JP2001230475A - Wavelength variable laser beam source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength variable laser beam source which can set the wavelength of laser beams to any value and can generate laser beams having an intensity as stable as that of coherent light. SOLUTION: A branch output 8 of a branch circuit 4 is outputted outside. The other branch output 7 is passed through a linear light amplifier 5 having a weak gain saturation characteristic, then is passed through a polarization controller 6-1 and is fed back to a semiconductor light amplification medium 1 to form a ring resonator. The polarization controller 6-1 is so adjusted that an effective go-around gain in the ring resonator may be maximum, the wavelength selection width of a wavelength variable optical filter 2 is so set that lasing may be conducted in at least two longitudinal modes of the annular optical resonator, and the gain of a linear light amplifier 5 is adjusted, to turn the semiconductor light amplification medium 1 into a gain saturated state. By adjusting the loop length of the ring resonator, the intervals of the optical frequency between the longitudinal modes of the wavelength variable laser light source is so set as not exceed the reciprocal of a carrier recovery time in the semiconductor light amplification medium 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunable laser light source used in an optical transmission system, and more particularly to a technique for providing a laser light source with low intensity noise that can arbitrarily set a wavelength.

[0002]

2. Description of the Related Art A variable wavelength light source is one of the elemental technologies of a wavelength division multiplexing (WDM) system in which a large number of signal lights are multiplexed on a wavelength axis and transmitted collectively. One of the requirements for the tunable light source is a light source that can freely set the wavelength according to the requirements of the optical transmission system.

As such a wavelength variable light source, a wavelength variable laser light source having a single oscillation frequency and a simple spectrum slice light source have been conventionally provided. A typical tunable laser light source having a single oscillation frequency is a laser oscillating device configured to include a light amplification medium and a tunable optical filter in a ring or Fabry-Perot optical resonator. In this laser oscillation device, it is necessary to narrow the selection width of the optical filter (evaluated by parameters such as full width at half maximum) compared to the axial mode of the resonator in order to cause laser oscillation at the center transmission wavelength of the wavelength variable optical filter. In addition, the laser length is strictly controlled to stabilize the laser oscillation state so that the optical length of the resonator becomes an integral multiple of the oscillation wavelength. The wavelength tunability is determined by the variable range of the wavelength tunable optical filter and the gain wavelength band of the laser medium.
A variability of about m is realized. However, if the resonator length slightly deviates from the optimum state, the oscillation becomes unstable rapidly, and large intensity noise is generated. Since such oscillation instability occurs only when a slight mechanical / thermal disturbance is applied to the laser resonator system, a stabilizing mechanism for preventing the disturbance is required to obtain a stable oscillation state.
However, these stabilizing mechanisms increase the size of the laser device, which hinders application to communication systems.

On the other hand, in order to overcome the above-mentioned problems of the conventional tunable laser light source, a simple spectrum slice light source having a simple configuration has been studied. This light source is a wavelength tunable light source that cuts out a white light source over a wide wavelength band with a wavelength tunable optical filter.Besides being able to set an arbitrary wavelength, the center wavelength is determined by a wavelength tunable optical filter that is a passive component. The feature is that long-term performance guarantee is also possible.

However, a simple spectrum slice light source is
That the minimum value of the spectral line width is limited by the transmission width of the wavelength tunable optical filter, and that the intensity itself of the slice light is weak because more light is removed by the filter,
In addition, since the temporal fluctuation of the number of photons detected by the filtering is developed, when the light intensity is attenuated, the intensity noise rapidly increases.

In order to solve these problems with a simple spectrum slice light source, the present inventors have proposed a spectrum slice light source having a ring-shaped resonator configuration as shown in FIG. 1]: Katagiri, Suzuki, Aida "Generation of stabilized monochromatic noise light" pp.531, 1
999 IEICE General Conference B-10-170).

