JP2001111493A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a signal level fluctuation suppressing effect as in a conventional example without providing an optical phase modulating means. SOLUTION: An optical transmitter is provided with a semi-conductor laser having a high relaxation vibration frequency in such a degree that a signal transmission characteristic is not deteriorated even when a coherence decay phenomenon occurs due to a reflection return light, a light re-injecting means for re-injecting a part of an output light from the semi-conductor laser to the semi-conductor laser in such a level that a coherence decay occurs in the laser and a light intensity modulating means for modulating intensity in the output light of the semi-conductor laser through the use of a data signal to be transmitted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter used in an optical communication system.

[0002]

2. Description of the Related Art Conventionally, coherent crosstalk has been known as a factor causing level fluctuation of signal light in an optical communication system. If the optical switch and the optical multiplexer / multiplexer are incomplete, light of the same wavelength is superimposed on the desired signal light as crosstalk light. When light of the same wavelength overlaps, the two interfere with each other, and the received light power is strengthened or weakened depending on the mutual phase relationship. Generally, since the phase of light fluctuates with time, the received light power also fluctuates accordingly. This is signal fluctuation due to coherent crosstalk.

[0003] As a method of suppressing the deterioration of a received signal due to coherent crosstalk, a technique of expanding the optical spectrum width of transmitted light by phase modulation has been used in PKPepeljugos.
ki and KYLau, ”Interferometric noise reduction i
n fiber-optic links by supreposition of high frequ
ency modulation, ”J. Lightwave Technology, vo1.10,
pp. 957-963, 1992. (Reference 1). As shown in FIG. 8, the transmitter is composed of a stationary light from a light source 1 and a light intensity modulator 3 to which a data signal 2 to be transmitted is applied.
, And phase-modulated by the optical phase modulator 5 to which the RF signal (f _p ) 4 is applied. Then, the receiver directly detects the intensity of the transmitted signal light. Thereby, only the intensity-modulated data signal is received.

[0004] The phase modulation is added to expand the optical spectrum of the signal light by the modulation sideband. When a light source having a wide optical spectrum is used for transmission light, the effect of reducing deterioration due to coherent crosstalk can be obtained.
The reason is as follows.

[0005] For example, the signal light has three values of f ₀ and f ₀ ± Δf.
It is assumed that it consists of two optical frequency components. This is an example where the optical spectrum extends from f ₀ −Δf to f ₀ + Δf. As described above, the deterioration due to coherent crosstalk is caused by signal fluctuation due to interference between the main signal light and the crosstalk light. This is expressed as
In the expression,

[0006]

[Number 1] received light intensity _{= | A S exp {i (} 2πf S t + φ S)} + A C exp {i (2πf C t + φ C)} | 2 = | A S | 2 + | A C | 2 + 2A S A _C cos {2π (f _S −f _C ) t + φ _S −φ _C }. Here, A is the amplitude of the optical electric field, f is the optical frequency, φ is the optical phase, and the subscripts s and c represent the main signal light and the crosstalk light, respectively. The third term is the interference term, usually f _S = f _C. In this case, the third term fluctuates in accordance with φ _S −φ _C , which causes deterioration of reception characteristics.

On the other hand, if the signal light is composed of three optical frequency components as described above, the third term becomes 2Δf, Δ
It has a frequency component of f, 0.

Here, Δf is assumed to be a frequency higher than the data signal band to be transmitted. Normally, in an optical receiving system, an electric filter that passes only a data signal band is used to remove an extra noise component. Therefore, if Δf is a frequency higher than the data signal band, 2
Δf and the frequency component of Δf are removed by an electric filter. That is, a part of the component that causes the deterioration of the reception characteristic is removed, and the degree of the deterioration is smaller than in the normal case.

In the above description, the case where the signal light is composed of three optical frequency components has been described as an example, but the same applies to the case where the optical spectrum is generally spread. That is, when the signal light spectrum is wider than the data signal band, part of the interference component between the main signal light and the crosstalk light is removed by the electric filter, and as a result, the coherent cross-talk is reduced as compared with the case where the light spectrum is not expanded. It is possible to reduce reception characteristic deterioration due to talk.

[0010] The article of the above-mentioned Document 1 proposes a method of broadening the optical spectrum of signal light by adding phase modulation, and as a result, suppressing deterioration of reception characteristics due to coherent crosstalk.

[0011]

In the above conventional example, phase modulation is applied to a signal light source, and an optical spectrum is expanded by a modulation sideband generated by the phase modulation. In this case, in order to obtain an optical spectrum width sufficient to suppress reception degradation, it is necessary to apply phase modulation at a frequency higher than the data signal band.
For this reason, a means must be provided for applying phase modulation at high speed, and there has been a problem that the configuration of the transmitter becomes complicated.

