Narrowband tunable photonic notch filter

Facile microfluidic fabrication of monodispersed self-coupling microcavity with fine tunability

Whispering gallery mode (WGM) resonators have received extensive attention because of their nonlinear optical application in lasers and sensors. Optical microcavities are excellent candidates for constructing powerful microlasers and label-free biosensors, owing to their low optical losses and small size. However, most of these microcavity syntheses rely on sophisticated fabrication methods and cannot be manipulated easily. To achieve facile and versatile microcavity fabrication, we present a robust microfluidics method for monodispersed self-coupling optical microcavity fabrication with a fine tunability. The microcavity polydispersity was less than 3%. The optical microcavity size could be varied from 10 to 30 µm with a steady quality factor (Q) of approximately 1000. The lowest laser threshold that we obtained was 0.82 µJ with a microcavity size of 20 µm. The doped fluorescent dye concentration can be tuned precisely from 0.001 to 0.05 wt% to explore an optimized fluorescent background. The experimental results and theoretical simulation match well in terms of Q and the electrometric resonance field intensity. Compared with previous precise and practical fabrication methods, we have demonstrated a facile approach for versatile optical microcavity fabrication. This method can vary the microcavity materials, size, doped fluorescent dye concentration, WGM resonance spectrum, Q factor, and laser threshold easily to adapt to various circumstances and specific applications.

On acceleration sensitivity of 2 μm whispering gallery mode-based semiconductor self-injection locked laser

While whispering gallery mode resonators are well known for their low acceleration sensitivity, there has not been much published experimental research on the subject. We performed environmental sensitivity tests of a 2 μm semiconductor distributed feedback (DFB) laser, self-injection locked to a high-Q crystalline whispering gallery mode resonator. Measured acceleration sensitivity of the laser is below 5×10^-11  g^-1 in the 1-200 Hz frequency bandwidth and thermal sensitivity does not exceed 12  MHz/°C. The laser's frequency noise is below 50  Hz/Hz^1/2 at 10 Hz, reaching 0.4  Hz/Hz^1/2 at 400 kHz. The instantaneous linewidth of the laser is improved by nearly 4 orders of magnitude compared to the free-running DFB laser and is measured to be 50 Hz at 0.1 ms measurement time. The Allan deviation of the laser frequency is on the order of 10^-9 from 1 to 1000 s. All these features make the laser attractive for metrology applications involving low-noise 2 μm seed lasers.

Synthetic-lattice enabled all-optical devices based on orbital angular momentum of light

All-optical photonic devices are crucial for many important photonic technologies and applications, ranging from optical communication to quantum information processing. Conventional design of all-optical devices is based on photon propagation and interference in real space, which may rely on large numbers of optical elements, and the requirement of precise control makes this approach challenging. Here we propose an unconventional route for engineering all-optical devices using the photon’s internal degrees of freedom, which form photonic crystals in such synthetic dimensions for photon propagation and interference. We demonstrate this design concept by showing how important optical devices such as quantum memory and optical filters can be realized using synthetic orbital angular momentum (OAM) lattices in degenerate cavities. The design route utilizing synthetic photonic lattices may significantly reduce the requirement for numerous optical elements and their fine tuning in conventional design, paving the way for realistic all-optical photonic devices with novel functionalities.

Design of all-optical devices rely on large numbers of optical elements and precise control makes this approach challenging. The authors demonstrate that optical devices such as quantum memory and optical filters can be realized using synthetic orbital angular momentum lattices in a single main degenerate cavity.

Lossless and high-resolution RF photonic notch filter

A novel technique to create a lossless and tunable RF photonic bandstop filter with an ultra-high suppression is demonstrated using the combination of an overcoupled optical ring resonator and tailored stimulated Brillouin scattering gain. The filter bandwidth narrowing is counterintuitively synthesized from two broad optical resonance responses. Through a precise amplitude and phase tailoring in the optical domain, the RF filter achieves a minimum insertion loss (<0  dB), a high isolation (>50  dB), and a tunable 3 dB bandwidth (60-220 MHz) simultaneously with wide frequency tunability (1-11 GHz). This ultra-low loss RF filter paves the way toward broadband advanced spectrum management with low loss, high selectivity, and improved signal-to-noise ratio.

