Tunable Single-Frequency Ring Laser

1. Field of Invention

This invention relates generally to single-frequency ring lasers and more particularly to tunable single-frequency solid-state lasers.

2. Description of Prior Art

Ultra-stable, single-longitudinal-mode lasers are the heart of coherent optical transmitters and receivers in coherent communication. They are also useful as local oscillators in high-powered lasers and laser amplifiers. These lasers are an essential part in coherent optical range finders, designators, sensors, and other measurement instruments. In coherent optical rangefinders, for example, it is very important to force such a laser to oscillate in a purely longitudinal mode and to tune its frequency rapidly over a limited frequency range in order to detect and determine the location and velocity of remote objects, using heterodyne FM-CW chirped techniques. Such techniques are well-known in FM-CW chirped radar applications, and are described in the literature, for example, by Skolnik, Introduction to Radar Systems, 2nd ed., Chapter 3, McGraw-Hill, New York, 1980.

Recent developments describe the use of solid-state lasers pumped by semiconductor laser diodes which provide very small, highly efficient, single-mode ultra-stable solid-state lasers. Such solid-state lasers are described, for example, in U.S. Pat. No. 4,578,793; Kane and Byer, "Monolithic, Unidirectional Single-Mode Nd:YAG Ring Laser," Optics Letters 10:65-67 (Feb. 1985); Kane et al., "Frequency Stability and Offset Locking of a Laser-Diode-Pumped Nd:YAG Monolithic Nonplanar Ring Oscillator," Optics Letters, 12:175-177 (March 1987); and Trutna et al., "Unidirectional Diode-Laser-Pumped Nd:YAG Ring Laser with a Small Magnetic Field," Optics Letters 12:248-250 (April 1987).

In the prior art, single-piece, solid-state non-planar as well as quasi-planar, internally reflecting, unidirectional ring lasers pumped by semiconductor lasers, have been shown to emit highly coherent single-frequency laser light. The advantage of the prior-art design configuration is its stability and simplicity due to the use of a single piece of solid-state laser material with magneto-optic properties and polished surfaces that create a unidirectional ring path for the laser beam. The internally reflective surfaces act to change the direction of the linear-polarized lasing light in order to compensate for the plane of polarization change induced by the Faraday rotation due to an externally applied magnetic field. Since the magneto-optic effect is non-reciprocal, the plane of polarization of a beam that traverses the ring in one direction, e.g. counter-clockwise, is designed to be aligned with the original polarized beam after completing one round. On the other hand, the beam which traverses the ring in the opposite direction, e.g., clockwise, increases its polarization rotation each time it completes a round trip in the cavity. A polarization-selective element in the cavity, aligned with the compensated, counter-clockwise, linear-polarized beam, induces differential losses between the two opposite traveling waves. The induced differential loss between the two contra-rotating beams occurs by use of a polarization-selective element, since one of the rotating beams (the clockwise beam in this example) has a plane of polarization which accumulates increasing deviation from the aligned polarizer orientation and thus an increased loss, each time the beam completes a full path within the cavity. On the other hand, for the polarization-compensated beam (in this example, the counter-clockwise beam) the plane of polarization remains aligned with the polarizer orientation, so that the beam does not suffer significant loss. The differential losses reinforce the counter-clockwise beam over the clockwise rotation, thereby creating a unidirectional traveling-wave lasing. The traveling-wave lasing configuration saturates the gain uniformly, thereby eliminating spatial hole-burning that creates multifrequency operation in a linear Fabry-Perot-type cavity. The gain is uniformly saturated since the electric field of the electromagnetic wave is travelling through the pumped volume. This is in contrast to the action in a Fabry-Perot configuration in which stimulated atoms spacially located across the nodes of the standing waves form unsaturated gain, in which case other longitudinal modes having different frequencies tend to oscillate and saturate this unsaturated gain.

The prior-art monolithic design, for example as shown in U.S. Pat. No. 4,578,793, and in Kane and Byer, "Monolithic, Unidirectional Single-Node Nd:YAG Ring Laser," Optics Letters, vol. 10, No. 2 (Feb. 1985), pp. 65-67, gives a highly stable single-frequency laser output. However, the stable monolithic design is inferior when fast frequency tuning is needed. Slow frequency tuning can be applied to the monolithic design by heating or cooling the solid-state lasing material. Fast frequency tuning over a very limited spectral range can be achieved by applying a variable pressure on the lasing material, such as Nd:YAG, using a piezo-electric transducer (PZT). See also A. Owyoung and P. Esherick, Optics Letters, vol. 12, p. 999 (1987), and T. J. Kane, Lightwave Electronics Corporation, Mountain View Calif., "Demonstration of Two Frequency-Locked Ring Lasers," exhibition shown at the Conference on Laser and Electro-Optics, Apr. 26-28, 1988, Anaheim, Calif.

