Diode-pumped photonic integrated titanium-sapphire waveguide amplifier

Abstract

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

Description

[0001] Diode-pumped photonic integrated titanium-sapphire waveguide amp 1 i f i e r

[0002] FIELD OF THE INVENTION

[0003] This invention relates to photonic circuit optical amp 1 i f i e r s .

[0004] BACKGROUND

[0005] Titanium doped sapphire (Ti:Sa) is an important laser gain medium that is particularly noteworthy for its large gain bandwidth. To date, most Ti:Sa lasers have been implemented in traditional bulk optics systems, but recent work has demonstrated an integrated optics Ti:Sa laser. However, work to date has not considered optical amplifiers in this material system and having an integrated optics configuration. Accordingly, it would be an advance in the art to provide photonic circuit Ti:Sa amplifiers.

[0006] SUMMARY

[0007] This work extends the previous integrated optics Ti:Sa laser work to amplifiers, and also considers several specific applications enabled by integrated optic Ti:Sa amplifiers. Here we enable, for the first time, ultra- wideband ampli fication of near infrared signals on a photonic integrated circuit . Currently, optical ampli fication on photonic integrated circuits is limited to wavelengths longer than 1000 nm . However, many applications require wavelengths shorter than 1000 nm . Moreover, currently ultra-wideband optical ampli fiers with high gain are not available on photonic integrated circuits , they are limited due to the use of semiconductor optical ampli fiers . This in turn prevents the reali zation of femtosecond mode- locked lasers on a chip or widely tunable lasers in the biological window .

[0008] An exemplary embodiment includes a nanophotonic optical waveguide reali zed in crystalline thin- film titanium- sapphire co-integrated with a semiconductor diode laser used to pump the titanium-sapphire waveguide . This nanophotonic titanium sapphire waveguide provides optical ampli fication of optical signals with a wavelength ranging from 700 nm to 1000 nm . Moreover, the titanium-sapphire waveguides do not show absorption when the material is not pumped and show passive propagation losses smaller than 0 . 45 dB / cm .

[0009] The titanium-sapphire waveguide could be an important product as a stand-alone optical ampli fier or as a wideband coherent light source for optical coherence tomography . Moreover, the wavelength range of 700 nm to 1000 nm is relevant for many applications such as augmented reality and virtual reality, light detection and ranging, microscopy and medical imaging .

[0010] The titanium-sapphire waveguide ampli fier enables optical gain on a chip across the entire 700 nm to 1000 nm wavelength spectrum . To illustrate , this coverage encompasses the crucial near-infrared window within biological tissue , playing a pivotal role in medical imaging and diagnostic equipment . Moreover, this extensive gain bandwidth facilitates efficient and distortion-free amplification of femtosecond pulses on a chip, a feature of significance in metrology and atomic optical clocks. Presently, waveguides featuring optical gain are primarily limited to semiconductor-based optical amplifiers. These semiconductor devices generally exhibit optical bandwidths on the order of tens of nanometers, occasionally extending up to 100 nm. Additionally, due to the meticulous engineering required for the layer stack of these diodes to match specific wavelengths, achieving coverage of any desired wavelength within the 700 nm to 1000 nm range covered by titanium-sapphire is not a straightforward task. Thus, titanium-sapphire waveguides outperform currently available optical gain waveguides, offering the potential for novel applications in photonic integrated circuits.

[0011] BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIGs. 1A-E show characterization results for Ti:Sa waveguide technology.

[0013] FIGs. 2A-G shows characterization results for photonic circuit Ti:Sa amplifiers.

[0014] FIGs. 3A-E show several exemplary amplifier configurations .

[0015] FIG. 4 shows an exemplary multi-stage amplifier.

[0016] FIG. 5 shows an exemplary narrow-linewidth photonic circuit laser enabled by photonic circuit Ti:Sa amplifier technology .

[0017] FIG. 6 shows an exemplary amplified spontaneous emission source enabled by photonic circuit Ti:Sa amplifier technology . DETAILED DESCRIPTION

[0018] Section A describes integrated photonic Ti : Sapphire technology, and section B describes several exemplary device configurations enabled by this integrated photonic Ti : Sapphire technology .

[0019] Al ) Introduction

[0020] Titanium : Sapphire ( Ti : Sapphire ) laser systems play an essential role in both fundamental research and technological applications , including the demonstration of the first optical frequency comb, generation of optical pulses as short as two oscillations of the electrical field, two-photon microscopy and optogenetics , and demonstrations of on-chip laser driven particle accelerators . As a solid- state ( i . e . , non-semiconductor ) gain medium, Ti : Sapphire has exceptional properties , possessing the widest gain bandwidth of any laser crystal ( 650 - 1100 nm) , a large emission cross-section, and a four-level structure . Consequently, mode-locked and continuous-wave Ti : Sapphire lasers are unmatched in performance , and are essential in disciplines such as quantum optics and atomic physics . However, the unrivaled performance of Ti : Sapphire lasers comes at a cost : commercial systems are bulky, expensive , and require high- power pump lasers , prohibiting their widespread use in many real-world applications that demand compactness and scalability .

