WO2004068191A2 - Apparatus and method for use in modulating a laser in phase and amplitude - Google Patents

Abstract

The present embodiments provide apparatuses and methods for use in optically communicating information and/or data, including modulating optical signals by applying a first voltage across a first birefringent optical element (410), applying a second voltage across a second birefringent optical element (412), and modulating a frequency of the optical signal by adjusting at least one of the first and second voltage potentials and producing a change in a sum voltage (V+) that is proportional to the sum of the first and second voltage potentials. The present embodiments can further modulate an amplitude of the optical signal by adjusting at least one of the first and second voltage potentials and producing a change in a voltage difference (V-) that is proportional to the difference between the first and second voltage potentials as the optical signal passes through the first and second elements.

Description

APPARATUS AND METHOD FOR USE IN MODULATING A LASER IN PHASE AND AMPLITUDE

TECHNICAL FIELD OF THE INVENTION

The present embodiments generally relate to data transfer over wired, wireless, and/or optical transmission channels. More particularly, this invention relates to the generation of modulated optical signals for the communication of information and/or data.

BACKGROUND OF THE INVENTION

The amount of information being communicated between entities, coiporations, companies and individuals has dramatically increased. Systems are continually upgraded or replaced in order to try and meet the demands of the amount of data to be communicated.

Many hard wired systems are being replaced by optical systems because optical communication provide greater data rates with greater reliable.

Optical systems can be costly to implement because they often require more components and/or more complex components in order to generate and communicate data. For example, a single driver of a hard wired system many require multiple optical drivers in order to generate the same communication signals. As such, the implementation and maintenance of optical systems can be prohibitively expensive.

SUMMARY OF THE INVENTION The present embodiments provide apparatuses, systems and methods for use in optically communicating information and/or data. In some embodiments, an apparatus is provided that includes first and second birefringent elements. The first birefringent element is positioned along a laser axis, where the first birefringent element comprises birefringent material that has first, second and third axes with the first axis being substantially parallel with a laser axis, and with first birefringent element having a length defined along the first axis and a thickness defined along the third axis. The second birefringent element is further positioned in optical alignment with the first birefringent element, the second birefringent element comprising birefringent material having first, second and third axes with the first axis substantially parallel with the laser axis, and the second axis being substantially parallel with the third axis of the first birefringent element and the third axis of the second birefringent element being substantially parallel with the second axis of the first birefringent element, with the second birefringent element having a length defined along the first axis and a thickness defined along the third axis. Further, the apparatus can be configured such that a length-to-thickness ratio of the first birefringent element is approximately equal a length-to- thickness ratio of the second birefringent element. In some implementations, the apparatus can further include a first electric field defined across the first birefringent element and generally along the third axis having a first voltage differential, and a second electric field defined across the second birefringent element along the third axis having a second voltage differential such that a voltage difference that is proportional to the difference between the first and second voltage differentials defines an optical carrier wavelength. A sum voltage that is proportional to the sum of the first and second voltage differentials defines a resonant axial mode wavelength such that the resonant axial mode wavelength overlaps with the carrier wavelength.

Some embodiments further provide apparatuses for use in generating an optical communication signal. These apparatuses can include means for generating a first electric field across a first means for providing birefringence of an optical carrier signal, means for generating a second electric field across a second means for providing birefringence of the optical carrier signal, means for defining an optical carrier wavelength that is proportional to the first and second electric fields, and means for defining a resonator axial mode wavelength that is proportional to the first and second electric fields such that the resonator axial mode wavelength overlaps with the optical carrier wavelength. The apparatuses can further include means for defining a voltage difference that is proportional to a difference between a first voltage potential associated with the first electric field and a second voltage potential associated with the second electric field, and means for amplitude modulating the optical carrier signal comprising means for modulating the voltage difference. Additionally, the apparatus can include means the for defining a sum voltage that is proportional to a sum of a first voltage potential associated with the first electric field and a second voltage potential associated with the second electric field and can further include means for phase modulating an optical carrier signal comprising means for adjusting the sum voltage. Some implementations of some embodiments provide a laser or laser source that comprise a gain medium positioned along a laser axis, a polarizer positioned along the laser axis and defining a direction of polarization, a first birefringent element and a second birefringent element. The first birefringent element is optically aligned with the laser axis and comprises birefringent material arranged with two of its differing dielectric axes offset from the direction of polarization. The first birefringent element has a length and a thickness. The second birefringent element is positioned optically aligned with the laser axis, where the second birefringent element comprises birefringent material arranged with two of its differing dielectric axes offset from the direction of polarization and oppositely aligned with the two differing dielectric axes of the first birefringent element. The second birefringent element has a length and a thickness such that a length-to-thickness ratio of the first birefringent element approximately equals a length-to-thickness ratio of the second birefringent element. The laser can further include, in some embodiments, a first modulator source coupled with the first birefringent element, where the first modulator controls a first voltage applied across the first birefringent element, and a second modulator source coupled with the second birefringent element, where the second modulator controls a second voltage applied across the second birefringent element, and a difference voltage proportional to the difference between the first and second voltages defined an optical carrier wavelength. Additionally, a sum voltage proportional to the sum. of the first and second voltages can be employed to define a resonator axial mode wavelength that overlaps with the carrier wavelength.

In some embodiments a method of generating an optical communication signal is provided, where the method includes generating a first electric field across a first birefringent element, generating a second electric field across a second birefringent element, defining an optical carrier wavelength that is proportional to the first and second electric fields, and defining a resonator axial mode wavelength that is proportional to the first and second electric fields such that the resonator axial mode wavelength overlaps with the optical carrier wavelength. These methods can further define a voltage difference that is proportional to a difference between a first voltage potential associated with the first electric field and a second voltage potential associated with the second electric field, and amplitude modulate an optical carrier signal comprising modulating the voltage difference. Still fiirther, some embodiments define a sum voltage that is proportional to a sum of a first voltage potential associated with the first electric field and a second voltage potential associated with the second electric field, and provide phase modulation of an optical carrier signal comprising adjusting the sum voltage. An alternative method can apply a first voltage across a first optical element, apply a second voltage across a second optical element, and modulate a frequency of an optical signal including adjusting at least one of the first and second voltage potentials and producing a change in a sum voltage that is proportional to the sum of the first and second voltage potentials as the optical signal passes through the first and second birefringent materials. Some implementations further modulate an amplitude of the optical signal including, adjusting at least one of the first and second voltage potentials and producing a change in a voltage difference that is proportional to the difference between the first and second voltage potentials as the optical signal passes through the first and second birefringent materials. Further, the first optical element can- be a first birefringent material and the second optical element can be a second birefringent material.

