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.

Description

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 ⁴I_15/2 and ⁴I_11/2 states.

FIG. 4 is an absorption spectra for possible wavelength transitions between Stark levels in the ⁴I_13/2 and ⁴I_9/2 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 ⁴I_11/2→⁴I_13/2 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 ⁴I_15/2 ground state to the ⁴I_11/2 upper lasing state, thereby creating the initial population inversion between the ⁴I_11/2 and ⁴I_13/2 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 ⁴I_15/2 ground state, the second state being one or more of the Stark levels of ⁴I_11/2 upper lasing state, the third state being one or more of the Stark levels of the ⁴I_13/2 lower lasing state, and the fourth state being on or more of the Stark levels of ⁴I_9/2 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 ⁴I_13/2 lower lasing state to the ⁴I_9/2 metastable state from where they non-radiatively decay to the ⁴I_11/2 upper lasing state. The present invention also takes into account that the Er ions from the ⁴I_13/2 lower lasing state may be recycled to a Stark level in the ⁴I_11/2 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 show 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 ⁴I_13/2 state. This is accomplished, first, by thermalizing down to lower Stark levels in the ⁴I_13/2 states, and second, by recycling up to one or more of the available Stark levels in the ⁴I_9/2 state, the ⁴I_11/2 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 ⁴I_13/2 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 ⁴I_13/2 lower lasing state to the ⁴I_9/2 state or higher lying states without having to completely decay to the ⁴I_15/2 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 ⁴I_15/2 ground state to the ⁴I_13/2 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 andb. exposing the medium to a second pump radiation of a second wavelength, such that a second erbium ion is excited from a third state to a fourth state;c. wherein a population of erbium ions in the second state is increased and a population of erbium ions in the third state is reduced.

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 the third state is an 4I13/2 lower lasing state and the fourth state is an 4I9/2 metastable state.

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

6. 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 whereinb. the second pump radiation has a wavelength region located between approximately 1610 nanometers to approximately 1680 nanometers.

7. 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 whereinb. the second pump radiation has a wavelength region located between approximately 1610 nanometers to approximately 1680 nanometers.

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

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

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

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

12. The method of claim 9, 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.