JPH0242836A - Transmitting device for frequency dividing multiplexing optical transmission - Google Patents

Abstract

PURPOSE:To eliminate a polarization controller in each laser for transmission, to eliminate necessity to control a polarizing surface and to cause a transmitting device to be compact by providing a polarizing rotator to generate a time change for the polarizing surface in one part of the output light of a frequency variable laser. CONSTITUTION:The signal lights of a laser 200 for transmission to have mutually different oscillating frequencies are synthesized by a first synthesizer 500 and coupled to fiber for transmission. However, one part of the light is made incident to a second synthesizer 510. For an output light from a frequency variable lase 100, a frequency is periodically changed and the output light is divided into two lights by a divider 300. Then, one of the divided lights is synthesized with the signal light by the synthesizer 510. In such a case, a polarizing rotator 900, whose rotating speed is larger than the frequency change speed of the laser 100, is provided to rotate the polarizing surface of the output light from the laser 100 which is made incident to the synthesizer 510. Since the polarizing surface of the output light from the laser 100 is always rotated even when the signal light is made incident from the laser 200 to the synthesizer 510 and changed by the temperature change, etc., of synchronization, the control of the polarizing surface in the signal light is eliminated and the device is made compact.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a transmitter for frequency division multiplexed optical transmission.

[Conventional technology]

2. Description of the Related Art Frequency division multiplexing (FDM) optical transmission, in which a plurality of optical signals having different frequencies are transmitted through a single optical fiber, is gaining importance as an optical communication method capable of transmitting extremely large-capacity signals. In FDM transmission, in order to obtain a large number of channels, it is necessary to bring the frequency spacing of optical signals closer together on the transmitting side, but if the frequency spacing varies over time, crosstalk may occur during demultiplexing on the receiving side. arise.

For this reason, control (frequency interval lock) for maintaining the frequency intervals of a plurality of optical signals on the transmitting side at a constant value is essential. Various types of frequency interval locking methods have been proposed, among which the ``optical reference pulse method'' proposed by Shimosaka et al. is an extremely stable method. This optical reference pulse method is described in detail in the Technical Report of the Institute of Electronics, Information and Communication Engineers (Shimosaka et al. C387-96, 1987), but an outline will be explained below.

FIG. 2 is a diagram showing the basic configuration of a transmitter using a frequency interval locking method based on the optical reference pulse method. In the figure, solid lines represent optical coupling and broken lines represent electrical coupling.

Frequency interval locking of the N transmitting lasers 200 is performed as follows.

The signal lights of the transmitting lasers 200 having different oscillation frequencies are multiplexed by the first multiplexer 500 and coupled to the transmission fiber, but a portion of the signal lights enter the second multiplexer 510. On the other hand, the frequency of the output light from the variable frequency laser 100 changes periodically, and this output light is sent to the demultiplexer 300.
The light beam is divided into two parts, one of which is combined with the signal light by the second multiplexer 510, and the beat signal of this combined light is received by the second optical receiver 610. At this time, the frequency intervals of the transmitting laser 200 are approximately determined by temperature control. Therefore, by extracting the low frequency component of the beat signal from the second optical receiver 610, an electric pulse train aligned on the time axis corresponding to each signal light is generated for each period of frequency change of the variable frequency laser. arise. The output light from the other variable frequency laser, which is split into two by the demultiplexer 300, is received by the first optical receiver 600 through an optical resonator 400 such as a Fabry-Perot interferometer. Corresponding to the Fries vector range of the optical resonator 400, the first light receiver 600 generates an electrical pulse train arranged at regular intervals on the time axis as the frequency of the variable frequency laser 100 changes over time. The time interval of the electrical pulse train from the first light receiver 600 matches the full spectral range of the optical resonator 400 and corresponds to a strict frequency interval.

