# Patent

## Title
Multi-wavelength pump method for improving performance of erbium-based lasers

## Abstract

A method for increasing the efficiency of generating lasers by pumping two separate wavelengths into an erbium-based medium to populate the 4I11/2 state and depopulate the 4I13/2 state. A first excitation wavelength region is located between approximately 955 nm to approximately 1100 nm. The second excitation wavelength region is located between approximately 1600 nm to approximately 1850 nm. This multi-wavelength pumping scheme may be operated in continuous wave or quasi-continuous wave mode.

## Background
CROSS-REFERENCES TO RELATED APPLICATIONS
This utility patent application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/812,355, filed Jun. 8, 2006.


FIELD OF THE INVENTION
This invention relates to the method of enhancing the performance of lasing media.
BACKGROUND
For certain applications, it is desirable to use mid-infrared lasers that directly emit radiation between wavelengths in the 2-5 micron region operating at pulse repetition frequency (PRF) ranging between 1 Hz-100 kHz or in continuous wave, and which can also be scalable in terms of their output power. Erbium-based (Er-based) lasers, operating on the 4I11/2→4I13/2 transition, have been shown to emit radiation between approximately 2.6 to approximately 3 microns. The terminal lasing state in the above-mentioned Er-based lasers self-terminates due to long fluorescent lifetime of the 4I13/2 lower lasing state ( ̃2.5-7.5 msec) relative to the 4I11/2 upper lasing state ( ̃0.09-1.5 msec). To a first order approximation, the highest pulse repetition frequency that can be achieved in these lasers is inversely proportional to the fluorescent lifetime of the lower lasing state (in this case, the 4I13/2 state). In the case of the nominally 50% doped Er:YAG laser medium which emit radiation at approximately 3 microns wavelength, the 2.5-7.5 msec fluorescent lifetime of the 4I13/2 state implies that the highest PRF operation is still less than 1 kHz. In order to achieve operation at PRF greater than few 100 Hz, the effective fluorescent lifetime of the 4I13/2 state in Er must be reduced to a value approximately equal to the reciprocal of the desired PRF value and/or the population of the 4I13/2 state must be significantly reduced to or below that of the population of the 4I11/2 state so as to minimize or eliminate the above-mentioned self-termination process. It has been well documented in published literature that the effective fluorescent lifetime of the 4I13/2 state in Er decreases with increasing Er doping concentration.
In addition, the approximately 3 micron lasing action (especially the continuous wave mode of operation) which occurs as a result of the 4I11/2→4I13/2 transition in Er, is highly dependent on the upconversion process as this energy transfer mechanism assists in alleviating the self-termination process (see FIG. 1). Two competing upconversion processes take place in Er. In one process, two Er ions in the 4I13/2 state interact such that one of the two Er ions is de-excited to the 4I15/2 ground state while the second Er ion is energized to the 4I9/2 state which via a fast non-radiative process decays to the 4I11/2 upper lasing state thus enhancing the population inversion between the upper and lower lasing states and the overall quantum efficiency of the lasing action. The energy given off by the Er ion decaying from the 4I13/2 state to the 4I15/2 ground state is identical to the energy gained by the Er ion in transitioning from the 4I13/2 state to the 4I9/2 state. The second upconversion process, involving two Er ions in the 4I11/2 upper lasing state, results in exciting one Er ion to the 4F7/2 state, while relaxing the second Er ion to the 4I15/2 ground state. The former upconversion process is beneficial in that it removes two Er ions from the 4I13/2 lower lasing state thereby assisting in the enhancement of the population inversion by depopulating of the 4I13/2 lower lasing state. The latter upconversion process, while not preventing the generation of the  ̃3 micron radiation, is undesirable in that it takes two Er ions out of the 4I11/2 upper lasing state thereby decreasing the net population inversion and thus negatively impacting the overall efficiency of the laser operation.
Much of the prior art, which aims to improve the lasing efficiency or enhance the PRF in Er-based lasers emitting at approximately 3 micron does so by attempting to promote the (beneficial) upconversion process involving two Er ions in the 4I13/2 state and/or depopulating the 4I13/2 state by codoping with certain other rare earth ions.
SUMMARY OF THE INVENTION
This invention is directed towards a method of improving lasing performance in terms of power scaling, PRF enhancement, or both, of an erbium-based medium comprising exposing the medium to a radiation of a first wavelength, such that a first erbium ion is excited from a first state to a second state and exposing the medium to a radiation of a second wavelength, such that a second erbium ion is excited from a third state to a fourth state; thereby populating the second state and reducing the population in the third state. In one embodiment, the first state is an 4I15/2 ground state, the second state is an 4I11/2 upper lasing state, the third state is an 4I13/2 lower lasing state, and the fourth state is one or more Stark levels in one of the higher states.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a primary energy transfer processes which occur in Er-based laser media.
FIG. 2 is a diagram symbolizing a dual-wavelength excitation of Er-based laser gain media emitting radiation in the approximately 2.5 to 3.1 micron wavelength region in keeping with one embodiment of the present invention.
FIG. 3 is an absorption spectra for possible wavelength transitions between Stark levels in the 4I15/2 and 4I11/2 states.
FIG. 4 is an absorption spectra for possible wavelength transitions between Stark levels in the 4I13/2 and 4I9/2 states.
FIG. 5 is a detailed transition diagram shown in FIG. 2 including the Stark levels shown in FIGS. 3 and 4, in keeping with one embodiment of the current invention.



