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Abstract
We present an intra-cavity frequency doubled Q-switched diode-pumped alexandrite ring-laser directly emitting in the UV at 386 nm. Using LBO as nonlinear crystal, the laser yields a pulse energy up to 3 mJ at 500 Hz with an excellent beam quality of M2 = 1.1. The pulse length is about 920 ns, allowing for very narrow bandwidth in single longitudinal mode operation. The optical-to-optical efficiency for the UV laser is > 9% and almost unchanged compared to the fundamental laser. First injection-seeding experiments show single longitudinal mode operation. The parameters of the laser are suitable for the use as an emitter in a multi-purpose atmospheric Doppler lidar system.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Corrections
10 April 2024: A typographical correction was made to Section 5.
1. Introduction
Given the increasing impact of the anthropogenic climate change, the acquisition of data to create more detailed climate models becomes crucial. Gathering data for atmospheric temperature distribution and wind velocities in the whole column of the atmosphere (from 1 km up to 100 km and above) is of particular interest, due to the scarcity of existing data.
One approach to gain such data is the use of Doppler-Mie (aerosols), -Rayleigh (air molecules) and -resonance lidar systems for different altitudes in a single general purpose lidar system [1–7]. For Doppler-resonance measurements, specific atomic lines such as the potassium line at 770 nm or the iron line at 386 nm are addressed [1–3]. The beam sources for such lidar systems are e.g. frequency doubled flashlamp-pumped Alexandrite lasers, addressing an iron resonance line at 386 nm [1], or diode-pumped alexandrite lasers, addressing a potassium resonance line at 770 nm [3,4]. The wide tunability of alexandrite allows for addressing different wavelengths with the same technological platform. For ground-based systems operating in remote areas, it is crucial to have an autonomously and maintenance-free running system, including the laser source. Therefore, rugged diode-pumped alexandrite lasers are a big enhancement compared to prior flashlamp-pumped systems. Furthermore, a change of pump source from flashlamps to laser diodes reduces the power consumption of the entire lidar system by a factor of 60 [4].
The iron resonance line at 386 nm has several advantages for lidar measurements. The annual concentration of iron is higher with less seasonal variation compared to potassium [2]. The wavelength lies within a deep Fraunhofer line, which reduces the solar background drastically. In combination with a narrow laser linewidth and matching filtering, high resolution daytime measurements can be performed [2]. Frequency doubled alexandrite lasers were used to address this iron resonance line [1]. Due to the much higher pulse energies of flashlamp-pumped alexandrite lasers (∼ 140 mJ in [1]) compared to diode-pumped alexandrite lasers (∼ 4.6 mJ in [5]) for the use in resonance lidar systems, efficient extra-cavity frequency conversion of a flashlamp-pumped alexandrite laser could be demonstrated with a high efficiency of 61% [1]. The much lower pulse energies of the diode-pumped alexandrite lasers are challenging for frequency conversion. In addition, long pulses of several hundred nanoseconds pulse duration are required to obtain a required laser bandwidth of less than 30 MHz for the application of resonance lidar systems [2]. The combination of low pulse energy and long pulse duration results in a low pulse peak power, making efficient frequency conversion challenging. With this limitation and the high complexity for resonance lidar emitters in general, diode-pumped alexandrite lasers, that meet the requirements for the use in iron resonance lidar systems, have not yet been demonstrated.
A new compact, rugged, maintenance-free iron lidar system, using frequency doubled, diode-pumped alexandrite lasers will be a big step forward compared to the shipping container-sized system using a frequency doubled flashlamp-pumped alexandrite laser [1] and to the currently deployed cubic meter sized K-lidar systems using diode-pumped alexandrite lasers [4].
Furthermore the new development of efficient iron lidar systems would be a possible alternative for the use in spaceborne wind lidar missions like AEOLUS follow on [2,6].
