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Abstract
We demonstrate a pulsed three-stage master oscillator power amplifier (MOPA) at a wavelength of 2047 nm and its application for mid-IR generation via pumping a ZnGeP2 (ZGP) optical parametric oscillator (OPO). A maximum MOPA output power of 19.8 W is reached while achieving a diffraction limited beam quality M2 <1.1. By applying the MOPA setup to pump a ZGP crystal in a linear OPO cavity, a combined signal and idler mid-IR output power of 8.1 W is reached. For the highest mid-IR output power, a mid-IR conversion efficiency of 44 % and beam quality factors M2 of 2.2 and 2.0 are determined for signal and idler, respectively.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mid-IR laser sources are an ongoing area of research related to applications in surgery, material processing and in defense. The lack of mid-IR laser sources capable of generating high-energy-pulses at a high-average-power drives the investigation of optical parametric oscillators (OPO) accessing the mid-IR band via nonlinear frequency conversion. $\text {ZnGeP}_\text {2}$ (ZGP) has been widely researched and possesses one of the highest nonlinear coefficients ($\mathrm {d}_\mathrm {36}$ = ${75}\;\textrm{pm/V}$) [1,2] but exhibits a strong defect driven absorption for wavelengths below ${2}\;\mathrm{\mu}\textrm{m}$ that delimits the wavelength range of possible pump sources [3]. Pumped by a $\text {Ho}^{\text {3+}}$:YAG based master oscillator power amplifier (MOPA) solid-state laser setup, ZGP has shown to enable a mid-IR output power of ${161}\;\textrm{W}$ for a beam quality $\mathrm {M}^2$ of 3.8. [4]. Another approach to pump ZGP OPOs beside solid-state lasers are fiber laser pump sources, which promise an advantageous average power scaling with the drawback of limited pulse energy. $\text {Tm}^{\text {3+}}$-doped silica fibers enable an efficient power scaling up to around ${2.07}\;\mathrm{\mu}\textrm{m}$ [5] and $\text {Ho}^{\text {3+}}$-doped silica fibers have shown to access wavelength up to ${2.2}\;\mathrm{\mu}\textrm{m}$ [6]. Only recently ${77}\;\textrm{W}$ in CW at ${2.2}\;\mathrm{\mu}\textrm{m}$ are demonstrated in a single oscillator based on a $\text {Tm}^{\text {3+}}$:$\text {Ho}^{\text {3+}}$-codoped silica fiber [7]. Fiber based Q-switched single oscillators [8–10] and fiber based pulsed MOPA systems [11–13] have been applied to pump ZGP OPOs but struggle to scale the mid-IR average output power. The fairly low conversion efficiencies <${32}\; \%$ achieved for approaches that try to scale the mid-IR output power above 4 W [8,9], linked to limited pump pulse energy, cause thermal lensing in the ZGP crystal resulting in thermal roll-off and a degradation of the mid-IR beam quality. Therefore, all the mid-IR beam quality values stated for fiber pumped ZGP OPOs are given for an average mid-IR output power below 4 W. Another more compact and robust approach compared to free-space lasers are all-fiber monolithic MOPA setups. Recently, a diode seeded fiber MOPA system achieved a ${59}\; \%$ mid-IR conversion efficiency by pumping a ZGP OPO ring resonator [14]. Compared to Gaussian pulse shapes, the favorable flat-top pump pulse leads to an improved utilization of the pump energy which reduces thermal effects. However, a slope efficiency around ${12}\; \%$ limits the scaling capability of the power amplifier stage and 3 W of mid-IR output power are reached.
In this work, we demonstrate a scalable all-in-fiber MOPA with a slope efficiency of ${40.5} \;\%$ in the power amplifier stage, designed to provide favorable OPO pump pulses. By applying this setup to pump a linear ZGP OPO, a conversion efficiency of ${44}\; \%$ and a maximum mid-IR output power of ${8.1}\;\textrm{W}$ are achieved while maintaining beam quality factors of 2.2 and 2.0 for the signal and idler, respectively. To our knowledge, this is the first time mid-IR output power values above 3 W are published for a ZGP OPO pumped by an all-in-fiber source. Further, this is the first time beam quality values are provided for mid-IR output power above 4 W considering fiber pumped ZGP OPOs in general.
