JP2970780B2 - Ultrashort optical pulse modulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrashort optical pulse modulation circuit capable of high multiplexing of optical pulses, and more particularly to a device for reducing the number of devices and further reducing the use of active elements such as optical switches. It is an improvement that eliminates system complexity, reduces power consumption, and suppresses optical power loss and S / N ratio degradation.

[0002]

2. Description of the Related Art First and second prior arts of an ultrashort optical pulse modulation circuit will be described with reference to FIGS.

In the first prior art, as shown in FIG.
An optical pulse train 2 emitted from a laser oscillator 1 is branched into N parallel light by a 1 × N-type optical branching device 3, and each optical pulse train 4 is separately modulated by an external modulator 5. By giving a relative time delay by the optical fiber 7 and joining them by the N × 1 type optical coupler 8, an N-multiplexed optical pulse train 9 is emitted.

Each of these N external modulators 5 is controlled by an output signal 12 of each stage of a shift register 11 based on an electric signal 10 input from the outside, and modulates the optical pulse train 4 for each pulse. .

The delay amounts of the N optical fibers 7 are t.c / n, 2t.c / n,... N.t.c / n, respectively.
Is set. Here, t is the light pulse width, c is the speed of light, n
Is the refractive index of the optical fiber core.

However, the first conventional technique has the following two problems. N external modulators 5 are required to multiplex optical pulses. Since a circuit is configured by the combination of the N-output optical splitter 3 and the N-input optical coupler 8, the power loss of the optical pulse is large.

On the other hand, in the second prior art, as shown in FIG. 7, an optical pulse train 2 having a repetition frequency V emitted from a laser oscillator 1 is intensity-modulated by an external modulator 5 and a modulated optical pulse train 6 is multiplexed. The module 13 is provided to those connected in N stages in series.

Here, each multiplexer module 13
Is composed of a 1 × 2 type optical switch 14, two optical fibers 15 for giving a relative delay amount, and a 2 × 1 type optical coupler 16. Then, in the multiplexer module 13 of the K-th stage (K = 1 to N), the optical switch 14 is driven at a period of V / 2 ^K , and the relative delay amount between the two optical fibers 15 is 2 ^K ｛(1 By setting / V) −t｝ c / n, the optical pulse train 17 emitted from the N-th stage is obtained as a group of 2 ^N pulses.

The second prior art can reduce the number of external modulators 5 and the like in terms of the device, and can reduce the power loss of an optical pulse, as compared with the first prior art. There is an advantage that you can.

However, the second prior art has the following problems. By using a large number N of the optical switches 14, it becomes necessary to set the timing of the switch control signal, etc.
The system becomes complicated, and the power consumption of the entire system increases. In addition, when the insertion loss and the S / N ratio of the optical switch 14 are large, the number of switches is large, so that the power loss and the waveform deterioration of the optical pulse in total are problematic.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the first and second prior arts, and has reduced the configuration of the apparatus and further reduced the use of active elements such as optical switches. It is an object of the present invention to provide an ultrashort optical pulse modulation circuit which has been improved so as to eliminate complexity, reduce power consumption, and suppress optical power loss and S / N ratio deterioration.

[0012]

An ultrashort optical pulse modulation circuit according to the present invention comprises a laser oscillator for outputting an optical pulse train having a period of T, and an optical pulse train output from the laser oscillator being output by an externally input electric signal. An external modulator that turns on and off and a 2 × 2 optical coupler that power-divides an input optical pulse train on a one-to-one basis. Two lights that provide a relative time delay between the two outputs of the optical coupler. It is characterized by comprising a cascade compressor connected by N stages by a fiber and connected to the external modulator, and an optical switch connected to the cascade compressor and cutting out only a necessary optical pulse group from the output of the cascade compressor. It is assumed that. In this case, if necessary, the first stage optical coupler of the cascade compressor may be a 1 × 2 type optical coupler instead of the 2 × 2 type, or the last stage optical coupler may be a 2 × 1 type optical coupler instead of the 2 × 2 type. It can be an optical coupler.

[0013]

The optical pulse train having the period T emitted from the laser oscillator is converted into a bit rate V (V = 1/1) by an external modulator.
After being modulated by the electric signal of T), an optical coupler passes through a cascade compressor connected in N stages, and only a necessary group of pulses is cut out by an optical switch. Here, in the cascade compressor, Tt is set by two optical fibers in the first stage, 2 (Tt) in the second stage, and 2 (Tt) in the Nth stage.
^A relative time delay of ^N (T−t) is given, and finally, an optical switch cuts out only a group of optical pulses whose light quantity has been correctly increased as the first pulse. Here, t is an arbitrary time smaller than T. Thereby, 2 ^N optical pulses are multiplexed at the interval t and output from the optical switch.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the embodiments shown in the drawings.