The light source shown in FIG. 5 includes a linear optical amplifier 5 having a small gain saturation characteristic, an isolator 3, a tunable optical filter 2, and a semiconductor optical amplifying medium 1 having a gain saturation characteristic.
, The splitter 4, the variable attenuator 20, and the polarization controller 6 are arranged in a ring shape. By controlling the transmission center wavelength of the wavelength tunable optical filter 2 by a filter control circuit 10 which controls the transmission center wavelength of the filter in response to an external wavelength designation 9, the first branch output 8 of one of the splitters 4 is output. It operates to generate light whose center wavelength matches the specified wavelength. The light generated by the linear optical amplifier 5 or the semiconductor optical amplifying medium 1 orbits the ring many times, so that light having a narrower spectral width and higher light output than the simple spectral slice light can be generated. is there. In order to increase the number of rounds, the polarization controller 6 optimizes the polarization state of the semiconductor optical amplifying medium 1 in which the polarization dependence is inevitable.

Further, in order to suppress the intensity noise caused by the fluctuation of the number of photons of the simple spectrum slice, the gain saturation characteristic of the semiconductor optical amplification medium 1 is used. This is to suppress the optical output by utilizing gain saturation for an excessive optical input, and has a mechanism in which the intensity is stabilized by the action as an optical limiter. To obtain a strong gain saturation effect, it is necessary to increase the gain of the linear optical amplifier 5 as much as possible to increase the optical input level to the semiconductor optical amplifying medium 1. On the other hand, the variable attenuator 20 reduces the effective gain of the loop. It lowers unnecessary laser oscillation of the ring resonator.

FIG. 6 shows a typical example of the wavelength spectrum of the spectrum slice light source improved in this way, and the accumulation 22 of the spontaneous emission light possessed by the linear optical amplifier 5 is well suppressed, and the spectrum width is narrow and the spectrum is high in intensity. This shows a characteristic in which the slice light 21 is generated.

[0010]

However, a conventional spectrum slice light source having a ring-shaped resonator configuration as shown in FIG. 5 has a reduced spectrum width and a high light intensity as compared with a conventional simple spectrum slice light source. However, as described below, there is a problem to be solved in that the use as a high-quality light source for optical transmission is limited due to the unique effect of intensity noise based on fluctuations in the number of photons. .

FIG. 7 is a graph obtained by normalizing the frequency of an event with respect to the number n of photons detected in a slot T when the observation time slot is T, that is, the probability density distribution is shown for various lights. It is a thing.

First, the simple spectrum sliced light 25 has a Bose-Einstein distribution whose frequency monotonously decreases with respect to the number n of photons due to chaotic behavior.
This indicates that even if the average value of the number of detected photons is a finite value of <n>, the probability that no photon is detected is the highest, and the sign of 1,0 is identified by the number of photons. The direct modulation / direct detection method causes a large error and cannot be tolerated.

On the other hand, a spectrum slice light source having a ring-shaped resonator configuration has a similar distribution shape although its photon distribution is wider than that of the Poisson distribution 23 corresponding to coherent light due to stabilization of intensity. It is shown that a super Poisson distribution 24 having

When the light intensity is attenuated, the spread of the Poisson distribution 23 corresponding to the coherent light does not change, but the distribution range of other super Poisson distributions 24 and Bose-Einstein distributions 25 increases. Such a spread of the distribution is reflected as light intensity noise during the limited observation time T.

For this reason, as shown in FIG. 8, in the case where the error rate (BER: bit error rate) of a wavelength-variable light source is measured by data coding using an intensity modulator, the intensity of the received light is increased. This results in a floor on which the error rate cannot be suppressed. (In the probability statistics, generally, when the population (here, the number of photons) is large, the distribution of the population is a Gaussian distribution. However, when the number of elements constituting the population is reduced, the specific A probability distribution appears, where the nature of light causes Bose-Einstein to Super Poisson and even Poisson distributions when the number of photons is extremely low, which is characterized by chaotic or coherent light. This is used to determine the state of light using measurement of the distribution shape.) Due to the relationship between the received light intensity and the error rate, the floor as shown in FIG. This is a characteristic of the spectrum sliced light that does not form the light. Such a floor is represented by the time slot T
And the barrier to long-span, high-bit-rate optical transmission, because the required light intensity increases exponentially for high-quality data transmission with extremely low error rates. Had become.

