JP4292470B2 - Optical signal generator - Google Patents

Description

  The present invention relates to an optical signal generator.

The pulse pattern generator is a device that generates various test optical signals. An optical pulse signal with time axis fluctuation (jitter) as one of the test optical signals (hereinafter referred to as an optical pulse signal with jitter). ), The following method using the pulse pattern generator has been adopted.

That is, a clock signal (electrical signal) having jitter is generated using an external PLL circuit, and this clock signal is input to the pulse pattern generator as a reference clock signal that serves as a reference for the operation of the pulse pattern generator. An attached optical pulse signal was generated. Such an optical pulse signal with jitter is used, for example, to evaluate the bit error rate (BER) of the test object. A technique for generating a jittered optical pulse signal using a pulse pattern generator is disclosed in, for example, the following publicly known documents.
JP 2003-139552 A

However, the conventional method for generating an optical pulse signal with jitter has the following problems.
(1) The clock system configuration (electric circuit configuration) is complicated.
(2) The amount of jitter added to the jittered optical pulse signal is limited by the sensitivity of the phase comparator and voltage controlled oscillator constituting the external PLL circuit.
(3) In order to generate a clock signal having a high speed, that is, a high frequency (for example, a clock signal having a repetition frequency of several to several tens GHz), it is necessary to use a phase comparator that operates at high speed. Such phase comparators are expensive and difficult to obtain.

(4) Because of a two-stage process in which a clock signal having jitter is generated as a preceding process and a jittered optical pulse signal is generated using the clock signal as a subsequent process, it is given to the jittered optical pulse signal. Jitter is greatly affected by the clock system electrical circuit inside the pulse pattern generator. If this clock electric circuit includes, for example, a PLL circuit (internal PLL circuit), jitter having a frequency component higher than the loop band of the internal PLL circuit cannot be added to the jittered optical pulse signal.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical signal generator capable of solving the above-described problems.

To achieve the above object, according to the present invention, an optical pulse generating means for generating an optical pulse signal having a constant repetition period, a jitter signal generating means for generating a jitter signal having a predetermined jitter, and the jitter signal are used. Polarization changing means for changing the polarization plane of the optical pulse signal, and an optical transmission medium in which the group delay speed in optical transmission depends on the polarization plane, and transmits the optical pulse signal output from the polarization changing means. The solution means that the polarization-dependent optical transmission medium is provided.

According to such a solution, since the jittered optical pulse signal is generated by directly adding the group velocity fluctuation to the optical pulse signal, the configuration of the clock system for generating the optical pulse signal can be simplified. In addition, jitter having a high frequency component can be added to the optical pulse signal .
In addition, since the PLL circuit is not included in the configuration for adding jitter to the optical pulse signal, it is possible to avoid the time axis fluctuation being limited by the PLL circuit.
Further, since it is not necessary to use a phase comparator that operates at high speed for the PLL circuit, it is possible to reduce the price.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram of an optical signal generator according to this embodiment. In this figure, reference numeral 1 is a reference oscillator, 2 is a pulse pattern generator (PPG), 3 is an electrical / optical converter, 4 is a jitter signal generator (time axis fluctuation signal generating means), and 5 is a polarization controller (biased). Wave changing means),
Reference numeral 6 denotes a polarization-dependent optical transmission medium, H denotes an optical pulse generation unit, and J denotes a jitter applying unit (time axis variation applying unit).

The reference oscillator 1 oscillates a reference signal a having a predetermined repetition period and outputs it to the clock input (terminal) of the pulse pattern generator 2. The pulse pattern generator 2 is configured so that the reference signal a
Is used as a reference clock to generate a data signal b having a fixed repetition period, and this data signal b is output to the electrical / optical converter 3. The electrical / optical converter 3 photoelectrically converts the data signal b (electric signal) to generate an optical pulse signal c having the same repetition period as the data signal b, and outputs the optical pulse signal c to the polarization controller 5.

The jitter signal generator 4 is a pulse-shaped jitter signal d having a predetermined time axis fluctuation (jitter).
(Time axis fluctuation signal) is generated and output to the polarization controller 5. The polarization controller 5
A polarization fluctuation optical pulse signal e is generated by changing the polarization plane of the optical pulse signal c based on the time axis fluctuation signal d, and is output to the polarization dependent optical transmission medium 6. Polarization-dependent optical transmission medium 6
The group delay rate in optical transmission depends on the polarization plane, that is, polarization mode dispersion (PDM)
The polarization fluctuation optical pulse signal e
Is transmitted to generate an optical pulse signal f with jitter and output to the outside.