[0021] Photonic integrated circuit technologies are revolutioni zing laser systems by delivering compactness , scalability, and cost-ef ficiency that is impossible to achieve in table-top systems , leveraging wafer-scale fabrication capabilities in material platforms such as silicon-on-insulator, thin- film lithium niobate , and silicon nitride. By integrating passive photonics elements with III- V semiconductor optical amplifiers, on-chip laser systems have already been realized in wavelength ranges optimized for telecommunications, and are now moving towards the near infrared (NIR) and visible range. However, integrated semiconductor laser and amplifier systems face several intrinsic limitations, most notably limited gain bandwidth and poor high-power handling capabilities due to two-photon and free-carrier absorption in high-confinement waveguides, limiting on-chip pulse energies to several picojoules. An emerging alternative to on-chip III-V systems are integrated solid-state gain media based on rare-earth ions. Recently, by implanting ultra-low-loss silicon-nitride waveguides with erbium ions, high-performance solid-state waveguide amplifiers and lasers have been demonstrated exceeding 100 pj pulse energies; however, these efforts, along with those in other rare-earth systems, are limited to wavelengths longer than 1 pm. Thus, the aforementioned applications that require the high-performance capabilities of Ti: Sapphire lasers have not been able to benefit from photonic circuit integration. An integrated solid-state laser and amplifier operating both in the visible and NIR wavelength range would have a sweeping impact in areas of science and technology where cost, size, and scalability considerations are critical .

[0022] Accordingly, efforts towards miniaturization of Ti: Sapphire lasers have a long history. Limitations in scalability for current Ti: Sapphire laser systems have been primarily due to the short fluorescence lifetime of the Ti3+ion, which, when combined with the large disparity between pump (490 - 532 nm) and lasing (650 - 1100 nm) wavelengths, increases system complexity and necessitates very high pump intensities before achieving appreciable amplification and lasing. To address this, approaches such as pulsed laser deposition of waveguides, laser written waveguides, optical fibers, and machined whispering-gallery mode lasers have been used to reduce mode volume and lower the lasing threshold. A recent demonstration of bonding Ti: Sapphire to a silicon nitride photonic chip to achieve lasing via evanescent coupling represents the first nanophotonic Ti: Sapphire laser, but this approach poses challenges for achieving substantial gain due to limited modal overlap. As a result, a practical on-chip Ti: Sapphire laser system has not yet been realized. To date, the highest single mode output power in an on-chip Ti: Sapphire laser is only 40 nW, and a tunable laser has yet to be demonstrated. Furthermore, no attempt to realize an integrated Ti: Sapphire amplifier has been reported.

[0023] In this work, we address this last challenge and demonstrate chip-integrated Ti: Sapphire waveguide amplifiers that deliver the power, stability, and tunability relevant for practical applications in research and technology. Our approach is based on an architecture that attains the fundamental limit of photonic mode confinement and overlap. We have developed a low-loss monocrystalline sapphire-on- insulator photonics platform (Q-factors > 106at 805 nm) , for example, enabling a sub-milliwatt threshold Ti: Sapphire microdisk laser. We then demonstrate the first integrated Ti: Sapphire broadband waveguide amplifier, realizing a record 64 dB / cm solid-state on-chip gain. The waveguide amplifier is capable of pulsed amplification up to 2.3 nJ, the first pulsed amplification on-chip below 1 pm, which furthermore exceeds the power handling of any nanophotonic waveguide amplifier by over an order of magnitude while maintaining a transform-limited pulse shape. A2 ) Titanium: sapphire-on-insulator photonics platform

[0024] FIG. 1A schematically shows the process flow for Ti:SaOI preparation. FIG. IB is an SEM image of a Ti:SaOI waveguide fabricated with electron-beam lithography. FIG. 1C is an SEM image of waveguide cross-section. FIG. ID shows simulated mode intensity profile at the pump (532 nm) and signal (800 nm) wavelength, with modal intensity overlap rp,sof 98.8%. FIG. IE show measured exponential decay of waveguide fluorescence with an optical lifetime of 3.09 ps .

[0025] The Ti:SaOI fabrication process is illustrated in FIG. 1A. A Ti: Sapphire wafer 102 is bonded to a sapphire carrier wafer 104 via a SiCt interface layer 106 that acts as the buried oxide layer. After bonding, the Ti: Sapphire layer is thinned via mechanical grinding and polishing, followed by reactive ion etching to a target thickness of < 1 pm. Using this approach, wafer-scale production of other high-quality monocrystalline photonics platforms, such as silicon-carbide-on-insulator, has already been demonstrated. The fabrication method does not constrain the thickness of the Ti: Sapphire layer, which can be precisely controlled via dry etching.

