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A sample of Fe:CdMnTe was grown from melt using the Bridgman technique and was polished and coated for use as a laser crystal. Optical spectroscopy techniques were used to determine the absorption and emission cross-sections of the sample at 80 K. A cryogenic free-running mid-IR laser was constructed in the X-cavity configuration. The laser output was centered at 5223 nm with a spectral width of 1 nm with a maximum average power of 810 mW. The laser exhibited a slope efficiency of 16.4% with respect to total pump power with a pulse repetition frequency of 400 Hz and a pulse duration of 220 μs.
© 2017 Optical Society of America
1. Introduction
Many chemical species exhibit optical absorption in the so-called molecular fingerprint region of the electromagnetic spectrum from 3 – 8 μm. Consequently, lasers operating in this region are of interest for a variety of military, scientific, and medical applications including remote chemical sensing, bench-top spectroscopy, and reaction diagnostics. Despite this interest, few coherent sources with significant output power in this waveband have been developed.
Recent progress with quantum cascade lasers (QCLs) has led to the development of laser sources operating in the 3 – 5 μm atmospheric transmission window [1]; however, these sources are limited in brightness by damage issues and are generally not capable of emitting energetic pulses. Nonlinear optical sources such as the optical parametric oscillator (OPO) can produce energetic pulses of radiation in this waveband [2–4]. However, the large quantum defect associated with OPOs leads to low slope efficiencies. Furthermore, relatively high intensities are required to activate the nonlinear effects involved, so continuous-wave operation of a mid-IR OPO is usually inefficient.
In contrast, transition metal (TM) doped II–VI materials are capable of high efficiency and high-power operation over a wide spectral range and a variety of pulse formats [5–7]. In such materials, Fe impurities incorporate substitutionally for the group II ion and assume a 2+ ionization state. The Fe2+ ion is tetrahedrally coordinated by group VI ions, giving rise to a symmetric Coulomb potential which splits the multiply degenerate 5D ground level of the ion into an upper 5T2 manifold and a lower 5E manifold. In Fe:ZnS and Fe:ZnSe, optical transitions between these two manifolds have been used to create efficient laser devices [8–12].
The output wavelength of such lasers is determined by the energy splitting between these manifolds which is inversely proportional to the inter-ionic distance between the impurity and the group VI element raised to the fifth power: a5 [13]. Thus, the output wavelength of Fe:ZnS lasers is typically shorter than that of Fe:ZnSe lasers. Similarly, telluride materials such as Fe:CdTe and Fe:ZnTe exhibit absorption and florescence bands centered at longer wavelengths than those of Fe:ZnS and Fe:ZnSe due to the increased inter-ionic distances associated with the more massive constituents [14, 15].
However, laser-quality Fe:CdTe and Fe:ZnTe are notoriously difficult to produce. Fortunately, recent studies have shown that ternary alloys of tellurium, such as CdxMn1−xTe (CMT) and CdxZn1−xTe, are suitable hosts for Fe2+ ions [16–19]. In this work, we demonstrate > 800 mW of average power at 5.2 μm from an Fe:CMT laser.
2. Sample preparation and analysis
A boule of Fe:CdxMn1−xTe was grown from melt by Brimrose Corporation using the Bridgman technique with a targeted Fe impurity concentration of 1019 ions/cm3 and a targeted alloying fraction of x = 0.85. Several cylindrical cross-sections with diameters > 25 mm were sliced from the boule. Square samples of approximately 20 × 20 × 2.65 mm3 were cut from each cylinder and polished on the largest faces. The index of refraction of one sample was measured on a J. A. Woollam Co. IR-VASE ellipsometer. This result appears in Table 1 alongside some literature values of relevant optical properties of CdMnTe with alloying fractions similar to the samples used in this work. From the Sellmeier equations of Schubert et al. [20], the measured index is shown to be consistent with an alloying fraction of x = 0.91.
Table 1. Some physical properties of CdMnTe and Fe:CMT.
The room-temperature absorption of the sample was spatially and spectrally resolved using an apertured Fourier transform interferometer (FTIR) addressing nine separate locations forming a 3 × 3 grid on the polished surface. The outer grid locations were observed to have excellent site–to–site transmission uniformity from 1 – 3 μm and exhibited strong attenuation beyond 3.3 μm due to the absorption by the Fe2+ ions. The optical transmission from 1 – 3 μm was reduced by a factor of five in the center location with respect to the outer locations.
The square sample was then diced into rectangular pieces of varying sizes. These samples were cut from the outer grid locations of the square cross-section. One of the samples was mounted in a closed-cycle helium cryostat and cooled to 80 K. The transmission spectrum was recorded using a Nicolet 6700 FTIR. The transmission spectrum was corrected for Fresnel reflection and the absorption coefficient α (see Fig. 1) was determined using Beer’s law:
where L is the crystal thickness and I/I0 is the measured transmission fraction.
