Saturable absorbers for Q-switching of middle infrared laser cavities

Abstract

A Q-switched laser includes a laser cavity including a cavity mirror and an output coupler mirror. The Q-switched laser also includes a doped laser gain material disposed in the laser cavity and a Q-switch including a saturable absorber comprising Fe.sup.2+:ZnSe or Fe.sup.2+:ZnS.

Claims

1. A Q-switched laser comprising: a laser cavity including a cavity mirror and an output coupler mirror; an erbium doped laser gain material in the laser cavity to produce laser light at a laser wavelength around 3 m under optical pumping; and a Q-switch including a saturable absorber comprising Fe.sup.2+:ZnSe or Fe.sup.2+:ZnS.

2. The Q-switched laser of claim 1 wherein the Q-switched laser is operable to lase at a wavelength ranging from 2.5 m to 4.0 m.

3. The Q-switched laser of claim 2 wherein the Q-switched laser is operable to lase at a wavelength ranging from 2.5 m to 3.4 m.

4. The Q-switched laser of claim 1 wherein the Fe.sup.2+:ZnSe or Fe.sup.2+:ZnS is single crystalline.

5. The Q-switched laser of claim 1 wherein the Fe.sup.2+:ZnSe or Fe.sup.2+:ZnS is polycrystalline.

6. The Q-switched laser of claim 1 wherein the saturable absorber is placed at Brewster's angle.

7. The Q-switched laser of claim 1 wherein the Q-switch is disposed between the cavity mirror and the doped laser gain material.

8. The Q-switched laser of claim 1 wherein the saturable absorber comprises a passive solid state saturable absorber.

9. The Q-switched laser of claim 1 wherein the erbium doped laser gain material comprises Er:YAG.

10. A Q-switched laser comprising: a laser cavity defined by a first cavity mirror and a second cavity mirror; an output coupler disposed in the laser cavity; an erbium doped laser gain material disposed between the first cavity mirror and the output coupler to produce laser light at a laser wavelength around 3 m under optical pumping; and a Q-switch including a saturable absorber comprising Fe.sup.2+:ZnSe or Fe.sup.2+:ZnS.

11. The Q-switched laser of claim 10 wherein the Q-switched laser is operable to lase at a wavelength ranging from 2.5 m to 4.0 m.

12. The Q-switched laser of claim 11 wherein the the Q-switched laser is operable to lase at a wavelength ranging from 2.5 m to 3.4 m.

13. The Q-switched laser of claim 10 wherein the Fe.sup.2+:ZnSe or Fe.sup.2+:ZnS is single crystalline.

14. The Q-switched laser of claim 10 wherein the Fe.sup.2+:ZnSe or Fe.sup.2+:ZnS is polycrystalline.

15. The Q-switched laser of claim 10 wherein the saturable absorber is placed at Brewster's angle.

16. The Q-switched laser of claim 10 wherein the Q-switch is disposed between the output coupler and the second cavity mirror.

17. The Q-switched laser of claim 10 wherein the saturable absorber comprises a passive solid state saturable absorber.

18. The Q-switched laser of claim 10 wherein the erbium doped laser gain material comprises Er:YAG.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is the absorption cross section of Fe.sup.2+ ions in ZnSe crystal.

(2) FIG. 2 is the absorption spectra of Fe.sup.2+:ZnSe and Fe.sup.2+:ZnS samples.

(3) FIGS. 3a, b, & c are the temporal profiles of single pulse (3a) output and multi-pulse outputs (3b,c) from the passively Q-switched Er:Cr:YSGG laser system utilizing a Fe.sup.2+:ZnSe saturable absorber.

(4) FIG. 4 shows Fe.sup.2+:ZnSe transmission versus incident 2.8 m photon flux, with the solid line theoretical fit of experimental results with Frantz-Nodvick equation for =0.610.sup.18 cm.sup.2.

(5) FIG. 5 is a schematic layout of a linear cavity design for Er laser Q-switch utilization of the Fe.sup.2+:ZnSe saturable absorber.

