JPH0362638A - Optical transmitter - Google Patents

Abstract

PURPOSE:To change only a stimulated light frequency or phase almost without a change in the strength of the stimulated light with respect to a change in a minute injection current by using an electrode split type distribution feedback semiconductor laser and an electrode split type reflection semiconductor laser. CONSTITUTION:The transmitter is provided with an electrode split DFB-LD 41, an FM modulation drive circuit 42, a semiconductor laser optical amplifier 43 and a polarization maintaining optical fiber 44. A polarizer 45 is arranged so as to pass only a light having a polarization of 45 deg. with respect to the main axis of the fiber 44 as shown in an English caption B. The fiber 44 and the polarizer 45 form an FM-AM conversion optical circuit. In the FM-AM conversion optical circuit, the polarization plane of the incident light is set with an angle of 45 deg. with respect to the major axis of the fiber 44 and the incident optical power is separated into two major axes of the fiber 44. a mutual time delay tau with points of a distance L is expressed as tau=L/C(n1-n2) to receive different refractive indexes of n1, n2 in the input lights separated into two, where C is the velocity of light in vacuum. An electrode split DBR-LD may be used in place of the electrode split DFB-LD 41.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical transmitter used in optical communication, and in particular,
The present invention relates to a high-output optical transmitter required in ultra-high-speed optical transmission repeaters.

[Conventional technology]

Direct intensity modulation of semiconductor lasers is used in the transmitting portion of repeaters used in conventional optical transmission systems.

On the other hand, the relay interval in the optical relay transmission system is determined by the level difference between the transmission output and the light receiving sensitivity.

Therefore, in order to extend the relay interval, there are two methods: increasing the optical transmission output and increasing the light receiving sensitivity. In order to increase the optical transmission output, there is a method of amplifying the light emitted from a directly intensity-modulated semiconductor laser using an optical amplifying section. As the optical amplifying section, a semiconductor laser amplifier, a rare earth doped optical fiber laser amplifier, etc. can be considered. Semiconductor laser amplifiers have the advantage of being able to be integrated with a semiconductor laser, which is a light source, and rare earth-doped optical fiber laser amplifiers have the advantage of being able to be integrated with a semiconductor laser, which is a light source.
It has an advantage in that it can be integrated with fiber components.

[Problem to be solved by the invention]

It is conceivable to increase the optical transmission output of about 1 mW in a conventional optical repeater to 10 mW or more using a semiconductor laser amplifier. However, although the aforementioned optical amplifier has a wide gain bandwidth of several THz, the average output optical power is only 1 THz.
If it exceeds mW, the signal gain is likely to decrease due to gain saturation. At this time, in a semiconductor optical amplifier, the transient response time for gain saturation, that is, the time required for the gain to reach a steady state with respect to an input optical signal that changes stepwise, is several hundreds of seconds.
It is known that ps.

Therefore, the gain for the mark signal light after spaces are continuous is greater than the gain when marks are continuous. Therefore,
When the transmission signal has a bit rate of several hundred Mb/s or more, the gain for each optical pulse differs depending on the pattern of the input signal light, resulting in a so-called pattern effect, which significantly increases the bit error rate of the transmission system.

In addition, in optical fiber amplifiers doped with rare earth elements, the transient response time for gain saturation ranges from about 10 μm to several 100 μm.
Although the pattern effect is negligible due to the slow speed of s, a drift in the output power occurs due to fluctuations in the mark rate of the signal series.

For these reasons, using an optical amplifier, pattern effects,
It is desired to realize an optical transmitter that can obtain a high optical transmission output of 10 IIW or more without causing drift or the like.

The present invention has been made in view of the above, and its object is to provide a high-power optical transmitter with an optical transmission output of 10 mW or more that does not cause pattern effects or output power drift.

[Means to solve the problem]

According to the invention, the above-mentioned objects are further related to the means set out in the appended claims.

That is, the present invention includes a light emitting section that includes a single longitudinal mode semiconductor laser and can modulate the angle of an output light wave, and a light emitting section that modulates only the angle of the light wave without changing the intensity of the light emitted from the light emitting section by inputting an electrical signal. An optical transmitter comprising: an electric circuit that drives the light emitting section; an optical amplifying section that amplifies the angle-modulated light output from the light emitting section; and an angle-to-amplitude conversion means for the light wave output from the optical amplifying section. It is a vessel.

