This page contains a Flash digital edition of a book.
INDUSTRY VCSELs


Figure 5. Energy efficiency (a) in units of femtojoules per bit and temperature (b) both versus error-free bit rate of state-of-the-art infrared VCSELs.


of the work at TU Berlin, wherein the VCSELs have broken every conceivable VCSEL energy efficiency record during the past several years at both 850 nm and at 980 nm (see Figure 5). By making independently similar changes to the epitaxial structure, including those previously listed, and by adding additional current spreading layers and proprietary changes to the quantum wells and distributed Bragg reflector (DBR) mirrors to increase the differential gain and the heat dissipation, the energy dissipation was decisively lowered in 2013 beyond the level requested by the ITRS road map for 2015.


We have strived to excel in this area, and earlier this year during the Photonics West 2014 conference (held during 1-6 February 2014, in San Francisco, CA) TU Berlin announced in partnership with IQE plc that error-free operation at speeds of up to 40 Gbit/s had been accomplished, with a record low energy dissipation (below 108 fJ/bit). This dissipated energy per bit at 40 Gbit/s is at least four times less than any other published result for VCSELs. For this work TU Berlin received the SPIE 2014 Green Photonics Award in Communications, after having previously already received this award in 2012.


In more recent work the 980 nm VCSELs that enabled this record also demonstrated extreme temperature stability during high-speed operation at 46 Gbit/s and temperatures of up to 85°C. These superb results were presented by the TU Berlin group at the IEEE ISLC in Palma de Mallorca, Spain during 6-11 September 2014.


The energy dissipation per bit is not fixed for a VCSEL, but depends on the transmission speed, the operating conditions, the intrinsic VCSEL materials and thus on the epitaxial design, and the other components along with the VCSELs that form a complete optical interconnect, such as the VCSEL driver electronics, the photoreceiver, and the modulation scheme. As demonstrated in 2013 when providing error- free operation at 25 Gbit/s, TU Berlin VCSELs have demonstrated that they can deliver a record-low dissipated energy of 56 fJ/bit. This remains to date the lowest reported value of dissipated energy at error-free operation for any semiconductor laser diode at any wavelength or bit rate, and it was achieved at a current density of


just ~10 kA cm-2


. This result demonstrates the suitability of


these devices for application in reliable, sustainable commercial ‘green’ optical interconnects.


Emerging new markets


Development of epitaxial processes to produce these highly complex structures, alongside refinements to the VCSEL architecture, will not just led to faster and more efficient lasers for datacentres – it will also help to drive the deployment of this class of laser in a wide range of new and emerging industrial, commercial, and consumer applications. Penetration into these new markets will be aided by establishing a European production capability that brings VCSEL manufacturing to a level comparable to LED and CMOS manufacturing. Such efforts are underway, with IQE plc taking part in a €€ 23 million programme announced this May, entitled VCSEL Pilot Line for IR Illumination, Datacom, and Power Applications (VIDaP).


Partners in this European project, which is funded by the European Commission, include Philips, STMicroelectronics, Sick and Sidel. Together, these firms will bring together existing high-volume production facilities at IQE with key end users to create a consortium delivering an end-to-end production supply chain. This should significantly reduce the cost-per-function for the VCSEL, by reducing GaAs processing costs whilst increasing device performance.


Figure 6. According to Philips, wafer costs should fall by a factor of three between 2012 and 2018, while the cost- per-Watt for high power VCSEL arrays should plummet by a factor of six over the same timeframe.


38 www.compoundsemiconductor.net October 2014 Copyright Compound Semiconductor


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76