The reference oscillator 1, the pulse pattern generator 2 and the electric / optical converter 3 constitute the optical pulse generator H in the present invention, while the jitter signal generator 4, the polarization controller 5 and the polarization dependent light. The transmission medium 6 constitutes a jitter applying unit J in the present invention.

In the optical signal generator configured as described above, an optical pulse signal is transmitted by transmitting the polarization-fluctuating optical pulse signal e generated by the polarization controller 5 based on the jitter signal d through the polarization-dependent optical transmission medium 6. A jittered optical pulse signal f is generated in which a jitter corresponding to the jitter signal d and the polarization mode dispersion of the polarization-dependent optical transmission medium 6 is added to c.

Next, a more specific configuration example (first configuration example) of the jitter applying unit J will be described with reference to the conceptual diagram of FIG. In the first configuration example, the polarization controller 5 is configured by an LN polarization modulator 5A and the polarization-dependent optical transmission medium 6 is configured by a PANDA fiber 6A as illustrated.

In this jitter applying section J, a pulse-like jitter signal d is converted into an LN polarization modulator 5 as shown in the figure.
By supplying to A, the polarization mode of the optical pulse signal c is controlled. LN polarization modulator 5
For example, the polarization of the optical pulse signal c incident on A is linearly polarized as shown in the figure. The polarization fluctuation optical pulse signal e is polarized when the jitter signal d is low level i) or c). Is the same as the polarization direction of the optical pulse signal c, and when the jitter signal d is high level b) and d), the polarization direction is rotated by 90 degrees with respect to the polarization direction of the optical pulse signal c.

The voltage value of the low level a), c) and the high level b), d) of the jitter signal d is
The polarization direction of the polarization fluctuation optical pulse signal e is adjusted in advance so as to be two polarization directions orthogonal to each other. Then, by arranging the PANDA fiber 6A so that the two polarization directions perpendicular to each other of the polarization fluctuation optical pulse signal e coincide with the low-speed axis and the high-speed axis of the PANDA fiber 6A, respectively, A jittered optical pulse signal f having a jitter caused by a difference in group velocity is generated.

Here, the jitter amount Δt added to the jittered optical pulse signal f is such that the refractive index difference between the two polarization directions of the PANDA fiber 6A is B, the light velocity of vacuum is c, and the fiber length of the PANDA fiber 6A is L. Is given by the following equation (1).
Δt = B · L / c (1)
For example, when the refractive index difference B is 3 · 10 ^ −4 and the fiber length L is 1 m (meter), the jitter amount Δt is specifically expressed by the following equation (2).
Δt = 3 · 10 ^ -4 / 3 · 10 ^ 8
= 1 · 10 ^ -12 = 1 psec (picoseconds) (2)

In this case, if the bit rate of the polarization fluctuation optical pulse signal e incident on the PANDA fiber 6A is 40 Gbps, one period of the optical signal is 25 [psec]. The jitter of 04 [UIpp] (= 1 [psec] / 25 [psec]) is added.

In the above description, the polarization of the optical pulse signal c is linearly polarized in a specific polarization direction.
It is widely known that circularly polarized light and elliptically polarized light input can be converted into linearly polarized light in a specific direction by arranging a combination of a 4 wavelength plate and a 1/2 wavelength plate before and after the LN polarization modulator 5A. It is clear that an example does not specify operating conditions. In addition, since an LN polarization modulator that provides an action equivalent to that of the quarter-wave plate and the half-wave plate for the optical pulse signal c is commercially available, a plurality of LN polarization modulators are connected in multiple stages. Therefore, it is possible to easily assume a system that can cope with circularly polarized light and elliptically polarized light input. Furthermore, the desired jitter amount [UIpp] can be arbitrarily set by arbitrarily selecting the refractive index difference B and the fiber length L of the polarization mode in the PANDA fiber 6A.

Next, a second configuration example relating to the jitter applying unit J will be described with reference to the conceptual diagram of FIG. In the second configuration example, as shown in the figure, the polarization controller 5 is configured by a half-wave plate 5B that is rotationally driven, and the polarization-dependent optical transmission medium 6 is configured by a PANDA fiber 6A.

