JP3199106B2 - Multi-wavelength light source and optical wavelength multiplex signal generation circuit using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength multiplexing technique used for optical fiber communication.

[0002]

2. Description of the Related Art In recent years, optical fiber communication technology has been developed for transmission capacity,
Significant progress has been made in relay distance. In particular, in optical wavelength division multiplexing communication in which a plurality of optical signals having different wavelengths are multiplexed and transmitted on a single optical fiber, the multiplexing / demultiplexing of a plurality of optical signals is performed using optical filters or optical couplers as optical signals. And does not require fast and complex signal processing,
There is a feature that large-capacity signal transmission can be performed with a simple device configuration.

In the optical wavelength division multiplexing communication, an optical wavelength division multiplexing signal generating circuit is required which modulates each of a plurality of different wavelengths (usually four or more wavelengths), multiplexes them, and inputs them to one optical fiber. Become. FIG. 5 shows a conventional example of an optical wavelength multiplex signal generation circuit. 5, 1-1 to 1-4 light source that outputs light having a wavelength of F ₁₀ ~F _40, 2-1~2-
Reference numeral 4 denotes an optical modulator, 3 denotes an optical multiplexer, and 4 denotes an optical output. For the optical multiplexer 3, an optical filter using a coupler or an arrayed waveguide is used. 5, frequency F ₁₀ to F ₄₀ are aligned at equal intervals on the normal frequency axis. This is because it is easier to design and manufacture an optical filter used for optical wavelength multiplexing / demultiplexing when the frequency interval is equal,
This is because the processing of the communication node can be simplified. The continuous light output from each of the light sources 1-1 to 1-4 in FIG. 5 is signal-modulated by the optical modulators 2-1 to 2-4, respectively, and output after being multiplexed by the optical multiplexer 3. As shown in FIG. 5 on the frequency axis, the frequency F ₁₀ to F ₄₀ are aligned at equal intervals, each wavelength component is assumed to have received a signal modulation. As described above, with the configuration of FIG. 5, it is possible to obtain an optical wavelength multiplexed signal in which each wavelength component is subjected to signal modulation.

[0004]

As described above, the configuration shown in FIG. 5 requires a number of light sources corresponding to the multiplex number. For example, in order to obtain a 16-wave multiplex signal, there is a problem that 16 light sources are required. In addition, an oscillation wavelength control circuit is required for each light source in order to accurately set each light source at an equal interval or an oscillation wavelength designed in advance. As the number of multiplexes increases, a large number of light sources are required. Problem. In the case of separating an optical wavelength division multiplexed signal, an optical wavelength demultiplexer such as an array waveguide for the separation has a slightly different absolute frequency and frequency interval depending on conditions at the time of device fabrication. For this reason, conventionally, there has been a problem in that means for controlling the oscillation frequency of the light source by the number of multiplexes is required in accordance with the conventional technique.

The present invention has been made under such a background, and uses a small number of light sources (two light sources) to generate a plurality (three or more) of wavelength multiplexed signal lights having equal wavelength intervals. It is an object of the present invention to provide a multi-wavelength light source that can perform the operation and an optical wavelength multiplex signal generation circuit using the same.

[0006]

[0007]

[0008]

According to a first aspect of the present invention, there is provided a first light source having an oscillation frequency F1 and a frequency F2.
A second light source having an oscillation frequency of
A plurality of frequency components having an interval equal to the frequency difference ΔF of 2
An optical non-linear medium for generating a light component;
Combining the lights respectively output from the light sources to form the optical non-linear medium
Multiplexing means for inputting to the above, the multiplexing means and the optical nonlinear medium
Multi-wavelength light source consisting of an optical amplification device inserted between
There are, is characterized in that this cascaded two or more pairs and the optical amplifier and the optical nonlinear medium as a set.

[0009] According to a second aspect of the invention, the oscillation frequency
A first light source which is F1 and a frequency F2 is an oscillation frequency.
And a frequency difference ΔF between the frequencies F1 and F2.
Light that generates multiple frequency components with the same spacing as
A nonlinear medium and the first and second light sources, respectively.
A multiplexing device that multiplexes the output light and inputs the multiplexed light to the optical nonlinear medium.
A step, inserted between the multiplexing means and the optical nonlinear medium
A multi-wavelength light source comprising: an optical amplifier; and an optical modulator inserted between a multiplexing unit that multiplexes the two lights and the optical amplifier.