Fig. 1 The un-corrected absorption and emission cross-sections of the Fe:CMT sample at 80 K. Absorption features due to atmospheric H2O and CO2 were verified using HITRAN Online [23].
Mid-IR fluorescence was generated in the sample via excitation with laser pulses centered at a wavelength of 3.45 μm, with an average duration of < 40 ns, and which were generated via optical parametric oscillation (OPO) in a MgO:PPLN grating (Λ = 30.5 μm) pumped by a Spectra-Physics T80 Nd:YVO4 laser operating at 1.064 μm. The decay traces were recorded directly using an InSb detector and a digital oscilloscope. The decay was fit with a single exponential and the fluorescence lifetime was determined to be 111 μs at 80 K and 78.5 μs at 10 K. The increase in the lifetime at 80 K is hypothesized to be due to absorption of the fluorescent radiation with subsequent re-emission. The details of this process are beyond the scope of this paper and will be discussed in future work.
The fluorescence was resolved using a Princeton Instruments SP-2500 spectrometer with a 300 g/mm grating blazed at 3 μm. The fluorescence of the sample was recorded using an InSb detector in combination with an SRS830 lock-in amplifier. The uncorrected fluorescence spectrum was transformed into an emission cross-section σem using the Füchtbauer–Ladenburg equation:
where If (λ) is the collected fluorescence spectrum. The radiative lifetime was assumed to be the measured value of the lifetime at 10 K. The integral reciprocity equation was then used to calculate the absorption cross-section: where Ia (ν) is the collected absorption spectrum, G1 = 10 is the degeneracy of the 5E manifold, and G2 = 15 is the degeneracy of the 5T2 manifold. The calculated absorption and emission cross-sections are shown in Fig. 1. An estimate of the average concentration Ncalc = α/σabs of Fe2+ ions can then be made (see Table 1). Errors in the measurement of τrad can drastically affect the value of the cross-section and the estimated concentration; thus the values presented in this work should be considered approximate. Further work is needed to refine the estimate.3. Apparatus
The laser sample used in this work was cut to 2.65×4×7 mm3 and polished on the 2.65×7 mm2 facets. The polished facets were anti-reflection (AR) coated for a pump wavelength of 4 μm and for a signal wavelength of 4.5 – 5.5 μm. For use in a laser resonator, the Fe:CMT sample was wrapped in indium foil and secured tightly to a copper cold finger within a liquid nitrogen dewar. The dewar was evacuated to approximately 1 mTorr and cooled with liquid nitrogen. The absorption coefficient of the sample was measured to be 5.5 cm−1 at the pump wavelength. From Beer’s law, it was determined that 90% of the pump pulse would be absorbed over the 4 mm length of the Fe:CMT crystal (ignoring saturation effects).
The laser resonator was constructed using the familiar X-cavity geometry of Kogelnik [24] as shown in Fig. 2. The focusing lens L1 had a focal length of 100 mm and mirrors M1 and M2 were separated by a distance equal to their radii of curvature (100 mm); thus, the laser mode was collimated in the folded legs of the resonator. Mirrors M1, M2, and M3 were coated as dichroic elements with > 98% transmission at 4 μm and a high-reflection (HR) band from 4.5 – 5.5 μm. Windows W1 and W2 were AR coated from 3 – 5.5 μm with approximately 95% transmission at 5.2 μm. The outcoupling mirror OC had a reflection of 65% at 5.2 μm.
The radius of the pump laser at the beamwaist was measured to be approximately 190 μm using the knife edge technique. The beamwaist of the resonator mode was calculated to be 95 μm using the self-consistent ABCD matrix method. Clearly, the size mismatch between the pump and resonator modes was significant. Unfortunately, improving the mismatch was not possible due to the limitations of available optics. Thus, the laser was not expected to run near the quantum limit of efficiency.
4. Results
The Fe:CMT laser was pumped at 4 μm by a Fe:ZnSe laser similar to [25] with a pulse repetition frequency (PRF) of 400 Hz and a pulse width of 220 μs. The laser operated in the free-running regime with no spectrally selective elements in the laser cavity and the instantaneous intensity of the output pulse closely followed that of the pump pulse. The pulse characteristics were recorded using a VIGO PVM-10.6 photodiode, which had a rise time of < 3.3 ns. The pulsetrain and a detailed view of an average pulse are shown in Fig. 3.
Fig. 3 The output pulse train of the laser (top). A detailed view of the pulse shape (bottom). Both traces are the triggered average of 10 frames.
