« journal navigation
Quasi-CW diode-pumped self-starting adaptive laser with self-Q-switched output
Optics Express, Vol. 15, Issue 10, pp. 6458-6463 (2007)
http://dx.doi.org/10.1364/OE.15.006458
Acrobat PDF (154 KB)
Article Navigation
▸ Article Outline
– Abstract
1. Introduction
2. Concept of the self-starting adaptive laser
3. Experimental self-starting adaptive laser system
4. Power scaling of an adaptive self-Q-switched laser system
5. Conclusion
– References and links
– Abstract
1. Introduction
2. Concept of the self-starting adaptive laser
3. Experimental self-starting adaptive laser system
4. Power scaling of an adaptive self-Q-switched laser system
5. Conclusion
– References and links
Article Tools
Full Text
Article Info
References
Cited By
Figures
Metrics
Download PDF
Viewing Equations in the
HTML Article
Optimal Viewing Recommendations
Abstract
An investigation is made into a quasi-CW (QCW) diode-pumped holographic adaptive laser utilising an ultra high gain (~104) Nd:YVO4 bounce amplifier. The laser produces pulses at 1064nm with energy ~0.6mJ, duration <3ns and peak power ~200kW, with high stability, via self-Q-switching effects due to the transient dynamics of the writing and replay of the gain hologram for each pump pulse. The system produces a near-diffraction-limited output with M2<1.3 and operates with a single longitudinal mode. In a further adaptive laser configuration, the output was amplified to obtain pulses of ~5.6mJ energy, ~7ns duration and ~1MW peak power. The output spatial quality is also M2<1.3 with SLM operation. Up to 2.9mJ pulse energy of frequency doubled green (532nm) radiation is obtained, using an LBO crystal, representing ~61% conversion efficiency. This work shows that QCW diode-pumped self-adaptive holographic lasers can provide a useful source of high peak power, short duration pulses with excellent spatial quality and narrow linewidth spectrum.
© 2007 Optical Society of America
1. Introduction
Success in power scaling of solid-state lasers whilst maintaining good beam quality
depends on the ability to find a solution to the problems associated with the
heating of the laser medium caused by the pumping. Such problems include a pump
dependent thermal lens, aberrations and stress-induced thermal birefringence which
can degrade the spatial quality of the laser system [1]. Investigations have shown that self-adaptive laser
resonators based on formation and diffraction from optically-induced gain gratings
(or holograms) offer considerable promise for high-average power scaling of
solid-state lasers with maintenance of high beam quality by adaptive correction of
the thermally-induced distortions [2–7].
R. P. Green, G. J. Crofts, and M. J. Damzen, “Holographic laser resonators in Nd:YAG,” Opt. Lett 19, 393–395 (1994). [PubMed]
In these systems, a gain grating is formed by spatial hole burning caused by
interference of coherent beams in the laser medium and modulation of the population
inversion. The adaptive resonator can be formed by gain grating formation in two
ways: (a) using an input beam in a self-intersecting loop geometry or (b) as a
self-starting adaptive oscillator by providing feedback from an output coupler and
thus requiring no external optical input [2–4]. The gain hologram encodes the distortions and
the oscillation, via diffraction from the hologram, creates a phase conjugate mode
with a distortion-corrected output. Much of the initial research involved flashlamp
pumping of Nd:YAG [2–4]. The advent of laser diodes allowed more
efficient pumping of solid-state media as the radiation typically has a better
spectral match to the absorption profile of the media. More recent investigations
into gain gratings have included quasi-continuous wave (QCW) diode-pumping studies
of degenerate four-wave mixing [5], a reciprocal dynamic holographic cavity [6] and efficient adaptive laser oscillators with continuous
wave (CW) diode-pumping [7].
