JP2007124019A - Optical transmitter and optical transmission method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter and an optical transmission method for preventing the interference of the signal light of other optical transmitters caused by leakage light from an optical transmitter having a plurality of variable-wavelength light sources. <P>SOLUTION: The optical transmitter uses the plurality of variable-wavelength light sources to extend a variable wavelength area. The optical transmitter has a first variable-wavelength light source for selecting a frequency at a frequency interval Δf, in a first variable-wavelength area to output a light signal; and a second variable-wavelength light source for selecting a frequency at the frequency interval Δf, in a second variable wavelength area to output the light signal. The difference between the frequency selected by the first variable-wavelength light source and that selected by the second one becomes an odd number of multiples of Δf/2, thus preventing the leakage light from one variable-wavelength light source from overlapping with the signal light of the other variable-wavelength light source, and preventing the interference to the signal light of the other variable-wavelength light source. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical transmitter and an optical transmission method in wavelength division multiplexing communication, and more particularly to an optical transmitter and an optical transmission method capable of varying an output wavelength.

In wavelength division multiplexing (WDM) communication, application of an optical transmitter capable of changing the wavelength of output light is being studied. FIG. 1 shows an optical transmission apparatus using a plurality of optical transmitters. The optical transmission device 10 includes 2n optical transmitters 100-1 to 2n, each of which can vary the wavelength, and an optical coupler 140 that multiplexes optical signals from these optical transmitters. Each optical transmitter 100 has frequencies f ₁ , f ₂ , f ₃ ,. . . , F _2n are provided with a wavelength tunable light source that can be set to either of them, and an optical signal whose intensity is modulated at the set frequency is output. These optical transmitters 100-1 to 2n are set to different frequencies and output 2n intensity-modulated optical signals. These intensity-modulated optical signals are combined by the optical coupler 120 and output to the transmission line as WDM signals having 2n wavelengths.

  In this way, by making the light source of each optical transmitter variable in wavelength, it is possible to freely set the wavelength with a single-configuration optical transmission device, and it is possible to make a single device type. By making a single device, it is possible to reduce the inventory of the device when the wavelength is increased. However, the wavelength variable light source has a more complicated structure than a single wavelength light source, and the cost reduction is an issue. For example, a tunable light source using a semiconductor is less expensive than a tunable light source using an external resonator. Examples of such a wavelength tunable light source include a distributed Bragg reflector laser (DBR-LD) and a distributed feedback laser (DFB-LD).

  In the DBR-LD, the refractive index changes according to the amount of current injected into the distributed reflection (DBR) region, and thereby the output wavelength can be changed. The DBR-LD can change the wavelength in the range of 10 nm by changing the refractive index, but when used in a wide variable wavelength range, the output wavelength changes due to the mode hop. Therefore, in order to use as a WDM light source, it is necessary to limit the wavelength variable width to about 3 nm.

  On the other hand, the refractive index of the DFB-LD is changed by temperature control, and the output wavelength can be changed by this. However, the wavelength variable width by the temperature control of the DFB-LD is also limited to 3 nm.

Kasaya et al., “High-Speed Wavelength-Switchable DFB Array Wavelength Selection Light Source”, 2003 Electronics Society Conference of IEICE, C-4-3, pages 279

  As described above, when DBR-LD or DFB-LD is used as the wavelength tunable light source of the optical transmitter, the wavelength tunable width is limited to about 3 nm. Therefore, there is a need for an optical transmitter with an expanded wavelength variable width. FIG. 2 shows a configuration of such an optical transmitter with an expanded wavelength variable width. The optical transmitter 200 includes a power source 210 having two channels, an LD array 220 including two DFB-LDs 221 and 222, a temperature setting unit 230 for controlling the temperature of the LD array, and outputs from the two DFB-LDs. The optical coupler 240 is configured to multiplex light and a modulator 250 that modulates output light from the optical coupler with a data signal.

  The two DFB-LDs 221 and 222 are configured as an LD array 220 on a single semiconductor chip, and are set to a predetermined temperature by a temperature setting unit 230. The two-channel power supply 210 supplies a bias power supply to only one of the DFB-LDs 221 and 222 by an external switching signal. The DFB-LD to which the bias power is supplied oscillates at the frequency set by the temperature setting unit 230 and outputs continuous light. Thus, the optical transmitter 200 can output an optical signal over a wide wavelength band by switching the power supply 210 and setting the temperature of the temperature setting unit 230.