FIG. 1 shows an ultrashort optical pulse modulation circuit according to an embodiment of the present invention. As shown in the figure, the ultrashort optical pulse modulation circuit 20 of this embodiment is configured by connecting the semiconductor laser oscillator 1, the external modulator 5, the cascade compressor 18, and the optical switch 19 in this order in series. Things.

The external modulator 5 is an optical intensity modulator that turns on or off an optical pulse train for each pulse in a modulatable band based on the input electric signal 5A. Here, assuming that the period of the ultrashort optical pulse train 2 from the semiconductor laser oscillator 1 is T, the bit rate V of the electric signal 5A is V = 1 / T. The optical pulse train 6 emitted from the external modulator 5 is input to the cascade compressor 18.

As shown in FIG. 2, the cascade compressor 18 includes N 2 × 2 optical couplers 21-1, 21-2, 21-2.
... 21-N is N-1 and one optical fiber pair for delay 22-
1, 22-2,..., 22-N-1. Here, the relative time delay of the optical fiber pair 21-j in j (j = 1, 2,..., N−1) stages is defined as j (Tt). t is an arbitrary time satisfying t <T. The 2 × 2 type optical fiber has a one-to-one power split of optical pulses from two input terminals to two output terminals. Therefore, the optical pulse trains at both output terminals are the same. The output terminal of the external modulator 5 is connected to one input terminal of the first stage 2 × 2 optical coupler 21-1. One output terminal of the 2 × 2 type optical coupler 21-N at the last stage is connected to an input terminal of the optical switch 19.

The optical switch 19 has a period of 2 ^N · from the optical pulse train 23 emitted from the cascade compressor 18.
Select only one optical pulse group at T and output optical pulse train 2
4 is applied.

Therefore, the above-mentioned ultrashort optical pulse modulation circuit 20
Then, modulation is performed as shown in FIG. Hereinafter, this will be described.

That is, an ultrashort optical pulse train 2 having a period T is emitted from the semiconductor laser oscillator 1, and the optical pulse train 2 is input to an external modulator 5. The external modulator 5 modulates the optical pulse train 2 for each pulse in a band that can be modulated based on the input electric signal 5A of the beatlet V.

Here, in order to make it easy to see the change after the modulation of the optical pulse train 2, it is assumed that all the bits are modulated by the input electric signal 5 A of high level (all 1). Therefore, the optical pulse train 2 input to the external modulator 5 and the optical pulse train 6 output from the modulator 5 are eventually the same.

Thereafter, as shown in FIG. 3A, when the optical pulse train 6 having a period T is input to the cascade compressor 18, the optical pulse train emitted by the second stage 2 × 2 optical coupler 21-2 becomes At each of the two output terminals, as shown in FIG. 3B, light pulses arranged at an interval t are emitted at a period T every two lines. Then, the N-th stage optical coupler 21-N
Pulse train 23 (cascade compressor 1
8), at both of the two output terminals, as shown in FIG. 3 (c), light pulses arranged at an interval t are emitted at a period T every 2 ^N pulses. The first-stage optical coupler 2
The same optical pulse train as the input terminal is emitted from the two output terminals 1-1, but these are given a relative delay of Tt by the optical fiber pair 22-1, and are applied to each input terminal in the second stage. Incident.

The optical switch 19 is shown in FIG.
Period 2 like^N・ Select only one light pulse group with T
t (2^N-1) A gate signal 1 having a pulse width of at least T and less than T
9A is given to cut out an optical pulse. This allows
The optical pulse train 24 emitted from the optical switch 19 is shown in FIG.
As shown in FIG. ^N
Only the multiplexed optical pulse group is obtained.

In the case where the time interval between the true light pulse and the dummy light pulse becomes short in the middle of the cascade compressor 18 and it becomes impossible to cut out at the switching speed of the optical switch 19, another one-way operation is performed. By intervening the optical switch and dividing the optical pulse into two stages, it is possible to obtain only a necessary optical pulse group.

It is sufficient to connect the output terminal of the external modulator 5 to only one of the two input terminals of the first-stage optical coupler 21-1 of the cascade compressor 18. Therefore, a 1 × 2 type optical coupler can be used as the first stage optical coupler instead of the 2 × 2 type optical coupler.

Similarly, the input terminal of the optical switch 19 is connected to the last (N-th) optical coupler 2 of the cascade compressor 18.
It is sufficient to connect only one of the two output terminals 1-N. Therefore, a 2 × 1 type optical coupler can be used as the last stage optical coupler instead of the 2 × 2 type optical coupler.

