JP2853684B2 - Wavelength multiplexing and tunable light sources - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength division multiplexing and wavelength tunable light source in a wavelength division multiplexing transmission system capable of transmitting a large amount of optical data.

[0002]

2. Description of the Related Art When light is applied to information processing, optical communication and optical link have been performed by utilizing the parallelism and space propagation property of light. These techniques are mainly based on amplitude modulation of light, and the information transmission capacity of light is greatly increased by incorporating wavelength information and phase information of the light. In particular, attempts to effectively use wavelengths as information resources have been made by various organizations as a wavelength division multiplexing (WDM) transmission system.

In such WDM transmission, what is important is a technique for fixing a reference wavelength. When the wavelength on the transmitting side changes, the wavelength is used as a kind of address,
Since the subsequent signal processing is performed, it is difficult to form the system itself. Therefore, a wavelength division multiplexed light source that can stably transmit at a constant center wavelength and at a wavelength interval is required.

As a conventional technique, as shown in FIG. 3, there is a distributed feedback (DFB) laser capable of outputting a predetermined wavelength by precisely forming a diffraction grating 35. The structure itself is typical,
What is important is that a high technology such as EB exposure is required when manufacturing the diffraction grating 35. The DFB laser manufactured in this manner having a desired wavelength interval is selected and used. In the standard of the present stage, about 20 wavelengths are multiplexed at an interval of 0.8 nm with a wavelength of 1.55 μm as a center.

It is also conceivable to decompose light emitted from one light source into light having a large number of specific wavelengths to increase the number of wavelengths. For example, as disclosed in JP-A-60-33538, it is possible to use a prism or a diffraction grating by decomposing it into multiple wavelengths.

On the other hand, instead of trying to obtain a fixed wavelength as described above, a wavelength-variable light source is prepared, and the amount of deviation from a reference value detected by a wavelength monitor is corrected and transmitted. There is a way. Therefore, a wavelength tunable light source is required. As shown in FIG. 4, in a distributed Bragg reflector (DBR) laser,
7, an electrode is provided in each of the phase adjustment region 48 and the DBR region 49, and the applied voltage is adjusted to provide a light source having a variable width. These are described in detail in JP-A-2-65189. As another variable light source, as shown in FIG. 5, a waveguide having an electro-optic effect is subjected to electric field control to change the refractive index to perform frequency modulation (Japanese Patent Application Publication No. 56-147).
123, and the transmission waveform is changed by changing the amount of wavelength chirping by the waveguide type intensity modulator 62a and the waveguide type phase modulator 63 as shown in FIG. There is. Common to these conventional examples is that phase modulation is performed by electric field control.
See, for example, -230329.

[0007]

However, none of the above reported examples can simultaneously perform wavelength multiplexing and wavelength tunable. As for the DFB laser of the first conventional example shown in FIG. 3, a high technology is required for the production, and the wavelength must be selected by the inspection. However, the yield is lowered, and the driving conditions and the characteristics are deteriorated. It is also difficult to control the wavelength over a long period. The use of a distributor such as a prism or a diffraction grating requires extremely accurate alignment of these optical components, and requires extra components.

In the case of a wavelength-variable light source, WDM requires transmission at several wavelength intervals, so that it is necessary to increase the variable width and to simplify its control. If an array having a small variable width is used in an array, the problem of the yield arises similarly to the above. In the second conventional example shown in FIG. 4, since the portion to be electrically controlled extends over three places, the precise control is extremely complicated, and it is difficult to tune the wavelength at high speed. A third conventional example shown in FIG. 5 and FIG.
In the fourth conventional example shown in FIG. 5, the variable width of the wavelength is very narrow.
There is a possibility that it becomes impossible to follow the wavelength-variable feedback due to the disturbance that occurs during communication.

A desirable light source in WDM transmission is to change a wavelength in accordance with feedback from a wavelength monitor so as to transmit multiple wavelengths at a fixed center wavelength and a fixed wavelength interval and to withstand environmental changes due to temperature changes and the like. A light source that can In addition, when the wavelength is changed, it is necessary to avoid performing electric control over a plurality of units in order to easily control the wavelength.

In view of the above, it is an object of the present invention to provide a light source capable of changing the wavelength by simple control and capable of outputting multiple wavelengths having a constant wavelength interval.

[0011]

In order to solve the above-mentioned problems, the present invention provides a plurality of waveguides each capable of controlling the phase of light having wavelength information, and arbitrarily controlling the phase by the waveguides. And a light amplifier mechanism for amplifying the controlled light. Further, using a material having an electro-optic effect for the plurality of waveguides,
This is a light source characterized by applying an electric field for phase control. Further, the light source is characterized in that a distortion is applied to the waveguide for the phase control.

