JP2000349713A - Wavelength variable type optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a wavelength variable type optical transmitter that can select signal light with an optional wavelength and can transmit the selected signal light that is optically modulated. SOLUTION: This wavelength variable type optical transmitter uses a tunable filter 24 to select light with a desired wavelength among lights with a broad wavelength band emitted from a broadband light source 20, an optical modulator 28 modulates the selected light with the selected wavelength and transmits a modulated information signal. Thus, the transmitter can transmit the information signal by using the wavelength requested by a receiver side. Since this wavelength variable type optical transmitter can select the optional wavelength and transmits the signal with the selected wavelength, problems such as deterioration due to crosstalk and the deterioration in communication quality are hardly caused. Since each optical transmitter transmits information by a different light with the different wavelength assigned differently to each optical transmitter, the secrecy of information can be maintained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunable optical transmitter.

[0002]

2. Description of the Related Art With the spread of multimedia, mainline systems,
Information transmission is rapidly increasing in access systems and datacoms.
It is desired to realize a large-scale WDM optical communication system. One of the key devices for realizing this system is a tunable optical transmitter, but it has not been realized yet.

FIG. 7 is a block diagram showing a conventional wavelength-division multiplexed optical transmitter.

In this wavelength-division multiplexed optical transmitter, a current I _L from a driving circuit (not shown) drives a broadband light source 40.
To emit light 42 having a broadband optical spectrum distribution and input it to the optical demultiplexer 43. Taken out by demultiplexing a signal light 44-1 to 44-n of the desired wavelength in the optical demultiplexer 43 λ ₁ ~λ _n, each wavelength lambda ₁ ~
optical signal light 44-1 to 44-n of lambda _n modulator array 45
Are input to the respective optical modulators configured in the above. A reverse bias voltage -Vm is applied to each optical modulator. Signal light 47-1 to 47- modulated by each optical modulator
n is input to each input terminal of an optical multiplexer 48 having n inputs and 1 output, multiplexed by the optical multiplexer 48, extracted as signal light 49, and input to the optical amplifier 50. This optical amplifier 50
When the current _Ik is injected from a driving circuit (not shown), the optically modulated and wavelength-multiplexed signal light 49 is optically amplified,
The amplified signal light 52 is transmitted to an optical receiver (not shown) through an optical transmission path.

FIG. 8 is a configuration diagram of a conventional variable wavelength light source.

This tunable light source comprises a semiconductor light emitting element 53 having a relatively wide band (narrower than the broadband light source of the optical transmitter shown in FIG. 7) and an optical switch 60.
This wavelength tunable light source propagates light emitted from the semiconductor light emitting element 53 into one (upper side in the figure) optical fiber 55-1 and transmits the propagated light of various wavelengths λ ₁ , λ ₂ , λ ₃ ,. Grating fibers 56-1 to 56- that reflect light
4, an external resonator is formed, and light of desired wavelengths λ _{1 to} λ ₄ is input to the optical fiber 57-1.
57-4 and input into the other (lower side in the figure) optical fiber 55-2. The output light is emitted in the direction of arrow 58.

Here, the wavelength of light traveling in the direction of arrow 58 is
The fixing table 54 for fixing the optical fiber 55-1 and the optical fiber 55-2 at a desired interval is indicated by arrows 59-1 and 59-2.
You can select by moving in the direction.

[0008]

However, a tunable optical transmitter capable of optically modulating and transmitting signal light of an arbitrary wavelength has not yet been reported.

Therefore, an object of the present invention is to solve the above-mentioned problems, to select a signal light having an arbitrary wavelength, and to modulate and transmit the selected signal light. To provide a transmitter.

[0010]

In order to achieve the above object, a tunable optical transmitter according to the present invention comprises a broadband light source, a tunable filter for selectively extracting light of a desired wavelength from light emitted from the wideband light source. And an optical modulator for modulating light of a desired wavelength and outputting a modulated signal light.

A tunable optical transmitter according to the present invention comprises: a broadband light source; a tunable filter for selectively extracting light of a desired wavelength from light emitted from the broadband light source; and a modulated signal light by modulating the light of the desired wavelength. An optical modulator to be output, adjusting means for adjusting the selected wavelength of the tunable filter, an optical tap circuit for extracting a part of the modulated signal light from the optical modulator, the modulated signal light extracted by the optical tap circuit and a reference wavelength And compare
A wavelength monitor circuit that feeds back the output signal to the adjusting means and controls the selected desired wavelength to be constant.

