The invention relates to light wavelength filtering and particularly to wavelength tuning an external cavity laser without mechanical motion.
An important property of external cavity lasers is wavelength tuning. To accomplish this, one or more optical components in the external cavity, such as a grating, focusing element or mirror, are typically translated or rotated. This motion causes the cavity to resonate at another wavelength. Unfortunately, this tuning mechanism suffers from the limitations inherent with motor-driven mechanical motion. Motor-generated heat can change the cavity optical path length via the thermal expansion or contraction of materials. This affects the cavity optical properties in an unpredictable manner. The resolution of mechanical wavelength tuning may also be limited by unreproducible mechanical motion and backlash always present in mechanical systems. Motors can also be bulky or, if miniaturized, expensive.
In accordance with one embodiment provided herein, a method of tunable wavelength filtering without requiring mechanical motion is provided. The method comprises receiving a light beam of wavelength within a range of wavelengths, dispersing the light beam at a wavelength-dependent angle, and propagating the light beam through an electro-optic device including an electrically-variable refractive index electro-optic element. The method further comprises applying a control voltage to the electro-optic device, causing tunable wavelength filtering dependent on the control voltage.
In accordance with another embodiment, an optical system is provided, comprising a dispersing element operable to disperse a light beam at a wavelength-dependent angle, and a variable-index electro-optic device positioned in the path of the light beam. The variable-index electro-optic device includes a variable-index electro-optic element having an electrically-variable refractive index, such that the variable-index electro-optic element is operable to perform wavelength-selective filtering of the light beam, dependent on the value of an applied control voltage.
In accordance with some embodiments, a system and method are provided which use electro-optic materials, e.g., liquid crystals, to accomplish wavelength tuning of an external cavity laser. In particular, wavelength tuning is accomplished by applying a control voltage to the electro-optic material, not by mechanical motion. Hence, the drawbacks inherent with mechanical tuning are avoided.
In accordance with some embodiments, in an external cavity laser, an optical gain medium, for example a light-emitting diode, emits a light beam within a range of wavelengths. The light beam is spectrally dispersed, for example, using a diffraction grating, and propagates through an electro-optic element located in the external cavity. The electro-optic element, for example, comprises a liquid crystal or other electro-optic material having an electrically variable refractive index. In some embodiments, the effective optical path length is tuned in response to an applied control voltage, such that the mode number of the cavity is electrically tuned. Additionally or alternatively to the above, an applied control voltage tunes the critical angle for total internal reflection, such that the desired oscillating wavelength is totally internally reflected and undesired wavelengths are partially segregated from the desired wavelength. In some implementations, a pair of such variable index electro-optic elements is located in the cavity, such that the first element partially segregates longer wavelengths and the second element partially segregates shorter wavelengths relative from the desired wavelength. Control voltages can be the same or determined independently, for example in response to feedback control signals. Numerical analysis shows that sufficient wavelength discrimination is provided to confine laser oscillation to one electrically tunable resonant wavelength.
For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
As illustrated in
Light emitted from optical gain medium 101 is collected and collimated by collimating element 102, which may be a reflecting paraboloid, a refractive lens, or a diffractive (e.g., Fresnel) element. The diameter of collimating element 102 should be large enough to collect over 90 percent of the optical flux (power) emitted by optical gain medium 101. The collimated light propagates along the z direction (indicated by the labeled directional arrow in
sin θ=λ/Λ,
where λ is the wavelength of light, Λ is the grating pitch, and θ is the angle between the diffracted propagation direction and the direction normal to the grating surface (z axis). In the example depicted in
After traversing the interior of variable index electro-optic element 110, light of all emitted wavelengths is next incident on second optical interface 111. Importantly, variable index electro-optic element 110 is shaped and oriented such that light of desired resonant wavelength λ0 is incident on second optical interface 111 at an angle near the critical angle θ1cr for total internal reflection (TIR). The critical angle is defined by the relation: n1(V) sin θ1cr=1, where n1(V) is the electrically-dependent refractive index of first variable index electro-optic element 110 adjacent second optical interface 111 and θ1cr is measured relative to the normal to second optical interface 111, in accordance with convention (see, for example, E. Hecht, “Optics,” Addison-Wesley, 1974, pp. 97-98).
