1. Introduction

To date, mid-infrared (mid-IR) sources produce relatively low output powers within the 3 – 4 µm wavelength region. Mid-IR applications such as remote chemical sensing and optical communication require higher output powers than what is currently available [1,2]. Transition-metal-doped chalcogenides have the potential to produce higher output powers that are of interest within the mid-IR region [3–6]. This work focused on the use of Co:CdTe to amplify the output power of an Interband Cascade Laser (ICL) at 3.7 µm.

The original work for this class of material identified the absorption, emission and lifetime features of chromium (Cr), cobalt (Co), nickel (Ni), and iron (Fe) doped into zinc chalcogenides to understand their potential as tunable mid-IR laser media, while additionally demonstrating a novel 2.35 µm Cr²⁺:ZnSe laser [3]. Since then, continuous-wave (CW), gain-switched, free-running (FR) and mode-locked lasing of transition-metal doped II-VI semiconductor materials have been reported [5]. Such progress has predominately focused on Cr²⁺- and Fe²⁺- doped zinc chalcogenide gain media, while other binary and ternary hosts such as CdTe, CdSe and CdMnTe doped with transition metal ions have shown promise for efficient lasing [5,7–11]. The absorption and emission properties of cobalt doped II-VI gain media lie within the 3 – 4 µm spectral region, but have historically been limited in research and development due to the absence of convenient 3 µm pump sources [12]. Table 1 lists a summary of progress in lasers based on transition-metal-doped cadmium-based crystal hosts.

Table 1. Progress summary of TM-doped cadmium chalcogenide lasers

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In addition to the work listed in Table 1, the spectroscopic literature on Co:CdTe has reported absorption peaks at 1808 nm on the ⁴A₂ → ⁴T₁(⁴F) transistion as well as 915 nm, 902 nm and 873 nm peaks on the ⁴A₂ → ⁴T₁ (⁴P) transistion [13]. A study featuring Co²⁺Cd_0.55Mn_0.45Te reported absorption peaks at 900 nm on ⁴A₂ → ⁴T₁ (⁴P), at 1900 nm on ⁴A₂ → ⁴T₁(⁴F), and broad mid-IR luminescence centered at 3600 nm following optical excitation at 1860 nm on the ⁴T₁(⁴F) → ⁴A₂ transistion [14].

Room-temperature and cryogenically cooled laser oscillation attempts of Co:II-VI gain media have been made with Co²⁺:ZnSe and Co²⁺:ZnS on the ⁴A₂ ↔ ⁴T₂, ⁴A₂ ↔ ⁴T₁(⁴F), and ⁴A₂ ↔ ⁴T₁(⁴P) transistions [15]. However, no optical gain or lasing has been reported for any type of Co:II-VI gain medium.

2. Sample and spectroscopy

The Co:CdTe sample was grown using the Bridgman vertical growth process by Brimrose Corporation of America. Temperature dependent absorption, emission and lifetime data was collected and analyzed throughout previous work [16–18]. Material advantages of Co:CdTe include an emission bandwidth of approximately 725 nm centered at 3720 nm, which is contained within the 3 – 5 µm atmospheric transmission window. Co:CdTe also features a maximum radiative lifetime of 625 µs, which is considerably longer than other transistion metal dopants, enabling the possibility of increased energy storage [3, 5, 6]. Additionally, Co:CdTe can be produced directly from a stoichometrically controlled melt, which is a simpler manufacturing method compared to what is required of zinc chalcogenides such as chemical vapor deposition and post growth diffusion processes. However, strong phonon interactions at room temperature require cryogenic operation to prevent the non-radiative transitions from depleting the excited ions. CdTe also has a low thermal conductivity of 6 W/m·K at room temperature and Co:CdTe has a significant decrease in emission intensity at temperatures greater than 77 K [16,19].

Figure 1 shows the emission and absorption cross-sections of Co:CdTe on the ⁴A₂ ↔ ⁴T₂ transition. The absorption and emission cross-sections were determined by the Integral Reciprocity and Füchtbauer–Ladenburg methods, respectively [10]. Table 2 lists the spectroscopic values of Co:CdTe that were measured at 77 K as part of this work.

 

Fig. 1 Absorption and emission cross-sections of Co²⁺:CdTe.