Therefore, the control circuit 700 adjusts the oscillation frequency of each transmitting laser 200 so that the position of the pulse train from the second light receiver 610 described above matches the position of the pulse train from the first light receiver 600 on the time axis. By applying electrical feedback for control, the frequency interval of the transmitting laser 200 can be matched to the Fries vector range of the optical resonator 400. This fleece vector range can be made very stable, allowing for stable frequency interval locking.

[Problem to be solved by the invention]

The conventional transmitter for FDM transmission using the optical reference pulse method has the following problems. That is, a polarization controller 800 is required for each laser. When attempting to obtain a beat signal by combining the output light of the variable frequency laser 100 and the signal light from the transmitting laser 200 in the second multiplexer 510, the polarization planes of both must match. . Therefore, as shown in FIG. 2, each of the variable frequency laser 100 and the transmitting laser 200 is provided with a polarization controller 800. This polarization controller 800 needs to be adjusted for each laser, especially in an FDM transmission transmitter having a signal light source of several dozen waves or more.
This adjustment was extremely complicated. Furthermore, since a large number of polarization controllers 800 are used, it is difficult to downsize the device.

The present invention improves the frequency interval locking method using the conventional optical reference pulse method, and provides a transmitting device for FDM transmission that does not require a polarization controller 800, does not require adjustment of the polarization plane, and can be miniaturized. It's about doing.

[Means to solve the problem]

The configuration of the present invention is a frequency division multiplexing optical transmission transmitter for transmitting a plurality of optical signals having different frequencies through a single optical fiber, wherein the frequency interval of the optical signals is determined by an optical reference pulse method. In the optical reference pulse method, a variable frequency laser whose frequency can be periodically changed is used to obtain a beat signal with the plurality of optical signals, and the A polarization rotator that temporally changes the polarization plane of a part of the output light of the frequency tunable laser, and the speed of the temporal change of the polarization plane is sufficiently larger than the frequency change speed of the frequency tunable laser. It is characterized by

〔Example〕

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a diagram showing the configuration of an embodiment of the present invention. This is a 30-channel FDM transmission transmitter that applies a frequency interval locking method using an improved optical reference pulse method. The basic configuration is the same as the conventional example shown in FIG. The difference is that there is no polarization controller 800 provided for each laser in the conventional example, and instead there is a polarization controller 800 that rotates the plane of polarization of the output light from the frequency variable laser 100 that enters the second multiplexer 510. The rotator 900 is introduced. Transmission laser 2
Even if the plane of polarization changes due to periodic temperature changes when the signal light from 00 enters the second multiplexer 510,
Since the polarization plane of the output light of the variable frequency laser 100 is constantly rotating, a beat signal can be obtained regardless of the state of the polarization plane of the signal light. Compared to the conventional example in which the plane of polarization is fixed, the level of the beat signal is theoretically degraded by 3dll, but the level of the beat signal required by this method may be very small (the level of the beat signal to the second multiplexer 510 is Transmission laser 20
Since the incident light level is -40 dBm or less), the signal level output to the transmission line is almost unchanged compared to the conventional example. Another feature of the present invention is that the rotation speed of the polarization rotator 900 is not required to be very fast. Regarding the rotation speed, the speed of frequency change of the variable frequency laser 100 (same as the control band of the entire control system) is P, (H
z), the rotation speed P2(H
z) should approximately satisfy P2>>PI -N-F. Here, N is the number of channels and F is the finesse of the optical resonator 400. Control band is l kHz
This is usually sufficient. Also, the number of channels N is several 10
If the channel and finesse are about 10, the rotation speed P2
It is sufficient if the frequency is about several MHz.

