examples in which the present invention is applied to a light source for WDM optical communication will be described. First, FIG. 1 shows a block diagram of a light source for WDM optical communication according to the first embodiment. The light source for WDM optical communication of this embodiment generates optical signal outputs for n channels from Ch1 to Chn.
[0011]
For this reason, the light source for WDM optical communication has n light source substrates Ch1 to Chn. Each of these light source substrates generates an optical signal for one channel out of n channels.
[0012]
That is, the light source substrate Ch1 is equipped with a light source LD1 that generates light of wavelength λ1. The light source substrate Ch1 further includes a temperature control circuit TEC1 and a superimposed signal generation circuit CW1. The output of the temperature control circuit TEC1 is given to the light source LD1. The temperature control circuit TEC1 changes the oscillation frequency of the light source LD by controlling the temperature of the laser chip of the light source LD. This operation will be described in detail later. The superimposed signal generation circuit CW1 gives a sine wave having a predetermined angular velocity ω1 to the light source LD1. As a result, the output of the light source LD1 is intensity-modulated. The optical signal having the wavelength λ1 emitted from the LD1 is given to the external modulator MOD1. At the same time, a part is taken out by the coupler CPL 1 and given to the multiplexer 1.
[0013]
A modulator driver DS1 is connected to the external modulator MOD1. The external modulator MOD1 modulates the light of wavelength λ1 emitted from the LD1 with transmission data. Transmission data for this modulation is given from the modulator driver DS1 to the external modulator MOD1. The output of the external modulator MOD1 is output to the outside as an optical signal output.
[0014]
Hereinafter, the light source substrates Ch2 to Chn also have the same configuration as the light source substrate Ch1. That is, the light source substrate Ch2 includes the light source LD2, the temperature control circuit TEC2, the superimposed signal generation circuit CW2, and the coupler CPL2. Here, LD2 generates light of wavelength λ2. Then, the superimposed signal generation circuit CW2 gives a sine wave having a predetermined angular velocity ω2 to the light source LD2. The same applies to the light source substrate Ch3 and below. Although not shown in the drawing, the light source for WDM optical communication of this embodiment naturally has external modulators MOD2 and MODn and modulator drivers DS2 and DSn corresponding to Ch2 and Chn.
[0015]
Optical signals of wavelengths λ1 to λn output from the respective light source substrates Ch1 to Chn are given to the multiplexer 1. The multiplexer 1 combines these optical signal outputs over all channels to obtain an optical multiplexed signal. This optical multiplexed signal is given to the dispersion generator 2 at the next stage.
[0016]
The dispersion generator 2 generates chromatic dispersion for the optical multiplexed signal from the multiplexer 1. The dispersion generator 2 can be configured using, for example, a dispersion compensating fiber (DCF), a chirped fiber Bragg grating (CFBG), or the like. Details of the configuration of the dispersion generator 2 will be described later. The optically multiplexed signal wavelength-dispersed in this way is supplied to the next-stage light receiver 3.
[0017]
The light receiver 3 converts the wavelength multiplexed optical multiplexed signal into an electrical signal. Therefore, the light receiver 3 includes a photoelectric conversion element. By this photoelectric conversion element, the optical signal is subjected to power-current conversion. The light receiver 3 further has a resistor for performing current-voltage conversion. In the optical receiver 3, the superimposed signal generation circuits CW1 to CWn demodulate the sine waves of the angular velocities (ω1 to ωn) intensity-modulated for the respective channels and multiplex the electric signals (voltages) for n channels. Is output as
[0018]
The amplifier 4 amplifies the electric signal converted by the light receiver 3. The output of the amplifier 4 is supplied to n phase difference detectors P1 to Pn. Each of these phase difference detectors includes a band pass filter BPF, a mixer MIX, and a low pass filter LPF. For example, the phase difference detector P1 includes a band pass filter BPF1, a mixer MIX1, and a low pass filter LPF1. It should be noted that the light receiver 3 and the amplifier 4 may be integrated, for example, like a transimpedance pin-PD amplifier.
[0019]
The band pass filter BPF extracts a sine wave having an angular velocity (ω1 to ωn) predetermined for its own channel from the electrical signal multiplexed for n channels. The mixer MIX mixes (multiplies) the sine wave of the angular velocity (ω1 to ωn) generated by the superimposed signal generation circuit CW of its own channel with the output of the bandpass filter BPF. As a result, the mixer MIX outputs a component whose angular velocity is doubled and a DC component. These components are given to the low-pass filter LPF. The low-pass filter LPF passes only the DC component.
