Transmitted and tuning characteristics of birefringent filters

Generating dual-wavelength VECSEL by selecting birefringence filter material and the application toward mid-infrared region via intracavity OPO

A method for realizing a synchronized watt-level dual-wavelength vertical-external-cavity surface-emitting-laser (VECSEL) by using a tilted birefringence filter (BRF) is demonstrated. It is verified that by selecting suitable BRF material with different refractive index differences between extraordinary wave and ordinary wave, the dual-wavelength emission with a free spectrum range from sub-THz to tens of THz can be achieved. The output characteristics of such a dual-wavelength VECSEL are thoroughly investigated including its wavelength tunability and power difference. Finally, the intracavity optical parametric oscillator is applied to efficiently convert the dual-wavelength laser toward the mid-infrared region. The gain competition and longitudinal mode hopping performances for the multi-wavelength mid-infrared output are explored.

Diffuse reflectance spectroscopy of the cartilage tissue in the fourth optical window

Studies of the optical properties of biological tissues in the infrared range have demonstrated significant potential for diagnostic tasks. One of the insufficiently explored ranges for diagnostic problems at the moment is the fourth transparency window, or short wavelength infrared region II (SWIR II). A Cr²⁺:ZnSe laser with tuning capability in the range from 2.1 to 2.4 µm was developed to explore the possibilities in this region. The capability of diffuse reflectance spectroscopy to analyze water and collagen content in biosamples was investigated using the optical gelatin phantoms and the cartilage tissue samples during their drying process. It was demonstrated that decomposition components of the optical density spectra correlated with the partial content of the collagen and water in the samples. The present study indicates the possibility of using this spectral range for the development of diagnostic methods, in particular, for observation of the changes in the content of cartilage tissue components in degenerative diseases such as osteoarthritis.

Broadly tunable two-color lasing of Cr:LiCAF with on-surface and off-surface optical axis birefringent filters: performance comparison

We studied the two-color lasing performance of a Cr:LiCAF laser using crystal quartz on-surface and off-surface optical axis birefringent filters (BRFs). Four different on-surface optical axis BRFs with thicknesses of 2 mm, 4 mm, 8 mm, and 16 mm, and three different off-surface optical axis BRFs with a diving angle of 25° and thicknesses of 2 mm, 4 mm, and 8 mm have been tested. Two-color lasing operation could be achieved in tens of different pairs of wavelengths using both types of BRFs. Regular on-surface optical axis BRFs provided two-color lasing in the 772-810 nm interval, with a discretely tunable wavelength separation of 1 to 37 nm (0.5 to 17 THz). In comparison, the off-surface optical axis BRFs enabled scanning of two-color lasing spectra in a much broader wavelength range between 745 nm and 850 nm with a discretely tunable wavelength separation of 0.8 to 99 nm (0.4 to 46 THz). The results clearly demonstrate the advantages of using off-surface optical axis BRFs to achieve two-color lasing with broadly tunable wavelength separation.

Compact millijoule Yb3+:CaF2 laser with 162 fs pulses

We report on a compact diode-pumped, chirped pulse regenerative amplifier system with a pulse duration of 162 fs and an output pulse energy of 1 mJ before as well as 910 µJ after compression optimized for the probing of ultrafast relativistic laser-plasma processes. A chirped volume Bragg grating (CVBG) acts as a combined pulse stretcher/compressor representing a robust solution for a CPA laser system in the millijoule range. Yb³⁺:CaF₂ is used as gain medium to support a large bandwidth of 16 nm (FWHM) when spectral gain shaping is applied. Chirped mirrors compensate for any additional dispersion introduced to the system.

Diving angle optimization of BRF in a single-frequency continuous-wave wideband tunable titanium:sapphire laser

In this study, the optimal condition of a multi-plate birefringent filter (BRF) used in a single-frequency continuous-wave (CW) tunable laser is theoretically and experimentally investigated. The dependence of the optimal condition on the diving angle of the BRF optical axis is first deduced. Based on the proposed optimal condition, the diving angle of the BRF optical axis is optimized to 29.1^°. Subsequently, a novel off-axis multi-plate BRF with a thickness ratio of 1:2:5:9 and the thinnest plate of 0.5 mm is designed and utilized in a tunable titanium:sapphire (Ti:S) laser. As a result, the operating wavelength of the Ti:S laser is successfully tuned from 691.48 to 995.55 nm by rotating the BRF 18^°. The obtained tuning slope efficiency and maximum tuning range are 16.9 nm/^° and 304.07 nm, respectively. The experimental results agree well with the theoretical analysis results, which provide a feasible approach for designing BRFs to satisfy the requirements of other single-frequency CW wideband tunable lasers.

