The World's Highest Electro-Optical Conversion Efficiency of 62% in Vertical Cavity Surface Emitting Lasers (VCSELs) Has Been Achieved

Furukawa Electric has achieved the world's highest electro-optical conversion efficiency of 62% in vertical cavity surface emitting lasers (VCSELs).

Since improvement in the electro-optical conversion efficiency directly results in the reduction in power consumption, use of the VCSEL developed here is expected to realize low power-consumption lasers. It is further anticipated that the laser would contribute to green IT and CO₂ emission reduction in future. This is because applications of low power-consumption lasers include high-power lasers for industrial processing as well as optical interconnection modules, which are gradually introduced into data servers, where such issues as processing capacity, power consumption and heat generation are becoming problematic due to the dramatic increase in data transmission capacity.

Developmental Background and Key Points

Despite the fact that the numerous features of VCSEL ^{(Note 1)} had been successfully validated, there remained room for improvement of operational power consumption, i.e., a key factor for realizing environment-harmonized products. To this end, Furukawa Electric drastically studied the laser design and chip fabricating method, thereby achieving the world's highest electro-optical conversion efficiency as high as 62%.

Key points in the development are as follows:

1) To increase lasing efficiency

The high-quality indium-gallium-arsenide (InGaAs) strained quantum well, which is actually used in the 980-nm laser for in-house optical amplifiers, was used for the active layer. For its crystal growth, the molecular beam epitaxy (MBE) technology was employed, enabling thickness control with the precision of atomic layer level. Use of this material allows for not only obtaining high-efficiency lasing characteristics but also achieving ultra-fast operation.

2) To reduce electrical loss and optical loss

Since in VCSELs, unlike in ordinary lasers, the direction of the current path coincides with that of the lasing cavity, the electrical loss is in a tradeoff relationship with the optical loss. In order for this tradeoff to be optimized, it is essential to quantitatively understand the fundamental properties of the material. We utilized the semiconductor laser processing technology, which we have long cultivated on pumping lasers for optical amplifiers, to study the reduction of electrical loss, thereby achieving electrical resistance reduction required for electrical loss reduction. As for the optical loss reduction being in a tradeoff relationship with the electrical loss, we took full advantage of the MBE technology to precisely control the layer thicknesses in the laser cavity, and succeeded in suppressing the optical loss while minimizing the electrical loss.

Characteristics of VCSEL Developed

Figure 1 shows the structure of VCSEL developed here, and Figure 2 its characteristics. In Figure 2, curve (1) represents the electrical current vs. optical output characteristics (input-output characteristics at 25°C and 90°C) with its optical output scaled on the left vertical axis. The slope of this curve, called slope efficiency, is approximately 1 W/A in this example, and the threshold current (the electric current with which lasing initiates) is 0.5 mA, an amply low value. Curve (2) shows the electric current vs. electric voltage characteristics. Curve (3) shows the electro-optical conversion efficiency (i.e., power conversion efficiency, PCE) represented by “optical output / electric current multiplied by electric voltage” with its value scaled on the right vertical axis. It can be seen that the efficiency is 62% in this example. Degradation in the conversion efficiency as electric current increases has been suppressed low, since both the electrical loss and the heat dissipation have been improved.

Currently, the PCEs of electric-to-optical converters, i.e., transceivers used in optical transmission equipment are generally 30 ~ 40%.

The highest PCE reported so far is 57% from Ulm University in Germany. But in this case, the efficiency significantly decreased as the electric current increased.

It is expected that the VCSEL developed here will be used as a laser light source of high efficiency to be utilized in parallel optical interconnection modules and high power processing machines.

Figure 1 Structure of 1060-nm VCSEL developed here.

Figure 2 Input-output characteristics and electro-optical conversion efficiency.

Photo of VCSEL Chip

VCSEL chips fabricated on a 3-in GaAs wafer. Several tens of thousands of chips can be fabricated on this wafer.

Remarks

This achievement has been presented as a post deadline paper to the 21st International Semiconductor Laser Conference held in Italy in the end of September.

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