2. State of the art
Before suitable high power laser diodes were available, alexandrite lasers were pumped by flashlamps and some research regarding frequency conversion was conducted. Extra-cavity frequency conversion of pulsed flashlamp-pumped alexandrite lasers was performed since 1988 [7,8]. A conversion efficiency of 31% (IR to UV) with a UV pulse energy of 105 mJ [7] was shown in 1988. In 2001 a conversion efficiency of 4% with a UV pulse energy of 186 mJ [8] was demonstrated for long pulses of 220 µs pulse duration. Due to the low repetition rate of the pumping flashlamps, the average power of these systems was at 0.42 W [7] and 0.19 W [8].
As reported in 2002, two injection-seeded, pulsed flashlamp-pumped alexandrite ring lasers were converted to the UV (372 and 374 nm) by extra-cavity frequency doubling with an LBO crystal for an iron Boltzmann lidar [9]. Each laser yielded up to 3 W with 100 mJ pulse energy with short pulses (60 ns) resulting in bandwidths of ∼850 MHz.
In 2004, a narrow band pulsed flashlamp-pumped alexandrite laser was extra-cavity frequency doubled, using a BBO crystal, for the use in the first mobile Doppler resonance lidar system [1]. A conversion efficiency of 61% with 85 mJ was shown and first measurements were conducted, showing the advantages of addressing iron instead of potassium or sodium atoms for daytime measurements without solar background [1,10]. A linewidth of 12 MHz with a Lorentzian line shape has been determined from mesospheric measurements in the UV with a pulse length of 250 ns, demonstrating that exceptional narrow UV-lasers can be built based on alexandrite.
Intra-cavity frequency conversion of pulsed flashlamp-pumped alexandrite lasers using an BBO crystal has been shown with an average UV power of 0.5 W (25 mJ UV pulse energy) in 1994 [11] and 0.9 W (90 mJ UV pulse energy) in 1998 [12].
In 2005 the first intra-cavity frequency conversion of a diode-pumped alexandrite laser in continuous wave operation was performed by Peng et al. [13]. A power of 0.67 W (bidirectional) was achieved, using an LBO crystal. The optical-to-optical efficiency (pump to UV) was 12.7%. The alexandrite laser without frequency conversion achieved an optical-to-optical efficiency (pump to IR) of 23.1%. In 2021 Song et al. achieved a UV power of up to 2.55 W by intra-cavity frequency doubling a continuous wave diode-pumped alexandrite laser, using a BBO crystal [14]. The optical-to-optical efficiency (pump to UV) was 7.9% for the UV wavelength and without frequency conversion the optical-to-optical efficiency (pump to IR) was 15.0%.
An extra-cavity frequency conversion of a pulsed diode-pumped alexandrite laser was performed in 2016 [15]. The output of a cavity-dumped Q-switched alexandrite laser with short pulses with a pulse duration of 3 ns (fundamental pulse energy of 395 µJ) was frequency doubled, using a BBO crystal. A conversion efficiency (IR to UV) of 47% was achieved with an output UV pulse energy of 184 µJ.
A Q-switched diode-pumped alexandrite laser at 5 kHz repetition rate with significant longer pulses of 124 ns was extra-cavity frequency doubled with an LBO and a BBO crystal respectively [16]. The laser was tuned over 49 nm and yielded up to 27 µJ at 379 nm with a conversion efficiency of 9.4%. Further improvement of the approach yields up to 0.66 mJ at 375 nm generated by extra-cavity SHG with an LBO crystal from 2 mJ pulse energy and 115 ns duration and M2 = 1.05 in the IR at 100 Hz, resulting in a conversion efficiency of 33% [17]. The line width in the UV is 1.2 nm, while the beam quality is unknown. The wavelength was tunable from 361 – 391 nm.
For low power extra-cavity frequency conversion, periodically poled waveguides can be used to increase the conversion efficiency. This was recently shown for a continuous wave diode-pumped alexandrite laser with a power of 185 mW [18]. A conversion efficiency (IR to UV) of 0.7% was achieved, resulting in a UV power of 1.3 mW.
