Patent Application
20150092808
The present teachings relate to lasers. In particular, the present teachings relate to semiconductor laser resonator devices fabricated in integrated circuits (ICs).
Laser devices that generate high coherence lasers are used in a wide range of applications. Typically, laser devices having large cavities are used for obtaining desired levels of coherence because larger cavities (in other words, larger energy storage capacity) directly reduce quantum phase noise and also render the generated laser frequency less sensitive to cavity vibration. However, this basic scaling rule of larger size is at odds with emerging trends that place focus on reducing the size of high coherence light sources in order to provide easier interaction or coupling with a variety of elements such as integrated circuits, tapered optical fibers, and miniature couplers. Unfortunately, such size reduction is often achieved at the expense of coherence, because quantum and technical noise contributions to laser coherence increase as laser-cavity form factor is decreased.
In the context of such efforts, while stimulated Brillouin scattering is a well-known effect, its application in microcavities for laser sources has only recently been documented in bulk-type CaF2 cavities and microsphere cavities [15, 16]. The principal challenge is to create sufficiently high-Q devices with the requisite mode spacing so as to precisely frequency match the narrow-band Brillouin gain spectrum. Specifically, the narrow linewidth of the Brillouin gain requires better than 1:1000 control of the resonator diameter to obtain a match and more realistically it requires 1:10,000 control for consistent low-threshold turn-on power. So far, the process of manufacturing resonator devices in larger volumes has relied on statistical procedures involving variation in size and shape to create accidental matches using different order of transverse modes among devices.
Generally, as part of this matching process, current manufacturing processes involve producing a batch of laser devices, say by using semiconductor manufacturing techniques, and then testing each of the devices in the batch of devices to identify a few devices that have an acceptable level of variance in performance characteristics between the devices. This process includes accepting a certain level of coherence that is a compromise value in comparison to large cavity devices.
Consequently, it is desirable to fabricate miniature laser devices that do not significantly sacrifice coherence for the sake of compactness. It is further desirable to provide such devices in the form of silicon-based integrated circuits with better manufacturing yield.
According to a first aspect of the present disclosure, a highly coherent chip-based laser generating system is provided. The system includes a disk resonator that incorporates a wedge structure fabricated from a silicon dioxide layer of a chip. The disk resonator is operable to generate a highly-coherent laser from a low-coherence optical pump input provided at an optical power level as low as 60 microwatts.
According to a second aspect of the present disclosure, a highly coherent laser generating system is provided. The system includes a disk resonator fabricated from a silicon substrate with a sub-micron cavity size control, the disk resonator operable to generate a highly-coherent laser using stimulated Brillouin scattering on a chip, the highly-coherent laser characterized in part, by a Schawlow-Townes noise level as low as 0.06 Hz2/Hz at a power level of about 400 μW.
According to a third aspect of the present disclosure, a method of manufacturing laser devices for one or more coherent laser generating systems, the method includes fabricating a batch of disk resonators from a silicon substrate, the fabricating comprising precision control of a diameter dimension to obtain a substantial similar Q factor in each disk resonator.
Further aspects of the disclosure are shown in the specification, drawings and claims of the present application.
Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale. Instead, emphasis is placed upon clearly illustrating various principles. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
Throughout this description, embodiments and variations are described for the purpose of illustrating uses and implementations of the inventive concept. The illustrative description should be understood as presenting examples of the inventive concept, rather than as limiting the scope of the concept as disclosed herein. For example, it will be understood that terminology such as, for example, optical, photonic, silica, silicon and chip are used herein as a matter of convenience for description purposes and should not be interpreted literally in a narrowing sense. Furthermore, a disk resonator may be alternatively referred to herein as a wedge, resonator, a circular wedge resonator, a waveguide resonator, a simulated Brillouin laser (SBL) device, a laser resonator device or other such terms; optical propagation may be alternatively referred to herein as photonic propagation or light propagation; a light source may be indicated as providing a laser; and input light power may be referred to as input optical power. A person of ordinary skill in the art will understand that these terms may be used interchangeably and as such must be interpreted accordingly. It will be also be understood that the drawings use certain symbols and interconnections that must be interpreted broadly as can be normally understood by persons of ordinary skill in the art. As one example, of such interpretation, the sloping surface on manufacturing stage 2 in
Attention is now drawn to
A high coherence laser is generated in accordance with the invention by a SBS process implemented in an ultra-high-Q disk resonator on a chip. By designing and fabricating the cavity size such that the cavity free-spectral-range matches the Brillouin shift frequency in silica, the Brillouin lasering process features maximum gain parameter and thus low threshold (as low as 60 microwatts), and high efficiency (90% overall efficiency and 95% differential efficiency) as described below in various exemplary embodiments. This approach makes SBL in microcavities a controllable and highly efficient process, in contrast to the statistical and uncontrollable approaches reported in [15, 16].
