High average power diamond Raman laser

High average power diamond Raman laser

Jean-Philippe M. Feve,* Kevin E. Shortoff, Matthew J. Bohn, and Jason K. Brasseur

Directed Energy Solutions, 890 Elkton Drive, Colorado Springs, Colorado 80907, USA *[email protected]

Abstract: We report a pulsed Raman laser at 1193nm based on synthetic

diamond crystals with a record output power of 24.5 W and a slope

efficiency of 57%. We compared the performance of an anti-reflection

coated crystal at normal incidence with a Brewster cut sample. Raman

oscillation was achieved at both room temperature and under cryogenic

operation at 77 K. Modeling of these experiments allowed us to confirm the

value of Raman gain coefficient of diamond, which was found to be 13.5 ±

2.0 cm/GW for a pump wavelength of 1030 nm.

© 2011 Optical Society of America

OCIS codes: (140.3550) Lasers, Raman; (140.3580) Lasers, solid-state; (140.3380) Laser

materials; (160.4330) Nonlinear optical materials; (140.3540) Lasers, Q-switched.

References and links

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#136114 - $15.00 USD Received 4 Oct 2010; revised 18 Nov 2010; accepted 4 Jan 2011; published 7 Jan 2011(C) 2011 OSA 17 January 2011 / Vol. 19, No. 2 / OPTICS EXPRESS 913

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1. Introduction

Raman lasers are attractive ways to generate new frequencies and to generate high brightness

Stokes beams due to the associated beam clean-up [1]. Among the various Raman-active

materials, diamond has attracted early attention due to its large Raman gain and wide

transparency range [2]. Diamond’s large thermal conductivity, κ = 2000 W/m.K [3], and low

thermo-optics coefficient, dn/dT = 7.9x106

[4], make it a material of choice for generating

very high power Stokes beams with excellent beam quality. Continuous progress in the

growth of low-birefringence, low-loss diamond crystals [5,6] has improved the Raman laser

output performances reported in past years [7]. High slope efficiencies, up to 84%, have been

reported under pulsed operation [8], with maximum pulse energy of 0.6 mJ [9]. Intra-cavity

Raman lasers using synthetic diamond crystals have been demonstrated recently, with time-

averaged Stokes powers of 375 mW in pulsed operation [10], and 200 mW in continuous-

wave regime [11], mostly limited by parasitic losses. Despite the favorable slope efficiency

reported in Chemical Vapor Deposition (CVD) diamond as compared with other Raman

crystals, the maximum Stokes output power published so far is limited to 2.2W [12].

In this paper, we report on higher average power pulsed Raman lasers. The optimized

thermal management of our experimental setup allowed us to generate a maximum Stokes

output of 24.5 W at a repetition rate of 40 kHz, only limited by optical damage to the anti-

reflection (AR) coated diamond crystal. Crystals with Brewster cut facets were also compared

to the AR coated diamond crystals. The external cavity Raman laser was pumped by a Q-

switched cryogenic Yb-doped YAG laser, delivering up to 340 W with diffraction-limited

beam quality, which is described in section 2. The design of the Raman cavity and the output

performances are detailed in section 3. One critical parameter for accurate modeling and

optimization of the Raman laser is the gain, gR. Published data range from 33 cm/GW [13] to

75 cm/GW for a pump wavelength of 532 nm [14]. Modeling of the Raman output

performances allows us to derive the value of the gain coefficient for diamond, leading to gR

(λp = 1030nm) = 13.5 ± 2.0 cm/GW, in agreement with recent data, as described in section 4.

Finally, we report on cryogenic operation of our Raman laser as a potential way for further

power scaling. The improved thermal performances at lower temperature have been used in

several laser materials in order to generate multi-kW beams with excellent brightness [15–17].

In the same way the thermal performance of diamond improves at lower temperature, κ =

9000 W/m.K [3] and dn/dT = 2x107

[18] at 80K. From our experiments, the Raman gain is

unchanged from 300 K to 80 K, which is, to our knowledge, the first measurement of a

cryogenic diamond Raman laser.


Our results represent an order of magnitude improvement in output performance for a

diamond Raman laser; the output power is more than double compared to the highest value

reported in any crystal Raman laser [19].

2. Cryogenically cooled Yb:YAG pump laser

2.1 Experimental setup

The design of the pump laser, shown on the left side of Fig. 1, was based on earlier work in

our group. The Yb:YAG crystals, cavity mirrors and acousto-optic Q-switch were identical to

the system reported in reference [17]. The crystals were mounted in a vacuum enclosure to

prevent condensation at cryogenic temperature. All surfaces were anti-reflection coated with

high-damage threshold coatings (Ion-Beam-Sputtering, from Advanced Thin Films, Boulder

CO), with typical residual reflection < 0.05%.