  The output light from these DFB-LDs is sent to the modulator 250 via the optical coupler 240 and is intensity modulated by the data signal. For example, if an electroabsorption (EA) modulator is used as the modulator, high-speed modulation up to 40 Gbps is possible. Further, if a semiconductor amplifier (SOA: Semiconductor Optical Amplifier) modulator is used, the modulation speed is limited to 10 Gbps or less, but the amplification function can amplify and output the output light. Furthermore, the optical coupler 240 and the modulator 250 can also be integrated on the same semiconductor chip as the DFB-LDs 221 and 222.

According to this configuration, the wavelength variable width of the optical transmitter can be expanded by using two DFB-LDs having different wavelength variable regions. In FIG. 2, only the DFB-LD 221 is driven by the two-channel power supply 210, the set temperature is adjusted to T _i by the temperature setting unit 230, and a signal of the frequency f _i is output. Here, the wavelength variable widths of the DFB-LDs 221 and 222 are the temperatures T ₁ , T ₂ , T ₃ ,. . . , T _i,. . . , T _n , frequencies f ₁ , f ₂ , f ₃ ,. . . , F _i,. . . , F _n and frequencies f _{n + 1} , f _{n + 2} , f _{n + 3} ,. . . , F _{n + i,.} . . , F _2n and the frequency intervals Δf are all equal. In this case, for a set temperature T _i, it holds the following relation.
f _{n + i} −f _i = nΔf (1)

Therefore, in FIG. 2, when the set temperature is T _i and the power supply 210 switches from the DFB-LD 221 to the DFB-LD 222, an optical signal having a frequency f _{n + i} is output from the DFB-LD 222.

  However, in this configuration, when the output of the DFB-LD to be turned off leaks due to a failure of the two-channel power supply 210, the leaked light may interfere with the signal light of other optical transmitters. . This problem will be described with reference to FIG. 3A shows an optical spectrum output from 2n optical transmitters having the configuration shown in FIG. 2, and FIG. 3B shows an output from the 2n optical transmitters by an optical coupler. The combined optical spectrum is shown.

As shown in FIG. 3 (a), each optical transmitter is controlled by a two-channel power source and a temperature setting unit from two DFB-LDs # 1 and # 2 having different wavelength tunable regions at a frequency f ₁ , with a frequency interval Δf. f ₂ , f ₃ ,. . . , F _i,. . . , F _{n + i,.} . . , F _2n signal light is output. In this case, in order to output the optical signal of the frequency f _i from the optical transmitter #i selects DFB-LD # 1 via the 2-channel power, the set temperature of the LD array through the temperature setting device 230 adjusted to T _i. At this time (when the set temperature _{T i),} the oscillation frequency of the DFB-LD # 2 not selected becomes _{f n + i.} Similarly, in order to output an optical signal of frequency _{f n + i} from the optical transmitter # n + i, select DFB-LD # 2 through the 2-channel power, set temperature to _{T i} via the temperature setter 230 adjust. At this time (when the set temperature _{T i),} the oscillation frequency of the DFB-LD # 1 not selected becomes _{f i.}

Here, when the two-channel power source of the optical transmitter # n + i fails and a bias current leaks to the DFB-LD # 1 of the optical transmitter # n + i, the frequency f _i from the DFB-LD # 1 of the optical transmitter # n + _i. Leaked light is output. The leakage light becomes interference to the signal light of the frequency _{f i} from DFB-LD # 1 of the optical transmitter #i. That is, the signal light and the leakage light are combined by the optical coupler, and the two lights overlap in the frequency domain as shown in FIG. 3B. As a result, these two lights are not subjected to wavelength demultiplexing by the receiving demultiplexer, but interfere with each other in the optical receiver to cause intensity noise, resulting in degradation of the signal-to-noise ratio (SNR).

  The present invention has been made in view of such a problem, and an object of the present invention is to obstruct the signal output of other optical transmitters by the leaked output from the optical transmitter having a plurality of wavelength variable light sources. It is an object of the present invention to provide an optical transmitter and an optical transmission method.

  In order to achieve the above object, the present invention provides an optical transmitter having a wavelength variable range expanded by switching a plurality of wavelength variable light sources, wherein the first wavelength variable is provided. The first wavelength tunable light source that outputs an optical signal by selecting a frequency at a frequency interval Δf in the band and the second that outputs an optical signal by selecting a frequency at a frequency interval Δf in the second wavelength tunable area And a difference between a frequency selected by the first wavelength tunable light source and a frequency selected by the second wavelength tunable light source is configured not to be an integral multiple of Δf. It is characterized by.

