For non-diffracting Gaussian beams, the threshold absorbed power can be obtained analytically from (1.6) and (1.23):
Figure 1.15 shows the variation of threshold absorbed power with
Wp
for the laser described above, and also the absorbed pump power required to maintain an output power of 2 W.Diffraction of the pump beam can be accounted for by assuming a pump intensity function
2r%
with
w /(z) = Wg/ 1 + A -l; % = ” ■ (1.27)
% y
wop
is the pump beam waist andXp
the pump wavelength. The absorption factore-^
remains valid since the beam is diffraction-limited; even if the waist is focussed to 5 pm, the far field diffraction angle is only 1.6“. The value of the overlap integral (1.24) obtained using (1.26) departs from that obtained by using (1.23) only for very small values of the pump mode waistwop.
This is intuitively obvious because significant spreading of the pump beam occurs only in the regime of relatively tight focussing. The departure ofJi
from the non-diflFracting case is also predictable; the fall off aswop
—> 0 is less rapid, the diffraction effect increasing the extent of the overlap. Figure 1.16 shows the variation of the overlap forwop
between 5 and 30 pm, with a signal mode waist of 350 pm. Forwop> 30
pm, the two calculations differ by only « 0.5%, and, since this is the domain whereJi
is maximum, the assumption of no diffraction of the pump beam is accurate for practical purposes.An estimate of the optimum confocal parameter for the case of a diffracting Gaussian pump beam can be made by defining an average pump mode waist within the crystal:
(128) Using (1.27), and minimising
<Wp>
in order to maximise overlap, the optimum pump mode waist isf IX
The optimum confocal parameter is therefore on the order of /, a result similar to that for optimum focusing in nonlinear optical parametric processes such as second
literatui'e [1.146- 150]. a C 1.25 D « 1 0.75 0.5 - € &
2
I
C 0.25 Q.I "
Dif{frac|ing Non-difflracti pump ng o >n o >ri C N C N C O mPump mode radius (pm)
Figure 1.16. Overlap integral for small pump mode radii calculated for diffracting and non-diffracting pump beams.
1.3.2 Photometric Aspects of End-Pumping with Diode-Laser Devices
1.3.2.1 Pump Beam Quality; Brightness, Etendue
Although the use of diode-laser-based devices as pump sources offers the potential advantages of compactness, reliability and efficiency, the output beams of these sources are generally poor in quality, being many times diffraction-limited and often highly astigmatic. This limits the extent of the spatial overlap of the pump mode with the fundamental TEM qo signal mode, and some measure of pump beam quality is
useful to assess the potential of a given source for end-pumping. Brightness and etendue are the principal parameters used for this.
Figure 1.17. Energy transmission from a surface element.
Figure 1.17 shows an emitting surface S, not necessarily plane, and a surface element dS under consideration. The amount of power radiated into elemental solid- angle dQ in the direction 6, (j) is
(fP = B{Ç,p,0,(l>).cosa.dSdQ.. (1.30)
^ ,7] are any convenient set of curvilinear coordinates on S. a is the angle between the normal to dS and the direction 6, (p A n the case of most pump devices we can assume that the brightness B is independent of position on S and is isotropic (independent of 6, (p
). B is then simply a constant. The etendue of the source is defined by [1.151]
(B = n^jj cos a. dS.dQ, (1.31)
and the brightness of a source of total power P is therefore
B =
(1.32)
Etendue is the volume of "radiance space" occupied by the output beam of the diode-laser device and governs the depth of focus possible in the gain medium: sources with low etendue can be focused to a small spot size over a greater distance. From (1.31) the characteristics of a good pump device are therefore a small emitting aperture and a low solid-angle. The emitting surface is plane in most cases, and we can assume for design purposes that a is small so that
(E = n"Sa.
(1.33)
A TEM(X) Gaussian beam from a bulk laser is diffraction limited (or close to the limit) and therefore has the highest brightness for a given power of any practical source:
^G aussian (1-34)
The etendue of a typical diode-laser device is tens of thousands of times greater than this diffraction-limited case.