2. Experimental Setup MOPA
The experimental setup of the monolithic fiber laser is shown in Fig. 1. A three stage polarization-maintaining (PM) MOPA enables the amplification of the seed pulses up to peak powers of several kW. A laser diode emitting at ${2047}\;\textrm{nm}$ is used to create ${50}\;\textrm{ns}$ pulses at a repetition rate of ${50}\;\textrm{kHz}$. By current modulation, the seed pulse can be formed to incorporate gain saturation effects of the subsequent amplifiers, which enables the design of pulse shapes optimized for the nonlinear frequency conversion in the OPO. This current modulation also introduces a time-dependent change of the charge carrier density inside of the semiconductor, which leads to a change in refractive index and an increase of the instantaneous emission frequency along the pulse. Stimulated Brillouin scattering (SBS) is the first effect limiting the power scaling of narrow-linewidth lasers; by applying a temporal chirp of the emission frequency, the effective fiber length for a build up of the Stokes wave is significantly reduced resulting in an increased SBS threshold. The first stage consists of a ${6.5}\;\textrm{m}$ long PM $\text {Ho}^{\text {3+}}$-doped silica fiber with a ${8}\;\mathrm{\mu}\textrm{m}$ core and a ${125}\;\mathrm{\mu}\textrm{m}$ cladding [15]. The active fiber is counter-pumped by a homemade $\text {Tm}^{\text {3+}}$-doped fiber laser delivering ${3.3}\;\textrm{W}$ at a pump wavelength of ${1993}\;\textrm{nm}$. A wavelength-division multiplexer (WDM) is used to launch the pump light into the core of the $\text {Ho}^{\text {3+}}$-doped fiber. Two isolators are placed around the first stage; the isolator before the active fiber is needed to protect the seed diode from residual pump light as well as amplified spontaneous emission (ASE) created by the first stage and the second isolator mitigates a degradation of the amplifier performance caused by ASE created in the second stage. The choice of a $\text {Ho}^{\text {3+}}$-doped silica fiber over a thulium-doped fiber for the first stage is motivated by the preferable spectral characteristics for wavelengths in the spectral region $\geq {2050}\;\textrm{nm}$, which have proven to enable high gain (>${40}\;\textrm{dB}$) amplification over a wide wavelength range from ${2050}\;\textrm{nm}$ up to ${2120}\;\textrm{nm}$ [16,17]. The achievable gain in high gain amplifiers is limited by parasitic lasing at the wavelength for which the inversion dependent effective cross-sections peaks. Since ${2050}\;\textrm{nm}$ is at the long wavelength edge of the thulium-emission cross section for doped silica fibers and high gain amplifiers are driven in the unsaturated regime with relatively high inversion, gain at shorter wavelengths is favored hindering the gain build up towards the desired >${40}\;\textrm{dB}$ at ${2050}\;\textrm{nm}$ [18,19]. Nevertheless, thulium-doped silica fibers are suitable to generate moderate gain at ${2050}\;\textrm{nm}$ in saturated amplifiers and the straight forward power scaling ability by clad-pumping lead to the choice of thulium-doped silica fibers in the second and the final power amplifier stage [5]. An acousto-optic modulator (AOM) is used to further manipulate the pulse shape by creating a designed transmission window which also suppresses the ASE in between pulses by ${40}\;\textrm{dB}$. The second stage amplifier is build in a counter-pumped arrangement. A ${5.8}\;\textrm{m}$ long PM $\text {Tm}^{\text {3+}}$-doped double-clad silica fiber with a ${10}\;\mathrm{\mu}\textrm{m}$ core and a ${125}\;\mathrm{\mu}\textrm{m}$ cladding is pumped by a ${793}\;\textrm{nm}$ laser diode providing ${8.9}\;\textrm{W}$. The pump light is launched into the cladding by a pump combiner. To remove the residual pump light, a cladding light stripper (CLS) is placed upstream of the active fiber. The spectral noise in the output of the second stage is filtered by a fiber Bragg grating (FBG) with a reflection bandwith of ${1}\;\textrm{nm}$ centered around the signal.