  According to a second aspect of the present invention, in the optical transmitter according to the first aspect, the frequency selected by the first wavelength variable light source and the frequency selected by the second wavelength variable light source. The difference is configured to be an odd multiple of Δf / 2.

  The invention according to claim 3 is the optical transmitter according to claim 1 or 2, wherein the first and second wavelength variable light sources include a DFB-LD that selects a frequency by temperature control. It is characterized by that.

  According to a fourth aspect of the present invention, there is provided the optical transmitter according to the first or second aspect, wherein each of the first and second wavelength variable light sources includes a DBR-LD that selects a frequency by current control. It is characterized by that.

  The invention according to claim 5 is the optical transmitter according to any one of claims 1 to 4, wherein the first and second wavelength variable light sources are integrated on a single semiconductor chip. It is characterized by that.

  The invention according to claim 6 is an optical transmission method in an optical transmitter in which a wavelength tunable range is expanded by switching a plurality of wavelength tunable light sources, and a frequency interval in the wavelength tunable range of the first wavelength tunable light source. Selecting a first frequency with Δf, and selecting a second frequency with a frequency interval Δf in a wavelength tunable region of the second wavelength tunable light source, wherein the second frequency is the first frequency Selecting so that the difference from the frequency does not become an integral multiple of Δf.

  The invention according to claim 7 is the optical transmission method according to claim 6, wherein the second frequency is selected such that a difference from the first frequency is an odd multiple of Δf / 2. It is characterized by that.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 4 shows an example of the optical spectrum of the optical transmitter according to the present invention. FIG. 4 (a) shows an optical spectrum output from 2n optical transmitters according to the present invention, and FIG. 4 (b) shows an output from the 2n optical transmitter according to the present invention combined by an optical coupler. The optical spectrum is shown. The configuration of the optical transmitter according to the first embodiment of the present invention is the same as that of FIG. 2, but the frequency arrangement for each set temperature of DFB-LD # 1 and DFB-LD # 2 of the optical transmitter is different.

In a first embodiment of the present invention, each frequency #n from the optical transmitter # 1 _{f 1, f 2, f 3} ,. . . , F _i,. . . , F _n , and the intervals Δf between the frequencies are all equal. Also, the frequencies of the optical transmitters # n + 1 to # 2n are f _{n + 1} , f _{n + 2} , f _{n + 3} ,. . . , F _{n + i,.} . . , F _2n , and the intervals Δf between the frequencies are all equal. However, the frequency _{f i} of the DFB-LD # 1 of the optical transmitter #i, the interval between the frequency _{f n + i} of the DFB-LD # 2 of the optical transmitter # n + i is the j is a natural number, is set as follows The
f _{n + i} −f _i = nΔf + (j−3 / 2) Δf (2)

In this case, as shown in FIG. 4 (a), each optical transmitter, the 2-channel power and temperature setter, DFB-LD # frequency _f 1 in the frequency interval Δf from _1, f _{2, f} 3,. . . , F _i,. . . , F _n signal light, and output from the DFB-LD # 2 at a frequency interval Δf of f _{n + 1} , f _{n + 2} , f _{n + 3} ,. . . , F _{n + i,.} . . , F _2n signal light is output. However, the frequency arrangement of DFB-LD # 1 and the frequency arrangement of DFB-LD # 2 are not an integer multiple of the frequency interval Δf, and an offset of Δf / 2 is provided. Therefore, in order to output an optical signal having the frequency f _i from the optical transmitter #i, the DFB-LD # 1 is selected via the two-channel power supply, and the set temperature is adjusted to T _i via the temperature setter. . At this time (at the set temperature T _i ), the oscillation frequency of the unselected DFB-LD # 2 is f _{n + i} − (j−3 / 2) Δf (= f _i + nΔf). FIG. 4 shows a case where j = 2. Similarly, in order to output an optical signal of frequency _{f n + i} from the optical transmitter # n + i, select DFB-LD # 2 through the 2-channel power, the set temperature via the temperature setter 230 _{T i} ' Adjust to. At this time (at the set temperature T _i ′), the oscillation frequency of the unselected DFB-LD # 1 is f _i + (j−3 / 2) Δf / 2 (= f _{n + i} −nΔf).

