Diode pumping photon integrated titanium-sapphire waveguide amplifier

Abstract

A photonic circuit Ti: sapphire optical amplifier is provided. Critical aspects of the technique include near perfect mode overlap between the signal and the pump in the amplifier waveguide, and low loss wavelength division multiplexing techniques for separating / combining the signal and the pump. Applications include various amplifier configurations, narrow linewidth photonic circuit lasers, and photonic circuit incoherent amplified spontaneous emission sources.

Description

Technical Field

[0001] This invention relates to photonic circuit optical amplifiers. Background Technology

[0002] Titanium-doped sapphire (Ti:Sa) is an important laser gain medium, particularly noteworthy due to its large gain bandwidth. To date, most Ti:Sa lasers have been applied to conventional bulk optical systems, but recent research has demonstrated integrated optical Ti:Sa lasers. However, current research has not considered optical amplifiers with integrated optical configurations within this material system. Therefore, providing photonic circuit Ti:Sa amplifiers would be a significant advancement in this field. Summary of the Invention

[0003] This study extends previous research on integrated optical Ti:Sa lasers to amplifiers and also considers several specific applications of integrated optical Ti:Sa amplifiers. Here, we achieve, for the first time, ultrawideband amplification of near-infrared signals on a photonic integrated circuit. Currently, optical amplification on photonic integrated circuits is limited to wavelengths longer than 1000 nm. However, many applications require wavelengths shorter than 1000 nm. Furthermore, high-gain ultrawideband optical amplifiers are currently unavailable on photonic integrated circuits due to limitations imposed by the use of semiconductor optical amplifiers. This, in turn, hinders the realization of femtosecond mode-locked lasers on-chip or wide-tunable lasers within biological windows.

[0004] An exemplary embodiment includes a nanophotonic optical waveguide implemented in a crystalline thin film of titanium-sapphire, integrated with a semiconductor diode laser for pumping the titanium-sapphire waveguide. This nanophotonic titanium-sapphire waveguide provides optical amplification of optical signals in the wavelength range of 700 nm to 1000 nm. Furthermore, the titanium-sapphire waveguide does not exhibit absorption when the material is not pumped, and it exhibits a passive propagation loss of less than 0.45 dB / cm.

[0005] Titanium-sapphire waveguides are important products, serving as stand-alone optical amplifiers or as broadband coherent light sources for optical coherence tomography. Furthermore, the 700 nm to 1000 nm wavelength range is crucial for many applications such as augmented and virtual reality, optical detection and ranging, microscopy, and medical imaging.

[0006] Ti:sapphire waveguide amplifiers achieve on-chip optical gain across the entire 700 nm to 1000 nm wavelength spectrum. For example, this coverage encompasses the near-infrared window, crucial within biological tissues, playing a key role in medical imaging and diagnostic equipment. Furthermore, this broad gain bandwidth facilitates efficient and distortion-free on-chip amplification of femtosecond pulses, a feature of significant importance in metrology and atomic optical clocks. Currently, waveguides characterized by optical gain are primarily limited to semiconductor-based optical amplifiers. These semiconductor devices typically exhibit optical bandwidths on the order of tens of nanometers, occasionally extending to 100 nanometers. Moreover, achieving coverage of any desired wavelength in the 700 nm to 1000 nm wavelength range covered by Ti:sapphire is not a straightforward task, as the layer stacking of these diodes requires precise engineering to match specific wavelengths. Therefore, Ti:sapphire waveguides outperform currently available optical gain waveguides, offering potential for novel applications in photonic integrated circuits. Attached Figure Description

[0007] Figures 1A-1E The characterization results of the Ti:Sa waveguide technology are shown.

[0008] Figures 2A-2G The characterization results of the photonic circuit Ti:Sa amplifier are shown.

[0009] Figures 3A-3E Several exemplary amplifier configurations are shown.

[0010] Figure 4 An exemplary multistage amplifier is shown.

[0011] Figure 5 An exemplary narrow-linewidth photonic circuit amplifier is shown, implemented using photonic circuit Ti:Sa amplifier technology.

[0012] Figure 6 An exemplary amplified spontaneous emission source is shown, implemented using photonic circuit Ti:Sa amplifier technology. Detailed Implementation

[0013] Chapter A describes the integrated photonic titanium:sapphire technology, and Chapter B describes several exemplary device configurations implemented using this integrated photonic titanium:sapphire technology.

[0014] A1) Introduction

[0015] Titanium:sapphire (Ti:Sapphire) laser systems play a vital role in fundamental research and technological applications, including the advent of the first optical frequency comb, the generation of light pulses as short as two electric field oscillations, two-photon microscopy and optogenetics, and the development of on-chip laser-driven particle accelerators. As a solid-state (i.e., non-semiconductor) gain medium, Ti:sapphire possesses exceptional properties, exhibiting the widest gain bandwidth (650–1100 nm) of any laser crystal, a large emission cross-section, and a quaternary structure. Therefore, mode-locked and continuous-wave Ti:sapphire lasers are unparalleled in performance and indispensable in disciplines such as quantum optics and atomic physics. However, the unparalleled performance of Ti:sapphire lasers comes at the cost of large, expensive commercial systems and the requirement for high-power pump lasers, hindering their widespread use in many practical applications that demand compactness and scalability.

