JP3435089B2 - Multi-frequency light source - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-frequency light source, and more particularly to a light source which performs frequency conversion with reference to the optical frequency of an incident light source and outputs the pulse train in time series.

[0002]

2. Description of the Related Art As the amount of communication information typified by the Internet has increased, research on optical frequency (wavelength) multiplex communication systems has been actively conducted and has been partially introduced. In order to cope with a further increase in communication volume, there is a trend toward narrowing the wavelength between communication channels and increasing the multiplicity. Further, in wavelength division multiplexing (WDM) communication, by using the optical frequency of the carrier itself as route (optical path) information, it becomes possible to switch routes using a wavelength selective element. By realizing it, it becomes possible to construct a large capacity and highly efficient communication network.

At present, as a signal optical frequency of a WDM (wavelength division multiplexing) communication system, light having an equal frequency interval (25 GHz or 100 GHz interval) with 193.1 THz or 192.5 THz as a reference channel, or an optical signal having a multiple of 25 GHz as an interval is used. It has been proposed to use light of equal frequency spacing.

In a WDM communication system incorporating the concept of an optical path, the optical frequency itself has route information and the interval between adjacent frequencies is narrow, so that strict control of the carrier optical frequency is essential. In order to construct a highly reliable communication system, a reference light source having high frequency accuracy and stability, and a measuring device capable of guaranteeing frequency accuracy are required. Further, it is considered that the multi-frequency light source plays an important role in realizing the reference light source and the measuring device.

A method using an optical ring circuit having an optical frequency converter and an optical delay element has been devised as a method for obtaining a multi-frequency light source with equal frequency intervals. The principle will be described below. That is, the light of the reference light source is pulsed and introduced into the optical ring circuit, and repeatedly circulated. The light that circulates in the optical ring circuit undergoes a frequency shift corresponding to the conversion frequency of the optical frequency converter for each circulation, so that it is possible to obtain light at equal frequency intervals while repeating the circulation. The frequency band of the multi-frequency light source using the optical ring circuit is given by [unit conversion frequency of frequency converter] × [number of cycles].

The method of obtaining a multi-frequency light source using this optical ring circuit has the following features. (1) Since the frequency shift amount of the frequency converter is electrically controlled, the frequency interval can be determined with high accuracy of the electric feedback circuit. It also shows high stability against temperature changes. (2) Since the light from the reference light source is frequency-shifted to generate new light, it is possible to produce a large number of highly accurate light sources from one highly stabilized reference light source.
Therefore, it can be realized at a lower cost than stabilizing a large number of lights individually. (3) Since light whose frequency is shifted at equal frequency intervals is output as pulsed light on the time axis, it is possible to extract a desired optical frequency by deciding the time slot and extracting the pulsed light. See Figure 7

As an example of another frequency light source with equal frequency intervals, FIG. 9 shows an acousto-optical shifter (Acousto) as a frequency converter.
-Optical ring circuit using optical frequency shifter (AOS) (for example, "K. Shimizu, T. Horiguchi, and Y. Koyamad
a, "Frequency translation of light waves by propaga
tion around an optical ring circuit containing afr
equency shifter: I.Expaeriment, "Appl.Opt.32,6718-
6726 (1993). "). In this example, the optical ring circuit includes a fiber coupler 4, a polarization controller 10, an acousto-optical optical frequency shifter (AOFS) 8, an optical amplifier 12, a variable optical bandpass filter 13, an optical delay line 9, and these. It is composed of connecting optical fibers. Reference numeral 2 is a light emitted from the frequency reference light source 1 , and 11 is a control circuit.

In FIG. 9, the reference pulse light emitted from the frequency reference light source 1 and cut out by the optical switch 3 is introduced into the optical ring circuit through the fiber coupler 4, and in the optical ring circuit, the acousto-optical optical frequency shifter. After undergoing frequency conversion by 8, a time delay is given by an optical delay line 9. Then, after being amplified by the optical amplifier 12, the optical ring circuit is circulated again. In the meantime, the optical frequency conversion corresponding to the shift frequency of the acousto-optical type optical frequency shifter 8 is performed every cycle. Then, a part of the light circulating in the optical ring circuit is guided to the outside of the optical ring circuit through the fiber coupler 4.

