This invention relates to an optical transmission system, and more particularly, to a configuration of an optical field transmitter suitable for transmission/reception of an optical multilevel signal transmitted via an optical fiber.
The amount of information that can be transmitted (transmission capacity) via one optical fiber has reached capacity of performance of an optical fiber amplifier because a wavelength bandwidth of the optical fiber amplifier has been almost used up owing to an increase in number of wavelength channels and a speedup of a modulation speed of an optical signal. In order to further expand the transmission capacity of the optical fiber, it is necessary to enhance use efficiency of the frequency bandwidth by devising a signal modulation format so that a large number of optical signals are packed in a limited frequency bandwidth.
In the world of radio communications, since 1960s, a multilevel modulation technology has realized transmission at such high efficiency that frequency use efficiency exceeds 10. There have conventionally been many studies of multilevel modulation which is regarded as promising also in the field of optical fiber transmission. For example, Non-patent Document 1 discloses quadrature phase shift keying (QPSK) for performing four-level phase modulation. In addition, Non-patent Document 2 discloses 32-level amplitude and phase modulation that is a combination of four-level amplitude modulation and eight-level phase modulation.
FIGS. 1A to 1D are explanatory diagrams illustrating a signal constellation of various known modulation formats on a complex plane used for the optical transmission. On the complex plane (complex plane, phase plane, IQ plane), there are plotted signal points of the various optical multilevel signals (complex display of an optical field at a decision timing).
FIG. 1A is an explanatory diagram for a signal point on the IQ plane. Each of the signal points may be displayed by complex Cartesian coordinates (IQ coordinates) or polar coordinates represented by an amplitude r(n) and a phase φ(n) illustrated in FIG. 1A.
FIG. 1B illustrates the four-level phase modulation (QPSK) in which four values (0, /2, π, and −π/2) are used as the phase angle φ(n) and two-bit information (00, 01, 11, 10) is transmitted per symbol.
FIG. 1C illustrates sixteen-level quadrature amplitude modulation (16QAM) widely used in radio communications. The 16QAM, in which signal points are arranged in lattice, allows four-bit information to be transmitted per symbol. In the example of FIG. 1C, the Q-axis coordinate represents a value of upper two bits (10xx, 113x, 01xx, 00xx), and the I-axis coordinate represents a value of lower two bits (xx10, xx11, xx01, xx00). It is known that in this signal constellation, a distance between the signal points increases to enhance the receiver sensitivity. It is reported that in optical communications, the quadrature amplitude modulation of this type can be realized by using a coherent optical receiver. For example, Non-patent Document 3 reports an experimental example of transmission/reception of a 64QAM signal using the coherent optical receiver. The coherent optical receiver uses a local light source disposed within the receiver in order to detect the phase angle of the optical signal.
Similarly, FIG. 1D illustrates sixteen-level amplitude and phase modulation (16APSK) widely used in radio communications.
Here, description is made of a coherent reception format which is one of conventional technologies for an optical multilevel receiver, for example, a coherent optical field receiver disclosed in Non-patent Document 4. FIG. 2 is a block diagram illustrating a configuration of a coherent optical field receiver of a polarization diversity type, which simultaneously receives information on two polarizations of the optical signal. The optical multilevel signal transmitted through an optical fiber transmission line as an input optical signal 101 is split into a horizontal (S) polarization component 105 and a vertical (P) polarization component 106 by a polarization beam splitter 102-1, which are input to coherent optical field receivers 100-1 and 100-2, respectively.
In the coherent optical field receiver 100-1, a local laser source 103 having a wavelength substantially the same as the input optical signal 101 is used as a reference of an optical phase. Local light 104-1 output from the local laser source 103 is split into two beams of local light 104-2 and local light 104-3 by a polarization beam splitter 102-2, which are input to the coherent optical field receivers 100-1 and 100-2, respectively.
Inside the coherent optical field receiver 100-1, an optical phase diversity circuit (PDC) 107 combines the S polarization component 105 of the input optical signal and the local light 104-2. The optical phase diversity circuit 107 generates an I (inphase) component output light 108 including an inphase component of the local light and the optical multilevel signal, and a Q (quadrature) component output light 109 including a quadrature component of the local light and the optical multilevel signal. Both the I component output light 108 and the Q component output light 109 are received by balanced optical receivers 110-1 and 110-2, respectively. The received optical signals are converted into electrical signals, which are then time-sampled by A/D converters 111-1 and 111-2 to become digitized output signals 112-1 and 112-2, respectively.