A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:

FIG. 1 depicts a simplified block diagram of an optical communication system that provides the mixing of digital and analog signals onto one or more optical links;

FIG. 2 depicts a simplified block diagram of a communication system according to some present embodiments; FIG. 3 depicts a simplified graphical representation of reference points in the field space for an N-level quadrature modulation (N-QAM);

FIG. 4 depicts a simplified block diagram of an optical source according to some present embodiments;

FIG, 5 depicts a simplified cross-sectional view of a birefringent element as may be utilized in the source of FIG. 4;

FIG. 6 depicts a simplified perspective view of a birefringent filter (BRF) according to some embodiments that can be utilized within the source of FIG. 4; FIG. 7 is a simplified cross-sectional view of a first birefringent element that can be implemented in the BRF of FIG. 6 showing the crystal orientation and an applied voltage according to some embodiments; and

FIG. 8 is a simplified cross-sectional view of a second birefringent element that can be implemented in the BRF of FIG. 6 showing the crystal orientation and an applied voltage according to some embodiments.

Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present embodiments provide apparatuses, systems and methods for use in telecommunications, optical networks, wireless communications, and other such applications. Preferred embodiments utilize one or more electro-optic birefringent filtering within an optical resonator. The present filtering technology can be utilized to simultaneously and independently control the optical carrier wavelength, the frequency and/or the modulation depth of an impressed FM signal, and the modulation amplitude of the output coupled laser intensity, thereby providing a modulated communication source. This source can provide both frequency and amplitude modulation with a high degree of linearity and efficiency with AM/FM orthogonality such that coupling (cross-talk) between the two modes of modulation is minimized or substantially prevented.

In some implementations, the birefringent filtering is implemented within a compact fiber-coupled optical resonator of a diode-pumped solid state laser. In some embodiments other types of lasers may be utilized. A birefringent filter (BRF) can be connected with FM and/or AM modulation control, which provides appropriate FM- and/or AM-modulated control signals to the BRF.

In many communication applications it can be beneficial to transmit mixed signals (i.e., both digital and analog) along a common communications link. For example, this mixed signal communication can be of paramount importance in military communication systems, while providing improved communication capabilities for substantially any communication system and other modulating systems. Within the context of optical links, the mixing of signals can be accomplished by taking advantage of the wavelength division multiplexing capabilities of optical links, with digital and analog signals carried over separate channels (wavelengths).

FIG. 1 depicts a simplified block diagram of an optical communication system 120 that provides the mixing of digital and analog signals onto one or more optical links 122. Digital and analog coding of information 124 and 126, respectively, are supplied to separate sources 130, 132. The sources can be implemented, for example, with a laser diode source. The digital data 124 drives a modulator or directly drives an optical source 130 to generate a first optical signal 140. The analog data 126 is mixed onto a high-frequency RF carrier 134 with the resulting signal either driving an external modulator or driving the optical source 132 directly to provide a second optical signal 142. These two signals are multiplexed, for example through wavelength division multiplexer 144, and forwarded to the one or more links 122.

At a receiver end, a demultiplexer 146 receives the mixed optical signal and demultiplexes the signals in the wavelength domain. Separate electronic channels 150, 152 are typically used to demodulate the received data streams 154, 156. An RF down-converter 160 is utilized on the analog signal 156. Previous systems were unable to provide for the simultaneous modulation of both amplitude and frequency with diode laser sources. These limitations of the drivers are part of the reasons for the separate driving sources 130, 132 which require the multiplexing and demultiplexing onto the link 122.

Alternatively, present embodiments provide sources that are capable of both AM and FM. modulation operation, each in either a digital or an analog mode. For example, optical sources of the present embodiments can generate digital signals modulated with an FM mode while, at the same time, generate analog signals modulated with an AM mode on to the same channel or link. Furthermore, the AM signals can be carried directly at the optical level, i.e., without resort to a radio frequency (RF) subcarrier. Thus, large bandwidth baseband modulation is possible, a bandwidth which, in principle, is limited only by the electrical characteristics of the internal modulator. Additionally, demodulation of the frequency-modulated optical signal can be accomplished in a straightforward fashion by way of an optical discriminator. Some preferred embodiments provide laser sources that can accommodate frequency, amplitude or both frequency and amplitude modulation. These sources preferably combine a high degree of linearity and efficiency with AM/FM orthogonality, such that there is little or no coupling (cross-talk) between the two modes of modulation. Together with low phase noise, the AM/FM orthogonality is typically paramount in applications that require the generation of arbitrary, independent amplitude and phase waveforms.

FIG. 2 depicts a simplified block diagram of a communication system 210 according to some present embodiments. The system employs a single optical source 212, such as a single laser source, that can be driven by one or both analog input signals 214 and/or digital input signals 216. The source modulates the optical source signal to provide one or both amplitude modulation and/or frequency modulation depending on the received data signals 214, 216 to generate a single output signal onto one or more links 220, A splitter 222 can be utilized as a received end to different from the amplitude modulated and frequency modulated signals for the retrieval of the desired data 214, 216. The source 212 according to present embodiments provides for a high degree of orthogonality between the amplitude modulated signal and the frequency/phase modulated signal, which provides a degree of flexibility and performance previously unavailable in communication links. For example, the communication source is capable of both AM and FM operation, each either in a digital or an analog mode. In some implementations, a mixed signal including both digital and analog communications can be transmitted along a single optical link. Furthermore, the FM signals can be carried directly at the optical level, and without the need to resort to an RF subcarrier. The modulation process can translate directly into the optical domain, for example, in one implementation two quadrature components may be encoded as parallel data streams that drive the AM and FM ports of the transmitter. Similarly, the source 212 can, in some embodiments, employ RF analog modulation techniques, which in some instance may be particularly advantageous for future generation systems. For example, a source can be utilized in a subcarrier multiplexed transmission system, in which the source 212 provides multi-level modulation formats, such as quadrature- amplitude modulation (QAM) due to the increased spectral efficiency that can be achieved. In a QAM link, the value of a given data bit can be encoded by modulating the field in both amplitude and phase such that points representing transmitted symbols form a rectangular lattice in phase space. In such a modulation scheme, the two orthogonal axes represent the in-phase and out-of-phase components of the electric field; i.e., E = ej + ie₂. FIG. 3 depicts a simplified graphical representation of reference points in the field space for an N-level quadrature modulation (N-QAM). The axes represent the components ei and e₂ of the electric field, with annular reference points defined about the component axes. By taking advantage of the ability to encode amplitude and frequency/phase independently, the present source technology translates the modulation process directly into the optical domain with two quadrature components being encoded as parallel data streams that drive the AM and FM ports of a transmitter.

