JPH08139401A - Modulation system of optical transmission equipment and optical communication system using the same - Google Patents

Abstract

PURPOSE: To realize modulation by applying a small current wherein the extinction ratio is large, and the spectrum spread is small. CONSTITUTION: In a semiconductor laser 1 provided with a plurality of electrodes, a reflector having reflection spectrum characteristics in which a plurality of high reflection peaks exist periodically on both sides of a center wavelength is installed. The oscillation wavelength is modulated between two wavelengths λ1 , and λ0 , by controlling and modulating injection currents into a plurality of the electrodes, in the manner in which switching is executed between two different longitudinal modes. The modulated light is passed a wavelength filter 2 having the function which selects only one wavelength out of the two wavelengths. Thereby a signal 6 which is subjected to intensity modulation is sent out, and detected in the receiving side by using a photodiode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a modulation system of an optical transmission device used in an optical communication system such as an optical local area network (LAN) system and an optical communication system using the same.

[0002]

2. Description of the Related Art FIG. 7 shows a conventional example of the present invention. Conventionally, in order to directly modulate the semiconductor laser, a method of injecting a bias current and superimposing the modulation current on the bias current to modulate the optical output of the laser has been adopted. In this case, high-speed modulation results in multimode oscillation. Distributed feedback type (DF
In the dynamic single mode laser such as B) or distributed reflection type (DBR), the spectrum broadening due to multimode oscillation is eliminated, and the spectrum width becomes the width of one oscillation mode. However, even in a dynamic single-mode laser, direct modulation causes a large variation in carrier density in the laser, which causes chirping to broaden the spectrum width and consequently limits the transmission band. Also, when performing wavelength division multiplexing communication, the degree of multiplexing is limited. On the contrary, if the modulation current is made small in order to narrow the spectrum width, a sufficient extinction ratio cannot be obtained and the transmission distance will be limited.

[0003]

As described above, in the conventional direct modulation system of the dynamic single mode semiconductor laser, the spectrum width is widened by the chirping, the transmission bandwidth, the transmission distance, and the number of channels at the time of wavelength multiplexing. There was a problem that such things were restricted.

Therefore, an object of the present invention is to provide an optical transmission device which realizes modulation with a large extinction ratio and a small spectrum spread by a minute current, a modulation system of the optical transmission device, and an optical communication system using the same. It is in.

[0005]

The present invention provides a sampled
In a semiconductor laser equipped with a reflector composed of a grating or a super-periodic structure diffraction grating, the longitudinal mode is switched by the injection current and a wavelength filter is provided on the output side of the laser, so that the extinction ratio is large with a small current. , Which realizes modulation with a small spectrum spread.

More specifically, the modulation method of the optical transmission apparatus of the present invention is provided with a reflector having a reflection spectrum characteristic in which a plurality of high reflection peaks are periodically present on both sides of a certain wavelength as a center. In a semiconductor laser having electrodes, the injection currents to the plurality of electrodes are controlled and modulated so as to perform switching between two different longitudinal modes, so that the oscillation wavelength is modulated between the two wavelengths. Out of 1
It is characterized in that an intensity-modulated signal is sent out by transmitting a wavelength filter having a function of selecting only a wavelength.

The reflector has a structure in which a region where the diffraction grating exists and a region where the diffraction grating does not exist are repeated in a certain cycle, or a region in which the pitch and the phase of the diffraction grating change continuously in a certain cycle. It may be a repeated structure.

Further, in the optical communication system of the present invention, in the modulation system of the above-mentioned optical transmission device, the wavelength of the one of the two longitudinal modes to be transmitted is made variable and injection is performed so as to perform switching between the two longitudinal modes. It is characterized in that a current is modulated and a signal of an arbitrary wavelength is detected on the receiving side by using a wavelength filter.

Further, the wavelength division multiplexing optical communication system of the present invention is characterized in that in the above optical communication system, a plurality of optical transmission devices and a plurality of wavelength filters on the receiving side are used for communication.

Further, the optical transmission device of the present invention comprises a semiconductor having a plurality of electrodes, which is provided with a reflector having a reflection spectrum characteristic in which a plurality of high reflection peaks periodically exist on both sides of a certain wavelength. Has a laser and a wavelength filter,
By controlling and modulating the injection current to multiple electrodes so as to switch between two different longitudinal modes,
It is characterized in that the oscillation wavelength of the semiconductor laser is modulated between two wavelengths, and the wavelength-modulated filter having a function of selecting only one of the two wavelengths is transmitted to transmit an intensity-modulated signal.

