The use of 1.55μm lasers for free space optical links has become well established. While one expects there to be
advantages to operating at 3.6μm, namely higher transmission through water vapor and reduced scintillation, the
availability of both lasers and detectors at the mid IR wavelength is not as mature as those available at the telecomm
choice of 1.55μm. However, there are potential schemes for using frequency conversion to probe the atmosphere in the
mid-ir but to detect back in the near-ir.
A sequence of experiments has been conducted, over a 16km one-way link across the Chesapeake Bay, to directly
compare the intensity variances and the power spectrum imposed by the atmosphere at the two wavelengths. An
interband cascade laser was used which operated at a wavelength of 3.6μm and had an output power of 100mW. The
1.55μm system used standard telecomm parts. Data were recorded simultaneously from both systems at 4kHz and were
digitized using a 16-bit card. A telescope measured the angle-of-arrival variance of the 1.55mm beam in order to give a
measure of the atmospheric structure constant C<sub>n</sub><sup>2</sup>. In addition a visibility monitor and weather station were usually
operational at each end of the link.
Electrically-pumped photonic-crystal distributed-feedback lasers with interband-cascade active regions operating in
single spectral mode at 3.3 μm are demonstrated. At 78 K, a stripe of width 400 μm emits up to 67 mW of cw power
into a single spectral mode with side-mode suppression ratio ≈ 27 dB. The full-width at half-maximum of the farfield
divergence angle is ≈ 0.5°, which combined with the near-field profile yields an effective <i>M<sup>2</sup></i> of 1.7-2.0.
Recent advances in the development of mid-IR antimonide type-II "W" interband cascade lasers have led to a
considerably improved high-temperature operation of the devices. We report an experimental investigation of four
interband cascade lasers with wavelengths spanning the mid-infrared spectral range, <i>i.e., </i>2.9-5.2 μm near room
temperature in pulsed mode. One broad-area device had a pulsed threshold current density of only 3.8 A/cm<sup></sup> at 78 K
(λ = 3.6 μm) and 590 A/cm<sup>2</sup> at 300 K (λ = 4.1 μm). The room-temperature threshold for the shortest-wavelength
device (λ = 2.6-2.9 μm) was even lower, 450 A/cm<sup>2</sup>. A cavity-length study of the lasers emitting at 3.6-4.1 μm
yielded an internal loss varying from 7.8 cm<sup>-1</sup> at 78 K to 24 cm<sup>-1</sup> at 300 K, accompanied by a decrease of the internal
efficiency from 77% to 45%. Preliminary cw testing led to a narrow-ridge device from one of the wafers with
emission at λ = 4.1 μm operating to 288 K, a new record for interband devices in this wavelength range.