Figure $1 is a diagram showing an example of the basic configuration of the present invention.

The example shown in the figure includes a light emitting section 11 that includes a single longitudinal mode semiconductor laser 1O and is capable of frequency modulation, and a light emitting section 11 that receives an electrical signal and modulates only the optical frequency without changing the intensity of the light emitted from the light emitting section. An electric circuit 12 (
(indicated as an FM drive circuit in the figure) and an optical amplifier 13 that amplifies the FM modulated light output from the light emitting section.
Then, the FM modulated light output from the optical amplifier 13 is inputted and divided into two equal parts by the branch circuit 14, and one half is sent to the delay circuit 1.
The above configuration shows a case in which frequency modulation is performed by the optical frequency-to-amplitude converter 17 which is delayed from the other by the optical frequency-amplitude converter 17 which is delayed from the other by the optical wave circuit 16. good. This is because in either case, the angle of the optical carrier wave is modulated.

PM-AM conversion can also be performed using a pine interferometer as the PM-AM conversion circuit.

That is, a light emitting section including a single longitudinal mode semiconductor laser and capable of phase modulation, and an electric signal inputted to drive the light emitting section for modulating only the phase of the light without changing the intensity of the light emitted from the light emitting section. an optical amplification section that amplifies the phase modulated light output from the light emitting section; and an optical amplification section that amplifies the phase modulated light output from the light amplification section; and after inputting the phase modulation light output from the optical amplification section and dividing it into two equal parts, one side is compared with the other. It is comprised of an optical phase-amplitude converter that delays the signal and converts it into a gold wave again.

FIG. 2 is a diagram showing an example in which an optical amplifier and an optical frequency-amplitude converter are integrated by adding a rare earth element to the optical frequency-amplitude converter.

That is, a light emitting section 21 that includes a single longitudinal mode semiconductor laser 20 and is capable of frequency modulation, and an electric signal is input to drive the light emitting section for modulating only the optical frequency without changing the intensity of the light emitted from the light emitting section. A miss circuit 22 (indicated as an FM drive circuit in the figure) for the purpose of An element-doped polarization-maintaining optical fiber 23, an excitation light source 24 that generates light having a wavelength that excites rare earth elements doped in the optical fiber, and means for inputting excitation light generated from the excitation light source into the optical fiber. 25, and a polarizer 26 arranged so that only the polarized component at 45 degrees with respect to the main axis of the output light of the optical fiber passes through.

[For production]

As the single longitudinal mode semiconductor laser, an electrode segmented layer distributed feedback semiconductor laser (DFB-LD), an electrode segmented layer distributed reflection semiconductor laser (D, BR-LD), etc. can be used. In these LDs, the oscillation light intensity hardly changes with respect to a small change in the injection current, and only the oscillation light frequency or phase changes significantly.

Therefore, the drive circuit may be a conventionally used normal LD drive circuit. Since the optical amplifier receives light whose intensity is constant and only the phase or frequency of the light is modulated, even when the gain of the optical amplifier is saturated,
No pattern effects occur.

Furthermore, as an FM-AM conversion or PM-AM conversion optical circuit, it is possible to use a Matsuhatsu Enda type optical interferometer that divides the input light into two equal parts, gives an appropriate time delay τ to one side, and then waves the other wave again. can.

The two electric fields E + E 2 immediately before the golden wave are E
, = Eo/J”E--cos(ω (-τ)
) ...... (1) E 2 = E 0 / fγ
・cos (ωt) ...... (2) Therefore, the light intensity after the golden wave is ■ = E, 1E, l2-cos2(ωt/2'
) ・−・−・・(3). However, Eo is the electric field strength of the amplifier output light.

Therefore, since the transmittance of this interferometer changes depending on the frequency ω of the input light, it is possible to convert a change in the frequency of the input light into a change in the intensity of the output light.

This interferometer can also be realized using a polarization-maintaining optical fiber and a polarizer placed at its output end. In this case, the polarization direction of the output light of the optical amplifier is set to 4
If the input light is incident at 5 degrees, the input light is separated into two lights E and E2 having the same polarization direction as the principal axis. Since the electric field at the output end is expressed similarly to equations (1) and (2), the transmitted light intensity of a polarizer arranged so that the transmitted light polarization is 45 degrees to the principal axis is also expressed by equation (3). It will be similar. FIG. 3 shows a time chart of the amplitude of the input electrical signal, the amount of frequency shift of the frequency-modulated light emitted from the light emitting section, and the light intensity after frequency-amplitude conversion.