[0016] Photonic integrated circuit technology is revolutionizing laser systems by leveraging wafer-level fabrication capabilities in material platforms such as silicon-on-insulator, lithium niobate films, and silicon nitride to achieve compactness, scalability, and cost-effectiveness that are unlikely to be realized in benchtop systems. On-chip laser systems have been realized in telecom-optimized wavelength ranges by integrating passive photonic elements with III-V semiconductor optical amplifiers, and these on-chip laser systems are now extending into the near-infrared (NIR) and visible light ranges. However, integrated semiconductor laser and amplifier systems face several inherent limitations, most notably the limited gain bandwidth due to two-photon and free carrier absorption in highly confined waveguides and poor high-power handling capabilities, which limit on-chip pulse energies to a few picojoules. An emerging alternative to on-chip III-V systems is integrated solid-state gain media based on rare-earth ions. Recently, high-performance solid-state waveguide amplifiers and lasers with pulse energies exceeding 100 pJ have been successfully fabricated by doping ultra-low-loss silicon nitride waveguides with erbium ions; however, these and other efforts in rare-earth systems are limited to wavelengths greater than 1 μm. Therefore, the application of the high-performance characteristics required by Ti:sapphire lasers cannot yet benefit from photonic circuit integration. Integrated solid-state lasers and amplifiers operating in both the visible and NIR wavelength ranges will have a profound impact on technological fields where cost, size, and scalability considerations are critical.

[0017] Accordingly, efforts toward miniaturization of Ti:sapphire lasers have a long history. The current limitations in the scalability of Ti:sapphire laser systems are primarily attributed to Ti... 3+The short fluorescence lifetime of ions, combined with the large difference between the pump wavelength (490–532 nm) and the laser wavelength (650–1100 nm), increases system complexity and necessitates very high pump intensities before significant amplification and laser emission can be achieved. To address this, methods such as pulsed laser deposition of waveguides, laser-written waveguides, optical fibers, and mechanically machined whispering corridor mode lasers have been employed to reduce mode volume and lower the laser emission threshold. The recent advent of bonding Ti:sapphire to silicon nitride photonic chips for attenuated coupling to achieve laser emission marks the birth of the first nanophotonic Ti:sapphire laser, but this approach still faces challenges in achieving significant gain due to limited mode overlap. Therefore, practical on-chip Ti:sapphire laser systems have not yet been realized. To date, the highest single-mode output power in on-chip Ti:sapphire lasers is only 40 nW, and tunable lasers have not yet been developed. Furthermore, no attempts have been reported to achieve integrated Ti:sapphire amplifiers.

[0018] In this study, we addressed this final challenge and demonstrated a chip-integrated Ti:sapphire waveguide amplifier that achieves capabilities, stability, and tunability relevant to practical applications in research and technology. Our approach is based on an architecture that reaches the fundamental limits of photonic mode constraint and overlap. We have already developed a low-loss, single-crystal sapphire photonic platform on insulators (Q-factor >10⁶ at 805 nm), which, for example, enables sub-milliwatt threshold Ti:sapphire microdisk lasers. Subsequently, we demonstrated the first integrated Ti:sapphire broadband waveguide amplifier, achieving a record-breaking 64 dB / cm solid-state on-chip gain. This waveguide amplifier is capable of pulse amplification up to 2.3 nJ, a first for on-chip pulse amplification below 1 μm, and further, it exceeds the power handling capability of any nanophotonic waveguide amplifier by an order of magnitude while maintaining a transformation-limited pulse shape.

[0019] A2) Titanium on Insulator: Sapphire Photonic Platform

[0020] Figure 1A The process flow for the preparation of Ti:SaOI is illustrated schematically. Figure 1B This is a SEM image of a Ti:SaOI waveguide fabricated using electron beam lithography. Figure 1C It is a SEM image of the waveguide cross-section. Figure 1D The simulated mode intensity distributions at the pump wavelength (532 nm) and signal wavelength (800 nm) are shown, with a modal intensity overlap Γp,s of 98.8%. Figure 1E The measured exponential decay of waveguide fluorescence with an optical lifetime of 3.09 μs is shown.

[0021] Ti:SaOI manufacturing process in Figure 1A The diagram shows a Ti:sapphire wafer 102 bonded to a sapphire carrier wafer 104 via a SiO2 interface layer 106, which acts as a buried oxide layer. After bonding, the Ti:sapphire layer is thinned by mechanical grinding and polishing, followed by reactive ion etching to a target thickness of less than 1 μm. Using this method, wafer-level production of other high-quality single-crystal photonic platforms, such as silicon carbide-on-insulator (SiC), has become possible. This manufacturing method does not restrict the thickness of the Ti:sapphire layer, which can be precisely controlled via dry etching.