By the way, when the substance is distorted or the pressure is changed, the refractive index is changed accordingly (photoelastic effect). When an ultrasonic wave propagates through a substance, the photoelastic effect causes periodic fluctuations in the diffraction index for light with the wavelength of sound as the period . When light is incident on this substance (medium), part of the incident light is diffracted by the periodic refractive index distribution created by the ultrasonic waves. This is the acousto-optic effect. At this time, the diffracted light undergoes Doppler shift,
The frequency changes. The application of this acousto-optical effect is
It is an acousto-optic optical frequency shifter called AOFS (Fig.
See 10).

As shown in FIG. 10, an AOFS (acoustic-optical type optical frequency shifter) 8 outputs light in different directions depending on the order of diffraction, so that carrier light or higher-order sidebands of higher order are unnecessary. It is easy to block with a simple beam stopper 101 and the like, and has a feature that unnecessary light is not output. In addition, AOFS generally has high frequency conversion efficiency. In FIG. 10, 102 is an acoustic element and 103 is incident light.

The method shown in FIG. 9 realizes a frequency shift of about 1.1 THz.

Further, FIG. 11 shows a conventional example of a multi-frequency light source using a phase modulator utilizing the electro-optical effect as a frequency converter (for example, "K. Shimizu, T. Horiguchi, and YK.
oyamada, "Bload-Band Absolute Frequency Synthesis
of Pulsed Coherent Lightwaves by Use od a Phase-Mo
dulation Amplified Optical Ping, "IEEE J. Quantum
Electron. 33, 1268-1277 (1997). ”). The feature of this example is that a phase modulator (EOM) 14 using an electro-optical effect is used as a frequency converter. EOM is the effect of changing the refractive index of a substance by applying an electric field,
That is, it is a modulator that changes the phase of light by utilizing the electro-optical effect. Materials having a large electro-optical effect include lithium niobate (LiNbO ₃ ) and semiconductors.

AO using ultrasonic Doppler shift
Compared to FS, EOM uses an electro-optical effect, which is a very fast phenomenon, so it has a modulation frequency about two orders of magnitude higher.
GHz). Therefore, the shift frequency per round of the optical ring circuit can be increased, and the WD
It is possible to improve the time efficiency when generating the light of the frequency interval used in the M communication.

However, in the frequency conversion using the EOM, since the frequency conversion is performed by taking out one of the modulation sideband waves, it is essential to remove unnecessary waves.

In the example shown in FIG. 11, in addition to the electro-optical modulator 14 (the amount of frequency shift is Δf ₁ ),
The Fabry-Perot resonator 15 which shifts the frequency also by the optical frequency shifter 8 (shift amount Δf ₂ ) and transmits only the light whose shift amount (Δf) is equal to Δf ₁ + Δf ₂
By using (Free Spectral range FSR = Δf), unnecessary light is removed. In FIG. 11, 4 is a fiber coupler, 10 is a polarization controller, 8 is an acousto-optic optical frequency shifter (AOFS), 12 is an optical amplifier, 13 is a variable optical bandpass filter, 9 is an optical delay line, and 11 is It is a control circuit.

[0016]

However, in the configuration shown in FIG. 9, amplified spontaneous emission (ASE) noise mainly generated in the optical amplifier 12 is a factor for limiting the frequency band. . Since ASE (spontaneous emission light) noise is accumulated along with the circulation, the signal-to-noise ratio (S / N ratio) is deteriorated and the number of circulations of the optical ring circuit, that is, the cover frequency band is limited. Also,
S / N ratio degradation of output light due to ASE accumulation is also occurring.

In the example shown in FIG. 9, a bandpass filter 13 for preventing ASE accumulation is introduced. The bandpass filter 13 has (1) low loss for signal light, (2) high blocking property for ASE noise, (3) high-speed and high-accuracy transmission frequency control, (4) frequency followability for frequency changes of signal light, etc. Is required. To realize the above characteristics (1) and (2),
A narrow band property of transmitting only signal light is required.

