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Optical Sensing & Optical Communication

Innovative DFB laser diode to address performance-sensitive applications in a cost-effective way

On: Feb 17, 2022

In: Optical Sensing & Optical Communication

By: Patrice Dionne

Edge-emitting distributed feedback laser (DFB) diodes are narrow 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.

For some applications, the DFB diode’s high levels of spontaneous emission result in frequency noise that can be prohibitive from a performance standpoint. Commercially available DFBs rarely exhibit linewidths below 500 kHz, which is still one to three orders of magnitude above the requirement of applications, such as distributed acoustic sensing (DAS) or light detection and ranging (LiDAR) based on coherent detection architectures.

To overcome these limitations, DFB laser diodes are often coupled with external cavities; however, this approach increases the level of complexity of the laser and affects the stability of the lasing mode.  

The dependence of the index of refraction of the gain medium on both carrier density and temperature also results in two response components that can become limiting factors in achieving (1) frequency noise-canceling feedback loops with high bandwidth and (2), highly linear frequency chirps using direct current drive approaches. For (1), this can represent a limiting factor to achieve high-precision frequency and phase-locking loops, which can benefit to applications, such as resonance fiber optic gyroscope (RFOG) [1,2], photonic quantum computing, biosensing, temperature, pressure, acoustics and strain sensing. As for (2), frequency chirp non-linearity directly impacts the range and precision performance levels of frequency modulated continuous wave (FMCW) LiDAR as it spreads the spectral content of the acquired beat signal.

TeraXion, in partnership with the National Research Council Canada, developed a custom DFB laser diode design, centered at 1550 nm, that can provide >100 mW output power and that presents significant improvements relative to the limitations described above.

Based on a unique epitaxial design, this small-sized and cost-effective monolithic laser displays a low-frequency noise with an intrinsic linewidth typically smaller than 20 kHz. It also exhibits a flat modulation response up to >100 MHz modulation frequencies. These properties, combined to the ability to minimize the loop delay through small form-factor integration approaches, enable impressive phase locking and frequency stability performance levels, as shown in the figures below.

Figure 1: 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

Figure 1: 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.  

Figure 2: TeraXion’s DFB frequency modulation bandwidth and frequency noise measurements

Figure 2: TeraXion’s DFB frequency modulation bandwidth and frequency noise measurements

(a)  Standard DFB vs TeraXion’s DFB responses to small modulation signals (frequency modulation magnitude & phase response). TeraXion’s DFB response is nearly flat, up to hundreds of MHz, whereas the standard DFB response dips well below 1 MHz.

(b)  TeraXion’s DFB power spectral density of frequency noise (PSDFN) associated with different operating modes.

The blue curve is associated with the free-running mode and shows an intrinsic frequency noise linewidth below 10 kHz (red arrow).

The orange curve shows the PSDFN when the laser is locked to a frequency discriminator, providing >2 order of magnitude phase noise reduction for noise spectral content up to 10 MHz.

The yellow curve presents the PSDFN of the beat signal of a slave and master DFB lasers locked in phase. The PSDFN peak frequency of this beat signal (blue arrow) can be seen as a correction loop bandwidth indicator. This indicator peaks at 1 MHz when using a standard DFB (equivalent electronic and loop delay, data not shown on the graph), which is two orders of magnitude below the feedback loop performance with TeraXion’s DFB.

Another TeraXion’s DFB feature associated with frequency modulation is its linearity with the drive current. At slower modulation rates (hundreds of kHz and below), as it is typically used in FMCW LiDAR systems, TeraXion’s DFB frequency modulation response has the advantage of being much more linear than standard DFBs (50-fold improvement). A comparison is presented in Figure 3.

Figure 3: Comparison of the frequency distortion of a commercial DFB laser and of TeraXion’s custom DFB laser over a frequency excursion of 1.4 GHz. TeraXion’s DFB exhibits a 50-fold linearity improvement over the standard DFB.

CONCLUSION

TeraXion has developed a unique monolithic DFB laser diode that provides three major improvements, compared to existing DFBs: (1) an intrinsic linewidth below 20 kHz, (2) a frequency modulation bandwidth above 100 MHz and (3) a 50-fold frequency response linearity improvement when using direct current drive.

This innovation is driven by the need to offer a small-sized and cost-effective laser technology that can fulfill demanding system requirements, reduce system complexity, and improve its stability. A laser product based on a cost-optimized packaging approach is under development.

Evaluation kit modules are offered for technology assessments: NLM Specsheet

 [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).

High Linearity DFB Laser Diode for FMCW LiDAR

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