JPH06152038A - Method and apparatus for converting optical phase or wavelength - Google Patents

Abstract

PURPOSE:To provide a method capable of changing a phase or a wavelength of a signal pulse of a high bit rate and an apparatus thereof. CONSTITUTION:An optical pulse having at least two wavelengths is generated from a Fabry-Perot type semiconductor laser 11, and ultrashort, light is pulsed through a dispersion shift fiber 12, an optical fiber amplifier 13, a band-pass filter 14 and an anomalous dispersion fiber 15, and the obtained ultrashort light pulsed raw is emitted into a traveling wave type semiconductor laser 16 as excitation light together with signal light 18. Therefore, a decrease in carrier due to an induction amplification of the long wavelength side is replenished with an induction absorption of the short wavelength side, and a carrier density in an active layer is rapidly returned to a steady state, and a phase or a wavelength corresponding to a signal light pulse of a high bit rate can be converted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical phase or wavelength conversion method and apparatus for changing the phase or wavelength of signal light.

[0002]

2. Description of the Related Art FIG. 2 is a sectional view showing an example of a traveling wave type semiconductor laser. In the figure, 1 is an n-InP substrate, 2 is an n-InGaAsP cladding layer having a bandgap of 1.1 eV, and 3 is undoped InG having a bandgap of 0.8 eV.
aAsP active layer, 4 is p- with a band gap of 1.1 eV
InGaAsP clad layer, 5 p-InGaAs ohmic contact layer, 6 AuGeNi electrode, 7 AuZnNi electrode formed in stripes on the waveguide, 8
Is an anti-reflection film of SiOx formed on the end face of the waveguide, 9 is an excitation light pulse, and 10 is a signal light pulse. Here, it is assumed that the signal light intensity is extremely smaller than the pump light intensity and the free carrier fluctuation due to the signal light can be ignored.

A forward current is injected between the electrodes 6-7, and a sufficient reflection distribution is formed inside the active layer 3. Therefore, light having a wavelength within the gain region is amplified. The pumping light pulse 9 has a wavelength in the gain region, and when guided through the active layer 3, it induces stimulated emission from the population inversion, so that the pumping light pulse 9 itself is amplified and free carriers inside the active layer 3 are amplified. Propagate while decreasing. Such a decrease in population inversion leads to a modulation of the refractive index through the free carrier plasma effect and the change in the band-to-band absorption spectrum. Therefore, when the pumping light pulse 9 and the signal light pulse 10 are made to enter at the same time, the phase or wavelength of the signal light pulse 10 changes as the refractive index changes.

When the signal light pulse 10 exists within the time of abrupt carrier fluctuation due to the pumping light pulse 9, the wavelength of the signal light pulse 10 shifts in proportion to the time change dn / dt of the refractive index n thereof. When the signal light pulse 10 is made incident immediately after the carrier fluctuation due to the pumping light pulse 9 is finished, the phase of the signal light pulse 10 changes by 2πΔnL / λ (where L is the resonator length and λ is Signal light wavelength).

[0005]

Problems to be Solved by the Invention FIGS. 3A, 3B, 3C are shown in FIG.
The changes over time in the excitation light intensity and carrier density in the active layer 3 at points A, B, and C are shown. Here, the pulse width of the excitation light pulse 9 is much shorter than the carrier life. As described above, the excitation light pulse 9 amplifies its light intensity as it propagates. Furthermore, since the induced transition probability is proportional to the electric field, the excitation light pulse 9 causes a large reduction in carriers at the output end rather than the incident end. The reduced carrier is replenished by injection from the electrode,
If the carrier lifetime is simply constant, the time change of the carrier density N at that time is expressed by N (t) = Jτ _s / Qd−ΔNe ^{−t / τ} s (1) and is determined by the carrier lifetime. It has a time constant. Here, J is the injection current density, τ _s is the carrier lifetime, Q is the elementary charge, and d is the thickness of the active layer.

Therefore, it takes several ns to return to the original state where the pumping light pulse 9 is incident, which causes crosstalk with respect to the signal light pulse 10 having a high bit rate, which is an extremely serious problem. Becomes There is also a method of increasing the recombination center by implanting protons and reducing the carrier lifetime, but as can be seen from equation (1), it causes a decrease in population inversion at the time of constant current injection, and it is expected to improve significantly. could not.

In view of the above-mentioned conventional problems, it is an object of the present invention to provide a method and apparatus capable of changing the phase or wavelength of a signal light pulse having a high bit rate.