As a wavelength-tunable band-pass filter 13 that has been conventionally devised, as shown in FIG. 12, (a) a wavelength-tunable filter 13a (b) which is tunable by controlling the incident angle by using the interference of the dielectric multilayer film 104. ) Having a wedge-shaped interference layer 105 and performing passage wavelength control by changing the incident position of light 13b (c) Using AOS 106 having an acousto-optic effect and performing wavelength control by the frequency of ultrasonic waves 13c, etc. There is.

However, the conventional wavelength tunable bandpass filter 13 has a problem that it is impossible to control the transmission frequency at high speed and with high precision, and the narrow band property must be sacrificed in order to secure a margin. .

On the other hand, in the example shown in FIG. 11, since the EOM 14 has a lower frequency conversion efficiency than the AOFS, it is necessary to increase the amplification factor of the optical amplifier 12. Along with this, the ASE (spontaneous emission light) power also increases, and the ASE is likely to be affected. In addition, the Fabry that is used to remove the fundamental wave and unwanted sidebands of the EOM output light
The number of turns is also limited by the accuracy of the Perot resonator 15. That is, the frequency error (f _m) of the frequency shift amount Δf per revolution and FSR (Free Spectral Range) is to be accumulated in each cycle, after n circulation becomes a n-f _m, Fabry-Perot The output light intensity will be significantly reduced by the resonator. Further, since a Fabry-Perot resonator having a periodic transmission frequency is used as a filter, there is also a problem that only optical frequencies at regular intervals can be generated.

An object of the present invention is to solve the problem of unnecessary light removal in an optical frequency successive conversion circuit such as an optical ring circuit,
To provide a multi-frequency light source.

[0022]

The present invention for solving the above problems has the following specific matters. (1) The invention according to claim 1 has a first means for emitting pulsed light serving as a frequency reference and an optical input port, and an optical frequency of the incident light with respect to the incident light from the optical input port. Schiff
And a bandpass optical filter whose passband changes according to
So as to function, the second means for emitting injection-locked laser oscillation light and the wavelength of the laser oscillation light emitted from the second means are set to the optical frequency of the laser oscillation light and the incident light. wherein as a difference between the optical frequency is within the injection locking range
Third means for controlling the optical frequency shift of the incident light and controlling the same, fourth means for delaying and emitting the input light, and the first means.
The fifth means for emitting combined light of the pulsed light emitted by the means and the light emitted by the fourth means, and the second means.
Means for converting the optical frequency of the laser oscillation light emitted from the means to emit the light to the fourth means, and the light emitted from the fifth means to the optical input of the second means. It is characterized in that at least means for making the light incident on the port is provided. (2) The invention according to claim 2 has first means for emitting pulsed light as a frequency reference, and an optical frequency of the incident light with respect to the incident light from the optical input port, which has an optical input port. Schiff
And a bandpass optical filter whose passband changes according to
And to function, and second means for emitting injection locking laser oscillation light, the wavelength of the laser oscillation light emitted from the unit of the second optical frequency of the laser oscillation light the incident light wherein as a difference between the optical frequency is within the injection locking range
Third means for controlling by following the optical frequency shift of incident light, fourth means for inputting and delaying the laser oscillation light emitted from the second means, and emitting the laser oscillation light, and first means A fifth means for emitting a combined light of the emitted pulsed light and a light emitted by the fourth means, and a sixth means for optically modulating the emitted light from the fifth means to emit a sideband wave. And a means for causing the light emitted from the sixth means to enter the light input port of the second means. (3) The invention according to claim 3 is characterized in that an acousto-optical shifter is used as the sixth means in claim 1. (4) The invention according to claim 4 is characterized in that an electro-optical modulator is used as the sixth means in claim 2. (5) The invention according to claim 5 is characterized in that an electroabsorption modulator is used as the sixth means in claim 2. (6) The invention according to claim 6 is the electro-optical modulator according to claim 4, wherein the light is branched into a plurality of optical paths, and the phase difference between the branched light and the modulation signals applied to the respective modulators. Is provided with a means for introducing. (7) The invention according to claim 7 is characterized in that the second means according to any one of claims 1 to 6 uses a laser whose oscillation wavelength can be adjusted. (8) The invention according to claim 8 is characterized in that a distributed reflection laser is used as the laser whose oscillation wavelength can be adjusted according to claim 7. (9) The invention according to claim 9 is characterized in that a super-periodic structure diffraction grating distributed reflection laser is used as the laser whose oscillation wavelength is adjustable in claim 7.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