Optical pump 11 may be implemented using a variety of light sources. In various embodiments, optical pump 11 is typically implemented using a low-coherence continuous wave (CW) laser, for example, an external cavity diode laser. Simulated Brillouin laser (SBL) disk resonator 12 is designed and fabricated such that its free-spectral-range (FSR) matches the Brillouin shift in silica. Therefore, both the pump and the stimulated Brillouin stokes are guaranteed to be both in resonance within the disk cavity and thus the Brillouin stokes laser can oscillate above pump threshold. The stimulated Brillouin laser (SBL) on a chip features high coherence, and is characterized, at least in part, by very low Schawlow-Townes noise (ST) noise Sv. In one exemplary embodiment, the ST noise, Sv, is less than 0.06 Hz2/Hz measured at an output power of approximately 400 μW. The Schawlow-Townes noise recorded in accordance with the invention (0.06 Hz2/Hz) may be the lowest fundamental noise of any-chip based laser. The ST noise, Sv, relates to the fundamental full-width-half-maximum linewidth Δv of the Lorentzian laser spectrum as: Δv=2π·Sv. Thus the fundamental laser linewidth of the SBL is 0.38 Hz. In comparison, state-of-the-art, high-coherence, monolithic semiconductor lasers feature a fundamental linewidth of 3 kHz [14] by using a long-cavity, corrugation-pitch-modulated laser structure.
Furthermore, the low-frequency technical noise generated in simulated Brillouin laser (SBL) disk resonator 12 is advantageously comparable to commercial fiber lasers. Such a performance in a chip-based device is attributable to a ultra-high-Q cavity [9, 10] that enables precise matching of the Brillouin gain shift to the free-spectral-range (FSR), thereby guaranteeing reliable oscillation. Specifically, simulated Brillouin laser (SBL) disk resonator 12 features more than 90% conversion of lower-coherence pump input (provided by optical pump 11) to high-coherence laser output, at threshold powers as low as 60 μW.
Elaborating upon the subject of noise, it may be pertinent to point out that frequency noise in laser sources has two general contributions: one associated with technical noise such as thermal drift and microphononics; and a second of fundamental origin, the Schawlow-Townes noise from spontaneous emission. Driven by the need for better frequency stability in applications like LADAR and optical communications [30-32], there has been increasing interest in laser sources that achieve stability more typical of fiber lasers, but in a compact footprint. Reduction of laser cavity size, however, typically works against this objective as the Schawlow-Townes linewidth scales inverse-quadratically with cavity Q factor at a given output power. This frequency-noise scaling feature of lasers is, in fact, responsible for the dominant role of Schawlow-Townes noise in telecommunications semiconductor lasers [12].
As described below in more detail, simulated Brillouin laser (SBL) disk resonator 12 is fabricated in a silicon-based integrated circuit (IC) thereby providing various benefits including size reduction, high yield, cost reduction, and ease of integration with various optical elements such as optical fibers and optical waveguides. In accordance with the invention, the manufacture of laser resonator device 12 involves techniques that provide not only a sub-micron level of precision in fabricating a diameter dimension of a circular resonator disk, but further result in the circular disk resonator providing Q factors as high as 875 million. The sub-micron precision in fabrication of the diameter, implemented for example, as a resonator size control to 0.3 micron, enables a precise frequency match to a desired Brillouin performance characteristic.
