Patent Application
20190157840
This invention relates generally to semiconductor lasers, and more particularly to ultra-low noise, narrow-linewidth, highly stable single-longitudinal-mode operation, high power, Bragg grating based semiconductor lasers.
Lasers with an ultra-low noise, including narrow linewidth operation, e.g. ˜1 kHz down to 1 Hz linewidth, are often required to support high performance optical communication systems and sensing systems, as well as low relative intensity noise (RIN) operations, e.g. <−155 dB/Hz. High power is also required for use in high performance systems without the need for optical amplification, or for limited booster amplification, with power levels from e.g. 50 mW up to 200 mW being required. Operating wavelengths can include a very wide range, ranging from ultraviolet (UV) e.g. 250 nm out to many microns, e.g. >10 microns.
Existing low-noise, narrow linewidth lasers such as solid-state lasers and fiber lasers have large size, large cost, limited operating wavelength ranges, and they are often unreliable and not suited for wide-scale commercial deployment. Semiconductor lasers have proven to be the best solution for wide-scale commercial deployment because they leverage the benefits of the semiconductor manufacturing process. Existing semiconductor-based laser systems cannot adequately support high performance systems with all the above requirements. Therefore, there is a need for an ultra-low noise, highly stable singlemode operation, and high power semiconductor-based laser to meet this long-felt need.
The following novel concepts according to embodiments of the present invention provide an ultra-low noise, highly stable singlemode operation, high power, semiconductor-based external cavity laser (ECL). Singlemode operation specifically refers to single-longitudinal mode or single-frequency operation consistent with a narrow linewidth laser. The concepts are applicable to both a hybrid integrated version using a Bragg grating based reflector and separate gain chip, or an integrated laser (monolithic or heterogeneously integrated).
To create an ultra-low phase (or frequency) noise laser, characterized by a very narrow linewidth as well as very small low-frequency phase (frequency) noise, the laser cavity should have a very long photon lifetime; that is, a long cavity and a high storage of photons compared to the number of photons leaving the cavity. Therefore, the first concept used in the basic design of this ultra-low noise laser according to an embodiment is to use a long external cavity to reduce the noise, linewidth, etc. The effects of using a long external cavity in laser are described in Refs. [1]-[3].
Using a broadband reflector with a long external cavity produces a narrow linewidth laser, however, the laser is not stable and is unlikely to operate in a single mode, and will generally mode-hop (jump) between different possible laser cavity modes with only small gain differences; this is the approach taken by many other researchers. In order to provide singlemode operation, a grating, such as a fiber Bragg grating (FBG), can be used to provide the feedback; the FBG provides a narrow reflection bandwidth, and provides mode control so that singlemode operation can be achieved.
Semiconductor lasers have their linewidth increased due to the alpha factor, α, also known as the linewidth enhancement factor, which increases the Schawlow Townes linewidth by a factor (1+α2), and so a low alpha factor will reduce the linewidth of such a laser (see Refs. [1] and [2]).
An important concept for reducing the laser linewidth is operation on the high-slope, long wavelength side of a grating reflector. This takes advantage of an effect described in Refs. [3]-[5], often called “Detuned Loading”, as described in those references; the use of a frequency dependent loss mechanism (e.g. a dispersive loss), together with the alpha-factor of the laser (the linewidth enhancement factor), to simultaneously increase the modulation bandwidth of the laser while reducing the chirp and noise. The long wavelength slope of the grating has the correct sign to work with the alpha-factor to reduce noise and make the laser stable. The opposite happens on the short wavelength side of the grating. An increased value of the frequency dependent loss (higher slope) has a strong effect in reducing noise, leading to designs with long gratings that have very high slope and very narrow bandwidth. In the design of hybrid-integrated laser according to an embodiment, all the design requirements are achieved at the same time; long cavity, detuned loading, and excellent mode selectivity, through very careful grating and laser cavity design. By designing the laser to operate with an extended singlemode operating range, the lasing mode can be set to be on the long wavelength slope of the grating, by controlling the cavity phase; the larger the singlemode range, the further to the long wavelength side the laser will operate, providing a lower linewidth output.
Using the design concepts described in the above embodiments, a laser cavity design including a high power gain chip (i.e. longer gain chip, such as 1-3 mm long) and an FBG external cavity can be achieved, with a high coupling efficiency fiber lens placed close to the FBG to couple light from the gain chip to the FBG efficiently.