Except for the work in [1,2,6,9] the resonator designs are rather simple, and the spectral and temporal parameters are not suitable for Doppler resonance lidar measurements. The results of the state of the art are summarized in Table 1.
Table 1. State of the art for frequency doubled alexandrite lasers fl:flashlamp, EC: extra-cavity, IC: intra-cavity, ?: unknown, -: not applicable
3. Design for frequency conversion of diode-pumped alexandrite laser
The iron resonance line at 386 nm can be achieved by frequency doubling the alexandrite laser presented in [5] operating at a fundamental wavelength of 772 nm. Since alexandrite is broadly tunable (700 nm – 800 nm), the wavelength-shift of 2 nm, from the former wavelength of 770 nm, can be implemented without changing the resonator’s design or components. The frequency conversion, namely second harmonic generation (SHG), can either be performed extra- or intra-cavity.
To optimize extra-cavity frequency conversion, it is crucial to achieve a maximum conversion efficiency. This is because any portion of the input signal that is not converted is essentially wasted for the intended application. In order to achieve high conversion efficiency, several factors need to be considered. Firstly, large pulse peak powers are desirable, which can be achieved by either shorter pulses or higher pulse energy. Additionally, using small beam diameters, long nonlinear crystals, and high effective nonlinear coefficients of the nonlinear material are also necessary for high conversion efficiency [19]. However, there are certain limitations that need to be taken into account. The available pulse energy is often limited, and a long pulse duration is necessary to achieve a narrow bandwidth. On the other hand, excellent beam quality allows for small focus diameters. It should be noted that using small diameters can lead to issues such as increased divergence, walk-off, and potential damage to the laser-induced threshold [19].
Exceptionally high nonlinear coefficients and the avoidance of walk-off are possible with periodically poled materials in quasi phase matching. A challenge is the need for a small period length, as explained in [18]. There is also a concern regarding UV-induced IR-absorption (UVIIRA) and laser-induced damage threshold (LIDT) issues when using high pulse energies [20].
Intra-cavity SHG has the advantage of power enhancement inside the cavity, which increases the conversion efficiency compared to extra-cavity SHG. Furthermore, the output coupling mirror is usually replaced by a dichroitic mirror, highly transmissive for the frequency doubled light and highly reflective for the fundamental light. Thus, the output coupling transmissivity of the resonator is given by the conversion efficiency while there is no other output coupling for the fundamental wavelength. Due to the quadratic dependence of the conversion efficiency on the fundamental intensity, the output coupling transmissivity of the resonator is not constant during the pulse build-up. This can lead to a pulse lengthening if the conversion efficiency is not matched to the working point of the resonator [21]. By matching the optimal conversion efficiency, the same energy can be extracted from the resonator in the fundamental and the frequency converted wavelength [22]. The output coupling transmissivity for the laser design without frequency conversion is 3% [5] which is therefore the designated conversion efficiency for intra-cavity SHG. Such a low conversion efficiency is easy to achieve even with low pulse energies, large beam diameters, and short conventional nonlinear crystals. Intra-cavity second harmonic generation (SHG) can pose an issue due to the nonlinear dynamics between the gain of the fundamental laser light and the SHG conversion efficiency. This can result in intensity noise (“green noise”) and other temporal pulse shape instabilities [23].
Due to the low pulse energies of diode-pumped alexandrite lasers, we opted for intra-cavity SHG. Calculations for extra-cavity and intra-cavity frequency conversion of the underlying laser design, used in this publication, are shown in [2].
The fundamental laser design and the pump laser correspond to the descriptions provided in more detail in [2,5]. A ring resonator with a single alexandrite crystal (0.2 at% Cr3+ concentration, 4 × 4 mm2 aperture, 7 mm length) is longitudinally pumped with a fiber-coupled diode module including backfolding of the pump light. The module generates a pulse peak power of 375 W at 638 nm out of a fiber with 800 µm core and NA of 0.22. Q-switching is achieved using an acousto-optic modulator (AOM), while unidirectional operation of the ring resonator is ensured through the implementation of a Faraday rotator, two waveplates, and thin film polarizers (TFPs). Two pairs of curved mirrors determine the laser mode size that is matched to the effective pump diameter in the alexandrite crystal and relatively large at the position of the critical intra-cavity components as AOM and Faraday rotator. For the frequency conversion, the operating point is optimized for a designated intensity at the position of the nonlinear crystal.
