JPH10285144A - Optical signal output device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the high density wavelength division multiplex system for a long distance by improving a wavelength deterioration and a transmission error rate while suppressing a spectral line width from being extended. SOLUTION: A data generating circuit 14 generates a binary data signal to be sent and a data modulator 12 applies digital modulation to a continuous laser beam from a laser 10. A phase shifter 22 delays a clock signal component extracted by a band pass filter(BPF) 20 and gives the result to a phase modulator 18 as a modulation signal. The phase modulator 18 applies phase modulation to the modulated data light synchronously with bits of the clock signal A polarized wave scrambler 24 scrambles the polarized wave output light from the phase modulator 18 with a sine wave signal whose frequency is sufficiently lower than a bit rate of digital modulation by the data modulator 12 and sufficiently higher (about 2 kHz or over) that a reply time of an optical fiber amplifier, several hundreds of Hz.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical signal output device, and more particularly, to an optical signal output device for outputting an optical signal suitable for a high-density wavelength division multiplexing transmission system.

[0002]

2. Description of the Related Art In an optical fiber transmission system, particularly a long-distance or ultra-long-distance optical fiber transmission system such as a transcontinental system, the temporal variation of transmission loss is caused by a signal-to-noise ratio (S / N ratio).
Is varied randomly. Random fluctuations in the signal-to-noise ratio cause a phenomenon known as signal fading, which increases the bit error rate (BER) of the digital signal.

The polarization dependence induced by optical fibers and / or other optical elements such as repeaters and amplifiers is:
It has been found that it greatly affects signal fading and S / N ratio fluctuation. For example, polarization hole burning Polarization Hole in an optical amplifier.
Also known as Burning (PHB). The polarization hole burning PHB is a phenomenon in which the amplification factor differs according to the polarization direction, that is, the noise light in the polarization direction orthogonal to the transmission light to be amplified is amplified more strongly than the noise light in the same polarization direction. .

As means for solving such signal fading, there has been proposed a configuration in which the polarization of digitally modulated light is modulated at the same clock frequency synchronized with the digital modulation. For example, Japanese Patent Application Publication No.
See US Pat. No. 662, U.S. Pat. No. 5,526,162. A configuration has also been proposed in which, before polarization-modulating digitally modulated light, phase modulation is performed at the same clock frequency synchronized with the digital modulation.

In such a configuration, since all polarization states occur in the bit, not only the PHB of the optical fiber amplifier, but also the polarization dependent loss Polarizat of the optical components.
Also, ion dependency loss (PDL) can be eliminated, and deterioration of transmission characteristics due to polarization dependency can be prevented. In addition, waveform degradation is reduced due to chirping accompanying high-speed polarization scrambling, and as a result, an error rate is reduced.

[0006]

According to the bit synchronous polarization scrambler method described in the above publication, the signal error rate of a long-distance optical amplification transmission system can be improved. Has the drawback that the line width is increased. In the wavelength division multiplexing transmission method, the wavelength interval between adjacent wavelengths is a major factor in determining the number of multiplexed wavelengths. That is, the conventional example has a drawback that it is not suitable for the high-density wavelength multiplexing system.

In the conventional example, since the degree of polarization and the prechirp are closely related to each other, it is difficult to simultaneously optimize the degree of polarization and the amount of prechirp.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical signal output device capable of suppressing expansion of a line width while suppressing deterioration of transmission characteristics due to polarization dependence.

Another object of the present invention is to provide an optical signal output device which can be used as a signal source in a long-distance or ultra-long-distance high-density wavelength division multiplexing optical fiber transmission system.

[0010]

SUMMARY OF THE INVENTION The present invention provides a laser light source, digital modulation means for digitally modulating input light, phase modulation means for phase-modulating input light in synchronization with a modulation bit of the digital modulation means, Polarization scrambling means for scrambling the polarization of the input light asynchronously with the digital modulation at a frequency sufficiently lower than the bit rate of the digital modulation in the digital modulation means and sufficiently higher than the response frequency of the optical amplifying means; Wherein the digital modulating means, the phase modulating means, and the polarization scrambling means are connected in series in a predetermined order. For example, the digital modulation means and the phase modulation means are arranged between the laser light source and the polarization scrambling means.

The digital modulation means and / or the phase modulation means can be integrated in the laser light source.

Since the optical signal is pre-chirped by the phase modulation synchronized with the bit of the digital modulation, the spread of the spectral line width can be suppressed. Further, since the final degree of polarization is determined by the polarization scrambling means, the amount of pre-chirp and the degree of polarization can be independently optimized.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of the present invention. Reference numeral 10 denotes a laser that generates a continuous laser wave, and its output light is applied to a data modulator 12. The data modulator 12 is composed of, for example, an electro-absorption optical modulator. The data generation circuit 14 generates a binary data signal to be transmitted in synchronization with the clock output from the clock generation circuit 16, and the data modulator 12
The continuous laser light from the laser 10 is digitally modulated by the data signal from the laser beam 4. The output light of the data modulator 12 is, for example, an NRZ signal, and is applied to the phase modulator 18. The phase modulator 18 is, for example, a lithium niobate (L
It is composed of an element using iNbO3) crystal. Lithium niobate crystals are ferroelectric crystals, and the refractive index changes when an electric field is applied. The change in the refractive index modulates the phase of light.

