JPS6281086A - Optical transmitter - Google Patents

Abstract

PURPOSE:To obtain an optical transmitter for optical fiber communication which can oscillate with a narrow spectrum even in rapid modulation and without causing deterioration in extinction ratio, by utilizing a saturatable absorber optically coupled with a semiconductor laser element. CONSTITUTION:A DFB laser 24 is biased to over an oscillation threshold value and modulated by an RZ code of 2 Gb/s so as to narrow the oscillation spectrum of the DFB laser 24. As a result, the extinction ratio of optical output 28 from a saturatable absorber 27 is improved to be 18:1 while the width of the divergent oscillation spectrum can be decreased up to 1Angstrom . This optical output 28 was transmitted on a 100km long single-mode fiber, and the code error rate characteristics of receiving sensitivity was determined. It was found that the deterioration in receiving sensitivity due to deterioration in extinction ratio and dispersion was as small as 0.5dB or less. Thus, the provision of the saturatable absorber 27 improves the extinction ratio of the modulated optical output of the DFB laser and decreases the width of oscillation spectrum, whereby light can be transmitted even on a long-distance single mode fiber without causing any deterioration in receiving sensitivity.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical transmitter used in an optical fiber communication system,
The present invention relates to an optical transmitter that operates with a high extinction ratio and a narrow spectrum width even during high-speed modulation.

[Conventional technology]

In recent years, in optical fiber communications, studies have been conducted on long-distance, large-capacity optical fiber communication systems that utilize the 1.5 μm wavelength band, which is the lowest loss band of single mode fibers.

By the way, in the wavelength band of 1.5 μm, a single mode fiber has a property in which the propagation speed varies depending on the wavelength, that is, chromatic dispersion. For this reason, long-distance, high-capacity optical fiber communication systems require a light source that oscillates in a single axis mode during high-speed modulation and has a small spectral width spread in order to eliminate the effects of wavelength dispersion of single-mode fibers. Currently, the above-mentioned light source that oscillates in a single-axis mode is a distributed feedback semiconductor laser (DFB laser) that has a diffraction grating inside the element and uses its wavelength selectivity to oscillate in a single-axis mode.
is being developed. The DFB laser is a semiconductor laser that oscillates in a single axis mode even during high-speed modulation, and the spread of the spectrum width during modulation is also relatively narrow.
single-mode fiber transmission has been realized (Fujita et al.
Study of high-speed long-distance optical fiber transmission using 1.5 μm band DFB-LD, 1985 National Conference of the Telecommunications Division of the Institute of Electronics and Communication Engineers (739).

In the above report example, the mark “1” and the space “0” are 2.
In order to reduce the spectral width broadening of the transmitted light that is rapidly modulated by the value, the DFB laser is operated with a bias above the oscillation threshold. As a result, the spectral width of the DFB laser during high-speed modulation is narrowed, and sensitivity deterioration due to wavelength dispersion after 60 km of single mode fiber transmission is suppressed to about 1 dB.

[Problem that the invention seeks to solve]

However, when the above-mentioned DFB laser is biased to a threshold value or higher to perform binary intensity modulation, the laser oscillates even in the space "0", so there is a problem in that sensitivity deteriorates due to a decrease in extinction ratio. Incidentally, in the example reported above, sensitivity deterioration of about 2 dB occurred due to a decrease in extinction ratio.

An object of the present invention is to provide an optical transmitter for optical fiber communication that oscillates with a narrow spectral width even during high-speed modulation and does not cause extinction ratio deterioration.

[Means for solving problems]

The optical transmitter of the present invention includes a semiconductor laser element as a transmission light source, a drive circuit for pulse modulating the semiconductor laser element, and a saturable absorber optically coupled to the semiconductor laser element, The saturable absorber improves the extinction ratio of pulse modulated light from the semiconductor laser element, and the optical output from the saturable absorber is used as transmitted light.