[0020]
Thus, each phase difference detector outputs a direct current component relating to its own channel. N comparators R1 to Rn are connected to the phase difference detectors P1 to Pn in a one-to-one correspondence with each other. Therefore, the direct current component which is the output of each phase difference detector P1-Pn is given to the corresponding comparators R1-Rn. These comparators R1 to Rn compare the output of the corresponding low-pass filter LPF with a reference voltage determined in advance for each channel. Then, the comparison result is used as a control signal and fed back to the temperature control circuits TEC1 to TECn of each channel.
[0021]
The operation of the first embodiment will be described below. In FIG. 1, n light sources LD1 to LDn oscillate optical signal outputs of wavelengths λ1 to λn, respectively. At this time, the superimposed signal generation circuits CW1 to CWn output optical signal outputs of the respective wavelengths (λ1 to λn) with sine waves of angular frequencies (ω1 to ωn) determined in advance for each channel, that is, SINω1t to SINωnt. The intensity is modulated. A part of the intensity-modulated optical signal output is separated by the couplers (CPL1 to CPLn) for each channel and sent to the multiplexer 1. The multiplexer 1 combines a part of this optical signal output over all channels to obtain an optical multiplexed signal.
[0022]
Here, the configuration and principle of the dispersion generator 2 will be described with reference to FIG. Here, FIG. 2A is a diagram showing a configuration of the dispersion generator. FIG. 2B is a diagram showing the characteristics, where the horizontal axis represents the wavelength λ and the vertical axis represents the delay time Td. In this embodiment, a case where CFBG is used as a dispersion generator will be described as an example.
[0023]
The dispersion generator 2 includes a circulator 21 and a CFBG 22. For example, when the circulator 21 has three apertures A, B, and C as shown in the figure, the optical signal input to the aperture A is output to the aperture B, and the optical signal input to the aperture B is Output to the opening C.
[0024]
The CFBG 22 is an optical fiber in which a large number of portions having a large refractive index and a small portion are repeatedly formed at a minute interval in a diffraction grating shape in the length direction in the core portion of the optical fiber. Suppose now that this CFBG 22 is connected to the opening B of the circulator 21. Furthermore, it is assumed that the minute interval of the diffraction grating of the CFBG 22 increases as the distance from the opening B increases.
[0025]
Assume that two optical signals having wavelengths λ1 and λ2 (λ1 <λ2) are input to the aperture A of the dispersion generator 2 having the above configuration. At that time, the λ1, λ2, and the two optical signals output from the aperture B to the CFBG 22 are reflected by a diffraction grating-like portion formed in the CFBG 22. Since the minute interval of the diffraction grating pattern increases as the distance from the aperture B increases, the signal with the wavelength λ2 is delayed more than the signal with the wavelength λ1 and returns to the aperture B. FIG. 2B shows the relationship between the wavelength and the delay time. It will be understood that the delay time Td increases as the wavelength λ increases.
[0026]
The optical multiplexed signal wavelength-dispersed by the dispersion generator 2 is converted into an electric signal by the light receiver 3, further amplified by the amplifier 4, and sent to the phase difference detectors P1 to Pn. When this optical multiplexed signal is converted into an electrical signal by the light receiver 3, the same result is obtained as when the optical signals of wavelengths λ1 to λn are also detected at the same time. Therefore, in the light receiver 3, a multiplexed electric signal in which n sine waves of the outputs SINω1t to SINωnt of the superimposed signal generation circuit CW are multiplexed is obtained. However, as described above, the delay time generated depending on the wavelength of the optical signal of each channel in the dispersion generator 2 is different. Therefore, the absolute phase of θ1 to θn is added to the optical signal of each channel. As a result, the output of the light receiver 3 becomes a multiplexed electric signal that is SIN (ω1t + θ1) to SIN (ωnt + θn).
[0027]
Each band pass filter (BPF1 to BPFn) extracts a sine wave having an angular velocity (ω1 to ωn) determined in advance for its own channel. Hereinafter, for convenience of explanation, explanation will be limited to Ch1. The electrical signal that has passed through the bandpass filter BPF1 is a signal approximated to SIN (ω1t + θ1). Here, θ1 is a value representing the delay time generated in the optical signal Ch1 by the dispersion generator 2 in absolute phase, and the relationship θ1 = 2π · Td / λ1 is established.
[0028]
The mixer MIX1 is provided with a signal approximated to SIN (ω1t + θ1) from the bandpass filter BPF1, and further with SINω1t from the superimposed signal generation circuit CW1. The mixer MIX1 mixes (multiplies) these. As a result, the mixer MIX1 outputs two components of SIN (2ω1t + θ1) and SINθ1 as main components. Here, for simplicity of explanation, it is assumed that the output of the mixer MIX1 is normalized to 1 by adjusting the amplitude. The low-pass filter LP1 that has received the two principal components passes only SINθ1. This SINθ1 is a level signal approximated to a direct current, and the level depends on the delay time Td1 of the wavelength λ1 of the optical signal of Ch1. The output of the low-pass filter LP1 is given to the comparator R1 as a level signal.
