JPH0786694A - Optical transmission device - Google Patents

Abstract

PURPOSE:To provide an optical transmission device which is capable of holding a narrow spectral line width and practically switching over a short circuit. CONSTITUTION:This invention relates to an optical transmission device which uses a multiple electrode DFB semiconductor laser for a receiving service. The optical transmission device uses a multiple electrode DFB semiconductor laser which is provided with an automatic frequency control circuit 12 which controls oscillation wavelength mainly based on the density of carriers and sweeps the oscillation wavelength over a required wavelength range by means of the current control device 12 and selects a specified wavelength in the wavelength range by means of the automatic frequency control circuit 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission device in a communication system such as optical frequency division multiplexing (optical FDM) communication.

[0002]

2. Description of the Related Art Conventionally, research and development of optical communication technology have been actively conducted. This is because optical signal transmission is superior to electrical signal transmission in terms of transmission speed and interference between signals. Under such circumstances, the optical FDM has been receiving attention in recent years. This is because the optical FDM is a communication system that realizes large-capacity transmission by utilizing multiplexing of light in the frequency space. In particular, coherent optical communication that utilizes the frequency and phase of light is promising as a next-generation communication method because it can achieve high density frequency multiplexing and can dramatically increase the amount of information transmission. .

By the way, in coherent optical communication, a light source that oscillates in a single longitudinal mode and has a narrow spectral line width is required. In the optical FDM communication system, in addition to the above characteristics, a wide range of wavelength tunable characteristics is required. The variable wavelength light source is used when selecting channels on the receiving side, and the wider the variable wavelength range, the more channels can be received. Further, in order to construct an n-to-n optical communication system, it is required that the channel switching time be short.

As the wavelength tunable semiconductor laser, a multi-electrode distributed feedback (multi-electrode DFB) semiconductor laser, a multi-electrode distributed Bragg reflection (multi-electrode DBR) semiconductor laser, etc. are known.

In a multi-electrode DFB semiconductor laser, the laser main body is divided into a plurality of regions in the cavity direction, and current is independently injected into each region to control the refractive index distribution inside the cavity and to generate an oscillation wavelength. The characteristic is that a high optical output and a narrow spectral line width can be realized over a wide wavelength tunable range.

However, in the multi-electrode DFB semiconductor laser, the refractive index depends on the carrier density and the temperature, but since the change amount of the carrier density due to the injection current in the active waveguide is small, the influence of the temperature rise accompanying the current injection is small. Cannot be ignored. In actual devices, a wide wavelength tunable range of several nm or more has been obtained, but the wavelength tunable amount by temperature is dominant.

Therefore, in the optical transmission device using the multi-electrode DFB laser, there is a problem that the speed of wavelength change is rate-controlled by the speed of temperature change, and the switching time of the channel is extended to about milliseconds.

On the other hand, the multi-electrode DBR semiconductor laser has a Bragg reflection region composed of a passive waveguide and a phase matching region in the cavity direction in addition to the active region, and a plurality of regions correspond to each region. It is provided with electrodes.

The wavelength tunable operation is performed by controlling the injection current to the Bragg reflection region and the phase matching region and changing the refractive index thereof. In the passive waveguide (Bragg reflection region, phase matching region), the carrier density of the waveguide can be greatly changed by the injection current. Therefore, rather than the contribution of temperature to the refractive index change in the Bragg reflection region and the phase matching region The contribution of carrier density is more dominant. For this reason, the carrier density changes at a high speed of about nanoseconds, and thus a multi-electrode DBR laser can realize a short channel switching time.

However, when the amount of injected carriers in the passive waveguide increases, the absorption loss increases, the optical output decreases, and at the same time the spectral line width increases. Therefore, the optical transmission device using the multi-electrode DBR laser has a problem that the wavelength variable range in which a narrow spectral line width can be maintained becomes small.

[0011]

As described above, in the conventional optical transmission device using the multi-electrode DFB semiconductor laser, a narrow spectral line width can be maintained over a wide wavelength tunable range, but the channel switching time is long. was there.

Further, in the conventional optical transmission device using the multi-electrode DBR semiconductor laser, there is a problem that the channel switching time is short, but the wavelength tunable range capable of maintaining a narrow spectral line width is narrow.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical transmission device capable of realizing a short channel switching time while maintaining a narrow spectral line width over a wide range. It is in.