[0022] Direct patterning transfer from photoresist to Ti:sapphire limits the resolution and sidewall angle of the structure, thus hindering the realization of many integrated photonic functions. To address this, we developed a patterning transfer process based on a 200 nm chromium secondary hard mask layer (deposited via electron beam evaporation). Patterning was defined in the chromium mask using electron beam photoresist (FOx-16, Corning) (see the "Methods" section). This process yielded waveguides with nearly vertical (11 degrees) sidewalls and sub-100 nm minimum feature sizes. Figures 1B-1C We note that this process is also compatible with deep ultraviolet lithography for high-throughput manufacturing. This highly constrained, near-single-mode waveguide geometry achieves nearly 100% broadband overlap between the pump mode and the laser emission mode (99.5% at 650 nm and 96.5% at 1100 nm). Figure 1D This is a key advantage of the Ti:SaOI laser platform. Crucially, as quantified by its quantum efficiency, the Ti:sapphire film maintains the quality of the bulk gain medium. The reduction in gain medium efficiency due to the introduction of a nonradiative decay rate is measured as a decrease in excited-state lifetime. We characterize the Ti:SaOI waveguide fluorescence in the time domain by utilizing weakly modulated pump excitation and detecting the emission rate using a superconducting nanowire single-photon detector (SNSPD), and observe pure single-exponential decay with a lifetime of 3.09 μs (…). Figure 1E This is consistent with the lifetime reported in the literature on Ti:TiOI. We conclude that the Ti:SaOI platform is characterized by excellent performance in both passive optical loss and active gain dielectric properties, making it ideal for amplifier fabrication.

[0023] A3) Ultra-wideband waveguide amplifiers in Ti:SaOI

[0024] Figure 2A An optical image of an 8 mm waveguide amplifier pumped from both ends (after 532 nm pump filtering) is shown. Figure 2BThe amplified spontaneous emission (ASE) spectrum collected from a 3 mm waveguide amplifier in the absence of an input signal is shown, along with the small-signal gain (dB / cm) measured for a 0.5 mm waveguide. The fit to the calculated gain line shape is shown as a solid line. Figure 2C The measured small-signal gain (with a pump power of 175 mW) and the measured passive waveguide loss (solid lines between dashed lines) are shown for an 8 mm amplifier. The standard error is shown by the dashed lines. Figure 2D The spectrum of the amplified CW output signal is shown at a peak recording output power of 60 mW. Figure 2E The normalized spectrum of the picosecond pulse before and after propagation in the amplifier is shown, indicating that no nonlinear distortion occurs. Figure 2F The spectrum of the amplified pulse (at a repetition rate of 1.25 MHz) is shown for different amplification levels. The inset shows the unamplified and maximum amplified outputs on a linear scale, indicating that the pulse remains transformation-limited and has only slight distortion at the tail.

[0025] Amplifiers are crucial partners for lasers, allowing for the compensation of losses in coherent signals and enabling power levels that surpass those of the laser source. Broadband amplifiers can also generate incoherent light even without an input signal, powering techniques such as optical coherence tomography (OCT). Solid-state amplifiers offer excellent power handling characteristics, and their media are characterized by low passive losses. Most commercial solid-state amplifier solutions are based on rare-earth dopants in crystals or glasses, limiting wavelengths longer than 1 μm. Integrating such amplifiers onto chips by implanting rare-earth ions into silicon nitride, lithium niobate, or alumina photonic devices is an active area of ​​research, but its inherently low gain, posing a major technical challenge, is limited. To date, on-chip optical gain elements below 1 μm have been limited to III-V semiconductors. Their heterogeneous integration with Si or SiN photonic devices has enabled breakthrough integrated laser technology; however, limited power handling capabilities and bandwidth constrain their applications and versatility. Using the Ti:SaOI platform, we demonstrate the first integrated solid-state amplifier operating in the sub-1 μm range, offering unparalleled power handling and bandwidth in both integrated and desktop cutting-edge systems.

[0026] Outside of nanophotonics, the use of Ti:sapphire as a general-purpose amplifier is limited to costly and specialized applications such as ultra-high pulse energy amplifiers. We demonstrate strong pump confinement in nanophotonics (with an effective mode area of ​​0.62 μm). 2 The near 100% mode overlap results in moderate power requirements for achieving saturation of the gain medium, thus enabling high gain even at low pump power. Ti:SaOI waveguide amplifiers ( Figure 2AIt is fully integrated and extremely compact (0.20 mm). 2 In the absence of an input signal, it has a frequency exceeding 100 THz. Figure 2B This is the widest amplified spontaneous emission source with a bandwidth of 150 mW, the widest to date. Such broadband sources, which can be easily coupled to optical fibers, could find applications in medical fields such as OCT. To measure the maximum gain achievable in Ti:sapphire nanophotonics, we first examined a short waveguide (485 μm in length) where pump absorption, passive losses, and spatial mode mixing effects were negligible. A commercial continuous-wave (CW) Ti:sapphire laser was used as the wavelength-tunable signal. At a pump power of 150 mW, we observed a peak gain of 64 dB / cm, and gains greater than 30 dB / cm across a 100 THz bandwidth. Figure 2B ).