One embodiment of the present invention provides a laser including: a semiconductor gain chip; an external cavity; and a first thermally conductive baseplate; wherein a first end of the gain chip has a high reflectivity facet forming a first end of the laser cavity; a second end of the gain chip has an low reflectivity facet, allowing light generated from the gain chip to be coupled with a first end of the external cavity; and a second part of the external cavity includes a Bragg grating which forms the second end of the laser cavity, supported by the first thermally conductive baseplate, the temperature of the Bragg grating being maintained through a feedback loop comprising a first thermo-electric cooler (TEC) and a first thermistor attached to the first thermally conductive baseplate; wherein the optical length of the external cavity is at least an order of magnitude greater than the optical length of the gain chip; wherein the Bragg grating occupies a majority of the length of the external cavity; and wherein the Bragg grating is apodized to control the sidemodes of the grating reflection.
The description of illustrative embodiments according to principles of the present invention is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description of embodiments of the invention disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not intended in any way to limit the scope of the present invention. Relative terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation unless explicitly indicated as such. Terms such as “attached,” “affixed,” “connected,” “coupled,” “interconnected,” and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. Moreover, the features and benefits of the invention are illustrated by reference to the exemplified embodiments. Accordingly, the invention expressly should not be limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features; the scope of the invention being defined by the claims appended hereto.
This disclosure describes the best mode or modes of practicing the invention as presently contemplated. This description is not intended to be understood in a limiting sense, but provides an example of the invention presented solely for illustrative purposes by reference to the accompanying drawings to advise one of ordinary skill in the art of the advantages and construction of the invention. In the various views of the drawings, like reference characters designate like or similar parts.
The Bragg grating 180 can be written directly into an optical fiber, as in a fiber Bragg grating (FBG), which can be accomplished with tremendous precision using industrial processes to create FBGs with extremely well controlled reflection properties, such as extremely narrow reflection bandwidth, grating apodization (to control sidelobes), and controlled grating chirp. The FBG is a preferred approach to develop ultra-low noise ECLs, due to the extreme control that can be used in fabricating these devices, leading to an extremely narrow reflection bandwidth with very sharp filter characteristics. Similar gratings are possible in silicon nitride based waveguides, silica and other low loss waveguides, using ultra-low kappa grating designs, which can also be used in these ultra-low noise lasers.
In order to provide a very stable singlemode operation device with ultra-low linewidth, a long grating or FBG is used in the laser cavity, providing a long cavity length, also providing a very narrow reflection bandwidth. When taken to the extreme, as in this concept, the FBG itself makes up a large portion of the laser cavity, at least 75%, providing excellent mode control and ensuring that only one mode can lase. The bandwidth of the FBG narrows as it is extended in length, countering the effect of the narrower mode spacing from the long laser cavity, while the proportion of the laser cavity within the FBG increases. The concept is to make the grating a sufficiently large part of the overall cavity length to ensure singlemode operation for a large part of the operating range of the laser, e.g. for high power gain chip physical lengths in the range of 600 microns to 1 mm or more (equivalent to optical length ˜2 mm to 3.5 mm in air), the FBG physical length should be significantly longer, e.g. ≥40 mm (equivalent to optical length ≥60 mm in air), i.e. in this case the FBG is ˜20× the optical length of the gain chip, to ensure a large singlemode operating range. The grating must start close to the gain chip, i.e. there should be no long region of the external cavity without a grating in it. As the grating is made longer, e.g. 50 mm or 100 mm, the percentage of the overall cavity being part of the grating increases, making the laser singlemode operation range even larger. Alternatively, the longer grating can support a longer gain chip (for higher power) or longer gain chip plus laser cavity region without a grating. Increasing the percentage of the laser cavity taken up by the grating, and fabricating a grating with as narrow a bandwidth as possible, creates the condition where for much of the operating range of the laser, i.e. for different cavity phases, the device has only one possible lasing mode and a very stable singlemode operation, providing a large singlemode operating range.
The optimum design for providing a wide singlemode operating region has the fiber lens at the edge of the FBG, or close to it. This can be accomplished practically by lensing the FBG fiber very close to the end of the FBG. The lensed fiber end provides the high coupling efficiency required for high power laser operation. The fiber lens is also AR coated to reduce unwanted optical reflections within the laser cavity, which can increase laser noise and reduce the singlemode operation range if significant.
A major issue with using a long grating or FBG in the laser cavity as described above is that the reflection characteristics of the long FBG (or integrated Bragg grating) become more sensitive to variations in the local grating sections along the length of the Bragg grating, especially as the FBG length is increased sufficiently to provide a very narrow bandwidth and also provide a large singlemode operating region. The longer the FBG, the narrower its bandwidth, and therefore the more sensitive it is to local variations which may occur from changes in temperature or strain along the length of the FBG. Variations in temperature along the length of a long FBG are inevitable when one end of the FBG is mechanically and thermally attached close to the gain chip, which itself is a heat source that increases in heat generation as its current bias is increased. For the long FBG lengths envisioned in this novel laser design, e.g. ≥40 mm in length, small variations in temperature, e.g. tenths of a degree, change the reflection peak wavelength along the length of the FBG and therefore broaden its bandwidth; which reduces the singlemode operating range of the laser. For this reason, lasers incorporating a Bragg reflector designed by other researchers have limited the length of the Bragg reflector. The following novel concepts according to embodiments of the present invention are proposed in order to overcome this limitation, and allow clean singlemode operation over a wide range for much longer FBG lengths, leading to lasers with narrower linewidth.