Also known in the prior art is a co-linear pumping technique where a laser diode is tuned to emit light in the exact absorption band of the solid-state laser material, as described, for example, in Kane et al, "Frequency Stability and Offset Locking of a Laser-Diode-Pumped Nd:YAG Monolithic Non-Planar Ring Oscillator," Optics Letters, vol. 12, no. 3 (March 1987), pp. 175-177. The laser diode light is focused on the same axis of the propagating light inside the cavity, colinear with the lasing beam. In the colinear pumping configuration the pumped volume overlaps exclusively with the lowest-order transverse-cavity mode, which gives rise to a lasing of single transverse mode, namely the TEM.sub.00 gaussian mode. Other prior art, pertaining to monolithic solid-state lasers pumped by laser diodes, is described by Trutna et al., "Quasiplanar Unidirectional Ring Laser," Paper WN2, presented at the Conference on Laser and Electro-Optics, Apr. 26-29, 1987, Baltimore, Md., and by Trutna et al., "Unidirectional, Diode-Laser-Pumped Nd:YAG Ring Laser with a Small Magnetic Field," Optics Letters, vol. 12 (April 1987), p. 248. This design has better performance due to exact compensation of the polarization plan rotation by the total internal reflecting facet of the prism.

It is an object of the present invention to provide a single-frequency unidirectional ring laser which can be quickly tuned.

It is a further object of the present invention to provide a compact, tunable, single-frequency, solid-state, unidirectional ring laser which can be quickly tuned.

It is yet another object of the present invention to provide a unidirectional, solid-state ring laser in which fast frequency tuning over a large frequency range is accomplished by means of quickly changing the optical path length traversed by the intra-cavity laser beam.

It is yet another object of the present invention to provide an electromechanical, electro-optical and magneto-optical means for changing the unidirectional ring intra-cavity optical-path length in order to achieve rapid frequency tuning over a large laser frequency range.

In accordance with the teachings of the present invention, a compact, solid-state ring laser is provided having a simple design with a minimum of two pieces which form the solid-state laser ring cavity. The two-piece design enables a rapid change of the laser-ring optical path length by applying electrical stimuli to one of the pieces forming the laser-ring cavity. Simple designs utilizing more than two pieces are also taught.

In the first embodiment of this invention, the two pieces forming the laser-ring cavity comprise two different materials that have complimentary properties. The first piece is formed of a solid-state laser material with magneto-optic properties. This first part acts as a lasing medium as well as a Faraday isolator by rotating the plane of polarization of light traversing within. The second piece is a highly transparent, quasi-planar reflector, such as a prism, made out of either low-cost glass, electro-optic material, magneto-optic material or a material having some combination of the above properties. In the case of the simple glass, quasi-planar, reflecting prism, this piece is mounted on a PZT. An applied tunable external voltage on the PZT moves the prism relative to the solid-state laser material, lengthening and shortening the optical path length of the ring and thereby tuning the laser frequency. In one embodiment, the intra-cavity beam is transmitted from piece 1 to piece 2 and back while the beam inclines to surface 3-1 and 3-2 in a Brewster angle, as shown in FIG. 1. Thus, surfaces 3-1 and 3-2 function as intra-cavity polarizers, and therefore these surfaces provide a simple internal polarizer with very low loss that improves the differential loss between the two light beams traversing oppositely within the ring cavity. The polarizer function can also be implemented by utilizing an appropriate polarization-preferred coating or multilayer coatings on surfaces 3-1 and 3-2 of FIG. 1, or by arranging the Brewster angles in any other combination inside the cavity to prefer one polarization over the other.

In a second embodiment of this invention, the second piece forming the laser ring cavity is attached to a PZT, the second piece being made of transparent magneto-optic material with high Verdet constant. Using high-Verdet-constant material enables larger rotation of the plane of polarization within the prism material, thus providing better isolation against the contra-rotating beam and back reflection into the laser ring cavity. Thus the external prism enables a rotation of the polarization plane out of the first piece, which is made of lasing material. The advantage of this embodiment is that a variety of laser materials which do not exhibit strong magneto-optic properties can now be used to form ring lasers in accordance with the teachings of this invention, thus allowing the emission of the lasers of this invention to be turned within a broader frequency spectrum than possible in the prior art.