[0026] Direct pattern transfer from photoresist to Ti: Sapphire limits the resolution and sidewall angle of structures, precluding the realization of many integrated photonics functionalities. To this end, we develop a pattern transfer process based on a secondary hard mask layer of 200 nm of chromium (deposited via electron-beam evaporation) . Electron-beam photoresist (FOx-16, Corning) is used to define the pattern in the Cr mask (see Methods) . This process results in waveguides with a near-vertical (11 deg) sidewalls and sub-100 nm minimum feature size (FIGs. 1B-C) . We note that this process is also compatible with deep UV photolithography for high-throughput fabrication. This high- confinement, nearly single-mode waveguide geometry achieves a near-unity broadband overlap of the pump and lasing modes (99.5% at 650 nm, 96.5% at 1100 nm, FIG. ID) , a crucial advantage of the Ti:SaOI laser platform. Of critical importance is that the thin film of Ti: Sapphire preserves the quality of the bulk gain medium, as quantified by its quantum efficiency. A reduction of the gain medium efficiency through introduction of non-radiative decay rates would be measured as a reduced excited state lifetime. We characterize the Ti:SaOI waveguide fluorescence in the time domain by exciting with a weak modulated pump and detecting emission rate using a superconducting nanowire single-photon detector (SNSPD) , and observe a purely single-exponential decay with a lifetime of 3.09 ps (FIG. IE) , consistent with lifetimes reported in literature for bulk Ti: Sapphire. We conclude that the Ti:SaOI platform features excellent metrics for amplifier fabrication, both in passive optical losses and in the active gain medium.

[0027] A3) Ultra-wide band waveguide amplifiers in Ti:SaOI

[0028] FIG. 2A shows an optical image of an 8 mm waveguide amplifier pumped from both ends (532 nm pump filtered) . FIG. 2B shows an amplified spontaneous emission (ASE) spectrum collected from a 3 mm waveguide amplifier when no input signal is present, and measured small-signal gain (dB / cm) for a 0.5 mm waveguide. The fit to the calculated gain profile is shown as a solid line. FIG. 2C shows measured small-signal gain for an 8 mm amplifier, with pump power of 175 mW, and measured passive waveguide losses (solid line between the dashed lines) . Standard error is shown via dashed lines. FIG. 2D shows the spectrum of the amplified GW output signal at peak recorded output power of 60 mW. FIG. 2E shows normalized spectra of a picosecond pulse before and after propagation in the amplifier, showing no nonlinear distortion. FIG. 2F shows the spectrum of the amplified pulses (at repetition rate of 1.25 MHz) for varying magnitude of amplification. The inset shows unamplified and maximally-amplified output on a linear scale, illustrating that the pulse remains transform-limited with minor distortion of the tails.

[0029] Amplifiers are essential companions to lasers, allowing for reversal of losses in coherent signals and enabling the reach of power levels beyond the capability of a laser source. In absence of an input signal, broad-band amplifiers also can generate incoherent light, powering technologies such as optical coherence tomography (OCT) . Solid-state amplifiers have excellent power-handling characteristics and their media feature low passive losses. Commercial solid- state amplifier solutions are mostly based on rare-earth dopants in crystals or glasses, which are limited to wavelengths longer than 1 pm. The integration of such amplifiers on-chip via implantation of rare-earth ions into silicon nitride, lithium niobate, or alumina photonics is an area of active research, but is limited by the intrinsically-low gain of such dopants, posing a major technological challenge. To date, the realization of on-chip optical gain elements below 1 pm has been limited to III-V semiconductors. Their heterogeneous integration with Si or SiN photonics has enabled break-through integrated laser technologies; however, limited power-handling and bandwidth constrain their applications and versatility. Using the Ti:SaOI platform, we demonstrate the first integrated solid- state amplifier operating below 1 pm, which possesses unmatched power handling and bandwidth among both integrated and table-top state-of-the-art systems. Outside of nanophotonics, the use of Ti: Sapphire as a versatile amplifier is limited to costly and specialized applications such as ultra-high pulse-energy amplifiers. We demonstrate that strong pump confinement (effective mode area 0.62 pm2) and near-unity mode overlap in nanophotonics results in modest power requirements to reach saturation of the gain medium, enabling high gain even at low pump power. The Ti:SaOI waveguide amplifier (FIG. 2A) is fully- integrated and ultra-compact (0.20 mm2) . In absence of an input signal, it is a source of bright amplified spontaneous emission with a bandwidth exceeding 100 THz (FIG. 2B) , the widest demonstrated to date. Such a broadband source, which can be readily coupled to an optical fiber, may find use in medical applications such as OCT. To measure the maximum gain achievable in Ti: Sapphire nanophotonics, we first examine a short (length of 485 pm) waveguide, where effects of pump absorption, passive losses, and spatial mode mixing are negligible. A commercial continuous-wave (GW) Ti: Sapphire laser is used as a wavelength-tunable signal. At a pump power of 150 mW, we observe peak gain of 64 dB / cm, and > 30 dB / cm across a 100 THz bandwidth (FIG. 2B) .

