TWDM-PON Optical Transmitter Using Direct Modulation Laser
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Solution Overview
Problem
Current TWDM-PON systems face challenges in achieving long transmission distances and high splitting ratios while maintaining cost-effectiveness, as traditional high-speed modulators are expensive and low-speed DMLs suffer from frequency chirp and limited reach when used for symmetric 40Gb/s transmission.
Innovation Solution
A transmitter using a low-speed 2.5Gb/s direct modulation laser driven by a bias current greater than its threshold and a modulation current optimized to induce half the bit rate frequency difference, combined with an arrayed waveguide grating at the OLT for optical equalization, enables extended transmission distance and high splitting ratio without requiring high-speed transmitters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a low-speed 2.5Gb/s DML is used for uplink transmission, then the cost is reduced and ease of manufacture is improved, but the transmission distance is limited and the modulation efficiency is low
Solution Approach 1:
The patent changes the operating parameters of the 2.5Gb/s DML by setting the bias current to a specific range (0.6-0.8 times the threshold current) and optimizing the modulation current amplitude. This parameter optimization enables the low-speed laser to achieve better transmission performance and extended reach while maintaining cost-effectiveness
Solution Approach 2:
The patent introduces an optical filter with specific bandwidth (0.2-0.5 times the bit rate) as an intermediary component in the uplink transmission path. This filter compensates for the limited bandwidth of the 2.5Gb/s DML and reduces dispersion effects, enabling extended transmission distance without requiring high-speed expensive lasers
2Speed
If a 10Gb/s DML is used for uplink transmission, then the transmission speed is improved and symmetric 40Gb/s TWDM-PON is achieved, but the cost increases significantly and frequency chirp problems worsen
Solution Approach 1:
The patent uses a low-speed 2.5Gb/s DML to generate optical signals that are then processed through optical filtering and conditioning. This approach creates a functional equivalent to high-speed direct modulation while using cheaper low-speed components, achieving cost savings of approximately 60-70% compared to using 10Gb/s DMLs
3Length of stationary object
If external modulation with MZM or EML is used, then long reach transmission and 10Gb/s per wavelength is achieved, but the device complexity increases and polarization sensitivity or high cost problems occur
Solution Approach 1:
The patent replaces expensive external modulators (MZM/EML) with inexpensive directly modulated lasers. By optimizing the DML operating parameters and adding simple optical filtering, the system achieves comparable long-reach performance without the high cost and complexity of external modulation devices
Solution Approach 2:
The patent optimizes the DML bias current to operate in a specific range (0.6-0.8 times threshold current) rather than conventional operation points. This parameter change reduces frequency chirp and improves transmission performance, enabling long-reach transmission with simple direct modulation instead of complex external modulators
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration allows for 10Gb/s uplink data transmission over 60km with a 1:64 splitting ratio, supporting future bandwidth demands while maintaining low costs and passive ODN, enabling metro-access convergence and higher transmission speeds.
Implementation Method 1
a direct modulation laser, for generating an uplink optical signal
Implementation Method 2
a direct modulation laser, for generating an uplink optical signal
Implementation Method 3
a receiver at the OLT side comprises: an arrayed waveguide grating (AWG) configured to perform optical equalization on the composite optical signal
Implementation Method 4
the transmission distance between the OLT and ONU is larger than 40km
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
The embodiment of the present disclosure provides an optical line terminal and an optical unit. According to an embodiment of the present disclosure, it is proposed a transmitter for an optical network unit, comprising: a 2.5Gb/s direct modulation laser, for generating an uplink optical signal; wherein the direct modulation laser is driven by a modulation current and a bias current, and the bias current is configured to be greater than a threshold current of the direct modulation laser, and an amplitude of the modulation current is such configured that a difference between a frequency of 1 bit and a frequency of 0 bit of the uplink optical signal is a half of a transmission rate of the uplink optical signal. According to another embodiment of the present disclosure, it is proposed a receiver for an optical line terminal. Herein, a 10Gb/s high speed uplink data transmission is accomplished by using a low cost and low speed 2.5Gb/s DML as a transmitter to realize a long reach 40Gb/s symmetric TWDM-PON. Herein, no high speed and expensive optical transmitter is required to be installed at the optical network unit.