A tunable optical transmitter according to the present invention comprises: a broadband light source; at least two tunable filters for selectively extracting light of a desired wavelength from light emitted from the broadband light source; At least two optical modulators for outputting the modulated signal lights, and an optical multiplexer for multiplexing the modulated signal lights from the two optical modulators.

In the wavelength tunable optical transmitter according to the present invention, the tunable filter may be a fiber-type or waveguide-type grating filter.

In addition to the above configuration, in the wavelength tunable optical transmitter according to the present invention, it is preferable that the broadband light source is a light source that emits incoherent light.

According to the present invention, light of a desired wavelength is selected by a tunable filter from light of a wide wavelength band emitted from a broadband light source, and the light of the selected wavelength is modulated by an optical modulator. Since the information signal can be transmitted, the information signal can be transmitted using the wavelength requested by the receiving side. In addition, since the wavelength tunable optical transmitter can select and transmit an arbitrary wavelength, problems such as deterioration of crosstalk and deterioration of communication quality hardly occur.
Since light of different wavelengths can be assigned to each optical transmitter to transmit information, confidentiality of information can be maintained.

According to the present invention, when the selected wavelength is controlled to be constant, the communication quality hardly deteriorates. Further, there is no deterioration of crosstalk with signal light from another optical transmitter. Further, information transmission can be performed by effectively utilizing wavelengths at very narrow wavelength intervals.

According to the present invention, a cheaper optical transmitter can be realized by using a tunable filter having a medium wavelength variable range.

According to the present invention, by using a tunable filter that can easily change the wavelength and can change the wavelength over a wider range,
A higher performance wavelength tunable optical transmitter can be realized.

According to the present invention, a light source having a wide band characteristic can be provided by using a light source that emits incoherent light. As a result, a wavelength can be selected from a wider wavelength range. In addition, since the broadband light source has high output characteristics, it can perform long-distance transmission, and can compensate for the optical loss of optical components such as an optical modulator and a tunable filter. It can be composed of optical components.

[0020]

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

FIG. 1 is a block diagram showing an embodiment of the wavelength tunable optical transmitter according to the present invention.

This optical transmitter selects a signal light having a desired wavelength from a wide wavelength range, modulates the signal light with an optical modulator, and transmits the modulated signal light. A tunable filter 24 for selectively extracting light of a desired wavelength from the light emitted from the broadband light source 20, a temperature controller 25 for adjusting the temperature of the tunable filter 24, and a temperature controller. And a light modulator 2 that modulates the light selected by the tunable filter 24 and outputs a modulated signal light.
8 and a drive circuit 29 for driving the optical modulator 28.

The broadband light source 20, a very wide wavelength range by current I _L is injected by the drive circuit 21 (3
The high-bandwidth light source 20 emits high-power light 23 over 0 nm or more (the broadband light source 20 will be described later in detail).

The light 23 emitted from the broadband light source 20 is input to a tunable filter 24. The tunable filter 24 may have the following configuration.

(1) A configuration in which a center wavelength is varied by using a fiber grating filter, attaching a heater to the filter, and adjusting the temperature of the heater. According to this configuration, the center wavelength is set from 0.05 nm / ° C. to 0.
It can be changed in the range of 1 nm / ° C.

(2) A configuration in which the fiber grating is fixed to a plate-like member such as a quartz plate or a ceramic plate with an adhesive, and the plate-like member is bent to apply tension to the fiber grating, thereby adjusting the center wavelength. According to this configuration, for example, the center wavelength can be changed in the range of 0.01 nm / g to 0.04 nm / g.

(3) A configuration using a grating having a waveguide structure instead of the fiber grating.

(4) A configuration in which an interference film filter is used and the center wavelength is changed by adjusting the incident angle of signal light to the interference film filter.

(5) A configuration in which a filter is formed in a multi-stage waveguide type Mach-Zehnder circuit, a heater is attached to the filter, and the temperature of the heater is adjusted to change the center wavelength.

In the tunable optical transmitter shown in FIG. 1, the center wavelength of the tunable filter 24 is selected by operating a temperature controller 25 for a heater attached to the tunable filter 24 by a drive circuit 26. You. As a result, a signal light 27 having a desired wavelength is extracted from the output end of the tunable filter 24 and input to the optical modulator 28. The optical modulator 28 is driven by a drive circuit 29.
As a result, the modulated signal light 30 modulated from the optical modulator 28
Is output. In such a configuration, which wavelength to select can be determined by a request from the receiving side, the intention of the transmitting side, and the like.

