Thulium-doped fluorozirconate fiber laser pumped by a diode laser source

A room temperature laser system for producing a CW laser emission at substially 2.3 microns is disclosed. In a preferred embodiment, the laser system comprises a laser diode source for producing a CW pump beam at a preselected wavelength; and a fiber laser doped with thulium activator ions sufficient to produce an output CW laser emission at a wavelength in the range of 2.2-2.5 microns when the fiber laser is pumped by the CW pump beam.

BACKGROUND OF THE INVENTION 
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. 
OBJECTS OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
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 
EQU I.sub.sat =hv.sub.p A/(.sigma..sub.p .tau.), (2) 
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.