A band pass filter (BPF) 20 extracts a sine wave component of the bit rate of the transmission data from the clock output from the clock generation circuit 16 and applies the sine wave component to the phase shifter 22. The phase shifter 22 outputs the output of the BPF 20
The signal is delayed so as to be synchronized with the digital modulation in the data generation circuit 14 and the data modulator 12 and applied to the phase modulator 18 as a modulation signal. Of course, the functions of the bandpass filter 20 and the phase shifter 22 may be integrated together with the clock generation circuit 16 as a synthesizer.

The phase modulator 18 modulates the phase of the output light of the data modulator 12, that is, the digitally modulated light, in accordance with the sine wave signal from the phase shifter 22. That is, the phase modulator 18
The output light of the data modulator 12 is phase-modulated in synchronization with the bits of the data modulated light output from the data modulator 12. Specifically, the phase modulation is set to zero in the middle of the rise, fall, and pulse of the optical pulse, so that the blue shift occurs at the rise of the pulse and the red shift occurs at the fall.

FIG. 2 is a waveform example showing the relationship between the output light of the data modulator 12 and the phase modulation by the phase modulator 18. 2A shows the output light of the data modulator 12, and FIG. 2B shows the modulated signal of the phase modulator 18 (the output of the phase shifter 22).

The output light of the phase modulator 18 is applied to a low-speed polarization scrambler 24. The polarization scrambler 24 scrambles the polarization of the output light of the phase modulator 18 according to the drive signal from the drive circuit 26 so that the polarization modulator is not in a specific polarization state. The driving signal output from the driving circuit 26 has a frequency (which is usually lower than the bit rate of the digital modulation in the data modulator 12) and higher than several hundred Hz which is the response time of the optical fiber amplifier. About 2
kHz or more). By setting the rate of the drive signal output from the drive circuit 26 sufficiently lower than the bit rate of the digital modulation in the data modulator 12, the spread of the spectral line width can be suppressed. If the frequency of the drive signal output from the drive circuit 26 is around several hundred Hz, which is the response time of an optical fiber amplifier (for example, an erbium-doped optical fiber amplifier), the optical fiber amplifier is turned into a scrambled polarization. Although the gain follows and the fluctuation of the optical signal level increases, the gain of the optical fiber amplifier can be increased to the scrambled polarization by setting it sufficiently higher than the response time of the optical fiber amplifier of several hundred Hz. As a result, the influence of PHB can be reduced or eliminated.

The polarization scrambler 24 can be realized, for example, by injecting light into a phase modulation crystal at an angle of 45 ° and modulating the phase of the incident light. It can also be realized by winding a fiber and driving the piezo element with a sine wave. In this embodiment, since the polarization scrambler 24 may be operated at a low speed, the latter mechanical configuration can be used.

The effectiveness of this embodiment was confirmed by experiments.
FIG. 3 shows a spectrum of an optical signal modulated at 5 Gbps. This corresponds to the spectrum of the output light from the data modulator 12. The vertical axis indicates light intensity, and the horizontal axis indicates wavelength. 20d
The B bandwidth is about 0.28 nm.

FIG. 4 shows a case where the bit-synchronous polarization scrambling described in the above publication is applied.
2 shows a spectrum of an optical signal obtained by bit-synchronizing polarization scrambling of the light modulated by. The vertical axis indicates light intensity, and the horizontal axis indicates wavelength. The 20 dB bandwidth extends to about 0.5 nm. Therefore, when wavelength division multiplexing is performed at a narrow interval of about 0.5 nm, signal leakage easily occurs between adjacent wavelengths.

FIG. 5 shows the result of applying this embodiment.
The spectrum of an optical signal (specifically, output light of the polarization scrambler 24) modulated at 5 Gbps and further subjected to bit synchronous phase modulation and low-speed polarization scrambling is shown. The vertical axis indicates light intensity, and the horizontal axis indicates wavelength. The 20 dB bandwidth is about 0.29 nm, which is slightly wider than that in FIG. 3, because the bandwidth of the phase modulation is slightly wider than that of the simple data modulation. However, as compared with FIG. 4, the line width is sufficiently narrow, so that it is sufficiently applicable to high-density wavelength division multiplexing of about 0.5 nm.

In this embodiment, long-distance transmission characteristics of an optical signal can be improved by pre-chirping the optical signal by phase modulation synchronized with the bit. Since pure pre-chirp is provided by the phase modulator 18, unlike the bit-synchronous high-speed polarization scrambler described in the above publication, the expansion of the optical signal line width can be suppressed. This is because the modulation band of the phase modulation is significantly narrower than the modulation band associated with the bit synchronous high-speed polarization scrambling.

In this embodiment, the polarization scrambler 24
, Polarization hole scrambling reduces or eliminates polarization hole burning. The final degree of polarization is determined only by the polarization scrambler 24 and the phase modulator 1
8 is not affected. Therefore, in the present embodiment, the degree of polarization and the amount of pre-chirp can be independently adjusted and individually optimized.

In the above embodiment, the polarization scrambling is performed after the data modulation and the phase modulation. However, if the data modulator and the phase modulator have no polarization dependency, the data is modulated before the data modulation or before the phase modulation. Polarization scrambling may be performed. The digital modulation (data modulator 12) and the phase modulation (phase modulator 18) may be performed in reverse order as long as the phase modulation is synchronized with the bits of the digital signal light. That is, digital modulation using data to be transmitted after phase modulation may be performed. In this case, the phase modulation function can be built in the laser by using the DFB laser.

[0026]

As can be easily understood from the above description, according to the present invention, it is possible to improve the transmission characteristics, specifically, the waveform degradation and the transmission error rate while suppressing the expansion of the spectral line width, Alternatively, the high-density wave division multiplexing system can be applied to an ultra-long distance optical transmission system.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram of an embodiment of the present invention.

FIG. 2 is a waveform example showing a correspondence relationship between data modulation and phase modulation in the present embodiment.

FIG. 3 is an example of a spectrum of an optical signal modulated at 5 Gbps.

FIG. 4 is an example of a spectrum when an optical signal modulated at 5 Gbps is subjected to bit synchronous polarization scrambling.

FIG. 5 is a spectrum example when the present embodiment is applied to an optical signal modulated at 5 Gbps.

[Explanation of symbols]

 10: Laser 12: Data Modulator 14: Data Generation Circuit 16: Clock Generation Circuit 18: Phase Modulator 20: Band Pass Filter (BPF) 22: Phase Shifter 24: Polarization Scrambler 26: Drive Circuit

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/26 10/14 10/04 10/06 (72) Inventor Noboru Egawa 2-3-2 Nishishinjuku, Shinjuku-ku, Tokyo Kokusai Telegraph and Telephone Co., Ltd. (72) Inventor Shu Yamamoto 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Kokusai Telegraph and Telephone Co., Ltd. No. 2 International Telegraph and Telephone Corporation

Claims (8)

[Claims] 1. A laser light source, digital modulation means for digitally modulating input light, phase modulation means for phase modulating input light in synchronization with a modulation bit of the digital modulation means, and digital modulation in the digital modulation means. Polarization scrambling means for scrambling the polarization of the input light asynchronously with the digital modulation at a frequency sufficiently lower than the bit rate and sufficiently higher than the response frequency of the optical amplifying means; An optical signal output device, wherein the phase modulating means and the polarization scrambling means are connected in series in a predetermined order. 2. The optical signal output device according to claim 1, wherein said digital modulating means and said phase modulating means are arranged between said laser light source and said polarization scrambling means. 3. Digital modulation light output means for outputting digitally modulated laser light, phase modulation means for phase modulating input light in synchronization with digital modulation of the digital modulation light output means, A polarization scrambling means for scrambling the polarization of the input light asynchronously with the digital modulation at a frequency sufficiently lower than the bit rate of the digital modulation and sufficiently higher than the response frequency of the optical amplifying means. Characteristic optical signal output device. 4. The optical signal output device according to claim 3, wherein output light of said digital modulation light output means is input to said phase modulation means, and output light of said phase modulation means is input to said polarization scrambling means. 5. The optical signal output device according to claim 3, wherein output light of said digital modulation light output means is input to said polarization scrambling means, and output light of said polarization scrambling means is input to said phase modulation means. . 6. A phase modulation light output means for outputting continuous laser light phase-modulated at a predetermined phase modulation frequency; a digital modulation means for digitally modulating input light in synchronization with the predetermined phase modulation frequency; A polarization scrambling means for scrambling the polarization of the input light at a frequency sufficiently lower than the modulation frequency and sufficiently higher than the response frequency of the optical amplification means and asynchronously with the digital modulation of the digital modulation means. Characteristic optical signal output device. 7. The optical signal output device according to claim 6, wherein output light of said phase modulation light output means is input to said digital modulation means, and output light of said digital modulation means is input to said polarization scrambling means. 8. The optical signal output device according to claim 6, wherein output light of said phase modulation light output means is input to said polarization scrambling means, and output light of said polarization scrambling means is input to said digital modulation means. .