System and method for optical signal generation

Abstract

The present disclosure relates to systems and methods for optical signal generation. An optical signal generation system for optical signal generation includes a plurality of optical sources, a plurality of optical modulators, each optical modulator corresponding to one of the plurality of optical sources, and a synchronization driver coupled to each of the optical modulators. Each optical source can input a continuous wave (CW) to each optical modulator. The synchronization driver can provide a non-return-to-zero (NRZ) drive signal to each optical modulator, and the optical modulators can modulate each optical source signal received based on the NRZ drive signal transmitted to each optical modulator. Each modulated optical signal can be coupled into a transmission medium, thereby generating a multi-level pulse amplitude modulation (PAM) equivalent signal.

Description

Background Technology

[0001] Traditional optical modulation schemes, such as pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM), typically rely on linear modulation techniques that continuously modulate signal parameters within a certain range. While effective, these methods usually require complex and precise control systems to maintain signal integrity, especially at high data rates. A simpler, more cost-effective method is needed that reduces the reliance on precise analog ultra-high-speed signal processing without compromising the ability to meet ever-increasing bandwidth demands.

[0002] The subject of this disclosure relates to overcoming or at least reducing the impact of one or more of the problems described above. Summary of the Invention

[0003] This disclosure relates to the field of optical communications, and more specifically, to modulation techniques used in data encoding and transmission via optical fibers or free space. In some examples, this disclosure introduces novel methods and systems for generating optical signals that utilize multiple continuous-wave light sources with external modulation, wherein the modulated signals are synchronized and combined to generate multilevel optical signals, i.e., multilevel pulse amplitude modulation (PAM) equivalent signals.

[0004] These and other features of this disclosure will become more fully apparent from the following description set forth below and the appended claims.

[0005] The foregoing summary is not intended to outline every potential implementation or aspect of this disclosure. Attached Figure Description

[0006] To further illustrate the foregoing and other features of this disclosure, a more specific description of the subject matter will be presented by reference to specific examples of the subject matter shown in the accompanying drawings. It should be understood that these drawings depict only a few examples of the subject matter and should therefore not be considered as limiting its scope.

[0007] Figure 1 The overall architecture of a high-speed data transmission system according to an exemplary embodiment of the present disclosure is shown, illustrating the interconnection between a high-speed data source, a synchronous NRZ driver, a light source, a modulator, and a noise source.

[0008] Figure 2 Alternative configurations of the system according to an exemplary embodiment of the present disclosure are shown, highlighting the use of a noise source to modulate the attenuation of the modulator to further mitigate coherent interference.

[0009] Figure 3 A synchronous NRZ driver according to an example embodiment of this disclosure is shown.

[0010] Embodiments of this disclosure will be described more fully with reference to the accompanying drawings. These drawings may be merely schematic representations of current filters, components, devices, or methods, used to enhance understanding of the disclosed concepts. Detailed Implementation

[0011] The embodiments disclosed herein include an optical signal generation method and system, wherein the system may include: a plurality of light sources; a plurality of optical modulators, each optical modulator corresponding to one of the plurality of light sources; and a synchronization driver coupled to each of the optical modulators. The system is operable to: configure each light source in a continuous wave (CW) output state to generate a light source signal transmitted to each optical modulator; transmit a non-return-to-zero (NRZ) drive signal to each optical modulator using the synchronization driver; modulate each received light source signal using the optical modulator based on the NRZ drive signal transmitted to each optical modulator; and couple each modulated optical signal into a transmission medium, thereby generating a multi-level pulse amplitude modulation (PAM) equivalent signal.

[0012] Switch to the attached image. Figure 1 The overall architecture of a high-speed data transmission system according to an exemplary embodiment of this disclosure is shown, illustrating the interconnections between a high-speed data source, a synchronous non-return-to-zero (NRZ) driver, a light source, a modulator, a noise source, and optics. This disclosure provides a system for transmitting a high-speed PAM-n equivalent data stream using multiple externally modulated light sources, which are discrete devices or integrated on a single chip. The system has a modulator driven by a synchronous NRZ driver. The system incorporates a noise source to modulate different aspects of the signal at the light source or modulator, thereby mitigating coherent interference in the signal output.