The cavity was end-pumped through 45° dichroic mirrors (M1 on Fig. 1). The pump

beams at 940 nm were delivered from water-cooled 400-W fiber-coupled diodes (IPG, model

DLM400, fibers core diameter 400μm, 0.22NA). These high-brightness diodes allowed us to

maximize the overlap of the pump beams with the fundamental mode of the cavity, which was

the main limiting factor in the performance of the earlier system. The double-sided end-

pumping geometry requires high isolation of the pump modules from back-propagating

power. This was achieved by the absorption of the pump beam through each YAG crystal,

>97%, and by the rejection of the laser signal at 1030 nm by mirrors M1, >99.95%. The total

length of the cavity was 133 cm which led stable operation of the fundamental mode over the

target operating power.

YAG

YAG

M1

AOM

M3

M2

M1

BWsDiode

Diode

PBSHWP

PD1

LN2

PD3PD2

PM2

PM1

OSA

CCD

PM3

VP

VE

VP

M6

M5

M4

M4

M4M4

M5

M6

CVD

coolant

L1

Pump Laser

VE

Raman Laser and diagnostics

lp lp

ls

ls

LPFLPF

Fig. 1. Schematic of the experimental setup. VE: vacuum enclosures, VP: vacuum pumps,

LN2: liquid nitrogen flow, PD: photodiodes, PM: power meters, M1-M6: mirrors (reflectivities

detailed in text). The left part is the pump laser. AOM: acousto-optic Q-switch, HWP: half-waveplate, PBS: polarizing beam-splitter, BWs: Brewster windows. The right part is the

Raman setup. CVD: diamond crystal, LPF: long-pass filters, L1: focusing lens, CCD: camera.

2.2 Output performance

The cavity was first characterized under continuous-wave (cw) operation, with the acousto-

optic Q-switch removed. The maximum output power at 1030 nm was 550 W, corresponding

to an optical efficiency of 79% with respect to the pump power delivered to the crystals. The

temperature of the crystals remained under 90 K at nominal operation, the output power did

not show any sign of thermal roll-over and was only limited by the available pump power.

The output beam was linearly polarized with an extinction ratio >15 dB over the entire range


from threshold up to the maximum output power. The beam quality was close to the

diffraction limit, with M2<1.1 up to an output power of 450 W. At 500 W, stronger thermal

lensing degraded the beam along one axis, leading to M2 = 1.1x1.2. The excellent brightness

of the laser is a direct consequence of the high thermal conductivity and low thermo-optic

coefficients of YAG at 77K [15,16]. The present results are competitive with the best

performance reported so far for a Q-switched Yb:YAG oscillator [20].

Q-switched operation was achieved with a repetition rate in the range of 40-100 kHz. The

beam characteristics were unchanged compared to cw lasing: for a given pump power, the

output power was only 1% lower. In the present work, we deliberately limited the output

power to 340 W to keep sufficient margin with respect to optical damage, even though higher

energy and lower repetition rate are achievable with the current setup. At 40 kHz and 340 W,

the pulse duration was 75 ± 5 ns, the pulse energy and peak power were 8.5 mJ and 91 kW

respectively. Use of a half-wave plate in conjunction with a polarizing beam splitter allow for

attenuation of the pump beam power without changing the beam quality or pulse temporal

shape. This performance and the diffraction-limited beam quality make it an ideal pump

source for the Raman laser cavity. The system has been operated reliably for several hundred

hours at nominal power without degradation and with minimal realignment.

3. Raman laser experiments

3.1 Description of the setup

The Raman laser cavity is shown on the right side of Fig. 1. For simplicity, the linear cavity

was comprised of two identical mirrors, so that the Stokes beam exited from both ends. The

cavity was 50 mm long, the mirrors had a radius of curvature of 200 mm. The mirror mounts

used piezo-electric actuators so that the cavity could be aligned while inside the vacuum

enclosure. The pump beam was focused by lens L1, the diameter at the location of the center

of the crystal was 304 μm, measured with a CCD camera; the pump mode was well matched

to the diameter of the fundamental mode of the cavity, 316 μm. Mirrors M6 had a reflectance

R = 83% for the Stokes beam, and high transmission at 1030 nm, T = 98%. The mirrors also

had low reflectance for the second Stokes beam (R = 6% at 1400 nm), so that only the first

Stokes would oscillate even at high power.

