Evan Hale

High brightness, sub-nanosecond, and compact passively Q-switched laser with intracavity volume Bragg gratings

A high brightness, high energy passively Q-switched (PQS) Nd:YAG laser is presented using volume transmitting Bragg gratings (TBGs) as angular filters. A planar cavity with a length of 1 cm, an 800 μm diameter pump beam, a 20% transmission Cr:YAG saturable absorber, and a 40% output coupling resulted in output pulse energies greater than 1 mJ and durations near 600 ps. In order to increase brightness without altering cavity length, a TBG with narrow angular selectivity placed in the cavity to suppress the higher order transverse modes allowing for near diffraction limited output.

High-energy wavelength tunable dual-channel Tm:YLF laser

A dual channel high energy solid state laser using intracavity volume Bragg gratings is presented. CW operation showed a maximum output power of 5.7 W with a slope efficiency of 48%. Implementation of a Cr:ZnSe saturable absorber with 80% transmission achieved q-switching regime with 160 ns long pulses and combined energy of 4 mJ. Each channel had less than 1 nm spectral width in the range between 1880 and 1908 nm. The spectral difference between the two channels was tuned from 5 to 20 nm, corresponding to 0.4 to 1.7 THz, providing an alternative method for generating narrowband terahertz radiation.

Femtosecond laser transverse mode conversion by an achromatic volume phase mask

Ultrafast femtosecond laser systems have enabled many breakthroughs in the fields of science and technology. However due to the large spectral bandwidth necessary for creating short pulses, it is quite difficult to manipulate their transverse mode structure. Here we present successful femtosecond transverse mode conversion from the fundamental mode TEM00 to modes TEM01, TEM10, and TEM11 with the use of an achromatic phase mask based on diffractive optics. The femtosecond source had ~12.8 nm of bandwidth and pulse duration of <100 fs.

Wavelength tunable dual channel solid state laser for terahertz difference frequency generation

The generation of tunable narrowband terahertz (THz) radiation has shown much interest in recent years. THz systems are used for rotational-vibrational spectroscopy, nondestructive inspection, security screening and others. Monochromatic THz emission has been generated by means of THz parametric oscillation, nonlinear difference frequency generation, and quantum cascade lasers. Intracavity difference frequency generation (DFG) in the nonlinear crystal gallium arsenide (GaAs) is known as an efficient way to generate a continuous wave THz radiation. A novel high power solid state resonator is presented with the use of volume Bragg grating (VBG) technology to create a dual channel system by spectral beam combination. The system consists of two separate Tm:YLF crystals and two VBGs for narrowband wavelength selection. At the end of the resonator both channels share common spherical mirrors, which provide feedback and focuses the beam for nonlinear purposes. This allows each channel to be independent in power and wavelength, eliminating gain competition and allowing individual wavelength tunability. The VBGs are recorded in photo-thermo-refractive glass, which has a high laser induced damage threshold and can withstand the high intracavity power present in the resonator. Tunability of the system has shown spectral spacing from 5 to 20 nm, 0.4 - 1.7 THz, and intracavity continuous wave power levels from 80 to 100 W. By placing the GaAs crystal near the waist, THz radiation can be extracted from the cavity.

High brightness sub-nanosecond Q-switched laser using volume Bragg gratings

The design of Q-switched lasers capable of producing pulse widths of 100’s of picoseconds necessitates the cavity length be shorter than a few centimeters. Increasing the amount of energy extracted per pulse requires increasing the mode area of the resonator that for the same cavity length causes exciting higher order transverse modes and decreasing the brightness of the output radiation. To suppress the higher order modes of these multimode resonators while maintaining the compact cavity requires the use of intra-cavity angular filters. A novel Q-switched laser design is presented using transmitting Bragg gratings (TBGs) as angular filters to suppress the higher order transverse modes. The laser consists of a 5 mm thick slab of Nd:YAG, a 3 mm thick slab of Cr:YAG with a 20% transmission, one TBG aligned to suppress the higher order modes along the x-axis, and a 40% output coupler. The gratings are recorded in photo-thermo-refractive (PTR) glass, which has a high damage threshold that can withstand both the high peak powers and high average powers present within the resonator. Using a 4.1 mrad TBG in a 10.8 mm long resonator with an 800μm x 400 μm pump beam, a nearly diffraction limited beam quality of M2 = 1.3 is obtained in a 0.76 mJ pulse with a pulse width of 614 ps.

Real-time spectrally-resolved imaging of the transverse modes in broad area diode lasers

We demonstrate a simple system which is capable of resolving and imaging, in real time, the lateral modes of a multimode broad area diode laser. This technique enables direct characterization of the intensity distribution of each mode of the laser and differs from prior work in that it provides both high spectral resolution (<; 3pm) and real-time imaging. This will allow for the first time direct observation of the dynamics of closely spaced lateral modes of in high power broad area diode lasers.

Higher order mode selection for power scaling in laser resonators using transmitting Bragg gratings

Power scaling using a higher order mode in a ribbon fiber has previously been proposed. However, methods of selecting the higher order mode and converting to a single lobe high brightness beam are needed. We propose using a multiplexed transmitting Bragg grating (MTBG) to convert a higher order mode into a single lobe beam. Using a ribbon fiber with core dimensions of 107.8 μm by 8.3 μm, we use the MTBG to select a higher order mode oscillating within the resonator with 51.4% efficiency, while simultaneously converting the higher order mode to a beam with diffraction limited divergence of 10.2 mrad containing 60% of the total power.

Achromatic phase elements based on a combination of surface and volume diffractive gratings

Phase masks are important optical elements that have been utilized for several decades in a large variety of applications. Recently, we demonstrated a new type of phase masks fabricated by encoding phase profiles into volume Bragg gratings, allowing these holographic elements to be used as phase masks at any wavelength capable of satisfying the Bragg condition of the hologram. Here, we present a new method of true achromatization of this type of phase masks that removes the need for angle tuning and is implemented by combining this holographic phase masks approach with a pair of surface diffraction gratings.

Characterization of laser diodes under short-pulsed conditions with high pulse energies

New applications require diode lasers to be driven with short pulses in the sub-micro second range. The goal is to minimize both the cost and size of the diode laser module by minimizing the number of laser bars required while maintaining the lifetime that is desired for the application. Products demanded by the market using such short pulses range from QCW stacks to fiber coupled modules. While many short pulsed applications use high fill factor bars, these bars are not suited for high brightness applications or coupling into small fiber cores. The focus of this work is the analysis of CW diode designs commonly used for high brightness fiber coupled modules under short pulsed conditions. Three key parameters need to be known in order to design a diode laser module that is suited for high peak powers. First is the damage threshold of the facet. The damage threshold determines the maximum power level at which the laser can be operated safely, considering a proper safety margin dependent on application. The damage threshold is a function of the input pulse width and amplitude. The second parameter which is influenced by the drive current is the slow axis divergence of the diode laser. Knowledge of this parameter is critical when designing the system optics. The third parameter is the effective emitter size which may increase with operating current. An increase in emitter size will lead to larger divergences after collimating optics for a given focal length lens and may result in a larger spot when coupling into an optical fiber. All these parameters have to be considered when designing a new product. Presented here is a study on these three critical parameters as a function of operating conditions. Results for different diode designs will be presented. The data presented includes damage thresholds, as well as near field and far field data at various operating currents. A design study for fiber coupled modules with high pulse energies based on the test results will be shown for various wavelengths.