The output spectrum of the laser was recorded using a Thorlabs OSA205 mid-IR optical spectrum analyzer (OSA). The output wavelength was determined to be 5.223 μm with a spectral full-width at half-maximum (FWHM) of 1 nm (see Fig. 4). This result is both notable and surprising in contrast to the broader emission typical of Fe:ZnSe lasers [26]. Ternary crystals are generally much more defective than their binary analogs, so local variations in crystal field effects should induce more broadening than observed in binary materials such as Fe:ZnS, Fe:ZnSe, and Fe:CdTe. The narrow spectral output of the laser is attributable to the fact that the crystal was grown from melt rather than doped via post-growth thermal diffusion. Transition metal ions doped into solid crystals by post-growth diffusion are known to accumulate near defects [27]. In contrast, the Bridgman technique incorporates active ions from the melt and does not depend on defects to admit the Fe2+ ion. Thus, the uniformity of doping is improved and the active ions occupy sites that are not in the neighborhood of defects that perturb the tetrahedral symmetry of the Fe2+ ion’s nearest neighbors. This effect was first noted recently in Cr:ZnSe doped via hot isostatic press by Stites et al. [28].
Fig. 4 The output spectrum of the Fe:CMT laser. The wavelength resolution of the OSA was 0.7 nm; thus, the emission line is barely resolved.
The output power was measured immediately after the output coupler. The slope efficiency was determined relative to the input power as measured immediately prior to the focusing lens L1. Intracavity absorption of the laser wavelength by the atmosphere and the significant transverse pump–mode mismatch mentioned in the previous section contributed to a slope efficiency that was substantially less than the quantum efficiency ηq = ηp/ηl = (4.05 μm)/(5.22 μm) = 77.6%. As shown in Fig. 5, the output power of the laser increased with a slope efficiency of 16.4% at low power and the threshold of the laser was approximately 50 mW. The maximum average power achieved was 810 mW at 8.05 W of input pump power, which corresponds to a pulse energy of 2.03 mJ, a peak power of 9.2 W, and a total efficiency of 10.1%.
Fig. 5 Power scaling characteristics of the Fe:CMT laser. Data points are shown in red and a linear fit to the data is shown in black.
Thermal effects are most likely responsible for the reduced slope efficiency at pump powers > 2 W. Two such effects are especially relevant. First, given the relatively low optical efficiency of this laser, it is likely that the maximum temperature of the sample in the lasing region rises to the point where thermally-activated nonradiative quenching (NRQ) begins to affect the lifetime of the Fe2+ ions, and thus reduces the laser gain of the sample. Second, the temperature gradient in the lasing region can induce a thermal lens that destabilizes the laser resonator. Thermal effects of this type in transition metal lasers using ZnSe as a crystal host have been studied extensively by Schepler et al. [29]. By comparison, the thermo-optic coefficient of CdMnTe is > 2× that of ZnSe and the thermal conductivity of CdMnTe is 1/8 that of ZnSe. The effect of Fe2+ impurities on these quantities is not known, but is assumed to be small. Thus, it is possible that both NRQ and thermal lensing are driving slope-efficiency rollover. These thermal effects can likely be mitigated in future work with optimization of the resonator which will lead to improvements in laser slope efficiency.
5. Conclusions and future work
In conclusion, we have reported demonstration of a free-running Fe:CdxMn1−xTe laser emitting at 5.223 μm. The spectral width of the output was 1 nm, which is substantially narrower than typical free-running Fe:ZnSe lasers. The maximum average output power was recorded to be 810 mW with a PRF of 400 Hz and an average pulse duration of 220 μs. The output pulse energy was calculated to be 2.03 mJ with a peak power of 9.2 W. The laser exhibited a slope efficiency of 16.4% with respect to total pump power at low power.
Future work will include optimization of the resonator optics for maximum pump–mode overlap, optimization of the output coupler reflectivity, and operation under a nitrogen purge to mitigate the effect(s) of atmospheric absorption. Given the low threshold of lasing obtained in this work, these improvements should enable an internal conversion efficiency that approaches the quantum limit. Additionally, spectral tuning across the fluorescence band from 4.6 – 5.4 μm should be possible using a blazed grating in the Littrow configuration in place of M3.
Since Fe:ZnS and Fe:ZnSe have been demonstrated at room temperature in the gain-switched regime, Fe:CMT should, in principle, be capable of room temperature oscillation in that regime, so this mode of operation could be pursued. Finally, CdMnTe alloys with increased fractions of Mn could be used. In particular, Fe:CdxMn1−xTe alloys with x < 0.55 promise to extend the output wavelength of this class of material beyond 6 μm.
Funding
We acknowledge and thank the Sensors Directorate and the Air Force Office of Scientific Research (AFOSR) for funding this effort. This research was performed while Thomas R. Harris held an NRC Research Associateship award at the Sensors Directorate.
Acknowledgments
We thank Gary Cook and Rita Peterson of the Sensors Directorate for technical discussions as well as Shekhar Guha of the Materials and Manufacturing Directorate for verifying the absorption uniformity of the Fe:CMT samples. Jonathan W. Evans thanks Nancy Giles of the Air Force Institute of Technology (AFIT), Sudhir Trivedi of Brimrose Corporation, and Ken Schepler of the University of Central Florida for technical discussions.
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