FIG. 2 is a block diagram showing another embodiment of the tunable optical transmitter according to the present invention.

The difference from the embodiment shown in FIG. 1 is that an external force controller 32 is used instead of the temperature controller 25.

This tunable optical transmitter uses a tunable grating filter (fiber type or waveguide type) 31 as a tunable filter, and applies an external force to the tunable grating filter 31 by the external force adjuster 32 as described above. The center wavelength is changed by adjusting. This external force adjuster 32 is operated by a drive circuit 33.

FIG. 3 is a block diagram showing another embodiment of the tunable optical transmitter according to the present invention.

The difference from the embodiment shown in FIG. 1 is that a part of the modulated signal light is extracted, the extracted modulated signal light is compared with a reference wavelength, and the output signal is fed back to the adjusting means. The point is that the selected desired wavelength is controlled to be constant.

The wavelength tunable optical transmitter comprises a broadband light source 2
0, a tunable filter 24 for selectively extracting light of a desired wavelength from the light emitted from the broadband light source 20, and an optical modulator 2 for modulating the light of the desired wavelength and outputting a modulated signal light.
8, a driving circuit 29 for driving the optical modulator 28, adjusting means for adjusting the selected wavelength of the tunable filter 24,
An optical tap circuit 34 for extracting a part of the modulated signal light from the optical modulator 28, the modulated signal light extracted by the optical tap circuit 34 is compared with a reference wavelength, and an output signal is fed back to the adjusting means to select a desired signal. A wavelength monitor circuit 36 for controlling the wavelength to be constant. The adjusting means includes a temperature controller 25 and a drive circuit 26 for driving the temperature controller.

This tunable optical transmitter is provided with an optical modulator 28.
The modulated signal light 30 output from the controller is input to the optical tap circuit 34 to output the modulated signal light 37, and a part of the signal light 35 is input to the wavelength monitor circuit 36. In addition,
As the wavelength monitor circuit 36, an ordinary circuit is used.
The wavelength monitor circuit 36 compares the extracted signal light with a signal light having a predetermined reference wavelength. As a result of the comparison, if the wavelengths are different, an error signal is generated at the output of the wavelength monitor circuit 36 and sent to the drive circuit 26. In the drive circuit 26, the temperature controller 25 is operated in accordance with the error signal, and the temperature is adjusted so that the wavelength of the signal light output from the tunable filter 24 matches a predetermined reference wavelength.

In FIG. 3, the optical tap circuit 34 may be provided between the tunable filter 24 and the optical modulator 28 to perform feedback control of the center wavelength.

FIG. 4 is a block diagram showing another embodiment of the tunable optical transmitter according to the present invention.

The difference from the embodiment shown in FIG. 1 lies mainly in that two tunable filters for selectively extracting light of a desired wavelength from light emitted from a broadband light source, and that light of a desired wavelength is modulated. This is characterized in that it is composed of two optical modulators for outputting modulated signal light and an optical multiplexer for multiplexing the modulated signal lights from both optical modulators.

The tunable optical transmitter comprises a broadband light source 200 for emitting light in two wavelength bands and two driving circuits 21
-1 and 21-2 to emit light of two wavelengths and convert one light 23-1 to one tunable filter 24-1.
Then, for example, a signal light having a wavelength λ ₁ is selected and input to one optical modulator 28-1 to take out a modulated signal light 30-1. A signal light having a wavelength different from ₁ , for example, a wavelength λ ₂ is selected and input to the optical modulator 28-2 to take out the modulated signal light 30-2, and these two modulated signal lights 30-1 and 30-2 are extracted. The optical signal is multiplexed by an optical multiplexer 38 and a modulated signal light 39 is output.

The wavelength tunable optical transmitter employs such a configuration to provide the tunable filters 24-1 and 24-2.
In 4-2, it is not necessary to use one that can change the center wavelength over a wide wavelength range. For example, when the wavelength range of the 1.5 μm band is used, the tunable filter 24-1 changes the range from 1.525 μm to 1.545 μm,
The tunable filter 24-2 can be assigned to change the range from 1.546 μm to 1.565 μm.

