JPH063722A - Optical signal generator - Google Patents

Abstract

PURPOSE:To generate a high speed optical signal by an element in a limited band and an amplifier. CONSTITUTION:This generator is an optical signal generator generating the optical signal by modulating consecutive light. The consecutive light is guided to an acoustooptical modulators 1(A)-1(D), one part of the light is diffracted by the modulators 1(A)-1(D) and the optical frequency thereof is changed. The light which passes without being diffracted by the modulators 1(A)-1(D) is guided to mirrors 3(A)-3(E) through phase shifters 2(A)-2(E), reflected by the mirrors 3(A)-3(E) to advance in an opposite direction, then the light diffracted by the modulators 1(A)-1(D) connects plural means led to the acoustooptical modulator at a following stage in a vertical array.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high speed optical signal generator used in optical communication systems and the like.

[0002]

2. Description of the Related Art An ordinary optical signal is obtained by modulating a laser beam which is a light source. For this purpose, a method of directly modulating a semiconductor laser, an external modulator composed of a dielectric crystal such as LiNbO ₃ is used. There is a method of using.

[0003]

In order to generate an optical signal by the conventional technique, the band of the modulator and the band of the amplifier required to drive the modulator are both higher than the modulation band of the generated optical signal. Must also be large. Therefore,
As the speed of optical signals increases, demands on the bandwidth of modulators and amplifiers become more severe.

An object of the present invention is to provide a technique capable of generating a high-speed optical signal by using an element and an amplifier having a limited band.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[0006]

In order to achieve the above-mentioned object, means (1) of the present invention is an optical signal generator for generating an optical signal by modulating continuous light. Is guided to the acousto-optic modulator, and a part of the light is diffracted by the acousto-optic modulator, and at the same time, the optical frequency changes,
The most main feature is that the light passed through the acousto-optic modulator without being diffracted is guided to a mirror through a phase shifter, reflected by the mirror, and travels in the opposite direction.

The means (2) of the present invention is an optical signal generator for generating an optical signal having a finite bit length by modulating continuous light, the continuous light being guided to an acousto-optic modulator. At the same time, a part of the light is diffracted by the acousto-optic modulator, and at the same time, the optical frequency is changed, and the light passing without being diffracted by the acousto-optic modulator is guided to a mirror through a phase shifter, and It is reflected and proceeds in the opposite direction,
The light diffracted by the acousto-optic modulator is connected to the acousto-optic modulator of the next stage in a plurality of cascades.

[0008]

According to the above-mentioned means, a large optical frequency conversion by the acousto-optical modulator is realized by connecting a plurality of acousto-optical modulators in cascade, and the amplitude and phase of the converted light of the frequency are realized. By controlling these so that they match the frequency spectrum of the desired optical signal, a desired high-speed optical signal can be generated.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a schematic diagram for explaining the configuration of an optical signal generator according to an embodiment of the present invention. In FIG.
1 (A) to 1 (D) are acousto-optic modulators, 2 (A) to 2
(E) is a phase shifter for adjusting the phase of light by adjusting the optical path length, 3 (A) to 3 (E) are mirrors, and 4 is a half mirror.

The optical signal that can be generated by the optical signal generator of this embodiment has a bit rate of F bits / second (bit / second).
s), which is an optical packet signal having a length of N (integer) bits. In order to generate such an optical signal by the optical signal generator of the present embodiment, it is necessary to connect N-stage acousto-optical modulators as described later, but in FIG. Only drawn. This optical packet is
It is assumed to be modulated by a Z (Non Return to Zero) code. It is known that such an optical packet is represented by combining frequency spectrum components as shown in equation (1).

[0012]

[Equation 1]

Here, f ₀ is the frequency of the carrier wave of light. am and φm are the amplitude and phase of the sidebands having the frequency f ₀ + mΔf, which depend on the bit pattern of the optical signal. However, Δf is equal to F / N, and c.c. means the complex conjugate.

In the embodiment of FIG. 1, each frequency component is generated by an acousto-optic modulator connected in series, and a phase shifter sets the phase φm of the frequency component to form a desired optical signal. .

For convenience of explaining the detailed operation of the optical signal generator of this embodiment, the acousto-optic modulator and the phase shifter of FIG. 1 will be named A, B, C and D from the left. Each acousto-optic modulator is driven by a sine wave with a frequency of Δf / 2, so that part of the light incident on the acousto-optic modulator is transmitted as it is, but part of it changes direction due to diffraction.
Moreover, the frequency of light increases by Δf / 2.

In the optical signal generator of this embodiment, continuous light is first incident on the acousto-optic modulator 1 (A) as shown in FIG. The acousto-optic modulator 1 (A) diffracts the incident light with the diffraction efficiency η ₁ and gives the diffracted light a frequency change of Δf / 2. This diffraction efficiency can be adjusted by changing the intensity of the drive signal (rf signal) that drives the acousto-optic modulator. The light that has passed through the acousto-optic modulator 1 (A) without being diffracted is guided to the mirror 3 (A) after undergoing a phase change of φ _{0 by} the phase shifter 2 (A), and is reflected there. After passing through the acousto-optic modulator 1 (A) again, it is taken out by the half mirror 4. The electric field amplitude of the light that follows such a path is expressed by the following equation.

