JPH10186425A - Light frequency tracking device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an full light processing type light frequency tracking device, in which the frequency interval wanted to be set is not limited by the band of an electric circuit, by providing a wavelength converter having specific functions. SOLUTION: First wavelength converter to n-th wavelength conveter respectively performing wavelength conversions two times are cascaded. A first four- wave mixture generating means 13-1 performs the wavelength conversion of an input signal light beam by a four-wave mixture with a first reference light beam. Here, the light frequency of the first reference light beam is set so that light beams are arranged in the order of the input signal light beam, the first reference light beam and a first wavelength converted light beam (an intermediate output light beam). A second four-wave mixture generating means 13-2 performs the wavelength conversion of the intermediate output light beam by a four-wave mixture with the intermediate output light beam and a second reference light beam. Here, the light frequency of the second reference light beam is set so that light beams are arranged in the order of the intermediate output light beam, the second reference light beam and a second wavelength converted light beam (an output light beam).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical frequency tracking device for generating light whose input signal light is controlled at a constant frequency interval. Such light is used as excitation light for a narrow-band optical amplifier using parametric amplification, stimulated Brillouin scattering, or the like, and is controlled so that the frequency interval of input signal light is constant at 10 GHz or more.

[0002]

2. Description of the Related Art Parametric amplification and stimulated Brillouin scattering in an optical fiber provide a narrow band gain at a specific optical frequency satisfying a law of conservation of energy with pump light. A narrow-band optical amplifier is configured using this property. In particular, parametric amplification is expected to be put to practical use as a low-noise optical amplifier.

[0003] In such a narrow-band optical amplifier, the optical frequency of the pump light must always be kept constant with respect to the input signal light. That is, when the optical frequency of the input signal light fluctuates with time, it is necessary to control the optical frequency of the pump light accordingly. Such an apparatus that generates pump light that is maintained at a constant frequency interval for input signal light whose optical frequency fluctuates with time is called an optical frequency tracking apparatus. However, the pump light is not necessarily CW light, and when the input signal light has a single protruding carrier component, it is also possible to convert the wavelength of the input signal light into the pump light (see References). References: K. Kubota and M. Ohtsu,
"Frequency Offset Locking of AlGaAs Semiconductor L
asers ", IEEE J. Quantum Electron., Vol.QE-23, pp.38
3-393, 1987).

FIG. 6 shows a configuration of a conventional optical frequency tracking device using a frequency discriminator. In the figure, the input signal light and the output light of the wavelength variable light source 51 are multiplexed by a multiplexer 52, and the multiplexed light is converted into an electric signal by a photoelectric converter 53 to become an intermediate frequency signal. This intermediate frequency signal is
The input signal light and the tunable light source 51 are input to a frequency discriminator constituted by a mixer 54 and a delay circuit (τ)
Is converted into the magnitude of the DC voltage. The wavelength controller 56 controls the excitation current and temperature of the variable wavelength light source 51 based on the DC voltage, and controls the output light frequency. Thereby, the output light frequency of the wavelength tunable light source 51 is controlled so as to have a predetermined frequency interval with respect to the input signal light. The output light of the wavelength tunable light source 51 is output via the splitter 57.

[0005]

However, in such a configuration of the conventional optical frequency tracking device, the frequency interval to be set is limited by the photoelectric converter 53 and the mixer 5.
4 band. An object of the present invention is to provide an all-optical processing type optical frequency tracking device in which a frequency interval to be set is not limited by a band of an electric circuit.

[0006]

SUMMARY OF THE INVENTION An optical frequency tracking apparatus according to the present invention generates a first wavelength-converted light by four-wave mixing of an input signal light and a first reference light having a constant wavelength. A second wavelength-converted light is generated by four-wave mixing of the wavelength-converted light and a second reference light having a constant wavelength, and the second wavelength-converted light is output light having a constant wavelength interval with respect to the input signal light. Is output. However, the input signal light, the first reference light, and the first wavelength-converted light are arranged on the wavelength axis.
In addition, the wavelengths of the first reference light and the second reference light are set for the input signal light so as to form an arrangement of the first wavelength converted light, the second reference light, and the second wavelength converted light.

Further, the two cascade-connected two wavelength converters constitute one wavelength converter, and a plurality of wavelength converters are cascaded using output light of the wavelength converter as input signal light of the next wavelength converter. You may connect. Here, the principle that the optical frequency tracking device of the present invention can obtain output light having a constant frequency interval with respect to input signal light will be described.