[Effect]

Next, the operation of the present invention will be described. As mentioned above, in order to narrow the oscillation spectrum of a high-speed modulated DFB laser, it is desirable to set the bias current of the DFB laser to a value higher than the oscillation threshold, but in this case, the extinction ratio of the modulated light deteriorates. . Therefore, in the present invention, a saturable absorber is used in order to improve the extinction ratio of modulated light of a DFB laser biased above a threshold value.

Fig. 6 shows the principle diagram. FIG. 6(a) shows the optical modulation output of a DFB laser biased above the threshold value and with a reduced extinction ratio. Figure 6(b) shows the predecessor output characteristics of the saturable absorber. Saturable absorbers have a constant light input
Above that, the light-absorbing type suddenly decreases, so its predecessor's output characteristic becomes as shown in FIG. 6(b). Now,
The optical modulation output shown in FIG. 6(a) is made incident on a saturable absorber having the characteristics shown in FIG. 6(b). At this time, the optical output from the saturable absorber becomes a waveform in which the optical output in the space "O" is small, as shown in FIG. 6(c).
Extinction ratio is improved. Furthermore, by using this saturable absorber, the oscillation spectrum width of a DFB laser that is modulated at high speed can be further narrowed.

FIG. 7(a) shows the oscillation spectrum of a DFB laser biased above the oscillation threshold and subjected to high-speed modulation with a binary random pattern. In this case, two laser oscillation peaks corresponding to the mark "1" and the space "0" of the modulation signal appear in the oscillation spectrum. Therefore, by using a saturable absorber to reduce the optical output in the space "0", the oscillation spectrum can be narrowed as shown in FIG. 7(b).

〔Example〕

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

FIG. 1 shows a configuration diagram of an optical transmitter according to a first embodiment of the present invention, which is composed of an integrated semiconductor laser element 32 and peripheral electric circuits.

The integrated semiconductor radar device 32 has an active layer 2 of InGaAsP with a wavelength composition of 1.55 μm on an n-1nP substrate 1 and an active layer 2 of InGaAsP with a wavelength composition of 1.3. After the light guide layer 3 of n-InGaAsP is epitaxially grown in order, a period of about 2
Forming a 400-meter diffraction grating 10, saturable absorption region 14
The part that becomes is configured as a flat state. Thereafter, a p-InP cladding layer 4 and a p"-In
Cap layers 5 of GaAsP are epitaxially grown in order, and a first . forming a second electrode 6.7, I
A third electrode 8 is formed under the nP substrate 1. A groove 9 deeper than the cap layer 5 is formed between the first and second electrodes 6, 7 to improve electrical isolation between the two electrodes.

Further, a peripheral circuit includes a drive electric circuit 1 for injecting a bias current 20 and a modulation current 21 into the laser region 13 described above.
1, and a control electric circuit 12 that injects a bias current 23 into the saturable absorption region 14 in order to control light absorption in the saturable absorption region 14.

The laser region 13 of this integrated semiconductor laser device 32 is
The diffraction grating 100 functions as a DFB laser that oscillates in a single axis mode by utilizing the wavelength selectivity. However, in this element, the resonator formed by the end face 17 of the laser region 13 and the end face 18 of the saturable absorption region 14 tends to cause hysteresis characteristics in the optical output.

In order to prevent this, in this embodiment, a non-reflection coating film 15 made of SiN and having a reflectance of 1% or less is formed on the end face 18 on the saturable absorption region 14 side.

Now, laser region 1 of this integrated semiconductor laser device 32
When a current is injected only into the drive circuit 12 through the drive electric circuit 12, single-axis mode oscillation occurs at a wavelength of around 1.55 μm. Further, the oscillation threshold current is 40 mA. The current vs. optical output characteristics of the integrated semiconductor laser device 32 at this time are shown in FIG.

