Thulium-Doped Fluorozirconate Fiber Laser Pumped by a Diode Laser Source

1. Field of the Invention

The present invention relates to lasers and particularly to a diode-pumped, fiber laser doped with thulium activator ions for producing an output CW laser emission at a wavelength of substantially 2.3 microns.

2. Description of the Prior Art

In low power applications, such as in telecommunications and in medical and sensing applications, the use of fiber lasers is becoming more and more important.

In a typical fiber laser a rare earth, such as erbium, neodymium, terbium or praseodymium, is doped into the core of an optical fiber to provide an active gain medium for the fiber laser. Typically, the optical fiber is comprised of silica. The input end of the fiber laser is pumped with optical radiation to produce lasing action in the fiber laser at a wavelength essentially determined by the dopant and the mirror reflectivities. The doped optical fiber is included in the laser resonant cavity of the fiber laser.

A major disadvantage of using a silica fiber as the host optical fiber for the dopant rare earth is that a silica fiber is not suitable for transmitting wavelengths longer than 2 microns. The reason for this is that there is too much attenuation of light in the silica fiber at wavelengths above 2 microns.

Intense research and development have been conducted in the area of fluorozirconate (ZrBaLaNa or ZBLAN) glasses to produce ultra-low-loss fibers for optical communications. Minimum transmission losses in ZBLAN fibers occur over the wavelength range between 2 and 3 microns. It is, therefore, highly desirable to develop ZBLAN rare earth fiber lasers in this wavelength range.

The first demonstration of a 2.3 micron, thulium-doped, ZBLAN fiber laser was demonstrated by Leon Esterowitz and Roger Allen, the inventors in the present application. Pulsed operation was achieved by pumping with a pulsed alexandrite laser. The work was presented at the Lasers '87 Conference in December 1987, at the IEEE/LEO's '88 Annual Conference in November 1988 and at the Conference on Lasers and Electro-Optics in April 1988, and was published in "Electronics Letters", Vol. 24, No. 17, pp. 1104-1106, Aug. 18, 1988. The fiber laser did not operate in the continuous wave mode and was inconveniently pumped by a large alexandrite laser.

Accordingly, one object of the invention is to provide a thulium-doped fluorozirconate fiber laser pumped by a laser diode source.

Another object of the invention is to provide a laser diode-pumped, CW, 2.3 micron, fiber laser and method for operating same.

Another object of the invention is to provide a room-temperature, laser diode-pumped, thulium-doped, fluoride fiber laser for producing a CW laser emission at substantially 2.3 microns.

Another object of the invention is to continuously pump a thulium-doped fiber laser with a CW pump radiation to enable the fiber laser to produce a CW laser radiation at substantially 2.3 microns.

Another object of the invention is to provide a fiber laser doped with thulium activator ions to produce an output CW laser emission at a wavelength in the range of substantially 2.2 to 2.5 microns when the fiber laser is pumped by a CW pump beam from a laser diode source.

A further object of the invention is to provide a continuous wave laser emission at substantially 2.3 microns in a thulium-doped fluorozirconate fiber laser.

These and other objects of the invention are achieved by providing a laser system comprising a laser diode source for producing a CW pump beam at a preselected wavelength, and a fiber laser doped with thulium activator ions to produce an output CW laser emission at a wavelength of substantially 2.3 microns when the fiber laser is pumped by the CW pump beam.

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 a preferred embodiment of the invention;

FIG. 2 illustrates an energy level diagram of trivalent thulium, indicating the 2.3 micron lasing transition; and

FIG. 3 illustrates the fiber laser output power at 2.3 microns as a function of pump power launched into the fiber 17 and with a 2% output coupling.

Referring now to the drawings, FIG. 1 illustrates a thulium-doped, fiber laser 11 pumped by a continuous wave (CW) laser diode source 13 in accordance with the invention. The fiber laser 11 is comprised of a host optical fiber 15 that is doped with thulium activator or lasant ions (not shown) to form a gain medium fiber or laser fiber 17. The laser fiber 17 is disposed in a linear resonant cavity 19 formed by flat mirrors 21 and 23 which are optically aligned with the laser fiber 17. The laser fiber 17 has an exemplary length of 35 centimeters and its ends are respectively butt-coupled to the sides of the mirrors 21 and 23 within the cavity 19.