Realization of compact Watt-level single-frequency continuous-wave self-tuning titanium: sapphire laser

Here, we present a compact Watt-level single-frequency continuous-wave (CW) self-tuning titanium:sapphire (Ti:S) laser, which is implemented using a three-plate Ti:S crystal as both a gain medium and frequency-tuning element. The thickness ratio of the three-plate Ti:S crystal is 1:2:4, of which the thinnest plate measured 1 mm. The optical axes lie on their own surfaces and parallel to each other. Based on the presented self-tuning crystal, a ring resonator is designed and built. The maximum wavelength tuning range of the single-frequency self-tuning Ti:S laser is 108.84 nm, as demonstrated experimentally by rotating the three-plate Ti:S crystal, indicating good agreement with theoretical prediction. To the best of our knowledge, this is the first study to report a single-frequency CW self-tuning Ti:S laser, which can provide a feasible approach for achieving a compact all-solid-state single-frequency CW-tunable Ti:S laser.

Off-surface optic axis birefringent filters for smooth tuning of broadband lasers

Transition metal-doped gain media, such as Ce:LiCAF, Ti:sapphire, Cr:ZnSe, and Fe:ZnSe, possess wide gain bandwidths that could provide ultra broadly tunable laser output with the usage of adequate intracavity tuning elements. Birefringent filters (BRFs) are a low-cost and easy to use solution for tuning. However, for ultrabroad gain media, regular on-surface optic axis BRFs could not provide smooth tuning of laser wavelength in the whole emission range. Basically, regular BRFs could not accommodate a large enough free spectral range with acceptable modulation depth variation while tuning, due to their slow tuning rates. Motivated by this, in this study, we have numerically investigated the effect of optic axis orientation on filter parameters for magnesium fluoride birefringent tuning plates. We have shown that a magnesium fluoride BRF with an optic axis diving by 30° into the plate could provide smooth tuning of ultra-broad laser gain media. A similar analysis has shown that for broadband tuning applications, the optimum optic axis diving angle lies around 25° for crystal quartz BRFs. The proposed filters have the potential to be useful in tuning of broadband lasers in continuous-wave, long-pulsed, and femtosecond operation regimes.

100-nm tunable femtosecond Cr:LiSAF laser mode locked with a broadband saturable Bragg reflector

We report broad tunability of a femtosecond (fs) diode-pumped Cr:LiSAF laser mode-locked with a broadband saturable Bragg reflector (SBR). The SBR had seven pairs of Al_xO_y(n∼1.5) and Al_0.17Ga_0.83As(n∼3.5) layers in its Bragg stack, enabling a 250 nm reflectivity bandwidth around 850 nm. A 6-nm-thick strained In_0.15Ga_0.85As quantum well placed between Al_0.17Ga_0.83As cladding layers was used as a broadband saturable absorber in the 800-920 nm wavelength range. The laser was pumped by 6 single mode diodes-four at 640 nn and two at 660 nm. Mode locking was self-starting and fs pulses could be continuously tuned from 800 nm to 905 nm by an intracavity birefringent filter with an out-of-plane optic axis. The pulse widths varied from 70 fs to 255 fs as the laser was tuned. The laser had an 85.5 MHz repetition rate and the output power varied from 80 mW to 180 mW with tuning.

Extending the narrow-bandwidth tunability of a thin disk Yb:YAG laser regenerative amplifier

We investigate the characteristics of a thin disk laser system based on the master oscillator-power amplifier design. The amplifier emits parasitic laser oscillation (PL) when the wavelength of the tunable narrow-bandwidth seed pulse is detuned more than 4 nm from the gain maximum at 1030 nm. For suppression of this unwanted PL, a birefringent filter (Lyot filter) was inserted into the amplifier cavity in order to generate wavelength selective losses, especially at the gain maximum of the Yb:YAG crystal. It is shown that the tunability range of the laser system can be extended by a factor of 4, when a properly chosen Lyot filter is applied and its angle of rotation is adapted to the seed wavelength of the system.

