JP3450573B2 - Semiconductor laser device, driving method thereof, and optical communication system using the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source device for optical communication for suppressing a dynamic wavelength fluctuation even at the time of high-speed modulation and stably realizing high-density wavelength-multiplexed optical communication, and an optical communication using the same. System, optical communication system, etc.

[0002]

2. Description of the Related Art In recent years, there has been a demand for expansion of transmission capacity in the field of optical communications, and development of optical frequency division multiplexing (optical FDM) transmission in which a plurality of wavelengths or optical frequencies are multiplexed in one optical fiber has been carried out. ing. In the optical FDM, it is important to reduce the wavelength interval in order to increase the transmission capacity as much as possible. For that purpose, it is desirable that the occupied frequency band or spectrum line width of the laser serving as the light source is small. However, it is pointed out that the direct intensity modulation method of the light source used for the current optical communication is not suitable for the optical FDM because the spectrum line width at the time of modulation spreads to about 0.3 nm. .

As a method for preventing the spectral line width of the light source from widening during modulation, there is an external modulation method, and Mach-Zehnder type modulators, semiconductor electroabsorption type modulators, etc. have been proposed and developed. In particular, a semiconductor electroabsorption modulator is easy to integrate with a semiconductor laser that is a light source, and therefore a modulator integrated laser has been developed because of its advantages such as small size and low loss. An example of the modulator integrated laser is shown in FIG. 9 (1
Proceedings of the 1992 IEICE Spring Conference C-20
3). A distributed feedback laser (DFB-LD) and an electro-absorption modulator (MD) using the quantum confined Stark effect (QCSE) with multiple quantum wells are integrated in series, and a voltage of several V is applied to obtain about 20 dB. The extinction ratio has been obtained. In FIG. 9, reference numeral 1001 denotes an n-side electrode, 1002
Is polyimide, 1003 is an n-InP substrate, 1004 is an InP buried layer, 1005 is a p electrode, and 1006 is a non-reflective coating.

[0004]

However, in the laser in which the modulator is integrated in series as in the conventional example, the wavelength fluctuation (chirping) at the time of modulation is about 0.03 nm, though smaller than that at the time of direct modulation. . This is because the amount of light returning to the active layer of the DFB laser changes when the modulator is driven when there is residual reflection on the exit end face on the modulator side. Therefore, it has been pointed out that it is necessary to suppress the residual reflectance to 0.1% or less by the antireflection coating 1006 on the exit end face of the modulator. In order to apply such a non-reflective coating, precise composition and thickness control of SiN film etc. are required, and productivity is not good.

Further, in the coupling portion of the laser and the modulator, since the waveguides having different compositions are coupled, the crystal growth technique for suppressing the coupling loss is required to have high precision. In particular,
When the modulator portion is regrown, a technique is required such that the active layer of the modulator and the active layer of the laser are accurately aligned with each other to suppress lateral growth on the wall surface (mesa portion) of the junction.

In addition, there is a propagation loss when passing through the modulator. That is, since the energy band gap of the active layer of the modulator is close to that of the active layer of the laser, there is absorption, and there is a loss even in the optical ON state (through state).
This loss and the extinction ratio of the modulator have a trade-off relationship, and there are restrictions in device design.

In view of these problems, the object of the present invention is to
By controlling the confinement of light in the optical waveguide and modulating the optical output, wavelength fluctuation during modulation is suppressed, and the optical output is high.
It is a highly productive semiconductor laser with integrated intensity modulator.
In order to improve the single mode property during intensity modulation, T
Providing a semiconductor laser having means for making gains of E mode and TM mode different (corresponding to claims 1 and 2) , and providing an optical communication system for performing high-density wavelength-division multiplexed optical transmission using the semiconductor laser as described above. That is (corresponding to claim 3) .