[0029]
The comparator R1 compares this level signal with a predetermined reference voltage Vr1. The reference voltage Vr1 is set to a level signal value corresponding to the delay time Td1 (FIG. 2B), that is, SINθ1 = SIN (2π · Td1 / λ1). The output of the comparator R1 is fed back to the temperature control circuit TEC1 as a control signal. Specifically, the temperature control circuit TEC1 operates as follows.
[0030]
When the level signal is smaller than the reference voltage Vr1 (point a in FIG. 2B), the temperature control circuit TEC1 controls the light source LD1 to decrease the oscillation frequency and increase the wavelength λ1 by Δλ. As a result, the delay time Td1 increases by ΔTd and moves toward the point c in FIG. On the other hand, when the level signal is higher than the reference voltage Vr1 (point b in FIG. 2B), the oscillation frequency of the light source LD1 is increased and the wavelength λ1 is controlled to decrease by Δλ. As a result, the delay time Td1 decreases by ΔTd and moves toward the point c in FIG.
[0031]
In this way, the wavelength of the optical signal output of the Ch1 light source LD1 is automatically controlled to λ1. As an actual experimental value, when Ch1 (λ1; 1.55 μm), the frequency of the superimposed signal generation circuit is 100 MHz, and the dispersion characteristic of the dispersion generator 2 is about 1000 pSec / nm, the comparator when λ1 changes by 1 pm The output fluctuation of R1 was about 1 mV. Similarly, the wavelengths of the light sources (LD2 to LDn) up to the following CH2 to CHn are also automatically controlled.
[0032]
As described above, the output wavelengths of the light sources LD1 to LDn are controlled based on the delay time generated by the dispersion generator 2. For this reason, it is not necessary to select the wavelength of the light source in a comb-like manner, and it is possible to select continuously and arbitrarily. Even if a delay time error occurs between the output wavelengths of the light sources LD1 to LDn due to an accuracy error of each component, it is easily achieved by individually adjusting the reference voltages applied to the comparators R1 to Rn. The error can be absorbed.
[0033]
Since a Mach-Zehnder type filter as conventionally used is not required, temperature control required for dithering is not required. As a result, there is no effect due to temperature changes on the peripheral devices. And since an expensive component part is not required compared with a prior art example, it contributes to the cost reduction of an apparatus.
[0034]
Next explained is the second embodiment of the invention. FIG. 3 is a block diagram of a light source for WDM optical communication according to the second embodiment. The light source for WDM optical communication has n light source substrates Ch1 to Chn. Each of these light source substrates generates an optical signal for one channel out of n channels.
[0035]
That is, the light source substrate Ch1 is equipped with a light source LD1 that generates light of wavelength λ1. The light source substrate Ch1 also includes a temperature control circuit TEC1 and a superimposed signal generation circuit CW1. The light source substrate Ch1 further includes an external modulator mod1. The output of the temperature control circuit TEC1 is given to the light source LD1. The optical signal having the wavelength λ1 emitted from the LD1 is given to the external modulator MOD1. At the same time, a part is taken out by the coupler CPL1 and given to the external modulator mod1. The superimposed signal generation circuit CW1 gives a sine wave having a predetermined angular velocity ω1 to the external modulator mod1. The output of the external modulator mod 1 is given to the multiplexer 1.
[0036]
In the first embodiment, the superimposed signal generation circuit CW1 directly modulates the intensity of the light source LD1. On the other hand, in the second embodiment, the coupler CPL1 separates and extracts a part of the optical signal output output from the light source LD1. The extracted optical signal output is externally modulated by the superimposed signal generation circuit CW1 via the external modulator mod1. That is, in the second embodiment, the optical signal output of the light source LD1 is a CW wave (unmodulated wave) that is not intensity-modulated. The light emitted from the light source LD1 is input to the external modulator MOD1 and modulated by the transmission signal, as in the first embodiment.
[0037]
In addition, the light source substrates Ch2 to Chn have the same configuration. Further, although not shown in the figure, the external modulator MOD2 or less MODn and the modulator driver DS2 or less DSn corresponding to Ch2 or less Chn are also the same.
[0038]
Further, the wavelengths given to the multiplexer 1 are multiplexed and input to the dispersion generator, and the subsequent operation is the same as in the first embodiment.