[0010] According to a third aspect of the invention, in the invention described in claim 2 is characterized in that this cascaded two or more pairs and the optical amplifier and the optical nonlinear medium as a set.

Further, an invention according to claim 4, wherein, in the multi-wavelength light source according to claim 2, connected in cascade which two or more sets of the above optical modulator and the optical amplifier and the optical nonlinear medium as a set It is characterized by:

[0012]

[0013]

[0014]

[0015]

[Function] Here, an optical nonlinear medium such as an optical fiber (hereinafter, referred to as an optical fiber)
An operation principle of the present invention will be described in which two lights having frequency components close to each other are input to generate a plurality of lights having new frequency components.

Generally, when three lights having frequencies f _p , f _q , and f _r and propagation constants β _p , β _q , and β _r are input to the optical fiber, the third order induced by these input lights is obtained. Light having a new frequency component is generated through the nonlinear polarization of. This is an optical non-linear phenomenon called four-wave mixing, are new and the frequency of the generated light and F _f, known that the relationship between the frequency and the formula of the input light is satisfied. _{F f = F p + F q} -F r (1)

Even if the frequency of two of the three lights is degenerated, new light is still generated. That is, assuming that two lights having the frequencies F _p and F _q are degenerate (that is, F _p = F _q ) in the equation (1), the frequency of the newly generated light is F _f = 2F _p − F _r (2). Therefore, according to the above configuration of the present invention, two lights having frequencies of F ₁ and F ₂ are input to the optical fiber.
So that the frequency difference from the frequency F ₁ (F ₂ -F ₁₎ apart new frequency components are generated in the optical fiber.

The power P _f of light generated by four-wave mixing is generally expressed by the following equation.

[0019]

(Equation 1)

Here, P _p , P _q , and P _r are the optical powers of the three input lights, α is the loss coefficient of the optical fiber, L is the optical fiber length, β _p , β _q , β _r , and β _f are It shows the propagation constants of three input lights and the generated four-wave mixing light.
△ β in equation (4) is called a phase mismatch amount. When △ β takes a small value, four-wave mixing light is efficiently generated.
In particular, the condition that △ β = 0 is called a phase matching condition, and when this condition is satisfied, the generation efficiency of four-wave mixing light is maximized. However, when a plurality of lights having different frequencies are input to the optical fiber, their propagation constants usually differ from each other due to the dispersion characteristics of the optical fiber, so that it is not possible to take a small value as Δβ.

However, when the frequencies of the plurality of lights are close to each other, the respective propagation constants have substantially the same value.
β can be reduced. Therefore, as can be seen from equation (4), when two lights having large optical power and close frequency components are incident on the optical fiber,
Four-wave mixing light having relatively large optical power is generated. When the optical power of the newly generated light is large, a new four-wave mixing light is further generated using the generated light and the originally input light as seeds. By repeating the process of generating four-wave mixing light by using the two lights originally input to the optical fiber as seeds and generating new four-wave mixing light by using the seeds as a seed, the frequency interval eventually becomes longer. A plurality of optical frequency components equal to the frequency difference (F ₂ −F ₁ ) between the _two initially input lights (frequency F ₁ , F ₂ ) can be generated.

Thus, according to the configuration of the present invention, three or more optical frequency components can be generated using two light sources. Moreover, the generated frequencies are always arranged at equal intervals on the frequency axis, and the interval has a property of being equal to the frequency difference (F ₂ −F ₁ ) between the two light sources. Therefore, there is an advantage that the absolute values of the frequency interval and the generated frequency can be varied by controlling only the oscillation frequencies of the two light sources.