[0008]

Means for Solving the Problems] To achieve the above object semiconductor laser device of the present invention, at least an active layer for emitting optical waveguide including a portion <br/> extends in the resonator direction, optical It is configured to be able to control the distribution of refractive index at least some of the portion near the optical waveguide in the vicinity of the waveguide
The active layer is TE, which is the two guiding modes of the optical waveguide.
Mode and TM mode have different gains and T in the active layer
Differentiating the gain between E mode and TM mode
Heavy hole ground level and electron
Wavelength corresponding to the energy band gap between ground levels
In the vicinity of the active layer so that the Bragg wavelength comes to the longer wavelength side
This is done by setting the pitch of the diffraction grating provided.
The TE mode threshold gain at the Ragg wavelength is TM
It is a distributed feedback laser that is much lower than the
And are characterized. In addition, a half of the present invention for achieving the above object
Conductor laser devices have at least a portion of the active layer for light emission.
The optical waveguide that includes it extends in the direction of the resonator,
Refraction in the optical waveguide and the neighboring part that is near at least a part
Configured to control the distribution of rates
The layer has two waveguide modes of the optical waveguide, TE mode and
The gain of TM mode is different, and TE mode in the active layer
And the gain of TM mode is different to TE mode
Heavy hole and electron ground levels that give gain.
Longer wavelength than the wavelength corresponding to the energy band gap between
Installed in series in the cavity direction so that the Bragg wavelength is on the long side.
This is done by setting the pitch of the worn grating.
The TE mode threshold gain at the Ragg wavelength is TM
It is a distributed reflection laser that is much lower than the laser diode.

[0009]

[0010]

In order to achieve the above object, the optical communication system of the present invention is such that a plurality of the semiconductor laser device and the receiving device described above are connected to one optical fiber, and the light of a plurality of wavelengths is intensity-modulated. The wavelength division multiplexing transmission is performed so that only the signal on the light of the desired wavelength is extracted through the wavelength tunable optical bandpass filter in the receiving device.

[0012]

The principle of the present invention will be described using a specific example.
The present invention solves the above problem by integrating a structure equivalent to a modulator in parallel with a laser. A specific device structure according to the present invention will be described with reference to FIG. Similar to a typical buried heterostructure DFB laser, the buried portion has a portion adjacent to the active layer of the laser and having a higher energy bandgap (lower refractive index) than the active layer. When a reverse electric field is applied to this vicinity (confinement control layer), the energy band gap is low (refractive index is high).
And approaches the energy band gap of the laser active layer. Then, the lateral confinement of the laser oscillation light deteriorates and the oscillation threshold value rises. Therefore, the optical output of laser oscillation is attenuated. At this time, even if a single crystal (bulk) is used as the confinement control layer, the multiple quantum well (MQ
W) may be used. In the case of the bulk, an effect called the Franz-Keldysh effect (FK effect) is generated, and the fundamental absorption edge is shifted to the long wavelength side by applying a voltage, so that the refractive index is increased. In the case of MQW, the exciton peak shifts to the long wavelength side due to QCSE, which increases the refractive index. There is also a method in which a current is passed through the confinement control layer instead of a voltage. The current injection causes the plasma effect to reduce the refractive index. This improves light confinement, lowers the threshold and increases light output. The above principle is substantially the same even if the configuration is slightly modified from that of FIG.

According to the method described above, the reflectance at the laser end face does not change as in the conventional example, so that the mode is stable and the chirping can be suppressed. Further, since the coupling between the laser active layer and the confinement control layer is not in the traveling direction of light, the manufacturing precision as in the conventional example is not required. Further, the propagation loss is smaller than that of the conventional example, and the maximum optical power is improved.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1 A first embodiment according to the present invention will be described. FIG. 1 is a sectional view of a DFB laser according to the present invention (a section taken along a plane perpendicular to the cavity direction), in which the active layer has a bandgap wavelength of 1.56 μm.
m, the confinement control layer has a bandgap wavelength of 1.45μ
The SCH-MQW is such that m. The layer structure will be described in detail below. In FIG. 1, 101 is n-InP serving as a substrate, 102 is an n-InP buffer layer in which a diffraction grating having a depth of 0.05 μm is formed, and 103 has a thickness of 0.1.
n-I with band gap wavelength λg = 1.25 μm at μm
nGaAsP lower optical guide layer, 104 is a well layer i
-In _0.53 Ga _0.47 As (thickness 7 nm), i-InGaAsP (λg = 1.25 μm, thickness 10) which is a barrier layer
nm) active layer having a multi-quantum well structure consisting of 10 layers, 10
5 is p-InGaAsP (λg = 0.02 μm thick)
1.25 μm) upper optical guide layer, 106 is p-InP
A clad layer, 107 is a p-In _0.53 Ga _0.47 As contact layer, 108 is a p-InP buried layer, 109 is a 0.2 μm thick p-InGaAsP (λg = 1.15 μm) layer.
m), i-In _0.53 Ga _0.47 As well layer (thickness 4 nm)
And i-InGaAsP (λg = 1.15 μm, thickness 10
nm) 10 barrier layers and 0.2 μm thick n-In
SCH-M made of GaAsP (λg = 1.15 μm)
QW confinement control layer, 110 is n-InP buried layer,
111/112 are Cr / AuZnNi which are p-side electrodes
/ Au layer, 113 and 114 are AuGe which are n-side electrodes
The Ni / Au layer and 115 are insulating films. The electrode in the p-InP clad layer 106 is formed so as to easily obtain ohmic contact.
The carrier concentration was partially increased by Zn diffusion.

