Multi-modulation-format compatible high-speed laser signal generation system and method

Abstract

The disclosure relates to a multi-modulation-format compatible high-speed laser signal generation system and method. The disclosure can greatly improve modulation format compatibility of a laser communication system and saves system upgrading costs and resource costs. The system mainly includes a control instruction unit, a serial-parallel changeover switch unit, a precoding switch unit, a power control unit, an IQ modulator, a narrow line width laser, and a bias point control unit, wherein the control instruction unit is respectively connected to the serial-parallel changeover switch unit, the precoding switch unit, the power control unit, and the bias point control unit; the serial-parallel changeover switch unit, the precoding switch unit, the power control unit, and the IQ modulator are electrically connected in sequence; the narrow line width laser is connected to an optical input end of the IQ modulator; and the bias point control unit is connected to the IQ modulator.

Claims

1. A multi-modulation-format compatible laser signal generation system, comprising: a control instruction unit, a serial-parallel changeover switch unit, a precoding switch unit, a power control unit, an IQ modulator, a narrow line width laser, and a bias point control unit, wherein the control instruction unit is electrically connected to the serial-parallel changeover switch unit, the precoding switch unit, the power control unit, and the bias point control unit; and the serial-parallel changeover switch unit, the precoding switch unit, the power control unit, and the IQ modulator are electrically connected in sequence, the narrow line width laser is connected to an optical input end of the IQ modulator, and the bias point control unit is electrically connected to the IQ modulator, wherein the control instruction unit is configured to specify a modulation format selected from intensity modulation (IM), binary phase shift keying (BPSK), differential phase shift keying (DPSK), quaternary phase shift keying (QPSK), and differential quaternary phase shift keying (DQPSK), and wherein the laser signal generation system is configured to perform IM, BPSK, DPSK, QPSK, and DQPSK.

2. The multi-modulation-format compatible laser signal generation system according to claim 1, further comprising a beam splitter connected to an optical output end of the IQ modulator, wherein, during operation, the beam splitter splits a laser into to two laser beams.

3. The multi-modulation-format compatible laser signal generation system according to claim 2, wherein the IQ modulator is a Mach-Zehnder modulator comprising two MZMs connected in parallel and a phase difference between an upper arm and a lower arm of IQ modulator is $\frac{\pi }{2},$ and; wherein the narrow line width laser is a 1550 nm band laser with a line width <100 KHz and the beam splitter is a 1550 nm band optical power beam splitter.

4. A method for generating a laser signal using a system that comprises a control instruction unit, a serial-parallel changeover switch unit, a precoding switch unit, a power control unit, an IQ modulator, a narrow line width laser, and a bias point control unit, the method comprising: [1] generating a modulation format instruction that specifies a modulation format of the laser signal; sending the modulation format instruction to flail the serial-parallel changeover switch unit, the precoding switch unit, the power control unit, and the bias point control unit; according to the specified modulation format, performing a serial-parallel changeover judgment on an input electrical signal in the serial-parallel changeover switch unit to produce a first electrical signal and sending the first electrical signal to the precoding switch unit; according to the specified modulation format, performing a precoding judgment on the first output electrical signal in the precoding switch unit to produce a second electrical signal, and sending the second electrical signal to the power control unit; according to the specified modulation format, performing power control on the second electrical signal in the power control unit so as to output a third electrical signal at a power level corresponding to the modulation format; and sending the third electrical signal to the IQ modulator, applying the third electrical signal to a laser signal from a narrow line width laser, and sending a control signal to the IQ modulator from the bias point control unit to cause the IQ modulator to perform modulation processing according to the specified modulation format, and outputting the laser signal.

5. The method according to claim 4, wherein, in the step, when the modulation format is QPSK or DQPSK, converting the input serial electrical signal into the first output electrical signal having two parallel electrical signals, and when the modulation format is IM, BPSK, or DPSK, transmitting the input serial electrical signal without conversion.