The different beam sampling for diagnostics used dichroic mirrors (M4, T = 99.9% @1030

nm, R = 99.95% @1193 nm) and uncoated wedges M5. The pump and Stokes spectra were

measured with a 0.007 nm resolution optical spectrum analyzer. Acquisition of the temporal

profiles was achieved with Silicon and InGaAs photodiodes, with rise times of <1 ns and <5

ns respectively. The power of the different beams was measured with thermal power-meters.

The overall accuracy of the measured threshold and slope efficiencies was estimated to be

better than 10%.

The 8 mm-long CVD diamond crystal (Element6, Cambridge MA) was oriented for

propagation along the <110> direction. Anti-reflection coatings were deposited on the 2x4

mm2 surfaces. The design of the coatings was optimized to maintain good adhesion under

thermal cycling over the range [77 K-600 K]. The reflectance was 0.6%, 0.08% and 1.6% at

1030 nm, 1193 nm and 1400 nm respectively. The 2mm edge was parallel to the [100] axis,

and the pump beam was polarized parallel to the 4mm edge, i.e. along [110].

One critical issue for power scaling is efficient thermal management of the gain medium.

The single crystal diamond was soldered on a polycrystalline diamond heat-spreader

(25.4x25.4x1 mm3). Silicon thermal sensors were epoxied on the top surface of the single

crystal diamond and on the heat-spreader, as shown on Fig. 2. This assembly was soldered on

the metalized top surface of a BeO heat-sink. The heat-sink was attached on a copper cooler,

where water or liquid nitrogen was circulated.


PZM

CL

HS

MTS

DH

CVD

Fig. 2. Photograph of the diamond crystal assembly in the Raman laser cavity. CVD: single

crystal diamond. DH: polycrystalline diamond heat-spreader. HS: ceramic heat-sink. TS:

temperature sensors (one on the top surface of the single crystal, one on the heat-spreader). M:

mirrors. PZM: mirror mounts with piezo-electric actuators. CL: cooler.

3.2 Optical quality of the diamond crystals

The optical quality of the single crystal diamond is an important parameter for the output

performance of the Raman laser. We characterized the total losses of the samples, using a

medium finesse cavity. Without crystal, the finesse was F = π/(1-R) = 6200 with R the

reflectance of the mirrors. Inserting the CVD diamond in the cavity, the finesse was

F’ = π/(1-R + L) = 605, leading to the total losses L = 0.5% at 1064 nm. Taking into account

the reflectance of 0.08% per surface, the corresponding higher bound for the loss coefficient

was 0.38%/cm, which includes absorption, scatter and losses due to wave-front distortion by

the diamond crystal; it was very close to the best quality reported for CVD diamond [6].

The birefringence of the crystals was Δn = 5x106

measured by Element6 using

birefringence microscopy (Metripol). We measured the spontaneous Raman gain in our

crystals as a function of the polarization of the pump and Stokes beams. As expected for

propagation along the [110] axis, a pump beam polarized along [100] generates a Stokes beam

polarized along [110] [21]. The extinction of the Stokes was better than 98%; this confirms

that the birefringence of our sample was acceptable for laser applications since the

depolarization was minimal along the 8 mm long crystal. We also verified that the same

extinction was obtained throughout the 2x4 mm2 surface of the crystal.

3.3 Laser performances at room temperature

The output performances of the Raman laser are shown on Fig. 3. The slope efficiency was

28.8% for one side, or 57.6% for the total Stokes output, which is lower than the one of Ref

[8]. However, the maximum output power measured with power-meter PM3 on Fig. 1 was

12.3 W at a repetition rate of 40kHz. This corresponds to a total Stokes power of 24.5 W,

which is approximately an order of magnitude higher than the results reported for any

diamond Raman laser so far [12]. The stability of the Stokes average power was 2.2% at

maximum output, as good as the stability of the pump laser, 2.5%. The maximum conversion

efficiency is 13%, low compared to other reported systems, because we operated the laser

only 1.5 times above threshold. The gain curve on Fig. 3 shows no sign of roll-over, so that

higher pump power would increase the efficiency. The bandwidth of the Stokes beam varied

between 0.042 and 0.048 nm over the entire range of output power; which is driven by the

linewidth of the pump, 0.045 nm. The Stokes beam was linearly polarized along [110],

parallel to the pump.