The present embodiments can provide AM and FM modulation for numerous optical signals generated from any number of devices, such as lasers. In some embodiments, diode-pumped solid-state (DPSS) laser configurations can be utilized in cooperation with intracavity electro-optic components as a means for providing various phase and amplitude modulation functions. A DPSS laser can be rapidly tuned across a spectral range (e.g., 1.5 μm regime), which can cover a desired band, such as a 40 nm band typically referred to as the erbium C-band (1.53-1.57 μm). Alternatively, other lasers can be utilized. The present embodiments preferably satisfy a plurality of conditions in implementing intracavity optical wavelength selection together with intracavity frequency modulation (IFM) and/or intracavity amplitude modulation of a single-mode solid-state laser oscillator. These conditions are relevant in attempting to achieve high fidelity performance in the absence of transient effects and/or strong coupling to the laser gain material, which in turn can produce strong amplitude and phase distortion in the FM/AM response, as well as undesirable FM/AM cross-talk. Further, some of these constraints can be particularly stringent if the desired modulation frequency range is to include a region where the candidate DPSS laser exhibits a classic relaxation oscillation (RO) response to an. impressed variation from the laser steady state (for example, some lasers can typically exhibit an RO resonance at a few hundred kilohertz). One condition that some embodiments attempt to ensure is that the selected intracavity frequency modulation mechanism applies a deterministic Doppler shift to a laser flux, such as a circulating laser flux, during each resonator pass. Some embodiments further attempt to ensure that the intracavity frequency modulation mechanism also shifts the cavity resonant frequency at substantially the same rate, and preferably precisely the same rate at which the circulating flux is Doppler shifted. An additional condition that some embodiments try and satisfy is that the intracavity frequency modulation mechanism does not introduce a significant amount of uncompensated loss-rate modulation (which can relate to FM-AM coupling).

The intracavity frequency modulation mechanism can in some embodiment be implemented through the physical motion, of a flat resonator end mirror in a direction normal to the mirror surface. Through this movement, it is apparent that the first two above identified conditions (applying a deterministic Doppler shift to a laser flux, and shifting the cavity resonant frequency at the same rate at which the circulating flux is Doppler shifted) can, in principle, be satisfied. In this implementation, a stable resonator mode waist may occur at the mirror surface and a mode k-vector remains normal to the mirror surface. When the mirror is moving with a normal linear velocity, the Doppler shift per reflection is substantially equal to the resonance frequency shift due to the cavity length change which occurs in a round-trip time. Consequently, in the limit, where the FM excursion within the effective atomic gain linewidth is negligible, the intracavity circulating flux remains resonant in the optical cavity and the gain material is not significantly perturbed by the IFM process. Similar arguments can be made for an intracavity index modulator which changes the optical cavity length by adjusting the average index of refraction of the resonator medium. For either case, in the absence of FM-AM coupling, high-fidelity IFM can be achieved, even in the frequency regime of the RO resonance. In typical applications intracavity frequency modulation mechanism introduce a finite amount of FM-AM coupling. This FM-AM coupling can in turn causing feedback which can produce FM distortion. For example, a mechanism that produces frequency modulation by means of optical cavity length modulation typically also produces a small amount of cavity loss-rate modulation due to that modulation. In the example of the moving cavity-end-mirror, the optical cavity loss rate is inversely proportional to the resonator cavity length, and as the length is made smaller the loss rate is increased. Further, the minimum loss rate modulation α_mm(t) is typically related to the frequency modulation Ω(t) by the coupling factor α₀/ω₀, where α₀ is the steady-state resonator loss rate and ω₀ is the optical angular frequency. For laser configurations utilized in some embodiments, this minimum FM-AM coupling factor has a magnitude of about 1 part in 10 million. However, even this small amount of coupling is sufficient in some implementation, if uncompensated, to produce RO resonant feedback FM distortion of a few decibels. If, in addition to this minimum FM-AM coupling, additional coupling occurs due to some unintended aspect of component or resonator design, and if this "collateral" coupling is large compared to the coupling factor ₀/ω₀ then significant FM distortion can occur. Therefore, in some embodiments optimum laser and intracavity frequency modulation mechanisms reduce and preferably minimize both the amount of collateral FM-AM coupling and the strength of the laser RO resonant FM response to residual FM-AM coupling. Further, laser RO resonance coupling investigations are often limited to power and gain medium inversion responses with respect to dissipative (gain) fluctuations, where such fluctuations can be introduced by various means. An investigation of this type is accomplished through an analysis which makes use of the classic "rate equations" in which the variables involve laser cavity photon density and laser gain medium population densities. In using this approach, information regarding the field, phase, frequency, and level population responses to reactive (index) fluctuations is usually lost. To preserve this information a description which is provided by the original laser field equations can be utilized. A solution of the field equations in the intracavity frequency modulation regime, using perturbation theory, involves an analytic procedure which can include: (1) solution of the equations under steady-state conditions, (2) linearization of the equations for small variations about the steady state, (3) introduction of appropriate time-dependent driving terms, (4) Fourier transformation of the variables and laser equations, (5) solution of five coupled equations for five variable response functions, and (6), if desired, transformation of the solutions back to the time domain. fn using field equation stability analysis for designing and characterizing high- fidelity frequency modulated lasers, it can be important to include substantially every significant form of FM-AM coupling that can be introduced by the selected intracavity frequency modulation mechanism. For example, it can be important to include coupling of the optical cavity length variation to the "filling factor", which is a measure of the overlap between the cavity mode field and the excited gain medium. As an illustration of the importance of including higher order collateral effects introduced by the intracavity frequency modulation mechanism, the combination of both the filling factor coupling and the minimum loss rate coupling, α_mm(t), described previously, leads to a substantially fully compensated laser response, whereas the inclusion of only one of these effects produces a laser response which can be significantly distorted near the RO resonance.

Further, once the laser equation parameters associated with optical length modulation, which are implicitly dependent upon the optical cavity length, have been modified to include the optical length modulation driving terms, it is beneficial to further consider higher order aspects of the laser or modulator designs that could lead (either directly or indirectly) to the collateral introduction of additional cavity loss-rate driving terms. For example, the mode waist at the gain medium decreases slightly as the cavity length is decreased, thereby slightly reducing the extent of the mode Field tail which extends into the outer regions of the active medium and which is less vigorously pumped. Other losses can also adversely affect FM-AM coupling effects. For example, a second class of higher order collateral FM-AM loss coupling effects can occur with a laser resonator according to some present embodiments that includes one or more birefringent elements. In these embodiments resulting birefringent resonator axial mode (x,y) resonances are split by the presence of the birefringent element. This split can leads to an off-resonance oscillation frequency condition that introduces small frequency-dependent optical losses. The inventors have determined, however, that such effects typically are comparatively small. As such, in many embodiments the dominant collateral driving teπns that can compromise the fidelity and quality of a modulated optical signal are those that arise from FM-AM coupling through the cavity loss rate α.

Alternatively and/or additionally, some present embodiments provide laser frequency modulation through an intracavity frequency modulation mechanism that varies an average index of refraction of the cavity medium such that a driving term is introduced into the laser description equations that is a small time-dependent variation of the reactive part of a polarization. An intracavity phase modulator, however, also can cause other ostensibly steady state constants (such as the cavity resonance frequency) to become time varying, thereby introducing collateral FM-AM coupling into the cavity loss rate.

Some implementations of present embodiments advantageously introduce a desired result into appropriate steady-state equations to provide a means for including an intracavity frequency modulation driving term into the variables of the laser equations. Further, the solution of such equations can be utilized to identify corresponding self-consistent variations of the laser field variables. Following this approach, the solution of the laser equations yields the time dependent forms of the variables which are self consistent to produce the desired frequency behavior. Utilizing the above noted method several laser configurations can be determined.