6. The method according to claim 4, wherein, in the step, when the modulation format is DPSK or DQPSK, performing differential precoding processing on the first output electrical signal; and when the modulation format is IM, BPSK or QPSK, transmitting the first output electrical signal without precoding.

7. A method according to claim 4, wherein the system further comprises a beam splitter connected to the IQ modulator, and the method further comprises: splitting the laser signal from by the IQ modulator into two laser beams in beam splitter, and sending one of the two laser beams to the bias point control unit.

8. The method according to claim 4, wherein the IQ modulator is a Mach-Zehnder modulator comprising two MZMs connected in parallel, and the method comprises adjusting the third electrical signal applied to the laser signal from a narrow line width laser so that a phase difference between an upper arm and a lower arm of IQ modulator is $\frac{\pi }{2}.$

9. The method according to claim 7, wherein the narrow line width laser is a 1550 nm band laser with a line width <100 KHz and the beam splitter is a 1550 nm band optical power beam splitter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of the principle of the present disclosure;

(2) FIG. 2a is a schematic diagram of the principle of an MZM;

(3) FIG. 2b is a modulation transmission curve diagram of an MZM; and

(4) FIG. 2c is a schematic diagram of the principle of an IQ modulator.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(5) Referring to FIG. 1, the present disclosure provides a specific embodiment of a multi-modulation-format compatible high-speed laser signal generation system, and the specific structure relationship is as follows.

(6) The system includes a control instruction unit, a serial-parallel changeover switch unit, a precoding switch unit, a power control unit, an IQ modulator, a narrow line width laser, a bias point control unit, and a beam splitter, wherein

(7) the control instruction unit is respectively connected to the serial-parallel changeover switch unit, the precoding switch unit, the power control unit, and the bias point control unit;

(8) the serial-parallel changeover switch unit, the precoding switch unit, the power control unit, and the IQ modulator are electrically connected in sequence; the narrow line width laser is connected to an optical input end of the IQ modulator; and the beam splitter, connected to an optical output end of the IQ modulator, divides a high-speed laser signal into two paths, where one path of signal is output, and the other path of signal is connected to the IQ modulator through the bias point control unit.

(9) In addition, what needs to be explained is that a small amount of high-speed laser signals are split by the beam splitter to perform closed-loop feedback control for a control unit in order to improve the control accuracy. In actual use, the beam splitter can also be eliminated, and the IQ modulator can be directly controlled by the bias point control unit.

(10) The IQ modulator in the embodiment is composed of two Mach-Zehnder modulators (MZM) connected in parallel in a Mach-Zehnder type, and there is a phase difference of

(11) $\frac{\pi }{2}$
between upper and lower arms; the narrow line width laser is a 1550 nm band laser with a line width ≤100 KHz; and the beam splitter is a 1550 nm band optical power beam splitter.

(12) What needs to be emphasized is that main technical indicators of the core component, i.e., the IQ modulator, in the system are as follows.

(13) As shown in FIG. 2a, when a Mach-Zehnder modulator (MZM) based on a lithium niobate (LiNbO3) crystal produces various code patterns, a refractive index of the LiNbO3 crystal changes on the basis of an electro-optic effect, and a phase size changes with the input voltage, as shown in the following equation:
E.sub.out=e.sup.jθE.sub.in,

(14) wherein E.sub.in is an input optical field, E.sub.out is an output optical field,

(15) $\theta =\pi \left(\frac{V_{in}}{V_{\pi }}\right),$
V.sub.in is a voltage value of an input electrical signal, and V.sub.π is an input voltage value when an optical signal is in π phase shift. The MZM divides an input optical wave into two beams with equal power, which are transmitted through two optical waveguides respectively. Since the two optical waveguides are made from electro-optical materials, refractive indexes of the optical waveguides change with a magnitude of an applied voltage, and further two beams of optical signals can have a phase difference at an output end. If an optical path difference between the two beams is an integer multiple of a wavelength, the output end will produce a coherent growth effect; and if the optical path difference of the two beams is a half-integer multiple of an optical wavelength, the output end will produce a coherent cancellation effect. Therefore, a signal can be modulated by controlling the magnitude of the applied voltage.

