LED wavelength and luminosity shifts due to temperature, dimming, aging, and binning uncertainty can cause large
color errors in open-loop light-mixing illuminators. Multispectral color light sensors combined with feedback circuits
can compensate for these LED shifts. Typical color light sensor design variables include the choice of light-sensing
material, filter configuration, and read-out circuitry. Cypress Semiconductor has designed and prototyped a color sensor
chip that consists of photodiode arrays connected to a I/F (Current to Frequency) converter. This architecture has been
chosen to achieve high dynamic range (~100dB) and provide flexibility for tailoring sensor response. Several different
optical filter configurations were evaluated in this prototype. The color-sensor chip was incorporated into an RGB light
color mixing system with closed-loop optical feedback. Color mixing accuracy was determined by calculating the
difference between (u',v') set point values and CIE coordinates measured with a reference colorimeter. A typical color
precision ▵u'v' less than 0.0055 has been demonstrated over a wide range of colors, a temperature range of 50C, and
light dimming up to 80%.
In this paper we present the development of semiconductor laser systems with output powers reaching 100 W and
linewidths down to 10 GHz. The combination of high power and narrow emission spectrum was achieved through
external resonator configurations based on volume Bragg gratings. By using Bragg gratings with extremely narrow
spectral selectivity we were able to narrower and lock emission spectra of diode lasers, with precise wavelength tuning
achieved by thermal control of the volume grating. The thermal coefficient of our volume gratings was approximately
8 pm/K, which was low enough to guarantee stable frequency operating regime. We implemented successfully two such
schemes for lasers generating at 780 nm and 1.55 μm as pumping sources for Rb vapor and Er-doped solid state lasers,
Diode pumped alkali-vapor (cesium, rubidium and potassium) lasers (DPALs) are attractive sources for high-power
applications due to their high quantum efficiency, excellent optical beam quality and reduced thermal load. DPALs
require optical pump sources that can reliably emit energy within the narrow (about 10 GHz) absorption bands of the
alkali vapor. Single laser diodes (LD) and laser bars (LB) integrated into wavelength selective external cavities with
volume diffraction gratings can simultaneously achieve narrow linewidths and high output power. A diode laser bar with
a volume Bragg grating output coupler emitting at 780 nm has demonstrated a CW output power up to 30 W with a slope
efficiency of 0.8 W/A, a spectral width (FWHM) below 10GHz, and a tunability over 400 pm. The output power of a
diode bar in an external cavity exceeded 90% of the output power of the free-running bar. More than 90% of the laser
emission was absorbed by Rb cell.
High wall-plug efficiency and a wide range of available wavelengths make laser diode arrays preferable for many high-power applications, including optical pumping of solid state lasers. Recently, we designed and fabricated InGaAsP/InP arrays operating at 1.5-μm and In(Al)GaAsSb/GaSb arrays operating at 2.3-μm. We have demonstrated a high continuous-wave (CW) output power of 25 W from a one dimensional laser array and a quasi-CW (q-CW) output power of 110 W from a two dimensional laser array both operating near 1.5-μm. We have obtained a CW output power of 10 W from the 2.3-μm laser array. The 1.5-μm arrays are suitable for resonant pumping of erbium doped solid-state lasers, which require high power optical sources emitting in the narrow erbium absorption bands. Long current-injection pulses produce a considerable temperature increase within the diode laser structure which induces a red-shift of the output wavelength. This thermal drift of the laser array emission spectrum can lead to misalignment with the erbium absorption bands, which decreases pumping efficiency. We have developed an experimental technique to measure the time dependence of the laser emission spectrum during a single current pulse. From the red-shift of the laser emission, we determine the temperature of the laser active region as a function of time.
The spacing between the individual laser emitters has an effect on the array heating. In steady state operation, this spacing is a contributing factor in the non-uniformity of the thermal field within the bar, and thus to the overall thermal resistance of the laser bar. Under pulse operation, the transient heating process can be divided into three time periods; each with its own heat transport condition. It was shown that in the initial period of time the heat propagates within the laser bar structure and the laser bar design (fill factor) strongly affects the active region temperature rise. In the later periods the temperature kinetics is insensitive to the fill factor. This analysis has been verified in experimental studies using the 1.5-μm laser arrays.