JPH02257109A - Optical demultiplexer - Google Patents

Abstract

PURPOSE:To reduce the size of the optical demultiplexer and to enhance the stability thereof and to improve inter-channel crosstalks and the light wave band width per wave by providing a light wave filtering means from which the signal light increased in the wavelength intervals between plural light waves is emitted and a diffraction grating by which the emitted signal light is demultiplexed by each of the different wavelengths. CONSTITUTION:The 1st signal light emitted from the optical fiber 1 is converted to collimated beams of light by a 1st optical system, for example, a collimating lens 2 and is branched in 2 directions by an optical branching means, for example, a beam splitter 3. One of the branched signal light is made incident to, for example, a Fabry-Perot resonator 5. The 1st signal light transmits this Fabry-Perot resonator 5, by which the mutual wavelength intervals of the light waves multiplexed on this signal light are increased. Since the signal light is demultiplexed to each of the different wavelengths by the diffraction grating in this way and, therefore, the size of the demultiplexer is reduced and the stability thereof is enhanced. In addition, the inter-channel crosstalks and the light wave band width per wave are improved.

Description

Detailed Description of the Invention [Object of the Invention] (Field of Industrial Application) The present invention relates to an optical demultiplexer used in an optical communication system.

(Prior art) In optical communication systems, optical multiplex transmission in which multiple light waves are propagated through optical transmission lines is often performed. To do this, an optical demultiplexer is generally used.

FIG. 7 is a diagram showing the configuration of a conventional optical demultiplexer. (1
) is an optical fiber that transmits signal light in which multiple light waves of different wavelengths are multiplexed.

The signal light emitted from the optical fiber (1) is converted into parallel light by a collimating lens (102), and then passed through a diffraction grating (10
3). Here, the signal light in which light waves are multiplexed is demultiplexed at different diffraction angles depending on its wavelength. The demultiplexed signal light is focused by a condensing lens (104) and received by a light receiving element (105).

By the way, there is a relationship as shown in the following equation (2) between the light receiving position interval in the light receiving element (105) and the wavelength interval of the light wave.

ΔX ratio fΔλ/S ・・・・・・■ Here, ΔX is the light receiving position interval, Δλ is the wavelength interval of the light wave,
f is the focal length of the lens, and S is the pitch of the diffraction grating.

■As can be seen from the equation, in order to demultiplex a signal light with a small Δλ without reducing ΔX, either the pitch S of the diffraction grating should be reduced, or the focal length f of the lens should be increased. It is.

However, in the case of a blazed diffraction grating with a wavelength of 1.5 C band and high efficiency, S reaches its limit at around 1/600 mn+, so in an optical demultiplexer that demultiplexes a small Δλ, the only option is to lengthen the focal length of the lens. Therefore, the device becomes large. For example, Δλ-0.5A, Δx=20IJM,
Substituting s = 1/600mm into formula ■, f = 66
cm.

In this way, in an optical demultiplexer that demultiplexes small Δλ, the focal length of the lens must be made long, which results in an increase in the size of the device. Such large-sized devices also pose problems in terms of stability against temperature changes.

Furthermore, when signal lights with small wavelength intervals are demultiplexed, there is a drawback that crosstalk between channels deteriorates, and the bandwidth of each light wave also decreases.

(Problems to be Solved by the Invention) As mentioned above, in conventional optical demultiplexers, the device is large in size in order to demultiplex signal lights with small wavelength intervals, and as a result, stability against temperature changes is affected. In addition, there were drawbacks such as deterioration of crosstalk between channels and a reduction in the bandwidth of each light wave.

Therefore, the present invention was made to eliminate these drawbacks.In order to achieve the above objects, the present invention is compact and highly stable, and improves inter-channel crosstalk and optical wave bandwidth per wave. An optical demultiplexer includes an optical system that emits signal light in which multiple light waves with different wavelengths are multiplexed;
a light wave filtering means from which a signal light in which the mutual wavelength spacing of a plurality of light waves multiplexed on the signal light is increased is emitted; It consists of a diffraction grating.