R. P. Green, G. J. Crofts, and M. J. Damzen, “Holographic laser resonators in Nd:YAG,” Opt. Lett 19, 393–395 (1994). [PubMed]
R. P. Green, G. J. Crofts, and M. J. Damzen, “Holographic laser resonators in Nd:YAG,” Opt. Lett 19, 393–395 (1994). [PubMed]
A. Brignon, G. Feugnet, J.-P. Huignard, and J.-P. Pocholle, “Multipass degenerate four-wave mixing in a diode-pumped Nd:YVO4 saturable amplifier,” J. Opt. Soc. Am. B 12, 1316–1325 (1995). [CrossRef]
O. Antipov, O. Eremeykin, A. Ievlev, and A. Savikin, “Diode-pumped Nd:YAG laser with reciprocal dynamic holographic cavity,” Opt. Express 12, 4314–4319 (2004). [CrossRef]
B. A. Thompson, A. Minassian, and M. J. Damzen, “Operation of a 33-W, continuous wave, self-adaptive, solid-state laser oscillator,” J. Opt. Soc. Am. B 20, 857–862 (2003). [CrossRef]
In this paper we present, for the first time to our knowledge, performance results of
a QCW diode-pumped self-starting holographic adaptive laser with a non-reciprocal
loop element and utilising an ultra-high gain Nd:YVO4 bounce amplifier. A
key advantage of using pulsed diode-pumping is that the transient dynamics of gain
grating formation can lead to a self-Q-switching of the adaptive laser, leading to
giant-pulse formation [3,4]. We demonstrate an adaptive Nd:YVO4 laser that
produces highly stable pulses of ~0.6mJ energy, <3ns duration
and ~200kW peak power for each pump pulse. Pump pulse rates up to 1kHz
were used. The system corrects for aberrations in the laser medium and the output
mode is near-diffraction-limited with M2<1.3 and operates with
a single longitudinal mode (SLM). In a slightly modified adaptive laser, the output
was amplified to obtain pulses of ~5.6mJ energy, <7ns duration
and ~1MW peak power, with equivalent spatial quality, SLM operation and
stability. Frequency doubled green radiation was obtained with an LBO crystal
producing 2.9mJ pulse energy at 532nm with ~61% conversion
efficiency.
K. S. Syed, R. P. Green, G. J. Crofts, and M. J. Damzen, “Transient modeling of pulsed phase conjugation experiments in a saturable Nd:YAG amplifier,” Opt. Commun. 112, 175–180 (1994). [CrossRef]
M. J. Damzen, R. P. Green, and K. S. Syed, “Self-adaptive solid-state laser oscillator formed by dynamic gain-grating holograms,” Opt. Lett. 20, 1704–1706 (1995). [CrossRef] [PubMed]
2. Concept of the self-starting adaptive laser
As stated, self-adaptive laser resonators based on formation and diffraction from
optically-induced gain gratings (or holograms) offer considerable promise for
high-average power scaling of solid-state lasers with maintenance of high beam
quality by adaptive correction of the thermally-induced distortions. The concept of
the self-starting adaptive laser with self-intersecting loop geometry is shown in Fig. 1.
Fig. 1. Schematic self-starting holographic resonator with intersecting beams
A1-A4, phase conjugate PPC and
non-phase conjugate PNPC outputs and non-reciprocal transmission
element NRTE.
The interacting fields A1-A4 are initiated by amplified
spontaneous emission whose source is the high gain amplifier, G. Any initial
spontaneous emission from G will induce weak gain gratings due to gain saturation in
the gain medium within the loop geometry. Weak diffraction from these initial
gratings will cause an enhancement of the amplified spontaneous emission, and
constructive interference within the cavity will give rise to an increase in the
diffraction efficiency of the gain gratings. These gratings are effectively volume
gain holograms that encode the spatial distributions and wave fronts of the
interacting fields and act as diffractive elements that allow build up of the
intracavity fields.
In this interaction, two important gratings are the transmission grating formed by
the interference term
(A1A3
*+A2
*A4),
shown in Fig. 1, and the reflection grating formed by the interference
term
(A1A4
*+A2
*A3).
