1. Introduction and background

Tunable mid-infrared (mid-IR) lasers are of great interest for a variety of applications which include, but are not limited to, remote sensing, medical applications and molecular spectroscopy [1,2]. Transition metal (TM) ions such as chromium (Cr), iron (Fe), nickel (Ni) and cobalt (Co) are known for the strong coupling of their energy levels to the vibrational motion of their crystal host, allowing for broad emission and absorption cross-sections. Cr-doped chalcogenide gain media were first reported to have broad emission characteristics by Deloach et. al. with an emission bandwidth of 900 nm centered at 2450 nm [3,4].

Historically, tunable Cr:ZnSe narrow-linewidth (NLW) lasers have been accomplished by the use of intracavity wavelength-selective elements or injection wavelength control (See Table 1). Even though each of these methods have demonstrated sucessful wavelength tuning and spectral bandwidth reduction of Cr:ZnSe lasers, they are not without their drawbacks. The use of diffraction gratings commonly introduce relatively high insertion losses within the cavity, which increases the lasing threshold, thus limiting the available output power and reducing sytem efficiency; while wavelength tuning by angular rotation also imposes constraints on the angular resolution. Such losses are associated with finite coating reflectivity and grating efficiency being less than unity [5]. Alternatively, injection wavelength control requires the use of a tunable seed source to be coupled into the laser oscillator, greatly increasing the complexity and cost of the system [5]. Therefore, the approach of Hot Isostatic Pressing (HIP) treatment as a line narrowing technique and a liquid-crystal (LC) etalon as a purely electronic tuning element described in this work, demonstrates continuous tunability and NLW emission without the drawbacks and constraints of historical configurations.

Table 1. Tuning of Solid-State Cr:ZnSe NLW lasers

View Table

The LC etalon electronically tunes the laser wavelength within the emission spectrum of an active gain medium by the induced birefringence. This allows for tuning of the transmitted light by applying voltage to the LC etalon cell, which rotates the crystal axes of the LC waveplate. When placed inside an existing laser cavity the transmission of the LC can be changed to achieve lasing at certain wavelengths while increasing losses at other wavelengths [13].

HIP treated Cr:ZnSe as an active gain medium has demonstrated significant improvements in NLW laser performance. This has been reported to be due to the reduction in inhomogeneously broadened characteristics that exists in thermally diffused Cr:ZnSe material [14]. Tuning of the CW HIP treated Cr:ZnSe NLW laser was initially accomplished by the use of an intracavity diffraction grating in Littrow configuration [14], in contrast to the purely electronic tuning of the HIP treated Cr:ZnSe NLW laser described in this work.

2. Experimental details

A 2.8 × 6.8 × 7.7 mm HIP treated polycrystalline Cr:ZnSe gain element was used in a z-cavity configuration. The gain element had a doping concentration of N_Cr = 6.0 x 10¹⁸ cm⁻³ with each facet of the laser crystal polished and uncoated. A 1908 nm, linearly polarized CW thulium (Tm) fiber laser was used to pump the laser cavity. The active gain medium was set at Brewster’s angle between two curved dichroic mirrors (Fig. 1, M1) to minimize the losses induced by Fresnel reflections and force the gain medium to be linearly polarized. A 75 mm plano convex lens with anti-reflection (AR) coating between 1700 nm – 3000 nm was used to mode match the pump laser beam into the cavity. The two dichroic mirrors (i.e., M1) had a radius of curvature of 5 cm and were used to collimate the beam between the high reflectivity mirror (HR) and output coupler (OC) as shown in Fig. 1. The flat dichroic mirrors (i.e., HR, OC) were AR coated between 1900 nm – 2100 nm and HR coated from 2300 nm – 2700 nm. The OC had 90 % reflectivity at 2000 nm – 3000 nm on one side and broadband AR coated at the same wavelength range on the other side. The HR mirror had > 99 % reflectivity at 2300 nm – 3000 nm and AR coated at 1900 nm – 2100 nm on the other side. The LC etalon was placed into a kinematic rotation mount in the collimated leg of the laser resonator, allowing angular adjustment along with off-axis angle rotation. Figure 1 shows a top-level diagram of the experimental setup used in this work.

