Resonantly Pumped, Erbium-Doped, 2.8 Micron Solid State Laser With High Slope Efficiency

1. Field of the Invention

The present invention relates to lasers and particularly to a resonantly pumped, erbium-doped, 2.8 micron solid state laser system with high slope efficiency at or near room temperature.

2. Description of the Prior Art

It is well known that the human body is comprised of approximately 70% water, with various human tissues containing about 60% to 90% of water, and bone and cartilage containing about 30% to 40% of water. Since the 2.8 micron wavelength has a substantially maximum absorption in water, this 2.8 micron wavelength is the ideal wavelength to use for a large variety of medical laser applications on the human body. The 2.8 micron wavelength also offers a controlled absorption or penetration depth of, for example, one micron in the human body. As a result, this 2.8 micron wavelength is extremely useful in surgical applications where very precise cuts in area and/or depth are needed while minimizing damage to good tissue, bone and/or cartilage adjacent to, or under, the area to be ablated. A 2.8 micron wavelength laser could be used for precise surgery in such exemplary applications as brain surgery, neurosurgery, ear surgery, eye surgery, plastic surgery, burn treatment, dentistry, and the removal of malignancies.

Current lasers for generating this 2.8 micron wavelength use a variety of host or lasant materials with various pumping techniques for exciting the lasant material. Typically these lasers are flashlamp pumped. Such flashlamp pumped lasers are large, inefficient and expensive.

The development of high power semiconductor lasers has led to renewed interest in resonant pumping of solid state lasers based on rare earth active ions. Most of this research has been confined to the use of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) semiconductor diode laser devices which perform at high power within the range of 750 to 870 nm (nanometers). These diode lasers have been used to pump Er.sup.3+ (erbium) at about 800 nm (as well as to pump Nd.sup.3+ at about 810 nm and Tm.sup.3+ at about 790 nm). One such diode laser pumped solid state laser is disclosed in U.S. Pat. No. 5,014,279, issued May 7, 1991 to Esterowitz et al. In this patent, an erbium-doped crystal laser is resonantly pumped by a pump beam at about 800 nm from an AlGaAs diode laser to enable the erbium-doped crystal laser to produce a laser emission at substantially 2.8 microns with about a 10% slope efficiency.

The 800 nm resonant pumping diagram for the 2.8 micron Er.sup.3+ 4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser is shown in FIG. 1. The .sup.4 I.sub.9/2 state is resonantly pumped by the 800 nm pump beam and the .sup.4 I.sub.11/2 upper laser state is populated as shown through the decay .sup.4 I.sub.9/2 .fwdarw..sup.4 I.sub.11/2. The decay of the .sup.4 I.sub.9/2 state is primarily non-radiative. The radiative decay rate from the .sup.4 I.sub.9/2 state is more than two orders of magnitude lower than the non-radiative decay rate. Therefore, the radiative decay processes .sup.4 I.sub.9/2 .fwdarw..sup.4 I.sub.13/2 and .sup.4 I.sub.9/2 .fwdarw..sup.4 I.sub.15/2, which would bypass the upper laser state and therefore reduce the efficiency of the 2.8 micron laser, can be ignored. However, there is a power loss experienced in the 800 nm resonant pumping scheme shown in FIG. 1 due to the .sup.4 I.sub.9/2 .fwdarw..sup.4 I.sub.11/2 phonon decay. This power loss reduces the slope efficiency in the 800 nm resonant pumping scheme of FIG. 1. More specifically, the maximum possible slope efficiency for a 2.8 micron laser pumped by the 800 nm resonant pumping scheme of FIG. 1 is given by .lambda..sub.pump /.lambda..sub.laser =28%.