The spatial forms of the fields are interrelated by loop boundary conditions,
feedback from the partially reflective output coupler (OC) and diffraction from the
gratings. The coherent mutual growth of the fields and the gratings requires a
self-consistency condition leading to mode formation. Diffraction from the
transmission grating forms a ring resonator and, with sufficiently high diffraction
efficiency, a threshold for laser oscillation is reached resulting in the
preferential build-up of a backward mode oscillating in the anti-clockwise
direction. Analysis of the self-consistency condition indicates preferential growth
for phase conjugate oscillation where A2 ∝
A1
* (and A4 ∝
A3
*).
Phase conjugate (PC) oscillation means that aberrations experienced by field
A1 in the gain medium and in the loop are compensated by the spatial
read-out of the gain holograms. A matching of the wave front of the mode
(A1 and A2 at the output coupler) to the plane output coupler
leads to an output mode with a high spatial mode quality and power PPC.
Optimisation of diffraction efficiency is achieved by use of the non-reciprocal
transmission element (NRTE). The NRTE attenuates in the forward (clockwise)
direction such that the relative strength of fields A1 and A3
may be optimised for efficient grating writing. In the QCW pumped regime, to
maximise output pulse energy it is important to hold-off the formation of the
gratings and hence prevent lasing until towards the end of each pump pulse. To delay
this time for grating formation, the forward transmission t+
needs to be set to a low value.
The NRTE also provides a relative non-reciprocal π-phase shift between
forward and backward loop directions. This phase shift compensates for the fact that
the saturable transmission gain grating is in antiphase to the intensity
interference pattern that forms it and hence is equivalent to the grating itself
being π-phase shifted. In the backward direction the transmission
t- is near unity allowing efficient build-up of the PC
aberration-corrected radiation PPC. A non-phase conjugate power
PNPC is emitted in the opposite output direction (see Fig. 1) with significant aberrations due to two passes
through the gain medium.
3. Experimental self-starting adaptive laser system
3.1 Experimental adaptive laser arrangement
Figure 2 shows the experimental self-starting adaptive
laser system. The system uses a QCW diode-pumped Nd:YVO4 crystal of
1.1 at. % neodymium doping in the form of an a-cut slab
(crystal is cut perpendicular to the c-axis which is the optic
axis) with dimensions 20 × 5 × 2mm. It is diode-pumped at
808nm and its main lasing transition is at 1064nm. The two 5 × 2mm
end faces are anti-reflection (AR) coated for 1064nm The slab is diode-pumped on
the 20 × 2mm front face, which is AR coated for 808nm, in a
side-pumped configuration. The pump diode has fast axis collimation and is
focused onto the front face with a f=50mm vertical cylindrical
lens (VCLD) producing a line focus with dimensions ~15
× 0.5mm. The light output from the laser diode is TM polarised and is
parallel to the c-axis of the Nd:YVO4 crystal, thus
accessing the high absorption coefficient of ~30cm-1 for
1.1 at. % Nd:YVO4. This results in strong absorption of pump power
with absorption depth ~330μm. The diode was driven in a
QCW mode at repetition rates up to 1kHz (limited by diode driver).
A bounce geometry was employed in the crystal, in which laser radiation can be
amplified by taking a path that experiences total internal reflection at the
pump face [8–10]. The bounce angle with respect to the
crystal’s pump face give considerable spatial averaging of the gain
and thermal non-uniformities seen by the amplified beam in the bounce plane.
However, the key significance to this geometry is its ability to produce
extremely high gain (~104) [11]. The self-adaptive laser cavity was formed by an output
coupler (OC) together with a self-intersecting loop produced by
high-reflectivity mirrors M1-M4. The non-reciprocal transmission element (NRTE)
was formed by a Faraday rotator and half waveplate placed between a pair of
polarisers. The reflectivity of the output coupler was chosen at a low value of
~0.2% and the forward transmission of the NRTE was set to a low value
<1% to inhibit the onset of self-Q-switching to the end of the pump
pulse.