(Operation) Signal light, in which a plurality of light waves with different wavelengths are multiplexed, is input to the light wave filtering means. The incident signal light passes through this light wave filtering means, thereby becoming a signal light in which the mutual wavelength spacing of a plurality of multiplexed light waves is increased, and inter-channel crosstalk is degraded by the diffraction grating. Demultiplexing is performed without any interference.

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

FIG. 1 is a diagram showing a first embodiment of an optical demultiplexer according to the present invention. (1) is an optical fiber, and the signal light transmitted through this optical fiber (1) is multiplexed with a plurality of light waves having different wavelengths. For example, in this signal light, N light waves having different wavelengths from #1 to #N are multiplexed at equal intervals as shown in FIG. Let the wavelength interval between these adjacent light waves be Δλ.

The first signal light, which is the signal light emitted from the optical fiber (1), is sent to a first optical system, for example, a collimating lens (2).
The light is converted into parallel light and split into two directions by a light branching means, for example a beam splitter (3). One of the branched signal lights is input to a light wave filtering means, for example, a Fabry-Perot resonator (5).

By transmitting the first signal light through this Fabry-Perot resonator (5), the mutual wavelength spacing of the light waves multiplexed with this signal light is increased.

For example, here, the resonant frequency of the Fabry-Perot resonator (5) is adjusted so that it has a characteristic of transmitting even-numbered light waves among the N light waves shown in FIG.

The signal light transmitted through the Fabry-Perot resonator (5) is incident on the diffraction grating (6), where the signal light is demultiplexed into wavelengths. The demultiplexed signal light is focused by a condensing lens (7) onto a light receiving element (8) on the focal plane.

The other signal light branched by the beam splitter (3) has its traveling direction changed by a mirror (4), and enters the light wave filtering means 1, for example, a Fabry-Perot resonator (9). This Fabry-Perot resonator (9) is similar to the Fabry-Perot resonator (5), and is slightly tilted to adjust the resonant frequency so that, for example, odd-numbered light waves in FIG. 2 are transmitted. do.

The signal light transmitted through the Fabry-Perot resonator (9) is demultiplexed by the diffraction grating (10), and the demultiplexed signal light is focused by the condenser lens (11) onto the light receiving element (12) on the focal plane. be done.

Here, for example, a photodetector array or the like is used as the light receiving elements (8) and (12). Alternatively, a semiconductor laser array, an optical waveguide array, etc. may be used instead of the light receiving element. When a semiconductor laser array is used, the device shown in FIG. 1 can also be used as an optical multiplexer.

When configured as above, a Fabry-Perot resonator (
5), (9) and diffraction gratings (6), (
1,0)), the wavelength interval of the light waves multiplexed to the signal light incident on the input signal light can be set to 2Δλ, so the above equation
Compared to an optical demultiplexer that separates Δλ from light, the focal length of the optical lens can be made smaller.

FIG. 3 is a diagram showing the configuration of a second embodiment according to the present invention.

The first signal light emitted from the optical fiber (1) is converted into parallel light by the collimating lens (2), and then polarized beam splitter (21), (2B), 45° Faraday rotator (23), , /2 wavelength plate (24), and mirrors (22) (25) , (27) , (28)
The light is incident on the first light circulator (2).

The first signal light emitted from the collimating lens (2) first enters the polarizing beam splitter (21).

This polarizing beam splitter (21) has a characteristic of transmitting signal light whose polarization direction is parallel to the plane of the paper and reflecting signal light whose polarization direction is perpendicular to the plane of the paper. The signal light (20a) transmitted through the polarizing beam splitter (21) is rotated by a 45° Faraday rotator (
23), and the polarization direction is rotated by 45°. When this signal light is incident on the 1/2 wavelength plate (24), the polarization direction is rotated by 45° in the opposite direction to the direction rotated by the 45° Farade single rotator (23). Therefore, 1/2
The polarization direction of the signal light (20a) after passing through the wave plate (24) is the same as before entering the 45° Farade single rotator (23).

Signal light (2I) reflected by polarizing beam splitter (2I)
0b) also has a 45° Hualade single rotor (23), 1/
A similar polarization plane conversion is performed by a two-wave plate (24).

