WO2007085982A2

WO2007085982A2 - A co-doped up-conversion laser system

Info

Publication number: WO2007085982A2
Application number: PCT/IB2007/050153
Authority: WO
Inventors: Ulrich Weichmann; Gero Heusler
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2006-01-26
Filing date: 2007-01-17
Publication date: 2007-08-02
Also published as: WO2007085982A3; TW200740059A

Abstract

The invention relates to a co-doped upconversion laser system comprising a host-material, whereas the host material is made of at least one crystal material and/ or of a glass material, which features a low phonon energy, whereas said host material comprises a dopant made of Erbium (Er3+), which is pumped by laser light with a single wavelength in the infrared wavelength range, whereas said host material furthermore comprises at least one co-dopant made of Terbium (Tb3+) or Samarium (Sm3+), in order to emit laser radiation in the range of at least one visible wavelength. This system provides a co-doped upconversion laser system featuring an elimination of coulour centres, a reduced re-absorption of the laser radiation and the emission of laser light in the range of visible wavelengths.

Description

A co-doped up-conversion laser system

A co-doped upconversion laser system is proposed comprising a host- material, whereas the host material is made of at least one crystal material and /or at least one glass material, which features a low phonon energy, whereas said host material comprises at least one dopant made of Erbium (Er³⁺), which is pumped by laser light with a single wavelength in the infrared wavelength range, whereas said host material furthermore comprises at least one co-dopant.

In recent years there is a need for efficient, inexpensive and reliable visible laser sources. The preferred field of applications of these laser sources are data storage systems, display technology, undersea communications, eye-safe measurements, ranging, spectroscopy, fibre-optics applications as well as projection systems. Upconversion lasers are suitable systems to fulfil most of the requirements and attracted a lot of attention in the research regarding optical light sources. The basic principle requires the successive absorption of two or more pump photons via intermediate long-lived levels, followed by the emission of laser radiation at a shorter wavelength than the pump wavelength. Mostly rare earth ions are used for such lasers. Special consideration deserves the proper choice of host materials. These should be characterised by low phonon energies, which is a requirement for the long lifetime of the intermediate levels as well as for the laser level. Besides crystalline hosts, most upconversion lasers were realised in ZBLAN-glass as a host material.

The US Patent 6 510 276 Bl describes a highly doped waveguide, which comprises a waveguide having a dopant disposed therein, said dopant having a concentration of between 1,001 and 500.000 ppm, and wherein said waveguide contains clusters of said dopant, wherein at least 50% of said dopant is in said clusters and wherein said clusters of said dopant enhance cross-relaxation between two elements of said dopant, whereas said dopant is Erbium (Er).

An optical amplifier and laser is disclosed in the United States Patent US 5 617 244 with a resonant cavity defined by a pair of mirrors butted to respective ends of a fluorozirconate optical fibre. The fibre has a numerical aperture of 0.205 and a LPn mode cut-off of about 2.0μm. The fibre is co- doped with Thulium ions to a concentration of about 0.1% and with Terbium ions to a concentration of about 1%. An optical pump source provides a pump signal at 775nm which excites the Thulium ions into the ¹G₄ energy level to provide lasing at about 475nm. The pump source is preferably a high power semiconductor laser.

One of the prominent examples for upconversion lasers is the EnZBLAN laser, whereby the doping ion is Erbium (Er). The laser is pumped by the resonant absorption of two infrared photons of similar wavelength. The intermediate state is the ⁴Ii i_/2 and the upper pump level is the ⁴F_7Z2 level. From this upper pump level non- radiative population transfer to the ⁴S_3/2- level occurs, which serves as the upper laser level for laser emission at 550nm to the ground state ⁴Ii_5/2.

This scheme is very attractive since it is possible to use a single pump wavelength around 970nm, where diode lasers are available. Not only the energies of the levels involved are very favourable here, but also the energy distances to the next lower levels. The energy distance from the ⁴S_3/2-level to the next lower level ⁴Fg_Z2 corresponds to 3100cm^"1 and the energy difference from the intermediate ⁴In_/2-level to the next lower level ⁴Ii_3/2is about 3600cm^"1. In a host material with low phonon energies like ZBLAN (phonon energies of 500-600Cm^"1) these levels are not likely to be depopulated by non-radiative transitions and exhibit sufficiently long lifetimes for up- conversion pumping and laser operation.