Here, when the two-channel power source of the optical transmitter # n + i fails at the set temperature T _i and a bias current leaks to the DFB-LD # 1 of the optical transmitter # n + i, the DFB-LD of the optical transmitter # n + i The leaked light of # _i + (j−3 / 2) Δf / 2 from # 1 is output. The leakage light is not an interference to an optical signal of a frequency _{f i} from DFB-LD # 1 of the optical transmitter #i. That is, even if these signal light and leakage light are combined by an optical coupler, the two optical signals do not overlap in the frequency domain as shown in FIG. 4B. As a result, the signal light is transmitted by the receiving-side demultiplexer, while the leaked light is attenuated by the receiving-side demultiplexer, and these lights do not interfere with each other in the optical receiver, and the SNR deteriorates. Can be prevented. Even if the crosstalk between channels of the duplexer on the receiving side is large and the leaked light cannot be sufficiently attenuated, the interference noise component due to the leaked light is generated outside the signal band of the signal light. Can be removed by a typical low-pass filter.

FIG. 5 shows the relationship between the output frequency of the optical transmitter according to the present invention and the set temperature. FIG. 5 shows a case where j = 2 in the equation (2). As shown, when the setting temperature is T _1, the output frequency of the DFB-LD # 1 is coincident with the transmission frequency of the demultiplexer of the optical receiver, the output frequency of the DFB-LD # 2 is Does not match. Next, when the set temperature is increased to T ₁ ′, the output frequency of DFB-LD # 1 does not match the transmission frequency of the duplexer of the optical receiver, but the output frequency of DFB-LD # 2 matches. Will come to do. Thus, in the first embodiment of the present invention, it is necessary to change the set temperature by the temperature setter at the same time as switching between DFB-LD # 1 and DFB-LD # 2 by the power source.

  FIG. 6 shows a configuration example of an optical transmitter according to the second embodiment of the present invention. In FIG. 6, unlike FIG. 2, DFB-LD # 1 (621) and DFB-LD # 2 (622) are configured on separate semiconductor chips 620-1 and 620-2, and each has a separate temperature setter 630. -1 and 630-2.

According to the present embodiment, DFB-LD # 1 and DFB-LD # 2 can be independently set in temperature, so that respective frequencies can be arbitrarily combined. For example, in the case of Example 1, the set temperature of _{T i,} and the output frequency of the DFB-LD # 1 and _{f i,} the output frequency of the DFB-LD # 2 inevitably _{f n + i - (j-} 3 / 2) Δf / 2 (= f _i + nΔf). However, in this embodiment, the optical transmitters # 1, # 2, # 3,. . . , #N are frequencies f ₁ , f ₂ , f ₃ ,. . . , F _n and optical transmitters # n + 1, # n + 2, # n + 3,. . . , # 2n are changed to frequencies f _2n , f _2n−1 , f _2n−2,. . . , F _{n + 1} can also be set.

  In this embodiment as well, it is necessary to change the set temperatures by the two temperature setters simultaneously with switching between DFB-LD # 1 and DFB-LD # 2 by the power source.

  Further, as shown in FIG. 7, DBR-LDs 721 and 722 may be used instead of DFB-LDs 621 and 622. In this case, DBR-LDs 721 and 722 are wavelength-set by separate power supply currents 730-1 and 730-2, respectively.

  FIG. 8 shows a configuration example of an optical transmitter according to the third embodiment of the present invention. In this embodiment, a switch 812 is used instead of the two-channel power supply 210 in FIG. 2 for switching between DFB-LD # 1 and DFB-LD # 2.

  The switch 812 includes a branching device and two switches. When the data signal is input to the switch 812, it is branched by the branching device and supplied to one of the DFB-LDs 221 and 222 via the switch. A bias current for oscillating and driving the DFB-LD is added to the data signal. Therefore, the DFB-LD selected by the switch 812 is current-driven (directly modulated) by this data signal, and outputs an intensity modulated signal having a frequency set by the temperature setter 830. The switching of the DFB-LD # 1 and DFB-LD # 2 by the switching signal and the temperature setting device, and the respective frequency settings are the same as in the first embodiment.

  According to this embodiment, the modulator (modulator 250 in FIG. 2) can be omitted by directly modulating the DFB-LD. This makes it possible to reduce the size and price of the optical transmitter.

  As shown in FIG. 9, a bias may be supplied to each DFB-LD from the two-channel power supply 910 without adding a bias current for oscillating the DFB-LD 921 and 922 to the data signal. . In this case, the switch 912 and the power supply 910 are operated in conjunction with each other by the switching signal so that the power is supplied to the DFB-LD to which the data signal is supplied.

  Furthermore, as shown in FIG. 10, two DFB-LDs 1021 and 1022 may be configured on separate semiconductor chips and controlled by separate temperature setting devices 1030-1 and 1030-2, respectively.