The most important point of the present invention is to apply the injection locking phenomenon to the laser as a filter, which is different from the prior art in that an active element is used as a filter. As described above, in the present invention, the bandpass optical filter to which the injection locking to the laser is applied is used. The injection locking phenomenon means the frequency f _SL of the self-oscillating laser (see FIGS. 13 (a) (b)). frequency f _ML closer to the reference 107) (FIG. 13
(a) (b) 108), the oscillation frequency is drawn into the incident light frequency f _ML (see 109 in FIGS. 13 (a) (b)), which is shown in FIG. 13 (b). The phenomenon is shown. Theoretically, the condition under which injection locking occurs is that the frequency f _SL and the frequency f _ML satisfy the relationship of the following expression (1) (for example, the document “CH. Henry, NAOlsson, and NKDutt”).
a, "Locking Range and Stability of Injection Locked
1.54 μm INGaAsP Semiconductor Lasers, "IEEE j. Qua
ntum Electron. 21, 1152-1156 (1985).) ”. Δω = (1 / τ) (I _in / I _out ) ^1/2 (1) where Δω = 2π | f _SL −f _ML |, and τ is the amount of light traveling back and forth in the laser. The required time, I _in, is the intensity of light incident on the laser, and I _out is the intensity of light output from the laser.

Looking at the injection locking phenomenon as a filter, (1) a narrow band about the laser line width (2) can follow the peak frequency of the incident light (3) amplifying the light intensity (4) polarization regeneration function (5 ) The feature is that high-speed transmission frequency (wavelength) control is possible. Of these, features (1), (3), and (4) are due to the fact that the laser output is used as the output of the filter, and feature (2) is that the laser and incident light frequency are predicted by the previous equation (1). Due to having a range. In addition, features (5)
Is different from a filter that adjusts the transmission frequency by using another mechanical or piezoelectric element, and can be electrically adjusted.

Furthermore, by using a wavelength tunable laser as the laser, adjustment of the transmission frequency over a wide range can be realized.

Based on the above, the operation of the present invention will be described.

The means for converting the optical frequency converts the frequency of the incident light into one or a plurality of different frequencies. On the other hand, in the means for injecting and locking the laser into the converted light, only the light that has received a desired frequency conversion amount from the frequencies converted by the means for converting the optical frequency is theoretically expressed by the above formula (1). ) Is adjusted so that it falls within the injection locking range as shown in (). Therefore, the laser is synchronized with the light that has undergone the desired frequency shift, and as shown in the above features (1) to (5), only the light that has undergone the desired frequency shift is amplified, and the other light is amplified. Will be reduced. On the other hand, the means for returning the output light of the laser to the means for converting the optical frequency again returns the laser output light in the injection locked state to the means for converting the optical frequency. Therefore, the circumferential repeatedly incident light
It becomes possible to undergo a wave number shift, and a multi-frequency light source that solves the problem of unnecessary light removal in the optical frequency successive conversion circuit which is the subject of the present invention is realized.

When an electro-optical modulator or an electro-absorption modulator is used as the means for converting the frequency, a plurality of modulation sidebands are generated at intervals of the frequency of the modulation signal with respect to the incident light frequency. . The means for injection-locking the laser with the converted light controls the laser in consideration of the frequency change amount and the conversion efficiency so that only the light of the frequency selected from the output light falls within the injection locking range. This selects only the desired light from among the multiple optical frequencies that are mixed into the converter output. By repeatedly frequency-shifting this light as described above, a multi-frequency light source that solves the problem of unnecessary light removal is realized.

