Polarization Controlling System for Lasers

This invention relates generally to lasers, and more particularly to optically pumped solid state lasers.

A large number of different kinds of solid state lasers have been discovered, distinguished from one another by host material, by active lasing ions with which the host is doped, and by output characteristics. (See U.S. Pat. No. 4,872,177 issued Oct. 3, 1989 and U.S. 4,860,304 to Mooradian issued Aug. 22, 1989).

Solid state optically pumped lasers comprise, in general, a solid state lasing material sometimes called the gain medium which is disposed within an optical cavity formed between two mirrors. An optical source, such as a laser diode or array of diodes, generates a pump beam which is focused onto the lasing material. Photon energy from the pump laser is absorbed by the gain medium. When a threshold level of absorbed light is achieved stimulated emission of light from the gain medium occurs. In the microchip laser of Mooradian the cavity length, or separation between mirrors, may be as small as 10 to 100 microns to obtain the desirable property that the resulting light oscillates in a single axial mode within the laser cavity.

However, in such microchip lasers and in many conventional lasers it is possible to obtain oscillation in two different polarization modes.

Polarization relates to the direction of the electric field vector E of a light wave. This vector is perpendicular both to the propagation vector k which describes the direction of travel of the wave and to the instantaneous direction of the magnetic field of the wave, H. The direction of the electric field vector is referred to as the "direction of polarization". Randomly polarized light, i.e. light from incandescent lamps, is unoriented and hence unpolarized, whereas light from lasers is generally highly oriented and hence "polarized".

In many lasers, including microchip lasers, two orthogonally polarized modes are present and have nearly the same threshold. This is due to the fact that a 90 degree rotation of a light wave about its propagation vector results in an optical field orthogonal to the original wave which sees the same amount of net gain (gain minus loss). As a result, the laser may oscillate in both polarizations, or may switch polarizations in response to a small amount of feedback provided by optics external to the laser cavity. One way of controlling the polarization of a laser is to introduce a polarizing element within the cavity, such as a Brewster's angle window. While this is a viable alternative for large lasers, it is not an acceptable solution for microchip lasers, which rely on a short cavity length to perform properly. Conventional polarizing elements made to be compact, flat, and thin for microchip applications are often too lossy to use intracavity.

The present invention provides a way of selecting the polarization of a laser without the use of intracavity polarizing elements. In addition it provides a method and apparatus for controllably switching a laser between two orthogonal polarizations. Polarization control of lasers is important because many optical systems (including systems with birefringent elements, gratings, optical surfaces which are not normal to the propagation vector of the light, nonlinear optical elements, or polarizing elements) have different properties for different polarizations of light. By switching a laser between orthoganal polarizations information can be encoded on the output light beam.

The invention comprises a polarization controlled laser formed by a gain cavity coupled to a polarizing cavity. The gain cavity is comprised of a laser gain medium disposed in a cavity formed between a pair of mirrors. Note: The term "mirror" is used herein, in a general sense, to include any surface which is partly, or highly, reflective at a particular wavelength. One of the mirrors is an input mirror for the gain cavity and is disposed adjacent to a pump source. The input mirror is highly reflective at the lasing wavelength of the gain medium and antireflective at the pump wavelength. The second, or more remote mirror from the pump, is partially transmitting and forms a first partial reflector for a polarizing cavity which is comprised of the first partial reflector, a spacer, a polarizing element or a birefringent element (which has a different refractive index for the two orthoganal polarizations), and a second partial reflector, assembled in that order. The first partial reflector provides the dominant part of the reflective feedback seen by the laser gain medium. The small amount of feedback provided by the second partial reflector is polarization dependent, however. The spacer is an optical element which has a variable optical length (physical length times refractive index). The optical length of the spacer determines whether the feedback from the second partial reflector in a given polarization is in phase or out of phase with the light reflected by the first partial reflector. By controlling the optical length of the spacer (electro-optically or piezoelectrically, for example) the reflected light from the second partial reflector in a given polarization can be made to add constructively or destructively to the light reflected by the first partial reflector.

The reflectivity of the second partial reflector is chosen to provide sufficient feedback to split the degeneracy of the two orthogonally polarized laser modes of the gain cavity, and to overcome the effects of feedback from the external optics. The polarization of the laser gain cavity that sees the highest net reflectivity from the two partially reflecting mirrors will experience the most net gain. As a result, it will be the first polarization mode to oscillate, and will deplete the gain for the orthogonally polarized mode. In this way, the polarization of the laser gain cavity is controlled by the polarizing cavity.

FIG. 1 is a schematic drawing of a laser in accordance with a first embodiment of the invention

FIG. 2 is a schematic drawing of a laser according to an alternative embodiment of the invention.

FIG. 3 is a plot of the reflectivity of the polarizing cavity for two orthogonally polarized light waves (solid and dashed lines) versus frequency.

The first embodiment of the invention will be described in connection with FIG. 1. Note that while this embodiment is illustrated in connection with a solid state laser, any suitable laser will benefit from the polarization control and switching capabilities of the invention.

The polarization controlled laser system of the invention comprises, in general, a pump source 4, a gain cavity 50 and a polarizing cavity 60. Pump source 4 may comprise any power source able to bring the gain medium 8 in cavity 50 to a level at which stimulated light emission occurs. Preferably source 4 is a diode laser or the like. Source 4 may be physically adjacent to input mirror 6 of cavity 60, as shown, or light from the source may be collimated and focused onto the gain medium by suitable lens elements (not shown).

Gain cavity 50 is comprised of input mirror 6, gain medium 8 and partial reflector 10.

Input mirror 6 is highly reflective to light at the wavelength of the laser emission from cavity 50 and antireflective at the wavelength of the light from pump 4. Mirror 6 may comprise a dielectric coating deposited on the gain medium 8 or on a thin glass sheet bonded to the medium or other suitable reflector.