J. E. Bernard and A. J. Alcock, “High-efficiency diode-pumped Nd:YVO4 slab laser,” Opt. Lett. 18, 968–970 (1993). [CrossRef] [PubMed]
G. Smith and M. J. Damzen, “Spatially-selective amplified spontaneous emission source derived from an ultra-high gain solid-state amplifier,” Opt. Express 14, 3318–3323 (2006). [CrossRef] [PubMed]
3.2 Performance results
The system is able to operate in a pulsed output mode. For pump pulses of
80μs duration and 10.6mJ energy, output pulse energy was
~0.6mJ with duration as short as <3ns and
~200kW peak power. The pulse output was virtually independent of
repetition rate up to the maximum available pump rate of 1kHz. The pulses have
good stability with standard deviation, σ~1.5% in pulse
energy and <0.5μs time jitter. The shortest pulses
observed had a FWHM of 2.7ns, as shown in Fig. 3(a), for the pump pulse parameters stated earlier.
Fig. 3. Experimental results: (a) temporal output showing self-Q-switched output
with 2.7ns FWHM, (b) spectrum measured using a FP etalon.
The low efficiency may be explained by a self-termination effect of the gain
grating forcing premature pulse emission reducing extraction of the pump energy.
All pulses are temporally clean and smooth without evidence of spectral
modebeating. Single longitudinal mode (SLM) operation is confirmed by use of a
Fabry-Perot (FP) etalon with free spectral range of 3.4GHz and finesse
~50 with single ring pattern shown in Fig. 3(b). The spatial output was characterized as shown
in Fig. 4. Figure 4(a) is a spatial profile of the output and Fig. 4(b) is a graph of beam radius through focus for the
horizontal and vertical components giving an M2<1.3 in
both planes. The profile shows some astigmatism but no attempt was made to
correct for this in the experiment. With suitable design its removal should be
readily obtained, e.g. with compensation block.
4. Power scaling of an adaptive self-Q-switched laser system
There are several ways to power scale an adaptive laser system. The method chosen in
this experiment was to use an adaptive laser system as a high quality source for a
second high gain bounce amplifier. Figure 5 shows the experimental power scaled system.
All experimental details of the self-starting adaptive resonator (shown in a
dotted-line box) are as before, apart from a small modification of intracavity
elements. Two vertical cylindrical lenses (VCL1 and VCL2) in the loop of focal
length f=50mm were used to match the laser mode with the gain
region in the vertical. The second amplifier also uses a Nd:YVO4 crystal
(1.1 at. % neodymium doping) in an a-cut slab with dimensions 25
× 5 × 2mm. The amplifier is pumped with a TM polarised diode
stack which is brought to a line focus on the front face of the crystal with a
f=25mm vertical cylindrical lens (VCLD2). The diodes
were connected electrically in series to the same QCW driver ensuring the pump
pulses to the loop and amplifier were synchronised. As the output coupler (OC)
reflectivity is low (~0.2%), an isolator (ISO) was included to reduce
amplified spontaneous emission (ASE) from the external amplifier adversely affecting
the operation of the adaptive oscillator. Vertical cylindrical lens VCL3
(f=50mm) matches the mode from the oscillator to the gain region in
the amplifier and VCL4 collimates the output to the diagnostics.
4.1 System performance
The system continues to operate in a pulsed output mode. For amplifier pump
pulses of 80μs duration and 26.5mJ energy, amplified output pulse
energy was ~5.6mJ with ~7ns duration and ~1MW
peak power. This corresponds to an amplifier extraction efficiency of
~19%. Pulse duration is several nanoseconds longer than the pulses
from the oscillator and may be caused by amplified spontaneous emission from the
high-gain amplifier system parasitically affecting the loop oscillator. Pulse
stability was again high with standard deviation, σ~2% in
pulse energy and <0.5μs time jitter. The spectral content
of the output was investigated by use of a FP etalon, showing SLM operation, in Fig. 6(a). Figure 6(b) is a graph of beam radius through focus for
the horizontal and vertical components, with M2<1.3 in
both planes.