[0008]

In order to achieve the above object, the present invention changes the refractive index of the active layer of a traveling wave type semiconductor laser by simultaneously injecting signal light and pumping light into the traveling wave type semiconductor laser. In the optical phase or wavelength conversion method for converting the phase or wavelength of the signal light by using a Fabry-Perot type semiconductor laser, a plurality of longitudinal modes are generated and a single optical pulse is generated on the time axis, and the optical pulses are grouped. The longitudinal modes are separated and compressed on the time axis by passing through a dispersion-shifted fiber with negative chromatic dispersion of velocity, converted into an optical pulse train, the optical pulse train is amplified through an optical fiber amplifier, and noise is removed through a bandpass filter. A weak mode component is removed, and the optical pulse train is passed through an anomalous dispersion fiber with positive chromatic dispersion of group velocity to form an ultrashort optical pulse train, and the ultrashort optical pulse train is excited. We propose an optical phase or wavelength conversion method simultaneously incident traveling wave type semiconductor laser to the signal light as the light. Further, an optical phase or wavelength conversion device for simultaneously injecting signal light and pumping light into a traveling wave semiconductor laser and changing the refractive index of the active layer of the traveling wave semiconductor laser to convert the phase or wavelength of the signal light. In which a Fabry-Perot type semiconductor laser, a dispersion shift fiber having a negative group velocity chromatic dispersion, an optical fiber amplifier, a bandpass filter, an anomalous dispersion fiber having a positive group velocity chromatic dispersion, and a traveling wave type semiconductor laser are provided. A single optical pulse containing a large number of longitudinal modes generated from a semiconductor laser on the time axis is passed through a dispersion shift fiber, an optical fiber amplifier, a bandpass filter, and an anomalous dispersion fiber in order to form an ultrashort optical pulse train. The optical phase or wavelength that is made to enter the traveling wave type semiconductor laser at the same time as the signal light using the generated ultrashort optical pulse train as the excitation light. To propose a conversion equipment.

[0009]

According to the present invention, the Fabry-Perot type semiconductor laser generates a single optical pulse containing a larger number of longitudinal modes on the time axis, and the optical pulse has a negative dispersion chromatic dispersion of the group velocity. Each longitudinal mode is separated and compressed on the time axis through to form an optical pulse train, the optical pulse train is amplified through an optical fiber amplifier, noise is removed through a bandpass filter, and a weak mode component is removed. An optical pulse train is made into an ultrashort optical pulse train by passing through an anomalous dispersion fiber with a positive chromatic dispersion of group velocity, and the ultrashort optical pulse train is made incident on the traveling wave type semiconductor laser at the same time as the signal light as pumping light, and The decrease in carriers due to the induced amplification of the excitation light is replenished by the induced absorption of the excitation light on the short wavelength side, and the carrier density in the active layer is rapidly returned to the steady state. Without being governed by the carrier lifetime as Yaria injection and enables high speed.

[0010]

FIG. 1 shows an embodiment of the present invention, in which 11 is a Fabry-Perot type semiconductor laser, 12 is a dispersion-shifted fiber having a normal dispersion in which the chromatic dispersion of the group velocity is negative, and 13 is An optical fiber amplifier, 14 is a bandpass filter, 15 is an anomalous dispersion fiber with positive chromatic dispersion of group velocity, 16 is a traveling wave type semiconductor laser, 17 is pumping light, 18
Is signal light, and 19 is output light.

First, a method of producing excitation light for realizing the present invention will be described with reference to FIG. When the Fabry-Perot type semiconductor laser 11 is driven by the gain switch method, it is output as a single optical pulse 21 including many longitudinal modes and on the time axis. When the optical pulse 21 propagates in the dispersion shift fiber 12 having a normal dispersion, the optical pulse of each longitudinal mode propagates farther toward the long wavelength side due to the group velocity dispersion of the fiber, so that it is separated on the time axis and the red shift chirp is generated. Are compensated and compressed (22).

However, each of these longitudinal modes reflects the gain distribution of the active layer and has a large difference in intensity on the wavelength axis, and fluctuations also occur on the time axis due to mode distribution noise between modes. When this optical pulse train 22 is amplified by the optical fiber amplifier 13, variations in the light intensity of each mode are suppressed on the wavelength axis and the time axis by the gain saturation effect there (23).

Further, by the bandpass filter 14,
It is necessary to eliminate spontaneous emission noise from the optical fiber amplifier 13 and to discard the weak mode at the tip on the time axis (24). This is because the pulse at the tip abruptly modulates the carriers inside the active layer, and the higher the intensity, the better.

The optical pulse train 24 is used as an anomalous dispersion fiber 1
Propagation in 5 causes a pull-shift chirp due to the self-phase modulation effect, and an optical soliton is formed in balance with the anomalous dispersion of the fiber. When the intensity of each mode becomes larger than this balance, higher-order solitons are induced, and due to their interference, they are extremely finely compressed and output at the end of the optimum long fiber. Further, the sign of the distribution is opposite to that of the dispersion shift fiber 12, and the interval between the modes is shortened.

As described above, the pumping light pulse train 25 having a pulse width and a pulse interval of 1 ps or less arranged on the time axis from the long wavelength side is formed. At this time, the pulse width and the pulse interval can be independently adjusted by the dispersion value and the length of the dispersion shift fiber 12 and the anomalous dispersion fiber 15.

Next, a wavelength conversion method using the excitation light will be described. FIG. 5 shows the correspondence between the wavelength of the pumping light pulse train and the gain spectrum of the traveling wave type semiconductor laser due to band filling. The inset in FIG. 5 is an optical pulse train separated on the time axis. The wavelength of each pulse matches the wavelength on the abscissa above it, and the backward pulse has a shorter wavelength.