(16) Supposing that the input optical field E.sub.in(t) can be expressed as:
E.sub.in(t)=|E.sub.0|e.sup.jω.sup.c.sup.t,

(17) wherein E.sub.0 is an amplitude of the input optical field, and ω.sub.c is a frequency of an optical signal carrier, then an output optical signal E.sub.out(t) after modulation of the MZM can be expressed as:

(18) $E_{\mathrm{out}}\left(t\right)=\frac{E_{in}\left(t\right)}{2}\left[e^{j{\phi }_1\left(t\right)}+\gamma e^{i{\phi }_2\left(t\right)}\right],$

(19) wherein

(20) $\gamma =\frac{\sqrt{\delta }-1}{\sqrt{\delta }+1}$
is a splitting ratio of the modulator, and δ is a DC extinction ratio of the modulator. For an ideal modulator, δ is infinite, so γ≈1, and then an output optical signal can be written as:

(21) $E_{\mathrm{out}}\left(t\right)=E_{in}\left(t\right)\cos \left(\frac{{\phi }_1\left(t\right)-{\phi }_2\left(t\right)}{2}\right)e^{j\frac{{\phi }_1\left(t\right)+{\phi }_2\left(t\right)}{2}}.$

(22) If input voltage (i.e., driving voltages of two LiNbO3 phase modulators) signals of two modulator arms are V.sub.1(t) and V.sub.2(t), a phase change of the output optical field caused by phase modulation can be expressed as:

(23) $E_{\mathrm{out}}\left(t\right)=E_{in}\left(t\right)\cos \frac{\pi }{V_{\pi }}\left(\frac{V_1\left(t\right)-V_2\left(t\right)}{2}\right)e^{j\frac{\pi }{V_{\pi }}\left(\frac{V_1\left(t\right)+V_2\left(t\right)}{2}\right)}.$

(24) Supposing that ϕ is the optical phase difference introduced by the two modulator arms in the case of no driving voltage, an optical signal field strength equation after modulation of the MZM can be obtained as:

(25) $E_{\mathrm{out}}\left(t\right)=E_{in}\left(t\right)\cos \left[\frac{\pi }{V_{\pi }}\left(\frac{V_1\left(t\right)-V_2\left(t\right)}{2}\right)+\varphi \right]e^{j\frac{\pi }{V_{\pi }}\left(\frac{V_1\left(t\right)+V_2\left(t\right)}{2}\right)},$

(26) in addition, V.sub.1(t)=v.sub.1 cos(2πf.sub.0t), and V.sub.2(t)=v.sub.2 cos(2πf.sub.0t+δ), wherein v.sub.1 and v.sub.2 are amplitudes of the driving voltages, δ is relative phase delay, and f.sub.0 is a modulation frequency. In order to prevent chirp during a modulation process, it needs to suppose V.sub.1(t)=V.sub.2(t), which can be achieved by setting v.sub.1(t)=v.sub.2(t) and δ=π.

(27) From the above analysis, a modulation transmission curve of the MZM can be obtained. Referring to FIG. 2b, the transmission curve has a lowest point (null point, identified by A), an orthogonal point (identified by B), and a highest point (identified by C).

(28) Referring to FIG. 2c, the IQ modulator used in the system is composed of two MZMs connected in parallel in a Mach-Zehnder type, and there is a phase difference of

(29) $\frac{\pi }{2}$
between upper and lower arms.

(30) The Modulation Principle Using the IQ Modulator

(31) An IM format uses a carrier amplitude of a signal element to transmit binary information. The presence or absence of the carrier amplitude represents binary data “1” and “0”. In an IM modulation process, the binary data “1” and “0” are directly loaded to the carrier amplitude, and a phase between carriers carrying “1” and “0” does not change. Therefore, in the IM modulation process, only one MZM is needed, and a bias point of the MZM is controlled at an orthogonal point of the MZM, and the required power is V.sub.π.