Attention is now drawn to
By proper control of taper loading, the simulated Brillouin laser (SBL) may be operated in two distinct ways: cascade or single-line. In the cascade mode of operation, the loading of a waveguide in simulated Brillouin laser (SBL) disk resonator 12 is kept low so that once oscillation on the first Stokes line occurs, this action can function as a pump wave for a second Brillouin wave and so on. In contrast to the cascade mode of operation, in the single-line mode of operation the waveguide loading is set to a value that critically couples the pump wave at the target laser output power. This operational mode advantageously suppresses oscillation on all modes leading to high side-mode suppression.
The threshold for SBL action is given by the following expression:
Beyond the importance of high cavity Q factor evident in this expression, it is very desirable to maintain a large SBL gain parameter, gb(Δω−ΩB) (where gain=gbPpump, Δω is the free-spectral range, and ΩB is the Brillouin shift). Because the gain spectrum is relatively narrow (typical full-width half maximum is 20-60 MHz [22, 29]), this requires a precise match of the free-spectral-range to the Brillouin shift. ΩB depends on the pump wavelength λp and phonon velocity Va through the relation ΩB/2π=2nVa/λp.
where τd is the Mach-Zehnder delay and Vpp is the peak-to-peak voltage of the detected MZI output over one fringe. This formula is used to plot the frequency-fluctuation spectral density in
The 1/f noise that appears at lower carrier offset frequencies is indicated in
In certain implementations, low coherence DFB lasers may be manufactured with the wavelengths set on the ITU grid by control of an integrated grating pitch. Final control may then be provided by temperature tuning of the fully packaged device. A DFB laser could be tuned through this same process to function as an SBL pump so that the emitted SBL wavelength resides at the desired ITU channel. In this way, the existing WDM infrastructure could be adapted for high coherence operation in optical QAM systems. The frequency noise levels demonstrated here exceed even state of the art monolithic semiconductor laser by 40 dB. Using the measured phase noise, it is estimated that Square 1024-QAM formats could be implemented using an SBL generated optical carrier at 40 GB/s.
Attention is now drawn to
The manufacture flow (indicated by arrow 160) begins with a silicon substrate 105. Though the description below may suggest fabrication of a single device from silicon substrate 105, it will be understood that multiple devices may be fabricated from a single silicon substrate 105. This form of batch manufacturing of devices is known in the art and will not be elaborated upon herein in the interests of avoiding distraction from the main aspects of the manufacture in accordance with the invention. It will be also understood that producing multiple devices from a single substrate 105 provides several advantages. Specifically, in accordance with the invention, the fabricating of a batch of devices is carried out with sub-micron control of a diameter dimension to obtain a substantial similar Q factor in each disk resonator. For example, various steps such as creating a mask by applying a photo-resist layer upon a silicon dioxide layer and the subsequent step of etching allow sub-micron control of the diameter dimension, thereby eliminating or minimizing the prior art process of relying on statistical procedures involving variation in size and shape to create accidental matches among resonator devices manufactured in larger volumes.
By proper control of both the wet and dry etching processes described below, the Q of the resulting resonator can be nearly 1 billion. Moreover, the lithography and etching process provides precise control over the cavity size, and more specifically allows fabrication of specific cavity sizes with sub-micro precision. Such precisely manufactured cavities are advantageous for executing SBS lasering operations. The cavity size control may be defined in terms of a diameter control, which is quantified by a ratio between an “uncertainty” of cavity size to a diameter of the cavity. A diameter control of 1:20,000 is deemed adequate to place the microcavity FSR within the Brillouin frequency shift for generating a laser in accordance with the invention.
Turning back to the manufacture flow shown in
Significantly, during this heating process, steam is introduced into the furnace. In other words, silicon substrate 105 is subjected to heat in a humid atmosphere for forming silicon dioxide layer 110. While the water constituent is desirable at this stage of manufacture, it is desirable that any residual moisture in the product be eliminated subsequently.
Consequently, the water content is purged by placing assembly 100 in a furnace operating at 1000 degrees Celsius for approximately a 24 hour period. This drying out process contributes to a significant level of reliability in obtaining a desirably high Q factor in the manufactured product.