[0013] Transmitter 100 includes a high-speed data source (HS data 101) input to a synchronous NRZ driver 103. The NRZ driver 103 generates multiple NRZ drive signals for an optical modulator 107 to modulate multiple light sources (LS1, LS2, ..., LSN) 105. Each light source 105 is coupled to a corresponding optical modulator 107 (MOD1, MOD2, ..., MODN), which receives the NRZ drive signals and modulates the optical signals received from the light source. The modulated signals can be focused by lens 113 and combined at transmission medium 115.

[0014] High-speed data source 101 may include any high-speed serial data generator, such as a serializer / deserializer (SerDes) module, a field-programmable gate array (FPGA), or an application-specific integrated circuit (ASIC) that generates signals to be transmitted through transmitter 100. Data from high-speed data source 101 may represent data expected to be transmitted from transmitter 100 to a remote receiver (not shown).

[0015] The driver 103 may include a standalone NRZ driver chip, a portion of an integrated transceiver module, or be implemented in an FPGA or ASIC. In an example embodiment, the driver 103 may reside on the same chip as the optical modulator 107. In another embodiment, the driver 103 and the optical modulator 107 may be on different chips. (See also...) Figure 3 The driver 103 is described further.

[0016] Light source 105 may include a laser diode, a light-emitting diode (LED), a liquid crystal source, or other semiconductor light source capable of generating an optical signal at a desired wavelength for optical communication. In one example scenario, light source 105 may include an on-chip VCSEL array that transmits the output optical signal to an optical modulator 107 on another chip, such as a silicon photonics chip. In another example scenario, light source 105 may include edge-emitting lasers that transmit the output optical signal to a modulator on a photonics chip via a grating connector or other optical connection device.

[0017] The current source 111 may include a circuit system for applying appropriate bias conditions to the light source 105 so that the light source outputs light at a desired optical power. In the example scenario, each light source 205 may be biased at the same output level or different output levels. In the example scenario, with each light source 105 biased at the same output, multiple output levels at the transmission medium can be enabled by summing the signal coupled to the transmission medium 115 by the optical modulator 107.

[0018] For example, configuring all modulators to "off" corresponds to "0," while "on" of one modulator to allow the light signal to pass corresponds to "1," two modulators "on" corresponds to "2," and so on up to the number of light source / modulator pairs. In this way, three light sources can be used to generate a PAM-4 equivalent signal, while the control requirements for the modulators are greatly simplified compared to traditional PAM-n systems, because the modulators only need to switch between ON and OFF states, instead of switching between multiple levels between these states.

[0019] In another example scenario, light source 105 can be biased so that the optical signal outputs are different from each other, such as in a binary weighted configuration with outputs of 1X, 2X, and 4X. In this case, combinations of the modulator's ON / OFF states can produce 0X (all modulators OFF), 1X (modulator of 1X source ON), 2X (modulator of 2X source ON), 3X (modulators of 1X and 2X sources ON), 4X (modulator of 4X source ON), and so on. This allows each optical modulator 107 to achieve a wide range of output signals by simply performing binary switching (ON / OFF).

[0020] Furthermore, because the modulation of the optical signal is performed by the optical modulator 107, the biasing and control of the light source 105 can be simpler, with each light source only configured to the desired CW current level. Some control may be required to address temperature variations, for example, by utilizing a monitoring photodiode adjacent to the light source 105 or a feedback loop from the optical modulator 107 connected to the controller 110 to the current source 109.