Another loss mechanism that results in a still lower slope efficiency for the Er.sup.3+ 800 nm resonant pumping scheme is illustrated in FIG. 2. Essentially, FIG. 2 illustrates the Er.sup.3+ concentration-dependent self-quenching process .sup.4 I.sub.9/2 +.sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.13/2 +.sup.4 I.sub.13/2. The non-radiative self-quenching process .sup.4 I.sub.9/2 +.sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.13/2 +.sup.4 I.sub.13/2 bypasses the .sup.4 I.sub.11/2 upper laser state and leads to a reduction in the pumping efficiency of the 800 nm-pumped 2.8 micron Er.sup.3+ laser. This self-quenching process is a phonon-assisted dipole-dipole interaction between nearby Er3+ ions in the crystal lattice. The probability for the occurrence of the self-quenching process increases at higher Er.sup.3+ concentrations due to the stronger dipole-dipole interaction for smaller separation between Er.sup.3+ ions. Therefore, the lifetime of the .sup.4 I.sub.9/2 state decreases at higher concentrations.

FIG. 3 illustrates the fluorescence decay from the .sup.4 I.sub.9/2 state for 4% and 30% concentrations of Er.sup.3+ in a YLF crystal host. The natural logarithm of the fluorescence intensity is shown plotted against time. The .sup.4 I.sub.9/2 state is excited by a pulsed dye laser having a pulse duration (.apprxeq.10 nano seconds) significantly shorter than the .sup.4 I.sub.9/2 lifetime. The .sup.4 I.sub.9/2 lifetimes are obtained from a linear least-squares fit (the solid line in FIG. 3) to the fluorescence data. Similar fluorescence decay data are obtained for two intermediate Er.sup.3+ concentrations, 8% and 16%, in the YLF crystal host. The .sup.4 I.sub.9/2 lifetimes for the 4%, 8%, 16% and 30% concentrations are given in the following TABLE 1.

The lifetimes for the 4% and 8% samples are very nearly the same, implying that for the 4% Er.sup.3+ the .sup.4 I.sub.9/2 decay is due almost entirely to phonon decay to the .sup.4 I.sub.11/2 state. (The radiative decay rate from the .sup.4 I.sub.9/2 state is negligible compared to the phonon decay rate.) The quantum efficiency for populating the .sup.4 I.sub.11/2 upper laser state is given by .tau.(C)/.tau.(0), where .tau.(C) is the .sup.4 I.sub.9/2 lifetime for Er.sup.3+ concentration C, and .tau.(0) is the limiting value of the lifetime for an arbitrarily small Er.sup.3+ concentration. That is, .tau.(0) is due to purely phonon decay. Since .tau.(4%).apprxeq..tau.(8%), it can be assumed that .tau.(0).apprxeq..tau.(4%). Using this approximation, the quantum efficiencies .tau.(C)/.tau.(0) for pumping the .sup.4 I.sub.11/2 upper laser state are given in the above TABLE 1.

The maximum possible slope efficiency for the 800 nm-pumped 2.8 micron Er.sup.3+ :YLF laser with Er.sup.3+ concentration C is (.tau.(C)/.tau.(0))(.lambda..sub.pump /.lambda..sub.laser). From TABLE 1, the maximum possible slope efficiency for the 800 nm pumping scheme of FIG. 1 is therefore 22% for an Er.sup.3+ concentration of 30%.

