The Electron-Multiplying Charge-Coupled Device (EM-CCD) shares a similar structure to the CCD except for the
inclusion of a gain register that multiplies signal before the addition of read-noise, offering sub-electron effective readnoise
at high frame-rates.
EM-CCDs were proposed for the dispersive spectrometer on the International X-ray Observatory (IXO) to bring
sub-300 eV X-rays above the noise, increasing the science yield. The high-speed, low-noise performance of the EMCCD
brought added advantages of reduced dark current and stray-light per frame, reducing cooling and filtering
requirements. To increase grating efficiency, several diffracted spectral orders were co-located so the inherent energy
resolution of the detector was required for order separation. Although the spectral resolution of the EM-CCD is
degraded by the gain process, it was shown that the EM-CCD could achieve the required separation.
The RIXS spectrometer at the Advanced Resonant Spectroscopy beamline (ADRESS) of the Swiss Light Source (SLS)
at the Paul Scherrer Institute currently uses a CCD, with charge spreading between pixels limiting the spatial resolution
to 24 μm (FWHM). Through improving the spatial resolution below 5 μm alongside upgrading the grating, a factor of
two energy resolution improvement could theoretically be made. With the high-speed, low-noise performance of the
EM-CCD, photon-counting modes could allow the use of centroiding techniques to improve the resolution. Using
various centroiding techniques, a spatial resolution of 2 μm (FWHM) has been achieved experimentally, demonstrating
the benefits of this detector technology for soft X-ray spectrometry.
This paper summarises the use of EM-CCDs from our first investigations for IXO through to our latest developments in
ground-based testing for synchrotron-research and looks beyond to future possibilities.
In the last decades, diode laser systems conquered the spectral range step-by-step from conventional gas lasers, wherever
they can match or outperform in optical specifications. Although highly anticipated in the ultraviolet wavelength range,
for instance in high-resolution lithography, biological and medical fluorescence applications or holography, cw single
frequency operation of sufficient power has been a challenge for diode or other solid state laser systems. Currently this
scope is still dominated by the HeCd gas laser, emitting at 325 nm with powers of up to 100 mW.
In this paper we present a diode laser system emitting at 325 nm offering the same output power by efficient second
harmonic generation (SHG) of a master oscillator power amplifier (MOPA) at 650 nm.
For the master oscillator a ridge waveguide diode is anti-reflection coated and used in an external cavity diode laser
(ECDL) with grating feedback in Littrow configuration. This setup features a MHz line width (coherence length of
100m), a coarse tuning range from 649 nm to 657 nm and a mode hope free tuning of 20 GHz. In a second step, we use a
tapered amplifier to boost the output from the ECDL to a level of 400 mW, for powering an efficient second harmonic
generation process in an enhancement cavity. Faraday isolators on both ends of the amplifier stage prevent back
reflection and stabilize the single mode operation of the system. Together with astigmatism compensation this yields to a
high spatial quality (M2<1.5) of the amplified beam. The frequency doubling is achieved by using a four mirror bow-tie
enhancement resonator fitted with a Beta-Barium Borate (BBO) crystal. The cavity length is actively locked to the laser
frequency using the Pound-Drever-Hall method.
With this set-up, stable and reliable laser operation is achieved. After a few minutes warm-up time, fixed frequency and
tunable UV output power of more than 100 mW could be generated, opening this important wavelength range for future
Optical techniques based on photon migration are rapidly emerging as a promising alternative and/or augmentation of existing medical imaging modalities. For example, real time studies of hemodynamic changes in brain tissue are possible as a step towards optical functional brain imaging. Time-resolved implementations of these techniques allow for discrimination between scattering and absorption and for depth resolution. They require sub-nanosecond pulsed light sources with high repetition rate and sufficient power for deep enough tissue penetration. Picosecond diode lasers satisfy the clinical demands of economy, compact size, and reliability almost perfectly. Today multi-channel diode laser devices are commercially available and are widely used in diffuse optical imaging and spectroscopy, in particular in optical tomography and breast cancer detection. However, the output powers of these devices are just about sufficient for moderate tissue penetration depths. An improvement that does not compromise the advantages of the diode laser sources is amplification of the diode laser output by means of solid state tapered amplifiers. We present an amplified light source for use in NIR diffuse optical spectroscopy and imaging, providing pulse widths as short as 100 ps, adjustable repetition rates up to 80 MHz, and peak power levels as high as 7 Watts, corresponding to average power levels exceeding 100 mW. In combination with time-resolved photon counting electronics matching the high throughput demands in conjunction with the new source, state-of-the-art systems for diffuse optical imaging can be built. System design features and possible application examples are presented.
Frequency conversion of near-infrared diode lasers provides an efficient method to generate tunable laser radiation in the near-UV, violet and blue-green spectral range. High-power, coherent fundamental laser sources such as master oscillator-power amplifier (MOPA) configurations are now state of the art and commercially available.
A new, highly efficient material for second-harmonic generation (SHG) is Bismuth Triborate ("BiBO", stoichiometry BiB3O6). The material has a high effective non-linearity deff, is non-hygroscopic and transparent for wavelengths between 286 nm and 2.5 μm. Compared to other non-linear crystals, "walk-off" effects between fundamental laser radiation and frequency-doubled beam are considerably lower. We used a BiBO crystal in a resonant doubling cavity to convert the output of a 780 nm, 900 mW tapered amplifier system. A maximum UV power of 400 mW (conversion efficiency 44%) was attained. This value is 3-4 times higher than previous results obtained with LBO or BBO crystals and, to the best of our knowledge, represents the highest tunable cw power of a frequency-converted diode laser.
Gain-switching of laser diodes might be the most convenient way to generate picosecond laser pulses. The outstanding features of gain-switched laser diodes are a rich choice of wavelength and an easy synchronization to an external trigger source. To broaden the field of applications we pushed the peak power to the 10 W level while maintaining the essential characteristics of the laser source. In a master oscillator power amplifier (MOPA) configuration a tapered amplifier is used to increase the output from 10 mW to 160 mW average power. Second harmonic generation is demonstrated in a single pass setup, which results in 6.5 mW average power at 532 nm with a repetition rate of 80 MHz.