In phase modulation using an electro-optical modulator or the like, the optical frequency of the incident fundamental wave is ω ₀ , the phase of light is φ, the depth of phase modulation is δ, the modulation frequency is ω _m , and the phase of modulation is Ψ. Then, the electric field E is the electric field amplitude E ₀ and the Bessel function J _i (i = 0,
It can be written as in the following equation (2) using E = E ₀ cos [ω ₀ t + φ + δ (sinω _m t + Ψ)] = E ₀ {J ₀ (δ) ・ cos (ω ₀ t + φ) + J ₁ (δ) ・ cos [(ω ₀ + ω _m ) t + ( φ + Ψ)] −J ₁ (δ) ・ cos [(ω ₀ −ω _m ) t + (φ−Ψ)] + J ₂ (δ) ・ cos [(ω ₀ + 2ω _m ) t + (φ + 2Ψ)] −J ₂ ( _{δ) · cos [(ω 0} -2ω m) t + (φ-2Ψ)] + J 3 (δ) · cos [(ω 0 + 3ω m) t + (φ + 3Ψ)] -J 3 (δ) · cos [(ω ₀ −3 ω _m ) t + (φ−3Ψ)] +…} Formula (2)

As a means for converting the frequency, when an electro-optical modulator having a means for branching light into a plurality of optical paths to give a phase difference between the light and the modulated signal and multiplexing again is used. as shown in FIG. 8 by the interference effect of light, it is possible to vary the intensity of the modulation sidebands and fundamental wave is output. Here, in FIG. 8A, when the light is split into two and only one side is phase-modulated, in FIG. 8B, the light is split into two and a phase difference of 90 degrees is obtained for both the light and the modulation signal. When modulated additionally, FIG. 8 (c) splits the light into two, and when modulated by adding a phase difference of 180 degrees to both the light and the modulated signal, FIG. 8 (d) splits the light into three. , Both light and modulated signal 12
It shows a case where modulation is performed by adding a phase difference of 0 degree. Therefore, it becomes easy to control the number of the modulation sidebands and the fundamental waves that fall within the injection locking range to one.

Here, an example using an acousto-optical type optical frequency shifter will be described as an embodiment of the present invention with reference to FIGS. 1, 14 and 15. In FIG. 1, the same functional portions as those in FIGS. 3, 4, 5, 6, 9 and 11 are designated by the same reference numerals.

In FIG. 1, the optical ring circuit includes a fiber coupler 4, an optical isolator 5, an optical circulator 6, a wavelength tunable laser (distributed reflection type laser) 7 via the optical circulator 6, an optical isolator 5, and an acousto-optical optical frequency. It is composed of a shifter 8, an optical delay line 9, a polarization controller 10 and an optical fiber 2 connecting them.

The light emitted from the frequency reference light source 1 is cut out by the optical switch 3 into a pulse shape that is shorter than the circulation time of the optical ring circuit and is used as reference light pulse light. The reference light pulse is introduced into the optical ring circuit through the fiber coupler 4 and injected into the wavelength tunable laser 7 through the optical circulator 6.

The wavelength tunable laser 7 is controlled by the control circuit 11 so that the difference between the oscillation frequency and the incident light frequency is within the injection locking range, and is injection locked with respect to the incident light. This output light is generated by the function of the optical circulator 6.
It is output to a different port from the incident. This output light is
As illustrated in FIG. 14, the circulating signal light is amplified and the noise level is reduced. The light exiting the optical circulator 6 is then guided to the acousto-optical optical frequency shifter 8. After undergoing a frequency shift by the acousto-optic optical frequency shifter 8, a delay time is added by the optical delay line 9, and the polarization controller 10 causes the frequency difference between the self-oscillation light and the circulating light to fall within the injection locking range. It is incident on the wavelength tunable laser 7 adjusted so that the wavelength tunable laser 7 is injection-locked with the frequency-shifted light.

As described above, by repeatedly circling the optical ring circuit and repeating the frequency shift, an optical pulse whose optical frequency changes at equal frequency intervals is obtained as an output.

FIG. 15 shows the signal-to-noise intensity ratio (S / N ratio) of the output light. With the output light of the conventional example, the S / N ratio deteriorates as the number of turns increases, whereas with the output light of the present embodiment, the S / N ratio is kept constant at a low value even when the number of turns increases. Be done.