SUMMARY OF THE INVENTION An object of the present invention is to provide a wavelength tunable laser capable of generating light stabilized as the photon number distribution approaches a Poisson distribution corresponding to coherent light in order to solve the above-mentioned problems of the prior art. It is to provide a light source.

[0017]

In order to achieve the above object, the invention of a wavelength tunable laser light source according to claim 1 is a semiconductor optical amplification medium having a high gain saturation characteristic having a negative feedback effect on light intensity. A ring in which an isolator for preventing unnecessary return light, a tunable optical filter, a splitter for extracting light, a linear optical amplifier having low gain saturation characteristics, and a polarization controller are arranged in a loop. A wavelength tunable laser light source having an optical resonator, wherein the polarization controller is adjusted so that the effective circulating gain in the ring optical resonator is maximized, and near a center transmission wavelength of the wavelength tunable optical filter. By setting the wavelength selection width of the wavelength tunable optical filter so as to oscillate in a plurality of axial modes having cross-correlation, and by adjusting the gain of the linear optical amplifier having the low gain saturation characteristic, A semiconductor optical amplifying medium having gain saturation in the gain saturation state, thereby a plurality of the axial mode, wherein the holding constant the light output from the branching unit correlated.

Here, by adjusting the length of the ring-shaped optical resonator, the optical frequency interval between the axial modes of the wavelength tunable laser light source can be adjusted to the carrier recovery time in the semiconductor optical amplifying medium having the high gain saturation characteristic. Is set so as not to exceed the reciprocal of.

Further, the length of the single mode optical fiber optically coupling each optical element of the ring type optical resonator or the length of the birefringent optical fiber holding polarization is adjusted. can do.

Further, a semiconductor optical amplifier is used as the linear optical amplifier, and a second polarization controller is arranged on an input side of the semiconductor optical amplifier.

The tunable optical filter is a tunable band-pass filter that performs continuously variable wavelength control, and sweeps the transmission peak wavelength of the tunable band-pass filter in synchronization with an external clock. it can.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings. (Basic Configuration of the Embodiment) First, the basic configuration of a wavelength tunable laser light source to which the present invention is applied and the operation principle thereof will be described.

According to the present invention, a wavelength tunable ring resonator laser using an optical amplification medium and a wavelength tunable optical filter is provided.
By oscillating multiple axial modes with cross-correlation near the center transmission wavelength of the tunable optical filter, it is possible to generate light that is stabilized as the photon number distribution approaches the Poisson distribution corresponding to coherent light. Realizing a wavelength tunable laser light source.

Specifically, first, the wavelength selection width (characterized numerically by a full width at half maximum) of the wavelength tunable optical filter is adjusted so as to oscillate in at least two axial modes. On the premise of such multi-mode oscillation, a semiconductor optical amplification medium having strong gain saturation characteristics is used in a ring resonator so that a plurality of modes have a negative cross-correlation so that the total number of photons is kept constant. It is placed on the light path.
Here, it is known that gain saturation gives a Langevin force for suppressing fluctuations in the number of photons.

FIG. 2 is a block diagram showing functions of a semiconductor optical amplifying medium having such a gain saturation characteristic. In FIG.
For the sake of simplicity, the functional configuration is shown divided into a linear gain portion (coefficient = Go) 12 having no saturation generated by current injection and a gain saturation portion 13 performing saturation by a feedback system. Accordingly, the net gain coefficient G (G = Pout / Pin: Pin = input light intensity, Pout =
Output light intensity), using incident light intensity P and saturated intensity P _s

[0026]

(Equation 1)

Can be written as This equation (1) means that the semiconductor optical amplifying medium has a negative feedback effect on the light intensity. Negative feedback has already been pointed out in reference [2], but its object is to suppress the quantum fluctuations of a pure single-mode laser and generate a photon-number squeezed state. There is no mention of the following inter-mode correlation of the multimode laser according to the present invention.
(Reference [2]: Y. Yamamoto, N. Imoto, and S. M.
achida, "Amplitude squeezing in a semiconductor la
ser using quantum nondemolition measurement and ne
gative feedback, "Phys. Rev. A, 33, pp.3243-3260 (1
986). )

In a wavelength tunable laser using a semiconductor optical amplifying medium having such a gain saturation characteristic, as shown in FIG. 3, a number of axial modes oscillate simultaneously around a transmission center wavelength 16 of the wavelength tunable optical filter. are doing. These axial modes have a negative correlation, and their sum is negligible for coherent interference.
That is, the total light output is constant.