In the following description, as illustrated in FIG. 1A, the optical field of the received optical multilevel signal 101 is represented as r(n)exp(jφ(n)), and the optical field of the local light 104-2 and the local light 104-3 is assumed to be 1 (originally, an optical frequency component is included, but the optical frequency component is omitted). Here, “r” represents an amplitude of the optical field, “φ” represents a phase of the optical field, and “n” represents a sampling number. The local light 104-2 and the local light 104-3 actually have random phase noise and a slight difference frequency component with respect to signal light. However, the phase noise and the difference frequency component exhibit temporally slow phase rotation, and may be eliminated by a digital signal processing. Therefore, the phase noise and the difference frequency component are ignored.
Each of the balanced optical receivers 110-1 and 110-2 performs homodyne detection on the input optical multilevel signal 101 with the local light 104-2, and outputs an inphase component and a quadrature component, respectively, of the optical field of the optical multilevel signal 101 by taking the local light as a reference. Therefore, the electrical signal 112-1 output from the A/D converter 111-1 is represented by Expression (1), and the electrical signal 112-2 output from the A/D converter 111-2 is represented by Expression (2). However, for simplification, constants including a conversion factor are all set to “1”.I(n)=r(n)cos(φ(n))  (1)Q(n)=r(n)sin(φ(n))  (2)
As described above, the coherent optical field receiver can easily obtain all information pieces indicating the optical field r(n)exp(φ(n)) (both I component and Q component) from the received optical multilevel signal 101, thereby allowing the optical multilevel signal reception.
A digital signal processing circuit 113, which is a complex field signal processing circuit, gives an inverse function to linear degradation (for example, chromatic dispersion) or the like exerted upon the optical signal during transmission, to thereby enable cancellation of influences thereof. Further, processings such as retiming and resampling are performed to output optical field components after the processings, that is, an inphase component 114-1 and a quadrature component 114-2.
As described above, the coherent optical field receiver 100-1 can obtain field information on the S polarization component of the input optical signal 101, but needs to receive the P polarization component as well because a polarization state of the optical signal changes at random during the optical fiber transmission. Therefore, the coherent optical field receiver 100-2 receives the P polarization component of the optical multilevel signal 101 in the same manner, and outputs the field information thereon as output signals 114-3 and 114-4.
A digital signal processing/symbol decision circuit 115 resolves the change of the polarization state by subjecting the above-mentioned I component and Q component of the respective polarizations output from the digital signal processing circuit 113 to conversion of the polarization state. Subsequently, the digital signal processing/symbol decision circuit 115 decides which symbol has been transmitted with high precision by comparing, for example, the signal constellation illustrated in FIG. 1C and the I component and the Q component of the respective polarizations. A decision result thereof is output as a multilevel digital signal 116.
By using the coherent optical field receivers described above, it is possible to obtain all the field information pieces on the received signal, which allows, in principle, any complicated multilevel signal to be received. However, the above coherent optical field receiver suffers from such a problem that the configuration of the receiver is extremely complicated and expensive. That is, there is a problem, among others, that because a local light emission source is disposed within the receiver, and a diversity configuration that receives both polarizations of S and P is provided, the scale of the receiver is doubled or more.
On the other hand, FIG. 3 is a block diagram illustrating a configuration of a phase pre-integration optical multilevel signal transmission system previously proposed by the inventors of this invention, which addresses the problem to be solved by the invention. This system easily realizes the optical multilevel transmission using optical delay detection with no local light emission source.
An unmodulated laser beam output from a laser source 210 is input to an optical field modulator 211 within a phase pre-integration optical field transmitter 200, and an optical field signal 213 subjected to required electric field modulation is output from an output optical fiber 212. An information signal to be transmitted is input to a digital information input terminal 201 as a parallel (for example, m-bit width) binary high-speed digital electric signal string. The input signal is converted into a complex multilevel information signal 203 within a complex multilevel signal generator circuit 202. The converted signal is a digital electric multilevel signal represented as (i, q) on a two-dimensional IQ plane, and a real part i and an imaginary part q of the signal are output every time interval T (=symbol time).
