JP2656967B2 - Optical car switch - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention is used for an optical communication device or an optical signal processing device such as an optical computer. In particular, the present invention relates to an optical car switch that performs high-speed switching without converting light into electricity without changing an optical signal.

<Prior Art> In the current optical communication device, an optical signal actually flows only in a transmission line portion. In a repeater, a multiplexing / demultiplexing device, and a switch, the optical signal is once an electrical signal. The optical signal is exchanged, processed like an electronic circuit, and then returned to an optical signal and transmitted again. The device configuration for performing such light / electric conversion and electric / light conversion is very complicated. Also, in the processing by the electronic circuit, 10 to several Gb
There is a fundamental processing speed limit for the bit rate per second. Due to the intervening electrical processing as described above, in order to solve the problem of the complexity of the device configuration and the limitation of the processing speed, without converting the optical signal to an electrical signal,
An optical signal processing technique for processing the optical signal as it is is required.

An optical switch is a basic component of such optical signal processing. Conventional technologies for achieving this include:
There are numerous types of optical switches. Among them, the optical fiber car switch, which focuses on the length of the optical fiber and the high power density, uses ultra-high speed operation and compatibility with optical fiber transmission lines. Very promising. A conventional example of such a fiber optic car switch is Morioka et al., Conference on Lasers and Electrooptics Technical Digest Series (1987), Volume 14, Ultra First Multi. / Demultiplexer Utilizing the Optical Car Effect in Polarization Mainting Single-Mode Fiber "(Mo
rioka et.al: “Ultrafast optical multi / demultiplexe
r utilizing the optical Kerr effect in polarizatio
n-maintaining single-mode fiber ", Technical Diges
t Series 1987 vol, 14, Conference on Lasers and Elec
tro-Optics, THO2, p.286, (1987)), a means using rotation of the polarization plane due to the optical Kerr effect in an optical fiber has been proposed.

This configuration is shown in FIG. A time-division multiplexed digital optical signal sequence 41 and a control optical signal 42 synchronized with the optical signal to be separated and having a polarization plane inclined by 45 degrees with respect to that of the optical signal sequence 41.
Are incident on the polarization-maintaining single-mode fiber 43 which is an optical Kerr medium. In order to compensate for the birefringence temperature characteristic of the polarization-maintaining single-mode fiber 43, the polarization-maintaining single-mode fiber 44 of the same length, having the same optical characteristics as that of the polarization-maintaining single-mode fiber 43, has its main axes inclined at 90 degrees to each other and fused. It is connected. The polarization plane of the separated optical signal is rotated by 90 degrees at the fiber output end with respect to that of the other optical signals due to the control optical signal 42 having a strong optical intensity. Therefore, both are separated by the polarizing beam splitter 46,
They are output as optical outputs 48 and 49, respectively. Light output 48
Is output without switching the optical signal train 41, and the optical output 49 is output when the optical signal train 41 is switched and the traveling direction changes by 90 °. The control optical signal 42 is filtered by the wavelength filter 45 and separated and output from the optical signal train 41 as control light 47.

Also, NJDoran et al., Optics Letters (1988), Volume 19, “Nonlinear Optical Loop Mirror” (NJDoran et.al: “Nonlinear-optical
loop mirror ", Optics Letters, vol.13, no, 1, pp.56-5
8, 1988) has been proposed which utilizes the difference in self-phase modulation between clockwise and counterclockwise light due to the optical Kerr effect in an optical fiber loop.

This configuration is shown in FIG. The two output ports 54 and 55 of the optical coupler 51 are connected to each other by an optical fiber 56,
It forms a loop mirror interferometer. Optical coupler 51
Is set to a value other than 1/2. Optical input 57 first
Input to the input port 52 of the
Then, the light is branched into 1-α to α, and propagates in the optical loop in the opposite direction to each other. Since the branching ratio α of the coupler deviates from 1/2, there is a difference between the clockwise light and the counterclockwise light in the magnitude of the self-phase modulation due to the optical Kerr effect. The electric field amplitude of the optical input 57 is E _IN , the wavelength is λ, and the nonlinear optical constant of the optical fiber is
Assuming that n, the loop length is L, and the speed of light in vacuum is c, the amplitude E _CW ,
The amplitude E _CCW of the electric field of the counterclockwise light is represented by the following equation.

