Optical frequency comb generator and method based on heterodyne phase locking

Abstract

The application discloses a kind of optical frequency comb generators and methods based on heterodyne phase-locked, including first laser, second laser, first Mach-Zehnder modulator, second Mach-Zehnder modulator, radio frequency signal generator, optical coupler, the radio frequency signal generator is connected the first Mach-Zehnder modulator and the second Mach-Zehnder modulator respectively, laser that the first laser emits is sent to the optical coupler after being modulated by the first Mach-Zehnder modulator, laser that the second laser emits is directly sent to the optical coupler, the optical coupler is coupled after sending to the second Mach-Zehnder modulator after receiving optical signal, generates optical frequency comb after being modulated by the second Mach-Zehnder modulator.The application has high bandwidth, simple and efficient and economic.

Description

Technical Field

[0001] This invention relates to optical frequency comb generation technology, and more particularly to an optical frequency comb generator and method based on heterodyne phase-locked loop. Background Technology

[0002] In recent years, optical frequency combs (OFCs) have played an important role in optical meteorology, spectroscopy, astronomy, and optical communications, possessing a flat spectral envelope and high equal-frequency spacing. Different laser sources can provide numerous techniques and mechanisms for generating OFCs, and each OFC has different applications. Among the various techniques for OFC generation, OFCs using electro-optic modulators (EOs) exhibit flexibility in repetition rate, making them a suitable choice for optical communications. Optical-optic modulator-based OFCs improve system performance, particularly supporting high-data-rate optical access and long-haul networks. However, existing OFCs suffer from narrow bandwidth, complex structure, and high cost. Summary of the Invention

[0003] Purpose of the invention: This invention addresses the problems existing in the prior art by providing a high-bandwidth, simple, efficient, and economical optical frequency comb generator and method based on heterodyne phase-locked loop.

[0004] Technical Solution: The optical frequency comb generator based on heterodyne phase-locked loop of the present invention includes a first laser, a second laser, a first Mach-Zehnder modulator, a second Mach-Zehnder modulator, a radio frequency signal generator, and an optical coupler. The radio frequency signal generator is connected to the first Mach-Zehnder modulator and the second Mach-Zehnder modulator respectively. The laser emitted by the first laser is modulated by the first Mach-Zehnder modulator and then sent to the optical coupler. The laser emitted by the second laser is directly sent to the optical coupler. The coupler couples the received optical signal and sends it to the second Mach-Zehnder modulator. After being modulated by the second Mach-Zehnder modulator, an optical frequency comb is generated.

[0005] Furthermore, the frequency difference between the lasers emitted by the first laser and the second laser is a preset value. Both the first laser and the second laser are continuous wave lasers. The radio frequency signal generator is used to emit a radio frequency signal with a 90° phase and transmit it to the first Mach-Zehnder modulator and the second Mach-Zehnder modulator. The optical coupler is used to directly superimpose the signal output by the first Mach-Zehnder modulator and the laser emitted by the second laser.

[0006] The optical frequency comb generation method based on heterodyne phase-locked loop described in this invention includes:

[0007] The first laser is used to emit the laser;

[0008] The laser emitted by the first laser is modulated by a first Mach-Zehnder modulator, which is driven by a radio frequency signal emitted by a radio frequency signal generator.

[0009] A second laser is used to emit the laser;

[0010] An optical coupler is used to couple the laser emitted by the second laser with the optical signal modulated by the first Mach-Zehnder modulator. The modulation of the second Mach-Zehnder modulator is driven by the radio frequency signal emitted by the radio frequency signal generator.

[0011] The optical signal output from the optical coupler is modulated using a second Mach-Zehnder modulator, and the modulated signal is the optical frequency comb.

[0012] Furthermore, the frequency difference between the lasers emitted by the first laser and the second laser is a preset value. Both the first laser and the second laser are continuous wave lasers. The radio frequency signal emitted by the radio frequency signal generator has a phase of 90°. The optical coupler directly superimposes the optical signal output from the first Mach-Zehnder modulator and the laser emitted by the second laser.

[0013] Beneficial Effects: Compared with existing technologies, the significant advantages of this invention are: This invention is a coherent optical communication system for downlink transmission, employing heterodyne phase-locked loop to generate two optical frequency combs. These two frequency combs are coherently connected in a novel manner, ultimately resulting in a high-bandwidth optical frequency comb covering more than half of the optical C-band. High-bandwidth frequency coverage in the optical C-band is essential for expanding the application boundaries of optical communication and networks. The device of this invention is simple, efficient, and more economical. Attached Figure Description

[0014] Figure 1 This is a structural block diagram of the optical frequency comb generator based on heterodyne phase-locked loop of the present invention;

[0015] Figure 2 yes Figure 1 The output results of the optical spectrometers are shown in the following figures: (a) output observed by optical spectrometer a; (b) output observed by optical spectrometer b; (c) output observed by optical spectrometer c. The enlarged view on the right shows detailed information on the flatness of the comb lines. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Example 1