Incidentally, such a multi-frequency light source can also be obtained by passing the output of a white light source through an optical filter having a periodic transmission frequency characteristic. In this case, the spectrum width of the generated light is limited by the optical filter. , And is generally wider than the spectrum width of the light output from the laser. In addition, there is a problem that the spectral components within the spectral width do not correlate with each other and become a noise source when a signal is transmitted. However, the spectral width of the light generated by the multi-frequency light source of the present invention is about the same as the spectral width of the light source used (usually a semiconductor laser).
Coherent light having sufficient spectral purity for signal transmission can be generated.

[0024]

【Example】

Embodiment 1 FIG. 1A is a block diagram showing a configuration of a first embodiment of a multi-wavelength light source according to the present invention.
(B), (c) is a figure which shows the modification of the structure shown to Fig.1 (a). 1A to 1C, 5-1 and 5-
2 generates a light controlled to frequencies F ₁ and F ₂ the oscillation frequency is close to each other the light source, 6 an optical multiplexer, 7 optical amplifiers, 8 optical fibers, 9 denotes an optical output, respectively. However, the multi-wavelength light source shown in FIG. 1A has the optical amplifier 7 omitted and the multiplexer 6 and the optical fiber 8 directly connected. The multi-wavelength light source shown in FIG. An optical amplifier 7 and an optical fiber 8 are connected after the multiplexer 6, and the multi-wavelength light source shown in FIG. One set is made up of a plurality of cascade-connected sets.

Therefore, in the multi-wavelength light source shown in FIG. 1A, the lights output from the light sources 5-1 and 5-2 are multiplexed by the optical multiplexer 6 and then directly input to the optical fiber 8. In the multi-wavelength light source shown in FIG. 1B, the light output from the light sources 5-1 and 5-2 is input to the optical amplifier 7 and the optical fiber 8 via the optical multiplexer 6. Output light 9
Then, in the multi-wavelength light source shown in FIG. 1C, the light output from the light sources 5-1 and 5-2 passes through the optical multiplexer 6 to a plurality of optical amplifiers 7, 7, 7,. Are input to the plurality of optical fibers 8, 8, 8,.

As described above in the "action" section, FIG.
In each configuration shown in (a) to (c), the optical fiber 8
Then, a new frequency component is formed at a frequency position that is an integer multiple of the frequency difference ΔF (F ₂ −F ₁ ) from the frequencies F ₁ and F ₂ by four-wave mixing. Since the generation efficiency of four-wave mixing is proportional to the optical power of the input light, the configuration shown in FIGS. 1B and 1C is newly generated by amplifying the input optical signal by the optical amplifier 7. The optical power of the frequency component can be increased, and a frequency other than F ₁ and F ₂ can be generated with higher efficiency than the configuration shown in FIG. As described above, according to the configuration of the present embodiment illustrated in FIGS. 1A to 1C, it is possible to generate a plurality of optical frequency components arranged at regular intervals equal to the frequency difference between two light sources. .

Embodiment 2 FIGS. 2A to 2C are views showing a second embodiment of the present invention. FIG. 2 (a) to (c)
In the figure, reference numeral 10 denotes an optical frequency selective demultiplexer, which can be realized by, for example, an array filter using an array waveguide or a combination of a coupler and a wavelength filter (hereinafter, referred to as an array filter 10). 11-1 to 11-4 are optical modulators,
12 is an optical multiplexer, 13 is an optical signal output, 14-1 to 14-
Reference numeral 4 denotes an optical amplifier. FIG. 1 (a)
The same components as those shown in (c) are denoted by the same reference numerals.

The embodiment shown in FIGS. 2A to 2C corresponds to FIG.
1 is an optical wavelength multiplex signal generation circuit using each of the multi-wavelength light sources according to the first embodiment of the present invention shown in (a) to (c). First, the output of the multi-wavelength light source according to the first embodiment of the present invention is input to an array filter 10 using an array waveguide. The array filter 10 demultiplexes frequency components arranged at equal intervals on the frequency axis of the input signal, and outputs the signals from different output ports. Therefore, only one frequency component having a different frequency is output from each of the four output terminals of the array filter 10. The optical components thus demultiplexed for each frequency are equalized in optical power by optical amplifiers 14-1 to 14-4, respectively, and are then modulated by optical modulators 11-1 to 11-4 in response to external signals. Each is modulated, multiplexed again by the optical multiplexer 12, and output. As a result, the output optical signal 13 is a signal obtained by multiplexing four optical signals arranged at equal intervals on the frequency axis.