 

Fig. 1 The experimental setup of the z-cavity resonator. The HIP treated Cr:ZnSe crystal is placed at Brewster’s Angle between two dichroic curved mirrors (M1) that collimate the beam between the two flat mirrors (HR) and (OC). The pump laser light is mode matched to the resonator by the focusing lens before being injected into the cavity. The liquid crystal etalon is mounted and rotated between HR and M1. The total cavity length of this setup was approximately 37 cm.

Download Full Size | PDF

The tuning wavelength measurements were collected at three different LC angles with an applied peak-to-peak voltage of 1 – 5 V by increments of 100 mV. The data sets were collected at normal incidence as well as 10° and 20° off-axis angles; which demonstrates angle tuning capabilities of the LC etalon, in addition to the novelty of purely electronic tuning. This is beneficial because the tuning range can be centered at the desired wavelength. The average input power of the CW Tm fiber laser was recorded at approximately 6.35 W. The spectral output of the tuning wavelength was observed and recorded using a Thorlabs, Inc. Optical Spectrum Analyzer (OSA 205) and a function generator was used to control the applied voltage to the LC etalon. The average output power was recorded using a power meter placed directly behind a long-pass 2000 nm spectral filter to block any unabsorbed pump light. The tuning wavelength and average output power were collected independently and post-processed as a function of applied voltage. The linewidth of the HIP treated NLW laser was measured using a Thorlabs SA200-18C Fabry Perot Interferometer, which had a free spectral range (FSR) of 1.5 GHz and Finesse of 200.

Figure 2 contains the measured transmission of linearly polarized light at normal incidence by the LC etalon used in this work. Note that the direction of the LC etalon is parallel to the polarization. The LC etalon is approximately 95 % transparent at Cr:ZnSe laser wavelengths, with a 9 THz nominal FSR. BEAM Engineering for Advanced Measurements Company currently has a proprietary right to the LC etalon used in this work. Additional product details will be available through BEAM Co.

 

Fig. 2 Measured transmittance of linearly polarized light through the LC etalon with an electric field application of 1 – 5 V_pp.

Download Full Size | PDF

3. Results and discussion

The emission bandwidth of conventional Cr:ZnSe is centered at approximatley 2450 nm [4]. Therefore, the average output power is expected to increase as it tunes closer to the center wavelength and vice versa as it tunes away from the center wavelength. Note that without the etalon the free-running laser operated at 2534 nm with an average output power of 750 mW. The redshift in emission for the Cr:ZnSe used in this work is a characteristic of the HIP treatment [14]. At normal incidence the LC etalon produced a tuning range of 130 nm between 2490 nm – 2623 nm. The HIP treated Cr:ZnSe laser output linewidth remained narrow as it was tuned across the gain spectrum (See Fig. 3(a)). The maximum average output power of 466 mW was recorded at 2490 nm by an applied voltage of 5 V. Figure 3(a) illustrates a few of the spectral outputs from the HIP treated Cr:ZnSe NLW laser as a function of the applied voltage, while Fig. 3(b) is the average output power and peak wavelength of each spectral output with respect to the applied voltage.

 

Fig. 3 Examples of the spectral output of the tuning wavelength by the applied voltage is displayed in (a). Each peak corresponds to the voltage applied to the LC etalon while mounted at normal incidence. The average output power and peak wavelength as a function of the applied voltage are displayed in (b).

Download Full Size | PDF

At a 10° off-axis angle the LC etalon also produced a 130 nm range of tunable wavelength. The angle of the LC etalon blue shifted the tuning range to 2473 nm – 2599 nm. The lower limit of the tuning range was blue shifted by 17 nm while the upper limit was blue shifted by 24 nm. This is to be expected as the AR coating of the LC etalon will move to shorter wavelengths as the off-axis angle increases. The maximum average output power was repeated at an applied voltage of 3.3, 3.9 and 4.0 V with a value of 471 mW. The peak wavelength for the maximum average output power was recorded at 2478 nm, 2478 nm and 2476 nm respectively. Figure 4(a) illustrates a few of the spectral outputs from the NLW laser as a function of the applied voltage to the LC etalon and Fig. 4(b) is the average output power and peak wavelength of each spectral output with respect to the applied voltage.

 

Fig. 4 Examples of the spectral output of the tuning wavelength by the applied voltage is displayed in (a). Each peak corresponds to the voltage applied to the LC etalon while mounted at a 10°off-axis angle. The average output power and peak wavelength as a function of the applied voltage are displayed in (b).

