We report demonstration of Watt level waveguide lasers fabricated using Ultrafast Laser Inscription (ULI). The waveguides were fabricated in bulk chromium and iron doped zinc selenide crystals with a chirped pulse Yb fiber laser. The depressed cladding structure in Fe:ZnSe produced output powers of 1 W with a threshold of 50 mW and a slope efficiency of 58%, while a similar structure produced 5.1 W of output in Cr:ZnSe with a laser threshold of 350 mW and a slope efficiency of 41%. These results represent the current state-of-the-art for ULI waveguides in zinc based chalcogenides.
© 2016 Optical Society of America
Compact, middle infrared (mid-IR) lasers operating in the 2 – 5 μm are a vital, enabling technology for a variety of applications including remote sensing, medical procedures, and spectroscopy. These applications drive the advancement and optimization of mid-IR sources, pushing for compact, efficient laser sources, which can be easily incorporated into existing systems. Transition metal (TM) ions doped into zinc based chalcogenides, notably zinc selenide (ZnSe) and zinc sulfide (ZnS), have many desirable qualities. The large absorption and emission cross-sections broadened by crystal field splitting of the allowed electronic levels of the TM ions produce a versatile medium capable of absorbing and emitting over a large spectral range . Specifically, chromium and iron dopants have been shown to readily emit over the 2 – 3 μm [2, 3] and 4 – 5 μm [4, 5] regions respectively. Additionally, these sources have been scaled to Watt level output power in non-guided wave devices. A summary of published power level records can be seen in Table 1.
However, many of the desired applications of these lasers are fielded in non-laboratory settings which are sensitive to vibration and environmental conditions. In order to improve the reliability of these systems, minimization of free-space optics are required. Thus, confining operation to a guided-wave configuration is desirable. Guided-wave operation is usually realized in a fiber form, but due to the low sublimation point of ZnSe (900°C ) and due to the absence of a glass-phase with crystalline materials, pulling of the material into a fiber is impossible with the current technology . However, other technologies exist to create ZnSe fibers with losses less than 1 dB/cm . However, these fibers have been limited to small lengths and do not support a Gaussian mode. An alternative method to create guided-wave structures was demonstrated by Okhrimchuk et al. , which uses femtosecond laser pulses to modify the refractive index of bulk material to create waveguide structures. This technology has been demonstrated in both Fe:ZnSe  and Cr:ZnSe  with limited output power. The current record powers obtained from these sources can be seen in Table 2. In this paper, we demonstrate power scaling of both Cr:ZnSe and Fe:ZnSe waveguides to power levels of 5.1 W and 1 W respectively. These improvements represent an increase in power output of 3× for Cr:ZnSe and 14× for Fe:ZnSe.
Fabrication of ULI waveguide structures relies on precise control of both an automated 3-axis translation stage and an ultrafast laser. Waveguides were inscribed in bulk Fe:ZnSe and Cr:ZnSe using a chirped pulse, amplified Yb-fiber laser (IMRA μJewel D1000) operating at 1047 nm. The waveguides were inscribed at 100 kHz repetition rate with a pulse width of 850 fs and 1–5 overwrites. The beam was focused into the sample using a 0.68 NA lens with a focal length of 4.1 mm. The sample was translated using an XYZ translation stage (Aerotech Model:A3200) moving with a velocity of 10 mm/s. The resultant cladding structure can be seen in Fig. 1. Similar structures have been demonstrated by other authors [13, 18]. The structure was designed to have a core diameter of 120 μm composed of 100 individual elements (i.e. the individual filamentation elements that radially surround the waveguide). The actual waveguide diameter was smaller than the designed diameter due to elongation of the individual waveguide elements. A similar structure was created inside of a bulk Fe:ZnSe sample except with a diameter of 80 μm. The material specific properties for Fe:ZnSe and Cr:ZnSe change the required pulse energy for modification significantly depending on the dopant type and concentration. Small changes in the intrinsic absorption of the sample can decrease the fluence at the focus of the beam. Decreased fluence compounded with the nonlinear self focusing caused by the large χ(3) coefficient (1 × 10−12 esu ), can result in poor quality waveguides if the inscription parameters are not precisely controlled. The pulse energy required to obtain modification in Cr:ZnSe was 2.4 μJ and 0.8 μJ for Fe:ZnSe. The absorption cross-sections of Cr:ZnSe and Fe:ZnSe can be seen in Fig. 2, where the green circles represent the absorption cross-section at the inscription laser wavelength. The parameters outlined in this section produced the optimal performance from the waveguides. Deviation from these parameters degraded laser performance.