To take advantage of power confinement in the host optical fiber 15, a quasi-single mode step index fiber 15 is used with an approximate 7.5 micrometer core radius and an index of refraction between the core (not shown) and cladding (not shown) of about 0.005, giving a cutoff wavelength of about 2.4 microns. The core index of refraction is about 1.5. The nominal diameter of the cladding is about 150 microns.

The host optical fiber 15 can be a fluoride fiber, such as a fluorozirconate fiber or a fluorophosphate fiber. As an alternative, the host optical fiber 15 could be a low-loss, water free, silica fiber. For purposes of this description, the host optical fiber 15 is a single-mode fluorozirconate glass fiber. This fiber material is referred to as ZBLAN, which is an acronym derived from the constituent parts of fluorozirconate, namely, ZrF.sub.4, BaF.sub.2, LaF.sub.3, AlF.sub.3 and NaF.

The host ZBLAN fiber 15 is doped with trivalent thulium activator ions (Tm.sup.3+) having a mole percentage in the broad range of 0.01% to 0.5%, a mole percentage in the preferred range of 0.04% to 0.25%, or a most preferred mole percentage of 0.1% in the host ZBLAN fiber 15. For a most preferred mole percentage of 0.1% of thulium ions in the fiber 15, the constituent ZBLAN parts of ZrF.sub.4, BaF.sub.2, LaF.sub.3, AlF.sub.3 and NaF would have the respective exemplary mole percentages of 53.8%, 20.0%, 4.04%, 3.16% and 18.87%, and the thulium activator ions would be in the compound TmF.sub.3 and would substantially have the mole percentage of 0.1% in the host ZBLAN fiber 15.

The laser diode source 13 supplies an exemplary 200 mW, continuous wave (CW), pump beam at an exemplary wavelength of 786 nm to cause the thulium-doped fiber laser 11 to produce a CW laser emission at a wavelength of substantially 2.3 microns. The laser diode source 13 is preferrably a GaAlAs laser diode array or a GaAlAs laser diode. Approximately 27 mW of the pump beam is collected and focused by convential optics 25 onto a spot approximately 8 micrometers by 25 micrometers at the surface of the end of the fiber laser 17 in optical contact with the mirror 21.

The input mirror 21 is transparent to the 786 nm wavelength of this exemplary 27 mW of pump power that is incident thereon, but is almost totally reflective to the substantially 2.3 micron, CW laser emission produced by the fiber laser 11 when it is pumped by the pump beam. The output mirror 23 is also highly reflective at 2.3 microns. However, mirror 23 is approximately 0.5% transmissive at the output wavelength of 2.3 microns. Consequently, mirror 23 also operates as an output coupler to output a portion of the substantially 2.3 micron laser emission developed by the fiber laser 11 when it is pumped by the pump power from the CW laser diode source 13.

As a result of the high transmissivity of the input mirror 21 to the 786 nm wavelength of the diode pump power incident thereon, approximately 60% of the exemplary 27 mW of pump power passes through the mirror 21 and is launched into the input end of the laser fiber 17. Approximately 88% of this launched power is absorbed by the thulium dopant at this low pump power. Upon being longitudinally pumped by this absorbed power, the thulium-doped, ZBLAN fiber laser 11 produces a CW laser emission at substantially 2.3 microns. A portion of this 2.3 CW laser emission passes through the partially transmissive mirror 23 (or output coupler) as the output CW laser emission at a wavelength of substantially 2.3 microns.

In an alternative arrangement, the CW laser diode source 13 can be an InGaAlP laser diode source which lases at a wavelength in the range of 680 nm to 690 nm. A pump beam at a wavelength in this range would still pass through the input mirror 21, be launched into the laser fiber 17 and be absorbed by the thulium-doped laser fiber 17, thereby causing the fiber laser 11 to produce an output CW laser emission at substantially 2.3 microns.

Referring now to FIG. 2, FIG. 2 illustrates an energy level diagram of the trivalent thulium (Tm.sup.3+) used in the ZBLAN fiber 15, indicating the 2.3 micron lasing transition. As indicated in FIG. 2, in response to the diode pump power at a wavelength of 786 nm (0.786 microns) from the CW laser diode source 13 (FIG. 1), electrons are pumped (or excited) from the .sup.3 H.sub.6 ground state level all the way up to the .sup.3 H.sub.4 level. This .sup.3 H.sub.4 level is the metastable level and is also the upper laser level lifetime. From the .sup.3 H.sub.4 level the electrons drop down to the .sup.3 H.sub.5 level, which is the lower laser level. Each electron that drops from the .sup.3 H.sub.4 level to the .sup.3 H.sub.5 level causes a photon to be emitted by the fiber laser 11 at a wavelength of substantially 2.3 microns. This 2.3 micron wavelength is the wavelength of the output laser emission from the mirror 23.