For intra-cavity frequency doubling, the LBO crystal is placed at the specified position shown in Fig. 1. The beam at this location is almost collimated with a diameter of ∼ 750 µm.
Fig. 1. Schematic setup of the ring cavity with numbered cavity elements: Optical fiber guiding the pump light (1), pump light collimation and focusing lenses (2), pump light backfolding unit (3), alexandrite crystal (4), flat, dichroitic pumping mirrors (5), flat folding mirrors (6), concave curved mirrors ROC = 1500 mm (7), concave curved mirrors ROC = 1200 mm (8), flat folding mirror on piezo actor for stabilization of the cavity length (9), flat output coupling mirror for UV (10), Faraday rotator (11), half-wave plates (12), AOM as Q-switch (13), thin-film polarizers (14), LBO crystal for second harmonic generation (15).
The output coupling mirror for the fundamental light in the IR (R@770 nm = 97%) is replaced by a mirror, highly reflective for the IR-light and highly transmissive for the frequency doubled light in the UV.
The LBO crystal (3 × 3 mm2 aperture, 5.0 mm length) is orientated for type I critical phase matching at a crystal temperature of 50 °C. The laser beam is normally incident with a wavelength of 772 nm (θ = 90° and ϕ = 34.4°). The crystal has an AR-coating for the fundamental and frequency doubled wavelength to reduce the intracavity losses to a minimum and it is mounted between two copper blocks. The temperature at the copper blocks is stabilized at the optimal temperature for UV emission.
Comparing to the properties of BBO, LBO outperforms BBO in several aspects: smaller deff (0.73 vs 1.99 pm/V), but larger spectral acceptance (733 vs 468 GHz·cm) and smaller walk-off (17.1 vs 69.9 mrad) [24]. LBO has also a higher bulk laser-induced damage threshold (LIDT) [25]. Considering the designated low conversion efficiency, LBO is the better choice for the design.
4. Results of laboratory experiments
To determine the losses introduced by the LBO crystal into the resonator, a Findlay-Clay analysis [26] is performed. For that, the laser threshold for operation at the fundamental wavelength is measured for six different output coupling reflectivities, with and without the LBO crystal inside the resonator, see Fig. 2. The LBO crystal introduces additional losses into the resonator of only about 0.6%. This is in accordance with the design parameters and crucial for efficient performance, because of the high influence of intra-cavity losses for low gain laser media.
Fig. 2. Findlay-Clay analysis to determine the resonator losses in the IR operation without (black) and with integrated LBO crystal (red), measured with same pump pulse duration of 101 µs.
The laser source operating at the fundamental wavelength at 500 Hz repetion rate yields 3.6 mJ with a pump pulse energy of 38.5 mJ (pump duration of 92 µs, alexandrite crystal temperature of 90 °C) in unseeded operation. Thus the optical-to-optical efficiency (pump to IR) equals 9.4%. The beam quality is M2 = 1.1 and the wavelength is λ = 773 nm. The pump and therefore the laser energy are slightly reduced compared to earlier publications [5] to prevent LIDT issues during the planned UV operation.
The experimental setup of the second harmonic generation in the laboratory achieves a UV output pulse energy of up to 3.0 mJ at 500 Hz repetition rate with a pump pulse energy of 33 mJ (82 µs pump duration, alexandrite temperature 82 °C) in unseeded operation. The pump duration was again reduced for the intracavity frequency conversion to increase the pulse-to-pulse energy stability. Thus the optical-to-optical efficiency (pump to UV) for the frequency doubled laser equals 9.1%, and is almost the same as for the fundamental wavelength with 84% of its pulse energy. Taking into account the electro-optical efficiency of the diode pump modules, the overall electro-optical efficiency of the UV emitting laser is 2%.
