WO2015020628A1

WO2015020628A1 - Non-lasing light source having amplified spontaneous emission and wavelength-selective feedback

Info

Publication number: WO2015020628A1
Application number: PCT/US2013/053638
Authority: WO
Original assignee: Avolonte Health LLC
Priority date: 2013-08-05
Filing date: 2013-08-05
Publication date: 2015-02-12

Abstract

A light source includes a waveguide having an optical path that extends longitudinally between a back facet and an emission facet. The optical path includes a gain region that generates light through spontaneous emission. At least some of the spontaneous emission light propagates to a distributed Bragg reflector (DBR) in the optical path. The distributed Bragg reflector has a relatively high reflectivity within a characteristic DBR wavelength range, and a relatively low reflectivity outside the DBR wavelength range. The spontaneous emission light within the DBR wavelength range reflects back to the gain region, is amplified by the gain region, and exits the waveguide through the emission facet to form the output light. The spontaneous emission light outside the DBR wavelength region can be attenuated, such as by an absorbing region in the optical path, and/or directed out of the waveguide through the back facet.

Description

NON-LASING LIGHT SOURCE HAVING AMPLIFIED

SPONTANEOUS EMISSION AND WAVELENGTH-SELECTIVE

FEEDBACK

TECHNICAL FIELD

[0001] The present invention relates to broadband mid-infrared

semiconductor light sources based on amplified spontaneous emission (ASE), also known as superluminescence.

BACKGROUND

[0002] The mid-infrared spectral region (2-20 μιη wavelength) is of significant interest for spectroscopic applications because many strong molecular absorption features fall within this range. However, thermal sources of mid- infrared light have insufficient brightness per unit optical bandwidth for many applications. In recent years, quantum cascade (QC) structures have emerged as an effective means of generating optical gain over extremely wide wavelength ranges in the mid-infrared. See, for example, Reference 1 , noted in a list of references at the end of the Detailed Description.

[0003] In contrast to conventional inter-band optical light emitters (LEDs and lasers), whose emission wavelengths are determined primarily by the semiconductor material's bandgap, QC devices operate on intra-band transitions, whose energies can be tailored over nearly the entire mid-infrared spectrum, independent of the bandgap energy, by altering the widths of a multiple quantum well structure. In recent years, tunable external-cavity quantum cascade lasers (QCLs) have been fabricated that provide room temperature, continuous- wave operation, with power in the tens or hundreds of milliwatts in a single spatial mode. These lasers can be tunable over a range of several hundred inverse centimeters (up to about 10 % of center frequency, and possibly more). There have been demonstrations of QC material with a gain spectrum covering over Such single-frequency lasers can operate with narrow linewidths, and hence exhibit a high degree of spatial and temporal coherence. Narrow linewidth QC lasers can be of great utility for applications such as probing narrow spectral lines in gasses.

[0004] For other applications, however, it is desirable to have a mid- infrared source with higher spectral brightness and better spatial coherence than a thermal source, but larger spectral bandwidth than a laser. In applications such as optical coherence tomography (OCT) or Fourier transform infrared spectroscopy (FTIR), it is necessary or desirable to have a broadband, low- temporal-coherence source with a single mode spatial profile. Furthermore, for spectroscopy of fluid species, it is only necessary to resolve absorption features with bandwidths on the order of 10 cm^"1, and in some cases 50-100 cm^"1. By probing such features with a low-coherence source with bandwidth greater than 1 cm^"1, the effect of parasitic Fabry- Perot etalons due to back reflections off of components within the optical train can be reduced.

[0005] In the near-infrared wavelength range, superluminescent diodes based upon amplified spontaneous emission have emerged as effective broadband sources. These devices are very sensitive to optical feedback, however, and care must be taken with optical isolation if they are operated at very high power levels. Attempts to make a superluminescent quantum cascade source emitting in the mid-infrared have been limited to an etched-facet device with relatively poor performance. See, for example, Reference 3. There have been no demonstrations of sources with smooth output spectrum at mW- level optical power. A smooth output spectrum is of intrinsic value for spectroscopic applications, and also is an indicator of low temporal coherence. The lack of a compact mid-infrared optical isolator has been one factor limiting explorations in this field, and hindering the development of highly integrated mid-IR systems. For many applications in spectroscopy where the highest possible optical output power is not essential, and where controlled spectral response and insensitivity to optical feedback are more important, the device described here can create a reduced-coherence light source with high brilliance and reduced feedback sensitivity. SUMMARY