^{_{E CW = α 1/2 E IN exp}} (iα | E IN | 2 2πn 2 L / λ) (1) E CCW = (1-α) 1/2 E IN exp (i (1-α) | E IN _{^{| 2 2πn 2 L / λ)}} (2) next, between the two phase differences φ, φ = (1-2α) | E iN | 2 2πn 2 L / λ (3) occurs. Since the electric field amplitude E _OUT of the optical output 59 is E _OUT = α ² E _CW − (1−α) ² E _CCW (4), the power of the optical output 59 output from the port 53
P _OUT is given by the following equation except for a non-essential proportional constant.

P _OUT = E _OUT ² = | E _IN | ² (1-2α (1-α) (1 + cos (φ))
(5) (3) As shown in equation, phi is | E _IN | is proportional to ² P
_OUT changes in accordance with | E _IN | ^{2 as} shown in FIG. 5B with the branching ratio α as a parameter. Therefore, the magnitude of P _OUT can be controlled (switched) by the input optical power | E _IN | ² . The optical output 58 is output from the port 52 by the switching.

<Problems to be Solved by the Invention> However, as in the first conventional example (FIG. 4), it is very difficult as a practical problem to completely compensate for the temperature characteristics of birefringence. Rotation of the waves and thus fluctuations of the switching operating point are inevitable. Although this method is effective under the assumption that the temperature fluctuation is uniform over the entire length of the fiber, the temperature fluctuation cannot be uniform in practice. Furthermore, it is insensitive to disturbances other than temperature fluctuations, for example, mechanical vibration and pressure. An increase in the fiber length is effective for the low input power operation, but the influence of disturbance becomes more remarkable as the fiber length increases, so that the fiber length cannot be unnecessarily increased.

On the other hand, in the second conventional example (FIG. 5), the interference between lights traveling in the same optical path in opposite directions is used.
Stable operation can be expected without being affected by disturbances other than rotation in the plane created by the optical loop. However, since the branching ratio of the coupler 51 is not 1/2, complete switching (a state in which the input power is all output to the second input port 53 and the extinction ratio becomes infinite) is possible in principle. Absent. Also | E
_OUT | ² / | E _IN | ² is the minimum | E _IN | ² from the condition of φ = π, | E _IN | ² = λ / 2 (1-2α) n ₂ L (6) since made, | E _OUT | ^{for 2} extinction ratio is increased, the alpha 1
/ 2, the input optical power required for switching | E
_IN | ² becomes large (see FIG. 5 (b)).

The present invention solves the above problems and provides an optical Kerr switch that is an ultra-high-speed optical switch that performs complete switching at low input power without being affected by temperature, mechanical vibration, and other disturbances. It is intended to be.

<Means for Solving the Problems> The optical car switch of the present invention has a branching ratio of 1 to 0 or 0 to 1 for control light, and a 1 to 1 for signal light. In the optical coupler having two input ports and two output ports, the two output ports are connected by an optical Kerr medium to form an optical loop, and control light and signal light are input to the two input ports. It is characterized by. Moreover, the group velocities of the signal light and the control light when propagating through the optical Kerr medium are substantially equal. Note that the signal light has information, and the control light is used for switching the signal light.

<Operation> Since the branching ratio of the optical coupler to the control light is 1: 0 or 0: 1, the control light pulse is one of clockwise light and counterclockwise light in which the signal light pulse is branched by the optical coupler. This light travels at the same speed and continues to provide cross-phase modulation at all times, but has little effect on the other light because it only passes instantaneously. Therefore,
A phase difference Δφ expressed by equation (7) is generated between the two.