Both signal lights are combined by a polarizing beam splitter (26), output from a boat (30), and input into a Fabry-Perot resonator (5). Here, the resonant frequency of this Fabry-Perot resonator (5) is set to, for example, #1, #4, etc. of N light waves multiplexed at equal intervals shown in FIG. #7.・・・
・Adjust so that it has a characteristic of transmitting light waves of −2 (1 (=constant)) to 3.

The signal light transmitted through the Fabry-Perot resonator (5) enters the diffraction grating (6) and is demultiplexed into wavelengths.

The demultiplexed signal light is focused onto a light receiving element (8) by a focusing lens (7).

On the other hand, the signal light reflected from the entrance surface of the Fabry-Perot resonator (5) is split into two signal lights by the polarizing beam splitter (26) from the boat (30) again.

Each signal light is incident on a 1/2 wavelength plate (24) and a 45° Faraday rotator (23). With a 1/2 wavelength plate (24),
The polarization direction of the signal light is rotated by 456 degrees, and further rotated by 45 degrees in the same direction by a 45 degree Farade rotator (23). Therefore, the polarization directions of the signal lights (20a) and (20b) transmitted through the 45° Farade single rotator (23) are perpendicular to the page and parallel to the page. Each signal light is combined by a polarizing beam splitter (2I), its traveling direction is changed by a mirror (27), and the signal light is input to a Fabry-Perot resonator (9) through a port (33).

The Fabry-Perot resonator (9) has the same characteristics as in the first embodiment and transmits light waves of non-adjacent wavelengths in FIG. The signal light transmitted through the Fabry-Perot resonator (9) is multiplexed and the wavelength interval between the multiplexed light waves is increased, and the signal light is incident on the diffraction grating (10). Here, this signal light is split into wavelengths and is focused onto a light receiving element (12) by a condensing lens (11).

The signal light reflected by the entrance surface of the Fabry-Perot resonator (9) enters the polarizing beam splitter (2) from the port 1-(33) again and is split into two directions.

The split signal light is sent to a polarization beam splitter (21).

After passing through a 45° Farade single rotator (23), a 1/2 wavelength plate (24), a polarizing beam splitter (2B), and a mirror (28), it is emitted from a bow (34). This signal light is split into wavelengths by a diffraction grating (35),
The light is focused onto a light receiving element (37) by a focusing lens (36).

In the case of the above configuration, the signal light emitted from the optical fiber (1) can be divided into three parts and demultiplexed by the diffraction grating, so the wavelength interval of the multiplexed light waves can be further increased. I can do it.

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

(60) is a semiconductor laser that generates coherent light. This coherent light is converted into parallel light by a second optical system, such as a collimating lens (6I), and then enters a beam splitter (3). On the other hand, optical fiber (
The signal light emitted from 1) is converted into parallel light by a collimating lens (2), and then enters a beam splitter (3), where it is mixed with the coherent light.

The mixed signal light is sent to a polarizing beam splitter (21),
(51,), mirrors (22), (52), and a 45° Faraday rotator (23).

The signal light first enters the polarizing beam splitter (2I) and is split into two signals.

Each signal light is incident on a 45° Faraday rotator (23), where the plane of polarization of the light wave is rotated by 45°.

Each signal light is input to a Fabry-Perot resonator (5).

This Fabry-Perot resonator (5) has the same characteristics as in the first embodiment, and the two signal lights transmitted through it are
The mutual wavelength spacing of the multiplexed light waves is increased, and the light waves are separated into wavelengths by a diffraction grating (6). The two branched signal lights are condensed by condensing lenses (43a) and (48b) and are incident on light receiving elements (44a) and (44J+).

The two signal lights reflected by the entrance plane of the Fabry-Perot resonator (5) are polarized again by the 45° Farade rotator (23), and then transferred to the polarizing beam splitter (21, , 2).
, a beam splitter (51), and a mirror (52) before entering the diffraction grating (10).

Demultiplexing is performed here, and each signal light is sent to a condenser lens (47
a) , (47b), and is focused on the element (48a),
(49b).

In the case of the above configuration, especially the semiconductor laser (60
) plays the role of a local oscillation signal, and local oscillation gain is obtained. Furthermore, since it has a polarization diversity configuration, it is possible to cope with polarization fluctuations due to the optical fiber (1).

