BC VCSELs for RF/Photonic Links

June 1, 2008

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

Bipolar cascade (BC) vertical-cavity surface emitting lasers (VCSELs) of a type intended specifically for use in radio-frequency (RF)/photonic links have been demonstrated. As used here, "RF/photonic link" signifies a communication link in which, at the transmitter, the continuous-wave output of a laser is modulated with an RF (typically, a microwave) signal and the resulting modulated laser beam is conveyed via free space or an optical fiber to a receiver, where the RF signal is detected. RF/photonic links offer advantages for short-range, wide-band communications and for diverse functions involved in phased arraying of large microwave antennas.

In comparison with conventional single laser diodes heretofore used in RF/ photonic links, BC VCSELs offer advantages of (1) better impedance matching with electronic drive circuitry and (2) greater modulation efficiency and, therefore, (3) lower insertion losses. In principle, one could obtain these advantages by constructing series stacks of conventional semiconductor lasers. However, the costs would be very high, and the solder bonds between individual lasers in stacks would induce large RF parasitic loss and increase sensitivity to thermomechanical stresses. Therefore, such series laser stacks would perform suboptimally and exhibit low reliability. By virtue of their integration into monolithic packages, BC VCSELs offer potential advantages of reduced packaging costs, greater reliability, and lower parasitic RF losses, in comparison with series stacks of conventional semiconductor lasers.

This Schematic Layer Diagram represents a single-stage BC VCSEL that was fabricated and tested. The curve qualitatively indicates the variation of the optical field with depth in the optical cavity, which is 21D2 wavelengths deep.

A single-stage BC VCSEL of the present type (see figure) includes a multiplequantum- well active region (AR) comprising In_0.2Ga_0.8As wells separated by GaAs barriers, a reverse-biased tunnel junction (TJ) comprising a layer of GaAs δ-doped with Si in one sublayer and doped with C in another sublayer, spacer layers of undoped GaAs, and an oxide aperture (OA), all incorporated into a resonator ("optical cavity") structure by sandwiching them between two distributed Bragg reflectors (DBRs) comprising multiple alternating n-doped layers of GaAs and Al_0.9Ga_0.1As. A multistage BC VCSEL is similar except that it incorporates multiple ARS, TJs, and OAs. The optical cavity must have a depth equal to an integer number of half laser wavelengths.

The present BC VCSELs were developed following a systematic approach to designing, fabricating, and characterizing the TJs (which are critical); incorporating the TJs into edge-emitting bipolar cascade lasers; and, finally, making the transition to VCSEL structures. In a departure from conventional practice, prior to fabrication and characterization of BC VCSELs, bipolar cascade light-emitting diodes that incorporate microcavities were investigated (the advantage of this departure was that it disentangled effects of microcavities from those of VCSEL cavities). Then best-performing design providing for electron-acceptor-doped (p-doped) OA microcavities was adopted for the microcavities in one-, two-, and threestage BC VCSELs to operate at a wavelength of 980 nm. These BC VCSELs were fabricated and tested. In the tests, the modulation frequency responses of the two-stage and three-stage VCSELs were found to be characterized by halfpower- falloff frequencies of 4.5 and 7.1 GHz, respectively, in response to smallsignal current injection at an operating temperature of -50°C. At room temperature, the half-power-falloff frequency of the three-stage device was 3.2 GHz.

This work was done by William J. Siskaninetz of the Air Force Institute of Technology for the Air Force Research Laboratory.