FIG. 4 is a diagram for explaining such a mechanism for stabilizing the optical output by the inter-mode correlation using three axial mode models. Different oscillation states A, B and C have the same sum. It is shown to have.

Such a multi-mode laser beam has a spectrum spread similar to that of a spectrum sliced light, but individual photons have high coherence and a much higher photon number density than the spectrum sliced light. , High output. In addition, such light essentially forms a laser field, so that the photon number distribution approaches a Poisson distribution from a super Poisson distribution. Therefore, in such a multi-mode laser beam, generation of intensity noise due to fluctuation in the number of photons as in a conventional spectrum slice is eliminated, and low intensity noise characteristics comparable to a single oscillation frequency laser are realized.

The above description will be quantified and considered. The suppression of the photon number fluctuation due to the Langevin force based on gain saturation is H, the intensity fluctuation of the laser beam circulating in the ring is Γ, the light intensity noise applied by the gain medium is Γ _c , the light intensity accompanying the fluctuation of the bias current Assuming that the noise and the enhancement factor are Γ _i and Γ, respectively,

[0032]

(Equation 2) Holds.

When the suppression current H and the enhancement factor ξ are simultaneously increased by increasing the injection current into the semiconductor optical amplifying medium, the intensity noise (intensity fluctuation) 漸 approaches Γ _i / H.
Normally, the ordering power (suppression) H is sufficiently larger than the light intensity noise Γ _i , so Γ _i / H can be regarded as zero. Therefore, the intensity noise Γ can be suppressed in a band in which the suppression H of the fluctuation of the number of photons due to the Langevin force is effective. The band where H is effective is approximately given as the reciprocal of the carrier recovery time. Therefore, as the carrier recovery time is shortened, the frequency band that can be suppressed rises, and it can be applied to data transmission at a high bit rate. In a semiconductor optical amplifying medium (semiconductor optical amplifier), the carrier recovery time can be reduced to several ps, and theoretically, a time slot (10 ps or less) is characterized by a high-speed optical signal of 100 Gbps or more.
The effect of reducing the fluctuation of the number of photons can be expected.

For the reasons described above, in the basic mode of the present invention, the polarization controller is adjusted so that the effective loop gain in the ring-shaped optical resonator is maximized, and at least two axial modes of the ring-shaped optical resonator are adjusted. By setting the wavelength selection width of the wavelength tunable optical filter so as to oscillate the laser, and adjusting the gain of the linear optical amplifier having low gain saturation characteristics to bring the semiconductor optical amplification medium into a gain saturation state, The axial modes are correlated to keep the light output from the splitter constant.

Further, in addition to the above-described basic function of the ring laser, in order to cope with a high bit rate, when a semiconductor optical amplifying medium having an extremely short carrier recovery time is used, another study is required. .

That is, the shorter the carrier recovery time is, the faster the fluctuation can be dealt with. However, in order to quickly suppress the fluctuation of the number of photons with the ring-shaped optical resonator laser, it is necessary to shorten the loop length and increase the loop response speed. There is. However, if the loop response speed is excessively increased, the time becomes shorter than the carrier relaxation time of the semiconductor optical amplifying medium, so that the fluctuations are accumulated and a sufficient suppression effect cannot be obtained. For this reason, it is necessary to make the loop response speed slower than the carrier recovery time. Therefore, it is necessary to provide an optimum loop length for the carrier recovery time of the semiconductor optical amplification medium to be used.

