Patent Application
20130044773
Embodiments of the present disclosure generally relate to optical sources and, more particularly, optical sources comprising a proximity-coupled laser source and waveguide for optical feedback control.
Although the various concepts of the present disclosure are not limited to lasers that operate in any particular part of the optical spectrum, reference is frequently made herein to frequency-doubled green lasers, where wavelength fluctuations of the diode IR source typically generate fluctuations of the frequency-converted green output power. Such fluctuations are often attributable to the relatively narrow spectral acceptance curve of the wavelength conversion device used in the frequency-converted laser—typically a periodically poled lithium niobate (PPLN) SHG crystal. If the aforementioned frequency-converted laser is used in a scanning projector, for example, the power fluctuations can generate unacceptable image artifacts. For the specific case when the laser comprises a two or three-section DBR laser, the laser cavity is defined by a relatively high reflectivity Bragg mirror on one side of the laser chip and a relatively low reflectivity coating on the other side of the laser chip. The resulting round-trip loss curve for such a configuration is proportional to the inverse of the spectral reflectivity curve of the Bragg mirror. Also, only a discrete number of wavelengths called cavity modes can be selected by the laser. As the chip is operated, its temperature and therefore the refractive index of the semiconductor material changes, shifting the cavity modes relative to the Bragg reflection curve. As soon as the currently dominant cavity mode moves too far from the peak of the Bragg reflection curve, the laser switches to the mode that is closest to the peak of the Bragg reflection curve since this mode corresponds to the lowest loss—a phenomenon known as mode hopping.
Mode hopping can create sudden changes in output power and will often create visible borders between slightly lighter and slightly darker areas of a projected image because mode hops tend to occur at specific locations within the projected image. Sometimes, a laser will continue to emit in a specific cavity mode even when it moves away from the Bragg reflection peak by more than one free spectral range (mode spacing)—a phenomenon likely related to spatial hole burning and electron-photon dynamics in the cavity. This results in a mode hop of two or more cavity mode spacings and a corresponding unacceptably large change in output power.
Optical feedback from the SHG crystal may create laser wavelength instability in the DBR laser. One method to limit the effect of optical feedback may be to wedge the input and/or output facets of the SHG crystal such that the beam produced by the DBR laser is not perpendicular to the input and/or output facets. However, the wedged facets may need to be orientated at a certain angle with respect to the optical axis of the system, which may add mechanical design constraints as symmetric designs may be desired to obtain increased stability. Even when the facets are wedged at a large angle, the present inventors have recognized that parasitic reflections from the SHG front facet may significantly affect the laser wavelength stability. According to the subject matter of the present disclosure, configurations and corresponding methods of operation are provided to address these and other types of power variations in frequency-converted optical sources.
In accordance with one embodiment, an optical source includes a laser source and a waveguide. The laser source includes a laser cavity having a laser optical path length extending from a DBR grating to a reflective laser output facet, and emits an output beam at a fundamental wavelength. The waveguide has an input facet and an output facet, and extends along a waveguide optical length from the input facet of the waveguide to the output facet of the waveguide. The input facet and the output facet of the waveguide are approximately normal with respect to an optical path of the output beam. The waveguide and the laser source are proximity coupled, and the waveguide optical length is an integer multiple of the laser optical path length.
In accordance with another embodiment, an optical source includes a laser source and a wavelength conversion device. The laser source includes a laser cavity having a laser optical path length extending from a DBR grating to a reflective laser output facet, and emits an output beam at a fundamental wavelength. The wavelength conversion device has an input facet, an output facet, and a waveguide extending from the input facet of the wavelength conversion device to the output facet of the wavelength conversion device. The input facet and the output facet of the wavelength conversion device are approximately normal with respect to the optical path of the output beam. The output facet of the wavelength conversion device has a reflectivity that is higher than a reflectivity of the reflective laser output facet of the laser source. The wavelength conversion device and the laser source are proximity coupled. A wavelength conversion device optical length extending along the waveguide is an integer multiple of the laser optical path length. The wavelength conversion device converts the output beam emitted by the laser source into a frequency-converted output beam having a converted wavelength that is shorter than the fundamental wavelength.
In accordance with yet another embodiment, an optical source includes a laser source and a wavelength conversion device. The laser source includes a laser cavity having a laser optical path length extending from a DBR grating to a reflective laser output facet, and emits an output beam at a fundamental wavelength. The wavelength conversion device has an input facet, an output facet, and a waveguide extending from the input facet of the wavelength conversion device to the output facet of the wavelength conversion device. The input facet and the output facet of the wavelength conversion device are approximately normal with respect to the optical path of the output beam. The reflective laser output facet, the input facet of the wavelength conversion device, and the output facet of the wavelength conversion device have a reflectivity such that R3>R1+R2+2(R1*R2)0.5, where R1 is a reflectivity of the reflective laser output facet of the laser source, R2 is a reflectivity of the input facet of the waveguide, and R3 is a reflectivity of the output facet of the waveguide. The wavelength conversion device and the laser source are proximity coupled. The wavelength conversion device optical length extending along the waveguide is an integer multiple of the laser optical path length. The wavelength conversion device converts the output beam emitted by the laser source into a frequency-converted output beam having a converted wavelength that is shorter than the fundamental wavelength.