A variation of this embodiment provides that both pieces have magneto-optic properties, thus allowing polarization rotation both inside and outside the lasing material. This enables much larger Faraday rotation of the plane of polarization, thus increasing the differential loss between the two contra-rotating beams transversing within the laser cavity. This increase in the isolation against external reflections is necessary since these reflections can be undesirably amplified in the opposite direction inside the ring, utilizing the gain arbitrarily, thus inducing intensity and correspondly frequency noise on the unidirectional laser beam.

In a third embodiment of this invention, the reflecting piece is made of an electro-optic material in which an applied voltage changes the refractive index along the direction of propagation of the light beam, thus varying the total optical-path length within the laser-cavity. This embodiment has the advantage that no mechanical movement is applied to the second piece. The first and second pieces can be attached together, for example, by means of optical cement, in order to prevent relative movement between the two pieces.

In a fourth embodiment of this invention, the second part is made of a magneto-optic material. When a variable magnetic field is applied to the second part, the magneto-optic properties cause a light beam having circular polarization which propagates in the second piece to change its phase, thus creating optical-path variations. The circular polarization of the light beam within the second piece is created by an additional quarter-wavelength plate located between the first and the second pieces. This quarter-wavelength piece can, if desired, be attached to the second or first piece. If desired, all the pieces can be attached together, for example by means of optical cement, in order to prevent relative movement.

In a fifth embodiment of this invention, the second piece is made of material (e.g., a birefringent material) where applied variable external pressure changes the refractive indices of the material along a preferred axis, thereby changing the optical-path length of the laser cavity. Such pressure can be applied by an attached piezo-electric transducer (PZT), or by other suitable means. Preferably, the external prism material itself can consist of a piezo-electric material which is transparent in the lasing spectrum, allowing a variable electric potential to be applied directly to the prism material in order to change its physical dimension slightly, thus enabling a rapid change of the optical-path length of the laser beam in the cavity. Two embodiments of this invention generalize the ways light is coupled out of the cavity and the ways pump light is coupled into the cavity.

In the general embodiment a combination of the above embodiments can be implemented to enable pure single-frequency tuning while preserving the unidirectional and single-polarization lasing properties.

FIG. 1a is a diagram of one embodiment of a two-piece, frequency-tunable, unidirectional ring resonator constructed in accordance with the teachings of this invention.

FIG. 1b depicts the implementation of the embodiment of FIG. 1a using three external mirrors, where at least one of the mirrors is movable.

FIG. 2a depicts the operation of one embodiment of the structure of FIG. 1 where the first piece is made of non-magneto-optic lasing material, the second piece is made of magneto-optic material, and the magnetic field applied to the second piece only.

FIG. 2b depicts the operation of another embodiment of the structure of FIG. 1 where both pieces are made of magneto-optic material and the magnetic field is applied to both pieces.

FIG. 3 depicts the operation of yet another embodiment of the structure of FIG. 1 where lasing-material piece 1 is made of magneto-optic material and piece 2 is made of an electro-optic material.

FIG. 4 depicts the operation of yet another embodiment of the structure of FIG. 1 where piece 1 is made of magneto-optic material and where the linear-polarized light is converted to circular polarization by the use of a quarter-wavelength plate.

FIGS. 5a and 5b are simplified schematic representations of another embodiment of this invention which generalize the way laser light is coupled out of the tunable unidirectional ring laser and how the sequence of reflection, refraction, and rotation is implemented.

FIGS. 6a and 6b are a simplified schematic representations of another embodiment of this invention which generalize the way pump light is coupled into the cavity.