Further, through the implementation of the method, high-fidelity FM performance can be achieved through careful laser/ΣFM mechanism design and control, and in particular, in the absence of collateral FM-AM coupling. For example, a desired laser resonator mode frequency can be precisely adjusted relative to the effective laser medium gain peak frequency. This adjustment can further be performed in a manner that produces a controllable off-resonance oscillation condition where ω_o<ω_o<ω_a. At the RO resonance angular frequency ω_ro the analytic solution for the normalized real part of the polarization response to the FM drive can be defined according to:

where Δω_a is the full width at half maximum transmission effective gain linewidth and ε_c is the axial cavity mode frequency pulling (ω₀ - ω_c). The first term is the value of the dc response. Therefore, if δ is equal to zero (signifying no collateral FM-AM coupling), then, when the oscillation frequency is on-resonance (ε_c = 0), the RO resonant reactive response is equal to the dc value. Therefore, substantially perfect FM fidelity is predicted. Additionally, some embodiments can include a control loop according to this model solution that can be implemented to compensate for residual collateral FM-AM coupling, where such compensation may be achieved by controlling the axial cavity mode frequency pulling. As described above, it has been shown that through appropriate selection of cavity parameters and/or mode-selecting elements, phase and amplitude modulation may be achieved in substantially a completely orthogonal fashion (without cross-talk).

Furthermore, the present embodiments, for example embodiments employing a laser source (e.g., co-doped Yb,Er DPSS laser) in cooperation with intracavity tuning elements, allow for both amplitude and frequency modulation, providing substantial advantages relative to alternate technologies. Some embodiments provide a source that can be used in extremely broadband applications, including the generation of optical arbitrary waveforms, mixed signal transmission, and optical QAM, all of which typically need a high degree of linearity and low noise. The high degree of FM fidelity that may be achieved through present embodiments employing well-designed DPSS lasers are fundamentally not typically achievable with directly modulated semiconductor lasers because strong coupling between the gain (drive current) and the medium index of a semiconductor laser insures that there also is strong AM- to-FM and FM-to-AM coupling. This AM/FM coupling leads to a highly non-uniform FM response with respect to drive frequency. Several publications provide support for this conclusion in that the measurements of FM response relative to various semiconductor lasers (see for example: S. Kobayashi, Y. Yama oto, M. Ito, and T. Kimura, "Direct frequency modulation in AIGaAs semiconductor lasers," IEEE J. Quantum Electron, vol. QE-18, no. 4, pp. 582-595, Apr. 1982; and D. Welford and S. B. Alexander, "Magnitude and phase characteristics of frequency modulation in directly modulated GaAlAs semiconductor diode lasers," J. Lightwave Technol., vol. LT-3, no. 5, pp. 1092-99, Oct. 1985). Additionally, two phenomena can contribute to a semiconductor laser FM response: a thermal effect, which typically dominates at low frequencies (<100 kHz); and a carrier density effect which typically dominates at higher frequencies. These two effects have different values of frequency shift versus modulation current. Moreover, the two effects tend to be out of phase with one another. The net result of these effects causes both a dip in magnitude and a 180° phase shift in the FM response. These effects typically occur beyond the thermal cutoff frequency, usually in the tens of kilohertz regime.

FIG. 4 depicts a simplified block diagram of an optical source 402 according to some present embodiments. This source is configured according to the above discussion and defined criteria to provide amplitude modulation (AM) and frequency modulation (FM), In some embodiments, the source 402 can simultaneously and independently control the optical carrier wavelength, the frequency and/or the modulation depth of an impressed FM signal, and the modulation amplitude of the output coupled laser intensity. Further, preferred embodiments provide both frequency and amplitude modulation with a high degree of linearity and efficiency with AM/FM orthogonality, such that the source exhibits little or no coupling (cross-talk) between the two modes of modulation.

The optical source 402 includes a laser resonator within a laser cavity 404 that defines a laser axis 416. A birefringent filter (BRF) 406 is maintained within the cavity. The BRF comprises first and second birefringent elements 410 and 412, respectively, and a polarization-selective element or polarizer 414. In some preferred embodiments, the first and second birefringent elements 410 and 412 comprise and/or are constructed as etalons with opposing surfaces that are substantially parallel, for example, the parallelism can be of about at least 0.5 milliradians, and in some preferred embodiments about at least 0.2 milliradians. Further, the etalon surfaces in some embodiments are coated with a reflective coating, or may be left uncoated to provide a finite reflectivity at the laser emission. Unlike conventional birefringent filters, the etalon surfaces in some embodiments are typically not coaled for anti- reflection at the laser wavelength. The thickness of the birefringent elements can be selected depending on design considerations such as the width of the passband desired. It may be advantageous to match the width of the passband to approximately coincident with the gain- bandwidth curve of a broadband laser medium. For example, to provide a passband of about 40 nm centered at about 1550 nm to match the gain-bandwidth curve of Er,Yb:glass, an etalon of lithium niobate (LN) could have a thickness of about 400 microns. Other thicknesses, however, can be utilized depending on implementation. For example, the thickness of some birefringent elements for some embodiments can be about 25 to 1000 microns, however, other thicknesses can be utilized.

Still referring to FIG. 4, the polarizer 414 can additionally be fabricated as an etalon in some preferred embodiments with substantially parallel opposing surfaces, with parallelism for example of about 0.2 milliradians. The polarizer can be configured through substantially any relevant material, such as glass, a Brewster plate (e.g., an optical material such as glass oriented at Brewster's angle), a POLARCOR, which is available from Corning, Inc. of Corning, NY., and other such relevant materials.

The first and second birefringent elements 410, 412 and the polarizer 414 are aligned along a laser axis that extends through the cavity 404. The etalon surfaces of the birefringent elements 410, 412 and/or polarizer 414 can be coated with a reflective coating, or may be left uncoated to provide a finite reflectivity at the laser emission. In some embodiments, unlike conventional birefringent filters, the surfaces are not coated for anti- reflection at the laser wavelength. In some embodiments, the source further includes a broadband gain medium or block 420 within the resonator. The gain medium can be a solid state laser material that is pumped by a suitable optical source such as a laser diode 422. In some preferred implementations, the gain material has a broad emission spectrum. For example, the broadband gain medium 420 can comprises glass co-doped with ytterbium (Yb³⁺) and erbium (Er³⁺) referenced as Er,Yb:glass. The gain medium in some preferred embodiments is also fabricated as an etalon with surface parallelism of opposing surfaces similar to those of the birefringent elements 410, 412 and/or the polarizer 414. Similarly, the etalon surfaces of the gain medium 420 can be coated with a reflective coating, or may be left uncoated to provide a finite reflectivity at the laser emission (e.g., highly reflective (HR) coatings and/or anti- reflective (AR) coatings). The pump radiation from the laser diode 422 is coupled to the gain medium by any suitable optical system. In the embodiment shown in FIG. 4, the laser diode radiation is focused from the laser diode 422 into the gain medium 420 through a ball lens 424.