[0006] A light source includes a waveguide having an optical path that extends longitudinally between a back facet and an emission facet. The optical path includes a gain region that generates light through spontaneous emission. At least some of the spontaneous emission light propagates to a distributed Bragg reflector (DBR) in the optical path. The distributed Bragg reflector has a relatively high reflectivity within a characteristic DBR wavelength range, and a relatively low reflectivity outside the DBR wavelength range. The spontaneous emission light within the DBR wavelength range reflects back to the gain region, is amplified by the gain region, and exits the waveguide through the emission facet to form the output light. The spontaneous emission light outside the DBR wavelength region can be attenuated, such as by an absorbing region in the optical path, and/or directed out of the waveguide through the back facet. The output light has a wavelength spectrum that corresponds to the DBR wavelength range.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Fig. 1 is a schematic side view of an example light source.

[0008] Fig. 2 is a schematic top view of the light source of Fig. 1.

[0009] Fig. 3 is a schematic top view of another example light source.

[0010] Fig. 4 is a plot of an example DBR reflectance spectrum, an example ASE spectrum, and an example light emission.

[0011] Fig . 5 is a schematic side view of an example light source.

[0012] Fig . 6 is a plot of an example DBR wavelength range.

[0013] Fig . 7 is a schematic side view of an example light source.

[0014] Fig . 8 is a schematic side view of an example light source.

[0015] Fig . 9 is a schematic side view of an example light source.

[0016] Fig . 10 is a schematic side view of an example light source.

[0017] Fig . 11 is a schematic side view of an example light source.

[0018] Fig . 12 is a schematic side view of an example light source.

[0019] Fig . 13 is a schematic side view of an example light source.

[0020] Fig . 14 is a schematic side view of an example light source. DETAILED DESCRIPTION

[0021] In this patent document, wavelengths of light are expressed in wavenumber, where a wavenumber is (1 / wavelength). Wavenumbers are often expressed in cm^"1, so that the wavenumber physically represents how many wavelengths fit into one cm. For example, the mid-infrared spectral region of 2 μιη to 20 μιη has a corresponding wavenumber range between 5000 cm^"1 and 500 cm^"1.

[0022] This disclosure describes a superluminescent mid-infrared light source based upon a quantum cascade semiconductor structure, with controlled spectral response and bandwidth. A distributed Bragg reflector (DBR) grating on the end of the waveguide opposite the emission facet, patterned by electron beam or optical lithography, provides selective optical feedback that determines the center wavelength and spectral width of the light emission. An absorbing waveguide section behind the DBR eliminates feedback from wavelengths within the quantum cascade gain structure but outside the DBR stop band. The incorporation of one or more techniques to mitigate or eliminate feedback from the emission facet and external optical components ensures that the device operates in the superluminescent regime, below the lasing threshold and without facet damage, even in the absence of an external magneto-optic isolator. One of the techniques to reduce feedback sensitivity includes the incorporation of a lossy waveguide segment behind the emission facet.

[0023] The proposed superluminescent mid-infrared source emits broadband, low-coherence light, and provides control over the tradeoff between output power and sensitivity to optical feedback. A distributed Bragg reflector can be added behind the gain region to determine both the center wavelength and spectral width of the emitted light.

[0024] An example device is a broadband quantum cascade epitaxial structure with large gain bandwidth (for example, >100 cm^"1). The structure is fabricated by molecular beam epitaxy (MBE), metal-organic vapor phase epitaxy / metal-organic chemical vapor deposition (MOVPE/MOCVD), or a similar technique. In one possible implementation, this gain structure consists of two or more cascaded active region designs, each designed for a different center wavelength, but overlapping in spectral coverage. See, for example, Reference 2. Each of these cascaded active region structures can utilize bound-to- continuum or similar broadband-transition schemes to achieve optimal spectral coverage. Alternatively, the example device can have an active region design that has sufficient gain bandwidth for the application of interest.

[0025] The device is fabricated using standard semiconductor processing techniques to form a ridge waveguide containing the epitaxial structure.

[0026] For optimal thermal performance, epitaxial regrowth of indium phosphide may be employed to form a buried heterostructure device.