Reference will now be made in detail to embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.
Referring initially to
The DBR laser diode 110 defines a laser cavity comprising a gain section 116, a phase section 114, and a wavelength selective DBR section 112 interposed between a relatively high reflectivity rear laser facet 119 and a relatively low reflectivity laser output facet 117 at the output of the DBR laser diode 110. The laser cavity defined by the DBR laser diode 110 provides an optical path length LLC of light propagating therein, which is the effective laser cavity length. It should be understood that throughout the figures the optical path length LLC of the laser cavity is labeled as the length of the DBR laser diode 110 for illustrative purposes only, and that the optical path length LLC of the laser cavity is the optical distance of light traveling therein, and may or may not be equal to the physical length of the DBR laser diode 110, and also may be altered by the control signals to the DBR laser diode 110.
Respective control electrodes 102, 104, 106, may be incorporated in the wavelength selective DBR section 112, the phase section 114, the gain section 116, or combinations thereof, and are merely illustrated schematically in
In one embodiment, the wavelength conversion device 120 comprises an SHG crystal having a frequency-converting waveguide 122 that extends from an input facet 121 to an output facet 123. As illustrated in
The waveguide 122 may be periodically poled to achieve quasi-phase matching to frequency-double an IR output beam emitted by the DBR laser diode 110. A frequency-converted output beam 124, which has a converted wavelength that is shorter than the wavelength of the IR output beam, is then emitted from the output facet 123 of the wavelength conversion device 120. In one embodiment, the frequency-converted output beam 124 has a wavelength that is in the green spectral range.
Although the waveguide 122 is illustrated as being a component of an SHG crystal, embodiments are not limited thereto. For example, the waveguide 122 may be incorporated into an optical fiber or other optical component.
The input facet 121 and/or output facet 123 of the wavelength conversion device 120 may be reflective such that portions of the IR beam emitted by the DBR laser diode 110 are reflected back into the laser. The output facet 123 of the wavelength conversion device may define an external cavity that may function as a Fabry-Perot cavity having an optical path length extending LEC from the reflective laser output facet 117 to the output facet 123 of the wavelength conversion device, depending on the reflectivity of the input and output facets 121, 123 of the wavelength conversion device 120. In one embodiment, the input facet 121 and the output facet 123 of the wavelength conversion device are normal with respect to the optical path of the IR beam and the frequency-converted output beam 124 (i.e., the input and output facets of the wavelength conversion device are straight facets).
A portion of the light from the laser cavity of the DBR laser diode 110 is emitted through the laser output face 117 and coupled to the wavelength conversion device 120, while the remaining light bounces back and forth in the laser cavity between the DBR grating, which acts as a mirror, and the laser output facet 117, each time passing through the gain medium of the gain section 116. Additionally, back reflections of light may be reflected from the input facet 121 and or the output facet 123 of the wavelength conversion device and re-enter the laser cavity. These back reflections may cause wavelength instability of the DBR laser diode 110, as well as the resulting frequency-converted output beam 124 emitted from the wavelength conversion device 120.
As described above, the cavity of a DBR laser may be closed by the grating of the wavelength selective DBR section on one side and the reflectivity of the laser output facet on the other side. The round-trip spectral gain curve may be expressed as:
RTG(λ)=G·DBR(λ)·Rff, Eq. (1),
where RTG(λ) is the round-trip gain, G is the gain section gain coefficient, and Rff is the laser front facet reflectivity. To determine the wavelength selected by the laser, the cavity modes may be calculated. The cavity modes of the laser diode are the wavelengths where the optical path over a round trip with in the laser cavity equals an integer times the wavelength. The cavity modes are calculated by determining the wavelengths that can create standing waves, i.e. wavelengths where there is a round trip light wave phase change of 2π. The wavelength emitted by the DBR laser diode is then given by the cavity mode that is closest to the RTG spectral curve. The wavelength difference between the various modes may be expressed as:
where L is the laser diode length, and n is the index of refraction of gallium arsenide (GaAs).
As an example and not a limitation, with a laser cavity physical length of 3 mm, mode spacing is about 0.06 nm. The expectation is then that the maximum wavelength fluctuations should be about ±0.03 nm, which would result in frequency-converted power fluctuations of about 4% when assuming a 0.24 bandwidth PPLN crystal as the wavelength conversion device. However, the present inventors have recognized that measured power fluctuations are much larger, and experimental results suggest that part of the wavelength fluctuations is due to instabilities induced by parasitic reflections on the input facet and/or output facet of the wavelength conversion device.
In one experiment, which consisted of generating variable amounts of back reflection from a wavelength conversion device and applying increasing gain current, about −110 dB of feedback resulted in a DBR laser diode that operated normally. The level of feedback was increased from about −110 dB to about −3 dB of back reflections. At −70 dB, some abnormal mode hops were discovered at low current. At −40 dB, abnormal mode hops were spread everywhere throughout the current range. The mode hop structure appeared to disappear at −18 dB and was replaced by smoother transitions. Finally, at −3 dB of feedback, the normal expected curve shape of the IR wavelength was totally disturbed and very large amplitude wavelength variations were evident. Accordingly, back reflections into the laser cavity may increase wavelength instability.
