Low power Mid-IR laser light exhibits much lower attenuation in propagation through the New York metro area when compared to Near-IR wavelengths. Depending on the type of atmospheric extinction we record a reduction of up to 800% in the exponential Beer-Lambert coefficient for Mid-IR light compared to Near-IR, thereby demonstrating the possibility of significantly increased deployable range and SNR of current communication systems by utilizing the Mid-IR spectrum.
We present and analyze transmission data from an outdoor collinear, coaxial, multi-wavelength laser test bed comparing 1.31&mgr;m, 1.55&mgr;m and 8&mgr;m through outdoor atmospheric fog and rain over a 550 m free space optical link across the Stevens Institute of Technology campus. This is achieved using lasers with average power ranging from 1 mW (Mid-IR QCL) to tens of milliwatts which have been normalized under lock-in detection.
We also present corroborating results from an indoor fog experiment simulating various fog types. Here we have also deconstructed Beer's attenuation coefficient and distinguish the contribution of scattering and absorption with a purpose-built polar nephelometer. Using Mie predictions we determine and measure the extent by which a Mid-IR system scatters light less under fog than a traditional Near-IR one, hence accounting for the performance enhancement in the metro-air test bed. We conclude finally that the Kruse-Mie prediction of insignificant Mid-IR-over-Near-IR-gain is strongly in error.
We consider the application of mid-infrared (MIR) wavelength quantum cascade lasers (QCL) as sources for free-space optical communications. QCL’s possess high modulation bandwidth and excellent optical performance in the atmospherically transparent MIR spectral range. In order to investigate this potential application area, we have performed a series of comparative evaluations on analog and digital free-space optical links operating in the near-infrared (NIR) (830nm, 1300nm and 1550nm) and mid-infrared (8μm). The measurements were made using well controlled atmospheric conditions in the 65ft long Pacific Northwest National Laboratory’s Aerosol Wind Tunnel Research Facility using water vapor, oil vapor and dust as the scattering media. We measured the transmitted intensity as a function of the density of scatterers in the tunnel. We also performed bit error rate analysis of signals transmitted at the DS-3 data rate. The QCL link consistently showed a higher performance level when compared to the NIR links for water fog, oil fog and dust scattering.
In this paper we present measured modulation responses on quantum-cascade lasers (QCL) up to 10 GHz. The obtained modulation response shows a flat response over the whole frequency range, proving the broadband capabilities of these devices. Even more striking is the absence of a strong resonance peak, which demonstrates the absence of relaxation oscillations, a feature which often limits the high speed performance of standard diode lasers. This property is quite attractive for use of these devices as high-speed data sources, particularly in applications where linearity is important. To address this possible application, the digital modulation properties of these devices were tested in a standard bit-error-rate-measurement at 2.5 GBit/s for cryogenically cooled QCL, showing the suitability of QCLs in digital as well as analog telecommunication application. In addition we present recent data showing bit error measurements and eye diagrams obtained for the pulsed mode operation of QCL near room-temperature.
We present experimental results for an optical free-space high-speed link using direct modulated mid-infrared (8.1 μm) quantum cascade lasers. A stable link was realized over a distance of 200m and QPSK encoded multimedia data were transmitted and received error-free, incorporating several hundred digitally encoded multimedia channels. The reliability of the system against weather influence (fog) was experimentally compared to that of a near-infrared (0.85 μm) link. Under clear weather conditions comparable results were obtained in both links. However, a higher stability of the MIR link was clearly observed under a dense fog situation.
Continuously tunable single-mode emission of high performance quantum cascade (QC) lasers is achieved by application of the distributed feedback (DFB) principle. The devices are fabricated either as loss-coupled or index-coupled DFB lasers. Single-mode tuning ranges of approximately equals 100 nm have been measured in both of the atmospheric windows at emission wavelengths around (lambda) approximately equals 5 micrometer and 8 micrometer. Linear thermal tuning coefficients of 0.35 nm/K and 0.55 nm/K have been obtained above 200 K for (lambda) approximately equals 5 micrometer and 8 micrometer, respectively. The side-mode suppression ratio is better than 30 dB. Pulsed single-mode operation has been achieved up to room temperature with peak power levels of 60 mW. The lasers also operated single-mode in continuous wave at temperatures above liquid Nitrogen temperature; a single-mode tuning range of 70 nm has been measured in the temperature range from 20 K to 120 K. The gas sensing capabilities of the QC-laser have also been demonstrated using both direct absorption and wavelength modulation techniques. A pulsed, room temperature, QC-DFB laser operating at (lambda) approximately equals 7.8 micrometer was used to detect N<SUB>2</SUB>O diluted in N<SUB>2</SUB>. The detection limit was found to be approximately equals 500 ppb- m. In addition, the high resolution capability of the QC-DFB lasers (at 77 K) has been demonstrated via continuous, rapid- scan, direct absorption measurement of the Doppler limited R(16.5) lambda doublet of NO at (lambda) approximately equals 5.2 micrometer.
As performance levels for sophisticated electronic materials and device processing increase so to does the demand for versatile methods for on-line real time sensing of many process parameters. To that end we have developed a series of tunable semiconductor laser spectrometers for use in detection schemes of gas phase chemical species via infrared absorption spectroscopy. We report here on characterization of dual modulation schemes for enhanced sensitivity, interference fringe free, absorption spectroscopy data.
We describe a diode laser frequency control scheme that dynamically locks the laser to an absorption resonance line while simultaneously allowing the quantitative measure of absorption strengths as weak as a few parts in 10<sup>6</sup>. The method exploits a novel modulation scheme, and is capable of locking the laser to both stable and transient species in an electrically noisy environment such as that found in plasma etching reactors. We present an experimental demonstration using a lead salt diode laser operating near an rt plasma etching diode reactor and in an industrial-type triode reactor.