The converted signal is input to a phase pre-integration unit 204. The phase pre-integration unit 204 digitally integrates only a phase component of the input signal with the time interval T, to thereby convert the input signal into a phase pre-integration complex multilevel information signal 205. When the input complex multilevel information signal 203 (i, q) is converted into polar coordinates on the complex plane, the signal can be represented by, for example, Expression (3) (j is an imaginary part unit). In this expression, n is a symbol number of the digital signal, r(n) is a symbol amplitude of the digital signal, and φ(n) is a phase angle.Ei(n)=i(n)+jq(n)=r(n)exp(jφ(n))  (3)
In this expression, the phase pre-integrated signal to be output can be represented in polar coordinates by Expression (4).Eo(n)=i′(n)+jq′(n)=r(n)exp(jθ(n))=r(n)exp(jΣφ(n))  (4)
In this expression, θ(n) is a phase angle of the output signal, and Σφ(n) is a value obtained by accumulating past phase angles φ(1) . . . φ(n).
The output signal is again converted into a Cartesian coordinate system, and then output as the phase pre-integration complex multilevel information signal 205. This signal is input to the sampling speed conversion circuit 206, and complements the sampling points so that the sampling speed becomes 2 samples/symbol or more. As a result, the Nyquist theorem is satisfied, and complete field equalization is enabled. Thereafter, an inverse function of degradation developed in an optical transmission line 214 by a preequalization circuit 207 is applied to the phase pre-integration complex multilevel information signal, and then divided into a real part i″ and an imaginary part q″. The divided signals are converted into high-speed analog signals by respective DA converters 208-1 and 208-2.
Those two analog signals are amplified by driver circuits 209-1 and 209-2, and then input to two modulation terminals I and Q of the optical field modulator 211. As a result, the optical field signal 213 can be generated with the preequalization phase integrated signals (i″(n), q″(n)) in the in-phase component I and the quadrate component Q of the optical field. The optical field of the optical field signal 213 is (i″(n)+jq″(n))esp(jω(n)), and ω(n) is an optical angular frequency of the laser source 210. That is, the optical field signal 213 is (i″(n), q″(n)) in the vicinity of the equalization low band where the optical frequency component is removed.
The optical field signal 213 is transmitted through the optical fiber transmission line 214, subjected to transmission degradation by chromatic dispersion of the optical fiber, and thereafter input to an incoherent optical field receiver 22 as a received optical field signal 221. The transmission degradation is mutually canceled by the inverse function applied by the preequalization circuit 207 in advance, and therefore the optical field of the receive signal is equal to the phase pre-integration complex multilevel information signal 205.
The received optical field signal 221 is split into three optical signal paths by an optical splitter 222. The split optical signals are input to a first optical delay detector 223-1, a second optical delay detector 223-2, and an optical intensity detector 225. The first optical delay detector 223-1 is set so that one delay time Td of two optical paths is substantially equal to a symbol time T of the received optical multilevel information signal, and so that a difference of optical phase between the optical paths becomes 0. Further, the second optical delay detector 223-2 is set so that one of two paths has a delay time Td=T, and so that an optical differential phase between those paths becomes π/2.
Two output lights of the first and second optical delay detectors 223-1 and 223-2 are converted into electric signals by balanced optical detectors 224-1 and 224-2, respectively. Thereafter, the converted electric signals are converted into digital signals dI(n) and dQ(n) by A/D converters 226-1 and 226-2, respectively. Further, an electric signal output from the optical intensity detector 225 is also converted into a digital signal P(n) by an AD converter 226-3.
Thereafter, the digital signals dI(n) and dQ(n) are input to an inverse tangential circuit 227. The inverse tangential circuit 227 conducts inverse tangent operation of two arguments with dI(n) as an X component and dQ(n) as a Y component, and calculates an phase angle of the digital signals dI(n) and dQ(n).
When the optical field of the received optical field signal 221 is described as r(n)exp(jθ(n)), dI can be represented by Expression (5) on the basis of the principle of the optical delay detection.dI=r(n)r(n−1)cos(Δθ(n)), dQ=r(n)sin(Δθ(n))  (5)
In this expression, Δθ(n) is a differential phase (θ(n)−θ(n−1)) from a symbol immediately before a received n-th optical field symbol. Because dI and dQ are a sine component and a cosine component of Δθ(n), respectively, the inverse tangential circuit 227 conducts inverse tangential (arc tan) operation of four quadrants so as to calculate Δθ(n).