Electro-optic materials belong to a class of optical materials whose refractive index can be varied by the application of a control voltage. Accordingly, refractive index n1(V1) within variable index electro-optic element 110 can be varied with the application of control voltage V1. Changing control voltage V1 applied within variable index electro-optic element 110 likewise changes critical angle θ1cr. Thus, varying applied voltage V1 controls the boundary (in wavelength terms) between the range of wavelengths of light incident on optical interface 111 totally internally reflected into and the range of wavelengths partially refracted out of variable index electro-optic element 110, as described below in more detail.
If the desired resonant wavelength of laser 100 is equal to λ0, optical gain medium 101 is capable of emitting light in a range of wavelengths including wavelengths longer (λL in
The light totally internally reflected from optical interface 111 with wavelength λL greater than or equal to λ0 propagates out through optical interface 113 of first variable index electro-optic element 110, is incident on optical interface 122 and is refracted into the interior of second variable index electro-optic element 120. Importantly, second variable index electro-optic element 120 is shaped and oriented such that critical angle θ2cr for TIR occurs near the angle of incidence for light of wavelength λ0 at second optical interface 121. Additionally, at optical interface 121, unlike at optical interface 111, light of wavelength λL longer than λ0 is incident at smaller angles relative the normal to optical interface 121 than light of wavelength λ0. Critical angle θ2cr such that n2(V2) sin θ2cr=1 is adjusted via the application of control voltage V2, such that only light with wavelength λS shorter or equal to λ0 is TIR reflected at optical interface 121. All longer wavelengths λL are partially refracted through optical interface 121 and thus are selectively partially segregated from desired wavelength λ0 and from shorter wavelengths in external cavity laser 100. Optical interface 121 alone accordingly behaves as a short wavelength pass filter.
Accordingly, only light of tunable wavelength λ0 propagates efficiently within external cavity laser 100 relative to longer and shorter wavelengths λL and λS. Optical interfaces 111 and 121 together behave as an electrically tunable bandpass wavelength filter. Relatively efficiently propagating light of wavelengthλ0 emerges through optical interface 123, is reflected from retro-reflector 105, and effectively retraces its path through cavity 100 back to gain medium 101. After retro-reflection, once again wavelengths λL and λS longer and shorter than λ0 are partially segregated from tunably selected wavelengthλ0 at respective optical interfaces 111 and 121 because of refraction and reflection near voltage-tunable critical angles θ1cr and θ2cr. In some embodiments, retro-reflector 105 can be integrally combined with optical interface 123 of electro-optic element 120. Alternatively, any of a wide variety of optical feedback elements, for example, prisms, TIR reflectors, planar mirrors, curved mirrors, and fiber Bragg gratings, may be used in place of retro-reflector 105.
It is convenient although not necessary for first and second variable index electro-optic elements 110 and 120 to be shaped and oriented such that light of desired resonant wavelength λ0 is normally incident on optical interfaces 113 and 122. Alternatively, first and second variable index electro-optic elements 110 and 120 can be combined into a single electro-optic element shaped and oriented such that optical interfaces 113 and 122 are eliminated and such that light of desired resonant wavelength λ0 is incident on each of optical interfaces 111 and 121 at angles near the critical angles for TIR. For convenience, first and second variable index electro-optic elements 110 and 120 can be prism-shaped. Alternatively, they can be configured in other two-dimensional or complex three-dimensional shapes with three-dimensional light propagation paths.
Accordingly, without loss of generality, the application of variable control voltages V1 and V2 to respective first and second variable index electro-optic elements 110 and 120 selectably tunes critical angles θ1cr and θ2cr at respective optical interfaces 111 and 121. This causes optical interfaces 111 and 121 together to behave as an electrically tunable bandpass wavelength filter, which tunably selects light at or adjacent a unique resonant wavelength λ0 to propagate with higher efficiency within external cavity laser 100 relative to longer and shorter wavelengths λL and λS. By varying control voltages V1 and V2, resonant wavelength λ0 within external cavity laser 100 is changed or tuned.