[0014]

In order to achieve the above object, an optical transmission apparatus of the present invention uses a transmission apparatus for transmitting an optical signal and a semiconductor laser as a light source for reception, and receives the optical signal. And a second oscillation wavelength control means for controlling the oscillation wavelength of the semiconductor laser according to temperature, and a second oscillation wavelength control means for controlling the oscillation wavelength of the semiconductor laser according to temperature. The first oscillation wavelength control means transiently sweeps the oscillation wavelength over a desired wavelength range, and the second oscillation wavelength control means transiently fluctuates this oscillation frequency to gradually change the desired wavelength in the range. It is characterized by sequentially selecting.

Here, the first oscillation wavelength control means for controlling the oscillation wavelength by temperature is, specifically, a current injection into the active waveguide region or a built-in heating means (such as a thin film resistor). Means In the case of current injection into the active waveguide region, since the carrier density changes at the same time as the temperature change, the effect of the temperature change should be dominant.

[0016]

In general, controlling the oscillation wavelength by temperature can maintain a narrow spectral line width over a wide wavelength tunable range,
It takes time to switch the oscillation wavelength. On the other hand, in the control of the oscillation wavelength by the carrier density, the oscillation wavelength switching time is short, but the wavelength variable range in which a narrow spectral line width can be maintained is narrow.

Therefore, according to the present invention, the first oscillation wavelength control means for controlling the oscillation wavelength by the temperature can sweep a wide wavelength range while keeping the narrow spectral line width, and control by the carrier density. The second oscillation wavelength control means can select a desired wavelength within the above wavelength range in a short time.

Therefore, according to the optical transmission device of the present invention using the semiconductor laser provided with the first and second oscillation wavelength control means, a short channel switching time is realized while maintaining a narrow spectral line width over a wide range. it can.

[0019]

Embodiments will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing the structure of a multi-electrode DFB semiconductor laser used in a receiver of an optical transmission device according to a first embodiment of the present invention.

In the figure, 1 indicates an n-type InP substrate,
A diffraction grating 2 is formed on the n-type InP substrate 1 except for a part in the cavity direction. On this diffraction grating 2,
An InGaAsP active layer (including an optical waveguide layer) 3 as an active waveguide is formed, and InGa as a passive waveguide is formed in a region where a diffraction grating is not formed in the resonator direction.
An AsP passive layer (including an optical waveguide layer) 4 is formed.

InGaAsP active layer 3 and InGaAsP
On the passive layer 4, a p-type InP clad layer 5 and a p-type InGaAs contact layer 6 are formed.
The GaAs contact layer 6 is formed separately for each electrode.

Four p-type ohmic electrodes 7 ₁ , 7 ₂ , 7 ₃ , 7 ₄ are provided in the cavity direction, and among these, the p-type ohmic electrode 7 ₂ is formed on the InGaAsP passive layer 4. There is. On both end surfaces formed by cleavage,
A low reflection film 9 made of SiNx and having a reflectance of 1% or less is coated.

The laser body is fixed on a heat sink (not shown) serving as a ground electrode by AuSn solder, and the heat sink is controlled to have a constant temperature by using, for example, a temperature sensor and a Peltier element. There is.

The p-type electrodes 7 ₁ , 7 ₂ , 7 ₃ , and 7 ₄ are connected to a power feed line formed on a ceramic substrate by bonding so that the injection current can be controlled independently for each electrode. . The p-type electrodes 7 ₁ , 7 ₃ , and 7 ₄ can be controlled by the current controller 12 (first oscillation wavelength control means), and the p-type electrode 7 ₂ can be controlled by the automatic frequency control circuit 11 (second oscillation wavelength control means). It is like this. Specifically, a part of the laser light 17 is introduced into the frequency detection device 10 via the beam splitter 13. Then, the frequency detection device 10 detects the frequency of the laser beam 17, and the detection result is fed back to the injection current to the electrode 7 ₂ via the automatic frequency control circuit 11. Further, the current control device 12 precisely controls the injection current into the InGaAsP active layer 3, and at the same time, sends a signal instructing the switching of the channel wavelength to the automatic frequency control circuit 1.
I am sending to 1.

Here, the current control device 12 specifically means a current injection into the active waveguide region or a built-in heating means (such as a thin film resistor). In addition,
In the case of current injection into the active waveguide region, since the carrier density changes at the same time as the temperature change, the effect of the temperature change should be dominant.