TABLE 1 also shows that the reduced efficiency for the 800 nm-pumped 2.8 micron laser due to the self-quenching process (previously discussed in relation to FIG. 2) can be avoided by using a low Er.sup.3+ concentration. However, this approach is not suitable for the cw-pumped 2.8 micron Er.sup.3+ laser due to the importance of the upconversion process .sup.4 I.sub.13/2 +.sup.4 I.sub.13/2 .fwdarw..sup.4 I.sub.9/2 .sup.4 I.sub.15/2 for cw operation of the .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 transition. This transition is nominally self-terminating due to the long lifetime (13.2 milliseconds or ms) of the .sup.4 I.sub.13/2 lower laser state relative to the lifetime (4.2 ms) of the .sup.4 I.sub.11/2 upper laser state. The upconversion process .sup.4 I.sub.13/2 +.sup.4 I.sub.13/2 .fwdarw..sup.4 I.sub.9/2 +.sup.4 I.sub.15/2, which is the inverse of the self-quenching process and is therefore increasingly efficient for higher Er.sup.3+ concentrations, effectively reduces the lower laser state lifetime and allows cw operation of the otherwise self-terminated 2.8 micron laser transition. This effect has been demonstrated for the cw-pumped 2.8 micron Er.sup.3+ :YLF laser, for which a slope efficiency of 0.7% was obtained for an 8% Er.sup.3+ concentration (See "CW and Pulsed 2.8 .mu.m Laser Emission from Diode-Pumped Er.sup.3+ :LiYF.sub.4 at Room Temperature" by G. J. Kintz, R. Allen, and L. Esterowitz, Appl. Phys. Letts., Vol. 50 (22), pp. 1553-1555 (June 1, 1987)), and a 10% slope efficiency was obtained for a 30% Er.sup.3+ concentration (See U.S. Pat. No. 5,014,279). This fundamental trade-off, i.e. higher cw efficiency due to the effective reduction in the lower laser state lifetime via the upconversion process for higher Er.sup.3+ concentration, and lower efficiency due to the self-quenching loss for higher concentration, can not be avoided in the 800 nm pumping scheme.

FIG. 4 illustrates the polarized absorption spectrum for 30% Er.sup.3+ :YLF in the 800 nm region. Since YLF is a uniaxial crystal, the absorption is shown for both the c-axis (the solid line) and the a-axis (the dotted line) polarizations. Note that the absorption spectrum in the 800 nm region is strongly polarized. The peak c-axis absorption is approximately five times stronger than the peak a-axis absorption. As a result of this weak a-axis absorption in the 800 nm region, a polarization-coupled beam-combining pumping scheme can not be employed in the 800 nm region. Also note the narrowness of the absorption spectrum for both polarizations in the 800 nm region. The strongest c-axis absorption peaks in the 800 nm region have a width of only 1 nm (FWHM or full width at half maximum).

Accordingly, one object of the invention is to efficiently generate a laser emission at a wavelength of substantially 2.8 microns with a high slope efficiency at or near room temperature.

Another object of the invention is to provide a resonantly pumped, 2.8 micron solid state laser system and method for operating same.

Another object of the invention is to provide a resonantly pumped 2.8 micron Er:YLF laser system at 970 nm with 17% slope efficiency.

Another object of the invention is to provide a 2.8 micron solid state laser that can be resonantly pumped with at least one InGaAs laser diode at 970 nm.

Another object of the invention is to provide a 2.8 micron solid state laser that can be resonantly pumped with a Ti:Sapphire pump laser at 970 nm.

Another object of the invention is to provide a continuous wave, laser diode pumped, erbium-doped, solid state laser system for producing a continuous wave laser emission at a wavelength of substantially 2.8 microns.

Another object of the invention is to provide a resonantly pumped Er:YLF continuous wave laser system at substantially 2.8 microns with at least a 10% slope efficiency, but preferrably with at least a 17% slope efficiency.

A further object of the invention is to provide a resonantly pumped, Er.sup.3+ -doped, solid state laser system for directly pumping the .sup.4 I.sub.11/2 upper laser state of the Er.sup.3+ with a pump beam at a wavelength of about 970 nm to cause the Er.sup.3+ -doped laser to produce a laser emission corresponding to the .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser transition and having a wavelength of substantially 2.8 microns.

These and other objects of the invention are achieved by providing a resonantly-pumped, solid state laser system which comprises an erbium-doped, 2.8 micron, solid state laser and a resonant pumping means for directly pumping the .sup.4 I.sub.11/2 upper laser state of the erbium with a pump beam at a wavelength of about 970 nm to cause the erbium-doped laser to produce a laser emission corresponding to the .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser transition having a wavelength of substantially 2.8 microns. The resonant pumping means can be, for example, at least one InGaAs laser diode or a Ti:Sapphire pump laser.

