KR0147762B1 - Multichannel Light Source Using Multimode Laser - Google Patents

Abstract

The present invention relates to a light source in which the optical frequencies are regularly aligned in the WDM transmission technology, and a plurality of optical frequencies (7-1 to 7-) aligned at regular intervals using a multimode laser (1) employing a Fabry-Perot resonator. 5) and each of these optical frequencies are separated by WDM, and different optical amplifications are performed for each optical frequency, thereby obtaining a light source in which a constant light output and an optical frequency are aligned.

Description

Multichannel Light Source Using Multimode Laser

1 is a block diagram of a multi-channel light source according to a first embodiment of the present invention.

FIG. 2 is a graph showing optical frequency characteristics of the multimode laser and reverse wavelength multiplexer shown in FIG.

3 is a block diagram of a multi-channel light source according to a second embodiment of the present invention.

4 is a block diagram of a multi-channel light source according to a third embodiment of the present invention.

* Explanation of symbols for main parts of the drawings

1: multimode laser 2: reverse wavelength multiplexer

3: Wavelength control circuit 4: Photodetector

5,6-1 to 6-5,14-1 to 14-4,15,17,21-1 to 21-4: Optical fiber

7: metal wire 9: optical isolator

10: multi-channel light source 13-1 to 13-4: external light modulator

11-1 ~ 11-4: optical fiber with optical amplification

12: optocoupler or wavelength multiplexing element

16: Optical star coupler 18: Here light source

20-1 ~ 20-4: Semiconductor Laser Amplifier / Modulator

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-channel light source used in the construction of a multi-channel optical communication network by wavelength multiplexing and a multi-channel transmission method, and more particularly, to a design method of a multi-channel light source in which optical frequencies are uniformly aligned.

In general, in order to construct an optical communication network by wavelength multiplexing or to complete an optical transmission system by wavelength multiplexing, an optical channel set having a constant wavelength or optical frequency interval is required. As a light source for an optical channel, a semiconductor laser having a distributed feedback (DFB) and a distributed bragg reflector (DBR) structure is currently widely used. This type of laser can generate light at a single optical frequency and can be modulated at high speeds.

There are two main ways to construct a set of optical frequencies.

One is to select only to generate a predetermined optical frequency through several tests in the manufacturing process of the semiconductor laser, and the other is to set a given optical frequency by varying the optical frequency of the tunable laser which can change the optical frequency within a certain range. To construct.

The first method is probabilistically difficult due to the manufacturing characteristics of the semiconductor laser, and this situation becomes more severe as the number of optical frequencies increases. As a result, the price of the multichannel light source becomes very high.

The second method requires the installation of a wavelength control circuit in order to maintain a single optical frequency in each optical channel, and needs to constantly monitor and adjust whether the optical frequency interval of the output light is constant. In addition, as the number of wavelength control circuits are needed as the number of optical channels, the volume is increased by that amount, and improvement in maintenance and cost is required.

The above-described prior art has a disadvantage in that it is inherently expensive because many expensive high-precision DFB or DBR semiconductor lasers are used. In addition, in terms of constituting a set of light sources having a constant optical frequency interval, there is a disadvantage that the configuration itself is difficult or it is difficult to maintain a constant optical frequency interval with respect to changes in the surrounding environment and characteristics of the laser itself. These shortcomings are hardly applicable because the cost per subscriber increases significantly, especially when low speed transmission is required and the operating environment is poor, such as in an optical subscriber network.

Accordingly, an object of the present invention is to provide a multi-channel light source suitable for optical wavelength multiplexing communication at a lower cost by improving the characteristics of the optical frequency interval.

In order to achieve the above object, in the multi-channel light source of the present invention, multi-mode laser means for generating light including a plurality of optical frequencies,

An optical isolator means for inputting the output from the multimode laser means to transmit the reflected wave generated outside the laser to the reverse wavelength multiplexing means without affecting the laser longitudinal mode characteristics;

A reverse wavelength multiplexing means for receiving the light output from the optical isolator means and sequentially separating only the optical frequencies placed at regular frequency intervals and outputting the optical fiber composed of n pieces;

Light detecting means for detecting an optical frequency connected to one of the n optical fibers and transmitted thereto;

A wavelength control means for outputting an electrical control signal for wavelength adjustment to the multi-mode laser means by inputting the output from the light detection means.

Hereinafter, embodiments of the present invention will be described in more detail with reference to the accompanying drawings.

1 is a block diagram of a multi-channel light source according to a first embodiment of the present invention, a multi-mode laser (1) having a FP (Fabry-Perot) resonator structure,

An inverse multiplexer 2 whose output of the multimode laser 1 is connected by an optical fiber 5 to an input terminal via an optical isolator 9,

A plurality of optical fibers 6-1 to 6-5 connected to each output terminal of the reverse wavelength multiplexer 2,

A photodetector 4 connected with one of the plurality of optical fibers,

A wavelength control circuit 3 to which an output of the photodetector 4 is connected by a metal line 7;

An electrical output terminal of the wavelength control circuit 3 was provided with the multimode laser 1 connected by a metal line 7.

In the FP resonator, two plane mirrors are placed in parallel, and resonant frequencies 8-1 to 8-5 are periodically arranged as shown in FIG.

Here, the frequency interval Δf of two adjacent resonance frequencies is expressed as follows.

Where c is the speed of light in free space, L is the spacing of two planar mirrors, and n is the refractive index of the material in the resonator.