According to the present invention, a multiplexed wavelength can be provided, and a wavelength shift caused by a change in environment can be controlled by a feedback signal from a wavelength monitor by a simple phase control in the waveguide. Since it can be changed back to the reference value, WDM
It is possible to provide a transmission light source having a constant wavelength and a certain wavelength interval that are essential for transmission. As a result, the transmission / reception system using the wavelength as a kind of address and carrying information on the wavelength functions, so that a large-capacity communication which is dramatically increased as compared with the related art becomes possible. It also provides a light source that is highly resistant to environmental changes, and promises to maintain a stable communication environment for a long time.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic view of a multi-wavelength variable light source shown for explaining one embodiment of the present invention. As shown in the figure, the entire configuration is an optical amplifier 11, a slab type waveguide 12, an arrayed waveguide 13, a slab type waveguide 14, a waveguide array 15, and an optical amplifier 1 corresponding to each of them.
6. The end faces of the optical amplifiers 11 and 16 on the slab type waveguide side are coated with an anti-reflection film, and the emission end faces of the laser light of the optical amplifier 11 are coated with a reflection film, and the reflectance is, for example, 70%. . The end face of the optical amplifier 16 at the other end is coated with a highly reflective film, and has a reflectance of, for example, 99%. The optical amplifier 16 is formed so as to individually emit lasers having different wavelengths in an array, and it is also possible to replace the coating film on the end surface with that described above and use the emission end surface. The reflectivity of the reflective film is determined by giving priority to the relation between the laser oscillation threshold and the laser output power. When a sufficient optical amplification factor can be obtained, only one optical amplifier is required, and the laser light emitting end face of the optical amplifier 11 is a cleaved surface or coated with a reflective film having a reflectance of 70%, for example. . The other end face becomes a fiber end or a waveguide end, and is coated with a highly reflective film. Furthermore, the end face of the optical amplifier 11 is coated with a highly reflective film,
The end of the fiber or the end of the waveguide may be coated with, for example, a reflective film having a reflectivity of 70% to form a laser light emitting end face.

The arrayed waveguide 13 is made of a material whose refractive index changes when an electric field is applied by, for example, an electro-optic effect. Specifically, the lithium niobate (L)
iNbO ₃ ) or lithium tantalum oxide (L
iTaO ₃ ) is adopted. An electrode 17 is formed on each of the waveguides of the arrayed waveguide 13 so that the electrodes have the same length. When an electric field is applied and the refractive index of the waveguide changes, the optical path length changes, so that the effective length of each waveguide in the arrayed waveguide 13 changes. As a result, the light amplified by the optical amplifier repeats reflection at each amplifier end, and the laser light is output using this as a resonator. A multiplexed wavelength is obtained by the arrayed waveguide 13 in the resonator, and the wavelength can be made variable by phase modulation by the electric field of the arrayed waveguide 13. Since the wavelengths are multiplexed from the beginning, the wavelength tunable range does not need to be extremely large. Arrayed waveguide 13
Can supply wavelengths at regular intervals in any case, and it is one of the great advantages that it is only necessary to make the whole shift variable.

Here, the refractive index of the slab type waveguide is ns,
The effective refractive index of the arrayed waveguide 13 is nc, the pitch between the waveguides is d, the wavelength of light is λ, the applied electric field is E, the length of each waveguide in the arrayed waveguide 13 is L, and each waveguide is Assuming that the difference between the lengths is ΔL, the light diffracted at an angle θ from the exit of the waveguide has a phase of nc · L + ns · d · sin θ = m · λ (m is the order of diffraction). Satisfy the relationship. nc is a function of the electric field E when using the electro-optic effect. When it is determined which row of the waveguide row 15 has a resonator of which wavelength, θ is determined. The order of analysis is also determined by the relational expression of △ λ = λ / N · m from the wavelength interval △ λ in wavelength multiplexing and the number N of the arrayed waveguides 13, so that the wavelength of the light to be optically amplified is determined. , Nc corresponding to the λ is determined. If the electric field E is added so that the refractive index becomes nc, the wavelength can be changed to a desired value (reference value). Electro-optical crystal material constituting the arrayed waveguide 13 is the electric field dependence of the refractive index change with respect to ordinary _{rays, △ n 0 = - (n} 0 3/2) · r 13 · E ( where, n ₀ = 2.286, r ₁₃ = 8.6 × 10 ⁻¹² m / V,
(E is an electric field), the desired change in the refractive index can be obtained by controlling the applied amount of the electric field, in other words, the voltage applied to the crystal constituting the arrayed waveguide 13. Control can be performed so that nc is calculated as described above.

