Diode-pumped Nd:YAG master oscillator power amplifier with high pulse energy, excellent beam quality, and frequency-stabilized master oscillator as a basis for a next-generation lidar system

High-peak-power electro-optically Q-switched laser with a gradient-doped Nd:YAG crystal

We report on a high-peak-power electro-optically Q-switched laser emitting a near-diffraction-limited beam profile at 1064 nm by using a gradient-doped Nd:YAG crystal. The gradient-doped crystal features a unique combination of a reduced thermal lens effect through effectively spreading the heat load distribution within its volume. Its performance is compared with those of Nd:YAG crystals with uniform volume doping distribution operating in the Q-switched regime with the same laser configuration, demonstrating the higher average and peak power achievable with the gradient-doped crystal. The maximum average output power amounts to 6.9 W at a pulse repetition rate of 2 kHz, which corresponds to a maximum peak power of ∼585kW. Compared to homogeneous dopant crystals, the slope efficiency and average output power increased by 30.8% and 21.1%, respectively.

Space-borne profiling of atmospheric thermodynamic variables with Raman lidar: performance simulations

The performance of a space-borne water vapour and temperature lidar exploiting the vibrational and pure rotational Raman techniques in the ultraviolet is simulated. This paper discusses simulations under a variety of environmental and climate scenarios. Simulations demonstrate the capability of Raman lidars deployed on-board low-Earth-orbit satellites to provide global-scale water vapour mixing ratio and temperature measurements in the lower to middle troposphere, with accuracies exceeding most observational requirements for numerical weather prediction (NWP) and climate research applications. These performances are especially attractive for measurements in the low troposphere in order to close the most critical gaps in the current earth observation system. In all climate zones, considering vertical and horizontal resolutions of 200 m and 50 km, respectively, mean water vapour mixing ratio profiling precision from the surface up to an altitude of 4 km is simulated to be 10%, while temperature profiling precision is simulated to be 0.40-0.75 K in the altitude interval up to 15 km. Performances in the presence of clouds are also simulated. Measurements are found to be possible above and below cirrus clouds with an optical thickness of 0.3. This combination of accuracy and vertical resolution cannot be achieved with any other space borne remote sensing technique and will provide a breakthrough in our knowledge of global and regional water and energy cycles, as well as in the quality of short- to medium-range weather forecasts. Besides providing a comprehensive set of simulations, this paper also provides an insight into specific possible technological solutions that are proposed for the implementation of a space-borne Raman lidar system. These solutions refer to technological breakthroughs gained during the last decade in the design and development of specific lidar devices and sub-systems, primarily in high-power, high-efficiency solid-state laser sources, low-weight large aperture telescopes, and high-gain, high-quantum efficiency detectors.

Frequency and timing stability of an airborne injection-seeded Nd:YAG laser system for direct-detection wind lidar

We report on the design and performance of the laser deployed in the airborne demonstrator Doppler wind lidar for the Aeolus mission of the European Space Agency (ESA). The all-solid-state, diode-pumped and frequency-tripled Nd:YAG laser is realized as a master oscillator power amplifier (MOPA) system, generating 60 mJ of single-frequency pulses at 355 nm wavelength, 50 Hz repetition rate and 20 ns pulse duration. For the measurement of the Doppler frequency shift over several accumulated laser shots, the frequency stability of the laser is of crucial importance. Injection-seeding, in combination with an active cavity control based on the Ramp-Delay-Fire technique, provides a pulse-to-pulse frequency stability of 0.25 MHz measured at 1064 nm under laboratory conditions. This value increases to 0.31 MHz for airborne operation in a vibration environment that has been characterized by multiple acceleration sensors during different flight conditions. In addition, a pure Ramp-Fire setting was tested for comparison leading to a frequency stability of 0.16 MHz both in airborne operation and on ground. The laser cavity control electronics also have to provide a trigger signal for the lidar detection electronics, about 60 μs prior to the expected laser pulse emission and with high timing stability. An in-flight timing stability of below 100 ns was measured decreasing to 20 ns for a shorter pre-trigger time of 10 μs.

