Optical: systems and elements – Optical amplifier – Multiple pass
Reexamination Certificate
1999-11-03
2002-05-07
Tarcza, Thomas H. (Department: 3662)
Optical: systems and elements
Optical amplifier
Multiple pass
Reexamination Certificate
active
06384966
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a multiple pass optical amplifier and a method for amplifying electromagnetic radiation, particularly light, and especially laser light.
BACKGROUND OF THE INVENTION
There are an increasing number of applications for laser systems where the power of the laser beam must be maximized, and one attractive method for reaching high power levels is to amplify a laser beam generated in a separate laser by passing it through a laser gain material such as Nd:YAG or Nd:YVO
4
. Many such amplifier systems have been demonstrated including straight forward single pass, and zig-zag designs. In order to maximize the energy extracted from a laser amplifier, of whatever design, multiple co-linear passes of the amplifier can be made. These multi-pass amplifier designs typically use polarization rotating devices to enable separation of the input and output laser beams. For example it is possible to obtain two passes of the amplifier using an optical polarizer and a quarter-wave plate, or using an optical isolator. To increase the efficiency of the amplifier further it is possible to obtain four co-linear passes of the gain material, for example, using an optical polarizer, a quarter-wave plate and an optical isolator, as proposed in U.S. Pat. No. 5,268,787 (McIntyre).
Although the multiple pass amplifier designs described above can in themselves increase the efficiency of optical amplifiers, this design in itself fails to address two important issues which can have a serious detrimental effect on amplifier performance.
One such issue is thermal depolarization which is significant in many solid state laser gain media. Thermal depolarization arises as a result of thermal stress due to absorption of pump or other energy. This induces stress-related birefringence in the gain material, which can cause rotation of the polarization of the amplified laser beam in the gain medium. Various techniques to compensate for thermal depolarization in laser resonators have been reported. In particular in W. A. Clarkson, N. S. Felgate, and D. C. Hanna, “Simple method for compensation of thermally-induced birefringence in high-power solid-state lasers”, EuroCLEO 98, a quarter-wave plate, aligned to give zero phase retardation for the favored laser polarization, is inserted into the laser cavity. Further in C. A. Denman and S. I. Libby, “Birefringence Compensation using a Single Nd:YAG Rod”, Advanced Solid State Lasers 1999, a 45-degree Faraday rotator is inserted into the laser cavity and used in combination with an optical polarizer. In both of these approaches the thermally induced birefringence of an initial pass is substantially cancelled in the next pass. In order to achieve this the amplified beam must experience birefringence distribution in the following pass that is, to the extent possible, the same as that in the initial pass. Usually this is done by making the beam re-trace the initial beam path. However, 4-pass amplifiers constructed in this way have the disadvantage that unwanted lasing might occur.
Apart from thermally induced birefringence another important issue, especially in the case of four-pass amplifiers, is the avoidance of unwanted lasing in the amplifier. This can occur when a stable optical cavity is formed by the mirrors in the amplifier system used to reflect the beam through its multiple passes of the gain medium. Lasing is particularly likely to occur in a 4-pass amplifier, when the thermally induced birefringence is large, and thus can rotate the polarization of the light on each pass through the gain medium to such a degree that light can be “trapped” in the amplifier rather than exiting through the polarizing beamsplitter after making a pass through the system. Thus thermal depolarization in the gain medium can be significant enough to allow lasing to occur where the presence of a polarizing beamsplitter and a polarization rotating device would otherwise prevent repeated passes along the same beam path.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a multiple-pass optical amplifier and a method for amplifying light where thermal birefringence is compensated. It is a further object of the invention to provide a multiple-pass optical amplifier and a method for amplifying light where undesired lasing in the amplifier is avoided.
For studying the effect of thermal-stress-induced birefringence in a laser gain medium, the effect of the birefringence on a linearly polarized input beam can be considered. In passing through the birefringent gain medium (or any other birefringent component), the linearly polarized input beam is transformed into an elliptically polarized beam. Here we note that the direction of rotation of the polarization (i.e., left-handed or fight-handed) depends on the orientation of the fast and slow birefringent axes in the gain medium relative to the input polarization. Hence if the fast and slow birefringent axes are exchanged, then the direction of rotation is reversed. In order to compensate for the thermal-stress-induced birefringence, the elliptically polarized beam should be transformed back into one which is again linearly polarized. This could, at least in principle, be achieved by passing the beam through a device which exhibits a birefringence which is exactly equal and opposite to that of the gain medium. This means that at every point the two orthogonal birefringent axes are exchanged. However, the birefringence (or inverse birefringence) distribution in a typical laser rod is a function of position in the rod; it depends on the thermal load, the rod type etc. Therefore, it is not easily reproduced, and thus it is not generally possible to use such a separate device.
Instead of introducing a separate device in which the birefringent axes are exchanged compared to those in the gain medium, the invention exploits the birefringence distribution already present in the gain medium. This is done by modifying the polarization state of the elliptically polarized beam in a way (or ways) which is equivalent to exchanging the birefringent axes in the gain medium, and by passing the beam for a second time through the gain medium. In other words, the polarization state of the beam is modified so that it is the same as that which would have been obtained if the birefringent axes of the gain medium had been exchanged for the first pass.
According to the invention, the multiple pass optical amplifier for an incident light beam comprises:
an optical gain material;
means for reflecting a light beam which has passed once through said gain material back into said gain material; means for modifying the polarization state of a light beam after passing through said gain material for a first time and before passing through said gain material for a second time with respect to two orthogonal axes in a way which is equivalent to exchanging said two orthogonal axes; and
means for separating a light beam which has passed twice through said gain material from said incident light beam.
According to the invention, the method for amplifying an incident light beam comprises the steps of:
passing said incident light beam through an optical gain material;
reflecting the light beam which has passed once through said gain material back into said gain material;
modifying the polarization state of the light beam after passing through said gain material for a first time and before passing through said gain material for a second time with respect to two orthogonal axes in a way which is equivalent to exchanging said two orthogonal axes;
passing said incident light beam through said gain material for a second time; and
separating the light beam which has passed twice through said gain material from said incident light beam.
A first exemplified embodiment of a 2-pass amplifier according to the invention rotates all polarization components of the elliptically polarized beam through 90°. This is equivalent to rotating the birefringent axes of the gain medium through 90°, which has the same effect as exchanging the two orthogonal axes o
Hughes Deandra M.
Tarcza Thomas H.
Time-Bandwidth Products AG
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