Stephen Chakmakjian

LED illumination control and color mixing with Engineered Diffusers

We propose and demonstrate the use of Engineered Diffusers for control and distribution of LED light for general lighting applications. These diffusers are based on refractive microstructures and enable the efficient use of energy by controlling light propagation and directing it to specific regions of space. The microstructures are generally microlens-based arrays with each microlens elements individually designed to meet the desired scatter properties. In addition to light control, Engineered Diffusers can be used for RGB mixing to produce white light with variable color temperature, depending on the RGB content of the source. A single Engineered Diffuser component can be used for efficient color mixing and illumination control. We also discuss the fabrication of Engineered Diffusers by means of a single-point laserwriting method with capability to manufacture the deep refractive structures needed for LED beam shaping.

Deterministic microlens diffuser for Lambertian scatter

We demonstrate the application of deterministic microlens diffusers to generate Lambertian scatter in transmission with high efficiency. Two identical surface diffuser elements are used in tandem to spread light with constant radiance. Design methods are discussed and experimental demonstration is presented.

Multimode Instabilities of Homogeneously Broadened Lasers

The cw homogeneously broadened ring laser appears at first glance to be an ideal laser system in which simple rate equation theory applies. A series of experiments in several different laboratories has shown that this is not-the case, and that in fact such lasers exhibit a number of instabilities. HILLMAN et al. [1] have demonstrated a multimode instability in which a dye laser discontinuously switches from single frequency to two-frequency operation. The separation between the two frequency components is the Rabi frequency or a subharmonic of it, and is sometimes as large as 50 THz. RAYMER et al. [2] have observed many-mode operation of a dye laser with strong intermode competition. MANDEL et al. [3] and KUHLKE et al. [4] have observed a different sort of instability in which the ring laser operates in a single longitudinal cavity mode, but switches between clockwise and counterclockwise direction of propagation around the ring. In the present paper we will describe another dynamical instability in which the laser operates in two longitudinal cavity modes, but switches between the two directions of propagation.

Multimode Instabilities In Cw Dye Lasers

New experimental results are presented which show the behavior of a homogeneously broadened ring dye laser both below and above the threshold for the onset of the two-frequency instability. It is shown that the spectrum of the output of the laser is extremely sensitive to the detuning of the initial lasing frequency from the center of the gain curve. Spectral splittings as large as 340 A are reported. The close connection between this instability and the production of femtosecond mode-locked laser pulses is discussed.

Recent advances in microlens-based projection display screens

In the past few years the microdisplay-based projection television market has been quickly spreading over what used to be the domain of CRT technology. The demand for larger display sizes and improved image quality, together with more accessible pricing is aggressively driving the transition to microdisplays. The screen is the component of the projection system that directly conveys the visual information to the user, responsible for distributing the luminous energy from the light engine. However, the projection screen is far from a simple diffuser and todays performance requirements for display systems require sophisticated screens to meet the resolution, transmission efficiency, and contrast required by visual displays.

Lightguide with controlled numerical aperture for general illumination

Lightguide devices are commonly used in a large number of applications such as light-delivery systems, illumination, and displays. The general approach is to outcouple light from the lightguide without concern for its propagation properties within the guiding material. We introduce the concept of a lightguide with controlled numerical-aperture as it propagates within the guiding substrate and demonstrate the advantages with this approach, compared to other methods commonly used. We also demonstrate the application of our lightguide technology for general illumination with controlled light distribution and high efficiency.

Near Threshold Behavior of Multimode CW Dye Lasers with an Amplitude Modulated Pump

Most applications of lasers today require amplitude stability of the laser. The amplitude stability of the laser is affected by multiplicative fluctuations. These multiplicative fluctuations arise from technical noise sources (e.g. pump fluctuations, mirror vibrations, dust and other gain or loss fluctuations). It is possible to perform accurate studies of these multiplicative effects in lasers since the experimentalist can precisely vary control parameters such as the pump power, cavity loss, and the strength of the multiplicative fluctuations. An effective method of characterizing the response of a laser to multiplicative fluctuations is to measure the power spectrum of the intensity of a laser influenced by wideband multiplicative noise. The intensity of a single mode laser behaves as a low-pass amplifier to such amplitude fluctuations of the pump. Furthermore, the bandwidth of the response decreases as the intensity of the pump is reduced to its threshold value. This low-pass behavior of the intensity of a laser operating near its first threshold is a manifestation of a universal property of the dynamics of nonlinear systems operating near a critical point. These dynamics are present not only in the laser’s intensity but also in the inversion of the atoms and also the absorption of the pump laser. In these experiments we weakly amplitude modulate the power of the pumping argon ion laser and measure the response of a dye laser to this modulated pump. The output power, the fluorescence from the atoms in the upper lasing level, and the fraction of the pump laser power that is absorbed are all measured as a function of the modulation frequency.

Excited-State Relaxation Dynamics in a Continuous-Wave Dye Laser

A recently observed instability in a continuous-wave dye laser1–3 is remarkable for its simple experimental signatures and its elusion from theoretical description. The essential signatures of the experiment axe: (1) the threshold for instability is much less than nine times above threshold, (2) a bichromatic spectrum emerges above the instability threshold quenching the single frequency operation, and (3) the output power of the dye laser experiences discontinuous jumps and hysteresis as the pump power is varied about the instability threshold. A recently developed theory,4–6 that includes a band of levels in the lower lasing state, successfully predicts a lower instability threshold, the development of a bichromatic spectrum, and for a certain range of parameters, predicts discontinuous jumps in the output intensity at the second instability threshold. Specifically, when the energy decay rate from the excited state exceeds the energy decay rate of the lower states, the theory predicts discontinuous intensity jumps at the second instability threshold. The theory does not predict discontinuous jumps when the energy decay rate from the excited state is less than that of the lower states. The combination of these theoretical predictions and the results of the experiments led to the hypothesis that there may be long-lived states in the lower lasing band of the dye molecule. In this paper we will discuss experiments that directly investigate this possibility. There is also the possibility that the theory may need some slight modifications to better model the experimental conditions. For instance, as originally developed, the theory does not consider spontaneous emission from the excited state into the lower band.