FIG. 5A is a plan view of a broadband light source used in the tunable optical transmitter according to the present invention, and FIG.
5A is a left side view, FIG. 5C is a right side view of FIG. 5A, and FIG. 5D is a sectional view taken along line AA of FIG. 5A.

[0044] The broadband light source is a light source for a wavelength 1.5μm band, slab-shaped n-type InP lower clad layer 2 on the substrate surface of the n ⁺ -type InP substrate 1 having a lower electrode 8 on the back surface, InGaAsP active layer (Hereinafter referred to as "active layer")
3 and p-type InP upper first cladding layer (hereinafter referred to as “upper first
It is called “cladding layer”. ) 4 are sequentially stacked, and a p-type In having a substantially rectangular cross section is formed on the upper first cladding layer 4.
A P upper second cladding layer (hereinafter, referred to as “upper second cladding layer”) 5 and an InGaAsP contact layer (hereinafter, referred to as “contact layer”) 6 are sequentially formed to form a stacked body.

An upper second cladding layer 5 having a substantially rectangular cross-sectional shape;
SiO ₂ layer 13 and polymer layer (polyimide layer) on the side surface of contact layer 6 and the exposed upper surface of upper first cladding layer 4
14 are sequentially formed. The upper electrode 7 is formed on the contact layer 6 having a substantially rectangular cross section. Non-reflective coating layers 12-1, 1 are provided on both end faces 10, 11 of the laminate.
2-2 are formed. The upper second clad layer 5, the contact layer 6, and the upper electrode 7 have substantially the same width and are formed in the same pattern, and the upper electrode 7 on one end face (the left end face in the figure) 10 is connected to the other end. A non-excited absorption region 18 is formed so as to be interrupted in the vicinity of the end surface of FIG. 1, and a region 15 having the same width Wi and a length Li is provided after the absorption region 18 (on the right side in the figure). (The right end surface in the figure) The width is tapered to 11 and the width W is
o, and is configured to have a region 16 with a length Lo.

In this light source, when the current _IL is injected into the terminal 9, spontaneous emission light is generated in each region of the active layer 3 substantially immediately below the upper second cladding layer 5. The spontaneous emission light generated in the active layer 3 in the region 15 is guided and propagated in the active layer 3 substantially immediately below by the substantially rectangular cross-sectional shape of the upper second cladding layer 5, but is amplified by stimulated emission with the propagation. . When the spontaneous emission light propagates through the region 16 having the length Lo, the light output becomes extremely high, and is emitted from the other end face 11 in the direction of the arrow 17.

Here, since the light spreads in the region 16 in a tapered shape as the light propagates, the power density is reduced at the emission end, and acts to suppress the gain saturation. Both end faces 10,
Since the antireflection coating layers 12-1 and 12-2 are formed on the laser beam 11, laser oscillation due to reflection from the end faces 10 and 11 can be suppressed, and incoherent light can be emitted in the direction of arrow 17. Can be.

If both end faces 10 and 11 are formed obliquely (θ: 2 to 10 °), laser oscillation is substantially “0”.
Can be suppressed.

[0049] In this configuration, by increasing the value of the current I _L, it is possible to increase the light output. Broadening the light source can be realized by forming the active layer 3 with a multiple quantum well structure. For example, a non-uniform quantum well structure in which the thickness of the well layer is different may be used. As a specific example, when a seven-period structure is used as a multiple quantum well structure, a well layer having a seven-period structure (having a film thickness of about 2 to 7 n
m, InGaAs layer) and a barrier layer (about 8 nm thick, I
The thickness of the well layer (nGaAsP layer) is sequentially increased from 7 nm to 2n.
Forming with a layer changed to m can achieve a wider band.

FIG. 6A is a plan view of a broadband light source used in the tunable optical transmitter shown in FIG. 4, FIG. 6B is a left side view of FIG. 6A, and FIG. 6A is a right side view of FIG. 6A, and FIG. 6D is a sectional view taken along line BB of FIG. 6A.

This broadband light source is a light source for extracting two lights. That is, the upper second section having a substantially rectangular cross-sectional shape
Cladding layers 5-1 and 5-2, contact layers 6-1 and 6-
2 and the upper electrodes 7-1 and 7-2 are separately provided in parallel from one end face 10 to the other end face 11. By providing two in parallel in this way, incoherent light can be emitted in the directions of arrows 17-1 and 17-2. It is possible to control the optical output and bandwidth characteristics by adjusting the current I _L1, I _L2 to be injected into the terminal 9-1 and 9-2 independently. In addition, 12-
2a and 12-2b are anti-reflection coating layers. The broadband light source may further extract two or more lights.