The temporal profiles on Fig. 4 show a very efficient depletion of the pump beam; this can

also be seen on Fig. 3 where the transmitted pump power remains constant once the threshold


of the Raman laser has been reached. The duration of the Stokes pulses was 29 ns, the

maximum peak power was 19.2 kW.

0

20

40

60

80

100

120

140

0

2

4

6

8

10

12

14

16

0 50 100 150

Tra

nsm

itte

d p

um

p p

ow

er

(W)

Sto

ke

s o

utp

ut

po

we

r (W

, o

ne

sid

e)

Incident pump power (W)

Fig. 3. Stokes output power (blue symbols, measured at PM3 on Fig. 1) and transmitted pump

power (red symbols, measured by PM2 on Fig. 1) as a function of the pump power incident on

the crystal. Results from 3 data sets are shown as different symbols. The dashed lines are calculated from the model in section 4 with a Raman gain coefficient gR = 15cm/GW.

0.0

0.5

1.0

-150 -100 -50 0 50 100 150

Am

pli

tud

e (a

.u.)

Time (ns)

0.0

0.5

1.0

-150 -100 -50 0 50 100 150

Am

pli

tud

e (a

.u.)

Time (ns)

0.0

0.5

1.0

-150 -100 -50 0 50 100 150

Am

pli

tud

e (a

.u.)

Time (ns)

Fig. 4. Measured temporal profiles: incident pump (left, blue, photodiode PD1 in Fig. 1), transmitted depleted pump (center, red, photodiode PD2) and output Stokes (right, green,

photodiode PD3). The total output Stokes power was 24.5W.

Figure 3 shows no saturation and no sign of thermal roll-over. The heat load in the

diamond crystal is given by RPPPQ ss totalsp inputptotalsps 11,,, ll . The first

term is the quantum defect and the last two terms are the absorption of the incoming pump

beam and of the intra-cavity Stokes power. Figure 5 shows the linear rise of the temperature

measured by the two thermal sensors as a function of the heat load in the crystal. The

temperature difference between the single crystal and the heat-spreader has a slope of 0.116

K/W. The value of this temperature rise can be calculated from the thermal resistance of the

solder. A perfect indium bond would have a resistance of 3.1x106

W.m2/K at room

temperature [22]; this would correspond to a slope of 0.097 K/W for our crystals. The good

agreement with the measured data shows that efficient heat transfer is achieved through our

assembly, and confirms the potential for further power scaling.

No second Stokes was detectable in any of our experiments. The Stokes output power was

limited by optical damage to the anti-reflection coatings; the maximum output pulse energy

was 0.6 mJ, corresponding to a fluence of 4.6 J/cm2 on the diamond faces. Coatings with an

optimized design can withstand 20 J/cm2 for 100ns pulses; mirrors with lower reflectivity

would also reduce the circulating Stokes intensity and allow further power scaling.


0

0.2

0.4

0.6

0.8

1

295

296

297

298

299

300

0 1 2 3 4 5

Tem

pe

ratu

re r

ise

he

atsp

read

er

to c

ryst

al (

K)

Dia

mo

nd

tem

pe

ratu

re (K

)

Calculated heat load (W)

Fig. 5. Measured temperature on the top surface of the diamond crystal (red) and temperature

difference between the diamond crystal and heat-spreader (blue), as a function of the calculated

heat load in the crystal.

3.4 Brewster-cut crystal

Brewster cut crystals could be an interesting alternative to anti-reflection coated diamonds; it

was used in some of the recent works [8,10,11]. One of our single crystals was cut for

propagation along the <110> direction with incidence at Brewster angle, 67.4°. The optical

length was 6.3 mm for that sample. The pump beam was polarized along [110]. The linear

cavity used 2 mirrors with R = 98.8% at 1193 nm, and T = 98% at 1030 nm. For a repetition

rate of 50kHz, the threshold was 334W and the slope efficiency 58%. The maximum Stokes

output power generated from this cavity was only 2.8 W, limited by the high intra-cavity

Stokes power due to the high reflectance of the mirrors.

The same cavity was tested with the anti-reflection coated crystal, the corresponding

threshold was 119 W. The larger threshold for the Brewster cut crystal is a result of the

shorter crystal length, and the larger beam size due to oblique incidence. For the 50 mm-long

cavity with mirrors radius of curvature of 500 mm, the waist diameter is 406μm for normal

incidence, and 406 x 978 μm2 at Brewster angle. The expected ratio of the Raman laser

threshold is 3.04threshold thresholdBrewster AR AR Brewster nP P L L which is in good agreement with the

measured ratio of 2.81. This confirms that no significant depolarization losses occurred in our

crystals, as could be expected from the low birefringence. However, the Brewster cavity could

be configured using a 4-mirror geometry with cylindrical optics to achieve a circular beam at

the focus by invoking astigmatism correction at the concave turning mirrors [23].