In this embodiment, the array filter 1 is controlled only by controlling the oscillation frequencies F ₁ and F ₂ of the two light sources 5-1 and 5-2.
All frequencies can be matched to the zero characteristic. Therefore, according to the optical wavelength multiplex signal generation circuit of this embodiment, it is possible to efficiently generate an optical wavelength multiplex signal with simple control. As in the first embodiment shown in FIG.
The optical wavelength multiplex signal generation circuit shown in FIG. 2B and FIG.
As compared with that shown in FIG. 2 (a), F ₁ at a higher efficiency,
It can be generated frequencies other than F _2.

[Embodiment 3] FIGS. 3A to 3C are views showing a configuration of a third embodiment of the present invention. In FIG. 3, reference numeral 15 denotes an optical modulator. Further, the other components are denoted by the same reference numerals as the corresponding components shown in FIG. The multi-wavelength light source shown in FIGS.
The optical modulator 15 is arranged at the output end of the optical multiplexer 6 of the multi-wavelength light source shown in (a) to (c). As described in the section of “action”, the optical power of the four-wave mixing light largely depends on the optical power of the input light. The greater the optical power of the input light, the more efficiently four-wave mixing light is generated. However, optical power that can be input to an optical fiber is generally limited by stimulated Brillouin scattering of the fiber. It is known that the intensity of input light can be reduced due to stimulated Brillouin scattering by preliminarily performing intensity modulation or phase modulation on light input to an optical fiber and broadening the spectrum width. Therefore, the light input to the optical fiber 8 can be increased by pre-modulating the light input to the optical fiber 8 with the configuration shown in FIG. 3 and broadening the spectrum width. However, in the case where the intensity modulation is performed here, it is assumed that the modulation is performed so that the RZ signal (all-mark signal) is equivalent to the signal speed when the signal modulation is performed later. In an extreme case, the light input to the optical fiber 8 may be a short optical pulse train. Further, in the configuration shown in FIG. 3C, a plurality of optical modulators 15 can be provided in combination with the optical amplifier 7 and the optical fiber 8.

As described above, according to the configuration of the present embodiment, the optical power that can be input to the optical fiber 8 can be increased by broadening the spectral width of the two lights input to the optical fiber 8, thereby increasing the number of light. The generation efficiency of the wavelength light source can be increased. The two light sources 5-1 and 5
The same effect as described above can also be obtained when the output current is frequency-modulated or phase-modulated by modulating the -2 drive current or by providing an external modulator. FIG. 3B and FIG.
In the multi-wavelength light source shown in FIG. 3C, one or a plurality of optical amplifiers 7 are arranged at the output end of the optical modulator 15, so that FIG.
Can generate frequencies other than F ₁ and F ₂ with higher efficiency than those shown in FIG.

Embodiment 4 FIG. 4 is a view for explaining a fourth embodiment of the present invention. As described in the section of “action”, it is important to reduce the amount of phase mismatch Δβ in order to efficiently generate four-wave mixing light. The gist of the present embodiment is to reduce Δβ and increase the generation efficiency of four-wave mixing light by controlling the dispersion characteristics of the optical fiber used to generate four-wave mixing light. In FIG. 4, (a) is a normal single mode optical fiber, (b)
Represents the dispersion-shifted optical fiber, and (c) and (d) represent the refractive index distribution of the dispersion-flattened optical fiber, respectively. The graph in FIG. 4E shows the dispersion characteristics of the above four types of optical fibers.

An ordinary single mode optical fiber has a dispersion characteristic having a positive slope with respect to the wavelength, and the wavelength at which the dispersion value becomes zero (called the zero dispersion wavelength) is determined by the material of the optical fiber and the waveguide. And is around 1310 nm (see the curve (a) in FIG. 4E). On the other hand, FIG.
It is known that the optical fiber having the refractive index distribution of (b) shifts the zero-dispersion wavelength, and is called a dispersion-shifted fiber. In this dispersion-shifted fiber, the wavelength at which the transmission loss is minimized is around 1550 nm, so that a dispersion-shifted fiber in which the zero dispersion wavelength is shifted to 1550 nm is realized. As shown as a curve (b) in FIG. 4E, the dispersion-shifted fiber also has a dispersion characteristic having a positive slope with respect to the wavelength.