[0027] To achieve higher output power and reduced beam divergence in one axis, one end of the waveguide may be adiabatically tapered such that single lateral mode emission is maintained, as has been demonstrated in quantum cascade lasers. See, for example, Reference 4. While the (single-transverse- mode) QC gain waveguide acts as a spatial filter to suppress high-order

(undesired) transverse modes, the tapered structure allows for amplification to higher power levels than may otherwise be practical at the output facet for a strictly single-mode waveguide design.

[0028] The waveguide/epitaxial structure design of the present device may differ from that of a standard quantum cascade laser, as higher waveguide loss can be tolerated, or even beneficial, as it increases the noise figure. As in doped- fiber amplifier devices, an increased noise figure results in an increase in ASE power for a fixed level of net device gain. When increased waveguide loss can be tolerated, highly-doped regions or metal layers may be positioned closer to the active region of the epitaxy, allowing more efficient electrical injection or better thermal properties, for example. Hence, the optimization of waveguide design for feedback-insensitive ASE output is inherently different than the optimization of waveguide design for highly efficient laser output. Most/all teaching in the art of QC devices, semiconductor lasers, and semiconductor amplifiers emphasizes design solutions that optimize laser output power and/or efficiency, and tend to teach against the design goals of this device. See, for example References 7-8.

[0029] A wavelength- selective feedback element may be incorporated on one end of the laser waveguide. This could be an etched grating followed by overcoating to form a distributed Bragg reflector (DBR). The reflectance spectrum of the mirror can be tailored by choice of etch depth, grating duty cycle, and overcoating material in order to adjust the mode index contrast. The length of the DBR region (e.g., number of periods) will further determine the spectral width and strength of the feedback. The DBR has a grating period that can be uniform, or can be non-uniform. Non-uniform grating periods can include chirped, or superimposed grating structures that have multiple grating periods, or any other continuous or discontinuous grating function that is not strictly periodic. An additional design freedom that may be employed is linear or non-linear chirping of the DBR grating period. This enables arbitrarily wide reflection bandwidths, with independent control of grating reflectivity. In principle, the shape (reflectivity vs. wavelength) of the reflection spectrum can be used as a design parameter to compensate for the QC gain spectrum, or to achieve a desired output spectral shape.

[0030] With the grating present, light generated by spontaneous emission in the gain region of the waveguide will be amplified as it propagates through the waveguide. Light propagating toward the DBR will be reflected if it is in the range of wavelengths reflected by the DBR, otherwise, it will not be reflected.

[0031] The DBR segment is either sufficiently long such that light that is not reflected is substantially attenuated inside the grating, or one of the several techniques listed below is employed behind the DBR, at the end of the waveguide farthest away from the emission facet.

[0032] The DBR segment can include a length of electrically unpumped (or pumped at a substantially reduced current density) waveguide that is sufficiently long such that the waveguide losses (due to free-carrier absorption in the doped semiconductor regions, as well as metal losses, and other loss mechanisms) are sufficient to substantially attenuate the beam.

[0033] The waveguide width can adiabatically taper (narrow) to decrease the optical confinement of the mode, thus decreasing the modal gain while increasing the waveguide losses.

[0034] The rear laser facet can be formed by patterning (by optical or e- beam lithography) and reactive ion etching, instead of cleaving. The rear facet pattern can be made deliberately rough, patterned with a non-planar shape, or tilted by an angle large in comparison to the mode divergence, in order to decrease the amplitude of the mode-matched portion of reflections off the facet. Furthermore, the plasma etching process will result in a rougher facet, which will decrease the mode-matched reflections.

[0035] An electrode may be used to inject current into the DBR region, either in order to decrease optical losses or to change the refractive indices in the DBR by thermal heating, which would tune the center wavelength of the reflectance spectrum.

[0036] Light reflected by the grating will be amplified as it propagates toward the emission facet of the device.

[0037] As the gain spectrum of the QC material will be substantially broader than the DBR stop band width, a significant fraction of light generated by spontaneous emission inside the waveguide will be at wavelengths outside the desired range selected by the DBR. Some of this out-of-band light will be propagating in the direction of the emission facet and will be amplified by the time it reaches the facet. However, the intensity of this light will be less than that of the in-band light, because some of the in-band spontaneous emission will experience substantial double-pass amplification by being reflected off the back DBR. The higher the gain in the active region, the better the suppression ratio will be.

[0038] To further suppress out-of-band ASE, a DBR band pass filter can be fabricated in the waveguide on the emission facet end of the gain region.