[0016] Here, in this DFB laser has become a multi-quantum well layer of lattice-matched systems, because the transition energy E _hh0 -E _e0 is smaller than the transition energy of E _LH0 -E _e0, the gain of the TE mode TM It is bigger than the mode. Also, the distributed feedback wavelength by the diffraction grating, that is, D
The pitch of the diffraction grating was set so that the oscillation wavelength of the FB laser was slightly longer than the wavelength of 1.56 μm corresponding to the transition energy (E _hh0 -E _e0 ) between the ground level of heavy holes and electrons. . As a result, there was almost no gain in the TM mode, and oscillation occurred only in the TE mode.

In the above structure, the electrode 111 and the electrode 1
When a forward bias current is passed between 14
It oscillates at 0 mA. Here, when a reverse electric field is applied between the electrodes 112 and 113, the refractive index of the confinement control layer 109 is increased, the confinement in the active layer 104 is deteriorated, the threshold value is increased, and the optical output is decreased. . FIG. 2 shows the relationship between the applied voltage of the reverse electric field and the optical output when the bias current to the laser is 40 mA. It can be seen that the optical output is attenuated by about 20 dB when 5 V is applied. It is efficient to apply the electric field to both the left and right sides, but only one may be applied. The electrodes 111 and 113 may be short-circuited.

Therefore, when a digital signal with an amplitude of 5 V is applied between the electrode 112 and the electrode 113, intensity modulation with an extinction ratio of about 20 dB can be performed. At this time, the modulation band is DC to 3 GHz
Met. The dynamic fluctuation of the oscillation wavelength, that is, the chirping was about 0.02 nm. However, the end surface treatment of the element at this time is a non-reflective coating with a reflectance of about 1%, and it can be seen that the manufacturing accuracy is not required as compared with the conventional example.

In this embodiment, the unstrained MQW is used as the active layer 104 of the laser, but the bandgap wavelength is 1.5.
InGaAsP single layer of 5 μm, compressive strain MQW (for example, In _0.7 Ga _0.3 As (thickness 3 nm) / InGaAs)
P) may be used to further increase the gain difference between the TE mode and the TM mode. It is also possible to use the tensile strain MQW to increase the gain of the TM mode and perform intensity modulation of the TM mode oscillation. Confinement control layer 1
09 has an MQW structure, but has a bandgap wavelength of 1.4
The FK effect may be used as a 5 μm InGaAsP single layer. In FIG. 1, the confinement control layer 109 and the active layer 1
04 are almost the same height, but there are some deviations,
The confinement control layer may be curved. Of course, it is also useful to introduce a λ / 4 shift structure into the diffraction grating or to improve the single mode property by applying an antireflection coating to the end face.

According to the present embodiment, the reflectance at the laser end face does not change as in the prior art, so that the mode is stable and the chirping can be suppressed as described above. Further, since the coupling between the laser active layer and the active layer (confinement control layer) of the modulator is not in the traveling direction of light, the manufacturing precision as in the conventional example is not required. Further, the propagation loss is smaller than that of the conventional example, and the maximum optical power is improved.