In the example shown in Figure 2, the polarization-maintaining optical fiber itself is improved by doping rare earth elements such as erbium and neodymium, which serve as the laser medium, into the polarization-maintaining optical fiber for FM-AM conversion and PM-AM conversion. It is used as an optical amplifier.

That is, the rare earth atoms excited by the excitation light having a shorter wavelength than the signal light form a population inversion at the level corresponding to the signal light wavelength, so that the input signal light is optically amplified by stimulated emission. Therefore, optical amplification and FM-AM actual conversion or PM
-AM real conversion is done simultaneously.

The difference with conventional technology is that instead of using an optical amplifier for intensity-modulated light, it uses frequency-modulated or phase-modulated light with a constant optical intensity, which is then converted to optical intensity. It is in.

Therefore, even if the optical amplifier is used in a high output region where gain saturation occurs, the input light intensity remains constant, making it possible to avoid gain fluctuations due to the input signal pattern, and to increase the power output without causing pattern effects. Output light can be obtained.

Furthermore, since it is sufficient to drive the LD of the light emitting part with a minute current with an amplitude of several mA, damping due to the relaxation of minority carriers in the active layer that occurs when driving with a large amplitude of the amplitude number + a + A in the conventional direct modulation method of light intensity of a semiconductor laser can be avoided,
The high frequency response characteristics of the LD can be improved.

Furthermore, since the drive amplitude is minute, the load on the drive electric circuit is small and it is advantageous for speeding up, which is a feature not found in conventional optical transmitters.

〔Example〕

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

FIG. 4 is a diagram showing an embodiment of the present invention.

In the same figure, 41 is an electrode split type I) FB-LD, 42
43 is an FM modulation drive circuit, 43 is a semiconductor laser optical amplifier (the output light of the amplifier is indicated by the letter A in the figure),
44 is a polarization-maintaining optical fiber, and 45 is a polarization-maintaining optical fiber with respect to the main axis of the polarization-maintaining optical fiber, as shown by the letter B in the figure.
This is a polarizer arranged so that only light having 5 degrees of polarization passes through. Polarization maintaining optical fiber 44 and polarizer 45
This constitutes an FM-AM conversion optical circuit. The electrode split type DFB-LD41 may be an electrode split type DBR-LD.

In the F M - A M conversion optical circuit, the plane of polarization of the incident light is set at 45 degrees with respect to the main axis of the polarization-maintaining optical fiber, and the power of the incident light is separated into the two main axes of the polarization-maintaining optical fiber. 2
The input light is separated into two parts, each with a different refractive index n,
The mutual time delay τ at a point far away to feel In2 is T=L/C(n, -12). However, C is the light intensity in vacuum.

The electric field E + E2 on the two principal axes at the distance is E, = E, / (“r − cos(ω (−τ
〉 )E2=E0/J″″′r◆cos (ωt), so the intensity of light transmitted through the polarizer placed at the fiber output end■
is 10E 212~cos2 (ω t/2).

however, E.

is the electric field strength of the amplifier output light degree.

Therefore, the clock frequency f. 10GHz.

If the frequency deviation is 20GHz, τ is 25ps
Therefore, L is determined to be approximately 75a+.

However, n and -n2 were set to 10-4. Electrode split type DF
Since the FM modulation efficiency of B-LD is usually about 56 Hz/mA, the drive current amplitude may be about 4a+A.

In a semiconductor laser amplifier whose end face reflectance is suppressed to 0.01% or less, the unsaturated gain is 25 dB, and the optical output is about 7 LIA, which is 3 dB lower than the unsaturated gain.
If the amplifier input power is 0.1 mW, the gain is 20 dB.
, an optical output of 10 mW can be obtained.

A phase modulation method can also be adopted with the same configuration.

Phase modulated light can be generated by direct modulation of a single longitudinal mode semiconductor laser, or by using an external modulator using an electro-optic effect. Conversion from phase modulation to intensity modulation light can be achieved by making light in a certain time slot interfere with light in the previous time slot in the same configuration as the FM-AM conversion circuit.

Therefore, if the clock frequency f0 is 10 GHz, the relative delay amount τ is 100 ps, and L is determined to be 300 m.