[0039] It may be desirable for this filter to diffract light out of the waveguide, rather than to reflect the undesirable light backwards into the waveguide, where it would be amplified.

[0040] It may also be desirable to locate this band pass filter after a lossy waveguide section, so as to attenuate any back-reflected light at undesirable wavelengths.

[0041] A waveguide directional coupler is a very simple mechanism to provide a large, and easily engineerable, wavelength-dependent loss. This could be used to pass a wavelength range near the DBR reflection spectrum, while substantially attenuating (via leakage to a secondary waveguide) all other wavelengths within the QC gain spectrum. Such a device could be integrated between the gain/ASE section and the output facet. The result would be an improved output spectrum with substantially reduced pedestal. Roughly 20dB of suppression would be highly practical. Because directional couplers have a roughly sinusoidal response vs. wavelength, they are tolerant to slight manufacturing variations, and hence, do not require exquisite matching of the DBR reflection spectrum to the filtering spectrum.

[0042] To limit optical feedback from the emission facet, one or several of the following may be employed.

[0043] By lithographically patterning the waveguides at an angle relative to the crystal plane of the substrate, when the devices are cleaved, the waveguide facet will be angled relative to the orientation of the waveguide, such that light reflected from the facet is not coupled back into the waveguide. See, for example, Reference 6.

[0044] Deposition of a single- or multi- layer dielectric anti-reflection coating upon the facet, in order to decrease the amount of reflection due to the high index contrast of the semiconductor/air interface. These anti-reflection coatings should offer low reflectance (ideally < 1 %, or more preferably <0.25% or even <0.1%) over the entire (ideally) gain spectrum of the epitaxy, or at least, over the spectrum of the DBR reflectivity. This coating is additionally beneficial because it results in increased output power, as compared with an uncoated device. See, for example, Reference 6.

[0045] Including a length of electrically unpumped (or pumped at a reduced current density) waveguide that is sufficiently long such that the waveguide losses (due to free-carrier absorption in the doped semiconductor regions, as well as metal losses) attenuate the beam to an extent such that the single-pass losses decrease the optical output power by a tolerable amount, but the double-pass losses (encountered when an optical element is placed in front of the laser, such as a lens, reflects light back into the cavity) are large enough to limit the amount of optical feedback into the gain medium.

[0046] As an alternative to (iii) above, including a length of waveguide with an electrode on top that permits biasing independently from the main gain region of the laser. This electrode can be left unbiased, or can be biased at a desired setting below the transparency current. This allows dynamic adjustment of the optical losses in the waveguide segment, allowing the beam to be attenuated to an extent such that the single-pass losses decrease the optical output power by a tolerable amount, but the double-pass losses (encountered when an optical element is placed in front of the laser, such as a lens, reflects light back into the cavity) are large enough to limit the amount of optical feedback into the gain medium.

[0047] In conjunction with the independently biased QC waveguide segment being operated as an integrated variable optical attenuator, a feedback system can be constructed in which the device output is of some combination of a spectrometer, power meter, and a computer or microcontroller can be implemented to dynamically adjust both the gain section drive current and attenuator section biasing such that the device is maintained below the lasing threshold with the desired spectral properties.

[0048] Because the gain spectrum of the material is broader than the emission wavelength of the device described here, several of these devices, each emitting at different center wavelengths, can be monolithically integrated on the same substrate, by lithographically defining the DBR regions on each waveguide to have a different periodicity or duty cycle, such that the adjacent waveguide devices emit light with different center wavelengths within the gain spectrum of the underlying quantum cascade epitaxy.

[0049] These multiple waveguides, sharing a common epitaxy but distinct DBR regions, could be combined into a common output waveguide via an integrated waveguide element, such as a directional coupler, Y-junction, MMI- combiner, etc. See, for example, References 9-11.

EXAMPLE

[0050] One possible implementation is depicted in Fig. 1 (side view) and Fig. 2 (top view). The device includes a 10 micron wide waveguide containing the epitaxial quantum cascade active region, patterned at an angle of about 5 degrees relative to the cleaved crystal facet that forms the front facet of the device. An absorbing region, approximately 500 microns long, is situated behind the front facet. Electrical contact is made to this region by a gold contact, allowing the loss of this segment to be adjusted as desired. This segment acts to reduce the sensitivity of the device to light reflected by components in the optical path. Behind the absorber region is the electrically- pumped gain region, approximately 500 microns long, in which the amplified spontaneous emission occurs. Behind the gain region is the etched DBR grating to provide optical feedback over the desired wavelength range. The center wavelength of the DBR stop band can be tuned by current injection, which changes the effective refractive indices in the DBR region. Behind the DBR is a passive absorber section, at least 250 microns long, to absorb the majority of the light that is not reflected by the DBR grating. The metal on top of this passive absorber increases the waveguide losses. The back facet of the device is deliberately roughened, or coated with an antireflection coating to prevent any additional optical feedback.