[0030] To achieve practical levels of amplification, we use a spiral waveguide with a length of 8 mm, where the pump is nearly fully absorbed. We note that, due to the four-level structure of Ti3+, no signal re-absorption takes place in weakly-pumped sections of the waveguide, and material gain is always positive. The small-signal (3 pW input power) amplification of an 8 mm Ti:SaOI waveguide is shown in FIG. 2C, reaching peak on-chip gain of > 20 dB . Here, the maximum gain was limited by parasitic lasing caused by reflections from the waveguide facets, similar to that reported for on-chip Erbium amplifiers in the literature. Higher gain can be achieved via a reflection-suppressing design or packaging. FIG. 2D shows large-signal amplification, with an output CW power of 60 mW at 790 nm.

[0031] High-performance mode-locked lasers have long eluded photonic integration due to the challenge of amplifying ultra-short, high peak-power optical pulses. These lasers are pivotal for advancing diverse fields such as supercontinuum and optical frequency comb generation, two- photon microscopy, and dual-comb metrology. Here, we show that the combination of high gain-per-unit-length, absence of two-photon absorption, weak Kerr nonlinearity (n2 = 3 x ICT20m2 / W) , and excellent power-handling of Ti:SaOI enables unprecedented distortion-free pulsed amplification on-chip. We use a commercial picosecond mode- locked Ti: Sapphire laser as the input signal. To assess the degree of passive nonlinear distortion of a pulse propagating in the amplifier, we compare the pulse spectrum before and after passing through the waveguide, without amplification. Negligible spectral distortion is observed for an input pulse energy of 120 pj (FIG. 2E) . The shape of the output pulse for varying pump power is shown in FIG. 2F, and shows minimal change in shape at peak gain of 17 dB, which corresponds to a pulse energy of 2.3 nJ and peak power of 1.0 kW. This is over an order of magnitude greater than the highest integrated pulsed amplification demonstrated to date, based on rare-earth waveguides, and is the only integrated high-power transform-limited amplifier for any wavelength. The Ti:SaOI amplifier constitutes the first realization of on-chip pulsed amplification below 1 pm, and demonstrates the potential of the platform for the realization of a high-performance integrated mode-locked laser. FIG. 2G shows the dependence of amplification and output pulse energy on the repetition rate. The vertical dashed line indicates the lifetime of the gain medium (l / i) . A4 ) Discussion

[0032] In this work, we have demonstrated chip-integrated, broadband tunable , and scalable Ti : Sapphire ampli fiers in a monocrystalline sapphire-on-insulator photonics platform . The demonstration of a Ti : SaOI waveguide ampli fier ( > 20 dB of on-chip gain, and peak gain of 64 dB / cm) opens doors to ampli fication of both continuous-wave and pulsed light within the ultra-wide bandwidth of 650 - 1100 nm . Owing to the excellent power-handling and weak nonlinearity of sapphire , we have shown trans form-limited ampli fication of picosecond pulses up to 2 . 3 nJ pulse energies , constituting the first demonstration of integrated high-energy pulse ampli fication in the visible to near- IR and the first demonstration of pulse ampli fication exceeding 150 pj in any nanophotonic waveguide ampli fier . In comparison, this level of performance can never be attained with heterogeneously- integrated I I I-V semiconductor ampli fiers due to their short upper-state li fetime and two-photon absorption . This work addresses one of the maj or challenges in the development of on-chip mode-locked lasers and sel f-referenced optical frequency combs in a sought-after wavelength range previously out-of-reach . Through nanophotonic confinement , highly-ef f icient generation of ultra-broad ASE shown in this work could serve as a versatile fiber-coupled source for high-resolution optical coherence tomography .

[0033] The integration of Ti : Sapphire technology on-chip is a decisive step towards massively-scalable Ti : Sapphire systems , both monolithic and integrated with passive materials . Strides in heterogeneous laser integration have already enabled the combination of other novel on-chip laser technologies with large-scale foundry photonics processing . The approach presented in this work is compatible with co- integration in platforms such as silicon nitride and lithium niobate to realize sub-micron wavelength frequency-agile

[0034] Ti: Sapphire lasers, integrated frequency-doubled lasers, on- chip supercontinuum generation, and integrated optical parametric amplifiers.

[0035] A5) Methods

[0036] A5.1) Device fabrication.

[0037] For amplifier and waveguide demonstration, we use an a- plane (1120) Ti: Sapphire wafer die, bonded to undoped sapphire via an interfacial SiCt layer which serves as the buried oxide layer. The bonded Ti: Sapphire dies are then ground and polished in a precision lapping system (PM5, Logitech) , followed by thinning via reactive-ion etching in BCla plasma (PlasmaTherm Versaline TCP) to reach a target thickness of 450 nm.