[0027] To achieve a practical amplification level, we used an 8 mm long helical waveguide, in which the pump was almost completely absorbed. We noted that due to Ti... 3+ The four-stage structure exhibits no signal reabsorption in the weak pump region of the waveguide, and the material gain remains consistently positive. Small-signal (3 μW input power) amplification in an 8 mm Ti:SaOI waveguide is achieved. Figure 2C The figure shows that it achieves a peak on-chip gain of >20 dB. Here, the maximum gain is limited by parasitic laser emission caused by waveguide end-face reflections, similar to what has been reported in the literature for on-chip erbium amplifiers. Higher gains can be achieved through designs or packaging that suppress reflections. Figure 2D This demonstrates large-signal amplification, with an output CW power of 60 mW at 790 nm.

[0028] For a long time, high-performance mode-locked lasers have hindered photonic integration due to the challenge of amplifying ultrashort, high-peak-power optical pulses. These lasers are crucial for advancing numerous fields such as supercontinuum and optical frequency comb generation, two-photon microscopy, and dual-comb metrology. Here, we demonstrate the high unit-length gain, lack of two-photon absorption, and weak Kerr nonlinearity (n² = 3 × 10⁻⁶) of Ti:SaOI. -20 m 2 The combination of ( / W) and excellent power handling capabilities enables unprecedented distortion-free on-chip pulse amplification. We used a commercial picosecond mode-locked Ti:sapphire laser as the input signal. To evaluate the degree of passive nonlinear distortion of the pulse propagating in the amplifier, we compared the pulse spectrum (without amplification) before and after passing through the waveguide. For an input pulse energy of 120 pJ, negligible spectral distortion was observed. Figure 2E The shape of the output pulse for different pump powers is as follows: Figure 2F The figure shows the minimum shape change at a peak gain of 17 dB (corresponding to a pulse energy of 2.3 nJ and a peak power of 1.0 kW). This is more than an order of magnitude higher than the highest integrated pulse amplification demonstrated to date based on rare-earth waveguides, and is the only integrated high-power conversion-limited amplifier for any wavelength. The Ti:SaOI amplifier constitutes the first realization of on-chip pulse amplification below 1 μm and demonstrates the potential of this platform for realizing high-performance integrated mode-locked lasers. Figure 2G This shows the dependence of amplified and output pulse energy on repetition rate. The vertical dashed line indicates the lifetime (1 / τ) of the gain medium.

[0029] A4) Discussion

[0030] In this study, we have demonstrated an on-chip integrated, broadband tunable, and scalable Ti:SaOI amplifier in a single-crystal insulator-on-sapphire (SOI) photonic platform. The advent of the Ti:SaOI waveguide amplifier (with on-chip gain greater than 20 dB and peak gain of 64 dB / cm) opens the door to amplification of continuous waves and pulsed light over an ultrawide bandwidth of 650–1100 nm. Due to sapphire's excellent power handling capabilities and weak nonlinearity, we have demonstrated transform-limited amplification of picosecond pulses up to 2.3 nJ, representing the first demonstration of integrated high-energy pulse amplification in the visible to near-infrared range, and the first demonstration of pulse amplification exceeding 150 pJ in any nanophotonic waveguide amplifier. In contrast, this level of performance has never been achieved with such amplifiers due to the short up-state lifetime and two-photon absorption of heterogeneously integrated III-V semiconductor amplifiers. This research addresses one of the major challenges in developing on-chip mode-locked lasers and self-referenced optical frequency combs in a previously unattainable and highly sought-after wavelength range. Through nanophoton confinement, the highly efficient generation of ultrawideband ASE shown in this study can serve as a universal fiber coupling source for high-resolution optical coherence tomography.

[0031] Integrating Ti:sapphire technology onto chips is a key step toward scalable Ti:sapphire systems (both monolithic and integrated with passive materials). Significant advances in heterogeneous laser integration have enabled the combination of other novel on-chip laser technologies with large-scale photonics foundry processes. The approach proposed in this study is compatible with synergistic integration across platforms such as silicon nitride and lithium niobate to achieve submicron wavelength frequency-tunable Ti:sapphire lasers, integrated frequency-doubled lasers, on-chip supercontinuum generation, and integrated optical parametric amplifiers.

[0032] A5) Method

[0033] A5.1) Device Manufacturing

[0034] For amplifier and waveguide demonstrations, we used an a-facet (1120) Ti:sapphire wafer die bonded to undoped sapphire via an interface SiO2 layer that acts as a buried oxide layer. The bonded Ti:sapphire die was then ground and polished in a precision grinding system (PM5, Logitech), followed by thinning via reactive ion etching in BCl3 plasma (PlasmaTherm Versaline ICP) to achieve a target thickness of 450 nm.