Second Embodiment A second embodiment according to the present invention is a laser having substantially the same structure as that of the first embodiment (the composition of the layer is slightly different), and a change in the refractive index of the confinement control layer in the buried hetero structure is caused by current injection. It is something to do. The difference from Example 1 is that the confinement control layer is a layer having a bandgap wavelength of 1.48 μm. As a result, when no current is injected into the confinement control layer, the difference in band gap between the active layer and the confinement control layer is small, and the effect of optical confinement is small, so that oscillation can be suppressed. In this layer structure, a bias current (near the threshold current) that does not oscillate when no current is injected into the confinement control layer is injected into the laser active layer.
When the current is injected into the confinement control layer, the refractive index is reduced by the plasma effect, the lateral confinement is improved, the threshold gain is reduced, and laser oscillation occurs at about 40 mA. This allows intensity modulation.

Compared with the first embodiment, the DC current constantly injected into the laser active layer is smaller than the threshold value, so that the power consumption can be reduced.

Embodiment 3 In the third embodiment according to the present invention, the laser of Embodiment 1 has multiple electrodes in the cavity direction. FIG. 3 shows a sectional perspective view of a three-electrode type laser according to this embodiment (showing the right half of the device). A λ / 4 shift 407 is provided in the center of the electrode 402, and both end faces are antireflection coated.
The electrode length is (401, 404), (402, 405),
(403, 406) 300μm, 100μ respectively
m and 300 μm. The oscillation wavelength can be changed by changing the ratio of each current. In this element, the electrode 4
As the current I1 of 01, the current I2 of electrode 402, and the current I3 of electrode 403, the oscillation wavelength can be continuously changed by about 2 nm by changing the value of I2 / (I1 + I2 + I3) from 0.1 to 0.5. it can. This allows the device to be used as a light source for wavelength division multiplexing transmission.
In FIG. 3, the same reference numerals as those in FIG. 1 indicate the same functional units.

The intensity modulation drive can be performed by applying a voltage signal having an amplitude of 8 V only to the electrode 405.
The voltage amplitude is larger than that in the first embodiment because the region where confinement control is performed is short only in the central portion. However, since the parasitic capacitance is reduced as compared with the first embodiment, the modulation band is extended and the modulation up to 10 GHz becomes possible.

Embodiment 4 The fourth embodiment according to the present invention is applied to a distributed Bragg reflector (DBR) laser. FIG. 4 is a DBR according to this embodiment.
A cross-sectional view parallel to the resonator of the laser, and a cross section perpendicular to the cross-section, that is, a buried heterostructure is almost the same as that of the first embodiment. The electrodes 511, 512, and 513 (only the laser injection electrode is shown) are divided into three, which are the laser injection electrode and the electric field application electrode, like the third embodiment, and correspond to the active region, the phase adjustment region, and the DBR region, respectively. . When an electric field is applied to the buried layer in the DBR region to weaken the lateral confinement, the amount of light distributed and reflected in the active region decreases, the threshold value increases, and the light output attenuates.

In this device, the layer structure of the active region having the active layer 515 is almost the same as that of the first embodiment. In the DBR region, the InP substrate 516 is etched and then the diffraction grating is formed, and InGaA having a band gap wavelength of 1.4 μm is formed.
sP light guide layer 501, p-InP clad layer 502,
The p-InGaAs contact layer 503 is regrown. Correspondingly, unlike the first embodiment, the confinement control layer is an MQW having a bandgap wavelength of 1.3 μm.

In the present device, the injection currents in the DBR region and the phase adjustment region (between the electrodes 513 and 514 and between the electrodes 512).
And the electrode 514) change the oscillation wavelength to about 3 nm.
Can be changed. Therefore, it can be used as a light source for wavelength division multiplexing transmission as in the third embodiment.

Fifth Embodiment A fifth embodiment according to the present invention is the same DBR as the fourth embodiment.
Although it is a laser, it changes the bandgap wavelength of the optical waveguide itself in the DBR region by an electric field. Figure 5
FIG. 4B is a cross-sectional view of the DBR laser according to the present embodiment in the cavity direction. An optical guide layer 601 having a bandgap wavelength of 1.25 μm and an InGaAs / InGaAs having a bandgap wavelength of 1.45 μm are formed on a diffraction grating formed on a substrate 616.
MQW layer of P10 layer (confinement control layer 109 of Example 1)
602, InP clad layer 603, InGaA
The s contact layer 604 has been regrown. Unlike the first to fourth embodiments, the lateral burying structure is composed only of high-resistance InP. The layer structure of the active region having the active layer 615 is substantially the same as that of the first embodiment.