In this configuration, because the phase preintegration is conducted at the transmit side as described above, a phase angle of the received optical field signal can be represented by Expression (6).θ(n)=Σφ(n)  (6)
Hence, an output signal of the inverse tangential circuit 223 can be represented by Expression (7), and a phase component φ(t) of the original complex multilevel information signal 203 can be extracted.Δθ(n)=Σφ(n)−Σφ(n−1)φ(n)  (7)
On the other hand, an output signal P of the optical intensity detector is input to a square root circuit 228 so as to obtain an original electric field amplitude represented by Expression (8) as an output.r(n)=sqrt(P(n))  (8)
For that reason, the obtained amplitude component r(n) and phase component φ(n) are input to a Cartesian coordinate converter circuit 229 so as to reproduce an original digital electric multilevel signal 230 represented by Expression (9) from a reproduced complex information output terminal 225.(i,q)=r(n)exp(Δθ(n))  (9)
FIGS. 4A and 4B are explanatory diagrams of signal constellation in the phase preintegration transmission system.
For example, as the complex multilevel information signal 203 in FIG. 3, a 16QAM signal illustrated in FIG. 4A can be used. The 16QAM signal has 16 signal points a to p in the figure, and the respective points can be represented by Expression (10) as described above.Ei(n)=r(n)exp(jφ(n))  (10)
Those signal points are generated in a random order, and hence the phase integrated signal of the 16QAM signal has triple concentric circles having various phase angles Σφ(n) as illustrated in FIG. 4B. This is because in the phase integrated signal, the signal points of the original 16QAM signal have three amplitude levels (outermost peripheral points (a, d, m, p), intermediate points (b, c, h, l, o, n, e, i), and innermost peripheral points (f, g, k, j)).
FIG. 5 is a block diagram illustrating configurations of the complex multilevel signal generator circuit 202 and the phase pre-integration unit 204 in the conventional phase pre-integration optical field transmitter 200 of FIG. 3 in more detail.
The complex multilevel signal generator circuit 202 allocates a complex multilevel information signal to the input binary high-speed digital signal string of the m-bit width. For example, in the case of m=4 bits, the information signal has 2^4=16 states. For that reason, the complex multilevel signal generator circuit 202 allocates an input signal to any one of 16 points a to p of FIG. 4A, and outputs the Cartesian coordinates (i, q) as the complex multilevel information signal 203. The complex multilevel information signal 203 is input to a polar coordinate converter circuit 240 in the phase pre-integration unit 204, and converted into an amplitude information signal 241 represented by Expression (11) and a phase information signal 242 represented by Expression (12).r(n)=sqrt(i^2+q^2)  (11)φ(n)=arctan(q,i)  (12)
Subsequently, the phase information signal 242 is input to a phase pre-integration circuit 243. The phase pre-integration circuit 243 includes a delay circuit 249 with a delay time T and an adder circuit 248. The phase pre-integration circuit 243 repeats the operation of adding the input digital phase signal φ(n) to an integrated value Σφ(n−1) delayed by a time T to repeat the operation of obtaining the integrated phase 244 (Σφ(n)). Then, a phase pre-integration information signal 245 of polar coordinates which is new complex information having the amplitude value r(n) as the amplitude component and the phase integrated value Σφ(n) as the phase component is configured. Thereafter, this signal is input to a Cartesian coordinate converter circuit 246, and again converted into a phase pre-integration information signal 247 (i′, q′) of the Cartesian coordinate display.
Non-patent Document 1: R. A. Griffin, et. al., “10Gb/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs/A1GaAs Integration,” OFC2002, paper PD-FD6, 2002
Non-patent Document 2: N. Kikuchi, K. Mandai, K. Sekine and S. Sasaki, “First experimental demonstration of single-polarization 50-Gbit/s 32-level (QASK and 8-DPSK) incoherent optical multilevel transmission,” in Proc. Optical Fiber Communication Conf. (OFC/NFOEC), Anaheim, Calif., Mar. 2007, PDP21.
Non-patent Document 3: J. Hongou, K. Kasai, M. Yoshida and M. Nakazawa, “1 Gsymbol/s, 64 QAM Coherent Optical Transmission over 150 km with a Spectral Efficiency of 3 Bit/s/Hz,” in Proc. Optical Fiber Communication Conf. (OFC/NFOFEC), Anaheim, Calif., Mar. 2007, paper OMP3.
Non-patent Document 4: M. G. Taylor, “Coherent detection method using DSP to demodulate signal and for subsequent equalization of propagation impairments,” paper We4.P.111, ECOC 2003, 2003