Even though discrimination against wavelengths λL and λS longer and shorter than λ0 near voltage-selectable critical angles θ1cr and θ2cr is a gradual function of wavelength, it is typically sufficient to ensure single-mode oscillation in external cavity laser 100. Numerical analysis using the well-known Fresnel equations shows that, for an example of center wavelength λ0=1.59 μm, no more than 10 modes propagate in the top 10 percent of the cavity efficiency curve. Experience has shown further that, if ten or fewer modes propagate in the top 10 percent of the cavity efficiency curve, then nonlinear mode competition for the population inversion in optical gain medium 101 will limit actual oscillation within the cavity to a single dominant mode only. Accordingly, the method described above provides tuning of external cavity laser to desired resonant wavelength λ0 through application of variable control voltage to variable index electro-optic elements 110, 120, without requiring mechanical motion.
Additional variable index electro-optic element 210 enables optical path length tuning by varying refractive index n3(V3) of additional variable index electro-optic element 210 via application of variable control voltage V3, and hence changing the optical path length (physical path length L210 multiplied by refractive index n3(V3)) of light propagating within additional variable index electro-optic element 210. By placing additional variable index electro-optic element 210 within the cavity of external cavity laser 200, the optical path length of the cavity can be tuned for light propagating within the cavity. The mode number n(m) associated with resonant light within the cavity is directly related to resonant wavelength and cavity path length through the expression n(m)=(cavity optical path length)/(wavelength). For example, the mode number n(m) of resonant wavelength λ0 within external cavity laser 200 can be tuned electrically by varying control voltage V3 applied to additional variable index electro-optic element 210. Thus, resonant wavelength λ0 within external cavity laser 200 can be tuned electrically via applying variable control voltages V1 and V2 to variable index electro-optic elements 110 and 120, for example, while keeping mode number n(m) constant via tunable control voltage V3 applied to additional variable index electro-optic element 210, without requiring mechanical motion. Alternatively, mode number n(m) can be tuned independently by varying control voltage V3, regardless of any ability to provide wavelength tuning by applying variable control voltages V1 and V2.
Alternatively, the effectively optical path length within the cavity of external cavity laser 200 can be tuned by translating retro-reflector 105 parallel to the optical path of light of resonant wavelength λ0, i.e., perpendicular to the line formed by the locus of all bottom or top apex points of the sawtooth retro-reflector profile. This translation does not affect the directionality of the resonant light, but it changes the cavity optical path length, causing tuning of the resonant mode number n(m). Although this technique has the disadvantage of requiring mechanical motion, it can, for example, be used to provide coarse mechanical mode number tuning optionally in conjunction with applying variable control voltage V3 to provide fine electrical mode number tuning.