As illustrated in
Additionally, the modulation frequency increases as the distance of the external reflective surface from the DBR laser diode increases. The consequence is that the DBR laser diode may become unstable and start mode hopping very often. Accordingly, simulations and experimentation suggest that feedback as low as 0.01% may be enough to create laser instabilities. Even with wedged and anti-reflective (AR) coated crystals, such reflectivity levels may be difficult to achieve.
The present inventors have recognized that when the external reflective surface (e.g., the output facet 123 of the waveguide 122/wavelength conversion device 120 depicted in FIG. 1) is substantially the same as the optical path of the laser cavity (i.e., a cavity matching condition), the period of the gain curve modulation is substantially the same as the cavity mode inter-distance.
As with
Referring now to
Referring once again to
To obtain a cavity matching condition, the optical length of the waveguide 122 within the wavelength conversion device 120 (or other optical component) should be an integer multiple of the optical path length of the laser cavity LLC. In this condition, the output facet 123 of the waveguide 122 (or wavelength conversion device 120) is positioned such that the output facet 123 of the waveguide 122 defines an external cavity having an optical path length LEC extending from the reflective laser output facet 117 to the output facet 123 of the waveguide 122, wherein the external optical path length LEC is an integer multiple of the optical path length of the laser cavity LLC (i.e., a cavity matching condition). In one embodiment, the waveguide optical length is substantially equal to twice the laser optical path length within the laser cavity LLC.
The optical length of the waveguide should be as close to an integer multiple of the optical path length of the laser cavity LLC as possible. The optical length of the waveguide and the distance of the waveguide from the DBR laser diode should be such that wavelength fluctuations of the IR output beam due to mode hopping are within a frequency conversion bandwidth of the waveguide and wavelength conversion device when the waveguide is incorporated into a frequency-converting system. In one embodiment, the optical length of the waveguide is such that wavelength fluctuations due to mode hopping are less than ±0.05 nm. To achieve stable operation of the DBR laser diode, in one embodiment the optical length of the waveguide is within about 500 μm of an integer multiple of the laser optical path length within the laser cavity LLC.
The output facet 123 of the waveguide 122 should be the dominant reflective surface such that the external cavity is defined by the laser output facet 117 and the output facet 123. Therefore, the output facet 123 of the waveguide 122 should be more reflective to the IR output beam of the DBR laser diode 110 than the input facet 121 as well as the laser output facet 117. In one embodiment, the output facet 123 of the wavelength conversion device 120 has a reflectivity that is higher than the reflectivity of the laser output facet 117 combined with the input facet 121 of the wavelength conversion device 120.
In one particular embodiment, the reflectivities of the laser output facet 117, the input facet 121 and the output facet 123 satisfy the following relationship:
R
3
>R
1
+R
2+2(R1*R2)0.5, Eq. (3),
where R1 is a reflectivity of the reflective laser output facet, R2 is a reflectivity of the input facet of the waveguide, and R3 is a reflectivity of the output facet of the waveguide.
As an example and not a limitation, in one embodiment R1 is about 0.5%, R2 is about 0.1%, and R3 is about 15%. It should be understood that other reflectivities satisfying Eq. (3) may be utilized. To achieve these reflectivities, the laser output facet 117 and the input facet 121 of the waveguide 122 (or wavelength conversion device 120) may be coated with an anti-reflectivity coating. For example, an anti-reflectivity coating may be provided to the input facet 121 such that the input facet has a reflectivity of less than about 0.3%. The output facet 123 may or may not be coated with an anti-reflective coating to achieve the desired reflectivity.
Curve 194 is the result of the simulation when assuming that the dominating mirror is the laser output facet of DBR laser diode, wherein r1/r2/r3=2.5%/0.1%/0.5%, respectively. Curve 196 was calculated assuming that the external cavity was dominated by the output facet of the SHG crystal, wherein r1/r2/r3=0.5%/0.1%/15%, respectively. As can be seen, the second case results in mode hops that remain of small amplitude although the cavity matching condition is wrong by half a millimeter. Accordingly, high reflectivity factors on the output facet of the SHG crystal may help to relax the manufacturing and alignment tolerances of the wavelength conversion device to achieve stable lasing operation.
It is to be understood that the preceding detailed description is intended to provide an overview or framework for understanding the nature and character of the subject matter as it is claimed. It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
It is noted that terms like “preferably,” and “typically,” when utilized herein, are not intended to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. Further, it is noted that reference to a value, parameter, or variable being a “function of” another value, parameter, or variable should not be taken to mean that the value, parameter, or variable is a function of one and only one value, parameter, or variable.
For the purposes of describing and defining the present invention it is noted that the terms “substantially,” “approximately” and “about” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation. e.g., “substantially above zero,” varies from a stated reference, e.g., “zero,” and should be interpreted to require that the quantitative representation varies from the stated reference by a readily discernable amount.
