Innovative DFB laser diode to address performance-sensitive applications in a cost-effective way
On: Jul 29, 2024
In: Optical Sensing & Optical Communication
Narrow Linewidth DFB Laser Diode with Unique Modulation Response
Edge-emitting distributed feedback laser (DFB) diodes are small-sized laser sources that can be produced in high volumes at a low cost. They are widely deployed in telecommunication systems and have proven to be reliable over years of continuous operation.
DFB diodes typically exhibit high levels of spontaneous emission resulting in frequency noise that may negatively affect the performance of some applications. Commercially available DFBs rarely exhibit linewidths below 500 kHz, which is still 1-3 orders of magnitude above the requirement for applications such as distributed acoustic sensing (DAS) or light detection and ranging (LiDAR) based on coherent detection architectures. To overcome these limitations, light emitting diodes are often optically coupled to external cavities; however, this approach increases the level of complexity of the laser and affects the stability of the lasing mode. This ends up limiting the frequency modulation amplitude because of mode hopping.
In standard DFB laser diodes, the dependency of the gain medium’s index of refraction over both its carrier density and its temperature is a limiting factor to maintain an effective frequency modulation response up to high modulation frequencies. This limits the maximum locking loop bandwidth that can be achieved when locking the laser on a frequency reference or when locking different lasers together in phase. This also restricts high amplitude and linear frequency chirps at fast repetition rates in system architectures applying direct modulation to the laser electrode.
Solving the modulation response limitation is then beneficial to many applications. Frequency and phase locking loops are used in resonance fiber optic gyroscope (RFOG) [1,2], photonic quantum computing, quantum sensing, atomic clocks and biosensing. Triangular and sawtooth frequency waveforms are used in metrology applications based on interferometry, frequency modulated continuous wave (FMCW) lidar and optical frequency domain reflectometry (OFDR) for distributed sensing of temperature, strain or acoustics (DAS) [3]. Improving the frequency response of the laser and its native level of frequency noise has positive impacts on measurement resolution, repeatability and range.
TeraXion, in partnership with a III-V fab, developed a proprietary DFB laser diode design, centered at 1550 nm, that provides >150 mW output power and features significant linewidth and modulation response improvements relative to standard DFB lasers. Based on a unique epitaxial and longitudinal design, this small-sized and cost-effective monolithic laser exhibits an intrinsically narrow linewidth (< 20 kHz typical) and a flat modulation response up to >100 MHz modulation frequency (see figure 1 below).
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Figure 1: TeraXion proprietary DFB laser diode intrinsic narrow linewidth and flat modulation response properties.
a) Power spectral density of frequency noise (PSDFN) of TeraXion’s DFB laser diode vs a standard DFB laser.
b) Frequency modulation response of TeraXion’s DFB laser diode vs a standard DFB laser (frequency modulation magnitude & phase response dependency over the modulation frequency). TeraXion’s laser response is nearly flat, up to hundreds of MHz, whereas the standard DFB laser response dips around 100 kHz modulation frequency.
These properties, combined to the ability to minimize the loop delay through small form-factor integration approaches, enable impressive laser phase locking and frequency locking of the laser as shown in the figures below:
Figure 2: Differential frequency noise of TeraXion laser in a RFOG configuration
Left: example of compact integration of TeraXion’s DFBs (red arrows) with a silicon photonic chip (SiP, blue arrow) to achieve a high-performance phase locked multi-frequency laser source for RFOG application.
Right: differential frequency noise of two lasers locked on the clockwise and anti-clockwise resonances of a RFOG. Both the intrinsic narrow linewidth and the flat frequency modulation response properties of the laser are required to achieve this unmatched level of performance at the system level.
Figure 3: PSDFN of TeraXion’s DFB laser locked on a frequency discriminator (LXM-U product exhibiting a typical linewidth < 0.1 kHz) vs a standard DFB laser locked using similar frequency locking loop electronics (NLL product). TeraXion’s laser design optimizes the locking performance which translates into an ultra-low frequency noise floor and a locking bandwidth pushed by two orders of magnitude.
While modulating the frequency of a DFB laser diode by applying current fluctuations at the laser electrode represents a simple and cost-effective modulation method, obtaining high amplitude frequency chirps that are highly linear at fast repetition rates remains a challenge. Thanks to its flat frequency modulation response, TeraXion laser can easily be linearized up to hundreds of kHz repetition rate using pre-distortion of the drive signal. The natively low level of phase noise of the laser is also preserved under modulation. This allows generating high quality frequency waveforms that maximize the measurement range, resolution and repeatability.
Figure 4: Linearization of TeraXion DFB laser using pre-distortion of the drive current. A single iteration allows to reach 0.02% non-linearity residue for 2 GHz chirps at 100 kHz repetition rate.
Figure 5: High amplitude chirp (7.5 GHz at 10 kHz repetition rate) linearization using pre-distortion of the drive current. The correction converges with two iterations (k=2) and reaches 0.04% non-linearity residue.
Conclusion
TeraXion has developed a unique monolithic semiconductor DFB laser that provides two major improvements, compared to existing DFBs: an intrinsic linewidth below 20 kHz and a flat frequency modulation response up to > 100 MHz modulation frequency. This innovation is driven by the need to offer a small-size and cost-effective laser technology that can fulfill demanding system requirements, reduce system complexity and improve its stability and performance. This proprietary DFB laser diode is integrated into the LXM turn-key laser module product, which exhibits a <0.1 kHz linewidth in the ultra narrow configuration (LXM-U).
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Bibliography
[1] Ayotte, S. et al., "Compact silicon photonics-based multi laser module for sensing," Proc. SPIE 10537, 1053717 (2018).
[2] Ayotte, S. et al., "Compact silicon photonics-based laser modules for FM-CW LIDAR and RFOG," Proc. SPIE 11284, 1128421 (2020).
[3] Cardin, V. et al., "Narrow-linewidth semiconductor laser with highly linear frequency modulation response for coherent sensing, " Proc. SPIE 12905, 129050F (2024)