Next, another embodiment of the present invention will be described with reference to FIG. In this embodiment, a plurality m of the ultrashort optical pulse modulation circuits 20 shown in FIG. 1 are used.

That is, as shown in FIG. 4, m ultrashort optical pulse modulation circuits 20-1, 20-2,..., 20-m are each composed of m-1 optical fibers 25 having a relative time delay.
-1, 25-2,... 25- (m-1) in parallel,
It is connected to an mx1 type optical coupler 26. Each ultrashort optical pulse modulation circuit emits an optical pulse train 25 shown in FIG. Further, each optical fiber 25-1, 25-
2,... 25- (m−1) are 2 ^N · t and 2 ^N ·
.. 2 ^N · t · (m−1). Here, 2 ^N · t is the time width of a group of 2 ^N optical pulses.

Therefore, each ultrashort optical pulse modulation circuit 20-
1, 20-2,..., 20-m
Are the optical fibers 25-1, 25-2,... 25- (m-
1), and the time-series optical pulse trains 24, 24-1, 24-2,... 24-(m-
1), and when they are combined and emitted by the mx1 type optical coupler 26, a continuous optical pulse train 27 shown in FIG.

That is, in the embodiment shown in FIG. 4, the light pulse emitted from another ultrashort light pulse modulation circuit within a blank time width of the light pulse train 24 emitted from a certain ultrashort light pulse modulation circuit. By multiplexing the groups, an ultra-high-speed optical highway having an information speed approximately equal to the reciprocal of the optical pulse width is realized in principle. The multiplicity may be determined by adjusting the amount of delay, the number of ultrashort optical pulse modulation circuits 20, and the like so that the optical pulse trains do not overlap.

[0031]

As described above in detail with reference to the embodiments, in the present invention, multiplexing is performed cumulatively by a cascade compressor. Therefore, as compared with the parallel type multiplexing as in the first related art, the device configuration such as the external modulator is greatly reduced, and the overall size can be reduced. Furthermore, compared to the same serial processing multiplexing as in the second prior art, the use of active elements such as optical switches is minimized, so that the system required for timing setting and the like becomes more complicated. The power consumption of the entire system can be reduced. Further, since the power loss of the optical pulse and the deterioration of the S / N ratio in the optical switch can be suppressed, the insertion loss and the waveform deterioration of the entire system are reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an ultrashort optical pulse modulation circuit according to one embodiment of the present invention.

FIG. 2 is a diagram showing a configuration example of a cascade compressor.

FIG. 3 is a diagram showing a relationship between each optical pulse and a gate signal to an optical switch.

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

FIG. 5 is a diagram illustrating a relationship between an optical pulse train after emission from each ultrashort optical pulse modulation circuit and an optical pulse train after emission from an m × 1 type optical coupler;

FIG. 6 is a diagram showing a first related art.

FIG. 7 is a diagram showing a second conventional technique.

[Explanation of symbols]

 Reference Signs List 1 laser oscillator 5 external modulator 5A electric signal 18 cascade compressor 19 optical switch 19A gate signal 20, 20-1 to 20-m ultrashort optical pulse modulation circuit 21-1 to 21-N 2 × 2 type optical coupler 22-1 -22- (N-1) Optical fiber (delay line) 25-1-25- (m-1) Optical fiber (delay line) 26mx1 type optical coupler

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification code FI H04J 14/08 (56) References JP-A-3-117236 (JP, A) JP-A-64-51834 (JP, A) JP-A-64-249827 (JP, A) JP-A-2-281825 (JP, A) JP-A-2-107703 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H04B 10 / 00-10/28 H04J 14/00-14/08 G02B 6/00

Claims (3)

(57) [Claims] 1. A laser oscillator that outputs an optical pulse train having a period T, an external modulator that turns on and off an optical pulse train output from the laser oscillator by an external input electric signal, and a pair of an input optical pulse train. Power branch to 1 2
A cascade compressor connected to the external modulator by connecting a × 2 type optical coupler by N stages by two optical fibers having a relative time delay between two outputs of the optical coupler; An optical switch that is connected to the cascade compressor and that extracts only a required group of optical pulses from the output of the cascade compressor. 2. The ultrashort light pulse modulation circuit according to claim 1, wherein the first stage optical coupler of the cascade compressor is a 1 × 2 type optical coupler instead of a 2 × 2 type optical coupler. 3. The ultrashort optical pulse modulation circuit according to claim 1, wherein the last stage optical coupler of the cascade compressor is a 2 × 1 type optical coupler instead of a 2 × 2 type optical coupler.