Coherent light generators – Particular beam control device – Mode discrimination
Reexamination Certificate
1997-05-21
2002-04-16
Leung, Quyen (Department: 2881)
Coherent light generators
Particular beam control device
Mode discrimination
C372S032000
Reexamination Certificate
active
06373868
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to laser pump/cavity/amplifier configuration design, and more particularly to realizing single longitudinal mode operation and intracavity frequency conversions for diode-pumped solid-state lasers, as well as optical multipass constructions for pumping laser media and fiber lasers, and for the use of multipass optical amplifier.
BACKGROUND OF INVENTION
Since the so-called “green problem” was discovered by T. Baer in 1986, it has become well known and has long plagued the stability of the CW intracavity harmonic generation of diode-pumped solid-state (DPSS) lasers. The essential difficulty in solving the “green problem” results from that, there is a persistent obstacle in effectively obtaining single longitudinal mode CW operation due to the spatial hole-burning effect in solid-state lasers. The related critical design issues are extremely tough. For the past decade, much research has attempted to solve this problem to obtain stable green light. Almost every effort has been made and nearly every way has been tried. However, none of true CW devices or designs has been successful by far with a regular standing-wave cavity. Only ring or very short cavity configurations have been used for this purpose, but they have appreciable inconveniences and limitations.
Baer, many AMOCO scientists and others did primary works and made some detailed reviews to the “green problem” in their papers and patents, such as the U.S. Pat. No. 5,164,947 (1992) and paper “Intracavity Doubling of CW Diode-pumped Nd:YAG laser with KTP,” IEEE J. QE-28, 1148(1992). Baer recognized the “green problem” and pointed out that there was a fundamental barrier to successful multimode operation of intracavity doubled lasers. (Now, the “multimode operation” should be corrected to be “a few modes operation”.) AMOCO and other scientists examined and worked out several important problems, including minimizing spatial hole burning effect with the “twisted mode” technology and the various polarization related problems, such as modifying the polarization of the laser modes in the doubling crystal to reduce the likelihood of chaotic amplitude fluctuations.
Controlling spatial hole burning can greatly reduce the possibility of amplitude oscillations. However, weak residual spatial hole burning resulted from imperfect “twisted mode” operation can still cause oscillations. In spite of those intense efforts, there remains a determining approach required to achieve dynamically stable single-mode operation with the use of a regular standing-wave cavity when the spatial hole-burning effect is present. What is needed is to provide a powerful form of wavelength selectivity to clamp the peak position of the operating frequency and prevent the laser operation from mode hopping and shifting to wavelengths outside the phase matching curve while controlling appreciable losses to the system.
On the other hand, an etalon within a cavity is commonly used to further control and suppress the harmful mode operation. Etalons typically have the highest spectral mode discrimination. However, the insertion of an etalon often leads to large passive losses and significantly reduces output power. This is especially true, for example, when the etalon is of high-finesse type, or the cavity has a small spatial mode waist and, hence, large beam divergence, and these effects are worse when the etalon is titled. Therefore, as simply inserting an etalon to a laser cavity, these characteristics often lead to the failure of laser operation.
AMOCO scientists realized and considered this key factor and were very close to success. In fact, there was almost one step behind to win the battle of the “green problemn”. Although they did not cross this decisive step, they have demonstrated several important concerns over the unsolved difficulties inherent in the “green problem” under the condition of single-mode operation. Following are the major concerns in their paper.
(1) “The intracavity harmonic generation laser is much more sensitive to component quality and the associated insertion loss than are most other lasers. To build an efficient intracavity harmonic generation laser, one needs to find some forms of mode selectivity with low loss, which is a significantly difficult task. On the other hand, if enough constraints are placed on the cavity without introducing appreciable losses to the system, stable and efficient operation of intracavity harmonic generation lasers is possible.”
(2) “The doubling efficiency is extremely sensitive to the finesse of the laser cavity so that all these controls must be introduced into the laser cavity without adding appreciable loss to the system.”
(3) “The principle difficulty with this design is that combining a polarizer and a highly birefringent element with a relatively small mode radius (w=100 um) can lead to significant losses. It is found that the green and 1064 nm output from cavities containing Brewster plates are often substantially smaller than those in similar cavities without Brewster plates. A 100 um beam has a far-field divergence angle of 3.4 mr; the off-axis components of the beam are appreciably depolarized by the angle-dependent refractive index. The phase shifts are only a fraction of a wave, but in the presence of a polarizer, these correspond to losses on the order of a fraction of a percent.”
In conclusion, their major point focuses on that, a relatively small mode waist can lead to significant insertion losses for the intracavity optical elements, particularly for an inserted etalon or Brewster plate in the present case.
SUMMARY OF THE INVENTION
In order to overcome the dominant difficulties in the prior art, the present invention offers two solutions for obtaining a dynamically stable single-mode operation with regular standing-wave cavities. These two different methods can be used separately, or collectively at the same time to be more powerful and effective.
(1) Cavity designs with a beam expander are applied to render a large mode waist and an improved beam divergence, so as to significantly reduce the insertion losses for intracavity optical elements, typically for a tilted etalon and Brewster plate; and
(2) A pump head with a thin gain zone is applied to minimize the spatial hole-burning effect. The effect caused by a thin gain region is equivalent to that caused by short cavity configurations in which longitudinal modes are separated substantially, so that the required resolving-power of a frequency-selective form will be largely relaxed, and it becomes possible to use a spectral filter with low insertion losses, such as a birefringent filter or a low-finesse etalon, in realizing single-mode operation.
Further, the thickness of a thin gain zone is considered a critical factor to minimize the spatial hole-burning effect. To construct a thin gain zone at the end of a solid-state laser medium, there are several practical limitations to the commonly-used pumping schemes. The conventional side-pumping schemes are not capable of producing such a thin gain region. On the other hand, the constraints on the end-pumping scheme result from the need of a laser medium with a very large absorption coefficient and a very limited thickness. Therefore, a need still exists in the art to provide an effective method to serve this purpose. Consequently, in the present invention the multipass waveguide pump head has been developed for producing a thin gain zone within a laser medium.
Besides, once successfully solving the “green problems” for the intracavity second harmonic generation and obtaining a stable green light output, a new and promising way appears for the development of intracavity third and fourth harmonic generations, which can directly and effectively produce CW UV coherent light from one-single-stage cavity with using two or three nonlinear crystals in a serial manner. This offers a much more attractive solution than the external resonant cavity frequency doubling technology in the art. Nevertheless, such a design used to be considered infeasible i
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