[0038] Amplifiers are fabricated via electron-beam lithography. A chromium hardmask (200 nm) is deposited on the Ti:SaOI film by electron-beam evaporation. Chromium is chosen to allow for etching of sapphire with sufficient selectivity (4:1) . Electron-beam photoresist (FOx-16, Corning) is spun afterwards and a 50 keV electron beam lithography system (Voyager, Raith) is used to define the pattern. Limitations of the lithography system necessitated confinement of devices to a 0.5 x 0.5 mm area to avoid stitching errors. The pattern is transferred into the Cr mask via a CI2 and O2 plasma, and subsequently into sapphire via a BCI3 etch. Subsequently, the Cr hard mask is stripped through wet etching via Chromium etchant and Piranha. Devices are then capped with an initial layer of flowable oxide (FOx-16, Dow Corning) , followed by an additional SiO2 layer through high-density plasma chemical vapor deposition (HDP-CVD, PlasmaTherm) , and were annealed at 800°C in air. Facets for edge-coupling of devices were created either through wafer saw dicing followed by f ocused-ion-beam milling or through laser stealth dicing.

[0039] A5.2 ) Characterization of the waveguide amplifier.

[0040] Light is coupled from free-space on- and off-chip via microscope objectives (M-plan NIR 100, Mitutoyo) . The temperature of the sample substrate is maintained at 290 K throughout experiments. The 532 nm pump source is a frequency-doubled diode-pumped solid-state laser (Verdi V10, Coherent) , and is delivered onto the chip either from one or both waveguide facets, or in one direction counterpropagating with respect to the signal. Unless otherwise stated, single-side pumping is used. For continuous-wave amplification experiments, a commercial Ti: Sapphire laser (SolsTiS, M Squared) is used, which has a tuning range of 700-1000 nm. Small-signal gain-per-unit-length is measured for an input power of 0.10 mW. For characterization of small-signal gain in the 0.485 mm long waveguide (FIG. 2B) requiring resolution of < 0.1 dB, the pump and signal are chopped and the amplification is extracted from the timedependent signal recorded on a silicon photodetector. For small-signal gain characterization in 8 mm amplifiers, an optical spectrum analyzer (Yokogawa AQ6370D) is used to obtain output signal with and without the pump laser. This measurement does not take into account the passive waveguide losses. Wavelength-dependent passive waveguide loss is measured from statistical analysis of waveguides of different lengths. Cumulative off-chip amplification is represented by the difference between measured on-chip amplification and passive loss at the given wavelength in FIG. 2C. To record maximum amplified power output (FIG. 2D) , the double-side pumped scheme is used. To infer the output power at the chip, the measured output power is corrected for objective losses (transmission of 0.85) . For the pulsed- amplif ication experiment, a picosecond mode-locked Ti: Sapphire laser (Tsunami, Spectra-Physics ) is used. The sech2fit to the spectrum of the pulse reveals a pulse duration of 2.2 ps, in agreement with autocorrelator measurement. To measure the dependence of amplification and output pulse energy on the repetition rate, the intrinsic repetition rate of the laser (80 MHz) is reduced via an electro-optic modulator pulse picker (Conoptics) with a measured rejection ratio of 6.2 x 103.

[0041] B) Exemplary device configurations

[0042] Bl) Amplifier configurations

[0043] FIGs. 3A-E show several exemplary amplifier configurations enabled by the above-described technology.

[0044] The example of FIG. 3A is a photonic integrated optical amplifier including: a substrate 302; a first wavelength division multiplexer (WDM) 306 integrated onto the substrate in a planar technology configuration; an amplifier waveguide 304 connected to the first WDM that is integrated onto the substrate in a planar technology configuration and which includes a titanium doped sapphire gain medium; and a first pump input waveguide 318 integrated onto the substrate in a planar technology configuration and connected to the first WDM.

[0045] The unique gain characteristics of Ti: Sapphire greatly simplify the design and fabrication complexity of suitable WDMs in the above-described material system, allowing them to be done by those of ordinary skill in the art. Because the peak pump and lasing wavelengths (490 nm and 800 nm, respectively) are so disparate , the WDM design becomes readily achievable , even for the broad lasing bandwidth of Ti : Sapphire ( 700 nm - 1000 nm) . This is because the pump and lasing wavelengths have signi ficantly di f ferent coupling strengths , owing to the large di f ference in wavelength scales causing strongly di f fering amounts of modal confinement . This disparity between pump and lasing wavelength is a special property of Ti : Sapphire that is not present in many other optically-pumped gain media, and allows for the ef ficient routing of pump and signal in a standalone ampli fier device .

[0046] The example of FIG . 3A further includes a semiconductor diode laser 314 butt-coupled to the first pump input waveguide 318 . The semiconductor diode laser is preferably an InGaN diode laser or a GaN diode laser having an emission wavelength in a range from 480 nm to 540 nm .

[0047] The design goals of a standalone nanophotonic waveguide ampli fier di f fer signi ficantly from those of a photonic integrated laser cavity, which contains an active waveguide gain element .