[0051] Fig. 3 presents the side view of an alternative implementation, in which the gain region is tapered to achieve higher output power. In this implementation, the facets are not angled relative to the waveguide. In this implementation, the antireflection coating on the rear facet is replaced by a tapered rear waveguide section that adiabatically increases the waveguide loss by decreasing the modal confinement, ensuring that no light is reflected from the back of the waveguide.

[0052] Fig. 4 illustrates how the output spectrum of the light is determined by the combination of the gain spectrum of the material (manifested in the amplified spontaneous emission spectrum in the absence of optical feedback) and the DBR reflectance spectrum. The reflected light experiences large double- pass gain and constitutes the majority of the emitted light.

[0053] We now discuss the sensitivity of the designs described above to optical feedback. In a worst-case scenario of 100% optical feedback, based on the design described above, it is possible to construct an implementation that operates without reaching the laser threshold.

[0054] We consider a representative set of parameters such as a QC gain region with 30dB of distributed gain (single-pass), and lOdB of distributed loss (single-pass), hence 20dB of net single-pass gain. On the back end, an ideal Bragg reflector with nearly 100% reflectivity over a 10cm^"1 bandwidth is chosen. If the absorbing section in front of the gain region is designed or biased to achieve a distributed 20dB of waveguide loss, no amount of optical feedback can be a problem, whether it comes from the output facet reflectivity or the downstream optical setup.

[0055] The ASE output power can be enhanced while still ensuring total feedback insensitivity if the following two conditions are met. First, the "back end" Bragg reflector has <100% peak reflectivity, and is protected from feedback from the back facet, such as by an absorbing waveguide, AR-coating, and/or an angled-facet. Second, the QC device has more distributed gain and more distributed loss (i.e. worse "noise figure").

[0056] In a realistic scenario, optical feedback (due to facet reflectivity or system reflections) on the order of a few percent or less can be expected, and as such, the loss of the front absorbing region can be decreased, which will increase the ASE output power while still maintaining insensitivity to optical feedback.

[0057] Fig. 5 shows an example light source 500. The light source 500 includes a waveguide 110 extending longitudinally between a back facet 112 and an emission facet 114. The back facet 112 and the emission facet 114 define an optical path 116 therebetween. The waveguide 110 is configured to transmit light along the optical path 116. The emission facet 114 is configured to at least partially transmit light therethrough to form output light 118.

[0058] The light source 500 includes a first gain region 120 disposed in the optical path 116. The first gain region 120 is configured to produce light through spontaneous emission, to transmit light therethrough, and to amplify light transmitted therethrough. The amplified light is below a lasing threshold and has an intensity insufficient to trigger stimulated emission.

[0059] The light source 500 includes a distributed Bragg reflector (DBR) 130 disposed in the optical path 116 between the first gain region 120 and the back facet 112. The distributed Bragg reflector 130 is configured to reflect light from the first gain region 120 within a DBR wavelength range, and at least partially transmit light from the first gain region outside the DBR wavelength range. In a round-trip of the optical path 116, a cumulative optical loss can exceed an optical gain of the first gain region 120.

[0060] The light source 500 includes a first absorbing region 140 disposed in the optical path 116 between the distributed Bragg reflector 130 and the back facet 112. The first absorbing region 140 is configured to at least partially absorb light transmitted through the distributed Bragg reflector 130. The absorbing region 140 can be passive or active.

[0061] In some examples, the first gain region 120 has a gain wavelength range that corresponds to amplified spontaneous emission in the first gain region 120. In some examples, the DBR wavelength range 202 has a width that corresponds to a number of grating periods in the distributed Bragg reflector 130. In some examples, the DBR wavelength range 202 has a center wavelength that corresponds to a grating period in the distributed Bragg reflector 130. In some examples, the DBR wavelength range 202 is a subset of the gain wavelength range.