The drawbacks of the material systems relating to the prior art can be described by three main characteristics:

At first excited state absorption (ESA) towards higher lying levels can lead to the emission of UV-radiation and the subsequent formation of colour centres in the host material. Such phenomena are described for Tm:ZBLAN lasers and were also observed in EnZBLAN lasers. The consequence of colour centres is an increased threshold and decreased conversion efficiency for the laser. Secondly, the lower laser level is one of the sublevels of the ground state. Therefore the laser is effectively a three level laser and re-absorption of the laser radiation may occur, which decreases the efficiency of the material system.

Thirdly, the EnZBLAN laser is most efficient in the green wavelength range around 550nm. Laser operation at other wavelengths in the visible requires the excitation of higher lying electronic states, which is not efficiently possible by using said material system.

The invention has the objective to eliminate the above-mentioned disadvantages. In particular, it is an objective of the present invention to provide a co- doped upconversion laser system which features an elimination of colour centres, a reduced re-absorption of the laser radiation, a high lifetime of the electronic states involved and the emission of laser light in the range of visible wavelength.

This objective is achieved by a co-doped upconversion laser system as taught by claim 1 of the present invention. The preferred embodiments of the invention are defined in the sub-claims.

The invention discloses, that said host material comprises at least one co- dopant made of Terbium (Tb³⁺) or Samarium (Sm³⁺), in order to emit laser radiation in the range of at least one visible wavelength.

With respect to the energy-level diagram the corresponding material combination leads to different advantages. The co-dopands are characterized by a large energy gap between the respective upper laser level and the next lower level. This has the consequence, that the upper laser levels are not depopulated by non- radiative energy transfer and exhibit a long lifetime. This situation is much more favourable than the situation in the pure Er- system. Advantageously the dopant Erbium (Er) is combined with the co-dopant

Terbium (Tb) or Samarium (Sm). The material combination according to the present invention leads to the possibility of laser operation at different wavelengths in the visible wavelength range. To achieve this in the pure EnZBLAN laser is rather involved and implies the excitation of higher lying levels. In the Tb- and Sm-co-doped EnZB LAN-lasers other wavelengths are available, where in the Sm-laser a wavelength can be generated that is very close to the well-known Na-D-lines. Additionally the Tb- co- doping system features the advantage, that not only the Ar-ion- laser wavelength at 488nm, but also the three primaries for RGB can be obtained. The Tb-codoped EnZBLAN - laser is therefore an ideal candidate to be applied as a light source for display applications.

The Tb co- doped system leads to a further advantage as compared to the pure EnZBLAN laser. Several states of Er with energies above 3eV are in resonance with Tb-levels. Energy transfer between these two systems helps to avoid the population of high-lying levels that lead to the emission of UV-radiation and subsequent formation of colour centres. Co-doping of the EπZBLAN-laser with Tb is therefore an effective way to circumvent the problem of photo-bleaching. In another preferred embodiment the host material is made of the group of crystal materials characterized by relatively low phonon energies, comprising Y₂O₂, YLF, YAP, YAG, - material, which means, that the range of suitable materials is enlarged.

As a preferred embodiment of the present invention the upconversion laser material is brought to the form of a waveguide, whereas the waveguide is formed as a fiber or a planar waveguide. Therewith the system may be applied in many different fields comprising fiber systems or waveguides for micro-systems or projection systems.

According to preferred dopant concentrations the co- dopants Terbium (Tb) and /or Samarium (Sm) feature a doping level comprising a factor of 0.1 to 10 related to the doping level of the dopant Erbium (Er). Both, the dopant Er as well as the co-dopants Tb or Sm are in their triple-ionic state.

The co- doped upconversion laser system mentioned above can be used in a variety of applications amongst them systems being data storage systems, display technology, undersea communications, fibre-optics applications as well as projection systems.

The aforementioned components, as well as the claimed material components and the components to be used in accordance with the invention in the described embodiments, are not subject to any special exceptions with respect to the size, shape, material selection as well as the technical concept such that the selection criteria known in the pertinent field can be applied without limitations.

Additional details, characteristics and advantages of the objective of the invention are disclosed in the sub-claims and the following description of the respective -figures which are exemplary fashions- show preferred embodiments of the co- doped upconversion laser system according to the invention, which will be described in conjunction with the accompanying figures, in which:

Fig. 1 shows the energy level diagram of a co- doped upconversion laser system with a dopant Erbium (Er³⁺) as an exciting material and a Terbium (Tb³⁺), used for the laser transition; and Fig. 2 shows the energy level diagram of a co- doped up-conversion laser system with a dopant Erbium (Er³⁺) as an exciting material and a co-dopant Samarium (Sm³⁺), used for the laser transition.