Download Full Size | PDF

At steeper off-axis angles, the optical path length through the LC etalon becomes longer, which can cause the laser to operate at higher orders of the LC etalon. The higher order of the etalon broadens the transmission peak, which is likely due to the lower effective finesse caused by the changes to the angular reflectivity of the dielectric coating. When the LC etalon was set to a 20° off-axis angle a 116 nm tunable wavelength range was measured in the first order mode between 2455 nm – 2571 nm and a small tuning range between 2643 nm – 2650 nm was measured in the adjacent etalon transmission peak (see Fig. 5). In this case, the wavelength tuning range blue shifted in the initial etalon transmission peak as it did in the previously mentioned cases. However, at 3.5 V, the laser begins to operate in the adjacent etalon mode (i.e., the adjacent transmission peak) of the LC etalon at 2643 nm and an average output power of 315 mW. The wavelength remains fairly constant as the applied voltage increases to 5 V. At 20°, the maximum average output power was recorded at 364 mW by an applied voltage of 3.2 V.

 

Fig. 5 Examples of the spectral output of the tuning wavelength by the applied voltage is displayed in (a). Each peak corresponds to the voltage applied to the LC etalon while mounted at a 20° off-axis angle. The average output power and peak wavelength as a function of the applied voltage are displayed in (b).

Download Full Size | PDF

Note, the approximate 130 nm tuning range for the Normal, 10° and 20° mechanical angles were limited by the choice to operate the etalon at a maximum voltage of 5 V_pp. The device is capable of voltage tuning up to 20 V. However, the relative tuning becomes progressively smaller with the increased voltage steps. The operating voltage was limited to 5 V to minimize any risk of accidental damage. The composite tuning wavelength range through a combination of angular and voltage tuning was 195 nm. The wavelength was tuned between 2490 nm – 2623 nm, 2473 nm – 2599 nm and 2455 nm – 2571 nm at normal incidence, 10° and 20° off-axis angles, respectively.

Figure 6 shows the observed emission linewidth structure by the HIP treated Cr:ZnSe laser illustrated in Fig. 1. The HIP treatment is of significant importance because it allows inherent narrow linewidth operation without the loss of power, tuning range or system efficiency [7,14]. In contrast to other line narrowing techniques such as intracavity diffraction gratings or etalons. This is understood to be due to the removal of defect structures that influence the local crystal fields associated with the Cr ions [15]. The removal of crystal defects reduces the broadband spectral output observed by commercially sourced Cr:ZnSe laser material and increases the spectral brightness [14]. Therefore, in this work the LC etalon has a sole purpose of electronically tuning the HIP treated Cr:ZnSe laser within the 2 – 3 μm spectral range, while the HIP treatment acts as the line narrowing element.

 

Fig. 6 The spectral output of the HIP treated Cr:ZnSe NLW laser at normal incidence.

Download Full Size | PDF

The integrated linewidth of the Cr:ZnSe laser was approximatley 900 MHz, in agreement with the previously published result [7]. However, the emission linewidth contained either three or four longitudinal modes, depending on the overlap of the FSR with the longitudinal mode structure. The measured linewidth for each longitudinal mode of ≤ 7.5 MHz was resolution limited by the scanning Fabry-Perot etalon used to collect the measurements.

The scope of this work is to demonstrate the purely electronic tuning of a mid-IR laser by a novel LC etalon, which was highlighted in the discussion above. However, it is also worth mentioning other effects observed by the intracavity etalon. When the LC etalon was assembled and tuned to the same wavelength as the free-running laser, a 2.8% difference in slope efficiency was observed. Note that when linearly polarized light is transmitted through the LC etalon, the transmission depends on the wavelength and applied voltage. Therefore, the losses observed at the free-running laser wavelength will operate at the peak transmission of approximately 95% of that particular wavelength.

4. Conclusions

This work demonstrates the purely electronic use of a low-loss proprietary LC etalon developed by BEAM Engineering. The LC etalon was used to tune a continuous-wave HIP treated Cr:ZnSe NLW laser. The etalon exhibited damage free operation with intracavity intensity of approximately 6 W/cm², and a continuous tuning range of 130 nm. This tuning range can be extended further through additional mechanical tuning of the etalon, allowing up to 195 nm of total tuning to be achieved.