JP2006114584A - Optical subcarrier transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical sub-carrier transmitter which may be driven with an output higher than that of the optical sub-carrier transmitter of prior art and moreover having digital signal of small amplitude. <P>SOLUTION: The optical sub-carrier transmitter comprises a current input means 105 for inputting DC current to a gain region 101, by optically coupling a field-absorbing type light modulating region 102 for changing its light absorbing coefficient accompanying the applied field to one end side of the gain region 101; forming a Bragg reflector region 103 designed to result in the light spectrum formed of major two vertical modes of the output light to the upper surface of the other end side of the gain region 101, and by optically coupling the regions 102 and 103 sandwiching the gain region 101; a high-frequency wave input means 106 for externally inputting high-frequency signal to the field-absorbing type optical modulator region 102; and modulating current input means 108, 109 for inputting the bias signal modulated with digital signal to the Bragg reflector region 103. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical subcarrier transmitter. For example, it is used for an optical fiber wireless link that transmits an optical signal superimposed with a wireless signal through an optical fiber.

In recent years, mobile communication services such as mobile communication and personal communication have rapidly spread, and wireless access systems based on wireless LAN have also spread in private networks.
In these radio access systems, the use of millimeter waves is being studied as a method for increasing the transmission signal capacity and exploiting radio frequency resources.
As a method of efficiently distributing this millimeter-wave radio signal from a centralized base station to multiple antennas, a method of distributing an optical signal (optical subcarrier) superimposed with a radio signal via an optical fiber (optical fiber radio link) is drawing attention. Has been.

As a method for superimposing a millimeter-wave band radio signal on an optical signal in this optical fiber radio link, a method using a semiconductor mode-locked laser that can efficiently generate optical subcarriers synchronized with a reference signal is conceivable.
Normally, a mode-locked laser is tuned by a single frequency reference signal, and a digital signal to be transmitted is not superimposed on an output optical subcarrier.
Conventionally, in order to superimpose a digital signal on an optical subcarrier, the output light of a mode-locked laser is intensity-modulated with a digital signal using an external optical modulator such as a Mach-Zehnder optical modulator or an electroabsorption optical modulator. It was.

However, the module-to-module connection between the mode-locked laser and the external optical modulator is too expensive to be practical, and the optical output is greatly reduced due to loss of the external optical modulator itself and coupling loss with the fiber. There was a drawback that it would.
On the other hand, a method of significantly reducing costs by monolithically integrating a semiconductor mode-locked laser and an electro-absorption optical modulator for encoding with a digital signal into a single module has also been reported (non-) Patent Document 1).

A configuration example of a conventional optical millimeter wave transmitter is shown in FIG.
The optical millimeter-wave transmitter shown in FIG. 4 has a semiconductor mode-locked laser 304 and an encoding electroabsorption optical modulator 305 formed on the same semiconductor substrate.
The semiconductor mode-locked laser 304 includes a gain region 301, an electroabsorption optical modulator region 302 formed adjacent to the left side of the gain region 301, and a Bragg reflector 303 formed at the right end of the gain region 301. The
The gain region 301 and the Bragg reflector 303 are electrically connected to the current source 307 and the current source 308, respectively, and the electroabsorption optical modulator region 302 is electrically connected to the oscillator 306.

When a high-frequency signal having a frequency close to the resonance frequency of the semiconductor mode-locked laser 304 is applied from the oscillator 306 to the electroabsorption optical modulator region 302 while a direct current is input to the gain region 301 and the Bragg reflector 303, the high-frequency signal Is generated from the semiconductor mode-locked laser 304.
On the other hand, the encoding electroabsorption optical modulator 305 is electrically connected to the digital signal source 310 and the voltage source 311 via the bias circuit 309, and has a digital signal source together with a reverse bias applied from the voltage source 311. When a digital signal is input from 310, the intensity of the optical pulse train propagating through the encoding electroabsorption optical modulator 305 is modulated by this digital signal.