FIG. 1a is a simplified schematic representation of a first embodiment of a laser constructed in accordance with this invention. In this embodiment, piece 1 comprises lasing material and piece 2 serves as the quasi-planar reflecting prism. Of importance, gap 3 exists between piece 1 and piece 2, with piece 1, piece 2 and gap 3 forming the ring cavity. Surfaces 3-1 and 3-2 adjacent gap 3 are polished and serve as partial polarizers of the light beams passing within the laser cavity. Surface 4 of piece 1 serves as the output mirror of the ring laser. As shown in FIG. 1a, energy from a pump source is applied as a light beam 5 and an output laser beam is provided along line 6. Piece 2 includes three surfaces which change the plane of polarization of the intra-cavity light beam by reflecting this beam at points B, C, and D, out of the plane ABD, as shown in the side view of FIG. la. The applied magnetic field is shown as vector H. Faraday rotation of the plane of polarization is performed inside the magneto-optic lasing material, such as Nd:YAG, by applying a magnetic field as described. The beam is inclined in a Brewster angle to surfaces 3-1, 3-2 facing gap 3, and thus there is minimal loss for the polarization component in plane ABD. Therefore polarization in plane ABD is preferred in the lasing.

Piece 2 is a prism polished to reflect the intra-cavity beam in a quasi-planar path while changing its plane of polarization to compensate for the Faraday rotation which occurs inside piece 1. Piece 2 is attached to an electromechanical transducer (not shown), such as a PZT. When electrical voltage is applied to the electromechanical transducer, piece 2 changes its position relative to piece 1, thus changing the optical path length traversed by the intra-cavity light beam. The pump energy is applied by, for example, a laser diode tuned to the absorption band of the solid-state lasing material via output mirror 4. Output mirror 4 has an antireflecting coating in the pump energy regime and a high reflecting coating in the lasing spectral regime, thereby coupling only a small portion of the intra-cavity light to the outside of the cavity. The multilayer coating can be designed to act as a partial polarizer in order to create lasing in a preferred polarization. The coating can also be designed to introduce a known polarization change to the reflected intra-cavity lasing beam, thus again compensating the Faraday rotation in one direction of rotation, thereby increasing the differential loss between the two counter-rotating beams. In this case surfaces 3-1, 3-2 near gap 3 can be polished at an angle which enables different transmission angles of the beam traversing from piece 1 to piece 2 and back. This is needed when parallel beams are desired to be transferred from piece 1 to piece 2, for example, by way of a retardation plate, which are commercially available. An antireflecting coating applied to surfaces 3-1, 3-2 can also be designed to act as a partial polarizer for either one of the beam polarizations. Both the laser diode and the lasing material are mounted on a thermoelectric cooler, or any other heat stabilizer, to enable adjustment of their emission spectra, as is well known in the art. The curvatures of surface 4 are designed to form the desired transverse-mode shapes inside the laser cavity. If desired, different curvatures can be used for the two axes of symmetry, or a single curvature for both axes can be used for simplicity.

Without losing generality, piece 2 can be mounted to more than a single electro-mechanical transducer to adjust other degrees of freedom, and the movement can also be supplied to piece 1 of the solid-state lasing material by attaching it to one or more electromechanical transducers (such as PZTs) while holding piece 2 constant or, in another form of relative movement, applying mechanical movement to both piece 1 and piece 2.

The output light is coupled out from the cavity either directly through an antireflection coating or a Brewster-mounted window, or coupled to an optical fiber using one or more lenses in the path of output beam 6. If desired, a prism can be used to correct for astigmatism caused by output coupling mirror 4 of the laser. If desired, both the output light of the diode laser and the solid-state laser are monitored by photodiodes (not shown) to provide control signals to allow for closed-loop intensity stabilization.

Without losing generality, the function of the above-described piece 2 in the embodiment of FIG. 1a can be implemented using three tilted mirrors mounted to reflect the light in a ring configuration at points B, C, and D, while one or more of the mirrors are attached to an a electro-mechanical transducer to allow mirror movement and thus enable optical path tuning by external means, as seen in FIG. 1b.

FIGS. 2a and 2b are simplified schematic representations of alternative embodiments of this invention. Since the embodiments of FIGS. 2a and 2b are similar to the embodiment of FIG. 1a, only the main differences from the embodiments of FIG. 1 are described. In the embodiment of FIG. 2a the Faraday rotation of the plane of polarization is performed outside the lasing material of piece 1, i.e. in the quasi-planar retro-prism of piece 2. The prism of piece 2 is made of magneto-optic material (such as Hoya FR-5 glass) transparent in the lasing wavelength of the solid-state laser material. The same electromechanical approach as in the embodiment of FIG. 1a is used in order to tune the optical-path length of the cavity and thus the laser frequency.

FIG. 2b depicts an embodiment where both the lasing material and the retro-prism of piece 2 have magneto-optic properties. In this embodiment, Faraday rotation is applied to the plane of polarization in both piece 1 and piece 2.