[0039]
In this embodiment, a part of the optical signal output of the light source is separated and extracted by a coupler without directly modulating the intensity. The superposed signal generation circuit externally modulates the separated optical signal output via the external modulators mod1 to modn, so that the oscillation operation of the light source is stabilized and a more stable control effect can be obtained. In addition, it is easy to add this frequency stabilization device to an existing WDM optical communication light source as a retrofit, and it is easy to upgrade the existing equipment.
[0040]
Next explained is the third embodiment of the invention. In this embodiment, the phase difference is not detected for each channel, but the common hardware is used for each channel. A block diagram of the third embodiment is shown in FIG.
[0041]
As shown in FIG. 4, in the light source for WDM optical communication according to the third embodiment, a channel selection unit 8 is connected to the subsequent stage of the light receiver 3 and a delay time detection unit 7 is connected to the subsequent stage. The output of the delay time detector 7 is given to the comparator R1. A reference voltage 9 is further applied to the comparator R1. The output of the comparator R1 is once held in the memory buffer 6. The output of the memory buffer 6 is given to each light source substrate Ch1 to Chn as a control signal for controlling the temperature control circuit TEC of each channel. A sine wave having angular frequencies ω1 to ωn is input to the channel selection unit 8 from the superimposed signal generation circuit mounted on each of the light source substrates Ch1 to Chn. Then, a selection signal that indicates which of the plurality of reference voltages is to be selected is supplied from the channel selection unit 8 to the reference voltage 9.
[0042]
Details of the channel selection unit 8, the delay time detection unit 7, the comparator R1, the reference voltage generation unit 9, and the memory buffer 6 according to the third embodiment will be described below with reference to FIG.
[0043]
The output of the amplifier 4 is input to the channel selector 8. In this channel selector 8, the input from the amplifier 4 is connected to one of a plurality of bandpass filters. That is, the channel selection unit 8 has the same number of bandpass filters as the number of channels from BPF1 to BPFn. A switch SW1 is disposed on the input side of these bandpass filters, and a switch SW2 is disposed on the output side. The switches SW1 and SW2 connect the selected bandpass filter to the amplifier 4 and the delay time detector 7.
[0044]
The channel selection unit 8 has a switch controller 5 for switching the switches SW1 and SW2. The channel selector 8 further includes a switch SW3 for selecting a sine wave given from the plurality of superimposed signal generation circuits CW. The sine wave selected by the switch SW3 is given to the delay time detector 7. Switching of the switch SW3 is also performed by the switch controller 5.
[0045]
The delay time detection unit 7 includes a mixer MIX1 and a low-pass filter LPF1. The mixer MX1 is connected to the outputs of a plurality of bandpass filters BPF. The output of the mixer MX1 is connected to the comparator R1. The reference voltage generator 9 gives 1 of the plurality of reference voltages to the comparator R1. Therefore, the reference voltage generation unit 9 includes a plurality of reference voltages Vr1 to Vrn. Further, a switch SW4 for selecting the reference voltages Vr1 to Vrn is provided. The switch SW4 is switched according to a selection signal given from the switch controller 5.
[0046]
Hereinafter, the switch switching operation will be described. First, when the frequency control of Ch1 is performed, the switch controller 5 connects the switches SW1 and SW2 to the bandpass filter BPF1, the switch SW3 to the superimposed signal generation circuit CW1, and the switch SW4 to the reference voltage Vr1. As a result, a signal wave approximated to SIN (ω1t + θ1) is given to the first input of the mixer MX1. Further, SINω1t is given to the second input of the mixer MX1 from the superimposed signal generation circuit CW1 through the switch SW3. The mixer MIX1 mixes (multiplies) these, and outputs the two components SIN (2ω1t + θ1) and SINθ1 as main components.
[0047]
The output of the mixer MX1 is given to the low pass filter LPF1. The low-pass filter LP1 passes only SINθ1. The output of the low-pass filter LP1 is given to the comparator R1 as a level signal. The comparator R1 compares this level signal with the reference voltage Vr1 given from the reference voltage generator 9. The output of the comparator R1 is fed back to the temperature control circuit TEC1.
[0048]
When the frequency control of Ch1 is thus performed, the switch controller 5 next performs the frequency control of Ch2. Similarly, frequency control of each channel is performed. After frequency control is performed for all channels, the operation returns to Ch1 again and the frequency control operation is repeated. The memory buffer 6 stores the control signal for each channel for one period. That is, the contents of the memory buffer 6 are rewritten when the frequency control is next performed for a certain channel. Since execution of frequency control requires a certain amount of time, the memory buffer 6 is necessary to absorb this time. The above control is based on experience that frequency fluctuation does not occur rapidly and intermittent control is sufficient.