Wavelength tuning in embodiments of the invention is achieved by application of a control voltage without requiring mechanical motion. As a result, the adverse effects associated with mechanical tuning, such as thermal issues, non-repeatable motion, and backlash, are avoided. Also, an electrically-controlled external cavity laser does not require a bulky motor for mechanical tuning and is, hence, more easily miniaturized. Further, wavelength and mode number can simultaneously be controlled electrically. Control voltages, for example V1, V2, V3, applied individually to variable index electro-optic element 110, 120, and/or 210 can be equal or unequal in value to one another, and can be individually or cooperatively controlled conventionally, programmably and/or through feedback signals derived from photodetectors or other appropriate sensors (not shown in
Transparent electrically-conducting film layers 304 and 305 connected through conductors 314 and 315 to a variable voltage source (not shown) apply a variable voltage across liquid crystal layer 302. The voltage electrically tunes the refractive index n1(V1) of liquid crystal layer 302. This in turn provides a tunable critical angle θ1cr for TIR at optical interface 306 between liquid crystal layer 302 and outer low-index dielectric layer 303, where θ1cr satisfies the relation n1(V1) sin θ1cr=nL. As depicted in
Similar to combining first and second variable index electro-optic elements 110 and 120 into a single variable index electro-optic element as described in connection with
Refractive index n1(V1) of liquid crystal layer 302 is electrically tunable over a range of approximately 1.5 to 1.7. Optical-grade dielectric materials suitable for dielectric layer 303 have refractive indices smaller than the minimum index in the range for the liquid crystal material. Candidates include, for example, silicon dioxide (SiO2) and lithium fluoride (LiF), having respective refractive indices of 1.45 and 1.38. Transparent conducting film layers 304 and 305 can be made, for example, of indium tin oxide (ITO), which is 50% transparent at a wavelength of 1.5 micrometers (μm) and 90 percent transparent at visible and near infrared (<1.0 μm) wavelengths. Layered structure 300 is configured so that transparent conducting film layers 304, 305 are spaced away from TIR interface 306 and therefore produce no adverse effect on the optical properties of the interface. Refracted light escapes from the external cavity of the laser if outer conducting film layer 305 has a rough surface that causes diffuse reflection and scattering. Refracted light likewise escapes if outer conducting film layer 305 is absorbing, or if it is specularly reflecting but non-parallel to the plane of optical interface 306, and thus deflects incident light either in or out of the optical plane of the external cavity.
In addition to liquid crystals, the embodiments can employ other electro-optic materials, for example lithium niobate or other electro-optic crystals, that provide a substantially transparent optical medium across the wavelength range of interest and have electrically-dependent refractive indices. Liquid crystals exhibit a greater coefficient of refractive index change relative to control voltage than do other materials, such as lithium niobate, but have the drawback of scattering light. For example, light scattering through a thickness greater than or equal to 5 mm of liquid crystal is observed to degrade light propagation efficiency by at least 50 percent relative to the same path length through a non-scattering medium. Hence, liquid crystals are particularly advantageous in thin layers. Again in accordance with numerical analysis results, wavelength discrimination in layered structure 300 in an external cavity laser is expected to be slightly inferior to that described above in connection with long optical-path, low-scatter media in
More generally, it is possible to move the center wavelength of the pass band to shorter or longer wavelengths by varying the refractive indices of the two liquid crystal-based elements unequally. This could be useful in external cavity laser tuning and in other situations where a dynamic bandpass filter is desired, for example, in receivers where the certain incoming wavelengths are selected, or to select wavelengths to be re-routed in an optical switch or in an add-drop optical multiplexer. Likewise, individual long wavelength pass and short wavelength pass filters represented by curves 401 and 402 can be useful to select wavelengths to be re-routed in a switch or in an add-drop optical multiplexer.
A range of embodiments alternative to that depicted in
Furthermore, external cavity laser 500 may optionally include additional variable index electro-optic element 210 to provide mode number tuning by electrically varying refractive index n3), as described in connection with
This application is related to concurrently filed, co-pending and commonly assigned U.S. patent application Ser. No. ______ [Attorney Docket 10030129-1], titled “EXTERNAL CAVITY LASER IN WHICH DIFFRACTIVE FOCUSING IS CONFINED TO A PERIPHERAL PORTION OF A DIFFRACTIVE FOCUSING ELEMENT”; concurrently filed, co-pending and commonly assigned U.S. patent application Ser. No. ______ [Attorney Docket 10030130-1], titled “USING RELAY LENS TO ENHANCE OPTICAL PERFORMANCE OF AN EXTERNAL CAVITY LASER”; concurrently filed, co-pending and commonly assigned U.S. patent application Ser. No. ______ [Attorney Docket 10030131-1], titled “METHOD OF ENHANCING WAVELENGTH TUNING PERFORMANCE IN AN EXTERNAL CAVITY LASER”; and co-pending and commonly assigned European Patent Application No. 02 017 446.2, titled “WAVELENGTH TUNABLE LASER WITH DIFFRACTIVE OPTICAL ELEMENT,” filed Aug. 3, 2002, the disclosures of all of which are hereby incorporated herein by reference.