In this way, in an element in which the semiconductor mode-locked laser 304 and the electroabsorption optical modulator 305 are monolithically integrated, an optical subcarrier signal encoded by a digital signal can be generated by one element. The cost can be greatly reduced as compared with the case where the optical modulator is connected between modules.
However, in this case as well, as in the case where the modules are connected, the loss of the modulator itself is large, so that the optical output is greatly reduced as compared with the mode-locked laser alone.
In addition, since the electroabsorption optical modulator 305 is driven, it is necessary to supply a large amplitude digital signal to the element.
K. Sato, I. Kotaka, Y. Kondo, M. Yamamoto Actively mode-locked strained-InGaAsP multiquantum-well lasers integrated with electroabsorption modulators and distributed Bragg reflectors, "IEEE J. Select, Topics in Quantum Electron., Vol. 2 , (3), pp. 557-565, 1996.

As described above, in the optical subcarrier transmitter in which the conventional semiconductor mode-locked laser and the electro-absorption optical modulator for encoding are integrated, the loss of the modulator itself is large. There was a drawback that the output was greatly reduced.
Further, in order to drive the electroabsorption optical modulator, there is a problem that it is necessary to supply a digital signal having a large amplitude to the element.
An object of the present invention is to provide an optical subcarrier transmitter that can be driven by a digital signal having a higher output and a smaller amplitude than those of a conventional optical subcarrier transmitter.

  The optical subcarrier transmitter according to claim 1 of the present invention for solving the above-mentioned problems is mainly composed of an electroabsorption optical modulation region in which an absorption coefficient of light changes in accordance with an applied electric field, and an optical spectrum of output light. A Bragg reflector region designed to consist of a longitudinal mode of a book, optically coupled with a gain region interposed therebetween, and a current input means for inputting a direct current into the gain region; and the electroabsorption optical modulation A high-frequency signal input means for inputting a high-frequency signal from the outside to the storage area; and a modulation current input means for inputting a bias current modulated by a digital signal to the Bragg reflector area.

  An optical subcarrier transmitter according to a second aspect of the present invention for solving the above-described problem is the optical subcarrier transmitter according to the first aspect, wherein instead of optically coupling the gain region and the Bragg reflector region, the gain subregion is on the gain region. The Bragg reflector region is formed in part.

  An optical subcarrier transmitter according to a third aspect of the present invention for solving the above-described problems is the chirped Bragg reflector according to the first or second aspect, wherein the Bragg reflector region has a diffraction grating period changed at a constant rate. It is characterized by using.

  An optical subcarrier transmitter according to claim 4 of the present invention for solving the above-mentioned problems is characterized in that, in claim 1, 2 or 3, a multiple quantum well structure is used for the absorption layer of the electroabsorption optical modulation region. And

  An optical subcarrier transmitter according to claim 5 of the present invention for solving the above-mentioned problems is characterized in that, in claim 1, 2, 3 or 4, a multiple quantum well structure is used for the absorption layer in the gain region. .

According to the optical subcarrier transmitter of the present invention, it is possible to realize an optical subcarrier transmitter that can be driven by a digital signal having a higher output and a smaller amplitude than the conventional optical subcarrier transmitter.
Even with a semiconductor mode-locked laser with a conventional structure, modulation of the injection current of the Bragg reflector portion was possible, but the repetition frequency only slightly changed according to the modulation.
On the other hand, since the present invention uses a semiconductor mode-locked laser that generates two longitudinal mode oscillations, it is possible to modulate the intensity of the optical subcarrier.

  The best mode for carrying out the present invention is Examples 1 and 2 described below.

A first embodiment of an optical subcarrier transmitter according to the present invention is shown in FIG.
A semiconductor mode-locked laser 104 shown in FIG. 1 is configured by forming a multiple quantum well electroabsorption optical modulation region 102 and a gain region 101 on the same semiconductor substrate.
The gain region 101 has a structure in which an InGaAsP / InGaAsP multiple quantum well having a center wavelength of 1.55 μm is sandwiched from above and below by an InGaAsP optical confinement layer having a center wavelength of 1.3 μm, and is further sandwiched from above and below by an InP cladding layer.