These and other objects, features and advantages of the invention, as well as the invention itself, will become better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein like reference numerals designate identical or corresponding parts throughout the several views and wherein:

FIG. 1 illustrates the 800 nm resonant pumping diagram for a 2.8 micron Er.sup.3+ laser with a .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser transition;

FIG. 2 illustrates the Er.sup.3+ concentration-dependent, self-quenching process of .sup.4 I.sub.9/2 +.sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.13/2 +.sup.4 I.sub.13/2 ;

FIG. 3 illustrates the fluorescence decay from the .sup.4 I.sub.9/2 state for 4% and 30% concentrations in YLF;

FIG. 4 illustrates the polarized absorption spectrum for 30% Er.sup.3+ :YLF in the 800 nm region;

FIG. 5 illustrates the 970 nm resonant pumping diagram for a 2.8 micron Er.sup.3+ laser with a .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser transition;

FIG. 6 illustrates the polarized absorption spectrum for 30% Er.sup.3+ :YLF in the 970 nm region;

FIG. 7 illustrates a preferred embodiment of the invention; and

FIG. 8 illustrates an exemplary plot of the performance of the 970 nm-pumped, 2.8 micron, Er.sup.3+ :YLF laser output power as a function of the 970 nm pump power.

As stated before, the invention is a resonantly-pumped, solid state laser system which comprises an erbium-doped, 2.8 micron, solid state laser and a resonant pumping means for directly pumping the .sup.4 I.sub.11/2 upper laser state of the erbium with a pump beam at a wavelength of about 970 nm to cause the erbium-doped laser to produce a laser emission corresponding to the .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser transition having a wavelength of substantially 2.8 microns.

FIG. 5 illustrates the 970 nm resonant pumping diagram for a 2.8 micron Er.sup.3+ laser with a .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser transition. As shown in FIG. 5, the .sup.4 I.sub.11/2 upper laser state is pumped directly by a 970 nm pump beam. The maximum possible slope efficiency for a 2.8 micron laser pumped by this 970 nm resonant pumping scheme of FIG. 5 is given by .lambda..sub.pump /.lambda..sub.laser =35%, while the maximum possible slope efficiency for a 2.8 micron laser pumped by the previously discussed 800 nm resonant pumping scheme of FIG. 1 is 28%. By directly pumping into the .sup.4 I.sub.11/2 upper laser state of the Er.sup.3+ laser, the 970 nm pumping scheme of FIG. 5 has a higher slope efficiency than the 800 nm pumping scheme of FIG. 1, because both the loss due to the phonon decay in the 800 nm pumping scheme of FIG. 1 and the Er.sup. 3+ concentration-dependent self-quenching process .sup.4 I.sub.9/2 +.sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.13/2 +.sup.4 I.sub.13/2 of FIG. 2 are avoided completely.

The above-mentioned self-quenching process further reduces the slope efficiency of the 800 nm pumped 2.8 micron Er.sup.3+ :YLF laser. From the discussion of TABLE 1 above, it will be recalled that the maximum possible slope efficiency for the 800 nm-pumped 2.8 micron Er.sup.3+ :YLF laser with an Er.sup.3+ concentration of 30% is 22%. Therefore, the maximum possible slope efficiency of 35% for the 970 nm pumping scheme of FIG. 5 is more than 50% higher than the maximum possible slope efficiency of 22% for the 800 nm pumping scheme in 30% Er.sup.3+ :YLF.

Thus, as discussed above, the 2.8 micron Er.sup.3+ :YLF .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser has a higher slope efficiency for resonant pumping into the 970 nm absorption band than for the 800 nm pump band. The improved efficiency is due to the lower energy of the 970 nm pump radiation, and the avoidance of the self-quenching process .sup.4 I.sub.9/2 +.sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.13/2 +.sup.4 I.sub.13/2, which partially bypasses the upper laser state for 800 nm pumping. These advantages result from the direct pumping of the .sup.4 I.sub.11/2 upper laser state in the 970 nm pumping scheme of FIG. 5.