The source or laser cavity 404 is typically defined between the gain medium 420 and the polarizer 414. More specifically between a first external surfaces or face 430 of the gain medium and a external end face or surface 432 of the polarizer 414. These external surfaces 430, 432 can be reflective and/or include a reflective coating such that the laser resonator is defined between surfaces, hi some embodiments, the first surface 430 of the gain medium 430 can include a high reflective (HR) coating, and the first surface 432 of the polarizer 414 can include an HR-coating. The polarizer end surface 432 is coated in some embodiments so as to provide an output coupling of about ~1 percent. The polarizer can be formed from substantially any relevant material such as glass and other such material. For example, the polarizer can be formed from erbium-doped-glass, where a magnitude of coupling is near-optimum for uniform laser perfonnance across the telecommunications C- band. hi some alternative embodiments, separate mirrors or other reflective elements or surfaces can be positioned proximate the edge surfaces 430, 432 to provide the desired reflection.

In preferred embodiments, electric fields are applied to both, the first and second birefringent elements 410, 412 while propagating the laser through the elements. The apphed voltage across birefringent elements allows the laser output to be quickly tuned over a large spectral range. The source 402 can include first and second modulator controllers 440, 442. The modulator controllers can include substantially any suitable drivers, electronics, sensors, and other devices and components that may be beneficial in controlling the wavelength and modulation. The first modulator 440 applies an electric field defined by a first voltage potential across the first birefringent element 410. Similarly, the second modulator 442 applies an electric field defined by a second voltage potential across the second birefringent element 412. In some embodiments, the first and second modulator controllers can be incorporated and/or implemented in a single device. In some alternative embodiments, a single controller or single modulator is utilized to adjust the electric field and/or voltage potential across both the first and second birefringent elements 410 and 412.

The present embodiments provide for the timing of the laser to achieve the desired amplitude and/or frequency (phase) modulation. The laser tunability is obtained through the BRF that comprises the first and second birefringent elements 410, 412 and the polarizer 414. In some embodiments the birefringent elements are two lithium niobate (LN) crystals and the polarizer 414 can be a linear polarizer. The birefringent elements can be formed of other materials providing the desired birefringent effects such as yttrium orthovanadate (YNO ), crystal quart, mica, lithium borate (LBO), barium borate (BBO), potassium niobate (KΝbO₃), potassium titanyl phosphate (KTP), and other similar materials. Some other factors that may be considered in selecting a particular material (and/or determining a thickness) are the material quality, the material birefringence, the temperature dependence of the indices of refraction and thermal expansion, spectral bandwidth of the gain medium, longitudinal mode spacing of the laser cavity, and loss that may resulting in the suppression of adjacent longitudinal modes. To achieve accurate and/or proper operation of the BRF the two birefringent elements 410, 412 are oriented such that light and/or laser propagation is along a first axis, such as along a-axes. FIG. 5 depicts a simplified cross-sectional view of a birefringent element, such as the first element 410. Illustrated in FIG. 5 is the relationship between a preferred polarization 510 and the dielectric axes of the birefringent material. The preferred polarization 510 is determined by the polarizer 414 (see FIG. 4). The birefringent element 410 defines three dielectric axes, including an a-axis, a b-axis, and a c-axis. In some embodiments described herein, the birefringent element 410 comprises a uniaxial birefringent crystal (e.g. LN) that has two ordinary axes (the a- and b-axes) and an extraordinary axis (the c-axis). As is known, the two ordinary axes can have a similar index of refraction, while the extraordinary axis has a different index of refraction. In other embodiments, another type of crystal (e.g. a biaxial crystal) or another type of birefringent material may be used to provide birefringent properties.

In one embodiment, the a-axis (e.g. one of the ordinary axes) of the birefringent element is aligned with the cavity axis 416. The b-axis (e.g. the other ordinary axis), shown at 514 and the c-axis (e.g. the extraordinary axis) shown at 516, which has a differing index from the b-axis, are at right angles to each other, and oriented such that the plane containing both of these axes is aligned substantially peφendicular with the cavity axis. However, in some embodiments, the b- and c- axes of the birefringent element do not have to be precisely peφendicular to the cavity axis. The birefringent element is configured so that the b- and c- axes 514 and 516 of the birefringent element form an approximately 45° angle with respect to the preferred polarization axis 510 as defined by the polarizer 414.

Further, the two birefringent elements 410 and 412 are aligned with their a-axes parallel and generally coaxial with the laser axis 416 (see FIG. 4). FIG. 6 depicts a simplified perspective view of a BRF 406 according to some embodiments that can be utilized within the source 402 of FIG. 4, where the BRF can include two birefringent elements whose extraordinary and ordinary axes are oppositely aligned. Referring to FIGS. 4 and 6, the first birefringent element 410 can be further oriented with a c-axis being peφendicular with the drawing plane and a b-axis is parallel with the same drawing plane, while the c-axis of the second birefringent element 412 is parallel with the same drawing plane and the b-axis is perpendicular with the drawing plane. In some alternative embodiments, the orientation of the b- and c-axes of the first and second birefringent elements are reversed (i.e., 410= c-axis is parallel with drawing surface and b-axis is peφendicular, while 412= c-axis is peφendicular and b-axis is parallel). Similarly, the polarizer 414 has a preferred orientation relative to the laser axis and the axes of the first and second birefringent elements 410, 412. In some prefened embodiments a linear polarizer axis 610 of the polarizer 414 is oriented at about 45° to the drawing plane. Hence, a wave having its E-field parallel to the polarizer axis enters the polarizer 414 with equal components along the b- and c-axes. To maximize the linear electro- optic effect, the electric fields applied to the first and second birefringent elements, from the first and second modulators 440, 442, are typically applied along the c-axes of both elements. In some embodiments, the modulators or other controllers can further provide temperature control to adjust and/or maintain a temperature of the birefringent element 412, 412 (and/or the polarizer 414).

Reference is now made to FIGS. 7 and 8 in conjunction with FIG. 6, FIGS. 7 and 8 are, respectively simplified cross-sectional views of first and second birefringent elements taken peφendicular to the optical axis 416 and along the a-axis, showing the crystal orientation and the applied voltage according to some embodiments. The drivers 440, 442 (see FIG. 6) are electrically connected to the first birefringent element 410 using a first pair of electrodes including a first electrode 710 and a second electrode 712 situated opposite to the first electrode, to thereby apply a first electric field across the first birefringent element 410. Similarly the drivers 440, 442 are electrically connected to apply a second electric field across the second birefringent element 412 by a second pair of electrodes including a third electrode 810 and a fourth electrode 812. Both the first and second electric fields are applied peφendicular to the optical axis 416, although typically not in alignment with each other. In some preferred embodiments the voltages are applied along the c-axis of each of the birefringent elements.

Each of the birefringent elements is aligned with their crystal axes in a particular orientation with respect to the optical axis and a fixed polarization shown at 720. The fixed polarization 720 is typically determined by the polarizer 414. In some embodiments, one of the crystal axes in each of the birefringent elements is aligned with the optical axis, and the other two crystal axes (each having a differing index) are oppositely aligned with respect to each other (i.e. the optical axes are rotated about 90°). For the example depicted in FIGS. 7 and 8, the a-axis of both crystals aligned with the optical axis. The c-axis of the first birefringent element 410 is approximately aligned with the b-axis of the second birefringent element 412 (i.e., both are aligned at an approximately equal angle 722 with respect to the fixed polarization). Similarly, the b-axis of the first birefringent element is approximately aligned with the c-axis of the second birefringent element (i.e., both are aligned, at an approximately equal angle 724) with respect to the fixed polarization, h some embodiments, the angles 722 and 724 are about 45°, and the angular separation between the c- and b-axes is about 90°.

