Birefringent filter with tilted optic axis for tuning dye lasers: theory and design

Wavelength-switchable watt-level continuous wave near-infrared Pr3+:LiYF4 lasers pumped by an InGaN laser diode

We report a high-performance wavelength-switchable near-infrared Pr3+:LiYF4 (Pr:YLF) laser by InGaN laser diode (LD) pumping. The 895, 922, and 924 nm lasers with low emission cross sections in the Pr:YLF crystal have been successfully realized using a birefringent filter Lyot as well as designing and optimizing optical thin films and the laser resonant cavity. The maximum output powers of the 895, 922, and 924 nm lasers are 2.01, 1.92, and 1.95 W, respectively. As far as we know, these are the highest power for Pr:YLF lasers at 895, 922, and 924 nm so far. The beam quality M x2 and M y2 factors are measured to be 1.85 and 1.71 at 895 nm, 1.94 and 1.67 at 922 nm, and 1.76 and 1.60 at 924 nm, respectively. The laser output power fluctuates within ±3%. In addition, the transmittance of the Lyot is theoretically calculated to achieve laser wavelength switching. The successful realization of the wavelength-switchable watt-level continuous wave near-infrared Pr:YLF laser can provide many practical applications in biomedicine and other fields.

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.

Continuous-wave Tm:YLF laser with ultrabroad tuning (1772-2145 nm)

We report detailed experimental data aiming for rigorous investigation of Tm:YLF laser performance, especially with a focus on tuning behavior. Continuous-wave (cw) lasing performance of Tm:YLF crystals with thulium dopings in the 2-6% range is investigated under diode and Ti:Sapphire pumping at 792 nm and 780 nm, respectively. While employing the c-axis, we have achieved cw lasing thresholds below 20 mW, laser output power up to 1.42 W, and laser slope efficiencies as high as 70% with respect to absorbed pump power. The passive loss of the Tm:YLF crystal is estimated to be as low as 0.05% per cm, corresponding to a crystal figure of merit above 10000. Via employing this low-loss crystal and a 2-mm thick off-surface optical axis birefringent filter (BRF) with strong sideband rejection, a record cw tuning range covering the 1772-2145 nm interval is demonstrated (except a small gap between 1801-1815nm region). Detailed lifetime and emission cross section measurements have been performed to explain the observed performance, and strategies for further performance enhancement are discussed.

Wavelength extension and power improvement of red Pr3+:YLF lasers

High-power red lasers (mainly at 639 and 670 nm) based on Pr³⁺:YLF crystals have been presented in many works. However, the spectral resources of Pr³⁺:YLF in the red region have not been fully developed to obtain lasers due to their relatively low emission cross sections and the irrepressible strong emission at ∼639 nm. In this work, we propose a scheme to further develop the spectral resources of Pr³⁺:YLF in the red region and improve the red laser powers based on this crystal. The laser wavelengths are obtained from 634.5 to 674.7 nm (continuous tunings are achieved at some wavebands). To the best of our knowledge, the output powers obtained at 638.7, 644.6, 670.1, and 674.7 nm (2.88 W, 1.87 W, 3.55 W, and 1.73 W, respectively) are the highest to date. Furthermore, lasing originating from the ³P₂ energy level of Pr³⁺:YLF (∼653 nm) is realized for the first time.

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.

Tunable sub-300 fs soliton and switchable dual-wavelength pulse generation from a mode-locked fiber oscillator around 2.8 µm

In this Letter, we demonstrate, for the first time, to the best of our knowledge, tunable soliton and switchable dual-wavelength pulse generation from a nonlinear polarization rotation mode-locked Er³⁺-doped fluoride fiber oscillator around 2.8 µm, employing a LiNbO₃ birefringent plate (BFP)-based Lyot filter. In the single soliton state, the wavelength can be continuously tuned in the region of 2752.4∼2807.2nm (∼55nm) with a pulse width between 199 and 270 fs, by rotating the BFP with the aid of its spectral filtering effect. At higher pump levels, two kinds of dual-wavelength synchronous mode-locking states (i.e., 2746.4/2782.0 and 2787.2/2825.2 nm) are observed, and can be flexibly switched by adjusting the rotation angle of the BFP. This approach opens up new opportunities for the development of versatile mid-infrared ultrashort laser sources.