The samples, obtained from IPG Photonics, were doped with 8.2 × 1018cm−3 of chromium and 1.4 × 1019cm−3 of iron ions respectively. The sample dimensions were 9 mm × 7 mm × 3 mm for the Cr:ZnSe sample and 10.1 mm × 9 mm × 1.8 mm for the Fe:ZnSe sample. The samples were produced using commercial Chemical Vapor Deposition (CVD) growth with post-growth thermal diffusion of the doping ion. The inscription laser’s wavelength is denoted by the circular marker on the graph. Upon closer examination of the transmission spectra, it can be seen that Cr:ZnSe has a much higher absorption at 1047 nm than Fe:ZnSe. The difference in absorption can explain the difference in the pulse energy required to produce modification. It should be noted that the required pulse characteristics for inscription in Fe:ZnSe follow the parameters required for inscription of undoped ZnSe more closely than the parameters required for inscription in Cr:ZnSe.
3. Experimental setup
Optimal operation parameters for Cr:ZnSe and Fe:ZnSe have been investigated by many authors [20–23]. The results were taken into consideration to obtain optimal operation of the waveguide lasers. Unfortunately, Fe:ZnSe only operates efficiently in a CW configuration at cryogenic temperatures due to thermal quenching of the radiative lifetime . In previous demonstrations of waveguide lasers in Fe:ZnSe and Cr:ZnSe, laser mirror coatings were kept on external optics to allow versatility for testing [14, 16]. However, building upon previous research, we can determine the ideal operating parameters for each material (i.e. optimal out-coupler reflectivity, operating temperature, tolerable losses etc.) [3,14]. Knowledge of the ideal operating parameters allows for minimization of free-space optics to prevent unnecessary losses from external optics, thus cavity mirrors were directly coated onto the end facets of the samples. Figure 3 shows the cavity configurations for Cr:ZnSe and Fe:ZnSe waveguide power scaling. For Fe:ZnSe, L1 and L2 were 4 cm focal length broadband (2 – 5 μm) anti-reflective (AR) lenses, W1 and W2 were broadband (2 – 5 μm) AR coated windows, and the solid and dotted lines represent the dichroic incoupler and outcoupler coatings on the end face of the crystal. The incoupler coating was designed to be AR at 2.94 μm and HR at 4 – 4.2 μm. The outcoupler coating was designed to be 40% reflective at 4 – 4.2 μm. The Fe:ZnSe crystal was wrapped in indium and placed in a vacuum dewar cooled by liquid nitrogen to 77 K, which is represented by the black box in Fig. 3. Similarly, the Cr:ZnSe setup consisted of dichroic incoupler and outcoupler coatings in addition to the broadband AR 10 cm focal length lenses L1 and L2. The incoupler coating was designed to be AR at 1.9–2.1 μm and HR at 2.3–3 μm. The outcoupler was designed to be AR at 1.9 – 2.1 μm and 70% reflective at 2.3 – 3 μm. The Cr:ZnSe sample was wrapped in indium to improve thermal contact with the heatsink. The heatsink was chilled to 11°C, which was slightly above the point where water vapor started to condense on the sample. The pump sources used for pumping of Cr:ZnSe and Fe:ZnSe were a thulium fiber laser operating at 1.9 μm and a erbium fiber laser operating at 2.9 μm. The samples were mounted on a 5-axis translation stage capable of pitch and yaw control in addition to XYZ translation. The 5-axis stage allowed for fine adjustment of the input pump coupling into the waveguide to maxamize the laser output. Nominally, the pump laser was focused to a 1/e2 diameter of 80 μm for the Cr:ZnSe waveguide and 50 μm for the Fe:ZnSe waveguide. The focal spot sizes were kept smaller than the waveguide diameter to prevent interaction of the pump beam with the cladding structure, which could cause increased laser losses due to absorption or scattering from the cladding. Small adjustments were made to the distance between the pump focusing lens and the sample to maximize the output power from the laser.