Δφ = 4πn ₂ | E _PMP | ² L / λ (7) where n ₂ is a nonlinear Kerr constant, E _PMP is a control optical electric field amplitude, L is an optical loop length, and λ is a wavelength of control light. .
On the other hand, the branching ratio of the optical coupler to the signal light is 1
Since the ratio is one, the clockwise light and the counterclockwise light branched by the optical coupler have the same amplitude. Therefore, of the signal light, the optical power output from the input port to which the light enters is represented by P ₀₁ (electric field amplitude E ₀₁ ). If the optical power output from the other input port is P ₀₂ (electric field amplitude E ₀₂ ), then P ₀₁ = | E ₀₁ | ² = | E _PRB | ² (1 + cosΔφ) / 2 (8) P ₀₂ = | E ₀₂ | ² = | E _PRB | ² (1−cosΔφ) / 2 (9), and when the phase difference Δφ is π + 2nπ (n is an integer), all of the signal light pulses are output from the input port instead of the input port. Output to two input ports to achieve full switching.

^{_{| E 01 | 2 / | E}} PRB | 2 is 1 and becomes minimum | E _PRB | ² is, [Delta] [phi = [pi
| E _PRB | ² = λ / 4n ₂ L (10) from the condition (1), complete switching is possible with half (1-2α) power of the second conventional example.

Since the light branched into two by the coupler passes through exactly the same optical path (opposite to each other), disturbances other than the control light only change the phases of the two, and do not affect the switching. In addition, since the length of the optical Kerr medium can be increased, low power operation can be performed from the equation (1). Since the switch originally uses the optical Kerr effect, a switching operation in a femto (10 ⁻⁵ ) second region is possible.

Embodiment An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1A shows an embodiment of the present invention. As shown in the figure, the optical coupler 106 has two input ports 106a and 106b and two output ports 106c and 106d. Optical fiber 1
Reference numeral 10 optically connects the two output ports 106c and 106d of the optical coupler 106. The optical fiber 110 functions as an optical Kerr medium, and can achieve high power density by confining light to the core with low loss. The optical coupler 106
As shown in FIG. 1B, the wavelength characteristic has a branching ratio of 0: 1 for the control light wavelength 118 and a 1: 1 branching ratio for the signal light wavelength 119. I have. More specifically, when the control light pulse 102 is input to the input port 106b, most of the control light pulse 102 is output from the output port 106d, and when the signal light pulse 101 is input to the input port 106a, the signal light pulse is output. There is a characteristic that 101 is almost equally divided and output from the two output ports 106c and 106d.

Further, in the present embodiment, the optical couplers 107 and 109, the control light input port 113, the signal light input / output port 114, and the signal light output port
115 and a control light output port 116 are provided. As shown in FIG. 1 (c), the control light wavelength
A wavelength characteristic having a branching ratio of 0 to 1 for 18 and a branching ratio of 1 to 0 for the signal light wavelength 119 is used. Therefore, the control light pulse 102 input from the control light input port 113
Passes through the optical couplers 109 and 106 without branching, propagates counterclockwise through the loop formed by the optical fiber 110, and similarly passes through the optical couplers 106 and 107 without branching,
It is output as a control light pulse 105 from the control light output port 116. On the other hand, the signal light pulse 101 is
After being passed through the optical coupler 106, the light is divided into two, a counterclockwise light 111 and a clockwise light 112, by the optical coupler 106, propagating in the opposite directions, and merging again with the optical coupler 106.

First, considering the case where there is no control light pulse 102, since the phase difference between the counterclockwise light 111 and the clockwise light 112 is zero, all the signal light pulses 101
Is output as a non-switched signal light pulse 103. Next, considering the case where a control light pulse 102 exists and completely overlaps with one of the signal light pulses 101 in time, in this case, the control light pulse 102 propagates counterclockwise through the optical loop composed of the optical fiber 110. Of the clockwise light (signal light) 111, a pulse overlapping with the control light pulse 102 is always subjected to phase modulation by the control light pulse 102 during propagation in the optical loop. However, clockwise light (signal light) 112 that propagates clockwise in the optical loop only instantaneously passes the control light pulse 102, and does not receive phase modulation. This is shown in FIG.