The reduction of the spectral noise content by the FBG improves the performance of the subsequent power amplifier stage by significantly decreasing the ASE which restrains the gain buildup in between pulses. More importantly, it also reduces the spectral content $\pm {15}\;\textrm{nm}$ around the signal wavelength of the pulse. These wavelengths are located in the gain region of modulation instability (MI). Since the targeted signal wavelength lies in the anomalous dispersion regime of silica fibers, self-phase modulation which accompanies high intensities can interact with dispersion, leading to significant gain in the spectral region around the signal and finally to a transformation of the signal power into the sidebands. Reducing the initial spectral power in the MI gain bandwidth, delays the onset of the spectral degradation caused by MI and therefore enables higher pulse peak powers, while increasing the spectral quality. A tap coupler is used to monitor the input pulse launched into the power amplifier as well as to detect the possible onset of SBS in the backward propagating light. The power amplifier stage consists of a PM $\text {Tm}^{\text {3+}}$-doped double-clad silica fiber with a ${25}\;\mathrm{\mu}\textrm{m}$ core and a ${400}\;\mathrm{\mu}\textrm{m}$ cladding, which is placed into a water bath kept at ${19}\;^{\circ}\textrm{C}$. A (6+1)x1 pump combiner is used to launch the power of two ${793}\;\textrm{nm}$ laser diodes into the cladding of the fiber. To prevent damage to the endface of the fiber, an anti-reflection (AR) coated end cap is spliced to a passive delivery fiber connected to the active fiber. Two dichroic mirrors separate the collimated signal beam from the residual pump light of the power amplifier.
3. Results MOPA setup
Fig. 2. Wide spectrum of the seed laser pulse, measured for a resolution of ${1}\;\textrm{nm}$. The insets shows the respective pulse shape and the narrow spectrum of the seed laser pulse, measured for a resolution of ${0.05}\;\textrm{nm}$.
Fig. 3. Spectra measured at different positions of the amplifier chain: (Red) after the acousto-optic-modulator (AOM), (Blue) after the pump combiner (PC) of the second stage. (Resolution ${1}\;\textrm{nm}$).
The pulsed seed laser delivers a maximum output power of up to ${2}\;\textrm{mW}$. By applying a linear increasing current to the seed laser, the output pulse shown in the left inset of Fig. 2 is generated, resulting in ${2.9}\;\mathrm{\mu}\textrm{W}$ of average power. The spectrum shown in Fig. 2 is averaged over multiple repetitions and measured with a resolution of ${1}\;\textrm{nm}$. For this resolution, an optical signal to noise ratio (OSNR) of ${44}\;\textrm{dB}$ is determined. The narrow spectrum of the seed pulse is shown in the right inset of Fig. 2, measured with a resolution of ${0.05}\;\textrm{nm}$. A full width at half-maximum (FWHM) of ${0.18}\;\textrm{nm}$ is measured. Two processes dominate the semiconductor laser frequency chirp: By increasing the injection current, the charge-carrier density is raised which leads to a adiabatic reduction of the refractive index and a nearly instantaneous frequency up-chirp. Over time the temperature of the laser diode chip and its surrounding increases, leading to a longer optical path length which results in a frequency down-chirp. Since the shortest time constant for this thermal effects found for DFB diode lasers are in the range of ten up to several tens of nanoseconds, the adiabatic frequency up-chirp dominates the laser frequency for the investigated ${50}\;\textrm{ns}$ pulses with linear increasing driving current [20,21] The first stage is characterized after the AOM, which filters the ASE accumulated in between the pulses and is used to manipulate the pulse shape. At a pump power of ${3.3}\;\textrm{W}$ and a pump wavelength of ${1993}\;\textrm{nm}$, an average output power of ${13.7}\;\textrm{mW}$ is measured, which refers to a pulse energy of ${0.27}\;\mathrm{\mu}\textrm{J}$. This corresponds to a total gain of ${36.7}\;\textrm{dB}$ relating to the output signal of the seed diode. Considering the insertion loss of the AOM (${3.4}\;\textrm{dB}$) and the isolator (${0.9}\;\textrm{dB}$), the total gain of the first stage, stated after the WDM, equates ${41}\;\textrm{dB}$. The spectrum measured after the AOM is shown in Fig. 3, illustrated in red and given for a resolution of ${1}\;\textrm{nm}$. The relative amount of noise in the spectrum is ${0.5}\; \%$ of the overall spectral power and the OSNR is around ${44}\;\textrm{dB}$. This value equates to the OSNR of the seed spectrum in Fig. 2, which indicates the succesfull filtering of ASE. The spectrum is defined by the spectral distribution of the seed laser emission and the wavelength specific gain of the first stage. The WDM filter significantly suppresses wavelengths below ${2030}\;\textrm{nm}$.