An object of the present invention is to provide a technique capable of obtaining a signal level fluctuation suppressing effect based on the principle of the above-mentioned conventional example without providing an optical phase modulation means. The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

[0013]

The outline of a typical invention among the inventions disclosed in the present application will be briefly described as follows. (1) A semiconductor laser having a high relaxation oscillation frequency such that signal transmission characteristics are not deteriorated even if a coherence collapse phenomenon caused by reflected return light occurs, and a part of output light from the semiconductor laser is subjected to coherence collapse by the semiconductor laser. An optical transmitter comprising: a light re-injection unit for re-injecting the semiconductor laser at a raised level; and a light intensity modulation unit for intensity-modulating the output light of the semiconductor laser with a data signal to be transmitted.

(2) a semiconductor laser having a terminal for inputting a current to be injected and having a relaxation oscillation frequency high enough to prevent signal transmission characteristics from deteriorating even if coherence collapse occurs due to reflected return light; An optical transmitter comprising: a light re-injection unit for re-injecting a part of output light from a laser at a level at which the semiconductor laser causes coherence collapse; and an injection current modulation unit for modulating the injection current by a transmission data signal. It is.

Hereinafter, the present invention will be described in detail along with embodiments (examples) with reference to the drawings. In all the drawings for describing the embodiments (examples), those having the same functions are denoted by the same reference numerals, and their repeated description will be omitted.

[0016]

(Embodiment 1) FIG. 1 is a block diagram showing a schematic configuration of an optical transmitter according to Embodiment 1 of the present invention, wherein 11 is a semiconductor laser, 12 is an optical coupler, 13
Is a light intensity modulator, 14 is a reflection point, 15 is a transmission data signal, and 16 is a transmission signal light.

As shown in FIG. 1, the optical transmitter according to the first embodiment has an optical intensity modulator in which output light from a semiconductor laser 11 is branched into two by an optical coupler 12 and one of which is applied with a transmission data signal 15. The light intensity is modulated by 13, and the other is guided to the reflection point 14. The reflection point 14
For example, this is realized by means such as coating the end face of the optical fiber with a reflective film. The light reflected at the reflection point 14 is re-input to the semiconductor laser 11 following the same path. Here, it is assumed that the polarization state of the re-input light coincides with the polarization of the laser oscillation light. Such a setting can be realized by setting the optical path to a polarization maintaining fiber.

FIG. 1 is configured so that reflected return light is injected into the semiconductor laser 11. By the way, it is known that when there is reflected return light of a certain level or more in the semiconductor laser 11, a phenomenon called “coherence decay” occurs and the optical spectrum of the oscillated light broadens. FIG. 2 shows an example in which the inventor actually observed the optical spectrum spread at the time of coherence collapse. FIG. 2 (a) shows the case where there is no reflection, and FIG. 2 (b) shows the case where there is reflection. When the reflected light of −15 dB was re-injected into the semiconductor laser 11 called DFB-LD, an optical spectrum spread of 25 GHz was observed.

On the other hand, it is known that when coherence decay occurs, the optical spectrum is broadened and the relative intensity noise of the laser beam increases. For this reason, coherence decay is generally considered undesirable for signal transmission. However, if a semiconductor laser with a high relaxation oscillation frequency is used, good signal transmission characteristics can be obtained even if there is an increase in relative intensity noise due to reflected return light.
ng and K. Petermann, ”Noise characteristics of PCM-
modulated single-mode semiconductor laserdiodes wi
th distant optical feedback, ”IEE Proceedings, vo
1.137, Pt. J, pp. 385-390, 1990. (Reference 2). Note that the relaxation oscillation frequency is a kind of resonance frequency unique to the semiconductor laser.

FIG. 3 shows the results of an experiment conducted by the inventor to confirm the above-mentioned reference 2. FIG. 3 shows that the laser light in the coherence decay state due to the reflected return light is 2.
This is the result of externally modulating with a 5 Gbit / s pseudo random signal and measuring the bit error rate. The measurement is performed while changing the injection current to the semiconductor laser. It is known that the relaxation oscillation frequency of a semiconductor laser depends on an injection current. The higher the injection current, the higher the relaxation oscillation frequency. In other words, measuring by changing the injection current is equivalent to measuring by changing the relaxation oscillation frequency.

Referring to FIG. 3 (reception light power vs. bit error rate characteristic diagram), the bit error rate characteristic is greatly degraded at a low injection current, but deteriorates at a high injection current (ie, a high relaxation oscillation frequency). It can be seen that no error rate characteristics can be obtained. This result indicates that the use of a semiconductor laser having a high relaxation oscillation frequency can provide good signal transmission characteristics even in a coherence collapse state. Incidentally, the relaxation oscillation frequency of the semiconductor laser used in the experiment at an injection current of 60 mA was 7 GHz. The above characteristics arise from the fact that the increase in relative intensity noise due to coherence decay occurs largely near the relaxation oscillation frequency.