Referring back to FIG. 6, a control system and/or path 620 can also be included i some embodiments where the output is monitored (e.g., by a device associated with the source or a device receiving the modulated optical signal can return data utilized in control). A system controller and/or sensor(s) 622 can be included to provide control instructions for the adjustment of modulation and/or the modulators 440, 442 can receive feedback data, and determine and implement changes, as well as monitor changes to verify accuracy of changes. The control can provide a control loop that at least in part can provide some compensation for residual collateral FM-AM coupling. Further, the controller 622 can in some embodiments receive external signals for additional control and or adjustment of modulation. Some embodiments additionally and/or alternatively can provide some tuning through temperature control, such as the temperature control described in co-pending U.S. Patent Application Serial No. 10/138,091, filed May 3, 2002, entitled TUNABLE SNGLE FREQUENCY FILTER FOR LASERS.

As a laser tuning element, the BRF 406 of some present embodiments functions as an intracavity axial resonator mode-selection filter with a resolution being one axial mode over a bandwidth of many tens of nanometers or more. A spectral resolution of the unique dual-crystal BRF is determined, at least in part, by a difference in the lengths (l) of the two crystals comprising the first and second birefringent elements, while a tuning voltage is inversely proportional to the sum of crystal lengths.

In determining an accurate design and modulation mechanisms for a laser source according to present embodiments, several variables and parameters are determined and/or defined. Initially, within a birefringent element (e.g., an LN crystal) at a temperature T, and for a free-space wavelength λ, voltage dependent indices of refraction along the c- and b-axes are given by:

^2t , and Eq. 2a

n_b = n_bϋ - »^■

2t , Eq. 2b where n_c(T,λ) and ?-_&(T,λ) are the respective indices of refraction of a birefringent element

(such as an LN birefringent element), "t" is the crystal thickness along the c-axis, and r₃₃ and r₁₃ are the linear electro-optic coefficients for the optical field along the c- and b-axes, respectively.

Further, a two-pass optical path length through the birefringent element, as measured along the c- and b-axes of the birefringent element crystal (e.g., through the axes of the first birefringent element 410), respectively, are therefore expressed as:

Eq. 3a n_b (T,λ)r^L n_e (T,λ)r„V₂L_

OPL_b 2(n_b(T,X)L_x +n_c(T,X)L₂) ~-

Eq. 3b

The subscripts 1 and 2 refer to parameters of the first and second birefringent elements 410 and 412, respectively. The temperatures of the two crystals are assumed equal (however, this is not required), but the voltage, length, and thickness of each birefringent crystal can be set independently. When the optical path difference, OPL_c - OPLb, is equal to an integral number of wavelengths, radiation at that wavelength will return to the polarizer with substantially the same phase relationship that emerged from the polarizer (assuming no other resonator birefringent components), i.e., kλa = OPL_c - OPLb. The wavelength λ_B is referred to as the birefringent filter wavelength of the k order. A resonator axial mode corresponding to such a wavelength becomes highly selected at a lowest-loss resonator mode. Ifr_eff is defined by:

^Teff ~ ⁿ. ^r3_ ^{' •}»* ^r._ Eq. 4 then the BRF wavelength is given by:

kλ = 2{n_b(T,λ_B)- n_c(T,λ_B))(L₂ -J,) + v_

In terms of the sum and difference crystal voltages (defined by V₊ = (V_ + Vj)/2 and (V, - V /2), a desired BRF wavelength, λβi, is selected by applying a difference voltage V. to the birefringent elements 410, 412 which is given by:

From Equation 6, it is apparent that, if the iength-to-thickness ratios of the birefringent elements 410 and 412 are substantially equal, and preferably precisely equal, then wavelength selection tuning of the BRF becomes substantially independent of the sum (bias) voltage V+. Consequently, the difference voltage V. can be used to select the optical carrier wavelength λβi, while the sum voltage V₊ can be used to control the resonator axial mode wavelength such that it overlaps with the carrier wavelength λ_Bj. Moreover, the sum voltage V₊ also can be used to provide frequency modulation of the laser output without or only minimally introducing AM-FM cross-talk. Conversely, modulation of the difference voltage V. can be used to modulate the output power via a variation in the output coupling. In practice, however, the length-to-thickness ratios 412/t and 410/t] are not precisely equal. Therefore, the mathematical distinction between the two ratios can be maintained for the purpose of evaluating thickness sensitivity. In some embodiments, a design goal, nevertheless, is to provide length-to-thickness ratios 412/t₂ and 410/tι that are closely equal. In the implementation of many preferred embodiments, the precise zero-voltage operating wavelength is not critical. Thus, the remaining free parameter becomes the free spectral range of the birefringent filter. However, the birefringent filter spectral range still should be maintained wide enough to prevent multiple wavelength oscillation within the gain bandwidth (e.g., erbium gain bandwidth). As such, some embodiments set a target birefringent filter spectral range of about 40 nm, however, other ranges can be utilized depending on implementation and desired modulation.

It has been shown that the present embodiments provide for wavelength selective properties of the BRF and under desired conditions the operating wavelength is independent of bias or sum voltage. The present embodiments further allow for the selection and/or adjustment of the resonator modes. In some preferred implementations, the resonator modes respond to an applied voltage, allowing for the frequency modulation of a laser. Adding the optical path length (OPL) associated with the remainder of the cavity (e.g., cavity 404 of FIG. 4) to the OPL of the BRF, the resonance condition for the c-component of polarization becomes:

mλ_B =∑OPL + 2{n_c(T,λ_B)L_] ~n_b(T,λ_B)L₂) + V_(r^ - r_l3ef) - V,( ^_ff + r_l3eff) , Eq. 7

where T OPL is the OPL of the cavity less the BRF and m is the longitudinal mode order. Further, the linear electro-optic coefficients are equal to r_neff - r^n_c ³(T,λ_Bi)L_x lt , and ^ru_ejf - ^r _\_ⁿ _b (T,λ_Bi)L₂ lt₂. Prior studies of birefringent Fabry-Perot resonators have shown that axial modes which are coincident with BRF resonances have identical c~ and b- polarization component wavelengths. As such, it is generally unnecessary to consider the OPL of the b-component.