In this device, current injection into the active region (electrode 611
And the electrode 614) to obtain DBR oscillation, but when an electric field is applied to the DBR region (between the electrode 613 and the electrode 614), confinement deteriorates and the amount of light distributed and reflected in the active region decreases, and the threshold value becomes It rises and the light output is attenuated.

As described above, in this device, since the intensity is modulated by the electric field in the DBR region, the wavelength variable operation is not performed. Further, regarding the phase adjustment region, the electrode 612 is used in the initial setting such that the DBR oscillation becomes stable, instead of adjusting when the wavelength is variable. Alternatively, it can be used to prevent current from flowing from the active region when applying a reverse electric field to the DBR region.

Sixth Embodiment Although the first to fifth embodiments have shown the embodiments of the dynamic single mode laser having the diffraction grating, the idea of the present invention is that the Fabry-Perot laser without the diffraction grating and cleaved at both end surfaces. Can also be applied to. The structure is almost the same as in FIG.
03 was thinned to 0.02 μm, and no diffraction grating was produced. Although the single mode property is not good, intensity modulation can be performed as in the first embodiment. The present embodiment can be applied to simple optical communication that does not require wavelength multiplexing, spatial propagation optical communication, and the like.

Embodiment 7 FIG. 6 shows a sectional view of a DFB laser which is a seventh embodiment of the present invention. The structure is almost the same as that of the first embodiment. In the present embodiment, in order to improve the electrical insulation between the active layer 104 and the confinement control layer 109, 0.3 μm after the mesa etching.
The high resistance InP layer 901 having a thickness of m is grown on the bottom surface and the side surface, and then the buried growth is performed as in the first embodiment.
For the growth of InP, it is sufficient to use that the growth rate of the side surface and the bottom surface of the mesa become substantially equal by growing by the low pressure MOCVD method when the waveguide direction of the light in the waveguide is the reverse mesa direction.

In this embodiment, since the leakage current can be reduced, the injection efficiency of the laser into the active layer 104 is improved, the threshold current can be reduced, and the optical output power can be increased.

Embodiment 8 This embodiment is an example in which the intensity-modulated optical transmission using the semiconductor laser according to the present invention is applied to a wavelength division multiplexing optical LAN system. . FIG. 7 shows a configuration example of an optical-electrical conversion unit (node) connected to each terminal in this case, and FIG. 8 shows a configuration example of an optical LAN system using the node.

An optical signal is taken into a node by using an optical fiber 701 connected to each unit as a medium, and a branching unit 702 partially receives the wavelength tunable filter 70.
It is incident on 3. A fiber Fabry-Perot filter is used as the wavelength tunable filter, but a Mach-Zehnder filter, an interference film filter, or the like may be used instead. The receiving device 703 extracts only an optical signal having a desired wavelength and performs signal detection. On the other hand, when the optical signal is transmitted from the node, the wavelength tunable DFB laser of the third embodiment or the wavelength tunable DBR laser 704 of the fourth embodiment is intensity-modulated and is incident on the optical transmission line via the branching unit 707. At this time, in order to avoid the influence of the returning light to the laser, the isolator 7
You may enter 06.

Depending on the terminal, it may be necessary to transmit only a single wavelength, and in that case, the DFB laser of the first embodiment or the DBR laser of the fifth embodiment is used. On the contrary, when it is necessary to further widen the wavelength tunable range, a plurality of wavelength tunable lasers may be provided.

As a network of the optical LAN system,
What is shown in FIG. 8 is a bus type, and nodes can be connected in the directions A and B, and a large number of networked terminals and centers can be installed. However, in order to connect a large number of nodes, it is necessary to install an optical amplifier on the transmission line to compensate for the attenuation of light. Also, by connecting two nodes to each terminal and using two transmission lines, DQDB
It enables bidirectional optical transmission by the method.

In the intensity modulation according to the present invention, since the wavelength variation at the time of modulation is about 0.02 nm as described in the first embodiment, when the wavelength variable width is 2 nm, 2 / 0.02 = 100.
An optical network system can be constructed by high-density wavelength division multiplexing transmission of channels. Further, the network may have a loop type in which A and B in FIG. 8 are connected, a star type, or a combination thereof.

[0039]

According to the structure of the present invention as described above, the following effects can be obtained.