Implementation of a SVWP-based laser beam shaping technique for generation of 100-mJ-level picosecond pulses

We present implementation of the energy-efficient and flexible laser beam shaping technique in a high-power and high-energy laser amplifier system. The beam shaping is based on a spatially variable wave plate (SVWP) fabricated by femtosecond laser nanostructuring of glass. We reshaped the initially Gaussian beam into a super-Gaussian (SG) of the 12th order with efficiency of about 50%. The 12th order of the SG beam provided the best compromise between large fill factor, low diffraction on the edges of the active media, and moderate intensity distribution modification during free-space propagation. We obtained 150 mJ pulses of 532 nm radiation. High-energy, pulse duration of 85 ps and the nearly flat-top spatial profile of the beam make it ideal for pumping optical parametric chirped pulse amplification systems.

Evaluating the beam quality of double-cladding fiber lasers in applications

We put forward a new β_FL factor, which is used exclusively in fiber lasers and is suitable to assess beam quality and choose the LP₀₁ mode as the new suitable ideal beam. We present a new simple measurement method and verify the reasonability of the β_FL factor in experiment in a 20/400 μm fiber laser. Furthermore, we use the β_FL factor to evaluate the beam quality of a 3-kW-level fiber laser. It can be concluded that β_FL is a key factor not only for assessing the performance of the high-power fiber laser that is our main focus, but also for the simple measurement.

8.4mJ, 10kHz, 3.6ns, Nd:YVO4 slab amplifier

A diode-pumped nanosecond eight-pass amplifier with Nd:YVO₄ crystal was developed. Combined with the advantages of direct pumped and Innoslab structure, a high energy pulse was obtained by using the wedged multi-folded configuration with two plane mirrors. The 0.4mJ Q-switched seed at the pulse repetition frequency of 10kHz was amplified to 8.4mJ with the pump power of 330W, and the corresponding optical-to-optical efficiency was 29.8%. The M2 factors in the horizontal direction and vertical direction were 1.48 and 1.39, respectively. And no remarkable of ASE was observed.

Simple method for locking birefringent resonators

We report on a simple method of locking a laser to a birefringent cavity using polarization spectroscopy. The birefringence of the resonator permits the simple extraction of an error signal by using one polarization state as a phase reference for another state. No modulation of the light or the resonator is required, reducing the complexity of the laser locking setup. This method of producing an error signal can be used on most birefringent optical resonators, even if the details of birefringence and eigenpolarizations are not known. This technique is particularly well suited for fiber ring resonators due to the inherent birefringence of the fiber and the unknown nature of that birefringence. We present an experimental demonstration of this technique using a fiber ring.

Method to improve beam quality by compensating spherical aberrations in master oscillator power amplifier laser systems

A method is presented for beam quality improvement, in master oscillator power amplifier laser systems. Intensive study was first carried out with the beam wavefront evolution in a laser resonator. When the laser beam propagates inside the resonator, the spherical aberration coefficient of the beam wavefront can change sign, i.e., the negative spherical aberration coefficient can turn to positive, and vice versa. This process also occurs when the beam propagates outside the resonator in a free space. The laser beam, from an oscillator with negative spherical aberration, was found to be well-compensated by the positive spherical aberration of a strongly pumped laser rod in a laser amplifier. The laser beam quality M² factor has been significantly improved, from 2.2 to 1.4, while the output power has been scaled from 31 W up to 60 W.

Pulsed Yb³⁺-doped fiber laser operating at 1011 nm by intra-cavity phase modulation

A 1011 nm pulsed Yb³⁺-doped fiber laser is experimentally demonstrated by employing a commercially available LiNbO₃ phase modulator (PM) in the linear cavity. The resonator is built up with a section of normal single-cladding Yb³⁺-doped fiber, a PM, and a pair of fiber Bragg gratings. Active mode-locked stable trains of pulses with 2 and 1.4 ns are generated at repetition rates of 30.2478 and 60.4956 MHz, respectively. The maximum average output power is 10.6 mW at pump power of 200 mW, with the slope efficiency of 13.3%. Relaxation-oscillation-modulated pulses with width of 2 μs are obtained at a repetition rate of 27.778 kHz.