A self-tunable Titanium Sapphire laser by rotating a set of parallel plates of active material

In a recent work, the authors reported the experimental demonstration of wavelength tuning in a single birefringent plate of Ti:sapphire crystal based on its own birefringence properties. In that device, the thickness of the active plate, limited by the width of the single order tuning spectral region, imposed a strong constraint in the power performance of the laser. The aim of this work is to overcome this limitation by using a set of several identical birefringent plates so that the wavelength tuning of the laser is obtained by synchronously rotating the plates in their own plane. A discussion about the laser performance is presented.

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.

K5Nd(MoO4)4: a self-tunable laser crystal

We report for the first time to our knowledge an experimental demonstration of wavelength self-tuning in a K5Nd(MoO4)4 broadband-emitting laser crystal (KNM), as well as a theoretical treatment of the system based on its birefringence properties. The self-frequency-tuning of the laser along the full spectral range of the KNM crystal was obtained by rotating a birefringent gain plate in its own plane. The gain plate was placed inside a resonator at Brewster's angle. The tuning characteristics of the spectral filter were obtained by use of the Jones vector formalism. The calculated wavelength-selective tuning precisely matches the experimental observations.

Nearly-off-axis transmissivity of Solc birefringent filters

The characteristics of light propagating near the axis of a birefringent filter are studied. A generalized formulation to describe the nearly-off-axis transmissivity of a Solc birefringent filter is derived. On this basis, the polarization conoscopic figures of Solc filters with different numbers of birefringent plates are simulated. Furthermore the variation of spectral transmission with angle of incidence is analyzed, and the field-of-view transmissivity and the spectral transmissivity averaged with respect to the spread of incident light are given. Primary experiments for verification are also demonstrated.

Method for determining the optical axis and (n(e), n(o)) of a birefringent crystal

There is a phase difference between s and p polarizations when a circularly polarized heterodyne light beam is reflected from a birefringent crystal. It can be measured accurately with a common-path heterodyne interferometric technique. We have derived an equation that describes the relationship between the phase differences and n(e), n(o), and alpha. Two groups of solutions for (n(e), n(o)) can be obtained from this equation by the phase measurements performed at three incident angles under moderate conditions. Each group consists of three pairs of solutions for (ne), n(o)). Finally, by justifying with physical conditions, we obtained the correct solution for (n(e), n(o)). Azimuth angle alpha of the birefringent crystal optical axis can also be determined. And the feasibility of this method is demonstrated.

Spectroscopic ellipsometry of anisotropic materials: application to the optical constants of HgI2

A variable angle-of-incidence spectroscopic fixed-polarizer, rotating-polarizer, fixed-analyzer ellipsometer (PRPSE) across a spectral range from 300 to 800 nm is used to determine the optical properties of anisotropic uniaxial tetragonal red mercuric iodide (HgI(2)). For the first time, to our knowledge, the bulk crystal HgI(2) surface measured by ellipsometry was not subjected to potassium iodide cutting or etching. Measurements were made at an air-HgI(2) interface with the optic axis parallel to the sample surface. To determine the optical constants, we varied both the angle of incidence and the azimuth of the optic axis with the plane of incidence. The detailed formulas needed for reliable procedures for analyzing the data are presented. The ordinary and extraordinary complex indices of refraction, (n(o)--ik(o)) and (n(e)--ik(e)), respectively, are determined. Good agreement between PRPSE and the prism technique for the refractive index is observed. The surface aging effects of the ellipsometric parameters of HgI(2), during 30 h of exposure to air, were detected by PRPSE.

Tunable liquid-crystal filter for solar imaging at the He i 1083-nm line

A Lyot-Ohman filter for imaging near the solar He i 1083-nm line is described. Fast and continuous spectral tunability is provided by nematic liquid crystals. This solid-state filter has a free spectral range of 2.35 nm and a spectral resolution of 0.135 nm at the operating wavelength of 1083 nm. A wide-fielded design was used for both static and electro-optic retarder elements, facilitating use in fast imaging systems. A first-light He i image of the Sun is presented.