(32) A BPSK modulation format uses an absolute carrier phase of a signal element to transmit binary information. Binary data are represented by “0” or “π” phase of two data bits, and the amplitude and frequency of a signal remain unchanged during the modulation process, but a phase of an optical carrier carrying the binary data is required to be “0” or “π”. When a bias point of the MZM is at a lowest point, an output optical signal will have an additional n phase shift compared to an input optical signal, but the power required at the lowest point is higher, being 2V.sub.π.

(33) A DPSK modulation format uses a relative phase, which is “0” or “π”, of a carrier between elements before and after a modulation signal to transmit binary information, which is similar to the BPSK modulation process, so that the MZM is also in a lowest point bias state, and the required power is 2V.sub.π. However, because the relative phase between the elements before and after the modulation signal, the binary data need to be differentially pre-coded during modulation. The pre-coding rule is d.sub.k=a.sub.k⊕d.sub.k-1, wherein a.sub.k is original data, d.sub.k is coded binary data, and ⊕ denotes an exclusive OR logic operation. The original data a.sub.k=d.sub.k⊕d.sub.k-1 can be decoded using the same rule at a receiving end. A pre-coded data sequence {d.sub.k} is modulated to a laser carrier to form an optical signal with the DPSK modulation format.

(34) A QPSK modulation format uses 4 different phases of a carrier to characterize input digital information. Initial phases of a signal are:

(35) 0$\frac{\pi }{4},\frac{3\pi }{4},\frac{5\pi }{4},\mathrm{and}\frac{7\pi }{4}.$
A QPSK signal is essentially an orthogonal linear combination of two paths of BPSK, so that two MZMs are required. Each MZM is in a lowest point bias state, and the required power is 2V.sub.π. Meanwhile, a phase difference between the two MZMs is

(36) $\frac{\pi }{2},$
and an IQ modulator can be used to replace the two parallel MZMs with

(37) $\frac{\pi }{2}$
phase difference.

(38) A DQPSK modulation format also uses a relative phase difference Δθ of elements before and after a modulation signal to represent digital information, and Δθ takes 4 different phase values 0,

(39) $\frac{\pi }{2},-\frac{\pi }{2},$
and π. Similarly, since the relative phase difference between the elements before and after the modulation signal is used, it is necessary to perform differential precoding first during modulation. The coding rules are:
u.sub.k=(I.sub.k⊕Q.sub.k)(I.sub.k⊕u.sub.k-1)+(I.sub.k⊕Q.sub.k)(Q.sub.k⊕v.sub.k-1)
v.sub.k=(I.sub.k⊕Q.sub.k)(I.sub.k⊕v.sub.k-1)+(I.sub.k⊕Q.sub.k)(Q.sub.k⊕u.sub.k-1)

(40) wherein ⊕ represents logical exclusive OR, I.sub.k∈(0,1) and Q.sub.k∈(0,1) are raw binary data, u.sub.k∈(0,1) and v.sub.k∈(0,1) are pre-coded binary data, and a DQPSK signal is essentially an orthogonal linear combination of two paths of DPSK, which is similar to the QPSK process.

(41) It can be seen from the above principle that the modulation of a BPSK communication system is similar to that of a DPSK communication system, and the difference is that DPSK requires differential coding of an original electrical signal before modulation. A QPSK communication system is composed of two orthogonal BPSK signals, and a DQPSK communication system is composed of two orthogonal DPSK signals. By using this corresponding relation, the generation of satellite-borne high-speed laser signals compatible with multiple communication systems based on a single IQ modulator can be realized.