Here, line (a) in FIG. 2 represents the end surface 17 of the laser region 13.
Line (C) in FIG. 2 is the second optical output 16 from the end face 18 of the saturable absorption region 14. The active layer 2 in which no current is injected in the saturable absorption region 14 acts as a saturable absorber whose light absorption decreases as the light input increases, so that the current versus light output of the second light output 16 decreases. A king occurs in the characteristics as shown by line (C) in FIG. On the other hand, when a current of 6 mA is injected into the saturable absorption region 14 via the control electric circuit 12, the amount of light absorbed by the saturable absorption region decreases, and the current versus first output characteristic of the second optical output 16 decreases. looks like the line (b> in Figure 2).Next, this integrated semiconductor laser device 32 is connected to a bit array) 2Gb/s RZ
The case of modulation using a code will be explained. In this case, the bias current to the laser region 13 is set to 50 mA, which is higher than the oscillation threshold, so that the oscillation spectrum of the first optical output 19 from the Rade region 13 is narrowed, and the injection current to the saturable absorption region is 6 mA. And so. FIG. 3 shows the oscillation spectra of the first optical output 19 and the second optical output 16 when modulated with a 2 Gb/s RZ code. The oscillation spectrum (line (a) in FIG. 3) of the first optical output 19 from the laser region 13 is a bimodal oscillation spectrum corresponding to the mark "1" and the space "0" of the modulation signal, and the oscillation The width of the spectrum was about 4 people. On the other hand, in the oscillation spectrum of the optical output 16 from the saturable absorption region 14 (line (b) in FIG. 3), the spectrum in the space "0" where the optical output is small is attenuated, so the base width of the oscillation spectrum is approximately The number has narrowed to 1.5 people.

Figure 4 shows the first signal when modulated with a 2Gb/s RZ code.
The optical modulation waveforms of the optical output 19 and the second optical output 16 are shown. Since the optical modulation waveform (line (a) in FIG. 4) of the first optical output 19 from the laser region 13 has a bias current equal to or higher than the oscillation threshold, the extinction ratio is in a poor state of 4:1. On the other hand, the extinction ratio of the second optical output 16 from the saturable absorption region 14 is improved to 15:1 because the optical output of the space "0" is attenuated.

Actually, the optical output 16 from the saturable absorption region 14 of this integrated semiconductor laser device 32 is inputted into a single mode fiber via an isolator and an objective lens, and the optical output is 2Gb/s.
As a result of conducting a 100 km single mode fiber transmission experiment with an RZ code of
It was about 5 dB or less. Also, the wavelength is 1.5. A single mode fiber with wavelength dispersion of 18 ps/km/nm at
Sensitivity degradation due to dispersion after 00 km transmission is 0.5d
I was able to make it as small as B.

FIG. 5 is a block diagram of an optical transmitter according to a second embodiment of the present invention. Here, a DFB laser 24 that oscillates in a single axis mode in the 1.55 μm band is used as a light source, and a saturable absorber 2
For No. 7, a multiple quantum well (MQW) structure made of InGaAS and InAl1AS was used. This saturable absorber 27 is made of InAl2AS with a thickness of 1 μm on an InP substrate.
form a layer, on top of which each rnc has a thickness of 80 people
It has a structure in which aAs layers and rnAlAs layers are grown alternately over 30 periods, and an absorption peak associated with excitons exists around a wavelength of 1.55 μm.

In FIG. 5, the optical output 3o from the DFB laser 24 is transmitted through an objective lens 25 and an isolake 26 for preventing return light.
The light is incident on the saturable absorber 27 via.

The optical output 28 from the saturable absorber 27 enters a single mode fiber 34 for transmission via an objective lens 33. Furthermore, the optical output 31 from the isolator 26 is incident perpendicularly to each layer of the saturable absorber 27. With the above configuration, the DFB laser 24 was biased above the oscillation threshold and modulated with a 2 Gb/s RZ code so that the oscillation spectrum of the DFB laser 24 was narrowed. As a result, the optical output 29, which is the modulated light from the DFB laser 24, had an extinction ratio of 5=1 and a base spread of the oscillation spectrum width of 4.