  While the present invention has been described with respect to several embodiments, the embodiments described herein are merely illustrative in view of many possible forms to which the principles of the present invention can be applied. It is not intended to limit the scope of the invention. The embodiments illustrated herein can be modified in configuration and details without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

It is a schematic diagram which shows the structure of the optical transmitter using a some optical transmitter. It is a schematic diagram which shows the structure of the optical transmitter which expanded the wavelength variable width. FIG. 3A is a diagram showing the frequency arrangement of a conventional optical transmitter. FIG. 3A shows an optical spectrum at the output of each optical transmitter, and FIG. 3B shows the output of each optical transmitter combined by an optical coupler. The optical spectrum is shown together with the transmission characteristics of the duplexer on the receiving side. FIG. 4A is a diagram showing the frequency arrangement of an optical transmitter according to an embodiment of the present invention, FIG. 4A shows an optical spectrum at the output of each optical transmitter, and FIG. 4B shows the output of each optical transmitter. The optical spectrum after being multiplexed by the optical coupler is shown together with the transmission characteristics of the receiving-side duplexer. It is a figure which shows the adjustment of the frequency by the temperature setting of the optical transmitter by one Example of this invention. It is a schematic diagram which shows the structure of the optical transmitter by another Example of this invention. It is a schematic diagram which shows the structure of the optical transmitter by another Example of this invention. It is a schematic diagram which shows the structure of the optical transmitter by another Example of this invention. It is a schematic diagram which shows the structure of the optical transmitter by another Example of this invention. It is a schematic diagram which shows the structure of the optical transmitter by another Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical transmitter 100-1-2n Optical transmitter 140 Optical coupler 200 Optical transmitter 210 Two channel power supply 220 LD array 221 DFB-LD # 1
222 DFB-LD # 2
230 Temperature Setting Unit 240 Optical Coupler 250 Modulator 600 Optical Transmitter 610 Two Channel Power Supply 620-1, 620-2 LD Chip 621 DFB-LD # 1
622 DFB-LD # 2
630-1, 630-2 Temperature setter 640 Optical coupler 650 Modulator 700 Optical transmitter 710 Two channel power supply 720-1, 720-2 LD chip 721 DBR-LD # 1
722 DBR-LD # 2
730-1, 730-2 Power supply current 740 Optical coupler 750 Modulator 800 Optical transmitter 812 Switch 820 LD array 821 DFB-LD # 1
822 DFB-LD # 2
830 Temperature setter 900 Optical transmitter 910 Two-channel power supply 912 Switch 920 LD array 921 DFB-LD # 1
922 DFB-LD # 2
930 Temperature setter 940 Optical coupler 1000 Optical transmitter 1012 Switcher 1020-1, 1020-2 LD chip 1021 DFB-LD # 1
1022 DFB-LD # 2
1030-1, 1030-2 Temperature setter 1040 Optical coupler

Claims (7)

An optical transmitter that expands a wavelength tunable range by switching a plurality of wavelength tunable light sources,
A first wavelength tunable light source that selects a frequency at a frequency interval Δf in the first wavelength tunable region and outputs an optical signal;
A second wavelength tunable light source that selects a frequency at a frequency interval Δf in the second wavelength tunable region and outputs an optical signal, and a frequency selected by the first wavelength tunable light source, An optical transmitter characterized in that a difference from a frequency selected by a wavelength tunable light source is not an integral multiple of Δf. The optical transmitter according to claim 1,
The light characterized in that the difference between the frequency selected by the first tunable light source and the frequency selected by the second tunable light source is an odd multiple of Δf / 2. Transmitter. The optical transmitter according to claim 1 or 2,
The first and second wavelength tunable light sources each include a DFB-LD that selects a frequency by temperature control. The optical transmitter according to claim 1 or 2,
The first and second wavelength tunable light sources each include a DBR-LD that selects a frequency by current control. An optical transmitter according to any one of claims 1 to 4,
The optical transmitter characterized in that the first and second wavelength variable light sources are integrated on a single semiconductor chip. An optical transmission method in an optical transmitter in which a wavelength tunable range is expanded by switching a plurality of wavelength tunable light sources,
Selecting a first frequency at a frequency interval Δf in the wavelength tunable region of the first wavelength tunable light source;
The step of selecting the second frequency at a frequency interval Δf in the wavelength tunable range of the second wavelength tunable light source, wherein the second frequency is such that the difference from the first frequency is not an integral multiple of Δf. An optical transmission method comprising the steps of: The optical transmission method according to claim 6.
The optical transmission method according to claim 1, wherein the second frequency is selected such that a difference from the first frequency is an odd multiple of Δf / 2.

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