Therefore, in a further aspect of the present invention, by adjusting the length of the ring-shaped optical resonator constituting the wavelength tunable laser light source, the optical frequency interval between the axial modes of the wavelength tunable laser light source is adjusted to the gain saturation characteristic. Is set so as not to exceed the reciprocal of the carrier recovery time in the semiconductor optical amplification medium having

(First Embodiment) FIG. 1 shows the configuration of a wavelength variable laser light source according to a preferred embodiment of the present invention. The tunable laser light source shown in FIG. 1 includes a semiconductor optical amplifier 1, an isolator 3, a tunable optical filter 2, a splitter 4, a linear optical amplifier 5, and a polarization controller 6-1 having a high gain saturation characteristic. It constitutes a ring-shaped optical resonator arranged in a (loop shape). In order to realize such an arrangement, each optical element is usually optically coupled using a single-mode optical fiber, but a birefringent fiber that maintains polarization can also be used.

Here, the isolator 3 arranged at the optical output port of the semiconductor optical amplifier 1 in the loop determines the direction of the light circulating around the ring, and also prevents oscillation of the optical amplification medium 1 due to unnecessary return light. Plays a role. Various wavelength selection filters such as a dielectric multilayer optical filter and an acousto-optic optical filter can be used for the wavelength tunable optical filter 2.
Through the filter control circuit 10, the wavelength is selected based on the incident angle and transmission position of the beam and the frequency of the applied RF signal.

The splitter 4 is used to extract light (second branch output 8) from the loop. The other first branch output 7 output from the splitter 4 passes through the polarization controller 6-1 via the linear optical amplifier 5 having a weak gain saturation effect, and returns to the semiconductor optical amplification medium 1. The linear optical amplifier 5 usually has
Although a rare earth element-doped fiber optical amplifier can be used, a semiconductor optical amplifier having extremely high saturated light intensity can be used instead. However, in the case where the latter semiconductor optical amplifier is used as the linear optical amplifier 5, the second polarization controller 6-2 may be arranged on the incident side to control the polarization plane and obtain the maximum gain. Required. The polarization controller 6-1 is arranged on the incident side (optical input port) of the semiconductor optical amplification medium 1 for the same reason.

Then, the polarization controller 6-1 is adjusted so that the effective loop gain in the ring resonator is maximized, and the wavelength tunable optical filter is oscillated in at least two axial modes of the ring type optical resonator. By setting the wavelength selection width of 2 and adjusting the gain of the linear optical amplifier 5 having a low gain saturation characteristic to bring the semiconductor optical amplification medium 1 into a gain saturated state, a plurality of axial modes are correlated and branched. The light output 8 from the vessel 4 is kept constant. Further, by adjusting the loop length of the ring-shaped optical resonator constituting the wavelength tunable laser light source, the optical frequency interval between the axial modes of the wavelength tunable laser light source can be adjusted so that the carrier recovery in the semiconductor optical amplification medium 1 having gain saturation characteristics. Set to not exceed the reciprocal of time.

In the ring laser configured as described above, when a wavelength designation 9 is externally received, the filter control circuit 10 receives the signal and controls the transmission center wavelength of the tunable optical filter 2 in accordance with the designated wavelength. The laser light having the center wavelength corresponding to the wavelength specification
From the branch output 8. This output 8 is a laser beam whose intensity is as stable as the coherent light.

[0043]

As described above, according to the present invention,
By setting an arbitrary wavelength, it is possible to generate a laser beam having an intensity as stable as coherent light, and it is possible to generate light that is stabilized as the photon number distribution approaches the Poisson distribution corresponding to the coherent light A tunable laser light source can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a wavelength tunable laser light source (ring laser) according to a preferred embodiment of the present invention.

FIG. 2 is a block diagram showing functions of a semiconductor optical amplification medium having a gain saturation characteristic, which is a component of the present invention.

FIG. 3 is a characteristic diagram showing an optical spectrum in a case where a number of axial modes oscillate simultaneously near a transmission center wavelength of a wavelength tunable optical filter in a wavelength tunable laser light source according to the present invention in a relationship between wavelength and intensity.

