JPH06350174A - Communication pulse light source device - Google Patents

Abstract

PURPOSE:To reduce a frequency fluctuation between pulses and suppress pulse expansion after an optical fiber transmission and an increase in timing jitters by a method wherein a fluctuation of optical frequency between beam pulses is detected and a feedback loop into a change part of a refractive index is arranged so as to minimize the fluctuation portion. CONSTITUTION:An optical interferometer 7 acts as an optical frequency-strength converter that a change portion of the optical frequency is converted into a change of strength of output beams as to a certain longitudinal mode of an inputted light wave. The thus-generated outputs of the optical interferometer 7 are converted into electric signals by an optical converter 8 and sent to an electric circuit part 9. The electric circuit part 9 transmits signals to a refractive index change part 3 so that input electric signals can be constant, namely frequency of beam pulses can be constant. An increase in line width, namely a fluctuation of the optical frequency between pulses can be suppressed by modulating an effective resonator length by changing the refractive index within the optical reso-l nator 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse light source device for communication, and more particularly to a short optical pulse light source required in a long distance ultra large capacity optical transmission system and optical signal processing. .

[0002]

2. Description of the Related Art In a large-capacity optical transmission system currently in practical use, a direct modulation method of a semiconductor laser (LD) is used. Recently, a transmission method using a short optical pulse such as an optical soliton has been studied for the purpose of constructing an optical transmission system having a higher speed and a larger capacity. Currently, as such a short optical pulse light source for future communication, a gain switching method for a semiconductor laser, a mode locking method for a semiconductor laser, a harmonic mode locking method for a fiber ring laser, etc. are mainly studied.

In the above-mentioned gain switching method for semiconductor lasers, the optical phase between pulses fluctuates irregularly.

The harmonic mode-locking method for the fiber ring laser is a mode-locking method in which a mode locker is driven at a frequency that is a multiple of the fundamental frequency of the laser resonator.
As is well known, in the mode-locking method of a normal fiber ring laser, the cavity length is several meters or more, and the basic repetition frequency is 200 MHz or less. Therefore, the harmonic mode-locking method is indispensable for generating a pulse having a repetition frequency of several gigahertz or more. However, even in such a harmonic mode-locking method for a fiber ring laser, when a plurality of longitudinal modes constituting an optical pulse compete with each other, the optical phase between the pulses fluctuates irregularly. for that reason,
When the optical pulse generated by these methods is used for the signal source of the optical fiber transmission system, the wavelength jitter and the optical nonlinear effect of the optical fiber increase the timing jitter, and the transmission distance is limited (reference: Ming Ding , “Limits of lo
ng-distance soliton transmission in optical fibers
withlaser diodes as pulse sources ”, IEEE Photon.
Tech Lett., 4 (6), pp.667-669).

On the other hand, in the mode-locking method of the semiconductor laser, not only can the optical pulse train in which the optical phases of the pulses are aligned is generated by the mode-locking method of the fundamental frequency, but also an optical resonator can be monolithically formed. By configuring, there is a possibility that a stable pulse light source device that does not suffer from thermal or mechanical disturbance can be configured.

[0006]

By the way, in mode-locking of the fundamental frequency, the resonator length becomes short in inverse proportion to the repetition frequency, and becomes 5 mm or less at 10 GHz or more.
According to the well-known theory for determining the laser line width by Shallow Towns, the line width of laser oscillation is inversely proportional to the cavity length. Therefore, when the cavity length of the laser light source becomes short, the line width increases.

This increase in line width mainly corresponds to an increase in optical phase noise (fluctuation of oscillation optical frequency) in the case of continuous oscillation operation in a single longitudinal mode, but in the case of mode-locked pulse oscillation operation, It corresponds to the increase of the optical phase fluctuation existing over a plurality of light pulses, that is, the fluctuation of the optical frequency between the pulses.

Therefore, the mode-locking method for high-speed pulses has a problem that the optical frequency fluctuation between pulses is large, which leads to an increase in timing jitter after optical fiber transmission.

Further, in the light source for communication, it is necessary to generate an optical pulse in synchronization with an external electric signal, so that the active mode locking method is essential among the mode locking methods. In the conventional active mode-locking method for semiconductor lasers, the carrier density is modulated by injecting an external current signal into a gain medium to alternately give a gain and a loss to an optical wave in a resonator passing through the gain medium (optical gate Operation), mode synchronization was realized. However, the fluctuation of the carrier density in the gain medium is
When the gain is modulated, the refractive index of the gain medium fluctuates at the same time, and the fluctuation of the oscillation wavelength within each pulse (chiaping) is also induced. This makes the envelope of the optical spectrum wider than necessary compared to the pulse width,
There is a problem that it causes an excessive spread of the pulse after the optical fiber transmission.