The FM deviation Δv can thus be calculated based on the preceding equations, and with the knowledge that the bias voltage ₊ is used to effect frequency modulation-, and that the right-hand side of Equation (6) is simply the resonator OPL (ROPL). Therefore, the FM deviation can be determined according to:

Additionally andor alternatively, amplitude modulation (AM) of a source can be implemented according to present embodiments by varying the difference voltage J⁷,. Under quasi-static conditions, where modulation rates are relatively small in comparison to the laser relaxation frequency (e.g., «1 MHz in some implementations), the varying of the difference voltage causes either a shift in the laser operating wavelengtii (via Equation 4) or to a pulling of the oscillation frequency due to the mismatch between a cavity longitudinal mode and a gain center. This, in turn, results in an increased loss rate together with strong AM-FM coupling. At relatively large modulation rates, however, the intracavity field wavelength typically cannot respond to the instantaneous modulation. Rather, the operating wavelength remains generally fixed and the effective output coupling is modulated by the failure of the returning field to satisfy the roundtrip phase shift condition that is implicit to Equation 5. Therefore, the effective transmission of the BRF/polarizer combination acquires the time dependence of the modulating signal and can be expressed as: πkλΛ s Eq. 9

- * J which, when using the small angle approximation in conjunction with Equation 6, can be manipulated to yield the change in transmission defined according to:

When the change in transmission is evaluated according to one implementation of a source according to some embodiments (where 412 is about 2 mm, t₂ is about 250 microns, and λa is about 1550 nm) the ΔJ =4.3X10 |ΔF_ j . As such, the application of about 15 volts produces a one-percent change in resonator output coupling, a change which, in turn, will deliver tens of milliwatts of optical power. The lengths, thickness, wavelengths and other parameters can very depending on the implementation. For example, some embodiments utilize birefringent crystals having lengths of about 0.5 to 4 mm, with thicknesses from about 10-1000 microns, while some preferred embodiments have lengths of between 1-2 mm and thicknesses of between 100-200 microns.

The present embodiments provide sources, such as laser sources, that are capable of providing both amplitude and frequency modulation on a single optical signal. Further, the modulation can be performed on substantially any frequency, such as frequencies less than 100 MHz and frequencies in the gigahertz range. Some laser implementations according to some present embodiments employ lumped-element electro-optic devices such that the source provides modulation bandwidths of a few gigahertz (AM), together with FM excursions as large as several tens of gigahertz. The present embodiments provide compact, fiber-coupled optical resonator laser communication sources that employ unique intracavity dual-crystal electro-optic birefringent filtering. Use of this filter technology allows for the independent control of the optical carrier wavelength, the modulation amplitude of the output coupled laser intensity, and the frequency modulation depth of an impressed FM phase modulation. Some embodiments are implemented through compact, diode-pumped solid-state laser (DPSSL) technology. Further, some embodiments provide laser source devices that include narrow spectral linewidth (e.g., <100 kHz, preferably <10 kHz), high fiber-coupled power (e.g., >10 mW, preferably >50 mW), wide spectral tunability (e.g., >5 nm, preferably >40 nm), high degree of polarization (e.g., >300:1), and operating at substantially any wavelength (e.g., around 1550 nm), with substantially any bandwidth (e.g., >1.0 gigahertz, preferably > 2.5 gigahertz), and in some embodiments leverages WDM technologies. Additionally, the present embodiments provide AM/FM orthogonality through the use of an intracavity electro- optic tuning mechanism that can simultaneously modulate both the resonator gain and the longitudinal mode structure. The BRF according to some embodiments, such as the BRF shown in FIGS. 4 and

56, includes a plurality of birefringent crystals and a linear polarizer situated along a laser axis within the laser cavity. The birefringent crystals (e.g., 410 and 412) typically comprise a similar material (e.g., LN). A first crystal and a second crystal can be arranged with their crystal axes in a particular configuration, for example, the crystals can be arranged so that the a-axis of each crystal is aligned with the laser axis, the b-axis of the first crystal is aligned with the c-axis of the second crystal, and the c-axis of the second crystal is aligned with the b-axis of the first crystal. The linear polarizer has an etalon configuration, and the polarizer is oriented so that its dielectric axis is about 45° with respect to the plane defined by the b-axis of the first crystal and the c-axis of the second crystal.

As a result of this configuration, a wave having its E-field parallel to the polarizer axis enters the second crystal with equal components along the b- and c-axes. To maximize the electro-optic effect, a modulating electric field can be applied along the extraordinary axis, c-axes, of both crystals. A first modulator can apply a modulated electric field along the c-axis in the first crystal, and/or a second modulator can apply a modulated electric field along the c-axis of the second crystal. Applying the electric field along the c-axis of, for example, an LN birefringent element, both ordinary and extraordinary refractive indexes are modified, thus allowing electric field tuning. As a laser tuning element, the BRF Junctions as an intracavity axial resonator mode- selection filter, where, in one example, its resolution being one axial, mode over a bandwidth of many tens of nanometers. Its spectral resolution can be determined based on the difference in the lengths between the first and second crystals, while the desired tuning voltage generally is inversely proportional to the sum of crystal lengths. hi some embodiments, it is beneficial to provide birefringent crystals where the length-to-thickness ratios of the first and second crystals are approximately equal (410l\\ = 412/X_). When this criteria is establish, the wavelength selection tuning of the BRF becomes substantially independent of the sum (bias) voltage applied to the crystals.

Further, with the length-to-thickness ratios of the first and second crystals being substantially equal, the difference voltage (V-;) can be used to select the optical carrier wavelength (λβj), while the sum voltage (V_+.) can be used to control the resonator axial mode wavelength such that it overlaps with the optical carrier wavelength. Moreover, the sum voltage can also be used to provide frequency modulation of the laser output substantially without introducing AM-FM cross-talk. Additionally and/or alternatively, modulation of the difference voltage can be used to modulate the output power through a variation in the output coupling. Accordingly, the tuning mechanism of the filter according to some preferred embodiments can simultaneously modulate both the resonator gain and the longitudinal mode structure by appropriately modulating the first and second modulators. The BRF of the present embodiments can be employed in numerous and varied applications. Some of those applications can include, but are not limited to, telecommunications, optical networks, wireless communications, phased array radar, precision metrology, LIDAR, optical fiber sensors for acoustic and seismic sensing, and other such applications.

While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.

Claims

CLAIMSWhat is claims is:

1. An apparatus for use in providing modulated optical communication signal, comprising: a first birefringent element positioned with a laser axis, where the first birefringent element comprises birefringent material having first, second and third axes with the first axis being substantially parallel with a laser axis, and the first birefringent element has a length defined along the first axis and a thickness defined along the third axis; and a second birefringent element positioned in optical alignment with the first birefringent element, the second birefringent element comprises birefringent material having first, second and third axes with the first axis substantially parallel with the laser axis, and the second axis being substantially parallel with the third axis of the first birefringent element and the third axis of the second birefringent element being substantially parallel with the second axis of the first birefringent element, and the second birefringent element has a length defined along the first axis and a thickness defined along the third axis; wherein a length-to-thickness ratio of the first birefringent element is approximately equal a length-to-thickness ratio of the second biref ingent element.

2. The apparatus of claim 1, further comprising: a first electric field defined across the first birefringent element and generally along the third axis having a first voltage differential; and a second electric field defined across the second birefringent element along the third axis having a second voltage differential such that a voltage difference that is proportional to the difference between the first and second voltage differentials defines an optical carrier wavelength.