DETAILED DESCRIPTION OF THE EMBODIMENTS
The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments and is not intended to represent the only forms in which these embodiments may be constructed and/or utilized. The description sets forth the functions and the sequence of steps for constructing and operating the exemplary embodiments. It is to be understood, however, that the same or equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the specification.
As shown in FIG. 1, the lasing action occurs due to the 4I11/2→4I13/2 transition. Process (a) is the beneficial upconversion process. Process (b) is the undesired but weaker upconversion process. Process (c) is the cross-relaxation process.
One method, in keeping with the present invention, involves excitation of the Er-based laser gain medium in two wavelength regions, as shown in FIGS. 2 and 5. In this embodiment, the first excitation wavelength region λ1 is located between approximately 955 nm to approximately 1100 nm and excites the Er ions from the 4I15/2 ground state to the 4I11/2 upper lasing state, thereby creating the initial population inversion between the 4I11/2 and 4I13/2 states as a necessary condition to initiate the lasing action, as shown in FIG. 3.
By the foregoing multi-wavelength pumping method of this embodiment, one or more first erbium ions are excited from a first state to a second state, and one or more second erbium ions are excited from a third state to a fourth state. The first state being one or more of the Stark levels of 4I15/2 ground state, the second state being one or more of the Stark levels of 4I11/2 upper lasing state, the third state being one or more of the Stark levels of the 4I13/2 lower lasing state, and the fourth state being on or more of the Stark levels of 4I9/2 metastable state.
As shown in FIG. 4, the second wavelength region λ2 is between approximately 1600 to approximately 1850 nm to recycle the Er ions from the 4I13/2 lower lasing state to the 4I9/2 metastable state from where they non-radiatively decay to the 4I11/2 upper lasing state. The present invention also takes into account that the Er ions from the 4I13/2 lower lasing state may be recycled to a Stark level in the 4I11/2 state above the one involved in the approximately 3 micron generation resulting in the most efficient overall energy conversion efficiency, or to one of the higher states (states 3-6 in FIG. 1) resulting in a lower overall energy conversion efficiency, still a significant improvement over an approach where no λ2 pumping is involved. In either case, however, this multi-wavelength pumping method is to depopulate the 4I13/2 state.
As shown in FIG. 5, for instance, Stark levels and their associated energy level values (in units of cm−1) of the several energy states in Er:YAG provide a number of wavelengths potentially involved in depopulating the 4I13/2 state. This is accomplished, first, by thermalizing down to lower Stark levels in the 4I13/2 states, and second, by recycling up to one or more of the available Stark levels in the 4I9/2 state, the 4I11/2 state (not shown), or one of the higher states (also not shown), such as the higher states shown as states 3-6 in FIG. 1. The downward pointing arrow λ3 indicates the dominant 2.94 μm laser transition in nominally, 50% doped Er:YAG.
In one embodiment, this multi-wavelength pumping method permits operation in high PRF mode, such as greater than 1 kHz. In another embodiment, this multi-wavelength pumping method can be operated in a continuous wave mode or a quasi-continuous wave mode. In one embodiment, the first excitation wavelength may be exposed to the medium before the second excitation wavelength. In one such embodiment, exposure to the first pump radiation may precede exposure to the second pump radiation by approximately 100 microseconds or greater.
In another embodiment, the second excitation wavelength may be exposed to the medium prior to the first excitation wavelength. In another embodiment, both excitation wavelengths may be applied simultaneously.
This embodiment differs significantly from other existing methods to depopulate the 4I13/2 state in that it does not involve or rely upon any upconversion process. In addition, this embodiment works with commercially available Er-doped materials that are routinely used to generate the approximately 3 micron radiation, thus eliminating the need for any codoped Er-based media. This approach allows for high PRF operation by directly recirculating the Er ions in the 4I13/2 lower lasing state to the 4I9/2 state or higher lying states without having to completely decay to the 4I15/2 ground state thereby increasing the overall efficiency of the laser. The second wavelength λ2 may be selected by applying the selection rules from quantum mechanics and by avoiding or at least minimizing wavelengths which would excite the Er ions from the 4I15/2 ground state to the 4I13/2 state or coincide with the wavelengths that represent typical eye-safe wavelength generating transitions (most notably the approximately 1618 nm and approximately 1645 nm wavelengths associated with eye-safe Er:YAG lasers). The recirculation rate and thus the PRF value is proportional to pump energy and temporal pump pulse format of the power operating at the wavelength λ2.
In closing, it is to be understood that the embodiments described herein are illustrative of the principles of the present invention. Other modifications that may be employed are within the scope of the invention. Thus, by way of example, but not of limitation, alternative configurations may be utilized in accordance with the teachings herein. Accordingly, the drawings and description are illustrative and not meant to be a limitation thereof.