High-power Ti:sapphire laser at 820 nm for scanning ground-based water-vapor differential absorption lidar

The Ti:sapphire (TISA) laser transmitter of the mobile, three-dimensional-scanning water-vapor differential absorption lidar (DIAL) of the University of Hohenheim is described in detail. The dynamically-stable, unidirectional ring resonator contains a single Brewster-cut TISA crystal, which is pumped from both sides with 250 Hz using a diode-pumped frequency-doubled Nd:YAG laser. The resonator is injection seeded and actively frequency-stabilized using a phase-sensitive technique. The TISA laser is operating near 820 nm, which is optimum for ground-based water-vapor DIAL measurements. An average output power of up to 6.75 W with a beam quality factor of M2<2 is reached. The pointing stability is <13 μrad (rms), the depolarization <1%. The overall optical-optical conversion efficiency is up to 19%. The pulse length is 40 ns with a pulse linewidth of <157 MHz. The short- and long-term frequency stabilities are 10 MHz (rms). A spectral purity of 99.9% was determined by pointing to a stratus cloud in low-elevation scanning mode with a cloud bottom height of ≈2.4 km.

Hybrid master oscillator power amplifier high-power narrow-linewidth nanosecond laser source at 257 nm

We report on a high-power narrow-linewidth pulsed laser source emitting at a wavelength of 257 nm. The system is based on a master oscillator power amplifier architecture, with Yb-doped fiber preamplifiers, a Yb:YAG single crystal fiber power amplifier used to overcome the Brillouin limitation in glass fiber and nonlinear frequency conversion stages. This particularly versatile architecture allows the generation of Fourier transform-limited 15 ns pulses at 1030 nm with 22 W of average power and a diffraction-limited beam (M(2)<1.1). At a repetition rate of 30 kHz, 106 μJ UV pulses are generated corresponding to an average power of 3.2 W.

Detailed performance modeling of a pulsed high-power single-frequency Ti:sapphire laser

Differential absorption lidar (DIAL) is a unique technique for profiling water vapor from the ground up to the lower stratosphere. For accurate measurements, the DIAL laser transmitter has to meet stringent requirements. These include high average power (up to 10 W) and high single-shot pulse energy, a spectral purity >99.9%, a frequency instability <60 MHz rms, and narrow spectral bandwidth (single-mode, <160 MHz). We describe extensive modeling efforts to optimize the resonator design of a Ti:sapphire ring laser in these respects. The simulations were made for the wavelength range of 820 nm, which is optimum for ground-based observations, and for both stable and unstable resonator configurations. The simulator consists of four modules: (1) a thermal module for determining the thermal lensing of the Brewster-cut Ti:sapphire crystal collinear pumped from both ends with a high-power, frequency-doubled Nd:YAG laser; (2) a module for calculating the in-cavity beam propagations for stable and unstable resonators; (3) a performance module for simulating the pumping efficiency and the laser pulse energy; and (4) a spectral module for simulating injection seeding and the spectral properties of the laser radiation including spectral impurity. Both a stable and an unstable Ti:sapphire laser resonator were designed for delivering an average power of 10 W at a pulse repetition frequency of 250 Hz with a pulse length of approximately 40 ns, satisfying all spectral requirements. Although the unstable resonator design is more complex to align and has a higher lasing threshold, it yields similar efficiency and higher spectral purity at higher overall mode volume, which is promising for long-term routine operations.

Variable pulse repetition frequency output from an optically injected solid state laser

An optically injected solid state laser (OISSL) system is known to generate complex nonlinear dynamics within the parameter space of varying the injection strength of the master laser and the frequency detuning between the master and slave lasers. Here we show that within these complex nonlinear dynamics, a system which can be operated as a source of laser pulses with a pulse repetition frequency (prf) that can be continuously varied by a single control, is embedded. Generation of pulse repetition frequencies ranging from 200 kHz up to 4 MHz is shown to be achievable for an optically injected Nd:YVO4 solid state laser system from analysis of prior experimental and simulation results. Generalizing this to other optically injected solid state laser systems, the upper bound on the repetition frequency is of order the relaxation oscillation frequency for the lasers. The system is discussed in the context of prf versatile laser systems more generally. Proposals are made for the next generation of OISSLs that will increase understanding of the variable pulse repetition frequency operation, and determine its practical limitations. Such variable prf laser systems; both low powered, and, higher powered systems achieved using one or more optical power amplifier stages; have many potential applications from interrogating resonance behaviors in microscale structures, through sensing and diagnostics, to laser processing.