The gain medium 8 is preferably a solid state material, such as Nd:YAG or Nd pentaphosphate or the like. The polarizing cavity 60 is comprised in general of a partial reflector or mirror 10, variable optical length spacer or element 12, thin polarizer 18 and output mirror 20. Partial reflector 10 is positioned adjacent to spacer 12. The spacer 12 is preferably transparent to the laser radiation from cavity 50. The first partial reflector 10 may also be formed by a dielectric coating on the spacer 12. The transparent spacer 12 may be an electro-optic material (such as LiTaO.sub.3 or GaAlAs) in which the refractive index can be controlled electrically, or a transparent piezoelectric material (such as PLZT or Quartz) in which the length of the spacer can be controlled electrically. In either case, the spacer 12 is processed so as to be conductive along the surfaces 14 and 16 while being insulating in the bulk. The conductive surfaces 14 and 16 may be produced through the deposition of a transparent, conductive material (such as Indium Tin Oxide) on the surface of the spacer 12. Alternatively, the spacer 12 may be a semiconducting material (as in the case of GaAlAs or Si), in which case the conductive layers 14 and 16 could be produced by growing thin conductive semiconductor layers on both sides of an insulating substrate.

A thin polarizer 18 is attached to the side of the spacer 12 remote from the first partial reflector 10. The polarizing cavity is completed by attaching a second partial reflector, or mirror, 20 to the free side of the polarizer 18. The second partial reflector 20 may also be formed by dielectric deposition directly onto the polarizer 18. The optical length of the spacer 12 in the polarizing cavity 60 is controlled through the application of a voltage (not shown) between contacts 22 and 24 formed on the conductive surfaces 14 and 16 of the spacer 12.

The polarizing cavity 60 can be used as the output coupler of a laser. The first partial reflector 10 faces the laser gain cavity 50, and provides the dominate part of the reflectivity seen by the laser gain cavity 50. The small amount of feedback provided by the second partial reflector 20 is polarized, however. The reflectivity of the second partial reflector 20 is chosen to provide sufficient polarized feedback to split the polarization degeneracy of the two orthogonally polarized laser modes, and overcome the effects of feedback from external optics. The optical length of the spacer 12 (physical length times refractive index) determines whether the polarized feedback from the second partial reflector 20 is in phase or out of phase with the light reflected by the first partial reflector 10. By controlling the optical length of the spacer 12, the polarized component of the reflected light can be made to add constructively or destructively to the light reflected by the first partial reflector 10, and the polarization of the laser can be controlled. Note that the polarizer 18 may be placed anywhere between the first partial reflector 10 and the second partial reflector 20. Note also that if a reflective polarizing element (a material which reflects one polarization and transmits the other) is used the second partial reflector 20 may not be required. If the polarizer 18 is strongly reflecting (a very good reflective polarizer) it may be appropriate to use the polarizing cavity as an end mirror of a laser cavity with a different output coupler.

In a variation on the above described embodiment the spacer 12 may comprise a piezoelectric material which is not transparent to the laser radiation. In this option a hole 210 (shown by the dotted lines in FIG. 1) is cut in the spacer 12 and the conductive surfaces 14 and 16 to provide a via for the laser radiation. In this case the conductive surfaces 14 and 16 need not be transparent. Alternatively, with an appropriate spacer material, such as a material with a transverse piezoelectric or electro-optic effect, the conductive surfaces 14 and 16 may be deposited on surfaces of the spacer 12 parallel to the cavity axis, in which case they need not be transparent.

In another embodiment of the invention, as shown in FIG. 2, a piezoelectric or electro-optic birefringent element 310 is placed between two partially reflecting mirrors 10' and 20' so that the net reflectivity of the etalon formed by the two partially reflecting mirrors is different for the two orthogonal polarizations of the laser as a result of the coherent addition of the light reflected by the two partial reflectors. Note that elements with counterparts in FIG. 1 contain the same numeral designation with a prime suffix and need not be described further herein. The optical length of the birefringent element 310 may be changed piezoelectrically or electro-optically (for one or both polarizations) in order to change the phase relationship between the light reflected by the two partial reflectors and thereby control the polarization of the laser.

In yet another embodiment the polarizing cavity may be formed by a partially reflecting mirror, a spacer, a birefringent element and a second partially reflecting mirror in such a way that the optical length of the spacer is changed to control the net reflectivity of the composite mirror in the two orthogonal polarizations. In this embodiment the piezoelectric or electro-optic element of the previous embodiments is replaced by two elements, a spacer having a controllable optical length and a birefringent element.

Any of the above embodiments can be used with a laser that supports several longitudinal cavity modes that are equally spaced, or spaced by multiples of a common frequency. In such a case the free spectral range of the etalon formed by the two partially reflecting surfaces should be approximately equal to the cavity mode spacing, or said common frequency, as illustrated in FIG. 3 which shows the reflectivity of a birefringent etalon in two orthogonal polarizations (solid and dashed lines), where the length of the etalon was chosen so that the net reflectivity of the etalon is low for one of the polarizations (the solid line) and high for the other (the dashed line), at all the potential lasing frequencies of a laser (indicated by the large vertical tic marks at the top of the figure). The polarization of the resulting laser structure can be switched by moving either the resonant transmission frequencies of the polarizing cavity (for one or both of the orthogonal polarizations) or by changing the frequencies of the longitudinal modes of the laser.

Having shown a number of embodiments, those skilled in the art will realize many variations are possible which will still be within the scope and spirit of the claimed invention. Therefore it is the intention to limit the invention only as indicated by the scope of the claims and equivalents thereof.