[0049]
As described above, in this embodiment, attention is paid to the fact that the mixer and the low-pass filter included in the n phase difference detectors P1 to Pn are components having the same characteristics in all the channels. Similarly, the comparators R1 to Rn are components having the same characteristics in all channels. Therefore, these common parts are shared by switching between all channels. As a result, the number of parts can be reduced, which can contribute to the cost reduction of the apparatus.
[0050]
Next, a fourth embodiment of the present invention will be described with reference to FIG. Also in this embodiment, the light source for WDM optical communication includes n light source substrates Ch1 to Chn. Here, the light source substrate Ch1 will be described as an example. The light source substrate Ch1 includes a light source LD1 that generates light having a wavelength λ1 and a temperature control circuit TEC1. The optical signal of wavelength λ1 emitted from the light source LD1 is output as the main signal. The light source substrate Ch1 is further mounted with a coupler CPL1 that extracts a part of the optical signal emitted from the light source LD1. That is, in the fourth embodiment, the main signal and the optical signal extracted by the coupler CPL1 are CW light. Hereinafter, the light source substrates Ch2 to Chn have the same configuration.
[0051]
Optical signals taken out by the couplers from the respective light source substrates Ch1 to Chn are given to the optical switch SW5. The optical switch SW5 selects one of the optical signals given from the respective light source substrates Ch1 to Chn and connects it to the external modulator MOD. The external modulator MOD amplitude-modulates the applied CW light. For this reason, a superimposed signal generation circuit Fs is connected to the external modulator MOD.
[0052]
The output of the external modulator MOD is given to the dispersion generator 2 at the next stage. Here, the wavelength-dispersed optical signal is given to the light receiver 3 at the next stage. The optical signal is converted into an electric signal by the light receiver 3, and the electric signal is amplified by the amplifier 4 and then given to the phase difference detector P <b> 1. Since the configuration of the phase difference detector P1 is the same as that in the first or second embodiment, detailed description thereof is omitted.
[0053]
A comparator R1 is connected to the subsequent stage of the phase difference detector P1. A plurality of reference voltages Vr1 to Vrn are supplied to the comparator R1 from the reference voltage generator 9. The output of the comparator R1 is once given to the memory buffer 6 as a control signal and fed back to the temperature control circuits TEC1 to TECn on the respective light source substrates Ch1 to Chn. Since this operation is the same as that described in the third embodiment, a detailed description thereof will be omitted.
[0054]
Also in the fourth embodiment, the switch controller 5 switches the switches. That is, the switch controller 5 connects the switch SW5 to the light source board Ch1 and the switch SW4 to the reference voltage Vr1 when performing frequency control of Ch1. As a result, as in the third embodiment, frequency control can be performed for each channel.
[0055]
In this embodiment, the external modulator for performing amplitude modulation on the CW light of each channel and the superimposed signal generation circuit Fs are also shared. As a result, the number of parts can be further reduced. Further, the optical switch SW5 to the memory buffer 6 can be configured as one module, which contributes to the downsizing of the device.
[0056]
Next explained is the fifth embodiment of the invention. FIG. 7 shows a block diagram of a light source for WDM optical communication according to this embodiment. The WDM optical communication light source of the fifth embodiment has the same configuration as that of the fourth embodiment. However, the difference is that the frequency controller 10 is connected to the superimposed signal generation circuit Fs. Another difference is that the oscillation frequency of the superimposed signal generation circuit Fs is variable. Furthermore, a difference is that a fixed reference voltage Vref is applied to the comparator R1. Furthermore, in the fifth and subsequent embodiments, it is assumed that the wavelengths of the plurality of light sources are arranged at equal intervals, or are arranged at unequal intervals on a fixed grid interval. On the other hand, in the form up to the fourth embodiment, there is no such restriction between a plurality of wavelengths.
[0057]
The superimposition signal generation circuit Fs supplies a sine wave having a period designated by the frequency controller 10 to the external modulator MOD1 and the mixer MIX1. Hereinafter, the operation of this embodiment will be described with reference to FIG. FIG. 8 illustrates the DC output characteristics of the phase difference detector P1. That is, the vertical axis in FIG. 8 represents the output voltage of the phase difference detector P1, and the horizontal axis represents the wavelengths of the light sources LD1 to LDn on the light source substrates Ch1 to Chn.
[0058]
If the slope of the delay characteristic of the dispersion generator 2 is A (psec / nm) and the frequency interval of the light source of each channel is B (nm), the delay time difference between the channels Y = A · B (psec). . Further, as described above, since a sine wave having a fixed period is given to the mixer MIX1, the DC output characteristic of the phase difference detector P1 also becomes a periodic function as shown in FIG. Therefore, by appropriately setting the frequency of the superimposed signal, the period Ts of the superimposed signal and the delay time difference Y (the period of the DC output characteristic of the phase difference detector) can be made equal. At this time, the interval between the DC output characteristics of the phase difference detector P1 and the reference voltage is constant B (nm) regardless of the level of the reference voltage Vref.