Therefore, there is a phase difference Δφ expressed by equation (7) between the two.
Occurs, and the power of the control light pulse 102 becomes Δφ = π + 2nπ
When (n is an integer), the signal light pulses 101 are all switched from the signal light output port 115 to the signal light 104.
Is output as In this way, the control light pulse 102
Depending on the presence or absence, the signal light pulse 101 is switched to the port 114 and the port 115 and output.

Since the magnitude of the cross-phase modulation due to the optical Kerr effect strongly depends on the polarization state of the control light pulse 102 and the signal light pulse 101, it is necessary to use a polarization maintaining fiber in order to keep both polarization states constant. It is valid. Further, in this embodiment, the signal light 10 is controlled so that the group velocities of the signal light and the control light substantially match.
1 wavelength, control light 102 wavelength, optical Kerr medium (optical fiber 11
The zero-dispersion wavelength of 0) is set, and by doing so, the degradation of the switching window width due to dispersion can be suppressed.

Further, in the above-described embodiment, the wavelength characteristic of the branch ratio of the coupler is used to make the branch characteristics of the signal light and the control light different in the coupler, but a coupler having a polarization dependence in the branch ratio is used. Thus, one of the two orthogonal polarizations may be used for signal light and the other may be used for control light.

When the wavelengths of the signal light and the control light are different, the overlap between the two gradually shifts (walk-off) during fiber propagation due to the difference in the group velocities of the two, and the cross-phase modulation is not effectively performed. In such a case, even if the fiber length is increased, the power required for switching is not reduced. Therefore, it is desirable that the group velocities of both the signal light and the control light are equal at the wavelength. For this purpose, as shown in FIG. 3, the dispersion shift fiber may be designed so that the signal light and the control light are arranged on both sides of the zero dispersion wavelength.

<Effects of the Invention> As is apparent from the above description, according to the present invention, the control light has a branching ratio of 1 to 0 or 0 to 1,
2 input ports 2 with a 1: 1 split ratio for signal light
The two outputs of the optical coupler at the output port are connected with an optical Kerr medium to form an optical loop, and a loop mirror type optical Kerr switch is configured to reduce the effects of temperature, mechanical vibration, and other disturbances. There is an effect that it is possible to obtain an ultra-high-speed optical switch that performs complete switching in a femtosecond region with low input power without receiving the same. If a fiber is used in which the group velocities of the signal light and the control light are equal to each other at the wavelength, the walk-off is released, and the fiber length is increased to further reduce the input power.

[Brief description of the drawings]

FIG. 1A is a configuration diagram showing an optical car switch according to an embodiment of the present invention, FIG. 1B is a characteristic diagram showing a branching ratio characteristic of a switching optical coupler, and FIG. ) Is a characteristic diagram showing a branching ratio characteristic of an optical coupler that guides light,
FIG. 2 is a characteristic diagram showing the relationship between clockwise signal light, counterclockwise signal light and control light, FIG. 3 is a characteristic diagram showing dispersion characteristics of fibers having the same group velocity of signal light and control light, and FIG. FIG. 5 is a configuration diagram showing a first conventional example, FIG. 5 (a) is a configuration diagram showing a second conventional example, and FIG. 5 (b) is a characteristic diagram showing characteristics of the second conventional example. In the drawing, 101 is a signal light, 102 and 105 are control light pulses, 103 is a non-switched control light pulse, 104 is a switched control light pulse, 106, 107 and 109 are optical couplers, 110 is an optical fiber, 111 is a counterclockwise signal light, and 112 is Clockwise signal light, 106a and 106b are input ports, and 106c and 106d are output ports.

Claims (1)

(57) [Claims] An input port has two input ports and two output ports. A signal light indicating information is input to one of the input ports, and the signal light is switched to one of the input ports. An optical coupler having a characteristic that the control light is input, and the input signal light is almost equally divided and output from two output ports, and most of the input control light is output from one output port; An optical Kerr medium optically connecting two output ports of the coupler, wherein the group velocities of the signal light and the control light when propagating through the optical Kerr medium are substantially equal. .