According to this multi-electrode DFB semiconductor laser,
For example, when the injection current into the electrode 7 ₂ is 0 mA and no carrier is injected into the passive waveguide, the injection current into the electrodes 7 ₁ and 7 ₄ is constant and the injection current into the electrode 7 ₃ is 400 mA.
By changing the mA, the spectral line width is 1MHz
Continuous tunable operation over 5 nm can be obtained while keeping below.

FIG. 2 shows the change over time in the oscillation wavelength when the injection current to the electrode 7 ₃ (active waveguide) is changed stepwise. Here, the oscillation wavelength is changed by utilizing the refractive index change in the active waveguide, and the wavelength tunable operation is obtained by the temperature change. Therefore, there is a time of 1 millisecond before switching the oscillation wavelength, regardless of the wavelength variable amount. In addition, the time variation of the oscillation wavelength has been gradually changing after it suddenly rises.

Therefore, as shown in FIG. 3, when the injection current to the electrode 7 ₃ is linearly changed, the oscillation wavelength can be changed almost linearly with time. For example, 1
If a wavelength sweep of 5 nm is performed in milliseconds, in coherent optical communication, frequency multiplexing can be performed with a narrow channel interval of about 0.1 nm. Therefore, a large number of channels up to 50 channels can be obtained during a wavelength sweep of 5 nm. Will cross the wavelength.

In this way, by utilizing the change in the refractive index in the active waveguide and sweeping the wavelength in the predetermined oscillation wavelength range so as to cross each channel wavelength, injection into the electrode 7 ₂ (passive waveguide) is performed. By changing the current as shown in FIG. 5, the oscillation wavelength can be swept while adjusting the oscillation wavelength to each channel wavelength. For example, 50 channels can be switched by accessing each channel for 20 microseconds while sweeping the oscillation wavelength by 5 nm over 1 millisecond.

In this channel switching, since the refractive index change in the passive waveguide is based on the carrier density change, the channel switching time is as short as about nanosecond.
Therefore, for example, even when the time slot assigned to each channel is as short as 20 microseconds, a sufficiently short channel switching time can be realized.

Further, since the wavelength change amount due to the carrier density change of the InGaAsP passive layer 4 is as small as the channel interval or less and the injection current to the electrode 7 ₂ is very small, a narrow spectral line width of 3 MHz or less is obtained for all channels. Obtainable.

Thus, according to the present embodiment, while the injection current to the active waveguide is changed to perform the wavelength sweep over a wide range, the injection current to the passive waveguide is controlled to switch each channel at high speed. It is possible to access a large number of channels while maintaining a narrow spectral line width.

Next, a first modification of the wavelength variable operation in the multi-electrode DFB semiconductor laser of FIG. 1 will be described with reference to FIG. FIG. 4 shows the relationship between the injection current into the InGaAsP active layer 3 (active waveguide) and the InGaAsP passive layer 4 (passive waveguide) and the time change of the oscillation wavelength.

In an actual system, wavelength sweep over a wide range is repeated, and therefore, for example, channel 1
It is necessary to sweep the oscillation wavelength while sequentially switching from to channel n and then return the oscillation wavelength to the wavelength of channel 1.

To realize this, as shown in FIG.
After switching the channel from the long wavelength side channel 1 to the short wavelength side channel n sequentially, by controlling the injection current to the active waveguide and utilizing the excessive response of the temperature change, the oscillation wavelength is 100 microseconds. Can be swept up to channel 1 on the long wavelength side. 1 for each channel in 1 millisecond
If you decide to access each time, the dead time is 10
% Will be suppressed.

Next, a second modification of the wavelength variable operation in the multi-electrode DFB semiconductor laser of FIG. 1 will be described with reference to FIG. In this modification, in order to eliminate the dead time, as shown in FIG. 8, the oscillation wavelength is swept while sequentially switching from channel 1 to channel n, and then the oscillation wavelength is switched while sequentially switching from channel n to channel 1. Is sweeping. In addition, the wavelength sweep time is set to 500 microseconds one way by controlling the injection current to the active waveguide, and at least once in at least 1 millisecond,
You can access any channel.

