High-power, rare-earth-doped crystal amplifier based on ultra-low-quantum-defect pumping scheme Utilizing single or low-mode fiber lasers

Abstract

A high average and peak power single transverse mode laser system is operative to output ultrashort single mode (SM) pulses in femtosecond-, picosecond- or nanosecond-pulse duration range at a kW to MW peak power level. The disclosed system deploys master oscillator power amplifier configuration (MOPA) including a SM fiber seed, outputting a pulsed signal beam at or near 1030 nm wavelength, and a Yb crystal booster. The booster is end-pumped by a pump beam output from a SM or low-mode CW fiber laser at a pump wavelength in a 1000-1020 nm wavelength range so that the signal and pump wavelengths are selected to have an ultra-low-quantum defect of less than 3%.

Claims

1. A high power single mode (SM) laser system comprising: a master oscillator power amplifier (MOPA) configuration including: a single mode (SM) seed outputting pulsed signal beam at or around a 1030 nm wavelength λs, a booster including an ytterbium (Yb) doped crystal or Yb crystal ceramic receiving the pulsed signal beam; a singlemode (SM) or low-mode (LM), continuous-wave (CW) fiber laser outputting a high brightness pump beam at a wavelength λp in a 1000 1020 nm wavelength range to end-pump the booster, wherein the pulsed signal and pump beams propagate substantially coaxially or collinearly overlapping one another in a range between 80 and 100% so as so as to provide an ultra-low quantum defect; and first and second wavelength discriminators flanking the booster, wherein the first and second wavelength discriminators each being a dichroic mirror or volume Bragg grating (VBG).

2. The high power SM laser system of claim 1, wherein the ultra-low quantum defect is less than 3%.

3. The high power SM laser system of claim 1 or 2, wherein the booster includes an Yb:YAG crystal, and the Yb crystal ceramic is Yb.sub.2O.sub.3 ceramic, the booster being shaped as a plate or rod.

4. The high power SM laser system of claim 1 or 2, wherein the wavelength λp of the high brightness pump beam varies in a 1006-1010 nm wavelength range, the ultra-low quantum effects being between 2 and 2.5%.

5. The high power SM laser system of claim 1, wherein the SM or LM CW fiber laser is operative to output the pump beam of up to a few kWs.

6. The high power SM laser system of claim 1, wherein the SM seed is configured as one or more laser diodes or a SM fiber laser operative to output a train of ultrashort signal light pulses in a fs-, ps- or ns- pulse duration range.

7. The high power SM laser system of claim 1, wherein the signal beam at an output of the booster characterized by: an average pulse power which varies from several hundred watts to kWs, and an energy per pulse in several hundred micrjoules to several milljoules range.

8. The high power SM laser system of claim 1, wherein the SM seed includes a mode-locked fiber laser.

9. The high power SM laser system of claim 1, wherein the SM seed operates in a pure pulsed mode or burst mode.

10. The high power SM laser system of claim 1, wherein the CW high brightness fiber laser outputs the pump beam such that the pump beam is coupled into one of opposite ends of the booster or both ends.

11. A booster comprising: an ytterbium (Yb) bulk amplifier receiving a SM signal beam at or around a 1030 nm wavelength; and a single or low-mode, continuous-wave (CW) fiber laser outputting high brightness pump beam coupled into one of opposite or both facets of the Yb bulk amplifier at a wavelength λp in a 1000-1020 nm wavelength range, wherein the SM signal and pump beams propagate overlapping one another at more than 80% so as to provide an ultra-low quantum defect of less than 3%; and further comprising at least one pre-amplifying stage.

12. The booster of claim 11, wherein the Yb bulk amplifier is configured to be a slab or rod, the ultra-low quantum defect being between 2 and 2.5%.

13. The booster of claim 11, wherein the Yb bulk amplifier includes a Yb:YAG or Yb.sub.2O.sub.3 ceramic.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above structural aspects of the present disclosure are explained in more detail hereinafter in conjunction with the following drawings, in which:

(2) FIG. 1 is an absorption cross-sections of Yb-doped YAG.

(3) FIG. 2 is an emission cross-section of Yb-doped YAG rod or slab.

(4) FIG. 3 is an optical schematic of the disclosed structure with an ultra-low quantum-defect pumping.

(5) FIGS. 4A-4B are respective output power and gain characteristics plotted against absorbed pump power at a 1010 nm.

(6) FIGS. 5A-5B are respective output/power and gain characteristics plotted against absorbed pump power at a 1006 nm.

(7) FIG. 6 illustrates a modification of the structure shown in FIG. 3.