FIG. 4 is a conceptual diagram showing that the sum of the three axial modes having a negative correlation (total light intensity) is constant in the wavelength tunable laser light source according to the present invention, in terms of wavelength and intensity.

FIG. 5 is a block diagram showing a configuration example of a conventional high-output, narrow-spectrum-width spectrum slice light source having a ring-shaped configuration.

FIG. 6 is a characteristic diagram showing a typical spectrum of light output from the ring-shaped spectrum slice light source of FIG. 5 in relation to wavelength and intensity.

FIG. 7 shows various types of light (chaotic light, stabilized chaotic light,
FIG. 3 is a characteristic diagram showing a photon distribution with respect to (coherent light).

FIG. 8 is a characteristic diagram showing a relationship between received light intensity and error rate (BER) of data transfer using stabilized chaotic light.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Semiconductor optical amplifying medium 2 Wavelength variable optical filter 3 Isolator 4 Branching device 5 Linear optical amplifier 6 Polarization controller 6-1 Polarization controller 6-2 Second polarization controller 7 First branch output 8 Second 9 Wavelength designation 10 Filter control circuit 12 Linear gain portion (linear amplification portion) 13 Gain saturation portion 14 Optical output 15 Injection current 16 Transmission center wavelength of tunable optical filter 17 Transmission wavelength characteristic 18 Axis mode 19 Oscillation wavelength region 20 Variable attenuator 21 Spectrum slice light 22 Cumulative spontaneous emission light 23 Coherent light (Poisson distribution) 24 Stabilized spectrum slice light (Super Poisson distribution) 25 Simple spectrum slice light (Bose-Einstein distribution)

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Kenichi Suzuki 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Masamitsu Fujiwara 2-3-3, Otemachi, Chiyoda-ku, Tokyo No. 1 Nippon Telegraph and Telephone Corporation (72) Noboru Kochio No. 2-3-1, Otemachi, Chiyoda-ku, Tokyo Japan F-term (reference) 5F072 AB13 HH06 JJ05 JJ13 JJ20 KK15 KK30 LL17 YY15 5F073 AB28 AB30 BA01 EA29

Claims (5)

[Claims] 1. A semiconductor optical amplifying medium having a high gain saturation characteristic having a negative feedback effect on light intensity, an isolator for preventing unnecessary return light, a wavelength tunable optical filter, and a splitter for extracting light. And a linear optical amplifier having low gain saturation characteristics; and a wavelength tunable laser light source having a ring-shaped optical resonator in which a polarization controller is arranged in a loop, wherein the effective round-trip gain in the ring-shaped optical resonator is The polarization controller is adjusted to be the maximum, and the wavelength selection width of the tunable optical filter is set so that the laser oscillates in a plurality of axial modes having a cross-correlation near the center transmission wavelength of the tunable optical filter. Setting, and adjusting the gain of the linear optical amplifier having the low gain saturation characteristic to bring the semiconductor optical amplifying medium having the high gain saturation characteristic into a gain saturated state, whereby a plurality of the Tunable laser, wherein a mode is kept constant light output from the branching unit correlated. 2. Adjusting the length of the ring-shaped optical resonator so that the optical frequency interval between the axial modes of the wavelength tunable laser light source is reduced by the carrier recovery time in the semiconductor optical amplification medium having the high gain saturation characteristic. 2. The tunable laser light source according to claim 1, wherein the wavelength is set so as not to exceed a reciprocal. 3. A length of a single mode optical fiber optically coupling each optical element of the ring type optical resonator or a length of a birefringent optical fiber holding polarization. The tunable laser light source according to claim 2. 4. The semiconductor device according to claim 1, wherein a semiconductor optical amplifier is used as the linear optical amplifier, and a second polarization controller is arranged on an input side of the semiconductor optical amplifier. Tunable laser light source. 5. The tunable optical filter according to claim 1, wherein the tunable optical filter is a tunable band-pass filter that performs continuously variable wavelength control, and sweeps a transmission peak wavelength of the tunable band-pass filter in synchronization with an external clock. 5. The tunable laser light source according to any one of 1 to 4.