The wavelength of the input signal light is converted by four-wave mixing of the input signal light and the first reference light. Here, on the optical frequency axis, the first signal light, the first reference light, and the first wavelength-converted light (hereinafter, referred to as “intermediate output light”) are arranged in the order of the first.
The optical frequency of the reference light is set. The optical frequency of the intermediate output light becomes lower (or higher) as the optical frequency of the input signal light becomes higher (or lower). Further, the wavelength of the intermediate output light is converted by four-wave mixing of the intermediate output light and the second reference light. Similarly, the optical frequency of the second wavelength-converted light (hereinafter referred to as “output light”) becomes higher (or lower) as the optical frequency of the intermediate output light becomes lower (or higher).

As described above, by performing the wavelength conversion twice, the direction and amount of fluctuation of the optical frequency of the input signal light and the direction and amount of fluctuation of the optical frequency of the output light become the same, and the input signal light is changed. The frequency interval between the light and the output light can be kept constant. Similarly, the output light after the input signal light has been subjected to a total of even number of wavelength conversions always maintains a constant frequency interval with respect to the input signal light. By making this output light the final output light, it is possible to obtain output light having a constant frequency interval with respect to the input signal light.

[0010]

FIG. 1 shows an embodiment of an optical frequency tracking device according to the present invention. In this embodiment, a configuration is shown in which a first wavelength converter to an n-th wavelength converter (n is a natural number) that perform wavelength conversion twice are connected in cascade.
The first wavelength converter includes a first reference light output unit 11-1 that outputs a first reference light having a constant wavelength, an input signal light, and a first reference light.
A first multiplexing means 12-1 for multiplexing the reference light, a first four-wave mixing generating means 13-1 for generating four-wave mixing by both lights, and an intermediate output obtained by wavelength-converting the input signal light. First wavelength selecting means 14-1 for selectively passing only light
And first optical amplification means 15-1 for amplifying the intermediate output light
A second reference light output unit 11-2 for outputting a second reference light having a constant wavelength, a second combining unit 12-2 for combining the intermediate output light and the second reference light, A second four-wave mixing generating means 13-2 for generating four-wave mixing with the light of the second wavelength, a second wavelength selecting means 14-2 for selectively passing only the output light obtained by wavelength-converting the intermediate output light, A second light amplification unit 15-2 for amplifying light. The same applies to the second to n-th wavelength converters.

The feature of the present embodiment is that the first four-wave mixing generating means 13-1 and the second four-wave mixing generating means 13-2 for performing wavelength conversion by four-wave mixing are provided in each wavelength converter. In this case, the wavelength conversion is performed an even number of times in total. FIG. 2 shows a first operation example in one wavelength converter.
In the figure, the horizontal axis indicates the optical frequency. Since the wavelength is inversely proportional to the optical frequency, when viewed in terms of wavelength, the left side in the figure is the long wavelength side and the right side is the short wavelength side. For example, the low frequency side is the long wavelength side.

Here, a case where a frequency offset lock is applied to the low frequency side of the input signal light will be described. (a) Relationship between input signal light and output light at time t _{1 In} the first four-wave mixing generating means, the input signal light and the reference light are four-wave mixed. The optical frequency of the reference beam is fixed to [nu _1. ν ₁ needs to be lower than the lowest possible optical frequency of the input signal light. It is also assumed that the optical frequencies of the input signal light and the reference light have a difference of ΔFa. In the first wavelength selection means, only the four-wave mixing component on the low frequency side is taken out, and this is used as intermediate output light. It has an optical frequency of ν ₁ -ΔFa. This intermediate output light is input to the second four-wave mixing generator.

In the second four-wave mixing generating means, the intermediate output light and the reference light are four-wave mixed. The optical frequency of this reference light is fixed at ν ₂ . ν ₂ needs to be lower than the lowest optical frequency that the intermediate output light can take. It is also assumed that the optical frequencies of the intermediate output light and the reference light have a difference of ΔFb. In the second wavelength selecting means, only the four-wave mixing component on the low frequency side is taken out and used as output light. It has an optical frequency of ν ₂ -ΔFb. Here, since ΔFa + ΔFb = ν ₁ −ν ₂ , the frequency interval between the input signal light and the output light is 2 (ν ₁ −
ν ₂ ).

[0014] In (b) the time t ₂ the input signal light and the output light of the relationship between the time t ₂ in, and the optical frequency of the input signal light is increased by alpha. On the other hand, the optical frequencies ν ₁ and ν _{2 of the} two reference lights
Since is kept constant, the optical frequency of the intermediate output light decreases by α as compared with the time t _1, and the optical frequency of the output light increases by α as compared with the time t ₁ . Accordingly, the optical frequencies of the input signal light and the output light both increase by α, and the frequency interval is kept constant at 2 (ν ₁ −ν ₂ ). Since this operation does not depend on the sign of α, it is possible to obtain output light that is frequency offset locked with respect to the input signal light.