FIG. 3 is a simplified schematic representation of another embodiment of this invention. In this embodiment, Faraday rotation occurs in the lasing material of piece 1, where piece 2 is made of an electro-optic material aligned such that applied external voltage changes the index of refraction along the propagation direction of the intra-cavity light beam, thus tuning the effective optical path length seen by the lasing light. In FIG. 3, piece 2 is attached to piece 1 by means of optical cement or can be separated with Brewster-angled planes. Without loosing generality, piece 2 can be constructed from an electro-optic material attached to a magneto-optic material, thus performing the Faraday rotation as well as the path-length tuning outside of the lasing material of piece 1, or combined with a magneto-optic laser material serving as piece 1, as described in the embodiment of FIGS. 2a and 2b. The more detailed descriptions of the embodiments of FIG. 1 are also applicable to this embodiment.

FIG. 4a is a simplified schematic representation of another embodiment of this invention, in which the static Faraday rotation occurs in the lasing material of piece 1. Quarter-wave plate 41 is mounted between piece 1 (the lasing material) and the magneto-optic material of piece 2, which serves to convert the linearly polarized light beam coming from piece 1 to a circularly polarized light beam transferred to piece 2. The applied variable magnetic field changes the phase of the light in piece 2, thus changing the total optical-path length traversed by the lasing beam, thereby creating a frequency change of the laser light. When the light is reflected back from piece 2, towards piece 1 the beam passes once again through the quarter-wave plate 41, and thus becomes linearly polarized. In this way the laser output power is linearly polarized when coupled out of piece 1 at point A, and is circularly polarized when coupled out from piece 2 at point C. In a variation of this embodiment the variable magnetic field is applied to piece 2 together with a static magnetic field that creates constant polarization change that compensates for the Faraday rotation created inside piece 1. The constant polarization change can also be implemented in piece 2 by using the quasi-planar reflector design for piece 2. When non-magneto-optic lasing material is used for piece 1, a combination of constant magnetic field together with a variable magnetic field operating on piece 2 can be used to create both the constant Faraday rotation for the unidirectional operation together with the alternating magnetic field which tunes the lasing frequency. As in the embodiment of FIG. 2, an additional existing or induced either static or dynamic birefringent property in one or more of the pieces can enhance the optical path change thus inducing larger frequency change.

FIG. 5a is a simplified drawing depicting another embodiment of this invention. It depicts certain ways in which the laser light may be coupled out of the tunable unidirectional ring laser. The output coupling techniques depicted in FIG. 5a can be applied to each of the previous embodiments described. In the embodiment of FIG. 5a , surface 7 of piece 2 serves as the output coupling mirror of the ring laser. The light is coupled out from the laser along path 6 from point C. This coupling scheme becomes possible in this invention due to the use of a new non-reciprocity scheme where the new sequence is MIRROR-FARADAY ROTATION-MIRROR-FARADAY ROTATION-MIRROR as shown in FIG. 5a, thus: INTERNAL REFLECTIVE MIRROR B-FARADAY ROTATION-MIRROR C (not internally reflective and serving also as a coupling output)-FARADAY ROTATION-INTERNAL REFLECTIVE MIRROR D. It is important to understand fully the difference in this invention over the prior art in this important point. In this invention the mirror reflections which bend the light beam path out-of-plane occur prior to the Faraday rotations, in contrast with previous art where the Faraday rotation occurs prior to the mirror reflections: FARADAY ROTATION-MIRROR-FARADAY ROTATION-MIRROR-FARADAY ROTATION-MIRROR-FARADAY ROTATION (as described in U.S. Pat. No. 4,578,793).

FIG. 5b is a simplified schematic representation of another embodiment where the out-of-plane rotation is performed by out-of-plane REFRACTIONS in points B and C instead of total internal reflections as described in FIG. 5a. Here the sequence is REFRACTION B-FARADAY ROTATION-MIRROR C-FARADAY ROTATION-REFRACTION D. The improvements of FIGS. 5a and 5b are as follows:

1. The middle mirror need not be a total internal reflection mirror, thus allowing extraction of the laser beam out of the cavity at a smaller angle with reduced astigmatism.

2. Complete cancellation of the Faraday rotations and the reciprocal rotations between the planes of incidence of the middle mirror and the other two mirrors is made possible by the topology which results in improved isolation.