[0017]

(2) (1-η ₁ ) ² exp [j2πf ₀ t + φ ₀ ] + c.c. (2) However, it is assumed that the frequency of the incident continuous light is equal to f ₀ . Next, the light diffracted by the acousto-optic modulator 1 (A) is guided to the acousto-optic modulator 1 (B). In the acousto-optic modulator 1 (B), the incident light is diffracted with the diffraction efficiency η ₂ , and the diffracted light is further frequency-shifted by Δf / 2. The light that is transmitted without being diffracted by the acousto-optic modulator 1 (B) is converted into φ ₁ by the phase shifter 2 (B).
Is given a phase change of and is reflected by the mirror 3 (B),
The original path is reversed and the half mirror 4 takes it out. The electric field amplitude is as follows.

[0018]

[Equation 3]

Similarly, when the electric field amplitude of the light passing through the (m + 1) th stage acousto-optic modulator and reflected by the mirror and traveling backward in the original path is m + 1≤N-1,

[0020]

[Equation 4]

When m + 1 = N,

[0022]

[Equation 5]

It is The light extracted from the half mirror 4 is the sum of equations (2) to (5). Comparing these sums with Equation (1), the diffraction efficiency of N acousto-optic modulators

[0024]

[Equation 6]

Both can be made equal by appropriately adjusting. In this way, the optical signal represented by the equation (1) can be generated by the configuration of FIG.

The optical signal generated by the optical signal generator of this embodiment has a band of F = NΔf. However,
The bandwidth of the acousto-optic modulator required for the configuration of this embodiment and the drive signal (rf signal) amplifier required to drive it is at most Δf / 2. Therefore, it is possible to generate a high-speed optical signal by using an element and an amplifier having a limited band. However, the bit length of the optical signal generated in this embodiment is limited to N bits. However, an ATM (Asyncronous Transfer) that handles a signal for each packet having a certain bit length
Mode) In a network, such a modulation method is convenient.

In the above embodiment, the case where the optical signal modulated by the NRZ code is generated has been described, but in this case, in order to generate the optical signal of N bit length, the acousto-optic modulator of N stages is used. Cascade connection was required. However, the number of acousto-optic modulator stages required to generate an optical signal having an N-bit length is not necessarily equal to N. For example, when an optical signal modulated by the RZ (Return to Zero) coding method is generated, the required number of stages is 2N. This is because the optical signal modulated by the RZ code is 2 times larger than the optical signal modulated by the NRZ code.
It comes from having double the number of sidebands. In addition to this, when the one-sided sideband communication known as a known technique is used, the number of stages required for generating an optical signal may be half of this.
The one-side sideband communication is a method of transmitting only one half of the sideband by utilizing the fact that the sideband of a normal modulation signal is symmetrical. In this case, the required number of stages is NRZ
In the case of the optical signal modulated by the code, N / 2 stages are required, and in the case of the RZ code, N stages are required. As described above, the acousto-optic modulator required to generate an optical signal having a constant bit length varies depending on the modulation system or transmission system adopted at that time.

Although the present invention has been specifically described based on the above embodiments, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the invention. Is.

[0029]

As described above, according to the present invention, a high-speed optical signal can be generated by using a device and an amplifier having a limited band. By applying this, it can be used to generate high-speed signals in a packet switching network, an ATM network, and the like.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining a configuration of an optical signal generator according to an exemplary embodiment of the present invention.

[Explanation of symbols]

1 (A) to 1 (D) ... Acousto-optic modulator, 2 (A) to 2
(E) ... Phase shifter, 3 (A) to 3 (E) ... Mirror, 4 ... Half mirror.

Claims (2)

[Claims] 1. An optical signal generator for generating an optical signal by modulating continuous light, wherein the continuous light is guided to an acousto-optic modulator, and a part of the continuous light is diffracted by the acousto-optic modulator. At the same time, the optical frequency is changed, and light passing through the acousto-optic modulator without being diffracted is guided to a mirror through a phase shifter, reflected by the mirror, and traveled in the opposite direction. And an optical signal generator. 2. An optical signal generator for generating an optical signal having a finite bit length by modulating continuous light, the continuous light being guided to an acousto-optic modulator, and the acousto-optic modulator At the same time that a part of the light is diffracted, the optical frequency is changed, and the light passed through without being diffracted by the acousto-optic modulator is guided to a mirror through a phase shifter, reflected by the mirror, and travels in the opposite direction. An optical signal generator, wherein means for guiding the light diffracted by the acousto-optic modulator to the acousto-optic modulator of the next stage is connected in a plurality of cascades.