3 (injection current = 51 mA)
FIG. 4 shows the noise spectrum of the laser light intensity at 60 mA). The noise spectrum has a shape having a peak at a certain frequency, and this peak frequency corresponds to the relaxation oscillation frequency. That is, it is understood that the increase in the relative intensity noise occurs around the relaxation oscillation frequency. Therefore, when the relaxation oscillation frequency is high, the noise power concentrates on the high frequency, and as a result, the noise in the data signal band (2 GHz or less in the above experiment) is small enough not to deteriorate the signal transfer characteristics.

Using the above-described results, the spread of the optical spectrum in coherence decay can be applied to suppression of reception characteristic degradation due to coherent crosstalk. That is, a semiconductor laser having a high relaxation oscillation frequency such that signal transmission characteristics are not degraded even when coherence collapse occurs is provided, and reflected coherence is applied to the semiconductor laser to cause coherence collapse. Then, an optical transmitter is configured using the laser light as a light source (FIG. 1). Since the signal light to be outputted has a wide optical spectrum spread, it is possible to suppress the reception characteristic deterioration due to the coherent crosstalk according to the principle described in the section of the related art.

The relaxation oscillation frequency of the semiconductor laser is
It is determined by injection current, differential gain coefficient, carrier lifetime, resonator length, and the like. By devising these parameters, a high relaxation oscillation frequency can be obtained.

(Embodiment 2) In the first embodiment, the signal light intensity is modulated by the light intensity modulator. In the second embodiment, as shown in FIG. 5, the injection current to the semiconductor laser 11 is directly modulated. The configuration is such that: In FIG. 5, reference numeral 17 denotes a bias current, 18 denotes an inductance, and 19 denotes a capacitor. The inductance 18 and the capacitor 19 constitute an electric filter.

(Embodiment 3) In Embodiment 1, the output light from one end face of the semiconductor laser 11 is branched into two, one for signal transmission and the other for reflected return light.
As shown in FIG. 6, the output light from one end face is used for signal transmission, and the output light from the opposite end face is used for reflected return light.

Fourth Embodiment In the third embodiment, the signal light intensity is modulated by the light intensity modulator 13.
Has a configuration in which the injection current to the semiconductor laser 11 is directly modulated as shown in FIG.

Although the present invention has been described in detail based on the embodiments, the present invention is not limited to the above-described embodiments, and it is needless to say that various changes can be made without departing from the gist of the present invention. is there.

[0029]

As described above, according to the present invention,
The optical spectrum of the signal light can be broadened without providing the phase modulation means as in the related art, and the deterioration of reception characteristics due to coherent crosstalk can be suppressed with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a schematic configuration of an optical transmitter according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an example of actually observing the spread of an optical spectrum at the time of coherence collapse.

FIG. 3 is a diagram illustrating a received optical power versus bit error rate characteristic of an optical transmitter.

FIG. 4 is a diagram showing the results of an experiment performed by the inventor to confirm the indication of Document 2.

FIG. 5 is a block diagram showing a schematic configuration of an optical transmitter according to a second embodiment of the present invention.

FIG. 6 is a block diagram illustrating a schematic configuration of an optical transmitter according to a third embodiment of the present invention.

FIG. 7 is a block diagram illustrating a schematic configuration of an optical transmitter according to a fourth embodiment of the present invention.

FIG. 8 is a block diagram showing a schematic configuration of a conventional optical transmitter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Light intensity modulator, 3 ... Optical phase modulator, 4 ...
RF signal (f _p ), 5: transmission data signal, 11: semiconductor laser, 12: optical coupler, 13: optical intensity modulator, 14
... Reflection point, 15 ... Transmission data signal, 16 ... Transmission signal light,
17: bias current, 18: inductance, 19: capacitor.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) H04B 10/02 10/18

Claims (2)

[Claims] 1. A semiconductor laser having a high relaxation oscillation frequency such that signal transmission characteristics are not degraded even when a coherence collapse phenomenon due to reflected return light occurs, and a part of output light from the semiconductor laser is cohered by the semiconductor laser. An optical transmitter, comprising: a light re-injection unit for re-injecting light into the semiconductor laser at a level at which collapse occurs; and a light intensity modulation unit for intensity-modulating an output light of the semiconductor laser with a data signal to be transmitted. 2. A semiconductor laser having a terminal for inputting a current to be injected and having a relaxation oscillation frequency high enough to prevent signal transmission characteristics from deteriorating even if coherence collapse occurs due to reflected return light; Light re-injection means for re-injecting a part of output light from the semiconductor laser at a level at which the semiconductor laser causes coherence collapse, and injection current modulation means for modulating the injection current by a transmission data signal. Optical transmitter.