[0011]

Embodiment 1 FIG. 1 shows the structure of this embodiment, and FIG. 2 shows the characteristics of the laser used in this embodiment. In recent years, in order to expand the variable width of a wavelength tunable laser, a semiconductor laser has been proposed which has a plurality of reflection peaks and is provided with a reflector that can arbitrarily design the intervals of the reflection peak wavelengths by the structure of a diffraction grating.
For example, sampled grating (V. Jayar
aman Appl. Phys. Lett. , 60 (1
9), 1992 (p.2321)), super-periodic structure diffraction grating (Kano et al., IEEJ, OQE 92-131, 1992).
(P.39)) having a reflector constituted by
FB or DBR laser.

The laser used in this embodiment is a two-electrode DFB whose reflector is a sampled grating.
Laser. The structure of the sampled grating is shown in FIG. In the sampled grating, a portion where the diffraction grating exists and a portion where the diffraction grating does not exist are repeated at a certain period (sampling period length Z ₀ ) as shown in the figure. The characteristics of the reflector are shown in FIG. Wavelength interval Δ of the reflection peak of the reflector composed of sampled grating
λ is determined by the sampling cycle length Z ₀ as follows. Δλ = λ ² / 2μ _g Z ₀ where μ _g is the mode group refractive index (mode group)
index). Therefore, the sampling cycle length Z
By selecting ₀ , it becomes possible to manufacture a reflector having an arbitrary reflection peak wavelength interval.

This semiconductor laser selects one longitudinal mode by slightly shifting the interval between the reflection peak wavelengths of the reflectors in the two regions and injecting a current into each to change the refractive index. Since the intervals of the reflection peak wavelengths are slightly different, different longitudinal modes can be selected one after another by a slight current change due to the vernier effect, and a large wavelength change can be obtained. The oscillation wavelength can be finely adjusted by controlling the injection current after selecting one longitudinal mode.

In the present embodiment, the reflection peak intervals are 10 each.
It was designed to be nm and 11 nm. FIG. 2C shows the relationship between I and the oscillation wavelength λ when the injection current I to the electrode in one region of the laser of this embodiment is changed. λ is I
The wavelength jump is about 10 nm when the longitudinal mode shifts to the next mode.

FIG. 3 shows a laser modulation method. Injecting a bias current 1 ₀ to one electrode of the semiconductor laser, by heavy responsibility the modulation current [Delta] I = (1 ₁ -1 _0), the output light is modulated between a wavelength lambda ₁ and lambda _0. This output light is converted into a wavelength filter (half-value width of 10 nm) according to the wavelength interval between λ ₁ and λ _0.
By transmitting only λ ₁ at a level of about ₁ ), it is possible to extract a signal with power of 1,0.

A normal laser also has a plurality of longitudinal modes, but in a practical structure, the longitudinal mode interval is as narrow as several Å, and it is difficult to separate with a wavelength filter with a high extinction ratio. Further, in this case, when the vertical modes are switched, since the vertical modes are close to each other, competition occurs between a plurality of vertical modes, and noise is generated, which causes instability. Also, DFB or D
Even with a BR laser, a plurality of modes can be selected in the same manner, but the mode interval is also as narrow as several Å, and it is difficult to separate with a wavelength filter at a high extinction ratio.

FIG. 1 shows a block diagram of the first embodiment. Figure 2
The injection current injected into one region of the semiconductor laser 1 having the characteristics as shown in (c) is modulated by the method shown in FIG. 3, and the oscillation wavelength is modulated between wavelengths λ ₁ and λ ₀ . This light 7 is converted into a signal 6 having a power of 1 and 0 by transmitting only λ ₁ using the wavelength filter 2 having the filter characteristic 8. The light 6 transmitted through the wavelength filter 2 is coupled to the fiber 4 using the lens 3 and transmitted, and then the signal is detected by the photodiode 5.

Since the wavelength filter 2 of this embodiment separates light separated by 10 nm, it is possible to use a filter having a relatively wide transmission band width such as a dielectric multilayer film filter. However, as described above, since the wavelength interval can be selected by the sampling period length Z ₀ of the diffraction grating, the wavelength interval Δλ is designed according to the form of the system, and the wavelength filter 2 is selected accordingly.