(6) FIG. 6 is a schematic layout of a folded cavity design for Er laser Q-switch utilization of the Fe.sup.2+:ZnSe saturable absorber.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(7) In our experiments, the undoped polycrystalline and single crystalline samples of ZnSe were grown by chemical vapor deposition. Doping of the 1-3 mm thick ZnSe polycrystalline and single crystalline wafers was performed by after growth thermal diffusion of Fe from the metal or gas phase in quartz evacuated ampoules. Alternatively, Fe doped thin films of the ZnS and ZnSe the crystals were grown by pulsed laser deposition on ZnS/Se substrates. In addition, Fe:ZnS and ZnSe were fabricated by hot pressing of ZnS and ZnSe powders containing iron. Demirbis et al estimated the diffusion coefficient for iron and chromium ions to be 7.9510.sup.10 cm.sup.2/s and 5.4510.sup.10 cm.sup.2/s, respectively at 1000 C. In our preparation, the sealed ampoules were placed in a furnace and annealed at 820-1120 C. for 5-14 days. Once removed from the furnace and cooled, doped crystals were extracted from the ampoules and polished. This method of production of transition metal doped crystals is covered in U.S. Pat. No. 6,960,486 commonly owned by the assignee of this application and which is incorporated by reference herein for all purposes.

(8) The Q-switched regime of operation for a Er:Cr:YSGG laser system has two distinctive qualities: large amplitude pulses and temporally short pulses with respect to free running oscillation. Both of these qualities are needed for medical applications as well as to ensure efficient Q-switched operation of Fe.sup.2+:ZnSe lasers at room temperature. The absorption spectra of Fe.sup.2+:ZnSe and Fe.sup.2+:ZnS .sup.5E.fwdarw..sup.5T.sub.2 transitions are depicted in FIG. 2. These transitions feature a broad absorption centered at 3 m with FWHM of approximately 1400 nm. Further, the absence of exited state absorption makes polycrystalline Fe.sup.2+:ZnSe a very good candidate for a passive Q-switch for an Er laser.

(9) In our experiments a flashlamp pumped Er:Cr:YSGG laser was used as a test bed for passive Q-switching. Many cavity designs were tested, however in all cavity designs the laser head includes a 73 mm long Er:Cr:YSGG crystal with a 3 mm diameter in a gold elliptical pumping chamber pumped with a xenon flashlamp. FIG. 5 schematically illustrates a linear design with a 100% reflective mirror, HR, and an output coupler (OC) with reflectivity of 83% or 40%. The HR was placed approximately 70 mm from the end of the Er:Cr:YSGG laser crystal and the OC was placed approximately 50 mm from the laser crystal. The Fe.sup.2+:ZnSe was sample placed between 17-65 mm from the high reflector in the cavity. The laser was pulsed at 10 Hertz. Input power was determined by directly measuring the voltage across the capacitor driving the flashlamp. The output was measured with a Molectron EPM 1000 power meter or a JR-09 joule meter. For this cavity, at maximum pump energy of 31 J, an output energy of 0.5 J was achieved in a free-running mode.

(10) Using a 481 mm 90% initial transmission at 2.8 m, Fe:ZnSe placed at the Brewster angle Q-switched operation was achieved. We obtained single giant pulse lasing with a pulse duration of approximately 65 to 100 ns FWHM measured with a pyroelectric detector with a rise time of approximately 15 ns (See FIG. 3a). A maximum output energy of 5 mJ for 80% OC and approximately 7 J pump energy was achieved. The ratio of energy of single giant pulse to the respective free-running energy approached 20% and could be further increased with improvements of Fe:ZnSe quality.