4. Raman gain coefficient

4.1 Raman laser model

The output performances of the Raman laser were modeled by numerical integration of the

time-dependent Raman laser equations [24], with the addition of a third equation for the

second Stokes field. The spatial overlap of the pump beam with the fundamental mode of the

cavity is calculated from Ref [25]. The Stokes field was initialized to one photon per mode,

and numerical integration was achieved with a 4th order Runge-Kutta solver (Matlab ode45).

The losses in the diamond crystal were treated as bulk losses. The input parameters of the

model included the duration of the pump pulses, the pump beam diameter, the reflectivities of

the mirrors at all wavelengths. All values were available from our measurements, so the only

adjustable parameter was the Raman gain coefficient. The model was used to fit the measured

output performances of the Raman laser; Fig. 3 shows a good agreement for both the


transmitted pump power and the generated Stokes. Figure 6 shows the calculated temporal

profiles, averaged over 3 ns in order to match the resolution of the photodiodes. The model is

in good agreement with the data in Fig. 4; the duration and build-up time of the Stokes pulses

are well calculated. The fast oscillations are due to the beating of the longitudinal modes. The

depletion of the transmitted pump pulses is also well modeled.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

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plit

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e (a

.u.)

Time (ns)

0.0

0.1

0.2

0.3

0.4

0.5

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plit

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e (a

.u.)

Time (ns)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-150 -100 -50 0 50 100 150

Am

plit

ud

e (a

.u.)

Time (ns)

Fig. 6. Calculated temporal profiles averaged over 3ns: incident pump (left, blue), transmitted

depleted pump (center, red) and output Stokes (right, green). The total output Stokes power

was 25W.

4.2 Raman gain coefficient of CVD diamond

The above fit gives an estimate of the Raman gain coefficient of gR = 15 cm/GW. A second

data set was measured with a different cavity in order to confirm the gain coefficient with

different parameters. The mirrors M6 had higher reflectivity, R = 98.8% at 1193 nm. Their

radius of curvature was 500 mm, corresponding to a beam waist diameter 406 μm. The pump

beam was focused to a waist of 423x368 μm, the pulse duration was 85 ns (FWHM) and the

repetition rate was 50 kHz. The measured threshold was 119 W; using the above model a

good fit of the output performances was achieved with gR = 12 cm/GW. The average value of

the gain coefficient is then gR = 13.5 cm/GW. The associated uncertainty can be estimated

from the threshold of the Raman laser, since Pth(T + L)/gR; the relative error for the

transmission of the output coupler is ΔT/T = 6%, the error on the losses is ΔL/L = 25% and the

threshold is measured with an accuracy of ΔPth/Pth = 5% (from the calibration of the meter

and the repeatability between several experimental runs). The overall uncertainty for our

measurement of the Raman gain coefficient is 15%, so that gR = 13.5 ± 2.0 cm/GW.

There is a significant variation among the published values of the Raman gain of diamond,

due to variations in the quality of the samples as well as differences in the measurement

conditions. Spontaneous Raman scattering was used for relative comparison of different

crystals [14]; normalizing to CaCO3, Ba(NO3)2 or KGd(WO4)2 leads to gR = 53 ± 20cm/GW

for λp = 0.532μm. The dispersion of the Raman gain coefficient can be deduced from Miller’s

delta [26]. Using the Sellmeier equations of diamond [27], the above value corresponds to gR

= 20.5 ± 7.7cm/GW for λp = 1.03μm. Reference [27] reports gR>8cm/GW for λp = 1.06μm

using stimulated Raman scattering. The more recent reference [13] lists gR = 12cm/GW for λp

= 1.06μm. As a conclusion, our measurement is at the lower end of the published values of gR,

compatible with all reported uncertainties.

5. Cryogenic operation

As mentioned above, the thermal performances of diamond improve at low temperature,

which could allow one to generate high power Stokes beam with high brightness. However no

high power Raman laser operating at low temperature has been reported so far. We used the

above Raman laser cavity described in section 3.1 in order to study the output performances

with cryogenic cooling. In the present experiment, the pump beam diameter was 323 μm, the

repetition rate was 40 kHz and the pulse duration 85 ns. The threshold was 150 W and the

maximum output power was 20.6 W. The data of Fig. 7 have been fitted with the model of

section 4 in order to derive the Raman gain coefficient; the value of gR = 15 cm/GW is similar

to the one at room temperature derived from Fig. 3. No direct measurement of the Raman gain


at 80K had been published earlier; however, the Raman gain bandwidth was found to have no

measurable variation between 80K and 300K [28], which is in good agreement with the

present results.