An electro-absorption optical modulator region 102 is formed adjacent to the left side of the gain region 101 and is optically coupled to the left end side of the gain region 101.
The electroabsorption optical modulator region 102 has a structure in which an InGaAsP / InGaAsP multiple quantum well having a center wavelength of 1.48 μm is sandwiched from above and below by an InP cladding layer.
A Bragg reflector 103 is formed on the upper surface of the right end portion of the gain region 101 and is optically coupled to the right end side of the gain region 101.
The Bragg reflector 103 forms a diffraction grating by processing the interface between the InGaAsP optical confinement layer having a center wavelength of 1.3 μm and the InP cladding layer.

The gain region 101 is electrically connected to the current source 105, and the electroabsorption optical modulator region 102 is electrically connected to the oscillator 106.
The Bragg reflector 103 is connected to the current source 107 and the digital signal source 108 via the bias circuit 109, and a bias current modulated by the digital signal is input to the Bragg reflector 103.

Accordingly, a high-frequency signal having a frequency close to the resonance frequency of the semiconductor mode-locked laser 104 or 1 / n (n is a natural number) is input to the oscillator 106 while a direct current is input to the gain region 101 and the Bragg reflector 103. Is applied to the electroabsorption optical modulator region 102, an optical pulse train synchronized with the high-frequency signal is generated from the semiconductor mode-locked laser 104.
In this embodiment, the resonance frequency of the semiconductor mode-locked laser 104 is about 125 GHz, the frequency of the high-frequency signal output from the oscillator 106 is 25 GHz (about 1/5 of the resonance frequency of the semiconductor mode-locked laser 104), and a pulse train of 125 GHz is used. Generated.

In addition, the signal speed of the digital signal input to the Bragg reflector 103 is 10 Gbit / s.
The Bragg reflector 103 changes the center wavelength (Bragg wavelength) of the reflection spectrum by changing the amount of injected current. At this time, the half-width of the reflection spectrum of the Bragg reflector 103 is mainly 2 in the output light spectrum of the mode-locked laser. When the amount of current injected into the Bragg reflector 103 is changed by designing it to be composed of a single longitudinal mode, as shown in FIG. 2, the number of main longitudinal modes included in the spectrum is two (injection current I ₁ ) And one (injection current I ₂ ).

Specifically, in this embodiment, the coupling constant of the Bragg reflector is 100 cm ⁻¹ and the length is 100 μm.
125 GHz optical subcarriers are generated in the 2 mode, and simple CW light is generated in the 1 mode (there is no 125 GHz component). Therefore, the upper and lower levels of the modulation current input to the Bragg reflector 103 are set to I ₁ and I By setting to ₂ , 125 according to the current modulation
The intensity modulation of the optical subcarrier of GHz is realized.
By modulating (encoding) the optical subcarrier by such a method, it is not necessary to use an intensity modulator having a large loss, and an optical subcarrier having a large output can be generated.

Further, as shown in FIG. 2, the difference in injection current required to change the Bragg wavelength of the Bragg reflector 103 by about half the mode interval of the 125 GHz mode-locked laser (about 0.5 nm) is about 5 to 10 mA. Therefore, even if the termination method is adjusted to match 50Ω, a digital signal may be applied with an amplitude of about 0.25 to 0.5V.
On the other hand, when an optical subcarrier is modulated using an electroabsorption optical modulator as in the conventional example, a voltage amplitude of at least about 2 V is required. Therefore, a digital signal necessary for modulation can be obtained by using the present invention. Can be suppressed to 1/4 or less.

In this embodiment, the Bragg reflector 103 is formed on the gain region 101. However, the Bragg reflector 103 may be formed on a transparent waveguide that does not generate gain or on the same layer structure as the electroabsorption optical modulator. The effect is obtained.
In this embodiment, the multiple quantum well structure is used for the absorption layer of the electroabsorption optical modulation region 102, but a bulk absorption layer may be used.

However, in this case, the efficiency of synchronization with a high-frequency electric signal is somewhat worse than when a multiple quantum well structure is used.
Further, in this embodiment, a multiple quantum well structure is also used for the gain region 101, but this can also be replaced by a bulk active layer.
However, in this case, there is a drawback that the threshold current becomes larger than when an active layer having a multiple quantum well structure is used.
Furthermore, although the carrier frequency is set to 125 GHz in this embodiment, an optical subcarrier transmitter can be configured with the same configuration even at different frequencies.