Fig. 4. Spectrum of the signal after the tap coupler measured for a resolution of ${0.1}\;\textrm{nm}$. The left inset shows the wide spectrum down to ${1900}\;\textrm{nm}$. The inset on the right shows the respective pulse shape, illustrated in green.
The output power after the second stage, given after the pump combiner and measured for ${8.9}\;\textrm{W}$ of pump power at a pump wavelength of ${793}\;\textrm{nm}$, results in ${1.85}\;\textrm{W}$. The corresponding averaged spectrum is shown in Fig. 3 depicted in blue. The amount of noise in the output spectrum increased to ${2.7} \;\%$. Taking the additional noise into account, a signal gain of ${21.2}\;\textrm{dB}$ and an extracted pulse energy of ${36}\;\mathrm{\mu}\textrm{J}$ result. Three different kind of noise attributions can be identified. First there is ASE created in the $\text {Tm}^{\text {3+}}$-doped silica fiber spanning the range from ${1850}\;\textrm{nm}$ up to ${2030}\;\textrm{nm}$. This noise is dominated by an accumulation in between pulses and will degrade the inversion build up in consecutive amplifier stages. The second noise contribution is linked to the spectrum of the pulse and is caused by the amplification of the output spectrum from the first stage, starting from ${2030}\;\textrm{nm}$ up to ${2120}\;\textrm{nm}$. Third, specific noise arises for wavelengths in close vicinity of the signal, caused by an additional gain related to modulation instabilities (MI) which is concomitant to high signal intensities. This additional gain increases the noise content around the signal; the ratio of the signal to the highest peak in the MI gain bandwith is ${35}\;\textrm{dB}$ and therefore significantly degraded compared to the spectrum after the first stage. The signal power after filtering the output of the second stage is reduced to ${785}\;\textrm{mW}$, measured after the tap coupler. The relating spectrum, measured with a resolution of ${0.1}\;\textrm{nm}$, is shown in Fig. 4. Clearly visible is the remnant noise of the MI in the second stage. As the wide spectrum in the inset reveals, no further noise can be seen. The shape of the pulse launched into the third stage amplifier is shown in the right inset of Fig. 4, illustrated in green.
Fig. 5. Signal output power after third stage versus the launched pump power at ${793}\;\textrm{nm}$. The inset on the left illustrates the pulse shape at the maximum output power, colored in red. The inset on the right shows the respective spectrum, measured for a resolution of ${0.1}\;\textrm{nm}$.
Fig. 6. The beam diameter of the focused signal beam is given around the focal spot. Continuous lines represent the fit to the measured data points.