4.2 Second harmonic generation with LBO crystal
A second harmonic experiment was performed to demonstrate the utility of such a
high peak power pulse source with near-diffraction-limited beam quality. The
amplified adaptive laser output was focused into a non-critically phase-matched
LBO crystal with a f= 100mm spherical lens for second harmonic
(SH) conversion into green (532nm). Figure 7 is a graph of SH output pulse energy against
input pulse energy from the adaptive laser (solid squares), together with the
conversion efficiency (open squares).
As shown, up to 2.9mJ of green is produced at a 1064nm input energy of 4.7mJ,
corresponding to ~61% conversion efficiency.
5. Conclusion
QCW diode-pumping of a self-adaptive holographic laser with a non-reciprocal loop
element is investigated for the first time. The laser produces pulses of
~0.6mJ energy, <3ns duration and ~200kW peak power
via self-Q-switching effects due to the transient dynamics of writing and replay of
the gain holograms for each pump pulse. The output pulses are stable with standard
deviation, σ~1.5% in energy and <0.5μs
time jitter. The output mode is near-diffraction-limited with
M2<1.3 and operates with a SLM. The adaptive laser output was
amplified to obtain pulses of ~5.6mJ energy, <7ns duration and
~1MW peak power. The amplification process maintains the output spatial
quality at M2<1.3 and SLM operation. Up to 2.9mJ pulse energy
of frequency doubled green (532nm) radiation was obtained, using an LBO crystal,
with ~61% conversion efficiency. This shows that QCW diode-pumped
adaptive holographic lasers are a useful source of high peak power, short duration
pulses with high spatial quality and narrow linewidth spectrum.
Acknowledgements
The authors acknowledge support from Electromagnetic Remote Sensing Defence
Technology Centre and from UK Engineering and Physical Sciences Research Council
under grant number GR/T08555/01.
References and links
W. Koechner, Solid-State Laser Engineering (Springer-Verlag, Berlin, 1999). | |
R. P. Green, G. J. Crofts, and M. J. Damzen, “Holographic laser resonators in Nd:YAG,” Opt. Lett 19, 393–395 (1994). [PubMed] | |
K. S. Syed, R. P. Green, G. J. Crofts, and M. J. Damzen, “Transient modeling of pulsed phase conjugation experiments in a saturable Nd:YAG amplifier,” Opt. Commun. 112, 175–180 (1994). [CrossRef] | |
M. J. Damzen, R. P. Green, and K. S. Syed, “Self-adaptive solid-state laser oscillator formed by dynamic gain-grating holograms,” Opt. Lett. 20, 1704–1706 (1995). [CrossRef] [PubMed] | |
A. Brignon, G. Feugnet, J.-P. Huignard, and J.-P. Pocholle, “Multipass degenerate four-wave mixing in a diode-pumped Nd:YVO4 saturable amplifier,” J. Opt. Soc. Am. B 12, 1316–1325 (1995). [CrossRef] | |
O. Antipov, O. Eremeykin, A. Ievlev, and A. Savikin, “Diode-pumped Nd:YAG laser with reciprocal dynamic holographic cavity,” Opt. Express 12, 4314–4319 (2004). [CrossRef] | |
B. A. Thompson, A. Minassian, and M. J. Damzen, “Operation of a 33-W, continuous wave, self-adaptive, solid-state laser oscillator,” J. Opt. Soc. Am. B 20, 857–862 (2003). [CrossRef] | |
J. E. Bernard and A. J. Alcock, “High-efficiency diode-pumped Nd:YVO4 slab laser,” Opt. Lett. 18, 968–970 (1993). [CrossRef] [PubMed] | |