[0021] The optical modulator 107 may include an electro-optic modulator, an acousto-optic modulator, or other means that can modulate light based on an electrical input. In one example, the optical modulator 107 includes a Mach-Zehnder interferometer modulator integrated into a silicon photonic chip, although it could also be of other modulator types (such as thin-film lithium niobate, liquid crystal modulators, or similar devices). In the example scenario, the optical modulator 107 can be configured by an applied bias. Since the optical modulator 107 only needs to switch between ON and OFF states, there is no need to worry about any nonlinearity issues between the modulator's attenuation characteristics and the applied bias.

[0022] Noise source 109 may include a random number generation circuit system, for example, which may be operable to provide a small signal current with random intensity for providing a noise signal on the current applied to light source 105. Example options for noise source 109 are a white noise generator, a pseudo-random binary sequence generator, a thermal noise source, a shot noise source, or a 1 / f noise source, and may be digital or analog.

[0023] Figure 1 The controller 110, lens 113, and transmission medium 115 are also shown. Depending on the specific requirements of the optical system for coupling the signal to the transmission medium 115, lens 113 may include a focusing lens or a gradient refractive index lens. Lens 113 can focus the optical signal 112 received from the optical modulator 107 onto the transmission medium 115, thereby generating a synthesized PAM-n signal.

[0024] The transmission medium 115 may include single-mode or multimode optical fiber, or any other suitable optical waveguide. In another example scenario, the transmission medium 115 may include free space, such that the optical signal 112 is focused onto the input of the receiver. In one example, the transmission medium 115 may include a single-core optical fiber for multi-level signal generation, thereby providing a simple optical connection mechanism to combine multiple light source outputs into a single optical signal for transmission through the fiber. In another example scenario, the transmission medium 115 may include free space.

[0025] Combining light sources with approximately equal wavelengths can cause unmanaged coherent interference, resulting in fluctuations in output amplitude. In one example, this can be mitigated or eliminated by introducing a small-amplitude noise source into the bias current of each light source. A noise source 109 at each light source 105 modulates the DC bias applied to the light source 105. In the example scenario, to mitigate coherent interference in the transmission medium 115, a noise source 109 is introduced for each light source 105. The noise source 109 modulates the DC bias of the light source 105, causing random chirping of the wavelength of the light source 105, thereby reducing or eliminating coherent interference.

[0026] The main advantage of this binary source modulation and multi-level signal generation scheme is the simplification of high-speed signal generation. From the transmitter's perspective, the synthesized signal appears as PAM-n, without the need to configure a single source or modulator with multiple (n) levels to generate a PAM-n equivalent signal. By utilizing multiple CW lasers with external binary modulation, the system reduces the need for complex control electronics and precise analog modulation, while still supporting signal generation exceeding 100 GBaud.

[0027] Although Figure 1 A single lens is shown, but lens 113 may alternatively comprise multiple lenses, depending on the focusing requirements of the received beam. In another example implementation, multiple lenses may be combined into a single optical component, or the lenses may be omitted depending on the direction and shape of the beam output from the light source of the light modulator 107. In yet another example, the lens may be one or more waveguide combiners or one or more etched waveguides in silicon photonics technology. Another example is a grating connector and a Mach-Zehnder Y combiner for each light source.

[0028] The controller 110 may include, for example, circuitry for configuring the driver 103, noise source 109, and current source 111, and thus may include a processor and associated electronics for controlling the output current applied by the current source 111 to the light source 105. Additionally, the controller 110 may communicate with the driver 103 to provide an NRZ drive signal to the optical modulator 107. The controller 110 may also receive feedback from a monitoring photodiode (not shown) at any location in the emission path for dynamically configuring the light source 105 and the optical modulator 107 to a desired state. Although the controller 110 is shown as a single block, any number of individual circuit blocks may be used, depending on the location of the controlled elements and whether they are on-chip or off-chip.

[0029] In operation, current source 111 can use small random fluctuations (on the order of a few tenths of a dB in signal modulation) in the current supplied by noise source 109 to bias light source 105 to the ON state. Although this noise is added to the output PAM-n signal, the higher modulation depth and increased output power from multiple CW sources mitigate the effects of this intentionally introduced noise, while reducing or eliminating coherent interference. The electrical data signal, HS data 101, can be transmitted to driver 103, which provides a synchronized NRZ drive signal to optical modulator 107.