Specific parts will be explained below. As the polarization rotator 900 of the present invention, a waveguide-based element using lithium niobate was used. Regarding this element, A, Sm1t
International Conference on Optical Waveguides (Topical
Meeting on Integrated an
d Guided-Wave Optics, I
GWO' 88. MB2, 1988)
It is stated in Specifically, it consists of a mode converter that converts the TE/TM mode of the incident light, and a phase shifter that corrects the positional shift of the TE/TM mode of the incident light. Both have waveguides and electrodes formed on a LiNbO3 substrate, and utilize the electro-optic effect of LiNbO3. By appropriately adjusting the high-frequency electric field applied to the mode converter and phase shifter, it is possible to output light incident in any polarization state as rotating linear polarization. This polarization rotator 900 has a band of 100 MHz or more, which is sufficient for use in this embodiment. Other parts are almost the same as the conventional example. As the frequency variable laser 100, electrically 300
The transmission laser 2 is a Bragg reflection type semiconductor laser with variable frequency distribution in the 1 and 5 μm band whose frequency can be changed over GHz.
00 is a 1.5 μm band distributed reflection semiconductor laser, the optical resonator is finesse 10, the Frey spectral range (corresponding to the frequency interval) is 8Gl (z Fabry-Perot resonator, and the first multiplexer 500 is The control circuit 700 is exactly the same as the conventional example, and the transmitting laser is controlled so that the pulse train from the first optical receiver 600 coincides with the pulse train from the second optical receiver on the time axis. The oscillation frequency of each laser is controlled by changing the current flowing through the 200.Specifically, the two counters into which the above two pulse trains enter, an exclusive OR gate that combines the outputs of the two counters, and this gate. The control circuit 700 has a control band of approximately 1 kllz.
The rotation speed of the polarization rotator 900 was set to 5 MHz. All lasers are temperature controlled to ±0.1°C.

Frequency control of 30 waves (frequency interval is 8 GHz) was performed using this FDM transmission transmitter. Frequency interval variation is 1%
It was below.

In this embodiment, a lithium niobate waveguide type element is used as the polarization rotator 900, but other polarization rotators, such as a type that controls stress applied to a fiber, may be used. However, in this case, the rotational speed is slower than in the actual case, so it is necessary to reduce the entire control band.

Further, although a distributed feedback laser was used as the transmitting laser 200, frequency control would be easier if a variable frequency laser was used. Further, although the embodiment is designed with a transmission system in which a signal is transmitted by directly frequency modulating the transmitting laser 200 and received by optical heterodyne detection, it is also applicable to direct detection. In this case, the transmitting laser 2
As long as the intensity modulation is introduced immediately after 00, the other configurations may be the same.

〔Effect of the invention〕.

As described above, according to the present invention, the polarization controller 800 is not required for each transmitting laser, and therefore FDM using a frequency interval locking method that does not require adjustment of the polarization plane.
A transmitter for transmission can be realized. In a transmitting device consisting of a transmitting laser with 30 wavelengths in the 1.5 μm band, 8 GHz
We were able to lock the frequency at a frequency interval of , and suppress the fluctuation to less than 1%. Furthermore, in the present invention, only one polarization rotator 900 is required instead of a large number of polarization controllers 800, so that the entire device can be reduced in size to about half that of the conventional device.

[Brief explanation of the drawing]

FIG. 1 is a block diagram that does not show an embodiment, and FIG. 2 is a block diagram showing a conventional example. 100... Frequency variable laser, 200... Transmission laser, 300... Demultiplexer, 400... Optical resonator,
500.510... Multiplexer, 600.610... Light receiver, 700... Control circuit, 800... Polarization controller, 900... Polarization rotator. Agent Patent Attorney Shintori Uchihara

Claims (1)

[Claims] A variable frequency laser, a plurality of transmitting lasers having different oscillation frequencies, a first multiplexer that multiplexes light from each of the transmitting lasers, an optical resonator, and a plurality of transmitting lasers having different oscillation frequencies; a polarization rotator that divides the light from the variable frequency laser into two, and makes one of the lights enter the optical resonator and the other light enter the polarization rotator. a first light receiver that receives the light that has passed through the optical resonator; a second multiplexer that multiplexes the light that has passed through the polarization rotator and the light that has passed through the first multiplexer; At least a second light receiver that receives light from the second multiplexer, and a control device that changes the oscillation frequency of the transmitting laser based on the signals from the first and second light receivers. A transmitter for frequency division multiplexed optical transmission, characterized by: