Patent Publication Number: US-2004052532-A1

Title: Optical transmission system and optical transmitter and methods thereof

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001] This application claims priority to Japanese Patent Application No. 2002-262337, filed Sep. 9, 2002, the entire contents of which are incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002] This invention generally relates to an optical transmission system, an optical transmitter and methods thereof and more specifically relates to an optical transmission system and optical transmitter and methods thereof using carrier suppressed return to zero (CS-RZ) signals.  
       BACKGROUND OF THE INVENTION  
       [0003] In the field of optical fiber communication, dense wavelength division multiplexing (DWDM) is used for expanding transmission capacity and has been developed for narrowing wavelength intervals.  
       [0004] As a method to realize the DWDM, single sideband (SSB) transmission or vestigial sideband (VSB) transmission has been proposed. This is described, for instance, in U.S. Pat. Nos. 6,088,147, 6,141,141, and 6,262,834, and US Patent Application Publication No. 2002/0075546.  
       [0005] A system to angle-modulate an optical-intensity-modulated signal with its carrier suppressed is described in U.S. Pat. No. 6,211,996.  
       [0006] In addition, the CS-RZ signal has characteristics of narrow bandwidth and therefore its application to the optical DWDM can is expected. For example, see Y. Miyamoto et al., “320 Gbit/s (8×40 Gbit/s) WDM transmission over 367 km zero-dispersion-flattened line with 120-km repeater spacing using carrier-suppressed return-to-zero pulse format”, Optical Society of America (OSA) Trends in Optics and Photonics (TOPS), vol. 30, Optical Amplifiers and Their Applications, (1999). The text of this paper is incorporated in this specification for reference.  
       [0007]FIG. 8 is a spectrum example of a CS-RZ signal light. By reversing phases of optical pulses locating adjacent timeslots, RZ pulse signal which optical carrier (having a frequency f0) is suppressed is generated as shown in FIG. 8. From this point of view, the CS-RZ signal is considered as a composition of two NRZ signals as shown in FIG. 9. In FIGS. 8 and 9, the horizontal axis expresses frequency (wavelength), and the vertical axis expresses optical intensity.  
       [0008] Conventionally, it has been considered that it is necessary to transmit the spectrum of the frequencies f1, f2 located on both sides of a carrier frequency f0 together in order to make a bandwidth of CS-RZ signal narrower as shown in FIG. 10 (See, for example, N. Yoshikane et al., “50 GHz-spaced 55×42.7 Gb/s transmission over 2500 km using a SPM-based all-optical reshaper”, Optical amplifiers and their applications topical meeting (OAA 2002), PDP6). FIG. 10 shows a spectrum example obtained by symmetrically extracting both upper spectrum components (having a frequency f1) and lower spectrum components (having a frequency f2) from the CS-RZ optical signals having optical spectrum shown in FIG. 8. In FIG. 10, the horizontal axis expresses frequency (wavelength) and the vertical axis expresses optical intensity.  
       [0009] When a bit rate of CS-RZ optical signal is expressed as B (bps), the following equations are obtained:  
         f 1 −f 0 =B/ 2  
         f 0 −f 2 =B/ 2.  
       [0010] However, a demand for even narrower bandwidth has been increasing to improve the transmission characteristics, to improve the WDM density and/or to extend transmission distance.  
       SUMMARY OF THE INVENTION  
       [0011] In the invention, a CS-RZ optical pulse train is data-modulated according to a data at a data rate (B) to generate modulated lights including a first component of a carrier frequency (f0), a second component of a frequency (f0−B/2) in which a half of the data rate (B/2) is subtracted from the carrier frequency (f0), and a third component of a frequency (f0+B/2) in which a half of the data rate (B/2) is added up to the carrier frequency (f0). A band limit filter extracts the first, second, and third components from the modulated light and outputs the extracted components to an optical fiber transmission line. Here, when a detune amount of the band limit filter relative to the modulated light is expressed as A and a filter bandwidth of the band limit filter is expressed as W, a condition expressed as 0&lt;A×B/W 2 &lt;0.225 is satisfied. More preferably, it should be 0.09&lt;A×B/W 2 &lt;0.2. The most preferable condition is that A×B/W 2  substantially becomes 1.5. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of explanatory embodiments of the invention in conjunction with the accompanying drawings, in which:  
     [0013]FIG. 1 is a schematic block diagram of a first explanatory embodiment according to the invention;  
     [0014]FIG. 2 is an optical spectrum example of output light from an optical bandpass filter  18 ;  
     [0015]FIG. 3 is a measured example showing the relationship between a detune amount of an optical bandpass filter and a Q 2  value;  
     [0016]FIG. 4 is an eye pattern example in a conventional symmetrical band limitation;  
     [0017]FIG. 5 is an eye pattern where Δλ=−0.04 nm according to the embodiment;  
     [0018]FIG. 6 is a measured example showing the relation between a detune amount of the optical bandpass filter  18  and an improved Q 2  amount;  
     [0019]FIG. 7 is a schematic block diagram of an explanatory embodiment of WDM transmitter;  
     [0020]FIG. 8 is an optical spectrum example of a CS-RZ signal;  
     [0021]FIG. 9 is an example of optical spectrum of two no-return-to-zero (NRZ) signals composed of the CS-RZ signal; and  
     [0022]FIG. 10 is an optical spectrum example in a conventional symmetrical band limitation. 
    
    
     DETAILED DESCRIPTION  
     [0023] Embodiments of the invention are explained below in detail with reference to the drawings.  
     [0024]FIG. 1 shows a schematic block diagram of an explanatory embodiment according to the invention. An optical transmitter  10  comprises a laser diode (LD)  12 , a CS-RZ pulse train shaper  14 , data modulator  16 , and an optical bandpass filter (O-BPF)  18  functioning as a band limit filter.  
     [0025] The LD  12  outputs continuous wave (CW) light having a wavelength λs (frequency f0). The CS-RZ pulse train shaper  14  generates a CS-RZ optical pulse train at a bit rate B out of the CW light from the LD  12 . Such apparatus is described in the above paper by Y. Miyamoto et al.  
     [0026] The data modulator  16  intensity-modulates the optical pulse train output from the CS-RZ pulse train shaper  14  according to a data D to be transmitted. For example, the data modulator 16 blocks the optical pulse output from the CS-RZ pulse train shaper  14  when the data D shows a binary digit “0” and transmits the optical pulse output from the CS-RZ pulse train shaper  14  when the data D shows a binary digit “1”.  
     [0027] The output light from the data modulator  16  comprises, for example, an optical spectrum shown in FIG. 9. The optical bandpass filter  18  functioning as a band limit filter transmits the output light from the data modulator  16  in such manner that both of the upper component of optical carrier frequency f0 and the upper component of frequency f1 (=f0+B/2) entirely pass through and a small amount of lower component of frequency f2 (=f0−B/2) pass through. FIG. 2 shows an optical spectrum example of the output light from the optical bandpass filter  18  relative to the optical spectrum shown in FIG. 8. The horizontal axis expresses wavelength (frequency), and the vertical axis expresses optical intensity. That is, in the conventional example, the center frequency of the optical bandpass filter is equalized to the optical carrier frequency f0 so that the upper component of frequency f1 and the lower component of frequency f2 equally transmit. However, in the embodiment, the center frequency of the optical bandpass filter  18  is shifted from the optical carrier frequency f0 so that the upper components (or the lower components) entirely pass through the filter and only a small amount of the lower components (or the upper components) pass through the filter.  
     [0028] The output light of the optical bandpass filter  18  enters an optical fiber transmission line  20  as the output light from the optical transmitter  10 . An optical receiver  22  receives the signal light propagated in the optical fiber transmission line  20  and restores the data D.  
     [0029] The influence by detune of the optical bandpass filter  18  functioning as a band limit filter has been experimented. FIG. 3 shows a measured result when the embodiment is applied to a 42.7 Gb/s CS-RZ optical signal. A fourth Gaussian filter was used as the optical bandpass filter  18 . The horizontal axis expresses the difference Δλ between the center wavelength of the optical bandpass filer  18  and the optical carrier wavelength of the CS-RZ optical signal and the vertical axis expresses Q 2  obtained after the signal propagated on a predetermined distance. The condition that Δλ=O shows a conventional example. As obvious from FIG. 3, in the band of the bandpass filter  18 , Q 2  is more improved on the condition that −0.055&lt;αλ&lt;0 compared to the case of Δλ=0. When Δλ=−0.04 (nm), Q 2  becomes maximum.  
     [0030] When the bandwidth of the optical bandpass filter  18  is set to another value, even if the component of frequency f2 is more reduced, a greater Q 2  value is obtained compared to the symmetrical band limiting. That is, in the embodiment, it is sufficient if the optical bandpass filter  18  passes at least one of the side-band components.  
     [0031]FIG. 4 shows an eye pattern where Δλ=0, namely an eye pattern in the symmetric band limiting. Here, an obtained result was Q 2 =23.74 (dB). FIG. 5 shows an eye pattern in the asymmetric band limiting (Δλ=−0.04 (nm)). Here, an obtained result was Q 2 =28.11 (dB).  
     [0032]FIG. 6 shows a measured result of the function of the optical bandpass filter  18  for a 42.7 Gbit/s CS-RZ optical signal. The horizontal axis expresses a value in which a detune amount A of the optical bandpass filter  18  relative to modulated light (the output from the data modulator  16 ) is normalized by a data rate B of a data D to be transmitted and a filter bandwidth W. Specifically, the horizontal axis is expressed as (A/W)×(B/W). The vertical axis expresses an improvement amount (dB) of Q 2  value compared to the Q 2  value obtained when the detune amount A is 0.  
     [0033] The filter bandwidth of the optical bandpass filter  18  is set to 45 GHz, 60 GHz, and 70 GHz. The solid line, the broken line, and the alternate long and short dash line show results of the 45 GHz case, the 70 GHz case, and the 60 GHz case respectively. It shows that regardless of the filter bandwidths, Q 2  is most improved when (A/W)×(B/W) becomes 0.15.  
     [0034] Within a range of 0&lt;A×B/W 2 &lt;0.225, Q 2  is improved compared to a conventional symmetry limitation (a detune amount is 0). Within a range of 0.09&lt;A×B/W 2 &lt;0.2, Q 2  is improved by 2 dB or more compared to a conventional symmetry limitation (a detune amount is 0).  
     [0035] In the case that the filter bandwidth is 45 GHz, although Q 2  becomes 19.9 dB at the detune amount of 0, it is improved to 23.5 dB when the detune is set to the optimum value. In the case that the filter bandwidth is 60 GHz, although Q 2  becomes 17.1 dB at the detune amount of 0, it is improved to 24.5 dB when the detune is set to the optimum value. In the case that the filter bandwidth is 70 GHz, although Q 2  becomes 19.5 dB at the detune amount of 0, it is improved to 24.5 dB when the detune amount is set to the optimum value.  
     [0036]FIG. 7 shows a schematic block diagram of an embodiment of WDM optical transmitter. In this embodiment, 2n wavelengths are multiplexed.  
     [0037] Laser diodes  30   1 - 30   2n-1  respectively output continuous wave (CW) lights of odd number wavelengths λ 1 -λ 2n-1 . CS-RZ pulse train shapers  32   1 - 32   2n-1  having the same configuration with the CS-RZ pulse train shaper  14  generate CS-RZ optical pulse trains having a bit rate B from the CW output lights from the laser diodes  30   1   30   2n-1  respectively. Data modulators  34   1 - 34   2n-1  intensity-modulate the optical pulse trains output from the CS-RZ pulse train shapers  32   1 - 32   2n-1  according to data D 1 -D 2n-1  to be transmitted. The functions of the CS-RZ pulse train shapers  32   1 - 32   2n-1  and the data modulators  34   1 - 13   2n-1  are identical to those of the CS-RZ pulse train shaper  14  and data modulator  16  respectively. An arrayed waveguide grating (AWG)  36   a  multiplexes the output lights from the data modulators 34 1 - 34   2n-1 . The arrayed waveguide grating (AWG)  36   a  functions as a multiplexer to wavelength-division-multiplex the signal lights of the odd number wavelengths λ 1 -λ 2n-1 . When the odd number wavelengths λ 1 -λ 2n-1  alone are multiplexed, it is not necessary to limit its bandwidth because spectra of adjacent wavelengths do not superimposed.  
     [0038] The configuration for even number wavelengths λ 2 -λ 2n  is basically the same to the above-described configuration for the odd number wavelengths λ 1 -λ 2n-1 . That is, the laser diodes  30   2 - 30   2n  respectively output continuous wave (CW) lights of the even number wavelengths λ 2 -λ 2n . CS_RZ pulse train shapers  32   2 - 32   2n  having the same configuration with the CS-RZ pulse train shaper  14  generate CS-RZ optical pulse trains having a bit rate B from the CW output lights from the laser diodes  30   2 - 30   2n  respectively. Data modulators  34   2 - 34   2n  intensity-modulate the optical pulse trains output from the CS-RZ pulse train shapers  32   2 - 32   2n  according to data D 2 -D 2n , to be transmitted. An arrayed waveguide grating (AWG)  36   b  multiplexes the output lights from the data modulators  34   2 - 34   2n .  
     [0039] The functions of the CS-RZ pulse train shapers  32   1 - 32   2n  and the data modulators  34   1 - 34   2n  are identical to those of the CS-RZ pulse train shaper  14  and data modulator  16  respectively. The arrayed waveguide grating  36   b  functions as a multiplexer to wavelength-division-multiplex the signal lights of the even number wavelengths λ 2 λ 2n . When the even number wavelengths λ 2 -λ 2n  alone are multiplexed, it is not necessary to limit its bandwidth because spectra of adjacent wavelengths do not superimposed.  
     [0040] A wavelength interleaver  38  multiplexes output lights from the arrayed waveguide gratings  36   a  and  36   b . The wavelength interleaver  38  comprises optical bandpass filtering characteristics. Therefore, by controlling center wavelengths in the optical bandpass filtering characteristics in the wavelength interleaver  38 , it is possible to apply a desired detune to both signal lights having the odd number wavelengths λ 1 -λ 2n-1  and even number wavelengths λ 2 -λ 2n  in the lump. That is, the wavelength interleaver  38  functions as the optical bandpass filter  18 .  
     [0041] As readily understandable from the aforementioned explanation, according to the invention, the transmission characteristics of a CS-RZ signal can be largely improved. Since the transmission characteristics are improved keeping a spectrum width per channel narrow, the multiplexing density of WDM can be increased and the transmission distance can be extended.  
     [0042] While the invention has been described with reference to the specific embodiment, it will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiment without departing from the spirit and scope of the invention as defined in the claims.