4. Special considerations
Several factors must be taken into consideration when applying focused, laser power to the end facet of a crystal. First, the intensity must be kept below the damage threshold of the coating material. Following Equation 1, the intensity of a given spot size increases linearly as a function of power for a given spot size, where P is the incident power, I is the intensity, and r is the radius of the pump spot. With a conservative estimate of 200 kW/cm2 damage threshold for CW operation of the optical coatings, the power has to be kept under 19 W for a spot size of 80 μm (Cr:ZnSe) and 10 W for a spot size of 50 μm (Fe:ZnSe). In practice, these values will be less due to localized heating and differential thermal expansion of the thin film coating and the bulk laser sample.Equation 2 shows the threshold pump power for a simple four-level laser, where h is Plank’s constant, ν is the frequency of the pump laser, A is the area of the pump spot, l is the passive losses for the resonator, η is the conversion efficiency, τ is the upper state lifetime, and σ is emission cross-section of the material. The graph shown in Fig. 4 shows an approximation of the pump power required to obtain lasing from samples of Cr and Fe:ZnSe. Published values for the cross-section and lifetime were used [1,25], as shown in Table 3. In addition, a equivalent loss of approximately 1 dB/cm was estimated for both waveguide samples [14, 16]. Threshold pump values of 350 mW and 50 mW were estimated for Cr:ZnSe and Fe:ZnSe respectively.
5. Cr:ZnSe results
Utilizing the setup shown in Fig. 3, the Cr:ZnSe sample was pumped with a Tm:fiber laser (IPG Photonics TLR-40-1910-LP). An Electrophysics PV-320 camera was used to maximize the pump light coupled into the waveguide structure. The sample was mounted on a 5-axis translation stage to maximize the light coupled into the waveguide structure. Using a dichroic splitting mirror, AR at 1.9 μm and HR at 2.3 – 3μm and 45 angle of incidence, the fluorescence signal from the waveguide was maximized using an extended range InGaAs detector. Lasing occurred around at 400 mW pump for Cr:ZnSe. The sample was then translated perpendicular to the pump to couple into adjacent waveguides to identify the best performing waveguide (i.e. the waveguide that produces the highest output power). The adjacent waveguides had a variety of different inscription parameters including number of overscans, scan speed, pulse energy, and core size. Figure 5 shows the output of the best performing waveguide. A maximum output of 5.2 W was obtained from 11.95 W of input power, with a slope efficiency of 41%. Additionally, Fig. 5 shows no thermal rollover indicating that these waveguide structures are limited by the damage threshold of the coating. It should be noted that the input power was referenced to the power incident on the front face of the crystal, not the absorbed power. Unfortunately, at 12.2 W of input power the sample coating was damaged, which rendered the waveguide inoperable. Additionally, we can compare the Cr:ZnSe results obtained from this work to the work previously investigated by Berry et al. , which identified thermal quenching as the limiting factor in power scaling of Cr:ZnSe waveguide lasers. Several factors affect the internal temperature of the waveguide. The external cooling of the sample lowers the internal temperature of the waveguide allowing higher operating powers to be reached before thermal quenching occurs. Previously, the sample was cooled to 15°C, while in this work the sample was cooled to 11°C. In addition, the extraction efficiency of the work demonstrated in this paper was increased 18% compared to the previous work . The increased rate of energy extraction from the gain medium also decreases the heat load on the waveguide. The output mode of the waveguide can be seen in Fig. 6(A), which was a scaled and resized image of the mode. From Fig. 6(A), we can see that the output mode of the Cr:ZnSe waveguide laser was highly multimode. Translating the beam vertically out of the waveguide, Fig. 6 (B), shows the unscaled, unguided spot from the pump laser transmitted through the same optics. The unguided beam shown in Fig. 6(B) produced no laser output from the system. From direct measurement of output beam divergence using a scanning slit profilometer, we can estimate the numerical aperature (NA) of the waveguide to be 0.37. For a symmetric, cylindrical waveguide the number of guided modes can be estimate by the normalized frequency of the waveguide, Equation 3. In Equation 3,V is the normalized frequency, a is the radius of the core, λ is the wavelength of the guided light, and ncore and nclad are the indices of refraction for the core and cladding respectively. It should be noted that is equal to the NA of the waveguide. Equation 4 provides an estimate of the number of supported modes for the waveguide structure referred to as N. The number of guided modes of the waveguide, using 0.37 for the NA, was ≈ 1000. In practice, this number is much less due to the increased interaction with the cladding region for higher order modes and due to the asymmetry of the waveguide profile.
