JPH08264872A - Synchronous short optical pulse train generator - Google Patents

Abstract

PURPOSE: To sufficiently converge a pulsed light with reduced loss. CONSTITUTION: An optical pulse generator is formed in combination of a semiconductor laser 101 accompanied with a spot size converter and provided with an antireflection coating 102, and a light guided part 103 coated with a semipermeable film 104. A plurality of the pulse generators are provided to form an optical pulse train generator 105. Each pulse generator has a high frequency modulator 106 and a phase delay unit 107. The light reflected on the film 104 is circulated (reflected) in an external resonator, and each time it is modulated by the laser 101 with the converter, the pulse width is sharpened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronous short optical pulse train generator used as a transmission source for time division multiplexing in ultra high speed optical communication.

[0002]

2. Description of the Related Art FIG. 6 shows a conventional synchronous short optical pulse train generator. In the figure, 1 is a semiconductor laser having an anti-reflection coating 2 on one end face, 3 is a semi-transparent mirror, and 4 is a lens.
Are formed, and a plurality of the short light pulse generation units 5 are required. A high frequency modulator 6 for driving the plurality of short optical pulse generators 5 and a phase delay device 7 for generating signals from the high frequency modulators 6 having different phases are provided in each of the short optical pulse generators 5. Has been added.

In such a configuration system, the gain of the semiconductor laser (semiconductor gain section) 1 reaches a peak at a modulation frequency corresponding to an integral multiple of the round-trip time of the cavity length of the short optical pulse generator 5. A high frequency signal that slightly exceeds the value is applied to the semiconductor laser 1. Under such an operating condition, every time the optical pulse passes through the semiconductor gain section 1, the central portion of the pulse receives a larger gain than both tails, so that the pulse becomes sharper each time it is modulated. Furthermore, since the light intensity is strong near the peak of the optical pulse, the gain in the gain region sharply decreases due to the transmission of the peak (gain saturation). Therefore, only the first half of the pulse is amplified, and the effect of cutting the latter half is also added, making the pulse even sharper.
A short light pulse of psec width is obtained. At this time, it is possible to obtain a synchronized pulse train by adjusting the delay amount of the phase delay device 7 so that the pulse trains generated from the plurality of short optical pulse generators 5 do not overlap.

[0004]

However, in the conventional synchronous short optical pulse train generator, in order to create the short optical pulse generator 5, each resonator length or phase delay device 7 to the phase of the semiconductor laser 1 is changed. Due to the variation occurring in the delay section length, variation occurs in the pulse transmission frequency and the delay time. For example, when the external cavity length is L, the time interval t = 2L / c (c: speed of light) between short optical pulse peaks is 100 ps.
ec short pulse train generator (external cavity length L = 1.5c
In m), the pulse peak interval fluctuates by several tens of psec when the fluctuation of the external resonator length ΔL = 1 μm, and there is a high possibility that a bit error will occur due to the overlapping of a plurality of generated short optical pulse trains.

Further, in the optical coupling mechanism between the semiconductor gain section 1 and the external semi-transparent mirror 3 via the coupling lens 4, since the coupling efficiency is low, the pulsed light has a large loss during the circulation in the external resonator, and the pulsed light There are problems that the light is not sufficiently narrowed and that a stable pulse signal cannot be obtained due to strict tolerance for axis deviation.

[0006]

In order to solve the above-mentioned problems, a synchronous short optical pulse train generator of the present invention comprises a semiconductor gain section, a spot size conversion section associated with the semiconductor gain section, and the spot size conversion section. Is optically coupled with one end face,
A plurality of short optical pulse generators composed of an optical waveguide having a semi-transmissive film on the other end face, a plurality of high frequency modulators for generating high frequency modulation signals, and high frequency modulators generated by the high frequency modulators. It is characterized in that it comprises a plurality of phase delay devices which make the phases of the signals different and apply the high-frequency modulated signals having the different phases to the short optical pulse generator.

Furthermore, the synchronous short optical pulse train generator of the present invention has a short optical pulse generator in which the semiconductor gain section, the spot size conversion section, and the optical waveguide section are integrally formed on a semiconductor substrate. It is characterized by

[0008]

The phase of the high frequency signal generated by the high frequency modulator is shifted by a phase delay unit for a fixed period of time and applied to the semiconductor gain section. The applied high-frequency signal enters the optical waveguide through the spot size conversion unit and is split into transmitted light and reflected light. The reflected light circulates (reflects) in the external resonator and is modulated by the semiconductor gain unit. The pulse width becomes sharper each time.

[0009]

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

<First Embodiment> FIG. 1 shows a synchronous short optical pulse train generator according to a first embodiment of the present invention. As shown in the figure, a semiconductor laser (semiconductor gain section) 1 with a spot size conversion section having an antireflection coating 102 on one end surface 1
01 and the optical waveguide portion 103 having a semi-transmissive film 104 coated on one end face thereof to form a short optical pulse generation portion. A plurality of the short light pulse generators are integrated on the same substrate to form the short light pulse train generator 105. Further, a high frequency modulator 106 for driving each short optical pulse generator and a phase delayer 107 for generating signals from the high frequency modulator 106 having different phases are provided in each short optical pulse generator. Has been. Then, an external resonator having the semi-transmissive film 104 and the cleavage plane of the semiconductor laser 101 as two reflecting mirrors is formed.