Alternatively, the light can be coupled out of the laser at either points B or D (depending on the direction of rotation of the light beam) by utilizing a partially reflective surface at refraction point B or D, respectively, and a fully reflective mirror at point C (as shown in FIG. 5b). The advantage of causing the output light from the laser to emanate from points B or D is that another pumping source can be added to pump the laser material of piece 1 in the direction designated as path 8, thus increasing the pumping power. Alternatively, the output light can be coupled out by inserting a partially reflecting mirror in space 3 of FIG. 5a, or as shown schematically in FIG. 5b, by polishing the surfaces in a tilted manner so that part of the light is reflected out of the cavity along path 9 in FIG. 5b. By applying appropriate polarization selective coatings, the output coupling mirror can suppress unwanted polarization components circulating in the cavity.

FIGS. 5a and 5b, and also FIG. 4, describe another completely new embodiment of this invention. Here the out-of-plane total internal reflections or the out-of-plane total internal refractions at points B and D are designed with the correct tilt to act as elliptical retarders in the general case. Here we make use of elliptical retarders in conjunction with Faraday non-reciprocal rotators, inversions and reciprocal rotations, and polarizers to generate non-reciprocal frequency shifts and differential losses between the two counter-propagating circulations. Prior-art designs are based on linear polarization which is only effective in generating differential losses. The improvement due to separating the two circulations in frequency is improved suppression of the circulation with the higher losses, by reduction of the stray coupling between the circulations due to the frequency mismatch. The stray coupling between circulations is due to scattering of light in the ring path, and also due to small back-reflections of the output beam back into the laser which feed back into the other circulation. The separation of the two frequencies is accomplished in piece 2 by a constant Faraday rotation, and the frequency tuning is accomplished by an additional alternating magnetic field. Piece 1 can act as an additional Faraday isolator when the lasing material has magneto-optic properties. FIGS. 5a and 5b also show another new embodiment with a structure comprising two parts: (1) The first part (piece 1) generates non-reciprocal differential losses between the two counter-propagating circulations making use of linear polarizers (in between the pieces or surface 3-1), Faraday rotations, and inversion and reciprocal rotations. (2) The second part (piece 2) makes use of circular retarders in conjunction with Faraday non-reciprocal rotators and inversions and reciprocal rotations, to generate non-reciprocal frequency shifts between the two circulations.

In another embodiment of the invention, the differential-loss generating part is the same piece in which the laser-gain action and Faraday isolation occurs, and the second part which generates the frequency shift is either (1) a second single-piece material or (2) a second single-piece material in conjunction with a third piece (a discrete quarter-wave plate) in between (see FIG. 4).

Yet another embodiment makes use of the above-described embodiments which are based on circular or elliptical light propagation, to also achieve frequency tuning by varying the magnetic field to vary the amount of Faraday rotation.

Yet another embodiment makes use of the above described embodiments to achieve all three effects: (1) differential loss between the two lowest loss counter-propagating polarizations states; (2) frequency separation between those polarizations states; and (3) frequency tuning between those polarizations states. This results in a tunable single frequency unidirectional laser.

Yet another embodiment exists in which the ability to tune frequency is obtained by one or a combination of the following: (1) magneto-optic tuning (applying variable magnetic field); (2) opto-mechanical tuning (inducing relative displacement between the two pieces); (3) elasto-optic tuning applied to some or all of the individual pieces (changing the refractive index by the applied pressure; and (4) electro optic tuning (applying variable electric field to change the optical path length inside the cavity).

Novel aspects of this invention include the following:

1. Separation of laser gain and non-reciprocal isolation in different pieces.

2. Use of linear polarizers at the interfaces between the pieces to act as a differential polarization loss elements:

2.1 Brewster polarizers.

2.2 Coating polarizers, which discriminate between the two polarizations upon tilted incidence.

3. The combination of reciprocal rotation and inversion (of the polarization state with respect to a transverse axis) which is essential for the non-reciprocal operation is achieved by:

3.1 At least two out of plane REFRACTIVE bends, and at least one reflective bounce.

3.2 At least three REFLECTIVE out of plane bounces.

4. The combination of reciprocal rotation and inversion as described in both cases in 3, above, makes use of a topology in which the optical path follows the new sequence REFRACTION-FARADAY ROTATION-MIRROR-FARADAY ROTATION-REFRACTION (for the case 3.1) or MIRROR-FARADAY ROTATION-MIRROR-FARADAY ROTATION-MIRROR (for the case 3.2) in which the mirror bounces which bend the ray path occur prior to the Faraday rotations, in contrast with prior art where the Faraday rotation occurs prior to the mirror bounces: (FARADAY ROTATION-MIRROR-FARADAY ROTATION-MIRROR-FARADAY ROTATION-MIRROR-FARADAY ROTATION (in U.S. Pat. No. 4,578,793).