Next, with reference to FIG. 2, an embodiment of the present invention in which injection modulation is performed on the modulation sideband will be described.
2, the same functional parts as those in FIGS. 1, 3, 4, 5, 6, 9 and 11 are designated by the same reference numerals.

The optical ring circuit shown in FIG. 2 includes a fiber coupler 4, an electro-optical modulator 14, an optical isolator 5, an optical circulator 6, and a wavelength tunable laser (distributed reflection laser).
7, an optical delay line 9, a polarization controller 10 and an optical fiber 2 connecting them.

The light emitted from the frequency reference light source 1 is cut out by the optical switch 3 in a pulse shape that is shorter than the circulation time of the optical ring circuit. The obtained reference light pulse is sent to the fiber coupler 4
Through the optical ring circuit. The light in the optical ring circuit is modulated by the electro-optical modulator 14.
The modulator output light includes several-order modulated sidebands and a fundamental wave that has not undergone frequency conversion.

The wavelength tunable laser 7 is follow-controlled by the control circuit 11 so as to be injection-locked only to the light having a desired frequency shift from the light output from the modulator. Therefore, the tunable laser 7 is connected to the electro-optical modulator 14 by the tunable laser 7.
Only light that has undergone a desired frequency shift with respect to the light incident on is output, and frequency conversion is realized.
At this time, at the same time, amplification of the converted light and reduction of noise light other than the converted light can be achieved. Output light from the tunable laser 7, by passing through the optical delay line 9, given a time delay.

After the polarization is adjusted by the polarization controller 10 so that the efficiency of the electro-optical modulator 14 and injection locking is maximized, a part of the light is output from the optical ring circuit through the fiber coupler 4. And the rest goes around the optical ring circuit again. By repeating the circulation of the optical ring circuit, a repeated frequency shift is received and a multi-frequency light source is realized.

Here, when the wavelength tunable laser 7 is controlled by using the control circuit 11, if it is always synchronized with the modulation sideband of the same order, light having equal frequency intervals can be obtained as an output. On the other hand, it is also possible to control the injection locking by selecting the order of the modulation sideband for each circulation. In this case, W
Like unequal frequency spacing carrier light in DM communication,
Light is obtained that is spaced at multiples of a certain frequency (corresponding to the modulation frequency).

In the present example shown in FIG. 2, the electro-optical type optical modulator 1 is used as means for performing optical modulation and generating a modulation sideband.
4 is used, it is also possible to use the electroabsorption type optical modulator 16 as a means for generating a modulation sideband as shown in FIG.

Further, as shown in FIG. 4, by using the means 17 for branching light into a plurality of optical paths to add a phase difference between the light and the modulation signal, it is easy to select a modulation sideband having a desired frequency change amount. It becomes possible to That is, in FIG.
As means for performing optical modulation and generating a modulated sideband, a phase modulator 17 that splits incident light into a plurality of optical paths and can independently apply phase modulation to each of them is used.

This phase modulator 17 includes a plurality of phase modulators 1.
8 and the phase of the light is adjusted using the variable DC voltage source 21. Further, the high frequency signal generated by the high frequency generator 20 is applied to each phase modulator 18 after a phase difference is added by the phase shifter 19. As described above, by making it possible to adjust the light passing through each phase modulation section 18 and the phase of modulation in each phase modulation section 18, the above equation (2)
Φ and Ψ inside can take arbitrary values. This makes it possible to cancel a maximum of "the number of branches of the path-1" optical frequencies by the interference effect at the time of multiplexing. Further, by canceling the fundamental wave and the unnecessary sidebands existing in the vicinity of the desired modulation sideband, it becomes easy to inject and lock the laser into the desired modulation sideband.

Here, the DC voltage is changed to introduce a phase difference into the light passing through each phase modulator 18, but it is also possible to use a multimode interference coupler for the branching and combining of the light. .

In each of the above-mentioned embodiments, the optical circulator 6 is used to perform injection locking to the wavelength tunable laser 7, but it is of course possible to adopt a configuration in which the laser incident end and the laser emitting end are different. .