[0030] The ON / OFF modulator needs to be synchronously driven in NRZ mode to achieve PAM-n at the combined position. This, in turn, requires phase management of the drive signal from driver 103 on each path. This is done using... Figure 3 The driver shown is used to implement this.

[0031] Figure 2 Alternative configurations of the system according to an exemplary embodiment of the present disclosure are shown, highlighting the use of a noise source to modulate the attenuation of the modulator to further mitigate coherent interference. Figure 2 Components with similar numbers in the middle can be with Figure 1 The components are basically similar. (Refer to...) Figure 2 The image shows a transmitter 200, which includes a high-speed data (HS data) source 101 input to a synchronous NRZ driver 103. The NRZ driver 103 generates multiple non-return-to-zero (NRZ) drive signals for an optical modulator 107 to modulate multiple light sources (LS1, LS2, ..., LSN) 105. Each light source 105 is coupled to a corresponding optical modulator 107 (MOD1, MOD2, ..., MODN), which receives the NRZ drive signals and modulates the optical signals received from the light source 105. The modulated optical signals 112 can be focused by a lens 113 and combined at a transmission medium 115.

[0032] exist Figure 2 In an alternative configuration, each light source 105 is driven by a current source 111, but the optical modulators 107 are also modulated by a noise source 209. This noise source modulates the attenuation of the modulator, thereby reducing or eliminating coherent interference in the output signal, rather than... Figure 1 The bias current of the center-directing light source 105 adds noise. The system still uses a synchronous NRZ driver 103 to ensure precise timing of the drive signal, where the driver 103... Figure 3 It is shown in more detail below.

[0033] Transmitter 200 and Figure 1 The main difference between the transmitters 100 is that the noise source is located at the optical modulator 107, where noise applied to the optical modulator 107 modulates a random signal on the output signal of the optical modulator 107. This can be configured, for example, by applying a random noise signal to the bias signal of one arm of a Mach-Zehnder modulator, thereby applying a small phase change to that arm. Although noise is added to the output PAM-n signal, the higher modulation depth and increased output power from multiple CW sources mitigate the effects of this intentionally introduced noise, while reducing or eliminating coherent interference in the output signal.

[0034] This figure depicts an alternative configuration (200) of a system that uses a non-return-to-zero (NRZ) signal to generate a combined pulse amplitude modulation (PAM-n) equivalent signal. Similar to... Figure 1 As described herein, the system begins with a high-speed data source (HS data 101), which feeds data to a driver 103, which generates multiple NRZ drive signals. These signals are used to modulate the output of multiple light sources (LS1 to LSN 105) via optical modulators (MOD1 to MODN 107).

[0035] In this configuration, noise is injected into the modulation process itself, rather than using separate low-level random noise sources (109) directly connected to each light source (e.g., Figure 1 (As seen in the image). This is achieved through a noise injection circuit (209) connected to the modulator input. The injected noise is used to introduce slight random modulation directly into the modulated signal, thereby ensuring minimal variation in the output of the light source. This method effectively prevents coherent interference when the light signals from the light source are combined.

[0036] Each light source 105 is still biased by current source 111 to maintain stable operation. The modulated optical signal 112 from optical modulator 107 is combined using lens 113, which focuses the signal into a single PAM-n equivalent signal. The combined signal is then transmitted through transmission medium 115.

[0037] The system remains under the control of the control unit 110, which ensures that the components are properly coordinated to produce the desired PAM-n equivalent output signal.

[0038] Figure 3 A synchronous NRZ driver according to an example embodiment of this disclosure is shown. (Refer to...) Figure 3 The diagram illustrates a driver 103 comprising multiple independent drivers 309, repeated n times, where n depends on the number of desired synchronization signals. A common reference clock 311 can be coupled to each driver 309, wherein each driver 309 includes a phase-locked loop (PLL) 313, programmable frequency dividers 315 and 319, a voltage-controlled oscillator (VCO) 317, and a retimer latch 321.