(1.30) can be useful for calculating the intensity profiles of pumping devices. Figures 1.18 and 1.19 show the intensity profiles of a fibre-coupled diode-laser system at
5 mm and 10 |xm from the fibre-tip respectively. B is assumed to be a constant.
1.32.2 Brightness Theorems
Pump light from diode-laser based devices is normally coupled into the end of a laser rod via some optical system, for example low /-number lenses. Two important theorems govern the radiation through an optical system and into the laser rod itself. The first is the brightness theorem [1.152] which states that if the object and image spaces have equal refractive index, the brightness of the light distribution produced by an optical system B i cannot be greater than the original source brightness Bq, and the two can only
be equal if the losses in the system are negligible:
r» Y
^ Bg.
(1.35)
v^oy
(1.32) and (1.35) have the important consequence that if the losses in the optical system are zero so that no power is lost, the etendue is a constant. In practice this means that focused pump spot size and divergence angle are not independent variables but are
3 < 1 .3 9 20 8 - 0 . 4 6 4 0 2 2 . 3 2 0 1 3 I P o sitio n (mm) OJ
Figure 1.18. Far-field intensity pattern from a fibre-coupled diode-laser.
0 . 1 8 2 4 2 0 . 0 6 0 8 0 . 3 0 4 0 4 I
P o sitio n (mm)
Figure 1.19. Near-field intensity pattern from a fibre-coupled diode-laser.
constrained by (1.33). The second, related theorem [1.153] states that the brighmess of a collection of mutually incoherent (but otherwise identical) sources cannot be increased by a passive optical system to a level greater than the single brightest source. There are, however, certain cases where the total brightness of a laser array can exceed the
brightness of the individual elements. First, if mutual coherence is established across the array, the entire source behaves as a single spatial mode and the phase and amplitude can in principle be made uniform. This is limited to individual low-power diode-laser arrays; high power diode-bars consist of arrays of arrays, and this method cannot be applied to them.
A second way to increase radiance is to use lasers with different average properties, such as wavelength or polarisation. Passive optical elements such as diffraction gratings and polarising beam splitters can then be used to multiplex the individual beams.
1.3.2.3 Photometric Requirements o f Diode-Laser Pumps
Given the poorer beam quality of diode-laser pumps, optimum focusing is more difficult to describe quantitatively. Most attempts have been extensions of Gaussian beam models [1.154]. A simple model which gives the brightness requirements of the pump source assumes that the pump mode is entirely contained within the signal mode over a fixed crystal length L [1.155], as shown in Figure 1.20. The pump mode is assumed to have the form
Wp(z) = Wop+lziap (1.36)
where wop is the waist radius at the centre of the crystal and Op is the corresponding divergence angle. The non-diffracting signal mode has a 1/e^ radius Wg. Optimum focusing is achieved by minimising the pump radius at the ends of the crystal, Wp(L/2).
An azimuthally symmetric pump with an emitting aperture of radius r and divergence a , has an etendue € = {1- c o s a ) ~ ( 7 m r o f . (1.37) Pum c beam 2m ►
Figure 1.20. Pump beam and laser mode in the gain medium.
Minimizing Wp(L/2) using (1.36) subject to the constraint (1.37), the optimum pump spot size and divergence are
(1.38)
One result implied by (1.38) is that if the emitting dimension of the pump source were to be increased (to obtain higher power, for example) at constant far-field angle and fixed L,
then the pump beam could be focused to a spot that increases with only the square root of the emitting dimension. This seems counter-intuitive, but occurs because L is not increased as well. The etendue clearly limits the minimum spot size. Using (1.38),
lor
[w,(L/2)]^^ = e ‘' ^ J ^ . (1.39)
^ 7m
This is therefore the maximum Wg for given L and C. Increasing L from the fixed value used to obtain (1.39) may improve interaction length but there will be a trade off eventually with actual overlap and hence threshold and slope efficiency. If Wg is fixed there is a limit on the etendue of the pump source in order to preserve the overlap defined above:
A.