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Table 2. Co²⁺:CdTe spectroscopy data collected at 77 K

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3. Experimental details

The Co:CdTe crystal was diced to 14 mm × 1.5 mm × 7 mm, polished and anti-reflection (AR) coated between 2.8 – 4.6 µm. The sample was then sputtered with approximately 250 nm of unprotected gold on the rear face to create a highly reflective surface. The sample was mounted into a side-facing dewar, which contained a broadband AR coated sapphire window. It was then cooled to 77 K by liquid-nitrogen (LN₂) and pumped by a 2825 nm single-mode erbium fiber laser. The pump beam was collimated with a 40 mm plano convex (PCX) lens and reduced from a 6 mm to 3 mm beam by a Keplerian telescope, to better match the ICL beam diameter. The ICL was thermoelectrically cooled and operated at 25°C. An aspheric lens with a focal length of 4 mm was used to collimate the ICL beam. The incoming beams were focused into the dewar and onto the gain medium by a 100 mm PCX lens, to a calculated pump and signal spot size of 98 µm and 130 µm respectively. The optical chopper was set to 11 Hz, with a measured duty cycle of 17%. The dewar was set at an angle of approximately 45° to allow the reflected light to be directed away from the incoming laser beams, where the seed signal was then isolated using another dichroic mirror (CaF₂, AR at 2936 nm 15 – 20°) and 3000 nm long-pass filter as shown in Fig. 2. A LN₂ cooled InSb detector and spectrometer were used to collect the spectral intensity of the seed signal. The spectrometer had a reciprocal linear dispersion of 20 nm/mm and the entrance/exit slit widths were to 200 µm. The internal grating featured a groove density of 300 g/mm and was blazed at 3000 nm. An InSb detector and oscilloscope were used to directly observe the output of the amplifier.

 

Fig. 2 Double-pass Co:CdTe Mid-IR Laser Amplifier.

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4. Results and discussion

The optical gain of the Co:CdTe mid-IR amplifier was measured by both spectral and temporal methods. The spectral method consisted of measuring the area under the spectrum of the seed signal with and without the pump present. Similarly, the temporal method consisted of calculating the area under the detector signal of the seed trace with and without the pump present. Figure 3(a) shows a broadened spectral gain centered at 3810 nm with a bandwidth of approximately 72 nm and Fig. 3(b) shows the chopped temporal gain, with a pulse width of approximately 16 ms. Trace A in both Fig. 3(a) and Fig. 3(b) represent the seed signal without the pump. Trace B is the amplified seed signal with the pump. The maximum spectral gain was observed at an instantaneous pump power of 3.25 W and the maximum temporal gain was observed at an instantaneous pump power of 4.25 W. Traces C and D correspond to pump powers of 6.35 W and 8.82 W, respectively. Note, at such instantaneous pump powers, the gain is quickly suppressed by thermally induced losses at the seed wavelength.

 

Fig. 3 Gain measurements of the double-pass Co:CdTe mid-IR laser amplifier. Trace A is the seed signal without the pump. Trace B is the maximum amplified seed signal. Traces C and D are the measured seed signals at instantaneous pump powers of approximately 6 and 8 W, respectively.

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Table 3 lists the incident beam values that were recorded and calculated at the setpoint where a maximum gain was observed for each data collection method. The temporal and spectral gains were recorded at different setpoints, with a maximum gain of 2.8 for each data collection process (see Fig. 4). The CW output power of the ICL was measured at approximately 20 mW, corresponding to 3 mW chopped at a 17% duty cycle.

Table 3. Incident pump and signal values recorded at gain maximum

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Fig. 4 Optical gain as a function of instantaneous pump power.

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Fig. 4 shows the optical gain with respect to the incident pump power. The observed decrease in gain at pump powers > 4.25 W is due to thermal broadening of the absorption spectrum, as well as the reduced emission intensity and flouorescence lifetime of Co:CdTe at temperatures greater than 77 K. Thermal modeling predicts a theoretical on-axis temperature rise of approximately 94 K or greater from the heat induced by the pump laser [17]. Due to the low thermal conductivity of the material, such a temperature rise will broaden the absorption bandwidth, increase the absorption coefficient at the seed wavelength and suppress the overall gain. For this reason, the seed did not experience gain at relatively high pump powers.

The temperature dependent emission intensity of Co:CdTe is significantly higher at lower temperatures, while the temperature dependent absorption coefficient of the seed wavelength is reduced at lower temperatures [16]. Therefore, it is hypothesized that cooling the Co:CdTe crystal to temperatures below 77 K should mitigate the thermal effects that appear to limit the achievable gain within the crystal. However, such experiments would require more complicated cooling systems and operating at such low temperatures is considered to be impractical for many solid-state laser applications. In addition to cooling the sample to lower temperatures, other cooling mechanisms and heat sink geometries could potentially mitigate the thermal effects within the crystal. For example, cooling the front and rear faces simultaneously, while end pumping the Co:CdTe sample should allow for better thermal management.

5. Conclusions and future work

This work demonstrated a peak gain of 2.8 spectrally and temporally by amplifying a mid-IR ICL using a cryogenically cooled Co:CdTe crystal. It was observed that even small temperature rises from 77 K can be detrimental to the optical gain of Co:CdTe due to the low emission intensity at higher temperatures and thermal broadening of the temperature dependent absorption bandwidth. However, mid-IR amplification at 3.7 µm was successfully demonstrated. Future work will consider optimization of the experimental parameters, multi-pass configurations to increase the extraction efficiency and different thermal management techniques to mitigate the steady-state and time dependent temperature rise of the crystal.