In manufacture stage 1, an assembly 120 is formed by applying a photo-resist layer 115 upon silicon dioxide layer 110 in the form of a suitable pattern. Various patterns may be used, including patterns used for fabricating a batch of resonator devices. Specifically, in one exemplary embodiment, a circular pattern is used to obtain a waveguide such as the disk resonator in accordance with the invention. The application of photo-resist layer 115 may be carried out in different ways. For example, photo-resist layer 115 may be patterned using a stepper on thermally grown silicon dioxide layer 115 having a thickness in the range of 2-10 microns.
In manufacture stage 2, assembly 120 is immersed in a suitable etchant, such as, for example, a buffered hydrofluoric acid solution. After immersion for a certain period of time, the etchant acts upon silicon dioxide layer 110 to not only expose a portion of the top surface of silicon substrate 105, but also to form a wedge structure 135 below photo-resist layer 115. In contrast to the exposed top surface of silicon substrate 105, which is relatively smooth, the sloping surface of the wedge structure 135 has a certain roughness as well as a deformity in the form of what is referred to herein as a “foot region” 130.
The surface roughness of wedge structure 135, as well as the presence of foot region 130, is undesirable because both these features create a negative impact on light propagated inside the wedge structure 135, more specifically on the Q factor of the finished product. As is known, the Q factor defines a photon storage time in a waveguide structure, with a higher Q indicating a larger storage time.
Consequently, manufacturing stage 2 is followed by manufacturing stage 3, wherein the period of time selected for manufacturing stage 2 is specifically extended in order to obtain a higher Q by eliminating foot region 130 and producing a smooth, linear surface having a desirable slope angle 125 in the wedge structure 135.
The desirable slope angle 125 ranges from about 7 degrees to about 90 degrees. Prior art solutions have expressly aimed at manufacturing and using angles below 27 degrees. However, such small angles do not provide satisfactory performance in terms of obtaining high Q values, and furthermore suffer from certain handicaps during manufacture as well as in product structure. One of the handicaps associated with prior art manufacturing is the need to include a reflow process in order to obtain high Q in a lossy foot wedge region. As for product structure, one of the handicaps pertains to a relatively large spacing requirement between two adjacent elements such as between a pair of waveguide channels. This spacing aspect may be better understood from additional description below.
Slope angle 125 may be empirically controlled in various ways. In one implementation, slope angle 125, as well as the surface roughness of the wedge structure 135, is controlled by selecting an appropriate adhesion factor incorporated into photoresist layer 115. The adhesion factor incorporated into photoresist layer 115 may be varied by using various types of adhesion promoters. In other words, for a given period of time, to obtain a larger slope angle, a promoter that provides higher adhesion may be used because such a promoter prevents fast penetration between the photoresist layer 115 and silicon dioxide layer 110.
In manufacture stage 4, photo-resist layer 115 is removed by employing a cleaning process, after which assembly 145 is exposed to a xenon difluoride (XeF2) environment that etches a portion of silicon substrate 105 and forms a support pillar that supports wedge structure 135. The removal of the portion of silicon substrate 105 results in an undercut below wedge structure 135, the undercut serving to isolate light 111 propagating inside wedge structure 135 from coupling into silicon substrate 105. The undercut may be set to various values. For example, in one embodiment, the undercut is set to about 100 microns for a 1 mm diameter wedge structure, and in another embodiment, the undercut is set to over 150 microns for a 7.5 mm diameter wedge structure.
A disk resonator fabricated upon a chip, and having a Q factor of about 875 million may be manufactured via the method of manufacture described above, wherein a disadvantageous prior art reflow step has been eliminated. This Q factor is achievable in an important size range of diameters greater than 500 microns, and may be used for realizing microwave rate FSR. A disk resonator fabricated using this method of manufacture not only provides a new benchmark for Q factor on a chip, but also provides precise control of micro cavity size. This precise control of micro cavity size permits placing of the micro cavity FSR within the Brillouin frequency shift for generating a laser in accordance with the invention.
Furthermore, it will be understood that while the method described above described the creation of a silicon dioxide layer upon a silicon substrate, the method may be similarly applied using substrates other than silicon.