FIG. 5 shows an embodiment in which an acousto-optical shifter is used in the case where the incident end and the emitting end of the laser are different. In FIG. 5, the optical ring circuit is a fiber coupler 4,
The optical isolator 5, the variable wavelength laser (distributed reflection type laser) 7, the optical isolator 5, the acousto-optic type optical frequency shifter 8, the optical delay line 9, the polarization controller 10 and the optical fiber 2 connecting them are included. The light emitted from the frequency reference light source 1 is cut out by the optical switch 3 controlled by the control circuit 11 into a pulse shape that is equal to or less than the circulation time of the optical ring circuit, and is used as reference light pulse light. The reference light pulse is introduced into the optical ring circuit through the fiber coupler 4 and injected into the wavelength tunable laser 7. The wavelength tunable laser 7 includes a control circuit 1
The difference between the oscillation frequency and the incident light frequency is controlled by 1 so as to fall within the injection locking range, and injection locking is performed with respect to the incident light. This output light is guided to the acousto-optical optical frequency shifter 8 and, after undergoing frequency shift by the acousto-optical optical frequency shifter 8, a delay time is added by the optical delay line 9,
The polarization controller 10 enters the wavelength tunable laser 7 adjusted so that the frequency difference between the self-oscillation light and the circulating light is within the injection locking range, and the wavelength tunable laser 7 is injection locked to the frequency-shifted light. It By repeating the circulation of the optical ring circuit and the frequency shift in this way, an optical pulse whose optical frequency changes at equal frequency intervals is obtained as an output.

In each of these embodiments, the optical amplifier is not used, but the optical amplifier 12 may be arranged in the optical ring circuit as illustrated in FIG. The optical ring circuit shown in FIG. 6 is different only in that an optical amplifier 12 is introduced in addition to the optical ring circuit configuration of FIG.

In FIG. 6, when the loss of the optical ring circuit cannot be compensated only by the amplifying operation of the injection-locked laser 7, the optical amplifier 12 can be used to compensate the loss. In this case, the ASE that was a problem in the conventional example
The problem of (spontaneous emission) accumulation is the injection-locked laser 7
Since is used as a filter, it is effectively removed.

FIG. 14 shows incident light (dotted line: ASE and signal light from the optical amplifier are mixed) which is actually incident on the filter using injection locking, and outgoing light (solid line) from the filter. It can be seen that the / N ratio is improved.

The optical isolator 5 of each of the above embodiments is desired to be used when the incident end and the emitting end are different, but when the optical circulator 6 is used, the same function is obtained without using the optical isolator 5. Can be realized.

Further, although the distributed reflection type laser is used as the wavelength tunable laser 7, it is also possible to use the super-periodic structure diffraction grating distributed reflection type laser capable of varying the wavelength in a wider range.

[0055]

As described above, according to the present invention, a multi-frequency light source having a high signal-to-noise intensity ratio can be obtained by using the injection locking phenomenon to the laser as a filter.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example of a multi-frequency light source using an acousto-optical type optical frequency shifter as an embodiment example of the present invention.

FIG. 2 is a diagram showing a configuration example of a multi-frequency light source using an electro-optical type optical modulator as an embodiment example of the present invention.

FIG. 3 is a diagram showing a configuration example of a multi-frequency light source using an electroabsorption type optical modulator as an embodiment example of the present invention.

FIG. 4 is a diagram showing a configuration example of a multi-frequency light source using a multi-electrode electro-optical light modulator as an embodiment example of the present invention.

FIG. 5 is a diagram showing a configuration example of a multi-frequency light source using a laser having different entrance ends and exit ends as an embodiment example of the present invention.

FIG. 6 is a diagram showing a configuration example of a multi-frequency light source using an optical amplifier as an embodiment example of the present invention.

FIG. 7 is a diagram showing output light of an optical ring circuit.

FIG. 8 is a diagram showing an output light spectrum of a multi-electrode electro-optical light modulator.

FIG. 9 is a diagram showing a conventional example of an optical ring circuit using an acousto-optical optical frequency shifter.

FIG. 10 is a schematic diagram of an acousto-optical element.

FIG. 11 is a diagram showing a conventional example of an optical ring circuit using an electro-optical type optical modulator.

FIG. 12 is a diagram showing a conventional example of a bandpass filter.