Sub-200 fs, 344 MHz mode-locked Tm-doped fiber laser

We demonstrate a compact, self-starting mode-locked thulium-doped fiber laser based on nonlinear polarization evolution (NPE), with a fundamental repetition rate of ∼344MHz and a pulse duration of ∼160fs. The generated pulses centered at ∼1975nm have a maximum output power of ∼560mW, corresponding to a pulse energy of ∼1.63nJ. To the best of our knowledge, the achieved repetition rate represents the highest value of fundamentally NPE mode-locked fiber lasers at ∼2µm, while the average output power is also higher than the previously reported 2 µm ultrafast single-mode fiber oscillators. The timing jitter in the integrated range [5 kHz, 10 MHz] and the integrated relative intensity noise in the range [10 Hz, 10 MHz] reach ∼35fs and ∼0.009%, respectively. Our high-performance laser is an ideal candidate for various applications, including mid-infrared frequency metrology and high-speed optical sampling.

Efficient framework for the examination of the field of view sensitivity of a field-widened birefringent interferometer

An efficient approach is presented that allows the field of view sensitivities of a field-widened birefringent interferometer constructed from several stacked birefringent slabs to be examined. The approach utilizes a Jones matrix framework that is valid for birefringent slabs that have their optic axis parallel to the surface of the slab. It neglects Fresnel effects and multiple reflections, but accounts for birefringent splitting and does not neglect higher-order angular effects. The simplified approach allows the angular sensitivity of the optical path difference near the field-widened configuration to be examined in the presence of misalignment and mismatches between the components. Understanding these effects is critical to developing wide-field interferometers that can be utilized for imaging purposes. Here, we present the developed framework and apply it to examine the field of view effects of a three-element field-widened static birefringent interferometer that is being developed for the measurement of upper atmospheric winds. We examine the sensitivity of the device to rotational misalignment, mismatches, and wavelength shifts. Comparisons among the modeled interference fringes, output from Zemax optical design software, and lab observations are used to validate the approach. It is also shown that the approach accurately simulates parasitic fringes associated with unwanted coupling between extraordinary and ordinary waves at the interfaces.

Widely tunable dual-wavelength operation of Tm:YLF, Tm:LuAG, and Tm:YAG lasers using off-surface optic axis birefringent filters

In this work, we have demonstrated dual-wavelength continuous-wave laser operation in diode-end-pumped Tm:YLF, Tm:LuAG, and Tm:YAG lasers. A 3-mm-thick quartz birefringent filter with an optical axis 45° to the surface plane was exploited for achieving broadly tunable two-color laser operation. By using the different orders of the filter with varying filter width and free spectral range values, dual-wavelength operation has been achieved in 11, 12, and 8 different wavelength pairs in Tm:YLF, Tm:LuAG, and Tm:YAG, respectively. Fine tuning of the rotation angle of the birefringent filter enabled control of laser power in each line. To our knowledge, this is the first report of multicolor laser operation in these gain media, and the technique used is applicable to other laser operation regimes including mode-locking.

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.

Diode-pumped Nd:YVO4 laser with discrete multi-wavelength tunability and high efficiency

Selectable and discretely tunable multi-wavelength diode-pumped Nd:YVO₄ laser operating at 1064.0, 1073.1, and 1085.2 nm was demonstrated using a single intracavity birefringent plate. Under 11.2 W of absorbed pump power, the maximum output power of more than 3.8 W for any of the three wavelengths was achieved with slope and optical-to-optical efficiencies of more than 45% and 35%, respectively.

Wavelength tuning in cryogenically cooled lasers based on Er-doped orthovanadates

Continuous tuning between 1590 and 1612 nm has been demonstrated in cryogenically cooled lasers with composite active media consisting of Er:YVO₄, Er:LuVO₄, and Er:GdVO₄ crystals resonantly pumped by a fiber-coupled laser diode emitting at 1529 nm. Wavelength tuning was implemented using an intracavity single-plate birefringent filter.

Observation of the Sagnac Effect in the Magnetization of Magnetic Nanoparticles in Water

A square optical loop used to observe the Sagnac effect was constructed by using a 632.8-nm He-Ne laser, a polarizing beam splitter and three mirrors, combined with a detection system. When a magnetic field was applied from the transversal direction to the beam of the loop, which passed through the sample cell containing magnetic nanoparticles (MNPs) in water, Sagnac interference was observed depending on the concentration of MNPs, indicating evolution of the magnetic birefringence. A possible analytical use of the magnetic Sagnac effect has been suggested.

High-power continuous-wave dual-wavelength operation of a diode-pumped Yb:KGW laser

High-power dual-wavelength diode-pumped Yb:KGW laser using a single birefringent filter plate was demonstrated. Two oscillating wavelengths maintained the same polarization and stable dual-wavelength operation at 1014.6 and 1041.3 nm (7.57 THz of frequency offset) with 3.4 W of average output power and a diffraction-limited beam profile was obtained. Dual-wavelength laser operation at shorter- or longer-wavelength pairs with lower average output power could also be realized for other output-coupling transmissions.