[0048] In a waveguide laser configuration, the active gain element is part of a passive cavity . In this case , the main obj ective is to minimi ze the round-trip losses of the waveguide laser cavity, which in turn enables a narrow optical linewidth, a low lasing threshold, and high conversion ef ficiency from pump input to laser output . Moreover, the gain element inside a waveguide laser cavity is optimi zed to suppress spontaneous emission and often requires only modest gain . For example , titanium-sapphire laser crystals in free-space laser cavities are relatively short , and factors such as thermal load and parasitic losses are often the main design challenges . Moreover, in waveguide laser cavity, in the absence of an optical isolator, the gain medium is impinged by a lasing mode on both sides. This prevents tailoring the modal size, by utilizing tapering into a multimode waveguide, to the power build up as the signal traverse the gain waveguide.

[0049] In contrast, a standalone waveguide amplifier is designed to provide significantly higher gain, up to 20 or 30 dB and the modal cross-section of the amplifying waveguide can be tailored at each point to optimize amplification and power handling as the amplifier is only traversed in a single direction by the signal wavelength. This includes using a multimode waveguide with tapering sections to enable high power handling. It is preferred for the small-signal gain of a standalone waveguide amplifier to be 10 dB or more. Here the "small-signal gain" is the unsaturated gain provided by the amplifier. For power amplifiers, the gain seen in operation is typically less than the small-signal gain, but the small-signal gain is still a definable specification of such an amplifier. Waveguide tapering as described above can be used to make the signal mode larger, thereby reducing signal saturation in the amplifier. In integrated optics configurations like those considered herein, it is possible and often preferred to selectively launch a signal into a selected mode of a multi-mode waveguide. For example, this can be done by selectively coupling a single-mode waveguide to the desired mode of the multi-mode waveguide with an adiabatic taper.

[0050] The amplifier's role is to boost a wide range of signals, from femtosecond pulses to intense CW signals and spectrally complex inputs. Therefore, the design focuses on maximizing gain and / or power output or pulse energy, rather than minimizing losses or maintaining the narrow linewidth required in lasers. Finally, external ampli fier waveguide design is heavily influenced by in- and out-coupling challenges not present for a gain element inside a monolithic waveguide laser cavity . These challenges include the ef fective coupling of light from a wide range of input sources , handling higher powers , ef fective combination with high-intensity pump signals , and managing parasitic losses and reflections . This concretely involves designing optimi zed edge-coupling waveguide facets or grating couplers , developing antireflection coatings , or implementing on-chip anti-reflection structures .

[0051] The example of FIG . 3A further includes a signal input waveguide 310 integrated onto the substrate in a planar technology configuration . In this example , the signal input waveguide 310 is connected to the first WDM 306 , and the first WDM is configured to combine signal and pump in the ampli fier waveguide . This is an example of co-propagating signal and pump .

[0052] FIG . 3B shows a first example of counter-propagating signal and pump . Here the signal input waveguide 310 is connected to an end of the ampli fier waveguide 304 opposite the first WDM 306 . In this example , the first WDM 306 is configured to separate output signal from backscattered pump, and the output signal is provided in a signal output waveguide 312 .

[0053] The on-chip signal input waveguide 310 is optional , as seen in the counter-propagating signal and pump example of FIG . 3C . Here the signal input is directly coupled to ampli fier waveguide 304 .

[0054] It is often preferred to include a second WDM in the ampli fier configuration, so that one WDM combines pump and signal onto the ampli fier waveguide at the input , and the other WDM separates pump and signal at the output. FIG. 3D shows an example. Here we have a second WDM 308 integrated onto the substrate in a planar technology configuration. The second WDM 308 is connected to the amplifier waveguide 304 such that the amplifier waveguide 304 is between the first WDM 306 and the second WDM 308. Signal output waveguide 312 is integrated onto the substrate in a planar technology configuration and connected to the second WDM 308. The second WDM 308 is configured to separate pump from amplified signal and is configured to provide the amplified signal in the signal output waveguide 312.

[0055] Amplifier configurations having two WDMs can be bidirectionally pumped, as in the example of FIG. 3E . Here we have a second pump input waveguide 320 integrated onto the substrate in a planar technology configuration and connected to the second WDM 308, and the second WDM 308 is configured to provide pump to the amplifier waveguide 304 (from second pump source 316) .

[0056] In these examples, the titanium-sapphire waveguide amplifier is implemented as a stand-alone chip optical amplifier. It features 1 or 2 pump diodes, which are commercially available with output power 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 pulsed) in the wavelength range of 700 nm to 1000 nm. Thanks to the low passive losses of the titanium- sapphire waveguides, nearly lossless WDM couplers can be realized that separate (or combine) the pump and signal wavelength .