[0059]
This is shown in FIG. FIG. 8 shows two types of reference voltages Vref1 and Vref2. It will be understood that the crossing interval between the DC output characteristic of the phase difference detector P1 and the reference voltage is constant B (nm) regardless of the reference voltage. The frequency Fs of the superimposed signal at this time can be expressed by the following equation (1).
Fs = (1 / Ts) = (1 / Y) = 1 * 10 ^ 12 / A · B (Hz)
Formula (1)
[0060]
As described above, in the fifth embodiment, the oscillation frequency of the superimposed signal generation circuit is made variable, and the frequency controller for controlling the oscillation frequency is provided. Then, the frequency of the superimposed signal Fs was set to a specific frequency determined by the wavelength intervals of the plurality of light sources and the period of the group delay characteristic and phase difference detection characteristic of the dispersion generator. Thereby, in the wavelength control process of each channel, the reference voltage given to the comparator R1 can be shared. Furthermore, when changing the wavelength interval of each multiplexed light, it is only necessary to change the frequency of the superimposed signal without resetting the reference voltage for each light individually.
[0061]
FIG. 9 shows another example of the phase difference detector P1 for the fifth embodiment. FIG. 9A is a block diagram showing another example of the phase difference detector P1, and FIG. 9B shows its DC output characteristics.
[0062]
In the DC output characteristic shown in FIG. 8, the interval between the DC output characteristic and the reference voltage is constant regardless of the value of the reference voltage Vref. However, for example, when the reference voltage is Vref2, the amount of change (slope) at the intersection is small. In such a case, the detection sensitivity of the phase difference is deteriorated, and the accuracy of frequency control may be reduced. Therefore, in this modification, a configuration is provided in which the accuracy of control can be made constant regardless of the level of the reference voltage.
[0063]
That is, in the phase difference detector P1 according to this modification, the limiter amplifier LIM1 is disposed at the subsequent stage of the band pass filter BPF. The output of the limiter amplifier LIM1 is input to the mixer MIX1. Further, the phase difference detector P1 inputs the output of the superimposed signal generator Fs to the limiter amplifier LIM2. Then, the output of the limiter amplifier LIM2 is given to the mixer MIX1.
[0064]
In these limiter amplifiers LIM1 and LIM2, the inputted sine wave is converted into a pulse wave of duty 50. These pulse waves are input to the mixer MIX1. As a result of the multiplication of these pulse waves by the mixer MIX1, the output of the mixer MIX1 becomes a rectangular wave as shown in FIG. 9B. This is the output of the phase difference detector P1. Therefore, regardless of the level of the reference voltage, the amount of change (slope) at the intersection point is constant. Therefore, the detection sensitivity of the phase difference is constant, and the accuracy of frequency control is ensured.
[0065]
A further modification of the fifth embodiment will be described with reference to FIG. In this modified example, the inverting switch 11 is arranged at the subsequent stage of the phase difference detector P1. The switch controller 5 also controls the inverting switch 11 together with the optical switch SW5. The inverting switch 11 switches the output side of the output of the low-pass filter LPF and the reference voltage. In this modification, the oscillation frequency of the superimposed signal generation circuit Fs is set to Fs / 2. Further, the output of the superimposed signal generation circuit Fs is given to the limiter amplifier LIM2 via the phase adjuster 12. The phase adjuster 12 delays the output of the superimposed signal generation circuit Fs by a fixed time.
[0066]
In general, an oscillator having a low oscillation frequency is easy to configure, but the higher the oscillation frequency, the more expensive the oscillator and the greater the complexity of the oscillator configuration. Therefore, in this modified example, the frequency of the superimposed signal is halved to reduce the circuit scale. The operation of this modification will be described below with reference to FIG. FIG. 11A is a diagram showing the DC output characteristics of the phase difference detector P1, and FIG. 11B is a waveform diagram for explaining the operation of the inverting switch 11. FIG.
[0067]
First, the reference voltage Vref is determined so as to be in the middle of the amplitude of the phase difference detection voltage characteristic. This is the state indicated by the one-dot chain line in FIG. Next, the phase adjuster 12 adjusts the delay amount so that the intersection of the phase difference detection voltage characteristic and the reference voltage matches the target multiplexed light wavelength. This is the state shown by the solid line in FIG.