This is unexpected as the cavity losses are raised by 50% by inserting the LBO crystal and the efficiency should be reduced [22]. One possibility is that the output coupling mirror with 3% transmission for the fundamental wavelength is suboptimal, reducing the extractable energy but improving the conversion efficiency towards the optimal output coupling ratio. Another explanation could be that the longer pulse duration for IR emission, as described in [4], is unfavorable at the current operating point.
In Fig. 3, the pulse energy of the second harmonic output beam is observed over 25 minutes. Initially, the pulse energy remains low for about 7 seconds before reaching 3.0 mJ after approximately 8 seconds. This behavior suggests a potential thermal effect in the alexandrite crystal, influencing the laser mode size and overlap with the pump beam. The absolute temperature, regulated by the peltier element, also impacts the laser gain. Occasionally, the pulse energy drops back to its initial level during stable laser operation. Despite the lack of an enclosure and potential disturbances from clean airflow in the laboratory, the pulse-to-pulse stability remains at approximately 1% [5]. The instabilities are probably emphazised by the coupling between laser and SHG process and the additional air perturberance at the LBO crystal at elevated temperature.
Fig. 3. Pulse energy of the frequency doubled beam a) directly after switching on the laser and b) during the time up to 25 minutes.
The temporal shape of the fundamental IR and of the frequency doubled UV laser pulse is given in Fig. 4. The temporal shape of the UV pulse is smooth and without fluctuations. The average full width at half maximum (FWHM) IR pulse duration is 1.23 µs and the average FWHM UV pulse duration is 0.92 µs. For this measurement we used a photodiode with a bandwidth of 350 MHz and an oscilloscope with a bandwidth of 1 GHz. As the conversion process is strongly dependent on the fundamental intensity, the lower parts at the end of the IR pulse are not converted to the UV. No pulse lengthening is observed, indicating that the conversion efficiency is chosen correctly [21].
A caustic of the frequency doubled output beam, focused by a lens with f = 300 mm, is given in Fig. 5. The resulting beam qualities are M2x = 1.1 in x-direction and M2y = 1.1 in y-direction. The beam quality and size in the x- and y-directions are consistently excellent, resulting in negligible walk-off effects in the LBO crystal [24]. Moreover, the beam quality of the UV output, after frequency doubling, remains unchanged compared to the fundamental beam quality. This is in contrast to other studies on intra-cavity second harmonic generation (SHG) of diode-pumped alexandrite lasers [15].
Fig. 5. Beam caustic of the frequency doubled output beam with the inset of the intensity profile at designated positions.
Those lasers are operating in cw in contrast to the pulsed laser presented here, possibly leading to different thermal effects in the alexandrite crystals. Also the use of short LBO (5 mm) instead of longer BBO (10 mm) crystals significantly reduces the walk-off effect. The emitted beam is stigmatic with a round gaussian beam profile without any beam shaping behind the laser source.
All experiments are performed at a fundamental wavelength of 773 nm in unseeded operation resulting in a frequency converted wavelength of 386.5 nm, which is not exactly matching the iron resonance line. Figure 6 shows the spectrum of the fundamental and frequency doubled wavelength during operation in the UV. The spectrum shows a substructure with three peaks in the IR and a relatively narrow envelope (0.18 nm FWHM width), despite the broad bandwidth of alexandrite and no bandwidth-limiting elements or seeding. The substructure probably originates from an etalon effect caused by one of the transmissive optics. The frequency doubled spectrum shows five peaks that match the three peaks in the IR and the mixture of those.
Fig. 6. Fundamental spectrum in the IR (red) and frequency doubled spectrum in the UV (blue) without seeding.