In this embodiment, wavelength switching can be performed with a minute current. Therefore, the carrier density in the laser does not fluctuate much, chirping is suppressed, the spread of the oscillation spectrum can be reduced, and high-speed modulation can be performed. Further, since Δλ = (λ ₁ −λ ₀ ) can be designed by the sampling period length Z ₀ of the diffraction grating,
A wide choice can be made and it is easy to separate the powers of λ ₁ and λ ₀ . As a result, it becomes possible to easily perform high-speed modulation with a high extinction ratio and a small spectrum width.

[0020]

Second Embodiment The wavelength tunable laser used in this embodiment has the same characteristics as the laser used in the first embodiment.

FIG. 5 shows a laser modulation method. As in Example 1, the bias current 1 ₀ is injected into one side of the electrode of the semiconductor laser 1, as signal 1, the modulation current [Delta] I = (I ₁ -
By modulating with 1 ₀ ), the output light becomes a signal modulated between wavelengths λ ₁ and λ ₀ . Similarly, by injecting a bias current 1 ₀ to the semiconductor laser 1, as the signal 2, the modulation current Δ
By modulating with _{I = (1 2 -1 0)} , the output light is modulated signals between the wavelength lambda ₂ and lambda _0. In this way, the wavelength of the output light can be changed by changing the modulation current. By cutting λ ₀ of this output light with a wavelength filter (half-value width of about 10 nm) according to the wavelength interval between λ ₁ and λ ₀ , it is possible to extract a signal with power of 1.0.

FIG. 4 shows a block diagram of the second embodiment. Figure 2
The semiconductor laser 1 having the characteristics as shown in FIG. 5C is modulated by the modulation current (I _n −I ₀ ) by the method of FIG. 5, and the output light is a signal 7 ′ modulated between wavelengths λ ₀ and λ _n. (Signal n).
By transmitting only this λ _n to this light 7 ′ using the wavelength filter 2 having the filter characteristic 8 ′, the power is 1,
0 signal 6 '. The light transmitted through the wavelength filter 2 is
After transmitting the fiber 4 using the lens 3, the branch 1
0 is distributed to n pieces. The distributed light becomes a signal 6'having a power of 1 and 0 by transmitting only the wavelength λ _{n by the} reception filter 11 having the characteristic 12.

In this case, the reception filter 11 separates wavelengths separated by only about 1Å, so that a wavelength filter having a small transmission bandwidth is required. For example, it is a DFB filter, a Fabry-Perot filter, a Mach-Zehnder type filter, or the like. The light transmitted through the reception filter 11 is received by the photodiode 5 and the signal 6'is detected.

Further, by making the wavelength filter 11 a variable filter having a tuning function, the channel can be freely selected on the receiving side.

Also in this embodiment, since modulation can be performed with a minute current, carrier density fluctuation is small, chirping is suppressed, spectrum spread can be suppressed, and high-speed modulation can be performed. Further, since the spectrum spread is small, a large number of wavelengths can be taken even when different signals are assigned for each wavelength, and a signal with a good extinction ratio can be obtained. As a result, the extinction ratio is high,
Moreover, high-speed communication can be performed.

[0026]

Third Embodiment The wavelength tunable laser used in this embodiment has the same characteristics as the laser used in the second embodiment.
In the present embodiment, the modulation method of the second embodiment is applied to wavelength division multiplexing communication.

The laser modulation method is the same as that in the second embodiment. Injecting a bias current 1 ₀ to one electrode of the semiconductor laser (LD1) 1, as a signal I, the modulation current [Delta] I = (I ₁
By modulating with −I ₀ ), the output light has wavelengths λ ₁ and λ _0.
It becomes a signal modulated between. Semiconductor laser (LD2)
Injecting a bias current 1 ₀ to 1, as the signal 2, by modulating the modulation current _{ΔI = (I 2 -I 0)} , the output light is modulated signals between the wavelength lambda ₂ and lambda _0. Like this
The wavelength of output light can be changed by changing the modulation current.