FIG. 6 illustrates the polarized absorption spectrum for 30% Er.sup.3+ :YLF in the 970 nm region. The absorption spectrum in the 970 nm region of FIG. 6 has two advantages over that in the 800 nm region shown in FIG. 4.

In regard to a first advantage to pumping at 970 nm, it will be recalled that the absorption in the 800 nm region is strongly polarized and that the peak c-axis absorption is five times stronger than the peak a-axis absorption, as shown in FIG. 4. On the other hand, as shown in FIG. 6, the polarization is much less pronounced in the 970 region. A polarization-coupled, beam-combining pumping scheme can therefore be employed in the 970 nm region shown in FIG. 6, but the a-axis absorption in the 800 nm region shown in FIG. 4 is too weak to facilitate such a pumping scheme.

The second advantage for pumping at 970 nm lies in the broad and smooth character of the absorption spectrum for both the c-axis (the solid line) and the a-axis (the dotted line) polarizations in the 970 nm region shown in FIG. 6. For both the c-axis and the a-axis, the primary absorption feature in the 970 nm region is broader than 5 nm (FWHM). In contrast, the strongest c-axis absorption peaks in the 800 nm region have a width of only 1 nm (FWHM), as shown in FIG. 4.

Referring now to FIG. 7, a preferred embodiment of the invention is shown. In FIG. 7, a pump laser 11 transmits a cw (continuous wave) pump beam at a wavelength of substantially 970 nm (nano meters). This 970 nm pump beam is longitudinally focused by an optical system 13 into an erbium-doped, yttrium lithium fluoride (Er.sup.3+ :YLF) crystal or rod 15 of a solid state laser 17 to resonantly pump the Er.sup.3+ :YLF crystal 15. In response to this 970 nm pump beam the Er:YLF crystal 15 produces an output cw laser emission at substantially 2.8 microns with a high slope efficiency.

The pump laser 11 can comprise one or more single lasers or even one or more laser diode arrays, with each laser diode preferrably being a strained-layer indium gallium arsenide (InGaAs) diode for providing a laser emission at a wavelength of substantially 970 nm. Strained-layer InGaAs diode lasers have been shown to have lower threshold current densities and less susceptibility to damage than AlGaAs diode lasers. In addition, the pump laser 11 can be a Ti:Sapphire (titanium sapphire) pump laser, or any other suitable optical source, for producing a 970 nm pump beam.

The optical system 13, which can be a focusing lens or other suitable optical arrangement, is utilized to collect and focus the pump beam from the pump laser 11 into the crystal 15 to essentially match the 970 nm pump beam to the cavity mode of the erbium-doped crystal 15.

The Er:YLF crystal 15 is a 30% erbium-doped LiYF.sub.4 crystal, 8 mm long, with high reflectivity coatings (not shown) on opposite front and back end surfaces 19 and 21, respectively, of the crystal 15 to form a monolithic laser cavity (not shown). The front end surface 19 of the crystal 15 is flat and is high reflectivity (HR)-coated at the 2.8 micron laser wavelength and anti reflectivity (AR)- coated at the 970 nm pump wavelength. The back end surface 21 of the crystal 15 is concave with an 1 cm (centimeter) radius of curvature and is the output coupler of the solid state laser 17 with a 99.7% reflectivity at 2.8 microns and 100% reflection for the 970 nm pump beam.

In the embodiment shown in FIG. 7, the yttrium lithium fluoride (YLF) material of the crystal 15 forms the laser host material or lasant material which is doped with a 30% concentration of erbium, which is the activator material. Upon being pumped by the pump beam at the wavelength of 970 nm, the Er:YLF crystal 15 produces a laser emission at substantially 2.8 microns with a slope efficiency of at least 10%, but preferrably at least 17% (to be discussed).

At the 30% erbium concentration in the YLF host material of the crystal 15, an advantageous upconversion process takes place and aids in the laser operation of the crystal 15. In addition, better mode matching is also achieved in the crystal 15 at this 30% concentration of the erbium activator in the crystal 15.