Various configurations of this sloping surface will now be described in order to highlight the effects of slope angle 125 (in wedge resonator 150) upon photonic propagation through wedge structure 135 of circular wedge resonator 150.
When slope angle 125 is relatively acute (less than 27 degrees, typically around 7 degrees) as shown in embodiment 365, a distorted mode profile 340 is created during a fundamental TE mode of photonic propagation.
When slope angle 125 is greater, not only does mode profile 340 suffer less mode compression, but the widths of surfaces 310 and 315 may be made smaller in width than those of embodiment 365.
When slope angle 125 approaches 90 degrees, mode profile 340 suffers reduced mode compression and the widths of surfaces 310 and 315 become more optimally minimal. Consequently, in certain embodiments in accordance with the invention it is preferable to incorporate slope angles ranging from about 60 degrees to about 90 degrees. However, in certain other embodiments, it may be preferable for various other reasons, to incorporate slope angles down to around 7 degrees.
The description above that is made with reference to various figures illustrates certain aspects of the invention. Additional description provided below provides further insights into certain benefits and features.
Prior art approaches have taken into consideration the Q factor in a wedge-shaped resonator device fabricated of silica on a silicon wafer [6]. Q factors as high as 50 million were obtained in these prior art approaches, which primarily focused on reducing optical loss by isolating the propagation mode from the lithographic blemishes near the outer rim of the resonator by using a shallow wedge angle. In contrast to the Q values obtained in the prior art solutions, the approach described herein results in an optical Q that is boosted by a factor of 15× beyond the earlier achieved values. An exemplary Q factor of 875 million has been achieved using various embodiments described herein. This Q factor corresponds to an equivalent waveguide loss of 0.03 dB/m.
While the devices in accordance with the invention may resemble the earlier geometry to some extent, these new devices derive ultra-high Q performance from overall changes to the etch process and the oxide growth. Specifically, the physical principle at work in the earlier structures, a shallow wedge angle to provide roughness isolation, is no longer required. Indeed, and in contrast to earlier work, larger wedge angles are desirable. For example, these larger wedge angles can range from 27 degrees to 90 degrees as described herein.
The ability to lithographically define ultra-high Q resonators as opposed to relying upon the reflow process enables a multi-order-of-magnitude improvement in control of resonator diameter and free-spectral-range (FSR).
All patents and publications mentioned in the specification may be indicative of the levels of skill of those skilled in the art to which the disclosure pertains. All references cited in this disclosure are incorporated by reference to the same extent as if each reference had been incorporated by reference in its entirety individually.
It is to be understood that the disclosure is not limited to particular methods or systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. The term “plurality” includes two or more referents unless the content, clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains.
The examples set forth above are provided to give those of ordinary skill in the art a complete disclosure and description of how to make and use the embodiments of the waveguides of the disclosure, and are not intended to limit the scope of what the inventors regard as their disclosure. Modifications of the above-described modes for carrying out the disclosure may be used by persons of skill in the video art, and are intended to be within the scope of the following claims.
A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, other embodiments are within the scope of the following claims.
The present application claims priority to U.S. Provisional Patent Application No. 61/498,442 entitled “Highly Coherent, Microcavity Brillouin Laser on Silicon” filed on Jun. 17, 2011 which is incorporated herein by reference in its entirety. The present application may also be related to: i) U.S. Provisional Patent Application No. 61/498,444 entitled “Chip-Base Frequency Comb with Microwave Repetition Rate” filed on Jun. 17, 2011, ii) U.S. Provisional Patent Application No. 61/496,440 entitled “Ultra Low Loss Wedge Waveguide and Resonator on a Chip” filed on Jun. 13, 2011, iii) U.S. Pat. No. 8,045,834, and iv) U.S. Utility patent application Ser. No. 13/494,707 entitled “Silica-on-Silicon Waveguides and Related Fabrication Methods” filed on Jun. 12, 2012, all of which are also incorporated herein by reference in their entirety.
This invention was made with government support under FA9550-10-1-0284 awarded by the United States Air Force. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61498442 | Jun 2011 | US |