FIG. 6 is a schematic diagram showing a schematic configuration of an n-to-n optical communication system according to the second embodiment of the present invention. In the figure, 21 _{1 to} 21 _n indicate optical transmitters, and these optical transmitters 21 _{1 to} 21 _n are connected to the optical receivers 22 _{1 to} 22 _n via a star coupler 20. The optical receivers 22 _{1 to} 22 _n use the multi-electrode DFB semiconductor laser of the previous embodiment as a local oscillator. That is, while maintaining a narrow spectral line width over a wide range,
Semiconductor lasers that can realize a short channel switching time are used.

The optical transmitters 21 _{1 to} 21 _n are respectively
The optical signals of the wavelengths λ _{1 to} λ _n , which are transmission signals, are transmitted. The optical signals of the wavelengths λ _{1 to} λ _n are frequency-division multiplexed by the star coupler 20 and sent to the optical receivers 22 _{1 to} 22 _n .

FIG. 7 shows the relationship between the transmission signals (channels ch _{1 to} ch ₂ ) from the star coupler 20 and the oscillation wavelength of the multi-electrode DFB semiconductor laser. The transmission signal from each channel is synchronized with the adjacent channel so as to be shifted by one time slot. Here, regarding the optical receiver 22 ₁ , if the oscillation wavelength is swept in a stepwise manner as shown in FIG. 7, the optical receiver 22 ₁ can receive a transmission signal from each channel in a time division manner.

[0040] That is, the receiving apparatus 22 ₁ first receives the transmission signal from the channel ch ₁ from the laser beams of the emission wavelength lambda ₁ at time t _1, then channel from the laser beams of the emission wavelength lambda ₂ at time t ₂ after receiving the transmission signal from the ch _2, channel c from the laser beams of the emission wavelength lambda ₂ at time t ₃
The transmission signal from h ₃ is received, and thereafter, the transmission signal from each channel is similarly received. Similarly, the receiving device 22
_{2 to} 22 _n can also receive transmission signals from all channels.

In the optical communication system of this embodiment, it is necessary to synchronize the transmission signals from each channel in the entire network, and it is necessary to transmit the control signal for this purpose to each channel from the controller in the entire network. For this purpose, for example, one channel wavelength may be assigned for this control signal, or the control signal may be transmitted as an intensity modulation signal during the dead time.

Further, it is not necessary for the wavelength variable amount due to the carrier density change of the InGaAsP passive layer 4 to be less than the channel interval, and the injection current to the passive waveguide should be changed within a range in which a narrow spectral line width can be maintained. You can

For example, by controlling the injection current to the passive waveguide as shown in FIG. 9, the time slots assigned to the channels ch _i-1 and ch _{i + 1} are used for communication with the channel ch _i .

Also, the time slots assigned to each channel do not have to be the same length for all channels, nor do the wavelength wavelength sweeps need to be linear with time. The period for wavelength sweeping while switching channels does not have to be constant.

Channels ch ₁ to c
The wavelength sweep time to h _n and the wavelength sweep time from channel ch _n to channel ch ₁ may be changed. FIG. 10 is a schematic diagram showing the structure of a multi-electrode DFB semiconductor laser used in the receiver of the optical transmission apparatus according to the third embodiment of the present invention. The parts corresponding to those of the multi-electrode DFB semiconductor laser of FIG.
Detailed description is omitted.

InGaAsP is formed on the n-type InP substrate 1.
An active layer (including an optical waveguide layer) 3, an InGaAsP passive layer (including an optical waveguide layer) 4 ₁ , 4 ₂ are formed, and InGaA is formed.
The diffraction grating 2 is formed below the sP passive layer 4 ₂ .
InGaAsP active layer 3, InGaAsP passive layer 4 _1,
4 ₂ is semi-insulating InP after being etched into a mesa shape
Embedded by layer 14.

InGaAsP active layer 3, InGaAsP
On the passive layers 4 ₁ and 4 ₂ and the semi-insulating InP layer 14,
A p-type InP clad layer 5 is formed, and a p-type I
nGaAs contact layer 6 and p-type ohmic electrode 7
₁ , 7 ₂ and 7 ₃ are formed separately corresponding to the InGaAsP layers 41, 42 and 3 in the resonator direction. A Pt thin film resistor 16 for heating is formed by the SiO ₂ film 15 so as to be insulated from the p-type InP clad layer 5.