[0068]
Here, since the frequency of the superimposed signal generator Fs is halved, as understood from FIG. 11A, the point where the reference voltage Vref intersects in the region where the phase difference detection voltage is increased is an odd channel. For this reason, the points that cross the reference voltage Vref in the region where the phase difference detection voltage is reduced are the stabilization points for the even channels. Therefore, it is necessary to invert the control signal applied to the temperature control circuit TEC between the odd channel and the even channel.
[0069]
Therefore, the switch controller 5 connects the inverting switch 11 to the A side when connecting odd-numbered light source substrates, that is, Ch1, Ch3, Ch5... To the external modulator. On the other hand, when connecting even-numbered light source substrates, that is, Ch2, Ch4, Ch6... To the external modulator, the inverting switch 11 is connected to the B side.
[0070]
As a result, as shown in FIG. 11B, the output voltage of the comparator R1 is determined. For example, when odd channel control is performed, the output of the phase difference detector P1 is applied to the non-inverting input of the comparator R1, and the reference voltage is applied to the inverting input of the comparator R1. Here, for example, when the output frequency of the light source LD is shifted to the longer wavelength side from the stabilization point, the comparator R1 outputs a positive voltage (see FIG. 11B). In response to this, negative feedback is provided to the temperature control circuit TEC. As a result, the temperature control circuit TEC lowers the temperature of the Peltier element arranged immediately below the light source LD. Therefore, the oscillation frequency of the light source LD is increased and the wavelength is shortened.
[0071]
On the other hand, when the output frequency of the light source LD is shifted to the short wavelength side from the stabilization point in the control of the odd channel, the comparator R1 outputs a negative voltage (see FIG. 11B). In response to this, positive feedback is provided to the temperature control circuit TEC. As a result, the temperature control circuit TEC increases the temperature of the Peltier element disposed immediately below the light source LD. Therefore, the oscillation frequency of the light source LD is lowered and the wavelength is lengthened.
[0072]
This is the reverse of the case described above when even channel control is performed. By performing control in this way, it is possible to use the areas on the increasing side and decreasing side of the rectangular wave that is the output of the phase difference detector P1.
[0073]
In this modification, the frequency of the superimposed signal can be halved by using the areas on the increasing side and the decreasing side of the rectangular wave. Therefore, the configuration of the oscillator can be simplified. If an inverting switch is added to an existing system, the wavelength multiplexing interval can be made close to the previous half. In other words, a system that multiplexes twice as many wavelengths as before without changing the configuration of other parts.
[0074]
Finally, another structure of the light source substrate will be described with reference to FIG. This configuration uses the backward emission light of the light source LD. That is, the light source LD mounted on each light source substrate emits front light and back light. The front light is output as it is as the main signal.
[0075]
On the light source substrate, a lens for condensing the backlight is disposed. The back light collected by this lens is coupled to the fiber FB. This fiber FB is guided to the optical switch SW. According to this configuration, the coupler CPL is not necessary. For this reason, the effect that the loss does not occur in the main signal can be obtained. In addition, when a polarization-dependent device such as an LN modulator is connected after the light source LD, an expensive constant polarization coupler is required as a coupler.
[0076]
Although the wavelength division multiplexing light source has been described above, the present invention is not limited to the wavelength division multiplexing light source. Even when n = 1, the present invention can be applied in the same manner as described above. Moreover, although the above description demonstrated limited to the structure provided with a circulator and CFBG22 as a dispersion | distribution generator, this invention is not limited to such a structure. Any element having chromatic dispersion characteristics may be used, and for example, a dispersion compensating fiber used for compensating chromatic dispersion generated in an optical communication network may be used.
[0077]
Further, the superimposed signal generation circuit has been described as a part that modulates the intensity of the output of the light source with a sine wave having a predetermined angular velocity (ω1 to ωn) for each channel. It is not limited. The intensity modulation may be any marker that can identify the output of the light source for each channel. For example, it may be a pulse signal having a different repetition frequency for each channel. However, the phase detector in this case needs to detect a positional deviation of pulse signals having different repetition frequencies for each channel.
[0078]
If the number of channels of the optical switch and the number of reference voltages are set in advance, it is easy to increase the number of multiplexing later. In addition, the number of multiplexing can be easily added.
[0079]
【The invention's effect】
As described above in detail, in the present invention, the output wavelength of the light source is controlled based on the delay time generated by the dispersion generator. For this reason, it is not necessary to select the wavelength of the light source in a comb-like manner, and it is possible to select continuously and arbitrarily.
[Brief description of the drawings]
FIG. 1 is a block diagram of a light source for WDM optical communication according to a first embodiment of the present invention.
FIG. 2 is a diagram illustrating the configuration and principle of a dispersion generator.