The details of the waveguide design when the wavelength band is in the 1.55 μm band will be described. The distance from the optical amplifier 11 to the arrayed waveguide 13, that is, the slab waveguide 1
As shown in FIG. 2, the focal length of No. 2 is changed according to the wavelength interval of the wavelength information to be wavelength-multiplexed. Here, a system in which the wavelength interval is set to 1 nm and the wavelength multiplicity is 28 will be constructed. Then, from FIG. 2, the focal length of the slab type waveguide 12 is determined to be 2179 μm, and the diffraction order m is determined to be 42. From the intensity distribution of the diffracted light, the diffracted light from the optical amplifier 11 is 750 at the entrance of the arrayed waveguide 12.
At least 94 waveguides are required because they extend to μm, but here, 201 waveguides will be manufactured in consideration of the margin. If ΔL is 34 μm, the length of the arrayed waveguide 13 is 1 cm, and the diameter of the waveguide is 1
μm, it is 1n on the long wave side due to some environmental change.
When it is desired to return the wavelength shifted by m to the reference value, it is sufficient to apply a voltage of 100 V to the electrode 17 based on the electric field dependence of the refractive index described above. If you use extraordinary rays,
About 1/3 of that is sufficient. At this time, the array pitch of the arrayed waveguides 13 is 8 μm, and this distance is a distance sufficient to prevent crosstalk between the waveguides. Lithium niobate may cause optical damage to light having a short wavelength in the visible region, but such a problem does not occur in the long wavelength region standardized in actual wavelength division multiplexing transmission. The optical amplifier may have any configuration as long as it can amplify the wavelength in the wavelength range to be used. Further, as the method of performing the phase control, the method using the electric field control is taken as an example this time. However, for example, the phase modulation may be applied by inducing distortion in the waveguide using a piezo element. In the present embodiment, the waveguide is formed using an inorganic material, but this may be replaced with a semiconductor.

[0017]

As described above, according to the present invention, while being a multi-wavelength light source suitable for WDM transmission, a wavelength tunable function for returning a wavelength to a reference value with respect to a wavelength shift caused by an environmental change or the like is provided. A provided light source can be provided. As a result, a stable WDM system can always be constructed, and large-capacity communication using wavelengths as addresses becomes possible.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a multi-wavelength variable light source for explaining an embodiment of the present invention.

FIG. 2 is a diagram showing a focal length (mm) and a diffraction order (m) of a slab type waveguide obtained from a wavelength interval and a wavelength multiplicity.

FIG. 3 is a distributed feedback type (DFB) that can output a fixed wavelength by precisely forming a conventional diffraction grating.
FIG. 3 is an explanatory view of a main part of a laser.

FIG. 4 is an explanatory view of a main part of a semiconductor laser in which electrodes are provided in each of a gain region, a phase adjustment region, and a DBR region of a conventional DBR laser, and a voltage tunable light source is obtained by adjusting an applied voltage thereof.

FIG. 5 is a diagram illustrating the principle of a conventional optical frequency modulator that performs frequency modulation by changing the refractive index by controlling the electric field of a waveguide having an electro-optic effect.

FIG. 6 is a diagram showing a conventional example of an optical modulator that changes a wavelength chirping amount and changes a transmission waveform by using a waveguide type intensity modulator and a waveguide type phase modulator.

[Explanation of symbols]

 Reference Signs List 11 optical amplifier 12 slab type waveguide 13 arrayed waveguide 14 slab type waveguide 15 waveguide array 16 optical amplifier 17 electrode 31 substrate 32 buffer layer 33 active layer 34 guide layer 35 diffraction grating 41 substrate 42A diffraction grating 42 waveguide layer 43 Active layer 44 Cladding layer 45 Electrode 46 Common electrode 47 Gain region 48 Phase adjustment layer 49 DBR region 40 High resistance semiconductor layer 51 Electro-optic crystal 52 Optical waveguide region 53 Electrode 54 Electrode 61 Substrate 62a Waveguide intensity modulator 63 Waveguide Type phase modulator 64 electric signal 65 amplifier 66 electric signal variable means 67 delay means 68 optical waveguide 69 optical waveguide

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-64-70703 (JP, A) JP-A-5-323246 (JP, A) JP-A-7-218935 (JP, A) JP-A-7- 92506 (JP, A) JP-A-3-236647 (JP, A) JP-A-64-76012 (JP, A) Proceedings of the 1996 IEICE General Conference, Electronics 1 (March 11, 1996) p. 194 Hiroaki Sanjo et. al. , “C-194 Equally spaced multi-wavelength light source using semiconductor mode-locked laser and arrayed waveguide grating filter” (58) Fields investigated (Int. Cl. ⁶ , DB name) H01S 3/08 H04B 10 / 04

Claims (3)

(57) [Claims] 1. A semiconductor device comprising: a plurality of waveguides each capable of controlling a phase of light including wavelength information; and at least one optical amplifier for amplifying light whose phase is arbitrarily controlled by the waveguide. Wavelength multiplexing and tunable light sources. 2. The wavelength division multiplexing apparatus according to claim 1, wherein the plurality of waveguides are formed of a material having an electro-optic effect, and include a plurality of waveguides to which an electric field is applied for phase control. Tunable light source. 3. The wavelength-division multiplexed and tunable light source according to claim 1, wherein the plurality of waveguides include a plurality of waveguides in which distortion is applied to the waveguides as a phase control method.