## Claims

1. A method of improving lasing efficiency of an erbium-based medium comprising:
a. exposing the medium to a first pump radiation of a first wavelength, such that a first erbium ion is excited from a first state to a second state,
b. thereby increasing the population of erbium ions in the second state, creating a population inversion necessary for lasing between the second state as the upper lasing state and a third state as the lower lasing state, said third state being different in energy level than the first state, and
c. exposing the medium to a second pump radiation of a second wavelength, such that a second erbium ion is excited from the third state to a fourth state, thereby reducing the population of erbium ions in the third state, reducing a bottleneck created by erbium ions populating the third state, and improving the population inversion necessary for lasing between the second state and the third state, wherein the third state is an 4I13/2lower lasing state and the fourth state is an 4I9/2 metastable state or a Stark level in the 4I11/2 state above said second state, and
wherein the lasing efficiency between the second state as the upper lasing state and the third state as the lower lasing state is improved.

2. The method of claim 1, wherein the medium is exposed to the first pump radiation and the second pump radiation generally simultaneously.

3. The method of claim 1, wherein the first state is an 4I15/2 ground state and the second state is an 4I11/2 upper lasing state.

4. The method of claim 1, wherein
a. the first pump radiation has a wavelength between approximately 955 nanometers to approximately 1100 nanometers and wherein
b. the second pump radiation has a wavelength between approximately 1600 nanometers to approximately 1850 nanometers.

5. The method of claim 1, wherein
a. the first pump radiation has a wavelength region located between approximately 959 nanometers to approximately 985 nanometers and wherein
b. the second pump radiation has a wavelength region located between approximately 1610 nanometers to approximately 1680 nanometers.

6. The method of claim 1, wherein
a. the first pump radiation has a wavelength region located between approximately 959 nanometers to approximately 976 nanometers and wherein
b. the second pump radiation has a wavelength region located between approximately 1610 nanometers to approximately 1680 nanometers.

7. The method of claim 1, wherein the first pump radiation and the second pump radiation are continuous waves.

8. The method of claim 1, wherein the first pump radiation and the second pump radiation are pulsed.

9. The method of claim 8, wherein the medium is exposed to the first pump radiation before to exposing the medium to the second pump radiation.

10. The method of claim 9, wherein the exposure to the first pump radiation precedes exposure to the second pump radiation by approximately 100 microseconds.

11. The method of claim 8, wherein the first pump radiation and the second pump radiation are pulsed with a pulse repetition frequency ranging between approximately 1 Hz to approximately 100 kHz.