The output spectrum of the Cr:ZnSe waveguide laser can be seen in Fig. 7. The output wavelength was centered at 2522 nm with a spectral bandwidth of 20 nm. Compared to typical bulk lasers with no line narrowing mechanisms (i.e. etalons, tuning elements, filters), the spectral bandwidth is decreased by approximately 30 nm .
In this case, damage occurred to the waveguide coating at an input power of 12.2 W, which approximately corresponds to an intensity of 200 kW/cm2 at a spot size of 80 μm. The difference in the specified coating damage threshold and the actual damage threshold is likely due to localized heating and differential thermal expansion of the coating and the sample.
6. Fe:ZnSe results
Utilizing the setup shown in Fig. 3, the Fe:ZnSe sample was aligned to an Er:fiber laser operating at 2.94 μm (Coractive ILM series) capable of 2 W of output power. The Er:fiber output was focused into the 80 μm diameter Fe:ZnSe waveguide using a 4 cm focal length lens, which produced an approximate spot size of 50 μm. A maximum output power of 995 mW was obtained at 1.9 W of input pump power producing a slope efficiency of 58%, Fig. 8. The laser exhibited single mode output shown in the inset picture of Fig. 8 with a mode field diameter of approximately 70 μm.
The output spectra of the Fe:ZnSe waveguide laser can be seen in Fig. 9. The output of the laser was centered at 4070 nm with a spectral bandwidth of 70 nm. The spectral output of this laser is broader than previously demonstrated Fe:ZnSe waveguide lasers . This is likely due to decreased interaction with the cladding structure in this device as compared to previous devices. There is a general trend when moving towards larger diameter waveguides, that the spectral output of the laser broadens. This has been shown to occur in Cr:ZnSe lasers, moving from 2 nm  to 10 nm , and even greater in this work (20 nm). In addition, Fe:ZnSe was initially demonstrated with an asymmetric 151 μm × 40 μm  cladding profile, which produced output spectra with a FWHM of 10 nm. In this work, the cladding profile was increased to an 80 μm × 65 μm diameter asymmetric profile, which increased the spectral bandwidth to 70 nm. The Fe:ZnSe waveguide exhibited increased slope efficiency compared to the Cr:ZnSe waveguide. The increased performance is due to the increased upper-state lifetime of the Fe:ZnSe sample, which increases the gain of the system. Additionally, the longer wavelength laser emission reduces scattering losses proportional to 1/λ4 and decreased waveguide diameter allows for an increased intensity profile for the pump and laser mode, thus more strongly stimulating laser emission.
In conclusion, we have demonstrated power scaling of both Cr:ZnSe and Fe:ZnSe waveguide lasers to record levels. The Cr:ZnSe laser produced 5.2 W of output power with a slope efficiency of 41%. This represents a 3x improvement over previously demonstrated waveguide lasers. The Fe:ZnSe laser produced 995 mW of output into a single mode with a slope efficiency of 58%. This output power represents a 14x improvement to previously published values.
This work was funded by the U.S. Air Forces and Sensors directorate (contract FA8650-12-D-1377), EOARD (Grant Number FA8655-1-3026), and EPSRC (Grant number EP/G030227/1). A. Lancaster acknowledges support from EPSRC studentship EP/K502844/1. Sean McDaniel, Gary Cook and Jonathan Evans thank Patrick Berry and Ronald Stites of AFRL for valuable technical discussion.
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