Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
1999-04-29
2001-04-17
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular beam control device
Nonlinear device
C372S022000, C359S328000, C359S330000
Reexamination Certificate
active
06219363
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of frequency conversion of the radiation of a pulsed optical parametric oscillator (OPO). It also relates to sum or difference frequency mixing in an optically nonlinear crystal. In addition, the invention relates to the use of a nonlinear OPO crystal pumped through a pulsed laser-pump radiation.
REVIEW OF RELATED TECHNOLOGY
Pulsed optical parametric oscillators having pulses in the nanosecond range (10
−9
s) are coherent radiation sources that can be tuned over a wide wavelength range and have high average powers and high pulse energies in the visible and near-infrared spectral range. With the methods of frequency conversion in optically non-linear crystals, it is possible to expand the spectral range of OPOs into spectral ranges of interest for many scientific, technical and medical applications.
Frequency doubling (SHG, Second Harmonic Generation), sum frequency mixing (SFM) and difference frequency mixing (DFM) are available as nonlinear processes for the frequency conversion of OPO radiation for expanding the spectral range. In this connection, see, for example, the book by V. G. Dmitriev, G. G. Gurzadyan, D. N. Nikogosyan, “Handbook of Nonlinear Optical Crystals,” Springer Verlag [publisher], Berlin, Heidelberg, N.Y., 1997, which is incorporated herein by reference.
These processes are characterized by the second-order nonlinear susceptibility in nonlinear optical materials. A coupling occurs among three waves whose angular frequencies (of the generated wave &ohgr;
new
, the OPO wave &ohgr;
OPO
, and the so-called pump wave &ohgr;
p
) must correspond to the energy conservation condition:
{tilde over (&ohgr;)}
neu
={tilde over (&ohgr;)}
p
±{tilde over (&ohgr;)}
OPO
(1)
If the sign is positive, sum frequency mixing (SFM) or “up-conversion” is taking place. If the sign is negative, difference frequency mixing (DFM) or “down-conversion” is taking place. The special case of &ohgr;
p
=&ohgr;
OPO
is referred to as frequency doubling (SHG) or “Second Harmonic Generation.”
In the idealized case of interaction of plane waves, the power of the generated wave P
new
can be written as an approximation:
P
neu
=
8
⁢
π
⁢
⁢
d
eff
⁢
L
2
⁢
P
p
⁢
P
OPO
ϵ
0
⁢
cn
neu
⁢
n
p
⁢
n
OPO
⁢
λ
neu
2
⁢
A
⁢
sin
2
⁡
(
&LeftBracketingBar;
Δ
⁢
⁢
k
&RightBracketingBar;
⁢
L
2
)
.
(
2
)
Here d
eff
represents the effective nonlinear coefficient, L represents the crystal length, P represents the powers of the pump and OPO radiation, s
0
represents the dielectric constant, c the speed of light in a vacuum, n the refractive index of the involved waves, &lgr;
new
the wavelength of the generated wave, A the beam area and &Dgr;k the wave vector difference. The following applies for the power of a pulsed radiation having the pulse duration &tgr; and the energy E: P=E/&tgr;.
The nonlinear frequency conversion is the most efficient when the wave vector difference becomes zero. This is referred to as phase matching.
The conversion efficiency &eegr; is defined by:
η
=
P
neu
P
p
⁢
P
OPO
.
(
3
)
It is readily apparent that the power of the generated radiation is a function of the product of the powers of the radiated waves.
The standard external frequency conversion (i.e., frequency conversion occurring outside of the resonator) is a conventional technology that has been used frequently for lasers, but also for OPOs. This technology is known in connection with OPOs from the essay by G. Haub, M. J. Johnson and B. J. Orr, “Spectroscopic and Nonlinear-Optical Applications of a Tunable &bgr;-Barium Borate Optical Parametric Oscillator,” J. Opt. Soc. Am. B 10, 1765 (1993). From the paper by J. Clark, B. Johnson and V. Newell, “Frequency Doubling of Narrowband High Energy Optical Parametric Oscillators,” Proc. SPIE 2379, 256 (1995), it is known that products in which an external frequency doubling of OPOs is used are already commercially available.
The known methods have considerable disadvantages, however. A high conversion efficiency of the nonlinear frequency conversion is desirable, because it contributes significantly to the overall efficiency of the light source, that is, the ratio of the used electrical power to the generated light power. A higher overall efficiency means a higher light efficiency with the same power, or an energy savings with comparable output values. Therefore, a lower outlay for machinery and more cost-effective operation are usually attained, which also results in lower initial costs.
In theory, conversion efficiencies of 100% can be attained with the aid of nonlinear frequency conversion. In practice, however, the attained conversion efficiencies tend to be far lower. Numerous reasons account for this fact:
Normally, real beams cannot be described by ideal, plane wave fronts. The temporal and spatial coherence of pulsed radiation sources, particularly OPOs, is often comparatively poor.
Only a portion of the incident radiation is converted if the pulse lengths of the beams to be mixed are either different or have timing jitter.
Because nonlinear frequency conversion necessarily places many demands on a suitable nonlinear material (e.g. transparency or low internal losses, the possibility of phase matching, high optical, mechanical and thermal damage thresholds, etc.), usually not all properties can be optimized. Often, technological limits are also established. Thus, crystals frequently cannot be produced with high optical quality or with larger dimensions.
The crystals that are suitable for nonlinear frequency conversion have so-called acceptance bandwidths for the divergence and the spectral width of the radiation. These acceptance bandwidths decrease as the crystal length increases. As a result, in divergent or spectrally-broadband radiation sources, only a portion of the incident radiation can be used for nonlinear frequency conversion. This means that limits are placed on the focussing of the incident radiation for increasing the power density. Pulsed OPOs are spectrally-broadband if no special devices have been implemented to reduce their spectral width, and are frequently comparatively divergent.
A further disadvantage of OPOs having conventional external frequency conversion is the energy stability, which is generally clearly worse than that of the pump source.
Moreover, the beam quality and the divergence of OPOs having a conventional external frequency conversion, particularly frequency doubling, are often inadequate for applications.
For pulsed OPOs, there is only one description of an experimental setup for the special case of frequency doubling (SHG) that is integrated into the resonator: L. R. Marshal, A. Kaz, O. Aytur, “Continuously Tunable Diode-Pumped UV-Blue Laser Source,” Opt. Lett. 18, 817 (1993). Furthermore, theoretical calculations have been proposed for the integrated sum frequency mixing of continuous-wave (cw) or cw-modelocked OPOs on the basis of a plane wave model; see G. T. Moore, K. Koch, “Optical Parametric Oscillation with Intracavity Sum-Frequency Generation,” IEEE J. Quantum Electron. QE29, 961 (1993).
OBJECTS AND SUMMARY OF THE INVENTION
It is the object of the invention to provide a method and a device for nonlinear frequency conversion of the radiation of a pulsed optical parametric oscillator with a high conversion efficiency and better energy stability, with the beam quality and the divergence of the OPO satisfying the requirements set by applications.
In accordance with the invention, this object is accomplished in a method of the type mentioned above in that the frequency mixing is integrated into the OPO resonator. According to the invention, the basis of the proposed method and device is the integration of frequency mixing into the OPO resonator. In contrast to the conventional external frequency mixing, the method and the device for executing the method lead to an increase in the conversion efficiency of the nonlinear frequency conversion of the
Fix Andreas
Wirth Martin
Browdy and Neimark
Deutsches Zentrum fur Luft-und Raumfahrt e.V.
Jr. Leon Scott
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