[0035] The amplifier was fabricated using electron beam lithography. A chromium hard mask (200 nm) was deposited on a Ti:SaOI thin film via electron beam evaporation. Chromium was chosen to allow etching of sapphire at a sufficient selectivity ratio (4:1). Electron beam lithography resist (FOx-16, Corning) was then spin-coated, and the pattern was defined using a 50 keV electron beam lithography system (Voyager, Raith). The limitation of the lithography system necessitated that the device be constrained to a 0.5 × 0.5 mm area to avoid stitching errors. The pattern was transferred to the Cr mask via Cl2 and O2 plasma, followed by transfer to the sapphire via BCl3 etching. The Cr hard mask was then removed by wet etching with a chromium etchant and Piranha. The device was then covered with an initial layer of flowable oxide (FOx-16, Dow Corning), followed by deposition of an additional SiO2 layer via high-density plasma chemical vapor deposition (HDP-CVD, PlasmaTherm), and annealed in air at 800 °C. The edge coupling facets for devices are generated by focused ion beam milling after wafer sawing or by laser stealth cutting.

[0036] A5.2) Characterization of waveguide amplifiers

[0037] Light was coupled from the chip and free space outside the chip via a microscope objective (M-plan NIR 100, Mitutoyo). The sample substrate temperature was maintained at 290 K throughout the experiment. The 532 nm pump source was a frequency-doubled diode-pumped solid-state laser (Verdi V10, Coherent), delivered to the chip either from a single-sided waveguide endface, a double-sided waveguide endface, or in the opposite direction to signal propagation. Single-sided pumping was used unless otherwise specified. For continuous-wave amplification experiments, a commercial Ti:sapphire laser (SolsTiS, M Squared) with a tuning range of 700–1000 nm was used. Small-signal gain per unit length was measured for an input power of 0.10 mW. To characterize a 0.485 mm long waveguide requiring a resolution less than 0.1 dB, [further details needed]. Figure 2BThe small-signal gain, pump, and signal in the amplifier were chopped and amplified from the time-varying signal recorded on the silicon photodetector. For small-signal gain characterization in the 8 mm amplifier, the output signal was obtained with and without a pump laser using an optical spectroscopy analyzer (Yokogawa AQ6370D). Passive waveguide loss was not accounted for in this measurement. Wavelength-dependent passive waveguide loss was measured based on statistical analysis of waveguides of different lengths. Figure 2C In this context, the cumulative off-chip amplification is represented by the difference between the on-chip amplification measured at a given wavelength and the passive loss. This is used to record the maximum amplified output power (…). Figure 2D A dual-pumping scheme was used. To infer the output power at the chip, the measured output power was corrected for objective loss (transmittance 0.85). For the pulse amplification experiment, a picosecond mode-locked Ti:sapphire laser (Tsunami, Spectra-Physics) was used. The pulse spectrum was analyzed using... 2 The fitting revealed a pulse duration of 2.2 ps, consistent with autocorrelation measurements. To measure the dependence of amplified and output pulse energy on repetition rate, the inherent repetition rate of the laser was reduced (80 MHz) using an electro-optic modulator pulse selector (Conoptics), where a suppression ratio of 6.2 × 10⁻⁶ was measured. 3 .

[0038] B) Exemplary Device Configuration

[0039] B1) Amplifier Configuration

[0040] Figures 3A-3E Several exemplary amplifier configurations implemented using the techniques described above are shown.

[0041] Figure 3A An example is a photonic integrated optical amplifier, which includes: a substrate 302; a first wavelength division multiplexer (WDM) 306 integrated onto the substrate in a planar configuration; an amplifier waveguide 304 connected to the first WDM, the amplifier waveguide 304 being integrated onto the substrate in a planar configuration and including a titanium-doped sapphire gain medium; and a first pump input waveguide 318 integrated onto the substrate in a planar configuration and connected to the first WDM.

[0042] The unique gain characteristics of Ti:sapphire greatly simplify the design and fabrication complexity of suitable WDMs in the material systems described above, allowing those skilled in the art to perform the work. Due to the significant difference between the peak pump wavelength and the laser wavelength (490 nm and 800 nm, respectively), WDM design becomes readily achievable even for the wide laser bandwidth (700 nm – 1000 nm) of Ti:sapphire. This is because the pump wavelength and laser wavelength have significantly different coupling strengths, stemming from the large difference in wavelength scale leading to strongly different modal constraints. This difference between the pump wavelength and the laser wavelength is a unique property of Ti:sapphire, which is not present in many other optically pumped gain media, and allows for efficient routing of pump and signal in independent amplifier devices.

[0043] Figure 3A Examples also include a semiconductor diode laser 314 coupled end-to-end to a first pump input waveguide 318. The semiconductor diode laser is preferably an InGaN diode laser or a GaN diode laser having a generation wavelength in the range of 480 nm to 540 nm.

[0044] The design goals of stand-alone nanophotonic waveguide amplifiers differ significantly from those of photonic integrated laser cavities that include active waveguide gain elements.

