100 W average power 10.6 µm isolator based on the interband Faraday effect in InSb

Note: A high transmission Faraday optical isolator in the 9.2 μm range

“…The cryogenic temperature isolator of Ref. [8] clearly exhibit the best figures of merit, but a rather low isolation ratio, with the drawback of a complicated setup. Due to the high B-field and good wafer quality, our isolator shows improved performance as compared to previously published ones at room temperature.…”

“…The alternative solution relies on the Faraday effect, and was studied between the 60's and the 90's by several authors in the context of CO 2 laser applications. Mid-infrared Faraday isolation has been demonstrated using several Faraday media and different processes such as free carrier contribution at room temperature or interband and free carrier spin contributions at cryogenic temperatures [5][6][7][8][9][10][11]. Isolation ratios up to 30 dB and insertion losses of 1.5 dB (71 % transmission) have been reported.…”

Magneto-Optical Properties of InSb for Infrared Spectral Filtering

“…Such materials have been used for decades to implement large Verdet coefficients (V = ∆θ/|B|L) -the constant of proportionality between the change in polarization angle (∆θ), magnitude of the applied magnetic field (|B|), and material thickness (L) [1]. In the farinfrared (FIR) range of interest for thermal imaging (λ = 8 µm -12 µm), the best-performing Faraday rotators are doped semiconductors where the magneto-optical coupling is dominated by the free carrier effect [2][3][4][5]. Taking the Drude model for a free-electron gas, it can be shown that the Verdet coefficient varies linearly with the doping concentration (N ) and inversely with the square of the electron effective mass (m * ) as shown in Equation 1.…”

“…At shorter wavelengths approaching the electronic band gap, free-carrier rotation is reduced and interband absorption dominates. Since interband rotation opposes free carrier rotation and is independent of doping, there will exist a doping region where rotation is very low and a single doping where it is zero [4,5,7]. Additionally, the Moss-Burstein effect will modify the optical band gap, altering the usable range of wavelengths.…”

“…Verdet coefficient versus doping), while others are more complicated (e.g. temperature dependencies or absorption versus doping) or may depend heavily on sample preparation and purity [2,4,8]. Therefore, an experimental effort is warranted to determine the optimal doping for maximizing the FOM.…”