Tunable megahertz bandwidth microwave photonic notch filter based on a silica microsphere cavity

We propose and experimentally demonstrate a tunable microwave photonic notch filter with a megahertz order bandwidth based on a silica microsphere cavity coupled by an optical microfiber. The silica microsphere with a quality factor of hundreds of millions offers a full width at half-maximum bandwidth down to the order of megahertz in the transmission spectrum. Due to the coupling flexibility between the microcavity and the optical microfiber, the bandwidth and suppression ratio can be tuned and optimized to get a rejection ratio beyond 30 dB. The tunability of over 15 GHz is also achieved. To the best of our knowledge, this single-stopband microwave photonic filter has the narrowest bandwidth filter that has ever been experimentally demonstrated. This microwave photonic notch filter shows distinct advantages of high selectivity, compactness, flexibility, and low insertion loss.

Millisecond Photon Lifetime in a Slow-Light Microcavity

Optical microcavities with ultralong photon storage times are of central importance for integrated nanophotonics. To date, record quality (Q) factors up to 10^{11} have been measured in millimetric-size single-crystal whispering-gallery-mode (WGM) resonators, and 10^{10} in silica or glass microresonators. We show that, by introducing slow-light effects in an active WGM microresonator, it is possible to enhance the photon lifetime by several orders of magnitude, thus circumventing both fabrication imperfections and residual absorption. The slow-light effect is obtained from coherent population oscillations in an erbium-doped fluoride glass microsphere, producing strong dispersion of the WGM (group index n_{g}∼10^{6}). As a result, a photon lifetime up to 2.5 ms at room temperature has been measured, corresponding to a Q factor of 3×10^{12} at 1530 nm. This system could yield a new type of optical memory microarray with ultralong storage times.

Surface plasmon polariton scattering by subwavelength silicon wires

Surface plasmon polariton scattering from 2D subwavelength silicon wires is investigated using the finite-difference time-domain method. It is shown that coupling an incident surface plasmon polariton to intercavity modes of the particle can dramatically change transmitted fields and plasmon-induced forces. In particular, both transmission and optical forces are highly sensitive to the particle size that is related to the excitation of whispering gallery modes or standing wave modes depending on the particle shape and size. These features might have potential sensing applications.

Mode-locked ultrashort pulse generation from on-chip normal dispersion microresonators

We describe generation of stable mode-locked pulse trains from on-chip normal dispersion microresonators. The excitation of hyperparametric oscillation is facilitated by the local dispersion disruptions induced by mode interactions. The system is then driven from hyperparametric oscillation to the mode-locked state with over 200 nm spectral width by controlled pump power and detuning. With the continuous-wave-driven nonlinearity, the pulses sit on a pedestal, akin to a cavity soliton. We identify the importance of pump detuning and wavelength-dependent quality factors in stabilizing and shaping the pulse structure, to achieve a single pulse inside the cavity. We examine the mode-locking dynamics by numerically solving the master equation and provide analytic solutions under appropriate approximations.

Measurement of temperature profile induced by the optical signal in fiber Bragg gratings using whispering-gallery modes

The temperature sensitivity of whispering-gallery mode resonances of an optical fiber is exploited to measure thermal effects induced by an optical signal of moderate power along a fiber Bragg gating (FBG). The UV inscription technique used for the fabrication of FBG introduces a permanent change in the absorption coefficient of the fiber; thus, thermal effects are expected. The resonance wavelength shift of whispering-gallery modes provides information about the temperature change in the fiber, point to point. We present the experimental characterization of the thermal effects in FBG as a function of the wavelength and the power of the launched optical signal through the grating.