On the other hand, since the saturable absorber 27 absorbs the optical output corresponding to the space "0", the optical output 28 from the saturable absorber 27 has an improved extinction ratio of 18:1 and a narrower oscillation spectrum width. The spread has narrowed to just one person. As a result of transmitting this optical output 28 as transmission light through a 100 km single mode fiber and examining the reception sensitivity and bit error rate characteristics,
Sensitivity degradation due to extinction ratio degradation and dispersion is 0.5d each.
It was small, below B.

As mentioned above, by using a saturable absorber, it is possible to improve the extinction ratio of the optically modulated output of the DFB laser and reduce the oscillation spectrum width. Transmission light without deterioration can be obtained.

In the above embodiment, a DFB laser that oscillates in the 1.5 μm band is used as the light source, but the light source is not limited to this, and for example, a distributed feedback laser (DBR laser) that oscillates in a single axis mode may be used.
, Fabry-Perot type semiconductor laser, etc. may be used.

In addition, the oscillation wavelength of the light source is not limited to the 1.5 μm band, but the wavelength 1
．． A 3 μm band may also be used. In addition, although we used a saturable absorber made of a semiconductor, the material of the saturable absorber is not limited to semiconductors, but any material that exhibits the effect of saturable absorption at the oscillation wavelength of the laser can be used. But that's fine.

〔Effect of the invention〕

The optical transmitter according to the present invention oscillates with a narrow spectrum width even during high-speed modulation, and can obtain an optical modulation waveform with a high extinction ratio.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an optical transmitter according to a first embodiment of the present invention, FIG. 2 is a diagram showing current vs. optical output characteristics of this optical transmitter, and FIG. 3 is an oscillation diagram of the optical transmitter. 4 is a diagram showing the optical modulation waveform of the optical transmitter; FIG. 5 is a block diagram of the optical transmitter according to the second embodiment of the present invention; FIGS. FIG. 7 is a diagram for explaining the present invention in detail. 1 ・・・・・・・・・・・・・・・ n-1nP substrate 2 ・・・・・・・・・・・・ Active layer 3 ・・
・・・・・・・・・・・・ Light guide layer 4 ・・・・
・・・・・・・・・・・・ Clad layer 5 ・・・・・・
・・・・・・・・・ Cap layer 6.7.8 ・・・
Electrode 9・・・・・・・・・・・・ Groove 10 ・・・・・・・・・・・・ Diffraction grating 1
1 ・・・・・・・・・・・・・・・ Drive electric circuit 12 ・・・・・・・・・・・・・・・ Control electric circuit 13 ・・・・・・・・・・・・・・・ Laser region 14 ・・・・・・・・・・・・・・・ Saturable absorption region 15 ・・・・・・・・・・・・・・・
Anti-reflection coating film 16, 19.28.29.30.
31... Light output 17, 18...
End face 20 ・・・・・・・・・・・・・・・ Bias current 21 ・・・・・・・・・・・・・・・ Modulation current 23 ・・・・・・・・・・・・・・・ Control current 24 ・・・・・・・・・・・・・・・ DFB
Laser 25, 33... Objective lens 26... Isolator 27...・ Saturable absorber 32 ・・・・・・・・・・・・ Integrated semiconductor laser element 34 ・・・・・・・・・・・・
... Single mode fiber agent Yoshiyuki Iwasa, patent attorney Figure 2 Figure 3 Figure 4 (C)
(b) Sales power of 100 million seconds and 2000 books
Photo 6 of Kashoji Elementary School

Claims (1)

[Claims] (1) The saturable absorber includes a semiconductor laser device as a transmission light source, a drive circuit for pulse modulating the semiconductor laser device, and a saturable absorber optically coupled to the semiconductor laser device. An optical transmitting device characterized in that the extinction ratio of pulse modulated light from the semiconductor laser element is improved and the optical output from the saturable absorber is used as transmitted light.