(8) FIG. 7A is the output power of the disclosed bulk amplifier plotted against dopant concentration or crystal length.

(9) FIG.7B illustrates the dependence of the output power of differently dimensioned disclosed bulk amplifiers from a total pump power.

SPECIFIC DESCRIPTION

(10) FIGS. 1 and 2 illustrate absorption and emission cross-sections of Yb-ion doped gain media. Typically the pump wavelength is selected in 940-980-nm range which can be obtained from widely-available high power laser diodes. The extraction wavelength is set at or around the gain peak of 1030 nm±less than 5 nm. In contrast, the present disclosure teaches another, unconventional approach of pumping in 1 μm wavelength range with a particular interest in 1000-1020 nm and advantageously a 1006-1010 nm wavelength range utilizing SM high brightness fiber laser(s).

(11) FIG. 3 illustrates an exemplary laser system 10 illustrating the present disclosure and operable as a standalone amplifying system, such as a high power amplifier or booster, or part of a larger, more complicated amplifying system. Configured to have a master oscillator power amplifier (MOPA) architecture or be a standalone booster amplifier, laser system 10 is operative to output ultrashort pulses of signal light at 1030 nm wavelength in a fs-, ps- or ns-pulse duration range which is of particular interest for many industrial applications. The desired pulse duration is provided by a seed 12 operative to output signal light at the desired signal light wavelength and pulse repetition rate. The seed 12 is preferably a pulsed, SM fiber laser, operating in a pure pulse or burst regimes. The fiber laser/oscillator may be mode-locked. The configuration of seed 12 may also include, in addition to the fiber oscillator, one or multiple pre-amplifying stages. The use of high power diode laser without the fiber oscillator is conceivable, but less effective since the brightness of diode laser cannot be compared to that of the fiber oscillator. The linewidth depends on the pulse duration but preferably varies in a 4-7 nm range.

(12) As the output signal light propagates over a light path, it impinges upon optics L.sub.1 14 which focuses the signal light inside of Yb:YAG crystal 16 which may have various geometrical shapes and dimensions including rods and thin slabs. The latter can be plate-shaped body having a small width, for example 2 mm, and a relatively great length reaching, for instance, 6 cm. Besides the Yb:YAG crystal, the use of Yb oxide ceramics, such as Yb.sub.2O.sub.3, particularly when the above-mentioned plate configuration is used, maybe highly beneficial because of the possibility of a very high dopant (Yb) concentration.

(13) The Yb booster is operative to amplify SM signal light to kW-MW peak power levels and energy per pulse ranging between several hundred miCrjoules to several millJoules. Such a high peak-power output with diffraction-limited beam is a result of a pump arrangement (pump) 18 outputting a pump beam which is coupled into one of opposite facets of the Yb crystal booster while propagating either in the same direction as or in direction counter to that of the signal beam. The configuration of pumping the Yb booster at the opposite ends thereof is also possible. Regardless of the propagation direction, the signal and pump beams propagate in a collinear manner overlapping one another in a range varying between 80% and, under certain conditions, 100% with the overlap exceeding 90% obviously being advantageous.

(14) The pump 18 is configured with a fiber laser, in this case, doped with Yb ions and operates in CW regime to output a high-brightness pump light in the 1000-1020 nm wavelength range. The M.sup.2 of the pump light varies from 1 to 10 with a range between 1 and 2 being preferable. Accordingly, pump 18 may be configured as a single mode or low mode CW fiber laser outputting pump high brightness light beam through focusing optics L.sub.2 and L.sub.3 20 before it impinges upon a filter or wavelength discriminator F1.

(15) The filter F1 is configured as a dichroic mirror, as shown, or volume Bragg grating (VBG) 22 with the latter being particularly practical when signal and pump beams propagate at respective wavelengths λs and λp that are very close to one another. Regardless of a particular configuration and beam propagation direction, filter 22 is transparent to the signal light and reflects the pump light such that both signal and pump beams propagate in the above describer collinear and sometimes coaxial manner. In reality, the signal and pump beams may diverge from one another at a very small angle less than 1° which is not detrimental to the overall performance of the disclosed system. The overlapped beams simultaneously impinge against one of the faucets of amplifier with pump 18 and amplifier 16 being the end pumping configuration. The amplifier 16 may include multiple crystals or ceramic parts defining respective amplifying cascades 16 and 26 or single cascade 16 as shown in FIG. 6. If the multi-cascaded amplifying arrangement is deployed, another focusing lens L.sub.4 24 is installed so as to focus the beams inside the downstream crystal amplifying cascade 26.