In Fig. 5, the signal output power of the third stage is shown versus the launched pump power at a pump wavelength of ${793}\;\textrm{nm}$. At a maximum pump power of ${54.7}\;\textrm{W}$ an output power of ${19.8}\;\textrm{W}$ is measured, while ${5.2}\;\textrm{W}$ of the pump power is residual. A pulse energy of ${396}\;\mathrm{\mu}\textrm{J}$ is resulting. The slope efficiency of ${40.5}\; \%$ is determined by a linear fit to the data, illustrated by the dotted line. The onset of SBS oberserved in the spectrum of the backward reflected signal, measured at the tap coupler, prevented a further scaling in pulse energy. The pulse shape at the maximum output power is shown in the inset on the left, depicted in red. Gain saturation significantly changes the pulse shape inducing a peak of ${9.2}\;\textrm{kW}$ at the beginning of the pulse. The spectrum at the maximum signal power is illustrated in the right inset for a resolution of ${0.1}\;\textrm{nm}$. A spectral analysis reveals that ${97.5} \;\%$ of the output power is in the range of ${1}\;\textrm{nm}$ around the signal peak. By comparing this spectrum to the input spectrum in Fig. 4, a clear increase of noise caused by modulation instabilities is found. No sign of ASE is detected for a wide spectrum analysis at lower wavelengths. The output beam is characterized by measuring the beam waist of the focused beam around the focus spot, shown in Fig. 6. At the maximum output power of ${19.8}\;\textrm{W}$, beam quality factors of 1.04 $\mathrm {M}_\mathrm {x}^2$ and 1.06 $\mathrm {M}_\mathrm {y}^2$ are found. The polarization extinction ratio (PER) is subject to a fluctuation on long time scales with local minima ${>11}\;\textrm{dB}$, attributed to temperature fluctuations in the cooling setup. Nevertheless, the PER is above ${>15}\;\textrm{dB}$ for a time span up to ${10}{\min }$ which is found to be sufficient enough for pumping the OPO setup.
Fig. 7. Experimental setup of the doubly resonant linear $\text {ZnGeP}_\text {2}$ (ZGP) optical parametric oscillator (OPO) cavity.
4. Experimental setup linear ZGP OPO
In the following, the described MOPA setup is applied to pump a linear ZGP OPO, illustrated in Fig. 7. The emission from the MOPA setup at a wavelength of ${2047}\;\textrm{nm}$ is launched into an isolator to prevent back reflections of the OPO resonator mirrors from entering the third stage amplifier. An attenuator consisting of a halfwave plate combined with a polarizer enables to adjust the pulse energy. A further half-wave plate is used to adjust the polarization axis of the light launched into the ZGP crystal. By focusing the pump beam at ${2047}\;\textrm{nm}$ by a lens with a focal length of ${200}\;\textrm{mm}$, a focal spot diameter of ${325}\;\mathrm{\mu}\textrm{m}$ is created inside the ZGP crystal. Since the isolator, as well as the attenuator, both add losses to the system, the maximum pump power launched into the OPO resonator is reduced to ${18.3}\;\textrm{W}$. Referring to the focal diameter, a peak fluence of ${0.88}\;\textrm{J} /\textrm{cm}^{2}$ inside the ZGP crystal results. The linear resonator consists of a mirror highly reflective in the 3-${5}\;\mathrm{\mu}\textrm{m}$ wavelength band used to launch the pump at ${2047}\;\textrm{nm}$ and a mirror with ${50}\; \%$ reflectance for the targeted mid-IR wavelengths used as an outcoupling mirror. Between the two mirrors, a ${2}\;\textrm{cm}$ long anti-reflection coated ZGP crystal is placed, cut at an ${57}^{\circ}$ angle with respect to the c-axis for type-I phase matching and water cooled at ${19}^{\circ}\textrm{C}$. A dichroic mirror, reflective at wavelengths around ${2}\;\mathrm{\mu}\textrm{m}$ but highly transmissive in the 3-${5}\;\mathrm{\mu}\textrm{m}$ wavelength band is used to separate the residual pump light from the signal and idler. By rotating the ZGP crystal a final internal phase matching angle of ${57.8}^{\circ}$ is set providing phase matching for signal and idler wavelengths of ${3800}\;\textrm{nm}$ and ${4440}\;\textrm{nm}$ which results in a walk-off of ${10.5}\;\textrm{mrad}$ and ${10.3}\;\textrm{mrad}$, given in the respective order. For the nonlinear coefficient $\mathrm {d}_\mathrm {36}$ = ${75}\,{\textrm{pm}/\textrm{V}}$ and the given phase matching angle, an effective nonlinear coefficient of $\mathrm {d}_\mathrm {eff}$ = ${67.6}\,{\textrm{pm}/\textrm{V}}$ is calculated.