FIG. 3 is a block diagram of a light source for WDM optical communication according to a second embodiment of the present invention.
FIG. 4 is a block diagram of a light source for WDM optical communication according to a third embodiment of the present invention.
FIG. 5 is a diagram showing details of a third embodiment.
FIG. 6 is a block diagram of a light source for WDM optical communication according to a fourth embodiment of the present invention.
FIG. 7 is a block diagram of a light source for WDM optical communication according to a fifth embodiment of the present invention.
FIG. 8 is a diagram illustrating output characteristics of a phase difference detector.
FIG. 9 is a block diagram showing another example of a phase difference detector and output characteristics.
FIG. 10 is a diagram showing a modified example of the fifth embodiment.
FIG. 11 is a diagram for explaining the operation of the modified example shown in FIG. 10;
FIG. 12 is a block diagram illustrating another example of a light source substrate.
[Explanation of symbols]
1 multiplexer
2 Dispersion generator
3 Receiver
4 Amplifier
5 Switch controller
6 Memory buffer
10 Frequency controller
11 Reverse switch
BPF1 to BPFn Bandpass filter
Ch1 to Chn Light source board
CPL1-CPLn coupler
DS1-DSn modulator driver
LD1 to LDn Light source
LPF1 to LPFn Low pass filter
MIX1-MIXn mixer
MOD1-MODn External modulator
P1-Pn phase difference detector
R1-Rn comparator
TEC1 to TECn Temperature control circuit
Vr1 to Vrn Reference voltage

Claims (6)

A light source that outputs the modulated light;
A superimposed signal generation circuit that modulates the intensity of the output light of the light source at an arbitrarily set constant angular velocity;
A dispersion generator that receives an optical signal intensity-modulated by the superimposed signal generation circuit and generates a delay according to the wavelength of the optical signal;
A phase difference detector for detecting a phase difference between the optical signal input to the dispersion generator and an optical signal output from the dispersion generator;
A frequency stabilization device for a light source, comprising a control circuit for controlling a wavelength of output light of the light source so that the phase difference becomes a predetermined reference value. A plurality of light sources that output modulated light of a specific wavelength;
A superimposed signal generation circuit that modulates the intensity of the output light of each light source at a different angular velocity;
An optical signal whose intensity is modulated by the superimposed signal generation circuit is input, and a multiplexer that multiplexes these optical signals;
A dispersion generator that receives the output of the multiplexer and generates a delay according to the wavelength of each optical signal;
A phase difference detector for detecting a phase difference between the optical signal input to the dispersion generator and an optical signal output from the dispersion generator;
A frequency stabilization device for a light source, comprising a control circuit for controlling a wavelength of output light of the light source so that the phase difference becomes a predetermined reference value. The frequency stabilization device for a light source according to claim 1 or 2, wherein the phase difference detector includes:
A mixer for mixing the optical signal input to the dispersion generator and the signal output from the dispersion generator;
A filter that extracts a phase difference component from the output of the mixer;
The output of the filter is compared with a reference value preset for each frequency of each light source, and the difference is output as a control signal for controlling the wavelength of the modulated light output from the light source. A frequency stabilization device for the light source. The frequency stabilization device for a light source according to claim 2, wherein the phase difference detector includes:
A plurality of bandpass filters which are provided with outputs of the dispersion generator and separate and extract a plurality of output lights intensity-modulated at the respective angular velocities;
A switch for selecting the outputs of the plurality of bandpass filters and supplying the same to the mixer;
A mixer for mixing a signal input to the dispersion generator and a signal output from the switch;
A filter that extracts a phase difference component from the output of the mixer;
The output of this filter is compared with a reference value preset for each frequency of each light source, and the difference is output as a control signal for controlling the wavelength of the modulated light output from each light source. A frequency stabilization device for the light source. A plurality of light sources that output modulated light of a specific wavelength;
A switch for selecting the output of these light sources,
A superimposed signal generation circuit for intensity-modulating the output of the switch at a predetermined angular velocity;
A dispersion generator that receives the output of the superimposed signal generation circuit and generates a delay according to the wavelength of each optical signal;
A phase difference detector for detecting a phase difference between the optical signal input to the dispersion generator and an optical signal output from the dispersion generator;
A frequency stabilization device for a light source, comprising a control circuit for controlling a wavelength of output light of the light source so that the phase difference becomes a predetermined reference value. In the frequency stabilization apparatus of the light source of Claim 4 or Claim 5,
The switch has more input side and output side terminals than the number of the plurality of light sources or the plurality of band pass filters,
The phase difference detector has a reference voltage larger than the number of the plurality of light sources or the plurality of bandpass filters.

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