Continuously tunable Yb:KYW femtosecond oscillator based on a tunable highly dispersive semiconductor mirror

The optimized nonuniform growth process was used to achieve spatially dependent reflectivity and dispersions characteristics in a highly dispersive semiconductor mirror. The mirror, together with a semiconductor saturable absorber mirror (SESAM), was used to demonstrate a tunable femtosecond Yb:KYW oscillator. In the passive modelocking regime the laser could be continuously tuned over 3.5 nm spectral band around 1032 nm with high resolution, maintaining the average output power above 140 mW.

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.

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.

Precise and versatile formula for birefringent filters

In an investigation of extraordinary- (E-) ray behavior and the index of refraction for E waves in a uniaxial crystal, a precise and versatile formula for birefringent filters, based on the exact construction of the optical path difference, is set up with neither the approximation Δn = n(o) - n(e) ≤ (or n(e)), nor the ambiguity sin(θ)sin(r(ω)) = n(e). The exact construction gives the correct variation of the position and the dimension in each path, yielding the path difference while the filter is tuning. The formula is applicable not only to a filter with its optical axis parallel to the entrance surface (PAPS) but also to a filter with its axis inclined to the surface (PAIS). Also, the formula indicates that a FAIS allows laser wavelengths to be tuned over a wider range than does a FAPS. The origin of the wider range is interpreted to be the greater variation in the index for the FAIS while the filter is tuning. With the help of the formula we design a FAIS for tuning a cw Ti:sapphire laser.

Four-plate birefringent filter for high-gain pulsed dye laser tuning

A complete quantitative analysis of a multistage birefringent filter has been used to design a four-plate filter for a high-gain dye laser. It is shown that both S and P polarization modes must be considered at all stages. The four-plate design, with thickness ratios of 1:2:4:8, succeeds where typical commercial designs (1:2:15 and 1:4:16) fail when used in a high-gain cavity. Calculations have been verified by performance measurements for a quartz filter. Design specifications are presented.

Representation of birefringent filters by directly composed 4 × 4 matrices

The interaction of an obliquely incident plane wave with an arbitrary birefringent plane structure can be described exactly by a so-called 4 × 4 layer matrix. However, its formation is rather complex, and therefore a simplified method to set up this matrix is proposed. It is deduced from the boundary conditions for the tangential components of the E and the H fields at the interfaces of the birefringent layer and the phase shifts of the four waves within the layer between both interfaces. The refractive indices and the directions of polarization of the four waves are calculated from the well-known dispersion relation in the principal coordinate system and a coordinate transformation. Transmission curves of Lyot filters made of uniaxial and biaxial materials are calculated in the cases of plane waves of finite or infinite lateral extension that are incident under Brewster's angle upon the filter positioned outside or within a laser resonator. The special examples given for a quartz filter and a YAlO(3) filter are characterized by a high selectivity within a broad bandwidth if Fabry-Perot effects can be neglected. Transmission curves of a quartz filter were verified by measurements.

Explicit Jones transformation matrix for a tilted birefringent plate with its optic axis parallel to the plate surface

It is shown that, for a tilted birefringent plate (BP) with its optic axis parallel to the plate surface, an explicit Jones transformation matrix can be derived without neglecting the birefringent splitting.Differences in calculating the desired thickness(es) of a BP by use of different formulas of phase difference between ordinary and extraordinary waves are also presented.

Jones matrices of tilted birefringent plates in atunable dye laser: modes at low gain and temperature effects

The Jones matrices of tilted birefringent plates that are used as a tunable device in a laser cavity are established in order to compute the resonance modes. The results are compared with experimental spectra of a dye laser that contains this device at low gain. The temperature effect on the filter is also described.

Birefringent filter with arbitrary orientation of the optic axis: an analysis of improved accuracy

The transmission characteristics of a birefringent quartz filter inserted under Brewster's angle into a linear laser resonator is investigated theoretically and experimentally for arbitrary orientations of the optic axis. Exact expressions for filter curves of zero losses as functions of wavelength and orientation of the optic axis are given and confirmed by measurements. The Jones matrix of birefringent filter, which describes the propagation of plane waves through the filter with improved accuracy, is derived. Transmission curves are calculated, showing that the optimum selectivity strongly depends on the orientation of the optic axis. For a special filter the results are verified by measurements.