When the frequency interval to be finally set is large, the efficiency of four-wave mixing is reduced in a configuration with one wavelength converter. However, the frequency interval is increased stepwise by using a plurality of wavelength converters. A configuration that finally achieves the frequency interval to be set may be used. When the frequency interval to be finally set is close to the full width of the fluctuation of the optical frequency of the input signal light or is 100 GHz or less, the reference light is shifted to the low frequency side (or high frequency side) with respect to the input signal light. Side) and setting the reference light to the high frequency side (or low frequency side) with respect to the intermediate output light, it is possible to finally reach the frequency interval to be set. The following is an example.

FIG. 3 shows a second operation example in one wavelength converter. The optical frequency of the intermediate output light output from the first four-wave mixing generating means is ν ₁ -ΔFa. By setting the reference light input to the second four-wave mixing generating means on the high frequency side with respect to the intermediate output light, the optical frequency of the output light becomes ν ₂ + ΔFb. Here, ΔFa−ΔFb = | ν
₁₋ ν ₂ |, the frequency interval between the input signal light and the output light is | 2 (ν ₁ −ν ₂ ) |. As a result, it is possible to obtain output light whose frequency offset is locked at a small frequency interval with respect to the input signal light.

Similarly, by combining the frequency offset lock on the low frequency side and the frequency offset lock on the high frequency side, the frequency interval finally set can be reached. The following is an example.
FIG. 4 shows an operation example in two wavelength converters. First
The operation of the wavelength converter is the same as that of FIG. 2, the output light thereof is frequency offset-locked to the low frequency side of the input signal light, and the frequency interval is 2 (ν ₁ −ν ₂ ). The operation of the second wavelength converter is the same as that of FIG. 2 except that the selected four-wave mixing component is on the high frequency side, and the output light is shifted to the high frequency side of the output light of the first wavelength converter. It is offset locked and its frequency interval is 2 (ν ₄ −ν ₃ ). Therefore, the frequency interval between the output light of the second wavelength converter and the input signal light is | 2 (ν ₁ −ν ₂ ) −2 (ν ₄ −ν ₃ ) |.

By making the output light of the second wavelength converter a final output, it is possible to obtain output light whose frequency offset is locked at a small frequency interval with respect to the input signal light.

[0019]

FIG. 5 shows a specific embodiment of the wavelength converter. Here, the correspondence with the embodiment shown in FIG. 1 is shown. A first wavelength stabilized light source 21-1 is used as the first reference light output unit 11-1. First multiplexing means 12-
As 1, the first optical coupler 22-1 is used. As the first four-wave mixing generating means 13-1, a first wavelength multiplex coupler 23-1, a first pump light source 24-1 and a first distributed gain fiber 25-1 are used. It should be noted that a distributed gain fiber doped with a rare earth element over several kilometers efficiently generates four-wave mixing even for input light having a wavelength apart from the zero-dispersion wavelength, and is suitable for this purpose. A first optical bandpass filter 26-1 is used as the first wavelength selection unit 14-1. As the first optical amplification unit 15-1,
The second optical amplifier 27-1 is used.

As the second reference light output means 11-2, a second wavelength stabilized light source 21-2 is used. A second optical coupler 22-2 is used as the second multiplexing means 12-2. As the second four-wave mixing generating means 13-2, a second wavelength multiplex coupler 23-2, a second pump light source 24-2, and a second distributed gain fiber 25-2 are used. It should be noted that a distributed gain fiber doped with a rare earth element over several kilometers efficiently generates four-wave mixing even for input light having a wavelength apart from the zero-dispersion wavelength, and is suitable for this purpose. Second
The second optical bandpass filter 26-2 is used as the wavelength selecting means 14-2. The second optical amplifier 27-2 is used as the second optical amplifier 15-2.

Hereinafter, a case where the frequency offset lock is applied to the input signal light on the low frequency side as shown in FIG. 2 will be described. The optical frequency of the input signal light and f _in + α. α takes a positive or negative value and represents fluctuation of the optical frequency. The frequency interval to be finally achieved is f _ofs (= 2 (ν ₁ −
ν ₂ )), which is sufficiently larger than the amplitude of α.