[0062] In some examples, where the emission facet 114 produces feedback into the optical path 116, the feedback is lowered by at least one of an anti- reflection coating, an angling away from perpendicularity to the optical path, or curvature. In some examples, where the back facet 112 produces feedback into the optical path 116, the feedback is lowered by at least one of an anti-reflection coating, an angling away from perpendicularity to the optical path, curvature, or surface texturing.

[0063] In some examples, the first gain region 120 has a first gain wavelength range that corresponds to amplified spontaneous emission in the first gain region 120. In some examples, the DBR wavelength range 202 is a subset of the first gain wavelength range.

[0064] In some examples, the distributed Bragg reflector 130 is tunable, so that the DBR wavelength range has a controllable center wavelength.

[0065] Fig. 6 shows an example reflectivity 200 of a distributed Bragg reflector (DBR), such as the DBR 130 depicted in Fig. 5. The DBR 130 can have an associated DBR wavelength range 202. The output light 118 depicted in Fig. 5 can have a spectrum that corresponds to the DBR wavelength range 202.

[0066] Figs. 7 through 14 include various combinations of elements in the optical path 116.

[0067] Fig. 7 shows a light source 700, in which the distributed Bragg reflector 130 and the first gain region 120 are disposed within the optical path 116. The light source 700 is devoid of an absorbing region. [0068] Fig. 8 shows a light source 800, in which the first absorbing region 140 and the first gain region 120 are disposed within the optical path 116. The light source 800 is devoid of a distributed Bragg reflector.

[0069] Fig. 9 shows a light source 900, in which a second absorbing region 910 is disposed in the optical path 116 between the first gain region 120 and the emission facet 114. A single-pass absorption of the second absorbing region 910 can be greater than a single-pass gain of the first gain region 120. In some examples, the first absorbing region 140 is passive. In some examples, the second absorbing region 910 is active.

[0070] Fig. 10 shows a light source 1000, in which a second gain region

1010 is disposed in the optical path 116 between the first gain region 120 and the emission facet 114. The first gain region 120 has a first gain wavelength range that corresponds to amplified spontaneous emission in the first gain region 120. The second gain region 1010 has a second gain wavelength range that corresponds to amplified spontaneous emission in the second gain region 1010. The first gain wavelength range overlaps with the second gain wavelength range.

[0071] Fig. 11 shows a light source 1100, which includes a first absorbing region 140, a first gain region 120, and a second gain region 1110 in the optical path 116. The light source 1100 is devoid of a distributed Bragg reflector.

[0072] Fig. 12 shows a light source 1200, which includes a first absorbing region 140, a first gain region 120, and a second absorbing region 1210 in the optical path 116. The light source 1200 is devoid of a distributed Bragg reflector.

[0073] Fig. 13 shows a light source 1300, which includes a waveguide directional coupler 1310 disposed in the optical path 116 between the first gain region 120 and the emission facet 114. The light source 1300 includes a first absorbing region 140 in the optical path 116. The light source 1300 is devoid of a distributed Bragg reflector.

[0074] Fig. 14 shows a light source 1400, which includes a waveguide directional coupler 1410 disposed in the optical path 116 between the first gain region 120 and the emission facet 114. The light source 1400 includes a distributed Bragg reflector 130 in the optical path 116. The light source 1400 is devoid of an absorbing region. REFERENCES

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[0094] The above Detailed Description includes references to the accompanying drawings, which form a part of the Detailed Description. The drawings show, by way of illustration, specific embodiments in which the device and method can be practiced. These embodiments are also referred to herein as "examples." Such examples can include elements in addition to those shown or described. However, the inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein.

[0095] In this document, the terms "a" or "an" are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of "at least one" or "one or more." In this document, the term "or" is used to refer to a nonexclusive or, such that "A or B" includes "A but not B," "B but not A," and "A and B," unless otherwise indicated. In this document, the terms "including" and "in which" are used as the plain-English equivalents of the respective terms "comprising" and "wherein." Also, in the following claims, the terms "including" and "comprising" are open-ended, that is, a system, device, kit, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms "first," "second," and "third," etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.