Controling the coupling properties of active ultrahigh-Q WGM microcavities from undercoupling to selective amplification

Ultrahigh-quality (Q) factor microresonators have a lot of applications in the photonics domain ranging from low-threshold nonlinear optics to integrated optical sensors. Glass-based whispering gallery mode (WGM) microresonators are easy to produce by melting techniques, however they suffer from surface contamination which limits their long-term quality factor to a few 10⁸. Here we show that an optical gain provided by erbium ions can compensate for residual losses. Moreover it is possible to control the coupling regime of an ultrahigh Q-factor three port microresonator from undercoupling to spectral selective amplification by changing the pumping rate. The optical characterization method is based on frequency-swept cavity-ring-down-spectroscopy. This method allows the transmission and dispersive properties of perfectly transparent microresonators and intrinsic finesses up to 4.0 × 10⁷ to be measured. Finally we characterize a critically coupled fluoride glass WGM microresonator with a diameter of 220 μm and a loaded Q-factor of 5.3 × 10⁹ is demonstrated.

Frequency agile microwave photonic notch filter with anomalously high stopband rejection

We report a novel class microwave photonic (MWP) notch filter with a very narrow isolation bandwidth (10 MHz), an ultrahigh stopband rejection (>60 dB), a wide frequency tuning (1-30 GHz), and flexible bandwidth reconfigurability (10-65 MHz). This performance is enabled by a new concept of sideband amplitude and phase controls using an electro-optic modulator and an optical filter. This concept enables energy efficient operation in active MWP notch filters, and opens up a pathway toward enabling low-power nanophotonic devices as high-performance RF filters.

Si₃N₄ ring resonator-based microwave photonic notch filter with an ultrahigh peak rejection

We report a simple technique in microwave photonic (MWP) signal processing that allows the use of an optical filter with a shallow notch to exhibit a microwave notch filter with anomalously high rejection level. We implement this technique using a low-loss, tunable Si₃N₄ optical ring resonator as the optical filter, and achieved an MWP notch filter with an ultra-high peak rejection > 60 dB, a tunable high resolution bandwidth of 247-840 MHz, and notch frequency tuning of 2-8 GHz. To our knowledge, this is a record combined peak rejection and resolution for an integrated MWP filter.

Laser emission from the whispering gallery modes of a graded index fiber

Whispering gallery mode (WGM) laser emission has been observed from rhodamine B doped polymer optical graded index (GI) fiber by transverse pumping with a frequency doubled Q-switched Nd:YAG laser. The propagation and confinement of these modes were also observed. A variation in the free spectral range from 0.29 to 1.24 nm is obtained along the length due to the confinement of WGMs in the GI fiber.

Tunable narrowband fiber laser with feedback based on whispering gallery mode resonances of a cylindrical microresonator

Narrowband filtering based on whispering gallery modes of a slightly tapered cylindrical optical microresonator was used to implement a tunable narrowband erbium-doped fiber laser. The laser can be set to emit a single longitudinal cavity mode (single frequency), although the laser cavity is a few meters long. In the single-frequency regime the laser can emit a maximum power of 0.380 mW with a linewidth <35 kHz and a signal-to-noise ratio exceeding 50 dB. Tunability is achieved by sliding the excitation point along the microcylinder. A tuning range of 1.16 nm is demonstrated.

Metallic diffraction grating enhanced coupling in whispering gallery resonator

For the first time, metallic diffraction grating is investigated to enable efficient coupling in the whispering gallery resonator (WGR). Six-fold field enhancement in the resonator is achieved with respect to their dielectric counter-parts. This higher coupling efficiency is attributed to the surface plasmon excitation which drives the whispering gallery mode along the grating. Fano resonances have been observed in optical reflection. With the metallic grating, single-port end-fire WGR configuration becomes possible - a scheme that has not been demonstrated in any other WGR coupling devices. Hence, it serves as a prototype for portable whispering gallery devices potentially useful in sensing, switching and nonlinear applications.

Excitation and interrogation of whispering-gallery modes in optical microresonators using a single fused-tapered fiber tip

We show that whispering-gallery modes (WGMs) in optical microresonators can be excited and detected using a fused-tapered fiber tip (FTFT). The fabrication of FTFTs is simple and inexpensive; they are robust and allow the excitation and interrogation of the resonances with a single fiber. Excitation of high-Q WGMs in silica microcylinders and microspheres is demonstrated.