For the use in a Doppler resonance lidar system, the spectral width should be < 30 MHz [27] and for Doppler Mie as narrow as possible with a known spectral shape [2]. This is achieved by injection seeding, using a SLM diode laser at the fundamental wavelength in the IR and actively controlling the cavity length. A resonator mirror is placed on a piezo actor that is used for an advanced version of the ramp-and-fire technique presented in [28] and also used in [2,4,5].
The available seed laser is limited to the resonance wavelength of potassium at 770 nm. The change of the wavelength from 773 nm to 770 nm has no significant influence on the laser or frequency conversion process or parameters as tested in unseeded operation.
First tests for single longitudinal mode operation show that injection seeding with a SLM diode laser in the IR in combination with stabilizing the cavity length (same stabilization technique as described in [5]) results in a slight shift and significantly narrowing of the spectrum in the UV, see Fig. 7. The measured linewidth of the seeded UV emission is < 20 pm, which is the limit of the spectrometer in the UV spectral region.
The exact linewidth of the UV emission cannot be measured yet with the available equipment. The IR spectral bandwidth of a similar laser source with the same resonator design and stabilization technology described in [5] was measured to be 3 MHz. Measurements with a matched narrow-bandwidth confocal etalon will allow for the determination of the exact linewidth in the UV. These measurements require a high spectral stability of the laser emission which can only be achieved with an enclosed laser system that is not disturbed by air movement and disturbances.
5. Summary and outlook
We present, to our knowledge, the first demonstration of an intra-cavity frequency doubled, diode-pumped, Q-switched alexandrite laser operating at 385 nm with a spectral linewidth of less than 20 pm. This laser achieves at 386.5 nm a maximum unseeded output pulse energy of 3.0 mJ at 500 Hz, corresponding to an average power of 1.5 W. Remarkably, the frequency doubled laser maintains excellent beam quality (M2 = 1.1) and high optical-to-optical efficiency (pump to UV > 9%) compared to the fundamental wavelength. First tests with injection seeding, using a SLM diode laser in the IR in combination with stabilizing the cavity length show significant narrowing of the spectral linewidth to < 20 pm, limited by the resolution of the spectrometer.
The demonstrated parameters, achieved through the adaptation of the complex laser design of the potassium resonance lidar [5], make them well-suited for the intended application in an atmospheric Doppler lidar system, especially with the inclusion of matched intra-cavity frequency conversion. The high electro-optical efficiency of 2% of the laser operating in the UV also fulfills the requirements of an autonomous, compact and efficient lidar system.
To further explore the use of resonance lidar systems that focus on an iron resonance line at 386 nm, it is necessary to employ a different seed laser for the investigations. Furthermore, stable single longitudinal mode operation must be achieved, and the exact spectral bandwidth needs to be measured. Based on the demonstrated design, rugged prototypes will be built to be integrated in novel Doppler Mie, Rayleigh, and iron resonance lidar systems with multiple fields of view, currently also under development. These lidars will be used to demonstrate the potential of a lidar array for simultaneous measurements at different locations with an extensive field campaign throughout Europe [29].
Due to the demonstrated performance of the intra-cavity frequency doubling and the tuneability of the fundamental alexandrite laser, other resonance lines within the UV region are addressable with this technology platform. Interesting tracers are Ca+ at 393 nm [30,31] as the only metallic ion in the upper atmosphere (80–120 km) observable from ground, N2+ at 391 nm to extend the measurement altitude (up to 300 km) [32] and metastable Helium at 389 nm for the thermosphere [33].
Recently, measurements of the ratio between metals in stratospheric sulfuric acid particles indicate that a significant fraction originates from reentering space debris [34]. The influence of this level of metallic content on the properties of stratospheric aerosol and the consequences for the climate are unknown and needs to be investigated continuously over longer time scales. The presented technology can measure aluminum as one of the most important materials in space crafts at a resonance line at 394 nm continuously from ground with comparably low effort [35].
Funding
Horizon Europe Food, Bioeconomy, Natural Resources, Agriculture and Environment (101086317).
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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