5. Results Linear ZGP OPO
In Fig. 8, the combined mid-IR output power (signal + idler) is shown versus the launched pump power at ${2047}\;\textrm{nm}$. A threshold of ${5.2}\;\textrm{W}$ and a slope of ${61} \;\%$ is found. At the maximum pump power of ${18.3}\;\textrm{W}$ a mid-IR output power of ${8.1}\;\textrm{W}$ is reached, resulting in a ${44} \;\%$ conversion efficiency. At this mid-IR output power, around ${6}\;\textrm{W}$ of the pump power is residual. The insets show the pulse shapes of the combined signal and idler, denoted as mid-IR, as well as the launched and the residual pump power, for different pump power levels. The inset on the right describes the pulses for a pump power of ${8.4}\;\textrm{W}$, with ${4.7}\;\textrm{W}$ of residual pump and a combined mid-IR power of ${2}\;\textrm{W}$. Ripples are visible in the pulse shape of the mid-IR as well as the residual pump. We link these ripples to the frequency chirp along the pulse. The seed diode laser of the MOPA setup introduces a frequency chirp, which also causes a change in phase over time. The pump light launched into the OPO resonator now varies from being favorable to unfavorable in terms of contribution to the mid-IR conversion, which leads to the periodic ripples in the pulse shapes. The inset on the left shows the respective pulse shapes at the maximum launched pump power. The build up time decreased, but the residual pump has nearly the same constant power level towards the end of the pulse. Noticeably, the ripples in the pulse shapes lost its prominence. This behavior is attributed to the broadened bandwidth for signal and idler, which increases with pump intensity. More and more wavelengths can favorably contribute to the conversion, accordingly the ripples on the pulse shape average out. In order to investigate the beam quality factor $\mathrm {M}^2$ of signal and idler at the maximum output power of ${8.1}\;\textrm{W}$, the mid-IR light is separated by wavelength specific dichroic filters and the beam diameters for signal and idler are separately measured around the focus spot, shown in Figs. 9 and 10, respectively. By applying a fit to the measured data points, plotted as continuous lines, beam quality factors of 2.2 (signal) and 2.0 (idler) are determined.
Fig. 8. Mid-IR output power of the ZGP OPO versus the launched pump power at ${2047}\;\textrm{nm}$. The insets show the respective pulse shapes for the mid-IR, launched, and residual pump power.
Fig. 9. The beam diameter of the focused signal beam is given around the focal spot. Continuous lines represent the fit to the measured data points.
Fig. 10. The beam diameters of the focused idler beam is given around the focal spot. Continuous lines represent the fit to the measured data points.
6. Conclusion
In conclusion, we developed an all-in-fiber MOPA seeded by a laser diode at ${2047}\;\textrm{nm}$, providing suitable OPO pump pulses with ${50}\;\textrm{ns}$ pulse width. An output power of ${19.8}\;\textrm{W}$ is reached and the slope efficiency of ${40.5}\; \%$ for the power amplifier stage ensures a future scaling in average power. The onset of SBS detected in the backward spectrum limits the pulse energy to ${396}\;\mathrm{\mu}\textrm{J}$. At the maximum output power, a long time PER ${>11}\;\textrm{dB}$ and a diffraction-limited beam quality are measured. Targeted for mid-IR generation, this MOPA setup is applied to pump a ZGP crystal placed in a linear OPO cavity. A slope efficiency of ${61}\; \%$ is measured for the mid-IR conversion. At the maximum launched pump power of ${18.3}\;\textrm{W}$, a combined signal and idler mid-IR output power of ${8.1}\;\textrm{W}$ is reached, corresponding to a ${44} \;\%$ conversion efficiency. At the maximum mid-IR output power, beam quality factors $\mathrm {M}^2$ of 2.2 and 2.0 are determined for signal and idler, respectively. Further research based on the demonstrated work will focus on scaling the mid-IR output power of ZGP OPOs pumped by fully integrable all-in-fiber pump sources up to several tens of Watt.
Funding
Bundesamt für Ausrüstung, Informationstechnik und Nutzung der Bundeswehr; Bundesministerium der Verteidigung.
Acknowledgments
We acknowledge the support of Artur Schander and the mechanical workshop of the IOSB, who fabricated special optomechanical components for the experimental setup.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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