(42) According to the description of a structure of the system of the present disclosure and the description of the technical principle of the IQ modulator, a high-speed laser signal generation method using the system is now introduced. The method includes the following steps:

(43) step [1] a control instruction unit selects among five modulation formats: IM, BPSK, DPSK, QPSK, and DQPSK according to communication requirements, and sends corresponding modulation format instructions to a serial-parallel changeover switch unit, a precoding switch unit, a power control unit, and a bias point control unit;

(44) step [2] after receiving the modulation format instruction provided in the step [1], the serial-parallel changeover switch unit performs serial-parallel changeover judgment and processing on an input high-speed electrical signal, wherein

(45) a specific process of the serial-parallel changeover judgment and processing is as follows:

(46) if the communication system is one of QPSK and DQPSK, one path of input serial high-speed electrical signal is converted into two paths of parallel electrical signals to be output, and

(47) if the communication system is one of IM, BPSK and DPSK, the input high-speed electrical signal is transparently transmitted;

(48) step [3] after receiving the modulation format instruction provided in the step [1], the precoding switch unit performs precoding judgment and processing on a high-speed electrical signal generated in the step [2], wherein

(49) a specific process of the precoding judgment and processing is as follows:

(50) if the communication system is one of DPSK and DQPSK, one path of input high-speed electrical signal is pre-coded based on “exclusive OR” logic and then output, and

(51) if the communication system is one of IM, BPSK and QPSK, the input high-speed electrical signal is transparently transmitted;

(52) step [4] after receiving the modulation format instruction provided in the step [1], the power control unit performs power control on a high-speed electrical signal generated in the step [3], if the received communication system instruction is IM, the power control unit causes an output electric signal power to be V.sub.z, and if the received communication system instruction is one of BPSK, DPSK, QPSK and DQPSK, the power control unit causes the output electric signal power to be 2V.sub.π, wherein V.sub.π is a half-wave voltage value of the IQ modulator;

(53) step [5] the IQ modulator loads a high-speed electrical signal generated in the step [4] to a 1550 nm band laser output by a narrow line width laser to form and output a 1550 nm band high-speed laser signal; and

(54) step [6] after receiving the modulation format instruction provided in the step [1], the bias point control unit uses the power of the high-speed laser signal generated in the step [5] as a reference value to perform feedback control on a bias point of the IQ modulator. Referring to FIGS. 2a-2c and Table 1 for the feedback control process.

(55) If the received modulation format instruction is IM, the bias point control unit causes an MZM1# in the IQ modulator to be at an orthogonal point B of the transmission curve, and an MZM2# is suspended;

(56) if the received modulation format instruction is one of BPSK and DPSK, the bias point control unit causes the MZM1# in the IQ modulator to be at a lowest point A of the transmission curve, and the MZM2# is suspended; and

(57) if the received modulation format instruction is one of QPSK and DQPSK, the bias point control unit causes the MZM1# and MZM2# in the IQ modulator to be at the lowest point A of the transmission curve; and the above bias point control process ensures that the IQ modulator outputs the 1550 nm band high-speed laser signal with good modulation performance in accordance with a specified modulation format.

(58) TABLE-US-00001 TABLE 1 Communication Bias Point Control Unit Power Control No. System Value Unit Value 1 IM MZM1#-orthogonal point V.sub.π MZM2#-suspended 2 BPSK MZM1#-lowest point 2V.sub.π MZM2#-suspended 3 DPSK MZM1#-lowest point 2V.sub.π MZM2#-suspended 4 QPSK MZM1#-lowest point 2V.sub.π MZM2#-lowest point 5 DQPSK MZM1#-lowest point 2V.sub.π MZM2#-lowest point

(59) It can be seen that through combination of different control parameters, generation of a high-speed laser signal compatible with five communication systems of IM, BPSK, DPSK, QPSK and DQPSK is realized.

(60) The above embodiments show that the multi-modulation-format compatible small lightweight high-speed laser signal transmission solution provided by the present disclosure realizes the generation of the high-speed laser signal compatible with five modulation formats of IM, BPSK, DPSK, QPSK and DQPSK. By the combination of different control parameters, the sharing rate of core devices is greatly increased, the modulation format compatibility of the high-speed laser communication system is expanded, and the complexity of system implementation and the number of devices are effectively reduced, not only can the compatibility and scalability of the system be ensured, but also the existing system upgrading costs and resource costs are saved.