Some of the improvements made available by the invention are:

1. The middle mirror need not be a total internal reflective mirror, thus allowing extraction of the laser beam out of the cavity at a smaller angle with reduced astigmatism.

2. Complete cancellation of the Faraday rotations and the reciprocal rotations between the planes of incidence of the middle mirror and the other two mirrors is made possible by the topology which results in improved isolation.

3. Use of elliptical retarders in conjunction with Faraday non-reciprocal rotators and inversions and reciprocal rotations, and polarizers to generate non-reciprocal frequency shifts and differential losses between the two counter-propagating circulations. Prior-art designs are based on linear polarization which is only effective in generating differential losses. The improvement due to separating the two circulations in frequency is improved suppression of the circulation with higher losses, by reduction of the stray coupling between the circulations due to the frequency mismatch. The stray coupling between circulations is due to scattering of light in the ring path, and also due to small back-reflections of the output beam back into the laser which feed back into the other circulation.

4. Embodiment of some of the elements mentioned in 5, above, in particular the elliptical retarders as total internal reflections in a single piece out of the two or more pieces described above.

5. A structure comprising two parts:

(a) One part generates non-reciprocal differential losses between the two counter-propagating circulations making use of linear polarizers, Faraday rotations, and inversion and reciprocal rotations.

(b) The second part makes use of circular retarders in conjunction with Faraday non-reciprocal rotators and inversions and reciprocal rotations, to generate non-reciprocal frequency shifts between the two circulations.

6. An embodiment of the differential-loss generating part of as one piece where the laser gain action occurs, and the second part which generates the frequency shift is either a second single piece material or a second single piece material in conjunction with a third piece in between: a discrete quarter-wave plate.

7. Making use of the embodiments 5,6,7,8 which are based on circular or elliptical light propagation, to achieve frequency tuning by varying the magnetic field to vary the amount of Faraday rotation.

8. Use of embodiments 5,6,7,8 to achieve all three effects:

(a) Differential loss between the two lowest loss counter-propagating polarizations states.

(b) Frequency separation between those polarizations states.

(c) Frequency tuning between those polarizations states. This results in a tunable single frequency unidirectional laser.

9. Tunability (of frequency) is obtained by either or a combination of:

(a) Magneto-optic tuning (applying variable magnetic field)

(b) Opto-mechanical tuning (inducing relative displacement between the two pieces).

(c) Elasto-optic tuning applied to some or all of the individual pieces

FIG. 6a describes another embodiment of this invention which generalizes the way pumped light can be applied to the lasing material of piece 1. In FIG. 6a high-power, broad-area laser diode or laser-diode arrays 9 are utilized to pump the lasing material of piece 1 from its side. This enables greater pump power to be coupled into the laser cavity, either by pulse operation or CW. When the unidirectional ring laser operates continuously in a single-frequency mode due to the end pumping, this existing frequency will force the laser to continue to oscillate in a single-frequency mode when higher pump power is applied from its side. This is similar to an injection locked phenomenon known in the prior art.

FIG. 6b shows another embodiment where larger power can couple to the ring laser by using the zig-zag laser configuration in conjunction with the ring configuration to create high power diode pumped tunable unidirectional single mode ring solid state laser. The advantage of this configuration is that at each reflection a pumped light from laser diode or laser diode array can be matched with the intra-cavity beam, this give high efficiency of pumping. In the lasing material the light travels in a zig-zag path and is coupled to the second piece which functions according to the different combinations of the above described embodiments.

In any of the above-described alternative embodiments, instead of only a static magnetic field a combination of static and dynamic magnetic fields are applied in order to operate the magneto-optic material in the unsaturated magneto-optic regime. In this way, a small variation of the plane of polarization of the lasing beam is created, thereby inducing a small optical path length change due to existing birefringent properties of either the lasing material of piece 1 or piece 2, or any additional piece inserted in the cavity. The above birefringent properties of either piece 1, piece 2, or any additional piece can also be induced in a material by pressing either one of the pieces or both constantly or alternating. The more detailed descriptions of FIG. 1a are also applicable to this embodiment.

All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be practiced within the scope of the appended claim.