It should be noted at this time that experiments and calculations have indicated that the optimum percent concentration of the erbium activator (or erbium activator ions) is between 25% and 65% erbium in YLF. It should also be noted that, in addition to YLF, other laser host materials can be employed in the crystal 15, such as BaY.sub.2 F.sub.8 (barium yttrium fluoride), GGG (gadolinium gallium garnet), GSGG (gadolinium scandium gallium garnet), LLGG (lanthanum lutetium gallium garnet), GSAG (gadolinium scandium aluminum garnet), NaLiF.sub.4 (sodium lithium fluoride) and YSGG (yttrium scandium gallium garnet). Each of these laser host materials would also be doped with a percent concentration of erbium activator ions between 25% and 65%.

By the use of the term "percent concentration of erbium activator ions" it is meant the percent of substitution of the yttrium ions in YLF, BaY.sub.2 F.sub.8, or YSGG by the erbium (Er) activator ions, or the percent of substitution of the gadolinium ions in GGG, GSGG, LLGG, or GSAG by the Er activator ions, or the percent of substitution of the sodium ions in NaLiF.sub.4 by the Er activator ions. For example, with a 30% concentation of erbium activator ions selected, erbium would replace 30% of the yttrium in the LiYF.sub.4 or BaY.sub.2 F.sub.8, or would replace 30% of the gadolinium in the GSGG or GSAG.

Referring now to FIG. 8, the 970 nm-pumped 2.8 micron Er.sup.3+ :YLF cw laser performance is shown. The 2.8 micron output power is plotted versus the incident 970 nm pump power. The 970 nm-pumped 2.8 micron Er.sup.3+ :YLF laser is therefore significantly more efficient than the above-discussed prior art 800 nm-pumped laser, for which a 10% slope efficiently has been achieved. The expected advantage for the 970 nm pumping scheme over the 800 nm pumping scheme, due to the direct pumping of the .sup.4 I.sub.11/2 upper laser state in the 970 nm pumping scheme, is therefore reflected in the improved 2.8 micron laser efficiency.

The 2.8 micron Er.sup.3+ :YLF .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser has been shown to have higher efficiency for resonant pumping into the 970 nm absorption band than for the 800 nm pump band. This improved efficiency is due to the lower energy of the 970 nm pump radiation, and the avoidance of the self-quenching .sup.4 I.sub.9/2 +.sup.4 I.sub.15/2 .fwdarw..sup.4 I.sub.13/2 +.sup.4 I.sub.13/2 which partially bypasses the .sup.4 I.sub.11/2 upper laser state for 800 nm pumping. These advantages result from the direct pumping of the .sup.4 I.sub.11/2 upper laser state in the 970 nm pumping scheme. There are several other advantages of the 970 nm pumping scheme in addition to the ones discussed above. The 970 nm absorption is stronger and broader than the 800 nm absorption, as discussed in relation to FIGS. 4 and 6. This is an important consideration for pumping with high power diode laser arrays. Also, with a 970 nm pumping of a 2.8 micron Er.sup.3+ :YLF .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser, the Er.sup.3+0 concentration can be increased, thereby achieving stronger pump absorption, without reducing the .sup.4 I.sub.11/2 upper laser level pumping quantum efficiency. With 800 nm pumping, concentration quenching reduces the pumping quantum efficiency with increased Er.sup.3+ concentration.

Therefore, what has been described in a preferred embodiment is a resonantly pumped, solid state laser system which comprises an erbium-doped, 2.8 micron, solid state laser and a resonant pumping means for directly pumping the .sup.4 I.sub.11/2 upper laser state of the erbium with a pump beam at a wavelength of about 970 nm to cause the erbium-doped laser to produce a laser emission corresponding to the .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 laser transition and having a wavelength of substantially 2.8 microns at a slope efficiency of at least 10%, but preferrably at least 17%.

It should therefore readily be understood that many modifications and variations of the present invention ar possible within the purview of the claimed invention. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.