[0045] In waveguide laser configurations, the active gain element is part of the passive cavity. In this case, the primary objective is to minimize the round-trip loss of the waveguide laser cavity, which in turn achieves a narrow optical linewidth, a low laser emission threshold, and high conversion efficiency from pump input to laser output. Furthermore, the gain element within the waveguide laser cavity is optimized to suppress spontaneous emission and typically requires only modest gain. For example, the Ti:sapphire laser crystal in a free-space laser cavity is relatively short, and factors such as thermal load and parasitic losses are often major design challenges. Additionally, in a waveguide laser cavity, in the absence of an optical isolator, the gain medium is subjected to incident laser emission modes on both sides. This prevents the use of graded-multimode waveguides to tailor mode dimensions based on power accumulation as the signal travels through the gain waveguide.

[0046] In contrast, standalone waveguide amplifiers are designed to provide significantly higher gain (up to 20 or 30 dB), and because the amplifier is traversed by the signal in only a single direction, the modal cross-section of the amplifying waveguide can be customized at each point to optimize amplification and power handling capabilities. This includes using multimode waveguides with tapered regions to achieve high power handling capabilities. A small-signal gain of 10 dB or higher is preferred for standalone waveguide amplifiers. Here, "small-signal gain" refers to the unsaturated gain provided by the amplifier. For power amplifiers, the gain seen in operation is typically smaller than the small-signal gain, but the small-signal gain is still a definable specification for such amplifiers. Waveguide tapering, as described above, can be used to make the signal modes larger, thereby reducing signal saturation in the amplifier. In integrated optical configurations, such as those considered herein, selectively injecting the signal into a selected mode of the multimode waveguide is possible and often preferred. This can be achieved, for example, by selectively coupling a single-mode waveguide to a desired mode of the multimode waveguide using an adiabatic cone.

[0047] The role of an amplifier is to amplify a wide range of signals, from femtosecond pulses to high-intensity continuous wave signals, as well as inputs with complex spectra. Therefore, the design focuses on maximizing gain and / or output power or pulse energy, rather than minimizing losses or maintaining the narrow linewidth required by the laser.

[0048] Finally, external amplifier waveguide design is heavily influenced by the challenges of incident and outgoing coupling, challenges that do not exist for gain elements within a monolithic waveguide laser cavity. These challenges include: effectively coupling light from a wide range of input sources, handling higher power, effectively combining with high-intensity pump signals, and managing parasitic losses and reflections. Specifically, this involves designing optimized edge-coupled waveguide endfaces or grating couplers, developing anti-reflective coatings, or implementing on-chip anti-reflective structures.

[0049] Figure 3A Examples also include a signal input waveguide 310 integrated into a substrate using a planar technology configuration. In this example, the signal input waveguide 310 is connected to a first WDM 306, and the first WDM is configured to combine the signal and pump within an amplifier waveguide. This is an example of signal and pump co-propagation.

[0050] Figure 3B A first example of backpropagation of the signal and pump is shown. Here, the signal input waveguide 310 is connected to the end of the amplifier waveguide 304 opposite to the first WDM 306. In this example, the first WDM 306 is configured to separate the output signal from the backscattered pump and to provide the output signal in the signal output waveguide 312.

[0051] The on-chip signal input waveguide 310 is optional, such as in... Figure 3C This is seen in an example of backpropagation of the signal and pump. Here, the signal input is directly coupled to the amplifier waveguide 304.

[0052] Including a second WDM in the amplifier configuration is generally preferred, such that one WDM combines the pump and signal onto the amplifier waveguide at the input, and the other WDM separates the pump and signal at the output. Figure 3D An example is shown. Here, we have a second WDM 308, which is integrated onto the substrate in a planar configuration. The second WDM 308 is connected to an amplifier waveguide 304 such that the amplifier waveguide 304 is positioned between the first WDM 306 and the second WDM 308. A signal output waveguide 312 is integrated onto the substrate in a planar configuration and is connected to the second WDM 308. The second WDM 308 is configured to separate the pump and amplified signals and is configured to provide the amplified signal in the signal output waveguide 312.

[0053] As in Figure 3E In the example, the amplifier configuration with two WDMs allows for bidirectional pumping. Here, we have a second pump input waveguide 320, which is integrated onto the substrate in a planar configuration and connected to a second WDM 308, and the second WDM 308 is configured to provide pump (from the second pump source 316) to the amplifier waveguide 304.

[0054] In these examples, the Ti:sapphire waveguide amplifier is implemented as a stand-alone optical amplifier. It is characterized by one or two pump diodes, which are commercially available and have an output power of up to 300 mW. This allows for high-gain (>20 dB) and high-output-power (>60 mW) amplification of incident optical signals (continuous wave or pulse) in the wavelength range of 700 nm to 1000 nm. Thanks to the low passive loss of the Ti:sapphire waveguide, a virtually lossless WDM coupler that separates (or combines) the pump wavelength and the signal wavelength can be realized.