On the other hand, as shown in FIGS. 4 (c) and 4 (d), it is known that by multiplexing the cladding of the optical fiber and selecting an appropriate structural parameter, the structural dispersion can be reversed from the material dispersion. I have. If the parameters shown in FIGS. 4C and 4D are set so that the material dispersion is offset by the structural dispersion in a certain wavelength range (for example, 1550 nm band), the dispersion characteristics over a wide wavelength range as shown in the graph of FIG. Can be made flat and almost zero. In particular, the optical fiber having a refractive index distribution shown in FIG. 4C called a W-shaped fiber has a feature that light is strongly confined in the core and the optical power density in the optical fiber can be increased.

In each of the above-described embodiments, if the optical fiber 8 used for generating four-wave mixing light is a W-shaped fiber as described above, the dispersion value can be made zero regardless of the wavelength in the used wavelength range. The phase matching condition can always be satisfied. That is, the wavelength interval between the input lights 5-1 and 5-2,
The phase matching condition is always satisfied irrespective of the absolute wavelength, and four-wave mixing light can be generated efficiently.

[0036]

As described above, according to the present invention, 2
There is an effect that a plurality of light beams at equal frequency intervals can be easily obtained by using a plurality of light sources.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration (a) and modifications (b) and (c) of a first embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration (a) and modified examples (b) and (c) of a second embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration (a) and modified examples (b) and (c) of a third embodiment of the present invention.

FIG. 4 is a diagram for explaining a fourth embodiment of the present invention.

FIG. 5 is a diagram showing a conventional example of an optical wavelength multiplex signal generation circuit used for optical wavelength multiplex communication.

[Explanation of symbols]

5-1 and 5-2: the light source 6 respectively generating the frequencies F ₁ and F ₂ are close to each other: the optical multiplexer 7: optical amplifier 8: optical fiber 9: Light Output 10: array filter 11 using an array waveguide -1 to 11-4: Optical modulator 12: Optical multiplexer 13: Output optical signal 14-1 to 14-4: Optical amplifier 15: Optical modulator

Continuation of the front page (56) References JP-A-7-58388 (JP, A) JP-A-59-90826 (JP, A) JP-A-6-194697 (JP, A) JP-A-7-79212 (JP) , A) Japanese Patent Publication No. 62-49997 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04J 14/00-14/08 H04B 10/00-10/28 G02F 1/35

Claims (4)

(57) [Claims] An interval between a first light source having an oscillation frequency F1 and a second light source having an oscillation frequency of F2 is equal to the frequency difference ΔF between the frequencies F1 and F2.
And an optical nonlinear medium that generates a plurality of frequency components, and light output from the first and second light sources, respectively.
Multiplexing means for multiplexing and inputting to the optical nonlinear medium, and optical amplification inserted between the multiplexing means and the optical nonlinear medium
A multi-wavelength light source, comprising: a set of the optical amplifying device and the optical non-linear medium; 2. A first light source having an oscillation frequency F1 and a second light source having an oscillation frequency of F2 have an interval equal to the frequency difference ΔF between the frequencies F1 and F2.
And an optical nonlinear medium that generates a plurality of frequency components, and light output from the first and second light sources, respectively.
Multiplexing means for multiplexing and inputting to the optical non- linear medium, and optical amplification inserted between the multiplexing means and the optical non-linear medium
A multi-wavelength light source, comprising: a light modulating device interposed between a multiplexing means for multiplexing the two lights and an optical amplifying device. 3. The multi-wavelength light source according to claim 2, wherein the optical amplifier and the optical nonlinear medium are one set and two or more sets are cascaded. 4. The multi-wavelength light source according to claim 2 , wherein said optical modulator, said optical amplifying device, and said optical nonlinear medium are connected to each other by one.
A multi-wavelength light source comprising two or more cascade-connected sets.