The output Stokes power in Fig. 7 shows thermal saturation at high power. The

temperature of the diamond crystal varied between 86.5 K and 90.5 K over the range of

Stokes power; it increased linearly at low power, and it had a super-linear behavior at high

power. The increased thermal lensing also resulted in larger instability of the Stokes average

power, 8%, almost 4 times more than at room temperature. The increased sensitivity to

thermal effects is explained by the thermal properties of diamond. For pulsed operation, the

figure of merit in Table 1 is actually lower at cryogenic temperature due to the lower heat

capacity. Because the time constant for heat diffusion is longer than the Stokes pulses the

temperature rise is mostly localized inside the interacting beams. A given heat load results in a

stronger thermal lensing at cryogenic temperature; reduced overlap between pump and Stokes

beams reduces the efficiency. This is no longer true in cw operation, where the figure of merit

is significantly higher at 80 K compared to room temperature. In that case, cryogenic

operation will lead significant brightness enhancement under power scaling.

0

50

100

150

200

0

2

4

6

8

10

12

14

0 50 100 150 200 250

Tra

nsm

itte

d p

um

p p

ow

er

(W)

Sto

ke

s o

utp

ut

po

we

r (W

, o

ne

sid

e)

Incident pump power (W)

Fig. 7. Measured Stokes output power (blue symbols) and transmitted pump power (open red symbols) as a function of the pump power incident on the crystal at cryogenic temperature. The

dashed lines are calculated from the model in section 4 with a gain coefficient gR = 15cm/GW.

Table 1. Thermal properties of diamond at room temperature and at 80K. Figures of

merit for pulsed and c.w. regimes are normalized to their values at room temperature

Temperature 80K 300K

Diamond

properties Conductivity κ (W/m.K) a

Heat capacity Cp (J/g.K) b Thermo-optic coefficient dn/dT

9000

0.0085

2 107 c

2200

0.5 7.9

106 d

Pulsed regime Heat diffusivity α = κ /ρ.Cp (m2/s)

Thermal time constant τ = πw2/2α (μs) e

Figure of Merit FOMpulsed = Cp /(dn/dT)

0.3 0.12

0.67

0.00125

29

1 c.w. Figure of Merit FOMc.w. = κ /(dn/dT) 160 1

a from Ref [3]; b from Ref [29]; c from Ref [18]; d from Ref [4]; e from Ref [30]. with beam waist radius w = 158μm

6. Conclusion

We developed a pulsed Raman laser based on synthetic diamond crystals pumped by a high-

brightness Q-switched cryogenic Yb:YAG master oscillator. The maximum output power of

24.5 W was the highest reported for such lasers, with a corresponding slope efficiency of


57%. Beyond the ten-fold improvement of the output performances, the present experiments

also provide additional information for the design of high power diamond Raman lasers. We

compared the performances of an anti-reflection coated crystal with a Brewster cut sample;

the latter had a laser threshold almost 3 times higher due to oblique incidence and shorter

optical path length. Raman oscillation was achieved at both room temperature and under

cryogenic operation at 77K. The laser threshold was unchanged which indicates that the

Raman gain does not vary significantly over that range of temperature. Modeling of these

experiments allowed us to confirm the value of Raman gain coefficient of diamond; all sets of

data could be fitted with no adjustable parameters, the corresponding Raman gain was 13.5 ±

2.0cm/GW for a pump wavelength of 1030 nm, which agrees with previously published

values and the given experimental uncertainty. The present experiment also confirms that the

diamond heat-spreader and assembly techniques lead to efficient heat transfer and cooling of

the single crystal diamond, and that further power scaling can be achieved.

Acknowledgements

This material is based upon work supported by the Defense Advanced Research Projects

Agency under SPAWAR/SYSCEN Pacific Contract No. N66001-09-C-2079. Any opinions,

findings and conclusions or recommendations expressed in this material are those of the

author(s) and do not necessarily reflect the views of DARPA or SPAWAR/SYSCEN Pacific.

This material has been approved for public release with unlimited distribution under case

reference 16328.


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High average power diamond Raman laser