If a material capable of generating optical gain exists in the resonator, it operates with the FP laser 1, and each resonance frequency corresponds to the longitudinal mode of the FP laser having a finite linewidth. If the light gain distribution according to the optical frequency of the light gain material is substantially parabolic, the optical frequency characteristics as shown in FIG. For simplicity, five longitudinal modes (7-1 to 7-5) were shown.

The reverse wavelength multiplexer 2 adopted in the present invention is an interference type, and receives light including a plurality of optical frequencies, and sequentially separates only the optical frequencies arranged at regular frequency intervals and presents them to each output terminal. If light having an optical frequency characteristic as shown in FIG. 2 (b) is input, light of the optical frequency 8-1 is propagated to the optical fiber 6-1, and the optical frequency 8- is transmitted to the optical fiber 6-2. Only one light frequency propagates in the optical fiber connected to each output terminal of the reverse wavelength multiplexer 2 in such a manner that the light of 2) propagates. The frequency spacing of the reverse wavelength multiplexer (2) is designed to match the frequency spacing between the longitudinal modes of the multimode laser, and is not affected by the external environment such as temperature, humidity, or influenced in the same shape as the multimode laser (1). It is designed to receive.

Therefore, when the alignment error of Figs. 2 (a) and 2 (b) becomes zero, each longitudinal mode of the multi-mode laser 1 appears separately at each output terminal of the inverse wave multiplexer 2. The vertical mode of each output stage is used as an optical channel for wavelength multiplexing, and an external modulation scheme is applied.

The absolute longitudinal mode position of the multimode laser 1 varies with the strength of the excitation light and electrical signals, such as temperature, voltage or current, so that the alignment error is adjusted. When the alignment error is 0, the output power 6-5 of the reverse wavelength multiplexer 2 shows the maximum light output. When the alignment error is not 0, the light output becomes the minimum. Therefore, the output of the photodetector 4 is received as an input. Generate an electrical control signal for adjustment. This control signal is converted into a parameter capable of changing the absolute position of each longitudinal mode by the internal circuit of the multimode laser 1. In this way, the alignment error always remains zero. The optical isolator 9 serves to isolate the reflected wave generated outside the laser from affecting the laser longitudinal mode characteristic.

FIG. 3 illustrates a method of maintaining a constant size and external modulation using optical fiber optical amplifications 11-1 to 11-4 when optical powers included in the various modes of the multimode laser 1 are different. It is shown. Since the amplification rate of the optical fiber optical amplifier depends on the intensity of the excitation light, it is adjusted by the length of the optical fiber or the coupling ratio of the optical coupler 12. A wavelength division multiplexer may be used instead of the optical coupler 12.

4 shows an example of simultaneously performing optical amplification and modulation using semiconductor laser amplifiers / modulators 20-1 to 20-4.

As described above, using a multi-channel light source using the multi-mode laser of the present invention has the following effects.

Since each channel of the multi-channel light source is aligned at regular intervals, there is no need for optical frequency alignment unlike conventional methods. Therefore, a control circuit conforming to the optical frequency alignment becomes unnecessary, and the transmission characteristic deterioration which may be caused by incomplete alignment can be reduced.

In addition, when the output of the light source according to the present invention is multi-stage diverged and optically amplified, the optical frequency set can be used in duplicate, thereby reducing the cost per channel significantly.

Claims (11)

In a multi-channel light source with a constant optical frequency, a multimode laser means for generating light including a plurality of optical frequencies, and a reflected wave generated outside the laser by inputting output from the multimode laser means Optical isolator means for transmitting to the reverse wavelength multiplexing means so as not to affect the mode characteristics, and the light is output from the optical isolator means in order to separate only the optical frequencies arranged at regular frequency intervals to n optical fibers A reverse wavelength multiplexing means for outputting, a light detecting means for detecting an optical frequency connected to one of the n optical fibers, and an output from the light detecting means as an input to the multimode laser means Wavelength control number to output electrical control signal for wavelength adjustment The multi-channel light source, comprising a step of having a. The multi-channel light source according to claim 1, wherein the multimode laser means and the optical isolator means are connected to each other by an optical fiber. The multi-channel light source according to claim 1, wherein the optical isolator means and the reverse wavelength multiplexing means are connected to each other by an optical fiber. The multi-channel light source of claim 1, wherein the light detecting means and the wavelength control means are connected to each other by a metal line. The multi-channel light source of claim 1, wherein the wavelength control means and the multi-mode laser means are connected to each other by a metal line. The multi-channel light source according to claim 1, wherein only one optical frequency of light propagates in the optical fiber connected to each output terminal of the reverse wavelength multiplexing means. The multi-channel light source according to claim 1, wherein the reverse wavelength multiplexing means has an interval of optical frequencies output to the optical fiber coinciding with an interval between longitudinal modes of the multimode laser means. The multi-channel light source according to claim 1, wherein the optical fiber connected to the light detecting means has a maximum light output when the alignment error is zero and a minimum light output when the alignment error is not zero. The multi-channel light source according to claim 1, wherein the reverse wavelength multiplexing means optically amplifies different amplification rates for each channel in order to make the optical power of each channel (long mode) constant. The method of claim 1, wherein when the optical power of each channel (long mode) output from the reverse wavelength multiplexing means is different from each other, an external modulation is maintained by maintaining a constant size using an optical fiber having an optical amplification function. Multichannel light source. The multi-channel light source according to claim 1, wherein the optical amplification and modulation of the optical power of each channel (vertical mode) output from the reverse wavelength multiplexing means are performed simultaneously using a semiconductor laser.