The first concept according to an embodiment of the present invention incorporates the FBG and gain chip on the same long thermally conductive baseplate, as shown in
The second design concept according to an embodiment of the present invention places the gain chip and FBG on separate baseplates (or heatsinks) 310, 340, as shown in
Using the two separate baseplates as described above, and extending the length of the FBG ≥40 mm, the laser can be provided a good singlemode operating range even as a short section of the fiber cavity between the fiber lens and the start of the grating is made up of fiber without a grating. This short section of fiber without a grating can be extended until the laser singlemode range becomes too limited; the longer the FBG, the longer the non-grating fiber section can be made. By increasing the FBG length, the non-grating fiber length 410 can be extended until it covers the distance from the attached fiber lens end, across to the second baseplate, and into the FBG holder 420 as shown in
In order to extend the singlemode operating range of the laser, the effect of sidemodes of the FBG grating need to be considered. The reflection spectrum of a uniform 40 mm FBG provides the narrowest bandwidth for this grating length, however, the sidemodes are large and affect the singlemode operation range of the laser, as well as its noise performance. The closest sidemode on the long wavelength side of the grating is close to the lasing wavelength, especially when the lasing mode is positioned on the long wavelength side of the grating reflection spectrum in order to reduce the laser linewidth. To extend the singlemode operating range of the laser, the sidemodes of the grating reflection must be reduced. One way to accomplish this is to apodize the grating, i.e. to vary the amplitude along the grating with e.g. a Gaussian profile. The power reflectivity spectrum 520 of a 40 mm FBG that is apodized using a Gaussian amplitude profile is shown in
In order to control the position of the lasing mode relative to the FBG reflection spectrum, placing it at the optimum position for singlemode stability and optimum noise performance (narrow linewidth and low RIN), i.e. on the long wavelength side of the reflection peak, it would be useful to have a feedback control loop to measure some aspects of the laser operation and use that information to control the mode position. This feedback loop would provide continuous singlemode laser operation over the life of the laser. An alternative approach is to set the laser at the correct operating mode position at the start of its operating life, with the laser design and component reliability optimized to prevent significant movement of the lasing mode from the correct position on the FBG spectrum over the life of the component.
The very narrow optical filter of the FBG and the novel laser cavity design ensure that only one mode can lase, and if that mode is locked to a specific position on the FBG reflection spectrum, through an electronic feedback loop, then the laser will always stay in that same single mode over the life of the component. The feedback mechanism according to an embodiment of the present invention uses the optical output power of the laser, which has passed through the FBG (a narrow filter), as well as the back facet monitor (BFM) current that measures the light from the gain chip HR facet (no filter) at the other end of the laser. Taking the ratio of the BFM value to the output power provides a signal on which to lock the laser; a fixed value for this ratio will set the laser at a specific position on the FBG spectrum. Comparing the BFM and output power signals allows the elimination of laser intensity variations from the feedback signal, as intensity variations are in both signals. An example of the measured singlemode power (SM) 620, the multimode power (NM) 630, the BFM output 610 and the ratio of BFM/SM 640 are shown in
Locking the laser to a specific value of BFM/SM using a fast feedback loop, which uses the large slope of the FBG filter on the long wavelength side of the FBG spectrum as a frequency discriminator to convert laser output frequency changes to amplitude changes, will also reduce the frequency noise of the laser at frequencies within the feedback loop bandwidth. Reducing the low frequency phase/frequency noise of the laser is important for a number of sensing applications, and in coherent mixing applications.
Note that in
While the present invention has been described at some length and with some particularity with respect to the several described embodiments, it is not intended that it should be limited to any such particulars or embodiments or any particular embodiment, but it is to be construed with references to the appended claims so as to provide the broadest possible interpretation of such claims in view of the prior art and, therefore, to effectively encompass the intended scope of the invention. Furthermore, the foregoing describes the invention in terms of embodiments foreseen by the inventor for which an enabling description was available, notwithstanding that insubstantial modifications of the invention, not presently foreseen, may nonetheless represent equivalents thereto.
This application claims the benefit of U.S. Provisional Patent Application No. 62/377,760 filed on Aug. 22, 2016. The disclosure of U.S. Provisional Patent Application 62/377,760 is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62377760 | Aug 2016 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15683380 | Aug 2017 | US |
| Child | 16237643 | US |