In this second configuration example, the polarization direction of the optical pulse signal c is controlled by rotating the half-wave plate 5B at a constant angular velocity. For example, when the polarization of the optical pulse signal c is linear polarization as shown in the figure, the polarization direction of the polarization fluctuation optical pulse signal e is rotated at an angular velocity twice as large as the half-wave plate 5B is rotated. Become. When the arrangement of the low speed axis and the high speed axis of the PANDA fiber 6A is set as shown in the figure, the jittered optical pulse signal f has jitter due to the difference in the group velocity between the two polarization directions. Is added. The jitter amount Δt of the jittered optical pulse signal f is given by the above equation (1).

According to the second configuration example, since jitter can be added to the jittered optical pulse signal f by continuously rotating the half-wave plate 5B, the jitter signal generator 4 is not required, and the apparatus configuration is simpler. It becomes.

Furthermore, a third configuration example relating to the jitter applying unit J will be described with reference to the conceptual diagram of FIG. In the third configuration example, the polarization controller 5 is configured by a Faraday rotator 5C, and the polarization-dependent optical transmission medium 6 is configured by a PANDA fiber 6A.

In this third configuration example, the polarization direction of the optical pulse signal c is controlled by supplying the pulse-like jitter signal d generated by the jitter signal generator 4C to the Faraday rotator 5C. When the polarization of the optical pulse signal c is linear polarization as shown in the figure, the polarization fluctuation optical pulse signal e has a polarization direction that is the same as the polarization direction of the optical pulse signal c when the jitter signal d is low level i) or c). When the jitter signal d is high level b) and d), the polarization direction is the optical pulse signal c.
The rotation direction is 90 degrees with respect to the polarization direction.

The voltage value of the low level a), c) and the high level b), d) of the jitter signal d is
The polarization direction of the polarization fluctuation optical pulse signal e is adjusted in advance so as to be two polarization directions orthogonal to each other. Then, by arranging the PANDA fiber 6A so that the two polarization directions perpendicular to each other of the polarization fluctuation optical pulse signal e coincide with the low-speed axis and the high-speed axis of the PANDA fiber 6A, respectively, A jittered optical pulse signal f having a jitter caused by a difference in group velocity is generated.

According to the present embodiment, unlike the conventional method of generating an optical pulse signal with jitter using a conventional pulse pattern generator, it is not necessary to use a PLL circuit, that is, a phase comparator that operates at high speed. However, it is not limited by the sensitivity of the phase comparator or voltage controlled oscillator, and the cost of the optical signal generator can be reduced. Also, there is an advantage that the clock system configuration (electric circuit configuration) is extremely simple compared to the prior art.

It is a block diagram which shows the function structure of one Embodiment of this invention. It is a conceptual diagram which shows the 1st structural example of the jitter provision part J in one Embodiment of this invention. It is a conceptual diagram which shows the 2nd structural example of the jitter provision part J in one Embodiment of this invention. It is a conceptual diagram which shows the 3rd structural example of the jitter provision part J in one Embodiment of this invention.

Explanation of symbols

1 ... Reference oscillator 2 ... Pulse pattern generator (PPG)
3 ... Electric / optical converter 4 ... Jitter signal generator (time axis fluctuation signal generation means)
5 ... Polarization controller (polarization changing means)
6 …… Polarization-dependent optical transmission medium J …… Jitter adder (time axis fluctuation applying means)
H: Optical pulse generation means

Claims (5)

An optical pulse generating means for generating an optical pulse signal having a constant repetition period;
Jitter signal generating means for generating a jitter signal having a predetermined jitter;
Polarization changing means for changing the polarization plane of the optical pulse signal based on the jitter signal;
An optical transmission medium in which a group delay rate in optical transmission depends on a polarization plane, and a polarization-dependent optical transmission medium that transmits an optical pulse signal output from the polarization changing unit;
An optical signal generator characterized by comprising:   2. The optical signal generator according to claim 1, wherein the polarization changing means is an LN polarization modulator.   2. The optical signal generator according to claim 1, wherein the polarization changing means is a half-wave plate that is rotationally driven.   2. The optical signal generator according to claim 1, wherein the polarization changing means is a Faraday rotator.   The optical signal generator according to claim 1, wherein the polarization-dependent optical transmission medium is a PANDA fiber.

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