A. Minassian, B. A. Thompson, and M. J. Damzen, “Ultrahigh-efficiency TEM00 diode-side-pumped Nd:YVO4 laser,” Appl. Phys. B 76, 341–343 (2003). [CrossRef] | |
A. Minassian, B. A. Thompson, and M. J. Damzen, “High-power TEM00 grazing-incidence Nd:YVO4 oscillators in single and multiple bounce configurations,” Opt. Commun. 245, 295–300 (2005). [CrossRef] | |
G. Smith and M. J. Damzen, “Spatially-selective amplified spontaneous emission source derived from an ultra-high gain solid-state amplifier,” Opt. Express 14, 3318–3323 (2006). [CrossRef] [PubMed] |
OCIS Codes
(140.3480) Lasers and laser optics : Lasers, diode-pumped
(140.3530) Lasers and laser optics : Lasers, neodymium
(140.3540) Lasers and laser optics : Lasers, Q-switched
(140.3580) Lasers and laser optics : Lasers, solid-state
(190.4380) Nonlinear optics : Nonlinear optics, four-wave mixing
(190.5040) Nonlinear optics : Phase conjugation
ToC Category:
Lasers and Laser Optics
History
Original Manuscript: February 9, 2007
Revised Manuscript: March 27, 2007
Manuscript Accepted: March 28, 2007
Published: May 11, 2007
Citation
G. Smith and M. J. Damzen, "Quasi-CW diode-pumped self-starting adaptive laser with self-Q-switched output," Opt. Express 15, 6458-6463 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-10-6458
Sort: Year | Journal | Reset
References
- W. Koechner, Solid-State Laser Engineering (Springer-Verlag, Berlin, 1999).
- R. P. Green, G. J. Crofts and M. J. Damzen, "Holographic laser resonators in Nd:YAG," Opt. Lett 19, 393-395 (1994). [PubMed]
- K. S. Syed, R. P. Green, G. J. Crofts and M. J. Damzen, "Transient modeling of pulsed phase conjugation experiments in a saturable Nd:YAG amplifier," Opt. Commun. 112, 175-180 (1994). [CrossRef]
- M. J. Damzen, R. P. Green and K. S. Syed, "Self-adaptive solid-state laser oscillator formed by dynamic gain-grating holograms," Opt. Lett. 20, 1704-1706 (1995). [CrossRef] [PubMed]
- A. Brignon, G. Feugnet, J.-P. Huignard and J.-P. Pocholle, "Multipass degenerate four-wave mixing in a diode-pumped Nd:YVO4 saturable amplifier," J. Opt. Soc. Am. B 12, 1316-1325 (1995). [CrossRef]
- O. Antipov, O. Eremeykin, A. Ievlev and A. Savikin, "Diode-pumped Nd:YAG laser with reciprocal dynamic holographic cavity," Opt. Express 12, 4314-4319 (2004). [CrossRef]
- B. A. Thompson, A. Minassian and M. J. Damzen, "Operation of a 33-W, continuous wave, self-adaptive, solid-state laser oscillator," J. Opt. Soc. Am. B 20, 857-862 (2003). [CrossRef]
- J. E. Bernard and A. J. Alcock, "High-efficiency diode-pumped Nd:YVO4 slab laser," Opt. Lett. 18, 968-970 (1993). [CrossRef] [PubMed]
- A. Minassian, B. A. Thompson and M. J. Damzen, "Ultrahigh-efficiency TEM00 diode-side-pumped Nd:YVO4 laser," Appl. Phys. B 76, 341-343 (2003). [CrossRef]
- A. Minassian, B. A. Thompson and M. J. Damzen, "High-power TEM00 grazing-incidence Nd:YVO4 oscillators in single and multiple bounce configurations," Opt. Commun. 245, 295-300 (2005). [CrossRef]
- G. Smith and M. J. Damzen, "Spatially-selective amplified spontaneous emission source derived from an ultra-high gain solid-state amplifier," Opt. Express 14, 3318-3323 (2006). [CrossRef] [PubMed]
Cited By |
 Alert me when this paper is cited |
OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.
« Previous Article | Next Article »
OSA is a member of 