The present invention is not limited to the above embodiment.
First, as a broadband light source, in addition to a superluminescent diode that emits incoherent light as shown in FIGS. 5A to 5D and FIGS. 6A to 6D,
A light emitting diode or the like may be used. In addition, as a method of adjusting the center wavelength of the tunable filter, the temperature may be changed while applying an external force to adjust the center wavelength. Conversely, the temperature may be maintained at a predetermined value. The center wavelength may be adjusted by changing the external force.
The wavelength band is not limited to the 1.5 μm band, and a 1.3 μm band or a shorter wavelength band may be used. That is, the broadband light source may be formed using a GaAs substrate instead of the InP substrate.

As described above, according to the present invention, (1) an information signal can be transmitted by selecting an arbitrary wavelength.

(2) A wavelength can be selected and transmitted according to a request from the receiving side.

(3) Crosstalk and communication quality hardly deteriorate.

(4) Since signal light having different wavelengths can be assigned to each optical transmitter and an information signal can be transmitted,
The confidentiality of information can be maintained.

(5) Since the selected wavelength can be controlled to be constant, the communication quality hardly deteriorates.

(6) A signal light having a desired wavelength can be selected from a very wide wavelength range, and the signal light can be used as a carrier to carry an information signal.

(7) An inexpensive optical transmitter can be provided.

(8) A high-output optical transmitter can be provided.

[0061]

In summary, according to the present invention, the following excellent effects are exhibited.

It is possible to select a signal light having an arbitrary wavelength, and to provide a wavelength tunable optical transmitter capable of optically modulating and transmitting the selected signal light.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a tunable optical transmitter according to the present invention.

FIG. 2 is a block diagram showing another embodiment of the tunable optical transmitter according to the present invention.

FIG. 3 is a block diagram showing another embodiment of the tunable optical transmitter according to the present invention.

FIG. 4 is a block diagram showing another embodiment of the tunable optical transmitter according to the present invention.

5A is a plan view of a broadband light source used in the wavelength tunable optical transmitter of the present invention, FIG. 5B is a left side view of FIG.
(C) is a right side view of (a), and (d) is a sectional view taken along line AA of (a).

6A is a plan view of a broadband light source used in the wavelength tunable optical transmitter shown in FIG. 4, FIG. 6B is a left side view of FIG. 6A, and FIG. 6C is a right side view of FIG. Figure, (d) is B- of (a).
It is a B sectional view.

FIG. 7 is a block diagram showing a conventional wavelength-division multiplexing optical transmitter.

FIG. 8 is a configuration diagram of a conventional variable wavelength light source.

[Explanation of symbols]

 Reference Signs List 20 broadband light source 21, 26, 29 drive circuit 24 tunable filter 25 temperature controller 28 optical modulator

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/14

Claims (5)

[Claims] 1. A broadband light source, a tunable filter for selectively extracting light of a desired wavelength from light emitted from the broadband light source, and an optical modulator for modulating the light of the desired wavelength and outputting a modulated signal light. A tunable optical transmitter, comprising: 2. A broadband light source, a tunable filter for selectively extracting light of a desired wavelength from the light of the broadband light source, an optical modulator for modulating the emitted light of the desired wavelength and outputting a modulated signal light, Adjusting means for adjusting the selected wavelength of the tunable filter, an optical tap circuit for extracting a part of the modulated signal light from the optical modulator, and comparing the modulated signal light extracted by the optical tap circuit with a reference wavelength. And a wavelength monitor circuit that feeds back an output signal to the adjusting means to control the selected desired wavelength to be constant. 3. A broadband light source, at least two tunable filters for selectively extracting light of a desired wavelength from light emitted from the broadband light source, and modulating the light of the desired wavelength to output a modulated signal light. A tunable optical transmitter, comprising: at least two optical modulators; and an optical multiplexer for multiplexing modulated signal lights from both optical modulators. 4. The tunable optical transmitter according to claim 1, wherein the tunable filter is a fiber type or a waveguide type grating filter. 5. The tunable optical transmitter according to claim 1, wherein the broadband light source is a light source that emits incoherent light.