3. The apparatus of claim 2, wherein a sum voltage that is proportional to the sum of the first and second voltage differentials defines a resonant axial mode wavelength such that the resonant axial mode wavelength overlaps with the canier wavelength.

4. The apparatus of claim 3, wherein modulation of the voltage difference further defines a modulation of an optical signal, and the sum voltage further defines a frequency modulation of the optical signal.

5. The apparatus of claim 4, wherein the optical signal includes an analog and digital signal.

6, The apparatus of claim 1, further comprising: a first electric field defined across the first birefringent element and generally along the third axis having a first voltage differential; and a second electric field defined across the second birefringent element along the third axis having a second voltage differential such that a voltage difference that is proportional to the difference between the first and second voltage differentials defines amplitude modulation of an optical carrier signal and adjustments to a sum voltage that is proportional to the sum of the first and second voltage differentials defines frequency modulation of the optical carrier signal.

7. The apparatus of claim 1, further comprising: a polarizer aligned with the laser axis and defining a direction of polarization, wherein the second and third axes of the first and second birefringent elements are offset from the direction of polarization.

8. The apparatus of claim 7, further comprising: a first mirror and a second mirror aligned along the laser axis and defining an optical cavity between the first and second mirrors; and a gain material aligned along the laser axis, and the gain material comprises an etalon configuration including opposing sides that are at least partially reflective.

9. The apparatus of claim 1, wherein the first birefringent element comprising an etalon configuration comprising opposite surfaces that are both at least partially reflective, and the first birefringent element further comprising a birefringent material arranged with its dielectric axes offset from a direction of polarization; and wherein the second birefringent element comprising an etalon configuration comprising opposite surfaces that are both at least partially reflective, and the second birefringent element further comprising a birefringent material arranged with its dielectric axes offset from the direction of polarization.

10. An apparatus for use in generating an optical communication signal, comprising: means for generating a first electric field across a first means for providing birefringence of an optical carrier signal; means for generating a second electric field across a second means for providing birefringence of the optical carrier signal; means for defining an optical carrier wavelength that is proportional to the first and second electric fields; and means for defining a resonator axial mode wavelength that is proportional to the first and second electric fields such that the resonator axial mode wavelength overlaps with the optical carrier wavelength.

11. The apparatus of cl aim 1 , further comprising: means for defining a voltage difference that is proportional to a difference between a first voltage potential associated with the first electric field and a second voltage potential associated with the second electric field; and means for amplitude modulating the optical carrier signal comprising means for modulating the voltage difference.

12. The apparatus of claim 10, further comprising: means for defining a sum voltage that is proportional to a sum of a first voltage potential associated with the first electric field and a second voltage potential associated with the second electric field; and means for phase modulating an optical carrier signal comprising means for adjusting the sum voltage.

13. The apparatus of claim 12, further comprising: means for defining a voltage difference that is proportional to a difference between a first voltage potential associated with the first electric field and a second voltage potential associated with the second electric field; and means for amplitude modulating the optical carrier signal comprising means for modulating the voltage difference such that the optical canier signal is both phase modulated and amplitude modulated.

14. A laser comprising: a gain medium positioned along a laser axis; a polarizer positioned along the laser axis and defining a direction of polarization; a first birefringent element optically aligned with the laser axis, where the first birefringent element comprises birefringent material arranged with two of its differing dielectric axes offset from the direction of polarization, and the first birefringent element has a length and a thickness; and a second birefringent element is optically aligned with the laser axis, where the second birefringent element comprises birefringent material arranged with two of its differing dielectric axes offset from the direction of polarization and oppositely ahgned with the two differing dielectric axes of the first birefringent element, and the second birefringent element has a length and a thickness such that a length-to-thickness ratio of the first birefringent element approximately equals a length-to-thickness ratio of the second birefringent element.

15. The laser of claim 14, further comprising: a first modulator source coupled with the first birefringent element, where the first modulator controls a first voltage applied across the first birefringent element; and a second modulator source coupled with the second birefringent element, where the second modulator controls a second voltage applied across the second birefringent element such a difference voltage proportional to the difference between the first and second voltages defined an optical carrier wavelength.

16. The laser of claim 15, wherein a sum voltage proportional to the sum of the first and second voltages defines a resonator axial mode wavelength that overlaps with the carrier wavelength.

17. The laser of claim 16, wherein the first and second modulators control the first and second voltages such that a modulation of the voltage difference provides a modulation of signal intensity of an optical signal, and a variation of the sum voltage provides frequency modulation of the optical signal.

18. The laser of claim 16, wherein the firsf: birefringent element comprises an etalon configuration, and the first birefringent element comprises an etalon configuration.

19. A method of generating an optical communication signal, comprising: generating a first electric field across a first birefringent element; generating a second electric field across a second birefringent element; defining an optical canier wavelength that is proportional to the first and second electric fields; and defining a resonator axial mode wavelength that is proportional to the first and second electric fields such that the resonator axial mode wavelength overlaps with the optical canier wavelength.

20. The method of claim 19, further comprising: defining a voltage difference that is proportional to a difference between a first voltage potential associated with the first electric field and a second voltage potential associated with the second electric field; and amplitude modulating an optical carrier signal comprising modulating the voltage difference.

21. The method of claim 19, further comprising: defining a sum voltage that is proportional to a sum of a first voltage potential associated with the first electric field and a second voltage potential associated with the second electric field; and phase modulating an optical canier signal comprising adjusting the sum voltage.

22. The method of claim 21 , further comprising: defining a voltage difference that is proportional to a difference between a first voltage potential associated with the first electric field and a second voltage potential associated with the second electric field; and amplitude modulating the optical carrier signal comprising modulating the voltage difference.

23. The method of claim 22, wherein the defining of the optical carrier wavelength comprises adjusting the difference voltage when a length-to-thickness ratio of the first birefringent element is substantially the same as a length-to-thickness ratio of the second birefringent element; and wherein the defining the resonator axial mode comprises adjusting the sum voltage such that the resonator axial wavelength overlaps with the optical carrier wavelength.

24. A method for use in modulating optical signals, comprising: applying a first voltage across a first optical element; applying a second voltage across a second optical element; and modulating a frequency of an optical signal comprising: adjusting at least one of the first and second voltage potentials and producing a change in a voltage summary that is proportional to the sum of the first and second voltage potentials as the optical signal passes through the first and second elements.

25. The method of cl aim 24, further compri sing: modulating an amplitude of the optical signal comprising: adjusting at least one of the first and second voltage potentials and producing a change in a voltage difference that is proportional to the difference between the first and second voltage potentials as the optical signal passes through the first and second elements where the first and second elements comprise birefringent materials.

26. The method of claim 25, wherein the first optical element is a first birefringent material and the second optical element is a second birefringent material.

27. The method of claim 25, further comprising: defining an optical carrier wavelength comprising adjusting a difference voltage that is proportional to the difference between the first and second voltages; and defining a resonator axial mode wavelength comprising adjusting a sum voltage that is proportional to the sum of the first and second voltages such that the resonator axial mode wavelength overlaps with the optical canier wavelength.