One-Watt level mid-IR output, singly resonant, continuous-wave optical parametric oscillator pumped by a monolithic diode laser

We report more than 1.1 Watt of idler power at 3373 nm in a singly resonant optical parametric oscillator (SRO), directly pumped by a single-frequency monolithic tapered diode laser. The SRO is based on a periodically poled MgO:LiNbO3 crystal in a four mirror cavity and is excited by 8.05 W of 1062 nm radiation. The SRO pump power at threshold is 4 W. The internal slope-efficiency and conversion efficiency reach 89% and 44% respectively. The signal and idler waves are temperature tuned in the range of 1541 to 1600 nm and 3154 to 3415 nm respectively. To the best of our knowledge, this is the highest output obtained for a diode pumped optical parametric oscillator (OPO), and the first time a SRO is directly pumped by a monolithic tapered diode laser.

Fast-switching system for injection seeding of a high-power Ti:sapphire laser

A high frequency switching and tunable seed laser system has been designed and constructed for injection seeding of a high-power pulsed Ti:sapphire laser. The whole laser system operates as the transmitter of a scanning, ground-based, water-vapor differential absorption lidar (DIAL). The output of two seed lasers can be tuned in the wavelength range of 815-840 nm up to the power of 20 mW and switched between the online and offline wavelengths of the DIAL at frequencies of 0-1 kHz. The frequency stability of online and offline seed lasers is better than +/-20 MHz rms and the mode-hop-free tuning range is greater than 40 GHz with external cavity diode lasers. The advantage of this system for efficient injection seeding of the Ti:sapphire cavity is that it is modular, robust, fully fiber-coupled, and polarization maintaining.

Development of an eye-safe solid-state tunable laser transmitter in the 1.4-1.5 microm wavelength region based on Cr4+:YAG crystal for lidar applications

An experimental optimization of the efficiency of a gain switched tunable Cr4+:YAG laser at 10 Hz is described. The thermal lensing during pulsed operation was measured. Optimal performance occurred at a crystal temperature of 34 degrees C and resulted in an output energy of approximately 7 mJ and a pulse duration of approximately 35 ns. Tunability in the range of 1350-1500 nm, spectral linewidth of approximately 200 GHz, and M2<4 are demonstrated. The main laser material parameters are estimated. Such a laser could be employed in a laboratory-based nonscanning lidar system if a narrowband birefringent filter is installed. The tunability will permit the improvement of the Cr4+:YAG transmitter for water-vapor differential absorption lidar if injection seeding is applied.

Diode-pumped Nd:YAG laser with 38 W average power and user-selectable, flat-in-time subnanosecond pulses

A diode-pumped injection-seeded Nd:YAG laser system with an average output power of 38 W is described. The laser operates at 300 Hz with pulse energies up to 130 mJ. The temporal pulse shape is nominally flat in time and the pulse width is user selectable from 350 to 600 ps. In addition, the spatial profile of the beam is near top hat with contrast <10%.

Spaceborne profiling of atmospheric temperature and particle extinction with pure rotational Raman lidar and of relative humidity in combination with differential absorption lidar: performance simulations

The performance of a spaceborne temperature lidar based on the pure rotational Raman (RR) technique in the UV has been simulated. Results show that such a system deployed onboard a low-Earth-orbit satellite would provide global-scale clear-sky temperature measurements in the troposphere and lower stratosphere with precisions that satisfy World Meteorological Organization (WMO) threshold observational requirements for numerical weather prediction and climate research applications. Furthermore, nighttime temperature measurements would still be within the WMO threshold observational requirements in the presence of several cloud structures. The performance of aerosol extinction measurements from space, which can be carried out simultaneously with temperature measurements by RR lidar, is also assessed. Furthermore, we discuss simulations of relative humidity measurements from space obtained from RR temperature measurements and water-vapor data measured with the differential absorption lidar (DIAL) technique.