(16) FIGS. 4A-4B illustrate respective output power and gain of the crystal amplifier relative to 1010 nm pump power. The graphs are obtained at lower pulse repetition rate (prr) of 0.1 MHz (blue dots) and higher prr of 1 MHz (red dots). It can be seen that the seed average power affects the attainable gain and output power from the crystal amplifier. FIGS. 4C-4D illustrate the same tendencies at 1006 nm pump.

(17) FIGS. 5A, 5B illustrate amplified spectra at the output of the disclosed system at respective 1010 and 1006 nm wavelength of pump light at 1 MHz prr. As can be seen, blue curve corresponds to maximum pump power which is 183.5 W for 1010 nm ump wavelength and 93.5 for 1006 nm pump wavelength in one of many experimental setups.

(18) FIG. 6 illustrates a modification of system 10. Here instead of two amplifying cascades, each having with a pair of crystals, one amplifying cascade with a single crystal 30 is used. Generally the length of the crystal and dopant concentration may be selected in respective wide ranges. For optimization purposes, increasing the length of the crystal necessitates lowering the dopant concentration and vice versa. Generally, the concentration range may include any reasonable percentage number usually limited by about 20%, whereas the crystal length may be as long as tens of centimeters, with the best results obtained so far with the crystal length less than 1 centimeter. The rest of the shown components remain identical to those used in FIG. 3. Similar to the embodiment of FIG. 3, signal and pump light beams at respective 1030 and 1010 nm wavelengths propagate coaxially along the light path downstream from filter 22 which can be either a dichroic mirror or VBG.

(19) Extensive experiments with both schematics of FIGS. 3 and 6 show that amplified signal light is more powerful at 1006 nm pump wavelength than at 1010 nm pump wavelength, which is well understood since inversion is higher at the shorter pump wavelengths, provided, as here, all other conditions are the same. These conditions include seed power, absorbed pump power and seed/pump beams sizes, The measured absorption coefficient in Yb: YAG 16 of of FIG. 3 is about 1,5x higher at 1006 nm pump wavelength than at 1010 nm pump wavelength. Importantly, the detrimental lensing effect is minimal for both 1006 and 1010 nm pump wavelengths which is readily explained by a small quantum effect at both disclosed pump wavelengths. The beam size remains essentially the same the change of less than 3%-within a seed power range from 2 W to 7W and/or pump power range between 93 W and 180 W. As can be well understood, the pump power may be unlimited and reach kW levels. The pulse energy is a function pump power and also varies within a broad range energy from several hundred micrjoules to several milljoules. pis The schematic shown in FIG. 6 In contrast, the thermal lens effect certainly manifests itself at 969 rim pump wavelength of FIG. 6. The beam size changes about 21% with pump power increase from 0 to 97.5 W at both low and high prr.

(20) Based on the observed data, it is clear how to improve the characteristics of disclosed system 10 of FIGS. 3 and 6 including output power. One useful modification includes increasing the crystal length. Still another modification leading to improved results is the increased dopant concentration. Both of the above tendencies are illustrated in FIG. 7A. FIG. 7B illustrates that the output system power grows as the total pump power at the output of the fiber laser pump increases. The red curve corresponds to the crystal length greater than that of the blue curve.

(21) Furthermore, increasing the power and brightness of the SM or low mode fiber pump allows relatively small cross-section (area) of the gain region in the amplifier crystal, i.e. the operation at power density well above I.sub.sat. For Gaussian beams at 1-um wavelength and beam diameter ≥0.2 mm the divergence is inconsequential if the crystal length varies from 20 to 80 mm. It is clear that pumping with highly bright diffraction-limited high power beams are critical to the improved efficiency of Yb-amps.

(22) It is understood that the host medium for Yb ions is not limited to YAG and can include a great variety of crystals. The non-limiting list of the host crystals may include garnets (LuAG, GGG etc), tungstates (e.g. KGW, KYW, KLuW), vanadates (YVO.sub.4, YGdO4), fluorides (YLF, LuLiF, CaF2 etc.), borates (BOYS, GdCOB), apatites (SYS), sesquioxides (Y.sub.2O.sub.3, Sc.sub.2O.sub.3) and others. Furthermore, other rare earth ions and respective crystals can be used for the resonant pumping characterized by a high power SM fiber pump and low quantum defect.

(23) The foregoing description and examples have been set forth merely to illustrate the main concept of the disclosure—using SM high power, bright CW pump lasers, which output a pump beam coaxially propagating with a signal light beam, for energizing bulk amplifiers. The structural specifics disclosed here are not intended to be limiting. Accordingly, disclosure should be construed broadly to include all variation within the scope of the disclosed concept.