The characteristics are not greatly influenced by the deviation from the design value in the laser fabrication, the wavelength fluctuation at the time of modulation is suppressed, and the semiconductor laser of the intensity modulator integrated type having a high optical output can be stably manufactured with a high yield. , When intensity modulation with semiconductor laser
TE mode and TM to improve the single mode property of
The mode gains can be different (claims 1 and 2).
Corresponding to). With the semiconductor laser of the present invention, low cost and multiplicity
High wavelength division multiplexing optical transmission can be realized (corresponding to claim 3) .

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a DFB laser (Example 1) according to the present invention.

FIG. 2 is a diagram showing the relationship between light output and applied voltage.

FIG. 3 is a three-electrode DFB laser according to the present invention (Example 3).
FIG.

FIG. 4 is a vertical sectional view of a wavelength tunable DBR laser (Example 4) according to the present invention.

FIG. 5 is a vertical sectional view of a DBR laser (Example 5) according to the present invention.

FIG. 6 is a cross-sectional view of a DFB laser (Example 7) according to the present invention.

FIG. 7 is a diagram showing a configuration example of an optical-electrical conversion unit used in an optical LAN system using a laser according to the present invention.

FIG. 8 is a diagram illustrating a network of an optical LAN system.

FIG. 9 is a view showing a conventional example of an intensity modulator integrated semiconductor laser.

[Explanation of symbols]

101, 516, 616 Substrate 102 Buffer layer 103, 105, 501, 601 Light guide layer 104, 515, 615 Active layer 106, 502, 603 Cladding layer 107, 503, 604 Contact layer 108, 110 Buried layer 109, 602 MQW layer 111, 112, 113, 114, 401, 402, 4
03,404,405,406,511,512,51
3, 514, 611, 612, 613, 614 Electrode 115 Insulating film 407 λ / 4 shift part 701 Optical fiber 702, 707 Optical branching device 703 Wavelength selective receiver 704 Semiconductor laser 706 of the present invention Optical isolator 901 High resistance InP insulating layer 1001 n-side electrode 1002 polyimide 1003 n-InP substrate 1004 InP burying layer 1005 p electrode 1006 anti-reflection coating

Front page continued (51) Int.Cl. ⁷ identification code FI H04J 14/00 H04B 9/00 E 14/02 // H01L 27/15 (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (3)

(57) [Claims] 1. An optical waveguide including an active layer for light emission in at least a part thereof extends in a resonator direction, and a refractive index distribution in the vicinity part in the vicinity of at least a part of the optical waveguide and the optical waveguide. The active layer is configured so that the gains of the two waveguide modes of the optical waveguide, that is, the TE mode and the TM mode, are different, and the TE layer and the TM mode have different gains in the active layer. Of the diffraction grating provided in the vicinity of the active layer so that the Bragg wavelength is on the longer wavelength side than the wavelength corresponding to the energy band gap between the heavy hole ground level and the electron ground level that gives a gain to the TE mode. A semiconductor laser characterized by being a distributed feedback laser which is made by setting a pitch and has a threshold gain of TE mode at Bragg wavelength much lower than that of TM mode. The device. 2. An optical waveguide including an active layer for light emission in at least a part thereof extends in the resonator direction, and a refractive index distribution in the vicinity part in the vicinity of at least a part of the optical waveguide and the optical waveguide. The active layer is configured so that the gains of the two waveguide modes of the optical waveguide, that is, the TE mode and the TM mode, are different, and the TE layer and the TM mode have different gains in the active layer. However, the diffraction provided in series in the cavity direction so that the Bragg wavelength is on the longer wavelength side than the wavelength corresponding to the energy band gap between the heavy hole ground level and the electron ground level that gives a gain to the TE mode. This is done by setting the pitch of the grid,
The TE mode threshold gain at the Bragg wavelength is TM
A semiconductor laser device characterized by being a distributed Bragg reflector laser whose mode is much lower than the mode. 3. A plurality of the semiconductor laser device and the receiving device according to claim 1 or 2 are connected to one optical fiber, and light of a plurality of wavelengths is intensity-modulated and transmitted, and a wavelength tunable light is received in the receiving device. An optical communication system characterized by performing wavelength division multiplex transmission so that only a signal on which light of a desired wavelength is carried out is taken out through a bandpass filter.