[0057] Any number or configuration of pump stages can be employed. FIG. 4 shows an example further including at least one intermediate WDM (422, 424 etc.) integrated onto the substrate in a planar technology configuration and disposed on the amplifier waveguide 304 between the first and second WDMs (306, 308) , where 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, etc. For simplicity, the example of FIG. 4 shows all pumps co-propagating with the signal, but any combination of co- and counter-propagating pumps can be employed.

[0058] Here a multi-stage amplifier configuration is considered. This multi-stage amplifier allows for optical output powers exceeding 100 mW of continuous wave power. This amounts to combining the output of N pump laser diodes at different stages of the waveguide amplifier. Moreover, as the current of each pump diode can be individually controlled, the amplification profile along the wavelength can be used to optimize the noise figure and gain.

[0059] B2) Narrowband source example

[0060] The above described technology can be used to make a narrowband optical source, as seen in the example of FIG. 5. This example further includes a waveguide optical filter 502 integrated onto the substrate in a planar technology configuration. The signal input waveguide 310 is connected to a first port of the waveguide optical filter, and the signal output waveguide 312 is connected to a second port of the waveguide optical filter, as shown. Thus a photonic integrated laser is provided having the waveguide optical filter 502 as an intracavity filter. Practice of this embodiment does not depend critically on the kind of waveguide optical filter employed. In this example, the waveguide optical filter 502 includes passive waveguide resonators 504 and 506 which can provide a narrow passband in transmission as known in the art .

[0061] In this example , the titanium-sapphire waveguide is cointegrated on a photonic integrated circuit together with ultra-low-loss dielectric waveguides ( this can be silicon nitride , silicon carbide , . . . ) to provide wideband gain to the laser cavity reali zed in an ultra-low-loss waveguide platform . For example , in silicon nitride ultra-narrow wavelength filters can be reali zed such that tunable lasers with a tuning range exceeding 50 THz in the 700 nm to 1000 nm wavelength range are provided . Moreover, such lasers would have extremely narrow optical linewidths making them an ideal candidate for applications in quantum optics , atomic timekeeping systems and optical metrology ( LIDAR, spectroscopy, sensing, etc . ) . Integration of such optical ampli fiers with nonlinear photonic platforms like lithium niobate may provide frequency agile CW tunable lasers .

[0062] B3 ) Broadband source example

[0063] The above-described ampli fier technology can also be adapted to provide a wide band incoherent light source , as seen in the example of FIG . 6 . This example is a photonic integrated wide band incoherent light source including : a substrate 302 ; a wavelength division multiplexer (WDM) 306 integrated onto the substrate in a planar technology configuration; an ampli fier waveguide 304 connected to the WDM, where the ampli fier waveguide is integrated onto the substrate in a planar technology configuration, and the ampli fier waveguide includes a titanium doped sapphire gain medium; a first waveguide beam dump 602 integrated onto the substrate in a planar technology configuration and having back reflection of 1 % or less in a wavelength range from 700 nm to 1000 nm; a second waveguide beam dump 604 integrated onto the substrate in a planar technology configuration and having back reflection of 1 % or less in a wavelength range from 700 nm to 1000 nm; a pump input waveguide 318 integrated onto the substrate in a planar technology configuration, where the WDM 306 is connected to the pump input waveguide 318 , the ampli fier waveguide 304 , the first waveguide beam dump 602 and the second waveguide beam dump 604 such that radiation incident on the WDM 306 in a wavelength range from 700 nm to 1000 nm ( from pump input waveguide 318 or ampli fier waveguide 304 ) is directed to the first waveguide beam dump 602 or to the second waveguide beam dump 604 ; and an anti-reflection coating 606 disposed at an output end of the ampli fier waveguide . Here output radiation from the ampli fier waveguide 304 is incoherent ampli fied spontaneous emission .

[0064] Fabrication of suitable waveguide beam dumps in the above-described material system having at least the speci fied performance can be done by those of ordinary skill in the art .

[0065] This example can further include a semiconductor diode laser 314 butt-coupled to the pump input waveguide 318 . Preferably the semiconductor diode laser is an InGaN diode laser or a GaN diode laser having an emission wavelength in a range from 480 nm to 540 nm . The ampli fier waveguide can be multi-mode at one or more wavelengths of the incoherent ampli fied spontaneous emission .

[0066] In this example , a titanium-sapphire waveguide is used to implement an ultra-wideband incoherent light source . The titanium-sapphire waveguide can achieve a 10 dB emission bandwidth exceeding 250 nm centered at 800 nm . This bandwidth is unattainable with current superluminescent diodes and allows for superior resolution of optical coherence tomography systems . A WDM (wavelength division multiplexing) coupler is added to ef fectively scatter the spontaneous emission in the slab of the chip without reflections . Combined with an anti-reflection coating on the output facet ( for example 150 nm of SiOx) ultra-low reflections can be achieved eliminating ripple of the wideband incoherent light , which is important for applications in optical coherence tomography . Moreover, the spectrum can be further shaped by adding wavelength filters in the titanium sapphire waveguide platform as needed .