In this embodiment, the short optical pulse train generator 105 is used.
The phase delay unit 107 outputs a high-frequency signal at a modulation frequency corresponding to an integral multiple of the round-trip time of the external resonator length, and at which the gain of the semiconductor laser (semiconductor gain unit) 101 peaks and slightly exceeds the threshold value. The semiconductor laser 10 with the spot size conversion unit, the phase of which is shifted by a fixed time by the
Applied to 1. The light incident on the optical waveguide 103 via the spot size converter is split into a transmitted light and a reflected light by the semi-transmissive film 104, and the reflected light circulates within the external resonator (reflects between two reflecting mirrors). The pulse width becomes sharper every time the semiconductor gain section 101 is modulated.

FIG. 2 shows a schematic view of a semiconductor laser 101 with a spot size converter. The active region 201 of the semiconductor gain unit 101 is followed by the spot size conversion unit waveguide region 2
02 is formed, and the spot size is controlled by changing the waveguide width or the layer thickness in a tapered shape.

FIG. 3 shows the optical coupling characteristics of the semiconductor laser 101 with the spot size conversion section and the optical waveguide 103. Coupling efficiency is improved by several orders of magnitude compared to ordinary semiconductor lasers, and horizontal and vertical tolerances (defined at 1 dB deterioration)
Is ± 2 μm, and the optical coupling characteristics are improved.

FIG. 4 shows an actual measurement example of the synchronous short optical pulse train obtained in this embodiment. An extremely stable synchronous short optical pulse train having a pulse peak interval of 20 psec and a pulse half-value width of 2.5 psec is obtained from each short optical pulse generator. Was given.

<Second Embodiment> FIG. 5 shows a short optical pulse train generator used in a synchronous short optical pulse train generator according to a second embodiment of the present invention. In the second embodiment, the semiconductor gain section, the spot size conversion section and the optical waveguide section are integrally formed on the semiconductor substrate. That is, the semiconductor gain section 301 is optically coupled to the optical waveguide section 303 via the spot size conversion section 302. Semi-transmissive film 1 formed on the end face of the optical waveguide section 303
04 and an external resonator having the cleavage planes of the semiconductor gain unit 301 as two reflecting mirrors are formed to form one short optical pulse generation unit. Such short light pulse generators are integrated on a semiconductor substrate in an array. Such a structure can be easily manufactured by a manufacturing technique used for manufacturing a normal semiconductor laser.

Comparing the structure of the second embodiment with that of the first embodiment shown in FIG. 1, it corresponds to the gap between the optical waveguide 103 and the semiconductor laser 101 having the spot size conversion portion in FIG. Since there is nothing, the semiconductor gain unit 30
The optical coupling between 1 and the optical waveguide 303 becomes stronger. Further, the optical axis adjustment and the fixing work between the optical waveguide 103 and the semiconductor laser 101 having the spot size conversion unit are not necessary, the processing cost is suppressed, and the reliability is improved.

[0017]

As described above, in the present invention, the optical coupling mechanism is composed of the spot size conversion section and the optical waveguide section which are attached to the semiconductor gain section. The spot size of the emitted light of the semiconductor laser is usually as small as 1 μm or less,
With this structure, a spot size comparable to that of an optical waveguide portion or an optical fiber forming an external resonator can be obtained by optical coupling, and a low loss optical coupling of 1 dB or less can be achieved with the optical waveguide portion without a lens system. Will be realized. Therefore, the loss of the pulsed light while circulating in the external laser resonator is reduced, and the pulsed light is sufficiently narrowed. Further, since the tolerance for the axis deviation becomes loose, a stable pulse signal can be obtained, and the processing cost such as the optical axis adjustment and the fixing work can be suppressed.

Further, according to the present invention, since the spot size conversion portion and the optical waveguide portion constituting the semiconductor laser and the external resonator can be formed on the same substrate by a photolithography process, the fluctuation of the refractive index and the resonance. It is possible to reduce variations in device length and easily create a pulse generator with the same modulation frequency using the same photomask. Therefore, a compact synchronous short optical pulse train generator with excellent stability and mass productivity is provided. Will be realized. Further, if the drive circuit is also formed on the same substrate, the stability of the phase delay time is improved.

[Brief description of drawings]

FIG. 1 is a block configuration diagram showing a synchronous short optical pulse train generator according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing a semiconductor laser with a spot size conversion unit used in the first embodiment.

FIG. 3 is a characteristic diagram showing a comparison of actual measurement examples of optical coupling characteristics of a semiconductor laser with a spot size converter and a semiconductor laser without a spot size converter.

FIG. 4 is a waveform chart showing an actual measurement example of a synchronous short optical pulse train obtained in the first embodiment.

FIG. 5 is a configuration diagram showing a short light pulse train generation unit used in the second embodiment.

FIG. 6 is a block diagram showing a conventional synchronous short optical pulse train generator.

[Explanation of symbols]

 101 Semiconductor Laser with Spot Size Converter 102 Non-Reflective Coating 103 Optical Waveguide 104 Semi-Transparent Film 105 Short Optical Pulse Train Generator 106 High Frequency Modulator 107 Phase Delay 301 Semiconductor Gain Unit 302 Spot Size Converter 303 Optical Waveguide

Claims (2)

[Claims] 1. A semiconductor gain section, a spot size conversion section associated with the semiconductor gain section, and an optical waveguide section that is optically coupled to the spot size conversion section at one end face and has a semi-transmissive film on the other end face. A plurality of short optical pulse generators, a plurality of high-frequency modulators that generate a high-frequency modulation signal, and a high-frequency modulation signal having a different phase by changing the phase of each high-frequency modulation signal generated by the high-frequency modulator. A short optical pulse train generator for synchronization, comprising a plurality of phase delay devices for applying signals to the short optical pulse generator, respectively. 2. The synchronization according to claim 1, wherein the semiconductor gain section, the spot size conversion section, and the optical waveguide section have a short optical pulse generation section integrally formed on a semiconductor substrate. Short optical pulse train generator.