[0096] The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) can be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features can be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter can lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various

combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

What is claimed is:

1. A light source, comprising:

a waveguide extending longitudinally between a back facet and an emission facet, the back facet and the emission facet defining an optical path therebetween, the waveguide configured to transmit light along the optical path, the emission facet configured to at least partially transmit light therethrough to form output light;

a first gain region disposed in the optical path, the first gain region configured to produce light through spontaneous emission, to transmit light therethrough, and to amplify light transmitted therethrough, the amplified light being below a lasing threshold and having an intensity insufficient to trigger stimulated emission; and

a distributed Bragg reflector (DBR) disposed in the optical path between the first gain region and the back facet, the distributed Bragg reflector configured to reflect light from the first gain region within a DBR wavelength range, and at least partially transmit light from the first gain region outside the DBR wavelength range;

wherein the output light has a spectrum that corresponds to the DBR wavelength range; and

wherein in a round-trip of the optical path, a cumulative optical loss exceeds an optical gain of the first gain region.

2. The light source of claim 1, further comprising:

a first absorbing region disposed in the optical path between the distributed Bragg reflector and the back facet, the first absorbing region configured to at least partially absorb light transmitted through the distributed Bragg reflector.

3. The light source of claim 2, further comprising:

a second absorbing region disposed in the optical path between the first gain region and the emission facet, wherein a single-pass absorption of the second absorbing region is greater than a single-pass gain of the first gain region;

wherein the first absorbing region is passive; and

wherein the second absorbing region is active.

4. The light source of claim 1,

wherein the first gain region has a gain wavelength range that corresponds to amplified spontaneous emission in the first gain region;

wherein the DBR wavelength range has a width that corresponds to a number of grating periods in the distributed Bragg reflector;

wherein the DBR wavelength range has a center wavelength that corresponds to a grating period in the distributed Bragg reflector; and

wherein the DBR wavelength range is a subset of the gain wavelength range.

5. The light source of claim 1 , wherein the emission facet produces feedback into the optical path, the feedback being lowered by at least one of an anti-reflection coating, an angling away from perpendicularity to the optical path, or curvature.

6. The light source of claim 1,

wherein the first gain region has a first gain wavelength range that corresponds to amplified spontaneous emission in the first gain region; and wherein the DBR wavelength range is a subset of the first gain wavelength range.

7. The light source of claim 1 , wherein the distributed Bragg reflector is tunable, so that the DBR wavelength range has a controllable center wavelength.

8. The light source of claim 1 , wherein the back facet produces feedback into the optical path, the feedback being lowered by at least one of an anti- reflection coating, an angling away from perpendicularity to the optical path, curvature, or surface texturing.

9. The light source of claim 1, further comprising:

a second gain region disposed in the optical path between the first gain region and the emission facet;

wherein the first gain region has a first gain wavelength range that corresponds to amplified spontaneous emission in the first gain region;

wherein the second gain region has a second gain wavelength range that corresponds to amplified spontaneous emission in the second gain region; and wherein the first gain wavelength range overlaps with the second gain wavelength range.

10. The light source of claim 1, further comprising a waveguide directional coupler disposed in the optical path between the first gain region and the emission facet.

11. A light source, comprising:

a first gain region disposed in the optical path, the first gain region configured to produce light through spontaneous emission, to transmit light therethrough, and to amplify light transmitted therethrough, the amplified light being below a lasing threshold and having an intensity insufficient to trigger stimulated emission;

a first absorbing region disposed in the optical path between the first gain region and the back facet, the first absorbing region configured to at least partially absorb light emerging from the first gain region;

12. The light source of claim 11, further comprising:

13. The light source of claim 11, wherein the emission facet produces feedback into the optical path, the feedback being lowered by at least one of an anti-reflection coating, an angling away from perpendicularity to the optical path, or curvature.

14. The light source of claim 11, wherein the back facet produces feedback into the optical path, the feedback being lowered by at least one of an anti- reflection coating, an angling away from perpendicularity to the optical path, curvature, or surface texturing.

15. The light source of claim 11, further comprising a distributed Bragg reflector (DBR) disposed in the optical path between the first gain region and the first absorbing region, the distributed Bragg reflector configured to reflect light from the first gain region within a DBR wavelength range, and at least partially transmit light from the first gain region outside the DBR wavelength range.

16. The light source of claim 15,

wherein the DBR wavelength range has a center wavelength that corresponds to a grating period in the distributed Bragg reflector; and wherein the DBR wavelength range is a subset of the gain wavelength range.

17. The light source of claim 15,

18. The light source of claim 11, wherein the first absorbing region is passive.

19. The light source of claim 11, wherein the first absorbing region is active.

20. The light source of claim 11, further comprising a waveguide directional coupler disposed in the optical path between the first gain region and the emission facet.