[0067] Finally, the incoherent light can be coupled to a nanophotonic waveguide with dimensions that can be tailored to the needed applications . This will allow, for example , high ef ficiency coupling of the output to a single mode fiber or other outcoupling as needed .

Claims

CLAIMS1 . A photonic integrated optical ampli fier comprising : a substrate ; a first wavelength division multiplexer (WDM) integrated onto the substrate in a planar technology configuration; an ampli fier waveguide connected to the first WDM, wherein the ampli fier waveguide is integrated onto the substrate in a planar technology configuration, and wherein the ampli fier waveguide includes a titanium doped sapphire gain medium; and a first pump input waveguide integrated onto the substrate in a planar technology configuration and connected to the first WDM .2 . The photonic integrated optical ampli fier of claim 1 , further comprising a semiconductor diode laser butt-coupled to the first pump input waveguide .3 . The photonic integrated optical ampli fier of claim 2 , wherein the semiconductor diode laser is an InGaN diode laser or a GaN diode laser, and wherein an emission wavelength of the semiconductor laser is in a range from 480 nm to 540 nm .4 . The photonic integrated optical ampli fier of claim 1 , wherein the ampli fier waveguide is multi-mode at a signal wavelength .5 . The photonic integrated optical ampli fier of claim 1 , further comprising a signal input waveguide integrated onto the substrate in a planar technology configuration6 . The photonic integrated optical ampli fier of claim 5 , wherein the signal input waveguide is connected to an end of the ampli fier waveguide opposite the first WDM, and wherein the first WDM is configured to separate signal from backscattered pump .7 . The photonic integrated optical ampli fier of claim 5 , wherein the signal input waveguide is connected to the first WDM, and wherein the first WDM is configured to combine signal and pump in the ampli fier waveguide .8 . The photonic integrated optical ampli fier of claim 7 , further comprising : a second WDM integrated onto the substrate in a planar technology configuration; wherein the second WDM is connected to the ampli fier waveguide such that the ampli fier waveguide is between the first WDM and the second WDM; a signal output waveguide integrated onto the substrate in a planar technology configuration and connected to the second WDM; wherein the second WDM is configured to separate pump from ampli fied signal and is configured to provide the ampli fied signal in the signal output waveguide .9 . The photonic integrated optical ampli fier of claim 8 , further comprising : a second pump input waveguide integrated onto the substrate in a planar technology configuration and connected to the second WDM, wherein the second WDM is configured to provide pump to the ampli fier waveguide .10 . The photonic integrated optical ampli fier of claim 8 , further comprising at least one intermediate WDM integrated onto the substrate in a planar technology configuration and disposed on the ampli fier waveguide between the first and second WDMs , wherein each intermediate WDM is configured to provide pump to the ampli fier waveguide .11 . The photonic integrated optical ampli fier of claim 8 , further comprising : a waveguide optical filter integrated onto the substrate in a planar technology configuration; wherein the signal input waveguide is connected to a first port of the waveguide optical filter ; wherein the signal output waveguide is connected to a second port of the waveguide optical filter ; whereby a photonic integrated laser is provided having the waveguide optical filter as an intracavity filter .12 . A photonic integrated wide band incoherent light source comprising : a substrate ; a wavelength division multiplexer (WDM) integrated onto the substrate in a planar technology configuration;an ampli fier waveguide connected to the WDM, wherein the ampli fier waveguide is integrated onto the substrate in a planar technology configuration, and wherein the ampli fier waveguide includes a titanium doped sapphire gain medium; a first waveguide beam dump integrated onto the substrate in a planar technology configuration and having back reflection of 1 % or less in a wavelength range from 700 nm to 1000 nm; a second waveguide beam dump integrated onto the substrate in a planar technology configuration and having back reflection of 1 % or less in a wavelength range from 700 nm to 1000 nm; a pump input waveguide integrated onto the substrate in a planar technology configuration; wherein the WDM is connected to the pump input waveguide , the ampli fier waveguide , the first waveguide beam dump and the second waveguide beam dump such that radiation incident on the WDM in a wavelength range from 700 nm to 1000 nm is directed to the first waveguide beam dump or to the second waveguide beam dump ; and an anti-reflection coating disposed at an output end of the ampli fier waveguide ; whereby output radiation from the ampli fier waveguide is incoherent ampli fied spontaneous emission .13 . The photonic integrated wide band incoherent light source of claim 12 , further comprising a semiconductor diode laser butt-coupled to the pump input waveguide .14 . The photonic integrated wide band incoherent light source of claim 13 , wherein the semiconductor diode laser isan InGaN diode laser or a GaN diode laser, and wherein an emission wavelength of the semiconductor laser is in a range from 480 nm to 540 nm . 15 . The photonic integrated wide band incoherent light source of claim 12 , wherein the ampli fier waveguide is multi-mode at one or more wavelengths of the incoherent ampli fied spontaneous emission .

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