[0018] This embodiment provides an optical frequency comb generator based on heterodyne phase-locked loop, such as... Figure 1 As shown, the system includes a first laser L1, a second laser L2, a first Mach-Zehnder modulator MZ1, a second Mach-Zehnder modulator MZ2, a radio frequency (RF) signal generator (RFG), and an optical coupler Cp. The RFG is connected to both the first and second Mach-Zehnder modulators MZ1 and MZ2, respectively, and is used to drive the two modulators with a generated 90° phase RF signal. The laser emitted by the first laser L1 is modulated by the first Mach-Zehnder modulator MZ1 and then sent to the optical coupler Cp. The laser emitted by the second laser L2 is directly sent to the optical coupler Cp. The optical coupler Cp superimposes and couples the received optical signals before sending them to the second Mach-Zehnder modulator MZ2, where they are modulated to generate an optical frequency comb.

[0019] The frequency difference between the two lasers is set to a preset value, specifically 1.38 THz. Each laser generates a laser signal, and the optical signal sidebands are coupled to an adjacent carrier wave via an optical coupler. The coupling of adjacent frequency components of the laser signals generated by the two lasers is considered a heterodyne phase-locked loop (PLL) mechanism. This is because the heterodyne concept involves mixing two frequencies to combine two in-phase signals to generate a frequency. This is then passed through a second Mach-Zehnder modulator (MZ2) to generate the maximum possible OFC line. The resulting OFC covers a bandwidth of approximately 3.2 THz, with 2.74 THz occupied by a flat combline located in the optical C-band. The radio frequency (RFG) generator operates at 20 GHz with a 90° phase, used to delineate the frequency interval between the comblines, thereby generating a signal with... Figure 1 The aforementioned OFC line of excellent quality.

[0020] Let the optical field of the first laser L1 be defined as E L:1 =E1cosω0t, the light field of the second laser L2 is defined as E L:2 =E2cosω0t, where E1,2 represents the amplitude of the laser-emitted light field, and ω0 is the angular frequency of the carrier signal. Similarly, the RF signal generated by the RF signal generator RFG is used to drive the Mach-Zehnder modulator, and can be given as V RF (t)=V1cosω c t, where V1 represents the amplitude, ω c This represents the corresponding frequency of the RF signal. The output of the first Mach-Zehnder modulator MZ1 can be denoted as...

[0021]

[0022] In the formula, ω0 and ωc These are the angular frequencies of the carrier wave and the radio frequency signal, respectively. The phase shift is caused by the bias voltage, γ1=πV1 / V π1 It is the modulation index of two Mach-Zehnder modulators, V π1 This is the half-wave voltage of MZ1, and the same applies to MZ2. After coupling the optical signal emitted by the second laser L2 with the output of MZ1, the resulting combined signal is: After introducing it into MZ2, the final output is:

[0023]

[0024] Expanding equation (2) using the Jacobian expansion yields the final output:

[0025]

[0026] In equation (3), Jn represents the nth order of the Bessel function, n represents the order of the sideband induced by the connected optical frequency comb, and the harmonic components have a fixed frequency interval ω with the frequency generated by the microwave source. c / 2π, it can be deployed in the field of optical communication to support high data rates. The final output optical power is

[0027]

[0028] In equation (4), P0 is the power of the photocarriers, and N, M are the order of the sidebands. The synthesis of many modulation sidebands gives the intensity of the nth comb line. Since the saturated input power limits the driving oscillation signal of the intensity modulator, the modulation index of the Mach-Zehnder modulator is fixed. Therefore, the intensity of each line can be affected by the phase difference between the lines, the modulation index, and the DC bias of the Mach-Zehnder modulator, which also controls the smoothness.

[0029] The invention is then verified through simulation. The center frequencies of lasers L1 and L2 are F1 = 193.08 THz and F2 = 194.46 THz, respectively. Each laser has an operating power of 1.3 dBm and a linewidth of 10 MHz. Figure 1The generator shown deploys three optical spectrum analyzers labeled a, b, and c: one for the output of MZ1 (a), one for the output coupled to laser L2 (b), and one for the final output (c). Laser L2 is connected to the band generated by laser L1. Therefore, based on the heterodyne phenomenon of mixing two frequencies (F1, F2), many comb lines are generated, which can be given as: F1 + F2 + N(F1 ± F2). L1 is connected to MZ1, driven by a 20 GHz RF signal, and the symmetry factor and extinction ratio of MZ1 are carefully controlled. The Mach-Zehnder modulator is a photoelectric intensity modulator based on the interference principle, consisting of a pair of 3 dB couplers connected to a waveguide of the same length. At the output of MZ1, approximately 45 comb lines are generated, of which 35 tones are very stable with an amplitude difference of 0.1 -6 dBm, such as... Figure 2 As shown in (a), the OFC generated by MZ1 is coupled to the second laser through an optical coupler Cp with a coupling coefficient of 0.5, and the output is as follows. Figure 2 As shown in (b), after coupling the two signals, a wide frequency band is achieved with over 160 comb lines, 137 of which are very flat and can be deployed in optical communications. The lowest frequency of the comb line is 192.26 THz when the TNR is above 40 dBm, while the highest frequency is 194.98 THz. Therefore, the realized bandwidth of the comb is approximately 2.74 THz, covering most of the C-band optics. Currently produced OFCs exhibit an amplitude difference of 0.1–6 dB between the high-frequency and low-frequency comb lines, and 0.1–3 dB for the rest. Detailed information on comb line flatness is as follows... Figure 2 As shown on the right. In summary, by adding one laser beam, the TNR value of the generated DEO comb exceeds 40 dB. On the other hand, if a single laser source is configured with two identical or different cascaded modulators, the number of carriers will be around 60 or less. Introducing a second laser source at the optical coupler not only helps to provide more bandwidth but also helps to generate more comb lines. This scheme can generate 137 healthy and stable carriers with an effective bandwidth of 2.74 THz.