(1.40)
Ag is the cross-sectional area of the signal mode. The corresponding minimum brightness in air of the pump is
4ÛP
Azimuthal symmetry of the pump is also an important factor in beam quality. For example, fibre-coupling of a diode-laser bar may be a better alternative to pumping with just a diode-bar alone, although other issues such as overall laser efficiency also need to be considered. The etendue of an asymmetric pump source with perpendicular emitting dimensions rx, ry and emission angles Ox, 0 y can be approximated by
€ « nf7trjy.7iO^O^ = inTur^aJinjur^a^) = (1.42)
The effect of astigmatism can be modelled if and are made unequal. For example, in the case of a high-power diode-laser linear bar, the output beam is diffraction limited in the plane perpendicular to the junction, and typically more than 800 times diffraction- limited in the plane parallel to the junction. If
where Q is an asymmetry factor greater than 1, and the minimum brightness is calculated as before, the result is
4LPQ
A /» '
(1.41)
(1.43) In other words the required pump brightness is Q times larger than than for a symmetric pump beam.
1.22.4 Pump Focusing Criteria for Diode-Pumped Solid-State Lasers
Generally, the signal mode radius Wg in the gain medium is determined by resonator design. For example, in the case of intracavity-doubled lasers, the resonator
geometry is often fixed by arranging for a given waist size and confocal parameter in the nonlinear medium. Even if this is not the case, Wg is usually only slightly variable for a given resonator configuration. Given that for a particular pump source the etendue C is also a constant, only the focusing, crystal length and pump wavelength are generally available as variables in laser head design. Although in section 1.3.2.3 the crystal length
L was fixed to give pump brightness requirements, here it is assumed to be a variable since commercial crystals can be obtained with any desired length. For maximum efficiency and output, the usual design aim is to maximise the physical pump-signal interaction length by varying the focusing, subject to the constraint of constant etendue (1.37) and the requirement that all the pump mode be contained inside the laser mode, as in Figure 1.20. If the pump spot size and divergence are arranged as wop. Op , and the pump and signal waists are made equal at the front end of the laser rod, then the length over which the pump is contained inside the signal is
(1.44) tan0p
Since etendue is constant for all focusing, the condition
€ = (n7TWop0p)^ (1.45)
can be used with (1.44) to find the (optimum) value of wop which maximises /. (1.45) can be used to find the corresponding optimum Op.
Figure 1.21 shows the focusing behaviour for a circular symmetric source of radius 300 jim and numerical aperture 0.37 when used to pump a signal mode of radius 750 |im. (These are the characteristics of the B030 fibre-coupled diode-laser system available from OptoPower Corporation.) The graph also shows the percentage fill factor as the pump spot size increases, i.e. the percentage of the signal mode volume overlapped with the pump beam. Variation of the overlap length with pump spot size is shown in Figure 1.22. This graph has a simple intuitive explanation. Initially, as the pump spot size increases from a very small value, the pump divergence falls rapidly, as shown in Figure 1.21. The pump spot therefore moves further into the crystal, increasing the overlap length. As the pump spot size increases further, however, the pump divergence decreases much more slowly and the pump spot has to move back toward the front end of the laser rod if pump and signal are to maintain equal waists at the front surface. The benefit of a lower etendue is shown in Figure 1.23, which shows the maximum overlap length as the signal mode size is varied, using the OptoPower B030 system and the SDL 3450 P6
fibre-coupled diode system. The latter has a much lower etendue as the fibre has a 250 |im radius core and a numerical aperture of 0.2. Geometrically, the SDL system has the advantage, giving a longer interaction length. (However, this may not be the only consideration as lower etendue means that the system is capable of delivering less power. Nonetheless, if the two are run at full power, the 12 W SDL device is more than twice as
w 100 fg ctpr 60 750 |0,tn 20 >oo o oo
§
I
ooI
oo Pum p sp ot w aist (jiim)Figure 1.21. Pump focusing behaviour and % fill factor v. pump spot radius.
2.5 q. ■g 1-5 s - , s CL
I
CL 0.5/ r;
i Sis nal75nod0 pi3 ra( n iius
§ § § ooTt oo g §VO
Pum p sp ot w aist (jLim)
Figure 1.22. Pump overlap length
V. pump spot radius. 12.5 SOL 3453 P6 2.5 Powe o o oo