FIG. 13 is a schematic diagram of an injection locking phenomenon.

FIG. 14 is a diagram showing an input / output spectrum of a bandpass filter using injection locking.

FIG. 15 is a diagram showing a comparison between the signal-to-noise intensity ratio of the present invention and a conventional example.

[Explanation of symbols]

1 Frequency reference light source 2 optical fiber 3 optical switch 4 Fiber coupler 5 Optical isolator 6 Optical circulator 7 Tunable laser 8 Acoustic-Optical Optical Frequency Shifter 9 Optical delay line 10 Polarization controller 11 Control circuit 12 Optical amplifier 13 Variable optical bandpass filter 14 Electro-optical type optical modulator 15 Fabry-Perot Resonator 16 Electro-absorption optical modulator 17 Phase modulator having a plurality of phase modulators 18 Phase modulator 19 Phase shifter 20 High frequency generator 21 Variable DC voltage source

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yuzo Yoshikuni 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nihon Telegraph and Telephone Corporation (56) Reference JP-A-56-115592 (JP, A) Kai 55-61082 (JP, A) U.S. Pat. No. 5,153,933 (US, A) Coppin, P.M. et al. , E Electronics Letters, Vol. 26, No. 1, pp. 28-30. Shimizu, K .; et al. , IEEE Journal of Quantum Electronics, Vol. 33, No. 8, pp. 1268-1277. (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 3/00-3/30 H01S 5/00-5/50

Claims (9)

(57) [Claims] 1. A first means for emitting pulsed light as a frequency reference; and an incident light from the optical input port, which has an optical input port and passes by following an optical frequency shift of the incident light. Band is strange
Second means for emitting injection-locked laser oscillation light so as to function as a bandpass optical filter for converting the wavelength of the laser oscillation light emitted from the second means to the optical frequency of the laser oscillation light. the optical frequency shift of the incident light so that the difference between the optical frequencies of said incident light is within the injection locking range and
A third means for controlling the input light, a fourth means for delaying and emitting the input light, a pulsed light emitted by the first means, and a light emitted by the fourth means. And a sixth means for converting the optical frequency of the laser oscillation light emitted from the second means to emit the light to the fourth means, A multi-frequency light source, comprising at least a means for causing light emitted from the means to enter the light input port of the second means. 2. A first means for emitting pulsed light as a frequency reference, and an optical input port, which passes incident light from the optical input port while following an optical frequency shift of the incident light. Band is strange
Second means for emitting injection-locked laser oscillation light so as to function as a bandpass optical filter for converting the wavelength of the laser oscillation light emitted from the second means to the optical frequency of the laser oscillation light. the optical frequency shift of the incident light so that the difference between the optical frequencies of said incident light is within the injection locking range and
Means for controlling the laser oscillation light emitted from the second means, inputting and delaying the laser oscillation light emitted from the second means, and emitting the delayed laser light; and pulsed light emitted from the first means. And fifth means for emitting a combined light of the light emitted by the fourth means, sixth means for optically modulating the emitted light from the fifth means, and emitting a sideband wave, 6. A multi-frequency light source, comprising at least means for causing light emitted from the means of 6 to be incident on the light input port of the second means. 3. The multi-frequency light source according to claim 1, wherein an acousto-optical shifter is used as the sixth means. 4. The multi-frequency light source according to claim 2, wherein an electro-optical modulator is used as the sixth means. 5. The multi-frequency light source according to claim 2, wherein an electroabsorption modulator is used as the sixth means. 6. The electro-optical modulator comprises means for branching light into a plurality of optical paths, and means for introducing a phase difference between the branched light and a modulation signal applied to each modulator. The multi-frequency light source according to claim 4. 7. The multi-frequency light source according to claim 1, wherein the second means uses a laser whose oscillation wavelength can be adjusted. 8. The multi-frequency light source according to claim 7, wherein a distributed reflection laser is used as the laser whose oscillation wavelength can be adjusted. 9. A multi-frequency light source according to claim 7, wherein a super-periodic structure diffraction grating distributed reflection laser is used as the laser whose oscillation wavelength can be adjusted.