[0030] Example 2

[0031] Figure 2 yes

[0032] This embodiment provides a method for generating optical frequency combs based on heterodyne phase-locked loops, including:

[0033] The first laser is used to emit the laser;

[0034] The laser emitted by the first laser is modulated by a first Mach-Zehnder modulator, which is driven by a radio frequency signal emitted by a radio frequency signal generator.

[0035] A second laser is used to emit the laser;

[0036] An optical coupler is used to couple the laser emitted by the second laser with the optical signal modulated by the first Mach-Zehnder modulator. The modulation of the second Mach-Zehnder modulator is driven by the radio frequency signal emitted by the radio frequency signal generator.

[0037] The optical signal output from the optical coupler is modulated using a second Mach-Zehnder modulator, and the modulated signal is the optical frequency comb.

[0038] The frequency difference between the lasers emitted by the first and second lasers is a preset value, specifically 1.38 THz. Both the first and second lasers are continuous wave lasers. The radio frequency signal emitted by the radio frequency signal generator has a 90° phase. The optical coupler directly superimposes the optical signal output from the first Mach-Zehnder modulator and the laser emitted by the second laser.

[0039] This embodiment corresponds one-to-one with Embodiment 1. For any details not covered herein, please refer to Embodiment 1. Further details will not be repeated here.

Claims

1. An optical frequency comb generator based on heterodyne phase-locked loop, characterized in that: The system includes a first laser, a second laser, a first Mach-Zehnder modulator, a second Mach-Zehnder modulator, a radio frequency (RF) signal generator, and an optical coupler. The RF signal generator is connected to both the first and second Mach-Zehnder modulators. The laser emitted by the first laser is modulated by the first Mach-Zehnder modulator and then sent to the optical coupler. The laser emitted by the second laser is sent directly to the optical coupler. The optical coupler couples the received optical signal and sends it to the second Mach-Zehnder modulator, where it is modulated to generate an optical frequency comb.

2. The optical frequency comb generator based on heterodyne phase-locked loop according to claim 1, characterized in that: The difference in laser frequency emitted by the first laser and the second laser is a preset value.

3. The optical frequency comb generator based on heterodyne phase-locked loop according to claim 1, characterized in that: Both the first laser and the second laser are continuous wave lasers.

4. The optical frequency comb generator based on heterodyne phase-locked loop according to claim 1, characterized in that: The radio frequency signal generator is used to transmit a radio frequency signal with a phase of 90° and transmit it to the first Mach-Zehnder modulator and the second Mach-Zehnder modulator.

5. The optical frequency comb generator based on heterodyne phase-locked loop according to claim 1, characterized in that: The optical coupler is used to directly superimpose the signal output from the first Mach-Zehnder modulator and the laser emitted by the second laser.

6. A method for generating optical frequency combs based on heterodyne phase-locked loop, characterized in that... include: The first laser is used to emit the laser; The laser emitted by the first laser is modulated by a first Mach-Zehnder modulator, which is driven by a radio frequency signal emitted by a radio frequency signal generator. A second laser is used to emit the laser; An optical coupler is used to couple the laser emitted by the second laser with the optical signal modulated by the first Mach-Zehnder modulator. The second Mach-Zehnder modulator is driven by the radio frequency signal emitted by the radio frequency signal generator. The optical signal output from the optical coupler is modulated using a second Mach-Zehnder modulator, and the modulated signal is the optical frequency comb.

7. The optical frequency comb generation method based on heterodyne phase-locked loop according to claim 6, characterized in that: The difference in laser frequency emitted by the first laser and the second laser is a preset value.

8. The optical frequency comb generation method based on heterodyne phase-locked loop according to claim 6, characterized in that: Both the first laser and the second laser are continuous wave lasers.

9. The optical frequency comb generation method based on heterodyne phase-locked loop according to claim 6, characterized in that: The radio frequency signal emitted by the radio frequency signal generator has a phase of 90°.

10. The optical frequency comb generation method based on heterodyne phase-locked loop according to claim 6, characterized in that: The optical coupler directly superimposes the optical signal output from the first Mach-Zehnder modulator and the laser emitted by the second laser.

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