Distributed Bragg Reflector (DBR) laser diodes are a class of single-frequency monolithic semiconductor lasers. Monolithic semiconductor lasers in general are small in size, mechanically robust, and have good power conversion efficiency.
They also provide opportunities for hybrid integration with photonic integrated circuits (PICs). This application note provides an overview of DBR lasers, their principles of operation, their modal behavior, and their diverse applications.
Principles of DBR Laser Operation
A DBR laser is a semiconductor laser with one or several quantum wells embedded within a p-n junction as the active medium. It contains Bragg reflectors at either one or both ends of the laser diode. These Bragg reflectors are gratings that act as wavelength-selective mirrors with reflectivity optimized at a specific wavelength.
DBR lasers primarily operate based on the Fabry-Perot (FP) resonance effect with at least one distributed Bragg reflector acting as an end mirror. The interaction between the FP modes and the DBR mirror reflectivity selects the longitudinal modes supported by the cavity. The mode that aligns best with the gain spectrum reaches the threshold first and initiates lasing.
DBR lasers typically have an emission wavelength ranging from 630 nm to 4000 nm, depending on their design and intended applications. They are known for their high output power, narrow linewidth (often less than 1 MHz), and operation in a single TEM00 mode.
Linewidth of a DBR laser
In a well-designed DBR laser diode, the photolithographically defined and etched horizontal waveguide and the vertical waveguide defined by the epitaxially deposited thin semiconductor layers of different material compositions only support single transverse mode (TEM00), and the wavelength-selective Bragg reflector selects only one supported longitudinal cavity mode at a time, effectively suppressing other frequencies. With optimized Bragg reflector design, not only the laser wavelength can be accurately defined but also the laser intrinsic linewidth can be minimized.
In contrast to distributed feedback (DFB) laser diodes where the wavelength selective grating is distributed over the whole gain section, DBR lasers have a distinct physical separation between the grating and gain sections. The grating acts as a passive component outside the gain medium. This key structural difference results in some major differences in the modal behavior between these two laser diode types.
Wavelength Tuning and Mode Hopping
A DFB laser with an uniformly distributed Bragg grating supports two competing modes, separated by a stop band. This mode degeneracy leads to mode competition. The dominance of one mode over the other is defined by the phase of the light at the facets. The laser cavity length can’t be controlled at an accuracy to match to the desired phase, therefore the dominating mode can’t be accurately controlled.
To overcome the mode degeneracy problem, several techniques such as /4-shifted gratings have been developed, but they typically come with other problems such as spatial hole burning. With careful design, DFB lasers operating as single-frequency lasers over a rather large operating range can be manufactured. However, as temperature and/or bias current change DFB lasers typically suffer from some mode hops, where the lasing wavelength abruptly changes.
DBR lasers’ emission wavelength can be tuned using injection current or laser temperature. In well-designed DBR lasers, the emission is single-frequency over the whole operating current and temperature range.
In DBR lasers the gain section and grating section are electrically decoupled from each other. Due to the passive grating section without any current injection, changes in the gain section injection current do not significantly affect the grating’s reflectivity.
Both the gain section’s spontaneous emission wavelength as well as the grating section’s maximum reflectivity redshift with increasing temperature, but the temperature gradient of the spontaneous emission wavelength is much larger. This means that at a certain point, the laser emission jumps into the next mode supported by the device cavity.
The mode-hops of DBR lasers are much more deterministic and predictable than those of DFB lasers, and more dictated by the design than by statistical variations in the manufacturing process. However, the mode-hop spacing is shorter, meaning shorter mode-hop free tuning range. The mode-hop-free tuning range of DBR lasers is limited by the device cavity’s free spectral range and the active area heating rate.
Typical spectra under current tuning of a DBR laser
Due to these differences in their mode-hop behavior, DBR lasers are typically better suited for applications where deterministic mode-hop behavior is more critical and the requirements for a wide mode-hop free tuning range are not that strict, whereas DFBs are better suited for applications where wider wavelength tuning of several nm is required, and user can tolerate with some less predictable mode-hops that are typical for each individual chip instead of each design.
DBR Laser applications
DBR lasers find application in a wide range of fields due to their stable, single-mode operation and precise output characteristics. Some key applications include:
1. Sensing
- DBR lasers are employed in high-resolution spectroscopy and sensing applications to detect and measure various physical and chemical parameters, such as temperature, pressure, and gas concentration.
- They are used in optical biosensors for medical diagnostics and drug discovery.
2. Quantum
- In laser cooling experiments, DBR lasers are used to cool, repump and trap atoms or molecules to near absolute zero temperatures.
3. Raman Spectroscopy
- The narrow linewidth of DBR lasers makes them well-suited for Raman spectroscopic applications, enabling precise measurements of molecular vibrations.
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