Figure 1 shows the energy level diagram 1 of Er/Tb co- doping and figure 2 shows the energy level diagram 13 of Er/Sm co- doping. On the left side of the scheme is shown an excitation energy scale 11 and on the right side is shown the wave number 10. The co-dopants Sm and Tb can easily be excited via the Er ⁴F_7/2-state at about 21000cm^"1, whereas the Er- dopant is at first excited from the ground state 2 by a ground state absorption (GSA) 3 to a first excitation level 5, followed by a subsequent excited state absorption (ESA) 4 to a second excitation level 6.

The energy level diagrams 1, 13 feature a non-radiative transition 7 to the exited state of the co-dopant, whereas the laser transitions in Sm couple the ⁴Gs_/2- level with H-states and the ⁵D₄-state with F-states in the case of Tb. These transitions should therefore be much more probable than the laser transition in Er, where the ⁴S_3/2-level is coupled with the ⁴Ii_5/2-level. Therefore the laser action according to the laser transition 8, 8a, 8b and 8c occurs entirely in the co-dopand, while the laser pumping is done via the Er-ion, indicated by the ground state absorption 3 and the excited state absorption 4. This excitation scheme is somewhat comparable to the He/Ne-laser and provides the advantage of a true four-level-laser scheme.

The laser transitions 8, 8a, 8b and 8c occur between the co- dopant excited state and the lower laser level 9, 9a, 9b. The transition between one of the lower laser levels 9, 9a, 9b and the ground state 2 are characterised by a non- radiative relaxation 12, whereas the relaxation 12 is featured by a fast relaxation. Thereby a four level laser scheme is realised.

Also indicated in the figure 1 and 2 is the possibility of laser operation at different wavelengths in the visible wavelength range. To achieve this in the pure

EnZBLAN laser is rather involved and implies the excitation of higher lying levels. In the Tb- and Sm-co-doped EnZB LAN-lasers other wavelengths are available, whereas in the Sm-laser a wavelength can be generated that is very close to the well-known Na-D- lines and in the Tb-codoped system, red, green and blue wavelengths can be generated as shown in Fig. 1.

The present invention is not limited by the embodiments described above, which is represented as an example only and can be modified in various ways within the scope of protection defined by the appended patent claims. Thus the invention is also applicable to different combinations of the described materials, in particular the doping ions.

LIST OF NUMERALS

1 energy level diagram with Er-Tb- doping

2 ground state 3 ground state absorption (GSA)

4 excited state absorption (ESA)

5 first excitation level

6 second excitation level

7 non- radiation transition to the co- dopant exited state 8 laser transition

9 lower laser level

10 wave number

11 excitation energy

12 non- radiation relaxation 13 energy level diagram with Er-Sm- doping

Claims

CLAIMS:

1. L A Co- doped upconversion laser system comprising: a host- material, whereas the host material is made of at least one crystal material and /or at least one glass material, which features a low phonon energy, whereas said host material comprises at least one dopant made of Erbium (Er³⁺), which is pumped by laser light with a single wavelength in the infrared wavelength range, whereas said host material furthermore comprises at least one co-dopant made of Terbium (Tb³⁺) or Samarium (Sm³⁺), in order to emit laser radiation in the range of at least one visible wavelength.

2. A co- doped upconversion laser system according to claim 1, characterised in that the host material is made of the group of glass materials, comprising ZBLAN, telluride, germanates, quartz or phosphates.

3. A co- doped upconversion laser system according to claim 1, characterised in that the host material is made of the group of crystal materials, comprising Y₂O₂, YLF, YAP or YAG- material.

4. A co- doped upconversion laser system according to one of the previous claims, characterised in that the upconversion laser material is brought to the form of a waveguide, whereas the waveguide is formed as a fiber or a planar waveguide.

5. A co- doped upconversion laser system according to one of the previous claims, characterised in that the co- dopants Terbium (Tb³⁺) or Samarium (Sm³⁺) feature a doping level comprising a factor of 0,1 to 10 related to the doping level of the dopant Erbium (Er³⁺). A Co- doped upconversion laser system according to one of the previous claims, characterised in that the system is applied in display and/ or image- projection systems.