FIG. 6 shows the configuration of the third embodiment. Figure 2
A signal obtained by modulating a semiconductor laser (LD1) 1 having the characteristics as shown in (c) with a modulation current (I ₁ -I ₀ ) by the method of FIG. 5 and modulating output light between wavelengths λ ₀ and λ _1. 7 '(Signal 1)
And Similarly, the LD2 is also modulated by the modulation current (I ₂ −I ₀ ), and the output light is a signal 7 ′ modulated between wavelengths λ ₀ and λ _2.
(Signal 2). Similarly, let LDn be a signal 7 ′ ((signal n) that is modulated between wavelengths λ ₀ and λ _n . Signals 1 to n are multiplexed using a multiplexer 9 to obtain a filter characteristic 8 ′. Λ ₀ is cut by the wavelength filter 2 which it has, and the fiber 4 is transmitted, and then it is divided into n pieces by the branching device 10. The distributed light is transmitted by the reception filter (Fn) 11 only at the wavelength λ _n. , A signal 6'having a power of 1, 0.

The reception filter 11 in this case is the same as the second embodiment.
Similarly to the above, in order to separate wavelengths separated by only about 1Å, a wavelength filter having a characteristic 12 with a small transmission bandwidth is required. Light 6'transmitted through the reception filter 11
Is received by the photodiode 5 and a signal is detected.

In this embodiment, since the oscillation wavelength of each laser 1 is variable, the receiving side can be freely selected.
Furthermore, by making the wavelength filter 11 variable, the transmission side and the reception side can freely select each other.

Also in the present embodiment, since the modulation can be performed with a minute current, the variation of the carry density is small, the chirping is controlled, the spectrum spread can be suppressed, and the high speed modulation can be performed. Further, since the spectrum spread is small, a signal with a good extinction ratio can be obtained even when wavelength division multiplexing is performed. As a result, it becomes possible to easily perform high-speed WDM communication with a high extinction ratio.

[0032]

Since the present invention is configured as described above, it has the following effects. 1. Since modulation can be performed with a minute current, chirping is suppressed, spectrum spread is suppressed, and high-speed modulation can be easily realized. 2. A good extinction ratio can be easily obtained despite modulation with a minute current. 3. Since the spread of the spectrum width can be suppressed, a good extinction ratio can be obtained even at the time of wavelength multiplexing.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a first embodiment.

FIG. 2 is a diagram showing a configuration and a reflection characteristic of a reflector of a laser used in Example 1, and a current-wavelength characteristic of the laser.

FIG. 3 is a diagram illustrating a laser modulation method according to the first embodiment.

FIG. 4 is a configuration diagram of a second embodiment.

FIG. 5 is a diagram illustrating a laser modulation method according to a second embodiment.

FIG. 6 is a configuration diagram of a third embodiment.

FIG. 7 is a diagram illustrating a conventional example.

[Explanation of symbols]

 1 Semiconductor Laser 2 Wavelength Filter 3 Lens 4 Optical Fiber 5 Photodiode 6, 6'Signal After Passing Wavelength Filter 7, 7'Light from Laser 8, 8'Filter Characteristic 9 Multiplexer 10 Brancher 11 Reception Filter 12 Reception Filter characteristics

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H04B 10/04 10/06

Claims (5)

[Claims] 1. A semiconductor laser having a plurality of electrodes, comprising a reflector having a reflection spectrum characteristic in which a plurality of high reflection peaks are periodically present on both sides of a certain wavelength as a center, and a semiconductor laser having a plurality of electrodes is provided. A wavelength filter having a function of modulating and oscillating an injection current between two wavelengths by controlling and modulating the injected current so as to perform switching between two different longitudinal modes, and selecting only one of the two wavelengths. A modulation system for an optical transmission device, which transmits an intensity-modulated signal by transmitting light. 2. The modulation system for an optical transmission device according to claim 1, wherein the reflector has a structure in which a region where the diffraction grating exists and a region where the diffraction grating does not exist are repeated in a certain cycle. 3. The modulation system for an optical transmission device according to claim 1, wherein the reflector has a structure in which regions in which a pitch and a phase of a diffraction grating continuously change are repeated in a certain cycle. . 4. The modulation method for an optical transmission device according to claim 1, wherein a wavelength of one of two longitudinal modes to be transmitted is made variable, and an injection current is modulated so as to perform switching between the two longitudinal modes, An optical communication system characterized by detecting a signal of an arbitrary wavelength using a wavelength filter on the receiving side. 5. The wavelength division multiplexing optical communication system according to claim 4, wherein communication is performed by using a plurality of wavelength filters on the optical transmission device and the receiving side.