A second embodiment of the optical subcarrier transmitter according to the present invention is shown in FIG.
In this embodiment, as shown in FIG. 3, a chirped Bragg reflector 110 is used instead of the Bragg reflector 103 of the first embodiment. Other configurations are the same as those of the optical subcarrier transmitter shown in FIG.
The chirp Bragg reflector 110 is obtained by changing the period of the diffraction grating of the Bragg reflector at a constant rate, and has a feature that the delay amount due to reflection changes according to the wavelength.

By using this chirp Bragg reflector 110, the effective resonator length (that is, the repetition frequency) of the mode-locked laser becomes wavelength-dependent, and conversely, the two longitudinal modes used when the repetition frequency is determined. Mode selectivity occurs in which is uniquely determined.
Therefore, by replacing the Bragg reflector 103 used in the first embodiment of the present invention with the chirp Bragg reflector 110, there is an advantage that the two-mode oscillation state in FIG.

In addition, since different repetition frequencies can be dealt with by changing the oscillation wavelength, there is also an advantage that mode locking can be achieved in a wider frequency range as compared with the case where a Bragg reflector without chirping is used.
Specifically, the mode synchronization range when using a normal Bragg reflector is about 0.2% of the repetition frequency, whereas the mode synchronization range when using a chirp Bragg reflector is the repetition frequency. About 2.5%, which is 10 times or more.

  As described above, the present invention is characterized in that, in an optical subcarrier transmitter that generates an optical signal on which a radio signal is superimposed, a modulation current is input to a Bragg reflector region of a semiconductor mode-locked laser, Therefore, it is possible to realize an optical subcarrier transmitter that can be driven by a digital signal with high output and small amplitude.

  INDUSTRIAL APPLICABILITY The present invention can be widely used in an optical fiber wireless link for transmitting an optical signal on which a wireless signal is superimposed by an optical fiber, and a wireless access system using a wireless LAN.

It is a schematic block diagram which shows 1st embodiment of this invention. It is a graph which shows a mode that the modulation | alteration of an optical subcarrier is performed by 1st embodiment of this invention. It is a schematic block diagram which shows 2nd embodiment of this invention. It is a block diagram of the conventional optical subcarrier transmitter.

Explanation of symbols

101 Gain region 102 Multiple quantum well electroabsorption optical modulator region 103 Bragg reflector 104 Semiconductor mode-locked laser 105 Current source 106 Oscillator 107 Current source 108 Digital signal source 109 Bias circuit 110 Chirp Bragg reflector 301 Gain region 302 Electroabsorption type Optical modulator region 303 Bragg reflector 304 Semiconductor mode-locked laser 305 Encoding electro-absorption optical modulator 306 Oscillator 307 Current source 308 Current source 309 Bias circuit 310 Digital signal source 311 Voltage source

Claims (5)

 An electroabsorption type optical modulation region in which the light absorption coefficient changes according to the applied electric field, and a Bragg reflector region designed so that the optical spectrum of the output light consists of two main longitudinal modes are divided into gain regions. Current input means for optically coupling with each other and inputting a direct current to the gain region, high frequency signal input means for inputting a high frequency signal from the outside to the electroabsorption optical modulator region, and the Bragg reflector An optical subcarrier transmitter comprising modulation current input means for inputting a bias current modulated with a digital signal into a region.  2. The optical subcarrier transmission according to claim 1, wherein the Bragg reflector region is formed in a part on the gain region, instead of optically coupling the gain region and the Bragg reflector region. vessel.  The optical subcarrier transmitter according to claim 1 or 2, wherein a chirped Bragg reflector whose period of the diffraction grating is changed at a constant rate is used as the Bragg reflector region.  4. The optical subcarrier transmitter according to claim 1, wherein a multi-quantum well structure is used for an absorption layer in the electroabsorption optical modulation region. 5. The optical subcarrier transmitter according to claim 1, wherein a multiple quantum well structure is used for the absorption layer in the gain region.

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