According to the multi-electrode DBR semiconductor laser configured as described above, the oscillation wavelength control by the temperature change is performed by the current injection into the Pt thin film resistor 16, and the oscillation wavelength control by the carrier density change is performed by the passive waveguide. It is performed by current injection. The current injection into the passive waveguide is performed by the current injection into the electrode 7 ₁ or the electrode 7 ₂ . Further, the injection currents to the electrodes 7 ₁ and 7 ₂ may be controlled simultaneously. The specific operation method is the same as in the first embodiment.
Various wavelength variable operations are possible.

The present invention is not limited to the above embodiment. For example, in the above embodiment, InGaA
Although the case of the sP-based semiconductor laser has been described, the present invention is InGaAlAs-based, AlGaInAs-based, and AlG.
It can be applied to various material systems such as aInP system, InGaAsSb system, ZnCdSSe system.

Further, the substrate is not limited to the n-type substrate, and the end face may not be coated with antireflection coating.
Furthermore, the lateral structure of the semiconductor laser is not limited to the semi-insulating layer embedded structure, and the active layer and the passive layer may be composed of a bulk material or a quantum well structure.

Furthermore, the positions of the passive waveguide and the active waveguide in the resonator direction, the position where the diffraction grating is provided, the number of divided electrodes, the position where the heating means is provided, the number of heating means, etc. can be changed as appropriate. In addition, various modifications can be made without departing from the scope of the present invention.

[0052]

As described above in detail, according to the present invention, the first oscillation wavelength control means for controlling the oscillation wavelength mainly by the temperature can sweep a wide wavelength range while keeping the narrow spectral line width. The desired wavelength in the above wavelength range can be selected in a short time by the second oscillation wavelength control means which is controlled mainly by the carrier density.
Therefore, it is possible to realize a short channel switching time while maintaining a narrow spectral line width over a wide range.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a structure of a multi-electrode DFB semiconductor laser used in a receiver of an optical transmission device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between an injection current in an active region and an oscillation wavelength.

FIG. 3 is a diagram showing a relationship between an injection current in an active region and an oscillation wavelength.

FIG. 4 is a diagram showing a relationship between an injection current and an oscillation wavelength in an active region and a passive conducting waveguide.

FIG. 5 is a diagram showing a relationship between an injection current and an oscillation wavelength of an active region and a passive conducting waveguide.

FIG. 6 is a schematic diagram showing a schematic configuration of an n-to-n optical communication system according to a second embodiment of the present invention.

FIG. 7 is a diagram showing a relationship between a transmission signal and an oscillation wavelength.

FIG. 8 is a diagram showing a relationship between an injection current and an oscillation wavelength of an active region and a passive conducting waveguide.

FIG. 9 is a diagram showing a relationship between an injection current and an oscillation wavelength of an active region and a passive conducting waveguide.

FIG. 10 is a schematic diagram showing the structure of a multi-electrode DFB semiconductor laser used in a receiver of an optical transmission device according to a third embodiment of the present invention.

[Explanation of symbols]

1 ... n-type InP substrate 2 ... diffraction grating 3 ... InGaAsP active layer 4, 4 _1, 4 ₂ ... InGaAsP passive layer 5 ... p-type InP cladding layer 6 ... p-type InGaAs contact layer 7 _1, 7 _2, 7 _3, 7 ₄ ... p-type ohmic electrode 8 ... n-type ohmic electrode 9 ... low reflection film 10 ... frequency detection device 11 ... automatic frequency control circuit (second oscillation wavelength control means) 12 ... current control device (first oscillation wavelength control means) ) 13 ... Beam splitter 14 ... Semi-insulating InP layer 15 ... SiO ₂ film 16 ... Pt resistance thin film 17 ... Laser light 20 ... Star coupler 21 _{1 to} 21 _n ... Optical transmitting device 22 _{1 to} 22 _n ... Optical receiving device

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

Claims (2)

[Claims] 1. A transmission device for transmitting an optical signal, and a light receiving device for receiving the optical signal, wherein a semiconductor laser is used as a light source for reception, and the semiconductor laser has an oscillation wavelength whose temperature is a temperature. And a second oscillation wavelength control means for controlling the carrier density. The first oscillation wavelength control means transiently sweeps the oscillation wavelength over a desired wavelength range. At the same time, the oscillation frequency is transiently changed by the second oscillation wavelength control means, and desired wavelengths are sequentially selected step by step. 2. The optical transmission device according to claim 1, wherein the optical signal is a frequency division multiplexed signal.