The first wavelength-stabilized light source 21-1 outputs, for example, reference light having an optical frequency ν ₁ = f _in −f _ofs / 4. The reference light and the input signal light are supplied to the first optical coupler 22-
They are multiplexed by 1. The first excitation light source 24-1 is 1.48 μm
Alternatively, 0.98 μm excitation light is output. The pump light and the output light of the first optical coupler 22-1 are coupled to the first wavelength multiplex coupler 23-
1 and is incident on the first distributed gain fiber 25-1. The output of the first distributed gain fiber 25-1 includes the input signal light, the reference light, the pump light, and the two types of four-wave mixing components, but is output by the first optical bandpass filter 26-1. Wave mixing component (optical frequency = 2 × ν ₁ −f
_in -α) and transmits the first optical amplifier 27-
Amplify by 1. The output of the first optical amplifier 27-1 is an intermediate output light.

The second wavelength-stabilized light source 21-2 outputs reference light having an optical frequency ν ₂ = ν ₁ −f _ofs / 2 = f _in −3 × f _ofs / 4. The reference light and the intermediate output light are multiplexed by the second optical coupler 22-2. The second excitation light source 24-2 includes: 1.
An excitation light of 48 μm or 0.98 μm is output. The pump light and the output light of the second optical coupler 22-2 are multiplexed by the second wavelength division multiplexing coupler 23-2, and are combined into a second distributed gain fiber 25-2.
Is incident on. The output of the second distributed gain fiber 25-2 includes the intermediate output light, the reference light, the pumping light, and the two types of four-wave mixing components, and is output by the second optical bandpass filter 26-2. wave mixing components (optical frequency = f _in + α
Only + 2 × (ν ₂ −ν ₁ ) = f _in + α−f _ofs ) is transmitted and amplified by the second optical amplifier 27-2 to obtain output light.

[0024]

As described above, the optical frequency tracking device of the present invention can set the frequency interval with respect to the input signal light with a large degree of freedom without being limited by the band of the electric circuit. Further, the present invention is particularly effective in a case where a pump light having a predetermined frequency interval with respect to an input signal light is generated in a narrow band amplifier using parametric amplification or stimulated Brillouin scattering.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an optical frequency tracking device of the present invention.

FIG. 2 is a diagram showing a first operation example in one wavelength converter.

FIG. 3 is a diagram showing a second operation example in one wavelength converter.

FIG. 4 is a diagram showing an operation example in two wavelength converters.

FIG. 5 is a block diagram showing a specific configuration of a wavelength converter according to an embodiment.

FIG. 6 is a block diagram showing a configuration of a conventional optical frequency tracking device using a frequency discriminator.

[Explanation of symbols]

 REFERENCE SIGNS LIST 11 reference light output means 12 multiplexing means 13 four-wave mixing generating means 14 wavelength selecting means 15 optical amplifying means 21 wavelength stabilizing light source 22 optical coupler 23 wavelength multiplexing coupler 24 pumping light source 25 distributed gain fiber 26 optical bandpass filter 27 optical amplifier

Claims (3)

[Claims] An input signal light whose wavelength varies with time;
A first wavelength conversion unit that generates a first wavelength-converted light by four-wave mixing with a first reference light having a constant wavelength, and a first wavelength conversion unit that generates the first wavelength-converted light and the second reference light having a constant wavelength. A second wavelength conversion unit that generates a second wavelength-converted light by four-wave mixing, and outputs the second wavelength-converted light as output light having a certain wavelength interval with respect to the input signal light; The input signal light, the first reference light, and the first wavelength-converted light are arranged on a wavelength axis, and the first wavelength-converted light, the second reference light, and the second wavelength-converted light are arranged. An optical frequency tracking device, wherein wavelengths of the first reference light and the second reference light are set for the input signal light so as to form a light arrangement. 2. The first wavelength converter and the second wavelength converter according to claim 1 are one wavelength converter, and the output light of the wavelength converter is used as the input signal light of the next wavelength converter. An optical frequency tracking device comprising a plurality of wavelength converters connected in cascade. 3. The optical frequency tracking device according to claim 1, wherein each of the wavelength conversion units includes: a reference light output unit that outputs a reference light having a constant wavelength; and an input signal light or a first wavelength conversion unit. Multiplexing means for wavelength-multiplexing and outputting the light and the reference light, and inputting the output light of the multiplexing means to generate four-wave mixing by each wavelength component, and using the four-wave mixing component as wavelength-converted light Four-wave mixing generating means for outputting, of the wavelength-converted light output from the four-wave mixing generating means, only the input signal light or the wavelength-converted light whose wavelength is symmetric with the first wavelength-converted light with respect to the reference light. Wavelength selecting means for selectively transmitting as the first wavelength-converted light for the input signal light or the second wavelength-converted light for the first wavelength-converted light; and optically amplifying and outputting the output light of the wavelength selecting means Optical amplification means An optical frequency tracking device characterized in that: