Linearly frequency-swept laser source has various applications in optical frequency-domain reflectometry (OFDR) and optical coherence tomography (OCT), particularly in frequency modulated continuous wave (FMCW) light detection and ranging (LiDAR) technology. In addition to estimating the target's location and velocity in a single measurement, FMCW LiDAR uses the coherent detection method to extract frequency information, eliminating interference from other light detection and ranging emitters and sunlight.
The fundamental component of FMCW LiDAR is the linear frequency swept laser source. The distance that can be recorded by light detection and ranging is governed by the consistency of the laser source, whereas the spatial resolution is dictated by its frequency-swept period.
While a laser source with great coherence often does not have acceptable frequency tuning characteristics, a laser that enables a wide frequency swept period typically has poorer phase noise properties. The measurement range and spatial resolution also constrain the laser's nonlinearity in the frequency swept process. Therefore, the key to frequency modulated continuous wave light detection and ranging research is a frequency swept laser source with high linearity, great coherence, and a frequency swept period that matches the requirements.
Approaches for Linear Frequency Swept Implementation
Laser implementation of linear frequency swept utilizes two techniques: internally modulated and externally modulated approaches.
The most effective technique to implement a high linearity frequency swept laser source is employing the externally modulated semiconductor laser. However, this approach has limitations in generating a linear frequency swept electrical signal due to the costly arbitrary waveform generator (AWG).
The majority of semiconductor lasers do not exhibit linear relation between tuning signal and optical frequency for internal modulation technique. The tuning signals with high repetition rate force lasers to present nonlinear properties; therefore, several linearization techniques are required to achieve linear frequency swept. However, the internal modulation technique has relatively easy integration and does not require costly AWGs.
Optical Phase-Locked Loop and Iterative Pre-Distortion Algorithm
Frequency swept linearization of semiconductor lasers can be achieved via resolving the nonlinear relation between the laser output frequency and modulation current. For this purpose, two solutions are available, i.e. employing an optical phase-locked loop or an iterative pre-distortion algorithm.
When an iterative pre-distortion algorithm is applied, the frequency swept nonlinearity is improved; however, the broadband random frequency noise cannot be suppressed.
The optical phase-locked loop suppresses random frequency noise but has a limitation of phase-locked bandwidth; hence an extensive range of nonlinear frequency errors cannot be tracked.
The Focus of the Study
This study addresses the issues discussed above by suggesting a pre-distortion algorithm-integrated digital phase-locked-loop frequency swept linearization implementation that may be utilized to enhance the coherence and linearity of a frequency-swept laser source.
The pre-distortion drive current generated by the iterative pre-distortion algorithm is used for frequency swept nonlinearity correction for a wide range. In contrast, the residue frequency swept nonlinearity is suppressed by utilizing the phase-locked loop. This technique for linear frequency swept does not have any specific laser requirements.
A 1550 nm narrow linewidth directly modulated distributed feedback laser diode (DFB-LD) was applied in the experiment with 175 kHz linewidth and 60mW output power. There was a small delayed fiber length of 0.95 m between arms of the Mach-Zehnder interferometer, which ensures the correction of errors initiated by laser nonlinearity.
The optical phase-locked loop's maximum locking range was ±200 kHz, and the maximum frequency swept period was 70 GHz. Laser frequency swept signal's linearity was determined via frequency spectrum measurement. Moreover, Hilbert transform was used to derive the laser's actual frequency swept curve.
The researchers acquired the beat frequency signal via oscilloscope and then calculated its time-frequency curve via a zero-crossing algorithm. They observed the beat signal's real-time spectrum with an electrical signal spectrum analyzer.
Significant Findings of the Study
Using a directly modulated distributed feedback laser diode, the researchers achieved a 31.1 GHz frequency swept period in 120 μs. Pre-distortion drive signal's amplitude was ±85 mA and a bias current of 200mA.
A program control circuit board was used to implement these parameters. The root mean square (RMS) value was less than 1.4MHz for the frequency swept nonlinearity after phase-locking. A frequency modulated continuous wave light detection and ranging system showed long-term stability and high performance.
This system is advantageous due to its cost-effectiveness since it does not need tedious data post-processing, complicated linearization setups, and expensive linear frequency swept lasers. The theoretical calculations suggest that a detection distance of 272 m is achievable using this LiDAR system. However, this research does not include the laser's dynamic linewidth before and after calibration since it is out of its current scope.
Peng Li, Yating Zhang, JianquanYao (2022) Linear frequency swept laser source with high swept slope based on digital optical phase-locked loop. Optics Communications. https://www.sciencedirect.com/science/article/pii/S0030401822005466