[0010]

FIG. 1 is a block diagram showing an example of the basic configuration of the present invention. Referring to this figure,
The configuration of the present invention will be described. That is, the communication pulse light source device of the present invention includes a gain medium section 1 having a gain in a certain wavelength range, an optical gate section 2 in which an optical absorption rate in the wavelength range is changed by an applied voltage, and an optical gate section 2 from the outside. An optical signal having a refractive index changing section 3 whose refractive index changes according to the signal of 1. and an optical signal having a frequency substantially the reciprocal of the optical circulation (reciprocating) time of the optical resonator 4. To the optical gate unit 2, and a part of the output light of the optical resonator 4 is incident through the optical branching unit 6, and the center frequency of a certain longitudinal mode spectrum. Optical interferometer 7 designed to be converted into a change in optical power and output, a photoelectric converter 8 for detecting the output light incident on the optical interferometer 7, and the photoelectric converter 8 Input the output of An electrical circuit portion 9 is fed back to the refractive index change portion 3 of the optical resonator 4 to small, it is characterized in that it consists of.

In the above structure, the optical interferometer is preferably a Fabry-Perot interferometer or a Mach-Zehnder interferometer.

[0012]

The optical pulse emitted from the optical resonator 4 is partially taken out by the optical splitter 6 and guided to the optical interferometer 7. The optical interferometer 7 functions as an optical frequency-intensity converter that converts a change in the optical frequency of one longitudinal mode of an input light wave into a change in the intensity of output light.
FIG. 2 schematically shows how the conversion is viewed in the optical frequency region. The center of the spectrum of one longitudinal mode of the mode-locked output pulse is set at the position of the shoulder of the optical frequency-transmittance curve of the optical interferometer. At this time, it is necessary to take care not to mix the output from other longitudinal modes with the transmitted output light of the longitudinal mode spectrum of interest. The weak fluctuation of the optical frequency in this attention mode is converted into the intensity fluctuation of the output light of the optical interferometer.

The output of the optical interferometer 7 thus generated is converted into an electric signal by the photoelectric conversion unit 8 and sent to the electric circuit unit 9. The electric circuit section 9 sends a signal to the refractive index changing section 3 of the optical resonator 4 so that the input electric signal becomes constant, that is, the frequency of the optical pulse becomes constant. The increase of the line width, that is, the fluctuation of the optical frequency between the pulses can be suppressed by modulating the effective resonator length by changing the refractive index in the optical resonator 4. Therefore, this feedback loop outputs an optical pulse train in which fluctuations in the optical frequency of the output pulse train are suppressed.

A gain medium portion 1 and an optical gate portion 2 are formed in the optical resonator 4 on the semiconductor substrate. The active mode-locking method modulates the optical wave inside the resonator at a frequency that is almost the reciprocal of the optical round-trip time in the optical resonator (within an error of about ± 5%), and excites multiple resonator modes in the same phase. This is a method of generating an optical pulse. In the present invention, unlike the conventional semiconductor laser mode-locking method in which the injection current of the gain medium is directly modulated, the injection current to the gain medium is constant and the optical gate unit 2 provided in a region different from the gain medium is driven by a drive signal. Since it is driven by, there is no fluctuation in the refractive index of the gain medium. In addition, the optical gate unit 2 is provided with a so-called EA (Electroabsorption) modulator that modulates the band structure near the semiconductor conduction band and the valence band according to the applied electric field and changes the absorptance with respect to the incident light wave. , Its refractive index change is so small that it can be ignored. Therefore, since there is no portion where the refractive index fluctuates inside the optical resonator 4, it is possible to reduce the chaping caused by the refractive index fluctuation that has occurred in the conventional gain medium to a negligible amount. Further, in the EA modulator, the gate width for light is compressed to about 1/10 of the half cycle of the applied sine wave voltage due to the non-linearity of the applied voltage and the light transmission characteristic, and compared with the modulation of the gain medium. It is possible to generate shorter light pulses.

Further, since the optical gate section 2 can be monolithically integrated with the gain medium section 1, it is possible to provide a simple light source device having a small volume which is not subjected to thermal or mechanical disturbance.

The difference between the present invention and the prior art is that, firstly, a change in the optical frequency between optical pulses is detected, and a feed pack loop to the refractive index changing portion is set so that the change is minimized. The installation is to suppress the frequency fluctuation between optical pulses. Secondly, the optical gate portion in the optical resonator is provided separately from the gain medium, and the monolithic EA modulator is provided, whereby the chapering is eliminated. By these, the present invention,
It is applicable to a long-distance high-speed optical transmission system as a light source, and this point is different from the prior art.

[0017]

Embodiments of the present invention will be described in detail below with reference to the drawings.