[0039] The driver system 103 includes a reference clock CLK_ref 311, which provides a stable timing source for the multiple drivers 309. The reference clock is fed into a phase-locked loop (PLL) 313 in each NRZ signal path. The use of a common reference across all PLLs ensures that all NRZ signals are aligned in the time domain.

[0040] PLL 313 controls the phase after dividing the VCO 317 signal by a factor of M using divider 315. Therefore, VCO 317 is operating at a frequency M times higher than the frequency seen by PLL 313 from reference clock 311. This VCO frequency is also divided by divider 319, for example, by a factor of M, so that the retimer latch 321 can modulate its phase by a step size of M, dividing the VCO frequency by M.

[0041] PLLs are essential components in various electronic and communication systems, particularly for clock generation, synchronization, and jitter reduction. When used in conjunction with or within a retimer, a PLL plays a fundamental role in ensuring a stable, synchronized output signal that is precisely aligned with the desired timing. A PLL may include a phase detector that compares the phase of the input signal with the phase of a feedback clock signal generated by the VCO 317 via a frequency divider 315. The phase detector produces an output signal proportional to the phase difference between these two signals.

[0042] The PLL 313 may also include a charge pump that converts the phase error signal from the phase detector into a corresponding current signal. This current is then used to charge or discharge the loop filter, controlling the frequency of the VCO 317. The loop filter smooths the output of the charge pump to provide a stable control voltage to the VCO 317 and is typically a low-pass filter that removes high-frequency noise and determines the dynamic response of the PLL.

[0043] PLL 313 also includes a feedback loop provided by frequency divider 315 and returned to PLL 313 as shown in the signal path. The feedback loop consists of the VCO output after passing through frequency divider 315, which is then fed back to the phase detector of PLL 313. This loop continuously adjusts the VCO 317 output to match the phase and frequency of the input signal (reference clock 311). Frequency divider 315 can be fixed or programmable, allowing the PLL to lock to different input frequencies while generating a range of output frequencies. Once the feedback clock signal is aligned with the input signal in phase and frequency, the PLL is said to be "locked." The VCO output then provides a stable and synchronized clock signal that can be used by other components in the system, such as the retimer latch 321 used to configure the timing of the NRZ signal.

[0044] The VCO 317 in each block is driven by the same reference source and is therefore synchronized with it. The VCO operates at RF frequencies ranging from several GHz. A programmable divider 319 connected to the retimer latch 321 allows the NRZ signals to be offset relative to each other, the offset being limited only by the VCO frequency. The VCO 317 generates a clock signal whose frequency is controlled by the input voltage from the PLL 313. The frequency of this clock signal is adjusted until it is locked to the frequency of the input signal (reference clock 311). The frequency range, linearity, and sensitivity to the control voltage of the VCO are important parameters. The VCO 317 must be able to generate a clock signal with minimal jitter and phase noise.

[0045] The adjusted clock signal is used to retime the incoming NRZ signal using the retimer latch 321. The retimer latch 321 ensures that each NRZ signal is time-aligned, thereby correcting the approximate phase alignment of the NRZ signals.

[0046] Multiple instances of this driver 309 are used to process all NRZ signals (NRZ 1, NRZ 2, NRZ 3, ..., NRZ n) independently. Alignment processing ensures that all NRZ signals are synchronized when they arrive at the optical assembly point in the transmission medium.

[0047] Retimer latches are crucial components in high-speed digital communication systems, especially in applications involving serial data transmission. A retimer latch may include an input data buffer, a sample latch, an output data buffer, error detection and correction, and control logic, although not all of these components may be necessary in the specific application. The input data buffer (First-In-First-Out or FIFO) can temporarily store high-speed serial input data, providing resilience against temporary data rate mismatches between the input and output rates.

[0048] The sampling latch can capture data bits using a recovered and stable clock signal from divider 319. This component effectively "retires" the data, thereby reducing jitter and aligning data transitions with clock edges. An output data buffer can hold the retired data before it is sent to the next stage of the communication system. This buffer helps manage timing differences between the retired data and the next processing element. Optional error detection and correction circuitry in the retimer latch 321 can identify and correct any bit errors that may occur during transmission or retiring to ensure data integrity. Finally, control logic can manage the operation of the retimer latch 321, including enabling / disabling the retiring function, adjusting the phase of the clock signal, and coordinating data transmission through the system.

[0049] This time alignment process ensures that all NRZ signals are aligned in the time they drive. Figure 1 and Figure 2 The modulator is synchronized, enabling the formation of coherent and well-timed PAM-n signals for optical transmission.

[0050] During operation, driver 103 provides time alignment control for multiple NRZ signals to ensure the correct formation of combined pulse amplitude modulation (PAM-n) signals, such as... Figure 1 and Figure 2 As described in the diagram. By retiming the data, the retimer latch 321 ensures that timing differences introduced during transmission do not cause errors, thereby maintaining the integrity and reliability of the high-speed data communication system. Time alignment is important because each NRZ signal may have phase mismatch.

[0051] NRZ signals NRZ 1 to NRZ n can all be received by their corresponding drivers 309 and synchronized into retimed NRZ output signals. These retimed NRZ output signals are used to drive optical modulator 107, which modulates the NRZ data onto the optical signal. Because each light source is maintained under CW conditions and each optical modulator is independently controlled to ON or OFF instead of multiple levels, the modulator control performance requirements for generating the PAM equivalent output signal are significantly reduced.

[0052] In the disclosed example, the optical generation system includes multiple light sources, multiple optical modulators, and a synchronization driver. Each light source inputs continuous wave (CW) light to its corresponding optical modulator. The synchronization driver drives each optical modulator using an NRZ signal. The output of each optical modulator is coupled to a single transmission medium, thereby generating an optically equivalent pulse amplitude modulation (PAM) signal in that single transmission medium. Multiple current sources can simultaneously supply current to their respective light sources to generate CW light.

[0053] A noise source can be connected to each current source and can provide a noise signal to the current applied to each light source. A noise source can also be connected to each optical modulator and can provide a noise signal to the bias applied to each optical modulator. The synchronization driver may include a phase-locked loop (PLL) and a retimer latch for each optical modulator. A single clock reference can be connected to each PLL.

[0054] A synchronous driver can operate to provide drive signals for multiple optical modulators by synchronizing the received NRZ data signal with a frequency-divided clock signal provided by the PLL and based on a single clock reference in each retimer latch. The transmission medium may include a single-core optical fiber. Lenses can focus each modulated optical signal into the transmission medium. Multiple light sources may include vertical-cavity surface-emitting lasers (VCSELs) and edge-emitting lasers.

[0055] In other disclosed examples, the optical signal generation system includes: multiple light sources; multiple optical modulators, each corresponding to one of the multiple light sources; and a synchronization driver coupled to each of the optical modulators. The system can operate to: configure each light source in a continuous wave (CW) output state to generate a light source signal transmitted to each optical modulator; transmit a non-return-to-zero (NRZ) drive signal to each optical modulator using the synchronization driver; modulate each received light source signal using the optical modulator based on the NRZ drive signal transmitted to each optical modulator; and couple each modulated optical signal into a transmission medium, thereby generating a multi-level pulse amplitude modulation (PAM) equivalent signal.

[0056] The system may include a current source that configures each light source to a CW output state. A noise source may be coupled to each current source and provide a noise signal to the current applied to each light source. A noise source may also be coupled to each optical modulator and provide a noise signal to the bias applied to each optical modulator. A synchronization driver may include a phase-locked loop (PLL) and a retimer latch for each optical modulator. A single clock reference may be coupled to each PLL.

[0057] A synchronous driver can operate to provide drive signals for multiple optical modulators by synchronizing the received NRZ data signal with a frequency-divided clock signal provided by the PLL and based on a single clock reference in each retimer latch. The transmission medium may include a single-core optical fiber. Lenses can focus each modulated optical signal into the transmission medium. Multiple light sources may include vertical-cavity surface-emitting lasers (VCSELs) and edge-emitting lasers.

[0058] In the accompanying drawings, similar features are always indicated by the same or similar reference numerals.

[0059] The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concept conceived by the applicant. It will be understood with respect to the benefits of this disclosure that, in any other embodiment or aspect of the disclosed subject matter, the features described above according to any embodiment or aspect of the disclosed subject matter may be used alone or in combination with any other described features.

Claims

1. A system for generating optical signals, comprising: Multiple light sources Multiple optical modulators, and Synchronous drive, Each light source inputs continuous wave CW light into its corresponding optical modulator. The synchronous driver uses a non-return-to-zero (NRZ) signal to drive each optical modulator. The output of each optical modulator is coupled into a single transmission medium, thereby generating an optically equivalent pulse amplitude modulated (PAM) signal in the single transmission medium.

2. The system according to claim 1, comprising a plurality of current sources, each current source supplying current to a corresponding light source to generate the CW light.

3. The system according to claim 2, wherein, A noise source is connected to each current source and provides a noise signal to the current applied to each light source.

4. The system according to claim 1, wherein, A noise source is connected to each optical modulator and provides a noise signal to the bias applied to each optical modulator.

5. The system according to claim 1, wherein, The synchronous driver includes a phase-locked loop (PLL) and a retimer latch for each optical modulator.

6. The system according to claim 5, wherein, A single clock reference is connected to each PLL.

7. The system according to claim 6, wherein, The synchronous driver is operable to provide drive signals for the plurality of optical modulators by synchronizing the received NRZ signal with a frequency-divided clock signal based on the single clock reference and provided by the PLL in each retimer latch.

8. The system according to claim 1, wherein, The transmission medium includes a single-core optical fiber.

9. The system of claim 1 further includes a lens that focuses each modulated optical signal into the transmission medium.

10. The system according to claim 1, wherein, The multiple light sources include vertical cavity surface-emitting lasers (VCSELs) or edge-emitting lasers.

11. A method for generating an optical signal, comprising: In a system that includes multiple light sources, multiple optical modulators, and synchronous drivers: Each light source inputs continuous wave CW light to the corresponding optical modulator; The synchronous driver is used to drive each optical modulator with a non-return-to-zero (NRZ) signal. as well as The output of each optical modulator is coupled to a single transmission medium, thereby generating an optically equivalent pulse amplitude modulated (PAM) signal in the single transmission medium.

12. The method of claim 11, further comprising configuring each light source to generate the CW light using a current source coupled to each light source.

13. The method of claim 12, further comprising using a noise source to couple a noise signal to a current applied to each light source by the current source coupled to each light source.

14. The method of claim 11, further comprising providing a noise signal to the bias applied to each optical modulator using a noise source.

15. The method according to claim 11, wherein, The synchronous driver includes a phase-locked loop (PLL) and a retimer latch for each optical modulator.

16. The method of claim 15, further comprising coupling a single clock reference to each PLL.

17. The method of claim 16, further comprising using the synchronous driver to provide drive signals for the plurality of optical modulators by synchronizing the received NRZ signal with a frequency-divided clock signal based on the single clock reference and provided by the PLL in each retimer latch.

18. The method according to claim 11, wherein, The transmission medium includes a single-core optical fiber.

19. The method of claim 11, further comprising using a lens to focus each modulated optical signal into the transmission medium.

20. The method according to claim 11, wherein, The multiple light sources include vertical cavity surface-emitting lasers (VCSELs) or edge-emitting lasers.

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