As described above, since the first pulse of the pumping light consumes the internal carriers by the amplification process, when the pulse propagates in the waveguide, the carrier density distribution becomes as shown in the upper side of FIG. The shift of the signal light wavelength is due to the change over time of the carrier density.

Now, what matters is how quickly the carrier is returned from the consumed state to the steady state carrier density. At point A near the input point of the waveguide, the decrease of carriers is small and the gain spectrum as shown in a of FIG. 5 is obtained. Therefore, the second and third pulses still have a slight gain and are not absorbed and are guided. A shorter wavelength pulse propagating in the waveguide but further behind is absorbed at point A. Therefore, the carrier density at point A is replenished with carriers due to this induced absorption, and the state rapidly shifts to a steady state.

At point B, the carrier density is considerably reduced and the gain spectrum is as shown in FIG. 5b, so that the absorbed wavelength shifts to the long wavelength side, and the mode not absorbed up to that point is absorbed. Will be done. Therefore, similarly, the carrier density at point B also rapidly shifts to the steady state. At the point C, the output pumping light is as shown in the lower side of FIG. 6 because it enters the wavelength range where it is absorbed after the second pulse and is absorbed in the active layer.

As described above, the carriers consumed in the first pulse are supplemented by the second pulse and below, but not all are absorbed in the vicinity of the input end of the waveguide, and the second and subsequent pulses are their own. As it is absorbed in the wavelength sensitive area,
The entire active layer rapidly shifts to a steady state.

[0021]

As described above, according to the present invention, a Fabry-Perot type semiconductor laser generates an optical pulse having at least two wavelengths, and by using the optical pulse, the redundant carrier lifetime is controlled. Since the recovery of the carrier density in the active layer is caused in the entire waveguide by the pumping light itself, phase or wavelength conversion can be performed with respect to the ultrahigh-speed, high-repetition signal light pulse train.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an example of an optical phase or wavelength conversion device of the present invention.

FIG. 2 is a configuration diagram showing an example of a traveling wave type semiconductor laser.

FIG. 3 is an explanatory diagram showing temporal changes in excitation light intensity and carrier density in the active layer 3 at points A, B, and C in FIG.

FIG. 4 is an explanatory diagram of a method for producing excitation light.

FIG. 5 is an explanatory diagram showing a correspondence relationship between a wavelength of a pumping light pulse train and a gain spectrum of a traveling wave type semiconductor laser.

FIG. 6 is an explanatory diagram showing a carrier density distribution in an active layer and an intensity change due to propagation of excitation light.

[Explanation of symbols]

11 ... Fabry-Perot type semiconductor laser, 12 ... Dispersion shift fiber, 13 ... Optical fiber amplifier, 14 ... Band pass filter, 15 ... Anomalous dispersion fiber, 16 ... Traveling wave type semiconductor laser, 17 ... Excitation light, 18 ... Signal light, 19 ... Output light.

Claims (2)

[Claims] 1. An optical phase in which signal light and pumping light are simultaneously incident on a traveling wave type semiconductor laser and the refractive index of the active layer of the traveling wave type semiconductor laser is changed to convert the phase or wavelength of the signal light. In the wavelength conversion method, a single optical pulse that includes a larger number of longitudinal modes than the Fabry-Perot semiconductor laser and is generated on the time axis is generated, and the optical pulse is passed through a dispersion-shifted fiber whose group velocity chromatic dispersion is negative. The longitudinal mode is separated and compressed on the time axis to form an optical pulse train, the optical pulse train is amplified through an optical fiber amplifier, noise is removed through a bandpass filter, and weak mode components are removed. An ultra-short optical pulse train is passed through an anomalous dispersion fiber with positive chromatic dispersion of velocity, and the ultra-short optical pulse train is used as pumping light to the traveling wave type semiconductor laser simultaneously with the signal light. Optical phase or wavelength conversion method characterized by morphism. 2. An optical phase or an optical phase for simultaneously injecting signal light and pumping light into a traveling wave type semiconductor laser and changing the refractive index of the active layer of the traveling wave type semiconductor laser to convert the phase or wavelength of the signal light. The wavelength converter is equipped with a Fabry-Perot semiconductor laser, a dispersion shift fiber with a negative group velocity chromatic dispersion, an optical fiber amplifier, a bandpass filter, an anomalous dispersion fiber with a positive group velocity chromatic dispersion, and a traveling wave semiconductor laser. , An ultra-short optical pulse train that contains a large number of longitudinal modes generated from a Fabry-Perot semiconductor laser and passes a single optical pulse on the time axis sequentially through a dispersion shift fiber, an optical fiber amplifier, a bandpass filter, and an anomalous dispersion fiber. The resulting ultrashort optical pulse train was made to enter the traveling wave type semiconductor laser at the same time as the signal light as the excitation light. Light phase or wavelength conversion device according to symptoms.