FIG. 5 is a diagram showing another embodiment of the present invention.

In the figure, the letter A indicates the input of the polarization-maintaining optical fiber, and the letter B indicates the output of the polarizer 54. In the case of this embodiment as well, the configuration is almost the same as that in Fig. 4, but the FM
- Since the polarization-maintaining optical fiber that performs AM conversion also functions as an optical amplifier, the semiconductor optical amplifier that was necessary in the case of FIG. 4 becomes unnecessary. Further, 51 is a split electrode type DFB-LD,
52 is an FM modulation drive circuit, 53 is a polarization-maintaining optical fiber doped with erbium, which is one of the rare earth elements, and 54 is a polarization-maintaining optical fiber having a polarization angle of 45 degrees with respect to the main axis of the polarization-maintaining light 7. A polarizer 55 arranged to pass only light is an excitation light source for exciting the erbium-doped optical fiber 53 to generate a gain for the signal light, and has an oscillation wavelength of 1.
A 48 μm optical output semiconductor laser can be used. The excitation light and the signal light are converted into gold waves by a gicroic mirror 56 and input into an erbium-doped polarization maintaining optical fiber for FM-AM conversion and optical amplification. Clock frequency l0G
When H2 and the frequency shift amount are 20 GHz, the length of the polarization-maintaining light 7-eyeper is determined to be approximately 75ω.

Since the excitation light output is about 100+eW, the concentration of erbium added is about 30 ppm.

At this time, the gain is 10 dB for the optical amplifier input optical power of 1 flW, so the output optical power is 10 mW.
is obtained.

〔Effect of the invention〕

As described above, according to the present invention, a high-output optical intensity signal can be obtained without causing pattern effects even when a semiconductor laser amplifier is used in the gain saturation region.

Therefore, the present invention can be expected to be applied to repeaters in future optical relay transmission systems.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of the basic configuration of the present invention, and FIG. 2 is a diagram showing an example in which an optical amplifier and an optical frequency-amplitude converter are integrated.
Figure 3 is a time chart showing the light intensity after frequency-amplitude conversion, Figure 4 is a diagram showing the configuration of one embodiment of the present invention, and Figure 5 is a diagram showing the configuration of another embodiment of the present invention. It is a diagram. 10.20...Single longitudinal mode semiconductor laser, 11 21...Light emitting section, 12
, 22 . . . Electric circuit for driving the light emitting section, 13 . . . Optical amplification section, 14 ・
... Branch circuit, 15 ... Delay circuit, 16 ... Gold wave circuit, 1
7... Optical frequency-amplitude conversion section, 23.
...Rare earth element-doped polarization preserving light 7 AINO (,24
...Excitation light source, 25 ...Input means, 26 45 54 ...
Polarizer, 41.51 ・・・・・・Electrode split type DFB
-LD. 42, 52...FM modulation drive circuit,
43... Semiconductor laser optical amplifier, 4
4...Polarization maintaining optical fiber, 53
...Erbium-doped polarization preserving light 7 AINO<, 5
5...Excitation LDjle, source, 56
・・・・・・Guicroic mirror

Claims (1)

[Claims] 1. A light emitting section that includes a single longitudinal mode semiconductor laser and is capable of modulating the angle of an output light wave, and an electric signal is input to the light emitting section to modulate only the angle of the light wave without changing the intensity of the emitted light. an electric circuit for driving the light emitting section, an optical amplifying section for amplifying the angle-modulated light output from the light emitting section, and an angle-to-amplitude conversion means for the light wave output from the optical amplifying section. An optical transmitter characterized by: 2. The optical angle-amplitude conversion means includes a polarization-maintaining optical fiber arranged so that the polarization plane of the input light is inputted at 45 degrees with respect to the principal axis, and a polarization-maintaining optical fiber arranged so that the polarization plane of the input light is inputted at 45 degrees with respect to the principal axis, and of the output light of the optical fiber.
2. The optical transmitter according to claim 1, comprising a polarizer arranged so as to pass only a 5 degree polarized wave component. 3. The optical transmitter according to claim 1, wherein the optical angle/amplitude converting means comprises means for dividing the input light into two, delaying one half by a specific amount, and converting the input light into two waves again. 4. The optical transmitter according to claim 2, wherein an optical fiber doped with a rare earth element is used as the polarization maintaining optical fiber.