Laser Diodes: Monolithically unified high-power tapered laser enables true single-mode

ADAM ERLICH

Multimode semiconductor laser diodes with >10 W output are readily available. However, single-mode lasers that can operate at such power levels without optical pumping have only previously been created in university or government laboratories. Sheaumann Laser (Marlborough, MA) is working in combination with the University of Massachusetts Lowell (Lowell, MA) to create a commercially viable, monolithic master oscillator power amplifier (MOPA) in the form of a tapered laser with power levels of more than 8 W with true single-longitudinal-mode operation.¹ This is one of the first-ever attempts to develop a manufacturable, robust, and reliable commercial tapered laser, especially at this power. 

This tapered laser is a monolithically integrated photonic device capable of true single-mode output at high power. It includes a master oscillator section consisting of a seed laser with an integrated Bragg reflector and a power amplifier section.

Demand is rapidly accelerating for high-power, compact, reliable single-mode semiconductor lasers for applications including lidar, medical, laser pumping, and Raman spectroscopy.^2,5 Currently, single-mode sources that are available in the market do not have sufficient power, are complex to integrate, are large, and are too expensive.^3,4 Sheaumann’s new monolithic tapered laser provides true single-mode operation without the drawbacks of existing solutions.

The tapered laser is unique because it is a high-power, single monolithic device, making it compact and low-cost to manufacture in volume. An added benefit of the monolithic structure is higher overall efficiency than a diode-pumped solid-state (DPSS) laser. This is because a DPSS laser is optically pumped, whereas a tapered laser directly converts electricity into laser light. 

Development details

Although there are other tapered laser amplifiers available as standalone solutions, they require further integration and do not result in the advantages of a monolithic device. The unified structure reduces the footprint by about a factor of two, while reducing the cost of the package. In addition, the integrated structure prevents mechanical alignment errors between separate components and makes it easier to integrate within a fiber-coupled package. Creating this structure and making it all work together is a complex development process.

This integrated system required extensive modeling and empirical work to simultaneously optimize epitaxy and fabrication for each of the three devices that compose the tapered laser. For example, maintaining good mode structure involved an optimization of the epitaxial layers across the various elements of the device. Designing the right gratings within the distributed Bragg reflector (DBR) section was equally important. This design was complicated because optimal parameters, such as duty cycle and etch depth, needed to be determined to produce the proper beam performance, not just for the DBR as an independent section, but also for the interaction between the DBR and the amplifier. This is because the DBR section and amplifier section act together as a larger cavity length than the DBR alone.

Another critical aspect in the development of the tapered laser was properly implementing electrical connections and power management. The master oscillator section consists of a seed laser with an integrated Bragg reflector and a power amplifier section. This section of the chip requires milliamps of current to operate, whereas just adjoining the oscillator is a tapered amplifier section that operates at amperes of current. Providing contacts to both sides while minimizing crosstalk is essential, as otherwise failure of the seed laser can result. Managing these different current requirements on a very small piece of semiconductor real estate required innovative approaches both in terms of power management and development of good electrical connections.

One of the primary advantages of the monolithic structure is that it reduces the footprint of the package and prevents mechanical alignment errors between separate components. A challenge that was overcome by the engineers at Sheaumann in the development of the integrated device was getting stable mode behavior with good beam quality, while simultaneously attaining high-power operation. Furthermore, this power had to be achieved with an acceptable threshold current and slope efficiency.

Reliability and lifetime are directly related to proper thermal management. A high-powered semiconductor laser requires creative packaging and cooling techniques to function reliably. Creative solutions related to connecting the submount to the heatsink were developed to dissipate the heat. Furthermore, the ability to develop facet coatings that can survive high power levels was essential to ensure lifetime and reliability.

Meticulous design, planning, and execution are required to develop a unified structure with separate device elements made as one unit, particularly when it is produced across an entire wafer. Sheaumann developed a process flow to accommodate the needs of each of the elements at the subcomponent level and at the wafer level. 

Technology advantages

There are a number of real-world benefits to the MOPA tapered laser vs. competing technologies. Because the tapered laser is a single integrated system, it can be packaged into a compact form factor that can be fiber-coupled. Although a DPSS laser can achieve high-power, single-mode operation, the DPSS is typically at least an order of magnitude larger and less efficient than a tapered laser.

Currently, some of the most commonly used sources for true single-mode operation include DBR diode lasers, distributed feedback (DFB) lasers, ridge lasers, and broad-area emitters.⁴ When compared against monolithic MOPAs, DBR lasers demonstrate significant spectral mode-hopping and higher levels of thermal drift at 500 mW of spectral single-mode operation.² Although ridge lasers provide good beam quality, they cannot deliver high power levels. And broad-area emitters provide high power, but with poor beam quality and with unwanted thermal effects such as catastrophic optical damage (COD).^1,4 In contrast, tapered lasers deliver both high power and a high-quality beam.

Sheaumann is currently developing a 1060 nm tapered laser, and other wavelengths from 780 to 1080 nm are possible. The company will soon be able to grow on indium phosphide (InP) wafers, expanding the wavelength range up to 1875 nm. Sheaumann can customize these tapered lasers and its other laser diode products and reliably manufacture them in high volume.

ACKNOWLEDGEMENT

The author would like to thank the Sheaumann team, who developed the technology and/or assisted in writing this article. 

REFERENCES

1. J. M. G. Tijero et al., IEEE Photon. Technol. Lett., 19, 20, 1041–1135 (2007).

2. N. Werner et al., Proc. SPIE, 10553, 105531D, 1–9 (2018).

3. C. Rablau, Proc. SPIE, 11443, 111430C, 1–14 (2019).

4. G. Kochem, M. Haverkamp, and K. Boucke, Proc. SPIE, 6997, 69971A (2008); https://doi.org/10.1117/12.780241.

5. J. A. Beil et al., Proc. SPIE, 10514, 10514U, 1–7 (2018).

Adam Erlich is director of sales and marketing at Sheaumann Laser, Marlborough, MA; e-mail: [email protected]; www.sheaumann.com.