(Embodiment 1) FIG. 3 shows a first embodiment of the present invention. The semiconductor gain medium part 1 is formed on the same semiconductor substrate 10.
1, EA gate section (optical gate section) 12, refractive index changing section 1
3 are monolithically integrated. In the case of generating a pulse in the 1.5 μm band, the gain medium 11 has n-
InGaAsP layer, InGaAsP / InGaAs multiple quantum well (MQW)
Layer, a p-InGaAsP layer, and a p-InP layer are grown, and an electrode 11a is placed on the grown layer, and the EA gate portion 12 is
This can be realized by growing an n-InAlAs layer, an InGaAs / InAlAs-MQW layer, a p-InAlAs layer, and a p-InGaAs layer, and disposing an electrode 12a on the grown layer. Further, the refractive index changing portion 13 can be realized by forming a laminated structure in which the MQW layer is omitted from the layer structure of the gain medium portion 11 and providing the control electrode 13a on the upper portion. Further, the substrate 10 is provided with a diffraction grating 15 close to the waveguide layer of the refractive index changing portion 13 and along the input / output direction of the resonator 14, whereby the device is provided with a pitch of the diffraction grating 15. It has a distributed Bragg reflection structure that reflects only the wavelengths that match the (peak-to-peak spacing). This is to oscillate in the wavelength region where the reflectance is the highest in the gain wavelength region, so that the oscillation wavelength region can be adjusted by controlling the pitch of the diffraction grating 15. The refractive index of the refractive index changing portion 13 is changed by the current injected from the electric circuit portion 16 through the control electrode 13a provided above the refractive index changing portion 13, and the effective pitch of the diffraction grating 15 is changed. You can control. The frequency of the signal applied to the EA gate unit 12, which is the optical gate unit, is a frequency that is approximately the reciprocal of the optical circulation time of the optical resonator 14, and an error of about ± 5% is allowed. The length of the optical resonator 14 is about 2.5 mm
Then, an optical pulse train with a repetition frequency of 20 GHz can be generated. If the signal applied to the EA gate unit 12, which is the optical gate unit, is a sine wave of 20 GHz, its gate width will be about 5 ps due to the nonlinearity of the EA transmission characteristic, so that an optical pulse with a pulse width of 1 ps or less can be expected. .

The light pulse leaked from the high reflection film 17 formed on the back surface of the substrate 10 passes through a lens system 18 for converting into a parallel beam and is guided to a Fabry-Perot interferometer 19. This Fabry-Perot interferometer 19 has a free spectral range of 500 GHz with a reflecting mirror spacing of 0.3 mm.
In addition, the optical frequency resolution is set to about 100 MHz, and the longitudinal mode of mode-locked pulses at 20 GHz intervals is set to 1
For one, the change in the optical output depending on the fluctuation amount of the optical frequency can be extracted. The output light of the Fabry-Perot interferometer 19 is photoelectrically converted by the photoelectric conversion unit 20, and an electric signal corresponding to the fluctuation amount of the optical frequency between the pulses is sent to the electric circuit unit 16. In this electric circuit section 16, about 100MHz
The above high-frequency components are removed by a high-frequency removal filter (low-pass filter) so that the fluctuation amount of the components of 100 MHz or less that reflects the fluctuation of the optical frequency is reduced.
An electric current for feed-packing is injected into the refractive index changing section 13.

(Second Embodiment) FIG. 4 shows a second embodiment of the present invention. In the figure, the same components as those in FIG. 1 are designated by the same reference numerals to simplify the description.

In the present embodiment, a saturable absorber 21 for pulse narrowing is added to the structure of the first embodiment so that collision mode locking occurs, and the output pulse of the output pulse is generated by the data given by the electric signal. The optical EA modulator 22 for performing intensity modulation is monolithically integrated outside the optical resonator 24. That is, the saturable absorber 21 is provided at the center of the optical resonator 24 in the axial direction, and the optical resonator 2
4 is provided with the first optical gate portion 23a and the second optical gate portion 23b at both ends. As a result, two optical pulses simultaneously exist in the optical resonator 24 and collide with the saturable absorber 21 at the center of the optical resonator 24.
The effect of absorption saturation doubles, resulting in shorter light pulses. If the length of the optical resonator 24 is about 2.5 mm and the gate drive frequency is 40 GHz, the repetition frequency is 40 GHz.
The optical pulse train can be generated. Optical resonator 24
Since the same gain saturation effect is given to the two optical pulses in the optical resonator 24, the positions of the gain medium portions 25a and 25b in the axial direction of the optical resonator 24 are exactly 4 minutes from the end of the optical resonator 24. It is provided at position 1. The conversion of the fluctuation of the optical frequency into the light intensity is executed in two steps here. This state is shown in FIG. First, the optical pulse emitted from the high-reflection film 17 shown on the left side of FIG. 4 has a resonator length of about 0.15 mm and a finesse (ratio of free spectral region and frequency resolution) of about 25. One of the longitudinal modes of the mode-locking pulse is extracted by the Perot interferometer 26a. Then, the one extracted longitudinal mode is incident on the second Fabry-Perot interferometer 26b adjusted so as to lie just on the shoulder of the transmittance curve. The second Fabry-Perot interferometer 26b has a resonator length of 10 mm and a finesse of 500 (frequency resolution conversion; 30 in order to efficiently convert fluctuations of a frequency of about 10 MHz into intensity fluctuations.
MHz). With this configuration, 1MHz
The frequency fluctuation can be converted into a light intensity change of about 10%. Thus, by performing the frequency-intensity conversion with the two Fabry-Perot interferometers 26a and 26b,
Compared with the conversion method using one Fabry-Perot interferometer in the first embodiment, the degree of freedom in designing the individual interferometers 26a and 26b can be increased, and more stable frequency-intensity conversion can be performed. Further, the Fabry-Perot interferometer and the electric circuit section are separate parts from the mode-period optical resonator in the configuration of FIG. 3, but the OEIC (Opt-Electric
It is also possible to form an integrated circuit (a circuit in which an optical circuit and an electronic circuit are monolithically integrated) and form them on the same semiconductor substrate as the mode-locked optical resonator.

