Ramgopal Madugani

Terahertz tuning of whispering gallery modes in a PDMS stand-alone, stretchable microsphere

We report on tuning the optical whispering gallery modes (WGMs) in a poly dimethyl siloxane-based (PDMS) microsphere resonator by more than 1 THz. The PDMS microsphere system consists of a solid spherical resonator directly formed with double stems on either side. The stems act like tie-rods for simple mechanical stretching of the microresonator, resulting in tuning of the WGMs by one free spectral range. Further investigations demonstrate that the WGM shift has a higher sensitivity (0.13 nm/μN) to an applied force when the resonator is in its maximally stretched state compared to its relaxed state.

Optomechanical transduction and characterization of a silica microsphere pendulum via evanescent light

Dissipative optomechanics has some advantages in cooling compared to the conventional dispersion dominated systems. Here, we study the optical response of a cantilever-like, silica, microsphere pendulum, evanescently coupled to a fiber taper. In a whispering gallery mode resonator, the cavity mode and motion of the pendulum result in both dispersive and dissipative optomechanical interactions. This unique mechanism leads to an experimentally observable, asymmetric response function of the transduction spectrum, which can be explained using coupled-mode theory. The optomechanical transduction and its relationship to the external coupling gap are investigated, and we show that the experimental behavior is in good agreement with the theoretical predictions. A deep understanding of this mechanism is necessary to explore trapping and cooling in dissipative optomechanical systems.

Linear Laser Tuning Using a Pressure-Sensitive Microbubble Resonator

The tunability of an optical cavity is an essential requirement for many areas of research. Here, we use the Pound-Drever-Hall technique to lock a laser to a whispering gallery mode (WGM) of a microbubble resonator, to show that linear tuning of the WGM, and the corresponding locked laser, display almost zero hysteresis. By applying aerostatic pressure to the interior surface of the microbubble resonator, optical mode shift rates of around 58 GHz/MPa are achieved. The microbubble can measure pressure with a detection limit of 2 × 10-4 MPa, which is an improvement made on pressure sensing using this device. The long-term frequency stability of this tuning method for different input pressures is measured. The frequency noise of the WGM measured over 10 min for an input pressure of 0.5 MPa has a maximum standard deviation of 36 MHz.

Cavity ring-up spectroscopy for dissipative and dispersive sensing in a whispering gallery mode resonator

In whispering gallery mode resonator sensing applications, the conventional way to detect a change in the parameter to be measured is by observing the steady-state transmission spectrum through the coupling waveguide. Alternatively, sensing based on cavity ring-up spectroscopy, i.e. CRUS, can be achieved transiently. In this work, we investigate CRUS using coupled mode equations and find analytical solutions with a large spectral broadening approximation of the input pulse. The relationships between the frequency detuning, coupling gap and ring-up peak height are determined and experimentally verified using an ultrahigh Q-factor silica microsphere. This work shows that distinctive dispersive and dissipative transient sensing can be realised by simply measuring the peak height of the CRUS signal, which may improve the data collection rate.

Packaged Optical Add-Drop Filter Based on an Optical Microfiber Coupler and a Microsphere

In this letter, a packaged add-drop filter composed of a silica microsphere resonator with a diameter of 80 μm and an optical microfiber coupler with a waist diameter of 1 μm is investigated. A one-step fabrication process using UV curable epoxy is shown to stabilize the coupling between the microsphere and the microfiber coupler, which is used for the add and drop ports. The packaged microsphere-microfiber coupler device has a high Q-factor of 1 × 107 in the add-drop filter configuration. This device has excellent features, such as improved ease of fabrication, multiple resonant peaks, and high mechanical stability.

Hollow whispering gallery resonators

In recent years, whispering gallery mode (WGM) devices have extended their functionality across a number of research fields from photonics device development to sensing applications. Here, we will discuss some such recent applications using ultrahigh Q-factor hollow resonators fabricated from pretapered glass capillary. We will discuss device fabrication and different applications that can be pursued such as bandpass filtering, nanoparticle detection, and trapping. Finally, we will introduce our latest results on visible frequency comb generation.

Whispering gallery resonators for optical sensing

In recent years, whispering gallery mode devices have extended their functionality across a number of research fields from photonics to sensing applications. Here, we will discuss environmental sensing applications, such as pressure, flow, and temperature using ultrahigh Q-factor microspheres fabricated from ultrathin optical fiber and microbubbles fabricated from pretapered glass capillary. We will discuss device fabrication and the different types of sensing that can be pursued using such systems. Finally, we will introduce the concept of using cavity ring-up spectroscopy to perform dispersive transient sensing, whereby a perturbation to the environment leads to a frequency mode shift, and dissipative transient sensing, which can lead to broadening of the mode, in a whispering gallery mode resonator.

Development of packaged silica microspheres coupled with tapered optical microfibres

In this research, we present a packaged add–drop filter composed of a silica microsphere resonator and a strongly coupled optical microfiber coupler. A one-step fabrication process using UV curable epoxy is shown to stabilize the microsphere resonator coupled to the microfiber coupler, which is used as add and drop ports. A high Q-factor of 3×107 is obtained at around 780 nm from the packaged microspheres coupled with the microfiber coupler in the add–drop configuration.

Asymmetric response function of the transduction spectrum for a microsphere pendulum

The optical response of a silica microsphere pendulum evanescently coupled to a tapered optical fiber is studied. The pendulum oscillation modulates the microsphere’s whispering gallery mode (WGM) resonance frequencies (dispersive shift) and the external coupling rate (dissipative effect). These effects combine to give an observable modulation in the transmitted optical power so that the tapered fiber also acts as the mechanical motion transducer. This unique mechanism leads to an asymmetric response function of the transduction spectrum, i.e. the amplitude of the transmitted noise depends on laser detuning. This phenomenon can be explained using coupled mode theory with a Fourier transformation. The transduction of the mechanical motion and its relation to the external coupling gap was experimentally investigated and showed good agreement with the theory.

Optical WGMs THz tuning and mechanical modes in a PDMS double-stem resonator

2 THz tuning of optical WGMs is observed in a stretchable PDMS microresonator, demonstrating sensitivities as high as 0.13 nm/μN. The structure also provides the possibility of observing mechanical modes with frequencies in kHz range.