The electrons that dropped to the .sup.3 H.sub.5 level very quickly relax to the .sup.3 F.sub.4 level, depopulating the .sup.3 H.sub.5 lower laser level. Since the fiber laser 11 is being continuously pumped by the CW pump power from the CW laser diode source 13, the above-described operation continuously repeats. As a result, a CW laser emission is developed at substantially 2.3 microns between the .sup.3 H.sub.4 and .sup.3 H.sub.5 levels.

If the losses in the cavity 19 are too high, then the long lifetime of the .sup.3 F.sub.4 level would prevent CW operation (although pulsed operation would still be possible) since the pump powers employed would deplete the ground state and populate the .sup.3 F.sub.4 level. It is desired to keep excess cavity losses at 3% or less to generate CW laser operation. These losses are primarily contributed by the respective optical contacts between the ends of the fiber 17 and the mirrors 21 and 23, scattering and impurity absorption losses.

FIG. 3 shows laser efficiency data obtained in the system of FIG. 1 for a 2% output coupling. From the near linear portion of the curve corresponding to pump powers less than about 10 mW, a slope efficiency of approximately 10% is estimated. It can be noticed from the curve that the output appears to approach saturation with increasing pump powers. This saturation is most likely caused by the relatively long lifetime of the .sup.3 F.sub.4 level such that, when pumping above threshold, this level becomes rapidly populated by decay from the .sup.3 H.sub.5 terminal laser level. For an optically thin medium the population of the .sup.3 F.sub.4 level above threshold may be given approximately by ##EQU1## Here the definition of a saturation flux, I.sub.sat, is given by

where I.sub.p is the pump power above threshold, hv.sub.p is the pump photon energy, .sigma..sub.p is the pump absorption cross section, N is the total ion concentration, A is the core cross-sectional area of the fiber 17, and .tau. is the lifetime of the .sup.3 F.sub.4 level. From Eq. (2), using values of hv.sub.p =2.5.times.10.sup.-19 J, .tau.=9.8 ms, and .sigma.=3.5.times.10.sup.-21 cm.sup.2, a saturation flux, I.sub.sat, of approximately 15 mW can be calculated. The pump fluences used in FIG. 3 are clearly of the same order as the saturation flux. Therefore, from Eq. (1), a departure from the low-power linear behavior due to depopulation of the ground state level or manifold should be expected.

In order to obtain CW operation, it is necessary to understand the population dynamics of the relevant thulium energy levels. High losses in the laser cavity 19, even under high pump intensity excitation will cause the laser to self terminate (and end CW operation) due to rapid filling of the .sup.3 F.sub.4 level and depletion of the .sup.3 H.sub.6 ground state. Introducing an index matching fluid (e.g. a paraffin oil) between the ends of the fiber 17 and the associated mirrors 21 and 23 further decreases the losses to 3% or less in the cavity 19 and significantly aids the successful generation of the 2.3 micron CW laser emission.

It should be noted that CW outputs as high as 1 mW at 2.3 microns have been obtained using a single-mode, thulium-doped ZBLAN fiber laser 11 pumped by a GaAlAs diode source 13. It should also be noted at this time that the system of FIG. 1 can produce an output CW laser emission at a wavelength in the range of 2.2 to 2.5 microns when the fiber laser is pumped by the CW laser diode source 13, because of the different Stark levels (not shown) that are involved in the Tm.sup.3+ energy level diagram of FIG. 2 and because of the slight differences in the previously-described, alternative fiber materials that could be used as the host optical fiber 15 in FIG. 1. Finally, it is estimated that the system of FIG. 1 will produce a slope efficiency of approximately 10%.

Therefore, what has been described in a preferred embodiment of the invention is a room-temperature, diode-pumped, ZBLAN fiber laser doped with thulium activator ions for producing an output CW laser emission at a wavelength of substantially 2.3 microns.

It should therefore readily be understood that many modifications and variations of the present invention are 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.