(Third Embodiment) FIG. 6 shows a third embodiment of the present invention. In this embodiment, the structure of the second embodiment is a ring resonator structure. Assuming that the total length of the ring-shaped optical resonator 30 is L, the space between the saturable absorber 31 and the gain medium part 32 and the space between the gain medium part 32 and the optical gate part 33 are set to L / 4, respectively. Rotate in the opposite direction in the resonator 30 2
The two pulses are designed to overlap in the saturable absorption section 31 and the optical gate section 33 and enter the gain medium section 32 at equal intervals. A pulse train having a diameter of 3 mm and a repetition frequency of 10 GHz is generated. Optical gate unit 33
Drives at 10 GHz. The right-handed output light is emitted from the linear optical waveguide 34 provided in the vicinity of the ring-shaped optical resonator 30, modulated by the optical modulator 35 by the data signal, and transmitted. The other left-handed output light is used as monitor light for suppressing fluctuations in frequency between pulses. In the figure, the same reference numerals as those in FIGS. 3 and 4 denote the same components, 36 is a refractive index changing portion of the ring-shaped optical resonator 30, and 37 is a semiconductor substrate.

[0023]

Since the device of the present invention has no chipping and little frequency fluctuation between pulses, it can be effectively used as an optical pulse light source for communication with little increase in pulse spread and timing jitter after optical fiber transmission. is there.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a basic configuration of a pulse light source device according to the present invention.

FIG. 2 is a graph for explaining one of the actions of the pulse light source device according to the present invention.

FIG. 3 is a configuration diagram showing a first embodiment of a pulse light source device according to the present invention.

FIG. 4 is a configuration diagram showing a second embodiment of the pulse light source device according to the present invention.

FIG. 5 is a graph for explaining the operation of the second embodiment of the present invention.

FIG. 6 is a configuration diagram showing a third embodiment of the pulse light source device according to the present invention.

[Explanation of symbols]

 10 semiconductor substrate 11 gain medium part 11a electrode 12 EA gate part (optical gate part) 12a electrode 13 refractive index changing part 13a electrode 14 optical resonator 15 grating 16 electric circuit part 17 high reflection film 18 lens system 19 Fabry-Perot interferometer 20 photoelectric conversion unit 21 saturable absorption unit 22 optical EA modulation unit 23a first optical gate unit 23b second optical gate unit 24 optical resonator 25a gain medium unit 25b gain medium unit 26a first Fabry-Perot interferometer 26b Second Fabry-Perot interferometer 30 Ring-shaped optical resonator 31 Saturable absorption section 32 Gain medium section 33 Optical gate section 34 Optical waveguide 35 Optical modulation section 36 Refractive index changing section 37 Semiconductor substrate

Claims (1)

[Claims] 1. A gain medium section having a gain in a certain wavelength range, an optical gate section in which a light absorption rate in the wavelength range is changed by an applied voltage, and a refraction in which a refractive index is changed by an external signal. An optical resonator in which a rate changing unit is arranged and formed inside the optical resonator, and an optical gate unit for applying an electrical signal having a frequency that is the reciprocal of the optical circulation (round-trip) time of the optical resonator to the optical gate unit. A drive circuit, an optical interferometer that converts a change in center frequency of the output light from the optical resonator into a change in optical power, and a photoelectric detector that detects output light of the optical interferometer. A communication unit, comprising: a conversion unit; and an electric circuit unit that inputs an output of the photoelectric conversion unit and feeds back the output to the refractive index changing unit of the optical resonator so that a fluctuation component of the signal is reduced. Pulse light source device