(11) A multi-pulse regime was also obtained using either the 83% or the 40% OC, yielding multiple pulses depending on pump power although better performance was obtained using the 40% OC. The threshold for lasing with this OC was approximately 9 J. The five pulse regime shown in FIG. 3b represents a nearly ideal train of pulses with little energy difference from pulse to pulse. The pump energy for five pulses was 14 J. Multi-pulse output with a maximum of 19 pulses was obtained with 85 mJ total output energy at pump energy of 30 J with a 40% OC as shown in FIG. 3c. Utilization of a 50% initial transmission Fe:ZnSe sample, yielded 9 mJ output energy using a 40% OC and 42 J pump energy.

(12) Altering the cavity to a folded cavity scheme using three mirrors and two output beams allows the effective reflectance of the OC to be tuned with angle (see FIG. 6). Also this design reduced the photon flux upon the Fe.sup.2+:ZnSe sample allowing a sample with a high initial transmission to be more effectively used as a passive Q-switch with little difficulty. The HR was located approximately 115 mm from the laser crystal. The cavity was folded at approximately 45 degrees using a 40% reflecting OC as the folding mirror at approximately 180 mm from the front of the laser crystal. A 82% reflecting mirror was used as the second HR. The Fe.sup.2+:ZnSe sample was placed on this side as a passive Q-switch. The pulse repetition rate was reduced to 4 Hz to deal with thermal lensing problems. Using this setup enabled maximum Q-switched single pulse energy of 13 mJ with 65 ns FWHM using 30 J of pump energy. Similar results on Cr:Er:YSGG cavity Q-switching were obtained with the use of single thermally diffused Fe:ZnSe crystals as well as with hot-pressed ceramic Fe:ZnSe and thin films of Fe:ZnSe grown by pulsed laser deposition. Thus we propose these Fe.sup.2+:ZnSe materials for use as a passive Q-switch, particularly for Er lasers.

(13) Further, Fe.sup.2+:ZnS, having similar spectroscopic properties to Fe.sup.2+:ZnSe, is known to have the larger bandgap (3.84 vs. 2.83 eV), better mechanical and optical damage characteristics, better overlap of absorption band with the Cr:Er:YSGG lasing wavelength, higher cross-section of absorption at 2.8 m, as well as lower thermal lasing dn/dT (+4610.sup.6 vs. +7010.sup.6/ C.). Therefore, the intracavity energy and power handling capability of this material should lie higher; making Fe.sup.2+:ZnS very attractive for high energy, high power applications. Parallel experiments to those with Fe:ZnSe have been performed using Fe:ZnS, fabricated similarly to Fe:ZnSe by after growth thermo-diffusion. A 581 mm sample of Fe.sup.2+:ZnS with an absorption coefficient of 6 cm.sup.1 and an initial transmission of 75% at 2.8 m was utilized as a passive Q-switch. Using a linear cavity design placing the Fe.sup.2+:ZnS sample at the Brewster angle between the HR and Er:Cr:YSGG crystal, with an 80% reflectance OC, Q-switching experiments were performed. Approximately 5 mJ per pulse was obtained. Similar results on Cr:Er:YSGG cavity Q-switching were obtained with the use of single thermally diffused Fe:ZnS crystals as well as with hot-pressed ceramic Fe:ZnS and thin films of Fe:ZnS grown by pulsed laser deposition. Thus we propose these Fe.sup.2+:ZnS materials for use as a passive Q-switch, particularly for Er lasers.

(14) The Q-switched output of the Er:Cr:YSGG laser was used for saturation studies of Fe:ZnSe. The saturation curve of Fe:ZnSe was measured (FIG. 4). It's fitting with the Frantz-Nodvick equation results in, absorption cross section of 0.610.sup.18 cm.sup.2, which is of the same order of magnitude as the absorption cross-section obtained from spectroscopic measurements (1.010.sup.18 cm.sup.2. Hence, the described Fe-doped ZnSe and ZnS crystals are very promising as passive Q-switches for mid-IR Er lasers operating over the 2.5-4.0 m spectral range.

(15) Although the invention has been described in various embodiments it is not so limited but rather enjoys the full scope of any claims granted hereon.