Furthermore, one half-wave plate can be omitted, reducing costs.

FIG. 5 shows a fourth embodiment of the present invention. In this example, the first
Fabry-Perot resonator (5) in the figure.

Instead of (9), beam splitter ([12),
A Matsuhatsu Ender interferometer consisting of (ee), mirrors (83), and (84) is used. Since the Matsuhatsu Ender interferometer can also periodically demultiplex wavelengths, it can be used in the same way as the Fabry-Perot resonator. However, it differs from the Fabry-Bello resonator in that, as shown in Figure 6, the transmission intensity characteristics of the light that passes through the Masso-Hunninder interferometer are sinusoidal, and the Matsoh-Hunninder interferometer has two output ports. Yes, the first port (67)
The signal light of the wavelength canceled by is emitted from the second port (68), and the signal light of the wavelength canceled by the second port (68) is emitted from the first port (67). be. The signal light emitted from the eleventh port enters the first diffraction grating and is demultiplexed into wavelengths. The signal light emitted from the second port is similarly incident on the second diffraction grating and demultiplexed. To finely adjust the wavelength characteristics of the Matsuha Fender interferometer, a parallel plate (65) may be inserted into the optical path of the Matsuha Fender interferometer, and the angle of the parallel plate (65) may be finely adjusted.

In the wooden embodiment, since there is no need to use an optical circulator, the number of parts can be reduced.

[Effects of the Invention] As described above, according to the present invention, among a plurality of multiplexed light waves, a signal light whose mutual wavelength interval has been increased is emitted from the light wave filtering means, and this signal light is Since the light is separated into different wavelengths by the diffraction grating, it can be made compact and highly stable, and the crosstalk between channels and the optical wave bandwidth per wave can be improved.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of the first embodiment of the present invention, and FIG.
The figure shows the wavelength spacing between light waves multiplexed in the signal light used in the present invention, Figure 3 shows the configuration of the second embodiment of the present invention, and Figure 4 shows the configuration of the third embodiment of the present invention. FIG. 5 is a diagram showing the configuration of the fourth embodiment of the present invention, FIG. 6 is a diagram showing the transmitted intensity characteristics of light transmitted through the Matsuhatsu Ender interferometer, and FIG. 7 is a diagram showing the configuration of the fourth embodiment of the present invention. 1 is a diagram showing a configuration of a conventional example of the present invention. (1)...Optical fiber (2)...Collimating lens (3)...Beam splitter (4) Mira (5), (9)...Fabry-Perot resonator. (6), (1,0)...diffraction grating (7),
(Il,)...Condensing lens (8), (12)...
Light receiving element

Claims (4)

[Claims] (1) A first optical system from which a first signal light multiplexed with a plurality of light waves with different wavelengths is emitted; It is characterized by comprising a light wave filtering means from which the signal light is emitted so as to increase the mutual wavelength interval of the light waves, and a diffraction grating from which the signal light emitted from the light wave filtering means is split into different wavelengths. Optical demultiplexer. (2) an optical branching means for branching the first signal light into a plurality of directions; and mutual wavelengths of some of the light waves among the plurality of light waves multiplexed into the signal light branched by the optical branching means. The method includes a plurality of light wave filtering means from which signal light is emitted with increasing intervals, and a plurality of diffraction gratings from which the signal light emitted from the plurality of light wave filtering means is split into different wavelengths. The optical demultiplexer according to claim (1). (3) The light wave filtering means is a Fabry-Perot resonator, and an optical circulator having a plurality of output ports is inserted between the first optical system and the light wave filtering means, and a part of the plurality of ports is inserted. The optical demultiplexer according to claim 1, further comprising a diffraction grating that demultiplexes the signal light emitted from the optical fiber into different wavelengths. (4) The light wave filtering means is a Mach-Zehnder interferometer, and a parallel plate is inserted into the optical path of the Mach-Zehnder interferometer, and the first and second
The first port in which the multiplexed signal light is demultiplexed into different wavelengths by increasing the wavelength spacing between the two ports.
The optical demultiplexer according to claim 1, further comprising a second diffraction grating and a second diffraction grating.