[0055] Any number of pump stages or any pump stage configuration can be used. Figure 4 An example is shown that further includes at least one intermediate WDM (422, 424, etc.), which is integrated onto a substrate in a planar configuration and disposed on an amplifier waveguide 304 between the first and second WDMs (306, 308), wherein each intermediate WDM is configured to provide pump to the amplifier waveguide. Here, additional pump sources 402, 404, etc. are coupled to the WDMs via pump waveguides 412, 414. For simplicity, Figure 4 The example shows that all pumps propagate with the signal, but in reality any combination of pumps propagating in the same direction and in the opposite direction can be used.

[0056] Here, we consider a multi-stage amplifier configuration. This multi-stage amplifier allows for continuous-wave optical output power exceeding 100 mW. This is equivalent to combining the outputs of N pump laser diodes in different stages of the waveguide amplifier. Furthermore, since the current of each pump diode can be individually controlled, the noise figure and gain can be optimized using amplification distribution along the wavelength.

[0057] B2) Narrowband source example

[0058] As in Figure 5 As seen in the example, the techniques described above can be used to fabricate narrowband light sources. This example further includes a waveguide optical filter 502, which is configured and integrated onto a substrate using a planar technique. As shown, a signal input waveguide 310 is connected to a first port of the waveguide optical filter, and a signal output waveguide 312 is connected to a second port of the waveguide optical filter. Thus, a photonic integrated laser with a waveguide optical filter serving as an intracavity filter is provided. Implementation of this embodiment does not critically depend on the type of waveguide optical filter employed. In this example, the waveguide optical filter 502 includes passive waveguide resonators 504 and 506, which provide a narrow passband in transmission known in the art.

[0059] In this example, a titanium-sapphire waveguide is integrated with an ultra-low-loss dielectric waveguide (which could be silicon nitride, silicon carbide, etc.) within a photonic integrated circuit, providing broadband gain for a laser cavity implemented in an ultra-low-loss waveguide platform. For example, ultra-narrow wavelength filters can be implemented in silicon nitride, providing tunable lasers with a tuning range exceeding 50 THz in the 700 nm to 1000 nm wavelength range. Furthermore, such lasers exhibit extremely narrow optical linewidths, making them ideal candidates for applications in quantum optics, atomic timing systems, and optical metrology (LIDAR, spectroscopy, sensing, etc.). Integration of such optical amplifiers with nonlinear photonic platforms (such as lithium niobate) can provide frequency-flexible CW tunable lasers.

[0060] B3) Broadband Source Example

[0061] The amplifier technology described above can also be adapted to provide broadband incoherent light sources, such as in Figure 6As seen in the example. This example is a photonic integrated broadband incoherent light source comprising: a substrate 302; a wavelength division multiplexer (WDM) 306 integrated onto the substrate in a planar configuration; an amplifier waveguide 304 connected to the WDM, wherein the amplifier waveguide is integrated onto the substrate in a planar configuration and the amplifier waveguide includes a titanium-doped sapphire gain medium; a first waveguide beam collector 602 integrated onto the substrate in a planar configuration and having a back reflectivity of 1% or less in the 700 nm to 1000 nm wavelength range; a second waveguide beam collector 604 integrated onto the substrate in a planar configuration and having a back reflectivity of 1% or less in the 700 nm to 1000 nm wavelength range; and a pump input waveguide 318 integrated onto the substrate in a planar configuration, wherein the WDM 306 is connected to the pump input waveguide 318, the amplifier waveguide 304, the first waveguide beam collector 602, and the second waveguide beam collector 604, such that radiation incident on the WDM 306 in the wavelength range of 700 nm to 1000 nm (from the pump input waveguide 318 or the amplifier waveguide 304) is directed to the first waveguide beam collector 602 or to the second waveguide beam collector 604; and an anti-reflection coating 606 is disposed at the output end of the amplifier waveguide. Here, the output radiation from the amplifier waveguide 304 is incoherent amplified spontaneous emission.

[0062] Those skilled in the art can fabricate a suitable waveguide beam collector in the material system described above, which has at least the specified properties.

[0063] This example may further include a semiconductor diode laser 314, which is coupled end-to-end to a pump input waveguide 318. Preferably, the semiconductor diode laser is an InGaN diode laser or a GaN diode laser having an emission wavelength in the range of 480 nm to 540 nm. The amplifier waveguide may be multimode at one or more wavelengths of incoherent amplified spontaneous emission.

[0064] In this example, a Ti:sapphire waveguide is used to realize an ultrawideband incoherent light source. The Ti:sapphire waveguide achieves a transmit bandwidth exceeding 250 nm and 10 dB at a center wavelength of 800 nm. This bandwidth is unattainable with current superluminescent diodes and allows for excellent resolution in optical coherence tomography (OCT) systems. A wavelength division multiplexing (WDM) coupler is added to effectively scatter spontaneous emissions within the chip's slabs without reflection. Combined with an anti-reflective coating (e.g., 150 nm SiOx) on the output facets, ultra-low reflection is achieved, eliminating ripples in the broadband incoherent light, which is crucial for OCT applications. Furthermore, spectral shaping can be further refined as needed by adding wavelength filters to the Ti:sapphire waveguide platform. Finally, the incoherent light can be coupled to a nanophotonic waveguide with dimensions tailored to the required application. This allows for efficient coupling, for example, output to single-mode fiber or other forms of coupled output as needed.

Claims

1. A photonic integrated optical amplifier, comprising: substrate; The first wavelength division multiplexer (WDM) is integrated onto the substrate using a planar configuration. An amplifier waveguide, connected to the first WDM, wherein the amplifier waveguide is integrated onto the substrate in a planar configuration, and wherein the amplifier waveguide comprises a titanium-doped sapphire gain dielectric; and A first pump input waveguide is integrated onto the substrate in a planar configuration and connected to the first WDM.

2. The photonic integrated optical amplifier of claim 1, further comprising a semiconductor diode laser, the semiconductor diode laser being coupled end-to-end to the first pump input waveguide.

3. The photonic integrated optical amplifier of claim 2, wherein the semiconductor diode laser is an InGaN diode laser or a GaN diode laser, and wherein the emission wavelength of the semiconductor laser is in the range of 480 nm to 540 nm.

4. The photonic integrated optical amplifier as claimed in claim 1, wherein the amplifier waveguide is multimode at the signal wavelength.

5. The photonic integrated optical amplifier of claim 1, further comprising a signal input waveguide, the signal input waveguide being integrated onto the substrate in a planar configuration.

6. The photonic integrated optical amplifier of claim 5, wherein the signal input waveguide is connected to the end of the amplifier waveguide opposite to the first WDM, and wherein the first WDM is configured to separate the signal from the backscattered pump.

7. The photonic integrated optical amplifier of claim 5, wherein the signal input waveguide is connected to the first WDM, and wherein the first WDM is configured to combine the signal with the pump in the amplifier waveguide.

8. The photonic integrated optical amplifier of claim 7, further comprising: The second WDM is integrated onto the substrate using a planar technology configuration; The second WDM is connected to the amplifier waveguide such that the amplifier waveguide is located between the first WDM and the second WDM; A signal output waveguide, configured using planar technology, is integrated onto the substrate and connected to the second WDM; The second WDM is configured to separate the pump from the amplified signal and is configured to provide the amplified signal in the signal output waveguide.

9. The photonic integrated optical amplifier of claim 8, further comprising: A second pump input waveguide, configured in a planar technique, is integrated onto the substrate and connected to the second WDM, wherein the second WDM is configured to provide pump to the amplifier waveguide.

10. The photonic integrated optical amplifier of claim 8, further comprising at least one intermediate WDM, the at least one intermediate WDM being integrated onto the substrate in a planar configuration and disposed on the amplifier waveguide between the first WDM and the second WDM, wherein each intermediate WDM is configured to provide pump to the amplifier waveguide.

11. The photonic integrated optical amplifier of claim 8, further comprising: A waveguide optical filter is integrated onto the substrate using a planar configuration. The signal input waveguide is connected to the first port of the waveguide optical filter; The signal output waveguide is connected to the second port of the waveguide optical filter; Thus, a photonic integrated laser having the waveguide optical filter as an intracavity filter is provided.

12. A photonic integrated broadband incoherent light source, comprising: substrate; A wavelength division multiplexer (WDM) is integrated onto the substrate using a planar technology configuration; An amplifier waveguide is connected to the WDM, wherein the amplifier waveguide is integrated onto the substrate in a planar configuration, and wherein the amplifier waveguide comprises a titanium-doped sapphire gain dielectric. A first waveguide beam collector is integrated onto the substrate in a planar configuration and has a back reflectivity of 1% or less in the wavelength range of 700 nm to 1000 nm. The second waveguide beam collector is integrated onto the substrate in a planar configuration and has a back reflectivity of 1% or less in the wavelength range of 700 nm to 1000 nm. The pump input waveguide is integrated onto the substrate using a planar configuration. The WDM is connected to the pump input waveguide, the amplifier waveguide, the first waveguide beam collector, and the second waveguide beam collector, such that radiation incident on the WDM in the wavelength range of 700 nm to 1000 nm is directed to the first waveguide beam collector or to the second waveguide beam collector. as well as An anti-reflection coating is applied at the output end of the amplifier waveguide; Therefore, the output radiation from the amplifier waveguide is incoherent amplified spontaneous emission.

13. The photonic integrated broadband incoherent light source of claim 12, further comprising a semiconductor diode laser, the semiconductor diode laser being coupled end-to-end to the pump input waveguide.

14. The photonic integrated broadband incoherent light source of claim 13, wherein the semiconductor diode laser is an InGaN diode laser or a GaN diode laser, and wherein the emission wavelength of the semiconductor laser is in the range of 480 nm to 540 nm.

15. The photonic integrated broadband incoherent light source of claim 12, wherein the amplifier waveguide is multimode at one or more wavelengths of the incoherent amplified spontaneous emission.

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